Cement and zeolite stabilization/solidification of heavy metal-contaminated sediments: 841-Day leaching characteristics, mechanisms, and microstructure

The long-term effectiveness of stabilized/solidified sediments (S/S sediments) is increasingly attracting attention. This study investigated the long-term leaching characteristics and mechanisms of S/S sediment through an 841-day tank leaching test, considering the influence of cement content, curing time, and zeolite. The results indicate significant correlations among pH, heavy metals, TN, NH₃-N, and COD. The specimens with 6 % cement cured for 30 days (C6(30)) demonstrated considerable heavy metal stabilization, with stabilization rates for Cr, Ni, Cu, Zn, As, and Pb reaching 99.81 %, 99.06 %, 98.93 %, 99.61 %, 97.58 %, and 99.97 %, respectively. Compared to C6(30), partial replacement of cement with 10 % zeolite (C5 + Z0.5(30)) not only more effectively stabilized heavy metals except As, but also reduced the release of COD and NH₃-N by 4.23 % and 10.04 %, respectively. However, there was a risk of TN, NH₃-N, and COD exceeding permissible limits during long-term leaching. Microscopic analysis results suggested that hydration products and low porosity contributed to stabilization of heavy metals. Leaching mechanisms was revealed that surface wash-off controls the leaching of Cr and Pb, while diffusion controls the leaching of Ni, Cu, Zn, As, COD, TN, and NH₃-N. Considering stabilization performance, cost and carbon emissions, C5 + Z0.5(30) is an effective strategy for reducing long-term environmental risks of S/S sediments.