Impact of deashing treatment on biochar physicochemical properties and sorption mechanisms of naphthalene and 1-naphthol

Abstract

To elucidate the mechanisms controlling mineral effects of biochars on sorption of organic pollutants (OPs), sorption kinetics and isotherms of naphthalene and 1-naphthol on both pristine and deashed rice straw-derived biochars obtained at 200, 400 and 600 ºC were investigated. The organic carbon-normalized distribution coefficients (Koc) and equilibrium sorption capacity (Qe) of naphthalene and 1-naphthol by biochars generally increased after deashing. This was principally because mineral removal caused less exposure of polar groups, especially the O-alkyl components, on biochar surfaces and enhancement of hydrophobic sorption domains, thus mainly facilitating hydrophobic interactions between the compounds and aromatic components within biochars relative to the alkyl carbon components. The removal of minerals in biochars, which reduced the surface polar groups, promoted diffusion of naphthalene molecules toward the surface hydrophobic domains, thereby increasing its sorption rate constant for the fast-sorbing fraction on biochars (kfast). In contrast, the kfast values of 1-naphthol decreased after deashing, which could be ascribed to the weakened fast sorption via H-bond between 1-naphthol and mineral O-containing surfaces or polar groups on biochar surfaces. Both compounds exhibited lower rate constants for the slowly sorbing fraction (kslow) on deashed biochars, which had higher aromaticity and larger porosity and surface area than the raw biochars and inhibited the diffusion of sorbates into condensed organic moieties and pores. Results of this work are critical for better understanding on the impact of minerals, bulk/surface polarity, and sorption domain arrangement of biochars on their efficacy for OPs removal and predicting the fate and risk of OPs.