ARSENIC REMOVAL BY PHYTOFILTRATION AND SILICON TREATMENT A POTENTIAL SOLUTION FOR LOWERING ARSENIC CONCENTRATIONS IN FOOD CROPS
Arifin Sandhi
April 2017
TRITA-LWR PHD-2017:02 ISSN 1650-8602 ISBN 978-91-7729-332-3
Arifin Sandhi TRITA-LWR PHD-2017:02
© Arifin Sandhi 2017 Doctoral Thesis in Land and Water Resources Engineering Department of Sustainable Development, Environmental Science & Engineering KTH Royal Institute of Technology SE-100 44 STOCKHOLM, Sweden Reference to this publication should be written as: Sandhi, A (2017) “Arsenic removal by phytofiltration and silicon treatment-A potential solution for lowering arsenic concentrations in food crops” TRITA-LWR PHD-2017:02
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Dedicated to My Parents & the millions of people affected by arsenic contamination around the globe
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SUMMARY IN SWEDISH Halvmetallen arsenik (As) tillhör ett av de grundämnen som blivit ett globalt hälsohot. Mer än 100 miljoner människor runt om i världen har redan direkt eller indirekt påverkats av denna metall som en förorening. Befolkningen i många utvecklingsländer får sitt dricksvatten från lokala brunnar med arsenikhaltigt vatten och där reningsmetoder saknas. Förutom till dricksvatten, används grundvatten till jordbruket och om det är förorenat med arsenik kan det tas upp i livsmedelsgrödor. Vid odling av ris är risfälten ofta vattendränkta för att skapa förutsättning för tillväxt och omfattande användning av grundvatten påverkar upptaget av arsenik i riskorn. Ett antal förorenade områden finns särskilt i södra Asien, där innehållet av arsenik i ris lokalt är relativt högre än ris från andra delar av världen. Jämfört med maximalt tillåtet innehåll i dricksvatten (10 mikrogram/L eller 10 ppb), så finns inte regler för tillåtet innehåll i livsmedel i flertalet utvecklingsländer. I avhandlingen undersöktes innehållet av arsenik i hybridiserade sorter av ris och i lokala rissorter (Oryza sativa) för att bestämma variationen beroende på vilka jordbruksmetoder (t.ex. bevattningskrav) som använts. Resultaten visade för såväl riskorn som skal att det beror på den s.k. ackumuleringsfaktorn (AF). Detta AF värde verkade påverkas av den specifika odlingsjorden och koncentration av arsenik i denna samt de olika rissorterna. Genom screening av rissorternas AF värde kan man få en effektiv strategi för att minska upptaget av arsenik i livsmedelskedjan.
Följaktligen bör också innehållet av arsenik i bladgrönsaker uppmärksammas särskilt då de konsumeras som råa grönsaker (t.ex. sallad). Användningen av kisel (Si), ett av de mest förekommande grundämnena i jordskorpan och med en kemi som liknar arsenikens, har befunnits kunna minska upptaget i spannmålsgrödor t exvete. I avhandlingen undersöktes hur koncentrationen av arsenik i sallad (Lactuca sativa) påverkades av tillförsel av kisel och det konstaterades att det kunde minska koncentrationen i den ätliga delen av salladen. Slutsatsen var att kisel kan tillföras jorden för att arsenikupptaget ska minska till grödor och grönsaker, särskilt i förorenade områden. Det är känt att vissa växter har särskilt stor förmåga att ta upp arsenik och ansamla det i vävnaden. Denna egenskap kan utnyttjas vid en saneringsmetod av arsenikförorenad mark eller vatten som kallas fytoremediering. Jämfört med mekanisk och kemisk sanering, har växtbaserade saneringstekniker blivit uppmärksammade på grund av att de är ekonomiska och har fördelar ur hållbarhetsperspektiv. Bland de växtarter som inte undersökts så mycket är fytoremediering mha undervattensväxter s.k. fytofiltrering. En akvatisk mossart (Warnstorfia fluitans) som växer i gruvområden i norra delen av Sverige, där hög koncentration av arsenik konstaterades i vattensystemen, studerades med avseende på fytofiltrering. I avhandlingen undersöktes därför om denna vattenmossa har förmåga att binda arsenik som är löst i vatten och
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om dess upptag från vattnet kan bidra till att minska innehållet av arsenik i livsmedelsgrödor.
Vattenmossan (W. fluitans) odlades i vatten med låg koncentration av arsenik (74 µg As/L) och försöket visade ett snabbt avlägsnande (upp till 82%) inom kort tid. Mossan befanns fast fixera oorganiska arsenikformer (arsenat och arsenit) i sin växtvävnad. Slutsatsen av försöket var att denna vattenmossa har hög potential för fytofiltrering av arsenik i synnerhet i tempererade områden. När det gäller effekter av vattnets pH, temperatur och syresättning på mossans arsenikupptag så visade det sig att låga pH-nivåer (pH 2,5) orsakade stress hos mossan och lägre upptag oavsett oorganisk form av arsenik. Förmågan att hålla kvar arsenik i vävnaden testades vid både låga (12°C) och höga (30°C) vattentemperaturer. Inga tecken syntes på att arsenik skulle frisläppas utom när vattnet innehöll arsenat efter behandlingstiden 96 timmar. Lågt syreinnehåll i vattnet visade sig påverka arsenik upptaget positivt jämfört med hög syresättning. Möjligheten att använda den studerade vattenmossan som fytofilter i en anlagd våtmark för att rena vattnet innan det används för salladsodling, visade att metoden avsevärt skulle minska innehållet av arsenik i ätbara delar.
Slutsatsen är att lämpliga strategier att minska koncentrationen av arsenik i livsmedel t ex ris, är att tillämpa screening av AF-värdet på grödor i hot-spot områden eller att tillföra kisel under odlingen. Förutom dessa verkar den studerade vattenmossan ha potential att ackumulera arsenik från förorenat vatten, åtminstone i tempererade klimatområden, och därmed bidra till en hållbar och miljövänlig lösning för att minska innehållet i grödor vid bevattning.
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ACKNOWLEDGMENT First, I would like to warmly acknowledge my supervisor at Stockholm University, Maria Greger, who has always been full of creativeness, motivation and support. The research question that I focused on in my PhD thesis was very unique and we could not find any external agency funding in the beginning, despite intense efforts. Maria realised the magnitude and significance of this problem and in discussions she encouraged me to take on the challenge. I been working with her since my Master’s studies at Stockholm University in 2009 and she is really a great group leader and the best supervisor any student could ask for. I was fortunate to have such a mentor in the initial period of my research career. I strongly believe the passion for research in my mind was greatly nourished by her continuous support and encouragement. I wish to be a similar example to my future students, providing the same level of inspiration, motivation and kindness that she gave to me.
I also want to thank my principal supervisor at KTH, Gunno Renman, for providing real-time support at a very crucial time in my PhD studies, without you it would have been almost an impossible task to move forward. Gunnar Jacks, for providing practical help during the initial period of my PhD studies at KTH and also during the reviewing process for the rice manuscript.
Tommy Landberg, I deeply acknowledge your support since my postgraduate studies started in Sweden. Your kind smile and help in the learning process in the laboratory made me confident in different kinds of critical analysis, especially arsenic speciation in plant samples. I am sure that in my future career, I will always miss your smile in the laboratory. My former colleague from the Plant Metal Research Group at Stockholm University, Claes Bergqvist, I owe you great thanks. You have given me so much practical information about arsenic analysis. Besides the laboratory, you were one of the best office roomies in my whole life. I will miss your cheerful talk in lunch hours. Tariq Javed, thanks a lot for your kind support during your time in the Plant Metal group. Sylvia Lindberg, I will never forget your caring smile during my whole study period. Special thanks to Hsinyu Wang for your outstanding work and help during one of my experiments. You did great work during your exchange study time in our group. A number of guest researchers worked in our Plant Metal group during my PhD studies, I am really thankful to Kader vai, Kabir vai, Marek, Basir, Jieyuan, Maria for your nice company.
Staffan Mellvig, thank you very much for your time and helpful advice. You inspired me a lot to go to the gym after work! Eva Roubeau, your exchange time in our Plant Metal group was so memorable and joyful, many thanks from me. Nicoleta and Lillvor, thanks a lot for your help in the laboratory.
Thanks to Olle Wahlberg, Dept. of Chemical Engineering, KTH, for giving me the chance to take part in the water chemistry course, which I enjoyed very much.
I also acknowledge my co-authors in my scientific articles: Meththika Vithanage, Beata Dabrowska, Arun Mukherjee and Prosun Bhattacharya.
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Head of the department Ove Eriksson, former head Peter Hambäck and head of Plant Physiology Division Lisbeth Jonsson from the Dept. of Ecology, Environment and Plant Sciences (DEEP), Stockholm University, are greatly acknowledged for giving me their permission to continue my research work and to use the facilities at SU.
The faculties at DEEP, Stockholm University, especially Ulla, Jonas, Clare, Mats, Barbro, Ellen, Sara, Lotta, Lina, and PhD student Konrad, Pil, Thomas, Van, Chen, Olle and Marco, all of you are greatly acknowledged for your support and good company during my time in DEEP. Thanks also to DEEP staff who provided administrative and logistics support during my studies, especially Siw, Åsa, Erica, Johan, Lars, Erik, Susanne, Anna, Ingela, Ahmed and Peter.
Special thanks to my work colleagues and friends Tobias Robinson, Clara Neuschütz and Daniel Nordstrand for making my study period a good time.
To my former and present colleagues from Land and Water Resources Engineering Division, KTH, Jon Petter, Helfrid, Ashis, Carin, Alireza, Agnieszka, Lily, Zou, Rajabu, Ezekiel, Lea and so many others! Thanks a lot – I had a really good time there.
I was lucky to have an amazing set of Bangladeshi friends in Stockholm: Shammeda, Sandipda, Zamiul Vai, Surovi Vabi, Badrul Vai, Shatu Vabi, Bipu Vai, Hasan Vai, Fahima Vabi, Tammim, Emon Vai, Rekha Apu and others. Interacting with all of you gave me the much needed work-life balance during my PhD studies. Moreover, I am really grateful to Muradul Islam – your positive advice has helped me a lot since I first arrived in Sweden.
I would like to acknowledge my father (Muhammad Abdul Malek), and my beloved mother (Shahana Parveen Dora) and younger brother (Dr. Sudipta Moitry). The family is a place for me where I get the energy for my work and life. I will always treasure many sweet memorable moments with my family. I especially mention my mom as she inspired me to pursue PhD studies, which she once dreamed of doing. I have learnt one important life lesson from her: ‘Never give up’.
I must say I am lucky to have a wonderful wife, Nasrin Haque Shampa, who has been a very caring & loving partner during my PhD. She worked hard as a medical doctor and also did most of the household work when I was occupied in research in the last two years of my PhD studies. Without her patience and sacrifice, I could not have completed this thesis.
I would like to dedicate this thesis to the millions of people around the globe who are still suffering from arsenic contamination in water and soil. I believe our findings on remediation of arsenic from the water system in an eco-friendly, sustainable approach can assist in future reduction of arsenic pollution in the northern hemisphere.
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Part of the research in this thesis was performed at the Plant Metal Group Laboratory, Department of Ecology, Environment and Plant Sciences, Stockholm University, Sweden. Funding was provided by the Swedish Development Agency (SIDA). Parts of the project were funded by C.F. Lundströms Stiftelse, the Environment Foundation (Miljöfonden) of the Swedish Engineers’ Association. The Knut and Alice Wallenberg Foundation is also gratefully acknowledged for a travel grant in 2013.
Stockholm, 2017
Arifin Sandhi
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TABLE OF CONTENTS Summary in Swedish ...... v Acknowledgment ...... vii Table of contents ...... xi List of appended papers and my contributions ...... xiii Nomenclature and abbreviations ...... xv Abstract ...... xvii 1. Introduction ...... 1 2. Aim and Scope ...... 2 3. Background ...... 3 3.1. Arsenic chemistry 4 3.2. Arsenic content in water and food 5 3.3. Effect of arsenic on human health 7 3.4. Availability of arsenic species to plants 7 3.5. Arsenic accumulation in plants 9 3.5.1. Terrestrial plants 9 3.5.2. Aquatic plants 10 3.6. Remediation of arsenic contamination 11 3.6.1. Conventional remediation 11 3.6.2. Phytoremediation 12 3.6.3. Arsenic phytofiltration and aquatic macrophytes 13 3.7. Effect of abiotic factors on arsenic uptake 14 3.7.1. pH level 15 3.7.2. Temperature regime 16 3.7.3. Oxygen supply 16 3.7.4. Silicon application 17 3.8. Reducing arsenic content in the diet 17 4. Materials and methods ...... 19 4.1. Experimental design and comments on materials and methods 19 4.1.1. Arsenic geochemistry in Sweden and Bangladesh 19 4.1.2. Selection of plant species 20 4.1.3. Arsenic analysis 21 4.1.3.1. Soil 21 4.1.3.2. Water 21 4.1.3.3. Plant materials 21 4.1.3.3.1. Rice grain and husk 21 4.1.3.3.2. Lettuce 22 4.1.3.3.3. Moss 23 4.1.3.4. Arsenic species analysis 23 5. Results and discussion ...... 25 5.1. Selection of crop cultivars and plants for low arsenic accumulation 25 5.1.1. Arsenic content in rice cultivars in arsenic hotspots and uptake in humans 25 5.1.2. Arsenic accumulation in lettuce 27 5.2. Effect of silicon on arsenic species uptake in lettuce 27 5.3. Lowering the arsenic concentration in irrigation water and soil 28 5.3.1. Arsenic phytoremediation and the phytofiltration potential of aquatic moss 28 5.3.2. Arsenic removal efficiency and bioconcentration factor 29 5.3.3. Arsenic absorption-adsorption capacity in moss 30 5.3.4. Arsenic species effect on arsenic uptake on moss 31 5.3.5. Effect of abiotic factors on arsenic uptake in moss 32 5.3.5.1. Variation with pH level in water 32
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5.3.5.2. Effect of temperature regime 33 5.3.5.3. Influence of oxygenation 34 5.3.6. Feasibility of aquatic moss as a phytofilter candidate 34 5.3.6.1. Combined cropping method 34 5.3.6.2. Moss-based constructed wetland system 35 6. Conclusions ...... 37 References ...... 39
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LIST OF APPENDED PAPERS AND MY CONTRIBUTIONS Paper I
Vithanage, M., Dabrowska, B., Mukherjee, A.B., Sandhi, A. & Bhattacharya, P. (2012). Arsenic uptake by plants and possible phytoremediation applications: A brief overview. Environmental Chemistry Letters 10(3), 217-224. Reprinted with permission from Springer.
I was involved in writing the phytoremediation part of this paper.
Paper II
Greger, M., Bergqvist, C., Sandhi, A. & Landberg, T. (2015). Influence of silicon on arsenic uptake and toxicity in lettuce. Journal of Applied Botany and Food Quality 88, 234-240. Reprinted with permission.
I performed part of the experimental work, arsenic species extraction and analysis, and minor of the writing.
Paper III
Sandhi, A., Greger, M., Landberg, T., Jacks, G., & Bhattacharya, P. Arsenic concentrations in local aromatic and high-yielding hybrid rice cultivars and the potential health threat: A study in an arsenic hotspot. Accepted for publication in Environmental Monitoring and Assessment, Reprinted with permission from Springer. DOI: 10.1007/s10661-017-5889-3
I was involved in planning, did the field work, sampling, laboratory work and data analysis and carried out most of the writing.
Paper IV
Sandhi, A., Landberg, T. & Greger, M. Phytofiltration of arsenic by aquatic moss (Warnstorfia fluitans). Submitted to Environmental Pollution.
I was involved in planning and performed almost all experimental and laboratory work and data analysis and most of the writing.
Paper V
Sandhi, A., Landberg, T. & Greger, M. Effect of pH, temperature regime and oxygenation on arsenic phytofiltration by aquatic moss (Warnstorfia fluitans). Submitted to Ecological Engineering.
I was involved in planning and performed all experimental and laboratory work and data analysis and most of the writing.
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Other relevant peer-reviewed publications not appended to this thesis
Sandhi, A., Bhattacharya, P., Greger, M. & Jacks, G. (2013). Arsenic in irrigation water: A threat for rice cultivation? In: Proceedings of 12th International Conference on the Biogeochemistry of Trace Elements (ICOBTE), Athens, USA, p. 463.
Greger, M., Sandhi, A., Nordstrand, D. & Bergqvist, C. (2010). Water cleaning from toxic elements using phytofiltration with E. canadensis. In: Proceedings of 4th International COST Action 637 Conference, Kristianstad, Sweden, pp. 183-187.
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NOMENCLATURE AND ABBREVIATIONS AAS Atomic absorption spectrophotometry AF Accumulation factor As Arsenic BCF Bioconcentration factor BGS British Geological Survey DEEP Department of Ecology, Environment and Plant Sciences DMA Dimethylarsenic acid DW Dry weight EU European Union FW Fresh weight (of biological samples) HPLC High pressure liquid chromatography ICP-OES Inductively coupled plasma-optical emission spectroscopy MIP Major intrinsic protein PC Phytochelation SGU Geological Survey of Sweden Si Silicon
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ABSTRACT Use of arsenic-rich groundwater for crop irrigation can increase the arsenic (As) content in food crops, e.g. rice and lettuce, and act as a carcinogen, compromising human health. Phytofiltration is a possible technique for removing arsenic from irrigation water, but aquatic plants or macrophytes with good capacity for removing arsenic first need to be identified. The aquatic moss species Warnstorfia fluitans grows naturally in mining areas in northern Sweden, where high concentrations of arsenic occur in lakes and rivers. This species was selected as a model for field, climate chamber and greenhouse studies on factors governing arsenic removal and arsenic phytofiltration of irrigation water, thus reducing arsenic concentrations in food crops. The arsenic and silicon (Si) concentrations in soil, water and plant samples were measured by AAS (atomic absorption spectrophotometry), while arsenite and arsenate species were determined using AAS combined with high pressure liquid chromatography (HPLC) with an anion exchange column. A review of the literature showed that successful arsenic phytoremediation depends on the plant species used and the heavy metal and metalloid content of the contaminated medium. The arsenic content in grains of hybrid and local aromatic rice (Oryza sativa) cultivars with differing arsenic accumulation factor (AF) values was investigated in an arsenic hotspot in Bangladesh with different soil arsenic concentrations. The results showed that arsenic AF was important in determining arsenic-safer rice cultivars for growing in an arsenic hotspot. The study based on silicon effect on arsenic uptake in lettuce showed that arsenic accumulation in lettuce (Lactuca sativa) could be reduced by silicon addition, with arsenite shoot/root distribution being particularly strongly influenced. The aquatic moss had good phytofiltration capacity, with fast arsenic removal of up to 82% from a medium with low arsenic concentration (1 µM). Extraction analysis showed that inorganic arsenic species were firmly bound inside moss tissue. Absorption of arsenic was relatively higher than adsorption in the moss. Regarding effects of different abiotic factors, plants were stressed at low pH (pH 2.5) and arsenic removal rate was lower from the medium, while arsenic efflux occurred in arsenate-treated medium at low (12°C) and high (30°C) temperature regimes. Besides these factors, low oxygenation increased the efficiency of arsenic removal from the medium. Finally, combining W. fluitans as a phytofilter with a lettuce crop on a constructed wetland significantly reduced the arsenic content in edible parts (leaves) of lettuce. Thus W. fluitans has great potential for use as an arsenic phytofilter in temperate regions. It could also be a feasible solution for reducing the arsenic content in irrigation water used for food crops.
Keywords: aquatic moss, grain, rice, lettuce, macrophyte, phytoremediation, speciation, temperature, oxygenation, wetland.
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1. INTRODUCTION The presence of heavy metals and metalloids in water is considered a major water quality problem that leads to a number of harmful effects in the biosphere. Among the different metal(loid) contaminants in water, arsenic (As) has entered the spotlight because of its ubiquitous nature in various parts of the world and carcinogenic effects on humans (Mandal & Suzuki, 2002). Many global water systems are overwhelmed by arsenic pollution, which is consequently one of the greatest concerns for human health quality in some regions. A number of eco-toxicological effects of arsenic pollution have been identified through experimental analysis and regional surveys (Chen et al., 2015). The arsenic water quality problem has become a global concern as it leads to biological, societal and economic consequences for regions such as South Asia, South East Asia, South America and Central America. For the above-mentioned reasons, sustainable management of water resources is becoming a world-wide priority. The importance of water quality is also reflected in the recently announced sustainable development goals (SDG) of the United Nations, according to which sustainable management of water resources (Goal no 6.3) is important, as the availability of clean water is inextricably tied to health, food security, climate change and the resilience of ecosystems (United Nations, 2015).
Arsenic accumulation in food crops and vegetables occurs mainly in two ways, either when grown in arsenic-contaminated soils or by application of As-rich groundwater for irrigation (Heikens et al., 2007). One study found that Asian-Americans tend to eat more rice than the average American, and this high consumption of rice could increase the arsenic content in the body (Gilbert-Diamond et al., 2011). Another investigation on the arsenic content in rice cultivars found that the concentration present in some cultivars can subsidize between 33 and 317% of the maximum tolerable daily intake (MTDI) of arsenic in Bangladesh (Williams et al., 2005). The arsenic content in rice grains is mainly taken up from paddy soil and/or from arsenic-rich soil (Meharg & Rahman, 2003). High arsenic accumulation have been also found in cereals (wheat) and few leafy vegetables besides rice (Huq et al., 2006; Greger & Landberg, 2015). Therefore, screening of arsenic concentrations in different kinds of rice cultivars and reduction of arsenic content in the vegetables by using antagonistic element or additive could be a useful option for arsenic mitigation strategies in human food chain.
A number of conventional remediation techniques can be applied for arsenic removal from soil and water systems. However, most of these treatment techniques are expensive and not eco-friendly and usually not designed for irrigation water purposes. For these reasons as mentioned earlier, developing a sustainable, environment friendly management of irrigation water resources is becoming a worldwide priority.
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This thesis concentrates on complementing the scientific knowledge regarding reductions of arsenic content in the irrigation water by developing eco-friendly water filtration management and screening low arsenic uptake cereal varieties and application of additive for reduction of arsenic accumulation in the vegetables for less arsenic ingestion in humans.
2. AIM AND SCOPE Apart from elevated arsenic concentrations in drinking water, elevated levels of arsenic in food crops are a threat to human health. The overall aim of this thesis work was therefore to improve knowledge about the accumulation of arsenic in food plants and to study possible ways to decrease this accumulation by using aquatic plant-based phytofiltration techniques and silicon application. In this thesis, I addressed the following general research questions, in order to meet the overall aim:
Is it possible to decrease arsenic intake from food through choice of crop cultivar? Is it possible to decrease arsenic uptake and the effects of arsenic by silicon application? Is it possible to use phytoremediation to remove arsenic? Is phytofiltration using water moss a way forward to achieving clean irrigation water and thereby growing food crops with low arsenic content?
Specific objectives of the different studies in the thesis were:
To study the variation in arsenic accumulation pattern in rice varieties frequently grown in arsenic hotspots and the effect of silicon application on inorganic arsenic species uptake in lettuce.
To evaluate the efficiency of aquatic moss in removing arsenic from the water and determine how different abiotic factors affect the accumulation pattern in the aquatic system.
The studies presented in Papers I-V examined the following questions:
Which phytoremediation applications are currently known for arsenic uptake by plants? (Paper I) Does silicon reduce inorganic arsenic species accumulation in lettuce? Are inorganic arsenic species affected by silicon addition? (Paper II)
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Do hybrid rice cultivars accumulate more arsenic than local rice cultivars? (Paper III) Are local aromatic rice cultivars safer for human consumption than hybrid cultivars? (Paper III) How is arsenic removal efficiency correlated with arsenic uptake pattern in aquatic moss species at different levels of arsenic concentration in water? (Paper IV) What are the differences between absorption and adsorption processes in moss species when they are exposed to arsenic? (Paper IV) What are the effects of abiotic parameters (pH, temperature regimes and oxygenation level) on arsenic uptake and inorganic arsenic speciation in the moss plant? (Paper V)
3. BACKGROUND In view of the scope of this research work, a multidisciplinary approach was required. The focus was on plant ecology, plant physiology and eco-engineering, but also on chemistry and life science. This chapter provides some background based on a literature review and discusses the main factors in relation to the objectives of the thesis.
Arsenic is an ubiquitous element ranked 20th in terms of abundance of elements, while it is the 14th most common element in sea water and the 12th most common in the human body (Mandal & Suzuki, 2002). The average arsenic concentration in crustal and igneous rocks ranges from 1 to 3 mg As/kg, whereas the concentration in sedimentary rock is in a higher range (0.3 to 500 mg As/kg) (Butcher, 2009). The inorganic arsenic species are considered to be class 1 carcinogenic elements and chronic arsenic. Poisoning effects have been detected in different regions of South East Asia (Bangladesh, India, Pakistan, Myanmar, Nepal, Pakistan, Thailand), South America (Bolivia, Argentina), Australia, Canada and few parts of the United States of America (Bhattacharya et al., 2007; Heikens et al., 2007; Butcher, 2009; Zhao et al., 2010). These regions are mainly As-contaminated for geogenic reasons, such as release of arsenic contained in iron oxides into groundwater, weathering of volcanic rock and ashes and geothermal activities (Butcher, 2009; Benner, 2010). On the other hand, a number of anthropogenic activities such as mining, smelting, fossil fuel burning and industrial disposal are significant anthropogenic causes that are mainly responsible for arsenic contamination hotspots in other regions (Wang & Mulligan, 2006).
In Sweden, about 26% of all contaminated sites (approximately 1500 sites) have been found to have arsenic as the primary contaminant or to have arsenic-enriched soil due to industrial activities such as wood impregnation, sawmills, glassworks and
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Figure 1. Arsenic (As) concentration in the Ap horizon of soils in different regions of Sweden, 2013. (Geological map modified with permission from SGU; Ladenberger et al., 2013)
metal industries (Forslund et al., 2010). Northern Sweden has more arsenic-contaminated sites than other areas of Sweden due to large- scale mining activities. It has been estimated that 0.6 million tons of arsenic have been deposited over the years by various industrial processes, including mining and wood impregnation (Routh et al., 2007). The median arsenic concentration in agricultural soil and grazing soil is relatively high in Sweden compared with other Nordic countries such Norway and Finland. According to Swedish Geological Survey (SGU), areas in Bergslagen, Jämtland (alum shale), Skellefteå (Västerbotten) and Boden (Norrbotten) have high levels of arsenic present in sulphidic minerals (Ladenberger et al., 2013) (Figure 1).
3.1. Arsenic chemistry This carcinogenic metalloid is an ubiquitous element in the Earth’s crust, even though it is only present as a trace element. Geochemical sources of arsenic-contaminated soil includes arsenic-rich parent rocks, as arsenic substitutes for silicon (Si), aluminium (Al) and iron (Fe) in silicate minerals (Fitz & Wenzel, 2002). Both inorganic and organic forms of arsenic are present in soil (Figure 2) (Heikens et V - al., 2007). The major inorganic forms are arsenate, As (H2AsO4 ), III and arsenite, As (H3AsO3). Arsenic has four different oxidation states: -3, 0, +3, and +5, with the latter two primarily being present
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Figure 2. The major arsenic (As) compounds present in the environment. These include inorganic arsenic species (left hand box) and organic species (right hand box). (Modified from Zhao et al., 2010) in the soil system (Zhao et al., 2010). Redox potential is one of the important factors controlling inorganic arsenic speciation in soil. Along with inorganic arsenic species, organic (methylated) forms are also present in soil, e.g. monomethylarsonic acid (MMA:
CH3AsO(OH)2), dimethylarsinic acid (DMA: (CH3)2AsOOH) and trimethylarsine oxide (TMAO: (CH3)3AsO). These organic arsenic species are generally present as minor constituents in soil, but can also occur in high concentrations in soil due to flooded conditions, especially MMA and DMA (Fitz & Wenzel, 2002; Heikens et al., 2007)(Fig. 2).The organic species of arsenic can also be produced from inorganic arsenic species by methylation mediated by soil microorganisms and algae (Zhao et al., 2010).
Besides a number of biotic and abiotic factors, the pH of the medium and redox conditions are reported to be the two most important factors responsible for inter-conversion of arsenate and arsenite (Zhao et al., 2009). In aerobic soil conditions (high redox potential), arsenate is the dominant inorganic arsenic species, while in anaerobic or submerged soil (low redox potential) arsenite is more abundant (Fitz & Wenzel, 2002). Presence of phosphate-
phosphorus (PO4-P) in soil also plays an important role for arsenic behaviour, as phosphate is an analogue of arsenate. It has been suggested that addition of phosphate to aerobic soil helps to increase the mobility and availability of arsenate (Heikens et al., 2007). 3.2. Arsenic content in water and food Arsenic is mobilised into the water system either by geogenic or anthropogenic factors. The concentration of arsenic in the water system is greatly influenced by factors such as: sources of arsenic, mobility of arsenic species and local geological conditions. More
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specifically, arsenic is mobilised into groundwater as arsenite under moderately reducing conditions with reduction of ferric oxyhydroxides (Bhattacharya et al., 1997).
In arsenic-rich regions and regions influenced by mining, the groundwater and geothermal water can contain up to 85 000 µg/L, whereas low concentration of arsenic are found in natural lakes, rivers and sea water (Smedley & Kinniburgh, 2002). The mobilisation of arsenic in the groundwater as arsenite is mainly due to anaerobic conditions in the water system (Reynolds et al., 1999; Smedley & Kinniburgh, 2002).
Due to the carcinogenic effects in humans, the World Health Organization (WHO) has set a maximum limit for arsenic concentration in drinking water of 10 µg/L (ppb) (WHO, 2011). Most countries around the globe follow this WHO guideline, but in Bangladesh the maximum limit for arsenic concentration in drinking water has been left at 50 µg/L or 50 ppb (BGS and DPHE, 2001). In Sweden, the maximum limit for arsenic concentration in water differs depending on intended use; for drinking water it is 10 µg/L, while for other water 50 µg/L is the maximum concentration permitted in Swedish water resources (Naturvårdsverket, 2011). The Food and Agriculture Organization (FAO) has set a guideline value whereby the maximum arsenic content in irrigation water is 100 µg/L (Ayers & Westcot, 1985). However, to the best of my knowledge, there is no separate maximum limit for arsenic concentration in irrigation water in Bangladesh.
The type 1 carcinogenic metalloid arsenic enters the human food chain not only via drinking water, but also via different food components. A number of studies have found that using arsenic- rich groundwater for irrigation increases the arsenic concentration in food crops, and especially in rice (Abedin et al., 2002; Meharg & Rahman, 2003; Williams et al., 2005; Norton et al., 2009). Besides rice, a few other foodstuffs (mainly leafy vegetables such as arum (Colocasia gigantea), gourd leaf (Cucurbita lagenaria), water spinach (Ipomoea aquatic), lettuce (Lactuca sativa) etc. have high levels of arsenic in their consumable parts (Huq et al., 2006; Paper II). In the past two decades, a wide range of studies have found high levels of arsenic loads in rice grains in the laboratory and under field conditions. However, while there are world-wide maximum arsenic concentration guidelines for water, corresponding maximum arsenic concentration guidelines for foodstuffs are currently not applied globally. The reason for the delay in establishment of maximum limits for arsenic concentrations in foodstuffs is a lack of concern about the potential arsenic pathway through the human food chain. However, it has been discovered recently that rice-based food products (e.g. infant formula) could work as a pathway of high-level arsenic ingestion in the human food chain, even in areas not contaminated with arsenic (Ljung et al., 2011).
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While the guidelines for safe drinking water are widely applied, only a few regions in the developing countries have introduced a maximum limit for arsenic concentration in food. In 2013, China introduced a regulation limiting the maximum total arsenic concentration in rice and vegetables to 0.5 mg/kg, while inorganic arsenic concentration in rice should be less than 0.2 mg/kg (Jennifer & Jie, 2014). Australia previously introduced a maximum limit on arsenic concentration in vegetables of 1 mg/kg on a fresh weight basis (Huq et al., 2006). Lastly, in early 2017, Sweden introduced a maximum limit for inorganic arsenic concentration in rice grains and rice-based products. This limit is in compliance with European Union (EU) regulations and sets the inorganic arsenic concentration in rice and vegetables to a maximum of 0.2-0.3 mg As/kg, while baby food may only contain a maximum of 0.1 mg As/kg on a wet weight basis (Ankarberg et al., 2015).
3.3. Effect of arsenic on human health Arsenic enters the human food chain mainly in two ways: through using arsenic-rich groundwater as a drinking water source and through food consumption. A relatively low percentage of arsenic in the food chain comes from the food compared with drinking water. A number of chronic poisoning symptoms can arise in the human body, such as skin lesions and problems in lung, liver, blood circulation, immune system etc., due to long-term ingestion and exposure to arsenic (Saha et al., 1999; Gilbert-Diamond et al., 2011). Most human organs are affected by arsenic exposure, but the kidney is the most sensitive organ to toxicity (Jomova et al., 2011). The earliest adverse health effect reported due to continuous drinking of arsenic-rich groundwater in South Asia was pigmentation in the human skin and keratosis (Guha Mazumder & Dasgupta, 2011). Besides skin pigmentation, one of the important diseases in humans caused by arsenic, arsenicosis, is mostly seen in affected regions in South Asia (Bhattacharya et al., 2007). A recent study also found a strong link between arsenic exposure and child mortality (Carbonell- Barrachina et al., 2012). Another study has found that arsenic exposure in pregnant women also increases the risk of infant mortality and has negative implications for child growth and development (Vahter, 2009). 3.4. Availability of arsenic species to plants Plants generally require a number of essential elements for maintaining growth and development of their physiological systems. These include copper (Cu), which is required for the photosynthesis process, zinc (Zn), which is required for DNA catalytic binding, and silicon (Si), which is required for fighting abiotic stresses (Clemens et al., 2002; Paper II). However, owing to lack of specificity in the metal uptake and distribution system in plants, a number of non- essential elements (e.g. cadmium (Cd), arsenic) can accumulate in plant tissues. Arsenic present in the soil, which as mentioned comes
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Figure 3. Molecular structure of arsenite and silicic acid. from either geogenic or anthropogenic sources, becomes available to plants for uptake. In nature, inorganic arsenic species such as arsenate and arsenite are available for plant uptake and plants can accumulate both of these from environmental media such as soil and water. However, the uptake pathways of these two inorganic arsenic species in the plant system are completely different from each other.
Arsenate (pentavalent inorganic arsenic species) mainly accumulates in the plant through phosphate transport systems, because arsenate appears to be an analogue of phosphate. More than 100 phosphate transport systems have been detected in plants and most of these are found in the root zone (Raab et al., 2007; Zhao et al., 2009).
However, under reducing conditions in soil, e.g. under flooded conditions, arsenite (trivalent inorganic arsenic) becomes the dominant arsenic species available for plants and accumulates in plant tissues by using the aquaporin system (Zhao et al., 2010). In submerged plant species or plant species that can grow under flooded conditions (e.g. rice), studies show that arsenite can also be transported through the silicon transport systems (Ma et al., 2006). The reason why arsenite can share silicon transporters is because arsenite and silicic acid have two important similarities: they have high pKa (9.2 and 9.3 for arsenious acid and silicic acid, respectively) and their molecular structure is tetrahedral in shape (Zhao et al., 2009) (Figure 3).
The detoxification of inorganic arsenic species inside plant tissues differs depending on their valence state. Arsenate does not bind to thiol ligands, whereas arsenite has a high binding affinity to sulph- hydryl (-SH) groups of peptides such as glutathione (GSH) and phytochelatins (PCs) (Raab et al., 2007; Zhao et al., 2009). As regards transformation of inorganic arsenic species inside the plant, some plants have an excellent capability for reduction of arsenic species, so arsenite is mainly present in the tissues even when the plant is exposed to arsenate. Both glutathione and enzymatic processes are responsible for this inorganic arsenic species reduction (Rahman & Hasegawa, 2011).
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3.5. Arsenic accumulation in plants 3.5.1. Terrestrial plants Accumulation of arsenic in plants varies due to plant accumulation features, habitat and strategies for heavy metal uptake (Figure 4). Depending on the type of heavy metal uptake, especially arsenic uptake, plant species are divided into three groups (Figure 5): Hyperaccumulators: plants can accumulate high amounts of metals in their aboveground parts; accumulators: plant metal uptake reflects the concentration in the growth medium; and excluders: plants have the ability to prevent metal uptake in their tissues (Baker, 1981; Paper I). The first arsenic hyperaccumulator plant species to be identified, Chinese brake fern (Pteris vittata), was discovered in 2001. It was found to accumulate 22.63 mg As/kg within six weeks from an arsenic-contaminated soil (1500 mg As/kg) (Ma et al., 2001). A further 12 arsenic hyperaccumulator plant species have since been discovered and all of these are fern species (Pteridaceae family) (Zhao et al., 2009; Zhao et al., 2010). As an intermediate form between arsenic hyperaccumulator and excluder plant species, a number of plant species can accumulate arsenic in their tissues when grown in relatively highly arsenic-contaminated soil conditions (e.g. Holcus lanatus and Cytisus striatus) (Tripathi et al., 2007). Among the terrestrial plant species, a number of rice cultivars can grow in submerged and flooded conditions and they can accumulate relatively high levels of arsenic compared with other cereal crops (Zhao et al., 2009). Two main factors are responsible for high As uptake in rice: 1) Due to growth under submerged conditions, arsenite bioavailability is higher than arsenate bioavailability; and 2)
Terrestrial Emergent Submerged Figure 4. Arsenic accumulation points (black dots) in different type of plant species in terrestrial, emergent and submerged conditions.
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rice is also considered a high silicon accumulator plant species and arsenite can follow the silicon transport system (Zhao et al., 2010).
3.5.2. Aquatic plants In a similar way to terrestrial plant species, aquatic macrophytes, especially wetland plant species, can also be exposed to soil redox conditions that influence arsenic speciation in the aquatic system (Carbonell et al., 1998) (see Figure 4). In the natural water system, the inorganic arsenic species are associated with a number of phytoplankton which convert methyl-As species or higher orders of organo-arsenical compounds, such as As-sugars, whereas organic arsenic species are also transformed to inorganic arsenic species with the help of bacteria (Rahman & Hasegawa, 2011).
Arsenic uptake in aquatic plant species is also quite similar to that in terrestrial plant species. A number of studies have suggested that the arsenic uptake mechanism in aquatic macrophytes uses three different strategies: 1) Active transport through phosphate uptake pathways; 2) passive uptake through aqua-glyceroporins; and 3) physiochemical adsorption through root surfaces (Meharg & Hartley-Whitaker, 2002; Tripathi et al., 2007; Zhao et al., 2009; Rahman & Hasegawa, 2011). A wide range of aquatic macrophytes have been shown to achieve high levels of heavy metal and metalloid uptake in their tissues from aquatic systems. In many previous studies on heavy metal pollution in water, macrophyte species such as Elodea canadensis, Eichhornia crassipes, Ceratophyllum demersum etc. have been used as biomonitor species (Prasad, 2007). A number of macrophyte species have been used for different kinds of heavy metal accumulation and biomonitor-related studies, such as Microspora, Lemna minor, Fontinalis antipyretica Hedw., Hydrilla verticillata, Spirodela polyrhiza L., Azolla caroliniana and Vallisneria natans (Lour.) (Fritioff & Greger, 2003; Samecka-Cymerman et al., 2005;
Figure 5. Arsenic accumulation pattern in different classes of plants (hyperaccumulators, accumulators, excluders).
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Rahman & Hasegawa, 2011; Srivastava et al., 2011; Zhang et al., 2011: Chen et al., 2014; Rofkar et al., 2014). The arsenic detoxification process inside aquatic plants species is not very different from that in terrestrial plant species. A study based on Hydrilla verticillata and arsenic uptake has shown that thiol compounds including cysteine, glutathione and phytochelatins increase at same rate, which is a similar strategy as seen in arsenic- tolerant terrestrial plant species (Srivastava et al., 2007; Rahman & Hasegawa, 2011).
Among these phytoremediation techniques, rhizofiltration could be an effective strategy for removal of arsenic from the water system. In general, arsenic species availability varies with the oxygenation status of the water system. A number of aquatic plants have shown potential for arsenic accumulation from contaminated water systems. These include duckweed, lesser duckweed, Petries starwort, water spinach, water fern, Brazilian waterweed etc. (Rahman & Hasegawa, 2011). From this perspective, screening aquatic plant species for their capacity for arsenic removal from irrigation water and growing the best-performing species in contaminated aquatic environments could be an effective environmental management strategy to reduce the arsenic content in crops and vegetables. Macrophytes or other aquatic plant species in particular, rather than terrestrial plants, could be most suitable for use in phytofiltration/rhizofiltration techniques to remove arsenic from contaminated water resources.
3.6. Remediation of arsenic contamination 3.6.1. Conventional remediation A number of chemical and mechanical techniques are available for removal of arsenic from soil and water systems. For soil remediation, treatments includes isolation, immobilisation and stabilisation, toxicity reduction, physical separation and extraction for removal of arsenic (Mulligan et al., 2001). Among in situ soil treatments, stabilisation of soil arsenic is one of the major treatment processes, where chemical amendments such as goethite (α- FeOOH) (crystallised iron oxide) are used for binding and co- precipitation with arsenic, leading to a reduction in the arsenic concentration in plant shoots (Hartley & Lepp, 2008). Capping and soil replacement are other methods that have been used for arsenic remediation in soil (Tokunaga & Hakuta, 2002). Compared with these methods, the soil washing technique has gained more priority, as it can give a permanent result within a limited time frame (Jang et al., 2005; Amofah et al., 2011). In Sweden, arsenic-rich soils are treated by excavation, removal and disposal in landfill, while for mine tailings a ‘wet and dry care’ process that includes cover with a protective layer of glacial till and an elevated groundwater level is generally applied (Carlsson et al., 2003; Amofah et al., 2010).
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While only a narrow range of arsenic remediation techniques are available for soil-based pollution, a wide range of technologies for decreasing arsenic concentrations in water systems have been developed. Arsenic oxidation or precipitation is one of the simplest and relatively low-cost technologies available to remove arsenate species, by oxidation (Borho & Wilderer, 1996; Leupin & Hug, 2005). Adding chemicals to arsenic-rich water to promote coagulation is another process for removing arsenic from water, but the removal rate is relatively low (Hansen et al., 2006; Mohan & Pittman, 2007). Different types of ion exchange and membrane technologies can be used for removing arsenic from water systems, but both of these have relatively high operating costs (Mohan & Pittman, 2007). Besides the above-mentioned techniques, arsenic adsorption has recently attracted attention as it uses different types of material, including biological material, mineral oxides and activated carbon, to remove arsenic from water systems (Dambies et al., 2002; Mohan & Pittman, 2007).
3.6.2. Phytoremediation In comparison with the conventional removal techniques for heavy metals and metalloids from the environment, the use of plant-based remediation is a relatively new environmental management approach. Phytoremediation involves the use of living plant species for extraction, stabilisation and/or filtration of different kinds of heavy metals and metalloids from the environmental matrix (soil, water, air) (Salt et al., 1995; Clemens et al., 2002). In other words, it can be defined as using green plants to engulf, eliminate or render environmental contaminants harmless (Cunningham & Berti, 1993). The major benefits of using phytoremediation technology is that it is an economically viable and environmentally friendly solution for treating heavy metal and metalloid pollution (Wan et al., 2016). Depending upon application type and plant uptake features, phytoremediation is classified into several different categories, such as phytoextraction, phytostablisation, phytofiltration and phytovolatilisation (Salt et al., 1998) (Figure 6).
In phytoextraction, the plants mainly accumulate metals from the soil by using their root system and it is then transferred to harvestable parts. One example of successful application of a phytoextraction technique can be found in Sweden, where willow (Salix spp.) was grown to lower the cadmium content from the soil and the willow biomass was then harvested for bioenergy purposes (Landberg & Greger, 1996; Greger & Landberg, 1999, 2015).
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Figure 6. Different types of phytoremediation techniques: phytoextraction, phytostablisation, phytofiltration and heavy metal transport.
In phytostablisation, plants stabilise heavy metals in their belowground parts (Salt et al., 1998). This technique is mainly intended for mine tailing areas, where the content of heavy metals is relatively high (Mendez & Maier, 2008). Moreover, it is always recommended that native plant species are used in the phytostabilisation process and that invasive species are avoided as they can cause biodiversity problems in local ecosystems (Stoltz & Greger, 2002; Mendez & Maier, 2008; Paper I).
Phytoremediation applications for arsenic removal were first highlighted around a decade ago, after the discovery of the first arsenic hyperaccumulator Pteris vittata, which can be used for arsenic phytoextraction from contaminated fields (Ma et al., 2001; Singh & Ma, 2006). Apart from such phytoextraction of arsenic, most arsenic phytoremediation-related studies focus on removal of arsenic from water systems rather than soil, because arsenic-rich groundwater carries more of a health threat than arsenic-rich soil. 3.6.3. Arsenic phytofiltration and aquatic macrophytes Phytofiltration is one of the major phytoremediation techniques and involves use of aquatic plants for removal of heavy metals from aquifers by accumulation and absorption processes (Brooks &
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Robinson, 1998). In general, submerged-type aquatic plant species can accumulate heavy metals in all tissues (Rahman & Hasegawa, 2011). It has also been demonstrated that if the submerged plant species lacks cuticle in the leaves, then the surface area for heavy metals and metalloid accumulation is increased (Srivastava et al., 2011). According to the literature, research on arsenic uptake in macrophytes first started in the early 1990s. Prior to that, in 1982, the aquatic plant hornwort (Ceratophyllum demersum) was shown to increase its arsenic accumulation capacity with increasing arsenic concentration in the experimental medium (Liddle, 1982; Robinson et al., 2003). Another two macrophyte species, Azolla sp. and Spirodela polyrhiza L., have also been shown to accumulate relatively high amounts of arsenic (approx. 900 mg As/kg dry weight) when they are exposed to arsenic-rich water (Zhang et al., 2011). Besides these, watercress (Lepidium sativum) has also been shown to be capable of relatively high accumulation of arsenic in different plant parts (Robinson et al., 2003). In aquatic macrophytes, a relatively less toxic effect of metal accumulation is seen and, as they grow in submerged conditions, they are recommended for use in phytofiltration of rice fields, as most rice varieties can grow in submerged conditions (Zhang et al., 2011). A number of features of macrophytes contribute to their potential as candidate phytofiltration species for arsenic-rich groundwater systems (Brooks & Robinson, 1998; Robinson et al., 2006; Riis et al., 2010; Srivastava et al., 2011; Iriel et al., 2015):
1) High accumulation of arsenic from water to concentrations of 100-1000 mg As/kg in plant tissues. 2) Rapid removal of arsenic from water (e.g. 1 mg As/week by Vallisneria gigantea). 3) Large surface area for high levels of direct uptake of metalloids (Hydrilla sp.) 4) Higher bioaccumulation coefficient compared with terrestrial plant species. 5) Can grow in low nutrient, light and temperature conditions (species such as aquatic moss, Warnstorfia fluitans (Hedw.)). 6) Not used as a food or feed for humans or livestock.
Growing aquatic macrophytes in constructed wetlands could be another potential practical application for removal of heavy metals and metalloids from runoff water or stormwater (Fritioff & Greger, 2003; Prasad, 2007). 3.7. Effect of abiotic factors on arsenic uptake The abiotic factors considered here are mainly present in the natural habitats for macrophytes, especially in temperate regions. The habitats considered here are mainly continental standing water, running water, ponds, lakes, wetlands etc.
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Figure 7. Prediction of inorganic arsenate as a function of pH. (Modified from Medusa software)
3.7.1. pH level The availability of inorganic arsenic species (arsenate and arsenite) and their predominance in the environment are greatly influenced by both redox conditions and pH level of the medium (Zhao et al., 2010). In particular, the speciation of inorganic arsenic greatly depends on the pH level of the water medium. Arsenite (trivalent) is mainly found as uncharged species at neutral pH, whereas arsenate is found as negatively charged species at pH 2.3 or above (Lizama et al., 2011). While arsenic is considered to be one of the stable anions, with changes in pH the predominance of arsenate varies in the aquatic medium. As the chemical equilibrium diagram
generator shows (Figure 7), the neutral species arsenate (H3AsO4) − dominates at pH 1-2, the negatively charged form (H2AsO4 ) is found at pH 3-6 and the bivalent form becomes negatively charged 2− HAsO4 in the pH range 7-10 (Pennesi et al., 2012; Medusa, nd).
Beside these changes, the arsenic detoxification process at the cellular level can be controlled by the application of sulphur (S). The main pathway of arsenic detoxification in the plant cell occurs via chelation using reduced sulphur-containing ligands, involving phytochelatins and glutathione (Leão et al., 2014). The microbial activity of sulphate-reducing bacteria is controlled by pH and sulphate reduction is observed in acidic conditions (pH 3-4) (Lizama et al., 2011). The effect of pH on arsenic accumulation in macrophyte species has not been studied in detail. However, one study based on arsenite uptake in coontail (Ceratophyllum demersum L.) showed that accumulation of arsenic was highest at pH 5 and declined as the pH value increased (Khang et al., 2012). Therefore when applying macrophyte-based phytoremediation for removal of arsenic from arsenic-rich water systems, the pH level is one of the influential factors that needs to be addressed (Tu & Ma, 2003).
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3.7.2. Temperature regime Compared with the pH status of the aquatic medium, the effect of temperature of the medium on As accumulation in either terrestrial or aquatic plant species has not been widely investigated. In general, the water temperature may have an influence on water chemistry, metal solubility and metal uptake in the plant species, but there is no direct effect on metal solubility in water from seasonal variations in water temperature (Zumdahl, 1992; Fritioff et al., 2005). Regarding temperature effects on aquatic plant species, it has been found that higher temperature has a strong effect on growth and development, giving extensive biomass production in macrophyte species (Marschner, 1995). It has also been suggested that an increase in biomass of the plant species can increase total metal uptake when the plant species is grown in less metal-contaminated areas, as growth rate surpasses metal uptake rate in tissues, especially e.g. for cadmium uptake in Pinus sylvestris (Ekvall & Greger, 2003). It has also been suggested that lower temperature could decrease the permeability of cell membrane lipids, which could affect metal uptake in the plant species used for phytoremediation (Fritioff et al., 2005). Low cell membrane permeability in the low temperature range can cause low metal uptake in plant tissues (Marschner, 1995). Regarding arsenic accumulation in plant species, it has already been well documented that arsenate uptake occurs in the plant root by using the phosphate transporter system. One study also found that the phosphate uptake process in the plant system varies with different temperature levels (Jonasson & Chapin, 1991; Zhao et al., 2009).
Thus, from the above-mentioned facts, it could also be presumed that arsenate uptake in plant tissues is indirectly influenced by water temperature. In temperate regions, the changes in temperature in different seasons play a vital role, as growth and production of biomass by the phytoremediation plant species could be affected by this fluctuation in temperature. In a study on seasonal variation in uptake of arsenic, it was found that Carex stricta and Spartina pectinata took up relatively more arsenic in summer time compared with spring (Rofkar & Dwyer, 2011). Moreover, another study found that arsenic accumulation in seaweed (Fucus spiralis) increased twofold when it was exposed to 30 °C water temperature compared with 16 °C (Klumpp, 1980). In general, it could be assumed that arsenic uptake in aquatic plants reaches an optimum level when the temperature reaches relatively high levels. As sulphur reduction rate declines with decreasing temperature, temperature could also indirectly affect the arsenic detoxification process in the plant system (Lizama et al., 2011).
3.7.3. Oxygen supply The availability of oxygen in the aquatic environment is considered one of the major abiotic factors that controls the speciation of inorganic arsenic species in the medium. Arsenite becomes slowly
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oxidised in the presence of oxygen (Lizama et al., 2011). However, wetland plant species can also change their physiological features depending on the oxygen content in the aquatic medium. In general, aerenchyma is well developed in wetland plant species and aerenchymal development could be further improved for some plant species in water-logged conditions that provide less resistance to transfer of oxygen and other substances between root and shoot parts (Li et al., 2011). Besides aquatic plant species, rice has its own oxidising ability and it can change arsenic speciation on the root surface when the roots are exposed to water-logged conditions (Liu et al., 2006).
3.7.4. Silicon application A number of elements that can influence arsenic uptake and detoxification processes in plant tissues have been detected. These include iron, phosphorus (P), sulphur and silicon (Zhao et al., 2010). Among these elements, the effect of silicon on arsenic accumulation in the plant was demonstrated recently and it has been found that both silicic acid and arsenite have similar physiochemical structure (Meharg & Meharg, 2015). A number of studies have found that application of additional silicon during rice cultivation either in the field or in a hydroponics system reduces arsenic uptake and translocation to shoot and grain (Bogdan & Schenk, 2008; Fleck et al., 2013; Meharg & Meharg, 2015). Many studies examining the effect of silicon on arsenic uptake have been performed in various rice species, since rice is a silicon accumulator plant species and uptake of arsenite is higher when plants are grown in water-logged conditions (Bogdan & Schenk, 2012; Yamamoto et al., 2012). There are few studies on the effects of silicon on arsenic uptake in other plants, but those available show that silicon could mitigate arsenic effect in this regard (Marmiroli et al., 2014; Paper II).
3.8. Reducing arsenic content in the diet Due to lack of centralised water supply systems, most of the population in rural areas of countries in South Asia, such as Bangladesh, depend on groundwater for their daily drinking water. Arsenic enters the human food chain mainly by the use of arsenic- rich groundwater for drinking water. A study has shown that in an arsenic-affected area of Bangladesh, 80% of the regional population, or 35 million people, depend on groundwater as their main drinking water source and are exposed to arsenic toxicity (von Brömssen et al., 2007).
However, a number of mitigation strategies have been implemented in arsenic-affected areas all over the world. In Bangladesh, a frequently cited example of an arsenic-affected area, a number of measures and strategies have been used as arsenic mitigation policies, such as deep tubewells, dug wells, piped water systems,
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rainwater harvesting, household arsenic filters, community arsenic filters, river sand water filters and so on (Khan & Yang, 2014).
Compared with the number of mitigation strategies for lowering high arsenic concentrations in drinking water, methods for decreasing the arsenic content in cereal grains and vegetables have still not been the subject of much research in either the developed or the developing countries. A number of studies have shown that high arsenic content in food grains and vegetables is the second largest source of arsenic intake in humans (Tripathi et al., 2007; Gilbert-Diamond et al., 2011; Ljung et al., 2011; Carbonell- Barrachina et al., 2012, Bergqvist et al., 2014). However, despite this risk to the human food chain from consuming arsenic-rich food, globally only a few regions have introduced limits for maximum tolerable inorganic As concentrations in foodstuffs. Existing maximum limits on inorganic As content in foodstuffs were discussed in a previous section of this thesis (section 3.2).
Of all food grains and foodstuffs, rice (Oryza sativa) contributes most arsenic to the human food chain. Hence reducing the arsenic content in rice has received relatively great attention compared with the content in other food crops. A study has found that presence of organic matter and water regime in the cultivation area can influence arsenic uptake in rice grains (Norton et al., 2013). Besides this, identification of low arsenic accumulating rice cultivars and optimisation of irrigation technique (sprinkler irrigation and intermittent irrigation) are two strategies that can lower the arsenic concentration in rice grains (Norton et al., 2009; Spanu et al., 2012). Moreover, the study described in Paper II of this thesis found that application of silicon can reduce the arsenic concentration in lettuce exposed to arsenite.
Thus, there is still a major knowledge gap regarding how to devise an eco-friendly sustainable solution for lowering the arsenic concentration in foodstuffs that are frequently cultivated in arsenic- contaminated areas.
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4. MATERIALS AND METHODS 4.1. Experimental design and comments on materials and methods
4.1.1. Arsenic geochemistry in Sweden and Bangladesh In this thesis, two different locations were used for field investigations: Bangladesh and Sweden. The arsenic contamination problems and the occurrence of arsenic in soil and groundwater differ between Sweden and Bangladesh. In Bangladesh, high levels of arsenic in the groundwater were first detected in 1993, in Nawabgonj District (Smith et al., 2000). The presence of high arsenic concentrations in the groundwater in this Bengal delta region has emerged due to geogenic reasons (Bhattacharya et al., 1997; Smedley & Kinniburgh, 2002; Heikens et al., 2007). The frequent use of groundwater-based irrigation water for hybrid high- yielding rice cultivation also influences the transfer from arsenic-rich groundwater to upper soil levels and further to the rice grains (Dittmar et al., 2010; Khan et al., 2010). An area called Matlab was chosen for investigating the arsenic content in rice grain because it is already considered an arsenic hotspot in Bangladesh and a number of mitigation strategies have been implemented in this area to solve the groundwater arsenic problem (von Brömssen et al., 2007).
In Sweden, geogenic factors but mostly anthropogenic factors are the main reasons for arsenic contamination of soil and water. The Swedish Geological Survey (SGU) has reported a number of areas (including agricultural land and grazing land) in Sweden that have elevated levels of arsenic in the soil. SGU also suggested that sulphide oxidation is one of the major geogenic reasons for arsenic contamination in different regions (e.g. Bergslagen, Jämtland (alum shale), Skellefteå (Västerbotten) and Boden) (Ladenberger et al., 2013). Alum shale is sedimentary rock formed under marine anoxic conditions, also called ‘black shale’. Exposure to air and water influences the weathering of the alum shale, a process which can be divided into two steps: 1) Oxidation of sulphide minerals by oxygen
with release of H2SO4 and free metal cations; and 2) destabilisation of silicate minerals and kerogen by the H2SO4 released (Falk et al., 2006). Besides this geogenic process, using arsenic for wood impregnation is also considered one of the major previous anthropogenic reasons for high arsenic concentrations in the environment in Sweden. Moreover, mining, smelting, glassworks and metal industries also release arsenic to soil and water in Sweden (Forslund et al., 2010). It is estimated that 0.6 million tons of arsenic have been deposited in northern Sweden over the years by mining and wood impregnation activities, which are concentrated in that region (Routh et al., 2007). As a result, the arsenic content in water and soil in northern Sweden is relatively high compared with that in the rest of the country.
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4.1.2. Selection of plant species The selection of plant material for the studies is this thesis was based on their capacity for arsenic accumulation from the medium. Rice was chosen for some of the investigations based on the fact that in Bangladesh, rice cultivation depends on a groundwater-based irrigation system. The high content of arsenic in the groundwater in different parts of Bangladesh is already well documented and was mentioned in earlier sections of this thesis. The accumulation of arsenic in rice grains through arsenic-rich groundwater in Bangladesh was initially discovered around 15 years ago (Meharg & Rahman, 2003). Since then, a number of investigations have found high arsenic concentrations in different kinds of rice species cultivated in arsenic-contaminated areas (Williams et al., 2006; Norton et al., 2009; Panaullah et al., 2009; Ahmed et al., 2010; Dittmar et al., 2010; Roberts et al., 2011). However, to the best of my knowledge, there is no information about the arsenic concentration in local aromatic rice cultivars grown in arsenic- hotspot areas such as Matlab in Bangladesh.
As a model for leafy vegetable plant species, the selection of lettuce (Lactuca sativa) had a number of advantages, such as widely available, short growth period, high level of water uptake and easily grown in a hydroponics system. Some studies have also confirmed that arsenic accumulation can occur in lettuce (Gusman et al., 2013; Caporale et al., 2014). However, another important reason for using lettuce as a model vegetable was that most people eat it as a raw vegetable in their diet. Hence the possibility of arsenic exposure in the human food chain is possibly higher from consumption of lettuce than from consumption of cooked foodstuffs.
The aquatic moss Warnstorfia fluitans was chosen for tests as a potential arsenic phytofiltration candidate because it is one of the native plant species in the northern Sweden. The average temperature in the region varies from -12 °C to 20 °C throughout the year, in two main seasons: a four-month warm season and an eight-month cold season. This aquatic moss mainly grows in lakes and streams in the boreal and arctic region. The most important feature of the arctic lakes that comprise the moss habitat are poor- nutrient conditions, with low concentrations of nitrogen and phosphorus. Warnstorfia fluitans grows as moss mats and very often these mats are formed in the bottom of the lake, in shady conditions. Increased nutrient run-off and higher light density could increase the growth of this aquatic moss (Riis et al., 2010). There is a relatively high content of arsenic present in the groundwater and soil system in the region where the moss occurs naturally. Therefore, this native aquatic moss species has already been exposed to a high level of arsenic in the local ecosystem. Preliminary pilot studies on arsenic uptake in aquatic moss in laboratory conditions showed good potential for arsenic uptake from water. Another major factor in its suitability for phytotechnology is that using local plant species
20 Arsenic phytofiltration and silicon application-lowering arsenic concentrations in food crops
is always recommended, to prevent invasive species in local ecosystems causing biodiversity problems at different trophic levels (Mench et al., 2010).
4.1.3. Arsenic analysis 4.1.3.1. Soil Soil samples (paddy soil samples) were collected from Matlab, Bangladesh (Paper III). Each of the soil samples was collected from 0-15 cm depth in rice fields, all plants and other debris were removed and the soil samples were stored in plastic bags until they reached the analytical laboratory in Sweden. After cleaning, the soil samples were oven-dried at 60 °C for 72 hours and then wet
digested with 7M HNO3 in a wet digestion block according to (Stoltz & Greger, 2006; Bregqvist & Greger, 2012). The wet- digested soil samples were analysed for total arsenic in by inductively coupled plasma-optical emission spectrometry (ICP-OES) (Thermo iCAP 7600). 4.1.3.2. Water Water samples were collected from different aquatic moss experiments where arsenic uptake by the moss was studied (Papers IV, V). The water samples were collected at different time intervals according to the plan and design of the experiments. The collected water samples were stored in a cold room (8 °C) until analysis for total arsenic. Arsenic concentration in the water samples was measured by atomic absorption spectrometry (AAS) (Varian SpectAA 55B, VGA-77). The chemicals used for analysis of total arsenic content in the water samples were: sodium borohydride (3%; Merck) and sodium hydroxide (2.5%; EKA Chemicals) and hydrochloric acid (6 M; VWR International) was used for hydride generation. The detection limit for analysis was 7 µg As/ L. To compensate for the effect of background matrix, standard addition was also applied during analysis of total arsenic content in the water samples. 4.1.3.3. Plant materials 4.1.3.3.1. Rice grain and husk Mature rice grain samples from hybrid rice types (BRRI dhan 32, BRRI dhan 29) and a local aromatic rice cultivar (Kalijira) were collected from an arsenic hotspot in Matlab, Bangladesh (Paper III) (Figure 8).
The rice grains were cleaned of different kind of debris in the laboratory of the Plant Metal Group, Department of Ecology, Environment and Plant Sciences (DEEP), Stockholm University, Sweden. The rice grains were de-husked by hand and then divided into two fractions: de-husked grain and husk. The de-husked grain and husk samples were placed in an oven at 60 °C for 72 hours. After oven drying, the samples were wet digested in a stepwise heated digestion block. The wet digestion procedure was performed
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Figure 8. Rice cultivars in the field: a) hybrid type BRRI dhan 32, b) hybrid type BRRI dhan 29 and c) the local aromatic rice cultivar Kalijira. (Photo: A. Sandhi)
according to Bergqvist & Greger (2012) using HNO3:HClO4 (7:3, v/v) for 19 hours in a heating programme with the temperature increasing up to 225 °C. The wet-digested plant samples were kept in a cold room until analysis for total arsenic content using AAS. 4.1.3.3.2. Lettuce The lettuce (Lactuca sativa), seedlings were germinated from seed (var: Amerikanischer brauner, Nelson Garden Ab, Sweden) in a vermiculate medium and placed in a climate chamber (Supplier: Labrum; www.Labrum.se). The seedlings were kept growing inside the climate chamber with Hoagland nutrient solution until the start of the experiment (Figure 9). After different experiments on inorganic arsenic species uptake in lettuce plants, the plant parts were separated into root and shoot fractions.
Depending upon total arsenic analysis and arsenic speciation analysis, different plant parts were processed (for details, see Paper II). For total arsenic analysis, the lettuce root and shoot parts were wet digested in a similar wet digestion process as applied to rice grains (section 4.1.3.3.1 of this thesis). For arsenic speciation analysis, the fresh plant parts were kept at -80 °C, where all plant physiological activities ceased, until use.
Figure 9. Lettuces growing in a climate chamber in a) a hydroponics medium and b) soil.
22 Arsenic phytofiltration and silicon application-lowering arsenic concentrations in food crops
Figure 10. a) Moss mats growing in the greenhouse at the Department of Ecology, Environment and Plant Sciences, Stockholm University and b) moss material before analysis. (Photo: A. Sandhi)
4.1.3.3.3. Moss The moss mats were grown under an adequate oxygen supply on sediment collected from a wetland located in northern Sweden (N65°20,943’ E18°35.587), and kept in a greenhouse at DEEP, Stockholm University, before being used in various investigations (Figure 10).
For the experiments, moss was collected from the greenhouse (Papers IV, V). After the experiments, the moss samples were collected, washed and dried with paper napkins and preserved for chemical analysis (total arsenic content and arsenic speciation), using the procedure applied for rice and lettuce samples (see sections 4.1.3.3.1-2 of this thesis).
For the study examining arsenic adsorption-absorption capacity in moss (Paper IV), the fresh moss samples were dried at 60 °C overnight to stop physiological processes.
4.1.3.4. Arsenic species analysis The arsenic species analysis technique used on plant samples was adapted from previous studies (Mir et al., 2007; Bergqvist & Greger, 2012). The only deviation from Bergqvist & Greger (2012) was that the plant samples were centrifuged at 2000 rpm for 10 minutes. The arsenic extraction efficiency varied between the different plant species tested (i.e. lettuce and moss) (Papers II, IV and V). The differences in inorganic arsenic extraction from different plant species can provide important information about variations in arsenic bonding in the plant material. For example, arsenic species extraction efficiency was high in lettuce but differed greatly between lettuce roots (range 58-96.2%) and shoots (range 12-31%). In the moss biomass, extraction efficiency was very low (2-8.8%) in different inorganic arsenic treatments (Paper IV) and only slightly
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higher (3.3-24.7%) for inorganic As species at different pH levels (Paper V). The reason for the low arsenic extraction efficiency in moss biomass compared with lettuce biomass (both root and shoot) could be that accumulated arsenic species in the moss were firmly bound inside plant tissue. This firmly bound arsenic could not be extracted with a technique using methanol (MeOH) and hydrochloric acid (HCl). However, the effectiveness of the arsenic extraction technique used in experiments can be taken to be acceptable, as it extracted both inorganic and organic arsenic species from lettuce material (Paper II).
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5. RESULTS AND DISCUSSION Arsenic accumulation mainly occurs in different crops and vegetables either grown in arsenic-rich soil or grown using arsenic- rich irrigation water and arsenic enters the human food chain through consumption of these foodstuffs. Thus there are potential three ways to lower the arsenic concentration entering the human body through food consumption: 1) Selection of crop cultivars or plant species with low arsenic accumulation; 2) use of an additive or element that could reduce arsenic availability and arsenic accumulation in plants; and 3) lowering the arsenic concentration in irrigation water and soil.
5.1. Selection of crop cultivars and plants for low arsenic accumulation
5.1.1. Arsenic content in rice cultivars in arsenic hotspots and uptake in humans Rice is considered the second most important pathway of arsenic accumulation in the human body (Gilbert-Diamond et al., 2011; Ljung et al., 2011). The analytical results for the Matlab area of Bangladesh (Paper III) showed that while the arsenic concentration in soil on which a local aromatic rice (LAR) cultivar was grown was significantly higher than that in soil on which a hybrid rice (HYV) cultivar was grown, the arsenic content in whole grain of these two rice types was not significantly different (Table 1 in Paper III). A number of previous studies have shown that soil arsenic concentration and arsenic extractability from soil have a significant effect on the arsenic concentration in different parts of plants, especially in the case of rice (Giri et al., 2012; Bergqvist, 2013; Lin et al., 2015). Moreover, a recent study confirmed that the arsenic concentration in rice is more dependent on the oxic conditions of the soil than the arsenic concentration in irrigation water (Wu et al., 2017).
A study on arsenic concentrations in soil and vegetables performed at a location close to the study area in Bangladesh investigated in this thesis reported similar arsenic concentrations in soil ranging from 7 to 27 mg/kg (Das et al., 2004; cf. Paper III). The arsenic in rice grains is mainly taken up from paddy soil and it is also possible to increase the arsenic concentration in rice grain when it is cultivated in arsenic-rich soil (Meharg & Rahman, 2003). Therefore, screening arsenic concentrations in different kind of rice cultivars could be considered an effective option for identifying safe rice types that lower arsenic intake in the human food chain. A recent study showed that genetic diversity is one of the major reasons for differences in arsenic uptake rate in rice cultivars grown in different areas and that it is possible to develop low arsenic accumulating rice varieties (Norton et al., 2012).
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On the other hand, the arsenic distribution in the different rice grain fractions (de-husked grain and husk) of the rice types analysed in this thesis was not consistent with previous findings on genetic differences. The arsenic accumulation factor (AF) was found to be higher for the hybrid (HYV) rice types compared with the local aromatic (LAR) type, but the arsenic concentration in de-husked grain was higher in LAR than in the HYV rice types (Figures 2 & 3 in Paper III). This variation in arsenic distribution in rice grain parts (de-husked grain and husk) could be because the husk part of the LAR rice type was more strongly attached to the grain. The rice de- husking process was performed manually (by hand with tweezers) in the laboratory, which could have played an important role for the high arsenic content in de-husked grain of LAR rice. A predictive calculation based on arsenic content in the de-husked rice showed that arsenic ingestion in humans could be increased 3-4 fold due to consumption of the LAR rice type rather than a HYV type (Table 2 in Paper III). However, according to the regulations on maximum limits for arsenic concentrations in rice and rice-based products introduced recently in the EU and Sweden, the arsenic content in the de-husked HYV rice grain studied did not exceed the maximum arsenic content limit for milled rice (Ankarberg et al., 2015).
Overall, the results in this thesis showed that the arsenic accumulation factor (AF) of rice varieties should be considered an important factor that can control arsenic uptake in rice grain, especially in arsenic hotspots. It appears that the arsenic entering the human food chain through rice consumption could be mitigated if policy makers were to focus on, and recommend, arsenic AF values for different rice varieties grown in arsenic hotspots. Besides this, the rice milling process could also play an effective role by polishing rice to reduce the husk part with its associated arsenic. An investigation based on arsenic concentrations in rice cultivars from different countries found that the arsenic content in rice can contribute an estimated 33-317% to maximum tolerable daily intake (MTDI) of arsenic, especially in Bangladesh (Williams et al., 2005). By using the rice AF for arsenic as recommended in this thesis, it could also be possible to identify safe rice cultivars that would contribute less arsenic to the human food chain. In addition, knowledge of the arsenic AF value of different rice types could help local farmers and agronomists to select and cultivate less arsenic- accumulating varieties on arsenic-rich agricultural soils.
Moreover, the arsenic content in cooked grains of hybrid and local rice types also needs to be investigated in future, as it has been reported that the content in cooked rice can vary due to cooking methods and to the arsenic content in the water used for cooking (Sengupta et al., 2006; Signes et al., 2008).
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5.1.2. Arsenic accumulation in lettuce Most of the studies to date on arsenic accumulation in foodstuffs have mainly focused on arsenic concentrations in cereal grains. Only a few studies have investigated the arsenic content in vegetables (as discussed earlier in this thesis). Application of the two inorganic arsenic species (arsenate and arsenite) to lettuce in high concentrations resulted in a reduction in fresh weight of the lettuce (Table 2 in Paper II). Similarly, another investigation found that application of arsenic reduced the biomass of lettuce (Caporale et al., 2014). Besides this growth inhibition effect, the distribution of arsenic species varied in root and shoot parts of the lettuce in Paper II. With both arsenic species applied, the amount of accumulated arsenic stored in shoot parts was approximately half the arsenic amount stored in the root (Table 3 in Paper II). However, the overall accumulated amount was found to be lower when the lettuce was treated with arsenite compared with arsenate. The reason for higher arsenate uptake compared with arsenite in lettuce shoots could be explained by arsenite being kept in root cells, where phytochelatins (PCs) detoxified and bound it, whereas arsenate was translocated to the shoot (Meharg & Hartley-Whitaker, 2002). Currently, there is no maximum value for arsenic concentration in fresh vegetables in either the EU or globally. In comparison with the newly established maximum arsenic concentration in rice grain in the EU and Sweden (Ankarberg et al., 2015), the arsenic content in the shoots of lettuce exceeded that maximum level when they had been treated with arsenate for four days (Paper II).
The arsenic speciation data also provided the information that arsenic extraction efficiency varied between the root and shoot parts of lettuce, with lower arsenic species concentrations being found in shoots compared with roots (Table 4 in Paper II). This lower arsenic species concentration in shoots can be explained by protection of the photosynthetic apparatus from the toxicity of inorganic arsenic species, resulting in lettuce having a tendency to store most of the accumulated arsenic in the root. A previous study on application of arsenic-rich irrigation water to lettuce grown in soil also found that more arsenic was stored in the root parts compared with the shoot parts (Caporale et al., 2014). Plant species generally have a strong tendency to restrict metal transfer to aboveground parts, as energy production by photosynthesis (green part of shoot, leaves) occurs mainly in the aboveground parts of the plant. An investigation on cucumber (Cucumis sativus) also showed that it stored less arsenic in the shoot, in order to protect the photosynthetic apparatus from the toxic effect of arsenic (Uroic et al., 2012). 5.2. Effect of silicon on arsenic species uptake in lettuce The results of the investigation presented in Paper II showed that addition of silicon reduced the arsenic concentration in the lettuce root, especially when arsenite was applied as the treatment in the growth medium (Table 3 in Paper II). This lower uptake could be explained by that fact that addition of silicon modifies the cell wall
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and improves metal tolerance in the plant in several ways, e.g. by 1) reducing accumulation of metals in plants; 2) stimulating the antioxidant system; 3) altering the photosynthesis mechanism; and 4) complexing heavy metals (Liu et al., 2013). Besides this, the arsenic species analysis confirmed that addition of silicon in the growth medium significantly reduced arsenate and arsenite concentrations in the shoot of lettuce when arsenate was applied in the treatment (Table 4 in Paper II).
A recent study showed that silicon has the capability to reduce arsenite uptake and transport in rice grains (Fleck et al., 2013). However, the effect of silicon on arsenic uptake in various vegetable crops had not been studied prior to Paper II. Recently, silicon has gained attention for its protective properties against metal toxicity (e.g. Cd, manganese (Mn), Al, zinc (Zn), chromium (Cr), As) in plants and it has also been discovered that silicon helps in the arsenic detoxification process inside plant tissues (Zhao et al., 2010; Liu et al., 2013).
In the work presented in this thesis, it was clearly shown that silicon lowers net arsenic uptake in the lettuce plant (Table 3 in Paper II). Similar findings have been made in a previous study (Bogdan & Schenk, 2008). Another study examining the effect of silicon on cadmium and arsenic uptake in different field crops (e.g. potato, carrot, onion, wheat) showed that it lowered accumulation of these two heavy metals in the edible parts of these field crops (Greger & Landberg, 2015). A recent review of the beneficial effects of silicon for crops recommended it as a ‘silver bullet’ (Meharg & Meharg, 2015). Therefore, addition of silicon to the growth medium can decrease the toxic effect of inorganic arsenic species on lettuce, and can also indirectly help to mitigate arsenic-related human health risks arising from consumption of lettuce as a regular dietary component.
5.3. Lowering the arsenic concentration in irrigation water and soil 5.3.1. Arsenic phytoremediation and the phytofiltration potential of aquatic moss
Rather than conventional remediation methods, ion exchange and membrane technologies are mainly used for arsenic removal from water. However, due to the high operating costs of those methods, an oxidation and precipitation process for arsenic removal from the water system has attracted attention.
A number of phytoremediation techniques are available for heavy metal and metalloid management in the environment (Figure 6). Analysis of different conventional and phytoremediation techniques revealed that phytoremediation has strong potential for removal of arsenic from contaminated sites and is beneficial from the economic
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as well as the ecological point of view (Paper I). In the case of phytoremediation techniques, phytofiltration of arsenic is interesting as the active biosorption occurs in the living biomass. It is a matter of concern that arsenic phytoremediation from soil has been the main focus of research in the past few decades rather than phytofiltration of water. The first arsenic hyperaccumulator plant identified, Pteris vittata, can be used for removal of arsenic from the soil (Ma et al., 2001).
High arsenic content in the groundwater becomes a major problem for soil and other crops due to deposition when arsenic-rich groundwater is applied as irrigation water. Some previous research has shown that arsenic phytofiltration could be applicable to irrigation water. A study based on arsenic removal using dried root powder of water hyacinth (Eichhornia crassipes) showed that 93% of arsenite and 95% of arsenate could be removed from the water medium (200 µg As/L) (Al Ramalli et al., 2005). However, more research is needed regarding phytofiltration of arsenic when present at lower concentrations in water to confirm its promise in wider applications (Mohan and Pittman, 2007). Rahman and Hassegawa (2011) reviewed a number of macrophyte plant species that can accumulate high levels of arsenic from water systems and could be used in various phytoremediation techniques and recommended phytofiltration as a promising eco-friendly management technique that can be applied for different kinds of contaminated water systems.
5.3.2. Arsenic removal efficiency and bioconcentration factor The uptake of arsenic in the model moss used in this study (Warnstorfia fluitans) from contaminated water mainly depended on the arsenic concentration in the water. It was shown that the moss could remove relatively higher amounts of arsenic from the water system when the arsenic concentration in the medium was lower (1 µM) (Figure 1 in Paper IV). The reason for choosing the arsenic concentration range tested in Paper IV was the arsenic concentration in the particular area located in northern Sweden, which ranged between (100 and 200 µg As/L) and the poor nutrient level in the aquatic medium. The 1 µM arsenate solution contained 74 µg As/L. On the other hand, the WHO maximum arsenic content in drinking water is 10 µg As/L or 10 ppb. However, arsenic removal (%) from the water system with high As concentration (10 µM and 100 µM) was found to have an lower pace initially, but by the end of the experiment it had significantly reduced the arsenic concentration in the medium (Table 2 in Paper IV). The reason for this change in removal efficiency over time is possibly that in the initial period of treatment the moss needs to adapt to the high concentration in the medium. However, the arsenic concentration in the moss reached 4.5 mg/g DW within 192 h of treatment with 100 µM arsenate solution and it was significantly higher than in the control (zero arsenate) treatment (Table 1). The moss was able to
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Table 1. Arsenic (As) concentration (µg/g) and bioconcentration factor (BCF, %) in moss treated with 1, 10 and 100 µM arsenate and 0.1 and 10% Hoagland solution. n=4, mean±standard error
accumulate 9 mg As/g dry weight from the aquatic medium at its collection point in Northern Sweden.
A number of physiological and biochemical effects on macrophytes are generally observed when they are exposed to heavy metal- contaminated media (Mishra & Tripathi, 2008). However, W. fluitans did not show any physiological symptoms due to arsenic exposure, based on the fact that no significant biomass change occurred (Table 1 in Paper IV). From arsenic removal kinetic experiments, it was found that the fastest (up to 82%) removal from the medium occurred when the moss was treated with 1 µM arsenic (0.074 µg/L) and the bioconcentration factor (BCF) value was highest at the lowest concentration tested (Table 1). Greger (2004) suggested that that low concentrations of heavy metals in the medium result in the highest AF values in plant species.
Moreover, a study based on eelgrass (Vallisneria gigantea) showed high arsenic removal at lower arsenic concentration in the medium (Iriel et al., 2015). However, the treated aquatic moss in this thesis did not show any kind of toxic side-effect in response to different concentrations of arsenic (1-100 µM). The aquatic moss used was collected from a wetland exposed to high levels of arsenic. It can be speculated that the outstanding arsenic uptake capacity of the moss in experiments is related to development of arsenic tolerance at its site of origin.
5.3.3. Arsenic absorption-adsorption capacity in moss In addition to studies on arsenic removal kinetics, it was found that arsenic absorption (uptake by living cells) was significantly higher
30 Arsenic phytofiltration and silicon application-lowering arsenic concentrations in food crops
(p>0.05) than adsorption (uptake by dead cells) in the aquatic moss (Table 5 in Paper IV). The high arsenic uptake in the living moss could be because the mat consists of both living and dead biomass in a mixture. Analysis of absorption and adsorption processes in a single moss mat showed that in the living cell, both absorption and adsorption of arsenic occurred. The arsenic concentration in living part was significantly higher than in the dead tissue of the same moss mat (Figure 4 in Paper IV).
Differences in morphological structure could be a reason for the high arsenic absorption in moss compared with terrestrial plant species. In broad terms, W. fluitans has a thallus structure (no distinct root and stem structure), so the metalloid translocation barrier between root and shoot found in terrestrial plant species does not exist in moss. Therefore, the high arsenic uptake, high BCF at low arsenic concentration and low As leakage (efflux) to the medium are strong arguments to consider W. fluitans as a promising new arsenic phytofilter candidate, especially for temperate regions.
5.3.4. Arsenic species effect on arsenic uptake on moss The effect of arsenic species on arsenic accumulation in W. fluitans moss followed a similar pattern to that reported for arsenic accumulation and species extraction in Swedish native submerged plant species (Bergqvist & Greger, 2012; Paper IV). The arsenic speciation results showed that more arsenate than arsenite was taken up (Table 4 in Paper IV). Arsenic extraction from the moss pellet after speciation analysis showed that the inorganic arsenic species were more loosely bound in arsenite-treated medium compared with arsenate-treated (Figure 11). However, arsenic speciation as a function of pH of the medium showed a different trend, with the extracted arsenic species being
Figure 11. Mean arsenic (As) species (arsenate and arsenite) concentration (mg/kg) and total As concentration (mg/kg) in moss pellets after treatment with 10 µM arsenate (As(ate)) or arsenite (As(ite)) for 72 hours. n=4, bars show standard error. (Note: log scale on y-axis)
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arsenite, except in one sample (Table 1 in Paper V). The reason for these differences in arsenic species extraction between Paper IV and Paper V could be that the different pH levels in Paper V stressed the moss and altered the rate of recovery of arsenic species from the moss tissue. In general, arsenic species extraction from moss in both studies showed that the arsenic species were tightly bound in the moss pellet (remaining biomass after extraction). This high amount of arsenic remaining in the moss pellet indicated that most accumulated arsenic in the moss was strongly bound within lipids, insoluble cellulose, pectin or lignin present in the plant cell (Koch et al., 2000). This strong binding of inorganic arsenic could be beneficial in preventing arsenic efflux into the growth medium and has potential for use in constructed wetlands.
5.3.5. Effect of abiotic factors on arsenic uptake in moss
5.3.5.1. Variation with pH level in water The most important fact regarding the effect of pH on inorganic arsenic species that needs to be high-lighted is that arsenite remains uncharged under neutral pH conditions. However, due to oxidation, arsenite can be transformed into arsenate in the medium, and that may affect the pH level (Lizama et al., 2011). Concerning the effect of different pH levels in the medium on arsenic species (arsenate or arsenite) accumulation (mg/kg) in the moss, it was found that in low pH conditions (pH 2.5), the remaining arsenic concentration had not changed much after 48 hours of the experiment (Figure 2 in Paper V). Despite arsenic remaining in the water, arsenic concentration in the moss varied significantly with pH (Figure 3 in Paper V). In that study, the lowest arsenic content (%) remaining in water was found at relatively neutral pH (pH 6.5). A study based on the aquatic macrophyte coontail (Ceratophyllum demesum L.) also suggested that this species could take up relatively more arsenic in neutral pH conditions, and that uptake declined with increasing pH level (Khang et al., 2012). Another previous study found the highest bioavailability of arsenate when the soil pH came closer to neutral level (pH 5.5) (Marin et al., 1993). The arsenic accumulation trend in the moss at different levels of pH in Paper V was in the following order: pH 6.5 > pH 9.5 > pH 2.5. The low arsenic accumulation in the moss at low pH can be explained as physiological stress caused by acidic pH conditions in the medium. Besides this, low arsenic uptake in low pH conditions could also be explained by low pH damaging cell membrane permeability, reducing arsenate affinity inside plant cells and ultimately causing leakage (efflux) of accumulated arsenic into the medium (Wells & Richardson, 1985).
Despite high arsenic accumulation in the moss from arsenate- treated medium, recovery of inorganic arsenic species was lower than with the arsenite-treated medium (Table 1 in Paper V). Most of the extracted inorganic arsenic species recovered from the moss
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at different pH levels was in the form of arsenite, with one exception. Another study also found that arsenate supplied in the medium could be rapidly converted to arsenite (Xue et al., 2012). However, a survey on arsenic species accumulation in different submerged plant species in Sweden found that accumulation of arsenic species followed the order arsenate>arsenite (Bergqvist & Greger, 2012). Compared with that study, the opposite effect of pH on arsenic species in W. fluitans was found in this thesis. The reason for these different results could be that the moss in this thesis was treated with arsenic species in laboratory conditions, which differ from natural conditions. The reasons for the high arsenite extraction from the moss by the arsenic speciation analysis could be that: 1) Accumulated arsenite species are loosely bound with moss internal tissue; 2) arsenite is more bioavailable in the medium than arsenate species; and 3) arsenite could be detoxified and bound by phytochelation inside moss tissue.
5.3.5.2. Effect of temperature regime Studies on the effect of temperature regime on arsenic accumulation in the moss (Paper V) showed that the arsenic accumulation process was not interrupted when the temperature levels were varied. Besides, arsenic removal from the water system was not significantly different between inorganic arsenic species (arsenate or arsenite) treatments of the water medium. The amount of arsenic remaining (%) in the water medium showed a similar trend for 10, 20 and 30 °C treatments, although an arsenic efflux effect was observed for the arsenate-treated medium after 48 h (Figure 4 in Paper V). The arsenic accumulation in moss showed no significant difference between arsenate and arsenite treatment compared with the control (zero) treatment (Figure 5 in Paper V). In general, it could be assumed from the results obtained that the temperature of the aqueous medium has no effect on arsenic uptake in the moss. Other published studies on the effect of temperature regime on arsenic uptake in plant species are few in number. However, one study has shown that phosphorus accumulation in plants fluctuates with temperature (Jonasson & Chapin, 1991). Since arsenate uptake follows the same phosphorus transporter pathways in different plant species, temperature could be expected to have an effect on arsenate accumulation in moss. However, in Paper V, the effect of temperature regime on arsenic uptake in moss growing in arsenate-treated medium was not significant. Another study has found that lower temperatures can reduce cell (cell membrane lipid) permeability, which can directly affect heavy metal uptake in plant tissues (Lynch & Steponkus, 1987; Fritioff et al., 2005). In Paper V, the lowest arsenic uptake in moss for both inorganic arsenic species treatments was found at the lowest temperature (12 °C) regime tested, but uptake was not significantly different between the other (20 or 30 °C) temperature regimes (Figure 5 in Paper V).
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5.3.5.3. Influence of oxygenation Inorganic As species accumulation in moss showed a trend for the highest As removal (%) from water under low oxygenation conditions in both treatments (arsenate, arsenite) (Figure 6 in Paper V). The remaining As (%) in the medium did not show any kind of arsenic efflux process from moss. In general, arsenic non- hyperaccumulator plant species show active efflux (50-80%) of their accumulated arsenic (Moreno-Jiménez et al., 2012). Therefore, lack of arsenic efflux by the moss indicates that lowering the oxygenation of the water medium under natural condition will result in higher arsenic removal by this macrophyte. The availability of oxygen in the medium is considered to be one of the important factors for the conversion of inorganic arsenic species in the medium (Zhao et al., 2010). It has also found that the accumulated arsenate can be reduced to arsenite and stored inside plant cell vacuoles (Moreno- Jiménez et al., 2012).
5.3.6. Feasibility of aquatic moss as a phytofilter candidate 5.3.6.1. Combined cropping method A study of arsenic content in lettuce in the presence of different amounts of aquatic moss (0 g/L, 5 g/L or 20 g/L water) in a hydroponic medium was performed (1-L bucket growth medium) for one week. Net arsenic uptake in lettuce showed a significantly (p<0.05) strong positive trend for an arsenic reduction in lettuce tissues when co-cropped with moss, depending on the content of aquatic moss in the growth medium (Figure 12).
The difference in net arsenic uptake in the lettuce could be explained by the prevalent force competition between plant species when they are grown in a given volume of water. Craine & Dybzinski (2013) suggested that the presence of different plant species in a certain area would create nutrient stress as the companion crop limited nutrient resources. Generally, two co-cropped plant species are exposed to more nutrient or element stress and competition when their nutrient absorption sites are situated close together.
Figure 12. Mean net arsenic (As) uptake (mg/g DW root) in lettuce treated with no moss (control 0) or 5 g or 20 g moss in hydroponic medium. n=3, different letters (a-b) shows significance between treatments (p<0.05).
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Figure 13. Schematic drawing of the moss-based wetland system treating water for irrigation of lettuce.
5.3.6.2. Moss-based constructed wetland system Another pilot study was performed to investigate the potential for arsenic removal in response to time and water flow in a moss-based constructed wetland system. The water flow from the inlet (1 µM arsenate) to the outlet had two different flow rates: 1.5 mL/min (low speed) or 5 mL/min (high speed) (Figure 13). The water was filtered through the moss in the wetland and was used for irrigation of the lettuce plants for 10 days.
The arsenic concentration in the lettuce leaves (edible part) showed a reduction compared with that in lettuce grown using contaminated water (Figure 14).
Figure 14. Mean total arsenic (As) concentration (µg/g) in lettuce leaves after application of treated water from the constructed wetland system at low flow speed (replicates low1, low2) or high flow speed (replicates high1, high2) and application of contaminated water with no moss (control), n=3, bars show standard error.
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These unpublished data about the arsenic content in the edible part of lettuce give a strong indication that using moss-based wetlands for contaminated water treatment could reduce the arsenic content in the irrigation water, which would consequently reduce the arsenic content in food crops.
36 Arsenic phytofiltration and silicon application-lowering arsenic concentrations in food crops
6. CONCLUSIONS The data presented in this thesis revealed a number of ways in which arsenic concentrations could be reduced in the human food chain. The conclusions of this thesis are as follows:
• Arsenic accumulation factor (AF) of rice varieties should be considered an important factor that can control arsenic uptake in rice grain, especially in arsenic hotspots. It is recommended that mean rice arsenic AF value be used to identify safe rice cultivars, which could contribute to less arsenic in the human food chain. In addition, knowledge of the arsenic AF value of different rice types could help local farmers and agronomists to select and cultivate less arsenic-accumulating varieties in arsenic-rich agricultural fields.
• To achieve low arsenic AF in the edible parts of crops and vegetables, application of silicon to the growing crop is recommended. The beneficial effect of silicon in lowering arsenic accumulation in lettuce was evident in the edible parts of the plant, especially following arsenite treatment of the growth medium.
• The literature recommends phytoremediation techniques for arsenic removal and this thesis showed that phytofiltration is a promising eco- friendly management technique that can be applied for different kinds of contaminated water systems.
• A new arsenic phytofilter candidate, the aquatic moss Warnstorfia fluitans, was identified and a series of experiments proved its capacity to remove arsenic from polluted water. This macrophyte showed fast arsenic uptake from water and accumulated both inorganic arsenic species (arsenate, arsenite), which were strongly bound inside plant cells. High bioconcentration factor and combined absorption-adsorption features of W. fluitans mean that it can be recommended as a potential arsenic phytofilter candidate, especially in mining areas situated in temperate regions. There are a number of positive features (sustainable, eco-friendly, economical) and only two features can be considered small drawbacks in the use of W. fluitans as phytofilter plant species such as (time required and fluctuations in arsenic absorption).
• To increase the benefits obtained from a sustainable eco-technology such as phytoremediation, the environmental conditions, especially abiotic factors (temperature, oxygenation, pH regime), need to be optimised.
• A pilot project combining lettuce cropping and a W. fluitans wetland treatment system showed that arsenic concentration in water can be reduced, which leads to less arsenic accumulation in the edible parts of vegetables irrigated with the treated water. Hence using an aquatic moss-
37 Arifin Sandhi TRITA-LWR PHD-2017:02
based phytofiltration technique could be a sustainable solution to reduce the arsenic concentration in irrigation water.
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REFERENCES Abedin, M. J., Cresser, M. S., Meharg, A. a, Feldmann, J., & Cotter- Howells, J. (2002). Arsenic accumulation and metabolism in rice (Oryza sativa L.). Environmental Science & Technology, 36(5), 962–8. Retrieved from Ahmed, Z. U., Panaullah, G. M., Gauch, H., McCouch, S. R., Tyagi, W., Kabir, M. S., & Duxbury, J. M. (2010). Genotype and environment effects on rice (Oryza sativa L.) grain arsenic concentration in Bangladesh. Plant and Soil, 338(1–2), 367–382. Al Rmalli, S. W., Harrington, C. F., Ayub, M., & Haris, P. I. (2005). A biomaterial based approach for arsenic removal from water. Journal of Environmental Monitoring, 7(4), 279–282. Amofah, L. R., Maurice, C., & Bhattacharya, P. (2010). Extraction of Arsenic from Soils Contaminated with Wood Preservation Chemicals. Soil and Sediment Contamination: An International Journal, 19(2), 142–159. Amofah, L. R., Maurice, C., Kumpiene, J., & Bhattacharya, P. (2011). The influence of temperature, pH/molarity and extractant on the removal of arsenic, chromium and zinc from contaminated soil. Journal of Soils and Sediments, 11(8), 1334–1344. Ankarberg, E. H., Foghelberg, P., Gustafsson, K., Nordenfors, H., & Bjerselius, R. (2015). Inorganic Arsenic in Rice and Rice Products on the Swedish Market 2015. Swedish National Food Agency. Report no. 16/2015. pp 1–33. Ayers, R. S., & Westcot, D. W. (1985). Water quality for agriculture. Irrigation and Drainage Paper 29 Rev. 1. FAO, Rome. 174 p. Baker, A. J. M. (1981). Accumulators and excluders -strategies in the response of plants to heavy metals. Journal of Plant Nutrition, 3(1–4), 643–654. Bergqvist, C., & Greger, M. (2012). Arsenic accumulation and speciation in plants from different habitats. Applied Geochemistry, 27(3), 615–622. Bergqvist, C. (2013). Arsenic accumulation in plants for food and phytoremediation: Influence by external factors. PhD Thesis, Stockholm University. http://doi.org/ISBN 978 - 91 - 7447 - 653 - 8 Bergqvist, C., Herbert, R., Persson, I., & Greger, M. (2014). Plants influence on arsenic availability and speciation in the rhizosphere, roots and shoots of three different vegetables. Environmental Pollution, 184, 540–546. BGS and DPHE. (2001). Arsenic contamination of groundwater in Bangladesh. (D. G. Kinniburgh & P. L. Smedley, Eds.). Keyworth: British Geological Survey. Retrieved from http://www.bgs.ac.uk/research/groundwater/health/arsenic/Ban gladesh/home.html Bhattacharya, P., Chatterjee, D., & Jacks, G. (1997). Occurrence of Arsenic-contaminatedGroundwater in Alluvial Aquifers from Delta
39 Arifin Sandhi TRITA-LWR PHD-2017:02
Plains, Eastern India: Options for Safe Drinking Water Supply. International Journal of Water Resources Development, 13(1), 79–92. Bhattacharya, P., Welch, A. H., Stollenwerk, K. G., McLaughlin, M. J., Bundschuh, J., & Panaullah, G. (2007). Arsenic in the environment: Biology and Chemistry. Science of the Total Environment, 379(2–3), 109–120. Bogdan, K., & Schenk, M. K. (2008). Arsenic in Rice (Oryza sativa L.) Related to Dynamics of Arsenic and Silicic Acid in Paddy Soils. Environmental Science & Technology, 42, 7885–7890. Bogdan, K., & Schenk, M. K. (2012). Arsenic mobilization in rice (Oryza sativa) and its accumulation in the grains. Journal of Plant Nutrition and Soil Science, 175(1), 135–141. Borho, M., & Wilderer, P. (1996). Optimized removal of arsenate(iii) by adaptation of oxidation and precipitation processes to the filtration step. Water Science and Technology, 34(9), 25–31. Brooks, R. R., & Robinson, B. H. (1998). Aquatic phytoremediation by accumulator plants. In R. R. Brooks (Ed.), Plants that hyperaccumulate heavy metals : their role in phytoremediation, microbiology, archaeology, mineral exploration and phytomining Wallingford: CAB International. pp. 203– 226. Butcher, D. (2009). Phytoremediation of Arsenic: Fundamental Studies, Practical Applications, and Future Prospects. Applied Spectroscopy Reviews, 44(6), 534–551. Caporale, A. G., Sommella, A., Lorito, M., Lombardi, N., Azam, S. M. G. G., Pigna, M., & Ruocco, M. (2014). Trichoderma spp. alleviate phytotoxicity in lettuce plants (Lactuca sativa L.) irrigated with arsenic-contaminated water. Journal of Plant Physiology, 171(15), 1378– 1384. Carbonell-Barrachina, A., Wu, X., Ramírez-Gandolfo, A., Norton, G. J., Burló, F., Deacon, C., & Meharg, A. (2012). Inorganic arsenic contents in rice-based infant foods from Spain, UK, China and USA. Environmental Pollution, 163, 77–83. Carbonell, A., Aarabi, M. ., DeLaune, R. ., Gambrell, R. ., & Patrick Jr, W. (1998). Arsenic in wetland vegetation: Availability, phytotoxicity, uptake and effects on plant growth and nutrition. Science of The Total Environment, 217(3), 189–199. Carlsson, E., Öhlander, B., & Holmström, H. (2003). Geochemistry of the infiltrating water in the vadose zone of a remediated tailings impoundment, Kristineberg mine,northern Sweden. Applied Geochemistry, 18(5), 659–674. Chen, G., Liu, X., Xu, J., Brookes, P. C., & Wu, J. (2014). Arsenic Species Uptake and Subcellular Distribution in Vallisneria natans (Lour.) Hara as Influenced by Aquatic pH. Bulletin of Environmental Contamination and Toxicology, 92(4), 478–482. Chen, G., Shi, H., Tao, J., Chen, L., Liu, Y., Lei, G., … Smol, J. P. (2015). Industrial arsenic contamination causes catastrophic changes in freshwater ecosystems. Scientific Reports, 5(17419), 1–7.
40 Arsenic phytofiltration and silicon application-lowering arsenic concentrations in food crops
Clemens, S., Palmgren, M. G., & Krämer, U. (2002). A long way ahead: understanding and engineering plant metal accumulation. Trends in Plant Science, 7(7), 309–315. Craine, J. M., & Dybzinski, R. (2013). Mechanisms of plant competition for nutrients, water and light. Functional Ecology, 27(4), 833–840. Cunningham, S. D., & Berti, W. R. (1993). Remediation of contaminated soils with green plants - An overview. In Vitro Cellular & Developmental Biology-Plant, 29(4), 207–212. Dambies, L., Vincent, T., & Guibal, E. (2002). Treatment of arsenic- containing solutions using chitosan derivatives: uptake mechanism and sorption performances. Water Research, 36(15), 3699–3710. Das, H., Mitra, A., Sengupta, P., Hossain, A., Islam, F., & Rabbani, G. (2004). Arsenic concentrations in rice, vegetables, and fish in Bangladesh: a preliminary study. Environment International, 30(3), 383–387. Dittmar, J., Voegelin, A., Maurer, F., Roberts, L. C., Hug, S. J., Saha, G. C., … Kretzschmar, R. (2010). Arsenic in soil and irrigation water affects arsenic uptake by rice: complementary insights from field and pot studies. Environmental Science & Technology, 44(23), 8842– 8848. Ekvall, L., & Greger, M. (2003). Effects of environmental biomass- producing factors on Cd uptake in two Swedish ecotypes of Pinus sylvestris. Environmental Pollution, 121(3), 401–411. Falk, H., Lavergren, U., & Bergbäck, B. (2006). Metal mobility in alum shale from Öland, Sweden. Journal of Geochemical Exploration, 90(3), 157–165. Fitz, W. J., & Wenzel, W. W. (2002). Arsenic transformations in the soil–rhizosphere–plant system: fundamentals and potential application to phytoremediation. Journal of Biotechnology, 99, 259–278. Fleck, A. T., Mattusch, J., & Schenk, M. K. (2013). Silicon decreases the arsenic level in rice grain by limiting arsenite transport. Journal of Plant Nutrition and Soil Science,176(5), 785–794. Forslund, J., Samakovlis, E., Johansson, M. V., & Barregard, L. (2010). Does remediation save lives? - on the cost of cleaning up arsenic- contaminated sites in Sweden. The Science of the Total Environment, 408(16), 3085–3091. Fritioff, A., & Greger, M. (2003). Aquatic and terrestrial plant species with potential to remove heavy metals from storm-water. International Journal of Phytoremediation, 5(3), 211–224. Fritioff, Å., Kautsky, L., & Greger, M. (2005). Influence of temperature and salinity on heavy metal uptake by submersed plants. Environmnetal Pollution, 133(2), 265–274. Gilbert-Diamond, D., Cottingham, K. L., Gruber, J. F., Punshon, T., Sayarath, V., Gandolfi, J., … Karagas, M. R. (2011). Rice consumption contributes to arsenic exposure in US women. Proceedings of the National Academy of Sciences of the United States of America, 108(51), 20656–20660.
41 Arifin Sandhi TRITA-LWR PHD-2017:02
Giri, P. K., Bhattacharyya, K., Sinha, B., & Mazumdar, D. (2012). Study of the Suitability of Selected Extractants for Determination of Plant- Available Arsenic in Some Inceptisols of West Bengal, India. Communications in Soil Science and Plant Analysis, 4319(43), 2449–2466. Greger, M., & Landberg, T. (1999). Use of Willow in Phytoextraction. International Journal of Phytoremediation, 1(2), 115–123. Greger, M. (2004). Metal Availability, Uptake, Transport and Accumulation in Plants. In MNV Prasad (Ed.), Heavy Metal Stress in Plants. Springer, Berlin Heidelberg. pp. 1–27. Greger, M., & Landberg, T. (2015). Silicon Reduces Cadmium and Arsenic Levels in Field-Grown Crops. Silicon, 1–5. Guha Mazumder, D., & Dasgupta, U. B. (2011). Chronic arsenic toxicity: studies in West Bengal, India. The Kaohsiung Journal of Medical Sciences, 27(9), 360–70. Gusman, G. S., Oliveira, J. A., Farnese, F. S., & Cambraia, J. (2013). Arsenate and arsenite: the toxic effects on photosynthesis and growth of lettuce plants. Acta Physiologiae Plantarum, 35(4), 1201– 1209. Hansen, H. K., Núñez, P., & Grandon, R. (2006). Electrocoagulation as a remediation tool for wastewaters containing arsenic. Minerals Engineering, 19(5), 521–524. Hartley, W., & Lepp, N. W. (2008). Remediation of arsenic contaminated soils by iron-oxide application, evaluated in terms of plant productivity, arsenic and phytotoxic metal uptake. Science of The Total Environment, 390(1), 35–44. Heikens, A., Panaullah, G. M., & Meharg, A. A. (2007). Arsenic behaviour from groundwater and soil to crops: impacts on agriculture and food safety. Reviews of Environmental Contamination and Toxicology, 189, 43–87. Holmstro, H., Carlsson, E., Öhlander, B., & Holmström, H. (2003). Geochemistry of the infiltrating water in the vadose zone of a remediated tailings impoundment, Kristineberg mine, northern Sweden. Applied Geochemistry, 18(5), 659–674. Huq, S. M. I., Joardar, J. C., Parvin, S., Correll, R., & Naidu, R. (2006). Arsenic contamination in food-chain: transfer of arsenic into food materials through groundwater irrigation. Journal of Health, Population, and Nutrition, 24(3), 305–316. Iriel, A., Lagorio, M. G., & Fernández Cirelli, A. (2015). Biosorption of arsenic from groundwater using Vallisneria gigantea plants. Kinetics, equilibrium and photophysical considerations. Chemosphere, 138, 383–389. Jang, M., Hwang, J. S., Choi, S. Il, & Park, J. K. (2005). Remediation of arsenic-contaminated soils and washing effluents. Chemosphere, 60(3), 344–354. Jennifer, C., & Jie, M. (2014). China’s Maximum Levels for Contaminants in Foods. Global Agricultural Information Network (GAIN), Report no. CH14058, pp. 1-19.
42 Arsenic phytofiltration and silicon application-lowering arsenic concentrations in food crops
Jomova, K., Jenisova, Z., Feszterova, M., Baros, S., Liska, J., Hudecova, D., … Valko, M. (2011). Arsenic: toxicity, oxidative stress and human disease. Journal of Applied Toxicology : JAT, 31(2), 95–107. Jonasson, S., & Chapin, F. S. III (1991). Seasonal uptake and allocation of phosphorus in Eriophorum vaginatum L measured by labelling with 32P. New Phytologist, 118(2), 349–357. Khan, M. A., Islam, M. R., Panaullah, G. M., Duxbury, J. M., Jahiruddin, M., & Loeppert, R. H. (2010). Accumulation of arsenic in soil and rice under wetland condition in Bangladesh. Plant and Soil, 333(1–2), 263–274. Khan, N. I., & Yang, H. (2014). Arsenic mitigation in Bangladesh: An analysis of institutional stakeholders’ opinions. Science of The Total Environment, 488, 493–504. Khang, H. V., Hatayama, M., & Inoue, C. (2012). Arsenic accumulation by aquatic macrophyte coontail (Ceratophyllum demersum L.) exposed to arsenite, and the effect of iron on the uptake of arsenite and arsenate. Environmental and Experimental Botany, 83, 47–52. Klumpp, D. W. (1980). Characteristics of arsenic accumulation by the seaweeds Fucus spiralis and Ascophyllum nodosum. Marine Biology, 58(4), 257–264. Koch, I., Wang, L., Ollson, C. a., Cullen, W. R., & Reimer, K. J. (2000). The Predominance of Inorganic Arsenic Species in Plants from Yellowknife, Northwest Territories, Canada. Environmental Science & Technology, 34(1), 22–26. Ladenberger, A., Andersson, M., Reimann, C., Tarvainen, T., Filzmoser, P., Uhlbäck, J., … Sadeghi, M. (2013). Geochemical mapping of agricultural soils and grazing land ( GEMAS ) in Norway , Finland and Sweden – regional report. SGU-report no. 2012:17, Uppsala. pp 1-160. Landberg, T., & Greger, M. (1996). Differences in uptake and tolerance to heavy metals in Salix from unpolluted and polluted areas. Applied Geochemistry, 11(1–2), 175–180. Leão, G. A. A., Oliveira, J. A. A., Farnese, F. S. S., Gusman, G. S. S., & Felipe, R. T. A. T. A. (2014). Sulfur metabolism: Different tolerances of two aquatic macrophytes exposed to arsenic. Ecotoxicology and Environmental Safety, 105, 36–42. Leupin, O. X., & Hug, S. J. (2005). Oxidation and removal of arsenic (III) from aerated groundwater by filtration through sand and zero- valent iron. Water Research, 39(9), 1729–1740. Li, H., Ye, Z. H., Wei, Z. J., & Wong, M. H. (2011). Root porosity and radial oxygen loss related to arsenic tolerance and uptake in wetland plants. Environmental Pollution, 159(1), 30–37. Liddle, J. R. (1982). Arsenic and other elements of geothermal origin in the Taupo Volcanic Zone. PhD Thesis, Massey University, Palmerston North, New Zealand. Liu, J., Zhang, H., Zhang, Y., & Chai, T. (2013). Silicon attenuates cadmium toxicity in Solanum nigrum L. by reducing cadmium uptake and oxidative stress. Plant Physiology and Biochemistry, 68, 1–7.
43 Arifin Sandhi TRITA-LWR PHD-2017:02
Liu, W. J., Zhu, Y. G., Hu, Y., Williams, P. N., Gault, G., Meharg, A., … Smith, F. (2006). Arsenic sequestration in iron plaque, its accumulation and speciation in mature rice plants (Oryza Sativa L.). Environmental Science and Technology, 40(18), 5730–5736. Lizama A., K., Fletcher, T. D., & Sun, G. (2011). Removal processes for arsenic in constructed wetlands. Chemosphere, 84(8), 1032–1043. Ljung, K., Palm, B., Grandér, M., & Vahter, M. (2011). High concentrations of essential and toxic elements in infant formula and infant foods - A matter of concern. Food Chemistry, 127(3), 943–951. Lynch, D. V, & Steponkus, P. L. (1987). Plasma Membrane Lipid Alterations Associated with Cold Acclimation of Winter Rye Seedlings (Secale cereale L. cv Puma). Plant Physiology, 83, 761–767. Ma, J. F., Tamai, K., Yamaji, N., Mitani, N., Konishi, S., Katsuhara, M., … Yano, M. (2006). A silicon transporter in rice. Nature, 440(7084), 688–691. Ma, L. Q., Komar, K. M., Tu, C., Zhang, W., Cai, Y., & Kennelley, E. D. (2001). A fern that hyperaccumulates arsenic. Nature, 409(6820), 579. Mandal, B., & Suzuki, K. T. (2002). Arsenic round the world: a review. Talanta, 58(1), 201–235. Marmiroli, M., Pigoni, V., Savo-Sardaro, M. L., & Marmiroli, N. (2014). The effect of silicon on the uptake and translocation of arsenic in tomato (Solanum lycopersicum L.). Environmental and Experimental Botany, 99, 9–17. Marschner, H. (1995). Mineral nutrition of higher plants (2nd ed.). Academic Press Limited, Cambridge. Marin, A. R., Masscheleyn, P. H., & Patrick, W. H. (1993). Soil redox- pH stability of arsenic species and its influence on arsenic uptake by rice. Plant and Soil, 152(2), 245–253. Medusa (nd). Hydra/Medusa software, Chemical Equilibrium Diagrams, Retrieved from http://www.kemi.kth.se/medusa. Meharg, A. A., & Hartley-Whitaker, J. (2002). Arsenic uptake and metabolism in arsenic resistant and nonresistant plant species. New Phytologist, 154, 29–43. Meharg, A. A., & Rahman, M. M. (2003). Arsenic contamination of Bangladesh paddy field soils: implications for rice contribution to arsenic consumption. Environmental Science & Technology, 37(2), 229– 34. Meharg, C., & Meharg, A. A. (2015). Silicon, the silver bullet for mitigating biotic and abiotic stress, and improving grain quality, in rice? Environmental and Experimental Botany, 120, 8–17. Mench, M., Lepp, N., Bert, V., Schwitzguébel, J.-P., Gawronski, S. W., Schröder, P., & Vangronsveld, J. (2010). Successes and limitations of phytotechnologies at field scale: outcomes, assessment and outlook from COST Action 859. Journal of Soils and Sediments, 10(6), 1039–1070.
44 Arsenic phytofiltration and silicon application-lowering arsenic concentrations in food crops
Mendez, M. O., & Maier, R. M. (2008). Phytostabilization of mine tailings in arid and semiarid environments--an emerging remediation technology. Environmental Health Perspectives, 116(3), 278–283. Mir, K. A., Rutter, A., Koch, I., Smith, P., Reimer, K. J., & Poland, J. S. (2007). Extraction and speciation of arsenic in plants grown on arsenic contaminated soils. Talanta, 72(4), 1507–1518. Mishra, V. K., & Tripathi, B. D. (2008). Concurrent removal and accumulation of heavy metals by the three aquatic macrophytes. Bioresource Technology, 99(15), 7091–7097. Mohan, D., & Pittman, C. U. (2007). Arsenic removal from water/wastewater using adsorbents-A critical review. Journal of Hazardous Materials, 142(1–2), 1–53. Moreno-Jiménez, E., Esteban, E., & Peñalosa, J. M. (2012). The Fate of Arsenic in Soil-Plant Systems. Springer New York. pp. 1–37. Mulligan, C. N., Yong, R. N., & Gibbs, B. F. (2001). Remediation technologies for metal-contaminated soils and groundwater: An evaluation. Engineering Geology, 60(1–4), 193–207. Naturvårdsverket. (2011). Environmental Quality Criteria Groundwater. Kalmer. Retrieved from https://www.naturvardsverket.se/Documents/publikationer/620- 6033-3.pdf Norton, G. J., Islam, M. R., Deacon, C. M., Zhao, F.-J., Stroud, J. L., McGrath, S. P., … Meharg, A. (2009). Identification of low inorganic and total grain arsenic rice cultivars from Bangladesh. Environmental Science & Technology, 43(15), 6070–6075. Norton, G. J., Pinson, S. R. M., Alexander, J., McKay, S., Hansen, H., Duan, G.-L., … Price, A. H. (2012). Variation in grain arsenic assessed in a diverse panel of rice (Oryza sativa) grown in multiple sites. The New Phytologist, 193(3), 650–664. Norton, G. J., Adomako, E. E., Deacon, C. M., Carey, A.-M., Price, A. H., & Meharg, A. A. (2013). Effect of organic matter amendment, arsenic amendment and water management regime on rice grain arsenic species. Environmental Pollution, 177, 38–47. Panaullah, G. M., Alam, T., Hossain, M. B., Loeppert, R. H., Lauren, J. G., Meisner, C. a., … Duxbury, J. M. (2009). Arsenic toxicity to rice (Oryza sativa L.) in Bangladesh. Plant and Soil, 317(1–2), 31–39. Pennesi, C., Vegliò, F., Totti, C., Romagnoli, T., & Beolchini, F. (2012). Nonliving biomass of marine macrophytes as arsenic(V) biosorbents. Journal of Applied Phycology, 24(6), 1495–1502. Prasad, M. N. V. (2007). Aquatic Plants for Phytotechnology. In S. N. Singh & R. D. Tripathi (Eds.), Environmental Bioremediation Technologies. Springer Berlin Heidelberg. pp. 259–274. Raab, A., Williams, P. N., Meharg, A., & Feldmann, J. (2007). Uptake and translocation of inorganic and methylated arsenic species by plants. Environmental Chemistry, 4(3), 197–203. Rahman, M. A., & Hasegawa, H. (2011). Aquatic arsenic: phytoremediation using floating macrophytes. Chemosphere, 83(5), 633–646.
45 Arifin Sandhi TRITA-LWR PHD-2017:02
Reynolds, J. G., Naylor, D. V., & Fendorf, S. E. (1999). Arsenic Sorption in Phosphate-Amended Soils during Flooding and Subsequent Aeration. Soil Science Society of America Journal, 63(5), 1149–1156. Riis, T., Olesen, B., Katborg, C. K., & Christoffersen, K. S. (2010). Growth Rate of an Aquatic Bryophyte (Warnstorfia fluitans (Hedw.) Loeske) from a High Arctic Lake: Effect of Nutrient Concentration. ARCTIC, 63(1), 100-106. Roberts, L. C., Hug, S. J., Voegelin, A., Dittmar, J., Kretzschmar, R., Wehrli, B., … Ali, M. A. (2011). Arsenic dynamics in porewater of an intermittently irrigated paddy field in Bangladesh. Environmental Science & Technology, 45(3), 971–976. Robinson, B., Duwig, C. C., Bolan, N., Kannathasan, M., & Saravanan, A. (2003). Uptake of arsenic by New Zealand watercress (Lepidium sativum). The Science of the Total Environment, 301(1–3), 67–73. Robinson, B., Kim, N., Marchetti, M., Moni, C., Schroeter, L., Van Den Dijssel, C., … Clothier, B. (2006). Arsenic hyperaccumulation by aquatic macrophytes in the Taupo Volcanic Zone, New Zealand. Environmental and Experimental Botany, 58(1–3), 206–215. Rofkar, J. R., & Dwyer, D. F. (2011). Effects of Light Regime, Temperature, and Plant Age on Uptake of Arsenic by Spartina Pectinata and Carex Stricta. International Journal of Phytoremediation, 13(6), 528–537. Rofkar, J. R., Dwyer, D. F., & Bobak, D. M. (2014). Uptake and Toxicity of Arsenic, Copper, and Silicon in Azolla caroliniana and Lemna minor. International Journal of Phytoremediation, 16(2), 155–166. Routh, J., Bhattacharya, A., Saraswathy, A., Jacks, G., & Bhattacharya, P. (2007). Arsenic remobilization from sediments contaminated with mine tailings near the Adak mine in Västerbotten district (northern Sweden). Journal of Geochemical Exploration, 92(1), 43–54. Routh, J., Saraswathy, A., & Collins, M. D. (2007). Arsenicicoccus bolidensis a novel arsenic reducing actinomycete in contaminated sediments near the Adak mine (northern Sweden): impact on water chemistry. The Science of the Total Environment, 379(2–3), 216–25. Saha, J. C., Dikshit, A. K., Bandyopadhyay, M., & Saha, K. C. (1999). A Review of Arsenic Poisoning and its Effects on Human Health. Critical Reviews in Environmental Science and Technology, 29(3), 281–313. Salt, D. E., Blaylock, M., Kumar, N. P., Dushenkov, V., Ensley, B. D., Chet, I., & Raskin, I. (1995). Phytoremediation: a novel strategy for the removal of toxic metals from the environment using plants. Bio/technology (Nature Publishing Company), 13(5), 468–474. Salt, D. E., Smith, R. D., & Raskin, I. (1998). Phytoremediation. Nnual Review of Plant Physiology and Plant Molecular Biology, 49, 643–668. Samecka-Cymerman, A., Kolon, K., & Kempers, A. J. (2005). A comparison of native and transplanted Fontinalis antipyretica Hedw. as biomonitors of water polluted with heavy metals. The Science of the Total Environment, 341(1–3), 97–107.
46 Arsenic phytofiltration and silicon application-lowering arsenic concentrations in food crops
Sengupta, M. K., Hossain, M. A., Mukherjee, A., Ahamed, S., Das, B., Nayak, B., … Chakraborti, D. (2006). Arsenic burden of cooked rice: Traditional and modern methods. Food and Chemical Toxicology, 44(11), 1823–1829. Signes, A., Mitra, K., Burlo, F., & Carbonell-Barrachina, A. A. (2008). Effect of cooking method and rice type on arsenic concentration in cooked rice and the estimation of arsenic dietary intake in a rural village in West Bengal, India. Food Additives & Contaminants. Part A, Chemistry, Analysis, Control, Exposure & Risk Assessment, 25(11), 1345– 1352. Singh, N., & Ma, L. Q. (2006). Arsenic speciation, and arsenic and phosphate distribution in arsenic hyperaccumulator Pteris vittata L. and non-hyperaccumulator Pteris ensiformis L. Environmental Pollution, 141(2), 238–246. Smedley, P. L., & Kinniburgh, D. G. (2002). A review of the source, behaviour and distribution of arsenic in natural waters. Applied Geochemistry, 17(5), 517–568. Smith, A. H., Lingas, E. O., & Rahman, M. (2000). Contamination of drinking-water by arsenic in Bangladesh: a public health emergency. Bulletin of the World Health Organization, 78(9), 1093–1103. Spanu, A., Daga, L., Orlandoni, A. M., & Sanna, G. (2012). The role of irrigation techniques in arsenic bioaccumulation in rice (Oryza sativa L.). Environmental Science & Technology, 46(15), 8333–8340. Srivastava, S., Mishra, S., Tripathi, R. D., Dwiveldi, S., Trivedi, P. K., & Tandon, P. K. (2007). Phytochelatins and Antioxidant Systems Respond Differentially during Arsenite and Arsenate Stress in Hydrilla verticillata ( L.f.) Royle. Environmental Science & Technology, 41(8), 2930–2936. Srivastava, S., Shrivastava, M., Suprasanna, P., & D’Souza, S. F. (2011). Phytofiltration of arsenic from simulated contaminated water using Hydrilla verticillata in field conditions. Ecological Engineering, 37(11), 1937–1941. Stoltz, E., & Greger, M. (2002). Accumulation properties of As, Cd, Cu, Pb and Zn by four wetland plant species growing on submerged mine tailings. Environmental and Experimental Botany, 47(3), 271–280. Stoltz, E., & Greger, M. (2006). Influences of wetland plants on weathered acidic mine tailings. Environmental Pollution, 144, 689– 694. Tokunaga, S., & Hakuta, T. (2002). Acid washing and stabilization of an artificial arsenic-contaminated soil. Chemosphere, 46(1), 31–38. Tripathi, R. D., Srivastava, S., Mishra, S., Singh, N., Tuli, R., Gupta, D. K., & Maathuis, F. J. M. (2007). Arsenic hazards: strategies for tolerance and remediation by plants. Trends in Biotechnology, 25(4), 158–165. Tu, S., & Ma, L. Q. (2003). Interactive effects of pH, arsenic and phosphorus on uptake of As and P and growth of the arsenic hyperaccumulator Pteris vittata L. under hydroponic conditions. Environmental and Experimental Botany, 50(3), 243–251.
47 Arifin Sandhi TRITA-LWR PHD-2017:02
United Nations. (2015). Transforming our world: the 2030 Agenda for Sustainable Development. Retrieved from http://www.un.org/ga/search/view_doc.asp?symbol=A/RES/70 /1&Lang=E Uroic, M. K., Salaün, P., Raab, A., & Feldmann, J. (2012). Arsenate Impact on the Metabolite Profile, Production, and Arsenic Loading of Xylem Sap in Cucumbers (Cucumis sativus L.). Frontiers in Physiology, 3, 55(1-23). Vahter, M. (2009). Effects of arsenic on maternal and fetal health. Annual Review of Nutrition, 29, 381–399. von Brömssen, M., Jakariya, M., Ahmed, K. M., Hasan, M. A., Sracek, O., Jonsson, L., … Jacks, G. (2007). Targeting low-arsenic aquifers in Matlab Upazila, Southeastern Bangladesh. Science of The Total Environment, 379(2), 121–132. Wan, X., Lei, M., & Chen, T. (2016). Cost–benefit calculation of phytoremediation technology for heavy-metal-contaminated soil. Science of The Total Environment, 563, 796–802. Wang, S., & Mulligan, C. N. (2006). Occurrence of arsenic contamination in Canada : Sources , behavior and distribution. Science of The Total Environment, 366, 701–721. Wells, J. M., & Richardson, D. H. S. (1985). Anion accumulation by the moss Hylocomium splendens: uptake and competition studies involving arsenate, selenate, selenite, phosphate, sulphate and sulphite. New Phytologist, 101(4), 571–583. WHO. (2011). Guidelines for Drinking-water Quality. World Health Organization. WHO Press. Retrieved from http://www.who.int Williams, P. N., Price, A. H., Raab, A., Hossain, S. A., Feldmann, J., & Meharg, A. A. (2005). Variation in arsenic speciation and concentration in paddy rice related to dietary exposure. Environmental Science & Technology, 39(15), 5531–5540. Williams, P. N., Islam, M. R., Adomako, E. E., Raab, A, Hossain, S. A, Zhu, Y. G., … Meharg, A. A. (2006). Increase in rice grain arsenic for regions of Bangladesh irrigating paddies with elevated arsenic in groundwaters. Environmental Science & Technology, 40(16), 4903–4908. Wu, C., Huang, L., Xue, S.-G., Pan, W.-S., Zou, Q., Hartley, W., & Wong, M.-H. (2017). Oxic and anoxic conditions affect arsenic (As) accumulation and arsenite transporter expression in rice. Chemosphere, 168, 969–975. Xue, P., Yan, C., Sun, G., & Luo, Z. (2012). Arsenic accumulation and speciation in the submerged macrophyte Ceratophyllum demersum L. Environmental Science and Pollution Research, 19(9), 3969–3976. Yamamoto, T., Nakamura, A., Iwai, H., Ishii, T., Ma, J. F., Yokoyama, R., … Furukawa, J. (2012). Effect of silicon deficiency on secondary cell wall synthesis in rice leaf. Journal of Plant Research, 125(6), 771– 779. Zhang, X., Hu, Y., Liu, Y., & Chen, B. (2011). Arsenic uptake, accumulation and phytofiltration by duckweed (Spirodela polyrhiza L.). Journal of Environmental Sciences, 23(4), 601–606.
48 Arsenic phytofiltration and silicon application-lowering arsenic concentrations in food crops
Zhao, F. J., Meharg, A. A., & Mcgrath, S. P. (2009). Arsenic uptake and metabolism in plants. New Phytologist, (181), 777–794. Zhao, F., Mcgrath, S. P., & Meharg, A. A. (2010). Arsenic as a food chain contaminant : mechanisms of plant uptake and metabolism and mitigation strategies. Annual Review of Plant Biology, 61, 535–559. Zumdahl, S. S. (1992). Chemical Principles (1st ed.). Mishawaka: D.C. Heath and Co., USA.
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