Processes releasing arsenic to groundwater in the Caldes de Malavella geothermal area, NE Spain

Arsenic concentrations exceeding the World Health Organization drinking water guideline (10 μg/L) have been measured in thermal and non-thermal groundwaters from the Caldes de Malavella geothermal area (La Selva graben, NE Spain). The CO₂-rich Na-HCO₃ thermal waters (up to 60 °C at the spring) have elevated arsenic concentrations ([As_T] from 50 to 80 μg/L). The non-thermal waters are of Ca-Na-HCO₃-Cl type and have [As_T] between <1 and 200 μg/L, defining a hot-spot distribution. The present-day contribution of As from CO₂-rich thermal waters to non-thermal aquifers is very limited, as shown by the concentration of geothermal tracers such as Li and B. Redox-controlling processes appear to govern the mobility of As in the non-thermal waters. Arsenate is clearly predominant in most oxidizing groundwaters (>85% of As(V) over total As), whereas reducing, high-As groundwater reaches up to 100% in arsenite. The reductive dissolution of Fe(III) oxyhydroxides and the coupled release and reduction of adsorbed As explain the elevated dissolved arsenite (up to 190 μg/L) and Fe (up to 14 mg/L) content in the more reducing non-thermal groundwater. Conversely, the high levels of nitrate (up to 136 mg/L) ensure an oxidizing environment in most non-thermal groundwaters ([As_T] between <1 and 60 μg/L). Under these conditions, Fe(III) oxyhydroxides are stable and As release to groundwater is not related to their dissolution. Instead, dissolved arsenate concentrations up to 60 μg/L are explained by a competition for sorption sites with other species, mainly bicarbonate and silicic acid, while arsenate desorption due to pH increase is not considered a major process.     

Phosphorus limitation in nitrifying groundwater filters

Phosphorus limitation has been demonstrated for heterotrophic growth in groundwater, in drinking water production and distribution systems, and for nitrification of surface water treatment at low temperatures. In this study, phosphorus limitation was tested, in the Netherlands, for nitrification of anaerobic groundwater rich in iron, ammonium and orthophosphate. The bioassay method developed by Lehtola et al. (1999) was adapted to determine the microbially available phosphorus (MAP) for nitrification. In standardized batch experiments with an enriched mixed culture inoculum, the formation of nitrite and nitrate and ATP and the growth of ammonia-oxidizing bacteria (AOB; as indicated by qPCR targeting the amoA-coding gene) were determined for MAP concentrations between 0 and 100 μg PO₄-P L⁻¹. The nitrification and microbial growth rates were limited at under 100 μg PO₄-P L⁻¹ and virtually stopped at under 10 μg PO₄-P L⁻¹. In the range between 10 and 50 μg PO₄-P L⁻¹, a linear relationship was found between MAP and the maximum nitrification rate. AOB cell growth and ATP formation were proportional to the total ammonia oxidized. Contrary to Lehtola et al. (1999), biological growth was very slow for MAP concentrations less than 25 μg PO₄-P L⁻¹. No full conversion nor maximum cell numbers were reached within 19 days. In full-scale groundwater filters, most of the orthophosphate was removed alongside with iron. The remaining orthophosphate appeared to have only limited availability for microbial growth and activity. In some groundwater filters, nitrification was almost totally prevented by limitation of MAP. In batch experiments with filtrate water from these filters, the nitrification process could be effectively stimulated by adding phosphoric acid.     

Implications of organic matter on arsenic mobilization into groundwater: Evidence from northwestern (Chapai-Nawabganj), central (Manikganj) and southeastern (Chandpur) Bangladesh

Boreholes (50 m depth) and piezometers (50 m depth) were drilled and installed for collecting As-rich sediments and groundwater in the Ganges, Brahmaputra, and Meghna flood plains for geochemical analyses. Forty-one groundwater samples were collected from the three areas for the analyses of cations (Ca²⁺, Mg²⁺, K⁺, Na⁺), anions (Cl⁻, NO₃⁻, SO₄²⁻), total organic carbon (TOC), and trace elements (As, Mn, Fe, Sr, Se, Ni, Co, Cu, Mo, Sb, Pb). X-ray powder diffraction (XRD) and X-ray fluorescence (XRF) were performed to characterize the major mineral and chemical contents of aquifer sediments. In all three study areas, results of XRF analysis clearly show that fine-grained sediments contain higher amounts of trace element because of their high surface area for adsorption. Relative fluorescent intensity of humic substances in groundwater samples ranges from 30 to 102 (mean 58 ± 20, n = 20), 54-195 (mean 105 ± 48, n = 10), and 27-243 (mean 79 ± 71, n = 11) in the Ganges, Brahmaputra and Meghna flood plains, respectively. Arsenic concentration in groundwater (20-50 m of depth) ranges from 3 to 315 μg/L (mean 62.4 ± 93.1 μg/L, n = 20), 16.4-73.7 μg/L (mean 28.5 ± 22.4 μg/L, n = 10) and 4.6-215.4 μg/L (mean 30.7 ± 62.1 μg/L, n = 11) in the Ganges, Brahmaputra and Meghna flood plains, respectively. Specific ultra violet adsorption (SUVA) values (less than 3 m⁻¹ mg⁻¹ L) indicate that the groundwater in the Ganges flood plain has relatively low percentage of aromatic organic carbon compared to those in the Brahmaputra and Meghna flood plains. Arsenic content in sediments ranges from 1 to 11 mg/kg (mean 3.5 ± 2.7 mg/kg, n = 17) in the three flood plains. Total organic carbon content is 0.5-3.7 g/kg (mean 1.9 ± 1.1 g/kg) in the Ganges flood plain, 0.5-2.1 g/kg (mean: 1.1 ± 0.7 g/kg) in the Brahmaputra flood plain and 0.3-4.4 g/kg (mean 1.9 ± 1.9 g/kg) in the Meghna flood plain. Arsenic is positively correlated with TOC (R² = 0.50, 0.87, and 0.85) in sediments from the three areas. Fourier transform infrared (FT-IR) analysis of the sediments revealed that the functional groups of humic substances in three areas include amines, phenol, alkanes, and aromatic carbon. Arsenic and Fe speciation in sediments were determined using XANES and the results imply that As(V) and Fe(III) are the dominant species in most sediments. The results also imply that As (V) and Fe (III) in most of the sediment samples of the three areas are the dominant species. X-ray absorption fine structure (EXAFS) analysis shows that FeOOH is the main carrier of As in the sediments of three areas. In sediments, As is well correlated with Fe and Mn. However, there is no such correlation observed between As and Fe as well as As and Mn in groundwater, implying that mobilizations of Fe, Mn, and As are decoupled or their concentrations in groundwater have been affected by other geochemical processes following reductive dissolution of Fe or Mn-hydroxides. For example, dissolved Fe and Mn levels may be affected by precipitation of Fe- and Mn-carbonate minerals such as siderite, while liberated As remains in groundwater. The groundwaters of the Brahmaputra and Meghna flood plains contain higher humic substances in relative fluorescence intensity (or fluorescence index) and lower redox potential compared to the groundwater of Ganges flood plain. This leads to the release of arsenic and iron to groundwater of these three plains in considerable amounts, but their concentrations are distributed in spatial variations.     

As oxidation by Thiomonas arsenivorans in up-flow fixed-bed reactors coupled to As sequestration onto zero-valent iron-coated sand

The combined processes of biological As^III oxidation and removal of As^III and As^V by zero-valent iron were investigated with synthetic water containing high As^III concentration (10 mg L⁻¹). Two up-flow fixed-bed reactors (R1 and R2) were filled with 2 L of sieved sand (d = 3 ± 1 mm) while zero-valent iron powder (d = 76 μm; 1% (w/w) of sand) was mixed evenly with sand in R2. Thiomonas arsenivorans was inoculated in the two reactors. The pilot unit was studied for 33 days, with HRT of 4 and 1 h. The maximal As^III oxidation rate was 8.36 mg h⁻¹ L⁻¹ in R1 and about 45% of total As was removed in R2 for an HRT of 1 h. A first order model fitted well with the As^III concentration evolution at the different levels in R1. At the end of the pilot monitoring, batch tests were conducted with support collected at different levels in R1. They showed that bacterial As^III oxidation rate was correlated with the axial length of reactor, which could be explained by biomass distribution in reactor or by bacterial activity. In opposition, As^III oxidation rate was not stable in R2 due to the simultaneous bacterial As^III oxidation and chemical removal by zero-valent iron and its oxidant products. However, a durable removal of total As was realized and zero-valent iron was not saturated by As over 33 days in R2. Furthermore, the influence of zero-valent iron and its oxidant corrosion products on the evolution of As^III-oxidizing bacteria diversity was highlighted by the molecular fingerprinting method of PCR-DGGE using aoxB gene as a functional marker of aerobic As^III oxidizers.     

Anoxic oxidation of arsenite linked to denitrification in sludges and sediments

In this study, denitrification linked to the oxidation of arsenite (As(III)) to arsenate (As(V)) was shown to be a widespread microbial activity in anaerobic sludge and sediment samples that were not previously exposed to arsenic contamination. When incubated with 0.5 mM As(III) and 10 mM NO₃⁻, the anoxic oxidation of As(III) commenced within a few days, achieving specific activities of up to 1.24 mmol As(V) formed g⁻¹ volatile suspended solids d⁻¹ due to growth (doubling times of 0.74-1.4 d). The anoxic oxidation of As(III) was partially to completely inhibited by 1.5 and 5 mM As(III), respectively. Inhibition was minimized by adding As(III) adsorbed onto activated aluminum (AA). The oxidation of As(III) was shown to be linked to the complete denitrification of NO₃⁻ to N₂ by demonstrating a significantly enhanced production of N₂ beyond the background endogenous production as a result of adding As(III)-AA to the cultures. The N₂ production corresponded closely the expected stoichiometry of the reaction, 2.5 mol As(III) mol⁻¹ N₂-N. The oxidation of As(III) linked to the use of common-occurring nitrate as an electron acceptor may be an important missing link in the biogeochemical cycling of arsenic.     

1,4-Dioxane biodegradation at low temperatures in Arctic groundwater samples

1,4-Dioxane biodegradation was investigated in microcosms prepared with groundwater and soil from an impacted site in Alaska. In addition to natural attenuation conditions (i.e., no amendments), the following treatments were tested: (a) biostimulation by addition of 1-butanol (a readily available auxiliary substrate) and inorganic nutrients; and (b) bioaugmentation with Pseudonocardia dioxanivorans CB1190, a well-characterized dioxane-degrading bacterium, or with Pseudonocardia antarctica DVS 5a1, a bacterium isolated from Antarctica. Biostimulation enhanced the degradation of 50 mg L⁻¹ dioxane by indigenous microorganisms (about 0.01 mg dioxane d⁻¹ mg protein⁻¹) at both 4 and 14 °C, with a simultaneous increase in biomass. A more pronounced enhancement was observed through bioaugmentation. Microcosms with 50 mg L⁻¹ initial dioxane (representing source-zone contamination) and augmented with CB1190 degraded dioxane fastest (0.16 ± 0.04 mg dioxane d⁻¹ mg protein⁻¹) at 14 °C, and the degradation rate decreased dramatically at 4 °C (0.021 ± 0.007 mg dioxane d⁻¹ mg protein⁻¹). In contrast, microcosms with DVS 5a1 degraded dioxane at similar rates at 4 °C and 14 °C (0.018 ± 0.004 and 0.015 ± 0.006 mg dioxane d⁻¹ mg protein⁻¹, respectively). DVS 5a1 outperformed CB1190 when the initial dioxane concentration was low (500 μg L⁻¹, which is representative of the leading edge of plumes). This indicates differences in competitive advantages of these two strains. Natural attenuation microcosms also showed significant degradation over 6 months when the initial dioxane concentration was 500 μg L⁻¹. This is the first study to report the potential for dioxane bioremediation and natural attenuation of contaminated groundwater in sensitive cold-weather ecosystems such as the Arctic.     

Mineralogical and geochemical controls of arsenic speciation and mobility under different redox conditions in soil, sediment and water at the Mokrsko-West gold deposit, Czech Republic

Naturally contaminated soil, sediment and water at the Mokrsko-West gold deposit, Central Bohemia, have been studied in order to determine the processes that lead to release of As into water and to control its speciation under various redox conditions. In soils, As is bonded mainly to secondary arseniosiderite, pharmacosiderite and Fe oxyhydroxides and, rarely, to scorodite; in sediments, As is bonded mainly to Fe oxyhydroxides and rarely to arsenate minerals.The highest concentrations of dissolved As were found in groundwater (up to 1141 μg L^− 1), which mostly represented a redox transition zone where neither sulphide minerals nor Fe oxyhydroxide are stable. The main processes releasing dissolved As in this zone are attributed to the reductive dissolution of Fe oxyhydroxides and arsenate minerals, resulting in a substantial decrease in their amounts below the groundwater level. Some shallow subsurface environments with high organic matter contents were characterized by reducing conditions that indicated a relatively high amount of S^− 2,0 in the solid phase and a lower dissolved As concentration (70-80 μg L^− 1) in the pore water. These findings are attributed to the formation of Fe(II) sulphides with the sorbed As. Under oxidizing conditions, surface waters were undersaturated with respect to arsenate minerals and this promoted the dissolution of secondary arsenates and increased the As concentrations in the water to characteristic values from 300 to 450 μg L^− 1 in the stream and fishpond waters. The levels of dissolved As(III) often predominate over As(V) levels, both in groundwaters and in surface waters. The As(III)/As(V) ratio is closely related to the DOC concentration and this could support the assumption of a key role of microbial processes in transformations of aqueous As species as well as in the mobility of As.     

Mobilization of arsenic and other naturally occurring contaminants in groundwater of the Main Ethiopian Rift aquifers

This study investigates the mechanisms of arsenic (As) and other naturally occurring contaminants (F⁻, U, V, B, and Mo) mobilization from Quaternary sedimentary aquifers of the Main Ethiopian Rift (MER) and their enrichment in the local groundwater. The study is based on systematic measurements of major and trace elements as well as stable oxygen and hydrogen isotopes in groundwater, coupled with geochemical and mineralogical analyses of the aquifer rocks. The Rift Valley aquifer is composed of rhyolitic volcanics and Quaternary lacustrine sediments. X-ray fluorescence (XRF) results revealed that MER rhyolites (ash, tuff, pumice and ignimbrite) and sediments contain on average 72 wt. % and 65 wt. % SiO₂, respectively. Petrographic studies of the rhyolites indicate predominance of volcanic glass, sanidine, pyroxene, Fe-oxides and plagioclase. The As content in the lacustrine sediments (mean = 6.6 mg/kg) was higher than that of the rhyolites (mean: 2.5 mg/kg). The lacustrine aquifers of the Ziway-Shala basin in the northern part of MER were identified as high As risk zones, where mean As concentration in groundwater was 22.4 ± 33.5 (range of 0.60–190 μg/L) and 54% of samples had As above the WHO drinking water guideline value of 10 μg/L. Field As speciation measurements showed that most of the groundwater samples contain predominantly (∼80%) arsenate-As(V) over arsenite-As(III) species. The As speciation together with field data of redox potential (mean Eh = +73 ± 65 mV) and dissolved-O₂ (6.6 ± 2.2 mg/L) suggest that the aquifer is predominantly oxidative. Water-rock interactions, including the dissolution of volcanic glass produces groundwater with near-neutral to alkaline pH (range 6.9–8.9), predominance of Na–HCO₃ ions, and high concentration of SiO₂ (mean: 85.8 ± 11.3 mg/L). The groundwater data show high positive correlation of As with Na, HCO₃¯, U, B, V, and Mo (R² > 0.5; p < 0.001). Chemical modeling of the groundwater indicates that Fe-oxides and oxyhydroxides minerals were saturated in the groundwater, suggesting that the As reactivity is controlled by adsorption/desorption processes with these minerals. The data show that As and other oxyanion-forming elements such as U, B, Mo, and V had typically higher concentrations at pH > ∼8, reflecting the pH-dependence of their mobilization. Based on the geochemical and stable isotope variations we have established a conceptual model for the occurrence of naturally occurring contaminants in MER groundwater: 1) regional groundwater recharge from the Highland, along the Rift margins, followed by lateral flow and water–rock interactions with the aquifer rocks resulted in a gradual increase of the salinity and naturally occurring contaminants towards the center of the valley; and (2) local δ¹⁸O-rich lake water recharge into adjacent shallow aquifers, followed by additional mobilization of As and other oxyanion-forming elements from the aquifer rocks. We posit that the combined physical-chemical conditions of the aquifers such as oxidizing state, Na–HCO₃ composition, and pH>∼8 lead to enhanced mobilization of oxyanion-forming elements from Fe-oxides and consequently contamination of local groundwater. These geochemical conditions characterize groundwater resources along the Eastern African Rift and thus constitute a potential threat to the quality of groundwater in larger areas of Eastern Africa.     

Modeling the decay of ammonium oxidizing bacteria

A bench-scale sequencing batch reactor was used to study factors affecting the endogenous decay of the ammonium oxidizing biomass (AOB) in different operating conditions. AOB decay was very sensitive to oxygen concentration, and increased up to 0.4 d⁻¹ for oxygen concentration of 7 mg O₂ L⁻¹. The decay in anaerobic conditions was shown to be very low (0.03 d⁻¹) when compared to literature data.The effect of nitrite and nitrate on AOB decay was also studied. The correlation was quite weak suggesting that both nitrate and nitrite absence had little impact on decay which is contrary to what is typically assumed in some of the existing process models. A simple expression for the decay of AOB was proposed, calibrated and validated using the results of batch kinetic tests and of the continuous sequencing batch reactor monitoring.     

Preparation and evaluation of iron–chitosan composites for removal of As(III) and As(V) from arsenic contaminated real life groundwater

A study on the removal of arsenic from real life groundwater using iron–chitosan composites is presented. Removal of arsenic(III) and arsenic(V) was studied through adsorption at pH 7.0 under equilibrium and dynamic conditions. The equilibrium data were fitted to Langmuir adsorption models and the various model parameters were evaluated. The monolayer adsorption capacity from the Langmuir model for iron chitosan flakes (ICF) (22.47 ± 0.56 mg/g for As(V) and 16.15 ± 0.32 mg/g for As(III)) was found to be considerably higher than that obtained for iron chitosan granules (ICB) (2.24 ± 0.04 mg/g for As(V); 2.32 ± 0.05 mg/g for As(III)). Anions including sulfate, phosphate and silicate at the levels present in groundwater did not cause serious interference in the adsorption behavior of arsenate/arsenite. The column regeneration studies were carried out for two sorption–desorption cycles for both As(III) and As(V) using ICF and ICB as sorbents. One hundred and forty-seven bed volumes of As(III) and 112 bed volumes of As(V) spiked groundwater were treated in column experiments using ICB, reducing arsenic concentration from 500 to <10 μg/l. The eluent used for the regeneration of the spent sorbent was 0.1 M NaOH. The adsorbent was also successfully applied for the removal of total inorganic arsenic down to <10 μg/l from real life arsenic contaminated groundwater samples.     

Remediation of groundwater contaminated with MTBE and benzene: the potential of vertical-flow soil filter systems

Field investigations on the treatment of MTBE and benzene from contaminated groundwater in pilot or full-scale constructed wetlands are lacking hugely. The aim of this study was to develop a biological treatment technology that can be operated in an economic, reliable and robust mode over a long period of time. Two pilot-scale vertical-flow soil filter eco-technologies, a roughing filter (RF) and a polishing filter (PF) with plants (willows), were operated independently in a single-stage configuration and coupled together in a multi-stage (RF + PF) configuration to investigate the MTBE and benzene removal performances. Both filters were loaded with groundwater from a refinery site contaminated with MTBE and benzene as the main contaminants, with a mean concentration of 2970 ± 816 and 13,966 ± 1998 μg L⁻¹, respectively. Four different hydraulic loading rates (HLRs) with a stepwise increment of 60, 120, 240 and 480 L m⁻² d⁻¹ were applied over a period of 388 days in the single-stage operation. At the highest HLR of 480 L m⁻² d⁻¹, the mean concentrations of MTBE and benzene were found to be 550 ± 133 and 65 ± 123 μg L⁻¹ in the effluent of the RF. In the effluent of the PF system, respective mean MTBE and benzene concentrations of 49 ± 77 and 0.5 ± 0.2 μg L⁻¹ were obtained, which were well below the relevant MTBE and benzene limit values of 200 and 1 μg L⁻¹ for drinking water quality. But a dynamic fluctuation in the effluent MTBE concentration showed a lack of stability in regards to the increase in the measured values by nearly 10%, which were higher than the limit value. Therefore, both (RF + PF) filters were combined in a multi-stage configuration and the combined system proved to be more stable and effective with a highly efficient reduction of the MTBE and benzene concentrations in the effluent. Nearly 70% of MTBE and 98% of benzene were eliminated from the influent groundwater by the first vertical filter (RF) and the remaining amount was almost completely diminished (∼100% reduction) after passing through the second filter (PF), with a mean MTBE and benzene concentration of 5 ± 10 and 0.6 ± 0.2 μg L⁻¹ in the final effluent. The emission rate of volatile organic compounds mass into the air from the systems was less than 1% of the inflow mass loading rate. The results obtained in this study not only demonstrate the feasibility of vertical-flow soil filter systems for treating groundwater contaminated with MTBE and benzene, but can also be considered a major step forward towards their application under full-scale conditions for commercial purposes in the oil and gas industries.     

Evaluation of autotrophic growth of ammonia-oxidizers associated with granular activated carbon used for drinking water purification by DNA-stable isotope probing

Nitrification is an important biological function of granular activated carbon (GAC) used in advanced drinking water purification processes. Newly discovered ammonia-oxidizing archaea (AOA) have challenged the traditional understanding of ammonia oxidation, which considered ammonia-oxidizing bacteria (AOB) as the sole ammonia-oxidizers. Previous studies demonstrated the predominance of AOA on GAC, but the contributions of AOA and AOB to ammonia oxidation remain unclear. In the present study, DNA-stable isotope probing (DNA-SIP) was used to investigate the autotrophic growth of AOA and AOB associated with GAC at two different ammonium concentrations (0.14 mg N/L and 1.4 mg N/L). GAC samples collected from three full-scale drinking water purification plants in Tokyo, Japan, had different abundance of AOA and AOB. These samples were fed continuously with ammonium and ¹³C-bicarbonate for 14 days. The DNA-SIP analysis demonstrated that only AOA assimilated ¹³C-bicarbonate at low ammonium concentration, whereas AOA and AOB exhibited autotrophic growth at high ammonium concentration. This indicates that a lower ammonium concentration is preferable for AOA growth. Since AOA could not grow without ammonium, their autotrophic growth was coupled with ammonia oxidation. Overall, our results point towards an important role of AOA in nitrification in GAC filters treating low concentration of ammonium.     

Influence of manganese and ammonium oxidation on the removal of 17 alpha-ethinylestradiol (EE2)

Flow-through reactors with manganese oxides were examined for their capacity to remove 17α-ethinylestradiol (EE2) at μg L⁻¹ and ng L⁻¹ range from synthetic wastewater treatment plant (WWTP) effluent. The mineral MnO₂ reactors removed 93% at a volumetric loading rate (B_V) of 5 μg EE2 L⁻¹ d⁻¹ and from a B_V of 40 μg EE2 L⁻¹ d⁻¹ on, these reactors showed 75% EE2 removal. With the biologically produced manganese oxides, only 57% EE2 was removed at 40 μg EE2 L⁻¹ d⁻¹. EE2 removal in the ng L⁻¹ range was 84%. The ammonium present in the influent (10 mg N L⁻¹) was nitrified and ammonia-oxidizing bacteria (AOB) were found to be of prime importance for the degradation of EE2. Remarkably, EE2 removal by AOB continued for a period of 4 months after depleting NH₄⁺ in the influent. EE2 removal by manganese-oxidizing bacteria was inhibited by NH₄⁺. These results indicate that the metabolic properties of nitrifiers can be employed to polish water containing EE2 based estrogenic activity.     

Magnetic binary oxide particles (MBOP): a promising adsorbent for removal of As (III) in water

Magnetic binary oxide particles (MBOP) synthesized using chitosan template has been investigated for uptake capacity of arsenic (III). Batch experiments were performed to determine the rate of adsorption and equilibrium isotherm and also effect of various rate limiting factors including adsorbent dose, pH, optimum contact time, initial adsorbate concentration and influence of presence cations and anions. It was observed that uptake of arsenic (III) was independent of pH of the solution. Maximum adsorption of arsenic (III) was ∼99% at pH 7.0 with dose of adsorbent 1 g/L and initial As (III) concentration of 1.0 mg/L at optimal contact time of 14 h. The adsorption equilibrium data fitted well to Langmuir and Freundlich isotherm. The maximum adsorption capacity of adsorbent was 16.94 mg/g. With increase in concentration of Ca²⁺, Mg²⁺ from 50 mg/L to 600 mg/L, adsorption of As (III) was significantly reduced while for Fe³⁺ the adsorption of arsenic (III) was increased with increase in concentration. Temperature study was carried out at 293 K, 303 K and 313 K reveals that the adsorption process is exothermic nature. A distinct advantage of this adsorbent is that adsorbent can readily be isolated from sample solutions by application of an external magnetic field. Saturation magnetization is a key factor for successful magnetic separation was observed to be 18.78 emu/g which is sufficient for separation by conventional magnate.     

Effects of free cyanide on microbial communities and biological carbon and nitrogen removal performance in the industrial activated sludge process

The changes in process performance and microbial communities under free cyanide (CN⁻) were investigated in a lab-scale activated sludge process treating industrial wastewater. The performance of phenol degradation did not appear to be adversely affected by increases in CN⁻ concentrations. In contrast, CN⁻ was found to have an inhibitory effect on SCN⁻ biodegradation, resulting in the increase of TOC and COD concentrations. Nitratation also appeared to be inhibited at CN⁻ concentrations in excess of 1.0 mg/L, confirming that nitrite-oxidizing bacteria (NOB) is more sensitive to the CN⁻ toxicity than ammonia oxidizing bacteria (AOB). After CN⁻ loads were stopped, SCN⁻ removal, denitrification, and nitrification inhibited by CN⁻ were recovered to performance efficiency of more than 98%. The AOB and NOB communities in the aerobic reactor were analyzed by terminal restriction fragment length (T-RFLP) and quantitative real-time PCR (qPCR). Nitrosomonas europaea lineage was the predominant AOB at all samples during the operation, but an obvious change was observed in the diversity of AOB at the shock loading of 30 and 50 mg/L CN⁻, resulting in Nitrosospira sp. becoming dominant. We also observed coexisting Nitrospira and Nitrobacter genera for NOB. The increase of CN⁻ loading seemed to change the balance between Nitrospira and Nitrobacter, resulting in the high dominance of Nitrobacter over Nitrospira. Meanwhile, through using the qPCR, it was observed that the nitrite-reducing functional genes (i.e., nirS) were dominant in the activated sludge of the anoxic reactor, regardless of CN⁻ loads.     

Health risk assessment of inorganic arsenic intake of Cambodia residents through groundwater drinking pathway

In order to compare the magnitudes and health impacts of arsenic and other toxic trace elements in well water, groundwater and hair samples were collected from three areas with different arsenic exposure scenarios in the Mekong River basin of Cambodia. Ampil commune in Kampong Cham province was selected as an uncontaminated area, Khsarch Andaet commune in Kratie province was selected as a moderately contaminated area, and Kampong Kong commune in Kandal Province was selected as an extremely contaminated area. Results of ICP-MS analyses of the groundwater samples revealed that As, Mn, Fe and Ba concentrations were significantly different among the three study areas (Kruskal-Wallis test, p < 0.0001). Out of 46 observed wells in the Kandal province study area, 100% detected As > 50 μg L⁻¹ and Fe > 300 μg L⁻¹; 52.17% had Mn > 400 μg L⁻¹ and 73.91% found Ba > 700 μg L⁻¹. In the Kratie province study area (n = 12), 25% of wells showed elevated arsenic levels above 10 μg L⁻¹ and 25% had Mn > 400 μg L⁻¹, whereas samples from Kampong Cham province study area (n = 18) were relatively clean, with As < 10 μg L⁻¹. A health risk assessment model derived from the USEPA was applied to calculate individual risks resulting from drinking groundwater. Computational results indicated that residents from Kandal Province study area (n = 297) confronted significantly higher non-carcinogenic and carcinogenic risks than those in Kratie (n = 89) and Kampong Cham (n = 184) province study areas (Kruskal-Wallis test, p < 0.0001). 98.65% of respondents from the Kandal province study area were at risk for the potential non-cancer effect and an average cancer risk index was found to be 5 in 1000 exposure. The calculations also indicated that, in the Kratie province study area, 13.48% of respondents were affected by non-cancer health risks and 33.71% were threatened by cancer, whereas none of respondents in the Kampong Cham province study area appeared to have non-carcinogenic effect. Positively significant correlations of the arsenic content in scalp hair (As_h) with both arsenic levels in groundwater (As_w) (r_s (304) = 0.757, p < 0.0001) and individual average daily doses (ADD) of arsenic (r_s (304) = 0.763, p < 0.0001) undoubtedly indicated that arsenic accumulation in the bodies of Cambodia residents in the Mekong River basin was mainly through a groundwater drinking pathway. To the best of our knowledge, this is the first comprehensive report comparing individual health risk assessments of arsenic exposure through a groundwater drinking pathway to enriched arsenic levels from groundwater in the Mekong River basin, Cambodia. This study indicates that elevated arsenic concentrations in groundwater may lead to thousands of cases of arsenicosis in the near future if mitigating actions are not taken.     

Occurrence of arsenic in core sediments and groundwater in the Chapai-Nawabganj District, northwestern Bangladesh

Groundwater and core sediments of two boreholes (to a depth of 50 m) from the Chapai-Nawabganj area in northwestern Bangladesh were collected for arsenic concentration and geochemical analysis. Groundwater arsenic concentrations in the uppermost aquifer (10-40 m of depth) range from 2.8 μg L⁻¹ to 462.3 μg L⁻¹. Groundwater geochemical conditions change from oxidized to successively more reduced, higher As concentration with depth. Higher sediment arsenic levels (55 mg kg⁻¹) were found within the upper 40 m of the drilled core samples. X-ray absorption near-edge structure spectroscopy was employed to elucidate the arsenic speciation of sediments collected from two boreholes. Environmental scanning electron microscopy and transmission X-ray microscopy were used to investigate the characteristics of FeOOH in sediments which adsorb arsenic. In addition, a pH-Eh diagram was drawn using the Geochemist's Workbench (GWB) software to elucidate the arsenic speciation in groundwater. The dominant groundwater type is Ca-HCO₃ with high concentrations of As, Fe and Mn but low levels of NO₃⁻ and SO₄²⁻. Sequential extraction analysis reveals that Mn and Fe hydroxides and organic matter are the major leachable solids carrying As. High levels of arsenic concentration in aquifers are associated with fine-grained sediments. Fluorescent intensities of humic substances indicate that both groundwater and sediments in this arsenic hotspot area contain less organic matter compared to other parts of Bengal basin. Statistical analysis clearly shows that As is closely associated with Fe and Mn in sediments while As is better correlated with Mn in groundwater. These correlations along with results of sequential leaching experiments suggest that reductive dissolution of MnOOH and FeOOH mediated by anaerobic bacteria represents an important mechanism for releasing arsenic into the groundwater.     

Assessment of arsenic exposure from groundwater and rice in Bengal Delta Region, West Bengal, India

Arsenic (As) induced identifiable health outcomes are now spreading across Indian subcontinent with continuous discovery of high As concentrations in groundwater. This study deals with groundwater hydrochemistry vis-à-vis As exposure assessment among rural population in Chakdaha block, West Bengal, India. The water quality survey reveals that 96% of the tubewells exceed WHO guideline value (10 μg/L of As). The groundwaters are generally anoxic (−283 to −22 mV) with circum-neutral pH (6.3 to 7.8). The hydrochemistry is dominated by HCO₃⁻ (208 to 440 mg/L), Ca²⁺ (79 to 178 mg/L) and Mg²⁺ (17 to 45 mg/L) ions along with high concentrations of As_T (As total, below detection limit to 0.29 mg/L), Fe_T (Fe total, 1.2 to 16 mg/L), and Fe(II) (0.74 to 16 mg/L). The result demonstrates that Fe(II)-Fe(III) cycling is the dominant process for the release of As from aquifer sediments to groundwater (and vice versa), which is mainly controlled by the local biogeochemical conditions. The exposure scenario reveals that the consumption of groundwater and rice are the major pathways of As accumulation in human body, which is explained by the dietary habit of the surveyed population. Finally, regular awareness campaign is essential as part of the management and prevention of health outcomes.     

Sources and controls for the mobility of arsenic in oxidizing groundwaters from loess-type sediments in arid/semi-arid dry climates - evidence from the Chaco-Pampean plain (Argentina)

In oxidizing aquifers, arsenic (As) mobilization from sediments into groundwater is controlled by pH-dependent As desorption from and dissolution of mineral phases. If climate is dry, then the process of evaporative concentration contributes further to the total concentration of dissolved As. In this paper the principal As mobility controls under these conditions have been demonstrated for Salí River alluvial basin in NW Argentina (Tucumán Province; 7000 km²), which is representative for other basins or areas of the predominantly semi-arid Chaco-Pampean plain (1,000,000 km²) which is one of the world’s largest regions affected by high As concentrations in groundwater. Detailed hydrogeochemical studies have been performed in the Salí River basin where 85 groundwater samples from shallow aquifers (42 samples), deep samples (26 samples) and artesian aquifers (17 samples) have been collected. Arsenic concentrations range from 11.4 to 1660 μg L⁻¹ leaving 100% of the investigated waters above the provisional WHO guideline value of 10 μg L⁻¹. A strong positive correlation among As, F, and V in shallow groundwaters was found. The correlations among those trace elements and U, B and Mo have less significance. High pH (up to 9.2) and high bicarbonate (HCO₃) concentrations favour leaching from pyroclastic materials, including volcanic glass which is present to 20-25% in the loess-type aquifer sediments and yield higher trace element concentrations in groundwater from shallow aquifers compared to deep and artesian aquifers. The significant increase in minor and trace element concentrations and salinity in shallow aquifers is related to strong evaporation under semi-arid climatic conditions. Sorption of As and associated minor and trace elements (F, U, B, Mo and V) onto the surface of Fe-, Al- and Mn-oxides and oxi-hydroxides, restricts the mobilization of these elements into groundwater. Nevertheless, this does not hold in the case of the shallow unconfined groundwaters with high pH and high concentrations of potential competitors for adsorption sites (HCO₃, V, P, etc.). Under these geochemical conditions, desorption of the above mentioned anions and oxyanions occurs as a key process for As mobilization, resulting in an increase of minor and trace element concentrations. These geochemical processes that control the concentrations of dissolved As and other trace elements and which determine the groundwater quality especially in the shallow aquifers, are comparable to other areas with high As concentrations in groundwater of oxidizing aquifers and semi-arid or arid climate, which are found in many parts of the world, such as the western sectors of the USA, Mexico, northern Chile, Turkey, Mongolia, central and northern China, and central and northwestern Argentina.