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 (S 0 ) 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 Cu 2+ -EDTA method in removing S 0 interference is unsatisfactory due to the inability to maintain sufficient Cu 2+ concentrations during alkaline digestion. The toluene-mercury method, utilizing hot toxic toluene for pre-extraction of S 0 , makes the operation complex and require higher personal safety protection. This paper proposes a simple method for eliminating sulfides and S 0 interference using Zn 2+ , only involving the addition of Zn 2+ before alkaline digestion without extra operation. Experimental results indicate that at pH > 12.5, S 0 begins to produce a large amount polysulfides (S n 2- ) during the alkaline digestion, which in turn reduces Cr(Ⅵ). At the same pH, Zn 2+ predominantly exists as aqueous Zn(OH) 4 2- and instantly eliminates the S 0 -generated S n 2- by the formation of ZnS precipitates. Experiments also indicate that the addition of Zn 2+ exerts no significant influence on the method detection limits of the USEPA Method 3060a & Method 7196a. • Element sulfur produce more S n 2- when pH > 12.5 during alkaline digestion. • Zn 2+ can maintain a sufficiently high solubility during alkaline digestion. • Zn 2+ can more effectively eliminate the influence of S 0 -generated S n 2- than Cu 2+ • The adsorption of soil significantly increase the demand of Zn 2+ to be added. • The addition of Zn 2+ has no effect on the method detection limit.