Multifunctional phytorestoration of contaminated urban waste sites using Erythrina variegata: integrating soil restoration, heavy metal attenuation, and biomass valorization

Introduction The restoration of heavy metal-contaminated urban waste-dumping sites requires sustainable approaches that improve soil functionality while supporting contaminant attenuation, biomass production, and resource recovery. The novelty of this study lies in the integrated field-based evaluation of Erythrina variegata L. (Indian coral tree) for phytorestoration, combining soil quality assessment, heavy metal dynamics, biomass production, bioenergy potential, and techno-economic feasibility. Its performance was compared with that of naturally colonizing Calotropis procera W.T. Aiton and Datura innoxia Mill. Methods Over one year, soil physicochemical properties, heavy metal concentrations, plant growth, biomass production, estimated metal uptake, calorific value, energy yield, and techno-economic parameters were assessed under field conditions. Principal component analysis (PCA) and hierarchical cluster analysis (HCA) were employed to evaluate relationships among metal-related indices and similarities among sampling sites. Results Soil electrical conductivity decreased from 171.8 to 120.9 μS cm -1 , while bulk density declined from 1.39 to 1.02 g cm⁻³. Soil organic carbon increased from 1.93% to 2.80%, and total nitrogen increased from 0.33% to 0.78%. Soil chromium (Cr), nickel (Ni), and lead (Pb) concentrations declined substantially, whereas cadmium (Cd) showed comparatively limited change. C. procera exhibited the highest estimated uptake of Cr (1.008 mg plant -1 ), Cd (0.289 mg plant -1 ), and Pb (0.676 mg plant -1 ), while E. variegata showed the highest estimated Ni uptake (0.657 mg plant -1 ). E. variegata produced the highest dry biomass (358.83 ± 214.77 g plant -1 ) and estimated energy yield (4.14 ± 1.86 GJ ha -1 ), whereas C. procera recorded the highest calorific value (20.09 ± 1.74 MJ kg -1 ). The first two PCA components explained 77.90% of the total variance. The techno-economic model projected annual revenue of ₹46,412 ha -1 and net profit of ₹12,412 ha -1 for E. variegata . Discussion and Conclusion Overall, this study provides the first field-based evidence of the biomass and bioenergy potential of E. variegata in a heavy metal-contaminated urban waste-dumpsite and highlights its potential integration into urban forestry and phytorestoration. Further replicated, controlled, and long-term studies are required to validate contaminant attenuation and economic performance.