Occurrence characteristics and ecological risk assessment of heavy metals in fly ash of Ningdong Energy Chemical Industry Base

In order to expand the resource utilization of fly ash, research was conducted based on the fly ash of Yuanyang Lake Power Plant in the Ningdong Energy Chemical Industry Base. Specifically, the physical and chemical properties of fly ash were characterized and analyzed by means of scanning electron microscope, energy spectrum analysis and X-ray diffraction. Besides, the total amount, effective state and content in different forms of Pb, Cr, Ni, Cu, Cd and As in fly ash were determined through acid digestion, DTPA extraction and 5-step Tessier sequential extraction. In addition, the ecological risk of heavy metals in fly ash was assessed with the risk assessment coding method (RAC), the ratio of secondary phase and primary phase (RSP) and the potential ecological risk index. The results show that: (1) The fly ash is in good microstructure at small density, with rich contents of rapidly available potassium and phosphorus. (2) The total average value of the six heavy metal elements in fly ash is higher than the background value of soil environment in Ningxia, but less than the screening value of soil pollution risk of other agricultural land (the basic items). The times of out-of-limit is ranked as Pb, Cu, Cr, As, Cd and Ni in order. The available contents of the six heavy metals do not exceed the background value of soil environment in Ningxia and the screening value of soil pollution risk of other agricultural land. (3) According to the RAC assessment results, the ecological risk is high for As, medium and high for Cd, Cu and Ni, low for Cr and zero for Pb. Meanwhile, the heavy metal As is slightly polluting, while Pb, Cr, Ni, Cu and Cd are non-polluting in accordance with the RSP assessment results. Moreover, the results of potential ecological risk index method show that the total amount and effective RI index of the six elements in fly ash are at grade I, with slight ecological hazard. Especially, Cd is the main contributing element of ecological risk. Therefore, fly ash could be used as the cemented filling material in coal mines for large-scale resource utilization.