Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 765–775
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Chelating stability of an amphoteric chelating polymer flocculant with Cu(II), Pb(II), Cd(II), and Ni(II) Lihua Liu ⇑, Yanhong Li, Xing Liu, Zhihua Zhou, Yulin Ling Key Laboratory of Theoretical Chemistry and Molecular Simulation of Ministry of Education and Hunan Province, Hunan Province College Key Laboratory of QSAR/QSPR, School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan, Hunan 411201, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Determination of the ultraviolet
spectra of ACPF and its chelates with heavy metals. Determination of the compositions and stability constant of the chelates. The chelating precipitates are very stable at pH above 5.6. A chelating mechanism of ACPF with heavy metal ion was proposed. The stability mechanism of the chelating precipitates was explored at different pH.
a r t i c l e
i n f o
Article history: Received 5 November 2012 Received in revised form 8 September 2013 Accepted 25 September 2013 Available online 4 October 2013 Keywords: Stability constant Amphoteric chelating polymer flocculant Heavy metal ions Ultraviolet spectrophotometric method Leaching-out characteristics
a b s t r a c t The absorption spectra of Cu2+, Pb2+, Cd2+, and Ni2+ chelates of an amphoteric chelating polymer flocculant (ACPF) were measured by ultraviolet spectrophotometry, and their compositions and stability constants (b) were calculated. ACPF exhibited three apparent absorption peaks at 204, 251, and 285 nm. The ACSS group of ACPF reacted with Cu2+, Ni2+, Pb2+, and Cd2+ to form ACPF–Cu2+, ACPF–Ni2+, ACPF–Pb2+, and ACPF–Cd2+ chelates, respectively, according to a molar ratio of 2:1. The maximum absorption peaks of ACPF–Cu2+, ACPF–Ni2+, ACPF–Pb2+, and ACPF–Cd2+ appeared at 319, 326, 310, and 313.5 nm, respectively. The maximum absorption peaks of the chelates showed significant red shifting compared with the absorption peaks of ACPF. The b values of the ACPF–Cu2+, ACPF–Pb2+, ACPF–Cd2+, and ACPF–Ni2+ chelates were (1.37 ± 0.35) 1012, (3.26 ± 0.39) 1011, (2.05 ± 0.27) 1011, and (3.04 ± 0.45) 1010, respectively. The leaching rate of heavy metal ions from the chelating precipitates decreased with increasing pH. ACPF–Cu2+, ACPF–Ni2+, ACPF–Pb2+, and ACPF–Cd2+ were very stable at pH P 5.6. Cu2+, Ni2+, Pb2+, and Cd2+ concentrations in the leaching liquors were lower than the corresponding limits specified by the Integrated Wastewater Discharge Standard of China. Ó 2013 Elsevier B.V. All rights reserved.
Introduction The dangers posed by toxic heavy metals to the environment are an increasing global concern [1]. Heavy metal wastewater damages the ecological environment and seriously threatens human health. Thus, the exploration and development of effective ⇑ Corresponding author. Tel.: +86 731 58291625. E-mail addresses: [email protected], [email protected] (L. Liu), [email protected] (Y. Li), [email protected] (X. Liu), [email protected] (Z. Zhou), [email protected] (Y. Ling). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.09.066
removal methods and separation technologies for toxic heavy metal ions in wastewater has attracted significant research attention. Many heavy metal wastewater treatment methods, including chemical precipitation [2], ferrite method [3], chelation–precipitation method [4,5], ion exchange [6], adsorption [7], membrane separation [8], electrochemical method [9], and biological flocculation [10], have been developed. Among these methods, chelation–precipitation offers excellent results, lower costs, and better suitability for large-scale heavy metal wastewater treatment [4,5]. The treatment effects of chelation depend on the performance of the chelating flocculant. Chelating agents that have been recently developed
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and applied can be classified as either small molecule chelating agents or polymer chelating agents. Small molecule chelating agents mainly include aminocarboxyl chelating agents and dithiocarbamates (DTCs), such as ethylenediamine-tetraactic acid, sodium diethyl dithiocarbamate [11], disodium N,N-bis-(dithiocarboxy) piperazine [12], and sodium 1,3,5-hexahydrotriazinedithiocarbamate [13]. Polymer chelating agents can be further divided into water-soluble polymer chelating agents and water-insoluble chelating resins. Chelating resins are usually obtained by introducing chelating groups to cross-linked resins, such as by anchoring the chelating agent N,N0 -di(carboxymethyl) dithiocarbamate to chloromethylated PS-DVB [14] or by introducing iminodiacetic acid into poly(styrene-co-GMA) [15]. These resins are mainly used for adsorption and ion exchange. Watersoluble polymer chelating agents mainly originate from the introduction of hydroxyl, carboxyl, iminodiacetic acid, xanthan, or DTC groups to the chelating agent. Water-soluble polymer chelating agents include N-methyl hydroxamic acid [16], phosphonic acyl methyl [17], DTC groups in polyethyleneimine chains [18], and DTC groups in polyacrylamide chains [19]. Chelating agents containing DTC groups, which have strong binding capacity for heavy metal ions [20], can efficiently remove various heavy metal ions, making them suitable for various heavy metal wastewater treatments [4,5,11–13,18,20]. However, the performance of these chelating agents and the stability of the resulting chelates, especially polymer chelating agents with heavy metal ions, are rarely reported [21,22]. The lack of a theoretical basis for analyzing and comparing differences in the performance of various chelating agents toward heavy metal ions limits their application. In a previous study, we successfully synthesized a novel amphoteric chelating polymer flocculant [poly (dimethyldiallylammonium chloride-co-acrylamide-graft-triethylenetetraminedithiocarbamate)] (ACPF) and applied it successfully to the treatment of wastewater containing Ni2+ and Cu2+ (Fig. 1) [4,5]. However, the properties and chelating capacity of ACPF toward heavy metal ions have not been systematically investigated. In this study, the chelating ability of ACPF toward Cu2+, Pb2+, Cd2+, and Ni2+ ions is investigated by ultraviolet (UV) spectrophotometry. The composition and stability constants (b) of the chelates are discussed and their stability at different pH is studied. The results can provide a theoretical basis and guidance for analyzing and comparing differences in the performance, development, application, and improvement of chelating agents.
0.1 mol/L borax solution was prepared by dissolution of 19.0685 g of borax in deionized water and dilution to 500 mL with water. ACPF was prepared according to a method described previously [5]. Deionized water was used in all experiments. Experiment principles Considering the chelation reactions that occur between the ACSS groups of the ACPF chain and divalent heavy metals [4,5], the chelation reaction equation can be expressed as follows (omitting the polymer chain):
M2þ þ n CSS ! MðCSSÞn
nþ2
ð1Þ
b can be expressed as follows [23]:
b¼
½ðMðCSSÞn Þnþ2
ð2Þ
½M2þ ½CSS n
If A is the absorbance of the chelate (M(CSS)n)n+2 and cM2þ is much larger than cCSS , the following equation can be obtained:
Amax ¼ e
cCSS n
ð3Þ
At equilibrium:
½ðMðCSSÞn Þnþ2 ¼
A
ð4Þ
e
½M2þ ¼ cM2þ ½ðMðCSSÞn Þnþ2 ¼ cM2þ
A
ð5Þ
e
½CSS ¼ cCSS n½ðMðCSSÞn Þnþ2 ¼ cCSS n
A
e
ð6Þ
Thus:
b¼
A=e ðcM2þ A=eÞðcCSS nA=eÞn
ð7Þ
where e is the molar absorptivity, cM2þ and cCSS are the heavy metal ion and ligand concentrations, respectively, and n is the complex ratio. e can be obtained by determination of the saturated absorbance (Amax) and then calculation according to Eq. (3). A is the absorbance of the chelate solution at different concentration ratios (cM2þ =cCSS ) at the maximum absorption wavelength [23], and n is obtained from the turning point of the A—cM2þ =cCSS curve. A, n, e, cM2þ , and cCSS may be substituted into Eq. (7) to calculate b.
Experimental Determination of the UV of ACPF and its chelates Materials Analytical grade nickel sulfate (98.5%, Chengdu Xi-ao Huaye Chemicals Ltd., Chengdu, China), copper sulfate (99.5%), lead nitrate (99.0%), cadmium chloride (98.5%), calcium chloride (99.5%), and magnesium chloride (99.5%) (Sinopharm Chemical Reagent Beijing Co. Ltd., Beijing, China) were used as purchased. A
Fig. 1. Structure of ACPF.
Determination of the UV spectrum of ACPF was conducted as follows. Approximately 5 mL of 0.1 mol/L borax buffer solution (pH 9.25) and specific amounts of the ACPF solution were pipetted into a 50 mL volumetric flask, brought to volume with deionized water to prepare an ACPF solution of 20 106 mol/L ACSS, and then held at room temperature for 5 min. The UV spectrum of ACPF was determined using a Lambda 35 UV spectrophotometer (Perkin Elmer Co., USA) with deionized water as the reference solution. Determination of the UV spectra of the ACPF–Cu2+, ACPF–Ni2+, ACPF–Pb2+, and ACPF–Cd2+ chelates was conducted via the same method used to determine the ACPF spectrum except that 20 106 mol/L ACPF–Ni2+, ACPF–Cu2+, ACPF–Pb2+, or ACPF–Cd2+ solutions were used as test samples and an ACPF solution of 20 106 mol/L ACSS was used as the reference. The properties of the spectrophotometer used were as follows: wavelength accuracy: ±0.1 nm; wavelength repeatability: Cd (144 pm) > Cu (117 pm) > Ni
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Table 1 Stability constant of ACPF–Cu2+ chelate. cCSS / Amax (lmol L1) 10
0:1—0:5 cCSS / bi 1012 b Amax 1 1012 (lmol L )
cCu2þ = Ai cCSS
0.1140 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.70
0.0225 1.3761 0.0337 1.3723 0.0447 1.3758 0.0555 1.3801 0.0657 1.3732 0.0751 1.3717 0.0839 1.3699 0.0891 1.3720 0.0938 1.3670 0.1031 5.0565 0.1039 4.0229 0.1110 80.3819
1.37
20
0:1—0:5 cCSS / bi 1012 b Amax 1 1012 (lmol L )
cCu2þ = Ai cCSS
0.2273 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.70
0.0453 0.0679 0.0905 0.1128 0.1349 0.1562 0.1756 0.1910 0.2007 0.2071 0.2118 0.2212
1.3648 1.3782 1.3756 1.3781 1.3792 1.3718 1.3701 1.3797 1.3772 1.5270 1.8681 7.9138
1.38
30
0:1—0:5 1012 bi b 1012
cCu2þ =cCSS Ai
0.3400 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.70
0.0679 1.3672 1.37 0.1018 1.3711 0.1357 1.3785 0.1695 1.3806 0.2030 1.3798 0.2360 1.3757 0.2673 1.3689 0.2934 1.3698 0.2993 1.3668 0.3193 1.7401 0.3227 1.6132 0.3553 13.5675
Table 2 Stability constant of ACPF–Ni2+ chelate. cCSS / (lmol L1)
Amax
cNi2þ = Ai cCSS
10
0.1230 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.70
0.0170 0.0245 0.0311 0.0369 0.0423 0.0467 0.0504 0.0541 0.0575 0.0608 0.0646 0.0751
0:1—0:5 cCSS / bi 1010 b 1 1010 (lmol L )
Amax
3.0032 3.0984 3.0760 3.0523 3.0985 3.0715 3.0054 3.0374 3.0865 3.1717 3.4519 5.0829
0.2273 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.70
3.06
20
cNi2þ = Ai cCSS
(115 pm). However, their corresponding atomic electronegativities (Pauling Scale) are 1.91, 2.00, 1.69, and 1.87, respectively. Therefore, the stability order of the chelates is difficult to explain if only radius and electronegativity are considered. Chelate stability also depends on spatial structure, which is affected by various factors, such as ligand structure, central ion bonding orbital configuration and its spatial orientation, and metal ion radius. The differences in these chelates are mainly embodied in the last three aspects. A chelate is more stable when the tension formed in its spatial structure is low. Cu2+, Ni2+, and Pb2+ form chelates with a planar square structure in a dsp2 orbital configuration (Fig. 5a). Yang et al. [27] demonstrated that ACSS is a rigid bidenate ligand with a small pitch, and the distance SAS in the four-member ring is 281 pm. Moreover, the skeleton of each metal chelate is not evidently deformed. Studies have shown that the bond length of CAS with a partial double bond is approximately 172 pm [28–32]. Considering these data, we can calculate that the bond angle of SACAS is 109.55°. We can also consider the MAS bond length in the chelates of dithiocarbamates with Cu2+, Ni2+, and Pb2+ (mean bond length: CuAS 230 pm [28,29]; NiAS 244 pm [30]; PbAS 271 pm [32,33]) to determine the bond angles of SAMAS: 75.31°; 70.31°; and 62.45°, respectively. These angles deviate from the theoretical angle between adjacent dsp2 orbitals of 90°. The degree of deviation increases in the same trend as the above order; this result indicated that the strains of the metal chelate rings similarly increase. Therefore, ACPF–Cu2+ is the most stable among the three chelates. ACPF–Cu2+ is more stable than ACPF–Ni2+; this finding can also be attributed to the sum of the first and the second ionization potentials, which are 28.06 and 25.80 eV for Cu and Ni, respectively. A larger sum of these ionization potentials corresponds to a more stable chelate. This phenomenon fits Irving–Williams sequence [34]. ACPF–Cu2+ is also more stable than ACPF– Pb2+ because of the strains of the metal chelate rings; this result is
0.0439 0.0639 0.0828 0.1000 0.1147 0.1269 0.1376 0.1468 0.1548 0.1595 0.1683 0.1872
0:1—0:5 cCSS / bi 1010 b 1 1010 (lmol L )
Amax
3.0489 2.9836 3.0061 3.0775 3.0517 2.9805 2.9854 3.0182 3.0797 2.8956 3.3142 5.1930
0.3690 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.70
3.03
30
cNi2þ = Ai cCSS 0.0698 0.1033 0.1353 0.1650 0.1915 0.2137 0.2327 0.2483 0.2604 0.2683 0.2916 0.3103
0:1—0:5 bi 1010 b 1010 2.9808 2.9917 3.0471 3.0553 3.0753 3.0057 3.0319 3.0770 3.0759 2.9091 4.8701 6.6040
3.04
also attributed to the higher atomic electronegativity and shorter covalent radius of Cu than those of Pb. Hence, the nucleus of Cu exhibits a strong attraction to bonding electrons, resulting in a more stable ACPF–Cu2+ than ACPF–Pb2+. Based on the strains of metal chelate rings, ACPF–Ni2+ should be more stable than ACPF–Pb2+. However, this result has not been obtained possibly because the radius of Pb2+ is larger than that of Ni2+, resulting in larger deformation and polarization capability. Therefore, the sum of the polarization of Pb2+ and proteiform S is high, producing a stronger bond between Pb2+ and S. By contrast, Cd2+ forms chelates with a tetrahedral structure in sp3 orbital configuration (Fig. 5b), the angle between these orbitals is 109°280 . The bond length of Cd–S is approximately 172 pm calculated using the previously described method, but the distance between Cd and C in ACSS is approximately 200 pm (Fig. 5b (I)), which is less than the sum of the covalent radius (221 pm). Therefore, the angle between these orbitals is compressed. Calculated using CdAS bond length (258 pm [31]), the bond angle of SACdAS is approximately 66.98°; this angle shows a higher deviation from the angle between orbitals (109°280 ). Therefore, tension is relatively large. The molecular chain elicits an inhibitory effect on the formation of chelates; for instance, chelating groups should adjust to a suitable position when the ACSS groups used to form chelates are found on adjacent positions; as a result, a specific tension is obtained (Fig. 5a (I)). However, the ACSS groups are more difficult to adjust to fit the dsp2 orbital in a planar square structure than those that should be fitted in the sp3 orbital (Fig. 5). The spatial orientation of the orbitals is uniform because of the Cd2+ bonding in sp3 orbitals. Therefore, such phenomenon causes ACSS to form a complex with Cd2+ from different directions. For instance, adjusting the chelating groups to a suitable position is not as difficult as forming a planar square structure (Cu2+, Ni2+, and Pb2+) when the ACSS groups used to form chelates are located on adja-
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L. Liu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 765–775 Table 3 Stability constant of ACPF–Pb2+ chelate. cCSS / (lmol L1)
Amax
10
0.1861 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.70
0:1—0:5 cCSS / bi 1011 b 1 1011 (lmol L )
cPb2þ = Ai cCSS
0.0321 3.2270 0.0476 3.2722 0.0624 3.2598 0.0762 3.2477 0.0886 3.2306 0.0994 3.2576 0.1084 3.2956 0.1152 3.2229 0.1210 3.2720 0.358 7.4883 0.1462 15.5146 0.1492 13.6994
3.25
20
Amax
0:1—0:5 cCSS / Bi 1011 b 1 1011 (lmol L )
cPb2þ = Ai cCSS
0.3356 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.70
0.0663 3.2662 0.0992 3.2852 0.1315 3.2416 0.1631 3.2824 0.1931 3.2419 0.2205 3.2497 0.2436 3.2568 0.2612 3.2534 0.2739 3.2834 0.2914 5.4003 0.3124 18.0938 0.3245 51.0089
3.26
30
Amax
cPb2þ = Ai cCSS
0.5040 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.70
0.1003 0.1501 0.1997 0.2487 0.2964 0.3416 0.3814 0.4118 0.4316 0.4456 0.4676 0.4793
0:1—0:5 bi 1011 b 1011 3.2872 3.2953 3.2005 3.2639 3.2345 3.2643 3.2755 3.2865 3.2096 3.3892 7.2606 9.7987
3.26
Table 4 Stability constant of ACPF–Cd2+ chelate. Amax cCSS / (lmol L1) 10
cCd2þ = Ai cCSS
0.0512 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.70
0:1—0:5 1011 cCSS / Amax bi 1011 b (lmol L1)
0.0095 2.0571 0.0141 2.0735 0.0183 2.0450 0.0222 2.0355 0.0256 2.000 0.0286 2.0066 0.0312 2.0812 0.0332 2.0856 0.0348 2.0680 0.0370 2.4949 0.0401 4.0279 0.0453 12.6751
2.05
20
cCd2þ = Ai cCSS
0.1021 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.70
cent positions (Fig. 5b (II)). Therefore, the hindrance is smaller, resulting in a more stable ACPF–Cd2+ than ACPF–Ni2+ (the atomic electronegativities of Cd and Ni are approximately equal). For ACPF–Cd2+ and ACPF–Pb2+, the angle between the orbitals in ACPF–Cd2+ exhibits a high compression degree, although adjusting the chelating groups to a suitable position is easier in ACPF–Cd2+ than in ACPF–Pb2+ (Fig. 5a and b (II)). Moreover, the atomic electronegativity, deformation, and polarization capability of Pb are larger than those of Cd, hence ACPF–Pb2+ is more stable than ACPF–Cd2+. Therefore, the stability order of ACPF–Cu2+, ACPF–Ni2+, ACPF–Pb2+, and ACPF–Cd2+ is presented as the above order. The above results showed that the stability of the chelate can be compared; the selectivity and priority of Cu2+, Pb2+, Cd2+, and Ni2+ in mixed ion solutions can be easily drawn. Effect of Ca2+ or Mg2+ on the heavy metal ion removal Fig. 6 shows the changes in the removal rates of heavy metal ions in solutions with the dosage of Ca2+ or Mg2+. The initial pH values of the simulated heavy metal wastewater samples are as follows: Cu2+, 5.61; Ni2+, 6.73; Cd2+, 6.28; Pb2+, 5.72. The structures of the chelates were characterized by FTIR and are shown in Fig. 7. Fig. 6 shows that the removal rates of heavy metal ions are similar with or without Ca2+ and Mg2+. These results indicate that addition of Ca2+ and Mg2+ has no obvious effect on the chelation and flocculation performance of ACPF. Considering that ACSS is a soft base, Ca2+ and Mg2+, which are hard acids, are not easily coordinated with the ACSS groups of ACPF [35] and cannot compete these groups with heavy metal ions. Fig. 7 shows that the IR spectra of the chelates changed significantly compared with the IR spectrum of ACPF. The main peak assignments have been shown in a previous study [5]. Peaks at approximately 3300–3400 cm1 became weaker and smaller,
0:1—0:5 cCSS / Amax bi 1011 b 1 1011 (lmol L )
0.0200 2.0120 0.0299 2.0781 0.0396 2.0235 0.0489 2.0782 0.0576 2.0711 0.0654 2.0847 0.0718 2.0628 0.0767 2.0395 0.0803 2.0198 0.0851 2.8392 0.0902 5.1795 0.0975 26.6730
2.05
30
cCd2þ = Ai cCSS
0.1536 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.70
0:1—0:5 bi 1011 b 1011
0.0304 2.0721 0.0455 2.0315 0.0605 2.0824 0.0752 2.0843 0.0893 2.0168 0.1025 2.0913 0.1136 2.0425 0.1222 2.0582 0.1280 2.0678 0.1334 2.4526 0.1416 6.2014 0.1476 16.7807
2.06
which may be attributed to the formation of chelates with increasing hydrophobicity, leading to elimination of the OAH group of water absorbed from air during pellet-making. Most of the other peaks weakened, deformed, and shifted, while others disappeared. For example, peaks at approximately 1600 cm1 became weaker and shifted, that at approximately 1310 cm1 disappeared, that at approximately 1100 cm1 blue-shifted, and that at approximately 950 cm1, which are assigned to the characteristic peaks of the ACSS group, became weaker and blue-shifted. These changes indicate the occurrence of chelation reactions. Compared with Fig. 7(a), the largest difference between ACPF and its chelates is the appearance of new peaks at approximately 300–400 cm1 in Fig. 7(b–e). These new peaks can be assigned to the characteristic peaks of ACSS group complexes [27,36]. Fig. 7(b) shows that the peak at 397.23 cm1 can be assigned to the characteristic peak of CuAS of ACPF–Cu2+. Peak at 386.19 cm1 can be assigned to the characteristic peak of NiAS of ACPF–Ni2+ in Fig. 7(c). Peak at 342.36 cm1 can be assigned to the characteristic peak of PbAS of ACPF–Pb2+ in Fig. 7(d). Peak at 369.44 cm1 can be assigned to the characteristic peaks of CdAS of ACPF–Cd2+ in Fig. 7(e). The IR spectra of the chelating precipitates prepared from ACPF with Cu2+, Ni2+, Cd2+, and Pb2+ in the presence of Ca2+ or Mg2+ were also obtained. Interestingly, the spectra are basically similar to those in absence of Ca2+ or Mg2+. These results indicate that Ca2+ or Mg2+ does not participate in the chelation reactions, consistent with the flocculation test results previously obtained.
Dissolution stability of heavy metal ions in the chelates Fig. 8 shows the dissolution of ACPF–Cu2+ (Cu content, 13.87%), ACPF–Ni2+ (Ni content, 13.19%), ACPF–Pb2+ (Pb content, 34.10%), and ACPF–Cd2+ (Cd content, 21.58%) at pH 3.0, 4.0, 5.6, 7.0, and
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Fig. 5. The bonding pattern of a central ion with the ACSS groups of ACPF: (a) dsp2 orbital configuration; (b) sp3 orbital configuration.
Fig. 6. Effect of Ca2+ and Mg2+ on the removal rates of (a) Cu2+, (b) Ni2+, (c) Pb2+, and (d) Cd2+.
9.0. The leaching-out rates of heavy metal ions within 60 d are shown in Table 5. Fig. 8 shows that the dissolution concentrations of heavy metal ions in the four chelates increased with increasing dissolution time at the same pH and with decreasing pH at constant dissolution
time. The dissolution concentrations changed minimally after 30 d. ACPF–Cu2+, ACPF–Ni2+, ACPF–Pb2+, and ACPF–Cd2+ were very stable at pH 5.6, 7, and 9. After leaching for 60 d, the dissolution concentrations of Cu2+, Ni2+, Pb2+, and Cd2+ were 0.051, 0.126, 0.205, and 0.091 mg/L, respectively, at pH 5.6, lower than the limits
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Fig. 7. IR spectra of (a) ACPF, (b) ACPF–Cu2+, (c) ACPF–Ni2+, (d) ACPF–Pb2+, and (e) ACPF–Cd2+.
set by the National Integrated Wastewater Discharge Standards of China (Cu: 0.5 mg/L; Ni: 1.0 mg/L; Pb: 1.0 mg/L; Cd: 0.1 mg/L). After 60 d, the dissolution quantity of the ions was minimal and the dissolution rates were lower than 0.05% in neutral (pH 7) and alkaline (pH 9) conditions (Table 5). The dissolution concentrations of Cu2+, Ni2+, Pb2+, and Cd2+ significantly increased at pH 4.0 and especially increased at pH 3.0. The measured dissolution concentrations of Cu2+, Ni2+, Pb2+, and Cd2+ in the chelates after 60 d were 0.553, 1.152, 2.011, and 0.861 mg/L, respectively, higher than the corresponding discharge standards. However, the dissolution rates of Cu2+, Ni2+, Pb2+, and Cd2+ at 0.40%, 0.87%, 0.59%, and 0.40%, respectively, were not very high. Table 5 shows that the dissolution rates of heavy metal ions in the chelate precipitates decreased with increasing b of the chelate at the same pH and leaching time. The results show that the four chelating precipitates were very stable at pH values greater than 5.6. The dissolution concentrations of the ions were all lower than the corresponding discharge standards after long-term soaking. Dissolved ions easily exceeded standards after long soaking periods in strongly acid environments. The dissolution of metal ions from the chelates neither exceeded
standard limits nor caused secondary pollution because the natural environment is rarely strongly acidic. Proposed chelation model of ACPF with heavy metal ions The results show that the ACSS groups of ACPF chelate with Cu , Ni2+, Pb2+, and Cd2+ to form stable chelates at a ACSS to heavy metal ion molar ratio of 2:1. The ACSS source indicates that the ACSS groups can chelate with heavy metal ions in several binding modes (Fig. 9). For instance, (1) one heavy metal ion can chelate two ACSS groups from the same side chains; (2) one heavy metal ion can chelate two ACSS groups from different side chains of the same main chain; (3) one heavy metal ion can chelate two ACSS groups from different side chains of different main chains. Fig. 9 shows that several ACSS groups do not combine with heavy metal ions because of steric hindrance and spatial mismatches and, instead, form flocs with excess negative charges. The positive charges of the polymer chains can effectively neutralize excess negative charges to promote floc formation and growth. Different ACPF molecular chains can form larger aggregates through chelation of 2+
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Fig. 8. Leaching-out characteristics of (a) ACPF–Cu2+, (b) ACPF–Ni2+, (c) ACPF–Pb2+, and (d) ACPF–Cd2+.
the ACSS groups of ACPF with the same heavy metal ion. Microflocs can also form larger, thicker, and denser flocs through the remaining ACSS groups by chelating with the same heavy metal ion, increasing the settling velocity of the flocs.
Mechanism of stability of the chelating precipitates at different pH Section of Dissolution stability of heavy metal ions in the chelates shows that the four chelating precipitates presents good dissolution stabilities at pH above 5.6. Heavy metal ions dissolving out from the chelating precipitates initially undergo solvation of the chelation moiety, leading to dissolution of the solvation moiety. The affinity of the chelating precipitates to the solvent water is very low because of the chelation moiety formed from the hydrophilic ACSS groups of ACPF and M2+, as well as from other insoluble groups of ACPF molecular chains. Therefore, water molecules cannot easily infiltrate the chelation region for effective solvation. Moreover, dissociation equilibrium also exists in the solvation region, as shown in Eq. (8) (omitting the polymer chain). The degrees of dissociation of the four chelates are very low because they have large b. In addition, the dissociated M2+ must diffuse away from the solvation region into the main body of the liquid phase to move the dissociation equilibrium toward the right side of Eq. (8), thus increasing the dissolution of M2+ from the chelating precipitates. However, the periphery of the solvation region is often hydrophobic, which causes difficulties in dissociating M2+ into the main body of the liquid phase by diffusion.
Table 5 Leaching-out rate of heavy metal ions over 60 days. pH
Dissolution rate/%
3.0 4.0 5.6 7.0 9.0
ACPF–Cu2+
ACPF–Ni2+
ACPF–Pb2+
ACPF–Cd2+
0.40 0.22 0.037 0.0037 0.0030
0.87 0.47 0.095 0.046 0.041
0.59 0.43 0.060 0.023 0.019
0.40 0.23 0.051 0.023 0.018
Therefore, minimal heavy metal ion elution occurs and the chelating precipitates exhibit good stability. The increase in dissolution rate under low pH is attributed to H+ attacking the ACSS group to form ACSSH with a low dissociation degree [37], which induces the dissociation equilibrium of Eq. (8) to move toward the right. However, the dissolution rate remains low because the solvation and diffusion of dissociated ions do not easily occur. S
S M
C S
S C
C S H+
S
S C SH
+
M2+
ð8Þ
L. Liu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 765–775
775
Fig. 9. Chelating mechanism of ACPF with heavy metal ions.
Conclusions (1) ACPF has a strong chelating ability toward Cu2+, Pb2+, Cd2+, and Ni2+. The ACSS groups of ACPF combine with the four heavy metal ions to form stable chelates according to a molar ratio of 2:1. b values of ACPF–Cu2+, ACPF–Pb2+, ACPF–Cd2+, and ACPF–Ni2+ are (1.37 ± 0.35) 1012, (3.26 ± 0.39) 1011, (2.05 ± 0.27) 1011, and (3.04 ± 0.45) 1010, respectively, in 0.1 mol/L borax buffer solution. Ca2+ and Mg2+ do not participate in the chelation reactions of ACPF with Cu2+, Pb2+, Cd2+, and Ni2+. (2) Two ACSS groups chelating with the same heavy metal ion can originate from the same side chain and different side chains of similar or different main chains. The positive charges of the polymer chains can effectively neutralize excess negative charges. The ACSS groups of different molecular chains or microflocs can chelate with the same heavy metal ion to cause aggregation of molecular chains or microflocs, promoting floc formation and growth. (3) ACPF–Cu2+, ACPF–Ni2+, ACPF–Pb2+, and ACPF–Cd2+ are very stable at pH above 5.6. Cu2+, Ni2+, Pb2+, and Cd2+ concentrations in the leaching liquids are lower than the corresponding limits specified by the Integrated Wastewater Discharge Standard of China. The dissolution rates of heavy metal ions significantly increase but are not high at pH 3.0.
Acknowledgements This research was supported by the National Nature Science Foundation of China (Grant No. 51078141) and Hunan Province Science and Technology Research Program (Grant No. 2010FJ3030). References [1] M.M. Matlock, K.R. Henke, D.A. Atwood, J. Hazard. Mater. B92 (2002) 129–142. [2] I. Giannopoulou, D. Panias, Hydrometallurgy 90 (2008) 137–146. [3] T.G. Timoshenko, G.N. Pshinko, B.Y. Kornilovich, V.A. Bagrii, A.L. Makovetskii, J. Water Chem. Technol. 29 (5) (2007) 246–253. [4] L.H. Liu, J. Wu, X. Li, Y.L. Ling, Chin. J. Environ. Chem. 30 (4) (2011) 843–850.
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Chelating stability of an amphoteric chelating polymer flocculant with Cu(II), Pb(II), Cd(II), and Ni(II) Lihua Liu ⇑, Yanhong Li, Xing Liu, Zhihua Zhou, Yulin Ling Key Laboratory of Theoretical Chemistry and Molecular Simulation of Ministry of Education and Hunan Province, Hunan Province College Key Laboratory of QSAR/QSPR, School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan, Hunan 411201, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Determination of the ultraviolet
spectra of ACPF and its chelates with heavy metals. Determination of the compositions and stability constant of the chelates. The chelating precipitates are very stable at pH above 5.6. A chelating mechanism of ACPF with heavy metal ion was proposed. The stability mechanism of the chelating precipitates was explored at different pH.
a r t i c l e
i n f o
Article history: Received 5 November 2012 Received in revised form 8 September 2013 Accepted 25 September 2013 Available online 4 October 2013 Keywords: Stability constant Amphoteric chelating polymer flocculant Heavy metal ions Ultraviolet spectrophotometric method Leaching-out characteristics
a b s t r a c t The absorption spectra of Cu2+, Pb2+, Cd2+, and Ni2+ chelates of an amphoteric chelating polymer flocculant (ACPF) were measured by ultraviolet spectrophotometry, and their compositions and stability constants (b) were calculated. ACPF exhibited three apparent absorption peaks at 204, 251, and 285 nm. The ACSS group of ACPF reacted with Cu2+, Ni2+, Pb2+, and Cd2+ to form ACPF–Cu2+, ACPF–Ni2+, ACPF–Pb2+, and ACPF–Cd2+ chelates, respectively, according to a molar ratio of 2:1. The maximum absorption peaks of ACPF–Cu2+, ACPF–Ni2+, ACPF–Pb2+, and ACPF–Cd2+ appeared at 319, 326, 310, and 313.5 nm, respectively. The maximum absorption peaks of the chelates showed significant red shifting compared with the absorption peaks of ACPF. The b values of the ACPF–Cu2+, ACPF–Pb2+, ACPF–Cd2+, and ACPF–Ni2+ chelates were (1.37 ± 0.35) 1012, (3.26 ± 0.39) 1011, (2.05 ± 0.27) 1011, and (3.04 ± 0.45) 1010, respectively. The leaching rate of heavy metal ions from the chelating precipitates decreased with increasing pH. ACPF–Cu2+, ACPF–Ni2+, ACPF–Pb2+, and ACPF–Cd2+ were very stable at pH P 5.6. Cu2+, Ni2+, Pb2+, and Cd2+ concentrations in the leaching liquors were lower than the corresponding limits specified by the Integrated Wastewater Discharge Standard of China. Ó 2013 Elsevier B.V. All rights reserved.
Introduction The dangers posed by toxic heavy metals to the environment are an increasing global concern [1]. Heavy metal wastewater damages the ecological environment and seriously threatens human health. Thus, the exploration and development of effective ⇑ Corresponding author. Tel.: +86 731 58291625. E-mail addresses: [email protected], [email protected] (L. Liu), [email protected] (Y. Li), [email protected] (X. Liu), [email protected] (Z. Zhou), [email protected] (Y. Ling). 1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.09.066
removal methods and separation technologies for toxic heavy metal ions in wastewater has attracted significant research attention. Many heavy metal wastewater treatment methods, including chemical precipitation [2], ferrite method [3], chelation–precipitation method [4,5], ion exchange [6], adsorption [7], membrane separation [8], electrochemical method [9], and biological flocculation [10], have been developed. Among these methods, chelation–precipitation offers excellent results, lower costs, and better suitability for large-scale heavy metal wastewater treatment [4,5]. The treatment effects of chelation depend on the performance of the chelating flocculant. Chelating agents that have been recently developed
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and applied can be classified as either small molecule chelating agents or polymer chelating agents. Small molecule chelating agents mainly include aminocarboxyl chelating agents and dithiocarbamates (DTCs), such as ethylenediamine-tetraactic acid, sodium diethyl dithiocarbamate [11], disodium N,N-bis-(dithiocarboxy) piperazine [12], and sodium 1,3,5-hexahydrotriazinedithiocarbamate [13]. Polymer chelating agents can be further divided into water-soluble polymer chelating agents and water-insoluble chelating resins. Chelating resins are usually obtained by introducing chelating groups to cross-linked resins, such as by anchoring the chelating agent N,N0 -di(carboxymethyl) dithiocarbamate to chloromethylated PS-DVB [14] or by introducing iminodiacetic acid into poly(styrene-co-GMA) [15]. These resins are mainly used for adsorption and ion exchange. Watersoluble polymer chelating agents mainly originate from the introduction of hydroxyl, carboxyl, iminodiacetic acid, xanthan, or DTC groups to the chelating agent. Water-soluble polymer chelating agents include N-methyl hydroxamic acid [16], phosphonic acyl methyl [17], DTC groups in polyethyleneimine chains [18], and DTC groups in polyacrylamide chains [19]. Chelating agents containing DTC groups, which have strong binding capacity for heavy metal ions [20], can efficiently remove various heavy metal ions, making them suitable for various heavy metal wastewater treatments [4,5,11–13,18,20]. However, the performance of these chelating agents and the stability of the resulting chelates, especially polymer chelating agents with heavy metal ions, are rarely reported [21,22]. The lack of a theoretical basis for analyzing and comparing differences in the performance of various chelating agents toward heavy metal ions limits their application. In a previous study, we successfully synthesized a novel amphoteric chelating polymer flocculant [poly (dimethyldiallylammonium chloride-co-acrylamide-graft-triethylenetetraminedithiocarbamate)] (ACPF) and applied it successfully to the treatment of wastewater containing Ni2+ and Cu2+ (Fig. 1) [4,5]. However, the properties and chelating capacity of ACPF toward heavy metal ions have not been systematically investigated. In this study, the chelating ability of ACPF toward Cu2+, Pb2+, Cd2+, and Ni2+ ions is investigated by ultraviolet (UV) spectrophotometry. The composition and stability constants (b) of the chelates are discussed and their stability at different pH is studied. The results can provide a theoretical basis and guidance for analyzing and comparing differences in the performance, development, application, and improvement of chelating agents.
0.1 mol/L borax solution was prepared by dissolution of 19.0685 g of borax in deionized water and dilution to 500 mL with water. ACPF was prepared according to a method described previously [5]. Deionized water was used in all experiments. Experiment principles Considering the chelation reactions that occur between the ACSS groups of the ACPF chain and divalent heavy metals [4,5], the chelation reaction equation can be expressed as follows (omitting the polymer chain):
M2þ þ n CSS ! MðCSSÞn
nþ2
ð1Þ
b can be expressed as follows [23]:
b¼
½ðMðCSSÞn Þnþ2
ð2Þ
½M2þ ½CSS n
If A is the absorbance of the chelate (M(CSS)n)n+2 and cM2þ is much larger than cCSS , the following equation can be obtained:
Amax ¼ e
cCSS n
ð3Þ
At equilibrium:
½ðMðCSSÞn Þnþ2 ¼
A
ð4Þ
e
½M2þ ¼ cM2þ ½ðMðCSSÞn Þnþ2 ¼ cM2þ
A
ð5Þ
e
½CSS ¼ cCSS n½ðMðCSSÞn Þnþ2 ¼ cCSS n
A
e
ð6Þ
Thus:
b¼
A=e ðcM2þ A=eÞðcCSS nA=eÞn
ð7Þ
where e is the molar absorptivity, cM2þ and cCSS are the heavy metal ion and ligand concentrations, respectively, and n is the complex ratio. e can be obtained by determination of the saturated absorbance (Amax) and then calculation according to Eq. (3). A is the absorbance of the chelate solution at different concentration ratios (cM2þ =cCSS ) at the maximum absorption wavelength [23], and n is obtained from the turning point of the A—cM2þ =cCSS curve. A, n, e, cM2þ , and cCSS may be substituted into Eq. (7) to calculate b.
Experimental Determination of the UV of ACPF and its chelates Materials Analytical grade nickel sulfate (98.5%, Chengdu Xi-ao Huaye Chemicals Ltd., Chengdu, China), copper sulfate (99.5%), lead nitrate (99.0%), cadmium chloride (98.5%), calcium chloride (99.5%), and magnesium chloride (99.5%) (Sinopharm Chemical Reagent Beijing Co. Ltd., Beijing, China) were used as purchased. A
Fig. 1. Structure of ACPF.
Determination of the UV spectrum of ACPF was conducted as follows. Approximately 5 mL of 0.1 mol/L borax buffer solution (pH 9.25) and specific amounts of the ACPF solution were pipetted into a 50 mL volumetric flask, brought to volume with deionized water to prepare an ACPF solution of 20 106 mol/L ACSS, and then held at room temperature for 5 min. The UV spectrum of ACPF was determined using a Lambda 35 UV spectrophotometer (Perkin Elmer Co., USA) with deionized water as the reference solution. Determination of the UV spectra of the ACPF–Cu2+, ACPF–Ni2+, ACPF–Pb2+, and ACPF–Cd2+ chelates was conducted via the same method used to determine the ACPF spectrum except that 20 106 mol/L ACPF–Ni2+, ACPF–Cu2+, ACPF–Pb2+, or ACPF–Cd2+ solutions were used as test samples and an ACPF solution of 20 106 mol/L ACSS was used as the reference. The properties of the spectrophotometer used were as follows: wavelength accuracy: ±0.1 nm; wavelength repeatability: Cd (144 pm) > Cu (117 pm) > Ni
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Table 1 Stability constant of ACPF–Cu2+ chelate. cCSS / Amax (lmol L1) 10
0:1—0:5 cCSS / bi 1012 b Amax 1 1012 (lmol L )
cCu2þ = Ai cCSS
0.1140 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.70
0.0225 1.3761 0.0337 1.3723 0.0447 1.3758 0.0555 1.3801 0.0657 1.3732 0.0751 1.3717 0.0839 1.3699 0.0891 1.3720 0.0938 1.3670 0.1031 5.0565 0.1039 4.0229 0.1110 80.3819
1.37
20
0:1—0:5 cCSS / bi 1012 b Amax 1 1012 (lmol L )
cCu2þ = Ai cCSS
0.2273 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.70
0.0453 0.0679 0.0905 0.1128 0.1349 0.1562 0.1756 0.1910 0.2007 0.2071 0.2118 0.2212
1.3648 1.3782 1.3756 1.3781 1.3792 1.3718 1.3701 1.3797 1.3772 1.5270 1.8681 7.9138
1.38
30
0:1—0:5 1012 bi b 1012
cCu2þ =cCSS Ai
0.3400 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.70
0.0679 1.3672 1.37 0.1018 1.3711 0.1357 1.3785 0.1695 1.3806 0.2030 1.3798 0.2360 1.3757 0.2673 1.3689 0.2934 1.3698 0.2993 1.3668 0.3193 1.7401 0.3227 1.6132 0.3553 13.5675
Table 2 Stability constant of ACPF–Ni2+ chelate. cCSS / (lmol L1)
Amax
cNi2þ = Ai cCSS
10
0.1230 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.70
0.0170 0.0245 0.0311 0.0369 0.0423 0.0467 0.0504 0.0541 0.0575 0.0608 0.0646 0.0751
0:1—0:5 cCSS / bi 1010 b 1 1010 (lmol L )
Amax
3.0032 3.0984 3.0760 3.0523 3.0985 3.0715 3.0054 3.0374 3.0865 3.1717 3.4519 5.0829
0.2273 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.70
3.06
20
cNi2þ = Ai cCSS
(115 pm). However, their corresponding atomic electronegativities (Pauling Scale) are 1.91, 2.00, 1.69, and 1.87, respectively. Therefore, the stability order of the chelates is difficult to explain if only radius and electronegativity are considered. Chelate stability also depends on spatial structure, which is affected by various factors, such as ligand structure, central ion bonding orbital configuration and its spatial orientation, and metal ion radius. The differences in these chelates are mainly embodied in the last three aspects. A chelate is more stable when the tension formed in its spatial structure is low. Cu2+, Ni2+, and Pb2+ form chelates with a planar square structure in a dsp2 orbital configuration (Fig. 5a). Yang et al. [27] demonstrated that ACSS is a rigid bidenate ligand with a small pitch, and the distance SAS in the four-member ring is 281 pm. Moreover, the skeleton of each metal chelate is not evidently deformed. Studies have shown that the bond length of CAS with a partial double bond is approximately 172 pm [28–32]. Considering these data, we can calculate that the bond angle of SACAS is 109.55°. We can also consider the MAS bond length in the chelates of dithiocarbamates with Cu2+, Ni2+, and Pb2+ (mean bond length: CuAS 230 pm [28,29]; NiAS 244 pm [30]; PbAS 271 pm [32,33]) to determine the bond angles of SAMAS: 75.31°; 70.31°; and 62.45°, respectively. These angles deviate from the theoretical angle between adjacent dsp2 orbitals of 90°. The degree of deviation increases in the same trend as the above order; this result indicated that the strains of the metal chelate rings similarly increase. Therefore, ACPF–Cu2+ is the most stable among the three chelates. ACPF–Cu2+ is more stable than ACPF–Ni2+; this finding can also be attributed to the sum of the first and the second ionization potentials, which are 28.06 and 25.80 eV for Cu and Ni, respectively. A larger sum of these ionization potentials corresponds to a more stable chelate. This phenomenon fits Irving–Williams sequence [34]. ACPF–Cu2+ is also more stable than ACPF– Pb2+ because of the strains of the metal chelate rings; this result is
0.0439 0.0639 0.0828 0.1000 0.1147 0.1269 0.1376 0.1468 0.1548 0.1595 0.1683 0.1872
0:1—0:5 cCSS / bi 1010 b 1 1010 (lmol L )
Amax
3.0489 2.9836 3.0061 3.0775 3.0517 2.9805 2.9854 3.0182 3.0797 2.8956 3.3142 5.1930
0.3690 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.70
3.03
30
cNi2þ = Ai cCSS 0.0698 0.1033 0.1353 0.1650 0.1915 0.2137 0.2327 0.2483 0.2604 0.2683 0.2916 0.3103
0:1—0:5 bi 1010 b 1010 2.9808 2.9917 3.0471 3.0553 3.0753 3.0057 3.0319 3.0770 3.0759 2.9091 4.8701 6.6040
3.04
also attributed to the higher atomic electronegativity and shorter covalent radius of Cu than those of Pb. Hence, the nucleus of Cu exhibits a strong attraction to bonding electrons, resulting in a more stable ACPF–Cu2+ than ACPF–Pb2+. Based on the strains of metal chelate rings, ACPF–Ni2+ should be more stable than ACPF–Pb2+. However, this result has not been obtained possibly because the radius of Pb2+ is larger than that of Ni2+, resulting in larger deformation and polarization capability. Therefore, the sum of the polarization of Pb2+ and proteiform S is high, producing a stronger bond between Pb2+ and S. By contrast, Cd2+ forms chelates with a tetrahedral structure in sp3 orbital configuration (Fig. 5b), the angle between these orbitals is 109°280 . The bond length of Cd–S is approximately 172 pm calculated using the previously described method, but the distance between Cd and C in ACSS is approximately 200 pm (Fig. 5b (I)), which is less than the sum of the covalent radius (221 pm). Therefore, the angle between these orbitals is compressed. Calculated using CdAS bond length (258 pm [31]), the bond angle of SACdAS is approximately 66.98°; this angle shows a higher deviation from the angle between orbitals (109°280 ). Therefore, tension is relatively large. The molecular chain elicits an inhibitory effect on the formation of chelates; for instance, chelating groups should adjust to a suitable position when the ACSS groups used to form chelates are found on adjacent positions; as a result, a specific tension is obtained (Fig. 5a (I)). However, the ACSS groups are more difficult to adjust to fit the dsp2 orbital in a planar square structure than those that should be fitted in the sp3 orbital (Fig. 5). The spatial orientation of the orbitals is uniform because of the Cd2+ bonding in sp3 orbitals. Therefore, such phenomenon causes ACSS to form a complex with Cd2+ from different directions. For instance, adjusting the chelating groups to a suitable position is not as difficult as forming a planar square structure (Cu2+, Ni2+, and Pb2+) when the ACSS groups used to form chelates are located on adja-
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L. Liu et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 118 (2014) 765–775 Table 3 Stability constant of ACPF–Pb2+ chelate. cCSS / (lmol L1)
Amax
10
0.1861 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.70
0:1—0:5 cCSS / bi 1011 b 1 1011 (lmol L )
cPb2þ = Ai cCSS
0.0321 3.2270 0.0476 3.2722 0.0624 3.2598 0.0762 3.2477 0.0886 3.2306 0.0994 3.2576 0.1084 3.2956 0.1152 3.2229 0.1210 3.2720 0.358 7.4883 0.1462 15.5146 0.1492 13.6994
3.25
20
Amax
0:1—0:5 cCSS / Bi 1011 b 1 1011 (lmol L )
cPb2þ = Ai cCSS
0.3356 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.70
0.0663 3.2662 0.0992 3.2852 0.1315 3.2416 0.1631 3.2824 0.1931 3.2419 0.2205 3.2497 0.2436 3.2568 0.2612 3.2534 0.2739 3.2834 0.2914 5.4003 0.3124 18.0938 0.3245 51.0089
3.26
30
Amax
cPb2þ = Ai cCSS
0.5040 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.70
0.1003 0.1501 0.1997 0.2487 0.2964 0.3416 0.3814 0.4118 0.4316 0.4456 0.4676 0.4793
0:1—0:5 bi 1011 b 1011 3.2872 3.2953 3.2005 3.2639 3.2345 3.2643 3.2755 3.2865 3.2096 3.3892 7.2606 9.7987
3.26
Table 4 Stability constant of ACPF–Cd2+ chelate. Amax cCSS / (lmol L1) 10
cCd2þ = Ai cCSS
0.0512 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.70
0:1—0:5 1011 cCSS / Amax bi 1011 b (lmol L1)
0.0095 2.0571 0.0141 2.0735 0.0183 2.0450 0.0222 2.0355 0.0256 2.000 0.0286 2.0066 0.0312 2.0812 0.0332 2.0856 0.0348 2.0680 0.0370 2.4949 0.0401 4.0279 0.0453 12.6751
2.05
20
cCd2þ = Ai cCSS
0.1021 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.70
cent positions (Fig. 5b (II)). Therefore, the hindrance is smaller, resulting in a more stable ACPF–Cd2+ than ACPF–Ni2+ (the atomic electronegativities of Cd and Ni are approximately equal). For ACPF–Cd2+ and ACPF–Pb2+, the angle between the orbitals in ACPF–Cd2+ exhibits a high compression degree, although adjusting the chelating groups to a suitable position is easier in ACPF–Cd2+ than in ACPF–Pb2+ (Fig. 5a and b (II)). Moreover, the atomic electronegativity, deformation, and polarization capability of Pb are larger than those of Cd, hence ACPF–Pb2+ is more stable than ACPF–Cd2+. Therefore, the stability order of ACPF–Cu2+, ACPF–Ni2+, ACPF–Pb2+, and ACPF–Cd2+ is presented as the above order. The above results showed that the stability of the chelate can be compared; the selectivity and priority of Cu2+, Pb2+, Cd2+, and Ni2+ in mixed ion solutions can be easily drawn. Effect of Ca2+ or Mg2+ on the heavy metal ion removal Fig. 6 shows the changes in the removal rates of heavy metal ions in solutions with the dosage of Ca2+ or Mg2+. The initial pH values of the simulated heavy metal wastewater samples are as follows: Cu2+, 5.61; Ni2+, 6.73; Cd2+, 6.28; Pb2+, 5.72. The structures of the chelates were characterized by FTIR and are shown in Fig. 7. Fig. 6 shows that the removal rates of heavy metal ions are similar with or without Ca2+ and Mg2+. These results indicate that addition of Ca2+ and Mg2+ has no obvious effect on the chelation and flocculation performance of ACPF. Considering that ACSS is a soft base, Ca2+ and Mg2+, which are hard acids, are not easily coordinated with the ACSS groups of ACPF [35] and cannot compete these groups with heavy metal ions. Fig. 7 shows that the IR spectra of the chelates changed significantly compared with the IR spectrum of ACPF. The main peak assignments have been shown in a previous study [5]. Peaks at approximately 3300–3400 cm1 became weaker and smaller,
0:1—0:5 cCSS / Amax bi 1011 b 1 1011 (lmol L )
0.0200 2.0120 0.0299 2.0781 0.0396 2.0235 0.0489 2.0782 0.0576 2.0711 0.0654 2.0847 0.0718 2.0628 0.0767 2.0395 0.0803 2.0198 0.0851 2.8392 0.0902 5.1795 0.0975 26.6730
2.05
30
cCd2þ = Ai cCSS
0.1536 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.70
0:1—0:5 bi 1011 b 1011
0.0304 2.0721 0.0455 2.0315 0.0605 2.0824 0.0752 2.0843 0.0893 2.0168 0.1025 2.0913 0.1136 2.0425 0.1222 2.0582 0.1280 2.0678 0.1334 2.4526 0.1416 6.2014 0.1476 16.7807
2.06
which may be attributed to the formation of chelates with increasing hydrophobicity, leading to elimination of the OAH group of water absorbed from air during pellet-making. Most of the other peaks weakened, deformed, and shifted, while others disappeared. For example, peaks at approximately 1600 cm1 became weaker and shifted, that at approximately 1310 cm1 disappeared, that at approximately 1100 cm1 blue-shifted, and that at approximately 950 cm1, which are assigned to the characteristic peaks of the ACSS group, became weaker and blue-shifted. These changes indicate the occurrence of chelation reactions. Compared with Fig. 7(a), the largest difference between ACPF and its chelates is the appearance of new peaks at approximately 300–400 cm1 in Fig. 7(b–e). These new peaks can be assigned to the characteristic peaks of ACSS group complexes [27,36]. Fig. 7(b) shows that the peak at 397.23 cm1 can be assigned to the characteristic peak of CuAS of ACPF–Cu2+. Peak at 386.19 cm1 can be assigned to the characteristic peak of NiAS of ACPF–Ni2+ in Fig. 7(c). Peak at 342.36 cm1 can be assigned to the characteristic peak of PbAS of ACPF–Pb2+ in Fig. 7(d). Peak at 369.44 cm1 can be assigned to the characteristic peaks of CdAS of ACPF–Cd2+ in Fig. 7(e). The IR spectra of the chelating precipitates prepared from ACPF with Cu2+, Ni2+, Cd2+, and Pb2+ in the presence of Ca2+ or Mg2+ were also obtained. Interestingly, the spectra are basically similar to those in absence of Ca2+ or Mg2+. These results indicate that Ca2+ or Mg2+ does not participate in the chelation reactions, consistent with the flocculation test results previously obtained.
Dissolution stability of heavy metal ions in the chelates Fig. 8 shows the dissolution of ACPF–Cu2+ (Cu content, 13.87%), ACPF–Ni2+ (Ni content, 13.19%), ACPF–Pb2+ (Pb content, 34.10%), and ACPF–Cd2+ (Cd content, 21.58%) at pH 3.0, 4.0, 5.6, 7.0, and
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Fig. 5. The bonding pattern of a central ion with the ACSS groups of ACPF: (a) dsp2 orbital configuration; (b) sp3 orbital configuration.
Fig. 6. Effect of Ca2+ and Mg2+ on the removal rates of (a) Cu2+, (b) Ni2+, (c) Pb2+, and (d) Cd2+.
9.0. The leaching-out rates of heavy metal ions within 60 d are shown in Table 5. Fig. 8 shows that the dissolution concentrations of heavy metal ions in the four chelates increased with increasing dissolution time at the same pH and with decreasing pH at constant dissolution
time. The dissolution concentrations changed minimally after 30 d. ACPF–Cu2+, ACPF–Ni2+, ACPF–Pb2+, and ACPF–Cd2+ were very stable at pH 5.6, 7, and 9. After leaching for 60 d, the dissolution concentrations of Cu2+, Ni2+, Pb2+, and Cd2+ were 0.051, 0.126, 0.205, and 0.091 mg/L, respectively, at pH 5.6, lower than the limits
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773
Fig. 7. IR spectra of (a) ACPF, (b) ACPF–Cu2+, (c) ACPF–Ni2+, (d) ACPF–Pb2+, and (e) ACPF–Cd2+.
set by the National Integrated Wastewater Discharge Standards of China (Cu: 0.5 mg/L; Ni: 1.0 mg/L; Pb: 1.0 mg/L; Cd: 0.1 mg/L). After 60 d, the dissolution quantity of the ions was minimal and the dissolution rates were lower than 0.05% in neutral (pH 7) and alkaline (pH 9) conditions (Table 5). The dissolution concentrations of Cu2+, Ni2+, Pb2+, and Cd2+ significantly increased at pH 4.0 and especially increased at pH 3.0. The measured dissolution concentrations of Cu2+, Ni2+, Pb2+, and Cd2+ in the chelates after 60 d were 0.553, 1.152, 2.011, and 0.861 mg/L, respectively, higher than the corresponding discharge standards. However, the dissolution rates of Cu2+, Ni2+, Pb2+, and Cd2+ at 0.40%, 0.87%, 0.59%, and 0.40%, respectively, were not very high. Table 5 shows that the dissolution rates of heavy metal ions in the chelate precipitates decreased with increasing b of the chelate at the same pH and leaching time. The results show that the four chelating precipitates were very stable at pH values greater than 5.6. The dissolution concentrations of the ions were all lower than the corresponding discharge standards after long-term soaking. Dissolved ions easily exceeded standards after long soaking periods in strongly acid environments. The dissolution of metal ions from the chelates neither exceeded
standard limits nor caused secondary pollution because the natural environment is rarely strongly acidic. Proposed chelation model of ACPF with heavy metal ions The results show that the ACSS groups of ACPF chelate with Cu , Ni2+, Pb2+, and Cd2+ to form stable chelates at a ACSS to heavy metal ion molar ratio of 2:1. The ACSS source indicates that the ACSS groups can chelate with heavy metal ions in several binding modes (Fig. 9). For instance, (1) one heavy metal ion can chelate two ACSS groups from the same side chains; (2) one heavy metal ion can chelate two ACSS groups from different side chains of the same main chain; (3) one heavy metal ion can chelate two ACSS groups from different side chains of different main chains. Fig. 9 shows that several ACSS groups do not combine with heavy metal ions because of steric hindrance and spatial mismatches and, instead, form flocs with excess negative charges. The positive charges of the polymer chains can effectively neutralize excess negative charges to promote floc formation and growth. Different ACPF molecular chains can form larger aggregates through chelation of 2+
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Fig. 8. Leaching-out characteristics of (a) ACPF–Cu2+, (b) ACPF–Ni2+, (c) ACPF–Pb2+, and (d) ACPF–Cd2+.
the ACSS groups of ACPF with the same heavy metal ion. Microflocs can also form larger, thicker, and denser flocs through the remaining ACSS groups by chelating with the same heavy metal ion, increasing the settling velocity of the flocs.
Mechanism of stability of the chelating precipitates at different pH Section of Dissolution stability of heavy metal ions in the chelates shows that the four chelating precipitates presents good dissolution stabilities at pH above 5.6. Heavy metal ions dissolving out from the chelating precipitates initially undergo solvation of the chelation moiety, leading to dissolution of the solvation moiety. The affinity of the chelating precipitates to the solvent water is very low because of the chelation moiety formed from the hydrophilic ACSS groups of ACPF and M2+, as well as from other insoluble groups of ACPF molecular chains. Therefore, water molecules cannot easily infiltrate the chelation region for effective solvation. Moreover, dissociation equilibrium also exists in the solvation region, as shown in Eq. (8) (omitting the polymer chain). The degrees of dissociation of the four chelates are very low because they have large b. In addition, the dissociated M2+ must diffuse away from the solvation region into the main body of the liquid phase to move the dissociation equilibrium toward the right side of Eq. (8), thus increasing the dissolution of M2+ from the chelating precipitates. However, the periphery of the solvation region is often hydrophobic, which causes difficulties in dissociating M2+ into the main body of the liquid phase by diffusion.
Table 5 Leaching-out rate of heavy metal ions over 60 days. pH
Dissolution rate/%
3.0 4.0 5.6 7.0 9.0
ACPF–Cu2+
ACPF–Ni2+
ACPF–Pb2+
ACPF–Cd2+
0.40 0.22 0.037 0.0037 0.0030
0.87 0.47 0.095 0.046 0.041
0.59 0.43 0.060 0.023 0.019
0.40 0.23 0.051 0.023 0.018
Therefore, minimal heavy metal ion elution occurs and the chelating precipitates exhibit good stability. The increase in dissolution rate under low pH is attributed to H+ attacking the ACSS group to form ACSSH with a low dissociation degree [37], which induces the dissociation equilibrium of Eq. (8) to move toward the right. However, the dissolution rate remains low because the solvation and diffusion of dissociated ions do not easily occur. S
S M
C S
S C
C S H+
S
S C SH
+
M2+
ð8Þ
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775
Fig. 9. Chelating mechanism of ACPF with heavy metal ions.
Conclusions (1) ACPF has a strong chelating ability toward Cu2+, Pb2+, Cd2+, and Ni2+. The ACSS groups of ACPF combine with the four heavy metal ions to form stable chelates according to a molar ratio of 2:1. b values of ACPF–Cu2+, ACPF–Pb2+, ACPF–Cd2+, and ACPF–Ni2+ are (1.37 ± 0.35) 1012, (3.26 ± 0.39) 1011, (2.05 ± 0.27) 1011, and (3.04 ± 0.45) 1010, respectively, in 0.1 mol/L borax buffer solution. Ca2+ and Mg2+ do not participate in the chelation reactions of ACPF with Cu2+, Pb2+, Cd2+, and Ni2+. (2) Two ACSS groups chelating with the same heavy metal ion can originate from the same side chain and different side chains of similar or different main chains. The positive charges of the polymer chains can effectively neutralize excess negative charges. The ACSS groups of different molecular chains or microflocs can chelate with the same heavy metal ion to cause aggregation of molecular chains or microflocs, promoting floc formation and growth. (3) ACPF–Cu2+, ACPF–Ni2+, ACPF–Pb2+, and ACPF–Cd2+ are very stable at pH above 5.6. Cu2+, Ni2+, Pb2+, and Cd2+ concentrations in the leaching liquids are lower than the corresponding limits specified by the Integrated Wastewater Discharge Standard of China. The dissolution rates of heavy metal ions significantly increase but are not high at pH 3.0.
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