Toluene-mercuric modified USEPA Method 3060A to eliminate interference of sulfide-based reductants with Cr(VI) determination

The validity of USEPA Method 3060A as universal Cr(VI) analysis method for remediated soil is controversial. We investigated soil Cr(VI) remediation performance by commonly used reductants (FeSO4, CaSx, Na2S) under different operating conditions (dosage, curing time and degree of mixing) using Method 3060A, and developed modified 3060A specific for sulfide-based reductants. Results showed that Cr(VI) was primarily removed during analysis stage rather than remediation stage. Thereinto, chemical dosage played a much more important role than curing time and degree of mixing. Besides, soil Cr(VI) concentration decreased to below the detection limit with residual reductant content increasing. Comparing standard and toluene-mercuric modified 3060A, Cr(VI) removal efficiency decreased from 100 % to 38.9–45.4 %, 67.1–68.8 % and 94.1–96.3 %, corresponding to mixing degree of 33 %, 67 % and 100 %, for treated soil using 1× and 2× the molar stoichiometric ratio of CaSx. Subsequently, the optimization mechanism was revealed. Elemental sulfur, remediation product of sulfide-based reductants, was removed from soil by toluene preventing its disproportionation to sulfide at Method 3060A stage. Sulfide was fixed by mercuric oxide in species of mercuric sulfide. This method also proved suitable for different types of soils. Therefore, an effective way for scientific evaluation of soil Cr(VI) remediation was provided in this study. [Display omitted] •Excess residual reductants caused soil Cr(VI) content below the detection limit.•Toluene-mercuric modified Method 3060A improved accuracy of soil Cr(VI) analysis.•Elemental sulfur was removed from the soil by toluene.•Sulfide was transformed to mercuric sulfide by mercuric oxide.