Iron oxide nanoparticles: applicability for heavy

Arab Journal of Nuclear Sciences and Applications, 45 (2) 335-346 (2012)

Iron oxide nanoparticles: applicability for heavy metal removal from contaminated water K. A. Al-Saad, M. A. Amr, D. T. Hadi, R. S. Arar, M. M. AL-Sulaiti, T. A. Abdulmalik, N. M. Alsahamary, J. C. Kwak Department of Chemistry and Earth Sciences, Qatar University, P.O. Box 2713, Doha, Qatar

ABSTRACT

Due to the infinite size of nanoparticles, the surface area is relatively large and, as a result, they usually have high reactivity and sorption to various heavy metals. In this work, we investigated the sorption behavior of the iron oxide (α-Fe2O3) nanoparticles and its applicability to purify water from the aluminum (Al III), arsenic (As III), cadmium (Cd II), cobalt (Cd II), copper (Cu II), and nickel (Ni II). A batch experiment was performed, in which aqueous solutions of the metallic ions were prepared. The adsorption behaviors of the α-Fe2O3 nanoparticles towards the metallic ions were studied under different conditions of contact time, pH, temperature, α-Fe2O3 dosage and metal concentration. 10 mL of aqueous solutions contaminated with each metal were artificially prepared and treated with the nanoparticles. The adsorption behaviors study was performed by changing one of the conditions while keeping the others fixed. The fixed conditions were: metal concentration = 1 ppm; αFe2O3 dosage = 0.35 g; contact time = 30 minutes; temperature = 211C; and pH 7. According to the results, maximum percent removals (%) for all metals tested were reached within a short period of time (5 minutes). The maximum percent removal (%) of both Cu (II) and As (II) reached more than 95%, while the other metal had percent removal between 35% and 65%. Increasing the pH of solution led to increase of the percent removal for all metals except Al which had plateau shape with pH, reaching a maximum percent removal at pH 5 and decreasing back at higher pH. Key words: removal metals iron oxide nanoparticles ICP-MS INTRODUCTION

Huge amount of metals-contaminated water are daily released to the environment worldwide as a result of industrial activities. Human exposure to toxic metals may cause many infections and diseases. Over years, the symptoms and infections from these toxic metals started to be significant issue, making it crucial for scientists to find innovative, economical approaches to purify water from metal contaminants. -335-


There are many traditional methods used to remove metals from water. Chemical precipitation (hydrolysis) is the most widely used for metal removal from inorganic effluent [Wang et. al., 2004]. It is summarized in the equation below: Mn+ + n (OH)-  M(OH)n  where Mn+ and OH- represent the dissolved metal ion and the precipitant, respectively. While M(OH)n is the insoluble metal hydroxide, and n is the oxidation state. Raising the pH significantly improves the heavy metal removal by chemical precipitation [Barakat, 2011]. This method, however, is not practical when trace quantity of metals are to be removed. Distillation is well known methods, but required huge amount of energy [Bloomfield, 2001]. Freezing salty water to produce fresh water is a process used by Eskimos for thousands of year to obtain drinking water. When salty water freezes slowly, a highly ordered crystal is formed. Lower overall potential energy is obtained when ice contains no impurities. Therefore, salts are excluded from the crystalized ice and remained in the dissolved water. This process is suitable in cold climates where refrigeration is a natural phenomenon [Bloomfield, 2001]. Reverse osmosis uses a semipermeable membrane surface that allows only certain molecules to pass through it. Unlike the spontaneous osmosis process, the reverse osmosis is used to extract fresh water from salt water. High pressure is applied on the salt-water side, so the water molecules flow from the salt-water side to the fresh water side, moving reversibly from high to low concentration. [Büchner et al, 1989]. Complexation and Chelation was also applied for the purification of water from heavy metals, many metals tend to have coordination bond with electron donor (ligand) to reach their stability. The metal-ligand complex formed can then be filtered using 0.2μm filter and removed from the water. However, complexation increases heavy metals seeping during filtration and reduce the efficiency of removing biological waste, also it helps to algal growth [Manahan, 1991]. In addition, ion exchange is used often as one of the wellknown methods for water deionization. However, it requires very high energy if ought to be used for a large scale purification. Recently, adsorption by nanoparticles has been proposed as an innovative treatment method for the purification of water from heavy metals with low cost and no residual metal sludge. The aim of this work is to investigate the applicability of iron oxide nanoparticles (-Fe2O3) to purify metal-contaminated water. The adsorption behavior of iron oxide nanoparticles towards six metallic ions (Al (III), As (III), Cd (II), Co (II), Cu (II), and Ni(II)) has been investigated under different conditions (pH, contact time, concentration of metals, temperature, and adsorbent dosage).

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EXPRIMENTAL

Materials The aqueous solutions of heavy metals were purchased from AccuStandard®, USA. The concentration of each solution was 1000 ppm. Iron oxide nanoparticle (αFe2O3) was purchased from Nanostructured & Amorphous Materials, Inc, USA. Polytetrafluoroethylene (PTFE) tubes and 0.2 μm filters were purchased from CellStar, Greiner Bio-One, Germany. Freshly prepared ultrapure deionized water (18 MΩ) was used to prepare all solutions. Samples preparation and nanoparticles treatment was carried out using PTFE tubes. Instrumentations Heavy metal analysis were carried out using inductively coupled plasma quadruple mass spectrometry (ICP-QMS, Agilent 7500 ce) were used for elemental analysis. The operational conditions of ICP-MS in this experiment are summarized in Table1.

Table 1. Operating parameters of ICP-MS Agilent 7500 ce. RF power

1500 W

Plasma gas flow

15 L/min

Auxiliary gas flow

1 L/min

Carrier gas flow

1.25 L/min

Sampling depth

7 mm

Torch injector internal diameter

2.5 mm

Interface

Ni (1 mm sampler: 0.4 mm skimmer )

Ion lens voltages

optimized for sensitivity in 10 ng/ml tune solution (Li, Y, Ce, Tl)

Octopole bias

- 17 V

Quadrupole bias

-13.5 V

The nanoparticle (-Fe2O3) used in the adsorption study was characterized by Scanning Electron Microscope (SEM, FEI NOVA NANO 450). The SEM was used to find the dimension of -Fe2O3. The instrument is also equipped with Energy Dispersive X-ray, which was used to determine the elemental composition of the nanoparticles. Adsorption study Contaminated aqueous solutions were artificially prepared (1 ppm) by adding the metals into the ultrapure deionized water in PTFE tube. The weight of the nanoparticles (0.35 g) was measured in 15 mL polytetrafluoroethylene (PTFE) tubes. Then, the contaminated aqueous solutions were added to each tube. The time measuring at the moment the contaminated water is added. The adsorption behavior was studied under five -337-


different conditions of: 1. contact time, 2- different pH, 3. different metal concentration, 4. different dosage of iron particles, and 5. different temperature. First, the effect of contact time was examined to find the suitable contact time at which the nanoparticles are saturated and the adsorption is at equilibrium. The artificially contaminated water (10 mL) were treated with 0.35 g of nanoparticles in room temperature (21.5  1°C) after different periods of contact times. The times of the treatments were: 5 min, 10 min, 15 min, 20 min, 25 min, 30 min, 1 h, 5 h, 10 h, 24 h, 30 h. It was found that 30 min was the best period of time in which maximum percent removal (%) was reached, so it was used during the study of the other behaviors. The pH was adjusted by adding 0.1 M HCl and 0.1 M NaOH solutions into the ultrapure deionized water. Eight solutions of different pH (from pH 3 to pH 10) were prepared. 10 mL of solutions containing 1 ppm of metals were treated for 30 min with 0.35 g of nanoparticles in room temperature (21 ± 1C) and at PH 7. The concentration of metals was measured by ICP-MS before and after the treatment. The percent removal (%) of heavy metals was calculated. Three trails for each pH were conducted. For studying the effect of concentration, four different concentrations were prepared (100 ppb, 300 ppb, 500 ppb, and 700 ppb) in addition to the original, which contained (1000 ppb or l ppm). All prepared solutions were treated for 30 min with 0.35 g of nanoparticles in room temperature and at PH 7. Three trial were made for each concentration. To determine the Effect of different dosage of iron oxide nanoparticles, three different masses of nanoparticles (0.1 g, 0.2 g and 0.5 g) were used to treat 10 mL of the metal solution (1 ppm). Three trails for each mass were conducted. In the temperature study, the solutions in PTFE tubes were tested in refrigerator (6.3 ± 1 °C), room temperature (21.9 ± 1 °C), sunlight (34.4 ± 2 °C), and furnace (63.5 ± 2 °C). 10 mL of 1 ppm of metals were treated with 0.35 g of nanoparticles and kept for 30 min. After the times end, the solutions were centrifuged between 5 to 30 min and then filtered using 0.2 m filters. The filtrate was forced through the filter using needles. Finally the samples were measured using Inductively Coupled Plasma Mass Spectrometry (ICP-MS). RESULTS AND DISCUSSION Characterization of nanoparticles SEM Experiment Scanning electron microscope shows that the dimensions of the nanoparticles were between 25-55 nm (Fig 1). Fig. 2. shows the Energy Dispersive X-ray (EDX) diagram of iron oxide nanoparticle (-Fe2O3). According to the diagram, there is no contamination with all the metal tested except Al.

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Fig . 1 The diameter of the nanoparticles was obtained by scanning electron microscope between 25-55 nm.

Fig 2. EDX diagram of iron oxide nanoparticle (-Fe2O3).

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Adsorption Results Effects of contact time: The concentrations of heavy metals were determined before treatment and after different time of treatments with nanoparticles, varying between 1.0 and 30 minutes. The percent metal removal (%) was calculated using the equation: Removal (%) 

C  Ce 100 C

where Co is the initial concentration and Ce is the concentration at equilibrium, after treatment with nanoparticles. The percent removal (%) for all metals reached maximum within five minutes and it was relatively high for Cu and As, above 95% (Fig. 3). This indicates that after five minutes, the concentration of metals were at equilibrium (Ce), at which the amount of metal desorbed is equal the amount of metal adsorbed. Previous study by Savina et al (2011) using 0.5 gram of Fe3O4 adsorbent in 500 mL showed maximum removal at ~ 80% for As (III) (2ppm) within 5 h. Compared to many metal (including Zn (II) Co(II) and Ni(II)), the percent removal of As and Cu is relatively high. Compared to the other metal, Al ranged the removal of Al is not reproducible which could be due to the Al impurity found on the nanoparticles used. This was evidenced by the EDX diagram (Fig.2), which shows the presence of 0.24% of Al.

Fig 3. Percent removal (%) of metals versus contact time. 10 mL of aqueous solution (1ppm) were treated with 0.35g of nanoparticle.

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Effect of pH: The percent removals of metals as a function of pH are shown in Fig. 4. As pH increased, the percent removal for most metals increased reaching a maximum level except for Al (III) which was optimum at pH 5 (98%) and decreased back at higher pH. At pH higher than 5, the percent removal of Al decreased back to 75%. This is likely due to formation of Al(OH)4 in the alkali media. Othman et al (2010) studied the removal of Al by two chelating resins (iontosorb oxin and polyhydroxamic acid). The result was qualitatively similar to our result. The adsorption of Al increased on both adsorbents by changing the pH from 2 to 6. It reached the maximum removal (100%) at pH 6 and remained at high level until pH 8, but declined back at pH higher than 8. Othman et al (2010) attributed the decrease of adsorption at high pH (pH 9) to the formation of aluminum hydroxide [Al(OH)n]. The cation Al3+ remained positively charged at pH less than 4. According to Othman et al (2010), after pH 4, [Al(OH)2]+ formed and at pH 6, [Al(OH)3] started to form. After pH 8 complex anion [Al(OH)4]- formed. Since iron oxide particles are also negatively charged at high pH [Huang and Chen, 2009], repulsion between [Al(OH)4]- and the iron oxide are evidently the reason for the sudden decline on the removal % at pH higher than 8 [Othman et al, 2010]. Unlike Al (III), As (III) remained positively charged and, as a result, the more alkali the solution the higher the removal of As3+.

Fig 4. Percent removal (%) at different pH. 10ml of aqueous solution (1ppm) were treated with 0.35g of nanoparticle.

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The adsorption behavior under different pH shown from our results as well other results from previous authors indicated, in general, that there is electrostatic interactions between the adsorbent and the metal (adsorbates). According to the zeta potential for Fe3O4 which is measured by Huang and Chen (2009) at various pH, it was indicated that the nanoparticle is positively charged at pH below 4.6, and negatively charged at pH above 4.6. This is due to the higher concentration of protons (H+) that surrounds the nanoparticles at low pH. Therefore, positive metal will have higher electrostatic repulsion with the adsorbent surface at lower pH and, as a result, the adsorption of positive metals will reduce. Increasing pH will decay the competition between H+ and the positive metals for surface sites and the adsorption of the positive metals with iron oxide nanoparticles will increase. According to Othman et al (2010) however, Al(III) exists as Al3+ at extremely low pH and exists as [Al(OH)4]- at high pH (at alkali solution). Therefore, Al3+ undergoes electrostatic repulsion with H+ surrounding the nanoparticles at low pH, whereas at high pH, [Al(OH)4]- undergoes electrostatic repulsions with the negatively charged nanoparticles. It was observed in general that the adsorption of negative metallic ions decreases with increasing the pH, unlike the positive metallic ions [Tuutija rvi et al (2009), Huang and Chen (2009), Hu et al (2006)]. For example, study by Tuutija rvi et al (2009), on the effect of pH on adsorption of As (V), indicated that the adsorption decreases with increasing the pH. That is because the As (V) exists in aqueous solution as (H2AsO3, H2AsO3- , H2AsO4−, HAsO42−) not as As3+ ion. Also, it was observed by Huang and Chen (2009) and Hu et al (2006) that Cr (VI), which existed as CrO42- and Cr2O7-2 , has lower adsorption at higher pH. On the other hand, it was observed by the same researchers that Cr(II), which exists as Cr2+ has higher adsorption at higher pH. These results, therefore, support the existent of electrostatic interaction between the iron oxide and metals. Although, Huang and Chen used amino-functionalized magnetic nanoadsorbent, their results resembled our work, since both adsorbents are negatively charged at high pH. In study carried out by Savina et al (2011), using α-Fe2O3, showed similar results in which the absorption of the As(III) increased with increasing the pH from 3 to 10 but it decreased after pH 10. The author [Savina et al (2011)] explained this change in absorption due to that in alkaline pH (higher than 10), an anionic species of As3+ will dominate. The effect of pH on the concentrations level of metals were also examined in blank solutions, where no iron oxide nanoparticles were present (results not shown). The concentration of metals remained unchanged, indicating that changes in the metal concentration was attributed to electrostatic interaction of nanoparticles with the metals. This interaction is influenced by the pH level of the aqueous solution. Effect of metal concentration:

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Increasing the concentration of the metal didn’t effect much the adsorption of the metal As (III) and Cu (II) (Fig. 5). The % removal remained around 95% for both.

Fig 5. Percent removal (%) of metal versus concentration. 10ml of aqueous solution were treated with 0.35 g -Fe2O3 nanoparticles at room temperature and at pH7. Effect of iron oxide nanoparticles dosage: The adsorption efficiency increased with increasing the amount of nanoparticle (Fig. 6). This is due to increase in surface area where the adsorption takes places. About 0.2 gram of the iron oxide nanoparticles was enough to remove about 100% of As (III) and Cu (II) at concentration equal 1ppm in 10 mL aqueous solution.

Fig 6. Percent removal (%) versus -Fe2O3 dosages. 10ml of aqueous solution (1ppm) were treated at room temperature and at pH7. Contact time = 30 minutes. -343-


Effect of temperature: The percent removals of metals as a function of temperature are shown in Fig. 7. The adsorption of As(III) and Cu (II) did not change noticeably as a function of temperature. Al (III) did not show reproducible result because of the impurity of Al coming from nanoparticles which was obviously prominent at relatively high temperature, so it was eliminated. Study carried out by Shen et al (2009), on the adsorption of the elements (Cu2+, Cr6+, Ni2+, and Cd2+) by Fe3O4, showed similar result, in which the adsorption of metals increase of as temperature increased.

Fig 7. Percent removal (%) versus different temperature (°C). 10ml of aqueous solution (1ppm) were treated with 0.35g at pH7. Contact time = 30 minutes. CONCLUSIONS

The result showed that the Fe2O3 is more efficient to remove As (III) and Cu (II) than the other metal tested. For all metals tested, maximum adsorption is reached for all metals within only 5 minutes. According to the pH effect on the adsorption behaviors, it was evidenced that there was electrostatic interaction between the -Fe2O3 nanoparticles and the metals tested. We can conclude that the Fe2O3 nanoparticles are positively charged (coated by H+) at low pH and negatively charged at relatively high pH. Al (III) exist as Al3+ at low pH and [Al(OH)4]- at higher pH. Therefore it undergoes electrostatic repulsion with the positively charged particles at low pH and also undergoes repulsion with the negatively charged particles at relatively high pH. ACKNOWLEDGEMENT

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This article was made possible by a UREP award [UREP10-028-1-004] from the Qatar National Research Fund (a member of The Qatar Foundation). The statements made herein are solely the responsibility of the authors.

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