Transformation of ferrihydrite to hematite and goethite in the presence of various surfactants.

The transformations of ferrihydrite (Fh) to more stable minerals (e.g., hematite (Hm) and goethite (Gt)) can be significantly affected by the co-existing substances. In this work, the widely used sodium dodecyl sulfonate (SDS), polyvinylpyrrolidone (PVP), and cetyltrimethylammonium bromide (CTAB) were respectively chosen as representatives of anionic, nonionic, and cationic surfactants to investigate the effects of surfactants on the transformation of Fh. X-ray diffraction and kinetics data showed that Hm was the dominant transformation product while Gt was the minority. All of the co-existing surfactants inhibited the transformation of Fh to more stable minerals. In addition, the transmission electron microscopy images revealed that the morphologies of the resulting products were evidently affected by the co-existing surfactants. Hm particles formed in the presence of SDS have larger size than those formed without surfactant. This phenomenon might be attributed to the factor that the adsorbed SDS facilitated the aggregation of Fh, forming large Hm particles. In addition, the products in the presence of PVP or CTAB were changed from rhombic particles to hexagonal or pentagon-like particles. In all, this study clarified the effects of three types of surfactants on the phase transformation of Fh, which can help us understand the effects of artificial chemicals on natural minerals.