The conversion and migration behavior of phosphorus speciation during pyrolysis of different sludges

The study was enforced to probe the conversion and migration behavior of phosphorus speciation in sludge and the biochar received from pyrolysis of municipal sludge (MS), town sludge (TS), and slaughterhouse sludge (SS). This study creatively used fractionation of soil phosphorus to further differentiate speciation of phosphorus in three sludges (MS, TS, and SS). According to the x-ray diffraction (XRD) analysis and sequential extraction, the study proved the dependence of P speciation conversion on pyrolysis temperature and different types of raw sludge. The results of P-fractionation indicated that Ca-bound IP (Ca-IP) content in all biochars significantly increased at pyrolysis temperature of 350–800 ℃, and the proportion of soluble and loosely bound IP (SL-IP), aluminum-bound IP (Al-IP), and Fe-bound IP (Fe-IP) of MS and SS decreased. The difference is that the Al-IP in the TS increased slightly as the pyrolysis temperature increased. Among the three kinds of sludge, the Olsen-P of TS is the lowest because the content of Olsen-P in sludge will decrease with the decrease of pH in the process of sewage treatment after acidification. In addition, XRD patterns of three sludges and biochar further confirmed the low crystallinity of AlPO₄ minerals. Through in-depth research on the environmental behavior of phosphorus, this study might additionally provide essential knowledge for the recovery and utilization of phosphorus in sludge.