Co-pyrolysis of iron-rich sewage sludge and potassium phosphate to prepare biochars: P fractionation and alleviated occlusion

摘要

An iron-rich sewage sludge was employed to prepare nutrient-enhanced biochars through co-pyrolysis with K3PO4. Phosphorus availability was evaluated using the Hedley fractionation. In co-pyrolyzed biochars, the combined proportion of short- and medium-term P was -70 , with a short- to medium-term ratio of 1:3.4-4.3. The extrinsic P distributed in H2O-, NaOH- and HCl-P pools at proportions of -30 , -50 and -20 , respectively. The molar ratio of NaOH-P to Fe reached 1:1.7, suggesting a great potential of sludge Fe-oxides for P binding. The binding of extrinsic P was through the formation of iron hydroxyphosphate and adsorption mechanisms. By chemical extractions and instrumental analysis, this study illustrated an occluding effect for P extraction after pyrolysis, caused by the dehydration and crystallization of sludge Fe-oxides. The co-pyrolysis strategy effectively alleviated this effect due to the elimination of excessive Fe-oxides and improvement of P releasability. Consequently, it was superior in terms of the overall P availability. In addition, -80 of K was available within medium-term. Co-pyrolyis with phosphate exhibits a good prospect for the recycling of iron-rich sewage sludge.