The effect of the zero charge point and surface functionality on the kinetics and adsorption capacity of picloram on carbon sorbents of various origins

Abstract

This paper presents the results of a comparative study of the adsorption of the persistent organochlorine herbicide picloram (4-amino-3,5,6-trichloropyridine-2-carboxylic acid) on a series of carbon sorbents with different surface prop-erties. The aim of the work was to establish the relationships between acid-base characteris-tics, textural parameters of sorbents and their adsorption activity, as well as to determine the dominant sorption mechanisms. Four carbon materials were used as research objects: activated carbon from coconut shell (AUCO), mesoporous sorbent (MNUM-2), activated carbon from macadamia shell (MO800-60) and macroporous Sibunite. Their specific surface area (SBET from 420 to 1150 m2/g), porous structure, zero charge point (pHpzc from 4.80 to 8.50), and surface chemical composition were determined by low-temperature nitrogen adsorption, potentiometric titration, FTIR spectroscopy, and X-ray microprobe anal-ysis. The kinetics and equilibrium of picloram adsorption were studied under static conditions at initial con-centrations of 20–200 mg/l. It was found that the time to reach adsorption equilibrium (te) has an inverse correlation with the pHpzc value of the sorbent. For the main sorbent MNUM-2 (pHpzc = 8.50), the te was 8 hours, while for the acidic AUCO (pHpzc = 4.80) it was 200 hours. This indicates the dominant role of elec-trostatic interactions at the kinetic stage of the process. The equilibrium adsorption capacity (qe) does not correlate directly with pHpzc or SBET. The maximum capacity (207 mg/g at C0 =100 mg/l) was shown by AUCO, which is explained by the predominance of specific interactions (hydrogen bonds and π-π-stacking) between its acidic surface groups and functional fragments of the picloram molecule. For MNUM-2, the high capacity is due to the synergy of favorable electrostatics and a developed surface. It is concluded that the surface functionality, assessed through pHpzc, is a key factor controlling the kinetics of adsorption, while the equilibrium capacity is determined by the synergy of textural parameters and the ability of the surface to specific interactions. The results allow us to propose criteria for the targeted selection of carbon adsorbents: materials with high pHpzc for rapid removal of pollutants, and microporous sorbents with an acidic surface for deep cleaning.