Elimination of the interference of sulfides by zinc ions on the determination of hexavalent chromium in soil by Method 3060a & Method 7196a

The sulfides are recognized as the most promising reducing agents for remediation of chromium-contaminated soils due to their strong reducing ability and low cost. However, the residual sulfides and elemental sulfur (S⁰) from the oxidation of sulfides significantly interfere with the determination of hexavalent chromium (Cr(Ⅵ)) in soil, seriously hindering the engineering application and experimental researches. The effectiveness of the Cu²⁺-EDTA method in removing S⁰ interference is unsatisfactory due to the inability to maintain sufficient Cu²⁺ concentrations during alkaline digestion. The toluene-mercury method, utilizing hot toxic toluene for pre-extraction of S⁰, makes the operation complex and require higher personal safety protection. This paper proposes a simple method for eliminating sulfides and S⁰ interference using Zn²⁺, only involving the addition of Zn²⁺ before alkaline digestion without extra operation. Experimental results indicate that at pH > 12.5, S⁰ begins to produce a large amount polysulfides (S_n^2-) during the alkaline digestion, which in turn reduces Cr(Ⅵ). At the same pH, Zn²⁺ predominantly exists as aqueous Zn(OH)₄^2- and instantly eliminates the S⁰-generated S_n^2- by the formation of ZnS precipitates. Experiments also indicate that the addition of Zn²⁺ exerts no significant influence on the method detection limits of the USEPA Method 3060a & Method 7196a.