Abstract
In this work, chitosan was modified by the esterification with L-alanine in the presence of H2SO4 as a catalyst to increase the number of amino groups with the aim of increasing the adsorption efficiency. Chitosan (CS) and chitosan-O-alanine (CS-Aln) were characterized and investigated by elemental analysis, Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD). The results obtained from elemental analysis and IR indicated the presence of sulfuric acid after neutralization as a crosslinker between CS-Aln chains. Also CS-Aln is more amorphous than CS due to the ionic bonds of crosslinker. The removal of three heavy metals (Mn2+, Pb2+ and Al3+) and total organic carbon (TOC) from wastewater by CS and CS-Aln in the batch mode has been studied at different adsorbent dosages, temperatures and contact times. The maximum metal ions removal efficiency using CS achieved 99.6%, 99.1% and 98.9%, respectively, while by using CS-Aln 95.3%, 99.3% and 98.9% were achieved. However, the maximum adsorption capacity of TOC by CS achieved 50 mg/g and 89 mg/g by CS-Aln. The total maximum adsorption capacity of CS-Aln is higher than CS.
Introduction
The current shortages of potable water around the world and the looming water scarcity, especially in the developing countries, is the greatest problem facing humanity in the 21st century and possibly beyond [1], [2]. According to the UN, 1.2 billion people do not have access to clean drinking water, and half of the world’s population lacks the technology for water purification such as India, countries in South-East Asia, Northern Africa and North-East Africa. Nearly 60% of illness around the world are due to contaminated water and lack of sewage treatment [3].
About 70% of the Earth’s surface is covered with water and of this 97.5% of the water is present in seas and oceans and 1.73% is present in the form of glaciers. Only 0.77% of fresh water is available on the Earth’s surface for human and agricultural use. However, the availability and application of water is uneven around the world; in water-stressed countries such as India, China as well as in countries in South-East Asia, Northern Africa and North-East Africa, the situation is especially acute [4]. Clean water is very essential to human health and is also used in pharmaceutical and foods industries. It is also a critical component in a variety of key industries including electronics, pharmaceuticals and the food industry [5]. At present, the world is facing formidable challenges in meeting rising demands for clean water and the available supplies of fresh water are decreasing due to (i) extended droughts, (ii) population growth, (iii) more stringent health based regulations [6], and competing demands from a variety of users [5]. Scientific and technological advances relating to water treatment has been tasked over the last several decades with designing methods for producing safe drinking water throughout the world, and also in the search for sources of fresh water [7]. Wastewater treatment is a source of fresh water. Wastewater contains many kinds of heavy metals, such as Mn, Pb, Al, etc., which are considered persistent, bioaccumulative and harmful substances. Due to the serious threat to human health and ecological systems, these contaminants must be removed from wastewaters before discharge into the environment. Several methods are currently used for the removal of heavy metal ions from aqueous wastes (chemical precipitation, ion exchange, electrochemical treatment, membrane technologies, adsorption on activated carbon, etc.). Each of these methods has its own merits and demerits. But the cost-effective technologies for the removal of heavy metals from wastewater have been directed towards adsorption using natural biosorbents [8]. Adsorption using low-cost natural adsorbents such as agricultural waste [9], clay materials and seafood processing waste are some of the few promising alternatives which can be applied to wastewaters with low concentrations of heavy metal ions [10].
Chitosan, a good biosorbent has been used extensively for wastewater treatment due to its high hydrophilicity, nontoxicity, abundance in nature, biocompatibility and antibacterial properties. Chitosan has the highest sorption capacity for many metal ions due to the several amino and hydroxyl groups in its structure. Also, when the deacetylation degree of chitosan increases, the ability for metal ion removal increases owing to the free –NH2 groups [11]. In this work, chitosan has been chemically modified to improve the sorption capacity as in the literature [12] by esterification with L-alanine as in Scheme 1 to increase the free amino groups in this chain, and consequently increase heavy metal ion removal from waste water.
Reaction mechanism of esterification of chitosan with L-Alanine.
Experimental
Preparation of chitosan (CS)
Chitosan was prepared from shrimp shells with 110 kDa of molecular weight and 85% of degree of deacetylation in our lab as reported in the literature [13], [14].
Preparation of chitosan-alanine (CS-Aln)
CS-Aln was prepared according to reference [15]. Eighteen millimolar of chitosan flakes (3 g calculated as glucosamine units) was suspended in 100 mL of distilled water. To this solution 36 mmol of L-alanine (4.8 g) was added with the ratio (1 chitosan: 2 L-alanine). Then, 5 mL of H2SO4 (2M) was added dropwise at room temperature. The mixture stirred at 80°C for 4 h using a condenser to prevent evaporization and subsequently cooled at room temperature. Then pH was adjusted to 7 by neutralization with NaHCO3. The desired compound was precipitated in acetone, filtered and washed with acetone to remove the unreacted acid. Finally, the precipitants were soxhlet-extracted with acetone for 2 days, then oven-dried overnight at 60°C, giving the titled compounds.
Specification of wastewater sample
The characterization of collected wastewater samples at 25°C and pH 7.44 were 15.98 NTU of turbidity value, conductivity 523 μS/cm, TDS 420 mg/L, 24.2 mg/L of Al3+, 5.4 mg/L of Mn2+, 2.33 mg/L of Pb2+ and 40 mg/L of TOC, in addition, a few concentrations of different metal ions did not exceed 2.5 ppm.
Adsorption procedure
The heavy metals and TOC adsorption processes were carried out in batch mode by mixing various amounts of CS and CS-Aln (0.05–0.2 g) with 200 mL of sewer water in jars at different times (30, 60, 90 and 120 min) and pH 7. At the end of the adsorption experiments, the solution samples were centrifuged at 4000 rpm for 10 min, and then the metal concentration was determined using a Perkin–Elmer 560 Atomic Adsorption Spectrophotometer and the TOC was measured using an Analytik jena multi N/c 2100s analyzer. In order to determine the effect of temperatures they were established at 22, 30 and 40°C.
Data processing
The percent removal (%) and the equilibrium adsorption capacity (mg g−1) of metal ions in the solution are calculated using the eq. (1) and eq. (2), respectively.
where η(%) is the percent removal, qe (mg g−1) is the amount of adsorbed metal ions per unit biopolymer mass at equilibrium, and C0 (mg L−1) is the initial concentration of the metal ions solution. Ce (mg L−1), M (g) and V (L) are the equilibrium concentration of the metal ions solution, the mass of adsorbent and the volume of metal ions solution, respectively.
Results and discussion
Chemical characterization
Elemental analysis
The elemental analysis of CS and CS-Aln are listed in the Table 1. It is clear from this table that the containment of the CS-Aln on the sulfur contents indicates the strong interaction between amino groups in CS-Aln chains and sulfuric acid that has not been removed by neutralization with sodium bicarbonate.
Elemental analysis of CS and CS-Aln.
| Sample | C (%) | H (%) | N (%) | S (%) |
|---|---|---|---|---|
| CS | 35.7 | 7.8 | 6 | – |
| CS-Aln | 32.7 | 10.96 | 6.87 | 3.4 |
Determination of degree of esterification (DS)
The DS could be determining by elemental analysis as has published in the literature [16], [17], [18], [19]. Inukai et al. determined the DS according the following:
where (C/N)m is the C/N of the modified CS, (C/N)0 is the C/N of the original CS, and a and n are the number of nitrogen and carbon introduced after CS modification, but in case of this work which contains sulfur atoms we can modify the Eq.(4) to become:
where (C/N+S)m is the carbon percent divided on summation of nitrogen and sulfur percent of the modified CS, (C/N)0 is the C/N of CS, a is the number of nitrogen and sulfur, and n is the number carbon that are introduced after CS modification, respectively.
The DS values obtained was 0.14 for CS-Aln.
Determination of substituted degree of sulfuric acid (SDS)
Similar to calculating the DS, the substituted degree of SDS can be calculated as in Eq. (3):
also modify Eq. (5) by the removal the right side because the ratio of sulfur in chitosan originally is zero to become:
The DSS values obtained was 0.17 for CS-Aln.
FT-IR
Figure 1 shows the IR data of the CS virgin. The broad and strong band ranging from 3200 to 3500 cm−1 is attributed to the presence of OH and NH2 groups, which is consistent with the peaks at 1074 and 1152 cm−1 assigned to alcoholic C6–O and C–N–H stretching vibrations. The peaks at 2920 and 2875 cm−1 are assigned to asymmetric and symmetric CH2 groups. The peaks at 1590 and 1430 cm−1 are characteristic of NH2 and CH2 deformation, respectively. The small peak at 1235 cm−1 is attributed to the C–O–C stretching vibration.
FT-IR of CS and CS-Aln.
For Fourier transform infrared (FTIR) spectrum of CS-Aln, the appearance of new absorption peak at 1678 cm−1 with a shoulder at 1703 cm−1 is attributed to the characteristic peaks of NH2 and esters group of alanine, respectively. Also the disappearance of the characteristic peak of C6–OH of CS at 1027 cm−1, indicates that the esterification reaction occurred between alanine and C6–OH of CS [20]. On the other hand, the characteristic peak of –NH2 of CS at 1597 cm−1 has shifted to 1545 and 1510 cm−1, indicating that the NH2 groups of CS-Aln has protonated to NH3+ by the remaining sulfuric acid. The strong absorption peak at 1470 cm−1 corresponds to the CH3 symmetric stretch of alanine [21]. The absorption peaks at 923 and 841 cm−1 are assigned to C–C–N symmetric stretching vibrations. Also, the disappearing of OH stretching peak of L-alanine at 3066 cm−1 indicates the esterification has taken palace.
X-ray diffraction (XRD) study
The crystalline properties of the CS and CS-Aln could be identified by X-ray diffraction (XRD) patterns as can be seen in Fig. 2. It was observed that the diffractogram of commercial CS consisted of one major crystalline peak located at around 20.2°. The peak registered near 20° is reported to be the indication of the relatively regular crystal lattice of the chitosan, which indicated the partial crystallinity [22]. Compared with CS, the diffractogram of CS-Aln exhibited some changes in its peaks, diffraction angles, peak intensity, and peak width. The appearance peaks around 11°, 16°, 18° and 24° related to L-alanine [23], [24], while in the peak at around 20.2°, its intensity decreased and its width increased. It is well known that peak intensity and width in the XRD pattern had close correlations with crystallite size [25]. The new and broader peaks implied that there was a low crystalline phase in CS-Aln matrix. The change of these characteristic peaks in the XRD pattern of the samples demonstrated that CS transited to a low crystalline state through the esterification process. This was attributed to the ionic bonds between sulfuric acid and amino groups which destroy the intra- and inter-molecular hydrogen bonds of CS lattice chains and leads to a decrease in the crystallinity [22].
XRD pattern of CS and CS-Aln.
Adsorption results
Effect of adsorbent dosage on metal removal
Figure 3 illustrates the effect of CS and CS-Ala dosages on the adsorption of Mn2+, Pb2+ and Al3+ from wastewater. It can be seen that by increasing the adsorbent dosage, the removal efficiencies steadily increase for all three metals. The increase in adsorption with adsorbent dosage can be attributed to the higher adsorbent surface and availability of more adsorption sites for metal ions. However, with the increasing amount of adsorbent, the adsorption capacity would decrease. This may be attributed to overlapping or aggregation of adsorption sites that results in the increase of diffusion path length and decreases the total adsorbent surface area which is available to the metal ions [6].
Effect of adsorbent dosage on heavy metal adsorption by CS and CS-Aln (pH 7, 30 min and T=22°C).
Effect of adsorption time on metal removal
Figure 4 shows the removal of metals (Mn2+, Pb2+ and Al3+) as a function of contact time from wastewater by CS and CS-Aln. It is observed from this figure that the divalent metals (Mn, Pb) adsorption are at a maximum after 30 min, but trivalent metal (Al) takes 120 min to reach to the adsorption maximum, this result could be attributed to the faster adsorption of divalent metals than trivalent ones [26]. Beyond the equilibrium time, adsorption is found to be nearly constant, which can be attributed to unfilled surface sites are available at the initial stage of adsorption, and once the equilibrium is attained, the remaining unfilled sites are difficult to fill, which is probably caused by the repellent forces between the heavy metal molecules on the biosorbents surface and bulk solution.
Effect of contact time on heavy metal adsorption by CS and CS-Aln (CAdsorbent=1 g/L, pH 7, and T=22°C).
Effect of temperature on metal removal
The studies relating to the effect of temperature on the adsorption process was carried out at three different temperatures (22, 30 and 40°C) with a metal concentration of 5.4 mg/L of Mn2+, 2.33 mg/L of Pb2+ and 24.2 mg/L of Al3+, respectively, as shown in Fig. 5. Increasing the temperature of solutions would increase the mobility of metal ions and also produces a swelling effect within the internal structure of CS and CS-Aln. For CS, the adsorption of divalent metals (Mn and Pb) decreased with increasing temperatures, due to the physisorption which occurred. But in case of CS-Aln, the adsorption of these metals decrease at 30°C and increase again at 40°C may be attributed to the chemical reaction which takes place between the functional groups of the adsorbate and adsorbent molecules at 40°C.
Effect of temperatures on heavy metal adsorption by CS and CS-Aln (pH 7, 30 min and CAdsorbent=1 g/L).
However, the adsorption of trivalent metal (Al) on both CS and CS-Aln decrease with increasing temperature due to the physisorption [27].
Isotherm studies
The bio-sorption isotherm demonstrates how metal ions can be distributed between the liquid and solid phases at various equilibrium concentrations, and how efficient the interaction between the biosorbent and adsorbate is [28]. Several isotherm models such as Langmuir and Freundlich, have been used to describe the experimental data to optimize the design of the biosorption system for the removal of metal ions from solutions.
In the Langmuir model, a uniform energy of biosorption and a single layer of adsorbed solute at a constant temperature are assumed. The monolayer biosorption in Langmuir model is the most frequently employed isotherm and is given as follows [29], [30]:
where qe, Ce, Qmax, and KL are the amount of solute adsorbed at equilibrium (mg/g), the concentration of adsorbate at equilibrium (mg/L), maximum biosorption capacity (mg/g), and the Langmuir constant (L/mg), respectively. To study the applicability of the Langmuir isotherm model for the biosorption of metals onto chitosan and CS-Aln, the linear plot of Ce/qe against Ce was plotted and the values of Qmax, KL, and R21 (correlation coefficient) are shown in Table 2.
Linearized isotherm coefficients of metal removal by CS and CS-Aln.
| Metals | Langmuir | ||
|---|---|---|---|
| Qmax | KL | R12 | |
| CS | |||
| Mn | 14.07 | 11.65 | 0.9125 |
| Pb | 8.86 | 15.03 | 0.9944 |
| Al | 24.45 | 7.26 | 1 |
| CS-Aln | |||
| Mn | 11.61 | 2.46 | 0.9565 |
| Pb | 8.85 | 3.96 | 0.9608 |
| Al | 23.15 | 216.85 | 1 |
The results in Table 2 showed that the correlation coefficients, R2, for the Langmuir isotherm model >0.9, which revealed that the number of biosorption sites on CS and CS-Aln were limited and metal molecules formed a monomolecular layer on the biosorbent at saturation point, and therefore followed the Langmuir equation.
On the other hand, the difference between the rates of the Qmax of three kinds of metals is shown in Table 2, this may attributed to the difference between ionic radii of these three metals as follows: Al (1.25 Å)>Mn (1.4 Å)>Pb (1.8 Å). It is clear from the results that with increasing the atomic radii decrease the Qmax value. It is a reasonable interpretation for this is that the metal ion with smaller size [like Al(III) and Mn(II) compared with Pb(II)] can approach the active sites of CS and CS-Aln more, enhancing the binding and decreasing the disorder of the system more [31], [32].
As can be seen from Table 2, the maximum adsorption capacities of CS for all metals are slightly higher than CS-Aln, this is attributed to presence of sulfuric acid that blocked some amino groups in CS-Aln and consequently decreased the adsorption efficiency.
Removal of TOC
The maximum adsorption capacity of TOC was also calculated from the Langmuir model in Eq. (8). It has achieved 51 mg/g by CS and 89 mg/g by CS-Aln, due to the presence of polarity in the CS-Aln structure that resulted from ionic bonds between sulfuric acid and amino groups which can attract and bind with polar groups in TOC [33].
Calculation of total maximum adsorption capacity
The total maximum adsorption capacity of CS and CS-Aln are shown in Fig. 6, and was calculated by Doskočil and Pekař [34]. As can be seen from this figure, the total maximum adsorption capacity of CS-Aln to metals and TOC together are higher than CS, due to its chemical structure that possess free amino groups from an alanine ester to support adsorption of metals, and sulfuric groups in chitosan units to support adsorption of TOC.
Adsorption capacity (Qmax) and total adsorption capacity (total Qmax) by CS and CS-Aln for metals and TOC (CAdsorbent=1 g/L, pH 7, at T=22°C).
Conclusion
The esterification of chitosan with L-alanine was carried out in presence of sulfuric acid as a catalyst, but after neutralization of the solution by sodium bicarbonate some of sulfuric acid stayed between the chains. This presence of sulfuric acid partially decreased the crystallinity properties of CS-Aln, due to formation of ionic bonds which disrupt the inter- and intra-molecular hydrogen bonds between the chains, and also blocked the amino groups leading to a slight decrease of metal ions removal from wastewater than chitosan. On the other hand, these sulfuric groups increased the polarity of CS-Aln as a result of the formation of ionic bonds leading to increasing TOC removal.
Article note:
A collection of invited papers based on presentations at the 12th Conference of the European Chitin Society (12th EUCHIS)/13th International Conference on Chitin and Chitosan (13th ICCC), Münster, Germany, 30 August–2 September 2015.
Acknowledgments
This work has been supported by Egyptian Petroleum Research Institute (EPRI).
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