Molecular selective fractionation of biochar-derived DOM in FeCr co-precipitation and post-adsorption systems: Its roles in carbon sequestration and Cr(III) immobilization

Abstract

FeCr co-precipitation is one of the most important pathways for Cr(III) immobilization, which is inevitably influenced by the coexisting dissolved organic matter (DOM) in aqueous solutions. However, the role of the molecular composition of DOM in carbon sequestration and Cr(III) immobilization remains poorly understood. This study highlights the role of biochar-derived DOM in Fe/Cr co-precipitation and post-adsorption, demonstrating that both the molar C/Fe ratio and DOM type significantly influenced organic carbon (OC) sequestration and Cr(III) immobilization in these processes. At a C/Fe ratio below 4, particularly at a ratio of 2, the Cr(III) removal efficiencies in systems containing hydrochar DOM (DOM_HC) and pyrochar DOM (DOM_PC) were 34.9 % and 41.5 % higher than those without DOM. However, when the C/Fe ratio exceeded 2, iron (oxy)hydroxides became unstable due to charge neutralization, resulting in reduced Cr(III) removal efficiency. The differences in lignin and polysaccharide content between DOM_HC and DOM_PC led to variations in OC sequestration between co-precipitation and post-adsorption. Compared with post-adsorption, co-precipitation resulted in higher OC content (1.5 to 1.7 times) and retained specific DOM components such as lignin, tannins, and polysaccharides. Two-dimensional Fourier-transform infrared correlation spectroscopy (2D-FTIR-COS) revealed that polysaccharides co-precipitated with Fe/Cr faster than aromatic compounds, while aromatics dominated in the post-adsorption. The neutralization of acidic groups in DOM during the co-precipitation process enhanced the stability of Fe/Cr-DOM complexes, leading to greater Cr(III) immobilization in the form of Cr(OH)₃-DOM. These findings provide valuable insights for environmental remediation strategies and enhance the understanding of carbon cycling dynamics in contaminated aquatic systems.