Jinglei Cui,Qian Wang,Jianming Gao,Yanxia Guo,Fangqin Cheng

State Environmental Protection Key Laboratory of Efficient Resource Utilization Techniques of Coal Waste,Institute of Resources and Environmental Engineering,Shanxi Collaborative Innovation Center of High Value-added Utilization of Coal-related Wastes,Shanxi University,Taiyuan 030006,China

Keywords:Rare earth elements Selective adsorption SBA-15 Modification

ABSTRACT Rare earth elements (REE) are strategic resources and the recycling of REE in alternative resources is urgent and gets increasingly attention.However,the separation of REE in these alternative resources is still a challenge due to the low concentration of REE and multi coexisted ions in acidic system.In this study,the species distribution of REE within the pH 0-8.0 was calculated.The SBA-15 originated from coal fly ash was modified by two steps with (3-aminopropyl) triethoxysilane (APTES) and diethylenetriaminepentaacetic dianhydride(DTPADA)to obtain DTPADA-SBA-15 adsorbent,which was applied to the selective adsorption of REE.The results showed that DTPADA-SBA-15 possessed excellent adsorption performance on the selective adsorption of REE,including Eu,Gd,Tb,Nd and Sm,in acidic solution (pH 2)with multi competing ions.The FT-IR and Zeta potential characterization verified that the chemical adsorption through the coordination of O in DTPADA-SBA-15 with REE was dominant at lower pH value.The study of adsorption kinetics indicated that the adsorption of rare earth metal ions followed pseudosecond-order kinetic,of which the adsorption process followed the Langmuir isotherm model.

1.Introduction

The rare earth elements(REE)are strategic metal resources and are necessities in new materials industry and modern high-tech industry,and its demand increases rapidly [1].With the extensive exploitation of REE minerals,the amount of REE resources reduces drastically and cannot meet the demand for industry[2].Therefore,searching for more resource contained rare earth is required.Typical alternative sources of REE include brine,coal byproduct,mining wastewater and tailing[3].Among them,the content of REE in the bauxite slag and coal fly ash reached industrial mining grade[4].The total amount of rare earth in these sources is large and valuable for recycling,which has been widely concerned by researchers.These rare earth elements usually enter the liquid phase system in the form of ions along with the separation of other elements,such as the waste liquid generated in rare earth mining,and the extraction waste liquid generated during the extraction of alumina from coal fly ash[5].The concentration of REE ions(III)in the liquid phase is usually at the range of tens to hundreds mg·L-1,and inevitably coexists with multiple metal ions.It is a scientific and technical challenge for efficient separation of REE (III) [6].

Currently,many technologies for rare earth metal recycling were developed,likely chemical precipitation [7],chemical liquid-liquid extraction [8],ion exchange [9] and membrane separation [10],by which high separation efficiency can be achieved in industrial production.However,these technologies are usually suitable for the separation of liquid phases with high concentration REE.Compared with the above separation technology,adsorption is more efficient for the recovery of metal ions with low concentration.Moreover,the adsorption operation is simple and the adsorbent can be recycled conveniently,which meets the requirements of sustainable development [11].

The adsorption method achieves the purpose of recovering REE(III) by forming a complex between the adsorbent and the rare earth metal ions [12].Metal ions are generally combined with the adsorbent through a weak van der Waals force in physical adsorption,while chemical adsorption occurred by forming a chemical bond between the adsorbent and the metal ions.Physical adsorption is dominant over activated carbon,clay and zeolite adsorbents.Typically,such adsorption is not effectiveness to separate REE in alternative resources as the extreme low rare earth concentration (mg·L-1level) [13].Additionally,physical adsorption is not selective for rare earth metals over these adsorbents.To address these problems,researchers exploited adsorbent with inorganic (metal)-organic hybrid structures to absorb REE (III)by the complexation of chemical groups with REE(III)under acidic conditions.Louet al.[14] used a carboxylic acid-modified metalorganic framework (MIL-101) to adsorb and separate rare earth metal ions.The results showed that the adsorbent had a high adsorption capacity and selectivity for Sc (III) in the presence of competing ions Cu (II),Zn (II),and Mn(II).However,the synthesis process for adsorbent is complicated and its stability in acidic liquid-phase is insufficient.In order to resolve the problem,researchers developed inorganic-organic hybrid materials based on silica,metal oxides and carbon materials to adsorb REE(III)efficiently.Sunet al.[15] obtained nano core-shell carbon materials with abundant pores and large specific surface area through carbonization of polydopamine.The results displayed that the groups of amino and carbonyl located in the surface of carbon materials are conducive to adsorb REE (III).Additionally,the graphene with more active groups was used to synthesize carbon-organic hybrid materials to further improve the adsorption capacity [16].

Recently,silica-based adsorption materials have attracted more attentions due to its preferable stability under acidic conditions and easily modified properties [17-21].Jonathanet al.[22]adopted two-step surface modification method to graft carboxyl and phosphate on the surface of the mesoporous silica,and used it to adsorb REE(III).Their results showed that the adsorbent have better selectivity for the lanthanide ions in real brine.Gaoet al.[23] used phosphorous acid modified mesoporous SBA-15 to adsorb Gd (III),the adsorption capacity reach 1.3 mmol·g-1.Maet al.[24] found that lysine-modified SBA-15 can adsorb Sc (III)selectively and its adsorption capacity reach 35.29 mg·g-1.Although varying degrees of improvement has achieved over silica-based adsorbent,the fact that the selective adsorption of REE (III) with preferable adsorption capacity,particularly in acidic solution (typically pH ＜4),is still a big challenge.

In this study,species distribution of REE (III) in aqueous solution was calculated by software package Visual MINTEQ.The SBA-15 material was synthesized from sodium silicate produced by coal fly ash.Then,the coal fly ash based SBA-15 was modified by APTES and DTPADA.The surface modified SBA-15 (DTPADASBA-15) was applied to the selective adsorption of Eu (III),Gd(III),Tb(III),Nd(III)and Sm(III)in acidic solution.The Zeta potential and FTIR results implied that the coordination of O in DTPADASBA-15 with REE3+favored the selective adsorption REE (III).The DTPADA-SBA-15 appeared preferable adsorption capacity and selectivity with a series of competing ions (Mg2+,Ca2+,Al3+and Fe3+).Additionally,the recycling test showed that the adsorbent owned preferable stability and reusability in adsorption process.Moreover,the study of adsorption kinetics and the adsorption isotherms were performed to deeply understand the adsorption process by DTPADA-SBA-15 adsorbent.

2.Experimental

2.1.Materials

The coal fly ash (CFA) was obtained from Pingshuo Antaibao Thermal Power Plant,Shanxi province,China.The chemical component was characterized by X-ray fluorescence method and the result was shown in Table S1(as seen in Supplementary Material).

Anhydrous ethanol(AR)was purchased from Sinopharm Group Chemical Reagent Co.Ltd.P123 (EO20PO70EO20) was purchased from Aldrich Chemistry (USA).(3-Aminopropyl)triethoxysilane(APTES,AR) and diethylenetriaminepentaacetic dianhydride(DTPADA,AR)were all purchased form Alfa Aesar.Propylphosphonic anhydride(T3P,AR)and 4-dimethylaminopyridine(DMAP,AR)were purchased from Shanghai Macklin Biochemical Technology Co.LTD.N,N-Dimethylformamide (DMF) was purchased from Shanghai Aladdin Biochemical Technology Co.LTD.The rare earth elements (Eu,Nd,Gd,Sm and Tb,1000 mg·L-1in 1 mol·L-1HNO3)used for adsorption were purchased from Shanghai Aladdin Biochemical Technology Co.,LTD.NaOH (AR),HNO3(AR),Mg(NO3)2·6H2O (AR),Ca(NO3)2·6H2O (AR),Fe(NO3)3·6H2O (AR),Al(NO3)3·6H2O (AR) for the adsorption study were purchased from Sinopharm Group Chemical Reagent Co.Ltd.

2.2.The preparation and surface modification of coal fly ash based SBA-15

12 g CFA was added into 60 ml HCl(9 mol·L-1),the mixture was sealed in an autoclave with teflon lining and reacted at 160 °C for 4 h.After reaction,the autoclave was cooled,filtrated,wash for 5 times and then dried at 105 °C overnight.The obtained solid was mixed with 10% (mass) NaOH solution,the solid-to-liquid ratio was set to 1 g/3 ml.The mixture was reacted at 100 °C for 30 min to obtain sodium silicate solution.The chemical composition of sodium silicate solution was shown in Table S2 (in Supplementary Material).

2 g P123 was mixed with 60 g HCl solution(2 mol·L-1),the mixture was stirred at 30°C for 4 h.Then 7.4 g sodium silicate solution prepared from CFA and 22 g water were added at room temperature.After stirring for 24 h at 40 °C,the solution was sealed in an autoclave and reacted for 24 h at 100°C.After reaction,the mixture was filtrated,washed for 5 times and then dried at 105 °C overnight,then the solid was calcined at 550 °C for 5 h to obtain SBA-15 material.

The preparation of DTPADA-SBA-15 was shown in Fig.1.0.5 g as-prepared SBA-15 was dispersed in 50 ml anhydrous ethanol and stirred for 30 min,then 5 ml APTES was added,the modification was performed at 80°C with refluxing for 24 h.After that,the solid was centrifuged,washed by anhydrous ethanol for 5 times and dried.The prepared SBA-15 was denoted as NH2-SBA-15.1.6 g DTPADA,0.7 g DMAP,1.4 g NH2-SBA-15 and 1.7 g T3P were dispersed in 35 ml DMF,then the solution was stirred for 24 h at room temperature.The mixture was centrifuged and washed by DMF for 4 times,then the solid was dried.The obtained materials were denoted DTPADA-SBA-15.

2.3.Characterizations

The coal fly ash was ground to below 200 mesh,dried at 120°C for 2 h,and the chemical composition was analyzed by S8 Tiger XRay Fluorescence Spectroscopy (Bruker,Germany).

X-ray diffraction(XRD)analysis of the samples were carried out on a D8 ADVANCE A25 X-ray diffractometer (Bruker) with nickel filtered Cu Kα radiation.The tests were performed with a scanning angle(2θ)of 0°-10°or 10°-80°,a scanning speed of scanning speed 1 second per step,step length 0.02°.

Fig.1.The preparation DTPADA-SBA-15 by the modifying of SBA-15.

The specific surface area was determined by N2physisorption at-196 °C using an ASAP-2020 instrument (Micromeritics,USA).Samples were pre-degassed at 60 °C for 8 h under vacuum before measurement.The specific surface area was calculated following the multi-point BET (Brunauer-Emmett-Teller) procedure.

Fourier Transform Infrared spectroscopy(FTIR)characterization was carried out using a Nicolet iS50 attenuated total internal reflectance Fourier transform infrared spectroscopy (ATR-FTIR,Thermo Fisher Scientific,USA) with a scan range of 400-4000 cm-1,a resolution of 4 cm-1.

Thermogravimetric analysis(TGA)of NH2-SBA-15 and DTPADASBA-15 was performed with a SETSYS Evolution 16/18 instrument(France) at a heating rate of 10 °C·min-1from 25 °C to 1000 °C under air atmosphere.

Zeta potential of the samples was performed by a Malvern NanoZS90 instrument.At room temperature,0.25 mg SBA-15 or DTPADA-SBA-15 was mixed with deionized water to prepare a suspension (10 ml),and the adsorption pH (equilibrium value) was adjusted with HNO3solution (2 mol·L-1) or NaOH solution(2 mol·L-1) in the range of 1-10,the Zeta potential of the particle suspension was verified at different pH values.

2.4.The adsorption of REE (III)

The adsorption was performed in 20 ml glass tubes at 60 r·min-1at room temperature.The pH value(equilibrium value)for adsorption was adjusted by HNO3solution (2 mol·L-1) or NaOH solution(2 mol·L-1),then the adsorbent was added into the REE (III) solution with stirring.After adsorption,the solution was filtered by 45 μm filter.The concentration of metal ions was determined by inductively coupled plasma-atomic emission spectroscopy (ICPOES,Thermo-Fisher iCAP Model 6000,USA).

The selective adsorption of REE (III) with competing ions contained two parts.For the first part,the competing ions Ca2+,Mg2+,Al3+and Fe3+were mixed to the solution which contained a selected REE (III).The concentration of each competing ion was equal and was set to 10,50,100,200 mg·L-1,respectively,whereas the concentration of REE(III)was 10 mg·L-1.Then the solution was applied to the adsorption study.Subsequently,the distribution coefficient (Kd) and separation factor () were calculated.In the second part,a selected competing ion was added into the solution contained a selected REE (III).The concentration of REE ion was 10 mg·L-1.

The following formulates were used to evaluate the adsorption process.

Whereqeis the adsorption capacity;mis the mass of adsorbent in adsorption;c0andceare the initial and final concentration(mg·L-1)of REE (III) or competing ions;v is the volume of the adsorption solution (L);Kdis the distribution coefficient for REE (III) and competing ions,α is the separation factor of REE(III)and the competing ions (M).

3.Results and Discussion

3.1.Physicochemical properties of as-prepared adsorbents

The synthesis materials were analyzed by XRD,and the results were shown in Fig.2.All XRD patterns showed three diffraction angles centered at 0.9°,1.5°,1.8°,which related to (100),(110)and (200) crystallographic planes,respectively [25].These peaks assigned to the hexagonal mesoporous structure of SBA-15,confirming that the highly ordered p6mm hexagonal symmetrical mesoporous structure was reserved after the modification [26].

The N2adsorption/desorption isotherms were shown in Fig.3.All the samples displayed the type IV adsorption/desorption isotherm with a H1 hysteresis loop defined by International Union of Pure and Applied Chemistry (IUPAC),which are the typical features of meso cylindrical pores in SBA-15.The results also indicated that the mesopores were reserved after the modification.The specific surface area and pore parameters were shown in Table 1.The specific surface area of NH2-SBA-15 decreased as the modification of APTES,and it declined further due to the grafting of DTPADA to NH2-SBA-15.Accordingly,the volume of pores decreased as the blocking caused by the grafting groups of APTES and DTPADA.Additionally,the amount of ligand loading of the adsorbents was evaluated by thermogravimetric analysis (as seen in Fig.S1 in Supplementary Material) and the results were shown in Table 1.The relative lower ligand loading of DTPADA was likely due to the steric hindrance hinderance created by greater grafting densities [27].

Table 1 Specific surface area and pore parameters of SBA-15 and modified SBA-15

The synthesized materials were characterized by ATR-FTIR,and the results were shown in Fig.4.The peaks centered at 1041 cm-1and 802 cm-1for all samples are assigned to the antisymmetric and symmetric vibrations of Si-O-Si,respectively,which are main structures of SiO2·nH2O.For the DTPADA-SBA-15 sample,the peak centered at 1636 cm-1assigned to the -CO-NH-group,which verified the successful grafting of DTPADA to NH2-SBA-15 [28].

Fig.4.The FTIR spectra of SBA-15,NH2-SBA-15 and DTPADA-SBA-15.

In addition,the apparent morphology of the adsorbent before and after the modification was observed by scanning electron microscope (Fig.S2),and it was found that the adsorbent was in the shape of a long rod before and after the modification.At the same time,it can be seen by transmission electron microscope that the pore structure before and after modification has not changed significantly (Fig.S3).

Fig.2.The XRD patterns of synthesized SBA-15 materials.

Fig.3.The N2 adsorption/desorption isotherms of SBA-15 (a),NH2-SBA-15 (b) and DTPADA-SBA-15 (c).

3.2.Species distribution of REE (III) in aqueous solution

The species distribution was key influence factor for the selective adsorption of REE(III).Therefore,the thermodynamic equilibrium species distribution of REE (III) were calculated by software package Visual MINTEQ and the results were shown in Fig.5.REE3+(Eu3+,Gd3+,Nd3+,Sm3+,Tb3+) and REE(NO3)2+are the major REE (III) species in nitric acid with pH below 6.0.The fraction of REE3+decreased with the proportion of REE(NO3)2+increased when the pH value below 2.The proportion of REE3+declined significantly with the increase of pH value from 6.0 to 8.0.For Sm3+,Nd3+,Eu3+,Gd3+and Tb3+,REE(OH)2+species appeared when the pH values are 5.6,6.1,5.2,5.0 and 5.2,respectively,and the fraction of REE(OH)2+increased gradually with the increase of pH value until a precipitate was formed.The precipitation rate was higher than 95% for Sm3+,Nd3+,Eu3+,Gd3+and Tb3+when the pH value reached 7.5,7.8,7.5,7.2 and 7.6,respectively.These results indicated that the adsorption pH was preferably selected below 6.0.

3.3.The adsorption behaviors of REE (III)

3.3.1.The effect of pH on the adsorption of REE (III)

The pH affects both of properties of REE (III) and the functional groups on the adsorbent surface.Therefore,the effects of pH on the adsorption were studied (Fig.6).It was found that the pH value strongly affected the adsorption of REE(III).The maximum adsorption was obtained at the equilibrium pH of 5.0 and 6.0 for SBA-15 and NH2-SBA-15 adsorbents,respectively,while the adsorption declined drastically with the decrease of equilibrium pH.To further investigate the effect of pH,the Zeta potential measurement was carried out at different pH (Fig.7).The isoelectric point is at the range of 2.0-3.0,4.0-5.0 and 3.0-4.0 for SBA-15,NH2-SBA-15 and DTPADA-SBA-15,respectively,implying that the surface of adsorbents was with negative charge when the pH was greater[29].Therefore,a strong interaction was presence between the surface of SBA-15 and the rare earth element ions,which will improve the adsorption.However,the adsorption over DTPADA-SBA-15 reached a higher value at the equilibrium pH of 2.0,even the surface of DTPADA-SBA-15 was full of positive charges when the equilibrium pH below 3.0.This result reflected that the chemical adsorption was appeared between the rare earth metal ions and DTPADA-SBA-15,further indicating that the surface functional groups of DTPADA-SBA-15 are further indicating that the surface functional groups of DTPADA-SBA-15 are chemically bonding with rare earth ions.The electron-rich oxygen in the DTPADA ligand is likely to bond with REE (III),which makes the chelation occur to adsorb REE (III) at a lower pH [30].

3.3.2.The effect of contact time and initial concentration of REE(III)on the adsorption

The effect of contact time on the adsorption performance of DTPADA-SBA-15 was studied(Fig.8).The all selected REE(III)represented that it can be irrespective for the contact time as high removal efficiency (＞96%) was observed within short contact time(5 min),indicating a fast adsorption of DTPADA-SBA-15 to REE(III).After that,the removal increased slightly with extending the contact time and the adsorption can reach the equilibrium within 50 min for Eu (III),Gd (III),Tb (III) and Nd (III).

Fig.9 showed the influence of REE (III) concentration on its adsorption over DTPADA-SBA-15.As the initial solution concentra-tion increased,the adsorbed REE (III) gradually increased until it reached equilibrium.Accordingly,the maximal adsorption capacities were 32.1 mg·g-1,31.4 mg·g-1,29.1 mg·g-1,28.5 mg·g-1and 25.7 mg·g-1for Eu (III),Tb (III),Gd (III),Sm (III) and Nd (III),respectively.

3.3.3.The effect of competing ions on the adsorption of REE (III)

A series of metal,such as alkali metals and transition metal,inevitably coexist with REE in alternative resources,and the concentration of these metal ions often reaches several times that of REE(III).For example,CFA was considered one of the most promising alternative resources of REE,the acid leaching process was indispensable in the recovery of REE [30,31].In this technology,the major components of CFA consisted with Al3+,Fe3+and Mg2+will enter into the leaching solution and are the competing ions for selective adsorption of REE (III) [32-34].Additionally,Ca2+has similar oxophilicity and ionic radius with REE ions,which is also a common metal exist in REE-containing sources [35-37].Therefore,Ca2+,Mg2+,Al3+and Fe3+were selected as representative competing ions to investigate the effect on the adsorption.The four kinds of competing ions(Ca2+,Mg2+,Al3+and Fe3+)with every single concentration 1-20 times that REE(III)(10 mg·L-1)were mixed and the mixtures were applied for selective adsorption by DTPADA-SBA-15 (Fig.S4).The removals of rare earth ions were all above 99%(Fig.6(c)),and the removal decreased obviously with the adding of competing ions (each single concentration 10 mg·L-1).The further decline of removal efficiency was observed with the increase of competing ions.Meanwhile,it was found the removal for the Fe3+and Al3+was higher than that of Ca2+and Mg2+.To further understand the effect of competing ions on the adsorption,the separation factor between REE (III) and the competing ions (）was calculated,and the results were shown in Fig.10.It was observed that the α of REE (III) and Ca2+or Mg2+was much higher than that of Al3+or Fe3+,indicating that selective adsorption of REE (III) was more efficient with the competing ions Ca2+or Mg2+.However,thevalue declined drastically when the Ca2+concentration reached 10 times that of REE (III),implying the negative effect of high concentration of competing ions on the selective adsorption.Additionally,to further understand the negative effect of the competing ions,a competing ion was mixed with REE (III) solution and the mixture was applied for adsorption with DTPADA-SBA-15 adsorbent,respectively (Fig.S5).It was observed that the removal efficiency of REE (III) did not decline when the competing ion was Ca2+or Mg2+.However,the adsorption efficiency of REE (III) decreased markedly while Fe3+was added in the solution with a low concentration (10 mg·L-1).It was suggested that the Fe3+owned strong coordination ability with COOor -NH2group [38-40],which will compete with REE (III) in the adsorption and declined the adsorption efficiency.

Fig.5.Species distribution of REE (III) in aqueous as a function of pH value (the concentration of REE (III) 10 mg·L-1,25 °C).

Fig.6.The adsorption of REE(III)by SBA-15(a),NH2-SBA-15(b)and DTPADA-SBA-15 (c) (25 mg adsorbent, V 10 ml,the concentration of REE (III) 10 mg·L-1,25 °C,contact time 180 min).

Fig.7.The Zeta potential of adsorbents as a function of pH value (the mass of adsorbent 25 mg, V=10 ml,25 °C).

Fig.8.The effect of contact time on the REE (III) adsorption (25 mg adsorbent,V=10 ml,the concentration of REE (III) 10 mg·L-1,pH 2,25 °C).

Fig.9.The effect of REE(III) concentration on the adsorption (25 mg adsorbent,V=10 ml,pH 2,25 °C,contact time 180 min).

Fig.10.The separation factor of REE (III) adsorption with competing ions (a) Eu (III),(b) Tb (III),(c) Gd (III),(d) Sm (III) and (e) Nd (III) (25 mg adsorbent, V=10 ml,the concentration of REE(III) 10 mg·L-1,pH 2,25 °C,contact time 180 min).

3.3.4.The recycling of DTPADA-SBA-15 adsorbent for REE (III)adsorption

The recycling of DTPADA-SBA-15 adsorbent for REE(III)adsorption was investigated by 10 recycling adsorption runs(Fig.11).The elution of adsorbent after each run was performed by nitric acid(0.5 mol·L-1) for 3 h.A slight decrease of the removal efficiency was observed after 4 recycling runs.The results indicated that the adsorbent possessed preferable stability and reusability for the REE (III) adsorption in acidic solution.

3.4.Adsorption kinetics

To study the effect of adsorption parameters,particularly on the chemical adsorption,the adsorption kinetics were studied over DTPADA-SBA-15.The experiments were carried out at 298 K and the concentration of REE(III)was 10 mg·L-1.The adsorption kinetics were analyzed by the pseudo-first-order and pseudo-secondorder model (details in Supplementary Material) and the results were shown in Fig.12.

The correlation coefficientR2and the adsorption rate constant for pseudo-first-order and pseudo-second-order model was list in Table 2.The adsorption of rare earth metal ions followed pseudosecond-order kinetics with higherR2values than that obtained from the pseudo-first-order equation.The pseudo-second-order model assumes that the adsorption rate is determined by the square value of the number of unoccupied adsorption vacancies on the adsorbent surface [41].The adsorption process complied with the chemical adsorption which refers the electronic sharing or transferring between adsorbent and REE (III) [42].

To study the controlling step,the interparticle diffusion kinetic model(Weber and Morris model)were applied for the experimental data (Fig.S6 and Table S3) [43].The adsorption process contained the surface adsorption and the diffusion in pore (internal diffusion).In this model,the slope of the fitted straight line represented the intraparticle diffusion rate constant.It was found that the two-stage fitting lines did not pass through the origin,implying that the internal diffusion is not the only step to control the adsorption,and other adsorption stages controlled the adsorption process coordinately with internal diffusion [44].

Fig.12.The pseudo-first-order (a) and pseudo-second-order (b) fitting of the adsorption data on DTPADA-SBA-15.

Table 2 The adsorption kinetic parameters for DTPADA-SBA-15

Fig.13.The adsorption data of REE (III) onto DTPADA-SBA-15 fitting by Freundlich (a) and Langmuir (b) isotherm model.

Fig.14.The FTIR spectra of DTPADA-SBA-15 before and after the adsorption of Eu(III).

3.5.Adsorption isotherms

To investigate the metal uptake properties of the adsorbent with a series of metal ion concentrations,the adsorption isotherm was performed,and the data was fitting by Langmuir and Freundlich isotherm models (details in the supporting information),respectively (Fig.13 and Table S4).The results suggested that the adsorption process is fitted to the Langmuir isotherm model as theR2values are higher than that of Freundlich isotherm model.Therefore,it is likely that the adsorption site is uniformly distributed on the surface of DTPADA-SBA-15,and the adsorption of the selected REE (III) was single layer [45,46].Additionally,theRLwas applied to evaluate the adsorption ability for Langmuir isotherm model [47].The affinity of the adsorbent to REE ions is strong when theRLvalue is in the range of 0-1.In this study,theRLvalues of DTPADA-SBA-15 adsorbent for the selected REE are all in the range of 0-1 (Table S3),indicating that the DTPADASBA-15 adsorbent has strong affinity to the selected REE (III).

3.6.Adsorption mechanism

Typically,the adsorption mechanism includes electrostatic adsorption,coordination and ion exchange.FTIR spectroscopy was selected to identify the interaction between Eu (III) and the adsorbent of DTPADA-SBA-15.Initially,the adsorbent was mixed with Eu (III) solution and the adsorption process were maintained for 180 min,then the adsorbent was filtered,washed and dried at room temperature before the test of FTIR(see Fig.14).The adsorption band at 547 cm-1assigned to the vibration of the Eu—O bond was obviously,indicating that the chemical coordination was occurred between the carboxylic acid group in DTPADA-SBA-15 and Eu (III).It is reflected that the chelation of O atoms in carboxylic acid group and metal ions enables the effective separation of REE(III) with a lower pH of the solution [48].

4.Conclusions

In this work,the coal fly ash based SBA-15 was prepared,followed by an efficient adsorbent for REE (III) adsorption was synthesized by surface modification.The successful incorporation of an organic carbon chain (DTPADA) and amino group (APTES) onto the surface of SBA-15 was verified by ATR-FTIR and TG analysis.It was observed that the highly ordered structure of SBA-15 was reserved after modification,whereas the volume of pores decreased as the blocking caused by the modification.The results showed that the DTPADA-SBA-15 adsorbent exhibited preferable adsorption capacity to REE (III) (Eu,Gd,Tb,Nd and Sm) and the maximum adsorption capacity reached 32 mg·g-1in acidic solution.The adsorbent displayed high adsorption selectivity with high separation factor (α) value when the competing ions were Mg2+,Ca2+and Al3+.However,the addition of Fe3+decreased the adsorption efficiency as its strong coordination ability with carboxylic acid group.The adsorption capacity of DTPADA-SBA-15 was retained after 10 cycling run,indicating a better reusability in acidic solution.The adsorption kinetics suggested that the adsorption of REE(III)followed pseudo-second-order kinetic.Meanwhile,the adsorption isotherms implied that the adsorption process is well fitted to the Langmuir isotherm model.The Zeta potential and FTIR characterization proved that the chemical adsorption was dominant for DTPADA-SBA-15 and which was favored the selective adsorption REE(III)by the coordination of O atoms in carboxylic acid group with REE3+at lower pH value.

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

The authors gratefully thanks for the National Natural Science Foundation of China (U1810205),Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi Province (2020L0022),Key Research and Development Program of Shanxi Province (201903D311006).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.07.033.