The Effect of Varying Metal Ion Concentration in Soil on Accumulation of the Ions in Parts of Some Vegetables
Chapter One
AIM AND OBJECTIVES OF THE STUDY
This study was conducted to:
- Determine the uptake of lead, nickel, cadmium, chromium and arsenic by lettuce, okra and pepper at different concentrations in the
- To determine the distribution/accumulation of these metals in the different parts of the plants
CHAPTER TWO
LITERATURE REVIEW
UPTAKE OF METALS BY PLANTS
Trace elements such as Cd, Cu, Ni, Pb and Zn are common pollutants in urban industrial soils (Dudka et al, 1996). These elements are often adsorbed or occluded by carbonates, organic matters, Fe-Mn oxides and primary or secondary minerals (Adriano, 1986; Ross, 1994). Since plants take up most nutrients from the soil solution, it is often assumed that the dissolved trace metals are readily available to organisms (Barber, 1984). Thus, determination of total dissolved metals and their chemical speciation may provide useful information on metal bioavailability and toxicity (Knight et al, 1998).
Trace metals are distributed in the soil solution as free ions (Mn+) in form of inorganic and organic complexes. It is generally accepted that, in the short term, plant growth and metal uptake are related to free ion activities (Sparks 1983, Me Bride 1994). In experiments conducted using soil or nutrient solution, the free ion model was able to predict metal uptake by plants (Pavan et al 1982; Bell et al 1991).
Several studies have shown that constructed wetlands are very effective in removing heavy metals from polluted waste waters (Larsen and Schierup, 1981; Nixon and Lee, 1986; Hansen et al, 1998).
Wetland vegetation plays a major role in the removal of toxic trace elements from waters passing through the wetland. Wetland plants remove trace elements by several processes.
- Uptake and accumulation in their
- Physto stabilization of the element into unavailable form; and
- Phyto stimulation of hizo sphere micro-organisms that mineralize, sequester and stabilize the elements (e.g. Se, Hg) into the atmosphere (Terry and Zayed, 1994; Rugh et al, 1996; Berti and Cunningham 1997).
Different wetland plants species differ however, in their abilities to take up and accumulate various trace elements in their tissues (Rai et al, 1995). Different plant species can have varying abilities to take up trace metals (Martin et al, 1996). Since some plants transfer more metals to foliage than others, the effects of bioavailability of metals could not be ascertained. More recently, Falih (1998a) reported that Cd and Pb were accumulated in bio mass of Rhodotorula minute in amounts between 4 and 5mg/g dry weight at a metal initial concentration of 400mg/l in the medium.
According to Zayed et al, (1998), cauliflower, collard etc accumulated Cr mostly in the roots, with concentration exceeding 160 mg/kg Dw. Application of high cadmium content in corn leaves from 0.5 to 20 mg/kg Dw. (Jones et al, 1975).
Leafy vegetables such as lettuce, spinach, beet greens as well as tobacco were found to contain high cadmium levels (Dowdy and Larson, 1975; Charney and Honrich, (1978). The source of lead found in some plants is mostly from the soil (De Dolp et al, 1970) and it is absorbed through the roots of the plants. Half of the lead found in rye grass and radish leaves originated from the soil. Distribution of lead within the plant is variable with plant species.
Motto et al (1970) observed that the lead occurring in plants was more concentrated in the roots and leaves than in the fruits, gratins or tubers. Also, the concentration of lead in the leaves increased as the soil concentration of the elements increases (Motto et al, 1970).
Twelve leafy vegetables were planted on field plots and were treated with 356 mgkg-‘ Zn per acre and control (normal water). The results showed that of the crops grown, only Swiss orchards and spinach had tendency to accumulate zinc. The concentrations of zinc in Swiss orchard and spinach were 80 mg/kg and 139 mg/kg respectively in the control plants, while in the zinc treated plots (436 mg/kg Zn acre), the zinc concentrations in Swiss orchard and spinach were 862 mg/kg and 340 mg/kg respectively. (Boawn, 1971).
CHAPTER THREE
EXPERIMENTAL
SAMPLE COLLECTION AND TREATMENT
COLLECTION OF THE SOIL
Soil sample was collected from a location around the Department of Crop production, Ahmadu Bello University, Zaria in March, 2005 before the commencement of the rainy season.
COLLECTION OF THE SEEDS
The lettuce seeds (Lactuca satiuaty, okra seeds (Abelmoschus esculentum) and pepper seeds (Caspicum annum) were of improved variety purchased from an Agro-Allied Shop in Zaria.
SOIL TREATMENT/ANALYSIS
Soil samples were collected from a virgin land close to the Department of Crop Protection, A.B.U. Zaria, before the commencement of the rainy season. The soil was thoroughly mixed to ensure homogeneity and composite samples were taken for the analysis of the physicochemical properties.
Two kilograms each of the soil was weighed into 48 plastics containers and soaked with distilled water and allowed to stand for three days in the Biological Gardens in Federal College of Education, Zaria.
The seeds were initially planted in a nursery in biological gardens in F.C.E, Zaria for about two weeks after which they were carefully transplanted into the 48 plastics containers and safely transported into the glass house in the Biological Sciences Department of A.B.U, Zaria.
CHAPTER FOUR
RESULTS AND DISCUSSION
PHYSICO-CHEMICAL ANALYSIS OF THE SOIL USED
The physicochemical properties of the soil were carried out in the soil science laboratory in Ahmadu Bello University Zaria, before planting the three
(3) vegetable plant (viz, lettuce, okra and pepper) as shown in Table 4.1. The soil test value suggested that the pH values were weakly acidic, and the textural class is loamy with average organic matter content. The above classes of soil are suitable for the growing of the three (3) vegetable plants (Gruben and Tahir, 2004).
CHAPTER FIVE
CONCLUSION
The conclusion drawn from this study is that, of all the vegetable plants studied (Lettuce, Okra and pepper) lettuce accumulated the highest concentration of Lead (35.07µg/g) in the Roots, followed by Okra (0.28µg/g) in the roots and pepper (0.25µg/g) in the roots. The absorptions of other metals in these vegetable plants are relatively lower. It is known fact that cadmium exists mainly as free ions at acidic pH, where as the other four metals occur mostly as either organic or inorganic complexes.
Results obtained for Lead, Nickel, Cadmium, Chromium and Arsenic on the tables of lettuce, okra and pepper, indicate that the vegetable plants have a way of controlling metal uptake from the soil to root, stem, Leaves and to the fruits. It is also a clear fact with the result that the concentration of these metals in soils does not necessarily determine the concentration in the vegetable plant.
The vegetable plant took what it needed for its metabolic activities irrespective of the level or concentration of the metals in the soil.
RECOMMENDATIONS
- Generally, all the vegetable plants lettuce, okra and pepper, has accumulated little or much of this trace element Pb, Ni, Cd, Cr and As in their roots, leaves or fruits, and this trace metals at high concentrations becomes dangerous to the human health. Therefore, it is recommended that care should be taken to properly investigate where this vegetable plants are grown, since soil polluted with this metals may accumulate it.
- In this study the analysis of the metal concentration in the different parts of the vegetable plants was carried out using Atomic Absorption Spectrometric method (A. A. S) and Energy dispersive x-ray fluorescence. (ED-XRF) other methods of analysis of the metal concentration such as differential pulse poloragraphy, (DPP). Neutron activation analysis and induction coupled plasma atomic emission spectrophotometry (ICP – AES) t.c could be employed.
- The present study is limited to only a few metals and crops. Therefore, the study should be extended to cover other toxic metals and other different vegetable
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