Biochar for future and futuristic biochar

Biochar,a close-to-natural product derived from renewable biomass resources,has proven to be a promising carbon-negative material for achieving sustainable development goals and improving the future well-being of ecosystems and human life.Over the past thousands of years,advances in biochar research could generally be divided into three stages: the historical application stage,the recognition and investigation stage,and the rapid engineered development and advancement stage.In this perspective,we describe the development history of biochar,discuss the features of futuristic biochar,and propose new research directions for biochar in the future.

Benefits of charcoal or biochar in agriculture soils,labeled as‘agrichar’,were discovered at the historical application stage(before 2006),marked by articles calling for a brighter future for biochar and emphasizing its application to smallholders,published simultaneously in Nature(Hayes,2006;Woodset al.,2006).The basic physicochemical properties of biochar were preliminarily characterized,and its potential application in agriculture for long-term carbon storage,improving soil health,and increasing crop yield was discussed.The potential value of biochar in ameliorating barren or acid soils attracted the most attention.Meanwhile,many advantages of biochar application remained largely unquantified.Later,biochar research progressed into the recognition of its value and entered a stage of further investigation(2006—2015),during which the definition of biochar was modified and scientifically established by the International Biochar Initiative in 2015(IBI,2015).Besides studying the production and characterization of different kinds of biochar,its application in diverse agricultural and environmental aspects became apparent.The new application landscapes of biochar,such as reducing soil erosion and topsoil loss,mitigating greenhouse gas emissions,improving nutrient storage,cycling,and utilization efficiencies,altering soil microbial community structures,diversity,and functions,alleviating aluminum toxicity,and immobilizing/transforming heavy metals and organic contaminants,as well as controlling non-point source pollution,were identified (Glaseret al.,2009;Woolfet al.,2010;Abivenet al.,2014).Thus,biochar had expanded from agricultural science into environmental science during this period.With the emergence of new ideas,technologies,and applications,the rapid engineered development and advancement stage of biochar has emerged since 2015,leading to the recognition of the multifunctional values of biochar.

Biochar,with a plethora of feedstock materials,advanced pyrolysis processes,and multiple functions,has been widely proposed and applied in diverse fields,demonstrating improved performance (Chenet al.,2019;Lehmannet al.,2021;Xiaet al.,2023).Firstly,at the rapid engineered development and advancement stage of biochar,the feedstocks used for producing biochar have expanded.Marine biomass,food waste,and animal carcasses have been explored as promising precursors for the low-cost preparation of nutrient-rich and specific targeted biochar,such as nitrogen-doped biochar,sulphur-doped biochar,and metallic oxide-porous biochar composites (Elkhalifaet al.,2019;Heet al.,2022).Secondly,many controllable engineered pre/post-modification methods for biochar,such as ballmilling,co-pyrolysis with alkali/salt/acid/minerals,loading of functional groups/materials,immobilization of microorganisms,and enrichment of nutrients,have been proposed to enhance its performance in nutrient use efficiency,microbial inoculation,contaminant immobilization/removal,carbon sequestration,solid waste management,greenhouse gas emission mitigation,hydrogen storage,and electrochemical capacitors(Akhilet al.,2021;Awasthi,2022;Busset al.,2022;Chenget al.,2022;Mongaet al.,2022;Bolanet al.,2023).Multifunctional biochar would be much better for simultaneously solving conventional soil health and environmental contamination problems.Recently,biochar-supported composites have been proposed,such as biochar-mineral,biochar-microorganism,biochar-enzyme,biochar-fertilizer,biochar-nano zero-valent iron(e.g.,magnetic biochar),and biochar-metal organic framework composites,which have been assisting in expanding the benefits of other functional materials (Yeet al.,2017;Shuklaet al.,2021;Linet al.,2023).Thirdly,the field of biochar application has been further broadened and expanded.For example,biochar can be used as a solid-phase microextraction coating for ultratrace analysis of contaminants,as a construction material for increasing strength and reducing thermal conductivity and bulk density of fresh mortars,as an electrode material for improving energy density,power density,and cyclic stability in supercapacitors,as a constructed wetland substrate for enhancing the removal efficiency of contaminants,as a feed supplement for improving animal health and nutrient intake efficiency,as a sorbent for capturing carbon dioxide,and as a redox catalyst for degrading organic contaminants(Jiet al.,2021;Bolanet al.,2022;Leganet al.,2022;Ozkanet al.,2022;Khedulkaret al.,2023;Wuet al.,2023).The formation of an application web of high value-added biochar can help to drive the development of the biochar industry and generate income including carbon credit.Moreover,based on new technologies/ideas and long-term application studies,the internal mechanisms and processes by which biochar functions have gradually been elucidated in detail,which is helpful for controlling its specific performance(Hagemannet al.,2017;Wenget al.,2017;Yanget al.,2021;Wenget al.,2022).

During the extremely complex and multiphase carbonization process,the potential value of numerous liquid and gaseous byproducts has been explored at the rapid engineered development and advancement stage of biochar.High-value recycling of biochar byproducts is important for reducing costs throughout the industrial chain and promoting a circular economy.In the pyrolysis process,gas condensation-derived liquid byproducts are quite numerous,usually called wood vinegar,which can be used as soil conditioners,bactericides,herbicides,and deodorants(Liuet al.,2021).Interestingly,even more liquid byproducts could be produced in the hydrothermal carbonization process (González-Ariaset al.,2022;Jiet al.,2023).Derived from these liquid byproducts,artificial humic acid has been proposed and used as an effective and environmentally friendly remediation agent,demonstrating commercial value and even greater effectiveness than biochar(Bentoet al.,2020;Yanget al.,2023).Different hydrothermal processes generate various humic acid-like materials,and the associated mechanism investigation and application are ongoing.Therefore,in the process of preparing biochar,the yield and properties of liquid byproducts should be emphasized.In addition,byproducts from the gaseous phase during pyrolysis process also require recycling or pretreatment before release.

Mass production is crucial for large-scale application of biochar,but it has always posed a huge challenge.Emerging technologies,such as microwaves and plasma carbonization,have the potential to improve the biochar production process with the advantages of rapid production,low energy consumption,low cost,excellent properties,high yield,and precise control (Dermawanet al.,2022;Norbertoet al.,2023).Some modified methods for producing engineered,specialty,and purpose-specific futuristic biochar can also be considered,designed to be compatible with pyrolyzing equipment,thus facilitating secondary processing and mass production of high value-added biochar.Mobile equipment may prove useful in reducing the collection and transportation costs of biowaste and enablingin-situcarbonization and application simultaneously.In addition,artificial intelligence technology can be adapted to achieve automation and advancements in biochar production,significantly reducing labor costs.

In the next few years,machine learning can be widely utilized to optimize the carbonization process and predict the correlation between specific production parameters and purpose-specific applications.Prior to large-scale implementation,it is crucial to prepare green and safe biochar with adequate functional groups or nutrients.Additionally,studying the actual and long-term interactions between biochar,crops,microorganisms,and the environment is essential to avoid possible negative effects,particularly concerning the emerging nanobiochar.Conducting life cycle assessments and economic analyses will be necessary to evaluate and optimize the environmental benefits and economic feasibility of biochar applications.Lastly,there is a need for worldwide standards,databases,and research networks to regulate the quality control,production,and application of futuristic biochar.In the future,efforts should be directed towards achieving waste-free,safe,and cost-effective mass production of high-quality tailored futuristic biochar for diverse applications,heralding a new era of biochar for a better environment.

ACKNOWLEDGEMENT

The authors acknowledge the financial support of the Youth Innovation Promotion Association,Chinese Academy of Sciences(No.2021309)and the National Natural Science Foundation of China(No.42007124).