The trees and tree leaves are absorbing heavy metals from the constituents of environment. The waste materials of industries like fly ash of thermal power plant, slag of steel plant and engine oils of automobile industries are contaminating the soils of these areas. The waste materials contain heavy metals which increases the toxicity of soil. These heavy metals may be retained in soils in the form of oxide, hydroxide, exchangeable cations and or bound to organic metal in soil. These are then absorbed by trees and tree leaves. Although some heavy metals at low concentration are essential micro-nutrients for plants, but at high concentration they may cause metabolic disorders and growth inhibition for most of the plants. When these types of tree leaves are directly or indirectly consumed by us, it causes several problems & diseases in our body. The contamination beyond the limit effects badly in our ecosystem. Exposure to different heavy metals causes various health problems viz-hypertension, fever, kidney disorder, cramp and DNA damage in living cells.
In the present work, an attempt has been made to see the qualitative and quantitative extent of absorption of heavy metals in soils and tree leaves collected from different industrial regions like: Thermal Power Plant, Steel Plant and Auto-mobile Industries Area. The collected samples have been digested in appropriate acid medium under optimized condition followed by their analysis in ICP-OES.
a) Sample Preparation:
Soils: The powered soils collected from different region are treated with HCl & HF in a Teflon beaker on a hot plate. After proper digestion they are baked and made solution with HCl and water to analyze for most of the heavy metals except As and Hg. For these two, the soil is directly boiled with HNO3 in a water bath at as low as 65°C.
Tree leaves: The collected leaves are cleaned properly by water and dried in air. Then they are made ash in a platinum basin using 2 ml of conc. HNO3 & then digested with conc HCl to analyze for most of the heavy metals except As and Hg . For As & Hg, the leaves are directly boiled with HNO3 in a water bath at as low as 65°C.
To ensure the best resolution available, an ICP-OES (Make: Perkin Elmer, Optima 7000 DV) was used for this study. Calibration was carried out by Merck Multi-element standards dissolving them in similar acid medium by the same procedure as used for the samples. In all the cases reagent blanks were also made under identical condition During calibration, the correlation co-efficient values have been found to be greater that 0.999- for all the parameters revealing a true linear curve in all the cases. The characteristics of the Instrument and of the operating conditions are given in Table 1 below:
Results & Discussion:
The heavy metal contents in soil are summarized in Table 2. The concentration of Cr, Cu, Fe, Mn, Pb, Cd, Zn, As, Hg was ranged from 20-200, 13-32, 39013-62910, 604-809, 2.5-10, 0.2-0.6, 9-15, 0.9-1.5, 2.2-7 mg/L respectively. Cr, Fe, Mn concentrations are greater in the soil of steel industry. The concentration of Pb, Hg, As are also in higher side in the auto-mobile industrial area than the other places.
The heavy metal contents in leaves is summarized in Table 3. The concentration of Cr, Cu, Fe, Mn, Pb, Cd, Zn, As, Hg was ranged from 10-72, 11-52, 720-5123, 23-103, 1.8-12, 0.1, 9-15, 1-2, 1.9-10 mg/Kg respectively. The concentration of Hg & Zn is higher around Thermal power plant and steel plant than in the soil of these area. The concentration of Pb in leaves is also found higher in thermal power plant & Auto-mobile industries. Figure 1 shows a comparison in bar graph for each heavy metal content between soil and leaves near the thermal plant, steel plant and auto-mobile industries.
A Scientific Term called Bio-accumulation Factor may be derived by calculating the ratio of heavy metal contents in leaves and heavy metal contents in corresponding soil. If the bio-accumulation factor is greater than 1 then it is harmful for our ecosystem and environment. Thus, in case of Thermal Power Plant bio-accumulation factor of Cr, Cu, Fe, Mn, Cd <1 and for Pb, Zn, As,Hg it is > 1. For Steel plant bio-accumulation factor of Cr, Fe, Mn, Pb, Cd, Hg <1 and for Cu, Zn, As, it is > 1. For auto-mobile industry bio-accumulation factor of Cr, Cu, Fe, Mn, Cd Zn, < 1 and for Pb, As,Hg it is>1 as revealed from this short study. Table 4 the Bio-accumulation Factors derived for the individual heavy metals under different industrial regions whereas, Fig. 2 depicts their hazards above the optimum level.
The result of this study shows that the extent of absorption of individual heavy metals vary significantly in case of soil and leaves grown on them depending on the nature of Industrial Regions. Calculation of Bio-accumulation Factor gives a direct indication about the type of heavy metals which are prone to cause significant hazard in that particular Industrial area. By this work we could gain some brief idea about the nature of heavy metals in our ecosystem.
The work has been carried out by Mr. Susovan Pal and Ms. Priya Das, MSK Central Laboratory, Kolkata