Sorption behavior of heavy metal oxyanions on iron-oxyhydroxysulfate minerals

The sorption reaction of toxic ions on high surface minerals is a matter of great concern because of potential usage in environmental remediation. Iron oxide and hydroxide such as goethite are known to have high sorption capacity due to their high specific surface areas and can be used for the sorption of heavy metals. Schwertmannite, commonly found iron-oxyhydroxysulfate in acid mine drainage, is also a good candidate for the heavy metal sorption due to its high specific surface area. Heavy metals existing as oxyanions are known to have high affinity to schwertmannite compared with other heavy metals. The different sorption mechanisms of oxyanions are thought to be due to its substitution for sulfate in the mineral structure. However, schwertmannite has low crystallinity and its structure is not well known, and as a result, more systematic information of the sorption on the schwertmannite is needed. The sorption of several different oxyanions on the schwertmannite was studied. Schwertmannite was synthesized by the reaction of 1L of solution containing 10 g FeSO4 and 5 mL H2O2(30%) for 24 h. The sorption experiment was conducted at pH 4, 25 oC for 24 h. Synthetic schwertmannite was reacted in 100 ml solution at various concentrations of AsO42-, SeO32-, SeO42-, MoO42-, and CrO42-. Those oxyanions show different sorption isotherms. AsO42-, SeO32-, and MoO42- show high sorption on schwertmannite and most ions in solution are sorbed at low concentrations. However, the sorption of each oxyanion does not increase much over certain concentrations and those concentrations are different for each oxyanion with the concentration order of MoO42- > SeO32- > AsO42-. In the case of AsO42-, the sorption does not increase at all over 0.7 mmol/g. Other two oxyanions show lower sorption in all concentration ranges, indicating that the sorption behavior of SeO32-, and CrO42- is different from AsO42- which is known to replace sulfate in the crystal structure. Thermal analysis results will be also presented to show the different sorption behaviors of those oxyanions.