Effect of natural organic matter on arsenic release from soils and sediments into groundwater

Complexation of arsenite with dissolved organic matter: conditional distribution coefficients and apparent stability constants

The complexation of arsenic (As) with dissolved organic matter (DOM), although playing an important role in regulating As mobility and transformation, is poorly characterized, as evidenced by scarce reporting of fundamental parameters of As-DOM complexes. The complexation of arsenite (AsIII) with Aldrich humic acid (HA) at different pHs was characterized using a recently developed analytical technique to measure both free and DOM-bound As. Conditional distribution coefficient (KD), describing capacity of DOM in binding AsIII from the mass perspective, and apparent stability constant (Ks), describing stability of resulting AsIII-DOM complexes, were calculated to characterize AsIII-DOM complexation. LogKD of AsIII ranged from 3.7 to 2.2 (decreasing with increase of As/DOM ratio) at pH 5.2, from 3.6 to 2.6 at pH 7, and from 4.3 to 3.2 at pH=9.3, respectively. Two-site ligand binding models can capture the heterogeneity of binding sites and be used to calculate Ks by classifying the binding sites into strong (S1) and weak (S2) groups. LogKs for S1 sites are 7.0, 6.5, and 5.9 for pH 5.2, 7, and 9.3, respectively, which are approximately 1-2 orders of magnitude higher than for weak S2 sites. The results suggest that AsIII complexation with DOM increases with pH, as evidenced by significant spikes in concentrations of DOM-bound AsIII and in KD values at pH 9.3. In contrary to KD, logKs decreased with pH, in particular for S1 sites, probably due to the presence of negatively charged H2AsO3- and the involvement of metal-bridged AsIII-DOM complexation at pH 9.3.

Speciation and evaluation of Arsenic in surface water and groundwater samples: a multivariate case study

The principal object of the current study was to estimate total arsenic and its inorganic speciation in different origins of surface water (n=480) and groundwater (n=240) of Sindh, Pakistan. This study provided a description based on the evaluation of physico-chemical parameters of collected water samples and possible distribution of As with respect to its speciation. The concentration of total inorganic As (iAs) and its species (As(3+) and As(5+)) for the surface and underground water was reported in terms of basic statistical parameters, principal component analysis, cluster analysis, metal-to-metal correlations and linear regression analyses. The chemical correlations were observed by PCA, which were used to classify the samples by CA, based on the PCA scores. Standard addition method confirmed the accuracy; the recoveries of As(3+) and iAs were found to be >98%. The concentration of As(5+) in the water samples was calculated by the difference of the total inorganic arsenic and As(3+). The results revealed that the groundwater of the understudied area was more contaminated as compared to the surface water samples. The mean concentration of As(3+) and As(5+) in the surface water and groundwater samples were in the range 3.0 to 18.3 and 8.74-352 microg/L, respectively.

Formation of binary and ternary colloids and dissolved complexes of organic matter, Fe and As

Natural organic matter can change As speciation via redox reactions and complexation influencing its mobility and toxicity. Here we show that binary and ternary colloids and dissolved complexes of As(V), Fe and organic matter (OM) form at environmentally relevant conditions and analyzed these colloids/complexes using ATR-FTIR- and Mossbauer-spectroscopy. Dissolved Fe-OM complexes and ferrihydrite-OM colloids were formed by reacting OM with ferrihydrite (Fe(OH)(3)). Mossbauer-spectroscopy showed that 95% of the Fe in the Fe-OM fraction were present as ferrihydrite-OM colloids while the remaining 5% were in the dissolved fraction. In As(V) plus Fe-OM systems (containing both dissolved and colloidal Fe-OM), 3.5-8 microg As(V)/mg OC was bound to the Fe-OM complexes/colloids compared to <0.015 microg As(V)/mg OC in As-OM systems (without Fe). Upon filtration of As-Fe-OM complexes/colloids with a 3 kDa filter, approximately 6% As was found in the dissolved fraction and approximately 94% As in colloidal Fe-OM. This suggests that As(V) is associated with Fe-OM mainly via ferrihydrite-OM colloids but to a small extent also in dissolved Fe-OM complexes via Fe-bridging. Since As-contaminated soils and aquifers contain Fe(III) minerals and OM, colloids of As with OM-loaded ferrihydrite and complexes of As with dissolved Fe-OM have to be considered when studying As transport.

Trace element mobility in a contaminated soil two years after field-amendment with a greenwaste compost mulch

Application of greenwaste compost to brownfield land is increasingly common in soil and landscape restoration. Previous studies have demonstrated both beneficial and detrimental effects of this material on trace element mobility. A pot experiment with homogenised soil/compost investigated distribution and mobility of trace elements, two years after application of greenwaste compost mulch to shallow soils overlying a former alkali-works contaminated with Pb, Cu and As (approximately 900, 200 and 500 mg kg(-1), respectively). Compost mulch increased organic carbon and Fe in soil pore water, which in turn increased As and Sb mobilization; this enhanced uptake by lettuce and sunflower. A very small proportion of the total soil trace element pool was in readily-exchangeable form (<0.01% As, <0.001% other trace elements), but the effect of compost on behaviour of metals was variable and ambiguous. It is concluded that greenwaste compost should be applied with caution to multi-element contaminated soils.

Dissolved organic matter sources and consequences for iron and arsenic mobilization in Bangladesh aquifers

Iron (Fe) and dissolved organic matter (DOM) cycling have been implicated in arsenic mobilization via microbially mediated Fe oxide reduction. To evaluate the sources and multiple roles of DOM in Bangladesh aquifers, we conducted spectroscopic analyses on various types of surface water and groundwater samples from a site representative of aquifer chemistry and hydrology. Surface water contained humic substances with oxidized quinone-like moieties and high concentrations of labile microbially derived DOM. In contrast, in shallow groundwater where dissolved iron and arsenic concentrations were high, the quinone-like moieties of humic substances were more reduced, with less abundant labile DOM than that of surface water. Instead, DOM at these depths was characterized by terrestrial (plant/soil) signatures. A sediment microcosm experiment demonstrated that Fe(II) and terrestrially derived DOM were released from sediment over time. The results provide new evidence to support a dual role of natural DOM in Bangladesh aquifers (1) as a labile substrate for Fe- and humic-reducing bacteria and (2) as an electron shuttle via humic substances to enhance microbial iron reduction. Fluorescence index, amino acid-like fluorescence, and redox index may serve as useful indicators of the type of DOM likely to be involved in Fe solubilization and potentially As mobilization reactions.

Association of individual soil mineral constituents and heavy metals as studied by sorption experiments and analytical electron microscopy analyses

Sorption characteristics of bulk soil samples and discrete soil mineral constituents were studied by Cu, Zn and Pb batch sorption experiments and analytical electron microscopy analyses. Copper and zinc sorbed mostly on soil mineral constituents, while lead was associated mainly to soil organic matter. Additionally, the competitive situation resulted in increase of the role of iron oxides in Pb sorption. Close association of iron oxides and silicates resulted in significant change in their sorption capacities for all the studied metals. The alkaline conditions due to the calcite content in one of the studied soil samples resulted in both increased role of precipitation for Pb and Cu and elevated sorption capacity for Cu by discrete mineral particles. Using the analytical electron microscopy analyses the sorption characteristics of metals were supported by particular data. When the methods used in this study are combined, they become an extremely powerful means of getting a deeper insight into the soil-metal interaction.

Effect of natural organic matter on arsenic mobilization from mine tailings

This research study was to elucidate the mechanisms of arsenic mobilization from mine tailings in the presence of natural organic matter (NOM). Humic acid (HA) was chosen as a model for NOM. The introduction of the HA at a low mass ratio (below 2mg added HA/g mine tailings) inhibited arsenic mobilization under acidic conditions. Arsenic mobilization increased with increasing mass ratio. When pH was above 7, the addition of HA enhanced arsenic mobilization significantly. A mobilization isotherm was developed to predict arsenic mobilization from the mine tailings in the presence of HA. It was indicated that HA sorption to the mine tailings was essential for arsenic mobilization. HA might enhance arsenic mobility through formation of aqueous complexes, competition for adsorption and electrostatic interactions. Capillary electrophoresis analyses indicated that arsenic redox reactions might not have a significant effect on arsenic mobilization in this study. The mobilization of co-existing metals could enhance arsenic mobilization by helping incorporating it into soluble complexes in the presence of HA.

Arsenic redox changes by microbially and chemically formed semiquinone radicals and hydroquinones in a humic substance model quinone

Arsenic is a redox-active metalloid whose toxicity and mobility strongly depends on its oxidation state, with arsenite (As(III)) being more toxic and mobile than arsenate (As(V)). Humic substances (HS) are also redox-active and can potentially react with arsenic and change its redox state. In this study we show that semiquinone radicals produced during microbial or chemical reduction of a HS model quinone (AQDS, 9,10-anthraquinone-2,6-disulfonic acid) are strong oxidants. They oxidize arsenite to arsenate, thus decreasing As toxicity and mobility. This reaction depends strongly on pH with more arsenite (up to 67.3%) being oxidized at pH 11 compared to pH 7 (12.6% oxidation) and pH 3 (0.5% oxidation). In addition to As(III) oxidation by semiquinone radicals, hydroquinones that were also produced during quinone reduction reduced As(V) to As(III) at neutral and acidic pH values (less than 12%) but not at alkaline pH. In order to understand redox reactions between arsenite/arsenate and reduced/oxidized HS, we quantified the radical content in reduced quinone solutions and constructed Eh-pH diagrams that explain the observed redox reactions. The results from this study can be used to better predict the fate of arsenic in the environment and potentially explain the occurrence of oxidized As(V) in anoxic environments.

Retention of As and Sb in ombrotrophic peat bogs: records of As, Sb, and Pb deposition at four Scottish sites

Possible postdepositional As migration in ombrotrophic peat bogs was investigated by comparing depth profiles of As with those of Sb and Pb, two elements considered to be essentially immobile in peat, and those of redox-sensitive, potentially mobile nutrient elements such as Mn, Fe, P, and S in 210Pb-dated cores from four Scottish bogs. Concentration profiles of As were similar to those of Sb and Pb rather than these other elements, indicating that As is bound strongly to organic matter and is relatively immobile in ombrotrophic peat. Historical records of atmospheric anthropogenic As, Sb, and Pb deposition during the industrial and postindustrial periods were derived, site-specific maxima (up to 1.55, 1.33, and 45 mg m(-2) y(-1), respectively) occurring between the late 1890s and 1960s, reflecting emissions from diverse sources such as mining and smelting, coal combustion, and also, in the case of Pb, exhaust emissions from the use of leaded gasoline. Since the mid-1980s, fluxes of Pb decreased (4-7 fold) more rapidly than those of As and Sb (2-3 fold), attributable to both the gradual elimination of leaded gasoline and recent new sources of the latter elements. Relative trends in derived anthropogenic As, Sb, and Pb deposition largely agreed with other Scottish peat and moss archive records, direct measurements of deposition, and UK emissions, i.e., four different types of data source.

Enhanced arsenic removals through plant interactions in subsurface-flow constructed wetlands

Arsenic (As) removal in pilot-scale subsurface-flow constructed wetlands (CWs) was investigated by comparing between CW units with vetiver grasses (CWplanted) and CW units without vetiver grasses (CWunplanted) in order to determine the roles of vetiver grasses affecting As removal. Based on the data obtained from 147 days of experiment, it is apparent that CWplanted units could remove As significantly higher than those of CWunplanted units with approximately 7-14%. Although analysis of As mass balance in CW units revealed that only 0.5-1.0% of total As was found in vetiver grasses, the As retained within bed of the CWplanted units (23.6-29.7 g) was higher than those in the CWunplanted units (21.3-26.8 g) at the end of the experiment, illustrating the effect of vetiver grasses on As accumulation in the CW units. Determination of As in different fractions in the CW bed suggested that the main mechanism of As retention was due mainly to As entrapment into the porous of bed materials (50-57% of total fraction), this mechanism is likely not affected by the presence of vetiver grasses. However, fraction of As-bound in organic matters that could be released from plant roots decomposition indicated the increase adsorption capacity of CW bed. In addition, organic sulfides produced from their root decomposition could help remove As through the precipitation/co-precipitation process. Under reducing condition in those CWplanted units, As could be leached out in the form of iron and manganese-bound complexes.

Enhanced mobilization of arsenic and heavy metals from mine tailings by humic acid

Arsenic and heavy metal mobilization from mine tailings is an issue of concern as it might pose potential groundwater or ecological risks. Increasing attention recently has been focused on the effects of natural organic matter on the mobility behavior of the toxicants in the environment. Column experiments were carried out in this research study to evaluate the feasibility of using humic acid (HA) to mobilize arsenic and heavy metals (i.e., Cu, Pb and Zn) from an oxidized Pb-Zn mine tailings sample collected from Bathurst, New Brunswick, Canada. Capillary electrophoresis analyses indicated that arsenate [As(V)] was the only extractable arsenic species in the mine tailings and the addition of HA at pH 11 did not incur the oxidation-reduction or methylation reactions of arsenic. A 0.1% HA solution with an initial pH adjusted to 11 was selected as the flushing solution, while distilled water (initial pH adjusted to 11) was used as the control to account for the mobilization of arsenic and the heavy metals by physical mixing and the effect of pH. It was found that the HA could significantly enhance the mobilization of arsenic and heavy metals simultaneously from the mine tailings. After a 70-pore-volume-flushing, the mobilization of arsenic, copper, lead and zinc reached 97, 35, 838 and 224 mg kg(-1), respectively. The mobilization of arsenic and the heavy metals was found to be positively correlated with the mobilization of Fe in the presence of the HA. Moreover, the mobilization of arsenic was also correlated well with that of the heavy metals. The mobilization of co-existing metals to some extent might enhance arsenic mobilization in the presence of the HA by helping incorporate it into soluble aqueous organic complexes through metal-bridging mechanisms. Use of HA in arsenic and heavy metal remediation may be developed as an environmentally benign and possible effective remedial option to reduce and avoid further contamination.

Speciation and surface structure of inorganic arsenic in solid phases: a review

Accurate determination of individual arsenic species is critical because the toxicology, mobility, and adsorptivity of arsenic vary substantially with its chemical forms and oxidation states. Separation techniques together with techniques for chemical identification make it possible to determine the combinational forms and oxidation states of arsenic in solid phases. Selective sequential extraction is often employed to determine operationally defined fractions, but it has a poor precision and selectivity. Direct methods, based on X-ray techniques and vibrational spectroscopy, have been developed to analyze the valence, local coordination, protonation, and other properties of arsenic in solid phases. Extensive research studies in the literature have been performed to elucidate the interfacial reactions between inorganic arsenic and solid surfaces of sulfides, and Fe, Al, and Mn (hydro)oxides. Outer-sphere and inner-sphere complex (monodentate mononuclear, bidentate mononuclear, and bidentate binuclear complex) models have been proposed to interpret the sorption mechanisms. The nature of the surface complexes has been inspected by spectroscopic methods but remains controversial. This paper focuses on the recent advancement in arsenic speciation in solid phases and covers relevant methodological, analytical and modeling aspects. The identification of arsenic species in natural materials, however, is complicated by the presence of multiple species, and the applications of instrumental methods are usually limited due to their comparatively high detection limits. Development of advanced in-situ methods with high sensitivity, therefore, is required.

Arsenic removal by native and chemically modified lactic acid bacteria

Arsenic in drinking water is a major health problem globally. Simple, novel methods are needed for its removal from water, especially in rural areas. For this purpose, the potential of different microbes in toxin and heavy metal removal from water has gained interest. This study focused on the arsenic removal capacity of three Lactobacillus strains in their native and chemically modified forms. Both native and methylated forms of all three strains were not efficient in arsenic removal. Aminated Lactobacillus casei DSM20011 was observed to remove As(V) but not As(III) in water. Removal was fast, dependent on pH and As(V) concentration. The highest removal percentage 38.1+/-9.0% was observed at the lowest As(V) concentration (100 microg/l) studied at pH 7. The maximum As(V) removal capacity, calculated from Langmuir isotherm, was 312+/-68 microg As(V)/g dry biomass. Interactions between As(V) and the bacteria were weak, demonstrated by release of removed As(V) when contact time was prolonged. Desorption with 1.5 mM HNO3 and NaOH released all bound As(V) indicating that removal occurred at the bacterial surface.

Natural attenuation processes for remediation of arsenic contaminated soils and groundwater

Arsenic (As) contamination presents a hazard in many countries. Natural attenuation (NA) of As-contaminated soils and groundwater may be a cost-effective in situ remedial option. It relies on the site intrinsic assimilative capacity and allows in-place cleanup. Sorption to solid phases is the principal mechanism immobilizing As in soils and removing it from groundwater. Hydroxides of iron, aluminum and manganese, clay and sulfide minerals, and natural organic matter are commonly associated with soils and aquifer sediments, and have been shown to be significant As adsorbents. The extent of sorption is influenced by As speciation and the site geochemical conditions such as pH, redox potential, and the co-occurring ions. Microbial activity may catalyze the transformation of As species, or mediate redox reactions thus influencing As mobility. Plants that are capable of hyperaccumulating As may translocate As from contaminated soils and groundwater to their tissues, providing the basis for phytoremediation. However, NA is subject to hydrological changes and may take substantial periods of time, thus requiring long-term monitoring. The current understanding of As NA processes remains limited. Sufficient site characterization is critical to the success of NA. Further research is required to develop conceptual and mathematical models to predict the fate and transport of As and to evaluate the site NA capacity. Engineering enhanced NA using environmentally benign products may be an effective alternative.

Occurrence of arsenic contamination in Canada: sources, behavior and distribution

Recently there has been increasing anxieties concerning arsenic related problems. Occurrence of arsenic contamination has been reported worldwide. In Canada, the main natural arsenic sources are weathering and erosion of arsenic-containing rocks and soil, while tailings from historic and recent gold mine operations and wood preservative facilities are the principal anthropogenic sources. Across Canada, the 24-h average concentration of arsenic in the atmosphere is generally less than 0.3 microg/m3. Arsenic concentrations in natural uncontaminated soil and sediments range from 4 to 150 mg/kg. In uncontaminated surface and ground waters, the arsenic concentration ranges from 0.001 to 0.005 mg/L. As a result of anthropogenic inputs, elevated arsenic levels, above ten to thousand times the Interim Maximum Acceptable Concentration (IMAC), have been reported in air, soil and sediment, surface water and groundwater, and biota in several regions. Most arsenic is of toxic inorganic forms. It is critical to recognize that such contamination imposes serious harmful effects on various aquatic and terrestrial organisms and human health ultimately. Serious incidences of acute and chronic arsenic poisonings have been revealed. Through examination of the available literature, screening and selecting existing data, this paper provides an analysis of the currently available information on recognized problem areas, and an overview of current knowledge of the principal hydrogeochemical processes of arsenic transportation and transformation. However, a more detailed understanding of local sources of arsenic and mechanisms of arsenic release is required. More extensive studies will be required for building practical guidance on avoiding and reducing arsenic contamination. Bioremediation and hyperaccumulation are emerging innovative technologies for the remediation of arsenic contaminated sites. Natural attenuation may be utilized as a potential in situ remedial option. Further investigations are needed to evaluate its applicability.