Reactivity of sugar cane bagasse as a natural solid phase extractor for selective removal of Fe(III) and heavy-metal ions from natural water samples

2.1 Development of the sorbent SCB

SCB was first washed thoroughly with distilled water to remove the dust particles, then soaked over night in 0.1 N NaOH solution and again washed well with DDW. SCB was then soaked in 0.1 N CH₃COOH for a period of 2–3 h to remove the traces of NaOH (Rao et al., 2002). It was thoroughly washed again with DDW till the wash water became colorless and then filtered, well dried, powdered and sieved before use. The average particle size was 0.75 mm.

2.2 Metal binding capacities of SCB

The metal uptake capacities of SCB towards Fe(III), Co(II), Cu(II), Zn(II), Cd(II) and Pb(II) ions were determined in triplicate under static conditions by the batch equilibrium technique. SCB (100 mg) was added to a mixture of 100 μl of 0.1 M of the tested metal ion and a buffer solution of pH 1.0–7.0 – except with Fe(III) the pH used were in the range 1.0–4.0 – the total volume was completed to 50 ml by DDW in a 100 ml measuring flask. This mixture was mechanically shaken for 60 min at room temperature to attain equilibrium. The SCB was separated by filtration, washed with DDW and the unretained metal ion in the filtrate was determined by complexometric EDTA titration or FAAS (Soliman et al., 2004). For pH adjustment, solutions of 0.1 M HCl and 0.1 M NaOH were used. The equation of metal uptake capacity could be obtained as follows: $$M_c=(X-Y)/Z$$ where X refers to the initial concentration of metal ion in μmol, Y equals concentration of the unsorbed metal ion in the filtrate in μmol, Z = mass of the SCB in g, M_c = metal capacity value in μmol g⁻¹.

Moreover, the effect of contact time on Fe(III) uptake was determined under the same batch conditions, but at different shaking periods (10, 30, 45, 60, 90, 120 and 150 min) and at the pH 3.0 (Soliman et al., 2004). The effects of Fe(III) concentration (10.0–100.0 μmol) and weight of phase (25.0–150.0 mg) on its uptake efficiency by SCB were also systematically investigated.

2.3 Instrumentation

Infrared spectra of SCB before and after Fe(III) sorption were obtained using FT-IR model 410 JASCO (Japan). A Fisher Scientific Accumet pH-meter model 825 calibrated against two standard buffer solutions at pH 4.0 and 9.2 was used for all pH-measurements. Atomic absorption measurements were performed with Aquanova-Genway spectrometry.

3.1 FT-IR analysis of SCB before and after Fe(III) sorption

SCB as a lignocellulosic compound is generally considered as structures built by cellulose molecules, organized in microfibrils and surrounded by hemicellulosic materials (xylans, mannans, glucomannans, galactans, and arabogalactans), lignin and pectin along with small amounts of protein (Asheh and Duvnjak, 1997). The FT-IR spectra of SCB before and after sorption of Fe(III) were performed to determine the vibration frequency changes in their functional groups within the range of 4000–400 cm⁻¹. The spectral measurements support the complex nature of the examined adsorbent. SCB before Fe(III) sorption showed an absorption peak around 3403.7 cm⁻¹ indicates the existence of free and intermolecular bonded hydroxyl groups. The peaks observed at 2918.7 cm⁻¹ can be assigned to stretching vibration of the C–H group. The presence of strong C–O band at 1058.7 cm⁻¹ due to –OCH₃ group in addition to the peaks at 613.2 and 833.1 cm⁻¹ related to bending modes of aromatic compounds confirm the presence of lignin structure incorporated sugarcane bagasse (Garg et al., 2008). Moreover, a weak intensity band located at 1704.8 cm⁻¹ was argued to stretching vibration of carboxylic groups of galactouronic acid (Pehlivan et al., 2008). Comparing and assignments of SCB before and after Fe(III) sorption is too important to indicate the functional groups responsible for Fe(III) binding. Generally, the binding of metal ions to biosorbents may be proceed via complexation with functional groups, ionic exchange, surface precipitations and chemical reaction with surface sites (Torresdey et al., 2004). Regarding FT-IR for SCB after Fe(III) uptake, it was found that, oxygen containing functional groups vis, methoxy –OCH₃, carboxy –COOH and phenolic –OH groups are affected after uptake process. This is judging from shifts in their position, shape or band intensity from 1058.7, 1704.8 and 3403.7 cm⁻¹ to 1046.2, 1682.6 and 3406.6 cm⁻¹ of ν(–O–C), ν(–COOH) and ν(–OH) for SCB before and after Fe(III) sorption, respectively. The results indicate the participation of these groups via oxygen for Fe(III) binding to SCB in agreement with Person principal for hard-soft acids and bases (Pearson, 1963).

3.2 Effect of pH on metal ion binding capacities using SCB

SCB mainly contains cellulose (45%), hemi-cellulose (28%) and lignin (18%). This variety of chemical composition and consequently functional groups make SCB shows different efficiencies for removal of heavy-metal ions over a wide range of pH (Garg et al., 2008). This is clear as shown Fig. 1 for the uptake of the studied metal ions as a function of pH (1.0–7.0). Therefore, at low pH values (pH <2.0) SCB showed very low tendency for uptake of all the investigated metal ions due to protonation of its functional groups or competition of H⁺ with metal ions for binding sites (Mohan and Singh, 2002). These uptake values increased with increasing pH from 2.0 to 7.0. The binding capacities reached their maximum values around pH 6.0 and then retained almost constant. Maximum sorption capacities were 15.5 μmol g⁻¹ at pH 5.0 for Co(II), 86.0, 70.0 and 87.0 μmol g⁻¹ at pH 6.0 for Cu(II), Cd(II) and Pb(II), respectively, and 81.0 μmol g⁻¹ at pH 7.0 for Zn(II). The predominant metal species at pH between 2.0 and 7.0 are positively charged [Mⁿ⁺ and M(OH)⁽ⁿ⁻¹⁾⁺], therefore, uptake of metals may proceed through Mⁿ⁺ exchange process with acidic sites H⁺, complexing with functional groups and/or chelation (Rengaraj et al., 2001).

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Figure 1

Metal uptake capacity (μmol g⁻¹) as a function of pH using SCB.

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The uptake behavior of Fe(III) by SCB as a function of pH was unique compared to the other metal ions (Fig. 1). Its uptake reached to maximum value (100 μmol g⁻¹⁾ at pH 2.0 and then retained constant at pHs 2.5, 3.0, 3.5 and 4.0. This behavior reflects the pronounced capability of Fe(III) ions to overcome the competition arisen by H⁺ ions at low pH values for binding to oxygen donor sites in SCB.

Finally, the decreasing affinity of SCB for binding to the studied metal ions can be arranged based on their uptake values according to the following order: Fe(III) > Cu(II) > Pb(II) > Zn(II) > Cd(II) > Co(II).

3.3 Optimum conditions for binding of Fe(III) using SCB

Considering the biological important of Fe(III), the study will be directed to optimize the uptake properties of Fe(III) after choosing pH 3.0 for this purpose.

3.3.1

3.3.1 Effect of shaking time

Shaking time is an important factor in the process of evaluation of the SCB. The batch experiments were carried out at different contact times 10, 30, 45, 60, 90, 120 and 150 min using mechanical shaker with a fixed sorbent mass (100 mg), Fe(III) concentration (100 μl) at a pH of 3.0 and 50.0 ml contact solution. Results of percentage extraction of Fe(III) as a function of shaking time intervals is represented in Fig. 2. The equilibrium is reached within the first 60 min of contact and reached a saturation level. In the beginning, the ions adsorbed, occupied selectively the active sites on SCB. As the contact time increased the active sites on the sorbent were filled. The results which plotted in Fig. 2 clearly indicate that the equilibrium for Fe(III) is attained in 60 min of contact, (100.0%). These results of percentage extraction and time of attained equilibrium are higher and faster, respectively, than the values reported by other authors (Bansode et al., 2003).

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Figure 2

% Extraction of Fe(III) as a function of shaking time using SCB.

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3.3.3

3.3.3 Effect of weight of SCB

The weight of SCB adsorbent was varied from 50.0 to 150.0 mg keeping all the other experimental variables, viz., pH 3.0 initial concentration (10.0 μmol), and contact time 60 min. Fig. 3 presents the adsorbent weight profile versus Fe(III) adsorbed per unit mass. It may be observed that on increasing the adsorbent weight, the percentage extraction of Fe(III) increased up to a weight of 100.0 mg remained constant.

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Figure 3

Effect of SCB weight on % extraction of Fe(III).

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3.4 Binding performance (sorption isotherms)

The purpose of the adsorption isotherms is to relate the adsorbate concentration in the bulk and the adsorbed amount at the interface (Demirbas et al., 2002). The simplest adsorption isotherm is based on the assumptions that every adsorption site is equivalent and the ability of a particle to bind is independent of whether or not adjacent sites are occupied (Shukla et al., 2005). To find out the mechanistic parameters associated with Fe(III) sorption, the results obtained by the adsorption experiments were analyzed by the well-known models given by Freundlich and Langmuir models. The sorption isotherms for binding of Fe(III) with SCB were determined by the batch technique at pH 3.0. Thus, solutions of iron(III) in the concentration range: (4.0 × 10⁻⁴–2.0 × 10⁻³ M) were shaken for 60 min with a constant weight 100.0 mg of SCB.

The basic assumption of Freundlich isotherm is that if the concentration of the solute in the solution at equilibrium, C_e was raised to the power 1/n, the amount of solute sorbed being q_e, then $C_e^{1/n}{q}_e$ was a constant at a given temperature. This model is more widely used but provides no information on the monolayer biosorption capacity (Pehlivan et al., 2008) and it assumes neither homogeneous site energies nor limited levels of sorption. The non-linear form (Fig. 4) of Freundlich equation expressed as follows:

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Figure 4

Adsorption isotherm of Fe(III) using SCB phase.

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The Langmuir equation, which is valid for monolayer sorption onto a completely homogeneous surface with a finite number of identical sites and with negligible interaction between adsorbed molecules is given by the following equation:

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Figure 5

Langmuir plot for the sorption of Fe(III) on SCB at constant temperature.

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The different parameters of Freundlich and Langmuir isotherms constants are collected in Table 1. Data were fitted with non-linear regressions using a least-square fitting program (origin 6.0) to obtain the best estimate of all constants for the Freundlich and Langmuir isotherms.

3.5 Kinetic studies

The sorption kinetic data of Fe(III) measured on SCB was analyzed in terms of pseudo-second-order sorption equation (Reddad et al., 2002). This equation can be written as:

The following equation can be obtained on rearranging Eq. (5) into a linear form as follows:

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Figure 6

Pseudo-second-order kinetic plot for the sorption of Fe(III) on SCB.

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3.6 Separation and selectivity studies

The values of distribution coefficients K_d = (amount of the metal ion sorbed by the SCB in μmol g⁻¹)/(amount of the metal ion unsorbed in solution in μmol ml⁻¹) and selectivity parameter $S=\log K_{d_{\text{Fe}\left(\text{III}\right)}}-\log K_{d_{\text{M}}}^{n+}$ , where Mⁿ⁺ [Co(II), Cu(II), Zn(II), Cd(II) and Pb(II)], for separation of Fe(III) from the investigated metal ions are given in Table 2. It can be deduced that the separation process is possible and successful at pHs 2.0 and 3.0 ( $K_{d_{\text{Fe}\left(\text{III}\right)}}=49\text{,}950$ , S ⩾ 2.0). As the S value increase the separation process is favorable with minor or no interferences. The order of increasing interference is as follows: at pH 2.0, Co(II) < Cd(II) < Zn(II) < Pb(II) ≅ Cu(II) and at pH 3.0, Co(II) < Zn(II) < Cd(II) < Cu(II) < Pb(II).

4.1 Batch studies

Uptake experiments were carried out using different natural water samples: DDW, DTW, NRW, GW and NDW. A 100.0 mg of SCB was conditioned with 50.0 ml of water sample spiked Fe(III) at concentration of 2.5 and 5.0 ppm (adjusted at pH 2.0 by fine addition of concentrated HCl) with shaking for 60 min. Ten milliliters of the solution (free from the suspended solid phase) were taken at the end of the experiment where the residual concentration of metal ion was determined via FAAS. The percentage recovery values of Fe(III) are combined in Table 3.

4.2 Column studies

A glass column of 30 cm length and 2 cm internal diameter (ID) was used. SCB (100.0 mg) was put between two layers of glass wool, the first at the bottom to avoid loss of sorbent when the sample solution passes through the column and the second at the top to retain it. Then, 50 ml solution of each of the natural water sample (DDW, DTW, NRW, GW and NDW) containing 2.5 and 5.0 ppm of Fe(III) (adjusted at pH 2.0 by fine addition of concentrated HCl) passed through the column with flow rate of 1.0 ml/min. Values of Fe(III) recovered are given in Table 4.

The quantitative recovery values of Fe(III) as shown in Tables 3 and 4 indicate the suitability and validity of using SCB for selective removal of Fe(III) from natural water samples either by batch or column techniques.