Biochar derived from shell waste: Adsorption of phosphorus from aqueous solutions and potential application as slow-release fertilizer

The utilization of waste biomass to develop green and efficient phosphorus adsorbents is of significant importance for the recycling of phosphorus in eutrophic water bodies. In this study, a novel shell-modified rice straw biochar (SRSB) was fabricated via a facile one-step pyrolysis of invasive Pomacea canaliculata shells and rice straw. Systematic optimization of the feedstock mass ratio yielded a composite with significantly enhanced phosphate adsorption capacity. The research results indicated that when the mass ratio of Pomacea canaliculata shells to rice straw was 3:1 and the mixture was pyrolyzed at 800°C (labeled as SRSB3:1), the biochar exhibited excellent phosphorus adsorption performance over a broad pH range, with a maximum phosphorus adsorption capacity of 251.87 mg/g. The thermodynamic and kinetic studies revealed that the adsorption of phosphorus by SRSB3:1 was a spontaneous monolayer chemical adsorption process. Moreover, characterization suggested that SRSB3:1 reacted with phosphorus through a Ca-P chemical reaction, forming Ca 5 (PO 4 ) 3 OH. Additionally, seed germination and plant pot experiments demonstrated that phosphorus-saturated SRSB3:1 (labeled as SRSB3:1-P) effectively promoted seed germination and growth. This study presents a triple-win strategy for the harmless utilization of invasive species, efficient phosphorus recovery from wastewater, and the production of value-added biochar-based fertilizers, contributing to sustainable waste valorization and eutrophication mitigation.