Sorption behavior of three aromatic acids (benzoic acid, 1-naphthoic acid and 9-anthroic acid) on biochar: Cosolvent effect in different liquid phases

Influence of the cosolvent on the sorption of organic acids on biochar has not been well understood. For this purpose, the sorption (log K_m, L kg⁻¹) of three aromatic acids (benzoic acid (BA, pK_a = 4.20), 1-naphthoic acid (1-NAPA, pK_a = 3.70), and 9-anthroic acid (9-ANTA, pK_a = 3.65) was evaluated as a function of methanol volume fraction (f_c = 0.0, 0.25, and 0.5), liquid pH (2.5 and 7.0), ionic composition (CaCl₂ and KCl) and ionic strength (0.005 M, 0.5 M, and 1 M CaCl₂). A giant Miscanthus-derived biochar (ZPC of 2.86) was used as the sorbent. For all solutes, the sorption coefficients (log K_m) measured at pH 2.5 (i.e., pH < pK_a) tended to decrease with increasing f_c, as expected from the cosolvency model, while the result obtained at pH 7.0 was not fully explained by the same model. The log K_m of 1-NAPA in the CaCl₂ system was always greater than in the KCl system (p < 0.05) and the impact became pronounced at high pH (>pK_a) with increasing f_c. Increasing the Ca²⁺ concentration at f_c = 0.0 (from 0.005 M to 1 M) enhanced the value by 0.32 log unit of K_m. These phenomena indicate a significant role of dissolved Ca²⁺ in the liquid phase, most likely due to the formation of cation bridges between aromatic carboxylates and the biochar surface (i.e., [R-COO^--Ca²⁺]-{Biochar⁻}). A decrease in the dielectric constant of the methanol mixture could fortify the formation of this bridge. Regardless of the degree of cosolvency power (σ), as the number of aromatic rings of solutes increases, K_m decreases in the order BA > 1-NAPA > 9-ANTA, where f_c = 0.0. In conclusion, the sorption potential of biochar can be significantly weakened by increasing pH and f_c, and in the absence of a divalent cation.