The influences of temperature and hydrogen ion concentration on wastewater containing cobalt ions were investigated by constructing Pourbaix diagrams for the Co0-H2O system at temperatures of 288, 298, 308, and 318 K with varying pH levels. The majority of cobalt in water has been observed to exist in the form of Co2+ ions. Co0 and Co2+ are mutually interchangeable in acidic conditions depending on the reduction potential. Here, the conversion of Co2+ to Co(OH)2 and to Co(OH)3 was observed when pH was increased to around pH 5.5 to 6.5; the pH at which the change occurred decreased as the temperature increased. The changes in reactivity between the chemical species Co0, Co2+, Co(OH)2, and Co(OH)3 were quantified by measuring the area of stable conditions in the Pourbaix diagrams. The ratios of stable regions between Co(OH)3/Co2+, Co(OH)2/Co2+, Co0/Co2+, Co(OH)2/Co0, and Co(OH)2/Co(OH)3 were observed to increase as the temperature increases. Therefore, the reactivity between these species was considered to in crease with temperature. These results indicate that it is necessary to consider the effect of temperature for the effective treatment of wastewater that contains cobaltions.
