A study published in Scientific Reports by Laleh Divband Hafshejani and Mehri Saeidinia systematically investigated how the timing and sequence of aluminum modification affect the final biochar structure and its performance in nitrate removal. The authors synthesized rapeseed-derived biochars using three distinct pathways: modification before 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 (MP), modification after pyrolysis (PM), and modification after pyrolysis followed by re-pyrolysis (PMP). Comprehensive analysis showed that the synthesis pathway critically determined the structural properties and adsorption efficiency.

Nitrate contamination in rivers and groundwater, largely stemming from industrial and intensive farming activities, is a serious and growing threat to both human health and aquatic life. High nitrate levels in drinking water have been linked to conditions like methemoglobinemia, thyroid disorders, and an increased risk of cancer. While conventional water treatment technologies exist, they are often associated with high costs and energy use or result in secondary pollution. Adsorption using biochar, a carbon-rich material produced from 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, has emerged as a particularly efficient and low-cost solution due to its simple operation and potential for regeneration. However, unmodified biochar is typically ineffective at removing negatively charged nitrate ions because of its own negatively charged surface. To overcome this, researchers have explored various chemical modification methods, with the incorporation of metal elements like aluminum being a highly effective strategy.

The PM biochar, which involved pyrolysis followed by aluminum modification, demonstrated superior performance, achieving the highest nitrate removal efficiency. Characterization confirmed that this sequence resulted in the highest specific surface area, a favorable mesoporous structure with the smallest average pore diameter, and the highest aluminum content. This post-pyrolysis modification effectively anchored active aluminum species onto the carbon framework, forming stable Al-O bonds and enriching the surface with Lewis acidic sites that act as binding centers for nitrate ions. In contrast, the MP biochar, modified before pyrolysis, exhibited significantly lower removal efficiency. The study suggests that the aluminum salts decomposed or volatilized during the high-temperature pyrolysis step, leading to the absence of detectable surface aluminum and a reduction in surface-active sites. The PMP biochar, subjected to a second heating step, showed a moderate performance, suggesting the re-pyrolysis partially altered the key functional groups necessary for nitrate binding.

Further experiments using the optimal PM biochar explored the influence of operational parameters. The adsorption efficiency was found to be strongly dependent on factors such as initial nitrate concentration, contact time, and solution 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. While a higher initial concentration and adsorbent dose increased removal, the effect of competing ions was also significant. The presence of common multivalent anions, specifically carbonate and sulfate, had the strongest inhibitory effect, reducing nitrate removal. Regeneration tests confirmed the biochar’s potential for sustainable use: the PM biochar retained approximately 76% of its initial adsorption capacity after five adsorption-desorption cycles.

To complement the experimental work, three machine learning models—Random Forest (RF), Support Vector Regression (SVR), and Linear Regression (LR)—were employed to predict nitrate removal efficiency based on operational conditions. The RF model significantly outperformed the others, achieving the highest predictive accuracy. This robust performance highlights the non-linear, multivariate nature of the adsorption process and confirms the value of machine learning for optimizing treatment strategies. Sensitivity analysis further reinforced the importance of contact time and solution pH as the most influential factors. This work successfully integrates systematic chemical modification with advanced machine learning, providing a data-driven blueprint for designing next-generation, high-performance biochar adsorbents for sustainable water purification.

Source: Hafshejani, L.D. & Saeidinia, M. Integrating chemical modification pathways and machine learning for optimization of nitrate removal by rapeseed (Brassica napus L.) biochar. Scientific Reports (2025).