Preparation of an avocado seed hydrochar and its application as heavy metal adsorbent: Properties and advanced statistical physics modeling

•Avocado seed hydrochar was prepared for heavy metal adsorption.•An heterogeneous monolayer model was used to interpret heavy metal adsorption.•Carboxylic and phenolic functionalities from hydrochar surface governed the adsorption process.•Adsorption mechanism of heavy metals on hydrochar was analyzed and explained. The preparation, characterization and adsorption properties of an avocado seed hydrochar for heavy metal removal are reported in this paper. Equilibrium adsorption of Ni2+, Pb2+ and Cu2+ ions on this hydrochar was experimentally quantified at pH 5 and 298–313 K and results were used to interpret the role of surface functionalities via statistical physics calculations. A heterogeneous monolayer model that assumed the presence of both phenolic and carboxylic functional groups on hydrochar surface was utilized to estimate the steric and energy parameters of the heavy metal adsorption mechanism. Overall, the maximum heavy metal adsorption capacities of this hydrochar ranged from 0.12 to 0.35 mmol/g at tested operating conditions where the highest adsorption was obtained for Ni2+. Statistical physics calculations indicated a multi-ionic adsorption process of these heavy metals where the carboxylic functional groups played a relevant role despite they represented only ~37% of total acidic functional groups available on the hydrochar surface. In fact, the adsorption capacities of carboxylic groups outperformed those of phenolic functional groups where this behavior could be associated to polar character and dissociation degree of these oxygenated surface functionalities. Calculated adsorption energies indicated an endothermic adsorption of these metals where electrostatic interactions and van der Waals forces were involved. This paper contributes with new findings to characterize and understand the adsorption of heavy metal ions on surfaces of adsorbents prepared from hydrothermal carbonization of lignocellulosic biomasses.