Production of hydrochar by low-temperature hydrothermal carbonization of residual biomass from cocoa production for mercury adsorption in acidic aqueous solutions

Abstract

Hydrothermal carbonization (HTC) of waste biomass is a promising alternative to produce efficient and environmentally friendly sorbents. Lowering the temperature and selecting the right biomass can provide economic, environmental and technical advantages, improving process efficiency and product yield. In this work, the properties of hydrocarbon (HC) produced from residual biomass from cocoa (Theobroma cacao L.) production were evaluated using HTC at a temperature of 130 °C and a reaction time of 4 h. The effect of HC was also evaluated. The effect of HC on mercury (Hg) adsorption, conductivity and pH of acidic aqueous samples (pH 3 to 4) was also evaluated. The obtained hydrocarbon was characterized by Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM), which confirmed the presence of the main functional group's characteristic of hydrochar and the formation of porous structures. The results show that high Hg removal rates ranging from 75.5 to 89.4 % with adsorption capacities up to 174.74 μg g−1 with a short residence time (30 min) were achieved with the produced HC. Under the operating conditions established during the HTC process, the yield of HC produced with low reaction severity reached 88.5 %. The characteristics of the starting biomass and the obtained HC slightly modified the pH of the solutions and decreased the electrical conductivity. A comparative analysis was performed and showed no significant differences between the adsorbent masses used for removal. It is possible to produce HC efficient in Hg removal at very low actual concentrations (μg L−1), from cocoa production residues with HTC processes at lower temperatures than those generally employed (180 °C–250 °C) and in short reaction times. These operating conditions could become a promising alternative to address the problem of Hg water contamination without the need for energy intensive processes with technology accessible to communities and regions with limited resources.

Keywords

• Theorbroma cacao L
• Heavy metals
• Agricultural waste
• Water treatment
• Waste valorization