Arsenic contamination of groundwater is a serious issue in many countries. We investigated the removal mechanism of arsenite (As(III)) through coprecipitation with ferrihydrite to establish an optimum wastewater treatment method. We conducted adsorption and coprecipitation experiments with different As/Fe molar ratios to identify the differences between the adsorption and coprecipitation processes. Precipitates from these experiments were analyzed by using X-ray diffraction and X-ray absorption fine structure analysis to determine local structural changes of ferrihydrite during coprecipitation. As(III) removal via coprecipitation with ferrihydrite was caused by surface complexation at a lower initial As/Fe molar ratio (< 1.0). Surface complexation and amplification of the octahedral structure of the ferrihydrite were the removal mechanisms at higher initial As/Fe molar ratios (> 1.0). Our results contribute to developing a more efficient wastewater treatment method for As(III) removal.
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