Environ. Sci. Technol. 2009, 43, 6793–6798
Simple and Efficient Sonochemical Method for the Oxidation of Arsenic(III) to Arsenic(V) BERNAURDSHAW NEPPOLIAN, AUGUSTINE DORONILA, FRANZ GRIESER, AND MUTHUPANDIAN ASHOKKUMAR* Particulate Fluids Processing Centre, School of Chemistry, University of Melbourne, Parkville, Melbourne, Victoria 3010, Australia
Received March 24, 2009. Revised manuscript received June 10, 2009. Accepted July 22, 2009.
A simple and efficient sonochemical method has been developed for the oxidation of acutely toxic As(III) to the less toxic As(V). The OH radicals produced during acoustic cavitation readily oxidize As(III) to As(V) in an aqueous environment. The effects of various operational parameters of ultrasonication, such as power density and pulse mode, on the oxidation of As(III) were studied. Pulsed sonication showed a profound effect on the oxidation of As(III) to As(V) in comparison to the continuous mode of operation, that consequently reduced the reaction time and energy requirement for the process. Increasing acoustic amplitude led to an enhanced rate of oxidation of As(III). In the presence of As(III), the production of H2O2 was significantly reduced compared to that observed during the sonication of water alone, which suggests that OH radicals were involved in the oxidation process. The oxidation of As(III) was independent of the initial pH of the solution in the range 3 to 10.6. Continuous sparging of nitrogen through the reaction solution noticeably reduced the oxidation rate of As(III), indicating that dissolved oxygen is involved in the oxidation process to a certain extent. It is concluded that the sonochemical treatment process is a simple and possibly cost-effective technique for the oxidation of As(III) to As(V) without assistance of any external chemicals or catalysts.
Introduction
Experimental Section
Arsenic is a toxic metalloid, naturally found in ores and soil in many parts of the earth, and is released into the groundwater through natural processes (1, 2). People living in Southeast Asian countries (e.g., Bangladesh, Nepal, West Bengal in India), in particular, as well as Vietnam, and western U.S. have been adversely affected by the arsenic content in the groundwater (3-7). Long-term exposure to arsenic can affect many important organs in the human body apart from its carcinogenic effect (6, 8). Among the two different naturally existing oxidation states of arsenic (+III, +V), As(III) is highly toxic and much more mobile than As(V) (6, 7, 9, 10). Many reports have discussed the oxidation of As(III) to As(V) by using different advanced oxidation processes (AOPs), including photocatalysis, Fenton reactions, ozonation, etc. (6, 7, 11-22). Once As(III) is oxidized to As(V), it can be easily removed from the groundwater by * Corresponding author phone: +61-3-83447090; fax: +61-393475180; e-mail:
[email protected]. 10.1021/es900878g CCC: $40.75
Published on Web 08/05/2009
other methods, such as adsorption onto activated carbon, iron-coated activated carbon, TiO2, and zerovalent iron (23-28). Recently, the use of photocatalysts and zerovalent iron adsorption have received considerable attention for the oxidation of As(III) because both methods can oxidize As(III) as well as simultaneously adsorb As(V). Ryu and Choi (6), Lee and Choi (19), and Dutta et al. (7) have extensively studied the photocatalytic method of oxidation of As(III) using TiO2, as the photocatalysts, in which they have reported the role and importance of OH radicals and other radicals. Xu et al. (20) have also investigated the role of OH radicals involved during TiO2 photocatalysis. Kanel et al. (27) and Giasuddin et al. (28) have undertaken a detailed study on the application of zerovalent iron on the oxidation and removal of arsenic. Neppolian et al. (12) have employed photochemical oxidation of arsenic using the peroxydisulfate ion as an oxidizing agent. Recently, sonochemistry based methods have been widely employed to degrade organic pollutants either alone or together with select oxidants (29-33). During ultrasonication of an aqueous solution, thermolytic cleavage of water molecules occurs within the cavitation bubbles that are produced, leading to the formation of hydrogen and hydroxyl radicals (34). Of the two radicals, the hydroxyl radical is a powerful nonselective oxidant for the complete degradation of many organic pollutants. Development of new techniques for effective oxidation of organics or any pollutant without using various added chemicals, catalysts, photocatalysts, or powerful light sources, in the case of photocatalysts, is highly desirable. In using sonolysis, there is no need to use any added chemicals, UV light source, or catalysts for the oxidation of organic pollutants; a simple ultrasonic transducer is sufficient for the complete oxidation of organics in water within a relatively short period of time under ambient conditions. This is the main advantage for using a sonolytic method for the oxidation of organic pollutants. To the best of our knowledge, there have been no previous reports on the sonolytic oxidation of As(III) to the less toxic As(V), and hence this study is mainly focused on the effect of ultrasonication on the oxidation of As(III) using various operational parameters such as acoustic amplitude and pulse mode, in addition to other commonly considered solution parameters, for the effective oxidation of As(III).
2009 American Chemical Society
Chemicals and Reagents. All the chemicals used in this study were obtained from Aldrich and were used without further purification, while the solutions were prepared with 18 MΩ cm deionized water from a water purification system (Millipore, Synergy). For pH adjustment, 0.1 M HCl and 0.1 M NaOH were used. A stock solution containing 0.134 mM of arsenic(III) was prepared and diluted to the required initial concentration. Estimation of H2O2. The estimation of the amount of H2O2 produced sonochemically was carried out with 18 MΩ cm deionized water under experimental conditions similar to that of the As solutions, using standard methodology (35, 36). Iodide reagent was prepared by mixing equal volumes (1 mL) of solution A (0.4 M KI, 0.05 M NaOH, 0.00016 M (NH4)6Mo7O24 · 4H2O) and solution B (0.1 M KHC8H4O4). One mL of the sonicated sample was added to the iodide reagent, and after mixing, the sample was kept for 1 min and then the absorbance of I3- was measured spectrophotometrically using ε ) 26,400 M-1 cm-1 at 353 nm. VOL. 43, NO. 17, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Effect of the different initial concentrations of As(III) on the oxidation of As(III) to As(V). Experimental conditions: pH ) 7, tip diameter ) 19 mm, power ) 36 W. Reactor Setup and Analysis Methods. The reactor (29) was made up of a borosilicate glass chamber with 85 mL capacity equipped with an ultrasonic horn-type transducer (Branson Digital Sonifier-450, tip diameter ) 19 mm or 10 mm, USA). The reactor had a double wall with cooling water from a cooler unit circulating between the walls to maintain a constant temperature at 20 °C. Sonication at 20 kHz was carried out with a Branson Digital Sonifier at a calorimetrically measured acoustic power range of 34-36 W. At regular time intervals, around 2-3 mL samples were taken out of 75 mL of the initial As(III) solution from the reactor, filtered using a 0.25 µm membrane filter (Acrodisc, Pall corporation, USA) with a disposable syringe filter unit (TERUMO, Philippines). Then the filtrate was collected using an anion-exchange cartridge (Supelclean LCSAX SPE 3 mL) that separated the As(III) and As(V) (the anion exchange cartridge retains As(V) while leaving As(III) in the filtrate). The As(III) concentration was then measured using an ICP-OES instrument, Varian Vista Pro with a VGA77 hydride for arsenic. The stability of the SPE cartridge has already been described elsewhere (12). Mass balance on As(III) and As(V) indicated that no arsenic was unaccounted for in the experiments.
Results and Discussion The sonication of As(III) solutions (75 mL) over the concentration range of 0.0013 to 0.0268 mM was carried out and the results are shown in Figure 1. It can be seen that a rapid and, in some instances, complete oxidation of As(III) to As(V) was achieved within 15-50 min of reaction time. The results shown in Figure 1 clearly reveal that As(III) can be readily oxidized into As(V) through the sonochemical oxidation process. Further experiments on the effect of the initial concentration of arsenic, estimation of H2O2 produced in the reaction system, consumption of H2O2 during the oxidation of As(III), the effect of pH, influence of dissolved oxygen and nitrogen, and other operating variables of ultrasonication were performed and are discussed later. Reaction Mechanism of the Sonolytic Oxidation of As(III). Acoustic cavitation in aqueous solutions is known to generate highly reactive free radicals such as OH and H radicals as shown in reactions 1-4 (29, 34). The H radical is largely converted to OH radical through the endothermic reaction 4 (37-39) within the bubble. In the As system, As(III) undergoes oxidation by donating an electron to the OH 6794
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FIGURE 2. Aqueous yield of H2O2 (µM) from sonolysis in the absence and presence of As(III) with different concentrations from 0.0067 to 0.0268 mM. Experimental conditions: pH ) 7, tip diameter ) 19 mm, power ) 36 W. radical (reaction 5), as explained by Dutta et al. (7) and Xu et al. (20). The resulting As(IV) is highly unstable and readily undergoes further oxidation with either another OH radical or with dissolved oxygen forming the corresponding stable and less mobile As(V) ion (reactions 6 and 7) (19). In addition to OH radicals, the superoxide radical anion, also an efficient reactive radical, may also oxidize As(III) into As(IV) as shown in reaction 8. Thus, As(III) is rapidly oxidized to As(V) by reactions 5-8. thermolysis
H2O 98 HO· + H·
(1)
HO· + H· f H2O
(2)
HO· + HO· f H2O2
(3)
H· + H2O f HO· + H2
(4)
))))))
where “))))))” refers to ultrasonication As(III) + HO· f As(IV) + HO-
(5)
As(IV) + HO· f As(V) + HO-
(6)
·
As(IV) + O2 f As(V) + O2·
As(III) + O2- + 2H+ f As(IV) + H2O2
(7) (8)
To support the proposed mechanism, the amount of hydrogen peroxide generated in the absence and presence of As(III) was quantitatively measured by monitoring the concentration of hydrogen peroxide generated (Figure 2). In the absence of any other solute, a portion of the OH radicals generated during acoustic cavitation react among themselves and produce hydrogen peroxide. However, in the presence of As(III) reaction between As(III) and OH radicals competes with the formation of hydrogen peroxide. Hence, the observation of a reduced amount of hydrogen peroxide formation in the presence of As(III) would provide strong support to the proposed mechanism of As(III) oxidation by OH radicals. The amount of H2O2 was measured iodimetrically (35, 36) in the absence and presence of As(III) with different concentrations (0.0067-0.0268 mM) and the results are shown in Figure 2.
TABLE 1. Amount of As(III) Oxidized into As(V) after Sonication
FIGURE 3. Effect of benzoic acid (BA) on the oxidation of As(III) to As(V). Experimental conditions: [As(III)] ) 0.0101 mM, [BA] ) 0.5 mM and 1.0 mM, pH ) ∼7, tip diameter ) 19 mm, power ) 36 W. It can be seen in Figure 2 that the amount of hydrogen peroxide produced in the presence of As(III) was below the detection limit during the first 10 min of sonication from 0.0134 mM As(III) concentration. This is consistent with the domination of the reaction between As(III) and OH radicals. Considering the results shown in Figure 1, more than 40% of As(III) was converted to As(V) within the first 10 min of sonication (0.0134 mM As(III)). In other words, at shorter sonication times (
