Design of porous strong base anion exchangers bearing N,N-dialkyl 2-hydroxyethyl ammonium groups with enhanced retention of Cr(VI) ions from aqueous solution
Introduction
Among the heavy metal ions present in the waste waters, chromium, especially Cr(VI), is one of the most dangerous metal ion, considered as a powerful carcinogenic agent [1], [2], [3], [4], [5], the main sources of Cr(VI) being the paints and pigments production, leather tanning, metal finishing, electroplating industry, wood preservation, and so on. According to the World Health Organization (WHO) drinking water guidelines, the maximum allowable limit for total chromium is 0.05 mg/L [1], because, above this level, Cr(VI) may cause dramatic health problems [1], [6]. Therefore, to avoid the dangerous impact of Cr(VI) on human health and on the environment, as well as from the economic considerations, it is essential to remove or recover Cr(VI) from the waste waters before disposal.
Beside silica based materials [4], functionalized sands [7], and biosorbents [8], [9], [10], [11], [12], a large variety of synthetic sorbents have been lately designed with the final aim to increase the efficiency in the removal of Cr(VI) [6], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]. Synthetic sorbents offer the possibility to introduce almost any reactive functionality which could be involved in the retention of heavy metal ions and, furthermore, could be preserved for a long time with no changes in their structure. Thus, Rajiv Gandhi et al. synthesized some amino-functionalized resins based on acrylonitrile/divinylbenzene/vinylbenzyl chloride and have found that the sorption of Cr(VI) was influenced by the medium pH and slightly by the presence of co-ions [6]. Neagu and Mikhalovsky have prepared some strong base anion exchangers (SBAEx) based on poly(4-vinylpyridine) and discussed their selectivity towards chromium anions in the chromium/sulfate binary sorption systems [15]. Pehlivan and Cetin have found the maximum adsorption of Cr(VI), at an optimum pH of 5.0, onto Lewatit MP 64 and Lewatit MP 500 was 0.40 and 0.41 mmol Cr(VI)/g resin, respectively [16]. Shi et al. have evaluated the removal of Cr(VI) by three commercial macroporous weak base anion exchangers, two based on styrene-divinylbenzene (S-DVB) matrix and one acrylic, and found a strong influence of temperature on the sorption capacity, this being lower for the acrylic resin at room temperature but higher at 60 °C, compared with S-DVB resins [18].
As reported in literature, SBAEx demonstrated high sorption capacity for Cr(VI) oxyanions [14], [15], [16], [19]. The synthesis of some composite sorbents having SBAEx microspheres entrapped in chitosan/poly(vinyl amine) cryobeads and their application in the removal of Cr(VI) oxyanions have been recently reported by our group [25]. However, the retention of Cr(VI) oxyanions on the SBAEx microspheres, as a function of their structure and morphology, has been not studied yet. Therefore, the purpose of this work is to investigate the correlation between the structure of some novel porous SBAEx obtained by the functionalization of porous S-DVB matrix with N,N-dialkyl 2-hydroxyethyl ammonium chloride, the alkyl substituents being either methyl or ethyl, and their efficiency in the removal of Cr(VI) from aqueous solutions. For the synthesis of the porous matrix, n-butyl alcohol (n-BA) was used as porogen, in different proportions, while the content of DVB was kept constant at 8 wt%. To decide on the best sorption conditions, the influence of initial pH, sorbent dose, initial concentration of Cr(VI) ions, contact time, temperature, and presence of other anions was surveyed. The sorption experimental data were fitted with several kinetic and isotherm models to identify the controlling mechanism of the sorption of Cr(VI) oxyanions. This is the first systematic investigation on the influence of the size of the alkyl substituents in the N,N-dialkyl 2-hydroxyethyl ammonium groups of the porous SBAEx on the sorption of Cr(VI) oxyanions reported to date.
Section snippets
Materials
S (99% purity) (Arpechim, Romania) and DVB (54.5% o-DVB, m-DVB, p-DVB, 36.2% ethylstyrene and 9.3% inert compounds) (Viromet Co., Romania) were distilled under reduced pressure before use. n-BA, chloroform, and methanol, purchased from Chemical Company, were used as received. Paraformaldehyde [HO(CH2O)nH], FeCl3 and trimethyl chlorosilane (TMCS) (Fluka Chemical Co., Buchs, Switzerland) were used as received. N,N-dimethyl 2-hydroxyethyl amine (DMHEA) and N,N-diethyl 2-hydroxyethyl amine (DEHEA)
Structural characterization
Structural characterization of the sorbents before and after loading with Cr(VI) was performed by FTIR spectroscopy. Fig. 1 presents FTIR spectra of SBAExE:0.44, before and after loading with Cr(VI) (spectra A and B), and of the SBAExM:0.44 loaded with Cr(VI) (spectrum C).
The absorption peak located at 3436 cm− 1 in spectrum A is assigned to–OH stretching vibration; the weak peak at 3023 cm− 1 is assigned to the aromatic CH stretching, and those at 2922 and 2854 cm− 1 correspond to CH and CH2
Conclusions
Removal of Cr(VI) from synthetic aqueous solutions was conducted with porous strong base anion exchangers bearing N,N-dialkyl 2-hydroxyethyl ammonium groups obtained by functionalization of some porous S-DVB copolymers prepared in the presence of n-BA as porogen. It was found that the sorption capacity of Cr(VI) oxyanions was strongly dependent on the size of the alkyl substituents, being higher when the alkyl was methyl than when the substituent was ethyl. Thus, the sorption capacity at
Acknowledgements
The results presented in this manuscript have been financed by a grant of the Romanian National Authority for Scientific Research, CNCS – UEFISCDI, Grant number PN-II-ID-PCE-2011-3-0300. The authors thank to Dr. Florica Doroftei for the SEM/EDX measurements.
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