Chemical Transformations of 2D Kaolinic Clay Mineral Surfaces from Sulfuric Acid Exposure

A combined experimental and computational approach is used to investigate the chemical transformations of kaolinite and metakaolin surfaces when exposed to sulfuric acid. These clay minerals are hydrated ternary metal oxides and are shown to be susceptible to degradation by loss of Al as the water-soluble salt Al₂(SO₄)₃, due to interactions between H₂SO₄ and aluminum cations. This degradation process results in a silica-rich interfacial layer on the surfaces of the aluminosilicates, most prominently observed in metakaolin exposed to pH environments of less than 4. Our observations are supported by XPS, ATR-FTIR, and XRD experiments. Concurrently, DFT methodologies are used to probe the interactions between the clay mineral surfaces and H₂SO₄ as well as other sulfur-containing adsorbates. An analysis performed using a DFT + thermodynamics model shows that the surface transformation processes that lead to the loss of Al and SO₄ from metakaolin are favorable at pH below 4; however, such transformations are not favorable for kaolinite, a result that agrees with our experimental efforts. The data obtained from both experimental techniques and computational studies support that the dehydrated surface of metakaolin interacts more strongly with sulfuric acid and provide atomistic insight into the acid-induced transformations of these mineral surfaces.