Enhanced transformation of phosphorus (P) in sewage sludge to hydroxyapatite via hydrothermal carbonization and calcium-based additive

Abstract

Phosphorus (P) recovery from sewage sludge is one of the promising substitutes to the phosphate ores to alleviate the shortage of P supply in the future. In this study, how hydrothermal carbonization (HTC) temperature functioned in the migration and transformation of P from sewage sludge during HTC process was clarified. The enhancement effect of Ca-based additive on the transformation of P to hydroxyapatite via HTC was systematically studied. Additionally, leaching characteristics of P in the hydrochars with Ca-based additive were analyzed to assess their P-bioavailability. Results indicated that favoring by increased pH in the process water, HTC temperature played a significant role in the migration and transformation of P during HTC. Therefore, higher HTC temperature was beneficial for P enrichment and transformation to apatite phosphorus in the hydrochars. Both alkaline environment and existence of sufficient calcium ions were essential for enhanced formation of apatite phosphorus during HTC. Adding CaO could stimulate almost complete transformation of non-apatite inorganic phosphorus to apatite phosphorus such as hydroxyapatite, causing a maximum increase of apatite phosphorus by 252%. Concurrently, P-bioavailability by 2% (w/w) formic acid extraction of the hydrochars increased by 233% at CaO = 4%. These findings were confirmed by XRD analyses. Ca-associated apatite phosphorus such as Ca5(PO4)3OH and Ca2P2O7 ∙ 2H2O, was detected in the hydrochars. With the assistance of thermochemical calculation, transformation pathways of P after CaO addition have been proposed, which coincided with our experimental results well. The stable P-containing minerals of hydroxyapatite (Ca5(PO4)3OH) was calculated at equilibrium state.