Decoloration of Acid Red 88 Using Synthetic-Zeolite-Based Iron as a Heterogeneous Photo-Fenton Catalyst

Decoloration and mineralization of an azo dye, Acid Red 88, were conducted using synthetic-zeolite-based Fe as a heterogeneous photo-Fenton catalyst in the presence of ultraviolet (UV) rays and H2O2. Under the optimal conditions (pH = 5.5, 17.6 mM H2O2, and 4 W m UVC), 100% decoloration and 90% total organic carbon removal of 0.12 mM Acid Red 88 were achieved in 120 min. The effects of initial pH with time, as well as oxidation, were studied in a batch reactor. It was found that high decoloration was achieved using a heterogeneous Fenton method at pH 7 and lower. The catalyst also had the advantages of low leaching of Fe ions and maintenance of a high decoloration in consecutive catalytic treatments. Zeolite-based Fe was successfully used repeatedly (up to three consecutive cycles) for decoloration. A high rate of decoloration was also achieved in the case of continuous operation, although ≤0.4 mg L of Fe were leached into the treated water.


Introduction
In the textile industry, dyeing consists of three processes: preparation, dyeing, and finishing. Large quantities of dyes and other organic matter are therefore present in effluents from the textile industry, resulting in contamination of water. There are currently environmental standards for chemical oxygen demand (COD) and biochemical oxygen demand (BOD), but chromaticity is regulated to a lesser degree. However, when color remains in the effluent, even if COD and BOD meet the environmental standards, there is still a strong impression that the discharge is contaminated. Active-carbon adsorption and biodegradation are usually used as the treatments for effluent coloration, but these treatment methods have problems with respect to running costs and efficiency (Muhammad et al., 2009;Nillson et al., 2006;Suntud et al., 2007).
For environmental protection, there are a number of methods (chemical, physical, and biological) for treating discolored azo dye effluents from various industries. Biological and physical treatment methods are not satisfactory because they simply transfer the pollutants from one phase to another, and the equipment involved in these processes is expensive. In recent years, treatments using advanced oxidation processes, i.e., chemical methods, have attracted much attention. Advanced oxidation processes such as homogeneous photo-Fenton reactions can produce hydroxyl radicals, which are powerful oxidants for organic degradation (Idil, 2007;Huseyin et al., 2005;Papadopoulos et al., 2007).
Although the homogeneous photo-Fenton reaction is very powerful in the degradation of organic compounds, it has significant disadvantages such as the need for recovery of the Fe sludge after treatment and the narrow pH range in which the reaction proceeds. To overcome these drawbacks, much effort has been made to develop heterogeneous catalysts for the reactions; these catalysts contain Fe clusters or Fe oxides (Jiyun et al., 2005;2009;Mesut et al., 2008;Marco et al., 2006). where TOC i is the initial absorbance of the sample and TOC t is the absorbance at time t.

Fe Catalyst
A 0.02 M ferric solution was prepared by dissolving ferric nitrate in distilled water. Zeolite (5.0 g) was added to 50 mL of freshly prepared 0.02 M ferric solution, followed by shaking for 6 h at 160 rpm in a water bath at 60 °C. After shaking, the mixture was filtered using a 0.45-μm membrane filter and dried at 30 °C. The whole procedure was repeated four times, and finally Fe 3 -zeolite was collected as a brown powder 4.3 g). The Fe 3+ attachment was determined to be 0.60 mmol g −1 .
The XRD patterns of H-zeolite and Fe-zeolite are shown in Figure 5. Diffraction lines for iron oxide are not observed in the XRD pattern of Fe-zeolite. This is not unusual as the diffraction lines of iron oxide are expected to broaden and be buried in the background noise of the XRD pattern (Chen HY et al., 2000).

Decoloration Experiments
Fe 3+ -zeolite (0.298 g) and Acid Red 88 (0.100 g) were added to 1 L of distilled water adjusted to pH 3.0 using H 2 SO 4 . Then 1 mL of H 2 O 2 was added to the solution, and the liquid was irradiated with UV light (4.0 Wm −2 ). The intensity of the UV lamp is shown in Figure 6. The intensity is strong at 283 nm. Mixing was performed at room temperature at 120 rpm. At each sampling, 30 mL of solution were removed and 1 mL of 0.2 M NaOH solution was added. The solution was then filtered and the TOC and absorbance were measured using a total carbon analyzer (TOC-V, Shimadzu, Japan) and spectrophotometer (V-630Bio, Jasco, Japan), respectively. The pH-dependence tests were performed using 0.1 M H 2 SO 4 and 0.1 M NaOH. After decoloration, the Fe-zeolite was separated using a 0.45-μm membrane filter and dried at 30 °C for further use. For comparison, a Fenton reaction was performed as a control experiment, as follows. Ferric sulfate (0.05 g) and Acid Red 88 (0.100 g) were added to 1 L of distilled water, and the pH was adjusted to 3 by adding H 2 SO 4 , followed by addition of 1 mL of 30% H 2 O 2 solution. Mixing was performed at room temperature and 120 rpm. At each sampling, 30 mL of solution were removed and 1.0 mL of 0.2 M NaOH solution was added.  Table 2. The test was carried out in a transparent glass fluidized-bed reactor of inner diameter (ID) 2 cm and a Plexiglass tube of length 10 cm, which was fused to a 6-cm ID and 5-cm long tube to form a 150-mL reactor body. Approximately 0.8 g of the Fe 3+ -zeolite were soaked in deionized water to facilitate swelling, and then packed into the reactor. The dye solution (0.12 mM Acid Red 88 and 17.6 mM H 2 O 2 at pH 5.5) was fed through the bottom of the column at a desired flow rate using an Iwaki PST-100N peristaltic pump. Two small UV lamps were placed in parallel at a position 50 mm from the bottom of the wall, giving irradiation of 0.5 W m −2 . A cross-flow membrane filter was installed so that the Fe 3+ -zeolite could not flow out. At set time intervals, samples were collected using a Biorad Model 2110 Fraction Collector in 8 m-L plastic tubes and analyzed for absorbance at 514 nm and Fe 2+ ions.

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