Xiaohui Tang

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.

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.

Preparative separation of arsenate from phosphate by IRA-400 (OH) for oxygen isotopic work.

The paper reports about a series of tests carried out to find out the optimal conditions for the preparative separation of arsenate and phosphate from natural waters, using the anion exchange resin Amberlite IRA-400 (OH). Freundlich isotherms have been constructed on basis of data obtained by stirring different amounts of resin (0.05-1.00 g) with solutions containing 1mg/L As and 10mg/L P in form of arsenate and phosphate and the effect of pH and P/As ratio on adsorption was investigated. It was found that at these concentrations 0.5 g of IRA-400 (OH) can adsorb quantitatively arsenate and phosphate within 1h. In a range of 3.6-11.1, pH seems to have no influence on the adsorption behavior of the resin, but at pH 1.5 the adsorption of both arsenate and phosphate drops to values close to zero. Experiments with solutions with P/As ratios in a range between 1 and 30 have shown that the concentration ratios have also little effect on adsorption. An efficient selective desorption of the anions could be achieved with 2 mol/L HNO3 or HCl, but the use of HCl is impracticable if the separation aims at precipitating arsenate for oxygen isotopic work. The reported adsorption/ desorption properties of the resin are supported also by data obtained by investigating the resin particles with a scanning electron microscope equipped with a fluorescence detection device.

Selective Separation and Preconcentration of Arsenite from Arsenic Enriched Natural Waters with Three Different Adsorbents

Arsenite (AsO33-) is the dominant arsenic (As) oxyanion in most of As rich groundwaters, generating serious health problems worldwide. It is assumable that the isotopic composition of the associated oxygen may broaden our understanding about the causes, which led to the appearance of this kind of contamination. The aim of this study was to work out an efficient method for the selective extraction of arsenite from natural water samples, in a form adequate for isotopic measurements. Three adsorbents, i. basic yttrium carbonate (BYC), ii. porous polymer beads loaded with monoclinic hydrous zirconium oxide (MHZR), and iii. nanoparticlate Fe2O3 (NFO) were tested for their suitability to separating AsO33- from coexisting anions that may interfere with the isotopic measurement. Results show that 95.1%, 99.3%, and 98.2% of AsO33- were adsorbed selectively and separated from SO42- and Cl- in water by BYC, MHZR, and NFO, respectively. Using different desorption agents, 95.0% and 87.5% of AsO33- were stripped from BYC in 0.05 M HNO3 and NFO in 2 M NaOH, respectively, but only 27.5% from MHZR. By adding AgNO3 in excess to these solutions, 98.3 ± 1.28% (n = 4) and 95.7 ± 4.81% (n = 4) of the AsO33-, desorbed from the BYC and NFO, were precipitated as Ag3AsO3, respectively. The purity of the precipitates was checked with SEM-EDX.