The pervasive issue of nitrate contamination in water and soil ecosystems is a direct consequence of anthropogenic activities, notably the excessive application of fertilizers and pesticides on agricultural lands. Globally, nitrogen fertilizer use is approximately 133 kg N ha^-1 \year^-1. The resultant high levels of nitrate, which possesses a strong soluble affinity in water, lead to the contamination of surface and groundwater systems through surface runoff and leaching. The health and environmental risks are significant; high nitrate consumption is linked to “blue baby syndrome” and certain cancers in humans, while in water bodies, it causes eutrophication. Given that the World Health Organization (WHO) and the European Union (EU) set the maximum permissible limit for nitrate in drinking water at 50 \ mg L^-1, the need for efficient remediation is critical. A review by Rakesh Kumar, Atiqur Rahman, Jasmeet Lamba, Sushil Adhikari, and Henry Allen Torbert in the journal Biochar explores the practical applications and economic feasibility of using biochar to combat this pollution.

Among various remediation technologies, which include separation-based methods like reverse osmosis and reduction-based methods like biological denitrification, adsorption stands out for its effectiveness, simplicity, and low costBiochar, a carbon material derived from the 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 of various biomasses, is an exceptionally promising adsorbent due to its porous structure, high surface area, and abundant functional groups When applied as a soil amendmentA soil amendment is any material added to the soil to enhance its physical or chemical properties, improving its suitability for plant growth. Biochar is considered a soil amendment as it can improve soil structure, water retention, nutrient availability, and microbial activity.   More, biochar helps reduce nitrate loss, enhancing nitrogen use efficiency and restricting percolation in subsurface systems The mechanisms of nitrate adsorption by biochar involve electrostatic attraction, ligand exchange, complexation, inner pore filling, and intraparticle diffusion.

In treating naturally contaminated water, raw biochar often shows negligible remediation, but chemically modified biochar achieves high performance. Similarly, in column experiments treating synthetic wastewater, lanthanum and cationic surfactant modified biochar achieved a maximum nitrate removal capacity. Furthermore, fixed-bed column studies focused on reducing nitrate leaching from agricultural soil demonstrated that biochar made from Brazilian pepperwood and peanut hull reduced leaching by up to 34% in sandy soil. In clay soil, rice husk biochar reduced nitrate leaching compared to loamy sand at low application rates by significantly enhancing soil properties like cation exchange capacity and water holding capacityWater holding capacity is the amount of water that soil can retain. Biochar can significantly increase the water holding capacity of soil, improving its ability to withstand drought conditions and support plant growth.   More.

One of the most robust and high-efficacy treatment strategies involves integrating biochar into constructed wetlands (CWs). CWs, particularly the horizontal flow (HF) type, naturally promote denitrification, but their efficiency can be limited by the scarcity of electron donors, especially when treating wastewater with a low carbon-to-nitrogen ratio. Biochar can solve this problem by supplying labile carbon, which enhances the denitrification process. Studies on this integrated approach have shown exceptional results. The removal was primarily attributed to microbial activities, accounting for 92.69% of the total removal. In a separate pilot-scale column experiment, a woodchip bioreactor pre-treatment followed by a biochar-amended filter media achieved 98\% nitrate removal.

Considering the practical feasibility and cost implications, traditional nitrate treatment methods often incur high installation and operational costs. For example, reverse osmosis (RO) systems were estimated to cost households $4-164$ per year for point-of-use treatment, but the biological treatment approach is generally less expensive for community-level water supply due to lower initial and operation costs. In contrast, the synthesis cost for high-capacity modified adsorbents, like amine-functionalized biogas residue, was reported as low. Economic analysis highlights that incorporating cost abatement policies, such as the polluter pay principle and pollution control subsidies, alongside biochar application for reforestation and organic farming can lead to significant cost-benefits by reducing eutrophication and restoring water quality.

Overall, both raw and engineered biochar show strong potential for nutrient management and environmental sustainability. However, future research needs to address knowledge gaps, including the lack of standardized economic cost and benefit analyses for biochar applications across different treatment units and the long-term monitoring of non-point source pollutant loss in agricultural subsurface flows. Policy interventions are recommended to support in-situ biochar application in agriculture, leveraging its role in minimizing nitrate leaching and functioning as a slow-release fertilizer.

Source: Kumar, R., Rahman, A., Lamba, J., Adhikari, S., & Torbert, H. A. Harnessing biochar for nitrate removal from contaminated soil and water environments: Economic implications, practical feasibility, and future perspectives. Biochar, 7(1): 94 (2025).