Study on the Physical, Chemical and Radioactive Gas—Radon (222Rn) Exhalation Properties of Bleached Sandstone in the Northeast Edge of the Ordos Basin

The northeast margin of Ordos Basin, China has a large area with ‘bleached sandstone’, a unique rock characterised by a low diagenetic degree and poor cementation degree. Long-term wind and hydraulic erosion have led to significant soil erosion, vegetation degradation and low ecological carrying capacity in this region, with huge amounts of sediments being transported to the Yellow River. Therefore, it is critical to investigate the weathering mechanism and physicochemical characteristics of bleached sandstone. In this study, the pore structure, elemental composition, mineral composition and radon exhalation characteristics of the bleached sandstone were analysed using low-temperature nitrogen adsorption instrument, inductively coupled plasma mass spectrometer, polarising microscope and environmental radon detector, respectively. Moreover, the physicochemical characteristics of the bleached sandstone were analysed to determine the correlation among them. The results demonstrated that the degree of pore development in bleached sandstone is significantly higher than that in original rock, and the content of mesopores of size from 2 to 50 nm is the highest. These pores were found to be predominantly slit pores with parallel plate structure. The mineral composition is primarily quartz, feldspar, clay minerals and muscovite, with quartz being the highest. Quartz and feldspar exhibited significant weathering, broken crystals and more impurities. A large amount of biotite was altered to form muscovite. The intergranular spaces of bleached sandstone are filled with heterobase, primarily composed of clay minerals. Additionally, clay minerals formed a ring band structure around the particle skeleton. Bleached sandstone was found to be rich in P, Ba, Mn, Sr, Zr and other elements, with Ba showing the highest proportion. The bleached sandstone is enriched in toxic elements Be, As, Cd, Sb, Hg, Tl and ²³⁸U compared with the original rock, indicating that the latter has a certain degree of adsorption. Among them, Be has the highest enrichment degree. Moreover, it is enriched with light rare earth and deficient in heavy rare earth. δEu and δCe exhibit a slight deficiency. Although the rate of radon exhalation in bleached sandstone is low, it is higher than that of the original rock. The high porosity and micropore content can promote radon exhalation. High ²³⁸U content can enhance radon generation, which exhibited a positive correlation with radon exhalation rate. The research results can provide a reference for ecological management, development and utilisation of bleached sandstone in this region.