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

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; 0.5M NaHCO₃ pH 8.5; 0.1M NaOH, 1M HCl) was applied for P speciation over 56days of composting of either agro-industrial or urban wastewater sludge with green waste treated (AIC_Fe+; SSC_Fe+) or not (AIC_Fe-; SSC_Fe-) with FeSO₄ (2%v/v). Composting strongly reduced the H₂O-P, promoting the organic-P (P_o) mineralization from the labile fraction (H₂O+NaHCO₃ 40%), in addition to the increases of NaHCO₃- and HCl-extractable inorganic-P (P_i) in both AIC_Fe- and SSC_Fe- (+20% on average). The FeSO₄ treatment did not negatively affect the process, reducing the P_o 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 AIC_Fe-: NaOH (41)=NaHCO₃ (38)>HCl (18)>H₂O (3); in AIC_Fe+: NaOH (53)>NaHCO₃ (24)=HCl (22)>H₂O (2); in SSC_Fe-: NaOH (46)>NaHCO₃ (29)>HCl (21)>H₂O (4) and in SSC_Fe+: NaOH (66)>NaHCO₃ (13)>HCl (20)>H₂O (1). Composting reduced the more easily leachable fraction (labile-P_o), 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.