Treatment azo dye by persulfate oxidation activated by nano-zero valent iron and UV-light



Persulfate (PS) is activated by nano zero valent iron without and with UV lights that produces free radicals ( ^• OH, SO ₄ ^−• ) in the water environment. These radicals decompose organic matter in the water. Investigation of decomposition kinetics of methyl orange (MO) in the activated persulfate systems by nano-zero valent iron (ZVI) without UV lights: nZVI/PS/MO; with UV lights: nZVI/PS/MO/UV. The results showed that the decomposition of MO in the activated persulfate systems obeys the first pseudo order kinetics. The first order apparent reaction rate constants in these systems were calculated. The decomposition efficiency of MO in the systems with UV lights was higher than that in the systems without UV lights.

Keywords : activated persulfate, hydroxyl radical, sulfate radical, azo dye, nano zero valent iron, UV.

Advanced oxidation processes (AOPs) have been studied and applied to treat wastewater and contaminated groundwater in the world. The AOPs is based on the in-situ free radicals which are generated in reaction. These free radicals have high oxidation activity like hydroxyl radicals ^ OH (E ^ = 2.8V) and sulfate radicals SO ₄ ^ (E ^ = 2.6V). The free radicals selectively react to all most organic compounds in the water, decomposing and converting the organic compounds into non-toxic or less toxic substances to humans and the environment. The advanced oxidation processes are known as: Fenton, Fenton — photo, peroxon, catazon, Fenton- electrochemical,…The ^ OH radicals are usually produced by activating hydrogen peroxide or ozone with various activating agents such as: transition metal ions, temperature, UV radiation,… [2], [4], [10], [12], [13], [19], [26]. Recent scientific announcements by scientists on the researching and application of other oxidants, such as persulfate and peroxymonopersulfate are also suitable for wastewater treatment containing persistent organic pollutant [8], [14], [15], [17]. If these oxidants are activated, they also produce free radicals which are higher oxidation activity than the original ones. Persulfates, peroxymonopersulfate are not stronger than hydrogen peroxide and ozone, but they are more durable than hydrogen peroxide and ozone in solution, better soluble in water than ozone. Specially, the process of activating the persulfate produces free radicals SO ₄ ^ and free radicals ^ OH [8], [18], [25].

Azo dyes occupy more than 50 % of the dye global trade. Some azo dyes have been found to cause cancer, mutations in genes and are banned worldwide. However, they are still produced and used on a large scale in the dyeing industry now. Because they are low production cost, easy to synthesize and some good color properties. The bonds in the azo molecules are quite stable, showing the ability to decompose and accumulate in the environment [16], [19], [24].

The textile industry consumes a large amount of clean water and also discharges a similar amount of wastewater which is complex composition and properties. This wastewater contains residual dyes from dyeing process (occupying about 10 to 15 % of the dye initial amount) and has color, temperature, content of COD, BOD and surfactants being very high [7], [16], [19], [24]. Therefore, the treatment of azo dye wastewater is necessary and need to investigate. Some azo dyes were used in these experiments those are methyl orange.

Materials and methods

Chemicals

– Methyl orange (MO), purity of 99 %, Merck, Germany;

– Nano — zero valent iron powder (nZVI), purity of 99 %, d< 80 m, Fisher, Belgium;

– Sodium persulfate Na ₂ S ₂ O ₈ (PS), purity of 99 %, Across, Belgium;

– Sodium hydroxide NaOH, purity of 99 %, Merck, Germany;

– Sulfuric acid H ₂ SO ₄ , purity of 99 %, Merck, Germany;

– Acetonitrile, ethanol, methanol with cleanliness for HPLC analysis, Fisher, Belgium;

– Twice distilled water.

Instruments

– High performance liquid chromatography (HPLC), Agilent 1100 Model G1314A Variable Wavelength UV-VIS Detector;

– pH-OAKLON machine, accuracy of 0.01, serie 510, USA;

– CHYO analytical balance, accuracy of 0.1mg, Japan;

– SB-348A air compressor, 1.5 L/min, China;

– UV light, capacity 15W, intensity of 875 Lux, wavelength 254 nm, USA;

– HY-5 shaking machine, shaking amplitude of 20mm, speed of 60 rpm, China;

– The system of self-made device of UV activated persulfate for oxidizing the AZOs: MO, AY, BT and waste water of textile dyeing villages.

Fig. 1. Diagram of a reaction device for UV heated activated persulfate process

1. Electricity supply 220V
2. Air compressor
3. Air duct
4. Glass tube
5. Quartz tube
6. UV light
7. Thermal bath
8. Heat bar
9. Thermometer

+ Operation principle:

Survey solution containing MO dye, nZVI powder and persulfate solution were added to the glass tube (4). The quartz tube (5) is put in the glass tube (4), inside the quartz tube is a UV lamp (6). The solution is stirred by an air compressor (2) through an air duct (3). The device works when the air compressor and the UV lamp are on.

In the event that heating is required to change the temperature of the reaction system, the UV activated persulfate device is placed in a thermostatic bath with water (7), with a heating bar (8) and the temperature adjusted and monitored (Fig. 1).

In the case that heating is required to change the temperature of the reaction system, the photo catalytic device is placed in a thermostatic bath with water (7), with a heating bar (8) and the temperature adjusted and monitored (Fig. 1).

Analytical methods

The concentration of MO in solution is determined by the HPLC. Retention times (t _R ) of MO 2.6 minutes. The pH solutions were adjusted to be at pH= 4.5, temperature t= 25 ^ C. Samples of survey solutions were collected in test tubes with a volume of 4 mL in test tubes always containing 1 mL of 0.01 M Na ₂ S ₂ O ₃ solution, capping and shake well. After that, they are taken to determine the concentration of azo dyes with corresponding standards on HPLC machine (Agilent 1100 Model G1314A Variable Wavelength UV-VIS Detector)

The reaction rate of AZOs decomposition is calculated and based on the change of AZOs concentration over time, the formula:

(1.1)

In which: r: Reaction rate;

C _t , C ₀ : Concentration of MO at time t and initial time;

t: Time variation.

Assuming the MO decomposition kinetics follows the pseudo first-order kinetics, reaction rate r, with the apparent reaction rate constant k _p , the MO decomposition reaction rate is calculated according to the formula:

r = k _p .C(1.2)

From formula (2.4) and (2.5) having:

(1.3)

or

(1.4)

To study the kinetics of the decomposition reaction of AZOs according to equation (2.7). It is need to graph the dependency ln(C/C ₀ ) — t. The slope of the line (1.74 is the apparent rate constant k _p of the MO decomposition reaction in units (minutes ^-1 ).

Results and discussions

The kinetic of the MO decomposition in systems without UV and with UV

These are the nZVI/PS systems and ZVI/PS/UV systems to study the decomposition kinetic in Fig 2,3.

Fig. 2. The MO decomposition kinetics equations in the nZVI/PS/MO systems (The conditions: C _ZVI = 0.5 g/L, C _PS = 1.0 mM, C _MO = 0.1 mM, pH= 4.5)

Fig. 3. The MO decomposition kinetics equations in the nZVI/PS/MO/UV systems (The conditions: C _ZVI = 0.5 g/L, C _PS = 1.0 mM, C _MO = 0.1 mM, pH= 4.5, I= 785 Lux, = 254 nm)

The experiments show that the decomposition efficiency of MO in nZVI/PS/UV systems and nZVI/PS are quite high. The nZVI and the UV are activating PS to generate free radicals SO ₄ ^−• , ^• OH. These free radicals have strong oxidation activity, have the effect of decomposing AZOs at high rate compared to normal oxidizing compounds, suitable for studies. The MO decomposition effect in the system with UV is higher than one without UV. Because the systems could happen reactions to form free radicals SO ₄ ^−• , ^• OH [5], [20], [22], [24], [26].

Fe ⁰ + S ₂ O ₈ ^2-  2SO ₄ ^2- + Fe ²⁺ , k= 1.5.10 M ^-1 .s ^-1

Fe ²⁺ + S ₂ O ₈ ^2-  Fe ³⁺ + SO ₄ ^ + SO ₄ ^{2 } , k= 2.0.10 ¹ M ^-1 s ^-1

Fe ²⁺ + SO ₄ ^  Fe ³⁺ + SO ₄ ^{2 } , k= 3.0.10 ⁸ M ^-1 s ^-1

SO ₄ ^ + SO ₄ ^  S ₂ O ₈ ^2- , k= 5.5.10 ⁸ M ^-1 s ^-

SO ₄ ^ + H ₂ O  HSO ₄ ^- + HO ^ , k = 2.10– ³ M ^-1 s ^-1

SO ₄ ^ + HO ^  HSO ₄ ^- + 1/2O ₂ , k= 1.5.10 ⁷ M ^-1 s ^-1

SO ₄ ^ + S ₂ O ₈ ^2-  S ₂ O ₈ ^{ -} + SO ₄ ^2- , k= 6.1.10 ⁵ M ^-1 s ^-1

HO ^ + HO ^  H ₂ O ₂ , k= 5.5.10 ⁹ M ^-1 s ^-1 ,

HO ^ + S ₂ O ₈ ^2-  S ₂ O ₈ ^{ -} + OH ^ , k= 1.2.10 ⁷ M ^-1 s ^-1

HO ^ + Fe ²⁺  HO ^ + Fe ³⁺ , k= 4.3.10 ⁸ M ^-2 s ^-1

Fe ²⁺ + H ₂ O ₂  Fe ³⁺ + HO ^ + OH ^ , k= 63.0 M ^-2 s ^-1

S ₂ O ₈ ^2- + UV  2SO ₄ ^ , k = 5.7.10– ⁵ s ^-1

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