The global increase in sewage sludge (SS) production and the escalating issue of phosphorus-induced eutrophication in water bodies present significant environmental challenges. Addressing these concerns, a recent study in Results in Engineering by Domagoj Nakić, Katarina Licht, Hana Posavčić, and Ivan Halkijević, investigates the potential of pristine 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 derived from experimental SS gasificationGasification is a high-temperature, thermochemical process that converts carbon-based materials into a gaseous fuel called syngas and solid by-products. It takes place in an oxygen-deficient environment at temperatures typically above 750°C. Unlike combustion, which fully burns material to produce heat and carbon dioxide (CO2), gasification More as an efficient adsorbent for phosphate removal from synthetic wastewater. This innovative approach offers a unified solution to both waste management and water pollution without requiring additional pretreatment of the biochar.
The study’s key findings demonstrate that pristine SS biochar exhibited remarkable phosphate adsorption capacities, ranging from 5.31 to 17.60 mg/g. Under optimal conditions, specifically using 0.75 g of biochar per 100 mL of solution with an initial phosphate concentration of 40 mg/L, the biochar achieved an impressive removal efficiency of up to 99.50%. These results are particularly notable as they match or even exceed the performance of many chemically modified biochars reported in previous studies, highlighting the effectiveness of this untreated material.
Response Surface Methodology (RSM) was employed to optimize the adsorption process, revealing that biochar mass, initial phosphate concentration, and treatment time were critical factors influencing removal efficiency. Higher biochar mass led to greater phosphate adsorption, with an optimum dosage identified around 0.55 g, beyond which no significant increase in removal was observed as phosphate ions were completely adsorbed. The study found that lower initial phosphate concentrations resulted in higher removal efficiencies. Adsorption also increased with longer treatment duration, with a rapid removal rate of 80% achieved within the first 6 hours using 0.75 g of biochar, gradually slowing thereafter due to biochar saturation.
One significant and practical finding was that 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, when varied between 3 and 7, had no significant effect on phosphate removal efficiency. This can be attributed to the biochar’s high point of zero charge (pHzpc) of approximately 10.5, ensuring its surface remained positively charged throughout the tested pH range and favoring the adsorption of negatively charged phosphate species through electrostatic attraction. Additionally, the biochar’s chemical composition, rich in metal oxides like CaO (14.7%), Fe2O3 (10.5%), and Al2O3 (15.6%), suggests that non-electrostatic mechanisms such as ligand exchange and surface precipitation also contributed to phosphate removal, mechanisms that are less sensitive to pH changes in the near-neutral range. This broad pH effectiveness is advantageous for real-world wastewater treatment applications.
The adsorption process conformed to a pseudo-second-order (PSO) kinetic model and aligned well with the Langmuir isotherm. The PSO model fit (R² > 0.99) suggests that chemisorption, controlled by the availability of high-affinity surface sites, is the predominant adsorption mechanism. The Langmuir isotherm fit (R² = 0.995) indicates monolayer adsorption onto a homogeneous set of active sites, implying specific interactions between phosphate ions and the biochar surface.
Furthermore, 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 tests of the biochar confirmed very low concentrations of heavy metals (such as As, Cd, Cu, Ni, Pb, and Zn) in the eluate, with many falling below detection limits. This indicates that the metals are tightly bound within the biochar matrix, minimizing the risk of secondary pollution and supporting its environmental safety for wastewater treatment or soil remediation. The material’s porous structure, featuring numerous small channels and irregular cavities observed through SEM, provides a large specific surface area and a high number of active sites for adsorption.
This study represents a significant breakthrough by demonstrating that SS biochar from an innovative gasification process can effectively remove phosphates without additional modification, offering both economic and environmental benefits. Future research will focus on testing the biochar’s effectiveness in real wastewater, its reusability across multiple adsorption-desorption cycles, economic viability, and its potential repurposing as a slow-release fertilizer for agriculture or a supplementary construction material.
Source: Nakić, D., Licht, K., Posavčić, H., Halkijević, I. (2025) Biochar from Experimental Sewage Sludge Gasification as an Adsorbent for Phosphate Removal.
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