Quantifying the immobilization mechanisms of heavy metals by Calcium Silicate Hydrate (C-S-H): The case of Cu2+

Understanding the interactions between calcium silicate hydrate (C-S-H) and heavy metals is vital for optimizing the use of solid waste in cementitious materials. This paper introduces an innovative approach to elucidate the immobilization mechanism of Cu²⁺, a representative heavy metal. The roles of Ca²⁺ and OH⁻ in Cu²⁺ immobilization were comprehensively investigated through being treated by C-S-H samples with varying interlayer Ca²⁺ contents via meticulous immersion in ammonium chloride (NH₄Cl). Our results indicate that Cu²⁺ immobilization primarily occurs via complete substitution of interlayer Ca²⁺ and partial substitution of intralayer Ca²⁺, with contributions of 40 % and 59 %, respectively, for a Ca/Si ratio of 1.2. Minor mechanisms like OH⁻ complexation and surface adsorption account for the remaining immobilization. The formation of Si-O-Cu complexes was confirmed by X-ray photoelectron (XPS) and Extended X-ray Absorption Fine Structure (EXAFS) analysis. This study provides a quantitative methodology that informs strategies for sustainable cementitious waste utilization.