In a critical review published in Biochar, Rakesh Kumar, Atiqur Rahman, Jasmeet Lamba, and colleagues explored the extensive potential and practical feasibility of using biochar to remove excessive nitrate (NO3−) from contaminated soil and water environments. Nitrate, a ubiquitous contaminant primarily sourced from agricultural fertilizers and industrial activities, poses a serious threat to human health (e.g., “blue baby syndrome” and stomach cancer) and causes eutrophication in water bodies. This review emphasizes how both raw and engineered biochar, particularly when integrated with other treatment technologies, can provide cost-effective solutions for environmental sustainability.

The paper highlights that conventional methods for nitrate removal, such as reverse osmosis and ion exchange, often face challenges due to elevated installation and operational costs. Biochar stands out as a promising, eco-friendly adsorbent. Its effectiveness stems from several key mechanisms including electrostatic attraction (due to positively charged surfaces, particularly in modified biochar), ligand formation, surface complexation, and pore filling. Notably, one urban stormwater study implementing iron filings as a filter material achieved high nitrate removal efficiency, ranging from 74%−100% due to electrochemical reduction. Generally, modified or engineered biochars, often achieved through physical or chemical activation, possess higher adsorption performance than raw biochar precursors. For instance, La-coupled cationic surfactant modified biochar demonstrated a maximum NO3−​ removal capacity of 18.99 mg g−1 in fixed-bed column studies. Biochar’s practical application spans both ex-situ (e.g., wastewater treatment) and in-situ (e.g., 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) remediation. A major focus is on integrating biochar into constructed wetlands (CWs), which are known for their robustness, low cost, and high treatment efficacy. The addition of biochar can compensate for the common scarcity of organic carbon sources (electron donors) needed for the biological denitrification process in CWs. One study on a horizontal subsurface flow wetland amended with Fe-modified biochar achieved a maximum NO3−​−N removal efficiency of 95.3%. The corresponding N removal rate reached 2.52 g N/(m3⋅d) due to enhanced microbial activities. Another study involving NaOH-modified corn straw biochar with plants reported a high NO3−​ removal rate of >80%.

When used as a soil amendment (an in-situ application), biochar incorporation significantly reduces NO3−​ leaching into groundwater, a major source of contamination. Soil column studies show that biochar from Brazilian pepperwood and peanut hull reduced NO3−​ leaching in sandy soil by approximately 34%. This retention of NO3−​ in the soil profile not only prevents groundwater contamination but also enhances nitrogen use efficiency, thereby increasing soil fertility and plant growth.

Considering the economic implications, biochar offers a low-cost synthesis alternative compared to highly expensive conventional technologies like reverse osmosis and electrodialysis. For example, the synthesis cost for amine-functionalized biogas residue was reported as low as $2.89 per kg. The review suggests that implementing the polluter pay principle and offering subsidies for pollution control could encourage the large-scale, cost-effective application of biochar for environmental remediation. Future research is recommended to focus on optimizing biochar’s role in mitigating non-point source pollution from agricultural surface and subsurface flows and standardizing the reporting of synthesis and performance costs to facilitate commercial scaling.

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