Removal of 2-chlorophenol from Aqueous Solution Using
Silica-supported Cu(II) Catalyst
Hrishikesh Sarma and K G Bhattacharyya
Department of Chemistry,
Gauhati University, Guwahati 781014, Assam, INDIA. email: [email protected].
(Received on: June 4, 2016)
ABSTRACT
Microbiological oxidation as is the common effluent treatment practice in organochemical and petroleum-based industries is not effective in mineralizing chlorinated phenols, which are persistent and toxic to organisms as well as humans even at low concentrations. Effluents containing these recalcitrant organics require tertiary treatment and catalytic wet oxidation has shown tremendous promise in this regard. In the present work, commercial silica impregnated with Cu(II) was used as a catalyst for oxidation of 2-chlorophenol (2-CP) in water with or without the presence of the common chemical oxidant, H2O2. The catalyst was prepared by
refluxing silica (60-120 mesh) with conc. H2SO4 and then with aqueous
CuSO4.5H2O for 6 h. The structural features of the catalysts were determined with
XRD and FTIR measurements and the amount of Cu(II) entering into silica matrix was estimated with atomic absorption spectrometry. The material was calcined at 873 K before using as a catalyst. The oxidation was carried out in a high-pressure stirred reactor at different temperatures and other process variables. As much as 77.3 % conversion of 2-CP could be achieved. Results showed that with increasing temperature, catalyst load and mole ratio of H2O2 in the feed, the oxidative
conversion could be further improved both in presence of the chemical oxidant and without it. Effects of various reaction conditions, kinetics of the catalytic process and the probable mechanism of oxidation have been discussed.
Keywords: XRD and FTIR measurements, 2-chlorophenol (2-CP), spectrometry .
1. INTRODUCTION
environment for long periods. 2-Chlorophenol and 2,4- dichlorophenol have both been recognized as priority pollutants by USEPA since 1976.1
Several successful processes can be applied at various stages of water treatment, and undoubtedly catalytic wet oxidation (CWO), using pure oxygen or air for oxidant, has proven its worth for wastewater treatment, particularly when organic contaminants at low or moderate loads are concerned. CWO is usually practiced between 100 and 300 ◦C and at pressures in the 1–10MPa range. Another far more economical alternative route to supercritical wet oxidation is to opt for a liquid-phase oxidant, such as hydrogen peroxide, that would allow the (wet peroxidation) conditions to decrease dramatically to atmospheric pressure and sub-boiling water
temperature2.
The degradation of pollutants by catalytic wet peroxide oxidation (CWPO) using the Fenton’s reagent is a well known process, the major drawback being the need to recover the iron catalyst at the end of treatment. To overcome this, new heterogeneous catalysts have been developed and studied since some years ago, which involve the incorporation of iron species into a solid matrix3-4.
These pollutants originate from a large number of different sources, the most important among them being the effluents of the petroleum and petrochemical industries, Kraft Mills, Olive Oil production, and various chemical manufacturing industries such as those making phenolic resins, herbicides, pesticides, solvents, paints, plastics and other chemicals5-6.
Microbiological oxidation as is the common effluent treatment practice in organochemical and petroleum-based industries is not effective in mineralizing chlorinated phenols, which are persistent and toxic to organisms as well as humans even at low concentrations7-8.
While it appears that alumina and silica supports improve the performance of heterogeneous iron-containing catalysts, the mechanism through which this occurs is not well understood. Possible explanations for the higher efficiency of iron/silica catalysts include less efficient scavenging of hydroxyl radicals by silica relative to iron oxide surfaces and more oxidant production due to the better dispersion of iron on the surface. In addition, alumina, as a Lewis acid, could facilitate the reduction of Fe(III) to Fe(II) by H2O2, usually
the rate limiting step in the Fenton’s reagent chain reaction, and thus accelerate activation of H2O2 9-10.
Effluents containing these recalcitrant organics require tertiary treatment and catalytic wet oxidation has shown tremendous promise in this regard. The present work evaluates the suitability and efficiency of catalyst obtained by incorporating Cu(II) into Silica by impregnation with respect to wet oxidation of 2-Chlorophenol(2-CP) in water with or without the presence of the oxidising agent,H2O2. The reaction conditions were varied to
2. MATERIALS AND METHOD 2.1. Chemicals used
The following commercially available chemicals were used without further purification: (i) Silica Gel (60-120 Mesh) (E. Merck)
(ii) CuSO4. 5H2O (E. Merck)
(iii) H2SO4 (98%, BDH Analytical Grade)
2.2. Synthesis of Cu (II) supported on Silica
It is synthesized by refluxing Silica Gel (60-120 mesh) with 0.5M H2SO4 for 6 hr,
filtered, washed, dried in oven at 120 0C and then refluxed with 100 ml 1M Cu(SO4)2.5H2O
for 6hr,filtered, washed, dried and calcined at 773 K.
2.3. Characterization
Cu (II)-Silica synthesis was characterized by FT-IR measurements (Perkin-Elmer Spectrum RXI, range 4400–440 cm−1) using KBr self-supported pellet technique. The percentage of Cu(II) entering into silica was determined with atomic absorption spectrophotometer (Agilent Spectra AA 240). The catalysts were further characterized by XRD measurements (Philips Analytical, PW 1710, Cu K_ radiation) and comparing the same with known XRD patterns of silica.
2.4. Wet Oxidation of 2-CP
Catalytic oxidation was carried out in a high-pressure stirred reactor (Toshniwal Instruments, India) with equal volumes (25 mL each) of the reactant (2-CP: 2×10−3 M) and H2O2 (2×10−3M), catalyst load of 2 g/L at 353K under an autogenous pressure of 0.2 MPa
and stirrer speed of 180 rpm for 6 h. The reactants were mixed together at room temperature, introduced into the reactor and then the heating was started to obtain the desired temperature. When evaluating the effects of a particular variable, appropriate changes were made in the values of the variable. When no H2O2 was used, the total volume was kept at 50 mL. After
the reaction was over, the mixture was centrifuged (Remi Research Centrifuge, R24) and the unconverted reactant was estimated in the supernatant layer spectrophotometrically (Hitachi UV-visible U3210).
3. RESULTS AND DISCUSSION 3.1. Characterization of the catalysts
3.1.1. FT-IR study
The FTIR analysis of Cu(II)Silica shows a broad band of stretching vibration of hydroxyl group on its surface at 3430.33 cm-1. The bands at 1071.1 cm-1 and 805.51 cm-1 are assigned to Si-O stretching vibration and Si-O-Si bending vibration. The band at 1649.23 cm-1 corresponds to –OH bending vibration. The band at 520.12 cm-1 corresponds to Cu-O.
4000 3500 3000 2500 2000 1500 1000 500
0 10 20 30 40 50 60 % T
Wavenumber (cm-1)
Cu(II)-Silica
Silica Gel
Fig1: FTIR spectra for Silica (bottom) and Cu(II)-Silica(top)
3.1.2. XRD study
Fig.2 shows X-ray powder diffraction pattern silica and supported Cu(II). X-ray powder diffraction of silica impregnated with Cu shows one amorphous peak located at 2θ = 21.8o XRD pattern of the modified silica completely matched with that of the parent silica,
which indicates that the modification has no obvious effect on the parent silica structure. There was also no new phase formation during heat treatment and modification. The XRD pattern of the modified silica indicates that the silica structure remains intact after loading copper metals on to it. No peaks related to the metals were found from the diffractograms.
0 10 20 30 40 50 60
0 50 100 0 20 40 60 80 100 120
0 10 20 30 40 50 60
In
te
nsi
ty
2-Theta(o)
Silica Cu(II)-Silica
3.1.3. AAS measurement
AAS measurement shows the amount of Cu in Silica supported copper catalyst is 5.27 mg g-1
3.2. Wet oxidation of 2-CP
3.2.1. Blank Experiment
Before investigating the effectiveness of the catalyst for wet oxidation of 2-CP in water (2 X 10-3 M), a set of blank experiment were carried out for the following system. (i) Reaction without catalyst and H2O2
(ii) Reaction with H2O2 (mole ratio 1:1) and without catalyst
(iii) Reaction without H2O2 and with catalyst (2 gL-1 )
(iv) Reaction with H2O2 (mole ratio 1:1) and silica as catalyst (2 gL-1)
Under the same condition of temperature and pressure and time interval(5h), no measurable conversion were observed for (i) and (iii), however, a small amount of decomposition observed in (ii) 10.4% and (iv) 36.1% due to presence of H2O2.
.
3.2.2. Effect of Reaction time
In the time interval of 30-300 min 38.9 to 82.3% conversion of 2-chlorophenol could be observed. The equilibrium time was found to be 210 min.
Fig3: Effect of reaction time on oxidation of 2-CP with Cu(II)-Silica as catalyst with H2O2 (reactant: H2O2
mole ratio 1:1) at 343K (catalyst load 2g/L, 2-CP 2 x 10-3M)
3.2.3. Effects of reactant concentration
When the concentration of reactants were increased from 2x10-4 M to 12x10-4 M keeping the concentration of H2O2 constant in the reaction mixture, the conversion comes
Fig3: Effect of reactant concentration on wet oxidation of 2-CP for the catalyst Cu(II)-Silica at 343K (catalyst load 2g/L) for reaction time of 300 min
3.2.4. Effect of Catalyst load
Five different catalyst loadings of 2, 4, 6, 8 and 10 g/L were used to carry out the oxidation reactions at constant reaction time of 300 min and reactant: H2O2 mole ratio of 1:1.
The results (Fig. 4) show that the catalyst load did not have much influence on the conversion.
Therefore the possibility of either reduction in the active phase or conversion of transition metal cation to lower oxidation state may be ruled out. The catalyst load of 2 g/L was found to show maximum percentage of conversion
Fig.4: Effect of catalyst load on wet oxidation of 2-CP (2x 10-3M) using Cu(II) -Silica as catalyst with H 2O2
(reactant: H2O2 mole ratio 1:1) at 343K for reaction time of 300 min
3.2.5. Effect of mole ratio of the reactants
To investigate the effects of increasing amount of H2O2, the mole ratio of hydrogen
peroxide and the reactant was increased from 1:1 to 20:1 with respect to H2O2 for a constant
number of moles of H2O2, up to 91.3% conversion of 2- chlorophenol could be achieved. It
was obvious that a large excess of H2O2 did not particularly help the conversion. At the
comparatively high temperature H2O2 itself may undergo some amount of decomposition
aided by the presence of the catalyst. Such decomposition has been recognized as a factor determining the overall rate of reaction as well as the product yield.
Fig.5: Effects of mole ratio of feed on catalytic wet oxidation of 2-CP (2x10− 3M) using Cu(II)-Silica as the catalysts with H2O2 at 343 K (reaction time 300 min, catalyst load 2 g/L).
3.2.6. Kinetic Study
The reaction rates of oxidative degradation have been tested for first order kinetics, by plotting log Ct versus time from the first order relation:
Ct = C0e-kt or, log Ct = log C0 – (k/2.303) t
The plots of log Ct (average of three different sets of measurements under identical
conditions) vs. reaction time gives linear plots indicating first order kinetics. The slope of the plot gives the apparent kinetic constant, k.
Fig 6: First order degradation of reactants 2-CP (2 X10-3) by wet oxidation over impregnated Cu(II)-Silica
The plots are linear with regression coefficient of 0.971 for 2-chlorophenol. The first order rate coefficient (k) is 3.5 x 10-3.
4. CONCLUSION
Incorporation of transition metal Cu(II) into silica could convert it into a very active catalyst for treating water contaminated with 2-chlorophenol. The results of the present study shows that the conversion of 2-CP increases smoothly with increase in the reaction time from 30 to 300 min. A small amount of catalyst (2g/L) was sufficient for maximum oxidative destruction of 2-CP in water. The main advantage of the present work is that 2-CP could be oxidized to harmless end products which do not have persistent nature and are likely to undergo complete mineralization.
REFERENCES
1. Pera-Titus M., Garcya-Molina V., Banos M.A., Gimenez J., Esplugas S., Appl. Catal. B:
Environ. 47, 219–256 (2004).
2. Nicolas Crowther, Fa¨ıçal Larachi; Applied Catalysis B: Environmental 46, 293–305 (2003).
3. Quintanilla A., Fraile A.F., Casas J.A., Rodriguez J.J.; J. Hazard. Mater. 146,582 (2007). 4. Melero J.A., Calleja G., Martinez F., Molina R., Lazar K., Micropor. Mesopor. Mater.
74, 11 (2004).
5. Chaliha A., Bhattacharyya K. G.; Chemical Engineering Journal 139, 575–588 (2008). 6. Bauer R., Waldner G., Hager S., Malato S., Maletzky P.; Catal. Today. 53, 131–144
(1999).
7. Oxana P. T., Andrey N. Zagoruiko; Chemical Engineering Journal, 14, 201 -205 (2015). 8. J. Arana, E. Tello-Rendon, J.M. Dona-Rodriguez, C. Valdes do Campo, J.A.
Herrera-Melidan, O. Gonzalez-Diaz, J. Perez-Pena, , Water Sci. Technol. 44, 229–236 (2001). 9. Lim H, Lee J, Jin S, Kim J, Yoon J, Hyeon T.; Chemical Communications; 4:463–465
(2006).
