Enhanced Tolerance of Ornamental Plants for Industrial Wastewater Contaminated with Heavy Metals

Abstract

With the improvement of living standards, the consumption and demand for the products and services have also increased. This not only increase the stress on already available limited natural resources by over exploitation at unsustainable pace, but also pollute environmental matrices. The prime notorious contaminants that are introduced into the enviromnent include a range of organic and inorganic substances, along with heavy metals (HMs). Due to non-degradable nature of HMs and capacity to bioaccumulate and biomagnify in the food chain, a proper consideration for their disposal is needed. In recent decade, the use of green technologies, like constructed wetlands and green/blue green infrastructures have been promoted, due to their 10w recurring cost and enviromllental friendliness. Much of the attention has been given to crop plants, despite the risk of bioaccumulation. Little is known that how ornamental plants (OPs) will behave upon exposure to HMs. Using OPs in phytoremediation have dual advantages i.e. they can remediate the HMs contaminated site with no risk of bioaccumulation in comparison to crop plants, and simultaneous beautification of the contaminated site. The irrigation with treated wastewater or grey water will increase the chances ofHMs exposure to OPs, that can lead to economic losses if a non-tolerant OP is used for phytoremediation or aesthetics improvement. For this purpose, Nicotiana alata L. and Petunia hybrida L., two widely cultivated ornamental plants were subjected to experiments. These plants were exposed to selected HMs, Cd, Cr, Cu, Ni, and Pb, in hydroponic conditions, to identify their intrinsic potential to tolerate and uptake HMs. A good OPs candidate for tolerance must be capable of resisting and performing well upon exposure to multiple HMs, as in field mostly the contamination of a site is heterogeneous. The potential of selected OPs was tested to tolerate multi-metal exposure with the aid of soil conditioners and bacterial augmentation in pot experiments. The physiological, biochemical, enzymatic, stress injury parameters of plants and HMs compartmentalization were examined. It was identified that N alala L. and P. hybrida L. can accumulate Cd, Cr, Ni and Pb in aerial parts (In shoot for N alata, while in leaves for P. hybrida), and Cu in roots. Higher levels of stress injury and increased production of enzymes was observed upon HMs exposure (versatile response for each HM). The results indicated that both OPs can tolerate low to medium levels of HMs. It was identified that the combined use of biochar, compost, and peat moss (5% each in soil) can improve the plant growth, and simultaneously reduce the HMs mobility. The effects of bacterial augmentation with commercially available soil conditioners (compost and peat moss) also showed significant positive impact on plant performance in multi-HMs wastewater. We conclude that the tolerance of N alata L. and P. hybrida L. can be enhanced with the use of soil conditioners and bacterial augmentation. Both plants hold the potential for use in phytostabilization of HMs.