Ambient-temperature precipitation of Zn ions from aqueous solutions as ferrite-type compounds

The magnetite-forming reaction from aqueous solutions at 25 °C was systematically studied in our earlier publications. On this basis, the present work investigated the conditions conducive to the incorporation of target metal ions into the structure of ferrites at ambient temperature. The Zn-bearing ferrite system was considered as the first case study. The formation of the solids was followed by monitoring the oxidation–reduction potential (ORP), measured against an Ag/AgCl reference electrode, and rate of proton release during the aerial oxidation of the suspensions at constant pH. Only mildly oxidizing conditions, represented by an ORP value of −150 mV and a moderate oxidation rate of Fe(II) species were conducive to the formation of crystalline Zn-bearing ferrites at 25 °C. It was also found that increasing the Fe(II)/Zn mole ratio in starting solutions enhanced the stability of the corresponding ferrite. When the formation of ferrite was incomplete, aging of the sludges in their mother liquors at 25 °C promoted the crystallinity of the precipitates. The mentioned conditions favored the progress of the ferrite-forming reaction, which involves the oxidation of Fe(II) entities, subsequent hydrolysis of produced Fe(III) species and dehydration of the intermediate. Furthermore, the formation of the ferrite permitted the elimination of Fe and Zn ions from starting solutions to sufficiently low concentration.