Enhanced Cd2+ Adsorption by Na2SeO3-Modified Grapevine Pruning Waste-Derived Biochar (Se-GBC): Synthesis, Characterization, and Mechanisms

Abstract

The utilization rate of grapevine pruning waste for resource recovery is low, with most material being disposed of through incineration, creating an urgent need for high-value transformation. To develop an adsorbent with high Cd²⁺ adsorption performance, three biochar samples derived from grapevine pruning waste (GBC) were prepared at relatively low pyrolysis temperatures (300, 400 and 500℃) and modified via a facile impregnation method using trace Na₂SeO₃ at varying dosages. The loaded selenium exists predominantly as surface Se-O functional groups, with minor CaSe and SeO₂ crystalline phases detected by XRD. The modification slightly reduced the specific surface area but significantly enlarged the average pore diameter, creating a macroporous structure that facilitates Cd²⁺ mass transfer. The enrichment of oxygen-containing functional groups (OFGs) arises from selenium-containing species introduced during impregnation, which promote the retention and formation of surface oxygen groups. The results indicate that Cd²⁺ adsorption onto GBCs follows a monolayer adsorption model involving both physical and chemical processes. Among the samples, GBC pyrolyzed at 400 °C, and modified at a Se:GBC mass ratio of 1:1500 (Se-GBC400) exhibited the highest efficiency, achieving a maximum adsorption capacity of 24.52 mg/g under the conditions of initial Cd²⁺ concentration of 50 mg/L, adsorbent dosage of 0.1 g/40 mL, pH 6.0, and 25 °C, showing a 61.03% increase in adsorption capacity compared to the unmodified biochar. The loading of Na₂SeO₃ increased the proportion of OFGs such as C-O, C = O, and -COOH on the GBCs surface, thereby enhancing complexation and ion exchange with Cd²⁺, promoting the formation of insoluble precipitates and effectively immobilizing Cd²⁺. These findings provide a scientific basis for the resource utilization of grapevine pruning waste in heavy metal remediation and clarify the adsorption mechanism of Se-GBC400 for Cd²⁺.