Modification and Application of Albizia lebbeck Sawdust For The Sorption of Lead (II) and Copper (II) From Aqueous Solutions

Introduction

Disposal of untreated industrial wastes is the major source of heavy metals contamination, which causes severe water, soil and air pollution. The persistent nature of heavy metals results their accumulation in plants and animal tissues and ultimately enter the food chain. When heavy metals enter into human body through contaminated foods, they are bio-amplified and cause serious threat to the public health.^1-2 From various industrial activities lead (Pb) and copper (Cu) are exposed to the environment and cause detrimental effects such as malfunction of brain, cognitive disorder to the children, renal diseases, anemia and loss of reproductive power.³ Many techniques such as precipitation, ion-exchange, membrane separation, electrodialysis, extraction etc. are reported for removing heavy metals.^4,5 However, these techniques are not attractive because of high cost and difficulties associated with the handling of toxic sludges produced during the processes^1,4,6. Adsorption processes have drawn much attention due to their high efficacy in removing heavy metals⁴ and availability of a wide variety of adsorbents such as activated carbon,⁷ clay minerals,^8-9 zeolites,¹⁰ metal oxides,^11-12 agricultural wastes,^13-20 biomass²¹ polymeric materials,²² which demonstrated significant adsorption capacities for heavy metals. Although activated carbon possesses very high adsorption capacities for various organic compounds and dyes, it is not that effective in removing heavy metals. Moreover, activated carbon is too expensive and thus has found less application in removing heavy metals.⁴ On the other hand, the use of untreated plant wastes were reported to have poor sorption capacities and releases soluble organic substances and make the treated water unsuitable for uses.^23,24 During chemical treatment of plant materials most of the soluble organic materials are removed and simultaneously the metal sorption capacity is enhanced.⁴

Sawdust is a by-product of wood industries and can be obtained at negligible cost. Earlier sawdust of different plants were chemically modified using a variety of reagents and applied for sequestration of metals ions, e.g. alkali treated sawdusts demonstrated adsorption capacities of 6.92 mg/g and 15.8 mg/g of Cu²⁺ and Zn²⁺ respectively,²⁵ while acid treated sawdust showed Cu²⁺adsorption capacities of 3.6-13.95 mg/g.^26-28 Few research groups adopted treatment method using formaldehyde in sulfuric acid to enhance the adsorption capacity of sawdust but ended with poor results.^29-31 Recently we observed that sawdust impregnated with nano-zerovalent iron showed significant sorption of As(III) from aqueous solution.³² Other recent studies showed that sawdust,³³ nano-sawdust,³⁴ sawdust-kaolinite composite³⁵ could be used as adsorbents for heavy metals. Modification of pure cellulose were reported to have good to excellent adsorption capacities of heavy metals, e.g etherification of cellulose with epichlorohydrin followed by epoxide ring opening by polyethyleneimine afforded an adsorbent that showed moderate adsorption capacities of 25 mg/g and 15 mg/g of Pb²⁺ and Cu²⁺ respectively.³⁶ An adsorbent prepared by oxidation of pure cellulose with periodate and hydroxamic acid demonstrated excellent Cu²⁺ sorption capacity of 246 mg/g.³⁷

To the best of our knowledge no report is available on the development of adsorbent for heavy metal by modification of sawdust using etherification followed by ring opening by an alkyl amine strategy. In the present investigation modification of sawdust of Albizia lebbeck was carried out by treatment with sodium methylate, followed by etherification using epichlorohydrin and oxirane ring opening by n-dodecylamine to afford a material, which was applied for the sorption of Cu²⁺ and Pb²⁺ ions.

Materials and Methods

Materials and Reagents

Sawdust of Albizia lebbeck (Local name Koroi) obtained from a sawmill located at Mirpur in Dhaka Bangladesh was washed with water and sun-dried for 3 days followed by drying in an oven at 50 ^oC for 72h and cooled in desiccators. The dried material was sieved and the fraction with a particle size between 0.2 and 0.6 mm was used for the preparation of chemically modified adsorbent. Chemical analyses of dried sawdust were carried out using literature methods³⁸ and found that it contained 7.25% extractible materials, 68.7% cellulose and 17% lignin.

Sodium methylate, dimethylsulfoxide (DMSO), epichlorohydrin, ethanol, n-dodecylamine, copper sulfate pentahydrate, lead nitrate, sodium hydroxide and nitric acid were purchased from Aldrich and used as received. Stock solutions of the metals were obtained by dissolving required amounts of CuSO₄.5H₂O and Pb(NO₃)₂ in deionized (DI) water. A small volume (10-20 µl) of 0.1M NaOH or 0.1M HNO₃ were added to the metal ions solutions to adjust thepH.

Preparation of the Adsorbent

Modification of sawdust was carried out following a literature procedure.³⁶ In brief, sawdust (5.0 g) was suspended in DMSO (100 ml), purged with N₂ and stirred in a hot plate magnetic stirrer for 2 h at 60 ^oC. The mixture was cooled to room temperature, treated with sodium methylate (15.0 g) and stirred for 1 h in N₂ atmosphere. The resulting solids were separated by filtration and washed with DI water (200 ml) and dried at 50 ^oC for 3 hours to afford the material NaO-SD (4.5 g). To the suspension of NaO-SD (3.0 g) in DMSO (50 ml) epichlorohydrin (30 ml) was added and stirred for 2 hours at 50 ^oC in N₂ atmosphere. Solid materials thus obtained were separated by filtration and washed with DI water and dried at 50 ^oC for 3 hours to afford ECH-SD (3.12 g). ECH-SD (1.4 g) was suspended in DMSO (30 ml), n-dodecylamine (3.2 g) was added in portions and heated at 100 ^oC with stirring for 18 hours. The final material was separated by filtration and washed with DI water and dried in an oven at 50 ^oC for 3 hours and cooled in desiccators to afford the sorbent DDA-SD (1.4 g).

Instruments

The functional groups present in the sorbent (DDA-SD) were identified by FTIR-ATR (Shimadzu, Japan) and the surface morphology of it was determined using a scanning electron microscope (Philips XL30, FEI, USA). The surface area was determined by BET analysis, which was performed on a Quantachrome Autosorb-1. For BET experiment DDA-SD sample were out gassed at 120 °C for 24 hours before analysis and Krypton gas was used as the adsorbent. To obtain a value of surface area, 11 points were measured in the standard range of (P/P_o) = 0.05 to 0.3.  The pH of the solutions was measured with a pH meter (HI 2211-02, Hanna, Romania). Batch adsorption experiments were performed under shaking in an orbital Shaker (SSL1, Stuart UK). Metal ions concentrations of the filtered solutions (Whatman 41) were measured with Atomic Absorption Spectroscopy (AAS) (Varian AA 240 FS, USA).

Batch Sorption Experiments

Metal uptake capacities of the DDA-SD were determined by adding 100 mg of DDA-SD into 100 mL of metal ion solution taken in a conical flask. The experiments were carried out at 25 ^oC under shaking at 180 rpm in an orbital shaker. After completion of the experiments the solutions were filtered with Whatman 41 filter paper and the filtrates were then analyzed with AAS. The sorption capacity, qe was determined according to the equation (1).

Where Ci and C_e stand for initial and equilibrium concentrations of the metal ion (g/L), m represents mass of the adsorbent (g) and V is the volume of the solution (L). 

To observe the effect of pH on the sorption process 100 ml of a metal ion solutions were taken in different conical flasks, pH were adjusted 1.9-6 for Pb(II) and 3-10.2  for Cu(II) by adding dilute nitric acid or sodium hydroxide and 100 mg of DDA-SD was added to each flask and left under shaking for 1 hour. Afterwards sorbents were separated by filtration and sorption capacities were calculated using the equation (1).

Effect of dosage was investigated by adding different amounts of the DDA-SD (50-350 mg) to 100 ml metal ion solutions of fixed concentration (25.0 ppm) in different conical flasks, pH were adjusted 5.8 and 7.1 for Pb²⁺ and Cu²⁺ respectively. Effect of contact time was monitored by adding fixed amount of the sorbent (500 mg) to metal ion solutions of concentrations of 23.5, 38.0 ppm for Cu²⁺ and 9.5, 28.5 ppm for Pb²⁺ and pH were maintained at 5.8 for Pb²⁺ and 7.1 for Cu²⁺ and left under shaking. Aliquots of 2 ml were drawn at different time intervals and metal ion concentrations were measured by AAS.

Desorption Study

Desorption study was conducted by soaking 20 mg of the exhausted DDA-SD in 20 mL of 0.1M HNO₃ or 0.1M HCl and left under shaking for 3 hours. An aliquot of 0.5 mL was drawn in every 30 min and metal ion concentrations were analyzed using AAS.

Results and Discussion

Synthesis of the Adsorbent

The major chemical constituents of sawdust are cellulose, lignin and hemicellulose. The reactions occurred during the modification process are represented in the scheme 1. The primary hydroxyl group of the β unit (1-4 linked D-glucose) of cellulose reacted with sodium methylate to afford Cell-CH₂-ONa (NaO-SD). During treatment with sodium methylate in DMSO a considerable amount of extractible materials, hemicellulose and lignin were removed from the cellulosic material ³⁸. Cell-CH₂-ONa then reacted with epichlorohydrin to afford cellulose derivative containing oxirane moiety (ECH-SD). The oxirane ring was opened by reaction with n-dodecyl amine to afford the sorbent (DDA-SD).

Scheme 1: Modification of Sawdust Click here to View Scheme

Characterization

FTIR Analysis

FTIR analysis of NaO-SD, ECH-SD and DDA-SD were carried out using ATR sampling technique and vibrational frequencies are summarized in the table 1.

Figure 1: FTIR spectra of NaO-SD, ECH-SD and DDA-SD Click here to View Figure

Table 1: FTIR peaks for synthesized materials

Material	νcm-1
NaO-SD ECH-SD DDA-SD	3587-3033, 2925-2825, 1590, 1508, 1458, 1420, 1325, 1262, 1218, 1016, 638. 3700-3400, 2976, 2888, 1458, 1388, 1250, 1155, 1073, 954, 821, 638. 3700-3490, 2976, 2888, 1458, 1388, 1250, 1155, 1073, 954, 821, 669, 638.

For NaO-SD the prominent peaks appeared at 1016 (strong), 1250, 1458, 2825-2925 and 3033-3587cm^-1 which are assignable to the C-O, C-C, C-H, and OH stretching vibrations of the cellulose moiety. For ECH-SD the C-O, C-C, C-H, and OH stretching vibrations were observed at 1073, 1250, 1458, 2888, 2976 and 3400-3700 cm^-1, while those peaks for DDA-SD were at 1073, 1250, 1458, 2888, 2976 and 3490- 3700 cm^-1. New peaks at 954 and 821 cm^-1 were present in case of ECH-SD and DDA-SD which are assignable to the presence of the oxirane moiety.

The presence of an epoxy group in the ECH-SD was further confirmed by a chemical method³⁶. On the treatment of ECH-SD with sodium thiosulfate the oxirane ring breaks down and liberates an equimolar amount of NaOH which can be quantified by titration with standardized HCl in presence of phenolphthalein indicator. Using this strategy the amount of epoxide ring (mg) per gm of DDA-SD was determined by using the following equation (2). The amount of epoxide moiety was found to be 0.37 mg/g which was in good agreement with the previous result³⁶.

Scheme 2:  Reaction of epoxide ring with sodium thiosulfate Click here to View Scheme

Where V = Volume (mL) of HCl required, M = molarity of HCl, W= Weight (gm) of treated sawdust (ECH-SD).

SEM Analysis

SEM images of NaO-SD and DDA-SD revealed similar morphology with rough surfaces in both cases (Fig. 2).

Figure 2: SEM images of Na-SD (top) and DDA-SD (bottom) Click here to View Figure

Surface Area

The surface area of NaO-SD and DDA-SD were measured by BET analysis and found to be 0.75 m²/g and 0.73 m²/g respectively, which revealed that the materials have comparable surface areas.

Effect of pH

Depending on the pH of the solution the surface charge of an adsorbent varies significantly, which plays a vital role in the sorption process. The effect of pH on the sorption of Pb²⁺ and Cu²⁺ was studied by adding a constant dosage of DDA-SD (20 mg/20 ml) into and metal ion solutions of fixed concentrations (50 ppm for Pb²⁺ and 5 ppm for Cu²⁺) in different conical flasks maintaining pH in the range of 1.9 to 10. The result showed that with increasing pH the sorption of Pb²⁺ increased dramatically and reached the maximum at pH 5.9. Effect of further increase of pH was not monitored as Pb²⁺ precipitated beyond the pH of 5.9. In case of Cu²⁺ the maximum sorption was achieved at pH 7.1 and it remained same up to the pH 10. It is assumed that at low pH some of the NH groups of DDA-SD were protonated, which hindered the chelation of metal ions and resulted lower sorption (Scheme 3).

Scheme 3: Sorption mechanism at different pH Click here to View Scheme

Figure 3: Effect of pH on sorption of Pb2+ and Cu2+on DDA-SD. Click here to View Figure

Effect of Dosages

The effect of dosages on the sorption of Pb²⁺ was carried out using DDA-SD dosages ranging from 0.5 g/L to 3.5 g/L at pH 5.85 and at room temperature. The experiments were carried out under shaking in an orbital shaker at rpm 180 for 1hr. The change in removal efficiency (%R.E) and sorption capacity of DDA-SD for Pb²⁺ with the adsorbent dosages are shown in the figure 4.

Figure 4: Effect of dosages of DDA-SD on the Pb2+ sorption Click here to View Figure

It is clear that sorption capacity decreased with the increasing amounts of dosage and maximum adsorption capacity was obtained at a dosage of 0.5g/L (Figure 4). This is because almost all the active sites of sorbent was efficiently occupied by Pb²⁺ ions and at higher dosage although the number of active sites increased a situation was created where a surplus of adsorption sites existed and resulted lower sorption capacity.

Effect of Time

The equilibration time is considered as the optimum contact time, which was determined by measuring the sorption capacities at different time. In a conical flask 500 mL of the metal ion solution was taken, the pH was adjusted (5.9 for Pb²⁺ and 7.1 for Cu²⁺) and 500 mg of DDA-SD was added. The solution was left under shaking and an aliquot of 2 mL was withdrawn at different time intervals, filtered, analyzed by AAS and the results are shown in the Fig. 5. It was evident that for all experiments the equilibration time was 60 minutes. It was also found that the sorption capacity of DDA-SD increased with increasing metal ion concentration, thus sorption capacity of Pb²⁺ increased from 7.97 mg/g to 17.47 mg/g as the concentration was raised from 9.5 ppm to 28.5 ppm, while that for Cu²⁺ increased from 10.32 mg/g to 17.8 mg/g when the concentration was increased from 23.5 to 38 ppm. The higher sorption capacity at higher concentration is attributed to the higher probability of collision between sorbate and sorbent surface.³⁹

Figure 5: Effect of concentration and time on sorption of Pb2+ and Cu2+on DDA-SD. Click here to View Figure

Adsorption Isotherms

The relationship between the amount of sorbate attached on the surface of a sorbent and its concentration in the equilibrium solution at a constant temperature is represented by sorption isotherm. Fig. 6 shows the equilibrium sorption data for Pb²⁺ and Cu²⁺ on DDA-SD.

Figure 6: Sorption isotherms for Pb2+ and Cu2+on DDA-SD. Click here to View Figure

The relationship between the equilibrium sorption capacities, q_e (mg/g) with the metal ion concentrations at equilibrium, C_e (mg/L) can be obtained by using Langmuir (equ.3) and Freundlich (equ. 4) isotherm models.

Where C_e is the equilibrium concentration of sorbate (mg. L^-1); q_e is the equilibrium sorption capacity (mg. g^-1); q_max is the maximum monolayer sorption capacity (mg. g^-1) and K_L is Langmuir constant (L. mg^-1). 

From the slope and intercept of the Langmuir plot the values of K_L and q_max were calculated and shown in the Fig. 7 and table 2.The maximum sorption capacities were 17.37 mg/g and 9.4 mg/g for Pb²⁺ and Cu²⁺ respectively. It was clear that sorption process of both Pb²⁺ and Cu²⁺on DDA-SD followed Langmuir isotherm which was attributed to the fact that the sorption occurred through the formation of monolayer at homogeneous surface of the sorbent and there was no transmigration of sorbate particles during the process⁴⁰.

Figure 7: Langmuir isotherms for sorption of Pb2+ and Cu2+on DDA-SD. Click here to View Figure

Table 2:  Sorption parameters of Langmuir isotherms

Parameters	Pb2+	Cu2+
qmax Langmuir constant, K (L/mg) R2	17.37 0.743 0.9819	9.40 0.745 0.9904

A dimensionless parameter known as separation factor (R_L) was obtained from the Langmuir constant, K_L using the equation (4):

Where Ci is the initial concentration of the metal ion (mg/L) and K_L is the Langmuir constant (L/mg).

Table 3: Separation factor (R_L) for the sorption processes

Metal	Initial conc.(ppm)	RL Value
Pb2+	9.5 10.5 18.0 28.5 34.6	0.1241 0.1136 0.0695 0.0451 0.0374
Cu2+	20.6a	0.0612

^aLangmuir isotherm was obtained from experiments  using fixed  initial concentration and different dosages of DDA-SD.

R_L values indicate the feasibility of the sorption process, thus R_L= 0, 0<R_L<1, R_L= 1 and R_L>1 indicate that the process is irreversible, favourable, linear and unfavourable respectively. The values of R_L in the present study were between 0 and 1(Table 3) and indicated that the sorption process was favourable.

The sorption data were then evaluated for Freundlich model using the equation (5).

The Freundlich constant, K_F indicates sorption capacity, while 1/n is a function of the energy of sorption process. It is evident from the Fig. 8 that the sorption process did not follow Freundlich isotherm model indicating that the processes did not proceed through the formation of a multilayer on heterogeneous surface.⁴⁰

Figure 8: Freundlich isotherms for sorption of Pb2+ and Cu2+ on DDA-SD. Click here to View Figure

Sorption Kinetics

To understand the kinetics of the sorption of Pb²⁺ and Cu²⁺ on DDA-SD, the pseudo first order (equ. 6) and pseudo second order (equ. 7) kinetic equations were tested for the sorption data.

According to the equation 6, log (q_e-q_t) versus t was plotted and observed that the sorption data for neither Pb²⁺ nor Cu²⁺ (not shown) matched with the first order kinetics (Fig. 9).

Figure 9: First order kinetics for the sorption of Pb2+ on DDA-SD. Click here to View Figure

The second order kinetics model was tested by plotting t/q_t versus t and it was evident that the sorption of both Pb²⁺ and Cu²⁺ on DDA-SD perfectly followed this kinetic model (R² = 0.9966 to 0.9998 ). This means that the sorption rate depends on the metal ion concentration as well as the number of available active sites of the sorbent. The sorption capacities of DDA-SD were calculated from the pseudo-second-order kinetics plots and listed in the table 4 and found that the sorption capacities were similar to those obtained from Langmuir isotherm model.

Figure 10: Second order kinetics for the sorption of Pb2+ (top)Cu2+ (bottom) on DDA-SD. Click here to View Figure

Table 4: Adsorption capacities of DDA-SD

Metal	Initial conc. (ppm)	R2	Ads. capacity, qe (mg/g)
Pb2+	28.5 18.0 9.5	0.9966 0.9875 0.9983	18.0 14.0 8.0
Cu2+	38.0 23.4	0.9994 0.9998	17.7 11.1

 

Regeneration

In this study, regeneration of the sorbent was carried out by soaking 20 mg of exhausted DDA-SD  in 20 mL 0.1M HNO₃ or 0.1M HCl and leaving it for 30 minutes followed by wash with DI water. The desorption ratio (DR) was calculated as follows:

After 30 min the desorption ratio was 0.97 for both the acid solutions used for regeneration purpose. Further increase in time decreased the desorption ratio, which may be due to the re-adsorption of the metal ions on regenerated DDA-SD.

The regenerated DDA-SD was reused for sorption of  Pb²⁺ and it was found that in case of 34.0 ppm  solution the removal efficiency of regenerated DDA-SD was 30.4%, 29% and 27 % for 1^st, 2^nd and 3^rd recycle while removal efficiency of fresh DDA-SD was 49.5%. The decrease of   removal efficiency of the regenerated DDA-SD is attributed to the reduction of active site due to protonation of the NH functional group in the acidic solution, which is assumed to be responsible for metal ion sorption.

Figure 11: Sorption of Pb2+ ion on fresh and recycled DDA-SD. Click here to View Figure

In the present investigation sawdust of Albizia lebbeck was modified chemically and the sorption characteristics of the material were studied under equilibrium conditions using Pb²⁺ and Cu²⁺ solutions. The optimum pH for the sorption of Pb²⁺ and Cu²⁺ were 5.9 and 7.1 respectively. The sorption data fitted well to Langmuir model and did not fit Freundlich model at all. The maximum sorption capacities of Pb²⁺ and Cu²⁺ on DDA-SD were 17.37 and 9.4 mg/g respectively. The sorption data fitted well with the second order kinetic model. The exhausted DDA-SD was regenerated by treating with either 0.1M HNO₃ or 0.1M HCl followed by washing with DI water and the regenerated DDA-SD showed acceptable sorption capacities.

Acknowledgement

The research was supported by the Commonwealth Scholarship Commission in the UK.  Thanks to the Ministry of Science and Technology, Bangladesh for financial support to one of the researchers. The authors are grateful to the Department of Applied Chemistry and Chemical Engineering, University of Dhaka, Bangladesh and the Division of Materials, Mechanics and Structures, Faculty of Engineering, University of Nottingham, UK.

Conflict of Interest

The authors declare that there is no conflict of interest.

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