Porous Organic Polymer Synthesized by Green Diazo-Coupling Reaction for Adsorptive Removal of Methylene Blue.

A porous organic polymer (marked as DT-POP), which contains abundant free phenolic hydroxyl groups, is synthesized by the well-known green azo-coupling reaction in water, characterized, and utilized as an effective adsorbent for the elimination of methylene blue (MB) from water solutions. The presence of permanent mesopores, abundant active functional groups, and π-electron enrichment ascribed to phenyl rings make DT-POP an efficient adsorbent for MB due to strong hydrogen bonding, π-π, and electrostatic interactions with the cationic dye MB. DT-POP with high stability and high adsorption capacity can be reused many times and thus shows high applicability in pollutant disposal.

Introduction

The shortage and pollution of water resources have received extensive attention recently. Water pollution by the disposal of dyes, including methylene blue (MB), has an adverse impact on ecosystems and human health.[1] Therefore, it is necessary to establish efficient techniques to remove dyes from contaminated water sources. Many techniques can be adopted for elimination of dyes from contaminated water, including membrane separation, biodegradation, and adsorption.[2−4] Among these methods, adsorption is known as the easiest, most effective, and economical way for dye capture. The key feature of adsorption in treating wastewater is to choose suitable adsorbents. Hence, new adsorption materials with high adsorption capacity, low cost, high chemical stability, and easy regeneration are urgently demanded for dye removal.

Porous organic polymers (POPs), a novel type of porous substances built from organic monomers via polycondensation reaction to form covalent bonds, have attracted wide attention in the fields of gas storage, adsorption, and catalysis thanks to their low density, variable structures, high porosity, large specific surface areas, and excellent physicochemical stability.[5−10] Recently, POPs have become promising candidates for wastewater treatment.[11−14] For example, Liu et al.[11] synthesized through polycondensation two nitrogen-rich POPs for highly efficient adsorption of iodine. Most POPs are synthesized by condensation reactions, such as the Schiff base formation under organic solvents and high-temperature conditions.[15−17] The diazo-coupling reaction is mild in aqueous solutions for the synthesis of POPs by the use of different monomers, such as tri/diamines and tri/diphenols.[18−20] For example, Das and co-workers prepared a novel POP based on the diazo-coupling reaction of 2,6,14-triaminotriptycene with 1,4-dihydroxybenzene, and the POP can adsorb cationic MB with the capacity of 250 mg·g–1.[19] However, the use of POP-based adsorbents to treat wastewater is still in its infancy. Thus, much work needs to be done, especially a systematic study of the adsorption process and mechanism.

Phloroglucinol with abundant hydroxyl groups is an inexpensive commercial reagent, and 2,6-diaminoanthraquinone is an aromatic compound containing diamine. Here, an azo-based porous POP (marked as DT-POP) was conveniently synthesized by the diazo-coupling of two monomers in aqueous solutions under mild conditions and used as an adsorbent to investigate the adsorption performance and mechanism toward MB.

Results and Discussion

Characterization of DT-POP

The FT-IR spectra of DT-POP with and without adsorption of MB are displayed in Figure a. The wide adsorption band at 3426 cm–1 comes from −OH stretching vibration.[18,20] The peaks at 1295 and 1672 cm–1 stand for the stretching vibrations of C–N and C=O, respectively.[18] The peak at 1588 cm–1 corresponds to C=C.[21] The peak at 1476 cm–1 is caused by the asymmetric vibrating of N=N, indicating that the coupling reaction occurred and the framework structure of DT-POP was formed.[18] After adsorption of MB, the adsorption bands at 1595, 1388, and 1328 cm–1 stand, respectively, for the stretching vibrations of C=N, C–N, and the −CH3 symmetric deformation from MB which also appears.[22] These characteristic peaks indicate that MB is adsorbed on DT-POP. The stretching vibration of −OH in DT-POP is obviously red-shifted and widened after adsorption, indicating that there is a hydrogen bond formation between the phenolic hydroxyl of DT-POP and the nitrogen atom in phenothiazine of MB.[12,20] The C–N stretching vibrating of −N(CH3)2 and the −CH3 symmetric deformation vibration at 1397 and 1357 cm–1 in MB were red-shifted, and sodium element content in DT-POP decreased after adsorption, as revealed by energy-dispersive X-ray spectroscopy (EDS). These results verify that −N(CH3)2+ of MB can electrostatically interact with phenolate of DT-POP.[4] The stretching vibration of C=C and N=N in DT-POP migrated from 1588 to 1595 cm–1 and 1476 to 1486 cm–1 after MB adsorption, respectively. These changes may be attributed to π–π interaction in MB and DT-POP during the adsorption.[29]

Figure 1

(a) FT-IR spectra of the samples. (b) N2 adsorption–desorption isotherms and pore size distribution (inset) of DT-POP.

The isotherm of DT-POP was detected by N2 ad/desorption analysis at 77 K (Figure b). DT-POP exhibits a typical IV pattern, indicating the presence of mesopores in DT-POP. The pore size layout curve (inset in Figure b) clearly validates its mesoporous structure. The Brunauer–Emmett–Teller (BET) specific surface area of DT-POP is slightly smaller than a reported value (133.87 vs 192.93 m2·g–1).[18] However, the Barrett–Joyner–Halenda (BJH) adsorption cumulative volume is slightly larger than a reported value (0.51 vs 0.41 cm3·g).

The morphology of DT-POP before and after adsorption was characterized by scanning electron microscopy (SEM)–EDS (Figure ). The images showed an amorphous agglomerate morphology and no remarkable differences in morphology or size for both samples (Figure a,b). After adsorption, the sulfur element content increased obviously from the EDS data (Figure c,d), indicating that MB was adsorbed on the surface of DT-POP. The powder X-ray diffraction (PXRD) patterns further confirmed the amorphous structure of DT-POP (Figure S1a).

Figure 2

SEM images/EDS spectra of DT-POP without (a,c) and with (b,d) adsorption of MB.

The thermal stability of DT-POP was tested by thermogravimetric analysis (TGA). DT-POP has high thermal stability from the TGA curve (Figure S1b) and only about 12% of weight was lost at 200 °C, which was probably ascribed to the loss of solvent molecules in the pores of DT-POP. Its weight loss rate was rapid from 200 to 400 °C and then dropped sharply after 400 °C, which may be due to the degradation of the DT-POP skeleton and partial carbonization. Its thermal stability is similar to that reported in the literature.[18]

Adsorption Isotherms

The adsorption isotherms (Figure a) showed that the adsorbing ability of DT-POP for MB was strengthened rapidly and then slowly increased to a plateau with the rise of MB concentration, indicating that increase in concentration within a certain range promoted the increase of adsorption capacity. To clarify the adsorbing performance of MB onto DT-POP, we described the adsorption procedure by classic Langmuir (eq ) and Freundlich (eq ) equations.[23,24]where Qo (mg·g–1) and qe (mg·g–1) represent the theoretical maximal quantity and actual adsorbed quantity of MB on DT-POP, respectively; Ce (mg·L–1) represents the final MB concentration in solution under equilibrium; b (L·mg–1) and KF (mg1–1/ L1/ g–1) stand for the constants of the two models, respectively; and n is adsorption intensity of MB on DT-POP at an equilibrium state.

Figure 3

(a) Adsorption isotherms of MB onto DT-POP. Linear fitting of equilibrium adsorption data with (b) Langmuir and (c) Freundlich equations. (m = 10 mg; V = 25 mL; Co = 50–400 mg·L–1; t = 8 h; T = 298, 303 and 308 K).

The fitting curves by the two models are shown in Figure b,c, and the relevant indices are summarized in Table . Clearly, the adsorption isotherms are more in line with the Langmuir equation due to the larger correlation coefficient (R2) (Table ), indicating that the absorption of MB by DT-POP belongs to the monolayer coverage.

Table 1

Parameters of Langmuir and Freundlich Equations for MB on DT-POP

		Langmuir model			Freundlich model
T (K)	qe (exp)	Qo (mg·g–1)	b (L·mg–1)	R2	KF	1/n	R2
298	346.10	348.43	0.2719	0.99974	148.46	0.16865	0.78461
303	374.56	377.36	0.3829	0.99990	168.02	0.16375	0.77040
308	404.66	406.50	0.5041	0.99987	189.09	0.15862	0.80510

The Qo computed by the Langmuir model is 348.43 mg·g–1 at 298 K, indicating that DT-POP is an excellent adsorbent for MB. Table compares the adsorbing ability of DT-POP with other adsorbents. The uptake capacity of DT-POP is higher than that of most other reported adsorbents. In addition, Qo increases from 348.43 at 298 K to 406.50 mg·g–1 at 308 K, suggesting that the adsorption of MB on DT-POP is favorable at high temperatures.

Table 2

Adsorption Capacity of DT-POP and Other Reported Adsorbents

adsorbents	adsorption capacity (mg g–1)	references
triptycene based-hydroxyl-azo-polymer	250	(12)
bifunctional anionic metal–organic framework	149	(25)
melamine-formaldehyde-tartaric acid resin	60.6	(26)
porous poly(imide-ether)s	166.8	(27)
TPT-DMBD-COF	45.45	(28)
tannin-based magnetic POPs	1832	(29)
DT-POP	346.1	this study

Adsorption Thermodynamics of DT-POP for MB

The thermodynamic parameters (ΔG, ΔH, and ΔS) of MB adsorbed onto DT-POP were determined from the isotherms data by eqs and 4(4,30)

The results are listed in Table . Adsorption of MB onto DT-POP is spontaneous due to the negative ΔG. The ΔS is 252.80 J·mol–1·K–1, which is favorable for adsorption due to the increase of randomness during the adsorption. The positive ΔH indicates that the adsorption is endothermic, as revealed by the increase of adsorption capacity with the temperature rise.

Table 3

Thermodynamic Parameters of MB Adsorption onto DT-POP

T (K)	ΔG (kJ·mol–1)	ΔH (kJ·mol–1)	ΔS (J·mol–1·K–1)	R2
298	–28.18	47.132	252.80	0.99431
303	–29.51
308	–30.71

Adsorption Kinetics

The adsorption kinetics of MB on DT-POP was investigated by exploring the influence of contact time (Figure ). The adsorption capacity of DT-POP enhanced quickly at first and then rose slowly until balancing because the adsorbing positions were slowly taken by MB with the prolonging of contact time (Figure a). The adsorption was balanced after 120 min.

Figure 4

(a) Adsorption uptake of MB versus contact time. Fitting plots of (b) pseudo-second-order and (c) pseudo-first-order kinetic equations. (m = 10 mg; V = 25 mL; Co = 150 mg·L–1; T = 298 K, t = 5–360 min).

To further understand the kinetic mechanism, we fitted the kinetic data by using the pseudo-2nd-order (eq ) and pseudo-1st-order (eq ) equations.[31−33]where qe and q (mg·g–1) represent the quantities adsorbed under balance and at time t, respectively; and k2 (g mg–1·min–1) and k1 (min–1) represent the constants of the two models, respectively.

The fitting plots and corresponding parameters from both models were displayed in Figure b,c and Table . Results showed that eq better described MB adsorption onto the DT-POP due to the higher R2. Moreover, the qe (cal.) (317.46 mg·g–1) computed by eq is also closer to the experimental qe(exp) (307.28 mg·g–1).

Table 4

Parameters of Two Kinetic Models for MB onto DT-POP

		pseudo-first-order			pseudo-second-order
T (K)	qe(exp)	qe(cal.) (mg·g–1)	k1 (min–1)	R2	qe(cal.) (mg·g–1)	k2 (mg·g–1·min–1)	R2
298	307.28	102.26	0.00862	0.72962	317.46	3.467 × 10–4	0.99949

Effect of pH and Ionic Strength

To clarify the adsorption mechanism, we studied the zeta potential of DT-POP and adsorption capacity for MB at different pHs. The adsorption capacity of MB on DT-POP was enhanced rapidly with an increment from pH 2 to 4 and slightly climbed with a rise of pH (Figure a). The zeta potential of DT-POP dropped from +11.7 to −35.7 mV with the pH increasing from 2 to 12 (Figure b). The surface of DT-POP is positively charged at pH 2, which is unfavorable for the adsorption of cationic MB because of the same charge repulsion. However, the large adsorbing ability for MB at pH 2 implies that electrostatic interaction is not the only mechanism.[20]

Figure 5

(a) Effect of pH on adsorption capacity; (b) zeta potential of DT-POP at varying pH (m = 10 mg; V = 25 mL; Co = 150 mg·L–1; h = 3 h; T = 298 K; pH = 2–10).

Figure a displays the influence of ionic strength on the MB adsorption onto DT-POP in varying contents of NaCl solution. The adsorption capacity at 0.15 M NaCl remains up to 89.6% of that without NaCl. In other words, the effect of salt concentration on adsorption is very weak, indicating that ion exchange is not the main adsorption mechanism.[29]

Figure 6

(a) Effect of NaCl concentration on uptake quantity, (b) adsorption capacity of MB onto DT-POP for several adsorption–desorption cycles (m/V = 0.4 g·L–1; V = 25 mL; Co = 150 mg·L–1; h = 8 h; T = 298 K).

Regeneration of DT-POP

To test whether DT-POP can be regenerated, we carried out four adsorption–desorption cycles (Figure b). Results showed that the adsorbing capacity of the second cycle was less than that of pristine DT-POP and then a slow decrease of adsorption capacity was found in the following second cycles. The reusability experiments prove that DT-POP has high stability and can be reused for many cycles.

Conclusions

DT-POP containing phenolic (−OH) and azo (N=N) moieties was successfully prepared by diazo-coupling reaction using water as a solvent under mild conditions. DT-POP showed excellent stability in water under different pHs, large specific surface area, with mesopores and abundant functional groups, which make it an excellent adsorbent with high recyclability and uptake capacity for the elimination of cationic MB from water solutions. The high adsorbing performance depends mainly on the strong hydrogen bonding between the phenolic −OH of DT-POP and nitrogen atom of MB, and π–π interaction of DT-POP and MB. Overall, DT-POP is potentially a versatile adsorbent that is applicable in environmental repair.

Materials and Methods

Materials

All chemicals were directly used from commercial sources: phloroglucinol (≥99.0%) and 2,6-diaminoanthraquinone (97%) (both Aladdin) and NaNO2 (≥99.0%), Na2CO3 (≥99.8%), HCl (36.0–38.0%), NaOH (≥96.0%), methanol (≥99.5%), and MB (Ind) (all Sinopharm, Shanghai, China).

The stock solution of MB was made by dissolving an appropriate quantity of MB in deionized water and was diluted to prepare solutions at the desired concentrations.

Synthesis of DT-POP

The DT-POP was prepared by the diazo-coupling reaction according to a reported procedure with minor modification.[18] Typically, 3.574 g of 2,6-diaminoanthraquinone (1.5 mmol) was added into a blend of concentrated hydrochloric acid (60 mL) and deionized water (50 mL) in a 2 L beaker and magnetically stirred at 0–5 °C for 20 min. Then the solution containing deionized water (300 mL) and sodium nitrite (3.1 mmol, 2.140 g) was slowly added into the above solution under an ice bath. Next, the mixture was adjusted to neutral pH by adding 950 mL of sodium carbonate (80 g) solution. Finally, 300 mL of a solution composed of 1.262 g of 1,3,5-trihydroxybenzene (1.0 mmol) and 3.160 g of sodium carbonate (3.0 mmol) was slowly dropped into the above solution under stirring below 5 °C. A sepia powder was acquired by filtration, washed with deionized water, tetrahydrofuran, and deionized water, stirred for another 12 h at ambient temperature, and freeze-dried. The synthetic process for the DT-POP is illustrated in Scheme .

Scheme 1

Synthesis Route of DT-POP

Instruments and Characterization Methods

PXRD was observed by a Bruker D8 XRD instrument. TGA was performed on a Universal V4.5A TA Instruments device at a heating speed of 10 °C·min–1 with nitrogen protection. The surface morphology and element analysis of DT-POP before and after adsorption were obtained on a SEM instrument (FEI, Quanta 400) containing an EDS detector. The porosity of DT-POP was measured by using the N2 adsorption–desorption isotherms with a Quantachrome Autosorb iQ2 analyzer at 77 K. The BJH pore size layout and BET specific surface area were detected. Fourier transform infrared (FT-IR) spectra were observed using a Nicolet 8700 spectrometer.

Adsorption and Regeneration Experiments

The batch adsorption of MB on DT-POP was tested in triplicate by putting DT-POP (10 mg) into an MB solution (25 mL) shaken at the rate of 110 rpm in a thermostatic bath. After adsorption, the samples were filtered with 0.22 μm filter membranes and the remaining MB content in the filtrate was tested by a UV-1100 spectrophotometer (Shanghai Meipuda Instrument Co., Ltd, Shanghai) at 665 nm. The uptake amount (qe, mg·g–1) of MB by DT-POP can be calculated as follows[34]where Co and Ce (mg·L–1) mean the original and final at equilibrium MB concentrations in the solution, respectively; and m (mg) and V (mL) stand for the mass of DT-POP and the volume of solution, respectively.

Adsorption isotherms were separately obtained at 25, 30, and 35 °C for 8 h with 50–400 mg·L–1 initial MB concentrations.

Adsorption kinetics experiments were carried out at varying time intervals with initially 150 mg·L–1 MB at 25 °C.

To study the effect of pH on MB adsorption onto DT-POP, we adjusted the initial pH of the MB solution (150 mg·L–1) from 2 to 12 by diluting with NaOH or HCl solutions.

The regeneration of DT-POP was further evaluated by using ethanol/HCl (1 M) (3:1) solution as the eluent to remove MB from the MB-loaded DT-POP. In each cycle, both adsorption and desorption were carried out at 25 °C for 3 h, and the solid–liquid ratio was 0.4 g·L–1.