Phytoremediation of inorganic contaminants in vellayani wetland ecosystem

Abstract

An investigation entitled ―Phytoremediation of inorganic contaminants in Vellayani wetland ecosystem‖ was carried out at the Department of Soil Science and Agricultural Chemistry, College of Agriculture, Vellayani, during 2013-16. The objective of the study was to track the potential sources of contaminants threatening the Vellayani wetland ecosystem and suggest a viable phytoremediation technology. The experiment comprised of four parts. The first part involved a peripatetic survey in the catchment of Vellayani lake along the rivulets that contribute water to Palappoor, Pallichalthodu, Reservoir bund, Arattukadavu RB, Valiyavilagam, Mannamvarambu and Manamukku sites. Three rivulets per site were identified, and geocoded water and sediment samples were drawn from five sampling points under each rivulet during pre and post monsoon seasons of 2014-15. Among the physical properties of water colour, turbidity and suspended solids were above the maximum permissible limit (MPL). Chemical properties viz., pH, EC, NO3-N, NH4-N, P, Fe, Al, Pb, Cd and BOD of water showed significant difference among the sites during both the seasons but COD was significant only for post monsoon. P content and BOD exceeded the MPL and Al the desirable limit. Coliforms were detected at all sites during both seasons and were above the MPL. Texture of the sediment varied from sandy clay to sandy clay loam. EC, OC, NO3-N, NH4-N and P contents of sediment showed a decreasing trend during the post monsoon season. Arattukadavu RB was the most contaminated site followed by Palppoor and Manamukku the least contaminated site preceeded by Mannamvarambu and Valiyavilagam with respect to water and sediment quality. No pesticide residue was detected in water and sediment. The highest plant density was noticed for Cynadon dactylon L. In the second part, potential sources of contaminants were identified as automobile workshops/servicing centres, domestic wastes from hotels/houses and sewage. Out of the 29 species of dominant macrophytes, highest biomass was recorded by Limnocharis flava L. (Buch.). In shoot, the highest concentration for Fe was recorded by M. vaginalis, Panicum repens L. for Aland S. trilobata for Pb and Cd. In root, the highestconcentration for Fe, Al, Pb and Cd was showed by Eichhornea crassipes Mart., Monochoria vaginalis (Burm.f.), Colacasia esculenta L. and Sphagneticola trilobata L. respectively. Sediment from Arattukadavu recorded the highest contents for Fe, Al, Pb and Cd and count for bacteria, fungi and actinomycetes. It was observed from the study that concentration of Fe, Al, Pb and Cd decreased with the distance from the source of contamination. Higher quantities of Fe was extracted by E.crassipes and M.vaginalis; Al by M. vaginalis and L. flava and Pb and Cd by L.flava and E.crassipes from the wetland ecosystem. In the third part, four pot culture experiments were carried out with graded doses of Fe (1000, 2000 and 3000 mg kg-1), Al (750, 1000 and 1250 mg kg-1)and Pb and Cd (50, 75 and 100 mg kg-1) to determine the hyperaccumulation ability of selected macrophytes based on a screening trial. The macrophytes were grown in sediment and water collected from the Vellayani wetland ecosystem for a period of 45 days. From the study it was observed that E. crassipes was the best hyperaccumulator for Fe, M. vaginalis for Al and L. flava for both Pb and Cd and can be identified as the best phytoextractors for the same. Root was the major retention site for all the metals. Among the rhizosphere microbes, fungi maintained a positive relation with levels of of Fe, Al, Pb and Cd and a negative relation by bacteria and actinomycetes. In the fourth part, the macrophytes showing highest hyperaccumulation ability for each metal (based on part III) were raised in pots containing the respective metals (2000 mg Fe kg-1, 1000 mg Al kg-1 and 75 mg Pb /Cd kg-1) for 60 days. The plants were harvested and the biomass was put to different disposal methods viz., composting, vermicomposting, ashing and production of biochar and was used for the pot culture experiments with amaranthus. The treatment effects were significant for biomass production and metal extraction. Among the four disposal methods, vermicomposting had resulted the highest biomass production for all except Pb. Regarding the metal extraction by amaranthus, application of ash (T4) showed the highest removal and the least by the biochar (T5). The metal retention in soil was highest for biochar and least for ash. Loss of metals from the processed materials was also lowest for biochar. Thus the best disposal technique of phytoextractors /hyperaccumulators is conversion to biochar. The viable phytoremediation technology is to raise suitable phytoextractors / hyperaccumulators in the contaminated area and dispose them through biochar production.