Effect of Salinity and pH on the Accumulation of Heavy Metals in Sunflower (Helianthus Annus) Plant

Keywords: heavy metal toxicity, phytoremediation, vermicompost, physico-chemical factors

Introduction

The main sources of contamination in soil are industrial and domestic waste disposal. Of the various industries, tannery waste water and sludge introduce heavy metals into the soil. Soil contamination with heavy metals has become a worldwide problem, leading to loss in agricultural yield and hazardous health effects as they enter the food chain (Pergent and Pergent-Martini, 1999). The plants growing under such heavy metal contaminated soil accumulate high amount of toxic metals which in turn are being assimilated and transferred within the food chains by the process of biomagnifications (Pergent and Pergent-Martini, 1999). There are several reports on the accumulation of metals in the plants growing on soils receiving industrial wastes (Barman et al., 1999; Nan and Cheng, 2001). The role of plants in phytoremediation of contaminated soil and wastewater has recently been explored (Salt et al., 1995; Sinha, 1999; Lasat, 2002; Sinha et al., 2002).

Heavy metals poison us by disrupting our cellular enzymes, which run on nutritional minerals such as magnesium, zinc, and selenium. Toxic metals kick out these minerals and bind their receptor sites, causing diffuse symptoms by affecting nerves, hormones, digestion and immune function. Conventional technologies, such as ion exchange or lime precipitation, are often ineffective and/ or expensive, particularly for the removal of heavy metal ions at low concentrations (below 50 mg / L). Cadmium is a toxic metal observed in many agricultural soils which is due to long term use of phosphatic fertilizers and sewage sludge application. The role of plants in phytoremediation of contaminated soil and wastewater has recently been explored (Salt et al., 1995; Sinha, 1999; Lasat, 2002; Sinha et al., 2002).

Phytoremediation is an aesthetically pleasing mechanism that can reduce remedial costs, restore habitat and cleanup contamination in place rather than entombing it in place or transporting the problem to another site. Phytoextraction is partially a component of phytoremediation in which metal accumulating plants are used to extract metals from soil and concentrate them into the harvestable parts of root or above ground shoots so as to recover the metal through smelting process.

Materials and Methods

Collection of control soil

Top soil upto 15 cm depth, were collected from the fertile agricultural lands of Kaniyambadi village. The samples were air dried, crushed to powder and sieved in 0.5 mm mesh. The sieved soil samples were stored in polythene bags. 50 g of the various soil samples were mixed with 500 ml of distilled water and stirred using a magnetic stirrer for 2 hours. The extracted soil solutions were used for further analysis. The various physico-chemical factors were analyzed using the soil extracts.

Chemicals

Analar sample of Cadmium chloride was used as a source of Cd.

Vermicompost

Vermicompost was prepared with cow dung using earthworm species Eurdilus euginae.

Plant species used for phytoremediation

Sun flower (Helianthus annuus) seeds were procured from Tamil Nadu Agricultural University, Coimbatore. The seeds were cold treated (10°C) for 3 days to break dormancy and synchronize germination. Seeds were germinated in roll towels and germinating seedlings of similar size were sown in the control and experimental pots and watered. On sixtieth day the plants were harvested for experiments.

Experimental procedure

The seedlings were exposed to different concentrations of heavy metal Cadmium to find the toxicity. The concentration of low toxicity was chosen for further studies.

To study the effect of salinity on the metal accumulation by sunflower plant, the soil samples were prepared as shown in the following table 1.

To study the effect of pH on the metal accumulation by sunflower plant, the different samples were prepared as shown in the following.

• Garden soil (GS) + sunflower plant+100mg/Kg Cd - pH - 5
• 50%Garden soil +50% vermicompost (VC) +100mg/Kg Cd + sunflower plant - pH - 5
• Garden soil + sunflower plant+100mg/Kg Cd - pH - 6
• 50%Garden soil + 50% vermicompost+100mg/Kg Cd + sunflower plant - pH - 6
• Garden soil + sunflower plant + 100mg/Kg Cd - pH - 7
• 50%Garden soil + 50% vermicompost+100mg/Kg Cd + sunflower plant - pH - 7
• Garden soil + sunflower plant + 100mg/Kg Cd - pH - 8
• 50%Garden soil + 50% vermicompost + 100mg/Kg Cd + sunflower plant - pH - 8

Plants were harvested and roots washed with running tap water. Roots, shoots and leaves were then separated and oven dried for 3 days at 80°C. The sample was then ground into fine powder and used for metal analysis.

Physico-chemical characteristics of soil samples

Physico-chemical factors such as pH, Electrical conductivity (EC), moisture, porosity, specific gravity, calcium, nitrogen, phosphorous, potassium (NPK values) were analyzed for soil.

Fig. 1 · A few physico chemical parameters of the garden soil
and 50 % garden soil + 50% vermicompost combination

Heavy metal analysis of soil samples

Soil samples were air dried and crushed to pass through 0.2 mm sieve and stored in polythene bags for analysis. Representative samples from this lot were used throughout the study. 10 grams of each soil was treated with 2ml of perchloric acid and 5 ml of con. HNO₃ and stored in vials for further analysis.

Digestion of the sample

0.5 g of each sample was treated with 3ml of Merk Hydrofluoric Acid, 1 ml of Merk Perchloric Acid and 7 ml of 65% Suprapur Merk Nitric acid. The water used for washing and dilution was milli Q element distilled water from Millipore. This mixture was digested in Ethoplus high performance microwave lab station. The digested sample was made upto 25 ml and analyzed for the metal Cadmium by Atomic Absorption Spectroscopy (Varian AAA 220 FS). The whole analysis was conducted in clean air room of class 10,000 (APHA, 1990).

Results and Discussion

Table 2 and Fig.1 represent the physico-chemical characteristics of the Garden soil, 50% Garden soil (GS) + 50% vermicompost (VC). The EC of GS and GS + VC were recorded 0.249 and 0.924 mmho/cm². The porosity of the 50% GS + 50% VC was recorded the highest (2.82). The organic matter of the 50% GS + 50% VC was found to be the highest when compared to the control GS. This is because the vermicompost added has higher concentration of humic and fulvic acids which increases the organic carbon nitrogen content of the soil. Hence, addition of vermicompost increases the organic matter content of both GS and PS. Total nitrogen recorded 7527 mg/kg, 8215 mg/kg in GS and GS + VC respectively.

The pH is raised (7.9) by the addition of VC. All the factors such as electrical conductivity, percentage of organic matter, porosity, specific gravity, total nitrogen, total phosphorus, total potassium and calcium have been increased in the garden soil and heavy metal polluted soil by the addition of vermicompost. This shows that soil amendment with humus material like vermicompost improves the soil properties showing improvement in the fertility of the soils for the cultivation of crops. Such types of increase in NPK values have been reported by scientists. The pH is an important factor for soil because Cr(III) could be oxidized to more toxic Cr(VI) at pH > 7.The organic matter content had increased uniformly in all the soil samples due to the addition of organic substrates like vermicompost

When conducting phytoremediation to remove heavy metals from soil, the first factor to be considered is to select the species with high biomass and then that with greater uptake ability. Furthermore, plants having a larger biomass could yield better covering and revegetating benefits.

At the same time, the plants seem to survive and accumulate the metals. Addition of vermicompost increases this efficiency. We know heavy metals like Cr, Cd, Zn, Fe, Al, Pb, As are highly reactive and toxic to living cells. Some heavy metals, particularly Cu, Zn and Fe are essential micronutrients involved in various physiological processes but become toxic above certain threshold concentrations. By accumulating these metals, plants have developed complex mechanisms by which they control the uptake and accumulation of heavy metals (Cobbett and Goldsbrough, 2001)

Effect of pH and Salinity on phytoremediation

Although adverse effects of salt accumulation in soil and crops have been investigated intensively, relatively little attention has been paid to the influence of either soil or irrigation water salinity on heavy metal uptake by crops. A very important question raised regarding plant–soil–metal interactions is how much soil pH affects the Ni and other metals by hyper accumulator plants. High human impact (urban, industrial and agricultural activities, handicrafts and traffic) on these territories leads to the pollution of sands and irrigated lands with pesticides, nitrates, organic pollutants and various heavy metals.

Table 3 and Fig.2 represents the effect of salinity on the uptake of heavy metal cadmium in sunflower plant. The increasing the concentration of sodium chloride from 60 mg/L to 240 mg/L there is a steady increase in the metal bioaccumualtion both in the roots and shoots. Comparing the bioaccumulation of heavy metal cadmium by the roots and shoots of sunflower plant it is shown that the shoots accumulate more metal than roots.

Fig. 2 · Percentage Bioaccumulation of Cadmium
by Sun Flower at different Salinity Values

Table 4 and Fig. 3 represent the effect of pH in the bioaccumulation of heavy metal cadmium by the heavy metal hyper accumulator sunflower plant. When the pH was raised from 4 to 8 there was regular decrease in the heavy metal uptake. From this results it is evident that pH of the soil has a profound effect on the metal uptake by the plants. At lower pH values the metal ions shows greater cation exchange capacity (CEC) and become more available in the aqueous medium there by making the metal to be more bioavailable to the plant.

pH dramatically effects the CEC of soil by limiting the available exchange sites at low pH. H+ bind to soil particles tighter than other cations, thus, any metal bound to a soil particle will get booted off in the presence of excess H⁺.

At low pH (<6), H⁺ is in excess and replaces all other cations on the micelle, thus making them bioavailable. At high pH (>7), cations are less bioavailable because they have less competition from H⁺ for available binding sites. Many cations bind to free hydroxyl groups (OH^–) and form insoluble hydrous metal oxides, which are unavailable for uptake, such as CdCO₃.

Fig. 3 · Effect of pH and Soil Amendment on
the Percentage Bioaccumulation of Cadmium
by Sunflower Plant

Adding sand to the soil will decrease the overall cation exchange capacity since sand does not bind cations. The problem with this is that the CEC of the clay is still the same and since it is not being removed, the net gain in availability of metals is small. Organic matter is commonly added to soil to lower pH. This is seemingly a good way to increase Cd bioavailability, but could actually backfire since Cd binds to organic matter with high affinity Thus, addition of organic matter could actually decrease Cd bioavailability, despite the decrease in pH conferred (Table 3 and Fig. 3).

Conclusion

These technologies will prove useful in environmental cleanup procedures and subsequent restoration of soil fertility. Hence, attempts on remediation of polluted soil by phytoremediation methods fulfills the National policy of “Eco-friendly” development, and create socio-economic, scientific developments among the Environmentalists, Agriculturalists and Scientists who involve in the application of this technique. Above all, this low cost technique will help the poor farmers to reuse the lands for agricultural purposes.

Acknowledgement

We express our deep and profound sense of gratitude to Global Institute of Engineering and Technology for providing the necessary facilities throughout this work.

References

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