Stabilization of hydrophobic organic compounds in sediment using alkali-activated rice husk biochar: Enhancement of sorption by dissolved organic matter

Polycyclic aromatic hydrocarbons and phthalate esters are hydrophobic organic compounds (HOCs) that pose strong risks to aquatic and sediment ecosystems. The aim of this study was to develop a material for stabilizing (sorbing) HOCs in contaminated sediment. Activated biochar with a high surface area (called RH–NaOH) was successfully synthesized by activating rice husk biochar with an alkali (NaOH). The kinetics of HOC sorption to the RH–NaOH were described well by a three-stage intraparticle diffusion model. The sorption isotherms were adequately described by the Sips sorption model. The presence of dissolved organic matter (DOM) at environmentally relevant concentrations enhanced HOC sorption to the RH–NaOH by as much as a factor of 10, and this was mainly caused by the fulvic-acid- and humic-acid-like fractions of the DOM. The degree of sorption enhancement was predicted by the integrated volume of the fulvic-acid- and humic-acid-like fraction regions of the excitation–emission matrix that was used to characterize the DOM. The polycyclic aromatic hydrocarbon and phthalate ester stabilization efficiencies when 150 d was allowed for equilibration between the sediment and RH–NaOH to be reached were 96.5% and 94.6%, respectively. The stabilization tests indicated that HOC sorption proceeded via multiple stages with different kinetics. These stages were attributed to complex processes involving HOC and DOM sorption and desorption, HOC–DOM interactions, and HOC and HOC–DOM mixture diffusion into pores. The RH–NaOH has great potential for use as a sorbent for stabilizing HOCs in contaminated sediment.