Silicon (Si) modification of biochars from different Si-bearing precursors improves cadmium remediation

Abstract

Silicon (Si) is the main inorganic component in biochar, but its key role in pollutant removal and the corresponding mechanisms are still unclear. In this study, rice husk and wood biochars were modified with calcium silicate (CaSiO3) by ball milling method and used to investigate the removal efficiency of cadmium (Cd) from water and the removal mechanisms underlying the role of dissolved Si in biochar. The modification of biochar resulted in significant decrease in specific surface area, but increase in oxygen-containing functional groups (i.e., Si-O, Si-C). The Cd(II) removal efficiency was improved significantly by the modified wood biochar (MSiWB) from 23 % to 57 %, but only slightly (albeit from a higher baseline level) by the modified rice husk biochar (MSiHB) from 95 % to 98 %. It’s the first time to quantitatively verify a significant (p < 0.05) positive relationship between the dissolved Si in biochar and Cd(II) removal capacity, proving the crucial role of Si in Cd(II) removal via complexation with Si-O-Si groups or cation exchange with silicate minerals. Quantitative analysis of elements and particle size distribution of biochars before and after adsorption showed that the cation exchange of Ca2+ and physical attachment of Cd compound precipitation dominated Cd(II) removal by wood-derived biochar, while the carbon-Si coupled structure of rice husk derived biochar plays an important role in the chemical adsorption of Cd(II). This study provides an important reference for understanding the role of Si in biochar to improve Cd remediation.