Effects of acid leaching on halloysite

A characteristic iron-containing halloysite sample from Turkey was subjected to acid leaching using organic and inorganic acids for removing iron impurities. The aim of this study was to compare the raw hydrated and dehydrated halloysites with the leached products. Hydrochloric acid and oxalic acid were chosen as leaching agents for the removal of iron impurities at 80 °C for 2.5 h. The physicochemical properties of the acid-treated halloysite were analyzed by XRF, XRD, FTIR, TGA, DTA, SEM, and TEM. The XRF results showed that the acid treatment caused the dissolution of minor amounts of Al3+ ions from the clay layer. The XRD results indicated that the crystalline structure was unchanged after the leaching. However, it was observed that the hydrated (1 nm) halloysite readily lost its interlayer water and was transformed to the dehydrated (0.7 nm) form. In addition, no visible effects of the acid treatment on the tubular structure of halloysite were detected in the SEM and TEM images. Typical pores of varying dimensions were observed in all of the samples regardless of their form or treatment. Furthermore, the TG-DTA and FTIR analysis results were similar for both the raw and the acid-treated samples.