Valorizing nickel slag waste for sustainable mine land rehabilitation strategies: A circular economy approach to synergize green material engineering and ecosystem restoration

This study rehabilitates degraded post-nickel mining land using nickel slag waste within a circular economy framework, evaluating it as a sustainable planting media component to reduce topsoil reliance. A greenhouse experiment tested six media compositions combining nickel slag, overburden soil, manure, and topsoil (e.g., slag:manure at 40:10%, 80:20%). Growth of four tree species (Falcataria moluccana [Jeungjing], Myristica fragrans [Nutmeg], Anthocephalus macrophyllus [Red Jabon], Melaleuca leucadendron [Cajuput]) and three cover crops (Centrosema pubescens [Centro], Cymbopogon nardus [Citronella], Brachiaria decumbens [Bede Grass]) was assessed over 17–20 weeks. Media properties (texture, pH, CEC, nutrients) and plant parameters (root length, biomass, shoot-root ratio [SRR]) were analyzed using ANOVA. Findings showed species-media synergy: Red Jabon thrived best in 40% slag + 10% manure; Citronella excelled in 80% slag + 20% manure, yielding 3.7x and 6.6x higher biomass than Centro and Bede Grass respectively, with an ideal SRR (2.45) for stabilization. Jeungjing root length (4.93 cm) and dry weight (0.7 g) increased in overburden + 20% slag + 30% manure versus manure alone. Conversely, Cajuput root growth decreased (3.4 cm) in slag-topsoil-manure (20:15:15%) due to low P/K absorption. Nickel slag improved soil fertility: C-organic, CEC, total N, and P increased significantly after 20 weeks via litter decomposition, though plant uptake varied. Conclusion: Nickel slag (up to 80%) effectively substitutes topsoil, enhancing revegetation and waste repurposing. Species-specific formulations are critical, with Red Jabon and Citronella highly adaptable to slag media. Practical protocols: use 40% slag + 10% manure for Red Jabon; 80% slag + 20% manure for Citronella; avoid slag-topsoil-manure (20:15:15%) for Cajuput. Valorization replaces scarce topsoil, reducing costs. The framework integrates local resources (slag, manure) and native species, diverting slag from landfills, reducing fertilizer dependency, and enabling self-sustaining nutrient cycles.