Elementary Iodine-Doped Activated Carbon As an Oxidizing Agent for the Treatment of Arsenic-Enriched Drinking Water

water

Article Elementary Iodine-Doped Activated Carbon as an Oxidizing Agent for the Treatment of Arsenic-Enriched Drinking Water

Fabio Spaziani 1,2,*, Yuli Natori 1, Yoshiaki Kinase 1, Tomohiko Kawakami 1 and Katsuyoshi Tatenuma 1

1 KAKEN Inc., Mito Institute 1044 Hori, Mito, Ibaraki 310-0903, Japan 2 ENEA Casaccia, Via Anguillarese 301, 00123 Roma, Italy * Correspondence: [email protected]

Received: 18 July 2019; Accepted: 23 August 2019; Published: 27 August 2019

Abstract: An activated carbon impregnated with elementary iodine (I2), named IodAC, characterized by oxidation capability, was developed and applied to oxidize arsenite, As(III), to arsenate, As(V), in arsenic-rich waters. Batch and column experiments were conducted to test the oxidation ability of the material. Comparisons with the oxidizing agents usually used in arsenic treatment systems were also conducted. In addition, the material has been tested coupled with an iron-based arsenic sorbent, in order to verify its suitability for the dearsenication of drinking waters. IodAC exhibited a high and lasting oxidation potential, since the column tests executed on water spiked with 50 mg/L of arsenic (100% arsenite) showed that 1 cc of IodAC (30 wt% I2) can oxidize about 25 mg of As(III) (0.33 mmol) before showing a dwindling in the oxidation ability. Moreover, an improvement of the arsenic sorption capability of the tested sorbent was also proved. The results conﬁrmed that IodAC is suitable for implementation in water dearsenication plants, in place of the commonly used oxidizing agents, such as sodium hypochlorite or potassium permanganate, and in association with arsenic sorbents. In addition, the well-known antibacterial ability of iodine makes IodAC particularly suitable in areas (such developing countries) where the sanitation of water is a critical topic.

Keywords: elementary iodine; activated carbon; arsenic; water puriﬁcation; drinking water

1. Introduction Water is essential to life, and ensuring safe drinking water is a priority in any country. The health risks associated with the consumption of unsafe drinking water are related to infectious diseases but also to environmental components such as ﬂuoride, arsenic, lead, cadmium, nitrates and mercury [1]. There is an urgent need to develop suitable water treatment strategies especially for small rural and remote communities in low-income developing countries [2]. However, providing safe drinking water is also a crucial task in the managing of emergency situations (such as earthquake, ﬂooding, etc.). Therefore, even in the so-called “more developed countries”, bacteria and chemical contamination must be constantly monitored and, if detected, removed. Arsenic is one of the most widespread drinking water-related concerns, due to its toxicity, and because this element occurs naturally in many groundwater sources around the world [3–6]. Soluble arsenic can be found in water in both inorganic and organic forms. Inorganic arsenic compounds (such as those predominantly found in groundwater) are highly toxic, while organic arsenic compounds (mainly found instead in seafood) are less harmful to health. Moreover, inorganic arsenic can occur in several oxidation states (+5, +3, 0 and 3), depending on the physic-chemical − characteristics (pH and Eh) of the water, but in natural water systems it is mostly found as trivalent

Water 2019, 11, 1778; doi:10.3390/w11091778 www.mdpi.com/journal/water Water 2019, 11, 1778 2 of 14 arsenic (As(III), arsenite) or pentavalent arsenic (As(V), arsenate). Under oxidizing conditions, such as the ones typically found in surface waters, the major species is As(V), mainly present as its oxyanionic 2 forms (H2AsO4−, HAsO4 −) at the common pH range of natural waters. In contrast, under mildly reducing conditions, such as in groundwater, As(III) is the thermodynamically stable form, which, at the common pH values found in natural waters, occurs as non-ionic form of arsenious acid (H3AsO3)[7–9]. The health risks associated with the long-time consumption of arsenic contaminated water are well documented [10,11], and the World Health Organization (WHO), European Union (EU) and the US Environmental Protection Agency (US-EPA) have consequently defined the arsenic standard for drinking water as 10 µg/L. The toxicity of inorganic arsenic is related to its aptitude to combine with sulfhydryl groups in proteins and, therefore, to accumulate in the body. In this regard, it is important to notice that arsenite is potentially more toxic since it has a higher affinity for protein than arsenate [12]. The natural occurrence of arsenic in groundwater is mainly linked to the influence of geothermal systems, such as mixing with arsenic-rich deep-rising fluids [13,14] or to the release/mobilization of the element from the minerals in the aquifer [15]. These mechanisms are the result of a complex conjunction of stratigraphy, hydrogeology, and geochemistry, therefore the administration of groundwater in the affected area is a very challenging and difficult task. Fortunately, several methods for the removal of arsenic from contaminated water have been proposed, including adsorption, coagulation/flocculation, chemical coprecipitation, membrane filtering, and ion exchange. Among these technologies adsorption is distinguished as the most promising one, as it is cost-effective and efficient even at low concentrations. Due to its ease of operation, lower costs and lower amounts of resultant sludge, adsorption is widely used as the preferential method for the removal of arsenic and other heavy metals. Furthermore, there are several types of adsorbents available, such as metal oxides and hydroxide, mineral clay, activated carbon and biochar [16–20]. Owing to the slow redox transformations, the prevailing forms of inorganic arsenic, As(III) and As(V), are present in both reduced and oxidized environments [21]. However, it is important to notice that the most commonly used sorbent materials are more effective on As(V) [22]; indeed, as already noticed, As(III) basically occurs in non-ionic form at the common pH range and therefore is less adsorbed than the As(V) anions on mineral surfaces [12]. Consequently, to maximize the removal performance, a pre-oxidation step is frequently applied. The predominantly used oxidizing agents are liquid chemicals (sodium hypochlorite, potassium permanganate) or gases (ozone, chlorine), and their use implies a rigorous dosage and sometimes a handling-related danger (e.g. chlorine). In this paper we introduce an innovative “elementary iodine impregnated activated carbon” (IodAC from now on), developed in our laboratories, which shows potential applicability in the field of water treatment, combining in a solid media the disinfectant capability and also the abovementioned oxidation capability useful in the arsenic removal systems.

2. Materials and Methods

2.1. Materials Used for IodAC Preparation

IodAC was manufactured by means of the impregnation of elementary iodine (I2) on granular activated carbon, through polyiodide wet treatment. The polyiodide chemical precursor was NaI3, while the activated carbon was Shirasagi LH2 C-32/60ss supplied by Osaka Gas Chemicals Co., Ltd. (Osaka, Japan). NaI3 was prepared from the reaction between NaI and I2 in aqueous solution. The activated carbon was then added to the NaI3 solution in order to achieve the impregnation of I2. Lastly, the mixture was filtered, washed with distilled water (to remove the unreacted iodine) and dried to yield the final material IodAC. Various types of IodAC, with different loads of polyiodide, were produced in our laboratory and tested for a wide range of purposes and applications. In this report, for preliminary investigation of water treatment usages, we are discussing IodAC(30) and IodAC(10), which means that 100 g of Water 2019, 11, 1778 3 of 14

IodAC respectively hold 30 g and 10 g of I2 (the type of IodAC used is speciﬁed in each paragraph of the results).

2.2. Materials Used for the Experimental Procedures The arsenic oxidation capabilities of IodAC were tested on As(III) enriched distilled water and As(III) enriched groundwater. This differentiation was important to compare the IodAC oxidation performance in a laboratory matrix and in a natural matrix. The groundwater was selected and analyzed prior to the enrichment, in order to assure the absence of preexisting arsenic. Sodium meta-arsenite (NaAsO , assay 90%) from Sigma-Aldrich (St. Louis, MO, USA) was 2 ≥ used as the source of As(III). It was dissolved in distilled water, to obtain a 1000 mg/L As(III) stock solution, and the final pH was adjusted to 7 using hydrochloric acid (HCl, reagent grade, 37%) from Sigma-Aldrich (St. Louis, MO, USA). Small amounts of the stock solution were then diluted, using distilled water or the abovementioned arsenic-free groundwater, to prepare the As(III) enriched solutions. In addition, to compare IodAC with the oxidizing agents commonly employed for water treatment, sodium hypochlorite (NaClO, 5 g/L) and potassium permanganate (KMnO4, 0.02 mol/L) were also used. The chemicals were reagent grade, purchased from Kanto Chemical Co., Inc. (Tokyo, Japan). Appropriate dosages, calculated to fulfill the stoichiometric oxidant demand required by the tests, were added to the experiment solutions (more details in the results section). All the tests were conducted at room temperature and the experimental solutions had initial conditions of T = 20 ◦C and pH = 6.5.

2.3. Characterizations Techniques Direct analysis of arsenate content, As(V), was performed by ion chromatography, using a Dionex-DX-120 (Sunnyvale, CA, USA) equipped with an AS12A analytical column. An ICP-MS, ELAN-DRC-II from Perkin-Elmer (Waltham, MA, USA), was used to analyze the total arsenic (such as to check the initial amount of As(III) in the stock solution and in the experimental solutions before the experiments). SEM-EDS, JSM-6510 from JEOL Ltd. (Tokyo, Japan), was used to investigate the morphological features of IodAC and to conﬁrm the iodine assimilation on the activated carbons.

2.4. Arsenic Oxidation: Batch Experiments Amounts of 0.1 cc and 0.2 cc of IodAC were added to 100 ml of 50 mg/L As(III) solutions (prepared with distilled water or groundwater) and shaken for 4 hours in polypropylene containers. 1 ml aliquots were sampled after 10 minutes, 1 hour, 3 hours and 4 hours. Each sample was ﬁltered through a 0.45 µm syringe ﬁlter and analyzed for total arsenic (ICP-MS) and arsenate (ion chromatography). A similar batch protocol was applied also for tests aimed to compare the IodAC oxidation ability with some commonly used oxidizing agents.

2.5. Arsenic Oxidation: Rapid Small-Scale Column Experiments A 5 ml syringe, with a diameter of 1.2 cm, was packed with 3 cc of IodAC, to create a 2.5 cm long packed column. A cotton filter was placed at the end of the column, to avoid loss of IodAC. The 50 mg/L As(III) solution was continuously flowed in the column, from the top to bottom, using a peristaltic pump set at 10 ml/min. The solution passed in the column, exiting at the bottom nozzle, and was collected in portions of 150 ml for periodic pH monitoring. Moreover, after completing each 150 ml recovery, the following 1 ml of solution exiting from the column was sampled from the nozzle in a plastic vial for arsenic chemical analysis. The test was discontinued when the oxidation efficiency on the 50 mg/L As(III) solution felt lower than 90% (meaning an arsenate concentration amount lower than 45 mg/L). Water 2019, 11, 1778 4 of 14

2.6. Combined Use with Arsenic Sorbent Based on the results of the oxidation experiments, in order to test the applicability in a full “arsenic treatment system”, that is a system able both to oxidize arsenite and to remove arsenic, we also executed batch and column tests using IodAC in combination with iron-impregnated activated carbon (FeAC). The preparation of FeAC was conducted using Fe(NO ) 9H O, following a method 3 3· 2 based on impregnation by iron-salt evaporation similar to the one illustrated by Chen et al. (2007) [23]. The batch experiment compared the kinetics of arsenic adsorption from an arsenite contaminated groundwater, with or without preliminary oxidation by IodAC. The column experiments, instead, tested the real-time synergy of the two mechanisms through a “sequential column experiment”, where arsenite contaminated groundwater was first flowed in a column filled with IodAC and then in a column filled with FeAC.

2.7. Influence of Coexisting Anions on Oxidation Ability The effects of coexisting anions were studied using 50 mg/L As(III) solutions, each of them spiked 2 with 100 mg/L of one the following anions: Cl−, NO3−, SO4 −,H2PO4−). An additional sample, used as reference, was characterized by the same amount of As(III) but without any coexisting anion inside. After the addition of IodAC, the solutions were agitated for 4 hours, filtered through a 0.45 µm syringe filter and used for arsenate estimation.

2.8. Statistical Analyses of Data All tests were conducted in triplicate (n = 3). All data given in text and figures are expressed as the mean of the results; the error bars in figures are expressed as standard deviation. All statistical significance was noted at α = 0.05.

3. Results

3.1. IodAC Characterization Figure1 shows the SEM images of IodAC. The production process did not a ffect the surface of the activated carbon, and no superficial differences were observed comparing the IodAC and the activated carbon before the impregnation process. The EDS results, showed the presence of iodine on the granules, confirming the success of the impregnation. Other physical and chemical characteristics of IodAC are summarized in Table1.

Table 1. Main characteristics of IodAC.

Properties IodAC Color, form Black, granular Granular size 0.25~50.5 mm Bulk density 0.50~0.56 g/cc Element composition Carbon (C), Iodine (I) Heat-resistance 184 ◦C (I2 gradually volatile at higher temperature) Weight loss by drying 5.0% ≤ pH 4.0~6.0 (Measured by pressing the electrode against the IodAC soaked in water) Strong in acid Chemical durability Weak in high concentrated alkaline, alcohol, org. solvents, reductants Adsorption capacity I2 30 g/AC 100 g (for IodAC(30)) Corrosive Low < 0.02 mmpy (SUS316 in seawater) Solubility in water Iodide (I−) < 0.05%, elementary iodine (I2) < 0.05% BET surface area 1620 m2/g Pore volume and size Pore volume of 0.6 mL/g, <2 µm pore; pore volume of 0.75 mL/g, <30 µm pore Ash 2.0 ≤ Water 2019, 11, 1778 5 of 14 Water 2019, 11, x FOR PEER REVIEW 5 of 14

Figure 1. SEM images (various magniﬁcations) and EDS elemental analysis of elementary iodine impregnatedFigure 1. SEM activated images carbon (various (IodAC). magnifications) and EDS elemental analysis of elementary iodine impregnated activated carbon (IodAC). 3.2. Batch Test Results TheOther batch physical tests, and expressing chemical the characteristics kinetics of arsenic of IodAC oxidation are summarized by IodAC, were in Table executed 1. on 100 ml of 50 mg/L As(III) (corresponding to 5 mg of As(III) in the solution). IodAC(30) was used. Preliminary tests were performedTable in1. distilledMain characteristics water in order of IodAC. to understand the oxidation capability of theProperties material. The results showed that 0.1 cc of IodAC canIodAC achieve the oxidation of up to 2.5–2.9 mg of arsenicColor, form (around 50–58% of the total amount in solution) Black, and granular that the process is almost completed withinGranular 3 hours. size Using the amount of 0.2 cc, all the As(III) in 0.25~50.5 the solution mm (5 mg) was oxidized (Figure2). BulkTaking density into account these results, new batch tests were0.50~0.56 performed, g/cc using 0.1 cc and 0.2 cc of ElementIodAC, composition both in distilled water and in natural groundwater. Carbon The(C), Iodine results (I) showed that IodAC was also eﬃHeat-resistancecient on arsenite oxidation in a natural184°C aqueous (I2 gradually matrix. volatile at higher temperature) Weight loss by drying ≤ 5.0% pH 4.0~6.0 (Measured by pressing the electrode against the IodAC soaked in water) Strong in acid Chemical durability Weak in high concentrated alkaline, alcohol, org. solvents, reductants Adsorption capacity I2 30 g/AC 100 g (for IodAC(30)) Corrosive Low < 0.02 mmpy (SUS316 in seawater)

Solubility in water Iodide (I−)< 0.05%, elementary iodine (I2)<0.05% BET surface area 1620 m2/g Pore volume and size Pore volume of 0.6 mL/g, <2 μm pore; pore volume of 0.75 mL/g, <30 μm pore Ash ≤ 2.0

3.2. Batch Test Results

Water 2019, 11, x FOR PEER REVIEW 6 of 14

The batch tests, expressing the kinetics of arsenic oxidation by IodAC, were executed on 100 ml of 50 mg/L As(III) (corresponding to 5 mg of As(III) in the solution). IodAC(30) was used. Preliminary tests were performed in distilled water in order to understand the oxidation capability of the material. The results showed that 0.1 cc of IodAC can achieve the oxidation of up to 2.5–2.9 mg of arsenic (around 50–58% of the total amount in solution) and that the process is almost completed within 3 hours. Using the amount of 0.2 cc, all the As(III) in the solution (5 mg) was oxidized (Figure 2). Taking into account these results, new batch tests were performed, using 0.1 cc and 0.2 cc of WaterIodAC,2019, 11 both, 1778 in distilled water and in natural groundwater. The results showed that IodAC was also6 of 14 efficient on arsenite oxidation in a natural aqueous matrix.

FigureFigure 2. 2.Kinetics Kinetics of of the the arsenite arsenite (50(50 mgmg/L)/L) oxidationoxidation batch test test in in distilled distilled water water and and groundwater groundwater using IodAC(30). using IodAC(30).

3.3.3.3. Comparison Comparison with with Common Common Oxidizing Oxidizing AgentsAgents It isIt is di difficultfficult to to discuss discuss a a clear clear comparison comparison between IodAC IodAC and and the the oxidizing oxidizing agents agents commonly commonly used in water treatments, since they have a different state: solid (IodAC), liquid (NaClO, KMnO4) used in water treatments, since they have a different state: solid (IodAC), liquid (NaClO, KMnO4) and gas (O3). The consequent solid-liquid interaction between the activated carbon and the arsenic and gas (O3). The consequent solid-liquid interaction between the activated carbon and the arsenic solution is affected by various factors (such as surface area), while aqueous ions in a liquid-liquid solution is affected by various factors (such as surface area), while aqueous ions in a liquid-liquid interaction tend to react faster. Therefore, it was not a surprise to notice that sodium hypochlorite interaction tend to react faster. Therefore, it was not a surprise to notice that sodium hypochlorite and and potassium permanganate showed a faster, almost instant, oxidation. potassium permanganate showed a faster, almost instant, oxidation. The experiments were conducted again on 50 mg/L arsenic (100% As(III)) solution (distilled The experiments were conducted again on 50 mg/L arsenic (100% As(III)) solution (distilled water). water). The dose of each oxidizing agent used in the experiments, taken directly from the stock The dose of each oxidizing agent used in the experiments, taken directly from the stock solution at solution at the concentration mentioned in the Materials and Methods section, were calculated to thetheoretically concentration achieve mentioned the oxidation in the of Materials 50% of the and As(III) Methods (for the section, comparison were with calculated 0.1 cc of to IodAC(30)), theoretically achieveand 100% the oxidationof the As(III) of 50% (for of the the co As(III)mparison (for with the comparison 0.2 cc of IodAC( with 0.130)). cc The of IodAC(30)),kinetics of arsenite and 100% ofoxidation the As(III) are (for summarized the comparison in Figure with 3. 0.2 cc of IodAC(30)). The kinetics of arsenite oxidation are summarizedThe slower in Figure oxidation3. process of IodAC, however, is balanced by the long-term oxidation capabilityThe slower of the oxidation material, process which of IodAC,will be however,exposed in is balancedthe column by theexperiments long-term section. oxidation Also, capability the ofchemical the material, hazard which related will to be the exposed use of concentrated in the column liquid experiments oxidants section.causes many Also, difficulties the chemical in areas hazard related to the use of concentrated liquid oxidants causes many difficulties in areas such as developing countries where specialized technical staff is rare. Indeed, a liquid-phase or a gas-phase oxidant (such as O3 or Cl2) needs to be regularly added, exactly at the appropriate dosage, using a dosing device; moreover, since all the above mentioned oxidants are potentially toxic and harmful, special precautions must be observed in all the handling operations. Therefore, any malfunction of the dosing device can produce an overdosage of toxic chemicals in the water. Water 2019, 11, x FOR PEER REVIEW 7 of 14

such as developing countries where specialized technical staff is rare. Indeed, a liquid-phase or a gas- phase oxidant (such as O3 or Cl2) needs to be regularly added, exactly at the appropriate dosage, using a dosing device; moreover, since all the above mentioned oxidants are potentially toxic and Waterharmful,2019, 11 ,special 1778 precautions must be observed in all the handling operations. Therefore, any7 of 14 malfunction of the dosing device can produce an overdosage of toxic chemicals in the water.

FigureFigure 3. 3. ComparisonComparison of ofthe the progress progress of oxidation of oxidation by IodAC(30) by IodAC(30) and by the and commonly by the commonlyused oxidizing used oxidizingagents. agents.

3.4.3.4. Column Column Experiments Experiments Results Results OurOur column column experiments experiments can can bebe consideredconsidered a “stres “stresss test”, test”, since since the the amount amount of of arsenite arsenite (50 (50 mg/L mg /L As(III))As(III)) was was considerably considerably higher higher thanthan thethe concentrationsconcentrations usually found found in in natural natural waters. waters. Such Such high high concentrationconcentration was was also also used used in in order order toto notnot excessivelyexcessively protract the the experiment experiment time. time. InIn the the distilled distilled water water based based experiment,experiment, and using using IodAC(30), IodAC(30), an an oxidation oxidation efficiency efficiency on on As(III) As(III) greatergreater than than 90% 90% was was measured measured upup toto 1.51.5 L (resulting in in a a total total of of about about 75 75 mg mg of of As(III) As(III) oxidized, oxidized, andand in in an an oxidizing oxidizing capability capability ofof aboutabout 2525 mg of As(III) for for 1 1 cc cc of of IodAC). IodAC). After After that, that, the the oxidation oxidation effiefficiencyciency gradually gradually decreased decreased and and fell fell lowerlower thanthan 50%50% at around 2.5 2.5 L L (Figure (Figure 4a).4a). Groundwater affected by arsenic pollution is usually characterized by an arsenic concentration Groundwater affected by arsenic pollution is usually characterized by an arsenic concentration in in the range of ppb and shows a percentage of As(III) lower than the 100% used in our experiments. the range of ppb and shows a percentage of As(III) lower than the 100% used in our experiments. As an As an example, the data concerning the groundwater of central Italy shows a mean arsenic example, the data concerning the groundwater of central Italy shows a mean arsenic concentration of concentration of 23 μg/L [5]; therefore, assuming a hypothetical water with a similar arsenic 23 µg/L[5]; therefore, assuming a hypothetical water with a similar arsenic concentration (20 µg/L) and concentration (20 μg/L) and a 100% presence of As(III), 1 cc of IodAC(30) could theoretically oxidize a 100% presence of As(III), 1 cc of IodAC(30) could theoretically oxidize about 1250 liters of such water. about 1250 liters of such water. According to the available world data [24], even in the countries most Accordingvulnerable to theto arsenic available occurrence world data in [24groundwater,], even in the including countries the most regions vulnerable where to the arsenic incidence occurrence of inreducing groundwater, conditions including boost the the regions portion where of As(III), the incidencethe concentration of reducing of the conditions element is boost still much the portion lower of As(III),than the the amount concentration used in ofour the experiments. element is still much lower than the amount used in our experiments. NoNo significant significant di differencesfferences were were observedobserved when the experiment was was repeated repeated using using the the arsenite arsenite solutionsolution prepared prepared with with real real groundwater, groundwater, resulting resulting in a veryin a similarvery similar progress progress of the oxidationof the oxidation efficiency. efficiency.Additional similar column test was conducted on IodAC(10), to verify a proportional relation betweenAdditional the iodine similar load andcolumn oxidation test was capability. conducted The on amount IodAC(10), of material to verify used a proportional to fill the column relation was, alsobetween in this the case, iodine 3 cc, load but theand concentration oxidation capability. of the arsenite The amount solution of material was set used to 25 to mg fill/L, the instead column of was, the 50 mgalso/L usedin this in case, the IodAC(30)3 cc, but the experiments. concentration The of the lower arsenite arsenite solution concentration was set to was25 mg/L, adopted instead in orderof the to avoid50 mg/L an excessive used in the rapid IodAC(30) decrease experiments. in the oxidation The power.lower arsenite The results concentration showed a verywas adopted good proportional in order comparisonto avoid an with excessive IodAC(30), rapid since decrease the oxidizing in the ox poweridation of power. IodAC(10) The fellresults below showed the 90% a aftervery flowinggood aroundproportional 900 ml comparison of arsenite with solution IodA (FigureC(30), since4b). Itthe corresponds oxidizing power to about of IodAC(10) 7.5 mg of fell arsenic below oxidation the 90% capabilityafter flowing each around 1 cc of material900 ml of (closearsenite to solution the expected (Figure 1/3 4b). of theIt corresponds result attained to about with 7.5 IodAC(30)). mg of arsenic oxidation capability each 1 cc of material (close to the expected 1/3 of the result attained with IodAC(30)).

Water 2019, 11, 1778 8 of 14 Water 2019, 11, x FOR PEER REVIEW 8 of 14

Figure 4. As(III)As(III) oxidation oxidation rate performance in column st study:udy: (a) (a) Arsenate Arsenate concentration in the output water using 3 cc of IodAC(30); ((b)b) arsenate concentr concentrationation in the output water using 3 cc of IodAC(10).

3.5. Integration of IodAC in Arsenic Adsorption System As noticednoticed in thein introduction,the introduction, the common the common arsenic removalarsenic techniquesremoval showtechniques better performancesshow better onperformances arsenate. Therefore, on arsenate. it is importantTherefore, toit checkis importan the eﬀtective to check contribution the effective of IodAC contribution in the improvement of IodAC in ofthe arsenic improvement removal. of arsenic removal. A batch procedure was used as the ﬁrstfirst approach to testtest thethe integrationintegration of arsenitearsenite oxidation with arsenic adsorption. The batch experiment was carried out with conditions similar of the ones conducted to check check the the kinetics kinetics of of arsenite arsenite oxidat oxidation.ion. Amounts Amounts of of0.2 0.2 cc ccof ofIodAC(30) IodAC(30) and and 0.2 0.2 cc ccof ofFeAC FeAC were were introduced introduced in 100 in 100 ml mlof groundwater of groundwater (naturally (naturally free free of arsenic) of arsenic) spiked spiked with with arsenite arsenite (50 (50mg/L) mg /andL) and mixed mixed for for4 hours. 4 hours. The The quantity quantity of of IodAC(30) IodAC(30) was was decided decided on on because because the oxidation experiments revealed that such an amount allowed the complete oxidation of of arsenite arsenite in in 4 4 hours. hours. Aliquots of 1 ml were sampled after 10 minutes, 1 hour, 3 hours and 4 hours, filtered through a 0.45 μm syringe filter and analyzed for total arsenic. The same experiment was carried out introducing

Water 2019, 11, 1778 9 of 14

Aliquots of 1 ml were sampled after 10 minutes, 1 hour, 3 hours and 4 hours, filtered through a 0.45 µm syringeWater 2019 filter, 11, and x FOR analyzed PEER REVIEW for total arsenic. The same experiment was carried out introducing9 of only14 FeAC in the arsenite spiked groundwater. The analysis results allowed calculation of the percentage of only FeAC in the arsenite spiked groundwater. The analysis results allowed calculation of the arsenicpercentage adsorption of arsenic in each adsorption sampling in step.each Thesampling oxidation step. ofThe IodAC oxidation sensibly of IodAC improved sensibly the eimprovedfficiency of arsenicthe efficiency adsorption of arsenic of FeAC adsorption (Figure5 ).of FeAC (Figure 5).

FigureFigure 5. 5.Comparison Comparison of of the the kinetic kinetic ofof arsenicarsenic adsorption on on iron-impregnated iron-impregnated activated activated carbon, carbon, with with oror without without the the use use of of IodAC. IodAC.

AsAs final final test, test, a real-timea real-time column column experiment experiment was was executed executed according according toto thethe schemescheme inin FigureFigure6 6..A specialA special “arsenic “arsenic speciation speciation cartridge”, cartridge”, purchased purchased from from MetalSoft MetalSoft Center (Highland (Highland Park, Park, NJ, NJ, USA), USA), waswas used used to to check check the the oxidation oxidation of of arsenite arsenite toto arsenatearsenate (by IodAC(30)), while while a acolumn column filled filled with with FeACFeAC was was used used for for arsenic arsenic adsorption. adsorption. TheThe speciationspeciation cartridge cartridge is is able able to to remove remove the the only only arsenate arsenate and does not adsorb arsenite. Therefore, the difference between the total arsenic concentrations in the and does not adsorb arsenite. Therefore, the difference between the total arsenic concentrations in starting solution water and in the solution exiting from the cartridge, reveal the residual arsenite the starting solution water and in the solution exiting from the cartridge, reveal the residual arsenite concentration (that is expected to be zero due to the oxidation capability of IodAC). A valve, installed concentration (that is expected to be zero due to the oxidation capability of IodAC). A valve, installed on the output line of the IodAC column, allowed switching the routing of the outgoing solution in on the output line of the IodAC column, allowed switching the routing of the outgoing solution in the the FeAC column or in the speciation cartridge. The valve was set to continuously send the output of FeACthe IodAC column column or in the in speciationthe FeAC column; cartridge. every The 150 valve ml pumped was set tofrom continuously the starting sendsolution the and output flowed of the IodACin the column IodAC inand the FeAC FeAC column, column; anevery aliquot 150 (1 mlml) pumped was collected from from the starting the output solution of the and FeAC flowed columns in the IodACand the and valve FeAC was column, switched an aliquot in order (1 to ml) collect was collectedanother aliquot from the from output the output of the FeACof the columnsspeciation and thecartridge valve was (the switched first 5 ml in discarded order to collect for washing another and aliquot the next from 1 theml collected output of for the analysis). speciation Then, cartridge the (thevalve first was 5 ml switched discarded again for to washing restore the and flow the nextin the 1 FeAC ml collected column. for The analysis). tested water Then, was, the again, valve the was switchednatural againgroundwater to restore spiked the with flow 100 in theμg/L FeAC of As(III). column. The starting The tested concentration water was, of again,arsenite the was natural set groundwaterto 100 μg/L since, spiked according with 100 toµ theg/L specifications of As(III). The of the starting maker, concentration the speciation of cartridge arsenite is was designed set to to 100 µgcope/L since, with according concentrations to the of specifications arsenate <500 of μ theg/L. maker,The results the speciation showed a cartridgegreat efficiency is designed in real-time to cope witharsenic concentrations removal, and of arsenatedemonstrated<500 µtheg/ L.synergy The results of the showed oxidation a greatand adsorption efficiency inmechanisms: real-time arsenic The removal,oxidation and of demonstrated arsenite by IodAC the synergy was fully of accomplished the oxidation again and adsorption (arsenite <0.1 mechanisms: μg/L in all the The steps), oxidation and of arsenitethe residual by IodAC concentration was fully of accomplished arsenic from againthe FeAC (arsenite output<0.1 wasµ galways/L in all lower the steps),than 1 andμg/L the (ICP-MS residual concentrationanalysis). The of arsenictest was from discontinued the FeAC after output the was completing always lower of ten than sampling 1 µg/Ls, (ICP-MSsince the analysis). aim of the The experiment was not to check the lasting of the removal capacity of the FeAC but only to confirm the attainment of the process and the applicability of IodAC together with an arsenic sorbent.

Water 2019, 11, 1778 10 of 14 test was discontinued after the completing of ten samplings, since the aim of the experiment was not to check the lasting of the removal capacity of the FeAC but only to conﬁrm the attainment of the process andWater the 2019 applicability, 11, x FOR PEER of REVIEW IodAC together with an arsenic sorbent. 10 of 14

Figure 6. Scheme of the combined (oxidation and arsenic adsorption) column experiment. Figure 6. Scheme of the combined (oxidation and arsenic adsorption) column experiment. 3.6. Effect of Coexisting Anions 3.6. Effect of Coexisting Anions The chemical composition of natural water is the result of the dissolution of several salts. Therefore, The chemical composition of2 natural water is the result of the dissolution of several salts. many anions (such as Cl− and SO4 −) coexist in water with the arsenite oxyanions. Due to the similar − 2− anionicTherefore, nature, many it isanions important (such to as test Cl theand e ffSOect4 of) coexist such coexisting in water with anions the on arsenite the IodAC oxyanions. system andDue toto checkthe similar any lowering anionic onnature, the arsenite it is important oxidation to power. test the As effect an example, of such fromcoexisting the point anions of view on the of arsenicIodAC adsorption,system and theto check effect any of coexisting lowering anionson the arsenite on the arsenite oxidation removal power. performances As an example, of manyfrom the sorbents point isof wellview documented of arsenic adsorption, [25,26]. the effect of coexisting anions on the arsenite removal performances of manyThe sorbents test was is well conducted documented using 0.1[25,26]. cc of IodAC(30) in all the solutions described in the related paragraphThe test of thewas Materials conducted and using Methods 0.1 cc section. of IodAC(30 After) thein all completing the solutions of the described 4 hours agitation,in the related the results,paragraph expressed of the Materials as the percentage and Methods comparison section. of Af theter As(V)the completing concentration of the in 4 the hours spiked agitation, solutions the andresults, in the expressed reference as solution, the percentage showed comparison that no significant of the As(V) or marginal concentration variations in inthe arsenite spiked oxidation solutions eandfficiency in the occurred reference (Figure solution,7). showed that no significant or marginal variations in arsenite oxidation efficiency occurred (Figure 7).

Water 2019, 11, 1778 11 of 14 Water 2019, 11, x FOR PEER REVIEW 11 of 14

Figure 7. As(III) oxidation efficiency in presence of coexisting anions. Figure 7. As(III) oxidation efficiency in presence of coexisting anions. 3.7. Mechanism of Arsenic Oxidation 3.7. Mechanism of Arsenic Oxidation The chemical mechanism related to arsenite oxidation is described in the following equation: The chemical mechanism related to arsenite oxidation is described in the following equation: + H3AsO3 + I2 + H2O <—> H3AsO4 + 2 H + 2 I−. H3AsO3+I2+H2O <—> H3AsO4+2 H++2 I−. Therefore, iodide release and acidification must be expected as a consequence of using IodAC. Therefore, iodide release and acidification must be expected as a consequence of using IodAC. Indeed, we noticed an acidification of the water during the above mentioned experiments. During Indeed, we noticed an acidification of the water during the above mentioned experiments. During the column tests with IodAC(30), the pH of the output solution sampled in the various steps was the column tests with IodAC(30), the pH of the output solution sampled in the various steps was 4.36 4.36 0.21. However, it must be noted that the huge amount of arsenite, in the order of tens of mg/L, ± 0.21.± However, it must be noted that the huge amount of arsenite, in the order of tens of mg/L, that that triggered the reaction, is unlikely to be found in real groundwater. Under natural amounts of triggered the reaction, is unlikely to be found in real groundwater. Under natural amounts of arsenite arsenite occurrence, in the order of µg/L, the acidification and release of iodide are expected to be occurrence, in the order of μg/L, the acidification and release of iodide are expected to be insignificant. According to the column tests conducted on 100 µg/L arsenite spiked groundwater (the insignificant. According to the column tests conducted on 100 μg/L arsenite spiked groundwater (the IodAC + FeAC experiments), the pH variation was negligible ( 3% from the starting value of pH = 6.5) IodAC + FeAC experiments), the pH variation was negligible± (±3% from the starting value of pH = and the concentration of iodine in the output water was below 1 µg/L (ICP-MS determination). 6.5) and the concentration of iodine in the output water was below 1 μg/L (ICP-MS determination). Indeed, the release of iodine can be easily controlled using activated carbon, such as the FeAC Indeed, the release of iodine can be easily controlled using activated carbon, such as the FeAC used in our experiments (that adsorbed both arsenic and iodine) or also using a final stage of used in our experiments (that adsorbed both arsenic and iodine) or also using a final stage of filtration/purification trough standard activated carbon. An indicative scheme of such type of system filtration/purification trough standard activated carbon. An indicative scheme of such type of system could be the one proposed in Figure8, which includes IodAC for water sanitization and arsenite could be the one proposed in Figure 8, which includes IodAC for water sanitization and arsenite oxidation, an arsenic adsorbing media, and a final treatment such as a “guard column” made of oxidation, an arsenic adsorbing media, and a final treatment such as a “guard column” made of activated carbon (to trap residual iodine) and also a neutralizing filter (if pH adjustment is required). activated carbon (to trap residual iodine) and also a neutralizing filter (if pH adjustment is required). The abovementioned components could be integrated, as example, in a multilayer cartridge for a rapid The abovementioned components could be integrated, as example, in a multilayer cartridge for a and onsite water purification. rapid and onsite water purification.

Water 2019, 11, 1778 12 of 14 Water 2019, 11, x FOR PEER REVIEW 12 of 14

Figure 8. Model for the application of IodAC in an integrated, multipurpose, water treatment system orFigure cartridge. 8. Model for the application of IodAC in an integrated, multipurpose, water treatment system or cartridge. 4. Conclusions 4. Conclusion We demonstrated the applicability of IodAC in the oxidation of arsenite in groundwater. The resultsWe of demonstrated the column tests the showedapplicability that theof IodAC two types in the of oxidation IodAC tested, of arsenite IodAC(30) in groundwater. and IodAC(10), The haveresults an of oxidation the column efficiency tests showed of over that 25 mg the of two As(III) types and of overIodAC 7.5 tested, mg of As(III)IodAC(30) for 1and cc ofIodAC(10), material. Thus,have IodACan oxidation can enhance efficiency the eofffi ciencyover 25 of mg arsenic of As(III) adsorbing and over media 7.5 such mg asof ironAs(III) oxide foror 1 cc iron-modified of material. activatedThus, IodAC carbons. can enhance the efficiency of arsenic adsorbing media such as iron oxide or iron- modifiedMoreover, activated the resultscarbons. show that the main advantages of using IodAC in place of the oxidants commonlyMoreover, used forthe arseniteresults show oxidation that (suchthe main as NaClO, advantages KMnO of4, Clusing2,O 3IodAC) are the in long-lasting place of the oxidizing oxidants power,commonly the solid-state used for arsenite and the oxidation absence of (such direct as chemicalNaClO, KMnO hazards.4, Cl Such2, O3) characteristics are the long-lasting allow aoxidizing simpler designpower, of the the solid-state water treatment and the systemabsence (no of direct need for chem a dosingical hazards. device Such for the characteristics oxidant), and allow easy a and simpler safe transportdesign of andthe water handling. treatment system (no need for a dosing device for the oxidant), and easy and safe transportThrough and additionalhandling. experiments not exposed in this paper, we also verified the disinfectant propertyThrough of IodAC additional related experiments to the presence not of exposed iodine. Tea-bagin this likepaper, packs wefilled also withverified IodAC the weredisinfectant placed inproperty the oxygen of IodAC filter related of three to fish the tanks,presence with of twoiodine goldfish. Tea-bag inside, like packs so that filled the with water IodAC circulating were placed in the filterin the passed oxygen through filter of the three IodAC fish packs.tanks, with A similar two goldfish fish tank, inside, without so that the IodACthe water pack, circulating was used in as the a reference.filter passed After through several the days IodAC the biofouling packs. A similar appeared fish on tank, the walls without of the the reference IodAC pack, fish tank wasbut used not as in a thereference. ones equipped After several with IodAC days the packs. biofouling Detailed appeared results concerning on the walls these of the experiments reference willfish betank discussed but not inin anotherthe ones paper. equipped with IodAC packs. Detailed results concerning these experiments will be discussedIn conclusion, in another IodAC paper. could be used as part of an integrated and multipurpose system for water treatment,In conclusion, such as in IodAC small could or home be used treatment as part units of an in integrated the developing and multipurpose areas of the world,system aimedfor water to achievetreatment, both such the as water in small sanitization or home and treatment the arsenic-decontamination. units in the developing areas of the world, aimed to achieve both the water sanitization and the arsenic-decontamination. Author Contributions: Conceptualization, K.T.; Investigation, F.S.; Project administration, K.T.; Supervision, Y.N., Y.K.,Author T.K. Contributions: and K.T.; Writing—original Conceptualization, draft, F.S.; K.T.; Writing—review Investigation, F.S.; & editing, Project F.S. administration, K.T.; Supervision, Y.N., Y.K., T.K. and K.T.; Writing – original draft, F.S.; Writing – review & editing, F.S. Funding: This research received no external funding. ConflictsFunding: of This Interest: researchThe received authors no declare external no conflictfunding. of interest. Conflicts of Interest: The authors declare no conflict of interest.

References

Water 2019, 11, 1778 13 of 14

References

1. Edokpayi, J.; Rogawski, E.; Kahler, D.; Hill, C.; Reynolds, C.; Nyathi, E.; Smith, J.; Odiyo, J.; Samie, A.; Bessong, P.; et al. Challenges to sustainable safe drinking water: A case study of water quality and use across seasons in rural communities in Limpopo province, South Africa. Water 2018, 10, 159. [CrossRef][PubMed] 2. Huang, J.; Huang, G.; An, C.; He, Y.; Yao, Y.; Zhang, P.; Shen, J. Performance of ceramic disk filter coated with nano ZnO for removing Escherichia Coli from water in small rural and remote communities of developing regions. Environ Pollut. 2018, 238, 52–62. [CrossRef][PubMed] 3. Schreiber, M.; Simo, J.; Freiberg, P. Stratigraphic and geochemical controls on naturally occurring arsenic in groundwater, eastern Wisconsin, USA. Hydrogeol. J. 2000, 8, 161–176. [CrossRef] 4. Rahman, M.M.; Sengupta, M.K.; Ahamed, S.; Chowdhury, U.K.; Lodh, D.; Hossain, A.; Das, B.; Roy, N.; Saha, K.C.; Palit, S.K.; et al. Arsenic contamination of groundwater and its health impact on residents in a village in West Bengal, India. Bull. World Health Organ. 2005, 83, 49–57. [PubMed] 5. Angelone, M.; Cremisini, C.; Piscopo, V.; Proposito, M.; Spaziani, F. Influence of hydrostratigraphy and structural setting on the arsenic occurrence in groundwater of the Cimino-Vico volcanic area (central Italy). Hydrogeol. J. 2009, 17, 901–914. [CrossRef] 6. Piscopo, V.; Armiento, A.; Baiocchi, A.; Mazzuoli, M.; Nardi, E.; Piacentini, S.M.; Proposito, M.; Spaziani, F. Role of high-elevation groundwater flows in the hydrogeology of the Cimino volcano (central Italy) and possibilities to capture drinking water in a geogenically contaminated environment. Hydrogeol. J. 2018, 26, 1027–1045. [CrossRef] 7. Cullen, W.R.; Reimer, K.J. Arsenic speciation in the environment. Chem. Rev. 1989, 89, 713–764. [CrossRef] 8. Yao, S.; Liu, Z.; Shi, Z. Arsenic removal from aqueous solutions by adsorption onto iron oxide/activated carbon magnetic composite. J. Environ. Health Sci. Eng. 2014, 12, 58. [CrossRef] 9. Feenstra, L.; Van Erkel, J.; Vasak, L. Arsenic in Groundwater: Overview and Evaluation of Removal Methods; IGRAC Report nr. SP 2007-2; International Groundwater Resources Assessment Centre: Utrecht, The Netherlands, 2007. 10. Tondel, M.; Rahman, M.; Magnuson, A.; Chowdhury, I.A.; Faruquee, M.H.; Ahmad, S.A. The relationship of arsenic levels in drinking water and the prevalence rate of skin lesions in Bangladesh. Environ. Health Perspect. 1999, 107, 727–729. [CrossRef] 11. Hughes, M.F. Arsenic toxicity and potential mechanisms of action. Toxicol. Lett. 2002, 133, 1–16. [CrossRef] 12. Payne, K.B.; Abdel-Fattah, T.M. Adsorption of Arsenate and Arsenite by Iron-Treated Activated Carbon and Zeolites: Effects of pH, Temperature, and Ionic Strength. J. Environ. Sci. Health 2005, 40, 723–749. [CrossRef] 13. Ballantyne, J.M.; Moore, J.N. Arsenic geochemistry in geothermal systems. Geochim. Cosmo-Chim. Acta 2008, 52, 475–483. [CrossRef] 14. Webster, J.G.; Nordstrom, D.K. Geothermal Arsenic. In Arsenic in Ground Water, Chapter: 4; Weclh, A.H., Stollenwerk, K.G., Eds.; Kluwer Academic Publishers: Norwell, MA, USA, 2003; pp. 101–125. 15. Smedley, P.L.; Kinniburgh, D.G. A review of the source, behaviour and distribution of arsenic in natural waters. Appl. Geochem. 2002, 17, 517–568. [CrossRef] 16. Liu, R.X.; Wang, Y.X.; Tang, H.X. Removal of Arsenate by a New Type of Ion Exchange Fiber. Environ. Sci. 2002, 23, 88–91. 17. Gholami, M.M.; Mokhtari, M.A.; Aameri, A.; Alizadeh Fard, M.R. Application of Reverse Osmosis Technology for Arsenic Removal from Drinking Water. Desalination 2006, 200, 725–727. [CrossRef] 18. Mohan, D.; Pittman, C.U. Arsenic Removal from Water/Wastewater Using Adsorbents: A Critical Review. J. Hazard. Mater. 2007, 142, 1–53. [CrossRef] 19. Shahrin, S.; Lau, W.J.; Goh, P.S.; Jaafar, J.; Ismail, A. Adsorptive removal of As(V) ions from water using Graphene Oxide-Manganese Ferrite (GMF) and Titania Nanotube-Manganese Ferrite (TMF) hybrid nanomaterials. Chem. Eng. Technol. 2018, 41, 2250–2258. [CrossRef] 20. Yildiz, S. Artificial neural network approach for modeling of Ni(II) adsorption from aqueous solution by peanut shell. Ecol. Chem. Eng. S 2018, 25, 581–604. [CrossRef] 21. Nicomel, N.R.; Leus, K.; Folens, K.; Van Der Voort, P.; Du Laing, G. Technologies for Arsenic Removal from Water: Current Status and Future Perspectives. Int. J. Environ. Res. Public Health 2016, 13, 62. [CrossRef] 22. Sorlini, S.; Gialdini, F. Conventional oxidation treatments for the removal of arsenic with chlorine dioxide, hypochlorite, potassium permanganate and monochloramine. Water Res. 2010, 44, 5653–5659. [CrossRef] Water 2019, 11, 1778 14 of 14

23. Chen, W.; Parette, R.; Zou, J.; Cannon, F.S.; Dempsey, B.A. Arsenic removal by iron-modified activated carbon. Water Res. 2007, 41, 1851–1858. [CrossRef] 24. Herath, I.; Vithanage, M.; Bundschuh, J.; Maity, J.P.; Bhattacharya, P. Natural Arsenic in Global Groundwaters: Distribution and Geochemical Triggers for Mobilization. Curr. Pollut. Rep. 2016, 2, 68–89. [CrossRef] 25. Su, C.M.; Puls, R.W. Arsenate and Arsenite Removal by Zerovalent Iron: Effects of Phosphate, Silicate, Carbonate, Borate, Sulfate, Chromate, Molybdate, and Nitrate, Relative to Chloride. Environ. Sci. Technol. 2001, 35, 4562–4568. [CrossRef] 26. Jiang, Y.; Yanagita, T.; Mitani, T. Effects of Coexisting Anions on Arsenite Adsorption of Newly Developed Iron Hydroxide. J. Jpn. Soc. Water Environ. 2014, 37, 169–176. [CrossRef]

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).