Nutrient Recovery as Struvite from Hydrothermal Carbonization and Wet Oxidation Aqueous-Phase Byproducts

Abstract

Abstract Hydrothermal carbonization (HTC) is a promising pathway to convert wet wastes into energy-rich products. To achieve scalability, further valorization of the aqueous-phase (AP) byproduct is required. For example, HTC AP contains organics that necessitate treatment as well as valuable inorganics for struvite production (MgNH4PO4·6H2O, a slow-release fertilizer). However, direct struvite crystallization from HTC AP suffers from organic and inorganic impurities (specifically, Ca) co-precipitation. To selectively improve the quality of struvite precipitate, this study evaluated wet oxidation (WO) pretreatment for organics degradation before crystallization. Two experimental approaches were evaluated: (1) direct crystallization from HTC AP and (2) crystallization from WO-treated HTC AP (WO AP). The second approach was selected to aim at improved struvite product quality by mitigation of C-organics and other inorganic co-precipitation. HTC was conducted at 190 °C for 30 min with a biomass-to-water ratio of 1:10, while WO was performed at 200 °C and 30 min under 650 psi of O2. Crystallization experiments were performed at pH 9 with mixing at 200 rpm. Phosphorus recovery from HTC AP reached ∼90%, with ∼40% magnesium recovery, whereas WO AP achieved >90% recovery of both phosphorus and magnesium. XRD analysis showed the HTC AP precipitate consists of ∼88% struvite and ∼12% sodium- and carbonate-substituted hydroxyapatite crystalline phases, while the WO AP precipitate was predominantly a struvite crystalline phase. Further, elemental analysis revealed ∼5 wt % carbon and significant inorganic impurities (Ca, Na, and K) in HTC AP precipitates, compared to only ∼1 wt % carbon and minimal impurities in WO AP precipitates. Chemical and functional group characterization further confirmed that WO pretreatment effectively mitigated organic and inorganic co-precipitation, resulting in a higher-purity struvite product.