Methodological aspects of extracting heavy metals from soils and sediments

Soil and sediment contamination by heavy metals (HMs) can create a significant risk to human health. Quite a few human activities produce waste, much of which is discharged in soils as well as rivers and other water bodies where they accumulate in sediments. The behavior of pollutants in the terrestrial ecosystems is characterized by their fractional composition rather than their total content in the soil and sediments. To determine HMs there are different sample preparation techniques: sifted through a sieve with a hole diameter of 1 mm (AAB; McLaren, Crawford 1973; Miller et al, 1986; etc), of 2 mm (EDTA, EDTPA, etc), of 0.25 mm (Tessier et al, 1979). Another problem is the readsorption of metals that depends on the extraction conditions. Due to the fact that there are a number of difficulties in comparing the results obtained by different methods of extraction.

The main objective of this work was to study the influence of sample preparation and readsorption processes on the extractability of HMs from soil and bottom sediments in the model experiment. The experimental design included the control (original uncontaminated soil - Haplic Chernozem), treatments with the addition of Cu, Ni, Zn, Cd and Pb at a rates of 2, 10 and 20 maximum permissible concentration. The metal compounds extracted with the 1 N CH₃COONH₄ (AAB) are classified as exchangeable. Different sample preparation techniques has been used: the air-dry soil was sieving through a sieve with holes in 1 mm and with holes in 0.25 mm. The assessment of HM readsorption processes in soil was based on the comparative analysis of the results of multiple extraction of metals by AAB static extraction (shake for 1 hour and set aside for a day, 10 times) and dynamic conditions (10 times continuous processing).

It was found that the extraction of HM during sample preparation through a sieve of 0.25 mm was higher than through a sieve of 1 mm (to 3-17%). This is due to the larger surface of soil particles. These differences were manifested both in unpolluted soil and sediments and at different levels of their pollution. With the increasing contamination level the differences were more noticeable. Under static conditions a single AAB extraction does not extract the entire stock of mobile forms of HMs. Dynamic extraction of heavy metals from the soil and sediments, when conditions do not allow to achieve equilibrium, the processes of metal readsorption are eliminated, which leads to greater HM extraction from the soil and sediments.

Thus, the state of the analyzed sample has a significant influence of HMs extraction. To analyze and compare the results of fractionation of HM compounds from soils, it is necessary to take into account the sample preparation used and extraction time required in each method.

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