INTRODUCTION:

Redox polymers that are based on polyamines and various quinones contain, within their structure, nitrogen atoms with lone-electron pairs, and such active groups as С═О, ─NH─, ─ОН. These groups are capable of forming both intra-molecular and inter-molecular hydrogen bonds and afford that the macromolecules obtain, apart from redox properties, a potential capability to form complexes. The research into redox polymers’ complex-forming properties is of practical interest not only in order to solve various hydrometallurgy-related issues (such as concentration and separation of metal ions from industrial solutions) and environmental issues (such as treatment of industrial and domestic waste water from heavy metal ions), but also in order to obtain polymer-metallic complexes, which could be applied in a variety of areas of use.

The ions of metals in water solutions exist, as a rule, in the form of compounds completely or substantially dissociated. Therefore, their removal from industrial and domestic waste water is limited to adsorbing any metal-containing cations or anions.

This article sets forth the results of a research in respect of sorption properties in redox polymers based on weak-base anionites – such as aminated polyvinylchloride [1], Polyethyleneimine [2, 3] and chloranil in relation to chromium (VI) ions.

The problem with treating any waste water containing high toxic and expensive hexavalent chromium salt causes difficulties for many companies producing or using these compounds [4, 5]. When such waste water gets into water reservoirs or basins, it causes, apart from general toxicity, a mutation impact on the living organisms. The maximum permissible concentration (MPC) of chromium (VI) is set at 0.1 mg/l for sanitary and domestic water reservoirs.

For the meantime, it is known that, at the chromium ore mining sites, such MPC critically exceeds the sanitary standards. Thus, in the surroundings of Aktobe where the largest chromium deposits are mined, chromium concentration exceeds 20-fold the MPC [6].

Ion-exchange technique and sorption technique have become widely used to remove heavy metals from compounds and industrial waste water [7-12]. The advance of ion-exchange technology depends substantially on the synthesis of multiform and relatively inexpensive ion-exchange resins with improved properties.

On the basis of polyethyleneimine (PEI) and aminated polyvinylchloride (АPVC) with chloranil (CA), we have synthesized new redox polymers PEI-CA and АPVC-CA, which, in addition to their redox properties, are capable of forming complexes and exchanging ions.

The purpose of work is to research into the sorption of chromium (VI) by redox polymers PEI-CA and АPVC-CA.

MATERIALS AND METODS:

The synthesis of chlorine-containing redox polymers based on PEI and APVC was carried out by condensation of polyamines (PA) with chloranyl in various reaction conditions [3].

The pH level was adjusted by adding ammonia, sodium hydroxide or sodium acetate. As soon as the reaction terminated, the polymer was separated, cleaned with methyl (ethyl) alcohol in a Soxhlet extractor, treated by 4% NaOH, washed until the wash water showed neutral reaction, where after the key physical and chemical properties were identified.

The progress of the reaction was assessed in view of the data obtained by way of elemental analysis and IR spectroscopy, as well as by referring to redox capacity (RC) and static exchange capacity (SEC) upon the main groups of final products, and the values of potentiometric acid-base and redox titration measured by DL50 titration apparatus Mettler Toledo at 25⁰.

Redox polymers PEI-CA and АPVC-CA are dark brown grains, insoluble in organic solvents, of an irregular form, varying in size from 0.5 to 1.0 mm, and having a static exchange capacity for 0.1N HCl at 3.7 and 2.3 mg-eq/g, respectively.

The sorption of ions of chromium by redox polymers PEI-CA and АPVC-CA in OH-form was surveyed in static conditions, stirring occasionally, at the temperature of 20±2°C, at the sorbent/solution ratio of 1:400, from sample K₂Cr₂O₇solutions containing from 0.205 to 2.080 g/l of chromium. The acidity of the solutions was interchanged occasionally from pH 1.1 to 5.1, adding 0.1N H₂SO₄ or potassium hydrate (KOH). The time of contact with K₂Cr₂O₇ solutions ranged from 1 hour to 168 hours. A “chemically pure” K₂Cr₂O₇ type was used to prepare the solution.

Sorption capacity (SC) was calculated based on the difference between the initial concentration and the equilibrium concentration of the solutions. To do so, classical polarography was applied at 0.5 M NH₄Cl upon the wave reconstruction Cr₂O₇^2-(Е_1/2= - 0.17 V), at the 0.1 N KOH background. Polarograms were recorded by a universal polarography device PU-1, in a thermostatically-controlled cell at 25±0.50⁰С, applying a dropping mercury electrode (DME). Oxygen was removed from the solutions under analysis by argon blowing for 5 minutes. A saturated calomel electrode was used as a reference electrode.

RESULTS AND DISCUSSION:

In surveying the sorption of chromium (VI) by redox polymers PEI-CA and АPVC-CA, it was found that SC depends on the concentration of K₂Cr₂O₇ sample solutions (Fig. 1).

Fig.1 Isotherm curves. Cr₂O₇^2- sorption by redox polymers

PEI-CA (1) and АPVC-CA (2). Contract Time 7 days.

The form of the isotherm curves showing chromium (VI) ions sorption indicates that redox polymer PEI-CA is the most suitable for extraction.

One of the main factors, on which both the capacity and the selectivity of ion-exchange resins in respect of the ions of metals in a solution depend, is pH of the solutions. pH governs the degree of dissociation (or, in other words, the degree of protonation) of the functional groups of ion-exchange resins, and the forms in which the ions of metals exist in solution. As can be seen in Figure 2, the adsorption capacity of redox polymer PEI-CA remains unchanged within the pH 3-5.1 range. When acidity (pH <3) is increasing, the SC of that redox polymer rises, reaching the maximum of 416.0 mg/g at pH 1.1. It is known that redox processes develop in acidic conditions, the more intensively the less pH is [11]. In such conditions, an assumption can be made that the portion of polymerized chromium ions is increasing, and, as a result, a larger number of chromium ions in the forms of Cr₃O₁₀^2-,Cr₄O₁₃^2-and Cr₅O₁₆^2- is adsorbed per unit of sample surface. This also facilitates increasing the degree of extraction of chromium from solutions.

Fig.2 Impact of K₂Cr₂O₇(C_Cr=2.08 g/l) acidity on sorption of
chromium (VI) ions by redox polymer PEI-CA. Contract Time 7 days.

The SC of well-known high-base anionites AMP and AV-17 is 234.6 and 240.4 mg/g, respectively [12,13].

Fig. 3 shows how the SC of redox polymer PEI-CA depends on the time of its contact with K₂Cr₂O₇solution (pH 1.1, С_Cr = 2.08 g/l).

Fig.3 Kinetic curve sorption of Cr (VI) Ions by redox polymer PEI-CA from K₂Cr₂O₇solution (pH 1.1, C_Cr=2.08 g/l)

As can be seen from Figure 3, equilibrium is reached in 1 hour, i.e. the redox polymer has fairly high kinetic properties.

The degree of extraction of Cr (VI) ions from K₂Cr₂O₇ solutions containing 0.20-0.53 g/l of chromium is 78%. When the concentration of metal is increased up to 2.08 g/l, the degree of extraction of Cr (VI) ions drops to 50%.

Redox polymers based on vinyl ether of monoethanolamine and linear spatial 1,4-naphthoquinone [9] have high sorption properties in respect of chromium (III) ions [13]. SC reaches 800-873 mg/g. However, the extraction of hexavalent chromium ions have not been the subject of our research.

It is difficult to extract chromium from waste water because its valence varies. Depending on the method of treatment, chromium is either reduced to trivalent, or oxidized to hexavalent.

In cleaning the chromium electrolytes, it has been found that anionites AB-17x8 AB and AB-28, the exchange capacity of which in respect of chromate-ion is 130 and 116 mg/g, respectively, are the best for adsorbing the ions of chromium (VI) in acidic environment, due to their sorption properties and chemical stability [14]. The PEI-based redox polymer that we synthesized shows a significantly higher sorption activity in respect of chromium (VI) ions than the anionites.

Consequently, the research into the sorption of Cr (VI) ions by redox polymers PEI-CA and АPVC-CA shows that the most promising ion-exchange resin for the extraction of Cr (VI) ions from waste water is redox polymer PEI-CA, which, due to its specific structure, has high sorption and kinetic properties.

ABBREVIATIONS:

PEI	Polyetheleneimine
APVC	Aminated polyvinyl chloride
PA	Polyamine
CA	Chloranyl
EA	Ethyl alcohol
IR	Infrared spectroscopy
ORC	Oxidation-reduction capacity (mg-equ/g)
SEC	Static exchange capacity (mg-equ/g)
SC	Sorption capacity (mg/g)
AMP	Strong-basic anion exchanger
AB-17	Industrial strong-basic anion exchanger
AB-28	Industrial strong-basic anion exchanger

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Received on 17.02.2015 Modified on 05.03.2015

Asian J. Research Chem. 8(10): October 2015; Page 609-612

DOI: 10.5958/0974-4150.2015.00096.6