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Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND EXPERIMENTAL METHODS
  5. RESULTS AND DISCUSSIONS
  6. CONCLUSIONS
  7. AKNOWLEDGMENTS
  8. REFERENCES

A new chelating terpolymer resin of sulphanilic acid–dithiooxamide–formaldehyde (SDTOF) was synthesized. Dithiooxamide–formaldehyde (DTOF) was prepared by the reaction of dithiooxamide and formaldehyde. These resins were characterized using Fourier transform infrared spectroscopy, proton nuclear magnetic resonance, and scanning electron microscopy. Chelating resins beads were applied for adsorption of nickel (II) ions by batch and column techniques. Sorption experiments were performed by varying pH, agitation time, sorbent dosage and initial concentration of nickel (II) ion solution. SDTOF and DTOF resins showed adsorption capacity of 188.3 and 99.8 mg g−1, respectively. Nickel adsorption isotherms data were fitted to Langmuir isotherm. Kinetic studies showed the adsorption process followed pseudosecond-order rate model. Desorption of Ni(II) ions was done using 0.1 M HCl, HNO3, and ethylenediamine tetraacetic acid solutions. The reusability of SDTOF and DTOF resins for the removal of Ni(II) ions was also determined after 10 sorption−desorption cycles. POLYM. ENG. SCI., 55:163–172, 2015. © 2014 Society of Plastics Engineers


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND EXPERIMENTAL METHODS
  5. RESULTS AND DISCUSSIONS
  6. CONCLUSIONS
  7. AKNOWLEDGMENTS
  8. REFERENCES

Ni(II) ion and its various compounds were used in various industries such as metal plating facilities, mining operations, fertilizer industries, batteries, paper industries and pesticides, and wastewaters are directly or indirectly discharged into the environment. Removal of Ni(II) from industrial effluents has a primary importance because contamination of wastewater causes very serious health and environmental problems. Excessive accumulation of nickel might bring about serious lung and kidney problems [1-3]. Generally, nickel exists as Ni(II) in aqueous solution. It is reported that Ni(II) concentration in wastewaters varies from a low value of 0.5 mg L−1 to a high value of 1000 mg L−1 while the maximum permissible concentration of Ni(II) in aqueous media is only 0.02 mg L−1 [4-6].

Until now, many studies have been carried out on heavy metal removal by various methods such as precipitation, reduction flocculation, membrane filtration, reverse osmosis, solvent extraction and electrolysis, biological treatment, ion-exchange processes, and adsorption. Among them, adsorption technique has advantage of high efficiency, adsorbate specificity, and cost effectiveness [7-9]. Also, adsorption has the additional advantages of applicability at very low concentrations, suitability for using batch and continuous processes, ease of operation, little sludge generation, and possibility of regeneration and reuse [10].

Many critical reviews available on heavy metal removal from aqueous solutions using various adsorbents [6, 11-13] recommend the need for additional research to optimize the procedures for the synthesis of adsorbents followed by their application in heavy metal removal process. The existing literature gives only limited information on the removal of Ni(II) by various adsorbents from aqueous solutions as compared with the data available for other heavy metals such as Hg(II), Cd(II), Pb(II), and Cr(VI) [14, 15]. The adsorptive removal of Ni(II) onto various adsorbents such as bentonite clay [16], kaolinite, and montmorillonite [17] have been reported, whereas studies on the applicability of activated carbons for the adsorption of Ni(II) from aqueous solutions are rare in the literature [18, 19]. A reactively fibrous adsorbent prepared by graft copolymerization of methacrylic acid/acrylamide monomer mixture onto poly(ethylene terephthalate) fiber showed adsorption capacity 43.48 mg g−1 for Ni(II) [20]. Histidine-modified chitosan and sodium polyacrylate grafted activated carbon showed the capacity for nickel adsorption 55.6 and 55.7 mg g−1, respectively [1, 21], whereas activated carbon obtained from sugarcane bagasse showed higher adsorption capacity of 140.85 mg g−1 for the removal of Ni(II) from aqueous solutions [22].

Thiourea functional group has been widely used in chelating resins for the adsorption of several metal ions. Dithiooxamide (DTO) has similar molecular structure to thiourea (thiooxamide) molecule and it includes more sulfur (S) donor atoms. 2,4-Dihydroxyacetophenone–dithiooxamide–formaldehyde terpolymer is an example of chelating resin functionalized with DTO and used for adsorption of Fe(III), Cu(II), Ni(II), Co(II), and Zn(II) ions [23], polystyrene–divinylbenzene functionalized with DTO has been used for removal of Cu(II), Zn(II), Cd(II), and Pb(II) ions [24], and DTO groups incorporated into a matrix of polystyrene–divinylbenzene have been used for Pd(II) adsorption [25]. Dithiooxamide–formaldehyde (DTOF) chelating resin has been used for the adsorption of silver metal ion. DTOF resin can be synthesized similar to thiourea formaldehyde resin, and it can be used in the separation of silver ions [26].

This work aimed to synthesize a new sulphanilic acid–dithiooxamide–formaldehyde (SDTOF) terpolymer resin. The synthesized resin was applied for adsorption of nickel (II) ions comparing with the conventional DTOF chelating resin. The parameters affecting adsorption process as initial pH, agitation period, sorbent dosage, and initial concentration of Ni(II) ions were studied. Also, different isotherms and kinetic models were studied.

MATERIALS AND EXPERIMENTAL METHODS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND EXPERIMENTAL METHODS
  5. RESULTS AND DISCUSSIONS
  6. CONCLUSIONS
  7. AKNOWLEDGMENTS
  8. REFERENCES

Materials

All chemicals used were of analytical grade and supplied by Sigma. The chemicals used in this study were nickel (II) nitrate hexahydrate as a source of nickel ions. Sulphanilic acid, DTO, and formaldehyde (37%) for synthesis of DTOF and SDTOF resins. Hydrochloric acid and sodium hydroxide were used for pH adjustment.

Synthesis of Dithiooxamide Formaldehyde Copolymer Resin

DTOF resin was prepared by condensing DTO and formaldehyde solution. In this experiment, 0.1 mol (12 g) of DTO was added to 25 mL 0.1 M NaOH solution and then the mixture was poured into 0.15 mol (12 mL) formaldehyde solution (37%). pH was adjusted to 10 with 0.1 M NaOH solution and temperature was raised to 80°C. After complete dissolution, pH was adjusted to 2 with 0.1 M HCl solution, while the mixture was stirred at the same temperature. The orange-colored DTOF resin condensates converted gradually into reddish brown precipitate and the reaction was allowed to stand for 6 hr. The precipitated resin was filtered, washed with acid and water, dried at 80°C and ground into fine particles with mortar.

Synthesis of SDTOF Terpolymer Resin

The SDTOF terpolymer resin was prepared by condensation polymerization of sulphanilic acid (0.1 mol, 17.3 g ) and DTO (0.1 mol,12 g) with formaldehyde (0.2 mol, 16 mL). The pH of the reaction mixture was adjusted to 2 using 0.1 M hydrochloric acid and the reaction temperature was raised up to 140 °C. The reaction mixture begins to condensate and the precipitated resin was formed gradually. The reaction was carried out for 6 hr. The resulting product was then cooled, poured into crushed ice with constant stirring and left overnight. The orange brown-colored resin obtained was washed with warm water and acetone and then filtered off to remove the unreacted monomers. Finally, the terpolymer resin was air dried. The dried resin was further purified by dissolving it in 8% NaOH and regenerated in 1/1 (v/v%) HCl/water solution [11, 12]. This process was repeated twice to separate the pure polymer.

Instrumentation and Characterization of the Synthesized Adsorbents

Nickel (II) nitrate hexahydrate solution of 200 mg L−1 concentration was prepared in deionized water as stock solution. A single beam Shimadzu UV-2401PC spectrophotometer with 1 cm cell was used for measuring all absorption data. pH Meter (pH30-GS Glass pH sensor electrode) with a combined double junction glass electrode was used for pH determination. The pH adjustments were carried out using dilute NaOH and HCl solutions. The Fourier transform infrared spectroscopy (FT-IR) spectrum of the synthesized resin sample was scanned in KBr pellets in the range of 4000–400 cm−1 using a JASCO 6100 FT-IR spectrophotometer, Nicolet. The proton nuclear magnetic resonance (1H–NMR) spectrum of the resin was recorded in DMSO-d6 solvent using a Varian 1H-Gemini 200 spectrometer using tetramethyl silane as an internal reference. The surface morphology of the DTOF and SDTOF resins and their complexes with nickel ion was analyzed by scanning electron microscopy (SEM) using JEOL JXA-840A Electron Probe Microanalyzer on gold coated specimens. The SEM observation was carried out with secondary electrons imaging and acceleration of the electron beam at 10 kV.

Adsorption Studies

Batch equilibrium method was used to perform sorption experiments. Stock solution of nickel (II) nitrate hexahydrate containing 200 mg L−1 was prepared and this was used for sorption experiments. These experiments were carried out in duplicate by mixing 25 mg of sorbent with 20 mL of stock nickel ion solution. The contents were agitated thoroughly using a magnetic stirrer at a speed of 300 rpm. After equilibrium, the sample was centrifuged and the filtrates were analyzed by a UV–visible spectrophotometer and the Ni(II) ion concentration was found by forming complex with dimethyl glyoxime at 470 nm by dimethylglyoxime method [27]. In this method: definite weight of nickel (II) nitrate hexahydrate was dissolved in distilled water, 2 mL concentrated HCl was added and different concentrations of nickel ion solution were prepared by dilution of the basic nickel ion solution and placed in separated flasks. To each flask of working nickel standard solution, 10 mL ammonium citrate, 5 mL iodine solution and 20 mL dimethylglyoxime solution dissolved in methanol and the ingredients were mixed well to give red to brownish red color. To prepare calibration curve, the prepared standards were ready to be used after 10 min but not later than after 30 min. Samples were transferred to the cuvette (absorption cell) and absorption at λmax = 470 nm was measured. The linear calibration curve was drawn as the function of absorption in different concentration of nickel ions (mg mL−1). The unknown concentrations of nickel ion solution undergoes the same steps and the concentrations were read from calibration curve.

The effect of pH on the adsorption of Ni(II) ions was studied in a pH range of 2–6. The pH of the initial 20 mL solution of stock nickel ion solution was adjusted to the required pH value using appropriate buffers. The effect of agitation period was studied at room temperature and under stirring rate of 300 rpm with 25 mg of adsorbent. The influence of sorbent dosage was conducted in a range of 0.6–3.6 g under the optimum pH and agitation period. Different concentration of nickel solution from the range of 50–1000 mg L−1 was studied under the optimum pH, sorbent dosage and agitation period as has been determined from the previous step.

The amount of metal ion adsorbed was calculated according to the following equation:

  • display math(1)

where Qe is the amount (mg g−1) of metal ions adsorbed by DTOF copolymer or SDTOF terpolymer resins, Co and Ce are the metal ion concentrations (mg L−1) in the solution initially and after adsorption, respectively, V the volume (L) of the solution, and m is the mass (g) of adsorbent used.

Isotherm Studies

Freundlich, Langmuir, Temkin, and Dubinin–Radushkevich isotherms were adopted to quantify the sorption capacity of the sorbents studied for nickel (II) ion removal. In order to investigate and analyze these isotherms, 20 mL of nickel (II) ion solution was mixed with different amounts of the studied resins (0.6–3.6 g L−1). The mixture was stirred for 360 min for DTOF resin and 240 min for SDTOF resin at 25°C, the pH was 3.0 for DTOF resin and 4.0 for SDTOF resin and the initial nickel (II) ion concentration was 1000 mg L−1. The mixture was filtered and the filtrate was analyzed for nickel ion concentration using UV-visible spectrophotometer. The data were fitted into Langmuir, Freundlich, Temkin, and Dubinin-Raduskevich isotherms.

Kinetic Studies

Adsorption kinetic studies were carried out for both DTOF and SDTOF resins, 0.1 g of the DTOF or SDTOF resins were added into 100 mL of a nickel (II) ion solution with initial concentration, 1000 mg L−1 and pH of 3.0 for DTOF resin and 4.0 for SDTOF resin. The mixture was stirred magnetically at 25°C and samples were taken from the solution at desired time intervals for the analysis of nickel (II) ion concentrations in the solution using UV-visible spectrophotometer.

Column Studies

The column breakthrough experiments were carried out in a glass column of dimensions 10 cm height and 2.5 cm diameter. An amount of 1 g of sorbent particles were packed within the column between two filterable membranes at the top and bottom end to prevent the sorbent from floating. The feeding solution containing Ni(II) was pumped continuously through the column at desired volumetric flow rate using a peristaltic pump. Effluent samples were collected at different intervals. The flow rate and the feed concentration of Ni(II) ions solution were (10.0–15.0 mL min−1) and 200 mg L−1, respectively.

Desorption Experiments

For these studies, various desorbing agents including ethylenediamine tetraacetic acid (EDTA), HCl, and HNO3 with concentration of 0.1 M were used. 0.1 g of sorbent was loaded with 20 mL of 200 mg L−1 Ni(II) solution under the optimum conditions. Ni(II) loaded sorbents were filtered and washed with distilled water to remove any unadsorbed Ni(II). The amount of Ni(II) adsorbed was determined by measuring the residual nickel concentration. The sorbents were agitated with 25 mL of 0.1 M EDTA, HCl, and HNO3 solutions and the amount of desorbed Ni(II) was determined as before. Desorption efficiency for Ni(II) is calculated as the ratio between the amount of Ni(II) desorbed and amount of Ni(II) adsorbed.

RESULTS AND DISCUSSIONS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND EXPERIMENTAL METHODS
  5. RESULTS AND DISCUSSIONS
  6. CONCLUSIONS
  7. AKNOWLEDGMENTS
  8. REFERENCES

Preparation and Characterization of DTOF and SDTOF Resins

The DTOF and SDTOF resins are reddish brown in color. DTOF resin was synthesized by hydroxymethylation of amino groups of DTO in basic aqueous solution followed by condensation in acidic aqueous solution and SDTOF resin was also prepared by hydroxymethylation of amino groups of DTO in basic medium followed by condensation with sulphanilic acid in acidic medium. As illustrated in the experimental section, the chemical reaction of the reactants and the chemical structure of the products (DTOF copolymer and SDTOF terpolymer resins) are represented in Scheme 1.

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Scheme 1. The suggested chemical structure of DTOF copolymer and SDTOF terpolymer resins.

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The structures of DTOF copolymer and SDTOF terpolymer resins are assigned on the basis of FT-IR, and 1H-NMR spectroscopy techniques.

FT-IR Spectral Studies

Figure 1 shows FT-IR spectra of DTOF and SDTOF resins before and after Ni(II) ion loading in the range of 4000–400 cm−1. The characteristic peaks for DTOF and SDTOF resins are: stretching vibrations of secondary amine at 3423 cm−1 for SDTOF resin; stretching band of N–(C[DOUBLE BOND]S)– at 1666 cm−1 for DTOF resin and at 1652 cm−1 for SDTOF resin; vibration band of C[DOUBLE BOND]S at 1084 cm−1 for both DTOF and SDTOF resins. On the other hand, the most characteristic peaks for SDTOF resin are: a strong band at 1298 cm−1 is due to C-N stretching of (Ar-NH2) [28]. The consistent appearance of band at 960 cm−1 is attributed to –C[DOUBLE BOND]S [29]. A peak at 2829.5 cm−1 may be assigned to aromatic ring stretching modes [30]. Finally, the peak appeared at 2993 cm−1 may be attributed to –CH2 linkage present in the terpolymer resin [26].

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Figure 1. FT-IR spectra of DTOF, DTOF-loaded Ni2+, SDTOF, and SDTOF-loaded Ni2+ resins.

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The mechanism of interaction between adsorbate (metal ion) and active groups or functional groups on the surface of adsorbents (DTOF and SDTOF resins) can be studied on the basis of FT-IR spectroscopy and depends on the chemical structure of DTOF and SDTOF resins. DTOF resin contains sulfur and nitrogen as active centers for adsorption of nickel(II) ions, whereas SDTOF resin has in addition to sulfur and nitrogen moieties, SO3H, primary and secondary amine groups and these functional moieties affect adsorption mechanism of nickel ions. it is proposed that the mechanism of adsorption may relate to the dispersion forces, complexation, ion exchange, hydrogen bond, or electrostatic interaction [31]. Figure 1 represents interpretation on the mechanism of nickel adsorption onto DTOF and SDTOF resins. There is considerable decrease in the band intensity at 1666 and 1652 cm−1 of N–(C[DOUBLE BOND]S)– for DTOF and SDTOF resins, respectively, after Ni2+ ion loading which give possibility of forming surface complex in which a lone pair of electrons from the N and S atoms were shared with the nickel ions and the noticed shift in the bands at 1298, 3423, and 2800 cm−1, for C-N stretching of Ar-NH2, stretching vibrations of secondary amine and OH of sulphonic acid group, respectively, indicates that adsorption at primary, secondary amine nitrogen and sulphonic acid group follows ion exchange mechanism.

1H-NMR Spectral Study

1H-NMR spectrum of DTOF copolymer and SDTOF terpolymer resins is depicted in Fig. 2 and the spectral data are given in Table 1. The multiplet signals observed in the range of 6.8–7.2 ppm are indication of the presence of aromatic protons [32] of sulphanilic acid moiety for SDTOF terpolymer resin. An intense signal appeared in the region of 4.8 ppm is assigned to the –NH2 protons of Ar-NH2 group in the terpolymer resin [30]. The signals appeared in the region of 2.3–4.91 ppm is due to the methylene proton of Ar-CH2 bridge in the SDTOF terpolymer resin [33]. The signal in the region of 7.9 ppm is due to the presence of –NH bridge in the terpolymer [34]. A signal appeared triplet in the range of 7.4–7.5 ppm is due to the presence of the amido proton of –CH2–NH–CS linkage [23]. The intense signal at 9.6 ppm is attributed to proton present on –SO3H groups.

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Figure 2. 1H-NMR spectra of DTOF copolymer and SDTOF terpolymer resins.

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Table 1. 1H-NMR spectral data of DTOF copolymer and SDTOF terpolymer resins in DMSO-d6.
Chemical shift, δ (ppm)Nature of proton assigned
DTOF copolymer resinSDTOF terpolymer resin
6.8–7.2Aromatic proton (Ar–H)
4.8Ar–NH2
2.3–4.91Ar–CH2– moiety
9.6Proton of SO3H– group
7.0–7.77.9Amido proton of NH of –CH2–NH–CS– linkage
Scanning Electron Microscopy (SEM)

SEM images of DTOF, SDTOF, and DTOF-loaded Ni2+ and SDTOF-loaded Ni2+ resins are shown in Fig. 3a-d. Figure 3a and b shows structural changes for DTOF resin before and after nickel loading, respectively. Before nickel adsorption, the DTOF resin structure seems porous and after nickel adsorption, the structure of DTOF resin becomes more tighter. The morphology of SDTOF resin before and after nickel adsorption is illustrated in Fig. 3c and d, SDTOF resin showed tightly smooth structure before adsorption while after adsorption, its structure clearly changed and nickel metal ions appeared on the surface as aggregates with different particle sizes.

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Figure 3. SEM images of (a) DTOF, (b) DTOF-loaded Ni2+, (c) SDTOF, and (d) SDTOF-loaded Ni2+.

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Factors Affecting Removal Efficiency

Effect of pH of Solution

Figure 4 represents the effect of solution pH on the sorption capacity of sorbents (DTOF copolymer and SDTOF terpolymer resins). It was regarded that the removal of nickel ion from aqueous solution is dependent on pH of the solution and in many cases it altered the surface charge of the sorbent. Therefore, the sorption capacity of the DTOF copolymer and SDTOF terpolymer resins were performed at different pH levels in the range of 1.5–6.0. All the variables such as, contact time, sorbent dosage and initial Ni(II) concentration kept constant. It was observed that maximum sorption capacity was observed at pH 3.0 (95.1 mg g−1) for DTOF copolymer resin and at pH 4.0 (185.7 mg g−1) for SDTOF terpolymer resin. So, optimum pH was found to be 3.0 for DTOF resin and 4.0 for SDTOF resin. For DTOF copolymer resin, the sorption capacity increased gradually from pH 1.5 to 3.0, after pH 3.0, sharp decrease in sorption capacity was observed. For SDTOF terpolymer resin, sorption capacity was found to increase from pH 1.5 up to pH 4.0 followed by gradual decrease in sorption capacity at pH (4.0–6.0). it is clear that only a narrow pH range 3.0–4.0 is favorable for the adsorption of Ni(II) on SDTOF terpolymer resin.

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Figure 4. Effect of pH on the adsorption capacity of DTOF and SDTOF resins (agitation time, 24 hr, sorbent dosage, 1 g L−1, Co, 200 mg L−1).

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Effect of Agitation Period

The effect of agitation period on the adsorption of Ni(II) onto DTOF copolymer and SDTOF terpolymer resins is depicted in Fig. 5. For DTOF copolymer resin, the adsorption capacity of the resin for Ni(II) increases gradually with time to approach maximum value and finally attain equilibrium at nearly 6 hr. For SDTOF terpolymer resin, it was found that the time required for equilibrium attainment was reduced in comparison with DTOF copolymer resin, the maximum adsorption capacity of SDTOF resin for Ni(II) was achieved after 4 hr and this could be attributed to the addition of sulphanilic acid moiety which increases chelating sites, so, better results were obtained in case of applying SDTOF terpolymer resins.

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Figure 5. Effect of agitation time on the adsorption capacity of DTOF and SDTOF resins (pH, 3.0 for DTOF resin, 4.0 for SDTOF resin, sorbent dosage, 1 g L−1, Co, 200 mg L−1).

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Effect of Sorbents Dosage

Figure 6 shows the effect of DTOF and SDTOF resins concentration (g L−1) on the adsorption capacity for Ni(II). The adsorption capacity decreased as DTOF and SDTOF resins concentration increased. It is well known that, adsorption capacity decreased as adsorbent concentration increased because of the increase in unsaturated sorption site for metal ions [35]. In the present study, the adsorbent dosages were varied from 0.6 to 3.6 g L−1 for Ni(II) solution with initial concentration 200 mg L−1, while all the other variables such as pH and agitation time were kept constant. It was concluded that optimum concentration for DTOF and SDTOF resins was chosen as 1 g L−1 which shows adsorption capacity of 95.1 and 185.7 mg g−1 and this was efficient in terms of economical process.

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Figure 6. Effect of adsorbent dosage on the adsorption capacity of DTOF and SDTOF resins (pH, 3.0 for DTOF resin, 4.0 for SDTOF resin, agitation time, 6 hr for DTOF resin and 4 hr for SDTOF resin, Co, 200 mg L−1).

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Effect of Initial Nickel Concentration

The initial Ni(II) ion concentration has a pronounced effect on the removal of adsorbate species from aqueous solution. The effect of initial Ni(II) concentration on adsorption capacity was studied in the range of 50–1000 mg L−1. These experiments were performed under constant variables, pH (3.0 for DTOF resin and 4.0 for SDTOF resin), agitation period (6 hr for DTOF resin and 4 hr for SDTOF resin) and adsorbent dosage of 1 g L−1. Figure 7 represents data of adsorption capacity at initial Ni(II) concentration range (50–1000 mg L−1). From this plot, it was found that as the initial Ni(II) ions concentration increased from 50 to 1000 mg L−1, the adsorption capacity of DTOF and SDTOF resins for Ni(II) ions increased from 22.3 to 266.3 mg g−1 and from 40.2 to 330.7 mg g−1 resin, respectively. This may be explained as a higher Ni(II) ion concentration led to more binding sites on the surface of DTOF and SDTOF resins and increased driving force to overcome the mass transfer resistance of Ni(II) ions between the aqueous and solid phases resulting in higher chance of collision and intensive interaction between the Ni(II) ions and the resins. SDTOF terpolymer resins showed better adsorption capacity values than DTOF copolymer resin due to addition of sulphanilic acid moiety in the terpolymer resin which introduce another chelating sites (NH2 and SO3H) through terpolymerization reaction, this resulted in increasing concentration of chelating sites per gram of resin, hence, the adsorption efficiency increased.

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Figure 7. Effect of initial Ni(II) concentration on the adsorption capacity of DTOF and SDTOF resins (pH, 3.0 for DTOF resin, 4.0 for SDTOF resin, agitation time, 6 hr for DTOF resin and 4 hr for SDTOF resin, sorbent dosage, 1 g L−1).

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Adsorption Isotherms

Freundlich Isotherm

The Freundlich isotherm [36] is expressed as follows:

  • display math(2)
  • display math(3)

where qe is the amount of nickel adsorbed per unit weight of the DTOF and SDTOF resins (mg g−1), Ce the equilibrium concentration of nickel in solution (mg L−1), while k and n are the constants of Freundlich equation incorporating adsorption capacity and intensity in L·g−1, respectively. The constants were calculated from the slope and intercept of the plot of log qe versus log Ce as shown in (Fig. 8) and the values are listed in Table 2. The values of 1/n confirm the unfavorable conditions for adsorption.

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Figure 8. Adsorption isotherms of Ni(II) on (a) DTOF and (b) SDTOF resins.

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Table 2. Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich isotherm constants for the adsorption of Ni(II) ion onto DTOF and SDTOF resins.
 Freundlich isothermLangmuir isotherm
1/nnK (mg g−1)R2qm (mg g−1)b (L mg−1)RLR2
DTOF copolymer resin20.50.10.9790.0370.70.0070.998
SDTOF terpolymer resin0.3752.660.250.9770.005250.00010.999
 Temkin isothermDubinin–Radushkevich isotherm
A (L mg−1)BR2qm (mg g−1)KR2
DTOF copolymer resin1.820.40.744109.434.5 × 10−60.662
SDTOF terpolymer resin4.823.90.652192.126.1 × 10−30.664
Langmuir Isotherm

Langmuir isotherm [37] model is expressed in the form of following equation:

  • display math(4)
  • display math(5)

where qm is the amount of adsorbate at complete monolayer coverage (mg g−1), which gives the maximum sorption capacity of sorbent; and b (L mg−1) is the Langmuir isotherm constant that relates to the energy of adsorption calculated from the slope and intercept of the plot Ce/qe versus Ce (shown in Fig. 8) and the values are shown in Table 2. The essential characteristics of the Langmuir isotherm can be expressed in terms of the dimensionless constant separation factor or equilibrium parameter RL [38]:

  • display math(6)

where Co is initial Ni(II) concentration for adsorption study and was determined as 200 mg L−1. The RL value was calculated and is listed in Table 2. The RL value lying between 0 and 1 indicated that the conditions were favorable for adsorption.

Temkin Isotherm

This model assumes that heat of adsorption of all molecules in the layer would decrease linearly rather than logarithmic with coverage [39, 40]. As implied in the equation, its derivation was carried out by plotting the quantity sorbed qe against ln Ce (as shown in Fig. 8) and the constants were determined from the slope and intercept and the results are represented in Table 2. The model is given by the following equation [39]:

  • display math(7)
  • display math(8)

A =Temkin isotherm equilibrium binding constant (L g−1), B = Temkin isotherm constant, R = universal gas constant (8.314 J mol−1 K−1), and T = temperature at 298 K.

Dubinin–Radushkevich Isotherm

The Dubinin–Radushkevich isotherm is more general than the Langmuir isotherm, because it does not assume a homogeneous surface or constant sorption potential [41, 42]. The Dubinin–Radushkevich equation is

  • display math(9)
  • display math(10)

where qe = amount of adsorbate in the adsorbent at equilibrium(mg g−1), qm = theoretical isotherm saturation capacity (mg g−1), K = Dubinin–Radushkevich isotherm constant (mol2 kJ−2) and inline image, ε is Dubinin–Radushkevich isotherm constant. The values of qm and K were determined by plotting ln qe versus ε2. Based on the values of coefficient of regression (R2), it is possible to suggest the best isotherm model to explain the adsorption of Ni(II) onto DTOF and SDTOF resins. From the figure it is clear that Langmuir isotherm model is the fittest one as compared with other models.

Kinetic Models

In order to investigate the adsorption of Ni(II) ions onto DTOF and SDTOF resins, the following kinetic models are generally used to test experimental data.

Pseudofirst-Order Model

A simple pseudofirst-order kinetic model is represented by the Lagergren equation [43], as given subsequently:

  • display math(11)

where qe and qt refer to the amount of metal ion adsorbed per unit weight of adsorbent (mg g−1) at equilibrium and at any time t (min). The first-order adsorption rate constant K1 (min−1) can be obtained from the plot of ln (qeqt) versus t.

Pseudosecond-Order Model

The pseudosecond-order model [44] is expressed by the following equation:

  • display math(12)

A plot of t/qt against t provides second-order adsorption rate constants K2 (g mg−1 min−1) a value from the slopes and intercepts.

Intraparticle Diffusion Model

The intraparticle diffusion model [45] is characterized by a linear relationship between the amounts adsorbed and the square root of the time and is expressed as:

  • display math(13)

where Kid is initial rate of the intraparticle diffusion (mg (g min0.5)−1). The value of Kid can be obtained from the slope of the plot of qt versus t0.5.

The three models were illustrated in Fig. 9 and the results were represented in Table 3. The corresponding parameters of equations of the different kinetic models were determined by linear regression. From these results, it can be concluded that the pseudosecond-order kinetic equation provided the best model for describing the adsorption of the nickel onto DTOF and SDTOF polymer resins. For the intraparticle diffusion plot there are two portions, the first is curved due to boundary layer effect and the second is linear which indicates intraparticle or pore diffusion and this suggests that the adsorption process proceeds by surface adsorption, intraparticle diffusion and ion exchange.

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Figure 9. (a) First-order, (b) second-order, and (c) intraparticle diffusion models for Ni2+ adsorption onto DTOF and SDTOF resins.

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Table 3. Kinetic constants for the adsorption of nickel ion onto DTOF and SDTOF resins.
 Pseudofirst-order modelPseudosecond-order modelIntraparticle diffusion model
K1 (min−1)R2K2 (g mg−1 min−1)R2Kid (mg g−1 min−0.5)R2
DTOF resin2.01 × 10−60.8975.2 × 1080.9780.010.765
SDTOF resin1.9 × 10−40.9926.7 × 1080.9995.43 × 10−30.71

Column Studies

The experimental solution containing Ni(II) of initial concentration 200 mg L−1 was introduced into the glass column at a flow rate of 10.0–15.0 mL min−1 using peristaltic pump. The retention time was about 8 hr, the effluent from the outlet of the column was analyzed for Ni(II) concentration. Figure 10 represents the breakthrough curve. From this curve, the adsorption capacities of DTOF and SDTOF resins were found to be 94.5 and 187.8 mg g−1, which is in good agreement with the adsorption capacity calculated using batch adsorption technique.

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Figure 10. Breakthrough curve for the adsorption of Ni(II) onto DTOF copolymer and SDTOF terpolymer resin.

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Desorption Studies

Desorption studies help to recover the adsorbed Ni(II) from sorbents and also help to regenerate the sorbents so that it can be used again to adsorb metal ions. Various chemical reagents including HCl, HNO3, and EDTA were used for the elution of previously adsorbed Ni(II) onto DTOF copolymer and SDTOF terpolymer resins. Desorption experiments were performed at room temperature by using 0.1 M HCl, 0.1 M HNO3, and 0.1 M EDTA solutions. As shown in Fig. 11, HNO3 gave the best desorption efficiency for SDTOF terpolymer resin, whereas EDTA showed maximum desorption efficiency for DTOF copolymer resin. The desorption efficiency for DTOF resin was 54.5, 77.6, and 90% for HCl, HNO3, and EDTA, respectively, while, SDTOF resin showed desorption efficiency, 44.4, 93.34, and 80.32% for HCl, HNO3, and EDTA, respectively.

image

Figure 11. Desorption efficiency of nickel (II) ion for various desorbing agents [concentration of desorbents: 0.1 M, volume of solution: 100 mL, weight of adsorbent with Ni(II): 0.1 g].

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Reusability Studies

The reusability of adsorbent is considered very important from economical point of view. The regeneration of 0.1 g of DTOF and SDTOF resins was achieved by desorbing the adsorbed nickel (II) and this was done by using 0.1 M EDTA and 0.1 M HNO3 for DTOF and SDTOF resins, respectively. The initial concentration of nickel(II) ion solution was 1000 mg L−1. The adsorption and desorption were carried out several times and Table 4 represents the effect of regeneration cycles on the desorption efficiency of resin. It was regarded that up to the sixth cycle, the loss in the desorption efficiency for the two resins was very small (85.5 and 90.3% for DTOF and SDTOF resins, respectively).

Table 4. Effect of number of adsorption–desorption cycles on the desorption efficiency of DTOF and SDTOF resins.
DTOF resinSDTOF resin
Number of cyclesDesorption efficiency (%)Number of cyclesDesorption efficiency (%)
190192.9
289.41292.7
388.3392.3
488492
586.3591.8
685.5690.3

CONCLUSIONS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND EXPERIMENTAL METHODS
  5. RESULTS AND DISCUSSIONS
  6. CONCLUSIONS
  7. AKNOWLEDGMENTS
  8. REFERENCES

The highest adsorption capacity of DTOF copolymer and SDTOF terpolymer resins for Ni(II) was 99.8 and 188.3 mg g−1, respectively. This was achieved at pH 3.0 for DTOF copolymer resin and 4.0 for SDTOF terpolymer resin. All adsorption process was completed in 360 min for DTOF resin and 240 min for SDTOF resin. The optimal concentration for DTOF and SDTOF resins was chosen as 1 g L−1, while the initial concentration of Ni(II) was 1000 mg L−1. At all conditions, SDTOF terpolymer resin showed higher adsorption capacity than DTOF copolymer resin, the incorporation of chelating groups included in sulphanilic acid moiety can increase the adsorption capacity of DTOF copolymer resin for Ni(II). Four adsorption isotherm models were studied and the sorption data fitted into Langmuir, Freundlich, Temkin, and Dubunin–Radushkevich isotherms out of which Langmuir adsorption model was found to be have the highest regression value and hence the best fit. The adsorption process could be represented by a pseudosecond-order rate model for both DTOF and SDTOF resins. The column studies data showed a good agreement with the results obtained by batch equilibrium method. In addition, previously adsorbed Ni(II) onto DTOF and SDTOF resins could be easily desorbed by EDTA and HNO3, respectively. Ni2+-loaded resins were reused for six adsorption–desorption cycles.

AKNOWLEDGMENTS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND EXPERIMENTAL METHODS
  5. RESULTS AND DISCUSSIONS
  6. CONCLUSIONS
  7. AKNOWLEDGMENTS
  8. REFERENCES

The authors thankfully acknowledge Prof. Dr. Hassan S. Emira for providing pH meter instrument. The authors are also grateful to Dr. Mona S. Hashem for UV-visible measurements.

REFERENCES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND EXPERIMENTAL METHODS
  5. RESULTS AND DISCUSSIONS
  6. CONCLUSIONS
  7. AKNOWLEDGMENTS
  8. REFERENCES