1.9.2 Metals Removal
The plants require some heavy metals up to a certain limit (e.g., Zn, Cu, Mo, Ni, or Mn) for their growth and development but above that limit these metals become toxic to the plants and influence negatively the metabolic functions (Andresen et al., 2013). Toxicity of heavy metals causes Reactive Oxygen Species (ROS) production, affecting physiological processes such as photosynthesis, respiration, and cell disintegration and causing even the death of the plants (Zhang et al., 2017). Some plants have tolerant capacity toward heavy metals due to presence of anthocyanins, thiols, and anti oxidants (Leao et al., 2014). Other like duckweeds has extraordinary efficiency to recover very fast toward high heavy metals exposure (Ekperusi et al., 2019; Ansari et al., 2020). Bioremediation by phytoremediation involves extracting pollutants from the polluted environment and concentrating them in plants which should be removed from the environment to be restored (Haldar and Ghosh, 2020). Through the food chain, metals and metalloids stored in plant tissues can also be consumed by aquatic organisms and accumulate in their tissues.
Heavy metal can also precipitate and be immobilized in bottom sediment: this is only apparent self-purification. It is known that bottom sediment can act as a sink for metals. However, metals precipitated in this way can be released massively again, when there is a change in pH or redox potential, as in the case of eutrophic or dystrophic waterbodies.
The higher concentrations of trace metals accumulated were recorded in the roots and leaves of common reed Phragmites communis and spiny naiad Najas marina (Baldantoni et al., 2004). A pilot SSF-CW planted with the Mexican sword plant (Echinodorus palaefolius) in a zeolite substrate makes it possible to immobilize 91.8% of the Hg of an effluent highly spiked at 14.94 mg/L in three days (Prasetya et al., 2020). It should be pointed out that wetlands are hot spots for the Hg methylation, which diminishes the interest of this phytoremediation technique for mercury pollution (Wang et al., 2020b).
Free-floating aquatic plants develop submerged roots that capture heavy metals in the water and accumulate them in their roots, runners, and leaves (Galal et al., 2018; Ali et al., 2020). Free-floating aquatic plants grow relatively fast and are easy to cultivate. They have the great advantage of rapidly building up a biomass that can be easily harvested continuously, thus allowing a continuous export of accumulated pollutants out of the water body. Water hyacinth (Eichhornia crassipes) has been identified to be quite efficient in the removal of heavy metals from AMD (Mang and Ntushelo, 2019). E. crassipes exhibits high metal bioconcentration factors (Mn = 199,567 > Pb = 19,605 > Cd = 3403 > Zn = 1913). The translocation factor is less than 1, suggesting a weak ability to transfer metals from roots to runners and leaves. The roots accumulate more Pb, Zn, Cd, and Mn than runners and leaves because of stronger metal tolerance of roots (Jamuna and Noorjahan, 2009). Water lettuce (Pistia stratiotes) has the ability to accumulate high concentrations of Pb, Cu, Cr, Ni, and Co in its roots and leaves. The bioconcentration factor of investigated heavy metals is greater than 1,000, except for Cr (65) and Pb (241), while the translocation factor does not exceed one, except for Pb (1.3) and Cu (37.8). Its roots are bioaccumulators of Cu, Ni, and Co (Galal et al., 2018).
Evaluation of the synergy of Pb and Cr(VI) on their simultaneous accumulation in the watercress (Nasturtium officinale) in CWs shows a cumulative accumulation in stems, leaves, and roots. The highest concentration of both metals occurs in the roots (Pb> Cr). The higher the concentration of Cr(VI) ions, the more metal the N. officinale absorbs, the higher the metal tolerance index and the lower the translocation factor. In the CW, removal rates were >99.9% (100 mg/L) for Pb and 95% (28.5 mg/L) for Cr (Marquez-Reyes et al., 2020).
Lemna minor has the ability to remove soluble Pb and Ni (Axtell et al., 2003) and Cr(VI) (Uysal, 2013; Sinha et al., 2018). Wolffia globosa is also reported for its accumulation capability for Cd and Cr (Boonyapookana et al., 2002). Metal removal efficiency by common duckweed (L. minor) from landfill leachate is larger than 70% for Ni, Pb, Cu, and Zn, with a maximum value for Cu of 91%. Bioconcentration factors are lower than 1 with a maximum for Cu (0.84) and Pb (0.81), indicating a low metal bioaccumulative capacity of L. minor (Daud et al., 2018). Intermediate duckweed (Spirodela intermedia) was identified as a good Hg accumulator. Bioaccumulation of Hg takes place mainly in the roots (bio-concentration factors larger than 1,000), leaves accumulating less (bio-concentration factor larger than 200), for a concentration of 1 mg/L of Hg in 96 hrs. In all cases, translocation factors lower than 1 indicates an absence of Hg translocation (de la Fourniere et al., 2019). Floating ferns, such as Carolina mosquitofern Azolla caroliniana, showed that the plants have the potential to purify water polluted by Hg and Cr (Bennicelli et al., 2004). Water spangles, Salvinia minima, can be used as a phytoremediating agent for the heavy metal removal in effluents. S. minima grows in low concentrations of Cd, Ni, Pb, and Zn and adsorbs or accumulates these metals in its tissues in higher concentrations, without necessarily growing (Iha and Bianchini, 2015). Floating fern S. natans shows some capacities to accumulate metals, as shown by the concentration factors calculated in relation to the surrounding water sampled in the slightly contaminated Yi River (Shandong): Cr: 4.3; Ni: 2.3; Cu: 12.4; Zn: 29.0; and Pb: 73.2 (Li et al., 2019). High bioaccumulation factors (Cu: 9,526; Ni: 13,810; and Zn: 42,438) measured on S. natans collected in recreational ponds in a residential area near Wrocław (Poland) confirm the high accumulative capacity of Salvinia spp. (Polechonska et al., 2019).
In submerged aquatic plants, the adsorption of metals mainly takes place through the fully submerged leaves, the main function of roots being to anchor the plant in the bottom sediment. This constant submersion allows the plant to present a greater surface area of biomass in contact with the water to accumulate heavy metals. Hornwort (Ceratophyllum demersum) is an almost rootless plant; some leaves develop into root-like organs (rhizoids) capable of attaching the stems to the sediment. Measurements of the heavy metal accumulation capacity of C. demersum in a wetland in Himalayan Kashmir show high bio-concentration factors for Cd (>1,300) and Co (>3,600) and, to a lesser extent, for Cr (689), Pb (294), Ni (196), and Cu (85). The results confirm the high capacity of C. demersum to bio-accumulate Co and Cd (Ahmad et al., 2016). C. demersum shows in natural waterbodies high bioaccumulation factors for heavy metals: Cd (>24,700); Cr (>33,900); Cu (>3,200); Ni (>29,600); Pb (1677); and Zn (>14,100), indicating a considerable capacity to absorb trace metals. Its properties make C. demersum a particularly relevant indicator in the monitoring of heavy metal pollution (Dogan et al., 2018; Polechonska et al., 2018). Measurements of the heavy metal accumulation capacity of Potamogeton natans in a wetland in Himalayan Kashmir show high bio-concentration factors for Cd (1028) and, to a lesser extent, for Co (744), Pb (712), and Ni (363). The results confirm the high capacity of P. natans to bio-accumulate Cd (Ahmad et al., 2016). Pondweed (Potamogeton crispus), a rooted submerged angiosperm, sampled in the Yi River (Shandong) can accumulate relatively large amounts of heavy metals as shown by these concentration factors calculated in relation to sediments: Cr: 9.2; Ni: 11.8; Cu: 9.4; Zn: 18.8; and Pb: 2.7. Factors are closely related to sedimentary metal concentrations (Li et al., 2019).
Aquatic plants can also be used to extract radionuclides from water bodies (Vanhoudt et al., 2018). Watermilfoil (Myriophyllum spicatum) is a promising candidate for the removal of 59Co and 133Cs from contaminated water bodies, even in the presence of their 137Ce and 60Co radioisotopes. M. spicatum removes more than 90% and 60% of stable 59Co and 133Cs radioisotopes within 24 hours, even in the presence of 60Co and 137Cs. The bio-accumulation factors for the whole plant are 27.13 for 137Ce and 10.80 for 60Co (Saleh et al., 2020).
1.9.3 Organic Micropollutant Removal
Urban
