Stability of arsenopyrite and As(III) in low-temperature acidic solutions

Arsenopyrite is one of the most important pri-mary arsenic mineral. In the Eh-pH diagram of the As-O2-S-H2O system, if the total arsenic concentration (TAs) is taken to be 0.75 mg/L, the total sulfur con-centration, 32 mg/L, the temperature, 25℃and the pressure, one atmosphere pressure for the discrimina-tion of arsenic species, it may be found that under hy-pergene conditions, arsenopyrite is a moderately stable mineral. Only in the strongly alkaline and reducing environment can arsenopy…     

Arsenic — a Review. Part II: Oxidation of Arsenic and its Removal in Water Treatment

In natural waters arsenic normally occurs in the oxidation states +III (arsenite) and +V (arsenate). The removal of As(III) is more difficult than the removal of As(V). Therefore, As(III) has to be oxidized to As(V) prior to its removal. The oxidation in the presence of air or pure oxygen is slow. The oxidation rate can be increased by ozone, chlorine, hypochlorite, chlorine dioxide, or H₂O₂. The oxidation of As(III) is also possible in the presence of manganese oxide coated sands or by advanced oxidation processes. Arsenic can be removed from waters by coprecipitation with Fe(OH)₃, MnO₂ or during water softening. Fixed‐bed filters have successfully been applied for the removal of arsenic.The effectiveness of arsenic removal was tested in the presence of adsorbents such as FeOOH, activated alumina, ferruginous manganese ore, granular activated carbon, or natural zeolites. Other removal technologies are anion exchange, electrocoagulation, and membrane filtration by ultrafiltration, nanofiltration or reverse osmosis.     

Mycorrhizal Colonization Affects the Survival of Vetiveria zizanioides (L.) Nash Grown in Water Containing As(III)

The presence of arsenic (As) in water is of great public concern. Arsenic exists in three common valence states viz., As(0) metalloid arsenic, As(III) (arsenite) and As(V) (arsenate). Arsenite [As(III)] is the most toxic form among arsenicals which, predominates in anaerobic conditions, generally in flooded soils and in the water with high BOD. Experiments were conducted to investigate the effect of As(III) on the mycorrhization in vetiver (Vetiveria zizanioides (L.) Nash) grass in hydroponics. Studies showed significant alteration in the mycorrhizal colonization in the roots of vetiver exposed to higher concentrations of As(III) starting from 1.0, 2.0, 3.0, 4.0 to 5.0 mg/L prepared in 5% Hoagland nutrient solution without addition of phosphate ions. Considerable reduction in the mycorrhizal intensity (M) was observed in all the treatment sets as compared to the control suggesting a negative impact of the As(III) on the mycorrhizal association. Simultaneously, the study also showed that, As(III) is toxic to the vetiver plants having mycorrhizal association however plants with non‐mycorrhizal (cleansed) roots were found to be able to survive for a longer period exposed to As(III).     

The enrichment and removal of arsenic (III) from water samples using HFSLM

At specific concentrations oxyanions such as arsenic pose a major threat to human beings and to the environment because of their ability to biomagnify. The World Health Organization has set the drinking water standard at 10 μg l⁻¹ for arsenic. It is in this regard that novel and cheaper methods to detect oxyanions and remove them from the environment are developed. In this work, we have developed sample preparation methods involving solid phase and liquid membrane for the extraction and enrichment of these oxyanions in aqueous environments. Various parameters which govern their optimal extraction and enrichment have been optimised. The manipulation of the liquid membrane extraction process was utilised to selectively extract arsenic (III) which is more toxic as compared to arsenic (V). The liquid membrane consisted of a combination of two organic solvents, and n-undecane and di-n-hexyl ether were used in a combination at various ratios of the two liquid membranes. The means of detection was by graphite furnace atomic absorption spectroscopy (GFAAS) as well as inductively coupled plasma optical emission spectroscopy (ICP-OES). The applicability of the method was tested in real wastewater samples where it was able to remove at least 50% of As (III) with enrichment factors of up to 20.     

Biogeochemical controls of arsenic occurrence and mobility in the Indian Sundarban mangrove ecosystem

This study aims to investigate the control of arsenic distribution by biogeochemical processes in the Indian Sundarban mangrove ecosystem and the importance of this ecosystem as an arsenic source for surrounding coastal water. The As(V)/As(III) ratio was found to be significantly lower in both surface and pore waters compared to sea water, which could be attributed to biogeochemical interconversion of these arsenic forms. The biological uptake of arsenic due to primary and benthic production occurs during the post-monsoon season, and is followed by the release of arsenic during the biochemical degradation and dissolution of plankton in the pre-monsoon season. These results suggest that arsenic is immobilized during incorporation into the arsenic-bearing initial phase, and unlikely to be released into pore water until the complete microbial degradation of arsenic-bearing organic compounds.     

Arsenic — a Review. Part I: Occurrence,Toxicity, Speciation,Mobility

In natural waters arsenic concentrations up to a few milligrams per litre were measured. The natural content of arsenic found in soils varies between 0.01 mg/kg and a few hundred milligrams per kilogram. Anthropogenic sources of arsenic in the environment are the smelting of ores, the burning of coal, and the use of arsenic compounds in many products and production processes in the past. A lot of arsenic compounds are toxic and cause acute and chronic poisoning. In aqueous environment the inorganic arsenic species arsenite (As(III)) and arsenate (As(V)) are the most abundant species. The mobility of these species is influenced by the pH value, the redox potential, and the presence of adsorbents such as oxides and hydroxides of Fe(III), Al(III), Mn(III/IV), humic substances, and clay minerals.     

Treatment of Arsenic Contaminated Groundwater Using Calcium Impregnated Granular Activated Carbon in a Batch Reactor: Optimization of Process Parameters

This paper is an experimental investigation into the removal of arsenic species from simulated groundwater by adsorption onto Ca²⁺ impregnated granular activated carbon (GAC‐Ca) in the presence of impurities like Fe and Mn. The effects of adsorbent concentration, pH and temperature on the percentage removal of total arsenic (As(T)), As(III) and As(V) have been discussed. Under the experimental conditions, the optimum adsorbent concentration of GAC‐Ca was found to be 8 g/L with an agitation time of 24 h, which reduced As(T) concentration from 188 to 10 μg/L. Maximum removal of As(V) and As(III) was observed in a pH range of 7–11 and 9–11, respectively. Removal of all the above arsenic species decreased slightly with increasing temperature. The presence of Fe and Mn increased the adsorption of arsenic species. Under the experimental conditions at 30°C, the maximum percentage removals of As(T), As(III), As(V), Fe, and Mn were found to be ca. 94.3, 90.6, 98.0, 100 and 63%, respectively. It was also observed that amongst the various regenerating liquids used, a 5 N H₂SO₄ solution exhibited maximum regeneration (ca. 91%) of the spent GAC‐Ca.     

Modeling of the Removal of Arsenic Species from Simulated Groundwater Containing As,Fe, and Mn: A Neural Network Based Approach

The present paper deals with the modeling of the removal of total arsenic As(T), trivalent arsenic As(III), and pentavalent arsenic As(V) from synthetic solutions containing total arsenic (0.167–2.0 mg/L), Fe (0.9–2.7 mg/L), and Mn (0.2–0.6 mg/L) in a batch reactor using Fe impregnated granular activated charcoal (GAC‐Fe). Mass ratio of As(III) and As(V) in the solution was 1:1. Multi‐layer neural network (MLNN) has been used and full factorial design technique has been applied for the selection of input data set. The developed models are able to predict the adsorption of arsenic species with an error limit of ?0.3 to +1.7%. Combination of MLNN with design of experiment has been able to generalize the MLNN with less number of experimental points.     

Quantitative Separation of As(III) and As(V) from a Synthetic Water Solution Using Ion Exchange Columns in the Presence of Fe and Mn Ions

The quantitative separation of As(III) from a water sample containing As(III) and As(V) in the presence of Fe and Mn in an ion exchange resin (AG1 X8) column for the speciation of arsenic is described. Individual and combined effects of Fe and Mn on the separation of As(III) from the solution have been studied separately. In absence of Fe and Mn, the ratio between the As(T) concentration in the eluent and the As(III) concentration in the original sample has been found to be 0.9717 under optimum process conditions. The presence of Fe(II) in the water sample increased the As(T) concentration in the eluent whereas Mn(II) decreased it. Combined effects of Fe and Mn on the percentage increment in the eluent arsenic concentration have been expressed by additive and interactive models. The interactive model has been developed by a statistical software with a 95 % confidence level. In most of the cases the error on the determination of the As(III) concentration had a minimum when using the interactive model.     

Effective Precipitation of Arsenic from Aqueous Solution by Iron(III) Sulfate

A study of the removal of As(V) from aqueous solution by Fe₂(SO₄)₃ has been carried out to establish optimum parameters for the process. Optimum arsenic removal is obtained at pH = 5, and mole ratio Fe(III)/As(V) = 7. Minimum arsenate solubility is obtained from sediments precipitated at pH = 5 and Fe/As = 7…8.     

The behaviors and sources of dissolved arsenic and antimony in Bohai Bay

In this paper, we determined the concentrations of antimony species (antimonite (Sb(III)), antimonate (Sb(V)) and dissolved inorganic antimony (DISb)) and arsenic, in Bohai Bay seawaters, as well as the relationships of the analytes with environmental factors such as seawater characteristics (e.g., suspended particulate material (SPM), salinity and total organic carbon (TOC)), heavy metals, nutrients and phytoplankton species, and evaluated the sources of arsenic and antimony. Dissolved arsenic and antimony concentrations in the surface waters were ranging spatially from 1.03 to 1.26 ng/ml and 0.386 to 1.075 ng/ml, with mean values of 1.18 and 0.562 ng/ml, respectively. Sb(V) as the prominent chemical species constituted about 89%. Regarding arsenic concentrations in the surface waters, there was a tendency for a small variation. However, antimony species concentrations were much variable than arsenic. The highest arsenic and antimony concentrations were found near the Haihe Estuary. These distribution patterns were controlled mainly by environmental factors, biological activities and sources. In this region, DISb and Sb(V) negatively correlated with salinity. Besides, arsenic and antimony correlated well with the nutrients, chlorophyll a and phytoplankton, implying that arsenic and antimony had been involved in biological cycling. In addition, according to our estimate, about 333.5×10⁸ mg/year of arsenic and 454.2×10⁸ mg/year of antimony reached Bohai Bay via rivers.     

Use of the Anion‐Exchange Resin Amberlite IRA‐93 for the Separation of Arsenite and Arsenate in Aqueous Samples

The method described uses the separation of As(III) and As(V) species in aqueous samples by means of the anion‐exchange resin Amberlite IRA‐93. The samples were acidified using acetic acid and passed through a glass column filled with pre‐treated Amberlite IRA‐93 resin. As(III) was poorly adsorbed on the anionic exchanger material, whereas As(V) was retained. The arsenic concentration was measured in the column effluent by graphite furnace AAS (GF‐AAS). The retained As(V) was eluted from the column using 1 M NaOH. Prior to the determination of the As(V) concentration in the NaOH eluate, the eluate was passed through a glass column filled with a cation‐exchange resin (Amberlite 200) to remove sodium ions and minimize the Na⁺ interference with the AAS determination. After calibration the method was applied to the separation of As(III) and As(V) species in two aqueous extracts of arsenic contaminated soils. The results were compared with those obtained from an on‐line separation and determination of As(III) and As(V) in the aqueous soil extracts using a state of the art HPLC‐ICP‐MS system.     

Inorganic As speciation and bioavailability in estuarine sediments of Todos os Santos Bay, BA, Brazil

The spatial distribution of As (total As, As (III) and As (V)) in estuarine sediments from the main tributaries of Todos os Santos Bay, BA, Brazil, was evaluated under high and low flow conditions. The concentrations of As were determined using a slurry sampling procedure with hydride generation atomic absorption spectrometry (HG-AAS). The highest concentrations were observed at estuary mouths, and exceeded conservative lower threshold value (Threshold Effects Level; TEL). Due to the oxic conditions and abundance of Mn and Fe (oxyhydr)oxides in the sediments, most inorganic arsenic in the Subaé and Paraguaçu estuaries was present as As (V). Nevertheless, the concentration of As (III) at several locations along the Jaguaripe River were also above the TEL value, suggesting that As may be toxic to biota. In the Subaé estuary, antropogenic activities are the main source of As. At the Jaguaripe and at Paraguaçu estuaries, nevertheless, natural sources of As need to be considered to explain the distribution patterns.     

Detoxification of Arsenite through Adsorption and Oxidative Transformation on Pyrolusite

Adsorption and oxidative transformation processes critically affect the mobility and toxicity of arsenic (As) in the environment. In this study, the detoxification of arsenite through adsorption and oxidation by pyrolusite was systematically investigated. Disappearance of aqueous As(III) in the solution can be efficiently achieved using pyrolusite. The As(III) oxidative transformation product arsenate or As(V) was obtained both in the solution and on the pyrolusite surface. The arsenic species adsorbed on pyrolusite exist in two forms: As(III) and As(V). Furthermore, over 64.8% of the adsorbed As cannot be desorbed. They were fixed more stably in the structure of the mineral to achieve a safer removal. Lower As(III) initial concentration increased As(III) detoxification rates. Elevating the reaction pH from 4.5 to 7.9 elicited a slight effect on the disappearance rate of As(III). Efficient As(III) detoxification can be achieved by pryrolusite within the studied pH range. The addition of low‐molecular‐weight carboxylic acids decreased the detoxification rate of As(III) through competition for active sites on pyrolusite. Co‐existing divalent metal ions, such as Ca²⁺, Ni²⁺, and Mn²⁺, also decreased the detoxification rate of As(III). However, the trivalent ion Cr³⁺ largely increased the detoxification rate through co‐precipitation and adsorption processes.     

Removal of Arsenic from Simulated Groundwater Using GAC‐Ca in Batch Reactor: Kinetics and Equilibrium Studies

This paper deals with kinetics and equilibrium studies on the adsorption of arsenic species from simulated groundwater containing arsenic (As(III)/As(V), 1:1), Fe, and Mn in concentrations of 0.188, 2.8, and 0.6 mg/L, respectively, by Ca²⁺ impregnated granular activated charcoal (GAC‐Ca). Effects of agitation period and initial arsenic concentration on the removal of arsenic species have also been described. Although, most of the arsenic species are adsorbed within 10 h of agitation, equilibrium reaches after ～24 h. Amongst various kinetic models investigated, the pseudo second order model is more adequate to explain the adsorption kinetics and film diffusion is found to be the rate controlling step for the adsorption of arsenic species on GAC‐Ca. Freundlich isotherm is adequate to explain the adsorption equilibrium. However, empirical polynomial isotherm gives more accurate prediction on equilibrium specific uptakes of arsenic species. Maximum specific uptake (q_max) for the adsorption of As(T) as obtained from Langmuir isotherm is 135 µg/g.     

上覆水pH值和总磷浓度对含铁盐的高砷沉积物中砷迁移转化的影响

含铁盐的高砷沉积物中,上覆水p H值和总磷浓度的变化对砷迁移的影响尚不明确.以含铁盐的高砷沉积物为研究对象,通过实验观测其暴露在不同p H值的上覆水及经过不同初始p H值、不同初始总磷浓度的溶液淋洗后,沉积物中砷的迁移规律.结果表明:当沉积物暴露在p H=10和11的上覆水时,分别在第11 d和第6 d后,沉积物中的砷溶出速度突然增大,说明当沉积物处于强碱性上覆水环境中,碱性越强,砷的溶出速度越快、溶出量越大.从室内实验条件的角度来看,上覆水的碱化有可能会增加沉积物中砷溶出的风险.而上覆水p H值在4~8之间时,上覆水的酸碱度对沉积物砷的迁移影响较小.此外,当上覆水总磷浓度不高于0.5 mg/L时,磷的竞争吸附对砷的迁移影响较小.     

Stable isotope-guided analysis of biomagnification profiles of arsenic species in a tropical mangrove ecosystem

We performed stable carbon and nitrogen-guided analyses of biomagnification profiles of arsenic (As) species, including total As, lipid-soluble As, eight water-soluble As compounds (arsenobetaine (AB), arsenocholine (AC), tetramethylarsonium ion (TETRA), trimethylarsine oxide (TMAO), dimethylarsinic acid (DMA), monomethylarsonic acid (MMA), arsenate (As[V]), and arsenite (As[III])), and non-extracted As in a tropical mangrove ecosystem in the Ba Ria Vung Tau, South Vietnam. Arsenobetaine was the predominant As species (65-96% of water-soluble As). Simple linear regression slopes of log-transformed concentrations of total As, As fractions or individual As compounds on stable nitrogen isotopic ratio (δ15N) values are regarded as indices of biomagnification. In this ecosystem, lipid-soluble As (slope, 0.130) and AB (slope, 0.108) were significantly biomagnified through the food web; total As and other water-soluble As compounds were not. To our knowledge, this is one of the first reports on biomagnification profiles of As compounds from a tropical mangrove ecosystem.     

Capability of Pentavalent Arsenic Bioaccumulation and Biovolatilization of Three Fungal Strains under Laboratory Conditions

In order to control and remediate arsenic (As) contaminated soil, sediment or water, fungi are used to investigate their potential accumulation and volatilization of As. In this study, after cultivation for 2 days, the dry weights of mycelia for Trichoderma asperellum, Fusarium oxysporum and Penicillium janthinellum all show an increased trend when the As(V) concentration ranges from 0–50, 0–50, 0–80 mg/L, respectively. When the culture system is loaded with 2500 μg As(V), which represents 50 mg/L As, and cultivated for 5 days, P. janthinellum presents the highest efficiency of 87.0 μg/g for As bioaccumulation, and the order of the efficiency for As bioaccumulation is P. janthinellum > T. asperellum > F. oxysporum. However, the order of the amount of volatilized As is F. oxysporum > P. janthinellum > T. asperellum, and the highest amount of volatilized As is observed for F. oxysporum at 181.0 μg. Thus, the ability of As bioaccumulation and biovolatilization for T. asperellum and P. janthinellum is reported for the first time in this study.     

Arsenic Cycling in Hydrocarbon Plumes: Secondary Effects of Natural Attenuation

Monitored natural attenuation is widely applied as a remediation strategy at hydrocarbon spill sites. Natural attenuation relies on biodegradation of hydrocarbons coupled with reduction of electron acceptors, including solid phase ferric iron (Fe(III)). Because arsenic (As) adsorbs to Fe‐hydroxides, a potential secondary effect of natural attenuation of hydrocarbons coupled with Fe(III) reduction is a release of naturally occurring As to groundwater. At a crude‐oil‐contaminated aquifer near Bemidji, Minnesota, anaerobic biodegradation of hydrocarbons coupled to Fe(III) reduction has been well documented. We collected groundwater samples at the site annually from 2009 to 2013 to examine if As is released to groundwater and, if so, to document relationships between As and Fe inside and outside of the dissolved hydrocarbon plume. Arsenic concentrations in groundwater in the plume reached 230 µg/L, whereas groundwater outside the plume contained less than 5 µg/L As. Combined with previous data from the Bemidji site, our results suggest that (1) naturally occurring As is associated with Fe‐hydroxides present in the glacially derived aquifer sediments; (2) introduction of hydrocarbons results in reduction of Fe‐hydroxides, releasing As and Fe to groundwater; (3) at the leading edge of the plume, As and Fe are removed from groundwater and retained on sediments; and (4) downgradient from the plume, patterns of As and Fe in groundwater are similar to background. We develop a conceptual model of secondary As release due to natural attenuation of hydrocarbons that can be applied to other sites where an influx of biodegradable organic carbon promotes Fe(III) reduction.     

Iron oxide formation in artificial ground waters

Artificial ground water containing 40 mg/l Ca and varying concentrations of Fe(II), Fe(III) and Si were rapidly oxidized with air. The ferrihydrite forming is similar to those found in natural Finish ground waters.