Transforming microalgae into high-performance biochar for circular and sustainable wastewater treatment

The growing rates of industrialization and urbanization have worsened the problem of wastewater pollution, and traditional methods of treatment are limited by energy use, cost of operation, and low effectiveness against emerging pollutants. Microalgae-derived biochar is becoming a multi-purpose and sustainable adsorbent based on its good biochemical structure, good mineral content, and tunable surface characteristics. The current review critically discusses the recent advances in thermochemical conversion of microalgal biomass using pyrolysis, hydrothermal carbonization, and torrefaction, coupled with physical and chemical activation processes, to obtain adsorbents with high performance in wastewater remediation. Biomass characteristics, conversion, strategies of activation, and adsorption mechanisms have been discussed systematically with a concentration on ion exchange, electrostatic attraction, π-π interaction, hydrogen bonding, or surface complexation. Microalgae-derived biochars consistently outperform lignocellulosic biochars, exhibiting adsorption capacities of up to ~480 mg/g for Cd²⁺ and near-complete removal of dyes and selected pharmaceuticals. Kinetic and isotherm analyses reveal that adsorption typically follows Langmuir and pseudo-second-order models, indicating monolayer chemisorption and strong surface–pollutant interactions. A framework of feedstock-to-function that connects microalgal characteristics, thermochemical treatment, and surface functionality to rational adsorbent design has been established in a circular bioeconomy context. To aid the transition of the laboratory scale study to the industrial scale, future research studies need to focus on optimization of conversion and regeneration, thorough techno-economic and life-cycle analysis, as well as performance validation within a real wastewater matrix to enable the laboratory scale research to be translated to the industrial scale.