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Halide removal from water using silver doped magnetic-microparticles
Journal of Environmental Management 253 (2020) 109731
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Research article
Halide removal from water using silver doped magnetic-microparticles ~ alver a, M. Sa �nchez-Polo a, *, J. Rivera-Utrilla a, M.V. Lo �pez-Ramo �n b, A.M.S. Polo a, J.J. Lopez-Pen a M. Rozal�en a b
Department of Inorganic Chemistry, Faculty of Science, University of Granada, Campus Fuentenueva s/n, ES18071, Granada, Spain Department of Inorganic and Organic Chemistry, Faculty of Science, University of Ja�en, Campus Las Lagunillas s/n, ES23071, Ja�en, Spain
A R T I C L E I N F O
A B S T R A C T
Keywords: Halide removal Silver microparticles Advanced oxidation process
This study proposes the use of new materials based on core-shell structure magnetic microparticles with Ag0 (Ag (0)-MPs) on their surface to remove bromides and chlorides from waters intended for human consumption. Hydrogen peroxide was used as oxidizing agent, Ag(0)-MPs is thereby oxidized to Ag (I)-MPs, which, when in contact with Cl and Br ions, form the corresponding silver halide (AgCl and AgBr) on the surface of Ag-MPs. The concentration of Cl and Br ions was followed by using ion selective electrodes (ISEs). Silver microparticles were characterized by high-resolution scanning electron microscopy and X-ray photoelectron spectroscopy, while the presence of AgCl and AgBr on Ag-MPs was determined by microanalysis. We analyzed the influence of operational variables, including: hydrogen peroxide concentration in Ag-MP system, medium pH, influence of Cl ions on Br ion removal, and influence of tannic acid as surrogate of organic matter in the medium. Regarding the influence of pH, Br and Cl removal was constant within the pH range studied (3.5–7), being more effective for Br than for Cl ions. Accordingly, this research states that the system Ag-MPs/H2O2 can remove up to 67.01% of Br ions and 56.92% of Cl ions from water (pH ¼ 7, [Ag-MPs]0 ¼ 100 mg L 1, [H2O2]0 ¼ 0.2 mM); it is reusable, regenerated by radiation and can be easily removed by applying a magneti cally assisted chemical separation process.
1. Introduction The removal of halides before the conventional disinfection of water (chlorination, chloramination, or chloride dioxide) is important, because their presence gives rise to disinfection byproducts (DBPs) through reaction with the natural organic matter (NOM) present, espe cially humic compounds, giving rise to the formation of byproducts that are toxic for human health (Mansouri et al., 2015; Shah and Mitch, 2012). Most DBPs are halogenated, hence the need to reduce Br and I ion concentrations in water before disinfection treatments (Liu et al., 2018). Although some brominated DBPs (bromate, bromodichloro methane, dibromochloromethane, and bromoform) are regulated by the International Standards for Drinking-Water of the World Health Orga nization, the analysis of these DBPs and its potential toxicity provides a serious challenge to researchers because NOM is inherently complex, variable and typically poorly characterized. Therefore, alternatives to the use of chlorine have been proposed for disinfection such as advanced oxidation processes (AOPs), including the combined use of UV radia tion/H2O2 and O3/H2O2. However, each disinfection process generates
its own specific DBPs; therefore, the main approach to the minimization of DBP formation is to remove halide ions from the medium before any disinfection process. There are several approaches for halide removal from aqueous media but the most attractive and highly selective method is precipitation based on the low solubility of metal halides, such as AgI. However, this precipitate could form small crystalline particles and colloids, which are very difficult to remove from the solution (S� anchez-Polo et al., 2007). Many studies attempted to solve this drawback using silver containing adsorbents as it is summarized in Table 1. On one side, there is currently a major increase in the use and environmental application of micro and nanomaterials, especially magnetic ones. Thus, magnetite has been tested as an adsorbent in polluted water treatment (Horst et al., 2015), and the size reduction was found to increase its arsenic adsorption capacity >100 fold (Yean et al., 2005). Similar results were obtained by Shen et al. (2009), who observed that the adsorption of metallic ions on Fe3O4 nanoparticles increased with smaller size, while Zhao et al. (2010) proposed the use of magnetic nanoparticles with iron hydroxide to remove fluoride ions. Despite
* Corresponding author. E-mail address: [email protected] (M. S� anchez-Polo). https://doi.org/10.1016/j.jenvman.2019.109731 Received 28 May 2019; Received in revised form 2 October 2019; Accepted 16 October 2019 Available online 27 October 2019 0301-4797/© 2019 Elsevier Ltd. All rights reserved.
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Table 1 Adsorbents for the removal of halide from aqueous phase. Adsorbent type
Surface area (m2/g)
[Ag] (%)
pH
Adsorption capacity (mg/g)
Reference
Ag@activated carbon Ag-doped carbon aerogels Ag2O grafted titanate nanoalumina 3D Ag2O–Ag/TiO2 MIL-101 (Cr)–SO3Ag AgCl@calcium alginate Ag@Cu2O AgI-doped MIL-101 Ag-cloths AgCu/Cu2O hybrid Ag2O@Mg(OH)2 Ag–Ag2O modified on carbon spheres Ag2O–Ag2O3@ZIF-8 Nano silver anion exchanger Synthetic zeolite-based AgNPs
941 428 143 154 – – – 2513 <5 – 78.2 – 370 – 34–51
1.05 10 – 5.82 30.44 10.89 1 2 – – 34.74 6.62 20 6.8 2.15
7–8 7 – – 7.5 6 7 3 6.9–7.8 3 7 2.1 – 7 2.5
25.4–38.1 0.25 428 208 244 133 25.4 43.2 1.12–1.66 65.9 369 375 232 313 19.54–20.44
Hoskins et al. (2002) S� anchez-Polo et al. (2007) Bo et al. (2013) Liu et al. (2015) Zhao et al. (2015)
mentioned advantages, the nanosized materials must be removed care fully due to its potential toxicity and persistence in the environment. In a recent study, Feng et al. (2018) have demonstrated both size and coating have remarkable impact on the cellular uptake, cytotoxicity, distribu tion and clearance of magnetic nanoparticles. On the other side, magnetic microparticle composites have proven to be effective in separating selectively dissolved pollutants from solution ~ ez, 2000). The main advantage lies in magnetically (Kaminski and Nun assisted chemical separation (MACS) process, which utilizes magnetic microparticles coated with a selective extractant (e.g., solvent extractant or ion-exchange material) for the efficient recovery of dissolved trans uranics, fission products, and hazardous species from acidic aqueous ~ ez, 2000). wastes (Kaminski and Nun With this background, this study proposes the use of magnetic mi croparticles composite with Ag(0) on their surface (Ag-MPs), as selective extractant, and their subsequent oxidation with H2O2 to remove Br and Cl ions from the medium through the formation of the corresponding silver halides adsorbed on their surface for their subsequent elimination by applying an external magnetic field. The main objectives of this research were: i) to study halide (chloride and bromide) removal from waters using Ag-MPs; ii) to analyze the influence of operational vari ables (pH, initial H2O2 concentration, etc.) on halide ion removal from water; iii) to evaluate the regeneration of Ag-MPs by solar or ultraviolet (UV) radiation; and iv) to study the cytotoxicity of Ag-MPs.
Mao et al. (2016a) Mao et al. (2016b) Polo et al. (2016) Mao et al. (2017) Chen et al. (2018) Yu et al. (2018) Chen et al. (2019) Li et al. (2019) Tauanov and Inglezakis (2019)
MPMS XL type Magnetometer equipped with EVERCOOL. It was spe cifically configured to study the magnetization (M) and susceptibility (χ) of small experimental samples in a wide range of temperatures (2–400 K) and magnetic fields (5 to 5 T). The surface chemical properties of the material were analyzed by XPS, acquiring the data using a Kratos Axis Ultra DLD spectrometer equipped with Al Kα source. A surface area of 300 � 700 μm was studied at a power of 600 W. Photoemitted electrons were collected at an inci dence angle of 20� and pressure of 10 9 mbar. The general XPS spectrum was obtained with a resolution of 50 eV. The background signals of spectra were adjusted according to the Lorentz-Gauss method. Spectra were calibrated considering the bond energy corresponding to C1s, 284.5 eV, as reference peak. XPS spectra were analyzed using CasaXps version 2.3.16 software. Peaks of the different bond energies were deconvoluted using Gaussian functions and a program of curve adjust ment by square minimums. Backgrounds of peaks of interest were sub tracted using Shirley’s method (Shirley, 1972). Solid samples were examined for mineralogical composition by X-ray diffraction (XRD) in powder mounts, using a BRUKER D8 VENTURE diffractometer (dual radiation Cu and Mo founts) equipped with a PHOTON 100 (CMOS) area detector. 2.3. Halide ion determination N sensors ion-selective electrodes (ISEs) were used for the halide ion determination: model CNT ISE C35 for chlorides and model CNT ISE C80 for bromides.
2. Materials and methods 2.1. Materials and reagents
2.4. Study of Agþ ion leaching as a function of medium pH
All reagents used (sodium chloride, potassium bromide, hydrogen peroxide, 3-(N-morpholino) propanesulfonic acid [MOPS]) were of high purity and supplied by Sigma Aldrich. Ag-MPs were provided by NanoMyP® (Nanomaterials y Polímeros S.L, Spain). These microparti cles have a core-shell structure, with a magnetite core and a shell formed by methacrylic-type polymers. Ag(0) nanoparticles (’20 nm) formed by Ag(I) reduction were deposited on the shell. All solutions used were prepared with ultrapure water using a Milli-Q® equipment (Millipore).
Agþ ion leaching was studied by placing 100 mg L 1 Ag-MPs in contact with 1.0 � 10 3 mol L 1 H2O2 at different pH values. The con centration of Agþ in the medium was measured by using silver ISEs. 2.5. Study of the influence of operational variables on halide ion removal from the medium using the Ag-MPs/H2O2 system The silver deposited on the surface of Ag-MPs is Ag0 and requires oxidation by H2O2 in order to remove halide ions from the medium. We studied the influence on the process of the initial H2O2 concentration by placing 100 mg L 1 Ag-MPs in contact with initial H2O2 concentrations ranging from 5.0 � 10 4 to 2.0 � 10 3 M in a total volume of 10 mL Milli-Q water under constant agitation, with an initial Br or Cl con centration of 1.0 � 10 3 M. The influence of pH on halide removal from the medium was studied in experiments with pH values of 3.0, 5.0, or 7.0 at an initial H2O2 concentration of 1.0 � 10 3 mol L 1 and Ag-MPs concentration of 100 mg L 1.
2.2. Ag-MPs characterization UV–Visible spectroscopy analysis was conducted using a CARY 100 spectrophotometer with a resolution of 1 nm in the wavelength range between 220 and 750 nm. Ag-MPs morphology was studied by HRSEM using a Hitachi S-510 microscope. Before image acquisition, samples were coated with a light gold layer to provide conductive properties, and elemental microanal ysis of their surface (EDX) was conducted. The magnetic susceptibility study used a SQUID QUANTUM DESIGN 2
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Journal of Environmental Management 253 (2020) 109731
Fig. 1. High resolution spectra of Ag0 and Agþ by XPS.
We studied the influence of the presence of Cl ions on Br ion removal from the medium by maintaining a fixed initial concentration of Br ions (10 � 10 3 mol L 1) and H2O2 (1.0 � 10 3 mol L 1) and vary ing the initial concentration of Cl ions (2.0 � 10 4, 5.0 � 10 4, and 1.0 � 10 3 mol L 1).
We examined Ag-MPs regeneration by solar or UV radiation using the solar reactor and UV reactor described previously elsewhere (Polo et al., 2017).
The culture medium was EMEM (EBS), 2 mM Glutamine, 1% nonessential amino acids, 1 mM sodium pyruvate, and 10% fetal bovine serum. Cell viability was evaluated by the MTS reduction reaction, in which the amount of formazan generated is directly proportional to the num ber of viable cells. We seeded 10,000 cells/well in a plate of 96 flatbottom wells (ThermoFischer Scientific-Nunclon 96 flat), adding con centrations of 10, 30, or 50 mg L 1 Ag-MPs to the medium at 24 h. Cytotoxicity was measured 24 after by adding MTS and measuring the absorbance at 590 nm on an Infinite 200 PRO Nano Quant absorbance reader.
2.7. Study of Ag-MPs cytotoxicity
3. Results and discussion
Ag-MPs cytotoxicity was determined by the MTS method, based on the conversion of a tetrazolium salt by cell enzymes into formazan, a product that is soluble in the culture medium. Accordingly, we placed increasing Ag-MPs concentrations dissolved in phosphate-buffered sa line in contact with culture of the 293-human embryo kidney cell line (Reference Nº ECACC: 85120602 [lot CB2737]) supplied by Cell Bank of the Center for Scientific Instrumentation of the University of Granada.
3.1. Chemical, textural, and morphological characterization of Ag-MPs
2.6. Study of Ag-MPs regeneration by solar or UV radiation
XPS spectra (Fig. 1) demonstrate the presence of Ag0 and Agþ on the surface of Ag-MPs, with a higher percentage of Ag0 than of Agþ, hence the need to oxidize the silver by H2O2 addition to increase the per centage surface Agþ, facilitating halide ion removal from the medium. The morphology of Ag-MPs was studied by HRSEM, observing
Fig. 2. SEM analysis of Ag-MPs, a) [Ag-MPs]0 ¼ 100 mg L 3
1
, B) EDX spectrum of point 4, located on the Ag-MPs.
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Journal of Environmental Management 253 (2020) 109731
3.3.2. Study of the influence of initial H2O2 concentration on the removal of Br and Cl ions We demonstrated above that part of the silver on Ag-MPs surface is present as Ag0. It is therefore necessary to utilize H2O2 as oxidizing agent. Table 2 exhibits the variance in percentage removal of Br and Cl ions as a function of the initial H2O2 concentration. The results indicate that higher initial H2O2 concentrations also increase the per centage removal of Cl and Br ions, up to a maximum of 67.01% for Br ion and 56.92% for Cl ion at an initial H2O2 concentration of 5.0 � 10 4 mol L 1. Higher initial H2O2 concentrations lower the per centage Cl or Br ion removal because they favor the formation of superoxide radicals, thereby reducing the number of Agþ reactive cen ters. This was also observed in several studies (Wang et al., 2013; He et al., 2011, 2012a, 2012b) in which the reduction of Agþ to Ag0 by action of the superoxide radical was detected as H2O2 concentration increases, following the reactions:
Fig. 3. Measurement of the magnetic susceptibility of Ag-MPs.
spherical-shape microparticles on the images (Fig. 2a). The presence of Ag on the surface of the material and Fe in the core was confirmed by energy dispersive X-ray spectroscopy (EDX) (Fig. 2b). An estimation of microparticle size distribution by image analysis using “imageJ” soft ware showed an average size of 3.5 � 1.6 μm (Fig. 2c). The measurement of magnetic susceptibility (Fig. 3) shows the hys teresis cycle at 25 � C, observing that the synthetized microparticles are soft and readily-magnetized materials.
Ag0n
MPs þ 2H2 O2 →Ag0n
1
MPs
Ag0n
1
MPs
Agþ þ O⋅2 →Agn
Agn
1
MPs
Agþ →Ag0n
1
1
MPs
Agþ þ O⋅2 þ 2H2 O MPs Ag0
Agþ þ O2
(1) (2) (3)
As shown in Table 2, the Ag-MPs/H2O2 system is more effective for Br ion than Cl ion removal, which can be attributed to the different solubility products of these silver halides. These results are similar to those described previously for commercial silver nanoparticles (Polo et al., 2017).
3.2. X ray diffraction study Fig. 4 reveals the phase purity of the starting material (AgMPs). Despite the small size of AgMPs, they are well crystallized. The diffraction peaks at 38.2� , 45.9� and 64.7� , corresponding to Ag (1 1 1), (2 0 0) and (2 2 0) reflections, indicate the formation of Ag microcrystals with cubic face centered structure (Altomare et al., 2004). Moreover, after H2O2 treatment XRD patterns confirm that crystalline structure remains unaltered. 3.3. Study of the influence of operational variables 3.3.1. Study of the leaching of silver from Ag-MPs as a function of medium pH The results of Ag-MPs characterization show that part of the surface Ag is present as Ag0, therefore requiring its oxidization to favor halide removal from the medium. We explored whether silver was leached from Ag-MPs surfaces during oxidization as a function of the medium pH at a fixed initial H2O2 concentration. The results (Fig. 5) indicate that the surface silver was not leached at acid pH values, while the value of Agþ at medium pH of 7.0 was 4.62 mg L 1, which then remained constant. This may be attributable to the formation of silver hydroxy-complexes.
Fig. 5. Study of the leaching of Ag-MPs surface silver as a function of medium pH. [Ag-MPs]0 ¼ 100 mg L 1, [H2O2]0 ¼ 1.0 � 10 3 mol L 1, (■) pH ¼ 3.0, (▴) pH ¼ 5.0, and (●) pH ¼ 7.0.
Fig. 4. XRD patterns of AgMPs before and after treatment with H2O2. 4
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5.0 � 10 4 mol L 1, and 1.0 � 10 3 mol L 1). Fig. 12 depicts the results, observing that the percentage Br ion removal was lower at higher initial concentrations of Cl ions in the medium reduce, being 5% when the Cl ion concentration was equal to the Br ion concentration (1.0 � 10 3 mol L 1). Solutions were prepared based on NaCl and KBr, calculating the diffusivity coefficient for each ion and taking values at 25 � C in accordance with the literature (Vanýsek, 2000), D0Naþ ¼ 1.33 � 10 5 cm2 s 1, D0Cl ¼ 2.03 � 10 5 cm2 s 1, D0Br ¼ 2.08 � 10 5 cm2 s 1, D0Kþ ¼ 1.96 � 10 5 cm2 s 1. Diffusivity values were lower for the Cl ion than for the Br ion, which may be attributable to the smaller size of Cl versus Br ions, allowing them to capture a larger number of binding sites on the microparticle surface to form the corre sponding AgCl precipitate.
Table 2 Influence of initial H2O2 concentration on the removal of Br and Cl from the medium. [Ag-MPs]0 ¼ 100 mg L 1, [Cl ]0 ¼ 1.0 � 10 3 mol L 1, [Br ]0 ¼ 1.0 � 10 3 mol L 1, pH ¼ 7.0. [H2O2] (mol L 1) 1.0 � 10 2.0 � 10 5.0 � 10 1.0 � 10 2.0 � 10
4 4 4 3 3
% Br removal
% Cl removal
5.30 11.60 67.01 53.05 54.14
– 7.04 56.92 34.91 37.69
HRSEM images reveal the presence of silver halide deposits on the surface of Ag-MPs. As an example, Fig. 6 depicts the results obtained by EDX microanalysis of Ag-MPs saturated with Cl , confirming that this deposit corresponds to AgCl. This is in accordance with Li et al. (2010) who showed that AgNPs aggregated in aqueous solution containing chloride formed a secondary phase attributed to the formation of AgCl precipitate at the surface of AgNPs. The preferential formation of AgCl and corresponding AgBr versus aqueous complexes, such as AgClaq,
3.3.5. Influence of the presence of dissolved organic matter on Br and Cl ion removal Finally, we studied the influence of the presence of dissolved organic matter (DOM) on the efficacy of the Ag-MPs/H2O2 system, using tannic acid (TAN) as reference compound. As in other natural organic matter (NOM), TAN is present in many types of water, mainly in forest regions. TAN has simpler structure and properties than general NOM, so the research with TAN would be more reproducible. Presence of DOM can considerably modify the effectiveness of AgMs because it can be absorbed on its surface blocking active surface sites and hindering the access of bromide and chloride ions. The results (Fig. 13) demonstrated the influence of its presence, with a reduction in percentage halide ion removal at higher initial TAN concentrations, as it was previously observed by Polo et al. (2017). This behavior can be explained by the consumption of H2O2 during oxidization of the dissolved organic matter in the medium, reducing the number of effective (e.g., Agþ) sites on the Ag-MPs surface. No adsorption of TAN was observed on the Ag-MPs under any of the experimental conditions.
AgCl2 , AgCl23 , is showed in Fig. 7, supporting the proposed mechanism. Thus, the fraction of silver aqueous species varies with the concentration of chloride Fig. 7a and bromide Fig. 7b but only for high concentrations the formation of aqueous complexes competes with the deposition of AgCl on the Ag(0)NPs surface. 3.3.3. Influence of medium pH In a previous paper (Polo et al., 2017) our group demonstrated that silver nanoparticles (AgNPs) oxidization depends on the medium pH, and we therefore explored whether the same was true for Ag-MPs. Re sults obtained (Fig. 8) indicate that the percentage halide ion removal depends on the medium pH. In this experiment, we used a spectropho tometric method to study the reaction of Ag-MPs with hydrogen peroxide at different pH values (Baga et al., 1988). Fig. 9 depicts the variance of the absorption spectrum of Ag-MPs in the presence of H2O2 and at different pH values, observing that the absorbance of Ag-MPs is lower at acid pH, suggesting that Ag-MPs oxi dization is favored at this pH. In addition, SEM images of Ag-MPs as a function of pH (Figs. 10 and 11) show that the microparticles undergo a nucleation process at higher pH, reducing the effectiveness of halide ion removal at increased me dium pH values.
3.4. Regeneration of Ag-MPs by solar or UV radiation An important factor in the consideration of a novel material for water treatment is its regeneration and reutilization capacity. In the case of AgMPs, these particles can be regenerated after their utilization to remove halide ions by irradiation with solar or UV radiation. This is because of the plasmon resonance in the visible region of nanoparticles of noble metals caused by the collective oscillations of surface electrons (Papa vassiliou, 1979; Mock et al., 2002; Huang et al., 2014; Djuhana et al., 2016), resulting in properties that completely differ from those of the bulk materials (Schürch et al., 2002; Nehl et al., 2006). Ag-MPs were irradiated for 90 min with solar radiation and with UV radiation. Fig. 14 depicts the percentage halide removed after multiple regeneration cycles. We also measured the presence of Agþ ions after
3.3.4. Influence of the presence of Cl ions on Br ion removal A further factor to be considered is the effect of the presence of Cl ions on Br ion removal. Accordingly, we varied the initial Cl ion concentrations present in the medium (2.0 � 10 4 mol L 1,
Fig. 6. HRSEM image of Ag-MPs showing the presence of AgCl on their surface. A) HRSEM image of the surface of Ag-MPs, B) EDX analysis of a deposit on the surface of Ag-MPs. [Ag-MPs]0 ¼ 100 mg L-1, [H2O2]0 ¼ 1.0 � 10-3 mol L-1, [Cl-]0 ¼ 1.0 � 10-3 mol L-1, pH ¼ 7.0. 5
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a)
b)
c)
d)
Fig. 7. Ag speciation diagrams as a function of the concentration of chloride (a), bromide (b), and solution pH (c) and (d).
Fig. 8. Influence of pH on Br and Cl [Br ]0 ¼ 1.0 � 10 3 mol L 1, [Cl ]0 ¼ 1.0 � 10 3 mol L 1, mg L 1, [H2O2]0 ¼ 1.0 � 10 3 mol L 1.
ion removal. [Ag-MPs]0 ¼ 100
Fig. 9. Influence of pH on Ag-MPs oxidization with hydrogen peroxide. [AgMPs] ¼ 100 mg L-1, [ H2O2] ¼ 10-3 mol L-1. 6
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Fig. 10. SEM image of Ag-MPs at pH ¼ 3.0. [Ag-MPs]0 ¼ 100 mg L-1.
Fig. 11. SEM image of Ag-MPs at pH ¼ 7.0. [Ag-MPs]0 ¼ 100 mg L 1.
Fig. 12. Influence of the presence of Cl ions on Br ion removal by the AgMPs/H2O2 system. [Ag-MPs]0 ¼ 100 mg L 1, [H2O2]0 ¼ 1.0 � 10 3 mol L 1, [Br ]0 ¼ 1.0 � 10 3 mol L 1, (●) [Cl ]0 ¼ 0.0 mol L 1, (▴) [Cl ]0 ¼ 1.0 � 10 3 mol L 1, (■) [Cl ]0 ¼ 5.0 � 10 4 mol L 1, (▾) [Cl ]0 ¼ 2.0 � 10 4 mol L 1, pH ¼ 7.0.
Fig. 13. Influence of TAN on Br and Cl ion removal by the Ag-MPs/H2O2 system. [Ag-MPs]0 ¼ 100 mg L 1, [H2O2]0 ¼ 1.0 � 10 3 mol L 1, [Br]0 ¼ 1.0 � 10 3 mol L 1, [Cl ]0 ¼ 1.0 � 10 3 mol L 1, pH ¼ 7.0.
each regeneration cycle, finding no Agþ ion leaching into the medium at any time. The different behavior of UV and solar radiation is directly related to the radiant energy emitted by the lamp. Thus, in the case of the UV lamp it only emits energy at 254 nm, and solar lamp presents a wide wave length emission spectrum. The resonance plasmon of Ag is close to 450–500 nm and, therefore, it is easier to regenerate Ag-NPs using solar radiation. Moreover, the results obtained after photoregeneration show that: i) Ag-MPs can be regenerated by radiation; and ii) they lose efficacy to
remove halide with each regeneration process, more markedly when UV radiation is used. 3.5. Study of the cytotoxicity of Ag-MPs The toxicity of Ag-MPs has been reported by several authors (Allen et al., 2010; Kahru and Dubourguier, 2010; Gunsolus et al., 2015; Kwok et al., 2016) although the concentration at which toxicity is reached is not known and the mechanisms involved are poorly understood. Their toxicity is reported to be influenced by particle size, coating material, 7
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Fig. 14. Ag-MPs regeneration using UV and solar radiation. [Ag-MPs]0 ¼ 100 mg L
Absorbance
%Viability
Cytotoxicity
0.00 10.00 30.00 50.00
1.50 1.55 1.62 1.76
100.00 103.34 108.08 117.71
Viable Viable Viable Viable
, [H2O2]0 ¼ 5.0 � 10
4
mol L 1, [Cl ]0 ¼ 1.0 � 10
3
.
Baga, A.N., Johnson, G.R.A., Nazhat, N.B., Saadalla-Nazhat, R.A., 1988. A simple spectrophotometric determination of hydrogen peroxide at low concentrations in aqueous solution. Anal. Chim. Acta 204, 349–353. https://doi.org/10.1016/S00032670(00)86374-6. Bo, A., Sarina, S., Zheng, Z., Yang, D., Liu, H., Zhu, H., 2013. Removal of radioactive ioden from water using Ag2O grafted titanate nanolamina as efficient adsorbent. J. Hazard Mater. 246–247, 199–205. https://doi.org/10.1016/j. jhazmat.2012.12.008. Chen, Y.Y., Yu, S.H., Yao, Q.Z., Fu, S.Q., Zhou, G.T., 2018. One-step synthesis of Ag2O@ Mg(OH)2 nanocomposite as an efficient scavenger for iodine and uranium. J. Colloid Interface Sci. 510, 280–291. https://doi.org/10.1016/j.jcis.2017.09.073. Chen, J., Gao, Q., Zhang, X., Liu, Y., Wang, P., Jiao, Y., Yang, Y., 2019. Nanometer mixed-valence silver oxide enhancing adsorption of ZIF-8 for removal of iodine in solution. Sci. Total Environ. 646, 634–644. https://doi.org/10.1016/j. scitotenv.2018.07.298. Djuhana, D., Putra, M.H., Imawan, C., Fauzia, V., Harmoko, A., Handayani, W., Ardani, H., 2016. Numerical study of the plasmonic resonance sensitivity silver nanoparticles coated polyvinyl alcohol (PVA) using Bohren-Huffman-Mie (BHMie) approximation. AIP Conf. Proc. 1729, 020023. https://doi.org/10.1063/1.4946926. Feng, Q., Liu, Y., Huang, J., Chen, K., Huang, J., Xiao, K., 2018. Uptake, distribution, clearance, and toxicity of iron oxide nanoparticles with different sizes and coatings. Sci. Rep. 8, 2082. https://doi.org/10.1038/s41598-018-19628-z. Gunsolus, I.L., Mousavi, M.P., Hussein, K., Bu⋅⋅hlmann, P., Haynes, C.L., 2015. Effects of humic and fulvic acids on silver nanoparticle stability, dissolution, and toxicity. Environ. Sci. Technol. 49, 8078–8086. https://doi.org/10.1021/acs.est.5b01496. He, D., Jones, A.M., Garg, S., Pham, A.N., Waite, T.D., 2011. Silver nanoparticle-reactive oxygen species interactions; application of a charging-discharging model. J. Phys. Chem. C 115, 5461–5468. https://doi.org/10.1021/jp111275a. He, D., Garg, S., Waite, T.D., 2012. H2O2-mediated oxidation of zero-valent silver and resultant interactions among silver nanoparticles, silver ions, and reactive oxygen species. Langmuir 28, 10266–10275. https://doi.org/10.1021/la300929g. He, W., Zhou, Y.T., Wamer, W.G., Boudreau, M.D., Ying, J.J., 2012. Mechanism of the pH dependent generation of hydroxyl radicals and oxygen induced by Ag nanoparticles. Biomaterials 33, 7547–7555. https://doi.org/10.1016/j.biomaterials.2012.06.076. Horst, M.F., Lassalle, V., Ferreira, M.L., 2015. Nanosized magnetite in low cost materials for remediation of water polluted with toxic metals, azo-and antraquinonic dyes. Front. Environ. Sci. Eng. 9, 746–769. https://doi.org/10.1007/s11783-015-0814-x. Hoskins, J.S., Karanfil, T., Serkiz, S.M., 2002. Removal an sequestration of iodide using silver-impregnated activated carbon. Environ. Sci. Technol. 36, 784–789. https:// doi.org/10.1021/es010972m. Huang, Y.F., Zhang, M., Zhao, L.B., Feng, J.M., Wu, D.Y., Ren, B., Tian, Z.Q., 2014. Activation of oxygen on gold and silver nanoparticles assisted by surface plasmon resonances. Angew. Chem., Int. Ed. Engl. 53, 2353–2357. https://doi.org/10.1002/ anie.201310097. Jiravova, J., Tomankova, K.B., Harvanova, M., Malina, L., Malohlava, J., Luhova, L., Panacek, A., Manisova, B., Kolarova, H., 2016. The effect of silver nanoparticles and silver ions on mammalian and plant cells in vitro. Food Chem. Toxicol. 96, 50–61. https://doi.org/10.1016/j.fct.2016.07.015. Kahru, A., Dubourguier, H.C., 2010. From ecotoxicology to nanoecotoxicology. Toxicology 269, 105–119. https://doi.org/10.1016/j.tox.2009.08.016. Kaminski, M.D., Nu~ nez, L., 2000. Separation of uranium from nitric- and hydrochloricacid solutions with extractant-coated magnetic microparticles. Separ. Sci. Technol. 35, 2003–2018. https://doi.org/10.1081/SS-100102086. Kwok, K.W., Dong, W., Marinakos, S.M., Liu, J., Chilkoti, A., Wiesner, M.R., Chernick, M., Hinton, D.E., 2016. Silver nanoparticle toxicity is related to coating materials and disruption of sodium concentration regulation. Nanotoxicology 10, 1306–1317. https://doi.org/10.1080/17435390.2016.1206150. Li, X., Lenhart, J.J., Walker, H.W., 2010. Dissolution-accompanied aggregation kinetics of silver nanoparticles. Langmuir 26, 16690–16698. https://doi.org/10.1021/ la101768n. Li, B., Li, M., Zhang, J., Pan, Y., Huang, Z., Xiao, H., 2019. Adsorption of Hg (II) ions from aqueous solution by diethylentriaminepentaacetic acid modified cellulose. Int. J. Biol. Macromol. 122, 149–156. https://doi.org/10.1016/j.ijbiomac.2018.10.162.
Table 3 Absorbances obtained at different Ag-MPs concentrations and the percentage cell viability. [Ag-MPs] (mg L 1)
1
and aggregation behavior, and it would therefore vary among different types of Ag-MPs and according to the type of cell that come into contact with them (Jiravova et al., 2016). We studied Ag-MPs toxicity using a cytotoxicity assay, finding that Ag-MPs are not toxic at the concentra tions studied (Table 3), with the increase in percentage viability indi cating a protective effect. 4. Conclusions In summary, results obtained in this study showed that the Ag-MPs synthesized are effective for halide ion removal after their oxidization with an oxidizing agent such as hydrogen peroxide. The effectiveness of the Ag-MPs/H2O2 system depends on the me dium pH, being favored at acid and neutral pH values, the initial amount of Cl ions interferes with the elimination of Br ions and on the pres ence of dissolved organic matter, which interferes with Ag-MPs oxi dization. Consequently, setting up proper conditions we have managed to remove up to 67.01% of Br ions and 56.92% of Cl ions by this system. Moreover, Ag-MPs can be regenerated with UV or solar radiation, although the efficacy of the Ag-MP/H2O2 system is reduced with each regeneration cycle. The cytotoxicity study indicates that Ag-MPs are not toxic at the concentrations studied. Acknowledgments The authors are grateful for the financial support of the Ministry of Science, Innovation and Universities (CTQ2016-80978-C2-1-R). References Allen, H.J., Impellitteri, C.A., Macke, D.A., Heckman, J.L., Poynton, H.C., Lazorchak, J. M., Govindaswamy, S., Roose, D.L., Nadagouda, M.N., 2010. Effects from filtration, capping agents, and presence/absence of food on the toxicity of silver nanoparticles to Daphnia magna. Environ. Toxicol. Chem. 29, 2742–2750. https://doi.org/ 10.1002/etc.329. Altomare, A., Caliandro, R., Camalli, M., Cuocci, C., Silva, I., Giacovazzo, C., Moliterni, A.G.G., Spagna, R., 2004. Space-group determination from powder diffraction data: a probabilistic approach. J. Appl. Crystallogr. 37, 957–966. https:// doi.org/10.1107/S0021889804023982.
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Liu, S., Wang, N., Zhang, Y., Ki, Y., Han, Z., Na, P., 2015. Efficient removal of radioactive iodide ions from water by three-dimensional Ag2O-Ag/TiO2 composites under visible light irradiation. J. Hazard Mater. 284, 171–181. https://doi.org/10.1016/j. jhazmat.2014.10.054. Liu, Z.Q., Shah, A.D., Salhi, E., Bolotin, J., von Gunten, U., 2018. Formation of brominated trihalomethanes during chlorination or ozonation of natural organic matter extracts and model compounds in saline water. Water Res. 143, 492–502. https://doi.org/10.1016/j.watres.2018.06.042. Mansouri, F., Kalankesh, L.R., Hasankhani, H., 2015. Removal of humic acid from contaminated water by nano-sized TiO–SiO. Adv. Biol. Res. 9, 58–65. https://doi. org/10.5829/idosi.abr.2015.9.1.14564. Mao, P., Liu, Y., Jiao, Y., Chen, S., Yang, Y., 2016. Enhanced uptake of iodide on Ag@ Cu2O nanoparticles. Chemosphere 164, 396–403. https://doi.org/10.1016/j. chemosphere.2016.08.116. Mao, P., Qi, B., Liu, Y., Zhao, L., Jiao, Y., Zhang, Y., Jiang, Z., Li, Q., Wang, J., Chen, S., Yang, Y., 2016. AgI doped MIL-101 and its adsorption of iodine with high speed in solution. J. Solid State Chem. 237, 274–283. https://doi.org/10.1016/j. jssc.2016.02.030. Mao, P., Liu, Y., Liu, X., Wang, Y., Liang, J., Zhou, Q., Dai, Y., Jiao, Y., Chen, S., Yang, Y., 2017. Bimetallic AgCu/Cu2O hybrid for the synergetic adsorption of iodide from solution. Chemosphere 180, 317–325. https://doi.org/10.1016/j. chemosphere.2017.04.038. Mock, J.J., Barbic, M., Smith, D.R., Schultz, D.A., Schultz, S.J., 2002. Shape effects in plasmon resonance of individual colloidal silver nanoparticles. J. Chem. Phys. 116, 6755–6759. https://doi.org/10.1063/1.1462610. Nehl, C.L., Liao, H., Hafner, J.H., 2006. Optical properties of star-shaped gold nanoparticles. Nano Lett. 6, 683–688. https://doi.org/10.1021/nl052409y. Papavassiliou, G.C., 1979. Optical properties of small inorganic and organic metal particles. Prog. Solid State Chem. 12, 185–271. https://doi.org/10.1016/0079-6786 (79)90001-3. Polo, A.M.S., Velo-Gala, I., S� anchez-Polo, M., von Gunten, U., L� opez-Pe~ nalver, J.J., Rivera-Utrilla, J., 2016. Halide removal from aqueous solutions by novel silverpolymeric materials. Sci. Total Environ. 573, 1125–1131. https://doi.org/10.1016/j. scitotenv.2016.08.071. Polo, A.M.S., L� opez-Pe~ nalver, J.J., Rivera-Utrilla, J., von Gunten, U., S� anchez-Polo, M., 2017. Halide removal from waters by silver nanoparticles and hydrogen peroxide. Sci. Total Environ. 607–608, 649–657. https://doi.org/10.1016/j. scitotenv.2017.05.144.
S� anchez-Polo, M., Rivera-Utrilla, J., Salhi, E., von Gunten, U., 2007. Ag-doped carbon aerogels for removing halide ions in water treatment. Water Res. 41, 1031–1037. https://doi.org/10.1016/j.watres.2006.07.009. Shah, A.D., Mitch, W.A., 2012. Halonitroalkanes, halonitriles, haloamides, and Nnitrosamines: a critical review of nitrogenous disinfection byproduct formation pathways. Environ. Sci. Technol. 46, 119–131. https://doi.org/10.1021/es203312s. Shen, Y.F., Tang, J., Nie, Z.H., Wang, Y.D., Ren, Y., Zuo, L., 2009. Preparation and application of magnetic Fe3O4 nanoparticles for wastewater purification. Separ. Purif. Technol. 68, 312–319. https://doi.org/10.1016/j.seppur.2009.05.020. Shirley, D.A., 1972. High-resolution X-ray photoemission spectrum of the valence bands of gold. Phys. Rev. B 5, 4709–4714. https://doi.org/10.1103/PhysRevB.5.4709. Schürch, D., Currao, A., Sarkar, S., Hodes, G., Calzaferri, G., 2002. The silver chloride photoanode in photoelectrochemical water splitting. J. Phys. Chem. B 106 (49), 12764–12775. https://doi.org/10.1021/jp0265081. Tauanov, Z., Inglezakis, V., 2019. Removal of iodide from water using silver nanoparticles-impregnated synthetic zeolites. Sci. Total Environ. 682, 259–270. https://doi.org/10.1016/j.scitotenv.2019.05.106. Vanýsek, P., 2000. Ionic conductivity and diffusion at infinite dilution. In: Lide, D.R. (Ed.), CRC Handbook of Chemistry and Physics, 83th edition. CRC Press, Boca Raton. 5-95-5-97. Wang, G.L., Zhu, X.Y., Dong, Y.M., Jiao, H.J., Wu, X.M., Li, Z.J., 2013. The pH-dependent interaction of silver nanoparticles and jydrogen peroxide: a new platform for visual detection of iodide with ultra-sensitive. Talanta 107, 146–453. https://doi.org/ 10.1016/j.talanta.2012.12.029. Yean, S., Cong, L., Yavuz, C.T., Mayo, J.T., Yu, W.W., Kan, A.T., Colvin, V.L., Tomson, M. B., 2005. Effect of magnetite particle size on adsorption and desorption of arsenite and arsenate. J. Mater. Res. 20, 3255–3264. https://doi.org/10.1557/ jmr.2005.0403. Yu, K., Ki, X., Chen, L., Fang, J., Chen, H., Li, Q., Chi, N., Ma, J., 2018. Mechanism and efficiency of contaminant reduction by hydrated electron in the sulfite/iodide/UV process. Water Res. 129, 357–364. https://doi.org/10.1016/j.watres.2017.11.030. Zhao, X., Wang, J., Wu, F., Wang, T., Cai, Y., Shi, Y., Jiang, G., 2010. Removal of fluoride from aqueous media by Fe3O4@Al(OH)3 magnetic nanoparticles. J. Hazard Mater. 173, 102–109. https://doi.org/10.1016/j.jhazmat.2009.08.054. Zhao, X., Han, X., Li, Z., Huang, H., Liu, D., Zhong, C., 2015. Enhanced removal of iodide from water induced by a metal-incorporated porous metal-organic framework. Appl. Surf. Sci. 351, 760–764. https://doi.org/10.1016/j.apsusc.2015.05.186.
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Research article
Halide removal from water using silver doped magnetic-microparticles ~ alver a, M. Sa �nchez-Polo a, *, J. Rivera-Utrilla a, M.V. Lo �pez-Ramo �n b, A.M.S. Polo a, J.J. Lopez-Pen a M. Rozal�en a b
Department of Inorganic Chemistry, Faculty of Science, University of Granada, Campus Fuentenueva s/n, ES18071, Granada, Spain Department of Inorganic and Organic Chemistry, Faculty of Science, University of Ja�en, Campus Las Lagunillas s/n, ES23071, Ja�en, Spain
A R T I C L E I N F O
A B S T R A C T
Keywords: Halide removal Silver microparticles Advanced oxidation process
This study proposes the use of new materials based on core-shell structure magnetic microparticles with Ag0 (Ag (0)-MPs) on their surface to remove bromides and chlorides from waters intended for human consumption. Hydrogen peroxide was used as oxidizing agent, Ag(0)-MPs is thereby oxidized to Ag (I)-MPs, which, when in contact with Cl and Br ions, form the corresponding silver halide (AgCl and AgBr) on the surface of Ag-MPs. The concentration of Cl and Br ions was followed by using ion selective electrodes (ISEs). Silver microparticles were characterized by high-resolution scanning electron microscopy and X-ray photoelectron spectroscopy, while the presence of AgCl and AgBr on Ag-MPs was determined by microanalysis. We analyzed the influence of operational variables, including: hydrogen peroxide concentration in Ag-MP system, medium pH, influence of Cl ions on Br ion removal, and influence of tannic acid as surrogate of organic matter in the medium. Regarding the influence of pH, Br and Cl removal was constant within the pH range studied (3.5–7), being more effective for Br than for Cl ions. Accordingly, this research states that the system Ag-MPs/H2O2 can remove up to 67.01% of Br ions and 56.92% of Cl ions from water (pH ¼ 7, [Ag-MPs]0 ¼ 100 mg L 1, [H2O2]0 ¼ 0.2 mM); it is reusable, regenerated by radiation and can be easily removed by applying a magneti cally assisted chemical separation process.
1. Introduction The removal of halides before the conventional disinfection of water (chlorination, chloramination, or chloride dioxide) is important, because their presence gives rise to disinfection byproducts (DBPs) through reaction with the natural organic matter (NOM) present, espe cially humic compounds, giving rise to the formation of byproducts that are toxic for human health (Mansouri et al., 2015; Shah and Mitch, 2012). Most DBPs are halogenated, hence the need to reduce Br and I ion concentrations in water before disinfection treatments (Liu et al., 2018). Although some brominated DBPs (bromate, bromodichloro methane, dibromochloromethane, and bromoform) are regulated by the International Standards for Drinking-Water of the World Health Orga nization, the analysis of these DBPs and its potential toxicity provides a serious challenge to researchers because NOM is inherently complex, variable and typically poorly characterized. Therefore, alternatives to the use of chlorine have been proposed for disinfection such as advanced oxidation processes (AOPs), including the combined use of UV radia tion/H2O2 and O3/H2O2. However, each disinfection process generates
its own specific DBPs; therefore, the main approach to the minimization of DBP formation is to remove halide ions from the medium before any disinfection process. There are several approaches for halide removal from aqueous media but the most attractive and highly selective method is precipitation based on the low solubility of metal halides, such as AgI. However, this precipitate could form small crystalline particles and colloids, which are very difficult to remove from the solution (S� anchez-Polo et al., 2007). Many studies attempted to solve this drawback using silver containing adsorbents as it is summarized in Table 1. On one side, there is currently a major increase in the use and environmental application of micro and nanomaterials, especially magnetic ones. Thus, magnetite has been tested as an adsorbent in polluted water treatment (Horst et al., 2015), and the size reduction was found to increase its arsenic adsorption capacity >100 fold (Yean et al., 2005). Similar results were obtained by Shen et al. (2009), who observed that the adsorption of metallic ions on Fe3O4 nanoparticles increased with smaller size, while Zhao et al. (2010) proposed the use of magnetic nanoparticles with iron hydroxide to remove fluoride ions. Despite
* Corresponding author. E-mail address: [email protected] (M. S� anchez-Polo). https://doi.org/10.1016/j.jenvman.2019.109731 Received 28 May 2019; Received in revised form 2 October 2019; Accepted 16 October 2019 Available online 27 October 2019 0301-4797/© 2019 Elsevier Ltd. All rights reserved.
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Table 1 Adsorbents for the removal of halide from aqueous phase. Adsorbent type
Surface area (m2/g)
[Ag] (%)
pH
Adsorption capacity (mg/g)
Reference
Ag@activated carbon Ag-doped carbon aerogels Ag2O grafted titanate nanoalumina 3D Ag2O–Ag/TiO2 MIL-101 (Cr)–SO3Ag AgCl@calcium alginate Ag@Cu2O AgI-doped MIL-101 Ag-cloths AgCu/Cu2O hybrid Ag2O@Mg(OH)2 Ag–Ag2O modified on carbon spheres Ag2O–Ag2O3@ZIF-8 Nano silver anion exchanger Synthetic zeolite-based AgNPs
941 428 143 154 – – – 2513 <5 – 78.2 – 370 – 34–51
1.05 10 – 5.82 30.44 10.89 1 2 – – 34.74 6.62 20 6.8 2.15
7–8 7 – – 7.5 6 7 3 6.9–7.8 3 7 2.1 – 7 2.5
25.4–38.1 0.25 428 208 244 133 25.4 43.2 1.12–1.66 65.9 369 375 232 313 19.54–20.44
Hoskins et al. (2002) S� anchez-Polo et al. (2007) Bo et al. (2013) Liu et al. (2015) Zhao et al. (2015)
mentioned advantages, the nanosized materials must be removed care fully due to its potential toxicity and persistence in the environment. In a recent study, Feng et al. (2018) have demonstrated both size and coating have remarkable impact on the cellular uptake, cytotoxicity, distribu tion and clearance of magnetic nanoparticles. On the other side, magnetic microparticle composites have proven to be effective in separating selectively dissolved pollutants from solution ~ ez, 2000). The main advantage lies in magnetically (Kaminski and Nun assisted chemical separation (MACS) process, which utilizes magnetic microparticles coated with a selective extractant (e.g., solvent extractant or ion-exchange material) for the efficient recovery of dissolved trans uranics, fission products, and hazardous species from acidic aqueous ~ ez, 2000). wastes (Kaminski and Nun With this background, this study proposes the use of magnetic mi croparticles composite with Ag(0) on their surface (Ag-MPs), as selective extractant, and their subsequent oxidation with H2O2 to remove Br and Cl ions from the medium through the formation of the corresponding silver halides adsorbed on their surface for their subsequent elimination by applying an external magnetic field. The main objectives of this research were: i) to study halide (chloride and bromide) removal from waters using Ag-MPs; ii) to analyze the influence of operational vari ables (pH, initial H2O2 concentration, etc.) on halide ion removal from water; iii) to evaluate the regeneration of Ag-MPs by solar or ultraviolet (UV) radiation; and iv) to study the cytotoxicity of Ag-MPs.
Mao et al. (2016a) Mao et al. (2016b) Polo et al. (2016) Mao et al. (2017) Chen et al. (2018) Yu et al. (2018) Chen et al. (2019) Li et al. (2019) Tauanov and Inglezakis (2019)
MPMS XL type Magnetometer equipped with EVERCOOL. It was spe cifically configured to study the magnetization (M) and susceptibility (χ) of small experimental samples in a wide range of temperatures (2–400 K) and magnetic fields (5 to 5 T). The surface chemical properties of the material were analyzed by XPS, acquiring the data using a Kratos Axis Ultra DLD spectrometer equipped with Al Kα source. A surface area of 300 � 700 μm was studied at a power of 600 W. Photoemitted electrons were collected at an inci dence angle of 20� and pressure of 10 9 mbar. The general XPS spectrum was obtained with a resolution of 50 eV. The background signals of spectra were adjusted according to the Lorentz-Gauss method. Spectra were calibrated considering the bond energy corresponding to C1s, 284.5 eV, as reference peak. XPS spectra were analyzed using CasaXps version 2.3.16 software. Peaks of the different bond energies were deconvoluted using Gaussian functions and a program of curve adjust ment by square minimums. Backgrounds of peaks of interest were sub tracted using Shirley’s method (Shirley, 1972). Solid samples were examined for mineralogical composition by X-ray diffraction (XRD) in powder mounts, using a BRUKER D8 VENTURE diffractometer (dual radiation Cu and Mo founts) equipped with a PHOTON 100 (CMOS) area detector. 2.3. Halide ion determination N sensors ion-selective electrodes (ISEs) were used for the halide ion determination: model CNT ISE C35 for chlorides and model CNT ISE C80 for bromides.
2. Materials and methods 2.1. Materials and reagents
2.4. Study of Agþ ion leaching as a function of medium pH
All reagents used (sodium chloride, potassium bromide, hydrogen peroxide, 3-(N-morpholino) propanesulfonic acid [MOPS]) were of high purity and supplied by Sigma Aldrich. Ag-MPs were provided by NanoMyP® (Nanomaterials y Polímeros S.L, Spain). These microparti cles have a core-shell structure, with a magnetite core and a shell formed by methacrylic-type polymers. Ag(0) nanoparticles (’20 nm) formed by Ag(I) reduction were deposited on the shell. All solutions used were prepared with ultrapure water using a Milli-Q® equipment (Millipore).
Agþ ion leaching was studied by placing 100 mg L 1 Ag-MPs in contact with 1.0 � 10 3 mol L 1 H2O2 at different pH values. The con centration of Agþ in the medium was measured by using silver ISEs. 2.5. Study of the influence of operational variables on halide ion removal from the medium using the Ag-MPs/H2O2 system The silver deposited on the surface of Ag-MPs is Ag0 and requires oxidation by H2O2 in order to remove halide ions from the medium. We studied the influence on the process of the initial H2O2 concentration by placing 100 mg L 1 Ag-MPs in contact with initial H2O2 concentrations ranging from 5.0 � 10 4 to 2.0 � 10 3 M in a total volume of 10 mL Milli-Q water under constant agitation, with an initial Br or Cl con centration of 1.0 � 10 3 M. The influence of pH on halide removal from the medium was studied in experiments with pH values of 3.0, 5.0, or 7.0 at an initial H2O2 concentration of 1.0 � 10 3 mol L 1 and Ag-MPs concentration of 100 mg L 1.
2.2. Ag-MPs characterization UV–Visible spectroscopy analysis was conducted using a CARY 100 spectrophotometer with a resolution of 1 nm in the wavelength range between 220 and 750 nm. Ag-MPs morphology was studied by HRSEM using a Hitachi S-510 microscope. Before image acquisition, samples were coated with a light gold layer to provide conductive properties, and elemental microanal ysis of their surface (EDX) was conducted. The magnetic susceptibility study used a SQUID QUANTUM DESIGN 2
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Fig. 1. High resolution spectra of Ag0 and Agþ by XPS.
We studied the influence of the presence of Cl ions on Br ion removal from the medium by maintaining a fixed initial concentration of Br ions (10 � 10 3 mol L 1) and H2O2 (1.0 � 10 3 mol L 1) and vary ing the initial concentration of Cl ions (2.0 � 10 4, 5.0 � 10 4, and 1.0 � 10 3 mol L 1).
We examined Ag-MPs regeneration by solar or UV radiation using the solar reactor and UV reactor described previously elsewhere (Polo et al., 2017).
The culture medium was EMEM (EBS), 2 mM Glutamine, 1% nonessential amino acids, 1 mM sodium pyruvate, and 10% fetal bovine serum. Cell viability was evaluated by the MTS reduction reaction, in which the amount of formazan generated is directly proportional to the num ber of viable cells. We seeded 10,000 cells/well in a plate of 96 flatbottom wells (ThermoFischer Scientific-Nunclon 96 flat), adding con centrations of 10, 30, or 50 mg L 1 Ag-MPs to the medium at 24 h. Cytotoxicity was measured 24 after by adding MTS and measuring the absorbance at 590 nm on an Infinite 200 PRO Nano Quant absorbance reader.
2.7. Study of Ag-MPs cytotoxicity
3. Results and discussion
Ag-MPs cytotoxicity was determined by the MTS method, based on the conversion of a tetrazolium salt by cell enzymes into formazan, a product that is soluble in the culture medium. Accordingly, we placed increasing Ag-MPs concentrations dissolved in phosphate-buffered sa line in contact with culture of the 293-human embryo kidney cell line (Reference Nº ECACC: 85120602 [lot CB2737]) supplied by Cell Bank of the Center for Scientific Instrumentation of the University of Granada.
3.1. Chemical, textural, and morphological characterization of Ag-MPs
2.6. Study of Ag-MPs regeneration by solar or UV radiation
XPS spectra (Fig. 1) demonstrate the presence of Ag0 and Agþ on the surface of Ag-MPs, with a higher percentage of Ag0 than of Agþ, hence the need to oxidize the silver by H2O2 addition to increase the per centage surface Agþ, facilitating halide ion removal from the medium. The morphology of Ag-MPs was studied by HRSEM, observing
Fig. 2. SEM analysis of Ag-MPs, a) [Ag-MPs]0 ¼ 100 mg L 3
1
, B) EDX spectrum of point 4, located on the Ag-MPs.
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3.3.2. Study of the influence of initial H2O2 concentration on the removal of Br and Cl ions We demonstrated above that part of the silver on Ag-MPs surface is present as Ag0. It is therefore necessary to utilize H2O2 as oxidizing agent. Table 2 exhibits the variance in percentage removal of Br and Cl ions as a function of the initial H2O2 concentration. The results indicate that higher initial H2O2 concentrations also increase the per centage removal of Cl and Br ions, up to a maximum of 67.01% for Br ion and 56.92% for Cl ion at an initial H2O2 concentration of 5.0 � 10 4 mol L 1. Higher initial H2O2 concentrations lower the per centage Cl or Br ion removal because they favor the formation of superoxide radicals, thereby reducing the number of Agþ reactive cen ters. This was also observed in several studies (Wang et al., 2013; He et al., 2011, 2012a, 2012b) in which the reduction of Agþ to Ag0 by action of the superoxide radical was detected as H2O2 concentration increases, following the reactions:
Fig. 3. Measurement of the magnetic susceptibility of Ag-MPs.
spherical-shape microparticles on the images (Fig. 2a). The presence of Ag on the surface of the material and Fe in the core was confirmed by energy dispersive X-ray spectroscopy (EDX) (Fig. 2b). An estimation of microparticle size distribution by image analysis using “imageJ” soft ware showed an average size of 3.5 � 1.6 μm (Fig. 2c). The measurement of magnetic susceptibility (Fig. 3) shows the hys teresis cycle at 25 � C, observing that the synthetized microparticles are soft and readily-magnetized materials.
Ag0n
MPs þ 2H2 O2 →Ag0n
1
MPs
Ag0n
1
MPs
Agþ þ O⋅2 →Agn
Agn
1
MPs
Agþ →Ag0n
1
1
MPs
Agþ þ O⋅2 þ 2H2 O MPs Ag0
Agþ þ O2
(1) (2) (3)
As shown in Table 2, the Ag-MPs/H2O2 system is more effective for Br ion than Cl ion removal, which can be attributed to the different solubility products of these silver halides. These results are similar to those described previously for commercial silver nanoparticles (Polo et al., 2017).
3.2. X ray diffraction study Fig. 4 reveals the phase purity of the starting material (AgMPs). Despite the small size of AgMPs, they are well crystallized. The diffraction peaks at 38.2� , 45.9� and 64.7� , corresponding to Ag (1 1 1), (2 0 0) and (2 2 0) reflections, indicate the formation of Ag microcrystals with cubic face centered structure (Altomare et al., 2004). Moreover, after H2O2 treatment XRD patterns confirm that crystalline structure remains unaltered. 3.3. Study of the influence of operational variables 3.3.1. Study of the leaching of silver from Ag-MPs as a function of medium pH The results of Ag-MPs characterization show that part of the surface Ag is present as Ag0, therefore requiring its oxidization to favor halide removal from the medium. We explored whether silver was leached from Ag-MPs surfaces during oxidization as a function of the medium pH at a fixed initial H2O2 concentration. The results (Fig. 5) indicate that the surface silver was not leached at acid pH values, while the value of Agþ at medium pH of 7.0 was 4.62 mg L 1, which then remained constant. This may be attributable to the formation of silver hydroxy-complexes.
Fig. 5. Study of the leaching of Ag-MPs surface silver as a function of medium pH. [Ag-MPs]0 ¼ 100 mg L 1, [H2O2]0 ¼ 1.0 � 10 3 mol L 1, (■) pH ¼ 3.0, (▴) pH ¼ 5.0, and (●) pH ¼ 7.0.
Fig. 4. XRD patterns of AgMPs before and after treatment with H2O2. 4
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5.0 � 10 4 mol L 1, and 1.0 � 10 3 mol L 1). Fig. 12 depicts the results, observing that the percentage Br ion removal was lower at higher initial concentrations of Cl ions in the medium reduce, being 5% when the Cl ion concentration was equal to the Br ion concentration (1.0 � 10 3 mol L 1). Solutions were prepared based on NaCl and KBr, calculating the diffusivity coefficient for each ion and taking values at 25 � C in accordance with the literature (Vanýsek, 2000), D0Naþ ¼ 1.33 � 10 5 cm2 s 1, D0Cl ¼ 2.03 � 10 5 cm2 s 1, D0Br ¼ 2.08 � 10 5 cm2 s 1, D0Kþ ¼ 1.96 � 10 5 cm2 s 1. Diffusivity values were lower for the Cl ion than for the Br ion, which may be attributable to the smaller size of Cl versus Br ions, allowing them to capture a larger number of binding sites on the microparticle surface to form the corre sponding AgCl precipitate.
Table 2 Influence of initial H2O2 concentration on the removal of Br and Cl from the medium. [Ag-MPs]0 ¼ 100 mg L 1, [Cl ]0 ¼ 1.0 � 10 3 mol L 1, [Br ]0 ¼ 1.0 � 10 3 mol L 1, pH ¼ 7.0. [H2O2] (mol L 1) 1.0 � 10 2.0 � 10 5.0 � 10 1.0 � 10 2.0 � 10
4 4 4 3 3
% Br removal
% Cl removal
5.30 11.60 67.01 53.05 54.14
– 7.04 56.92 34.91 37.69
HRSEM images reveal the presence of silver halide deposits on the surface of Ag-MPs. As an example, Fig. 6 depicts the results obtained by EDX microanalysis of Ag-MPs saturated with Cl , confirming that this deposit corresponds to AgCl. This is in accordance with Li et al. (2010) who showed that AgNPs aggregated in aqueous solution containing chloride formed a secondary phase attributed to the formation of AgCl precipitate at the surface of AgNPs. The preferential formation of AgCl and corresponding AgBr versus aqueous complexes, such as AgClaq,
3.3.5. Influence of the presence of dissolved organic matter on Br and Cl ion removal Finally, we studied the influence of the presence of dissolved organic matter (DOM) on the efficacy of the Ag-MPs/H2O2 system, using tannic acid (TAN) as reference compound. As in other natural organic matter (NOM), TAN is present in many types of water, mainly in forest regions. TAN has simpler structure and properties than general NOM, so the research with TAN would be more reproducible. Presence of DOM can considerably modify the effectiveness of AgMs because it can be absorbed on its surface blocking active surface sites and hindering the access of bromide and chloride ions. The results (Fig. 13) demonstrated the influence of its presence, with a reduction in percentage halide ion removal at higher initial TAN concentrations, as it was previously observed by Polo et al. (2017). This behavior can be explained by the consumption of H2O2 during oxidization of the dissolved organic matter in the medium, reducing the number of effective (e.g., Agþ) sites on the Ag-MPs surface. No adsorption of TAN was observed on the Ag-MPs under any of the experimental conditions.
AgCl2 , AgCl23 , is showed in Fig. 7, supporting the proposed mechanism. Thus, the fraction of silver aqueous species varies with the concentration of chloride Fig. 7a and bromide Fig. 7b but only for high concentrations the formation of aqueous complexes competes with the deposition of AgCl on the Ag(0)NPs surface. 3.3.3. Influence of medium pH In a previous paper (Polo et al., 2017) our group demonstrated that silver nanoparticles (AgNPs) oxidization depends on the medium pH, and we therefore explored whether the same was true for Ag-MPs. Re sults obtained (Fig. 8) indicate that the percentage halide ion removal depends on the medium pH. In this experiment, we used a spectropho tometric method to study the reaction of Ag-MPs with hydrogen peroxide at different pH values (Baga et al., 1988). Fig. 9 depicts the variance of the absorption spectrum of Ag-MPs in the presence of H2O2 and at different pH values, observing that the absorbance of Ag-MPs is lower at acid pH, suggesting that Ag-MPs oxi dization is favored at this pH. In addition, SEM images of Ag-MPs as a function of pH (Figs. 10 and 11) show that the microparticles undergo a nucleation process at higher pH, reducing the effectiveness of halide ion removal at increased me dium pH values.
3.4. Regeneration of Ag-MPs by solar or UV radiation An important factor in the consideration of a novel material for water treatment is its regeneration and reutilization capacity. In the case of AgMPs, these particles can be regenerated after their utilization to remove halide ions by irradiation with solar or UV radiation. This is because of the plasmon resonance in the visible region of nanoparticles of noble metals caused by the collective oscillations of surface electrons (Papa vassiliou, 1979; Mock et al., 2002; Huang et al., 2014; Djuhana et al., 2016), resulting in properties that completely differ from those of the bulk materials (Schürch et al., 2002; Nehl et al., 2006). Ag-MPs were irradiated for 90 min with solar radiation and with UV radiation. Fig. 14 depicts the percentage halide removed after multiple regeneration cycles. We also measured the presence of Agþ ions after
3.3.4. Influence of the presence of Cl ions on Br ion removal A further factor to be considered is the effect of the presence of Cl ions on Br ion removal. Accordingly, we varied the initial Cl ion concentrations present in the medium (2.0 � 10 4 mol L 1,
Fig. 6. HRSEM image of Ag-MPs showing the presence of AgCl on their surface. A) HRSEM image of the surface of Ag-MPs, B) EDX analysis of a deposit on the surface of Ag-MPs. [Ag-MPs]0 ¼ 100 mg L-1, [H2O2]0 ¼ 1.0 � 10-3 mol L-1, [Cl-]0 ¼ 1.0 � 10-3 mol L-1, pH ¼ 7.0. 5
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a)
b)
c)
d)
Fig. 7. Ag speciation diagrams as a function of the concentration of chloride (a), bromide (b), and solution pH (c) and (d).
Fig. 8. Influence of pH on Br and Cl [Br ]0 ¼ 1.0 � 10 3 mol L 1, [Cl ]0 ¼ 1.0 � 10 3 mol L 1, mg L 1, [H2O2]0 ¼ 1.0 � 10 3 mol L 1.
ion removal. [Ag-MPs]0 ¼ 100
Fig. 9. Influence of pH on Ag-MPs oxidization with hydrogen peroxide. [AgMPs] ¼ 100 mg L-1, [ H2O2] ¼ 10-3 mol L-1. 6
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Fig. 10. SEM image of Ag-MPs at pH ¼ 3.0. [Ag-MPs]0 ¼ 100 mg L-1.
Fig. 11. SEM image of Ag-MPs at pH ¼ 7.0. [Ag-MPs]0 ¼ 100 mg L 1.
Fig. 12. Influence of the presence of Cl ions on Br ion removal by the AgMPs/H2O2 system. [Ag-MPs]0 ¼ 100 mg L 1, [H2O2]0 ¼ 1.0 � 10 3 mol L 1, [Br ]0 ¼ 1.0 � 10 3 mol L 1, (●) [Cl ]0 ¼ 0.0 mol L 1, (▴) [Cl ]0 ¼ 1.0 � 10 3 mol L 1, (■) [Cl ]0 ¼ 5.0 � 10 4 mol L 1, (▾) [Cl ]0 ¼ 2.0 � 10 4 mol L 1, pH ¼ 7.0.
Fig. 13. Influence of TAN on Br and Cl ion removal by the Ag-MPs/H2O2 system. [Ag-MPs]0 ¼ 100 mg L 1, [H2O2]0 ¼ 1.0 � 10 3 mol L 1, [Br]0 ¼ 1.0 � 10 3 mol L 1, [Cl ]0 ¼ 1.0 � 10 3 mol L 1, pH ¼ 7.0.
each regeneration cycle, finding no Agþ ion leaching into the medium at any time. The different behavior of UV and solar radiation is directly related to the radiant energy emitted by the lamp. Thus, in the case of the UV lamp it only emits energy at 254 nm, and solar lamp presents a wide wave length emission spectrum. The resonance plasmon of Ag is close to 450–500 nm and, therefore, it is easier to regenerate Ag-NPs using solar radiation. Moreover, the results obtained after photoregeneration show that: i) Ag-MPs can be regenerated by radiation; and ii) they lose efficacy to
remove halide with each regeneration process, more markedly when UV radiation is used. 3.5. Study of the cytotoxicity of Ag-MPs The toxicity of Ag-MPs has been reported by several authors (Allen et al., 2010; Kahru and Dubourguier, 2010; Gunsolus et al., 2015; Kwok et al., 2016) although the concentration at which toxicity is reached is not known and the mechanisms involved are poorly understood. Their toxicity is reported to be influenced by particle size, coating material, 7
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Fig. 14. Ag-MPs regeneration using UV and solar radiation. [Ag-MPs]0 ¼ 100 mg L
Absorbance
%Viability
Cytotoxicity
0.00 10.00 30.00 50.00
1.50 1.55 1.62 1.76
100.00 103.34 108.08 117.71
Viable Viable Viable Viable
, [H2O2]0 ¼ 5.0 � 10
4
mol L 1, [Cl ]0 ¼ 1.0 � 10
3
.
Baga, A.N., Johnson, G.R.A., Nazhat, N.B., Saadalla-Nazhat, R.A., 1988. A simple spectrophotometric determination of hydrogen peroxide at low concentrations in aqueous solution. Anal. Chim. Acta 204, 349–353. https://doi.org/10.1016/S00032670(00)86374-6. Bo, A., Sarina, S., Zheng, Z., Yang, D., Liu, H., Zhu, H., 2013. Removal of radioactive ioden from water using Ag2O grafted titanate nanolamina as efficient adsorbent. J. Hazard Mater. 246–247, 199–205. https://doi.org/10.1016/j. jhazmat.2012.12.008. Chen, Y.Y., Yu, S.H., Yao, Q.Z., Fu, S.Q., Zhou, G.T., 2018. One-step synthesis of Ag2O@ Mg(OH)2 nanocomposite as an efficient scavenger for iodine and uranium. J. Colloid Interface Sci. 510, 280–291. https://doi.org/10.1016/j.jcis.2017.09.073. Chen, J., Gao, Q., Zhang, X., Liu, Y., Wang, P., Jiao, Y., Yang, Y., 2019. Nanometer mixed-valence silver oxide enhancing adsorption of ZIF-8 for removal of iodine in solution. Sci. Total Environ. 646, 634–644. https://doi.org/10.1016/j. scitotenv.2018.07.298. Djuhana, D., Putra, M.H., Imawan, C., Fauzia, V., Harmoko, A., Handayani, W., Ardani, H., 2016. Numerical study of the plasmonic resonance sensitivity silver nanoparticles coated polyvinyl alcohol (PVA) using Bohren-Huffman-Mie (BHMie) approximation. AIP Conf. Proc. 1729, 020023. https://doi.org/10.1063/1.4946926. Feng, Q., Liu, Y., Huang, J., Chen, K., Huang, J., Xiao, K., 2018. Uptake, distribution, clearance, and toxicity of iron oxide nanoparticles with different sizes and coatings. Sci. Rep. 8, 2082. https://doi.org/10.1038/s41598-018-19628-z. Gunsolus, I.L., Mousavi, M.P., Hussein, K., Bu⋅⋅hlmann, P., Haynes, C.L., 2015. Effects of humic and fulvic acids on silver nanoparticle stability, dissolution, and toxicity. Environ. Sci. Technol. 49, 8078–8086. https://doi.org/10.1021/acs.est.5b01496. He, D., Jones, A.M., Garg, S., Pham, A.N., Waite, T.D., 2011. Silver nanoparticle-reactive oxygen species interactions; application of a charging-discharging model. J. Phys. Chem. C 115, 5461–5468. https://doi.org/10.1021/jp111275a. He, D., Garg, S., Waite, T.D., 2012. H2O2-mediated oxidation of zero-valent silver and resultant interactions among silver nanoparticles, silver ions, and reactive oxygen species. Langmuir 28, 10266–10275. https://doi.org/10.1021/la300929g. He, W., Zhou, Y.T., Wamer, W.G., Boudreau, M.D., Ying, J.J., 2012. Mechanism of the pH dependent generation of hydroxyl radicals and oxygen induced by Ag nanoparticles. Biomaterials 33, 7547–7555. https://doi.org/10.1016/j.biomaterials.2012.06.076. Horst, M.F., Lassalle, V., Ferreira, M.L., 2015. Nanosized magnetite in low cost materials for remediation of water polluted with toxic metals, azo-and antraquinonic dyes. Front. Environ. Sci. Eng. 9, 746–769. https://doi.org/10.1007/s11783-015-0814-x. Hoskins, J.S., Karanfil, T., Serkiz, S.M., 2002. Removal an sequestration of iodide using silver-impregnated activated carbon. Environ. Sci. Technol. 36, 784–789. https:// doi.org/10.1021/es010972m. Huang, Y.F., Zhang, M., Zhao, L.B., Feng, J.M., Wu, D.Y., Ren, B., Tian, Z.Q., 2014. Activation of oxygen on gold and silver nanoparticles assisted by surface plasmon resonances. Angew. Chem., Int. Ed. Engl. 53, 2353–2357. https://doi.org/10.1002/ anie.201310097. Jiravova, J., Tomankova, K.B., Harvanova, M., Malina, L., Malohlava, J., Luhova, L., Panacek, A., Manisova, B., Kolarova, H., 2016. The effect of silver nanoparticles and silver ions on mammalian and plant cells in vitro. Food Chem. Toxicol. 96, 50–61. https://doi.org/10.1016/j.fct.2016.07.015. Kahru, A., Dubourguier, H.C., 2010. From ecotoxicology to nanoecotoxicology. Toxicology 269, 105–119. https://doi.org/10.1016/j.tox.2009.08.016. Kaminski, M.D., Nu~ nez, L., 2000. Separation of uranium from nitric- and hydrochloricacid solutions with extractant-coated magnetic microparticles. Separ. Sci. Technol. 35, 2003–2018. https://doi.org/10.1081/SS-100102086. Kwok, K.W., Dong, W., Marinakos, S.M., Liu, J., Chilkoti, A., Wiesner, M.R., Chernick, M., Hinton, D.E., 2016. Silver nanoparticle toxicity is related to coating materials and disruption of sodium concentration regulation. Nanotoxicology 10, 1306–1317. https://doi.org/10.1080/17435390.2016.1206150. Li, X., Lenhart, J.J., Walker, H.W., 2010. Dissolution-accompanied aggregation kinetics of silver nanoparticles. Langmuir 26, 16690–16698. https://doi.org/10.1021/ la101768n. Li, B., Li, M., Zhang, J., Pan, Y., Huang, Z., Xiao, H., 2019. Adsorption of Hg (II) ions from aqueous solution by diethylentriaminepentaacetic acid modified cellulose. Int. J. Biol. Macromol. 122, 149–156. https://doi.org/10.1016/j.ijbiomac.2018.10.162.
Table 3 Absorbances obtained at different Ag-MPs concentrations and the percentage cell viability. [Ag-MPs] (mg L 1)
1
and aggregation behavior, and it would therefore vary among different types of Ag-MPs and according to the type of cell that come into contact with them (Jiravova et al., 2016). We studied Ag-MPs toxicity using a cytotoxicity assay, finding that Ag-MPs are not toxic at the concentra tions studied (Table 3), with the increase in percentage viability indi cating a protective effect. 4. Conclusions In summary, results obtained in this study showed that the Ag-MPs synthesized are effective for halide ion removal after their oxidization with an oxidizing agent such as hydrogen peroxide. The effectiveness of the Ag-MPs/H2O2 system depends on the me dium pH, being favored at acid and neutral pH values, the initial amount of Cl ions interferes with the elimination of Br ions and on the pres ence of dissolved organic matter, which interferes with Ag-MPs oxi dization. Consequently, setting up proper conditions we have managed to remove up to 67.01% of Br ions and 56.92% of Cl ions by this system. Moreover, Ag-MPs can be regenerated with UV or solar radiation, although the efficacy of the Ag-MP/H2O2 system is reduced with each regeneration cycle. The cytotoxicity study indicates that Ag-MPs are not toxic at the concentrations studied. Acknowledgments The authors are grateful for the financial support of the Ministry of Science, Innovation and Universities (CTQ2016-80978-C2-1-R). References Allen, H.J., Impellitteri, C.A., Macke, D.A., Heckman, J.L., Poynton, H.C., Lazorchak, J. M., Govindaswamy, S., Roose, D.L., Nadagouda, M.N., 2010. Effects from filtration, capping agents, and presence/absence of food on the toxicity of silver nanoparticles to Daphnia magna. Environ. Toxicol. Chem. 29, 2742–2750. https://doi.org/ 10.1002/etc.329. Altomare, A., Caliandro, R., Camalli, M., Cuocci, C., Silva, I., Giacovazzo, C., Moliterni, A.G.G., Spagna, R., 2004. Space-group determination from powder diffraction data: a probabilistic approach. J. Appl. Crystallogr. 37, 957–966. https:// doi.org/10.1107/S0021889804023982.
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