Duckweeds for Remediating Water Contaminated with Heavy Metals: History
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Subjects: Ecology
Contributor:
Yuzhen Zhou
,
Anton Stepanenko
,
Olena Kishchenko
,
Jianming Xu
,
Nikolai Borisjuk

Tiny aquatic plants from the Lemnaceae family, commonly known as duckweeds, are often regarded as detrimental to the environment because of their ability to quickly populate and cover the surfaces of bodies of water. The global distribution of duckweeds and their tolerance of ammonia, heavy metals, other pollutants, and stresses are the major factors highlighting their potential for use in purifying agricultural, municipal, and some industrial wastewater. In summary, duckweeds are a powerful tool for bioremediation that can reduce anthropogenic pollution in aquatic ecosystems and prevent water eutrophication in a simple, inexpensive ecologically friendly way.

  • heavy metals
  • duckweed
  • wastewater remediation
  • water pollutants

1. Heavy Metals

Heavy metals are released into the environment from natural and anthropogenic sources, predominantly from mining and industrial activities. After entering the water environment, they accumulate in aquatic organisms, affecting their normal physiological and metabolic activities. Because they pose a threat to human health via the food chain and have serious impacts on the ecological environment, the removal of toxic pollutants is extremely important to minimize potential threats. Conventional techniques for the remediation of heavy metals are generally costly, time-consuming, and generate the problem of sludge disposal [1]. An environmentally friendly and economical treatment technology for the remediation of wastewater polluted with heavy metals is needed [2]. Duckweeds are relatively tolerant to heavy metals and able to take up many heavy metal ions, including those of cadmium, chromium, copper, iron, mercury, manganese, nickel, palladium, lead, and zinc [3][4][5][6][7][8][9][10][11][12]. Therefore, duckweed also has potential uses for monitoring and remediating heavy metals [13]. As a floating plant, duckweed can rapidly absorb heavy metals due to its special morphology and high growth rate [14]. In addition, duckweed can resist the toxicity of heavy metals through chelation and compartmentalization in vacuoles, effectively removing heavy metals in water through biological adsorption and intracellular accumulation [15].
Different duckweed species have different tolerances to various heavy metals, and their biomass, photosynthetic pigments, and antioxidant enzyme activities are significantly different. The toxic effect of heavy metals on duckweed is the main factor limiting the application of duckweed. Therefore, identifying duckweed species that can tolerate specific heavy metals, have suitable bioaccumulation ability, and have suitable resistance will help to improve the phytoremediation of heavy metals in polluted water by duckweed.
Some researchers found that mixing different species of duckweed and coculturing duckweed with microorganisms or other plants can affect the absorption of heavy metals. Due to differences in tolerance and accumulation ability of different duckweed species for various heavy metals, the coculture of different duckweed species can improve both biomass and antioxidant enzyme activity, reducing the toxicity of heavy metals to duckweed and thus aiding the removal of heavy metals from polluted water [16]. By coculturing L. punctata and L. minor or individually in the medium with different concentrations of copper (Cu), Zhao (2015) found that coculturing produced better remediation effect than did single cultures at low Cu concentration; however, the single culture system was more effective at higher Cu concentration [17]. Duckweed can partly neutralize the toxic effect of high Cu concentrations by enhancing the activity of antioxidant enzymes, thus limiting the absorption of Cu.
The ability of duckweed to absorb heavy metals is also affected by the particular microorganisms symbiotically associated with the duckweed. Stout et al. (2010) showed that axenic duckweed, L. minor, accumulated slightly more Cd than did plants inoculated with bacterial isolates, suggesting that bacteria serve a phytoprotective role in their relationship with L. minor by preventing toxic Cd from entering plants [18].
Due to their ability to absorb heavy metals from the environment, duckweeds have been proposed for removing heavy metal contamination from wastewater. Bokhari et al. (2016) found that L. minor could effectively remediate both municipal and industrial wastewater [11], with removal rates of cadmium, copper, lead, and nickel all above 84%. In addition, because dried duckweed power has a large specific surface area and high porosity, duckweed can also be processed into dry powder and used as a potential new adsorbent. Chen et al. (2013) found that the lead ion (Pb2+) adsorption capacity of dried powder of L. aequinoctialis was more than 57 mg/g [19]. Nie et al. (2015) compared the removal rate of uranium ion (U4+) by live L. puntata and its dry powder and found that the removal rate of 5 g/L U4+ was nearly 96% by 1.25 g/L dry powder at pH 5, which is higher than that (79%) by 2.5 g/L (FW, fresh weight) live L. puntata [20]. Li et al. (2017) studied the adsorption of cadmium ion (Cd2+) in the aquatic environment by the dry powder of S. polyrhiza and L. puntata and found that the removal rates of Cd (50 mg/L) by the two kinds of dry powder of duckweed were 83% (L. punctata) and 96% (S. polyrhiza), respectively [21].

2. Metalloids: Boron and Arsenic

Boron (B) is an essential nutrient for plants but is toxic at high concentrations [22][23]. A study of the toxic effect of B (0.5–37 mg/L) on duckweed revealed that S. polyrhiza showed significantly reduced frond production and growth rates while significantly increasing the production of abnormal fronds. The authors concluded that S. polyrhiza could not remove significant amounts of B from the treatment solutions and, as a result, cannot be used as an effective component of B bioremediation systems [24]. Growing L. gibba at B concentrations of 0.3–10 mg/L showed no change in biomass production and a significant accumulation of B in fronds. At the same time, duckweed effectively reduced the B content in the environment in concentrations up to 2.0 mg/L [25]. A study of B toxicity using L. minor and L. gibba, with the aim of using them for phytoremediation and biomonitoring, revealed that significant inhibition of plant growth began at a B concentration of 16 mg/L. L. minor was more sensitive to B than L. gibba. The activity of the antioxidant enzymes superoxide dismutase, ascorbate peroxidase, and guaiacol peroxidase can serve as biomarkers for B-rich environments [26]. In another study, the combined use of L. gibba and chitosan beads effectively removed B from drinking water [27].
L. gibba showed the greatest potential to remove boron from irrigation water with B concentrations of 5.58–17.39 mg/L using a batch reactor. It was capable of removing 19–63% of the B from irrigation water, depending upon the level of contamination or initial concentration [28]. L. gibba and L. minor in the form of duckweed-based wastewater treatment systems coupled with microbial fuel cell reactor was shown to be an efficient method to simultaneously remove B from domestic wastewater/irrigation water and generate electricity [29][30]. In these studies, a monoculture of L. gibba showed the highest efficiency of B removal. Part of the research focused on the possibilities of B accumulation by duckweed under salt stress. Salt stress significantly affects the growth and B accumulation of L. minor. It was shown that only 7.9% to 15.5% of B was accumulated by L. minor during cultivation at NaCl concentration in a range of 0–200 mM. Finally, the authors concluded that L. minor is suitable for the accumulation of B when NaCl is below 100 mM [31]. Similar results were also shown for S. polyrhiza [32]. Thus, to date, information on the possibility of using duckweed for B removal is very limited, focusing on only three species, of which only L. gibba showed a sufficiently high potential for phytoremediation.
Arsenic (As) is present in the environment in inorganic and organic form and exists in four oxidation states—arsenate (As(V)), arsenite (As(III)), arsenic (As(0)), and arsine (As(-III)) [33]. Aquatic As phytoremediation approaches continue to be actively pursued [34][35]. Among 36 duckweed species, L. gibba, L. minor, S. polyrhiza, W. globosa, W. australiana, and L. valdiviana have been reported to remove As from water. The potential of duckweed for phytoremediation of As was first demonstrated in 2004 in waters from abandoned uranium mines. L. gibba revealed high arsenic bioaccumulation coefficients in wetlands of two former uranium mines in eastern Germany and under laboratory conditions. The potential extractions from mine surface waters using L. gibba were estimated to be 751.9 kg As/ha·yr [36]. In another study, L. gibba accumulated 10 times more As than background concentrations in the tailing waters of an abandoned uranium mine, reducing arsenic on average by 40.3% in the solutions [37].
L. minor showed high As accumulation (641 ± 21.3 nmol/g FW) when grown on As concentrations of 25–80 µM under laboratory conditions [38]. In another study, L. minor showed a removal rate of 140 mg As/ha·d, with a recovery of 5% when grown under a concentration of 0.15 mg/L [39]. The study of biological responses of L. minor revealed that both the duration of exposure and the concentration of inorganic As had a strong synergistic effect on antioxidant enzyme activity. L. minor showed a higher accumulation of As(III) compared to As(V) from polluted water [40]. A study of the accumulation of As by aquatic plants in running waters showed that L. minor is one of the top three studied species regarding arsenic accumulation (430 mg/kg DW). Higher values were observed only for Callitriche lusitanica and Callitriche brutia [41]. In hydroponics, L. minor revealed maximum removal of more than 70% As at a low concentration (0.5 mg/L) on day 15 of the experiment [42]. Another finding revealed that chelating agents had positive effects on As(III) or As(V) accumulation in L. minor [43].
For L. valdiviana, the As was only absorbed by the plant after a decline in the phosphate levels of the medium [44]. Concentrations greater than 1 mg/L As in the nutrient solution caused deleterious effects in L. valdiviana and compromised their phytoremediation capacity of water contaminated with As [44]. In addition, for L. valdiviana, As accumulation was dependent on pH. L. valdiviana accumulated 1190 mg/kg As (dry weight) from the aqueous media and reduced its initial concentration by 82% when cultivated between pH 6.3 and 7.0 [45].
At concentrations of 1.0, 2.0, and 4.0 µM As and dimethylarsinic acid, S. polyrhiza showed a significant level of As bioaccumulation, using different mechanisms for the degradation of arsenate vs. arsenite [46]. In addition, the uptake of inorganic arsenic (As (V) and As (III)) by S. polyrhiza was higher compared to the organic As sources, monomethylarsonic and dimethylarsinic acid. The addition of EDTA increased the uptake of inorganic As into the plant tissue, but the uptake of organic arsenic was not affected [47]. The study of the stability of S. polyrhiza at As (V) concentrations of 1, 5, 10, and 20 µM revealed an increase in the fresh biomass, photosynthetic pigments, and total protein contents of S. polyrhiza at lower concentrations of As (V) after 1 d of exposure, followed by a decrease in biomass with an increase in metal concentration [48]. In another study, S. polyrhiza showed the ability to survive in high concentrations of As (V) solution in hydroponics by decreasing As concentration, with a removal rate of 41% [49].
W. globosa accumulated 2–10 times more As than S. polyrhiza/L. minor and Azolla species [50]. At the low concentration range, the uptake rate was similar for arsenate and arsenite, but at the high concentration range, arsenite was taken up at a faster rate [50]. W. globosa was more resistant to external arsenate than arsenite but showed a similar degree of tolerance. A more detailed study of the mechanisms of As assimilation in W. globosa demonstrated an important role of phytochelatins in detoxifying As and enabling As accumulation [51]. A study conducted using W. australiana revealed the importance of microbial agglomerations for As assimilation. Sterile W. australiana did not oxidize As(III) in the growth medium or in plant tissue, whereas W. australiana with phyllosphere bacteria displayed substantial As(III) oxidation in the medium [52].

This entry is adapted from the peer-reviewed paper 10.3390/plants12030589

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