Char derived from food waste based solid digestate for phosphate adsorption

Abstract

Recovery of phosphorus from waste streams is desirable as natural reserve of phosphorus is being depleted. This study explores the potential of phosphate adsorption using chars produced from anaerobically digested food waste. Two types of feedstocks, dried solid digestate (DSD) and pelletized solid digestate (PSD) are used to produce chars via pyrolysis between 300 and 900 °C. The maximum P sorption capacity of DSD char (100.7 mg-P/g-char) was more superior than that of PSD char (66.0 mg-P/g-char). The X-ray powder diffraction (XRD) result suggests that P was mainly captured by Ca in the chars via the formation of hydroxyapatite. The P speciation and exterior surfaces in chars before and after P exposure (P-laden chars) are quantitatively characterized using 31P solid-state nuclear magnetic resonance spectroscopy and transmission electron microscope equipped with energy dispersive X-ray spectroscopy. Orthophosphate was found to be dominant in both digestate chars and P-laden chars, and the P capture role of nano-sized CaO formed from digestate pyrolysis at 700 °C was observed. The results suggest that chars produced from food waste digestate can reclaim exogenous phosphate. In addition, the P-laden chars rich in hydroxyapatite can be applied as a slow-release P fertilizer or soil remediation agents.