Eco-friendly reuse of agricultural wastes to produce biocomposites with high potential in water treatment and fertilizers

Large quantities of orange peel wastes are generated and require adequate management and disposal into the environment. Agricultural waste valorization is a strong research topic, due to the increasing need for achieving a sustainable environment. Especially, the development of adsorbent materials applied to wastewater treatment for efficient phosphorus recovery and reuse systems is a challenge of current interest in research. In this work, a novel low-cost biocomposite was obtained by calcination of orange peel (OP) and eggshell (ES), which is highly effective for P recovery from synthetic aqueous solutions and domestic wastewater. After calcination at 700 °C, the material with a ratio by weight of OP/ES = 1 showed a high adsorption capacity (Q_max = 315.25 mg P/g) compared to other materials previously reported. The XRD and FTIR characterization results of the materials after the P removal showed that brushite (Ca(HPO₄)•2H₂O) is the main calcium phase obtained at pH 6, while apatite (Ca₅(PO₄)₃OH) is favored at a pH higher than 8. Moreover, the ESOP adsorbents showed a high P adsorption from wastewater, with 98.6% of P recovery as apatite. Additionally, Ca₅(PO₄)₃OH and Ca(HPO₄)•2H₂O phases exhibited a high solubility of P in 2% formic acid releasing up to 97.50 and 194.22 mg P/g. The results of this work indicate that calcined ESOP is a promising material for P recovery from aqueous solution, forming calcium phosphates with great potential to be used as alternative phosphate fertilizer, contributing to implementing the 3R slogan “Reduce, Reuse, Recycle” based on a circular economy.