Competitive adsorption of naphthalene and phenanthrene on walnut shell based activated carbon and the verification via theoretical calculation†
Walnut shell based activated carbon (WAC) was prepared via microwave-assisted KOH activation. The adsorption behaviors towards naphthalene (NAP) and phenanthrene (PHE) over WAC were studied, both in single- and binary-compound systems. Characterization results reveal the excellent microporous structure of WAC, with a micropore specific surface area of 438.5 m2 g−1. The functional groups of walnut shell precursor surface were activated through microwave irradiation. In both systems, the pseudo-second-order model can better describe the adsorption kinetic data of PAHs over WAC at all experimental conditions. Mass transfer mechanism analysis shows that film diffusion was the rate-limiting step during the adsorption process. The adsorption amount of PAHs on WAC decreased as pH values increased, and the equilibrium data can be fitted by the Freundlich isotherm model well. In binary-component systems, the presence of PHE prominently restrained the adsorption towards NAP, and the Sheindorf–Rebhun–Sheintuch (SRS) model can fully fit the adsorption equilibrium experimental values of PAHs over WAC. In addition, the preferential adsorption behavior of PHE over WAC also was confirmed by theoretical calculations. The π–π complex between the active sites on the WAC surface and π-electrons of benzene rings from PAHs may play a major role in competitive adsorption. These results indicated that WAC was a potentially low-cost adsorbent for PAH elimination.