In developing countries, pollution caused by industrial activity has encouraged interest in finding more efficient technologies for wastewater treatment. Aromatic acids such as benzoic acids (BA) and its derivatives are common contaminants in industrial effluents. Benzoic acid (BA) is used in food items as preservatives to retard their microbiological, enzymatic, or chemical degradation. The aqueous wastes from these food industries may contain dissolved BA. If it exist in significant amount, it may cause acute toxicity in living organisms in water-bodies and can also affect human health . Some countries have discarded the use of BA as a food additive, even in traces . Therefore, such compounds need to be abolished from wastewater before the water is discharged. Removal of benzoic acid from aqueous solution is a vital topic, and several researches have been carried out in this area. Conventional physicochemical methods for BA removal suffer from major shortcomings due to their limited effectiveness and high operational costs , . Further, the conventional processes only transfer the contaminants from one phase to another causing secondary problems like sludge disposal or regeneration of used adsorbents. Thus, current work has been aimed to develop other methods for the removal of organic pollutant from the environment. Currently, advanced oxidation processes (AOPs) are gaining prominence for removing broad spectrum of persistent organic pollutants. In water treatment using AOPs, oxidative degradation of organic pollutants in wastewater is carried out using hydroxyl (•OH) radicals produced by combination of oxidants like hydrogen peroxide, ozone, ultraviolet (UV) light, a semiconductor photo catalyst, ultra-sound, or Fenton reagent . This literature survey indicated a growing interest among researchers in this approach due to its ability to overcome the severe limitations of conventional photo-Fenton, which are essentially related to expensive pH adjustments. These processes have merits, such as shortening of the processing time and operational ease, and show potential for applications in effluent processing and deodorization.
The objective of the present work is focused on the comparative assessment of degradation of these pollutants by AOPs (UV/H2O2, UV/TiO2, Fe(III)-oxalate complexes) studied. In this study optimization of peroxide concentration, TiO2 concentration, ZnO concentration, Fe (III) – ligand ratio, pH and pollutant removal rate is examined. Further, pseudo first order rate kinetics is derived to determine degradation efficiency.
FeCl3•6H2O, H2O2, H2SO4, NaOH, Oxalic acid dehydrate, 2- hydroxyl benzoic acid(salicylic acid-SA), p- nitro benzoic acid (pNB), p- amino benzoic acid (pAB), BA, m- nitro benzoic acid (mNB), m- amino benzoic acid (mAB), m- hydroxyl benzoic acid (mHB), p- hydroxyl benzoic acid (pHB) were all analytical grade. All stock solutions were prepared in deionised water. Sodium hydroxide and Sulphuric acid were used for pH adjustment. Glass distilled water is used for the entire study. All experiments were carried out in batch mode in a 1.0 liter jacketed thermostatic photo reactor (diameter 81mm and height 320 mm) equipped with low pressure mercury lamp (8 W, UV-C, manufacture by Philips, Holland) placed axially at its center. The UV lamp was encased in a quartz tube to protect it from direct contact with an aqueous solution flowing through the annulus between the inner surface of the vessel and the outer surface of the quartz tube, located at the axis of the vessel. The reactor was placed over a magnetic stirrer and a teflon coated magnetic needle was used for stirring the solution at the constant rpm using a dimmerstat. All the reactions were performed at atmospheric pressure. Constant reactor temperature was maintained by circulating water from a cryostat bath (Fourtech Systems, Mumbai, India). A gas tight syringe is used to collect the sample at desired intervals from the sample-port of the reactor.
Experiments were carried out in batch mode. For studying the photodegradation of organic acids, the photoreactor was charged with 800 ml of 3×10-4 mol/L solution of the pollutant in deionised water and the required concentrations of iron salt FeCl3 as a source of Fe (III) were added. A 30% aqueous hydrogen peroxide solution was injected into the reactor at required concentrations and the UV-light was switched on at the same time. During the experiments, aliquot were withdrawn from the reactor at desired time intervals and directly used for further analysis. The change in pollutant concentration was measured with UV-visible double beam spectrophotometer (Spectrascan UV 2600, Chemito, India). The experimental data of time versus [Ct/Co], (where Ct is the concentration of pollutant at time‘t’ and Co is the initial concentration of pollutant) is used to study the reaction kinetics of degradation of pollutants.
Results and Discussions
Optimization experiments on degradative oxidation of substituted BA were carried out and were analyzed by pseudo first order kinetic model. The Hammett equation was used to determine the effect of the substituent on the observed changes in reactivity of the system. Hence, to understand the mechanistic difference between aromatic substitutions and kinetics, reaction rate constants of disubstituted benzene were studied, keeping one substituent specifically –COOH.
Optimization of initial pH
The pH value has a decisive effect on the oxidation potential of •OH (Kim et al., 1997). To determine the optimum pH for the degradation of organic acids under direct degradation, experiments were carried from acidic to basic range. At high pH values, the OH. radical concentration is high and hence the chances of recombination are also high . Maximum degradation efficiency was achieved at pH of 4 (Table 1).
The pH value around 4-5 was the best for the Fenton and the H2O2/UV processes. Hydrogen peroxide was most stable in the pH range 3-4, but its decomposition rate rapidly increased with increasing pH above pH 5 (Meeker et al., 1965). Our results are in good agreement with those of previous reports (Meeker et al., 1965; Feuerstein, 1981; Bigda, 1995). [2-HBA] removal efficiency decreased at higher pH due to the decomposition of hydrogen peroxide (Feuerstein, 1981), and the deactivation of the ferrous catalyst which might be due to the formation of ferric hydroxyl complexes (Bigda, 1995).
Optimization of initial H2O2 / COD ratio
In order to fix the optimum ratio of H2O2/COD, studies were conducted for the ratio varying from 2.0 to 7.0 (Table 1). As shown in Table 1, the degradation efficiency increased when H2O2 concentration increases which is explained by the effect of the additionally produced OH radicals. However, above the optimum H2O2 concentration, the reaction rate levels off and sometimes is negatively affected, by the progressive increase of the hydrogen peroxide. This may be due to auto decomposition of H2O2 to oxygen and water and recombination of OH radicals. Excess of H2O2 will react with OH competing with organic pollutants and consequently reducing the efficiency of the treatment, the H2O2 itself contributes to the OH radical scavenging capacity. Thus, the higher the H2O2 concentration, the more favored the occurrence of auto-scavenging reactions.
The degradation of pollutants increases with increasing H2O2 / COD ratio. This is because in the H2O2 /UV process, hydroxyl radicals generated from the direct photolysis of hydrogen peroxide were the species mainly responsible for pollutants elimination , . From the study, it is found that for the H2O2 / COD ratio 4.0 – 5.0, all the acids are efficiently degraded. At lower and higher ratio, degradation rate is reduced. Therefore, H2O2 should be added at an optimal ratio of 4.0 – 5.0 to achieve the best degradation .
Optimization studies of TiO2 photocatalyst
Heterogeneous photocatalytic oxidation using TiO2, ZnO as photocatalyst in solar light has emerged as a promising route for the successful degradation of persistent organic pollutants –. To optimize the catalyst suspension concentration, the effect of photocatalyst dosages from 10 to 90mg on the degradation of SBA in aqueous solution was studied. The results were illustrated in Table 1. As the dosage of photocatalyst increased, the efficiency increases due to the increase in the total surface area, in turn increasing the number of active sites which are available for the photocatalytic reaction. The fragmentation of catalyst which produces higher surface area may be helpful in increasing the degradation rate. Thereafter with further increase in catalyst loading the degradation rate starts declining because of the decreased light penetration, the increased light scattering and the loss in surface area occasioned by agglomeration (particle-particles interactions) at high solid concentration. This lowers down the rate of degradation above an optimum catalyst loading.
Optimization studies of Fe (III): Ligand ratio
Fe (III) may complex with certain target compounds or their byproducts, especially those acting as polydentate ligands. These complexes typically have higher molar absorption coefficients in the near-UV and visible regions than do the aquo complexes. Their excitation leads to the production of Fe2+ and a ligand radical with quantum yields that are wavelength dependent. An interesting and potentially useful modification of the photo-Fenton reaction takes advantage of the photo ability of Fe(III)–oxalate complexes (Hislop and Bolton, 1999; Safarzadeh-Amiri et al., 1997, 1996), which is efficient up to 500 nm. Ferric oxalate is commonly used as a chemical actinometer . Thus, it provides a continuous source of Fenton’s reagent. The hydroxyl radical is a strong oxidant and reacts indiscriminately with organic pollutants and starts a cascade of oxidation reactions that can ultimately lead to total mineralization of organic pollutants. Table 1 illustrates the effect of (Fe3+/oxalate) weight ratio on the kinetics of substituted phenol degradation efficiency keeping other operating variables unchanged. The best degradation rate has been observed for Fe3+/oxalate 1:2 ratio.
Effect of Molecular Structure on Reactivity
Substituents influence reactivity by altering electron density at the reaction site and by steric effects. Electronic effects are the easiest to be evaluated for aromatic compounds with substituents in meta and para positions, where steric influences (ortho positions, which are more difficult to predict) are minimized. Reactions favored by high electron density are accelerated by EDG, while EWG will accelerate reactions favored by low electron density. Highest rate was observed for p-NB, and the lowest one was for p-AB.
Comparison of different AOP
A comparison of various AOPs studied in terms of degradation rate is illustrated in Table 1. From the table, it is observed that the mineralization achieved by UV in combination with peroxide and photocatalytic process is almost very little because the compound undergoes photochemical reaction as a consequence of light absorption. During photochemical treatments, electronically excited states of SBA are generated where the molecule undergoes intramolecular transformations and stabilizes itself with different electron distributions, followed by decomposition to radical or molecular products contributing rarely to compound removal. In this sense, the light absorbed by the compound can be generally considered as wasted light. However, the time required for degradation is still long. This is in agreement with the observations of , that the presence of an electron rich atmosphere in the aromatic ring, making it more resistant to hydroxyl radical attack. The pseudo first order rate constant increased from UV/ H2O2 to complexes indicating that Fe (III)-oxalates as the powerful oxidant in degrading SBA. The degradation rate followed the order:
Fe(III)- Oxalate > UV+H2O2+TiO2 > UV+H2O2+ZnO
Among all the processes Fe (III)-oxalates process is found to be more efficient in degrading SBA (90%) which is almost ten times higher than other, as evidenced by the kinetic constant values. Another important factor that can help to understand our results is the reactivity between Fe (II)-oxalates and H2O2. For all the Fe : ligand ratios studied, there is a certain amount of free oxalate available to complex with Fe(II) produced from the photolysis of Fe(III)-oxalates . As stated in the literature, Fe (II)-oxalate reacts much faster with H2O2 than uncomplexed Fe (II), yielding _OH radicals fast as well. Therefore, the inclusion of Fe (III)-complexes in investigations dealing with the environmental fate of emerging pollutants in natural water bodies is strongly recommended.
Effect of substituent and the reactivity
The initial rate of the reaction is affected more by the nature of the substituents atoms than by the number of these atoms in the molecules . Since –NO2 was an electron withdrawing group, the substitution of such group on the aromatic ring decreased the electron density by σ-electron withdrawing conductive effect; at the same time, the substitution of –NH2 and -OH on the aromatic ring increased the electron cloud by π-electron donating conjugative effect, and the conjugative effect could counteract the negative impact of the conductive effect to some extent. But, the substitution of -OH at ortho position on the aromatic ring decreased the oxidation rate constant by steric hindrance effect . EWG in the aromatic ring at ortho- and para-positions make the favorable conditions for OH● radicals which are then supposed to attack the aromatic centers. Therefore, the degradation kinetics decreases accordingly following the order:
p-nitro benzoic acid> m- nitro benzoic acid > m- hydroxy benzoic acid > benzoic acid> m- amino benzoic acid > p- hydroxy benzoic acid > o-hydroxy benzoic acid> p- amino benzoic acid. The reason for the difference in the degradation of o-hydroxy benzoic acid and other could be due to the fact that benzoic acid is in the dimeric form but o-hydroxy benzoic acid is less prone to form dimers due to intramolecular, rather than intermolecular hydrogen bonds. In case of –NH2 and -OH, one of the substituent increases the ∏- electron density of the ring by resonance, while the other one withdraws electron by induction. As a result, Kmeta is greater than K para.
The effectiveness of oxidative degradation in SBA removal from synthetic wastewater has strongly dependent on pH of the solution. Experimental results revealed that the efficiency of removal in acidic environment is higher than in basic. Initial hydrogen peroxide loading has a positive impact on the removal efficiency of SBA, increasing the ratio of H2O2/COD from 2 to 5 resulted an increase in SBA removal. The substrate degradation efficiency by UV+ H2O2 combination was optimum at H2O2/COD ratio of 5. It is worth noting that as more H2O2 has been added to solution, the increment of increasing in SBA removal becomes lower. The efficiency of SBA removal has been significantly affected by the initial FeSO4/Oxalic weight ratio, as this ratio increased, the degradation increased correspondingly. Results showed that the optimum addition of ferrioxalate complexes (in the ratio 1:2) enhanced the degradation by (20–30%) at 120 min of illumination time. It is the current opinion that this approach may boost the use of conventional photo Fenton in real applications. The advantages of such step over other processes are efficiency in destroying aromatic compounds, easy handling of the method, can be applied at room temperature, less energy demand and harmless products. Hence, combination of complexation based AOP with biological treatment is a promising alternative in developing a potential wastewater purification method.