Changes in the fractional composition of heavy metals in soils under the influence of the mining and metallurgical industry (based on the example of the Novosibirsk agglomeration)

Introduction. Soil contamination with various eco-toxicants is a serious problem in cities located near mining and metallurgical enterprises, or in cases where the mining and metallurgical enterprise is a city forming enterprise, having an adverse effect on public health and the state of the environment. This problem is also relevant for the Novosibirsk agglomeration, which is one of the largest industrial centers in Siberia. In Novosibirsk there are several heat and power organizations, numerous non-ferrous metallurgy enterprises - the only tin smelting plant in the country (JSC Novosibirsk Tin Plant), plants for the production of rare metals, a refinery that pollutes the environment with emissions containing heavy metals (HM). Issues of soil contamination with heavy metals are particularly complex, since they are not only of anthropogenic origin, but are also natural components of the soil. Research methods and materials. Samples of anthropogenically transformed soils were selected in the “mining and metallurgical enterprise-environment” system on the territory of Novosibirsk and its immediate surroundings. The total content of HM (Cu, Zn, Cd, Pb) was determined after dry ashing (450 0C) by optical atomic emission spectrometry using an argon arc double-jet plasmatron, a spectrometer (PGS-2) and a multichannel emission spectrum analyzer (MAES). The fractional composition of metal compounds was studied using the Tessier method. To assess the mobility and bioavailability of HMs in soil, the risk assessment code (RAC) (also known as the mobility factor (MF)) was used. This value is calculated as a percentage of the sum of exchangeable and carbonate-related fractions to the sum of HM fractions. Research results and discussion. Significant variability was observed in the total content of heavy metals in the studied samples; the concentrations of Cu and Pb varied in the greatest range (V more than 50%). Nonparametric analysis of variance did not reveal statistically significant differences between the elemental chemical composition of background and urban soils, between the identified clusters of anthropogenically transformed soils, and also between different types of soils. The main factors of differentiation of HMs (except for Pb) are the content of silt and physical clay, and for Pb - the content of carbonates and acidity. Cation exchange capacityis important for the accumulation of Cu, Zn, and Corg- Zn, Cd. The purpose of territories plays an important role in the distribution of Cu and Zn. RAC values for background soils are lower than for anthropogenically transformed soils. Class II includes Cu in all studied soils, Pb and Zn in background soils (the latter is also in the “OCR” cluster). Class III includes Pb, Zn in urban soils and Cd in all studied soils, with individual Cd samples coming very close to the lower limit of class IV – 29%. In our studies, the relative content of all fractions in the samples obeyed the law of normal distribution. The low scatter of HM values in the residual fraction indicates the stability of the forms of HM compounds associated with this fraction, which are not significantly influenced by factors other than their initial content in the soil-forming rock. The residual fraction of HMs in background soils decreased in the following order (%): Cd (71-73) > Zn (68-70) > Cu (58-62) > Pb (42-46). It is worth emphasizing that the high proportion of residual Zn fraction – about 70% – also confirms its geogenic origin. For Cd, the relative content of exchangeable and carbonate-related forms of compounds in all background soils was lower than in anthropogenically transformed soils, and those associated with organic matter and Fe and Mn oxides were practically no different. The average relative Cd content associated with different fractions was in the following order: F5 > F3 ~ F2 ~ F1 > F4 and F5 > F3 ~ F2 ~ F4 > F1 in urban and background soils, respectively, and varied slightly within individual fractions. Zinc forms the following series of fractions (%): F5 > F3 > F4 > F2 > F1, and in anthropogenically transformed soils the mobility is higher. The average relative content of Cu associated with different fractions in all studied soils was in the following order: F5 > F4 ~ F3 > F2 > F1. Conclusions. The results of assessing the change in the fractional composition of HM in soils under the influence of the mining and metallurgical industry in the Novosibirsk agglomeration revealed that the studied soils were characterized by significant differences in the degree of buffering with respect to pollution by HM. Almost all low-buffer sandy loam soils exceed the approximately permissible concentrations (APC) for Zn, including background areas, which is probably explained by the natural geogenic origin of the element and low APC values. Statistically significant differences in the total content of HM were not revealed, possibly due to high variability and abnormal distribution of metals. At the same time, the fractional composition of various HM differs significantly from each other. Calculation of the risk assessment code values for the two most mobile fractions showed that Cu poses the least environmental hazard (class II), while in the case of Cd, the risk assessments are close to hazard class IV, despite the highest proportion of inaccessible forms of metal in the soil. Thus, the relative content of HM in inaccessible forms found in the residual fraction decreases in the series Cd > Zn > Cu > Pb in all the studied soils. The share of Cu in the fraction associated with organic matter (F3) practically coincides with the fraction associated with Fe and Mn oxides (F4), confirming the high affinity of this element to organic matter and Fe and Mn oxides. The latter prevail in the fractional composition of Pb. Taking into account the above, the recommended methodology for assessing the distribution and accumulation of potentially hazardous forms of HM compounds in soils can help in making significant decisions on the protection, restoration and promotion of sustainable development of the “mining and metallurgical enterprise-environment” systems not only in Novosibirsk, but also in other cities of the Russian Federation, in which mining and metallurgical enterprises are city-forming.