Abstract
The increasing global generation of organic waste, including food waste (FW), emphasizes the pressing need for sustainable and efficient strategies for managing waste. Traditional disposal methods are increasingly becoming environmentally and economically unsustainable due to the volume of FW produced. Thermochemical recycling approaches such as gasification, pyrolysis, and incineration have been explored as potential valorisation techniques for FW. However, these methods are often limited by the relatively high moisture present in FW, which necessitates energy-intensive and costly pre-drying steps. In contrast, hydrothermal carbonisation (HTC) offers a promising alternative: it can process wet biomass without drying. HTC converts FW into hydrochar (HC), a carbon-rich solid material with numerous purposes. This study focuses on the valorisation of FW through HTC, aiming to produce hydrochar for potential environmental applications.
Industrial processes have led to the discharge of heavy metals (HMs), which are amongst the major contaminants in natural water systems. Because these metals are non-biodegradable, they accumulate over time, posing substantial risks to human health and ecosystems. Traditional means for treating HM-contaminated wastewater, such as flocculation, membrane filtration, oxidation, and chemical precipitation, are often constrained by the high operational cost as well as the production of toxic sludge that necessitates further treatment and disposal. As an eco-friendly and sustainable option, biomass-derived adsorbents have gained attention for their effectiveness in metal removal. Among them, HC, produced via HTC, stands out as a result of its porous structure, unique surface functional groups, and elevated carbon content, making it a really effictive for adsorbing HMs from water.
Butternut waste (BW) and potato peels (PP) were utilised as FW feedstocks for the HTC process. A central composite design (CCD) supported by response surface methodology (RSM) was used to examine the influence of four process parameters: temperature (140-300℃), residence time (22-248 minutes), solid-to-liquid (S/L) ratio (1:11-1:15), and butternut waste to potato peel (BW/PP) ratio on three responses: HC yield, calorific value (CV), and surface area (SA). Temperature, S/L ratio, and BW/PP ratio significantly influenced HC yield, while residence time had minimal effect. CV was primarily affected by temperature, with other variables showing limited impact. SA was influenced significantly by both temperature and BW/PP ratio. The RSM showed the optimum conditions to be 220℃ for 113 minutes with a 1:13 S/L ratio and 0.5:0.5 BW/PP ratio, resulting in HC yield, CV, and SA of 36%, 33.8MJ/kg,
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and 9.76 m2/g, respectively. Characterisation of selected HC samples revealed that increasing temperature and time led to lower atomic O:C and H:C ratios, indicating enhanced dehydration and decarboxylation. Higher temperatures disrupted the HC carbon structure, and prolonged reaction times led to greater fragmentation and collapse of the HC’s structural integrity.
The scope of this research entailed the use of HC prepared from FW for removing HMs (Pb2+, Cu2+, and Cr3+) from synthetic wastewater. The HC synthesised at the optimum conditions underwent chemical activation using KOH to improve its adsorption properties. Several techniques were used to characterise the activated HC. The results indicated an increased SA (859.04 m2/g) and high porosity. The activated hydrochar (AHC) exhibited an increase in carbon content (approximately 10-15%), a more stable carbon structure (lower H:C and O:C ratios), fewer volatile impurities, an enhanced abundance of O₂-containing functional groups, and smaller particle sizes (in a range of 40-50 μm) compared to the untreated HC. The adsorption efficiency was significantly influenced by operational parameters, which include adsorbent dosage, temperature, contact time, solution pH, and initial HM concentration. The study demonstrated that AHC derived from FW is an effective adsorbent for removing HMs in single-metal and multi-metal systems. Adsorption in single-metal systems obeyed the Freundlich adorption isotherm (AdI) model, signifying a multilayer adsorption on a heterogeneous surface, whereas multi-metal systems adhered to the Langmuir AdI model, suggesting monolayer adsorption on a homogeneous surface. Higher adsorption capacities were observed in single-metal systems, likely due to the absence of competing ions. Kinetic modelling showed that both systems aligned with the pseudo-2nd-order (PSO) model, indicating chemisorption as the dominant mechanism. Furthermore, the thermodynamic analysis revealed that the adsorption process was spontaneous and endothermic, accompanied by increased disorder at the solid-liquid interface. The AHC exhibited the highest affinity for HMs in the order: Pb2+ > Cu2+ > Cr3+.