Excess phosphorus (P) in agricultural runoff is a major environmental issue in the USA, leading to harmful algal blooms and water quality degradation. Yet, phosphorus is a finite and essential resource for agriculture, with global stocks projected to be depleted within 50–100 years at current consumption rates. Addressing both P pollution and the increasing demand for P fertilizer is crucial for sustainable development. A review by Soni Kumari, Younsuk Dong, and Steven I. Safferman, published in Applied Water Science, explores the effectiveness of unmodified and modified biochars in capturing and recovering phosphorus from waste streams for agricultural reuse.

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 has emerged as an environmentally friendly adsorbent due to its large surface area, high porosityPorosity of biochar is a key factor in its effectiveness as a soil amendment and its ability to retain water and nutrients. Biochar’s porosity is influenced by feedstock type and pyrolysis temperature, and it plays a crucial role in microbial activity and overall soil health. Biochar More, and numerous functional groups. This review highlights how biochar’s properties, pyrolysis conditionsThe conditions under which pyrolysis takes place, such as temperature, heating rate, and residence time, can significantly affect the properties of the biochar produced.   More, 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 levels, and the presence of coexisting ions influence its P adsorption capacity. The study reveals that modified biochar significantly outperforms unmodified biochar in P removal. For instance, Mg-laden bamboo biochar achieved a maximum phosphate adsorption capacity of 370 mg/g at 600°C, a notable increase from 344 mg/g at 400°C. This enhanced capacity is often attributed to increased surface area and a more developed mesopore structure at higher 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 temperatures. In contrast, unmodified wheat straw biochar showed only 1.57 mg/g adsorption capacity.

The pH of the solution plays a critical role in P adsorption. Lower (acidic) pH values tend to result in a positively charged biochar surface, enhancing the electrostatic attraction of negatively charged phosphate ions and increasing adsorption. For example, studies with Mg-laden biochar observed maximum phosphate adsorption when the pH was between 2.5 and 5.5. However, at higher pH levels, the biochar surface becomes negatively charged, repelling phosphate and reducing adsorption efficiency.

The presence of other ions in water can either help or hinder phosphate removal. Carbonate anions, for example, significantly reduce phosphate adsorption capacity due to competition for binding sites. On the other hand, certain ions like calcium (Ca2+) and ammonium (NH4+​) can improve P removal by forming precipitates such as calcium phosphate or struvite.

The mechanisms behind phosphorus adsorption on biochar are complex, involving electrostatic interactions, ion exchange, ligand exchange, and surface precipitation. Modified biochars, particularly those treated with metals like magnesium, aluminum, iron, and lanthanum, exhibit improved adsorption due to the incorporation of metal oxides and functional groups. For example, a lanthanum-iron-modified biochar (La-Fe-BC) demonstrated a remarkable phosphate uptake of 330.86 mg/g.

After P adsorption, the P-laden biochar can be used as a slow-release fertilizer, promoting soil fertility and plant growth. Studies have shown that applying P-laden biochar can significantly increase seed germination rates and plant 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. For example, mung bean seed germination increased from 30.0% in sand to 46.7% with P-laden biochar amendment. This approach aligns with the circular economy, reducing reliance on non-renewable phosphate rock resources.

Despite the promising results, challenges remain in scaling up biochar-based technologies. These include concerns about biochar stability, the risk of metal 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 from modified biochars, and reduced adsorption efficiency in real wastewater due to competing ions and organic matter. While some modified biochars, especially those with iron and aluminum, show good stability and minimal metal leaching, magnesium and calcium-based biochars may require additional stabilization.

Further research is needed, particularly continuous flow and column studies, to bridge the gap between laboratory findings and real-world applications. A deeper understanding of the long-term stability and leaching behavior of doping elements is also crucial for the safe and effective implementation of biochar in sustainable phosphorus management.

Source: Kumari, S., Dong, Y., & Safferman, S. I. (2025). Phosphorus adsorption and recovery from waste streams using biochar: review of mechanisms, modifications, and agricultural applications. Applied Water Science, 15(162).