Influence of soil pH and application rate on the oxidation of calcium sulfite derived from flue gas desulfurization

Calcium sulfite hemihydrate (CaSO(3).0.5H2O), a common byproduct of coal-fired utilities, is fairly insoluble and can decompose to release toxic SO2 under highly acidic soil conditions; however, it can also oxidize to form gypsum. The objective of this study was to examine the effects of application rate and soil pH on the oxidation of calcium sulfite under laboratory conditions. Oxidation rates measured by release of SO4-S to solution decreased with increasing application rate. Leachate SO4-S from soils amended with 1.0 to 3.0 g kg-1 CaSO3 increased over a 21 to 28 d period before reaching a plateau. At 4 g kg-1, maximum SO4-S release was delayed until Week 7. Oxidation and release of SO4-S from soil amended with 3.0 g kg-1 calcium sulfite increased markedly with decreasing soil pH. After only 3 d incubation, the concentrations of SO4-S in aqueous leachates were 77, 122, 170, 220, and 229 mg L-1 for initial soil pH values of 7.8, 6.5, 5.5, 5.1, and 4.0, respectively. At an initial soil pH value of 4.0, oxidation/dissolution did not increase much after 3 d. At higher pH values, oxidation was maximized after 21 d. These results suggest that autumn surface applications of calcium sulfite in no-till systems should permit ample time for oxidation/dissolution reactions to occur without introducing biocidal effects related to oxygen scavenging. Soil and annual crops can thus benefit from additions of soluble Ca and SO4 if calcium sulfite is applied in advance of spring planting.