A study of the mechanisms of adsorption of nickel ions by different types of soils in Udmurtia

Abstract

The share of soils contaminated with nickel in Russia (2.8%) among other heavy metals (HM) is very significant, which is due to the diversity of anthropogenic sources of the metal. In order to predict the migration of an element in the soil and development of technologies for land reclamation under emergency conditions, the study of the process of its absorption by the humus horizon of soils is required. The presented study is devoted to the investigation of the specific and nonspecific adsorption of nickel based on the exam-ple of typical soils of the Udmurt Republic.
The object of the study was the upper humus horizons of three types of soils of Udmurtia: sod-strong podzolic lightly washed soil, sod-carbonate lightly washed soil, and grey forest podzolic soils. In order to minimize the effect of ion exchange reactions on specific sorption, the soils were preliminarily saturated with calcium from an aqueous 0.01 M CaCl2 solution, after the samples were dried. The soil suspension was obtained by mixing air-dry soil samples with a solution with a variable nickel concentration in a ratio of 1:10, stirred on a rotator, and allowed to stand for two days. The equilibrium concentration of nickel ions in the soil filtrate was determined by atomic absorption spectrometry with electrothermal atomization.
According to the Giles classification, the isotherms of ion-exchange and specific sorption of nickel belong to the Langmuir type and indicate a high affinity of metal ions to the active centres of the soil absorb-ing complex (SAC) of all studied soils. With an increase in the metal concentration from 1.4 to 7.0 mM, the maximum decrease in the degree of nonspecific sorption was recorded for sod-strong podzolic soil by almost two fold, while for specific adsorption, the value decreased by 1.2 times. Among the studied soil samples, only in the sod-strong podzolic the predominance of the specific mechanism of nickel sorption over the ion-exchange one was recorded in the entire range of concentrations of the introduced metal, which determines its more strong fixation by SAC.At a high concentration of the introduced metal (1.4-7.0 mM) for sod-carbonate and grey forest soils, a slight predominance of ion-exchange adsorption compared to the specific one was recorded.
It was found that, for almost all soils, with an increase in nickel concentration, the pH of the soil so-lution decreases. Only in the case of ion-exchange sorption of sod-strong podzolic soil was an unchanged pH recorded, which indicates the absence of exchange reactions of nickel ions with the protons of SAC.