Phytoremediation ability of various aquatic and semi aquatic weeds.
Arsenic (As) is the one the most toxic element present in earth which poses a serious threat to the environment and human health. Arsenic contamination of drinking water in South and Southeast Asia reported one of the most threatening problems that causes serious health hazard of millions of people of India and Bangladesh. Further, use of arsenic contaminated ground water for irrigation purpose causes entry of arsenic in food crops, especially in Rice and other vegetable crops. Currently various chemical technologies utilized for As removal from contaminated water like adsorption and co-precipitation using salts, activated charcoal, ion exchange, membrane filtration etc. are very costly and cannot be used for large scale for drinking and agriculture use. In contrast, phytoremediation utilizes green plats to remove pollutants from contaminated water using various mechanisms such as rhizofiltration, phytoextraction, phytostabilization, phytodegrartion and phytovolatilization. A large numbers of terrestrial and aquatic weed flora have been identified so far having hyper metal, metalloid and organic pollutant removal capacity. Among the terrestrial weed flora Arundo donax, Typha latifolia, Typha angustifolia, Vetivaria zizinoids etc. are the hyper As accumulator. Similarly Eicchornea crassipes (Water hyacinth), Pistia stratiotes (water lettuce), Lemna minor (duck weed), Hyrdilla verticillata, Ceratophyllum demersum, Spirodella polyrhiza, Azola, Wolfia spp., etc. are also capable to extract higher amount of arsenic from contaminated water. These weed flora having As tolerance mechanism in their system and thus remediate As contaminated water vis-à-vis continue their life cycle. In this chapter we will discuss about As extraction potential of various aquatic and semi aquatic weeds from contaminated water, their tolerance mechanism, future scope and their application in future world mitigating As contamination in water resources.
Arsenic (As) is the one the most toxic element present in earth which poses a serious health hazard to animal and human health. Generally arsenic is present in the earth crust in the form minerals, especially associated with iron pyrite and zinc ores. Arsenic contamination occurs through both by natural as well as anthropogenic processes . Unlike other toxic heavy metals (Cadmium, mercury and chromium) arsenic contamination in environment predominately occurs through natural biogeochemical process  and some manmade activities play important role (triggering the process) in that process. Anthropogenic activities such as coal mining and burning smelting of As containing metal ores and other industrial activities are also responsible for distribution of arsenic in the environment . Arsenic contamination of drinking water in South and Southeast Asia reported one of the most threatening problems that causes serious health hazard of millions of people of India and Bangladesh . The source of As contamination in water in those countries were due to two different natural processes; oxidation of arsenopyrite minerals lies below ground water table due to water mining process and reduction of As containing iron hydroxides . Arsenic exists in the nature in −3, 0, +3 and + 5 oxidation states and environmental forms include arsenious acids, arsenic acids, arsenites, arsenates, methylarsenic acid, dimethylarsinic acid, arsine, etc. Two inorganic forms are very common in natural waters: arsenite (AsO33−) and arsenate (AsO43−), referred to as arsenic (III) and arsenic (V). Pentavalent (+5) or arsenate species are AsO43−, HAsO42−, H2AsO4− while trivalent (+3) arsenites include As(OH)3, As(OH)4−, AsO2OH2− and AsO33−. The solubility of inorganic species depends on pH and redox potential of the environment and arsenite (As3+) is the most soluble form inorganic As. Pentavalent species or arsenate (As5+) predominate in oxygen rich aerobic environments, where as trivalent arsenites (As3+) dominant in moderately reducing anaerobic environments such as groundwater .
Arsenic concentration in drinking water reported more than 50 μg L−1 in many areas in the world , whereas maximum permissible limit set by World Health Organization (WHO) is 10 μg L−1. The use of arsenic contaminated ground water for irrigation purpose causes build up of As in soil and leads to entry of As in food crops, especially in rice and vegetables [7, 8]. This causes serious health hazard, in those As containing areas. In Southeast Asian countries like Bangladesh, Eastern parts of India (West Bengal and Bihar) and Vietnam, rice is consumed as major staple food and is very efficient in As translocation in grains . Thus rice crop play a major pathway for As entry in human body living in those contaminated areas apart from drinking water. Thus remediation of arsenic contaminated water is important for environmental point of view. Various technologies are for remediation of arsenic contaminated water like ion exchange, electro dialysis, membrane filtration, adsorption and coagulation-flocculation generates lot of arsenic enriched waste. That waste material generally dumped or disposed in nearby surroundings, from where arsenic can also come back to soil and water by leaching thus making system susceptible to arsenic contamination. Along with above mentioned problem, huge cost is involved in this existing arsenic remediation technology. That necessitates finding out an alternate low cost technology which can take care of arsenic contaminated water.
Phytoremediation is an alternate and low cost technology that utilizes green plant to extract arsenic from water and store it vegetative cells. Phytoremediation process includes phytoextraction, phytostabilization, phytovolatilization, phytotransformation, and rhizofiltration . Researchers find out that plants uptake arsenic by roots through phosphate uptake pathway and transfer it their above ground parts (shoot and leave). But how much amount of arsenic translocated from source (water) to sink (plant parts) depends on phytoremediation efficiency of the plant concern. However, more than 90% of total arsenic accumulated into the plant is stored in roots.
The plants utilized for phytoremediation have some criteria like (1) plant have higher specific growth rate under contaminated environment, (2) higher translocation capability of the toxic element concerned . Metal translocation capability depends on factors like (1) bio concentration factor (BCF) and (2) translocation factor (TF). Plants having BCF >1 are ideal for Phytoremediation. Chinese brake fern (
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 . 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.
Rhizofiltration: Plants uptake toxics substances by their roots through adsorption or absorption process and sequester in their root system. Aquatic plants mainly exhibited this process.
Phytoaccumulation or phytoextraction: Plants uptake toxic substances by their root system and translocated to other plant parts such as stem and leave or other modified plant parts. This mechanism mainly exhibited this process are suitable for remediation of contaminated soil.
Phytostabilization: In this process, plants restrict movement of toxic substances in soil or water, thus reduced their availability to plants. In this method, plants do not uptake toxic substances from environment. Rather, plants secrets some root exudates or photochemicals which form stable chemical bond with toxic substances and increases its stability in environments.
Phytodegradation or Phytotransformation: In this process, plants uptake toxic substance from soil or water and degrade these primary toxic substances into nontoxic forms. A large number of metabolic and physiological factors are involved in this process.
Phytovolatilization: Plant uptake toxic substances by their root system and translocated to their aerial plant parts especially in leaves; and release toxic substances in the form of vapor which may not be toxic as their primary source.
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
The capability of removing arsenic from contaminated water was earlier observed by Misbahuddin and Fariduddin  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) . 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 . 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 (
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 . 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,
3.2 Phytoremediation of arsenic by semi aquatic weeds
Some semi aquatic weed such as
3.3 Phytoremediation by submerged aquatic weeds
Among the submerged aquatic weeds
In natural conditions, submerged weeds grow in water bodies in association with floating macrophytes. Use of Combinations of submerged and floating weeds found more effective for phytoremediation purpose than submerged and floating weeds alone. Research work carried out using Hydrilla, Certophyllum, lemna and Wolfia at various combinations showed that Ceratophyllum + lemna combination (3326 μg) combination removed maximum total As followed by
|Name of the plants||Key findings||Reference|
|Removed 600 mg As ha−1 day−1 within 21 days with 18% removal efficiency when As was applied @ 0.15 mg L−1|||
|Removal rate 140 mg As ha−1 day−1 within 21 days with 5% removal efficiency when As was applied @ 0.15 mg L−1|||
|Removed relatively higher As3+ (17408 μg g−1) and lower As5+ (8674 μg g−1As) from As containing solutions (64 μM As each)|||
|Accumulates 1120 μg g−1 As in |||
|Accumulates 50 μg g−1 As in roots|||
|Removed sum total 8546 μg (348 μg g−1) of As from contaminated water (As concentration 1500 μg L−1)|||
|Accumulates 525 μg g−1 (dry weight baisis) from 250 μM As5+ solution for 7 days|||
|Accumulates 1000 mg kg−1 (dry weight basis) from As contaminated environment|||
|Accumulates 1000 mg kg−1 (dry weight basis) from As contaminated environment|||
|Vallisnaria natans||Accumulates 1000 mg kg−1 (dry weight basis) from As contaminated environment|||
|Extract 12.94 mg kg−1 total As (dry weight basis) from pulp paper industry effluents|||
|Accumulates As at the rate 9 mg kg−1 with TF = 4.93 and BF = 15.00 for the arsenic containing solution 600 μg L−1.|||
|Phragnites austratlis||Accumulates 32.5 mg kg−1 As in root|||
4. Mechanisms of arsenic uptake and detoxification in aquatic weeds
4.1 Mechanisms of arsenic uptake in aquatic macrophytes
Three pathways for arsenic uptake in marine macrophytes have been described – (i) active uptake through phosphate uptake transporters, (ii) passive uptake through aquaglyceroporins, and (iii) physicochemical adsorption on root surfaces. Plants mainly uptake As(V) through phosphate uptake transporters [63, 64]. As(III), DMAA and MMAA gets into the plants by passive mechanism through the aquaglyceroporin channels .
4.1.1 Active uptake through phosphate uptake transporters
As(V) and phosphate are chemical analogs, and compete for uptake carriers in the plasmalemma . As a result, as the phosphate content rises, more As (V) is required to be desorbed in the solution. Mkandawire and Dudel.  and Rahman et al.  showed that As (V) is taken up by aquatic plants through the phosphate uptake pathway, it competes with phosphate for uptake in tissues of
4.1.2 Passive uptake through aquaporins/aquaglyceroporins
Physiological studies indicate that these arsenic species are transported in rice through aquaporins /aquaglyceroporins via passive uptake mechanisms [66, 67]. Molecular studies revealed that Nodulin26-like intrinsic membrane proteins (NIPs), one of the major subfamilies of aquaporins transporters that promote the transport of neutral molecules like water, glycerol, and urea, are responsible for transporting As(III) into rice roots . Aquaporins and aquaglyceroporins are two of three subfamilies of water channel proteins (WCPs), the transmembrane proteins that have a specific three-dimensional structure with a pore that permeates water molecules , which are permeable to water, glycerol, and/or other small, neutral molecules. Glycerol and As(III) compete for uptake in rice (
4.1.3 Physicochemical adsorption on root surfaces
Arsenic is adsorbing and accumulating on the surfaces of aquatic plants due to suspended iron oxides (Fe-plaque). Robinson et al.  discovered a strong association between arsenic and iron concentrations in aquatic plants, which is believed to be due to arsenic adsorption on plant surfaces’ iron oxides. Rahman et al.  investigated arsenic species adsorption on precipitated iron oxides on
4.2 Arsenic metabolism and detoxification in aquatic macrophytes
Arsenic occurs primarily as As (V) in an oxic environment and as As (III) in a reduced environment . In plants, As (V) and phosphate share the same transporter, while As(III) enters plant cells through NIPs’aquaporins [57, 64]. Because of their distinct molecular properties, these two types of arsenic elicit different biochemical responses in aquatic plants . As (V) has no affinity for thiol ligands, while As(III) has a strong affinity for peptides with sulfhydryl (-SH) groups, such as glutathione (GSH) and phytochelatins (PCs) [64, 72]. Even though plants had been exposed to As, arsenic speciation in plant tissues indicates that arsenic is primarily present in the As(III) oxidation state (V).This suggests that As(V) is effectively reduced to As(III) in plant cells after uptake, and that most plants have high As(V) reduction competence . The reduction of As(V) to As(III) is mediated by GSH  and by enzyme , which is thought to be a detoxification mechanism of the plants. As(V) and As(III) have been shown to generate reactive oxygen species (ROS) within cells when they are taken up , and plants counteract the generation of ROS by various enzymes and cellular compounds . The GSH can act as an antioxidant and is required for the synthesis of Phytochelatins which are required for metalloid chelation .
The mechanism of arsenic accumulation and detoxification was studied by many others in aquatic plant
5. Biotechnological interventions for phytoremediation
Plants have been utilized for phytoremediation of toxic metals and metalloids, however due to heavy metal phytotoxicity to plants; this process has been slow and largely rendered ineffective . Natural heavy metal hyperaccumulators are also available, however, they are limited to specific geo-climatic conditions and also lack the crucial biomass required for efficient phytoremediation. Phytoremediation has a lot of potential using genetic engineering technologies to improve plant tolerance and heavy metal accumulation. Furthermore, various new studies using omics technologies such as genomics, transcriptomics, proteomics, and metabolomics to elucidate the genetic determinants and pathways involved in heavy metal and metalloid tolerance in plants have been identified. Presently there are three main biotechnological approaches for the phytoremediation of heavy metals and metalloids are currently being used to engineer plants for phytoremediation of heavy metals and metalloids: (1) manipulating metal/metalloid transporter genes and uptake systems; (2) enhancing metal and metalloid ligand production; (3) conversion of metals and metalloids to less toxic and volatile forms  (Figure 3).
5.1 Manipulating metal/metalloid transporter genes and uptake system
Enhanced heavy metal tolerance and bioaccumulation has been attained in different plant species by genetic manipulation of metal transporter genes. For example, the overexpression of full length
Recent research findings have revealed arsenite is transported in plants by proteins belonging to the aquaporins [83, 84]. It is observed that in efficient arsenic hyperaccumulators such as
Genome-wide gene expression analysis in
5.2 Enhancing metals and metalloids ligand production
Complexation of Arsenic with phytochelatins (PCs), or metallothionein (MTs) or glutathione (GSH) is an proficient way to detoxify As(III), since these complexes are sequestered in the vacuoles, this process is catalyzed by the homologs of multidrug resistance proteins (MRPs) [92, 93]. Enhancing the accumulation or synthesis of PCs and/or GSH and/or MTs may be one way to increase phytoremediation of arsenic. The overexpression of
Arsenic (As) tolerance in plants can also be increased by modifying GSH and PCs. Dhankher et al.  transferred and co-expressed two bacterial genes,
5.3 Conversion of metals and metalloids to less toxic and volatile forms
There are several reports for developing phytoremediation strategies for heavy metals with the help of biotechnological interventions by conversion of these metals to less toxic and volatile forms. It is observed that many organisms, including bacteria, fungi, and animals, methylate arsenic. Methylated arsenic have been discovered in several plant species, including rice grain [100, 101], and suggest that this is the process is a result of endogenous methylation by the plants themselves. The final product of this pathway is the gas trimethylarsine (TMAs(III)), that can be volatilized from the plant. Qin et al.  have cloned a gene encoding an As(III)-S-adenosylmethionine methyltransferase (arsM) from the soil bacterium
Contamination of soils and water by arsenic is one the serious threat for food security and human health in throughout the world. Some severe skin and other diseases occur due to continuous consumption of As contaminated foods and water. This necessitates a suitable technology to handle arsenic contaminated water carefully, so that above mentions points can be satisfied. Phytoremediation of arsenic contaminated water by aquatic and semi aquatic weeds offers low cost, economically feasible and eco-friendly technology to remove arsenic from contaminated water for long term. Some weeds have tremendous potential to accumulate higher amount of arsenic in their plant parts such as Eichhornia crassipes, Hydrilla verticillata, Spirodella polyrhiza, Arundo donax and Vetivaria spp. More specifically semi aquatic weeds like Arundo donax and Vetivaria sp. (perennial) can be used with in combination with Eicchornia, Spirodella and Hydrilla to remove arsenic more efficiently from treatment tanks or constructed wetland system. Although management of plant biomass will be another concern for disposal, but these plant materials can be used for making fiber (water hyacinth), handcraft items (Arundo and Typha stems) and biofuel purpose. Moreover, with advancement of molecular genetics in future As tolerance genes can be transferred to food crops (specially rice) which can store huge amount of As in their roots or very low transfer co-efficient from root to grain so that transgenic rice crops will able to grow using As contaminated water and contribute in food security in upcoming days.