PHYTOREMEDIAL POTENTIALS OF Ipomoea aquatica AND Colocasia esculenta IN SOILS CONTAMINATED WITH HEAVY METALS THROUGH AUTOMOBILE PAINTING, REPAIRING AND SERVICE CENTRES

Heavy metal pollution is one of the biggest environmental issues at present. The study was conducted with the objectives of evaluation of soil contamination of heavy metals with Cu, Zn, Pb, Mn, Cr and Fe by automobile repairing, painting and service centres in Kandy area, Sri Lanka, and the phytoremediation potential of using Ipomoea aquatica and Colocasia esculenta in such soils. Soil and plant samples from ten sites associated with these industries were analyzed for the Cu, Zn, Pb, Mn, Cr and Fe concentrations and Bioconcentration (BCF) and the translocation factors (TF) were determined. The soils associated with these nutrients were highly contaminated with all the heavy metals studied and both plant species accumulated these heavy metals in different concentrations. The automobile repairing centres caused highest pollution by Cu, Mn and Cr, automobile painting centres caused highest pollution by Pb and Zn, and automobile service stations caused highest pollution by Cr and Fe. Fe concentration was the highest in contaminated soils. Heavy metals were accumulated in all plant parts, roots containing the highest concentrations. Considering the TF and BCF, I. aquatica was identified to be suitable for phytoextraction of Cu and Mn, while C. esculenta was found to be suitable for phytostabilization of Cu, Pb, Mn, Fe and Zn. Uncontrolled release of waste containing heavy metal pollutants, and consequences of I. aquatica and C. esculenta growing in such contaminated sites may cause heavy health hazards.


Introduction
Heavy metals are amongst the most toxic contaminants in the environment.They are defined as elements with metallic properties and an atomic number more than 20 (Tangahu et al., 2011).Anthropogenic activities such as industries, energy production, constructions, vehicle exhaust, waste disposal, as well as coal and fuel combustion cause production of heavy metals (Li et al., 2001;Bai et al., 2008).Dispersion of these heavy metals to non contaminated areas occurs in many different ways, while contributing towards contamination of the ecosystem (Gaur and Adholeya, 2004).A large number of sites worldwide have already been contaminated with high concentrations of heavy metals, making these sites unsuitable for any potential use.This problem is more severe in developing countries such as Sri Lanka, where strict monitoring mechanisms on the waste disposal to the environment from industries do not exist.Therefore, it is an urgent matter to find effective and affordable remediation technologies to clean the contaminated environment.
Heavy metals usually do not undergo biodegradation, instead accumulate in living organisms causing bioaccumulation followed by health issues (Pehlivan et al., 2009).Their soil residence may have negative effects on plant growth, ground cover and soil microflora (Roy et al., 2005).Therefore, heavy metals should be removed physically or be transformed into nontoxic compounds.
Though several methods have been used to clean up the environment, most of them are costly and do not yield optimum results.Remediation of heavy metal contaminated soil involves chemical and thermal methods or excavation and subsequent disposal to landfill sites.However, most of these technologies are expensive, technically complex and difficult to apply (Rukhshaee et al., 2009); hence their implementation in countries such as Sri Lanka is not feasible.In this context, phytoremediation can be considered as an effective and affordable solution to remove heavy metals from contaminated soils.
Phytoremediation is the technology that uses selected plants to clean up contaminants from soils, sediments and water (Tangahu et al., 2011).This technology uses exceptional metal accumulating capacity of some plants, which are known as 'hyperaccumulators' that tolerate high concentrations of heavy metals in their systems (Chaney, 1983;Baker et al., 2000).These hyperaccumulators have extremely high capacity to uptake metals, together with the translocation, bioaccumulation and constant degradation abilities (Hinchman et al., 1998).Plant species that are able to survive in contaminated soils rich in Zinc (Zn), Copper (Cu), Lead (Pb), Cadmium (Cd), Nickel (Ni), Chromium (Cr), and Arsenic (As) are divided into two main groups; pseudometallophytes that grow on both contaminated and noncontaminated soils and the absolute metallophytes that grow only on metalcontaminated and naturally metal-rich soils (Baker, 1987).
Many plant species have been identified of having hyperaccumulation properties.Ipomoea reptans (Water spinach), Eichhornia crassipes (water hyacinth), Salvinia molesta, Ipomoea aquatica (Kankun) and Colocasia esculenta (Habarala) are some examples that have the potential of hyperaccumulation of heavy metals.Some of these species are found to be quite effective in remediating contaminated areas (Reeves and Baker, 2000;Mahamud et al., 2008;Bindu et al., 2010;Kruatrachue et al., 2015;Mazumdar and Das, 2015).Results of many studies have proven that I. aquatica and C. esculenta have the potential to remediate contaminated soil, water and sediments.C. esculenta is also a promising plant species for remediation of waste water polluted with Pb and Cd (Bindu et al., 2010).
There are many large and small scale industries in Sri Lanka where the wastes containing high amounts of heavy metals are directly released to the environment.Some of the above mentioned hyper accumulators grow in such environments naturally; hence, they can be used effectively for phytoremediation.
Present study was executed with the objective of assessing the heavy metal accumulation of soils in land areas near automobile painting, servicing and repairing centres in the Kandy area of Sri Lanka, and to evaluate the phytoremediation potential of I. aquatica and C. esculenta of such soils.

Material and Methods
This study was carried out from July to September 2011 at the Department of Crop Science, Faculty of Agriculture, and Department of Geology, Faculty of Science, University of Peradeniya.Sampling was done in the Kandy area of Sri Lanka, located at 7.2955˚N and 80.6356˚E at an average elevation of 506 m.Locations of three common small-scale industries, i.e. automobile painting, automobile repairing and automobile service centres were selected for the study, since there is no monitoring mechanism of the release of waste to the environment from these industries.All these locations were within or in close proximity to the Kandy municipal area and located in lowlands which were prone to water logging during the rainy season.Ten of such sites, including four automobile repairing, three automobile painting and three automobile service centres were selected for the study.
I. aquatica and C. esculenta were found to be commonly growing in all the above sites.Soil and plant samples of I. aquatica and C. esculenta were collected from lower elevation of these sites, where stored water flows in.Samples were collected in four replicates from each site.Soil and plant samples were also collected from upper elevations of the industries where no contamination from industries was evident due to stored water runoff and these were considered as controls.The plant samples were oven dried at 73 0 C to a constant weight.Dry weight was taken using an analytical balance.Dried plant samples were ground using a grinder and sieved through using 1mm mesh sieve.The soil was air dried to a constant weight, ground using a mortar and a pestle, and sieved using a 1mm mesh sieve.
The ability to translocate metals from roots to above ground parts of the plants were evaluated by means of the bioconcentration factor (BCF), which is defined as the ratio of metal concentration in the roots to that in soil ([metal] root/ [metal] soil).The translocation factor (TF) was calculated as the ratio of metal concentration in the shoots to that in roots ([Metal] shoot/ [Metal] root) (Stephen et al., 2013).
Data were analyzed using Analysis of Variance and the mean separation was done using Duncan's multiple range test, using SAS statistical package.

Results
The mean concentrations of different heavy metals in the soil and plant samples collected from the three different types of industrial sites are given in Table 1, and their concentration as a percentage of the concentration in the control sites are given in Table 2. Table1.The concentration of different heavy metals in plant and soil samples from different experimental sites and control sites.Results showed that soils and plants of each site contained high concentrations of the heavy metals studied (Cu, Pb, Mn, Cr and Fe) compared to the control (Table 1).The mean accumulation of Cu, Pb, Mn, Cr, Fe in soil were 672%, 837%, 2015%, 228% and 11180% respectively compared to the control, irrespective of the industry.When taken as an average in each industry, the highest contaminations of Cu, Mn and Cr were from the automobile repairing centres compared to the other two industries.Automobile painting centres mostly caused pollution by Pb, whilst pollution by Cr and Fe was mostly by the automobile service stations compared to the other industries considered.Of all the heavy metals considered, the highest concentrations were with Fe, followed by Mn (Table 2).The only heavy metal that did not show a difference with the industries and control was Zn; The Zn accumulation in different plant species and industries did not show any trend.All other heavy metal concentrations in plants were higher than in the control.However, the differences in the concentrations of heavy metals between different industries were not statistically significant.

Heavy metals in the soil and plants
All the three industries caused pollution of the soil by heavy metals.Heavy metals such as Pb are known to be present in fuel, which is used in its purification (Harrison and Laxen, 1981;Culbard et al., 1988;Ho and Tai, 1988).It is also contained in paints.Hence, a most probable source of such contamination of Pb may be waste gasoline.Scraping of the old paint, removal of used oils, washing of different automobile parts and ad-hoc releasing them to the environment may have caused these contaminations in the soil.The accumulation of some of these heavy metals was quite alarming, such as Fe and Mn, since they were present in extremely high concentrations compared to the control.These metals are very commonly used in various alloys in automotive industries and spare parts, and could contaminate the soil upon their cleaning and rusting.Contamination of water bodies by these heavy metals may be possible, which may lead to more severe long lasting problems.The statistical non-significance of the differences in heavy metals between different industries may be due to the variation between the different locations.
According to the percentages given in table 2, it is very clear that all three industrial sites have high concentrations of heavy metals compared to the control, indicating that industries have released their waste directly to the environment.The highest percentage was recorded for Fe, followed by Mn and Cu.When heavy metals present in the soil, they were taken up by both plant species studied, and accumulated, as shown in Tables 1  and 2. This was true for all the heavy metals studied, as the concentrations were highest than in the control, indicating bioaccumulation (Table 1).In many situations, the plants contained higher concentrations of heavy metals than the soil in the same locations.Both plant species appeared to be healthy under high concentration of heavy metals without showing any toxicity symptoms.
Though both plant species were exposed to same metal concentrations, their capabilities of accumulation varied with the metal.When the average value of the concentration of each heavy metal of the three locations was considered, the concentrations of Cu, Pb and Cr were higher in C. esculenta (145, 7, 52 mg/kg respectively) than in I. aquatica (130, 4, 49 mg/kg respectively), and the Mn, Fe and Zn concentrations were greater in I. aquatica ( 524, 20492, 110mg/kg respectively) than in C. esculenta (432, 20189, 105 mg/kg respectively), irrespective of the industry.In I. aquatica, Mn, Fe, Cu, Zn, Pb and Cr concentrations were in the range of 325-760 ppm,17574-22234 ppm 83-206 ppm, 69-192 ppm, 2.38-4.67 ppm and 41.00-56.50ppm respectively and in C. esculenta they were in the range of 275-692 ppm, 7336-45014ppm, 89-243 ppm and 47-187ppm, 6.00-8.65 ppm and 43.92-58.00ppm respectively.The Cu concentrations were larger than in the soil in both these species, and the Mn concentration was larger in I. aquatica than in the soil.
The ratio of different heavy metals in the roots of the two species considered was close to 1, indicating similar capacities of accumulation of the heavy metals by both species.However, there were two exceptions, i.e., near the automobile repairing industry where Pb accumulation was approximately 3.6 times greater in C. esculenta than in I. aquatica, and near the automobile repairing and painting industries Fe accumulation was about 2.7-2.9 times higher in C. esculenta than in I. aquatica.

Heavy metal accumulation and translocation
Results showed that after uptake of heavy metals from the contaminated soil into the roots of the plants, they were translocated to different plant parts in lower concentrations.The fact that all heavy metal concentrations in leaves were significantly lower than the concentrations in roots and stems suggested that the metals were bound to the root cells and their translocation to the leaves was limited.These results confirm some other earlier studies (Bindu et al., 2010).The higher concentrations of these heavy metals also suggest that these two species can be used in phytoremediation of the above metals.These have been suggested as potential phytoremediators of heavy metals such as Fe, Mn, Cr, Zn, Pb, Cu, Ni, and Cd in some other studies too (Reeves and Baker, 2000;Mahamud et al., 2008;Bindu et al., 2010;Kruatrachue et al., 2015;Mazumdar and Das, 2015).
The BCF and TF values were calculated to be used to evaluate the ability of plants to accumulate heavy metals (BCF) and to translocate (TF) (Table 3).These values are useful in determining whether these species could be used for phytoextraction and phytostabilization purposes as per Yoon et al. (2006).Plants with both BCF and TF values greater than one have the potential to be used in phytoextraction.In addition, plants with BCF greater than one and TF less than one have the potential to be used for phytostabilization (Yoon et al., 2006).Phytoremediation technologies involve several mechanisms of phytoextraction, phytostabilization, rhizofiltration and phytovolatilization (Anon., 2009).Phytoextraction is the uptake of contaminants by plant roots and translocating them to shoots that can be harvested and burned gaining energy and recycling the metals from ash.Phytostabilization is defined as immobilization of contaminants in the soil and groundwater through absorption and accumulation, adsorption onto roots or precipitation within the root zone preventing their migration in soil (Ibeanusi, 2004;Erdei et al., 2005;Erakhrumen and Agbontalor, 2007).Therefore, ability of plants to accumulate metals from soils can be estimated using BCF, and their ability to translocate metals from roots to shoots can be measured using TF (Deng et al., 2004;Yoon et al., 2006).
In the present study, values of both BCF and TF were greater than 1 were recorded in I. aquatica, for Cu and Mn (Table 3).When the BCF values were greater than 1, the TF values were less than 1 in C. esculenta for Cu, Pb, Fe and Zn.Therefore, it can be inferred that amongst the tested plant species, I. aquatica was suitable for phytoextraction of Cu and Mn, while C. esculenta is suitable for phytostabilization of Cu, Pb, Mn, Fe and Zn.
I. aquatica and C. esculenta were used in this study, since these were the only common species to the sites examined.There were many other species growing in these sites which were not common in both sites.These plants also did not show any apparent toxicity symptoms despite quite high concentrations of heavy metals.Testing such species for their phytoremediation potential will also be very useful.However, I. aquatica and C. esculenta should be used for phytoremediation purposes with caution, since both are edible species.I. aquatica is a commercially cultivated popular green vegetable in Sri Lanka, which has a high domestic demand as well as a demand from hotels and restaurants.C. esculenta is also consumed as a root vegetable in rural areas.Consumption of these plant species growing in polluted soils could have direct impacts on health of local people.Green vegetables such as I. aquatica growing wild in abandoned lands are collected by vendors due to the high prices during certain periods.Therefore, strict measures should be adopted against using these species growing in contaminated soils for human consumption.The study also emphasizes the importance of implementation of regulatory measures against environmental pollution as the uncontrolled release of pollutants to the environment by various small scale industries cause accumulation of heavy metals in soil and plants, and may cause irreversible damage to the environment including the soil, its fauna, flora and humans.

Conclusion
This study revealed that automobile repairing, painting and service centres cause high pollution of soils in Kandy, due to heavy metal contamination.The heavy metals such as Cu, Pb, Mn, Cr, Fe and Zn were present in such contaminated soils.It was also revealed that two edible plants, I. aquatica and C. esculenta absorb these heavy metals and accumulate them.Concentration of all the above mentioned heavy metals were higher in roots than in shoots indicating that these metals were bound to the root cells and their translocation to the leaves was limited.It was also found out that I. Aquatica is suitable for phytoextraction of Cu and Mn, while C. esculenta is suitable for phytostabilization of Cu, Pb, Fe and Zn.People should be cautioned about using these species growing in contaminated soils as a food, since they can contain high levels of heavy metals.

Figure 1 .
Figure 1.The concentrations of heavy metals in different parts of I. aquatica and C. esculenta plant species (a) Cu, (b) Pb, (c) Mn, (d) Cr, (e) Fe and (f) Zn

Table 2 .
The concentration of heavy metals in plants and soil samples in experimental sites as a percentage of their concentration compared to control sites.Fe and Zn were less than the concentrations of Cu, Pb and Mn, comparable to that of the roots and stems.

Table 3 .
Bioconcentration (BCF) and translocation factors (TF) calculated for different heavy metals for I. aquatica and C. esculenta grown in heavy metal contaminated soil.