Investigation of sorption mechanism on arsenic co-precipitation with ferrihydrite for quantitative modelling of AMD treatment

Sorption mechanism of dilute As(V) co-precipitation with ferrihydrite was investigated for quantitative modelling toward an effective treatment of acid mine drainage (AMD). In this study, co-precipitation experiments were exactly distinguished from simple adsorption ones. Several kinds of experimental investigations were conducted for both experiments respectively; sorption characteristics observation, X-ray diffraction (XRD) analysis and X-ray Absorption Fine Structure (XAFS) analysis. We compared experimental results between co-precipitation experiments and simple adsorption ones. These results suggested that the sorption mechanism in co-precipitation process consists of not only simple two-dimensional adsorption onto the surface of ferrihydrite but also some three-dimensional uptake with ferrihydrite. We also evaluated the sorption mechanism by analysis of XRD and XANES. XRD or XANES spectra showed that surface precipitation of poorly crystalline ferric arsenate was formed when the initial As/Fe molar ratio was > 0.5, beyond the thermodynamic bulk precipitation range for ferric arsenate. From XANES analysis on As K edge, we also obtained the ratio of simple adsorption of As(V) to ferrihydrite and surface precipitation of ferric arsenate in co-precipitates quantitatively. We also attempted to obtain quantitative models for As(V) sorption in co-precipitation or simple adsorption with ferrihydrite. The surface complexation models could quantitatively represent experimental results in adsorption experiments. On the other hand, the surface precipitation models considering with poorly crystalline ferric arsenate formation was newly constructed for As(V) co-precipitation with ferrihydrite. As a result, calculations of the surface precipitation models were in good agreement with the experimental results of As(V) co-precipitation and surface precipitation models were capable to predict As(V) co-precipitation.