Huaming Guo

Groundwater geochemistry and its implications for arsenic mobilization in shallow aquifers of the Hetao Basin, Inner Mongolia

Arsenic concentrations in shallow groundwaters from the Hetao Basin of Inner Mongolia range between 0.6 and 572 microg/L. High As groundwaters generally occur in the shallow alluvial-lacustrine aquifers, which are mainly composed of black (or dark grey) fine sands in a reducing environment. They are characterized by high concentrations of dissolved Fe, Mn, HCO(3)(-), P and S(2-), and low concentrations of NO(3)(-) and SO(4)(2-). Low SO(4)(2-) coupled with high S(2-) suggests that SO(4)(2-) reduction has been an active process. In the reducing groundwaters, inorganic As(III) accounts for around 75% of total dissolved As. Total As contents in the sediments from three representative boreholes are observed to be 7.3-73.3 mg/kg (average of 18.9 mg/kg). The total As is mildly-strongly correlated with total Fe and total Mn, while a quite weak correlation exists between total As and total S, suggesting that the As is associated with Fe-Mn oxides, rather than sulfides in the sediments. It is found in the sequential extraction that chemically active As is mainly bound to Fe-Mn oxides, up to 3500 microg/kg. The mobilization of As under reducing conditions is believed to include reductive dissolution of Fe-Mn oxides and reduction of adsorbed As. Although exchangeable As is labile and very vulnerable to hydrogeochemical condition, the contribution is relatively limited due to the low concentrations. The competition between As and other anions (such as HPO(4)(2-)) for binding sites on Fe-Mn oxides may also give rise to the release of As into groundwater. Slow groundwater movement helps accumulation of the released As in the groundwaters.

Hydrogeochemical processes in shallow quaternary aquifers from the northern part of the Datong Basin, China

Groundwater is the most important source of water supply in Datong city. However, the levels of shallow Quaternary groundwaters from urbanized areas have been declining continuously and groundwater quality deteriorating in recent years. Understanding the geochemical evolution of groundwater is important for sustainable development of the water resources in Datong. Mineral hydrolysis of alumino-silicate minerals such as plagioclase and clinopyroxene, is the primary process controlling the concentration of H4SiO4 in the study area. Speciation calculations using the geochemical modeling code PHREEQC indicated that hydrolysis of bedrock, mainly composed of basalt and metamorphic rocks, is the major hydrogeochemical process controlling groundwater chemistry. The study area can be divided into 3 hydrogeochemical zones: A. Recharge (unimpacted) zone, B. Intermediate (industry-impacted) zone, and C. Discharge (agriculture-impacted) zone. Ion exchange and industrial and/or agricultural contamination contribute to the increase of Na+ from Zone A to Zone C, where the concentration of NO3- is up to 461.5 mg/l with a mean value of 101.5 mg/l, indicating that agricultural practice seriously affects groundwater. Sulfate concentration in groundwaters in an alluvial fan at Datong is extremely high, up to 1172.9 mg/l, and shows a close relationship with the concentrations of trace elements, especially Ni and Co, indicating that coal mining is the main contamination source for groundwater from the alluvial fan, in addition to agricultural activities.

Hydrogeological and biogeochemical constrains of arsenic mobilization in shallow aquifers from the Hetao basin, Inner Mongolia.

Little is known about the importance of drainage/irrigation channels and biogeochemical processes in arsenic distribution of shallow groundwaters from the Hetao basin. This investigation shows that although As concentrations are primarily dependent on reducing conditions, evaporation increases As concentration in the centre of palaeo-lake sedimentation. Near drainage channels, groundwater As concentrations are the lowest in suboxic-weakly reducing conditions. Results demonstrate that both drainage and irrigation channels produce oxygen-rich water that recharges shallow groundwaters and therefore immobilize As. Groundwater As concentration increases with a progressive decrease in redox potential along the flow path in an alluvial fan. A negative correlation between SO₄²⁻ concentrations and δ³⁴S values indicates that bacterial reduction of SO₄²⁻ occurs in reducing aquifers. Due to high concentrations of Fe (> 0.5 mg L⁻¹), reductive dissolution of Fe oxides is believed to cause As release from aquifer sediments. Target aquifers for safe drinking water resources are available in alluvial fans and near irrigation channels.

Enhancement of Arsenic Adsorption during Mineral Transformation from Siderite to Goethite: Mechanism and Application

Synthesized siderite was used to remove As(III) and As(V) from water solutions under anoxic conditions and oxic conditions. Results showed that As adsorption on synthetic siderite under anoxic conditions was around 10 mg/g calculated with Langmuir isotherm. However, the calculated As adsorption on synthetic siderite under oxic conditions ranged between 115 and 121 mg/g, which was around 11 times higher than that under anoxic conditions. It was found that 75% siderite was transformed into goethite during oxic adsorption. However, synthetic goethite had lower As adsorption capacity than siderite under oxic conditions, although its adsorption capacity was a little higher than siderite under anoxic conditions. It suggested that the coexistence of goethite and siderite bimineral during mineral transformation probably contributed to the robust adsorption capacity of siderite under oxic conditions. Results of extended X-ray absorption fine structure (EXAF) spectroscopy indicated both As(III) and As(V) formed inner-sphere complexes on the surface of As-treated solid regardless of substrates, including the bidentate binuclear corner-sharing ((2)C) complexes and the monodentate mononuclear corner-sharing ((1)V) complexes. Monodenate ((1)V) and bidentate ((2)C) complexes would be related to high As adsorption capacity of siderite under oxic conditions. It showed that more Fe atoms were coordinated with As atom in the monodentate complexes and the bidentate complexes of As(V)/As(III)-treated siderite under oxic conditions, in comparison with As(V)/As(III)-treated siderite under anoxic conditions and As(V)/As(III)-treated goethite. Calcinations of natural siderite resulting in the coexistence of goethite and siderite greatly increased As adsorption on the solid, which confirmed that the coexistence of bimineral during mineral transformation from siderite to goethite greatly enhanced As adsorption capacity of siderite adsorbent. The observation can be applied for modification of natural siderite for As removal from high As waters.

Arsenate removal from aqueous solution using synthetic siderite.

The study was carried out to evaluate the feasibility of synthetic siderite for As(V) removal from aqueous solution. Batch experiments were performed to investigate effects of various experimental parameters such as contact time (10 min-8 h), initial As(V) concentration (0.5-60.0 mg/L), temperature (15, 25, 35 and 45 degrees C), pH (2.0-10.0) and the presence of competing anions on As(V) adsorption on the synthetic siderite. Kinetic data reveal that the uptake rate of As(V) was rapid at the beginning and 90% adsorption was completed within 10 min at 45 degrees C and equilibrium was achieved within 3 h. The adsorption process was well described by pseudo-second-order kinetics model. The adsorption data better fitted Langmuir isotherm at low temperatures (i.e., 15 and 25 degrees C), while Freundlich isotherm at relatively high temperatures (35-45 degrees C). The maximum adsorption capacity calculated from Langmuir isotherm model was up to 31 mg/g. Thermodynamic study indicates an exothermic nature of adsorption and a spontaneous and favorable process. The optimum pH for As(V) removal was broad, ranging from 3.0 to 10.0. The As(V) adsorption was impeded by the presence of SiO(3)(2-), followed by PO(4)(3-) and NO(3)(-). The adsorption process appeared to be controlled by the chemical process. The high As uptake may attribute to both coprecipitation of As with goethite and lepidocrocite forming during the reaction and subsequent adsorption of As on these minerals.

Removal of arsenite from water by synthetic siderite: Behaviors and mechanisms

Synthetic siderite has been used as adsorbent for As(III) removal in this study. Effects of contact time, temperature, pH, co-existing anions on As(III) adsorption were intensively investigated. Adsorption mechanisms were also studied using the X-ray absorption technique. Results show that the maximum adsorption capacity is up to 9.98 mg g(-1) at 25°C at a siderite dosage of 2 g L(-1). Adsorption kinetics agrees with the Lagergren pseudo-second order model. Arsenic(III) adsorption can be better described by Langmuir isotherm model for As(III) adsorption at 55°C, indicating that the coverage of the adsorption sites is in the form of monolayer, although Freundlich isotherm yields a better fit to the experimental data at 25, 35 and 45°C. Thermodynamic study indicates that As(III) adsorption on the synthetic siderite is spontaneous and endothermic in nature. The adsorption capacity is enhanced with the increase in reaction temperature. The adsorption is independent on solution pH between 3.0 and 9.6. The presence of NO(3)(-), SO(4)(2-), PO(4)(3-) or SiO(3)(2-) with element concentrations less than 20 mg L(-1) does not have adverse effect on As(III) adsorption. XANES spectra indicate that As mainly occurs as As(V) in the As adsorbed-materials, and the fraction of oxidized As(III) increases with the decrease in As(III) concentration. The formation of Fe hydroxide minerals (such as lepidocrocite and goethite) followed by As(III) oxidation and adsorption is shown to be the main mechanism of As(III) removal by the synthetic siderite.

Arsenic contamination of the soil–wheat system irrigated with high arsenic groundwater in the Hetao Basin, Inner Mongolia, China

As one of the most important crop in the world, wheat (Triticum aestivum L.) was irrigated with low As water and high As water. However, little is known about As cycling in the soil-wheat-water system. Two wheat fields (site G and site Y), irrigated with high dissolved As (178 μg L(-1)) groundwater and low dissolved As (8.2 μg L(-1)) surface water, respectively, were systematically sampled in the Hetao Basin, including irrigation water, soils and plants. The annual As (including dissolved As and suspended As) input per m(2) was estimated at 140 and 36.7 mg in site G and site Y, respectively. Topsoils of site G contained relatively higher As content (average 18.8 mg kg(-1)) than those of site Y (13.8 mg kg(-1)). Arsenic content of wheat grains in site G is systematically higher than in the site Y, which were positively correlated with non-specifically sorbed-As and amorphous Fe/Al oxide-bound As in topsoils. Arsenic-contaminated groundwater led to As accumulation in irrigated soils and the increase in As bioavailability, and subsequently resulted in the increase in As content of wheat grain. It suggested that less problematic water resources should be used for wheat irrigation in order to avoid As accumulation in the soil-plant system.

Arsenic mobilization in aquifers of the southwest Songnen basin, P.R. China: evidences from chemical and isotopic characteristics.

High As groundwater has widely been found in the inland basins of China. Little is known about distribution and mobilization mechanisms of high As groundwater in the Songnen basin, where groundwater is the major source for drinking and irrigation. Eighty-seven groundwater samples, three surface water samples and sixty-three sediment samples were taken from the southwest of the Songnen basin, in order to investigate spatial distribution and constrains of groundwater As. Results showed that high As groundwater was generally of Na-Mg/Ca-HCO3 type, which had relatively low Eh values and neutral-weakly alkaline pH. High As groundwater was characterized by low concentrations of NO3(-) and SO4(2-), and high concentrations of Fe, Mn, and H2S. Around 65.5% of sampled shallow groundwater and 96% of sampled deep groundwater had As concentrations greater than 10 μg/L. Sediments had higher total As contents and higher Fe/Mn oxide-bound As contents in high As groundwater area than in the low As groundwater area. Distribution of groundwater As was dependent upon hydrogeologic settings, redox potential, microbial degradation of organic carbon, and precipitation of pyrite, siderite, and calcite. Along the groundwater flow path, As concentration showed an increasing trend. High As groundwater was mainly distributed in the low-lying areas. Reducing conditions were the major causes for As mobilization in the aquifers, which led to more As released from the sediments with higher contents of Fe/Mn oxide-bound As in higher As groundwater area. Results of (13)CDOC and (13)CDIC showed that dissimilatory Fe(III) reduction coupled with microbial degradation of dissolved organic carbon would be related to As mobilization in the aquifers. Although both Fe and As were released during these redox processes, pyrite, siderite and calcite precipitation would be the sink of dissolved As, which resulted in weak correlation between dissolved Fe and As.

Arsenate reduction and mobilization in the presence of indigenous aerobic bacteria obtained from high arsenic aquifers of the Hetao basin, Inner Mongolia

Intact aquifer sediments were collected to obtain As-resistant bacteria from the Hetao basin. Two strains of aerobic As-resistant bacteria (Pseudomonas sp. M17-1 and Bacillus sp. M17-15) were isolated from the aquifer sediments. Those strains exhibited high resistances to both As(III) and As(V). Results showed that both strains had arr and ars genes, and led to reduction of dissolved As(V), goethite-adsorbed As(V), scorodite As(V) and sediment As(V), in the presence of organic carbon as the carbon source. After reduction of solid As(V), As release was observed from the solids to solutions. Strain M17-15 had a higher ability than strain M17-1 in reducing As(V) and promoting the release of As. These results suggested that the strains would mediate As(V) reduction to As(III), and thereafter release As(III), due to the higher mobility of As(III) in most aquifer systems. The processes would play an important role in genesis of high As groundwater.

Contrasting distributions of groundwater arsenic and uranium in the western Hetao basin, Inner Mongolia: Implication for origins and fate controls.

Although As concentrations have been investigated in shallow groundwater from the Hetao basin, China, less is known about U and As distributions in deep groundwater, which would help to better understand their origins and fate controls. Two hundred and ninety-nine groundwater samples, 122 sediment samples, and 14 rock samples were taken from the northwest portion of the Hetao basin, and analyzed for geochemical parameters. Results showed contrasting distributions of groundwater U and As, with high U and low As concentrations in the alluvial fans along the basin margins, and low U and high As concentrations downgradient in the flat plain. The probable sources of both As and U in groundwater were ultimately traced to the bedrocks in the local mountains (the Langshan Mountains). Chemical weathering of U-bearing rocks (schist, phyllite, and carbonate veins) released and mobilized U as UO2(CO3)2(2-) and UO2(CO3)3(4-) species in the alluvial fans under oxic conditions and suboxic conditions where reductions of Mn and NO3(-) were favorable (OSO), resulting in high groundwater U concentrations. Conversely, the recent weathering of As-bearing rocks (schist, phyllite, and sulfides) led to the formation of As-bearing Fe(III) (hydr)oxides in sediments, resulting in low groundwater As concentrations. Arsenic mobilization and U immobilization occurred in suboxic conditions where reduction of Fe(III) oxides was favorable and reducing conditions (SOR). Reduction of As-bearing Fe(III) (hydr)oxides, which were formed during palaeo-weathering and transported and deposited as Quaternary aquifer sediments, was believed to release As into groundwater. Reduction of U(VI) to U(IV) would lead to the formation of uraninite, and therefore remove U from groundwater. We conclude that the contrasting distributions of groundwater As and U present a challenge to ensuring safe drinking water in analogous areas, especially with high background values of U and As.