关键词: 工业固体废物, 环境安全风险, 土壤污染, 磷石膏, 重金属

Abstract:

Phosphogypsum (PG) is an industrial by-product gypsum obtained from wet process production of phosphoric acid, and it is one of the largest solid waste emissions in the chemical industry. With the development of phosphate and compound fertilizer industry at the late 1950s, the PG waste residue discharged from the production process had continuously increased in China. To solve the environmental problems caused by massive PG stockpiling, at present, PG was mainly used as for building materials, such as cement retarder, and gypsum-building materials, as well as for filling mine pits and road construction. Some researches have also tried to use PG instead of natural gypsum for saline-alkali land amendment in agriculture since 1990s. However, several harmful elements in phosphate rock were preserved or enriched in PG in the process of wet process phosphoric acid production, resulting in high concentration of hazardous pollutants, including heavy metals, fluoride (F), and radioactive element radium ( ²²⁶Ra) in PG, and to some extent, even far beyond the limits of national standards for soil environmental quality and for groundwater quality. For example, the concentrations of heavy metals Cr, As, Cd and Hg in PG exceed the limits (about 20%-67%) of Soil Environmental Quality Risk Control Standard for Soil Contamination of Agricultural Land (GB 15618-2018); and the leachable concentrations of Hg, Cd, Pb, Ni, Cr and Be in PG exceeded the limits (IV-V) of Standards for Groundwater Quality (GB/T 14848-2017). The concentrations of total F and water-soluble F in PG exceeded the critical values of total and soluble F (about 89% and 100%, respectively) in soils of endemic fluorosis-affected areas in China, and the leachable concentration of F in PG exceeded the limit (V) of Standards for Groundwater Quality, and some exceeded the limit of Identification Standards for Hazardous Wastes Identification for Extraction Toxicity (GB 5085.3-2007). According to the concentration of pollutants in PG, as compared with the limits of Soil Environmental Quality Risk Control Standard (GB 15618-2018), the long-term impacts of PG application (if PG input was at rate of 22.5 t·hm ^-2·a ^-1, and with no leaching) on the accumulations of pollutants in the soil could be estimated. For instance, the accumulation of Cd, Hg, F and ²²⁶Ra from un-contaminated to contaminated soils would need 1, 5, 4 and 25 years, respectively. Some field experiments have shown that PG could lead to the risk of excessive pollutants (such as F, As, Cd, Hg, Pb and Zn) enriched in some agricultural products. In 2017, the Identification Standards for Solid Wastes General Rules (GB 34330-2017) was issued in China, which stipulated clearly that those by-products produced in the production process, including PG produced in the inorganic chemical production process, were solid wastes; and those directly used for soil amendment, land reconstruction, land restoration and other land use methods by solid waste disposal were still managed as solid wastes. In order to ensure soil health, food safety, and environmental quality, it was suggested that those industrial waste like PG without any harmless treatment of pollutants, and with harmful elements far beyond the limit standard should not be allowed to directly use as for soil remediation or conditioning in the farmlands by solid waste disposal methods, to prevent hazardous pollutants from entering food chain and harming to human health.

Key words: industrial waste, environmental safety risk, soil pollution, phosphogysum, heavy metals