[Adsorption Performance of Walnut Green Husk Biochar for Heavy Metals]

Abstract

The biochars of WP300, WP500, and WP700 were prepared by pyrolyzing walnut green husk under 300℃, 500℃, and 700℃ with the oxygen-free condition for removing Pb²⁺, Cu²⁺, and Cd²⁺ in an aqueous solution. The results revealed that WP500 prepared under the medium pyrolysis temperature achieved the best adsorption performance for heavy metals, and the highest removal efficiency was reached when the solution pH was 8, in which the removal efficiency of Pb²⁺, Cu²⁺, and Cd²⁺ were 97.87%, 99.78%, and 71.15%, respectively. The required biochar dosage for heavy metal removal varied under different adsorption conditions. In the single-metal system, the optimal dosage for WP500 in the Pb²⁺, Cu²⁺, and Cd²⁺ solutions was 1.3 g·L^-1, 2.1 g·L^-1, and 1.9 g·L^-1, respectively, whereas in the pollution metals system, the optimal biochar dosage was 5.1 g·L^-1. In addition, the adsorption capacity of WP500 for the three heavy metals followed the order of Pb²⁺>Cu²⁺>Cd²⁺ under the single and combined-metals system, indicating that there were no synergistic or antagonistic effects among these three adsorbates. The fitting results of the adsorption isotherm model suggested that various immobilization methods existed in adsorption process between WP500 and Pb²⁺, Cu²⁺, and Cd²⁺. The kinetic fitting results suggested that the main reaction between WP500 and Pb²⁺, Cu²⁺, and Cd²⁺ was chemical adsorption. The mechanisms of WP500 for heavy metals involved pore-filling, electrostatic attraction, ion-exchange, mineral precipitation, complexation, and π-π electron donor-accepter interaction. To conclude, this study offered a new insight for the resource utilization of the waste walnut green husk.

Keywords: adsorption isotherms; adsorption kinetics; cadmium; combined pollution system; copper; lead; walnut green husk biochar.