Thermophysical and molecular mechanisms of a novel and cost-efficient Cr-contaminated soil stabilization by steam flash heating

[Display omitted] •Innovative steam flash heating for effective Cr(VI) reduction and immobilization.•SFH improved the molecular contact and mass transfer to accelerate Cr(VI) reduction.•First report using soil organic matter to reduce Cr(VI) with the aid of steam flash heating.•Steam flash heating triggered MnOx reduction to maintain long-term Cr stability. Chemical stabilization is a frequently used method to stabilize heavy metals in soils, however suffering from the reagent overdosage and exorbitant cost due to the insufficient reactions between chemical stabilizers and heavy metals. In this work, we innovated an economy and energy efficient stabilization technique, namely steam flash heating (SFH), and investigated its stabilization performance and mechanisms for hexavalent chromium [Cr(VI)]. SFH can efficiently promote Cr(VI) reduction by both soil organic matter and chemical stabilizers. From thermophysical analysis, SFH efficiently enhanced the molecular contact and accelerated mass transfer in porous soils, resulting in the promoted transformation of Cr(VI) into OM-Cr(III), Fh-Cr(III), and Cr(OH)3 from Cr/Fe/S/C XANES results. By replacing partial chemical stabilizers with soil organic matter in Cr(VI) reduction, SFH decreased nearly 70.7% of cost and 38.1% of CO2 emission. Besides, Mn XANES results unraveled that MnOx was reduced by soil organic matter and chemical stabilizers with the aid of SFH, and this process decreased the risk of Cr(III) re-oxidation and benefited long-term Cr stability. These results proved that SFH could efficiently immobilize Cr with low energy consumption, low economic cost, high stabilizer efficiency, and low re-mobility risk, exhibiting applicable potential in the remediation of Cr-contaminated soils. This study revolutionized chemical stabilization by SFH and unraveled the underlying mechanisms of Cr immobilization in contaminated soils.