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REMOVAL OF HEAVY METALS FROM INDUSTRIAL

WASTE WATER

1

CONTENTS

INTRODUCTION

MATERIALS

1.HEAVY METALS

2.SOURCES OF HEAVY METALS

3.TOXICITY OF HEAVY METALS

METHODS

1.CONVENTIONAL METHOD

2.ADSORPTION METHOD

3.MEMBRANE FILTRATION METHOD

4.ELECTRO DIALYSIS

5.PHOTO CATALYSIS

ADVANTAGES

CONCLUSION

REFERENCES

2

INTRODUCTION

Rapid industrial development have led to the recognition and increasing understanding of interrelationship between pollution, public health and environment. Industrial development results in the generation of industrial effluents, and if untreated results in water, sediment and soil pollution. (Fakayode and Onianwa, 2002; Fakayode, 2005). Industrial wastes and emission contain toxic and hazardous substances, most of which are detrimental to human health (Jimena et al.,2008; Ogunfowokan et al., 2005; Rajaram etal., 2008). The key pollutants include heavy metals, chemical wastes and oil spills etc.

Due to the discharge of large amounts of metal-contaminated wastewater, industries bearing heavy metals, such as Cd, Cr, Cu, Ni, As, Pb, and Zn, are the most hazardous among the chemical-intensive industries. Because of their high solubility in the aquatic environments, heavy metals can be absorbed by living organisms. Heavy metals from industrial processes are of special concern because they produce water or chronic poisoning in aquatic animals (Ellis, 1989). While some heavy metals are purely toxic with no cellular role (Shi et al.,2002),other metals are essential for life at low concentration but become toxic at high concentrations (Badar et al. 2000), high concentration of all heavy metals inhibits activity of sensitive enzymes (Koropatnick and Leibbrandt 1995). Heavy metals are not biodegradable and tend to be accumulated in organisms and cause numerous diseases and disorders.

Heavy metals can damage the cell membranes, alter enzymes specificity, disrupt cellular functions and damage the structure of the DNA. Toxicity of these heavy metals occurs through the displacement of essential metals from their native binding sites or through ligand interactions. Also, toxicity can occur as a result of alterations in the conformational structure of the nucleicacids and proteins and interference with oxidative phosphorylation and osmotic balance. Once they enter the food chain, large concentrations of heavy metals may accumulate in the human body. If the metals are ingested beyond the permitted concentration, they can cause serious health disorders (Babel and Kurnia wan 2004). Therefore, it is necessary to treat metal-contaminated wastewater prior to its discharge to the environment.

3

Heavy metal removal from inorganic effluent can be achieved by conventional treatment processes such as chemical precipitation, ion exchange, and electrochemical removal. These processes have significant disadvantages, which are, for instance, incomplete removal, high-energy requirements, and production of toxic sludge (Eccles, 1999).

Recently, numerous approaches have been studied for the development of cheaper and more effective technologies, both to decrease the amount of wastewater produced and to improve the quality of the treated effluent.

Adsorption has become one of the alternative treatments, in recent years, the search for low-cost adsorbents that have metal-binding capacities has intensified. (Leung et al., 2000). The adsorbents may be of mineral, organic or biological origin, zeolites, industrial by-products, agricultural wastes, biomass, and polymeric materials (Kurniawan etal 2005). Membrane separation has been increasingly used recently for the treatment of inorganic effluent due to its convenient operation. There are different types of membrane filtration such as ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) .Electro treatments such as electrodialysis (Pedersen,2003) has also contributed to environmental protection. Photocatalytic process is an innovative and promising technique for efficient destruction of pollutants in water (Skubal et al.,2002). Although many techniques can be employed for the treatment of inorganic effluent, the ideal treatment should be not only suitable, appropriate and applicable to the local conditions, but also able to meet the maximum contaminant level (MCL) standards established.

4

MATERIALS

1. HEAVY METAL

A heavy metal is a member of a loosely defined subset of elements that exhibit metallic properties. It mainly includes the transition metals, some metalloids, lanthanides, and actinides. Many different definitions have been proposed—some based on density, some on or atomic weight and some on chemical properties, or toxicity (John H. Duffus ,2002). The term heavy metal has been called a considered to be as toxic metal. Heavy metals are generally considered to be those whose density exceeds 5 g per cubic centimeter.

Most commonly releasing heavy metals are arsenic, chromium, copper, mercury, cadmium, lead. These are causing hazardous affect on humans and other living organisms.

HEAVY METALS RELEASING INDUSTRIES

Industrial wastewater streams containing heavy metals are produced from different industries. Electroplating and metal surface treatment processes generate significant quantities of wastewaters containing heavy metals (such as cadmium, zinc, lead, chromium, nickel, copper, vanadium, platinum, silver, and titanium) from a variety of applications. These include electroplating, electroless depositions, conversion-coating, anodizing-cleaning, milling, and etching. Another significant source of heavy metals wastes result from printed circuit board (PCB) manufacturing. Tin, lead, and nickel solder plates are the most widely used resistant overplates. Other sources for the metal wastes include; the wood processing industry where a chromated copper-arsenate wood treatment produces arsenic-containing wastes; inorganic pigment manufacturing producing pigments that contain chromium compounds and cadmium sulfide; petroleum refining which generates conversion catalysts contaminated with nickel, vanadium, and chromium; and photographic operations producing film with high concentrations of silver and ferrocyanide. All of these generators produce a large quantity of wastewaters, residues, and sludges that can be categorized as hazardous wastes requiring extensive waste treatment (Sorme and Lagerkvist, 2002).

Pharmaceuticals, leather processing industries, pesticide manufacturing industries, agro chemicals, paint industries etc., are some of the industries that release more amount of heavy metals in to the environment.

5

2. NATURE OF HEAVY METALS

Heavy metals have high solubility, and are having acidic or neutral pH . These heavy metals are generally present in very low concentration. Because of high solubility these are easily taken by living organisms and get accumulated in the body. On increasing pH to basic and by changing concentration of metal to more amount metal gets precipitated and can be easily removed from water.

3.TOXICITY OF HEAVY METALS

Arsenic is usually regarded as a hazardous heavy metal even though it is actually a semi-metal. Heavy metals cause serious health effects, including reduced growth and development, cancer, organ damage, nervous system damage, and in extreme cases, death. Exposure to some metals, such as mercury and lead, may also cause development of autoimmunity, in which a person’s immune system attacks its own cells. This can lead to joint diseases such as rheumatoid arthritis, and diseases of the kidneys, circulatory system, nervous system, and damaging of the fetal brain. At higher doses, heavy metals can cause irreversible brain damage. Children may receive higher doses of metals from food than adults, since they consume more food for their body weight than adults.

Table 1: heavy metal affects

Heavy

Toxicities

MLC

metal

 

 

 

 

 

Arsenic

Skin manifestations, visceral cancers, vascular disease

0.050

 

 

 

Cadmium

Kidney damage, renal disorder, human carcinogen

0.01

 

 

 

Chromium

Headache, diarrhea, nausea, vomiting, carcinogenic

0.05

 

 

 

Copper

Liver damage, Wilson disease, insomnia

0.25

 

 

 

Nickel

Dermatitis, nausea, chronic asthma, coughing, human

0.20

 

carcinogen

 

 

 

 

Zinc

Depression, lethargy, neurological signs and increased thirst

0.80

 

 

 

Lead

Damage the fetal brain, diseases of the kidneys, circulatory

0.006

 

system, and nervous system

 

 

 

 

tMercury

Rheumatoid arthritis, and diseases of the kidneys, circulatory

 

 

system, and nervous system

0.00003

 

 

 

6

Wastewater regulations were established to minimize human and environmental exposure to hazardous chemicals. This includes limits on the types and concentration of heavy metals that may be present in the discharged wastewater. The MCL standards, for those heavy metals, established by USEPA

REMOVAL OF HEAVY METALS:

Waste water containing heavy metals

 

 

 

↓

 

 

 

 

 

Treatment

 

 

 

 

 

↓

 

 

↙

↙

 

↓

↘

↘

Conventional

electro dialysis

adsorption

photo catalytic

membrane

separation

 

 

 

 

 

↓

 

 

↓

 

↓

1. Chemical Precipitation

 

1.biological material

1.micro filtration

2 .Ion exchange

2.modified natural material

 

2.ultra filtration

3.electro winning

3.agricultural waste

3.nano filtration

4. reverse osmosis

7

METHODS

1. CONVENTIONAL PROCESS

Here pH, concentration are increased , so solubility is decreased. The conventional processes for removing heavy metals from wastewater include many processes such as chemical precipitation, flotation, adsorption, ion exchange, and electrochemical deposition. Chemical precipitation is the most widely used for heavy metal removal from inorganic effluent .

TREATMENT BY HYDROXIDE PRECIPITATION

As metals enter the treatment process, they are in a stable, dissolved aqueous form and are unable to form solids. The goal of metals treatment by hydroxide precipitation is then to adjust the pH (hydroxide ion concentration) of the water so that the metals will form insoluble precipitates. Once the metals precipitate and form solids, they can then easily be removed, and the water, now with low metal concentrations, can be discharged.

Metal precipitation is primarily dependent upon two factors: The concentration of the metal, and the pH of the water. Heavy metals are usually present in wastewaters in dilute quantities (1 - 100 mg/L) and at neutral or acidic pH values (< 7.0). Both of these factors are disadvantageous with regard to metals removal. However, when one adds caustic to water which contains dissolved metals, the metals react with hydroxide ions to form metal hydroxide solids. The high pH corresponds to high hydroxide concentrations.

By simply adjusting the pH from 6.8 to 8.6 has effectively precipitated most of the dissolved metal from the water. Since all metals show similar effects, it is clear that the adjustment of pH is critical when the metal is to be removed from the wastewater. All metals have the same minimum solubility. Therefore in a wastewater where multiple metals are present, as a general rule, pH should be adjusted to an average value.

8

A)Chemical precipitation:

 

Metal

+ hydoxide

→ metal hydroxide

 

(from caustic)

precipitates

Fig 1:The Processes of a Conventional Metals Precipitation Treatment Plant

9

1. RAPID MIX

The goal of the rapid mix operation is to first raise the wastewater pH to form metal hydroxide particles, as discussed above. After the addition of caustic, the next step is to add aluminum or iron salts, or organic polymers (coagulants) directly to the wastewater. These polymers attach to the metal solids particles. The small metal hydroxide particles become entangled in these polymers, causing the particle size to increase (form flocs), which promotes settling.

+

=

Metal hydroxide

polymer

metal hydroxide entrapped in polymer

2. SEDIMENTATION

Once particles become enmeshed in the polymer, they are allowed to settle so that they are removed from the wastewater. The particles settle since they are heavier than water. This settling occurs in the sedimentation tanks. Sedimentation tanks, in contrast to rapid mixing units, are designed to have no mixing, to produce a calm flow for settling.

Fig 2:process of sedimentation

10

A Sedimentation Basin with Metal Hydroxide Sludge Formation

The pH during sedimentation must be maintained at approximately 9.0 to ensure that none of the metal hydroxides redissolve and become soluble in the water. A detention time of 1.5 to 3 hours is usually adequate to accomplish efficient settling.

3. FILTRATION

The filtration unit is designed to trap those particles that did not settle in the sedimentation basin (because they were too small) or did not have sufficient time to settle and were carried out of the basin.

Water entering the filtration unit is passed through silica sand, diatomaceous earth, carbon, or cloth to capture the remaining metal hydroxide particles. Metal particles stick to the filtering material and are removed from the water. Filtration completes the metal treatment process. Only now should the pH be reduced for discharge, if necessary, or pH can now be adjusted for water reuse. Figure 11 depicts a typical filtration unit.

INLET

TRANSFERRED

SOLIDS

EFFLUENT

Fig:3 Filtration Unit for Metals Removal

11

As filtration progresses and more metal hydroxides and other solids clog the filter material, pressure drop through the filter rises and some solids may pass through the filter. When either of these two situations occurs, the filter must be backwashed by reversing the flow of water through the filter. This backwash water is sent back to the rapid mix tank for mixing with the incoming water since it contains a significant concentration of solids from the dislodging that has occurred. Furthermore, the pH of this water (since it will be diluted with incoming water) may drop significantly and pose the problem of redissolving all of the metal hydroxides solids

4. SLUDGE TREATMENT

The solids produced in the sedimentation stage (and possibly solids from filtration) are denoted as a sludge and periodically removed. In diatomaceous earth and fiber filters, the entire filter media (diatomaceous earth, filter cartridge) is dumped with the captured metal hydroxide solids. This sludge may be sent to a dewatering stage to remove excess water and leave only solids. The water from the dewatering stage may not be completely free of metals and should be piped to the rapid mix tank.

The sludge now contains the precipitated metal hydroxide solids, made up of identifiable quantities of heavy metals, which are regulated according to state and federal guidelines. The solids produced from heavy metal wastewater treatment must then be disposed of as a hazardous waste.

M2++2(OH)−↔M(OH)2↓

where M2+ and OH− represent the dissolved metal ions and the precipitant, respectively, while M(OH)2 is the insoluble metal hydroxide. Adjustment of pH to the basic conditions (pH 9– 11) is the major parameter that significantly improves heavy metal removal by chemical precipitation .

Lime and limestone are the most commonly employed precipitant agents due to their availability and low-cost in most countries (Mirbagherp and Hosseini, 2004 and Aziz et al., 2008). Lime precipitation can be employed to effectively treat inorganic effluent with a metal concentration of higher than 1000 mg/L. Other advantages of using lime precipitation include the simplicity of the process, inexpensive equipment requirement, and convenient and safe operations. However, chemical precipitation requires a large amount of chemicals to reduce metals to an acceptable level for discharge.

12

Other drawbacks are its excessive sludge production that requires further treatment, slow metal precipitation, poor settling, the aggregation of metal precipitates, and the long-term environmental impacts of sludge disposal

ION EXCHANGE FOR HEAVY METAL REMOVAL

Ion exchange is a reversible chemical reaction wherein an ion (an atom or molecule that has lost or gained an electron and thus acquired an electrical charge) from a wastewater solution is exchanged for a similarly charged ion attached to an immobile solid particle. These solid ion exchange particles are either naturally occurring inorganic zeolites or synthetically produced organic resins. The synthetic organic resins are the predominant type used today because their characteristics can be tailored to specific applications.

An organic ion exchange resin is composed of high molecular weight polyelectrolytes that can exchange their mobile ions for ions of a similar charge from the wastewater.

Ion exchange is another method used successfully in the industry for the removal of heavy metals from effluent. An ion exchanger is a solid capable of exchanging either cations or anions from the surrounding materials. Commonly used matrices for ion exchange are synthetic organic ion exchange resins. The disadvantage of this method is that it cannot handle concentrated metal solution as the matrix gets easily fouled by organics and other solids in the wastewater. Moreover ion exchange is nonselective and is highly sensitive to the pH of the solution.

ELECTRO WINNING

Electrolytic recovery or electro-winning is one of the many technologies used to remove metals from process water streams. This process uses electricity to pass a current through an aqueous metal-bearing solution containing a cathode plate and an insoluble anode. Positively charged metallic ions cling to the negatively charged cathodes leaving behind a metal deposit that is strippable and recoverable. A noticeable disadvantage was that corrosion could become a significant limiting factor, where electrodes would frequently have to be replaced.

13

2.ADSORPTION ON NEW ADSORBENTS

Adsorption is transfer of ions from water to the soil i.e. from solution phase to the solid phase. Adsorption actually describes a group of processes, which includes adsorption and precipitation reactions. Recently, adsorption has become one of the alternative treatment techniques for wastewater laden with heavy metals. Basically, adsorption is a mass transfer process by which a substance is transferred from the liquid phase to the surface of a solid, and becomes bound by physical and/or chemical interactions (Kurniawan and Babel, 2003).

Various low-cost adsorbents, derived from agricultural waste, industrial by-product, natural material, or modified biopolymers, have been recently developed and applied for the removal of heavy metals from metal-contaminated wastewater. In general, there are three main steps involved in pollutant sorption onto solid sorbent: (i) the transport of the pollutant from the bulk solution to the sorbent surface; (ii) adsorption on the particle surface; and (iii) transport within the sorbent particle. Technical applicability and cost- effectiveness are the key factors that play major roles in the selection of the most suitable adsorbent to treat inorganic effluent.

ADSORPTION ON MODIFIED NATURAL MATERIALS:

Natural zeolites gained a significant interest, mainly due to their valuable properties as ion exchange capability. Among the most frequently studied natural zeolites, clinoptilolite was shown to have high selectivity for certain heavy metal ions such as Pb(II), Cd(II), Zn(II), and Cu(II). It was demonstrated that the cation-exchange capability of clinoptilolite depends on the pre-treatment method and that conditioning improves its ion exchange ability and removal efficiency (Babel and Kurniawan, 2003; Bose et al., 2002). The ability of different types of synthetic zeolite for heavy metals removal was recently investigated. The role of pH is very important for the selective adsorption of different heavy metal ions ( Barakat, 2008a). Basaldella et al. (2007) used NaA zeolite for removal of Cr(III) at neutral pH, while Barakat (2008a) used 4A zeolite which was synthesized by dehydroxylation of low grade kaolin.

14

Barakat reported that Cu(II) and Zn(II) were adsorbed at neutral and alkaline pH, Cr(VI) was adsorbed at acidic pH while the adsorption of Mn(IV) was achieved at high alkaline pH values. Nah et al. (2006) prepared synthetic zeolite magnetically modified with iron oxide (MMZ). MMZ showed high adsorption capacities for the Pb(II) ion and a good chemical resistance in a wide pH range 5–11.

The natural clay minerals can be modified with a polymeric material in a manner that this significantly improves their capability to remove heavy metals from aqueous solutions. These kinds of adsorbents are called clay–polymer composites (Vengris et al., 2001, Sölenera et al., 2008 and Abu-Eishah, 2008).

Different phosphates such as calcined phosphate at 900 °C, activated phosphate (with nitric acid), and zirconium phosphate have been employed as new adsorbents for removal of heavy metals from aqueous solution (Aklil et al., 2004, Moufliha et al., 2005 and Pan et al., 2007). Fig. 2 shows the adsorption isotherm of Pb(II), Cu(II), and Zn(II) onto calcined phosphate at pH 5

Table 2:Adsorption capacities of modified natural materials for heavy metals.

Adsorbent

 

Adsorption capacity (mg/g)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Pb2+

Cd2+

Zn2+

Cu2+

Cr6+

Ni2+

 

 

 

 

 

 

 

 

 

 

 

Zeolite, clinoptilolite

 

1.6

2.4

0.5

1.64

 

0.4

 

 

 

 

 

 

 

 

 

 

 

Modified zeolite, MMZ

 

123

 

 

 

 

8

 

 

 

 

 

 

 

 

 

 

 

 

HCl-treated clay

 

 

 

63.2

83.3

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Clay/poly(methoxyethyl)acrylamide

 

81.02

 

20.6

29.8

 

80.9

 

 

 

 

 

 

 

 

 

 

 

 

 

85.6

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Calcined phosphate

 

155.0

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Activated phosphate

 

4

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Zirconium phosphate

 

398

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

15

ADSORPTION ON INDUSTRIAL BY-PRODUCTS

Industrial by-products such as fly ash, waste iron, iron slags, hydrous titanium oxide, can be chemically modified to enhance its removal performance for metal removal from wastewater. (Lee et al. (2004) . green sands, another by-product from the iron foundry industry, for Zn(II) removal. Feng et al. (2004) investigated Cu(II) and Pb(II) removal using iron slag. A pH range from 3.5 to 8.5 [for Cu(II)] and from 5.2 to 8.5 [for Pb(II)] was optimized. Fly ashes were also investigated as adsorbents for removal of toxic metals. Gupta et al. (2003) explored bagasse fly ash, a solid waste from sugar industry, for Cd(II) and Ni(II) removal from synthetic solution at pH ranging from 6.0 to 6.5. Alinnor (2007) used fly ash from coal-burning for removal of Cu(II) and Pb(II) ions. Sawdust treated with 1,5-disodium hydrogen phosphate was used for adsorption of Cr(VI) at pH 2 Uysal and Ar, 2007. Iron based sorbents such as ferrosorp plus (Genç-Fuhrman et al., 2008) and synthetic nanocrystalline akaganeite (Deliyanni et al., 2007) were recently used for simultaneous removal of heavy metals.

Ghosh et al. (2003) and Barakat (2005) studied hydrous titanium oxide for adsorption of Cr(VI) and Cu(II), respectively. Barakat reported that, the adsorbed Cu(II) aqueous species can undergo surface hydrolysis reaction as pH rises. This yields a series of surface Cu(II) complexes such as TiO–CuOH+, TiO–Cu(OH)2, and TiO–Cu(OH)3− species.

Fig 4: The adsorption mechanism of Cu(II) on hydrous TiO2

16

ADSORPTION ON MODIFIED AGRICULTURE WASTE:

Recently, a great deal of interest in the research for the removal of heavy metals from industrial effluent has been focused on the use of agricultural by-products as adsorbents. The use of agricultural by-products in bioremediation of heavy metal ions, is known as bio- sorption. This utilizes inactive (non-living) microbial biomass to bind and concentrate heavy metals from waste streams by purely physico-chemical pathways (mainly chelation and adsorption) of uptake (Igwe et al., 2005). New resources such as hazelnut shell, rice husk, pecan shells, jackfruit, maize cob or husk can be used as an adsorbent for heavy metal uptake after chemical modification or conversion by heating into activated carbon. Ajmal et al. (2000) employed orange peel for Ni(II) removal from simulated wastewater. They found that the maximum metal removal occurred at pH 6.0. The applicability of coconut shell charcoal (CSC) modified with oxidizing agents and/or chitosan for Cr(VI) removal was investigated by Babel and Kurniawan (2004). Cu(II) and Zn(II) removal from real wastewater were studied using pecan shells-activated carbon (Bansode et al., 2003) and potato peels charcoal (Amana et al., 2008). Bishnoi et al. (2003) conducted a study on Cr(VI) removal by rice husk-activated carbon from an aqueous solution. They found that the maximum metal removal by rice husk took place at pH 2.0. Rice hull, containing cellulose, lignin, carbohydrate and silica, was investigated for Cr(VI) removal from simulated solution (Tang et al., 2003). To enhance its metal removal, the adsorbent was modified with ethylenediamine. The maximum Cr(VI) adsorption of 23.4 mg/g was reported to take place at pH 2.

a) Adsorption by agricultural waste

agricultural waste (Rice husk, hazel nut etc.,)

↓

Burning in furnace

↓

produces activated carbon

↓

used as adsorbent

17

Activated carbon has large sized pores.

ADSORPTION BY BANANA PEELS:

science shows banana peels can pull heavy metal contamination from river water. The metal was removed from the water and remained bonded to the banana peels. Collected banana peels were cut it into small pieces (< 5 mm), washed three times with tap water and three times with distilled water to remove external dirt. Wetted banana peels were kept in air for removing the free water from the surface and dried in oven for 24 hours at 105°C.

The dried banana peels were grounded into powder and kept in an air tied bottle prior to Banana peels is a low cost and readily available materials for preparing bioadsorbent. This study has explored the economically viable bio adsorbents for copper removal from water. The banana peel could be regenerated and reused till seven times without reducinefficiency the experiments.

Banana peel as an adsorbent:

Dried and ground banana peels

↓

Then combined them in flasks of water with known concentrations of metals

↓

Metals removed

They also built water filters out of peels and pushed water through them.

Adsorption by Moringa

This study evaluated the efficacy of moringa seeds (Moringa oleifera Lam.) as an adsorbent material for removing toxic heavy metals such as cadmium, lead, and chromium from contaminated solutions. Different by-products of the seed processing were used as adsorbents. These include, the Whole Seed Powder (WSP), the Residue after Coagulant Extraction (RaCE) and an Activated Carbon (AC) prepared from the seed husk. Adsorption studies for the removal of Cr(III) and Cr(VI) were carried out in batch experiments and the effects of adsorbent dosage, contact time, pH and initial chromium concentration were analysed.

18

Experimental results showed that maximum removal of Cr (III) and Cr (VI) was observed at pH 7 and pH 2, respectively. The percentage removals of Cr (III) by WSP, RaCE and AC were: 97, 94 and 99.9%, respectively. And the percentage removals of Cr (VI) by RaCE and AC were 47 and 83.2%, respectively. RaCE showed similar adsorption capacity to the WSP, which indicates that it is possible to extract a coagulant and use the waste product for adsorption. By using the RaCE, residual dissolved organic carbon in the treated water was significantly reduced compared to using the WSP. These results indicate that biomaterials can be considered as potential adsorbents for heavy metals removal from water or wastewater systems

Moringa as adsorbent

Dried and powdered moringa seeds

↓

Placed in contaminated water

↓

Sediment settles at bottom

Orange peels, potato peels can also be used in metal removing process as adsorbents.

Adsorption by biological wastes

Some of the used alga wastes were, Spirogyra species (Gupta et al., 2006), Ecklonia maxima (Fenga and Aldrich, 2004), Ulva lactuca (El-Sikaily et al., 2007), Oedogonium sp. and Nostoc sps. (Gupta and Rastogi, 2008), and brown alga Fucus serratus (Ahmady- Asbchin et al., 2008). On the whole, an acidic pH ranging 2–6 is effective for metal removal by adsorbents from biological wastes. The mechanism of up-taking heavy metal ions can take place by metabolism-independent metal-binding to the cell walls and external surfaces (Deliyanni et al., 2007). This involves adsorption processes such as ionic, chemical and physical adsorption. A variety of ligands located on the fungal walls are known to be involved in metal chelation. These include carboxyl, amine, hydroxyl, phosphate and sulfhydryl groups. Metal ions could be adsorbed by complexing with negatively charged reaction sites on the cell surface. Table 3 shows the adsorption capacities of different biosorbents.

19

Table 3: Adsorption capacities of some agricultural and biological wastes for heavy metals.

Adsorbent

 

Adsorption capacity (mg/g)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Pb2+

Cd2+

Zn2+

Cu2+

 

Cr6+

Ni2+

 

 

 

 

 

 

 

 

 

 

Maize cope and husk

 

456

493.7

495.9

---

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Orange peel

 

 

 

 

 

 

 

158

 

 

 

 

 

 

 

 

 

 

 

Coconut shell charcoal

 

 

 

 

 

 

3.65

 

 

 

 

 

 

 

 

 

 

 

 

Pecan shells activated carbon

 

 

 

13.9

31.7

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Rice husk

 

 

2.0

 

 

 

0.79

 

 

 

 

 

 

 

 

 

 

 

 

Modified rice hull

 

 

 

 

 

 

23.4

 

 

 

 

 

 

 

 

 

 

 

 

Spirogyra (green alga)

 

 

 

 

133

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Ecklonia maxima – marine alga

 

235

 

 

90

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Ulva lactuca

 

 

 

 

 

 

112.3

 

 

 

 

 

 

 

 

 

 

 

 

Oedogonium species

 

145

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Nostoc species

 

93.5

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Bacillus – bacterial biomass

 

467

85.3

418

381

 

39.9

 

 

 

 

 

 

 

 

 

 

 

 

ADSORPTION ON MODIFIED BIOPOLYMERS AND HYDROGELS

Biopolymers are industrially attractive because they are, capable of lowering transition metal ion concentrations to sub-part per billion concentrations, widely available, and environmentally safe. Another attractive feature of biopolymers is that they posses a number of different functional groups, such as hydroxyls and amines, which increase the efficiency of metal ion uptake and the maximum chemical loading possibility. New polysaccharide-based-materials were described as modified biopolymer adsorbents (derived from chitin, chitosan, and starch) for the removal of heavy metals from the wastewater. There are two main ways for preparation of sorbents containing polysaccharides: (a) crosslinking reactions, a reaction between the hydroxyl or amino groups of the chains with a coupling agent to form water-insoluble crosslinked networks (gels); (b) immobilization of polysaccharides on insoluble supports by coupling or grafting reactions in order to give hybrid or composite materials (Crini, 2005).

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Chitin is a naturally abundant mucopolysaccharide extracted from crustacean shells, which are waste products of seafood processing industries. Chitosan, which can be formed by deacetylation of chitin, is the most important derivative of chitin. Chitosan in partially converted crab shell waste is a powerful chelating agent and interacts very efficiently with transition metal ions (Pradhan, 2005). Recently other modified chitosan beads were proposed for diffusion of metal ions through crosslinked chitosan membranes (Lee et al., 2001). The excellent saturation sorption capacity for Cu (II) with the crosslinked chitosan beads was achieved at pH 5. Liu et al., (2003) prepared new hybrid materials that adsorb transition metal ions by immobilizing chitosan on the surface of non-porous glass beads. Column chromatography on the resulting glass beads revealed that they have strong affinities to Cu (II), Fe (III) and Cd (II). Yi et al. (2003) proposed the use of chitosan derivatives containing crown ether. The materials had high adsorption capacity for Pb (II), Cr (III), Cd (II) and Hg (II). The materials can be regenerated and their selectivity properties were better than crosslinked chitosan without crown ether.

The sorption mechanism of polysaccharide-based-materials is different from those of other conventional adsorbents. These mechanisms are complicated because they implicate the presence of different interactions. Metal complexation by chitosan may thus involve two different mechanisms (chelation versus ion exchange) depending on the pH since this parameter may affect the protonation of the macromolecule (Crini, 2005).

Chitosan is characterized by its high percentage of nitrogen. present in the form of amine groups that are responsible for metal ion binding through chelation mechanisms.

Amine sites are the main reactive groups for metal ions though hydroxyl groups, especially in the C-3 position, and they may contribute to adsorption. However, chitosan is also a cationic polymer and its pKa ranges from 6.2 to 7. Thereby, in acidic solutions it is protonated and possesses electrostatic properties. Thus, it is also possible to sorb metal ions through anion exchange mechanisms. Sorbent materials containing immobilized thiacrown ethers were prepared by immobilizing the ligands into sol–gel matrix (Saad et al., 2006). The competitive sorption characteristics of a mixture of Zn(II), Cd(II), Co(II), Mn(II), Cu(II), Ni(II), and Ag(I) were studied. The results revealed that the thiacrown ethers exhibit highest selectivity toward Ag(I).

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Table 4: Adsorption capacities of modified biopolymers for heavy metals (Crini, 2005).

Adsorbent

 

Adsorption capacity (mg/g)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Pb2+

Cd2+

Zn2+

Cu2+

Cr6+

As5+

 

 

 

 

 

 

 

 

 

 

Cross linked chitosan

 

 

150

 

164

 

230

 

 

 

 

 

 

 

 

 

 

Cross linked starch gel

 

433

 

 

135

 

 

 

 

 

 

 

 

 

 

 

 

Alumina/chitosan composite

 

 

 

 

200

 

 

 

 

 

 

 

 

 

 

 

 

Hydrogels, which are crosslinked hydrophilic polymers, are capable of expanding their volumes due to their high swelling in water. Accordingly they are widely used in the purification of wastewater. Various hydrogels were synthesized and their adsorption behavior for heavy metals was investigated. Kesenci et al. (2002) prepared polyhydrogel (ethyleneglycol dimethacrylate-co-acrylamide) beads with the following metals in the order Pb(II) > Cd(II) > Hg(II); Essawy and Ibrahim (2004) prepared polyhydrogel (vinylpyrrolidone-co-methylacrylate) with Cu(II) > Ni(II) > Cd(II); while Barakat and Sahiner (2008) prepared poly(3-acrylamidopropyl) trimethyl ammonium chloride hydrogels for As(V) removal. The removal is basically governed by the water diffusion into the hydrogel, carrying the heavy metals inside especially in the absence of strongly binding sites. Maximum binding capacity increases with pH increase to >6.

Fig 5: Three-dimensional network formation of cationic hydrogel (Barakat and Sahiner, 2008).

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3.MEMBRANE FILTRATION

Membrane filtration has received considerable attention for the treatment of inorganic effluent, since it is capable of removing not only suspended solid and organic compounds, but also inorganic contaminants such as heavy metals. Depending on the size of the particle that can be retained, various types of membrane filtration such as ultrafiltration, nanofiltration and reverse osmosis can be employed for heavy metal removal from wastewater.

Depending on size of particles , various filters are used for separation.

 

membrane filtration

 

a)Micro

b) ultra

c) nano

 

osmosis

↙

↙

↘

d)

reverse↘

Pore size:

size:

size:

 

size:

10-0.1

0.1-0.01

0.01-0.001

0.001-0.0001

 

 

microns

 

 

Fig 6: Micro filtration

Fig 7: ultrafiltration

23

Fig 8: Nanofiltration

Fig 9: Reverse osmosis

Ultrafiltration (UF) utilizes permeable membrane to separate heavy metals, macromolecules and suspended solids from inorganic solution on the basis of the pore size (5–20 nm) and molecular weight of the separating compounds (1000–100,000 Da). These unique specialties enable UF to allow the passage of water and low-molecular weight solutes, while retaining the macromolecules, which have a size larger than the pore size of the membrane. The application of both reverse osmosis (RO) and nanofiltration (NF) technologies for the treatment of wastewater containing copper and cadmium ions was investigated (Abu Qdaisa and Moussab, 2004). The results showed that high removal efficiency of the heavy metals could be achieved by RO process (98% and 99% for copper and cadmium, respectively). NF, however, was capable of removing more than 90% of the copper ions existing in the feed water. Lv et al. (2008) investigated amphoteric polybenzimidazole nanofiltration hollow fiber membrane for both cations and anions removal NF membranes perform separation in between those of UF and RO ones. The molecular weight of the solute that is 90% rejected by NF membrane range from 200 to 1000 Da with pore diameters varying from 0.5 to 2 nm (Lv et al., 2008 and Khedr, 2008). A multiple membrane process was developed for selective separation to reduce cost and mitigated the increasing heavy metal pollution.

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The process was divided into three stages: firstly, microfiltration (MF) and UF were used to separate the possible organic and suspended matters, secondly, electrodialysis (ED) was carried out for effective desalination, and thirdly, the concentrate from ED was treated by NF and RO separately to increase the recovery rate of water. Results showed that filtration characteristics of UF membrane here was not so good as is usually, even if compared with MF membrane. And RO performed better than NF in wastewater separation, especially in anti-compacting ability of membrane (Zuoa et al., 2008).

Polymer-supported ultrafiltration (PSU) technique has been shown recently to be a promising alternative for the removal of heavy metal ions from industrial effluent (Rether and Schuster, 2003). This method employs proprietary water-soluble polymeric ligands to bind metal ions of interest, and the ultrafiltration technique to concentrate the formed macromolecular complexes and produce an effluent, essentially free of the targeted metal ions (Fig. 8). Advantages of the PSU technology over ion exchange and solvent extraction are the low- energy requirements involved in ultrafiltration, the very fast reaction kinetics, all aqueous based processing and the high selectivity of separation if selective bonding agents are applied. Polyamidoamine dendrimers (PAMAM) have been surface modified, using a two- step process with benzoylthiourea groups to provide a new excellent water-soluble chelating ion exchange material with a distinct selectivity for toxic heavy metal ions. Studies on the complexation of Co(II), Cu(II), Ni(II), Pb(II) and Zn(II) by the dendrimer ligand were performed using the PSU method. The results show that all metal ions can be retained almost quantitatively at pH 9. Cu(II) form the most stable complexes with the benzoylthiourea modified PAMAM derivatives (can be completely retained at pH >4), and can be separated selectively from the other heavy metal ions investigated (Fig. 9).

Fig 10: Process of ultrafiltration

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Another similar technique, complexation–ultrafiltration, proves to be a promising alternative to technologies based on precipitation and ion exchange. The use of water-soluble metal-binding polymers in combination with ultrafiltration (UF) is a hybrid approach to concentrate selectively and to recover valuable elements as heavy metals. In the complexation – UF process cationic forms of heavy metals are first complexed by a macroligand in order to increase their molecular weight with a size larger than the pores of the selected membrane that can be retained whereas permeate water is then purified from the heavy metals (Petrov and Nenov, 2004, Barakat, 2008b and Trivunac and Stevanovic, 2006). The advantages of complexation–filtration process are the high separation selectivity due to the use of a selective binding and low-energy requirements involved in these processes. Water-soluble polymeric ligands have shown to be powerful substances to remove trace metals from aqueous solutions and industrial wastewater through membrane processes. Carboxyl methyl cellulose (CMC) Petrov and Nenov, 2004 and Barakat, 2008b, diethylaminoethyl cellulose (Trivunac and Stevanovic, 2006), and polyethyleneimine (PEI) Aroua et al., 2007 were used as efficient water-soluble metal-binding polymers in combination with ultrafiltration (UF) for selective removal of heavy metals from water. Barakat (2008b) investigated the removal of Cu(II), Ni(II), and Cr(III) ions from synthetic wastewater solutions by using CMC and polyethersulfon ultrafiltration membrane.

A new integrated process combining adsorption, membrane separation and flotation was developed for the selective separation of heavy metals from wastewater (Mavrov et al., 2003). The process was divided into the following three stages: firstly, heavy metal bonding (adsorption) by a bonding agent, secondly, wastewater filtration to separate the loaded bonding agent by two variants.crossflow microfiltration for low-contaminated wastewater (Fig. 10), or a hybrid process combining flotation and submerged microfiltration for highly contaminated wastewater (Fig. 11), and thirdly, bonding agent regeneration. Synthetic zeolite R selected as a bonding agent, was characterized and used for the separation of the zeolite loaded with metal (Mavrov et al., 2003). Bloocher et al. (2003) and Nenov et al. (2008) developed a new hybrid process of flotation and membrane separation by integrating specially designed submerged microfiltration modules directly into a flotation reactor. The feasibility of this hybrid process was proven using powdered synthetic zeolites as bonding agents. The toxic metals, copper, nickel and zinc, were reduced from initial concentrations of 474, 3.3 and 167 mg/L, respectively, to below 0.05 mg/L, consistently meeting the discharge limits.

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Fig 11: The integrated processes combining metal bonding and separation by cross flow membrane filtration (for low-contaminated wastewater) (Mavrov et al., 2003).

Fig 12: The integrated processes combining metal bonding and separation by a new hybrid process (for highly contaminated wastewater) (Mavrov et al., 2003).

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4. ELECTRODIALYSIS

Electrodialysis (ED) is a membrane separation in which ionized species in the solution are passed through an ion exchange membrane by applying an electric potential. The membranes are thin sheets of plastic materials with either anionic or cationic characteristics. When a solution containing ionic species passes through the cell compartments, the anions migrate toward the anode and the cations toward the cathode, crossing the anion exchange and cation-exchange membranes (Chen, 2004), Fig. 12 shows the principles of electrodialysis.

Fig 13: Electrodialysis principles (Chen, 2004). CM – cation-exchange membrane, D – diluate chamber, e1 and e2 – electrode chambers, AM – anion exchange membrane, and K – concentrate chamber.

Some interesting results were reported by Tzanetakis et al. (2003), who evaluated the performance of the ion exchange membranes for the electrodialysis of Ni(II) and Co(II) ions from a synthetic solution. Two cation-exchange membranes, perfluorosulfonic Nafion 117 and sulfonated polyvinyldifluoride membrane (SPVDF), were compared under similar operating conditions. By using perfluorosulfonic Nafion 117, the removal efficiency of Co(II) and Ni(II) were 90% and 69%, with initial metal concentrations of 0.84 and 11.72 mg/L, respectively.

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Effects of flow rate, temperature and voltage at different concentrations using two types of commercial membranes, using a laboratory ED cell, on lead removal were studied (Mohammadi et al., 2004). Results show that increasing voltage and temperature improved cell performance; however, the separation percentage decreased with an increasing flow rate. At concentrations of more than 500 ppm, dependence of separation percentage on concentration diminished. Using membranes with higher ion exchange capacity resulted in better cell performance. Electrodialytic removal of Cd(II) from wastewater sludge, was studied (Jakobsen et al., 2004). During the remediation a stirred suspension of wastewater sludge was exposed to an electric dc field. The liquid/solid (mL/g fresh sludge) ratio was between 1.4 and 2. Three experiments were performed where the sludge was suspended in distilled water, citric acid or HNO3 (Fig. 13). The Cd(II) removal in the three experiments was 69%, 70% and 67%, respectively.

Fig 14: Electrodialytic remediation of cadmium from wastewater sludge (Jakobsen et al., 2004) (AN: anion exchange membrane, CAT: cation-exchange membrane, (a) stirrer).

ED process was modeled based on basic electrochemistry rules and copper ion separation experimental data (Mohammadi et al., 2005). The experiments were performed for zinc, lead and chromium ions. It was found that performance of an ED cell is almost independent on the type of ions and only depends on the operating conditions and the cell structure.

In spite of its limitation, ED offers advantages for the treatment of wastewater laden with heavy metals such as the ability to produce a highly concentrated stream for recovery and the rejection of undesirable impurities from water. Moreover, valuable metals such as Cr and Cu can be recovered. Since ED is a membrane process, it requires clean feed, careful operation, periodic maintenance to prevent any stack damages.

29

5. PHOTOCATALYSIS

In the recent years, photocatalytic process in aqueous suspension of semiconductor has received considerable attention in view of solar energy conversion. This photocatalytic process was achieved for rapid and efficient destruction of environmental pollutants. Upon illumination of semiconductor–electrolyte interface with light energy greater than the semiconductor band gap, electron–hole pairs (e−/h+) are formed in the conduction and the valence band of the semiconductor, respectively (Herrmann, 1999). These charge carriers, which migrate to the semiconductor surface, are capable of reducing or oxidizing species in solution having suitable redox potential. Various semiconductors have been used: TiO2, ZnO, CeO2, CdS, ZnS, etc. As generally observed, the best photocatalytic performances with maximum quantum yields are always obtained with titanium dioxide

Fig 14: The conceptual reaction path of photocatalysis over TiO2

The mechanism of photocatalysis over titanium dioxide particle was reported (Zhang and Itoh, 2006). The generated electron–hole pairs must be trapped in order to avoid recombination. The hydroxyl ions (OH−) are the likely traps for holes, leading to the formation of hydroxyl radicals which are strong oxidant agents, while the traps for electrons are adsorbed oxygen species, leading to the formation of superoxide species (O2−) which are unstable, reactive and may evolve in several ways.

30

Barakat et al. (2004) studied the photocatalytic degradation using UV-irradiated TiO2 suspension for destroying complex cyanide with a con-current removal of copper. Results revealed that free copper (10−2 M) was completely removed in 3 h. The co-existence of Cu(II) and CN− enhanced the removal efficiency of both CN− and copper; the removal (%) increased with increase of Cu:CN− molar ratio reaching a complete removal for both copper and cyanide at a ratio of 10:1 .

Table 5: The main advantages and disadvantages of the various physico-chemical methods for treatment of heavy metal in wastewater.

 

Treatment

 

Advantages

 

 

 

Disadvantages

 

 

method

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1

Chemical

 

Low capital cost, simple operation

Sludge

generation,

extra

 

precipitation

 

 

 

 

 

operational cost for

sludge

 

 

 

 

 

 

 

disposal

 

 

 

 

 

 

 

 

 

2

Adsorption

with

Low-cost,

easy

operating

Low selectivity, production of

 

new adsorbents

conditions, having wide pH range,

waste products

 

 

 

 

high metal-binding capacities

 

 

 

 

 

 

 

 

 

 

 

3

Membrane

 

Small space

requirement,

low

High operational cost due to

 

filtration

 

pressure,

high

separation

membrane fouling

 

 

 

 

selectivity

 

 

 

 

 

 

 

 

 

 

 

 

4

Electrodialysis

 

High separation selectivity

 

High operational cost due to

 

 

 

 

 

 

 

membrane fouling and energy

 

 

 

 

 

 

 

consumption

 

 

 

 

 

 

 

5

Photocatalysis

 

Removal of metals and organic

Long duration time,

limited

 

 

 

pollutant simultaneously,

less

applications

 

 

 

 

harmful by-products

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

31

CONCLUSION

Although many techniques can be employed for the treatment of wastewater laden with heavy metals, it is important to note that the selection of the most suitable treatment for metal-contaminated wastewater depends on some basic parameters such as pH, initial metal concentration, the overall treatment performance compared to other technologies, environmental impact as well as economics parameter such as the capital investment and operational costs. Finally, technical applicability, plant simplicity and cost-effectiveness are the key factors that play major roles in the selection of the most suitable treatment system for inorganic effluent. All the factors mentioned above should be taken into consideration in selecting the most effective and inexpensive treatment in order to protect the environment

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REFERENCES

New trends in removing heavy metals from industrial wastewater M.A. Barakat

Removing Heavy Metals from Wastewater David M. Ayres et al

Bioremediation of industrial toxic metals with gum kondagogu(Cochlospermum gossypium): A natural carbohydrate biopolymer V T P Vinod1 and R B Sashidhar2

removal of heavy metals from water with microalgal resins

IonExchange-forheavymetalremoval by Wastech

INTERNATIONAL JOURNAL OF ENVIRONMENTAL SCIENCES Volume 2, No 2, 2011

Cyanide and heavy metal removal Steven A. Holtzman 1994

Removal of Arsenic in Water Using Polypyrrole and its Composites Hossein Eisazadeh

Simultaneous heavy metal removal mechanism by dead macrophytes

Patricia Miretzky, Andrea Saralegui, Alicia Ferna´ndez Cirelli

‘Is That a Banana in Your Water’ Anne Minard

Removal of heavy metals from wastewater by membrane processes: a comparative study Abu Qdaisa and Moussab, 2004

Removal of Zn, Cd, and Pb ions from water by Sarooj clay Abu-Eishah, 2008

Microbial and plant derived biomass for removal of heavy metals from wastewaterAhluwalia and Goyal, 2006

Biosorption of Cu(II) from aqueous solution by Fucus serratus: surface characterization and sorption mechanisms Ahmady-Asbchin et al., 2008

Adsorption studies on Citrus reticulata (fruit peel of orange) removal and recovery of Ni(II) from electroplating wastewaterAjmal et al., 2000

Cr(VI) removal from synthetic wastewater using coconut shell charcoal and commercial activated carbon modified with oxidizing agents and/or chitosan, Babel and Kurniawan, 2004

Adsorption behavior of copper and cyanide ions at TiO2–solution interface, Barakat, 2005

Adsorption of heavy metals from aqueous solutions on synthetic zeolite, Barakat, 2008a

Hybrid flotation—membrane filtration process for the removal of heavy metal ions from wastewater, Bloocher et al., 2003

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