Geoenvironmental properties of a Cr(VI)-contaminated soil treated by alkali-activated GGBS under freeze-thaw cycles: Insights into Cr species transformation and microscopic mechanism

Abstract

Stabilization/solidification is the most frequently used method for treating soils contaminated by heavy metals; however, degradation of the treatment will occur under freeze-thaw (F-T) cycles. In this paper, a low-carbon emission by-product, ground granulated blast furnace slag (GGBS), was adopted as a binder to treat Cr(VI)-contaminated soil after alkali excitation. Built on the usage scenarios of subgrade materials, the impact of F-T cycles and initial water content on the geoenvironmental properties of the treated soils, including leaching toxicity, unconfined compressive strength (UCS), pH, Eh, and permeability, were discussed. To investigate the mechanisms of the changing properties, this study analyzed the chemical morphology of Cr, the micromorphology of the reaction products, and the pore characteristics. The results demonstrated that negative impact of F-T cycles on treatment effectiveness was low at the optimal water content. After 28 F-T cycles, the Cr(VI) component increased by 6.4 %, and the leached Cr concentration showed a significant increase, especially for specimens with low water content. A new solid phase with mixed valence Mn(III/IV), mainly composed of birnessite and manganite, was observed via microscopic analysis. During the first 3 F-T cycles, the content of hydration gel increased by 0.18 %, and the cumulative pore volume decreased such that the UCS increased by an average of 1.2 MPa. This study demonstrated that a few F-T cycles would result in a secondary alkali-activated GGBS reaction, enhancing the treatment effect. However, additional F-T cycles would create an oxidizing environment under which the initially precipitated Cr(III) would react with manganese oxide, resulting in more Cr(VI) released. The degree of reoxidation was closely related to the initial water content of the solidified soil.