Effect of chelating agents on reactive dyebaths color removal with high pH ozone oxidation

Reaktif boya banyolarında kullanılan iyon tutucuların yüksek pH' da ozon oksidasyonu ile renk giderimi üzerine etkisi

Abstract

Color is one of the characteristic parameters of textile industry wastewaters. Although there are several physical-chemical methods applied for color removal such as adsorption, membrane processes and chemical oxidation, chemical oxidation has become the method of choice due to its high efficiency and ease of operation. Ozone is one of the most effective oxidant used for this purpose. Ozone oxidation of textile industry wastewaters, spent and simulated dye baths has been a focus of investigations (Davis et al., 1982; Grau, 1991; Carriére et al., 1993; Namboodri et al., 1994a,b). Mechanism and kinetics of ozonation of synthetic dye solutions were also extensively studied (Snider and Porter,1974; Perkins et al., 1980; Teramoto et al., 1981; Perkins et al., 1995). Actual dye baths involve the use of several assisting chemicals in addition to dyes. Basic chemicals used in reactive dyeing are NaCl, Na2CO3 and sequestering agents. NaCl is used to facilitate diffusion of the dye to the fiber. Na2CO3 mostly has a dual purpose: it helps the fixing of the dye and increasing and buffering the pH. Sequestering agents are chelate forming ligands that bind the metal ions such as $Cu^{2+}$, $Fe^{2+}$, and prevent their interference with dye. Kabdaşlı et al. (2002) studied the effect of assisting chemicals in the dye bath on ozonation of dyestuffs. They found out that sequestering agents exerted a significant effect on the ozone oxidation of the dye by reducing the reaction rate and increasing the amount of ozone being used, while NaCl and Na2CO3 had negligible effect on color removal. Commonly used conventional sequestering agents such as NTA (nitrilo triacetic acid) and EDTA (ethylenediamine tetraacetic acid) contain aminopolycarboxylate groups. Recently a new generation sequestering agents that are based on phosphonic acid derivatives has been introduced. The advantage of these compounds in addition to their capability of strongly binding the metal ions, is their stability at high temperatures employed in reactive dyeing. The study aimed to asses several aspects of oxidation dye bath assisting chemical, by ozonation emphasizing their effect on decolorization at high pH. In the first stage of experiments dyestuff and chelating agents were separately oxidized by ozone at high pH. In the second stage, oxidation of the combination of dyestuff and chelating agent was carried out. The dyestuff used in the experiments was Procion brand Navy HEXL that involved the bifunctional group bis(amino chlorotriazine) and have been frequently used for dyeing cellulosic fibers. The chelating agents selected were, EDTA which has been used commonly as a conventional sequestering agent and a new generation sequestering agents EDTMPA (ethylenediamine tetramethylene phosphonic acid) which has been actually used in textile dyeing and had a similar structure to EDTA. EDTA and EDTMPA concentrations were selected considering the actual concentrations used in the reactive dye baths. Oxidation experiments were conducted at pH 10.5 which was also reflected the actual pH of the spent dye bath. Oxidation of the Procion Navy HEXL was relatively fast providing over 50 % color removal in 3 minutes and reaching almost complete removal in 10 minutes. COD removal efficiency was 72% at the end of the 15 minute reaction time. At the end of the experiment a specific ozone consumption of 0.8 mg $O_3/mg$ COD removed was determined. The evaluation of the single dye ozonation results showed that pseudo-first order kinetic expression was applicable with respect to the COD. 80 % and 43 % COD removal efficiencies were obtained in 90 minutes by the ozonation of 2.293 mM EDTMPA and EDTA solutions respectively. The ozone utilization ratios were of 2.8132 mg $O_3/mg$COD removed for EDTMPA and 3.32 mg $O_3/mg$COD removed for EDTA at the end of the oxidation period. The evaluation of the results showed that pseudo-first order kinetic was also applicable for the oxidation of selected chelating agents with respect to COD. The rate constants were 0.0291 and 0.0258 1/min for EDTMPA and EDTA respectively. As can be seen from the results it can be concluded that EDTMPA could oxidize more rapidly than EDTA. Results of ozone oxidation of combination of the dye and chelating agents at pH 10.5 showed that when they were existed together oxidation rate of both reduced. The same degree of color removal (below 100 Pt-Co) was obtained in 30 minutes for EDTMPA and 60 minutes for EDTA versus 10 minutes when dye existed singly. COD removal characteristics of the systems were also modified when the dye and chelating agents existed in combination as well as pseudofirst order reaction rates.

Özet

Bu çalışmada, tekstil endüstrisinde yaygın olarak kullanılan reaktif boyar maddelerden Procion Navy HEXL ile boyama proseslerinde iyon tutucu olarak kullanılan EDTA ve EDTMPA’nın yüksek pH’da ozon ile renk ve organik madde giderimi üzerine etkileri deneysel olarak incelenmiş ve anılan iyon tutucuların ortamda ayrı ayrı ve Procion Navy HEXL ile birlikte bulunmaları durumunda oksidasyon karakterindeki değişimleri karşılaştırmalı olarak değerlendirilmiştir. 15 dakikalık reaksiyon süresi sonunda %72 KOİ giderim verimi elde edilmiş ve rengin tamamına yakını giderilmiştir. EDTMPA ve EDTA’nın ortamda tekil olarak bulunmaları durumunda 90 dakikalık ozonlama sonunda, EDTMPA ve EDTA için sırasıyla 2.81 ve 3.32 mg $O_3/mg$ giderilen KOİ ozon tüketim oranında, % 80 ve % 43 KOİ giderim verimleri elde edilmiştir. Boyar madde ve iyon tutucuların ortamda tekil ve birlikte bulunmaları durumunda organik maddenin ozon ile oksidasyonun görünür birinci derece reaksiyon kinetiğine uyum sağladığı belirlenmiştir. Elde edilen KOİ giderim hızları EDTMPA’nın (0.0291 1/dak) EDTA ya (0.0258 1/dak) nazaran daha hızlı okside olduğunu göstermiştir. Yüksek pH’da boyar maddenin ortamda EDTA ile birlikte bulunması durumunda 90 dakikalık reaksiyon süresi sonunda % 78 lik KOİ giderimi 2.88 mg $O_3/mg$ giderilen KOİ ozon tüketim oranında gerçekleşmiştir. Ortamda boyar madde ve EDTMPA’nın beraber bulunması durumunda ise bu değerler sırası ile % 90 ve 2.40 mg $O_3/mg$ giderilen KOİ olarak bulunmuştur. İyon tutucunun boyar maddenin kromofor grubunun oksidasyonu engellenmediği ancak bu grubun aynı seviyede oksidasyonu için gerekli süreyi uzattığı tespit edilmiştir. Aminopolikarboksilat yapısına sahip ve iyon tutucu olarak kullanılan yardımcı kimyasal maddelerin renk giderim hızını, fosfonik asit yapısına sahip iyon tutuculara nazaran daha fazla yavaşlattığı saptanmıştır.