Thermodynamic Modeling of Thioarsenic Species Distribution in High As Groundwater in Hetao Plain*

Thioarsenic has gained increasing attention as a newly identified arsenic species. This paper selected Hetao Plain as the study area. Based on the field survey data and the thermodynamic reaction equation, the distribution characteristics of thioarsenic speciation in high-arsenic groundwater were simulated and analyzed. The results show that the major of arsenic speciation is arsenite, followed by thioarsenate and arsenate, and thethioarsenite concentrations are extremely low. Among them, monothioarsenate and trithioarsenate are the predominant thioarsnate species, with monothioarsenite being the dominant thioarsenite. In the buried depth of the range of 5~40 m, the content of various arsenic species does not change significantly with increasing depth. In the range of 40~80 m buried depth, the content of arsenite decreases with the increase of depth, and the content of thioarsenite increases with depth. Sulfide concentration has a significant effect on the distribution of arsenic species. When the concentration of sulfide is less than 5 μg/L, the various arsenic species do not change regularly with the increase of sulfide concentration. When the sulfide concentration is more than 5 μg/L, the content of arsenite and arsenate decreases with the increase of sulfide concentration, and the content of thioarsenate and thioarsenite tends to increase, and mutual transformation happens between different thioarsenite and thioarsenate species.     

Arsenic distribution, speciation and solubility in shallow groundwater of Owens Dry Lake, California

Generation of dust particles from the Owens Lake playa creates a severe air pollution hazard in the western United States. Much of the dust produced from the dry lakebed is derived from salts formed by evaporation of saline groundwater that often contains high concentrations of dissolved arsenic (As). The objectives of this research were to study the spatial distribution of dissolved arsenic in the shallow groundwater, and to examine factors affecting arsenic solubility and speciation. Evapoconcentration, redox potential, pH, and mineral solubility were examined as factors regulating arsenic biogeochemistry. Dissolved arsenic concentrations ranged from 0.1 to 96 mg L⁻¹ and showed a general increase from the shoreline to the center of the lakebed. Arsenic concentrations were strongly correlated to electrical conductivity (EC) and δD suggesting that evapoconcentration is an important process regulating total As concentrations. Arsenite [As(III)] was the dominant form of inorganic arsenic at Eh values less than about −170 mV while arsenate [As(V)] was predominant at higher Eh values. Organic arsenic was negligible (<0.21%) in all shallow groundwater samples. Dissolved arsenic concentrations do not appear to be strongly regulated by solid-phase reactions. Solid-phase arsenic concentrations generally ranged between 4.0 and 42.6 mg kg⁻¹ and a maximum concentration range (20 to 40 mg kg⁻¹) was reached as solution concentration increased up to 80 mg L⁻¹, indicating minimal sorption and/or precipitation of arsenic. Chemical equilibrium modeling indicated that orpiment (As₂S₃) was the only solid phase with a positive saturation index (indicating over-saturation), but only at high arsenic and sulfide concentrations. The findings of this research are important for assessing the potential environmental impacts of elevated arsenic concentrations on dust mitigation efforts taking place at Owens Dry Lake.     

高温富硫化物热泉中硫代砷化物存在形态的地球化学模拟:以云南腾冲热海水热区为例

天然水环境中地质成因砷的存在是世界范围内对人类威胁极大的环境问题之一.在高温富硫化物地热水中,硫代砷化物是砷的主要存在形态之一.在国内尚无硫代砷化物定量检测方法的背景下,以云南腾冲地热带的热海水热区为典型研究区,基于不同类型硫代砷化物的最新化学热力学数据wateq4f.dat,利用水文地球化学模拟软件PHREEQC开展了不同类型热泉中砷的存在形态的地球化学模拟.结果表明,热海热泉中砷的主要形态是硫代砷酸盐,砷酸盐和亚砷酸盐次之,硫代亚砷酸盐则含量极低;在各类硫代砷酸盐中,按平均百分含量降序依次为:一硫代砷酸盐→三硫代砷酸盐→四硫代砷酸盐→二硫代砷酸盐.pH、Eh和总硫化物含量是热泉中砷的形态分布的控制性因素.在酸性条件下,砷以硫代砷酸盐和亚砷酸盐为主要存在形式;而在中性/偏碱性条件下,砷的形态则以硫代砷酸盐为主,砷酸盐次之.偏还原环境和高硫化物含量是硫代砷化物、特别是三硫代砷酸盐和四硫代砷酸盐稳定存在的有利条件.      

西藏搭格架高温热泉中砷的地球化学异常及其存在形态

西藏搭格架高温热泉是我国大陆少有的大型间歇性喷泉,砷元素作为对人类威胁极大的环境问题普遍存在于热泉之中,搭格架高温热泉中砷元素质量浓度最高已达到了9.75 mg/L,其对地表水和浅层地下水的污染不容忽视。硫代砷是富含硫化物热泉中砷的存在形态之一,鉴于国内相关研究较少,本文对西藏搭格架地热区的热泉样品进行了水化学分析,并利用水文地球化学模拟软件PHREEQC开展了对热泉中砷元素存在形态的地球化学模拟。结果表明：西藏搭格架热泉中砷元素的存在形态有亚砷酸盐、砷酸盐和硫代砷,其中亚砷酸盐与砷酸盐是砷的主要存在形态,且在pH影响下两者之间存在相互转化关系;各种硫代砷按质量浓度由高至低依次为一硫代砷酸盐、三硫代砷酸盐、二硫代砷酸盐、一硫代亚砷酸盐、四硫代砷酸盐;硫代砷形态占总砷浓度比例主要受热泉中硫化物质量浓度、Eh（氧化还原电位）和pH等因素的控制,在硫化物质量浓度总体偏低的情况下,硫化物质量浓度的上升可促进其他形态的砷向硫代砷形态转化,强还原性环境有利于硫代砷形态的存在;此外,在中性环境下,硫代砷占总砷浓度比例随pH上升亦有上升趋势。     

Predicting arsenic concentrations in the porewaters of buried uranium mill tailings

The proposed JEB Tailings Management Facility (TMF) to be emplaced below the groundwater table in northern Saskatchewan, Canada, will contain uranium mill tailings from McClean Lake, Midwest and Cigar Lake ore bodies, which are high in arsenic (up to 10%) and nickel (up to 5%). A serious concern is the possibility that high arsenic and nickel concentrations may be released from the buried tailings, contaminating adjacent groundwaters and a nearby lake. Laboratory tests and geochemical modeling were performed to examine ways to reduce the arsenic and nickel concentrations in TMF porewaters so as to minimize such contamination from tailings buried for 50 years and longer. The tests were designed to mimic conditions in the mill neutralization circuit (3 hr tests at 25°C), and in the TMF after burial (5–49 day aging tests). The aging tests were run at, 50, 25 and 4°C (the temperature in the TMF). In order to optimize the removal of arsenic by adsorption and precipitation, ferric sulfate was added to tailings raffinates¹ having Fe/As ratios of less that 3–5. The acid raffinates were then neutralized by addition of slaked lime to nominal pH values of 7, 8, or 9.Analysis and modeling of the test results showed that with slaked lime addition to acid tailings raffinates, relatively amorphous scorodite (ferric arsenate) precipitates near pH 1, and is the dominant form of arsenate in slake limed tailings solids except those high in Ni and As and low in Fe, in which cabrerite-annabergite (Ni, Mg, Fe(II) arsenate) may also precipitate near pH 5–6. In addition to the arsenate precipitates, smaller amounts of arsenate are also adsorbed onto tailings solids.The aging tests showed that after burial of the tailings, arsenic concentrations may increase with time from the breakdown of the arsenate phases (chiefly scorodite). However, the tests indicate that the rate of change decreases and approaches zero after 72 hrs at 25°C, and may equal zero at all times in the TMF at 4°C. Consistent with a kinetic model that describes the rate of breakdown of scorodite to form hydrous ferric oxide, the rate of release of dissolved arsenate to tailings porewaters from slake limed tailings: (1) is proportional to pH above pH 6–7; (2) decreases exponentially as the total molar Fe/As ratio of tailings raffinates is increased from 1/1 to greater than 5/1; and (3) is proportional to temperature with an average Arrhenius activation energy of 13.4 ± 4.2 kcal/mol.Study results suggest that if ferric sulfate and slaked lime are added in the tailings neutralization circuit to give a raffinate Fe/As molar ratio of at least 3–5 and a nominal (initial) pH of 8 (final pH of 7–8), arsenic and nickel concentrations of 2 mg/L or less, are probable in porewaters of individual tailings in the TMF for 50 to 10,000 yrs after tailings disposal. However, the tailings will be mixed in the TMF, which will contain about 35% tailings with Fe/As = 3.0, and 65% tailings with Fe/As = 5.0–7.7. Thus, it seems likely that average arsenic pore water concentrations in the TMF may not exceed 1 mg/L.     

微生物对砷的地球化学行为的影响--暨地下水砷污染机制的最新研究进展

砷在自然界中广泛存在,近年来砷污染对人类健康造成的危害越来越引人关注。微生物在自然界中长期与砷共存,进化出不同的生物转化机制,在自然水体中微生物主要参与砷的不同氧化价态之间的转化过程,即As(V)和As(III)之间的氧化还原作用。砷酸盐异化还原菌（Dissimilatory Arsenate Respiring Prokaryote, DARP）可以将As(V)还原为As(III),化能自养亚砷酸盐氧化菌（Chemoautotrophic Arsenite Oxidizer, CAO）和异养亚砷酸盐氧化菌（Heterotrophic Arsenite Oxidizer, HAO）可以将As(III)氧化为As(V)。这些砷代谢微生物在分类和代谢能力上都具有很大的多样性,它们广泛参与了砷的生物地球化学循环的关键步骤,对特定环境条件下砷的地球化学行为产生重要影响,进而参与了砷的全球循环。在盐碱湖莫诺（Mono）湖中砷的不同价态分层存在,CAO与DARP的紧密偶联共同参与了莫诺湖中的砷的地球化学循环。在孟加拉三角洲的地下含水层中,微生物参与了将砷从固相迁移到水相的关键步骤,最终导致了地下水中的砷污染。     

Speciation of arsenic in sulfidic waters

Formation constants for thioarsenite species have been determined in dilute solutions at 25°C, ΣH₂S from 10^-7.5 to 10^-3.0 M, ΣAs from 10^-5.6 to 10^-4.8 M, and pH 7 and 10. The principal inorganic arsenic species in anoxic aquatic systems are arsenite, As(OH)₃ ⁰, and a mononuclear thioarsenite with an S/As ratio of 3:1. Thioarsenic species with S/As ratios of 1 : 1,2 : 1, and 4 : 1 are lesser components in sulfidic solutions that might be encountered in natural aquatic environments. Thioarsenites dominate arsenic speciation at sulfide concentrations > 10^-4.3 M at neutral pH. Conversion from neutral As(OH)₃ ⁰ to anionic thioarsenite species may regulate the transport and fate of arsenic in sulfate-reducing environments by governing sorption and mineral precipitation reactions.     

Evidence suggesting anaerobic oxidation of the bisulfide ion in Chesapeake Bay

Upper Chesapeake Bay bottom waters are stratified in the summer. In the water column below the pycnocline, anoxic and sulfidic conditions exist. Hydrogen sulfide concentrations approach 60 μM or greater and elemental sulfur is also present. Water samples brought on board ship, exposed to light, and not treated with formaldehyde show rapid sulfide decomposition which is significantly faster than sulfide oxidation by molecular oxygen. The data presented show evidence for anaerobic, sulfide oxidation. The kinetics of the decomposition are consistent with possible biological mediation. Hydrogen, peroxide produced by microorganisms may be the chemical oxidant responsible for the oxidation. Alternately, solid metal oxides such as colloidal manganese oxide phases may be reponsible.     

Arsenic Speciation in a Contaminated Gold Processing Tailings Dam

Gold recovery in ores containing arsenopyrite releases significant amounts of arsenic into the environment due to mineral processing and oxidation during storage. The extent of arsenic weathering in a tailings dam has been investigated. Speciation of As in surface and pore waters and pond sediments showed that for gold tailings in the dam, As enrichment took place in the pore water relative to the surface water. In pond sediments As was predominantly present as residual arsenopyrite and partly as a substance co-precipitated with iron hydroxide. The arsenic release from the sediment results from a reductive dissolution of the arsenopyrite and Fe oxides. In the surface water, arsenate and arsenite are the main arsenic species (arsenate is dominant), but in the pore waters methylation processes play a significant role. Arsenic transport is accompanied by the transformation of As into the less toxic compounds (methylated species) co-existing with the most toxic species (arsenite).     

Removing arsenic and hydrogen sulfide production using arsenic-tolerant sulfate-reducing bacteria

Environmental remediation technologies that involve the use of sulfate-reducing bacteria constitute a feasible alternative to the remediation of sites polluted with heavy metals and metalloids. The present study evaluates hydrogen sulfide production and arsenic removal by two microbial consortia (C1 and C2) in batch systems exposed to different arsenic concentrations and oxidation states. We identify the following three consecutive stages of arsenate removal: (1) hydrogen sulfide production/accumulation, (2) arsenate reduction to arsenite associated with the incomplete oxidation of hydrogen sulfide to elemental sulfur and (3) arsenic polysulfide precipitation as the main arsenic removal mechanism from aqueous solution. Kinetic parameters are determined in regard to the arsenic oxidation state through the fit of hydrogen sulfide production. The r _max reached by C1 and C2 is increased seven- or eightfold when 250 mM As[+5] was used instead 250 mM As[+3]. Arsenic removal by extracellular precipitation of arsenic polysulfides associated with elemental sulfur precipitation detected through scanning electron microscopy coupled to energy-dispersive X-ray spectroscopy (SEM–EDS) can explain the exceptional value of r _max observed at 250 mM during As[+5] exposition.     

An evaluation of copper remobilization from mine tailings in sulfidic environments

A laboratory-based assessment of copper remobilization from Cu-rich mine tailings exposed to anoxic, sulfide rich waters was performed. The results from incubation experiments, conducted over a 20 day period, were compared to thermodynamic modelling calculations of copper speciation in sulfidic waters. The tailings materials were observed to react rapidly with added sulfide, consuming 159 μmol HS⁻ g⁻¹ (dry wt) within a 24 h period. The consumption of sulfide was attributed to a two stage process involving the reduction of Fe-hydroxy phases by sulfide followed by reaction with available Fe²⁺ and Cu²⁺ resulting in the formation Fe- and Cu-sulfide phases. During incubation experiments, the dissolved copper concentrations in the absence of sulfide were approximately 0.31 μmol l⁻¹, whereas in the presence of sulfide (0.5–5 mM) concentrations were typically 0.24 μmol l⁻¹. The experiments did not indicate enhanced solubility owing to the formation of soluble copper sulfide species. The predictions (based on the most recent thermodynamic data for aqueous Cu-sulfide and Cu-polysulfide species) did not accurately explain the laboratory observations. Model predictions were greatly influenced by the assumptions made about the oxidation state of copper under anoxic conditions and the solid sulfide phase controlling copper solubility. The study emphasizes the limitations of modelling copper speciation in sulfidic waters and the need for laboratory or field verification of predictions.     

Regulation of bacterial sulfate reduction and hydrogen sulfide fluxes in the central namibian coastal upwelling zone

The coastal upwelling system off central Namibia is one of the most productive regions of the oceans and is characterized by frequently occurring shelf anoxia with severe effects for the benthic life and fisheries. We present data on water column dissolved oxygen, sulfide, nitrate and nitrite, pore water profiles for dissolved sulfide and sulfate,³⁵S-sulfate reduction rates, as well as bacterial counts of large sulfur bacteria from 20 stations across the continental shelf and slope. The stations covered two transects and included the inner shelf with its anoxic and extremely oxygen-depleted bottom waters, the oxygen minimum zone on the continental slope, and the lower continental slope below the oxygen minimum zone. High concentrations of dissolved sulfide, up to 22 mM, in the near-surface sediments of the inner shelf result from extremely high rates of bacterial sulfate reduction and the low capacity to oxidize and trap sulfide. The inner shelf break marks the seaward border of sulfidic bottom waters, and separates two different regimes of bacterial sulfate reduction. In the sulfidic bottom waters on the shelf, up to 55% of sulfide oxidation is mediated by the large nitrate-storing sulfur bacteria, Thiomargarita spp. The filamentous relatives Beggiatoa spp. occupy low-O₂ bottom waters on the outer shelf. Sulfide oxidation on the slope is apparently not mediated by the large sulfur bacteria. The data demonstrate the importance of large sulfur bacteria, which live close to the sediment-water interface and reduce the hydrogen sulfide flux to the water column. Modeling of pore water sulfide concentration profiles indicates that sulfide produced by bacterial sulfate reduction in the uppermost 16 cm of sediment is sufficient to account for the total flux of hydrogen sulfide to the water column. However, the total pool of hydrogen sulfide in the water column is too large to be explained by steady state diffusion across the sediment-water interface. Episodic advection of hydrogen sulfide, possibly triggered by methane eruptions, may contribute to hydrogen sulfide in the water column.     

Mobilization and speciation of arsenic from hydrothermally altered rock in laboratory column experiments under ambient conditions

This paper describes the mobilization and speciation of As found in hydrothermally altered rock under oxic column conditions. The altered rock sample was obtained from a tunnel project located in the Nakakoshi area of Hokkaido, Japan, whose geology is represented by slate, shale and sandstone. This area has undergone silicification, pyritization and argillic alteration resulting in As-enrichment of the rock. Results of the column experiments show that the infiltration rate, bulk density and rock bed thickness affected the duration of water residence, which in turn influenced the pH of the rock–water system. Coexisting ions most notably Ca²⁺ at amounts greater than ca. 50 mg/L retarded the mobilization of As. Mobilization of As from the rock with time occurred in two stages: stage 1 (weeks 1–20) with higher As leaching and stage 2 (weeks 20–76) characterized by nearly constant As release. In addition, pore water As concentrations revealed that the columns developed into two regions: the top half where most of the leaching occurred and the bottom part dominated by adsorption. Thus, the mechanism controlling the mobilization of As from the rock is a combination of one or more of the following processes: dissolution of soluble As-bearing fractions, pyrite oxidation and adsorption reactions. Arsenite (As[III]) was the dominant species in the effluent at the start of the experiment in columns with shorter water residence time and lower pH conditions (<8). On the other hand, arsenate (As[V]) was the major inorganic species released from the rock at higher pH (8–9.5) and when the system was close to equilibrium. Speciation of As with depth also indicated that As[III] disappeared around the bottom half of the columns, probably as a result of adsorption and/or oxidation. Arsenic speciation is partially controlled by the pH dependent adsorption of As species. The important adsorbent phases in the rock included Fe–Al oxides/oxyhydroxides, clay minerals and organic matter, which permitted the columns to attenuate additional As loadings including As[III]. Implications of these results on the design of a novel disposal method for these altered rocks include the enhancement of As adsorption through the addition of natural or artificial adsorbents and the utilization of a covering soil with low permeability to minimize rainwater infiltration into the rock.     

Speciation and natural attenuation of arsenic and iron in a tidally influenced shallow aquifer

The mobility of subsurface arsenic is controlled by sorption, precipitation, and dissolution processes that are tied directly to coupled redox reactions with more abundant, but spatially and temporally variable, iron and sulfur species. Adjacent to the site of a former pesticide manufacturing facility near San Francisco Bay (California, USA), soil and groundwater arsenic concentrations are elevated in sediments near the prior source, but decrease to background levels downgradient where shallow groundwater mixes with infiltrating tidal waters at the plume periphery, which has not migrated appreciably in over two decades of monitoring. We used synchrotron X-ray absorption spectroscopy, together with supporting characterizations and sequential chemical extractions, to directly determine the oxidation state of arsenic and iron as a function of depth in sediments from cores recovered from the unsaturated and saturated zones of a shallow aquifer (to 3.5 m below the surface). Arsenic oxidation state and local bonding in sediments, as As-sulfide, As(III)-oxide, or As(V)-oxide, were related to lithologic redox horizons and depth to groundwater. Based on arsenic and iron speciation, three subsurface zones were identified: (i) a shallow reduced zone in which sulfide phases were found in either the arsenic spectra (realgar-like or orpiment-like local structure), the iron spectra (presence of pyrite), or both, with and without As(III) or As(V) coordinated by oxygen; (ii) a middle transitional zone with mixed arsenic oxidation states (As(III)–O and As(V)–O) but no evidence for sulfide phases in either the arsenic or iron spectra; and (iii) a lower oxidized zone in the saturated freshwater aquifer in which sediments contained only oxidized As(V) and Fe(III) in labile (non-detrital) phases. The zone of transition between the presence and absence of sulfide phases corresponded to the approximate seasonal fluctuation in water level associated with shallow groundwater in the sand-dominated, lower oxic zone. Total sediment arsenic concentrations showed a minimum in the transition zone and an increase in the oxic zone, particularly in core samples nearest the former source. Equilibrium and reaction progress modeling of aqueous-sediment reactions in response to decreasing oxidation potential were used to illustrate the dynamics of arsenic uptake and release in the shallow subsurface. Arsenic attenuation was controlled by two mechanisms, precipitation as sulfide phases under sulfate-reducing conditions in the unsaturated zone, and adsorption of oxidized arsenic to iron hydroxide phases under oxidizing conditions in saturated groundwaters. This study demonstrates that both realgar-type and orpiment-type phases can form in sulfate-reducing sediments at ambient temperatures, with realgar predicted as the thermodynamically stable phase in the presence of pyrite and As(III) under more reduced conditions than orpiment. Field and modeling results indicate that the potential for release of arsenite to solution is maximized in the transition between sulfate-reduced and iron-oxidized conditions when concentrations of labile iron are low relative to arsenic, pH-controlled arsenic sorption is the primary attenuation mechanism, and mixed Fe(II,III)-oxide phases do not form and generate new sorption sites.     

A geochemistry study of arsenic speciation in overburden from Mae Moh Lignite Mine, Lampang, Thailand

The aim of this study was to investigate the geochemical characteristics of arsenic in the solid material samples of the Mae Moh Mine and also the Mae Moh power plants fly ash samples were systematically studied. Arsenic concentration in overburden, coal lignite and fly ash are variable (depending on source of solid samples). The results show that the strata of overburden, J seam of coal and fly ash are rich in arsenic and also relatively soluble from fly ash; it occurs as a surface precipitate on the ash particle. The experimental study on speciation in the strata also indicates that the arsenic speciation of Mae Moh solid samples are mainly arsenate, As (V), which are approaching exceed 80%. Arsenic content in the main of overburden is in the range of 14.3–888.8 mg/kg, which is larger than the arsenic background soil values. Solid materials polluted wastewater; the arsenic speciation was present predominantly as arsenate in the surface water of a series of Mae Moh solid materials basins.     

Arsenic enrichment in unconfined sections of the southern Gulf Coast aquifer system, Texas

Groundwater arsenic concentrations exceeding the federal drinking water standard are common in the southern Gulf Coast aquifer system in Texas, including in aerobic, unconfined groundwater which provides much of the municipal and domestic water supplies for the region. The objective of this study was to determine geochemical factors affecting the occurrence and distribution of groundwater As in unconfined portions of the southern Gulf Coast aquifer system through a comparative transect study of groundwater across three major hydrostratigraphic units (the Catahoula Formation, Jasper aquifer and Evangeline aquifer) and analysis of regional water quality data. Results show that As concentrations decrease with increasing distance from the Catahoula Formation, which is consistent with Miocene volcanic ash as the main source of As to groundwater in the region. Arsenic concentrations correlate with V, SiO₂ and K, all of which were released during weathering of volcanic sediments and their degradation products. In all three units, carbonate weathering and active recharge in the unconfined zones result in circum-neutral pH and oxidizing groundwater, which are typically amenable to As immobilization by adsorption of arsenate onto mineral oxides and clays. However, As concentrations exceed 10 μg/L in approximately 30% of wells. Silica that was co-released with As may compete for sorption sites and reduce the capacity for arsenate adsorption.     

砷在石膏中固定机制:掺杂态和表面吸附沉淀态以及其在砷污染控制中的作用

石膏是矿山开采及冶炼等工业过程产生的大宗固体废弃物。工业活动产生的废液普遍有高含量的砷等有毒元素,这导致所产生的石膏也含有较高浓度的砷等有毒元素。研究砷在石膏中地球化学行为和归趋对含砷石膏的砷污染控制具有重要的理论和实际意义。然而目前对含砷石膏中不同形态的砷的定量测定和分析尚存在问题。本文在不同pH值的条件下共沉淀砷和石膏,利用电感耦合等离子体质谱(ICP-MS)、同步辐射X-射线吸收近边光谱(XANES)和电子顺磁共振(EPR)对石膏中掺杂态和表面吸附沉淀态的砷进行定量分析。ICP-MS的结果表明随着pH从2升高到12 和14,石膏中砷的含量由57×10^-6 增加到 67 470×10^-6和63 980×10^-6。同步辐射X-射线吸收近边光谱和电子顺磁共振光谱分析表明石膏样品中主要含有五价砷。在2≤pH≤7.5时,固体样品中同步辐射吸收边后的峰形状和掺杂态砷的形状类似,而在pH≥8时,其边后峰的形状发生明显的变化;粉末电子顺磁共振(EPR)定量分析表明在2≤pH≤7.5时砷在石膏中的含量和ICP-MS的分析结果一致,而在pH≥8时其含量明显小于ICP-MS的分析结果。这些结果揭示了在2≤pH≤7.5时,砷在石膏中主要以掺杂态的形式存在,而在pH≥8时大部分砷是以吸附态或表面沉淀的形式存在。五价砷在石膏中的含量和固定机制随着pH值的变化而变化,其研究对了解尾矿中石膏对砷污染的控制作用具有重要作用。此外,研究石膏中由辐射导致的g约为2.33的[AsO₃]^2-自由基电子顺磁共振特征峰,有助于补充和完善石膏的电子顺磁共振特征谱在地质测年及辐射剂量学中的应用。     

Sources and behavior of arsenic and trace elements in groundwater and surface water in the Poopó Lake Basin,Bolivian Altiplano

Water management in semiarid and arid catchments such as the Poopó Lake Basin requires improved understanding of the complex behavior of the various contaminants, which affect the drinking water quality and considered as crucial for sustainable development of the region. Mechanisms of arsenic (As) release in the surface and groundwater were studied. Hydrochemical data for surface water (4 samples) and groundwater (28 samples) were collected in a small watershed in the Poopó catchment at the highland of the Bolivian Andes (Altiplano). All of them show high electrical conductivity values and moderately oxidizing conditions. The surface water contains high concentration of sulfate and the trace elements As, Zn and Pb in the zone affected by acid mine drainage. There is a large variability of the concentration of As and of the trace elements in the groundwater in the five different regions within the Poopó catchment. The metal concentrations sensitive to changes of redox state and results of speciation modeling suggest that As (V) is a predominant aqueous species, which conforms to the prevailing oxidizing conditions in the shallow groundwater environment. Two generalized trends for As distribution were identified in groundwater: (a) high concentrations are found in the arid zone (100–250 μg/L) in the southern (region III) and in the northwestern (region V) regions, and (b) low concentrations (<50 μg/L) are found in the remaining part of the basin (region I, II and IV). However, the spatial distribution within these regions needs to be investigated further. A conclusion from the present study is that there are multiple sources of As as well as other trace elements (such as Cd, Mn and Zn) in the Poopó Lake Basin. Among the sources and the processes which led to the mobility of As and other trace metals in the region are: (a) weathering of sulfide minerals, (b) oxidation of pyrite and/or arsenopyrite in mineralized areas and (c) desorption from hydrous ferric oxide (HFO) surfaces. In non-mining areas, volcanic ash is suggested to be a significant source of As.     

The chemistry of Offatts Bayou,Texas: A seasonally highly sulfidic basin

Offatts Bayou basin was created by use of this are as a borrow pit for landfill by the city of Galveston, Texas, in the first half of this century. Restricted exchange of water with the adjacent West Bay results in this basin changing, on a seasonal time scale, between oxic winter and highly sulfidic summer (greater than 500 μM ΣH₂S) bottom water conditions. It is, therefore, a “natural laboratory” for the investigation of the behavior of toxic metals in an estuarine environment where redox conditions undergo major variations. Here we report the first study of the chemistry of Offatts Bayou’s waters and sediment-associated trace metals. The high concentrations of dissolved sulfide in bottom water during summer cause a loss of macrofauna from most of the water deeper than about 4 m and the bottom of the basin. The potential exists for major mortality of organisms living in the oxic surface waters if rapid mixing of waters were to occur during the summer. Reactions of toxic metals with sulfides are probably the dominant influence on their potential bioavailability in this type of environment, as evidenced by large seasonal changes in concentrations of sedimentary sulfide minerals and associated trace metals. The trace metals As, Cu, and Hg are dominantly found in the pyrite phase (greater than 75% pyritization), Ag and Mn are moderately pyritized (40% to 60%), and Zn is not strongly associated with pyrite (less than 20% pyritization).