Separation of Pb2+ from Mg2+ by modified sugarcane bagasse under batch and column conditions: Effect of initial concentration ratio

Jun-xia Yu; Wan-li Xiong; Quan Sun; Jing Zhu; Ru-an Chi; Yue-fei Zhang

doi:10.1016/j.arabjc.2016.07.005

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Original article

12 (

); 4438-4445

doi:

10.1016/j.arabjc.2016.07.005

Separation of Pb²⁺ from Mg²⁺ by modified sugarcane bagasse under batch and column conditions: Effect of initial concentration ratio

Jun-xia Yu, Wan-li Xiong, Quan Sun, Jing Zhu, Ru-an Chi^⁎, Yue-fei Zhang

Hubei Novel Reactor & Green Chemical Technology Key Laboratory, Key Laboratory for Green Chemical Process of Ministry of Education, School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Wuhan 430074, China

⁎Corresponding author. Fax: +86 02787194980. rac_wit@163.com (Ru-an Chi)

Received: 2016-2-13, Accepted: 2016-7-10, Published: 2016-12-01

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

Effect of initial concentration ratio $(C_{0}^{Mg} : C_{0}^{Pb})$ on separation of Pb²⁺ from Mg²⁺ by the modified sugarcane bagasse was carried out under batch and column conditions. For comparison, the adsorption performance of the modified SCB (sugarcane bagasse) for Pb²⁺ in one component system was studied under the two conditions. Amount of Pb²⁺ and Mg²⁺ adsorbed on the column in the binary system was calculated through the elution curves. Results showed that the adsorption capacity of Pb²⁺ $(q_{e}^{Pb})$ decreased while that of Mg²⁺ $(q_{e}^{Mg})$ increases with the increase of $C_{0}^{Mg} : C_{0}^{Pb}$ , and good linear relationships between $C_{0}^{Mg} : C_{0}^{Pb}$ and $q_{e}^{Mg} / q_{e}^{Pb}$ were obtained under both conditions. According to the linear equations, mass ratio of metal ions adsorbed on the modified SCB was calculated. It was observed that mass ratio was higher than 95% for Pb²⁺ at $C_{0}^{Mg} : C_{0}^{Pb} < 0.022$ while for Mg²⁺ at $C_{0}^{Mg} : C_{0}^{Pb} > 10.01$ under batch condition. Under the column condition, high mass ratio (>95%) for Pb²⁺ and Mg²⁺ was obtained at $C_{0}^{Mg} : C_{0}^{Pb} < 1.24$ and $C_{0}^{Mg} : C_{0}^{Pb} > 585.5$ , respectively. The above results showed that modified SCB could be used to separate Pb²⁺ and Mg²⁺ when $C_{0}^{Mg} : C_{0}^{Pb} < 1.24$ (Pb²⁺ adsorbed selectively) or $C_{0}^{Mg} : C_{0}^{Pb} > 10.01$ (Mg²⁺ adsorbed selectively). These findings would provide theoretical guidance for separation of metal ions.

Keywords

Sugarcane bagasse

Pb2+

Mg2+

Separation

Biosorption

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1 Introduction

With the development the society, large amounts of wastewater containing heavy metals were discharged from printing, electroplating, leather tanning, mining and pigments industries (Ghaedi et al., 2006; Anayurt et al., 2009). Among these heavy metal ions, lead in particular is the most important pollutants, which would cause severe damage to the human kidney, nervous system, reproductive system, liver, and brain even at low concentration (Xu et al., 2008). Moreover, it is non-biodegradable and tends to accumulate in organisms as part of the food chain (Motsa et al., 2011; Deng et al., 2013). Efforts for effective lead removal from wastewater have been continuously made for decades. A variety of methods, including chemical precipitation, membrane process, coagulation, phyto-extraction, and adsorption had been reported to remove lead from aqueous solution (Isaac et al., 1997). Among these methods, adsorption is recognized as an efficient and economic method.

Recently, more and more attention was paid on biosorbents, especially on the agricultural waste such as sugarcane bagasse, leaf and orange peel due to its low cost and ready availability (Yuvaraja et al., 2014; Madala et al., 2015; Lasheen et al., 2012; Hossain et al., 2012; Wan Ngah and Hanafiah, 2008 ). However, the application of untreated agricultural waste has drawbacks such as low adsorption capacity and poor adsorption selectivity (Nguyen et al., 2013). In order to improve its adsorption capacity for metal ions, different kinds of modified biosorbent were prepared. Yu et al. (2015a) had reported that the adsorption capacity of sugarcane bagasse for Pb²⁺ increased from 0.04 to 1.06 mmol g⁻¹ after modification by pyromellitic dianhydride. Zheng et al. (2011) had reported that the maximum sorption capacity of cotton for Pb(II) increased to 2.45 mmol g⁻¹ after grafted by polyacrylic acid sodium. Feng et al. (2011) found that the grafted copolymerization of orange peel increased the metal uptake of Pb(II) up to 4.2 times. The adsorption capacity of the sorbents for Pb²⁺ increased significantly after modification by functional groups. Even so, the application of biosorbent still encounters many problems, especially the selective adsorption from the co-ions. In real industrial wastewater, it contains not only heavy metals but also high concentration of other metal ions, such as Mg²⁺, Ca²⁺, K⁺ and Na⁺. Contreras et al. (2006) had reported that Pb²⁺ binding decreased as the comparative concentration ratio of Mg²⁺ increased to a certain value. Sawalha et al. (2009) found that the binding of Pb²⁺ would be hindered by combined concentrations of Ca and Mg higher than 40.0 mmol L⁻¹. Reddy et al. (2010) explored that as the initial concentration ratio of the alkali metal ions increased, the uptake of Pb(II) ions by modified Moringa oleifera leaves decreased. Results above showed that concentration ratio of co-ions had great effect on the adsorption of Pb²⁺ in the binary system. However, the effects of concentration ratio on the adsorption of co-ions still needed to be investigated in details, and the optimum conditions for Pb²⁺ and common co-ions separation by the modified sorbent under batch and column conditions should be determined.

In previous study, pyromellitic dianhydride modified SCB with high adsorption capacity for heavy metal ions had been prepared (Yu et al., 2015b). In this study, Mg²⁺ was chosen as the model co-ions for it’s a common interfering ion, and effects of $C_{0}^{Mg} : C_{0}^{Pb}$ on Pb²⁺ and Mg²⁺ adsorption in the Mg/Pb binary system were carried out on the modified SCB under the batch and column conditions. For comparison, the adsorption kinetic and isotherm of Pb²⁺ in one component system were studied under the two conditions. Breakthrough curves of Pb²⁺ at different $C_{0}^{Mg} : C_{0}^{Pb}$ in the binary system were depicted. The amount of Pb²⁺ and Mg²⁺ adsorbed on the modified SCB fixed bed column was calculated through the elution curves by using ethylenediaminetetraacetic acid disodium salt as the eluent. The relationship between $C_{0}^{Mg} / C_{0}^{Pb}$ and the adsorption capacity ratio of the two metal ions $(q_{e}^{Mg} / q_{e}^{Pb})$ was discussed. The optimum condition for Pb²⁺ and Mg²⁺ separation under the batch and column conditions was concluded.

2 Materials and methods

2.1

2.1 Materials

Sugarcane bagasse was dried and ground to pass through a 100-mesh sieve, and then it was washed with ethanol and distilled water, and dried at 60 °C for 24 h before use. Pyromellitic dianhydride (PMDA) and ethylenediaminetetraacetic acid disodium salt were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Solutions of lead and magnesium were prepared using the corresponding metal nitrate.

2.2

2.2 Surface modification

The modified SCB was prepared according to Yu et al. (2015a). A 1.0 g of PMDA and 1.0 g of SCB were added into a round-bottomed flask containing 30 mL of N,N-dimethylacetamide. After stirring at 50 °C for 4 h, the modified SCB was obtained, and it was washed in order with NaOH (0.01 mol L⁻¹) and water. Then it was dried and stored in a desiccator before use.

2.3

2.3 Batch adsorption experiment

Adsorption performances including isotherm and kinetic experiment were carried out at 28 °C and 150 rpm on an orbital shaker. In the isotherm experiment, 0.005 g of the modified or unmodified SCB was added into a 20 mL of Pb²⁺ solution with initial concentration ranged from 0.05 to 1.21 mmol L⁻¹ at pH ranged from 5.0 to 5.15 adjusting by using HCl and NaOH solution. In the kinetic experiment, 0.010 g of the modified SCB was added into 80 mL of Pb²⁺ solution with the initial concentration of 1.00 mmol L⁻¹ (pH = 5.0). Effect of Mg²⁺ on the adsorption of Pb²⁺ was carried out in the binary system. In this system, initial concentration of Pb²⁺ was fixed at 0.12 mmol L⁻¹, and concentration of Mg²⁺ ranged from 0.001 to 37.6 mmol L⁻¹. The residual concentration of metal ions after adsorption was determined by atomic absorption spectrophotometer (AA6300, Shimadzu, Japan). The adsorption isotherms of Pb²⁺ and Mg²⁺ were fitted by Langmuir model (Eq. (1))

(1)

q_{e} = \frac{q_{m} K_{L} C_{e}}{1 + K_{L} C_{e}}

where q_e is the amount of dyes adsorbed per gram of sorbent at equilibrium (mmol g⁻¹), q_m is the maximum amount of adsorption (mmol g⁻¹), K_L is the Langmuir biosorption constant (L mmol⁻¹), and C_e is the equilibrium concentration of substrates in the solution (mmol L⁻¹).

2.4

2.4 Fixed bed column adsorption experiment

The fixed bed column adsorption experiment was conducted at room temperature in a glass column with an internal diameter of 1 cm and length of 20 cm. 1.0 g of the modified SCB was accurately weighed and soaked in 100 mL of distilled water for 30 min before being poured slowly into the column. After the adsorbent had settled, the column was pumped with distilled water for 30 min (bed height: 12.1 cm). In the dynamic adsorption experiment, the solution of metal ions was pumped using a peristaltic pump (YZ1515X, Longer pump), connected with Teflon tubes from the sample tank to the bottom of the column at the designated flow rate (up flow pattern). Flow rate of the sample in the experiment was fixed at 6.25 mL min⁻¹. Solution pH was in the range of 5.0–5.15. The concentration of Pb²⁺ used in the experiment was all fixed at 0.12 mmol L⁻¹. In the binary system, mixture solutions of Mg²⁺ and Pb²⁺ with mass concentration ratios of 1:1, 10:1, 20:1, 50:1 and 100:1 were pumped into the fixed bed column. Samples were collected at the exit of the column at different time intervals and analyzed for metal ion concentrations by using atomic absorption spectrophotometer. The amount of metal ions adsorbed on the fixed bed column at time t (q_t, mmol g⁻¹) could be calculated by the following equation (integrated by using the software origin 8.0):

(2)

q_{t} = \frac{ν \int_{t = 0}^{t = t} (C_{0} - C_{t}) dt}{m}

where ν (mL min⁻¹) is the volumetric flow rate. C_t (mmol L⁻¹) is the concentration of metal ion in the effluent at time t. C₀ (mmol L⁻¹) is the inlet concentration of metal ion, and m is the mass of the modified SCB used in the fixed bed column.

2.5

2.5 Fixed bed column desorption experiment

After exhaustion of the modified SCB fixed bed column, desorption experiment was carried out by pumping ethylenediaminetetraacetic acid disodium salt (EDTA-2Na, 0.02 mol L⁻¹) as eluent at flow rate of 6.25 mL min⁻¹. Concentrations of Mg²⁺ and Pb²⁺ in the eluate at time t ( $C_{t}^{'}$ , mmol L⁻¹) were determined. The amount of metal ions desorbed at time t ( $q_{t}^{'}$ , mmol g⁻¹) could be calculated by Eq. (3).

(3)

q_{t}^{'} = \frac{ν \int_{t = 0}^{t = t} C_{t}^{'} dt}{m}

3 Results and discussion

3.1

3.1 Batch adsorption experiment in one component system

Adsorption isotherms of Pb²⁺ showed that the adsorption capacity was increased obviously after modification. According to Langmuir model, the maximum amount of metal ions adsorbed on the unmodified and modified SCB was 0.18–0.74 mmol g⁻¹, respectively (data was not shown). The increased adsorption capacity was due to the introduction of the large amount of carboxyl groups, which provide the active sites for Pb²⁺ adsorption (Yu et al., 2013). Adsorption kinetic of Pb²⁺ on the modified SCB showed that the adsorption is a rapid process, and it could complete within 60 min (data was not shown). According to the pseudo-second order model, the adsorption rate was 1.04 g mmol⁻¹ min⁻¹.

3.2

3.2 Effects of the initial concentration ratio on Pb²⁺ and Mg²⁺ separation under the batch condition

In this experiment, the initial concentration of Pb²⁺ was fixed at 0.12 mmol L⁻¹, and concentration of Mg²⁺ ranged from 0.001 to 37.6 mmol L⁻¹. Inset in Fig. 1 shows the amount of Mg²⁺ and Pb²⁺ adsorbed on the modified SCB in the binary system. The adsorption amount of Mg²⁺ increased with the increase of $C_{0}^{Mg} : C_{0}^{Pb}$ , while that of Pb²⁺ decreased with it. The increased Mg²⁺ adsorption was due to its high initial concentration providing an important driving force to overcome the mass transfer resistance between the aqueous and solid phase (Aksu and Tezer, 2000). The decrease in Pb²⁺ adsorption was due to competitive inhibition effect. The inhibition of Mg²⁺ on the adsorption of Pb²⁺ had been reported by Spinti et al. (1995) and Chandra et al. (2003).

Linear relationship between initial mass concentration ratio and adsorption capacity ratio of the two metal ions under batch condition. Inset: amount of Mg2+ and Pb2+ adsorbed on the modified SCB at different initial concentration ratios of the two metal ions.

It was interesting to find that $q_{e}^{Mg} : q_{e}^{Pb}$ increased linearly with the increase of $C_{0}^{Mg} : C_{0}^{Pb}$ (shown in Fig. 1). Similar results had been observed in the previous study (Yu et al., 2015c, 2012). Since the adsorption capacity of the two metal ions was decided by their initial concentration ratio, the mass ratio of Pb²⁺ and Mg²⁺ adsorbed on the modified SCB was calculated from the values of $C_{0}^{Mg} : C_{0}^{Pb}$ (Eqs. (4) and (5)) and shown in Fig. 6.

(4)

{Mass ratio of Pb}^{2 +} = \frac{1}{1.0117 + 1.896 C_{0}^{Mg} / C_{0}^{Pb}}

(5)

{Mass ratio of Mg}^{2 +} = \frac{0.0117 + 1.896 C_{0}^{Mg} / C_{0}^{Pb}}{1.0117 + 1.896 C_{0}^{Mg} / C_{0}^{Pb}}

C_{0}^{Mg} : C_{0}^{Pb} < 0.022

, the mass ratio of Pb²⁺ was higher than 95%, while at

C_{0}^{Mg} : C_{0}^{Pb} > 10.01

, the mass ratio of Mg²⁺ was higher than 95%. That’s means Pb²⁺ could be adsorbed selectively from the mixture solution at

C_{0}^{Mg} : C_{0}^{Pb} < 0.022

, while Mg²⁺ could be adsorbed selectively at

C_{0}^{Mg} : C_{0}^{Pb} > 10.01

. The two metal ions could be separated by the modified SCB when

C_{0}^{Mg} : C_{0}^{Pb} < 0.022

C_{0}^{Mg} : C_{0}^{Pb} > 10.01

. At

0.022 < C_{0}^{Mg} : C_{0}^{Pb} < 10.01

, the two metal ions interfered each other seriously, and Mg²⁺ and Pb²⁺ could not be separated directly by modified SCB under the batch condition.

3.3

3.3 Adsorption of Pb²⁺ on the modified SCB fixed bed column in one component system

Fig. 2 shows the breakthrough curve of Pb²⁺ on the modified SCB fixed bed column. It was observed that the breakthrough curve showed an obvious “S” shape. Pb²⁺ was removed completely from the aqueous solution at the beginning due to the large amount of unoccupied active sites (carboxyl groups), and C_t/C₀ was much lower than 0.05 in this period. With the occupation of carboxyl groups, large amount of Pb²⁺ could not be adsorbed and begin to flow out of the column, and C_t/C₀ increased sharply to 1.0 till the column exhausted. The amount of Pb²⁺ adsorbed at time t (q_t) was calculated according to Eq. (1) and shown in Fig. 2. It was observed that the saturated capacity of the column was 0.78 mmol g⁻¹, which was close to that obtained in the batch experiment.

Breakthrough and adsorption kinetic curves of Pb2+ on the modified SCB fixed bed column. Inset: variation of pH during the dynamic adsorption process (amount of sorbent = 0.5 g, inlet concentration of Pb2+ = 0.12 mmol L−1, flow rate = 6.25 mL min−1).

Inset in Fig. 2 shows the variation of pH during this dynamic adsorption process. It was observed that pH dropped firstly to a platform at about 4.0 and then increased slowly to the initial pH. As the initial rate of adsorption was faster, the amount of binding metals onto the sorbent was larger. Hydrogen ions were released into solution through the reaction (R1), and the values of pH decreased rapidly at the initial process. Similar reports had been reported by Vilar et al. (2008) and Han et al. (2006).

(R1)

2 R – COOH + {Pb}^{2 +} \to 2 RCOO – Pb + 2 H^{+}

By using Eq. (6), the amount of H⁺ released was calculated to be 0.956 mmol g⁻¹, which demonstrated that 0.478 mmol g⁻¹ of active site was provided by –COOH and the other amount of 0.302 mmol g⁻¹ (total amount: 0.78 mmol g⁻¹) was provided by –COO⁻ and the other functional groups.

(6)

q_{t}^{H} = \frac{ν \int_{t = 0}^{t = t} (C_{0}^{H} - C_{t}^{H}) dt}{m}

3.4

3.4 Effects of the initial concentration ratio on Pb²⁺ adsorption on the modified SCB fixed bed column

The effect of the initial mass concentration ratio on the adsorption of Pb²⁺ in the binary system was conducted on the fixed bed column. Fig. 3a shows that breakthrough curves were dispersed and breakthrough occurred more slowly at low $C_{0}^{Mg} : C_{0}^{Pb}$ , while sharply breakthrough curves were obtained at high $C_{0}^{Mg} : C_{0}^{Pb}$ . The breakthrough time (C_t/C₀ = 0.05) of Pb²⁺ decreased obviously from 300 min to 210 and 5 min when $C_{0}^{Mg} : C_{0}^{Pb}$ increased form 1:1 to 10:1 and 100:1, and the amount of Pb²⁺ adsorbed decreased from 0.66 to 0.50 and 0.13 mmol g⁻¹ (shown in Fig. 3b).

Effects of the concentration ratios on the (a) breakthrough curve and (b) adsorption kinetic of Pb2+ on the modified SCB fixed bed column (amount of sorbent = 0.5 g, inlet concentration of Pb2+ = 0.12 mmol L−1, C 0 Mg : C 0 Pb = 1 : 1 , 10:1 and 100:1, flow rate = 6.25 mL min−1).

In order to describe the fixed-bed column behavior and scale it up for industrial applications, the breakthrough curves of Pb²⁺ were fitted by Bohart-Adams model (Eqs. (7)–(9)) (Baral et al., 2009; Aksu and Gonen, 2004; McKay and Bino, 1990 ).

(7)

\frac{C_{t}}{C_{0}} = \frac{e^{k_{AB} C_{0} t}}{e^{k_{AB} N_{0} Z / U} - 1 + e^{k_{AB} C_{0} t}} = \frac{1}{1 + (e^{k_{AB} N_{0} Z / U} - 1) \times e^{- k_{AB} C_{0} t}} = \frac{1}{1 + b \exp (- kt)}

(8)

b = e^{k_{AB} N_{0} Z / U} - 1

(9)

k = k_{AB} C_{0}

where k_AB (L mg⁻¹ min⁻¹) is the kinetic constant, N₀ (mg L⁻¹) is the maximum volumetric sorption capacity, U (cm min⁻¹) is the linear flow rate, and Z (cm) is the bed depth in the column. Calculated constants are listed in Table 1. It was observed that the model fitted the curves well. k_AB obtained in the single ion system was higher than that obtained in the binary system, demonstrating that the presence of Mg²⁺ inhibits the adsorption of Pb²⁺. Since all the breakthrough curves matched the Bohart-Adams model well, the attempt to find the relationship between q_t and time t was tried by using a modified Bohart-Adams model (Eq. (10)).

(10)

q_{t} = \frac{q_{m}}{1 + b^{'} \exp (- k^{'} t)}

where b′ and k′ (min⁻¹) are the adsorption constants, and q_m is the calculated adsorption capacity of the sorbent in the dynamic system. The estimated constants are listed in Table 1. High values of R² were obtained in all the systems. The calculated q_m at

C_{0}^{Mg} : C_{0}^{Pb} = 1 : 1

, 10:1 and 100:1 were 0.66, 0.49 and 0.13 mmol g⁻¹, which were very close to the experimental results: 0.66, 0.50 and 0.13 mmol g⁻¹, demonstrating that the adsorption kinetic curves could be predicted well by the modified Bohart-Adams model.

Table 1 Calculated parameters by using Bohart-Adams and modified Bohart-Adams models for the dynamic adsorption of Pb²⁺ on the modified SCB fixed bed column.

Constants	Concentration ratio $(C_{0}^{Mg} : C_{0}^{Pb})$
Constants	Single ion	1:1	10:1	100:1
Breakthrough curves (Bohart-Adams model)
R²	0.983	0.988	0.977	0.975
b	3.3 × 10⁶ ± 4.4 × 10⁶	2159.6 ± 1134.3	689.75 ± 445	6.99 ± 1.25
k (min⁻¹)	0.03 ± 0.003	0.019 ± 0.001	0.022 ± 0.002	0.022 ± 0.002

q_t curves (modified Bohart-Adams model)
R²	0.994	0.995	0.994	0.977
q_m (mmol g⁻¹)	0.79 ± 0.01	0.66 ± 0.004	0.49 ± 0.004	0.13 ± 0.001
b′	12.18 ± 1.17	11.08 ± 0.97	10.51 ± 1.03	7.68 ± 1.5
k′ (min⁻¹)	0.01 ± 0.0004	0.11 ± 0.0004	0.015 ± 0.0006	0.03 ± 0.003

3.5

3.5 Desorption of Pb²⁺ and Mg²⁺ from the saturated fixed bed column

In order to further verify the adsorption capacity of Pb²⁺ and determine the amount of Mg²⁺ adsorbed on the column, desorption of the saturated columns was conducted by using EDTA-2Na (0.02 mol L⁻¹) as the eluent. Desorption of metal ions by EDTA-2Na was due to its strong coordination ability with Pb²⁺ and Mg²⁺ (lg K_Pb-_EDTA = 18.04, lg K_Mg-_EDTA = 8.7). The adsorbed metal ions (through electrostatic attraction with the carboxyl groups) on the sorbent surface would coordinate with EDTA-2Na and be released from the sorbent surface into the eluent. Fig. 4a and b depicts the desorption curves of the two metal ions from the saturated column obtained at different $C_{0}^{Mg} : C_{0}^{Pb}$ . The completion of Pb²⁺ desorption was after 10 min, while that for Mg²⁺ was after 40 min. The fast desorption rate of Pb²⁺ may be due to its higher coordinating ability to EDTA. The desorption amount of the two metal ions (q_t′) at different $C_{0}^{Mg} : C_{0}^{Pb}$ was calculated according to Eq. (2) and shown in inset of Fig. 4a and b. The obtained desorption amounts of Pb²⁺ were very close to those obtained in the adsorption process, demonstrating the high desorption efficiency of EDTA-2Na. It was observed that when $C_{0}^{Mg} : C_{0}^{Pb}$ varied from 1:1 to 10:1 and 100:1, q_t′ of Pb²⁺ decreased from 0.60 to 0.49 and 0.12 mmol g⁻¹, while that of Mg²⁺ increased from 0.058, 0.26 and 0.41 mmol g⁻¹, respectively. The key role of the initial concentration on the adsorption of the two metal ions was further confirmed.

Desorption curves of (a) Pb2+ and (b) Mg2+ from the saturated column obtained at C 0 Mg : C 0 Pb = 1 : 1 , 1:10 and 1:100 (inset: desorption kinetic curves of Pb2+ and Mg2+), and desorption kinetic curves of Pb2+ and Mg2+ at (c) C 0 Mg : C 0 Pb = 20 : 1 and (d) C 0 Mg : C 0 Pb = 50 : 1 (inset: desorption curves of Pb2+ and Mg2+).

Since $q_{e}^{Mg} : q_{e}^{Pb}$ increased linearly with the increase of $C_{0}^{Mg} : C_{0}^{Pb}$ in the batch system, the relationship was also tested in the column system. Fig. 5 depicts the effects of $C_{0}^{Mg} : C_{0}^{Pb}$ on the values of $q_{e}^{Mg} : q_{e}^{Pb}$ . High values of R² demonstrated the equation fitted the data well. In order to further examine the model accuracy, comparison of theoretical predictions with experimental values of Pb²⁺ and Mg²⁺ sorption on the fixed bed column was carried out. Fig. 4c and d shows desorption curves and kinetics of the two metal ions from the saturated column obtained at $C_{0}^{Mg} : C_{0}^{Pb} = 20 : 1$ and 50:1. It was observed that the desorption amount of Pb²⁺ and Mg²⁺ at $C_{0}^{Mg} : C_{0}^{Pb} = 20 : 1$ was 0.21 and 0.22 mmol g⁻¹, while that at $C_{0}^{Mg} : C_{0}^{Pb} = 50 : 1$ was 0.14 and 0.25 mmol g⁻¹, respectively. The predicted and experimental values $q_{e}^{Mg} : q_{e}^{Pb}$ are shown in Fig. 5. Results showed that $q_{e}^{Mg} : q_{e}^{Pb}$ calculated were in excellent consistent with those obtained experimentally, demonstrating the feasibility of the linear equation under column condition. According to the linear equation, mass ratio of Pb²⁺ and Mg²⁺ was calculated by Eqs. (11) and (12) and is shown in Fig. 6. The mass ratio of Pb²⁺ adsorbed was higher than 95% when $C_{0}^{Mg} : C_{0}^{Pb} < 1.24$ , while that of Mg²⁺ was higher than 95% when $C_{0}^{Mg} : C_{0}^{Pb} > 585.5$ . Pb²⁺ and Mg²⁺ could be adsorbed selectively from the mixture solution at $C_{0}^{Mg} : C_{0}^{Pb} < 1.24$ and $C_{0}^{Mg} : C_{0}^{Pb} > 585.5$ , respectively. The two metal ions could not be separated by the column when $1.24 < C_{0}^{Mg} : C_{0}^{Pb} < 585.5$ .

(11)

{Mass ratio of Pb}^{2 +} = \frac{1}{1.011 + 0.034 C_{0}^{Mg} / C_{0}^{Pb}}

(12)

{Mass ratio of Mg}^{2 +} = \frac{0.011 + 0.034 C_{0}^{Mg} / C_{0}^{Pb}}{1.011 + 0.034 C_{0}^{Mg} / C_{0}^{Pb}}

Fig. 7 compares the mass ratio of metal ions adsorbed on the modified SCB under the batch and column conditions, respectively. It was observed that different

C_{0}^{Mg} : C_{0}^{Pb}

range for Pb²⁺ and Mg²⁺ separation (mass ratio >95%) was obtained, which may be due to the different experimental condition. In batch experiment, concentration of the metal ions decreased gradually with the completion of the adsorption, while inlet concentration was kept at a constant value in the whole process under the column experiment. However, the reason why different separation and enrichment effects obtained under the two conditions still needed to be investigated comprehensively. Either in batch or in column conditions, initial concentration ratios played a key role in the adsorption of the two metal ions. Pb²⁺ could be easily adsorbed selectively from the mixture solution at low

C_{0}^{Mg} : C_{0}^{Pb}

under the column condition due to the fact that the metal has the larger atomic weight (207.2) and the more electronegative (Pb: 2.33 and Mg: 1.31) and the smaller hydrated ionic radii (Pb: 0.401 nm and Mg: 0.428 nm). Mg²⁺ could be easily separated at high

C_{0}^{Mg} : C_{0}^{Pb}

values under batch condition due to the strong salt-effect. By considering the results obtained in the batch and dynamic conditions, it was preferred to separate Pb²⁺ and Mg²⁺ under column condition when

C_{0}^{Mg} : C_{0}^{Pb} < 1.24

(Pb²⁺ adsorbed selectively), and separate the two metal ions under batch condition when

C_{0}^{Mg} : C_{0}^{Pb} > 10.01

(Mg²⁺ adsorbed selectively). Pb²⁺ and Mg²⁺ could not be separated directly by the modified SCB in both conditions when

1.24 < C_{0}^{Mg} : C_{0}^{Pb} < 10.01

. These findings illustrated that separation experimental condition was decided by the initial concentration ratio of the two metal ions.

Linear relationship between initial mass concentration ratio and adsorption capacity ratio of the two metal ions under column condition.

Mass ratio of metal ions adsorbed on the modified SCB under the batch and column conditions.

Comparison of mass ratio of Pb2+ and Mg2+ (>95%) obtained under batch and column condition.

4 Conclusions

Batch adsorption experiment showed that adsorption of Pb²⁺ decreased while Mg²⁺ increased with the increase of $C_{0}^{Mg} / C_{0}^{Pb}$ . A good linear relationship between $C_{0}^{Mg} / C_{0}^{Pb}$ and $q_{e}^{Mg} / q_{e}^{Pb}$ was obtained. The slope and intercept of the fitted linear equation were 1.896 and 0.0117, respectively. Column experiments also illustrated that the initial concentration ratios played a key role in adsorption of the two metal ions. Breakthrough curves of Pb²⁺ at different $C_{0}^{Mg} : C_{0}^{Pb}$ in the binary system fitted Bohart-Adams model well and the adsorption kinetic curves of Pb²⁺ fitted the modified Bohart-Adams model well. The amount of Pb²⁺ and Mg²⁺ adsorbed on the modified SCB fixed bed column in the binary system was calculated through the elution curves. Also good linear relationship between $C_{0}^{Mg} / C_{0}^{Pb}$ and $q_{e}^{Mg} / q_{e}^{Pb}$ was obtained under the column condition. However, the slope and intercept of the fitted equation varied to 0.011 and 0.034, respectively. The different linear equations obtained under the different conditions illustrated that different mass ratio of Pb²⁺ and Mg²⁺ would be obtained at the same $C_{0}^{Mg} / C_{0}^{Pb}$ . According to the values of mass ratio, it was concluded that Pb²⁺ and Mg²⁺ could be separated by the modified SCB at $C_{0}^{Mg} : C_{0}^{Pb} < 0.022$ or $C_{0}^{Mg} : C_{0}^{Pb} > 10.01$ under batch condition, and the range changed to $C_{0}^{Mg} : C_{0}^{Pb} < 1.24$ or $C_{0}^{Mg} : C_{0}^{Pb} > 585.5$ under the column condition. Separation of Pb²⁺ and Mg²⁺ would be preferred to conduct at column condition when $C_{0}^{Mg} : C_{0}^{Pb} < 1.24$ (Pb²⁺ adsorbed selectively), while conducted at batch condition when $C_{0}^{Mg} : C_{0}^{Pb} > 10.01$ (Mg²⁺ adsorbed selectively).

Acknowledgments

The work is financially supported by National Natural Science Foundation of China (No. 51574182), the Key Project of Chinese Ministry of Education (No. 213024A) and the program for excellent young scientific and technological innovation team of Hubei Provincial Department of Education, China (No. T201506).

References

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Show Sections

[1] Anayurt R.A., Sari A., Tuzen M., . Equilibrium, thermodynamic and kinetic studies on biosorption of Pb(II) and Cd(II) from aqueous solution by macrofungus (Lactariusscrobiculatus) biomass. Chem. Eng. J.. 2009;151:255-261.
[Google Scholar]

[2] Aksu Z., Tezer S., . Equilibrium and kinetic modeling of adsorption of Remazol Black B by Rhizopus arrhizus in a batch system: effect of temperature. Process Biochem.. 2000;36:431-439.
[Google Scholar]

[3] Aksu Z., Gonen F., . Biosorption of phenol by immobilized activated sludge in a continuous packed bed: prediction of breakthrough curves. Process Biochem.. 2004;39:599-613.
[Google Scholar]

[4] Baral S.S., Das N., Ramulu T.S., Sahou S.K., Das S.N., Chaudhury G.R., . Removal of Cr(VI) by thermally activated weed Salvinia cucullata in a fixed-bed column. J. Hazard. Mater.. 2009;161:1427-1435.
[Google Scholar]

[5] Contreras C., De La Rosa G., Peralta-Videa J.R., Gardea-Torresdey J.L., . Lead adsorption by silica-immobilized humin under flow and batch conditions: assessment of flow rate and calcium and magnesium interference. J. Hazard. Mater. B. 2006;133:79-84.
[Google Scholar]

[6] Chandra K., Kamala C.T., Chary N.S., Anjaneyulu Y., . Removal of heavy metals using a plant biomass with reference to environmental control. Int. J. Miner. Process.. 2003;68:37-45.
[Google Scholar]

[7] Deng Y.H., Gao Z.Q., Liu B.Z., Hu X.B., Wei Z.B., Sun C., . Selective removal of lead from aqueous solutions by ethylenediamine modified attapulgite. Chem. Eng. J.. 2013;223:91-98.
[Google Scholar]

[8] Feng N., Guo X., Liang S., Zhu Y., Liu J., . Biosorption of heavy metals from aqueous solutions by chemically modified orange peel. J. Hazard. Mater.. 2011;185:49-54.
[Google Scholar]

[9] Ghaedi M., Asadpour E., Vafaie A., . Simultaneous preconcentration and determination of copper, nickel, cobalt, lead, and iron content using a surfactant-coated alumina. Bull. Chem. Soc. Jpn.. 2006;79:432-436.
[Google Scholar]

[10] Hossain M.A., Ngo H.H., Guo W.S., Nguyen T.V., . Palm oil fruit shells as biosorbent for copper removal from water and wastewater: experiments and sorption models. Bioresour. Technol.. 2012;113:97-101.
[Google Scholar]

[11] Han R., Zou W., Li H., Li Y., Shi J., . Copper(II) and lead(II) removal from aqueous solution in fixed-bed columns by manganese oxide coated zeolite. J. Hazard. Mater. B. 2006;137:934-942.
[Google Scholar]

[12] Isaac R.A., Gil L., Cooperman A.N., Hulme K., Eddy B., Ruiz M., Jacobson K., Larson C., Pancorbo O.C., . Corrosion in drinking water distribution systems: a major contributor of copper and lead to wastewaters and effluents. Environ. Sci. Technol.. 1997;31:3198-3203.
[Google Scholar]

[13] Lasheen M.R., Ammar N.S., Ibrahim H.S., . Adsorption/desorption of Cd(II), Cu(II) and Pb(II) using chemically modified orange peel: equilibrium and kinetic studies. Solid State Sci.. 2012;14:202-210.
[Google Scholar]

[14] Motsa M.M., Mamba B.B., Thwala J.M., Msagati T.A.M., . Preparation, characterization, and application of polypropylene–clinoptilolite composites for the selective adsorption of lead from aqueous media. J. Colloid Interf. Sci.. 2011;359:210-219.
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[15] Madala S., Reddy M.V.N., Vudagandla S., Abburi K., . Modified leaf biomass for Pb(II) removal from aqueous solution: application of response surface methodology. Ecol. Eng.. 2015;83:218-226.
[Google Scholar]

[16] McKay G., Bino M.J., . Fixed bed adsorption for the removal of pollutants from water. Environ. Pollut.. 1990;66:33-53.
[Google Scholar]

[17] Nguyen T.A.H., Ngo H.H., Guo W.S., Zhang J., Liang S., Yue Q.Y., Li Q., Nguyen T.V., . Applicability of agricultural waste and by-products for adsorptive removal of heavy metals from wastewater. Bioresour. Technol.. 2013;148:574-585.
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[18] Reddy D.H.K., Harinath Y., Seshaiah K., Reddy A.V.R., . Biosorption of lead from aqueous solutions using chemically modified Moringa oleifera tree leaves. Chem. Eng. J.. 2010;162:626-634.
[Google Scholar]

[19] Sawalha M.F., Peralta-Videa J.R., Sanchez-Salcido B., Gardea-Torresdey J.L., . Sorption of hazardous metals from single and multi-element solutions by saltbush biomass in batch and continuous mode: interference of calcium and magnesium in batch mode. J. Environ. Manage.. 2009;90:1213-1218.
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[20] Spinti M., Zhuang H., Trujillo E., . Evaluation of immobilized biomassbeds for removing heavy metals from wastewaters. Water Environ. Res.. 1995;67:943-952.
[Google Scholar]

[21] Vilar V.J.P., Loureiro J.M., Botelho C.M.S., Boaventura R.A.R., . Continuous biosorption of Pb/Cu and Pb/Cd in fixed-bed column using algae Gelidium and granulated agar extraction algal waste. J. Hazard. Mater.. 2008;154:1173-1182.
[Google Scholar]

[22] Wan Ngah W.S., Hanafiah M.A.K.M., . Removal of heavy metal ions from wastewater by chemically modified plant wastes as adsorbents: a review. Bioresour. Technol.. 2008;99:3935-3948.
[Google Scholar]

[23] Xu H.Y., Yang L., Yang P., Liu Y., Peng M.S., . Kinetic research on the sorption of aqueous lead by synthetic carbonate hydroxyapatite. J. Environ. Manage.. 2008;86:319-328.
[Google Scholar]

[24] Yuvaraja G., Krishnaiah N., Subbaiah M.V., Krishnaiah A., . Biosorption of Pb(II) from aqueous solution by Solanum melongena leaf powder as a low-cost biosorbent prepared from agricultural waste. Colloid. Surf. B: Biointerf.. 2014;114:75-81.
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[25] Yu J.X., Wang L.Y., Chi R.A., Zhang Y.F., Xu Z.G., Guo J., . Adsorption of Pb²⁺, Cd²⁺, Cu²⁺ and Zn²⁺ from aqueous solution by modified sugarcane bagasse. Res. Chem. Intermed.. 2015;41(3):1525-1541.
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[Google Scholar]

[27] Yu J.X., Wang L.Y., Chi R.A., Guo J., Zhang Y.F., Xu Z.G., Xiao C.Q., . Competitive adsorption of Pb²⁺ and Cd²⁺ on magnetic modified sugarcane bagasse prepared by two simple steps. Appl. Surf. Sci.. 2013;268:163-170.
[Google Scholar]

[28] Yu J.X., Cai X.L., Feng L.Y., Xiong W.L., Zhu J., Xu Y.L., Zhang Y.F., Chi R.A., . Synergistic and competitive adsorption of cationic and anionic dyes on polymer modified yeast prepared at room temperature. J. Taiwan Inst. Chem. Eng. 2015
[CrossRef] [Google Scholar]

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[Google Scholar]

[30] Zheng Y.Q., Deng S.B., Niu L., Xu F.J., Chai M.Y., Yu G., . Functionalized cotton via surface-initiated atom transfer radical polymerization for enhanced sorption of Cu(II) and Pb(II) J. Hazard. Mater.. 2011;192:1401-1408.
[Google Scholar]

Separation of Pb²⁺ from Mg²⁺ by modified sugarcane bagasse under batch and column conditions: Effect of initial concentration ratio