A quantitative modeling for simultateous removal of As(V) and Hg(II) using neutralization and co-precipitation process with ferrihydite

T. Kato, T. Uchida, M. Yagisawa, S. Hobo and *C. Tokoro

Department of Science and Engineering, Waseda university

3-4-1 Okubo, Shinjuku-ku

Tokyo, Japan 169-8555

(*Corresponding author: )

ABSTRACT

Wastewater generated in mining, such as acid mine drainage and mineral processing wastewater, often contains several kinds of inorganic toxic elements. Co-precipitation with ferrihydrite is widely known as a robust and effective process for simultaneous removal of several kinds of inorganic toxic elements. However, quantitative knowledge for the removal has been limited.

In this study, quantitative modeling was conducted for simulated wastewater containing As(V), Zn(II) and Hg(II) as toxic elements, and Fe(III), Na(I) and sulfate as coaxing ions. Objective of this modeling was to predict pH or residual concentration of all elements in solution after neutralization/co-precipitation treatment. To accomplish this, removal mechanism of As(V) and Hg(II) by ferrihydrite was investigated firstly for simulated wastewater containing only As(V) or Hg(II). From experimental results, it was confirmed that removal mechanism of As(V) and Hg(II) by co-precipitation method is not only surface complexation but also surface precipitation. However, surface complexation is main mechanism for As(V) and Hg(II) removal under the target level regulated in Japan. Next, surface complexation model was constructed for removal of As(V) and Hg(II) by ferrihydrite. The specific surface area and site density were determined from reference value proposed by Dzombak and Morel (1990). Comparing between experimental and calculated results, it was confirmed that our constructed model could successfully represent experimental results.

Furthermore, the surface complexation model was coupled with chemical equilibrium calculation and applied to neutralization process of the simulated wastewater containing As(V), Hg(II), Zn(II), Na(I), Fe(III) and sulfate. As a result, our constructed model could successfully represent pH edge for As(V), Hg(II), Zn(II), Fe(III) and SO42- removal. In addition, our constructed model could represent ion balance as Fig. 1. Thus, our constructed model is useful to estimate the amount of neutralizer that needed to adjust target pH.

Fig. 1 Ion balance results of neutralization treatment

KEYWORDS

Ferrihydrite, Surface complexation, modeling, mercury, arsenic, sulfate