Experimental Study Of The Oxidation-Acid Erosion Reaction In Black Shale

Black shales are dark-colored sedimentary rocks that contain significant amounts of sulfide minerals, which formed in reducing environment. Due to high levels of sulfide （pyrite）, black shales are susceptible to chemical weathering when exposed to oxidation environment. As a kind of sulfide mineral, pyrite with high chemical activity, content is more than other sulfide. It plays an important role in the processes of black shale chemical weathering.Oxidation-acid erosion reaction is the main form of black shale chemical weathering, throughout the whole process of chemical weathering. In different stages of oxidation-acid erosion, the formation of the water environment characteristics, the product features and generate the weathering products of space and time sequence are different, embodies the characteristics of chemical weathering of black shales. Oxidation-acid erosion reaction contains two aspects, mineral oxide formation of acidic water and acidic water continues to mineral dissolution, both contact each other, promote each other, resulting in mineral weathering, strength decreases, structural damage.This paper takes Qingxi Formation of black shale in guangxi sanjiang area as the research object. The dynamic and static water circumstances are considered in oxidation-acid erosion experiment. Under the static immersion condition, the different weathering degree and immersion depth are designed in this oxidation-acid experiment. Based on thermodynamics theory and hydro-geochemical simulation software （Phreeqc）, the chemical weathering characteristics of black shale are explored in this paper. The main conclusions are summarized as follow:（1） The engineering geology characteristics were collected from the study area. The mineral and chemical compositions of black shale and weathered samples are measured by X-ray diffractometer （XRD）, X-ray fluorescence spectrometer （XRF）, respectively. The results show that the shale samples contain mainly illite, muscovite, quartz and pyrite and other minerals. However, the rozenite and copiapite are persisted in white and yellow weathered samples, respectively. The surface waters in the study area are tested in laboratory, showing that they are all strongly acidic environment and belong to water type of SO₄^2--Fe²⁺/Fe3、Ca²⁺、Mg²⁺.（2） The water-rock chemistry device and half soak two methods were simulated under the condition of two kinds of black shale oxidation-acid erosion experiments. The former is mainly discussed the water-rock chemistry characteristics under the advection state, the latter focuses on the solution ion composition variation characteristics and weathering product combination characteristics and formation sequences in the oxidation-acid erosion process. pH meter, EC meter, ORP meter three kinds of water quality monitoring equipment to test the experiment process under the two conditions real-time monitoring, when experiment began, three sets of data appear different characteristics:pH value decreased and then increased and finally stabilized, EC value increased and then decreased and finally stabilized, ORP value decreased in the whole process.（3） In the process of oxidation-acid erosion of black shale might involve chemical reaction equation is analyzed. Using the enthalpy, entropy and gibbs free energy thermodynamic theory in the process of chemical weathering of black shales spontaneity to calculate. Results show that the oxidation of pyrite in the standard state into acid process will occur spontaneously, and the higher the temperature the more promotion.（4） The mineral saturation index and dissolution/precipitation processes are simulated through the Phreeqc software in different condition. In this process, the dissolved characteristics of ablite, K-feldspar, and illite are calculated under different pH condition in Phreeqc software. The results showed that when pH> 4 contain iron content getting started to precipitate, the pH value in the process of from 2 to 5, aluminum silicate minerals dissolved content gradually reduced.