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Kinetic studies on the photocatalytic degradation of aliphatic carboxylic acids were carried out in a slurry photoreactor with in-situ monitoring, employing artificial UV light as the source of energy and nano-TiO2 powder as the catalyst. The influences on the photocatalytic degradation such as the initial concentration of reactant (C0), catalyst d...
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... result indicates that the k values of these aliphatic carboxylic acids increase with the increase of pH at low pH values, and decrease further on. A peak value of k appeared at pH value near the pKa value for monocarboxylic acids and between pK a1 and pK a2 for dicarboxylic acids (see in Table 4). Similar results were also reported in the degradation of formic acid [27][28][29]. ...
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... The size distribution of CDs was found to be influenced by the strength of the AHAs. Among the three alpha-hydroxy acids tested, GA had the highest pKa 1 value (3.7), [35] followed by CA (3.13) [36] and TA (2.98) [37] (Table 1). There is a trend and relationship between size and pKa (high pKa led to larger size), and therefore, pKa plays an important role. ...
The exhaustion of the ozone layer in the atmosphere has caused an escalation of dangerous UV radiation penetrating the earth's surface. Reactive oxygen species generated under UV exposure through photocatalytic reactions can lead to severe skin health risks, such as skin cancer and premature aging. Although chemical sunscreens are widely used for protection against UV radiation, their long‐term use poses safety concerns and causes environmental damage. In this regard, we have developed carbon dots (CDs) with excellent UV absorption along with extraordinary antioxidant activity, photostability, and biocompatibility from green tea as the main precursor and alpha‐hydroxy acids (AHA) as the co‐precursors, including gluconic acid (GA), citric acid (CA), and tartaric acid (TA). All CDs have a strong UV absorption, especially in the UVB and UVC regions, with G‐CDs having the highest UV absorption. The G‐CDs also have the highest antioxidant activity, which is also higher than that of their parent green tea, AHAs, and ascorbic acid, a well‐known antioxidant. The photostability and cell viability of the G‐CDs were also excellent. The potential of G‐CDs as multifunctional materials makes them a promising candidate for cosmetic applications, particularly for their ability to absorb UV radiation with exceptionally high antioxidant activity, photostability, and biocompatibility.
... These mechanisms occur simultaneously, but not to the same extent, and consequently, pathways and formed transformation products of organics would differ, affecting the overall quality of treated water in terms of toxicity and biodegradability [16][17][18]. Hence, Chen et al. [19] investigated the kinetics of photocatalytic degradation of aliphatic carboxylic acids by UV/TiO 2 ; the results revealed that the degradation mechanism and its efficiency depend on structural features of studied aliphatics, particularly the number of carboxylic groups-more -COOH groups would yield higher degradation rate. However, they did not provide deeper investigation of occurring mechanisms. ...
... Since CATS2D_01_DN has a positive index in Model (2), negatively contributing to the K value, compounds with carboxylic group are preferably degraded by O 2 • − . Although Chen et al. [19] emphasized the importance of carboxylic group moiety in photocatalytic degradation, a deeper correlation with the occurring mechanism and undergoing pathway was not provided. Actually, they assumed that this moiety was important within the adsorption step, which would then lead to direct degradation at the catalyst surface. ...
Although heterogeneous photocatalysis has shown promising results in degradation of contaminants of emerging concern (CECs), the mechanistic implications related to structural diversity of chemicals, affecting oxidative (by HO•) or reductive (by O2•−) degradation pathways are still scarce. In this study, the degradation extents and rates of selected organics in the absence and presence of common scavengers for reactive oxygen species (ROS) generated during photocatalytic treatment were determined. The obtained values were then brought into correlation as K coefficients (MHO•/MO2•−), denoting the ratio of organics degraded by two occurring mechanisms: oxidation and reduction via HO• and O2•−. The compounds possessing K >> 1 favor oxidative degradation over HO•, and vice versa for reductive degradation (i.e., if K << 1 compounds undergo reductive reactions driven by O2•−). Such empirical values were brought into correlation with structural features of CECs, represented by molecular descriptors, employing a quantitative structure activity/property relationship (QSA/PR) modeling. The functional stability and predictive power of the resulting QSA/PR model was confirmed by internal and external cross-validation. The most influential descriptors were found to be the size of the molecule and presence/absence of particular molecular fragments such as C − O and C − Cl bonds; the latter favors HO•-driven reaction, while the former the reductive pathway. The developed QSA/PR models can be considered robust predictive tools for evaluating distribution between degradation mechanisms occurring in photocatalytic treatment.
... To represent the kinetic behavior decomposition of TMP and MNZ, the Langmuir-Hinshelwood model was applied. The reaction obeyed pseudo-first-order kinetics, as indicated by the relation between ln (C o /C t ) and time, as well as the high coefficient R 2 > 0.95 (Chen et al. 2009). Figures 7b and 8b are used to calculate the rate constant values for all synthesized catalysts in both TMP and MNZ, and listed in Table 1. ...
A novel approach to synthesize porous Au/TiO2 nanocomposites has been achieved through a pyrolytic strategy by employing NH2-MIL-125(Ti) as a TiO2 precursor, and photo-deposition of Au nanoparticles (NPs) onto porous nanocrystalline TiO2 with varying Au contents (0.05–0.5%). TEM images of Au/TiO2 nanocomposites showed that TiO2 particles were spherical structures, highly dispersed, and homogeneous with diameters of 10–15 nm, and Au NPs (20–30 nm) were anchored onto porous TiO2 matrices with a uniform distribution. The synthesized Au/TiO2 nanocomposites were assessed through the degradation of two antibiotic models, metronidazole (MNZ), and trimethoprim (TMP), under visible light and compared with undoped TiO2 and commercial TiO2 (P-25). The synthesized Au/TiO2 photocatalyst revealed enhanced photocatalytic performance in the mineralization (80%) and degradation (100%) of MNZ and TMP in both water matrices compared to undoped TiO2 (60%, 76%) and commercial P-25 (48%, 65%). The obtained 0.1% Au/TiO2 nanocomposite could complete the mineralization of TMP and MNZ with rate constant values (4.47 × 10−3 min−1 and 5.23 × 10−1 min−1) owing to the large well-developed porosity and high surface area of TiO2 and the small size of Au NPs with high dispersity, surface plasmon resonance, and stability. The recyclability of the 0.1% Au/TiO2 nanocomposite exhibited high durability without the leaching or loss of photocatalytic performance after four cycles. Complete degradation was achieved within 100 min in the water matrix from real wastewater, indicating promising results for the degradation of pharmaceuticals in the different water matrices. The present work opens a new route to synthesize low-cost, effective, and high photocatalytic performance nanocomposites with a small Au content as a cocatalyst onto semiconductor materials.
... These photo-deposited metal nanoparticles on the TiO 2 surface act either as electron trappers or as sensitizers to enhance the lifetime of electrons and holes (Kaur, Singh, and Pal 2015). Enhanced photo-activity of metal-loaded TiO 2 was reported by Kaur and Pal (Kaur and Pal 2012) and a similar finding revealing the efficiency of metal-loaded TiO 2 for photodegradation was studied by Chen et al. (Chen et al. 2009). ...
Carboxylic acids are industrially important and specifically used for manufacturing of soaps and detergents and for various other chemical processes. As a result, waste water derived from such industries generally contains significant amounts of these acids posing threats to aquatic life, thus needing detoxification by suitable treatment methods. In the present work, the degradation of different aliphatic carboxylic acids by bare and Ag-loaded TiO2 under UV light irradiation was investigated. The acids were photo-decomposed to CO2 with a linear decrease in respective concentrations as a function of chain length/carbon atoms (C1-C8) and irradiation time. Small carboxylic acids, i.e., formic, acetic and propionic acid, exhibited faster rates of degradation than longer ones, i.e., butanoic, valeric and ethyl hexanoic acid, Ag loading did not significantly improve the TiO2 photocatalytic activity.
... Kinetics study in wastewater treatment is significant since it describes the reaction rate, which in turn defines the residence time of the wastewater in the system. The kinetic parameters of the common kinetic models including the Langmuir-Hinshelwood (L-H) [42], Freundlich-Hinshelwood (F-H) [42], and Langmuir-Freundlich-Hinshelwood (L-F-H) [43] models were calculated for the degradation of MB by EO and PEO using a non-linear fitting method and the results are shown in Table 1. ...
Stannic oxide modified Fe(III) oxide composite electrodes (SnO2/Fe2O3) were synthesized for simultaneously removing methylene blue (MB) and Cu(II) from wastewater using photoelectron catalytic oxidation (PEO). The SnO2/Fe2O3 electrodes were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), and photoelectrochemical techniques. The removal of MB and Cu(II) by PEO using the SnO2/Fe2O3 composite electrodes was studied in terms of reaction time, electric current density, and pH of the electrolyte. The kinetics of the reactions were investigated using batch assays. The optimal reaction time, pH, and electric current density of the PEO process were determined to be 30min, 6.0, and 10mA/cm(2), respectively. The removal rates of MB from wastewater treated by PEO and electron catalytic oxidation process were 84.87% and 70.64%, respectively, while the recovery rates of Cu(II) were 91.75% and 96.78%, respectively. The results suggest that PEO is an effective method for the simultaneous removal of MB and Cu(II) from wastewater, and the PEO process exhibits a much higher removal rate for MB and Cu(II) compared to the electron catalytic oxidation process. Furthermore, the removal of MB was found to follow the Langmuir-Freundlich-Hinshelwood kinetic model, whereas the removal of Cu(II) fitted well to the first-order reaction model.
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... One approach employed to study the effect of organic structure on either the photocatalytic oxidation rate or photocatalytic mineralization rate that further allows us to study the reaction mechanism is by examining the disappearance rate of parent substrate as well as the appearance of intermediate products [13][14][15][16]. In this study, the photocatalytic oxidation of a series of organic compounds, possessing different numbers of hydroxyl functional groups, was studied in an irradiated aqueous suspension of TiO 2 . ...
Global crisis of clean water supply has attracted great attention to reuse and recycle wastewater effluent. However, the effort has been limited by the presence of toxic organics in the effluent which in turn inhibits the public acceptance of recycling water. The existence of those organic pollutants, which cannot be eliminated by conventional primary and secondary treatment processes, can be problematic. Photocatalysis offers a promising process by breaking down low-level organic pollutants in water into nontoxic compounds such as CO2, H2O and mineral acids. The process involves excitation of a photocatalyst with near UV (300–400 nm) light to promote the formation of highly reactive chemical species such as holes, hydroxyl radicals, superoxide radical anions that are capable to mineralize the organic pollutants. An extensive variety of compounds can be satisfactorily degraded such as carboxylic acids, alcohols, aromatics, dyes, natural organic matters, etc.
The study shows the promising application of photocatalysis to treat water and wastewater with a series of organic compounds of carboxylic acids and alcohols was selected as the substrate model. The products resulted from the mineralization of the substrates have been identified and the effect of the formation of those intermediates to the overall mineralization rate has been discussed. The results showed the mineralization rates were greatly influenced by molecule structure of organics being degraded. For dicarboxylic acid group, the presence of hydroxyl functional groups increases the residence time of a carboxylate anion adjacent to the hydroxyl group and thus, rapid mineralization rate was observed. For alcohols, only one hydroxyl group attached to the surface and hence, the relative slow mineralization was recorded in the system. It can be concluded that the surface approach of the organic molecule determines the type of intermediates formed and thus, its overall mineralization rate.
... However, when the catalyst was overdosed, the intensity of incident light may be attenuated because of the decreased light penetration and increased light scattering, which levelled out the positive effect coming from the concentration increment and therefore the overall performance of the process may be reduced [13]. Some researchers claimed that the rate of photo-degradation may drop as a result of loss in surface area for lightharvesting occasioned by agglomeration of catalyst particles at higher catalyst concentrations than an optimum value [14][15][16][17]. The catalyst concentrations used in this study were higher than those used for photo-degradation studies conducted with plain TiO 2 which were in the range of 5-100 mg/L [13]. ...
... The aliphatic carboxylic acids are weak organic acids, so the existing speciation is partly dependent on pH value. The molecule configurations of carboxylic acids are the prevailing form at low pH value (pH < pK a ), whereas the carboxylic acid ions (RCOO -) are the main existingform at high pH value (pH > pK a ) [17]. The pK a value of formic acid is 3.74. ...
... In the present study, it has been found that increasing the reaction pH of formic acid solutions caused a decrease in the degradation efficiencies. These results showed that the formic acid can be photocatalytically degraded considerably faster at pH 3.0 indicating that the adsorption of the formic acid on the surface of SFT was probably the first step in the process of visible light induced photocatalytic degradation [17,19]. ...
In the present study, visible and UV-A light induced photocatalysis of aqueous solutions of formic and oxalic acid with S-doped, Fe-loaded TiO2 (SFT) was investigated. The effect of formic and oxalic acid concentrations (50 and 100 mg/L), initial pH (3 and 7) and catalyst concentrations (5 and 10 g/L) under visible or UV-A light on degradation rate was evaluated. The removal efficiencies of formic acid and oxalic solutions (50 mg/L) under UV light irradiation with SFT were 10% and 67%, respectively. During the experimental run conducted with formic acid solutions (50 mg/L), 53 and 40% dissolved organic carbon (DOC) removal efficiencies were achieved for 5 and 10 g/L catalyst concentrations at 120 min reaction time under visible light irradiation at pH 3.0, respectively. At the same experimental conditions the obtained DOC removal efficiencies for oxalic acid were 41% for 5 g/L and 51 % for 10 g/L catalyst concentration. The photocatalytic oxidation process under visible light was generally slower at pH 7 than at pH 3. Increasing the photocatalyst concentration from 5 to 10 g/L increased oxalic acid degradation rate, but decreased formic acid degradation under visible light irradiation. Our experimental results have clearly indicated that SFT has a potential as a photocatalyst that can be activated via visible light irradiation for the photocatalytic mineralization of the studied model pollutants.
... The decrease in mineralization rate at higher catalyst dosage may be attributed to the fact that at higher catalyst concentration, the tendency towards agglomeration increases and the increased turbidity of the suspension reduces the light transmission. [29] It is important to mention here that this optimum amount of catalyst depends on the initial concentration of the pollutant, geometry and the working conditions of the reactor and corresponds to the optimum of light absorption. [30] The optimum amount of 1 g L −1 TiO 2 has been suggested for the photocatalytic degradation of various herbicides. ...
Semiconductor-mediated hydrogen peroxide-assisted photocatalytic degradation of a selected herbicide, Bentazone (1) has been investigated in aqueous suspensions of TiO2 under a variety of conditions. The degradation was studied by monitoring the depletion in total organic carbon content as a function of irradiation time. The degradation kinetics was investigated under different conditions such as type of TiO2 (Anatase/Anatase-Rutile mixture), reaction pH, catalyst dosage and hydrogen peroxide (H2O2) concentration. The degradation rates were found to be strongly influenced by all the above parameters. Titanium dioxide Degussa P25 was found to be more efficient as compared with other two commercially available TiO2 powders like Hombikat UV100 and PC500 from Millennium Inorganic Chemicals. Gas Chromatography-Mass Spectrometry (GC-MS) analysis of the irradiated mixture of Bentazone (1) indicates the formation of several intermediate products which have been characterized on the basis of molecular ion/mass fragmentation pattern and also on comparison with the National Institute of Standards and Technology (NIST) library. Plausible mechanism for the formation of different products during photocatalytic treatment of Bentazone in the presence of TiO2 has been proposed. The use of H2O2 substantially increased the efficiency of TiO2 photocatalytic degradation.
... It was identified using mass spectrometric analysis that the photodegradation products of the H 2 O 2 /UV-C treatment were mainly hydroxylated compounds. Chen et al. (2009c) studied the kinetics of photocatalytic degradation of aliphatic carboxylic acids using artificial UV light and nano-TiO 2 powder as the catalyst. The results indicated that the experimental data fitted the Langmuir-Freundlich-Hinshelwood model, with the coefficient of determination (R 2 ) varying from 0.880 to 0.999. ...
This is a review of literature published in 2009 that covered issues related physico-chemical processes used to treat water and wastewaters. The review is divided into six sections, including coagulation/flocculation, sorption processes, filtration, sedimentation/flotation, oxidation, and air stripping.
