PHYTOEXTRACTION OF TOXIC HEAVY METALS FROM CONTAMINATED SOIL BY USING SELECTED PLANTS SPECIES

Abstract

Soil pollution has become a serious and challenging environmental problem all over the world because of rapid industrialization and urbanization. Various industries discharged their toxic heavy metals containing wastes directly into soil and water system that substantially enhanced the degradation process and significantly affected the ecosystem. These unwanted chemicals caused severe health problems when exceed the permissible limits. For this reason, the removal of toxic pollutants form environment has become the greatest challenge. Phytoextraction method based on the utilization of plants species has given a significant attention due to their efficient, rapid, easier, less expensive and environment friendly removal of toxic metals from contaminated soil. Due to the significance of phytoextraction technique, this research work was based on the exploitation of environmental friendly method for the removal of toxic heavy metal i.e. Pb, Cu, Zn and Cr from contaminated soil using biofuel plant species i.e. okra (Abelmoschus esculentus), oat (Avena sativa), cluster bean / guar (Cyamopsis tetragonoloba), soybean (Glycine max), niger (Guizotia abyssinica) and sesame (Sesamum indicum). In order to demonstrate that selected plant species showed better growth, several preliminary experiments were performed for metal tolerance in selected plants species before implication of phytoextraction on real field soil sample. For preliminary screening of plants species, the 12 weeks’ time period was considered to study plants for heavy metal uptake, translocation and accumulation potential. The comparison of plants growth parameters was assessed in terms of tolerance index, biochemical parameters and dry biomass yield per plant. After 12 weeks of experimental time period, plants were harvested from pots and separated into parts i.e. roots, stem, leaves and pods / fruit. The elemental analysis of soil and plant samples was conducted according to Standard Methods. Briefly, the heavy metals i.e., Pb, Cu, Zn and Cr toxicity on growth and biochemical parameters were determined. Plants were subjected to six Pb concentrations (100, 200, 400, 600, 800 and 1000 mg Pb kg-1 soil), six Cu concentrations (25, 50, 100, 150, 200 and 300 mg Cu kg-1 soil), eight Zn concentrations (50, 100, 200, 300, 400, 600, 800 and 1000 mg Zn kg-1 soil) and eight Cr concentrations (0.5, 2.5, 5, 10, 25, 50, 75 and 100 mg Cr kg-1 soil). For six plant species, seed germination and most of the growth parameters were significantly (p<0.05) reduced under high metal stress. Chlorophyll contents were also decreased with increased metal concentrations. Among the six plants species, significant highest metal accumulation i.e., Pb, Cu and Zn was found in the roots and shoots of A. sativa. Bioconcentration factor, bioaccumulation coefficient, translocation factor and highest values of phytoremediation ratio suggested that being more tolerant A. sativa could be an efficient plant for the reclamation of heavy metal contaminated soil. Whereas, for Cr contaminated soil the significant highest accumulation were in the order of C. tetragonoloba > A. sativa > A. esculentus > S. indicum > G. max > G. abyssinica. The bioconcentration factor, bioaccumulation coefficient, translocation factor and highest value of phytoremediation ratio suggested that C. tetragonoloba was suitable for prospective Cr phytostablization. In final approach, the four plant species i.e., A. esculentus, A. sativa, C. tetragonoloba and S. indicum were finally selected after screening because of their high metal tolerance in terms of growth, biochemical parameters and phytoextraction efficiency. For the application of phytoextraction technique, real field soil samples were collected from village Darya Khan Panhwar near Phuleli canal, Sindh, Pakistan. It was evaluated that under optimal conditions heavy metal i.e. Pb, Cu and Zn were found above the safe limits. The experiment was performed for 12 weeks. Subsequently, at the end of experimental period, plants were harvested and then analyzed. Among the four plants species investigated, the germination and growth parameters along with chlorophyll contents were significantly (p<0.05) higher in A. sativa followed by C. tetragonoloba, S. indicum and A. esculentus. Likewise, plants possess different phytoremediation potentials under given set of conditions and were successfully removed heavy metals up to the permissible limits recommended by WHO and EU standards.