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 ?g/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

Predicting Geogenic Arsenic Contamination in Shallow Groundwater of South Louisiana, United States

Groundwater contaminated with arsenic (As) threatens the health of more than 140 million people worldwide. Previous studies indicate that geology and sedimentary depositional environments are important factors controlling groundwater As contamination. The Mississippi River delta has broadly similar geology and sedimentary depositional environments to the large deltas in South and Southeast Asia, which are severely affected by geogenic As contamination and therefore may also be vulnerable to groundwater As contamination. In this study, logistic regression is used to develop a probability model based on surface hydrology, soil properties, geology, and sedimentary depositional environments. The model is calibrated using 3286 aggregated and binary-coded groundwater As concentration measurements from Bangladesh and verified using 78 As measurements from south Louisiana. The model’s predictions are in good agreement with the known spatial distribution of groundwater As contamination of Bangladesh, and the predictions also indicate high risk of As contamination in shallow groundwater from Holocene sediments of south Louisiana. Furthermore, the model correctly predicted 79% of the existing shallow groundwater As measurements in the study region, indicating good performance of the model in predicting groundwater As contamination in shallow aquifers of south Louisiana

Photochemical Production of Sulfate and Methanesulfonic Acid from Dissolved Organic Sulfur in Natural Waters

Despite its importance in biological processes and its influence on metal bioavailability, the biogeochemical cycle of dissolved organic sulfur (DOS) in aquatic systems is still poorly understood. Recent high-resolution mass spectrometry (HRMS) studies showed a selective loss of organic sulfur during photodegradation of dissolved organic matter (DOM), which was hypothesized to result in the production of sulfate. Here, we provide evidence of ubiquitous production of sulfate, methanesulfonic acid (MSA) and methanesulfinic acid (MSIA) during photodegradation of DOM samples from a wide range of natural terrestrial environments. We show that photochemical production of sulfate is generally at least one order of magnitude more efficient than the production of MSA and MSIA, as well as volatile S-containing compounds (i.e., CS2and COS). We also identify possible molecular precursors for sulfate and MSA, and we demonstrate that a wide range of relevant classes of DOS compounds (in terms of S oxidation state and molecular structure) can liberate sulfate upon photosensitized degradation. This work indicates that photochemistry plays a more significant role in the aquatic and atmospheric cycle of DOS than currently believed.</p

Colloidal Properties of Nanoparticular Biogenic Selenium Govern Environmental Fate and Bioremediation Effectiveness

Microbial selenium (Se) bioremediation is based on conversion of water soluble, toxic Se oxyanions to water insoluble, elemental Se. Formed biogenic elemental Se is of nanometer size, hampering straightforward separation from the aqueous phase. This study represents the first systematic investigation on colloidal properties of pure biogenic Se suspensions, linking electrophoretic mobility (ζ-potential) to column settling behavior. It was demonstrated that circumneutral pH, commonly applied in bioremediation, is not appropriate for gravitational separation due to the negative ζ-potential preventing agglomeration. Mono/di/trivalent counter cations and acidity (protons) were used to screen efficiently the intrinsic negative charge of biogenic Se suspensions at circumneutral pH. Fast settling was induced by La³⁺ addition in the micromolar range (86.2 ± 3.5% within 0.5 h), whereas considerably higher concentrations were needed when Ca²⁺ or Na⁺ was used. Colloidal stability was furthermore studied in different model waters. It was demonstrated that surface waters as such represent a fragile system regarding colloidal stability of biogenic Se suspensions (ζ-potential ∼ −30 mV), whereas dissolved organic matter increases colloidal stability. In marine waters, biogenic Se is colloidally destabilized and is thus expected to settle, representing a potential sink for Se during transport in the aquatic environment

Selenium Uptake and Methylation by the Microalga Chlamydomonas reinhardtii

Biogenic selenium (Se) emissions play a major role in the biogeochemical cycle of this essential micronutrient. Microalgae may be responsible for a large portion of these emissions via production of methylated Se compounds that volatilize into the atmosphere. However, the biochemical mechanisms underlying Se methylation in microalgae are poorly understood. Here, we study Se methylation by Chlamydomonas reinhardtii, a model freshwater alga, as a function of uptake and intracellular Se concentrations and present a biochemical model that quantitatively describes Se uptake and methylation. Both selenite and selenate, two major inorganic forms of Se, are readily internalized by C. reinhardtii, but selenite is accumulated around ten times more efficiently than selenate due to different membrane transporters. With either selenite or selenate as substrates, Se methylation was highly efficient (up to 89% of intracellular Se) and directly coupled to intracellular Se levels (<i>R</i>² > 0.92) over an intracellular concentration range exceeding an order of magnitude. At intracellular concentrations exceeding 10 mM, intracellular zerovalent Se was formed. The relationship between uptake, intracellular accumulation, and methylation was used by the biochemical model to successfully predict measured concentrations of methylated Se in natural waters. Therefore, biological Se methylation by microalgae could significantly contribute to environmental Se cycling

Formation and transformation of Fe(III)- and Ca-precipitates in aqueous solutions and effects on phosphate retention over time

Exfiltration of anoxic phosphate-rich groundwater into surface water leads to the oxidation of dissolved Fe(II) and the formation of Fe(III)-precipitates that can retain phosphate (PO₄) and thereby attenuate eutrophication. Fresh Fe(III)-precipitates transform into more stable phases over time, and retained PO₄ may be released again. In parallel, CO₂ outgassing can promote the formation of Ca-phosphates or -carbonates that also sequester PO₄. In laboratory experiments, we studied the effects of Mg, Ca, silicate (SiO₄) and PO₄ on these processes. Fresh Fe(III)-precipitates were formed in bicarbonate-buffered aqueous solutions at pH ∼ 7.0 via the oxidation of 0.5 mM Fe(II) in the presence of 0.15 or 0.025 mM PO₄, at Mg or Ca concentrations of 0, 0.4, 1.2 or 4 mM and in the absence or presence of 0.5 mM SiO₄. After CO₂ outgassing, the suspensions were allowed to age for 100 d at pH ∼ 8. Changes in the composition and structure of Fe(III)- and Ca-precipitates over time were probed with spectroscopic and microscopic techniques and were linked to variations in the retention of PO₄. The oxidation of Fe(II) led to effective PO₄ removal via the formation of Fe(III)-precipitates that consisted of amorphous (Ca-)Fe(III)-phosphate ((Ca)FeP), ferrihydrite (Fh) and, in SiO₄-free treatments, lepidocrocite (Lp). During aging, FeP and Fh that had formed in the absence of Mg, Ca and SiO₄ rapidly and nearly completely transformed into Lp. Via effects on molecular- and nanoscale precipitate structure, Mg slowed down FeP transformation into Fh, stabilized Fh, and decreased the crystallinity of Lp (in SiO₄-free suspensions), Ca stabilized CaFeP against transformation into Fh, and SiO₄ stabilized Fh and (Ca)FeP. Core/shell CaFeP/Fh particles formed in electrolytes that contained Ca and SiO₄ hardly transformed within 100 d. Calcite only formed at low dissolved PO₄ concentrations and, by incorporation of PO₄, contributed to PO₄ retention. Higher levels of dissolved PO₄ inhibited calcite formation but could induce Ca-phosphate precipitation. Differences in precipitate formation and transformation pathways and kinetics were reflected in the extents of PO₄ release over the 100-d aging period, ranging from rapid release of 77% of the total PO₄ in the treatment without Mg, Ca and SiO₄ at 0.15 mM total PO₄ to slow release of only 0.1% of the total PO at initial concentrations of 4 mM Ca, 0.5 mM SiO₄, and 0.025 mM PO₄. In summary, this study reveals the conditions and the extents and timescales over which Fe(III)- and Ca-precipitates form and transform and how these processes affect PO₄ immobilization in near-neutral natural waters. The detailed new insights into the coupling between Fe(III)- and Ca-precipitate formation and into the interdependent effects of Mg, Ca, SiO₄ and PO₄ are not only relevant with respect to PO₄ but also with respect to the cycling of trace elements in natural and engineered systems.ISSN:0016-7037ISSN:1872-953

Relationship between the efficiency of chemotrapping in nitric acid and boiling points of the studied volatile compounds.

<p>Error bars indicate the standard deviation of the measurements of triplicate samples.</p

Overview of the experimental set-ups for gas trapping experiments in the laboratory and in the field.

<p>(A) Schematic of the experimental set-up for the laboratory gas trapping experiments, with the separate in-situ production of volatile methylated As species in a gas-tight reaction vessel (left) and direct introduction of volatile methylated Se and S species (right), connected to (B), a set of glass impingers filled with concentrated nitric acid and <b>c</b>, schematic of the experimental set-up for the field gas trapping experiments, which consists of a flow-through box equipped with an air pump connected to the set of glass impingers (B). During field application, one impinger was connected to one flow-through box and the flow-through boxes were deployed in triplicate.</p

Studied volatile species, including their structure and boiling points, calculated total trapping efficiencies, and observed reactions products and structures after trapping and transformation in concentrated nitric acid.

a<p>Boiling point at 1 atm ^befficiency using a 30 mL·min⁻¹ N₂ gas flow, summed over three impingers, standard deviation from triplicate experiments.</p><p>Abbreviations: dimethyl selenide (DMSe), dimethyl diselenide (DMDSe), dimethyl sulfide (DMS), dimethyl disulfide (DMDS), monomethyl arsine (MMA), dimethyl arsine (DMA), trimethyl arsine (TMA), dimethyl selenoxide (DMSeO), methane seleninic acid (MSeA), dimethyl sulfoxide (DMSO), methane sulfonic acid (MSA), arsenate (As[V]), monomethyl arsonic acid (MMAA).</p