Dongmei Jia *,Huamin Cai ,Yongzheng Duan ,Jiangbao Xia ,Jia Guo

1 School of Chemical Engineering and Safety,Binzhou University,Binzhou 256603,China

2 School of Chemical Engineering and Pharmaceutical,Wuhan Institute of Technology,Wuhan 430073,China

3 Shandong Key Laboratory of Eco-Environmental Science for the Yellow River Delta,Binzhou University,Binzhou 256603,China

Keywords:D301 Iron oxalate Chromium Adsorption

ABSTRACT Chromium is a common harmful pollutant with high toxicity and low bearing capacity of soil and water.Excellent salinity resistance,a wide pH range,and high regeneration capacity were essential for qualified adsorbents used in removing hexavalent chromium(Cr(VI))from polluted water.Herein,iron oxalate modified weak basic resin(IO@D301)for the removal of Cr(VI)was prepared by the impregnation method.The IO@D301 was characterized by scanning electron microscope(SEM),Fourier transform infrared spectroscopy(FTIR),X-Ray diffraction(XRD)and X-ray photoelectron spectroscopy(XPS).Owing to abundant amine,carboxyl groups and iron ions existing on the surface,IO@D301 possesses high adsorption and salinity resistance capacity for Cr(VI).The maximum adsorption capacity of IO301 towards Cr(VI)reached 201.30 mg·g−1 at 293 K and a pH of 5.The adsorption equilibrium was well fitted by the Freundlich model,and the adsorption process was described by the pseudofirst-order kinetics model as spontaneous and exothermic.The mechanism may be identified as electrostatic attraction,coordination,and reduction,which was confirmed by FT-IR and X-ray photoelectron spectroscopy.

1.Introduction

Chromium is a common harmful pollutant with high toxicity and low-bearing capacity of soil and water[1,2],and it is listed as the toppriority toxic pollutant by the United States Environmental Protection Agency[3].Under natural conditions,chromium usually exists in two central oxidation states,trivalent chromium(Cr(III))and Cr(VI)[4].Cr(III)is less toxic and can be a nutrient for humans at low concentrations[5].The toxicity of Cr(VI)is much higher than that of Cr(III).Hexavalent chromium usually pollutes the environment in the form of solution,powder,or gas,causing damage to the nasal cavity,respiratory tract,and internal organs.Cr(VI)has strong carcinogenicity to the lung and nasal cavity.Therefore,the removal of chromium is mainly focused on Cr(VI)[6].According to the World Health Organization guidelines for drinking water quality,the content of Cr(VI)in drinking water should not exceed 0.05 mg·L−1[7].Accordingly,a safe and efficient treatment technology is essential to controlling Cr(VI)levels in drinking water.Many technologies,such as chemical reduction [8],membrane separation [9],ion exchange [10],chemical precipitation [11],photocatalytic degradation[12]and adsorption have been used in Cr(VI)removal[13].Among these conventional methods,adsorption has been recognized as one of the most promising technologies for the removal of Cr(VI)due to its high efficiency,simple operation,and low energy consumption[14].

Compared with conventional adsorbents such as activated carbon[15],carbon nanotubes[16],metal oxides[17],inorganic and organic composite materials[18]and MOFs[19],ion exchange resin has attract more attentions in Cr(VI)removal from aqueous solution because of its high selectivity,high adsorption capacity,and low production of sludge to enhance the adsorption capacity so that they can be applied extensively.Nonetheless,the adsorption capacity and reusability is not quite high due to its single adsorption mechanism.In order to enhance the adsorption performance,different modified methods were used along with ion exchange resin by introducing function groups and increasing surface area,such as poly-epichlorohydrin-dimethylamine modified D301[20],resin embedded into chitosan/poly(vinyl amine)cryogels [21],nanoscale hydrated metal oxides modified resin [22],ionic liquid modified Dowex1 × 8 resin [23] and zero-valent iron modified D301 [24].Therefore,seeking a novel efficient modified method of ion exchange resins is a very urgent strategy of Cr(VI)removal.

D301 resin is a weakly anion exchange resin with a tertiary amino group and mostly suitable for adsorption to metal ions,due to a high reactivity and a high chelating ability for heavy metal ions as already demonstrated.Different modified D301 resins adsorption to heavy metal ions reported in previous literature are summarized in Table 1.

Table 1Comparison of the maximum adsorption capacity for heavy metal ions with reported modified D301 resins

Table 2Isotherm parameters for Cr(VI)adsorption onto IO@D301

Iron oxides were demonstrated excellent adsorption properties for Cr(VI)[29,30]and carbonyl function groups can promote the formation of reducing hexavalent chromium to trivalent chromium and complexation of the reduced Cr(III) [31].However,there have not reported for the adsorption of Cr(VI)onto D301 resin modified with these two groups.

In this study,the macroporous polystyrene resin D301 was used as the carrier,and the iron oxalate was introduced into the outer surfaces of the macroporous resin using the impregnation method to prepare IO@D301.The IO@D301 was characterized and its adsorption property was examined in relation to a series of factors(initial concentration of Cr(VI),pH,contact time,and coexisting ions).The kinetic and thermodynamic parameters were also proposed in order to ascertain the adsorption behavior and mechanism using different kinetic and thermodynamic models.

2.Materials and Methods

2.1.Chemical reagents

D301 resin used in this work was purchased from Nankai Resin Co.,Ltd.(Tianjin,China).FeCl3,K2Cr2O7,NaOH and HCl were obtained from Tianjin Tianli Chemical Reagent Co.,Ltd.(Tianjin,China).H2C2O4was purchased from Aladdin.Stock solution (1000 mg·L−1) of K2Cr2O7was prepared by dissolving K2Cr2O7with ultrapure deionized water(18.2 MΩ·cm).Chemicals of analytical grade in this work were used directly without further purification.

2.2.Preparation of IO@D301

D301 resins were soaked with HCl (5 wt%) and NaOH (3 wt%)at room temperature for 8 h,respectively.Then were washed three times using deionized water.5.6 g (0.02 mol) of FeCl3·6H2O were dissolved in 100 ml mixed solvent(50 ml ethanol and 50 ml distilled water) in a 250 ml conical flask.3 g of pre-processed D301 resins were added to the above solution of FeCl3at 303 K for 12 h under vigorous stirring conditions.Then,2.7 g(0.03 mol)of H2C2O4dissolved in 25 ml ethanol and 25 ml distilled water was added into such mixed solution dropwise under vigorous stirring conditions.The resulting yellow resins was filtered off,and the product was washed with distilled water,and IO@D301 resins were obtained after dried at 313 K under vacuum for 24 h.

2.3.Characterizations

Scanning electron microscopy(SEM)(Hitachi SU8020,Japan)was used to record the size and morphology of the samples.The FTIR spectra of samples were recorded on a Nicolet 5700 instrument (Thermo Scientific,America).The IO@D301 samples were characterized by a surface area and porosity analyzer(TRISTAR 3000,MICROMERITICS).X-ray diffraction (XRD) patterns were recorded using a Y-2000 diffractometer at 40 kV and 40 mA with Cu Kαradiation(λ=0.1543 nm).X-ray photoelectron spectroscopy(XPS,ESCALAB 250Xi,Thermo Scientific,America)were performed using 200 W monochromated Al Kα(1486.6 eV)radiation.A 500-μm X-ray spot was used for XPS analysis and the base pressure in the analysis chamber was about 3×10−8Pa.

2.4.Adsorption test

The adsorption experiments were performed using a 250 ml conical flask with 50 ml Cr(VI)solution and 30 mg adsorbents in a water bath shaker at 120 r·min−1.The effect of certain factors on adsorption capacity,including initial concentration,temperature,contact time,pH and coexisting ions(sunch as Cl−and SO42−),were examined.The initial pH value of Cr(VI)solution was adjusted from 3 to 10 using a 0.1 mol·L−1HCl solution or a 0.1 mol·L−1NaOH solution.The concentration of Cr(VI)was analyzed using the UV–vis spectrophotometer (T6,Beijing Puxi General Instrument Co.,Ltd)at 540 nm.

The adsorption capacity(qe,mg·g−1)was calculated according the following equation:

where C0(mg·L−1)and Ce(mg·L−1)refer to the concentration of the Cr(VI)solution at start and at equilibrium,m(g)represents the mass of the adsorbent,and V(L)is the solution volume.

Furthermore,a mixed solution with 1 mol·L−1NaOH and 1 mol·L−1Na2CO3(volume ratio 1:1) was selected as the desorbent for investigating the reusability of IO@D301.The regenerated adsorbent was washed three times with deionized water and then washed with 20 ml desorbent for 1 h.

2.5.Adsorption model

2.5.1.Adsorption isotherm model

In order to determine the adsorption behavior of Cr(VI)on IO@D301,the adsorption isotherms were fitted by Langmuir[32],Freundlich[33]and Temkin[34]isotherm models,respectively.The Langmuir isotherm is shown in Eq.(2).

where qm(mg·g−1) and KL(L·mg−1) are the maximum adsorption capacity of adsorbent and the Langmuir constant,respectively.The Freundlich isotherm is shown in Eq.(3).

where KF(mg·g−1)is related to the relative adsorption capacity and n(g·L−1)is associated with the intensity of adsorption.The Temkin isotherm is shown in Eq.(4).

where B(kJ·mol−1)is the Temkin adsorption constant and KT(L·mg−1)is the equilibrium binding constant.

2.5.2.Adsorption thermodynamics model

Enthalpy change(ΔH)can be obtained by the Clausius–Clapeyron equation represented the Langmuir isotherm as shown in Eq.(5)[35].

where Ceis the equilibrium concentration;R(mole gas constant)and K0are constants while T is the thermodynamic temperature,the values of ΔH were obtained from the slope of the curve.

Gibbs free energy change (ΔG) is usually obtained by the Gibbs equation(Eq.(6)).

where x is the molar fraction of absorbate;q is the adsorption capacity.

Entropy change(ΔS)can be achieved by Gibbs-Helmholtz equation(Eq.(7))[36].

2.5.3.Kinetics model

The pseudo-first-order model(PFO)(Eq.(8)),the pseudo-secondorder model (PSO) (Eq.(9)) and the Weber and Morris internal diffusion model (WMID) (Eq.(10))

[37] were used to study the adsorption process and calculate the adsorption rates.

where qt(mg·g−1)and qe(mg·g−1)are the adsorption capacities at time t and equilibrium time,respectively;while k1(min−1),k2(g·mg−1·min−1) and kid(mg·min0.5·g−1) are the PFO,PSO and WMID rate constants,respectively.

3.Results and Discussion

3.1.Characterization of the adsorbent

The D301 and IO@D301 were characterized by FT-IR spectroscopy.As shown in Fig.1(a),the broad band at around 3400 cm−1was due to the--OH on the surface of the two adsorbents,and the C--H bond symmetry and asymmetric telescopic vibration characteristic peaks appeared at the band at around 2071–2925 cm−1[38].The characteristic peak at 1257 cm−1was the stretching vibration of amine group C--N in resin.In addition,the characteristic peaks at 1656 cm−1and 1404 cm−1in IO@D301 were the stretching vibration of C=O and--NH2,respectively.The stretching vibration peak of hydrated iron ions was about 3422 cm−1.These observations confirmed that iron oxalate was successfully loaded onto D301 resin.The XRD patterns of D301 and IO@D301 are illustrated in Fig.1(b).The main peak of D301 was observed at 2θ=19.7°,which correspond to intermolecular interactions of D301[39].The diffraction intensity of the main peak of IO@D301 was significantly increased than that of D301,indicating that the degree of crystallization of D301 after modification had been greatly improved.

The SEM images of D301 before loading iron oxalate have been reported in our research[40].After loading iron oxalate(Fig.1(c)and(d)),its surface was more rough than before loading,further confirming successful loading of the iron oxalate onto the resin,and this is good for Cr(VI)adsorption.

Fig.1.(a)The FT-IR spectra and(b)the XRD spectra of D301 and IO@D301,(c)and(d)the SEM images of IO@D301.

3.2.Effect of pH

A significant factor affecting Cr(VI) removal performance is pH,because it will affect the existing form of Cr(VI) ions in the solution and the surface charge of the adsorbent[41].The form of Cr(VI)exhibits different ionic species in different pH conditions.When 2.0

3.3.Effect of coexisting ions

It is known that any coexisting ions such as anions and metal ions in industrial wastewater can interfere with Cr(VI) adsorption [43].Therefore,the effect of ionic strengthon Cr(VI)removal by D301 and IO@D301 was investigated.As shown in Fig.2(b)and(c),the adsorption capacity of IO@D301 to Cr(VI)was higher than that of D301 whencoexisted,proving the introduction of iron oxalate enhanced the selectivity of D301.This is due to the complexes of chromate with Fe3+formed and the strong electrostatic attractions of amino and carboxyl to Cr(VI)[44].Moreover,for IO@D301 the Cl−suppressed the effect on Cr(VI)removal less thanThis is due to the more chargethe larger electrostatic attraction between the adsorbent and the anion,and the more competitive adsorption with Cr(VI) is,so the effect on Cr(VI) removal by the adsorbent is reduced[45].

Fig.2.(a)Cr(VI)removal by IO@D301 as a function of pH,(b),(c)and(d)Cr(VI)adsorption performance of D301 and IO@D301 in the presence of SO42−and Cl−.

Fig.3.(a)Adsorption isotherms of Cr(VI)onto IO@D301 at various temperatures.(b),(c)and(d)Fitting results for the Langmuir,Freundlich and Temkin isotherm model.

3.4.Adsorption isotherm

The effect of temperature on the uptake of Cr(VI)by IO@D301 was shown in Fig.3(a).With an increase in the temperature from 293 K to 313 K,the adsorption capacity of IO@D301 for Cr(VI) gradually decreased,indicating that the adsorption of Cr(VI)by IO@D301 is an exothermic process.A temperature of 293 K was selected for this study.It was reported that the adsorption capacity of D301 to Cr(VI)increased with the increase in temperature [46].Therefore,the temperature has obvious influence on the Cr(VI)adsorption of loaded iron oxalate.The adsorption capacity of loaded iron oxalate on surface of D301 to Cr(VI) increased with the decreasing of temperature.In addition,the uptake of Cr(VI) by IO@D301 increases as the initial concentration increases.An increase in the initial concentration of Cr(VI)results in an increase of free chromium ions,which makes it easier for the adsorbent to bind with chromium ions.

Langmuir,Freundlich,and Temkin isotherm models were used to correlate the adsorption experimental data of different temperatures(Fig.3 (b),(c) and (d)).The parameters of isotherm models were presented in Table 2.As shown in Table 2,the R2of the Freundlich model was higher than other models,indicating that the Freundlich model can better describe this adsorption process.The maximum adsorption capacity (qmax) of IO@D301 to Cr(VI) in this work was 201.30 mg·g−1at 293 K,which given by the Langmuir model.

3.5.Adsorption thermodynamics

The thermodynamic parameters including ΔG,ΔS and ΔH for the uptake of Cr(VI)by IO@D301 were calculated by Eqs.(5),(6)and(7),and the results were summarized in Table 3.

As shown in Table 3,the values of ΔG and ΔH were negative,which indicated that the adsorption process was spontaneous and exothermic.The values of ΔS were positive,indicating increased randomness at the solid–solution interface during chromium adsorption[47].

Table 3Thermodynamic parameters for Cr(VI)adsorption onto IO@D301

Fig.4.(a)Effect of contact time on Cr(VI)removal,(b)and(c)fitting results for the PFO and PSO kinetic model.(d)Cr(VI)adsorption performance of regenerated D301 and IO@D301.

3.6.Adsorption kinetics

Adsorption kinetics is a significant aspect of the adsorbent's performance [48].The effect of contact time on adsorption capacity is illustrated in Fig.4 (a),which shows that the adsorption rate increases very quickly during the first stage.With the accumulation of Cr(VI),the adsorption rate decreases slowly until it tends to reach equilibrium with Cr(VI) uptake of 77.16 and 122.19 mg·g−1for IO@D301 at 50 mg·L−1and 100 mg·L−1,respectively.This was probably due to the abundant active adsorption sites on the surface of IO@D301.As time increases,the concentration difference between the adsorbent and Cr(VI) gradually decreases,the driving force for mass transfer decreases,resulting in a decrease in the adsorption rate,and then the adsorption equilibrium is gradually reached.In addition,the adsorption capacity increases with the increase of the initial Cr (VI) concentration,because for a certain amount of adsorbent,the higher the concentration of Cr (VI) in the solution,the greater the concentration gradient,and the easier to be adsorbed.

The Cr(VI)adsorption kinetic data of IO@D301 were fitted by PFO(Fig.4 (b)),PSO (Fig.4 (c)) and WMID models.The resultant parameters and correlation coefficients were listed in Table 4.As shown in Table 4,the kinetic data fits better with the PFO model(R2＞0.932) than other two models (Table 4).It indicates that the adsorption of Cr(VI)on IO@D301 is more suitable for the PFO kinetic model[49].

Table 4Kinetic parameters for Cr(VI)adsorption onto IO@D301

3.7.Adsorption regeneration

Furthermore,the reusability of the D301 and IO@D301 adsorbents was considered in the experiments on Cr(VI)removal[50].As shown in Fig.4(d),the adsorption performance of D301 and IO@D301 for Cr(VI)gradually decreased with adsorbents recycle times increasing.For IO@D301,it may be caused by a small amount of iron oxalates desorbed from D301.After the seventh regeneration cycle,the removal efficiencies of D301 and IO@D301 were 23.7%and 42.5%,respectively[51].From the sixth to the seventh round,the adsorption capacity of D301 decreased sharply,but IO@D301 hardly changed.This may berelated to the enhancement of iron oxalate loaded on surface of D301.These results indicate that IO@D301 was easier to regenerate than D301.Therefore,IO@D301 can be regarded as an excellent adsorbent for the removal of Cr(VI).

Fig.5.Mechanism of chromium adsorption onto IO@D301 adsorbent.

From third to seventh cycle,the adsorption capacity of IO@D301 declined slightly and was obviously higher than that of D301.Therefore,iron oxalate loaded on D301 still contributes to Cr (VI) adsorption.Meanwhile,a small amount of iron oxalates may be desorbed from D301.

3.8.Adsorption mechanism

The mechanism of the reaction between IO@D301 and Cr(VI) is shown in Fig.5.The adsorption of Cr(VI) on IO@D301 was mainly chemical adsorption.The adsorption mechanism of Cr(VI) onto IO@D301 was supposed to include three components:(1) electrostatic attractions,(2) coordination bonding,and(3) reduction.The amino and carboxyl have strong electrostatic attractions to Cr(VI),the Fe3+of IO@D301 has intensive coordination interactions with the Cr(VI),and Cr(VI) can be reduced to Cr(III) by hydroxyl radicals in solution,which facilitate the removal of Cr(VI) from polluted water.

In order to further explore the adsorption mechanism,the FT-IR and XPS spectra of IO@D301 before and after adsorption were analyzed.The FT-IR spectra are shown in Fig.6(a).It can be seen that the intensity of amine group C--N bending vibration band (1268 cm−1) clearly declined after Cr(VI)adsorption[52,53].Moreover,the appearance of peaks at 808 cm−1and 942 cm−1confirmed that Cr(VI) was successfully adsorbed onto IO@D301 resin[54].

For further demonstration,XPS was used to analyze the chemical composition of IO@D301 before and after Cr(VI)adsorption.As shown in Fig.6 (b),compared with before adsorption,a peak at 578.08 ev appeared on IO@D301 after adsorption of Cr(VI)[55].

The C1s spectrum is shown in Fig.6(c).The C1s peak was divided into three peaks,which could be attributed to C--C/C=C at 283.74 eV,C--O at 285.12 eV,and C=O at 286.83 eV,respectively[56].After Cr(VI)adsorption,the peaks of C--C/C=C,C--O,and C=O decreased to 283.5,284,and 284.90 eV,which illustrated that the main factor is the combination of COOH−groups and Cr(VI)in the adsorption process.In theory,these protonated COOH−groups could participate in electrostatic interactions with metal ions[57,58].

The O1s spectrum is shown in Fig.6(d).It can be seen that the O1s spectrum of IO@D301 was divided into four peaks,at 530.20,531.60,532.80,and 534.10 eV,which were due to the Fe--O,C--O,C=O,and O=C--O,respectively [59].The change was not obvious after adsorption.

The Fe2p spectrum is shown in Fig.6(e).Characteristic Fe(II)and Fe(III)peaks were observed at about 711 and 725 eV,respectively[60].Thus,following the approach of Grosvenor,the Fe(2p 3/2) peak was fitted using four multiple peaks for Fe(II)and Fe(III)species,respectively.

The Cr2p analysis of the IO@D301 after the adsorption of Cr(VI)is shown in Fig.6(f).After adsorption,the Cr2p spectrum was divided into two peaks,Cr 2p 1/2(577.02 eV)and Cr2p 3/2(587.55 eV),both of which were separated into two doublet peaks after peak fitting[61,62].The peaks at binding energies of 577.68(2p 1/2)and 5589.36 eV (2p 3/2) were due to Cr(III).This indicates that the show that reduced of Cr(VI)was occurred on the surface of IO@D301.

4.Conclusions

In summary,a novel adsorbent for the removal of Cr(VI),IO@D301,was successfully prepared using the impregnation method.The IO@D301 adsorbent exhibited an excellent Cr(VI) adsorption capacity of 201.30 mg·g−1within 4 h at 293 K and pH 5.The Freundlich model fitted the equilibrium isotherm data well and the adsorption kinetic data were well described by the pseudo-firstorder model.This Cr(VI)-removal performance was superior to that of all other reported D301 resin adsorbents,which showed that the modified resin was an effective adsorbent for removal of Cr(VI)pollution.

Fig.6.(a)The FT-IR spectra of IO@D301 before and after Cr(VI)adsorption,(b)XPS spectra of IO@D301 before and after Cr(VI)adsorption,(c)C1s spectra before and after adsorption,(d)O1s spectra before and after adsorption,(e)Fe2p spectra before and after adsorption and(f)Cr2p spectra before and after adsorption.

Acknowledgements

The research has been funded by the National Key Research &Development Program of China(2017YFC0505904),National Natural Science Foundation of China(51808040),Taishan Scholars Program of Shandong Province.