Removal of pentachlorophenol pesticide from aqueous solutions using modified chitosan

Aparajita Shankar; Manasa Kongot; Vipin Kumar Saini; Amit Kumar

doi:10.1016/j.arabjc.2018.01.016

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Original article

13 (

); 1821-1830

doi:

10.1016/j.arabjc.2018.01.016

Removal of pentachlorophenol pesticide from aqueous solutions using modified chitosan

Aparajita Shankar^{^a,1}, Manasa Kongot^{^b,1}, Vipin Kumar Saini^{^a,⁎}, Amit Kumar^{^b,⁎}

a School of Environment and Natural Resources, Doon University, Dehradun, Uttarakhand 248012, India

b Centre for Nano and Material Sciences, Jain University, Jain Global Campus, Kanakapura, Ramanagara, Karnataka 562 112, India

⁎Corresponding authors. vksaini.senr@doonuniversity.ac.in (Vipin Kumar Saini), amit.kumar@jainuniversity.ac.in (Amit Kumar)

Received: 2017-10-17, Accepted: 2018-1-25, Published: 2018-01-01

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

1 Authors have contributed equally.

Abstract

Abstract

For the development of a bioadsorbent with enhanced adsorption properties to remove pentachlorophenol (PCP) pesticide from aqueous solutions, a modified chitosan material CHTA (1) was successfully obtained by crosslinking of chitosan (CHT) (I) with 2-hydroxy-1-naphthaldehyde. Functionalized chitosan CHTA was further modified by grafting with CuCl₂ to prepare CHTAC (2). The obtained products were characterized with the necessary chemical and spectroscopic techniques and were studied as an adsorbent for PCP removal from aqueous solutions. The adsorption capacity of 1 and 2 improved by 94% and 76% respectively as compared to CHT. Kinetics and thermodynamic studies were carried out to understand equilibrium data of uptake capacity and kinetic rate of adsorption. Desorption properties and reusability of prepared materials were evaluated up to three consecutive adsorption-desorption cycles with different desorbing agents.

Keywords

Pesticide removal by adsorption

Chitosan

Endocrine disruptor compound

Wastewater treatment

Pentachlorophenol removal

Surface modification

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1 Introduction

Pentachlorophenol (PCP) is one of the common pesticide and fungicide used in wood protection industry which contaminates soil and groundwater, especially around wood preserving facilities. Poisoning from PCP causes leukemia, peripheral neuropathy and several other neurological disorder (Ali et al., 2014). It is a widespread environmental pollutant and causes endocrine disruption (Orton et al., 2009). Therefore, PCP removal from aqueous solution is an objective of environmental and scientific concern. Adsorption based techniques are known for their efficiency, selectivity, and safety in water treatment applications. The cost and efficiency of these techniques are regulated by the adsorbents employed in the process (Ali and Gupta, 2007). It is well known that use of low-cost natural materials and by-products can improve the cost-effectiveness and sustainability of these techniques. But quite often, the adsorption properties of these low-cost materials are not comparable to the commercially available adsorbents (Gupta et al., 2009). Improvement or modifications in adsorption properties of these materials are therefore a requisite before perfoming their application studies.

Chitosan (a natural amino polysaccharide) has been used as a bioadsorbent for water treatment, biopesticide, antibacterial agent or as drug carrier (Ravi Kumar, 2000). Adsorption properties of chitosan are originated from a large number of primary and secondary hydroxyl (—OH) and primary amine (—NH₂) groups that act as active adsorption sites. As a result, its surface acts as a complexing medium for ionic species during adsorption process that distinguishes it from several other adsorbents. These properties, along with its biocompatibility and cost effectiveness makes it a suitable candidate for adsorptive removal of organic and inorganic pollutants such as dyes, phenols, pharmaceuticals, pesticides, fungicides and heavy metal ions (Wan Ngah et al., 2011) from wastewater.

The amino group on chitosan is particularly an advantageous functional group that can be modified chemically to obtain chitosan derivatives with desired adsorption characteristics. Several researchers worked largely on modification of chelating functionalities, mainly by increasing amino groups. For example, histidine, an amino acid with amino and imidazole side chain donor groups was added to improve chelation properties of chitosan (Beppu et al., 2004). Graftings of chitosan with cationic surfactant hexadecylamine (Vakili et al., 2016), amino silane and 3-aminopropyl triethoxysilane (APTES) (Vakili et al., 2015) were reported to improve the adsorption of reactive blue 4 dye. Uptake of cadmium from aqueous solution was improved by grafting with anionic surfactant sodium dodecyl sulfate (SDS) (Pal and Pal, 2017). Tannic acid, a polyphenol was used to modify chitosan, where improvement in adsorption of Al and Pb ions was reported (Badawi et al., 2017).

In an alternative way, chelating properties of chitosan have improved by modification of amino groups into Schiff base with the help of active carbonyl groups (Gavalyan, 2016). Chitosan modified by thiourea and glutaraldehyde was reported for adsorption of Hg ions (Donia et al., 2008). Effect of similar modification for adsorptive separation of UO₂⁺², was investigated by Elwakeel and Atia (Elwakeel and Atia, 2014). Schiff's base derivative of chitosan was prepared with 2-amino benzaldehyde, through an ion-imprinting technique, for selective removal of Pd which exhibited maximum adsorption capacity of 275 mg/g (Monier et al., 2016). Coupling of chitosan with 1-vinyl 2-pyrrolidone and 4-amino acetanilide was also studied for the adsorption of anionic reactive dye (methyl orange) (El-Sayed et al., 2016).

The adsorption potential of chitosan Schiff base derivatives is limited to the metal ions but not fully explored for emerging organic contaminants (Kyzas and Bikiaris, 2015). It is expected that if aryl groups are introduced to chitosan surface through Schiff base reaction, it may improve its adsorption properties towards organic contaminants. The modification of chitosan into Schiff base containing aromatic substituent groups followed by complexation with a transition metal ion can be the best approach for adsorption of organic contaminants. This grafting and cross-linking technique improves the stability and adsorption properties in extreme media conditions (Wu et al., 2016). Furthermore, the addition of metal sites (electron deficient) to adsorbent surface can improve its interaction with electron rich molecules, aromatic compounds for instance. In this context, copper is one such metal which can be used for grafting into the chitosan material. A chitosan based organic-inorganic hybrid modified with copper has been previously used for the oxidative degradation and adsorption of Reactive Blue 5 dye (Amreen et al., 2017). Hence it will be noteworthy to investigate if insertion of copper into chitosan material will enhance the PCP adsorption due to coordinatively unsaturated copper metallic site if any (Song et al., 2016).

In this study, the surface of chitosan flakes (CHT) was functionalized by two consecutive methods which involved cross-linking with 2-hydroxy-1-naphthaldehyde to obtain CHTA followed by further grafting with CuCl₂ to synthesize CHTAC. The prepared materials were used for the adsorptive removal of PCP from water samples in a batch process. The effects of modification were henceforth explored in terms of adsorption capacity and selectivity.

2 Experimental

2.1

2.1 Materials and methods

All the chemicals and solvents were of AR grade, which were used without further purification. Chitosan (CHT) flakes with >75% degree of deacetylation and 2-hydroxy-1-naphthaldehyde (98% pure) were obtained from Himedia Laboratories Pvt. Ltd, India. Copper chloride (97% pure), acetic acid (>98% pure), ethanol (99.9% pure), methanol (99% pure), and diethyl ether (99.5% pure) were obtained from S.D. Fine-Chem. Ltd., India. Pentachlorophenol (PCP) (97% pure) was obtained from Sigma-Aldrich. Chitosan and its modified derivatives (CHTA, and CHTAC) were characterized by different analytical techniques to analyze the change in their structural and surface properties. The specific surface area and pore volume were obtained from Microtrac BEL Belsorp Max, Japan. The Field Emission Scanning Electron Microscopy (FESEM) and Energy Dispersive X-ray Spectroscopic (EDAX) analysis were acquired from JEOL-JSM7100F. The ATIR spectra of materials were obtained from Bruker Alpha Single reflection ATR module equipped with ZnSe crystal. The point of zero charge pH_zpc of materials was measured by following a method, given elsewhere (Babić et al., 1999). Briefly, for each material, 11 Erlenmeyer flasks, each with 20 mL of Millipore water were taken and their pH was maintained from 2 to 12 by using sulfuric acid solution (5 N) and sodium hydroxide solution (1 N), whenever required. Consequently, 0.02 g of material was added into each of these 11 flasks and were stirred for overnight. The final pH of the solutions was recorded using Metrohm pH meter.

2.2

2.2 Chemical modification of chitosan (CHT)

2.2.1

2.2.1 Modification of CHT with 2-hydroxy-1-naphthaldehyde to prepare CHTA

The amine groups on CHT flakes were functionalized with 2-hydroxy-1-naphthaldehyde following a procedure (Scheme 1), reported elsewhere (Jiao et al., 2011; Sobahi et al., 2014). Briefly, about 7 g of CHT flakes were dissolved in 20 mL of 1% acetic acid solution in a round bottom flask at room temperature. Hot methanolic solution of 2-hydroxy-1-naphthaldehyde (8 g in 50 mL), was slowly added to the CHT solution. This reaction mixture was refluxed overnight with continuous stirring to obtain the modified chitosan material CHTA. The excess of aldehyde were removed by repeated wash with methanol. The purified material so obtained was then dried in an oven at 80 °C for 5 h and stored in a desiccator until further use.

Modification of chitosan (CHT) using 2-hydroxy-1-naphthaldehyde and CuCl2.

2.2.2

2.2.2 Modification of CHTA with copper chloride to prepare CHTAC

For incorporation of copper into the functionalized CHTA (Scheme 1), a clear solution of copper chloride (5 g in 20 mL ethanol) was added to CHTA (5 g) and the mixture was refluxed for 3 h. A dark green colored material CHTAC obtained, was filtered-off and washed repeatedly with ethanol and diethyl ether. The purified sample was then dried in an oven at 80 ^°C for 5 h and stored in a desiccator until further use.

2.3

2.3 Adsorption and desorption experiments

Most of the adsorption experiments were carried out with all the three chitosan samples in batch mode. In these experiments, the effects of different parameters like contact time, pH, temperature, and initial concentration on amount of adsorption was observed. For these studies, a stock solution of PCP, 1000 mg/L was prepared by using Millipore water and filtered through Durapore HVLP 0.45 µm 47 mm. For adsorption experiments, a definite amount of adsorbent was added to the standard PCP solution and then subjected to given adsorption conditions like pH, temperature, and duration. In the end of the experiment, the solution was filtered out with a syringe filter (Cole-Parmer Nylon 25 mm, 0.45 µm) and the concentration of remaining PCP in liquid phase was analyzed with UV spectrophotometer (Evolution 201, Thermo Scientific) at wavelength of 220 nm. Following mass balance equation (Eq. (1)) was used to calculate the amount of PCP adsorbed on the material:

(1)

q_{e} = \frac{C_{0} - C_{e}}{W} \times V

where, C₀ and C_e are the initial and equilibrium PCP concentrations (mg/L) respectively, V is the volume of solution (L), w is the mass of the adsorbent (g), and q_e is the amount of PCP adsorbed (mg/g).

The effect of contact time on adsorption was observed with all the three samples. In this experiment, 0.2 g of sample was added to 200 mL of 150 mg/L PCP solution in a conical flask of 250 mL capacity. The solution was stirred with a magnetic stirrer in a water bath at 293 K. While stirring, the aliquots of 3 mL were sampled periodically, up to 3 h, and analyzed for amount of adsorption. The effect of solution pH on the adsorption of PCP by all the three samples was investigated with pH gradient experiments. In this experiment, the pH of the PCP solution (100 mg/L) was varied from 2 to 12, using 0.1 N NaOH or 1.0 N HCl solutions. The adsorption experiments were carried out at 293 K, with adsorbent dose of 0.02 g in 20 mL of solution.

Similarly, the adsorption isotherms of all the three chitosan samples were obtained at 293, 303, and 313 K with typical batch scale equilibrium-adsorption experiments. Here, the batch contains 10 glass vials each of 50 mL capacity, fitted with PTFE caps. Each vial was filled with 20 mL of PCP solution with concentration ranging from 10 to 130 mg/L, and 0.02 g of adsorbent sample. The complete batch was stirred in a water bath, for next 3 h, on a multipoint magnetic stirrer, (Velp Scientifica). In the end, the equilibrium concentration of PCP in liquid phase was analyzed, as detailed above.

Desorption study was carried out to test the reusability of prepared materials for further adsorption cycles. The study involved two parts, in first a suitable desorbing agent was identified to achieve maximum desorption, and in second the adsorption-desorption cycles were carried out with selected desorbing agent. The objective of the second experiment is to follow the loss of adsorption capacity with successive cycles. For desorption test, five different desorbing agents namely HCl, HNO₃, NaOH, NaCl, and H₃PO₄, each with 0.1 N were used. In these experiments after adsorption, 0.05 g of exhausted adsorbent was added to 50 mL of each of the desorbing agent solution. The solution was then stirred for 3 h at room temperature and the change in concentration of supernatant was observed to calculate the amount of desorption (mg/g) as per the following equation,

(2)

q_{ed} = \frac{C_{ed}}{m} \times V_{d}

Here, the q_ed is the amount of desorption from a unit mass of chitosan materials in mg/g, and C_ed is the concentration of supernatant in mg/L after desorption, m is the mass of exhausted adsorbent in g and V_d is the volume of desorbing agent in L. The selected desorbing agent is used for reusability test, where 0.1 g of the chitosan sample is stirred with 50 mL of 100 mg/L, PCP solution. After adsorption, the exhausted sample is dried and desorbed with 50 mL of previously identified desorbing solution. After desorption, the sample was washed and dried again, before further adsorption. These adsorption-desorption cycles continued until the end of the third cycle. The change in adsorption and desorption amount after each cycle was calculated in terms of%, where, initial adsorption amount was taken as 100%. For instance, the% desorption was calculated as,

(3)

% Desorption = \frac{q_{ed}}{q_{ea}} \times 100

Here, the q_ea and q_ed are the amounts of adsorption and desorption with a unit mass of materials in mg/g. During this study, all the adsorption and desorption experiments were carried out, at least in duplicate, and from these experiments, the average value of amount adsorbed or desorbed is reported. The uncertainties in the results of batch experimental method were found below ±2%. The effect of this maximum value on the resultant parameter is below ±4% and does not have a significant influence on the interpretation of the results.

3 Results and discussion

3.1

3.1 Characterization of samples

Evaluation of structural properties is important for an adsorbent, therefore, FESEM micrographs and EDAX spectra of CHT, CHTA, and CHTAC are presented in Fig. 1. The micrographs show that the surface of all the three materials are non-homogenous and quite rough in nature and therefore provide a suitable surface area for adsorption. It also indicates that the structure remains unchanged even after modification and the only difference is the impregnation of aldehyde group and copper on the chitosan surface. Variation in surface morphology from CHT to CHTA and then to CHTAC can be observed from these images.

FESEM and EDAX spectra of chitosan and modified chitosan.

The EDAX analysis carried out for elemental analysis of CHT, CHTA, and CHTAC and also for surface chemistry shows that the modification with 2-hydroxy-1-naphthaldehyde increases the content of carbon on the surface. The wt.% of C in CHT is 32.6% and after the modification, it was increased to 46.1%. Similarly, the incorporation of Cu (9.1 wt.%) and Cl (8.6 wt.%) in CHTAC surface is also confirmed with these results.

The ATIR spectra of all the three samples are obtained and compared in Fig. 2. The analysis compares various functional groups, present on the material surface, before and after functionalization. In the case of CHT, the stretching at 1017 cm⁻¹ is attributed to C—O—C of the ring. The vibrational bands at 1648 cm⁻¹ and 1532 cm⁻¹ represent amide I band (the residual acetamido groups that remain after deacetylation of chitin during the production process of chitosan) and —NH₂ bending respectively. The C—H stretching can be assigned at 3081 cm⁻¹ and O—H group stretching can be confirmed from the vibrational peak at 3348 cm⁻¹.

In the IR spectra of CHTA, the stretching of C—O—C was confirmed at the peak of 1025 cm⁻¹. A new peak at 1635 cm⁻¹ was found which can be attributed to the imine bond (—C ⚌N) formation by the condensation between the free amine group of CHT and aldehyde group of 2-hydroxy-1-naphthaldehyde. The vibrational band in the region of 2807–3043 cm⁻¹ confirmed the presence of C—H stretching. The O—H stretching was confirmed from the peaks of 3489, 3411 cm⁻¹.

In the CHTAC sample, the absorption band at 615 cm⁻¹ confirms the presence of Cu—O stretching. The band at 1623 cm⁻¹ attribute to the imine bond (—C ⚌ N) whereas Ar—C—H stretching was observed in the band range of 2886–3027 cm⁻¹.

The results of point of zero charge experiments are presented in form of pH_ZPC plots (Fig. 3) where final pH of the solution is plotted as a function of initial pH of the solution, having material in contact. The change in surface chemistry on functionalization is evident from the difference in their isoelectric points.

The point of zero charge (pHzpc) of CHT, CHTA, and CHTAC.

The isoelectric points or pH_ZPC of CHT, CHTA, and CHTAC samples are found to be at 7.7, 6.7 and 4.0, respectively. It signified that these materials will retain positive charge below a pH then their pH_pzc and will retain a negative charge at pH above these values. This observation can be attributed to the dissociation of surface functional groups with an increase in solution pH (Kosmulski, 2009).

The low-temperature nitrogen adsorption/desorption isotherms were obtained for all the three materials. BET surface area and total pore volume were calculated from these isotherms (Table SI-1). The chitosan and modified materials are characterized by low values of the surface area ranging from A_BET = 2.43 – 0.37 m²/g. The total pore volume of CHTA (1.76 × 10⁻³ cm³/g) is higher compared to CHT (0.17 × 10⁻³ cm³/g) and CHTAC (0.71 × 10⁻³ cm³/g). Despite their low surface area, these materials are still useful for the effective removal of inorganic and organic contaminants (Wysokowski et al., 2014).

3.2

3.2 Rate of adsorption

Generally, the difference in adsorbent’s surface leads to a difference in adsorption capacity and rate of adsorption. Kinetic experiments were conducted to observe these differences with all the three materials. In these experiments, the progress of adsorption process was followed as a function of contact time. The results of these experiments are compared in Fig. 4.

Effect of contact time on the adsorption of PCP by CHT, CHTA, and CHTAC samples, with initial conc. 150 mg/L, at 20 °C.

All the samples show fast initial adsorption (up to 10–15 min) which is due to the availability of vacant sites for adsorption. Subsequently, the adsorption slows down gradually until a maximum adsorption capacity reached. Until that point, the duration of contact time is considered as equilibration time. After a certain time interval, there is no noticeable increase in adsorption capacity with each material. The maximum adsorption capacity of these materials follows an order of CHTA > CHTAC > CHT. The functionalization of chitosan surface has increased its adsorption capacity. The extent of increase is greater in the case of CHTA, as compared to CHTAC. In CHTA the addition of 2-hydroxy-naphthaldehyde through an imine linkage provide a suitable site for interaction with PCP. The —OH group connected to the aromatic ring has increased the interaction of CHTA with PCP molecules. In the case of CHTAC, the addition of electron deficient (Cu^II) metal center in the structure has caused lack of π electron density over the aromatic ring. This phenomenon reduces the interaction of PCP with the surface of CHTAC as compared to CHTA.

Kinetic modeling helps to determine the rate of adsorption of PCP on the adsorbent’s surfaces. For this purpose, adsorption data obtained from kinetic experiments (Fig. 4) were applied to different kinetic models to explain the rate of adsorption by these materials. Two kinetic models, namely pseudo-first order and pseudo-second order were employed in this process. In the pseudo-first order, we assume the adsorption to be completed in a single step and can be expressed by the Lagergren rate equation (Eq. (4)) (Lagergren, 1898):

(4)

\log (q_{e} - q_{t}) = \log q_{e} - \frac{k_{1}}{2.303} t

where q_e stands, for the amount of PCP adsorbed (mg g⁻¹) at equilibrium, q_t (mg g⁻¹) is the amount of PCP adsorbed at time t and k₁ is the rate constant (min⁻¹). The results of the fitting are compared in Table 1. The plot between log (q_e–q_t) and t showed low linear regression coefficient R² value ranging from 0.85 to 0.91, suggesting poor fit of pseudo-first order model to adsorption data. In addition, the calculated q_e value from this model does not concur with the experimental results. Altogether, these observations point that adsorption did not take place in single step with present adsorbate-adsorbent systems.

Table 1 Kinetic parameters for the adsorption of PCP on different chitosan samples.

Samples		Pseudo-first-order			Pseudo-second-order
	q_{e, exp}	q_{e, calc}	k₁ × 10²	R²	q_{e, calc}	k₂ × 10³	R²
	(mg g⁻¹)	(mg g⁻¹)	(min⁻¹)		(mg g⁻¹)	(g mg⁻¹ min⁻¹)
CHT	24.37	14.46	3.44	0.85	25.25	6.32	1.00
CHTA	39.07	21.90	3.10	0.88	40.02	4.78	1.00
CHTAC	35.37	22.05	3.03	0.91	36.63	3.80	1.00

Likewise, the pseudo-second order model Eq. (5) (Ho and McKay, 1999) can be expressed as:

(5)

\frac{t}{q_{t}} = \frac{1}{k_{2} q_{e}^{2}} + \frac{1}{q_{e}} t

where k₂ is the rate constant (g mg⁻¹ min⁻¹). The plot between t/q_t and t (Fig. SI-1) shows a straight line for all three adsorbents. The linear regression coefficient R² having value 0.998 (rounded off as 1.00) indicates best fit of the model. The values of calculated q_e,calc are in agreement with the experimental q_e,exp values (Table 1). The observations support the suitability of second order kinetics and thus suggests that the adsorption of PCP on three samples took place in two steps, which are possibly adsorption and diffusion.

During adsorption, the diffusion is a transport process, which involves the movement of species from the bulk of the solution to the solid phase. Besides, outer surface adsorption, there is a possibility of intraparticle diffusion from the outer surface into the pores of the materials (Wu et al., 2009). The overall adsorption process may indeed be controlled either by one or more steps, e.g. film or external diffusion, pore diffusion, surface diffusion and adsorption on the pore surface, or a combination of more than one step.

Usually, a process is diffusion-controlled when its rate depends on the rate at which adsorbate molecules diffuse towards one another. The likelihood of intraparticle diffusion was explored by using the intraparticle diffusion model. Following, Weber and Morris intraparticle diffusion model Eq. (6) (Weber and Morris, 1962) was applied to kinetic data:

(6)

q_{t} = k_{id} \cdot t^{0.5} + C

where k_id (mg/min^1/2) is the intraparticle diffusion rate constant and C (mg/g) is a constant related to the thickness of boundary layer. The value of C is directly related to the extent of boundary layer effect (Ho et al., 2000). If the plot of q_t against t^0.5 gives a straight line, then the sorption process is controlled only by intraparticle diffusion. However, if it shows multi-linear plots, then two or more steps influence the adsorption process. Fig. SI-2 shows plots for all the three materials, where every plot is composed of three different linear steps. It shows that the external resistance to mass transfer surrounding the particles is significant only in the early stages of adsorption, which is represented by a first sharper step. The second linear portion is the gradual adsorption stage with controlling intraparticle diffusion. The third step shows saturation of adsorbent surface (Li et al., 2012). The deviation of straight lines from the origin (Fig. SI-2) could be due to the difference in the rate of mass transfer in the initial and final stages of adsorption. Furthermore, such deviation indicates that the pore diffusion is not the only rate-controlling step.

3.3

3.3 Effect of pH

The effect of solution pH on adsorption of PCP by different chitosan samples is compared in Fig. SI-3. It shows that, the speciation of PCP and the surface properties of different chitosan samples determine the adsorption of PCP at various pH gradient. The PCP is a weak acid and its pKa is 4.7, which means at pH < 4.7 the PCP remain neutral (protonated) and at pH > 4.7, it deprotonates and remains as negatively charged species, PCP⁻. At low pH the neutral form of PCP is dominant; this form of PCP is strongly hydrophobic and causes its high adsorption under these conditions (Li et al., 2009; Zhou et al., 2014). As the pH of solution increases, the solubility of PCP starts increasing and the distribution of anionic PCP⁻ dominates (Mathialagan and Viraraghavan, 2009). The PCP adsorption by all the three materials fall steeply in the range of pH 3.0 to 8.0, which is due to high solubility and low hydrophobicity of anionic PCP⁻ species. The extent of adsorption by different chitosan samples is governed by their surface properties, or in other words, surface charge on the sample, which depends on both the pH of the solution and its pH_zpc. In case of CHT, (Fig. SI-3) the surface charge is positive at pH < 7.7, while tend to be negative at pH > 7.7. When pH is >8 more than 70% of PCP available as PCP⁻, the electrostatic repulsion between this anionic form and negatively charged CHT surface starts inhibiting adsorption. Nevertheless, PCP⁻ still remains adsorbed to negatively charged surface, which is possibly due to π–π interaction between negatively charged CHT and π electron cloud on PCP⁻ (Li et al., 2009). At pH < 7 presence of hydroxyl group and amino group on the surface provides suitable adsorption sites and weak hydrophilicity of CHT surface facilitate the adsorption to the negatively charged species. Under such conditions, the hydrogen bond formation between CHT surface and PCP could be responsible for higher adsorption. The CHTA and CHTAC samples follows, the same trend, (see Fig. SI-3) the relative amount of their adsorption is more evident at lower pH, where the effect of their surface charge is dominant on adsorption process.

3.4

3.4 Isotherm modeling

Adsorption isotherm studies can help in interpretation of relationship between equilibrium adsorption capacity and equilibrium concentration at a definite temperature. The variation in amounts of PCP absorbed per unit mass of CHT, CHTA, and CHTAC as a function of equilibrium PCP concentration in the solution at different temperature of 293 K, 303 K, and 313 K is presented in Fig. SI-4. These equilibrium adsorption data were applied to three different adsorption models, namely Langmuir, Freundlich, and Redlich-Peterson models. The parameters obtained from fitting of these models on adsorption data are compiled in Table 2. The Langmuir isotherm model is believed to be suitable for homogeneous adsorption where the adsorption process had identical activation energy and was applicable to monolayer adsorptions without any interaction among adsorbed species (Langmuir, 1916). The Langmuir equation (Eq. (7)) is given as follows:

(7)

\frac{C_{e}}{q_{e}} = \frac{1}{q_{\max} b} + \frac{C_{e}}{q_{\max}}

Here, q_e is the amount of PCP adsorbed on the adsorbent surface (mg/g), C_e is the concentration of PCP at equilibrium (mg/g), q_max stands for saturation adsorption capacity (maximum monolayer amount) of PCP absorbed on adsorbent surface, b is the Langmuir constant for adsorption energy (L/mg). The non-linear fitting of this equation can be observed in Fig. SI-4. The results of fitting shows (Table 2) decrease in adsorption energy ‘b’ and q_max with increase in temperature. For this model, the regression coefficient R² values varies from 0.91 to 0.98 collectively for all the three materials.

Table 2 Constants of Eqs. (7)–(9), derived from the fits of corresponding models to the experimental data for each chitosan sample.

Sample	Temp	Langmuir			Freundlich			Redlich peterson
	(K)	q_max	b	R²	K_f	1/n	R²	A	B	m	R²
		(mg/g)			K_f	1/n	R²	A	(L/g)	m	R²
CHT	293	21.29	0.04	0.98	2.56	0.42	0.95	2.53	0.66	0.64	0.99
	303	18.58	0.03	0.97	1.64	0.47	0.98	2.09	1.01	0.56	0.96
	313	13.13	0.03	0.91	1.21	0.46	0.98	1.28	0.62	0.64	0.99

CHTA	293	39.11	0.05	0.97	5.65	0.40	0.91	7.68	0.99	0.66	0.99
	303	31.91	0.04	0.94	3.71	0.43	0.94	1.46	0.07	0.91	0.98
	313	25.46	0.03	0.92	2.68	0.45	0.98	1.08	0.08	0.86	0.99

CHTAC	293	36.85	0.06	0.97	5.24	0.42	0.90	5.67	0.63	0.70	0.99
	303	32.19	0.04	0.95	3.41	0.46	0.94	3.50	0.58	0.65	0.99
	313	22.35	0.05	0.96	2.96	0.42	0.95	2.01	0.29	0.75	0.99

Unlike Langmuir model, the Freundlich isotherm model describes the multilayer adsorption on heterogeneous surface (Freundlich, 1906). The equation (Eq. (8)) is expressed as:

(8)

\log q_{e} = \log K_{f} + (\frac{1}{n}) {logC}_{e}

where, K_f is Freundlich constant and n denotes affinity of adsorption. The value of K_f and n is dependent on temperature, adsorbate, and properties of adsorbent. The non-linear fitting of this equation is presented in Fig. SI-4. Results show that both the parameters decrease with temperature similar to Langmuir isotherm model, here again the R², values for fitting varies from 0.91 to 0.98.

Both of the above models, give similar degree of fitting, it is therefore believed that adsorption of PCP on these materials involves homogenous as well as heterogeneous surfaces. In order to confirm this phenomenon, Redlich-Peterson model (Redlich and Peterson, 1959) was applied to above adsorption data. This model is a hybrid of both the Langmuir and Freundlich isotherms. The equation (Eq. (9)) for this model is:

(9)

q_{e} = \frac{{AC}_{e}}{1 + {BC}_{e}^{m}}

where A, B and m are the R–P constants. When the value of m is equal to 1, the above equation is reduced to the Langmuir isotherm and if in case the value of m is much bigger than 1, the above equation is reduced to a Freundlich isotherm. Examination of fitting parameters in Table 2 shows that Redlich-Peterson isotherm describes more accurately the adsorption of PCP on CHT, CHTA, and CHTAC over the concentration range studied. Since the method used to derive the isotherm parameters maximizes the linear coefficient of determination, it is unsurprising that in all cases, the Redlich–Peterson isotherms exhibit extremely high R² values (ranging from 0.96 to 0.99) indicating considerably better fit as compared to the preceding two-parameter isotherms.

From the Table 2, it is clear that the values of m are close to unity, which shows that the isotherms are approaching Langmuir form. At 293 K, the Langmuir and Redlich–Peterson models gave the highest R² value with each material, indicating that the adsorption isotherms of PCP on CHT, CHTA, and CHTAC can be best explained by these two isotherms. Further, the suitability of the Langmuir isotherm to fit the data at 293 K was confirmed by the exponent value m of the Redlich–Peterson model, which was equal to one. By taking into account the R² and q_max values projected by Langmuir model, it was observed that both the Langmuir and Redlich–Peterson isotherms could well describe the experimental adsorption data, however the Redlich–Peterson model was more suitable.

3.5

3.5 Thermodynamics of adsorption

Adsorption of PCP on the surface of chitosan and its modified forms varies with temperature ranging from 293 K to 313 K, as evident from Fig. SI-4. The maximum adsorption capacity was observed at lower temperature condition that indicates the exothermic nature of adsorption. With the help of adsorption isotherm data, thermodynamics of PCP adsorption was investigated to evaluate the thermodynamic feasibility and the spontaneous nature of the process. It involves estimation of thermodynamic parameters such as the change in Gibbs free energy (ΔG°), change in enthalpy (ΔH°) and the change in entropy (ΔS°). These parameters are estimated from the variation of the thermodynamic equilibrium constant ‘K’ as a function of adsorption temperature (Salvestrini et al., 2014). For a given adsorption reaction, ‘K’ is determined using Eq. (10):

(10)

K = \frac{a_{a}}{a_{s}} = \frac{ν_{s} q_{e}}{ν_{e} C_{e}}

where, a_a and a_s are the activity of PCP in adsorbed and solution phase; q_e is the surface concentration of PCP on the adsorbent; C_e is the PCP concentration in solution at equilibrium; ν_a and ν_s are the activity coefficient of PCP in adsorbed phase and solution phase respectively. The equilibrium constant K can be obtained by plotting ln(q_e/C_e) versus q_e, where the intercept at the vertical axis yields the values of K (Table SI-2) (Zhou and Zhou, 2014). The ΔG°, ΔH° and ΔS° were calculated using Eqs. (11)–(13).

(11)

Δ G ° = - RT \ln K

(12)

\ln K = \frac{Δ S °}{R} - \frac{Δ H °}{RT}

(13)

Δ G ° = Δ H ° - T Δ S °

where T is the solution temperature in Kelvin, and R is the universal gas constant (8.314 J mol⁻¹ K⁻¹). The values of Gibbs free energy change ΔG° were calculated from Eq. (10) and compiled in Table SI-2. Except CHT, at 303 and 313 K, other materials (CHTA and CHTAC) show negative values of ΔG° with all the three temperatures. It indicates the feasibility and spontaneous nature of adsorption process at these temperatures. These results show that the functionalization of chitosan surface increase the adsorption feasibility at higher temperature. Similarly, the ΔH° values were determined from the slope of the Van’t Hoff plot, between ln K and 1/T, on the basis of Eq. (11). (Fig. 5, Table SI-2). The ΔH° values were found to be −36.41 kJ mol⁻¹, −37.68 kJ mol⁻¹, and −26.22 kJ mol⁻¹ for the adsorption on CHT, CHTA, and CHTAC, respectively. All these values confirm the exothermic nature of the adsorption and the possibility of physical adsorption. The change in entropy ΔS°, during adsorption was calculated with Eq. (12). The negative values of ΔS° for adsorption on CHT (−122.84 J mol⁻¹ K⁻¹), CHTA (−118.83 J mol⁻¹ K⁻¹), and CHTAC (−79.80 J mol⁻¹ K⁻¹) suggest that there was a decrease in randomness at the solid/solution interface, and sufficient affinity of materials for PCP.

Van't Hoff plot for the estimation of thermodynamic parameters.

3.6

3.6 Desorption and reusability

The reusability of the developed adsorbents is a critical aspect of an adsorbent, to ensure its sustainable use in adsorption based techniques. The low-values of heat of adsorption indicate that the physical adsorption of PCP on chitosan based materials should be desorbed with a suitable desorbing agent. The effect of different desorbing agents was analyzed and compared on all the three spent adsorbent materials. Results showed that (Fig. SI-5), different desorbing agents show different% of desorption. The highest desorption with CHT (64%) and CHTA (90.3%) was achieved with NaOH, whereas with CHTAC (60.1%) it was achieved with H₃PO₄.

On the basis of above results, NaOH and H₃PO₄ are identified as suitable desorbing agents for respective chitosan materials and used in reusability tests. In these tests, three consecutive cycles of adsorption-desorption were performed on all the three materials using batch mode of operation. The results of these tests are presented in Fig. 6, where regaining of initial adsorption capacity after each cycle is compared. It clearly shows that for CHT and CHTAC, the loss of adsorption capacity is fast as compared to CHTA. In third cycles, the CHT and CHTAC samples can maintain only 41% and 35% of their original adsorption capacity, whereas CHTA can still retain about 80% of its original adsorption capacity. These results highlight the reusability of CHTA for PCP removal applications, in comparison to other two materials.

Effects of successive adsorption-desorption cycles on the reusability of prepared chitosan materials.

Inorder to further support the reusability and stability of the adsorbent material, EDAX analysis was performed for all the three materials after adsorption (Fig. SI-6a–c) and desorption (Figure Fig. SI-7a–c). The atomic% of carbon increased from 42.6% to 49.5% for CHT, 58.5% to 61.7% for CHTA, 50.4% to 58.9% for CHTAC and a new peak for chlorine was observed in CHT and CHTA after the PCP adsorption experiments which accounts for the adsorption of PCP. After the desorption experiments, for example using NaOH, it was seen that the peak for chlorine vanished in CHT and CHTA. The atomic weight% of carbon reduced and was nearly equal to the carbon wt% of the unused adsrobent materials which substantiates the retainment of adsorbent material in its original form without occurrence of any chemical reaction between PCP and adsorbent material. ATIR spectra was also recorded for the materials after adsorption (Fig. SI-8) and desorption (Fig. SI-9) experiments. All the three adsorbent materials showed characteristic stretching vibrations of the functional groups as that observed in the unused materials. A broad band was observed in the range of 3281–3322 cm⁻¹ in CHT, CHTA and CHTAC after adsorption of PCP. This can be accounted to the phenolic O—H group in PCP adsorbed on the surface of the materials. After desorption experiments, the broad band observed for O—H group from PCP disappeared and in addition, all the characteristic vibrations of the materials were within the expected range. The retainment of all the group-specific stertching vibrations in the materials after adsorption and desorption experiments and the appearance and disappearance of broad O—H peak before desorption and after desorption, respectively explains the concept of physical adsorption and that no chemical reaction occurred between the adsorbate PCP and the adsorbent materials.

4 Conclusion

The surface modification of chitosan using 2-hydroxy-1-napthaldehyde and copper chloride leads to promising materials CHTA and CHTAC respectively which were used as bioadsorbents for PCP removal from aqueous solutions. Crosslinking of chemically modified chitosan derivatives improved their interaction with PCP⁻ and thereby enhanced the ability to uptake PCP. At neutral pH, the highest adsorption capacity was exhibited by CHTA (39.1 mg/g) followed by CHTAC (35.4 mg/g) and CHT (24.4 mg/g). The rate of adsorption on all the three materials was found to follow a pseudo-second-order model. The adsorption capacity of materials decreased with increase in pH, owing to increase of PCP solubility. The amount of adsorption on chitosan and modified materials decreases with increase in temperature from 293 K to 313 K. The thermodynamic study revealed that the adsorption was favorable and exothermic in nature. Reusability of CHTA was comparatively better than CHT and CHTAC and the materials did not show any chemical modification upon adsorption. These results draw attention to the development of a method for the improved removal of a water contaminant like PCP by the surface functionalization of naturally available material like chitosan.

Acknowledgement

The Authors acknowledge the financial support and research infrastructure, provided by the School of Environment and Natural Resources, Doon University, Dehradun, Centre for Nano and Material Sciences, Jain University, Bangalore, DST India (SB/FT/CS-100/2013) (ECR/2017/001266), and UGC India (MRP-MAJOR-ENVI-2013-35206).

Conflict of interest

None.

References

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[CrossRef] [Google Scholar]
Monier M., Abdel-Latif D.A., Abou El-Reash Y.G., . Ion-imprinted modified chitosan resin for selective removal of Pd(II) ions. J. Colloid Interface Sci.. 2016;469:344-354.
[CrossRef] [Google Scholar]
Orton F., Lutz I., Kloas W., Routledge E.J., . Endocrine disrupting effects of herbicides and pentachlorophenol: In vitro and in vivo evidence. Environ. Sci. Technol.. 2009;43:2144-2150.
[CrossRef] [Google Scholar]
Pal P., Pal A., . Surfactant-modified chitosan beads for cadmium ion adsorption. Int. J. Biol. Macromol.. 2017;104:1548-1555.
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Song K.S., Kim D., Kyriaki P., Coskun Ali, . Synthesis of highly porous coordination polymers with open metal sites for enhanced gas uptake and separation. Appl. Mater. Interfaces. 2016;8:26860-26867.
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[CrossRef] [Google Scholar]
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Appendix A

Supplementary material

Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.arabjc.2018.01.016.

Appendix A

Supplementary material

Supplementary data 1

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Introduction

Experimental

Materials and methods
Chemical modification of chitosan (CHT)
Adsorption and desorption experiments

Results and discussion

Characterization of samples
Rate of adsorption
Effect of pH
Isotherm modeling
Thermodynamics of adsorption
Desorption and reusability

Conclusion

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[1] Ali I., Gupta V.K., . Advances in water treatment by adsorption technology. Nat. Protoc.. 2007;1:2661-2667.
[CrossRef] [Google Scholar]

[2] Ali U., Syed J.H., Malik R.N., Katsoyiannis A., Li J., Zhang G., Jones K.C., . Organochlorine pesticides (OCPs) in South Asian region: a review. Sci. Total Environ.. 2014;476:705-717.
[CrossRef] [Google Scholar]

[3] Amreen, N., Shiva, A., Shahid, S. N., Mohammad, S. A., Anu, S., Prabha, B., Dutta, P. K., 2017. Cu(II)-carboxymethyl chitosan-silane schiff base complex grafted on nano silica: Structural evolution, antibacterial performance and dye degradation ability, Int. J. Bio. Macromolecules. in press. https://doi.org/10.1016/j.ijbiomac.2017.11.112

[4] Babić B.M., Milonjić S.K., Polovina M.J., Kaludierović B.V., . Point of zero charge and intrinsic equilibrium constants of activated carbon cloth. Carbon. 1999;37:477-481.
[CrossRef] [Google Scholar]

[5] Badawi M.A., Negm N.A., Abou Kana M.T.H., Hefni H.H., Abdel Moneem M.M., . Adsorption of aluminum and lead from wastewater by chitosan-tannic acid modified biopolymers: isotherms, kinetics, thermodynamics and process mechanism. Int. J. Biol. Macromol.. 2017;99:465-476.
[CrossRef] [Google Scholar]

[6] Beppu M.M., Arruda E.J., Vieira R.S., Santos N.N., . Adsorption of Cu(II) on porous chitosan membranes functionalized with histidine. J. Membr. Sci.. 2004;240:227-235.
[CrossRef] [Google Scholar]

[7] Donia A.M., Atia A.A., Elwakeel K.Z., . Selective separation of mercury(II) using magnetic chitosan resin modified with Schiff's base derived from thiourea and glutaraldehyde. J. Hazard. Mater.. 2008;151:372-379.
[CrossRef] [Google Scholar]

[8] El-Sayed E.M., Tamer T.M., Omer A.M., Mohy Eldin M.S., . Development of novel chitosan Schiff base derivatives for cationic dye removal: methyl orange model. Desalin. Water Treat.. 2016;57:22632-22645.
[CrossRef] [Google Scholar]

[9] Elwakeel K.Z., Atia A.A., . Uptake of U(VI) from aqueous media by magnetic Schiff's base chitosan composite. J. Clean. Prod.. 2014;70:292-302.
[CrossRef] [Google Scholar]

[10] Freundlich H., . Over the adsorption in solution. J. Phys. Chem.. 1906;57:385-470.
[Google Scholar]

[11] Gavalyan V.B., . Synthesis and characterization of new chitosan-based Schiff base compounds. Carbohydr. Polym.. 2016;145:37-47.
[CrossRef] [Google Scholar]

[12] Gupta V.K., Carrott P.J.M., Ribeiro Carrott M.M.L., Suhas, . Low-cost adsorbents: Growing approach to wastewater treatment—a review. Crit. Rev. Environ. Sci. Technol.. 2009;39:783-842.
[CrossRef] [Google Scholar]

[13] Ho Y.S., McKay G., . Pseudo-second order model for sorption processes. Process Biochem.. 1999;34:451-465.
[CrossRef] [Google Scholar]

[14] Ho Y.S., Ng J.C.Y., McKay G., . KINETICS of pollutant sorption by biosorbents: review. Sep. Purif. Methods. 2000;29:189-232.
[CrossRef] [Google Scholar]

[15] Jiao T., Zhou J., Gao L., Xing Y., Li X., . Synthesis and characterization of chitosan-based Schiff base compounds with aromatic substituent groups. Iran. Polym. J.. 2011;20:123-136.
[Google Scholar]

[16] Kosmulski M., . pH-dependent surface charging and points of zero charge. IV. Update and new approach. J. Colloid Interface Sci.. 2009;337:439-448.
[CrossRef] [Google Scholar]

[17] Kyzas G., Bikiaris D., . Recent modifications of chitosan for adsorption applications: a critical and systematic review. Mar. Drugs. 2015;13:312.
[CrossRef] [Google Scholar]

[18] Lagergren S., . Zur theorie der sogenannten adsorption gelöster stoffe, kungliga svenska vetenskapsakademiens. Handlingar. 1898;24:1-39.
[Google Scholar]

[19] Langmuir I., . The constitution and fundamental properties of solids and liquids. Part I. Solids. J. Am. Chem. Soc.. 1916;38:2221-2295.
[CrossRef] [Google Scholar]

[20] Li J.-M., Meng X.-G., Hu C.-W., Du J., . Adsorption of phenol, p-chlorophenol and p-nitrophenol onto functional chitosan. Bioresour. Technol.. 2009;100:1168-1173.
[CrossRef] [Google Scholar]

[21] Li Y., Du Q., Liu T., Sun J., Jiao Y., Xia Y., Xia L., Wang Z., Zhang W., Wang K., Zhu H., Wu D., . Equilibrium, kinetic and thermodynamic studies on the adsorption of phenol onto graphene. Mater. Res. Bull.. 2012;47:1898-1904.
[CrossRef] [Google Scholar]

[22] Mathialagan T., Viraraghavan T., . Biosorption of pentachlorophenol from aqueous solutions by a fungal biomass. Bioresour. Technol.. 2009;100:549-558.
[CrossRef] [Google Scholar]

[23] Monier M., Abdel-Latif D.A., Abou El-Reash Y.G., . Ion-imprinted modified chitosan resin for selective removal of Pd(II) ions. J. Colloid Interface Sci.. 2016;469:344-354.
[CrossRef] [Google Scholar]

[24] Orton F., Lutz I., Kloas W., Routledge E.J., . Endocrine disrupting effects of herbicides and pentachlorophenol: In vitro and in vivo evidence. Environ. Sci. Technol.. 2009;43:2144-2150.
[CrossRef] [Google Scholar]

[25] Pal P., Pal A., . Surfactant-modified chitosan beads for cadmium ion adsorption. Int. J. Biol. Macromol.. 2017;104:1548-1555.
[CrossRef] [Google Scholar]

[26] Ravi Kumar M.N.V., . A review of chitin and chitosan applications. React. Funct. Polym.. 2000;46:1-27.
[CrossRef] [Google Scholar]

[27] Redlich O., Peterson D.L., . A useful adsorption isotherm. J. Phys. Chem.. 1959;63:1024.
[CrossRef] [Google Scholar]

[28] Salvestrini S., Leone V., Iovino P., Canzano S., Capasso S., . Considerations about the correct evaluation of sorption thermodynamic parameters from equilibrium isotherms. J. Chem. Thermodyn.. 2014;68:310-316.
[CrossRef] [Google Scholar]

[29] Sobahi T.R.A., Abdelaal M.Y., Makki M.S.I., . Chemical modification of chitosan for metal ion removal. Arab. J. Chem.. 2014;7:741-746.
[CrossRef] [Google Scholar]

[30] Song K.S., Kim D., Kyriaki P., Coskun Ali, . Synthesis of highly porous coordination polymers with open metal sites for enhanced gas uptake and separation. Appl. Mater. Interfaces. 2016;8:26860-26867.
[CrossRef] [Google Scholar]

[31] Vakili M., Rafatullah M., Ibrahim M.H., Abdullah A.Z., Salamatinia B., Gholami Z., . Chitosan hydrogel beads impregnated with hexadecylamine for improved reactive blue 4 adsorption. Carbohydr. Polym.. 2016;137:139-146.
[CrossRef] [Google Scholar]

[32] Vakili M., Rafatullah M., Salamatinia B., Ibrahim M.H., Abdullah A.Z., . Elimination of reactive blue 4 from aqueous solutions using 3-aminopropyl triethoxysilane modified chitosan beads. Carbohydr. Polym.. 2015;132:89-96.
[CrossRef] [Google Scholar]

[33] Wan Ngah W.S., Teong L.C., Hanafiah M.A.K.M., . Adsorption of dyes and heavy metal ions by chitosan composites: a review. Carbohydr. Polym.. 2011;83:1446-1456.
[CrossRef] [Google Scholar]

[34] Weber, W.J., Morris, J.C., 1962. Advances in water pollution research: Removal of biologically resistant pollutant from waste water by adsorption. Proceedings of the International Conference on Water Pollution Symposium, Pergamon Press, Oxford, UK, 231-266.

[35] Wu F.-C., Tseng R.-L., Juang R.-S., . Initial behavior of intraparticle diffusion model used in the description of adsorption kinetics. Chem. Eng. J.. 2009;153:1-8.
[CrossRef] [Google Scholar]

[36] Wu M.-T., Tsai Y.-L., Chiu C.-W., Cheng C.-C., . Synthesis, characterization, and highly acid-resistant properties of crosslinking β-chitosan with polyamines for heavy metal ion adsorption. RSC Adv.. 2016;6:104754-104762.
[CrossRef] [Google Scholar]

[37] Wysokowski M., Klapiszewski Ł., Moszyński D., Bartczak P., Szatkowski T., Majchrzak I., Siwińska-Stefańska K., Bazhenov V., Jesionowski T., . Modification of chitin with kraft lignin and development of new biosorbents for removal of Cadmium(II) and Nickel(II) ions. Mar. Drugs. 2014;12:2245-2268.
[CrossRef] [Google Scholar]

[38] Zhou L., Pan S., Chen X., Zhao Y., Zou B., Jin M., . Kinetics and thermodynamics studies of pentachlorophenol adsorption on covalently functionalized Fe₃O₄@SiO₂–MWCNTs core–shell magnetic microspheres. Chem. Eng. J.. 2014;257:10-19.
[CrossRef] [Google Scholar]

[39] Zhou X., Zhou X., . The unit problem in the thermodynamic calculation of adsorption using the langmuir equation. Chem. Eng. Commun.. 2014;201:1459-1467.
[CrossRef] [Google Scholar]

Removal of pentachlorophenol pesticide from aqueous solutions using modified chitosan

Abstract

Abstract

Keywords

Pesticide removal by adsorption

Chitosan

Endocrine disruptor compound

Wastewater treatment

Pentachlorophenol removal

Surface modification

1 Introduction

2 Experimental

2.1 Materials and methods

2.2 Chemical modification of chitosan (CHT)

2.2.1 Modification of CHT with 2-hydroxy-1-naphthaldehyde to prepare CHTA

2.2.2 Modification of CHTA with copper chloride to prepare CHTAC

2.3 Adsorption and desorption experiments

3 Results and discussion

3.1 Characterization of samples

3.2 Rate of adsorption

3.3 Effect of pH

3.4 Isotherm modeling

3.5 Thermodynamics of adsorption

3.6 Desorption and reusability

4 Conclusion

Acknowledgement

Conflict of interest

References

Appendix A

Supplementary material

Appendix A

Supplementary material

Supplementary data 1

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