Jianhua Chen *,Xue Sun ,Lijing Lin ,Xinfei Dong ,Yasan He

1 College of Chemistry and Environmental,Minnan Normal University,Zhangzhou 363000,China

2 Fujian Province University Key Laboratory of Modern Analytical Science and Separation Technology,Minnan Normal University,Zhangzhou 363000,China

1.Introduction

Phenols,especially nitrophenols,are classified as extremely toxic to human beings and to all aquatic life.These compounds exist in petrochemicals,olive oilfactories,photographic developers,oilre fining,plastics,leather,paints,pharmaceuticals,steel and other industry's waste waters and must be removed to meet the actual environmental regulations[1,2].Due to the high toxicity and ability to be accumulated in the human body,nitrophenols are listed as the main cause of cancercausing pollutants.Because of high chemical toxicity and difficult biodegradation,nitro compounds caused serious pollution to the environment.o-Nitrophenol(ONP)andp-nitrophenol(PNP)especially are designated as priority pollutants.Exposure to these substances,through inhalation,ingestion,eye or skin contact,and adsorption through the skin can cause skin and eye burns.Therefore,it is important to remove and recover these compounds from effluents before safely disposing into the surface waters.In the meantime,the pollution of nitrophenols is a prototype poison.Therefore,effective methods should be explored to solve the environmental pollution problem.Recently,different methods have been developed for the removal of nitrophenols from sewage and industrial waste waters,such as biodegradation process[3],denitrification process[4],membrane process[5],photocatalytic degradation[6-9],Fenton process[10,11],chemical and electrochemical techniques[12],and adsorption[13-16].Despite the availability of the above processes,the adsorption method is usually selected because of its high adsorption efficiency,ease of operation,and the availability of different adsorbents.However,the adsorbing capacity or selectivity is often unsatisfactory.As a result,we need to explore high adsorption efficiency material to solve the problem mentioned above.

Recently,metal-organic frameworks(MOFs),crystalline porous materials that belong to organic-inorganic hybrid compounds,are composed of metalions or metaloxide clusters and organic bridging ligands to construct variety of frameworks by selecting and combining building units[17].As a result,they have obtained great attention due to their large surface areas,low framework densities,and high pore volumes relative to other porous matrices[18].They have significant potential for use in a variety of applications including storage of gases[19],heterogeneous catalysis and selective gas separations(CO2/CH4)[20].However,only a few MOFs,such as MIL-101[21],ZIF-8[22],MOF-808[23],and MIL-96[24]are stable in water.As a result,the use of MOFs for water environmental pollutants treatment is rare[25].

In this study,we synthesized chromium(III)terephthalic acid(Cr-BDC)with large specific surface area(4128 m2·g-1).Its good thermal stability,solvent stability in water,π-π interactions/stacking,hydrogen bonding forming property,and pore/size-selective adsorption property[26],give it great promise for water treatment.The Cr-BDC was characterized by scanning electron microscopy(SEM),transmission electron microscope(TEM),X-ray diffraction(XRD),and Fourier transformed infrared spectroscopy(FT-IR),and the specific surface area and pore size were analyzed.Meanwhile,the Cr-BDC was used in adsorbing removal of ONP and PNP from aqueous solution.The results indicated that the Cr-BDC showed excellent adsorption capacity for ONP,which is more than ten times as much as that for PNP.

2.Experimental

2.1.Materials

Chromium(III)nitrate nonahydrate(Cr(NO3)3·9H2O),sodium hydroxide,and sodium acetate were purchased from West Long Chemical Co.,Ltd.Terephthalic acid ando-nitrophenol were obtained from Aladdin Corp(Shanghai,China).p-Nitrophenol,hydrochloric acid and ethanol were sourced from Medicines Reagents.All of them are of analytical grade and used without further purification.

2.2.Preparation of the Cr-BDC porous material

2.2.1.Synthesis of the Cr-BDC

The preparation of Cr-BDC particles follows as:4 g chromic(III)nitrate nonahydrate and 1.64 g terephthalic acid were added into 50 ml 0.05 mol·L-1sodium acetate solution under stirring,and a dark-green turbid mixture was acquired.Then the mixture was put into a 125 ml stainless steel autoclave with a polytetra fluoroethylene liner.The autoclave was heated at a heating rate of 4-5 °C·min-1from room temperature to 170°C and then held for 24 h.After natural cooling for 12 h,the particles were separated from the mixture by filtration with 0.4 μm membrane.

2.2.2.Subsequent purification of the Cr-BDC

The above samples underwent the subsequent process following these steps:the product was dispersed in 100 ml ethanol and sonicated for 30 min to remove the terephthalic acid.Then the final product was obtained by centrifugation.Finally,the product was dried under vacuum at 100°C over the night.

2.3.Characterization of the Cr-BDC

The chem-physical properties of the sample were determined by XRD(Bruker D8 Advance with CuKαradiation)and FT-IR(Nicolet 8700).The morphologies of the sample were observed by SEM(HITACHI S-4800),EDS and TEM(FEI Tecnai G20).The specific surface area and pore size distribution of sample were obtained by Brunauer Emmett Teller(BET,Micromeritics Gemini 2390T).

2.4.Adsorption experiments

The nitrophenol solutions were prepared in the concentration range of 100-500 mg·L-1in distilled water.For each adsorption equilibration experiment,50 mg of the adsorbent was added to 250 ml of solution(e.g.,nitrophenol concentrations of 100,200,300,400 and 500 mg·L-1)and shaken at150 r·min-1for24 h at35 °C with a thermostatic oscillator.The relationship between adsorption amount and temperature was determined using 0.2 g·L-1of adsorbent,100 mg·L-1nitrophenol solution at different temperatures(35,40,50,60°C).The pH of the solutions was adjusted using 1.0 mol·L-1HCl and/or NaOH solutions.Adsorption kinetics was carried out by withdrawing 5 ml of sample from the solution at regular intervals.The concentration of ONP and PNP was analyzed by UV spectrophotometer(UV-7502)at 400 nm and 414 nm,respectively[27].The amount of the adsorbed ONP and PNP onto Cr-BDC was calculated using the following equation:

whereqeis adsorbed amount of the adsorbate on the adsorbents(mg·g-1);C0andCeare the initial and equilibrium concentration of the adsorbate(mg·L-1),respectively;Vis the solution volume(ml)andmis the adsorbents dosage(g).

3.Results and Discussion

3.1.Properties of adsorbents

3.1.1.X-ray diffraction(XRD)

Fig.1 shows the XRD patterns of the as-synthesized Cr-BDC.The X-ray diffraction pattern showed that as-synthesized Cr-BDC produced many diffraction peaks(2.80°,3.20°,3.936°,5.791°,8.702°,17.305°),which matched well with the previously reported[28].Thus,the as-synthesized Cr-BDC particles were well-crystallized materials.

Fig.1.XRD patterns of Cr-BDC.

3.1.2.Scanning electron microscope(SEM)and transmission electron microscope(TEM)analysis

The morphology of the samples was further examined by SEM and TEM,and was shown in Fig.2(a)and(b),respectively.It can be found from Fig.2(b)that the as-prepared Cr-BDC particles are octahedral structure with a size of about 120 nm,which was smaller than that presented in previous literature[29,30].

3.1.3.Specific surface areas and pore size distributions

The pore structure and specific surface area of the as-prepared Cr-BDC were analyzed on the basis of nitrogen adsorption-desorption isotherms.As can be seen in Fig.3,the adsorption type of Cr-BDC is a Langmuir mono molecular layer adsorption,type-I.It can be seen that there are two different sizes of pore size.In the initial stage,with the increase ofP/P0,adsorption capacity of the Cr-BDC increased dramatically,indicating that there are a lot of microporous structures.Because of the two different sizes of cage-like structures of the material itself,there are two different slopes at lowrelative pressure between 0.05 and 0.22.The graph rises slowly at theP/P0range from 0.23 to 0.9,indicating that the material is gradually reaching saturation.The graph sharply rises at the time ofP/P0=1.0,which can be attributed to the aggregation of N2occurring in the voids originated from the accumulation of material.As can be seen from pore size distributions in Fig.3(b),pore sizes are concentrated in 1 nm and the specific surface area of Cr-BDC is 4128 m2·g-1.

Fig.2.(a)SEM image of Cr-BDC;(b)TEM image of Cr-BDC.

Fig.3.(a)N2 adsorption analysis the BET surface area of Cr-BDC;(b)pore size distribution of Cr-BDC.

3.1.4.Fourier transformed infrared spectrum(FT-IR)

The chemical property of Cr-BDC was studied by FT-IR spectroscopy,and was shown in Fig.4.The bands are presented in the vicinity of 1620 cm-1due to the skeleton of-O-C-O-vibration.The bands corresponding to the benzene ring can be seen in the spectral region of 1110-750 cm-1[31].The bands corresponding to stretching COO-vibrations appeared at 600 cm-1.

Fig.4.FT-IR spectra of Cr-BDC.

3.2.Adsorption test

3.2.1.The effect of pH

The effect of solution pH on the adsorption performance was investigated by keeping the solution pH in the range of 2.0 to 7.0,adsorbent concentration of 0.20 g·L-1and the origin ONP and PNP concentration of100 mg·L-1respectively.As seen in Fig.5,the effect of solution pHon adsorption efficiency is slight.The adsorption capacity of Cr-BDC for ONP or PNP depends on a number of factors such as the functional groups on the Cr-BDC,the nature of the adsorbate(its solubility,pKa,functional groups)and the solution conditions(pH,ionic strength,adsorbate concentration,oxygen availability).The possible interactions between the Cr-BDC surface and ONP or PNP are as following:(a)electron donor-acceptor interactions between the aromatic phenolic ring and the basic surface oxygen,such as carbonyl groups on the Cr-BDC[32];(b)strong π-π interaction formed between the Cr-BDC and ONP or PNP(In preparation of the Cr-BDC sample,terephthalic acid was used as a host material,which introduced a lot of benzene rings into the skeleton of the Cr-BDC structure.)[33];(c)electrostatic attraction and repulsion when ions are present.

The pHzpcof the Cr-BDC is 6.58,at which the adsorbent is neutral.The surface charge of the Cr-BDC is positive when the solution pH is below the pHzpcvalue while it is negative at a pH over the pHzpc.On the other hand,when the solution pH is below the pHzpcvalue,ONP(pKa7.22)and PNP(pKa7.15)are undissociated.Therefore,the effect of solution pH on adsorption capacity of the Cr-BDC for ONP or PNP is weak.When the solution pH is over the pHzpc,the surface of Cr-BDC is negative.Meanwhile,ONP or PNP began to dissociate and existed as anions.Therefore the negative charges(-COO-)on the Cr-BDC surface would result in electrostatic repel for dissociated ONP or PNP.As a result,the further experiments were performed under the original solution pH value(pH=6.0).

From Fig.5,one also can find that adsorption capacity of Cr-BDC for ONP is significant higher than that for PNP.This can be explained that ONP can form intra-molecular hydrogen bond between O-of the nitro group and the H of the hydroxyl group while PNP can form inter-molecular hydrogen bond,the latter is perhaps more easily influenced by the solution acidity[33].The solubility of nitrophenols also plays an important role in the adsorption[14].The solubility of ONP is less than that of PNP.This confirms that more-favorable adsorption involves the compound presenting a lower solubility in water.

Fig.5.Effect of solution pH on adsorption capacity:ONP and PNP initial concentration 100 mg·L-1,adsorbent dosage 0.2 g·L-1,temperature 308 K,stirring speed 150 r·min-1,contact time 24 h.

3.2.2.Thermodynamic study

The effects of temperature on the adsorption capacities ofCr-BDC for ONP and PNP were carried out and the results indicated that the adsorption capacity increased slowly with a higher temperature.The increase in ONP and PNP adsorption capacity of Cr-BTC with higher temperature indicated the endothermic nature of the adsorption process.

The thermodynamic model and the thermodynamic parameters(ΔH0,ΔS0,and ΔG0)can be calculated using the following equations:

Ris the universal gas constant(8.314 J·mol-1·K-1)andTis the absolute temperature(in Kelvin).Thermodynamic parameters were calculated according to Eqs.(2)-(4).As shown in Table 1,the negativeΔG0values indicate that the adsorption process is spontaneous,and the values decrease with an increase of temperature,indicating a better adsorption performance is obtained athigher temperature.The positive value of ΔH0confirms the endothermic nature of the overall adsorption process and the positive values of ΔS0suggest the affinity of the as prepared Cr-BDC for ONP and PNP as well as an increasing randomness at the solid/solution interface during the adsorption process.

Table 1Parameters obtained from different kinetic models

3.2.3.Adsorption isotherms

An adsorption isotherm expresses the relationship between the amount of adsorbate adsorbed per unit mass of adsorbent(qe,mg·g-1)and the concentrations of adsorbate in the bulk solution(Ce,mg·L-1)at a given temperature under equilibrium conditions.

Fig.6 shows adsorption isotherms of the Cr-BDC.To determine the parameters associated with ONP and PNP adsorption,the experimental data were analyzed using the Langmuir and Freundlich adsorption isotherm models,respectively.The Langmuir model assumes uniformenergies of adsorptive site on the surface of the adsorbent and no transmigration of adsorbate in the plane of the surface.The Freundlich isotherm is an empirical isotherm that can be used for non-ideal adsorption that is multilayer adsorption.Both of them are represented by the following equations Eqs.(5)and(6).

Fig.6.Adsorption isotherms of NOP and PNP on Cr-BDC composite:adsorbent dosage 0.2 g·L-1,temperature 303 K,stirring speed 150 r·min-1,contact time 24 h.

whereqeis the adsorption amount of ONP and PNP on adsorbent(mg·g-1)at an equilibrium state;qmaxis the maximum adsorption capacity of metals on adsorbent(mg·g-1);Ceis the equilibrium concentration of metals(mg·g-1);k1is the Langmuiradsorption constant,which is related to the adsorption energy;kfandnare Freundlich constants related to adsorption capacity and adsorption intensity,respectively.A linear fitting was applied to obtain the Langmuir and Freundlich isotherms parameters,as shown in Fig.7,and the relevant parameters are presented in Table 2.Taking into consideration the values of the correlation coefficient as a criterion for goodness of fit for the system,the Langmuir model shows less correlation than that of the Freundlich model,which indicated that ONP and PNP adsorbed onto the Cr-BDC are multilayer coverages.

Comparingqmaxvalues regarding ONP and PNP adsorption capacity of Cr-BTC with that of other adsorbents presented in Table 3,one can observe that Cr-BTC presents fairly high adsorption capacity of the reported materials.

Fig.7.Adsorption isotherm model:(a)The Langmuir isotherm;(b)The Freundlich isotherm.

Table 2Results and parameters associated with Langmuirand Freundlich models for ONP and PNP on Cr-BDC

Table 3Comparision of Maximum Adsorption Capacity of ONP and PNP on Various Adsorption.

3.2.4.Adsorption kinetics

The kinetics of adsorption plays an important role,from the point of view that it controls the process efficiency.Adsorption dynamics were investigated at a solution concentration of 100 mg·L-1and 35 °C.The experiment results indicate that adsorption rate was very fast.In just a few minutes(5 min),the adsorption capacity attainedqe-ONP=163 mg·g-1andqe-PNP=5 mg·g-1.This may be attributed to the large specific surface area of Cr-BDC,which plays an important part in this result.

In order to examine the adsorption mechanism of the Cr-BDC for nitrophenols,such as mass transfer and chemicalreaction,we examined

the Lagergren pseudo- first-order kinetic model,pseudo-second-order kinetic model and the intra-particular diffusion model[36]:

whereqeandqtdenote the amounts of adsorption atequilibrium and at timet(mg·g-1),k1andk2are the firstorder and second order rate constants(min-1),respectively.According to the kinetic Formulas(7)and(8),the experimental data were fitted and the linear plot oft/qtversus tyields a much high correlation coefficient than that of the linear plot of ln(qe-qt)versus t.This indicated that the pseudo-second-order model can better describe the adsorption process of ONP and PNP onto the Cr-BDC samples.

As the above two kinetic models were not able to explain the diffusion mechanism,therefore,the intra-particle diffusion kinetic model based on the equation proposed by Weber and Morris was tested.This empirical equation assumes that the adsorbate uptake by the adsorbent varies almost proportionally witht1/2rather than with the contact timet.According to the following Weber-Morris's equation[37]:

whereqtis the amounts of adsorption at timet(mg·g-1),andkdifis the intra-particular diffusion rate constant(mg·g-1·min-1/2).Ciis the intercept of stagei,giving an information about the thickness of the boundary layer;that is,the larger the intercept,the greater the boundary layer effect.This model is significant to know the rate determining step in the liquid adsorption systems.

The relation plots ofqtversustimet1/2are shown in Fig.8.It indicated that the plots did not go through the origin and included three stages[38,39]:The first portion of curve represents external surface adsorption or an instantaneous adsorption stage.It shows a fast initial uptake rate and accounts for approximately 79%of the overall adsorption capacity.This could be attributed to the large concentration gradient and availability of surface sites on Cr-BDC in the initial stage.The second stage is a gradual adsorption stage,which can be attributed to intra-particle diffusion as well as reduced surface sites on Cr-BDC.The third portion is a final equilibrium stage where the intra-particle diffusion starts to decelerate due to extremely low solute concentrations in the solution.

3.2.5.Reusability studies

Fig.8.The intraparticle diffusion model.(ONP and PNP initial concentration 100 mg·L-1,adsorbent dosage 0.2 g·L-1,stirring speed 150 r·min-1,contact time 24 h.)

Reusability was a key factor for an effective absorbent.The adsorbents with excellent reusability can reduce the adsorbent cost greatly,which was very important for industrial applications.In this study,the solvent elution was adopted and the nitrophenol-loaded Cr-BDC was regenerated with sodium hydroxide solution.The effects of the regeneration times on adsorption capacity and selectivity are shown in Fig.9.It can be found under 3 cycles of desorption-adsorption;the as-prepared Cr-BDC still remains a high adsorption capacity and good selectivity.

Fig.9.Adsorption-desorption cycles.(ONP and PNP initial concentration 100 mg·L-1,adsorbent dosage 0.2 g·L-1,stirring speed 150 r·min-1,contact time 24 h).

4.Conclusions

To remove ONP and PNP from wastewater,a novel adsorbent Cr-BDC was prepared.BET results indicated that the as-prepared Cr-BDC gets a large specific surface area of4128 m2·g-1,and pore sizes are concentrated in 1 nm,which is of benefit for using for wastewater treatment.Batch adsorption results indicated that the adsorption rate of ONP and PNP onto the Cr-BDC is rapid.The adsorption capacity of the Cr-BDC for ONP(310.0 mg·g-1)is excellent,and the adsorption capacity of the Cr-BDC for ONP is significantly higher than that for PNP.These results can be attributed to the fact that ONP can form the intramolecular hydrogen bonds,while PNP can only form the intermoleularhydrogen bonds.Also,solubility of nitrophenols in water is also an important factor affecting adsorption capacity.The adsorption kinetics of ONP and PNP on the Cr-BDC samples follow the pseudo-second-order kinetic model.Experiment results suggest that the as-prepared Cr-BDC could be used as an efficient adsorbent for removal of ONP from waste waters or separation of ONP/PNP mixtures.

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