Advancement pathway of biochar resources from macroalgae biomass: A review

Macroalgae, with short growth cycles, and high CO₂ fixation capacity, are third-generation biomass mainstays that play an essential role as a global carbon sink. Macroalgal biochar plays a crucial role in soil improvement, pollutant adsorption, electrodes, and capacitors, and it has already contributed to both ecological and economic fields. However, the physicochemical properties of macroalgal biochar are challenging to control, and the way macroalgal biochar is pyrolyzed and activated largely determines its physicochemical properties and areas of application. In this study, five standard methods (conventional pyrolysis, hydrothermal carbonization, microwave pyrolysis, co-pyrolysis, and drying) for the pyrolysis of macroalgal biochar and two activation methods (physical and chemical) are reviewed to screen for optimal preparation and activation methods under different conditions and in different application contexts. The conventional pyrolysis process is mature and simple, but the yield is low, which is suitable for industrial production. Hydrothermal carbonization can reduce the content of alkali metals in biochar without prior drying. Microwave pyrolysis has low energy consumption and uniform product properties, which is suitable for biochar with high stability requirements. Co-pyrolysis is a low-cost pyrolysis method if suitable co-pyrolysis materials can be found. The drying ash content is high, but the surface performance is weak, which is generally used as pretreatment. From laboratory to practical applications, macroalgal biochar still needs to be investigated in terms of cost reduction, yield improvement, and optimization of preparation and activation methods.