Floating Aquatic Macrophytes in Wastewater Treatment: Toward a Circular Economy
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Macrophytes have been used in the removal of toxic compounds individually or as components of constructed wetlands to purify various wastewater types [97]. Based on recent studies on the use of macrophytes, there is a trend toward the simultaneous removal of multiple pollutants [22,98]. Different macrophytes have been studied regarding their potential to remove various pollutants, organic matter, nutrients, heavy metals and pathogens. Factors such as plant tolerance, the feasible range of toxicants that plants can accumulate, the concentration of toxicants in the medium and environmental factors largely impact the remediation capability of macrophytes [99]. It is not always appropriate to use living aquatic macrophytes for the continuous removal of harmful pollutants. After becoming saturated with pollutants, pollutant uptake decreases, and the plant will eventually perish due to the detrimental effects of the pollutants on plant growth and metabolism [100]. Therefore, although macrophytes are widely used in wastewater treatment, the application of dead or inactive parts or any substances obtained from biological sources is a better choice in continuous wastewater treatment.
When the biomass is alive during the treatment of wastewater, the process is denoted as “bioaccumulation”, whereas the use of dead biomass is termed “biosorption” [101]. During the former process, the toxic compounds attach in an inter- and intracellular manner, whereas in biosorption, such attachment is extracellular. Absorption, a double-stage active process, is responsible for pollutant removal via bioaccumulation, whereas in biosorption, a single-stage passive process, adsorption controls pollutant removal. Further, while desorption is only partially possible for bioaccumulation, biosorption also includes desorption. Generally, the removal performance is higher in biosorption than in bioaccumulation [102,103].
Water hyacinth can effectively be used in the removal of pollutants by chemical, biological, mechanical or hybrid means [104]. It can eliminate inorganic nitrogen [nitrate (NO3-N), ammonium (NH4-N), and total N)] and phosphorus (PO4−3-P and total P) from nutrient-rich wastewater [96]. Duckweed is another promising macrophyte with high potential in the removal of a wide spectrum of pollutants (organic pollutants, heavy metals, agrochemicals, pharmaceuticals and personal care products, radioactive waste, nanomaterials and hydrocarbons) from wastewater and can thrive in highly contaminated water [96]. Duckweed has been used in the treatment of low-strength domestic wastewater to high/severe-strength industrial wastewater streams to obtain clean, non-potable water [105,106]. Some authors recommend the use of duckweed after the removal or conversion of organic sludge into simple organic and inorganic molecules as they can be easily taken up by this macrophyte [107]. Another aquatic macrophyte with a high potential for pollutant removal is Azolla [108]. In combination with Anabaena azollae, it efficiently removes nutrients even after complete nitrogen depletion. The species A. pinnata can effectively be used in the treatment of domestic and industrial effluents [8,109,110].
2.1. Heavy Metal Removal
Heavy metals are major pollutants in aquatic environments due to their high toxicity, non-degradable nature and bioaccumulation and biomagnification [10,112,113,114]. Aquatic macrophytes play a crucial role in the removal of heavy metals from the aquatic environment [115]. Table 2 shows the heavy metal uptake capacities of different FAMs, either via bioaccumulation or biosorption [101,116,117]. During the uptake of heavy metals at the whole plant and cellular level, plants absorb the metals based on the negative charges of their cell walls. Subsequently, the metals are transported into the cell cytoplasm and partitioned into cell organs or excreted [118]. Plants can accumulate 100,000 times higher concentrations of heavy metals compared to the effluent concentration [119].
Live water hyacinth can remove large amounts of heavy metals through absorption and translocation to shoots and other tissues [141]. Dried water hyacinth and ash obtained from water hyacinth have also been used to adsorb heavy metals from waste streams [121,142,143].
Jones, et al. [120] conducted a study in the British River and reported the most pronounced heavy metal removal (21 heavy metals) using water hyacinth. After an exposure period of 7 h, 63% of Al, 62% of Zn, 47% of Cd, 22% of Mn and 23% of As were removed. Under in situ conditions, the authors reported the removal of Mn, Zn and Cd at 6%, 11% and 15%, respectively.
Bais [144] explored the biosorption ability of the shoots and roots of water hyacinth in the rainy season as well as in winter and summer. Based on the findings, during winter, 31% of Cd was removed via the shoots and 41% via the roots. Bianchi, et al. [136] reported that A. filiculoides can efficiently eliminate Fe and Al, with removal rates of 92% and 96%, respectively, whereas only 10% of Cr could be removed.
When comparing the biosorption capacities of Azolla fliculoides and Hydrilla verticillata regarding the removal of Cu(II), Cr(VI), As(III) and Pb(II), Bind, et al. [145] found that Pb was effectively absorbed by both species, with removal rates of 81.4% and 84.3%, respectively, from a synthetic wastewater stream containing Pb at a concentration of 10 mg L1. The adsorption capacity followed the order Pb(II) > Cu(II) > As(III) > Cr(VI).
Chaudhary and Sharma [129] investigated the efficiency of Lemna gibba in removing Cr and Cd from solutions with varying concentrations under laboratory conditions. The experiments were carried out for 7 and 15 days, and the removal rates were 37.3% to 98.6% for Cr and 81.6% to 94.6% for Cd. The removal capacity of this species decreased with increasing metal concentrations.
Yilmaz and Akbulut [146] evaluated the efficiencies of two different species of duckweed, namely L. minor and L. gibba, regarding metal removal under aeration. The removal rates were Pb 57%, Ni 60%, Mn 60% and Cu 62% for L. minor and for L. gibba. Aeration and the combination of these species increased the removal rates.
2.2. Nutrient Removal
In water bodies, nutrients are essential for the survival of aquatic biomes. However, above certain thresholds, they can become toxic to various organisms. Since aquatic plants can thrive in high nutrient concentrations and produce large amounts of biomass, they remove nutrients from wastewater. Table 3 shows the nutrient uptake capacities of different FAMs. Nitrogen and phosphorous are the key nutrients, accompanied by carbon at a certain level. Excess nitrogen and phosphorous accumulation results in water eutrophication, with negative impacts on ecosystem health. To satisfy the physiological requirement of macro- and micronutrients and to support the epiphytic biofilm growing on the surface, macrophytes will consume nutrients through assimilative uptake, which is the direct method of nutrient removal [147]. Further, the macrophytes can additionally support the treatment system indirectly in nitrogen removal by enhancing the nitrification and denitrification process through generating a spatial oxygen gradient across the treatment system [148].
Kadir, et al. [156] carried out a preliminary study to determine the appropriate dilution of palm oil mill effluent (POME) to successfully grow L. minor and A. pinnata and to evaluate the corresponding nutrient removal rates. Both species showed high ammonia removal, with rates of 98% and 95.5%, respectively, in 5% POME. Phosphate removal was higher in 10% POME, with 93.3% removal by A. pinnata and 86.7% by L. minor. Overall, A. pinnata showed a significant nutrient reduction in 2.5% POME.
In another study, the authors performed a 2-week experiment to test the nutrient removal capacities of L. minor and A. filiculoides from textile, distillery and domestic wastewater mixtures. There were no significant differences in the nutrient removal rates between the species; A. filiculoides removed 94.6% of phosphorous, and L. minor removed 92% of phosphorous. Total nitrogen was more efficiently removed by A. filiculoides (94.6%) compared to L. minor (92%) [132]. Similarly, Verma and Suthar [164] investigated the capacity of L. gibba to treat sewage; L. gibba removed 42–64% of nitrate and 37–54% of total phosphorous.
Singh, et al. [165] investigated the potential of Eichhornia crassipes in removing nitrogen and phosphorous from glass industry effluent (GIE). This study was supported by a response surface methodology and an artificial neural network for optimization and prediction. Diluting the GIE to 60% and treating it with GIE showed the best results in terms of the removal of total Kjeldahl’s nitrogen (93.9%) and total phosphorus (87.4%).
2.3. Organic Contaminant Removal
Organic pollutants are broadly categorized into two major groups: oxygen-demanding waste and synthetic organic pollutants. Wastewater from municipalities and the food industry, paper mill effluent and animal farm wastewater contain more biodegradable compounds that can be degraded by microorganisms, resulting in a higher oxygen demand and, ultimately, in anoxic conditions. Plants can effectively remove simple organic matter, which requires high oxygen demand during decomposition, and their effectiveness was tested by several researchers. El-Kheir, et al. [166] used L. gibba to treat primary treated sewage and observed BOD (biological oxygen demand) and COD (chemical oxygen demand) decreases by 90.6% and 89.0%, respectively. Another study was carried out by Bhagavanulu, et al. [167] to evaluate the biosorption capacity of the root, stem and leaf powder of water hyacinth. Root and stem powder were effective in turbidity management. The maximum BOD reduction of 49.2% was observed when the root powder was used for 30 min. A COD reduction was observed when a combination of leaf and root powder, in equal amounts, was used. Sahi and Megateli [168] investigated the ability of L. minor to reduce the COD in real dairy wastewater (RDW) and synthetic dairy wastewater (SDW) over a period of 10 days, and this species was more effective in removing COD from RDW (60%) compared to SDW (55.5%). Mamat, et al. [169] determined the efficacy of Azolla pinata in the treatment of palm oil mill wastewater and reported that this species effectively removed 85.89% of the BOD and 80.58% of the COD.
Synthetic organic compounds are produced by synthetic detergents, agrochemicals, specific food additives, pharmaceuticals, synthetic fibers and plastics [170]. Organic pollutants of the category persistent organic pollutants (POPs) are more dangerous because they can remain in the food chain and have a longer half-life [171]. Endocrine disruptive chemicals are a subdivision of synthetic organic compounds with the capacity of creating hormonal imbalances and affecting reproduction development or behaviour in animals and causing irregular endocrine behaviour and cancer in humans [172]. The increase in the amounts of endocrine-disrupting chemicals in most waste streams has resulted in public concerns regarding their elimination [173]. Chlorophenols, bisphenol A, dichlorodiphenyltrichloroethane (DDT), chlorpyrifos, atrazine, 2, 4-D and glyphosate are widely available endocrine-disrupting chemicals [108,174].
Pharmaceuticals are another subclass of synthetic organic pollutants, and their use has increased recently. Anti-inflammatories, antidepressants, antiepileptics, lipid-lowering drugs, β-blockers, anti-ulcer agents, antihistamines and antibiotics [175] are organic pollutants derived from the pharma industry. As an example, diclofenac, a nonsteroidal anti-inflammatory drug, has gained attention because it persists in municipal wastewater [176].
Table 4 shows the capacities of different FAMs to take up organic pollutants. Campos, et al. [177] investigated the efficiency of E. crassipes and Cyprus isocladus in different constructed wetlands to eliminate endocrine disruptors from synthetic municipal wastewater. The removal rates varied from 9.0 to 95.6% for ethinyl estradiol, 29.5 to 91.2% for bisphenol A and 39.1 to 100.0% for the progestin levonorgestrel. Zazouli, et al. [108] reported the removal of bisphenol A by Azolla, with removal rates from 60 to 90%. In a study by Bianchi, et al. [136], by using L. minuta and A. filiculoids, diclofenac was removed at removal rates below 10%, whereas higher rates were observed for the removal of levofloxacin, with rates of 50% and 60%, respectively. Xia and Ma [178] investigated the removal of the phosphorus pesticide ethion with water hyacinth. The plant accounted for 69% of the removal of ethion from the waste stream through uptake and phytodegradation, but the roots and shoots emitted ethion at levels of 74–81% and 55–91%, respectively, in ethion-free medium after a growth period of 7 days. In a study by Balarak [179], 2-chlorophenol (2-CP) and 4-chlorophenol (4-CP) were removed from agropharma waste using Azolla, with removal rates of 71% and 85%, respectively. Garcia-Rodríguez, et al. [180] tested the potential of duckweed to remove carbamazepine, acetaminophen, propranolol, ibuprofen, diclofenac, caffeine, bisphenol A, and 17-a-ethinylestradiol from secondary treated wastewater, and the observed removal rates are promising.
Dyes are another group of organic pollutants in the wastewater stream and are mainly derived from industrial plants and households. According to Kant [196], around 8000 chemicals are associated with dyeing processes and pose risks to environmental and human health. For example, crystal violet, a commonly used dye, is mutagenic and carcinogenic. Color removal from dyes is a serious problem as it consumes more oxygen and increases the BOD value in the waste stream [197]. Kulkarni, et al. [197] studied the biosorption capacity of water hyacinth root powder for the decolorization of crystal violet dye and obtained a Langmuir monolayer biosorption capacity of 322.58 mg/g. These authors further examined the influence of initial pH, initial dye concentration, biosorbent dosage, contact time and temperature on dye removal and found that water hyacinth was an effective biosorbent to remove crystal violet dye from an aqueous solution. Nath, et al. [198] investigated the biosorption capacity of water hyacinth in the removal of industrial dyes such as methylene blue, Congo red, crystal violet and malachite green from aqueous solutions at laboratory scale and observed maximum removal rates of 90%, 88%, 92% and 90%, respectively. According to Padmesh, et al. [199], Azolla can efficiently be used in the removal of acid blue 15 and eliminated 61.3% of this dye from an aqueous solution through biosorption. Durairaj [200] and Imron, et al. [201] tested the effectiveness of L. minor in removing methylene blue and textile acid orange 10, respectively, with contact times of 1 day for methylene blue and 4 days for acid orange. The removal rates were 80.66% for methylene blue and 82.9% for acid orange.
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