Although biochar has been used to extract phosphate from wastewater, its efficacy is low due to the negative surface charge. Modified biochar has been proposed to improve phosphate removal from wastewater. However, it is unclear whether or not phosphorus (P) adsorbed on modified biochar is readily available for plants across a wide range of soil pH levels. To the authors’ knowledge, this is the first study to assess the phosphorus-releasing capacity of cation-coated biochar in soils with different pH values and phosphorus contents. Therefore, the objectives of this study were to: (i) determine whether the type of cation influences the phosphate removal efficiency of mineral-coated biochar; (ii) determine whether the interplay between soil pH and coated cation governs P availability from P-laden mineral-coated biochar. Accordingly, coffee husk biochar was coated with two different minerals, namely (i) MgCl2 and (ii) FeCl2. Uncoated biochar was used as a control. In comparison to uncoated biochar, biochar coated with MgCl2 enhanced phosphate removal from manure effluent by 35%, and biochar coated with FeCl2 by 25%. The highest P adsorption capacity (Qm) was found with biochar coated with MgCl2 and biochar coated with FeCl2 (9.8 10.8 mg P g-1, respectively), while uncoated biochar had a Qm of 5 mg P g-1. In comparison to untreated biochar, mineral-coated biochar increased phosphorus uptake by 95% in highly acidic soil (pH = 4), by 60–90% in moderately acidic soil (pH = 6), and by 40–75% in alkaline soil (pH = 8). Similarly, mineral-coated biochar increased water-soluble P, microbial biomass P, and plant biomass as compared to uncoated biochar. Overall, phosphorus release from biochar coated with MgCl2 reduced as soil pH increased, whereas it increased for biochar coated with FeCl2, demonstrating that soil pH and cation interaction have to be considered whenever designing P-loaded mineral-coated biochar as a fertilizer. It is also concluded that the application of P-loaded biochar greatly increases the phosphorus fertilizer value of biochar regardless of the soil pH it was applied to. It is further concluded that treated biochar can be used as a slow-release phosphorus fertilizer. For further understanding of phosphorus transformation in long-term applications, a comprehensive study was also suggested regarding the environmental implications and field application of this biochar-based fertilizer.

Enhanced Phosphorus Recovery from Wastewater Using Modified Biochar for Sustainable Phosphorus Fertilization

期刊论文
发表日期：

2025年8月21日

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Guta Amante,
Alemayehu Regassa Tolossa,
Abebe Nigussie

摘要

Although biochar has been used to extract phosphate from wastewater, its efficacy is low due to the negative surface charge. Modified biochar has been proposed to improve phosphate removal from wastewater. However, it is unclear whether or not phosphorus (P) adsorbed on modified biochar is readily available for plants across a wide range of soil pH levels. To the authors’ knowledge, this is the first study to assess the phosphorus-releasing capacity of cation-coated biochar in soils with different pH values and phosphorus contents. Therefore, the objectives of this study were to: (i) determine whether the type of cation influences the phosphate removal efficiency of mineral-coated biochar; (ii) determine whether the interplay between soil pH and coated cation governs P availability from P-laden mineral-coated biochar. Accordingly, coffee husk biochar was coated with two different minerals, namely (i) MgCl₂ and (ii) FeCl₂. Uncoated biochar was used as a control. In comparison to uncoated biochar, biochar coated with MgCl₂ enhanced phosphate removal from manure effluent by 35%, and biochar coated with FeCl₂ by 25%. The highest P adsorption capacity (Qm) was found with biochar coated with MgCl₂ and biochar coated with FeCl₂ (9.8 10.8 mg P g^-1, respectively), while uncoated biochar had a Qm of 5 mg P g^-1. In comparison to untreated biochar, mineral-coated biochar increased phosphorus uptake by 95% in highly acidic soil (pH = 4), by 60–90% in moderately acidic soil (pH = 6), and by 40–75% in alkaline soil (pH = 8). Similarly, mineral-coated biochar increased water-soluble P, microbial biomass P, and plant biomass as compared to uncoated biochar. Overall, phosphorus release from biochar coated with MgCl₂ reduced as soil pH increased, whereas it increased for biochar coated with FeCl₂, demonstrating that soil pH and cation interaction have to be considered whenever designing P-loaded mineral-coated biochar as a fertilizer. It is also concluded that the application of P-loaded biochar greatly increases the phosphorus fertilizer value of biochar regardless of the soil pH it was applied to. It is further concluded that treated biochar can be used as a slow-release phosphorus fertilizer. For further understanding of phosphorus transformation in long-term applications, a comprehensive study was also suggested regarding the environmental implications and field application of this biochar-based fertilizer.