The Suitability of an Iron Oxide‐Rich Gypsum By‐Product as a Soil Amendment

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AbstractAn industrial titanium dioxide process produces a gypsum by‐product rich in iron oxides (red gypsum). Dissolution of red gypsum when applied to soil may be affected by the presence of the oxides due to adsorption of sulfate. This may affect red gypsum's suitability as a sulfur source for crops compared with other forms of gypsum. In addition, the oxides could act as a sink for metal cations, again by adsorption. Adsorption behavior depends on the properties of the material. Characterization showed that the gypsum and iron oxide were present as discrete particles, and that these components had very large surface areas. The iron oxides were a mixture of crystalline and amorphous hydrated forms. They had a point of zero net charge (PZNC) of 6.5. The affinity of anions for adsorption on the iron oxide material was in the order: phosphate > sulfate > nitrate > chloride. Metal cation adsorption revealed an affinity order of Pb > Cu > Ni > Zn > Cd for the iron oxide material and Pb > Zn > Cd > Cu > Ni for red gypsum. In a soil column dissolution experiment, retention of SO2−4 by adsorption on the soil was not evident. Only a pelleted form of the red gypsum appeared to be effective at providing sulfate at a controlled rate. In contrast to sulfate, phosphate was strongly retained in the amended soil columns, while there was little retention of nitrate. Metal cation adsorption on the red gypsum suggested that it could be beneficial as an amendment in soils that have potentially toxic metals present.