A Novel Adsorbent Based on Lignin and Tannin for the Removal of Heavy Metals from Wastewater

 

Hassan T. Abdulsahib*, Abdulamir H. Taobi, Salah S. Hashem

Department of Chemistry, Science College University of Basrah, Basrah, Iraq

*Corresponding Author E-mail: lolaby2005@yahoo.com

 

ABSTRACT:

The adsorption of Cd, Co, Pb and Zn by novel adsorbent polymer based on Lignin and Tannin was investigated. The adsorption of Cd, Co, Pb and Zn by prepared polymer was studied. Polymer was characterized using FTIR, UV, GC-Mass, X-ray spectra DSC and TG. All characterization techniques confirm the existence of Lignin and Tannin. Adsorption of Cd, Co, Pb and Zn ions by prepared polymer was investigated under different conditions. The effect of pH, Dose of polymer and agitation time was studied. The removal efficiency under different conditions was evaluated using atomic adsorption spectroscopy. To improve uptake and make in an industry workable process, this study suggests that modification of the native polymer would be required.

 

KEYWORDS: Lignin, Tannin,  heavy metals, adsorption, wastewater.

 


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(1). 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(2). 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). 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. 1, connected with each other’s through various inter-unit linkages(3), 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(4). 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 formaldehyde free particleboards(5). 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(6).Although it is not the first time that lignin is used as gel precursor, few works exist about gels based on lignin(4).

 

Fig.-1 Schematic representation of structureal unit of lignin: (a) p-coumaryl alcohol (4-hydroxyl phenyl), (b) coniferyl alcohol (guaiacyl), (c) sinapyl alcohol (syringyl)

 

Tannins are high molecular weight polycyclic aromatic compounds widely distributed through the plant kingdom. Tannins can be classified into two groups(7), the proanthocyanidins (or condensed tannins) and the polyesters of gallic acid and (or) hexahydroxydiphenic acid (hydrolysable tannins, respectively, gallo- and ellagitannins). The co-occurrence of both kinds of tannins in the same plant or plant tissue is often observed. Tannins are found in the leaves, fruits, barks, roots and wood of trees(8). The structure of tannin is presented schematically in Figure 2. Complex polysaccharide tannin derivatives have been used extensively in potable water, wastewater and industrial effluent treatment applications(9). In addition tannin helps the filtration process(10) .

 

Figure 2 : Structure of Tannin

 

The idea of using a tannin to purify industrial wastewater, connected to the natural ability of polyphenols to entrapped metal ions with the easy removal system of such a new products(11). The scavenging behaviour of tannins for several kind of metal ions is well-known(12). The study of the adsorption by tannin of copper and lead, which are two of the most common metals in factory’s wastewater, afford to understand the general affinity of   tannin for metal ions (13). today they are also widely used in tannin-modified adhesive formulations (14), as adsorbents for pollution control of industrial effluent(15), and as flocculants (16). Their natural origin is as secondary metabolites of plants(17), occurring in the bark, fruit, leaves, etc. Tannin derivatives have been used extensively in potable water ,wastewater and industrial effluent treatment applications(18).

 

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.

 

MATERIALS AND METHODS:

Materials:

All reagents in this work were of analytic 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 lignin i.e. 4% (w/v) NaOH , HCl and 95% ethanol(2) reagents for preparation of Tannin formaldehyde, ie 10% NaOH and Formaline  (3) reagents for preparation of Lignin-Tannin polymer beads, i.e H3PO4, HCl, ethanolamine, NaHCO3 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 ZnCl2 crystal,  respectively (5) 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 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.

 

Isolation of Tannin:

The Laurus nobilis leaves was cut into pieces and powdered  using grinding machine. The powdered sample was sieved through a pair of 40 and 60 mesh sieve. In order to obtain maximum quantity of tannin, extraction was carried out at elevated temperature. The extraction of 50 gram of sample was carried out with water-ethanol mixture (1:1) in soxhlet apparatus. The tannin extract obtained from different cycles of reflux was collected in a flask and its volume was reduced in rotary evaporator. The concentrated tannin extract was dried at 50°C. The dried tannin extract which contains 64-67% tannin was ground in a morter with a pestle. The powdered sample was sieved (60 mesh) and stored in sample bottle.

 

Synthesis of Tannin formaldehyde:

TF resin was synthesized adopting the following procedure: 10 g tannin was dissolved in 50 ml water, the PH of solution was adjusted to (10-11) by added some drops from 10% NaOH solution, the solution then transferred to a 250 mL 3-necked round-bottom flask equipped with a reflux condenser, a thermometer and a magnetic stirring bar, the temperature of the solution raised to 80°C with stirring for 75 min. Afterwards the solution was allowed to cool to 60°C, then 40 ml from formalin solution was added, the temperature of the mixture was kept at 60°C; the reaction time was about 3 hrs.

 

Tannin- Lignin Polymer Synthesis:

Add 3 drops of O-phosphoric acid in a 250 mL 3-necked round-bottom flask containing 25 ml  ethanolamine, the temperature of the solution raised to 60°C with stirring for 30 min. after that an aqueous solution of 1 gm tannin-formaldehyde in 10 ml  ethanolamine  were added into the reactor by controlling the dropping speed with stirring for 30 min , and then the temperature of the solution raised to 100°C with stirring for 4 hrs . After cooling, solution was neutralized by sodium bicarbonate and then filtered and evaporated in oven. Then a 10 ml of 1% N-methyl-2-pyrroliodine were added, a magnetic stirring bar and mixed for 60 min or until dissolved. 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 3h at room temperature (25°C). Adjusted the pH  to 2 by HCl, the tannin-lignin was obtained.

 

Figure 3: Lignin-Tannin polymer structure.

 

Characterization techniques and instruments

Six methods were used for the characterization of the lignin, tannin  and tannin - 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 polymer 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 10° to 60°.

 

Study of heavy metal adsorption by synthesized Lignin – Tannin 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:

Ultraviolet –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: tannin and lignin was carried out in double beam UV-visible photometer , using dilute solution (3.5×10-3 ) .

 

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 tannin, 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.

 

The U.V. spectra for tannin, show an intense bands at λmax 270 nm for π→ π* transition due to the high conjugation between the π electrons of the benzene ring and the carbonyl group through the carbon-carbon double bond.

 

Fig(4): UV-Visible Spectra of Lignin and Tannin

 

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 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 5 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.

 

Conditions could be found for tannin that giving peaks at 647.2 m/z with retention times under our chromatography conditions around ~15 mins as shown in Fig. 6. Tannin consists of a ratio of gallic acid to glucose units of approximately 12:13. Therefore this tannin could be considered a hydrolysable tannin. The molecular weight of the obtained lignin was 2836.7

 

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

 

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

 

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 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 stretching 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 Tannin:

The poly phenolic tannin compounds have many characteristics bands at certain frequencies. Its FT-IR spectrum is shown in Fig.(7) broad peak at 3412 cm-1 is attributed to polymeric O-H group, the frequency at 2935 cm-1 corresponds to C-H stretching frequency and the peak at 1614 cm-1 has been assigned to C=O, and the wideness of the 1709 cm-1 band can be related to the presence of conjugated carbonyl groups. The presence of the functional group C-O-C in tannin is confirmed from the band at 1207 cm-1, C-H bending frequency is noted at 1340 cm-1. A notable band at 1031 cm-1 can be assigned to C-O stretching. At 759 cm-1 shows the result distortion vibration of C=C in benzene rings. Around 1449 cm-1 stretching vibrations of C-C aromatic groups appear in spectrum. The absorption band at 869 cm-1, corresponds to the characteristic absorption of β-D- glucose unit Table 3.2 shows the important main bands of tannin.

 

FTIR of Tannin-Formaldehyde:

The FT-IR spectra of these resins showed new intense bands in the region 2980-2840 cm-1 attributed to the C-H stretching vibration of the methylene group, which confirms the occurance of hydroxyl methylation for phenolic tannin. Another proof for the hydroxyl methylation is the intense band in the region 3600-3360 cm-1 due to the stretching vibration of –OH group intra and intermolecular hydrogen bonded of the methylol . The C-O stretching vibration of the hydroxyl groups showed very strong band in the region 1040 – 1020 cm-1 (figure 7).

 

Fig.(7) shows the same bands but with different intensities. The absorption intensity of –NH and –OH groups in regions (3200-3700 cm-1) from the obtained compound after the addition of hydroxyl amine to tannin formaldehyde is lower than that of –OH group from tannin formaldehyde which indicates a reaction occurred between hydroxylamine and  tannin formaldehyde.

 

 

 

FTIR of Tannin – lignin:

As shown in Fig.(7) , the absorption peak 3484 cm-1 is affected by extending vibration of O-H and N-H, the frequency at 2935 cm-1 corresponds to C-H stretching frequency and the peak at 1617 cm-1 has been assigned to C=O and the band at 1706 cm-1 can be related to the presence of conjugated carbonyl groups , the extending vibration bands and distortion vibration band of C=C in benzene are observed at 1518cm-1, 1451 cm-1 and 759 cm-1. Both characteristic absorption peaks of tannin and lignin can be observed in the FTIR spectrum of lignin- tannin  polymer indicating that tannin is modified by lignin successfully.

 

Fig.(7): FTIR of Lignin, Tannin and Lignin-Tannin 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. The samples of lignin were subjected to thermogravimetric analysis in order to study their thermal behavior. As shown in Fig.(8), the sample showed a weight loss around 4%wt. at 100oC that was associated to the moisture present in the lignin samples, that can be attributed to hemicelluloses degradation products. Between 200 and 300oC another weight loss was observed (39.98%wt.) that can be related to the presence of hemicelluloses.

 


 

Fig.(8): Thermogravimetric diagram of Lignin

 

Fig.(9): Thermogravimetric diagram of Tannin.

 


Lignin degradation occurred slowly in a wide range of temperatures with maximal mass loss rate between 350 and 650 oC which about 14%wt., this fact being associated to the complex structure of lignin with phenolic hydroxyl, carbonyl groups and benzylic hydroxyl, which are connected by straight links. Lignin samples presented high percentage of final residue (41.95%wt.) due to lignin aromatic polycondesations.

 

From thermogram of the tannin degradation, three distinct mass loss peaks can be seen in Fig.(9), a week peak centered at 150 oC where almost 2.35% of weight due the post curing, thermal reforming, preliminary oxidation steps and elimination of volatile fractions. The second peak is sharper and more pronounced and it is found at 305oC which about 63.3% where the degradation tannin begins and it could be the result of partial breakdown of the intermolecular bonding. Third degradation of tannin takes place after 450 oC with remark peak at 580 oC, in this section is seen a mass loss of 6.56% with 27.73% of carbon residue

 

Concerning tannin-lignin polymer, three stages of decomposition are shown in Fig.(10). The first stage at 100°C related to the loss of water molecules (4.75%). The second stage at 280°C can be related to the loss of buteraldehyde molecules (55.85% wt.) from the cleavage of the methylene group. The third stage is equivalent to 10.63% wt. loss at 500 oC corresponds to the depolymerization of tannin and lignin with remains 28.78% wt. of carbon.

 


 

Fig.(10): Thermogravimetric diagram of Lignin-Tannin polymer

 


Differential scanning calorimetry (DSC)

Differential scanning calorimetry can be used to measure a number of characteristic properties of a sample. This technique is used widely for examining polymeric materials to determine their thermal transitions. The sample undergoes a physical transformation such as phase transition which is exothermic or endothermic depending on the type of sample. DSC may also be used to observe more physical change such as glass transition temperature (Tg), crystallization temperature (Tc), melting of polymers (Tm), heat capacity, thermal of expansion and for studying polymer curing. From DSC thermo grams several parameters can also be determined like curing reactions, energy of curing, melting temperature, activation energy of curing, degree of crystallization, charging enthalpy and degree percentage of curing(124). Using it is possible to Glass transitions may occur as the temperature of an amorphous solid is increased. As the temperature increases, an amorphous solid will become less viscous. At some point the molecules may obtain enough freedom of motion to spontaneously arrange themselves into a crystalline form. This is known as the crystallization temperature (Tc). This transition from amorphous solid to crystalline solid is an exothermic process(the cross-linking of polymer molecules that occurs in the curing process), and results in a peak in the DSC signal that usually appears soon after the glass transition. As the temperature increases the sample eventually reaches its melting temperature (Tm). The melting process results in an endothermic peak in the DSC curve.

 

Lignin displayed a DSC curve (Figure11) with endothermic peak at 100ºC corresponding to the loss of hydration. When the 386.18ºC was reached, a sharp exothermic peak corresponding to the melting was apparent.


 

Fig.(11): DSC Thermogram of Lignin-Tannin and Lignin-Tannin polymer

 


Fig.(11) shows the DSC thermogram of tannin which showed two endothermic peaks, The first one is a wide peak which occur at 114.96 ºC corresponding to a dehydration. The second endothermic peak (232.6ºC) corresponded to the chemical bonds decomposition of tannin chains. At temperatures 364.62ºC the presence of one exothermic peak corresponding to the melting of tannin.

 

The DSC thermogram of lignin -tannin polymer (Fig. 11) showed an endothermic peak at 104.85 ºC corresponding to the loss of hydration and at about 205.10 ºC the glass transition temperature is appeared in a medium exothermic peak. When the range 205.10- 480 ºC was reached, a broad exothermic peak corresponds to the decomposition of the lignin -tannin polymer. 

 

X-ray Diffractometry:

X-ray spectroscopy is unarguably the most versatile and widely used means of characterizing materials of all forms. There are two general types of structural information that can be studied by X-ray spectroscopy: electronic structure (focused on valence and core electrons, which control the chemical and physical properties, among others) and geometric structure (which gives information about the locations of all or a set of atoms in a molecule at an atomic resolution). This method encompasses several spectroscopic techniques for determining the electronic and geometric structures of materials using X-ray excitation: X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy (XES), X-ray photoelectron spectroscopy (XPS) and X-ray Auger spectroscopy. Which type of X-ray spectroscopy is employed depends on whether the target information is electronic, geometric or refers to oxidation states. X-ray spectroscopy is thus a powerful and flexible tool and an excellent complement to many structural analysis techniques. The properties of polymers depended mostly on the molecular weight, polydispersity and crystallinity. XRD Commonly used to measure crystallinity, The crystallinity index (CI) can be calculated on the basis of X-ray diffractograms. Postulating the following equation for determining the crystallinity index (CI):

 

CI (%) = [(Im - Iam)/I110] × 100

 

Where: Im (arbitrary units) is the maximum intensity of the crystalline peak at around 2θ = 51°, and Iam (arbitrary units) is the amorphous diffraction at 2θ = 15°. In most cases, CI provides information about the crystal state. crystallinity could also be assigned from an X-ray diffractogram by dividing the area of the crystalline peaks by the total area under the curve (background area). In these calculations, the crystallinity percentage supplied information on relative crystallinity.

 

It was observed that the X-ray diffractogram (Fig.12) of lignin shows an almost amorphous structure (71%), the bands at 2θ = 42° and 51°. Figure (12) shows the X-ray diffraction patterns of the tannin sample showed strong reflections at 2θ around 42° and 2θ of 51°. Tannin have crystalline region and non-crystalline region. The crystallizations of tannin were 50%. In turn, the X-ray diffraction patterns of the Lignin-Tannin polymer showed that the bands at 2θ = 42° and 51° decreased significantly after cross linking (Fig.12), and that this was followed by a dramatic increase in the crystallinity percentage which was found to be (86%). It was therefore concluded that the crystallizations is influenced by components, reaction condition and so on.

 

Fig(12): X-Ray Spectra of Lignin, Tannin and Lignin-Tannin polymer

 

Treatment of an artificial solution by the prepared polymers:

A 100 ppm solution of Cd(II), Co(II), Pb(II) and Zn(II) were prepared by dissolving an accurate weight of the metal salt in distilled water. The metal content of the standard solution was then determined by using flame atomic absorption spectrometry.


 

 

 

Table(1) : The optimum conditions for the studied metal ions.

  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


The adsorption experiment of all polymers under investigation were prepared by mixing 50 ml of 100 ppm of Cd(II), Co(II), Pb(II) and Zn(II) ions separately with appropriate amounts of, chitosan- lignin, . The samples were subjected to stirring for a period of time then filtered, after filtration the samples were analyzed for their heavy metal ions content by using flame atomic absorption spectrometry at the optimum conditions for the studied ions listed in Table (1).

 

Preliminary experiments were carried out to assess the optimum conditions for the removal of Cd(II), Co(II), Pb(II) and Zn(II) ions from prepared solutions as well as from wastewater samples drained from Paper production factory. These conditions include: (1) the effect of pH, and (2) Amount of polymer used, (3) The effect of time of agitation.

 

The initial metal ions concentration of synthetic solution flow were 100 ppm of Cd(II), Co(II), Pb(II) and Zn(II) ions. In these experiments dry polymer powder were carefully transferred into four 100 ml glass-stoppered flask containing 50 ml of Cd(II), Co(II), Pb(II) and Zn(II) ions solutions separately and shaken at 175 rpm for 24 hrs. After filtration of the mixtures 25 ml aliquots were used to determine unreacted metal contents of the solutions. From the difference of the metal contents in the initial and final synthetic solutions, the removal efficiency of Lignin-Tannin polymer was calculated by using the following equation:

 

Removal efficiency  

 

Where, CO(mg/l) is the initial concentration of metal ions in the solution, Ci (mg/l) is the final concentration of metal ions in the solution.

 

Effect of pH on adsorption of metal ions using Lignin-Tannin polymer

The dependence of amount of adsorption of Cd(II), Co(II), Pb(II) and Zn(II) ions on pH is shown in Fig. (13) . The adsorption increased with increase of pH of the solution, in case of Cd(II) ions which show an optimum adsorption of 95 % at pH 6, while in case of Co(II) ions show an optimum adsorption 94 % at pH 6 . At pH 6 the optimum adsorption of  96 % in case of Pb(II) while it was found to be 94% in case of Zn(II).This could be explained on the premise that at low pH value, hydroxyl groups and amine groups in the tannin-lignin polymer easily form protonation that induced an electrostatic repulsion of Cd(II), Co(II), Pb(II) and Zn(II) ions. At low pH value of solutions H3O+ concentration increase and intensifies the competition between H3O+ and heavy metal ions for complexation sites. However, above pH 8 the amount of adsorption of M(II) decreased with increase pH value, owing to the interaction between OH- and M(II) ions in the solution to form M(OH)2. This behavior can be explained by the nature of Lignin-Tannin polymer at different pH values, the type of ionic state of functional group of polymer and the metal ions.

 

Fig (13) : Adsorption of metal ions using of Lignin-Tannin polymer as a function of pH.

 

 

Effect of amount of Lignin-Tannin polymer on the adsorption of metal ions

For Lignin-Tannin polymer, Fig. (14), presents Cd(II), Co(II), Pb(II) and Zn(II) ions removal efficiency as a function of polymer dosage. The dose of the Lignin-Tannin polymer was varied between 0.1 gm to 1 gm for 100 ppm of metal ions. Other operational parameters (pH, agitation time) were kept at the optimum value while agitation speed are kept at 175 rpm.

 

As shown in Fig. (14), the increase in dose of the Lignin-Tannin polymer increasing Cd(II), Co(II), Pb(II) and Zn(II) ions removal efficiency. These results are expected because more biding sites for ions are available at higher dose of Lignin-Tannin polymer. At the high removal efficiency of Lignin-Tannin polymer for all the studied ions, Lignin-Tannin 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.

 

Fig. (14) Effect of the weight of Lignin-Tannin polymer on metal ions adsorption

 

At the high removal efficiency of Lignin-Tannin polymer for all the studied ions, Lignin-Tannin 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 Lignin-Tannin polymer

The optimum period for the adsorption of Cd(II), Co(II), Pb(II) and Zn(II) ions using tannin-lignin polymer can be observed by looking for the behavior in adsorption of heavy metal ions solution after adding Lignin-Tannin polymer. Fig.(15) show the effect of agitation period on the adsorption of Cd(II), Co(II), Pb(II) and Zn(II) ions using Lignin-Tannin polymer . The adsorption of Cd(II), Co(II), Pb(II) and Zn(II) ions increases with agitation period and attain equilibrium at about 4 h /175 rpm for an initial concentration 100 ppm at pH 8 .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 using carboxylic starch resin 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 the adsorption rate is very high because of the large surface area of the beads available for adsorption. But after the coverage of this surface by the adsorbed metal ions as a monolayer its adsorption capacity was exhausted and the rate of adsorption will be 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 an initial concentration of 100 ppm were 98%, 97% , 98% and 99% for Cd(II), Co(II), Pb(II) and Zn(II) , respectively.

 

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

Desorption study for Lignin-Tannin polymer

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:   Co  >  Zn  >  Cd  > Pb 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 Lignin-Tannin 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) > Pb(II) > Cd(II) > Zn(II ).

 

Fig.(17) Treatment of industrial wastewater sample contains metal ions using Lignin-Tannin polymer

 

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:

An excellent method for metal ions removing using natural polymer based on Lignin-Tannin polymer was provides. 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. Co(II) > Pb(II) > Cd(II) > Zn(II). The batch method was employed parameters such as pH, polymer dose and agitation times were studied at an ambient temperature 25oC. 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 24.11.2014       Modified on 29.12.2014

Accepted on 15.01.2015      ©A&V Publications All right reserved

Res.  J. Pharmacognosy & Phytochem. 7(1): Jan.-Mar. 2015; Page 38-48

DOI: 10.5958/0975-4385.2015.00009.6