Study On The Transfer Characteristics Of Cobalt And Nickel In Soil-plant System And Their Health Risk Assessment

The contents and availability of cobalt and nickel in the agricultural soils in Fujian Province were investigated and the main soil factors influencing the accumulation in the soils and their availability, the transfer characteristics of Co and Ni from the soils to the edible parts of rice and 20 major vegetable species in the region and the toxic effects on the crops were studied. The environmental reference values of Ni and Co in the soils were estimated. The main results were as follows:1. The total Co concentrations of the soils ranged from 3.48 mg kg-1 to 21.70 mg/kg with an arithmetic mean of 12.30 mg/kg and the DTPA-extractable Co varied between 0.01 and 1.52 mg/kg with an average of 0.30 mg/kg. The total Ni contents of the soils ranged from 1.41 mg/kg to 79.24 mg/kg with a mean of 17.86 mg/kg and the DTPA-extractable Ni were between 0.03 and 24.84 mg/kg with an average of 1.16 mg/kg. The pH, clay, CEC and the contents of free Fe and total Mn of the soils influenced the accumulation of Ni and Co in the soils and their availability.2. The concentrations of Co in the edible parts of crops (FW) ranged from not detected to 104.66μg/kg. The Co concentrations of the different crops generally followed the order of fruit vegetables>leafy vegetables and rice>rhizome vegetables. The Ni concentration in the edible parts of the crops (FW) varied from not detected to 3.61 mg/kg. The concentrations of Ni in the crops generally followed the order:rice>fruit and leafy vegetables>rhizome vegetables.3. DTPA-extractable Co and Ni were better than total Co and Ni in indicating the bio-availabilitiy of Co and Ni in the soils. The concentrations of Co in the edible parts of the crops (FW) had close relationships with pH, CEC and the contents of free iron, organic matter and total Mn. The concentration of Ni in the edible parts of vegetables (FW) and rice had only close relationships with free iron content and pH. The soil-to-crop transfer factors of Co and Ni generally decreased with the increase of soil Co and Ni no mater based on total or available concentration.4. Low concentrations of Co and Ni in the culture solutions had hormesis effects to same vegetable species under water culture. However, when the concentrations of Co and Ni increased, toxic effects to the crops appeared. The typical Co and Ni toxicity symptoms were chlorosis in new leaves, withering in leaf margin or in whole leaf, growth inhibition, water loss or even death.5. Base on the apparent symptoms and the EC20 value (effective concentration causing a 20% reduction of shoot biomass) of each species, Early ripening No.5 (Brassica pekinensis Rupr.) and cucumber (Cucumis sativus L.) were selected as the most Co sensitive vegetables and Qingjiang Pakchoi (Brassica chinensis L..) and Water spinach (Lpomoea aquatica Forsk.) were as the most Ni sensitive vegetables. 6. Under soil culture condition, the shoot mass of cucumber with different dosage of Co addition decreased from 21.97% to 95.36% and the shoot mass of water spinach with different Ni treatments decreased from 29.59% to 93.84% compared to the control. High concentration of Co and Ni all influenced the uptake of other nutrients and their transfer from root to shoot, especially for iron and copper.7. The concentrations of Co in about four fifth of the vegetable soils and three quarter of the paddy soils were above the background value of Co in the surface soils of Fujian, and those of Ni in about half of the vegetable soils and the paddy soils were above the background value of Ni in the surface soils of Fujian, indicating the accumulation of Co and Ni in the soils as result from anthropogenic activities. However, the concentrations of Ni in most of the soils were below the Ni limits (China Environmental quality standard for soils and Standard for heavy-metal pollution classification for agricultural soils in Fujian Province).88.8% of the vegetables and 49.23% of the rice contained lower Ni than the interior-controlling standard of national food hygiene standard for Ni. According to the above results, the intake of Co through soil-plant path would not lead to human health risk. However, the daily intake for Ni would be of concern if all the sources of Ni exposure were taken into account,8. The limit values of Co and Ni for the soils were calculated based on the standard of national food hygiene standard (or reference value) and the EC10 values derived from the soil culture. The soil reference values of Co and Ni were developed according to the lower value of the two limit values. The soil reference values of Co were 97 mg/kg (total Co) and 2.7 mg/kg (DTPA-Co), respectively, and those of Ni were 36 mg/kg (total Ni) and 2.2 mg/kg (DTPA-Ni), respectively.