Heavy Metals from NEFA Recycling as a Road Base Material: Release Dynamics and Impacts on a Shallow Aquifer

Abstract

The recycling of fly ash for structural fill uses, such as road base materials, is the most promising avenue to dissipate fly ash. Traditional risk assessment methods do not take into account the ultra-alkaline nature of new emerging fly ash (NEFA) and may underestimate the risk of long-term release of heavy metals and contamination of shallow diving when fly ash is utilized as a road base material. In this study, carbonation experiments are used to reveal the heavy metal release characteristics of NEFA under natural aging conditions and to assess the environmental risk and regional variability characteristics of pollutant release to shallow aquifers under the new fly ash road utilization scenario based on process modeling and Monte Carlo methods. The results showed that the heavy metal release concentrations in carbonized NEFA increased by a factor of 1.17–114.56 with natural aging. This would result in a dramatic increase in the shallow aquifer contamination risk when this material is used as a road base in typical areas. Exposure concentrations of four heavy metals, Ni, Cu, Zn, and As, increased by 1.27–113.89 times, and Cd concentrations increased from 0 mg/L to 0.055 mg/L. Ground infiltration differences due to regional differences in rainfall and other factors lead to differences in the shallow aquifer contamination risk in different areas. Heavy metal exposure concentrations can vary by up to 1.55 times. The results of the study confirm the significant long-term increase in heavy metal release and risk under NEFA resource utilization conditions and the shortcomings of traditional methods in characterizing their release and risk dynamics. In response, it is recommended that attention be paid to the long-term risk of NEFA resource utilization and that a methodological system for characterizing the heavy metal release potential and risk assessment of NEFA be developed.