Abstract
Rainfall makes impacts on the process of solidification/stabilization (S/S) and the long-term safety of solidified matrix. In this study, the effect of rainfall on solidification/stabilization process was investigated by the rainfall test. The unconfined compressive strength (UCS) and toxicity characteristic leaching procedure (TCLP) were adopted to characterize the properties of S/S sediments before and after the rainfall test. The samples cured for 28 days were selected for semi-dynamic leaching tests with a simulated acidic leachant prepared at pH of 2.0, 4.0 and 7.0. The effectiveness of S/S treatment was evaluated by diffusion coefficient (De) and leachability index (LX). The results indicated that UCS decreased at maximum deterioration rate of 34.23% after 7 days of curing, along with the minimum rate of 7.98% after 28 days by rainfall, with greater than 14 days referred. The rainfall had little effect on the leaching characteristics of heavy metals during the curing process. However, the simulated acid rain made significant impacts on the leaching behaviours of the heavy metals in the S/S materials. All the values of cumulative fraction of leached heavy metals were less than 2.0%, exhibition of good stabilization of cement. Furthermore, the calculated diffusion coefficient (De) for Cu was 1.28 × 101 cm2 s–1, indicating its low mobility of heavy metal ions in S/S sediments. Furthermore, the calculated diffusion coefficients (Di) for Cd, Cu and Pb were 7.44 × 10−11, 8.18 × 10−12 and 7.85 × 10−12 cm2 s–1, respectively, indicating their relatively low mobility of heavy metal in S/S sediments.
Highlights
The contamination of river sediments, especially heavy metals, poses a serious threat to the environment in China [1,2]
After S/S treatment, the heavy metals are stored in the sediments matrix as metal-hydrated phases, calcium–metal compounds and metal hydroxides [4]
The production of carbonation has a positive impact on the mechanical strength of the solidified sediments if using the cement-based S/S [8,9], while acid rain has a adverse impact on the long-term safety of solidified heavy metal contaminated sediments [10]
Summary
The contamination of river sediments, especially heavy metals, poses a serious threat to the environment in China [1,2]. The production of carbonation has a positive impact on the mechanical strength of the solidified sediments if using the cement-based S/S [8,9], while acid rain has a adverse impact on the long-term safety of solidified heavy metal contaminated sediments [10]. If cementstabilized sediments are exposed to the acid rain, the hydrogen ions (H+) react with the cement hydration products spontaneously, such as portlandite (Ca(OH)2) and calcium silicate hydrate (C–S–H) in the sediments matrix [13] This corrosion has a negative effect on the solidified sediments structure [14]. Xue et al [17] conducted a 2-year semi-dynamic leaching test to investigate the leaching behaviour of lead in S/S waste These studies suggest that diffusion is mostly ‘controlling leaching mechanism’ of heavy metals contained in S/S materials. The leaching data for selected heavy metals (Cu, Cd and Pb) were elaborated with a diffusional leaching model based on the penetration theory, which fitted the release mechanisms of these heavy metals and predicted the long-term leaching behaviours of cement-based solidified sediments
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