INTRODUCTION:

Heavy metal contamination has been a critical problem mainly because metals tend to persist and accumulate in the environment. Copper, Nickel, Mercury, lead, Zinc, Arsenic etc. are such toxic metals which are being widely used. They are generated by dental operation, electroplating, tanning, textile, paper and pulp industry and are potentially toxic to humans. These heavy metals are used in many industries for different purposes and released to the environment with industrial wastage. Therefore the effluents being generated by these industries are rich in heavy metals should be treated before discharge in to the common waste water. On the other hand aquatic systems are particularly sensitive to pollution possibly due to the structure of their food chain. In many cases harmful substances enter the food chain and are concentrated in fish and other edible organisms⁽¹⁾. The current physico-chemical processes for heavy metal removal like precipitation, reduction, ion-exchange etc. are expensive and inefficient in treating large quantities. They also cause metal bearing sludges which are difficult to dispose off ⁽²⁾.

The application of biopolymers such as chitosan is one of the emerging adsorption methods for the removal of heavy metal ions, even at low concentrations ⁽³⁾. Chitosan is a type of natural polyaminosaccharide, synthesized from the deacetylation of chitin, which is a polysaccharide consisting predominantly of unbranched chains of (1→4)-2-acetoamido- 2-deoxy-d-glucose. Chitin is the second most abundant polymer in nature after cellulose. It can be extracted from crustacean shell such as prawns, crabs, fungi, insects and other crustaceans ⁽⁴⁾.Widely available biopolymers are also being used for adsorption mainly because they are a cheap resource or a freely available resource ⁽⁵⁾. Chitosan is a biopolymer, which is extracted from crustacean shells or from fungal biomass. The structure of chitosan is presented schematically in Figure 1.

Figure 1 Structure of chitosan

To improve chitosan’s performance as an adsorbent, cross-linking reagents such as glyoxal, formaldehyde, glutaraldehyde, epichlorohydrin, ethylene glycon diglycidyl ether and isocyanates have been used ⁽⁶⁾. Cross-linking agents do not only stabilize chitosan in acid solutions so that it becomes insoluble but also enhance its mechanical properties ⁽⁷⁾. Chitosan derivatives have been extensively investigated as adsorbents ⁽⁸⁾. Recently, chitosan composites have been developed to adsorb heavy metals from wastewater. Different kinds of substances have been used to form composite with chitosan such as montmorillonite ⁽⁹⁾, polyurethane ⁽¹⁰⁾, activated clay ⁽¹¹⁾, bentonite ⁽¹²⁾, poly vinyl alcohol, poly vinyl chloride, kaolinite ⁽¹³⁾, oil palm ash ⁽¹⁴⁾and perlite ⁽¹⁵⁾.

The high porosity of this natural polymer results in novel binding properties for metal ion such as cadmium, copper, lead, uranyl, mercury and chromium. Chitosan has been used for about three decades in water purification processes. ⁽¹⁶⁾. It has an amine functional group which is strongly reactive with metal ions ⁽¹⁷⁾.

Lignin is a phenolic, three-dimensional, cross-linked polymer occurring in plant tissues, and whose role is cementing cellulose fibers. It is based on three phenylpropanoid monomers, see Fig. 2, connected with each others through various inter-unit linkages ⁽¹⁸⁾, thus resulting in a complex macromolecular structure. In general, lignin is a waste material from the pulp and paper industry, and is most often used as fuel for the energy balance of pulping process ⁽¹⁹⁾. Yet, considerable effort has been made in the past for finding high value-added applications to lignin. For instance, it has been proved that glyoxalated lignin can be an effective precursor of adhesive resin for formaldehydefree particleboards ⁽²⁰⁾. In addition, potential health applications of lignin have been explored, and it was shown that lignin possesses high activity as binder of cholic acid sodium salt, and as antitumor and antivirus ⁽²¹⁾.Although it is not the first time that lignin is used as gel precursor, few works exist about gels based on lignin ⁽¹⁹⁾.

Fig.2. Schematic representation of the structural units of lignin: (a) p-coumaryl alcohol (4-hydroxyl phenyl), (b) coniferyl alcohol (guaiacyl), (c) sinapyl alcohol; (syringyl).

The aim of this study is to investigate the heavy metals removal from wastewater by adsorption and to evaluate factors affecting on the removal of heavy metals (Cd , Co , Pb and Zn) using as a bio-adsorption material.

EXPERIMENTAL:

MATERIALS:

All reagents in this work were of analytical grade and were used as received without further purification and then tested and prepared in order to be suitable for real experiments. The prepared reagent consist of: (1) reagent for isolation of chitosan, i.e. 45% (w/v) NaOH and 1 M HCl (2) reagent for isolation of lignin i.e. 4% (w/v) NaOH , HCl and 95% ethanol (3) reagents for preparation of chitosan – lignin polymer beads, i.e 1% acetic acid , HCl and buteraldehyde (4) Stock solution of 100 mg/ml Cd(II), Co(II), Pb(II) and Zn(II) from CdCl2 , CoCl2, Pb(NO3)2 and ZnCl2crystal, respectively (4) standard solutions for preparing standard curve for the determination of Cd(II), Co(II), Pb(II) and Zn(II) using atomic absorption spectrometer (AAS).

METHODS:

Isolation of Chitosan:

The shrimp shells which were used for chitosan isolation was purchased from local seafood processing industry. Chitin, isolated from shrimp shell, was ground to powder form. This powdered chitin was then deacetylated with NaOH (45% w/w) in 100°C water bath for 60 min and the reaction was terminated by an ice bath. Following that, the product was cleaned several times with deionized water until the pH of the suspension reached 7. The suspended particles were collected with a membrane filter and dried at 80°C for 48 h. The chitosan powder was modified with a novel method different from the previous one to achieve better performance.

Isolation of Lignin:

A 100 ml of black liquor was treated with a sufficient (4%) aqueous sodium hydroxide solution to cover it completely and heated under the reflux condenser at 100” for 4 hrs. The reaction mixture was filtered and the lignin precipitated by the addition of concentrated hydrochloric acid to the filtrate. The obtained lignin from extraction was purified by dissolving it in 500 ml of (2%) aqueous sodium hydroxide solution and adding to it 1 liter of (95 %) ethanol. The precipitate was filtered off, the filtrate was acidified with hydrochloric acid, and the alcohol was removed by distillation. The lignin was washed with water until the wash water was free of chlorides and dried in oven at 56°C for 3hrs. Yield, 45 gm. An amorphous brown substance was obtained.

Chitosan- Lignin Polymer Synthesis:

Place 2 gm of chitosan in a 250 mL 2-necked round-bottom flask containing 1% acetic acid , a magnetic stirring bar and mixed at 100 rpm for 60 min or until dissolved to make. after that an aqueous solution of 2 gm lignin in 25 ml distilled water were added, and then 0.5 ml of buteraldehyde was added into the reactor by controlling the dropping speed. The reaction was continued for 3hrs at room temperature (25°C). Adjusted the pH to 2 by HCl, the chitosan - lignin was obtained Fig(3).

Figure 3: Chitosan - lignin polymer structure.

Characterization techniques and instruments:

Six methods were used for the characterization of the chitosan , lignin and chitosan - lignin polymer:

The UV-visible spectra were recorded over the range of 200–700 nm using the T60 U PG Instrument Limited UV-visible spectrophotometer (UK).

Fourier transform infrared (FTIR) spectra were obtained with a FTIR- RX1 spectrometer (Perkim Elmer, USA) with samples incorporated into KBr discs in the range of 400 to 4000 cm^-1.

Gas chromatography-mass spectrometry (GC-MS) were performed using an Agilent Technologies 7890 GC with 5975 MSD1µL of reconstituted sample was injected through a 7683B Series Injector using a split mode of 50%. The GC separation was done using a DB5 column at a flow rate of 1mL/min He 99.999%. The oven temperature was programmed as follows: 50 °C (hold 1 min), 25 °C/min to 150 °C, 20 °C/min to 170 °C and 80 °C/min to 250 °C for 3 min. (The total run time was 10 min). Products were detected using a 5975C VLMSD with TripleAxis Detector (m/z 50-250).

Differential scanning calorimetry (DSC) experiments were carried out using a TA Instruments DSC 30 (Mettler Toledo , Switzerland ) Differential Scanning Calorimeter. Samples (5–10 mg) were loaded into standard aluminium pans and run using a heat/cool/heat cycle with a heating rate of 10 °C min^-1 and a cooling rate of 5 °C min^-1.

Thermogravimetric analysis (TGA) measurements were performed using a TA Instruments TGA (Mettler Toledo , Switzerland ) Thermogravimetric Analyzer. Samples (8–14 mg) were weighed out on platinum pans and heated to 600 °C at 10 °C min^-1under a nitrogen atmosphere. All thermal analysis employed duplicate runs for each sample. Working temperature range was 25–800°C with a efficiency of 10°C min⁻¹. Air was used as environmental medium at100 ml min⁻¹flux.

The crystallinity of the polymeric compounds in powder form was studied by X-ray diffraction method (Empyrean series 2) PAN analytical (Netherland) using Cu Kα radiation generated at 40 kV and 40 mA at scanning speed of 0.3 2⍬/ min within a range of 100 to 600.

Study of heavy metal adsorption by synthesized chitosan – lignin Polymer:

For the adsorption experiments, a 100 mg/l of cadimium , cobalt , lead, , and zinc solutions at different concentrations were prepared and different pH ( 2, 4, 6, 8) were tested. The pH of solution was adjusted to desired values with 0.1 N HNO3 and 0.1 N NaOH. Adsorption experiments were developed placing (0.1- 1) gm of the dry polymer and 50 ml of corresponding solution with metal ions in a 100 ml glass-stoppered flask. The mixture was shaken at 175 rpm for different mixing time (0.25, 0.5, 1, 2, 4, 6, 8, 24)hour using a thermostated shaker. The temperature was controlled at 25°C. Samples were filtered at equilibrium. The remaining concentration of metal ions was determined in the filterate by Atomic Absorption Spectrometry.

RESULTS AND DISCUSSION:

Ultra violet –visible study of the Compounds:

Ultraviolet/visible (UV-Vis) spectroscopy is useful as an analytical technique for two reasons. Firstly, it can be used to identify certain functional groups in molecules, and secondly, it can be used for assaying. UV-Vis spectroscopy involves the absorption of electromagnetic radiation from the 200–800 nm range and the subsequent excitation of electrons to higher energy states. The absorption of ultraviolet/visible light by organic molecules is restricted to certain functional groups (chromophores) that contain valence electrons of low excitation energy . The UV of the studied compounds: chitosan, tannin and lignin was carried out in double beam UV-visible photometer, using dilute solution (3.5×10^-3 ) .

UV-Vis spectra of chitosan are usually recorded in aqueous acetic acid solutions in a 1.0 cm quartz cell at ambient temperature. Chitosan include various ratios of two far-UV chromophoric groups, N-acetylglucosamine (GlcNAc) and glucosamine (GlcN); as a result, their extinction coefficients for wavelengths shorter than approximately 225 nm is non-zero. Because GlcNAc and GlcN residues show no evidence of interacting within the chitin/chitosan chain, the monomer units contribute in a simple, additive way to the total absorbance of this polymer at a particular wavelength. The UV spectra of chitosan was shown in Figure 4 and the λmax is 201 nm in 0.1 M HAc solution . The UV spectra for lignin is show an intense bands at λmax 281 nm as shown in Fig(4) . From the figure could conclude that the conversion of lignin is higher than chitosan , this is due to the presence of electron donating group on benzene ring i.e.(-OCH₃) for lignin which increase the electron density on the carbon-carbon double bond of lignin.

Fig(4): UV-Visible Spectra of chitosan, lignin and chitosan-lignin polymer

Gas Chromatography- Mass Spectrometry:

Mass spectrometry (MS) is a destructive analytical technique used for measuring the characteristics of individual molecules. The basic information obtained from mass spectrometric analysis is the molecular mass of a compound, which is determined by measuring the mass to charge ratio (m/z) of its ion. With the ionization method, full particulars about a molecule’s chemical structure can be found. MS can analyze chemicals with a wide mass range–from small molecules to complicated biomolecules such as carbohydrates, proteins, peptides or nucleic acids. The GC-MS analysis detected all organic species quantitatively. Each peak area in the chromatogram was proportional to the amount of the organic compounds forming that peak.

GC-MS spectra of chitosan is shown in Fig.5, from the mass spectra it is possible to identify volatile compounds obtained from chitosan. The peaks at m/z 537.9 with retention times under our chromatography conditions around ~15 mins is derived from glucosamine. The molecular weight of the obtained chitosan was 2561.1 . GC-MS provides a rapid and easy alternative to tedious chemical degradation procedures for analyzing the monolignol composition of lignin samples. It requires only a small amount of lignin (<1 mg). Compounds separated on a GC column can easily by identified from their mass spectra as being derived from p-hydroxyphenyl (H), guaiacyl (G), or syringyl (S) propane units. Figure 6 shows the chromatograms from the lignin . The peaks at m/z 970.1 with retention times under our chromatography conditions around ~6 mins. Lignin consists of a ratio of p-hydroxyphenyl to guaiacyl to syringyl-based units (H/G/S ratio) of approximately 12.4:13.8:1. Therefore this lignin could be considered a p-hydroxyphenyl-guaiacyl lignin. The molecular weight of the obtained lignin was 2818.5 .

Fig. (5): GC-MS spectra of Chitosan.

Fig. (6): GC-MS spectra of Lignin

Fourier Transformer Spectroscopy (FTIR):

Fourier transform infrared spectroscopy (FTIR) was used to determine the vibration frequency of the functional groups in the three different polymers. The spectra were measured by an FTIR spectrometer within the range of 400–4000 cm−1 wave number. The dry amount of polymers (about 0.1 g) was thoroughly mixed with KBr and pressed into a pellet and the FTIR spectrum was then recorded.

FTIR of Chitosan:

The characteristic IR absorption peaks of chitosan were observed (Fig. 7), which include a broad and strong band ranging from 3200-3700 cm^-1 (stretching vibration of O-H and extension vibration of N-H). The peaks located at 2920 and 2881 cm^-1 can be assigned to asymmetric and symmetric –CH₂ groups. The peak located at 1642 cm^-1 is characterstic of amine deformation. The prominent peak at observed at 1383 cm^-1 represents C-N stretching. The peak at 1164 cm^-1 can be attributed to the C-O-C stretching. The peak at 1022 cm^-1 is characteristic of C-O stretching vibration. The absorption band at 896 cm ^-1, corresponds to the characteristic absorption of β-D- glucose unit.

FTIR of Lignin:

The FT-IR spectra of lignin are shown in Figure 7 Around 3451 cm^-1 it can be observed a wide vibration caused by the stretching of the O-H group, the spectra presented band between 2937 and 2875 cm^-1 that corresponded to the vibration of C-H bond in methyl and methylene groups. Around 1462 cm^-1 streching vibrations of C-C aromatic groups appear in spectrum. Three typical vibrations appeared in aromatic compounds such as lignin, these bands were exhibited around 1512, 1462 and 1425 cm^-1. Therefore, phenylpropane units (lignin skeleton) were identified in all extracted lignins The vibration at around 1622 cm^-1 was associated to the C=O bond stretching. The most significant bands in lignin spectra were those that corresponded to its main substructures: guaiacylpropane (G), syringylpropane (S) and p-hydroxyphenylpropane (H)-such as the peak around 1033 cm ^-1 that was related to the breathing of the syringyl ring with C-O stretching and the bands at around 1215cm-1 (shoulder) that were associated to the breathing of the guaiacyl ring with C-O-C stretching. Around 1112 cm ^-1 a vibration can be distinguished that was caused by the deformation of the bond C-H in guaiacyl substructures and syringyl substructures. The vibration at around 1030 cm ^-1 was due to the deformation or the aromatic C-H linkages in guaiacyl substructures and as well it can be related to the deformation of the bond C-O in primary alcohols. Finally, at 760 cm^-1 shows the result distortion vibration of C=C in benzene rings.

FTIR of Chitosan – lignin Polymer:

The chitosan-lignin polymer has been synthesized by mannich reaction in acid medium using buteraldehyde. The cross linking was confirm by comparing the IR spectra of chitosan (Fig. 7) with that of lignin (Fig. 7). The IR spectrum of chitosan has strong peak around 3448 cm ^-1 due to the stretching vibration of O-H, the extension vibration of N-H and inter hydrogen bonds of polysaccharide . In lignin the strong peak around 3451 cm ^-1 could be assigned to the stretching vibration of O-H, The IR spectrum of chitosan-lignin (Fig. 7) has additional sharp absorption peaks at 1640 and 2988 cm^-1 (due to carbonyl stretching and asymmetrical stretching of methyl group, respectively). The extending vibration bands and distortion vibration band of C=C in benzene are observed at 1510 cm ^-1, 1400cm-1 and 650 cm^-1. Both characteristic absorption peaks of chitosan and lignin can be observed in the FTIR spectrum of chitosan-lignin po;ymerprovides a substantial evidence of cross linking of chitosan on to lignin.

Fig.(7): FTIR of Chitosan, Lignin and Chitosan-Lignin polymer

The Thermal Stability Study of the compounds:

In the present study the thermal stability characteristics of the compounds was investigated by TG and DTG technique. TG is one of the familiar techniques for systematic assessment of polymers thermal stability. It is very useful tool and helps to indicate the relative order of stability of various polymers. TG is defined as a continuous measurement of sample weight as a function of time or temperature at a programmed rate of heating. The resulting weight change v.s. temperature (or time) curve gives information about the thermal stability and decomposition of the materials.

The thermogravimetric analysis traces obtained for the polymers heated at a rate of 10°C/ min, which show the dependence of the mass loss of the sample expressed as a percentage of the initial mass and temperature. Also the first derivative is below of them.

From thermogram of weight loss vs. temperature one suggests a mechanism for the degradation of chitosan in the review of the decomposition temperature. Fig.(8) shows the dynamic thermogravimetric analysis of chitosan which showed wt.% loss of 3.15% at 100°C, which can be related to the loss of water molecule from the backbone chain of chitosan . The second loss peak of about 47.3% at 260°C correspond to the cleavage of the NH₂ and OH bond of chitosan moiety forming (NH₃, H₂O) molecules and the loss of this groups for each repeating unit .The third loss peak found in thermogram is proportional to 11.62% wt. loss at 480°C which are attributed to the cleavage of polymer and gaseous products leaving the carbon residue about 37.8% wt.

At the high removal efficiency of chitosan- lignin polymer for all the studied ions, chitosan- lignin polymer amount of 0.3 g is taken as optimum adsorbent dose because no appreciable change in the removal efficiency occurs at higher doses greater than 0.3 g. It is clear that the percent of removal of Cd(II), Co(II), Pb(II) and Zn(II) ions in better in case of synthesized polymer than in case of free chitosan and free lignin itself, as it take less amount of doses, this depends on other effects like pH and agitation time.

Effect of time of agitation on adsorption of metal ions using chitosan – lignin polymer:

The optimum period for the adsorption of Cd(II), Co(II), Pb(II) and Zn(II)ions on the synthesized polymer can be observed by looking for the behavior of adsorption of heavy metal ions solution after adding the synthesized polymer. Fig.(15) shows the effect of agitation period on the adsorption of Cd(II), Co(II), Pb(II) and Zn(II) ions using the synthesized polymer. The adsorption of Cd(II), Co(II), Pb(II) and Zn(II) ions increase with agitation period and attains equilibrium at about 3 h /175 rpm for an initial concentration 100 ppm at pH 6. This behavior may be explained by the availability of the active surfaces for the adsorption. Initially, the number of active sites available for adsorption on the adsorbent surface is high but this number starts to decrease with the progress of adsorption. Finally, adsorption will stop when all active surfaces are covered with the metal ions. This implies that the three heavy metal ions adsorbed on the synthesized polymer possibly by chemical adsorption because chemical adsorption takes places as a monolayer surface coverage rather than multilayer adsorption as in case of physical adsorption. Initially adsorption rate is very high because of the large surface area of the beads available for adsorption. But after the coverage of this surface area by the adsorbed metal ions as a monolayer its adsorption capacity is exhausted and the rate of adsorption controlled by the diffusion rate of adsorbate from external sites to the internal sites.

The experimental results obtained under the optimum conditions shows the highest removal efficiency at agitation time 4 hrs was 94%, 96% , 99.7% , 90% and 97% for Cd(II), Co(II), Pb(II) and Zn(II) , respectively.

Fig.(15) Effect of agitation time using of chitosan-lignin polymer on metal ions adsorption

Desorption study for chitosan – lignin polymers:

The desorption experiments were performed by suspending 0.3 gm of loaded polymers in 10 ml of 3 M HCl and shaking on shaker at 200 rpm at 25°C . After constant time intervals (0.5-24 hrs) the samples were filtered (Whatman filter paper No. 42) and the filterate was analyzed by flame atomic absorption spectrometer (FAAS) for the metal contents. Fig.(16) , shows the recovery percentage of the test metals from the synthesized polymer as a function of the contact time with (3M HCl) .The obtained results shows that the orders of recovery percentage of metal ions was in sequence: Pb > Cd > Co > Zn. This could be related to the strong binding between polymers and ions.

Fig(16): Effect of contact time on the recovery percentage of ions from chitosan-lignin polymer with (3M HCl)

Treatment of wastewater samples by the prepared polymers:

A wastewater sample drained from paper production factory for contains Cd(II), Co(II), Pb(II) and Zn(II)ions solution. The wastewater was treated with the optimum amount of the prepared polymer for 50 ml from wastewater samples and time of agitation as discussed previously with 175 rpm, agitation speed and pH was adjusted to be 6 . The concentration of metal ions of the wastewater before treatment was 8 ppm for Cd(II) ion, Co(II) ion was 5 ppm , Pb(II) ion was 12 ppm and Zn(II) ion was 25 ppm, the amount of the synthesized polymer was 0.3 gm.

The data of removal of each metal ion are given in Fig. (17). Inspection of the data for metal ions in wastewater samples before and after treatment given in Fig. (17) using the synthesized polymer, the order of removal of heavy metal ions in wastewater samples in a separately treatment was Co(II) > Zn(II) > Pb(II)> Cd(II).

ACKNOWLEDGMENT:

The authors gratefully acknowledge the contributions of Prof Dr. S. Archibald in the University of Hull, UK for his benefic contribution of this study.

CONCLUSION:

Natural polymer “Chitosan-Lignin polymer” based sorbent provides axcellant method for metal removing. The characterization of material gives information about molecular weight, crystallinity, good chemical and thermal stability which revels applicability towards metal removing . This low-cost adsorbents are effective for the removal of metal ions from aqueous solutions shows order of % removal efficiency i.e. Zn(II) > Co(II) > Cd(II) > Pb(II) . The batch method was employed parameters such as pH, polymer dose and agitation time were studied at an ambient temperature 25^oC. The optimum pH corresponding to the maximum adsorption of Cobalt, Cadimium, lead and Zinc removal was pH 6 Cobalt, Cadimium, lead and Zinc ions were adsorbed onto the adsorbents very rapidly within the 0.3 gm of polymer for 4 hrs.

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Received on 19.12.2014 Modified on 05.03.2015

Research J. Science and Tech. 7(1): Jan.-Mar. 2015; Page 35-46

DOI: 10.5958/2349-2988.2015.00007.8

Parameters Elements	Slit width(cm)	Lamp current(mA)	Wave length(nm)	Linearity ppm	Air flow rate (L/min)	Acetylene flow rate (L/min)
Cd(II)	0.2	4	228.8	5	8	2
Co(II)	0.2	4	242.5	5	8	2
Pb(II)	0.2	5	217.0	5	8	2
Zn(II)	0.2	5	213.9	4	8	2