Article pubs.acs.org/IECR
Hg2+ Adsorption from a Low-Concentration Aqueous Solution on Chitosan Beads Modified by Combining Polyamination with Hg2+Imprinted Technologies Xuejiao Tang,† Dong Niu,‡,§ Chengliang Bi,*,§ and Boxiong Shen† †
College of Environmental Science & Engineering, Nankai University, Tianjin 300071, People’s Republic of China Tianjin Port Free Trade Zone Environmental Monitoring Station, Tianjin 300308, People’s Republic of China § School of Environmental Science and Safety Engineering, Tianjin University of Technology, Tianjin 300384, People’s Republic of China ‡
ABSTRACT: Chitosan (CTS) derivative adsorbent [P-C-CTS-(Hg)] was prepared by employing the polyamination and Hg2+imprinted technologies for Hg2+ removal from a low-concentration aqueous solution (CHg2+ ≤ 40 mg·L−1). The prepared adsorbents were characterized by Fourier transform infrared spectroscopy, and their physical and chemical properties were compared. The adsorption capacity of P-C-CTS-(Hg) beads to Hg2+ was 2.2 times higher than that of CTS beads because of the increased number of amide groups as adsorption sites. The adsorption processes and mechanisms at pH from 1.5 to 5.5 were proposed according to the aqueous mercury species distributions. Langmuir and Freundlich isotherm models were applied to analyze the experimental data of Hg2+ adsorption by P-C-CTS-(Hg) and predict the relevant isotherm parameters. The best interpretation was given by the Freundlich isotherm equation. The kinetic adsorptions were well described by a pseudo-secondorder reaction model. Furthermore, the results of selectivity adsorption suggested that P-C-CTS-(Hg) possessed good adsorption selectivity for Hg2+ in the presence of Pb2+ because of the employment of Hg2+-imprinted technology. The regeneration and reutilization of P-C-CTS-(Hg) were investigated. The results showed that the adsorption capacity and adsorption property remained stable after eight reuse cycles.
1. INTRODUCTION Heavy-metal contamination in water, such as mercury (Hg), nickel (Ni), and chromium (Cr) from mining, manufacturing, and disposal, poses serious hazards to human health. They have high toxicity and carcinogenecity even at low concentrations.1,2 Mercury has long been of great concern because of its high toxicity, adverse environmental effects, bioaccumulative tendency, nonbiodegradability, difficulties in control, and growing discharging rates.3−5 The World Health Organization has announced a maximum acceptable Hg2+ concentration of 1 μg· L−1 in drinking water.6 Among the existing forms (Hg0, Hg+, and Hg2+) in aquatic systems, Hg2+ is the most stable and widely spread in the environment.7,8 Removal of Hg2+ from aqueous solutions is thus a significant environmental issue. The conventional methods for Hg2+ removal include chemical precipitation, solvent extraction, electrolysis, membrane separation, ion exchange, and adsorption.9,10 Chemical precipitation is the cheapest technique used to remove Hg2+ from industrial effluent, where the concentration and content of Hg2+ are both high. However, it will lead to hardening of the water quality and secondary pollution. Also it is infeasible to treat contaminated rivers on site. Even worse, it cannot completely remove Hg2+ from low-concentration aqueous solutions because of the water-soluble products. Adsorption is an efficient method to remove Hg2+ from aqueous solutions, avoiding secondary pollution. Also the adsorbents usually can be regenerated and reutilized.11,12 Various adsorbents, such as activated carbon, silica, metal oxides, agricultural byproducts, and modified polymers, have been studied.13−20 They all possess high © XXXX American Chemical Society
adsorption capacities and good adsorption properties for Hg2+ at high initial concentration (200−400 mg·L−1) in aqueous solutions. The adsorption behaviors and mechanisms of the absorbents were mostly discussed. However, there are few studies to figure out whether the Hg2+-contaminated water could directly meet the discharge standards by adsorption treatments. The small concentration gradient between the adsorbate in solution and adsorbate in the adsorbent may inhibit further adsorption of adsorbate from solution onto the adsorbents.21 It is thus difficult to directly reduce the concentration from high to extremely low by adsorbing Hg2+ onto adsorbents. So, it is more significant and practical to investigate the adsorption behaviors of adsorbents for Hg2+ in a low-concentration aqueous solution. Chitosan (CTS) has a positive environmental and economic impact because it is renewable and biodegradable.22 It is accepted that the amine sites (−H2N: groups) of CTS are the main reactive groups for capturing heavy-metal ions,23,24 where the free paired electrons of nitrogen may bind metal cations in neutral or moderately acidic media. Therefore, the adsorption capacity would be significantly enhanced by employing large numbers of −H2N: groups on CTS via polyamination. The increase of the −H2N: number could lead to an increase of the concentration gradient of adsorbate between solution and on Received: April 30, 2013 Revised: August 4, 2013 Accepted: August 19, 2013
A
dx.doi.org/10.1021/ie401359d | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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the adsorbent,18 which could improve the adsorption property and efficiency for Hg2+ adsorption. A molecularly imprinted polymer is a smart adsorbent that exhibits high affinity and selectivity.25 In the experimental study, Hg2+-imprinted CTS beads were selectively efficient to remove Hg2+ from aqueous solutions. The Hg2+-imprinted characters enable CTS to have better Hg2+ removal efficiencies compared with nonselective adsorbents. In the paper, polyamination and Hg2+-imprinted technologies were simultaneously employed to prepare CTS derivative adsorbent beads [P-C-CTS-(Hg)] for Hg2+ removal from aqueous solutions. The adsorption properties and behaviors of P-C-CTS-(Hg) were investigated in the diluted solution with a low initial concentration of Hg2+ (CHg ≤ 40 mg·L−1).
solution were determined by inductively coupled plasma optical emission spectrometry (ICP-OES; Varian Inc.). 2.4. Adsorption Experiments. The adsorption experiments for determining the capacities were carried out by oscillating the adsorbents in 20.0 mL of Hg2+ solutions of 60 mg·L−1 at a pH of 5.0 under room temperature for 24 h. The adsorbents were then taken out, and the residual Hg2+ solutions were adjusted to be at a pH of 2.0 by the diluted H2SO4 solution. The potential HgO precipitation thus dissolved, and the concentrations of the residual Hg2+ in solutions were determined by ICP-OES. The isotherm experiments were performed by shaking the adsorbents for 24 h at 298 K in a series of flasks containing 20.0 mL solutions where the initial Hg2+ concentrations ranged from 10 to 60 mg·L−1 at a pH of 3.0. Then the equilibrated Hg 2+ concentrations were determined. The adsorption kinetics and thermodynamics were accomplished by shaking the adsorbents in 80.0 mL of a Hg2+ solution of 20 mg·L−1 at a pH of 3.0 for 6 h at different temperatures from 298 to 323 K. An amount of 1 mL of the solutions was sampled at different time intervals, and the concentrations of Hg2+ in the samples were determined. The blank experiment was also taken at the same conditions without any adsorbent. Each batch of the experiment was carried out in triplicate, and the mean values showed a maximum standard deviation of ±5%. The adsorption capacities (Q) and moisture contents (W) of the adsorbents were calculated according to the following equations, respectively.
2. EXPERIMENTAL SECTION 2.1. Materials. Chitosan (CTS; deacetyl degree ≥92%) was purchased as an industrial-grade powder from Xiamen Sanland Chemical Reagent Co., Ltd. The stock solutions of Hg(NO3)2 (1.0 M) and Pb(NO3)2 (1.0 M) were purchased from Tianjin Zhongyi Standard Co., Ltd. The working solutions having different initial concentrations of metal ions were prepared by appropriate dilutions of the stock solutions immediately prior to their use. Acetic acid and sulfuric acid (H2SO4) were obtained from Tianjin Chemistry Reagent Factory. Sodium hydroxide (NaOH), ethanol, epichlorohydrin (ECH), and tetraethylenepentamine (TEPA) were obtained from Tianjin Guangfu Fine Chemicals Co., Ltd. All chemicals used were of reagent grade or better unless otherwise specified. The water used was deionized water. 2.2. Adsorbent Preparation. CTS powder was dissolved in a 4 wt % acetic acid solution. CTS beads with diameter of 2.0 mm were formed by dropping the CTS solution from an injector into a basic solution consisting of 100 mL of 10 wt % NaOH and 100 mL of anhydrous ethanol. After 24 h, the CTS beads were rinsed to be neutral for use. CTS-Hg beads were prepared by the same method as that for CTS beads except that the diluted acetic acid solutions contained Hg2+. CTS powders were dissolved in 4 wt % acetic acid solutions, which contained 0.01 and 0.1 g·L−1 Hg2+, respectively. The corresponding Hg2+ contents in CTS were 0.03 and 0.3 mg·g−1, respectively. The obtained beads were designated as CTS-Hg0.03 and CTS-Hg0.3, respectively. Cross-link reaction was carried out by immersing 10.0 g of CTS-Hg (CTS-Hg0.03 or CTS-Hg0.3) beads in 50 mL of an ECH solution of 3.5 wt % at 353 K for 5 h. After rinsing, the beads were designated as C-CTS-Hg. Then the obtained CCTS-Hg beads were polyaminated in 60 mL of a TEPA solution of 50.0 wt % at 333 K for 5 h. When heated, TEPA reacted with ECH molecules through a substitution reaction, where chloride was replaced by TEPA molecules and HCl went away into the solution. Thus, polyamination occurred. After rinsing, the P-C-CTS-Hg beads were obtained. The Hg2+imprinted and polyaminated CTS derivative beads [designated as P-C-CTS-(Hg)] were obtained by removing Hg2+ from the P-C-CTS-Hg beads in a diluted H2SO4 solution of 1 wt % under oscillation until there was no Hg2+ detected by a saturated H2S solution. After solidification in the diluted NaOH solution of 0.4 mol·L−1 for 1 h, the P-C-CTS-(Hg) beads were rinsed to be neutral for use. 2.3. Characterization Techniques. To investigate the reactions in the adsorbent preparation, the adsorbents were characterized by Fourier transform infrared spectrometry (FTIR; Nicolet MAGNA-560). The concentrations of Hg2+ in
Q (Q t ) =
(C0 − Ce) × 0.020 m0(1 − W )
W = 100 ×
m0 − m1 m0
(1)
(2)
where Q (mg·g−1) is the adsorption capacity of the adsorbent for 24 h and Qt (mg·g−1) is the adsorption capacity for t hours, W (%) is the moisture content of the adsorbent, C0 (mg·L−1) and Ce (mg·L−1) are the initial and equilibrated concentrations of Hg2+, respectively, and m0 and m1 are the mass of the adsorbent before and after drying at 323 K, respectively. 2.5. Desorption and Regeneration. The adsorbed Hg2+ was desorbed from the P-C-CTS-(Hg) beads in the diluted H2SO4 solution of 0.1 mol·L−1 under oscillation at 298 K until there was no Hg2+ detected. Then the beads were rinsed to be neutral. After solidification in the NaOH solution of 0.4 mol· L−1 for 4 h, the beads were rinsed to be neutral for use in the succeeding Hg2+ adsorption cycles. The “adsorption−desorption” procedure was repeated in eight cycles, and the regeneration and reutilization properties of P-C-CTS-(Hg) were investigated.
3. RESULTS AND DISCUSSION 3.1. Adsorption Capacities and Moisture Contents. The adsorption capacities and moisture contents of the adsorbents are listed in Table 1. It is known that the active amino groups (−H2N:) are the effective adsorption sites for Hg2+. As shown in Table 1, QC‑CTS‑Hg0.3 was smaller than QCTS because a portion of −H2N: groups in the C-CTS-Hg0.3 beads were “protected” by Hg2+ and occupied by ECH molecules. The decrease of the −H2N: group number and the increase of the molecular weight contributed to the relative decrease of the −H2N: group number per unit mass of C-CTS-Hg0.3. In polyamination, a large number of −H2N: B
dx.doi.org/10.1021/ie401359d | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Table 1. Adsorption Capacities for Hg2+ and Moisture Contents of the Adsorbents (CHg(initial) = 60 mg·L−1 at 298 K and a pH of 5.0)
groups were introduced and the adsorption sites for Hg2+ increased. Furthermore, Hg2+-imprinted technology plays an important role in improving the adsorption capacity of the adsorbent. So, Q P‑C‑CTS‑(Hg 0 . 3 ) was bigger than Q CTS . QP‑C‑CTS‑(Hg0.3) was bigger than QP‑C‑CTS‑(Hg0.03), which confirmed that Hg2+-imprinted technology enhanced the adsorption capacity because of protection by Hg2+ in the cross-link reaction. The original Hg2+ in P-C-CTS-Hg0.3 was desorbed in the adsorption experiment under oscillation, which resulted in a calculated value of QP‑C‑CTS‑Hg0.3 lower than zero. WC‑CTS‑Hg0.3 was lower than WCTS and WCTS‑Hg0.3, indicating that the cross-link reaction occurred successfully. The moisture content got lower because the molecular weight became higher after the cross-link reaction. For the same reason, increases of WP−C−CTS‑(Hg0.3) and WP−C−CTS‑(Hg0.03) were attributed to the fact that the molecular weight became lower after Hg2+ desorbed and the moisture content accordingly became higher. Comparisons of the adsorption capacities and moisture contents of the prepared adsorbents validated the synthesis of P-C-CTS-(Hg) in the proposed processes. P-C-CTS-(Hg0.3) reached a maximum adsorption capacity of 9.071 mg·g−1, 2.2 times higher than that of CTS. This was attributed to the fact that polyamination and Hg2+-imprinted technology were simultaneously employed. So, P-C-CTS-(Hg0.3) [shortened to P-C-CTS-(Hg) in the following text] was chosen to investigate the characteristics and adsorption behaviors for Hg2+ as follows. 3.2. FT-IR Characteristics. The FT-IR spectra of CTS, CTS-Hg, C-CTS-Hg, and P-C-CTS-(Hg) are shown in Figure 1. As shown in Figure 1, the broader and stronger peak at 3400−3200 cm−1 represents the overlapping peaks of the O−H and N−H groups. The stronger peaks appearing at 2930 and 2880 cm−1 correspond to C−H in the methyl and methylene groups. The peak at 1640−1600 cm−1 represent N−H in the
primary amide groups. The stronger and well-characterized N− H peak in the secondary amide groups appears at 1530−1500 cm−1. The peak appearing at 1420−1400 cm−1 is characteristic of C−N in the primary amide groups. The stronger peak appearing at 1300−1260 cm−1 corresponds to C−N in the secondary amide groups. Compared with the CTS spectra (Figure 1a), the peak of N−H in the primary amide groups appearing at 1640−1600 cm−1 in CTS-Hg (Figure 1b) becomes weaker and shifts to right. The change and shift indicates that a part of the amide groups (−H2N:) already chelated with Hg2+. These −H2N: groups were “protected” by Hg2+. Compared with the CTS-Hg spectra (Figure 1b), the peak of the C−O−C groups at 1070 cm−1 in C-CTS-Hg (Figure 1c) becomes stronger. This indicates that a cross-link reaction occurred between CTS-Hg and ECH, and the new groups (C−O−C) were introduced in C-CTS-Hg. Meanwhile, the peak of N−H at 1640 cm−1 disappears because part of the “unprotected” −H2N: groups in CTS-Hg was involved in the cross-link reactions. The peaks of C−H at 2932 and 2880 cm−1 become stronger because of the introduction of ECH molecules, verifying the occurrence of cross-link reactions. In the P-C-CTS-(Hg) spectra (Figure 1d), a stronger peak, which is characteristic of the N−H peaks in the primary amide groups, appears at 1600 cm−1. This indicates that Hg2+ was desorbed from the P-C-CTS-(Hg) beads. The peak at 1465 cm−1 becomes stronger because of the introduction of plenty of methylene groups from TEPA. It is suggested that polyamination occurred and the primary and secondary amide groups were introduced into P-C-CTS-(Hg). According to the results of the FT-IR spectra, the P-C-CTS(Hg) beads were successfully prepared in the proposed processes. These results are in accordance with the changes of the adsorption capacities and moisture contents of the prepared adsorbents. 3.3. Effect of the pH on Hg2+ Adsorption. The pH value can significantly affect the transition-metal species in aqueous solutions and change the surface functional groups of the adsorbents. So, the pH value determines the adsorption properties of transition-metal ions onto the adsorbents and influences the interactions between metal ions and adsorbents. The adsorption experiments of P-C-CTS-(Hg) at different pH values from 1.5 to 5.5 were conducted at 298 K with an initial Hg2+ concentration of 40 mg·L−1 for 24 h. The influences of the pH on the adsorption capacities of P-C-CTS-(Hg) are shown in Figure 2. It is illustrated that the adsorption capacities increased with increasing pH values at pH lower than 3.0 and then decreased with increasing pH values in the higher pH
Figure 1. FT-IR spectra for CTS derivative adsorbents.
Figure 2. Relationship of the pH values and adsorption capacities of PC-CTS-(Hg) (CHg(initial) = 40 mg·L−1 at 298 K).
CTS Q/mg·g−1 W/% Q/mg·g−1 W/%
4.164 95.63 P-C-CTS-Hg0.3
