Eutrophication, the excessive richness of nutrients in a body of water, is a major environmental issue caused in large part by phosphorus-containing wastewater from industrial and agricultural sources. While various methods exist for phosphorus removal, adsorption is a particularly practical option due to its simplicity, low cost, and environmental benefits. A recent study in Scientific Reports by Lihui Zhang and a team from Pingdingshan University addresses the challenge of enhancing biochar’s limited effectiveness for inorganic phosphorus removal. Their research presents a novel solution: a modified 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 created from plane tree bark, which demonstrates remarkable efficiency in both removing and recovering phosphorus from water.

The researchers prepared the plane tree bark biochar using a chemical-activation method with potassium carbonate (K2​CO3​) as the activation agent. Biochar has a porous surface and rich functional groups, making it an excellent adsorbent for organic pollutants. However, its inherent negative surface charge restricts its ability to adsorb negatively charged phosphate ions. To overcome this, the biochar was modified with Zinc-Aluminum layered double hydroxides (ZnAl-LDH) using a low-cost, high-yield in-situ co-precipitation method. This modification is crucial, as LDHs are known for their strong anion exchange capacity, surface complexation, and ability to reduce biochar agglomeration. The resulting composite material, with its combined porous and layered structures, was shown to be a highly effective adsorbent for phosphate.

Batch adsorption experiments revealed the composite’s impressive performance. Using just 10 mg of the sample, the researchers achieved an adsorption ratio of approximately 93% for a 25 mL solution containing 20 mg/L of phosphate (PO43−​). The maximum adsorption capacity, calculated using the Langmuir equation, was found to be 103.1 mg/g, suggesting that the adsorption primarily occurs as a single layer of molecules. The process was also exceptionally fast, with 60% of the phosphate removed within the first five minutes and an equilibrium state reached within two hours. This rapid initial adsorption is attributed to the high concentration of phosphate and the large number of available adsorption sites on the material at the beginning of the process.

A significant finding of the study is the material’s potential for phosphorus recovery and reuse. By soaking the phosphate-saturated biochar in a 5.5% sodium carbonate (Na2​CO3​) solution, nearly 60% of the absorbed phosphate could be recovered. The desorption mechanism involves the replacement of the adsorbed phosphate ions by the high concentration of carbonate ions in the solution. This ability to recover phosphorus is a key step toward a sustainable, circular economy model for wastewater treatment.

Further analysis of the adsorption mechanism showed that the process is best described by the pseudo-second-order kinetic model, which indicates that chemical adsorption is the dominant mechanism. FTIR and XPS analyses confirmed that the adsorption involves multiple mechanisms: interlayer anion exchange, where phosphate replaces sulfate ions between the hydrotalcite layers; surface complexation; and ligand exchange, where new bonds are formed between phosphate and the metal hydroxides (Zn-OH and Al-OH) on the material’s surface. This multifaceted approach to phosphorus removal and recovery makes the plane trees’ bark biochar/ZnAl-LDH composite a promising and practical solution for addressing water eutrophication.

Source: Zhang, L., Zhang, C., Zhang, C., Li, W., Zhou, Y., Wang, Y., & Du, G. (2025). Preparation of plane trees’ bark biochar/ZnAl-LDH and its adsorption performance for phosphate and recovery. Scientific Reports, 15(32105).