Mechanistic Insights into Surfactant-Induced Transformation of Chromium-bearing Schwertmannite and Chromate Redistribution during Dissolution and Recrystallization

Schwertmannite (Sch) is a typical metastable iron hydroxysulfate mineral found in acid sulfate soil and mine drainage, and it is susceptible to environmental factors and the transformation and concomitant release and re-adsorption of trapped heavy metals. Therefore, understanding the mechanism that induces Sch transformation and governs heavy metal redistributions is integral for both environmental science and geochemistry research. Given the widespread environmental accumulation of surfactants through human activities, an important yet understudied question arises: How do surfactants influence the transformation of Sch under such environmental conditions? To answer this query, this study investigated the transformation of chromium (Cr)-bearing schwertmannite (Sch-Cr) induced by anionic and cationic surfactants and the subsequent redistribution of Cr(VI). Both the surfactant type and pH affected the transformation of minerals. Specifically, at pH 3.0, goethite was the final product, in which the anionic and cationic surfactant promoted and inhibited mineral transformation, respectively. At pH 6.5, hematite was the final transformation product and both anionic and cationic surfactants inhibited the transformation of minerals. For the initial fixed Cr(VI), after 20 d of aging in the presence of sodium dodecylbenzesulfonate (SDBS), Cr(VI) mainly existed in the form of a solution, whereas in the presence of cetrimonium bromide (CTAB), Cr(VI) mainly existed in the newly formed secondary minerals. This study reveals the complex geochemical behavior of Sch under environmental disturbances in acid sulfate soils and provides insights into the use and potential exploitation of Sch as an adsorbent for heavy metal elements.