J. Cent. South Univ. (2012) 19: 1073-1080

DOI: 10.1007/s11771-012-1111-5

Effect of rhamnolipids on cadmium adsorption by

Penicillium simplicissimum

YUAN Xing-zhong(袁兴中)^{1, 2}, JIANG Li-li(姜丽丽)^{1, 2}, ZENG Guang-ming(曾光明)^{1, 2},

LIU Zhi-feng(刘智峰)^{1, 2}, ZHONG Hua(钟华)^{1, 2}, HUANG Hua-jun(黄华军)^{1, 2},

ZHOU Mei-fang(周梅芳)^{1, 2}, CUI Kai-long(崔凯龙)^{1, 2}

1. College of Environmental Science and Engineering, Hunan University, Changsha 410082, China;

2. Key Laboratory of Environment Biology and Pollution Control, Ministry of Education,

Hunan University, Changsha 410082, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2012

Abstract: The effect of rhamnolipids (RL) on Cd²⁺ adsorption by Penicillium simplicissimum (P. simplicissimum) was studied. The maximum adsorption capacities of Cd²⁺ were obtained at pH 6.0 for the intact P. simplicissimum and at pH 5.0 for the RL-pretreated P. simplicissimum, respectively. The adsorption equilibrium was reached after about 4 h. The experimental adsorption isotherms were in good agreement with the Langmuir model. The maximum adsorption capacities (q_max) for the intact P. simplicissimum and for the RL-pretreated P. simplicissimum were 51.6 and 70.4 mg/g, respectively. The interactions between Cd²⁺ and functional groups on the cell wall surface of the P. simplicissimum were identified by SEM, EDAX and FTIR analysis. It is indicated that carboxyl, amino and hydroxyl groups play major roles in the Cd²⁺ adsorption. The results suggest that the RL-pretreated P. simplicissimum is a promising candidate for the removal of Cd²⁺ from aqueous solutions.

Key words: adsorption; Penicillium simplicissimum; Cd²⁺; rhamnolipids

1 Introduction

The pollution of hazardous heavy metals on aquatic environment and human health is one of the important problems in the world. It has become increasingly prominent in dealing with the wastewater containing heavy metals. Recently, the technology of adsorption becomes more and more attractive around the world, owing to its advantages of extensive source of adsorbents, low cost, high speed of removal, large adsorption capacity and high selectivity [1-3].

It is well known that cell walls of both dead and alive microbial adsorption materials can be effective for metal ions adsorption [4]. However, there are some limitations for living cells in the application of metal ions adsorption, such as being sensitive to pH and metal toxicity. Non-activated dead cells may be preferential because of easier operations, broader application and better adsorption capacity. Microbial cell walls are porous and have many structures with negative charges, such as carboxyl, phosphoric and hydroxyl groups [5]. The adsorption depends on complexation of heavy metals with chemical groups on the cell wall surface, in form of ionic or covalent bond [6].

In recent years, several studies concerned about the pretreatment of bacteria which can increase the adsorption capacity for heavy metals [7-8]. Generally, biosurfactants can increase the membrane permeability by interacting with the lipid and protein on the cell membrane [9]. For example, rhamnolipids might disrupt membranes through increased membrane permeability [10]. The changes of permeability lead to releasing the charged substances, which may adhere on the cell wall surface to change the nature of cell surface charges.

However, previous investigations about the Cd²⁺ adsorption were mainly focused on the intact bacteria [5, 8, 11]. It failed in deeply investigating other fields, such as the technology of improving cell surface charges. Several studies have shown that biosurfactants can change the cell surface properties. For example, LIU et al [12] studied that the pretreatment of saponins changed the chemical structures of Penicillium simplicissimum       (P. simplicissimum), such as the shift of C=O in carboxyl or amine groups. Thus, the maximum Cd²⁺ adsorption capacity of P. simplicissimum pretreated with saponins (74.6 mg/g) was higher than that of intact  P. simplicissimum (51.6 mg/g).

Rhamnolipids (RL) is a kind of widely studied biosurfactants [13-15]. To authors’ knowledge, however, few studies have been done about the Cd²⁺ adsorption by microbial biosorbents pretreated with RL. In this work, rhamnolipids-pretreated P. simplicissimum, compared with intact biomass, was used as a novel type of adsorbent for Cd²⁺ adsorption from artificial wastewater. The objective of this work is to investigate the effect of biosurfactants on the Cd²⁺ adsorption and the possible mechanisms.

2 Materials and methods

2.1 Microorganism and pure culture

The strain P. simplicissimum was isolated from the soil samples of Yuelu Mountain woodland in Changsha. Pseudomonas aeruginosa AB93066 was obtained from the China Center for Type Culture Collection. The strain was maintained on potato dextrose agar (PDA) and stored at 4 °C. The 100 mL liquid medium in a 500 mL Erlenmeyer flask was sterilized at 115 °C for 30 min in the autoclave sterilizer. Then, the medium was cooled to room temperature. Each 100 mL liquid medium was inoculated with 1.0 mL fungal suspension containing approximately 1.0×10⁶ cells. The inoculated medium was cultured at 30 °C, 150 r/min for 3 d. The growth medium consisted of FeSO₄ 0.005 g/L, MgSO₄·7H₂O 0.25 g/L, NaHCO₃ 0.05 g/L, KH₂PO₄ 0.5 g/L, CaCl₂  0.1 g/L, NH₄Cl 2.0 g/L, KCl 0.1 g/L, NaCl 0.2 g/L, peptone 10.0 g/L, and glucose 20.0 g/L. The pH value was natural. Various concentrations of biosurfactant RL were added to the medium to affect the cells growth. The medium without RL was used as the control sample.

2.2 Preparation of biosorbents

The fungal mycelium was filtrated and collected after culture. The mycelium was washed twice with mineral salt medium (MSM). The MSM consisted of FeSO₄ 0.005 g/L, MgSO₄·7H₂O 0.25 g/L, NaHCO₃ 0.05 g/L, KH₂PO₄ 0.5 g/L, CaCl₂ 0.1 g/L, NH₄Cl 2.0 g/L, KCl 0.1 g/L, and NaCl, 0.2 g/L. The pH value was natural. Then, the mycelium was freeze-dried to constant mass. The mycelium was ground and screened with the sieve of 80 μm. The powder was marked separately, laid in the desiccators to be preserved as the biosorbents in the experiments.

2.3 Preparation of cadmium solutions

Cadmium nitrate (Cd(NO₃)₂·4H₂O, AR) was used to prepare 1 000 mg/L stock metal ions solution. Then, various amounts of Cd²⁺ stock solutions were added into the adsorption medium to get various concentrations (20-400 mg/L) of Cd²⁺. Before mixing with biosorbents, the pH value of each adsorption medium was adjusted to the desired values with 1 mol/L NaOH or 1 mol/L HNO₃.

2.4 Adsorption experiments

All the batch experiments were carried out in the  50 mL Erlenmeyer flasks containing 20 mL medium at 28 °C and 120 r/min on the water bath shaker.

The effects of the biosorbent dosage on the adsorption of Cd²⁺ were studied. The concentrations of biosorbents were 0.1, 0.2, 0.6, 1.0 and 2.0 g/L, respectively. The pH value of medium was adjusted to 4.0 with 1 mol/L HNO₃ at the beginning of the experiments. The initial concentration of Cd²⁺ was    20 mg/L. The operation time was 4 h. The effect of pH on the adsorption was carried out at pH 1.0-7.0. In the adsorption kinetics studies, samples were taken out at time intervals. In the adsorption isotherm studies, batch experiments were carried out with different initial concentrations of Cd²⁺. The medium was centrifuged at 10 000 r/min for 10 min after adsorption. The residual Cd²⁺ concentrations were analyzed with an atomic adsorption spectrometer (Agilent 3510, USA). All the adsorption experiments were performed in triplicate. The group of the medium without biosorbents was used as blank.

The amount of adsorbed Cd²⁺ was obtained using the following equation [16]:

Q=(c₀-c)V/m                                (1)

where Q is the amount of Cd²⁺ adsorbed onto the unit amount of the biosorbents (mg/g); c₀ and c are the concentrations of Cd²⁺ in the medium before and after adsorption, respectively (mg/L); V is the volume of the adsorption medium (L); m is the amount of the biosorbents (g).

2.5 Adsorption isotherms

In this work, Langmuir and Freundlich isotherms were applied to describe the Cd²⁺ adsorption equilibrium in the medium. The Langmuir adsorption isotherm is described by [17]

q_eq=q_maxbc_eq/(1+bc_eq)                          (2)

where q_eq and q_max are the equilibrium and maximum uptake capacities, respectively (mg/g); b is the equilibrium constant related to adsorption energy (L/mg); c_eq is the metal ion concentration at equilibrium (mg/L).

The Freundlich equation is commonly presented as [17]

                                (3)

where K_F and n are Freundlich isotherm constants related to adsorption capacity and adsorption intensity; q_eq and c_eq are stated as above, respectively.

2.6 SEM, EDAX and FTIR analyses

The surface structure of biosorbents was analyzed by scanning electron microscope (SEM Qutanta 200), which was coupled with energy dispersive X-ray analysis (EDAX Qutanta 200). The acceleration voltage was constant as 20 kV. The biosorbents samples before and after adsorption were coated with a thin layer of gold under vacuum to increase the electron conduction and improve the quality of the micrographs [17]. Then, the samples were mounted on a stainless steel stab with a double-stick tape.

The chemical characteristics of the samples were tested and studied by Fourier transform infrared spectrometer (FTIR WQF-410). The spectra were recorded on FTIR spectrometer with the samples prepared as Potasium bromide discs. All spectra were plotted using the same scale on the transmittance axis [17].

3 Results and discussion

3.1 Effect of biosorbent dosage on adsorption capacity

Biosorbent dosage is a significant factor to be considered for metal ions adsorption. It determines the sorbent-sorbate equilibrium of the system. The effect of the biosorbent dosage on the Cd²⁺ adsorption capacity was studied by varying the amount of biosorbent dosage from 0.1 to 2.0 g/L, while keeping the other parameters (pH, initial Cd²⁺ concentration and contact time) be constant.

The adsorption capacity (q) initially decreases quickly and finally reaches a lower level with increasing the biosorbent dosage. This effect is also reported in Refs. [18-19] for adsorption phenomenon of heavy metals. The results demonstrate that the biosorbent dosage strongly affects the adsorption of Cd²⁺ from aqueous solutions. Moreover, as the biosorbent dosage rises, the maximum adsorption capacity drops, indicating poorer biosorbents utilization (lower efficiency). The results can be explained as a consequence that a partial aggregation at higher biosorbent dosage results in a decrease in effective surface area, and a decrease in active sites [20] for the adsorption. Besides, FAN et al [11] also suggested that it is due to the interference between binding sites and higher biosorbent dosage or insufficiency of metal ions in solution with respect to available binding sites.

The addition of RL can improve the adsorption capacity for Cd²⁺ in various degrees. It is thought that 0.025% is best for the enhancement of the Cd²⁺ adsorption. Higher biosorbent dosages reduces adsorption capacity [21]. When the biosorbent dosages increases from 0.1 to 2.0 g/L, the adsorption capacity decreases from 30.6 to 4.9 mg/g and from 8.4 to      2.0 mg/g on the 0.025% RL-pretreated P. simplicissimum and the control sample, respectively. Therefore, the optimum biosorbent dosage is selected as 0.2 g/L for further experiments, considering the lower biosorbent dosage can fully present the difference of samples in the simulated environment.

3.2 Effect of pH on adsorption capacity

Earlier studies on metal adsorption revealed that pH value is an important parameter affecting the adsorption process. The effect of pH on the adsorption of Cd²⁺ is studied, as shown in Fig. 1. The adsorption capacities of pretreated biosorbents increase significantly as the pH value increases from 1.0 to 5.0, and then decreases at pH 5.0-7.0. However, the optimum condition for the control sample is at pH 6.0. Table 1 gives previously reported optimum pH of Cd²⁺ adsorption by fungal biomasses. It is indicated by the results that pretreatment of RL changes the optimum pH condition for Cd²⁺ adsorption, which may be caused by the altered characteristics of the cell walls of the biosorbents. The adsorption capacities of the pretreated biosorbents change slightly with RL concentrations, but they are higher than those of the control sample. Thus, it is reasonable to assume that the adsorption capacity increases due to the changes in adsorptive characteristics of the fungus as a result of RL pretreatment. The difference may be caused by the increased permeability of membrane after the pretreatment of RL [9-10], resulting in gathering a lot of negative charges on the cell wall surface.

The medium pH influences the solubility of metal ions and the ionisation state of the functional groups (such as carboxylate and phosphate groups) on the fungal cell walls [22]. Those functional groups carry negative charges, which promotes the metal ions adsorption [4]. In acidic conditions, protonations of the cell walls components affect adversely the adsorption capacity of metals, or influence the ion-exchange reaction equilibrium (Meⁿ⁺+nHRMeR_n+nH⁺). However, the effects are reduced with increasing pH value in the medium. Due to the deprotonation of binding sites, the negative charges density on the cell wall surface increase greatly, thus facilitating greater metal uptake. The decrease of the adsorption capacities at pH above 6.0 could be owing to the complexation of Cd²⁺ by OH— groups which would prevent the Cd²⁺ adsorption. According to the results of Fig. 1, subsequent experiments were carried out at optimum pH (5.0).

Fig. 1 Effect of pH on adsorption capacity by intact and rhamnolipids-pretreated P. simplicissimum (Volume of adsorption medium: 20 mL; adsorbent dosage: 0.2 g/L; initial concentration of Cd²⁺: 20 mg/L; reaction time: 4 h)

Table 1 Previously reported optimum pH of Cd²⁺ adsorption by fungal biomasses

3.3 Adsorption kinetics

The effect of reaction time on Cd²⁺ adsorption is shown in Fig. 2. It is shown that the adsorption capacities of Cd²⁺ increase quickly with reaction time, and then reach saturation. The process includes two phases: the rapid phase at which adsorption contributes significantly to adsorptive equilibrium, and the subsequent slower phase at which adsorption contributed relatively small. In the first 2 h, a large amount of Cd²⁺ is removed. The adsorption saturation is obtained after 4 h. There are obvious differences in adsorption capacities between RL-pretreated biosorbents and the control sample. Clearly, the maximum adsorption capacities are 21.8 and 6.2 mg/g for 0.025% RL-pretreated P. simplicissimum and the control sample, respectively.

Besides, the adsorption capacities of pretreated biosorbents demonstrate an initial quick increase in the first 3 h followed by a final stability, while that of the control sample is slower. Preliminary view is the environment for cell growth which is changed after the addition of rhamnolipids. The cell walls of many microorganisms consist of polysaccharides, proteins and lipids, offering a lot of functional groups capable of binding to metal ions. Those functional groups, such as amino, carboxylic and phosphate groups, differ in their affinity and specificity for metal binding. The equilibrium amount of metal ions bound onto the cell surface would be determined by the relative affinity of the sites [7]. Rhamnolipids erode microbial cell-surface integrity, causing the walls to become leaky to increase permeability of membrane [10], and leading to more adsorption sites exposed on the cell wall. Therefore, the pretreatment biosorbents exhibit higher adsorption capacity than the control sample in a certain concentration of Cd²⁺ solution.

Fig. 2 Effect of reaction time on adsorption capacity by intact and rhamnolipids-pretreated P. simplicissimum (Volume of adsorption medium: 20 mL; adsorbent dosage: 0.2 g/L; initial concentration of Cd²⁺: 20 mg/L; pH: 5.0)

3.4 Adsorption isotherms

The experiments were carried out containing various concentrations (20-400 mg/L) of Cd²⁺ in the medium under the determined pH value (5.0) and reaction time (4 h). The results are shown in Fig. 3.

The adsorption capacities increase with increasing the concentrations of Cd²⁺ from 20 to 200 mg/L, indicating that Cd²⁺ has a strong affinity to the biosorbents. At the same time, electrostatic interactions also increase gradually with increasing the concentration of Cd²⁺, resulting in easy occupation of adsorption sites by Cd²⁺. The adsorption is saturated when the Cd²⁺ concentration is higher than 200 mg/L. Meanwhile, the adsorption of Cd²⁺ on the 0.025% RL-pretreated biosorbent is approximately 36% higher than that of the control sample.

Fig. 3 Effect of initial concentration of Cd²⁺ on adsorption capacity by intact and rhamnolipids-pretreated  P. simplicissimum (Volume of adsorption medium: 20 mL; adsorbent dosage: 0.2 g/L; reaction time: 4 h; pH: 5.0)

In the adsorption experiments, the data are fitted by the linear regression method. The adsorption constants of Langmuir and Freundlich equations and their regression coef?cients (R²) are calculated, as listed in Table 2. As given in Table 2, the Langmuir model has higher regression coef?cients than Freundlich model, indicating that all the adsorption processes agree better with Langmuir than with Freundlich equation. The Langmuir isotherm is consistent with strong monolayer sorption onto surface containing finite number of identical sorption sites. It is assumed a monolayer adsorption happens whose energy is constant and there is no migration of adsorbate molecules in the surface plane. The Freundlich isotherm is purely empirical based on a heterogeneous surface [11]. The values of q_max (see Table 2) obtained from the Langmuir model for Cd²⁺ adsorption by both the intact and RL-pretreated        P. simplicissimum are close to the experimental values. This could be the evidence that the surface of biosorbents is homogenous. Meanwhile, many studies on heavy metal ions adsorption by fungus are better fit by Langmuir model than Freundlich model [24-25]. According to Langmuir isotherm, it is shown that 0.025% RL-pretreated P. simplicissimum is the greatest potential adsorbent to adsorb Cd²⁺, and the monolayer saturation adsorption capacity is 70.4 mg/g (see Table 2). LIU et al [12] found that the maximum Cd²⁺ adsorption capacity by 0.025% saponins-pretreated P. simplicissimun was 74.6 mg/g. The reason for the adsorption capacity difference may be that these two kinds of biosurfactants have different effects on cell surface charges of bacterial. RL is anionic surfactant. However, saponins belongs to non-ionic surfactant.

The higher values of b indicate the affinity of biosorbents to investigated metals and imply strong binding of metal ions [17]. Consequently, the preference of investigated biosorbents for Cd²⁺ adsorption is as follows: 0.025% RL-pretreated P. simplicissimum > 0.05% RL-pretreated P. simplicissimum > 0.005% RL- pretreated P. simplicissimum > The control sample.

3.5 SEM and EDAX analyses

According to the maximum adsorption capacities (q_max), as listed in Table 2, 0.025% RL-pretreated      P. simplicissimum is selected to investigate the adsorption mechanism in the following experiments, which is in comparison with the control sample. Two kinds of biosorbents were scanned by scanning electron microscope (SEM) before and after adsorption, and the SEM micrographs are presented in Fig. 4. As shown in Figs. 4(a) and (c), the biosorbent surface before adsorption is characterized markedly filamentous shape. Mycelium aggregates into clusters in the control sample, while the pretreated sample is evenly scattered. The even distribution of mycelium may be one reason for the enhancement of Cd²⁺ adsorption after being pretreated with RL. It is shown by Figs. 4(b) and (d) that the surface characteristic of mycelium changes after adsorption. The surface of control sample is covered by a layer of Cd²⁺ looking like mushroom, while that of the RL-pretreated P. simplicissimum is like sponge. These differences may be caused by the changed surface characteristics of cells by the pretreatment of RL.

Energy dispersive X-ray analysis (EDAX) is one of the useful tools to evaluate the chemical and elemental characteristics of the biosorbents [17]. The element analysis of biosorbents before and after Cd²⁺ adsorption was carried out by EDAX. The results are shown in   Fig. 5.

As shown in Fig. 5, the K⁺ peaks between 3 and   4 keV disappear after Cd²⁺ adsorption. These discoveries indicate that adsorption process also includes ion- exchange mechanism [17]. The element mass fractions comparisons of 0.025% RL-pretreated P. simplicissimum with that of the control sample are listed in Table 3. The mass fraction of C and P change indistinctly, that of N,  S, Cl and K increase, yet that of O decrease. It may be because amino group and sulfate group increase on the cell walls, leading to the increase of the interactions between binding sites and Cd²⁺ [26].

Table 2 Constants and correlation coefficients of Langmuir and Freundlich isotherms for adsorption of Cd²⁺ from aqueous solution

Fig. 4 Typical SEM micrographs of adsorbents (Volume of adsorption medium: 20 mL; adsorbent dosage: 0.2 g/L; reaction time: 4 h; pH: 5.0; initial concentration of Cd²⁺: 20 mg/L): (a) Control sample before Cd²⁺ adsorption [12]; (b) Control sample after Cd²⁺ adsorption [12]; (c) RL-pretreated P. simplicissimum before Cd²⁺ adsorption; (d) RL-pretreated P. simplicissimum after Cd²⁺ adsorption

Fig. 5 Typical EDAX spectra of adsorbents (Volume of adsorption medium: 20 mL; adsorbent dosage: 0.2 g/L; reaction time: 4 h; pH: 5.0; initial concentration of Cd²⁺: 20 mg/L): (a) Control sample before Cd²⁺ adsorption; (b) Control sample after Cd²⁺ adsorption;   (c) RL-pretreated P. simplicissimum before Cd²⁺ adsorption; (d) RL-pretreated P. simplicissimum after Cd²⁺ adsorption

Table 3 Element mass fraction in 0.025% RL-pretreated P. simplicissimum and in control sample

3.6 FTIR spectral analysis

The infrared spectra of 0.025% RL-pretreated     P. simplicissimum and the control sample before and after Cd²⁺ adsorption are shown in Fig. 6.

Fig. 6 FTIR spectra of adsorbents (Volume of adsorption medium: 20 mL; adsorbent dosage: 0.2 g/L; reaction time: 4 h; pH: 5.0; initial concentration of Cd²⁺: 20 mg/L): (a) Control sample before Cd²⁺ adsorption [12]; (b) Control sample after Cd²⁺ adsorption [12]; (c) RL-pretreated P. simplicissimum before Cd²⁺ adsorption; (d) RL-pretreated P. simplicissimum after Cd²⁺ adsorption

The functional groups about stretching vibration are alkane group (3 000-2 700 cm^-1), amino and hydroxyl groups (3 356 cm^-1), carbonyl group (1 653 cm^-1), ether group (1 300-1 000 cm^-1), benzene ring framework    (1 620-1 450 cm^-1), phosphate group (1 149 cm^-1), sulfate group (1 234 cm^-1) and halogenated hydrocarbon group (534 cm^-1) [26]. It could be seen that after Cd²⁺ adsorption, the absorbance peak of aromatic double bonds group stretching vibration shifts from 1 410 cm^-1 to 1 377 cm^-1 in Fig. 6(b). Furthermore, the absorbance peak of amino group (1 543 cm^-1), carboxyl group    (2 858 cm^-1) and the mixed absorbance peak of amino and hydroxyl groups (3 388 cm^-1) are moved to a certain extent as well.

The spectra in Fig. 6(c) are changed compared to those in Fig. 6(a). The absorbance peaks of amino     (1 547 cm^-1), hydroxyl (3 307 cm^-1) and carboxyl     (1 655 cm^-1) groups stretching vibration are moved to a certain extent. Moreover, two new absorbance peaks appear in the region of 1 379 cm^-1 and 1 317 cm^-1. LIU et al [12] discovered that the presence of saponins shifted the peaks at 3 356 cm^-1 (indicative of the overlapping of the O—H and N—H stretching) to 3 361 cm^-1. Obviously, the different shifts of the O—H and N—H stretching in P. simplicissimum pretreated with RL and saponins result in the different Cd²⁺ adsorption capacities.

It is estimated that the corresponding group is nitro group of fatty compound in the region of 1 379 cm^-1, and C—N group stretching vibration of aromatic, C—O group stretching vibration of carboxylic acid or S=O group stretching vibration in the region of 1 317 cm^-1. These results indicate that the addition of RL changes the chemical structures of the biomass. In addition, these changes also affect the adsorption of Cd²⁺. After Cd²⁺ adsorption, the spectrum of the RL-pretreated         P. simplicissimum is presented in Fig. 6(d). It is obvious that hydroxyl groups stretching vibration of carboxylic acid appears in the region of 2 858 cm^-1. Besides, the mixed absorbance peak of amino and hydroxyl groups stretching vibration shifts from around 3 406 cm^-1 to about 3 307 cm^-1.

Those changes observed in the spectrum indicate that those functional groups possible involve in adsorption [17]. The interactions of binding groups on biosorbents surface might be the crucial factors that lead to different adsorption capacities under the same conditions. On the basis of above, it is indicated that the important role is played by carboxyl, amino and hydroxyl groups in the process of Cd²⁺ adsorption [26].

4 Conclusions

1) The effect of rhamnolipids (RL) on Cd²⁺ adsorption by Penicillium simplicissimum is investigated, and the novel RL-pretreated P. simplicissimum is a promising candidate for the removal of Cd²⁺ from aqueous solutions.

2) After the pretreatment of RL, the permeability of membrane increases, resulting in gathering a lot of negative charges on the cell wall surface. Therefore, rhamnolipids change the optimum pH of Cd²⁺ adsorption by Penicillium simplicissimum from 6.0 to 5.0.

3) SEM, EDAX and FTIR analyses show the interactions between Cd²⁺ and functional groups on the cell walls of the biosorbents. Rhamnolipids change cell surface characteristics of P. simplicissimum. It is estimated that carboxyl, amino and hydroxyl groups are responsible for the process of adsorption. Meanwhile, the adsorption process also includes ion-exchange mechanism.

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(Edited by DENG Lü-xiang)

Foundation item: Project(50978087) supported by the National Natural Science Foundation of China; Project(CX2010B157) supported by the Hunan Provincial Innovation Foundation for Postgraduate, China

Received date: 2011-06-11; Accepted date: 2011-08-23

Corresponding author: YUAN Xing-zhong, Professor, PhD; Tel: +86-731-88821413; E-mail: yxz@hnu.cn