Optimized Synthesis of FeS Nanoparticles with a High Cr ( VI ) Removal Capability

FeS nanoparticles were synthesized using chemical precipitation method involving sulfide and ferrous solutions. Effects of important synthesis parameters including stabilizer, time taken for titration, horizontal oscillation speed, and initial salt concentration on the size of synthesized FeS nanoparticles were investigated by Orthogonal Array design. Increasing the CMC dosage significantly made the hydrodynamic diameter decrease between 0.05wt.% and 0.15 wt.% while Na 2 S titration, oscillation rate, and Na 2 S concentration did not show significant influence on the hydrodynamic diameter of FeS nanoparticles. The synthesized FeS nanoparticles were characterized by using XRD (X-ray diffraction), TEM (transmission electron microscopy), and XPS (X-ray photoelectron spectroscopy). The as-synthesized FeS nanoparticles had an average size of 25 ± 10 nm and had a better long-term stability after storage for 150 days compared to bare FeS particles. Because of the optimized process parameters, the synthesized FeS nanoparticles had a higher Cr(VI) removal capacity of 683mg per gram of FeS in comparison to the previously reported cases, and up to 92.48% Cr(VI) was removed from aqueous solutions. The small size, special surface property, and high reactivity make the synthesized FeS nanoparticles a promising tool for the remediation of Cr(VI) contaminated soil and groundwater.


Introduction
Nanomaterials have become one of the most promising materials to solve different problems in the various demanding fields [1][2][3][4][5]; in particular sulfur containing nanomaterials have been applied extensively in dechlorination of tetrachloroethene [6], cathode material for lithium batteries [7], catalytic applications [8], electrochemical DNA detection analysis [9], and so on.The successful synthesis and utilization of nanoscale metal sulfides have been accomplished such as CdS\ZnS\FeS\CuS [10][11][12][13].The size, shape, and other properties of those metal sulfide nanomaterials, which determined their applications, were greatly influenced by the synthesis methods and conditions [14][15][16].
FeS nanotubes [17], FeS nanosheets [18,19], and Fe/FeS nanoparticles [20] are synthesized with various methods such as sulfurization of hematite, solvothermal synthesis, soft-template method, and chemical reduction.During these processes, the frequent usage of toxic gases (H 2 S), complicated reactions, and unsafe reagents restricts their selection.Jeong et al. [13] produced nanosize mackinawite simply by mixing Na 2 S solution with FeCl 2 solution with an excess of sulfide, while FeS particles aggregated easily during sample drying process.Gong et al. [21] prepared FeS nanoparticle suspensions with CMC (Carboxymethyl Cellulose) as a stabilizer to efficiently immobilize Hg 2+ cation in water solution, and it was found that the size of FeS nanoparticles is an important factor related to the efficiency of chemical reaction or adsorption during their remediation process.In addition, the long-term stability of the nanoparticles may influence the effectiveness of their utilization as a remediation reagent; however the specific knowledge on how the synthesis process influences the size and the long-term stability of FeS nanoparticles is lacking and material's potential has not been realized.
Chromium is one of the notorious heavy mental contaminants in groundwater, surface water, and soil, and hexavalent chromium is the chemical form with the highest toxicity and mobility relative to other forms in different geological environments [22].It is hard to achieve an efficient remediation for Cr(VI) in groundwater and soil through traditional treatments.Reactive FeS solid is an excellent reductant with a strong adsorption capacity, which has been used to remove both metals and organics from groundwater [23].CMC stabilized FeS nanoparticles [24] and nanosized composites C/FeS/Fe [25] displayed an excellent Cr(VI) removal performance when applying to treat contaminated water.Nanoscale FeS may have a greater remediation potential in the Cr(VI) contaminated environment, such as soil or groundwater.
Herein, we attempted to find out the key factors during the synthesis of FeS nanoparticles and optimized the synthesis procedures to accomplish a higher Cr(VI) removal efficiency.The specific objectives of this study were to (1) identify the significant factors and optimize the synthesis conditions to obtain nanoparticles with a smaller size in order to increase their remediation efficiency; (2) examine the long-term stability of FeS nanoparticles in various conditions; and (3) test the effectiveness of the stabilized CMC-FeS nanoparticles for removal of Cr(VI) from water solutions.

Synthesis of FeS Nanoparticles.
The synthesis procedure of FeS nanoparticles in Figure 1 was adapted from Xiong's method [26], by which FeS nanoparticles were prepared using CMC as a stabilizer.CMC solutions (110 mL) with certain concentrations (0.05 wt.%, 0.1 wt.%, or 0.2 wt.%) were mixed with FeSO 4 aqueous solution (20 mL) in a 500 mL flask on a horizontal oscillator.Then Na 2 S solution (20 mL) with a concentration of 0.0213, 0.0426, or 0.0639 M was added to Fe 2+ -CMC solution through a constant flow pump and then oscillated continuously with certain rates (100, 200, or 250 rpm) for 5 min.The concentration of FeS nanoparticles was kept at 500 mg/L as a suspension.For comparison, bare FeS solution was also prepared following the same procedure without addition of a stabilizer.During the preparation, the whole system was deoxygenated by bubbling nitrogen gas (99.99% pure) at a flow rate of 5 L/min.
Stabilizer, time taken for titration, horizontal oscillation speed, and initial salt concentration were selected as the input factors in 3-level orthogonal tests in order to screen the key factors and optimize the synthesis conditions, and the ranges of the parameters were determined based on the practical conditions, preliminary test results, and previous studies as shown in Table 1.The hydrodynamic diameter of the synthesized FeS nanoparticles was deployed to evaluate the various combinations of the process conditions to obtain the optimal parameters for the synthesis.The process parameters of FeS nanoparticles were studied through an OA 9 (3 4 ) Orthogonal Array design as reported in Table 1.

Characterization of the Synthesized FeS Nanoparticles.
The morphology of the FeS particles was obtained using a TECNAI20TEM from FEI Company operated at 200 kV.Samples for TEM (transmission electron microscopy) were prepared by placing a droplet of dilute dispersed suspension on a carbon coated 200-mesh Cu TEM grid followed by vacuum drying.XRD (X-ray diffraction) analysis of synthesized nano-FeS was carried out using Jindao-7000 with Cu-K radiation and the angle 2 was measured between 3 ∘ and 80 ∘ in a step interval of 0.02 ∘ with a scan rate of 2 ∘ /min.The XRD patterns were processed using MDI Jade 6.5 software loaded with ICDD database (Materials Data Inc., Livermore, CA, USA).The elemental and species analysis of FeS nanoparticles was carried out by using a Thermo ESCALAB 250 Xray photoelectron spectrometer and monochromatic Al-K (ℎV = 1486.6eV) radiation.All the peak positions for analysis were calibrated using C1s peak at 284.5 eV.

Removal of Cr(VI) by the FeS Nanoparticles and Analysis.
Removal of Cr(VI) by synthesized FeS nanoparticles was conducted in 500 mL erlenmeyer flasks.Typically, bare FeS, synthesized FeS (0.05 wt.% CMC), and FeS (0.15 wt.% CMC) were added to each flask, respectively.Subsequently, Cr(VI) solutions with various concentrations were added to maintain an FeS-to-Cr(VI) molar ratio of 0.8.The final volume of the reaction system was 200 mL with an FeS concentration of 67.7 mg/L.The pH of the solution was kept at 5.6 with MES (4morpholineethanesulfonic acid) and NaOH buffer solutions.The flasks were sealed and agitated on a horizontal oscillator at 250 rpm and temperature of 25 ± 1 ∘ C. Samples were withdrawn at scheduled intervals (1,5,15,40,90, and 150 min) to determine the aqueous concentrations of chromium.The FeS nanoparticles, solids, and aqueous solutions after reactions were collected by filtering the suspensions using a 25 nm The hydrodynamic diameter of the synthesized FeS nanoparticles was determined by Nano Zetasizer ZS90 (Malvern Instruments, UK), which is based on DLS (dynamic lighting scanning) measurement.Cr(VI) and Fe(II) in water solutions were extracted using the diphenyl hydrazine method (GB/T7467-1987, Chinese National Standards) and the o-phenanthroline method (HJ/T345-2007, Chinese Environmental Protection Industry Standard) and then analyzed by ultraviolet spectrophotometer (T6 PERSEE, China).

Orthogonal Array Design of FeS Nanoparticles Synthesis.
The size of the nanoparticles could greatly influence the efficiency when they are utilized in environmental remediation, especially in groundwater or soil remediation.Generally, smaller nanoparticles possess higher remediation efficiency.It is well recognized that a larger surface area to volume ratio is attributed to a higher density of active reaction sites per unit mass [27].Gong et al. [21] indicated that increasing CMC concentration resulted in smaller FeS particles, which had a greater specific surface area and more sorption sites resulting in a proved Hg 2+ uptake.Hydrodynamic diameter was used for the optimization of process as the evaluation parameter.These process parameters were studied at three different levels as reported in Table 1 by OA 9 (3 4 ) Orthogonal Array design.Furthermore, the average hydrodynamic diameters of products obtained in all runs of orthogonal tests are given in the last column of Table 1.As per the results shown in Table 2, the hydrodynamic diameter of synthesized particles varied with different combinations, and the significance of the synthesis conditions in descending order was as follows: stabilizer (CMC) dosage, titration speed of Na 2 S, oscillation rate, and initial salt (Na 2 S) concentration.The optimal combination of factors was 0.2 wt.% CMC dosage, 250 rpm oscillation, 0.15 mL/s titration of Na 2 S solution, and 0.0213 M Na 2 S concentration.
Two groups of tests were conducted to observe the influence of the foremost two important factors (stabilizer dosage and titration speed of Na 2 S) on FeS nanoparticles. Figure 2(a) depicts the particle size profile with stabilizer dosage, which shows that increasing the dosage of CMC decreased particle size significantly.When the CMC dosage increased from 0.05 wt.% to 0.15 wt.%, hydrodynamic diameter declined by 110 nm; however hydrodynamic diameter of FeS nanoparticles did not decrease significantly anymore and remained constant after the dosage above 0.15 wt.%.
After the dosage of CMC, Na 2 S titration speed is the second influential factor in the particle size [28][29][30].However, it had a negligible effect on the hydrodynamic diameters of the nanoparticles, when adjusting the titration speed from 0.01 to 0.15 mL/s as shown in Figure 2(b).Our finding shows that the optimal synthesis conditions were CMC dosage at 0.15 wt.%, oscillation rate at 250 rpm, titration speed at 0.05 mL/s, and Na 2 S concentration at 0.0426 M.Under the optimal conditions, FeS nanoparticles were synthesized, characterized, and observed to check their long-term stability.Furthermore, the FeS nanoparticles were also synthesized at various stabilizer dosages under the optimal conditions and the capability of synthesized FeS nanoparticles was tested to remove Cr(VI) from aqueous solution.

Characterization of the Synthesized FeS Nanoparticles.
Figure 3 shows the XRD patterns of the stabilizer CMC and the synthesized FeS nanoparticles.The diffraction peaks at around 2 = 17.05 ∘ and 28.6 ∘ correspond to the (100) and (103) planes, respectively, which attributes to the troilite FeS (ICDD 37-0477) [31].No sharp crystalline peaks were present in the diffractograms of the synthesized FeS nanoparticles, indicating that the synthesized FeS nanoparticles had a low degree of crystallinity.Moreover, some weak diffraction peaks were present in CMC-FeS samples, which could be attributed to the diffraction of CMC and Fe 2 (SO 4 ) 3 in the FeS nanoparticles based on the material references in the ICDD database [32,33].The weak diffraction peaks may be due to the poor crystallinity of the synthesized nanoparticles, which could be a result of the inhibition of the FeS crystal growth by CMC [34].
Figure 4 shows the TEM images of the bare FeS particles and the synthesized FeS nanoparticles.Without using CMC, FeS solids appeared as aggregated flocs as shown in Figure 4(a).However, CMC stabilized FeS solids were well dispersed and appeared as sphere or ellipsoid shaped particles as shown in the TEM image in Figure 4(b).CMC molecules coated on the FeS cores in Figures 4(c) and 4(d) prevented the agglomeration of particles and produced a product with a smaller size and larger surface area.Particle size of stabilized FeS products was analyzed based on the TEM images [35], and the average size of the synthesized FeS nanoparticles was 25 ± 10 nm.
Figure 5 shows the Fe XPS analysis of FeS nanoparticles.In the C1s spectrum of the FeS nanoparticles (Figure 5(a)), the spectrum can be deconvoluted into three individual peaks.The sharp peak at around 284.5 eV corresponds to C=C bonds, called sp 2 carbon, while the other peaks at a higher energy come from C-OH bonds and C=O bonds [36,37].The Fe(2p 1/2 ) peak centered at 724.8 ± 0.4 eV and Fe(2p 3/2 ) peak centered at 710 ± 0.1 eV, which, respectively, belonged to Fe(III)-O and Fe(II)-S compounds [38].The intensity of the peak corresponding to Fe(II)-S is greater than Fe(III)-O, indicating that Fe(II)-S is the main compound in FeS nanoparticles.Fe(III)-O may be attributed to the partial oxidation of Fe(II) by O 2 in air or hydroxylation [39].As Figure 5(b) shows, the S(2p) spectrum is dominated by features at 162.8 ± 0.3 eV and 161.4 ± 0.1 eV, corresponding to polysulfide (S  2− ) and sulfide (S 2− ) anions, respectively [40,41].The S 2− peak demonstrates the main form of sulfur in FeS nanoparticles exists as S 2− anion.However, the existence of the peaks of S  2− reveals that some superficial oxidation occurred [42,43].

Long-Term Stability of FeS Nanoparticles.
The synthesized FeS nanoparticle was present as a suspension as shown in Figure 6 with an FeS concentration of 500 mg/L.A comparative analysis of the bare FeS and CMC stabilized FeS nanoparticles was carried out in sealed glass vessels in dark condition at room temperature.Bare FeS precipitated for a period of 0.5 h.After the storage of 24 h, some yelloworange substances appeared on the surface of precipitates, indicating that oxidation may have occurred with the generation of ferric oxides.Moreover, the CMC stabilized FeS nanoparticles remained dispersed with similar appearance, and precipitates or yellow-orange substances were not present after 5 months, while the whole surface of bare FeS was covered with yellow-orange substances.Compared to bare FeS, CMC stabilized FeS nanoparticles showed a great stability to meet the subsequent requirements for good dispersion and mobility for the remediation of contaminated soil or groundwater.
The mean hydrodynamic diameters of FeS (0.05 wt.% CMC) and FeS (0.15 wt.% CMC) nanoparticles were determined at desired time intervals (1 d, 60 d, 90 d, and 150 d). Figure 7 depicts the variation of the hydrodynamic diameters of FeS nanoparticles with the storage time.Lou [44] found that the viscosity of CMC solution reduces with time and then electrostatic attraction may get stronger than electrostatic repulsion, which may cause the increase of particle size.During the storage, particle dissolution or hydrolysis of CMC molecules may also cause the variation of the particle size [26].However, the CMC stabilized FeS nanoparticle's hydrodynamic diameter remained approximately constant during the storage of 150 days as shown in Figure 7.Not only could CMC effectively block the particle agglomeration during the preparation stage, but also CMC coating on FeS cores may promote the dispersion of nanoparticles with the effects of electrostatic repulsion and steric hindrance [45].
Besides, the Fe(II) in water solution was determined for bare FeS and FeS nanoparticles systems, respectively.

Removal Efficiencies of Cr(VI).
As observed in Figure 9, as-produced FeS nanoparticles achieved a higher removal efficiency of Cr(VI) than bare FeS solids during 150 min.Higher CMC dosage during nanoparticle synthesis increased the removal efficiency, approaching 92.48% Cr(VI) removal when the CMC dosage was increased to 0.15 wt.%; removal of 85.30% and 81.80% Cr(VI) was, respectively, achieved in comparison with 0.05 wt.% CMC stabilized and bare FeS.The as-produced FeS nanoparticles removed 683 mg per gram of FeS at pH 5.6, which has a higher capacity than those previously reported FeS materials, such as mackinawite (FeS) [46], C/FeS/Fe composite [25], and iron monosulfide (FeS) powder [47] under acidic conditions.Even compared to nanosized iron sulfide [48] with a Cr(VI) removal capacity of 506.5 mg Cr(VI) per gram of FeS at initial Cr(VI) concentration of 1.56 g/L at pH 3, as-produced FeS nanoparticles removed Cr(VI) from aqueous solutions with a higher removal efficiency.Since the core materials in our nanoparticles and the above reported [25,46,47] were all FeS, the higher efficiency of Cr(VI) removal by the as-synthesized FeS nanoparticles may be related to their smaller particle size and modified surface, which were produced by our optimized synthesis conditions.Moreover, hydroxyl groups in CMC molecules integrated with the Fe(II) to yield stable organic chelating ligands, which promote the redox reactions [49].Therefore, CMC coated FeS nanoparticles could have the special properties of smaller size and long-term stability and possess a higher removal efficiency of Cr(VI) in water solution.

Conclusions
Hydrodynamic diameter was chosen to evaluate the synthesis parameters, which included stabilizer, time taken for titration, horizontal oscillation speed, and initial salt concentration, where Orthogonal Array design was used to make the assessment.Our data shows that the significance of the synthesis conditions was in descending order: stabilizer (CMC) dosage, titration speed of Na 2 S, oscillation rate, and initial salt (Na 2 S) concentration, and increasing the CMC dosage significantly made the hydrodynamic diameter decrease between 0.05 wt.% and 0.15 wt.% while Na 2 S titration, oscillation rate, and Na 2 S concentration did not show significant influence on the hydrodynamic diameter of FeS nanoparticles.
The average size of the synthesized FeS nanoparticles was 25 ± 10 nm.CMC was a good stabilizer and it was coated on the surface of FeS cores.Compared to bare FeS particles (as control), as-synthesized FeS nanoparticles had a better long-term stability after 150-day storage.The synthesized FeS nanoparticles had a high Cr(VI) removal capacity of 683 mg per gram of FeS in a short period of 15 min, and up to 92.48% Cr(VI) was removed from water solutions.
Our optimized synthesis conditions, especially the proper dosage of CMC, resulted in the small size of 25 ± 10 nm and special surface properties of CMC coating of FeS nanoparticles.The as-synthesized FeS nanoparticles possessed a higher Cr(VI) removal capacity comparing to the reported materials, and it could be a promising tool in the remediation of soil and groundwater contaminated by other heavy metals although the current application is Cr(VI) removal.

Figure 2 :Figure 3 :
Figure 2: Particle sizes of synthesized FeS nanoparticles (a) with the dosage of CMC as the stabilizer and (b) with the titration speeds of Na 2 S solution.Oscillation rate = 250 rpm, and Na 2 S concentration = 0.0426 M. Data plotted as mean of duplicates and the error bars (calculated as standard deviation) indicate data reproducibility.

Figure 8 Figure 7 :Figure 8 :
Figure8shows the aqueous Fe(II) concentration for bare FeS and FeS nanoparticles systems with the storage time.There was 82.17% aqueous Fe(II) left within 1 day and about 50% aqueous Fe(II) was oxidized or transferred in bare FeS solids, while 90% Fe(II) still existed in CMC stabilized FeS

Figure 9 :
Figure 9: Removal of Cr(VI) in water solution by synthesized FeS nanoparticles.Initial Cr(VI) = 50 mg/L, FeS-to-Cr(VI) molar ratio = 0.8.Data plotted as mean of duplicates and the error bars (calculated as standard deviation) indicate data reproducibility.
All the chemicals used in this study were of analytical or higher grade except for CMC (M.W. = 300-800 in sodium salt) which was purchased from Shanghai Qiangshun Chemical Reagent Co., Ltd.Iron(II) sulfate heptahydrate (FeSO 4 ⋅7H 2 O) was purchased from Sinopharm Chemical Reagent Co., Ltd.Sodium sulfide nonahydrate (Na 2 S⋅9H 2 O) and potassium dichromate (K 2 Cr 2 O 4 ) were obtained from Chengdu Kelong Chemical Reagent Factory.

Table 1 :
OA 9(3 4) matrix for parameter optimization in the synthesis of FeS nanoparticles and DLS particle sizes (hydrodynamic diameter).

Table 2 :
ANOVA results for the synthesis of FeS nanoparticles using OA 9 (3 4 ) matrix.
3 B 3 C 3 D 1membrane filter of mixed cellulose esters (VSWP04700, Millipore).Control tests were carried out using 0.15 wt.% CMC solution only and all tests were conducted in duplicate.