Enhancing phosphate adsorption by Mg/Al layered double hydroxide functionalized biochar with different Mg/Al ratios

•Mg/Al-LDHs modified biochar with Mg/Al ratio 2, 3 and 4 were prepared.•Modification significantly improved biochar's phosphate adsorption performance.•Higher Mg/Al ratio increased LDHs interlayer space and biochar phosphate adsorption.•Adsorption maximum of 4:1 Mg/Al-LDHs biochar was 81.83 mg P/g at pH 3.•Inorganic anions decreased phosphate adsorption in order of F− > SO42 − > NO2− >Cl−.

Mg/Al ratio plays a significant role for anion adsorption by Mg/Al-layered double hydroxides (Mg/Al-LDHs) modified biochar. In this study, Mg/Al-LDHs biochar with different Mg/Al ratios (2, 3, 4) were prepared by co-precipitation for phosphate removal from aqueous solution. Factors on phosphate adsorption including Mg/Al ratio, pH, and the presence of other inorganic anions were investigated through batch experiments. Increasing Mg/Al ratio in the Mg/Al-LDHs biochar composites generally enhanced phosphate adsorption with Langmuir adsorption maximum calculated at 81.83 mg phosphorous (P) per gram of 4:1 Mg/Al-LDHs biochar at pH 3.0. The adsorption process was best described by the pseudo-second-order kinetic model. Solution pH had greater effects on the phosphate adsorption by Mg/Al LDHs biochar composites with lower Mg/Al ratios. The presence of other inorganic anions decreased the phosphate adsorption efficiency in the order of F− > SO42 − > NO2− >Cl−. Phosphate adsorption mechanism involves ion exchange, electrostatic attraction and surface inner-sphere complex formation. Overall, Mg/Al-LDHs biochar composites offer a potential alternative of carbon-based adsorbent for phosphate removal from aqueous solution.

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