Geochemical behavior of uranium and thorium during weathering and pedogenesis of carbonate rock: constraint from their speciation
Authors: Ma Qiang , Zhou Bing-jie , Feng Zhi-gang , Wang Xiao-long , Chen Rong , li Pei-shan , Huang Chong 
Published: April 2023

During weathering and pedogenesis of carbonate rock with poor-Uranium (U) and Thorium (Th), U and Th present the characteristics of strong leaching (especially U) and significant residual enrichment, the cause of which is still unclear. A weathering profile developed by dolomite in karst area of Guizhou province in southwest China was selected, which showed zonation characteristics of bedrock (Y), rock powder (Yf) and soil layer (T1 to T12) from the bottom to up. Through the determination of the occurrence speciation of U and Th in Y and weathering profile, combined with mineralogical, geochemical characteristics and element mass balance calculation, the constraints of U and Th speciation on the geochemical behavior of U and Th during the weathering of carbonate rock were revealed. The results show that U and Th in Y preferentially exist in acid insoluble phase, for example, the contents of U and Th in Y are 0.90 mg·kg-1 and 0.28 mg·kg-1 respectively, while those in acid insoluble matter are 2.34 mg·kg-1 and 2.57 mg·kg-1, respectively, but because the content of acid insoluble matter is extremely low (0.95%), the mass percentage of U and Th in the acid soluble phase in the whole rock is absolutely superior (96% and 86%, respectively). The U and Th in the acid-soluble phase of Y are mainly adsorbed on the crystal surface of carbonate minerals or existed in the cement, and the U and Th in the carbonate lattice only account for a small proportion. During the initial dissolution of Y to form the powdery rock layer (Yt), U and Th released from the surface of carbonate minerals and cements are in an alkaline environment rich in carbonate, and this part of U and Th are leaching out, resulting in strong loss of U and Th in the powdery rock layer ( the loss rates are 83% and 65% respectively). From the Yt to the overlying soil layer T1, the carbonate components are completely dissolved, and the U and Th released from the carbonate lattice show different behaviors which U is completely leached and Th tends to stay in the weathered residue. Thus, in the soil layer T1 formed by the interaction of Y (or Yt) and soil, the residual U is the inheritance of the U in the acid insoluble phase of Y; As for Th, it not only inherits the Th of acid insoluble phase of Y, but also superimposes the Th from carbonate lattice in Y. On the other hand, during the evolution process from Y to Y, and to soil layer T1, with the dissolution of carbonate, the acid insoluble phase also showed a significant tendency of chemical weathering. However, the U and Th in the Y acid insoluble phase are not leached with the decomposition of the acid insoluble phase, but are redistributed among the residual phases. As for the geochemical behavior of U and Th in the evolution of soil profile (T1~T12), it is subject to the occurrence speciation of U and Th in T1 and the change of U and Th occurrence speciation with the upward direction of soil profile. The U and Th released from the carrier minerals are mainly redistributed among the residual solid phases, which weakens the intensity of their further loss.