Biocharhas gained significant attention for its diverse environmental applications, f rom improving soil health to purifying water. But how exactly does it work its magic, especially when it comes to trapping pollutants? A new review paper by Prabhakar Sharma, Anand Kumar, and Jianying Shang, published in Total Environment Engineering, delves into the fascinating world of 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 colloids – microscopic biochar particles – and their intricate dance with co-existing contaminants in porous environments like soil.

The key to understanding biochar’s role lies in its “colloids” – particles ranging from nanometers to micrometers in size. These tiny fragments possess unique physical and chemical properties that dictate their behavior in porous media. The researchers highlight several mechanisms that govern how biochar colloids, and the contaminants they encounter, move or get stuck. These include advection (movement with the flow), dispersion (spreading out), diffusion (movement from high to low concentration), physical straining (getting trapped in small pores), and chemical sorption (sticking to surfaces).

The study emphasizes that a variety of factors influence these interactions. For instance, the size of the biochar colloids themselves is crucial: smaller particles tend to be more mobile and can travel further, while larger ones are more easily retained within the porous structure. The properties of the biochar, such as its vast surface area and intricate pore structure, also play a significant role, providing ample sites for contaminants to bind. This porous network acts like a sponge, drawing in pollutants.

The surface charge of biochar colloids, which can change depending on the acidity (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) of the environment, also dictates how they interact with charged contaminants and soil particles. For example, negatively charged biochar might attract positively charged contaminants, helping to immobilize them. Furthermore, whether biochar surfaces are hydrophobic (water-repelling) or hydrophilic (water-attracting) influences their affinity for different types of pollutants, with hydrophobic biochar being particularly effective at binding non-polar organic compounds.

Beyond the biochar’s own characteristics, the surrounding environment significantly affects its performance. Factors like solution pH, ionic strength (the concentration of dissolved salts), and the presence of other organic matter in the water all impact the stability and aggregation of biochar colloids. High ionic strength, for example, can cause biochar colloids to clump together, reducing their mobility and increasing their retention in the porous media. The type and concentration of cations (positively charged ions like calcium and magnesium) can also alter biochar’s surface charge and aggregation tendencies.

The movement of water itself, or hydrodynamics, also plays a critical role. High flow velocities can detach biochar colloids and associated contaminants from solid surfaces, leading to greater transport. Conversely, slower flows allow for more contact time and enhanced retention.

This comprehensive review also sheds light on how biochar directly impacts the behavior of various contaminants, including heavy metals, organic matter, and essential nutrients. Biochar effectively immobilizes heavy metals through processes like surface complexation and ion exchange, reducing their mobility and availability in the environment. It also readily adsorbs organic pollutants like pesticides and hydrocarbons. For nutrients, biochar helps retain them in the soil, minimizing leachingLeaching is the process where nutrients are dissolved and carried away from the soil by water. This can lead to nutrient depletion and environmental pollution. Biochar can help reduce leaching by improving nutrient retention in the soil.   More and improving their availability for plants.

The implications of this research are profound for environmental remediation and sustainable agriculture. By understanding the intricate mechanisms governing biochar colloid transport and retention, scientists can design more effective strategies for cleaning up contaminated sites and managing water quality. This knowledge allows for the targeted application of biochar to sequester specific pollutants, enhance soil health, and reduce the environmental risks associated with pollution.

As research continues, particularly on the long-term effects of biochar and its interactions with various contaminants under different environmental conditions, its potential to transform our approach to environmental management becomes increasingly clear.

Source: Sharma, P., Kumar, A., & Shang, J. (2025). Transport and retention of co-existing contaminants with biochar colloids in porous media: A review. Total Environment Engineering.