1 Introduction

Most techniques used to remove heavy metals from the environment are expensive, time taking and not environmentally friendly, but we require such technology that can reduce the burden of contaminants from the environment and is cost effective. For that purpose, phytoremediation is the most advanced and effective technology, which uses macrophytes to decontaminate environment (Jadia and Fulekar 2009; Farid et al. 2017a). Phytoremediation is a widely used technique to remove toxic chemicals from different growing mediums such as water, soil, physical and chemical conditions while using different plants (Wang et al. 2002; Farid et al. 2018a). There are different subgroups such as rhizofiltration, phytoextraction and phytostablization to remove or extract the heavy or toxic metals from the environmental media (Salt et al. 1995; Farid et al. 2017b). In developing countries like India and Pakistan, this technology is very suitable as it is cheap and has the potential to reap results so such plants are used that have the potential to survive and uptake metals from the contaminated medium (Farid et al. 2017c, 2019). These plants are called as hyperaccumulators (Ghosh and Singh 2005). Some heavy metals are essential for the environment, but with human activities their concentration increases and interact with plants animals and humans and disturb their normal functions causing abnormalities. Their natural path and concentration get increased (Bánfalvi 2011; Farid et al. 2017c). Most of the heavy metals released by industries into the environmental mediums cause water, air and soil pollution. Hexavalent chromium affects the plant growth, seed germination and photosynthetic process of plant and alters leaf profile (Zaheer et al. 2015). One of the most toxic elements for the plants and crops is nickel (Ni), and its concentration varies with soil solution so does its toxicity and its impacts on plant physiology (Hunter and Vergnano 1953). Phytoremediation is regarded as a new concept to remediate the environment by the use of plants as it is cheap, environment friendly and sustainable process to remove the contaminants from the environmental mediums. The most suitable plants in phytoremediation are aquatic plants as they can remove heavy metals from wastewater by up-taking them and no special environmental conditions are required (Singh et al. 2012; Sallah-Ud-Din et al. 2017). However aquatic plants have the potential or ability to accumulate heavy metals, but the level of uptake varies with each species. Various biological, physical and chemical processes take place during phytoremediation for the removal of heavy metals from water. To uptake heavy metals from the polluted water, various species of aquatic plants are introduced such as Lemna minor L. and microspora in the recent past. At certain level, L. minor has great potential to accumulate heavy metals and is easily available in brackish water (Sallah-Ud-Din et al. 2017). So due to its local availability, it is cost effective and widely used to extract heavy metals from polluted sites using phytoremediation (Hou et al. 2007).

Phytoremediation is considered as an advanced approach to remediate the contaminated water and soil mediums using plants. Hyperaccumulator plants are widely used to detoxify the polluted water and soil (Rizwan et al. 2017b). The purpose of phytoremediation is to eradicate or transport heavy metals from the water or soil to metal hyperaccumulator plants (Farid et al. 2020a). Phyto means light and the word remediation means able to cure the plants so the definition would be to cure plants by giving them light. It can also give us an idea of using plants to remediate the polluted water. Phytoremediation is a technique to use those plants that are capable enough to accumulate or uptake heavy metals making the environment less toxic (Farid et al. 2019). Generally, phytoremediation is based on phytoextraction, phytoaccumulation, phytofiltration, and phytovolatilization of heavy metals and is applied to a variety of organic and inorganic pollutants (Vamerali et al. 2010). Plants that are used in phytoremediation are used to degrade and break down the contaminant that helps the heavy metals present in wastewater to stabilize. New technology widely known as phytoremediation is used to remediate contaminated sites from organic pollutants. This technique is widely recommended for its low budget and sustainable energy benefits as it is economically feasible and more suitable for the environment. It also helped in reducing the transportation of contaminants from one media to another (Tangahu et al. 2011). Hyperaccumulator plants are naturally available in the environment and have high metal accumulation or transport capacity. These plants accumulate hundred times more than non-accumulators (Wuana and Okieimen 2011). This technique of phytoremediation is used at large scale to remediate the heavy metals from the contaminated site. Such techniques are less damaging to the environment as the process is carried out at topsoil and the remaining soil remains unaffected and is further used for agricultural purposes. Thus, phytoremediation is the best possible option as it does not damage the soil structure and even remove toxic metals, thus keeping the nutritional value of soil intact (Tangahu et al. 2011; Farid et al. 2018b).

Phytoremediation is applied to remove the toxic metals from environmental mediums like soil and water as plants accumulate or accept the heavy metals, thus remediating the environment. For this purpose, aquatic plants are used to remove the high concentration of heavy metals from the environment (Farid et al. 2015, 2019). Aquatic plants are easily preserved and low budgeted and store heavy metals in their body and still manage to survive. Duckweed is mostly present in ponds or marshy areas, and it is the free-floating plant that stays at the surface of the water. Because of its high potential to uptake or accumulate heavy metals, it has gained much importance. Almost 90% of lead can be extracted from wastewater. Duckweed increases in size if concentration of ammonia increases. It can grow at pH 6–9, and tolerance level ranges between 5 and 6 (Caicedo van der Steen et al. 2000; Singh et al. 2012).

2 Taxonomy

The present era of “resurgence of duckweed research and applications” (Lam et al. 2014; Zhao et al. 2012; Appenroth et al. 2017) is based on their practical application to remove, transfer, and uptake heavy metals. Lemnaceae comprises one of the fastest growing angiosperms (Sree et al. 2015b, 2016; Ziegler et al. 2015). In the past duckweed was used as human diet because of its high protein content in many parts of Asia (Bhanthumnavin and McGarry 1971; Cheng and Stomp 2009; Van der Spiegel et al. 2013; Appenroth et al. 1982). Due to their high protein content, they can replace soya bean products as an alternative. As exhibited by various research projects, duckweeds can be proved beneficial if given to domestic animals, e.g. sheep, cattle, rabbits, horses, waterfowls, fish and poultry (Van Dyke and Sutton 1977; Muztar et al. 1979; Landolt and Kandeler 1987; Hassan and Edwards 1992; Cheng and Stomp 2009; Anderson et al. 2011). Duckweed can also produce high starch content and can be used in biogas and biofuel plants, if suitable conditions are provided (Jain et al. 1992; Sree and Appenroth 2014; Su et al. 2014; Sree et al. 2015a; Cui and Cheng 2015). This would help in providing substitute for food crops that consume arable land and would be a cheap source for biogas plants. Duckweed plants are characterized by 37 species, and this is taken as new crop plants (Appenroth et al. 2013).

3 Heavy Metals Accumulation by Lemna minor

The most suitable plant species must have local adaptation, such as plant root depth, ability to toxic removal or detoxify heavy metals, fast growth rate and uptake large amounts of water for evapotranspiration (Ashraf 2010). Care should be taken in case of selection of plants because it must be considered that selected species is native to that place or not (Gordon 2003). Aquatic plant genera are known to gather metals from their surroundings, and it is well recognized that wetland species has been used for the phytoremediation of wastewater successfully. Aquatic blossoming florae can eliminate numerous metals from water including Eichhornia crassipes (water hyacinth), Hydrocotyle umbellata L. (pennywort) and L. minor (duckweed). The great water portion of aquatic flowers also confuses recollect of metals through burning. Many terrestrial plants roots grown hydroponically were recognized as very effective in captivating, engaged or precipitating toxic metals from contaminated wastewaters. This procedure was called rhizo-filtration. For the removal of Pb, some aquatic plant species are effectively used such as Duckweed (L. minor), Water hyacinth (Eichhornia crassipes) and Hydrilla (Hydrilla verticillata). Phytoextraction is effectively done if solubility and accessibility of metal in soil for root utilization are acquired. The bio-accessibility of metal mostly relies on soil properties like the ability of interchange negative ions, pipestone material, pH and soil biological stuff (Hou and Zhang 2007; Hou et al. 2007).

Extraction of metals and chemical contaminants from the soil by transferring of the contaminants into extractable parts of the plant, with the use of plants, is referred to as phyto-extraction. Preferably the hyperaccumulator plants with high mass are used in phytoremediation. These are the plants having potential to collect the huge portion of heavy metals in their above the surface parts unaccompanied by the pernicious impacts of heavy metals (Kamal et al. 2004). Involving different types of sunflower (for certain metals) and hydrangea (for aluminium [Al]), about 400 plant groups from 45 genera had been described as hyperaccumulators (Axtell et al. 2003). Aquatic macrophytes, among all the numerous plant species, gain greatest attention for phytoremediation systems. It is because of their accumulating capability of heavy metals up to 100,000 times larger than the amount of heavy metals present in wastewater. So, these types of macrophytes have been used for the treatment of heavy metals from different sources (Rai 2008).

Lemna minor consists of several small and free-floating plants on the upper surface of the water. The growth rate of L. minor is high and easily settled in every aquatic environment. It includes the plant family of Lemnaceae that are free-float and flowering plant. Duckmeat (Spirodela polyrhiza) and watermeal (Wolffia) are widely scattered on the ponds and used for the treatment of wastewater. These plant species are also a source of food for some animals that live in the aquatic environment (Krull 1970). L. minor grows very densely in nutrient-rich environment and forms a thick layer over each other. In aquatic systems, L. minor has been directly and indirectly affecting the properties of the wastewater. Duckweed is mostly used in phytoremediation for the phytoextraction of heavy metals from metal polluted sites.

Duckweed grows at an extensive range of temperature from 6 to 34 °C. Mostly, L. minor is found on municipal wastewater ponds (Sallah-ud-Din et al. 2017). L. minor has a great potential to accumulate heavy metals from the wastewater. It can remove about 90% of lead from the wastewater ponds. Duckweed grows under pH 6–9 and also has the capacity to tolerate a low pH level. With the increase in the ammonia concentration, the rate of growth is progressively increased (Akhtar et al. 2017). From some experimental results, it is shown that duckweeds have the potential to remove soluble Pb under various temperature ranges from 15 to 35 °C and under various pH level ranging from 5 to 8 under different Pb concentrations. It is reported that lead concentration was highest at pH 5 and maximum accumulation done less than 30 °C temperature (Gallardo et al. 1999).

4 Chemical Composition

With nutritional and environmental requirements met, duckweed plants grow very fast and can flourish for long (Cheng et al. 2002). Anyhow, growth rates of duckweed colonies will be reduced by a variety of problems: such as nutrient scarcity; toxins; higher levels of pH and temperature; overflown by overgrowth of the colony and competition from other plants for light and nutrients (Leng et al. 1995). There are many factors that affect the growth and composition of the plant. The levels of available nutrients, and species differences, can heavily influence both the quantity and quality of produced material. These differences may be interpreted in light of the existence of deficiency, optimal and toxic levels for nutrients. Little interest is shown in recent times in building an optimum nutrient range for growth of duckweed despite inconsistencies in published literature. Results from these studies showed that duckweed growth is affected by other populations primarily via nitrogen limits and increased pH. According to Leng et al. (1995) as a generalization, growth of duckweed is controlled by sunlight and temperature more than nutrient application in the water. Duckweeds can grow faster at high temperatures to trace quantity of P and N nutrients in water. Also, according to Culley et al. (1981), the reproduction and growth of duckweed are mostly influenced by the availability of macronutrients such as nitrogen, potassium and phosphorus in addition to micronutrients, light, wave action, temperature and density of plant can also tolerate pH from 3 to 10 with an optimum range of 5–7.

5 Ecological Consideration

Lemna minor has great ecological consideration due to its ability to withstand desiccation, quickly populate a new habitat and inhibit the other species competitively. In contrast to most plants, duckweeds can tolerate high concentrations of salts (up to 4000 mg/l total dissolved solids). Nutrients are equally mixed on the surface of duckweed leaf. Duckweed best grows over the pH 6.5–7.5 but can survive from pH 5 to 9 range. As a generalization, growth of duckweed is regulated by sunlight and temperature more than nutrient applications in the water. To find the nutrients like P and N, duckweeds grow faster at comparatively high temperature. Duckweed species have the ability to survive in severe conditions. Their growth rate, however, is highly dependent on the nutrient balances in the water. They can sustain and revive from nutrient loadings, higher temperatures, pH and nutrient balance. However, for duckweed to survive, four factors should be considered.

For the proper handling of crop, basic research is required that gives complete information about the life cycle of crops, when to fertilize, harvest and buffer, and which nutrients are required for the healthy growth. Strategic handling should be aimed at keeping a proper and dense cover of duckweed, a pH of 6–7 and low dissolved oxygen. In results the algal growth is being suppressed by absolute crop cover that used to minimize production of CO2 by algal respiration and further stop its effect on pH.

An application of nutrients along with waste organic material can be given to duckweed plants. The lowest budgeted and efficient sources are wastewater pollutants from food processing plants, extensively high poultry production, cattle feedlots and homes (Ahmad et al. 2020). Solid materials like food processing wastes, manure from livestock or night soil from rural areas can be added to a pond at certain levels by mixing it with water (Jabeen et al. 2016). For initial treatment, all wastewater containing night soil or manure has to go in an anaerobic pond for a few days, before applying it to fertilize duckweed. Reduction of solids and prevention of a floating mat are required when such type of nutrients is supplied.

6 Biochemical Attributes

All plants grown under different HMs stress show reduced chlorphyll contents. With the increased level of HMs concentration, the plants’ total chlorophyll content can be reduced due to the accumulation of toxic HMs. With the increased level of concentration of HMs, the total chlorophyll content of the plants decreases. The addition of citric acid along with lead concentration may promote the growth and development of plants. Citric acid provides support for the development and growth of the plants.

In all plants, the carotenoid content decreased with the addition of HMS at different concentrations. Higher the concentration of HMs, the lower will be the carotenoid content of the plants that are applied with only HMs. With the rise of HMs concentration, the plant carotenoid content can be reduced due to the accumulation of toxic HMs (Rizwan et al. 2017a). With the increasing concentration of HMs, the carotenoid content of the plants is decreased. The addition of citric acid along with HMs concentration may promote the growth and development of plants. Citric acid provides support for the development and growth of the plants (Ehsan et al. 2014; Farid et al. 2019).

In all plant samples four basic antioxidant enzymes such as catalysis (CAT), ascorbate peroxidase (APX), superoxide dismutase (SOD) and peroxidases (POD) were observed for the estimation of the HMs impact along with the addition of citric acid on the antioxidant potential of L. minor. In these plants, the concentration of HMs increases the electrical conductivity of plants that affects their development cycle. As the HMs concentration increased, the electrical conductivity is also increased. The plant samples that are applied with greater concentration of HMs were affected more and show a higher electrical conductivity stress. The controlled treatment has low oxidative stress as compared to all other samples because control system is not applied with any metal and acid concentration. Addition of citric acid along with lead treatment reduced the electrical conductivity (Habiba et al. 2015; Farid et al. 2018a, 2019).

The malondialdehyde stress increases with the increasing concentration of HMs into the plant samples due to the toxicity of HMs. The plant samples that are applied with greater concentration of HMs were affected more and show higher malondialdehyde stress. The control treatment has low oxidative stress as a comparison to all other samples because control system is not applied with any metal and acid concentration. The plant samples that were applied with HMs concentration along with citric acid have low effect because of the exogenous effect of citric acid on malondialdehyde (Shakoor et al. 2014; Farid et al. 2020b).

7 Significance of Duckweed Plant

For hundreds of years, duckweed has been used in Asian primary production systems to produce animal feed (Leng 1999). It has around 40 species worldwide, and the major ones are of the genus Lemna, Wolffia, Wolffiella and Spirodela (Les et al. 2002). The plant is rich in both macro- and micro-elements such as calcium and chlorine and has a protein content that ranges between 6.8% and 45.0% DM (Landolt and Kandeler 1987) but has been recognized by American researchers as ideally suited for heavy metal uptake. Rates of accumulation of heavy metals differ markedly between species, as does the relationship between concentration (in water) and take-up—in some cases it is direct, while in others reciprocal (Landolt and Kandeler 1987).

Duckweed plant species are recognized all over the world because of its potential to bear heavy metals (Radić et al. 2010). Promising nominees for this method are L. minor, Spirodela polyrhiza and Lemna gibba (Axtell et al. 2003; Charles et al. 2006; Zhang et al. 2011). L. minor (duckweed) can be highly helpful in phytoremediation as it can accumulate heavy metals, fast growth, cold-bearing potential and easy to harvest (Sharma and Gaur 1995; Radić et al. 2010). Plants of duckweed that are grown in freshwater exhibit low nutritional values than in domestic wastewater. In few countries Lemna gibba is imported and used as food for hens as a replacement of soya and fish because it is rich in protein.

8 Relationship Between Duckweed and Heavy Metals

Lemna minor commonly named as duckweed is a high accumulator plant that has potential to accumulate and degrade higher concentration level of metals when added in the watery solution (Axtell et al. 2003). The fresh weight of plant reduces due to the heavy metal concentration and accumulation and lowers the nutrient uptake in plants under heavy metal stress (Khaliq et al. 2016). The additional citric acid expressively enhances the heavy metal accumulation by Brassica napus (Afshan et al. 2015). It can also enhance the cadmium accumulation by stonecrops (Huang et al. 2013). The reliable application of citric acid makes it bio-available in soil and in hydroponic media (Shakoor et al. 2014). L. minor is a naturally occurring free drifting plant on the marshlands and swamps, formerly studied for its ability to uptake Cr, Cu, Cd, Pb and Ni grown hydroponically (Adrees et al. 2015; Farid et al. 2017c, 2018a; Sallah-Ud-Din et al. 2017; Rizwan et al. 2017a; Ahmad et al. 2020). The concentration of HMs at different levels severely affects the growth and development of plants. The addition of citric acid increased the uptake capacity on the increasing concentration of HMs in L. minor.

Distribution of heavy metals in entire plant tissue is a very major characteristic, and it is very useful for the suggestion of decontamination mechanisms. The role of L. minor is also recently investigated to know the potential of this plant for phytoextraction (Bokhari et al. 2016). In this study, results are compiled with the recent work of L. minor that plays a significant part in the phytoextraction of heavy metals in aqueous media.

One of the best-known plants to accumulate heavy metals are Lemna plants. So, the plants have the ability to treat diverse waste systems either it is industrial leachate or muncipal wastewater. The average life of L. minor is almost 5–6 weeks with a production rate of 0.45 fronds each day and its mass doubled in 2–3 days (Isaksson et al. 2007). Because of its faster growth, it can be the best possible solution for phytoremediation. The solubility and movement of heavy metals must be verified to ensure their availability in the environment.

9 Different Heavy Metals Stress on Physiology and Biochemical Attribute of Duckweed

The fresh weight, dry weight, root length and leaf area were investigated and highly observed that HMs increasing concentration reduced the growth of the whole plant. L. minor is best known for the HMs uptake from contaminated medium. The fresh weight of L. minor decreased with the increasing concentration of HMs. When HMs accumulate in the parts of a plant, it reduces the fresh weight of plant by disturbing its growth and development. Afterwards, the weight of L. minor increases with the addition of citric acid. Citric acid increases the growth and development of plants by giving simulative support to the plants. The dry weight of all plant decreases with the increase in the HMs concentration. The dry weight of all plants that are treated with the addition of citric acid along with other HMs concentration has more weight as compared to those that are only treated with different HMs. Due to the exogenous application of citric acid, the dry weight of plant increases with citric acid and HMs (Rizwan et al. 2017b).

Some other plant gorwth and health attributes such as electrolyte leakage, chlorophyll a, b, total and carotenoids have also been observed in various studies. It has been addressed in previous studies that the heavy metal accumulation and uptake navigates the growth and biomass of the plant by ending the mineral endorsement, which disturbs the metabolic processes of the plants (Gill et al. 2015). The stress of heavy metals is the main reason for the production of reactive oxygen species, which damaged the development and affected the growth of plants (Das et al. 2014). Addition of citric acid along with HMs treatment expressively increases the fresh and dry weight of plants along with the growth of L. minor, which promote it as a supportive character under HMs stress. According to the recent studies, citric acid plays an important role in nutrient uptake under heavy metal accumulation (Najeeb et al. 2011).

The leaf colour of L. minor plants was badly affected due to lead stress in aqueous media. Due to the treatment of HMs at different concentrations, the greenish colour of all plant reduced to pale yellowish colour. Addition of citric acid along with HM treatment recovered the growth and photosynthetic pigments of plants and reduced the effect of lead on them with its exogenous applications. The leaf area of all plants is gradually reduced under heavy metal stress. When treatment of different heavy metals is applied at different concentration level, the leaf area of plants is reduced. Greater the concentration of HMs, the lower will be the leaf area. Addition of citric acid with HMs treatment supports the growth and development of plant and increases the leaf area of plants as compared to the plants that are only treated with HMs.

10 Source for Domestic Animals

Duckweed has gained a lot of attention as a food source for ruminants, fowl, fish and humans around the globe, especially in developing nations (Iqbal 1999). This has been due to its high protein content, quality of high protein, yield of protein per growing area and low fibre (Cheng and Stomp 2009). Use of duckweed in animal diets as exclusive additional feed is widely reported.) Reeve and Black (1998) carried out feeding trials on poultry and showed enhanced layer performance and quality of eggs in chickens and ducks and no harmful effects, and as a result the animals consumed the duckweed willingly and it was very helpful to their growth. Studies regarding the utilization of duckweek plant as broiler feed has also being carried out by several authors at different levels. In broiler chickens the potential nutritional value of duckweed has been known (Haustein et al. 1994).

A prime limitation to fish farming is that meals are of high biological value with high protein content that are high priced and mostly unavailable. Duckweeds that grow on water with 10–30 mg NH3-N/L mostly have a high biological value with high protein content (Hillman and Culley 1978). Fresh duckweed is highly recommended for widespread fish farming systems that is used to remove waste by water exchange (Gaigher et al. 1984), and duckweed is efficiently converted to certain fish including carp and tilapia, i.e. the live weight (Van Dyke and Sutton 1977; Hepher and Pruginin 1979; Robinette et al. 1980; Hassan and Edwards 1992).

11 Conclusions

The present review showed that the application of citric acid along with heavy metals stress helped plants to produce more biomass and little damage to biological attributes. At heavy metal concentration without addition of chelates such as citric acid, the growth of duckweed was significantly reduced due to low nutrient availability. The biological attributes such as chlorophyll content, antioxidants and physiological characteristics were also negatively affected. However, as duckweed is locally available and a low budgeted plant it helps a lot in treating wastewater naturally.