Upload
others
View
5
Download
0
Embed Size (px)
Citation preview
1
CHAPTER 1
INTRODUCTION
1.1 INTRODUCTION TO HEAVY METALS
Cities and metropolis with uncontrolled growth of population
produce large quantities of waste water containing toxic compounds in
relatively small and confined areas either during the production, storage,
transport, use or disposal of industrial products, thus interfering with the eco
system. Among these compounds, heavy metals are considered as hazardous
pollutants due to their toxicity even at low concentration and their
nonbiodegradability. Increased industrialization and discharge of wastes has
resulted in an unprecedented increase in heavy metal influx into natural water
bodies. The alarmingly high levels of heavy metals in natural water bodies
pose a serious threat to human health, living resources and ecological
systems. The removal of toxic metal ions from waste water is an important
and widely studied research area in water treatment. It is, therefore, essential
to reduce the heavy metal concentration in effluents before they are
discharged into the water bodies. Therefore, in research priority is given to
regulating these pollutants at the discharge level. In recent years, the issues
regarding disposal and treatment of effluent containing heavy metal pollutants
has become a rising concern to the public. Several attempts have been made
for their elimination, the objective being to design an effective and economic
process.
2
Heavy metals include all metals with an atomic weight greater than
23 and a specific gravity of more than 5. The groups of transition and post
transition elements are referred to as heavy metals, which include cadmium,
chromium, copper, manganese, nickel, mercury, lead and zinc and the
metalloids arsenic and selenium. Most of these elements are classified as
priority pollutants by the United States Environmental Protection Agency
(USEPA) and have been grouped under dangerous substances by the
Commission of European Communities (CEC) (Athar and Vohra 1995).
These metallic elements are an intrinsic component of the
environment, and a variety of natural processes are responsible for their
widespread occurrence at trace levels in various parts of the biosphere.
Regardless of the source of generation, all heavy metals finally end up in the
surface and groundwater.
So, investigations are needed to device methods, which are
inexpensive, simple, easy to operate and maintain, for the removal of heavy
metals from waste water. Among the commonly used methods, adsorption is
the most effective and widely employed method to treat waste water
containing different heavy metals.
Considerable attention has been devoted to the study of removal of
heavy metals from synthetic waste water by the adsorption phenomenon using
adsorbents under optimum operating conditions.
1.2 SOURCES OF HEAVY METALS
Heavy metal pollution is mainly from five different sources through
which the metals and their ions come into the environment (Manivasakam
1987).
3
1. Geological weathering.
2. Industrial processing of ores, metals and metal compounds
3. Use of metal, metal components and compounds.
4. Animal and human excretions which contain heavy metals and
5. Leaching of metals from garbage and solid waste dumps.
Sometimes ground water may have a high concentration of heavy
metals depending on the lithosphere. Domestic waste water also contributes to
heavy metal concentration.
1.3 METHODS FOR THE REMOVAL OF HEAVY METALS
A number of specialized processes have been developed for the
removal of heavy metals from waste water (Manivasakam 1987, Calace et al
2003). These include chemical precipitation, coagulation-flocculation, electro
coagulation, cementation, ion exchange, membrane process, electro-flotation,
concentration, adsorption, absorption, electro-deposition, reverse osmosis,
solvent extraction, ion exchange process, evaporative recovery and biological
treatment.
1.4 ADVANTAGES OF ADSORPTION OVER OTHER METHODS
Conventional treatment technologies such as precipitation and
coagulation are the most widely used methods for removing heavy metals, as
insoluble hydroxide at alkaline pH or sometimes as sulphides. A major
problem with this type of treatment is the disposal of the precipitated
hydroxide (Wang and Chen 2009). These technologies are also less effective
and more expensive when situations involving high volumes but low metal
concentration (typically < 50 mg L-1) are encountered (Salinas et al 2000). Ion
exchange treatment which is the second most widely used method for heavy
metals removal does not appear to be practicable because it is not cost
effective (Ajmal et al 1998).
4
Chemical techniques such as ion exchange and solvent extraction
lack sufficiently high affinity and selectivity to reduce residual metal ion to
the level dictated by the government regulation. Techniques such as
membrane process, electrochemical treatment and evaporative recovery are
limited in their use due to their high operational and maintenance cost,
process complexity and low efficiency of heavy metal removal (Kapoor and
Viraraghavan 1995).
Cost effectiveness and adsorption properties are the main criteria
for choosing an adsorption process to remove heavy metals from aqueous
solution. Adsorption process has higher selectivity. Adsorption based
processes offer a more reliable and efficient removal of complex inorganic
and organic metals than many other conventional treatment methods. The
adsorption process achieves higher removal levels in a wide range of solution
conditions and generally reduces the quantity of sludge or solid residuals that
need to be disposed (Smith 1996).
The adsorption phenomenon has still been found to be
economically appealing for the removal of toxic metals from waste water by
choosing adsorbents under optimum operating conditions. Therefore, the
adsorption process is reported to be the best method for removal of metal ion
(Karabulut et al 2000, Cao et al 2010).
1.5 ADSORPTION PROCESS
Adsorption (Slejko 1985, Suzuki 1990, Noll et al 1992) involves
the preferential partitioning of substances from the gaseous or the liquid phase
accompanied by its accumulation or concentration onto the surface of a solid
substrate. The adsorbing phase is the adsorbent, and the material concentrated
or adsorbed at the surface of that phase is the adsorbate.
5
1.5.1 Gas Phase Adsorption
This is a condensation process where the adsorption forces
condense the molecules from the bulk phase within the pores of the adsorbent.
The driving force for adsorption is the ratio of the partial pressure and the
vapor pressure of the compound.
1.5.2 Liquid Phase Adsorption
The molecules go from the bulk phase to being adsorbed in the
pores in a semi-liquid state. The driving force for adsorption is the ratio of the
concentration to the solubility of the compound.
1.5.3 Adsorption Mechanisms
Adsorption processes are classified as either physical or chemical.
Physical adsorption occurs when the Vanderwaals forces bind
the adsorbing molecule onto the solid substrate; these intermolecular
forces are the same as the bond molecules to the surface of a liquid. It follows
that heats of adsorption are comparable in magnitude to latent heats (10 to
70 KJ mol-1). Species that are physically adsorbed to a solid can be released
by applying the heat; the process is reversible. An increase in temperature
causes a decrease in adsorption efficiency and capacity.
Chemical adsorption occurs when covalent or ionic bonds are
formed between the adsorbing molecules and the solid substrate. This
bonding leads to a change in the chemical form of the adsorbed compounds,
and is therefore not reversible. The bonding forces for chemical adsorption
are much greater than for physical adsorption. Thus, more heat is liberated.
With chemical adsorption, higher temperatures can improve performance.
6
1.6 LITERATURE REVIEW
Major trace elements generally present in various industrial
effluents (Dean et al 1972) are listed in Table 1.1. Sources, drinking water
standards and potential health effects (David and Bela 1997) of some widely
used heavy metals are given in Table 1.2.
Table 1.1 Major trace elements in industrial effluents
S.
No.Industry As Cd Cr Cu Fe Hg Mn Pb Ni Se Sn Zn
1. Paper and pulp - - X X - X - X X - - X
2. Organic chemicals - X X - X X - X - X X X
3. Inorganicchemicals
- X X - X X X - - X X X
4. Fertilisers X X X X X X X X X X - X
5. Petroleum refining - X X X X - - X X - - X
6. Metal finishing - X X X - X - - X X - -
7. Textile millproducts
- - X - - - - - - - - -
8. Leather tanningand finishing
- - X - - - - - - - - -
9. Pesticides X - - - - X - X - X - X
10. Paints and dyes - X X X - X - X - X - -
11. Batteries - X - - - X - X - X - X
12. Electrical andElectronics
- X - X - X - X - X - -
13. Explosives X - - X - X - X - - - -
14. Mining andmetallurgy
X X X X - X - - - X - X
15. Pharmaceuticals X - - - - - - - - - - X
X – Presence of heavy metals in industries.
Source: Trivedy, R.K. “Pollution management in industries”, Environmental Publication,
Karad, India, pp. 170, 1989.
7
Table 1.2 Sources, Drinking Water Standards and Potential Health
Effects of Heavy Metals
S.
No.Elements Sources
Max conta -
mination level
(mg L-1
)
Effects
1. Antimony Industry, type setting,enameled ware
0.006Shortens life span, heartdisease.
2. Arsenic Leaching, Weatheringprocess, Volcaniceruption
0.01Dermal pigments, Skinand lung cancer,Vascular diseases.
3. Barium Natural mineraldeposits, Oil/gasdrilling operation,Paints etc.
2.0
Affects circulatorysystems
4. Beryllium Coal combustion,Nuclear power plant,Rocket fuel, Ceramicunit
0.004
Acute and chronicrespiratory diseases, lungcancer, beryllosis.
5. Cadmium Coal combustion,Plating, Phophaticfertilizers, Water pipe,Tobacco smoke etc.
0.005
Cardiovasculardiseases,cancer,hypertension.
6. Chromium Anodising, Coolingtowers, Dye,Electroplating,inks,Paint, Tanning etc.
0.1
Cancer
7. Cobalt Alloys, Steel, Electro -plating,Glass enameletc.
0.05Cancer
8. Copper Paper and pulp,Electrical goods,Utensils, Electronics,Chemicals.
1.0
Cancer(suspected)
9. Iron Steel, Machinery, Dye,Textile, Medicines etc.
0.3Cancer(suspected)
8
Table 1.2 (Continued)
S.
No.Elements Sources
Max conta -
mination level
(mg L-1
)
Effects
10. Lead Battery industry, Autoexhausts, Paints etc
0.05
Affects nervous and renalsystem, head ache, braindamage, cancerconvulsion, blue linealong gums.
11. Manganese Metal alloys, Powerplants, Gasoline. 0.05
Nerve damage ( in traceamounts essential tohuman)
12. Mercury Chlo-alkali industry,Coal combustion,Electrical batteries etc
0.002Nerve damage,kidneyand brain damage, death.
13. Nickel Coal, Diesel oil, Metalplating, Steel and nonferrous alloys, Tobaccosmoke etc
0.1
Lung cancer, affectsrespiratory system.
14. Selenium Coal and oilcombustion, Glass andsulphur industry, Paperindustry etc.
0.05
Carcinogenic, causesdental caries (essential toman in trace amounts)
15. Zinc Galvanizing, alloys,rayon, paper industryetc.
5.0Cancer (suspected)
1.6.1 Heavy Metals
Several research studies of heavy metals adsorption onto various
adsorbents have been published. Several adsorbents such as sawdust (Ajmal
et al 1998), silica and iron oxide (Subramaniam et al 2003), sewage sludge
ash (Pan et al 2003), anatase type titanium dioxide (Kim et al 2003), olive
mill residues (Veglio et al 2003), inorganic colloids (Subramaniam and
9
Yiocoumi 2001), blast furnace sludge (Lopez et al 1998), functionalized silica
(Bois et al 2003), red mud and fly ashes (Apak et al 1998, Cho et al 2005),
peat (Gosset et al 1986), paper mill sludge (Calace et al 2003), activated
carbon (Goyal et al 2001, Monser and Adhoum 2002), calcined phosphate
(Aklil et al 2004), rice bran (Montanher et al 2005), vermiculite (Malandrino
et al 2006), anaerobic granular biomass (Hawari and Mulligan 2006), tea
waste (Amarasinghe and Williams 2007), poplar wood sawdust (Sciban et al
2007), commercial activated carbon (Srivastava et al 2008), activated carbon
developed from walnut, hazelnut, almond, pistachio shell, and apricot stone
(Kazemipour et al 2008), activated carbon from hazelnut husks (Imamoglu
and Tekir 2008), alumina (Mahmoud et al 2010), chitosan (Wu et al 2010), fly
ash (Ahmaruzzaman 2010) have been used for metal adsorption.
Petrov et al (1992) studied the adsorptive removal of several metal
ions such as Zn, Cd, Pb and Cu from aqueous solution on modifiedanthracite
prepared by thermal oxidation of anthracite in flowing air. The metal uptake
increased with increasing pH of the solution. The uptake was found to be only
slight at solution pH of 1 but it increased considerably in the pH range 3-4.
The oxidation of the carbon enhanced the uptake of the metal ions because of
the creation of oxygen surface groups on the anthracite surface. The presence
of the electrolyte in the solution decreased the uptake of metal ions.
Cheng and Wang (2000) studied the removal of Cu, Zn and Pb
from synthetic waste water using fixed bed granulated activated carbon. They
pretreated the carbon with deionised water and carried out the adsorption of
single-species (Cu, Zn, and Pb) and multi-species (Cu–Zn, Cu–Pb, and Cu–
Pb–Zn) metal ions. It was demonstrated that the breakthrough occurred more
slowly with an increase in the influent pH and a decrease in the flow rate.
Experiments on competitive adsorption illustrated that the removal of metal
ions decreased when additional metal ions were added.
10
The use of adsorbent produced by the chemical treatment of locally
available clay for the removal of some heavy metals from waste water has
been investigated by Vengris et al (2001). The modification of natural clay
was achieved by treating it with hydrochloric acid and subsequent
neutralisation of the resultant solution with sodium hydroxide. Acidic
treatment led to the decomposition of the montmorillonite structure. The
uptake capacity of the modified clay for nickel, copper and zinc increased
significantly. Batch and column sorption methods enabled the removal of
nickel, copper and zinc ions till the permissible sewerage discharge
concentration. They found that the breakthrough point at a flow rate of 2 mL
min-1 for copper ions occurred earlier than that for nickel and zinc. Also the
column uptake capacity at 40% breakthrough for nickel and zinc amounted to
1.15 and 0.92 meq g-1, respectively, and 0.75 meq g-1 for copper at 50%
breakthrough. The sorption process was reflected using the Langmuir-type
isotherm. The desorption rate of nickel, copper and zinc by water at pH 5 was
negligible.
Monser and Adhoum (2002) studied the removal of Cu, Zn, Cr and
CN from waste water onto modified activated carbon, which have tetra butyl
ammonium iodide and sodium diethyl dithio carbamate immobilized on their
surface. They found that the tetra butyl ammonium iodide – carbon adsorbent
had effective removal capacity approximately five times that of untreated
carbon. The sodium diethyl dithio carbamate – carbon column had an
effective removal capacity for Cu (4 times), Zn (4 times) and Cr (2 times)
greater than the untreated carbon.
Bayat (2002) compared two different Turkish fly ashes (Afsin-
Elbistan and Seyitomer) for their ability to remove nickel, copper and zinc
from aqueous solutions. The equilibrium time was found to be 2 h for all
metals on both the fly ashes. The maximum metal removal was found to be
11
pH 6 for Cu, pH 7 for Zn and pH 8 for Ni. The effectiveness of fly ash as an
adsorbent improved with increasing calcium (CaO) content. The adsorption
data fitted the Langmuir isotherm better than the Freundlich isotherm. He
found that the fly ash with high calcium content (Afsin-Elbistan) was as
effective as a metal adsorbent as activated carbon and, therefore, were good
prospects as adsorbents for these metals.
Wang et al (2006) observed that the fly ash modified by
hydrothermal treatment using NaOH solutions had better adsorbing capacity
for heavy metals and dyes. The XRD profiles revealed a number of new
reflexes, suggesting that a phase transformation had probably occurred. Both
heat treatment and chemical treatment increased the surface area and pore
volume. They found that the removal efficiency for copper and nickel ions
ranged from 30% to 90% depending on the initial concentrations. Also
increased adsorption temperatures enhanced the adsorption efficiency of both
the heavy metals.
Srivastava et al (2008) used activated carbon of commercial grade
for the adsorption of Cd, Ni and Zn, and found the BET surface area to be
171.05 m2 g-1. They found the adsorption to be a gradual process and that the
quasi-equilibrium condition was reached in 5 h. The effective diffusion
coefficient was of the order of 10 12 m2 s-1.
Poplar wood sawdust as a adsorbent was examined by Sciban et al
(2007) for the removal of copper, zinc and cadmium from electroplating
waste water. Langmuir, Freundlich, BET and competitive Langmuir (two
competing ions) isotherms were fitted to the experimental data and the
goodness of fit for adsorption was compared. The shapes of the isotherms
obtained fitted well with the multilayer adsorption. The adsorption of Cu ions
from the mixture (in waste water) was better than that from a single metal
solution. The adsorptions of Zn from waste water and from model water were
12
approximately equal, while that of Cd was significantly lower from the waste
water than from the model water. The selectivity of the sawdust for metal ion
adsorption was as follows: Cu > Zn > Cd.
Kazemipour et al (2008) investigated the adsorption of Cu, Zn, Pb,
and Cd that exist in industrial waste water onto the carbon produced from
nutshells of walnut, hazelnut, pistachio, almond, and apricot stone. All the
agricultural shells and stones used were ground, sieved to a defined size
range, and carbonized in an oven. The time and temperature of heating were
optimized at 15 min and 800oC respectively, to reach maximum removal
efficiency. The experiments were carried out using columns filled with a
predetermined amount of the adsorbent. The removal efficiency was
optimized based on the initial pH, flow rate, and dose of adsorbent. They
found that the maximum removal occurred at pH 6–10, flow rate of 3 mL min-1
and 0.1 g of the adsorbent. The adsorption capacity of the carbon decreased
on repeated use. They also studied the efficiency of the carbon to remove the
cations from real waste water produced by copper industries. The findings
showed that the removal efficiencies were much more in real samples.
1.6.2 Nickel, Copper and Zinc
The present work concentrates on the removal of heavy metals such
as copper, nickel and zinc from aqueous solutions and waste waters.
1.6.2.1 Nickel
Nickel may be found in waste water discharges from mining,
electroplating, pigments and ceramic industries, battery and accumulator
manufacturing (Parab et al 2006). Nickel is toxic to a variety of aquatic
organisms, even at very low concentration. An uptake of large quantities of
nickel may lead to higher instances of cancer, lung embolism, respiratory
failure, birth defects, asthma and chronic bronchitis, severe damage to kidney,
13
gastrointestinal distress (e.g. nausea, vomiting, diarrhea), allergic reactions
such as skin rashes and heart disorders. Exposure to nickel and its compounds
may result in the development of a dermatitis known as “nickel itch” in
sensitive individuals (USEPA 1986, ATSDR 1997). The properties of nickel
are summarized in Table 1.3.
Table 1.3 Properties of nickel
Properties Values
Atomic number 28
Atomic weight 58.71 g mol-1
Melting point 1453 C
Boiling point 2913 C
Density 8.908 g cm-3 at 20 C, 7.81 g cm-3 at meltingpoint
Electronegativity 1.91 (Pauling); 1.75 (Allred Rochow)
Ionic radius 0.69Å (Ni2+); 0.6Å (Ni3+)
Atomic radius 1.24Å
Covalent radius 1.15Å
Vapour pressure 237 Pa (at melting point)
Thermal conductivity 0.907 W (cm K)-1 (298 K)
Electrical resistivity 6.97x10-6 ohm cm (20 C)
Specific heat 0.44 J (g K)-1 (298 K)
HFusion 17.48 kJ mol-1
HVap 377.5 kJ mol-1
Energy of first ionization 735 kJ mol -1
Energy of second ionization 1753 kJ mol -1
Energy of third ionization 3387 kJ mol -1
The EPA stipulates that nickel in drinking water should not exceed
0.04 mg L-1 (Sheng et al 2004). In India, the acceptable limit of Ni in drinking
water is 0.01 mg L 1 and 2.0 mg L 1 for discharge of industrial waste water.
14
(Sharma et al 1992). Maximum contaminant limit for nickel in bottled water
has been fixed as 0.05 mg L-1 by European Economic Community (Demirbas
et al 2002). Hence, it is essential to remove Ni from industrial waste water
before releasing it into natural water sources.
A number of workers have used different adsorbent systems,
developed from various industrial waste materials, for the removal of Ni. Rice
hull (Suemitsu et al 1986), sphagnum peat (Viraraghavan and Drohamraju
1993), peat moss (Lo et al 1995), tea factory waste (Malkoc and Nuhoglu,
2005), blast furnace slag (Dimitrova 1996), apple waste (Maranon and Sastre
1991), peanut hull carbon (Periasamy and Namasivayam 1995), coir pith
activated carbon (Kadirvelu et al 2001), hazelnut shell activated carbon
(Demirbas et al 2002), bagasse fly ash (Gupta et al 2003), clay based beds
(Marquez et al 2004), saw dust (Shukla et al 2005), activated carbon from
waste apricot (Erdogan et al 2005), protonated rice bran (Zafar et al 2007),
Na-modernite (Wang et al 2007) have been investigated to remove Ni from
waste water. They all observed that a decrease in the adsorbent concentration
with constant Ni concentration, or an increase in the Ni concentration with
constant adsorbent concentration resulted in a higher nickel uptake per unit
weight of adsorbent. The effect of other operating variables, viz., solution pH,
temperature, particle size, etc., on the removal of nickel have been studied and
sorption characteristics have been evaluated using Freundlich, Langmuir,
Temkin and Dubinin–Radushkevich (D-R) adsorption isotherms.
Kadirvelu et al (2001) prepared activated carbon from coirpith by
chemical activation for the removal of Ni from aqueous solution. The specific
surface area was found to be 592 m2 g-1. They recovered Ni after adsorption
by treatment with HCl and confirmed the adsorption mechanism to be ion
exchange.
15
Gupta et al (2003) studied the removal of nickel from waste water
using bagasse fly ash, an industrial solid waste of the sugar industry. The
maximum adsorption of nickel occurred at a concentration of 12 mg L-1 and
90 % removal of nickel was possible in about 80 min. They found that the
maximum adsorption of nickel occurred at a pH value of 6.5 and the
adsorption data followed the Langmuir model better than the Freundlich
model. They also observed the adsorption process to be endothermic.
Gezici et al (2005) used the sodium form of insolubilized humic
acid as the solid phase in a column. Column operations were performed and
all of them were monitored continuously using a flow through cell-adapted
UV-Vis spectrophotometer. Sorption characteristics were evaluated using the
Freundlich, Langmuir, and Dubinin–Radushkevich (D-R) adsorption
isotherms, as well as by Scatchard plot analysis. The multilayer sorption was
found to be agreeable for Ni. From the D-R isotherm the mean free energy of
sorption (E) was calculated to be 6.65 kJ mol 1.
The potential to remove Ni from aqueous solutions using Na-
mordenite, a common zeolite mineral, was investigated by Wang et al (2007).
The maximum sorption capacity was found to be 5.324 mg g-1 at pH 6, the
initial concentration of 40 mg L-1 at temperature of 20oC. The activation
energy (Ea) was found to be 12.465 kJ mol-1 indicating a chemical sorption
process involving weak interactions between the sorbent and the sorbate.
They also observed that Ni adsorption by the Na-mordenite was not
completely attributed to ion exchange and when compared to other
adsorbents, Ni showed a lower affinity towards the clay mineral adsorbents.
Two Portuguese natural ball-clays named ZA-4 and NC were used
as bed filters by Marquez et al (2004) for the removal of Ni and they
compared the results with that of a commercial grade granular activated
carbon. They found that clay based materials showed higher removal
16
efficiency when compared to granular activated carbon (GAC). Higher cation
exchange capacity and development of surface negative charge on the clay
particles in contact with water also contributed to this promising performance,
despite the lower available specific surface area in comparison with granular
activated carbon.
Demirbas et al (2002) carried out the adsorption of Ni using
activated carbon prepared from hazelnut shell. They found that a contact time
of 180 min was required to reach equilibrium. The equilibrium data were
analysed using the Langmuir, Freundlich and Temkin isotherms.
The rice bran in its acid treated (H3PO4) form was used as a low
cost adsorbent for the Ni removal by Zafar et al (2007). The adsorption
characteristics of nickel on protonated rice bran were evaluated as a function
of pH, biosorbent size, biosorbent dosage, initial concentration of nickel and
time. Within the tested pH range (pH 1–7), they found that protonated rice
bran displayed more resistance to pH variation, retaining up to 102 mg g-1 of
the nickel binding capacity at pH 6. Kinetic and isotherm experiments were
carried out at the optimal pH 6. The equilibrium adsorption data fitted better
to the Langmuir adsorption isotherm model. The order of magnitude of the
Go values indicated an ion-exchange physiochemical sorption process.
Erdogan et al (2005) prepared activated carbon from waste apricot
using K2CO3 as the activating agent for Ni adsorption from synthetic waste
water. The activation temperature was varied in the temperature range of 400–
900oC and the N2 atmosphere was used with 10oC/min heat rate. They
observed the maximum surface area (1214 m2 g-1) and the micropore volume
(0.355 cm3 g-1) at 900oC. The adsorption parameters were determined using
the Langmuir model. The optimal conditions were determined to be; pH 5, 0.7
g (10 mL)-1 adsorbent dosage, 10 mg L-1 initial Ni concentration and 60 min
contact time.
17
1.6.2.2 Copper
Copper and its compounds are ubiquitous in the environment and
are thus found frequently in surface water. It is also a micronutrient in
agriculture and can, therefore, accumulate in surface waters. Copper bearing
mining wastes and acid mine drainage, discharge significant quantities of
dissolved copper into the waste water. The additional potential sources of
copper bearing waste include electrical apparatus, smelting, metal
electroplating baths, alloy industries, plumbing, roofing and building
construction, gasoline additive, cable covering ammunition, battery industries,
fertilizer industry, paints and pigments, municipal and storm run off
(Buchauer 1973, Dean et al 1972). The excessive intake of copper by man
leads to (WHO 1984) severe irritation of the nose, mouth and eyes, causes
headaches, dizziness, vomiting, diarrhea and widespread capillary damage,
brain damage, hepatic and renal damage and central nervous problems
followed by depression.
Though the maximum permissible concentration by the Indian
Council of Medical Research (ICMR), WHO, USPHS are 3.0 mg L-1, 1.5 mg
L-1 and 1.0 mg L-1 respectively, the maximum recommended concentration of
Cu2+ in drinking water by these agencies is 1.0 mg L-1 (Rao 1992).
Consequently, it is essential that potable waters be given some treatment to
remove copper before domestic use. As copper (Cu2+) is a highly toxic
element, the removal of Cu2+ from waste water has been the subject of many
studies. The properties of copper are summarized in Table 1.4.
Goyal et al (2001), Ajmal et al (1998), Larous et al (2005), Kim et
al (2003), pan et al (2003), Sebe et al (2004), Huang (2007), Subramaniam et
al (2003), Kahn and Khattak (1992), Low et al (1995), Mugisidi et al (2007),
Aman et al (2008), Demirbas et al (2008) studied the removal of Cu by
adsorption using adsorbents such as activated carbons, sawdust, anatase type
18
titanium dioxide (photocatalyst), sewage sludge ash, eelgrass, waste iron
oxide, silica and iron oxide, carbon black spheron – 9, coconut husk, modified
activated carbon, potato peel, hazelnut shell etc. They all found that the
percentage adsorption increases with increasing contact time, carbon dosage,
pH, temperature and decreases with an increase in the initial copper
concentration. Most of them used the Langmuir and Freundlich adsorption
isotherm to describe the adsorption process.
Table 1.4 Properties of copper
Properties Values
Atomic number 29
Atomic weight 63.546 g mol-1
Melting point 1083 C
Boiling point 2595 C
Density 8.95 g cm-3 at 20 C, 7.94 g cm-3 at meltingpoint
Electronegativity 1.90 (Pauling); 1.75 (Allred Rochow)
Ionic radius 0.96Å (Cu+); 0.73Å (Cu2+); 0.69Å (Cu3+)
Atomic radius 1.278Å
Covalent radius 1.17Å
Vapour pressure 5.05x10-2 Pa (at melting point)
Thermal conductivity 4.01 W (cm K)-1 (298 K)
Electrical resistivity 1.675x10-6 ohm cm (20 C)
Specific heat 0.3845 J (g K)-1 (298 K)
HFusion 13 kJ mol-1
HVap 306.7 kJ mol-1
Energy of first ionization 743.5 kJ mol -1
Energy of second ionization 1946 kJ mol -1
19
Goyal et al (2001) carried out adsorption studies in the
concentration range of 40 – 1000 mg/l. They found that at pH greater than 6,
the adsorption studies could not be carried out because of the precipitation of
copper hydroxide. Also, the uptake of Cu (II) ions by granulated activated
carbon and activated carbon fibers was influenced by the presence of acidic
surface groups. They found that the increase in adsorption on oxidation
depends on the nature of oxidative treatment while the decrease in adsorption
on degassing depends on the temperature of degassing.
Ajmal et al (1998), while studying the role of sawdust in the
removal of Cu(II) ions from a solution concentration of 17.054 mg (100 mL)-1,
found that the maximum adsorption occurred at pH 6. They observed that
total adsorption decreased with an increase in temperature at low
concentration and a reversal was observed in the adsorption capacity at higher
concentrations where the total adsorption increased with the temperature. The
effect of salinity on the adsorption of Cu was also tested and it was found that
the presence of NaCl in the range of 0.25 – 5.0 g (50 mL)-1 reduced the
adsorption of Cu from 81 to 10 %.
Kim and co-workers (2003) used anatase type titanium dioxide
photocatalyst particles for the adsorption of Cu (II) from an aqueous solution
of concentration 10 mg L-1. They observed that the pH value of the solution
changed to acidic pH during adsorption. The adsorption rate was rapid with
an increasing number of UV lamps of 254 nm.
Pan et al (2003), while studying the removal of Cu (II) ions from
waste water using Sewage Sludge Ash (SSA) observed that the chemical
composition of SSA was similar to that of fly ash. The precipitation of copper
hydroxide occurred when the equilibrium pH of waste water was above 6.2
and this observation was in agreement with the findings of Gilles Sebe et al
(2004).
20
Huang et al (2007) used waste iron oxide as a low cost adsorbent
for the treatment of waste water containing copper. XRD and SEM were used
to characterize the iron oxide material. The adsorption capacity was found to
be 0.21 mmol g 1 for 0.8 mmol dm 3 initial Cu2+ concentrations at pH 6.0 and
300 K. They observed that the adsorption data were well described by the
Freundlich model and the adsorption process to be endothermic.
Kahn and Khattak (1992) studied the removal of Cu(II) from
CuSO4 solution on carbon black spheron – 9 and observed that the adsorption
equilibrium was established within 1 h in the concentration range of
10 – 1000 ppm. The data was found to obey Langmuir and Freundlich
adsorption isotherms except for adsorption at higher pH values where
precipitation was thought to take place. The removal of Cu, Pb and Zn from
aqueous solutions by synthetic geolite was measured as a function of pH at
several temperatures. It was found that the adsorption was closely related to
cation hydrolysis.
Low et al (1995) examined the ability of coconut husk and its
reactive dye coated forms for the removal of copper from aqueous solution
and found that the dye coating of the husk enhanced the removal of copper.
They also found that the Langmuir isotherm fitted the equilibrium data for
both natural and dye-coated husk-Cu systems.
Mugisidi et al (2007) modified the activated carbon from coconut
shell using sodium acetate at concentrations of 10 % and 15 %, and used it in
a fixed-bed column to study the adsorption of copper ions. Synthetic waste
water containing 258 mg L-1 of Cu was passed through plain activated carbon
and modified activated carbon. The highest adsorption capacity was found for
the activated carbon modified by treatment with 15% sodium acetate, which
adsorbed 45 mg of Cu; that is 2.2 times as much as the untreated activated
carbon. After regeneration with 0.71M NaOH, they found that the activated
21
carbon modified by treatment with 15% sodium acetate was able to adsorb
60 mg of Cu; that is three times as much as the untreated activated carbon.
Aman et al (2008) used potato peels for the removal of Cu from
waste water. The optimum pH for adsorption onto potato peels charcoal was
found to be 6.0. They also observed that the rejuvenated material retained its
efficiency to adsorb copper during 5 repeated cycles.
Demirbas et al (2008) carried out the removal of copper from
aqueous solution and found that the surface of hazelnut shell exhibits negative
zeta potential value at all studied pH values. They also observed that the
hazelnut shell had no isoelectrical point in the studied pH range and the
adsorption was endothermic.
1.6.2.3 Zinc
Zinc is present in air, soil, water, and in almost all types of food.
Zinc is naturally released into the environment; however industrial activities
are mostly responsible for zinc pollution. Elevated levels of zinc come from a
variety of sources like mining and foundry activities, zinc, lead, and cadmium
refining, steel production, carbon combustion, and solid waste incineration.
Zinc is commonly used to coat iron and other metals for the prevention of
oxidation. Various zinc salts are industrially used in wood preservatives,
catalysts, photographic paper, accelerators for rubber vulcanization, ceramics,
textiles, fertilizers, pigments, and batteries (USDHHS, 1993). Water
reservoirs are contaminated by the run-off from these industries. Other
sources of metallic zinc traces in drinking water are water treatment processes
and pick-up of metallic ions during storage/distribution. The properties of
zinc are summarized in Table 1.5.
22
Table 1.5 Properties of zinc
Properties Values
Atomic number 30
Atomic weight 65.38 g mol-1
Melting point 419.5 C
Boiling point 907 C
Density 7.11 g cm-3 at 20 C
Electronegativity 1.65 (Pauling); 1.66 (Allred Rochow)
Ionic radius 0.74Å (Zn2+)
Atomic radius 1.38Å
Covalent radius 1.25Å
Vapour pressure 19.2 Pa (at melting point)
Thermal conductivity 1.16 W (cm K)-1 (298 K)
Electrical resistivity 6.024x10-6 ohm cm (20 C)
Specific heat 0.39 J (g K)-1 (298 K)
HFusion 7.28 kJ mol-1
HVap 114.2 kJ mol-1
Energy of first ionization 904.5 kJ mol -1
Energy of second ionization 1723 kJ mol -1
Energy of third ionization 3831 kJ mol -1
Zinc is not biodegradable and travels through the food chain via
bioaccumulation. Zinc causes various health problems, such as stomach
cramps, skin irritations, vomiting, nausea, anaemia, accumulative poisoning,
cancer, brain damage, etc. (Burrell 1974, Berman 1980). Very high levels of
zinc can damage the pancreas and disturb the protein metabolism thereby
causing arteriosclerosis. Extensive exposure to zinc chloride can cause
respiratory disorders. According to few surveys from the public health
services of different countries, a significant number of people have been
23
exposed to the hazards of excess metals in the municipal water supplies
(WHO 1971). Therefore, there is significant interest regarding zinc removal
from waste waters and its toxicity for humans at levels of 100–500mg (day)-1.
World Health Organization (WHO) recommends the maximum acceptable
concentration of zinc in drinking water as 5.0 mg L-1 (Kumar et al 2006).
Agrawal et al (2004) studied the removal of zinc from aqueous
solutions using sea nodule residue, a solid waste generated during the
processing of polymetallic sea nodules for copper, nickel, and cobalt
recovery. About 2.0 g of SNR was found to be sufficient to remove 99.8% of
200 mg L 1 zinc from 100 mL aqueous solution in 4 h, and the optimum pH
value for maximum adsorption was found to be 5.5. They also found the
adsorption of Zn to be an endothermic process and low value of the activation
energy indicated the adsorption to be physical in nature.
The factors affecting adsorption characteristics of Zn2+ on two
natural zeolites (Gordes and Bigadic zeolites) were investigated by Oren and
Kaya (2006). The results showed that the Zn2+ adsorption behavior of both
zeolites were highly dependent on the pH. The pH experiments showed that
the governing factors affecting the adsorption characteristics of all materials
was the competition of the H+ ions with Zn2+ ions (under pH 4), ion exchange
(pH 4–6), participation of zinc hydroxyl species in the adsorption and
precipitation onto the zeolite structure (pH 6–8). The results also revealed that
an increase in the initial concentration of Zn2+ in the system increased the
adsorption capacity to a degree, then it became constant at higher
concentrations. They found that the removal efficiency of Gordes zeolite was
two times higher than that of Bigadic zeolite.
Kaya and Oren (2005) carried out the adsorption of zinc from
aqueous solutions onto bentonite and found that the adsorption characteristics
of zeolites for zinc ions were very limited when compared with the natural
24
and Na-enriched bentonites. Previous investigations on bentonites showed
that the maximum adsorption capacity of Na-enriched bentonite and natural
bentonite was 54 and 24 mg g-1, respectively. Considering the zeolites, it was
6 and 3 mg g-1 for Gordes and Bigadic. The lower adsorption rates for zeolites
with respect to bentonites may be due to the difficulty in the penetration of
hydrated zinc ions into the zeolite channels. Hence, they concluded that the
adsorption may take place on the zeolite surface.
Amuda et al (2007) modified the activated carbon prepared from
coconut shell with chitosan and/or oxidizing agent (phosphoric acid) to
produce a composite adsorbent for the removal of zinc from industrial waste
water. Operational parameters such as pH, agitation time and adsorbent
concentration, initial ion concentration and particle size were also studied.
They observed that the Langmuir isotherm represented the experimental data
better than the Freundlich isotherm thus indicating the monolayer coverage of
the zinc (II) on the surface of the adsorbent. Desorption studies were carried
out with NaOH and substantial recovery of the metal was evident. It was also
suggested that the dominant sorption mechanism was ion exchange.
The dependence of Zn removal from aqueous solutions by mixed
mineral systems of kaolinite, montmorillonite, and goethite on surface charge,
surface chemistry, and the kinetic pattern of the sorbing ions over time was
investigated by Egirani et al (2005). Using an empirical model, they found
that the mineral mixing reduced the exchange of protons for adsorbing ions
and the acidity of the reactive sites, thus impeding Zn removal by proton
exchange. Based on the amount of Zn adsorbed on the mixed mineral
suspensions at ionic strength 0.01 to 0.1 M and pH 4, they suggested that Zn
removal from aqueous solution was both by inner and outer sphere
complexation. The behavior of the mixed suspensions in Zn sorption
25
suggested that different reactive sites were involved at the onset of sorption,
becoming similar to those of the single mineral components over time.
Janssen et al (2003) found that clay–Al hydroxide polymers could
bind heavy metals effectively, and played an important role in the adsorption
behavior and metal binding capacity of soils.
1.6.3 Adsorbents
Adsorbents (Noll et al 1992, Oscik et al 1982) are porous materials
that contain many miniscule internal pores. The most common industrial
adsorbents are activated carbon, silica gel, and activated alumina, because
they present enormous surface areas per unit weight. Activated carbon is
produced by roasting organic material to decompose it to granules of carbon -
coconut shell, sawdust, wood, and bone are some of the common sources.
Typical surface areas are 300 to 1500 m2 g-1. Silica gel is a matrix of
hydrated silicon dioxide. Silica is used to separate hydrocarbons. Typical
surface areas are 300 to 900 m2 g-1. Activated alumina is commonly used to
remove oxygenates and mercaptans from hydrocarbons and fluorides from
water. Typical surface areas are 200 to 400 m2 g-1. The adsorbents such as
zeolite, carbon molecular sieve, bone char, iron and manganese coated sand,
kaolinite clay, hydrated ferric oxide, activated bauxite, titanium oxide,
silicium oxide and other synthetic media are also widely used.
In the last few years many studies have focused on identifying
inexpensive but effective adsorbent material (Calace et al 2003, Kornold et al
1996, Davila et al 1992). It is quite well known that activated carbon can be
used and as a matter of fact, is recommended by the USEPA as the best
available technology to remove contaminants from water by adsorption
effectively (Gharaibeh et al 1998). Table 1.6 gives a comparison of the
26
adsorption capacity of Ni, Cu and Zn on different adsorbents taken from the
literature.
Table 1.6 Comparison of adsorption capacity of various adsorbents for
Ni, Cu and Zn
Adsorption Capacity
(mg g-1
)Adsorbent
Ni Cu Zn
Reference
A.C from apricot (AT 400oC) 17.04 - - Erdogan et al (2005)
A.C from apricot (AT 500oC) 22.47 - - Erdogan et al (2005)
A.C from apricot (AT 600oC) 19.37 - - Erdogan et al (2005)
A.C from apricot (AT 700oC) 35.59 - - Erdogan et al (2005)
A.C from apricot (AT 800oC) 32.36 - - Erdogan et al (2005)
A.C from apricot (AT 900oC) 101.01 - - Erdogan et al (2005)
Hazelnut husk A.C. (20oC) 5.757 - - Demirbas et al (2002)
Hazelnut husk A.C. (30oC) 7.181 - - Demirbas et al (2002)
Hazelnut husk A.C. (40oC) 10.109 - - Demirbas et al (2002)
Hazelnut husk A.C. (50oC) 11.64 - - Demirbas et al (2002)
Coirpith carbon 62.5 - - Kadirvelu et al (2001)
Peanut hull carbon 53.65 - - Periasamy andNamasivayam (1995)
Granular activated carbon 1.49 - - Periasamy andNamasivayam (1995)
Hazelnut husk A.C. - 6.645 - Imamoglu and Tekir(2008)
Spent activated clay - 10.9 - Weng et al (2007)
Activated poplar sawdust - 9.24 - Acar and Eren (2006)
Rubber wood sawdust A.C. - 5.729 - Kalavathy et al (2005)
Activated carbon - 3.56 - Machida et al (2005)
Activated carbon from sugarbeet pulp (AT 300 C)
- 68.03 - Ozer and Tumen(2003)
Activated carbon from sugarbeet pulp (AT 400 C)
- 71.99 - Ozer and Tumen(2003)
Activated carbon from sugarbeet pulp (AT 500 C)
- 79.99 - Ozer and Tumen(2003)
27
Table 1.6 (Continued)
Adsorption Capacity
(mg g-1
)Adsorbent
Ni Cu Zn
Reference
Activated carbon - 31.11 - Mohan and Singh(2002)
Rice hulls activated carbon - 3.92 - Teker et al (1999)
Carbon prepared from apricotstones
- 12.01 13.21 Budinova et al (1994)
Carbon prepared fromcoconut shells
- 11.10 - Budinova et al (1994)
Carbon prepared from lignitecoal
- 9.80 - Budinova et al (1994)
Carbon prepared from peanuthulls
- 89.29 - Periasamy andNamasivayam (1994)
Commercial activated carbon,India
- 2.74 - Periasamy andNamasivayam (1994)
Acid-treated coconut
shell carbon (ACSC)
- - 45.14 Amuda et al (2007)
Chitosan coated coconut shellcarbon (CCSC)
- - 50.93 Amuda et al (2007)
Chitosan coated ACSC
(CACSC)
- - 60.41 Amuda et al (2007)
Calcined phosphate - - 23.7 Aklil et al (2004)
Red mud - - 12.59 Lopez et al (1998)
Peat - - 9.28 McKay et al (1998)
Blast furnace slag - - 17.65 Gupta et al (1997)
Lignite - - 22.83 Allen and Brown(1995)
1.6.3.1 Activated carbon
Activated carbon (Noll et al 1992, David and Cooney 1987) is a
carbonaceous material which possesses a highly developed porosity, and hence a
large internal surface area. As a result of this, it is commonly used in a wide range
28
of applications, concerned principally with the removal of chemical species
by adsorption from the liquid or gas phase (Bansal et al 1988). Commercial
activated carbon has an internal surface area ranging from 500 to 1500 m2 g-1.
Related to the type of application, two major product groups exist:
Powdered activated carbon; particle size 1-150 micron
Granular activated carbon (granulated or extruded), particle size
in the 0.5-4 mm range
Activated carbon (AC) can be produced by heat treatment, or
“activation”, of raw materials such as wood, coal, peat and coconuts. During
the activation process, the unique internal pore structure is created, and it is
this pore structure, which provides activated carbon its outstanding adsorptive
properties.
Activated carbons have a number of unique characteristics: a large
internal surface area, dedicated (surface) chemical properties and good
accessibility of internal pores. According to IUPAC definitions three groups
of pores can be identified:
Macropores (above 50 nm diameter)
Mesopores (2-50 nm diameter)
Micropores (under 2 nm diameter)
Micropores generally contribute to a major part of the internal
surface area. Macro- and mesopores can generally be regarded as the
highways into the carbon particle, and are crucial for kinetics. The desired
pore structure of an activated carbon product is attained by combining the
right raw material and activation conditions.
29
1.6.3.2 Industrial applications
Activated carbon, finds numerous applications: decolourisation of
sugar and sweeteners, drinking water treatment, gold recovery, production of
pharmaceuticals and fine chemicals, catalytic processes, off gas treatment of
waste incinerators, automotive vapor filters, colour/odour correction in wines
and fruit juices, additive in liquorice, etc.
Activated carbon, because of its large surface area, a microporous
structure and a high degree of surface reactivity, has been considered to be
very good adsorbent for the adsorption of organics and inorganics from waste
water. It is found to be a better adsorbent for metal removal compared to
fuller’s earth or betonite. Fuller’s earth or betonite adsorbents are only
moderately effective at pH greater than 8 (Goyal et al 2001). Thus, a
considerable amount of work is being carried out for the removal of copper
ion from the aqueous phase using activated carbon.
1.6.4 Preparation of Activated Carbon
Activated carbon is one of the most widely used adsorbents. Over
the last few decades, adsorption systems involving activated carbon have
gained importance in the purification and separation processes on a large
scale. The high adsorptive capacities of AC are associated with their internal
porosity and related to properties such as surface area, pore volume and pore
size distribution. As is well known, the type of raw material employed and the
method of preparation dictate the type of porosity and chemical composition
of AC (Girgis et al 2002, Savova et al 2001). The characteristics of activated
carbon and adsorption capacity of AC depend on the physical and chemical
properties of the precursor as well as the activation method (Mattson and
Marck 1971). There are two methods of preparing activated carbon: physical
activation and chemical activation.
30
1.6.4.1 Physical activation
Physical activation (Smisek and Cerny 1970, Jankowska 1991)
involves two different and separate stages: pyrolysis or carbonization of the
precursor and controlled gasification of the resulting char. Carbonization is
the removal of the non-carbon species and the production of a mass of fixed
carbon (char) with a rudimentary porous structure. Experimental conditions of
carbonization (which is carried out in rotary kilns or multiple hearth
furnaces), especially the heating rate, the final temperature, and the residence
time, control the yield of the process but do not have much influence on the
porous texture of the char. As a result of the decomposition and deposition of
tar, the pores become partially filled or blocked by disorganized carbon. This
results in low adsorption capacity which has to be enhanced by activation via
partial controlled gasification with steam, carbon dioxide, or mixtures of the
two.
The development of porosity in a given char activated by steam or
carbon dioxide is different. In the first stages of the activation process, the
burning out of disorganized carbon results in the opening of the partially
blocked pores of the char and a subsequent increase in the micropore volume,
with no large differences between the two activating agents, although
somewhat larger development of mesoporosity occurs with steam. As the
degree of activation increases the differences in porosity created by the
activating agents become more pronounced and it is generally admitted that
carbon dioxide mainly develops the microporosity and that steam produces a
wider pore size distribution, with development of meso- and macroporosity.
There is an important point to be noted with respect to the porosity
development during physical activation of a char. The volume for the
different ranges of porosity may increase with increasing gasification up to
very large levels of burn-off, but the industrial production of activated carbon
31
is dominated by a compromise between porosity development and process
yield.
1.6.4.2 Chemical activation
Several activating agents have been reported for chemical
activation process; however the most important and commonly used
activating agents are phosphoric acid, zinc chloride and alkaline metal
compounds. Phosphoric acid and zinc chloride are used for the activation of
lignocellulosic materials, which have not been carbonized previously,
whereas, metal compounds such as potassium hydroxide are used for
activation of coal precursors or chars. The finely ground lignocellulosic
material is impregnated with a concentrated solution of a dehydrating
chemical, typically phosphoric acid or zinc chloride, to produce degradation
of the cellulosic material. The mixture is dried and heat treated at 400 -
700°C. These chemical agents may promote the formation of cross-links,
leading to the formation of a rigid matrix that is further less prone to volatile
loss and volume contraction upon heating to high temperatures (Wigmans
1989, Jagtoyen 1992). That is, these chemicals favor dehydration prior to
degradation and subsequent repolymerization, reducing the formation of tars
and other volatile products thus increasing the carbon yield. The product is
washed thoroughly to remove the remaining chemical from the carbon, dried,
and classified into the size range required. Carbon yields of 30-50 wt.% can
be obtained when using lignocellulosic materials, very high in comparison to
physical activation of the same precursor (the yield of carbonization is 25-30
wt.%, and assuming a 40% burn off upon gasification the overall yield would
be 15-18 wt.%).
32
1.6.4.3 Combination of chemical and physical activation
A combination of chemical and physical activation can be used to
prepare granular activated carbons with a very high surface area and porosity
adequate for certain specific applications such as gasoline vapor control, gas
storage, etc. Activated carbons of this type have been reported using
lignocellulosic precursors (e.g., olive or peach pits) chemically activated with
phosphoric acid or zinc chloride and later activated under a flow of carbon
dioxide. Uniform, medium-size microporosity and surface areas above 3600
m2 g-1 are obtained with this mixed procedure (Bansal 1988).
1.6.4.4 Advantages of chemical activation over physical activation
The advantage of chemical activation over physical activation is
that it can be performed in a single step and at relatively low temperatures
(usually < 500oC for activation of wood by phosphoric acid (Bansal et al
1988) and between 600 and 700oC for activation of lignocellulosic materials
impregnated with ZnCl2 (Reinoso and Sabio 1992)). The carbonization step
generates the porosity, which becomes accessible when the chemical is
removed by washing (Caturla et al 1991, Sabio et al 1995). Consequently, the
modification of the chemical/precursor ratio permits the adjustment of the
porosity in the final activated carbon. However, the most important
disadvantage of chemical activation is the incorporation of impurities, coming
from the activating agent, which may affect the chemical properties of the
activated carbon. Another disadvantage is the investment needed for the unit
for recovering the chemical used for impregnation.
1.6.4.5 Advantages of phosphoric acid over zinc chloride
The classical chemical used on a large scale for chemical activation
was zinc chloride due to its efficiency and simplicity of the process. However,
33
its use is on the decline, because of the problems of corrosion, ineffective
chemical recovery, and environmental disadvantages associated with zinc
chloride. This process produces activated carbons with large porosity,
although the pore size distribution is determined-for a given precursor-mainly
by the degree of impregnation (the larger the degree of impregnation, the
larger the average pore size of the final carbon). The AC obtained using zinc
chloride however cannot be used in pharmaceutical and food industries as it
may contaminate the products. Since then there have been many studies
reporting the activation of carbon using phosphoric acid.
Because of the disadvantages associated with zinc chloride,
phosphoric acid is used largely in industry to impregnate lignocellulosic
materials, mainly wood. Also, phosphoric acid induces important changes in
the pyrolytic decomposition of the lignocellulosic materials since it promotes
depolymerization, dehydration and redistribution of constituent biopolymers
(Jagtoyen and Derbyshire 1993), favoring the conversion of aliphatic to
aromatic compounds at temperatures lower than when heating in the absence
of an additive, thus increasing the yield. One of the reasons why activation
with phosphoric acid has become popular is because of the improvements
introduced in the process of acid recovery.
1.6.4.6 A review on activated carbon preparation
Several coals (Ehrburger et al 1986, Ahmadpour and Do 1996,
Teng et al 1998, Castello et al 2001), polymers (Park and Jung 2002, Puziy et
al 2002), and some agricultural by-products and forest wastes (Savova et al
2001, Garc a et al 2003, Villegas et al 1993) have been used as raw materials
to prepare AC.
Several studies of chemical activation have been conducted with
ZnCl2, which has been found to maximize the adsorptive capacity and bulk
34
density of activated carbons produced from cellulosic and lignocellulosic
materials (Reinoso and Sabio 1992, Caturla et al 1991, Kandiyoti et al 1984,
Teng and Yeh 1998).
Phosphoric acid activation has been used for a wide variety of
cellulosic precursors such as coconut shells (Laine et al., 1989), white oak
(Jagtoyen and Derbyshire 1993), peach stones (Sabio et al 1996), nut shells
(Toles et al 1998), cotton stalks (Girgis and Ishak 1999), almond shells (Bevia
et al 1984, Toles et al 2000), pecan shells (Dastgheib and Rockstraw 2001),
Arundo donax cane (Vernerson et al 2002), apple pulp (Garcia et al 2002),
peanut hull (Girgis et al 2002) and sugarcane begasse (Girgis et al 1994,
Ahmedna et al 2000).
Activated carbon from cheap and readily available sources such as
coal, coke, peat (Gosset et al 1986), heat treated sulphurised activated carbon
(Gomez et al 1998), sugarcane begasse pith (Krishnan and Anirudhan 2002),
renewable biosource (Basso et al 2002), rice husk (Kalderis et al 2008) have
been successively employed for removal of heavy metals.
A number of activation procedures have been reported in the
literature using phosphoric acid as an activating agent. To name a few, Corral
et al (2006) prepared powder activated carbon (AC) from vine shoots by the
method of chemical activation with phosphoric acid. After size reduction,
vine shoots were impregnated for 2 h with 60 wt % H3PO4 solution at room
temperature, 50 oC and 85oC. The three impregnated products were carbonised
at 400oC for 2h. The carbons were texturally characterised by gas adsorption
(N2, -196oC), mercury porosimetry, and density measurements. Better
developments of surface area and microporosity are obtained when the
impregnation of vine shoots with the H3PO4 solution is effected at 50oC and
for the products heated isothermally at 200 oC and 450oC.
35
Chestnut wood was used in the preparation of activated carbon
using the method of phosphoric acid-chemical activation by Gomez et al
(2005). The influence of heat treatment temperature (i.e., 300, 400, 500 and
600oC) and concentration of the solution of phosphoric acid (i.e., 1:1, 1:2 and
1:3 water/H3PO4 proportions) used in the impregnation of chestnut wood on
textural properties and fractal dimension were studied. The products obtained
were characterized by N2 adsorption at -196oC. The micropore volume (Vmi)
and the specific surface area (SBET) were found to increase with an increase in
temperature and acid concentration. However, Vmi and SBET decreased at
600oC with regard to 500oC. The micropore size distribution was similar in
the products prepared at 400 and 500oC, regardless of the acid concentration.
Activated carbons (ACs) have been prepared using chestnut, cedar
and walnut wood shavings from the furniture industry using phosphoric acid
(H3PO4) at different concentrations (i.e. 36 and 85 wt.%) as the activating
agent (Diez et al 2004). ACs have been characterized by N2 adsorption at 77 K.
Moreover, the fractal dimension have been calculated in order to determine
the AC surface roughness degree. The optimal textural properties of AC have
been obtained by chemical activation with H3PO4 36 wt %.
Guo and Rockstraw (2007) produced AC from rice hull by a one-
step phosphoric acid activation. The pore structure and surface chemistry in
the activation temperature range of 170–450oC was investigated and the
results showed that the development of porosity (extent of activation) was
negligible at activation temperatures below 300oC, and rapid evolution of
pores occurred at temperatures between 300 – 400oC. Porous activated carbon
with bimodel pore structure (pore < 1 nm and pore > 1 nm) and BET surface
area as high as 1295 m2 g-1 were obtained at 450oC. FTIR (Fourier transform
infrared spectroscopy) results revealed the existence of carbonyl-containing,
phosphorus-containing groups, and groups containing Si–O bond. Boehm
36
titration and FTIR results indicate that the surfaces of these carbons contain
both temperature-sensitive and temperature-insensitive groups.
However, there are only few publications reporting the preparation
of activated carbon from Hevea brasiliensis sawdust (Rubber wood saw dust),
Agave sisalana fiber (Sisal fiber) and Moringa oleifera wood (Drumstick
wood) using phosphoric acid as activating agent. One can design activated
carbon for adsorption of specific adsorbate, using approximate precursor and
by optimizing the activation process conditions.
Cost effectiveness, cheap availability, higher metal loading
capacity, relatively high surface area and high binding affinity were the main
criteria for choosing ACs to remove heavy metals from an aqueous solution.
Taking these criteria into consideration, the present study was carried out to
determine the feasibility of using the relatively common, cheap and thrown
away waste Hevea brasiliensis (Rubber wood) saw dust (HBSD), Agave
sisalina (Sisal) fiber (ASF) and Moringa oliefera (Drumstick) wood (MOW)
to prepare highly effective activated carbon with a large surface area. These
carbons were prepared using phosphoric acid as the impregnating agent by
chemical activation method followed by their characterization, for the
adsorption of nickel, copper and zinc from aqueous solutions and effluent, the
studies being carried out in both batch and continuous mode.
1.7 SCOPE OF THE PRESENT STUDY
Any carbonaceous material which is activated either chemically
or physically yields materials which are highly porous and having large
surface area, they are called activated carbon. Commercially available
activated carbons are expensive and hence are not often viable for treatment
of industrial effluents. In the present study it was therefore desired that
alternative raw materials be chosen that are region specific, easily available,
37
inexpensive and without much commercial applications. Activated carbons
prepared using phosphoric acid as the activating reagent was considered for
the removal of heavy metals from aqueous solutions and industrial effluent.
Heavy metals due to their high toxicity and carcinogenic effects need to be
removed from the effluent stream and if possible recovered for further use. In
the present study nickel, copper and zinc were the heavy metals identified for
adsorption on to activated carbons prepared from Hevea brasiliensis sawdust,
Agave sisalana fiber and Moringa oleifera wood. Batch and column studies
were carried out in order to examine the potential of these adsorbents for
adsorption of metal ions from solution. Effluent from electroplating industry
containing nickel ions was also treated using the above activated carbons.
1.8 OBJECTIVE OF THE PRESENT STUDY
In recent years heavy metal removal using adsorption process has
gained momentum as a means for reducing treatment costs.
The objective of the present work is to
Identify the prospects of using low cost substance as raw
materials for the production of adsorbents for removing heavy
metals such as copper, nickel, and zinc from waste water.
Produce activated carbon from Hevea brasiliensis sawdust
(HBSD), Agave sisalana fiber (ASF) and Moringa oleifera
wood (MOW) by chemical activation method using phosphoric
acid as activating agent.
Characterize activated carbon by means of iodine number,
methylene Blue number, methyl violet number, surface area,
yield, TGA analysis, SEM photographs etc.
38
Assess the precision and reproducibility of activated carbon.
Conduct the adsorption process for removing nickel, copper and
zinc from synthetic waste water onto activated carbon produced
from HBSD, ASF and MOW.
Obtain the kinetic data and equilibrium data in batch system by
studying the effects of different experimental parameters such
as agitation time, initial concentration of metal ions, the dosage
of activated carbon, temperature and pH on the adsorption
capacity.
Conduct column experiments to understand the adsorption
behavior in fixed bed column.
Recover the adsorbed metals from adsorbent and to regenerate
the adsorbent
Study the reusability of the regenerated adsorbent and also
sorption performance on repeated usage.