Liming Value Determination of a Calcareous, Gypsiferous Waste for Acid Sulfate Soil

Reclamation of acid sulfate soils requires the use of liming materials. SWAN-gypsum (secondary waste acid neutralization gypsum), a waste from a sulfuric acid, titanium oxide refining process, contains ≈10% CaCO₃, as well as CASO₄·2 H₂O (gypsum) and Fe oxides. This material was tested via laboratory incubation studies in comparison to laboratory grade CaCO₃ as a liming material for a silty clay material from a Typic Sulfaquept formed on dredged materials. A batch titration method (by shaking a set of soil samples amended with various levels of a potential liming material) was also tested. Closed, partially closed, and open systems were employed with the batch titration method. The ability of SWAN-gypsum to raise soil pH was comparable to its CaCO₃ content. The CaCO₃ occurs as unreacted aragonite that is Fe oxide coated as a result of the acid neutralization process that produces the waste. Gypsum and Fe oxides, also present in the waste, were shown to have little or no liming value with the soil material employed. Batch titration (open system, 24 h shaking) gave neutralization results similar to those from regular incubation for 45 d. The pH values achieved with increasing levels of SWAN-gypsum were lower with closed than with open systems at pH values above 6.6 (by 0.1 to 0.45 pH units, presumably due in part to differences in CO₂ pressure), but higher with the closed system at lower pH values (by 0.2 to 2.3 pH units, perhaps due to oxidation of sulfides with the open system). At pHs below neutral following open hatch titration, pHs measured before drying the materials, were higher than those measured after drying and rewetting by 0.2 to 1.5 pH units, whereas they were slightly lower before drying, by up to 0.2 pH units, at pHs above neutral. Additional factors (e.g., physical properties and other possible effects of Fe oxides and CASO₄·2 H₂O) that need to be considered prior to field utilization of SWAN-gypsum as a soil amendment are mentioned. The methods reported for testing the liming value of this waste should have value in testing the liming value of other potential acid neutralizing materials. Scientific Article no. A6407 and Contribution no. 8598 of the Maryland Agric. Exp. Stn.