Evaluation of the multiple-ion competition in the adsorption of As(V) onto reclaimed iron-oxide coated sands by fractional factorial design

This study describes the competitive effects of selected ions and natural organic matter on As(V) removal using reclaimed iron-oxide coated sands (RIOCS) in the single- and multi-ion systems. A 27−3 factional factorial experimental design (FFD) was employed for screening main competitive factors in this adsorption process. As a result, the inhibitive competition effects of the anions on As(V) removal in the single ion system were in the following sequence: PO43- > SiO32- > HCO3- > humic acid (HA) > SO42- > Cl−, whereas the cation Ca2+ was observed to enhance the As(V) removal. In addition, the optimum initial pH for As(V) removal in single-ion system was 5. Based on the estimates of major effects and interactions from the FFD, PO43-, SiO32-, Ca2+ and HA were important factors on As(V) removal in the multi-ion system. The promoters for the As(V) removal were found to be Ca2+ and, to a lesser extent, SO42-. The competitive effects of these ions on As(V) removal were in the order of PO43-, SiO32-, HA, HCO3-, and Cl−. In the single ion system, the efficiencies of As(V) removal range from 75% to 96%, much higher than those in the multi-ion system (44%) at the initial pH 5. Clearly, there were some complex anion interactions in the multi-ion system. To promote the removal of As(V) by RIOCS, it is proposed to lower the pH in the single-ion system, while in the multi-ion system, the increase of the Ca2+ concentration, or decreases of PO43-, SiO32- and HA concentrations is suggested.