Aquatic arsenic: Toxicity,speciation, transformations,and remediation

This paper reviews the current knowledge on the toxicity, speciation and biogeochemistry of arsenic in aquatic environmental systems. The toxicity of arsenic is highly dependent on the chemical speciation. The effects of pH, E_h, adsorbing surfaces, biological mediation, organic matter, and key inorganic substances such as sulfide and phosphate combine in a complex and interwoven dynamic fashion to produce unique assemblages of arsenic species. The number of different arsenic species found in environmental samples and an understanding of the transformations between arsenic species has increased over the past few decades as a result of new and refined analytical methods. Changes in arsenic speciation and in total arsenic content of foods upon processing have suggested possible risks associated with processed and unprocessed food. Arsenic removal from water using adsorbents, chemical oxidation, photolysis and photocatalytic oxidation techniques is also reviewed.     

Fractionation of radionuclide species in the environment

Naturally occurring and artificially produced radionuclides in the environment may be present in different physico-chemical forms (i.e., radionuclide species) varying in size (nominal molecular mass), charge properties and valence, oxidation state, structure and morphology, density, degree of complexation, etc. Low molecular mass (LMM) species are believed to be mobile and potentially bioavailable, while high molecular mass (HMM) species such as colloids, polymers, pseudocolloids and particles are considered inert. Due to time-dependent transformation processes such as mobilisation of radionuclide species from solid phases or interactions of mobile and reactive radionuclide species with components in soils and sediments, the original distribution of radionuclides deposited in ecosystems will change over time. To assess the environmental impact from radionuclide contamination, information on radionuclide species deposited, interactions within affected ecosystems and the time-dependent distribution of radionuclide species influencing mobility and biological uptake is essential. The development of speciation techniques to characterize radionuclide species in waters, soils and sediments should therefore be essential for improving the prediction power of impact and risk assessment models. The present paper reviews available fractionation techniques which can be utilised for radionuclide speciation purposes.     

Effect of indigenous bacterial activity on arsenic mobilization under anaerobic conditions

Batch biochemical leaching tests were carried out to investigate the mobility of arsenic from a contaminated soil collected from a French gold mining site. The specific objective of this research was to examine the effect of indigenous bacterial activity on arsenic mobilization under anaerobic conditions. In a first step, physical and chemical characterizations were performed to provide data concerning the liquid-solid partitioning and mobility of arsenic and other inorganic constituents. In a second step, batch bioleaching tests were conducted in shaker flasks to determine the effect of indigenous bacterial activity under different anaerobic conditions (i.e., addition of mineral nutrients and carbon sources) on arsenic mobilization. Results indicated that arsenic release during contact with deionized water was limited by its very low solubility in the interstitial solution and by the stability of the different arsenic compounds formed with the amorphous solid phases of the soil (mainly iron (oxy)hydroxides). However, an increased mobilization potential was observed over the long term under anaerobic conditions with indigenous bacterial activity enhanced by the addition of carbon sources.     

Arsenic contamination of the environment: a new perspective from central-east India

This paper reports a regional contamination of the environment in central-east India that does not share geology or boundary with the Bengal Delta Plain. About 30,000 people residing in 30 villages and towns are directly exposed to arsenic and more than 200,000 people are "at risk." Complete geographical extent of this contamination is being established, and this newly reported contaminated area could be quite large. This paper further reports that the mechanisms involved in arsenic mobilisation are complex and the two theories of arsenic mobilisation, i.e., pyrite oxidation and oxyhydroxides reduction, do not fully explain the high levels of arsenic contamination. This paper also proposes the "oxidation-reduction theory" for arsenic mobilisation where the arsenic originates from the arsenopyrite oxidation and the arsenic thus mobilised forms the minerals and gets reduced underground in favourable Eh conditions. The stoppage of water withdrawal from the contaminated sources did not result in lowering of arsenic levels as expected according to the heavy groundwater extraction theory (pyrite oxidation theory). Cases of arsenicosis in the region are on the rise and the switchover to less contaminated water has not reversed the arsenicosis progression in the affected persons even after 2 years. Surface water of the rivers is also being contaminated because of the probable dislocation of contaminated groundwater due to the heavy rains in monsoon season, which indicates that the river water could be a major carrier of arsenic in dissolved or adsorbed forms that may be a cause of contamination of the delta plains.     

Recent advances in the bioremediation of arsenic-contaminated groundwater

The biological treatment of groundwater is used primarily to remove electron donors from water sources, providing (biologically) stable drinking water, which preclude bacterial regrowth during subsequent water distribution. To the electron donors belong also the dissolved metal cations of ferrous iron and manganese, which are common contaminants found in most (anaerobic) groundwater. The removal of iron and manganese is usually accomplished by the application of chemical oxidation and filtration. However, biological oxidation has recently gained increased importance and application due to the existence of certain advantages, over the conventional physicochemical treatment. The oxidation of iron and manganese is accelerated by the presence of certain indigenous bacteria, the so-called "iron and manganese oxidizing bacteria." In the present paper, selected long-term experimental results will be presented, regarding the bioremediation of natural groundwater, containing elevated concentrations of iron and arsenic. Arsenic is considered as a primary pollutant in drinking water due to its high toxicity. Therefore, its efficient removal from natural waters intended for drinking water is considered of great importance. The application of biological processes for the oxidation and removal of dissolved iron was found to be an efficient treatment technique for the simultaneous removal of arsenic, from initial concentrations between 60 and 80 microg/l to residual (effluent) arsenic concentrations lower than the limit of 10 microg/l. The paper was focused on the removal of As(III) as the most common species in anaerobic groundwater and generally is removed less efficiently than the oxidized form of As(V). To obtain information for the mechanism of As(III) removal, X-ray photoelectron spectroscopy (XPS) analyses were applied and it was found that As(III) was partially oxidized to As(V), which enabled the high arsenic removal efficiency over a treatment period of 10 months.     

Soil-plant transfer of plutonium and americium in contaminated regions of Belarus after the Chernobyl catastrophe

Experimental data are presented for the soil to plant transfer of plutonium and americium into the main species of grass vegetation of Belarusian grasslands contaminated as a result of the Chernobyl catastrophe of 1986. The content of radionuclides in pore soil solutions and the total reserve of biologically available forms of plutonium and americium in rooting layers of different soil varieties have been established. The distribution coefficients of (239,240)Pu and 241Am between the solid phase and pore waters of soils have been evaluated. The migration ability and biological availability of radionuclides in soils with different structures of the absorbing complex have been analyzed for various landscape conditions. The dependence of soil to plant transfer of plutonium and americium on the content and composition of organic matter, and other characteristics of the soil complex has been studied. On the basis of these data, predictions of the contamination levels of the main grass species of natural and agricultural ecosystems by 241Am are presented.     

Organoarsenicals in poultry litter: Detection,fate, and toxicity

Arsenic contamination in groundwater has endangered the health and safety of millions of people around the world. One less studied mechanism for arsenic introduction into the environment is the use of organoarsenicals in animal feed. Four organoarsenicals are commonly employed as feed additives: arsanilic acid, carbarsone, nitarsone, and roxarsone. Organoarsenicals are composed of a phenylarsonic acid molecule with substituted functional groups. This review documents the use of organoarsenicals in the poultry industry, reports analytical methods available for quantifying organic arsenic, discusses the fate and transport of organoarsenicals in environmental systems, and identifies toxicological concerns associated with these chemicals. In reviewing the literature on organoarsenicals, several research needs were highlighted: advanced analytical instrumentation that allows for identification and quantification of organoarsenical degradation products; a greater research emphasis on arsanilic acid, carbarsone, and nitarsone; identification of degradation pathways, products, and kinetics; and testing/development of agricultural wastewater and solid treatment technologies for organoarsenical-laden waste.     

X-ray absorption spectroscopy studies of reactions of technetium, uranium and neptunium with mackinawite

Technetium, uranium and neptunium may all occur in the environment in more than one oxidation state (IV or VII, IV or VI and IV or V respectively). The surface of mackinawite, the first-formed iron sulfide phase in anoxic conditions, can promote redox changes so a series of laboratory experiments were carried out to explore the interactions of Tc, U and Np with this mineral. The products of reaction were characterised using X-ray absorption spectroscopy. Technetium, added as TcO4(-), is reduced to oxidation state IV and forms a TcS(2)-like species. On oxidation of the mackinawite in air to form goethite, Tc remains in oxidation state IV but in an oxide, rather than a sulfide environment. At low concentrations, uranium forms uranyl surface complexes on oxidised regions of the mackinawite surface but at higher concentrations, the uranium promotes surface oxidation and forms a mixed oxidation state oxide phase. Neptunium is reduced to oxidation IV and forms a surface complex with surface sulfide ions. The remainder of the Np coordination sphere is filled with water molecules or hydroxide ions.     

The determination of trace metal pollutants in environmental matrices using ion chromatography

A review is presented detailing the development of ion chromatography (IC) as a selective analytical tool for the determination of toxic metals and their organic species in many environmental sample matrices. A brief outline of ion chromatographic principles, together with an overview of the stationary phases used to separate metals, namely ion exchangers, modified ion pair sorbents and chelating ion exchangers, and the methods for detecting metal ions including hyphenation with spectroscopy and sample preparation schemes are also given, prior to a critical examination of developed methods for various metals including arsenic, chromium, cadmium, lead, mercury, beryllium, aluminium and uranium since 1990.     

Possible role of organic matter in radiocaesium adsorption in soils

The aim of this review is to examine the hypothesis that organic matter decreases the adsorption of radiocaesium on clay minerals. The factors that determine radiocaesium mobility and bioavailability in soil are briefly outlined to show why a relationship between soil organic matter content and enhanced Cs bioavailability is paradoxical. In all the investigations reviewed the ionic compositions of both the solid and the solution phases have been strictly controlled. We show that the addition of organic matter to reference clay minerals causes decreases of up to an order of magnitude in the distribution coefficient of radiocaesium. Similarly, the chemical removal of organic matter from the clay-sized fraction of soil usually leads to an increase in Cs adsorption. We suggest that the nature of the organic matter and its interaction with mineral surfaces are as important as the amount present.     

Aqueous, solid and gaseous partitioning of selenium in an oxic sandy soil under different microbiological states

The aim of this study was to investigate the role of microorganisms on the behaviour of selenium in natural soil maintained under strictly aerobic conditions. Six-day batch experiments were performed with soils constrained to different microbiological states, either by sterilisation or by adding organic substrates. Selenium was added to the soil as selenite. The distribution of selenium in the gaseous, liquid and solid phases of the batch was measured. Selenium partitioning between the various solid phases was investigated by chemical sequential extractions. Active microorganisms played major effects on the distribution of selenium within the soil. On the one hand, microorganisms could promote selenium volatilisation (in relatively small amounts), leading to the spreading of selenium compounds outside the soil. On the other hand, microbial activities increased both amount of selenium retained by the soil and the strength of its retention (less exchangeable selenium), making selenium less susceptible to remobilisation.     

Modelling the physico-chemical speciation of plutonium in the eastern Irish Sea: a further development

A numerical full three-dimensional model previously developed to simulate the physicochemical speciation of plutonium in the eastern Irish Sea has been improved. The model solves simultaneously the hydrodynamic equations, the suspended matter equation and the equations that give the time evolution of Pu concentrations in water, suspended matter and bottom sediments. It is considered that Pu may exist in each phase in two different oxidation states. Redox reactions are also considered. In the earlier version of the model, a one-step kinetic model was used to describe the transfers of radionuclides between the dissolved and solid phases. Although with this kind of model the contamination of the sediments can be properly simulated, it is clearly not able to describe the re-dissolution of radionuclides from a contaminated sediment once the external source to the sea is reduced. Thus, the model has been improved by substituting the one-step model with a two-step kinetic model consisting of two consecutive reversible reactions. Now it is possible to simulate both the sediment contamination and the re-dissolution processes.     

Factors influencing recovery and restoration following a chemical incident

Chemicals are an important part of our society. A wide range of chemicals are discharged into the environment every day from residential, commercial and industrial sources. Many of these discharges do not pose a threat to public health or the environment. However, global events have shown that chemical incidents or accidents can have severe consequences on human health, the environment and society. It is important that appropriate tools and technical guidance are available to ensure that a robust and efficient approach to developing a remediation strategy is adopted. The purpose of remediation is to protect human health from future exposure and to return the affected area back to normal as soon as possible. There are a range of recovery options (techniques or methods for remediation) that are applicable to a broad range of chemicals and incidents. Recovery options should be evaluated according to their appropriateness and efficacy for removing contaminants from the environment; however economic drivers and social and political considerations often influence decision makers on which remedial actions are implemented during the recovery phase of a chemical incident. To date, there is limited information in the literature on remediation strategies and recovery options that have been implemented following a chemical incident, or how successful they have been. Additional factors that can affect the approach taken for recovery are not well assessed or understood by decision makers involved in the remediation and restoration of the environment following a chemical incident. The identification of this gap has led to the development of the UK Recovery Handbook for Chemical Incidents to provide a framework for choosing an effective recovery strategy. A compendium of practical evidence-based recovery options (techniques or methods for remediation) for inhabited areas, food production systems and water environments has also been developed and is included in the chemical handbook. This paper presents the key factors that should be considered when developing a recovery strategy with respect to how these may impact on its effectiveness. The paper also highlights the importance of these factors through an evaluation of recovery strategies implemented following real chemical incidents that have been reported in the literature.     

Geochemical speciation of heavy metals in semiarid soils of the central Ebro Valley (Spain)

This study describes the chemical speciation of Fe, Mn, Zn and Cr in four selected soil types representative of the more abundant agricultural soils in the central Ebro river valley in Aragón. Gypsisols, Calcisols, Solonchaks and Solonetzs represent around 20% of the total soil surface in the region. A total of 12 selected sites have been sampled and five subsamples were taken in each site to create a composite sample. The four elements have been analysed by the sequential extraction procedure of Tessier et al. [Anal. Chem. 51 (1979) 844.] by emission atomic spectrometry of solid state (ICP OES). Very little amounts of Fe and Mn were retrieved from the exchangeable phase, the ready available for biogeochemistry cycles in the ecosystems. Therefore, low quantities of Fe and Mn can be taken up by plants in these alkaline soils. Cr was not detected in the bioavailable forms as well as Zn that was only present in negligible amounts in very few samples. The absence of mobile forms of Cr in all soils eliminates the toxic risk both in the trophic chain and from its migration downwards the soil profile. The largest contents of Fe, Zn and Cr were retrieved from the residual phase where metals are strongly bound to minerals, whereas Mn from the carbonate and oxide phases amounts 80% of its total content. Gypsisols and Calcisols have the lowest metal contents while the highest are found in Solonetzs and Solonchaks.     

The use of molecular techniques to characterize the microbial communities in contaminated soil and water

Traditionally, the identification and characterization of microbial communities in contaminated soil and water has previously been limited to those microorganisms that are culturable. The application of molecular techniques to study microbial populations at contaminated sites without the need for culturing has led to the discovery of unique and previously unrecognized microorganisms as well as complex microbial diversity in contaminated soil and water which shows an exciting opportunity for bioremediation strategies. Nucleic acid extraction from contaminated sites and their subsequent amplification by polymerase chain reaction (PCR) has proved extremely useful in assessing the changes in microbial community structure by several microbial community profiling techniques. This review examines the current application of molecular techniques for the characterization of microbial communities in contaminated soil and water. Techniques that identify and quantify microbial population and catabolic genes involved in biodegradation are examined. In addition, methods that directly link microbial phylogeny to its ecological function at contaminated sites as well as high throughput methods for complex microbial community studies are discussed.     

Trace metal occurrence and distribution in sediments and mangroves,Pumicestone region,southeast Queensland,Australia

The Pumicestone region is a unique catchment in northern Moreton Bay, southeast Queensland. The region supports a wide range of land-use activities as well as attractions such as nature conservation areas. One environmental aspect that has not previously been addressed in this area is the occurrence of minor and trace metals in estuarine sediments associated with the main estuaries of the region. The trace metals included in this investigation are: vanadium, chromium, molybdenum, cobalt, nickel, copper, zinc, cadmium, lead and arsenic. To determine and evaluate the occurrence and distribution of metals in the area, several components have been analysed: bedrock material, pre-industrial settings, recent estuarine sediments, soils of estuarine origin and mangrove pneumatophores. The 40 sites chosen for sediment and soil samples cover a variety of estuarine settings and represent a range of natural conditions in terms of channel and bank morphology, tidal energy, vegetation cover, relationship to bedrock, water salinity and land disturbance. The chemical, mineralogical and statistical analyses employed in this study enabled (a) establishment of background values for the area, (b) determination of relationships between metals and (c) identification of sites with anomalous metal concentrations. All the metals found in the sediments of the area are sourced from the geological bedrock. The dominant trace elements identified in sediments are Zn, V and Cr. The remaining metals are highly variable spatially. All trace metals are controlled by the presence of Fe and Mn oxides, and by the grainsize of the sediment. Typically, fine-grained Fe-rich materials tend to adsorb more trace metals than sandy sediments. In soils that have developed from estuarine muds, some metals such as Cr, Mo, Pb and As tend to be in larger quantities than in the estuarine counterparts. Some of the elements, which occur in significant amounts in the sediment, have been detected in mangrove tissue (Avicenniamarina) such as V, Cr, Zn, Fe and Mn. Of particular note is Cu, which is present in mangrove tissue in quantities many times exceeding the sediment concentration. The comparative analysis of pre-industrial settings and recent sediments and soils highlighted some areas of metal enrichment such as acid-affected sites where oxidation of pyrite has mobilised metals from sediments; these metals are then redistributed in Fe-rich surficial layers. Disturbed banks within the estuaries are also likely to have low levels of metal enrichment due to boating activities.     

砷污染与生态环境的关系

砷为广泛存在于自然界中的一种元素,在环境中表现出复杂的地球化学性质,不同形态的砷在大气、土壤及水体中的迁移和富集受许多物理化学因素的制约,并产生毒害程度不同的环境效应。总结砷的来源、其在不同环境介质中的存在状态及砷污染对生态环境的影响,提出应在了解不同介质环境中砷的赋存状态后,采用不同的治理方法有效地去除砷,以最大程度地降低砷对人体健康及生态环境造成的危害。     

Comparison of laboratory uranium sorption data with 'in situ distribution coefficients' at the Koongarra uranium deposit, Northern Australia

Distribution coefficients derived from laboratory sorption experiments are commonly used to model the migration of long-lived radionuclides in the environment. However, it has been suggested that field measurements in natural systems ('in situ distribution coefficients') may provide a more accurate indication of 'true' partitioning coefficients than laboratory experiments. In this paper, the relationship between field and laboratory sorption data for uranium is evaluated, using data from the Koongarra uranium deposit in Northern Australia. An extensive suite of laboratory sorption measurements and in situ partitioning data for U has been obtained at this site. A valid comparison can only be made when the calculation of field partitioning is based on U in 'accessible' phases (rather than total U in the solid) and U species in true solution (i.e. excluding particles). In this study, accessible U was estimated using a chemical extraction and the results were verified using an isotope exchange technique. A satisfactory correspondence between field and laboratory partitioning data was obtained when the pH values and partial pressures of CO2 in laboratory sorption experiments were similar to those found in the field. Under these conditions, the measured laboratory sorption ratios (Rd) and in-field partitioning values (Pacc) for U at Koongarra were in the range between approximately 1 x 10(3) and 2 x 10(4) ml/g. However, the distribution of U in solid and groundwater phases at Koongarra is extremely heterogeneous. This variability must be taken into account when modelling radionuclide migration at this site.     

A particle-tracking method for simulating the dispersion of non-conservative radionuclides in coastal waters.

A particle-tracking method has been used to simulate the dispersion of non-conservative radionuclides in the sea. Three dimensional turbulent diffusion and the interactions between water, suspended matter and bottom sediments are simulated using a stochastic method. Kinetic transfer coefficients, as in finite difference models, are used to describe the transfers between the liquid and solid phases. Deposition of suspended matter and erosion of sediment are also included in the model. The method has been applied to simulate the dispersion of 137Cs and (239,240)Pu in the English Channel and the results have been compared with those of a finite difference model. The results from both techniques are, in general, in good agreement.     

Chattanooga shale exploitation and the aquatic environment: The critical issues

Early identification of the critical environmental issues arising from new energy technologies is needed to ensure adequate consideration of these issues in all phases of research and development. This study examines the potential hazards to aquatic ecosystems from large-scale exploitation (190,000 Mg/day) of the Chattanooga Shale Formation, an immense reserve of oil shale and uranium in Kentucky, Tennessee, and Alabama. Using existing data on regional ecology, hydrology, mining operations, and raw and spent shale chemistry, we identified two major, related environmental issues: (1) the potential for extensive adverse effects on aquatic communities through degradation of water quality and habitat; and (2) the potential conflict between the requirements for shale exploitation, and the habitat and water quality needs of threatened or endangered species. Specific hazards to aquatic ecosystems include erosion, sedimentation, acid mine drainage, raw and spent shale leachates, and surface disposal of immense quantities of solid wastes. Twelve of 19 federally designated, threatened or endangered fish and mollusks in the shale-bearing region were identified as known or recent inhabitants of the counties believed to be most favorable for the exploitation of shale. Of these, five species occur as single populations or are limited to a single river system. The potential for adverse effects on these species is greatest in the counties near the Tennessee-Alabama state line. Future research needs include physical, chemical, and toxicological characterizations of shale leachates and studies of the transport and fate of leachable contaminants. Such research can provide the guidance necessary to minimize impacts on aquatic communities resulting from extraction, retorting, and disposal of shale.