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 and domestic wastewater is the main source of organic compounds such as halogenated aromatic compounds, BTEX compounds (benzene, ethyl-benzene, toluene, and three isomers of xylene), pharmaceuticals (e.g., antibiotics, hormones, cardiovascular drugs, antidepressants, anti-epileptics, and anti-cancer drugs) or personal care products (Gasperi et al., 2014; Becouze-Lareure et al., 2019). The presence of antibiotics can affect many aquatic species and generate the appearance or maintenance of antibiotic-resistant species. By phytodegradation macrophytes are able to metabolize contaminants (Black, 1995; Ansari et al., 2020). Microorganisms around plant roots play a significant role in organic pollutant biodegradation during rhizoremediation process (Zhang et al., 2014a). The presence of such active molecules can affect the plant rhizosphere and thus compromise the phytoremediation potential in CWs. Urban storm water runoff contains unburned hydrocarbon residues and combustion products such as PAHs (Gasperi et al., 2014). Bioremediation can be used as an alternative to physical and chemical methods taking advantage of the presence of natural ability of certain microbes to degrade hydrocarbons (El-Mufleh et al., 2014). Bioremediation can be achieved by biostimulation or bioaugmentation by adding microorganisms in the polluted environment (Amer et al., 2015). Degradation of the aromatic cycles requires their opening by oxidation. Degradation of PAHs involves enzymes such as dioxygenase, peroxidizes, and cytochrome P450 monooxygenases (Gaur et al., 2018). Nonspecific oxidoreductases emit free radicals oxidizing aromatic cycles by oxygen transferring, and cleave aromatic cycles, allowing then enzymatic degradation by bacteria (Cota-Ruiz et al., 2019).
Microbial degradation of PCBs follows two possible degradation pathways: anaerobic dechlorination and aerobic degradation (Jing et al., 2018). As with PAHs, the aerobic degradation of PCBs begins with a double oxygenation on the least chlorinated part of the biphenyl nucleus. This oxidation sequence leads to the opening of the phenolic ring and to the formation of cellular metabolism compounds: acetaldehyde and pyruvic acid. Chlorine will be released in the form of chloride (Cota-Ruiz et al., 2019). However, only molecules with three or less chlorine atoms can be degraded in this way (Sangely, 2010; Jing et al., 2018). Because of only a thin sedimentary aerobic surface layer, many aquatic environments are not able to perform any aerobic biodegradation. In anaerobic dechlorination process, a hydrogen atom replaces a chlorine atom. The PCB serves as a TEA in three possible positions of the chlorine atom, in order of preference: in the para, meta, and ortho positions. Biostimulation by pretreating the sediment with halogenated aromatic compounds (a kind of halopriming) induces the oxidoreductase enzymes in native dechlorinating microbial consortium (Jing et al., 2018). Biostimulation can also be achieved by installing an electron-accepting anode in the sediment (cf. § 1.10.3). Emergent plant species used for PCB phytoremediation include alfalfa Medicago sativa, Chinese bushclover Lespedeza cuneate, everlasting pea Lathyrus sylvestris, reed canary grass Phalaris arundinacea, cucurbits Cucurbitaceae sp., bur-reed Sparganium sp., Alaska willow Salix alaxensis, and white spruce Picea glauca (Jing et al., 2018). Stonewort Chara vulgaris, duckweed Lemna minor, water-milfoil Myriophyllum spicatum, and pond-weed Potamogeton perfoliatus showed their potential to remove detergents such as alkylbenzenesulfonate (Zhou et al., 2018; Liu et al., 2019; Ansari et al., 2020).
Plant uptake plays a dominant role in the elimination of clofibric acid and caffeine, and remains significant in the case of ibuprofen. Carbamazepine and MCPA (an herbicide) are relatively recalcitrant to all removal processes, although in hydroponic systems plant uptake accounts for half or more of the limited removal observed for these compounds (Zhang et al., 2014a). Using of mutual beneficial combination of plants and bacteria can overcome this obstacle. Plant-microbe partnership with soft rush (Juncus effuses) and Indian shot (Canna indica) can eliminate more than 80% for certain macrolides (Tai et al., 2017). Obviously, the elimination rates of biologically active molecules decrease as their concentration increases. Their concentration remain high, up to a concentration of a few µg/L. Microbes (rhizo- and endophytes) in this type of partnership not only directly degrade emerging pollutants but also accelerate plant growth by producing growth hormones and thus stimulate the bioremediation potential of CWs (Arslan et al., 2017; Nguyen et al., 2019a).
In a comparative study of the four major types of CWs, Ilyas and van Hullebusch (2020) noted better removal of 29 pharmaceuticals and 19 transformation products in hybrid systems, followed in descending order by VSSF-CWs, HSSF-CWs, and FSF-CWs. The coexistence of aerobic and anaerobic conditions and a longer HRT in hybrid CWs could explain the relatively good removal of diclofenac, acetaminophen, SMX, sulfapyridine, trimethoprim, and atenolol (Ilyas and van Hullebusch, 2020).
Treatment performances for pharmaceuticals are typically better in SSF-CWs, due to their large rhizospheric surface area, than in FSF-CWs (Zhang et al., 2014a). However, obviously because of direct exposure to sunlight, pharmaceuticals sensitive to photo-degradation degrade more easily in FSF-CWs than in SSF-CWs (Ilyas and van Hullebusch, 2020). For some pharmaceuticals, such as diclofenac, ketoprofen (non-steroidal anti-inflammatories), and triclosan, photo-degradation seems the predominant route of elimination. Indeed, diclofenac, highly sensitive to photo-degradation, is more easily removed in FSF-CWs: 65%, than in SSFCWs: <45% (Zhang et al., 2014a).
The bioconcentration factors are generally higher in the floating macrophyte species than in the submerged species. The more hydrophilic pharmaceuticals such as carbamazepine and diphenhydramine are more easily absorbed and transferred to leaf tissues (Pi et al., 2017). VSSF-CWs seem more efficiently remove highly biodegradable pharmaceuticals, compared to HSSF-CWs, probably because they enhance aerobic microbial biodegradation due to a better oxygenation (Zhang et al., 2014a). Xiong et al. (2018) summarize several novel approaches to the bioremediation of pharmaceuticals through the use of microalgae, including the construction of microbial consortia, acclimatization, and co-metabolism (Xiong et al., 2018)
As in the case of nitrogen pollution removal, different types of CWs combined in hybrid systems, to exploit the specific advantages of each system and to provide aerobic and anaerobic conditions at the same time, are more effective. Highly biodegradable pharmaceuticals, such as ibuprofen, ketoprofen, and salicylic acid, show high abatement rates