Monitoring of pH, Oxidation-Reduction Potential and Dissolved Oxygen to Improve the Performance of Dimethyl Phthalate Removal From Aqueous Solutions

Ali Reza Rahmani; Mohammad Taghi Samadi; Reza Shokoohi; Hassan Zolghadr Nasab

doi:10.17795/ajehe-4206

Monitoring of pH, Oxidation-Reduction Potential and Dissolved Oxygen to Improve the Performance of Dimethyl Phthalate Removal From Aqueous Solutions

Ali Reza Rahmani ¹ , Mohammad Taghi Samadi ¹ , Reza Shokoohi ¹ and Hassan Zolghadr Nasab ^{2
, *}

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Abstract

Since the process of heterogeneous Fenton is more complex and dynamic than the common Fenton process, control and online monitoring is entirely vital for optimum operation of this system. This study was aimed to investigate the effects of pH, oxidation-reduction potential (ORP) and dissolved oxygen (DO) variations on dimethyl phthalate (DMP) removal from aqueous solutions using the heterogeneous Fenton process with nano zero-valent iron (nZVI). Parameters affecting the process like contact time (5 - 120 minutes), pH (2 - 10), H₂O₂ concentration (0.01 - 1 mmol/L) nZVI content (0.01 - 0.5 g/L) and initial DMP concentration (2 - 50 mg/L) were also studied. It was found that, at optimum amounts (pH = 4, H₂O₂ concentration = 0.1 mmol/L, nZVI = 0.05 g/L, initial DMP concentration = 2 mg/L, and contact time = 60 minutes), approximately 98% of the pollutant was removed. This process could treat DMP well from aqueous environments and can be introduced as a cheap and effective method.

1. Introduction

Phthalic acid esters (PAEs) are an important group of persistent pollutants, widely employed for increasing flexibility, transparency, durability, and longevity of hard polyvinyl chloride (PVC) resins, and they are spread in the environment because of their week physical bonding in plastic structures, as they have been identified in soil, air, food, sewage sludge, landfill leachate, and even rain (1-3). PAEs are regarded as priority pollutants on account of having mutagenic and carcinogenic effects (2, 4).Dimethyl phthalate (DMP) is the most simple and common compound of PAEs which has low molecular weight and is considered as a good solvent. It is also applied in various industries like cosmetics, insect repellent, insecticide carries, and propellants (2, 5). This compound can disturb endocrine glands performance, natural growth, leukocyte chromosomes and so forth (2, 5-8). Since phthalates are hydrophobic, current conventional methods are not able to remove these compounds from water environments (1).

New and modern ways are required for the treatment of these compounds from the industrial effluents. Recently, coagulation, sedimentation, adsorption with chitosan and activated carbon, aerobic and anaerobic biological processes and advanced oxidation processes (AOPs) for removal of phthalates from aqueous solutions have been studied. However, lack of adequate removal, preparation, regeneration of absorbents cost, and sensitive microorganisms to condition changes such as methane production as a result of phthalate degradation are some disadvantages of these methods (9). Among these, advanced oxidation processes are highly effective in removal of water and wastewater because of the following advantages: high efficiency, easy operation, and high ability in removing unbiodegradable pollutants (3, 4).

One of the main mechanisms of AOPs is on the basis of active hydroxyl radical formation, able to treat target pollutants. Fenton process is an AOP capable of oxidizing well in the presence of iron ions (10, 11). Nonetheless, high volume of sludge production, lack of a reaction between ferric ions and H₂O₂, consumption of the generated OH^• and need for a permanent source of ferric ions are the major downsides of this process (12, 13). The heterogeneous Fenton process is a suitable alternative to the common method, which is aimed to use reversibly iron catalysts. Heterogeneous Fenton by means of catalysts of zero-valent iron (ZVI), Fe₃O₄ and Fe₃O₄/ Fe⁰ has been studied for treatment of various pollutants (14-16).

Nano zero-valent iron (nZVI) has a higher reaction power and specific surface area than ZVI. nZVI causes the reduction of intermediates and, in turn, these products are easily and quickly degraded over the Fenton process (17). It should be noted that online monitoring of the heterogeneous Fenton process with nZVI in order to control optimally the process nearly has not been investigated in previous studies; monitoring of economic parameters: oxidation-reduction potential (ORP), dissolved oxygen (DO), and pH can enhance the feasibility of using this process (18). When nZVI is added to wastewater to remove DMP, ORP can change rapidly and nZVI can consume DO. Moreover, reduction of DMP by nZVI results in H⁺ consumption or OH^- production. Therefore, online monitoring of the method is essential (18, 19).

The main objective of the study was to investigate DMP removal by means of the heterogeneous Fenton process. Furthermore, parameters of ORP, DO and pH were monitored during the process.

2. Materials and Methods

In this study, all the samples were prepared synthetically and all the chemicals (analytical grade) were purchased from Merck Co. (Germany). The standard stock solution was prepared by dissolving 0.832 mL of DMP solution (purity 99%) in 1000 mL dilution water and then the concentrations were prepared by serial dilutions of this stock. To adjust the pH, H₂SO₄ (0.1 N) and NaOH (0.1 N) were used, using a pH meter (HACH sension1 model). A mechanical mixer was used to provide enough mixing and the catalyst was separated from the samples by centrifugation as well as cellulose acetate filter. The samples taken out were analyzed by an HPLC device (Smart line auto sampler 3950 HPLC, A5005-1, KNAUER Company, Berlin, Germany) equipped with a UV detector (K2600-KNAUER, Germany) with C18 column (4.6 mm × 250 mm). The mobile phase was an acetonitrile and distilled water (0.01 M, pH = 6) in the ratio of 50:50 v/v. The volume of all injections was 40 µL (1). Moreover, the retention time was 5.1 ± 0.01. Figure 1 shows the HPLC chromatograph and the molecular structure of DMP. All the experiments were performed by changing pH values (2-10), H₂O₂ concentration (0.01 - 1 mmol/L), nZVI content (0.01 - 0.5 g/L) and initial DMP concentration (2 - 50 mg/L) and optimum amounts were attained by the one-at-a-time method. To increase the precision and confidence coefficient of the findings obtained, all the experiments were repeated three times and the averages were reported. The amount of DMP removal in this process was calculated through Equation 1:

(1)

Where C_i and C_e are, respectively, initial and equilibrium concentrations of DMP.

Figure 1. Chromatograph and Molecular Structure of Dimethyl Phthalate

3. Results and Discussion

3.1. pH Effect

Figure 2 presents the effect of pH solution on DMP removal by the process. As can be seen, with increasing the pH value from 2 to 4, the efficiency reached 85.7% in 60 minutes. The highest efficiency (88.5%) was attained in 120 minutes. However, the efficiency decreased with increasing the pH value. Since the highest efficiency was observed at pH of 4, it was chosen as the optimum pH. Since the pH of solution has a control effect on the activity of the catalysts and oxidant and predominant species of iron ion as well as H₂O₂ stability in the media, it is one of the most important factors in removing the pollutant in the Fenton process (20).

Figure 2. Effect of pH on Dimethyl Phthalate Removal, DMP concentration = 10 mg/L; H₂O₂ concentration = 0.1 mmol/L; nZVI dosage = 0.05 g/L.

The mechanism of oxidant production includes Fe⁰ oxidation and then the Fenton reaction. At first, Fe⁰ is oxidized by H₂O₂ (reaction 1) (21, 22). Next, the oxidants responsible for organic matters oxidation (mainly OH^•) are formed by the Fenton reaction (reaction 2). At pH values above 5, a weaker oxidant like ferric ion (for example FeO⁺²), which acts more selective than OH^•, can be created over reaction 3 (23):

(1) Fe⁰ + H₂O₂ + 2H⁺ → Fe²⁺ + H₂O

(2) Fe²⁺ + H₂O₂ → Fe³⁺ + OH^• + OH^-

(3) F²⁺ + H₂O₂ → Fe (IV) (e.g., FeO²⁺) + H₂O

A decrease in efficiency at high pH values can be attributed to H₂O₂ degradation; in addition to this, the oxidation potential of OH^• declines and ferric hydroxide complexes are formed resulting in a decrease in OH^• production (22). The degradation of DMP is very little at the pH value of 2, because at low pHs the stability of H₂O₂ is high and also H₃O⁺ is created which prevents H₂O₂ from reacting with Fe⁺². Therefore, an amount of OH^• is produced and, in turn, the efficiency decreases (24). According to the obtained results, the pH range between 4 and 6 had a desirable performance.

The results of similar studies accord with those of the current study. A study by Moon et al. in which the heterogeneous Fenton with nZVI was used to remove azo dye (Orange II), reported that at pH = 3, the highest efficiency occurred (17). Kallel et al. showed that the highest chemical oxygen demand (COD) removal happened at pH values from 2 to 4 when this method was utilized to treat olive mill wastewater (25). Xu et al. investigated 4-chloro-3-methyl phenol removal by means of heterogeneous Fenton with nZVI (23).

3.2. H2O2 Effect

With increasing the H₂O₂ concentration from 0.01 - 1 mmol/L, DMP removal increased from 47.8 to 88.5% (Figure 3). A 35% drop was seen in efficiency when the concentration of H₂O₂ was raised to 1 mmol/L. Therefore, the content of 0.1 mmol/L was selected as the optimum amount.

Figure 3. Effect of H₂O₂ Concentration on Dimethyl Phthalate Removal, pH = 4; DMP concentration = 10 mg/L; nZVI dosage = 0.05 g/L.

It should be noted that this variable is very important in Fenton process and is a key control parameter in wastewater treatment plants. The oxidation power of the process increases when there is an increase in OH^• from H₂O₂ degradation in the solution. At the beginning of the reaction, increasing H₂O₂ concentration had a positive effect on the efficiency, but a decline was observed in efficiency over time. Although in oxidation Fenton process hydroxyl radicals are mainly responsible for pollutant degradation, at high contents of H₂O₂ competitive reactions happen as follows (24):

(4) OH^• + H₂O₂ → HO₂^• + H₂O

(5) HO₂^• + H₂O₂ → OH^• +H₂O + O₂

(6) 2HO₂^• → 2H₂O₂+ O₂

(7) HO₂^• + OH^• → H₂O+ O₂

(8) 2OH^• → H₂O₂

It can be found from reactions 4 - 8 that some of hydroxyl radicals are consumed before DMP oxidation and it creates an inhibitory effect. The efficiency went down when the concentration of H₂O₂ exceeded 0.1 mmol/L. The bottom line is that for noticeable formation of hydroxyl radicals H₂O₂ should be at its optimized amount (24). The results of a study by Kallel et al. are in agreement with our findings (25). The concentration of 0.3 mmol/L of H₂O₂ was reported as the optimum amount in a study by Xu et al. who investigated 4-chloro-3-methyl phenol removal (23). In addition, Hou et al. found the economical and optimum amount of H₂O₂ for Rhodamine B treatment (26).

3.3. nZVI Effect

Scanning electron microscopy (SEM) images of nZVI before and after heterogeneous Fenton oxidation are shown in Figure 4. Based on the illustration, the structure of nZVI is compact, which is because of the magnetic force among fine particles. As can be seen, all the particles have spherical shapes. After the heterogeneous Fenton reaction, density decreased and iron oxide is formed as a result of nZVI corrosion in aqueous solutions. This leads to leakage of Fe⁺² and Fe⁺³ from the surface of nZVI and iron oxide (9). According to Figure 5, determined sites in Fourier transform infrared spectroscopy (FTIR) spectrum before and after heterogeneous Fenton oxidation include 3415 cm^-1 (stretching vibration of OH) and 1633, 1504, 1235, and 987 cm^-1 are for FeO bonds in Fe₂O₃ and Fe₃O₄ (9).

Figure 4. Scanning Electron Microscopy Images of nZVI, A, before and B, after the heterogeneous Fenton oxidation.

Figure 5. Fourier Transformed Infrared Spectra of nZVI, A, before and B, after the Fenton process.

The findings from the effect of nZVI concentrations on the efficiency are shown in Figure 6. With increasing the concentration of DMP to 0.05 g/L, the highest removal efficiency (51.7%) was seen and then it decreased. Therefore, 0.05 g/L of DMP was selected as the optimum amount.

Figure 6. Effect of nZVI on Dimethyl Phthalate Removal, pH = 4; DMP concentration = 10 mg/L; H₂O₂ concentration = 0.1 mmol/L.

Increasing nZVI and active sites results in more H₂O₂ degadation and consequently more hydroxyl radicals are produced, which enhances the DMP removal. Of course, at concentrations higher than the optimum amount, nZVI, which is added to the solution, is accumulated and it makes hydroxyl radicals to be scavanged and it finally decreases the efficiency (9).

Besides, when there is more nZVI in the solution, most of Fe⁺³ ions change to Fe (OH)⁺², which in acidic conditions are left out and the performance decreases (22). The study by Moon et al. showed that the efficiency reached from 30% to 95% with increasing the nZVI amount from 5 to 20 mg/L, but an increase in concentration to 100 mg/L did not change the results (17). Gulkaya et al. found that in Fenton process, up to 95% of COD was removed at the optimum amount of FeSO₄ (5.5 g/L) (27).

3.4. Effect of Initial Dimethyl Phthalate Concentration

As can be seen in Figure 7, with increasing the initial DMP concentration from 2 to 50 mg/L the efficiency decreased from 98.6% to 35.4% in 120 minutes.

Figure 7. The Effect of Initial Dimethyl Phthalate Concentration on Dimethyl Phthalate Removal, pH = 4; H₂O₂ concentration = 0.1 mmol/L; nZVI dose = 0.05 g/L.

The increase of pollutant molecules in the solution can make them to be absorbed on the nZVI surface and more active sites to be occupied. This causes the sites to be unavailable for H₂O₂ molecules and results in a decrease in hydroxyl radical formation on the surface of nZVI. Like our study, Xu et al. claimed that the increase of 4-chloro-3-methyl phenol caused a decrease in efficiency (23). Further, Babuponnusami et al. found that phenol removal by means of heterogeneous photo electro Fenton-like process decreased with increasing the initial phenol content (20).

3.5. Effects of H2O2, nZVI, and nZVI/H2O2

To evaluate the effect of each contributing parameter in the Fenton-nZVI process for removal of DMP, the effects of individual parameters were investigated and shown in Figure 8. DMP degradation in the presence of H₂O₂, nZVI, and nZVI/H₂O₂ was determined 10.1%, 53.5%, and 88.5%, respectively, indicating that the most degradation of DMP was achieved through the nZVI/H₂O₂ process due to the release of Fe²⁺/Fe³⁺ from nZVI followed by reaction of the spices with H₂O₂ and then the production of highly reactive hydroxyl radicals, which were able to degrade DMP (10).

Figure 8. Effects of H₂O₂, nZVI and nZVI/H₂O₂ on Dimethyl Phthalate Removal, DMP concentration = 2 mg/L; pH = 4, H₂O₂ concentration = 0.1 mmol/L; nZVI.

3.6. Oxidation-Reduction Potential Variations in Heterogamous Fenton Process With nZVI

There are two oxidation and reduction reactions. Therefore, the changes of ORP are not unusual and it is very necessary that this parameter is monitored. In addition, the changes of H⁺/OH^- and DO take place with reactions related to nZVI reduction or Fenton oxidation (18).

As can be clearly seen in Figures 9 and 10, after adding nZVI, ORP decreased. nZVI lowers ORP because it has strong oxidation power (E° = -0.4 V). There was no significant relationship between ORP and contact time, while the parameters of ORP and nZVI were entirely related together. At low and high contents of H₂O₂, the decrease of ORP is more noticeable (19).

Figure 9. The Variations of pH, Oxidation-Reduction Potential and Dissolved Oxygen on the Heterogeneous Fenton Process With nZVI at Different Amounts of nZVI, pH = 4; DMP concentration = 10 mg/L; H₂O₂ dose=0.1 mmol/L.

Figure 10. A, The effect of pH; B, ORP; C, DO, on heterogeneous Fenton process with nZVI at different contents of H₂O₂. pH = 4; DMP concentration = 10 mg/L; nZVI dose = 0.05 g/L.

With increasing the nZVI, the pH of the solution went up. Based on reactions 9 -11, nZVI consumes H⁺ ions and reacts with dissolved oxygen and releases OH^- ions. Fe⁺² ions from nZVI enter the next oxidation reactions and Fe⁺³ ions are generated.

(10) 2Fe⁰ +O₂ + H⁺ → 2Fe²⁺ + 4OH^-

(11) 2Fe²⁺ + 2H₂O → 2Fe³⁺ + H₂ +2OH^-

In aqueous solutions, DO is an ordinary oxidant and electron receiver. It is used on account of its reaction with electrons generated from nZVI corrosion in water (reaction 10). Figure 6 illustrates that by adding H₂O₂, DO increases. A very important point to make here is that at low concentrations of H₂O₂ the recompense of the reduced DO of the system is harder (18).

4. Conclusions

The results showed that the maximum efficiency of heterogeneous Fenton process in DMP removal was at pH = 4. In addition, the removal efficiency under optimal condition of reaction parameters is 92.8%. Variation monitoring of ORP, pH and DO of samples showed that increasing the time and nZVI led to decrease of ORP. Furthermore, there was no significant relationship between ORP and reaction time; however, the relationship between ORP and nZVI was significant. On the other hand, the addition of H₂O₂ to the system stabilized the decline ORP. Addition of nZVI to the system increased pH and decreased DO. However, reduction of DO over time slowly adjusted and even in some doses increased DO again. Addition of H₂O₂ resulted in the increase of DO. The results showed that there was a high correlation between nZVI dose, H₂O₂ concentration and removal efficiency.

Overall, the heterogeneous Fenton process with nZVI can degrade DMP in a low contact time, of course, at optimized amounts. More importantly, monitoring of the process parameters, pH, DO and ORP, can enhance the efficiency of the system and lead to a dramatic increase in costs.

Acknowledgements

References

1. Souza FL, Aquino JM, Irikura K, Miwa DW, Rodrigo MA, Motheo AJ. Electrochemical degradation of the dimethyl phthalate ester on a fluoride-doped Ti/beta-PbO2 anode. Chemosphere. 2014; 109: 187-94[DOI][PubMed]
2. Hou Y, Qu J, Zhao X, Liu H. Electrochemical incineration of dimethyl phthalate by anodic oxidation with boron-doped diamond electrode. J Environ Sci (China). 2009; 21(10): 1321-8[PubMed]
3. Li KL, Chen WQ, Li XT, Yuan BL. Degradation of dimethyl phthalate (DMP) in aqueous solution by UV/Si-FeOOH/H2O2. Huan Jing Ke Xue. 2010; 31(9): 2075-9[PubMed]
4. Chen CY, Wu PS, Chung YC. Coupled biological and photo-Fenton pretreatment system for the removal of di-(2-ethylhexyl) phthalate (DEHP) from water. Bioresour Technol. 2009; 100(19): 4531-4[DOI][PubMed]
5. de Souza F, Saez C, Canizares P, de Motheo A, Rodrigo MA. Electrochemical removal of dimethyl phthalate with diamond anodes. J Chem Technol Biotechnol. 2014; 89(2): 282-9[DOI]
6. Yuan BL, Li XZ, Graham N. Reaction pathways of dimethyl phthalate degradation in TiO2-UV-O2 and TiO2-UV-Fe(VI) systems. Chemosphere. 2008; 72(2): 197-204[DOI][PubMed]
7. Liang DW, Zhang T, Fang HH. Anaerobic degradation of dimethyl phthalate in wastewater in a UASB reactor. Water Res. 2007; 41(13): 2879-84[DOI][PubMed]
8. Koniecki D, Wang R, Moody RP, Zhu J. Phthalates in cosmetic and personal care products: concentrations and possible dermal exposure. Environ Res. 2011; 111(3): 329-36[DOI][PubMed]
9. Abdel daiem MM, Rivera-Utrilla J, Ocampo-Perez R, Mendez-Diaz JD, Sanchez-Polo M. Environmental impact of phthalic acid esters and their removal from water and sediments by different technologies--a review. J Environ Manage. 2012; 109: 164-78[DOI][PubMed]
10. Zha S, Cheng Y, Gao Y, Chen Z, Megharaj M, Naidu R. Nanoscale zero-valent iron as a catalyst for heterogeneous Fenton oxidation of amoxicillin. Chem Eng J. 2014; 255: 141-8[DOI]
11. Choi K, Lee W. Enhanced degradation of trichloroethylene in nano-scale zero-valent iron Fenton system with Cu(II). J Hazard Mater. 2012; 211-212: 146-53[DOI][PubMed]
12. Shukla P, Wang S, Sun H, Ang H, Tade M. Adsorption and heterogeneous advanced oxidation of phenolic contaminants using Fe loaded mesoporous SBA-15 and H2O2. Chem Eng J. 2010; 164(1): 255-60[DOI]
13. Weng C, Lin Y, Yuan H. Rapid decoloration of Reactive Black 5 by an advanced Fenton process in conjunction with ultrasound. Sep Purif Technol. 2013; 117(1): 75-82
14. Ay F, Kargi F. Advanced oxidation of amoxicillin by Fenton's reagent treatment. J Hazard Mater. 2010; 179(1-3): 622-7[DOI][PubMed]
15. Zhang S, Zhao X, Niu H, Shi Y, Cai Y, Jiang G. Superparamagnetic Fe3O4 nanoparticles as catalysts for the catalytic oxidation of phenolic and aniline compounds. J Hazard Mater. 2009; 167(1-3): 560-6[DOI][PubMed]
16. Costa RCC, Moura FCC, Ardisson JD, Fabris JD, Lago RM. Highly active heterogeneous Fenton-like systems based on Fe0/Fe3O4 composites prepared by controlled reduction of iron oxides. Appl Catal B. 2008; 83(1): 131-9[DOI]
17. Moon B, Park Y, Park K. Fenton oxidation of Orange II by pre-reduction using nanoscale zero-valent iron. Desalination. 2011; 268(1-3): 249-52[DOI]
18. Yu R, Chen H, Cheng W, Lin Y, Huang C. Monitoring of ORP, pH and DO in heterogeneous Fenton oxidation using nZVI as a catalyst for the treatment of azo-dye textile wastewater. J Taiwan Inst Chem Eng. 2014; 45(3): 947-54[DOI]
19. Yu R, Chi F, Cheng W, Chang J. Application of pH, ORP, and DO monitoring to evaluate chromium(VI) removal from wastewater by the nanoscale zero-valent iron (nZVI) process. Chem Eng J. 2014; 255: 568-76[DOI]
20. Babuponnusami A, Muthukumar K. Removal of phenol by heterogenous photo electro Fenton-like process using nano-zero valent iron. Sep Purif Technol. 2012; 98: 130-5[DOI]
21. Shih Y, Hsu C, Su Y. Reduction of hexachlorobenzene by nanoscale zero-valent iron: Kinetics, pH effect, and degradation mechanism. Sep Purif Technol. 2011; 76(3): 268-74[DOI]
22. Kallel M, Belaid C, Mechichi T, Ksibi M, Elleuch B. Removal of organic load and phenolic compounds from olive mill wastewater by Fenton oxidation with zero-valent iron. Chem Eng J. 2009; 150(2-3): 391-5[DOI]
23. Xu L, Wang J. A heterogeneous Fenton-like system with nanoparticulate zero-valent iron for removal of 4-chloro-3-methyl phenol. J Hazard Mater. 2011; 186(1): 256-64[DOI][PubMed]
24. Zhao X, Yang G, Wang Y, Gao X. Photochemical degradation of dimethyl phthalate by Fenton reagent. J Photochem Photobiol A Chem. 2004; 161(2-3): 215-20[DOI]
25. Kallel M, Belaid C, Boussahel R, Ksibi M, Montiel A, Elleuch B. Olive mill wastewater degradation by Fenton oxidation with zero-valent iron and hydrogen peroxide. J Hazard Mater. 2009; 163(2-3): 550-4[DOI][PubMed]
26. Hou MF, Liao L, Zhang WD, Tang XY, Wan HF, Yin GC. Degradation of rhodamine B by Fe(0)-based Fenton process with H2O2. Chemosphere. 2011; 83(9): 1279-83[DOI][PubMed]
27. Gulkaya I, Surucu GA, Dilek FB. Importance of H(2)O(2)/Fe(2+) ratio in Fenton's treatment of a carpet dyeing wastewater. J Hazard Mater. 2006; 136(3): 763-9[DOI][PubMed]

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Ali Reza Rahmani: [PubMed] [Scholar]
Mohammad Taghi Samadi: [PubMed] [Scholar]
Reza Shokoohi: [PubMed] [Scholar]
Hassan Zolghadr Nasab: [PubMed] [Scholar]

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