New Functionalized Magnetic Materials for As5+ Removal: Adsorbent

Apr 11, 2014 - This article is part of the Ganapati D. Yadav Festschrift special issue. .... Nina Nicomel , Karen Leus , Karel Folens , Pascal Van Der...
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New Functionalized Magnetic Materials for As5+ Removal: Adsorbent Regeneration and Reuse Juan Saiz, Eugenio Bringas, and Inmaculada Ortiz* Dept. Ingenierías Química y Biomolecular, ETSIIyT, Universidad de Cantabria, Avda Los Castros s/n, 39005 Santander, Spain S Supporting Information *

ABSTRACT: The presence of arsenic in natural water is one of the most important pollution problems worldwide. Functionalized magnetic silica/magnetite nanoparticles (M3) have been reported as effective materials for arsenate adsorption [Saiz et al., 2014]. Because the process economy might be limited by the solid reuse, this work aims at the analysis of the regeneration and reusability of arsenate loaded M3 materials. The influence on the desorption and readsorption efficacies of the type and concentration of the regeneration agent, HCl or NaOH, and the sorbent refunctionalization steps (F1 is protonation of amino groups, F2 is coordination of Fe3+) is analyzed. Desorption with HCl is concentration dependent with maximum efficacies at 0.25 mol L−1. Solutions of NaOH 10−3 mol L−1 provided the best desorption performance; however, the regeneration of the solid needed of two stages of refunctionalization (F1 and F2). Furthermore, regenerated materials under alkaline conditions reported adsorption yields of arsenic around 90%.

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materials,30,31 is a promising alternative to facilitate the recovery of the solid dispersed in the liquid phase by application of a high gradient magnetic field.5 In general, adsorption materials for metal removal should be stable, efficient, cost-effective, and reusable. Under the selected operation conditions, adsorbents have a finite removal capacity; when it is achieved, the material should be regenerated for reuse or managed at the end of its life depending on the process economy.32 Even though several end-of-life alternatives for exhausted adsorbents such as solid combustion have been reported in the literature, in the case of arsenic-loaded materials, thermal alternatives are not feasible because arsenic oxides are volatile and can easily escape to the atmosphere.33 On the other hand, regeneration processes aiming at restoring the sorbent close to its initial properties should be low-cost, allowing the solid reuse during the maximum number of cycles and, thus, decreasing the costs of the overall separation process. However, the eluted regeneration solution is a waste stream that should be managed. As arsenic has limited markets its recovery and further reuse from the regeneration solution may not be cost-effective, these streams are usually managed by the following strategies: (i) concentration and containment, (ii) dilution and dispersion, (iii) application of destructive techniques to remove the contaminant, and (iv) stabilization of the pollutant by material encapsulation, which is the most extensively reported option in the literature.33,34 In particular, arsenate desorption has been studied with different solutions, most of them based on the influence of pH on the process.4 Sodium hydroxide2−10,35 and strong mineral

INTRODUCTION Water treatment is one of the most important fields of adsorption application. This technology reports several advantages over other traditional water treatment alternatives such as precipitation because of its simple design, its easy operation and maintenance, and its ability to selectively remove the pollutants up to low concentration levels. Literature collects a large number of innovative adsorption materials with ability to remove heavy metals and organic compounds from aqueous solutions. In spite of the great importance of the adsorbent regeneration stage for the process economy, it is much less studied than the adsorption stage, and the available data are still rare in the case of the removal of heavy metals, such as arsenic solubilized in natural water.2−15 Groundwater contamination with arsenic (arsenate and arsenite) is a recognized environmental hazard that affects a large proportion of the world’s population that does not have access to adequate sources of water for drinking.16 The most affected areas are West Bengal, Bangladesh, Taiwan, Northern China, and Argentina due to the weathering of rocks, the improper management of industrial wastes, the agricultural use of arsenic, and so forth.4,17 Due to its toxicity for humans, the arsenic level in drinking water was limited by the WHO (World Health Organization) and the EPA (Environmental Protection Agency) as 10 μg L−1.18,19 Nowadays, the use of nanomaterials for the development of adsorption processes is an emerging area of study due to the unique characteristics of these materials because of their small size, large surface area, ease of functionalization, and so forth. Different nanoadsorbents have been reported as efficient materials to remove arsenic, that is, activated alumina, chitosan,14 nanozerovalent iron,20,21 maghemite and hematite nanoparticles,22−25 akaganeite,9 silica-based materials, or activated carbon.26−28 In addition, the design of nanoadsorbents incorporating magnetic properties, that is, using as adsorbent magnetite nanoparticles25,29 or magnetic composite © XXXX American Chemical Society

Special Issue: Ganapati D. Yadav Festschrift Received: March 3, 2014 Revised: April 10, 2014 Accepted: April 11, 2014

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acids3,11−15 are the most commonly used regeneration solutions to elute arsenic from loaded sorbents. Although the selection of the eluent depends on both the arsenic adsorption mechanism and the nature of the adsorbent,32 other operation variables, besides pH, may affect the efficiency of the desorption process, that is, the presence of competitive ions, the temperature, and the contact time between the solid and liquid phases.2 In a previous work, SiO2/Fe3O4 magnetic nanoparticles functionalized with aminopropyl groups incorporating Fe3+ were efficiently employed to adsorb As5+ and As3+ from polluted groundwater.1 In spite of other researchers who have analyzed the performance of similar materials for As5+ removal,31,36,37 most of them do not perform an exhaustive analysis of the regeneration and reuse steps.2,5 In this work, the desorption of arsenate from loaded magnetic Fe3O4@SiO2 nanoparticles is investigated as an essential step in the design of an adsorption process for the removal of this pollutant from groundwater. After having evaluated the performance of HCl and NaOH as desorption agents at different contact times, the influence on the efficiency of the adsorption/desorption process of both the desorption agent concentration and the unloaded material regeneration through refunctionalization (F1 is protonation of amino groups and F2 is coordination of Fe3+) is explored. Finally, the longterm performance of the process was evaluated in terms of the loss of adsorption capacity after several adsorption/desorption cycles, including intermediate regeneration steps.

onto the composite material followed by two sequential steps of functionalization: (i) washing with hydrochloric acid 0.1 M to generate the amine acid-functionalized material by protonation of the amino groups (functionalization F1; material M2) and (ii) Fe3+-organic coordination by contacting the material M2 with a solution of ferric chloride in 2-propanol (functionalization F2; material M3). The surface chemistry of the synthesized material was confirmed by Fourier transformed infrared spectroscopy and thermogravimetric analysis. The surface area analysis based on the nitrogen adsorption−desorption isotherm led to the following structural parameters: specific surface area, 293.6 ± 5.2 m2 g−1; average pore diameter, 2.87 nm; total pore volume, 0.21 cm3 g−1. The particle size determined by dynamic light scattering (ZetaSizer Nano, Malver Instruments) was 160.6 ± 15.6 nm.1 2.2. Desorption and Reusability Experiments. Initially, the assessment of employing HCl and NaOH as desorption agents and the need of material regeneration after the desorption step were evaluated by the analysis of the desorption and readsorption efficacies and the leaching of iron during the desorption step. As strong acids or bases might affect either the desorption yields or the silica-based material stability, the influence of the desorption agent concentration on the adsorption/desorption process was evaluated following the methodology reported for the viability analysis. In addition, the long-term performance of the process was evaluated through the decay of the arsenic removal efficacy after several adsorption/desorption cycles, including one or two regeneration steps. To evaluate the performance of the desorption process, the fresh material (M3) was first loaded with a certain amount of As5+ by contacting at room temperature 15 mg of solid M3 with 15 mL of arsenate (Na2HAsO4, Aldrich) solution with a concentration of 20 mg L−1 at pH ≈ 8. The experiments were performed in duplicate in glass contactors (15 mL) that were placed in a multisample stirrer (IKA RCT Basic) at 500 rpm with the temperature controlled by a probe submerged in one of the samples. Once equilibrium was reached, the solution and the adsorbent were separated being the supernatant characterized in terms of arsenic concentration by the molybdenum blue method that produces a molybdate−arsenate blue complex; its absorbance was measured at 868 nm wavelength using a UV−visible spectrophotometer (UV-1800; Shimadzu).1 After determining the residual arsenic concentration, the amount of arsenic adsorbed per unit mass, (qe, mg g−1) was calculated by mass balance using eq 1:

2. MATERIAL AND METHODS 2.1. Synthesis and Characterization of Materials. The method for the synthesis of magnetic Fe3O4@SiO2 composite nanoparticles functionalized with amino-alkyl alkoxysilanes groups incorporating Fe3+ was detailed in a previous work and is summarized in Figure 1.1

qe = qo +

(C i − Ce) ·Va ma

(1)

where qo is the initial concentration of arsenic on the solid surface (mg g−1), Ci and Ce are the initial and equilibrium arsenic concentrations in the solution (mg L−1), respectively, ma is the dry mass of contacted solid in the adsorption stage (g), and Va is the volume of arsenate solution (L). Desorption experiments were performed in a similar way than adsorption assays by contacting at room temperature 15 mg of the previously loaded material with a known amount of arsenate with 15 mL of the regeneration solution containing HCl (from 0.05 to 4 mol L−1) or NaOH (from 10−3 to 0.05 mol L−1) during the specified time. Water was used in blank experiments to quantify the potential effect of free-arsenic elution. Once the specified contact time was reached, phase

Figure 1. Synthesis procedure of functionalized nanoscale adsorbents.

In brief, the method consists of three steps. (i) Synthesis of superparamagnetic iron oxide nanoparticles (SPIONs) by coprecipitation in basic media of FeCl3·6H2O and FeCl2· 4H2O. (ii) Synthesis of magnetic silica nanoparticles (material M1) by coating SPIONs with a mesoporous silica layer obtained by hydrolysis and condensation of tetraethyl orthosilicate (TEOS) in the presence of the cationic surfactant CTABr. (iii) Postgrafting of amino-alkyl alkoxysilanes moieties B

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influence of the solvent on arsenic elution.2 The reusability of the previously unloaded materials and the quantification of iron leaching during the desorption step, as evidence of the loss of F2 functionalization, were also assessed. A brief summary of the viability results is next reported; however, a deeper analysis is developed in the Supporting Information section. Arsenate desorption is pH dependent whereas the influence of equilibration time is considered negligible (see Supporting Information Figure S1). The average As5+ desorption percentages obtained with 0.05 mol L−1 solutions of HCl and NaOH were 17.2% and 84%, respectively. Therefore, at the same concentration level, desorption in basic media is approximately 5 times more effective than in acidic conditions. From the reuse of the previously unloaded materials (Supporting Information Figure S2) it is observed that the solids washed with HCl 0.05 mol L−1 reported adsorption percentages of arsenate around 100% and close to the values obtained with the fresh solid M3. When the material treated with NaOH (0.05 mol L−1) was reused for adsorption purposes, the removal of arsenic was negligible as shown in Supporting Information Figure S2. This behavior was related to the deprotonation of the amino groups on the moieties grafted on the surface of the material fact that was also confirmed by the zeta potential measurements depicted in Supporting Information Figure S3. Therefore, it is concluded that after the desorption process with NaOH the functionalization F1 (see Figure 1) of the sorbent material is affected and, therefore, a refunctionalization stage with HCl is required to turn the amino groups into the acidic form. On the other hand, this stage is not needed for acid regeneration. Finally, the leaching of iron during the desorption stage (see Supporting Information Figure S4) occurred in a similar way under either acid or alkaline conditions with values of iron concentration varying from 2 to 7 mg L−1. It is, therefore, concluded that loaded materials subjected to acid and alkaline desorption should need of regeneration by F2 refunctionalization (see Supporting Information Figure S1). 3.2. Influence of the Desorption Agent Concentration. This section aims at evaluating the influence of the desoprtion agent concentration following the guidelines reported in Section 3.1. In the case of HCl, as the efficacy of the desoprtion process was demonstrated to be modest, the study is conducted by increasing the HCl concentration in the range between 0.05 and 4 mol HCl L−1. As was previously concluded, the reusability of the unloaded material was performed after regeneration by F2 refunctionalization. On the other hand, as it has been reported that the contact of mesoporous-silica-based materials with concentrated alkali solutions for a long time may affect the structure of the solid, thus limiting the applicability of the material,3,38 the influence of the NaOH concentration on the desorption process was carried at low concentration values in the range between 10−3 and 0.05 mol L−1 and reducing the contact time to the range between 15 and 60 min. The reusability experiments were conducted with unloaded materials with different levels of refunctionalization, F1 and F1+F2. Figures 2, 3, and 4 report the desorption, reusability, and iron leaching results, respectively, of the study performed at different HCl concentrations and different contact times. As shown in Figure 2, under the selected operation conditions arsenic desorption is affected by the hydrochloric acid concentration. The desorption percentage reaches a maximum value of 83.5% at an acid concentration of 0.25

separation and quantification of the arsenic content were carried out in the same way described above. The percentage of arsenate desorbed from the loaded material was calculated as follows: Desorption (%) =

Ced·Vd qe ·md

(2)

where Ced is the arsenate concentration in the regeneration solution at equilibrium conditions (mg L−1), Vd is the volume of the regeneration solution (L), qe is the arsenic concentration loaded on the solid before desorption calculated by eq 1 (mg g−1), and md is the mass of the loaded sorbent (g). Furthermore, the leaching or iron initially coordinated to the amino groups grafted on the solid surface was evaluated by determination of the total iron concentration in the regeneration solution by atomic absorption spectroscopy (AAS-3110, PerkinElmer). Occasionally, the total arsenic concentration was measured by ICP-OES spectroscopy (Plasma 400; PerkinElmer) in order to evaluate, by difference with the arsenate concentration, the presence of As3+ produced by reduction of As5+ species. Finally, the nanoparticles stability, before and after the desorption process, was investigated through surface zeta potential measurements (ZetaSizer Nano, Malver Instruments). The reusability of the previously unloaded material was evaluated by performing adsorption assays following the described methodology and employing the solid resulting from the desorption assays. Depending on the specific experiment the unloaded material was directly employed after water rinsing or after regeneration by one or two steps of refunctionalization, washing first with HCl 0.1 mol L−1 (F1) and then with FeCl3 0.1 mol L−1 (F2). In all cases, the sorbent is washed with deionized water and spin-dried after each step. The readsorption percentage of As5+ was calculated by the following expression: Adsorption (%) =

(C i − Ce) × 100 Ci

(3)

Having analyzed the regeneration/reusability conditions, namely, contact time, type, and concentration of the regeneration agent, and the degree of refunctionalization, the long-term performance of the adsorption/desorption process was evaluated in terms of the loss of the solid capacity after several loading and regeneration cycles. The analysis was performed working with solutions containing 5 mg As5+ L−1 and following the aforementioned guidelines and through two different approaches: (i) Evaluation of As5+ readsorption and desorption percentages after several adsorption/desorption cycles. (ii) Analysis of the process efficacy after several consecutive loading steps followed by a desorption step. Adsorption and desorption experiments were duplicated, with a mean value of the experimental error around 5%.

3. RESULTS AND DISCUSSION 3.1. Viability Results. After reviewing previous experimental conditions, in this work, the desorption performance from arsenate-loaded M3 materials (20.9 mg As5+ g−1) was studied at different contact times (1−15 h) employing HCl (0.05 mol L−1) and NaOH (0.05 mol L−1) as regeneration agents. Water rinsing was also performed to quantify the C

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performance of the desorption and reusability steps in the whole range of the operation conditions. At values of HCl concentration lower than 0.25 mol L−1, the release of iron and arsenic increases as the acid concentration grows. At stronger acid conditions, the concentration of iron in the desorption solution reaches values around 25 mg L−1, which are similar to the initial iron concentration on the solid, 31 mg L−1, reported by Yokoi et al.15 Finally, the potential damage of the chemical functionalization of materials contacted with concentrated HCl solutions was studied. As As5+ was not detected for HCl concentrations higher than 1 M (see Figure 2), the total arsenic concentration was measured by ICP-OES spectroscopy to evaluate the potential reduction of arsenate to either arsenite or arsine being the results depicted in Figure 5.

Figure 2. Influence of the HCl concentration and contact time on the desorption percentage of As5+ (qe = 20.34 ± 1.52 mg g−1; Vd = 15 mL; md = 15 mg).

Figure 3. Arsenate adsorption efficacy with fresh (material M3) and materials treated with HCl solutions with different concentrations (Ci = 20 mg L−1; Va = 15 mL; ma = 15 mg).

Figure 5. Comparison of As5+ and total arsenic concentrations after 15 h of desorption with solutions 1, 2, and 4 mol HCl L−1.

In the case of the desorption steps performed with HCl 1 mol L−1, the analytical results shown in Figure 5 reported a good agreement between the values of the total arsenic concentration and the concentration of arsenate; thus, no reduction of arsenate is taking place. On the other hand, at acid concentrations higher than 2 mol L−1, the reduction of arsenate to arsenite is confirmed by the difference between the As5+ concentration (≈0) and the total arsenic concentration (≈15.8 mg L−1). It is well known that in an excess of HCl (concentration ≈ 3 mol L−1), arsenate is reduced to arsenite in the presence of a reducing agent (usually the iodide anion) according to the following redox reaction:39,40

Figure 4. Influence of the HCl concentration and contact time on the leaching of iron from material M3.

H3AsO4 + 2H+ + 2I− → H3AsO3 + I 2 + H 2O

(4)

From the experimental proofs, namely, the reduction of arsenate to arsenite, the coloring of the desorption solution, and the loss of adsorption efficacy, it is pointed out that at high HCl concentrations a redox process likely affects the stability of the functional groups grafted onto the solid surface. However, further research focused on the analysis of the surface chemistry before and after the desorption stage is needed to confirm the proposed hypothesis by identifying the reducing groups involved in the process. It is also important to mention that mesoporous-silica based materials are attacked by mineral acids being the damage degree dependent both on the acid concentration and on the contact time.38,41 Therefore, a further long-term performance of the process is needed to confirm the stability of the desorption step. In the case of desorption under alkaline conditions, Pham et al.38 observed that under strong alkaline conditions silica particles are dissolved between 2% and 5% in less than 30 min. Furthermore, the presence of amino alkoxysilanes can increase the rate of dissolution of the silica up to three times because of

mol L−1 after 15 h. At HCl concentrations higher than 0.25 mol L−1, the amount of arsenic desorbed decreases as the acid concentration increases, reaching values close to zero and observing a light coloring of the desorption solution at HCl concentrations higher than 2 mol L−1. Similar conclusions can be found from the analysis of the reusability results depicted in Figure 3. The materials treated with solutions containing HCl concentrations lower than 1 mol L−1 keep the ability to adsorb arsenic due to the reasons reported in Section 3.1; however, the adsorption percentages decreased from 96.2% to 66.6% when the acid concentration varies from 0.05 to 1 mol L−1. On the other hand, HCl concentrations higher than 1 mol L−1 seem to damage the material structure or the chemical functionalization because the adsorption performance decreases up to values near zero. From the analysis of the results dealing with the leaching of iron during the regeneration process (Figure 4), it is difficult to establish a correlation between the leaching of iron and the D

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By comparison of the regeneration and reusability results under acid and basic conditions, it is concluded that (i) the desorption agent concentration and the contact time are lower than those reported in previous works to achieve similar efficacies (see Supporting Information Table S1), (ii) similar desorption yields (80−90%) are achieved with HCl (0.25 mol L−1) or NaOH (10−3 mol L−1), (iii) when NaOH is used in the desorption step the sorbent material should be regenerated by refunctionalization with HCl (0.1 mol L−1), and (iv) materials unloaded under either acid or alkaline conditions should be regenerated by washing with FeCl3 to reintroduce iron into the surface moieties. Although in terms of reagents consumption alkaline desorption seems to be more convenient, a further economical analysis should be performed. 3.3. Long-Term Regeneration and Reusability. This section evaluates the long-term performance of the regeneration process carried out with NaOH (10−3 mol L−1). The analysis performed following the aforementioned guidelines aims at the quantification of the loss of the solid capacity after several loading and regeneration cycles working with fresh solutions containing 5 mg of As5+ L−1 and solids subjected to one (F1) or two (F1+F2) refunctionalization steps. The adsorption and desorption percentages are evaluated through two different experimental approaches: (i) Several cycles of alternating adsorption and desorption stages with one (F1) or two (F1+F2) refunctionalization stages between them (Option 1; Figure 8).

the high localized pH at the silica surface caused by the natural basicity of the amino groups.38 It is concluded that the stability and long-term performance of the process requires the minimization of both the NaOH concentration and the contact time. Figure 6 shows the desorption results attained under alkaline conditions.

Figure 6. Influence of the NaOH concentration and contact time on the efficacy of the As5+ desorption step (qe = 21.79 ± 0.25 mg g−1; Vd = 15 mL; md = 15 mg).

The results confirm that the arsenate desorption from the loaded material can be effectively performed with 10−3 mol L−1 NaOH solutions in approximately 30 min achieving yields around 90%. It is also concluded that the desorption efficacy is not improved as the NaOH concentration increases up to 0.05 mol L −1 . The iron leaching increases as the NaOH concentration grows attaining mean iron concentrations of 1.7 and 0.4 mg L−1 at NaOH concentrations of 0.05 and 10−3 mol L−1, respectively. These values are lower than those the reported in Supporting Information Figure S4 for higher desorption times, thus confirming the importance of the contact time on the process stability. Finally, Figure 7 shows the reusability results obtained by employing the materials treated with NaOH in the adsorption of arsenate after either F1 or F1+F2 regeneration stages.

Figure 8. Evolution of the adsorption/desorption efficacies and the arsenic concentration on the material M3 following Option 1.

Figure 7. Comparisson of arsenate adsorption efficacy with regenerated (F1 or F1+F2) materials previously treated with solutions at different NaOH concentrations (Ci = 20 mg L−1; Va = 15 mL; ma = 15 mg).

(ii) Several cycles of consecutive loading stages followed by a desorption step with two (F1+F2) refunctionalization stages between them (Option 2; Figure 9). It is concluded that after five adsorption and desorption cycles with only one step of refunctionalization (F1) between stages, the adsorption and regeneration percentages decrease around 12.9% and 28.2%, respectively. Similar results are obtained if the whole refunctionalization (F1+F2) is performed, with the readsorption and desorption yields reduced by 5.7%

Under the selected operation conditions the readsorption of As5+ reached percentages around 90% independently of the contact time, the NaOH concentration and the degree of material regeneration. As it was previously discussed, a stronger influence of the degree of functionalization would be expected for longer use of the materials at higher arsenic concentrations on the solid. E

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84%, which are 5 times higher than those obtained in acidic media. (2) The regeneration with HCl in the range between 0.05 mol L−1 and 4 mol L−1 depends on the concentration level, with a maximum efficacy of 83.5% at 0.25 mol L−1. At concentrations higher than 2 mol L−1, the amount of desorbed arsenic decreases reaching values close to zero, a fact that is attributed to the potential modification of the chemical groups on the surface by a redox process. Although the step of amino groups reprotonation (F1) is not required, the loss of iron during the regeneration step suggested the need for an iron-based refunctionalization (F2) step. (3) Alkaline desorption was satisfactorily carried out with 10−3 mol L−1 of NaOH during 30 min achieving yields around 90%, which are not improved by increasing up to 50 times the NaOH concentration. On the other hand, the reusability of the material unloaded in basic media needs of regeneration by two refunctionalization steps to turn the amino groups into the protonated form (F1) and to coordinate the released iron (F2) into the surface functional groups. (4) After 5 cycles of adsorption/desorption performed using the conditions previously mentioned, the desorption efficacy decreased 26%, which was in agreement with the other values reported in literature, although the adsorption capacity of the material after several runs only diminished 5.7%, which was within the margin of the experimental error.

Figure 9. Evolution of the adsorption/desorption efficacies and the arsenic concentration on the material M3 following Option 2.

and 26%, respectively. The similarity of these results is in good agreement with the equilibrium data reported by Saiz et al.,1 which confirms the similar performance of the materials with different degree of functionalization at low arsenic concentrations based on the similar values of the Langmuir adsorption constants of both materials (KL,F1 = 0.344 L mg−1 and KL,F1+F2 = 0.383 L mg−1). Finally, Figure 9 depicts the process efficacy when the material M3 is subjected to three adsorption/desorption cycles consisting of six consecutive adsorption stages followed by the desorption stage and refunctionalization F1 and F2 between each cycle. From the adsorption results shown in Figure 9, it is observed that the arsenic concentration on the solid reaches values varying from 24 mg g−1 (cycle 1) to 18.1 mg g−1 (cycle 3). In each set of loading stages, the adsorption efficacy decreases from ≈100% to ≈10% due to the expected equilibrium limitations in the range of the operation conditions. Taking into account the six adsorption stages performed in each cycle, the overall adsorption percentages vary from 80% (cycle 1) to 60% (cycle 3). On the other hand, the regeneration efficacy decreases from 96% in the first cycle to 80% after three complete cycles. In spite of this loss of efficacy, the arsenic concentration on the solid after the third cycle was 4.3 mg L−1, with this value being lower than the solid concentrations observed at high adsorption yields.

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ASSOCIATED CONTENT

* Supporting Information S

Efficacy of arsenate desorption employing water, HCl, and NaOH (Figure S1). Comparison of arsenate adsorption efficacy with fresh solid and materials after desorption (Figure S2). Comparison of the surface zeta potential corresponding to fresh solids and materials after desorption (Figure S3). Influence of the contact time and the desorption agent on the leaching of iron (functionalization F2) from the material M3 (Figure S4). Literature information about arsenic desorption and readsorption employing nanoadsorbents (Table S1). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*I. Ortiz. E-mail: [email protected].

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Notes

The authors declare no competing financial interest.

CONCLUSIONS This research reports the analysis of the regeneration and reusability of magnetic Fe3O4@SiO2 composite nanoparticles functionalized with aminopropyl groups incorporating Fe3+ as adsorbents for arsenate removal from polluted groundwaters. First, the selection of the main operation conditions, namely, type and concentration of the desoprtion agent, HCl or NaOH, and influence of the solid refunctionalization (F1: protonation of amino groups and F2: coordination of Fe3+), that maximize the process efficacy has been analyzed and then the long-term stability of the adsorption/desorption stages has been assessed. The main results of this contribution can be summarized as follows: (1) For the same agent concentration (0.05 mol L−1), desorption in alkaline conditions leads to yields around

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ACKNOWLEDGMENTS Financial support from the Spanish Ministry of Economy and Competitiveness under the projects CTQ2008-00690 and CTQ2012-31639 (FEDER 2007-2013) is gratefully acknowledged.

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REFERENCES

(1) Saiz, J.; Bringas, E.; Ortiz, I. Functionalized magnetic nanoparticles as new adsorption materials for arsenic removal from polluted waters. J. Chem. Technol. Biotechnol. http://dx.doi.org/10.1002/jctb. 4331. (2) Tuutijärvi, T.; Vahalaa, R.; Sillanpitäa,̈ M.; Chen, G. Maghemite nanoparticles for As(V) removal: desorption characteristics and adsorbent recovery. Environ. Technol. 2012, 33, 1927. F

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dx.doi.org/10.1021/ie500912k | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX