Dynamics and mechanisms of atrazine adsorption on biogas-residue biochar with citric acid modification

Atrazine is a widely used high-risk herbicide. Biogas-residue biochar (BC) is an adsorption material with low cost and great potential; however, its use is limited due to its poor surface properties. An understanding on the adsorption dynamics and mechanisms of the chemical modification of atrazine on biogas-residue biochar is very limited. Here, we examined the adsorption characteristics of atrazine on citric acid-modified BC (CA-BC) by conducting batch adsorption experiments followed by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared (FT-IR), and X-ray photoelectron spectroscopy (XPS). The results showed that citric acid modification increased the surface area from 96.60 m2/g to 266.2 m2/g, decreased the pHzpc from 8.79 to 5.64, and effectively enhanced the adsorption capacity of CA-BC from 3.67 ± 0.31 mg g−1 to 5.94 ± 0.07 mg g−1. The adsorption kinetics and isotherm dynamics of atrazine on CA-BC were effectively fitted to the Elovich and Langmuir models, indicating that the adsorption process of CA-BC jointly involved physical and chemical adsorption. The dominant surface mineral of BC was whitlockite magnesian, which was dehydrogenated after modification and caused the development of its pore structure. The peaks corresponding to the oxygen-containing groups (hydroxyl and carboxyl groups) and cyclic structures increased after modification and decreased after adsorption. Based on these results, the adsorption mechanism of atrazine included microporous filling, electrostatic interactions and functional group interactions. Citric acid modification significantly enhanced the rate, maximal capacity, and stability of atrazine adsorption on biogas-residue biochar. Therefore, CA-BC could be considered an environmentally friendly material for controlling atrazine pollution. In general, our study provides a new approach for the high-value utilization of biogas residue and establishes a theoretical basis for improving the adsorption performance of atrazine using biogas residue biochar.