The content, chemical forms, and ecological environmental risks of trace elements in coal ash

The migration and accumulation mechanisms of trace elements (As, Pb, Cr, Cu, Ni, Zn, Mn) from coal fly ash in soil-plant systems critically constrain its sustainable utilization. Through integrated approaches including sequential chemical extraction, leaching experiments, electron probe microanalysis, and meta-analysis of bioconcentration data, this study reveals ash-type-dependent element levels with migration capacity: Mn > As > Zn > Cu > Ni > Cr > Pb, governed by their chemical speciation and mineral associations. Elements were classified into three activity groups based on activity and effective mass percentages, aligned with RAC risk levels. Divergent risk evaluations emerged: Nemerow index and ecological risk index indicated moderate pollution (Pz = 2.76) and slight risk (RI = 64.25) based on total content, highlighting differing RAC evaluation outcomes considering speciation. Meta-analysis of a robust bias-tested dataset demonstrated ash generally elevates trace elements in rice but not corn/wheat, with root accumulation exceeding shoots and application rates > 10–20 % significantly increasing concentrations. Crucially, by comparison between predicted values from pollution indices that based speciation concentrations and empirically measured elemental bioconcentration data, residual fractions (F3/F4/F5) positively correlated with plant bioaccumulation, whereas bioavailable fractions (F1/F2) dominated ecological hazards. These findings provide: (1) a speciation-based framework for environmental risk assessment prioritizing F1/F2, and (2) phytoremediation strategies targeting F3/F4/F5, enabling balanced coal ash reuse and risk mitigation through differentiated management of chemical fractions.