Lanthanum-Modified Steel Slag for Phosphate Removal: Adsorption Mechanism, Leaching Safety, and Scalability

Document Type : Original Research Paper

Authors

Department of Environmental Engineering, Graduate Faculty of Environment, University of Tehran, P.O. Box 14155-6619, Tehran, Iran

10.22059/poll.2025.401787.3101

Abstract

Phosphorus eutrophication threatens freshwater ecosystems and food security in semiarid agricultural regions. The potential of low-temperature (<400 °C) lanthanum modification of steel slag to achieve high adsorption capacity, environmental safety, and regenerability for phosphate removal is uncertain. We synthesized lanthanum-modified steel slag at 400 °C and conducted structural characterization. We performed batch and fixed-bed column adsorption tests, supplemented by the USEPA Method 1312 leaching assessment across pH 2–9, five-cycle regeneration studies, Visual MINTEQ geochemical modeling, and ISO 14044-compliant life-cycle cost analysis. The material showed a phosphate adsorption capacity of 3.21 ± 0.08 mg P/g at pH 7, a 90% improvement over pristine slag, maintained ≥80% performance across pH 5–9, and demonstrated lanthanum release of 0.0181 ± 0.0009 mg/L at pH 2—90.5% below regulatory guidelines and the lowest for rare-earth adsorbents. The five-cycle regeneration process retained 82 ± 2.1% of its capacity. Geochemical modeling revealed a dual-pathway mechanism: 60% reversible inner-sphere complexation and 40% irreversible lanthanum phosphate precipitation, accounting for superior performance. The manufacturing cost was USD 1.19 ± 0.12/kg, 60–70% lower than conventional rare-earth synthesis, yielding a net present value of USD 0.47 million and a 23.6% internal rate of return over ten years. This study established quantified design principles for low-temperature rare-earth adsorbents, facilitated decentralized phosphorus recovery in resource-limited regions, and demonstrated unprecedented lanthanum immobilization through triple-mechanism passivation combined with regenerability and economic viability in smallholder agricultural systems.

Keywords

Main Subjects

References

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Pollution
Volume 12, Issue 1
January 2026
Pages 420-432

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