Flower-like tin oxide membranes with robust three-dimensional channels for efficient removal of iron ions from hydrogen peroxide

Risheng Shen,Shilong Li,Yuqing Sun,Yuan Bai,Jian Lu,Wenheng Jing,*

1 State Key Laboratory of Materials-Oriented Chemical Engineering,Nanjing Tech University,Nanjing 210009,China

2 National Energy Group Science and Technology Research Institute Co.,Ltd.,Nanjing 210023,China

Keywords: Hydrogen peroxide SnO2 membrane Adsorption Hydrothermal

ABSTRACT Membrane technology has become the mainstream process for the production of electronic grade hydrogen peroxide(H2O2).But due to the oxidation degradation of the organic membranes(e.g.polyamide)by the strong oxidative radicals (e.g.?OH) generated via the activation of H2O2 by iron ions (Fe3+),the short effective lifetime of membranes remains a challenge.Inorganic nano tin oxide(SnO2)has great potential for the removal of Fe3+in strongly oxidative H2O2 because of its ability to stabilize H2O2 and preferentially adsorb Fe3+.Herein,we have designed for the first time a flower-like robust SnO2 membrane on the ceramic support by in situ template-free one-step hydrothermal method.The three-dimensional loose pore structure in the membrane built by interlacing SnO2 nanosheets endows the SnO2 membrane with a high specific surface area and abundant adsorption sites(-OH).Based on the coordination complexation and electrostatic attraction between the SnO2 surface and Fe3+,the membrane shows a high Fe3+removal efficiency(83%)and permeability(24 L?m-2?h-1?MPa-1)in H2O2.This study provides an innovative and simple approach to designing robust SnO2 membranes for highly efficient removal of Fe3+ in harsh environments,such as strong oxidation conditions.

1.Introduction

Electronic grade hydrogen peroxide is widely used in the electronics industry,for example as a cleaning agent for silicon wafers[1].The deposition of trace metal ions (Fe3+,Ca2+,K+,Na+,etc.) in H2O2on the surface of silicon wafers can adversely affect the electrical properties of silicon devices[2],it requires ultra-purification processes,such as distillation,adsorption,ion exchange,membrane technologies,etc.,to achieve electronic grade requirements[3].Because no auxiliary chemicals are required and zero effluent is generated,membrane technologies (mainly polyamide membranes)become the most desirable ultra-purification option.However,polyamide membranes can only maintain stability for three days when exposed to H2O2degradation,which is directly evidenced by an increase in permeate flux and a decrease in rejection levels [4].One of the main reasons for the degradation is the?OH radicals generated by the activation of Fe3+through the Fenton reaction [5–7].In order to extend the effective lifetime of reverse osmosis membranes and reduce the operation cost of H2O2ultra purification,developing a robust membrane capable of Fe3+removal in advance under oxidizing conditions is of great significance.

Inorganic metal oxide ceramic materials (such as zirconia and tin dioxide) are often researched as an adsorbent to remove Fe3+in H2O2due to their high anti-oxidation,heavy metal ion adsorption ability,and chemical tolerance [8,9].The construction of two-dimensional (2D) and three-dimensional (3D) tin oxide(SnO2) morphologies has attracted much more attention as it can possess higher specific surface area and surface adsorption activity,exhibiting better reactivity toward various applications such as photocatalysis [10],sensing [11],separation [12],etc.Benefiting from abundant adsorption sites and negatively charged properties,SnO2is often used for the removal of molecules and ions [13–15].

In this study,a flower-like SnO2membrane can be constructed to gain the coordination complexation and electrostatic attraction between the SnO2surface and Fe3+byin situtemplate-free onestep hydrothermal method for the first time,to realize H2O2stabilization and preferentially Fe3+adsorbing capacity.Thin nanosheets and the 3D channels structure create a high specific surface area and abundant adsorption sites(-OH).A high removal efficiency(83%)of Fe3+is achieved by the combined action of coordination complexation and electrostatic attraction.

2.Experimental

2.1.Synthesis of SnO2 nanosheets

SnO2nanosheets were prepared using a “bottom-up” approach with direct hydrothermal [16,17].Specifically,6 mmol?L–1SnCl2?2H2O (98%,Aladdin,China) was quickly dissolved in a 20 ml aqueous solution containing 5 ml anhydrous ethanol and 15 ml deionized (DI) water to form a precursor solution of SnO2by stirring.Then,45 ml 0.5 mol?L–1NaOH (98%,Aladdin,China)solution was slowly added to the solution.The obtained mixture was transferred into a 100 ml Teflon-lined stainless autoclave and set the filling volume of the solution to 25%,50%,75%,85%,and 95% respectively,sealed and maintained at 150 °C for 12 h,and then cooled down to room temperature.The precipitates were obtained by centrifuge and washed several times with water and ethanol respectively until Cl–could not be detected.The products were dried in the vacuum at 60 °C for 2 h.

2.2.Preparation of flower-like SnO2 ceramic membrane

The flower-like SnO2ceramic membrane was obtained using a one-step growth method(Fig.1).Porous α-Al2O3disks(Membrane Science&Technology Research Center,China:30 mm in diameter,3 mm in thickness,0.1 μm in average pore size) with robust mechanical stability were used as supports where the SnO2crystals grow.The other side of the support was covered by Teflon tape to prevent the crystals from growing.It was placed in the precursor solution synthesized in Section 2.1 for 12 h at 150 °C.Then the membrane was removed and rinsed in fresh ethanol for several hours,and dried at 60 °C.

Fig.1.Schematic illustration of the one-step in situ hydrothermal synthesis of SnO2 membranes.

2.3.Characterization

The morphology of SnO2nanosheets and element composition of SnO2membranes were characterized by field emission scanning electron microscopy (FESEM,S-4800,Hitachi,Japan) equipped with the energy dispersive spectrometer (EDX).X-ray diffraction(XRD,D8 Advance,Bruker,Germany) was conducted to qualitatively identify crystalline phases and quantitatively characterize the d-spacing of SnO2membranes.The nitrogen adsorption–desorption isotherms of SnO2nanosheets and SnO2nanoparticles were measured on an ASAP 2460 (Micromeritics,America) at–196 °C.The multipoint Brunner–Emmet–Teller (BET) method was carried out to calculate the specific surface area of the prepared material in theP/P0range of 0.05–0.25 and Barrett–Joyner–Halenda (BJH)was used to measure the corresponding pore diameter,pore volume atP/P0of 0.995.The zeta potential of the SnO2membrane was characterized by a Zetasizer (ZS90,Malvern,England).All metal ions were analyzed by inductively coupled plasma optical emission spectrometer (ICP–OES,Ametek,India).Fourier transforms infrared (FTIR,Thermo Nicolet8700,America) spectroscopy was used to identify chemical bonds in the SnO2and Fe3+using a Spectrum GX spectrometer(PerkinElmer,America)with a KBr pellet operating in transmittance mode.X-ray photoelectron spectroscopy (XPS,Thermo Kalpha,America) was recorded on an ESCALAB 250Xi.

A self-designed device with cross-flow conditions (Fig.S1,in Supplementary Material) was used to test the permeance and the solute removal efficiency.The feed solution was fed continuously using a plunger pump at a pressure between 0.1 and 1.0 MPa.The feed liquid was 0.4 mg?L–1Fe3+aqueous solution and 30%H2O2(AR,Yonghuachem,China),respectively,and the concentration of Fe3+was 0.06 mg?L–1in H2O2.The counter ion of Fe ion is Cl ion(c(Cl–)=0.76 mg?L–1)and the pH of 0.4 mg?L–1Fe ion solution simulated 30% H2O2solution,pH=3.6.The feed temperature was maintained at 25 °C.The retentate was recycled back to the feed container,and the permeate was at atmospheric pressure.The permeance (J,L?m-2?h-1?MPa-1) and the solute removal efficiencyR(%) were calculated based on the following equations:

In Eq.(1),V(L)represents permeation volume,A(m2)is the corresponding effective membrane filtration area,t(h)represents the filtration time,P(MPa) is the permeation pressure;In Eq.(2),CPandCFrepresent the solute concentrations of permeation and feed solution,respectively.

3.Results and Discussion

3.1.Characterization of SnO2 nanosheets

The morphology and purity of SnO2are influenced by the level of filling in the kettle liquid,as different filling levels correspond to different pressures and oxygen levels.The pressure corresponding to different filling levels is measured using an autoclave with a pressure gauge.The pressures are 0.25,0.3,0.4,0.5 and 0.6 MPa for 25%,50%,75%,85%and 95%fill levels respectively.In this work,five filler levels are set up to investigate the effect on the morphology and purity of the nanosheets.FESEM investigates the general morphologies of the prepared SnO2materials,and the results are demonstrated in Fig.2.As shown in Fig.2(a) and (b),the SnO2is presented as nanoparticles and did not form nanosheets.It may attribute to the low pressure in the kettle causing the seed crystals to dissolve and not grow.Increasing the pressure,SnO2nanosheets are successfully prepared (Fig.2(c)–(e)).It possesses spreading flower-shaped morphologies and very high density (Fig.2(b)).Interestingly,it is observed that the flower-shaped structures,almost triangular-shaped,are made of thin nanosheets which intermingle with each other in such a manner that one corner of individual nanosheet is connected with the side surface of the nanosheet (Fig.2(c)–(e)).This particular structure may be related to the oriented attachment of the crystal [18].The fast oriented attachment of the SnO2nanoparticles results in the formation of SnO2nanosheets.Subsequently,because of the surface energy minimization,the newly formed nanoparticles would spontaneously land on the as-formed sheets and preferential further grow to another nanosheet in the[0 0 1]direction,thus flower-like SnO2architectures are formed [19,20].The thickness of the nanosheets becomes thinner with increasing fill levels,70,25,and 15 nm respectively.

Fig.2.The SEM images (a–e) and XRD patterns (f) of SnO2 nanosheets in the filling level of the kettle liquid to 25%,50%,75%,85%,and 95% respectively.

To examine the crystallinity and crystal phases,the prepared SnO2is analyzed by the XRD pattern.Fig.2(f) shows the XRD patterns of the prepared SnO2nanosheets.The first four groups of identified peaks can be indexed to SnO2with a tetragonal rutile structure (JCPDF 41-1445).Except for SnO2,no other diffraction reflections are detected in the pattern,which confirms that the prepared samples are well crystalline and pure SnO2.The broadening of the peaks with increasing filling indicates a reduction in grain size,further suggesting that the synthesized products are nanoscale microcrystals.However,at a filling level of 95%,the characteristic (0 0 1) peak of SnO locates at 2θ=18.27°,which is attributed to the insufficient amount of oxygen in the kettle,resulting in the inability to fully oxidize the Sn2+to Sn4+.Considering the thickness and purity of the nanosheets,85% is a suitable level of filling of the kettle liquid for the preparation of SnO2nanosheets with a high specific surface area and no impurities.

3.2.Characterization of flower-like SnO2 membrane

The morphology of thein-situhydrothermally synthesized SnO2membrane is shown in Fig.3(a) and (b).The surface of the membrane is a flower-like structure (Fig.3(a)),and the thickness of the membrane is 2.7 μm (Fig.3(b)).By XRD characterization(Fig.3(c)),the characteristic peaks of both Al2O3and SnO2appear on the SnO2membrane,respectively,and the peak pattern is enhanced at the 2θ angle overlap of the two substances,with no impurity peaks appearing.This indicates that good crystallinity and impurity-free SnO2membranes are successfully prepared.Fig.3(d) displays the polyethylene glycol (PEC) retention of the SnO2membrane.The molecular weight(MWCO)of the membrane is 2686 Da,according to the formular=0.0262×-0.03,and the corresponding pore size is 2.7 nm [21].

Fig.3.The SEM images (a,b) and XRD patterns (c) of SnO2 membrane;the PEG retention of the SnO2 membrane (d).

3.3.Characterisation of SnO2 membrane for Fe3+ removal

Fig.4(a) and (b) shows the permeability and the removal efficiency of Fe3+in doped water and H2O2at 0.3 MPa.The SnO2membrane has a good removal effect on Fe3+in doped water,with a removal rate of over 97%(Fig.4(a)).Fig.4(b)shows a Fe3+removal efficiency (83%) and permeability (24 L?m-2?h-1?MPa-1) in H2O2.During this treatment,a layer of bronzing impurities deposit(Fig.S2(a)and(c)),and EDX characterization(Fig.S2(b))found that the impurities were iron precipitates [4].

Fig.4.The permeability and the removal efficiency of Fe3+ for doped water (a) and 30% H2O2 (b) at 0.3 MPa;effect of SnO2 membrane on the decomposition of H2O2 (c);removal efficiency (d) of SnO2 membrane for different metal ions in doped water.

Considering the process safety,the compatibility of H2O2and membrane materials is a prerequisite for purifying H2O2.Fig.4(c)explores the compatibility of the SnO2membrane with H2O2,which shows SnO2membrane has an inhibitory effect on H2O2decomposition.Fig.4(d) shows the removal efficiency of the SnO2membrane for different metal ions,and finds that the SnO2membrane has priority in removing Fe3+,Ca2+took second place,while K+and Na+are the lowest.The preferential adsorption of Fe3+may be related to its charges.Fig.S4 shows the wash-cycle stability test of the flower-shaped SnO2membrane.The membrane performance remained basically stable,but the adsorption residue of a small amount of impurities on the active sites on the membrane surface resulted in a slight decrease in both membrane flux and removal efficiency.

3.4.The mechanism of SnO2 membrane for Fe3+ removal

The specific surface area and the porous structure of SnO2nanosheets are characterized by nitrogen adsorption–desorption isotherms (Fig.5(a)).Compared to SnO2nanoparticles (AR,XFNANO,China) (Fig.S3),SnO2nanosheets show typical IV isotherms,and the hysteresis loops are of type H3[22],associating with the slit-like shape produced by layered packing.The major pore size distribution of SnO2ranges from 2 to 7 nm,suggesting the formation of a typical mesoporous structure [23].Moreover,the specific surface area of prepared SnO2is calculated to be 49 m2?g-1,much larger than that of the SnO2nanoparticles(5 m2?g-1).This significant increase in the specific surface area provides a large number of active sites for Fe3+adsorption and enhances adsorption efficiency.

Fig.5.Nitrogen adsorption–desorption isotherms,pore size distribution of SnO2 nanosheets(a);zeta potential of SnO2 nanosheets at different pH (b);FTIR spectra of SnO2 nanosheets before and after adsorption of Fe3+ (c);O 1s spectra of before and after adsorption (d);microscopic mechanism diagram of adsorption (e).

The microscopic mechanism of action needs to be explored to understand the process of removing Fe3+from SnO2membranes.Fig.5(b) shows the zeta potential of SnO2nanosheets at different pH,which reveals that SnO2is negatively charged at pH=2–10.This indicates that when the feed liquid is hydrogen peroxide(pH=3.6),the negative charge on the SnO2surface will allow Fe3+to migrate to the surface of the material through electrostatic attraction.The higher the charge,the stronger the electrostatic attraction [23].This mechanism explains the preferential adsorption phenomenon in Fig.4(d).Fig.5(c) shows the infrared spectra of SnO2before and after the adsorption of Fe3+.It can be seen that the bending of -OH (3443 cm-1) is partially weakened after the adsorption of Fe3+,which could be the result of the coordination complexation between hydroxyl groups on the surface of SnO2and Fe3+.To further explore the mechanism of the adsorption process.Fig.5(d) exhibits the chromatogram of O 1s before and after SnO2adsorption in XPS characterization.After the adsorption of Fe3+,Fe-O bonds appear at the corresponding position with a bond energy of 530.6 eV,indicating that Fe3+are adsorbed onto SnO2and complexed with hydroxyl groups on SnO2,which is consistent with infrared characterization analysis.

Fig.5(e) displays a microscopic mechanism diagram of adsorption.Benefiting from abundant adsorption sites (-OH) and negatively charged properties,the membranes exhibit H2O2stabilization and preferentially adsorbing capacity toward Fe3+based on coordination complexation and electrostatic attraction.

4.Conclusions

The synthesis of a flower-like SnO2ceramic membrane byin situtemplate-free one-step hydrothermal method is presented.Because of abundant adsorption sites (-OH) brought by the 3D pore structure of SnO2and negatively charged properties,SnO2membranes exhibit H2O2stabilization and preferentially adsorbing capacity toward Fe3+based on coordination complexation and electrostatic attraction,which could remove Fe3+with a high efficiency of more than 83% and permeability (24 L?m-2?h-1-?MPa-1) in H2O2.This study provides an innovative and practical approach for producing robust SnO2membranes with 3D channels for the highly efficient removal of Fe3+in harsh environments.

CRediT Authorship Contribution Statement

Risheng Shen:Methodology,Validation,Formal analysis,Investigation,Data curation,Writing–original draft,Writing–review&editing.Shilong Li:Methodology,Resources,Data curation,Writing– original draft,Validation.Yuqing Sun:Resources,Supervision,Project administration,Funding acquisition.Yuan Bai:Methodology,Resources,Formal analysis.Jian Lu:Investigation,Validation,Formal analysis.Wenheng Jing:Conceptualization,Writing–review&editing,Resources,Supervision,Project administration,Funding acquisition.

Data Availability

Data will be made available on request.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We sincerely appreciate the support from the National Key Research and Development Program (2021YFB3801303),the National Natural Science Foundation of China (21838005,21921006),the State Key Laboratory of Materials-Oriented Chemical Engineering (SKL-MCE-22A03),and the Key Research and Development Program of Jiangsu Provincial Department of Science and Technology (BE2022033-3).

Supplementary Material

Supplementary material to this article can be found online at https://doi.org/10.1016/j.cjche.2023.08.002.