Mechanistic insights into cadmium removal from environmental waters using silicon-enhanced lignin-derived hydrochar and pyrochar

Abstract

This study explores the improved cadmium (Cd2+) removal from environmental waters using silicon-modified hydrochar and silicon-modified pyrochar derived from industrial lignin. Despite the widespread use of lignin-based chars in environmental remediation, the specific role of silicon in enhancing these materials’ properties and their mechanisms for Cd2+ removal remain inadequately studied. Physicochemical analyses indicate that the introduction of silicon promotes the consumption of C–O bonds in hydrochar, facilitating the formation of Si–O bonds. In pyrochar, silicon introduction enhances the synthesis of silicate minerals. Results showed that silicon-modified pyrochar, in particular, exhibited a markedly higher Cd2+ adsorption capacity in environmental water, being 14.71, 16.83, and 1.87 times that of hydrochar, pyrochar, and silicon-modified hydrochar, respectively. Silicon-modified pyrochar also demonstrated excellent selective adsorption and reusability for Cd2+, showing optimal performance in actual water bodies. Notably, silicon-modified pyrochar maintained a high Cd2+ removal rate of up to 99 % after multiple cycles. Adsorption experiments revealed that the main mechanisms in lignin–based carbons are ion exchange and complexation precipitation, contributing about 70 % of the adsorption capacity. Specifically, silicon-modified hydrochar removes Cd2+ mainly through complexation precipitation with oxygen–containing groups (CO and SiO), while silicon-modified pyrochar relies mainly on mineral precipitation (Na2SiO3 and Na2CO3) and the ion exchange process. These findings underscore the transformative impact of silicon modification on lignin-derived chars, offering valuable insights for developing more effective and sustainable solutions for heavy metal remediation in aquatic environments.