COBALT(II) REMOVAL FROM WATER BY CHEMICAL-REDUCTION WITH SODIUM-BOROHYDRIDE

The stoichiometry and kinetics of Co2+ reduction with NaBH4 in aqueous solution have been studied. A substantial excess of borohydride is needed to achieve complete Co2+ removal from water, mainly due to borohydride consumption in the BH4-/H2O reaction unless the pH is sufficiently alkaline. In this case, cobalt precipitation occurs mainly as Co(OH)2. Absence of dissolved oxygen in the reactor is required to avoid rapid reoxidation.of reduced cobalt. The elemental analysis of the reduced species is consistent with Co2B. The ease of sludge reoxidation and the final boron concentration in the effluent resulting from the needed excess of borohydride are serious disadvantages of this technique for Co2+ removal. A kinetic model has been proposed to describe the behaviour of the system. The model reproduces reasonably well the experimental results, although some discrepancies were observed in the early stages of the process. These can be mainly attributed to mass-transfer limitations.