The Use of Copper oxides , Supported Onto Biogenic Silica from The Leaves of Dendrocalamus asper for the degradation of Congo red ANTI

Silica, which may be isolated from leaves, is one of the better known catalyst supports having a large surface area and high stability. Photocatalysts of the type CuOx@SiO2 were prepared using biogenic silica isolated from Dendrocalamus asper leaves, and evaluated for Congo red degradation. The photocatalysts of CuOx@SiO2 (CuOx=0.00%,0.25%, and 0.50%) was obtained by impregnating the silica in the Cu(NO3)·3H2O solution. A combination XRD and SEM-EDX techniques confirmed the formation of CuOx as the active phase of the CuOx@SiO2. The UV-Vis spectroscopy method was used to determine the maximum absorbance and bandgap of the catalysts. Photodegradation assays showed that copper oxides supported onto biogenic silica isolated from Dendrocalamus asper leaf was efficient for Congo red degradation, and that the material worked well in the presence of both UV and visible light.


INTRODUCTION
The textile industry in Indonesia has expanded considerably over the last two decades, [1][2][3][4] resulting in the discharge of a large volume of wastewater, containing organic contaminants, to the environment.A significant proportion of the organic compounds discharged can be classified dyes due to their strong absorption of visible light.This absorption of visible light effectively decreases the amount of light available for photosynthesis and can have a significant adverse impact on biological cycles.[6][7][8] Many wastewater treatment techniques have been developed over the centuries.Among these, the oxidative decomposition of the organic waste by hydroxyl radicals (OH•) has emerged as one of the most promising methods for wastewater 0][11] The OH• radical may be produced by UV or visible lights on semiconductors.The semiconductors generate electron-hole pairs, which may react with the surface hydroxyl groups, and oxygen molecules absorbed on the surface of the catalyst, thus yielding hydroxyl radical and the superoxide radical ion, respectively.Another possible mechanism involves the direct reaction between the photo-generated holes and the organic pollutant.3][14] Copper oxides are being studied as potential catalysts for the treatment of organic pollutants in wastewaters.6][17] In the current work we have explored the use of a biogenic silica support that is isolated from the Dendrocalamus asper leaves.This is readily available in Indonesia.
Here we describe the preparation of a copper oxide photocatalyst prepared using a biogenic silica support CuOx@SiO 2 and describe its performance using Congo red as a model for organic pollutants.

MATERIALS AND METHODS
Biogenic silica was isolated by calcining 100 g of dry Dendrocalamus asper leaves at 650 o C for 4 h, following by air cooling, and washing with 0.1 M HCl solution.The photocatalyst was prepared by mixing 1.9848 g of biogenic silica and 0.0152 g of copper nitrate [Cu(NO 3 )•3H 2 O (Merck & Co) (0.25 mol% copper in silica) in 5 mL of distilled water.
The mixture was then stirred for 2 h at which time 5 mL absolute ethanol was added and the solid isolated by filtration and air dried.The dry sample was then calcined at 800 o C for 4 h, resulting in biogenic silica supported copper oxide (CuOx@SiO 2 ).The catalysts with 0.5 mol% of copper were prepared by the same procedure except using 0.0304 g of copper nitrate.
The adsorption behaviour of the catalysts was determined as follows.A 10 mL of Congo red solutions with initial concentrations between 23 and 24 ppm were added to 0.02 g of catalysts powders and the mixture shaken for 24 h at which time the solid was removed from the solution by centrifuge at a rate of 1500 rpm for 10 minutes.The amount of Congo red adsorbed by the catalyst was estimated by the change in the absorption at 498.2 nm.
The band gap of the samples was estimated using UV-Vis spectroscopy.For these measurements the CuOx@SiO 2 samples were thoroughly mixed with triton-x and acetylacetonateto form a paste, which was then layered onto a glass slides.The slides were heated at 450 o C for 2 h to remove the organic material.
The photocatalytic activity of the catalysts was determined by measuring the concentration of the un-reacted Congo red estimated from the intensity at 498.2 nm.

RESULTS AND DISCUSSION
The crystalline materials CuOx@SiO 2 (CuOx=0.00%,0.25%, and 0.50%) were successfully prepared using an impregnation method.The XRD patterns of the samples are similar to that of cristobalite SiO 2 (Fig. 1).The positions of the peaks and their relative intensities are in good agreement with those in JCPDS card 76-0936.The extra peaks are not observed except for the trydimite SiO 2 (JCPDS card 42-1401).Lines shifting are observed indicating supported copper oxides perturb the SiO 2 lattice.This is though to be a consequence of copper oxides doped in the SiO 2 lattice.
The size of the CuOx@SiO 2 crystallites was estimated using the Scherrer equation. 21The Scherrer equation is a formula that relates the size of sub-micrometer, or crystallites, in a solid to the broadening of a peak in a diffraction pattern.
The Scherrer equation can be written as: (1)   Where τ is the mean size of the crystallite, which may be smaller or equal to the grain size; K is dimensionless shape factor, which has a typical value of about 0.9, but varies with the actual shape of the crystallite; λ is the X-ray wavelength; β is the line broadening at half the maximum intensity (FWHM), after subtracting the instrumental line broadening, in radians; and θ is the Bragg angle (in degrees).The crystallite size of the CuOx@SiO 2 crystallites is about 71nm (Table 1.), consequently these can be classified as nanoparticles.A combination of SEM and EDX methods were used to establish the distribution of Cu in the biogenic silica supported samples.The SEM micrographs of CuOx@SiO 2 (CuOx=0.00%,0.25% and 0.50%) are depicted in Fig. 2 and reveal considerable roughness and porosity.Both of these effects will increase the effective surface area of the sample, a property that is important in catalysis but they may also impact on the mechanical and thermal properties of the material. 23It is not possible to identify any copperoxides, from the SEM images, reflecting the uniform distribution of the Cu on the biogenic silica support and/or its a trial substitution into the silica lattice.EDX analysis confirms the presence of Cu in all the samples and indicates that the CuOx@SiO 2 samples have, within acceptable errors, the targeted Cu contents.Ta b l e 2 s u m m a r i s e s t h e U V-V i s spectroscopy of the sample and for each sample an electronic transition was obser ved.The Kubelka-Munk equation was used to calculate the band gap energy (Eg) for the CuOx@SiO 2 samples as depicted in Fig. 3.In general, the addition of CuOx lowers band gap energy which is expected to increase the photocatalytic activity of the materials when irradiated with visible light.
The adsorption of the Congo red by the CuOx@SiO 2 samples was estimated from the change in the intensity at 498.2 nm for samples after 24 hours in a closed chamber without a light source.These measurements used 0.02 g of CuOx@SiO 2 catalyst and the concentrations of Congo red emplo yed were 2, 4, 6, 8 and 10 (x10-6 mol/L).The catalyst adsorption capacity w a s c a l c u l a t e d u s i n g t h e L a n g m u i r a n d Freundlich equations.The Langmuir isotherm pattern was determined by correlating the Congo red concentration adsorbed per 1 g of catalyst (c/m) with Congo red concentration after 24 h (c).The adsorption capacity value (b) on the Langmuir isotherm is given by: (2) The Freundlich isotherm is obtained by plotting the log value of the amount of Congo red adsorbed per gram of catalyst (log x/m) with the log of Congo red concentration after 24 h (log c).The adsorption capacity (k) of the Freundlich isotherm was calculated by: (3) The R 2 values of the Langmuir adsorption isotherm pattern are higher than that of the Freundlich isotherm.Thus, we can conclude that adsorption on Congo red degradation by CuOx@SiO 2 catalysts follows Langmuir isotherm pattern.The SiO 2 has the lowest adsorption capacity (Table3).This is due to SiO 2 -supported copper oxide having a larger surface and a greater ability to adsorb CuOx@SiO 2 than the pure SiO 2 .
The photodegradation was performed using 0.02 g CuOx@SiO 2 for every 10 ml of 23 to 24 ppm Congo red solutions.The mixtures were placed in a shaker covered by closed box equipped with Evaco20Watt fluorescent light.Measurements were undertaken at variations of time ie.4, 8, 12, 16, 20 and 30 minutes.The absorbance was measured at a wavelength of 498.2 nm using Spectronic 20.The absorbances were then referred to the standard Congo red curve equation to obtain the Congo red concentration (Figure 4). Figure 4 shows the time dependence of the concentration of Congo red upon exposure to UV light.This proves that t h e degradation of Congo red is effectively performed by the CuOx@ SiO 2 catalysts, under UV light.Fig. 4 also indicates that the CuOx@SiO 2 with CuOx 0.25% and 0.50% are more efficient catalysts than SiO 2 alone.There is no significant difference between the effectivity of CuOx@SiO 2 with CuOx 0.25%, and 0.50%.
The photodegradation of Congo red under visible light was undertaken using the same procedure used for Congo red photodegradation under UV light, but the light source used was a Philips ML 100W/220-230E27.Fig. 5 illustrates that the concentration of Congo red solution is inversely proportional to the exposure time of visible light.The catalysts show a good performance undervisible light.The performance of CuOx@ SiO 2 with CuOx=0.50% in degrading Congo red is better than that of SiO2 and CuOx@SiO 2 CuOx= 0.25%.The reaction order was determined for the photo-decomposition of Congo red through consideration of the reaction rate.The largest value of R 2 was used to determine the order of reactions.The zero-order reaction was determined by integrating the equation of the reaction rate, as follows: (4) Where C t is the concentration of Congo red at time t, k is the rate constant, t is time and C 0 is the initial concentration of Congo red.By plotting C t vs. t, we obtain the equation of the line for the zero-order reaction (Table 4).
The first-order reaction was determined by integrating the rate of reaction to obtain the equation of the reaction, as follows: (5) (6)   By plotting ln C t vs. t, we obtain the equation of the line for the first-order reaction (Table 5).
The second-order reaction was determined by integrating the rate of reaction to obtain the equation of the reaction, as follows: (7)   By plotting 1/C t vs. t, we obtain the equation of the line for the second-order reaction.
By comparing the values of R 2 of the three reaction order, the rate of Congo red photo-degradation reaction using catalysts CuOx@ SiO 2 (CuOx=0.00%,0.25%, and 0.50%) under UV light and visible light is determined.As the second-order kinetics curve has the highest R 2 value than that of zero-and first-order, so the order of the reaction is second-order.
The decreasing trend of the concentration of Congo red in the degradation process using CuOx@ SiO 2 with CuOx=0.00%,0.25% and 0.50%, under UV light and visible light can be seen in Figure 6.
In the UV photodegradation of Congo red undoped SiO 2 was more effectively than the other catalysts.But, the CuOx@SiO 2 performs conversely, and the CuOx@SiO 2 with CuOx=0.50%indicates the most effective catalysis compare to the rest of catalysts.This is a consequence of the undoped SiO 2 having a band gap energy t h a t o n l y work sunder UV light, while CuOx@SiO 2 works on both UV and visible light.The CuOx is probably blocked and hence reduces the catalytic ability of the catalysts.