The Role of Black Carbon in Environmental Fate of Persistent Organic Pollutants (Pops) in Soils and Their Effect on Food Safety

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The role of black carbon in environmental fate of persistent organic pollutants (POPs) in soils and their effect on food safety

Rai S Kookana CSIRO Land and Water, PMB 2, Glen Osmond, 5064, Australia E-mail: [email protected]

Abstract

Partitioning of POPs into different compartments of environment (soil, air, water, sediment) determine their environmental fate, transport, accumulation and impact on biota and food quality. Physico-chemical properties of soil and sediments, especially the content and the chemistry of organic carbon, have a major bearing on not only the sorption and accumulation of POPs but also their degradation and plant uptake. It is now well known that “black carbon (BC)” (partially combusted material such as soot and charcoal) which has a strong affinity for POPs is ubiquitous in soils and sediments. However, the role of soil and sediment organic carbon, especially of BC, has not been fully appreciated or accounted for in assessment of distribution and impact of POPs on food safety. This paper demonstrates the importance of the chemistry of organic carbon as a strong moderating factors for several processes governing the accumulation, distribution and plant uptake of POPs, namely, sorption-desorption, degradation and bioavailability to plants/microorganisms. Recognizing that, as a response to climate change, biochar amendments of soils is attracting increasing interest globally and “black is seen as a new green”, the paper highlights the need for better understanding of the long term implications of BC in determining the fate and behaviour of POPs in soils and their potential impact on food safety.

Key words: Soil organic matter, black carbon, partitioning, plant uptake, food safety, bioavailability, POPs.

Abbreviations: BC, black carbon; HCB, hexachlorobenzene; HCH, hexachlorocyclo- hexane; Koc, organic carbon normalised sorption coefficient; MIR, midinfrared spectroscopy; NMR, nuclear magnetic resonance spectroscopy; OCDD, octachlorodibenzodioxin; PAH, polynuclear aromatic hydrocarbon, PCDD/Fs, dibenzo-p-dioxins and dibenzofurans; POPs, persistent organic pollutants; PCBs, polychlorinated biphenyls; SOC, soil/sediment organic carbon

Introduction Lohmann et al. 2007). Therefore, their partitioning properties play a crucial role in determining their Persistent organic pollutants (POPs) produced environmental fate, transport, accumulation, and used in industrialized nations are a cause of impact on biota and food safety. great concern globally due to their persistent, Soil has been identified as a one of the bioaccumulative and toxic nature and their most important repositories for fluxes of POPs propensity to travel long distances to affect even between soil and air and soil processes can remote, uninhabited parts of the globe. POPs have a large bearing on their future distribution have diverse physical-chemical properties and are patterns (Meijer et al. 2003, Sweetman et al. 2005, released in the environment from varied sources. Lohmann et al. 2007). While the importance of soil Based on their partitioning properties in different biogeochemistry and the role of soil or sediment environmental compartments (e.g. air, water, organic carbon (SOC) was recognized some thirty soil) POPs have been classified by Wania (2006) years ago (Kirckhoff et al. 1979), it is only in the as “swimmers” (e.g. HCHs), “single to multiple last decade, SOC has been recognized as the key hoppers” (e.g. PCBs, organochlorine pesticides, parameter governing spatial variability of POPs chlorobenzenes) and “flyers” (emerging polar in soils (Cousins et al. 1999) and partitioning in POPs such as perfluorinated octyl sulfonamides, the marine environment (Lohmann et al. 2005).

1 However, most POP studies so far have not to be expected depending on the partitioning and incorporated the complex biogeochemistry of SOC degradation processes and the inherent properties in terrestrial and aquatic ecosystems (Lohmann et of POPs. Meijer et al. (2002) using soils from al. 2007). a transect from the UK and Norway showed Lohmann et al. (2007) compared the that PCB concentrations varied up to 4 orders of predicted potential reservoirs of POPs with magnitude and SOC played an important role in the measured data on soils. The predicted soil their spatial distribution. Sweetman et al. (2005) maximum reservoir capacities for POPs ranged further examined the data and found the strongest from 60 ◦N to 80◦N, and were mainly a function of correlation between HCB concentrations in soil and SOC content and temperature (Fig. 1). However, the SOC, followed by PCB-52 (Fig. 2). However, the actual measurements by Meijer et al. (2003) the correlation between OCDD with SOC was the showed the maximum soil concentrations at 60 ◦N, weakest. HCB being the most volatile of the three, but ranging from 50 ◦N to 70 ◦N, suggesting that a near steady-state condition between air and SOC other processes such as differences in sources and via recurring soil-air exchange was suggested as the other biogeochemical and global dynamics were mechanism. However, for octachlorodibenzodioxin playing a role. Meijer et al., (2003) also highlighted (OCDD) the opposite was the likely scenario. the likelihood of a much retarded transport (grass- Heywood et al 2006 also reported no correlation hopping of POPs) due to strong sorption to SOC between SOC content and the sum of total or heavy than what has been taken into account in such PAH concentrations. A weak, but nonetheless predictions. Studies have shown that temperature significant (P < 0.05), positive correlation was and SOC are the drivers of the observed variability found with the total light PAHs congeners. Wilcke in the distribution of POPs. and Amelung (2000) found a strong positive correlation between total PAH concentration and Relationship of fate and distribution of SOC content in soil from remote sites in North POPs with soil organic carbon America. Notably the PAH profiles at these sites were dominated by lighter compounds. In all of the Due to their hydrophobic nature, most POPs above studies, the partition coefficient determined have a strong affinity for SOC. Therefore, global the strength of the relationship. surface soils have been recognized to serve as an Ren et al. (2007) assessed the distribution important reservoir for POPs, representing an of PCBs load in 52 Chinese surface soils (0- integration of several processes over a time scale of 20cm) collected from different regions and found years to decades (Sweetman et al. 2005). A strong a statistically significant but weaker (p <0.005) relationship between soil concentration and SOC is correlation with SOC (Fig. 3). However, the

Fig. 1. Predicted maximum soil reservoir capacity of POPs in top soils and surface mixed layer of ocean in comparison to that in the atmosphere. Source Lohmann et al. 2007 based on Dalla Valle et al. 2005.

2 correlation improved significantly (p <0.001) when The role of soil organic carbon the longitude was considered. They also noted chemistry in determining the a longitudinal fractionation of PCBs in Chinese fate and effects of POPs background/ rural soil from east to west (Fig. 3). Clearly, both the proximity of the source region and Not only the content of SOC but its chemistry SOC determined the distribution of PCBs in this has a strong influence on the environmental fate study. Recently Koblickova et al. 2008 reported and behaviour of POPs in soils and sediments. a similar relation between spatial distribution of Soil organic matter consists of heterogeneous PCBs and SOC in soils from the Czech Republic. mix of organic materials often classified as Considering the importance of SOC in an amorphous, gel-like “soft carbon” matrix dynamics, distributions and long range transport of or domain and a condensed, glasslike “hard POPs, in recent years several workers have called carbon” matrix or domain (Weber et al. 1992), for a greater understanding and accounting of role including altered and relatively unaltered aliphatic of SOC in fluxes, distributions and long range polymers, polysaccharides (e.g., cellulose), lignin transport of organic contaminants (Lohmann et al. and lignin degradation products, fats, proteins, 2007, Scheringer, 2009, Koblickova et al. 2008). pectines (Schumann 2006). Highly carbonaceous

Fig. 2. Correlation between soil concentrations of POPs (HCB and PCB-52) and soil organic carbon contents. Source: Sweetman et al. 2005.

Fig. 3. Correlation of total PCB load in 52 Chinese soils with soil organic carbon (upper) and with organic carbon and location (lower panel). Source: Ren et al. 2007

3 organic matter from partial combustion processes assessed the aquatic fate of dibenzo-p-dioxins and represents another important constituent of organic dibenzofurans (PCDD/Fs) and made a comparison matter in soils and sediments. This includes chars, of model predictions with observed organic carbon soots, and other highly carbonaceous material normalized sediment-water partition coefficients commonly termed as “black carbon” (Accardi-Dey (Koc) and dissolved water concentrations. They and Gschwend, 2003). Black carbon (BC) fraction found that for estimation of Koc or dissolved water in soils and sediments can be as high as 45-50% concentrations, the sorption models inclusive of of total organic carbon (Allen-King et al. 2002; black carbon provided greatly improved estimates Schmidt et al. 2002). compared to the amorphous carbon only model. Highly carbonaceous BC acts like activated In Fig. 5, the typical log Koc values observed for carbon and shows very high affinities for organic different chemistries of organic carbon found in compounds including POPs (e.g. Ahmad et al 2001; soils and sediments have been shown, highlighting Allen-King et al. 2002; Jonker and Koelmans, that Koc values can vary by several orders of 2002). Studies on sorption of pesticides in soils magnitude between humic acids and charcoal or have established a direct correlation between BC in soils and sediments (Kleineidam et al. 1999). aromaticity of SOC and sorption coefficients Furthermore, sorption on BC is generally highly of pesticides (Ahmad et al 2001, Ablemann et non-linear (indicated by decreasing Freundlich al. 2005). The relationship between partition exponent (n) with increase in aromaticity of SOC in coefficient of a pesticide and the aromaticity of Fig. 5) and has been found to be highly irreversible SOC, as determined by a solid state C13 nuclear leading to strong sorption-desorption hysteresis and magnetic resonance (NMR), has been shown reduced bioavailability (e.g. Braida et al. 2002; Yu in Fig. 4. Sorption of other organic compounds et al. 2006). Yu et al. (2006) assessed the sorption such as PAHs, chlorobenzenes and mono-ortho- and desorption behaviour of a pesticide in soils polychlorinated biphenyls (PCBs) in soils and that were amended with different types of biochars sediment has been reported to be up to 4 orders of at different amounts. They observed that as the magnitude higher than could be anticipated based content of biochar (especially the one produced at on the compounds’ hydrophobicities (Ahmad et al. higher temperature and with higher aromaticity) 2001, Allen-King et al. 2002; Jonker and Koelman, increased in the soil, the sorbed pesticide in soil 2002). became resistant to desorption, indicating effective In recent years, it is increasingly being sequestration of the sorbed residue by BC in soil recognized that BC is a key vector for transport (Fig. 6). The implications of BC on persistence and partitioning of POPs in marine environment of organic compounds in soils and food safety are (Lohmann et al. 2005). In sediments also, BC has discussed in the following section. been found to be the major sorbent for hydrophobic and planar POPs (Gustafson et al.1997; Implications for food safety Cornelissen et al. 2005). Armitage et al. 2008 Food safety of POPs is directly linked to the environmental fate and behaviour of POPs in soil which is strongly influenced by the content and chemistry of SOC. In addition to the effect on partitioning and accumulation of POPs, soil organic matter provides the substrate to support microbial activity and thereby plays a direct role

Fig. 4. Correlation between chemistry of soil organic carbon (aromaticity measured through solid state NMR spectroscopy) and partition coefficient of a pesticide. Source: Ahmad et al. 2001.

4 Fig. 5. Log sorption coefficients (log Koc) for different chemistries of organic carbon found in terrestrial and aquatic ecosystems. Adapted from K leineidam et al. 1999.

Fig. 6. Sorption desorption isotherms demonstrating the increased sorption (and its non-linearity) and sorption-desorption hysteresis with increase in black carbon content of soil (BC 850 is black carbon produced at 850OC). Source: Yu et al. 2006. in their persistence and degradation. However, for 1. SOC content and chemistry determines a contaminant to be degraded by microorganisms the partition and extent of accumulation of or to be taken up by plant, it needed to be released POPs in soil, as described in the sections into soil solution or sediment pore waters, in other above; words, it needs to be “bioavailable”. The organic 2. The release behaviour of POPs is related to matter in soil can influence the food safety of SOC and its chemistry which determines POPs through several direct and indirect ways, as its bioavailability in the environment; described in the following: 3. The flux or air-soil exchange of POPs

5 depends on the strength of partitioning and containing varying amounts of SOCs. The apparent accumulation of POPs in soils, which in accumulation of PAHs by wheat roots was found turn depends on SOC, as discussed above. to be positively and negatively correlated with Furthermore, both the content as well as dissolved organic matter and total organic matter, the chemistry of SOC can potentially affect respectively. These results were explained on the direct deposition of the airborne POPs - an basis that while total SOC reduced the extractability important pathway impacting food quality; through sorption, the dissolved organic matter 4. Microbial population density or soil’s mobilised the sorbed PAHs and facilitated their ability to degrade an organic compound availability to wheat roots, thus showing the is often linked to SOC and thereby contrasting role of the soil organic matter based on persistence of POPs depends on SOC. its chemistry. Also, the content and chemistry of SOC The role of SOC chemistry on persistence can determine the enantio-selective and plant uptake of pesticide residue from degradation of chiral POPs. Enantiomers contaminated soils was further demonstrated are often used as tracers for sources in recently through a study by Yu et al. (2009). They atmosphere of several POPs that are chiral amended soils with different amounts of two types in nature (Kurt-Karakus et al. 2005); of biochars (produced at different temperatures and 5. For plant uptake, the residues of POPs representing different physico-chemical properties) bound to SOC need to be first mobilised and assessed the plant uptake of two pesticides into soil solution for it to be phytoavailable. (of differing hydrophobicities) in both above and Phytoavailability of POPs has been shown below ground plant parts of spring onions (Allium to be related to SOC, as discussed below. cepa). They observed that the bioavailability of the Both extractability and bioavailability two pesticides to microorganisms for degradation of POPs in soils depend on soil organic matter as well as for plant uptake decreased as the content content and its chemistry. Tao et al. (2006) tested of the BC in soil increased. Figure 7 shows that the the hypotheses by assessing the extractability persistence of carbofuran insecticide increased with of polycyclic aromatic hydrocarbons (PAHs) by increased BC content in soil (indicating reduced wheat roots (Triticum aestivum L.) from artificially bioavailability to soil microorganisms), whereas contaminated soils in pot experiments. They added the decreased plant uptake of two insecticides with four PAHs (naphthalene, acenaphthylene, fluorene, increased BC contents by spring onions is shown in and phenanthrene) to 20 Chinese surface soils Fig. 8.

Fig. 7. Demonstration of increased persistence of carbofuran insecticide in soil with increase in contents of two types of black carbon content of soil (BC450 and BC850 are two wood biochars produced at 450O C and 8500C, respectively). Source: Yu et al. 2009.

6 Fig. 8. Demonstration of decreased plant uptake of two insecticides (carbofuran – solid bars and chlorpyrifos – empty bars) in above- and under-ground plant parts of spring onion with increase in contents of two types of black carbon content of soil (BC450 and BC850 are two wood biochars produced at 4500C and 8500C, respectively). Source: Yu et al. 2009.

The above studies highlight the importance 2009), Australia and elsewhere. of understanding the role of SOC and BC content The presence of legacy POPs in agricultural and chemistry in the plant uptake and food safety soils and potential implications for food safety, of POPs in soil. In this era of climate change, this is especially plants belonging to the Cucurbita genera becoming increasingly important for the prediction (Otani et al. 2007) has highlighted the need to and management of POPs residues in contaminated manage agricultural soils as source of POPs and soils. Let us explore this aspect further in the predict the extractability of residues by plants following section. based on soil physico-chemical properties such as SOC (Tao et al. 2006). The link between food Management of residues of POPs in safety and physico-chemical properties of soils as soils and food safety: demonstrated by several workers (Tao et al. 2006, A climate change link Yu et al. 2009) means that alteration of the nature of the SOC pool would have implication for the The foregoing discussion clearly demonstrates strength of binding and potential extractability of the importance of BC in sequestration of the the residues by plant roots. Therefore, predictive organic contaminants in terrestrial and aquatic tools incorporating the role of SOC and BC need ecosystems and how this could have bearing on the to be developed which could be used as a screening accumulation of POPs in soils and their potential tools to identify soils with potential concern for uptake by plants. The importance of biochar or BC food safety. in sequestration of carbon, reducing the emission To study soil organic matter chemistry, solid of greenhouse gases and improving the soil fertility state NMR has historically been employed as a has led to International Biochar Initiative (www. gold-standard. However, the technique involves biochar-international.org) promoting the biochar as labour-intensive sample preparation steps limiting a soil amendment which is increasingly attracting its broad scale applicability as a rapid screening the attentions of policy makers in USA (Bracmort, tool. Recently, however, studies have shown that

7 MIR spectroscopy can better predict sorption of uptake. BC is ubiquitous in soils and sediments organic contaminants in soils, by incorporating a and can make upto half of the total content of soil range of soil physico-chemical properties such as or sediment organic carbon mass. BC has a strong SOC content, its chemistry, soil mineralogy and affinity for POPs and can accumulate and render texture (Kookana, et al. 2008, Forouzangohar et the compound unavailable. However, the role of al. 2008). MIR spectroscopy is a rapid assessment soil and sediment organic carbon, especially of tool requiring less sample preparation and high BC, has not been fully appreciated in assessment throughput making it a cost-effective screening tool of distribution and impact of POPs on food safety. to incorporate soil physico-chemical properties in As shown above, the chemistry of SOC as a strong assessment of contaminant behaviour. moderating influence on a range of processes, Assuming that the fraction of bound organic namely, sorption-desorption, degradation, pollutants inaccessible to plant root may not be bioavailability to plants and soil microorganisms recovered from soil by certain mild chemical governing the accumulation, distribution and extraction, Tao et al. (2006) also explored the plant uptake of POPs. Since biochar amendments approach of using mild solvent extraction as a of soils is attracting increasing interest globally, predictive or management tool for assessment of in response to climate change, and “black is seen plant extractable pool of POPs in soil. They found as a new green”, there is an urgent need for better that the water or n-hexane-extractable PAHs were understanding of the long term implications of BC positively correlated to dissolved organic matter in determining the fate and behaviour of POPs fraction and negatively correlated to total organic in soils and their potential impact on food safety. matter, indicating mobilization and immobilization Combination of rapid assessment tools such as effects of the two different types of organic matter MIR spectroscopy and mild solvent extractants on soil PAHs, respectively. This, together with for identifying problem soils for food safety can other approaches, such as a rapid assessment of potentially be a useful management tool for food soil organic carbon content and chemistry through safety of plant products. MIR spectroscopy, may potentially provide a relatively rapid screening tool for identifying the References problem soils and predicting the potential uptake of contaminants by plants. Abelmann, K., Kleineidam, S., Knicker, H., In recent years substantial investment is Grathwohl, P., Ko¨gel-Knabner, I., 2005. being directed to conduct research on biochars in Sorption of HOC in soils with carbonaceous soils in USA, Australia and several other countries. contamination: influence of organic-matter “Black is now seen as a new green” as one of the composition. J. Plant Nutr. Soil Sci. 168, 293– articles in Nature (Marris, 2006) put it. Based 306. on limited research cited in the above sections, Accardi-Dey, A., Gschwend, P.M., 2003. biochar amendment can conceivably reduce the Reinterpreting literature sorption data bioavailability of POPs on one hand but can potential considering both absorption into organic serve as a source as well as a phase of accumulation carbon and adsorption onto black carbon. of POPs and other organic contaminants on the Environ. Sci. Technol. 37, 99–106. other. The implication of biochar amendment to soil Ahmad, R., Kookana, R.S., Alston, A.M., Skjemstad, and potential implications for the environmental J.O., 2001. The nature of soil organic matter accumulation, distribution and food safety of POPs affects sorption of pesticides: 1. Relationships needs to be understood. The long term effect of with carbon chemistry as determined by 13C biochar on the fate and behaviour of POPs deserves CPMAS NMR spectroscopy. Environ. Sci. urgent attention both as a potential management Technol. 35, 878–884. tool as well as a factor affecting the distribution and Allen-King, R.M., Grathwohl, P., Ball, W.P., global transport of POPs. 2002. New modeling paradigms for the sorption of hydrophobic organic chemicals to Conclusions heterogeneous carbonaceous matter in soils, sediments and rocks. Adv. Water Resour. 25, Physico-chemical properties of soil and sediments, 985–1016. especially the content and nature of BC, have a major Armitage, J.M., IT Cousins, NJ Persson, O. bearing on not only the sorption and accumulation Gustaffson, G. Cornelissen, T Saloranta, D of POPs but also their degradation and plant Broman and K Naes. 2008. Black Carbon-

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