Mechanisms of alachlor and pentachlorobenzene adsorption on biochar and hydrochar originating from Miscanthus giganteus and sugar beet shreds

This work investigates the adsorption mechanism of alachlor and pentachlorobenzene on chars (hydrochars and biochars) originated from Miscanthus giganteus and sugar beet shreds. Two different processes were used: hydrothermal conversion at three temperatures (180, 200, and 220 °C) and slow pyrolysis at 400 °C. Specific surface area (SSA) for all investigated chars ranged 3.87–260 m2/g, whereby biochars had a higher SSA. For hydrochars from sugar beet shreds, both the SSA (3.87–5.53 m2/g) and pore volume (0.023–0.0277 cm3/g) increased with increasing temperature, while the opposite trend was observed for hydrochars from miscanthus. The opposite trend could be a consequence of the different chemical composition of the used feedstock such as hemicelluloses and cellulose which can produce volatile organic products at higher temperatures and may cause the reduction of SSA. All adsorption isotherms were well described by the Freundlich model. The nonlinearity of the isotherms ranged from 0.450 to 0.986. Kd values for both investigated compounds followed the order: hydrochars from sugar beet shred < hydrochars from miscanthus < biochars, implying that the SSA of the chars affects the adsorption mechanism. In general, all the investigated adsorbents demonstrated higher adsorption affinity for pentachlorobenzene in comparison with alachlor, implying that hydrophobic interactions enhanced the adsorption of the more hydrophobic organic compound. In addition, for all hydrochars, adsorption affinities for the smaller pentachlorobenzene are significantly greater than for the larger alachlor, probably due to its ability to better penetrate into the pores of the adsorbents. This type of research is necessary to obtain safe adsorbents for water remediation