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 ne eded to achieve complete Co2+ removal from water, mainly due to borohy dride consumption in the BH4-/H2O reaction unless the pH is sufficient ly alkaline. In this case, cobalt precipitation occurs mainly as Co(OH )2. Absence of dissolved oxygen in the reactor is required to avoid ra pid reoxidation.of reduced cobalt. The elemental analysis of the reduc ed species is consistent with Co2B. The ease of sludge reoxidation and the final boron concentration in the effluent resulting from the need ed excess of borohydride are serious disadvantages of this technique f or Co2+ removal. A kinetic model has been proposed to describe the beh aviour of the system. The model reproduces reasonably well the experim ental results, although some discrepancies were observed in the early stages of the process. These can be mainly attributed to mass-transfer limitations.