Interfacial reactions clay minerals

It is notable that when performing reactions within a homogeneous solution, interfacial inorganic reactions often take place that are completely analogous to geochemical reactions. Both the reactions with clay minerals and with hydrotalcites are also well known in geochemistry. 

As a result of the interfacial processes on rocks and soils, the structure and chemical bonds of the sorbed compounds can be changed. For this reason, different chemical reactions can be initiated in which the components of rocks or soils act as catalysts. The most important mineral catalysts are zeolites and clay minerals. Naturally, the different oxides also have catalytic effects, and nowadays some of them are being artificially produced for catalytic purposes such as framework silicates (zeolites), the most effective and selective catalysts in organic syntheses. The catalytic applications of zeolites are too wide to summarize in this book, so we deal with the catalytic effects of clay minerals. 

Before discussing the interfacial reactions of montmorillonite, we have to define what we call montmorillonite in practice. As known, most minerals are not found in pure forms they are present in rocks together with other minerals. Montmorillonite is the main component of bentonite rocks, so bentonites with high montmorillonite content are frequently called montmorillonite. For scientific studies, we use bentonite with high montmorillonite content in natural form after purification with sedimentation. Usually, the fraction <2 pm is the clay fraction containing montmorillonite. 

Many of the important chemical reactions controlling arsenic partitioning between solid and liquid phases in aquifers occur at particle-water interfaces. Several spectroscopic methods exist to monitor the electronic, vibrational, and other properties of atoms or molecules localized in the interfacial region. These methods provide information on valence, local coordination, protonation, and other properties that is difficult to obtain by other means. This chapter synthesizes recent infrared, x-ray photoelectron, and x-ray absorption spectroscopic studies of arsenic speciation in natural and synthetic solid phases. The local coordination of arsenic in sulfide minerals, in arsenate and arsenite precipitates, in secondary sulfates and carbonates, adsorbed on iron, manganese, and aluminium hydrous oxides, and adsorbed on aluminosilicate clay minerals is summarized. The chapter concludes with a discussion of the implications of these studies (conducted primarily in model systems) for arsenic speciation in aquifer sediments. 

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