Phosphate adsorption by poultry manure biochar impregnated with different sources of magnesium

Rise in fertilizer prices and decline in fertilizer availability have prompted concerns about the sustainability and equity of fertilizer use. Utilizing P‑rich feedstocks for biochar production and further enriching them with P recovered from wastewater has the potential to transform conventional industrial P fertilizers. We explored different sources and levels of Mg as impregnation materials for poultry manure biochar production to optimize the retention of dissolved inorganic phosphate. Results demonstrated that using MgSO₄ and Mg(OH)₂ as Mg sources for producing Mg-impregnated biochars were more effective than MgCl₂, in terms of Mg yields and P retention capacity from solution. Although Mg impregnation increased the P sorption capacity, it can increase the Mg demand for P removal, by altered precipitation from struvite (NH₄MgPO₄∙6H₂O) to Mg₃(PO₄)₂. This study proposes an optimum Mg content of poultry litter biochar as ~10%w/w and a source as MgSO₄ and/or Mg(OH)₂, which was sufficient for >95% P recovery of a phosphate solution containing 50 mg P g L^-1. Maximum adsorption potential of Mg-doped poultry manure biochars was 110 mg P g^-1 being highest at 10% Mg, which indicates up to ~10% lower Mg needs or ~75 mg g^-1 greater sorption capacity than comparable approaches with minimal engineering process. Poultry manure exhibited the highest maximum P sorption capacity among other feedstocks (sawdust, pig manure, etc.) under given Mg contents, demonstrating its potential for P recovery with minimized engineering process.