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<p>Although recycling organic residues in farmlands introduces essential metals with agronomic benefits, it may also introduce toxic and biologically non-functional heavy metals that persist in the soil. Continuous application can lead to the accumulation of these metals at different depths, posing long-term environmental risks. This research investigates the depth-resolved distribution and ecological risks of five heavy metals (Pb, Cr, Cd, As, and Hg) in ferralitic soils of oil palm plantations that have been continuously subjected to anaerobically treated palm oil mill effluent (POME) applications. Samples of soil collected from different depths (0–75&#xa0;cm) were analysed using atomic absorption spectrophotometry. Indices, including the contamination factor (CF), degree of contamination (DC), pollution load index (PLI), and geoaccumulation index (Igeo), were applied, alongside mobility assessment, leachability modelling, and toxicological risk analysis. Concentrations in topsoils averaged 12.1&#xa0;mg/kg Pb, 10.2&#xa0;mg/kg Cr, and 0.47&#xa0;mg/kg Cd, while As and Hg were below detection thresholds. The most mobile and ecotoxic metal (20–30%) was found to be Cd, while Pb and Cr were highly immobilised. Although the predicted groundwater Cd concentrations (0.02–0.15&#xa0;µg L⁻<sup>1</sup>) and hazard quotients (&lt; 0.2) indicated a low immediate risk, the depth penetration of Cd warrants targeted monitoring, especially at mid-depth. This depth-resolved, toxicity-centric framework is highly recommended for monitoring heavy metals in farmlands fertilised with POME.</p>

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Depth-resolved distribution, mobility, and ecological risks of heavy metals in ferralitic oil-palm soils amended with palm oil mill effluent

  • 期刊论文

  • 发表日期：

    2026年1月16日

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  • Godwin O. Odigie,
  • Henry O. Orugba,
  • Wakil A. Shittu

摘要

Although recycling organic residues in farmlands introduces essential metals with agronomic benefits, it may also introduce toxic and biologically non-functional heavy metals that persist in the soil. Continuous application can lead to the accumulation of these metals at different depths, posing long-term environmental risks. This research investigates the depth-resolved distribution and ecological risks of five heavy metals (Pb, Cr, Cd, As, and Hg) in ferralitic soils of oil palm plantations that have been continuously subjected to anaerobically treated palm oil mill effluent (POME) applications. Samples of soil collected from different depths (0–75 cm) were analysed using atomic absorption spectrophotometry. Indices, including the contamination factor (CF), degree of contamination (DC), pollution load index (PLI), and geoaccumulation index (Igeo), were applied, alongside mobility assessment, leachability modelling, and toxicological risk analysis. Concentrations in topsoils averaged 12.1 mg/kg Pb, 10.2 mg/kg Cr, and 0.47 mg/kg Cd, while As and Hg were below detection thresholds. The most mobile and ecotoxic metal (20–30%) was found to be Cd, while Pb and Cr were highly immobilised. Although the predicted groundwater Cd concentrations (0.02–0.15 µg L⁻1) and hazard quotients (< 0.2) indicated a low immediate risk, the depth penetration of Cd warrants targeted monitoring, especially at mid-depth. This depth-resolved, toxicity-centric framework is highly recommended for monitoring heavy metals in farmlands fertilised with POME.