The solidification effect of contaminated soil degrades under wet-dry (W-D) cycles and acid rain. Acidic dry-wet cycle tests for Cr-contaminated soil solidified by alkali-activated granulated blast furnace slag (GGBS) are carried out. Toxic leaching test and accelerated leaching test are performed to study the leaching characteristic and mechanism. Scanning electron microscopy and energy spectrum analysis are used to investigate the microscopic mechanism. The long-term stability is evaluated through the apparent diffusion coefficient. The results show that a few W-D cycles at pH=7 will cause additional hydraulic reaction of GGBS and thus reduce the leaching concentration of total Cr and Cr(VI). Along with W-D cycles more AFt is generated. The expansion of AFt results in micro-fracture and thus more Cr leaching. In acidic W-D cycles, AFt dissolves first, releasing Cr immobilized by ion exchange. With the increasing acidity, C-S-H gels dissolve and more gypsum is generated, resulting in more micro-fractures. Consequently, the encapsulation effect weakens, resulting in more Cr leaching. However, the C-A-S-H gels remain stable. The slopes of the logarithmic curves of cumulative leached fraction versus time range from 0.373 to 0.675. The errors of fitting by a pure-diffusion analytical solution are mainly below 0.5%, indicating that diffusion is the dominant leaching mechanism. However, after 18 W-D cycles at pH=3, the effect of dissolution increases and the diffusion-dominated criteria are not satisfied. The mobility of Cr under neutral, weak acidic, and strong acidic W-D cycles is low, moderate, and high, respectively. It is necessary to take measures to reduce acid rain infiltration and W-D cycles when utilizing solidified soil. This research provides a reference for evaluating the long-term stability of solidified contaminated soil.

Wet-dry (W-D) cycles will cause the release of toxic Cr(VI) from stabilized/solidified soils, posing a threat to the surroundings. In this research, Cr(VI)-contaminated soil treated by alkali-activated granulated blast furnace slag (GGBS) was studied. Through accelerated leaching tests and Cr spatial distribution analysis, the effects of W-D cycle and rainfall pH on Cr mobility in the solidified soil were investigated. The mechanisms of Cr release were studied by chemical form analysis and a serious of micro analyses. The results showed that the leaching characteristics varied with W-D cycles and pH. Under a neutral pH, the cumulative leaching concentration of Cr decreased after 3 W-D cycles and then increased, and the leaching mechanism transitioned from surface wash-off to dissolution. Under acidic environments, the leaching concentration increased by 1-2 orders of magnitude along with the decomposition of hydration products. During the W-D cycles, Cr(VI) content in the solidified soil decreased from top to bottom, reflecting the top-down process of degradation, release, and migration. The W-D cycles had no obvious influence on valence state of Cr. After acidic W-D cycles, the fraction of soluble state Cr(VI) and adsorbed Cr(VI) increased significantly. The CrO4-jarosite phase exhibited good chemical stability in acidic conditions, indicating that chemical substitution is an important immobilization mechanism of Cr(VI). The microscopic morphology analysis showed that W-D cycle caused irreversible damage to the microstructure, and that in an acidic condition C-A-S-H gels were more stable than Aft and C-S-H gels. Along with the W-D cycles, the proportion of macropores increased and the interlayer/gel pores became larger, which significantly facilitated the release of Cr(VI).