Improvement of Saline-Alkaline Soil via Flue Gas Desulfurization Gypsum and Safety Analysis of the Associated Heavy Metals

Abstract Flue gas desulfurization gypsum (FGDG) is an effective soil amendment. However, some heavy metals present in FGDG may enter soil or groundwater due to surface rainfall, causing environmental risks. This study used a washing plant to investigate the improvement of saline-alkaline soil via treatment with different mass ratios of FGDG and analyzed the effects of FGDG treatment on the soil physicochemical properties, the distribution characteristics of heavy metals at different soil depths, and the changes in the heavy metal contents of the leachate to explore the migration characteristics of heavy metals and their influence on the groundwater environment. The results showed that an appropriate amount of FGDG could effectively improve the properties of saline-alkaline soil. For example, the soil bulk density decreased significantly, the porosity increased, and the infiltration rate increased by up to 3.42-fold with the addition of FGDG. The contents of exchangeable sodium (EX-Na+) in the soil layers analyzed (0-10 cm, 10-20 cm, and 20-30 cm) decreased by 26.4-40.4%, 16.4-34.4%, and 18.9-40.3%, respectively. After treatment with 6% FGDG, the alkalinity of the soil layers decreased by 67.7%, 61.6%, and 58.2%, respectively, and the pH was close to 7.30. The mass fraction of As in the soil first increased and then decreased with increasing amounts of FGDG, decreasing to the minimum under the treatment with 6% FGDG. The mass fraction of Hg in the soil was positively correlated with the amount of FGDG, while the mass fractions of other heavy metals in the soil layers decreased with increasing amounts of FGDG. The mass fraction of Hg in the leachate was also positively correlated with the amount of F GDG, and the increase in the mass fraction ranged from 0.6 to 30.2% under different treatments. The mass fractions of other heavy metals in the leachate reached the minimum value with the maximum amount of FGDG. The heavy metal contents in the soil conformed to the category 2 limits of the Environmental Quality Standard for Soils (GB 15618-1995), and the heavy metal contents in the leachate were far lower than the category 3 limits of the Standard for Groundwater Quality (GB14848-2017), indicating that the use of FGDG to treat soil does not pollute groundwater. The results expand the understanding of the safety of FGDG (in terms of the contained heavy metals) for improving saline-alkali soil and provide a reference for further environmental risk assessments when recycling FGDG.