Arsenic Forms In Aquifer Sediments And Their Impacts On Arsenic Enrichment In Groundwater From The Hetao Basin

High arsenic groundwater has caused serious public health problems in the Hetao Basin in Inner Mongolia. Reducing conditions drive geogenic release of As from sediments, which would be affected by mineralogical compositions and grain size distribution. However, it is unclear how mineralogical compositions and grain size distribution affect arsenic release from sediments. In order to solve those scientific issues, the Hetao Basin was selected as the study area to collect groundwater samples and sediment samples at depth-macthed intervals. Following conclusions are drawn in this thesis.(1) Groundwater was weakly neutral-alkaline, with As concentrations between 2.26 and 583 μg·L-1. Dissolved Fe, As and As(V) increased with the increase in sampling depths. In suboxic shallow groundwater(depths <14 m) with higher Eh values, As concentrations were generally lower than those in the reducing deep groundwater(depths >14 m) with relatively lower Eh values.(2) Sediment As was positively correlated with Fe, Mn, P and TOC, indicating that Fe and Mn oxides potentially had good affinity to As. Strongly adsorbed As and amorphous Fe oxyhydroxides-bound As were the major As forms in the sediments. Proportations of strongly adsorbed As and crystalline Fe oxyhydroxides-bound As were higher in clay and silty clay than in silt and fine sand. However, proportations of amorphous Fe oxyhydroxides-bound As were higher in silt and fine sand than in clay and silty clay. Arsenic tended to accumulate in sediments undergone strong chemical weathering and with high maturity.(3) Positive relationships were observed between dissolved As concentrations and contents of extractable As(including S1: weakly bound As, S2: strongly bound As, and F3: Fe/Mn oxide bound As) in aquifer sediments at depth-matched intervals in reducing and suboxic conditions. In reducing environments, As tended to be partitioned into the liquid phase, where S2-As showed a better correlation with dissolved As than S1-As. In the suboxic environment, As tended to be fixed in the solid phase, where S1-As of aquifer sediments was closely related to concentrations of dissolved As in groundwater.(4) Redox cyling processes led to As release from sediments. Results showed that more As was released from fine–grained size sand than the pristine sand. Thus, grain size had an impact on the rate of As adsorption and desorption. Sediments with smaller grain size adsorb and desorb As faster.(5) In relative to the pristine sediments, the used sediments after redox cycling experiments had lower contents of amorphous Fe oxyhydroxides-bound As decreased and higher contents of crystalline Fe oxyhydroxides-bound As, which indicated that successive redox oscillations increased the crystallinity in Fe(III)-oxyhydroxides and thus led to As accumulation in solutions.