Synergistic adsorption mechanism of heavy metals on zinc oxide/phosphate modified gel–biochar composites

In this study, waste catering oil was processed into a gel (NJ), sesame straw was processed into biochar (ZM), and the two were modified with zinc oxide nanoparticles and phosphoric acid to form NJZ and ZMP, respectively. Both materials were then coupled to obtain a polymer gel-biochar composite (N-Z). The materials were characterised by infrared spectroscopy and scanning electron microscopy, and the adsorption kinetics and isothermal adsorption characteristics of copper(II) ions (Cu²⁺) in water were analysed. The actual adsorption effect was tested by a soil column leaching experiment. The results showed that the modification and coupling processes strengthened the adsorption properties of the adsorbent materials. The Cu²⁺ adsorption processes of the five materials (NJ, NJZ, ZM, ZMP and N-Z) were consistent with the quasi-second-order adsorption kinetic model. The maximum Cu²⁺ adsorption capacities of NJ, NJZ, ZM, ZMP and N-Z were 6.38, 9.04, 25.41, 119.42 and 47.82 mg g^-1, respectively. Furthermore, a comparison of the parameters of Langmuir and Freundlich model equations revealed that the adsorption process was homogeneous and involved both single and double layers, which is more consistent with the Langmuir equation. The adsorption efficiency (% removal) of NJZ towards cadmium(II) ions leached from soil reached 90.67%, while that of N-Z reached approximately 50%. The lead(II) ion adsorption efficiencies of the five adsorbents were low. The adsorption efficiencies of ZMP and N-Z towards Cu²⁺ leached from soil were over 90%, which demonstrates their good application prospects.