Benign zinc oxide betaine-modified biochar nanocomposites for phosphate removal from aqueous solutions

Phosphate is one of the most costly and complex environmental pollutants that leads to eutrophication, which decreases water quality and access to clean water. Among different adsorbents, biochar is one of the promising adsorbents for phosphate removal as well as heavy metal removal from an aqueous solution. In this study, biochar was impregnated with nano zinc oxide in the presence of glycine betaine. The Zinc Oxide Betaine-Modified Biochar Nanocomposites (ZnOBBNC) proved to be an excellent adsorbent for the removal of phosphate, exhibiting a maximum adsorption capacity of phosphate (265.5 mg. g^-1) and fast adsorption kinetics (~100% removal at 15 min at 10 mg. L^-1 phosphate and 3 g. L^-1 nanocomposite dosage) in phosphate solution. The synthesis of these benign ZnOBBNC involves a process that is eco-friendly and economically feasible. From material characterization, we found that the ZnOBBNC has ~20-30 nm particle size, high surface area (100.01 m². g^-1), microporous (25.79 Å) structures, and 7.64% zinc content. The influence of pH (2-10), coexisting anions (Cl^-, CO₃^2-, NO₃^- and SO₄^3-), initial phosphate concentration (10-500 mg. L^-1), and ZnOBBNC dosage (0.5-5 g. L^-1) were investigated in batch experiments. From the adsorption isotherms data, the adsorption of phosphate using ZnOBBNC followed Langmuir isotherm (R² = 0.9616), confirming the mono-layered adsorption mechanism. The kinetic studies showed that the phosphate adsorption using ZnOBBNC followed the pseudo-second-order model (R² = 1.0000), confirming the chemisorption adsorption mechanism with inner-sphere complexion. Our results demonstrated ZnOBBNC as a suitable, competitive candidate for phosphate removal from both mock lab-prepared and real field-collected wastewater samples when compared to commercial nanocomposites.