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Removal of nonyl phenol ethoxylates in water by catalytic ozonation in presence of silica supported-cobalt nanoparticles

The catalytic ozonation of nonionic surfactant nonylphenolethoxylate (NPE) as pollutant in wastewater and its degradation in the presence of silica-supported Cobalt oxide nanoparticles was studied. Characterization of silica supportedcobalt oxide was made using XRD patterns and SEM profiles. The influence of pH, initial NPE concentration, ozonation time and catalyst contents in ozonation process was also investigated. Results show that NPE removals by using silica supported-cobalt oxide catalytic systems are higher than that of using single ozonation. About 99% NPE were removed within 10 min at 30 o C. Furthermore, in this condition more than 50% of total carbon of NPE was mineralized.


INTRODUCTION
Nonyl phenol Ethoxylates (NPEs) are nonionic surfactants belongs to alkyl phenol ethoxylates family having structure as following: Commercial NPEs has n value being from 2 to 16. They are widely used in industrial production such as agriculture, leather, metal, petroleum, pulp and paper, paints, adhesives, coatings, cleaners…NPEs decomposes in strong bases, strong acids or strong oxidizing agents. The shorter length of ethoxylate chain, the more toxic NPEs are. The decomposition of NPE could produce hydrophobic NP, NP1EO, NP2EO having smaller biodegradation rate [1][2] and the carboxylic acid nonylphenols (NP2EC or NP1EC), NPEO having toxicity higher NPE.
Among the methods for treating NPE and other pollutants, ozonation and other advanced oxidation processes (AOPs) are paid more attention since they are "environmental friendly". Based on hydroxyl-free radicals (*OH)

TAÏ P CHÍ PHAÙ T TRIEÅ N KH&CN, TAÄ P 19, SOÁ K6-2016
Trang 51 which are immediately generated during the reaction they can decompose the compounds into reaction products with less toxicological effect rather than simply separating them from the flow (such as adsorption or membrane processes). Ozonation is often applied in oxidation processes thanks to its advantages among other common oxidation agents. The oxidizing agent creates the hydroxyl radicals (*OHs) such as H 2  SEM images were used to characterise the catalyst.

Reagents
All chemicals were analytical grade and used without further purification. Nonylphenol Ethoxylate (NPE), silica were purchased from Sigma-Aldrich. Cobalt (II) nitrate hexahydrate, urea, acetonitrile, ammonium acetate, ammonium chloride were from Merck.

Synthesis of Co 3 O 4 -SiO 2
The material containing cobalt oxide supported on silica used as catalyst was prepare using sol-gel method [9]. In a typical experiment, 1g of silica was dispersed in 500mL of 10mM cobal tnitrate hexahydrate solution and sonicated for 3 min. After that, 10g of urea was added. The mixture was heated at 85 o C and stirred for 6 h.
During the reaction, the color of the mixture changed from pink to violet indicating the formation of α-cobalt hydroxide. The mixture was then cooled to room temperature, filtered using Munktell ® paper, washed with Millipore ® water several times and dried overnight in the oven. The obtained product was calcined in a muffle furnace (Lenton thermal ® ) under air at 500 o C for 3 hours at heating rate of 2 o /min. The as-prepared product would be Co 3 O 4 -SiO 2 .

Characterization of catalyst
Structural analysis of the synthesized samples was carried out using powder X-ray diffraction on a Brucker AXS D8 diffractometer over the 2θ range of 10-90 o and the scan rate was of 1 o /min. Copper was used as the target (Cu-Kα; λ = 1.5406 Å).
Morphological studies of the samples were carried out using Hitachi S-4800 II Field Emission Scanning Electron Microscope (SEM) with light element analysis using energy dispersive spectroscopy (EDS) operating at 10 kV.
Nitrogen adsorption-desorption measurement was also made at 77K using BET method for specific surface area of the catalyst.

Ozonation of NPE9 with Co 3 O 4 -SiO 2
Ozone was produced from dry air by use of Vina Ozone Generator model VN3 using Cold Plasma Technology.

Effect of initial pH on NPE catalytic ozonation
In the NPE ozonation, the hydroxide ions plays an important role in initiating ozone decomposition which involves a series of reactions as follows [9,10]: During the reaction as an oxidant the ozone molecule has selective electrophility for an interaction with amines, phenols and double bonds in aliphatic compounds. In appropriate medium, the ozone decomposition may also generate active secondary oxidants (mainly O 2and OH radicals have higher potential and no selectivity) to oxidize molecules. Ozone is more selective than hydroxyl radical but the latter is stronger in reaction. Thus the reaction paths of ozone and organic compounds determined pH which changed the redox reactions also the amount of OH radical [7,8]. The plots in Fig.5. showed the apparent effect of initial pH on the ozonation efficiency.
With Co 3 O 4 as catalyst the ozonation efficiency was better than that without catalyst at any pH.
After 10 minutes, with catalyst, the NPE ozonation yield reached 88% at pH=11 in comparison to 80% at pH=7 and 85% at pH=4. It is noted above that the mixed valency of cobalt atoms in Co 3 O 4 catalyst was important for electron transport. The ability of electrons to transform between various oxidation states of the metallic ions determined the efficiencies of catalysts in redox reactions. This result is similar to our previous work concerning OMS-2 as catalyst in ozonation [3].
For the ozone decomposition on the surface of Co 3 O 4 we propose following scheme: The Co 2+ ions could adsorb ozone so they accelerate ozone to react. In other words, the ozone decomposition could be favored by the presence of redox couple Co 2+ , Co 3+ controlled by pH values. In ozonation the transport of ozone into the solution was very important. An increase of ozone flowrate could conduct a positive reponse to ozone content in solution. This was an acceleration for the reaction and an increase of ozonation efficiency. As shown in Fig.6

Effect of initial NPE concentration
The initial NPE concentration in rivers, streams, ponds, lakes, was strongly various in the range of 5-30ppm. It can be seen on Fig.7. that in the same condition, on increasing initial NPE concentration, the decomposition efficiency decreases.
With a small NPE concentration of 5ppm, it took only 2 minutes to decompose almost completely NPE in water. In the same condition, NPE concentration increased fourfold, the ability to treat only reached 65%. It is noted that with an important content of NPE in medium, the rapid formation of foam caused great difficulty to the mass transfer in ozonation process.  The Langmuir adsorption isotherm of NPE on Co 3 O 4 -SiO 2 presented on Fig.9 showed the linear relation as equation y = 0,13x + 0,6 (R 2 = 0.975) where C is equilibrium concentration after adsorption and a is adsorption capacity.

Effect of ozonation time
The efficiency of NPEs ozonation gradually increased with the increase of reaction time and showed in Fig. 10. Besides, during the NPEs ozonation decomposition, the control of physical properties of the aqueous solution was fulfilled. The conductivity was slightly increased from 2 to 6μS/cm in 10 mins and pH was very slightly changed from 6.88 to 7.06 (Fig.11)

CONCLUSION
The