Flue Gas Desulfurization Gypsum Improving Saline-sodic Soil In Tidal Flats

With the rise of population and the reduction of land use area, large area of saline-sodic soil in reclaimed tidal flats become important reserve land resources in coastal area in China, but salinization in parts of reclaimed tidal flats is the biggest obstacle to vegetation reconstruction. The improvement of tidal flats is of great significance to the sustainable and healthy development of economy, land management, ecological environmental protection in coastal areas. It take decades to complete remediation of saline-sodic soil in tidal flats if rely on natural rainfall and vegetation succession.Gypsum has been used to improve soil for hundreds of years. Flue gas desulfurization gypsum (CaSO4·2H2O) is by-product of flue gas desulfurization from coal-fired power plant. If flue gas desulfurization gypsum could be used to improve saline-sodic soil in reclaimed tidal flats, it not only develop a new application field for flue gas desulfurization gypsum, also form a set of new technology to accelerate desalination process in tidal flats and improve soil quality, providing a new way to solving the lack of land resources in coastal areas and sustainable utilization of tidal flats.The purposes of the research in the laboratory and demonstration project are to confirm that (1) flue gas desulfurization gypsum could accelerate the desalination process of saline-sodic soil in reclaimed tidal flats, (2) promote natural succession of vegetation in tidal flats, (3) impact on halophytes growth in tidal flats, and (4) confirm the use safety of flue gas desulfurization gypsum, so as to verify the scientific hypothesis that flue gas desulfurization gypsum could shorten desalting process in tidal flats to several years by Na+/Ca2+exchange mechanism.Main results of this study are as follows:(1) Flue gas desulphurization Gypsum (FGD Gypsum) as soil conditioner accelerated desalting process of saline-sodic soil in reclaimed tidal flats through calcium (Ca2+)/sodium (Na+) exchange mechinism. Flue gas desulfurization gypsum at the rate of 60 Mg/ha reduced exchangeable sodium percentage (ESP) to around 10% in 2 years when the average annual rainfall was 1050 mm in coastal areas of Shanghai;(2) The desalting effect of flue gas desulfurization gypsum on reclaimed tidal flats was better in topsoil layer than subsoil layer.The exchangeable sodium percentage (ESP) decreased at different degrees in 0-10 cm topsoil layer 6 months after flue gas desulfurization gypsum application. When flue gas desulfurization gypsum was applied at the rate of 60 Mg/ha, the exchangeable sodium percentage of saline-sodic soil were both below 6%, and the pH values were close to neutral (7.0) in Nanhui Dongtan and Chongming Dongtan. The ESP in 10-30 cm subsoil layers reduced by different degree with the increase of flue gas desulfurization gypsum application rates. Compared with the datas of 6 months after flue gas desulfurization gypsum application, the desalting effect was more significant 18 months after the flue gas desulfurization gypsum application;(3) Flue gas desulfurization gypsum reduced pH value, the concentration of excessive salt ions (water-soluble Na+, Cl-, and CO32-+HCO3-), increased the concentration of Ca and S in tidal flats. The change of chemical properties in tidal flats, especially in topsoil layer, can provide the basis for ecosystem restoration in tidal flats;(4) Flue gas desulfurization gypsum promoted vegetation succession in tidal flats. The vegetation quadrat survey of 2 years showed that non-saline herbaceous plants appeared in the second year after flue gas desulfurization gypsum application, which were generally seen many years after reclamation. Flue gas desulfurization gypsum promoted the growth of arbor plants, improved survival rate, increased root length and plant height significantly. Survival rate of arbor plants is higher in flue gas desulfurization gypsum treatments with higher application rate, the survival rate of willow was close to 90% at the third year after planting;(5) Application of flue gas desulfurization gypsum in tidal flats has low environmental risk. The heavy metal content in flue gas desulfurization gypsum was lower, contents of water soluble Ca2+and SO42" were more, and it obtained obvious desalting effect only using flue gas desulfurization gypsum for one time. The heavy metal content except Hg in flue gas desulfurization gypsum generally was not high in China. In the study, even the application rate of flue gas desulfurization gypsum was up to 60 Mg/ha, Hg in the extracting solution of soil samples was still below the detection limit (n=22). primary standard of "Environment quality standard for soi (GB 15618-1995)" and "Environment quality standard for soil (GB 15618-2008, revised)", and the minimum value of secondary standard of "Environmental quality of soil inorganic pollutants" were recommend as the guidance of heavy metal limit in flue gas desulfurization gypsum. Hg in flue gas desulfurization gypsum should be given special attention to ensure the ecological safety, it should not be used if Hg content exceed the standard limit (National Public Project of Environmental Protection.The results provide experience and technical reference for large-scale use of flue gas desulfurization gypsum desalting saline-sodic soil in tidal flats.