Muti-scale analysis of solidification/stabilization (S/S) of Pb-contaminated dredged sediment using nano-SiO2 modified cement
Wei Zhang, Lei Lang, Zhen Qi, Yao-Yi Wang, Qiang Xue, Jiangshan Li
Abstract
The remediation of lead-contaminated dredged sediments (LDS) presents significant environmental challenges. This study investigates the solidification/stabilization (S/S) mechanisms of ordinary Portland cement (OPC) modified with nano-silica (NS) across a continuum from nanoscale interactions to macroscopic performance. For this, a series of macroscopic experiments was conducted to evaluate the mechanical performance and lead-encapsulation efficiency, including unconfined compressive strength (UCS) and toxicity characteristic leaching procedure (TCLP). Microstructural and phase transformations were characterized using X-ray diffraction, thermogravimetric analysis, and scanning electron microscope. Molecular dynamics simulations revealed the interactions between NS-modified cement, calcium silicate hydrates (C-S-H) gel, and Illite, focusing on interaction energies, atomic density distributions and structural changes. Macroscopic analyses demonstrated that increasing NS content from 0% to 8% improved Pb-immobilization rate from 88.7% to 97.6% and enhanced UCS from 764 kPa to 1358 kPa. These improvements were attributed to NS enhancing the microstructural integrity of C-S-H gel and filling pores in samples. Nanoscale simulations elucidated that Pb-stabilization occurs through coordination bonds with oxygen atoms in the C-S-H silicon chains and on Illite surfaces, complemented by the formation of stable Pb 3 (CO) 3 (OH) 2 precipitates. Additionally, the simulations revealed that Ca 2+ migration from hydration products to mineral surfaces generated substantial repulsive interaction energies, reducing Illite layer dispersion. However, the presence of Pb impeded further Ca 2+ migration, leading to expansion of the C-S-H gel, which collectively degraded the mechanical properties of the material. Furthermore, wet-dry and freeze-thaw cycles showed that after 10 cycles, UCS and TCLP results still met the United States Environmental Protection Agency standards, confirming long-term durability. This study provides a theoretical foundation for resource utilization of the contaminated sediments and offers a perspective for design of the cement-based curing agents, particularly in addressing variations in pollutant concentrations and environmental conditions, advancing the application of responsive and controlled release curing agents.