Rapid adsorption of acetochlor and metolachlor from aqueous solution onto solid waste coal fly ash

Coal fly ash (FA) as the solid waste was applied to explore the adsorption of acetochlor and meto- lachlor (herbicides). The effects of initial pH, contact time, temperature, and dosage of FA were explored. The physical and chemical characteristics of FA were analyzed by scanning electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy. The adsorption kinetics of FA to acetochlor and metolachlor were most consistent with the pseudo-secondary model of other kinetic models, with R2 values as high as 0.999 and the equilibrium time were attained after 10 and 15 min, respectively. Additionally, the maximum removal efficiencies of acetochlor and metolachlor (10 mg/L) by FA were 91.25% and 73.65%, respectively. Langmuir isotherm (for acetochlor, R2 = 0.9980; for metolachlor, R2 = 0.9913) was considered to be the better matched one than Freundlich, Dubinin–Radushkevich, and Brunauer–Emmett–Teller. The values of the saturated adsorption quantity got for acetochlor and metolachlor on FA calculated by the Langmuir model were 42.12 and 19.38 mg/g, respectively. The values of the saturated adsorption quantity predicted by the three-dimensional prediction model based on the pseudo-secondary dynamics were 42.76 and 16.57 mg/g, respectively, which were consistent with the Langmuir model. Moreover, the results of this paper suggested that there were a coexistence with physical and chemical adsorption processes of these two herbicides and the physical adsorption was the primary adsorption process of meto- lachlor on FA, while chemisorption played a dominant role in that of acetochlor on FA. The main adsorption mechanism might be the hydrogen bond and ligand exchange based on complexation.