J. Cent. South Univ. Technol. (2008) 15: 183-187

DOI: 10.1007/s11771-008-0035-6

Adsorption behavior of Pb²⁺ and Cd²⁺ ions on bauxite flotation tailings

WANG Yu-hua(王毓华)¹, LAN Ye(兰 叶)², HUANG Chuan-bing(黄传兵)³

(1. School of Resources Processing and Bioengineering, Central South University, Changsha 410083, China;

2. Beijing Wanhou Environment Technologies Development Co Ltd, Beijing 100085, China;

3. Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100086, China)

Abstract: The adsorption behavior of Pb²⁺ and Cd²⁺ ions on bauxite flotation tailings was investigated to demonstrate the adsorptivity of the bauxite flotation tailings. The adsorption percentage of Pb²⁺ and Cd²⁺ ions as a function of adsorbent dosage, solution pH value and shaking time were determined by batch experiments. The maximum adsorption percentage of 99.93% for Pb²⁺ ions and 99.75% for Cd²⁺ ions were obtained by using bauxite flotation tailings as adsorbent. The methods, such as zeta potentials, specific surface area measurements and the analysis of adsorption kinetics, were introduced to analyze the adsorption mechanisms of the Pb²⁺ ions on bauxite flotation tailings. The isoelectric point of bauxite flotation tailings shifts from 3.6 to 5.6 in the presence of Pb²⁺ ions. The specific surface area of bauxite flotation tailings changes from 12.57 to 20.63 m²/g after the adsorption of Pb²⁺ ions. These results indicate that a specific adsorption of the cation species happens on the surface of bauxite flotation tailings. Adsorption data of Pb²⁺ ions on the surface of bauxite flotation tailings can be well described by Langmuir model, and the pseudo-second-order kinetic model provides the best correlation for the adsorption data of Pb²⁺ and Cd²⁺ ions on bauxite flotation tailings.

Key words: adsorption; Pd²⁺; Cd²⁺; bauxite flotation tailings

1 Introduction

The species of Pb(Ⅱ) and Cd(Ⅱ) are hazardous substances that exist in aqueous waste streams of many industries, such as metal plating facilities and mining operations^[1]. Chemical precipitation, phytoextraction, reverse osmosis, electrodialysis, ion exchange and membrane filtration or adsorption have been developed over the years to remove these heavy metals from industrial waste water^[2]. However, the high price of the adsorbents is the disadvantage for adsorption treatment method. So, it is necessary to find some adsorbents with low cost and high efficiency for the removal of hazardous substances from waste water.

Silicate minerals show a great potential in the removal of hazardous substances from waste water due to the chemical and mechanical stability, high surface area and special structure. In recent years, silicate minerals have been used as adsorbents for the adsorption of hazardous substances from waste water, and much work on their adsorption properties has also been carried out^[3-7]. Aluminosilicate-tailings are the main waste generated during the production of aluminum from bauxite ore by the combined process of Bayer process and bauxite flotation. There are about 0.2 t of aluminosilicate-tailings generated while 1 t of bauxite ore are treated by flotation technology. It is well know that the main compositions of bauxite tailings are diaspore, kaolinlite, illite and pyrophyllite^[8-9]. Although some work on the utilization of bauxite flotation tailings, such as cement, absorbent materials, building materials and ceramics, have been carried out in recent years, a suitable utilization has not been identified yet. So it is necessary to find an application for bauxite flotation tailings in other industrial fields.

The preparation of adsorption materials from bauxite flotation tailings and the adsorption tests of hazardous substances ions in aqueous solution were involved in this work. The influences of adsorbent dosage, shaking time and solution pH value on the adsorption of Pb²⁺ and Cd²⁺ ions from aqueous solution were investigated by using bauxite flotation tailings as adsorbent. The adsorption mechanisms of Pb²⁺ ions on bauxite flotation tailings were also investigated by zeta potential, specific surface area(SSA) and the analysis of adsorption isotherm and kinetics.

2 Experimental

2.1 Materials and apparatus

The bauxite tailings samples were supplied by Zhengzhou Bauxite Flotation Plant, Henan, China. Samples were dried at 85 ℃ and screened by screen with screen hole size of 0.355 mm. A laser granulometer named CILAS-1064 was used for the measurement of

particle size of samples, and the average particle size of samples was 25.53 μm. The bauxite tailings samples were composed of 39.52% Al₂O₃, 28.89% SiO₂, 3.12% TiO₂, 0.46% MgO, 0.61% CaO, 7.31% total Fe, 0.13% S, 4.71% K₂O and 0.82% Na₂O (mass fraction). Diaspore, kaolinite, illite, anatase, hematite and quartz were also determined as the compositive minerals in samples by X-ray powder diffractometer (XRD, Shimadzu D/MAX- rA model).

2.2 Adsorption tests

Solutions of Pb²⁺ ions (40 mg/L) and Cd²⁺ ions (40 mg/L) were prepared from 1 g/L stock solution of each salt using distilled water, respectively. When the solution volume for both cations was fixed at 100 mL, the adsorption percentage of Pb²⁺ and Cd²⁺ ions as a function of adsorbent dosage, solution pH value and shaking time were investigated at a shaking speed of (170±3) min^-1 and (30±1) ℃. After the filtration, the concentrations of Pb²⁺ and Cd²⁺ ions in filtrate water were determined by inductively coupled plasma-mass(ICP) spectrometry. Solution pH value was modified by HCl and NaOH solutions (0.1-1.0 mol/L).

The adsorption percentage is determined as

                         (1)

where  η is the adsorption percentage of cations on tailings, %; c₀ and c_e are the initial and the equilibrium concentrations of cations, respectively, mol/L. The average of adsorption percentage is adopted everywhere in this work after three measurements.

2.3 Zeta potential measurements

A zeta meter named Brookhaven Zeta plus (USA) was used for the determination of zeta potentials. The samples were ground to 5 μm in an agate mortar. Suspensions containing solids 0.05% (mass fraction) were conditioned in a beaker for 15 min and pH value was measured at 25 ℃.

2.4 Specific surface area measurements

Specific surface areas of the bauxite flotation tailings were determined by N₂-BET adsorption tests.

3 Results

3.1 Effect of adsorbent dosages

Under a given initial condition, adsorbent dosage is an important parameter to determine the adsorbability of adsorbent. The influences of the dosage of bauxite tailings on the adsorption percentage of cations are shown in Fig.1, demonstrating that the adsorption of

Fig.1 Effect of bauxite tailings dosage on adsorption percentage of cations (solution: 100 mL; ion concentration: 40 mg/L (Pb²⁺ and Cd²⁺ ions); shaking time: 2 h; temperature: 303 K; pH: 5-6).

Pb²⁺ and Cd²⁺ ions increases with the increment of the bauxite tailings dosages.

3.2 Effect of shaking time

The effects of shaking time on the adsorption of Pb²⁺ and Cd²⁺ ions are shown in Fig.2, which indicates that the removal of cation by bauxite tailings is improved with increasing shaking time. The adsorption reaches equilibrium at about 1 h on bauxite tailings and the maximum adsorption percentage is close to 100% for both of Pb²⁺ and Cd²⁺ ions.

Fig.2 Adsorption percentage of cations as function of shaking time (solution: 100 mL; ion concentration: 40 mg/L (Pb²⁺ and Cd²⁺); adsorbent dosage: 0.5 g Pb²⁺ and 4 g Cd²⁺ ions; temperature: 303 K; pH: 5-6)

3.3 Effect of pH value

In the presence of bauxite flotation tailings, the influences of pH value on the adsorption of cations were also investigated in the pH range of 2-12. The results are shown in Fig.3.

Fig.3 Effect of pH value on adsorption percentage of cations in presence of bauxite tailings (solution: 100 mL; cations concentration: 40 mg/L (Pb²⁺ and Cd²⁺ ions); adsorbent dosage: 0.5 g Pb²⁺ ions and 4 g Cd²⁺ ions; shaking time: 1 h; temperature: 303 K)

It can be found from Fig.3 that the adsorption percentage of Pb²⁺ and Cd²⁺ ions on bauxite tailings increase as the pH value increases. The bauxite flotation tailings show good adsorbability for Pb²⁺ and Cd²⁺ ions. The adsorption percentage of Pb²⁺ ions by bauxite flotation tailings remains lower level at pH＜2.6, but it increases sharply and attains 99.8% at pH=6.1. A similar effect of pH value on the adsorption percentage of Cd²⁺ ions by bauxite tailings can also be observed. The low adsorption percentage of Pb²⁺ and Cd²⁺ ions at lower pH value may be caused by the competition adsorption of H⁺ on the available exchange sites of minerals. The hydroxylated surfaces of oxides develop a charge on the surface in aqueous solution through amphoteric dissociation as follows^[10]:

where  M represents Al, Fe and Si. These surfaces will be negatively charged at higher pH value and consequently will be favourable to the adsorption of Pb²⁺ and Cd²⁺ ions in the cationic form as PbOH⁺ and CdOH⁺.

4 Discussion

Because the adsorption curve of Cd²⁺ ions on bauxite flotation tailings is similar to that of Pb²⁺ ions, only adsorption mechanisms of Pb²⁺ ions were investigated.

4.1 Zeta potential measurement

The curves of pH—zeta potential of bauxite flotation tailings in the presence of 40 mg/L Pb²⁺ ions and distilled water are shown in Fig.4.

Fig.4 Zeta potentials vs pH value in solutions of 40 mg/L Pb²⁺ ions, containing solids 0.05%(mass fraction) at 298 K

Zeta potential increases in the presence of Pb²⁺ ions in the range of pH of 2-12. The increase of surface positive charge on bauxite tailings may be caused by the protonation of surface hydroxyl groups on bauxite tailings. The changes of surface charge result in the changes of zeta potential of bauxite tailings. Bauxite tailings carry net negative surface charges when pH value is over 3.6, and its isoelectric point(IEP) value is pH=3.6. The IEP of bauxite tailings changes from 3.6 to 5.6 in the presence of Pb²⁺ ions in the pH range of 2-12.The broken bonds on the surface of bauxite tailings are covalent Al—O bonds and Si—O bonds that are ≡Si—O^-1, ≡Si—OH and ≡Al—O^-1.25, ≡Si—O^-1, ≡Si—OH and ≡Al—O^-1.25 function keys^[11], and they can cause the absorption and electrostatic action with cationic species, such as Pb²⁺ and PbOH⁺.

4.2 Specific surface area

The specific surface area of the bauxite flotation tailings is 12.57 m²/g. However, a specific surface area of 20.63 m²/g after the adsorption of Pb²⁺ ions is obtained, which shows on another hand that there exist some species, such as PbOH⁺, on the surface of bauxite tailings, leading to the change in the specific surface area of the bauxite tailings.

4.3 Adsorption isotherm

The relationship between adsorption density of Pb²⁺ ions on bauxite tailings and its equilibrium concentra- tion in aqueous solution was described by Langmuir and Freundlich isotherm models. The experimental data conforming to the linear form of Langmuir model are expressed as the following equation:

                            (2)

where  c_e is equilibrium concentration of Pb²⁺ ions (mg/L) and q_e is the adsorption density of Pb²⁺ ions by per unit of tailings (mg/g). q_m and K_L are Langmuir constant related to adsorption capacity (mg/g) and the energy of adsorption (L/mg), respectively, q_m and K_L are evaluated from slope and intercept of the linear plots of c_e/q_e vs c_e, respectively (see Fig.5(a)).

The sorption equilibrium data was also applied to the Freundlich model in logarithmic form given as follows^[12-13]:

                          (3)

where  K_F (mg/g) and n are Freundlich constants related to adsorption capacity and adsorption intensity, respectively. K_F and 1/n are determined from the intercept and slope of linear plot of lg q_e vs lg c_e, respectively (see Fig.5(b)).

Fig.5 Langmuir (a) and Freundlich (b) plots for adsorption of Pb(Ⅱ) on bauxite flotation tailings

Adsorption equations were obtained from experimental data with Eqns.(2) and (3). The isotherm constants and correlation coefficients are calculated from the linear Langmuir and Freundlich plots by plotting c_e/q_e vs c_e and lg q_e vs lg c_e (Fig.5) and are listed in  Table 1.

Table 1 Langmuir and Freundlich constants and correlation coefficients for adsorption of Pb²⁺ ions

Langmuir model has better correlation than Freundlich model for the expression of the adsorption of Pb²⁺ ions on bauxite tailings. These results have also been reported in Ref.[14].

The K_F value of the Freundlich equation (Table 1)indicates that bauxite tailings have a very high adsorption capacity for Pb²⁺ ions. It was reported that when n is between 1 and 10, beneficial adsorptions of metal ions will be obtained^[15]. Here, n=4.02, which shows good adsorption of Pb²⁺ ions.

4.4 Adsorption kinetics

According to Fig.2, the adsorption rate of Pb²⁺ ions increases dramatically in the first 1 h, and then reaches equilibrium gradually at about 1 h. To analyze the adsorption rate of Pb²⁺ ions on bauxite tailings, the pseudo-first-order equation of Lagergren (Eqn.(4)) and the pseudo-second-order rate (Eqn.(5)) were evaluated based on the experimental data^[16]:

                               (4)

                            (5)

where  k₁ is the Lagergren adsorption rate constant (h^-1) and k₂ is the pseudo-second-order adsorption rate constant (g/(mg?h)), q_e and q_t are the amounts of Pb²⁺ ions absorbed (mg/g) at equilibrium and time t, respectively. Plots of lg [q_e/(q_e-q_t)] vs t and t/q_t vs t are shown in Fig.6. It can be seen from Fig.6 that the pseudo-second-order kinetic model provides a good correlation for the adsorption of Pb²⁺ ions on bauxite tailings in contrast to the pseudo-first-order model. This result is in good agreement with that in Refs.[17-18]. In addition, the correlation coefficients of the pseudo-second-order kinetic model are very high and the q_e values are close to the calculated q_e values (see  Table 2).

Table 2  Kinetic parameters of Pb²⁺ ions onto bauxite tailings

Fig.6 Pseudo-first-order (a) and pseudo-second-order (b) kinetic equation for adsorption of different concentrations of Pb²⁺ ions by natural bauxite tailings at (300±1) K

5 Conclusions

1) Bauxite tailings can effectively be used for the removal of cation ions from waste solution using adsorption method. The maximum adsorption percentages of Pb²⁺ and Cd²⁺ ions reach 99.93% and 99.75%, respectively.

2) The specific surface area of bauxite tailings shifts from 12.57 to 20.63 m²/g in presence of Pb²⁺ ions, and this shows that Pb²⁺ ions adsorb on the bauxite tailings. The IEP of bauxite tailings also changes from 3.6 to 5.6 correspondently.

3) Adsorption data of Pb²⁺ ions on bauxite tailings are well described by Langmuir model. Investigations of kinetic models show that pseudo-second-order kinetic model provides better correlation for the experimental data.

References

[1] MYROSLAV S, BOGUSLAW B, ARTUR P T. Study of the selection mechanism of heavy metal (Pb²⁺, Cu²⁺, Ni²⁺, and Cd²⁺) adsorption on clinoptilolite[J]. Journal of Colloid and Interface Science, 2006, 304(1): 21-28.

[2] MOORE J W, RAMAMORTHY S. Heavy metals in natural waters[M]. New York: Springer Verlag, 1994.

[3] BOWMAN R S, HAGGERTY G M, HUDLESTON R G. Sorption of nonpolar organic compounds, inorganic cations and inorganic oxyanions by surfactant-modified zeolites[M]. Washington: Americal Chemical Society, 1995: 54-64.

[4] PRADHAN J, DAS S N, THAKUR R S. Adsorption of hexavalent chromium from aqueous solution by using activated red mud[J]. Journal of Colloid and Interface Science, 1999, 217(1): 137-139.

[5] OSVALDO K J, LEANDRO V, ALVES G. Adsorption of heavy metal ion from aqueous single metal solution by chemically modified sugarcane bagasse[J]. Bioresource Technology, 2007, 98(6): 1291- 1297.

[6] ZHAO Xiao-rong, DU Dong-yun. Research on the adsorptive properties of rectorite to methylene blue[J]. Ion Exchange and Adsorption, 2003, 19(4): 337-342. (in Chinese)

[7] WANG Yu-hua, HU Yue-hua, LIU Xiao-wen. Flotation de-silicating from diasporic-bauxite with cetyl trimethylammonium bromide[J]. Journal of Central South University of Technology, 2003, 10(4): 324-328.

[8] LIU Wei-ping, YUAN Jian-xiong. The application of tailings in the non-organism nonmetal material[J]. China Mining, 2004, 13(11): 16-18. (in Chinese)

[9] WANG Jian-li, WANG Huai-de, HUANG Jian. Study of making absorption water compound materials with the gangue from bauxite benefication[J]. Light Metal, 2004(3): 9-10. (in Chinese)

[10] ABMED S M. Dissociation of oxide surfaces at the liquid solid interface[J]. Canadian Journal of Chemistry, 1966, 44: 1663.

[11] JIA Mu-xin. Study on surface properties of silicate minerals and their adsorption characteristics of metal ions[D]. Shenyang: School of Resources and Civil Engineering, Northeastern University, 2001: 92. (in Chinese)

[12] AHMET S, MUSTAFA T, MUSTAFA S. Adsorption of Pb(II) and Cr(III) from aqueous solution on Celtek clay[J]. Journal of Hazardous Materials, 2007, 144(1/2): 41-46.

[13] LI Hai-pu, HU Yue-hua, WANG Dian-zuo, XU Jing. Effect of hydroxamic acid polymerson reverse flotation of baccsite[J]. Journal of Central South University of Technology, 2004, 11(3): 191-294.

[14] CHEN Hao, WANG Ai-qin. Kinetic and isothermal studies of lead ion adsorption onto palygorskite clay[J]. Journal of Colloid and Interface Science, 2007, 307(2): 309-316.

[15] TAHIR S S, NASEEM R. Removal of Cr(III) from tannery wastewater by adsorption onto bentonite clay[J]. Separation and Purification Technology, 2007, 53(3): 312-321.

[16] HUANG Yao-hui, HSUEH C L, HUANG Chun-ping. Adsorption thermodynamic and kinetic studies of Pb(II) removal from water onto a versatile Al₂O₃-supported iron oxide[J]. Separation and Purification Technology, 2007, 55(1): 23-29.

[17] UNUABONAH E I, ADEBOWALE K O, OLU-OWOLABI B I. Kinetic and thermodynamic studies of the adsorption of lead (II) ions onto phosphate-modified kaolinite clay[J]. Journal of Hazardous Materials, 2007, 144(1/2): 386-395.

[18] BRIGATTI M F, LUGLI C, POPPI L. Kinetics of heavy metal removal and recovery in sepiolite[J]. Appl Clay Science, 2000, 16(1/2): 45-57.

(Edited by CHEN Wei-ping)

Foundation item: Project(2005CB623701) supported by the Major State Basic Research Development Program of China

Received date: 2007-09-12; Accepted date: 2007-10-26

Corresponding author: WANG Yu-hua, Professor; Tel: +86-731-8830541; E-mail: wangyh@mail.csu.edu.cn