Unravelling heterogenous adsorption performance of hydrochar particle and key properties in heavy metal immobilization relative to corresponding residual bulk hydrochar

Abstract

Immobilization performance of released hydrochar particle (HP) to metal is largely unknown and hinder accurate estimation to immobilization effect in hydrochar application. Herein, HPs derived from typical biomass, food waste (FW), and blue algae (BA), were collected to investigate their adsorption abilities, mechanisms towards typical Cd(II) relative to residual bulk hydrochar (BH) and unravel corresponding properties of this heterogeneity. The results were: 1) the micro-sized HP with poor porous structure, less O-containing functional groups (OFGs) and negative surface charge induced inferior adsorption capacities (0.65-0.81 times) but quicker adsorption rate (3.09-10.08 times) towards Cd(II) compared with BH, 2) cation bridge interaction in coagulation and settling processes unexpectedly facilitated adsorption performance of micro-sized and negative charged FWHPs added with counterions, 3) HP facilitated its electrostatic attraction with Cd(II) rather than common pore filling and surface complexation, especially bare aromatic structure in HP formed another cation-π interaction with Cd(II) highlighted by density functional theory (DFT) method. The special coagulation process and cation-π interaction of HP will facilitate its vertical co-migration ability but weaken immobilization stability towards metals. The distinct heterogeneities of adsorption ability of HP and BH and underlying key properties provide an in-depth understanding of hydrochar application for remediation of metal contaminants in soil/water environment.