Engineered hydrochar production methodologies, key factors influencing agriculture wastewater treatment, and life cycle analysis: A critical review

For intensive food production, a range of chemical compounds are used to increase production, reduce the amount of weeds, and prevent pest infestation. Therefore, agricultural wastewater discharge to water bodies creates human health and environmental risks. This highlights the need for technologies to remove organic and inorganic pollutants, where adsorption using carbon-based materials has emerged as a cost-effective and promising alternative for agricultural wastewater treatment with high removal efficacy and alignment with the circular economy concept by generating value-added products, achieving energy conservation and reducing the environmental footprint. Among the different adsorbent materials, hydrochar (HC) has attracted attention because, compared to the thermal processes used for synthesizing other carbon-based materials, it requires relatively milder production conditions and possesses higher adsorption capability for water pollutants. Although HC holds advantages for the adsorption of water pollutants, HC modification using different methods has been found to improve the properties, including adsorption capacity. Accordingly, engineered hydrochar (EHC) has attracted research attention. However, past research publications show that the investigations have focused on material characterization and removal rates, with little knowledge created of the environmental impacts of HC production, application, and disposal. This study reviews current knowledge on EHC synthesis, characteristics, water treatment applications, and life cycle analysis. Initially, production methodologies were investigated to understand their influence on key surface physical and chemical characteristics. This was followed by assessing EHC efficacy for water and wastewater treatment. Finally, the environmental footprint of EHC production, application, and disposal was evaluated to identify critical knowledge gaps and to provide recommendations for future research.