Use of Mg/Ca modified biochars to take up phosphorus from acid-extract of incinerated sewage sludge ash (ISSA) for fertilizer application

Recovery of phosphorus (P) from incinerated sewage sludge ash (ISSA) by biochar is an attractive solution for mitigating the P scarcity and transferring waste to resources. This work used Mg/Ca-modified biochars to take up P from the acid-extract from ISSA at low pH (<2), which simplified the previous P recycling process. The hypothesis is to produce a P-enriched post-sorption biochar that can be directly applied as a P fertilizer. Wastes of peanut shell and sugarcane bagasse were used to synthesize Mg/Ca-modified biochars at pyrolysis temperatures of 450 °C, 700 °C and 850 °C. Preliminary results indicated Mg-modified sugarcane bagasse biochar pyrolysed at 700 °C produced optimal P-absorption. This biochar was positively charged and had a high specific surface area (1440 m²/g), consistent with a layered porous structure. The optimal biochar showed rapid adsorption of P which could be described by the pseudo-second-order model. Successful adsorption of P from the acid-extract by the optimal biochar was mainly due to chemical precipitation and its adsorption capacity is 129.79 mg P/g biochar.