**1. Introduction**

Water contaminations, along with limited availability of water, have put a severe burden on the environment. Around 40% population of the world is facing the problem of water scarcity due to climate change, rapid urbanization, food requirement and unchecked consumption of natural resources [1,2]. During the past few decades rapid urbanization, industrialization, agricultural activities, discharge of geothermal waters and olive wastewater especially in olive-cultivating areas enhanced the discharge of polluted wastewater into the environment [3–6]. Wastewater carrying soaring concentrations of pollutants is immensely noxious for aquatic ecosystem and human health [7–9]. Reclamation of wastewater has been the only option left to meet the increasing demand of water in growing industrial and agricultural sectors [10].

Industrial and domestic untreated wastewater contains pesticides, oils, dyes, phenol, cyanides, toxic organics, phosphorous, suspended solids, and heavy metals (HMs) [11]. Heavy metals among these toxic substances can easily be accumulated in the surrounding environment [12]. Commercial activities such as metal processing, mining, geothermal energy plants, automotive, paper, pesticide manufacturing, tanning, dying and plating are held responsible for global contamination of heavy metals [13,14]. Removal of heavy metals from the wastewater is di fficult because they exist in di fferent chemical forms. Most metals are not biodegradable, and they can easily pass through di fferent trophic levels to persistently accumulate in the biota [15,16].

Removal of toxic pollutants is extremely important to minize the threat to human health and the surrounding environment. Removal of heavy metals achieved through various techniques such as reverse osmosis [17], ion exchange [18], chemical precipitation [19], adsorption and solvent extraction [20] include enormous operational and maintenance costs and are usually not environmentally friendly [19–22]. These conventional techniques for the remediation of heavy metals are generally costly and time-consuming. These treatment technologies require high capital investment and in the end, generate the problem of sludge disposal [23]. For the remediation of wastewater polluted with heavy metals contaminants, an environmentally friendly and economical treatment technology is needed [24,25]. The current study illustrates an environment-friendly technique phytoremediation for removal of contaminants on a long term basis. Furthermore, this review article summarizes the potential application of aquatic plants in phytoremediation for the treatment of wastewater.

#### **2. Heavy Metals in the Environment**

Anthropogenic and geological activities are the main source of heavy-metal pollution. Activities such as mining, military activities, municipal waste, application of fertilizer, discharge of urban e ffluent, vehicle exhausts, wastewater, waste incineration, fuel production, and smelting cause the production of metal contaminants [26,27]. Natural sources of heavy-metal pollution include erosion, weathering of rocks and volcanic eruption. Parent material during weathering is the primary and initial natural source of heavy metals [28].

Agricultural pesticides and utilization of fertilizers on agricultural soil have raised the concentration of Cd, Zn, Cu and As in soil [29]. A constantly increasing need for agricultural produce has increased the application of pesticides, fertilizers, and herbicides. This excessive use of these agrochemicals may result in the accumulation of these pollutants in plants and the soil as well [30]. Usage of phosphate fertilizer and inorganic fertilizers to control the diseases of crops, grain and vegetable sometime hold an uneven level of Ni, Pb, Zn, Cd, and Cr [31,32]. An enormous quantity of fertilizers is applied to deliver the K, P and N in order to improve the growth of crops, which in turn increase the incidence of cadmium, lead, iron and mercury in substantial high concentrations. Inputs of heavy metal to agricultural land through the excessive use of fertilizers is increasing apprehension about their probable hazard to the environment [33,34].

Wastewater irrigation leads to the buildup of various heavy metals like cadmium, lead, nickel, zinc, etc. Some of these metals like Zn, Cu, Ni, Cd and Pb are frequently present in the subsurface of the soil irrigated with untreated wastewater. Wastewater irrigation for long periods of time increase the concentration of heavy metal in the soil at toxic levels [35]. The unregulated dumping of municipal solid waste is also another main source of raised soil contamination load. Open dumps and land filling are the common practices using worldwide to dispose of municipal solid waste. Despite being a useful source of nutrients, these wastes are also a source of some harmful toxic metals as well. Precarious

and overload applications of fertilizers, pesticides and fungicides are very important sources of metal pollution [36]. Metal contamination can also be caused by transportation. Maintenance and deicing operations on roads also generate groundwater/surface contaminants. Corrosion, tread wear, and brake abrasion are well-recorded sources of heavy metals generation linked to highway traffic [37].
