Heavy Metals Removal on Dye – Modified Fly Ash Substrates

Maria Visa1

1Transilvania University of Barsov, RTD Dept. Renewable Energy Systems and Recycling, Romania, Eroilor 29, 500036 Brasov, Romania KEYWORDS: wastewater, zinc, lead, modified fly ash ABSTRACT The fly ash is a cheap waste materials which can be an efficient substrate in wastewater treatment for the immobilization of heavy metals, mainly Cd2+, Pb2+, Ni2+, Zn2+, Cu2+, and for dyes removal. The use of FA for heavy metals removal, from wastewaters resulted in the dye finishing industry, faces a supplementary problem, related to the dyes affinity for oxide surfaces. Working with industrial wastewaters proved that there is a competitive adsorption process involving heavy metal cations and dyes. Still, some dyes fixed on the FA surface can act as complexion agents, increasing the adsorption efficiency of the heavy metals. The paper presents a comparative study of the efficiency obtained when modifying fly ash, by washing with ultrapure water followed by alkali treatment (NaOH 2N) and by dyes immobilization. The Pb2+, Zn2+ removal efficiency on fly ash with and without dyes is investigated. The adsorption kinetic, the substrate capacities and the mechanisms are discussed correlated with the surface structure (XRD) and morphology (AFM). The results show that, fly ash with controlled surface properties allows the simultaneous removal of heavy metals and dyes, resulting waters that respect the discharge regulations.

INTRODUCTION The degree of pollutants persistence in the environment varies from a few days up to several years and can affected the humans and the biota. Heavy metals, like Cd2+, Pb2+, Ni2+, Zn2+, Cu2+, Cr3+ are among the most toxic pollutants, event hazardous, and can be found in waste waters, as resulting from mining, metallurgy, metal plaiting, smelting flux electrolysis, battery manufactories, petroleum refineries, paint manufacture, printing and photographic industries, etc. Most of industrial undertakings have developed small waste water treatment plants and are interested to develop various technologies targeting heavy metal removal from the wastewaters.

World of Coal Ash (WOCA) Conference - May 9-12, 2011, in Denver, CO, USA http://www.flyash.info/

If combined with dyes, the wastewater containing heavy metals becomes even more difficult to treat, because stabile and toxic compounds can develop between the dye and the cation. Undesired in solutions, this property can be exploited if fixing the dye on a substrate and activating it for cation adsorption. The conventional techniques used for heavy metal and dyes removal are expensive and with average efficiency and are running in sequential steps. The most commonly used methods for removal of heavy metal ions can be: chemical precipitation, solvent extraction, oxidation, reduction, dialysis/electro-dialysis, electrolytic extraction, reverse osmosis, ion-exchange, cementation, dilution, flotation, flocculation, sedimentation, soil flushing/washing chelating7, etc. For example, at industrial scale the ion-exchangers and activated carbon are used but there are certain disadvantages. The activated carbon is a great adsorbent for dyes but the efficiency in heavy metals removal is average and de-sorption and regeneration is difficult under common conditions, thus the overall process becomes expensive. Ion-exchange is an efficient method, with moderate selectivity since it can not only remove the heavy metal ions but exchange the Ca2+, Mg2+ ions13. Supplementary, the ion exchangers can load with dyes, thus modifying the surface and decreasing the process reversibility. These adsorbents are efficiency but do not represent a sustainable solution. Still, adsorption is an efficient process which can use various biological materials, mineral oxides, activated carbon, resins even waste materials. Many researches had investigated the adsorption capacity of less expensive materials such as red mud19, natural zeolite17, wood based biosorbents6, scrap rubber, bituminous coal, peat9, sugar beet pulp14, marine alga Padina16 or bone char4, etc. on heavy metals (cadmium, cooper, zinc, nickel, iron) and dyes removal. Ash, resulting from coal or biomass burning, is a mixture of oxides with less unburned carbon and other minority inorganic compounds thus has a predominant negative surface charge and represents a promising adsorbent. Many applications in wastewater treatment are already reported, most of them at pilot scale. Fly ash, the lightest fraction resulted in coal burning is also largely investigated for heavy metals and dyes adsorption1, 26, because the priority compounds from fly ash favour the heavy metals adsorption and are active sites in dyes’ immobilization. The ashes composition and surface aspect strongly depend on the type of coal, on the burning process and on the equipment design. Raw ash contains soluble compounds, therefore washing before use is compulsory. Previous studies proved that further conditioning by alkali treatment can be needed for enhancing the adsorption efficiency of heavy metals25, or multi-component systems of heavy metals and dyes24, on fly ash. It can be supposed that adsorbing a dye on the fly ash surface may result in an homogeneous, efficient adsorbent surface. The objective of the present study is to assess the ability of low cost fly ash in removing Pb2+ and Zn2+ from synthetic aqueous solutions. Fly ash was modified in two steps: (1) with NaOH 2N solution followed by (2) a treatment in methyl orange 0.2mM solution.

2. EXPERIMENTAL 2.1. THE SUBSTRATE The fly ash (FA) used in the experiments was obtained from lignite coal burned in the CPH plant CET Brasov, Romania. The major constituents of this fly ash are presented in Table 1. According to the ASTM standards12, the FA collected from the electro-filters of the plant is of F type (the sum of the major compounds, oxides SiO2, Al2O3 and Fe2O3, is over 70%).

Table 1. Fly ash composition

*LOI: loss of ignition

The pH value of the raw fly ash was evaluated by mixing 100 g of FA with 1000 mL ultra pure water, then stirring, at room temperature till constant pH and conductivity values (10.2, and respectively, 1.710 mS). The initial TDS value of the batch is 600 mg/L and after 24 h increases to 820 mg/L, being in the end (48 h) 850 mg/L. The soluble compounds: K2O, Na2O, MgO, CaO, were thus removed in solution and can be available by further separation and purification. Raw and washed FA shows a limited adsorption affinity for heavy metals as result of a high heterogeneity in the surface charge. Previous studies25 proved that negative surface charges can be obtained by using NaOH 2N solution as modifier. The FA, washed in ultra pure water, was further stirred 48 hours at room temperature with NaOH 2N alkaline solution (FA-NaOH) followed by filtration, washing and drying, at 105-120oC, till constant mass. The dried FA-NaOH was sieved and the 40 -100 µm fractions were selected for adsorption experiments. One half was used for Zn2+ and Pb2+ adsorption and other half was used to obtain a new substrate by mixing FA-NaOH with a Methyl orange MO (Merck) aqueous solution, for to 48 h, at room temperature, followed by filtration and flush with ultra pure water. Finally, the FA-NaOH-MO substrate was dried and used for the adsorption test of Zn2+ and Pb2+ cations. Both substrates were characterized using XRD (Bruker D8 Discover Diffractometer) for FA crystalline structure and AFM images (Ntegra Spectra, NT-MDT model BL222RNTE) were used for morphology studies (roughness surface, pore size distribution). 2.2 ADSORPTION EXPERIMENTS Kinetic and thermodynamic adsorption experiments were conducted in batch, at room temperature (20-250C), under (a) magnetic stirring and (b) under vessel rotation.

FA Composition [%]

SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O TiO2 MnO LOI*

53.32 22.05 8.97 5.24 2.44 2.66 0.63 1.07 0.08 1.58

The optimal contact time was evaluated on suspensions of 0.5 g of FA-NaOH or FA-NaOH-MO in 50mL solutions of Zn2+ (350 mg/L) respectively Pb2+ (1250 mg/L). Aliquots were taken each 10, 30, 45, 60, 90, 120 180, 240 min. when stirring was interrupted and the substrate was removed by filtration. The residual metal concentration from the supernatant were analyzed by AAS (Analytic Jena, ZEEnit 700), at λZn = 213.9 nm and λPb = 283.3 nm, after calibration. In all cases, the working pH was the natural value of the solutions. Preliminary experiments proved that heavy metals losses due to adsorption to the container walls and to the filter paper were negligible. 3. RESULTS AND DISCUSSIONS The XRD spectra, Fig. 1, show that the major components of fly ash are: carbon (graphite), carbon hexagonal (chaoite or white), SiO2 in various structures (cubic, rhombohedra) combined with Al2O3 as rhombo γ, mullite (3Al2O3

.2SiO5), along with γ-Al2O3, hematite (Fe2O3), TiO2 brookite, was also traced along with Cd, Cu, Zn (0.0742 mg/l), Pb (0.0094 mg/L) compounds. These components are characterizing the raw fly ash but are found also in the modified FA. The aluminium silicate, oxide components vary from one type of substrate to another, and the alkali treatment (FA-NaOH) can be responsible for partial dissolution processes.

20 30 40 50 60 70 80

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

5500

6000

Halloysite Al2Si

2O

5(OH)

4

maghemite γ Fe2O

3Cchaoite

hexagonal

Cchaoite

hexagonal

Cchaoite

hexagonal

FACET-NaOH-MO

FACET-NaOH

(2)

(1)

TiO2brookite

Cgrafit

HematiteAl

2O

3γ,ε

αSiO2 quartz

Kianite Al2SiO

5/Al

2O

3SiO

2

SiO2cristobalite

Arb

rita

ry u

nits [a

u]

2 theta [degree]

Fig. 1 XRD of (1). FA modified with NaOH 2N, (2). FA modified with NaOH 2N and MO 0.02 mM.

Heavy metals and dyes removal on FA are pH depended. Previous studies showed that the lowest heavy metals adsorption efficiency occurred when the FA was treated with HCl 2N23 ,this effect is caused by a positive surface charge leading to repulsions between the surface (≡ SiOH2

+) and metal ions21 . Supplementary, silica and alumina are forming alumino-silicates with pH-dependent structures. On the FA, during alkali conditioning with NaOH 2N solution for 48 h, several interactions are expected, according to the following reaction16:

NaOHaq + FAs → Naa(AlO2)b(SiO2)c·NaOH H2O (1) when on the FA surface can developed new active site (≡ SiO-) and (≡ AlO-) which allows metals to form complexes at the surface (eq. 2, 3)3,18:

2(≡ SiO-) +M2+ → (≡ Si-O)2M (2)

2(≡ AlO-) +M2+ → (≡ Al-O)2M (3)

The planar structure of Methyl orange molecule, with two aromatic rings can act as an electron donor in the interaction with the heavy metals cations, Fig. 2:

Fig 2. Chemical structure of Methyl orange in alkali solution Therefore, the alkali form of MO may act as a supplementary complexion agent, which, adsorbed on the FA surface can increase the affinity for heavy metals by terminal group

( −− 3SO ) and/or to decrease the pore size on the surface, Fig. 2.

0 50 100 150 200 250 300

FA CET NaOH

FA CET NaOH MO

Counts [a.u.]

Mesopores diameter [nm]

Fig.3. The pores distribution on FA surface

These chemical and structural changes are mirrored in morphology modifications, Fig 4 (a) and (b), resulting in large differences in the substrates’ affinity for heavy metals. The various roughness values appear due to a complex of factors, involving alkaline oxides leaching, methyl orange treatment, and the formation new structures with important role in the heavy metals adsorption.

(a) FA CET- NaOH 5 µm Average Roughness, 30.5 nm

(b) FA CET – NaOH- MO 5 µm Average Roughness, 15.6 nm

Fig. 4

3.1 UPTAKE KINETICS OF THE HEAVY METALS

For the metal removal kinetic studies, the metal uptake qe (mg/g) was evaluated

using the initial and current, t, heavy metal concentrations ( i

cationc and t

cationc ), in a

given solution volume, V, for a given amount of FA substrate, ms (eq. 4):

s

t

cation

i

cation

m

V)cc(eq

×=

- (4)

The kinetics of the heavy metals adsorption was modelled by the following equations: - The pseudo first-order, Lagergren equation11:

log (qe – qt) = log (qe) tKL

2,303− (5)

where KL is the Lagergreen constant, q0 is the initial uptake value and qt the current metal uptake. - The pseudo-second order kinetic equation, developed by Ho and McKay8:

e

2e2t q

t

qk

1

q

t+= (6)

where k2 is the equilibrium rate constant for the pseudo second-order adsorption (g mg-1 min-1) and can be evaluated from the slope of the plot. This model is based on

the assumption that the rate limiting step may by a chemical adsorption involving the valence forces through sharing or exchange of electrons between the adsorbent and the adsorbate8,10.The model is mentioned in literature for Cu2+, Co2+, Zn2+, Mn2+ adsorption on zeolites21, and of Pb2+

and Cu2+ on humic acid20. Other possible kinetic model which can be applied in adsorption processes on porous materials is the interparticle diffusion model. In this case, the amount of heavy metals ions adsorbed can be calculated with the equation2:

Ctkq 1/2

id += (7)

This model can be fitted well for, Pb2+ for adsorption, on both types of substrates. The linearization proved that the pseudo-second order kinetic describes well the adsorption mechanism both for Zn2+ and Pb2+, while the pseudo-first order kinetic, can describe only the adsorption processes of Zn2+ ions on FA-NaOH and the adsorption processes of Pb2+ ions on FA-NaOH-MO. The kinetic parameters are presented in Table 2. Table 2 Kinetic parameters of the heavy metal adsorption

n =1 n = 2 Interparticle Difusion

Type of FA

KL

[min-1]

R2 k2

[g mg-1 min-1]

qe

[mg/g]

R2 Kid

C R2

Lead (Pb2+)

FA-NaOH 0.03 0.868 0.064 94.34 0.998 3.62 54.51 0.998

FA NaOHMO

0.02 0.920 0.304 42.19 0.991 1.33 21.56 0.903

Zinc (Zn2+)

FA-NaOH 0.03 0.989 0.091 18.87 0.997 0.88 11.14 0.883

FA NaOHMO

- 0.758 0.507 8.49 0.976 0.37 4.53 0.845

The Pb2+adsorption capacity on both substrates was 94.339 mg-Pb2+ /g and 42.194 mg Pb2+ /g, showing a possible chemical adsorption, involving valence forces (through electrons sharing) between the adsorbent and the adsorbate.

The lead in aqueous solution may suffer solvation, hydrolysis or polymerization27. It has been reported that lead can form many polynuclear species such as: Pb2(OH)3+, Pb3(OH)4+ which can be adsorption etc. The Raman spectrum show that in aqueous solution the zinc cations are hexahidrated [Zn(H2O)6]

2+.

The Zn2+ cation can interact with the MO molecules, forming stable complexes on the FA surface. Adsorption studies carried out to estimate heavy metal removal from wastewater, using fly ash, showed that the efficiency follow the order15: Pb2 > Zn2+ >Cu2+ >Cd2+ >Ni2+. 3.2. OPTIMIZING THE ADSORPTION ON FA

The Pb2+and Zn2+ adsorption efficiency, η, on modify FA was evaluated to optimize the contact time and the amount of FA and was calculated with equation (8):

i

cation

e

cation

i

cation

c

cc 100)( ×−=η (8)

where i

cationc and e

cationc are the initial and equilibrated cations concentrations

(mg/L). The dynamic adsorption results are presented in Fig. 5a and 5b for the lead and zinc adsorption on FA-NaOH, compared with the results obtained using FA-NaOH - MO as adsorbents.

0 50 100 150 200 2500

10

20

30

40

50

60

70

80

Effic

ien

cy [

%]

Time [min]

Pb2+

/FACET-NaOH

Pb2+

/FACET-NaOH-MO

0 50 100 150 200 2500

10

20

30

40

50

60

70

Eff

icie

ncy [

%]

Time [min]

Zn/FACET-NaOH

Zn/FACET-NaOH-MO

Fig.5 a Lead immobilization Efficiency vs. contact time

Fig. 5 b Zinc immobilization Efficiency vs. contact time

0.0 0.5 1.0 1.5 2.0 2.5 3.00

20

40

60

80

100

Effic

ien

cy [%

]

mass [g]

Pb2+

/FA-NaOH

Pb2+

/FA-NaOH-MO

0.0 0.5 1.0 1.5 2.0 2.5 3.00

20

40

60

80

100

Effic

ien

cy [

%]

mass [g]

Zn/FACET-NaOH

Zn/FACET-NaOH+MO

Fig. 6 a Lead immobilization, efficiency vs. mass of FA

Fig. 6 b Zinc immobilization, efficiency vs. mass of FA

The low adsorption efficiency of Pb2+ and Zn2+ cations on FA-NaOH-MO comparative with the efficiencies on FA-NaOH show that a share of pores are occupied with

methyl orange molecules, as also supported by the AFM pictures and the average roughness values. Further, it may be concluded that in complex systems containing both the cation(s) and the dye, there are expected only average adsorption efficiencies. The effect of the amount of adsorbent on the uptake of Pb2+, Zn2+ cations is presented in Fig.6 a and 6 b. Increasing the amount of adsorbent from 0.5 g up to 3 g for 50 mL solution the initial pH increases up to 8 (bellow the precipitation value) while the adsorption efficiency of the heavy metals increase because exist more active centres in the system. The optimal adsorbant amount is found to be 1.5 g for Pb2+ cation removal respectively 1.25 g for Zn2+.

So the optimal parameters to remove the metals ions (Pb2+, Zn2+) are: Contact time, toptimal: 60 min Amount of substrate in 100 mL of solution, ms: 1.25 – 1.5 g. Table 3. Parameters of the adsorption isotherms

Langmuir Isotherm Freundlich Isotherm

Type of FA qmax

[mg/g]

a

[L/mg] R2 n KF R2

Lead (Pb2+)

FA-NaOH - - 0.063 1.042 1.88 10-5 0.937

FA-NaOH-MO 31.646 0.006 0.999 1.523 0.006 0.89

Zinc (Zn2+)

FA-NaOH - - 0.671 1.012 25.896 0.999

FA-NaOH-MO 6.269 0.031 0.999 7.337 0.002 0.980

The adsorption isotherm data were experimentally obtained and the absorption parameters were calculated considering the Langmuir and Freundlich equations, respectively, as given below: The Langmuir isotherm - linearization:

maxmaxeq

eq

qaqq

C eqC1+

⋅= (9)

Where qmax (mg/g) represents the monolayer adsorption capacity, a is a constant related to the free energy of adsorption, qeq and Ceq show the amount of metal adsorbed on.

The Freundlich isotherm - linearization:

eqF Clnkn

1lnqln eq += (10)

Where kF is Freundlich adsorption constants, being indicators of adsorption capacity and 1/n dimensionless parameter is a measure of the adsorption density. Table 3 presents the adsorption parameters for the heavy metal ions (Pb2+, Zn2+) calculated from the slope of the plots. The Langmuir model could better describe the lead and zinc adsorption on FA-NaOH-MO as result of a highly homogeneous substrate resulted after the dyes adsorption. The Freundlich model could better describe the adsorption of lead and zinc on FA-NaOH, the heterogeneous substrate. CONCLUZION The fly ash is a waste which can be used for the removal of aqueous lead and zinc ions, with good efficiency. Treating FA with alkaline NaOH solutions 2N the surface is modified by dissolution and re-precipitation reactions. By dissolution of acid oxides, the specific surface area is enhanced and activated and the efficiency of heavy metals removal increases. Adding dye to the FA surface results in a new surface, which is one more homogeneous but less negatively charged. The overall effect is a decrease in the affinity for heavy metals. The heavy metals removal was studied and the adsorption conditions were optimized in terms of contact time (60 min) and adsorbent mass in 100 mL solution (1.25-1.5 g). The pseudo-second order kinetics did well apply to the majority adsorption processes. The experimental results show that this can be an up-scalable solution and represent a first step in investigating the process of complex treatment of wastewaters containing dyes and heavy metals. AKNOWLEDGEMENT This work was supported by the Romanian National Research Plan, II, Programme Partnership, under the grant TECNANO-ECO, no.72-184/2008. REFERENCES [1] Ahmaruzzaman, M., A review on the utilization of fly ash, Progress in Energy and Combustion Science, 36, 2010, pp. 327 – 363.

[2] Allen, S.J., McKay, G., Khader, K.Z.H, Interparticle Diffusion of a basic dye during adsorption onto sphangum peat, Environment Pollution, 56, 1999, pp.39-50. [3] Chaizasith, S. P. Chaizasith, and C. Septhum, Removal of Cadmium and Nickel from aqueous solutions by adsorption onto treated fly ash from Thailand, J. Sc. Tech. 11 (2) , 2006, pp. 13-19. [4] Cheung, C.W.J., Porter F. and. Mckay, G, Sorption kinetic Analysis for the Removal of Cadmium ions from effluents using Bone Char, J. Water Research, 35, 3, 2001 pp. 605-612. [5] Choi J.H., S. Kim, Y.J. Kwon, W.J.Kim, , Adsorption behaviours of ETS+10 and its variant, ETAS-10 on the removal of heavy metals Cu2+, Co2+, Mn2+ and Zn2+ from waste water,J. Microporous and Mesoporous Materials, 96, (1-3), 2006, pp. 157-167. [6] Cochrane, E.L, Lu, Gibb, S,W., Villaescusa, I., A Comparison of Low-cost Biosorbent and Commercial Sorbents for the Removal of Copper from Aqueous Media, Journal Hazard Materials, 137, 2006, pp. 198-206. [7] Dinesh M., Kunwar P. Singh, Single – and multicomponent adsorption of cadmium and zinc using activated carbon derived from bagasse - an agricultural waste, J. Water Research , 36, 2002, pp. 2304-2318. [8] Ho, Y.S., G., McKay, Pseudo-second order model for sorption processes, Process Biochemistry, 34, (4)1999, pp.51- 465. [9] Ho, Y.S., McKay, G.J., Sorption of Cooper (II) from aqueous solution by peat, J. Water Air Soil Pollut, 158, 2004, pp.79-97. [10] Hui, K.S., Chao, C.Y.H.,., Kot S.C, Removal of mixed heavy metal ions in wastewater by zeolite 4A and residual products from recycled coal fly ash, Jour. of Hazardous Materials B 127, 2005, pp. 89-101. [11] Lagergren, S. About the theory of so-called adsorption of soluble substances, Kung. Sven. Veten. Hand. 24, 1898, pp.1-39. [12] Lee, W.K.W., Deventer, J.S.J., Structural reorganization of class F fly ash in alkaline silicate solutions,J. Colloids and Surfaces A, 211 (1) , 2002, pp.49-66. [13] Li, Q. H. Su, J. Li, T. Tan, Studies of adsorption for heavy metal ions and degradation of methyl orange based on surface of ion-imprinted adsorbent, Process Biochem. 42, 2007, pp. 379-383. [14] Mata, Y. N., Blázquez, M. L, Ballester A., González F, Muñoz, J. A, Sugar beet pulp pectin gels as biosorbent for heavy metals: Preparation and determination of biosorption and desorption characteristics, J. Chem Engineering, 150, 2009, pp. 289-301.

[15] Mathur A, Khare SK, Ruoainwar DC, Removal of heavy metals from main sewer –water of Varanasi city adsorption on fly ash and blast furnace slag, J.Indus. Pollut Countr (India), 1989, 5, pp. 52-7. [16] Pairat Kaewsarn, Qiming Yu, Cadmium (II)removal from aqueous solutions by pre-treated biomass of marine alga Padina sp., Environmental Pollution, 112, 2001, pp. 209-213. [17] Perić, J., Trgo, M., Medvidović, N.V., Removal of zinc, copper and lead by natural zeolite- a comparison of adsorption isotherms, J. Water Research, 38, 2004, pp.1893-1899. [18] Reyad, S., .Al-Harahsheh, A, Hami, M., Khlaifat, A. Preparation and determination of pulp pectin gels as biosorbent for heavy metals:J. Fuel 83 (7-8), 2004, pp. 981-985. [19] Shaobin W., Boyjoo Y., Choueib, A.,.Zhu, Z.H., Removal of dyes from aqueous solution using fly ash and red mud, J.Water Research, 39, 2005, pp. 129-138. [20] Shaobin, Wang, et. all, Single and co-adsorption of heavy metals on humic acid, Separation and PurificationTechnology, 58, 2008, pp.353-358. [21] Shawabkeh, R., Al-Harahsheh A., Hami M., Khlaifat A., Conversion of oil shale ash into zeolite for cadmium and lead removal from wastewater, J.Fuel 83, , 2004, pp. 981-985 [22] Taty – Costodes, V.C., Fauduet, H., Porte, C., Delacroix, A., Removal of Cd(II) and Pb(II) ions, from aqueous, by adsorption onto sawdust of Pinus sylvestris , J. Hazardous Materials 105, 2003, pp.121-142. [23] Visa M., Duta A., Advanced wastewater treatment for heavy metals immobilisation using modified fly ash, Proceedings of EMRS Fall Meeting, Warsaw, 2007, pp. 168. [24] Visa, M., Bogatu, C, Duta, A, Simultaneous adsorption of dyes and heavy metals from multicomponent solutions using fly ash, J. Appl. Surf. Sci. 256, 2010, pp. 5486, 5491. [25] Visa, M., Duta, A., Adsorption Mechanisms Of Heavy Metals, AIChE Conference Proceedings, Salt Lake City, USA, 2007. [26] Wang S.B., Boyjoo Y., Choueib A., Ng E., Wu H.W., Zhu Z.H., Role of unburnt carbon in adsorption of dyes on fly ash, J. of Chem. Tech. and Biotechnology, 80, 2005, pp. 1204-1209. [27] Yugui Tao, Lianbie Ye, Jun Pan, Yaoming Wang, Bin Tang, Rmoval of Pb(II) from aqueous solution on chitosan /TiO2 hybrid film, J. of Hazardous Materials, 161, 2009,pp. 718-722.