Effect of iron sulphate on the phosphorus speciation from agro-industrial sludge based and sewage sludge based compost

Composting is considered a suitable process for organic waste management, providing stable products that can be safely utilized as fertilizers, but little is still known about the variation of phosphorous (P) extractability during the stabilization process. In this work, sequential chemical extraction (SCE) with increasing strength extractants (H2O; 0.5 M NaHCO3 pH 8.5; 0.1 M NaOH, 1 M HCl) was applied for P speciation over 56 days of composting of either agro-industrial or urban wastewater sludge with green waste treated (AICFe+; SSCFe+) or not (AICFe−; SSCFe−) with FeSO4 (2% v/v). Composting strongly reduced the H2O-P, promoting the organic-P (Po) mineralization from the labile fraction (H2O + NaHCO3 40%), in addition to the increases of NaHCO3- and HCl-extractable inorganic-P (Pi) in both AICFe− and SSCFe− (+20% on average). The FeSO4 treatment did not negatively affect the process, reducing the Po mineralization during composting by increasing the NaOH-P, also protecting this fraction from fixation in the sparingly soluble fraction. The final P fractionation (%) was in AICFe−: NaOH (41) = NaHCO3 (38) > HCl (18) > H2O (3); in AICFe+: NaOH (53) > NaHCO3 (24) = HCl (22) > H2O (2); in SSCFe−: NaOH (46) > NaHCO3 (29) > HCl (21) > H2O (4) and in SSCFe+: NaOH (66) > NaHCO3 (13) > HCl (20) > H2O (1). Composting reduced the more easily leachable fraction (labile-Po), reducing the risk of P loss by increasing the long-term available P fraction (NaOH-P). This was enhanced by the FeSO4 addition. Further investigation into soil behaviour and plant availability of P from this source is needed.