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Loess clay based copolymer for removing Pb(II) ions

Journal of Hazardous Materials 227–228 (2012) 334–340

Contents lists available at SciVerse ScienceDirect

Journal of Hazardous

Materials

j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /j h a z m a

Loess clay based copolymer for removing Pb(II)ions

Yu-Feng He,Ling Zhang,Rong-Min Wang ?,Hui-Ru Li,Yan Wang

Key Laboratory of Eco-Environment-Related Polymer Materials of Ministry of Education,Key Laboratory of Polymer Materials of Gansu Province,Institute of Polymer,Northwest Normal University,Lanzhou 730070,China

h i g h l i g h t s

The loess clay based copolymer was prepared using functional monomers.

Characterization of the polymer adsorbent and the raw material were carried out. The adsorption behavior of the complex for Pb(II)ions was evaluated.

The removal rate of Pb(II)got to 99%and the adsorption capacity got to 356.9mg/g.

a r t i c l e

i n f o

Article history:

Received 19January 2012

Received in revised form 17May 2012Accepted 19May 2012

Available online 27 May 2012

Keywords:Loess

Polymer adsorbent Lead

In situ polymerization Kinetics

a b s t r a c t

Functional monomers,such as acrylic acid and 2-hydroxyethyl methacrylate were supported into loess clay in situ polymerization,which afforded loess clay based copolymer (LC/PAAHM),a new kind of poly-mer adsorbent for removing Pb(II)ions from aqueous solution.Characterization of the polymer adsorbent was carried out by different sophisticated methods,such as Fourier transformation infrared spectrome-try (FTIR),scanning electron microscopy (SEM),X-ray diffractometry (XRD),thermogravimetric analysis (TGA),and Zetasizer.Batch experiments were carried out to evaluate the factors affecting the removal ef?ciency,in which the pH,the adsorbent dosage,temperature and initial Pb(II)concentration all found in positive relevance to the increase of Pb(II)removal ef?ciency.The removal rate of Pb(II)got to 99%at room temperature and the adsorption capacity got to 356.9mg/g.The pseudo-?rst-order and pseudo-second-order kinetic models were applied to test the experimental data,and Langmuir and Freundlich models have been applied to study the adsorption equilibrium,respectively.

1.Introduction

At present heavy metals are among the most important pol-lutants in source and treated water therein is becoming a matter of environmental concern.Unlike organic pollutants,which are susceptible to biological degradation,metal ions do not degrade into any harmless end products [1]and tend to accumulate in liv-ing organisms,causing various diseases and disorders [2].Mining,leather industry,jewelry,chemical,paint,large scale electrical and electronics industries in industrial nations,and also arts and crafts in developing countries are the main source for metal containing waste pollution [3].Lead is one of the most useful metals and has been used as an additive of gasoline,paint,furniture,and porce-lain,etc.It can damage nervous connections (especially in young children)and cause blood and brain disorders.Lead poisoning

Abbreviations:LC,loess clay;LC/PAAHM,loess based poly(acrylic acid-2-hydroxyethyl methacrylate)complex;AA,acrylic acid;HEMA,2-hydroxyethyl methacrylate;GMA,glycidyl methacrylate.

?Corresponding author.Tel.:+869317970358.

E-mail address:wangrm@https://www.wendangku.net/doc/f51938871.html, (R.-M.Wang).

typically results from ingestion of food or water contaminated with lead.It can enter the human body through the uptake of food (65%),water (20%),and air (15%)[4].The limit for lead in drinking water is 50?g/L [5].The presence of lead has a potentially damaging effect on human physiology and other organism when the tolerance level is exceeded.

Many methods of treatment for heavy metal wastewater have been reported [6],such as precipitation,ion exchange,membrane separation,and electrolysis.However,these processes have some disadvantages,including low treatment ef?ciency for trace amount of heavy metal ion,high operational cost,and dif?cult further treat-ment due to generation of toxic sludge [7].Adsorption is also a method that is suitable for removing heavy metals from wastewa-ter and water supplies.Furthermore,this method had been proved to be superior to other methods for treatment of the water on the grounds of low cost,?exibility,practicality,and simplicity of oper-ation.The adsorption,with the selection of suitable adsorbents,can be highly ef?cient for the removal of various impurities/pollutants from wastewater [8].Some of the reported adsorbents are acti-vated carbon [9],zeolite [10],clay [11],sludge-soil [12],phosphate rock [13],calcite [14],biosorbent [15]and lignin [16].Now,in order to improve usability,it is very important to increase

Y.-F.He et al./Journal of Hazardous Materials227–228 (2012) 334–340335 adsorption capacity with cost down.In China,the Loess Plateau,

the soil of which has been called the“most highly erodible soil

on earth”[17]cover almost all the northwestern region,and parts

of others.As being rich in raw material and cheap,loess can be

used to remove the heavy metals from aqueous solution[18].For

instance,Tang et al.have found that the calcinated Chinese loess

can remove Zn(II)and the adsorption capacity was113.6mg/g[19].

Xing et al.have studied the removal of Pb(II)ions using red loess

[20].It demonstrated that the loess could be used as a possible

alternative low-cost adsorbent for the removal of Pb(II)ions from

aqueous solution.In the present study,the ability of loess clay to

adsorb Pb(II)ions from aqueous solutions was investigated.Fur-

thermore,the loess clay was modi?ed by in situ polymerization of

acrylic acid(AA)and2-hydroxyethyl methacrylate(HEMA),which

afforded loess based poly(acrylic acid-2-hydroxyethyl methacry-

late)complex(LC/PAAHM).In order to investigate the feasibility of

LC/PAAHM as highly ef?cient adsorbents for the removal of Pb(II)

from aqueous solutions,some in?uence factors,such as tempera-ture,contact time,pH value and adsorbent dosage were studied.

2.Materials and methods

2.1.Materials and reagents

Loess clay(LC),the starting material,was obtained for free from the local hill near Lanzhou of China.AA was distilled under reduced pressure before use.HEMA,glycidyl methacrylate(GMA) and ammonium persulfate are all analytical reagents and they are commercially available.

2.2.Preparation of polymer adsorbent(LC/PAAHM)

The raw material,LC,was ground,sieved(100mesh)and air-dried.The LC and AA(5.0g/7.2g)were dispersed in30mL of distilled water for1h at room temperature,and then sodium hydroxide(70%neutralization degree)was added in the mixture. After adding0.72g of HEMA in the precursor solution and stir-ring30min,0.72g of GMA as the cross-linking agent was put in.The solution was gradually heated to50?C under the pro-tection of nitrogen for30min.Then ammonium persulfate was introduced into the mixture under continuous stirring.After the mixture reacted for40min at85?C under nitrogen atmosphere,the loess based poly(acrylic acid-2-hydroxyethyl methacrylate)com-plex(LC/PAAHM)was obtained.Finally,the product was washed with distilled water for several times and dried at80?C in hot air oven until the weight of the product was constant.The material was sieved with a100mesh and stored in a tight plastic container.

2.3.Characterization of adsorbent

By use of an X-ray diffractometer(D/max-2400,Rigaku Cor-poration,Japan),we got X-ray diffraction(XRD)analysis of the adsorbents.A morphology analysis of certain features was visual-ized by using a SEM microscope(JSM-6701F,Japan Electron Optics Laboratory Corporation,Japan).FTIR spectra of the adsorbents were recorded between4000and400cm?1through the KBr method with a FTS3000spectrophotometer.TG/DTA plots of the samples were obtained using a Pyris Diamond instrument.The particle size of the adsorbents was also measured using a Nano series Zetasizer.

2.4.Adsorption studies

Batch adsorption experiments were carried out to determine the optimum pH for Pb(II)adsorption and the best adsorbent dosage. Tests were performed by shaking the amount of the adsorbent (0.01–0.20g)with50mL of Pb(II)aqueous solution of the desired

500

1000

1500

2000

2500

3000

3500

4000

Wavenumber(cm-1)

Fig.1.FTIR spectra of LC,LC/PAAHM and LC/PAAHM-Pb(loading Pb(II).

concentration in several250mL stoppered conical?asks.The test was carried out at various pH(2.0–8.0)and at room tempera-ture(25?C)for60min in a temperature controlled water bath shaker.The residual concentration of Pb(II)was measured by visible spectrophotography with adopting the xylenol orange as colored indicator.The removal ef?ciency and adsorption capacity were computed in the following equations:

Removal%=(

C0?C e

C0)×100%(1) Adsorption capacity=

(C0?C e)

V(2) where C0and C e are the initial and equilibrium concentration of metal ions(mg/L)in the solution.V is the volume of metal ions solution(L)and m is the weight of the adsorbent(g).

2.5.Desorption

The desorption of lead(II)ions from the LC/PAAHM was carried out by shaking Pb(II)-laden adsorbent in50mL of0.1mol L?1HCl solution at?xed intervals,and then?ltered.The concentration of desorbed lead(II)in the?ltrate was determined.

3.Results and discussion

3.1.Characterization of the adsorbents

The FTIR spectra of LC and LC/PAAHM are shown in Fig.1.The spectra of LC exhibit a broad absorption peak at about3470cm?1 and a sharp peak at797cm?1,indicative of the OH group and quartz,respectively.The peaks at3628,1037,694,538and 470cm?1correspond to the Si O Si stretching vibrations and bending vibrations.By contrast,the spectra of LC/PAAHM retain the structure of LC basically apart from the peaks at1564,1458-1421,1284and1371cm?1due to the COO?stretching vibrations of carboxylic acid,C O stretching vibrations and C H stretching from CH3.

Fig.2shows the thermogravimetric(TG)and differential ther-mal analysis(DTA)curves obtained from LC and LC/PAAHM under inert conditions.It was observed that the thermal stability of LC is higher than that of the LC/PAAHM.For LC/PAAHM,there are three stages of mass loss.The TG curve exhibits an initial small weight loss of18%starting at50?C and ending at350?C due to loss of

336Y.-F.He et al./Journal of Hazardous Materials 227–228 (2012) 334–340

TG(%)

800

700

600

500

400

300

200

100

-30

-20

-10

40 LC/PAAHM-DTA LC-DTA LC-TG

LC/PAAHM-TG

Temp.(Cel)

D T A (u V )

010

304050

7080

100Fig.2.Thermal analysis of LC and LC/PAAHM.

water molecules and unpolymerized monomer.The second mass loss (350–550?C)was ascribed to the decomposition of carboxyl group;and the last weight loss of 13.4%was due to the breakage of polymer chain and loss of constitution water after 550?C.DTA showed two well-differentiated exothermic peaks at 350–500?C,which coincided with the pyrolysis of the organic matter.

X-ray diffraction patterns of LC and LC/PAAHM are shown in Fig.3.It is obvious that no new phase formation occurs after the polymerization of acrylic acid onto LC.XRD data for LC indicate that the main clay mineral of loess is smectite,and the secondary is illite.XRD data for LC/PAAHM show that the diffraction peak of smectite disappears and the peak of illite moves left.This result can be attributed to LC modi?ed by AA and HEMA,which has larger interlamellar spacing.XRD studies have shown that,upon in-suit polymerization,a signi?cant decrease in crystallinity occurs.

SEM was used to examine the surface characteristics of LC and LC/PAAHM (Fig.4).Samples were mounted onto metal holders,coated with gold,and analyzed.The SEM analysis enables the direct observation of the changes in the surface microstructures of the

70605040302010I n t e n s i t y (c p s )

2-Theta (deg)

LC/PAAHM

Fig.3.X-ray diffraction patterns of LC and LC/PAAHM (S –smectite,I –illite,Q –quartz,Ca –calcite).

adsorbents.The surface morphology of LC is different from that of LC/PAAHM.In LC microphotographs,the unit particles can be clearly observed,and the interparticle gaps are loose.The SEM image of LC/PAAHM clearly shows that copolymer (PAAHE)has been deposited on the surface of the particles and in the interpar-ticle gaps.The PAAHE in LC/PAAHM are amorphous,reducing the degree of crystallinity and causing a homodisperse of particles.The particle size of LC and LC/PAAHM was also measured.The Z-average (d nm)of loess is 686,and the size is heterogeneous.The particle size of LC/PAAHM (1940nm)is about 2.8times of that of LC,and the size is homogeneous.This result illustrated that the copolymer binds to the loess clay.

3.2.Adsorption experiments

In order to investigate the absorbability of Pb(II)with differ-ent adsorbents,the experiments were conducted at initial lead concentration of 300mg/L.The selected adsorbents are LC,the loess based poly(acrylic acid)complex (LC/PAA),the loess based poly(2-hydroxyethyl methacrylate)complex (LC/PHEMA),and the loess based poly(acrylic acid-2-hydroxyethyl methacrylate)com-plex (LC/PAAHM).As shown in Table 1,the adsorption capacity of LC/PAAHM was higher than that of other adsorbents.This suggests the copolymer (PAAHM)can greatly improve the adsorptivity of LC.

3.2.1.Effect of pH on Pb(II)adsorption

The pH of the aqueous medium is an important factor that may in?uence adsorbate uptake;the pH of the solution affects the degree of ionization and speciation of various pollutants,which subsequently leads to a change in the reaction kinetics and equi-librium of the adsorption process [21].The effect of pH on the adsorption of Pb(II)by LC/PAAHM is presented in Fig.5.It can be

Table 1

The adsorbability of four sorbents.

Adsorbents

Removal (%)

Adsorption

capacity (mg/g)

9.914.8LC/PAA 81.3121.9LC/PHEMA 9.213.8LC/PAAHE

88.8

133.2

Cond:[Pb 2+]initial =300mg/L;[Adsorbent]=0.1g 100mL ?1;pH 5.0;Time:1h.

Y.-F.He et al./Journal of Hazardous Materials 227–228 (2012) 334–340

337

Fig.4.Scanning electron micrographs of LC and LC/PAAHM and

LC/PAAHM-Pb.

20406080

100pH

R e m o v a l r a t e /%

Fig.5.Effect of pH on the adsorption of Pb(II)ions (initial concentration =300mg/L,agitation speed =150rpm,temperature =20?C,contact time =60min and adsorbent dosage =0.05g/50mL).

seen that the removal rate increases with increasing pH,reaching a plateau value in the pH 5.0.On higher pH values a sharply decrease of adsorption for Pb(II)ions was observed.At lower pH values,the H +ions compete with Pb 2+ions for the active sites in the system.At higher pH values than 5,metal precipitation appeared and the adsorbent was deteriorated with the accumulation of metal ions [22,23].Therefore,pH 5was selected to be the optimum pH for further studies.

3.2.2.Effect of adsorbent dosage

The adsorption studies of Pb(II)ions on LC/PAAHM were done by varying the quantity of adsorbent from 0.01to 0.2g/50mL while keeping the value of the lead solutions constant at pH 5.The in?u-ence of adsorbent dosage in the removal rate of Pb(II)ions is shown in Fig.6.The adsorption of Pb(II)by LC/PAAHM increased from 11to 99%by increasing the sorbent dosage from 0.01to 0.05g/50mL.And after this,a slight decrease of adsorption for Pb(II)ions

was

020406080

1000.20

0.150.100.050.00

Adsorbent dosage/g

R e m o v a l r a t e /%

Fig.6.Effect of adsorbent dosage on the adsorption of Pb(II)ions (initial con-centration =300mg/L,agitation speed =150rpm,temperature =20?C and contact time =60

min).

060

120180240300360420

Time (min)

A d s o r p t i o n c a p a c i t y (m g /g )

450mg/L Fig.7.Effects of contact time and initial concentration on the adsorption of Pb(II)ions (adsorbent dosage =0.05g/50mL,pH 5,agitation speed =150rpm,tempera-ture =20?C).

observed.The increase in the adsorption percentage is due to the increase in active sites on the adsorbent and the greater surface area [24].But removal rate decreased with the increase in adsorbent dosage;and the decreasing thereof may be due to overlapping of adsorption sites as a result of over-crowding of adsorbent particles [25].

3.2.3.Effect of contact time and initial Pb(II)concentration

Fig.7shows the data from the kinetic studies of Pb(II)adsorp-tion by LC/PAAHM for different initial concentrations with time.The removal rate increased with the increase of initial metal ion con-centration.Pb(II)adsorption in the initial process is faster,which was completed after about 10min of contact time.After that,an equilibrium condition was gradually approached in a slow process.The rapid adsorption rate during the initial stage may be ascribed to the availability of suf?cient active sites.The amount of Pb(II)adsorbed at equilibrium increased from 235.5mg/g to 356.9mg/g as the initial concentration increased from 250to 500mg/L.The initial concentration might weaken the mass transfer resistances of the aqueous and solid phases.As a result of the above observa-tions,it was indicated that the removal of Pb(II)on LC/PAAHM was to be dependent.

3.2.

4.Effect of temperature

For adsorption of Pb(II)ions onto LC/PAAHM,adsorption exper-iments were investigated at 10,20,30and 40?C at optimum pH value of 5and adsorbent dose level of 0.05g/50mL.With the increase of temperature,the adsorption capacity at equilibrium increased from 280mg/g to 283mg/g.This result illustrated that the temperature can reduce liquid viscosity and increase diffusion processes.

338Y.-F.He et al./Journal of Hazardous Materials227–228 (2012) 334–340 Table2

Parameters of Langmuir and Freundlich isotherms.

Langmuir Freundlich

q m(mg/g)370.4K F((mg/g)(L/g)1/n)181.1

K a(dm3/mg)0.1511/n0.136

R20.999R20.816

3.3.Adsorption isotherm study

The Langmuir adsorption model proposed the existence of

monolayer coverage of the adsorbate onto surface of the adsorbent.

The model assumes uniform energy of the adsorption onto the sur-

face and adsorption occurs at speci?c homogeneous sites within

the adsorbent.The Langmuir isotherm is described as follows:[26]

C e q e =1

K a q m

q m

C e(3)

where q e is the amount adsorbed at equilibrium(mg/g)and C e is the equilibrium concentration(mg/L).q m and K a are the Langmuir con-stants related to the adsorption capacity and energy of adsorption or binding constant,respectively.

Regression coef?cient values(R2)indicated the best?tting of the data to the Langmuir equation.q m and K a were determined from the slopes and intercepts of the straight-line plot between C e/q e versus

C e(Fig.8)and are given in Table2.The correlation coef?cients is

0.999,indicating that the adsorption of Pb(II)onto LC/PAAHM can be adequately described by the Langmuir isotherm model.

The Freundlich model is an empirical equation that is used to describe heterogeneous systems.The equation is given as follows: [27]

log q e=log K F+1

n log C e(4)

where K F and n are Freundlich constants.

The K F gives an indication of the capacity of the adsorbent and n gives an indication of favorability.The calculated n and K F values according to the slopes and intercepts of the plot of log C e and log q e (Fig.8)are shown in Table2.The correlation coef?cients is0.816 (<0.999),indicating that the Langmuir isotherm is more suitable to describe the adsorption processes of Pb(II)onto LC/PAAHM than the Freundlich isotherm.

3.4.Adsorption kinetics

The adsorption process was also analyzed by using pseudo-?rst-order and pseudo-second-order models.The pseudo-?rst-order model can be explained as follows:[28]

log(q e?q t)=log q e?

2.303

t(5)

where q e is the amount of Pb(II)adsorbed per unit mass of adsor-bent at equilibrium(mg/g),q t is the amount of Pb(II)adsorbed at time t(mg/g),and k1is pseudo-?rst-order rate constant(min?1).By plotting log(q e?q t)as a function of the contact time t(Fig.9),the values of the calculated q e(q e,cal)and the rate constant k1can be obtained from the intercept and the slope of the plot,respectively.Table4

Adsorption capacities of Pb(II)on different clay adsorbents.

Adsorbents Adsorption

capacity

(mg/g)

References

Kaolinite31.8[29]

Diatomite24.0[30] Montmorillonite31.1[31]

Polyvinyl

alcohol-modi?ed

Kaolinite

56.2[32]

Red loess113.6[20]

LC14.8Present study LC/PAAHE133.3Present study

The pseudo-second-order kinetic model can be expressed as follows:

q t

k2q e2

q e(6) where k2is the pseudo-second-order rate constant(g/mg min).

The analyses of the data through the construction of plots of t/q t against the contact time t are shown in Fig.9.The values of k1,k2 and q e,cal along with those of the correlation coef?cient(R2)are summarized in Table3.

In the pseudo-second-order kinetics,the calculated q e,cal are nearly the same as experimental values(q e,exp(mg/g)),and the R2 were found to be all0.999.It indicated that the pseudo-second-order model is appropriate for the adsorption phenomena.

A comparison of the adsorption capacity of other reported clay adsorbents for removing Pb(II)ions is presented in Table4. Although their structures,functional groups,as well as the experi-mental conditions were different,their adsorption capacities could be qualitatively compared between different adsorbents.It is evi-dent that the adsorption capacity of LC/PAAHM exceeded other adsorbents.

3.5.Adsorption mechanism

In order to discuss the adsorption mechanism of LC/PAAHM,the difference of vibration absorption was investigated by FTIR spectra. Fig.1shows the FT-IR spectra of LC/PAAHM and loaded LC/PAAHM with Pb(II)ions(LC/PAAHM-Pb).The absorption band at1703cm?1 increased in the LC/PAAHE after adsorption.Meanwhile,the weak absorption bands at1421and1284cm?1disappeared.These bands correspond to C O.It demonstrated that the COO?and C O were the active sites.In other words,chelation was formed between car-boxyl groups of the LC/PAAHM and the lead(II)https://www.wendangku.net/doc/f51938871.html,bined with discussion of kinetic and isotherm,the adsorption process of Pb(II) ions on LC/PAAHM is speci?c adsorption.The coordinate bonds can be formed between Pb(II)ions and oxygen.A great change in the surface morphology of LC/PAAHM after adsorbing Pb(II)ions (LC/PAAHM-Pb,Fig.4)https://www.wendangku.net/doc/f51938871.html,pared with LC/PAAHM,the surface of LC/PAAHM-Pb shows slices structure,mainly due to for-mation of complex.This further showed that the chemical reactions existed wherein may play important roles in this process.

Table3

Kinetic parameters.

C0(mg/L)q e,exp(mg/g)Pseudo-?rst-order Pseudo-second-order

q e,cal(mg/g)k1(1/min)R2q e,cal(mg/g)k2(g/(mg min))R2

250235.5237.90.3380.9192238.1 1.8×10?50.9997 300284.2535.80.2460.7984294.1 1.2×10?50.9998 375327.5629.80.2500.7917333.30.9×10?50.9999 450340.5738.40.3280.9617344.8 1.0×10?50.9997 500356.9430.60.2170.8202357.1 1.0×10?50.9998

Y.-F.He et al./Journal of Hazardous Materials 227–228 (2012) 334–340

339

0.00

0.100.200.300.40

0.500.60200

150

100

C e

C e /q

2.362.402.442.482.52

2.562.60 2.5

2.01.51.0

logC e

l o g q e

Fig.8.Pb(II)adsorption isotherm for

LC/PAAHM.

-7

-5-3-11

3560

t (min)

l o g (q e -q t

)

0.00

0.06

0.120.18

0.24

0.30

t (min)

t /q t

375mg/L

450mg/L

500mg/L

Fig.9.Kinetic models for Pb(II)

adsorption.

5060708090100

Time (min)

D e s o r p t i o n r a t e (%)

Fig.10.Desorption kinetic curves of Pb(II)from LC/PAAHM-Pb.

3.6.Desorption properties

Desorption ratio was calculated from the amount of lead(II)ions initial adsorption of the adsorbent and the ?nal concentration in the desorption medium.The results were shown in Fig.10.The desorp-tion equilibrium of heavy metals could be reached after 20min and the desorption rate of Pb(II)was 99%.It indicated the LC/PAAHM could be recovered easily and be reused.

4.Conclusion

The acrylic acid and 2-hydroxyethyl methacrylate were used to modify the natural loess,and loess based poly(acrylic acid-2-hydroxyethyl methacrylate)complex (LC/PAAHM),a low-cost adsorbents for the Pb(II)from aqueous solution,was obtained.The

LC/PAAHM presented excellent adsorption activity for Pb(II)ions.The results con?rmed the Pb(II)removal ability of the LC/PAAHM,and a maximum removal ef?ciency of 99%was achieved under appropriate conditions.The equilibrium data were analyzed using various adsorption isotherms.The achieved experimental data were fairly well represented by the Langmuir adsorption isotherms.The best ?t was also obtained with a pseudo-second-order kinetic model while investigating the adsorption kinetics.

Acknowledgement

The project was supported by the NSFC (20964002;20274034),PCSIRT (IRT1177),Gansu Sci &Techn Support Project (1011GKCA017),the Funda Res Funds Gansu Univ (2010-176).

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