Co-hydrothermal carbonization of waste biomass and phosphate rock: promoted carbon sequestration and enhanced phosphorus bioavailability

Abstract Co-hydrothermal carbonization (co-HTC) of phosphorus rock (PR) and corn straw (CS) was investigated to prepare hydrochar-based materials as soil conditioners, focusing on the morphological transformation and solid–liquid migration of carbon and phosphorus. Various analytical methods, including elemental analysis, chemical quantification, FT-IR, XRD, 3D-EEM, TG, and XANES, were used to understand the synergistic interactions of PR and CS during co-HTC and determine the properties of the resultant products. The results indicated the acidic solution and humic acid-like substances produced by HTC of CS reduced the crystallinity of the PR and served as the activating agent for PR, allowing the PR to be easily dissolved and reconstituted, producing calcium carbonate and apatite-like materials, and the formation of C–O–PO3, C–PO3, C=O, and O=C–O chemical bonds. At 220 °C, adding 5% PR significantly promoted a 10.3% rise in the yield of CS hydrochar, a 4.3% rise in carbon recovery of CS, and a 4.8% rise in carbon sequestration potential of CS. The formation of Ca–P was notably promoted and the content of AP in co-HTC hydrochar was up to 89.9%, with 39% Hydro-P and 33% CaHPO4. In the case of artificial humic acid (HAa), its content was also remarkably increased by 5.9% in the hydrochar by co-HTC. In addition, the hydrochar produced by co-HTC of CS and PR was composed of carbon with an increased aromatic degree, rich organic matter, and biologically effective mineral nutrient elements and exhibited high stability. The present study provided a promising approach for value-added utilization of waste biomass and low-grade PR towards soil application. Graphical Abstract