Water is one of the most magical things on the planet and the driving force behind all of nature. At least 2 billion people worldwide drink contaminated water and it can spread diseases such as diarrhea, cholera, dysentery, typhoid, and polio. Each year, it is estimated that contaminated drinking water causes 485,000 diarrhoeal deaths. In 2017, 71% of the global population (5.3 billion people) drank from a safely managed drinking-water service, which was on-site, available when needed, and free of contamination. 90% of the world’s population (6.8 billion people) used some form of basic service. A basic service is an improved drinking water source within a round trip of 30 minutes to collect water. Climate change, increasing water scarcity, population growth, demographic changes, and urbanization already pose challenges for water supply systems. By 2025, half of the world’s population will be living in water-stressed areas. Re-use of wastewater, to recover water, nutrients, or energy, is becoming an important strategy. Increasingly countries are using wastewater for irrigation – in developing countries, this represents 7% of irrigated land. While this practice if done inappropriately poses health risks, safe management of wastewater can yield multiple benefits, including increased food production. 785 million people lack even a basic drinking-water service, including 144 million people who are dependent on surface water.
One of the burning problems of our industrial society is the high consumption of water and the high demand for clean drinking water. Groundwater is used for domestic, and industrial water supply and irrigation all over the world. Options for water sources used for drinking water and irrigation will continue to evolve, with an increasing reliance on groundwater and alternative sources, including wastewater. Although three-fourths of the earth is surrounded by sea only a little portion of it can be used for drinking purposes. The use of aquatic plants for water and wastewater treatment is increasing nowadays. The project aims to examine the phytoremediation potential of water hyacinth. Many researchers have used different plant species like water hyacinth, and water lettuce for the treatment of water.
In arid and semi-arid regions, where well-managed water transportation systems and related infrastructures are not available, groundwater serves as the chief source of drinking water. Groundwater is influenced by many factors, including the composition of precipitation, mineralogy of the aquifers, climate, topography, and anthropogenic activities. According to the United States Environmental Protection Agency (USEPA, 1993), groundwater becomes contaminated naturally or because of numerous types of human activities like residential, municipal, industrial and agricultural. In recent days, groundwater quality is decreasing day by day due to rapid urbanization and fast industrial growth. Unrestricted exploration of groundwater and excessive use of fertilizers and pesticides make possible the infiltration of detrimental constituents into the groundwater.
Phytoremediation uses plants to clean up contaminated soil and groundwater, taking advantage of plants’ natural abilities to take up, accumulate, and/or degrade constituents of their soil and water environments. Results of research and development into phytoremediation processes and techniques report it to apply to a broad range of contaminants including numerous metals and radionuclides and various organic compounds.
Contaminants that have been remediated in the laboratory and/or field studies using phytoremediation or plant-assisted bioremediation include:
• Heavy metals (Cd, Cr (VI), Pb, Co, Cu, Pb, Ni, Se, Zn)
• Radionuclides (Cs, Sr, Ur)
• Chlorinated solvents (TCE, PCE)
• Petroleum hydrocarbons (BTEX)
• Polychlorinated biphenyls (PCBs)
• Polynuclear aromatic hydrocarbons (PAHs)
• Chlorinated pesticides
• Organophosphate insecticides (e.g., parathion)
• Explosives (TNT, DNT, TNB, RDX, HMX)
• Nutrients (nitrate, ammonium, phosphate)
The principles of the phytoremediation system are to clean up contaminated water which includes identification and implementation of the efficient aquatic plant; uptake of dissolved nutrients and metals by the growing plants; and harvest and beneficial use of the plant biomass produced from the remediation system. The most important factor in implementing phytoremediation are-
The selection of an appropriate plant should have a high uptake of both organic and inorganic pollutants, grow well in polluted water, and be easily controlled in quantitatively propagated dispersion. The uptake and accumulation of pollutants vary from plant to plant and also from specie to specie within a genus.
The capture of groundwater (even contaminated groundwater) and utilization for plant processes.
The economic success of phytoremediation largely depends on photosynthetic activity and the growth rate of plants with low to moderate amounts of pollution.
These systems are generally cost-effective, simple, and environmentally non-disruptive.
Ecologically sound with low maintenance cost and low land requirements.
Groundwater Remediation Method
Surface water hemofiltration may be conducted in situ, with plants being grown directly in the contaminated water body. If groundwater is located within the rhizosphere (root zone), rhizofiltration of groundwater can also be in situ. Alternately, rhizofiltration may involve the pumping of contaminated groundwater into troughs filled with the large root systems of appropriate plant species. The large surface areas provided by these root systems allow for the efficient absorption of metals from the contaminated groundwater into root tissues. In addition to removal through absorption, metals are also removed from groundwater through precipitation caused by exudates (liquids released from plant tissues). These precipitates are filtered from the groundwater after it passes through the plant troughs and before treated water is removed from the process loop. Roots are harvested, and depending on the species of plant used, shoots may be transplanted to grow new roots. Plants can be replaced in the system to ensure constant operation results. Rhizofiltration using sunflowers has been used in the remediation of radionuclides from surface water near Chernobyl (strontium and cesium) and water using a rhizofiltration system, as described above, at a DOE facility in Ohio.
Surface water remediation via phytotransformation can be accomplished in situ in ponds or wetlands. In addition, groundwater can be remediated using phytotransformation in situ if the water table is within the zone tapped by deep-rooted plants such as poplars or ex-situ by pumping water to troughs or constructed wetlands containing appropriate plants. In the phytotransformation process, plants take up organic contaminants and degrade them to less toxic or non-toxic compounds. This technique is being tested on explosives-contaminated groundwater (TNT and RDX) at Milan Army Ammunition Plant in Tennessee by the U. S. Army Corps of Engineers Waterways Experimental Station (WES). In addition, an Environmental Security Technology Certification Program (ESTCP) project is testing the ability of trees with roots tapping groundwater to degrade TCE and hydrazine present in the aquifer. The U.S. Air Force is planning to evaluate phytoremediation through field studies followed by cell cultures and bio-chamber studies.
3. Plant-Assisted Bioremediation
This technique involves the installation of appropriate plants in areas in which near-surface bioremediation is being conducted. The plants provide carbonaceous material from liquids released from roots and through the decay of root tissue. In addition, oxygen released from the root systems of these plants increases the oxygen content in the bioremediation area. These additions to the soil as a result of plant activity increase the rates of microbial activity and thus the rates of contaminant degradation. The above-mentioned ESTCP project also involves the study of the beneficial effects of plant roots on the rate of in situ bioremediation by microorganisms.
Wastewater Remediation Method
This technology is applied for decreasing the bioavailability of contamination from the environment and stabilizing pollutants occurs more than removing them(commonly metallic elements) by plants (hydraulic control). Plants can help to stabilize pollutants by taking in an adsorption system or accumulating them in the root system. Indian mustard (Brassica juncea L.) was reported as a suitable plant species for the stabilization of mercury in soil and wastewater. The aquatic plant Hydrilla verticillata was reported as potential species for phytostabilization of wastewater, this plant has a high translocation factor (TF) and low bioconcentration factor (BCF) for toxic metals (Pb, Cr) (Ahmad et al., 2011). Rapeseeds (Brassica napus), sunflowers (Helianthus annuus), tomatoes (Solanum Lycopersicum), and soapworts (Saponaria officinalis) were reported as capable plants for phytostabilization with less than one bioaccumulation coefficients obtained in all of these plants.
Phytofiltration or rhyzofiltration is a green technology for removing contaminations by plant roots in aquatic media like groundwater, most wastewater, and extracted groundwater (Pivetz, 2001; Mukhopadhyay and Maiti, 2010). Terrestrial, aquatic, and wetland plants are suitable materials for phytofiltration and constructed wetlands are the best method for removing metallic elements from wastewater (Cheng et al., 2002). Limnocharis Flava (L.) was reported as suitable plant species for phytofiltration of low concentration Cd contaminated water (Abhilash et al. 2009) Wolffia globosa is a suitable nominated for arsenic metabolism studying via phytofiltration (Zhang et al. 2009). For the effective accumulation of cadmium and hyperaccumulation of arsenic, micranthemum umbrosum was introduced as a suitable macrophyte (Islam et al., 2013). Indian mustard (Brassica juncea (L.) Czern) could uptake 95% mercury from contaminated water via phytofiltration.
Phycoremediation is using macro and microalgae for bio-transforming or removing pollution from wastewater. Algae is a suitable plant for the decontamination of metallic elements, xenobiotics, and nutrients in various wastewater. Microalgae is a capable plant for the treatment of various types of wastewater such as industrial wastewater, domestic wastewater, and solid wastes both aerobically and anaerobically. Freshwater blue-green algae are used successfully for the treatment of dairy manure effluent(Mulberry et al., 2008). Sewage water treated by different algae and Chlorella Vulgaris could remove almost all of the contaminations and after the treatment process, it can be thrown in water bodies.
Phytoextraction /phytoaccumulation can be considered a suitable green technology for removing metallic elements from aquatic media (Wang et al., 2008). This kind of remediation is used for the accumulation of Zinc by duckwood (Lemna gibba) (Khellaf and Zerdaoui, 2009), Cadmium by water spinach (Ipomea aquatic), Chromium with small pondweed (Potamogeton pusillus) in presence of Cu2+ (Monferrán et al., 2012). recently reported study carried out for accumulation of Pb by Ceratophyllum demersum and Myriophyllum spicatum and finally introduced as phytoremediator and bioindicator of Pb. Water hyacinth (Eichhornia Crassipes) is used for removing heavy metals from coastal water, Crude oil from artificial wastewater amended by urea fertilizer (Ndimele and Ndimele 2013), and palm oil mill effluent treatment. Furthermore, heavy metals from industrial wastewater have been removed by vetiver (Chrysopogan zizanioides).
(The rate of metallic elements removal from polluted wetlands depended on plant species, climacteric conditions, the statute of substrates, type of element (Hg>Mn>Cd=Fe>Cr=Pb>Cu=Zn>Al>Ni>As), their ionic forms (Marchand et al., 2010).
Limitation of Phytoremediation
According to the information reviewed, general site conditions best suited for the potential use of phytoremediation include large areas of low to moderate surface soil (0 to 3 feet) contamination or large volumes of water with low-level contamination subject to low (stringent) treatment standards. Depth to groundwater for in situ treatment is limited to about 10 feet, but ex-situ treatment in constructed troughs or wetlands has also been investigated.
In the least developed countries, 22% of healthcare facilities have no water service, 21% have no sanitation service, and 22% have no waste management service. We forget that the water cycle and the life cycle are one. Audrey Hepburn said that water is life, and clean water means health. Climate change will lead to greater fluctuations in harvested rainwater. Management of all water resources will need to be improved to ensure provision and quality. UNICEF works with the government to provide sustained safe water access to 24 million people by 2030. Many activities can contaminate the groundwater quality of several areas and can cause health problems. Therefore, we can conduct to treat the groundwater as well as wastewater area using the phytoremediation technique.
Author is an Executive Editor,
The Environment review