Various human activities such as industrialization and urbanization accompanied by rapid population growth have added different types of pollutants into the freshwater ecosystems (CPCB 2008). Among these pollutants, heavy metals are of worldwide concern because of their toxic properties, tendency to accumulate in biota, and persistent nature (Rai et al. 1981; Lokeshwari and Chandrappa 2007; Mishra and Tripathi 2008; Chang et al. 2009; Sood et al. 2012; Verma and Suthar 2015). Heavy metals and metalloids such as arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), mercury (Hg), lead (Pb), zinc (Zn) and others can cause deleterious effects even at very low concentrations (Sood et al. 2012). Different sources such as smelters, tanneries, textile, fuel production, military activities, chemical industry, mining, application of fertilizer, urban sewage, waste incineration, and unsafe disposal of hazardous industrial wastes release various toxic heavy metals into the environment (Mishra and Tripathi 2008; Verma and Suthar 2015; Hargreaves et al. 2018; Ali et al. 2020). Some conventional techniques that are used for remediation of heavy metal pollutants include oxidation, chemical precipitation, adsorption, coagulation-flocculation, ion exchange, membrane filtration, ozonation, hydrogen peroxide-based methods,, photocatalytic degradation, reverse osmosis, electrodialysis and electrochemical methods. However, these are mostly metal specific, expensive, energy-intensive, and often have adverse effects on the environment (Mishra and Tripathi 2008; Olguin and Sánchez-Galván 2012; Lara et al. 2014; Rezaniaa et al. 2016). Under these circumstances, it is necessary to identify broad-spectrum, simple and cost-effective technologies for removal of toxic metals (Zhang et al. 2020). The idea of using plants to cleanup pollutants from the environment was introduced in 1983, though this method was being applied for the last 300 years (Ali et al. 2020). These technologies are also named as green remediation, botano-remediation, vegetative remediation, and agro remediation (Sarwar et al. 2017; Kushwaha et al. 2018). Therefore, phytoremediation technologies to clean up environmental contaminants from water, soil and air using living plants (Raskin and Ensley 2000; Terry and Banuelos 2000; Maletić et al. 2019; Moya et al. 2019) comprise viable, innovative, non-intrusive and cost-effective alternatives to other conventional methods (Singh et al. 1996; Miretzky et al. 2004; Putra et al. 2015). Plants that are used in the process of phytoremediation take up heavy metals through their roots and later transfer these contaminants to the above-ground parts of their body (Ashraf et al. 2018).
Aquatic macrophytes are of structural and functional significance in aquatic ecosystems because they provide stable habitats, and are a source of food and oxygen to the macroinvertebrate and fish fauna. They contribute to nutrient cycling, improve water quality by regulating oxygen balance and also play an important role in heavy metal accumulation (Srivastava et al. 2008; Dhote and Dixit 2009). Their high biomass yield, fast growth rate, high tolerance, ability to accumulate heavy metals, and direct exposure to contaminated water, facilitate their remediation ability, which enables them to act as natural water filtration systems (Sood et al. 2012). The roots, shoots, and leaves of aquatic macrophytes have the capacity to absorb heavy metals from aquatic environments, while their roots have the potential to sequester selected heavy metals (Mishra et al. 2009; Paiva et al. 2009; Muffarrege et al. 2010). Heavy metal uptake by aquatic plants involve three broad mechanisms: (i) accumulation, whereby plants concentrate and transport metals from water into plant body such as roots; (ii) binding of metals to cell walls of roots, thereby restricting their entry into the plant; and (iii) confining to the roots the metals that are still able to enter the plant, thus restricting their transport to shoots (Mishra and Tripathi 2008).
Various aquatic plants like duckweed (Lemna minor) (Mo et al. 1989), pennywort (Hydrocotyle umbellata) (Dierberg et al. 1987), water velvet (Azolla pinnata) (Jain et al. 1989), water hyacinth (Eichhornia crassipes) (Mishra and Tripathi 2008), water lettuce (Pistia stratiotes L.) (Putra et al. 2015), Ipomoea aquatica (Khumanleima Chanu and Gupta 2014; Bedabati Chanu and Gupta 2016; Haokip and Gupta 2020), and several other species have been reported to remove various heavy metals from solution. Enydra fluctuans DC. (Asteraceae), which is a floating or trailing macrophyte has been selected in the present study for testing its efficacy in removal of copper and lead from aqueous medium. This plant, which is commonly available in the study area of Barak Valley, South Assam, is also widely distributed in tropical Africa, South and South East Asia, and Australia. It is an edible, semi-aquatic herbaceous vegetable plant with serrate leaves (Rahman et al. 2002; Sannigrahi et al. 2011; Dua et al. 2015). The slightly bitter leaves are extensively used in traditional medicine to treat small-pox, inflammation and skin diseases (Kirtikar and Basu 2002). It is also used as an antibilious agent for treating liver disorders and neural diseases (Chopra et al. 2000; Kuri et al. 2014); anthelmintic and anti-inflammatory (Ali et al. 1972); analgesic (Rahman et al. 2002); and hypotensive (Kuri et al. 2014). Furthermore, it is packed with vitamins and other nutrients (Dua et al. 2015). The plant grows abundantly in rice fields, ditches, drains, natural channels and edges of fish ponds, where they propagate by fragmentation and often choke the water courses (Ali et al. 2013). However, its potential for removal of heavy metals from aqueous medium has not been assessed to any great extent, though it is known to have the ability to remove the metalloids arsenic and boron from water (Shaheen et al. 2006, 2007).
The two heavy metals copper and lead were selected for several reasons. Copper (Cu) is an essential micronutrient for the growth of plants and a constituent trace nutrient of the protein component of several enzymes, most of which are involved in catalyzing redox reactions, electron flow, etc. However, Cu can be toxic at high concentrations that are extremely harmful to both plants and animals (Devi and Prasad 1998; Wang and Dei 2001; Khumanleima Chanu and Gupta 2014). Due to direct exposure to its toxic effects, many aquatic plants are more sensitive to copper than their terrestrial counterparts (Fernandes and Henriques 1991). Cu is commonly used as a fungicide in tea plantations and different agricultural lands in the study area (Khumanleima Chanu and Gupta 2014). Lead at concentrations of more than 0.03 mg/100 gm (FAO/WHO 2001) can cause blood diseases and affect nervous system, teeth and gum, liver, pancreas, and bones in humans (Khan 2015). Lead is dense, soft, durable, and corrosion-resistant with a relatively low melting point, making it a major constituent of paint, ammunition, leaded glass, solder, storage batteries, etc. (Abadin et al. 2007). The principal consumption of lead (i.e., ~ 80% of the total use of lead) is for the lead-acid storage battery, and untreated wastewater containing lead used in recycling of batteries is released into the water bodies in the study area (Bedabati Chanu and Gupta 2016). In developing countries, there is a rapid increase in the demand for lead batteries due to an increase in the number of motorized vehicles, solar panels, telecommunication and computers (OK International 2021). A review of the available scientific literature reveals that the Cu and Pb accumulation and removal by E. fluctuans are not documented. Considering the above facts, the present paper is an attempt to critically evaluate the potential of this plant to accumulate Cu and Pb in its different tissues, and to remove these heavy metals from aqueous medium.