Phosphorus Recovery from P-Enriched Solution by Biochar Activated with Chloride Salts

Phosphorus (P) is a vital nutrient for plants, being involved in the biosynthesis of many metabolites involved in the functioning of plant metabolism. Despite its natural abundance, it can be considered a non-renewable resource since its predicted availability seems no longer than 300 years. Treated wastewater (TWW) appears to be a viable P source, and adsorption methods, including those involving biochar, offer cost-effective solutions for P recovering. The aim of this study was to evaluate the adsorption ability of biochar, obtained at 440 and 880 °C and activated with chloride salt solutions (CaCl2, AlCl3, FeCl3) at two different concentrations (0.5 and 2.0M), in adsorbing P from aqueous solution. The results indicate that biochar activation increased P adsorption, particularly for biochar obtained at 880 °C and treated with 2.0M solutions of AlCl3 and FeCl3, showing adsorption rates of up to 10 mg P g−1. This higher adsorption capacity is attributed to the higher amount of metals adsorbed on the surface of biochar B880 compared to B440, which was influenced by the higher cation exchange capacity of the former. In addition, the adsorption of P was significantly influenced by the pH of the solution in contact with the biochar and its point zero charge. The study showed that the P adsorption capacity of biochar is higher at lower pH levels, where the positively charged surface of the biochar facilitates electrostatic attraction with anionic P species. These results highlight the potential of metal-activated biochar in recovering P from aqueous solutions in a circular economy perspective.