ارزيابي خطرپذيري آلودگي عناصر سنگين در خاك ها و برخي محصولات كشاورزي در اطراف شهر و مناطق صنعتي زنجان

Risk Assessment of Heavy Metals Contamination in Soils and Selected Crops in Zanjan Urban and Industrial Regions

عناصر سنگین تركیباتی هستند كه به طور طبیعی در خاك وجود دارند یا در نتیجه فعالیت‌های انسان وارد خاك می‌شوند. مهم‌ترین مسیر در معرض قرار گرفتن عناصر سنگین مصرف روزانه مواد غذایی است. پژوهش حاضر با هدف تعیین غلظت عناصر سنگین در خاك و برخی محصولات كشاورزی اطراف مناطق شهری و صنعتی زنجان و احتمال خطرپذیری از مصرف آنها انجام گردید. تعداد 75 نمونه خاك سطحی (عمق 0 تا 10 سانتی‌متر) و 101 نمونه از محصولات كشاورزی اطراف مناطق شهری و صنعتی برداشت گردید و تجزیه‌های آزمایشگاهی در نمونه خاك‌ها و گیاهان مختلف انجام شد. میانگین غلظت كل و قابل جذب عناصر به ترتیب برای مس 4/52 و 47/4، روی 8/264 و 15/23، سرب 7/105 و 81/16، نیكل 7/47 و 20/1 و كروم كل 7/21 میلی-گرم بركیلوگرم خاك به دست آمد. همچنین غلظت عناصر سنگین در گیاهان مختلف نشان داد كه سبزیجات بیشترین سهم را در جذب و انباشت فلزات نشان می‌دهند. بالاترین مقادیر Zn (شاهی)، Pb (شوید)، Cu (تره)، Ni (ریحان) و Cr (ریحان) به ترتیب با غلظت‌های 25/150، 25/41، 13/23، 46/6 و 47/3 میلی‌گرم بركیلوگرم و كمترین مقدار فلزات مورد بررسی در میوه سیب و بذر گندم و جو مشاهده شد. به صورت كلی میزان جذب فلزات در گیاهان مختلف بدین صورت بدست آمد (Zn > >Pb >Cu >Ni >Cr). فاكتور تجمعی (BAF) فلزات در گیاهان به ترتیب Zn=Cu >Pb > >Cr >Ni حاصل شد. احتمال خطرپذیری (HQ) به بیماری‌های غیرسرطانی برای هر یك از عناصر (به جز سرب) در دو گروه سنی كودكان و بزرگسالان كم‌تر از یك به دست آمد كه نشان می‌دهد مصرف‌كنندگان از این محصولات در محدوده امن از نظر تأثیرات سوء بیماری‌های غیرسرطانی قرار گرفتند. همچنین مقدار احتمال خطرپذیری كل (THQ) در كودكان بیشتر از بزرگسالان بدست آمد.

Introduction: Heavy metals are types of elements naturally present in soil or enter into soil as a result of human activities. The most important route of exposure to heavy metals is daily intake of food. Crops grown in contaminated soil (due to mining activities, industrial operations and agriculture) may contain high concentrations of heavy metals. Also closeness to cities and industrial centers can have a great influence on the accumulation of heavy metals to agricultural products grown in the region. The study aimed to determine the concentration of heavy metals in soil and agricultural products around urban and industrial areas of Zanjan province (North West of Iran) and consumption hazard probability. Materials and Methods: Soil (75 samples of soil from a depth of 0 to 10 cm) and plant (101 samples) samples, in the summer 2011, were randomly taken from industrial areas as follow: tomatoes (Lycopersicum esculentum M), wheat seed (Triticum vulgare), barley seeds (Hordeum vulgare), alfalfa shoots (Medicago sativa L.), potato tubers (Solanumtuberosum L.), apple fruit, vegetables and fruits such as Dill (Aniethum graveolens L.), leek (Allium porrum L.), Gardencress (Barbara verna L.) and basil (Ocimum basilicum L.). Plant samples were then washed with distilled water, oven dried for48 hours at a temperature of 70 ´C until constant weight was attained and then they digested using 2 M hydrochloric acid (HCl) and nitric acid digestion in 5 M. Concentrations of heavy metals in the soil and crops were determined by atomic absorption spectrometry. DTPA extraction of metals by Lindsay and Norvell (1978) method and sequential extraction method by Tessier et al. (1979) were performed. Statistical analysis was accomplished using the software SPSS 16.0 and the comparison of mean values was done using the Duncan test at the 5% level of significance. Results and Discussion: The magnitude of variations for total copper was from 11.5 to 352.5 (average 52.4), zinc was from 96.3 to 1353.8 (average 264.8), lead was between 40.0 and 470.0 (average 105.7), nickel ranged from 12.8 to 77.0 (average 46.7) and chromium varied from 10.0 to 49.5 (average 21.7) mg kg-1. DTPA extracted heavy metals for copper varied from 1.50 to 21.23, averaging 4.47, zinc from 0.57 to 76.50 averaging 23.15, lead from 2.43 to 63.38 averaging 16.81 and nickel from 0.28 to 2.32 averaging 1.20 mg kg-1. Chemical changes in the different fractions were as follows: Cu (residual > bounded to organic matter > bounded to Fe-Mn oxides > bounded to carbonate > exchangeable fraction), Zn and Ni (residual > bounded to Fe-Mn oxides > bounded to carbonate > bounded to organic matter > exchangeable fraction,) and Pb (residual > bounded to Fe-Mn oxides > bounded to organic matter > bounded to carbonate > exchangeable fraction). The concentration of heavy metals in plant parts were high with respect to studied location. The highest amounts of Zn (Gardencress), Pb (Dill), Cu (Leek), Ni (Basil) and Cr (Basil), respectively were found to be 150.25, 41.25, 23.13, 6.46 and 3.47 mg kg-1 and the minimum amounts of the metals studied were found in fruits, wheat and barley grains. The total amount of metals in plants were as follow (Zn > > Pb > Cu > Ni > Cr). Bioaccumulation factor (BAF) of metals in plants were as Zn=Cu > Pb > > Cr > Ni. Hazard probability (HQ) in cancerous diseases for each element (except Pb) in both children and adults was less than unit. HQ content of Pb was much higher than the unit and for children and adults 9.07 and 6.94, respectively showing high contribution of Pb contamination of crops that threatens the consumer health in that location. The total amount of risk (THQ) in children was higher than that in adults. Conclusions: The results obtained in this study indicate that an urgent attention is required for consumer products related to public health, especially vegetables grown in the studied regions. Toxic effects of heavy metals have many deleterious effects which are more pronounced over time. With conventional monitoring of food quality produced in farms and presented in markets, excessive accumulation of heavy metals entering in to the human food chain can be prevented. Also, we can change the risk potential of heavy metals in the region by growing vegetables which accumulate heavy metals. Keywords: Agriculture products, Biological risk, Heavy metals, Soil, Zanjan province