Removing DOM from chloride modified hydrochar could improve Cu adsorption capacity from aqueous solution.

The behavior and composition of hydrochar-based dissolved organic matter (DOM) would affect the efficiency of copper (Cu) removal from wastewater through adsorption. In this study, the reed was hydrolyzed in the presence of feedwater with and without ZnCl, FeCl, and SnCl to produce pristine hydrochars (PHCs), which were named HO-HC, ZnCl-HC, FeCl-HC, and SnCl-HC. After removal of DOM, washed hydrochars (WHCs) were obtained, labelled as W-HO-HC, W-ZnCl-HC, W-FeCl-HC, and W-SnCl-HC. The release dynamics of DOM from PHCs were analyzed, and the adsorption behaviors of Cu on both PHCs and WHCs were investigated. The results showed that chloride-modifications were beneficial for the porosity, specific surface area (SSA), and functional groups of WHCs. Meanwhile, the quantity of hydrochar-based DOM was significantly affected by chloride-modifications. In particular, the relative contents of Ar-P and Fa-L in the DOM released from hydrochars varied with time and modification. Furthermore, the Qe of Cu adsorption on WHCs followed the order of W-SnCl-HC > W-FeCl-HC > W-ZnCl-HC > W-HO-HC at 15 °C. Compared to PHCs, the adsorption capacity of Cu on WHCs was improved by 7.15-119.77% at the temperature of 35 °C. Simultaneously, the adsorption capacity of Cu in WHCs showed a significant correlation with the SSA via physical adsorption (P < 0.05). Moreover, XPS analysis revealed that Cu adsorption also occurred via complexation and chelation through newly formed Cu-O group between W-SnCl-HC and Cu. Notably, the increase of Cu adsorption in WHCs was significantly correlated with the release of Fa-L and Ar-P from PHCs (P < 0.05). This study found that the content and composition of hydrochar-based DOM could be a major driving factor for Cu adsorption.