Khalid M.
Alotaibi
a,
Lewis
Shiels
b,
Laure
Lacaze
b,
Tanya A.
Peshkur
c,
Peter
Anderson
c,
Libor
Machala
d,
Kevin
Critchley
e,
Siddharth V.
Patwardhan
*f and
Lorraine T.
Gibson
*a
aDepartment of Pure and Applied Chemistry, WestCHEM, University of Strathclyde, 295 Cathedral Street, Glasgow, G1 1XL, UK. E-mail: lorraine.gibson@strath.ac.uk; Tel: +44 (0)141 548 2224
bDepartment of Chemical and Process Engineering, University of Strathclyde, 75 Montrose Street, Glasgow, G1 1XJ, UK
cScottish Environmental Technology Network (SETN), Faculty of Engineering, University of Strathclyde, 204 George Street, Glasgow, G1 1XW, UK
dRegional Centre of Advanced Technologies and Materials, Palacký University, Šlechtitelů 27, 78371 Olomouc, Czech Republic
eMolecular & Nanoscale Physics Group, School of Physics & Astronomy, University of Leeds, Woodhouse Lane, Leeds, LS2 9JT, UK
fDepartment of Chemical and Biological Engineering, University of Sheffield, Sheffield S1 3JD, UK. E-mail: s.patwardhan@sheffield.ac.uk; Tel: +44 (0)114 222 7593
First published on 13th September 2016
Iron has been used previously in water decontamination, either unsupported or supported on clays, polymers, carbons or ceramics such as silica. However, the reported synthesis procedures are tedious, lengthy (involving various steps), and either utilise or produce toxic chemicals. Herein, the use of a simple, rapid, bio-inspired green synthesis method is reported to prepare, for the first time, a family of iron supported on green nanosilica materials (Fe@GN) to create new technological solutions for water remediation. In particular, Fe@GN were employed for the removal of arsenate ions as a model for potentially toxic elements in aqueous solution. Several characterization techniques were used to study the physical, structural and chemical properties of the new Fe@GN. When evaluated as an adsorption platform for the removal of arsenate ions, Fe@GN exhibited high adsorption capacity (69 mg of As per g of Fe@GN) with superior kinetics (reaching ∼35 mg As per g sorbent per hr) – threefold higher than the highest removal rates reported to date. Moreover, a method was developed to regenerate the Fe@GN allowing for a full recovery and reuse of the adsorbent in subsequent extractions; strongly highlighting the potential technological benefits of these new green materials.
Iron compounds have shown great promise in selective removal of arsenic from water. It is known that iron oxides, hydroxides and oxyhydroxides (e.g. ferrihydrite) selectively adsorb As(V) through the formation of mono- and bi-dentate “Fe–As” complex as shown in exemplar reaction below.12,17,18
≡FeOH + H2AsO4− + H+ → ≡FeH2AsO4 + H2O | (1) |
The formation of these complexes is dependent on solution pH, arsenic concentration, iron surface chemistry and the presence of other metal ions.17 Nano-iron particles with large active surface areas have been shown to provide high arsenic adsorption capacity, presumably though increased surface area.19,20 However, the use of iron nanoparticle powders is found to be prone to aggregation which not only leads to reduced efficiency,21 but also restricts their application in water treatment systems due to a rapid loss of iron particles into the drinking water.22 This necessitates the use of a support which holds the iron-nanoparticle during its application. High surface area supports that have been used include zeolites, activated carbons, and mesoporous silica.22–24 When supported, aggregation of iron particles reduced significantly, the material had improved stability and better dispersion of iron nanoparticles leading to more efficient catalysts in decontamination applications.25–29 Porous silica has been of wide interest as a support for dispersing iron particles.30,31 Several methods have been proposed for the synthesis of mesoporous silica materials,32,33 but secondary pollution problems are often created at the end of the process when unused reactants (e.g. toxic alkoxysilanes) are discarded.34,35 Most reported syntheses involve lengthy, multistep procedures (11–72 h), high temperatures (80–120 °C) and extreme pH conditions (pH ∼ 1) creating additional problems. Thus, considerable efforts have been made to develop eco-friendly routes, taking into consideration the whole life-cycle of mesoporous silica materials from extraction of the raw material to disposal at the end of their life.
Porous silica materials can be prepared using bioinspired green routes that mimic the biomineralisation process.36,37 The popularity of this method is down to the ease of synthesis, the use of mild conditions and less toxic reagents. The understanding of the mechanism of biological silica formation in organisms has led to silica being produced in vitro under environmentally friendly mild reaction conditions, while maintaining a high level of control over the product.38 Here synthetic “additives” are used to produce silica rapidly and under mild conditions.39 Furthermore, bioinspired methods allow better control of the chemical and physical properties of the products with one-pot synthesis of hybrid materials such as optical materials, catalysts and biomaterials.38,40–43
In this study the versatility of green nanoparticles (GN) for water remediation is examined using arsenic as a target pollutant. The synthesis is simple, fast, and uses non-toxic reagents to produce porous green nanoparticles (GN) encapsulating iron. Although the preparation of nanoparticles encapsulated in silica using bioinspired routes has been demonstrated for quantum dots, iron oxide particles and gold particles,41,44–46 iron supported on silica (Fe@GN) has not been reported before, let alone for applications in environmental remediation. As shown in Table 1, there are significant advantages of the Fe@GN preparation method, compared to Fe-mesosilica (MS). The novel GN products were synthesised and characterised (see ESI†) in order to determine their chemical, textural and surface properties. The new materials were assessed at different extraction pH, before being used, and regenerated, for the removal of arsenic(V) from environmental samples under batch or continuous flow.
Conditions | Fe-MS | Fe@GN | |
---|---|---|---|
Step 1 | Chemicals | Alkoxysilane, surfactant, ethanol | Sodium silicate, additive, iron precursor, water |
T, t, pH | 60–100 °C, 2–5 d, pH 2 or 10 | 20 °C, 15 min, pH 7 | |
Step 2 | Chemicals | Iron precursor, acetone or ethanol | n/a |
t | 5–24 h | n/a |
Fe@GN was prepared by dissolving the desired amount of iron nitrate in water prior to addition to the silicate–PEHA mixture immediately before the pH was set to 7. After mixing the solution, the pH was adjusted to 7.0 ± 0.1 as above. The solution was left for approximately 15 min producing as-synthesized iron-silica precipitate which was collected by filtration, washed three times with deionized water and dried in air at room temperature (hereafter referred to as “D-Fe@GN”) before calcination at 550 °C for 5 h, to produce Fe@GN.
Elemental analysis was carried out using an Exeter Analytical CE440 elemental function to provide the functional group (carbon and nitrogen) and hydrogen content of the studied materials. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) data were obtained using an ABB MB3000 instrument and analysed as described elsewhere.47,48 The transmission 57Fe Mössbauer spectra were collected using a Mössbauer spectrometer in a constant acceleration mode with a 57Co(Rh) source. The isomer shift values were related to metallic α-Fe at room temperature (RT). The measurements were performed at RT and 5 K in a zero external magnetic field and at 5 K in an external magnetic field of 5 T, applied parallel to the direction of the gamma-rays propagation. Low temperature and in-field measurements were obtained using a cryomagnetic system by Oxford Instruments.
X-ray photoelectron spectroscopy data were obtained using a Thermo Electron Corporation ESCA Lab 250 instrument with a chamber pressure maintained below 1 × 10−9 mbar during acquisition. A monochromated Al Kα X-ray source (15 kV 150 W) irradiated the samples, with a spot diameter of approximately 0.5 mm. The spectrometer was operated in large area XL magnetic lens mode using pass energies of 150 and 20 eV for survey and detailed scans, respectively. The spectra were obtained with an electron takeoff angle of 90°. Charge compensation was applied using a low energy flood gun. High-resolution spectra were fitted using Avantage (Thermo VG software package) peak fitting algorithms.
To study the maximum adsorption capacity of arsenic on the prepared GN, the Langmuir and Freundlich isotherm models were applied to experimental data. Extraction experiments used 25 cm3 spiked solutions (40, 60, 80 or 100 μg cm−3) of As(V) and 50 mg of each GN adsorbent. Solutions were held at 20° C, pH 3 and stirred at 250 rpm for 120 min. The amount of As(V) extracted at equilibrium; qe (mg g−1) was calculated according to the following equation:
![]() | (2) |
![]() | (3) |
qe = KfCe1/n | (4) |
![]() | (5) |
With 50% Fe iron loading, five samples were prepared for decontamination assessment studies: 3 with iron (D-Fe@GN, Fe@GN and R-Fe@GN) and 2 without iron (D-GN and GN) where a prefix “D” indicates dried at room temperature, “R” indicates reduced and no prefix suggests calcined samples (see Table 2). The samples prepared without Fe (D-GN and GN) both demonstrated Type II isotherms, with the GN samples also illustrating a H4 hysteresis that was attributed to the presence of a small amount of mesopores (Fig. S1a and b†).53 The porosity results for all samples are summarised in Table 2. Sample D-GN had the lowest surface area at 12 m2 g−1 (and no pore volume or pore size, presumably due to template inclusion), which increased significantly, as expected, to 347 m2 g−1 after the removal of the bioinspired additive to produce a material with a pore size and pore volume of 3.5 nm and 0.23 cm3 g−1, respectively. When iron was incorporated into the synthesis mixture the silica framework of GN appeared to greatly alter and the materials produced illustrated Type IV N2 sorption isotherms, as shown in Fig. S1c† suggesting that all Fe containing GN were mesoporous. Even without the removal of the additives, D-Fe@GN had a surface area of approximately 244 m2 g−1 and an average pore size of approximately 18 nm; the latter being an interesting and remarkable feature that can provide accessibility to bulky analytes. This characteristic of D-Fe@GN could be attributed to the influence of the Fe3+ ions on silica condensation reactions, manifesting in a material with a significantly different microstructure compared to the D-GN sample.
Sample | Fe wt% | Finishing | Surface areaa (m2 g−1) | Pore sizeb (nm) | Pore volumec (cm3 g−1) |
---|---|---|---|---|---|
a Calculated by the BET model from sorption data in a relative pressure range from 0.05 to 0.25. b Calculated by the BJH model from the desorption branches of isotherms. c Calculated from N2 amount adsorbed at a relative pressure P/P0 of 0.99. | |||||
D-GN | 0 | Air dried | 12 | Nil | Nil |
GN | 0 | Calcined | 347 | 3.5 | 0.23 |
D-Fe@GN | 50 | Air dried | 244 | 18.6 | 0.75 |
Fe@GN | 50 | Calcined | 203 | 18.6 | 0.6 |
R-Fe@GN | 50 | Reduced | 129 | 18 | 0.47 |
The presence of nitrogen and carbon in the air dried samples confirmed the retention of PEHA in samples D-GN and D-Fe@GN, see elemental analysis data in Table 3. Interestingly, when the samples with and without iron were compared, the amine loading reduced from 4.49 for D-GN to 2.66 mmol g−1 for D-Fe@GN (reduction in both N and C content; Table 3). This implied that the presence of iron in the synthesis solution had a direct effect on amine loading, perhaps through ionic interactions. Calcination appeared to completely remove PEHA; carbon and nitrogen content reduced to undetectable levels (GN and Fe@GN). Furthermore, the reduction by sodium borohydride seem to have increased the hydrogen content, perhaps through hydration of the samples.
Sample | % C | % H | % N | L 0 (mmol g−1) |
---|---|---|---|---|
a Degree of residual PEHA (L0 millimoles of nitrogen per gram of silica). | ||||
D-GN | 9.36 | 3.06 | 6.29 | 4.49 |
GN | 0 | 1.08 | 0 | |
D-Fe-GN | 5.31 | 2.92 | 3.73 | 2.66 |
Fe-GN | 0 | 0.2 | 0 | |
R-Fe-GN | 0 | 1.08 | 0 |
In order to further investigate the composition of inorganic components and the oxidation state of iron, XPS analyses were performed. XPS analysis provided the evidence of silicon and iron in the samples (Table 4 and Fig. 1a). Upon chemical reduction, oxygen (not shown) and hydrogen content increased (Table 3), consistent with the possibility of hydration upon reduction. Further analysis of the Fe 2p1 peak for the R-Fe-GN (Fig. 1b) indicated that most of the Fe species are likely to be either in the Fe2+ or Fe3+ oxidation state, while the lack of a strong satellite peak suggests it is more likely Fe3+. A weak shoulder at binding energy of 707 eV indicates the presence of very little amount of Fe0 metal, if any. This result is surprising because it was expected that upon chemical reduction, most iron will be converted to zero valent metal. These results were further validated by using Mössbauer spectroscopy.
![]() | ||
Fig. 1 (a) Representative XPS survey scans for two iron containing samples. (b) High resolution Fe 2p1 spectrum for R-Fe-GN sample. |
Mössbauer spectroscopy provided additional information on the chemical nature of the iron in the samples (also see ESI†). Room temperature Mössbauer spectra (not presented) of the both samples (before and after the sodium borohydride treatment) indicated a presence of octahedral trivalent iron atoms in the structure of ferric oxide or (oxy)hydroxide. No zero valent and/or divalent iron were detected in the spectra, even for samples reduced using sodium borohydride for extended periods. More detail information was obtained from low temperature (T = 5 K) and in-field (B = 5 T) spectra (Fig. 2). The zero-field spectrum of the sample before the sodium borohydride treatment (Fig. 2a) reflects unusually low magnetic ordering temperature of the ferric oxide or (oxy)hydroxide phase and thus a presence of very small nanoparticles (<10 nm). The 5 K zero-field spectrum of the sodium borohydride treated sample (Fig. 2b) shows the presence of nanoparticles which were smaller in comparison with the “non-treated” sample. The superparamagnetic regime was confirmed by in-field Mössbauer spectra (Fig. 2c and d). The quadrupole shifts of the both sextets (Fig. 2c and d) were close to zero indicating amorphous Fe2O3 or Fe(OH)3. The Mössbauer spectroscopy data confirmed that nanoparticles of ferric oxide or hydroxide were incorporated, and were probably uniformly distributed, within the silica matrix. The long-term sodium borohydride treatment did not result in a reduction of the ferric oxide phase at all. It did however result in a particle size decrease.
Further characterisation of the D-GN and D-Fe@GN samples was performed using ATR-FTIR (Fig. 3a). The results supported the formation of silica (Si–O–Si peaks at ∼1100 cm−1 and 800 cm−1). The presence of PEHA was also detected from amine peaks located in the region of 1500–1700 cm−1. Although the shape of the siloxane peak at ∼1100 cm−1 was as expected for the sample without iron (D-GN), the shape of this peak was found to significantly change for the sample containing iron (D-Fe@GN). This suggested that the inclusion of iron affected the materials produced at a molecular level and further supported the observed differences in porosities discussed above between samples with and without iron. In order to further probe the effect of iron on silica, peak deconvolution protocol was applied to the ∼1100 cm−1 peak of both samples (Fig. S2a and b†). Analysis of the D-Fe@GN sample peak indeed highlighted drastic differences, in particular, the presence of additional peaks at ∼954 cm−1 and ∼870 cm−1. In the literature, these peaks have been commonly found in sol–gel materials and were attributed to Fe–O–Si/Fe–O–OH.54,55 Furthermore, as the iron content was increased from 0% to 50%, the area under the Fe–O–Si peak was found to increase (Fig. 3b), thus further strengthening the formation of iron oxide–silica composite material.
In summary, the materials characterisation suggested that the presence of iron profoundly affected the chemical nature of the GN samples, in addition to the physical properties, as observed from the porosity measurements. The surface area was found to depend on iron loading, calcination and chemical reduction. Further, chemical reduction increased the hydration of samples. It was clear that iron was not in a zero valent form, but rather in an FeII or, more likely, FeIII oxidation state, presented as well-dispersed, <10 nm particles of iron oxide, hydroxide and/or oxyhydroxide, in addition to Si–O–Fe.
The highest extraction efficiencies were observed at pH 3, which corroborates with the literature.17 Under acidic conditions, and specifically at pH 3, the dominant As(V) species is H2AsO4−.56 Under acidic pH condition, iron oxyhydroxides, such as those present in our samples, possess positive charge (typically >+1).17 Furthermore, the extraction efficiency of 100% at pH 3 may also indicate the possibility of co-operative effects from PEHA and iron towards the adsorption of this bulky inorganic anion. The surface amines, detected by FTIR and XPS, will be protonated at pH 3, further facilitating interactions between the negatively charged arsenic ions with the GN surface.57 It was interesting to note that R-Fe@GN exhibited the lowest arsenic extraction efficiency amongst the iron containing samples. Materials characterisation revealed that R-Fe@GN had lower surface area compared to the un-reduced Fe@GN sample (Table 2). Furthermore, XPS analyses suggested that upon reduction, some iron was lost, thus reducing the total iron content of the R-Fe@GN sample. These two observations – reduced surface area and reduced iron content – perhaps help explain why R-Fe@GN had the lowest arsenic extraction efficiency. This was further supported when the surface area and pore volume for all 3 iron containing samples were compared with extraction efficiencies (Fig. 4b). It was clear that both iron content and high surface area were crucial for As(V) removal – the former was known to actively interact with arsenic while the latter maximises the mass transport.
Method | Langmuir | Freundlich | |||
---|---|---|---|---|---|
q m (mg g−1) | b (L mg−1) | R L | K f (mg g−1) | n (L mg−1) | |
As made Fe-GNs | 69.64 | 0.09 | 0.2 | 14.84 | 2.81 |
A comparative evaluation of Fe-GNs and other iron containing low cost adsorbents for arsenic removal is listed in Table 6. It was observed that the adsorption capacity of D-Fe-GN (69.64 mg g−1) was higher than many other sorption materials reported in the literature for the removal of As(V) (e.g., the maximum adsorption capacity of the iron oxide nanoparticles immobilized on activated carbon was 35.34 mg g−1). Despite the extensive use of activated carbon (AC) in the water and wastewater treatments, AC is perhaps not ideal as it is difficult to separate powdered AC from aquatic system when it becomes exhausted.61 Furthermore, the regeneration of AC by chemical or thermal procedures are expensive and can result in sorbent loss.62 A number of candidates, e.g. Zr(IV)-loaded chelating resin, akaganeite nanocrystals and polyethylene mercaptoacetimide exhibit superior performance compared to GN, however, these materials have been reported to require several hours for equilibration. In contrast, D-Fe-GN provide a rapid adsorption platform, reaching 34.82 mg g−1 h−1 – a threefold increase when compared to the highest capacity reported in the literature. To the best of our knowledge, the highest iron loading reported in the literature for GAC was 33.6%,63 and as higher iron oxide loading provides better adsorption capacities, the material produced here with an iron loading of 50% is higher than any other loading reported in the literature.
Sorbent | Capacity (mg g−1) | Equilibration time (h) | Capacity (mg g−1 h−1) | Ref. |
---|---|---|---|---|
Activated carbon, Fe modified | 35.34 | 48 | 0.74 | 64 |
Clinoptilolite, Fe modified | 30.21 | 48 | 0.63 | 64 |
GAC-Fe (0.05 M) | 2.96 | 24 | 0.12 | 65 |
Fe10SBA-15 | 12.68 | 24 | 0.53 | 66 |
Cellulose loaded with iron oxyhydroxide | 15.6 | 24 | 0.65 | 67 |
Synthetic siderite | 31 | 3 | 10.33 | 68 |
Zr(IV)-loaded chelating resin (Zr-LDA) | 88.73 | 24 | 3.70 | 69 |
Akaganeite β-FeO(OH) nanocrystals | 141.3 | 24 | 5.89 | 70 |
Poly ethylene mercaptoacetimide | 105.75 | 20 | 5.29 | 71 |
Magnetite–maghemite nanoparticles | 6 | 3 | 2.00 | 72 |
D-Fe-GN | 69.64 | 2 | 34.82 | This work |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6sc02937j |
This journal is © The Royal Society of Chemistry 2017 |