Removal of arsenic from groundwater by granular titanium dioxide adsorbent

A novel granular titanium dioxide (TiO2) was evaluated for the removal of arsenic from groundwater. Laboratory experiments were carried out to investigate the adsorption capacity of the adsorbent and the effect of anions on arsenic removal. Batch experimental results showed that more arsenate [As(V)] was adsorbed on TiO2 than arsenite [As(III)] in US groundwater at pH 7.0. The adsorption capacities for As(V) and As(III) were 41.4 and 32.4 mgg(-1) TiO2, respectively. However, the adsorbent had a similar adsorption capacity for As(V) and As(III) (approximately 40 mgg(-1)) when simulated Bangladesh groundwater was used. Silica (20 mgl(-1)) and phosphate (5.8 mgl(-1)) had no obvious effect on the removal of As(V) and As(III) by TiO2 at neutral pH. Point-of-entry (POE) filters containing 3 l of the granular adsorbent were tested for the removal of arsenic from groundwater in central New Jersey, USA. Groundwater was continuously passed through the filters at an empty bed contact time (EBCT) of 3 min. Approximately 45,000 bed volumes of groundwater containing an average of 39 microgl(-1) of As(V) was treated by the POE filter before the effluent arsenic concentration increased to 10 microgl(-1). The total treated water volumes per weight of adsorbent were about 60,000 l per 1 kg of adsorbent. The field filtration results demonstrated that the granular TiO2 adsorbent was very effective for the removal of arsenic in groundwater.     

Desorption of arsenic from clay and humic acid-coated clay by dissolved phosphate and silicate

Arsenic (As) contaminated aquifers contain iron minerals and clays that strongly bind As at their surfaces. It was suggested that As mobilization is driven by natural organic matter (including fulvic acids (FA) and humic acids (HA)) present in the aquifers either via providing reducing equivalents for reductive dissolution of Fe(III) (hydr)oxides or via competitive desorption of As from the mineral surfaces. In the present study we quantified sorption of As(III) and As(V) to Ca(2+)-homoionized illite (IL) and to kaolinite (Kao) as well as to HA-coated clays, i.e., illite-HA (IL-HA) and kaolinite-HA (Kao-HA) at neutral pH. Clay-HA complexes sorbed 28-50% more As than clay-only systems upon addition of 100μM As(III)/As(V) to 0.5g of clay or HA-clay with Ca(2+) probably playing an important role for HA binding to the clay surface and As binding to the HA. When comparing sorption of As(V) and As(III) to clay and HA-clay complexes, As(V) sorption was generally higher by 15-32% than sorption of As(III) to the same complexes. IL and IL-HA sorbed 11-28% and 6-11% more As compared to Kao and Kao-HA, respectively. In a second step, we then followed desorption of As from Kao, Kao-HA, IL and IL-HA by 100 and 500μM phosphate or silicate both at high (0.41-0.77μmol As/g clay), and low (0.04 to 0.05μmol As/g clay) As loadings. Phosphate desorbed As to a larger extent than silicate regardless of the amount of As loaded to clay minerals, both in the presence and absence of HA, and both for illite and kaolinite. At high loadings of As, the desorption of both redox species of As from clay-HA complexes by phosphate/silicate ranged from 32 to 72% compared to 2-54% in clay only systems meaning that As was desorbed to a larger extent from HA-coated clays compared to clay only systems. When comparing As(III) desorption by phosphate/silicate to As(V) desorption in high As-loading systems, there was no clear trend for which As species is desorbed to a higher extent in the four clay systems meaning that both As species behave similarly regarding desorption from clay surfaces by phosphate/silicate. Similarly, no significant differences were found in high As-loading systems in the amount of As desorbed by phosphate/silicate when comparing Kao vs. IL and Kao-HA vs IL-HA systems meaning that both clay types behave similarly regarding desorption of As by phosphate/silicate. At low As loadings, up to 80% of As was desorbed by phosphate and silicate with no noticeable differences being observed between different As species, different types of clay, clay vs clay-HA or the type of desorbant (phosphate and silicate). The results of this study showed that HA sorption to Ca(2+)-homoionized clay minerals can increase As binding to the clay although the As sorbed to the clay-HA is also released to a greater extent by competing ions such as phosphate and silicate. Desorption of As depended on the initial loadings of As onto the clay/clay-HA. Based on our results, the effect of humic substances on sorption of As and on desorption of As by phosphate and silicate has to be considered in order to fully understand and evaluate the environmental behavior of As in natural environments.     

Evaluating a drinking-water waste by-product as a novel sorbent for arsenic

Arsenic (As) carcinogenicity to humans and other living organisms has promulgated extensive research on As treatment technologies with varying levels of success; generally, the most efficient methods come with a significantly higher cost burden and they usually perform better in removing As(V) than As(III) from solution. In the reported study, a novel sorbent, a waste by-product of the drinking-water treatment process, namely, drinking-water treatment residuals (WTRs) were evaluated for their ability to adsorb both As(V) and As(III). Drinking-WTRs can be obtained free-of-charge from drinking-water treatment plants, and they have been successfully used to reduce soluble phosphorus (P) concentrations in poorly P-sorbing soils. Phosphate and arsenate molecules have the same tetrahedral geometry, and they chemically behave in a similar manner. We hypothesized that the WTRs would be effective sorbents for both As(V) and As(III) species. Two WTRs (one Fe- and one Al-based) were used in batch experiments to optimize the maximum As(V) and As(III) sorption capacities, utilizing the effects of solid:solution ratios and reaction kinetics. Results showed that both WTRs exhibited high affinities for soluble As(V) and As(III), exhibiting Freundlich type adsorption with no obvious plateau after 2-d of reaction (15000 mg kg-1). The Al-WTR was highly effective in removing both As(V) and As(III), although As(III) removal was much slower. The Fe-WTR showed greater affinity for As(III) than for As(V) and reached As(III) sorption capacity levels similar to those obtained with the Al-WTR-As(V) system (15000 mg kg-1). Arsenic sorption kinetics were biphasic, similar to what has been observed with P sorption by the WTRs. Minimal (<3%) desorption of sorbed As(III) and As(V) was observed, using phosphate as the desorbing ligand. Dissolved Fe2+ concentrations measured during As(III) sorption were significantly correlated (r2=0.74, p<0.005) with the amount of As(III) sorbed by the Fe-WTR. Lack of correlation between Fe2+ in solution and sorbed As(V) (r2=0.2) suggests reductive dissolution of the Fe-WTR mediating As(III) sorption. Results show promising potential for the WTRs in irreversibly retaining As(V) and As(III) that should be further tested in field settings.     

Arsenic uptake and speciation and the effects of phosphate nutrition in hydroponically grown kikuyu grass (Pennisetum clandestinum Hochst)

Background

This work focuses on the accumulation and mobility properties of arsenic (As) and the effects of phosphate (P) on its movement in Pennisetum clandestinum Hochst (kikuyu grass), grown hydroponically under increasing arsenate (As(V)) concentrations. The uptake of both ions and the relative kinetics show that phosphate is an efficient competitive inhibitor of As(V) uptake. The P/As uptake rate ratios in roots indicate that P is taken up preferentially by P/As transporters. An arsenite (As(III)) efflux from roots was also found, but this decreased when the arsenate concentration in the solution exceeded 5???M.

Methods

Increases in both arsenite and arsenate concentrations in roots were observed when the arsenate concentration in the solution was increased, and the highest accumulation of As(III) in roots was found when plants were grown at 5???M As(V). The low ratios of As accumulated in shoots compared to roots suggest limited mobility of the metalloid within Kikuyu plants.

Results

The results indicate that arsenic resistance in kikuyu grass in conditions of moderate exposure is mainly dependent on the following factors: 1) phosphate nutrition: P is an efficient competitive inhibitor of As(V) uptake because of the higher selectivity of membrane transporters with respect to phosphate rather than arsenate; and 2) a detoxification mechanism including a reduction in both arsenate and arsenite root efflux.

Conclusions

The As tolerance strategy of Kikuyu limits arsenate uptake and As translocation from roots to shoots; therefore, this plant cannot be considered a viable candidate for use in the phytoextraction of arsenic from contaminated soils or water.     

Effects of compost and phosphate amendments on arsenic mobility in soils and arsenic uptake by the hyperaccumulator,Pteris vittata L

Chinese brake fern (Pteris vittata L.), an arsenic (As) hyperaccumulator, has shown the potential to remediate As-contaminated soils. This study investigated the effects of soil amendments on the leachability of As from soils and As uptake by Chinese brake fern. The ferns were grown for 12 weeks in a chromated-copper-arsenate (CCA) contaminated soil or in As spiked contaminated (ASC) soil. Soils were treated with phosphate rock, municipal solid waste, or biosolid compost. Phosphate amendments significantly enhanced plant As uptake from the two tested soils with frond As concentrations increasing up to 265% relative to the control. After 12 weeks, plants grown in phosphate-amended soil removed >8% of soil As. Replacement of As by P from the soil binding sites was responsible for the enhanced mobility of As and subsequent increased plant uptake. Compost additions facilitated As uptake from the CCA soil, but decreased As uptake from the ASC soil. Elevated As uptake in the compost-treated CCA soil was related to the increase of soil water-soluble As and As(V) transformation into As(III). Reduced As uptake in the ASC soil may be attributed to As adsorption to the compost. Chinese brake fern took up As mainly from the iron-bound fraction in the CCA soil and from the water-soluble/exchangeable As in the ASC soil. Without ferns for As adsorption, compost and phosphate amendments increased As leaching from the CCA soil, but had decreased leaching with ferns when compared to the control. For the ASC soil, treatments reduced As leaching regardless of fern presence. This study suggest that growing Chinese brake fern in conjunction with phosphate amendments increases the effectiveness of remediating As-contaminated soils, by increasing As uptake and decreasing As leaching.     

Evaluation of various chemical extraction methods to estimate plant-available arsenic in mine soils

Elevated levels of bioavailable As in mining soils, agricultural areas and human habitats may cause potential toxicity to human health, plants and microbe. Therefore, it is essential to determine proper soil chemical extraction method in order to estimate plant-available As in mining soils and protect agricultural and environmental ecosystems by evaluation of environmental risk and implementation of remediation measures. In this study, six single soil chemical extraction processes and four-step sequential chemical extraction protocol were used to determine the relative distribution of As in different chemical forms of soils and their correlations with total As in plants grown in mining areas and greenhouse experiments. The strongest relationship between As determined by single soil chemical extraction and As in plant biomass was found for sodium acetate and mixed acid extractant. The mean percent of total As extracted was: ammonium oxalate (41%)>hydroxylamine hydrochloride (32%)>mixed acid (16%)>phosphate (6%)>sodium acetate (1.2%)>water (0.13%). This trend suggests that most of the As in these soils is inside the soil mineral matrix and can only be released when iron oxides and other minerals are dissolved by the stronger chemical extractant. Single soil chemical extraction methods using sodium acetate and mixed acids, that extract As fractions complexed to soil particles or on the surface of mineral matrix of hydrous oxides of Fe, Mn and Al (exchangeable+sorbed forms) can be employed to estimate and predict the bioavailable As fraction for plant uptake in mining affected soils. In sequential chemical extraction methods, ammonium nitrate and hydroxylamine hydrochloride may be used to provide closer estimates of plant-available As in mining soils.     

Arsenic release from flooded paddy soils is influenced by speciation, Eh, pH, and iron dissolution

Arsenic (As) is highly mobilized when paddy soil is flooded, causing increased uptake of As by rice. We investigated factors controlling soil-to-solution partitioning of As under anaerobic conditions. Changes in As and iron (Fe) speciation due to flooded incubation of two paddy soils (soils A and B) were investigated by HPLC/ICP-MS and XANES. The flooded incubation resulted in a decrease in Eh, a rise in pH, and an increase in the As(III) fraction in the soil solid phase up to 80% of the total As in the soils. The solution-to-soil ratio of As(III) and As(V) (R_L/S) increased with pH due to the flooded incubation. The R_L/S for As(III) was higher than that for As(V), indicating that As(III) was more readily released from soil to solution than was As(V). Despite the small differences in As concentrations between the two soils, the amount of As dissolved by anaerobic incubation was lower in soil A. With the development of anaerobic conditions, Fe(II) remained in the soil solid phase as the secondary mineral siderite, and a smaller amount of Fe was dissolved from soil A than from soil B. The dissolution of Fe minerals rather than redox reaction of As(V) to As(III) explained the different dissolution amounts of As in the two paddy soils. Anaerobic incubation for 30 d after the incomplete suppression of microbial activity caused a drop in Eh. However, this decline in Eh did not induce the transformation of As(V) to As(III) in either the soil solid or solution phases, and the dissolution of As was limited. Microbial activity was necessary for the reductive reaction of As(V) to As(III) even when Eh reached the condition necessary for the dominance of As(III). Ratios of released As to Fe from the soils were decreased with incubation time during both anaerobic incubation and abiotic dissolution by sodium ascorbate, suggesting that a larger amount of As was associated with an easily soluble fraction of Fe (hydr) oxide in amorphous phase and/or smaller particles.     

Effects of root anatomy and Fe plaque on arsenic uptake by rice seedlings grown in solution culture

Hydroponic experiments were carried out to investigate the effects of root anatomy, induced by aeration and stagnation, and Fe plaque on arsenic (III&V) uptake and translocation by rice plants. The results showed that As uptake in rice plants (Gui Chao-2) treated by aeration was decreased due to lower root specific surface area. Rice roots with larger specific surface area tended to form more Fe plaque, and Fe plaque affected As uptake kinetics by changing As influx curves from linear to hyperbolic for As(III) and from hyperbolic to S-curve for As(V). Fe plaque increased As(III&V) adsorption and minimized the effects of root anatomy characteristics on As uptake into roots and subsequently translocation to shoots. Fe plaque increased As(III) uptake rate at As(III) concentrations of 0.5∼8 mg L⁻¹, reduced As(V) uptake rate at low As(V) concentrations (<2 mg L⁻¹), but increased As uptake rate at high As(V) concentrations (>6 mg L⁻¹).     

Effects of compost and phosphate on plant arsenic accumulation from soils near pressure-treated wood

Leaching of arsenic (As) from chromated copper arsenate (CCA)-treated wood may elevate soil arsenic levels. Thus, an environmental concern arises regarding accumulation of As in vegetables grown in these soils. In this study, a greenhouse experiment was conducted to evaluate As accumulation by vegetables from the soils adjacent to the CCA-treated utility poles and fences and examine the effects of soil amendments on plant As accumulation. Carrot (Daucus carota L.) and lettuce (Lactuca sativa L.) were grown for ten weeks in the soil with or without compost and phosphate amendments. As expected, elevated As concentrations were observed in the pole soil (43 mg kg(-1)) and in the fence soil (27 mg kg(-1)), resulting in enhanced As accumulation of 44 mg kg(-1) in carrot and 32 mg kg(-1) in lettuce. Addition of phosphate to soils increased As accumulation by 4.56-9.3 times for carrot and 2.45-10.1 for lettuce due to increased soil water-soluble As via replacement of arsenate by phosphate in soil. However, biosolid compost application significantly reduced plant As uptake by 79-86%, relative to the untreated soils. This suppression is possibly because of As adsorbed by biosolid organic mater, which reduced As phytoavailability. Fractionation analysis showed that biosolid decreased As in soil water-soluble, exchangeable, and carbonate fraction by 45%, whereas phosphate increased it up to 2.61 times, compared to the untreated soils. Our results indicate that growing vegetables in soils near CCA-treated wood may pose a risk of As exposure for humans. Compost amendment can reduce such a risk by reducing As accumulation by vegetables and can be an important strategy for remediating CCA-contaminated soils. Caution should be taken for phosphate application since it enhances As accumulation.     

Geochemistry of inorganic arsenic and selenium in a tropical soil: effect of reaction time, pH, and competitive anions on arsenic and selenium adsorption

Factors that can affect As and Se adsorption by soils influence the bioavailability and mobility of these elements in the subsurface. This research attempted to compare the adsorption capacities of As(III), As(V), Se(IV), and Se(VI) on a tropical soil commonly found in Singapore in a single-species system. The effect of reaction time, pH, and competitive anions at different concentrations on the adsorption of both As and Se species were investigated. The As and Se adsorption isotherm were also obtained under different background electrolytes. The batch adsorption experiments showed that the sequence of the As and Se adsorption capacities in the soil was As(V) > Se(IV) > As(III) > Se(VI). The adsorption kinetics could be best described by the Elovich equation. The adsorption of As(V), Se(IV), and Se(VI) appeared to be influenced by the variable pH-dependent charges developed on the soil particle surfaces. Phosphate had more profound effect than SO4(2-) on As and Se adsorption in the soil. The competition between PO4(3-) and As or Se oxyanions on adsorption sites was presumably due to the formation of surface complexes and the surface accumulation or precipitation involving PO4(3-). The thermodynamic adsorption data for As(V) and Se(IV) adsorption followed the Langmuir equation, while the As(III) and Se(VI) adsorption data appeared to be best-represented by the Freundlich equation.     

Potential for microbially mediated redox transformations and mobilization of arsenic in uncontaminated soils

Surface soil samples, which had no significant As contamination, were examined for As(V) reduction, As(III) oxidation and As mobilization capability. All five soil samples tested exhibited microbial As(V)-reducing activities both in aerobic and anaerobic conditions. Under aerobic conditions when As(V) reduction had almost ceased, oxidation of As(III) to As(V) occurred, whereas only As(V) reduction was observed under anaerobic conditions. In cultures incubated with As(III), As(III) was oxidized by indigenous soil microbes only under aerobic conditions. These results indicate that microbial redox transformations of As are ubiquitous in the natural environment regardless of background As levels. Mobilization through microbially mediated As(V) and Fe(III) reduction occurred both in the presence and absence of oxygen. Significant variation in dissolved As occurred depending on the Fe contents of soils, and re-immobilization of As arose in the presence of oxygen, presumably as a consequence of dissolved As(III) and Fe(II) oxidation. There was no apparent correlation between dissolved Fe(II) and As, suggesting that reductive dissolution of Fe(III) minerals does not necessarily determine the extent of As release from soils.     

Exposure of Brassica juncea (L) to arsenic species in hydroponic medium: comparative analysis in accumulation and biochemical and transcriptional alterations

Arsenic (As) contamination in the environment has attracted considerable attention worldwide. The objective of the present study was to see the comparative effect of As species As(III) and As(V) on accumulation, biochemical responses, and gene expression analysis in Brassica juncea var. Pusa Jaganath (PJn). Hydroponically grown 14-day-old seedlings of B. juncea were treated with different concentrations of As(III) and As(V). Accumulation of total As increased with increasing concentration of both As species and exposure time, mainly in roots. Reduction in seed germination, root–shoot length, chlorophyll, and protein content were observed with increasing concentration and exposure time of both As species, being more in As(III)-treated leaves. PJn variety showed that antioxidant enzymes (superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX)) and stress-related parameters (cysteine, proline, and malondialdehyde (MDA)) were stimulated and allows plant to tolerate both As species. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis in leaves showed significant changes in protein profile with more stringent effect with As(III) stress. Semiquantitative RT-PCR analysis showed regulation in expression of phytochelatin synthase (PCS), metallothionine-2 (MT-2), glutathione reductase (GR), and glutathione synthetase (GS) genes under both As(III) and As(V) stresses. Results suggested that accumulation and inhibition on physiological parameters differ according to the As species, while molecular and biochemical parameters showed a combinatorial type of tolerance mechanism against As(III) and As(V) stresses.     

Distribution of soil arsenic species, lead and arsenic bound to humic acid molar mass fractions in a contaminated apple orchard

Excessive application of lead arsenate pesticides in apple orchards during the early 1900s has led to the accumulation of lead and arsenic in these soils. Lead and arsenic bound to soil humic acids (HA) and soil arsenic species in a western Massachusetts apple orchard was investigated. The metal-humate binding profiles of Pb and As were analyzed with size exclusion chromatography-inductively coupled plasma mass spectrometry (SEC-ICP-MS). It was observed that both Pb and As bind "tightly" to soil HA molar mass fractions. The surface soils of the apple orchard contained a ratio of about 14:1 of water soluble As (V) to As (III), while mono-methyl (MMA) and di-methyl arsenic (DMA) were not detectable. The control soil contained comparatively very low levels of As (III) and As (V). The analysis of soil core samples demonstrated that As (III) and As (V) species are confined to the top 20 cm of the soil.     

Soil calcium significantly promotes uptake of inorganic arsenic by garland chrysanthemum (ChrysanthemumL coronarium) fertilized with chicken manure bearing roxarsone and its metabolites

Roxarsone (ROX), a widely used feed organoarsenic additive, occurs as itself and its metabolites in animal manure that is commonly land used as fertilizer. Soil property impacts arsenic (As) speciation and bioavailability. Fourteen soils across China were used to conduct culture experiments to investigate As uptake by garland chrysanthemum (ChrysanthemumL coronarium), with the soils fertilized with chicken manure bearing ROX and its metabolites. The results show As(III) was the sole As form in garland chrysanthemum shoots, and As(III) and As(V) occurred in roots. Only inorganic As was detected in all soils when the plants were harvested. Stepwise regression analysis shows soil-exchangeable Ca predominated shoot As(III) concentration (shoot As(III) = 1.60030 soil Ca, R ² = 0.8832***). Therefore, ROX is transferred into the human food chain finally as inorganic As in plants. Application of animal manure bearing ROX and its metabolites is not recommended in Ca-rich soils to avoid excess inorganic As dietary exposure.

     

Oxalate and root exudates enhance the desorption of p,p'-DDT from soils

The abiotic desorption of p,p'-DDT from seven Chinese soils spiked with p,p'-DDT and the effects of oxalate at 0.001-0.1M and the root exudates of maize, wheat, and ryegrass were evaluated using batch experiments. Soil organic carbon played a predominant role in the retention of DDT. Oxalate significantly increased the desorption of p,p'-DDT, with the largest increments ranging from 11% to 54% for different soils. Oxalate addition also resulted in the increased release of dissolved organic carbon and inorganic ions from soils. Root exudates had similar effects to those of oxalate. Root exudates significantly increased DDT desorption from the soils, and the general trend was similar among the plant species studied for all the soils (p > 0.05). Low molecular weight dissolved organic carbon amendments caused partial dissolution of the soil structure, such as the organo-mineral linkages, resulting in the release of organic carbon and metal ions and thus the subsequent enhanced desorption of DDT from the soils. The enhancing effects of oxalate and root exudates on DDT desorption were influenced by the contents of soil organic carbon and dissolved organic carbon in soils.     

Arsenic toxicity and accumulation in radish as affected by arsenic chemical speciation.

Arsenic (As) uptake by Rhapanus sativus L. (radish), cv. Nueva Orleans, growing in soil-less culture conditions was studied in relation to the chemical form and concentration of As. A 4 x 3 factorial experiment was conducted with treatments consisting of four As chemical forms [As(III), As(V), MMAA, DMAA] and three As concentrations (1.0, 2.0, and 5.0 mg As L-1). None of the As treatments were clearly phytotoxic to this radish cultivar. Arsenic phytoavailability was primarily determined by the As chemical form present in the nutrient solution and followed the trend DMAA < or = As(V) < or = As(III) < MMAA. Root and shoot As concentrations significantly increased with increasing As application rates. Monomethyl arsonic acid treatments caused the highest As accumulation in both roots and shoots, and this organic arsenical showed a higher uptake rate than the other As compounds. Inner root As concentrations were, in general, within the normal range for As contents in food crops but root skin As levels were close or above the maximum threshold set for As content in edible fruit, crops and vegetables. The statement that toxicity limits plant As uptake to safe levels was not confirmed in our study. If radish plants are exposed to a large pulse of As, as growth on contaminated nutrient solutions, they may accumulate residues which are unacceptable for animal and human consumption without exhibiting symptoms of phytotoxicity.     

Comparative effects on arsenic uptake between iron (hydro)oxides on root surface and rhizosphere of rice in an alkaline paddy soil

The iron (Fe) (hydro)oxides deposited around rice roots play an important role in arsenic (As) sequestration in paddy soils, but there is no systematic study on the relative importance of Fe (hydro)oxides on root surface and in rhizosphere soil in limiting As bioavailability. Twenty-seven rice genotypes were selected to investigate effects of Fe (hydro)oxides on As uptake by rice in an alkaline paddy soil. Results indicated that the As content was positively correlated with the Fe content on root surface, and most of As (88–97%) was sequestered by poorly crystalline and crystalline Fe (hydro)oxides in the alkaline paddy soil. The As sequestration by Fe (hydro)oxides on root surface (IAS_root 16.8–25.0 mg As/(g Fe)) was much higher than that in rhizosphere (IAS_rhizo 1.4–2.0 mg As/(g Fe)); therefore, in terms of As immobilization, the Fe (hydro)oxides on root surface were more important than that in rhizosphere. However, the As content in brown rice did not have significant correlation with the As content on root surface but was significantly correlated (R²?=?0.43, P?<?0.05) with the partition ratio (PR_As?=?IAS_root/IAS_rhizo) of As sequestration on root surface and in rhizosphere, which suggested that Fe (hydro)oxides on root surface did not play the controlling role in lowering As uptake, and the partition ratio PR_As would be a better indicator to evaluate effects of Fe (hydro)oxides around roots on As uptake by rice.

     

Removal of arsenic from water by electrocoagulation

In the present study electrocoagulation (EC) has been evaluated as a treatment technology for arsenite [As(III)] and arsenate [As(V)] removal from water. Laboratory scale experiments were conducted with three electrode materials namely, iron, aluminum and titanium to assess their efficiency. Arsenic removal obtained was highest with iron electrodes. EC was able to bring down aqueous phase arsenic concentration to less than 10 microgl(-1) with iron electrodes. Current density was varied from 0.65 to 1.53 mAcm(-2) and it was observed that higher current density achieved rapid arsenic removal. Experimental results at different current densities indicated that arsenic removal was normalized with respect to total charge passed and therefore charge density has been used to compare the results. Effect of pH on arsenic removal was not significant in the pH range 6-8. Comparative evaluation of As(III) and As(V) removal by chemical coagulation (with ferric chloride) and electrocoagulation has been done. The comparison revealed that EC has better removal efficiency for As(III), whereas As(V) removal by both processes was nearly same. The removal mechanism of As(III) by EC seems to be oxidation of As(III) to As(V) and subsequent removal by adsorption/complexation with metal hydroxides generated in the process.     

The oxidation states of arsenic in well-water from a chronic arsenicism area of northern Mexico

Aquifers in the Región Lagunera in northern Mexico are heavily contaminated with arsenic. The range of total arsenic concentrations in 128 water samples analyzed was 0.008 to 0.624 mg litre(-1), and concentrations greater than 0.05 mg litre(-1) were found in 50% of them. Approximately 400 000 people living in rural areas were exposed to high As concentrations. Most of the As was in inorganic form and pentavalent arsenic [As(V)] was the predominant species in 93% of the samples. In 36% of the samples, however, variable percentages (20-50) of trivalent As [As(III)] were found. Organic arsenicals were present in very small amounts. Since As(III) is several times more toxic than As(V), we suggest that periodic studies be performed on the As(III)/As(V) ratio in wells whose total As concentrations are above 0.05 mg litre(-1), in combination with epidemiological studies to evaluate possible differences in health effects produced by different As species.     

Arsenic hyperaccumulation by Pteris vittata from arsenic contaminated soils and the effect of liming and phosphate fertilisation

Pot experiments were carried out to investigate the potential of phytoremediation with the arsenic hyperaccumulator Pteris vittata in a range of soils contaminated with As and other heavy metals, and the influence of phosphate and lime additions on As hyperaccumulation by P. vittata. The fern was grown in 5 soils collected from Cornwall (England) containing 67-4550 mg As kg(-1) and different levels of metals. All soils showed a similar distribution pattern of As in different fractions in a sequential extraction, with more than 60% of the total As being associated with the fraction thought to represent amorphous and poorly-crystalline hydrous oxides of Fe and Al. The concentration of As in the fronds ranged from 84 to 3600 mg kg(-1), with 0.9-3.1% of the total soil As being taken up by P. vittata. In one soil which contained 5500 mg Cu kg(-1) and 1242 mg Zn kg(-1), P. vittata suffered from phytotoxicity and accumulated little As (0.002% of total). In a separate experiment, neither phosphate addition (50mg P kg(-1) soil) nor liming (4.6 g CaCO3 kg(-1) soil) was found to affect the As concentration in the fronds of P. vittata, even though phosphate addition increased the As concentration in the soil pore water. Between 4 and 7% of the total soil As was taken up by P. vittata in 4 cuttings in this experiment. The results indicate that P. vittata can hyperaccumulate As from naturally contaminated soils, but may be suitable for phytoremediation only in the moderately contaminated soils.