Carbon-ZnO Composite Synthesized from ZIF-8 Depositing Vegetable Biomass for Efficient Removal of Phosphate from Aqueous Solution

In order to combine the advantage of biochar and zeolitic imidazolate frameworks (ZIF-8), the carbon-ZnO composite were prepared by calcination the hybrid of two vegetable biomasses (rape, Ra, Brassia campestris L. and Chinese cabbage, Cc, Brassica rapa pekinensis) and ZIF-8. The characterization results showed Cc-ZnO and Ra-ZnO had the random, irregular and hollow structure. The adsorptive removal of carbon-ZnO composite for aqueous phosphate was performed using batch equilibrium method. Both Cc-ZnO and Ra-ZnO showed higher adsorption capacities than the pristine biochars and ZIF-8, and Ra-ZnO can remove phosphate quickly within 60 min. The adsorption kinetic data were consistent to the pseudo-second-order equation and the isothermal data followed the Langmuir and Freundlich models. The main interactions of phosphate adsorption by Cc-ZnO and Ra-ZnO were electrostatic attraction and surface complexation. The above results indicated the carbon-ZnO composite could be used as a promising adsorbent to treat wastewater containing phosphate.