Spatial variability and source analysis of typical soil trace elements at permafrost section along national highway 214 in the eastern Qinghai-Tibet Plateau

This paper attempts to reveal the enrichment status, spatial characteristics and material sources of typical soil trace elements at permafrost section along National Highway 214 on the Qinghai-Tibet Plateau. Therefore, the samples of typical trace elements in surface soil, being located at the northern slope of Bayan Kara Mountains, were collected and tested. The concentrations of typical trace elements in soil were analysed by mathematical statistics, spatial analysis and ecological assessment. The results show that: (1) the concentrations of As, Cd and Hg in the soil are higher than the local background values, and their degrees of variation were high. There was a certain degree of accumulation. Soil As and Hg elements constitute "slight pollution", indicating there is a none-to-slight ecological hazard. (2) The distributions of soil As, Cd, Pb and Zn concentrations are lower near the highway and increase with distance from it and then become relatively low further away. The distributions of Cr, Cu, Hg and Ni concentrations show no obvious trends in any direction. (3) The spatial heterogeneity of typical trace elements in soil is affected by soil organic matter (SOM), cation exchange capacity (CEC), pH, slope curvature and aspect. At the local scale, soil texture and topography were the main affecting factors. Concentrations of Cd, Cr, Cu, Ni, Pb and Zn were mainly affected by natural factors, while those of As and Hg were affected by both natural and human factors.

A review of the distribution coefficients of trace elements in soils: influence of sorption system, element characteristics, and soil colloidal properties

Knowledge about the behavior and reactions of separate soil components with trace elements (TEs) and their distribution coefficients (Kds) in soils is a key issue in assessing the mobility and retention of TEs. Thus, the fate of TEs and the toxic risk they pose depend crucially on their Kd in soil. This article reviews the Kd of TEs in soils as affected by the sorption system, element characteristics, and soil colloidal properties. The sorption mechanism, determining factors, favorable conditions, and competitive ions on the sorption and Kd of TEs are also discussed here. This review demonstrates that the Kd value of TEs does not only depend on inorganic and organic soil constituents, but also on the nature and characteristics of the elements involved as well as on their competition for sorption sites. The Kd value of TEs is mainly affected by individual or competitive sorption systems. Generally, the sorption in competitive systems is lower than in mono-metal sorption systems. More strongly sorbed elements, such as Pb and Cu, are less affected by competition than mobile elements, such as Cd, Ni, and Zn. The sorption preference exhibited by soils for elements over others may be due to: (i) the hydrolysis constant, (ii) the atomic weight, (iii) the ionic radius, and subsequently the hydrated radius, and (iv) its Misono softness value. Moreover, element concentrations in the test solution mainly affect the Kd values. Mostly, values of Kd decrease as the concentration of the included cation increases in the test solution. Additionally, the Kd of TEs is controlled by the sorption characteristics of soils, such as pH, clay minerals, soil organic matter, Fe and Mn oxides, and calcium carbonate. However, more research is required to verify the practical utilization of studying Kd of TEs in soils as a reliable indicator for assessing the remediation process of toxic metals in soils and waters.