A combination of conventional extraction and advanced analytical techniques afford a comprehensive understanding of phosphorus distribution and transformation in sewage sludge biochars

Abstract

Pyrolysis of sewage sludge (SS) to SS biochars (SBCs) is a promising method for maximizing the utilization of phosphorus (P) in SS. A comprehensive understanding of P distribution and transformation in SS and SBCs is fundamental for the recycling of P from these materials as a soil amendment. Most studies have focused on P itself in SS and SBCs, without examining the changes of closely related metals (e.g., Al, Ca, Fe and Mg) that are typically co-extracted with P. Thus, in this study, a series of investigations were performed in which an improved Hedley sequential extraction method of P was used together with advanced analytical techniques to determine the distribution and transformation of P in SS and SBCs. Increasing the pyrolysis temperature increased concentrations of total P and inorganic P in SS and SBCs. In addition, P and Al species in SS and SBCs were predominantly sodium hydroxide-soluble species—denoted NaOH-Al and NaOH-P—and increasing the pyrolysis temperature increased their concentrations in SBCs, especially for NaOH-Al, from 28.35 ± 2.56 mg/g to 38.18 ± 3.29 mg/g. Ca, Mg and Fe were predominantly extracted by HCl solution, showing that HCl-extracted P comprised not only Ca-P but also Fe-P, Mg-P and Al-P. Spectroscopic analyses revealed that the concentration of hydroxyapatite (HAp) in SS and SBCs increased with the pyrolysis temperature, and Al-P was the dominant species in both SS and SBCs. Thus the pyrolysis of SS affords SBCs that are a relatively stable pool of P for plants, enabling SBCs to serve as a slow-release P fertilizer in soil. These findings further contribute to fundamental knowledge on the recycling of P during integrated waste management.