Adsorption characteristics and removal mechanism of malathion in water by high and low temperature calcium–modified water hyacinth–based biochar

Calcium–modified water hyacinth biochar (WHCBC–400 and WHCBC–600) was prepared using water hyacinth as raw material and calcium chloride as modifier at two pyrolysis temperatures of 400 °C and 600 °C. The effectiveness for removal of malathion, a typical organophosphorus pesticide from water were comparatively examined by the prepared adsorbents. The impacts of the pyrolysis temperature on performance of the biochar and adsorption behavior of malathion on the modified biochar were systematically studied by surface analysis, removal efficiency tests under different solution conditions, as well as data simulations. The adsorption mechanisms and reusability of the modified biochar were also investigated and experimented, respectively. The results indicated that the WHCBC–600 had a larger specific surface area and pore volume than the WHCBC–400, while WHCBC–400 had a greater polarity and hydrophilicity. Both of the two prepared biochars followed pseudo–second–order kinetics and their adsorption isotherms conformed to the Sips model. The maximum adsorption capacities of WHCBC–400 and WHCBC–600 were 128 mg/g and 32.7 mg/g, respectively. It was found that the adsorption of malathion by the biochars were dominated by chemisorption. Diffusion at the liquid film was the rate–limiting step in the adsorption process. The difference in the form of the calcium loadings, polarity of biochar, and the rate–limiting step of adsorption were important factors affecting the adsorption capacity of the biochar. In comparison, the specific surface area and pore volume of the prepared absorbents had less significant effects on the adsorption capacity. The main adsorption mechanism of the two modified biochars was surface complexation. In addition, the solution pH and coexisting material had little effect on the adsorption of malathion by WHCBC–400 and WHCBC–600. Both of the modified biochar had good regeneration performance. The study provided a new adsorbent for the removal of malathion from water.