Phosphate removal from water using schwertmannite: The role of structural phosphate incorporation
Presenting person: Peter UhuegbueMo. 2021-05-17 (09:00-10:00), Zoom
Contact: Sven Frei
Schwertmannite is a nanocrystalline metastable iron (III) oxyhydroxysulfate mineral typically found in an acid sulfate-rich environment. Such systems have been reported to have varying concentrations of anions such as chromate, arsenate, and phosphate. Phosphate, on the other hand, is an essential nutrient to plants; however, an elevated concentration causes eutrophication of water bodies. The present study examined the effect of varying PO43- loading on schwertmannite’s sorption capacity as well as its stability over time at pH 3, 6, and 8. schwertmannite was synthesized via the H2O2 method. To also examine the bonding mechanism of phosphate in Schwertmannite, phosphate was coprecipitated with Schwertmannite. Following the adsorption isotherm experiment, phosphate was added at 0, 1, 10, and 20-mM concentration termed zero, low, medium, and high loading and was allowed to interact up to 1656 hours. The result of the adsorption experiments after 24 hours revealed that the maximum adsorption values are 1.51, 1.63, and 1.16 mmol g-1 at pH 3, 6, and 8, respectively. This maximum sorption occurred at maximum PO43- concentration, which decreased with increasing pH. A similar sorption behavior was observed with schwertmannite under a longer time scale (24 -1656 hours), where sorption increased with loading, which in turn controls sulfate release. Transformation of schwertmannite to goethite occurred in the zero-phosphate loading only after 1656 hours of aging at all pH values (more prominent at pH 3 and 8), with the formation of small goethite peaks revealed by XRD. This transformation was accompanied by SO42- release. In total, about 19 – 50%, 50 – 91%, 76 – 86% of the total sulfate ranging from zero to high phosphate loading was released after 1656 hours of aging at pH 3, 6, and 8, respectively. In contrast, high phosphate loading stabilizes the schwertmannite surface against transformation at all pH values. Here, PO43- may act as a barrier by blocking the reactive site of schwertmannite against dissolution and creating a strong Fe-O-P bond. Overall, the result showed that high PO43- loading potentially inhibit schwertmannite transformation both under acidic and alkaline medium. In particular, the amount of phosphate loading should be considered when Schwertmannite is used as a technical sorbent for phosphate removal from water.
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