Cadmium binding mechanisms and adsorption capacity by novel phosphorus/magnesium-engineered biochars

Abstract

Novel phosphorus/magnesium-engineered biochars were prepared from poultry litter and tested for their Cd2+ retention capacity, unraveling the adsorption mechanisms. Batch experiments were conducted to evaluate the adsorption ability of Cd2+ by biochars and a wide range of characterization techniques were used: scanning electron microscopy with energy-dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, surface area and pore volume, and X-ray photoelectron spectroscopy. Results showed that, in general, Cd2+ removal did not drastically change with initial pH variation and was relatively fast (up to 3 h); the pseudo-second-order kinetic model provided slightly better fitting to the data. Cadmium adsorption capacities of the P/Mg-engineered biochars were much higher than that of the unmodified biochar (up to 113.9 mg g−1), following the SIPS isotherm model. The surfaces of the biochars contain a rich variety of oxygen-containing functional groups as well as phosphate groups. Since the specific surface areas of the biochars are considered low (up to 25.19 m2 g−1), surface groups contributed more to Cd2+ retention. Biochars can be represented by type II isotherms with significant type H3 hysteresis patterns, which suggest the presence of asymmetrically slit-shaped pores. Complexation and precipitation were the predominant adsorption mechanisms. Thus, P/Mg-engineered biochars produced from poultry litter are considered effective and eco-friendly adsorbents for Cd2+ removal from aqueous medium, especially PLB-H3PO4-MgO, which is produced from low-cost materials.