High-Recovery Material for Mercury Ions Based on a Polyallylamine

Article pubs.acs.org/IECR

High-Recovery Material for Mercury Ions Based on a Polyallylamine Hydrogel with Thiourea Groups at Cross-Linking Points Daisuke Nagai,*,‡ Takehiro Daimon,‡ Satoshi Kawakami,§ and Kenji Inoue∥ ‡

Department of Chemistry and Chemical Biology, Graduate School of Engineering, Gunma University, 1-5-1 Tenjin-cho, Kiryu, Gunma 376-8515, Japan § DOWA ECO-SYSTEM Co. Ltd., 4-14-1, Sotokanda, Chiyoda-ku, Tokyo 101-0021, Japan ∥ Kaneka Corporation, 2-3-18, Nakanoshima, Kita-ku, Osaka 530-8288, Japan ABSTRACT: We have successfully synthesized a high-recovery material for mercury based on a polyallylamine (PAA) hydrogel bearing thiourea cross-linking points and pendant amino groups. Adsorption of HgII was not significantly influenced by pH, and the PAA hydrogel was also able to adsorb HgII at a wide pH range (1−9) with high adsorption efficiency (≥93%). Adsorption was fast in the first 10 min, with 84% adsorption efficiency, and equilibrium was reached within 30 min. Langmuir and Freundlich isotherm models were employed to correlate the experimental data. The adsorption of HgII by PAA hydrogel is well supported by a Langmuir isotherm model with a maximum adsorption capacity of 2.375 gHg/ggel, which is greater than those of other polymers in the literature. The PAA hydrogel was also to adsorb efficiently in a low concentration HgII aqueous solution (0.2 ppm). This polymer will be applicable as a high recovery material for mercury.

1. INTRODUCTION Heavy metal pollution represents an important environmental problem due to the toxic effects of metals even at very low concentrations. The accumulation of heavy metals throughout the food chain leads to serious ecological and health problems.1 Mercury is universally recognized as one of the most toxic, dangerous, and nonbiodegradable inorganic pollutants present in aquatic systems.2−8 Therefore, emission standards for mercury are more strictly defined (for example, 99.5%), sodium bromide (Wako Pure Chamical Industries, 99.9%), and sodium iodide (Wako Pure Chemical Industries, >99.9%) were used as received. Polyallylamine (PAA, 10.4 wt % aqueous solution, Mw = 25000) was supplied by Nitto Boseki Co., Ltd. (sample code PAA-25). Carbon disulfide (CS2, Wako Pure Chemical Industries, >96.0%) was distilled prior to use. Potassium hydroxide Received: Revised: Accepted: Published: 3300

September 19, 2013 January 21, 2014 February 4, 2014 February 4, 2014 dx.doi.org/10.1021/ie403118b | Ind. Eng. Chem. Res. 2014, 53, 3300−3304

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3. RESULTS AND DISCUSSION 3.1. Synthesis of PAA Hydrogels. PAA hydrogels were synthesized according to our previous reported procedure.15 A cross-linking reaction of PAA with carbon disulfide as a crosslinking agent in the presence of KOH afforded the corresponding PAA hydrogels bearing thiourea cross-linking points and pendant amino groups (Scheme 1). In this study,

(KOH, Wako Pure Chemical Industries, >85.0%) was used as received. 2.2. Instrumentation. IR spectra were recorded using a Jasco FT/IR-5000 spectrometer; values are given in cm−1. Flame atomic absorption spectrometry (AAS) was carried out using a Hitachi polarized Zeeman atomic absorption spectrometer (Z-2310). 2.3. Synthesis of PAA Hydrogel by Cross-Linking PAA with CS2 (Typical Procedure). Carbon disulfide (0.031 mL, 0.525 mmol, 40 mol % relative to PAA repeat units) was added to an aqueous solution of PAA (1.90 mL of 10.4 wt % aqueous solution, 3.50 mmol of the repeat unit), and the reaction mixture was stirred for 2 h at room temperature. KOH (0.18 g, 3.20 mmol) was then added to the mixture, which was stirred at 40 °C for 20 h. The resulting hydrogel was immersed in diethylether for 3 days to remove the unreacted CS2, and the unreacted PAA and potassium salts were removed by Soxhlet extraction with water. The hydrogel was dried to constant weight at 60 °C in vacuo to obtain the dry gel (208 mg, 90%). The sulfur contents of the PAA hydrogels were estimated by elemental analysis of sulfur (see Table 1). Feed ratio of CS2 relative to the repeating unit of PAA = 30 mol %; S = 0.1760. Feed ratio of CS2 = 40 mol %; S = 0.1866. Feed ratio of CS2 = 40 mol %; S = 0.2188.

Scheme 1. Synthesis of PAA Hydrogel

three types of hydrogel with different sulfur contents were synthesized by the use of 30, 40, and 50 mol % CS2 with respect to the repeating unit of PAA. Increasing the feed amount of CS2 resulted in increased sulfur contents in the PAA hydrogels (Table 1). 3.2. Effect of pH on HgII Adsorption. In general, the adsorption of metal ions to an adsorbent is influenced by pH, because the surface properties of the adsorbent and the form and ionic charge of the metal ions are dependent on pH.17 Mercury adsorption by polymers has been shown to be significantly affected by pH for adsorbents containing sulfur or amino groups.18,19 Accordingly, we first examined the effect of pH on the adsorption of HgII using PAA hydrogels. It is noteworthy that the adsorption of HgII was not influenced by pH to a significant extent, and the PAA hydrogels adsorbed HgII at a wide pH range (1−9) with high adsorption efficiency (Figure 1, ≥93%). No HgII ions were precipitated at any pH.

Table 1. Effect of Sulfur Content in PAA Hydrogel on HgII Adsorptiona entry

feed amount of CS2 relative to repeat unit of PAA (mol %)

sulfur content (wt%)b

adsorption (%)c

1 2 3

30 40 50

17.6 18.7 21.9

76 92 90

a

Conditions: metal ion aqueous solution, 10 mL (4.0 mM, pH 7) and PAA hydrogel (13 mg) at ambient temperature for 20 h. bCalculated on the basis of elemental analysis. cDetermined by AAS analysis.

IR (KBr): ν 1456 (−HN−CS−NH−), 1560 (−HN−C S−NH−), 1620 (−CH2NH2), 3420 (−CH2NH2, −HN−C S−NH−). 2.4. HgII Adsorption (Typical procedure). Adsorption experiments were individually conducted at ambient temperature. Thirteen mg of a powder consisting of the dried gel, ground by mortar and separated by a sieve to give sizes of less than 500 μm, was added to an aqueous HgII solution of the appropriate concentration (10.0 mL), and the mixture was stirred at ambient temperature. The resulting gels were separated by filtration (pore size of filter: 0.45 μm), and an aliquot (0.250 mL) of the filtrate was removed for sampling. After appropriate dilution, the metal concentration of the solution was determined by AAS. The amount adsorbed was calculated based on the following equation:

Figure 1. Effect of pH on HgII adsorption by PAA hydrogel. Conditions: metal ion aqueous solution, 10 mL (4.0 mM) and PAA hydrogel (13 mg) at ambient temperature for 20 h.

The adsorption efficiency decreased slightly around pH 6. This may have resulted from an interaction between the amino groups and the HgII ions. In the acidic region, the predominant mercury species is HgCl3−, and ion exchange between the protonated amino groups and HgCl3− is the main mechanism.20,21 In the basic region, coordination of the deprotonated amino groups to HgII is predominant. Around pH 6, both effects may be reduced. 3.3. Effect of Sulfur Content of PAA Hydrogels. Next, the effect of the sulfur content of the PAA hydrogel on the HgII

Adsorption amount (g metal /g Hg) = M of metal × recovered amount (mmol) /weight of gel used (g)

(1)

The pH of each solution was adjusted by HNO3 or NaOH aqueous solution. 3301

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significant advantage of PAA hydrogel for use in wastewater treatment processes and also implies that the adsorbent can potentially be used in continuous flow processes. The experimental adsorption kinetic data were fitted using pseudo-first- and pseudo-second-order kinetic models, with the following equations22,23

adsorption was examined (Table 1). An increase in the sulfur content to 18.7 wt % resulted in increased adsorption efficiency (entries 1 and 2) due to the high affinity of sulfur atoms toward HgII ions. However, when the sulfur content was further increased to 21.9 wt %, the adsorption was altered only by a small amount (entries 2 and 3). This was ascribed to both the adsorption ability and hydrophilicity of the PAA hydrogel. Increasing the sulfur content enhanced the adsorption ability, but the number of hydrophilic amino groups decreased, leading to low dispersibility of the gels in the aqueous solution of HgII ions. Hence, a PAA hydrogel with a sulfur content of 18.2 wt % was used in subsequent experiments. 3.4. IR Spectroscopy. IR spectra of the PAA hydrogel before and after the HgII adsorption showed that the absorption peaks corresponding to the thiocarbonyl groups (1546 cm−1) and allylamine moieties (1617 cm−1) shifted to higher wavenumbers (Figure 2), indicating that the thiocarbonyl

log(q1 − qt) = log q1 − k1t /2.303 (pseudo‐first‐order) (2) 2

qt = q2 k 2 t/(1 + q2k 2t ) (pseudo‐second‐order)

(3)

II

where both q1 and q2 are the amount of Hg adsorbed at equilibrium (gHg/ggel), qt is the amount of HgII adsorbed at a certain time (gHg/ggel), k1 is the pseudo-first-order rate constant (min−1), and k2 is the pseudo-second-order rate constant (min−1). The rate constants, experimental and predicted equilibrium uptakes, and corresponding correlation coefficients were calculated, and these are summarized in Table 2. In the Table 2. Constants for Kinetic Model of Adsorption qe (gHg/ggel) 0.592

q (gHg/ggel)

k (min−1)

pseudo-first-order model 0.413 k1 0.140 pseudo-second-order model 0.660 k2 0.4394

R2 0.981 0.998

pseudo-first-order model, the calculated q value deviated from the experimental qe value even though the correlation coefficient is 0.981, because the pseudo-first-order model only represented the kinetic experimental data for the rapid initial phase. However, the pseudo-second-order model is based on the adsorption capacity of the solid phase. In contrast to the pseudo-first-order model, it predicts the adsorption behavior over the entire study range.24 In our study, the pseudo-secondorder model produced better results. The correlation coefficient was 0.998, suggesting that the adsorption system was dominated by a chemical adsorption process that contributed to the fast adsorption kinetics. 3.6. Adsorption Isotherms. To optimize the design of an adsorption system to remove heavy metal ions, it is important to establish the most appropriate correlation for the adsorption isotherm. Langmuir and Freundlich isotherms have been used to model many adsorption processes. The Langmuir isotherm assumes monolayer coverage of the adsorbate over a homogeneous adsorbent surface, with the adsorption of each molecule onto the surface having equal adsorption activation energy, while the Freundlich isotherm supposes a heterogeneous surface and allows the expression of multilayer adsorption. The Langmuir and Freundlich isotherms are expressed as eqs 4 and 5, respectively

Figure 2. IR spectra of PAA hydrogel (a) before adsorption; (b) after adsorption (adsorption amount: 0.306 gHg/ggel); (c) after adsorption (adsorption amount: 0.613 gHg/gel).

sulfur and amino groups contributed to HgII adsorption. This may be attributed to the main mechanism, i.e., coordination of the lone pair of electrons on the sulfur and nitrogen atoms to the HgII ion. 3.5. Adsorption Kinetics. Because kinetics control the overall efficiency of the process, the kinetics of HgII adsorption by the PAA hydrogel were studied at ambient temperature. The kinetic curve showed that the adsorption was rapid in the first 10 min, with 84% removal efficiency, and equilibrium was reached within 30 min (Figure 3). The rapid rate represents a

Ce/qe = Ce/qm + 1/(bqm)

(4)

log qe = log K f + 1/n log Ce

(5)

where qm is the maximum adsorption amount (gHg/ggel), qe is the adsorption capacity at equilibrium (gHg/ggel), b is the equilibrium constant (L mg −1), Ce is the equilibrium concentration of substrates in the solution (mg L−1), Kf is the equilibrium constant indicating adsorption capacity, and n is the adsorption equilibrium constant. Figure 4 shows the adsorption isotherm of HgII on the PAA hydrogel. It can be seen that qe increased initially with

Figure 3. Adsorption kinetics of HgII by PAA hydrogel. Conditions: metal ion aqueous solution, 10 mL (4.0 mM, pH 7) and PAA hydrogel (13 mg) at ambient temperature from 1 to 120 min. 3302

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reactions in the system. This is not the case for solutions containing HgII ions, which are known to form very stable complexes with halide ions.35 The formation of such strong complexes results in a masking effect that can significantly affect the performance of an adsorbent. In this study, sodium halides (NaCl, NaBr, and NaI) were chosen as model salts for investigating the effect on the adsorption of HgII ions by the PAA hydrogel. The effect was studied by carrying out a series of adsorption experiments in solutions of HgII containing NaX at various concentrations. Table 5 shows the effect of NaX Table 5. Effects of Sodium Halide Salts on Adsorption of HgII by PAA Hydrogela Figure 4. Effect of remaining concentration of HgII on adsorption capacity of PAA hydrogel. Conditions: metal ion aqueous solution, 10 mL (pH 7) and PAA hydrogel (13 mg) at ambient temperature for 20 h.

entry

salt

concn (mM)

amount adsorbed (gHg/ggel)

adsorption (%)

1 2 3 4 5 6 7 8 9 10

− NaCl NaCl NaCl NaBr NaBr NaBr NaI NaI NaI

0 2 4 8 2 4 8 2 4 8

0.615 0.614 0.616 0.612 0.612 0.612 0.612 0.611 0.611 0.611

99 99 99 99 99 99 99 99 99 99

increasing Ce until equilibrium was reached, after which qe remained constant with further increases in Ce. The Langmuir and Freundlich adsorption constants evaluated from the isotherms, along with the correlation coefficients, are listed in Table 3. As can be seen, the Langmuir isotherm gave a better fit Table 3. Langmuir and Freundlich Isotherm Constants for HgII Adsorption by PAA Hydrogel Langmuir model

a

Conditions: metal ion aqueous solution, 10 mL (pH 7) and PAA hydrogel (13 mg) at ambient temperature for 20 h.

Freundlich model

b (L mg−1)

qm (gHg/ggel)

R2

n

Kf

R2

0.055

2.375

0.999

4.068

550.3

0.797

concentration on the adsorption of HgII by the PAA hydrogel. Surprisingly, the adsorption of HgII was very high in all cases, indicating that the HgII adsorption by the PAA hydrogel is not affected by the presence of halide ions because of its strong adsorptivity. 3.8. Adsorption in Low Concentration Solution of HgII. In practical use, the complete recovery of mercury from wastewater containing low concentrations is desirable. As described in the Introduction, emission standards for mercury are more strictly defined than those for other toxic metals such as cadmium and lead. To determine the adsorptivity of PAA hydrogels at low mercury concentrations, a recovery experiment was conducted using a low concentration of HgII (0.2 ppm) and PAA hydrogels. PAA hydrogels (13 mg) were added to an aqueous solution of HgII (0.2 ppm, 10 mL, pH 7) and stirred for 48 h at ambient temperature. After separation of the gels, it was found that the concentrations of HgII in the solutions had fallen below the AAS detection limit (0.0038 ppm). Thus, PAA hydrogel is expected to be useful as an adsorbent that can clear wastewater to the emission standard.

than the Freundlich isotherm, which showed that monolayer adsorption takes place on the surface of the PAA hydrogel. According to the Langmuir equation, the maximum adsorption capacity (qm) for HgII was 2.375 gHg/ggel. Table 4 compares the Table 4. Comparison of the Maximum Adsorption Capacities of Various Adsorbents for Mercury Ion entry

adsorbent

1

ethylenediamine-modified methyl methacrylate acrylamide-grafted sulfonamide-based resin chlorosulfonamidated polystyrene resin urea-grafted glycidyl methacrylate-based resin N-methacryloyl-(L)-cysteine modified PHEMA glutaraldehyde-cross-linked chitosan Triethylenetetramine modified polystyrene thiol-modified poly(GMA-DVB) thiol-grafted chitosan aniline/sulfonanisidine copolymer nanosorbents PAA hydrogel

2 3 4 5 6 7 8 9 10 11

qm (gHg/gpoly)

ref

0.700

25, 26

1.154 1.384 1.745

27 28 29

1.018

30

0.891 0.345 0.400 1.605 2.063

3 31 32 33 34

2.375

this work

4. CONCLUSIONS In summary, we have successfully synthesized a high-recovery material for mercury based on a polyallylamine hydrogel bearing thiourea cross-linking points and pendant amino groups. Adsorption of HgII was not significantly influenced by pH, and the PAA hydrogel was able to adsorb HgII at any pH with high adsorption efficiency (>93%). The maximum adsorption capacity calculated by the Langmuir isotherm model was 2.375 gHg/ggel, which is greater than those of other polymers in the literature. In addition, the PAA hydrogel adsorbed efficiently in low-concentration solutions of (0.2 ppm), indicating its high potential for practical use in the

maximum adsorption capacities of different types of adsorbent. Our polymer had a higher adsorption capacity (qm) than any other polymer reported in the literature sample we obtained. 3.7. Effect of the Presence of Salts. Some industrial wastewaters contain, in addition to toxic heavy metal ions, large quantities of other salts such as sodium chloride. Generally, the effect of this is a high ionic strength that slightly modifies the values of the equilibrium constants without introducing new 3303

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removal of mercury. This polymer is expected to be applicable as a high-recovery material for mercury.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +81-277-30-1485. Fax: +81-277-30-1409. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by Dowa Eco-System Co., Ltd. and Kaneka Corporation.

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REFERENCES

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