INTRODUCTION
Water is one of the most essential component for the survival of all living beings. Life without water seems impossible. Apart from drinking, there are many other uses of water from cleaning to food processing, industrial operations, agricultural work and many more. Although 70% of the earth’s surface is covered with water but only 0.3% of it is available for useful purposes. The other 99.7% is either present in the oceans, icebergs, soil or are present in the atmosphere in aerosol or vaporized form and hence cannot be used.[1] Moreover the rising global population and increased levels of pollution are contributing as the major sources for contamination of water. Hence, it is high time that we realize the alarming situation and first of all protect the water from contamination and try to safeguard and wisely use the small amount of water that is directly available for our sustenance. Secondly, water remediation methods should be adopted so that the water which is already contaminated can be reused and attain safe levels for several applications. Therefore, several water remediation methods have come into role and helps to improve the quality of potable water. There are a lot of contaminants of various types that are being discharged into the water but this review in particular discusses about the heavy metal contaminants (which have the ability to dissolve in wastewaters, get discharged into the surface waters, seep into groundwater and hence contaminating the drinking water) and their remediation methods.
1.HEAVY METALS
Heavy metals are set of metals or metalloids that have density 5 times higher than the density of water. Heavy metals are present on earth’s crust and whenever natural phenomena like volcanic activity, weathering etc. occurs it leads to heavy metal pollution. Anthropogenic activities like smelting, mining are also a major source for heavy metals release into the environment. They are commonly used in the industrial, agricultural, medical and various technological applications which increases their concentration into the environment. Some of the heavy metals are required in small amounts; they only become toxic when they are in excess amount.[3] Heavy metal pollution has become a global issue although it varies from place to place. For example, countries like Bangladesh, Nepal, India (West Bengal) faces serious arsenic contamination as weathering of rocks (which naturally contain arsenic) is common in these areas.[2] There are atleast 20 metals that are classified as toxic and their presence in the environment poses adverse effects on human health and the environment but the most common metals that pollute the water and cause immediate threat are: Arsenic, Lead, Mercury, Cadmium, Chromium. Table 1 summarizes about these metals.
Heavy Metal | Regulatory limits (mg/Kg) | Rank in ATSDR priority list of hazardous substances | Effects |
Arsenic | 0.39 | 1 | Can lead to arsenicosis, harmful impact on nervous system, renal system, gastrointestinal system. Can cause cancer, diabetes, skin lesions, cardiovascular disease. |
Lead | 400 | 2 | Causes severe damage to neurological system. |
Mercury | 260 | 3 | Impacts the digestive system, immune system, renal system, nervous system and ocular system. |
Cadmium | 85 | 7 | Leads to kidney dysfunction, severe impact on respiratory and skeletal system and on various enzymes. |
Chromium | 100 | 17 | Can cause cancer and harmful for renal system, respiratory system and immune system. |
2. REMEDIATION METHODS
Remediation methods for heavy metals present in water are aimed to reduce their concentration in water and minimize the harmful effects that they have on the human health and the environment. Selection of an appropriate remediation method depends on a lot of factors such as the extent of heavy metal contamination, regulatory limits set up for the heavy metals and characteristic of the site. They are broadly divided into three categories: (a) physical (b)chemical and (c)biological. Physical and chemical are together combined and can be put under a category of physiochemical methods and biological methods are biological methods are further divided into various types.
2.1 Physiochemical remediation methods
(A) Chemical precipitation
It is a widely used method for remediation of heavy metals present in water. Chemical precipitants are used which are also known as coagulants or flocculants. In this method, the dissolved contaminants are transformed into insoluble solid so that later they can be separated from the liquid phase by simple physical methods such as filtration and clarification. Requirement of the chemical is dependent upon the pH and alkalinity of water. Generally, precipitation of heavy metals was carried out using sodium hydroxide or lime during neutralization but the results were not satisfactory in many of the cases as complete hydroxide precipitation does not take place when complexing agents are present. However, in a study it was found that the use of granulated lime and granulated calcium carbonate as coagulants gave quite satisfactory results in which granulated lime as a coagulant was able to remediate 98% of arsenic and nickel from artificially contaminated water and granulated calcium carbonate remediated 97%. In an investigation it was discovered that 1, 3- benzenedaimidoethathiol dianion which is commercially known as Metx had the ability to irreversibly bind with soft heavy metals in aqueous solutions. It was found that Metx could remove over 90% of various heavy metals coming out from the acid mine drainage samples. Moreover, at a pH of 4.5 it had the capability to reduce the concentration of iron from 194ppm to below 0.009ppm. [4]
ADVANTAGES
∙ Low cost, simple pH adjustments, well established technology, availability of all the equipment and chemicals required for the method, low maintenance.
DISADVANTAGES
∙ Large volumes of sludge are generated leading to additional waste disposal costs, regulation requirements are not always met with the use of hydroxide and carbonate precipitation, pH sensitive method, requires the use of corrosive chemicals so safety needs to be kept in mind.
(B) Ion exchange
In this method ion from contaminated water is exchanged with the ion of similar charge attached to immobile solid particle and these particles are generally naturally occurring inorganic zeolites or synthetically produced organic resins. It is a reversible chemical reaction. This method has been used successfully many a times for remediating heavy metals. [4]
(C) Reverse osmosis
It is a membrane process, which acts as a molecular filter and removes 99% of all kind of dissolved minerals. When water passes through the membrane the water gets through it but the dissolved and particulate matter is left behind. It is a very effective and a well-known method as it even removes the ionic species from the solution. Also, the recovery of metals is easier in this method due to concentrated by-product solutions. [4]
ADVANTAGES
∙ Reduces the concentration of ionic contaminants as well as dissolved organic compounds.
DISADVANTAGES
∙ Membranes are relatively expensive both to procure and to operate, the process requires elevated pressures which makes it costly and sensitive to operating conditions.
2.2 Phyto Remediation methods
These methods entirely make use of the plants; to remediate substances in sludge, contaminated soil, sediment, groundwater, wastewater and surface water. These are also known as green remediation/Botano-remediation/ vegetative remediation or agro-remediation. Phyto remediation are broadly classified into the following categories: phytodegradation/phytotransformation, phytoaccumulation/ phytoextraction, phytostimulation/rhizostimulation, phytovolatilization, rhizofiltration and phytostabilisation. However, the heavy metals remediation from water can occur through any three of these methods: phytoextraction, rhizofiltration and phytostabalisation.
(i) Phytoextraction
It is a phytoremediation process in which plants roots absorb, translocate and store the contaminants along with other nutrients and water. Figure1 shows a schematic representation of phytoextraction process and the steps involved in it Different plants have different capacities to uptake and to withstand the level of pollutants so for this method we use the plants depending on the level of pollutants. Zinc, Copper and Nickel can be successfully phytoextracted if they contaminate the water and research is going on for the plants that can phytoextract Lead and Chromium. [4]
Figure 1: uptake of metals and accumulation in plants: (1) metal fraction sorbed at root surface, (2) movement of bio-available metal across cellular membrane into root cells, (3) fraction of metal absorbed into roots and then immobilized in the vacuole, (4) intracellular metal which is mobile crosses the cellular membranes into root vascular tissue (xylem), (5) translocation of metals from the root to aerial tissues i.e stems and leaves.[4]
(ii) Rhizofiltrarion
This process is quite similar to phytofiltration but the plants used in this process are raised in greenhouses and their roots are submerged into water. In this, the breakdown of organic contaminants takes place through microbial activity in the root zones of the plant or the rhizosphere (Figure 2). This process is basically used for remediation of contaminants in groundwater. Contaminants are either absorbed onto the surface of the roots or by the plant roots. Using this method several plant species have been able to successfully remediate toxic metals like Copper, Cadmium, Lead, Chromium, Nickel and Zinc from aqueous solutions. [4]
Figure 2: Schematic representation of Rhizofiltration process.[4]
(iii) Phytostabalization
It is referred as in place activation. In this process, plant roots limit the mobility of the contaminants and bioavailability in the soil. The process involves absorption and accumulation of contaminants by the plant roots. When the contaminants are immobilized their availability deceases in the food chain. The only requirement for plants to be used in this process is that should be able to tolerate the contaminants. Even if they do not remove any contaminants or
removes only some of them, but if they have the ability to withstand the contaminants for a long time and grow under such contaminated conditions it will still be useful for the phytostabalization method. [4]
Advantages of phytoremediation methods
∙ Economical, causes no harm to the environment, no requirement of any disposal sites, aesthetically pleasing, can treat sites contaminated with more than one type of contaminant.
Disadvantages of phytoremediation methods
∙ Dependent on the growing conditions of plant, requires need for agricultural equipment and their knowledge, remediation time required is long, leaching of soluble contaminants can cause environmental damage, shallowness and depth of roots decided the effectiveness as contaminants that are in reach of the roots can only be remediated.
CONCLUSIONS
Heavy metals leading to contamination of water is a serous issue and an area of concern globally. Hence, heavy metals remediation from water has become an important part of the wastewater treatment plants as the contaminated water can affect the living systems directly or indirectly. To provide safe drinking water and water safe to be used for other uses as well there is a need for several treatment processes to remove the heavy metal contaminants and to minimize the risks that they pose on the human health and the environment. Several remediation technologies are available for this purpose. As physical and chemical remediation methods are expensive and even generate a lot of secondary pollutants, therefore the focus has shifted towards greener and ecofriendly ways to remediate the contaminants through phytoremediation technique. But at present, there is no such technology; be it physical, chemical or biological method of heavy metal remediation that comes without any form of merits or demerits. Each method has its own advantages and disadvantages so now there is a need for integrated remediation technology which can have a great potential. This can only happen when there is further research on heavy metal remediation and only this way, we will be able to discover a method which will be low cost, safe for environment at the same time will give better remediation results.
REFERENCES
[1] Bhattacharya S, Gupta Bhushan A, Gupta A, Pandey A Introduction to water Remediation: Importance and methods 2017 -1-2.
[2] Tesh S, Scott T, Nano-Composites for Water Remediation: A Review 2014- 1.
[3] Chowdhury A, Datta R, Sarkar D, Heavy Metal Pollution and Remediation - 1.
[4] Akpor O. B. and Muchie M. Remediation of heavy metals in drinking water and wastewater treatment systems: Processes and applications 2010 3-8.
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