Porous Pr(OH)3 Nanostructures as High-Efficiency Adsorbents for Dye Removal
Porous Pr(OH)3Nanostructures as High-E?ciency Adsorbents for Dye Removal
Teng Zhai,?Shilei Xie,?Xihong Lu,?Lei Xiang,§Minghao Yu,?Wei Li,?Chaolun Liang,?Cehui Mo,§Feng Zeng,*,?Tiangang Luan,*,?and Yexiang Tong*,?
?Key Laboratory of Environment and Energy Chemistry of Guangdong Higher Education Institutes,MOE Laboratory of Bioinorganic and Synthetic Chemistry,School of Chemistry and Chemical Engineering,Institute of Optoelectronic and Functional Composite Materials and?Instrumental Analysis and Research Centre,Sun Yat-Sen University,Guangzhou510275,PR China
§Key Laboratory of Water/Soil Toxic Pollutants Control and Bioremediation of Guangdong Higher Education Institute,Department of Environmental Engineering,Jinan University,Guangzhou510632,PR China
*Supporting Information
in water treatment.
Heavy metal ions and organic dye removal have drawn increasing attention in recent years because of their long-term environmental toxicity and short-term public health damage.1?4 Various nanomaterials with speci?c morphology and structure have been widely explored as adsorbents or nanocatalysts to remove heavy metal ions and organic dyes.5?10One dimen-sional(1D)nanomaterials with porous structures hold great promise for the treatment of environmental pollutants because of their highly accessible surface area and unique porous structures.For example,porous layered-lanthanum nanowires prepared at160°C by a hydrothermal method showed a high adsorption capacity of470mg g?1in removing Congo red(a textile dye).11The porous titanate nanotubes synthesized via a hydrothermal method from110to～270°C can achieve an adsorption capacity of211mg g?1for basic green5and118mg g?1for basic violet10.12Unfortunately,most porous1D nanomaterials are typically obtained at high temperatures and/ or with hard templates such as ordered mesoporous silicas (OMSs),13?15polycarbonate membranes,16?18anodic alumina membranes(AAM),19?21and biological templates.22Most current research e?orts have been devoted to increasing the e?ciency of dye removal using nanomaterials.However,little attention has been paid to their environmental toxicity.23?25 Therefore,it is necessary and important to develop a facile, general,scalable method of preparing porous1D nanostruc-tures with highly desirable e?ciency as dye adsorbents in water treatment.Electrochemical deposition o?ers a simple,quick, and cost-e?ective method of preparing nanomaterials in terms of its low-temperature growth processes,arbitrary substrate shapes,and environmental friendliness.Moreover,it allows precise control of the morphology and structure by adjusting corresponding deposition parameters such as the current density,potential,electrolyte,and temperature.26?34Recently, we have demonstrated the electrochemical synthesis of porous 1D CeO2,35,36La(OH)3,37and Nd(OH)338nanomaterials and their implementation as adsorbents in water treatment.These porous1D CeO2,La(OH)3,and Nd(OH)3nanomaterials prepared by a simple electrochemical assembly process at low temperature(70°C)without any hard templates showed highly e?cient capabilities in removing organic dyes.Because the light rare earth elements La,Ce,Pr,and Nd have similar properties, in recent years praseodymium compounds have attracted great attention as luminescent materials,catalysts,high-k gate dielectric materials,and optical?lters.39?44In this article,we expanded the electrochemical method in the controlled growth of porous1D Pr(OH)3nanostructures and studied their dye adsorption properties and environmental toxicity.
Porous Pr(OH)3nanobelt arrays(NBAs),nanowire arrays (NWAs),nanowire bundles(NWBs),and nanowires(NWs)
Received:March30,2012
Revised:June30,2012
were successfully synthesized by simply varying the deposition parameters.The Pr(OH)3NWs yield the largest adsorption capacity of 837.4mg g ?1in removing Congo red,which is 5.6-fold larger than that of active carbon.The prominent performance of porous 1D Pr(OH)3adsorbents is attributed to the porous structure,abundant hydroxyl groups,and basic sites.In addition,environmental toxicity studies of porous Pr(OH)3nanostructures indicate their great potential as environmentally friendly adsorbents in water treatment.2.EXPERIMENTAL SECTION 2.1.Synthesis of Porous Pr(OH)3NBAs,NWAs,NWBs,and NWs.Pr(NO 3)3·6H 2O and ammonium acetate (CH
3COONH 4)were
obtained from Sinopharm Chemical Reagent Co.,Ltd.,China.All
reagents used were analytical grade and were used directly without any
puri ?cation.The electrochemical deposition was carried out in a
conventional three-electrode cell using a homemade HDV-7C potentialstatic apparatus via galvanostatic methods.The copper foil
(1.5cm ×3cm),a graphite rod,and a saturated Ag/AgCl electrode
are used as the working electrode,counter
electrode,
and reference
electrode,respectively.The Cu foil was cleaned ultrasonically in
distilled water,ethanol,and acetone and then rinsed in distilled water again before electrodeposition.The porous Pr(OH)3nanomaterials were obtained by electrodeposition in a solution of 0.01M Pr(NO 3)3·6H 2O +0.02M CH 3COONH 4with a current density of
1?4mA cm ?2for 120min.The reaction temperature was kept at 70°C.2.2.Characterizations.The surface morphology and the composition of the samples were analyzed by scanning electron microscopy (SEM,Quanta 400).The structures of the samples were investigated via X-ray di ?raction (XRD,Bruker,D8ADVANCE)with
Cu K αradiation (λ=1.5418?)and transmission electron microscopy (TEM,200KV,JEM2010-HR).For the SEM,XRD,and UV spectra
of porous Pr(OH)3samples,measurements were carried out directly on the samples without removing the substrates.For the measurement of the TEM,thin ?lms of products were exfoliated and ground into
powders and then some of the powder was dispersed into ethanol via
ultrasound and the dispersed samples were collected with carbon copper grids.Nitrogen adsorption/desorption isotherms at 77K were
conducted on an ASAP 2020V3.03H instrument.All samples (powders)were outgassed at 100°C for 180min under ?owing nitrogen before measurements.UV ?vis absorption spectra were
measured on a UV ?vis ?NIR spectrophotometer (UV,Shimadzu UV-3150a).2.3.Water Treatment.A scalpel has been used to scrape the deposition products o ?of the substrate.Five,ten,or twenty milligrams of as-prepared porous Pr(OH)3nanostructures were directly added to 50
mL of Congo red solution (100mg L ?1)under stirring.The 0.45μ
m membranes were used for ?ltration and were prepared by complying with the following steps before each water treatment experiment:?rst,the ?ltration of 4mL of a dye solution three times to
make sure that the membranes could be saturated by the dye solution
and then ?ltration by 4mL of distilled water three times to eliminate the dye solution,and ?nally 4mL of air several times until there is no
residual
distilled water.The
Congo red solution was collected via 0.45μm membranes at di ?erent time intervals.What is more,other dyes,including rhodamine B,methylene blue trihydrate,reactive yellow 86,reactive blue,and acidic blue 161have also been adopted in water
treatment under the same conditions.All dyes were purchased from Guangzhou Chemical Reagent Co.,Ltd.,China and used directly without any puri ?cation.The names and chemical structures of the dyes
are also shown in Table S1.2.4.Root Germination and Elongation.The brassica para-chinensis seed (Chinese
cabbage)is among the 10species recommend by the U.S.EPA 41for phytotoxicity.First,seeds were immersed in a 10%sodium hypochlorite solution for 10min to ensure surface sterility.43Then,they were soaked in distilled water as a control,a subsequent ?ltrate of wastewater treatment,and 10mg L ?1Pr(OH)3
NWs for 120min after being rinsed three times with deionized
water.Figure 1.SEM images of as-deposited Pr(OH)3(a)NBAs,(b)NWAs,(c)NWBs,and (d)NWs at di ?erent magni ?cations with current densities of 1?4mA cm ?2.
One piece of ?lter paper was put into each 100mm ×15mm Petri dish,and 5mL of a test medium was added.Seeds were then
transferred onto the ?lter paper,
with 20
seeds per dish and a 1cm or
larger distance between each seed.44Petri dishes were covered and sealed with tape and placed in the dark in a growth chamber at 25°C.
After 5
days,more than 98%of the control seeds had germinated and the roots were at least 20mm or longer.Then,the germination was halted and the seedling root length was measured with a millimeter ruler.2.5.Statistical Analysis.Each treatment was replicated three
times,and the results are presented as the mean ±SD (standard
deviation).The student ’s t test was utilized in the statistical
analysis
of
the experimental data.The statistical signi ?cance was accepted when the probability of the result,assuming the null hypothesis (p ),is less than 0.05.3.RESULTS AND DISCUSSION The cathodic electrodeposition of Pr(OH)3nanostructures was carried out in a solution containing 0.01M Pr(NO 3)3and 0.02M CH 3COONH 4with a current density of 1?4mA cm ?2for 120min at 70°C.To determine the crystal structure and composition,X-ray di ?raction patterns collected from the products prepared with di ?erent current densities are shown in Figure S1.All of the di ?raction peaks after the subtraction of the silicon substrate peaks can be indexed as the pure hexagonal phase of Pr(OH)3(JCPDF card 45-0086)with lattice constants of a =0.652nm and c =0.378nm.No impurity peaks are detected,suggesting that the products are pure.The broadened peaks suggest the relatively poor crystalline nature of the products.Figure 1shows the typical scanning electron microscopy (SEM)images of as-deposited Pr(OH)3with di ?erent current densities.From Figure 1a,Pr(OH)3NBAs with widths of 500?600nm,a thickness of ～100nm,and lengths of 4μm were obtained with a current density of 1mA cm ?2.Transmission electron microscopy (TEM)analysis in Figure 2a shows that many pores were uniformly distributed throughout these nanobelts.With the current density increased to 2mA cm ?2,small disordered and porous nanobelts with were obtained,as shown in Figures 1b and 2b.These porous nanobelts have a diameter of ～160nm,a thickness of 30?40nm,and a length of 6μm.When the current density further
increased to 3and 4mA cm ?2,Pr(OH)3NWBs and NWs were obtained,as presented in Figure 1c,d.TEM images (Figure
2c,d)also demonstrate they also have porous structures (several pores were highlighted by dashed lines).To further
investigate their porous structure and surface area,nitrogen
adsorption ?desorption isotherms (at 77K)collected from these samples are shown in Figure S2.The Brunauer ?Emmett ?Teller (BET)speci ?c surface areas for porous
Pr(OH)3NBAs,NWAs,NWBs,and NWs are about 26.1,31.3,42.1,and 74.9m 2g ?1,respectively.The average pore diameter is 13.3nm for NBAs,10.4nm for NWAs,8.9nm for
NWBs,and 9.5nm for NWs.
The as-prepared porous Pr(OH)
3nanostructures were used as adsorbents to investigate their potential applications in water
treatment.Congo red is a common dye in the textile industry
and has been recognized as an organic contaminant in wastewater in the textile industry.The absorption of Congo red solution was shown in photographs (Figures S3a,b and S4a,b)in the presence of Pr(OH)
3NWs,NBAs,NWAs,and NWBs,respectively,at di ?erent time intervals.Clearly,the color of Congo red completely disappeared after 2min when
using the NWs,whereas when using NBAs,NWAs,and NWBs,the color disappeared after 120,30,and 5min,respectively.This indicates that the porous Pr(OH)3NWs exhibit the best performance.For comparison,activated carbon with a large BET surface area of 1940m 2g ?1was used in the same test.Figure S3c shows the adsorption rate of activated carbon and various Pr(OH)
3nanostructures.Obviously,the adsorption rate of Congo red by Pr(OH)3nanostructures is much faster than that of activated carbon.From Figure S3d,the adsorption capacity of Congo red at 120min is about 81.1mg g ?1for activated carbon,250mg g ?1for Pr(OH)
3NWs,218mg g ?1
for Pr(OH)3NBAs,248.8mg g ?1for Pr(OH)3NWAs,and 246.5mg g ?1for Pr(OH)3NWBs,which further con ?rms the superior performance of these Pr(OH)3nanostructures in removing Congo red.Additionally,the adsorption capacity for the porous Pr(OH)3adsorbents is more than 250mg g ?1.To estimate the total adsorption capacity of the as-synthesized materials,we reduced the amount of each sample to 0.005g while keeping the other experimental conditions the same.The results are shown in Figure.3.Signi ?cantly,the adsorption capacity of porous Pr(OH)3NBAs,NWAs,NWBs,and NWs at 120min is 765.6,769.8,794.0,and 837.4mg g ?1,respectively.Note that the nanostructured powders may aggregate in solution and will decrease the di ?usion rate of active species.Considering the fact that our samples have similar morphol-ogies (porous structure with an average pore size of 9?13nm,which is much larger than that of dye molecules)and the same composition,their aggregation situations might be approximate in solution.A qualitative comparison of the surface area in solution between our samples is valid.Thus,the di ?erence for our samples in removing Congo red is believed to be due to their surface area.The adsorption capacity of Pr(OH)3NWs is 5.6times higher than that of active carbon (149mg g ?1),which is also substantially higher than the values recently reported for mesoporous Fe 2O 3(53mg g ?1,BET surface area 131m 2g ?1)8
and LHL-propionate porous lanthanum NWs (470mg g ?1,BET surface area 146.33m 2g ?1)11and is comparable to porous
CeO
2NWAs (987mg g ?1,BET surface area 98.1m 2g ?1).35It is well known that the surface area and surface
functional
Figure 2.TEM images of porous Pr(OH)3(a)NBAs,(b)NWAs,(c)NWBs,and (d)NWs.
groups are two key factors in the adsorption capability of nanomaterials.35?38,53A large surface area can provide more active sites on which to absorb dye molecules,and the surface functional groups may interact with dye molecules by hydrogen bonds and/or electrostatic forces.Congo red is one acid dye with NH 2groups.Pr(OH)
3has a large number of hydroxyl
groups and basic sites on its surface.In this regard,Pr(OH)3
may interact with the NH 2of dye molecule via Pr ?O ···H ?N and Pr ?O ?H ···N hydrogen bonds and/or basic sites via electrostatic forces with an amino cation (NH 3+).Compared to mesoporous Fe 2O 345and porous lanthanum NWs,11our
Pr(OH)3nanostructures have a smaller surface area of 26.1?75m 2g ?1.Therefore,their prominent adsorption capability may be largely ascribed to surface functional groups and basic sites.To understand the interplay between the absorption capability and the surface hydroxyl groups of Pr(OH)3,we have investigated their absorption capability as a function of surface hydroxyl groups (from Pr(OH)
3to PrO x ).According to the TG curves (Figure S5),Pr(OH)3NWs were annealed at 100,250,and 400°C in air for 180min to form di ?erent products.XRD patterns in Figure 4demonstrated that the
samples annealed at 100and 250°C are Pr(OH)
3(denoted as Pr-100)and PrOOH (denoted as Pr-250)whereas mixtures of Pr 2O 3and Pr 6O 11were annealed at 400°C (denoted as Pr-400).The HRTEM images and SAED patterns (Figure 4)also
con ?rm the crystalline transformation of the products during
the annealing process.However,the wirelike morphology did not signi ?cantly change and the porous structure was retained.The BET surface areas of Pr-100,Pr-250,and Pr-400are 141.9,85.7,and 76.7m 2g ?1,and their corresponding average
pore
Figure 3.Adsorption rate (a)and adsorption capacities (b)of a solution of Congo red (100mg L ?1,50mL)for Pr(OH)3NBAs,NWAs,NWBs,NWs,and activated carbon.The mass of the samples was 0.005
g.Figure 4.(a)XRD patterns of porous Pr(OH)3NWs and their calcined products at 100(Pr-100),250(Pr-250),and 400°C (Pr-400).HRTEM images of (b)Pr-100,(c)Pr-250,and (d)Pr-400.The insets in b ?d are the SAED patterns.
diameters are about 11.2,10.7,and 8.9nm,respectively.Figure 5a,b show the absorption rate and capability of the samples obtained at di ?erent annealing temperatures as a function of time.Pr-100exhibits excellent performance as as-prepared Pr(OH)3NWs because of its high surface area and abundant hydroxyl groups,which could eliminate 100%and reach 500mg g ?1of its adsorption capacity in 120min.On the contrary,with the annealing temperature increase,the absorption rate gradually decreased and the samples annealed at 400°C could remove only 52.5%after 120min.Photographs (Figure 5c ?e)also con ?rm the decreased adsorption rate after annealing at 250and 400°C.The surface area and pore size of Pr-400are similar to those of as-deposited Pr(OH)3NWs.However,their absorption rate and capability over 120min is much inferior to those of as-deposited Pr(OH)3NWs,and it took a longer time (840min)to achieve 500mg g ?1(Figure 5e).This indicates that the surface hydroxyl groups can promote the removal rate of dye molecules.To investigate the key sites of dye molecule interacting with porous Pr(OH)3nanoabsorbents,two other acidic dyes with
NH 2groups (i.e.,reactive yellow and reactive blue,one acidic dye with sulfonic groups (?SO 3?)and tertiary amine groups;acidic blue and two basic dyes;and rhodamine B and methylene blue trihydrate)were selected as model dyes under the same conditions in the presence of 0.02g of Pr(OH)3NWs.Surprisingly,the porous Pr(OH)3NWs could quickly remove almost 100%of reactive yellow and reactive blue,but almost no absorption for rhodamine B,methylene blue trihydrate,and acidic blue,indicating that porous Pr(OH)3nanoabsorbents interact with NH 2groups of the dye molecule via Pr ?O ···H ?N and Pr ?O ?H ···N hydrogen bonds and/or basic sites via electrostatic forces with the amino cation (NH 3+).Therefore,the high adaptability for porous Pr(OH)3nanostructures in removing Congo red is attributed to their large surface area,surface hydroxyl groups,and basic sites.Besides,it could be extended to other dyes with NH 2groups in
wastewater treatment.On the basis of the above results,the
schematic illustration of the possible adsorption mechanism for Pr(OH)3is proposed and shown in Figure 7.We used Congo red dye as the model dye in the illustration because it is known that the hydrogen bond is easily formed between these
substances with electronegative atoms such as ?uorine,oxygen or nitrogen,and hydrogen atoms.There are abundant OH groups on the surface of Pr(OH)
3nanostructures.They will absorb Congo red molecules via a hydrogen bond,which arises from the interaction between OH on the Pr(OH)3surface and NH 2on the Congo red molecule surface (illustrated by the dark dashed line).However,the porous architectures can not only provide more sites to which to absorb Congo red molecules but also facilitate the di ?usion of Congo
red
Figure 5.(a)Adsorption rate and (b)adsorption capacities of a solution of Congo red (100mg L ?1
,50mL)for Pr(OH)3NWs,Pr-100,Pr-250,and
Pr-400.The mass of the samples was 0.01g.Photographs of a solution of Congo red (100mg L ?1,50mL)in the presence of (c)porous Pr(OH)3NWs and (d)
Pr-250.Figure
6.Adsorption of ?ve model dyes with the same concentration
of 100mg L ?1in the presence of 0.02g of porous Pr(OH)3NWs.
molecules to the inside of Pr(OH)3nanostructures and thus enhance the absorption performance.Besides a high adsorption capability,environmental toxicity is the key factor in the application of adsorbents.However,only a few reports have been focused on the toxicity of praseodymium composites.45?47Herein,we also studied their environmental toxicity behaviors by root elongation experiments (details in Supporting Information).The brassica parachinensis seeds were adopted here because of their thinner skin among vegetable seeds.48,49The e ?ects of the subsequent solution of wastewater treatment (?ltrate)and porous Pr(OH)3NWs at 10mg L ?1on root elongation are shown in Figure 8.The seed germination rate is 98%,indicating that the seed germination is not a ?ected by the ?ltrate and porous Pr(OH)3NWs at 10mg L ?https://www.wendangku.net/doc/6f17007038.html,pared to the root elongation of the control (deionized water,22.9mm),the ?ltrate (21.9mm)and porous Pr(OH)3NWs at 10mg L ?1(21.5mm)demonstrated reductions of 4.4and 6.1%,respectively.The 10%e ?ective concentration (EC10value)is used as a screening value for the direct risk assessment of sites by legislators or those assessing sites for remediation.47Thus,the ?ltrate did not show any signi ?cant di ?erence (≤10%)from the control,showing that the ?ltrate has no e ?ect on the test seeds.51,52What is more,the ?ltrate may have a smaller reduction e ?ect (≤4.4%)on the root elongation of other plant seeds because of the higher sensitivity of brassica parachinensis seeds.These primary results indicate that these porous Pr(OH)3NWs hold great potential as candidates for
environmentally friendly adsorbents in water treatment.
Further
Figure 7.Schematic illustration of the adsorption mechanism between Pr(OH)3adsorbents and the Congo red dye
molecule.
Figure 8.E ?ects of a subsequent solution of wastewater treatment (?ltrate)and porous Pr(OH)3NWs at 10mg L ?1on root elongation.
research on Pr(OH)3NWs such as the toxicity to alga and human cells is carried out to con?rm that it can be elected as environmentally friendly adsorbents.
4.CONCLUSIONS
Porous Pr(OH)3nanostructures including NBAs,NWAs, NWBs,and NWs were successfully prepared via facile electrochemical deposition without any hard templates.These as-prepared porous NWs show high e?cient and selective adsorption properties in removing dyes with NH2groups because of their high surface area,porous structure,and abundant hydroxyl groups and basic sites on their surfaces. Moreover,porous Pr(OH)3NWs have negligible toxicity on root germination and elongation of the brassica parachinensis seeds.These?ndings suggest that the porous Pr(OH)3 nanostructures are promising as an environmentally friendly
dye adsorbent candidate in water treatment.
■ASSOCIATED CONTENT
*Supporting Information
Chemical structures of dye molecules.XRD and BET results of as-deposited samples.Photographs,adsorption capacities,and adsorption rate of the adsorption process.TG curves of Pr(OH)3NWs.This material is available free of charge via the
Internet at https://www.wendangku.net/doc/6f17007038.html,.
■AUTHOR INFORMATION
Corresponding Author
*(F.Z.)Fax:+862084112245.Tel:+862084110071.E-mail: ceszf@https://www.wendangku.net/doc/6f17007038.html,.(T.L.)Tel:+862084113785.E-mail:cesltg@https://www.wendangku.net/doc/6f17007038.html,.(Y.T.)Fax:+862084112245. Tel:+862084110071.E-mail:chedhx@https://www.wendangku.net/doc/6f17007038.html,. Notes
The authors declare no competing?nancial interest.■ACKNOWLEDGMENTS
Y.T.acknowledges the?nancial support of this work by the Natural Science Foundations of China(90923008and J1103305)and the Natural Science Foundation of Guangdong Province(9251027501000002).X.L.thanks the Academic New Artist Ministry of Education Doctoral Post Graduate
(China)for China Scholarship Council for?nancial support.■ABBREVIATIONS
NBAs,nanobelt arrays;NWAs,nanowire arrays;NWBs, nanowire bundles;NWs,nanowires;Pr-100/250/400,samples
annealed at100/250/400°C
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