Phytoremediation potential of native plants in fly ash-contaminated sites: a review on heavy metal uptake and soil restoration strategies

Fly ash generated from coal-fired thermal power plants represents one of the largest industrial solid wastes worldwide and poses significant environmental challenges due to its high alkalinity, poor nutrient status, and elevated concentrations of potentially toxic elements such as arsenic (As), cadmium (Cd), chromium (Cr), lead (Pb), nickel (Ni), and mercury (Hg). The uncontrolled disposal of fly ash threatens soil quality, groundwater resources, ecosystem stability, and human health, highlighting the urgent need for sustainable remediation strategies. Among the available technologies, phytoremediation using native plant species has emerged as an environmentally friendly and potentially lower-input because indigenous plants are naturally adapted to local climatic and edaphic conditions, enabling long-term establishment and ecological restoration with minimal external inputs. This review critically synthesizes current knowledge on the physicochemical characteristics of fly ash, heavy metal speciation and bioavailability, and the physiological, biochemical, molecular, and rhizosphere-mediated mechanisms governing heavy metal uptake, translocation, detoxification, and sequestration in native plant species. Unlike previous reviews that primarily provide descriptive accounts of phytoremediation, this review integrates plant functional traits, rhizosphere interactions, soil microbial dynamics, and ecosystem restoration processes into a comprehensive framework for selecting suitable native species for fly ash rehabilitation. The review further discusses recent advances in microbiome-assisted phytoremediation, biochar-assisted remediation, omics technologies, artificial intelligence-assisted species selection, remote sensing, and ecological risk assessment as emerging tools for improving remediation efficiency and long-term restoration monitoring. Existing knowledge gaps, including limited long-term field validation, biomass disposal challenges, insufficient economic assessment, and the need for standardized evaluation criteria, are critically examined. Finally, the review proposes future research priorities and emphasizes the integration of phytoremediation with sustainable land management and environmental policy to support resilient ecosystem restoration and the long-term rehabilitation of fly ash-contaminated landscapes.