I. INTRODUCTION:

Mat manufacturing units are generally operated as small scale industries or combined cottage industries. These utilize commercially available dyes for their dyeing unit pollutes the environment substantially by letting the water as such in fresh water channels. Treatment technologies extensively used for degradation of mat dyeing effluent includes electro-oxidation, bio-treatment, photochemical, and membrane processes, Biodegradation, Coagulation–flocculation, adsorption on activated carbon, Ozone treatment. Though Fenton’s reagent was discovered about 100 years ago, its application as an oxidizing process for destroying toxic organics was not reported to be applied until the late 1960s (Huang et al. [5]). Fenton reaction is known to be very effective in the removal of many hazardous organic pollutants from water. and wastewater. [9].

Fenton’s reagent is a mixture of H₂O₂ and ferrous iron, which generates hydroxyl radicals according to the reaction (Kitis et al. [6]; Yoon et al. [9]; Lu et al. [10])

Fe²⁺ + H₂O₂ → Fe³⁺ + OH^• + OH⁻ (1)

The ferrous iron (Fe2+) initiates and catalyses the decomposition of H₂O₂, resulting in the generation of hydroxyl radicals. The generation of the radicals involves a complex reaction sequence in an aqueous solution

OH^• + Fe²⁺ → OH⁻ + Fe³⁺ (2)

Fe³⁺ + H₂O₂ ↔ Fe–OOH²⁺ + H⁺ (3)

Fe–OOH²⁺→ Fe²⁺ + •O₂H (4)

Fe²⁺ + HO₂• → Fe³⁺ + HO₂^- (5)

Fe³⁺ + HO₂^• → Fe²⁺ + O₂ + H⁺ (6)

OH^• + Introduction

H₂O₂ → H₂O + HO₂^• (7)

Walling [15] simpliﬁed the overall Fenton chemistry [reaction (1)] by accounting for the dissociation water

2Fe²⁺ + H₂O₂ + 2H⁺ → 2Fe³⁺ + 2H₂O (8)

This equation suggests that the presence of H⁺ importance in the, maximum production of hydroxyl radicals.

The main advantage of Fenton oxidation is the complete destruction of contaminants to harmless compounds, e.g. CO2, water and inorganic salts. The Fenton reaction causes the dissociation of the oxidant and the formation of highly reactive hydroxyl radicals that attack and destroy the organic pollutants [9]. Another advantage of having Fenton’s process is the nature of non selectivity of the generated hydroxyl radicals to decompose numerous organic compounds [11].

The mat manufacturing dyeing waste water is characterized with high COD of 2000–3000 mg/L mainly because of presence of complex dyes. pH, concentration of Fe²⁺ and H₂O₂, reaction time and temperature are reported to affect the Fenton’s oxidation process. The effectiveness of these operating parameters in treatment of mat dyeing effluent is extensively studied in this paper.

2.1 MATERIALS AND METHODS:

2.1. Mat wastewater source and wastewater characterization:

The grass mat dyeing industry waste water for this study was collected from a dyeing (mat) industry located in Nagercoil, Tamilnadu. After collecting the sample Sodium azide (5 mg/l) was added to inhibit the biological activity during the storage at room temperature. The waste water characteristics are presented in Table 1.

Table 1 Dyeing wastewater characteristics

Parameter
BOD₅ (mg/l)	620
COD (mg/l)	2486
Suspended solids (mg/l)	68
NH4-N (mg/l)	0.784
Free Cl (mg/l)	–
Oil-grease (mg/l)	89
Total Cr (mg/l)	0.254
Sulfur, S^{2 −}(mg/l)	<0.006
pH	5.2
Conductivity (μ mhos/cm)	725

2.2 MATERIALS:

Hydrogen peroxide (H₂O₂, 35%; Merck), ferrous sulfate hepta-hydrated (FeSO₄ .7H₂O; Merck), of reagent grade were used without further puriﬁcation.

2.2 METHODS:

The experiments were conducted in batch reactors with 100 mL waste water sample. The pH adjustment was made by adding dilute Sulphuric acid (H₂SO₄) and Sodium hydroxide (NaOH) solutions. After adjusting the pH of wastewater to required level, required amounts of FeSO₄ 7H₂O and H₂O₂ were added to the waste water sample under continuous stirring. Then the reactants are allowed for oxidation under aeration through diffused aeration. Following 30 min of precipitation (FeOH), the supernatant was decanted. The pH of the decanted supernatant was adjusted to 7. After 2 h of precipitation, the supernatant was decanted for UV absorbance, COD measurements.

The extent of the oxidation of the mat dyeing wastewater was determined on the basis of COD. The oxidation efficiency was calculated by Eq. (9).

Oxidation efficiency (%) =
(9)

Where COD₀ is the initial value of COD and COD_t is the value of COD at time t.

3. RESULTS:

3.1 EFFECT OF pH:

Fenton oxidation is known to be a highly pH dependent process since pH plays an important role in the mechanism of OH^•production in the Fenton’s reaction [5–7]. The pH in the range of 2.0 – 5.0 was investigated in this study (Fenton reaction was reported to greater oxidation in acidic range). The initial COD concentration was 2468 mg/L. As shown in Fig 1, the COD removal efficiency was increased with increase in pH from 2 to 3, beyond that, increase in the pH results in decrease in COD removal efficiency. At high pH (pH > 4), the generation of OH^•gets slower because of the formation of the ferric hydroxo complexes; the complexes would further form [Fe(OH)₄] when the pH value rises to 9.0 [6]. On the other hand, at very low pH values (<2.0) the reaction got slowed down due to the formation of complex species[Fe(H₂O)₆]²⁺, which reacts more slowly with peroxide compared to that of [Fe(OH)(H₂O)₅]²⁺. In addition, the peroxide gets solvated in the presence of high concentration of H⁺ion to form stable oxonium ion [H₃O₂]⁺. An oxonium ion makes peroxide electrophilic to enhance its stability and presumably reduces substantially the reactivity with Fe²⁺ion [14]. According to literature [10], that investigation suggested that the Fe²⁺reacts very quickly with H₂O₂ to produce a large amount of OH (see Eq. (1)) and then Fe²⁺in the solution was rapidly consumed. The ^•OH can react rapidly with organic matter, therefore, the removal of COD and from mat wastewater was rapid.

Fig 1. Effect of pH on COD removal

3.2 EFFECT OF FeSO₄ CONCENTRATION:

As seen from Fig 2. as FeSO₄ increased from0.1 g to 0.5g, COD removal efficiency also increased from 52 % to 73%. Hence it can be said that higher the FeSO₄ dose leads to the generation of more OH^• radicals. Further addition of FeSO₄ leads to decline of COD removal efficiency from 73% to 61%. Since Fe²⁺ ions initially catalyzes the decomposition of H₂O₂ to form OH*, further it seems advantageous for some time after that the over addition will leads to scavenging of OH*and not results in proportional removal of COD from waste water. 3.3 Effect of H₂O₂Concentration:

Fig 2. Effect of FeSO₄ concentration on COD removal

3.3 EFFECT OF H₂O₂ CONCENTRATION:

The effect of H₂O₂ concentration on Fenton’s treatment over COD removal was studied by increasing the concentration of H₂O₂ from 1mL to 5mL. The results obtained are shown in Fig 3. As seen from Fig 3. COD removal efficiency was increased from 52% to 69% as a proportionate to H₂O₂ increased from 1mL to 3mL due to increase in formation of OH^• radicals. However for over dosage than 3 mL, no further increase in COD removal efficiencies was observed due to insufficient ferrous ion concentration became deficient for reacting with H₂O₂.

Fig 3. Effect of H₂O₂ Concentration on COD removal

3.4 KINETIC STUDIES:

Fig 4. Kinetic studies –Pseudo First order

Pseudo - First order equation

Fig 4 is a linearised plot for the Fenton’s reaction for temperature effect. The linearised plot shows that the assumption of pseudo first-order kinetic is valid as the data fit accordingly. Kinetic studies shows that the COD degradation using Fenton reagent follows first order with a rate constant of 0.012 min^-1

4. CONCLUSION:

The following general conclusion can be made from this study

a. The optimum pH range was observed to be 3 and the optimum dosage of FeSO₄/H₂O₂ was found to be 2.1.

b. Usage of lower dosage of Fenton’s reagent followed by biological treatment is suggested to be economic for economic and viable option for disposal to meet regulatory standards.

5. REFERENCES:

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Received on 28.08.2013 Accepted on 01.09.2013

Research J. Engineering and Tech. 4(4): Oct.-Dec., 2013 page 217-220