Assessment of Environmental Safety Thresholds for Electroplating Sludge-derived Ceramsite as Road Subbase Based on Dynamic Leaching Mechanisms

To evaluate the long term environmental risks of heavy metals in electroplating sludge ceramsites during their service as road sub base materials and to establish control limits, this study developed a dynamic risk assessment model by coupling rainfall infiltration with soil leaching dilution processes. Using ceramsites with a high electroplating sludge mass fraction(58%) as the research object, destructive leaching tests and 64 day dynamic leaching tests were conducted in accordance with EU standards(NEN 7371and NEN 7375) under simulated extreme acid rain conditions(pH =3.2, designed to simulate a worst-case environmental scenario).The results indicated that high-temperature sintering promoted the formation of stable mineral phases, such as the spinel group(e.g., magnetite), thereby achieving immobilization of Copper, Zinc, and Cadmium through lattice substitution and physical encapsulation. Calculations of the slope (rc) of leaching amount versus time in logarithmic coordinates confirmed that the release of these three heavy metals remained diffusion mechanism even under aggressive acidic conditions, whereas Nickel, Chromium, Lead, and Arsenic were governed by non-diffusion mechanism. The risk assessment model revealed that when groundwater was designated as the protection target, Copper, Zinc, Cadmium, Chromium, and Lead posed no environmental risk. However, Nickel and Arsenic exhibited potential risks: under dry season conditions, their maximum attainable concentrations in groundwater were 0.044 mg/L and 0.18 mg/L, respectively. These values exceed the Class III limits in the Standard for Groundwater Quality (GB/T 14848-2017) by 2.2fold and 18fold, respectively. In contrast, none of the heavy metals posed exceedance risks to the soil environment over a 30 year road service life. This study validates the assessment framework's sensitivity in identifying key risk factors, providing a scientific basis for the hierarchical management of electroplating sludge -derived resource products.