Separation and Purification Technology 95 (2012) 10–15
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Separation and Purification Technology
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Development of a cost-effective technique to remove the arsenic contamination from aqueous solutions by calcium peroxide nanoparticles ⇑ Ehsan Olyaie a, Hossein Banejad a, , Abbas Afkhami b, Alireza Rahmani c, Javad Khodaveisi d a Water Engineering Department, Faculty of Agriculture, Bu-Ali Sina University, Hamedan 65174, Iran b Faculty of Chemistry, Bu-Ali Sina University, Hamedan 65174, Iran c Department of Environmental Health Engineering, Faculty of Health and Research Center for Health Sciences, Hamedan University of Medical Science, Hamedan 4171, Iran d Young Researchers Club, Islamic Azad University, Bushehr Branch, Bushehr, Iran article info abstract
Article history: In the present study, we synthesized calcium peroxide nanoparticles and evaluated them as an innovative Received 2 February 2012 oxidant to remove As (III) from contaminated water samples. The CaO2 nanoparticles were 15–25 nm in Received in revised form 17 April 2012 diameter identified by TEM. Oxidation occurred within minutes and CaO2 nanoparticles effectively Accepted 19 April 2012 removed total As between natural pH conditions (6.5 and 8.5). Experiments were performed to investi- Available online 24 April 2012 gate the influence of CaO2 nanoparticles concentration, pH of solution and contact time on the efficiency of arsenic removal. Up to 88% removal efficiency for arsenic was obtained by nanoparticles dosage of Keywords: 40 mg/L at time equal to 30 min and pH 7.5. It could be concluded that the removal efficiency was Advance oxidation enhanced by increasing CaO nanoparticles dosage and reaction time, but decreased by increasing arsenic Arsenic removal 2 Calcium peroxide concentration and pH for nano sized CaO2. These results suggest that CaO2 nanoparticles may be used to Nanoparticles develop a simple and efficient arsenic (III) removal method. Water treatment Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction is due to the anthropogenic sources, existing in the earth’s crust and usually associated with several soil minerals. Mining, smelting and Natural arsenic (As) contamination in water supplies is a major herbicide industry are examples of the anthropogenic sources of ar- environmental and health problem on a global scale and has be- senic pollution [14]. come a challenge for the world scientists. Increasing concentra- As it is well known, most As removal technologies are efficient tions of arsenic in groundwater and surface water have been when the element is present in the pentavalent state. Aqueous ar- reported from many regions of the world in recent years [1–7]. senic is commonly found in two oxidation states: As(III) and As(V). Ingestion of geogenic arsenic from groundwater sources is mani- Arsenite [As(III)] is more mobile and more toxic than arsenate fested as chronic health disorder in most affected regions of the [As(V)] [15]. world [8]. Arsenic compounds have long-term carcinogenic prop- Arsenic is uniquely sensitive to mobilization (pH 6.5–8.5) under erties. They may cause skin alteration, damage to major body or- both oxidizing and reducing conditions among heavy metalloids gans and many types of cancer such as skin, lung and bladder [5]. cancer [9,10]. Predominant forms of As in natural ground and surface waters Keeping in view the toxic effects of arsenic on human and other (neutral pH) are arsenate (As(V), as oxianions H2AsO4 and 2 0 living organisms’ health, WHO has recently revised the maximum HAsO4 ) and arsenite (As(III), as the neutral H3AsO3 species) concentration limit for arsenic in drinking water by decreasing it [16–19]. The mobility of arsenical forms in waters is very depen- from 0.05 mg/L to 0.01 mg/L [11]. The US Environmental Protection dent on pH, Eh conditions and presence of different chemical spe- Agency (EPA) has adopted an arsenic maximum contaminant level cies. Consequently, removal methods must take into account of 0.01 mg/L [12]. Also, the guideline value of arsenic for irrigation these physicochemical properties. water is 0.1 mg/As/L as determined by FAO [13]. During the recent years, many remediation techniques such as Arsenic in groundwater is often associated with geologic sources. oxidation [20,21], precipitation/coprecipitation [22], coagulation But in some locations, the occurrence of arsenic in the environment [23,24], ion-exchange [25,26], sorption [27,28], membrane filtra- tion [29–31] have been studied to remove arsenic from water con- taining high initial arsenic concentrations. Although these methods ⇑ Corresponding author. Tel./fax: +98 811 8272071. E-mail addresses: [email protected], [email protected] (H. Bane- have been widely employed, they have several drawbacks such as jad). high operating and waste treatment costs, high consumption of
1383-5866/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2012.04.021 E. Olyaie et al. / Separation and Purification Technology 95 (2012) 10–15 11 reagents and large volume of sludge formation. Also, a significant (III) removal from contaminated water using synthesized calcium problem encountered in the removal of arsenic from groundwater peroxide nanoparticles. Also, for view of highly efficiency of CaO2 aquifers and municipal water systems is that arsenic exists as both nanoparticles, the effect of initial arsenic, CaO2 nanoparticles con- arsenic(III) and arsenic(V) compounds in water. Arsenic(III) com- centration, pH and reaction time on the total As removal efficiency pounds are primarily non-ionic whereas arsenic(V) compounds were investigated. The results of this research provide more infor- are ionic at natural water pH [32]. Arsenic(III) compounds or arse- mation to develop arsenic remediation techniques that are applica- nites are, therefore, not always readily removed from water by ble across a wide range of natural conditions without the need for methods that are very effective for removal of arsenic(V) com- additional treatments before or after arsenic removal. pounds or arsenates. Thus, it is necessary to oxidize any arsenites present to arsenates in order to effectively remove arsenic from 2. Materials and methods water to safe levels. Consequently, there is a growing interest in using low-cost 2.1. Reagents materials to remove arsenic from water based on chemical oxi- dation method. In situ chemical oxidation (ISCO) is being used All chemicals used in this study were analytical grade, obtained for ground water, sediment, and soil remediation. If adminis- from Merck which were used in their commercial forms without tered correctly, ISCO has the potential to be a low-cost, fast, further purification. Solutions were prepared with ultrapure de- effective, and relatively low maintenance remediation technology ionized water. To prepare the solution of As(III) in water, sodium
[33]. arsenite (Na3AsO3) was dissolved in de-ionized water. The stock For this purpose, several main species are used today in ISCO solutions were newly prepared every 2 weeks and refrigerated. industry such as permanganate, persulfate, hydrogen peroxide The pH of the solution was adjusted using 0.1 M HNO3/NaOH and calcium peroxide [34]. In addition, different oxidants like oxy- solution. gen (from air), ozone, hydrogen peroxide, liquid chlorine, perman- Inductively coupled plasma (ICP) was used for the determina- ganate and fenton’s reagent are used for arsenic removal based on tion of the concentration of total arsenic. XRD (Shimadzu XRD- oxidation method [35]. The major disadvantages of these tech- 6000 diffractometer) and TEM (Ziess, transmission electron micro- niques are high operating and maintenance costs, difficulty to store scope), were used for the characterization of the calcium peroxide or transport safely and significantly slow oxidation reaction. nanoparticles. Among these oxidants, calcium peroxide is an oxygen releasing Calcium chloride (Merck, 99.5%), hydrogen peroxide aqueous compound and effective oxygen source for ISCO [36]. solution H2O2 (Merck, 35%); polyethylene glycol 200 (PEG200), Calcium peroxide is one of the metallic peroxides which start to ammonia (25%), and sodium hydroxide, NaOH were used for the decompose after being placed in humid condition. The products of synthesis of calcium peroxide nanoparticles. The calcium peroxide this decomposing reaction are hydrogen peroxide and calcium nanoparticles were stabilized with PEG 200. hydroxide: In all experiments, pH was measured with a pH meter (290A and 410A) and the pH meter was calibrated with three buffers CaO2 þ 2H2O ! CaðOHÞ2 þ H2O2 ð1Þ (pH 4.0, 7.0, and 10.0) daily. In the presence of materials being able to oxidize, hydrogen peroxide provides the needed oxygen to oxide these materials 2.2. Synthesis of calcium peroxide nanoparticles [34].
Not so many recent studies on the use of calcium peroxide in The synthesis of CaO2 nanoparticles involved the following remediation have been done. Nevertheless, several studies have re- steps: three grams of calcium chloride was dissolved in 30 mL ported that the addition of calcium peroxide in saturated soil and distilled water, 15 mL ammonia solution (1 M) and one 120 mL ground water is a suitable choice for contaminant degradation. of PEG 200 was added to the stirring mixture. Then 15 mL of
Park et al. report on the use of calcium peroxide to increase the 30% H2O2 was added to the mixture by rate of three drops per remediation of soil [37]. The results showed that the rate of the minute. The preparation procedure was carried out in a continu- oxidation reaction between calcium peroxide and contaminant is ously stirred opened 250 mL glass beaker at room temperature. extremely slow [38,39]. In order to solve this problem, use of cal- The stirrer velocity was kept constant for all the experiments. cium peroxide in nano size can increase the ratio of surface to vol- After 2 h of stirring, a clear and colorless to yellowish solution ume, which increases the speed of reaction [40]. was obtained. Nanotechnology has provided several applications that can be In order to precipitate the product, NaOH solution (pH 13) was used to treat water and wastewater. Various nano-scale materials added to form a basic medium. This was done until a pH value of and nanotechnologies used in water treatment industry are in 11.5 was obtained. Upon adding the NaOH, the mixture was chan- development [41–43]. ged to a white color suspension. The white precipitate was sepa- Nanoparticles have unique physical and chemical properties rated by centrifuge and after the centrifugation process the and, therefore, have received substantial attention for their possi- powder was washed three times by NaOH solution. Finally, two ble applications in many science and engineering fields. One area additional washes by distilled water were done until final pH of of increased interest is the application of nanoparticles for environ- 8.4 for the residue water was obtained. The resultant precipitate mental clean-up [44]. The removal of arsenic by nanoparticles has was dried at 80 °C for 2 h in an evacuated oven [34]. In this re- shown promising results with titanium dioxide nanoparticles [45], search, the synthesis technique was based on precipitating the nanoscale zero valent iron [46,47] and modified zero valent iron insoluble peroxide from aqueous solution by adding H2O2 accord- particles [48]. The use of nanoparticles as a chemical oxidant is a ing to the following reaction: novel invention in oxidation technology. Before now, there is no re- port on using the calcium peroxide nanoparticles as a remediation CaCl2 þ H2O2 ! CaO2ðhydrateÞþ2HCl ð2Þ reagent. Main advantages of using calcium peroxide nanoparticles Addition of aqueous ammonia to neutralize the HCl forces the as an oxidant are simplicity, velocity and low-cost. Also, using this reaction to favor the precipitation of the peroxide hydrate by the method is spontaneous and no additional equipment or energy following equation: source is required. Then, the primary goal for this study was to de- velop a low-cost and effective technique as a novel ISCO for arsenic 2HCl þ 2NH3 ! 2NH4Cl ð3Þ 12 E. Olyaie et al. / Separation and Purification Technology 95 (2012) 10–15
2.3. Batch studies of arsenic removal by CaO2 nanoparticles result is shown in Fig. 2, the three dominant peaks: 2h = 30.1, 35.6, 47.3 match to the XRD of CaO2 (card number 03-0865). The All experiments were carried out in a series of 100 mL glass XRD result strongly proved that the CaO2 nanoparticles compound beakers. The beaker was mixed by magnetic stirrer (150 rpm) at was formed by this procedure. ambient temperature (22 ± 2 °C). After continuous stirring, over magnetic stirrer for a predetermined time interval, the aqueous samples in each bottle were centrifuged at 3000 rpm for 10 min 3.2. Effect of contact time on arsenic removal and the supernatant passed through a Whatman-42 filter paper (0.45 lm) before being analyzed for arsenic concentration. The Removal of arsenic at varying contact time of 5, 10, 20, 30 and remaining As concentration was determined by ICP (inductively 40 min, and different amounts of CaO2 were studied in neutral con- coupled plasma). Every experiment was run in triplicate and aver- dition (pH 7.5), at ambient temperature (22 ± 2 °C) and for initial age value is reported here. arsenic concentration of 0.4 mg/L, keeping all other parameters constant. The observed removal rates of As at different initial con- Solutions were allowed to react with CaO2 nanoparticles for a period of time (5, 10, 20, 30 and 40 min). The effect of pH on the centration are presented in Fig. 3. It is evident from this figure that removal of arsenic was studied by varying the initial pH of the the removal efficiency increased with the elapse of contact time. solutions (6.5, 7.5 and 8.5). The values of pH of the solutions were The change in the rate of removal might be due to the fact that initially all oxidant sources are available and also the reaction pro- adjusted by 0.1 M NaOH and/or HNO3 stock solutions. The effect of gress is high. Later the arsenic oxidation rate by oxidant is de- other parameters such as CaO2 nanoparticles dosage (0.4, 0.8, 2, 4 and 40 mg/L) and initial arsenic concentration (0.2, 0.4, 0.6, 0.8, 1 creased significantly, due to the decrease in the content of and 2 mg/L) were studied in terms of their effect on reaction oxidant as well as arsenic concentration. For a 0.4 mg/L initial As processes. concentration and CaO2 nanoparticles dosage of 0.4 mg/L, the re- moval of As increased from 58.11% to 69.00% during 5 to 40 min contact time. Also, for 40 mg/L CaO nanoparticles dosage, removal 2.4. Determination of arsenic 2 efficiency increased from 86.43% to 88.38% at the same contact time range. It is observed in Fig. 3 that after 30 min for 40 mg/L Arsenic concentration in the solution was determined with a dosage of nanoparticles, there is no significant change in percent- Perkin Elmer DRCe Inductively Coupled Plasma (ICP). The plasma age of removal of arsenic. It may be due to the overlapping of active is Ar gas with a nebulizer gas flow of 1.01 L/min, axillary gas flow sites at higher dosage. Thus, the contact time of 30 min was consid- of 1.80 L/min, coolant gas flow of 12.2 L/min, and Radio Frequency ered as the optimum time and used for further studies. The maxi- power of 1450 W. The stock solution was diluted to the desired cal- mum removal of arsenic was found to be about 88% in average and ibration standard concentrations of 0, 20, 100, 200, 300, 400, and hence the removal process is effective to bring the concentration of 500 lg/L. All calibration standards and sample bottles were arsenic very close to FAO permissible limit. washed with a detergent solution, rinsed with tap water, soaked in 1% (v/v) nitric acid. The correlation coefficient was generally 0.999. Total As concentration in the solution was measured by 3.3. Effect of solution pH on As removal ICP at 193.759 nm. To identify the pH effect as one of important factors on the As
3. Results and discussion removal using CaO2 nanoparticles, an experiment was conducted using a series of the solutions with initial arsenic concentration
3.1. Characterization of the calcium peroxide nanoparticles of 0.4 mg/L and 40 mg/L of CaO2 synthetic nanoparticles and differ- ent initial pH of 6.5, 7.5, and 8.5 in 5–40 min as contact time. The
The physical properties of the CaO2 nanoparticles, including percentage of arsenic removed as a function of initial pH is shown their morphology, mean size, and size distribution, were measured in Fig. 4. As shown in this figure, the arsenic removal efficiency by TEM with hydrolysis–precipitation method. Fig. 1 shows the insignificantly increased by decreasing pH in the investigated
TEM images of a typical CaO2 nanoparticles sample. As seen in range. This is consistent with the findings of Ndjou’ou and Cassidy Fig. 1, the nanoparticles are of uniform spherical shape and approx- [36], which showed that the yield of H2O2 and the dissolution rate imately 15–25 nm in diameter. It can be seen that the CaO2 parti- of CaO2 increased by decreasing pH. They reported that the % yield cles are faceted nanocrystals with low aggregation and low average of H2O2 decreased from 83% of the theoretical maximum size with a moderate size distribution. Chemical composition of (27.76 mmol) at pH 6 to 47% at pH 9, and was zero with unbuffered the CaO2 nanoparticles was determined by XRD. A representative CaO2. Values of pH lower than 6 were not tested in this study,
Fig. 1. TEM images of precipitated CaO2 with polyethylene glycol. The bars are 25 and 100 nm. E. Olyaie et al. / Separation and Purification Technology 95 (2012) 10–15 13
300
250
200
150
100
50
0 20 25 30 35 40 45 50
Fig. 2. XRD of CaO2 with polyethylene glycol.
Fig. 5. The effect of pH and initial As concentration on removal efficiency of As
using CaO2 nanoparticles.
Table 1 Change in pH during the removal process.
Initial concentration of Initial concentration of Initial concentration of 0.4 mg/L 0.8 mg/L 2 mg/L Initial pH Final pH Initial pH Final pH Initial pH Final pH 6.5 6.4 6.5 6.5 6.5 6.5 7.5 7.6 7.5 7.5 7.5 7.7 8.5 8.6 8.5 8.7 8.5 8.9
nanoparticles can be used for arsenite removal in the most natural waters pH range (6.5–8.5). At pH ranging from 6.5 to 8.5, the re- moval efficiency in detention time of 30 min and pH range 6.5– Fig. 3. Removal efficiency of As using CaO2 nanoparticles as a function of time and nanoparticles dosage. (As Initial concentration of 400 lg/L, pH = 7). 8.5 was 88.75–87.12%. This is studied in batch experiments using 4 mg of oxidant in 100 mL synthetic solution, at ambient tempera- ture, contact time of 30 min for initial arsenic concentration of 0.4 mg/L, 1 mg/L and 2 mg/L. The results are presented in Fig. 5. The pH of the solution after oxidation is measured, found to in- crease or decrease slightly without any regular trend (Table 1).
3.4. Effect of CaO2 nanoparticles concentration
The effect of CaO2 nanoparticles concentration on the removal of arsenic was carried out by 0.4 mg/L arsenic treated with varying
CaO2 nanoparticles concentration (0.4, 0.8, 2, 4 and 40 mg/L) in contact time of 30 min keeping all other parameters constant.
The effect of CaO2 nanoparticles concentration on arsenic removal efficiency by synthetic nanoparticles as a function of pH and CaO2 nanoparticles dosage shown in Fig. 6. It is clear from Fig. 6 that As
removal is directly proportional to the concentration of CaO2 nano- particles. It shows that the increase of CaO2 nanoparticles concen- tration greatly enhanced the removal efficiency. In detention time of 30 min and pH 7.5, about 87.86% As was removed when the
CaO2 nanoparticles mass concentration was 40 mg/L, but only 65.1% was removed when the CaO2 nanoparticles mass concentra- Fig. 4. Removal efficiency of As using CaO2 nanoparticles as a function of time and pH. (As initial concentration of 400 lg/L and oxidant 40 mg/L). tion was 0.4 mg/L.
3.5. Effect of initial arsenic concentration because the purpose was to investigate the use of CaO2 nanoparti- cles for quasineutral pH conditions. The removal of arsenic using synthetic nanoparticles was inves- It was determined that under the initial pH ranging from 6.5 to tigated by varying initial arsenic concentration, optimum oxidant 8.5, more than 87% of arsenite could be removed with an initial As dose (4 mg/100 mL) at ambient temperature (22 ± 2 °C) and con- concentration of 0.4 mg/L. This means that the synthetic CaO2 tact time of 30 min. The results are presented in graphical form 14 E. Olyaie et al. / Separation and Purification Technology 95 (2012) 10–15
applied to other contaminants such as heavy metals and used for treatment design, though much more work is needed to make these results applicable for wider ranges of data.
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