Zhibin Ma,Xueli Zhang,Guangjun Lu,Yanxia Guo,Huiping Song,Fangqin Cheng

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

Keywords:Circulating fluidized bed (CFB) coal fly ash Hydrothermal treatment Zeolite Pb2+Adsorption Waste treatment

ABSTRACT The utilization of coal fly ash derived from circulating fluidized bed combustion (CFBFA) still faces great challenges because of its unique characteristics.In this study,a zeolitic material with Na-P1 zeolite as the main phase was successfully synthesized via a hydrothermal method by using CFBFA as the raw material.The effects of hydrothermal temperature,time,and added CTAB amount on the characterizations of synthesized materials were investigated by XRD,SEM,and XPS.The properties of the optimal zeolitic material and its adsorption performance for Pb2+ in aqueous solution were evaluated.The influences of pH,initial concentration,dosage,and temperature on Pb2+ adsorption were also examined.Results revealed the following optimal parameters for the synthesis of zeolitic material:NaOH concentration of 2 mol·L-1,solid-to-liquid ratio of 1:10 g·ml-1,hydrothermal temperature of 110 °C,hydrothermal time of 9 h,and CTAB amount of 1 g (per 100 ml solution).The adsorption capacities of the zeolitic material reached 329.67,424.69,and 542.22 mg·g-1 when the pH values of aqueous solution were 5,6,and 7,respectively.The Pb2+removal efficiency can reach more than 99% in aqueous solution with the initial concentrations of 100-300 mg·L-1 under pH 6 and suitable adsorbent dosage.The adsorption and kinetics of Pb2+on the zeolitic material can be described by Langmuir isotherm and pseudo-second-order kinetic models,respectively.The ion exchange between Pb2+and Na+and chemisorption are the main adsorption mechanism.All these findings imply that the synthesis of low-cost adsorbent for Pb2+removal from weak acid and neutral aqueous solution provides a highly effective method to utilize CFBFA.

1.Introduction

As a new generation of clean and efficient solid fuel-fired power generation technology,circulating fluidized bed (CFB) combustion has attracted the attention of researchers and companies worldwide due to its many advantages,such as excellent fuel flexibility,cost-effectiveness in emission control,low-investment equipment,and good capability in scaled up capacity[1,2].Owing to the urgent demand to massively burn low-quality fuels and solid wastes,such as inferior coal,coal gangue,and coal sludge,more than 1000 CFB boilers have been employed by coal-fired power plants in the main coal-producing areas of China;consequently,approximately 120 million tons of coal fly ash derived from CFB boiler(CFBFA)are discharged each year[3].However,the utilization rate of CFBFA is currently low because of its unique characteristics.Compared with the ordinary coal fly ash derived from pulverized coal furnace (PCFA),CFBFA has high contents of anhydrite (CaSO4),free calcium oxide(CaO),and residual carbon [4,5].The excessive Ca-containing phases negatively affect the stability of building materials,thus greatly limiting the large-scale application of CFBFA in the building field [6,7].Large amounts of unused CFBFA are still dumped into ponds or stacked on the land,leading to serious environmental problems.Therefore,its utilization should be prioritized.

Metallic lead(Pb)and its compounds are widely used in battery production,machinery manufacturing,metallurgy,tetraethyllead manufacturing,chemical industries,and other fields [8].Although Pb brings good products to human beings,it also causes heavy metal pollution to environment.A large amount of Pb2+-containing wastewater is annually generated worldwide [9].As a heavy metal ion with high toxicity,Pb2+exposure can seriously damage human health,such as neurotoxicity,cardiovascular problems,kidney damage,and hormonal imbalances [10].Therefore,this element must be removed from wastewater to block its entry into the environment.Adsorption is a simple,cost-effective,and environmentally friendly method for heavy metal ion removal from wastewater [11].Zeolitic material is an ideal adsorbent for removing heavy metal ions from wastewater due to its excellent cation exchange capacity,negatively charged surface,thermal stability,and high affinity to heavy metal ions[12,13].Another attractive feature is that zeolitic adsorbent can be synthesized from various industrial solid wastes,such as,coal fly ash,coal gangue,biomass ashes,alum sludge,and bauxite residue [14,15].A meaningful practice is to convert CFBFA into zeolitic adsorbent for the highly efficient Pb2+removal from wastewater.

Various zeolitic materials have been synthesized using CFBFA as the raw material.Chareonpanichet al.[16] synthesized Na-A zeolite from CFBFA containing high crystalline silica contentviaan alkali fusion hydrothermal method.The mixture of ash and sodium carbonate(Na2CO3)was sintered at 900°C for 1 h to obtain the soluble sodium silicate (Na2SiO3),and zeolite A was then hydrothermally prepared from Na2SiO3and sodium aluminate (NaAlO2) at 120 °C for 4 h.Grelaet al.[17] investigated the effect of synthesis methods on the characteristics of zeolite prepared from CFBFA.The mixture of CFBFA and NaOH powder at a mass ratio of 0.83 was sintered at 550 °C for 4 h using alkali fusion,and the mixture of CFBFA and NaOH solution (3 mol·L-1) at a solid-to-liquid ratio of 1:10 g·ml-1was stored for 30 days (shaken every day) at 21 °C and processed using low-temperature synthesis.They found that sodalite was mainly generatedviaalkali fusion synthesis,and Na-X zeolite was obtained by low-temperature synthesis.The BET specific surface area of Na-X zeolite was larger than that of sodalite.Liuet al.[18] synthesized Na-A zeolite with high crystallinity from ultra-fine CFBFA by one-step hydrothermal method and discussed the effects of synthesis conditions on the crystallinity and purity of the product.The Na-A zeolite with the highest crystallinity of 97.9%was obtained under the following optimal conditions:NaOH solution concentration of 2.6 mol·L-1,crystallization temperature of 90 °C,crystallization time of 6 h.They found that the Na-A zeolite was under the metastable phase and easily transformed to sodalite and Na-P zeolite under high NaOH concentration and temperature and long crystallization time.Qiuet al.[19] modified CFBFA using NaOH solution (2 mol·L-1) at 100°C for 24 h and used it to remove Cd2+from aqueous solution.The results showed that some zeolites were generated on the particle surface,and the Cd2+in wastewater was removed through ion exchange and chemical precipitation.The maximum removal capacity reached 183.7 mg·g-1.Koukouzaset al.[20]treated CFBFA using NaOH solution(1 mol·L-1)at 90°C for 24 h,and the obtained zeolitic materials can thoroughly remove heavy metal ions including Cr,Pb,Ni,Cu,Cd,and Zn in wastewater,but the adsorbent dosage was as large as 200 g·L-1.To date,most studies on zeolite synthesis have used PCFA as the raw material.Only a few works on the synthesis of zeolitic adsorbents for heavy metal ion removal have employed CFBFA as the raw material.These two kinds of fly ash have quite different characteristics.For example,the PCFA particles are spherical,whereas the CFBFA particles are irregular in shape.The phases in PCFA are mainly mullite,quartz,corundum,and fused glass [21],and those in CFBFA mainly contain quartz,anhydrite,and unfused amorphous aluminosilicates [22].Therefore,the effects of synthesis conditions on the product phase and adsorption performance for heavy metal ions when using CFBFA as the raw material must be investigated.In addition,the Pb2+removal efficiency from aqueous solution by CFBFA-based zeolitic materials is rarely reported.

This study aims to synthesize a zeolitic adsorbentviaa hydrothermal method for the highly effective removal of Pb2+from aqueous solution by using CFBFA raw material under moderate conditions.The effects of hydrothermal temperature,time,and the added amount of hexadecyl trimethyl ammonium bromide(CTAB)as the modifier on the phase and micromorphology of zeolitic adsorbent were examined.The adsorption capacity of the adsorbent for Pb2+removal from aqueous solution was evaluated under different solution pH values,initial concentrations,adsorbent dosages,and temperatures.Adsorption isotherms and kinetics were also investigated.The adsorption mechanism of the synthesized adsorbent for Pb2+removal was proposed.The results provide theoretical guidance and technical support for the synthesis of low-cost zeolitic adsorbent using CFBFA as the raw material.Furthermore,this work is conductive to CFBFA recycling.

2.Materials and Methods

2.1.Materials

The CFBFA used in this study was obtained from the Pingshuo Coal Gangue Power Plant in Shanxi Province,China.The chemical reagents include hydrochloric acid (HCl,Beijing Chemical Works,36% -38 % (mass)),sodium hydroxide powder (NaOH,MACKLIN reagent,GR),hexadecyl trimethyl ammonium bromide(CTAB,C19-H42BrN,Sinopharm Chemical Reagent Co.,Ltd.,AR),and lead nitrate (Pb (NO3)2,Tianjin Fengchuan Chemical Reagent Technology Co.LTD,AR).

2.2.Synthesis of zeolitic material

Raw CFBFA was first pretreated by HCl solution to remove CaSO4,CaO,Fe2O3,and MgO.The ash was mixed well with 4.41 mol·L-1HCl solution at a liquid-to-solid ratio of 5 ml·g-1in a 250 ml three-necked,round-bottomed flask with a reflux condenser flask.The mixture was stirred at a constant rate of 300 r·min-1for 2 h in a water bath at 90 °C and filtered to obtain the pretreated CFBFA,which was washed repeatedly with deionized water to neutrality.In brief,10 g of the pretreated CFBFA,100 ml of NaOH solution(2 mol·L-1),and a certain amount of CTAB were mixed well in a 250 ml Teflon flask.The mixture was continuously stirred at 300 r·min-1for different times in an oil bath at a pre-set temperature and then vacuum filtrated to obtain the synthesized zeolitic material.Table 1 lists the detailed synthesis conditions.The products were fully washed with deionized water until pH 8 and dried at 105 °C for 6 h.

Table 1 Conditions for synthesis of zeolitic materials

2.3.Material characterization

X-ray fluorescence(XRF,S8 Tiger,Bruker,Germany)was used to measure the main chemical compositions of CFBFA and pretreated CFBFA.Powder X-ray diffraction (XRD,D2,Bruker,Germany) with Cu Kα radiation was carried out to determine the minerals in the specimens,which were scanned with a 2θ step size of 0.02° in the range of 10°-80°.The morphology and regional composition of the synthesized sample was detected by a scanning electron microscope with an energy-disperse X-ray spectrometer (SEMEDS,JSM-500HR,JEOL,Japan).Magic angle spinning nuclear magnetic resonance (MAS-NMR) analysis of27Al and29Si was performed on a 600 MHz wide bore spectrometer (BRUKER,AVANCEIII 14.2 T)to investigate the coordination of Al and Si in the adsorbent.N2adsorption was measured using the Accelerated Surface Area and Porosimetry System (Micrometric,ASAP 2460) at 77 K.The specific surface area and pore size distribution of the zeolitic adsorbent were determined using the Brunauer-Emmett-Teller(BET) method of multilayered adsorption.The valence states of surface elements of zeolitic adsorbent before and after adsorption were analyzed by X-ray Photoelectron Spectroscopy(XPS,K-Alpha+,Thermo scientific,America)with Al Kα source as X-ray excitation source.The concentration of Pb2+in aqueous solution was determined by a flame atomic absorption spectrophotometer (FAAS,TAS-990,PGENERAL,China).

2.4.Pb2+ adsorption experiments

Batch experiments were conducted to investigate the adsorption capacity of zeolitic material for Pb2+in aqueous solution.The Pb2+-containing solutions with various initial concentrations(50-500 mg·L-1) were prepared by dissolving Pb (NO3)2in deionized water.A certain amount of adsorbent and 50 ml of the solution with various concentrations of Pb2+were added into a 100 ml flask.The pH was adjusted to the desired value with 0.1 mol·L-1HCl or NaOH solution,and the mixture was shaken at 170 r·min-1in a shaker at a pre-set temperature for 60 min and centrifuged at 5000 r·min-1.The supernatant was detected by the FAAS.Table 2 lists the detailed conditions for adsorption experiments.Experiment D was conducted to determine the saturated adsorption capacity of the synthesized zeolitic materials prepared under various conditions.The effects of pH,initial Pb2+concentration,and adsorbent dosage on the adsorption capacity of adsorbents were studied in experiments E and F.Experiment G was performed to determine adsorption isotherms,experiment H was used to reveal the adsorption kinetics process,and the experiment I aimed to evaluate the removal efficiency of adsorbent for Pb2+under different dosages in solutions with specific Pb2+concentrations and reveal the effect of temperature on Pb2+removal.All adsorption experiments were conducted in triplicate with ±3%standard deviation.The results were presented as the average of three obtained values.

The equilibrium adsorption capacity (qe,mg·g-1) and removal efficiency (η,%) were calculated by the Eqs.(1) and (2),respectively:

wherec0andce(mg·L-1) are the initial and equilibrium concentrations of Pb2+,respectively.V(L) is initial volume of the aqueous solution andm(g) is the adsorbent dosage.

2.5.Desorption experiment

0.08 g of adsorbent was placed in 50 ml of 300 mg·L-1Pb2+solutions before shaking for 60 min at 25 °C.After adsorption,the adsorbent was collected through centrifugal separation and washed repeatedly with distilled water to remove non adsorbed Pb2+.Then the adsorbent was immersed into 50 ml of 0.1 mol·L-1NaCl solution under continuous stirring at 170 r·min-1for 6 h to desorb.The desorbed adsorbent was centrifuged and dried for next adsorption.

3.Results and Discussion

3.1.Properties of raw material

The main chemical composition,mineral phase,and micromorphology of the raw and pretreated CFBFA are shown in Table 3,Supplementary Material Fig.S1,and Fig.S2,respectively.The contents of SiO2and Al2O3account for more than 70% of the total CFBFA mass.The pretreatment has removed almost all the Fe2O3,CaO,SO3,K2O,and MgO particles and even part of Al2O3in the raw CFBFA.The pretreated ash mainly contains SiO2and Al2O3and a small amount of TiO2and residual carbon.

Table 2 Conditions for adsorption experiments

Table 3 Main chemical compositions of the raw and pretreated CFBFA

The crystalline minerals in raw CFBFA include quartz (SiO2,9.6%),anhydrite(CaSO4,8.7%),calcite(CaCO3,3.9%),hematite(Fe2O3,3.6%),and a small amount of lime(CaO,0.4%).Their total contents account for only 26.2%,and the rest is amorphous aluminosilicate[23].Except quartz,all the crystalline minerals are all removed by HCl pretreatment.Hence,the mineral phase in the pretreated CFBFA is mainly amorphous aluminosilicate as confirmed by the bump peak at 20°-28° (2θ).

Similar to those of the conventional CFBFA,the particles of raw CFBFA are irregular in shape,and the surface is relatively unconsolidated and rough.After the pretreatment,the particle surface of ash becomes loose,and some cracks appear on the surface.

3.2.Synthesis of zeolitic material

3.2.1.Effect of hydrothermal temperature

Hydrothermal temperature is one of the most important factors affecting the formation of zeolite.Fig.1 illustrates the effect of temperature on the mineral phase and adsorption capacity of zeolitic materials.A small amount of Na-P1 zeolite(Na3.6Al3.6Si12.4O32·14H2O)is formed at 90°C.Its content increases when the temperature is increased from 90 °C to 110 °C and then remains stable at 130 °C.The amorphous aluminosilicate in ash is first dissolved in NaOH solution to releasewhich combine with Na+to form amorphous sodium aluminosilicate hydrate (N-A-S-H) gel [18].This gel can be regarded as zeolite precursors,which are easily transferred into crystalline zeolites by elevating the reaction temperature [24].Therefore,with the increase in reaction time,the N-A-S-H gel is gradually transformed to Na-P1 zeolite above 90 °C.The quartz still exists in the zeoliticmaterials because its stable crystalline structure cannot be broken down by the solution with low NaOH concentration at this temperature range.Large amounts of Na-P1 zeolite are generated in the materials between 110°C and 130°C,and this finding is consistent with previous studies [25,26].

Fig.1.Effect of hydrothermal temperature on the mineral phase and Pb2+adsorption capacity of zeolitic materials (synthesis conditions:No.A in Table 1;adsorption conditions:No.D in Table 2).

The synthesized zeolitic materials present excellent adsorption capacity for Pb2+in aqueous solution.The materials prepared at 110°C and 130°C have higher adsorption capacities than that prepared at 90°C,indicating that the adsorption capacity is positively correlated with the amount of Na-P1 zeolite.The adsorption capacity of the material prepared at 110 °C is the highest at 424.69 mg·g-1.The material prepared at 130 °C has slightly lower adsorption capacity than that prepared at 110 °C,which may be related to the micromorphology of the material surface.On the basis of the adsorption capacity,the optimal hydrothermal temperature is 110 °C.

Fig.2 shows the micromorphology of the zeolitic materials prepared at different temperatures.No substantial change in the particle shape is observed between the material prepared at 90°C and the pretreated CFBFA,but the particle surface has many pores and becomes more porous than before.These finding imply that some aluminosilicate particles in the ash are dissolved by NaOH solution,but the amount of dissolvedandis not sufficient to form a large amount of Na-P1 zeolite at 90°C.When the temperature is increased to 110°C,large amounts of Na-P1 zeolite are generated,thus greatly changing the material morphology.The particle shape of the material becomes regular and spherical,and many wedge-shaped folds can be found on the surface.The particle shape becomes stable when the temperature is increased to 130°C.However,the number of wedge-shaped folds on the particle surface decreases,which may be responsible for the decrease in the adsorption capacity.

3.2.2.Effect of hydrothermal time

Figs.3 and 4 illustrate the effect of hydrothermal time on the mineral phase,adsorption capacity,and micromorphology of synthesized zeolitic materials.The amount of Na-P1 zeolite in the synthesized materials first increases when the hydrothermal time is increased from 6 h to 9 h and then decreases with the continuously prolonged time.Fig.4 (a) shows that some ash particles have not yet been converted to Na-P1 zeolite when the hydrothermal time is 6 h.With further zeolitization,the material particles containing Na-P1 zeolite crystals develop into regular spherical shape at 9 h.However,many spherical particles disappear at the hydrothermal time of 12 h,indicating that the Na-P1 zeolite has begun to transform to a stable zeolite phase.Liuet al.[18,27] found that as a metastable phase,Na-P1 zeolite may be transformed into sodalite with stable structure under high temperature or long hydrothermal time.Therefore,on the basis of Na-P1 zeolite amount and Pb2+adsorption capacity,the optimal hydrothermal time is 9 h.

3.2.3.Effect of added CTAB amount

The cationic surfactant CTAB is a common structure-directing agent and soft template for zeolite synthesis.The amount of added CTAB in raw materials substantially affects the zeolite formation.Fig.5 shows that only a small amount of Na-P1 zeolite is formed in the material under the absence of CTAB,whereas large amounts of Na-P1 zeolite are generated when CTAB is used in the raw materials.These findings reveal that CTAB promotes the formation of Na-P1 zeolite crystals.Theanddissolved in NaOH solution are easily adsorbed onto the positively charged CTAB micelles due to electrostatic attraction[28].Zeolite seeds are formed when the concentrations ofandreach a certain value,and Na-P1 zeolite crystals then grow onto the seeds.The formation of zeolite seeds is crucial to the conversion from amorphous precursor to Na-P1 zeolite.Therefore,many Na-P1 zeolite particles are generated when CTAB is used during synthesis because it induces the formation of zeolite seeds.The peak intensity of Na-P1 zeolite in the material hardly increases when added CTAB amount increases from 0.5 g to 1 g,but the Pb2+adsorption capacity increases greatly.This phenomenon occurs because added CTAB amount affects the particle size and micromorphology of the products.Fig.6 shows that the average particle size of the product decreases when the added CTAB amount increases.With further zeolitization,CTAB with the ammonium heads adsorb in the pores of the small zeolite crystals,while its tails outside inhibit the further growth and condensation of these zeolite crystals [29].Furthermore,the addition of organic matters in the solution may reduce the crystallization rate and produce small crystals grains because of organic steric hindrance [12,30].These processes lead to the decrease in average particle size.Many wedge-shaped folds appear on the spheric particles when CTAB is used due to its directional role on crystal growth.The rough surface increases the contact area between zeolite and Pb2+,thus promoting Pb2+adsorption on the zeolitic material.Therefore,the zeolitic material prepared with 1 g of CTAB(per 100 ml solution) exhibits the highest Pb2+adsorption capacity.

Fig.2.SEM images of zeolitic materials prepared at (a) 90 °C,(b) 110 °C,and (c) 130 °C.

Fig.3.Effect of hydrothermal time on the mineral phase and Pb2+ adsorption capacity of zeolitic materials (synthesis conditions:No.B in Table 1;adsorption conditions:No.D in Table 2).

3.3.Adsorption performance of the zeolitic material

The zeolitic material synthesized under the optimal conditions of hydrothermal temperature of 110 °C,time of 9 h,and addition of 1 g of CTAB was used as an adsorbent to remove Pb2+from aqueous solution.

3.3.1.Characterization of zeolitic adsorbent

According to the guidance from International Zeolite Association Framework Committee,the framework code of the synthesized Na-P1 zeolite is GIS.Na-P1 zeolite has GIS topological structure with interlaced eight-ring pore channels (0.31 nm × 0.4 4 nm in the [1 0 0] lattice plane and 0.26 nm × 0.49 nm in the[0 1 0] lattice plane) [12].

To explore the existing state of aluminum(Al)and silicon(Si)in the adsorbent,the27Al and29Si MAS NMR spectra of the adsorbent are presented in Fig.7(a)and(b),respectively.The significant peak centered at 47.87 represents the AlO4tetrahedron,which is primary building unit of zeolite [31].The negligible peak centered at -6.20 represents AlO6octahedron,which is not incorporated in the framework of zeolite.The peaks centered at 109.11,103.76,and 99.35 can be assigned to Q4(0Al),Q4(1Al),and Q4(2Al),respectively,which are involved in the framework of Na-P1 zeolite [32].The peak centered at 114.78 is attributed to quartz in the adsorbent [33].

The adsorption and desorption isotherms of N2at 77 K,as well as the pore size distribution of the adsorbent are shown in Fig.8.The isothermals belong to the type IV curve with an H3-type hysteresis ring according to the IUPAC classification.The pore diameter of the adsorbent is mainly distributed in the range of 2-14 nm,and the adsorption average pore diameter is 8.21 nm.These indicate that the synthesized adsorbent has a typical mesoporous structure.The BET surface area (SBET) and pore volume of the adsorbent are 43.96 m2·g-1and 0.0945 cm3·g-1,respectively.Compared with other types of zeolite,theSBETvalue of Na-P zeolite is relatively low,but the obtained SBETvalue of the adsorbent is satisfactory and comparable to that obtained in other procedures described in the literature.TheSBETvalues of Na-P zeolite synthesized by coal fly ash are generally in the range of 10-50 m2·g-1,such as 26.8[34],39[35],and 47 m2·g-1[36].TheSBETof Na-P zeolite may be increased by enhancing the synthesis temperature or doping nanoparticles.

Fig.5 demonstrates that a minor amount of unreacted quartz is present in the adsorbent,but the quartz and Na-P1 zeolite particles are separated seen from Fig.S3,indicating that the main chemical composition of the obtained Na-P1 zeolite can be determined by EDS result.Therefore,the Si/Al molar ratio of Na-P1 zeolite is approximately 1.63.The low Si/Al ratio of zeolite is particularly advantageous for ion exchange and chemisorption because the framework aluminum atoms can create negatively charged sites[14].

Fig.4.SEM images of zeolitic materials prepared at 110 °C for different times,(a) 6 h,(b) 9 h,and (c) 12 h.

Fig.5.Effect of added CTAB amount on the mineral phase and Pb2+ adsorption capacity of zeolitic materials (synthesis conditions:No.C in Table 1;adsorption conditions:No.D in Table 2).

Based on the above analysis,the content of Na-P1 zeolite is approximately 84% in the zeolitic adsorbent.The main impurity quartz is inert and does not adversely affect the use of the adsorbent.Considering the low cost and good adsorption performance of the zeolitic material synthesized in this study,the sample purity is satisfactory for the treatment of industrial wastewater.

3.3.2.Effect of pH

The pH of aqueous solution is one of the key factors affecting the adsorption performance of adsorbent.Fig.9 (a) shows that the adsorption capacity of the adsorbent for Pb2+gradually increases with the solution pH.Pb2+is highly soluble in acidic solution,and many H+ions occupy the adsorption sites of adsorbent.Hence,the adsorption capacity of Pb2+on the adsorbent is low when the solution pH is 3.Increasing the solution pH is conducive to Pb2+adsorption on the adsorbent,but Pb2+may precipitate to form Pb(OH)2when the pH is above 7[37].Therefore,subsequent adsorption experiments were conducted in solution with pH 6 to avoid Pb2+precipitation.

Fig.9 (b) shows the pH variation of solution with time during adsorption.The pH of the raw solution with 500 mg·L-1Pb2+is 4.68,which increases to 4.9 after the adsorbent is added into solution because the adsorbent is weak alkaline.The solution pH slightly increases after adsorption when the pH of the initial solution is not adjusted by NaOH solution.However,when the pH of the initial solution is adjusted to 6 by 0.1 mol·L-1NaOH solution,the solution pH decreases to 5.56 after adsorption.Therefore,Pb2+-precipitation is prevented because the solution pH during adsorption is always below 6.

3.3.3.Effect of adsorbent dosage

The adsorption capacity and removal efficiency of adsorbent for Pb2+are closely related to the initial Pb2+concentration of aqueous solution and adsorbent dosage.Fig.10 shows that with the increase in adsorbent dosage,the removal efficiency of Pb2+gradually increases and finally reaches more than 99% due to the large increase in adsorption sites in the wastewater.Only 1.6 g of adsorbent per liter of wastewater is needed to remove more than 99%of the Pb2+from the solution with initial Pb2+concentration of 300 mg·L-1.This finding implies that the synthesized material can efficiently remove Pb2+from wastewater.However,when the adsorbent dosage increases,some adsorption sites on the adsorbent cannot be utilized effectively because the Pb2+amount in the solution is limited.This phenomenon inevitably leads to a decrease in adsorption capacity [8,38].

Fig.6.SEM images of the zeolitic materials prepared with different amounts of added CTAB:(a) 0 g,(b) 0.5 g,and (c) 1 g.

Fig.7.(a) 27Al and (b) 29Si MAS NMR spectra of the zeolitic adsorbent.

Fig.8.(a) Nitrogen adsorption/desorption isotherms and (b) pore size distribution curve of the zeolitic adsorbent.

Fig.9.(a)Effect of pH on the adsorption capacity of Pb2+ on the zeolitic material;(b) pH variation of solution with time during adsorption.(adsorption conditions:No.E in Table 2).

Fig.10.Effect of adsorbent dosage on the adsorption capacity and removal efficiency of Pb2+under various initial concentrations(adsorption conditions:No.F in Table 2).

3.3.4.Adsorption mechanism

Langmuir and Freundlich isotherm models were used to describe the equilibrium adsorption for Pb2+on the adsorbent.The linear forms of Langmuir and Freundlich isotherm equations are as follows [39]:

whereceis the equilibrium concentration of Pb2+(mg·L-1),qeis the amount of Pb2+adsorbed at equilibrium (mg·g-1),qmis the maximum adsorption capacity(mg·g-1),andKL(L·mg-1)is the Langmuir isotherm constant.KF(mg·g-1)and1/n(dimensionless)are the constants of a Freundlich isotherm.

Fig.11 shows the Langmuir and Freundlich isotherm plots for Pb2+adsorption.The correlation coefficient of the Langmuir isotherm model(R2=0.9950)is higher than that of the Freundlich isotherm model (R2=0.9122),indicating that the former is more suitable for Pb2+adsorption than the latter.The maximum adsorption capacity calculated using the Langmuir model is 408.16 mg·g-1,which is close to the experimental value(424.69 mg·g-1).These findings indicate that the Pb2+adsorption onto the zeolitic material is mainly controlled by monolayer adsorption,and the synthetic material has a uniform surface and consistent particle mass [38].

Adsorption kinetics is helpful in scientifically designing the adsorbent and is important for its potential applications.The kinetics of Pb2+adsorption by the zeolitic material was investigated by using the pseudo-first-order and pseudo-second-order kinetics models.The equations of these models are expressed by Eqs.(5)and (6) [11]:

Fig.11.(a) Langmuir and (b) Freundlich isotherm plots for Pb2+ adsorption (adsorption conditions:No.G in Table 2).

Fig.12.(a) Pseudo-first-order and (b) Pseudo-second-order model plots for Pb2+ adsorption on the adsorbent (adsorption conditions:No.H in Table 2).

where,qeandqtare the amounts of Pb2+adsorbed per unit of the adsorbent (mg·g-1) at equilibrium time and at a given timet,respectively;k1andk2are the rate constant of pseudo-first-order and pseudo-second-order kinetic models,respectively,for adsorption.

Fig.13.XPS spectra of adsorbent before and after Pb2+ adsorption:(a) Si,(b) Al,(c) Na,and (d) Pb.

Fig.14.Effect of contact time on Pb2+removal efficiency under different initial concentrations and adsorbent dosages.

Fig.12 presents the pseudo-first-order and pseudo-secondorder model plots and the corresponding parameters for Pb2+adsorption on the adsorbent.The correlation coefficient of the pseudo-second-order model (R2=0.9958) is higher than that of the pseudo-first-order model (R2=0.9297),and the adsorption amount (452.49 mg·g-1) calculated by the pseudo-second-order model is closer to the experimental value (424.69 mg·g-1).Therefore,the pseudo-second-order model is suitable to describe the Pb2+adsorption process on the synthesized zeolitic material,and its main rate-controlling step is chemisorption [11,37].

XPS was used to investigate the interaction between Pb2+and adsorbent,and the results are shown in Fig.13.No changes are observed in the peaks of Si 2p and Al 2p in spectrum of the adsorbent before and after adsorption.After Pb2+adsorption,the peak of Na 1 s at approximately 1072 eV disappears.New peaks of Pb 4f5/2and Pb 4f7/2are detected on the adsorbent surface,indicating the ion exchange between Pb2+and Na+during adsorption.Na+and Pb2+have similar ionic size (Na+:0.99 nm;Pb2+:0.98 nm),which is beneficial for their ion exchange [40].Ion exchange is the predominant mechanism for heavy metal adsorption onto zeolitic materials [8,10,41].

3.3.5.Adsorption equilibrium time

Adsorption equilibrium time is one of the important parameters for evaluating the practicability of adsorbents.Fig.14 (a) and (b)show that more than 70% of Pb2+in the solution with initial concentrations of 100-200 mg·L-1are removed within 5 minutes.At 25 °C and 35 °C,the Pb2+removal efficiency on the adsorbent reaches 100% in 40 min,and the equilibrium time is reduced to 10 min at 45°C.Fig.14(c)shows that even in the solution with initial concentration of 300 mg·L-1,the removal efficiency of Pb2+can still reach more than 99%in 20 minutes at room temperature when the adsorbent dosage is 1.6 g·L-1.These findings indicate that the synthesized zeolitic material has a fast adsorption rate for Pb2+in aqueous solution.A high temperature is favorable for Pb2+adsorption onto the adsorbent,thus confirming that this process is endothermic and mainly occurs through chemisorption [27].

3.3.6.Reusability of the zeolitic adsorbent

The reusability of the zeolitic adsorbent was investigated through three adsorption-regeneration cycles.Fig.15 shows that with the increase of the cycle number,the adsorption capacity of Pb2+on the zeolitic adsorbent decreases dramatically.The removal efficiency of Pb2+decreases from 99.12% to 37.65% after three cycles.The Pb2+adsorbed on the zeolitic material cannot be desorbed completely,which indicates that some Pb2+were bound to the zeolitic material through the strong interaction of chemical adsorption or other reactions [11].The interaction between heavy metal ions and zeolitic materials needs further study.

Fig.15.Removal efficiency and adsorption capacity of Pb2+on the zeolitic material at varying regeneration cycles.

3.4.Comparison of various zeolitic adsorbents

Table 4 lists the main synthesis conditions and Pb2+adsorption capacities of various zeolitic materials reported in previous studies.Geet al.[37] and Buet al.[8] synthesized Na-X and Na-Y zeolites using coal gangueviaan alkali fusion hydrothermal method,and the adsorption capacities of the products for Pb2+reach as high as 457 and 482.1 mg·g-1,respectively.Uncalcined coal gangue contains some phases with stable chemical structure,such as clay minerals,and quartz.In particular,silicon and aluminum are difficult to be dissolved by alkaline solution during a direct hydrothermal treatment.Therefore,the mixture of coal gangue and solid alkali,such as NaOH powder,is fused between 400 °C and 850 °C to activate the stable minerals before the hydrothermal treatment and increase the yield and purity of zeolite.This method is also generally applied to synthesize zeolitic materials by using PCFA as the raw material because this ash contains a certain amount of mullite,a mineral with a stable structure.In addition to alkali fusion treatment,high pressure hydrothermal treatment can also accelerate the dissolution of silicon and aluminum and promote zeolite formation.However,the high energy consumption of these two methods will increase the synthesis cost of zeolitic adsorbent.By contrast,the silicon and aluminum in CFBFA are easily dissolved in NaOH solution during direct hydrothermal treatment because almost all of these particles exist in the form of amorphous aluminosilicate,except quartz.In the present work,a zeolitic material with Na-P1 zeolite as the main phase was synthesized using CFBFA as the raw material under relatively moderate condition.This material can be used as a low-cost and high-efficiency adsorbent to remove Pb2+from wastewater.

Table 4 Comparison of synthesis conditions and Pb2+ adsorption capacities of various synthetic zeolites

4.Conclusions

In this work,a zeolitic material with Na-P1 zeolite as the main phase was successfully synthesizedviaa hydrothermal method by using CFBFA as the raw material,and was employed as an adsorbent to remove Pb2+from aqueous solution.The hydrothermal temperature,time,and added CTAB amount have an important influence on the synthesis of this zeolitic material.The synthesized zeolitic material has a high Pb2+adsorption capacity under the following optimal operation conditions:NaOH concentration of 2 mol·L-1,solid-to-liquid ratio of 1:10 g·ml-1,hydrothermal temperature of 110 °C,hydrothermal time of 9 h,and added CTAB amount of 1 g (per 100 ml solution).The amount of Na-P1 zeolite and micromorphology of the zeolitic material are the most important factors affecting the Pb2+adsorption capacity.The adsorption capacities of the zeolitic material reach 329.67,424.69,and 542.22 mg·g-1when the pH values of aqueous solution are 5,6,and 7,respectively.More than 99% of Pb2+at concentrations of 100-300 mg·L-1in aqueous solution can be removed using a suitable adsorbent dosage.The adsorption process and kinetics of Pb2+on the zeolitic material are accurately described by Langmuir isotherm and pseudo-second-order kinetic models,respectively,implying that the adsorption yields a monolayer and occurs mainly through chemical adsorption.The adsorption process is endothermic and favorable at high temperature.The main adsorption mechanism is the ion exchange and chemisorption.These results indicate that the zeolitic material synthesized using CFBFA can be recommended as a low-cost and high-efficiency adsorbent for Pb2+removal from weak acid and neutral aqueous solutions.

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

This work was supported by National Natural Science Foundation of China (22078181,U1810205) and the Bidding Project of Shanxi Province (20191101007).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.05.043.