Soluble Calcium Amendment: Minimizing Negative Environmental Impacts.

Fluidized Gas Desulfurization Gypsum (FGDG) is a soluble Ca soil amendment which has been shown to have value in agriculture as a Ca and S fertilizer, as a soil aggregation promoter, as an inhibitor of phosphate runoff, as an amendment to reduce exchangeable Al in phytotoxic subsoils, and a Ca exchanger to remediate sodic soils. Another potential application is use as a Ca fertilizer for the extremely Ca deficient serpentine soils and mine wastes where Ca is needed to support agriculture or achieve phytostabilization.

Different methods have been used to manufacture FGDG which have important consequences to its composition. Before 2000, "old" FGDG was a mixture of fly ash and gypsum produced during removal of SO₂ during treatment of exhaust gases from coal combustion. That material often had substantial CaCO₃-equivalent and different sources were tested at both limestone rates and disposal rates of application. High rates of some sources caused boron toxicity, or raised soil levels of trace elements enough to cause concern to regulatory officials in the States. Use of FGDG in agriculture requires Beneficial Use Designation in States before permits for sale and use are allowed, so preventing these identified risks from "old" FGDG was important to marketing of FGDG.

The coal-fired electricity generating industry is increasingly required to remove SO₂ from exhaust gases and is generating increasing quantities of FGDG. The industry responded to the concerns of regulators by changing the exhaust treatment sequence so that fly ash was removed before SO₂ was removed and FGDG manufactured. Analysis has shown that this "new" FGDG is not practically enriched in most trace elements compared to mined gypsum or background US soils, or that presence of other elements in the FGDG prevents risk. This latter is the case for selenate present in FGDG at concentrations higher than background soils, but in a product which is very rich in sulfate such that plant uptake and other effects of the selenate are inhibited by the presence of sulfate. With the "new" FGDG, experimental evidence has not shown higher Se in crops than in control crops or crops fertilized with mined gypsum.

Other trace elements (Ag, As, Ba, Be, Cd, Co, Cu, Cr, Fe, Hg, K, Na, P, Pb, Mn, Ni, Se, Tl, V, Zn) have been evaluated and found to not be higher in FGDG than background US soils, and to not be increased in crops grown on FGDG treated soils, or to enter pasture runoff when FGDG and poultry litter are applied on pastures or incorporated into soils. Thus environmental testing has shown that the long list of potential concerns regarding "old" FGDG are clearly not a risk in the agricultural environment with the "new" FGDG.

One long term need for management of FGDG applications is exchange of Ca for Mg in the soil profile. Added Ca slowly causes leaching of Mg down the soil profile and out of the root zone, so that in light textured soils Mg deficiency might eventually occur to sensitive crops. Exchange of Ca for protons also lowers soil pH slightly which will need to be managed with the regular limestone applications which can also resupply the needed Mg. It may be possible for the industry to include some MgO with the CaO used in SO₂ removal so that the product maintains the fertility balance of amended soils.

Another potential risk is direct ingestion of FGDG by livestock and wildlife. Ruminant animals which consume excessive sulfate may suffer induced Cu deficiency and even sulfide toxicity, so it will be important to prevent animal contact with FGDG storage piles, and to wait for rainfall before allowing grazing of pastures where FGDG was surface applied. This potential risk is easily prevented by management.

Although FGDG is somewhat higher in Hg than mined gypsum, and background soils, research has not shown environmental Hg risks from FGDG applications. In addition, for many Hg mine wastes sites in the western US (which often are in serpentine mineralization which is so Ca deficient that plants cannot grow), FGDG amendment greatly aids revegetation. Because most Hg release from such Hg mine wastes results from photovolatilization, vegetative cover can remarkably reduce Hg emissions, providing an inexpensive way to reduce Hg risks from such mine sites.

Thus shift to production of FGDG post fly ash removal has alleviated previously identified concerns about FGDG use in agriculture. Extensive research conducted in the last decade is slowly being completed and published and will provide the technical basis for a formal risk assessment for beneficial use of "new" FGDG being conducted by US-EPA and USDA.