The study of the distribution of heavy metals contamination in serpentinite soils in the Virani area, Northwest of Mashhad

Heavy metals contamination of soils is one of the main environmental problems. Heavy metals are considered one of the most dangerous groups of pollutants because of their toxicity and stability. The biological and ecological importance of heavy metals is due to their characteristics, toxicity, persistence, and bioaccumulation. Heavy metal pollution not only directly affects the physical and chemical properties of the soil, but also reduces biological activity and access to nutrients in the soil and poses a serious threat to human health. The concentration of heavy metals in the soil varies depending on the type of parent rock, mineralization and soil formation conditions in different areas. soils from ultramafic rocks and mainly serpentinites from these rocks have a higher concentration of chromium, nickel and cobalt metals than non- serpentinite soils. Therefore, ophiolites can be named as one of the most important natural pollutants. The Virani area is located in Khorasan Razavi province, 15 km northwest of Mashhad. The purpose of this study is to investigate the status of heavy metal contamination in serpentinite soils resulting from the ophiolite complex of Mashhad.

In the present study, 20 soil samples (10-30 cm depth) were collected and the concentration of arsenic, cadmium, nickel, cobalt, coppe, chromium, zinc, manganese, molybdenum, lead, antimony, titanium, silver, aluminium and iron were measured using atomic absorption spectrophotometry. To prepare the samples, first 1 g of the sample powder was dissolved in nitric acid along with fluoric acid. The solution was heated in a Teflon dish at 60 ° C until completely evaporated and the sample dried. The sample was then dissolved in 100 ml of 4% HNo3 solution. Also, some other soil properties including PH, organic carbon, cation exchange capacity and soil texture proportions were measured. The pH of soil samples was measured using water and KCL 1 with a ratio of 1 to 5.2 soil solution. PH soil of the samples was measured by PHmeter modle UB-5, organic carbon was performed by the Walkley-Black method, which adjacent the soil with concentrated sulfuric acid and bichromate, and after the reaction, the remaining bichromate was calibrated with ferroammonium sulfate, and soil texture by hydrometric method. Based on geochemical data, index of enrichment factor (EF), geoaccumulation (Igeo), contamination factor (CF) and statistical analysis (cluster and principal component analysis) calculated for evaluating contamination in this area. The pearson correlation has used to find the association between the studied elements The statistical analysis and modeling were performed in SPSS, GIS and Excel software’s environment.

The results of measuring the physicochemical properties of the soil of the region showed that the value of PH all samples are neutral to relatively alkaline and varies from 7.71 to 7. 85. The studies samples show that the value of EC from 12.32 to 13.88 and organic carbon 0.842 to 0.971 percent. The soil samples were classified loam. Considering the relatively uniform PH in all samples and the similarity of organic carbon and cation exchange capacity in all samples, it can be concluded that the soil of the region is formed under similar physicochemical conditions and these factors have no effect on the distribution of heavy metals. The results of heavy metal analysis showed that aluminum has the highest concentration and cadmium has the lowest concentration in the soils of the region. The trend of changes in the concentration of heavy metals in the soil of the region can be more related to the rock units of the region. Serpentinite soils containing nickel, chromium and cobalt can be found in the structure of silicate minerals such as serpentine, talc and chlorite, as well as oxide minerals such as chromite, magnetite and various hydroxides. These minerals work better with heavy metals in the release of heavy metals. The calculated enrichment factor (EF) values vary 1.042 to 11.83. The results demonstrate than in the soil of the study area some heavy metals such silver, arsenic and antimony higher enrichment, chromium, nickel, lead moderate enrichment and zinc, copper, cadmium, iron, molybdnyum, cobalt, titanium, aluminium, manganes are enriched to a minor degree. The results geoaccumulation index (Igeo) than in the soil of the study area some heavy metals such silver moderate polluted, chromium, nickel, cobalt moderately polluted to un-polluted and cadmium, iron, zinc, copper, lead, titanium, antimony, molybdnyum, aluminium, manganes, arsenic are un-polluted. The spread of ultramafic and serpentinite rocks in the western and northwestern parts of the region has caused more pollution in these areas. The calculated contamination factor (CF) change from 0.39 to 4.617, indicates that the soil samples changes from low contamination to strong contamination. The samples are extremely polluted with silver. The contamination values of chromium, nickel, arsenic, lead, manganes are moderately contamination and zinc, copper, iron, aluminium, cadmium, titanium, antimony, cobalt are low contamination. The Pearson correlation has used to find the association between the studied elements. There is a high positive correlation between the chromium, nickel and cobalt elements. Investigation of the cluster analysis (CA) relationships between potentially toxic elements in the soils showed that the studied elements could divided in to two main cluster. The first and second branch can be divided into two sub-branches based on the branch weight and similarities. The cluster graph shows that chromium, nickel and cobalt were highly correlated with each other, indicating the same origin of the metals in the soils. To ensure, principal component analysis (CPA) was applied to find metal sources in the soils. These results show the geochemical behaviors and sources of chromium, nickel and cobalt are different from other elements. The elements can be originated from the mafic and ultramafic rocks in the western and northwestern if the area and the presence of these elements in the soil can attributed to their lithological origin. The entry of nickel into the circulatory system through agricultural products, respiration or consumption of groundwater with high levels of nickel can cause damage to internal organs, disorders of the body's defense system, various cancers of the blood and bone marrow, local infections, inflammation and chromium, Followed by neurological, digestive problems, liver damage, kidney damage, heart palpitations, skin damage and lung cancer. Biological methods such as phytoremediation, prevention of leafy crops such as cabbage and lettuce, prevention of livestock grazing and planting of resistant and serpentine tree is the region reduce, the negative effects of heavy metals in the soil.