Nickel and cadmium speciation in soils under long-term aerosol pollution

Coal mining and burning are major anthropogenic sources of atmospheric particles and heavy metals (HMs) (Wang et al., 2011).Coal dust contains a wide range of metal including Ni and Cd. Sequential extractions are the most used methods to estimate the mobility of metals closely related to bioavailability. The classic sequential extraction methods by Tessier (Tessier et al., 1979) are the most popular method of HMs. The aim of this work was to study the speciation of Ni and Cd in soils under anthropogenic contamination with combustion products from the Novocherkassk power plant (NPP).

The monitoring plots were arranged along predominant wind direction at 1.6 and 15 km from the emission source. The studied soils are represented by Haplic Chernozem. The properties of the soil were: pH - 7.3-7.4; 28.6-30.9% of silt, the content of organic carbon is 3.0-3.7%; carbonates - 0.3%; content of nonsilicate Fe – 3.8-3.9%; CEC – 35-37 cmol kg^–1. Areas located within 4 km from the power plants are subjected to the highest ecological disturbances; and a zone almost free from contamination is located beyond 15 km (Minkina et al., 2013).

It was found that the total content of Ni (39.0 mg kg^–1) and Cd (0.1 mg kg^–1) in the unpolluted soil far away from NPP (at 15 km) matching the background metal content in Haplic Chernozem was almost four times lower (145 mg kg^–1 and 3.8 mg kg^–1 accordingly) than in the soil located under the influence of aerosol emissions (at 1.6 km). In an uncontaminated soil occurring far from the emission source, 62–64% of total Ni and Cd fractions are concentrated in the residual fraction characterizing the metal bond with silicates. The following distribution of Ni among the fractions in the uncontaminated soil is noted: residual fraction > bound to organic matter > bound to Fe-Mn oxides > bound to carbonates > exchangeable. In uncontaminated soil, the following fractional distribution of Cd is observed: residual fraction> bound to Fe-Mn oxides > bound to organic matter > bound to carbonates > exchangeable.

Metals accumulate in the soil occurring near the power plant (at 1.6 km), which increases the total contents of Ni and Cd and their mobile (exchangeable and carbonate-bound) compounds in 18 and 33 times accordingly. With increasing pollution, the share of the residual fraction decreases (up to 42-47%) and the amount of the most mobile HM compounds increases. The high mobility in soils is established for Cd (exchangeable fraction was 9%). An increase in the Ni and Cd content in the soil increases its adsorption on the surface of Fe oxides (up to 20% and 27% accordingly). The role of soil organic matter in the absorption of Ni (up to 15%) is also noticeable.

Thus, the largest contributions to the adsorption and retention of metals are made by silicates, as well as nonsilicate Fe compounds for Cd and soil organic matter and nonsilicate Fe for Ni.

This work was supported by the Russian Science Foundation, project no. 19-74-00085