Sorption of ionic and nonionic organic solutes onto giant Miscanthus-derived biochar from methanol-water mixtures

The sorption of naphthalene (NAP) and 1-naphthoic acid (1-NAPA) onto giant Miscanthus–derived biochar was investigated in methanol volume fractions (f_c) of 0–0.6 as a function of ionic composition (5 mM CaCl₂ and 10 mM KCl) and liquid pH (2 and 7). The sorption onto biochar was nonlinear with 0.42 ≤ N ≤ 0.95; thus, a concentration-specific sorption constant (K_m) was compared. The K_m log linearly decreased with increasing f_c, except for 1-NAPA from a CaCl₂ mixture at pH 7. Isotherm data was fitted with a cosolvency sorption model through which the slope (ασ) of the inverse log linear K_m-f_c plot and empirical constant (α) were obtained. NAP sorption was well described by the cosolvency model with the α value being 0.41–0.53, indicating a methanol–biochar interaction favoring more sorption than the cosolvency based prediction. In particular, the slope (ασ) of 1-NAPA was lower than that of NAP, indicating less reduction of 1-NAPA sorption (i.e., lower α value) by methanol. In comparison with other sorbents, the α value was approximately intermediate between a humic substance and kaolinite clay. An analysis of FT-IR spectra suggested the transformation of O-containing functional groups by methanol, which will subsequently boost the π–π interaction between an organic solute and biochar. Moreover, Ca^{2 +}-induced sorption between anionic 1-NAPA and a negatively charged biochar surface was also fortified in the methanol mixture. The results revealed unexplored cosolvent effects on organic solute sorption onto biochar and identified the hydrophobic and hydrophilic sorption moieties of biochar as affected by the cosolvent.