Dichromate anion sorption from wastewaters using poly (vinyl alcohol) cryogels.

Croitoru, Catalin ; Patachia, Silvia ; Moise, Georgeta 等

1. INTRODUCTION

The dichromate anion [Cr.sub.2] [O.sub.7.sup.2-] is a toxic chemical species that is often found in wastewaters, as potassium dichromate, mainly resulting from wood, leather, textile and construction industry. It is also used in galvanization processes, for coating iron and zinc with a protective layer of chromium.

Potassium dichromate is one of the most common causes of dermatitis; chromium is highly likely to induce sensitization leading to dermatitis, especially of the hand and fore-arms, which is chronic and difficult to treat. It is also toxic, being accumulated in the organism, often leading to cancer or internal organs failure (Muhammad et al., 2001).

Traditionally, in wastewater purification plants, the dichromate ion is reduced to [Cr.sup.3+], a less toxic species, with the help of a reducing agent, such as sodium bisulphate. [Cr.sup.3+] is then precipitated as chromium hydroxide, by adjusting the pH of the water above 8, with sodium carbonate. This method is economically inefficient; consuming fairly large amounts of chemical reagents, with no possibility of recovery. Also it requires further steps in readjusting the pH of the water to its normal value, as high pH values disturb the aquatic flora and fauna. Other methods of dichromate removal employ the use of ion exchanging resins. This method is also less efficient, as the recovery of the ion exchanging resin must be done by addition of chemical reagents, with negative ecological impact. Less employed methods include photo oxidation or biodegradation (Sisti et al., 1996). Recently, large attention has been focused on polymeric membranes for removal of toxic species from wastewaters, such as heavy metals, dyes, and so forth. The polymeric membranes have the advantage to retain relatively large amounts of pollutants and in most cases they can be reused for several times. Furthermore, the continuous developments made in the field of polymeric materials obtaining lead to more economically efficient and ecologic methods of obtaining. Up to this extent, polymers such as cellulose, cellulose triacetate, chitosan or calix [4] arenes have been employed in the fabrication of polymeric materials with sorptive capacity for he dichromate anion. The efficiency of these materials has been reported as being up to 45%, with sharp decreasing on reusing (Muhammad et al., 2001).

In this work we have obtained a poly (vinyl alcohol) [PVA] based material with potential use as sorption substrate for the dichromate ion. PVA is a non-toxic, non-carcinogenic, biocompatible, biodegradable, water-soluble polymer, in consequence easy to handle and friendly for the environment (Patachia 2006). Physical crosslinking using freezing-thawing cycles has been used for the PVA cryogel obtaining. The method of physical crosslinking of PVA is often employed in pharmacy and medicine (Patachia 2003), as an alternative to chemical crosslinking which uses potentially toxic reagents. Additionally, PVA cryogels present good response to various external stimuli (Rot&Gupta, 2003), such as solution concentration, pH, ionic strength, temperature and so forth, so they can be used as a concentration sensor for dichromate. Our aims were to achieve good sorption capacity, and good reusability of the polymeric material.

2. EXPERIMENTAL

2.1 Materials

PVA 120-98 (1200 polymerization degree and 98% hydrolysis degree) was purchased from Chemical Enterprises Risnov, Romania. Potassium dichromate has been purchased from Sigma.

2.2 PVA cryogel obtaining

PVA solution has been prepared by dissolving the polymer powder in Milli-Q water, under magnetic stirring at room temperature, followed by heating at 75[degrees]C for 4h. The solid content of the obtained solution was 10%.wt. The PVA cryogel has been prepared by introducing a specific volume of PVA solution cooled to room temperature in a PVC cylindrical recipient and submitting it to freezing at -20[degrees]C for 12 hours, followed by thawing at room temperature (26[degrees]C) for 12 hours. The above mentioned freezing-thawing procedure has been repeated three times. After obtaining, the PVA cryogels have been immersed in distilled water for a week, to reach the swelling equilibrium.

2.3 Sorption of [Cr.sub.2] [O.sub.7.sup.2-] in the PVA cryogel

[Cr.sub.2] [O.sub.7.sup.2-] sorption has been studied by immersing weighted cryogel samples in a determined amount of 0.1 M [K.sub.2] [Cr.sub.2][O.sub.7] solution (pH~4).

At certain time intervals, 2 mL of [K.sub.2][Cr.sub.2][O.sub.7] solution were drawn and analyzed and the cryogel sample has been immersed in a fresh [K.sub.2][Cr.sub.2][O.sub.7] solution of 0.1 M.

[K.sub.2][Cr.sub.2][O.sub.7] has been determined by the spectrophotometric method using a SPECORD Carl-Zeiss Jena spectrophotometer (absorption maximum at 355 nm).

[K.sub.2][Cr.sub.2][O.sub.7] desorption from the cryogel has been studied by immersing the sample subjected earlier to absorption in a determined amount of distilled water. Solution samples have been drawn and analyzed as above, and after each determination the cryogels have been reimersed in a fresh amount of distilled water. To test the variation in sorption capacity, the PVA cryogels have been submitted to four sorption-desorption cycles.

3. RESULTS AND DISCUSION

Amount of [Cr.sub.2][O.sub.7.sup. 2-] sorbed in the PVA cryogels (in terms of sorbed [Cr.sub.2][O.sub.7.sup.2-] amount (g) reported to 1g of dry polymer [xerogel] is plotted in Fig.1 (sorption kinetic). The swelling of the polymeric matrix in contact with the potassium dichromate solution has been taken into consideration for this calculus. The percent of desorbed [Cr.sub.2][O.sub.7.sup. 2-], calculated as the amount of eliminated dichromate reported to the initially absorbed amount is plotted vs. time in Fig.2:

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

As it can be seen from Fig. 1, the PVA 120-98 cryogel absorbs the dichromate ion from the aqueous solution. FTIR spectroscopy performed on the dry PVA gel loaded with dichromate has indicated that the interactions between the polymer and [Cr.sub.2][O.sub.7.sup.2-] are of physical nature. This is extremely advantageous, as physical bonds are weaker, thus making the recovery of the material easier. As it can be seen from Fig. 2, almost the entire amount of dichromate loaded into the cryogel has been desorbed.

[FIGURE 3 OMITTED]

As it can be seen from fig. 3, the sorption capacity of the PVA 120-98 is almost unchanged during 4 repeated cycles of absorption, which makes this kind of material efficient for repeated use in sorption processes.

The pictures of the initial PVA cryogel and the PVA cryogel after dichromate absorption are presented in fig. 4:

[FIGURE 4 OMITTED]

4. CONCLUSIONS

PVA 120-98 cryogels have been obtained by the alternative freezing-thawing cycles method.

Sorption and desorption analysis of [Cr.sub.2][O.sub.7.sup.2-] have been performed. Studies have indicated that the cryogel absorbs dichromate from aqueous solutions and that the interactions between the anion and the polymer are of physical nature. The absorption capacity of the cryogel maintains at aproximetively the same value, when submitted to four absorption-desorption cycles. This makes the proposed material a good candidate for chromium sorption at industrial level. The eventual cryogel residues after complete usage could be reconverted to polymer solution by melting and reused. Further studies will be conducted having as aim the increasing of the sorption capacity, e.g. the use of a PVA with higher molecular mass, modifying the cryogel synthesis parameters (freezing temperature and time, number of alternative freezing-thawing cycles etc.)

5. ACKNOWLEDGEMENTS

We would like to acknowledge CNCSIS and ANCS for the financial support through the framework of TD161/2007 and IDEI 839/2009 grants. Also, we acknowledge the support of Eng. Chem. Terhu (Puchiu) Doina for her experimental work in the frame of our laboratory.

6. REFERENCES

Muhammad, M.; Baig, A.; Ishtiaq, H.; Ishtiaq, A.; Murtaza, M. & Haroon, S. (2001). Textile Wastewater Characterization and Reduction of its COD and BOD by Oxidation. Electronic Journal of Environmental, Agricultural and Food Chemistry, 1, Vol. 1, pp 804-811, ISSN: 1579-4377

Patachia, S. (2003). Blends based on poly (vinyl alcohol) and the products based on this polymer. Handbook of Polymer blends and composites, Vasile, C, Kulshreshtha, A.K., (Ed), pp 230-256, RAPRA Technology Ltd, ISBN 1-85957-2014, Shrewsbury, UK

Patachia, S.; Valente, A. & Baciu, C. (2006). Effect of non-associated electrolyte solutions on the behaviour of poly (vinyl alcohol)-based hydrogels. European Polymer Journal, Vol. 43, No. 2, pp 460-467, ISSN 0014-3057

Rot, I. & Gupta, M. N. (2003). Smart Polymeric Materials: Emerging Biochemical Applications. Chemistry&Biology, 10, No. 12, pp 1161-1171, ISSN 1074-5521

Sisti, F.; Allegretti, A. & Donati, E. (1996). Reduction of dichromate by Thiobacillus ferrooxidans. Biotechnology Letters, 18, No. 10, pp 1477-1480, ISSN 0141-5492