Binding characteristics of sulfonated biochar-DOM with minerals: Molecular fractionation and its facilitation to Cu(II) immobilization.

Biochar-derived dissolved organic matter (DOM) can interact with soil minerals, influencing the transport and immobilization of heavy metals. Sulfonated biochar (SBC), enriched with oxygen- and sulfur-containing functional groups, releases DOM (SBC-DOM) with a strong metal-binding capacity. This study investigated the fractionation behavior of SBC-DOM at mineral interfaces and its regulatory mechanism in Cu(II) immobilization, using ferrihydrite (Fh) and kaolinite (Kl) as representative minerals and Cu(II) as the target pollutant. The results showed that the adsorption capacity of Fh for SBC-DOM (5.35 mg C·g-1) was significantly higher than that of Kl (3.48 mg C·g-1). Due to its hydroxyl-rich and positively charged surface, Fh preferentially adsorbed high O/C (oxygen-rich) and high DBE (high aromaticity) fractions via ligand exchange, electrostatic attraction, and hydrogen bonding. In contrast, Kl selectively adsorbed CHON and hydrophobic low-O/C compounds through hydrophobic and electrostatic interactions. Mineral-driven fractionation markedly altered the composition of residual DOM and its complexation behavior with Cu(II). After fractionation by Fh, the Cu(II) complexation ability of protein-like components in DOMF was reduced, thereby lowering the risk of DOM-facilitated metal transport. Conversely, fractionation by Kl enhanced the Cu(II) complexation ability of both protein-like and humic-like components in DOMK. Furthermore, mineral-organic associations (MOAs) formed by SBC-DOM modification significantly improved Cu(II) immobilization, with the adsorption capacity of MOA-Fh increasing by 115.0%, whereas that of MOA-Kl increased by only 27.7%. This study provides molecular-level insights into interactions and fractionation within the sulfur-enriched, highly polar SBC-DOM-mineral-Cu(II) ternary system, offering a theoretical basis for applying sulfonated biochar in soil heavy metal remediation.