Introduction

Recently, a progressive increase in industrialization and urbanization has substantially increased aquatic environmental pollution by the discharge of industrial effluents containing highly toxic organic and inorganic pollutants. Dyes and heavy metals are the most important constituents among the toxic compounds present in the effluent (Gupta et al. 2009; Saad et al. 2010; Sajab et al. 2011; Siddiqi and Pathania 2002). They not only influence the quality of the water body, but also reduce the production and quality of crops and aquatic products, even threatening the health and life of animals and humans through the food chain (Cheng 2003; Duan et al. 2013). The presence of dyes in aquatic bodies increases chemical oxygen demand, color contents, and dissolved and suspended solids; impedes light penetration into water; interferes with photosynthesis of aquatic plants; hinders the growth of microbes; creates microtoxicity for fish and other organisms; etc. Unlike organic pollutants, the heavy metals are persistent in nature and do not undergo physical, chemical or microbial degradation; they have a tendency to accumulate in living organisms, having a severe impact on human health and on the sustainability of ecosystems because of their carcinogenic and mutagenic nature (Bailey et al. 1999; Cay et al. 2004; Roy et al. 2013). Due to the harmful effects of dye and heavy metal ion pollution in water, there is a pressing need to find efficient methods to combat this kind of pollution. Many approaches to the removal of dyes and heavy metal ions from aqueous solution have been carried out, such as adsorption, chemical precipitation, ion exchange, complexation, photodegradation, electrodialysis, reverse osmosis and ultra-filtration membrane separation (Bhattacharyya and Sharma 2005; Gupta et al. 2006; Gupta and Ali 2008; Wang and Zhu 2007). However, most of these techniques have several disadvantages such as inefficiency, high energy and chemical reagent requirements, generation of toxic dreg or other wastes as by-products that need careful treatment and disposal, high capital and operational costs, etc. (Demir et al. 2008). Nevertheless, adsorption is an efficient method that has been used extensively for the removal of dyes and heavy metal ions from industrial wastewater because of its economic viability and technical feasibility (Yin et al. 2008).

Activated carbon (Aber and Sheydaei 2012; Kim et al. 2013), graphene (Zhang et al. 2011; Zhao et al. 2011), sand (Gao et al. 2011), clay (Ozdes et al. 2011), oxides (Shi et al. 2011), composite fiber (Ji et al. 2012, 2013; Ren et al. 2011; Sui et al. 2012; Xia et al. 2013) and polymeric material (Jiang et al. 2012; Neghlani et al. 2011) have been utilized as adsorbents. However, most of these materials are non-renewable or non-biodegradable and may cause secondary pollution. Of course, several natural fiber-based materials have been explored for this purpose, for example, sugar cane bagasse (Gurgel et al. 2008a, b; Júnior et al. 2009; Karnitz Jr et al. 2007), kapok (Duan et al. 2013; Liu et al. 2012; Zheng et al. 2012), luffa cylindrical (Demir et al. 2008; Gupta et al. 2013), lignocellulosic jute fiber (Roy et al. 2012, 2013), wheat straw (Han et al. 2010), peanut shell (Liu et al. 2010), coconut shell (Shrestha et al. 2013), rice husk (Chowdhury et al. 2011), starch (Yin et al. 2008; Zhao et al. 2010b), etc. But most of these adsorbents can only cope with the pollutants with a single surface charge. Nevertheless, usually the constituents of industrial effluents are extremely complex and include both cationic and anionic pollutants. Therefore, the application fields of all the adsorbents mentioned above are seriously restricted.

With this in mind, amphiprotic adsorbents are gaining increasing attention because of their versatile adsorption capability for both cationic and anionic pollutants. Zhang et al. (2012) prepared a novel amphiprotic wheat straw-based adsorbent (AWS) used to adsorb various dyes from aqueous solutions. This AWS was proven effective in eliminating both cationic and anionic dyes [methylene blue (MB) and acid green 25], the maximum adsorption capacity of AWS for MB being around 138 mg/g and for acid green 25 around 360 mg/g. Xu and coworkers (Xu et al. 2005) crosslinked amphiprotic starch (CAS) with quaternary ammonium and carboxymethyl groups and used it to remove Pb2+ from aqueous solutions; the maximum adsorption capacity of CAS was around 53 mg/g for Pb2+.

Cellulose is the major component in cotton fibers and the most abundant renewable biopolymer in nature. Due to the characteristic properties of cellulose such as excellent hydrophilicity, biocompatibility and biodegradability, it can easily be chemically modified according to the introduction of various functional groups on hydroxyl (Qu et al. 2009). However, most of the modified cotton fibers were only used as adsorbents for the removal of cationic compounds (metal ions, etc.) instead of anionic pollutants. In this study, we synthesized an amphiprotic adsorbent based on cotton fiber that has a hollow, flat-banded structure. Specifically, cotton fibers were activated by dispersing them into the diluted NaOH aqueous solution; then activated cotton was chemically modified with the cationic monomer (3-chloro-2-hydroxypropyl trimethyl ammonium chloride, CTA) and anionic monomer (2-acrylamide-2-methyl propane sulfonic acid, AMPS) respectively. The resulting amphiprotic cotton fibers (AP-cotton) were used as the adsorbent for the removal of dyes and heavy metal ions from aqueous solutions (Scheme 1). Two typical azo Congo red (CR) and methylene blue (MB) dyes were chosen as the representatives of anionic and cationic pollutants, and Cu2+ and Pb2+ acted as the metal ions for the adsorption study. As shown in Scheme 1, the removal capacity of AP-cotton for dyes and metal ions is remarkable after amphiprotic modification. Furthermore, the influences of pH, adsorption time and initial concentration of dyes and metal ions in the adsorption behavior were investigated. Regeneration tests were carried out to evaluate the practical utility of this modified cotton fiber.

Scheme 1
scheme 1

Synthesis route and adsorption efficiency of AP-cotton

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Experimental

Materials

The cotton fibers used were provided by Suzhou Yintong Cotton Co., Ltd. CTA (60 wt%) and AMPS came from Sigma-Aldrich and were used without further purification. Sodium hydroxide (NaOH), hydrochloric acid (HCl), acetone, methanol, ethanol, diethyl ether and ammonium cerium nitrate [Ce(NH4)2(NO3)6], glacial acetic acid, copper chloride dehydrate (CuCl2 2H2O), lead chloride (PbCl2), CR and MB were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and used without any treatment.

Pretreatment of cotton fibers

Cotton fibers (10 g) were boiled in 500 ml NaOH aqueous solution (2 wt%) for 90 min. Then the fibers were filtered and transferred into 300 ml NaOH aqueous solution (18 wt%); the mixture was stirred at room temperature for 2 h. Then the fibers were filtered and washed three times with plenty of deionized water. After drying at 60 °C for 2 h, the activated cotton fiber (AC) was obtained.

Preparation of AP-cotton

ACs were cut into 5–10-mm-long segments by shears. The short fibers (8.0 g) were suspended in 80 ml acetone; under continuous stirring, 6.0 ml NaOH (10 wt%) and 16.0 g CTA respectively were added to the above-mentioned mixture dropwise. The resultant mixture was stirred at 45 °C for 3 h, and its pH value was regulated to be neutral. Then the fibers were filtered and washed with plenty of aqueous methanol solution (85 %) and absolute ethanol, followed by drying at 90 °C for 1 h. The product was named CTA-cotton (CC).

Then 5.0 g CC was added to 200 ml deionized water; the mixture was stirred at room temperature for 10 min under nitrogen atmosphere. Then 10.0 g Ce(NH4)2(NO3)6 was added and stirred for 30 min. Subsequently, 15.0 g AMPS was added to the above mixture and stirred for 1.0 h. The resultant system was stirred continuously at 50 °C for 3.0 h, then the fibers were filtered, and washed with plenty of deionized water, acetone and diethyl ether, followed by drying at 60 °C for 3 h. The product was CTA-AMPS-cotton; because of including both cationic groups and anionic groups, we named it as AP-cotton.

Characterization

The morphology of the fibers was studied using Hitachi S-4800 field emission scanning electron microscopy. The FT-IR spectroscopy measurements were made using a Nicolet 5700. X-ray diffraction studies were performed on an X-ray diffractometer (Brucker AXS, D8 Advance). XPS measurements were conducted with an Axis Ultra HAS system. The concentrations of metal ions were measured using a Thermo Scientific ICAP 6000 DUO inductively coupled plasma atomic emission spectroscopy. The concentration of CR and MB in the supernatant was estimated by UV/visible spectroscopy (Purkinje General, TU-1810) at 498 and 664 nm, respectively. The resultant standard curves of the relationship between the absorbance of dyes solutions and the dye concentration are shown in Figs. S1 and S2.

Adsorption and regeneration experiments

Adsorbents were added to different adsorbate solutions at a dosage of 1.0 g/l for Cu2+ and Pb2+, and 2.0 g/l for CR and MB, respectively. Initial pH values of the solution were calibrated using glacial acetic acid and NaOH solutions. The adsorption experiments were carried out at 30 °C. The adsorption capacity Qe (mg/g) was calculated according to the equation:

$$Q_{\text{e}} = \frac{{\left( {C_{0} - C_{\text{e}} } \right)V}}{m}$$
(1)

where C 0 and C e are the initial and equilibrium adsorbate concentration (mg/l), respectively. V is the volume of adsorbate solution (L), and m is the mass of adsorbent used for adsorption experiment (g). Three sets of experiments were conducted for complete investigation of Cu2+, Pb2+, CR and MB adsorption on AP-cotton. The details are given in Table 1.

Table 1 Experimental design for batch adsorption study
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For the regeneration study, the AP-cotton used for adsorption of Cu2+ or Pb2+ was placed with 1 M HCl solution at 303 K for 2 h for desorption. After being washed until no metal ions were detected in the filtrate, indicating complete desorption, then it was reemployed in metal ion removal (Duan et al. 2013). The AP-cotton was loaded with CR or MB solution, washed gently with water to remove any unadsorbed dye and dried. Then it was suspended in eluting solvents (0.1 M NaOH used for desorption of CR (Roy et al. 2013), and 0.1 M HCl used for desorption of MB (Liu et al. 2012) was agitated (120–140 rpm) at 303 K for 3 h. The desorbed dye concentration was quantified using a UV/visible spectrophotometer; the desorption was creased until the concentration of desorbed dye had no change. Then the regenerated AP-cotton was reused for the removal of dyes.

Results and discussion

Characterization of AP-cotton

The synthesis route and adsorption efficiency of AP-cotton are shown in Scheme 1. The main component of cotton fiber is cellulose, which accounts for over 94 % in ripe cotton. The other components were wax and ash, which were removed after treatment with NaOH solution. Then the obtained activated cotton (AC) was amphiprotically modified with CTA and AMPS respectively to improve the hydrophilicity and adsorption activity of the fibers. As a result, the AP-cotton product could adsorb both CR (anionic) and MB (cationic) as a versatile adsorbent, and its adsorption capacity for dyes and metal ions was obviously enhanced compared to raw cotton (RC).

SEM study of AP-cotton

Significant changes in the surface morphology of cotton fibers after amphiprotic modification with CTA and AMPS respectively were revealed by SEM (Fig. 1). The surfaces of raw cotton (Fig. 1a) and activated cotton (Fig. 1b) were smooth with few wrinkles. However, after the treatment of CTA, the surface morphology of CTA-cotton was observed to be wrinkled and porous (Fig. 1c, d). The enlarged image (Fig. 1d) shows the average pore size was 10–30 nm. With further treatment by AMPS, the porous nature remained without obvious change in the pore size, but some tablet shapes were observed on the surface of AP-cotton (Fig. 1e, f), which might be coated with AMPS molecules.

Fig. 1
figure 1

Surface morphology (SEM images) of raw cotton (a), activated cotton (b), CTA-cotton (c, d) and AP-cotton (e, f)

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FTIR analysis of AP-cotton

As shown in Fig. 2, the bands at 1,649.9 and 1,552.6 cm−1 in the spectrum of AP-cotton were attributed to the amide I (νC=O) and amide II (δN–H), respectively; their appearance confirms that the amide group was introduced onto the AP-cotton after modification with CTA and AMPS. The weakening and even disappearing of the band at 1,430.4 cm−1 with the process of modification from raw cotton to AP-cotton revealed the reduction in inter- or intramolecular hydrogen bonding between the hydroxyl groups of cellulose. This may mean that the crystallinity of the cotton fibers was reduced after amphiprotic modification. In addition, the unchanged characteristic absorption bands of cellulose at about 3,407, 2,903 and 1,060 cm−1 suggest that the main composition of the cotton fibers remained after chemical modification (Saad et al. 2010).

Fig. 2
figure 2

FT-IR spectra of raw cotton, CTA-cotton and AP-cotton

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XRD analysis of AP-cotton

The XRD pattern of AP-cotton is shown in Fig. 3 in order to further determine the change in crystallinity of cotton fibers after modification. The characteristic peaks of raw cotton were observed at 22.78 and 14.85 with relative intensities at 8,661 and 3,917 respectively. After cationic modification, the related characteristics peaks of the resultant CTA-cotton decreased obviously. More notably, the characteristic peak intensity of AP-cotton that resulted from the amphiprotic modification was found at 20.46 with relative intensities of 2,015, which decreased more significantly. This suggested that the incorporation of CTA and AMPS led to serious impairment of the fiber crystallinity. Therefore, the resultant AP-cotton developed a random microscopic structure with modification, which is consistent with the result of Fig. 2 and will be useful for the adsorption.

Fig. 3
figure 3

X-ray diffraction patterm of raw cotton, CTA-cotton and AP-cotton

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XPS characterization of AP-cotton

The chemical state of the elements in AP-cotton was further investigated by XPS (Fig. 4). The wide scan XPS spectrum (Fig. 4a) of raw cotton, CTA-cotton and AP-cotton showed photoelectron lines at binding energies of about 168, 285, 400 and 530 eV attributed to S2p, C1s, N1s and O1s, respectively. The high-resolution C1s peaks of raw cotton, CTA-cotton and AP-cotton are displayed in Fig. 4b–d. The C1s peak of raw cotton can be deconvoluted into two components (Fig. 4b): (1) the nonoxygenated C (284.6 eV); (2) the carbon in C–O (286.5 eV) (Wang et al. 2010; Zhao et al. 2010a). The deconvoluted C1s spectra of CTA-cotton contains three components (Fig. 4c): the nonoxygenated ring C (284.6 eV), C–N species (285.7 eV) and C–O species (286.5 eV) (Park et al. 2009), the C–N species coming from the cationic monomer CTA. The deconvoluted C1s spectra of the AP-cotton composite show four peaks at 284.6, 285.7, 286.5 and 287.8 eV, and they correspond to the C–C, C–N, C–O and C=O bonds, respectively (Fig. 4d) (Bai et al. 2009; Park et al. 2008). Apparently, the nitrogen and sulfur element existed in the AP-cotton because of the chemical interaction, which demonstrated that both the cationic monomer CTA and anionic monomer AMPS were introduced into the fibers successfully, and the final product was really AP-cotton.

Fig. 4
figure 4

XPS spectra of raw cotton, CTA-cotton and AP-cotton: a wide scan, b C1s spectra of raw cotton, c C1s spectra of CTA-cotton and d C1s spectra of AP-cotton

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Adsorption study of AP-cotton for dyes and metal ions

Effect of pH values on adsorption

The initial pH of the dye solution plays a significant role in the sorption process of dyes and metal ions by AP-cotton. The variation of the adsorption capacity for the adsorbent at different pH values can be attributed to the surface chemical character of both the adsorbent and adsorbate, specifically the surface charge of the adsorbent in aqueous solution at a certain pH. The effect of pH on the adsorption efficiency of AP-cotton toward CR, MB, Cu2+ and Pb2+ ions is presented in Fig. 5b. The CR removal increased as the initial pH of the solution increased from 2 to 4, but decreased drastically as the pH further increased from 4 to 13 (Fig. 5b). The adsorption capacity of the adsorbent is affected by the change of solution pH due to the protonation and deprotonation of the active functional groups of the adsorbent and adsorbate (Vieira et al. 2009). CR has a pKa value at 4.5–5.5, which can exist in both anionic form at basic pH and in cationic form at acidic pH (Purkait et al. 2007). Lower adsorption at pH 2 and pH 3 is due to the interionic repulsion between the positively charged CR molecule and adsorbent. At pH 4, the nitrogen atoms and sulfonate groups of the CR molecules become protonated (Aldegs et al. 2008), whereas at pH > 3.8 the surface charge of the adsorbent becomes negative because the pH point of zero net charge of AP-cotton is 3.8 (Fig. 5a). This results in an electrostatic attraction between the positively charged CR molecule and negatively charged adsorbent and leads to the maximum adsorption capacity of CR (around 241 mg/g) by AP-cotton at pH 4.0. On the contrary, as pH increases from 4 to 13, the negative surface charge of AP-cotton hinders the adsorption of CR by electrostatic repulsion between deprotonated CR molecules and negatively charged AP-cotton; consequently, the dye removal capacity decreases to 82.7 mg/g. However, it was clear that the adsorption capacity of AP-cotton for MB remarkably increased from 56 to 134 mg/g as the initial pH of MB solution increased from 2 to 13 (Fig. 5b). The surface of MB is always positively charged at acidic or basic pH. So lower adsorption before pH 3 is also because of the interionic repulsion between the positively charged MB molecules and AP-cotton, and the MB removal increased obviously as the pH increased from 4 to 13, attributed to the electrostatic attraction between positively charged MB molecules and negatively charged AP-cotton (Al-Ghouti et al. 2010). In addition, the effect of pH for the removal of Cu2+ and Pb2+ was similar to the tendency of MB removal (Fig. 5b), which was due to the same adsorption mechanism. The above evidence shows that the versatile AP-cotton can cope with both anionic and cationic molecules in their respective pH environment. Of course, we should note that a considerable amount of adsorption by AP-cotton indicates that the electrostatic mechanism is not the only mechanism for dye and metal ion adsorption; the physical forces are also responsible for adsorption (Chatterjee et al. 2007).

Fig. 5
figure 5

a The pH point of zero net charge of AP-cotton; b effect of pH values on the adsorption capacity of AP-cotton

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Adsorption kinetics

The adsorption kinetic study is a very important feature of water treatment as it depicts the adsorbate uptake, which in turn controls the residence time of adsorbate uptake at the solid-solution interface (Liu et al. 2012). AP-cotton allows the integrated adsorption process for the dyes and metal ions within 12 h. As shown in Fig. 6a, the adsorption equilibrium was reached within 4 h for CR and MB, 2 h for Cu2+, and 3 h for Pb2+, respectively. However, we should note that more than 50 % of each adsorbate was adsorbed by the AP-cotton within 1 h, whose fast adsorption was due to the electrostatic attraction between the adsorbate and adsorbent, and a considerable amount of adsorption before equilibrium may be attributed to other chemical interactions or physical forces, such as the hydrogen bond, complexation between the adsorbate and the active groups of AP-cotton or its porous, and the needed interaction time may be the result of the slower adsorption process of the dyes and metal ions by AP-cotton.

Fig. 6
figure 6

a Adsorption behavior of AP-cotton for CR, MB, Cu2+ and Pb2+, b pseudo-first-order kinetic plots of AP-cotton and c pseudo-second-order kinetic plots of AP-cotton

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Adsorption kinetic data were analyzed using pseudo-first-order (2) and pseudo-second-order (3) kinetic equations, which are given as (Cherifi et al. 2013; Roy et al. 2013):

$$\ln \left( {Q_{{ 1 {\text{e}}}} - Q_{t} } \right) = \ln Q_{{ 1 {\text{e}}}} - k_{1} t$$
(2)
$$\frac{t}{{Q_{t} }} = \frac{1}{{k_{2} Q_{{ 2 {\text{e}}}}^{2} }} + \frac{t}{{Q_{{ 2 {\text{e}}}} }}$$
(3)

where Q 1e or Q 2e represents the calculated values of the adsorption capacity of the AP-cotton at equilibrium (mg/g), Q t (mg/g) is the adsorption amount at time t (min), and k 1 (min−1) and k 2 [g/(mg min)] are the rate constants of pseudo-first-order and pseudo-second-order kinetics equations, respectively.

The corresponding parameters calculated and experimental adsorption capacities are obtained and tabulated in Table 2. It can be seen that the correlation coefficients from pseudo-second-order kinetics (Fig. 6c) were higher than those from pseudo-first-order kinetics (Fig. 6b), and the theoretical adsorption capacities (Q 2e) calculated from pseudo-second-order kinetic equation were close to the experimental adsorption capacities (Q exp), demonstrating pseudo-second-order kinetics could reasonably describe the adsorption process (Tang et al. 2012; Zhang 2011).

Table 2 Kinetic parameters and experimental adsorption capacities for the removal of CR, MB, Cu2+ and Pb2+ by AP-cotton
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Adsorption isotherm

As shown in Fig. 7a, the adsorption amount at equilibrium for each adsorbate increases dramatically with increasing initial concentration at first and then tends to level off.

Fig. 7
figure 7

a Influence of initial concentrations on the absorption capacity of AP-cotton, b Langmuir adsorption isotherm plots of AP-cotton and c Freundlich adsorption isotherm plots of AP-cotton

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Adsorption isotherms are used to describe how the adsorbate interacts with adsorbent, and the constants of the isotherm models are useful to determine the amount of adsorbent needed to adsorb a required amount of adsorbate (Tian et al. 2011). In this study, the Langmuir and Freundlich adsorption isotherms were constructed using the adsorption equilibrium data of CR, MB, Cu2+ and Pb2+ by the AP-cotton at initial adsorbate concentrations of 25–500 mg/l (Table 1). The Langmuir isotherm model presumes that the adsorption is limited to the formation of monolayer coverage of the adsorbate on the homogeneous adsorbent surface. The linearized Langmuir equation is given below (Ahmad 2009):

$$\frac{1}{{Q_{\text{e}} }} = \left( {\frac{1}{{bQ_{ \hbox{max} } }}} \right)\frac{1}{{C_{\text{e}} }} + \frac{1}{{Q_{ \hbox{max} } }}$$
(4)

where C e (mg/l) is the concentration at equilibrium, Q e (mg/g) is the adsorption amount at equilibrium, Q max is the maximum monolayer capacity of adsorbent (mg/g), and b is the Langmuir constant that represents the energy of the adsorption process (l/mg); both were computed from the slopes and intercepts of the straight line (Fig. 7b). In the Langmuir model, a dimensionless parameter called separation factor R L, which can indicate whether the adsorption process is favorable, is defined as follows:

$$R_{\text{L}} = \frac{1}{{1 + bC_{0} }}$$
(5)

As show in Table 3, the Q max of AP-cotton was 175.1 mg/g for CR, 113.1 mg/g for MB, 88.9 mg/g for Cu2+ and 70.6 mg/g for Pb2+. Having high correlation coefficient (R 2 > 0.99), the Langmuir constant b > 0 and the values of the separation factor R L are between 0 and 1, indicating that the Langmuir isotherm model described the adsorption process of AP-cotton for each dye and ions favorably. In relation to the majority of the adsorbents derived from natural materials in the literature, AP-cotton shows good performance compared with others (Table 4). The good adsorption capacity of AP-cotton first benefits from the hollow, flat-banded structure and excellent hydrophilicity of raw cotton, and the all-round adsorption capacity for various adsorbates is due to the amphiprotic modification of cotton fibers, which is a promising candidate modification method for the fabrication of natural material-based adsorbents for the removal of dyes or metal ions from waste water.

Table 3 Langmuir and Freundlich parameters for adsorption isotherms of AP-cotton
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Table 4 The Langmuir maximum capacity of various adsorbents for dyes or metal ions
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The Freundlich model is based on the assumption that multilayer adsorption occurs on a heterogeneous adsorption surface containing unequally available sites of different adsorption energies and is given by the relation (Hu et al. 2010):

$$\ln Q_{\text{e}} = \ln K_{\text{F}} + \frac{1}{n}\ln C_{\text{e}}$$
(6)

where K F is the Freundlich constant (mg/g), and n is the heterogeneity factor. The graphical presentation of the Freundlich isotherm model is expressed in Fig. 7c. The model parameters reflected that this isotherm model showed poorer fit to the experimental data as compared to the Langmuir isotherm under the studied initial concentration range of each adsorbate (Table 3).

Regeneration of AP-cotton

After six repititions of desorption and re-adsorption loop (Fig. 8), a small decrease was observed in the adsorption capacity for Cu2+ and Pb2+, and a slightly more obvious decrease in the AP-cotton adsorption capacity for CR and MB was observed. Even so, the adsorption capacity retained for each adsorbate was more than 90 % after six regenerations.

Fig. 8
figure 8

Adsorption capacity of the regenerated AP-cotton after repeated recycling

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Desorption studies of a spent adsorbent can provide important insight into the overall adsorption mechanism. Desorption of the adsorbate by water suggests the involvement of weak physisorption, whereas desorption by organic acids implies that the attachment of the adsorbate to the adsorbent is through chemisorption. If the adsorbate desorption occurs upon use of strong mineral acids or bases, the adsorption is due to an electrostatic interaction (Roy et al. 2013). All desorptions in this article were finished by 0.1–1 M HCl solution or 0.1 M NaOH aqueous solution, and the electrostatic interaction and chemisorption may be the main interactions between the adsorbate and adsorbent. This is consistent with the result from the effect of pH values on adsorption.

Conclusion

Cotton fiber, the most abundant renewable fiber in nature, and having excellent hydrophilicity for its hydroxyl groups, was chemically modified with the cationic monomer CTA and anionic monomer AMPS, respectively. The resultant amphiprotic adsorbent (AP-cotton) can cope with both anionic and cationic pollutants in aqueous solution. It could reach adsorption equilibrium within 4 h for CR and MB, 2 h for Cu2+ and 3 h for Pb2+, respectively. The Langmuir maximum adsorption capacities were 175.1 mg/g for CR, 113.1 mg/g for MB, 88.9 mg/g for Cu2+ and 70.6 mg/g for Pb2+, respectively, and its adsorption capacities retained for each adsorbate were more than 90 % after six regenerations. Due to the hollow, flat-banded structure and amphiprotic characteristic, AP-cotton exhibited an all-round adsorption capacity and regeneration potential for various adsorbates. All the results demonstrate this AP-cotton is a versatile, economical and environmentally friendly adsorbent, which is a promising candidate for practical application in the treatment of wastewater contaminated with dyes or heavy metal ions.