Chemical speciation of nickel in fly ash by phase separation and carbon paste electrode voltammetry

In a risk-based approach to cost-effective management of power plant emissions, chemical speciation of fly ash is required. A quantitative but indirect separation of nickel phases by sequential extraction was undertaken in conjunction with direct analysis of the sulfidic nickel phase by carbon paste electrode voltammetry (CPEV). Four ash samples produced in a laboratory combustion system from burning high- and low-sulfur residual oil at excess O(2) of 1 and 2-3 mol% were studied. The extractions yielded five phases of nickel. The CPEV analysis of sulfidic nickel in the ash and extraction residues was performed in pH 5 acetate. The anodic peak (-0.1 V) unique to Ni(3)S(2), distinguishing it from NiS and NiS(2), was found to be absent from these ash samples. The CPEV method was consistent with phase extraction, which showed NiSO(4) being predominant with very low proportions of sulfidic nickel.