Dechlorination of persistent organic pollutants in petrochemical wastewater sludge through a planetary ball mill using synthesized nanocomposite

ABSTRACT Numerous tons of hazardous wastes containing persistent organic pollutants (POPs) are stored in an unsafe manner in the petrochemical industries. The planetary ball mill causes the formation of mineral Cl compounds during the degradation of polychlorinated compounds in the presence of various reagents. In addition, the planetary ball mill has been applied for the synthesis of diverse organic-inorganic hybrid nanomaterials. In the present study, Fe-metal organic framework was prepared using a planetary ball mill, and then Fe@C nanocomposite with core/shell structure was synthesised by carbonisation. X-ray diffraction (XRD) and field emission scanning electron microscope (FESEM) and transmission electron microscope (TEM) were utilised to study the synthesised materials. The nanocomposite was used as a reagent to degrade persistent organic pollutants in the sludge of a petrochemical wastewater treatment plant by a planetary ball mill. CaO was also used to compare the effect of nanocomposites on system performance. Ion chromatography (IC) analysis was carried out to determine chlorine mineralisation. Furthermore, Fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy were applied to determine chemical bonds. The results illustrated a zero-valent iron nanoparticle in the core protected by carbon shell in a synthesised nanocomposite with a core/shell structure. Hence, nanocomposite was the most efficient additive, with 97% dechlorination efficiency in the first hour while the efficiency of the planetary ball mill in the presence of CaO was only 76% after 12 hours of milling. Furthermore, major changes occurred in the organic phases, carbon and chlorine bonds. Simultaneously, graphite and amorphous carbon were produced in the final products after milling. This indicates that aromatic structures and C-Cl bonds were broken down into inorganic compounds. Thus, this study presented a practical method of producing materials while degrading hazardous pollutants in petrochemical wastewater sludge through a planetary ball mill. Graphical abstract


Introduction
Increasing awareness of the adverse and sometimes irreversible effects of pollutants on human health and the environment on the one hand and the greater ability of scientific communities to identify these pollutants on the other hand has led governments to set stricter regulations for industrial outputs.Among these pollutants are polycyclic aromatic hydrocarbons (PAHs), persistent organic pollutants (POPs) such as polymers, and polychlorinated compounds (PCs) [1][2][3][4].Persistent organic pollutants are toxic and stable compounds that are not easily degradable and can thus accumulate in living tissues and finally be transported through the food chain [5,6].Hence, if these stable compounds are present in industrial waste, there is greater concern for their adverse effects on humans and the natural environment [7].The petrochemical industries produce a wide range of wastewater and sludge and so their wastes are classified as 'hazardous' due to the presence of persistent organic pollutants in wastewater.Statistical data shows that in the United States and China, approximately 30,000 tons and 3 million tons of petroleum sludge are produced annually by the petrochemical industry, respectively [8][9][10][11].Currently, few industrial technologies such as high temperature combustion, biodegradation and chemical decomposition are utilised to treat large amounts of these types of pollutants.These conventional technologies might be limited because of concern regarding the production of byproducts due to incomplete combustion, the presence of a high concentration of toxic substances that inhibit microbial activity in biodegradation, and the presence of resistant compounds that make chemical decomposition more difficult [12][13][14][15].
In recent years, several studies have been conducted on the effectiveness of mechanochemical methods to degrade persistent organic pollutants.Mechanochemical treatment is an efficient, safe, and inexpensive non-combustion method for POPs disposal [16,17].As a result, IUPAC (International Union of Pure and Applied Chemistry) has introduced this technology as one of the ten technologies that will change the world.In addition, USEPA (United States Environmental Protection Agency) in 2010 introduced the non-combustible method as one of the most valid and acceptable methods for removing hazardous pollutants at the industrial level.Due to the creation of greater energy, the planetary ball mill intensifies the mechanical forces and thus accelerates the degradation process of pollutants.However, the type of used reagent is very effective in its efficiency [18][19][20][21].In recent years, studies have been conducted at laboratory and industrial levels to remove POPs contaminants through mechanochemical methods.For instance, the Environmental Decontamination Ltd. (EDL) has developed a mechanochimical process able to treat soils contaminated with various POPs (such as DDT, Aldrin, Dieldrin, and Lindane) at the Fruitgrowers chemical company site in Mapua, New Zealand [22].The focus of the work by Yu et al. was mechanochemical destruction of mirex co-ground with iron and quartz in a planetary ball mill [23].In another study, Zhang et al. used a planetary ball mill to destroy pentachloronitrobenzene with reactive iron powder [24].
Metal-Organic Frameworks (MOFs) which are also known as Porosity Polymer Composites (PCPs) are a class of compounds that have coordination centres of binders (which are organic ligands) and metal ions.These materials have unique properties, including very high specific surface area, low density, high biocompatibility, and high mechanical and thermal stability.One of the most widely used MOFs, extensively used as a catalyst and adsorbent, is Mill-100(Fe) with metallic cores made of iron [25][26][27][28].In recent years, the production of MOFs using mechanochemical methods has received considerable attention from researchers because the fabrication of materials by mechanochemical methods is more advantageous than the fabrication of materials by conventional methods such as solvothermal method.Some of these advantages include lack of a need for using solvents and heating and the possibility of performing the process in the solid phase [29,30].In addition, recent studies have shown that MOFs can be converted by heat treatment to metal nanoparticles and carbon structures to form a core/shell nanocomposite [31].The Fe-Carbon nanocomposite (Fe@C) with a core/shell structure has an almost spherical morphology with the iron metal located in the core and and enveloped by carbon coils to protect it from corrosion and the asynchronous transfer of electrons [32,33].Recent studies show that Fe-Carbon nanocomposite synthesised by this method is cost-effective as it requires no additional carbon source and involves only a single calcination step [34,35].
Although studies on the removal of hazardous organic compounds using mechanochemical methods have been conducted in recent years, the significance of the present study is that for the first time, the sludge of petrochemical wastewater treatment plant was treated using this method.Furthermore, Fe@C nanocomposite with core/shell structure was produced from MOFs through the mechanochemical method.Hence, the main purpose was to investigate the dechlorination of polychlorinated pollutants in petrochemical wastewater treatment sludge in the presence of Fe@C nanocomposite through a planetary ball mill for the first time.A further objective of the present study was to identify and analyse synthesised Fe@C nanocomposite with core/shell structure obtained from MOFs by mechanochemical method.

Materials
Petrochemical wastewater treatment sludge was directly collected from the sludge disposal area of the Petrochemical Wastewater Treatment Plant in the southwest region of Iran which contains a wide range of organic compounds (Supplementary file, Table S1).Also, in (Supplementary file, Table S2) more information is shown according to X-ray fluorescence analysis of the raw sample.For the synthesis of nanocomposite 1,3,5 benzenetricarboxylic acid (H 3 BTC), iron(III) nitrate nonahydrate (Fe(NO 3 ) 3 .9H 2 O), and Tetramethylammonium hydroxide (TMAOH) were provided from Sigma Ltd company.To compare the effect of the nanocomposite on the efficiency of planetary ball mill for dechlorination of contaminants with other reagents, CaO was calcined at 900°C for 4 hours and then used.

Synthesis of nanocomposite
The MIL-100(Fe) was synthesised mechanochemically as follows: benzene-1,3,5-tricarboxylic acid (H 3 BTC) 0.471 g, Iron(III) nitrate nonahydrate (Fe(NO 3 ) 3 .9H 2 O) 1.29 g and 5 mL of an aqueous solution of Tetramethylammonium hydroxide (TMAOH) 0.22 M were added to a stainless steel vial [36].The mixture was milled in a planetary ball mill (PM-2400, Alborz Co., Karaj, Iran).The ball mill was equipped with balls weighing 14.6 g and measuring 1.8 mm in diameter, and placed in a 100 mL stainless steel vial.Ingredients were milled for 1 hour.The samples were washed 3 times and then air-dried at room temperature.The colour of the dry sample was light-orange (Supplementary file, Figure S1).Finally, to prepare the Fe@C nanocomposites, the sample of MIL-100(Fe) was carbonised at 800°C for 2 hours under the protection of N 2 [31,36].The resulting sample was reddish-brown (Supplementary file, Figure S2).

Mechanochemical procedure
A planetary ball mill (PM-2400, Alborz Co., Karaj, Iran) equipped with a pair of stainless steel vials (100 cm 3 volume) was used for the mechanical milling treatment of the pollutants in the presence of synthesised nanocomposites and CaO.Hence, synthesised nanocomposites and CaO were selected as reagents.The diameter of the stainless steel ball was 1.8 mm.Based on the results of previous studies on degradation of POPs, the optimal conditions were applied for maximum efficiency [37,38].Therefore, the same conditions (rotation speed of 400 rpm, charge ratio of 40:1 g, and reagent to pollutant ratio of 4:1 g) were used in the present study.The planetary ball mill operation was planned with a 15-minute break every 15 minutes of rotation to prevent overheating of the system.The maximum time for milling was considered to be 12 hours.To determine chloride ions in the mixture before and after milling, a portion of the sample (0.05 g) was added to 20 mL ultrapure water and then stirred for 20 min.After ultrasonic treatment for 30 minutes, the suspension was filtered with a 0.47 micrometre fibre filter.This procedure was repeated twice, and finally, the residue was analysed by ion chromatography (IC-732 Detector, Metrohm, Switzerland) to screen for chloride ions [37].The schematic of the methodology is shown in Scheme 1.

Characterisation of synthesised materials and milled products
To determine the characteristics and properties of synthesised MIL-100(Fe) and Fe@C nanocomposite, XRD analysis was performed with Cu Kα radiation at 40 kV and 30 mA using an X-ray powder diffractometer (Rigaku, Japan).SEM images using FESEM-TESCAN MIRA3 and FEI Quanta 200 scanning electron microscopes on an accelerating voltage of 30.0 Kv and transmission electron microscope (TEM) was performed using a FEI Tecnai G 2 spirit at acceleration voltage of 80 kV [39] to observe the micro-structural and morphological composition of the prepared Fe@C nanocomposites.A Fourier transform infrared (FTIR) spectrometer (Bomem, MB-Series, Japan) with the KBr disk method and Raman Spectrometer (DX 532, USA) with absorption wavelength at 532 nm were used for further analysis.

Properties of the synthesised materials
Since MIL-100(Fe) was made by a mechanochemical method in this study, XRD analysis was performed to evaluate its properties and compliance with the standard sample and XRD difarction paterns are shown in Figure 1.As shown in Figure 1, the results of the XRD analysis for the synthesised MIL-100(Fe) sample (Figure 1b) are in accordance with the standard diffraction pattern of MIL-100(Fe) (Figure 1a).Although in some cases peaks overlapped compared to standard sample peaks, synthesised sample diffraction pattern had higher intensity peaks.The XRD pattern (Figure 1b) of the synthesised sample completely matched the simulated one (Figure 1a) reported in the literatures [40][41][42].However, it appears that the material prepared by the mechanochemical method had more pore space because physical operation accelerates and improves the occurrence of chemical reactions.On the other hand, physical operations increase the chemical reactions of solvents and reactants, thus reducing the consumption of TMAOH and ultimately producing MIL-100(Fe) with a better and more regular structure [31,36,43].After the carbonisation of MIL-100(Fe) at 800°C for 2 hours in the flow of nitrogen gas to produce nanocomposites, FESEM and TEM analyses were performed and shown in Figure 2. The results of the FESEM analysis showed that the produced nanocomposites had almost spherical and uniform shapes with average sizes of less than 100 nm (Figure 2a).Furthermore, as shown in Figure 2b, the overall size of the iron nanoparticles was approximately 50 to 80 nanometres in the core and the thickness of the carbon coating was less than 10 nanometres which could greatly improve the stability of metal cores.The high temperature decomposed the organic ligands of the MOF, transforming them into a spherical hollow carbon structure with the iron at the centre of the carbon shell.As a result, Fe@C nanocomposite with core/shell structure was produced in diameters between 30 to 370 nm (Figure 2a).As it can be observed in Figure 2b, a thin layer, much thinner than the size of the Fe nanoparticles in the core, appeared to have formed in the nanocomposite structure [44,45].Figure 2c illustrates the XRD pattern of the prepared nanocomposite.Two well diffraction peaks confirmed the formation of zero-valent iron (JCPDS 06-0696).As a common impurity in Fe@C nanocomposites, Fe 3 C was also detected in the prepared nanocomposite (JCPDS 75-0910).Heat treatment above 800°C removed many of the impurities, but the diffraction peaks of γ-Fe 2 O 3 (JCPDS 39-1346), as expected, could be seen due to the presence of carboxylate ligand in MIL-100(Fe) as the source of oxygen.In addition, a small peak at 26.1° (JCPDS 35-0772) was assigned to the graphite, which indicated that the presence of iron oxide and iron species might catalyse the crystallisation of carbon components.

Dechlorination extent
Previous studies have confirmed that by increasing the rotation speed of the mill and charge ratio (i.e. the weight ratio of the balls to the material), the efficiency of the planetary ball mill for degradation of persistent organic pollutants increases [23,37,46].Therefore, in the present study, to evaluate the efficiency of the planetary ball mill for degradation of organic chlorine pollutants in the sludge of petrochemical wastewater, the rotation speed of the mill was adjusted at 400 rpm with a charge ratio of 40:1 g.Moreover, the weight ratio of reagent to pollutant was determined to be 4:1 g (a total of 5 g of chemicals per vial).The extent of dechlorination was also examined at 1, 3, 6, and 12 hours in the presence of CaO, Fe@C nanocomposite (with core/shell structure) and in the absence of reagents.All calculations were performed based on chloride ion changes in raw samples and refined samples at different experimental time which shows in Table 1.To determine the petrochemical wastewater treatment sludge sample's chemical composition, X-ray fluorescence (XRF) analysis was used, the data are presented in Supplementary Table S2, total chlorine had the highest concentration in the raw samples.In addition, based on the results of the ion chromatography analysis 1.2% of the chlorine in the raw sample was mineral chlorine, and the remaining was organic chlorine.According to the data in Table 1, the amount of chloride ion increased with increasing milling time in the presence of nanocomposite.
Detailed results of different dechlorination efficiency of the planetary ball mill in the presence of additives are displayed in Figure 3.As it can be seen in Figure 3a, the dechlorination efficiency of the planetary ball mill in the presence of Fe@C nanocomposite with core/shell structure was 97% in the first hour, but after 12 hours, it reached almost 98%.Figure 3b compares dechlorination efficiency for planetary ball mill with various reagents at different times.The results indicated that the dechlorination efficiency in the presence of CaO after 12 hours was 76%, but it was only 27% in the absence of reagents.The highest efficiency of the system was due to the presence of Fe@C nanocomposite with core/shell structure.In the first hour, almost all degradable organic chlorine compounds were converted to chloride ions, confirming that the size of the nanocomposite is very effective for increasing efficiency.However, iron, which was most likely present as zero-valent iron nanoparticles in the nanocomposite core, was effective for improving system efficiency [47][48][49].It is noteworthy that the carbon shell around iron in the nanocomposite structure prevented the exchange of iron electrons with oxygen and protects zero-capacity of iron [50,51].Furthermore, the energy generated in the planetary ball mill which occurred during friction or collision between balls and the vial wall caused the nucleophilic substitution of Cl − with H − to occur.Hence, the presence of zero- valent metal accelerated the MC destruction by providing free electrons [31,52].Due to the deep impact of the balls with the nanocomposite and the pollutants in the vial of the planetary ball mill, conditions are provided for chemical reactions and electron transfer.In the absence of a reagent, the efficiency of the planetary ball mill for dechlorination was 27%.Since the sludge of the petrochemical wastewater treatment plant contained different types of metals, the mechanical activities of the system accelerated chemical reactions causing the degradation of persistent organic pollutants.Based on previous studies, MC method is a completely viable non-combustion technology to destroy POPs that does not require heating or off gas treatment, thereby preventing secondary pollution [12].Although the results of the ion chromatography (Table 1) showed that most of the organic chlorine compounds were converted to inorganic chlorine, nevertheless the final degradation products after the MC treatment of the petrochemical wastewater treatment sludge have not yet been identified and warrant further investigation.
Because the mechanism of dechlorination in the planetary ball mill is not fully understood, many researchers believe that free radical generation is one of them.Several reactions have been proposed for degradation of POPs using a mechanochemical process.These reactions include dehydrohalogenation, oxidation and crosslinking A wide range of chemical reactions consisting of dechlorination, dehydrogenation, dimerisation and oxidation cleavage of the C-C bonds are involved in the degradation of polychlorinated organic pollutants in addition to some unknown reactions [53][54][55].Likewise, 98% efficiency of dechlorination was achieved after 12 h milling using Fe@C nanocomposite as an additive while chloride ions were not recovered completely.Since the efficiency of dechlorination was not 100%, it seems that much of the polychlorinated compounds were transformed into dechlorinated organic compounds and broken down into inorganic compounds and some parts of the organic chlorine converted into other forms than chloride and likely released into the air through MC treatment [16,37,[56][57][58].To better understand the effect of synthesised nanocomposites on increasing the removal efficiency of POPs by a planetary ball mill, Table 2 compares the efficiencies in the presence of various reagents studied by other researchers.As can be observed, in none of the previous studies was high efficiency achieved in 1 hour.

Characterisation of ground samples
Optical spectrophotometers such as FTIR and Raman are common methods for detecting heavy organic compounds in field samples.They can show functional groups and changes in carbon bands after degradation [23,59].Figure 4 shows the results of FTIR analysis in samples before and after the mechanochemical process in the presence of different reagents.The FTIR results for the sludge sample of the petrochemical wastewater treatment plant are shown in Figure 4.The spectrum indicated the presence of carbonchlorine bonds at 667 cm −1 and 602 cm −1 .In addition, bands that corresponded to C-H bonds (1025 cm −1 ), C-C bonds (1437 cm −1 and 1537 cm −1 ) were detected in the sludge sample of the petrochemical wastewater treatment plant.Additional band at 3437 cm −1 and 3425 cm −1 were observed which could be related to H 2 O, these peaks disappeared in the milled products [53,54].Comparing the results of the FTIR analysis of the milled sample in the presence of CaO with the results of the FTIR analysis of the raw sample, it was found that the peaks corresponding to chlorine compounds (667 cm −1 and 602 cm −1 ) disappeared but appeared in the new spectrum range between 700 cm −1 to 750 cm −1 , these could be associated with the mono-substitution of Cl in the benzene ring.The range of these peaks could be due to the presence of aromatic structures in the purified sample with CaO [37,60,61].In the refined sample with CaO, the intensity of peaks related to the C-Cl bond were reduced, and only the 1424 cm −1 and 3642 cm −1 peaks corresponding to the C-O and O-H bonds, respectively, were clearly visible [23,38,62].
During the process and operation of milling, many aromatic structure bonds were broken, and simple and branched compounds were produced with oxygen and hydrogen bonds.Furthermore, many aromatic compounds were degraded into fatty and esterified acidic compounds when C-O and O-H bonds formed.Since, in the spectrum, a band close to the (1150 cm −1 ) is seen, it can be concluded C-O-C or C-O-H bonds were present in the sample.Moreover, the peak intensity of the -Cl and C = C bonds decreased and disappeared due to the degradation of chlorinated and aromatic compounds present in the sludge sample of the petrochemical wastewater treatment plant [56,63,64].Comparing the results obtained after the mill operation in the presence of nanocomposite and CaO, it was notably observed that the intense peaks of 667 cm −1 , 602 cm −1 , 1025 cm −1 , 1244 cm-1, 1345 cm −1 , 2916 cm −1 , and 2852 cm −1 completely disappeared in refined samples with nanocomposite.The range of these peaks would be related to C-Cl bond, C-H, CH2bonds and organic chlorine compounds in the carbon chains which are in compliance with original persistent organic pollutants peaks [55,65,66].This finding was confirmed by the ion chromatography results (Figure 3) which reported that the dechlorination efficiency of the system in the presence of nanocomposite (97%) was higher than the dechlorination efficiency of the system in the presence of CaO (76%).
Figure 5 shows the Raman spectra of the final products of the planetary ball mill in the presence of two nanocomposites and CaO reagents.As shown in Figure 5, the peaks in the range of 900 cm −1 to 2000 cm −1 are related to = C-H bonds and alkynes, respectively.The highest intensity peaks at 1361 cm −1 and 1572 cm −1 indicate the formation of more amorphous carbon and C-H and C-C bonds.Therefore, the final products were special carbonic materials, which were amorphous and contained graphite carbon.Thus, after the degradation of organic chlorine pollutants and from the degradation of C-Cl bonds, aromatic structures were converted to graphite and eventually led to the production of amorphous carbon [53,56,67].Furthermore, comparing the results of the Raman analysis of the raw sample with the results of the milled sample in the presence of CaO and nanocomposite, it was observed that the intense peaks between 500 cm −1 to 1000 cm −1 and the intense peaks between 2000 cm −1 to 2500 cm −1 decreased in the refined samples with CaO and completely disappeared in the refined samples with nanocomposite.This affirms that aromatic structures were converted into defective graphite by the combination of shearing force and friction during the milling and in the presence of zero-valent iron in the core of the nanocomposite which could accelerat electron exchange by defective graphite layers [68,69].
The FESEM images of the final products of the planetary ball mill in the presence of additives are shown Figure 6.The FESEM micrographs of the final products of the planetary ball mill in the presence of reagents in Figure 6 demonstrates that the particle size became much smaller.This was as a result of the mechanochemical process which not only created and accelerated chemical reactions in the planetary ball mill but also created, due to the transfer of energy from the rotation of the mill, mechanical forces such as elastic deformations, plastic deformations, and fracture affecting the compounds in the vial and eventually reducing the particle size, increasing the 'surface to volume ratio' and producing active surfaces [12,70].The particle size of the final products of the planetary ball mill in the presence of Fe@C nanocomposite was in the range of 30 nm.In contrast, the particle size of the final products of the planetary ball mill in the presence of CaO was in the range of 50 nm.Furthermore, the final products in the presence of Fe@C nanocomposite were enwrapped and coagulated.

Conclusions
The present study confirmed an effective strategy to synthesise a high-quality metalorganic framework with a better and more regular structure and low cost and convenient mechanochemical method.Fe@C nanocomposite with core/shell structure was prepared from synthesised MIL-100(Fe) which had a very high efficiency as a reagent for increasing dechlorination efficiency through a planetary ball mill.Organic chlorine compounds present in the raw sludge samples from the petrochemical wastewater treatment plant might be associated with the presence of polychlorinated and polymer compounds.Additional analysis also revealed the presence of C-Cl bonds and aromatic carbon bonds.Both Fe@C nanocomposite and CaO applied as reagents in this study were effective in aiding degradation of pollutants by using a planetary ball mill.Furthermore, the Fe@C nanocomposite used as the reagent in the planetary ball mill was extremely effective in improving the efficiency of the system, reducing marginal costs such as energy consumption and depreciation of the mechanical components of the system.Moreover, after 1 hour of milling in the presence of the Fe@C nanocomposite, 97% of organic chlorine compounds were converted to water-soluble mineral chlorine.In addition, further analysis confirmed that many of the C-Cl and C = C structures associated with benzene rings collapsed and organic regular structures were disrupted.The system indicated maximum efficiency for Fe@C nanocomposite.Therefore, the protection of zero-valent iron in the nanocomposite core by carbon shell directly impacted efficiency.Finally, it can be concluded that the planetary ball mill was successful in constructing MOF and degradation of persistent organic pollutants in the petrochemical wastewater sludge.Not only were new materials (Fe@C nanocomposite with core/shell structure prepared from MIL-100(Fe)) produced, but also hazardous pollutants present in the sludge of petrochemical wastewater treatment plant were decontaminated.This synchronicity of creating some materials while destroying others can be applied to other industries in practice by performing more analyses.

Figure 2 .
Figure 2. (a) FESEM images of synthesised nanocomposite at 100 nm, (b) TEM images of synthesised nanocomposite at 50 nm and (c) XRD pattern of Fe@C nanocomposite.

Figure 3 .
Figure 3. Dechlorination efficiency of the planetary ball mill (a) in the presence of Fe@C nanocomposite, (b) comparison of dechlorination efficiency with different reagents (all operated under rotation speed 400 rpm, charge ratio 40:1 at different times).

Figure 4 .
Figure 4. FITR spectra of the petrochemical sludge and its residue milled in the presence of different reagents at 12 h.

Figure 5 .
Figure 5. Raman spectra of the petrochemical sludge and residue milled in the presence of different reagents at 12 h.

Figure 6 .
Figure 6.FESEM images of the final products of the planetary ball mill in the presence of (a) Fe@C Nanocomposite 1 µm, (b) Fe@C Nanocomposite 200 nm, (c) CaO 1 µm, and (d) CaO 200 nm.

Table 1 .
Ion chromatography analysis (wt%) of the petrochemical wastewater treatment sludge before and after treatment at different time.

Table 2 .
Summary of numerous reagents utilised in other studies that achieved best degradation results.