A Comprehensive Review of Characterization, Leaching, and Ecotoxicity of Coal Fly Ash to Aquatic Organisms

Upon entering aquatic ecosystems, coal fly ash (CFA) particles and their leachates distribute across all environmental compartments, from surface water to benthic sediments, rendering them accessible to aquatic organisms spanning multiple trophic levels. Although CFA is widely classified as non-hazardous waste, a growing body of toxicological evidence demonstrates its capacity to induce significant adverse biological effects. This discrepancy between laboratory-demonstrated toxicity and its current non-hazardous regulatory status highlights a critical knowledge gap in risk assessment frameworks and testing philosophies. Herein, we provide a comprehensive review of CFA characterization techniques, leaching methodologies and their environmental relevance, and the aquatic ecotoxicity of both particulates and leachates to primary, secondary, and tertiary consumers. Based on the findings from the review, we propose four paradigm shifts to more accurately characterize CFA ecotoxicological risks: from standardized leaching protocols to application-specific and maximum leachability tests, from single-species assays to multi-trophic-level bioassays, from single-metal analysis to comprehensive, whole-matrix testing that integrates particulate and dissolved fractions, and from standardizing the reporting of CFA provenance, coal types, and combustion conditions to meaningful meta-analyses and cross-study comparisons. It is believed that these suggestions will substantially advance the realistic ecotoxicological assessment of CFA and bridge the gap between laboratory toxicity data and regulatory classification.