Hydrochars production, characterization and application for wastewater treatment: A review

Hydrothermal carbonization (HTC) of wet solid wastes has been pointed out as an eco-friendly, flexible and highly efficient technology for the sustainable valorization of multiple sourced wastes. In this review paper, most recent studies on hydrochars (solid residue of the HTC process) production, characterization and application for wastewaters treatment was summarized and deeply discussed. The role of initial feedstock source nature and characteristics as well as the HTC experimental conditions including the temperature, the residence time and the pH media was assessed. Physical and chemical activation methods including the use of oxygen, steam, microwave, acids, alkaline, organics and salty solutions for the improvement of the physicochemical properties of the produced hydrochars are compared. The efficiency of these raw/modified hydrochars along with the involved mechanisms during organic (dyes) and mineral pollutants (heavy metals and nutrients) removal from aqueous solutions is also reviewed. Finally, this paper addresses the main challenges and also demonstrates insights on new directions for hydrochars research and development in the future. •Hydrochars showed a decrease in esters and hydroxyl groups compared to raw biomass.•Carbon content, minerals and polymeric structure influence hydrochars' adsorption capacity.•Solution temperature, pH and residence time impact hydrochar's efficiency.•Hydrochar's oxygen functional groups are reactive to heavy metals and organic molecules.•Post-activation improves significantly hydrochars' specific surface area.