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nickel boride nanoparticles coated resin: A novel adsorbent for arsenic(III) and arsenic(V) removal
Powder Technology 269 (2015) 470–480
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Nickel/nickel boride nanoparticles coated resin: A novel adsorbent for arsenic(III) and arsenic(V) removal Tülin Deniz Çiftçi ⁎, Emur Henden Department of Chemistry, Faculty of Science, Ege University, 35100 Bornova, İzmir, Turkey
a r t i c l e
i n f o
Article history: Received 3 July 2013 Received in revised form 12 September 2014 Accepted 21 September 2014 Available online 26 September 2014 Keywords: Nickel/nickel boride nanoparticles coated resin Arsenic removal Adsorption Water
a b s t r a c t A novel adsorbent named nickel/nickel boride nanoparticles coated resin has been introduced for the removal of both As(III) and As(V) from water. Nickel/nickel boride nanoparticles were formed on Purolite C-100 resin successfully. Perlite, pumice, zeolite and silica gel were also tried as the support material. However, nickel/nickel boride nanoparticles on those materials were not stable. Optimal preparation conditions for nickel/nickel boride nanoparticles coated resin were established. In the batch method, initial pH did not significantly affect the arsenic removal efficiencies for As(III) and As(V) in the pH range 3.3–11.5. As(III) and As(V) were removed quantitatively from the solutions at all pHs. 15% and 10% decreases were observed for As(III) removal efficiency when the solution contained phosphate and silicate respectively. However, none of the ions studied showed significant effect on the As(V) removal efficiency. Isotherm studies indicate that the Langmuir model fits the experimental data better than the Freundlich model. The isotherms also showed that the adsorption is favorable. Maximum arsenic adsorption capacities were calculated as 23.4 mg/g and 17.8 mg/g for As(III) and As(V), respectively. The kinetics of the adsorption process were tested for the pseudo-first order and pseudo-second order and intra-particle diffusion models. The comparison among the models showed that the pseudo second-order model best described the adsorption kinetics. As(III) and As(V) could be desorbed from the adsorbent without any efficiency loose using a mixture containing NaCl and NaOH and, therefore, the adsorbent could be used several times. Ni/NixBNPCR could reduce the concentration of both As(III) and As(V) below the maximum allowable concentration level (WHO limit, 10 μg/L) from such high arsenic containing waters. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Arsenic is a ubiquitous element and ranks 20th in natural abundance [1]. The presence of arsenic in natural water is mainly related to the process of leaching from the arsenic containing sources such as rocks and sediments [2]. Also biological actions and geochemical reactions help to mobilize arsenic into groundwater. Arsenic naturally occurs in over 200 different mineral forms, of which approximately 60% are arsenates, 20% sulfides and sulfosalts and the remaining 20% includes arsenides, arsenites, oxides, silicates and elemental arsenic (As) in the earth crust [3,4]. Two forms of arsenic are common in natural waters; arsenite 3− (AsO3− 3 ) and arsenate (AsO4 ), referred to as As(III) and As(V). Under atmospheric or more oxidizing environment, the predominant species is As(V) which exists mainly as H2AsO− 4 in the surface waters. As(III) is thermodynamically stable, predominant and exists mainly as H3AsO3 under mildly reducing conditions [5]. As(III) exists as neutral species, H3AsO3, in the pH range (6.5–7.5) of most natural waters, and this form of arsenic is more difficult to remove than As(V). ⁎ Corresponding author. Tel.: +90 232 3115447; fax: +90 232 3888264. E-mail addresses: [email protected] (T.D. Çiftçi), [email protected] (E. Henden).
http://dx.doi.org/10.1016/j.powtec.2014.09.041 0032-5910/© 2014 Elsevier B.V. All rights reserved.
Arsenic is toxic to both plants and animals and inorganic arsenicals are proven carcinogens in humans [6]. It is commonly accepted that inorganic As(III) compounds are approximately 60–80 times more toxic to humans than As(V) and inorganic arsenic compounds are about 100 times more toxic than organic ones [7,8]. The World Health Organization (WHO) revised the guideline for arsenic in drinking water as 10 μg/L in 1993 [9] and the United States Environmental Protection Agency (US-EPA) has implemented the reduction of permissible values of arsenic in drinking water from 50 to 10 μg/L in light of the epidemiological evidence to support the carcinogenic nature of the ingested arsenic and its connection with liver, lung and kidney diseases and other dermal effects [10]. Various techniques are employed for the removal of arsenic from water and wastewater. Precipitation, membrane processes, ion exchange and adsorption are the most common techniques [1–14]. Treatment cost, operational complexity of the technology, skill required to operate the technology and disposal of arsenic bearing treatment residual are the factors that should be considered before treatment method selection. Adsorption is an economical alternative to conventional metal removal techniques. Selective adsorption utilizing biological materials, mineral oxides, activated carbons and polymer resins has received
T.D. Çiftçi, E. Henden / Powder Technology 269 (2015) 470–480
increasing attention [15,16]. Also the unique structure and electronic properties of some nanoparticles can make them especially powerful adsorbents, such as nanoscale iron particles [17], amino-functionalized silica nano hollow sphere and silica gel [18] and clay nano-adsorbents [19]. Enhanced retention or solubilization of a contaminant may be helpful in a remediation setting. Remediation of arsenic has also been proposed using zero-valent iron and other classes of nanomaterials such as calcium peroxide nanoparticles [20], multiwall carbon nanotube-zirconia nanohybrid [21] and perlite incorporating γ-Fe2O3 and α-MnO2 nanomaterials [22]. Morgada et al. [23] investigated the effect of UV light and humic acid on As(V) removal using nano zerovalent iron. Also Deliyanni et al. [24] studied the removal of cadmium and arsenic using iron-based nano-adsorbents. Nanoparticles such as poly(amidoamine) dendrimers can serve as chelating agents, and can be further enhanced for ultrafiltration of a variety of metal ions (Cu(II), Ag(I), Fe(III), etc.) by attaching functional groups such as primary amines, carboxylates, and hydroxymates [25]. Self-assembled monolayers on mesoporous supports are nanoporous ceramic materials that have been developed to remove mercury or radionuclides from wastewater [26]. Nanoscale nickel/nickel borides appear to be promising materials for arsenic removal from drinking water because of its large active surface area and high arsenic adsorption capacity [27]. However, nano-nickel compounds usually appear as fine powders which cannot be applied in fix-bed columns unless they are of a granular shape. Therefore, in this study nickel/nickel boride nanoparticles have been formed on the cation exchange resin for fixed bed water treatment operations. The novel adsorbent named as nickel/nickel boride nanoparticles coated resin (Ni/NixB-NPCR) was prepared in our laboratory by reacting Ni(II) ion bound to resin with NaBH4. The adsorbent was found to be effective for both As(III) and As(V) removal. Another advantage of the adsorbent is that, As(III) and As(V) could easily be desorbed from the adsorbent making possible its use for several times. 2. Materials and methods 2.1. Reagents All the reagents were of analytical-reagent grade and glass distilled water was used throughout. 1000 mg/L arsenic(III) and arsenic(V) stock standard solutions were prepared by dissolving As2O3 (Merck) and Na2HAsO4·7H2O (Merck), respectively, in concentrated HCl (Merck) and diluting with distilled water. As(III) and As(V) stock solutions contained 2 mol/L HCl. More dilute standard solutions were prepared daily by dilution of the stock solutions. KI (Merck) and ascorbic acid (Merck) were used for arsenic determinations. 30 g/L Ni(II) solution was prepared by dissolving NiSO4·6H2O (Merck) in 0.01 mol/L HCl. 4% NaBH4 solution was prepared by dissolving NaBH4 (Merck, 98% pure) in distilled water just before use. The support material was Purolite C-100 cation exchange resin obtained from Purolite Company.
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2.3. Procedure for arsenic determination The operating conditions for the determinations of As(III) and As(V) using continuous flow and batch type hydride generation systems combined with atomic absorption spectrometer with a quartz tube atomizer were listed earlier [29]. Arsenic hydride was generated by reduction with NaBH4. For the determination of As(V) or total arsenic, As(V) was reduced to As(III) before NaBH4 reduction. For this purpose, 9 mL of the sample solution was mixed with 1 mL of 12 mol/L HCl and 2 mL of reducing agent (50% KI) and allowed to react for 15 min to reduce As(V) to As(III). After the addition of ascorbic acid, arsenic concentrations were measured by HGAAS. Continuous flow hydride generation system was used for the determination of total arsenic. Batch type hydride generation system was also used to determine the concentrations of As(III) and As(V) separately.
2.4. Optimization studies for the preparation of the adsorbent In the optimization studies, effect of the adsorbent preparation conditions on the arsenic removal efficiency and arsenic uptake was studied. For the uptake studies, 25 mL of 100 mg/L As(III) was added onto 0.1 g of the adsorbent and for the removal efficiency studies, 25 mL of 100 μg/L As(III) was added onto 0.4 g of the adsorbent. The solutions were shaken for 24 h at 25 °C using a shaker with thermostatic bath. Unadsorbed arsenic was measured.
2.5. Preparation of Ni/NixB nanoparticles coated resin (Ni/NixB-NPCR) Purolite C-100 cation exchange resin (0.3–1.2 mm) was ground and sieved to obtain 0.250–0.355 mm particles. Then, 0.5 g of the resin to be coated was weighed and added to a 50 mL beaker. 5 mL of 30 g/L Ni(II) solution was added to the beaker and the mixture was shaken for 20 min at 25 °C. After decanting the solution, the resin containing Ni(II) was washed with distilled water. Ni(II) bound to the resin was then reduced by adding 2.5 mL of 4% NaBH4 solution. When the gas bubbles were ceased, the solution was decanted and the resin coated with black Ni/NixB layer was obtained. The adsorbent was then washed several times with distilled water by decantation and dried at room temperature (20 °C).
2.6. Characterization of Ni/NixB-NPCR 25 mL of 500 mg/L As(III) and As(V) solutions were added onto 0.2 g of the adsorbents and shaken for 72 h. The particles were washed with distilled water and dried at room temperature. Parallel experiments were also carried out without arsenic. The surface morphology and chemical composition of the adsorbents were determined by SEMEDX, AFM and XPS analyses. Acquisition parameters for XPS analysis are shown in Table 1.
2.2. Apparatus pH was measured using Jenway 3040 model pH meter. Nuve ST-402 model shaker with thermostatic bath was used for shaking at fixed temperature. A GBC 904 PBT model atomic absorption spectrometer with HG 3000 model continuous flow hydride generation system and also a laboratory made batch type hydride generation system described earlier (HGAAS) [28] were used for arsenic determination. The characterization of Ni/NixB-NPCR was performed by using a scanning electronic microscope combined with X-ray energy dispersive spectrometer (SEM-EDX) (JEOL JSM-6060), atomic force microscopy (AFM) (Ambios Q-Scope 250) and Thermo Scientific K-Alpha model X-ray photoelectron spectroscopy (XPS). LW Scientific V325XS model spectrophotometer was used for boron determination.
Table 1 Acquisition parameters for XPS analysis. Parameter
Setting
Total acq. time No. of scans Source type Spot size Lens mode Analyzer mode Energy step size No. of energy steps
57.3 s 6 Al K alpha 400 μm Standard CAE: pass energy 30.0 eV 0.100 eV 191
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2.7. Adsorption studies The concentration of arsenic (mg/L) in the solution was determined before (C0) and after (Ce) the adsorption. The following equation was used for computing the percentage removal (R %) of arsenic: ðC0 −Ce Þ 100: R%¼ C0
ð1Þ
The amount of arsenic adsorbed by the adsorbent (qe) was calculated according to the following equation: qe ¼
ðC0 −Ce Þ V m
ð2Þ
where qe (mg/g) is the equilibrium adsorption capacity; V (L) is the volume of arsenic solution; m (g) is the mass of adsorbent used in the experiment. 3. Results and discussion The relative standard deviations for the determination of 30 μg/L arsenic using batch method and 20 μg/L arsenic with the continuous flow system were 3.30% (n = 6) and 1.40% (n = 7), respectively. The limits of detections (3 of the blank signal) were 1.2 μg/L and 0.5 μg/L for the batch and continuous flow techniques, respectively. 3.1. Selection of the support for nickel/nickel boride nanoparticles coating Nickel/nickel boride (Ni/NixB) containing nanoparticles have been recently found to be highly effective adsorbents for As(III) and As(V) [27]. However, their application may be limited to batch technique for arsenic removal in public water system and small scale water treatment plants due to their small particle size. Therefore, for column applications, coating the nanoparticles on perlite, pumice, zeolite, silica gel and Purolite C-100 resin supports was tried. The supports were coated with Ni/NixB nanoparticles as described earlier. It was observed that the nanoparticles were loosely bound to the support particles when perlite, pumice, silica gel and zeolite were used as the support. However, when the resin was used, all the resin particles were coated with the black Ni/NixB-NP and no loose particles were formed. Moreover, with the support other than the resin, the black nanoparticles were decomposed and the colors were turned to green for perlite, pumice, zeolite and silica gel in 24 h. However, the black coating on Purolite C-100 resin was stable for more than a year without taking any precaution to protect and, therefore, used in the following studies. 3.2. Optimization studies for the preparation of the adsorbent Effect of drying temperature, Ni(II) concentration, NaBH4 concentration and shaking time with Ni(II) on uptake and removal efficiency of As(III) was optimized. 3.2.1. Effect of drying temperature of the adsorbent For easy storage and handling, Ni/NixB-NPCR was dried. In order to understand the effect of drying temperature, prepared adsorbents were dried at room temperature (20 °C) and in an oven at 40, 60, 80 and 100 °C in air. For comparison, parallel experiments for arsenic adsorption were carried out using wet adsorbents. It was seen that nano-Ni/NixB barks were separated from the resin at the studied temperatures other than 20 °C. Decrease in the uptake and the removal efficiency of arsenic may mostly be due to the loss of the Ni/NixB particles as barks at high temperatures. When the drying temperature was increased from 20 °C to 100 °C, arsenic removal efficiency decreased from 99.7% to 92.4% (Fig. 1a) and arsenic uptake also decreased from
19.0 mg/g to 6.3 mg/g (Fig. 2a) so that room temperature (20 °C) was chosen as the drying temperature. Adsorbent dried at 20 °C was equally effective for arsenic removal compared with the wet adsorbent (18.8 mg/g uptake and 99.5% removal efficiency). 3.2.2. Effect of Ni(II) concentration used for the adsorbent preparation Solutions at different Ni(II) concentrations were used for the preparation of the adsorbents. 5 mL of 0; 1; 10; 30; 60 and 125 g/L Ni(II) solutions was added onto 0.5 g resin and the adsorbent was prepared as described earlier. After the arsenic adsorption on the prepared adsorbents, arsenic remaining in the solution was determined. As shown in Fig. 1b, removal efficiency reached a maximum value around 100% at Ni(II) concentration of 10 g/L and then, remained the same. Arsenic uptake reached a maximum (18.9 mg/g) at Ni(II) concentration of 30 g/L (Fig. 2b). Therefore, 30 g/L Ni(II) was chosen as the optimum Ni(II) concentration for the preparation of the adsorbent. 3.2.3. Effect of NaBH4 concentration used for the adsorbent preparation Solutions at different concentrations of NaBH4 were used for reducing Ni(II). 2.5 mL of 0.25; 0.5; 1.0; 2.0; 4.0 and 6.0% NaBH4 was added onto Ni(II) adsorbed resin (0.5 g). 10 min later, the solution was decanted and the adsorbent was washed with distilled water until achieving clear supernatant solution. The adsorbents were dried at room temperature. The adsorbents prepared by reducing Ni(II) with 0.25, 0.5 and 1.0% NaBH4 solutions were green or black–green colored possibly due to the uncompleted formation of nanoparticles. Maximum removal efficiency (99.6%) (Fig. 1c) and optimal arsenic uptake (19.2 mg/g) (Fig. 2c) were observed with 4% NaBH4 concentration. Therefore, 4% NaBH4 was chosen as the reducing agent in the adsorbent preparation. 3.2.4. Effect of shaking time of the resin in Ni(II) solution Purolite C-100 resin is a strong cation exchange resin and binds Ni(II) by exchanging with H+. Resin particles were shaken with Ni(II) before reducing Ni(II) with NaBH4 solution for different times (20; 60; 150; 390; 960 and 1350 min). Shaking time of the resin with Ni(II) solution did not have any significant effect on the removal efficiency and arsenic uptake as shown in Figs. 1d and 2d, respectively. Removal efficiencies varied in the range 97.1–100.2% and arsenic uptakes varied in the range 18.7–19.2 mg/g, when the shaking time was varied in the range 20–1350 min. 3.3. Characterization of Ni/NixB-NPCR 3.3.1. SEM-EDX and AFM analyses of Ni/NixB-NPCR Nanostructure of Ni/NixB-NPCR on the SEM image was not clear. SEM images, however, showed that some particles of the adsorbents were damaged and nickel/nickel boride nanoparticle barks were separated from the resin during the preparation of the adsorbent for SEMEDX analysis. AFM analysis showed that the diameter of the nanoparticles on the surface of the resin was in the range 18 to 35 nm (Fig. 3a). EDX analysis of Ni/NixB-NPCR was also carried out and found that the particles contained nickel, boron, carbon and oxygen (Fig. 3b). After the adsorption of As(III) and As(V), arsenic peaks were also observed in the EDX spectrum (Fig. 3c and d). 3.3.2. XPS analysis of Ni/NixB-NPCR In our previous paper, characterization of the Ni/NixB nanoparticles was done and the structure was suggested as Ni/NixB [27]. In the present study surface of Ni/NixB-NPCR was also examined using XPS. The results are shown in Fig. 4a–f. Fig. 4a, b and c shows the survey analysis, Ni 2p scan, and B 1s scan of the adsorbent, respectively. Fig. 4d shows the XPS survey analysis result of the adsorbent after the As(III) adsorption where the adsorbed arsenic can clearly be seen. No significant boron peak could be observed, possibly due to the high amount of arsenic sorption on the surface. While boron peak could not be determined
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Fig. 1. a) Effect of drying temperature of the adsorbent on the removal efficiency; b) Effect of Ni(II) concentration used for the adsorbent preparation on the removal efficiency; c) Effect of NaBH4 concentration used for the adsorbent preparation on the removal efficiency; and d) Effect of shaking time of the resin in Ni(II) solution on the removal efficiency.
after As(III) sorption by XPS which represents up to 10 nm depths, it was obvious in the EDX analysis (Fig. 3c) which represents up to 50 nm depth. Boron peak was also apparent in the EDX spectrum of the adsorbent before arsenic sorption. For the examination of the oxidation state of the adsorbed arsenic; the adsorbent without adsorbed arsenic, As(III) and As(V) adsorbed adsorbents were analyzed with XPS. It is known that generally As(III) binding energies are about 1 eV lower than that of As(V) [30]. In the present study binding energies of As 3d peaks were determined as 44.2 and 44.9 eV for As(III) and As(V), respectively (Fig. 4e and f). Also the peak shapes indicated the
presence of a single species, not a contribution of multiple species. It can, therefore, be said that arsenic species, As(III) and As(V), were adsorbed with their original oxidation states by Ni/Ni xBNPCR. Martinson and Reddy [30] have reported that, after the adsorption of As(III) and As(V) on CuO nanoparticles, As 3d peaks were seen at 45.0 and 45.2 eV, respectively. They suggest that the adsorption of As(III) on CuO nanoparticles involves a process of oxidation prior to adsorption. Sasaki et al. [31] also reported that, in the adsorption of As(III) on zero valent iron, As 3d peak was separated into two components with As(III) (43.8) and As(V) (45.1 eV); the predominant species was As(V). It means that As(III) was mainly
Fig. 2. a) Effect of drying temperature of the adsorbent on uptake; b) Effect of Ni(II) concentration used for the adsorbent preparation on uptake; c) Effect of NaBH4 concentration used for the adsorbent preparation on uptake; and d) Effect of shaking time of the resin in Ni(II) solution on uptake.
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Fig. 3. a) AFM image of Ni/NixB-NPCR; b) EDX image of Ni/NixB-NPCR (without arsenic); c) EDX image of Ni/NixB-NPCR containing As(III); and d) EDX image of Ni/NixB-NPCR containing As(V).
oxidized to As(V) during the adsorption on zerovalent iron, controversial to the adsorption by the present Ni/NixB-NPCR in this study. 3.3.3. Effect of washing on the chemical composition of the adsorbent Nickel and boron contents of Ni/NixB-NPCR were determined by dissolving the Ni/NixB coating with concentrated HCl. The average content of nickel in the Ni/NixB-NPCR dried at room temperature (20 °C) was determined as 49.20 ± 0.02 mg/g (n = 5) using flame AAS. Boron analysis was realized using azomethine spectrophotometric method and found to be 3.17 ± 0.32 mg/g (n = 5). It was observed that the boron content of the adsorbent was decreased by further elaborate washing of the adsorbent four times for 24 h with distilled water in a shaker. The boron content was decreased to 0.14 ± 0.01 mg/g (n = 5) after this washing step. Nickel content of the further washed adsorbent was determined as 50.50 ± 0.03 mg/g which was similar to that before the further washing. Ni/B mole ratio of the adsorbent before and after the elaborate washing step was then calculated as 2.9/1 and 64.7/1, respectively. This high mole ratio can be attributed to the conversion of nickel borides to mainly elemental nickel besides the remaining borides. Glavee et al. [32] reported that, when Ni(II) is reduced with NaBH4 in aqueous media, 2þ
−
2Ni ðaqÞ þ 4BH4 ðaqÞ þ 9H2 O→Ni2 BðsÞ þ 12:5H2 þ 3BðOHÞ3
ð1Þ
reaction occurs. It was suggested that upon air exposure, the nanoscale Ni2B is converted to Ni(S) + B2O3 probably by the equation, 4Ni2 B þ 3O2 →8NiðsÞ þ 2B2 O3 :
ð2Þ
Since B2O3 has high water solubility (36 g/L at 25 °C) boron content of the adsorbent in this study was possibly decreased by washing because of B2O3 dissolution. Similar results were also observed in the column study (Section 3.4). While the boron concentration of the effluent was 32.3 mg/L at the beginning (20 mL), it was decreased to below the
limit of detection (LOD for B by azomethine spectrophotometric method was 0.003 mg/L) after 500 mL effluent. Nickel concentration of the effluents was below the limit of detection (LOD for Ni by flame AAS was 0.02 mg/L). Arsenic removal efficiency and uptake were also determined before and after the further washing step. As(III) and As(V) removal efficiencies were about 100% for both adsorbents. As(III) and As(V) uptakes were 18.5 mg/g and 13.7 mg/g, and 12.2 mg/g and 9.9 mg/g before and after the further elaborate washing of the adsorbents, respectively. Although some decreases in the uptakes were observed after the elaborate further washing, the uptakes were still high. However, in the applications to real samples the elaborate washing steps of the adsorbent will be seldomly required. 3.4. Effect of pH on arsenic removal As(III) and As(V) solutions at various initial pH levels were used to determine the optimum pH for arsenic removal using Ni/NixB-NPCR. Unbuffered 100 μg/L As(III) or As(V) solutions with initial pH in the range of 3.3–11.5 were added onto the adsorbents and the mixtures were shaken for 24 h at 25 °C. pHs of the solutions were also measured after the adsorption. Arsenic removal efficiencies were calculated by measuring unadsorbed arsenic in the solution. In the batch method, initial pH did not significantly affect the arsenic removal efficiencies for As(III) and As(V) in the pH range 3.3–11.5 for Ni/NixB-NPCR quite contrary to that obtained with the nano zero-valent iron on activated carbon NZVI/AC [33], nano-iron (hydr)oxide coated granulated activated carbon Fe-GAC [34], and nanoscale zero-valent iron NZVI [35]. The removal efficiencies of Ni/NixB-NPCR varied with pH change in the range 97.0–98.9% for As(III) and 93.5–98.7% for As(V). The obtained values are shown in Fig. 5. Equilibrium pH measured after 24 h against the initial pH is also shown in Fig. 5. Under the experimental conditions used initial pH changed to around 9 at equilibrium. Stabilization of the pH at equilibrium explains
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Fig. 4. XPS analysis of Ni/NixB-NPCR: a) Survey analysis of the adsorbent; b) Ni 2p scan of the adsorbent; c) B 1s scan of the adsorbent; d) Survey analysis of As(III) sorbed adsorbent; e) As 3d scan of As(III) sorbed adsorbent; and f) As 3d scan of As(V) sorbed adsorbent.
why the variations in the initial solution pH did not significantly affect arsenic removal efficiencies. For the batch adsorption study, the adsorbent in the solution was shaken for 24 h and could be affected by the dissolved oxygen in the solution. Increasing the pH of the solution to around 9 can be explained by the possible formation of Ni(OH)2 on the adsorbent surface. Also a column experiment was done. 650 mL of distilled water (pH 7) was passed through 5 mL of Ni/NixB-NPCR packed column at a constant speed (1.5 mL/min) at room temperature. While the pH of the effluent was 8.5 at the beginning (20 mL), it was decreased to about 7 at 40 mL and stayed stable at this value thereafter. 3.5. Effect of adsorbent dose Fig. 5. Effect of the initial pH of the solution on As(III) and As(V) removal and the variation of the equilibrium pH with the initial pH of the solution (initial As concentration: 100 μg/L, sorption: 24 h at 25 °C).
The effect of the adsorbent dose on the removal efficiency of As(III) and As(V) is shown in Fig. 6. Beyond 3.0 g/L adsorbent dose the removal
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Fig. 6. Effect of the adsorbent dose on the removal efficiency of As(III) and As(V) (initial As concentration: 100 μg/L, sorption: 24 h at 25 °C).
efficiencies of both As(III) and As(V) were more than 91% for the initial As(III) and As(V) concentration of 100 μg/L. At 7.0 g/L adsorbent dose, 99.5% and 99.8% arsenic removal efficiencies were obtained for As(III) and As(V), respectively, and 10.0 g/L adsorbent dose was chosen as optimal.
3.6. Effect of column flow rate on arsenic removal efficiency Since the adsorbent particles used in this study are large enough, they will best be suited for column separation of arsenic. Therefore, optimization and application of the column were studied. 5 g of the adsorbent was packed into a 1.0 cm i.d. glass column as to have 8 mL packing volume and fitted with glass wool at both ends. 25 mL of 100 μg/L As(III) or As(V) solution was passed through the column at various speeds (0.5–7.0 mL/min) at room temperature. As(III) and As(V) in the effluent were determined. The removal efficiencies were about 100% for all the speeds used.
3.7. Effect of diverse ions on the adsorption of As(III) and As(V) The effect of some diverse ions such as phosphate, silicate, sulfate, nitrate and chloride on arsenic removal was examined. It was reported in the literature that while chloride and sulfate increase the arsenic adsorption, silicate and phosphate decrease when the iron ores were used as adsorbent [36]. In a similar study with nano zero-valent iron on activated carbon as adsorbent it was reported that phosphate and silicate markedly decreased the removal of both arsenite and arsenate, while sulfate, carbonate, oxalate and humic acid were insignificant [33]. In the present study, 25 mL of phosphate, chloride, sulfate, nitrate and silicate solutions containing 500 μg/L As(III) and As(V) was added onto 0.5 g adsorbent and shaken for 24 h at 25 °C. Arsenic removal efficiencies were determined. Effect of the diverse ions on As(III) and As(V) removal efficiencies by Ni/NixB-NPCR are shown in Fig. 7. Significant decreases at 15% and 10% were observed for As(III) removal efficiency when the solution contained 5 mg/L phosphate-P and 10 mg/L silicate-Si, respectively. However, none of the ions studied showed significant effect on the As(V) removal efficiency. 3.8. Adsorption isotherm studies In order to identify the adsorption type and determine the maximum arsenic adsorption capacities, adsorption isotherm graphs were drawn. 0.1 g of the adsorbents was weighed and 25 mL of 5; 20; 50; 100; 250 and 500 mg/L As(III) or As(V) solutions at pH 6.0 was added onto the adsorbents and shaken at 25 °C for 24 h. Unadsorbed arsenic in the solution phase was determined by HGAAS. Isotherm equations are; Ce 1 C ¼ þ e q bqm qm
log q ¼ log K þ
q: b: qm: Ce: K: n:
Fig. 7. Arsenic removal efficiencies in the presence of diverse ions a) for As(III), and b) for As(V) (initial arsenic concentration: 500 μg/L, sorption: 24 h at 25 °C).
Langmuir equation
1 log Ce n
Freundlich equation
ð3Þ
ð4Þ
Amount of adsorbed arsenic (mg) / amount of the adsorbent (g) Langmuir constant (L/mg) Adsorption capacity (mg/g) Equilibrium concentration of the solution (mg/L) Freundlich constant (mg/g) Freundlich constant (dimensionless).
Langmuir and Freundlich isotherm graphs are shown in Fig. 8a and b, respectively. The correlation coefficients for the linear regression fit of Langmuir model were found to be higher than those of Freundlich model. The Langmuir model assumes a homogeneous sorption surface
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Fig. 8. a) Langmuir isotherm for the adsorption of As(III) and As(V), and b) Freundlich isotherm for the adsorption of As(III) and As(V) (sorption: 24 h at 25 °C).
and active sites with uniform energies. The constants of adsorption isotherms are shown in Table 2. In order to predict the adsorption efficiency of the process, the dimensionless equilibrium parameter, RL, was determined by the following equation [37]: RL ¼
1 1 þ bC1
ð5Þ
where C1 is the initial concentration of arsenic and b is the Langmuir constant. Value of RL b 1 represents the favorable adsorption and value greater than one represents unfavorable adsorption [38]. The values of RL against C1 are shown in Fig. 9 for As(III) and As(V). The values indicate highly favorable adsorption. Nickel and boron contents of Ni/NixB-NPCR were indicated as 49.2 mg/g and 3.17 mg/g, respectively. Maximum arsenic adsorption capacities of the adsorbent were also determined as 23.4 mg/g for As(III) and 17.8 mg/g for As(V) from the Langmuir isotherms. Kanel et al. [35] reported that the capacity was 3.5 mg/g for As(III) using nanoscale zero-valent iron. Arsenic adsorption capacity of Ni/NixB-NPCR was compared with some reported adsorbents which were obtained by forming nano-structure on a support material and the comparison is presented in Table 3. It is seen that all of the adsorbents summarized in Table 3 have lower adsorption capacity than Ni/NixB-NPCR. Arsenic adsorption capacities for Ni/NixB-NPCR were also calculated for each Ni and B and shown in Table 4.
3.9. Adsorption kinetics 25 mL of 100 μg/L As(III) solution was added onto 0.25 g of the adsorbent and the mixture has been shaken for 10; 25; 60; 120; 300; 500; 960; and 1440 min at 25 °C. Arsenic remaining in the solution was determined by HG-AAS. The same procedure was also applied for As(V). As shown in Fig. 10a, during the adsorption of 100 μg/L As(III) and As(V) from 25 mL solution by 0.25 g of Ni/NixB-NPCR, arsenic concentrations were decreased below the maximum allowable contaminant level for drinking water (0.01 mg/L) for As(III) and As(V) in 270 and 400 min, respectively. Also the pseudo first-order, pseudo secondorder and intraparticle diffusion models were used to fit the experimental data. The pseudo first-order model of Lagergren is given as: log ðqe −qt Þ ¼ log qe −
k1 t: 2:303
The pseudo second-order model is expressed as: t 1 t ¼ þ : qt h qe
Table 2 Adsorption isotherm constants for As(III) and As(V) on Ni/NixB-NPCR. Species
qm (mg/g)
b (L/mg)
K (mg/g)
n
As(III) As(V)
23.4 17.8
0.060 0.031
1.79 0.89
1.98 1.79
ð6Þ
Fig. 9. RL values for the initial As(III) and As(V) concentrations.
ð7Þ
478
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Table 3 Comparison of the adsorption capacities of Ni/NixB-NPCR for the removal of As(III) and As(V) with some reported adsorbents. Adsorbent
pH
CAP-M CAZ-M CTS-g-PA F400-M NZVI/AC Ni/NixB-NPCR
Concentration
Temperature (°C)
500 μg/L 500 μg/L 25–80 mg/L 500 μg/L 2 mg/L 0–500 mg/L
7 7 7.2 7 6.5 6
25 25 30 25 25 25
The initial adsorption rate (h) is given as: 2
ð8Þ
The intraparticle diffusion equation can be described as: 0:5
þI
ð9Þ
where k1 (1/min), k2 (g/mg·min) and kint (mg/g·min0.5) are the rate constants of the pseudo first-order adsorption, the pseudo secondorder adsorption and the intraparticle diffusion, respectively. qe and qt (mg/g) are the amount of adsorbed arsenic on the adsorbent at equilibrium and time t, respectively [39]. The plots of the pseudo first-order adsorption, the pseudo second-order adsorption and the intraparticle diffusion models are shown in Fig. 10b, c and d, respectively. The slopes and intercepts of these plots were used for determining the constants k1, k2 and kint. The values of the constants are shown in Table 5. The linear fit indicates that the adsorption follows the pseudosecond order model best (R2 N 0.999 for both As(III) and As(V)). Also the theoretical qe values for As(III) adsorption (0.0101 mg/g) and As(V) adsorption (0.0103 mg/g) are very closed to the experimental value (0.0100 mg/g) with the pseudo-second order model. The correlation coefficients (R2) for the pseudo first-order model for both As(III) and As(V) were low (0.8339 and 0.9240, respectively). The results showed that the pseudo second-order adsorption mechanism was predominant and the overall rate of As(III) and As(V) adsorption process appeared to be controlled by chemical process [40]. A plot of qt versus t0.5 should give a straight line with a slope of kint and an intercept of C. If the linear fit of the data points is linear and passes through the origin (C = 0), then intraparticle diffusion is the sole rate-limiting step [41]. As shown in Fig. 10d, the plots for As(III) and As(V) were not linear implying that intraparticle diffusion was not only rate-limiting step. It was noted that the adsorption process tends to be followed by two phases. Two phases in the intraparticle diffusion plots suggest that the adsorption process proceeds by surface adsorption and intraparticle diffusion [42]. The intraparticle diffusion rate constants (kint1 and kint2) for each phase are shown in Table 5.
Table 4 Arsenic capacities calculated with respect to the amount of Ni/NixB-NPCR, nickel and boron contents. Capacities
Adsorbent (for each g) Ni (for each g) B (for each g) Ni (for each mol) B (for each mol)
Ref.
As(III)
As(V)
– – – – 18.2 23.4
0.370 0.526 6.56 1.250 12.0 17.8
[44] [44] [45] [44] [33] Present study
3.10. Regeneration studies of Ni/NixB-NPCR packed in a column
h ¼ k2 qe :
qt ¼ kint t
Adsorption capacities (mg/g)
As(III)
As(V)
23.4 mg 0.48 g 7.38 g 0.37 mol 1.06 mol
17.8 mg 0.36 g 5.61 g 0.28 mol 0.81 mol
Cumbal and SenGupta [43] reported that, Hybrid Anion Exchanger (HAIX) was amenable to efficient regeneration with 2% NaOH and 3% NaCl. Similarly, arsenic regeneration was tried by using the same mixture for Ni/NixB-NPCR packed in a column. Arsenic removal efficiencies were calculated by determining unadsorbed arsenic in the effluent and desorption efficiencies were calculated by determining arsenic in the eluate using 2% NaOH and 3% NaCl mixture as eluent. 25 mL of 100 μg/L As(III) or As(V) solution was passed through 8 mL of Ni/NixB-NPCR packed column at a constant speed (1.5 mL/min) at room temperature. The column was washed with distilled water and then 50 mL of 2% NaOH and 3% NaCl mixture was passed through the column at 1.5 mL/min to elute arsenic. In order to determine the reusability of the adsorbent, consecutive adsorption–desorption cycles were repeated six times by using the same adsorbent and the results are shown in Table 6. More than 96.1% of As(III) and 97.0% of As(V) were adsorbed by the adsorbent in the column and practically all of the adsorbed As(III) and As(V) were desorbed with a mixture of 2% NaOH and 3% NaCl. This is an advantage for this adsorbent that it can be used for several times without any efficiency loose. 3.11. Applications to real samples Ni/NixB-NPCR packed in a column was used in this study. 20 mL of distilled water, commercially bottled drinking water and arsenic spiked drinking water (10; 100 and 1000 μg/L As(III) and As(V)) were passed through 8 mL of Ni/NixB-NPCR packed column at a constant speed (1.5 mL/min) at room temperature. Unadsorbed As(III) and As(V) were determined in the effluent with HG-AAS. In the column studies, arsenic removal efficiencies were above 99% for both As(III) and As(V). Spiked drinking water samples containing 1000 μg/L As(III) and As(V) were passed through the column, and arsenic concentrations in the effluent were reduced to 4.9 and 9.9 μg/L for As(III) and As(V), respectively. It means that Ni/NixB-NPCR could reduce the concentration of arsenic below the maximum allowable concentration level from such high arsenic containing waters. Nickel and boron concentrations of the effluents were also determined using flame AAS and azomethine spectrophotometric methods, respectively. During the adsorption of As(III) and As(V), no detectable nickel and boron passing into the water were determined for all the water samples. 4. Conclusions Nickel/nickel boride nanoparticles were formed on Purolite C-100 resin and used for As(III) and As(V) removal from water successfully. Optimal conditions for the preparation of Ni/NixB-NPCR were determined. Optimum drying temperature was chosen as room temperature (20 °C). Initial pH did not significantly affect the arsenic removal efficiencies (~ 100%) for As(III) and As(V) in the pH range 3.3–11.5. 10.0 g/L adsorbent dose was selected as optimal. The adsorption
T.D. Çiftçi, E. Henden / Powder Technology 269 (2015) 470–480
479
Fig. 10. a) Effect of time on the adsorption of As(III) and As(V); b) Plots of pseudo-first order model; c) Plots of pseudo-second model; and d) Plots of intra-particle diffusion model (initial As concentration: 100 μg/L, sorption: at 25 °C).
equilibrium data fitted well to the Langmuir model and the kinetics of adsorption process shows good agreement with the pseudo secondorder equation. Maximum adsorption capacities were 23.4 mg/g and 17.8 mg/g for As(III) and As(V), respectively. Phosphate and silicate decreased As(III) removal efficiency by 15% and 10%, respectively. The other ions studied did not affect the As(III) removal efficiency. None of the ions studied affected significantly the As(V) removal efficiency. Desorption studies showed that As(III) and As(V) could be desorbed using a mixture of 2% NaOH and 3% NaCl from the adsorbent and, therefore, the adsorbent can be used several times. Ni/NixB-NPCR was developed for the first time in the literature. This novel adsorbent is effective for both As(III) and As(V) removal from water, is suitable for column studies, has high As(III) and As(V) removal efficiencies and high capacities, and could be regenerated. It is also stable for long use (more than a year). Acknowledgments This study was supported by the Research Fund of Ege University, İzmir, Turkey under grant number 2010.FEN.049. Also Tülin Deniz Çiftçi
wishes to thank The Scientific and Technological Research Council of Turkey for scholarship during this study. References [1] R.C. Weast, Handbook of Chemistry and Physics, CRC Publishing, USA, 1974. [2] F.N. Robertson, Arsenic in ground water under oxidizing conditions, south-west United States, Environ. Geochem. Health 11 (1989) 171–176. [3] C.K. Jain, I. Ali, Arsenic: occurrence, toxicity and speciation techniques, Water Res. 34 (2000) 4304–4312. [4] B.K. Mandal, K.T. Suzuki, Arsenic round the world: a review, Talanta 58 (2002) 201–235. [5] M.J. DeMarco, A.K. SenGupta, J.E. Greenleaf, Arsenic removal using a polymeric/inorganic hybrid sorbent, Water Res. 37 (2003) 164–176. [6] J.C. Ng, Environmental contamination of arsenic and its toxicological impact on humans, Environ. Chem. 2 (2005) 146–160. [7] J.F. Ferguson, J. Gavis, A review of the arsenic cycle in natural water, Water Res. 6 (1972) 1259–1274. [8] I. Villaescusa, J.C. Bollinger, Arsenic in drinking water: sources, occurrence and health effects (a review), Rev. Environ. Sci. Biotechnol. 7 (2008) 307–323. [9] Guidelines for Drinking Water Quality, World Health Organisation (WHO), Geneva, 1993. (http://www.who.int/water_sanitation_health/dwq/gdwq2v1/en/). [10] Rules and Regulations National Primary Drinking Water Regulations: Arsenic and Clarifications to Compliance and New Source Contaminants Monitoring, Federal Register 66 14, 6976–7066, The United States Environmental Protection Agency
Table 5 Parameters of pseudo-first order, pseudo-second order and intra-particle diffusion models for the adsorption of As(III) and As(V) on Ni/NixB-NPCR. Pseudo-first order k1 (1/min) As(III) As(V)
0.0053 0.0046
Pseudo-second order
qe (mg/g) 0.0041 0.0064
R
2
k2 (mg/g·min)
0.8339 0.9240
5.76 2.42
Intra-particle diffusion
qe (mg/g) 0.0101 0.0103
R
2
Kint1 (mg/g·min0.5)
0.9997 0.9992
0.0017 0.0008
I1 (mg/g) 0.0004 0.0005
Kint2 (mg/g·min0.5)
R21 0.9460 0.9330
−5
7.10 10.10−5
I2 (mg/g)
R22
0.0075 0.0065
0.9415 0.9233
Table 6 Regeneration studies with Ni/NixB-NPCR packed column. Removal efficiencies (%) Cycle As(III) As(V)
1 99.5 99.5
2 97.0 98.0
Desorption efficiencies (%) 3 96.1 97.4
4 99.4 100.1
5 99.5 99.5
6 96.9 97.0
1 99.5 100.9
2 99.3 100.1
3 99.5 100.3
4 100.6 101.1
5 101.5 101.4
6 101.3 99.9
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Powder Technology journal homepage: www.elsevier.com/locate/powtec
Nickel/nickel boride nanoparticles coated resin: A novel adsorbent for arsenic(III) and arsenic(V) removal Tülin Deniz Çiftçi ⁎, Emur Henden Department of Chemistry, Faculty of Science, Ege University, 35100 Bornova, İzmir, Turkey
a r t i c l e
i n f o
Article history: Received 3 July 2013 Received in revised form 12 September 2014 Accepted 21 September 2014 Available online 26 September 2014 Keywords: Nickel/nickel boride nanoparticles coated resin Arsenic removal Adsorption Water
a b s t r a c t A novel adsorbent named nickel/nickel boride nanoparticles coated resin has been introduced for the removal of both As(III) and As(V) from water. Nickel/nickel boride nanoparticles were formed on Purolite C-100 resin successfully. Perlite, pumice, zeolite and silica gel were also tried as the support material. However, nickel/nickel boride nanoparticles on those materials were not stable. Optimal preparation conditions for nickel/nickel boride nanoparticles coated resin were established. In the batch method, initial pH did not significantly affect the arsenic removal efficiencies for As(III) and As(V) in the pH range 3.3–11.5. As(III) and As(V) were removed quantitatively from the solutions at all pHs. 15% and 10% decreases were observed for As(III) removal efficiency when the solution contained phosphate and silicate respectively. However, none of the ions studied showed significant effect on the As(V) removal efficiency. Isotherm studies indicate that the Langmuir model fits the experimental data better than the Freundlich model. The isotherms also showed that the adsorption is favorable. Maximum arsenic adsorption capacities were calculated as 23.4 mg/g and 17.8 mg/g for As(III) and As(V), respectively. The kinetics of the adsorption process were tested for the pseudo-first order and pseudo-second order and intra-particle diffusion models. The comparison among the models showed that the pseudo second-order model best described the adsorption kinetics. As(III) and As(V) could be desorbed from the adsorbent without any efficiency loose using a mixture containing NaCl and NaOH and, therefore, the adsorbent could be used several times. Ni/NixBNPCR could reduce the concentration of both As(III) and As(V) below the maximum allowable concentration level (WHO limit, 10 μg/L) from such high arsenic containing waters. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Arsenic is a ubiquitous element and ranks 20th in natural abundance [1]. The presence of arsenic in natural water is mainly related to the process of leaching from the arsenic containing sources such as rocks and sediments [2]. Also biological actions and geochemical reactions help to mobilize arsenic into groundwater. Arsenic naturally occurs in over 200 different mineral forms, of which approximately 60% are arsenates, 20% sulfides and sulfosalts and the remaining 20% includes arsenides, arsenites, oxides, silicates and elemental arsenic (As) in the earth crust [3,4]. Two forms of arsenic are common in natural waters; arsenite 3− (AsO3− 3 ) and arsenate (AsO4 ), referred to as As(III) and As(V). Under atmospheric or more oxidizing environment, the predominant species is As(V) which exists mainly as H2AsO− 4 in the surface waters. As(III) is thermodynamically stable, predominant and exists mainly as H3AsO3 under mildly reducing conditions [5]. As(III) exists as neutral species, H3AsO3, in the pH range (6.5–7.5) of most natural waters, and this form of arsenic is more difficult to remove than As(V). ⁎ Corresponding author. Tel.: +90 232 3115447; fax: +90 232 3888264. E-mail addresses: [email protected] (T.D. Çiftçi), [email protected] (E. Henden).
http://dx.doi.org/10.1016/j.powtec.2014.09.041 0032-5910/© 2014 Elsevier B.V. All rights reserved.
Arsenic is toxic to both plants and animals and inorganic arsenicals are proven carcinogens in humans [6]. It is commonly accepted that inorganic As(III) compounds are approximately 60–80 times more toxic to humans than As(V) and inorganic arsenic compounds are about 100 times more toxic than organic ones [7,8]. The World Health Organization (WHO) revised the guideline for arsenic in drinking water as 10 μg/L in 1993 [9] and the United States Environmental Protection Agency (US-EPA) has implemented the reduction of permissible values of arsenic in drinking water from 50 to 10 μg/L in light of the epidemiological evidence to support the carcinogenic nature of the ingested arsenic and its connection with liver, lung and kidney diseases and other dermal effects [10]. Various techniques are employed for the removal of arsenic from water and wastewater. Precipitation, membrane processes, ion exchange and adsorption are the most common techniques [1–14]. Treatment cost, operational complexity of the technology, skill required to operate the technology and disposal of arsenic bearing treatment residual are the factors that should be considered before treatment method selection. Adsorption is an economical alternative to conventional metal removal techniques. Selective adsorption utilizing biological materials, mineral oxides, activated carbons and polymer resins has received
T.D. Çiftçi, E. Henden / Powder Technology 269 (2015) 470–480
increasing attention [15,16]. Also the unique structure and electronic properties of some nanoparticles can make them especially powerful adsorbents, such as nanoscale iron particles [17], amino-functionalized silica nano hollow sphere and silica gel [18] and clay nano-adsorbents [19]. Enhanced retention or solubilization of a contaminant may be helpful in a remediation setting. Remediation of arsenic has also been proposed using zero-valent iron and other classes of nanomaterials such as calcium peroxide nanoparticles [20], multiwall carbon nanotube-zirconia nanohybrid [21] and perlite incorporating γ-Fe2O3 and α-MnO2 nanomaterials [22]. Morgada et al. [23] investigated the effect of UV light and humic acid on As(V) removal using nano zerovalent iron. Also Deliyanni et al. [24] studied the removal of cadmium and arsenic using iron-based nano-adsorbents. Nanoparticles such as poly(amidoamine) dendrimers can serve as chelating agents, and can be further enhanced for ultrafiltration of a variety of metal ions (Cu(II), Ag(I), Fe(III), etc.) by attaching functional groups such as primary amines, carboxylates, and hydroxymates [25]. Self-assembled monolayers on mesoporous supports are nanoporous ceramic materials that have been developed to remove mercury or radionuclides from wastewater [26]. Nanoscale nickel/nickel borides appear to be promising materials for arsenic removal from drinking water because of its large active surface area and high arsenic adsorption capacity [27]. However, nano-nickel compounds usually appear as fine powders which cannot be applied in fix-bed columns unless they are of a granular shape. Therefore, in this study nickel/nickel boride nanoparticles have been formed on the cation exchange resin for fixed bed water treatment operations. The novel adsorbent named as nickel/nickel boride nanoparticles coated resin (Ni/NixB-NPCR) was prepared in our laboratory by reacting Ni(II) ion bound to resin with NaBH4. The adsorbent was found to be effective for both As(III) and As(V) removal. Another advantage of the adsorbent is that, As(III) and As(V) could easily be desorbed from the adsorbent making possible its use for several times. 2. Materials and methods 2.1. Reagents All the reagents were of analytical-reagent grade and glass distilled water was used throughout. 1000 mg/L arsenic(III) and arsenic(V) stock standard solutions were prepared by dissolving As2O3 (Merck) and Na2HAsO4·7H2O (Merck), respectively, in concentrated HCl (Merck) and diluting with distilled water. As(III) and As(V) stock solutions contained 2 mol/L HCl. More dilute standard solutions were prepared daily by dilution of the stock solutions. KI (Merck) and ascorbic acid (Merck) were used for arsenic determinations. 30 g/L Ni(II) solution was prepared by dissolving NiSO4·6H2O (Merck) in 0.01 mol/L HCl. 4% NaBH4 solution was prepared by dissolving NaBH4 (Merck, 98% pure) in distilled water just before use. The support material was Purolite C-100 cation exchange resin obtained from Purolite Company.
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2.3. Procedure for arsenic determination The operating conditions for the determinations of As(III) and As(V) using continuous flow and batch type hydride generation systems combined with atomic absorption spectrometer with a quartz tube atomizer were listed earlier [29]. Arsenic hydride was generated by reduction with NaBH4. For the determination of As(V) or total arsenic, As(V) was reduced to As(III) before NaBH4 reduction. For this purpose, 9 mL of the sample solution was mixed with 1 mL of 12 mol/L HCl and 2 mL of reducing agent (50% KI) and allowed to react for 15 min to reduce As(V) to As(III). After the addition of ascorbic acid, arsenic concentrations were measured by HGAAS. Continuous flow hydride generation system was used for the determination of total arsenic. Batch type hydride generation system was also used to determine the concentrations of As(III) and As(V) separately.
2.4. Optimization studies for the preparation of the adsorbent In the optimization studies, effect of the adsorbent preparation conditions on the arsenic removal efficiency and arsenic uptake was studied. For the uptake studies, 25 mL of 100 mg/L As(III) was added onto 0.1 g of the adsorbent and for the removal efficiency studies, 25 mL of 100 μg/L As(III) was added onto 0.4 g of the adsorbent. The solutions were shaken for 24 h at 25 °C using a shaker with thermostatic bath. Unadsorbed arsenic was measured.
2.5. Preparation of Ni/NixB nanoparticles coated resin (Ni/NixB-NPCR) Purolite C-100 cation exchange resin (0.3–1.2 mm) was ground and sieved to obtain 0.250–0.355 mm particles. Then, 0.5 g of the resin to be coated was weighed and added to a 50 mL beaker. 5 mL of 30 g/L Ni(II) solution was added to the beaker and the mixture was shaken for 20 min at 25 °C. After decanting the solution, the resin containing Ni(II) was washed with distilled water. Ni(II) bound to the resin was then reduced by adding 2.5 mL of 4% NaBH4 solution. When the gas bubbles were ceased, the solution was decanted and the resin coated with black Ni/NixB layer was obtained. The adsorbent was then washed several times with distilled water by decantation and dried at room temperature (20 °C).
2.6. Characterization of Ni/NixB-NPCR 25 mL of 500 mg/L As(III) and As(V) solutions were added onto 0.2 g of the adsorbents and shaken for 72 h. The particles were washed with distilled water and dried at room temperature. Parallel experiments were also carried out without arsenic. The surface morphology and chemical composition of the adsorbents were determined by SEMEDX, AFM and XPS analyses. Acquisition parameters for XPS analysis are shown in Table 1.
2.2. Apparatus pH was measured using Jenway 3040 model pH meter. Nuve ST-402 model shaker with thermostatic bath was used for shaking at fixed temperature. A GBC 904 PBT model atomic absorption spectrometer with HG 3000 model continuous flow hydride generation system and also a laboratory made batch type hydride generation system described earlier (HGAAS) [28] were used for arsenic determination. The characterization of Ni/NixB-NPCR was performed by using a scanning electronic microscope combined with X-ray energy dispersive spectrometer (SEM-EDX) (JEOL JSM-6060), atomic force microscopy (AFM) (Ambios Q-Scope 250) and Thermo Scientific K-Alpha model X-ray photoelectron spectroscopy (XPS). LW Scientific V325XS model spectrophotometer was used for boron determination.
Table 1 Acquisition parameters for XPS analysis. Parameter
Setting
Total acq. time No. of scans Source type Spot size Lens mode Analyzer mode Energy step size No. of energy steps
57.3 s 6 Al K alpha 400 μm Standard CAE: pass energy 30.0 eV 0.100 eV 191
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2.7. Adsorption studies The concentration of arsenic (mg/L) in the solution was determined before (C0) and after (Ce) the adsorption. The following equation was used for computing the percentage removal (R %) of arsenic: ðC0 −Ce Þ 100: R%¼ C0
ð1Þ
The amount of arsenic adsorbed by the adsorbent (qe) was calculated according to the following equation: qe ¼
ðC0 −Ce Þ V m
ð2Þ
where qe (mg/g) is the equilibrium adsorption capacity; V (L) is the volume of arsenic solution; m (g) is the mass of adsorbent used in the experiment. 3. Results and discussion The relative standard deviations for the determination of 30 μg/L arsenic using batch method and 20 μg/L arsenic with the continuous flow system were 3.30% (n = 6) and 1.40% (n = 7), respectively. The limits of detections (3 of the blank signal) were 1.2 μg/L and 0.5 μg/L for the batch and continuous flow techniques, respectively. 3.1. Selection of the support for nickel/nickel boride nanoparticles coating Nickel/nickel boride (Ni/NixB) containing nanoparticles have been recently found to be highly effective adsorbents for As(III) and As(V) [27]. However, their application may be limited to batch technique for arsenic removal in public water system and small scale water treatment plants due to their small particle size. Therefore, for column applications, coating the nanoparticles on perlite, pumice, zeolite, silica gel and Purolite C-100 resin supports was tried. The supports were coated with Ni/NixB nanoparticles as described earlier. It was observed that the nanoparticles were loosely bound to the support particles when perlite, pumice, silica gel and zeolite were used as the support. However, when the resin was used, all the resin particles were coated with the black Ni/NixB-NP and no loose particles were formed. Moreover, with the support other than the resin, the black nanoparticles were decomposed and the colors were turned to green for perlite, pumice, zeolite and silica gel in 24 h. However, the black coating on Purolite C-100 resin was stable for more than a year without taking any precaution to protect and, therefore, used in the following studies. 3.2. Optimization studies for the preparation of the adsorbent Effect of drying temperature, Ni(II) concentration, NaBH4 concentration and shaking time with Ni(II) on uptake and removal efficiency of As(III) was optimized. 3.2.1. Effect of drying temperature of the adsorbent For easy storage and handling, Ni/NixB-NPCR was dried. In order to understand the effect of drying temperature, prepared adsorbents were dried at room temperature (20 °C) and in an oven at 40, 60, 80 and 100 °C in air. For comparison, parallel experiments for arsenic adsorption were carried out using wet adsorbents. It was seen that nano-Ni/NixB barks were separated from the resin at the studied temperatures other than 20 °C. Decrease in the uptake and the removal efficiency of arsenic may mostly be due to the loss of the Ni/NixB particles as barks at high temperatures. When the drying temperature was increased from 20 °C to 100 °C, arsenic removal efficiency decreased from 99.7% to 92.4% (Fig. 1a) and arsenic uptake also decreased from
19.0 mg/g to 6.3 mg/g (Fig. 2a) so that room temperature (20 °C) was chosen as the drying temperature. Adsorbent dried at 20 °C was equally effective for arsenic removal compared with the wet adsorbent (18.8 mg/g uptake and 99.5% removal efficiency). 3.2.2. Effect of Ni(II) concentration used for the adsorbent preparation Solutions at different Ni(II) concentrations were used for the preparation of the adsorbents. 5 mL of 0; 1; 10; 30; 60 and 125 g/L Ni(II) solutions was added onto 0.5 g resin and the adsorbent was prepared as described earlier. After the arsenic adsorption on the prepared adsorbents, arsenic remaining in the solution was determined. As shown in Fig. 1b, removal efficiency reached a maximum value around 100% at Ni(II) concentration of 10 g/L and then, remained the same. Arsenic uptake reached a maximum (18.9 mg/g) at Ni(II) concentration of 30 g/L (Fig. 2b). Therefore, 30 g/L Ni(II) was chosen as the optimum Ni(II) concentration for the preparation of the adsorbent. 3.2.3. Effect of NaBH4 concentration used for the adsorbent preparation Solutions at different concentrations of NaBH4 were used for reducing Ni(II). 2.5 mL of 0.25; 0.5; 1.0; 2.0; 4.0 and 6.0% NaBH4 was added onto Ni(II) adsorbed resin (0.5 g). 10 min later, the solution was decanted and the adsorbent was washed with distilled water until achieving clear supernatant solution. The adsorbents were dried at room temperature. The adsorbents prepared by reducing Ni(II) with 0.25, 0.5 and 1.0% NaBH4 solutions were green or black–green colored possibly due to the uncompleted formation of nanoparticles. Maximum removal efficiency (99.6%) (Fig. 1c) and optimal arsenic uptake (19.2 mg/g) (Fig. 2c) were observed with 4% NaBH4 concentration. Therefore, 4% NaBH4 was chosen as the reducing agent in the adsorbent preparation. 3.2.4. Effect of shaking time of the resin in Ni(II) solution Purolite C-100 resin is a strong cation exchange resin and binds Ni(II) by exchanging with H+. Resin particles were shaken with Ni(II) before reducing Ni(II) with NaBH4 solution for different times (20; 60; 150; 390; 960 and 1350 min). Shaking time of the resin with Ni(II) solution did not have any significant effect on the removal efficiency and arsenic uptake as shown in Figs. 1d and 2d, respectively. Removal efficiencies varied in the range 97.1–100.2% and arsenic uptakes varied in the range 18.7–19.2 mg/g, when the shaking time was varied in the range 20–1350 min. 3.3. Characterization of Ni/NixB-NPCR 3.3.1. SEM-EDX and AFM analyses of Ni/NixB-NPCR Nanostructure of Ni/NixB-NPCR on the SEM image was not clear. SEM images, however, showed that some particles of the adsorbents were damaged and nickel/nickel boride nanoparticle barks were separated from the resin during the preparation of the adsorbent for SEMEDX analysis. AFM analysis showed that the diameter of the nanoparticles on the surface of the resin was in the range 18 to 35 nm (Fig. 3a). EDX analysis of Ni/NixB-NPCR was also carried out and found that the particles contained nickel, boron, carbon and oxygen (Fig. 3b). After the adsorption of As(III) and As(V), arsenic peaks were also observed in the EDX spectrum (Fig. 3c and d). 3.3.2. XPS analysis of Ni/NixB-NPCR In our previous paper, characterization of the Ni/NixB nanoparticles was done and the structure was suggested as Ni/NixB [27]. In the present study surface of Ni/NixB-NPCR was also examined using XPS. The results are shown in Fig. 4a–f. Fig. 4a, b and c shows the survey analysis, Ni 2p scan, and B 1s scan of the adsorbent, respectively. Fig. 4d shows the XPS survey analysis result of the adsorbent after the As(III) adsorption where the adsorbed arsenic can clearly be seen. No significant boron peak could be observed, possibly due to the high amount of arsenic sorption on the surface. While boron peak could not be determined
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Fig. 1. a) Effect of drying temperature of the adsorbent on the removal efficiency; b) Effect of Ni(II) concentration used for the adsorbent preparation on the removal efficiency; c) Effect of NaBH4 concentration used for the adsorbent preparation on the removal efficiency; and d) Effect of shaking time of the resin in Ni(II) solution on the removal efficiency.
after As(III) sorption by XPS which represents up to 10 nm depths, it was obvious in the EDX analysis (Fig. 3c) which represents up to 50 nm depth. Boron peak was also apparent in the EDX spectrum of the adsorbent before arsenic sorption. For the examination of the oxidation state of the adsorbed arsenic; the adsorbent without adsorbed arsenic, As(III) and As(V) adsorbed adsorbents were analyzed with XPS. It is known that generally As(III) binding energies are about 1 eV lower than that of As(V) [30]. In the present study binding energies of As 3d peaks were determined as 44.2 and 44.9 eV for As(III) and As(V), respectively (Fig. 4e and f). Also the peak shapes indicated the
presence of a single species, not a contribution of multiple species. It can, therefore, be said that arsenic species, As(III) and As(V), were adsorbed with their original oxidation states by Ni/Ni xBNPCR. Martinson and Reddy [30] have reported that, after the adsorption of As(III) and As(V) on CuO nanoparticles, As 3d peaks were seen at 45.0 and 45.2 eV, respectively. They suggest that the adsorption of As(III) on CuO nanoparticles involves a process of oxidation prior to adsorption. Sasaki et al. [31] also reported that, in the adsorption of As(III) on zero valent iron, As 3d peak was separated into two components with As(III) (43.8) and As(V) (45.1 eV); the predominant species was As(V). It means that As(III) was mainly
Fig. 2. a) Effect of drying temperature of the adsorbent on uptake; b) Effect of Ni(II) concentration used for the adsorbent preparation on uptake; c) Effect of NaBH4 concentration used for the adsorbent preparation on uptake; and d) Effect of shaking time of the resin in Ni(II) solution on uptake.
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Fig. 3. a) AFM image of Ni/NixB-NPCR; b) EDX image of Ni/NixB-NPCR (without arsenic); c) EDX image of Ni/NixB-NPCR containing As(III); and d) EDX image of Ni/NixB-NPCR containing As(V).
oxidized to As(V) during the adsorption on zerovalent iron, controversial to the adsorption by the present Ni/NixB-NPCR in this study. 3.3.3. Effect of washing on the chemical composition of the adsorbent Nickel and boron contents of Ni/NixB-NPCR were determined by dissolving the Ni/NixB coating with concentrated HCl. The average content of nickel in the Ni/NixB-NPCR dried at room temperature (20 °C) was determined as 49.20 ± 0.02 mg/g (n = 5) using flame AAS. Boron analysis was realized using azomethine spectrophotometric method and found to be 3.17 ± 0.32 mg/g (n = 5). It was observed that the boron content of the adsorbent was decreased by further elaborate washing of the adsorbent four times for 24 h with distilled water in a shaker. The boron content was decreased to 0.14 ± 0.01 mg/g (n = 5) after this washing step. Nickel content of the further washed adsorbent was determined as 50.50 ± 0.03 mg/g which was similar to that before the further washing. Ni/B mole ratio of the adsorbent before and after the elaborate washing step was then calculated as 2.9/1 and 64.7/1, respectively. This high mole ratio can be attributed to the conversion of nickel borides to mainly elemental nickel besides the remaining borides. Glavee et al. [32] reported that, when Ni(II) is reduced with NaBH4 in aqueous media, 2þ
−
2Ni ðaqÞ þ 4BH4 ðaqÞ þ 9H2 O→Ni2 BðsÞ þ 12:5H2 þ 3BðOHÞ3
ð1Þ
reaction occurs. It was suggested that upon air exposure, the nanoscale Ni2B is converted to Ni(S) + B2O3 probably by the equation, 4Ni2 B þ 3O2 →8NiðsÞ þ 2B2 O3 :
ð2Þ
Since B2O3 has high water solubility (36 g/L at 25 °C) boron content of the adsorbent in this study was possibly decreased by washing because of B2O3 dissolution. Similar results were also observed in the column study (Section 3.4). While the boron concentration of the effluent was 32.3 mg/L at the beginning (20 mL), it was decreased to below the
limit of detection (LOD for B by azomethine spectrophotometric method was 0.003 mg/L) after 500 mL effluent. Nickel concentration of the effluents was below the limit of detection (LOD for Ni by flame AAS was 0.02 mg/L). Arsenic removal efficiency and uptake were also determined before and after the further washing step. As(III) and As(V) removal efficiencies were about 100% for both adsorbents. As(III) and As(V) uptakes were 18.5 mg/g and 13.7 mg/g, and 12.2 mg/g and 9.9 mg/g before and after the further elaborate washing of the adsorbents, respectively. Although some decreases in the uptakes were observed after the elaborate further washing, the uptakes were still high. However, in the applications to real samples the elaborate washing steps of the adsorbent will be seldomly required. 3.4. Effect of pH on arsenic removal As(III) and As(V) solutions at various initial pH levels were used to determine the optimum pH for arsenic removal using Ni/NixB-NPCR. Unbuffered 100 μg/L As(III) or As(V) solutions with initial pH in the range of 3.3–11.5 were added onto the adsorbents and the mixtures were shaken for 24 h at 25 °C. pHs of the solutions were also measured after the adsorption. Arsenic removal efficiencies were calculated by measuring unadsorbed arsenic in the solution. In the batch method, initial pH did not significantly affect the arsenic removal efficiencies for As(III) and As(V) in the pH range 3.3–11.5 for Ni/NixB-NPCR quite contrary to that obtained with the nano zero-valent iron on activated carbon NZVI/AC [33], nano-iron (hydr)oxide coated granulated activated carbon Fe-GAC [34], and nanoscale zero-valent iron NZVI [35]. The removal efficiencies of Ni/NixB-NPCR varied with pH change in the range 97.0–98.9% for As(III) and 93.5–98.7% for As(V). The obtained values are shown in Fig. 5. Equilibrium pH measured after 24 h against the initial pH is also shown in Fig. 5. Under the experimental conditions used initial pH changed to around 9 at equilibrium. Stabilization of the pH at equilibrium explains
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Fig. 4. XPS analysis of Ni/NixB-NPCR: a) Survey analysis of the adsorbent; b) Ni 2p scan of the adsorbent; c) B 1s scan of the adsorbent; d) Survey analysis of As(III) sorbed adsorbent; e) As 3d scan of As(III) sorbed adsorbent; and f) As 3d scan of As(V) sorbed adsorbent.
why the variations in the initial solution pH did not significantly affect arsenic removal efficiencies. For the batch adsorption study, the adsorbent in the solution was shaken for 24 h and could be affected by the dissolved oxygen in the solution. Increasing the pH of the solution to around 9 can be explained by the possible formation of Ni(OH)2 on the adsorbent surface. Also a column experiment was done. 650 mL of distilled water (pH 7) was passed through 5 mL of Ni/NixB-NPCR packed column at a constant speed (1.5 mL/min) at room temperature. While the pH of the effluent was 8.5 at the beginning (20 mL), it was decreased to about 7 at 40 mL and stayed stable at this value thereafter. 3.5. Effect of adsorbent dose Fig. 5. Effect of the initial pH of the solution on As(III) and As(V) removal and the variation of the equilibrium pH with the initial pH of the solution (initial As concentration: 100 μg/L, sorption: 24 h at 25 °C).
The effect of the adsorbent dose on the removal efficiency of As(III) and As(V) is shown in Fig. 6. Beyond 3.0 g/L adsorbent dose the removal
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Fig. 6. Effect of the adsorbent dose on the removal efficiency of As(III) and As(V) (initial As concentration: 100 μg/L, sorption: 24 h at 25 °C).
efficiencies of both As(III) and As(V) were more than 91% for the initial As(III) and As(V) concentration of 100 μg/L. At 7.0 g/L adsorbent dose, 99.5% and 99.8% arsenic removal efficiencies were obtained for As(III) and As(V), respectively, and 10.0 g/L adsorbent dose was chosen as optimal.
3.6. Effect of column flow rate on arsenic removal efficiency Since the adsorbent particles used in this study are large enough, they will best be suited for column separation of arsenic. Therefore, optimization and application of the column were studied. 5 g of the adsorbent was packed into a 1.0 cm i.d. glass column as to have 8 mL packing volume and fitted with glass wool at both ends. 25 mL of 100 μg/L As(III) or As(V) solution was passed through the column at various speeds (0.5–7.0 mL/min) at room temperature. As(III) and As(V) in the effluent were determined. The removal efficiencies were about 100% for all the speeds used.
3.7. Effect of diverse ions on the adsorption of As(III) and As(V) The effect of some diverse ions such as phosphate, silicate, sulfate, nitrate and chloride on arsenic removal was examined. It was reported in the literature that while chloride and sulfate increase the arsenic adsorption, silicate and phosphate decrease when the iron ores were used as adsorbent [36]. In a similar study with nano zero-valent iron on activated carbon as adsorbent it was reported that phosphate and silicate markedly decreased the removal of both arsenite and arsenate, while sulfate, carbonate, oxalate and humic acid were insignificant [33]. In the present study, 25 mL of phosphate, chloride, sulfate, nitrate and silicate solutions containing 500 μg/L As(III) and As(V) was added onto 0.5 g adsorbent and shaken for 24 h at 25 °C. Arsenic removal efficiencies were determined. Effect of the diverse ions on As(III) and As(V) removal efficiencies by Ni/NixB-NPCR are shown in Fig. 7. Significant decreases at 15% and 10% were observed for As(III) removal efficiency when the solution contained 5 mg/L phosphate-P and 10 mg/L silicate-Si, respectively. However, none of the ions studied showed significant effect on the As(V) removal efficiency. 3.8. Adsorption isotherm studies In order to identify the adsorption type and determine the maximum arsenic adsorption capacities, adsorption isotherm graphs were drawn. 0.1 g of the adsorbents was weighed and 25 mL of 5; 20; 50; 100; 250 and 500 mg/L As(III) or As(V) solutions at pH 6.0 was added onto the adsorbents and shaken at 25 °C for 24 h. Unadsorbed arsenic in the solution phase was determined by HGAAS. Isotherm equations are; Ce 1 C ¼ þ e q bqm qm
log q ¼ log K þ
q: b: qm: Ce: K: n:
Fig. 7. Arsenic removal efficiencies in the presence of diverse ions a) for As(III), and b) for As(V) (initial arsenic concentration: 500 μg/L, sorption: 24 h at 25 °C).
Langmuir equation
1 log Ce n
Freundlich equation
ð3Þ
ð4Þ
Amount of adsorbed arsenic (mg) / amount of the adsorbent (g) Langmuir constant (L/mg) Adsorption capacity (mg/g) Equilibrium concentration of the solution (mg/L) Freundlich constant (mg/g) Freundlich constant (dimensionless).
Langmuir and Freundlich isotherm graphs are shown in Fig. 8a and b, respectively. The correlation coefficients for the linear regression fit of Langmuir model were found to be higher than those of Freundlich model. The Langmuir model assumes a homogeneous sorption surface
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Fig. 8. a) Langmuir isotherm for the adsorption of As(III) and As(V), and b) Freundlich isotherm for the adsorption of As(III) and As(V) (sorption: 24 h at 25 °C).
and active sites with uniform energies. The constants of adsorption isotherms are shown in Table 2. In order to predict the adsorption efficiency of the process, the dimensionless equilibrium parameter, RL, was determined by the following equation [37]: RL ¼
1 1 þ bC1
ð5Þ
where C1 is the initial concentration of arsenic and b is the Langmuir constant. Value of RL b 1 represents the favorable adsorption and value greater than one represents unfavorable adsorption [38]. The values of RL against C1 are shown in Fig. 9 for As(III) and As(V). The values indicate highly favorable adsorption. Nickel and boron contents of Ni/NixB-NPCR were indicated as 49.2 mg/g and 3.17 mg/g, respectively. Maximum arsenic adsorption capacities of the adsorbent were also determined as 23.4 mg/g for As(III) and 17.8 mg/g for As(V) from the Langmuir isotherms. Kanel et al. [35] reported that the capacity was 3.5 mg/g for As(III) using nanoscale zero-valent iron. Arsenic adsorption capacity of Ni/NixB-NPCR was compared with some reported adsorbents which were obtained by forming nano-structure on a support material and the comparison is presented in Table 3. It is seen that all of the adsorbents summarized in Table 3 have lower adsorption capacity than Ni/NixB-NPCR. Arsenic adsorption capacities for Ni/NixB-NPCR were also calculated for each Ni and B and shown in Table 4.
3.9. Adsorption kinetics 25 mL of 100 μg/L As(III) solution was added onto 0.25 g of the adsorbent and the mixture has been shaken for 10; 25; 60; 120; 300; 500; 960; and 1440 min at 25 °C. Arsenic remaining in the solution was determined by HG-AAS. The same procedure was also applied for As(V). As shown in Fig. 10a, during the adsorption of 100 μg/L As(III) and As(V) from 25 mL solution by 0.25 g of Ni/NixB-NPCR, arsenic concentrations were decreased below the maximum allowable contaminant level for drinking water (0.01 mg/L) for As(III) and As(V) in 270 and 400 min, respectively. Also the pseudo first-order, pseudo secondorder and intraparticle diffusion models were used to fit the experimental data. The pseudo first-order model of Lagergren is given as: log ðqe −qt Þ ¼ log qe −
k1 t: 2:303
The pseudo second-order model is expressed as: t 1 t ¼ þ : qt h qe
Table 2 Adsorption isotherm constants for As(III) and As(V) on Ni/NixB-NPCR. Species
qm (mg/g)
b (L/mg)
K (mg/g)
n
As(III) As(V)
23.4 17.8
0.060 0.031
1.79 0.89
1.98 1.79
ð6Þ
Fig. 9. RL values for the initial As(III) and As(V) concentrations.
ð7Þ
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Table 3 Comparison of the adsorption capacities of Ni/NixB-NPCR for the removal of As(III) and As(V) with some reported adsorbents. Adsorbent
pH
CAP-M CAZ-M CTS-g-PA F400-M NZVI/AC Ni/NixB-NPCR
Concentration
Temperature (°C)
500 μg/L 500 μg/L 25–80 mg/L 500 μg/L 2 mg/L 0–500 mg/L
7 7 7.2 7 6.5 6
25 25 30 25 25 25
The initial adsorption rate (h) is given as: 2
ð8Þ
The intraparticle diffusion equation can be described as: 0:5
þI
ð9Þ
where k1 (1/min), k2 (g/mg·min) and kint (mg/g·min0.5) are the rate constants of the pseudo first-order adsorption, the pseudo secondorder adsorption and the intraparticle diffusion, respectively. qe and qt (mg/g) are the amount of adsorbed arsenic on the adsorbent at equilibrium and time t, respectively [39]. The plots of the pseudo first-order adsorption, the pseudo second-order adsorption and the intraparticle diffusion models are shown in Fig. 10b, c and d, respectively. The slopes and intercepts of these plots were used for determining the constants k1, k2 and kint. The values of the constants are shown in Table 5. The linear fit indicates that the adsorption follows the pseudosecond order model best (R2 N 0.999 for both As(III) and As(V)). Also the theoretical qe values for As(III) adsorption (0.0101 mg/g) and As(V) adsorption (0.0103 mg/g) are very closed to the experimental value (0.0100 mg/g) with the pseudo-second order model. The correlation coefficients (R2) for the pseudo first-order model for both As(III) and As(V) were low (0.8339 and 0.9240, respectively). The results showed that the pseudo second-order adsorption mechanism was predominant and the overall rate of As(III) and As(V) adsorption process appeared to be controlled by chemical process [40]. A plot of qt versus t0.5 should give a straight line with a slope of kint and an intercept of C. If the linear fit of the data points is linear and passes through the origin (C = 0), then intraparticle diffusion is the sole rate-limiting step [41]. As shown in Fig. 10d, the plots for As(III) and As(V) were not linear implying that intraparticle diffusion was not only rate-limiting step. It was noted that the adsorption process tends to be followed by two phases. Two phases in the intraparticle diffusion plots suggest that the adsorption process proceeds by surface adsorption and intraparticle diffusion [42]. The intraparticle diffusion rate constants (kint1 and kint2) for each phase are shown in Table 5.
Table 4 Arsenic capacities calculated with respect to the amount of Ni/NixB-NPCR, nickel and boron contents. Capacities
Adsorbent (for each g) Ni (for each g) B (for each g) Ni (for each mol) B (for each mol)
Ref.
As(III)
As(V)
– – – – 18.2 23.4
0.370 0.526 6.56 1.250 12.0 17.8
[44] [44] [45] [44] [33] Present study
3.10. Regeneration studies of Ni/NixB-NPCR packed in a column
h ¼ k2 qe :
qt ¼ kint t
Adsorption capacities (mg/g)
As(III)
As(V)
23.4 mg 0.48 g 7.38 g 0.37 mol 1.06 mol
17.8 mg 0.36 g 5.61 g 0.28 mol 0.81 mol
Cumbal and SenGupta [43] reported that, Hybrid Anion Exchanger (HAIX) was amenable to efficient regeneration with 2% NaOH and 3% NaCl. Similarly, arsenic regeneration was tried by using the same mixture for Ni/NixB-NPCR packed in a column. Arsenic removal efficiencies were calculated by determining unadsorbed arsenic in the effluent and desorption efficiencies were calculated by determining arsenic in the eluate using 2% NaOH and 3% NaCl mixture as eluent. 25 mL of 100 μg/L As(III) or As(V) solution was passed through 8 mL of Ni/NixB-NPCR packed column at a constant speed (1.5 mL/min) at room temperature. The column was washed with distilled water and then 50 mL of 2% NaOH and 3% NaCl mixture was passed through the column at 1.5 mL/min to elute arsenic. In order to determine the reusability of the adsorbent, consecutive adsorption–desorption cycles were repeated six times by using the same adsorbent and the results are shown in Table 6. More than 96.1% of As(III) and 97.0% of As(V) were adsorbed by the adsorbent in the column and practically all of the adsorbed As(III) and As(V) were desorbed with a mixture of 2% NaOH and 3% NaCl. This is an advantage for this adsorbent that it can be used for several times without any efficiency loose. 3.11. Applications to real samples Ni/NixB-NPCR packed in a column was used in this study. 20 mL of distilled water, commercially bottled drinking water and arsenic spiked drinking water (10; 100 and 1000 μg/L As(III) and As(V)) were passed through 8 mL of Ni/NixB-NPCR packed column at a constant speed (1.5 mL/min) at room temperature. Unadsorbed As(III) and As(V) were determined in the effluent with HG-AAS. In the column studies, arsenic removal efficiencies were above 99% for both As(III) and As(V). Spiked drinking water samples containing 1000 μg/L As(III) and As(V) were passed through the column, and arsenic concentrations in the effluent were reduced to 4.9 and 9.9 μg/L for As(III) and As(V), respectively. It means that Ni/NixB-NPCR could reduce the concentration of arsenic below the maximum allowable concentration level from such high arsenic containing waters. Nickel and boron concentrations of the effluents were also determined using flame AAS and azomethine spectrophotometric methods, respectively. During the adsorption of As(III) and As(V), no detectable nickel and boron passing into the water were determined for all the water samples. 4. Conclusions Nickel/nickel boride nanoparticles were formed on Purolite C-100 resin and used for As(III) and As(V) removal from water successfully. Optimal conditions for the preparation of Ni/NixB-NPCR were determined. Optimum drying temperature was chosen as room temperature (20 °C). Initial pH did not significantly affect the arsenic removal efficiencies (~ 100%) for As(III) and As(V) in the pH range 3.3–11.5. 10.0 g/L adsorbent dose was selected as optimal. The adsorption
T.D. Çiftçi, E. Henden / Powder Technology 269 (2015) 470–480
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Fig. 10. a) Effect of time on the adsorption of As(III) and As(V); b) Plots of pseudo-first order model; c) Plots of pseudo-second model; and d) Plots of intra-particle diffusion model (initial As concentration: 100 μg/L, sorption: at 25 °C).
equilibrium data fitted well to the Langmuir model and the kinetics of adsorption process shows good agreement with the pseudo secondorder equation. Maximum adsorption capacities were 23.4 mg/g and 17.8 mg/g for As(III) and As(V), respectively. Phosphate and silicate decreased As(III) removal efficiency by 15% and 10%, respectively. The other ions studied did not affect the As(III) removal efficiency. None of the ions studied affected significantly the As(V) removal efficiency. Desorption studies showed that As(III) and As(V) could be desorbed using a mixture of 2% NaOH and 3% NaCl from the adsorbent and, therefore, the adsorbent can be used several times. Ni/NixB-NPCR was developed for the first time in the literature. This novel adsorbent is effective for both As(III) and As(V) removal from water, is suitable for column studies, has high As(III) and As(V) removal efficiencies and high capacities, and could be regenerated. It is also stable for long use (more than a year). Acknowledgments This study was supported by the Research Fund of Ege University, İzmir, Turkey under grant number 2010.FEN.049. Also Tülin Deniz Çiftçi
wishes to thank The Scientific and Technological Research Council of Turkey for scholarship during this study. References [1] R.C. Weast, Handbook of Chemistry and Physics, CRC Publishing, USA, 1974. [2] F.N. Robertson, Arsenic in ground water under oxidizing conditions, south-west United States, Environ. Geochem. Health 11 (1989) 171–176. [3] C.K. Jain, I. Ali, Arsenic: occurrence, toxicity and speciation techniques, Water Res. 34 (2000) 4304–4312. [4] B.K. Mandal, K.T. Suzuki, Arsenic round the world: a review, Talanta 58 (2002) 201–235. [5] M.J. DeMarco, A.K. SenGupta, J.E. Greenleaf, Arsenic removal using a polymeric/inorganic hybrid sorbent, Water Res. 37 (2003) 164–176. [6] J.C. Ng, Environmental contamination of arsenic and its toxicological impact on humans, Environ. Chem. 2 (2005) 146–160. [7] J.F. Ferguson, J. Gavis, A review of the arsenic cycle in natural water, Water Res. 6 (1972) 1259–1274. [8] I. Villaescusa, J.C. Bollinger, Arsenic in drinking water: sources, occurrence and health effects (a review), Rev. Environ. Sci. Biotechnol. 7 (2008) 307–323. [9] Guidelines for Drinking Water Quality, World Health Organisation (WHO), Geneva, 1993. (http://www.who.int/water_sanitation_health/dwq/gdwq2v1/en/). [10] Rules and Regulations National Primary Drinking Water Regulations: Arsenic and Clarifications to Compliance and New Source Contaminants Monitoring, Federal Register 66 14, 6976–7066, The United States Environmental Protection Agency
Table 5 Parameters of pseudo-first order, pseudo-second order and intra-particle diffusion models for the adsorption of As(III) and As(V) on Ni/NixB-NPCR. Pseudo-first order k1 (1/min) As(III) As(V)
0.0053 0.0046
Pseudo-second order
qe (mg/g) 0.0041 0.0064
R
2
k2 (mg/g·min)
0.8339 0.9240
5.76 2.42
Intra-particle diffusion
qe (mg/g) 0.0101 0.0103
R
2
Kint1 (mg/g·min0.5)
0.9997 0.9992
0.0017 0.0008
I1 (mg/g) 0.0004 0.0005
Kint2 (mg/g·min0.5)
R21 0.9460 0.9330
−5
7.10 10.10−5
I2 (mg/g)
R22
0.0075 0.0065
0.9415 0.9233
Table 6 Regeneration studies with Ni/NixB-NPCR packed column. Removal efficiencies (%) Cycle As(III) As(V)
1 99.5 99.5
2 97.0 98.0
Desorption efficiencies (%) 3 96.1 97.4
4 99.4 100.1
5 99.5 99.5
6 96.9 97.0
1 99.5 100.9
2 99.3 100.1
3 99.5 100.3
4 100.6 101.1
5 101.5 101.4
6 101.3 99.9
480
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