Review on Zinc Isotopes in River Systems: Distribution and Application

Zinc (Zn) is an essential trace element widely present in rivers and plays a crucial role in ecosystems. Human activities have led to the extensive migration and dissemination of Zn in the environment. When the accumulation of Zn in rivers reaches excessive levels, it transforms into a potentially harmful contaminant. Consequently, tracing Zn's origin by its isotope ratios (delta Zn-66) from natural and anthropogenic sources is essential for the management of and safeguarding against river pollution and comprehending Zn behavior in rivers as well. This review presents the advancements in Zn isotope research in river environments, including a summary of Zn isotope measurement, the distribution and controlling factors (mineral adsorption, precipitation, and biological effects) of Zn isotopes, and the applications of Zn isotopes in rivers. The median delta Zn-66 values for uncontaminated river water and suspended particulate matter (SPM) were 0.37 parts per thousand and 0.22 parts per thousand, respectively, close to the bulk silicate Earth (BSE) value of 0.28 +/- 0.05 parts per thousand. With respect to the contaminated river, delta Zn-66 falls within -0.73 parts per thousand to 1.77 parts per thousand, with it being heavily influenced by anthropogenic sources, such as the electroplating and metal processing industries. Apart from contamination identification, this review summarizes the river material exchange process and the delta Zn-66 values of rainwater (-0.2 parts per thousand similar to 0.38 parts per thousand), groundwater (-0.13 parts per thousand similar to 1.4 parts per thousand), lake water (-0.66 parts per thousand similar to 0.21 parts per thousand), and seawater (-1.1 parts per thousand similar to 0.9 parts per thousand), which facilitates a holistic understanding of the Zn cycle and isotope fractionation in the river system. In conclusion, Zn isotopes are an effective tool for tracing pollution sources and Zn migration processes, with enormous unexplored potential. Therefore, this review finally summarizes some challenges and future directions in current research on Zn isotopes.