Facile Functionalization of Natural Peach Gum Polysaccharide with Multiple Amine Groups for Highly Efficient Removal of Toxic Hexavalent Chromium (Cr(VI)) Ions from Water.

The development of low-cost adsorbent with excellent adsorption property remains a big challenge. Herein, the functionalization of natural peach gum polysaccharide (PGP) with multiple amine groups for the removal of toxic Cr(VI) ions from water was studied. The obtained PGP-NH2 gel exhibited high-removal efficiency (>99.5%) toward Cr(VI) ions, especially with relatively low initial concentration of Cr(VI) ions (≤250 mg/L). The influences of pH, ionic strength, contact time, initial concentration, and temperature on the adsorption of Cr(VI) ions were systematically investigated. The PGP-NH2 gel showed rapid adsorption rate and could reach adsorption equilibrium within about 40 min. The Cr(VI) ion uptake process could be described by pseudo-second-order kinetic and Langmuir isotherm models. The maximum adsorption capacity of PGP-NH2 gel could reach 188.32 mg/g. Thermodynamic investigation results indicated the spontaneous and exothermic characteristic of the uptake process. Moreover, the PGP-NH2 gel also exhibited favorable reusability, and 135.52 mg/g of adsorption capacity was retained even after being reused for five times. Considering its low cost and superior uptake property, the PGP-NH2 gel holds a great promise for employing as an adsorbent to treat Cr(VI) ion-containing wastewater.

Introduction

In the past decades, water pollution problem caused by the discharge of heavy-metal-ion-containing wastewater from various industries such as electroplating, leather tanning, petroleum refining, battery manufacturing, pigment, and plastic production has raised worldwide concerns.[1−4] Because the heavy metal ions can be accumulated via food chain and may cause carcinogenic and toxic effects on human healthy and ecological system, it is essential to eliminate toxic heavy metal ions from water before discharging into aquatic systems. Among various heavy metal ions, hexavalent chromium (Cr(VI)) ion is considered to be one of the most toxic heavy metal ions.[5−7] Excessive intake of Cr(VI) ions can cause serious diseases such as lung cancer, kidney damage, nervous system failure, pulmonary congestion, dermatitis, and kidney circulation. The U.S. Environmental Protection Agency (US-EPA) reported that the residual concentration of Cr(VI) ions for drinking water regulation limit is 50 μg/L.[8] Nowadays, various techniques such as ion exchange, chemical precipitation, membrane filtration, and electrodialysis techniques have been explored to remove Cr(VI) ions from water.[9−12] However, most of them suffer from the drawbacks of high cost and low removal efficiency. In contrast, the adsorption technique received special attention because of its attractive merits including low cost, ease of operation, high efficiency, insensitive to toxic pollutants, and without secondary pollution.[13−16] On the other hand, the recycle of chromium from wastewater also received intensive attention from the view of economic reason. In this regard, the chromium resource can be well-recycled from wastewater by the adsorption method.

Although a myriad of materials such as activated carbon,[17] montmorillonite,[18] and polymer gel[19] have been developed as adsorbents to adsorb Cr(VI) ions from water, much attention was paid on the use of renewable biomass such as chitosan,[20] husk biomass,[21] and fungal biomass[22] as an adsorbent to remove Cr(VI) ions from water because of its favorable biocompatibility, low cost, facile availability, and sustainable characteristic. Nevertheless, most of the reported biomass-based adsorbents showed low adsorption capacity (≤100 mg/g), slow adsorption rate, and poor reusability, which significantly limited their practical applications.[23]

Natural peach gum polysaccharide (PGP), secreted from the fruit and trunk of peach trees as a result of mechanical injury or microbial invasion, is a kind of heteropolysaccharide that generally consists of galactose, xylose, arabinose, uronic acid, and mannose.[24−26] Because peach trees are widely distributed in the world, the global PGP resource is abundant. Nevertheless, crude peach gum showed poor solubility in water because of its very high molecular weight and highly branched macromolecular structure.[26] Water-soluble PGP can be prepared by the hydrolysis treatment of the crude peach gum.[25−27] Until now, the PGP has been studied for applications in medicine, food industry, fluorescent materials, dye, and heavy metal ion adsorbents.[28−34] To our knowledge, the utilization of PGP as an adsorbent to remove Cr(VI) ions from aqueous solution has not been reported. So far, the development of biomass-based adsorbents that simultaneously possess rapid adsorption rate, superior adsorption capacity, and favorable reusability remains a big challenge.

To address this challenge, this study aims to fabricate high-performance Cr(VI) ion adsorbent from renewable PGP. Because amine groups have been demonstrated to show high binding capability toward Cr(VI) ions,[35−39] the synthesis of PGP-NH2 gel with multiple amine groups was performed. The employment of PGP-NH2 gel as an adsorbent to capture Cr(VI) ions from water was studied in detail. Various influence factors including solution pH, ionic strength, contact time, initial concentration of Cr(VI) ions, and temperature on the adsorption process were systematically evaluated. Moreover, the desorption and regeneration capability of PGP-NH2 gel was also examined. It is expected that the prepared PGP-NH2 gel could exhibit superior adsorption performance toward Cr(VI) ions.

Results and Discussion

Preparation and Characterization of PGP-NH2 Gel Adsorbent

Scheme shows the synthetic route to the PGP-NH2 gel. Water-soluble PGP with branched macromolecular structure and numerous hydroxyl groups can be easily prepared by hydrolysis of crude peach gum in acidic solution.[27,34] Considering amine groups could show strong interaction with Cr(VI) ions,[35−37] we think that the PGP gel with multiple amine groups should be suitable for the removal of Cr(VI) ions. On one hand, the presence of amine groups can enhance the adsorption capacity of PGP toward Cr(VI) ions. On the other hand, the PGP in the form of gel can be easily separated from water after adsorption. Benefiting from the numerous hydroxyl groups of PGP, the PGP-NH2 gel with multiple amine groups can be readily achieved by first reaction with epichlorohydrin to introduce epoxy groups and followed by reaction with diethylenetriamine (DEA) to cross-link the PGP macromolecular chains (Scheme ). It is worth noting that the prepared PGP-NH2 gel cannot be dissolved in water and can be easily separated from water by filtration treatment.

Scheme 1

Schematic Illustration of the Synthesis of PGP-NH2 Gel for the Removal of Cr(VI) Ions from Water

The photographs of crude peach gum, PGP, and PGP-NH2 gel are presented in Figure . As can be seen, white PGP powder with excellent water solubility can be prepared by hydrolysis treatment of brown crude peach gum. After the cross-linking reaction, PGP-NH2 gel with yellow color was obtained. X-ray photoelectron spectroscopy (XPS) spectrum of PGP shows that PGP is mainly composed of C and O elements, and no obvious N element is observed (Figure a). After introduction of amine groups, strong N 1s peak can be clearly observed in the XPS spectrum of PGP-NH2 gel. According to the XPS result, the content of N element associated with the content of amine groups is determined to be 10.86 mmol/g, which is close to the element analysis result (11.30 mmol/g) (see Table S1 in the Supporting Information). Such a high content of amine groups provides a favorable platform for the removal of Cr(VI) ions. Compared with the Fourier transform infrared (FTIR) spectrum of PGP (Figure b), the absorption bands centered at 3414 and 1041 cm–1 associated with the O–H and C–O vibrations, respectively, decreased significantly and a new absorption peak at 3209 cm–1 corresponding to the N–H vibration appeared for the PGP-NH2 sample. These results are due to the hydroxyl groups of PGP that have been reacted with epichlorohydrin and DEA to form amine groups.

Figure 1

Photographs of the crude peach gum (marked by dash line) (a), PGP (b), and PGP-NH2 gel (c).

Figure 2

XPS survey (a) and FTIR (b) spectra of PGP and PGP-NH2 samples. (c) Zeta potential values of PGP and PGP-NH2 in water as a function of pH values. (d) Swelling ratio of PGP-NH2 gel in water with increasing immersion time. SEM image (e) and N2 adsorption–desorption curve (f) of the freeze-dried PGP-NH2 sample.

In addition, the zeta potential value of PGP and PGP-NH2 samples was determined in the pH range of 2–9 (Figure c). Obviously, the PGP exhibited negatively charged characteristic in the whole pH range because of the presence of numerous oxygen-containing functional groups (e.g., −OH and −COOH).[27,34] In contrast, the PGP-NH2 sample showed positively charged value in the whole pH range, and the zeta potential value increased with the decrease of solution pH (e.g., 60.34 and 20.67 mV at pH 2 and 9, respectively). The positively charged surface of PGP-NH2 sample is attributed to the introduction of multiple amine groups. With decreasing pH, the amine groups are protonated, leading to a large zeta potential value. On the basis of XPS, FTIR, and zeta potential results, it is concluded that PGP-NH2 gel with numerous amine groups was successfully prepared. Considering adsorption applications, the swelling behavior of PGP-NH2 gel in aqueous solution was also investigated. As shown in Figure d, the swelling process of PGP-NH2 gel could reach equilibrium within about 5 min with the swelling ratio of 823%. The morphology of the PGP-NH2 gel after freeze-drying was observed by scanning electron microscopy (SEM) (Figure e). As can be seen, the PGP-NH2 sample possesses porous microstructure, which is constructed by the aggregation of cross-linked particles. It should be pointed out that the cross-linked particles possess smooth surface (see Figure S1 in the Supporting Information). In addition, the surface area of PGP-NH2 sample was determined to be about 2.38 m2/g based on the N2 adsorption–desorption curve, as shown in Figure f. It is expected that the porous microstructure associated with a relatively high surface area can offer a favorable platform for the following adsorption application.

Removal Efficiency Study

Before systematically investigating the adsorption performance of PGP-NH2 gel toward Cr(VI) ions, a simple adsorption test was performed by adding 50 mg of freeze-dried PGP-NH2 gel into 5 mL of aqueous solution of Cr(VI) ions (600 mg/L). As depicted in the inset of Figure a, the Cr(VI) ion solution with yellow color became colorless after adding PGP-NH2 gel for only 30 min, suggesting that the PGP-NH2 gel possesses high removal efficiency toward Cr(VI) ions. Then, the removal efficiency of PGP-NH2 gel was determined in detail by adding 50 mg of PGP-NH2 gel into 10 mL of Cr(VI) ions solution with an initial concentration from 50 to 1000 mg/L at pH 2.0 and room temperature. The PGP-NH2 gel exhibited very high removal efficiency (>99.95%) when the concentration of Cr(VI) ions was lower than 250 mg/L (Figure a). This is because the number of binding sites on the surface of PGP-NH2 gel is large enough to capture almost all of the Cr(VI) ions in water. Particularly, when the initial concentration of Cr(VI) ions was lower than 100 mg/L, the residual concentration of Cr(VI) ions in water after adsorption was lower than 50 μg/L, which could meet the standard of drinking water proposed by US-EPA. At a high initial concentration of Cr(VI) ions, the binding sites of PGP-NH2 gel cannot capture all of the Cr(VI) ions and thus the removal efficiency of PGP-NH2 gel decreased obviously. For example, the removal efficiency of PGP-NH2 gel dropped from 99.7 to 89.5% with increasing the initial concentration of Cr(VI) ions from 250 to 1000 mg/L.

Figure 3

(a) Removal efficiency of PGP-NH2 gel with diverse initial concentrations of Cr(VI) ions. Insets: Photographs of aqueous solution of Cr(VI) ions (600 mg/L, 5 mL) before and after adding 50 mg of PGP-NH2 gel for 30 min. (b) Adsorption capacities of PGP and amine-functionalized PGP gels toward Cr(VI) ions (C0 = 1000 mg/L).

To probe the influence of amine group content on the adsorption of Cr(VI) ions, PGP gels with a diverse content of amine groups were prepared by adjusting the weight feed ratio of DEA to PGP (fwt) (see Table S1 in the Supporting Information). In addition, the use of pure PGP for the removal of Cr(VI) ions was also studied. As depicted in Figure b, the pure PGP showed low adsorption capacity (20.6 mg/g) as compared with the amine-functionalized PGP gel (e.g., 173.3 mg/g for PGP-NH2 sample). This is possibly because of the negatively charged characteristic of PGP and the absence of effective functional groups to capture Cr(VI) ions. On the other hand, the adsorption capacity of amine-functionalized PGP gel enhanced obviously with the increase of the fwt value which is associated with the content of amine groups. This result demonstrated that the high adsorption capacity of amine-functionalized PGP gel toward Cr(VI) ions is attributed to the introduction of amine groups. Considering the cost and adsorption capacity together, the optimal fwt value was found to be 1.39 corresponding to the sample of PGP-NH2 gel.

In addition, the morphology of PGP-NH2 gel after the adsorption of Cr(VI) ions was characterized by SEM, as shown in Figure . The presence of the N element further confirms the successful introduction of amine groups. The strong Cr element signal suggests that the Cr(VI) ions were successfully adsorbed by the PGP-NH2 gel (Figure e). The corresponding energy-dispersive X-ray (EDX) analysis spectrum in Figure f further demonstrated that the PGP-NH2 gel possess excellent adsorption capability toward Cr(VI) ions. The content of Cr(VI) ions was calculated to be 168.56 mg/g according to the EDX result, which is well in accordance with the result measured by a UV–vis spectrophotometer (176.6 mg/g).

Figure 4

(a) SEM image and the corresponding C (b), O (c), N (d), and Cr (e) mapping images of PGP-NH2 gel after adsorption of Cr(VI) ions. (f) EDX spectrum of Cr(VI) ion-adsorbed PGP-NH2 gel shown in panel (a).

Effects of Solution pH and Ionic Strength

The solution pH is a very crucial parameter that can affect the removal performance of the adsorbent toward Cr(VI) ions.[39−41] On one hand, the variation of solution pH may cause the change of protonation degree of the amine groups associated with the surface charge of PGP-NH2 gel adsorbent. On the other hand, the change of solution pH can also influence the forms of Cr(VI) ions in water. It was reported that the Cr(VI) ions are mainly in the form of Cr2O72–, HCrO4–, and CrO42– ions in water depending on the solution pH (see eqs and 2).[8] At very acidic condition (e.g., pH in the range of 2–6), HCrO4– and Cr2O72– ions are the predominant forms. When the pH was higher than 6, the main form becomes CrO42– ions.

Herein, the influence of solution pH on the adsorption of Cr(VI) ions by PGP-NH2 gel was investigated at room temperature. As presented in Figure a, the adsorption capacity of PGP-NH2 gel gradually enhanced with the decline of solution pH. For example, the adsorption capacities of PGP-NH2 gel at pH of 2.0 and 9.0 were 173.6 and 52.8 mg/g, respectively. Other reported adsorbents with a positively charged surface showed a similar trend for the adsorption capacity with the increase of the solution pH.[40−42] This is because the amine groups can endow the PGP-NH2 gel with much more positively charged binding sites at low pH. The electrostatic attraction between the Cr(VI) ions (e.g., HCrO4– and Cr2O72– ions) and the amine groups of PGP-NH2 gel enhanced, leading to a significant enhancement of the adsorption capacity. With increasing solution pH (e.g., >6), the surface of the PGP-NH2 gel becomes less positively charged. In addition, increasing the concentration of OH– ions can cause competitive adsorption with Cr(VI) ions (e.g., CrO42– ions). On the basis of the above results, we can conclude that the adsorption of Cr(VI) ions by PGP-NH2 gel is mainly ascribed to the electrostatic attraction between the negatively charged Cr(VI) ions and the positively charged amine groups.

Figure 5

Effects of solution pH (a) and NaCl concentration (b) on the removal of Cr(VI) ions by PGP-NH2 gel (C0 = 1000 mg/L). The contact time is 2 h.

Considering that the industrial effluents generally contain various salts, the effect of ionic strength on the adsorption of Cr(VI) ions by PGP-NH2 gel was also studied. It was reported that high ionic strength could affect the adsorption process because the salt ions could compete with Cr(VI) ions.[43] Herein, NaCl was chosen as a representative salt. Obviously, the adsorption capacity of PGP-NH2 gel gradually declined from 173.6 to 132.4 mg/g with raising the concentration of NaCl from 0 to 1.0 M (Figure b). The adverse effect of ionic strength on the uptake of Cr(VI) ions is possibly attributed to the presence of ion exchange during the adsorption process. With the enhancement of ion strength, both the number of binding sites and the active Cr(VI) ions decreased. Under this circumstance, the electrostatic attraction between the negatively charged Cr(VI) ions and the positively charged amine groups becomes much weak, and thus inhibited the uptake of Cr(VI) ions by PGP-NH2 gel. Nevertheless, the adsorption capacity of the PGP-NH2 gel at high ionic strength remains high enough for using as an adsorbent to remove Cr(VI) ions from wastewater.

Adsorption Kinetics and Isotherms

For practical adsorption application, the adsorption rate is a crucial factor. In this study, the effect of contact time on the removal of Cr(VI) ions by PGP-NH2 gel was evaluated with the initial Cr(VI) ion concentration of 1000 mg/L at room temperature and at pH 2.0. The PGP-NH2 gel showed fast adsorption rate toward Cr(VI) ions in the first 20 min, and the adsorption process gradually reached equilibrium within about 40 min (Figure a). Such a high adsorption rate is possibly ascribed to the presence of strong electrostatic attraction between the negatively charged Cr(VI) ions and the positively charged PGP-NH2 gel. To gain insights into the adsorption mechanism, three well-known kinetic models including pseudo-first-order, pseudo-second-order, and intraparticle diffusion kinetic models were chosen to analyze the adsorption data (Figures a and S2 in the Supporting Information).[44−47] The kinetic parameters were calculated based on the slope and intercept values of the corresponding fitting plot. As presented in Table , the pseudo-second-order kinetic model exhibited the highest correlated coefficient value (R2 = 0.9971) as compared with that of pseudo-first-order (R2 = 0.9411) and intraparticle diffusion (R2 = 0.7288) kinetic models. Meanwhile, the calculated Qe value (182.48 mg/g) according to the pseudo-second-order kinetic model was close to the experimental Qe value (173.6 mg/g). All of these fitting results indicate that the pseudo-second-order kinetic model can be employed to describe the adsorption process. In addition, the fitting plot according to the intraparticle diffusion model does not pass the origin, indicating that the intraparticle diffusion was not the rate-controlling factor.[47] However, the fitting plot of the intraparticle diffusion kinetic model exhibits two obvious linear ranges (Figure S2c in the Supporting Information), which were caused by the surface adsorption and intraparticle diffusion, respectively.

Figure 6

(a) Effect of contact time on the adsorption of Cr(VI) ions onto the PGP-NH2 gel and the correlation of diverse kinetic models (C0 = 1000 mg/L, pH = 2.0). (b) Effect of initial concentration of Cr(VI) ions on the adsorption capacity of PGP-NH2 gel and the correlation of diverse isotherm models (pH = 2.0).

Table 1

Kinetic Parameters for the Adsorption of Cr(VI) Ions by PGP-NH2 Gel

pseudo-first-order model ln(Qe – Qt) = ln Qt – k1t			pseudo-second-order model t/Qt = 1/k2Qe2 + t/Qe			intraparticle diffusion model Qt = kit0.5 + C
k1 (min–1)	Qe (mg/g)	R2	k2 (mg–1 min–1)	Qe (mg/g)	R2	ki	C	R2
0.0634	89.76	0.9411	0.0016	182.48	0.9971	14.84	55.15	0.7288

To further probe the interaction between the PGP-NH2 gel and the Cr(VI) ions, the effect of initial concentration of Cr(VI) ions on the adsorption performance of PGP-NH2 gel was studied. As shown in Figure b, the PGP-NH2 gel exhibited high adsorption capacity at higher concentration of Cr(VI) ions. This is attributed to the factor that the higher concentration of Cr(VI) ions can offer stronger driving force to overcome the mass transfer resistance of Cr(VI) ions from water to the adsorbent phase. Because the adsorption isotherm can provide useful information on the adsorption mechanism and the adsorption properties of the adsorbent, three famous adsorption isotherm models including Langmuir, Freundlich, and Temkin isotherm models were employed to fit the adsorption data (Figures b and S3 in the Supporting Information).[48−51]

Table presents the values of isotherm parameters obtained from the fitting results. Obviously, the Langmuir isotherm was the best one to fit the adsorption data based on its high R2 value (0.9986) as compared with that of Freundlich isotherm (R2 = 0.8894) and Temkin isotherm (R2 = 0.9677). This result suggests that a monomolecular layer was formed on the surface of PGP-NH2 gel, and the surface binding sites were homogeneous with identical adsorption energy. On the basis of the fitting result of Langmuir isotherm model, the maximum adsorption capacity (Qm) of the PGP-NH2 gel toward Cr(VI) ions was calculated to be 188.32 mg/g, which is much higher than many reported biomass-based adsorbents, such as amino starch (12.12 mg/g),[14] guar gum–ZnO nanocomposite (55.56 mg/g),[16] fungal biomass (119.2 mg/g),[22] chitosan-based hydrogel (73.14 mg/g),[41] and chitosan–Fe(III) complex (173.1 mg/g)[52] (see Table ).

Table 2

Isotherm Parameters for the Adsorption of Cr(VI) Ions onto PGP-NH2 Gel Adsorbent

isotherm model	parameters
Langmuir: Ce/Qe = Ce/Qm + 1/QmKL	Qm (mg/g)	188.32
	KL (L/mg)	0.3488
	R2	0.9986
Freundlich: ln Qe = ln KF + bF ln Ce	KF (mg/g)	59.9086
	bF	0.2416
	R2	0.8894
Temkin: Qe = RT ln at/bt + RT ln Ce/bt	at	19.0823
	bt (J/mol)	103.1865
	R2	0.9677

Table 3

Comparison of the Maximum Monolayer Adsorption of Cr(VI) Ions on Various Biomass-Based Adsorbents

adsorbent	Qm (mg/g)	references
amino starch	12.12	(14)
cyanobacterium biomass	22.92	(15)
guar gum–ZnO nanocomposite	55.56	(16)
quaternary chitosan salt	68.3	(20)
bengal gram husk	91.64	(21)
fungal biomass	119.2	(22)
polyamide-modified corncob	131.6	(38)
biofunctionalized chitosan	182.0	(39)
chitosan-based hydrogel	73.14	(40)
surfactant-modified coconut coir pith	76.3	(41)
chitosan–Fe(III) complex	173.1	(51)
PGP-NH2 gel	188.32	this study

Adsorption Thermodynamics

The effect of temperature on the adsorption of Cr(VI) ions by the PGP-NH2 gel was also studied at 279, 293, and 313 K. As shown in Figure a, the adsorption capacity of PGP-NH2 gel toward Cr(VI) ions enhanced with the raising environmental temperature, suggesting the endothermic characteristic of the adsorption process. To gain more information on the thermodynamic parameters, van’t Hoff equation was employed to analyze the adsorption data as follows[53,54]where Qe (mg/g) is the amount of Cr(VI) ions adsorbed by PGP-NH2 gel, Ce (mg/L) is the residual concentration of Cr(VI) ions in water, ΔS (J mol–1 K–1) is change of entropy, R (8.314 J mol–1 K–1) is the ideal gas constant, ΔH (kJ mol–1) is the change of enthalpy, and T (K) is the Kelvin temperature. To probe the spontaneous property of this adsorption process, the variation of Gibbs free energy (ΔG, kJ mol–1) was calculated according to the following equation

Figure 7

(a) Adsorption capacities of PGP-NH2 gel toward Cr(VI) ions at different temperatures (C0 = 1000 mg/L, pH = 2). (b) Plots of ln Qe/Ce against 1/T for the adsorption of Cr(VI) ions onto the PGP-NH2 gel.

As shown in Figure b, the linear fitting plot of ln(Qe/Ce) against 1/T gives the R2 value of 0.9936, indicating that the adsorption data can be well-fitted by eq . The positive values of ΔS and ΔH in Table suggest that the adsorption of Cr(VI) ions by PGP-NH2 gel is a randomness increase and an endothermic process. Moreover, the negative value of ΔG demonstrates the spontaneous characteristic of the Cr(VI) ions adsorption process by PGP-NH2 gel.

Table 4

Thermodynamic Parameters for the Adsorption of Cr(VI) Ions by PGP-NH2 Gel Adsorbent

		ΔG (kJ mol–1)
ΔH (kJ mol–1)	ΔS (J mol–1 K–1)	279 K	293 K	313 K
7.476	28.31	–0.4237	–0.8201	–1.386

Desorption and Regeneration Study

Considering practical Cr(VI) ion adsorption applications, the adsorbent with superior desorption and regeneration capability is highly desired not only for reducing the cost of water treatment but also for recycling the chromium resource. Because the PGP-NH2 gel exhibited very low adsorption capacity at high pH, the desorption of Cr(VI) ion-adsorbed PGP-NH2 gel was conducted in NaOH solution (0.1 M). After desorption, the PGP-NH2 gel was reused to adsorb Cr(VI) ions again, and five consecutive cycles of desorption–adsorption were performed. As expected, the PGP-NH2 gel exhibited favorable desorption performance in alkaline solution and over 80% of adsorbed Cr(VI) ions could be desorbed even after five cycles of adsorption–desorption (Figure ). Meanwhile, the removal efficiency of desorbed PGP-NH2 gel gradually declined with increasing the desorption times. For instance, 76.8% of the removal efficiency was retained after five consecutive cycles of desorption–adsorption as compared with the original removal efficiency. Nevertheless, the adsorption capacity of regenerated PGP-NH2 gel can still reach 135.52 mg/g, which is higher than many reported biomass-based adsorbents (Table ). On the basis of the above results, the PGP-NH2 gel not only possesses excellent adsorption performance but also shows superior reusability for the removal of Cr(VI) ions from water.

Figure 8

Desorption and regeneration performance of PGP-NH2 gel after five cycles of desorption–adsorption.

Adsorption Mechanism Study

Because the XPS spectrum can provide useful information to identify the valence state of element, the Cr(VI) ion-adsorbed PGP-NH2 gel was characterized by XPS. As shown in Figure a, two obvious peaks at around 576.9 and 586.6 eV, respectively, associated with Cr 2p3/2 and Cr 2p1/2 peaks could be observed in the XPS survey spectrum, indicating the existence of Cr(III) ion in the sample. Moreover, both the Cr 2p3/2 peak and the Cr 2p1/2 peak could be fitted by two peaks corresponding to Cr(VI) and Cr(III) ions (Figure b), respectively, further confirming that a part of the adsorbed Cr(VI) ions have been reduced into Cr(III) ions.[55]

Figure 9

XPS survey (a) and XPS Cr 2p (b) spectra of PGP-NH2 gel after adsorption of Cr(VI) ions.

The studies on the adsorption performance of PGP-NH2 gel at diverse pH values and its desorption behavior suggest that the electrostatic attraction plays a crucial role in capturing negatively charged Cr(VI) ions from water (Scheme ). In addition, the decrease of the adsorption capacity at high ionic strength and the incomplete desorption of Cr(VI) ions from PGP-NH2 gel in alkaline solution indicated the presence of other affinity interactions such as ion exchange, hydrogen bonding, and van der Waals force during the adsorption process.[56] On the basis of the overall adsorption results, we deduce that the adsorption process of Cr(VI) by the PGP-NH2 gel takes place as follow: first, negatively charged Cr(VI) ions with different forms (e.g., Cr2O72–, HCrO4–, and CrO42– ions) in water were captured by the positively charged surface of PGP-NH2 gel mainly through electrostatic attraction (Scheme ). The surface Cr(VI) ions may be forced into the internal of PGP-NH2 gel by concentration gradient force. Then, the adsorbed Cr(VI) ions were fixed by electrostatic attraction and other affinity interactions such as hydrogen bonding and van der Waals force. Meanwhile, a part of Cr(VI) ions were reduced into Cr(III) ions and separated from the water phase.

Scheme 2

Schematic Illustration of the Adsorption Mechanism toward Cr(VI) Ions by the PGP-NH2 Gel

Conclusions

In summary, we have demonstrated that the PGP-NH2 gel with multiple amine groups could be utilized as an outstanding adsorbent for the removal of toxic Cr(VI) ions from the aqueous solution. The PGP-NH2 gel exhibited fast adsorption rate and high removal efficiency toward Cr(VI) ions, especially with the low initial concentration of Cr(VI) ions. A detailed adsorption study indicates that the adsorption process was spontaneous and endothermic and could be well-fitted by pseudo-second-order kinetic and Langmuir isotherm models. The maximum adsorption capacity of the PGP-NH2 gel can reach 188.32 mg/g. The high reusability of the PGP-NH2 gel was also confirmed by five cycles of desorption–adsorption. Adsorption mechanism study suggests that the main driving force for capturing Cr(VI) ions is the electrostatic attraction. Other interactions such as ion exchange, hydrogen bonding, and van der Waals force are also involved during the adsorption process. Therefore, this work presents a viable strategy to fabricate superior adsorbent from low-cost natural biomass for treating Cr(VI) ion-containing wastewater.

Experimental Section

Materials

Crude peach gum was obtained from the peach trees at Guilin University of Technology (Guilin, China). Water-soluble PGP was fabricated by the hydrolysis of crude peach gum according to our previous report.[27] DEA (97%), epichlorohydrin (99%), N,N-dimethylformamide (DMF, ≥99.5%), potassium dichromate (K2Cr2O7, ≥99.8%), diphenyl carbazide (DPCA), sulfuric acid (H2SO4, 95–98%), phosphoric acid (H3PO4, ≥85%), sodium chloride (NaCl, 99%), hydrochloric acid (HCl, 36–38%), hydrogen peroxide (H2O2, 25–28%), and sodium hydroxide (NaOH, ≥96%) were purchased from Aladdin Chemical Co. Ltd. (Shanghai China). Deionized water was used throughout the experiment.

Synthesis of PGP-NH2 Gel Adsorbent

Typically, the mixture of PGP (1.52 g), epichlorohydrin (3.94 g), and DMF (30 mL) was stirred at 95 °C for 90 min in a flask. After reaction, the unreacted epichlorohydrin was removed by precipitation in acetone. The obtained sediment and DEA (2.2 mL) were mixed in DMF (30 mL). After stirring at 90 °C for 2 h, the mixture was filtrated. The resulted solids were washed repeatedly with water and then were freeze-dried to afford PGP-NH2 gel. The swelling ratio of PGP-NH2 gel was determined based on the following equationwhere Wd and Ws are the weight of dried PGP-NH2 sample before and after immersing in water, respectively.

Batch Adsorption Experiments

Before adsorption experiments, a stock solution of Cr(VI) ions with the concentration of 2000 mg/L was prepared by dissolving 2.0 g of K2Cr2O7 in 1.0 L of water. The concentration of Cr(VI) ions after adsorption was determined by a UV–vis spectrophotometer using the standard DPCA method.[57] The pink complex solution formed between DPCA and Cr(VI) ions was determined at 540 nm. Before adsorption, the calibration curve was plotted. Typically, 50 mg of PGP-NH2 gel was added into 10 mL of Cr(VI) ions solution with specific concentration, and the mixture was shaken at certain temperature for 120 min. The concentration of Cr(VI) ions after adsorption was determined by the UV–vis spectrophotometer. HCl (0.5 M) and NaOH (0.1 M) were used to adjust the solution pH. The effect of initial concentration of Cr(VI) ions on the adsorption performance of PGP-NH2 gel was investigated by diluting the stock solution with water. The removal efficiency of PGP-NH2 gel toward Cr(VI) ions was calculated by the following equation

The adsorption capacity of PGP-NH2 gel adsorbent at time t (Q) and equilibrium (Qe) were calculated from the following equationswhere C0 (mg/L) is the initial concentration of Cr(VI) ions; C (mg/L) and Ce (mg/L) are the concentrations of Cr(VI) ions at time t and equilibrium, respectively; Q (mg/g) and Qe (mg/g) are the adsorption capacities of PGP-NH2 gel at time t (min) and equilibrium, respectively; V (L) is the volume of the solution; and m (g) is the mass of PGP-NH2 gel.