It is hereby certified that the work contained in the thesis entitled “MECHANISM INVESTIGATIONS IN ADVANCED HYBRID OXIDATION PROCESSES FOR THE DEGRADATION OF RECALCITRATE CONTAMINANTS”, by Sankar Chakma (Roll No) has been carried out under my supervision and that this work has not been submitted elsewhere for degree.
L IST OF F IGURES
297 Figure 9.8 Adsorption study of ARB and MB dyes on ZrFe2O5 and TiO2 powders 298 Figure 9.9 Time history of decolorization of ARB with various operating conditions:.
G ENERAL I NTRODUCTION AND L ITERATURE
R EVIEW
G ENERAL I NTRODUCTION
AND L ITERATURE R EVIEW
Sonolysis
In the subsequent compression phase, not all of these vapor molecules can diffuse back at the bubble level and undergo condensation. The molecules of pollution can also be trapped in the bubble and undergo thermal dissociation.
Fenton Process
This causes heating of the thin liquid layer at the bubble interface to moderate temperatures of ~ 500-600oC (Kotronarou et al., 1991). The latest variants of the Fenton processes are sono–Fenton or sono–photo–Fenton (Segura et al., 2009).
Photo-Ferrioxalate System
Higher concentrations of hydrogen peroxide can lead to the scavenging of the •OH radicals before their reaction with organic pollutants. This in-situ generated H2O2 can undergo photodecomposition under UVC light to generate additional •OH radicals.
Persulfate Oxidation Process
However, the influence of pH on the photolysis process is not very significant and it has been found that the optimal pH for photolysis or sono-photolysis reactions should be neutral (pH = 7). EP – electrode potential, US – ultrasound, PS – persulfate, NFDOHA - Perfluoroether carboxylic acids (CF3OC2F4OCF2COOH, CF3OC2F4OC2F4OCF2COOH, CF3OC3F6COOH, C2F5OC2F4OCF2COOH, C4F9OC2F4OC2F4OCF2COOH).
Table 1.5 summarizes the literature on the sonoenzymatic degradation of various organic pollutants. The relatively new AOP of sonolysis has also shown promise for effective pollutant degradation. Entezari MH, Petrier C, A combination of ultrasound and oxidative enzyme: sonoenzyme degradation of phenols in a mixture, Ultrason.
Mishra KP, Gogate PR, Intensification of sonophotocatalytic degradation of p-nitrophenol in pilot-scale capacity, Ultrason. Patidar R, Khanna S, Moholkar VS, Physical features of ultrasound-assisted enzymatic degradation of recalcitrant organic pollutants, Ultrason. Segura Y, Molina R, Martínez F, Melero JA, Integrated heterogeneous sono-photo Fenton processes for the degradation of aqueous phenolic solutions, Ultrason.
P HYSICAL M ECHANISM OF S ONO –F ENTON
P ROCESS
P HYSICAL M ECHANISM OF S ONO –F ENTON P ROCESS
Materials
Experimental setup
Synthesis unit (Millipore®, USA) was used as the liquid medium. explained later in this chapter. The neck of the conical flask used in the experiments was closed by means of a rubber stopper with metal tubes pierced through it. The outer end of the metal tube was connected to a nitrogen cylinder through two-stage pressure regulator.
The pressure of the exhaust gas from the cylinder, and therefore the static pressure on the dye solution, could be controlled via this regulator.
Experimental procedure
Depending on the pressure amplitude of the ultrasound wave and the static pressure in the environment, the radial movement of the bubble is characterized as stable cavitation and transient cavitation. In the subsequent stage of compaction, the compression of the bubble is dominated by the inertial force. The transient collapse of the bubble is extremely fast and adiabatic during which the bubble wall velocity reaches (or even exceeds) the speed of sound in the liquid medium.
However, in the final moments of bubble compression, the velocity of the bubble interface (or bubble wall) becomes extremely fast, and the time scale of bubble motion becomes smaller than the time scale of vapor diffusion to the bubble wall as well as the time scale of condensation (or phase change) at the bubble wall.
Estimation of physical and chemical effects of cavitation
Micro-convection is the oscillatory movement of liquid in the immediate vicinity of the bubble, caused by volume oscillations of the bubble. The size of the microcurrent velocity (u) is dependent on the pressure amplitude (PA) of the ultrasound wave as: u= PA/ρc. 2.4 & 2.5 that for both dyes most of the decolorization was achieved in the first 10 minutes of treatment in the categories involving Fenton's reagent.
On the other hand, the decolorization of the BLH dye remains unaffected by the deaeration of the medium.
Simulations results
Application of high static pressure in the medium (greater than or equal to the acoustic pressure amplitude) can aid in the separation of the effects of ultrasound and transient cavitation in the medium. Time history of (A) bubble radius, (B) temperature inside the bubble, (C) water vapor evaporation inside the bubble, (D) pressure inside the bubble, (E) microturbulence generated by the bubble, and (F) acoustic waves emitted by the bubble. Time history of (A) bubble radius; (B) temperature inside the bubble; (C) water vapor evaporation in the bubble; (D) pressure inside the bubble (E) microturbulence generated by the bubble; (F) acoustic waves emitted by the bubble.
Time history of (A) bubble radius; (B) temperature inside the bubble; (C) water vapor evaporation in the bubble; (D) pressure inside the bubble, (E) microturbulence generated by the bubble; (F) acoustic waves emitted by the bubble.
Analysis and discernment of the synergy in sono–Fenton process
The concentration of dye molecules in the solution is relatively small (10 mg/L), and therefore, the probability of dye-radical interaction also appears. The discrete production of •OH radicals in the reaction volume, together with the low dye concentration may lead to lower utilization of radicals produced by transient cavitation events for decolorization. The role of ultrasound and cavitation in the sono-Fenton hybrid process is purely physical, i.e.
Grčić I, Šipić A, Koprivanac N, Domagoj V, Global parameter of ultrasonic extraction (GPUE) in the reactors for wastewater treatment by sono-Fenton oxidation, Ultrason.
P HYSICAL M ECHANISM OF H YBRID AOP S ONOLYSIS + F ENTON + UV
P HYSICAL M ECHANISM OF H YBRID AOP S ONOLYSIS + F ENTON + UV
Experimental setup
The UV lamp was placed on top of the reaction beaker as shown in the schematic diagram in figure. The distance between the lamp and the surface of the reaction solution in the beaker was 14 cm. The UV lamp setup used in this study may result in relatively small access to UV radiation.
The acoustic pressure amplitude of the ultrasound waves generated by the transducers in the bath was determined using the calorimetric technique as ~190 kPa or 1.9 bar.
Experimental protocols and analysis
Typically, cavitation bubbles with initial sizes in the range of 2 to 10 µm undergo transient motion. Increasing the static pressure of the medium above the pressure amplitude of the ultrasound wave can thus eliminate transient cavitation in the medium. More details about the preliminary experiments are given in the next section (preliminary experimental result).
In the first set of experiments, BPA degradation was monitored by withdrawing 1 mL aliquots of the reaction mixture every 10 min.
The method for the determination of hydrogen peroxide (H2O2) generated by cavitation is described in the next section. In addition to these experiments with application of UV, we also performed experiments in the category sonolysis + UV, H2O2+UV (with mechanical agitation) and H2O2+sonolysis+UV. However, the net degradation of BPA obtained for these experimental categories was small (only in the range of 10–20%).
The thermal dissociation of water in the cavitation bubble during temporary collapse causes the formation of OH radicals, which are released into the medium.
Analytical technique for BPA
But at the highest concentration of H2O2 (11.75 mM) in the reaction mixture, the degradation rate decreases. The first-order pseudo-kinetic constant for BPA degradation in the four categories of the sono-Fenton process shows significant variation. Thus, the increase in the kinetic constant of the sono-Fenton process for the aerated medium is relatively smaller compared to the deaerated medium at increased static pressure.
The static pressure on the reaction mixture in the sono-Fenton process shows no influence on the extent of degradation.
M ECHANISTIC A NALYSIS OF H YBRID
S ONO –P HOTO –F ERRIOXALATE S YSTEM
M ECHANISTIC A NALYSIS OF H YBRID S ONO –P HOTO –F ERRIOXALATE S YSTEM
Experimental setup
The beaker was immersed in the water in the bath to about 3/4 of its height. The temperature of the sonication medium, and thus the reaction solution in the beaker, was maintained constant at 25±1oC by the circulation of cooling water from a water circulating bath (Amkette Analytics, Model: . WB2000). To avoid any artifact due to spatial variation of the ultrasound intensity and local acoustic pressure amplitude in the reaction solution/medium (Gogate et al., 2002; Moholkar.
The ultrasonic pressure amplitude in the reaction mixture in the beaker was also determined to be 1.9 bar using the calorimetric method.
Experimental procedure
The intensity of the acoustic energy emitted by transducers was 1.2 W/cm2 with volumetric energy dissipation of 8.71 W/L. The scheme of the experimental setup is available in our previous study (Chakma and Moholkar, 2014) as well as in the previous chapter (Chapter 3). Each of these techniques alters the predominant physical and chemical mechanism of the textile dye decolorization system.
Small aliquots (1 mL) of the reaction mixture were withdrawn at regular time intervals during treatment of the dye solution by mechanical stirring or ultrasonic sonication in any experimental category.
Kinetics of decolorization
In the subsequent compression phase, the vapor molecules diffuse to the bubble interface and condense. However, in the final moments of bubble collapse, the velocity of the bubble wall (or bubble interface) becomes extremely fast, that is, this phenomenon also causes a non-equilibrium phase change in the bubble and trapping of the vapor molecules.
During the bubble expansion, the gas dissolved in the liquid can also diffuse inside the bubble.
Physical and chemical effects of transient cavitation
Due to the very small volume of the bubble at the point of maximum compression and the very high temperature within the bubble, the reaction rates between the different chemical species present in the bubble are expected to be extremely fast. This essentially implies that layer and layer fraction of Fe3+ present in the reaction mixture forms a complex with oxalate, which undergoes dissociation in the presence of UV radiation to generate radicals. At CO2 24/Fe3+ ratios higher than 3, part of the oxalate remains unreacted in the system and can scavenge the •OH radicals generated in the system (equation 4.28) (Vedrenne et al., 2012; Monteagudo et al., 2013). ), resulting in a reduction in decolorization efficiency.
Addition of external H2O2 to the medium triggers Fenton-like reaction of Fe3+ with H2O2 running concurrently with the ferric oxalate reaction system.
Results of dye decolorization experiments
However, the pressure amplitude of the acoustic waves generated by the 5 m bubble is higher than the 10 m bubble. However, the qualitative similarity in the trends in the extent of discoloration highlights the similarity of the physical degradation mechanism of the two dyes. Most decolorization is achieved via the photo-ferrioxalate route, with ultrasound and cavitation contributing only in terms of the physical effect of generating convection (or agitation) in the environment.
For both dyes, the extent of decolorization is almost the same for saturated medium as well as unsaturated medium, where the content of dissolved oxygen in the medium is lowered.
Synergy of the hybrid AOPs
For the binary AOPs, the physical effect of ultrasound and cavitation (ie generation of micro-turbulence in the medium) aids and enhances the chemical effect of Fenton-like and photo-ferrioxalate system. But the trends in synergy are the same (ie, positive synergy for binary AOP and negative synergy for ternary AOP), indicating close similarity in the physical mechanism of degradation of the two dyes. In the present study, an attempt has been made to gain insight into physical mechanisms of binary and ternary hybrid AOPs with combination of sonolysis, Fenton-like system and photo-ferrioxalate system.
This result indicates similarities in the physical mechanism of decolorization/degradation of the two dyes, despite significant differences in the chemical structure.
M ECHANISTIC A NALYSIS OF S ONO –P HOTOLYSIS
