Volatile fatty acid release and metal ion concentration in hydrothermal carbonization liquid

Hydrothermal carbonisation (HTC) is an efficient technology for converting waste into carbon materials along with a significant by-product known as hydrothermal liquid (HL). This study examined the organic loads, nutrient-rich components, volatile fatty acid (VFA) release, and metal concentrations of HLs obtained under various reaction conditions and material formulations. Structural equation models (SEM) elucidated the relationship between the acidification system and metal concentrations in HL. The chemical oxygen demand (COD) in the HL varied from 4220 mg/L to 51,480 mg/L under different HTC conditions, increasing with reaction temperature, and the pH ranged from 3.5 to 5.6. Acetic acid was the predominant species in the VFAs of HL, the amount of which tended to decrease and then increase with increasing temperature. The metal ion concentrations in HL were K > Mg > Ca > Fe > Zn > Cu. The SEM indicated that COD and pH were the main factors influencing metal ion concentrations in HL. VFAs negatively affected the pH (P < 0.01), indirectly promoting metal ion dissolution, and positive interactions were observed among the metal ions. The concentration of VFAs in HL influences the pH and thus plays an important role in promoting the release of metals.