Oyster Shell Modified Tobacco Straw Biochar: Efficient Phosphate Adsorption at Wide Range of pH Values

In order to improve the phosphate adsorption capacity of Ca-loaded biochar at a wide range of pH values, Ca (oyster shell) was loaded as Ca(OH)₂ on the tobacco stalk biochar (Ca-BC), which was prepared by high-temperature calcination, ultrasonic treatment, and stirring impregnation method. The phosphorus removal performance of Ca-BC adsorption was studied by batch adsorption experiments, and the mechanism of Ca-BC adsorption and phosphorus removal was investigated by SEM-EDS, FTIR, and XRD. The results showed that after high-temperature calcination, oyster shells became CaO, then converted into Ca(OH)₂ in the process of stirring impregnation and had activated the pore expansion effect of biochar. According to the Langmuir model, the adsorption capacity of Ca-BC for phosphate was 88.64 mg P/g, and the adsorption process followed pseudo-second-order kinetics. The Ca(OH)₂ on the surface of biochar under the initial pH acidic condition preferentially neutralizes with H⁺ acid-base in solution, so that Ca-BC chemically precipitates with phosphate under alkaline conditions, which increases the adsorption capacity by 3-15 times compared with other Ca-loaded biochar. Ca-BC phosphate removal rate of livestock wastewater (pig and cattle farms) is 91~95%, whereas pond and domestic wastewater can be quantitatively removed. This study provides an experimental basis for efficient phosphorus removal by Ca-modified biochar and suggesting possible applications in real wastewater.