Stable incorporation of Co²⁺ into ferrite structure at ambient temperature: Effect of operational parameters

Removal of heavy metals (HM) from industrial wastewater is of primary environmental importance. The seeded ambient temperature ferrite process, in which heavy metals are removed from solution by their incorporation into a magnetite structure, is an attractive alternative for both complete metals separation and generation of stable magnetic sludge. Despite its potential, the effects of various operational parameters on the composition and stability of the final product are not fully understood to-date. The current paper addresses the combined effect of selected operational parameters on Co²⁺ incorporation efficiency. Co²⁺, which is used in many industries (e.g., plating, alloys manufacturing, catalytic converters, and paint pigments) and hence may pose a significant environment risk, was shown previously to be incorporated successfully into ferrite structures. Co²⁺-bearing ferrites were synthesized from Fe(II) solutions at ambient temperature (20 and 30°C), applying slow oxidation, pH 10.5, 20 g (as Fe) l^-1 ferrite seed, and different Fe²⁺ to Co²⁺ influent ratios. Pure magnetite was used as an initial seed, which was gradually exchanged, as the reaction proceeded, by the Co²⁺-bearing ferrite generated within the reactor (until full exchange at steady state). Under all conditions investigated >99% of the Co²⁺ was removed from solution (yielding Co ²⁺ levels below 20 μg/l in the effluent) while the generated precipitates were composed predominately of magnetite/ferrite (based on XRD analysis). Favorable results in terms of Co²⁺ incorporation efficiency and sludge stability were obtained when the following operational conditions were applied: 1:10 Co²⁺:Fe²⁺ molar influent ratio, 30°C, 1,500 mgFe²⁺ l^-1 metal intermediate concentration, and 16 h aging period (pH 10.5).