Stellerite-seeded facile synthesis of zeolite X with excellent aqueous Cd2+ and Ni2+ adsorption performance

Yinchang Pei ,Shengpeng Mo ,Qinglin Xie,2,*,Nanchun Chen,Zhongxin Yang ,Lili Huang ,Lili Ma

1 College of Environmental Science and Engineering,Guilin University of Technology,Guilin 541006,China

2 Collaborative Innovation Center for Water Pollution Control and Water Safety in Karst Area,Guilin University of Technology,Guilin 541006,China

3 College of Material Science and Engineering,Guilin University of Technology,Guilin 541006,China

Keywords:Zeolite X Stellerite zeolite Adsorption mechanism Cd2+ adsorption Ni2+ adsorption Ion exchange

ABSTRACT Zeolite X was synthesized by a two-step hydrothermal method using natural stellerite zeolite as the silicon seed,and its adsorption performance for Cd2+ and Ni2+ ions was experimentally and comprehensively investigated.The effects of pH,zeolite X dosage,contact time,and temperature on adsorption performance for Cd2+ and Ni2+ ions over were studied.The adsorption process was endothermic and spontaneous,and followed the pseudo-second-order kinetic and the Langmuir isotherm models.The maximum adsorption capacitiesfor Cd2+and Ni2+ions at 298 K were 173.553 and 75.897 mg·g-1,respectively.Ion exchange and precipitation were the principal mechanisms for the removal of Cd2+ ions from aqueous solutions by zeolite X,followed by electrostatic adsorption.Ion exchange was the principal mechanisms for the removal of Ni2+ ions from aqueous solutions by zeolite X,followed by electrostatic adsorption and precipitation.The zeolite X converted from stellerite zeolite has a low n(Si/Al),abundant hydroxyl groups,and high crystallinity and purity,imparting a good adsorption performance for Cd2+and Ni2+ ions.This study suggests that zeolite X converted from stellerite zeolite could be a useful environmentally-friendly and effective tool for the removal of Cd2+and Ni2+ions from aqueous solutions.

1.Introduction

With the development of industry,increasingly more heavy metal wastewater is being discharged into the environment.Among the various heavy metal ions,As,Pb,Hg,Cr,Cd,and Ni ions are considered to be very toxic[1].Heavy metal ions in wastewater not only harm plants and animals,but can enter the human body through the food chain,causing serious diseases [2].For example,Cd and Ni in water mainly originate from the effluent discharge from metallurgical,electroplating,battery,and other industries.The accumulation of Cd2+ions in the human body can lead to kidney and liver damage,leading to cartilage disease[3],whereas Ni2+ions can induce cancer and damage the reproductive system [4].Proper treatment of Cd2+and Ni2+ions in wastewater is crucial for the sustainable development of human and nature.The present methods for removing Cd2+and Ni2+ions from wastewater include chemical precipitation,electrochemical,biological treatment,solution extraction,and adsorption [4,5].The adsorption method is often used due to its advantages of simple operation,good treatment effect,and no secondary pollution.However,most adsorption materials have a poor adsorptive capacity for Cd2+and Ni2+ions in wastewater.Therefore,the development direction for future adsorption methods is to seek economical and efficient adsorbents.

Stellerite zeolite is a type of low-cost and easily stored natural zeolite,mainly composed of silica,alumina oxide,sodium oxide,and calcium oxide.It has a certain adsorption capacity for heavy metal ions [6,7].However,its small specific surface area,narrow pores,and impurities in the pores limit itsthe adsorption capacity[8,9].Therefore,the stellerite zeolite adsorption capacity can be improved by using different activation and modification methods.Among these methods,using stellerite zeolite as a seed to convert it to other zeolites with high adsorption capacities is a feasible method [10-12].For example,zeolite X is an excellent adsorbent due to its good removal of heavy metal ions.Zeolite X is an inorganic microporous material composed of SiO2,Al2O3,H2O,and alkali metal.The framework structure of zeolite X is composed of Si(Al)-O tetrahedra through the apex of the O atoms and are connected to each other into a three-dimensional space structure.After the Si atoms in the crystals are substituted by Al atoms,zeolite X needs Na+ions to compensate for the excess negative charge.Due to the weak binding of Na+ions with the zeolite X framework,Na+ions easily exchange with the surrounding heavy metal ions,imparting a strong ion exchange performance to zeolite X.Secondly,due to part of the skeleton O of Si(Al) O tetrahedral having a negative charge and the formation of a hydrated oxide cover layer on the zeolite X surface,the zeolite X surface is electronegative and exhibits,a strong electrostatic attraction to heavy metal ions.Moreover,there are many -OH groups on the zeolite X surface,and heavy metal ions react with the -OH groups to precipitate on the zeolite X surface;therefore,the adsorption properties of zeolite X can be further enhanced [13].Zhanget al.[14] used the traditional hydrothermal method to convert diatomite to zeolite X,obtaining a specific surface area of 543 m2·g-1and the achieving a zeolite X adsorption capacity for Cd2+ions of 67.130 mg·g-1.Baiet al.[15] converted oil shale ash into zeolite X using the alkali fusion hydrothermal method.The zeolite X adsorption capacity for Cd2+ions reached 169.490 mg·g-1,which was higher than that of modified oil shale ash.Hameedet al.[16]synthesized zeolite X using hydrothermal method with a pure chemical reagent as the raw material and glycine as the organic structure guiding agent,and the zeolite X adsorption capacity for Cd2+ions reached 62.814 mg·g-1.The obtained zeolite X could be used as an adsorbent to treat a low concentration of Cd2+ions in electroplating wastewater,completely removing the pollutants.Guo [17] adsorbed Ni2+ions in solution with commercial zeolite X as the adsorbent,and the adsorption capacity was approximately 25.000 mg·g-1.Zhanget al.[18] synthesized a product which had an abundant amount of zeolite X anda small amount of zeolite A using the hydrothermal method after calcination of fly ash generated in the coal gasification process.The adsorption capacity of the product for Ni2+ions reached 15.936 mg·g-1.Xiaoet al.[19]converted nephelinesyenite into zeolite X using the alkali fusion hydrothermal method,and the zeolite X adsorption capacity for Ni2+ions reached 49.700 mg·g-1.They treated high concentration electroplating wastewater containing Ni2+,Cu2+,and Zn2+ions using the zeolite X as an adsorbent.After pretreat the electroplating wastewater with Na2S,the removal rates of Ni2+,Cu2+,and Zn2+ions by zeolite X were 94.24%,99.97%,and 99.25%,respectively.Common adsorbents such as activated C [20],chitosan [21],and cyclodextrin [22],mainly remove heavy metal ions by the precipitation of surface functional groups.Ion exchange and electrostatic adsorption of these materials have a limited influence on the adsorption of heavy metal ions;therefore,the adsorption performance of aforementioned adsorbents for heavy metal ions is poor.Hence,it is usually necessary to obtain O-containing functional groups by modification and use ion exchange to enhance the adsorption performance of these materials for heavy metal ions.Zeolite X,as compared to the aforementioned adsorbents,does not need modification,can rely on its own special threedimensional space structure,and its physical and chemical properties,such as ion exchange,electrostatic attraction,and precipitation,play a simultaneous role in removing heavy metal ions [13].Therefore,zeolite X has the advantages of less interference,wider application range,and stronger adsorption capacity.Converting stellerite zeolite into zeolite X with a high adsorption capacity can realize,the low cost and fast adsorption of Cd2+and Ni2+ions in water.

The conversion of stellerite zeolite to other types of zeolites,such as zeolite X,Y,A,and P,is presently achieved by hydrothermal and alkali fusion-hydrothermal methods [11,23-25].In our previous study,stellerite zeolite was converted into zeolite X with high crystallinity by a two-step hydrothermal method [26],but its application to heavy metal ion adsorption was not studied.Because then(Si/Al) and surface properties of zeolite X prepared using different raw materials and synthesis methods are different,the zeolite X adsorption capacities for Cd2+and Ni2+ions differ.Additionally,many studies have mainly focused on the removal effect,adsorption kinetics,and adsorption isotherms of the zeolite X adsorption of Cd2+and Ni2+ions;however,there are almost no studies that examined the adsorption mechanism of zeolite X from the perspective of the structural changes before and after the adsorption of Cd2+and Ni2+ions.Therefore,studying the adsorption performance and mechanism of zeolite X for Cd2+and Ni2+ions has important reference significance for the study of other zeolites for the adsorption of heavy metal ions.

Thus,this study mainly aimed to investigate the zeolite X adsorption capacity for Cd2+and Ni2+ions.The adsorption kinetics,isotherms,and thermodynamics of the zeolite X adsorption of Cd2+and Ni2+ions were comprehensively studied.The potential of zeolite X was characterized by zeta potential before adsorption,and the before and after adsorption of products were characterized by X-ray powder diffraction (XRD),scanning electron microscopy(FESEM),Fourier transform infrared spectroscopy (FT-IR),and Xray photoelectron spectroscopy(XPS).Finally,the proposed mechanism for the zeolite X adsorption of Cd2+and Ni2+ions is presented and discussed.To the best of our knowledge,this study is the first propose the adsorption mechanism from the structural changes of zeolite X before and after the adsorption of Cd2+and Ni2+ions.

2.Materials and Methods

2.1.Materials and chemicals

Stellerite zeolite was obtained from Ziyuan (Guangxi,China),and was ground to ＜270 μm before use.Sodium silicate nonahydrate (Na2SiO3),sodium hydroxide (NaOH),and hydrochloric acid(HCl) were provided by Xilong Scientific Co.,Ltd.(Guangdong,China).Sodium aluminate (Al2O3,41%) was purchased from Sinopharm Chemical Reagent Co.,Ltd.(Beijing,China).Cadmium chloride hemidihydrate (CdCl2·2.5H2O) and Nickel chloride hexahydrate (NiCl2·6H2O) were purchased from Tianjin Damao Chemical Reagent Factory (Tianjin,China) and Rhawn Reagent Company (Shanghai,China),respectively.

2.2.Zeolite X synthesis

2.2.1.Traditional hydrothermal method

Zeolite X was synthesized from pure chemical reagents using a traditional hydrothermal method.The amount of Na2SiO3,NaAlO2,and NaOH solutions was determined according to the molar ratios of H2O/SiO2=80,Na2O/SiO2=1.5,and Al2O3/SiO2=0.33,respectively.Then,the NaAlO2and NaOH solutions were successively added to the Na2SiO3solution under vigorous magnetic stirring.After 1 h,the mixture solution was transferred into a flask with three necks.The gel solution was then subjected to aging for 48 h at room temperature.Finally,the gel solution was crystallized at 355 K for 6 h,and the product was filtered out.The product was washed to a pH of 7-8 with deionized water and dried at 353 K for 12 h.The obtained white solid was zeolite X.

2.2.2.Two-step hydrothermal method

Stellerite zeolite was converted to zeolite X by a two-step hydrothermal method.Initially,stellerite was added into a 15%HCl solution with a solid/liquid ratio of 1:3 at 363 K for 2 h under magnetic stirring.Subsequently,the dry acidified stellerite was mixed with a NaOH solution at 358 K for 15 min under magnetic stirring.The solid-liquid separation of the product was performed,and the concentration of amorphous Si in the alkali solution was calculated.The amount of alkali,NaAlO2,and NaOH solutions was determined according to the molar ratios of H2O/SiO2=55,Na2O/SiO2=1.1,and Al2O3/SiO2=0.33.Then,the NaAlO2and NaOH solutions were successively added to the alkali solution containing amorphous Si under vigorous magnetic stirring.After 1 h,the mixture solution was transferred into a flask with three necks.The gel solution was then subjected to aging for 48 h at room temperature.Finally,the gel solution was crystallized at 355 K for 4 h,and the product was filtered out.The product was washed to a pH of 7-8 with deionized water and dried at 353 K for 12 h.The obtained white solid was zeolite X.The relative crystallinity of the asobtained zeolite X was compared with the intensity of the standard reference zeolite X diffraction peaks.The standard reference zeolite X was prepared using the molar ratios of H2O/SiO2=55,Na2O/SiO2=1.1,Al2O3/SiO2=0.33,and was aged for 48 h,followed by a hydrothermal treatment at 355 K for 8 h.The relative crystallinity was calculated using Eq.(1):

whereXiis the relative crystallinity of the product;XFis the crystallinity of the standard reference zeolite X (the assumed value ofXFis 100%);∑I1and ∑I2are the sum of the intensity of the eight diffraction peaks of the product and standard reference zeolite X,respectively;andW1andW2are the full width half maximum of the(1 1 1)crystal face of the product and standardreference zeolite X,respectively.

2.3.Adsorption experiments

To study the effects of different influential parameters,such as the adsorption time,zeolite X dosage,pH,and heavy metal ions concentration,stock solutions of Cd2+and Ni2+ions were prepared by separately dissolving appropriate quantities of analytical grade CdCl2·2.5H2O and NiCl2·6H2O,followed by dilution with distilled water to the required concentrations.An ion solution(20 ml)with a certain concentration was placed in a 100 ml plastic centrifuge tube,and then 0.02 g of zeolite X was added.Then,the plastic centrifuge tube was immediately placed in a thermostat oscillator(150 r·min-1).Supernatants were withdrawn at different time intervals through a 0.45 μm membrane filter,and then the ion concentration in the supernatants was measured using inductively coupled plasma spectrometry (ICP-OES).All adsorption experiments were performed three times,and the average values were calculated and reported.The adsorption capacity (q) and removal efficiency (R) were calculated using Eqs.(2) and (3) [27]:

whereqtis the adsorption capacity at different times (mg·g-1);C0andCtare the initial and residual concentrations at different times(mg·L-1),respectively;andV(L) andm(g) are the solution volume and sorbents quantity,respectively.

2.4.Adsorption kinetic studies

To elucidate the kinetics mechanism that controls the adsorption process,the pseudo-first-order,pseudo-second-order,Elovich and intraparticle diffusion models were examined to suitably interpret the experimental results.

Pseudo-first-order model:

Elovich model:

Intraparticle diffusion model:

whereqeis the adsorption capacity at equilibrium(mg·g-1);tis the adsorption time (min);k1(min-1),k2(g·mg-1·min-1),andki(mg·g-1·min-1/2)are the rate constants of the models,respectively;and α1,β1,andCare constants [28,29].

2.5.Adsorption isotherm studies

To understand the adsorption process,the Freundlich,Langmuir and Temkin isotherms model were examined to suitably interpret the experimental results [28,29].

Freundlich isotherm model:

wherekF(mg·g-1) andn-1are the constant and heterogeneity factors of the Freundlich isotherms,respectively;the magnitude ofn-1specifies the sorption favorability;Ce(mg·L-1) andqm(mg·g-1) are the equilibrium concentration and the maximum adsorption capacity,respectively;kL(L·mg-1)is the Langmuir constant;RLis a separation factor,which is one of the most important dimensionless equilibrium parameters of the Langmuir model;and α2and β2(L·mg-1) are constants associated with the heat of adsorption and the largest binding energy dependent equilibrium constant of the Temkin isotherm,respectively.

2.6.Adsorption thermodynamic studies

The ΔH(change in enthalpy) and ΔS(change in entropy) were calculated from the slope and intercept of the straight line obtained by plotting lnkdversus1/Tusing the thermodynamics equation(Eq.(12))[30].Then,the ΔG(change in Gibbs free energy)was calculated using Eq.(13) [30].

wherekdis the distribution coefficient;Ris the molar gas constantR=8.314 J·mol-1·K-1;andTis the absolute temperature (K) [28-30].

2.7.Characterization

The crystal structure of the materials was characterized using XRD (X’Pert PRO,PANalytical,Holland) operated at 40 kV and 40 mA.The measurement was conducted at a 2θ angle of 5°-80°,with a step size of 0.02626° and a scan speed of 0.6565 (°)·s-1.The X’Pert Highscore Plus software was used to process the diffraction data of the products.The solid particle size and surface morphology of the products were analyzed using FESEM (S-4800,Hitachi,Japan).The chemical constituents of materials were analyzed using EDS (S-4800,Hitachi).The specific surface area and total pore volume of zeolite X was measured using the N2physisorption at 77 K (BET;ASAP 2020,Micromeritics,USA).The surface potential of zeolite X was analyzed using a nanoparticle and zeta potential analyzer (Nano-ZS,Malvern,Britain).The surface functional groups of the products were analyzed using FT-IR(IR Affinity,Shimadzu,Japan),with KBr as the reference material,in the range of 400-4000 cm-1and at a resolution of 0.5 cm-1.The metal ion content in the aqueous solutions was determined using ICP-OES (Optima 7000DV,PerkinElmer,USA).The elemental composition of the samples was determined using XPS (ESCALAB 250XI,Thermo Fisher Scientific,USA).

3.Results and Discussion

3.1.Crystal and morphological structures

Fig.1(a) shows that the main mineral compositions of the raw material were stellerite zeolite and quartz,indicating that the raw material is a natural mineral.The EDS analysis indicated that the main chemical constituents of the raw material were SiO2,Al2O3,and CaO.The Al2O3and SiO2contents in the raw material were 16.71%and 69.27%,indicating that the stellerite zeolite mineral was suitable to synthesize zeolite X(Table 1).The XRD analysis shows that the main phases of the acidized stellerite zeolite were amorphous silica and quartz.Because impurities such as CaO,were removed,the SiO2content increased from 69.27% to 91.63% after acidification(Table 1)[23].Quartz cannot be easily dissolved a traditional hydrothermal reaction;however,the amorphous silica of stellerite can be dissolved in the hot alkali solution,and then be used for conversion to zeolite X.Additionally,the Al2O3of stellerite zeolite after acidification was not detected as it was dissolved to form Al2O3in the subsequent operation.After the acidified stellerite was dissolved by alkali,only the diffraction peaks of quartz were found in the alkali dissolved residue;however,the broad peak of the amorphous substance disappeared (Fig.1(a)),indicating that amorphous silica was dissolved in the solution and the amorphous silicon in the solution existed in the form of sodium metasilicate.Fig.1(a) shows XRD analysis results of the product obtainedby the two-step hydrothermal synthesis.The diffraction peaks of the as-obtained product were located at 2θ=6.07°,9.95°,11.68°,15.38°,20.03°,23.26°,26.61°and 30.90°.The characteristic peak of the product corresponded to the standard PDF card(No.38-0237),indicating that the as-obtained sample was mainly composed of zeolite X.The aforementioned diffraction peaks correspond to the(1 1 1),(2 2 0),(3 1 1),(3 3 1),(4 4 0),(5 3 3),(6 4 2),and (7 5 1) crystal faces of zeolite X,respectively [31,32].The diffraction peaks of other types of zeolite were not present in the XRD spectrum,indicating that zeolite X was the main phase of the product and that the purity and crystallinity of the asobtained zeolite X was high.Additionally,the crystallinity of zeolite X was 98%.Fig.1(b) and Table 1 show that the pore size of the as-obtained zeolite X was meso-microporous,which is caused by the stacking of zeolite X crystals.The specific surface area and the total pore volume of zeolite X were 354 m2·g-1and 0.194 cm3-·g-1,respectively.Fig.1(c) and (e) shows the morphology of the raw material before and after the acid-treatment and of zeolite X.The stellerite zeolite presented a lamellar structure,and its edges were sharp.After acidification,the stellerite zeolite retained its lamellar structure.Fig.1(e) shows that the zeolite X was composed of uniformly distributed octahedral crystals.The above results demonstrate that the two-step hydrothermal method successfully converted the stellerite zeolite to high purity and crystallinity zeolite X.Generally,the higher the crystallinity of zeolite,the greater its ion exchange capacity and its ability to remove heavy metal ions.Therefore,the obtained zeolite X can be used to remove Cd2+ions and Ni2+ions from aqueous solutions.

Table 1Chemical compositions of stellerite zeolite before and after acidification and textural properties of zeolite X

Fig.1.(a) XRD patterns of the samples,(b) N2 adsorption-desorption isotherms of zeolite X,(c) and (e) FESEM images of the samples.

3.2.Adsorption experiments

3.2.1.Effect of pH and dosage on the adsorption performance

Fig.2 shows that the adsorption of Cd2+and Ni2+ions from the aqueous solution was strongly affected by the pH of the solution.When the pH was 2-3,the adsorption capacity for Cd2+and Ni2+ions increased rapidly.However,the adsorption capacity increased slowly when the pH increased.At a pH ＜4,the Al3+ions were dissolved by the high concentration of H+,destroying the zeolite X skeleton.In contrast,H+can compete with heavy metal ions for adsorption sites on the zeolite X surfaces,limiting the adsorption of Cd2+and Ni2+ions by zeolite X [33].When the pH value was increased to 5-8,the influence of H+decreased,and the competitive adsorption of H+and heavy metal ions decreased.When the pH value was adjusted to 6,the adsorption capacity for Cd2+and Ni2+ions reached a high value of 173.553 and 75.897 mg·g-1,respectively.At pH ＞7,the Cd2+and Ni2+ions and OH-formed Cd(OH)+(aq),Cd(OH)2(aq),Ni(OH)+(aq),Ni(OH)2(aq),Cd(OH)2(s),and Ni (OH)2(s) [33].Therefore,the removal of Cd2+and Ni2+ions under basic conditions can be mainly attributed to their adsorption by zeolite X and precipitation [33].The pH of the original Cd2+and Ni2+ions solutions was approximately 5.8,and industrial wastewater effluent pH is usually 6-9.Therefore,a pH of 6 was chosen as the optimum initial pH in this study.

Fig.2.Influence of solution pH on the(a)Cd2+and(b)Ni2+ions removal(contact time=180 min,zeolite X dosage=0.02 g,temperature=298 K,and initial Cd2+and Ni2+ions concentrations were 200 and 100 mg·L-1,respectively).

Fig.3 shows the effects of the zeolite X dosage on the adsorption of Cd2+and Ni2+ions.The removal rate of Cd2+and Ni2+ions increased and the adsorption capacity decreased with an increasing zeolite X dosage.When the adsorbent dose was ＜0.030 g,the removal rate increased rapidly because the number of adsorption sites increased.However,the binding sites overlapped at high zeolite X dosages,resulting in a reduced number of effective binding sites.The adsorption capacity for Cd2+and Ni2+ions reduced from 182.920 to 152.917 g·L-1and from 84.940 to 65.402 mg·L-1,respectively.Thus,considering the percentage removal,adsorption capacity,and economic cost,a zeolite X dosage of 0.02 g was selected for further investigation [28].

Fig.3.Influence of zeolite X dosage on the(a)Cd2+and(b)Ni2+ions removal(contact time=180 min,pH=6,temperature=298 K,and initial Cd2+and Ni2+ions concentration were 200 and 100 mg·L-1,respectively).

3.2.2.Adsorption kinetics

Fig.4 shows the influence of contact time on the adsorption capacity and the linear fitting of the adsorption kinetic models.Fig.4(a) shows that zeolite X has a high adsorption rate.The adsorption rates and capacities for Cd2+and Ni2+ions were 83.44% and 72.48% and 166.875 and 72.480 mg·g-1,respectively,within the first 30 min.When the adsorption time was extended,the adsorption capacity of zeolite X increased gradually and reached complete equilibrium within 180 min.Fig.4(b)-(e)shows the linear fitting of the adsorption kinetic models,and Table 2 and Table S1(Supplementary Materials)summarize the related parameters.The correlation coefficients (R2) of the pseudo-second-order kinetic model were 0.999,which is higher than that of the other models.Therefore,the pseudo-second-order kinetic model exhibited the best fitting for the experimental results.The intraparticle diffusion model was used to understand the factors controlling the adsorption process.The intercept of two fitted linear equations of the intraparticle diffusion model was not zero,indicating that the adsorption process was controlled by both external diffusion and intraparticle diffusion.

Fig.4.Influence of (a) contact time and the linear fitting of (b) the pseudo-first-order,(c) pseudo-second-order,(d) intraparticle diffusion,and (e) Elovich models (pH=6,zeolite X dosage=0.02 g,temperature=298 K,and initial concentration of Cd2+ and Ni2+ ions were 200 and 100 mg·L-1,respectively).

Table 2Correlation coefficients of the Cd2+ and Ni2+ ions adsorption onto zeolite X

3.2.3.Adsorption isotherms

Fig.5 shows the influence of the Cd2+and Ni2+ions initial concentration (C0) on the adsorption capacity and linear fitting of the adsorption isotherm models.Fig.5(a)shows the effect ofC0on the removal of Cd2+and Ni2+ions.When the temperature was 298 K andC0of Cd2+and Ni2+ions increased to 350 mg·L-1,the adsorption capacities for Cd2+and Ni2+ions increased to 189.030 and 81.000 mg·g-1,respectively.The highC0provided a large driving force to overcome the mass transfer resistance between the water and heavy metal ions.When the concentration of Cd2+and Ni2+ions was ＜200 mg·L-1and ＜100 mg·L-1,respectively,the adsorption capacity of zeolite X increased rapidly.When the concentration of Cd2+and Ni2+was ＞200 mg·L-1and ＞100 mg·L-1,the adsorption capacity of zeolite X increased gradually.Considering the optimal adsorption capacity for Cd2+and Ni2+ions,aC0of 200 and 100 mg·L-1,respectively,was selected for the subsequent experiments.Fig.5(a) shows that the adsorption capacity for Cd2+removal increased from 173.553 to 189.030 mg·g-1when the temperature was increased from 298 to 318 K and whenC0was 200 mg·L-1.Similarly,Fig.5(a)shows that the adsorption capacity for Ni2+removal increased from 75.897 to 81.000 mg·g-1when the temperature was increased from 298 to 318 K and whenC0was 100 mg·L-1.These results suggest that the Cd2+and Ni2+adsorption onto zeolite X was an endothermic process.Fig.5(b-d),Tables 3 and S2 provide the obtained linear fitting and equilibrium parameters at various temperatures.TheR2values of the Langmuir isotherm (Cd2+: 0.999 and Ni2+: 0.999) were markedly higher than those of the Freundlich (Cd2+: 0.697-0.764 and Ni2+: 0.968-0.973) and Temkin isotherms (Cd2+: 0.915-0.952 and Ni2+:0.979-0.988),indicating that the Cd2+adsorption mainly occurred through monolayer adsorption.Additionally,the adsorption capacity value obtained by the Langmuir isotherm was close to the experimental value indicating that the Langmuir isotherm exhibited the best fitting for the experimental results.The adsorption capacity and the constantkLof the Langmuir isotherm increased as the temperature increased,further indicating that zeolite X had a strong adsorption effect on the heavy metal ions.The values of the separation coefficient (RL) of the Cd2+and Ni2+ions were all between 0-1 (Table S2),indicating that the Langmuir isotherm adsorption process is the favorable adsorption process,and showing that the zeolite X obtained from stellerite conversion had a strong affinity for Cd2+and Ni2+ions,which is beneficial to adsorb Cd2+and Ni2+.

Table 3Correlation coefficients of the isotherm models for Cd2+ and Ni2+ions adsorption

The order of the adsorption capacities for Cd2+and Ni2+ions on the zeolite X is Cd2+＞Ni2+.The adsorption capacity is related to the type and pore size of zeolites,the hydration radius and hydration energy of heavy metal ions.When the hydration ion radius of the heavy metal ion is small,the heavy metal ion can be easily adsorbed by the zeolite X.The hydration radius of Ni2+(0.404 nm)is smaller than that of Cd2+(0.426 nm),but the adsorption capacity of Ni2+was lower than that of Cd2+.Because the hydration energy of Ni2+(2121 kJ·mol-1) is higher than that of Cd2+(1806 kJ·mol-1),Ni2+ions cannot easily enter the pore diameter of zeolite X,and the adsorption capacity is low [33].The adsorption capacity order of zeolite A synthesized by Baoet al.[34,35]for heavy metal ions is Pb2+＞Cu2+＞Cd2+＞Ni2+.The hydration ion radius of Pb2+,Cu2+,Cd2+,Ni2+are 0.401,0.419,0.426,and 0.404 nm,respectively,and the hydration energiesare 1481,2100,1806,and 2121 kJ·mol-1,respectively.The hydration energy of Ni2+is higher than that of other heavy metal ions,and the adsorption capacity of Ni2+onto zeolite A is the lowest.Thus,it was demonstrated that the adsorption capacity can be significantly influenced by the hydration enthalpy of heavy metal ions.

Fig.5.Influence of (a) the initial concentration of Cd2+ and Ni2+ ions and the linear fitting of the (b) Freundlich,(c) Langmuir,and (d) Temkin isotherms (contact time=180 min,pH=6,zeolite X dosage=0.02 g).

3.2.4.Adsorption thermodynamics

The obtained ΔH,ΔSand ΔGparameters due to the adsorption of Cd2+and Ni2+ions were estimated using Eqs.(12)and(13)[16].The lnKdversus1/Tplots of the case of zeolite X are linear as shown in Fig.6[16].Table 4 lists the obtained thermodynamic parameters due to the zeolite X adsorption of the Cd2+and Ni2+ions.When ΔH＞0,the adsorption process is an endothermic reaction,otherwise it is an exothermic reaction.The ΔHvalues for the zeolite X adsorption of Cd2+and Ni2+ions were 31.039 and 13.329 kJ·mol-1,respectively.Therefore,the zeolite X adsorption of Cd2+and Ni2+ions was an endothermic reaction.The ΔGvalues for the zeolite X adsorption of Cd2+and Ni2+ions were-6.670 to-4.298 kJ·mol-1and -1.150 to -0.240 kJ·mol-1,respectively.Hence,the negative sign indicates that removal of Cd2+and Ni2+ions using zeolite X was spontaneous.The ΔSvalues for the zeolite X adsorption of Cd2+and Ni2+ions were 118.581 and 45.532 J·mol-1·K-1,respectively.Hence,the positive sign indicates that the removal of Cd2+and Ni2+ions at the adsorbent/solution interface was very random[16].

Fig.6.Plot of lnkL vs. 1/T for Cd2+ and Ni2+ at different temperatures.

Table 4Thermodynamic parameters for the adsorption of Cd2+ and Ni2+ onto zeolite X

3.3.Reusability of zeolite X

The adsorbent after the adsorption of Cd2+and Ni2+ions can release heavy metal ions under certain conditions,introducing secondary pollution to water bodies.Therefore,desorption studies can help to elucidate the complete adsorption mechanism of zeolite X[36].HCl (0.05 mol·L-1),NaOH (0.05 mol·L-1),NaCl (1.0 mol·L-1)and H2O were selected as desorption agents to conduct regeneration experiments on the zeolite X after the adsorption Cd2+and Ni2+ions,and the experimental results are shown in Fig.7(a).The regeneration capacity of the four desorption agents on zeolite X was NaCl ＞HCl ＞NaOH ＞H2O.Heavy metal ions were desorbed from zeolite X by HCl,but the HCl damaged the zeolite X framework.Additionally,NaOH and H2O cannot reaction with heavy metal ions;therefore,they cannot easily to desorb heavy metal ions from zeolite X.Therefore,NaCl was selected as the desorption agent.

The reusability of zeolite X was determined by regenerating the used zeolite X three times in 1.0 mol·L-1NaCl and then by applying the recycled material to Cd2+and Ni2+ions removal.Fig.7(b)shows that the adsorption performance of zeolite X for Cd2+and Ni2+ions decreased with an increasing of the number of reuse cycles.The adsorption capacity of Cd2+ions on zeolite X in the three adsorption-desorption cycles were 78.640,68.770,and 57.040 mg·g-1,respectively.Similarly,the adsorption capacity of Ni2+on zeolite X were 71.090,61.516,and 67.658 mg·g-1,respectively.It is considered that zeolite X can be applied to the adsorption of aqueous Cd2+and Ni2+ions adsorption;however,the reusability of zeolite X for Ni2+ions adsorption is significantly better than that for Cd2+ions.The once-recycled zeolite X was mixed with a 1 mol·L-1HCl solution.When the Cd2+and Ni2+ions were completely dissolved from the zeolite X,the supernatant was analyzed using ICP,which showed that the content of Cd2+and Ni2+ions in the zeolite X was 90.372 and 2.220 mg·g-1,respectively.The Cd2+ions residue in the zeolite X was considerably higher than that of the Ni2+ions in the reusability experiments.This could be because Cd2+ions form a complex with zeolite X,and it is difficult for NaCl to completely exchange Cd2+ions out,which affects the recycling effect.

3.4.The adsorption mechanism

3.4.1.Zeta potential analysis

When ΔH＞40 kJ·mol-1,the adsorption process is dominated by chemisorption.When ΔH＜40 kJ·mol-1,the adsorption process is mainly dominated by physical adsorption.Table 4 shows that the ΔHvalues of the zeolite X adsorption of Cd2+and Ni2+ions were between 0 and 40 kJ·mol-1.Hence,zeolite X exerted a physical adsorption effect on the Cd2+and Ni2+ions.Fig.8 shows the variation trend of the zeta potential of zeolite X against the pH value.The Fig.8 shows that the zeta potential of the zeolite X at pH 2.0-10.0 is negative,indicating that the zeolite X surface was negatively charged,which enabled the electrostatic adsorption between the zeolite X and the Cd2+and Ni2+ions,which are positively charged.Additionally,combining Figs.2 and 8,it can be seen that the zeta potential of zeolite X increased with an increasing pH value,leading to an increase in the electrostatic adsorption of Cd2+and Ni2+ions,which is also an important reason why the adsorption capacity of zeolite X for Cd2+and Ni2+ions increased with an increasing of pH value.Therefore,it can be determined from Table 4,and Figs.2 and 8 that when ΔH＜40 kJ·mol-1,the physical adsorption of Cd2+and Ni2+ions by zeolite X was mainly electrostatic adsorption.

Fig.7.Reusability of the zeolite X for Cd2+ and Ni2+ ions removal (contact time=180 min,pH=6,zeolite X dosage=0.02 g,and temperature=298 K).

Fig.8.Variation trend of the zeta potential of zeolite X against the pH value.

3.4.2.XRD analysis

Fig.9(a) shows that the as-synthesized zeolite X exhibited a high crystallinity and purity (almost no impurities).The intensity of the diffraction peaks of the sample was lower than that of the zeolite X,mainly because the number of water molecules adsorbed in the zeolite X changed.Water molecules have a shielding effect on the diffraction peaks.The diffraction peak of zeolite X at 2θ=6.07° after the adsorption of Cd2+ions (zeolite X-Cd2+) and Ni2+ions (zeolite X-Ni2+) shifted toward a higher diffraction angle(zeolite X-Cd: 2θ=6.12°,zeolite X-Ni: 2θ=6.08°),indicating that Na+ions(0.102 nm)were replaced by smaller Cd2+(0.095 nm)and Ni2+(0.069 nm) ions,which reduced the cell size dimensions[28].XzeoliteX-Cd/XzeoliteXandXzeoliteX-Ni/XzeoliteXrepresent the relative crystallinity of zeolite X after exposure to the metal ion solutions.Based on the calculated values,the crystallinity loss was in the order of zeolite X-Cd ＞zeolite X-Ni.However,the diffraction peaks of zeolite X at 2θ=9.95° and 11.68° after the adsorption of Cd2+ions disappeared,indicating that the zeolite X phase changed significantly.The above XRD result shows that zeolite X transformed into a complex zeolite X containing Cd and H (zeolite (Cd,H),PDF card:01-089-6294),which is due to the Cd2+ions penetrating deeply into the interior of the zeolite X and substituting many of the Na ions,indicating that Cd2+ion adsorption occurred in the interior and outer surface of the zeolite X.According to a previous report[37],Cd2+ion coordination polyhedral are formed in the zeolite X,lead to geometric changes in the aluminosilicate framework of zeolite X.However,the number of diffraction peaks of zeolite X did not decrease after the adsorption of Ni2+ions,indicating that its crystal structure was not destroyed.Ni2+ions were mainly adsorbed on the outer surface of the zeolite X and a small part was adsorbed in the pore channels.The zeolite X-Ni corresponds to zeolite(Na,Ni),PDF card:01-087-1190.Therefore,it can be concluded that the Ni2+ions substituted the Na+ions of the super cages and sodalite cages,and then the Ni2+ions combined with the water molecules and O atoms in the framework structure to form complex clusters [38].

3.4.3.FT-IT analysis

Fig.9(b) shows that the T-O (T=Si,Al) bending vibrations of the internal tetrahedron in the zeolite X were observed at 464.33 cm-1.The absorbance band at 564.58 cm-1was attributed to the double ring vibration (D6R),and the absorbance band at 672.65 cm-1is due to the symmetric stretching of the internal tetrahedron.The symmetric stretching vibrations of the external tetrahedron were observed at 754.38 cm-1,and the absorbance band at 987.04 cm-1corresponded to the asymmetric stretching of internal tetrahedron.Finally,the band at 1636.12 cm-1was associated with the presence of zeolite water (bending vibration of water molecules) [28].The broad peak from 3479.10 cm-1was been associated with the stretching of the hydroxyl groups(-OH)of the zeolite X structure.Comparing the spectra of the zeolite X with those of zeolite X-Cd and zeolite X-Ni,it is clear that some peaks appeared or were shifted.The absorbance band at 987.04 cm-1,which was attributed to the Cd-O or Ni-O vibrations,shifted to 1000.12 cm-1(zeolite X-Cd)and 987.25 cm-1(zeolite X-Ni) because the vibration frequencies can be altered depending on the combination of cations with O,mass,charge and ion size [39].After the adsorption of Cd2+and Ni2+ions,new peaks were visible at 935.82 and 920.09 cm-1for zeolite X-Cd and zeolite X-Ni,respectively,which were assigned to Cd2+or Cd(OH)+and Ni2+or Ni(OH)+ions,respectively (Fig.9(b)) [39].The peak at 3479.10 cm-1shifted to 3558.86 cm-1(zeolite X-Cd) and 3399.80 cm-1(zeolite X-Ni)and the peak at 1636.12 cm-1shifted to 1634.71 cm-1(zeolite X-Cd) and 1634.85 cm-1(zeolite X-Ni)indicating that an attachment of Cd2+and Ni2+ions to the -OH groups in the zeolite X and sediment of Cd(OH)2and Ni(OH)2maybe be formed [28,39],and surface precipitation may be involved in the adsorption of Cd2+and Ni2+ions onto zeolite X.

Fig.9.(a) XRD and (b) FT-IR spectra of zeolite X,zeolite X-Cd,and zeolite X-Ni.

3.4.4.FESEM analysis

Figs.10 and 11 show that the morphology of zeolite X and the samples remained unchanged after adsorption,mainly presenting an octahedral shape.As is shown in Table S3 there was no Cd2+and Ni2+ions before adsorption.According to the EDS analysis,the actualn(Si/Al) of zeolite X was 1.18,which changed very little after adsorption.The Cd2+and Ni2+ions appeared after adsorption,and the mole ratio of Na decreased significantly.It was further confirmed that the adsorption of Cd and Ni on the zeolite X mainly occurred through ion exchange.The zeolite X converted from stellerite zeolite has a unique meso-microporous structure (specific surface area 353.767 m2·g-1and pore size 2.198 nm),which can promote the diffusion and transportation of heavy metal ions to the inner and outer surfaces of the zeolite X,and increase the heavy metal ions contact with the zeolite X surface.The Cd and Ni distribution maps show that,the Cd2+and Ni2+ions were evenly dispersed on the zeolite X surface.

Fig.10.FESEM images and EDS analysis of zeolite X-Cd.

3.4.5.XPS analysis

XPS analyses were conducted to further understand the mechanism of Cd2+and Ni2+ions removal by zeolite X.The distinct peak of Na 1s is visible in the zeolite X spectrum(Fig.12(a)).The intensity of the Na 1s peak decreased significantly after the adsorption experiments.This result indicates that Na+ions were exchanged with Cd2+and Ni2+in solution on zeolite X surface.Fig.12(b)shows the deconvoluted O 1s spectrum.The zeolite X peaks at 531.2,531.9,and 532.6 eV corresponded to the oxygen in Al-O-,Si-O-Si,and Si-OH,respectively [28].After adsorption of the Cd2+and Ni2+ions,the binding energy of 532.6 eV in the zeolite X shifted to a higher energy of 532.9 and 533.4 eV,and the binding energy of Si-O-Si shifted from 531.9 to 532.2 and 532.4 eV,because the replacement of the heavier atom causes the O 1s peak to shift to a higher binding energy[28].The Si 2p spectra could be deconvoluted into two SiOx(x=0-4) signal peaks positioned at 102.0 and 102.7 eV in zeolite X [28].After adsorption of Cd2+and Ni2+,the interaction between Si and the Cd2+and Ni2+ions caused the peak to shift to a higher binding energy (Fig.12(c)).As illustrated in Fig.12(d),two peaks appeared at 406.2 eV and 412.9 eV,which were assigned to Cd 3d5/2and Cd 3d3/2,respectively,demonstrating that Cd2+was successfully adsorbed on zeolite X [36].The high resolution XPS spectrum of Ni 2p (Fig.12(e))shows that four peaks appeared at 853.9,856.8,872.0,and 874.7 eV with two satellites at 863.3 and 880.6 eV which were attributed to Ni 2p3/2and Ni 2p1/2,respectively [39].Thus,these phenomena confirm that the Cd2+and Ni2+ions reacted with the-OH groups of zeolite X [28].The obtained results demonstrate that the Cd2+and Ni2+ions were attached to the zeolite X surface by surface precipitation [36].

Fig.11.FESEM images and EDS analysis of zeolite X-Ni.

Fig.12.(a)XPS wide scan showing the surface elemental composition of zeolite X,zeolite X-Cd,and zeolite X-Ni.High-resolution XPS of(b)O 1s,(c)Si 2p,(d)Cd 3d,and(e)Ni 2p of zeolite X after Cd2+ and Ni2+ ions adsorption.

It can be concluded from the reusability experiments and characterization analyses that three types of phenomenon occur during the zeolite X adsorption of Cd2+and Ni2+ions,namely ion exchange,electrostatic adsorption and precipitation (Fig.13)[36].The ΔHvalues were ＜40 kJ·mol-1,proving that physical adsorption occurred during the adsorption of Cd2+and Ni2+ions onto zeolite X.The zeta potential of zeolite X in the pH range of 2.0-10.0 was negative,helping zeolite X adsorb positively charged heavy metal ions by electrostatic attraction.Because the electrostatic attraction is a physical adsorption process,its effect on zeolite X adsorptionof Cd2+and Ni2+ions is relatively small.The zeolite X adsorption of Cd2+and Ni2+ions is mainly determined by chemical adsorption [36].The pseudo-second-order kinetics and the Langmuir isotherm models exhibited excellent correlation with the Cd2+and Ni2+ions adsorption onto zeolite X,proving that the chemisorption process is mainly responsible for the adsorption of Cd2+and Ni2+ions onto zeolite X.For the case of cations exchange,Na+was an important mechanism for the removal of Cd2+and Ni2+ions from the aqueous solution due to the high ion exchange capacity of zeolite X [36].This process can be expressed by Eq.(14) [36].Additionally,some Cd2+and Ni2+ions were adsorbed on the -OH group sites of zeolite X,as determined by the FT-IR and XPS analyses.Thus,the removal of Cd2+and Ni2+ions by zeolite X may occur through a precipitation process,which can be expressed as Eq.(15) [36].Furthermore,in the reusability experiment with NaCl,only some of the Cd2+and Ni2+ions adsorbed by the zeolite X by electrostatic adsorption and ion exchange could be desorbed.It was difficult for Na+ions to desorb some coordination polyhedral,formed by ion exchange,and sediments of Cd(OH)2and Ni(OH)2formed by precipitation,which impacted the recycling effect.This phenomenon demonstrates that ion exchange significantly influenced the adsorption of Cd2+and Ni2+ions.Concurrently,reusability experiments results demonstrate that Cd(OH)2and the coordination polyhedra of Cd2+significantly affected the desorption of Cd2+ions.Therefore,the main adsorption mechanism of zeolite X for Cd2+ions is ion exchange and precipitation.The reusability experiments results show that the Ni2+ions in zeolite X can be adequately exchanged by NaCl,while the main adsorption mechanism for Ni2+ions is ion exchange.

where M is the Cd or Ni ion.

3.5.Comparisonof heavy metal removal efficiency of zeolite X

Fig.14 shows XRD patterns and FESEM images of the samples synthesized by the two-step and traditional hydrothermal method.The XRD patterns exhibit the characteristic diffraction peaks of zeolite X at 2θ=6.07°,9.95°,11.68°,15.38°,20.03°,23.26°,26.61°,and 30.90°.The diffraction peaks were intact,and the strength of the diffraction peaks was high,indicating that the products had high purity and good crystallization.The intensity of diffraction peaks of zeolite X synthesized by the two-step hydrothermal method was higher,indicating that the crystallization was better than that of the zeolite X synthesized by the traditional hydrothermal method.The morphology of the products synthesized by the two methods was a typical octahedral structure,and crystals with a size of approximately 2 μm were evenly dispersed.When theC0of Cd2+and Ni2+ions solution was 200 and 100 mg·L-1,respectively,at 298 K,the adsorption capacity of the zeolite X synthesized by the traditional hydrothermal method for Cd2+and Ni2+ions was 55.210 and 36.557 mg·g-1,respectively(Table 5).The adsorption capacity of the products of the traditional hydrothermal method was lower than that of the two-step hydrothermal method,which may be caused by the lower relative crystallinity of the products of the traditional hydrothermal method.

Fig.13.Schematic diagram of the adsorption and desorption of Cd2+ and Ni2+ ions on zeolite X.

Fig.14.(a) XRD patterns and (b and c) FESEM images of the samples synthesized by different methods.

Table 5 shows the adsorption capacity of zeolite X prepared with different raw materials for Cd2+and Ni2+ions.As show in Table 5,the adsorption capacities of other reported zeolite X are in the range of 38.610-169.490 mg·g-1(Cd2+) and 15.936-49.700 mg·g-1(Ni2+).The adsorption performances of zeolite X to Cd2+and Ni2+ions decreased with an increasing number of reuse cycles(Fig.7(b)).The adsorption capacity of the third adsorption-desorption cycle of the zeolite X was used to compare it with other adsorbents.Accordingly,the adsorption capacity of zeolite X for Cd2+and Ni2+ions was 57.040 and 67.658 mg·g-1,respectively.The adsorption capacity of zeolite X obtained in this study was high,as compared to the other reported adsorbents (Table 5).Based on the analysis of the adsorption mechanism,we propose that the zeolite X adsorption capacity for Cd2+and Ni2+ions depends on the comprehensive influence of three factors,namely (1) then(Si/Al) of the zeolite X framework,(2) -OH groups of zeolite X,and (3) purity and crystallinity of zeolite X [41].Then(Si/Al) of the obtained zeolite X is relatively smaller than those of other studies,endowing it with a strong ion exchange performance and electrostatic attraction.Second,the FT-IR and XPS analyses showed that there are many -OH groups on zeolite X,which can form Cd(OH)2and Ni(OH)2sediments from the Cd2+and Ni2+ions,respectively,enhancing the adsorption effect.This point is rarely reported in other studies on the adsorption of Cd2+and Ni2+ions by zeoliteX.Third,the zeolite X converted from stellerite zeolite exhibited high crystallinity,high purity,and a large specific surface area,which is conducive to the dispersion of heavy metal ions and adsorption on the zeolite surface.Because then(Si/Al),the number of -OH groups,and the purity and crystallinity of zeolite X prepared using different raw materials and synthesis methods is different [41],therefore,the Cd2+and Ni2+ions adsorption capacities of zeolite X are different.In summary,the zeolite X which was converted from stellerite zeolite through the two-step hydrothermal synthesis method in this study had a good adsorption capacity,as compared to zeolite X synthesized from other pure reagents,natural minerals,and waste.

Table 5The zeolite X adsorption capacities X for Cd2+ and Ni2+ ions

4.Conclusions

Zeolite X with a significant adsorption affinity for Cd2+and Ni2+ions,was prepared using stellerite zeolite as the raw material.The adsorption of Cd2+and Ni2+ions was favourable at pH 6,because there was no competitive effect of H+to bind to the same adsorption active sites.Zeolite X exhibited a large adsorption capacity for the removal of Cd2+and Ni2+ions in a relatively shorter time.The pseudo-second-order kinetics model can best describe the adsorption kinetics.The analysis of the adsorption isotherm for Cd2+and Ni2+ions agreed well with the Langmuir isotherm models,and the adsorption process was endothermic and spontaneous.The maximum zeolite X adsorption capacity for Cd2+and Ni2+ions at 298 K was 173.553 and 75.897 mg·g-1,respectively.In addition to the adsorption of Cd2+and Ni2+ions by ion exchange and electrostatic interaction,zeolite X can also combine with Cd2+and Ni2+ions through the -OH groups to form sediments of Cd(OH)2Ni(OH)2,respectively,which then precipitate on the zeolite X surface.Due to the low hydration enthalpy,Cd2+ions can enter deeply into the interior of the zeolite X pores to form more coordination polyhedra and Cd(OH)2sediment,hindering its desorption by NaCl,which affects the use of recycled zeolite X.Based to the experiments and characterization analysis,ion exchange and precipitation were the principal mechanism in the removal of Cd2+ions from an aqueous solution by zeolite X,followed by electrostatic adsorption.Ion exchange was the principal mechanism in the removal of Ni2+ions from an aqueous solution by zeolite X,followed by electrostatic adsorption and precipitation.Based on the comparison of zeolite X synthesized using different raw materials,the zeolite X converted from stellerite zeolite presented a in lown(Si/Al),more hydroxyl groups,high crystallinity,high purity and large specific surface area,leading to a stronger adsorption performance.Therefore,the above finding suggests that zeolite X converted from stellerite zeolite by the two-step hydrothermal method can be used as a promising efficient absorbent for the elimination of Cd2+and Ni2+ions from wastewater.

Data Availability

Data will be made available on request.

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 the National Natural Science Foundation of China (51564008,41662005) and Natural Science Foundation of Guangxi Province (2019GXNSFBA245083).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2022.06.008.