Assessment of Heavy Metal and Pesticide Contamination in Banana Fields and Development of Phytoremediation System in Kozhikode District, Kerala, India

Analysis of soil samples collected from various banana fields in Kozhikode district was carried out to identify the extent of heavy metal and pesticide contamination. Analysis by atomic absorption spectrometry revealed contamination of lead in some of the banana fields (Omasseri, Balussery, Vengeri & Nanmanda) in the district. The Gas Chromatogram analysis revealed that the banana fields in Kozhikode district were free of organo-chloride and organo-phosphate pesticide contamination. Among the several ornamental plants evaluated for assessing lead phytoremediation potential, Dianthus chinensis L. showed the highest phytoaccumulation potential. Chrysanthemum indicum L., Ruellia simplex C. Wright and Lantana camara L.also showed high lead accumulation. Therefore, these ornamental plants could be used in phytoremediation to remove lead from contaminated soils.


INTRODUCTION
Mining, processing, and smelting activities have contaminated soil and water resources with heavy metals at various places in the world. Act of geochemical weathering processes upon metallurgical wastes and by-products instigate the process of carrying heavy metals from contaminated places and redistributing them into soils, streams, and groundwater of surrounding areas (Fuge et al.1993, Paulson 1997. Thus, soil and water resources, the health of the ecosystems, and human populations are adversely affected by heavy metals. Many reports have shown that short-term or long-term exposure to toxic metals results in the reduction of microbial diversity and activities in soil (Kelly et al. 2003).
Heavy metals include arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), iron (Fe), lead (Pb), mercury (Hg), platinum (Pt), silver (Ag), and the zinc (Zn) group elements. Heavy metal concentrations beyond certain limits have adverse health effects because they interfere with the normal functioning of living systems. Lead poisoning causes inhibition of hemoglobin synthesis, problems in the functioning of kidneys, reproductive systems, cardiovascular system, and acute and chronic damage to the central nervous system (CNS) and peripheral nervous system (PNS). Mercury poisoning in its organic forms, like monomethyl and dimethylmercury, causes erethism (an abnormal irritation or sensitivity of an organ or body part to stimulation), acrodynia (also called Pink disease, which is characterized by rash and desquamation of the hands and feet), gingivitis, neurological disorders, stomatitis, total damage to the brain and CNS, and are also connected with a congenital malformation (Duruibe et al. 2007).
Contamination by xenobiotic chemicals has been reported in a large number of hazardous waste sites. Considering the increasing contamination of the environment, many remediation technologies have been developed to treat soil, water, and air contaminated by various pollutants, including in situ and ex situ methods (Aboulroos et al. 2006). Soil remediation is the return of soil to a condition of ecological stability together with the establishment of plant communities it supports or supported to conditions prior to disturbance. Conventional technologies for the removal of metals from polluted soils involve transportation of soil to laboratories, soil washing with chemicals to remove metals, and finally replacing the soil at its original location after disposing of its hazardous waste (Francis et al. 1999). This decontamination strategy is an ex situ approach and can be very expensive and damaging to the soil structure and ecology (Salt et al. 1998). Immobilization of heavy metals through the addition of lime (Krebs et al.1999), phosphate (Ebbs & Kochian 1998), and calcium carbonate (Chen et al. 2000) has been suggested as remediation techniques. These remediation methods have the advantage of instantaneously reducing the risk factors arising from metal contamination, but these are considered temporary alternatives because the metals have not been fully removed from the soil environment. Conventional methods to remediate metal-contaminated soils like soil flushing, solidification/stabilization, vitrification, thermal desorption, encapsulation, etc. can be used at highly contaminated sites but these methods are not suitable for larger areas. Furthermore, these remediation techniques require high energy input and expensive machinery (Schnoor 1997). Conventional methods may destroy soil structure and decrease soil productivity (Leumann et al. 1995). Considering the problems associated with conventional methods, some organism-based remediation techniques, such as bioremediation was developed to degrade and detoxify certain contaminants. Compared to traditional methods, these biological systems are less amenable to environmental extremes but they are more cost-effective (Cunningham & Berti 1993). Over the past decade, there has been increasing interest in the development of plant-based remediation technologies which have the potential to be low-cost, low-impact, and environmentally sound, a concept called phytoremediation (Cunningham & Ow 1996).
Phytoremediation is applicable at sites containing organic, nutrient, or metal pollutants that can be absorbed by the roots of plants and sequestered, degraded, immobilized, or metabolized in place (Vijayan & Sushama 2018). Phytoremediation is accepted because of its cost-effectiveness, aesthetic advantages, and long-term applicability (Yadav et al. 2010). To obtain the optimum remediation for the prevailing conditions, a particular contaminated site may require a combination of procedures. Biological, physical, and chemical methods may be used in combination with one another to reduce contamination to a safe and acceptable level. Considering all these, the present study was conducted to identify the extent of heavy metal and pesticide contamination and the development of a phytoremediation system in banana fields of Kozhikode District.

Study Area
Kozhikode or Calicut district of Kerala state, India was selected as the study area for the present study. Bananas in their raw and value-added forms become an inseparable part of daily life and celebrations of the district.

Soil Samples
Soil samples collected from different banana fields of Kozhikode district were used for the study. Ten typical locations Omassery, Balussery, Kanthapuram, Vengeri, Vazhayur, Kakkur, Kuttiadi, Vanimel, Thiruvambadi, and Nanmanda were selected. In each of the selected banana field, soils were collected from a depth of 0-50 cm.

Experimental Protocol and Procedure
Test for heavy metals: Soil samples collected from 10 different banana fields of Kozhikode district were used for the present study. One gram of each soil sample was digested with repeated addition of 10ml nitric acid (HNO 3 ) and Hydrogen peroxide (H 2 O 2 ). The solutions were heated to boiling and filtered. 20ml of Hydrochloric acid (HCl) was added to the initial digestate and the sample was refluxed. After filtration, the digestate was diluted to a final volume of 100 mL. The heavy metal concentration in all the samples was analyzed by Flame Atomic Absorption Spectrophotometer at CWRDM (Centre for Water Resources Development and Management), Kozhikode, Kerala.
Test for pesticides: 5g of each soil sample collected from different places of Kozhikode district were taken. It was then shaken with 20ml of hexane for one hour. After filtration, the volume of the solutions thus obtained was made to 50ml each. The Pesticide concentration in all the samples was tested by a Varian-make CP-3800 gas chromatograph equipped with Ni63 ECD electron-capture detector at CWRDM (Centre for Water Resources Development and Management), Kozhikode, Kerala.
Phytoremediation: For the phytoremediation study, ten common species of ornamental plants were selected. Twenty plants of each species were collected and acclimatized in the college garden for two weeks. These plants were divided into two sets. The first set of ten plants of each species was kept as the control plants. The second set of ten plants of each species was kept for Lead treatment. The heavy metal treatment was carried out for 10 days after the acclimatization period. Each plant of the control set was given 100 mL of water regularly for 10 days. Each plant of the second set was treated with 100 mL of lead solution regularly for 10 days. The lead solution was prepared by dissolving 10 g of lead in 10 L of water. After the treatment period, the plants were taken out of the soil, washed with distilled water to remove the soil particles, dried in an oven for 48 hours at 80°C, and were ground into powder. 0.5 g ground powder of plants of each set were weighed accurately and digested with a 40 mL mixture of nitric acid (HNO 3 ) and perchloric acid (HClO 4 ) taken in the ratio of 4:1. The resulting mixtures were evaporated to dryness and were extracted with distilled water. The solutions were heated to boiling and filtered. The volume of the solutions thus obtained was made to 50 mL each. The metal ion concentrations in all the samples were analyzed by Flame Atomic Absorption Spectrophotometer at CWRDM (Centre for Water Resources Development and Management), Kozhikode, Kerala.

Soil Analysis
The soil in agricultural areas may be contaminated by toxic heavy metals, pesticides, etc. The presence of such toxic substances may cause various health and environmental issues. Results obtained in the present study by the analysis of soil samples collected from various banana fields in Kozhikode district were summarised in Tables 1 & 2.  The study revealed that in several places of Kozhikode district like Omassery, Balussery, Vengeri, and Nanmanda soil of banana fields was contaminated with lead. The amount of lead present was found to be 0.02 mg.kg -1 , 0.07 mg.kg -1 , 0.19 mg.kg -1 and 0.26 mg.kg -1 respectively. In places like Kanthapuram, Vazhayur, Kakkur, Kuttiadi, Vanimel, and Thiruvambadi, soil in banana fields was devoid of lead pollution. Mercury contamination was not at all observed in the soil of banana fields of Kozhikode district.
The present study also revealed that organochlorine and organophosphate pesticide contamination was absent in soils of banana fields of various places in Kozhikode district. This may be due to increased awareness among farmers about the harmful effects of synthetic pesticides and also may be due to legal restrictions enforced by the Government of Kerala.

Phytoremediation Studies
Results of soil analysis revealed the presence of lead contamination in some of the banana fields in Kozhikode district. Hence phytoremediation studies were conducted to identify suitable ornamental plants that can be used for phytoremediation of lead in soil. Results observed in comparative analysis of the accumulation of lead-in-control and experimental plants are summarized in Fig.1 and Table 3.
In control plants, the absorption level of lead was very low. This clearly showed that the concentration of lead in the soil used for the present study was very low. Among the various treated plants used for the study, the highest level of lead absorption was shown by D. chinensis L. (32.55 mg.kg -1 ), C. indicum L. (30.65 mg.kg -1 ), R. simplex C. Wright (23.11   mg.kg -1 ), and L. camara L. (22.83 mg.kg -1 ). Lead absorption of C. roseus (L.) G.Don (3.01 mg.kg -1 ) was found to be very low. Similar results showing a difference in lead absorption potential of different plant species were reported earlier by several workers (Ramana et al. 2008, Chirakkara & Reddy 2015, Adesodun et al. 2010.

Observations on Each Plant
Galphimia gracilis Bartl: The concentration of lead-incontrol and lead-treated G. gracilis plants were BDL (Below Detectable Level) and 3.79mg/kg respectively. Lead-treated G. gracilis plants showed yellowing of leaves within 10 days. White spots appeared on the leaves of treated D. pinnata Cav. plants within 10 days. Moreover, the leaves of treated plants were getting dried during the treatment, which showed that D. pinnata Cav plants cannot tolerate high lead content in the soil. Therefore in the present study, D. pinnata Cav plants were found to be less effective in the phytoremediation of lead-contaminated soil.