Hydrochar Nanocatalyst for Emerging Pollutant Remediation, Sustainable Development, and Circular Economy

Abstract

Hydrochar (HC), a carbon (C)-rich solid material produced using a hydrothermal technique at relatively low temperatures and pressures, has gained little attention regarding its use as a nanocatalyst in multiple wastewater treatment technologies. These HC-based nanocatalysts, catalyst support materials, and nano-adsorbents have the advantage of being cost-effective and chemically stable, having a porous structure, semiconductive properties, and oxygen (O)-rich surface functionality. Laboratory-scale trials that have successfully used HC catalysts to accelerate the kinetics of advanced oxidation processes (AOPs) (photocatalysis, electrochemical oxidation, Fenton catalysis) are here reviewed in depth, with a special focus on the mechanisms involved. The potential of HC catalysts in energy and valuables recovery using anaerobic digestion, bioelectrochemical systems, metal air batteries, transesterification, isomerization, and reforming processes are highlighted. In addition, the synthesis of HC from waste biomass (a zero-cost C precursor) would reduce the cost of expensive electrocatalysts, reduce greenhouse gas (GHG) emissions, ensure C capture, offer a low-C economy with less of a C footprint, and enhance environmental sustainability. This review provides a unique and comprehensive sustainability related perspective on HC and HC-based nanocatalysts that can be used to positively impact the United Nations’ Sustainable Development Goals (SDGs), especially SDGs 3, 6, 7, 11, and 13. Finally, a circular economy approach that includes HC and HC catalyst use in wastewater treatment and waste management systems is discussed, along with future perspectives and challenges.