Corn waste valorization to generate activated hydrochar to recover ammonium nitrogen from compost leachate by hydrothermal assisted pretreatment

Valorization of corn waste involves synthesis of hydrochar for use as a sorbent for ammonium nitrogen recovery from swine manure compost leachate. However, the inability to directly capture organic nitrogen and insoluble nitrogen, and the low sorption ability of hydrochar remain key issues. To overcome these issues, we used hydrothermal assisted pretreatment of compost leachate to promote the solubilization of nitrogen contained in feces, and the conversion of organic nitrogen into ammonium nitrogen in the compost leachate. The synthesis hydrochar was activated with KOH to enhance its sorption ability, and then characterized by SEM, FTIR, elemental analysis, specific surface area, pore volume and size analysis. The content of ammonium nitrogen in hydrolysis leachate at 210 °C increased by 22.3% compared with raw compost leachate. 3D-EEM analysis indicated that aromatic protein substances were rapidly hydrolyzed to gradually increase the content of ammonium nitrogen and produce considerable humic acids. The maximum adsorption capacity of ammonium nitrogen reached 140.3 mg/g at 45 °C for KOH-activated hydrochar of 260 °C. The Langmuir isotherm and pseudo second order kinetic models were good fit for the adsorption process of ammonium nitrogen at higher temperature (35 °C or 45 °C), and this reaction was mainly dominated by chemisorption. The adsorption of ammonium nitrogen was exothermic, spontaneous, and showed an increase in disorder at the solid–liquid interface. For resource recovery, the total release amount of ammonium nitrogen of five interval extractions could reach 12.2% of maximum adsorption capacity (140.3 mg/g) under alkaline (pH 8.0) condition. The nitrogen mass balance calculation revealed that 8.9% of total nitrogen in the compost leachate could be recovered.