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Waste-derived biochar for water pollution control and sustainable development

Abstract

Biochar, a carbon-rich material made from the partial combustion of biomass wastes, is an emerging material of interest as it can remediate pollutants and serve as a negative carbon emission technology. In this Review, we discuss the application of biochar in municipal wastewater treatment, industrial wastewater decontamination and stormwater management in the context of sustainable development. By customizing the biomass feedstock type and pyrolysis conditions, biochar can be engineered to have distinct surface physicochemical properties to make it more efficient at targeting priority contaminants in industrial wastewater treatment via adsorption, precipitation, surface redox reactions and catalytic degradation processes. Biochar enhances flocculation, dewatering, adsorption and oxidation processes during municipal wastewater treatment, which in turn aids sludge management, odour mitigation and nutrient recovery. The addition of biochar to sustainable drainage systems decreases potential stormwater impact by improving the structure, erosion resistance, water retention capacity and hydraulic conductivity of soils as well as removing pollutants. The feasibility of scaling up engineered biochar production with versatile, application-oriented functionalities must be investigated in collaboration with multidisciplinary stakeholders to maximize the environmental, societal and economic benefits.

Key points

  • Biochar, a type of partially combusted biomass, is a promising and carbon-negative solution for municipal and industrial wastewater treatment and stormwater management, as it can remove up to four times its own weight in carbon.

  • Biochar performance in the water–climate–energy nexus is governed by its properties, which can be engineered for different purposes during biomass feedstock selection and customized production. Mineral-rich biomass can produce biochar with higher nutrient and ash contents, whereas lignin- and cellulose-rich biomass can form biochar with higher aromatic carbon contents.

  • Multifunctional biochar can enhance sludge settleability, boost biological treatment and close resource loops by using sludge as feedstocks in municipal wastewater treatment.

  • Specific removal strategies, including precipitation, sorption and catalytic degradation, with appropriate design of engineered biochar, are needed for industry-specific wastewater treatment to target various pollutants and aquatic chemistry.

  • Engineered biochar can accelerate the attainment and harness the synergy of at least 11 of the 17 Sustainable Development Goals throughout the cradle-to-grave life cycle.

  • Partnership among interdisciplinary stakeholders, with strong policy support, a science-informed standardization system and state-of-the-art research advances, is the key to commercializing biochar for large-scale applications.

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Fig. 1: Feedstock and pyrolysis temperature determine biochar properties and applications.
Fig. 2: Biochar applications in wastewater treatment.
Fig. 3: Immobilization and biochar modification strategies for different industries.
Fig. 4: Relationships and roles of stakeholders over the emerging applications of biochar.

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Acknowledgements

The authors appreciate financial support from the Hong Kong Green Tech Fund (GTF202020153), Hong Kong Environment and Conservation Fund (ECF Project 101/2020) and Hong Kong Research Grants Council (PolyU 15222020) for this study.

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M.H. and Z.X. contributed equally to all aspects of the article. M.H., Z.X. and D.C.W.T. researched data for this review. M.H., Z.X., D.C.W.T., D.H. and Y.S.O. made a substantial contribution to the discussion of content. M.H., Z.X. and D.C.W.T. contributed to the writing and figure drafting of the review. D.H., B.G., X.C., Y.S.O., J.R., N.S.B. and D.C.W.T. reviewed and edited the manuscript before submission.

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Glossary

Sustainable Development Goals

(SDGs). Seventeen goals were adopted by the United Nations in 2015 to reconcile environmental health with socioeconomic development by 2030.

Pyrogenic carbonaceous material

Materials produced by thermochemical conversion and that contain organic carbon.

Amazonian dark earth

Also known as terra preta, a fertile black earth-like soil with a high concentration of pyrogenic carbon from in-field low-intensity burning of organic debris.

Sustainable drainage systems

Ecosystem-based drainage systems that absorb, retain and filter rainwater by soil and vegetation, allowing it to recharge to water bodies.

Thermal pyrolysis

Biomass pyrolysis under an oxygen-limited environment with a temperature of 300–900 °C, normally <700 °C.

Slow pyrolysis

Pyrolysis with a lower heating rate of 0.1 to 5 °C s–1 and longer holding time over 1 h, mainly used for biochar production.

Fast pyrolysis

Pyrolysis with a high heating rate over 100 °C s–1 and a shorter holding time of less than 2 s, used less for biochar production owing to lower yields.

Gasification

Partial biomass combustion with oxygen and activation agents at high pressure and temperature (greater than 700 °C) to produce syngas, with biochar as a co-product.

Microwave-assisted pyrolysis

Thermal pyrolysis with the assistance of 0.3–300-GHz microwaves, which improves the quality of bio-oil and biochar.

Activated carbon

A microporous carbon material with a high specific surface area, usually derived from charcoal.

Waste valorization

Upcycling waste materials and converting them to value-added useful materials.

Extracellular polymeric substances

A natural organic polymer with high molecular mass, a fundamental component of microorganism biofilms.

Reactive oxygen species

(ROS). Highly reactive chemicals derived from O2, such as hydroxyl radical, singlet oxygen and superoxide.

Disinfection by-products

Possible carcinogenic by-products that result from water disinfection processes.

ππ interaction

The interaction between the electron on π-bonds of the aromatic carbon in biochar and the electron on π-bonds from the organic pollutant.

π-bonds

π-bonds are covalent chemical bonds, in each of which two lobes of an orbital overlap with two lobes of an orbital on another atom, and in which this overlap occurs laterally.

Persistent free radicals

Stable surface radicals formed from incomplete combustion, which are persistent and stable in various environmental matrices.

Carbon defects

Carbon defects with an abnormal distribution of carbon atoms or heteroatoms on the graphitic layer, including vacancies, holes, topological defects, heteroatom doping or vacancies, edge defects and metal atom sites.

Electron–hole pairs

Energy from light can activate an electron to jump from the valence band to the conduction band, leading to the formation of holes in the valence band.

Photocatalyst bandgap

The gap between the valence band and conduction band of the photocatalyst, which represents the energy demand from light for the photocatalysis process.

Biochar colloids

Nanosized or microsized biochar particles released from bulk biochar, normally with higher mobility due to their small size.

Negative emission technologies

(NETs). Also known as carbon dioxide removal technologies for climate change mitigation.

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He, M., Xu, Z., Hou, D. et al. Waste-derived biochar for water pollution control and sustainable development. Nat Rev Earth Environ 3, 444–460 (2022). https://doi.org/10.1038/s43017-022-00306-8

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