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> or =Zn>Co> or =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 approximately Zn>Cd, and the displacement of Me(II) by Fe(II) was in order of Cd>Zn approximately Ni>Co. Fixed Fe(II) was in order of Fe+Co (20%)>Fe+Cd (6%)>Fe approximately Zn (4%)>Fe approximately Ni (4%) after 30 days. There was no fixation for the other metals in single or binary systems.