**2. Phytoremediation pathways**

The terminology "Phytoremediation" consists of two words, "Phyto" means "green plants" and "remediation" means "curative measures or restoration". The word "phytoremediation" was first given by Chaney [18]. In phytoremediation process, generally green plants are used which uptake toxic chemical substances (such as heavy metals and metalloids, pesticide residues etc.) from contaminated sites (soil and water) by various mechanisms and remove them from environment. Various crop and weed plants are found to be suitable for phytoremediation purpose. But research results indicated that weed flora had higher phytoremediation potential than cultivated crops (Example- Brassica sp). There are various pathways of phytoremediation process such as, rhizofiltration, phytoaccumulation or phytoextraction, phytostabilization, phytodegradation or phytotransformation and phytovolatilization etc.


Apart from this there are some other terminologies often used in phytoremediation process are bioconcentration factor (BCF) and translocation factor (TF).

BCF = toxic substance uptake by plant/toxic substance present in environment (soil or water).

TF = toxic substance present in shoot or stem/toxic substance present in roots or.

Toxic substance present in leaves/Toxic present in shoot or stem.

For, Hyper accumulator plants both BCF and TF is >1 is desired. In other words, plants suitable for phytoremediation, BCF >1 is always desirable. But for aquatic weeds, as their dominant pathways is rhizofiltration; their toxic substances BCF >1 but TF for root to shoot or shoot to leaves is <1.

## **3. Potential of various aquatic plants for phytoremediation**

#### **3.1 Phytoremediation by free floating aquatic weeds**

*Eichhornia crassipes: Eichhornia crassipes* is commonly known as water hyacinth, a free-floating perennial aquatic plant native to tropical and sub-tropical South America, and is now wide spread in all tropic climates. The genus Eichhornia comprises seven species of water hyacinth among which *E. crassipes* is the most common and have been reported to grow very first. However, its enormous biomass production rate, high tolerance to pollution and absorption capacity of heavy-metal and nutrient qualify it for use in wastewater treatment [19].

The capability of removing arsenic from contaminated water was earlier observed by Misbahuddin and Fariduddin [20] and they observed that water hyacinth can removes arsenic from water within 3–6 hr. exposure time. Amount of arsenic removed depends on number of the plant used, exposure time, presence of air and sunlight. They concluded that whole plants were more effective than fibruous roots alone. It was observed that dried roots of water hyacinth can rapidly reduces As content in contaminated water within below WHO recommended critical level (<10 μg Lg−1) [21]. A fine powder was prepared from dried roots of water hyacinth plants (obtained from Dhaka, Bangladesh) removed more than 93% arsenite and 95% of arsenate from a solution containing As @ 200 μg L−1 within 1 hr. exposure time [21]. Higher biomass production ability of water hyacinth allow it to remove As at higher rate (600 mg As ha−1 day−1) and greater efficiency (17%) compared to lower biomass producing aquatic macrophytes such as lesser duck weed (*Lemna minor*) which removed As at lower rate (140 mg As ha−1 day−1) and lesser efficiency (5%); though there was no difference in bioaccumulation capacity [13]. Similarly better As extraction capacity of water hyacinth (80%) compared to *Lemna minor* and Spirodella Polyrhiza from tropical coalmine effluent was also been reported [22] from India. Unlike lower biomass producing aquatic macrophytes, water hyacinth poses better As extraction ability compared to higher biomass producing vetivar grass [23]. Not only higher biomass, higher reproduction ability also plays an important role in As phytoremediation by water hyacinth. Water hyacinth was a suitable phytoremediation agent when As present in contaminated water at lower concentrations. When As was provided at lower concentrations @ 1 and 2 mg L−1, water hyacinth removed 90 and 65% of total As from contaminated solutions (1 and 2 mg L−1 respectively) provided respectively within 7 days [24] and maximum As stored in roots. Water hyacinth can extract higher amount As from contaminated water but their presence in water bodies reduces dissolved oxygen content (DOC), which makes its application for a larger water bodies a problematic pathway which needs to be taken care.

*Phytoremediation of Arsenic Contaminated Water Using Aquatic, Semi-Aquatic and Submerged... DOI: http://dx.doi.org/10.5772/intechopen.98961*

**Pistia stratiotes:** Pistia stratiotes is commonly called as water lettuce There are many previous studies indicated that Pistia stratiotes capable of removing toxic heavy metals from contaminated water [25–27], but there were few studies was done on As uptake by water lettuce. Earlier a field study carried out using *P. stratiotes* and results showed that *Pistia stratiotes* can remove As from contaminated water, along with higher bioconcentration factor (BCF) for root (8632) vis-à-vis lower BCF for leaf (2342) [28]. In a laboratory study it was demonstrated that maximum As removal efficiency of *P. stratiotes* was found at pH 6.5 and Pistia removed 87.5% of the metalloid provided in the solution [29]. From Laboratory study it was revealed that *P. stratiotes* can accumulate As efficiently when As was provided at lower concentrations, though total As uptake was increased with increase in As concentration in the solution [30]. Arsenite accumulation in *P. Stratiotes* was found more in root and less in leaves like water hyacinth. Arsenic accumulation in roots and leaves were respectively 1120 and 31.60 μg g−1 DW respectively when 10 μM As (As3+) solutions are employed [31]. When higher concentration of As solutions used (>20 μM), As toxicity symptoms like chlorosis, suppressed growth, lower photosynthetic rate, suppressed enzymatic activities and increased cell damage were observed in *P. stratiotes* [30, 31].

**Lemna, Spirodella and Wolfia**: Weeds belongs to Lemna, Spirodella and Wolfia are generally known as Duckweeds. Duckweeds are small free-floating aquatic weed plants which generally found in water bodies, mainly comprises of four genera*, Lemma, Spirodela, Wolfia*, and *Wolfiella,* and of 34 species. Among these Lemna, Spirodela, and Wolfia have been widely reported to accumulate arsenic from contaminated water [13, 32–34]. Research studies indicated that, total As accumulation in *Lemna gibba* was more in field condition compared to laboratory conditions due to higher exposure time in field condition [32]. However higher accumulation of As in plant parts is not always correlated with bio-concentration factor (BCF). It was found that total As accumulation plant parts may be higher in field condition, but higher BCF was obtained at laboratory conditions [32] due to better availability of external nutrients.

However nutrients like phosphate addition may suppressed As uptake by duckweeds as both phosphorus and arsenic belongs same group-V(b) element family in periodic table [33]. In most of the phytoremediation study carried out in laboratory condition, As is provided either in the form of arsenite (As3+) or arsenate (As5+). But some studies included dimethyl arsenic acid (DMAA), an organic form of arsenic for evaluation of As phytoremediation potential of duckweed species. In a lab study, *Spirodela polyrhiza* was exposed to two forms of As species, arsenate and DMAA with concentrations ranged from 1, 2, and 4 μM and their interaction with phosphate (100 to 500 μM) was studied [33]. Results obtained showed that arsenate uptake was affected by higher phosphate concentrations whereas DMAA uptake was not influenced by phosphate concentration indicating that *Spirodela polyrhiza* had separate mechanisms for DMAA uptake. Duckweeds showed contrasting As uptake behavior when provided in two separate inorganic forms (As5+ vs. As3+) and maximum As uptake was reported with arsenite form (As3+) [34]. Spirodela polyrhiza extracted 17408 and 8674 μg g−1 As (dry weight basis) respectively from solutions containing As in the form of As3+ and As5+ (64 μM As each) respectively within 6 days [34]. Maximum amount of As extracted by duckweeds is still questionable and it is varied with As exposure time, concentrations of As in contaminated solution, and research type (laboratory vs. field study). *Spirodela polyrhiza* reported to uptake 400 mg kg−1 As (dw basis) without showing any toxicity symptoms, but can accumulate up to 900 mg kg−1 As (dw basis) when subjected to 320 μM ml−1 As containing solutions [35]. Under natural condition, *Lemna minor* was found to accumulate 430 mg kg−1 As (dry weight basis) under As contaminated environment [36]. There are few studies on As uptake by Wolfia globosa (rootless duckweed). Wolfia globosa had been reported

to extract more than 1000 mg kg−1 (frond dry weight basis) from contaminated water [37]. Like other duckweeds, Wolfia globosa also uptake more arsenite form compared to arsenate form [37]. Later studies confirmed that Wolfia globosa produced phytochelatins which played an important role minimizing toxic effects of As in their body parts [38]. These above cited studies showed that Lemna minor, *Spirodela polyrhiza* and Wolfia globosa are suitable for phytoremediation of As from contaminated water.

**Salvinia**: Salvinia is a floating fern belongs to genus salviniaceae, commonly called as butterfly fern. The genus salviniaceae contains 12 different species, out of them only 3 had been investigated for As phytoremediation were namely *Salvinia molesta*, *Salvinia minima* and *Salvinia natans* [39–41]. *Salvinia minima* have been reported as an efficient scavenger of Pb (34 mg g−1 dw) and less efficient remover of As (0.05 mg g−1) from contaminated medium and uptake of both Pb and As increased with exposure time duration and concentration of the element in the medium concerned [40]. The plant showed toxicity symptoms when As3+ concentration was more than 100 μM and tolerates up to 300 μM. Addition of phosphate in solution, reduced As uptake of as occurred in other aquatic weed plant also been recorded in their study. Similarly negative impact of phosphate and iron on As uptake by Salvinia natans was observed [41]. Phosphate addition reduced As uptake when provided in the from arsenate (As5+), in contrast no impact when As was provided in the form of DMAA. Like other aquatic weeds (Eicchornia, Pistia and Spirodela), *Salvinia molesta* also showed As toxicity upon exposure to higher concentration. To counter As stress, antioxidant enzyme activities and reactive oxygen species (ROS) were increased in floating leaves [39]. These studies indicated that Salvinia can play an important role for As phytoremediation as it had own defense mechanism.

**Azolla**: Azolla is a small, free floating aquatic fern commonly found in paddy fields, ponds, river and lakes. There are numerous studies carried out globally showed that Azolla can remediate heavy metal toxicity from contaminated water [42–44]. But studies on As phytoremediation capability of azolla were scarce. In As contaminated area of Bangladesh, Mahmud et al. [45] evaluated 49 different plant species for As uptake and BCF; found that *Azolla pinnata* along with

**Figure 1.**

*Arsenic uptake pattern in different Azolla sp. (adapted from Zhang et al., 2009).*

*Phytoremediation of Arsenic Contaminated Water Using Aquatic, Semi-Aquatic and Submerged... DOI: http://dx.doi.org/10.5772/intechopen.98961*

*Eichhornia crassipes* and *Spirodella polyrhiza* showed higher BCF and TF in paddy field. Among 49 plant species, *Azolla pinnata* showed highest BCF 10.92 indicated its suitability to reduce As uptake by paddy plants in field condition. A study using Azolla conducted in China using 50 different strains of Azolla spp. based on their uptake and speciation [46]. As uptake was ranged from 29 to 397 mg kg−1 ; *A. caroliniana* accumulated maximum As followed by *A. macrophylla* and minimum accumulation was associated with *A. filiculoides* when all strains were grow in 50 μM As5+ solution for 10 days (**Figure 1**). Arsenic speciation in followed in the order of arsenate (As5+) > arsenite (As3+) > DMAA and MMAA accounting 50–60, 25–40 and 1–5% of total arsenic in *A. caroliniana* respectively. In contrast, asrenite (As3+) was dominant As species in *A. filiculoides* governs 55–69% of total As [46]. Another study was conducted on phytoremediation of As by *A. caroliniana* wild using various As concentrations (0, 0.25, 0.5, 1.0 and 1.5 mg L−1) and impact of As exposure on plant enzymatic properties were investigated [47]. Maximum As uptake (386 mg kg−1) was reported at highest As concentration (1.5 mg kg−1). It was observed that peroxidases, glutathione reductase, catalase and superoxide dismutase activities were enhanced at lower As doses and reduced at higher doses. In exposure to higher As concentration, thiol content and anthocyanin production were increased and correlated with higher As uptake.

#### **3.2 Phytoremediation of arsenic by semi aquatic weeds**

Some semi aquatic weed such as *Alternathera philoxeroides*, *Arundo donax, Vetivaria Zizinoids*, *Typha latifolia*, *Phragmites* spp. and *Canna* spp. had been widely reported to accumulate As in their body parts from contaminated soils and water [16, 17, 48–51]. *Alternanthera philoxeroides* had potential to extract As from contaminated water and stored in root system [52, 53]. Reports from previous studies indicates that As accumulation in *A. philoxeroides* followed in the order of root > stem > leaf and average BCF for root ranged from 106 to 191, when exposed to various doses of As containing solutions (1, 2 and 5 mg kg−1) under laboratory condition [52]. Under natural condition, *Alternanthera philoxeroides* observed to uptake 12.94 mg kg−1 total As dw from pulp paper industry water with average BCF- 3.58 and TF-0.51 [53]. Higher BCF under laboratory condition observed due to used of higher As containing solution and availability of external nutrients for weed plants which may trigger As uptake through phosphate uptake pathway.

*Arundo donax* is a perennial semi aquatic weed mostly found in submerged condition offer a tremendous potential to uptake As from contaminated water. Earlier research work showed that Arundo donax can grow efficiently up to 50–600 μg L−1 As concentration without showing any toxicity symptom and maximum As uptake, BCF (15), TF (4.93) were recorded at 600 μg L−1 [16]. Toxicity symptoms appeared when plants were exposed to solutions containing 1000 μg L−1 As [16]. Further, combined use of plant growth promoting rhizobacteria (PGPR) such as Stenotrophomonas maltophilia and Agrobacterium sp. increased bioaccumulation of As in roots of Arundo donax plant upon exposure to higher concentration As (20 mg kg−1) and enhanced overall phytoremediation efficiency of Arundo donax in presence of PGPR bacteria [51]. The As accumulation in *Phragnites austratlis* followed in the order of roots > rhizomes > leaves and maximum total As uptake was registered 32.5 mg kg−1 [54]. *V. zizinoids*, another semi aquatic weed reported to be capable of extracting As from contaminated water [17, 55]. In a hydroponic study (21 days), root to shoot As uptake it was increased with increase in As concentrations by *V. zizinoids* can uptake [17]. The BCF and TF for As were 10 and 0.86 indicates that *V. zizinoids* was an As hyper accumulator and stored higher proportion of As in their root system. Combined use of arbuscular

mycorrhizal fungi (Glomus spp.) enhanced As uptake capability and growth of vetivar grass *(Chrysopogon zizanioides*) [55]. *Typha latifolia* also had the potential to uptake higher proportion of As from contaminated environment (soil), but most of the studies conducted using *Typha latifolia* were focused in soil. Most of the studies showed that semi aquatic weeds store more As in their root system and lower in upper vegetative parts. Higher plant vigor, higher As extraction capacity and perennial nature make them suitable phytoremediation agent for constructed wetland system. Combined use of submerged weeds like Hydrilla, Ceratophyllum, Potamogeton along with semi aquatic weeds (*Arundo donax, Vetivaria zizinoids, Phragmites* spp. and Typha sp.) and PGPR like VAM, As oxidizing bacteria may be highly useful to treat and remediate As contaminated water in constructed wetland system. Semi aquatic weeds are highly efficient when As present in higher concentrations and when As concentration in the system become lower submerged weeds come to play their role, as they are highly efficient As remover at lower concentrations. Again use PGPR will increase overall phytoremediation efficiency. Future research may be undertaken in these aspects for better information and output.
