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Modified activated carbons with amino groups and their copper adsorption properties in aqueous solution
Modified Activated Carbons with Amino Groups and Their Copper Adsorption Properties in Aqueous Solution Mohammad Hassan Mahaninia, Paria Rahimian, Tahereh Kaghazchi PII: DOI: Reference:
S1004-9541(14)00209-2 doi: 10.1016/j.cjche.2014.11.004 CJCHE 141
To appear in: Received date: Revised date: Accepted date:
10 February 2014 24 March 2014 11 June 2014
Please cite this article as: Mohammad Hassan Mahaninia, Paria Rahimian, Tahereh Kaghazchi, Modified Activated Carbons with Amino Groups and Their Copper Adsorption Properties in Aqueous Solution, (2014), doi: 10.1016/j.cjche.2014.11.004
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ACCEPTED MANUSCRIPT Separation Science and Engineering
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Modified Activated Carbons with Amino Groups and Their Copper Adsorption Properties in Aqueous Solution Mohammad Hassan Mahaninia1,, Paria Rahimian2, Tahereh Kaghazchi3 1
Department of Chemistry, University of Saskatchewan, Saskatoon, Saskatchewan, Canada Department of Textile Engineering, Amirkabir University of Technology, Tehran, Iran 3 Department of Chemical Engineering, Amirkabir University of Technology, Tehran, Iran
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Abstract Activated carbons were prepared by two chemical methods and the
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adsorption of Cu (II) on activated carbons from aqueous solution containing amino
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groups was studied. The first method involved with chlorination of activated carbon following by substitution of chloride groups with amino groups, and the second involved with nitrilation of activated carbon with reduction of nitro groups to amino
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groups. Resultant activated carbons were characterized in terms of porous structure, elemental analysis, FTIR spectroscopy, XPS, Boehm titration, and pH zpc. Kinetic and
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equilibrium tests were performed for copper adsorption in the batch mode. Also,
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adsorption mechanism and effect of pH on adsorption of Cu (II) ions was discussed. Adsorption study shows enhanced adsorption for copper on the modified activated
AC
carbons, mainly by the presence of amino groups, and the Freundlich model is applicable for the activated carbons. It is suggested that binding of nitrogen atoms with Cu (II) ions is stronger than that with H+ ions due to relatively higher divalent charge or stronger electrostatic force. Keywords: activated carbon, amino groups, Cu (II) adsorption, Freundlich model
Corresponding author. E-mail address: [email protected]
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Article history: Received 10 February 2014 Received in revised form 24 March 2014
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Accepted 11 June 2014
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Available online xxxx
1 INTRODUCTION
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Activated carbon is a conventional porous adsorbent with unique physical properties
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such as relatively large surface area, high porosity, and variable pore size distribution and also contains oxygen functional groups, which have significant influences on the adsorption properties of this material [1-5. Activated carbon has recently attracted great
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attention in water and wastewater treatment processes due to its cost effectiveness, abundance, and suitable adsorptive properties [6-9]. Chiefly, one of the most important
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reasons for suitability of this adsorbent in such applications is its high ability toward
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organic substance removal, which makes it suitable for water purification [10-12]. However, this adsorbent has lower sorption affinity toward inorganic substances.
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Heavy metals are among the most toxic pollutants entering the environment in vast quantities through wastewaters of different industrial plants such as mining operations, electronics, tanneries, electroplating and petrochemical industries, as well as textile mill plants [13, 15]. Specially, copper is commonly used in electric and electroplating industries and agricultural poisoning [15] and can be found in the wastewater effluents of these industries. When present with high concentrations in human body, Cu (II) ions can cause liver and kidney damages. The legislation limits for discharge of copper are recommended to be less than 2 mgL−1 by World Health Organization [15].
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ACCEPTED MANUSCRIPT There are many processes for treatment of Cu (II) from contaminated wastewaters, including chemical precipitation, membrane filtration, reverse osmosis, ion exchange,
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and adsorption. Among various treatment technologies, activated carbon adsorption is commonly used due to its easy operation, porous surface structure and harmlessness to
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the environment. However, in general, activated carbons are more effective for
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adsorption of organic compounds rather than metal ions and inorganic pollutants, so developing methods for modification of activated carbons toward these materials should
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be considered.
Usually, original activated carbons contain some functional groups (mostly oxygen groups), and several techniques have been used to improve their structure by
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introducing other heteroatom-containing functional groups. For example, many investigations have been carried out on activated carbon to introduce functional groups,
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and it is expected to improve the adsorption of adsorbent towards specific adsorbate
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[16-19].
Amination of activated carbon, as one of the ways for modification of its
AC
structure, has been investigated in several researches, through the reaction with NH3 at elevated temperatures [20, 21], but amination through chemical reaction at low temperature has been rarely reported [22, 23]. Considering the limited researches about using these kinds of activated carbons for copper removal, amination of activated carbon and its influence on copper adsorption capacity are investigated in the present work. In this context, it is attempted to modify the surface of activated carbon by two methods, both introducing amino groups on activated carbon at low temperature. The first innovative method is based on the chlorination of activated carbon and then substitution of chloride groups with amino groups. The second method has been
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ACCEPTED MANUSCRIPT reported elsewhere [24], based on nitrilation of activated carbon and reduction of nitro groups to amino groups.
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Eventually, the adsorption capability of the modified adsorbents towards Cu (II) in aqueous phase is determined in the batch mode. Kinetic and equilibrium experiments
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are carried out in order to determine and compare the rate and capacity of Cu (II) adsorption and provide reasonable mechanisms involved in the copper adsorption by the
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newly designed sorbents.
2.1 Activated carbon sample
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2 MATERIALS AND METHODS
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Commercially activated carbon (prepared from Norit) with 425 kg·m-3 apparent density and 15 mm particle size (D50) was used as the carrier adsorbent for introduction
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of nitrogen groups, denoted as SAE. SAE was dried at 90 °C for 24 h after washing with
AC
impurities.
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10% HCl and deionized water separately in Soxhlet apparatus for 24 h to remove
2.2 Amination of Activated carbon 2.2.1 First method; chlorination of activated carbon and substitution of chloride groups with amino groups Fig. 1 shows the mechanisms of surface modification by the two methods. In the first method, amino groups are introduced to the activated carbon surface in a three-step procedure. The first step involves oxidation of carbon to increase oxygen functional groups. 10 g of carbon was oxidized with 100 ml of nitric acid (5 mol·L-1) at 84 °C for 5 h. Then the mixture was filtered and washed with deionized water in a Soxhlet
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ACCEPTED MANUSCRIPT apparatus till neutral pH was attained. The second step involves chlorination of activated carbon and transformation of oxygen groups into chloride groups. 10 g
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oxidized activated carbon was treated with 25 ml concentrated thionyl chloride at 84°C and refluxed for 24 h. Then the mixture was filtered and washed with deionized water in
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the Soxhlet apparatus till neutral pH was attained. In the final step, chloride groups were substituted into amino groups by another reaction; 10 g of chlorinated carbon along with
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50 ml of aqueous ammonia (28%) was placed in a 1000 ml flask and refluxed for 24 h.
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Then the mixture was filtered and washed with deionized water in Soxhlet apparatus till neutral pH was attained. It is necessary to note that after each step, the product was
AC
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PT
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dried at 90 °C. This carbon sample is hereafter abbreviated to ACN1.
Fig. 1 Synthesis of aminated activated carbon by two different methods
2.2.2 Second method; nitrilation of activated carbon and reduction of nitro groups to amino groups The second method for amination of activated carbon includes two stages. The first step involves a reaction for nitrilation of activated carbon. At 0 °C (an ice bath), 50 ml of concentrated (18 mol·L-1) sulfuric acid (H2SO4) was added slowly to 50 ml of concentrated (15.7 mol·L-1) nitric acid (HNO3). Then, 10 g activated carbon was slowly added to this acid mixture and stirred for 50 min in the ice bath. The mixture was
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ACCEPTED MANUSCRIPT filtered and washed with deionized water in the Soxhlet apparatus. The product was then dried at 90 °C. The second step was reduction of nitrobenzene to aniline by FeCl2.
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Reduction of nitrated activated carbon proceeded in a 1000 ml flask containing 100 ml of hydrochloric acid, 2 g of powdered iron, and 8 g of carbon with stirring at 80°C for
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24 h. The aminated activated carbon obtained was dried at 90 °C after separation of
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additional powdered iron with magnet and washing with deionized water in Soxhlet
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apparatus. This carbon sample is hereafter abbreviated to ACN2.
3 CHARACTERIZATIONS
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3.1 Point of zero charge measurements
Point of zero charge for produced carbons were measured according to the method
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suggested by Noh and Schwarz [25], which requires recording of the equilibrium pH
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after shaking of suspensions of carbon samples in distilled water for 24 h. The initial pH of the suspensions was selected in the range of 2-11. The fixed equilibrium value of pH
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was taken as the pHzpc.
3.2 Boehm titration The method introduced by Boehm [26] was used for determination of the amount of acidic functional groups available on the activated carbon surface. In this method, 0.5 g activated carbon sample was weighed into three 100 ml conical flasks. Then 30 ml of aqueous solutions of sodium hydrogen carbonate (0.1 mol·L-1), sodium carbonate (0.1 mol·L-1), and sodium hydroxide (0.1 mol·L-1) were added into the flasks. Suspended solutions were kept at 20 °C for 24 h . After filtration, 10 mL of withdrawn aliquots of
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ACCEPTED MANUSCRIPT products was titrated with 0.1 mol·L-1 hydrochloric acid. The amount of acidic surface functional groups such as carboxylic groups and phenolic hydroxyl groups was
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calculated using the titration data.
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3.3 Elemental analysis
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Elemental analysis of the produced samples was performed by an elemental analyzer (model Carlo Ebra 1106). Weighed the sample accurately on an aluminum foil and put
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it into the instrument. Prior to the flash combustion process, the system was purged with helium carrier gas. Flash combustion was then performed at 2073 K, and the gaseous combustion products were quantified using a thermal conductivity detector. Results
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were obtained as percentages of carbon, hydrogen, nitrogen, and the oxygen content
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was determined by difference. Also the amounts of volatile matter of all samples were measured. Samples were weighed, placed in a covered crucible, and heated in a furnace
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at (1173 ± 15) K. The samples were cooled and weighed. Loss of mass represents
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moisture and volatile matter. The remainder is coke (fixed carbon and ash).
3.4 Fourier transform infrared spectroscopy (FTIR) The surface organic FTIR spectra were taken using a Perkin Elmer spectrophotometer instrument (model Paragon 1000PC). Data acquisition was performed automatically using an interfaced computer and a standard software package. The samples were dried first under vacuum at 150C, ground with KBr salt followed by compression between two stainless steel cylinders to form a thin transparent solid film. The spectrometer
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ACCEPTED MANUSCRIPT collected 64 spectra in the range of 400-4000 cm-1, with a resolution of 4 cm-1 and 100
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scans.
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3.5 Determination of porous properties
The textural parameters of all samples were determined by nitrogen adsorption
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experiments at liquid nitrogen temperature (77 K) with a Quantachrom NOVA 1000 instrument equipped with a commercial software of analysis and calculation. The
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samples were first outgassed at 523 K for 3 h under the vacuum prior to the N2 adsorption/desorption tests. The specific surface area was calculated according to the Brunauer-Emmett-Teller (BET) equation, assuming a nitrogen molecule surface area of
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0.162 nm2. The total pore volume and the volume and surface area of micro- and
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mesopores were also determined.
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3.6 X-ray photoelectron spectroscopy measurement
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A Shimadzu XPS (AXIS-HS type) was employed to measure changes in surface functional groups before and after surface modification. The AlKRline was used as the exciting X-ray source (1486.6 eV).
3.7 Copper adsorption experiments The capacities of commercial activated carbon (SAE) and the aminated carbons (ACN1 and ACN2) to adsorb Cu (II) ions from aqueous solutions were determined through a series of batch mode adsorption experiments by a flame atomic adsorption spectrometer (GBC 906, Australia). The stock solution of 1000 mgL-1 Cu (II) was
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ACCEPTED MANUSCRIPT prepared by dissolving 1 g CuCl2 in deionized water acidified with 5 ml of concentrated HCl and diluting to 1 L volume. The kinetic and equilibrium experiments were carried
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out in order to determine and compare the rate and capacity of Cu (II) adsorption for the produced activated carbons. In the kinetic tests, 0.01 g of adsorbent was added to a
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number of 50-ml glass flasks containing 30 ml Cu (II) solution with the initial
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concentration of 40 mgL−1. The suspensions were shaken at 150 r·min-1 and 25C, and after 2, 5, 10, 30 and 60 min, and then after every 60 min (until 1440 min), 4 ml of the
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solution was sampled by a 10 mL plastic syringe and filtered by a Whatman NO. 42, and then analyzed for Cu (II) concentration using an atomic absorption spectrophotometer. The equilibrium isotherm of Cu (II) adsorption was established by
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adding adsorbents into a series of 100-mL glass bottles with the initial pH of ∼7.0. The suspensions were shaken at 150 r·min-1 and 25C for 24 h, and then the solutions were
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filtered and analyzed using an atomic absorption spectrophotometer. All the above-
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were reported.
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mentioned adsorption experiments were repeated three times and the average values
3.8 pH study
The uptake of Cu (II) as a function of pH was determined in the pH range of 2–6. The initial pH of solutions was adjusted by adding required amount of dilute NaOH and HCl solutions (0.1 mol·L-1). The suspensions were shaken at 200 r·min-1 for 24 h under 25 °C.
4 RESULTS AND DISCUSSION
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ACCEPTED MANUSCRIPT 4.1 Physico-chemical characterization The textural and chemical characteristics of the starting carbon and the samples after
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amination are presented in Table 1. The amination of SAE carbon by each procedure
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leads to a little decrease in the BET surface area of the modified carbons (ACN1 and ACN2) and the amounts of this decrease in both amination methods are almost the same
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and insignificant. This suggests that the aromatic hydrocarbon employed reactions to introduce amino groups on the carbon surface, but did not block the pores opening on
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the adsorbent surface. The value of pHpzc in the carbons containing amino group is higher than the virgin one. The amount of basic sites available on the surface of modified activated carbons determined by the Boehm titration was higher than that of
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the virgin sample and all of these results indicate that the addition of aminated groups
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raises the surface basicity significantly, suggesting the introduction of basic amino groups into the carbon surface. It clearly derivates from the results of Boehm titration
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that the amounts of lactone, hydroxyl and carboxyl groups decrease in the modified samples compared to SAE perhaps due to exchange between hydroxyl and amino
AC
groups in modified samples. Table 1 Textural and chemical characteristics of activated carbon samples Samples
SBET /m2·g-1
Hydroxyl group /mmol·g-1
Lactone group /mmol·g-1
Carboxyl group /mmol·g-1
Basic sites /mmol·g-1
pHpzc
SAE
1150
0.56
0.38
0.48
0.13
6.52
ACN1
957.3
0.10
0.28
0.27
0.62
7.88
ACN2
951.5
0.54
0.36
0.38
0.42
7.78
Table 2 gives the results of elemental analysis for all samples. The aminated samples present less volatile matter than the SAE (original carbon) due to the surface reactions.
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ACCEPTED MANUSCRIPT The results show increases in the nitrogen content of modified carbons compared to the virgin sample. It can be seen that both procedures have a relatively good ability to
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introduce nitrogen into the structure of activated carbons and can be considered as the efficient methods for amination of materials. Comparison of the two amination methods
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shows that ACN1 sample has more nitrogen content than ACN2, so that the first method
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is more successful in amination of carbons.
Proximate analysis ( dry
Ash
C
H
N
O
SAE
13.6
4.7
82.5
0.3
0.1
17.1
ACN1
12.3
4.1
79.6
0.6
1.6
18.2
ACN2
13.1
4.2
79.7
0.4
1.1
18.8
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VM
Ultimate analysis (dry ash free)/%
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basis)/%
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Table 2 Elemental analysis of the activated carbons
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Fig. 2 shows the surfaces of the modified activated carbons with amino groups and the virgin one studied using FTIR analysis and spectra. In order to get better comparison, the spectra are divided to several sub-regions and shown separately. Some peaks appear in the first sub-region, concerning C=O stretching (1750 cm-1), C=C stretching of aromatic rings and C-N stretching (1000-1350 cm-1) [27]. Unfortunately, no direct evidence could be observed for N-H stretching, because the most diagnostic signal from N-H stretching is expected at 3200-3400 cm-1, and in the present spectra it is actually lost within the broad O-H stretching bands in the main panel.
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Fig. 2 The FTIR spectra of virgin (SAE) and aminated activated carbons (ACN1 & ACN2)
Fig. 3 shows the X-ray photoelectron spectroscopy (XPS) measurements on
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N(1s). A peak assigned to the N-O bond and another one assigned to C-N-H are found
AC
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at about 405 and 399 eV, respectively [28].
Fig. 3 N(1s) XPS spectra of modified carbons
Table 3 shows the calculation results on the ratio of area under each of the peaks to that under the peak for C(1s) of samples. The intensity at the peak assigned to the N-
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ACCEPTED MANUSCRIPT O bond is very low in ACN1, while that at the peak assigned to the N-H bond increases for both modified activated carbons. This confirms that amination with the second
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method introduces nitro groups onto the activated carbon surface and they change to
Table 3 N/C values of modified carbons measured by XPS
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amino groups through its reduction.
399.6 eV (C-N-H)
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N(1s)/C(1s)
404.5 eV (N-O)
n.d.
ACN1
19.6 ×10-3
n.d.
ACN2
17.4 ×10-3
1.08 ×10-3
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4.2 Copper adsorption
n.d.
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SAE
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Fig. 4 illustrates the effect of solution pH on copper adsorption capacity of activated carbons. -NH2 binds with Cu (II) ions by coordination rather than ion-exchange process.
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The pH of a solution is an important evidence to be considered when using an activated carbon containing amino groups as a sorbent material. The efficiency of Cu (II) ion removal by activated carbon is dependent on pKa of the ligand and the stability constant of the metal-ligand complex as well as the pH of the solution [29]. All samples with weak adsorption of copper species at acidic pHs may be justified to the existence of large amount of hydronium ions (H3O+) that compete with the positively charged Cu (II) for the surface adsorbing sites. Thus the adsorption of Cu (II) will be decreased. The pH value cannot exceed 6 since precipitation of copper hydroxide will be accrued. The adsorption capacities of Cu (II) ions increased significantly as the pH value increased from 2 to 4 and remained approximately at 0.05 mmol·g-1 (equal around 3.177 mg·g-1)
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ACCEPTED MANUSCRIPT as the pH increased to 5.8. NH2-AC was able to remove 30%-90% of Cu (II) at pH 2-4, whereas the virgin activated carbon could remove mere amount of Cu (II) in the same
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pH range. The pH-dependent adsorption could be interpreted by the pH effects on the association/dissociation of surface functional groups, surface charges, formation of ion
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species, and interactions between functional group and metal ions, which was discussed in details [30, 31]. At low pH, the electrical repulsion between Cu (II) ions and
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positively charged function groups on the carbon surface would be responsible for the
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low Cu adsorption while at high solution pH, the carbon surface became more negatively charged due to the dissociation of functional groups, which could enhance the electrostatic interactions of Cu (II) ions with negative function groups and therefore
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make the copper adsorption convenient. With these explanations, other adsorption experiments were carried out at the initial pH of solution of 6.0 to reach the maximum
AC
CE
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copper adsorption capability.
Fig. 4 Adsorption capacity of Cu (II) as a function of solution pH
Fig. 5 show the percentage of Cu (II) (aq) removal from aqueous phase as a function of time. The initial adsorption rate of ACN1 and ACN2 samples are higher than that of SAE sample, although SAE has the highest BET surface area. Thus it may be concluded that the porous structure is not the most important factor determining the
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ACCEPTED MANUSCRIPT adsorption capacity of these activated carbons towards copper, and their surface chemistry and functionalities can also have a role. Amino groups present on the carbon
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surface substantially enhance the adsorption of Cu (II) (aq) under neutral or slightly basic conditions, with the carbon of highest nitrogen content (ACN1) having greater
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adsorption capacity. It can be seen that during the first hour of the process, almost 90% of removal capacity is achieved and then the adsorption gradually reaches a steady state
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ED
MA N
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in all of samples.
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Fig. 5 Cu (II) adsorption as a function of contact time (operating conditions: pH 6.0; T=25C)
In order to determine the Cu (II) sorption capacity of the activated carbons, the
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equilibrium adsorption tests were also carried out. The experimental results are shown in Fig. 6, by plotting the equilibrium adsorption capacity of adsorbents qe [= V(C0Ce)/M] versus Ce, where C0 and Ce are the initial and equilibrium concentrations of Cu (II) ion in the solution, V is the volume of solution and M is the mass of adsorbent. It can be seen that the amount of Cu (II) adsorbed by ACN1 and ACN2 samples at equilibrium conditions are much higher than that by the virgin sample. The higher adsorptive capacity of these samples can be related to the formation of strong Cu -bands with –NH2 functional group because of basic property of nitrogen groups. It has been found that NH2 group has a large capacity to make complex with metal ions.
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ACCEPTED MANUSCRIPT Therefore, activated carbon produced by the first method has a significant effect on the capacity to attract aqueous Cu (II) ions. The higher the nitrogen content of adsorbent,
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the larger the equilibrium adsorption capacity towards Cu (II) ions. Thus the adsorption capacity of ACN1 resulted from higher nitrogen content is higher than that of the other
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samples. In the case of ACN2 sample, as it is noted before, using nitric acid in the modification process reduces the pHzpc value and increases the surface acidity of the
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PT
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MA N
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sample, both having reverse effects on the adsorption capacity of ACN2.
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Fig. 6 Equilibrium isotherms of Cu (II) adsorption by SAE, ACN2 and ACN1 (a) t = 24 h, pH 6.0, T = 15C; (b) t = 24 h, pH 6.0, T = 25C
The experimental data on equilibrium study for the adsorption of Cu (II) onto the adsorbents are fitted with the most common 2-parameter adsorption isotherm model for adsorption from liquid liquids: Freundlich model. (1)
lg qe lg KF (1/n) lg Ce
The mechanism and the rate of adsorption are functions of constants 1/n and KF. The values of parameters and the correlation coefficients for two different temperature conditions (15 and 25C) are given in Table 4. According to the value of
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ACCEPTED MANUSCRIPT R2, it can be concluded that the fit of Freundlich equation for the adsorption of copper on all adsorbents is good, while the fitting for aminated samples is better than the virgin
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sample. For a good adsorbent with a favorable adsorption, 0.2 < 1/n < 0.8, and the values obtained for all samples situate in this range. The other constant of Freundlich
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model (KF) is indicative of the adsorption capacity of adsorbent towards adsorbate, and the aminated samples have higher values than the virgin sample. The higher the
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adsorption temperature , the higher the value of KF.
R2 0.97 0.99 0.99
1/n 0.46 0.75 0.72
25°C KF 0.13 0.14 0.21
R2 0.96 0.98 0.99
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SAE ACN1 ACN2
15°C KF 0.05 0.08 0.08
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1/n 0.59 0.67 0.67
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Table 4 Constants and correlation coefficients of Freundlich plots
4.3 Copper adsorption mechanisms
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Variety of interactions between metal ions and the carbon surface is possible, for example, formation of surface complexes [32], ion-exchange processes with the
AC
participation of strong surface acidic groups [33], and redox reactions with a change of metal valence [34]. In the case of the modified carbon, more specifically with the presence of amino groups, nitrogen atoms of the amino groups have free ion pairs of electrons that can potentially bind with Cu (II) ions or H+ ions to form a coordination complex through an electron pair sharing as illustrated in Fig. 7. Binding of nitrogen atoms with Cu (II) ions is speculated to be stronger than that with H+ ions due to relatively higher divalent charge or stronger electrostatic force [35, 36]. It is believed that a combination of more negatively charged surface and presence of amino groups as induced by the
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ACCEPTED MANUSCRIPT modification could be responsible for enhanced Cu (II) ion adsorption by ACN1 and
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ACN2.
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Fig.7 Formation of coordinating complex between amine groups and copper species
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It seems necessary to notice that besides ion exchange, physical adsorption in microand especially in meso-pores of the produced activated carbon has an important role in
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5 CONCLUSIONS
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the adsorption of copper species.
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In this work, introduction and characterization of amino groups (-NH2) onto the activated carbon structure were investigated through two different chemical reactions.
AC
The porous characterization of the modified carbons has shown slight decrease in surface area with efficient method for modification. Elemental analysis results, FT-IR spectra and also results of XPS substantiate that both procedures increase the nitrogen content and introduce amino groups on the surface of activated carbon successfully, while the first method introduces more amino groups on the activated carbon surface. Proved by Boehm titration and pHzpc, fixed amino groups on the surface of adsorbent give it a basic characteristic, with which modified carbons are much more effective than the unmodified AC in sequestering Cu (II) over the pH range from 2.0 to 5.8. This basic nature could strengthen the reaction of activated carbon surface with Cu (II) ions
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ACCEPTED MANUSCRIPT according to the acid-base Lewis theory. For the applicability of the amino-activated carbons for adsorption of copper from aqueous phase, it is found that the introduction of
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amino groups onto the carbon increases the adsorption capacity of Cu (II) ions
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considerably, because of a strong interaction of basic amino groups with Cu (II) ions.
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ACCEPTED MANUSCRIPT [13] Lokeshwari, H., Chandrappa, G.T., "Heavy Metals Content in Water, Water Hyacinth and Sediments of Lalbagh Tank, Bangalore (India) ", J. environ. sci. Eng., 48
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hulls ", J. hazard. Mater., 129 (1-3), 123–129(2006).
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[15] Water UK technical briefing note: copper in drinking water; November 2006. [16] Pevida, C., Plaza, M.G., Arias, B., Fermoso, J., Rubiera, F., Pis, J.J., "Surface modification of activated carbons for CO2 capture", Appl. Surf. Sci., 254 (22), 7165-
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7172(2008).
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[17] Pietrzak, R., Wachowska, H., Nowicki, P., Babeł, K., "Preparation of modified
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active carbon from brown coal by ammoxidation", Fuel process. technol., 88 (4), 409-
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[18] Soudani, N., Souissi-najar, S., Ouederni, A., "Influence of Nitric Acid Concentration on Characteristics of Olive Stone Based Activated Carbon", Chin. J. Chem. Eng., 21, 1425-1430(2013). [19] Szyma
ski, G. S., Grzybek, T., Papp, H., "Influence of nitrogen surface
functionalities on the catalytic activity of activated carbon in low temperature SCR of NOx with NH3", Catal. Today , 90 (1-2), 51-59(2004). [20] Biniak, S., Pakuła, M., Szyman´ski, G. S., wiaü tkowski, A. SÄ., "Effect of Activated Carbon Surface Oxygen- and/or Nitrogen-Containing Groups on Adsorption of Copper(II)Ions from Aqueous Solution", Langmuir, 15 (18), 6117-6122(1999).
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[22] Jia, Y. F., Xiao, B., Thomas, K. M., "Adsorption of metal ions on nitrogen surface
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functional groups in activated carbons", Langmuir, 18 (2), 470-478(2002). [23] Tanada, S., Kawasaki, N., Nakamura, T., Araki, M., Isomura, M., "Removal of
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carbon functionalized with amine ", Ind. Eng. Chem. Res., 43 (11), 2759-2764(2004).
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[25] Noh, J.S., Schwarz, J.A., "Estimation of the point of zero charge of simple oxides by mass titration", J. Colloid Interface Sci., 130 (1), 157-164(1989).
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[26] Boehm, H.P., "Chemical Identification of Surface Groups", In: advances in
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Catalysis, Academic Press, New York, 179-264(1966). [25] Zawadzki, J., "Infrared spectroscopy in surface chemistry of carbon", In: Thrower, P. A. (Ed.), Chemistry and Physics of Carbon. Marcel Dekker, New York, 147– 380(1989). [28] Jansen, R. J. J., Bekkum, H.V., "XPS of nitrogen-containing functional groups on activated carbon", Carbon, 33 (8), 1021-1027(1995). [29] Mellor, D. P., "Chemistry of Chelation and Chelating Agents", In: Levine, W. G. (Ed.), The Chelation of Heavy Metals, Pergamon Press: Oxford (1979).
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[31] Petrovic, M., Kastelan-Macan, M., Horvat, A.J.M., "Interactive Sorption of Metal
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[32] Corapcioglu, M. O., Huang, C. P., "The adsorption of heavy metals onto hydrous
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activated carbon" Water Res., 21 (9), 1031-1044(1987).
[33] Johnson Jr., J. S., Westmoreland, C. G., Sweeton, F. H., Kraus, K. A., Hagmann, E. W., Eatherly, W. P., Child, H. R., "Modification of cation-exchange properties of
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248(1986).
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Graphical abstract
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S1004-9541(14)00209-2 doi: 10.1016/j.cjche.2014.11.004 CJCHE 141
To appear in: Received date: Revised date: Accepted date:
10 February 2014 24 March 2014 11 June 2014
Please cite this article as: Mohammad Hassan Mahaninia, Paria Rahimian, Tahereh Kaghazchi, Modified Activated Carbons with Amino Groups and Their Copper Adsorption Properties in Aqueous Solution, (2014), doi: 10.1016/j.cjche.2014.11.004
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ACCEPTED MANUSCRIPT Separation Science and Engineering
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Modified Activated Carbons with Amino Groups and Their Copper Adsorption Properties in Aqueous Solution Mohammad Hassan Mahaninia1,, Paria Rahimian2, Tahereh Kaghazchi3 1
Department of Chemistry, University of Saskatchewan, Saskatoon, Saskatchewan, Canada Department of Textile Engineering, Amirkabir University of Technology, Tehran, Iran 3 Department of Chemical Engineering, Amirkabir University of Technology, Tehran, Iran
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2
Abstract Activated carbons were prepared by two chemical methods and the
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adsorption of Cu (II) on activated carbons from aqueous solution containing amino
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groups was studied. The first method involved with chlorination of activated carbon following by substitution of chloride groups with amino groups, and the second involved with nitrilation of activated carbon with reduction of nitro groups to amino
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groups. Resultant activated carbons were characterized in terms of porous structure, elemental analysis, FTIR spectroscopy, XPS, Boehm titration, and pH zpc. Kinetic and
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equilibrium tests were performed for copper adsorption in the batch mode. Also,
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adsorption mechanism and effect of pH on adsorption of Cu (II) ions was discussed. Adsorption study shows enhanced adsorption for copper on the modified activated
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carbons, mainly by the presence of amino groups, and the Freundlich model is applicable for the activated carbons. It is suggested that binding of nitrogen atoms with Cu (II) ions is stronger than that with H+ ions due to relatively higher divalent charge or stronger electrostatic force. Keywords: activated carbon, amino groups, Cu (II) adsorption, Freundlich model
Corresponding author. E-mail address: [email protected]
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Article history: Received 10 February 2014 Received in revised form 24 March 2014
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Accepted 11 June 2014
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Available online xxxx
1 INTRODUCTION
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Activated carbon is a conventional porous adsorbent with unique physical properties
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such as relatively large surface area, high porosity, and variable pore size distribution and also contains oxygen functional groups, which have significant influences on the adsorption properties of this material [1-5. Activated carbon has recently attracted great
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attention in water and wastewater treatment processes due to its cost effectiveness, abundance, and suitable adsorptive properties [6-9]. Chiefly, one of the most important
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reasons for suitability of this adsorbent in such applications is its high ability toward
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organic substance removal, which makes it suitable for water purification [10-12]. However, this adsorbent has lower sorption affinity toward inorganic substances.
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Heavy metals are among the most toxic pollutants entering the environment in vast quantities through wastewaters of different industrial plants such as mining operations, electronics, tanneries, electroplating and petrochemical industries, as well as textile mill plants [13, 15]. Specially, copper is commonly used in electric and electroplating industries and agricultural poisoning [15] and can be found in the wastewater effluents of these industries. When present with high concentrations in human body, Cu (II) ions can cause liver and kidney damages. The legislation limits for discharge of copper are recommended to be less than 2 mgL−1 by World Health Organization [15].
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ACCEPTED MANUSCRIPT There are many processes for treatment of Cu (II) from contaminated wastewaters, including chemical precipitation, membrane filtration, reverse osmosis, ion exchange,
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and adsorption. Among various treatment technologies, activated carbon adsorption is commonly used due to its easy operation, porous surface structure and harmlessness to
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the environment. However, in general, activated carbons are more effective for
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adsorption of organic compounds rather than metal ions and inorganic pollutants, so developing methods for modification of activated carbons toward these materials should
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be considered.
Usually, original activated carbons contain some functional groups (mostly oxygen groups), and several techniques have been used to improve their structure by
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introducing other heteroatom-containing functional groups. For example, many investigations have been carried out on activated carbon to introduce functional groups,
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and it is expected to improve the adsorption of adsorbent towards specific adsorbate
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[16-19].
Amination of activated carbon, as one of the ways for modification of its
AC
structure, has been investigated in several researches, through the reaction with NH3 at elevated temperatures [20, 21], but amination through chemical reaction at low temperature has been rarely reported [22, 23]. Considering the limited researches about using these kinds of activated carbons for copper removal, amination of activated carbon and its influence on copper adsorption capacity are investigated in the present work. In this context, it is attempted to modify the surface of activated carbon by two methods, both introducing amino groups on activated carbon at low temperature. The first innovative method is based on the chlorination of activated carbon and then substitution of chloride groups with amino groups. The second method has been
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ACCEPTED MANUSCRIPT reported elsewhere [24], based on nitrilation of activated carbon and reduction of nitro groups to amino groups.
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Eventually, the adsorption capability of the modified adsorbents towards Cu (II) in aqueous phase is determined in the batch mode. Kinetic and equilibrium experiments
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are carried out in order to determine and compare the rate and capacity of Cu (II) adsorption and provide reasonable mechanisms involved in the copper adsorption by the
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newly designed sorbents.
2.1 Activated carbon sample
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2 MATERIALS AND METHODS
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Commercially activated carbon (prepared from Norit) with 425 kg·m-3 apparent density and 15 mm particle size (D50) was used as the carrier adsorbent for introduction
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of nitrogen groups, denoted as SAE. SAE was dried at 90 °C for 24 h after washing with
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impurities.
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10% HCl and deionized water separately in Soxhlet apparatus for 24 h to remove
2.2 Amination of Activated carbon 2.2.1 First method; chlorination of activated carbon and substitution of chloride groups with amino groups Fig. 1 shows the mechanisms of surface modification by the two methods. In the first method, amino groups are introduced to the activated carbon surface in a three-step procedure. The first step involves oxidation of carbon to increase oxygen functional groups. 10 g of carbon was oxidized with 100 ml of nitric acid (5 mol·L-1) at 84 °C for 5 h. Then the mixture was filtered and washed with deionized water in a Soxhlet
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ACCEPTED MANUSCRIPT apparatus till neutral pH was attained. The second step involves chlorination of activated carbon and transformation of oxygen groups into chloride groups. 10 g
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oxidized activated carbon was treated with 25 ml concentrated thionyl chloride at 84°C and refluxed for 24 h. Then the mixture was filtered and washed with deionized water in
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the Soxhlet apparatus till neutral pH was attained. In the final step, chloride groups were substituted into amino groups by another reaction; 10 g of chlorinated carbon along with
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50 ml of aqueous ammonia (28%) was placed in a 1000 ml flask and refluxed for 24 h.
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Then the mixture was filtered and washed with deionized water in Soxhlet apparatus till neutral pH was attained. It is necessary to note that after each step, the product was
AC
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dried at 90 °C. This carbon sample is hereafter abbreviated to ACN1.
Fig. 1 Synthesis of aminated activated carbon by two different methods
2.2.2 Second method; nitrilation of activated carbon and reduction of nitro groups to amino groups The second method for amination of activated carbon includes two stages. The first step involves a reaction for nitrilation of activated carbon. At 0 °C (an ice bath), 50 ml of concentrated (18 mol·L-1) sulfuric acid (H2SO4) was added slowly to 50 ml of concentrated (15.7 mol·L-1) nitric acid (HNO3). Then, 10 g activated carbon was slowly added to this acid mixture and stirred for 50 min in the ice bath. The mixture was
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ACCEPTED MANUSCRIPT filtered and washed with deionized water in the Soxhlet apparatus. The product was then dried at 90 °C. The second step was reduction of nitrobenzene to aniline by FeCl2.
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Reduction of nitrated activated carbon proceeded in a 1000 ml flask containing 100 ml of hydrochloric acid, 2 g of powdered iron, and 8 g of carbon with stirring at 80°C for
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24 h. The aminated activated carbon obtained was dried at 90 °C after separation of
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additional powdered iron with magnet and washing with deionized water in Soxhlet
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apparatus. This carbon sample is hereafter abbreviated to ACN2.
3 CHARACTERIZATIONS
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3.1 Point of zero charge measurements
Point of zero charge for produced carbons were measured according to the method
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suggested by Noh and Schwarz [25], which requires recording of the equilibrium pH
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after shaking of suspensions of carbon samples in distilled water for 24 h. The initial pH of the suspensions was selected in the range of 2-11. The fixed equilibrium value of pH
AC
was taken as the pHzpc.
3.2 Boehm titration The method introduced by Boehm [26] was used for determination of the amount of acidic functional groups available on the activated carbon surface. In this method, 0.5 g activated carbon sample was weighed into three 100 ml conical flasks. Then 30 ml of aqueous solutions of sodium hydrogen carbonate (0.1 mol·L-1), sodium carbonate (0.1 mol·L-1), and sodium hydroxide (0.1 mol·L-1) were added into the flasks. Suspended solutions were kept at 20 °C for 24 h . After filtration, 10 mL of withdrawn aliquots of
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ACCEPTED MANUSCRIPT products was titrated with 0.1 mol·L-1 hydrochloric acid. The amount of acidic surface functional groups such as carboxylic groups and phenolic hydroxyl groups was
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calculated using the titration data.
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3.3 Elemental analysis
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Elemental analysis of the produced samples was performed by an elemental analyzer (model Carlo Ebra 1106). Weighed the sample accurately on an aluminum foil and put
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it into the instrument. Prior to the flash combustion process, the system was purged with helium carrier gas. Flash combustion was then performed at 2073 K, and the gaseous combustion products were quantified using a thermal conductivity detector. Results
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were obtained as percentages of carbon, hydrogen, nitrogen, and the oxygen content
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was determined by difference. Also the amounts of volatile matter of all samples were measured. Samples were weighed, placed in a covered crucible, and heated in a furnace
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at (1173 ± 15) K. The samples were cooled and weighed. Loss of mass represents
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moisture and volatile matter. The remainder is coke (fixed carbon and ash).
3.4 Fourier transform infrared spectroscopy (FTIR) The surface organic FTIR spectra were taken using a Perkin Elmer spectrophotometer instrument (model Paragon 1000PC). Data acquisition was performed automatically using an interfaced computer and a standard software package. The samples were dried first under vacuum at 150C, ground with KBr salt followed by compression between two stainless steel cylinders to form a thin transparent solid film. The spectrometer
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ACCEPTED MANUSCRIPT collected 64 spectra in the range of 400-4000 cm-1, with a resolution of 4 cm-1 and 100
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scans.
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3.5 Determination of porous properties
The textural parameters of all samples were determined by nitrogen adsorption
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experiments at liquid nitrogen temperature (77 K) with a Quantachrom NOVA 1000 instrument equipped with a commercial software of analysis and calculation. The
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samples were first outgassed at 523 K for 3 h under the vacuum prior to the N2 adsorption/desorption tests. The specific surface area was calculated according to the Brunauer-Emmett-Teller (BET) equation, assuming a nitrogen molecule surface area of
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0.162 nm2. The total pore volume and the volume and surface area of micro- and
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mesopores were also determined.
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3.6 X-ray photoelectron spectroscopy measurement
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A Shimadzu XPS (AXIS-HS type) was employed to measure changes in surface functional groups before and after surface modification. The AlKRline was used as the exciting X-ray source (1486.6 eV).
3.7 Copper adsorption experiments The capacities of commercial activated carbon (SAE) and the aminated carbons (ACN1 and ACN2) to adsorb Cu (II) ions from aqueous solutions were determined through a series of batch mode adsorption experiments by a flame atomic adsorption spectrometer (GBC 906, Australia). The stock solution of 1000 mgL-1 Cu (II) was
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ACCEPTED MANUSCRIPT prepared by dissolving 1 g CuCl2 in deionized water acidified with 5 ml of concentrated HCl and diluting to 1 L volume. The kinetic and equilibrium experiments were carried
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out in order to determine and compare the rate and capacity of Cu (II) adsorption for the produced activated carbons. In the kinetic tests, 0.01 g of adsorbent was added to a
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number of 50-ml glass flasks containing 30 ml Cu (II) solution with the initial
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concentration of 40 mgL−1. The suspensions were shaken at 150 r·min-1 and 25C, and after 2, 5, 10, 30 and 60 min, and then after every 60 min (until 1440 min), 4 ml of the
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solution was sampled by a 10 mL plastic syringe and filtered by a Whatman NO. 42, and then analyzed for Cu (II) concentration using an atomic absorption spectrophotometer. The equilibrium isotherm of Cu (II) adsorption was established by
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adding adsorbents into a series of 100-mL glass bottles with the initial pH of ∼7.0. The suspensions were shaken at 150 r·min-1 and 25C for 24 h, and then the solutions were
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filtered and analyzed using an atomic absorption spectrophotometer. All the above-
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were reported.
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mentioned adsorption experiments were repeated three times and the average values
3.8 pH study
The uptake of Cu (II) as a function of pH was determined in the pH range of 2–6. The initial pH of solutions was adjusted by adding required amount of dilute NaOH and HCl solutions (0.1 mol·L-1). The suspensions were shaken at 200 r·min-1 for 24 h under 25 °C.
4 RESULTS AND DISCUSSION
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ACCEPTED MANUSCRIPT 4.1 Physico-chemical characterization The textural and chemical characteristics of the starting carbon and the samples after
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amination are presented in Table 1. The amination of SAE carbon by each procedure
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leads to a little decrease in the BET surface area of the modified carbons (ACN1 and ACN2) and the amounts of this decrease in both amination methods are almost the same
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and insignificant. This suggests that the aromatic hydrocarbon employed reactions to introduce amino groups on the carbon surface, but did not block the pores opening on
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the adsorbent surface. The value of pHpzc in the carbons containing amino group is higher than the virgin one. The amount of basic sites available on the surface of modified activated carbons determined by the Boehm titration was higher than that of
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the virgin sample and all of these results indicate that the addition of aminated groups
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raises the surface basicity significantly, suggesting the introduction of basic amino groups into the carbon surface. It clearly derivates from the results of Boehm titration
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that the amounts of lactone, hydroxyl and carboxyl groups decrease in the modified samples compared to SAE perhaps due to exchange between hydroxyl and amino
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groups in modified samples. Table 1 Textural and chemical characteristics of activated carbon samples Samples
SBET /m2·g-1
Hydroxyl group /mmol·g-1
Lactone group /mmol·g-1
Carboxyl group /mmol·g-1
Basic sites /mmol·g-1
pHpzc
SAE
1150
0.56
0.38
0.48
0.13
6.52
ACN1
957.3
0.10
0.28
0.27
0.62
7.88
ACN2
951.5
0.54
0.36
0.38
0.42
7.78
Table 2 gives the results of elemental analysis for all samples. The aminated samples present less volatile matter than the SAE (original carbon) due to the surface reactions.
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ACCEPTED MANUSCRIPT The results show increases in the nitrogen content of modified carbons compared to the virgin sample. It can be seen that both procedures have a relatively good ability to
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introduce nitrogen into the structure of activated carbons and can be considered as the efficient methods for amination of materials. Comparison of the two amination methods
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shows that ACN1 sample has more nitrogen content than ACN2, so that the first method
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is more successful in amination of carbons.
Proximate analysis ( dry
Ash
C
H
N
O
SAE
13.6
4.7
82.5
0.3
0.1
17.1
ACN1
12.3
4.1
79.6
0.6
1.6
18.2
ACN2
13.1
4.2
79.7
0.4
1.1
18.8
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VM
Ultimate analysis (dry ash free)/%
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basis)/%
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Table 2 Elemental analysis of the activated carbons
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Fig. 2 shows the surfaces of the modified activated carbons with amino groups and the virgin one studied using FTIR analysis and spectra. In order to get better comparison, the spectra are divided to several sub-regions and shown separately. Some peaks appear in the first sub-region, concerning C=O stretching (1750 cm-1), C=C stretching of aromatic rings and C-N stretching (1000-1350 cm-1) [27]. Unfortunately, no direct evidence could be observed for N-H stretching, because the most diagnostic signal from N-H stretching is expected at 3200-3400 cm-1, and in the present spectra it is actually lost within the broad O-H stretching bands in the main panel.
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Fig. 2 The FTIR spectra of virgin (SAE) and aminated activated carbons (ACN1 & ACN2)
Fig. 3 shows the X-ray photoelectron spectroscopy (XPS) measurements on
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N(1s). A peak assigned to the N-O bond and another one assigned to C-N-H are found
AC
CE
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at about 405 and 399 eV, respectively [28].
Fig. 3 N(1s) XPS spectra of modified carbons
Table 3 shows the calculation results on the ratio of area under each of the peaks to that under the peak for C(1s) of samples. The intensity at the peak assigned to the N-
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ACCEPTED MANUSCRIPT O bond is very low in ACN1, while that at the peak assigned to the N-H bond increases for both modified activated carbons. This confirms that amination with the second
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method introduces nitro groups onto the activated carbon surface and they change to
Table 3 N/C values of modified carbons measured by XPS
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amino groups through its reduction.
399.6 eV (C-N-H)
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N(1s)/C(1s)
404.5 eV (N-O)
n.d.
ACN1
19.6 ×10-3
n.d.
ACN2
17.4 ×10-3
1.08 ×10-3
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4.2 Copper adsorption
n.d.
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SAE
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Fig. 4 illustrates the effect of solution pH on copper adsorption capacity of activated carbons. -NH2 binds with Cu (II) ions by coordination rather than ion-exchange process.
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The pH of a solution is an important evidence to be considered when using an activated carbon containing amino groups as a sorbent material. The efficiency of Cu (II) ion removal by activated carbon is dependent on pKa of the ligand and the stability constant of the metal-ligand complex as well as the pH of the solution [29]. All samples with weak adsorption of copper species at acidic pHs may be justified to the existence of large amount of hydronium ions (H3O+) that compete with the positively charged Cu (II) for the surface adsorbing sites. Thus the adsorption of Cu (II) will be decreased. The pH value cannot exceed 6 since precipitation of copper hydroxide will be accrued. The adsorption capacities of Cu (II) ions increased significantly as the pH value increased from 2 to 4 and remained approximately at 0.05 mmol·g-1 (equal around 3.177 mg·g-1)
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ACCEPTED MANUSCRIPT as the pH increased to 5.8. NH2-AC was able to remove 30%-90% of Cu (II) at pH 2-4, whereas the virgin activated carbon could remove mere amount of Cu (II) in the same
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pH range. The pH-dependent adsorption could be interpreted by the pH effects on the association/dissociation of surface functional groups, surface charges, formation of ion
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species, and interactions between functional group and metal ions, which was discussed in details [30, 31]. At low pH, the electrical repulsion between Cu (II) ions and
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positively charged function groups on the carbon surface would be responsible for the
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low Cu adsorption while at high solution pH, the carbon surface became more negatively charged due to the dissociation of functional groups, which could enhance the electrostatic interactions of Cu (II) ions with negative function groups and therefore
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make the copper adsorption convenient. With these explanations, other adsorption experiments were carried out at the initial pH of solution of 6.0 to reach the maximum
AC
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copper adsorption capability.
Fig. 4 Adsorption capacity of Cu (II) as a function of solution pH
Fig. 5 show the percentage of Cu (II) (aq) removal from aqueous phase as a function of time. The initial adsorption rate of ACN1 and ACN2 samples are higher than that of SAE sample, although SAE has the highest BET surface area. Thus it may be concluded that the porous structure is not the most important factor determining the
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ACCEPTED MANUSCRIPT adsorption capacity of these activated carbons towards copper, and their surface chemistry and functionalities can also have a role. Amino groups present on the carbon
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surface substantially enhance the adsorption of Cu (II) (aq) under neutral or slightly basic conditions, with the carbon of highest nitrogen content (ACN1) having greater
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adsorption capacity. It can be seen that during the first hour of the process, almost 90% of removal capacity is achieved and then the adsorption gradually reaches a steady state
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ED
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in all of samples.
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Fig. 5 Cu (II) adsorption as a function of contact time (operating conditions: pH 6.0; T=25C)
In order to determine the Cu (II) sorption capacity of the activated carbons, the
AC
equilibrium adsorption tests were also carried out. The experimental results are shown in Fig. 6, by plotting the equilibrium adsorption capacity of adsorbents qe [= V(C0Ce)/M] versus Ce, where C0 and Ce are the initial and equilibrium concentrations of Cu (II) ion in the solution, V is the volume of solution and M is the mass of adsorbent. It can be seen that the amount of Cu (II) adsorbed by ACN1 and ACN2 samples at equilibrium conditions are much higher than that by the virgin sample. The higher adsorptive capacity of these samples can be related to the formation of strong Cu -bands with –NH2 functional group because of basic property of nitrogen groups. It has been found that NH2 group has a large capacity to make complex with metal ions.
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ACCEPTED MANUSCRIPT Therefore, activated carbon produced by the first method has a significant effect on the capacity to attract aqueous Cu (II) ions. The higher the nitrogen content of adsorbent,
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the larger the equilibrium adsorption capacity towards Cu (II) ions. Thus the adsorption capacity of ACN1 resulted from higher nitrogen content is higher than that of the other
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samples. In the case of ACN2 sample, as it is noted before, using nitric acid in the modification process reduces the pHzpc value and increases the surface acidity of the
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sample, both having reverse effects on the adsorption capacity of ACN2.
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Fig. 6 Equilibrium isotherms of Cu (II) adsorption by SAE, ACN2 and ACN1 (a) t = 24 h, pH 6.0, T = 15C; (b) t = 24 h, pH 6.0, T = 25C
The experimental data on equilibrium study for the adsorption of Cu (II) onto the adsorbents are fitted with the most common 2-parameter adsorption isotherm model for adsorption from liquid liquids: Freundlich model. (1)
lg qe lg KF (1/n) lg Ce
The mechanism and the rate of adsorption are functions of constants 1/n and KF. The values of parameters and the correlation coefficients for two different temperature conditions (15 and 25C) are given in Table 4. According to the value of
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ACCEPTED MANUSCRIPT R2, it can be concluded that the fit of Freundlich equation for the adsorption of copper on all adsorbents is good, while the fitting for aminated samples is better than the virgin
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sample. For a good adsorbent with a favorable adsorption, 0.2 < 1/n < 0.8, and the values obtained for all samples situate in this range. The other constant of Freundlich
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model (KF) is indicative of the adsorption capacity of adsorbent towards adsorbate, and the aminated samples have higher values than the virgin sample. The higher the
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adsorption temperature , the higher the value of KF.
R2 0.97 0.99 0.99
1/n 0.46 0.75 0.72
25°C KF 0.13 0.14 0.21
R2 0.96 0.98 0.99
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SAE ACN1 ACN2
15°C KF 0.05 0.08 0.08
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1/n 0.59 0.67 0.67
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Table 4 Constants and correlation coefficients of Freundlich plots
4.3 Copper adsorption mechanisms
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Variety of interactions between metal ions and the carbon surface is possible, for example, formation of surface complexes [32], ion-exchange processes with the
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participation of strong surface acidic groups [33], and redox reactions with a change of metal valence [34]. In the case of the modified carbon, more specifically with the presence of amino groups, nitrogen atoms of the amino groups have free ion pairs of electrons that can potentially bind with Cu (II) ions or H+ ions to form a coordination complex through an electron pair sharing as illustrated in Fig. 7. Binding of nitrogen atoms with Cu (II) ions is speculated to be stronger than that with H+ ions due to relatively higher divalent charge or stronger electrostatic force [35, 36]. It is believed that a combination of more negatively charged surface and presence of amino groups as induced by the
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ACN2.
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Fig.7 Formation of coordinating complex between amine groups and copper species
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It seems necessary to notice that besides ion exchange, physical adsorption in microand especially in meso-pores of the produced activated carbon has an important role in
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5 CONCLUSIONS
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the adsorption of copper species.
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In this work, introduction and characterization of amino groups (-NH2) onto the activated carbon structure were investigated through two different chemical reactions.
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The porous characterization of the modified carbons has shown slight decrease in surface area with efficient method for modification. Elemental analysis results, FT-IR spectra and also results of XPS substantiate that both procedures increase the nitrogen content and introduce amino groups on the surface of activated carbon successfully, while the first method introduces more amino groups on the activated carbon surface. Proved by Boehm titration and pHzpc, fixed amino groups on the surface of adsorbent give it a basic characteristic, with which modified carbons are much more effective than the unmodified AC in sequestering Cu (II) over the pH range from 2.0 to 5.8. This basic nature could strengthen the reaction of activated carbon surface with Cu (II) ions
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ACCEPTED MANUSCRIPT according to the acid-base Lewis theory. For the applicability of the amino-activated carbons for adsorption of copper from aqueous phase, it is found that the introduction of
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amino groups onto the carbon increases the adsorption capacity of Cu (II) ions
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considerably, because of a strong interaction of basic amino groups with Cu (II) ions.
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Graphical abstract
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