Oxidation of Arsenic (III) in Ozone Assisted Microbubble System

Oxidation of Arsenic (III) in Ozone Assisted Microbubble System S. Khuntia*, S. K. Majumder** and P. Ghosh** *School of Engineering and Applied Science, Ahmedabad University, Commerce Six Road, Ahmedabad, Gujarat, India, 380009 (E-mail: Snigdha.khuntia@ahduni.edu.in) **Indian Institute of Technology Guwahati, Guwahati, Assam, India, 781039 (E-mail: skmaju@iitg.ernet.in; pallabg@iitg.ernet.in) Treatment of water and wastewater using ozone is gaining a lot of attention due to its high oxidizing power. One of the more toxic metal found in ground water is arsenic and its compounds which is well known as arsenic poisoning. Amongst all the species found, As(V) can be more easily removed from water by adsorptive methods. In this work, oxidation of more toxic As(III) to less toxic As(V) was studied in a pilot-plant by using ozone microbubbles. The microbubbles were effective in dissolving ozone in water. The oxidation was fast over a wide range of ph (e.g., 4 9). The role of hydroxyl radical in the oxidation of As(III) under acidic conditions was investigated by using 2-propanol as the hydroxyl radical scavenger. Under acidic conditions, the addition of 2- propanol slowed down the oxidation, which proves that hydroxyl radicals were involved in the oxidation process. Keywords Arsenic, Mass Transfer, Microbubble, Oxidation, Ozone INTRODUCTION Trivalent and pentavalent arsenicals (i.e., arsenite and arsenate) are commonly found in surface and ground waters as well as in soils and sediments. Different forms of inorganic and organic arsenic species enter into the human body mainly through food and water. Many acute and longterm ill-effects of arsenic on human body have been reported (DeSesso et al., 1998; Jain and Ali, 2000). Therefore, WHO has suggested a provisional upper limit of 0.01 mg L 1 arsenic in drinking water (Mohan and Pittman, 2007). However, the acceptable upper limits of arsenic in different countries vary from 0.01 to 0.05 mg L 1 (e.g., 0.01 mg L 1 in India and 0.05 mg L 1 in Bangladesh) (Karim, 2002). The widely-used removal technologies of arsenic are anion exchange, adsorption, reverse osmosis, coagulation/filtration, lime softening and oxidation/filtration. Oxidation of As(III) to As(V) followed by the removal of the latter is a suitable technology for elimination of arsenic from water. The major advantage of pre-oxidation is that it converts As(III) to As(V), which has a higher adsorption affinity towards many adsorbents. Various chemical oxidants have been used for this oxidation. The reactions of these chemical oxidants with As(III) are given Table 1. Use of ozone is a well-known method for removal of various organic and inorganic compounds present in wastewater (Khuntia et al., 2012). For an ozonation method to be effective, the gas must be dissolved in the aqueous phase as much as possible. The dissolution of ozone in water can be significantly enhanced by using microbubbles (Khuntia et al., 2013). The present work

thus focused on the ozonation of As(III) in a pilot-plant using a commercial microbubble generator (MBG). A wide range of feed concentrations of As(III) was used. The effects of ozone application rate, ph of the reaction medium and carbonate ion radicals on the efficiency of ozonation were investigated. The use of 2-propanol as OH radical scavenger was also studied. Table 1. Reactions of As(III) with various chemical oxidants. Chemical oxidant Reaction of As(III) with oxidant Mn(III) oxide (Driehaus et al., 2 2 H3AsO3 2MnOOH 2H HAsO4 2Mn 3H2O 1995) NaOCl (Ghurye and Clifford, 2001) H3AsO3 NaClO H2AsO4 Na Cl H KMnO4 (Lee et al., 2011) Chlorine dioxide (Ghurye and Clifford, 2001) Monochloroamine (Ghurye and Clifford, 2001) Mn(II) and O3 (Nishimura and Umetsu, 2001) Mn(IV) oxide (Manning et al., 2002) 3H3AsO3 2MnO4 3H2AsO4 2MnO 2H2O H H3AsO3 2ClO2 H2O H2AsO4 2ClO2 3H 5H3AsO3 2ClO2 H2O 5H2AsO4 2Cl 7H 2 H3AsO3 NH2Cl H2O HAsO4 NH4 Cl 2H 2 Mn O3 H2O MnO2 O2 2H 2MnO2 3O3 H2O 2MnO4 3O2 2H 3 Mn H3AsO4 MnAsO4 3H 2 H3AsO3 MnO2 2H H3AsO4 Mn H2O MATERIALS AND METHODS All chemicals used in this study are purchased from Merck, India. The Dowex 1 8 100200 resin (chloride form) was procured from Alfa Aesar, India. The details of the experimental setup including the generation of ozone microbubbles, destruction of excess ozone and detection of ozone concentration in the aqueous solution were discussed elsewhere (Khuntia et al., 2013). The reaction between As(III) and ozone took place in a 20 L polycarbonate reactor. A part of the reaction mass was continuously recirculated through the MBG thereby continuously providing ozone microbubbles in the reactor. All the experiments were repeated 3 times. As(III) and As(V) were separated by using Dowex resin (chloride form) by the method described by Ficklin (1983). Samples were withdrawn from the reactor by a glass pipette. At particular time intervals, 25 ml of sample was taken out of the reactor and acidified with 0.25 ml concentrated HCl. About 5 ml of the acidified solution was passed through the resin bed and the effluent liquid was collected in clean sample bottles. The resin allowed As(III) to pass through it, but completely

retained the As(V). Therefore, the effluent contained As(III) only. Arsenic concentration was measured by using an atomic absorption spectrophotometer (SpectrAA 240FS; Middleburg, the Netherlands) equipped with vapor generation assembly (which operated at 925 o C). RESULTS AND DISCUSSION Oxidation of As(III) to As(V). The oxidation of As(III) using ozone microbubbles at different initial arsenic concentrations is shown in Figure 1. Even at a low ozone application rate (e.g., 0.56 mg s 1 ) and a low arsenic concentration (e.g., 50 200 μg L 1 ), the oxidation of As(III) was fast and effective at ph 7. The time required to reach half of the initial As(III) concentration was below 15 min in all the four cases. More than 90% of As(III) was oxidized in just 25 min. The products of the reaction between As(III) and ozone change with ph, which are described by the following reactions (Ghurye and Clifford, 2001, 2004). At ph 6.5: H3AsO 3 O3 H2AsO 4 O 2 H (1) At ph 8.5: 2 H3AsO 3 O3 HAsO 4 O 2 2H (2) As more and more of As(III) was oxidized, the concentration of As(V) in the reactor increased. The total arsenic concentration was found to remain constant throughout the experiment, which therefore satisfied the overall material balance (Fig. 2). The rate of oxidation of As(III) increased with increasing ozone application rate at all ph. The increase in ozone application rate increased the concentration of ozone in the gas fed to the MBG, and therefore, the concentration of ozone in the aqueous phase increased. Therefore, the As(III) concentration profiles became steeper with increasing ozone application rate. Similar results were observed at other ph (e.g., 5 and 7 9) as well. At the ozone application rate of 1.7 mg s 1, the concentration of As(III) was reduced to 10 μg L 1 within 8 min. With the continuous supply of ozone in water in the form of microbubbles, the dissolved ozone concentration went on increasing and approached equilibrium. According to Eqs. (1) and (2), it is obvious that the generation of H + ion would decrease the solution ph during the oxidation. But at such low As(III) concentration, a significant change in ph was not observed. Figure 1: Oxidation of As(III) to As(V) with different initial As(III) concentrations Figure 2: Concentration profiles of As(III) and As(V) in the reactor

Effect of ph on As(III) oxidation. In most of the works reported, oxidation of As(III) has been carried out in the ph range of 39. Driehaus et al. (1995), have oxidized As(III) using manganese oxide in the ph range of 5 10. They did not observe any effect of ph on the rate of oxidation. On the other hand, Sorlini and Gialdini (2010), have reported variations in the rate of oxidation by using chlorine dioxide, hypochlorite, potassium permanganate and monochloramine in the ph range of 5.78. Hug and Leupin (2003), have reported that the oxidation of As(III) by H2O2 was slow at neutral and acidic ph. In our experiments with ozone microbubbles, the ph of the solution was varied between 5 and 9. The MBG used in this study was operable in this ph range. The manufacturer of the MBG recommended this range considering the internal materials of the equipment. Fig. 3 shows the effect of ph on the oxidation of As(III). About 98% oxidation of As(III) was completed within 30 min at all ph for 0.56 mg s 1 ozone application rate. The oxidation was slower at ph 7 at all As(III) concentrations. The oxidation was fast at ph 6 and 9. The oxidation rates were similar at ph 5 and 8. Similar results were observed for other ozone application rates (e.g., 1.1 and 1.7 mg s 1 ). The speciation of As(III) changes with solution ph (Moore et al., 1990). The dominant species, H3AsO3, dissociates into H2AsO 3, 2 HAsO 3 and 2 AsO 3 (Mohan and Pittman, 2007). The rates of reaction of these three forms of As(III) with ozone can be dependent on ph, which is the likely cause behind the faster oxidation of As(III) at ph 6 and 9. Small but noticeable changes occurred in the first 15 min of the reaction (Fig. 3). Moore et al. (1990), have reported similar results for the oxidation of As(III) with birnessite. Figure 3. Effect of ph on oxidation of As(III) Effect of 2-propanol as OH radical scavenger. Under certain conditions, ozone produces hydroxyl radicals in aqueous medium (Beltrán, 2004). The generation of hydroxyl radicals from ozone is an important phenomenon in the oxidation of any organic or inorganic compound. Because of its high standard redox potential (i.e., 2.8 V), the OH radical is capable of reacting with many compounds that are otherwise resistant towards oxidation by ozone (the standard redox potential of ozone is 2.07 V). Takahashi et al. (2007), have shown that hydroxyl radicals

are formed from ozone microbubbles under strongly acidic conditions in the presence of mineral acids (i.e., HCl, H2SO4, and HNO3). Highly alkaline condition favors the OH radical generation by the chemical reaction between ozone and hydroxide ions (Beltrán, 2004). But in acidic ph, OH radicals are generated due to the collapse of the microbubbles. The dissolved ions in water strongly affect the electrical properties of the gas water interface (Takahashi, 2005). The zeta potential of microbubbles changes from negative to positive under acidic conditions. Thus, the types of ions that accumulate at the interface during the collapse process might be related to the type of radicals generated. The extreme accumulation of ions around the ozone water interface (i.e., at the surface of the collapsing microbubbles) transforms ozone to the OH radicals. However, the exact mechanism by which the OH radicals are generated under strongly acidic condition is, precisely not known. Li et al. (2009), have investigated the generation of hydroxyl radicals by microbubbles at acidic ph. They have confirmed the generation of hydroxyl radicals at ph 2. The oxidation of As(III) during ozonation may occur by molecular ozone, by hydroxyl radicals, or by both. The presence of OH radicals at slightly acidic ph (e.g., 5 and 6) and the comparative roles of molecular ozone and these radicals in the oxidation of As(III) can be confirmed by using 2-propanol as the OH radical scavenger (Hug and Leupin, 2003). Figs. 4a and 4b depict the effect of 2-propanol at ph 5 and 6, respectively. At ph 6, the presence of 2-propanol at 0.01 M concentration had hardly any effect on the oxidation of As(III). However, when the concentration of 2-propanol was increased to 0.02 M, the reaction was slightly slowed down. At ph 5, the conversion of As(III) to As(V) was reduced from 70% to 60% after a reaction time of 5 min at 0.01 M concentration of 2- propanol. The conversion was reduced to 52% in presence of 0.02 M 2-propanol after the same reaction time. This appreciable effect of 2-propanol on the oxidation of As(III) by ozone microbubbles confirms the participation of hydroxyl radicals at acidic ph. Because 2-propanol acts as the scavenger of these radicals, it is apparent that the OH radicals play the role of an effective oxidant for As(III). Figure 4: Effect of 2-propanol on oxidation of As(III) at (a) ph 5, and (b) ph 6 (ozone generation rate = 1.1 mg s 1 ).

At ph 5 and 6, the reaction of As(III) is likely to occur with both OH radical and molecular ozone. Kläning et al. (1989), and Hug and Leupin (2003), have reported that the reaction of As(III) with OH radicals forms an As(IV) intermediate, which further oxidizes to As(V). Four different species of As(IV) [i.e., As(OH)4, As(OH)3O, HAsO3 and AsO3 2 ] are formed by the reaction between As(III) and the OH radicals, as follows. As(OH) 3 OH As(OH) 4 (3) As(OH) 2O OH As(OH) 3O (4) As(OH) 4 H2AsO 3 H2O (5) As(OH) 4 HAsO 3 H H2O (6) H2AsO 3 HAsO 3 H (7) 2 As(OH) 2O O AsO 3 H2O (8) The generation of the four As(IV) species depends on the ph. The species, As(OH)4, is observed at ph < 6, whereas As(OH)3O is found in the ph range of 8.5 to 10. The species, HAsO3 and AsO3 2, are formed at ph > 3. Therefore, in case of the generation of OH radicals from ozone microbubbles under acidic conditions, As(IV) would exist as As(OH)4, HAsO3 and AsO3 2. The species, As(OH)3O, is not expected to form under these conditions. All the As(IV) species further oxidize to As(V). CONCLUSIONS The oxidation of As(III) in a pilot-plant by using ozone microbubbles was fast. The rate of oxidation increased with increasing ozone application rate. The ph of the medium was a controlling factor for oxidation of As(III). At slightly acidic ph (e.g., ph 6), the conversion of As(III) to As(V) was fast as compared to ph 7. The variation in the speciation of As(III) with ph is the likely reason for this behavior. Both the molecular ozone and the hydroxyl radicals were involved in the oxidation process. The use of 2-propanol as the hydroxyl radical scavenger confirmed the participation of this radical in the oxidation of As(III). REFERENCES DeSesso J.M., Jacobson C.F., Scialli A.R., Farr C.H. and Holson, J.F. (1998). An assessment of the developmental toxicity of inorganic arsenic. Reproductive Toxicology 12 (4), 385 433. Driehaus W., Seith R. and Jekel M. (1995). Oxidation of arsenate(iii) with manganese oxides in water treatment. Water Research 29 (1), 297 305. Ficklin W.H., (1983). Separation of arsenic(iii) and arsenic(v) in ground waters by ionexchange. Talanta 30 (5), 371 373.

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