Optimizing adsorption of Cr(VI) from aqueous solutions by NiO nanoparticles using Taguchi and response surface methods.

In the present study, nickel oxide nanoparticles synthesized by the sol-gel method were used as an effective adsorbent for the removal of Cr(VI) from aqueous solutions. To do so, the effect of four parameters including the concentration of Cr(VI), the dosage of NiO, contact time, and pH on the removal of Cr(VI) by NiO nanoparticles were studied. In order to examine and describe the optimum conditions for each of the mentioned parameters, Taguchi and response surface methods were used. The results of the experiment using Taguchi and response surface methods indicated the greater effect of the NiO adsorbent parameter in comparison to the other parameters in the adsorption of Cr(VI) by NiO nanoparticles, and showed that the increase in contact time and pH does not affect the removal percentage of Cr(VI) significantly.

Photocatalytic degradation of chloramphenicol in an aqueous suspension of silver-doped TiO2 nanoparticles.

In this work, silver-doped TiO2 (Ag/TiO2) nanoparticles were synthesized using a photodeposition technique. The prepared Ag/TiO2 nanoparticles were characterized using TEM, SEM, XRD, and EDX techniques. The characterization of Ag/TiO2 nanoparticles using SEM and EDX techniques revealed the dispersion of Ag metal on the surface of TiO2. The photocatalytic activity of Ag/TiO2 and bare TiO2 in the presence of ultraviolet irradiation was investigated in the removal of chloramphenicol (CAP) as an antibiotic. CAP is a broad-spectrum antibiotic exhibiting activity against both Gram-positive and Gram-negative bacteria, as well as other groups of microorganisms. However, it is, in certain susceptible individuals, associated with serious toxic effects in humans including bone marrow depression, particularly severe in the form of fatal aplastic anaemia. The effects of the operational factors, such as doping content of Ag, photocatalyst dosage and calcination temperature were evaluated in the catalytic activity of Ag/TiO2. The results showed that the photocatalytic efficiency of TiO2 nanoparticles for the degradation of CAP, can be significantly improved by deposition an optimum amount of Ag nanoparticles (0.96 wt%) in the calcination temperature 300 degrees C. It was found that 900 mg/L of Ag/TiO2 is the optimum dosage in the removal of CAP with 20 mg/L initial concentration. The highest removal efficiency of CAP (-100%) at the optimum conditions was observed in 20 min. A mineralization study under optimum conditions showed about 88% reduction in total organic carbon after 120 min of irradiation time.

Synthesis of TiO2 nanoparticles in different thermal conditions and modeling its photocatalytic activity with artificial neural network.

Titanium dioxide (TiO2) nanoparticles were prepared by sol gel route. The preparation parameters were optimized in the removal of 4-nitrophenol (4-NP). All catalysts were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). An artificial neural network model (ANN) was developed to predict the photocatalytic removal of 4-NP in the presence of TiO2 nanoparticles prepared under desired conditions. The comparison between the predicted results by designed ANN model and the experimental data proved that modeling of the removal process of 4-NP using artificial neural network was a precise method to predict the extent of 4-NP removal under different conditions.

Influence of operational parameters and kinetics analysis on the photocatalytic reduction of Cr(VI) by immobilized ZnO.

In the present work the performance of immobilized ZnO on a glass plate with the heat attachment method has been described for photoreduction of Cr(VI) to the less harmful Cr(III) at different operational parameters. The photoreduction of Cr(VI) on the surface of the immobilized ZnO catalyst was studied as a function of the pH of solution, initial Cr(VI) concentration and ultraviolet (UV) light intensity. Results indicated that the reduction rate decreases with increasing initial concentration of Cr(VI) and initial pH of solution, whereas it increases with increasing UV light intensity. The photoreduction rate of Cr(VI) on the surface of the immobilized ZnO in the presence of O2 as mobile gas is more than Ar and N2. The reduction process of Cr(VI) by immobilized ZnO also could be done under visible light irradiation. Pseudo first-order kinetics were observed for the photoreduction of Cr(VI) at different operational conditions. With non-linear regression analysis a mathematical kinetics model was developed for the pseudo first-order constant (k(ap)) as a function of operational parameters.

Investigation of the efficiency of a tubular continuous-flow photoreactor with supported titanium dioxide nanoparticles in the removal of 4-nitrophenol: operational parameters, kinetics analysis and mineralization studies.

In this study, to investigate the application of heterogeneous photocatalysis in the removal of organic contaminants from aqueous media a novel tubular continuous-flow photoreactor with supported TiO2-P25 on glass plates was designed and constructed. The photoreactor comprises six quartz tubes and a UV lamp which was placed in the center of the quartz tubes. 4-nitrophenol (4-NP) as a most refractory pollutant was chosen as a probe pollutant to examine the photoreactor efficiency for environmental cleaning applications. Results of experiments show that the removal efficiency of 4-NP in this photoreactor is a function of photoreactor length, gas and liquid flow rates and 4-NP initial concentration. Kinetics analysis indicates that degradation of 4-NP in continuous-mode can be modeled with the Langmuir-Hinshelwood (L-H) model (k(L-H) = 1.5 mg L(-1) min(-1), K(ads) = 0.11 mg(-1) L). A design equation was obtained with a combination of L-H modified equation and tubular reactor design equation. This equation can be used for estimation of 4-NP concentration in different photoreactor lengths under various operational parameters. Mineralization study was followed through total organic carbon (TOC) analysis and measurement of nitrite and nitrate as final degradation products.

Synthesis of Mg-Doped TiO2 nanoparticles under different conditions and its photocatalytic activity.

In this study, TiO(2)- and Mg-doped TiO(2) nanoparticles with different dopant contents were prepared by sol-gel method. The prepared photocatalysts were characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer-Emmett-Teller (BET) and diffuse reflectance spectroscopy (DRS) techniques. The results of BET analysis indicated a pore diameter of 8 nm and surface area of 48.5 m(2) g(-1). XRD patterns of pure and doped TiO(2) nanoparticles at 450 °C revealed that all phases are anatase. The particle size obtained from TEM was less than 20 nm. The band gap energy of Mg-doped TiO(2) nanoparticles was lower than that of TiO(2) . The photocatalytic activity of the pure and doped nanoparticles has been compared in the removal of C.I. Acid Red 27 (AR27). The photocatalytic activity of Mg-doped (0.2 mol%) TiO(2) for the degradation of AR27 was higher than that of bare TiO(2) nanoparticles. Results of total organic carbon analysis and changes in the AR27 UV-Vis peaks indicated 99% mineralization and extinguishing of all peaks in UV and visible regions is possible with Mg-doped TiO(2) nanoparticles. Removal efficiency of AR27 was sensitive to the parameters such as catalyst dose, pollutant concentration and light intensity.

Sol-gel low-temperature synthesis of stable anatase-type TiO2 nanoparticles under different conditions and its photocatalytic activity.

In this work, TiO(2) nanoparticles in anatase phase was prepared by sol-gel low temperature method from titanium tetra-isopropoxide (TTIP) as titanium precursor in the presence of acetic acid (AcOH). The effects of synthesis parameters such as AcOH and water ratios, sol formation time, synthesis and calcination temperature on the photocatalytic activity of TiO(2) nanoparticles were evaluated. The resulting nanoparticles were characterized by X-ray diffraction, UV-Vis reflectance spectroscopy, transmission electron microscopy and Brunauer-Emmett-Teller techniques. Photocatalytic activity of anatase TiO(2) nanoparticles determined in the removal of C. I. Acid Red 27 (AR27) under UV light irradiation. Results indicate that with increasing AcOH/TTIP molar ratio from 1 to 10, sol formation time from 1 to 3 h and synthesis temperature from 0 to 25°C, increases crystallite size of synthesized nanoparticles. It was found that optimal conditions for low temperature preparation of anatase-type TiO(2) nanoparticles with high photocatalytic activity were as follows: TTIP:AcOH:water molar ratio 1:1:200, sol formation time 1 h, synthesis temperature 0°C and calcination temperature 450°C.

Enhancement photocatalytic activity of ZnO nanoparticles by silver doping with optimization of photodeposition method parameters.

The photocatalytic activity of silver-doped ZnO was tested by photocatalytic degradation of C.I. Acid Red 88 (AR88) as a model contaminant from monoazo textile dyes. Silver-doped ZnO was prepared by photodeposition (PD) method. Results show that silver-doped ZnO is more efficient than undoped ZnO at photocatalytic degradation of AR88. The PD method parameters such as irradiation time, calcination temperature and silver content of doping were effective on the photoactivity of silver-doped ZnO. Silver content had an optimum value of 0.5% for achieving high photocatalytic activity. In the real wastewater from a textile-dyeing factory color removal with pure and silver-doped ZnO after 18 min of irradiation were 45 and 90%, respectively.

Design equation with mathematical kinetic modeling for photooxidative degradation of C.I. Acid Orange 7 in an annular continuous-flow photoreactor.

The decolorization of C.I. Acid Orange 7 (AO7), an anionic monoazo dye of acid class was investigated using UV/H(2)O(2) process in an annular continuous-flow photoreactor (ACFP) as a function of oxidant, dye concentrations, reactor length and volumetric flow rate. The removal efficiency of AO7 was a function of operational parameters and increased with increasing initial concentration of H(2)O(2) but it was low at high flow rate and initial concentration of AO7. Results indicated that the decolorization rate was pseudo-first order kinetic with respect to the dye concentration. A rate equation for decolorization of AO7 was obtained by kinetic modeling. Design equation for ACFP reactor was obtained with combination of kinetic model and rearranged tubular reactor design equation. Design equation was used for predicting concentration of AO7 and also electrical energy per order (E(EO)) at different conditions. The calculated results obtained from design equation and kinetic model were in good agreement with experimental data.

Effect of operational parameters on degradation of Malachite Green by ultrasonic irradiation.

The aim of the present study was to apply ultrasonic technique to remove Malachite Oxalate Green (MG) from aqueous solution. An ultrasonic bath with frequency of 35 kHz was used to investigate the effect of different operational parameters such as MG concentration, power density, temperature, mechanical agitation and addition of EtOH, 2-PrOH and iso-BuOH. Decolorization of MG follows a first order kinetics and hydroxyl radicals have an important role in degradation of MG. The apparent reaction rate constant (k(ap)) was influenced by variation of operational parameters. The activation energy was 30.95 kJ/mol in temperature range of 21-34 degrees C, suggesting a diffusion-controlled reaction. Alcohols act as hydroxyl radicals scavengers having undesirable contribution. UV-vis spectral change of MG showed hypsochromic shift occurred with increasing sonication time, proposing N-demethylation process of MG.

Increasing photoactivity of titanium dioxide immobilized on glass plate with optimization of heat attachment method parameters.

In the present work the optimization of heat attachment method for increasing photoactivity of immobilized TiO2 on glass plate was investigated. Results show that sonication time, TiO2 suspension dosage, immobilization temperature, solvent type and immobilization replications are very effective on the photoactivity of immobilized TiO2 on glass plate on the removal of C.I. Acid Red 88 (AR88) and optimizing these parameters increases the photoactivity of immobilized catalyst. In other step, the effect of operational parameters such as light intensity and initial concentration of AR88 on the removal of AR88 was investigated with four times immobilized TiO2 on glass plate. Results show that removal rate decreases with increasing initial concentration of AR88 but increases with increasing UV-light intensity.

The effect of particle size and crystal structure of titanium dioxide nanoparticles on the photocatalytic properties.

Titanium dioxide nanoparticles with different average diameter of particles (8, 18 and 27 nm) and different crystal structure (anatase and rutile) were used for degradation of C.I. Acid Red 88 (AR88) as a model compound. The degree of degradation of AR88 by TiO(2) nanoparticles under irradiation of UV-light (254nm) with TiO(2)-UV100 (8 nm diameter and 100% anatase) is lower than TiO(2)-Merck (27 nm diameter and 95% anatase). TiO(2)-P25 with structure of anatase and rutile (80:20) combination shows higher photoactivity than other samples such as TiO(2)-UV100 and TiO(2)-Merck. Kinetics of removal of AR88 with all of nanosized-TiO(2) particles is pseudo-first-order. The apparent reaction rate constant (k(ap)) for removal of AR88 in the presence of different nanosized-TiO2 particles decreases with increasing of AR88 initial concentration. For TiO(2)-P25 and TiO(2)-UV100 a linear relation and for TiO(2)-Merck a nonlinear relation between k(ap) and light intensity were obtained.

Nonlinear regression analysis of kinetics of the photocatalytic decolorization of an azo dye in aqueous TiO2 slurry.

The kinetics of decolorization of an anionic monoazo dye of acid class named C.I. Acid Red 27 (AR27) was investigated in the UV/TiO2 process with nonlinear regression analysis. The experimental results indicated that the kinetics of decolorization of AR27 in this process fit well by pseudo-first order kinetics. With nonlinear regression analysis a model was developed for pseudo-first order rate constant (k(ap,UV/TiO2)) as a function of operational parameters such as TiO2 dosage, initial concentration of AR27, concentration of O2 and UV-light intensity (I0) as following: k(ap,UV/TiO2) = 0.0025 [TiO2](0.65) [AR27]0(-0.96) [O2](0.16)I0. This rate expression can be used for predicting k(ap,UV/TiO2) at different conditions.

Photooxidative degradation of Acid Red 27 (AR27): modeling of reaction kinetic and influence of operational parameters.

The decolorization and mineralization of Acid Red 27 (AR27), an anionic monoazo dye of acid class, were studied by UV/H202 process in laboratory and real samples. Effects of different process parameters such as initial H2O2 and AR27 concentrations, pH and EtOH as an electron scavenger have been studied. H2O2 and UV light have a negligible effect when they were used on their own. The decolorization rate follows pseudo-first order kinetic with respect to the dye concentration. The rate constant of the attack of *OH radicals to the AR27 has been estimated through the adoption of a simplified kinetic model (1.03 x 10(8)M(-1)s(-1)). This model allows predicting the pseudo-first order rate constant and concentration of AR27 in different illumination times for different initial concentrations of H2O2. Mineralization studies showed, the formation of sulfate ions, the decrease of pH and 85% of COD reduction occur in less than 60 min. In the real wastewater color removal could be achieved after only 75 min, whereas 45% of COD reduction occurs after 105 min of illumination.