Nickel is a widely used chemical species, but it is also known for its toxicity. The Pourbaix diagrams were constructed to investigate the influences of temperature and hydrogen ion on wastewater which contains nickel ion. Nickel exists as Ni3O4, Ni(OH)2, Ni2+, and Ni in the water of pH 0 to 14. In the temperature range of 288K to 318K, Ni(OH)2 was shown to have stable region from pH 5.7 to pH 14, and the area of stable region increases with temperature. The ratio of Ni(OH)2 to Ni and the ratio of Ni(OH)2 to Ni2+ wereexamined to be greater than the ratios of Ni3O4 to Ni and Ni3O4 to Ni2+, respectively. Also, the ratio of Ni(OH)2 to Ni2+ and the ratio of Ni3O4 to Ni2+ were shown to be proportional to the temperature, while Ni was not. The reduction-precipitation process of nickel ion to Ni(OH)2 by controlling the pH depending on the temperature following the value of pH 6.29 at 288K, pH 6.08 at 298K, pH 5.88 at 308K, and pH 5.70 at 318K, can be used to decrease concentration of nickel ion in wastewater and recover nickel from wastewater in the form of Ni(OH)2.
