Many methods of AOPs were continuously being evaluated for the remediation of emergent contaminants from environmental matrices. Although there are only limited number of studies concerning microplastics and many more AOP techniques are yet to be verified for the degradation of synthetic polymers (Kim et al., 2022). Ozone is one such oxidant which is predominantly used as such or in combination with hydrogen peroxide. The present study evaluated the efficiency of heterogenous catalytic ozonation method using conventional metal oxide catalysts to that of non-catalytic ozonation in oxidsing nano sized polystyrene particles in1 hour
Size distribution of PSNP
The study was conducted in the nano sized polystyrene particles in order to observe the oxidative changes in a minimum time period. The size distribution of the synthesized PSNP was estimated using DLS analysis and observed that almost all the particles comes under the size range of 200-2200 nm with an average size of 1050 ± 480.46 nm with an error margin of ±11.02% at 95 percent confidence level. Size distribution of particles as observed under DLS analysis is depicted in Fig 1. Very few particles having a higher size of 5000 to 10000nm were also found in the suspension.
Majority of PSNP were found to have a spherical structure when observed under optical microscope. In the similar method followed by Pawak et al. (2023), average size of the particles observed was 148 ± 22 nm. The difference in the size range of particles would be greatly due to the variations in the nanoprecipitation conditions like sample concentration, temperature and agitation speed. Many studies dealing with the structural and behavioural analysis of micro and nano plastics prefer using commercially available micro and nano materials (Amelia et al., 2022; Li et al., 2022). Purchased products are generally expensive but have the added advantage of uniform size and shape distribution and reduced processing time. Tanaka et al., 2021 devised methodologies for the synthesis of pure and uniform nano scale particles of major plastic polymers which can be used as an efficient alternative for the high cost commercial products.
Characterization of catalysts
Two types of metal oxide catalysts, Titanium dioxide (TiO2) and vanadium pentoxide (V2O5) were used in preforming the catalytic ozonation of synthetic polymer particles. Catalysts are materials which accelerates the speed of a chemical reaction without directly interfering with it. Hence it is important that the catalysts used in the present study should stay chemical and structurally intact even after the reaction. Both the catalyst at a concentration of 50mg/L was subjected to ozonation for 1 hour along with the PSNP particles. Catalysts after reactions were filtered out, dried and subjected to SEM and FTIR analysis along with untreated catalyst samples.
SEM analysis of samples before and after ozonation showed that there was no significant change in the morphology of the catalysts upon treatment. TiO2 particles had a rounded structure which was maintained even after ozonation. The reaction mixture containing TiO2 was found to form a membranous structure on the surface when kept for settling. Formation of similar structures were absent in case of the reaction solution containing V2O5. V2O5 particles were seen to have a needle like structure with angular phases which remain intact after the reaction. The SEM images of the catalysts taken before and after the ozonation is given in Fig 2.
EDAX detector in SEM enabled the elemental analysis of the catalysts. In case of both TiO2 and V2O5, presence of carbon (C) was detected in the samples after treatment. While TiO2 before treatment had an oxygen (O) atom concentration of 97.02 % with no traces of C, it have been reduced to 73.06% of O after treatment replaced by 25.33% of C atoms. For V2O5, C atom concentration of 12.27 % was observed even before the treatment which have been increased to 24.37% after ozonation. Around 6 % atom concentration of sodium (Na) was also found in the V2O5 samples after treatment. Traces of sulphur and silicon were also detected in the catalysts after ozone treatment.
No considerable variation were observed in the chemical composition and functional group distribution of TiO2 and V2O5 catalysts subjected to ozonation, analysed under FTIR spectroscopy. Most prominent peak in the TiO2 spectra at was seen at 1060cm-1 and another one at 770 cm-1, intensity of which remain intact even after the treatment. In case of V2O5, major peaks corresponds to metallic bonds were found at 984 cm-1 and 795 cm-1, of which the peak was slightly reduced after treatment for latter. Formation of minor peaks around 1720 cm-1 in the catalyst after treatment indicates the stretching vibrations of carboxylic acid groups. FTIR transmittance spectra of catalysts before and after ozone treatment is shown in Fig 3.
Both TiO2 and V2O5 act as a catalyst for ozonation reaction owing to its high surface area, by adsorbing the ozone gas resulting in surface activation of the compounds (Sahle-Demessie & Devulapelli, 2009). The adsorbed molecules further decompose to form ozone radicals or hydroxyl radicals in water. Water adsorbed both associatively and as radicals on titanium cation to oxidize its surface. Aminev et al. (2021) observed that the adsorption of different isotopic ozone mixtures on TiO2, causes shifting of vibrational frequencies which was not observed in the present study. The adsorption of gaseous species by TiO2 occur more in dry state which was eliminated as the catalyst was suspended in an aqueous solution.
Ozone oxidation of PSNP
Ozonation like other advanced oxidation processes was proven to have the ability to introduce oxygen containing groups in the chemical composition of synthetic polymers (Li et al., 2022). Oxidative transformation of polystyrene nanoparticles upon interaction with ozone gas was experimented under variable pH conditions of pH 5, 7 and 9. In all cases, it was observed that the pH of the reaction mixture fall to an acidic range of pH 3-4 after ozonation, irrespective of the initial value. Ozonation when coupled with other chemicals and processes can induce synergistic effect which enhances the oxidation of organic compounds (Amelia et al., 2022). This can in turn reduce the reaction time required to achieve particular targets, thus reducing the operational cost which is a major drawback of ozone based advanced oxidation process.
Primary evaluation on the effect of ozonation on PSNP was determining by the COD analysis. COD values of the treated samples revealed that the presence of organic compounds were least in case of ozone treatment at pH 9. COD removal efficiency of 58.1 percent and 62.4 percent was observed for the samples having PSNP concentration of 5mg/L and 15mg/L respectively upon one hour of ozonation. COD removal efficiency of samples under ozonation at different pH conditions is shown in Fig 4.
Samples were subjected to ozonation in the presence of metal oxide catalysts at a concentration of 50mg/L and tested their efficiency with respect to pH. It is evident from the results that ozone treatment in the presence of metal oxide catalysts considerably increases the COD removal for polymer samples. It was found that the highest COD removal efficiency was observed at pH 9 in the presence of V2O5 catalyst followed by TiO2 catalyst at pH 7. While TiO2 catalytic activity was favoured most at pH 7, V2O5 activity seemed to increase with the increase in pH. It was interesting to observe that the presence of TiO2 decreased the COD removal efficiency at pH 9 with regards to the non-catalytic conditions. COD removal obtained under catalytic and non-catalytic ozonation at different pH is given in Fig 5.
The oxidation of polystyrene particles under ozonation was further confirmed by TOC analysis of the treated samples along with the DLS analysis to observe the variation in particle size distribution. Ozone gas when dissolved in the water matrix dissociates to form molecular oxygen and hydroxyl radicals which induce oxidation of the organic contaminants present in the water specifically the polystyrene nanoparticles in this work. As oxidation proceeds, organic compounds present will get transformed into other lesser complex forms accompanied by the formation of new oxygen containing functional groups in its chemical frame work (Li et al., 2022). This ultimately results in the reduction of organic carbon in the samples due to carbon emission in the form of CO2 gas. Hence, reduction in total organic carbon corresponds to the removal of plastic polymers in the water sample (Kiendrebeogo et al., 2021). TOC removal efficiency was found to be higher for catalytic ozonation as compared to non-catalytic ozonation. Highest TOC removal efficiency of 97.78 % was observed in ozone treatment with TiO2 catalyst at pH 7. More than 90 percent removal efficiency was attained using both TiO2 and V2O5 at pH 9. Both TOC and COD analysis are analytical methods to assess organic content present in a water samples. While TOC measures the total concentration of organic carbon, COD gives the oxygen requirement for oxidation of all organic and inorganic substances present. It can be interpreted that both COD and TOC results show similar trend of organic carbon removal in the treatment of PSNP using ozone. Results of oxidation in presence of TiO2 reveals the possibility of using it as an efficient catalyst for ozone treatment irrespective of the pH of the samples to be treated. Although V2O5 found to be least suitable for catalysing ozone oxidation for samples having acidic pH. Fig 6 gives the TOC removal of the samples under different treatment conditions and clearly indicates the influence of catalysts in enhancing oxidation of PSNP.
Size distribution of the PSNP in the sample was considerably altered upon ozone treatment. DLS results revealed that there have been a shift in the particle size to a much wider range. While in the untreated sample majority of the particles were in the size range of 200 to 2200nm, the size range had shift to 50 to 3000nm in ozonized samples. Although the average value of particle size remain closer to that of initial conditions. A major change in the pattern of distribution occurred in case of catalytic ozonation. The dominant size range of particles falls below 500 nm in case of both TiO2 and V2O5 catalysed reactions. Comparison of size distribution of polystyrene nanoparticles before and after non-catalytic and catalytic ozone oxidation is depicted in Fig 7.
Under catalytic ozonation using TiO2, average size of the particles have been reduced to 220.13 ± 99.84 nm while it was 286.82 ± 103.99 nm in the presence of V2O5. From the data, it is evident that a size reduction of around 75 percent was obtained with the addition of catalysts in ozone oxidation reaction. The size reduction may be attributed to the breakdown of PSNP into further smaller particles. Modifications in the structural morphology of the microplastics when subjected ozonation was confirmed in SEM analysis in many studies (Luo et al., 2021; Lee et al., 2021). Here, the size of particles were below 500nm and were suspended in water, it is not preferable to SEM analysis for imaging.
When considering ozone mediated oxidation as a method of pollution control, it is of utmost importance that the reaction time should be as shorter as possible in order to balance the high operational cost and potential toxicity of ozone gas at higher concentrations (Tripathi & Hussain, 2022). This open up the need for catalytic ozonation methods which are aimed at obtaining required targets in a shorter period of time. The present study confined all the experiments to a fixed time period of 1 hour using a constant ozone dosage of 25mg/L. The only variable employed were the pH and the catalysts. Many studies have been conducted using catalysts coupling with advanced oxidation processes for the removal of toxic pollutants especially from water matrix (Gong et al., 2021; Atalay and Ersoz, 2016).Various compounds act as catalysts in order to accelerate the speed of chemical reactions. Pokkiladathu et al (2023) experimented the catalytic ozonation for the removal of bisphenol A from water using activated carbon based bimetallic nano composites and reported a 25% higher TOC removal efficiency as compared to non-catalytic conditions. Considering microplastic removal using advanced oxidation process majority of the studies focussed on the photocatalytic methods involving heterogenous catalysis (Nabi et al., 2020; Tofa et al., 2019). Even though some studies have been done on the interaction of ozone with microplastics (Solís-Balbín et al., 2023; Fitri et al., 2021), catalytic ozonation was not much explored.
Plastic polymers due to their high molecular weight and hydrophobicity are difficult get oxidized unless by the action of a strong oxidant (Du et al., 2021). Chemical reaction of ozone with polymeric compounds leading to molecular bond breakage and subsequent degradation can be termed as ozonolysis . Ozone is a powerful oxidising agent capable of breaking C-C and C-H bonds present in polystyrene. This can lead to chain scissoring causing reduction in length of polymer chain and molecular weight. Formation of oxygenated compounds also occur which increase the carbonyl index of the molecules. Increased carbonyl index is often considered as an indicator of the extent of oxidation occurred to a compound (Amelia et al., 2022). Ozone attack tend to initiate at carbon-carbon double bonds and further proceeds to the formation of carbonyl groups. Steps involved in the reaction of ozone with polystyrene can be assumed to be as follows;
The reactions will eventually leads to alterations in chemical and mechanical properties of the compounds like adhesion, tensile strength and flexibility. Chain scissoring and cross linking takes place by breakage of polymer chain at tertiary carbon atoms (Zhang et al., 2021) leading to formation of new degradation by products. Degradation pathway analysis of polystyrene under ozonation revealed the presence of formic acid, phenol, acetophenone, hydroquinone, methyl benzaldehyde, methyl acetophenone and phenyl propionic acid (Li et al., 2022). The degradation of PSNP in the presence of metal oxide catalyst can involve more complex reactions. Some of the possible reactions involves in the catalytic oxidation include; 1) Ozone dissolution in aqueous medium to form ozone molecules (O3) and hydroxide ions (OH-), 2) Catalytic activation of ozone molecules on the surface of catalyst, leading to formation of highly reactive oxygen species such as ozone radicals (·O3), 3) Reaction of radicals with polystyrene cause C-C and C-H cleavage, 4) Chain scissoring causing breakage of polymer chain and reduction in molecular weight, 5) Formation of oxygenated groups in the polymer structure like carbonyl or hydroxyl groups, 6) Release of gaseous by products including CO2 and other volatile compounds, 7) Change in physical properties and 8) overall enhancement in oxidation potential of ozone. The type of reactions and products formed can be varied depending on factors like reaction conditions, ozone and catalyst concentration and nature of target compound (Rodríguez et al., 2008).
Treating polyethylene (PE) particles using ozone for shorter durations was found to be insufficient to cause significant modifications (Zafar et al., 2021). Hence ozonation is commonly employed as a pre-treatment technique in the biodegradation of plastic polymers. Tian et al (2016) subjected polystyrene particles to ozonation followed by biodegradation using Penicillium variabile and found that the pretreatment had considerable enhanced mineralisation by fungi. An increase in weight loss and formation of carbonyl groups like ketone, aldehyde, carboxylic acids and alcohol upon ozone treatment was recorded by Fitri et al. (2021). Removal of polyvinyl alcohol (PVA), which is a water soluble polymer by catalytic ozonation using Cu-Mn-Al (CMA) catalyst was found to obtain 97.8% removal rate in 15 minutes at pH 7 and 99.3% removal in 10 minutes at pH 10 (Yan et al., 2020). The result is comparable to that of the present study, yielding 97.78 % removal using TiO2 catalyst at pH 7 in 1 hour considering the fact that polystyrene used was solid and water insoluble. The ozone treatment was also found to enhance the adsorption properties of plastic polymers (de Aragão Belé et al., 2021). Catalytic ozonation seems to be an effective method of advanced oxidation process in removing the micro and nano sized plastic particles from the water. It would be a better alternative as a heterogeneous catalytic process substituting the combination methods of ozone along with H2O2 (de Aragão Belé et al., 2021).
Possible errors in the plastic removal efficiency determination in the experiments is a matter of discussion. High TOC removal rates observed in case of catalytic ozone reactions can be linked to the adsorption of PSNP particles by the catalyst. Although the concern is of lesser significance as the FTIR results haven’t shown any considerable variations after the reaction. The formation of minor peaks at the carbonyl group region can be attributed to the presence of the degradation byproducts. Another drawback of the experiment is the usage of catalyst in suspended phase. Since the target compound also exist as suspended particles, the catalyst can interfere with the characterization techniques carried out in aqueous phase. In order to tackle this issue, blank samples of catalysts were performed in case of DLS analysis for particle size determination.