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

The removal of iron and various heavy-metal ions by their incorporation into stable ferrite-type precipitates at 25 degreesC is a promising alternative to clean up large volumes of polluted aqueous effluents. Our research work investigated the conditions to produce magnetite and various M-bearing ferrites (where M: Zn, Cu, Co, Ni, Cd, etc...) directly from aqueous solutions at 25 degreesC by simultaneous control of the oxidizing conditions and pH. The formation of the solids was followed by monitoring the oxidation-reduction potential (ORP) and rate of proton release during the aerial oxidation of the suspensions at constant pH (contact stage). Only mildly oxidizing conditions, represented by ORP values between -110 mV and -150 mV and a moderate oxidation rate of Fe(II) species were conducive to well-crystallized ferrites at ambient temperature. It was also found that increasing the Fe/M mole ratio in starting solutions enhanced the stability of the M-bearing ferrites at ambient temperature. When the formation of the ferrite was incomplete, aging of the sludges in their mother liquors at 25 degreesC promoted the crystallinity of the precipitates. The mentioned conditions favored the suitable progress of the ferrite-forming reaction involving the oxidation of Fe(II) entities, subsequent hydrolysis of produced Fe(III) species and de-hydration of the intermediate. Furthermore, the ferrite formation at low temperature permits the elimination of iron and co-existing heavy metal ions to sufficiently low concentration. In turn, the characterization of the precipitates evidenced their magnetic nature (room-temperature saturation magnetization above 60 emu/g) as well as the effective incorporation of target metal ions into the ferrite structure.