Evaluation of a ZnCl2-modified biochar derived from activated sludge biomass for adsorption of sulfamethoxazole

Sulfamethoxazole (SMX) was effectively removed from aqueous solution via adsorption to a chemically modified biochar created using a simple and rapid microwave (MW) pyrolysis of activated municipal wastewater sludge. Response surface methodology (RSM) was used to maximize SMX adsorption capacity optimizing three independent synthesis parameters including ZnCl2 concentration (0.5–1.5 mol/L), MW time (3–10 min), and MW power (400–1000 W). ZnCl2 modified biochar (ZnBC) synthesized under the optimized conditions (ZnCl2 concentration = 1.5 mol/L, MW time = 8 min, and MW power = 915 W) was analyzed by different characterization techniques including Brunauer Emmett and Teller (BET) surface area analysis, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Raman spectra, X-ray photoelectron spectroscopy (XPS), and point of zero charge (pHpzc). ZnCl2 impregnation coupled with a homogenous MW heating profile led to improved accessibility of surface functional groups with a specific surface area of 214 m2/g and robust porous structure with an average pore volume of 0.127 cm3/g. Adsorption experiments using the optimized ZnBC resulted in a 89% SMX removal efficiency and maximum SMX adsorption capacity of 50.6 mg/g. Isotherm and kinetic analysis suggested multilayer chemical adsorption of SMX over a heterogeneous biochar surface followed by heterogenous pore diffusion into the ZnBC pores. Results showed that acidic pH favors SMX removal through a spontaneous and exothermic process. The application of modified activated sludge-based biochar introduces a new sustainable method for treating pharmaceuticals and recycling waste sludges from wastewater treatment plants.