Cultivating sustainability: Harnessing biochar-derived composites for carbon-neutral wastewater treatment

Wastewater treatment plants (WWTPs) are responsible for greenhouse gas (GHG) emissions due to the release of CH4 and N2O from biological processes. To reduce their carbon footprint in WWTPs, novel solutions are needed to ensure a sustainable wastewater treatment. Biochar-derived composites (BDCs) can play a key role in minimizing GHG emissions. Biochar is a carbon-rich solid from biomass pyrolysis, which can be further modified with functional groups to enhance its adsorption capacity. They also can contribute to carbon sequestration and climate change mitigation by converting waste biomass into stable carbon forms. This work reviews recent progress in the application of BDCs for wastewater treatment and discusses their challenges or bottlenecks. While providing an overview of synthesis methods, adsorption mechanisms, and regeneration strategies of BDCs, and their technological benefits, this work also identifies knowledge gaps and research directions for BDCs in wastewater treatment. A literature survey of published articles (1998-2023) shows their promising findings in wastewater treatment applications. A complete removal of 100mg/L Cr(VI) was attained by 1.25g/L of BC-CMC-nZVI at pH 5.5, enabling it to meet the effluent limit mandated by legislation. Fe2O3@BC had a high Cr(VI) adsorption capacity (142.86mg/g), higher than those of magnetic BCs made by co-precipitation (23.85mg/g), impregnation-pyrolysis (43.122mg/g), and reductive co-deposition (58.82mg/g). Iron-loaded magnetic burley tobacco stem biochar achieved 54.92mg/g of Cr(VI) adsorption capacity, higher than that of raw burley tobacco biochar (3.84mg/g). This difference could be due to their benefits such as high surface area, porosity, exchange capacity, and tunable surface chemistry. With their advantages, BDCs are promising solutions for achieving carbon neutrality in wastewater treatment, one of the major sources of GHG emissions. By adopting BDCs as adsorbents, WWTPs not only can contribute to net zero emissions, but also can improve their environmental performance and economic viability.