Sorption kinetics of Fe(II), Zn(II), Co(II), Ni(II), Cd(II), and Fe(II)/Me(II) onto hematite

The reactions of Fe(II) and other divalent metal ions including Zn, Co, Ni, and Cd on hematite were studied in single and competitive binary systems with high sorbate/sorbent ratios in 10 mM PIPES (pH 6.8) solution under strict anoxic conditions. Adsorbed Me(II) was defined as extractable by 0.5 N HCl within 20 h, and fixed Me(II) was defined as the additional amount that was extracted by 3.0 N HCl within 7 days. Binary systems contained Fe(II) plus a second metal ion. The extent of uptake of divalent metal ions by hematite was in order of Fe⩾Zn>Co⩾Ni>Cd. For all metals tested, there was an instantaneous adsorption followed by a relatively slow stage that continued for the next 1–5 days. This sequence occurred in both single and binary systems, and could have been due to a variety of sorption site types or due to slow conversion from outer- to inner-sphere surface complexes due to increasing surface charge. Sorption competition was observed between Fe(II) and the other metal ions. The displacement of Fe(II) by Me(II) was in order of Ni∼Zn>Cd, and the displacement of Me(II) by Fe(II) was in order of Cd>Zn∼Ni>Co. Fixed Fe(II) was in order of Fe+Co (20%)>Fe+Cd (6%)>Fe∼Zn (4%)>Fe∼Ni (4%) after 30 days. There was no fixation for the other metals in single or binary systems.