**2. Chemical properties of the lanthanides and their distribution**

The REE are extremely similar to each other in electron shell structures and, accordingly, in chemical properties. Scandium and yttrium have one *d* and two *s* electrons on their outer electron levels. The lanthanides have also 4f sublevel, gradually filled from Ce to Lu. This structure of electron shells is the reason that 3+ is the main oxidation state for the REE. Although 4+ oxidation state is known for Ce, Pr, and Tb, and 2+ for Sm, Eu, and Yb, in natural conditions only Ce and Eu have oxidation state different from 3+ [4, 5].

La, Ce, Tb, and Er were isolated in pure form in the first half of the nineteenth century and the remaining lanthanides a bit later [6]. The difficulties in isolation of individual elements from rocks, containing their mixture, led to the formation of a sustainable opinion that these elements have similar chemical properties and geochemical behavior [7]. Investigations, carried out later, showed that in the lanthanide series, there are regular changes in chemical properties, associated with structural features of their electron shells. Unlike other heavy metals (HM), whose variation in chemical properties from element to element is not always obvious, in the lanthanide series, there are some interesting regularities.

In the La-Lu series, there is a gradual decrease in ionic radii (so-called lanthanoid compression). This is a consequence of the filling of the 4f level by electrons [5]. As a result, the stability of complex compounds increases, and the pH of the beginning of precipitation of lanthanide hydroxides decreases [4, 8, 9].

Since there are no cases of formation of own solid phases of the lanthanides under soil conditions, it can be assumed that the redistribution of the lanthanides between soil components during soil-forming processes and technogenic pollution is proceeding mainly due to ion exchange, sorption interactions, and complexation. The regularities of sorption of the lanthanides by the main soil components influencing the behavior of heavy metals—(hydr) oxides of iron and manganese, clay minerals, and organic matter—were studied [10–12]. The adsorption of the lanthanides increases significantly with increasing of pH of the soil liquid phase [13–15].

Investigations of lanthanide adsorption by various soil components showed the dependence of the amount of absorbed element on the ionic strength of the solution. In addition, under adsorption from solutions with high ionic strength, the amount of absorbed element increases with increasing atomic number of the element, corresponding to a change in ionic radii. This regularity is absent in adsorption from solutions with low ionic strength [13, 16].

and materials on their basis [1, 2]. The lanthanides are generally considered to be of low toxicity for living organisms. Moreover, the efficiency of lanthanide-containing fertilizers, widely used in agriculture in China, is proved [3]. However, environmental risks related to changes in the lanthanide content, mobility, and proportions in soils, caused by enhanced technogenic

Objective: to study the lanthanides in soils under anthropogenic impact of the Cherepovets

Tasks: (1). To study the total and acid-soluble form content of the lanthanides in soils. (2). To study the forms of the lanthanide compounds in soils using the sequential fractionation

The REE are extremely similar to each other in electron shell structures and, accordingly, in chemical properties. Scandium and yttrium have one *d* and two *s* electrons on their outer electron levels. The lanthanides have also 4f sublevel, gradually filled from Ce to Lu. This structure of electron shells is the reason that 3+ is the main oxidation state for the REE. Although 4+ oxidation state is known for Ce, Pr, and Tb, and 2+ for Sm, Eu, and Yb, in natural conditions

La, Ce, Tb, and Er were isolated in pure form in the first half of the nineteenth century and the remaining lanthanides a bit later [6]. The difficulties in isolation of individual elements from rocks, containing their mixture, led to the formation of a sustainable opinion that these elements have similar chemical properties and geochemical behavior [7]. Investigations, carried out later, showed that in the lanthanide series, there are regular changes in chemical properties, associated with structural features of their electron shells. Unlike other heavy metals (HM), whose variation in chemical properties from element to element is not always obvious,

In the La-Lu series, there is a gradual decrease in ionic radii (so-called lanthanoid compression). This is a consequence of the filling of the 4f level by electrons [5]. As a result, the stability of complex compounds increases, and the pH of the beginning of precipitation of lanthanide

Since there are no cases of formation of own solid phases of the lanthanides under soil conditions, it can be assumed that the redistribution of the lanthanides between soil components during soil-forming processes and technogenic pollution is proceeding mainly due to ion exchange, sorption interactions, and complexation. The regularities of sorption of the lanthanides by the main soil components influencing the behavior of heavy metals—(hydr) oxides of iron and manganese, clay minerals, and organic matter—were studied [10–12]. The adsorption of the lanthanides increases significantly with increasing of pH of the soil liquid

**2. Chemical properties of the lanthanides and their distribution**

only Ce and Eu have oxidation state different from 3+ [4, 5].

in the lanthanide series, there are some interesting regularities.

hydroxides decreases [4, 8, 9].

phase [13–15].

pollution of soils, are still unknown.

68 Lanthanides

steel mill (CSM), Vologda region, Russia

It can be assumed that the differences in individual chemical properties of lanthanides will have a great effect on formation of their compounds in soils, both in background conditions and under technogenic pollution [10–12].

The average content of lanthanides in different rocks depends on their origin and is described in detail in [17]. The most rich in lanthanides are acid magmatic and clay sedimentary rocks.

Another specific distribution feature of the lanthanides in the Earth crust and its individual components is the "sawtoothness" of the graphs, where the atomic number is plotted on x-axis and its content on y-axis (so-called geochemical spectra) (**Figure 1**). This kind of graphs clearly demonstrates one of the basic rules of geochemistry: the Oddo-Harkins rule states that an element with an even atomic number is more abundant than adjacent elements with odd atomic numbers.

Differences in the abundance of even and odd elements, as well as significantly higher abundance of light lanthanides, make difficult to compare characteristics of their behavior and location in natural objects, including soils. To exclude the influence of these differences, in geochemistry, it was suggested to normalize the content of elements in the studied objects to their content in standard objects.

Data on the content of lanthanides in geological rocks are usually to be normalized on composition of chondrites or shales [17–24]. However, chondrites are not suitable for studying lanthanides in soils of different compositions and origins.

**Figure 1.** Total content of the lanthanides in the surface horizons of soils in the CSM impact zone, mg/kg.

The clay of the Russian platform [19] is the best object for normalization of the content of lanthanides in the soils studied. The soils of the taiga-forest zone of the European part of Russia, formed on quaternary sediments as a result of the glacier activity, have similar elemental ratio for most of HM. These rocks were formed as a result of processing of the same source material by glaciers [25]. The main differences in the elemental composition of the sediments are related to their granulometric composition. They depend on the ratio of clay minerals with the maximum content of HM and quartz depleted by them.
