Current World Environment

Introduction

Pure water is essential for leading a harmonious life. Due to the vital role of water for humanity, it is very significant to maintain and improve the quality of water. Currently, because of rapid industrialization and technological advancements, the claim for usage of water is high in both the local and industrial regions¹. The availability of fresh water is becoming inadequate due to many man-made accomplishments. Because of the growing living standards, the increasing world population surges water supply². Nowadays the world is facing a great challenge of destruction of the environment particularly by water pollution ^2,3,4. The discharge of heavy metals and other toxic pollutants by human and industrial activities has become a major problem for both humans and aquatic life ⁴. There are some common pollutants like palladium (Pd), Mercury (Hg), Chromium (Cr), Cadmium (Cd) and Nickel (Ni) that are often detected in industrial effluents^5,6,7. Heavy metals are important in many aspects to mankind, especially in the manufacturing of accumulators, thermometers, and utensils. If these heavy metals present in higher concentrations beyond the limit, it is hazardous to the lives of both flora and fauna. Due to its bio-toxic effect, it is hazardous to the environment. Many techniques are existing to effectively remove heavy metals from industrial effluents. Some treatments are flocculation, coagulation, adsorption, membrane process, ion-exchange process, chemical precipitation, filtration, solvent extractions, photodegradation and electrodialysis^4,5,8. All these techniques have both merits and demerits. In recent days, there are numerous cost efficient and eco-friendly studies available in literature for the elimination of toxic heavy metals from the wastewater and to improve the quality of treated effluent^8,9.

Heavy metals 

They are trace elements such as Cadmium, Mercury, Arsenic, Nickel, Copper, Lead, Iron, Chromium, Zinc and Cobalt^{10, 11}. Heavy metals are toxic, carcinogenic, non-biodegradable and mutagenic. Heavy metals are toxic and hazardous to the environment when the concentration is beyond the acceptable limit^{11, 12, 13}. Lead, Cadmium, Mercury and Arsenic have no advantageous effects in humans¹⁴. Low concentrations of heavy metals are known to have neurotoxic and carcinogenic actions¹⁵.

Permissible Limits of Heavy metals

World Health Organization (WHO) is answerable for people health and several public health problems, healthy diet, food security and nutrition. WHO and EPA are the top most agencies of health organization in the world. These agencies also provide a guideline for drinking water. WHO and EPA study the drinking water and provide the acceptable limit of various heavy metals and other parameters in drinking water. The table below shows the permissible limit of different toxic heavy metals.

Table 1: Permitted limit of heavy metals (WHO and EPA) in ppm ^{4, 13, 16}

Applications of heavy metals

 Chromium is used in the process of electroplating, leather tanning, metal finishing, textile industry and photography industry⁴. Chromium is used in nuclear power plants, data storage and also in metallurgical processes¹⁵. Iron is used for coating metal corrosion and steel industry mining¹⁶.  Cobalt is used as a semiconductor. Co(II) is used in nuclear power plants, enamel painting on glass, manufacturing of aerospace materials, batteries and in the synthesis of vitamin B₁₂ ⁴.

Nickel is used in non-ferrous alloys, superalloy, electroplating industries, rechargeable batteries, catalysts, microphone capsules^{4, 6}. Copper is used in mining operations, pharmaceutical equipment manufacturing, paper industry and kitchenware manufacturing¹⁷. Zinc is used in batteries, anti-corrosion coating, cans, paints, cosmetics, rubber industries, pigments and as Zn alloys¹⁸. 

Arsenic is used in the industries of pesticides, mining, insecticides, ceramics, metallurgy, textile, veterinary medicine productions and also in the tanning process¹⁵. Cadmium is used in welding and electroplating processes. Cadmium plays a role in petroleum retaining, plastic stabilizers, coal combustion and nuclear fission plants⁴. Mercury is used in batteries, dental amalgams, photography, fuel combustion, pharmaceutical industries and textiles. It is also used as catalyst rectifiers, Hg vapour lamps and fungicides¹⁹.  Lead is used in the production of electronic products, paints and also used in the electroplating process²⁰. Lead also plays a role in the production of bullets and shot materials for soldiers ¹⁴.

Table 2: Applications of Heavy metals.

Toxicity and impacts

Various health-related pollutants, especially heavy metals surges in the environment due to their non-degradable, persistent and bioaccumulative nature. Long-term consumption and frequent exposure to toxic heavy metals can result in severe health problems ⁹. Heavy metals such as zinc subgroup ions, Hg²⁺ and Cd²⁺ will lead to serious extortions to human health because they are unsafe and have carcinogenic nature ^19,20.

Chromium 

Chromium (VI) in water are dangerous to the environment because of its high toxicity. Hexavalent- Chromium holds a high risk due to its water-soluble nature to enter into the living cells and cause acute health issues. It causes headache, diarrhoea, nausea, vomiting, carcinogenic and ulcer oedema²⁴, skin rashes and irritation, weakness of  immune systems and genetic constituents ⁹. It also produces lung tumours, allergic dermatitis ²². It can cause health disorders such as haemorrhage, pulmonary congestion and it affects human physiology ¹³.

Figure 1: Schematic representation of Chromium toxicity   Click here to view Figure

Iron

Iron shows a major role in the biological system of complex formation in hemoglobin ²⁷. High levels of iron cause side effects such as vomiting sensation, diarrhea or stomach upset ¹⁵.

Cobalt 

Exposure to high amounts of cobalt may cause problems to fetal development during pregnancy and sometimes it is carcinogenic ¹⁸. Cobalt may lead to physical and mental problems like vomiting, asthma, kidney congestion, skin degeneration and weight loss ¹⁵. 

Figure 2: Schematic representation of Cobalt toxicity   Click here to view Figure

Nickel

Toxic effects of Nickel cause damage to internal organs and mental and neural systems. Ni²⁺ is fatal to sensitive species of water living organisms ^{21, 28}. It causes dermatitis, headache, dizziness, nausea, vomiting.

Figure 3: Schematic representation of Nickel toxicity.   Click here to view Figure

Copper

If the copper intake is in excess, it accumulates in the liver causing gastrointestinal problems, kidney damage and anaemia ¹¹. The toxicity of copper leads to hair loss, headache ¹², liver failure, Wilson's disease and Insomnia ^9,11,14 . It also causes renal disorder and irritate the mouth, and also causes stomach ache, vomiting, diarrhea and headache ²².

Figure: 4 Schematic representation of Copper toxicity.   Click here to view Figure

Zinc

Zinc is non-biodegradable and highly toxic and has a reverse effect on humans. High exposure to zinc causes depression, lethargy and increased thirst ⁷. And it also causes skin irritations, stomach cramps, anemia⁴ and bloody urine¹¹. It leads to gastrointestinal distress and metal fume fever ⁹. 

Figure 5: Schematic representation of zinc toxicity.   Click here to view Figure

Arsenic

Exposure to arsenic affects the internal organs of the human body. Long term exposure causes skin, lung, kidney and bladder cancer ¹⁴. And also it causes muscle weakness, neurological disorders ¹³.

Figure 6: Schematic representation of arsenic toxicity   Click here to view Figure

Cadmium 

Itai-Itai disease is caused because of chronic cadmium toxicity. Cadmium induces gastrointestinal disorder, bronchitis, hypertension²³ and it is a human carcinogen, emphysema⁹, dyspnea and weight loss. Itai-Itai disease also causes cancer on lung, kidney liver and reproductive organs ^{13, 21, 27}.

Figure 7: Schematic representation of cadmium toxicity.   Click here to view Figure

Mercury 

Mercury causes retardation, genetic defects, teratogenic effects, rheumatoid arthritis, protoplasm poisoning ^{16, 17, 22}. It is a neurotoxin and high concentration of mercury causes dyspnea¹⁵, abnormal irritation, acrodynia, gingivitis, stomatitis, neurological disorders and congenital malfunction ²⁹. Inorganic mercury causes spontaneous abortion and hematochezia²¹.

Figure 8: Schematic representation of mercury toxicity.   Click here to view Figure

Lead

Lead poisoning has been accepted as one of the major public health risks. Pb²⁺ accumulates mainly in bones, kidneys, brain and muscle, acute and chronic damage to the central nervous system and peripheral nervous system ²⁸. It may cause anaemia, headache, irritability and muscle weakness, insomnia, hallucination, renal damage, dizziness, Malaise and anorexia³⁰. Series effect of lead toxicity of its teratogenic effect, gastrointestinal damage. Children are sensitive to this metal and leads to high growth rate and metabolism and mental retardation ¹³.

Figure 9: Schematic representation of Lead toxicity   Click here to view Figure

Sources of heavy metals in water samples

Heavy metal pollution continuously increases in the environment due to industrial development and urbanization. Due to the rapid development of industries, mining, electroplating, batteries, and paper industries, huge amounts of toxic metals are getting discharged into the environment. The metal plating industry is one of the major source that shed large amounts of wastewaters. Metal surface treatment processes and electroplating generate large quantities of wastewater containing heavy metals. 

Chromium

The main source by which chromium entered into the environment is several industrial actions such as metal smelting, tanning, electroplating, metallurgy, steel industries ²³. It is also released from dyes, textile industry and leather industry ^20,21.

Iron

The major source of iron are natural deposits, wastes from metal processing industries, ores, and corrosion of vessels and rods ²⁴. 

Cobalt

Cobalt is released from nuclear power plants and industries like petrochemical, metallurgical, electronics, dyes ¹⁴. 

Nickel

Nickel is spread via industrial process batteries manufacturing paint, mining, metal finishing & forging ¹⁷. Petroleum refining, electroplating are also other sources of nickel contamination ¹³.

Copper

Main sources of copper in industrial discharge are paints and pigments fertilizer industries, pulp, wood pulp production, metal cleaning, paperboard mills, mining, batteries, copper cooking pots ^{20, 27} . As a pollutant in food specifically shellfish, mushroom, chocolate, liver, nuts, electroplating industry ²².

Zinc

Sources of zinc include steel processing, mining and coal combustion, brass metal works, paper and pulp industries ³⁰. 

Arsenic

The major source of Arsenic in water is via dissolution of minerals, ores, sediments, bio- organisms, rocks, ground water, combustion of fossil fuels, mining and pesticides ¹⁹.

Cadmium

Cadmium is released via nuclear fission plants, refining pesticides, mining, plants and plastics, Cd and Ni batteries, fertilizers and electroplating ²⁹. It is also emitted by metal smelters and tobacco smoke ¹³. 

Mercury

Liberation of mercury into environments through the mining paper & paint industries ³⁰. And also includes fertilizer industries, Batteries, Textile photographic fossil fuel combustion, waste incineration scientific instruments, pharmaceutical industries ^{4, 22}. Agriculture industries, pulp and paper preservatives, catalysts in organic synthesis are also other means of mercury liberation into the environment ¹³. 

Lead

Lead enters into the atmosphere from windblown dust, biogenic material ²⁸.  Other sources of lead are industrial old lead pigment paints, fuel, mining sources, leaded gasoline,  batteries, explosives, electroplating, glass manufacturing industries ^{16, 19, 22}. 

Table 3: Sources of Heavy metals ^{20, 22, 28}

Methodology used for the elimination of heavy metals

Removal of toxic heavy metals from wastewater has become a major concern nowadays. Unproductive ways of dealing heavy metal ions may cause long-term risk to the ecosystem and humans. Number of techniques for the efficient elimination of heavy metals from wastewater are available.

Chemical precipitation 

This is the physicochemical process and a very flexible approach to various pollutants removals. In chemical precipitation, a reagent is added which reacted with heavy metal ions and resulting in the formation of insoluble compounds or insoluble precipitates ²⁹. Additional methods like sedimentation and filtration are required to remove the precipitates ²⁹. To reduce the solubility of the dissolved pollutants, it can be carried out by lowering the temperature or by adding some chemicals like sodium bicarbonate and ferric chloride to the solution²³. But chemical precipitation is not preferred due to increased cost. Chemical precipitation is applied for chromium and nickel plating industries ²⁴. The removal of the chemical precipitation process depends on pH, charge of the ions, temperature¹⁸, alkalinity, concentration of metal ions and phosphate level ³³. 

Figure 10: Schematic representation of chemical precipitation   Click here to view Figure

Hydroxide precipitation

Heavy metal ions have low solubility in alkaline medium, thus the precipitation technique is often organised in alkaline condition ²⁶. Lime, Limestone ²⁹ and NaOH ³³ are frequently used precipitants because they are cheap and available easily. Sodium carbonate, Calcium hydroxide , Ammonium hydroxide ³³, Magnesium hydroxide and Calcium chloride are other available precipitants used for the formation of hydroxide precipitates.

where Mⁿ⁺ denotes the metal ions, OH^- ions denotes the precipitants and M(OH)_n denotes the insoluble metal hydroxide precipitate ³³. Cr is removed using NaOH as precipitants ²⁶. Removal efficiency of Zinc is 99.3% by using lime as a precipitant ²³. The commonly used method is hydroxide treatment because cost of precipitant is cheap, relative simplicity and ease of automatic pH control³⁴. Some metal hydroxides are amphoteric, that leads to secondary precipitation in solution³³.

Sulphide precipitation

This method is frequently used method and precipitants are mostly FeS, CaS (solid)³⁴, BaS, Na₂S₂O₃²⁸, aqueous precipitants like NaHS, (NH₄)₂S and Na₂S ²³. Sulphide precipitates are non- amphoteric, so it has high removal efficiency ²⁹.

The sulphide precipitation overcomes the hydroxide precipitation because the solubility of sulphide precipitates are lower than the hydroxide precipitates³³. Other advantages are rapid reaction rate, better setting properties³³, reuse of sulphide precipitates by smelting and selective removal of metal ions²³. The factors that influence the sulphide precipitation are pH,  type of sulphur source, initial metal concentration, the precipitation agent, ratio of the two components ³².

Formation of colloidal precipitates ²⁹, toxic fumes of Hydrogen sulphide ³³ are the disadvantages of sulphide precipitation. It is not a conventional method for the mine water treatment, but it is widely used to eliminate heavy metals from the metal finishing industry. This process removes heavy metals like Lead, Chromium, Zinc, Nickel, cadmium and Mercury ³⁶.

Adsorption

A surface phenomenon where atoms or ions occupy to the active site of adsorbent, creating a layer of the adsorbate over the surface of the adsorbent is known as adsorption¹⁸. Adsorption is an effective purification and separation technique used in industries due to its flexibility in design and ease of operation ³⁷. Adsorption is a physicochemical technique extensively applied to eliminate heavy metals from the wastewater through the interaction between the adsorbents and pollutants ³⁹. Adsorption process increases with proliferation in the surface area of adsorbents ⁴⁰. The interaction between the adsorbent and adsorbate are classified into physical and chemical adsorption ³⁹. A very weak Vander Waals force of  attraction holds the adsorbate particles and adsorbent and the process is called physisorption and it has low adsorption capacity^41,42. If the adsorbate and adsorbent are bound due to chemical bonds like covalent or ionic, then it is chemisorption⁴³. Chemisorption has high efficiency but causes secondary pollution ^{45, 46, 47}.

Generally, when pollutants get adsorbed on the solid adsorbents, there are three steps involved ^49,52:

The transference of the adsorbate from the bulk solution to the surface of the adsorbent

Adsorption on the surface of the adsorbent

Transport of pollutants within the adsorbent 

Figure 11: Schematic representation of adsorption.   Click here to view Figure

Figure 12: Different between physical adsorption and chemical adsorption.   Click here to view Figure

All substances have an adsorption effect. But selection of adsorbents for the effective removal is an important component ⁵⁰. Adsorbents should have strong adsorption capacity, low equilibrium solubility, good selectivity and stability, ease of desorption⁵². The adsorption depends on time, quantity and nature of adsorbent, temperature, concentration of adsorbate, pH, etc.⁵⁴. 

Depending on the amount of adsorbate desorbed from the adsorbent surface, the reversibility of the adsorption process can be measured ^{55, 56}.

Adsorbents

To make an effective adsorption process, the selected adsorbent should be readily available, inexpensive, eco-friendly and abundant in nature⁵⁸. The elimination of heavy metals utilizing cheaper adsorbents is developing in recent days. There are several cheap adsorbents resulting from manufacturing byproducts, farm waste, biomaterials, reformed biopolymers, etc. that are slightly modified to effectively remove heavy metals from polluted wastewater⁵². Modified natural materials are used for adsorption such as clay, peat moss, zeolite, etc. Clinoptilolite and Peat moss are the most frequently used zeolite due to its specificity to some heavy metals such as lead, copper, cadmium and zinc ³⁴. Calcinated phosphates are used to adsorb Pb²⁺, Cu²⁺ and Zn²⁺⁷. 

Adsorption using biological wastes and modified agricultural wastes is called Biosorption. Orange peel is effectively used for the removal Ni²⁺⁷. Cr⁶⁺ are removed using almond shell, neem leaf powder⁹, crushed coconut shell, hazelnut shell, palm flower ¹⁷, groundnut hull ²⁹, activated rice husk ⁵⁶, UlvaLactuca ²⁹, rose wood ³⁷, eggshells ³⁷, etc. Cu²⁺ are adsorbed using sesame husk⁸¹, tea waste⁵⁹, marine green macroalgae⁴⁹, UlvaLactuca⁴⁹, etc. Musa paradisiaca peels are effevtively utilised to remove of Cd²⁺ and Pb^{2+ 54}. Some biosorbents utilized to remove heavy metals effectively are, CoriandrumSativum, grape stalk wastes³⁴, coffee husk, sugar beet peel in gels, citrus peels, potato peels ^49,56,59,60.  Agricultural waste is the most preferred adsorbent due to several reasons such as availability in abundance, economic and ecofriendly, efficient, unique chemical composition and low cost ⁵⁴. Heavy metals are also removed by using industrial solid wastes ^58,61. Cement bypass is used to treat tanneries effluent wastewater ⁶⁰. Treated waste newspaper has 72% efficiency to remove Chromium(VI)³⁸. Zn²⁺are removed using powdered waste sludge, dried marine green macroalgae⁶¹.

Ion exchange process 

The phenomenon used in the ion exchange process is, it draws dissolved ions present in the liquid state to the solid state⁶². This method is utilized only for low concentration of metals present in the solution and it is extremely sensitive to the pH of the aqueous phase. The resin eliminates metal ions in the dissolved form and releases other ions of like charges without altering the internal structure of the resin ^62,63. In the cationic resins H⁺ and Na⁺ are exchanged with Ni, Cu and Zn ions in the same resin. Similarly hydroxyl and chloride ions present in the anionic resin can be replaced by the chromate, sulphate, NO₃^-, CN^- and DOC present in the solution⁶⁴. Cation exchange resins called IRN77 and SKNI removes 95% of Chromium(III) from wastewater¹⁸. Among all the available kinds, synthetic polymers were preferred such as styrene- divinylbenzene, gel-like resins, macropore resin, etc.²⁶. 

Figure 13: Schematic representation of ion exchange process.   Click here to view Figure

Membrane process 

The membrane filtration process has been increasingly used to treat wastewater due to its convenient operation⁶⁹. A membrane adsorbent is ready by joining the functional groups to the surface and pore walls of the membrane. The aimed pollutants are preferentially adsorbed to the functional group site. When the polluted water moves via the membrane the functional active binding sites will be linked with the target pollutant to remove contaminants with a high adsorption rate and capacity⁷¹. Generally three types of membrane with minor differences are. There are many parameters that can disturb the membrane process like used materials, pore size and composition, which displays highly competent and feasible separation of heavy metals ⁷³. Membrane filtration is the most economic process because the water obtained from this process is of ultrapure water. Membrane process was also coupled with other techniques like ion- exchange process, adsorption, etc., ⁷⁰. To achieve the required filtration, membranes like cellulose, zeolite, polyamide, polyester, natural mineral based ceramic membranes and carbon membranes are used ⁶⁹. 

Figure 14: Schematic representation of membrane process.   Click here to view Figure

There are several membranes available to remove heavy metals. 

Cd is removed by montmorillonite kaolin, tobermorite, magnetitesiligagili and alumina membranes

Cu is removed by chitosan nanoparticles embedded ketoglutaric acid membranes 

Lead is removed by polymeric cation exchanger having nano- Zr

Arsenic removed by acid modified carbon membranes ⁶¹

Ultrafiltration

Ultrafiltration uses permeable membranes to distinct heavy metals and suspended solids (1000-10000 Da) on the basis of its pore size stretching from 5 to 20 nm. Ultrafiltration 30 is mainly used for product recovery and pollution control in the food, chemical, electronics, pharmaceuticals, metal plating industries due to its lower driving force, lesser space necessity and high loading density ²⁹.  More than 90% of removal efficiency can achieve by UF ⁷⁰.

Reverse osmosis

A process that utilizes pressure to move the solution through a membrane that has solute on one side and permits the pure solvent on the other side is known as Reverse osmosis (RO). RO is dependent on the solute concentration, pH, pressure and water flux rate³². According to various studies from the literature, the removal percentage of reverse osmosis has achieved 99.9% ⁷².

Nanofiltration

Nanofiltration performs separation between RO and UF. The molecular weight of the solute ranging from 200 to 1000 Da is rejected by a nanofiltration membrane with pore size ranging from 0.5 to 2 nm⁷⁴. Analysing the comparative study of RO and NF, RO performs superior than NF in wastewater separation due to its anti- compressing ability of membrane⁷⁵. Copper, Arsenic, Nickel and Chromium are removed from this process ³⁷. 

Solvent Extraction

The relocation of material from one solvent to another solvent in which the distribution coefficient is different is referred by extraction. Majority of the material can be extracted by repeated extraction²². Solvent Extraction is the cost effective energy efficient process to extract heavy metals from various complex leach products and industrial effluents using organic solvents ²⁹. Organic solvents like acetone, ethanol, hexane, methanol, acetonitrile, etc., are the most commonly used solvents ²⁴.  Solvent extraction is widely used in the field of biomedical industries, inorganic, organic chemistry labs and wastewater treatment ¹⁰. The solvent extraction process is used to separate metals like Zinc ⁷⁸, Nickel, lead, chromium ³⁰ and Cobalt ³⁴. 

Figure 15: Schematic representation of solvent extraction Process.   Click here to view Figure

Electrocoagulation

Electrocoagulation (EC) is an electrochemical approach that uses electrical current to remove metals from the solution. The oxidation occurs at anode and reduction occurs at cathode when electricity is applied ⁸¹. Electrocoagulation is one of the most promising techniques in the present era due to its high removal efficiency⁸¹.   This method is utilized to remove heavy metals from industries like steam cleaners, municipal sewage, palm oil effluents, pressure washers ⁸², electroplating, laundry, restaurant, poultry slaughterhouse ⁸², textile, paper, acid mine drainage and organic matters ⁸². Electrocoagulation process is used to remove suspended solids, dissolved metals, tanninsand dyes. EC process is able to eliminate heavy metals like Cr, Cd, Zn, Ni, Hg and Co ⁸³. pH, current density, conductivity ⁸³, gap between the electrodes ⁸⁸ and treatment time are the major factors that affect the performance of EC process ⁸⁹. Aluminium and Iron plates are widely used as sacrificial electrodes in EC processes ^61,66. But the usage of less reactive metal alloys like magnesium and aluminium alloy is a new trend ⁸⁴. Arsenic and Chromium are effectively removed using Iron electrode rather than Aluminium electrode ⁸⁵. By adjusting the pH and operating conditions, Cr (75%), Cu (97%), Zn (100%), and Ni (90%) removal efficiencies were obtained ⁸⁵. 

Figure 16: Schematic representation of electrocoagulation.   Click here to view Figure

Electrocoagulation mechanism 

Two electrodes were taken and current was applied between them and thereby the dissolution of the sacrificial anodes occurs and the supply ions to the wastewater was done by electrocoagulation. It allows settled, coagulated or soluble contaminants to develop aggregates⁸⁶. This process depends on the nature of aqueous solution, specifically conductivity ⁸⁶.

Steps involved in Electrocoagulation process are:

Oxidation of the “sacrificial electrodes”

Water molecules reduced at the negative electrode

The formed ions migrate to the oppositely charged electrodes

Metallic hydroxides are formed by the interaction of the positive ion and hydroxyl ion

Larger aggregates are formed by the absorption of pollutants into the hydroxide structures 

Removal of aggregates by flotation ⁷

Table 4: Heavy metal removal methodologies –uses and limitations

Reuse of treated water 

Water is an invaluable commodity that was once available almost for free cost. Water fulfills several needs in almost every industry. The utilised water from the industry comes out as wastewater with hazardous pollutants and leads the environment to danger. Wastewater is treated with various effective techniques ⁹⁴. The treated wastewater is almost free from pollutants and heavy metals are removed to the maximum extent. Reuse of the treated wastewater depends on factors such as waste volume, percentage of raw water, effluent standards, operation and maintenance costs. In the present era, reuse of treated wastewater is becoming a trend ⁹⁶. The applications of purified wastewater includes washing, coating, cooling, spraying, boiler water make-up etc. Treated wastewater by Electrocoagulation is used for recoveries of Pb and Zn ⁹². Treated textile wastewater is used for the irrigation of container grown ornamental shrubs ⁹⁷. Treated effluents from the dairy industry are used in cooling towers or boilers, washing the floors and external parts of trucks and rinsing outside areas in the dairy industry ⁹⁶. 

Summary

This review article elaborates on the properties and role of heavy metals in various fields. On the basis of the study, Mercury and Chromium (VI) is the most toxic heavy metal that contaminates the environment from various sources. Iron does not cause much impact on the environment and human beings. Mining is the major source that releases many heavy metals into the environment.  Various industries like textile, battery, electroplating are the other major sources that cause heavy metal pollution. The heavy metal contaminated wastewater is treated using various techniques like ion-exchange, adsorption, solvent extraction, electrocoagulation, membrane filtration and chemical precipitation. Among these techniques, Adsorption technique is used widely because of its simplicity. From the recent studies on adsorption, bio-sorbents are the extensively used adsorbent because of its easy availability. Among the three membrane filtration processes, reverse osmosis has a high removal efficiency of 99.9%. Electrocoagulation process has 90% removal efficiency to remove copper. 90% of Cr is removed using the ion- exchange process. Reuse of treated wastewater has various applications like growing shrubs, washing and cleaning.

Acknowledgement

The authors greatly acknowledge the Principal, Management and Department of Chemistry of Lady Doak College, Madurai for the support rendered for the completion of the work.

Funding sources

The author(s) received no financial support for the research, authorship, and/or publication of this review article. 

Conflict of Interest

The authors declare no conflict of interest.

Authors’ Contribution 

All the authors involved in reviewing and preparing the content for the review article.

Data Availability Statement

All data underlying the results are available as part of the article and no additional source data are required.

Ethics Approval Statement

Ethical review and approval were not required for the present review.

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