Microalgae, versatile microscopic organisms, are already integral to various industries, from food and cosmetics to pharmaceuticals and aquaculture. Beyond these applications, their potential extends to alternative uses, especially when cultivated in contaminated environments where direct food use is unsuitable. The residues from biofuel production, wastewater treatment, and nutraceutical extraction can be transformed into biochar—a stable, carbon-rich material produced through processes like pyrolysisPyrolysis is a thermochemical process that converts waste biomass into bio-char, bio-oil, and pyro-gas. It offers significant advantages in waste valorization, turning low-value materials into economically valuable resources. Its versatility allows for tailored products based on operational conditions, presenting itself as a cost-effective and efficient More, hydrothermal carbonization, and gasificationGasification is a high-temperature, thermochemical process that converts carbon-based materials into a gaseous fuel called syngas and solid by-products. It takes place in an oxygen-deficient environment at temperatures typically above 750°C. Unlike combustion, which fully burns material to produce heat and carbon dioxide (CO2​), gasification More. This approach not only adds value to waste streams but also enhances the sustainability of microalgae-based systems. Despite these advantages, challenges persist in microalgal biocharBiochar is a carbon-rich material created from biomass decomposition in low-oxygen conditions. It has important applications in environmental remediation, soil improvement, agriculture, carbon sequestration, energy storage, and sustainable materials, promoting efficiency and reducing waste in various contexts while addressing climate change challenges. More production, including high energy demands, variable feedstockFeedstock refers to the raw organic material used to produce biochar. This can include a wide range of materials, such as wood chips, agricultural residues, and animal manure. More quality, and the need for optimized processes. In a recent review published in Clean Technologies and Environmental Policy, Nivetha Sridhar and Rameshpathy Manian delve into recent advancements in microalgal biochar production, its challenges, and its diverse environmental benefits.

Microalgae are unicellular eukaryotes capable of photosynthesis, converting carbon dioxide into carbohydrates while releasing oxygen. They primarily comprise lipids, proteins, and carbohydrates, along with bioactive materials like polyunsaturated fatty acids, vitamins, and pigments, offering applications in pharmacology, cosmetics, and agro-fields. Microalgae can absorb significant amounts of carbon dioxide, contributing to nearly fifty percent of breathable air, making them an excellent biological carbon capture method. They grow faster than land plants and can be cultivated in various water bodies, reducing the need for arable land. While microalgae offer substantial environmental benefits, especially in heavy metal pollution remediation, their biomassBiomass is a complex biological organic or non-organic solid product derived from living or recently living organism and available naturally. Various types of wastes such as animal manure, waste paper, sludge and many industrial wastes are also treated as biomass because like natural biomass these More cultivated in polluted water or as residue from value extraction is unsuitable for dietary uses, necessitating other valorization methods like biochar production.

Biochar, a porous carbon substance formed from biomass in oxygen-deprived conditions, can be produced from diverse renewable resources such as agricultural wastes, wood, and microalgae grown in wastewater. It offers solutions for global issues like soil degradation, pollution, climate change, and carbon sequestration. Microalgae are particularly promising feedstocks due to their rapid biomass production and efficient conversion to volatile biochar. Microalgal biochar, compared to lignocellulosic biochars, generally has lower carbon content but higher nitrogen and oxygen content, along with an alkaline pHpH is a measure of how acidic or alkaline a substance is. A pH of 7 is neutral, while lower pH values indicate acidity and higher values indicate alkalinity. Biochars are normally alkaline and can influence soil pH, often increasing it, which can be beneficial More, lower carbon, but higher oxygen, nitrogen, and ashAsh is the non-combustible inorganic residue that remains after organic matter, like wood or biomass, is completely burned. It consists mainly of minerals and is different from biochar, which is produced through incomplete combustion.   Ash Ash is the residue that remains after the complete More content (18.6–58%). This results in a high oxygen-carbon ratio and a low carbon-nitrogen ratio, which are linked to hydrophilicity and hydrophobicity on the surface. The low carbon-nitrogen ratio promotes the adsorption of inorganic contaminants, while the high oxygen-carbon ratio aids in effective heavy metal adsorption.

Microalgal biochar exhibits enhanced properties, making it a promising alternative to lignocellulosic biochar. Its rapid growth rate (10–27 g/m² per day) and high yield (36–68%) directly impact carbon sequestration rates. It can be cultivated on non-arable land using waste materials, leading to more sustainable and cost-effective production. Microalgal biochar boasts a high surface area, up to 480 m²/g when pyrolyzed at higher temperatures (e.g., 700°C), facilitating better adsorption of pollutants for wastewater treatment and air purification. Its rich nutrient profile, particularly from species like Porphyridium cruentum and Nannochloropsis, includes vital elements like calcium, potassium, and magnesium, which improve soil nutrient retention and microbial activity. The lower ash content in microalgal hydrochar (e.g., from 44.66% to 14.45% in Scenedesmus after HCl washing) enhances its adsorption qualities. Furthermore, microwave pyrolysis can yield microalgal biochar with a fixed carbon content of up to 68.7%.

Various thermochemical methods are employed to produce biochar from microalgae, including torrefaction, pyrolysis (slow, fast, and microwave-assisted), and hydrothermal processes (carbonization and liquefaction), and gasification. Torrefaction, heating biomass between 200–300°C, increases carbon content and energy density. Pyrolysis, heating organic materials in an oxygen-depleted environment between 200–900°C, produces liquid, gaseous, and solid products. Microwave-assisted pyrolysis (MAP) is gaining popularity due to its rapid heating and efficiency, yielding up to 49.9% biochar under optimal conditions. Hydrothermal carbonization, a low-pressure process between 200–300°C, is ideal for wet biomass, while hydrothermal liquefaction converts wet microalgal biomass into biofuels and byproducts like charcoalCharcoal is a black, brittle, and porous material produced by heating wood or other organic substances in a low-oxygen environment. It is primarily used as a fuel source for cooking and heating.   More. Gasification, conducted at temperatures over 800°C in oxygen-poor atmospheres, produces gaseous fuels and a strong, resilient biochar.

Microalgal biochar has diverse environmental applications. In agriculture, its high nutrient content and ion exchange properties enable it to partially substitute artificial fertilizers, increasing plant nutrient availability and crop yield. It also lowers soil emissions of CO2 and CH4. For water treatment, microalgal biochar efficiently adsorbs heavy metals and organic pollutants from industrial effluents due to its high surface area and pore volume. It acts as a cost-effective alternative to activated carbonActivated carbon is a form of carbon that has been processed to create a vast network of tiny pores, increasing its surface area significantly. This extensive surface area makes activated carbon exceptionally effective at trapping and holding impurities, like a molecular sponge. It is commonly More, providing a greener, more efficient, and environmentally friendly solution. As a catalyst, microalgal biochar, often coupled with metals or other feedstocks, enhances catalytic activity and lowers activation energy in processes like catalytic pyrolysis for high-value chemical production. Its nanoporous structure and high adsorption capacity make it effective for ammonia removal in wastewater treatment. Microalgal biochar also shows promise in energy storage due to its high specific capacitance and stability, making it a potential supercapacitor material. Furthermore, its recalcitrant nature makes it an excellent material for carbon sequestration, persisting in soil for long periods and increasing the soil’s capacity to act as a carbon sink. Beyond these, microalgal biochar contributes to biofuel production by improving the mechanical strength and calorific value of fuel pellets.

Despite its vast potential, several challenges impede the widespread adoption of microalgal biochar. High processing expenses, particularly for harvesting and drying, limit economic viability. The energy demands of pyrolysis can outweigh its environmental benefits. The variability in microalgal biochar properties, dependent on feedstock type and production conditions, can affect its efficacy and hinder standardization. A shortage of comprehensive research on chemical changes and material uses further limits optimization. Lastly, the water content of microalgae significantly influences biochar characteristics and yields. Addressing these challenges through optimized production processes, technological advancements in energy recovery, and new functionalization methods will be crucial for realizing the full potential of microalgal biochar as a sustainable resource.

Source: Sridhar, N., & Manian, R. (2025). Advances in biochar production from microalgae: techniques, challenges, and environmental benefits. Clean Technologies and Environmental Policy.