Structure and properties of forming adsorbents prepared from different particle sizes of coal fly ash☆

Zhuannian Liu *,Yuan Liu

1 College of Geology and Environment,Xi'an University of Science and Technology,Xi'an 710054,China

2 School of Chemistry and Chemical Engineering,Xi'an University of Science and Technology,Xi'an 710054,China

Keywords:Coal fly ash Forming adsorbent Particle size Structure Methylene blue

ABSTRACT In this paper,different particle sizes of coal fly ash FA-R(D50=15.75 μm),FA-A(D50=3.61 μm)and FA-B(D50=1.73 μm)were treated with NaOH solution to prepare the forming adsorbents FFA-R,FFA-A and FFA-B.The structure and adsorption properties of the forming adsorbents for methylene blue(MB)from aqueous solution were examined.The results showed that the specific surface areas and adsorption capacities of the forming adsorbent for MB increased with decreasing particle size of raw coal fly ashes.The adsorption kinetic data of MB on FFA-R,FFA-A and FFA-B fitted the second-order kinetic model very well with the rate constants(k2)of 3.15×10?2,3.84×10?2 and 6.27×10?2 g·mg?1·min?1,respectively.The adsorption process was notonly controlled by intra-particle diffusion.The isotherms of MB on FFA-R,FFA-A and FFA-B can be described by the Langmuir isotherm and the Freundlich isotherm,and the adsorption processes were spontaneous and exothermic.

1.Introduction

The discharge of colored wastewater causes many significant environmental problems.Many physical-chemical methods have been tested to remove dye from aqueous solution,but only that of adsorption is considered superior to other techniques.Methylene blue is one kind of representative contaminant in industrial wastewater and shows poor biodegradability.Commercial activated carbon is a very effective adsorbent for removal of methylene blue from wastewater[1],but it is limited due to high costs.Some alternatives and low-cost adsorbents are used to remove methylene blue from aqueous solution,such as papaya seeds[2],mesoporous carbons[3],wood shavings[4],montmorillonite clay[5],natural zeolite[6],polyurethane foam[7], fly ash[8],beer brewery waste[9],palm kernel fibre[10],coal fly ash[11],cobalt-hectorite composite[12],grapheme[13],lotus leaf[14], NaOH-modified rejected tea[15],spinel magnesium aluminate nanoparticles[16], sericin biosorbent[17]and biomass[18].Coal fly ash is a solid waste from a coal- fired power plant.It was used as a low cost adsorbent to remove dyes from wastewater for its porous structure.Some methods were used to improve its adsorption capacities such as chemical modification[19]and heat treatment[20].Coal fly ash can be used to synthesize zeolite[21-23],synthesize molecular sieves[24]or develop other effective adsorbents[25,26].Our previous researches showed that the specific surface area and reactivity increase with decreasing particle size of coal fly ash[27].More smaller pores will be created when smaller coal fly ash particles closely packed together.Thus,the particle size will influence the pore structure and properties of the forming adsorbent.In order to explore the effect of particle size on the structure and adsorption capacity of the forming adsorbent from coal fly ash,three kinds of forming adsorbents were prepared from different particle sizes of coal fly ash.The structures of various adsorbents and their adsorption properties for methylene blue from aqueous solution were studied.

2.Materials and Methods

2.1.Materials

Raw coal fly ash(FA-R)was selected from a thermal power plant in Xi'an,China.The chemical composition of the raw coal fly ashes is shown in Table 1.As seen from Table 1,SiO2,Al2O3,Fe2O3and CaO are the main compositions of FA-R.FA-R was ball milled in a planetary high energy ball miller(Model BMP,China)for 3 h and 5 h and got ultra fine coal fly ashes FA-A and FA-B,respectively.The diameter distribution of coal fly ashes was shown in Fig.1 and the characteristic diameterof coal fly ashes is given in Table 2.As seen from Table 2,the average particle sizes(D50)of FA-R,FA-A and FA-B are 15.75,3.61 and 1.73 μm,respectively.Then,FA-R,FA-A and FA-B were reacted with 4.5 mol·L?1,350 ml NaOH solution in a stainless-steel beaker at 80 °C for3 h with stirring.After reaction,the mixture was extruded by a single screw extruder and cylindrical samples were obtained.These samples were dried at 110°C for 24 h,then washed thoroughly and oven dried.The forming adsorbents named FFA-R,FFA-A,and FFA-B were prepared.Methylene blue(MB)was used in the synthetic wastewater.All reagents used were of analytical reagent grade.

Table 1 Chemical composition of raw coal fly ash(%,by mass)

Fig.1.Size distribution of coal fly ashes.

Table 2 Characteristic diameter of coal fly ashes(μm)

2.2.Equipment

The chemical composition of raw coal fly ashes was analyzed by X-ray fluorescence(XRF)(PANalytical MiniPal-4,Phanake Ltd.,Netherlands).The particle size of coal fly ashes was measured by a laser particle size analyser(OMEC-LSPOP3-III model,China).X-ray diffraction(XRD)patterns were obtained with a model D/Max 2000PC diffractometer(Japan)using Cu Kαradiation at 40 kV and 40 mA.The SEM analyses were performed on a JSM-6460LV scanning electron microscope(Japan).The BET surface area and pore size were analyzed by Coulter SA 3100 surface area and pore size analyser(America).MB concentration was analyzed by a spectrophotometer(Model 724,China).

2.3.Kinetics studies

Batch studies were conducted in a temperature-controlled water bath shakerusing 100 ml(50 mg·L?1)of MB solution and a fixed adsorbent dosage of 1.0 g.The samples at different time intervals(10,30,60,90,120 and 150 min)were taken and filtered.The filtrates were analyzed for the concentration of MB left using a spectrophotometer.

2.4.Equilibrium experiments

Isotherm studies were conducted by a batch technique.100 ml MB concentrations of 50,100,200,400,600 and 800 mg·L?1solution were shaken with 1.0 g forming adsorbents at 298,308,and 318 K for 3 h using a temperature-controlled water bath shaker.At the end of a predetermined time,samples were filtered and the filtrates analyzed for residual MB.

3.Results and Discussion

3.1.Characterization

The SEM images of coal fly ashes and forming adsorbents were shown in Fig.2.It can be seen from Fig.2 that the particle size of FA-B is much smaller and more uniform than FA-A and FA-R[Fig.2(a-c)].After treated by NaOH solution,coal fly ash particles are deformed and changed into various shapes of crystals[Fig.2(d-i)].The XRD patterns of coal fly ashes and forming adsorbents are presented in Fig.3.As seen from Fig.3,the primary crystalline species in the raw coal fly ash sample are quartz(SiO2)and mullite(3Al2O3·2SiO2)as identified by the sharp peaks.The forming adsorbent was identified as a mixture of sodalite and indicated the conversion of coal fly ash into zeolite-like materials.The BET surface area,pore volume and pore size distribution of coal fly ash and forming adsorbents were given in Table 3.It can be seen from Table 3 that the BET surface areas of FFA-R,FFA-A and FFAB are 2.394,5.368 and 6.917 m2·g?1,respectively.

3.2.Kinetics studies

The kinetic model of MB adsorption onto forming adsorbents are shown in Fig.4.It can be seen from Fig.4 that the adsorbed amount of MB increases with contact time.The adsorptions are rapid for the first 30 min,and then proceeded at a slower rate until it finally reached equilibrium when contact time was about120 min.The adsorption capacity of MB onto forming adsorbent follows the order:FFA-B＞FFA-A＞FFA-R.When the particle size becomes smaller,more pores will be created and the pore diameter will become smaller,too.It can be seen from Table 3 that the specific surface area of forming adsorbents is increased from 2.394 m2·g?1(FFA-R)to 5.368 m2·g?1(FFA-A)and 6.917 m2·g?1(FFA-B),and the pore volume increases from 0.0033 ml·g?1(FFA-R)to 0.0045 ml·g?1(FFA-A)and 0.0072 ml·g?1(FFA-B)with decreasing average particle size(D50)of coal fly ashes.Higher specific surface areas,higher pore volume and more pores are beneficial to the adsorption process.

The kinetics of adsorption ofMB can be analyzed using the Lagergren first-order kinetic model and the pseudo second-order kinetic model for the determination of the rate constants[28,29].

A linear form of Lagergren first-order model is expressed as follows:

where qeand qt(mg·g?1)are the amounts adsorbed at equilibrium and time t(min),respectively,and k1(min?1)is the rate constant of the Lagergren first-order model.

The kinetic data are further analyzed using the pseudo second-order kinetics expressed as:

where k2(g·mg?1·min?1)is the rate constant of the pseudo secondorder model.If the second-order kinetics is applicable,then the plot of t/qtversus t should give a line relationship and there is no need to know any parameter beforehand.

Fig.2.SEM of coal fly ash and adsorbents.

Fig.3.XRD of coal fly ash and adsorbent.

Figs.5 and 6 are the Lagergren first-order and the pseudo secondorder plots for MB adsorption onto the forming adsorbents,respectively.Parameters obtained for the Lagergren first-order and the pseudo second-order kinetic models are presented in Table 4.As seen from Table 4,the regression correlation coefficients(R2)of MB onto FFA-R,FFA-A and FFA-B for the pseudo second-order kinetic model are all greater than the R2for the Lagergren first-order kinetic model.The data of equilibrium adsorbed amount calculated from the pseudo second-order kinetics(qe.c)are also close to the amounts adsorbed at equilibrium in experiments.This indicates that MB adsorptions onto the forming adsorbents obey the pseudo secondorder kinetics.The rate constants of the pseudo second-order model(k2)are 3.15 × 10?2,3.84 × 10?2and 6.27 × 10?2g·mg?1·min?1for FFA-R,FFA-A and FFA-B,respectively.

Further analysis of the adsorption kinetic data elucidated the diffusion mechanism.Transport from the bulk solution into the solid phase during diffusion is often the rate-limiting step in many adsorption processes.The intra-particle diffusion model based on the theory proposed by Weber and Morris was tested to identify the diffusion mechanism[30].The intra-particle diffusion model is represented as follows,where kidis the intra-particle diffusion rate constant(mg·g?1·min?1/2),and C is the intercept(mg·g?1).

Table 3 BET surface area,pore volume and pore size distribution of coal fly ash and forming adsorbents

Fig.4.Kinetic curve of MB adsorption onto forming adsorbents.

Fig.5.Lagergren first-order kinetics of MB onto forming adsorbents.

Fig.6.Pseudo second-order kinetics of MB onto forming adsorbents.

Fig.7.Plots qt vs.t1/2 of MB adsorption onto forming adsorbents.

The plots of qtversus t1/2are shown in Fig.7.The values of kidand C and the corresponding linear regression correlation coefficient R2are given in Table 5.As seen from Fig.7,a non-zero intercept of the regression line indicates that the adsorption process is not only controlled by intra-particle diffusion.From Tables 3 and 5,it can be seen that the proportion of pore size smaller than 10 nm increases from 50.7%(FFA-R)to 51.33%(FFA-A)and 57.14%(FFA-B)with decreasing particle size of coal fly ashes.The distance of adsorbate diffusion in pores becomes much longer and the intraparticle diffusion resistance increases with decreasing pore size.The intra-particle diffusion rate constants decrease from 0.158 mg·g?1·min?1/2(FFA-R)to 0.124 mg·g?1·min?1/2(FFA-A)and 0.107 mg·g?1·min?1/2(FFA-B),respectively.

Table 5 The intraparticle diffusion rate constants of MB adsorption onto forming adsorbents

3.3.Adsorption isotherm

In order to successfully represent the adsorption properties,the Langmuir and Freundlich adsorption isotherms were tested to fit the experimental data.The Langmuir isotherm model is given by Eq.(4).

where qeis the amount adsorbed at equilibrium(mg·g?1),Q0is the amount of monomolecular layer saturated adsorption(mg·g?1),Ceis the equilibrium concentration of solute(mg·L?1),and Q0and b are Langmuir constants related to adsorption capacity and energy,respectively.Q0and b can be calculated from the slope and intercept of the straight lines of the plot 1/qeversus 1/Ce.

The Freundlich isotherm model is represented by Eq.(5).

Table 4 Lagergren first-order and pseudo second-order adsorption rate constants of MB onto forming adsorbents

where KFand n are constants related to temperature and adsorbent specific surface area,which can be determined from the liner plot of lg qeversus lg Ce.The Freundlich adsorption constant(n)should be in a range of 1-10 for beneficial adsorption.

The adsorption isotherms for MB onto the forming adsorbents at 298,308 and 318 K are given in Fig.8.The Langmuir and Freundlich isotherm constants are listed in Table 6.As seen from Table 6,the good regression correlation coefficients are obtained for all adsorbents at different temperatures,suggesting that both the Langmuir and Freundlich isotherms are applicable to describe MB adsorption equilibrium.The amount of monomolecular layer saturated adsorption values(Q0)decreases with increasing temperature and confirms the exothermic nature of this adsorption process.In all cases,the Freundlich exponent 1＜n＜10 indicates favorable adsorption.

Fig.8.Isotherms of MB adsorption onto forming adsorbents.

The essential characteristics of a Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor or equilibrium parameter,RL,which is defined by Eq.(6).

where b is Langmuir constant and C0is the highest initial concentration of MB.The value of RLindicates the type of the isotherm to be either unfavorable(RL＞1),linear(RL=1),favorable(0＜RL＜1)or irreversible(RL=0)[31].The values of RLare also given in Table 6.As can be seen from Table 6,0＜RL＜1 for all forming adsorbents at different temperatures and indicates favorable adsorption for MB[32].

Table 6 Langmuir and Freundlich isotherm constants of MB adsorption onto forming adsorbents

The changes of free energy(ΔG0),enthalpy(ΔH0)and entropy(ΔS0)of MB adsorbed onto forming adsorbents can be calculated by Eqs.(7)and(8)[33].

where R is the gas constant(8.314 J·mol?1·K?1),T is the absolute temperature(K) ΔG0is the change of free energy(J·mol?1),b(L·mol?1)is the Langmuir constant,ΔH0is the change of enthalpy(J·mol?1),and ΔS0is the change of entropy(J·mol?1·K?1).Thus,a plot of ln b vs.1/T should be a straight line.ΔH0and ΔS0were obtained from the slope and intercept of this plot,respectively.

The ln b vs.1/T plots of MB onto forming adsorbents were presented in Fig.9.ΔG0,ΔH0and ΔS0valves are given in Table 7.The estimated values of ΔG0for adsorption MB were negative at different temperatures and indicated that a spontaneous process occurred.The negative value of ΔH0(?5.90,?2.70 and ?0.97 kJ·mol?1for FFA-R,FFA-A and FFA-B respectively)indicates that the adsorption of MB onto forming adsorbents is an exothermic process.

Fig.9.ln b vs.T?1 of MB adsorption onto forming adsorbents.

Table 7 Thermodynamics parameter of MB adsorption onto forming adsorbents

4.Conclusions

The structure and adsorption properties for MB adsorption onto the forming adsorbents FFA-R,FFA-A and FFA-B from different particle sizes of coal fly ashes FF-R,FF-A and FA-B with average particle sizes(D50)of 15.75,3.61 and 1.73 μm,respectively,were studied in this paper.The BET surface areas of FFA-R,FFA-A and FFA-B were 2.394,5.368 and 6.917 m2·g?1,respectively.The adsorption capacities of the forming adsorbents were in the order of FFA-R＜FFA-A＜FFA-B.Adsorption kinetic data were described by the pseudo-second-order equation.The adsorption process for MB onto the forming adsorbents can be described in terms ofboth the Langmuir and Freundlich isotherms,indicating that it is a spontaneous and exothermic process.The adsorption process is not only controlled by intra-particle diffusion.