Arsenic and Other Geogenic Contaminants in Groundwater - A Global Challenge.

Groundwater is a much safer and more dependable source of drinking water than surface water. However, natural (geogenic) hazardous elements can contaminate groundwater and lead to severe health problems in consumers. Arsenic concentrations exceeding the WHO drinking water guideline of 10 µg/L globally affect over 220 million people and can cause arsenicosis (skin lesions and cancers). Fluoride, while preventing caries at low concentrations, has detrimental effects when above the WHO drinking water guideline of 1.5 mg/L and puts several hundred million people at risk of dental and skeletal fluorosis. In this article, we report on the geochemistry and occurrence of arsenic and fluoride in groundwater and on the development of global and regional risk maps that help alert governments and water providers to take appropriate mitigation measures for the provision of safe drinking water. We then summarize research on the removal of arsenic and fluoride from drinking water, focusing on adapted technologies for water treatment. Finally, we discuss the applicability of various measures in a larger context and future challenges in reaching the goal of access to safe drinking water for all.

Multi-criteria assessment of community-based fluoride-removal technologies for rural Ethiopia.

Elevated concentrations of naturally-occurring fluoride in groundwater pose a serious health risk to millions of people living in the Ethiopian Rift Valley. In the absence of low-fluoride water resources of sufficient capacity, fluoride removal from drinking water is the accepted mitigation option. To date, five different community-level fluoride-removal technologies have been implemented in Ethiopia, although only a few units have been found in a functional state in the field. Which technology should be promoted and up-scaled is the subject of controversial debate amongst key stakeholders. This paper describes a multi-criteria decision analysis exercise, which was conducted with the participation of stakeholders in Ethiopia during a one-day workshop, to assess in an objective and transparent manner the available technology options. Criteria for technology comparison were selected and weighted, thus enabling the participants to assess the advantages and disadvantages of the different technologies and hear the views of other stakeholders. It was shown that there is no single most-preferable, technical solution for fluoride removal in Ethiopia. Selection of the most suitable solution depends on location-specific parameters and on the relative importance given to different criteria by the stakeholders involved. The data presented in this paper can be used as reference values for Ethiopia.

Groundwater arsenic contamination throughout China.

Arsenic-contaminated groundwater used for drinking in China is a health threat that was first recognized in the 1960s. However, because of the sheer size of the country, millions of groundwater wells remain to be tested in order to determine the magnitude of the problem. We developed a statistical risk model that classifies safe and unsafe areas with respect to geogenic arsenic contamination in China, using the threshold of 10 micrograms per liter, the World Health Organization guideline and current Chinese standard for drinking water. We estimate that 19.6 million people are at risk of being affected by the consumption of arsenic-contaminated groundwater. Although the results must be confirmed with additional field measurements, our risk model identifies numerous arsenic-affected areas and highlights the potential magnitude of this health threat in China.

Coupling predicted model of arsenic in groundwater with endemic arsenism occurrence in Shanxi Province, Northern China.

Statistical modeling has been used to predict high risk area of arsenic (As) hazard, but information about its application on endemic arsenism is limited. In this study, we aim to link the prediction model with population census data and endemic arsenicosis in Shanxi Province, Northern China. 23 explanatory variables from different sources were compiled in the format of grid at 1km resolution in a GIS environment. Logistic regression was applied to describe the relationship between binary-coded As concentrations data and the auxiliary predictors. 61 endemic arsenism villages were geo-located and combined with output maps of the prediction model. Linear regression was used to identify the relationship between arsenicosis occurrence rate and predictive As probability at village level. Our results show that 6 explanatory environmental variables were significantly contributed to the final model. Area of 3000 km(2) was found to have high risk of As concentrations above 50 µg L(-1). The linear regression indicates that 13% of the variation in arsenicosis occurrence rate can be predicted using predictive probability of As concentration above 50 µg L(-1) in Shanxi Province. These results suggest that As prediction model may be helpful for identifying As-contaminated area and endemic arsenism village.

Predicting the risk of arsenic contaminated groundwater in Shanxi Province, Northern China.

Shanxi Province is one of the regions in northern China where endemic arsenicosis occurs. In this study, stepwise logistic regression was applied to analyze the statistical relationships of a dataset of arsenic (As) concentrations in groundwaters with some environmental explanatory parameters. Finally, a 2D spatial model showing the potential As-affected areas in this province was created. We identified topography, gravity, hydrologic parameters and remote sensing information as explanatory variables with high potential to predict high As risk areas. The model identifies correctly the already known endemic areas of arsenism. We estimate that the area at risk exceeding 10 µg L(-1) As occupies approximately 8,100 km(2) in 30 counties in the province.

Environmental selenium research: from microscopic processes to global understanding.

Selenium is a natural trace element that is of fundamental importance to human health. The extreme geographical variation in selenium concentrations in soils and food crops has resulted in significant health problems related to deficient or excess levels of selenium in the environment. To deal with these kinds of problems in the future it is essential to get a better understanding of the processes that control the global distribution of selenium. The recent development of analytical techniques and methods enables accurate selenium measurements of environmental concentrations, which will lead to a better understanding of biogeochemical processes. This improved understanding may enable us to predict the distribution of selenium in areas where this is currently unknown. These predictions are essential to prevent future Se health hazards in a world that is increasingly affected by human activities.

Uptake of fluoride from aqueous solution on nano-sized hydroxyapatite: examination of a fluoridated surface layer.

Hydroxyapatite (Ca(10)(PO(4))(6)(OH)(2), HAP), both as a synthetic material and as a constituent of bone char, can serve as an effective and relatively inexpensive filter material for fluoride (F(-)) removal from drinking water in low-income countries. Fluoride uptake on HAP can occur through different mechanisms, which are, in principle, influenced by solution composition. Suspensions of HAP (2 g L(-1)) were equilibrated under controlled pH conditions (pH 6.5, 7.3, 9.5) at 25 °C for 28 d after the addition of different F(-) concentrations (0.5-7.0 mM). The reacted HAP solids were examined with Transmission Electron Microscopy (TEM), Fourier Transform Infrared Spectroscopy (FTIR), X-ray Photoelectron Spectroscopy (XPS), and Nano Secondary Ion Mass Spectroscopy (NanoSIMS). Fluoride uptake on HAP was dependent on pH, with the highest capacity at pH 6.5; the lowest uptake was found at pH 9.5. Under all experimental conditions, the thermodynamically stable mineral phase was fluorapatite, (Ca(10)(PO(4))(6)F(2), FAP). Fluoride uptake capacity was quantified on the basis of FTIR and XPS analysis, which was consistent with F(-) uptake from solution. The results of XPS and NanoSIMS analyses indicate that a fluoridated surface layer with a thickness of several nanometers is formed on nanosized HAP.

Solid solutions between CrO4- and SO4-ettringite Ca6(Al(OH)6)2[(CrO4)x(SO4)(1-x)]3*26 H2O.

Chromate is a toxic contaminant of potential concern, as it is quite soluble in the alkaline pH range and could be released to the environment. In cementitous systems, CrO4(2−) is thought to be incorporated as a solid solution with SO4(2−) in ettringite. The formation of a solid solution (SS) could lower the soluble CrO4(2−) concentrations. Ettringite containing SO4(2−) or CrO4(2−) and mixtures thereof have been synthesized. The resulting solids and their solubility after an equilibration time of 3 months have been characterized. For CrO4-ettringite at 25 °C, a solubility product log K(S0) of −40.2 ± 0.4 was calculated: log K(CrO4−ettringite) = 6log{Ca2+} + 2log{Al(OH)4(−)} + 3log{CrO4(2−)} + 4log{OH−} + 26log{H2O}. X-ray diffraction and the analysis of the solution indicated the formation of a regular solid solution between SO4- and CrO4-ettringite with a miscibility gap between 0.4 ≤ XCrO4 ≤ 0.6. The miscibility gap of the SO4- and CrO4-ettringite solid solution could be reproduced with a dimensionless Guggenheim fitting parameter (a0) of 2.03. The presence of a solid solution between SO4- and CrO4-ettringite results in a stabilization of the solids compared to the pure ettringites and thus in an increased uptake of CrO4(2−) in cementitious systems.

Statistical modeling of global geogenic fluoride contamination in groundwaters.

The use of groundwater with high fluoride concentrations poses a health threat to millions of people around the world. This study aims at providing a global overview of potentially fluoride-rich groundwaters by modeling fluoride concentration. A large database of worldwide fluoride concentrations as well as available information on related environmental factors such as soil properties, geological settings, and climatic and topographical information on a global scale have all been used in the model. The modeling approach combines geochemical knowledge with statistical methods to devise a rule-based statistical procedure, which divides the world into 8 different "process regions". For each region a separate predictive model was constructed. The end result is a global probability map of fluoride concentration in the groundwater. Comparisons of the modeled and measured data indicate that 60-70% of the fluoride variation could be explained by the models in six process regions, while in two process regions only 30% of the variation in the measured data was explained. Furthermore, the global probability map corresponded well with fluorotic areas described in the international literature. Although the probability map should not replace fluoride testing, it can give a first indication of possible contamination and thus may support the planning process of new drinking water projects.

Statistical modeling of global geogenic arsenic contamination in groundwater.

Contamination of groundwaters with geogenic arsenic poses a major health risk to millions of people. Although the main geochemical mechanisms of arsenic mobilization are well understood, the worldwide scale of affected regions is still unknown. In this study we used a large database of measured arsenic concentration in groundwaters (around 20,000 data points) from around the world as well as digital maps of physical characteristics such as soil, geology, climate, and elevation to model probability maps of global arsenic contamination. A novel rule-based statistical procedure was used to combine the physical data and expert knowledge to delineate two process regions for arsenic mobilization: "reducing" and "high-pH/ oxidizing". Arsenic concentrations were modeled in each region using regression analysis and adaptive neuro-fuzzy inferencing followed by Latin hypercube sampling for uncertainty propagation to produce probability maps. The derived global arsenic models could benefit from more accurate geologic information and aquifer chemical/physical information. Using some proxy surface information, however, the models explained 77% of arsenic variation in reducing regions and 68% of arsenic variation in high-pH/oxidizing regions. The probability maps based on the above models correspond well with the known contaminated regions around the world and delineate new untested areas that have a high probability of arsenic contamination. Notable among these regions are South East and North West of China in Asia, Central Australia, New Zealand, Northern Afghanistan, and Northern Mali and Zambia in Africa.

Sorption of Sb(III) and Sb(V) to goethite: influence on Sb(III) oxidation and mobilization.

Antimony is an element of growing interest for a variety of industrial applications, even though Sb compounds are classified as priority pollutants by the Environmental Protection Agency of the United States. Iron (Fe) hydroxides appear to be important sorbents for Sb in soils and sediments, but mineral surfaces can also catalyze oxidation processes and may thus mobilize Sb. The aim of this study was to investigate whether goethite immobilizes Sb by sorption or whether Sb(III) adsorbed on goethite is oxidized and then released. The sorption of both Sb(III) and Sb(V) on goethite was studied in 0.01 and 0.1 M KClO4 M solutions as a function of pH and Sb concentration. To monitor oxidation processes Sb species were measured in solution and in the solid phase. The results show that both Sb(III) and Sb(V) form inner-sphere surface complexes at the goethite surface. Antimony(III) strongly adsorbs on goethite over a wide pH range (3-12), whereas maximum Sb(V) adsorption is found below pH 7. At higher ionic strength, the desorption of Sb(V) is shifted to lower pH values, most likely due to the formation of ion pairs KSb(OH)6 degrees. The sorption data of Sb(V) can be fitted by the modified triple-layer surface complexation model. Within 7 days, Sb(III) adsorbed on goethite is partly oxidized at pH 3, 5.9 and 9.7. The weak pH-dependence of the rate coefficients suggests that adsorbed Sb(III) is oxidized by 02 and that the coordination of Sb(III) to the surface increases the electron density of the Sb atom, which enhances the oxidation process. At pH values below pH 7, the oxidation of Sb(III) did not mobilize Sb within 35 days, while 30% of adsorbed Sb(III) was released into the solution at pH 9.9 within the same time. The adsorption of Sb(III) on Fe hydroxides over a wide pH range may be a major pathway for the oxidation and release of Sb(V).

The challenge of micropollutants in aquatic systems.

The increasing worldwide contamination of freshwater systems with thousands of industrial and natural chemical compounds is one of the key environmental problems facing humanity. Although most of these compounds are present at low concentrations, many of them raise considerable toxicological concerns, particularly when present as components of complex mixtures. Here we review three scientific challenges in addressing water-quality problems caused by such micropollutants. First, tools to assess the impact of these pollutants on aquatic life and human health must be further developed and refined. Second, cost-effective and appropriate remediation and water-treatment technologies must be explored and implemented. Third, usage and disposal strategies, coupled with the search for environmentally more benign products and processes, should aim to minimize introduction of critical pollutants into the aquatic environment.

Iron-mediated oxidation of antimony(III) by oxygen and hydrogen peroxide compared to arsenic(III) oxidation.

Antimony is used in large quantities in a variety of products, though it has been declared as a pollutant of priority interest by the Environmental Protection Agency of the United States (USEPA). Oxidation processes critically affect the mobility of antimony in the environment since Sb(V) has a greater solubility than Sb(lll). In this study, the cooxidation reactions of Sb(lIl) with Fe(ll) and both O2 and H2O2 were investigated and compared to those of As(III). With increasing pH, the oxidation rate coefficients of Sb(lll) in the presence of Fe(ll) and O2 increased and followed a similar pH trend as the Fe(ll) oxidation by O2. Half-lives of Sb(lll) were 35 and 1.4 h at pH 5.0 and pH 6.2, respectively. The co-oxidation with Fe(ll) and H2O2 is about 7000 and 20 times faster than with Fe(ll) and O2 at pH 3 and pH 7, respectively. For both systems, *OH radicals appear to be the predominant oxidant below approximately pH 4, while at more neutral pH values, other unknown intermediates become important. The oxidation of As(lll) follows a similar pH trend as the Sb(lll) oxidation; however, As(lll) oxidation was roughly 10 times slower and only partly oxidized in most of the experiments. This study shows that the Fe(ll)-mediated oxidation of Sb(Ill) can be an important oxidation pathway at neutral pH values.

Solubility of antimony and other elements in samples taken from shooting ranges.

The aim of this study was to determine whether or not Sb and other elements (Ni, Cu, Bi, Tl, and Hg) originating from Pb alloy (2-5 wt. % Sb) bullets become more soluble as a result of weathering and what mechanisms possibly control their solubility. Samples were taken from bank material behind the targets at seven Swiss shooting ranges. The samples were dried, sieved, analyzed, and subjected to leaching experiments. Total average concentrations of Sb ranged from 0.5 to 13.8 g kg(-1). In the leaching experiments, Sb was almost exclusively present in solution as the oxidized species Sb(V) in concentrations of up to 5 mg L(-1). The Ca mineral Ca[Sb(OH)6]2 is suggested to control dissolved Sb(V) concentrations in soils at high concentrations. Oxalate extractions suggested that approximately 50% of Sb [predominantly Sb(V)] in the <0.5-mm fraction was adsorbed to Fe (hydr)oxides and possibly other minerals, such as calcite, that are soluble at pH 2. However, it is possible that only a fraction of the oxalate-extractable Sb(V) is reversibly bound to mineral surfaces. It was concluded that the release of Sb is significant and considerably higher than the other elements under investigation and that the mechanisms controlling Sb mobility should be further investigated.

Sorption of selenite and selenate to cement minerals.

The sorption of selenite and selenate to ettringite (3CaO x Al2O3 x 3CaSO4 x 32H2O), "monosulfate" (3CaO x Al2O3 x CaSO4 x 12H2O), and calcium silicate hydrate (C-S-H) was investigated in order to understand Se immobilization by cement in hazardous wastes. Sorption kinetics were fast with equilibrium between the minerals and Se species reached within 1 d. Selenite is suggested to sorb by surface reactions, and for ettringite, a sorption maximum of 0.03 mol kg(-1) was determined. Distribution ratios (Rd) for selenite were 0.18, 0.38, and 0.21 m3 kg(-1) for ettringite, monosulfate, and C-S-H, respectively. At high selenite additions, CaSeO3 was precipitated with a solubility product of Kso = 10(-7.27) (I = 0, 25 degrees C). Selenate sorbed only weakly to ettringite (Rd = 0.03), and no significant sorption to C-S-H was found. In contrast, sorption to monosulfate was strong (Rd = 2.06). With increasing selenate addition, XRD analyses revealed changes in the interlayer distance of monosulfate, in parallel with an increase of the ettringite fraction. Substitution of sulfate is suggested to be the relevant process. This indicates that selenate is sorbed more efficiently by monosulfate-rich cement, while the cement composition is of minor importance for selenite sorption.

Influence of biodegradation processes on the duration of CaCO3 as a pH buffer in municipal solid waste incinerator bottom ash.

The long-term leachability of heavy metals from municipal solid waste incinerator (MSWI) bottom ash is of concern because of its potential use as a secondary construction material. Calcite is the most important long-term buffer in MSWI bottom ash as it buffers solutions during percolation and is an important factor in the control of heavy-metal mobility. It has been argued that biodegradation of residual organic material in the MSWI is a significant source of acidity. Model calculations have therefore been carried out to determine the influence of biodegradation on the longevity of the calcite buffer. Using the program STEADYQL, which couples thermodynamic equilibrium with kinetically controlled reactions, solution composition was estimated at steady state. The concentration of Ca dissolved from calcite was estimated in the presence and absence of gypsum as a function of the reaction rate of a number of slow reactions: aerobic, ferrogenic, sulfogenic, and methanogenic biodegradation; diffusion of O2 into the system; degassing of CO2 out of the system; and dissolution of Ca silicate. It was found that, independent of the rate, the biodegradation of organic matter had little influence on the longevity of the calcite buffer (between 2,000 and 3,000 yr for a deposit of 1 m in depth), that anaerobic biodegradation may have a slight retarding effect, and that calcite dissolution due to acid input via precipitation was negligible (around 3% of the total at reference conditions for rainwater with a pH value of 4.3).