Dongmei Jia *,Aimin Li*,Changhai Li,Guoxia Liu ,Yuejin Li

1 State Key Laboratory of Pollution Control and Resources Reuse,School of the Environment,Nanjing University,Nanjing 210023,China

2 Department of Chemical Engineering,Binzhou University,Binzhou 256603,China

1.Introduction

Although atrazine(2-chloro-4-ethylamino-6-isopropylamino-striazine,Fig.1)is unsafe for both humans and the environment,it is still widely used as the herbicide throughout the world[1–3].The emission of atrazine in wastewater is 2–3 times its output.This wastewater,usually containing atrazine,sodium chloride,isopropylamine,monoethylamine and a high concentration of organic byproducts,causes serious environmental pollution.Because atrazine disrupts the production of normal human hormones,it may act as a significant carcinogen in both humans and laboratory animals[4–8].Therefore,some countries have set maximum contaminant levels(MCLs)for atrazine,such as 3 ppb according to the US Environmental Protection Agency.Therefore,effectively dealing with and reclaiming atrazine from wastewater have become important research topics.Various techniques such as reductive degradation and adsorption have been used to remove atrazine from both wastewater ef fluents and drinking water.Because atrazine is chemically and biologically stable,it is very difficult to remove it from water[9].Some studies have shown that adsorption is one of the most important techniques for removing atrazine from water.Many adsorbents have been investigated,including activated carbon[10–13],banana peel[14],zeolites[15,16],and lipoid adsorption materials[17–19].

Macroporous resins have been used to remove contaminants from water due to their huge specific surface areas,well-developed pore structures,safety,environmental friendliness and macromolecular compounds[20–22].Nanometalhydroxide has been widely used as environment function material due to their large specific area and high specific surface energy and excellentelectric charges or active functional groups.The coordinate bonding is surface metal oxyhydroxide particles with high affinity toward ligand sym-triazine group.Furthermore,application of metal hydroxides is limited by their fine particles and the fact that they need to be used in a fixed-bed.Therefore,the composite resin of iron or aluminum(hydroxides)and macroporous D301 probably may have excellent performance in adsorption of atrazine.In the present study,two composite macroporous resins were prepared to remove atrazine from water.The differences in the pore structures and surface chemistries between the resin and the associated composite resins were investigated.The adsorption behavior of atrazine on two types of resins was also studied.Wastewater usually contains sodium chloride close to its saturation point,and the presence of sodium chloride may affect the removal efficiency of atrazine.Therefore,in this study,the solubility of atrazine in various concentrations of sodium chloride in solution was investigated.

Fig.1.The structure of atrazine.

2.Materials and Methods

2.1.Materials

Atrazine was purchased from Shandong Qiaochang Chemical Industry Co.,Ltd.(Binzhou,China).All other chemical reagents were of analytical grade and obtained from Shanghai Chemical Reagent Station(Shanghai,China).D301 resin was purchased from Nankai Resin Co.,Ltd.(Tianjin,China).The desired pH was adjusted by adding either 1 mol·L?1HCl or 1 mol·L?1NaOH.The purities of the materials used for this study are listed in Table 1.

Table 1Sample description

2.2.Preparation of adsorbents

The D301 resin was washed with either1 mol·L?1HClor1 mol·L?1NaOH solution prior to the synthesis of the composite resins.Then the D301 resin was washed with deionized water until the pH of the solution reached 7.Finally,the suspension was dried.

In order to acquire two anion-exchange resins supported nanohydrous iron oxide and aluminum oxide particle(HIOD301 and HAOD301),two different salt solutions(Al and Fe salt solutions)were prepared separately.The Al salt solution was prepared by adding NaCl and AlCl3to HCl solution under magnetic stirring at 298 K.These salt solutions were stirred for another 12 h and aged at room temperature for 3 h.And D301 resin was dynamically soaked in either the Al salt solution for 72 h at 298 K.The mixtures were then filtered and rapidly added to a NaOH–NaCl solution to form HAOD301.Then the products were filtered and washed repeatedly with deionized water until their pH values were neutral.Finally,the composite resins were dried at 313 K for 24 h and stored in sealed bags.Similarly,the HIOD301 resin can be prepared using Fe salt solution.

2.3.Adsorption experiments

In order to test the effects of the composite resins on atrazine uptake,batch adsorption experiments for each resin were carried out in 250 ml stoppered conical flasks under constant shaking(180 r·min?1)in a thermostat shaker.All batch experiments were conducted under the same conditions:50 ml atrazine solution,0.05 g adsorbent dose,and 298 K.The pH effects were studied with an initialatrazine concentration of 4 mg·L?1at pH 4.0,6.0,8.0,10.0,and 12.0.Adsorption isotherms were determined with initial atrazine concentrations of 2.0,4.0,6.0,8.0,10.0,and 12.0 mg·L?1at 298 K and contact times of 24 h.Adsorption kinetic experiments were conducted with initial atrazine concentrations of 2.0,4.0,and 6.0 mg·L?1at different contact times.Coanion effects were analyzed with a co-anion system of Cl?at various concentrations of atrazine.All adsorption experiments were carried out in duplicate,and the averaged data were used to evaluate the adsorption performance.

The atrazine-uptake capacityqeat equilibrium(or at timet)of the sorbent was calculated from the following mass–balance relationship:

whereC0andCeare the initial and residual concentrations of atrazine in solution,respectively;Vis the test solution volume;andWis the mass of the adsorbent.

2.4.Characterization and analytical methods

Atrazine was measured with an ultraviolet(UV)spectrophotometer(Agilent 8453,Agilent Technologies,Santa Clara,CA).All samples were filtered with a 0.45 μm syringe-driven filter unit before the measurements.The morphological analysis of the composite resins was performed using field emission scanning electron microscopy(SEM,Sirion 200 FEI,Hillsboro,OR,USA).The iron species were identified with X-ray diffraction(XRD,D/MAX-2500/PC,Rigaku,Tokyo,Japan).The Brunauer–Emmett–Teller(BET)surface area of the HIOD301 and HAOD301 resins were measured using an automated gas sorption system(3H-2000PS2,Bei Shide Instrument—S&T Co.Ltd.,Beijing,China).

2.5.Solubility measurement

The solubility of atrazine was determined by the balance method.The setup for the solubility measurement was the same as that described in the literature[23].The amount of the solute in the saturated solution was determined by a gravimetric method.

3.Results and Discussion

3.1.Structural characterization of the composite resins

XRD patterns of the different composite resins are depicted in Fig.2.The general morphologies of HIOD301 and HAOD301 were investigated using SEM,and the results are shown in Fig.3(a)and(b),respectively.Fig.3(a)and(b)exhibits the evident morphological differences between the surfaces of HIOD301 and HAOD301 and depicts hydrous iron oxide and hydrous aluminum oxide distributed on the surface of D301.The porous structural parameters for the different samples,including the specific surface areas,total pore volumes,and average pore diameters,were obtained.The initial surface area of D301 resin was 29.46 m2·g?1,and its total pore volume was 0.13 ml·g?1.After loading with hydrous iron oxide and hydrous aluminum oxide,the surface areas became 35.80 and 32.85 m2·g?1,respectively.It may be caused by the nanohydrous oxides loading outer and inner surface of D301 resin.And the nanometal hydroxides have large specific area.

Fig.2.XRD spectra of HIOD301 and HAOD301.

3.2.Adsorption equilibrium

The effect of initial dye concentration(2.0–12.0 mg·L?1)on the adsorption capacity of HIOD301 and HAOD301 is given in Fig.4.The results revealed that the initial concentrations of atrazine changes appeared to have an appreciable impact on the adsorption capacity.It drives the mass transfer rate under a higher concentration gradient between atrazine solution and composite resin surface.Adsorption capacity of HAOD301 increased from1.34 to 7.03 mg·g?1,and the adsorption capacity of HIOD301 increased from 1.21 to 6.57 mg·g?1against an increase in atrazine concentration from 2.0 mg·L?1to 12.0 mg·L?1,respectively.Compared with D301,the adsorption capacity of atrazine on HIOD301 and HAOD301 increased 257%and 282%against the atrazine concentration(12.0 mg·L?1),respectively.

The adsorption isotherms were measured at different temperatures by varying the initial atrazine concentration(2–12 mg·L?1)for exploring the interaction mechanism of the adsorbent with the adsorbate(Fig.5).The isotherm data can be fitted by different isotherm models to optimize the adsorption system.In the present study,the equilibrium experimental data were compared using the following isotherm equations namely,Langmuir,Freundlich and Temkin.

Fig.4.Effect of initial concentration on the atrazine adsorption on D301,HIOD301 and HAOD301.(Temperature,298 K;agitation speed,180 r·min?1.) ■,D301;,HIOD301;,HAOD301.

The Langmuir model[24]is written as follows:

whereqe(mg·g?1)is the mass of atrazine adsorbed per unit mass of adsorbent,Ce(mg·L?1)is the equilibrium liquid-phase concentration,andqmax(mg·g?1)is a constant related to the area occupied by a monolayer of adsorbate,and therefore,reflects the maximum adsorption capacity.

The data listed in Table 2 show that the correlation coefficients of both HIOD301 and HAOD301 are very high,indicating good fits of the monolayer Langmuir model to the adsorption of atrazine by these two composite resins.The monolayer maximum adsorption capacities for atrazine by HIOD301 and HAOD301 were 133.04 and 53.89 mg·g?1at 298 K,respectively.

Fig.3.Morphology of resin(a)HIOD301 and(b)HAOD301.

The Freundlich isotherm is an empirical equation employed to describe heterogeneous systems.The linear form of the Freundlich equation[25]is as follows:wherek(mg·g?1)andnare Freundlich constants.Constantkis defined as an adsorption or a distribution coefficient,and it represents the mass of adsorbate adsorbed on an adsorbent for a unit equilibrium concentration.

As can be seen in Table 2,the values of correlation coefficientR2are greater than 0.984 for HIOD301 and 0.99 for HAOD301.Both the Langmuir and Freundlich isotherms fitted well with correlation coefficients greater than 0.983.The value of 1/nfell between 0 and 1,demonstrating that the surface of the adsorbent was heterogeneous.

The Temkin isotherm can be used to study the heat of adsorption and adsorbate–adsorbent interaction on the adsorbent surface[26,27].Eq.(4)gives the linear form of this isotherm:

Fig.5.Effect of temperature on the atrazine adsorption onto HIOD301(a)and HAOD301(b)

Table 2Isotherm parameters for atrazine adsorption onto the HIOD301 and HAOD301

whereBis the Temkin adsorption constant andKTis the equilibrium binding constant(L·mg?1).Based on the correlation coefficient,the results showed that only poor applicability was obtained by the Temkin isotherm(Table 2).

3.3.In fl uence of the initial pH on the adsorption of atrazine

The pH of a solution is considered to be one of the most important parameters controlling the adsorption process at water–adsorbent interfaces.It is apparent from Fig.6 that the HIOD301 and HAOD301 adsorption capacities for atrazine were dependent on pH.When the solution pH value increased from 4 to 10,the adsorption capacities of both HIOD301 and HAOD301 for atrazine decrease 67.3%to 69.0%,respectively.Such results may be caused by various factors such as the surface adsorption of the composite resins,electrostatic attraction,and coordination between the metal ions of the composite resins and atrazine.At pH 4,the adsorption of HIOD301 and HAOD301 to atrazine could be attributed to the surface properties of the adsorbents and ionization of the adsorbate molecules.The positive charge on the surfaces of the composite resins became increasingly intense because of the adsorption of H+ions.The electrostatic attraction between the positively charged composite-resin surfaces and negatively charged atrazine molecules increased,leading to increased adsorption capacities.However,the main form of atrazine was cationic,and the composite resins were protonated at pH 2;therefore,the adsorption capacities of the composite resins clearly decreased at this pH value.

Fig.6.Effect of pH on the atrazine adsorption on D301,HIOD301 and HAOD301.

3.4.Kinetic analysis

Fig.7 displays the extent of the adsorption of atrazine from an aqueous solution onto resin HIOD301 as a function of timetfor different initial concentrationsC0(4,6,and 8 mg·kg?1)at 298 K.We observed that there was a monotonic increase in the adsorption capacity with time for all the concentrations until the equilibrium adsorption amountqewas reached,and then the rate slowly increased.The amount of atrazine being adsorbed onto the adsorbent was in a state of dynamic equilibrium with the amount of atrazine being desorbed from the adsorbent.The time required to attain this state of equilibrium was termed the equilibrium time,and the amount of atrazine adsorbed at the equilibrium time reflected the maximumatrazine adsorption capacity of the adsorbent under these particular conditions.The results showed that the equilibrium time was different for different initial concentrations.It was clear that the equilibrium time increased with the initial solution concentration.Generally,the amounts of atrazine adsorbed increased rapidly in the 0–1.0 h reaction period and then more slowly in the 1.0–7.0 h reaction period,indicating that atrazine adsorption involved a multistep process.

To demonstrate the adsorption process of atrazine on composite resins,a kinetic investigation was carried out with a pseudo- firstorder model[28],pseudo-second-order model[29],and intraparticle diffusion model[30].The pseudo-second-order model assumes that chemisorption is involved in the rate-limiting step in adsorption systems.The intraparticle-diffusion model is commonly used in the analysis of liquid–solid adsorption.If intraparticle diffusion was involved in the adsorption process,then the plot of the square root of time versus the uptake(qt)would result in a linear relationship,and intraparticle diffusion would be the rate-limiting step if this line passes through the origin.The three models can be expressed as follows:

Fig.7.Effect of contact time on the adsorption of atrazine onto HIOD301(a)and HAOD301(b).(Temperature,298 K;agitation speed,180 r·min?1.)■,2.0 mg·L?1;,4.0 mg·L?1;,6.0 mg·L?1.

whereqerepresents the amount of atrazine adsorbed at equilibrium(mg·g?1);qtis the amount of atrazine adsorbed(mg·g?1)at certain timet;tis the time(h);k1(h?1)andk2[g·(mg·h)?1]are the rate constants of pseudo- first-order and pseudo-second-order adsorption,respectively;andktis the intraparticle-diffusion-rate constant[mg·(g·h0.5)?1].The coefficient of determination (correlation coefficient)(R2)of the kinetic models expresses the degree of conformity between the experimental data and theoretical values predicted by models.A relatively highR2value for the relationship between the measured and predicted atrazine adsorption data indicated that the model successfully described the kinetics.The model parameters calculated from the various models and their correlation coefficients are listed in Table 3.

From Table 3,we can conclude that the pseudo-second-order equation provided the best correlation coefficient and agreement between the calculatedqevalues and the experimental data,whereas the pseudo- first-order and intraparticle-diffusion equations did notprovide such good fits to the experimental data for the adsorption of atrazine.This fact suggested that the chemical adsorption was the rate-limiting step.The reaction mechanism may have partly resulted from the coordination reaction between atrazine and the metal groups present on the composite-resin surfaces.

3.5.The solubility of atrazine

For understanding the effect of adsorbate solubility on adsorption,the solubility of atrazine was measured in the temperature range 292–353 K in aqueous sodium chloride solutions of five different mass concentrations by using an equilibrium method.As shown in Fig.8,the solubility of atrazine in such solutions increased with temperature,but this increase varied according to the different mass concentrations of sodium chloride.According to Fig.8,the solubility of atrazine in aqueous sodium chloride solution decreased with increasing mass concentrations of sodium chloride.Sodium chloride has a salting-out effect on the solubility of atrazine,and therefore,it negatively impacted the adsorption of both HIOD301 and HAOD301 to atrazine.

Table 3Kinetic model parameters of atrazine adsorption onto the HIOD301 and HAOD301

Fig.8.Experimental mole fraction solubility(x)of atrazine vs.temperature(T)in aqueous sodium chloride solution.

3.6.Effects of sodium chloride

To analyze the effects of sodium chloride,experiments were carried out on six aqueous solutions containing different atrazine concentrations but similar composite-resin dosages,obtained by the addition of equivalent amounts of sodium chloride.Experimental results,described in terms of the adsorption capacities versus the ratios of the concentrations of sodiumchloride to atrazine,are reported in Fig.9.Increasing the sodium chloride to atrazine concentration ratio led to a significant decrease in the adsorption capacity.In fact,when the sodium chloride to atrazine concentration ratio was 6,the adsorption capacities of both HIOD301 and HAOD301 in the six aqueous solutions were less than 10.6%and 12.8%,respectively.However,the adsorption capacities of HIOD301 and HAOD301 were unaffected when the sodium chloride to atrazine concentration ratio was as great as 3.

3.7.Adsorption–regeneration cycles

Fig.10.Regeneration performance of HIOD301 and HAOD301(atrazine concentration,12 mg·L?1;temperature,298 K;agitation speed,180 r·min?1;pHwithoutany adjustment).

Fig.9.Effect of coexisting anion(Cl?)on atrazine adsorption capacity.■,D301;,HIOD301;,HAOD301(initial NaCl concentration,12 mg·L?1;atrazine concentration,2,4,6,8,10,12 mg·L?1;temperature,298 K;agitation speed,180 r·min?1).

In order to evaluate the reliability and feasibility of HIOD301 and HAOD301 for potential application,the cyclic adsorption–regeneration of HIOD301 and HAOD301 was performed using 8 wt%NaOH aqueous solution as the desorption agent,and the results are shown in Fig.10.As shown in Fig.10,atrazine could be effectively removed by HIOD301 and HAOD301 after 5 cycles,and the adsorption capacity of atrazine on HIOD301 and HAOD301 was decreased 7.9%and 10.2%,respectively.Therefore,due to the role of nanohydrous iron oxide or aluminum oxide particle and sufficient pore regions,HIOD301 and HAOD301 show attractive adsorption selectivity as wellas high and stable removal efficiency of atrazine.

3.8.Mechanism discussion

Fig.11 shows the adsorption pathway of atrazine on HIOD301.The mechanisms involved in the uptake of adsorbate by an adsorbent such as atrazine may be through specific adsorption processes.The main adsorption mechanism of atrazine onto HIOD301 composite resin may be the coordination structure with three N of ring in atrazine with hydrous iron oxide from the adsorbent surface.

Fig.11.The mechanism of atrazine adsorption onto HIOD301.

4.Conclusions

HIOD301 and HAOD301 were synthesized and analyzed along with resin D301 in order to compare their individual abilities to remove atrazine from water.The HIOD301,HAOD301,and D301 resins had similar BET surface areas and pore volumes.But atrazine adsorption isotherms showed that HIOD301 and HAOD301 had larger adsorption capacities than D301,suggesting that the hydrous metal oxide was advantageous for improved atrazine adsorption from water.The pH-dependence studies demonstrated that HIOD301 and HAOD301 strongly adsorbed atrazine at pH 4.The adsorption of atrazine fitted well to the Langmuir and Freundlich isotherm models.The kinetic results showed that the pseudo-second-order equation fitted the experimental data very well.Both HIOD301 and HAOD301 also displayed much greater effectiveness in selectively adsorbing atrazine over D301 in the presence of different concentrations of sodium chloride.

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