XRD/XAFS analysis were conducted to investigate the As(V) co-precipitation mechanism with ferrihydrite (γ-FeOOH). In As(V) co-precipitation with ferrihydrite, XRD results showed that As(V) complexation to the surface of ferrihydrite was dominant when the initial molar ratio of As/Fe was less than 0.25, whereas surface precipitation of amorphous ferric arsenate was formed when the initial molar ratio of As/Fe was more than 0.5.
Both of XANES and EXAFS analysis on K-edge of As showed As(V) co-precipitates with ferrihydrite was the mixture of As(V) adsorbed ferrihydrite and amorphous ferric arsenate. Molar ratio of amorphous ferric arsenate in As(V) co-precipitates was estimated to be more than 0.5 when the initial molar ratio of As/Fe was more than 0.5. These results are in good agreement with the XRD results. EXAFS analysis assuming one surface complex for As-Fe bond showed the coordination number of As to Fe in As(V) co-precipitates increased with increasing the initial molar ratio of As/Fe. Moreover, EXAFS analysis assuming three kinds of surface complexes for As-Fe bond showed the coordination number for 2.85 Å of the atomic distance of As-Fe increased and it for 3.24 Å of the atomic distance of As-Fe decreased with increasing the initial As/Fe molar ratio. All experimental data obtained in this study showed As(V) co-precipitation mechanism shifted gradually from As(V) complexation to the surface of ferrihydrite toward amorphous ferric arsenate with increase in the initial molar ratio of As/Fe.
