Abstract

This paper presents a new sorbent, agar-agar (AA), impregnated on porous activated carbon (AC) – and its Pb(II) sorption properties. The influence of impregnation ratio (AA/AC) on the Pb(II) ion sorption properties is studied in order to optimize this parameter. The developed AC-AA shows substantial capability to sorb Pb(II) ions from aqueous solutions and 75% represents the optimal impregnation ratio. The AC-AA sorbent with impregnation ratio of 75% was characterized by a liquid displacement method, point of zero charge pH (pHPZC), scanning electron microscopy and Fourier transform infrared spectroscopy. The effect of parameters such as sorbent dosage, pH, agitation time and initial Pb(II) concentration on Pb(II) removal were examined. In addition, sorption kinetics and sorption isotherms were determined. The maximum uptake of Pb(II) was about 242 mg/g at 25 °C, pH 5 and initial Pb(II) concentration of 100 mg/L. The kinetic data were fitted to the models of pseudo-first-order and pseudo-second-order, and the experimental results follow closely the pseudo-second-order model. The results also reveal that the experimental equilibrium is very close to those predicted by the Freundlich model. The developed AC-AA exhibits high Pb(II) sorption capacity, offering possibilities for future practical use.

INTRODUCTION

The problem of environmental pollution is still relevant because many industrial activities continue to generate various pollutants. Today, people introduce various notorious toxic heavy metals, dyes, organics and pharmaceuticals into the ecosystem in many ways such as in household products, municipal sewage, industrial wastewaters, mineral weathering, and underground toxic waste disposal into streams, lakes, rivers, and seas, thus making water bodies the sinks (Jiao et al. 2017). Heavy metals are still being used in various industries due to their technological importance. Yet, imperfect treatment of waste products from these industries will pose other issues to human health and the environment (Febrianto et al. 2009,). Among the heavy metals, lead is one of the most toxic elements, even at low concentrations. It affects the central nervous system, kidneys, liver, and gastrointestinal system, and it may directly or indirectly cause diseases such as anemia, encephalopathy, hepatitis, and the nephritic syndrome (Wang et al. 2010). The permissible level for lead in drinking water is 0.05 mg/L according to the US Environmental Protection Agency.

Several methods are used for the elimination of heavy metals such as ultrafiltration (Trivunac & Stevanovic 2006), coagulation–flocculation (Assaad et al. 2007), electrochemical treatment (Hunsom et al. 2005), ion exchange (Dąbrowski et al. 2004), membrane processes (Qdais & Moussab 2004), reverse osmosis (Ujang & Anderson 1996), and adsorption (Boudrahem et al. 2011a; Naushad et al. 2016; Alqadami et al. 2017b; Naushad et al. 2017). For low concentrations of metal ions in wastewater, the sorption process is recommended for their removal (Huang et al. 2016). The process of sorption implies the presence of a ‘sorbent’ solid that binds molecules by physical attractive forces, ion exchange, and chemical binding. It is advisable that the sorbent is available in large quantities, easily regenerable and cheap (Hashem 2007).

Activated carbons (granular or powdered) are the most widely used sorbents because of their excellent sorption capability for inorganic pollutants (Flores-Cano et al. 2016). The properties of activated carbons depend on the activation process and the nature of the source materials. Moreover, in both physical and chemical activation processes, knowledge of different variables is very important in developing the porosity of the carbons (Muthanna 2016). The high sorption capacities of activated carbons are related to properties such as surface area, pore volume, and porosity (Park et al. 2016a, 2016b). Particularly, the development of micropores and mesopores is of great importance because it allows the porous carbons to adsorb large amounts and various types of chemical compounds from gas or liquid streams.

Agar-agar (C12H18O9) is a porous galactose (galactan) polymer contained in the cell wall of certain red algae: the Gelidiaceae (Gelidium and Pterocladia) and Gracilariaceae (Gracilaria) (Rocha et al. 2014; Shankar & Rhim 2017) families. Agar-agar purified of all its mineral elements, called agarose, can be used according to its purity level in various applications in molecular biology (Asgari et al. 2014). In recent years, various types of biopolymer-based sorbents such as α-Fe2O3 impregnated chitosan beads (Liu et al. 2011), magnetic nanoparticles impregnated chitosan beads (Wang et al. 2014), graphene oxide/chitosan composite fibers (Li et al. 2014), activated carbon/calcium-alginate beads (Jung et al. 2016), calcium alginate/bentonite/activated carbon composite beads (Benhouria et al. 2015), and agar-agar/graphene composite (Chen et al. 2017) have been used as a sorbent material for the removal of heavy metals and dyes from wastewater.

A material that is a good adsorbent for one adsorbate may not be a good adsorbent for another. Different adsorbents must be tested because of their different surface properties to determine the optimal adsorbent conditions for the removal of heavy metals by adsorption. An agar-agar (AA) and activated carbon (AC) composite is developed to be used as an adsorbent for the removal of Pb2+ ions in aqueous solution. The choice of AA is due to its low cost, and the impregnation of AC with this biopolymer is being tested in order to try to improve the efficiency of the process with respect to the removal of Pb2+ ions and to reduce the cost of the adsorption process. The sorbents were characterized by Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), liquid displacement method and pH of the point of zero charge (pHPZC).

Batch sorption experiments were performed to evaluate the influence of sorbent dose, pH, contact time and initial Pb(II) concentration. The Langmuir and Freundlich isotherm models were applied to the experimental data. Kinetic studies were performed using pseudo-first-order and pseudo-second-order models.

MATERIALS AND METHODS

Sorbate and analytical measurements

All the compounds used to prepare reagent solutions were of analytic reagent grade. The stock solution of Pb(II) (1,000 mg/L) was prepared by dissolving a weighed quantity of lead nitrate (98.5% purity) in distilled water. Pb(II) solutions of concentration ranging from 10 to 100 mg/L were prepared by diluting the stock solution. Before mixing with the AC, the pH of each solution was adjusted to the required value by adding 0.1 M NaOH (97% purity) or 0.1 M HNO3 (52.5% purity). A Shimadzu AA6500 atomic sorption spectrophotometer equipped with a Zeeman atomizer and an SSC-300 autosampler was used to determine the residual concentration of the Pb(II) ions in the solution. All the instrumental conditions were optimized for maximum sensitivity as indicated by the manufacturer.

Sorbent preparation

The adsorbent used in this study was a commercial AC obtained from Panreac Quimica SA (E-08110 Montcada i Reixac, Spain). These physical properties are summarized in Table 1. Agar-agar of 100% purity was obtained from Merck Germany.

Table 1

Physical and chemical characteristics of AC and AC-AA 75%

ParameterACAC-AA 75%
Real density (g/cm31.04 1.38 
Apparent density (g/cm30.20 0.31 
Porous volume (cm3/g) 0.80 0.72 
Porosity (%) 80.8 77.5 
pHPZC 6.2 5.5 
Particle size (μm) ≤63 ≤63 
ParameterACAC-AA 75%
Real density (g/cm31.04 1.38 
Apparent density (g/cm30.20 0.31 
Porous volume (cm3/g) 0.80 0.72 
Porosity (%) 80.8 77.5 
pHPZC 6.2 5.5 
Particle size (μm) ≤63 ≤63 

The procedure to obtain an AC impregnated with AA (AC-AA) is described in the supplementary material (available with the online version of this paper). The impregnating agent was dissolved in 250 mL of hot water (85 °C) and mixed with 10 g of AC. The mixture was kept for 2 hours at 85 °C in order to facilitate the hydration of the AC and the swelling of the interior channels of sorbent structure, allowing a better access of the AA into the interior of the particles. The temperature was raised to 110 °C and the mixture was kept for 24 hours at this temperature to cause complete evaporation of the water, which will force the incorporation of AA into the interior of the particles. Impregnated composites with different impregnation ratios, 0%, 25%, 50%, 75% and 100%, were prepared. After each experiment, the composite was thoroughly washed with deionized water until a constant pH, and dried at 60 °C for 24 hours.

Batch sorption procedure

Batch experiments of sorption were performed in a batch reactor (1 L) placed in a temperature-controlled shaker (RCS LAUDA), at 25 ± 1 °C. A known mass of sorbent 0.1 to 0.3 g was left in contact with 250 mL of each solution (10 to 100 mg/L) and kept under an optimal agitation of 400 rpm for 1 hour. The initial pH value of the solution was adjusted with nitric acid (0.1 mol/L) or sodium hydroxide (0.1 mol/L). The pH value was chosen so that metallic species are present in their divalent form (2–6). Small-volume liquid samples were withdrawn at different time intervals. Samples were filtered to remove sorbent particles. Sorption capacity was calculated using the following equation:
formula
(1)
where qe is the amount of solute sorbed (mg/g); C0 is the initial metal ion concentration (mg/L); Ce is the equilibrium metal ion concentration (mg/L); V is the volume of the aqueous solution (L) and m is the weight of sorbent (g) used.

Characterization of the sorbent

Morphology of sorbent

The morphologies of the sorbents were analyzed by SEM (FEI QUANTA 250).

Liquid displacement method

A liquid displacement method was used to measure the density, porosity, and porous volume of this sorbent (Narbat et al. 2006). So, its apparent density (bulk density) was calculated by filling a calibrated cylinder with a given sorbent weight (mads) and tapping the cylinder until a minimum volume is recorded. This recorded volume represents the apparent volume (Vapp) of our sorbent. The apparent density of sorbent ρapp (g/cm3) was evaluated using the following equation (Narbat et al. 2006):
formula
(2)
However, its real density was established by filling a pycnometer with a definite mass of the sorbent (mads). Then, the pycnometer thus filled was adjusted to its volume (Vpyc) by adding a volume of a displacement liquid, which was methanol (99% purity), and was weighed (). The real density (g/cm3) was calculated using the following equations (Narbat et al. 2006):
formula
(3)
where: ; = 0.791 g/cm3; mT = mads + mmeth;

Vmeth, mmeth and are volume, weight and density of methanol, respectively.

Knowing the density of solvent, porous volume TPV (cm3/g) and porosity ɛ of the adsorbent were easily calculated using the following equations (Narbat et al. 2006):
formula
(4)
formula
(5)

Point of zero charge pH determination

The pHPZC of the sorbent sample was determined using the method reported by Boudrahem et al. (2011b). Aliquots of 50 mL of 0.01 M NaCl solution were prepared in different flasks. Their pH values (pHinitial) were adjusted to between 2 and 12 by adding 0.01 M solution of HCl or NaOH. A quantity of 0.15 g of sorbent was added to each flask and the whole was maintained under agitation for 48 hours. When the pH value remained constant, the final pH (pHfinal) was measured using a Hanna 211R pH meter. The pHPZC value is the point where the curve of pHfinal versus pHinitial crosses the line, that is, pHinitial = pHfinal.

Infrared spectroscopy

The nature of surface groups of sorbent was examined by IR analysis. FTIR spectra of adsorbent samples were obtained using the potassium bromide technique on an FTIR spectrometer (Shimadzu FTIR-8300) over the wavenumber range of 4,000 to 400 cm−1. Adsorbent was mixed with potassium bromide at a ratio of roughly 1/1000.

RESULTS AND DISCUSSION

Effect of the impregnating ratio on the sorption of Pb(II) ions

The efficiency of the modified AC has been tested by subjecting it to the sorption of Pb(II) ions under the following conditions: pH 6, sorbent dose 0.1 g/250 mL, agitation speed 400 rpm, initial concentration of Pb(II) 50 mg/L and 25 °C. The results are reported in Figure 1.

Figure 1

Effect of impregnation ratio on sorbed amount of Pb(II) by different sorbents. Conditions: pH 6, agitation speed 400 rpm, initial concentration of Pb(II) 50 mg/L and 25 °C.

Figure 1

Effect of impregnation ratio on sorbed amount of Pb(II) by different sorbents. Conditions: pH 6, agitation speed 400 rpm, initial concentration of Pb(II) 50 mg/L and 25 °C.

The results show that the impregnation of the AC with the solution of AA increases its effectiveness of elimination of the Pb(II) ions in aqueous solution. The amount of Pb(II) removal increases from 18 to 120 mg/g when the impregnation ratio increases from 0% to 75%. This behavior is due to the fact that surface functional groups involved in the sorption of Pb(II) ions increase with increasing impregnation ratio. The abundance of surface oxygenated functional groups and the relatively well-developed pore structure of AC-AA 75% are responsible for the high sorption capacity of the adsorbent. With the increase in AA concentration above 75%, there is a decrease in the sorption of Pb2+ ions probably due to a significant decrease in AC porosity with the use of large quantities of AA. So 75% is the optimal impregnation ratio. All sorption experiments were then conducted with AC-AA 75%.

Characterization

A scanning electron microscope, was used to observe the morphology and physical state of the surface of the CA and AC-AA 75% before and after adsorption of Pb(II). It is clear from these micrographs that before adsorption, the surface of the CA exhibits a large number of pores (Figure 2(a)). After impregnation of AC with AA (Figure 2(c)), the morphology of the surface has been completely modified. It can be seen that particles of AC are completely covered by a uniform layer of AA. The SEM micrographs of AC and AC-AA 75% after adsorption (Figure 2(b) and 2(d)), display practically the same morphology and adsorption of Pb(II) did not change significantly the adsorbent porosity.

Figure 2

SEM micrographs of: AC before Pb(II) adsorption (a), AC after Pb(II) adsorption (b), AC–AA 75% before Pb(II) adsorption (c), AC–AA 75% after Pb(II) adsorption (d).

Figure 2

SEM micrographs of: AC before Pb(II) adsorption (a), AC after Pb(II) adsorption (b), AC–AA 75% before Pb(II) adsorption (c), AC–AA 75% after Pb(II) adsorption (d).

The FTIR spectra of the AA, AC before and after adsorption, and the AC-AA 75% before and after adsorption are shown in the Figure 3. It can be seen that AC and AC-AA 75% before adsorption and AA display practically the same functional groups. The principal chemical groups of these adsorbents are presented in Table 2.

Table 2

Principal chemical groups of the AA, AC and AC-AA 75% before sorption of Pb2+

Functional groupsWavenumber assignments (cm−1)
–OH stretching vibrations 3,430 
–CH3 symmetrical valence vibration 2,350 
C=O stretching vibrations in the ketone, aldehyde, lactone, and carbonyl groups and the aromatic ring 1,700–1,500 
–CO stretching vibration in –COH 1,056 
Functional groupsWavenumber assignments (cm−1)
–OH stretching vibrations 3,430 
–CH3 symmetrical valence vibration 2,350 
C=O stretching vibrations in the ketone, aldehyde, lactone, and carbonyl groups and the aromatic ring 1,700–1,500 
–CO stretching vibration in –COH 1,056 
Figure 3

FTIR spectra of: AA (a), AC–AA 75% before sorption (b), AC before sorption (c), AC after sorption (d) and AC–AA 75% after sorption (e).

Figure 3

FTIR spectra of: AA (a), AC–AA 75% before sorption (b), AC before sorption (c), AC after sorption (d) and AC–AA 75% after sorption (e).

The intensity of the bands is relatively weaker in the case of AC-AA 75% compared to AA, which may be attributed to the interaction between the functional groups of AA and AC involved in binding of –COH, –OH, and –CO– groups during the impregnation process. We also note (Figure 4) that the intensity of the 3,430, 2,350, 1,700–1,500 and 1,056 cm−1 bands decreases after adsorption of the Pb2+ ions.

Figure 4

Sorption of Pb(II) onto AC-AA 75% at different: (a) sorbent dose, (b) initial pH, (c) contact time and initial Pb(II) concentration.

Figure 4

Sorption of Pb(II) onto AC-AA 75% at different: (a) sorbent dose, (b) initial pH, (c) contact time and initial Pb(II) concentration.

Effect of sorbent dose

The dose of sorbents was varied from 0.1 to 0.3 g/250 mL keeping all other parameters constant, viz. pH (6), initial Pb(II) concentration (50 mg/L) and contact time (60 min). Figure 4(a) shows the percentage of Pb(II) removal as a function of sorbent dose.

It can be seen that by increasing the adsorbent dose, the percentage of Pb(II) elimination increases with increasing sorbent dose until 0.15 g/250 mL. This can be attributed to the increased surface area and availability of more sorption sites resulting from the increase of the sorbent dose (Li et al. 2003; Acharya et al. 2009).

Increasing the sorbent dose to above 0.15 g/250 mL, the percentage of Pb(II) removal decreases. This result is due to the decrease in the total sorption surface area available for Pb2+ ions due to aggregation or overlapping of active sites (Naushad 2014).

Effect of pH

The pH of the solution has a significant impact on the uptake of Pb(II) ions, since it determines the surface charge of the sorbent, the degree of ionization, and the speciation of the sorbate. In order to establish the effect of pH on the sorption of Pb(II) ions, batch equilibrium studies at different pH values were carried out in the range of 2–8 (Figure 4(b)). We notice that the uptake is low at very acidic pH. However, with the increase in pH, a significant improvement in sorption is recorded. The maximum amount of Pb(II) adsorbed on AC-AA 75%, which is 125 mg/g, is obtained at pH 5.

At lower pH, sorbed amount of Pb(II) ions decreased because the surface area of the sorbent was more protonated and a competitive sorption occurred between H+ protons and positively charged Pb(II) at the surface sites (Boudrahem et al. 2009; Rafatullah et al. 2009). Therefore, H+ ions react with the anionic functional groups on the surface of AC-AA 75% and results in a reduction of the number of binding sites available for the sorption of Pb(II) ions. In an alkaline medium, Pb(II) tends to hydrolyze (PbOH)+, undergo precipitation (Pb(OH)2) and form soluble hydroxyl complexes (PbOH3), (Pb3OH4)2+, (Pb2OH)3+, etc. (Boudrahem et al. 2009).

Effect of contact time and initial concentration of Pb(II)

The rate of pollutant removal is of great significance for the development of adsorbents. In order to determine the equilibrium time for maximum absorption and to identify the kinetic model describing the sorption of Pb(II) ions on AC-AA 75%, the experiments were performed over a period of 60 min (Figure 4(c)). The sorption efficiency of Pb(II) gradually increases with increasing contact time and then reaches a plateau.

An increase of initial Pb(II) concentration leads to an increase in the sorption capacity of AC-AA 75%. Equilibrium uptake increases with the increasing initial metal ions concentration in the range of the concentrations investigated. This is a result of the increase in the driving force of the concentration gradient with an increase in the metal ions initial concentration. The majority of Pb(II) is removed within the first 10 min. According to the results, the duration of the experiment is set at 60 minutes for the rest of the tests to ensure that the equilibrium is reached. Similar results have also been reported for the removal of Pb(II) ions (Wang et al. 2010; Hayeeye et al. 2017). The sorbed amounts are found to be 23 and 242 mg/g for, respectively, initial concentrations of 10 and 100 mg/L.

Sorption isotherm (non-linear method)

Several models have been used in the literature to describe the experimental data of sorption isotherms and Langmuir and Freundlich models are the most frequently tested models. In the present work both models were used.

The Langmuir model (1918) suggested a theory to describe the monolayer coverage of sorbate over a homogeneous sorbent surface. Once a sorbate molecule occupies a site, no further sorption can take place at that site. The Langmuir isotherm model is given by the following equation (Boudrahem et al. 2015; Alqadami et al. 2017a):
formula
(6)
where is the amount of solute sorbed (mg/g), is the maximum sorption capacity (mg/g), b is the Langmuir constant and Ce is the equilibrium concentration of the solute in solution (mg/L).
The Freundlich isotherm model is given by the following equation (Boudrahem et al. 2017):
formula
(7)
where and n are Freundlich constants corresponding to the sorption capacity and sorption intensity, respectively.

The constants derived by fitting the two models are presented in Table 3. The correlation coefficients obtained from the Freundlich model being the highest (R2 > 0.96), this model is the one that best describes the adsorption of Pb(II) ions on sorbent.

Table 3

Langmuir and Freundlich constants for sorption of Pb(II) on AC-AA 75%

Langmuir 
 Maximum sorption capacity: qm 242.05 
 Langmuir constant: b 0.17 
 Correlation coefficient: R2 0.47 
Freundlich 
 Freundlich constant: KF 27.50 
 Sorption intensity: 1/n 0.76 
 Correlation coefficient: R2 0.96 
Langmuir 
 Maximum sorption capacity: qm 242.05 
 Langmuir constant: b 0.17 
 Correlation coefficient: R2 0.47 
Freundlich 
 Freundlich constant: KF 27.50 
 Sorption intensity: 1/n 0.76 
 Correlation coefficient: R2 0.96 

In order to check the validity of these models, it is interesting to recalculate the sorbed amount using the calculated parameters determined using the non-linear forms. The simulated curves determined using Freundlich and Langmuir isotherms are given in Figure 5.

Figure 5

Sorption isotherms for Pb(II) onto AC-AA 75%. Conditions: pH 5, agitation speed 400 rpm and 25 °C.

Figure 5

Sorption isotherms for Pb(II) onto AC-AA 75%. Conditions: pH 5, agitation speed 400 rpm and 25 °C.

The Freundlich isotherm generates a better fit of equilibrium data than the Langmuir isotherm.

The adsorption capacity of AC and AC-AA 75% were 18 mg/g and 242 mg/g, respectively.

The maximum amount of Pb(II) adsorbed by AC-AA 75% has been compared to those reported in the literature for other adsorbents (Table 4). The values are reported in the form of monolayer sorption capacity. It should be noted that the values and comparison reported for Pb(II) removal have only a relative meaning because of different testing conditions and type of precursors. The results obtained in this study show a good adsorption of Pb(II) ions onto AC-AA 75% compared to other materials. AC-AA 75% is an effective sorbent for Pb(II) ions from wastewater.

Table 4

Adsorption capacities of adsorbents for removal of Pb(II) ions

AdsorbentAdsorption capacity (mg/g)Optimum conditions
References
pHTime (min)T (°C)
AC 18 60 25 Present study 
AC-AA 75% 242 10 25 Present study 
SDS-AZS 18.38 40 25 Naushad (2014)  
MWCNTs/ThO2 25 5.5 50 45 Mittal et al. (2016)  
TIV 18.8 50 Naushad et al. (2015a)  
EDTA-Zr(IV) iodate composite cation exchange 26.04 50 45 Naushad et al. (2015b)  
PSTM nanocomposite cation exchanger 44.64 50 40 Bushra et al. (2015)  
Ash 588.235 40 Ghasemi et al. (2014a)  
nFe-A 833.33 40 Ghasemi et al. (2014a)  
Fig 80.65 50 60 Ghasemi et al. (2014b)  
Olive stones 115 5.8 15 25 Boudrahem et al. (2018b)  
AdsorbentAdsorption capacity (mg/g)Optimum conditions
References
pHTime (min)T (°C)
AC 18 60 25 Present study 
AC-AA 75% 242 10 25 Present study 
SDS-AZS 18.38 40 25 Naushad (2014)  
MWCNTs/ThO2 25 5.5 50 45 Mittal et al. (2016)  
TIV 18.8 50 Naushad et al. (2015a)  
EDTA-Zr(IV) iodate composite cation exchange 26.04 50 45 Naushad et al. (2015b)  
PSTM nanocomposite cation exchanger 44.64 50 40 Bushra et al. (2015)  
Ash 588.235 40 Ghasemi et al. (2014a)  
nFe-A 833.33 40 Ghasemi et al. (2014a)  
Fig 80.65 50 60 Ghasemi et al. (2014b)  
Olive stones 115 5.8 15 25 Boudrahem et al. (2018b)  

SDS-AZS, sodium dodecyl sulfate acrylamide Zr(IV) selenite; MWCNTs, multi-walled carbon nanotubes; TIV, iodovanadate cation exchanger; EDTA, ethylenediaminetetraacetic acid; PSTM, polyaniline Sn(IV) tungstomolybdate; nFe-A, Fe nanoparticles-loaded ash.

Sorption kinetic models

Kinetics is one of the key aspects used in the evaluation of sorption as a unit process. In this study, the kinetics of the Pb(II) sorption onto AC-AA 75% from the solution was investigated by two well-known kinetic models, namely, pseudo-first-order and pseudo-second-order. The non-linear forms of these two models are expressed by the following equations.

Pseudo-first-order (Boudrahem et al. 2018a)
formula
(8)
Pseudo-second-order (Boudrahem et al. 2018b)
formula
(9)
where qe1 and qe2 are the amount of Pb(II) sorbed at equilibrium (mg/g), qt is the amount of Pb(II) sorbed at time t (mg/g), k1 is the pseudo-first-order equilibrium rate constant (min−1) and k2 is the pseudo-second-order equilibrium rate constant (g mg−1 min−1). The constants of the pseudo-first-order and pseudo-second-order models and the correlation coefficients are given in Table 5. The results show that among these two models, the pseudo second-order kinetic model has the best correlation coefficient (R20.999) and the quantity of Pb(II) adsorbed at equilibrium calculated by the model (qe2) is the one corresponding to the value determined by the experiment (qe,exp).
Table 5

Pseudo-first-order and pseudo-second-order kinetic model constants of AC-AA 75% at different concentrations

Pseudo-first order kinetic model
C0 (mg/L)k1 (min−1)qe1 (mg/g)qe,exp (mg/g)R2
10 0.076 5.22 23 0.768 
30 0.113 19.16 73.1 0.901 
50 0.1 28.08 110 0.867 
70 0.085 33.35 161 0.779 
100 0.092 61.87 242 0.821 
Pseudo-second-order kinetic model
C0 (mg/L)k2 (g mg−1 min−1)qe2 (mg/g)qe.exp (mg/g)R2
10 0.051 23.25 23 0.999 
30 0.021 76.92 73.1 
50 0.013 111.11 110 
70 0.009 166.66 161 
100 0.005 250 242 0.999 
Pseudo-first order kinetic model
C0 (mg/L)k1 (min−1)qe1 (mg/g)qe,exp (mg/g)R2
10 0.076 5.22 23 0.768 
30 0.113 19.16 73.1 0.901 
50 0.1 28.08 110 0.867 
70 0.085 33.35 161 0.779 
100 0.092 61.87 242 0.821 
Pseudo-second-order kinetic model
C0 (mg/L)k2 (g mg−1 min−1)qe2 (mg/g)qe.exp (mg/g)R2
10 0.051 23.25 23 0.999 
30 0.021 76.92 73.1 
50 0.013 111.11 110 
70 0.009 166.66 161 
100 0.005 250 242 0.999 

Therefore, the pseudo-second-order kinetic model provided the best description of the Pb(II) adsorption.

Sorption mechanisms

The sorption mechanism involves chemical bonding and ion exchange. The sorption mechanisms can be explained by the presence of several interactions, such as complexation, ion exchange due to a surface ionization, and hydrogen bonds. In general, solution pH not only influences the properties of the adsorbent surface, but also affects the adsorbate speciation of different compounds in solution. The change in pH affects the adsorptive process through dissociation of functional groups on the solid surface active sites. Therefore, this leads to a change in reaction kinetics and equilibrium characteristics of the adsorption process.

At the end of each experiment, the pH of the equilibrium solution was systematically measured, which allowed us to observe a decrease in this parameter compared to the original pH. This reduction in pH is due to the dissociation of the functional groups located at the AC-AA 75%. Thus, some of the adsorbed Pb(II) ions can be attributed to an ionic exchange, and the other part of the adsorbed Pb(II) ions could be due to the various oxygen atoms of the AA functional groups, which have pairs of free electrons that can covalently bind to the Pb(II) ions. The same results are reported by Ghasemi et al. (2014a, 2014b).

The FTIR spectra of the AC-AA 75% have clearly elucidated the nature of the functional groups (–OH and –C=O) located on the surface and which are involved in the sorption mechanism for the removal of Pb(II) ions.

CONCLUSIONS

In this study, AC-AA 75% is employed as novel sorption material for the removal of Pb(II) ions from aqueous systems. FTIR spectra of the AC-AA 75% have clearly elucidated the nature of the surface functional groups (–OH and –C=O) which are involved in the sorption mechanism for the removal of Pb(II) ions. The impregnation ratio (AA/AC), the initial Pb(II) concentration, the absorbent dose and the pH play an important role in the adsorption process. The optimal values determined are respectively, 75%, 100 mg/L, 0.15 g/250 mL and pH 5. The sorption equilibrium is achieved in 10 min of contact time. The experimental data of Pb(II) ions sorption onto AC-AA 75% were well fitted by the Freundlich isotherm model. The sorption kinetics followed the pseudo-second-order model. These results show that AC-AA 75% is a potential sorbent for removing Pb(II) ions from aqueous solution.

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Supplementary data