Introduction

Using renewable feedstock to prepare polymeric materials is receiving tremendous attention from both academic and industrial fields because of their potential in replacing or partially replacing the petroleum-based materials. Rosin is a kind of natural resin produced from pine trees. The annual production of rosin was estimated more than 1 million metrics [1]. Resin acid (around 85%) bearing a bulky hydrophobic hydrophenanthrene structure, is the main component of rosin, which has been reported to be extensively used for adhesives tackifiers, coating, soaps, varnishes and so forth.

Recently, rosin has been reported to be used as a renewable feedstock for manufacture of polymeric materials [2] by free radical polymerization [3], click chemistry [4], ring-opening polymerization [5, 6], condensation polymerization [7], etc. Ganewatta and coworkers reported a surface-initiated atom transfer radical polymerization (ATRP) strategy toward surface-immobilized resin acid-derived cationic homopolymer using a novel rosin-derived acrylate monomer as feedstock. This homopolymer-grafted substrate exhibited strong antimicrobial activity against bacteria [8]. Liu and coworkers [9] adapted the “grafting from” ATRP for the preparation of two series of rosin-based graft copolymers. Cheng et al. [10] reported a series of grafted copolymers from rosin, fatty acid and ethyl cellulose by a “grafting from” reversible addition-fragmentation chain transfer polymerization (RAFT) of DAGMA (derived from rosin) and LMA (derived from fatty acid). The intrinsic rigidity and acidity derived from resin acid could give the polymer integrity with excellent mechanical properties, chemical stability and other performance advantages.

Dyes are extensively used in textile, paper-making, color photography, leather, food, printing and plastic industries for coloring [11]. However, these industries produce large amounts of dye-containing wastewater that is difficult to biodegrade in aqueous solution, leading to toxicity to humans and the environment [12]. Methylene blue (MB) and crystal violet (CV) are two common cationic dyes consisting of various aromatic amine groups, which are reported to be highly carcinogenic and mutagenic. As a result, various physical, chemical and biological methods, such as coagulation, biological oxidation, membrane filtration, biological oxidation, etc., have been developed for the removal of MB and CV from wastewaters to reduce their negative impact on the environment [13, 14].

Since most dyes have abundant aromatic rings, much effort has been devoted to develop the adsorption material containing the benzene ring, which is considered to be the dye adsorbent. It is reported that the benzene ring contributed the π–π stacking interactions existing between the adsorbent and adsorbate. In view of the aromatic rings in resin acid, rosin was reported to fabricate adsorption material for removal of phenolic compounds [15], metal ions [16, 17], bisphenol A [18], basic orange II [19], MB [20] and malachite green dye [21], etc. For example, Li and coworkers [22] converted rosin into ethylene glycol maleic rosinate acrylate that was used as a cross-linker to fabricate the rosin-based polymer microspheres (RBPMs) through suspension polymerization. RBPMs could be used as a polymeric adsorption material for concentrating coptidis total alkaloids and purifying berberine hydrochloride (BBH) from the coptidis rhizoma extracts. However, the reports on rosin-based polymers used for the adsorption of dyes (particularly for the adsorption of MB and CV) are rare.

Herein, we report a suspension polymerization strategy to synthesize a new type of carboxyl-containing cross-linked rosin-based microspheres (CCRMs) that is used for the adsorption of MB and CV from wasted water. Then, the adsorption performance of CCRMs was evaluated by detail investigation on the influence of different dyes, solution pH, contact time, contact temperature, and initial dye concentration on the MB or CV adsorption.

Experimental

Materials

2-Methacryloyloxyisopropanol ester of dehydroabietic acid (DAGMA) was prepared according to our previous work [23]. Dipropylene glycol diacrylate (DPGDA, > 80% (GC)), vanillin methacrylate (VMA), styrene (St), isobornyl methacrylate (IBOMA), benzoyl peroxide (BPO, AR), congo red (CR, > 98%), crystal violet (CV, 98%) and methylacrylic acid (MAA, AR, > 99.0%) were all purchased from Aladdin Chemistry Co. Ltd. (Shanghai, China), and used as received. Polyvinyl pyrrolidone (PVP, K30) and sodium dodecyl sulfate (SDS) were purchased from Sinopharm Chemical Reagent Co.Ltd. Rhodamine B (RhB, ≥ 99.0%) was purchased from Shanghai Macklin Biochemical Co. Ltd. Hydrochloric acid (HCl), ethyl acetate and 1-octane were purchased from Nanjing Chemical Reagent Co. Ltd. Methylene blue (MB, ≥ 98.5%), acid orange 74 (AO) and methyl orange (MO) were all analytical grades and provided by Tianjin Chemical Reagent Research Institute.

Characterization

The FTIR spectra (ATR) were recorded using a Nicolet iS50-spectrometer.Thermogravimetric analysis (TGA) was performed on a NETZSCH TG209F1 thermogravimetric analyzer from room temperature to 800 °C at a heating rate of 10 °C/min under nitrogen. Scanning electron microscopy (SEM) was performed with a S3400N electron microscope (Hitachi) working at 15 kV acceleration voltage. Brunauer–Emmett–Teller (BET) was carried out in an ASAP2020M adsorption apparatus (Micromeritics Instruments Corporation) at 77 K. An ultraviolet–visible spectrophotometer (UV–Vis, UV-1800PC, Shanghai MAPADA) was used.

Preparation of CCRMs

Suspension polymerization strategy was applied to synthesize CCRMs according to the previous report [24] (Scheme1). First, we prepared the oil phase by mixing DAGMA (1.0 g) or other monomer, MAA (0.5 g), BPO (0.1 g) and the cross-linker DPGDA (3.0 g) with a solvent mixture (22.5 mL) composed of ethyl acetate and 1-octane (volume ratio = 2/1). Meanwhile, PVP (1% by wt) and SDS (0.04% by wt) aqueous solutions were charged in a flask to form an aqueous phase. Subsequently, the aqueous phase was vigorously stirred at a rate of 400 r/min for 10 min, and the oil phase was dropped into the flask with a constant stirring (600 r/min) within 30 min at room temperature. The reaction was carried out at 90 °C for 2 h. The polymer particles were formed, followed by the extraction with ethanol and deionized water for 30 h to remove unreacted monomers and other substances. A 10% NaOH solution was used to treat the polymer particles and to ionize the –COOH groups in the microspheres to –COO− groups. Finally, deionized water was applied to wash the microspheres until the liquid was neutral. The obtained CCRMs were dried for further use. For comparative purpose, VMA, St and IBOMA were also used to prepare microspheres with the same procedure, respectively.

Scheme 1
scheme 1

A general strategy for CCRMs derived from rosin

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Adsorption studies

Adsorption behavior of CCRMs for dyes was investigated in batches. The method used for this study is described as follows: 20 mg of the microspheres and 20 mL of an aqueous of dye were charged in a 50 mL Erlenmeyer flask. The flask was oscillated at a speed of 150 r/min until reaching equilibrium. The dyes residual solution concentration was recorded with UV–Vis at the corresponding maximum absorption wavelengths at 590, 663, 478, 462 and 494 nm for CV, MB, AO, MO and CR, respectively. Equation (1) was used to calculate the adsorption capacity, qe(mg/g) at the equilibrium time (h):

$$q_{e} = \frac{V(c_{0} - c_{e} )}{m}$$
(1)

where qe is the adsorption capacity at equilibrium (mg/g), c0 and ce represent the initial and equilibrium concentrations (mg/L), respectively, m is the mass of adsorbent (g) and V represents the volume of the dye solution (L).

The effect of pH by mixing 20 mg of CCRMs microspheres into 20 mL dye solutions was studied. The pH values were set from 2.0 to 10.0 at 30 °C in 500 mg/L MB solution or 600 mg/L CV solution. pH of the solution was adjusted by dilute HCl or NaOH aqueous solutions.

To study the adsorption isotherms, 20 mg of CCRMs microspheres were added into 20 mL dye solutions with different initial MB and CV concentrations varied from 50 to 1000 mg/L at three temperatures (20, 30 and 40 °C) for 24 h.

Kinetic experiments were performed by varying initial dye concentrations (300, 500 mg/L) at 30 °C with adding 20 mg CCRMs microspheres. The concentration of dye solution after adsorption at predefined time intervals was analyzed. The adsorption capacity at time t (min) was calculated by the following equation:

$$q_{t} = \frac{{V(c_{0} - c_{t} )}}{m}$$
(2)

where c0 (mg/L) is the initial concentrations, ct is the concentration at time t (mg/L), V is the volume of the dye solution (L) and m is the mass of adsorbent (g).

Results and discussion

Characterization of polymer microspheres

We employed suspension polymerization to prepare the carboxyl-containing cross-linked rosin microspheres (CCRMs) using DAGMA as a monomer, DPGDA as a cross-linking agent, MAA as a functional monomer and BPO as an initiator. For comparative purpose, VMA, St and IBOMA were separately used to replace DAGMA as monomer for the preparation of microspheres. The SEM images were applied to study the morphologies of microspheres. Figures 1 and S1 (in supplementary material) illustrate the morphologies of different cross-linked microspheres synthesized from different monomers with the same polymerization process. CCRMs exhibit a regular spheres morphology and the numerous pores distributed on the surface of microspheres, whereas the counterpart derived from VMA, St or IBOMA showed a status of powder. Specially, CCRMs have large particle sizes (greater than 100 μm) and the larger pores. Moreover, the internal structure of microspheres appear rough with porous cavities, which can display more active site in the absorption process. Therefore, the as-synthesized CCRMs were supposed to have a great potential as a good adsorbent for removing organic dyes from polluted water.

Fig. 1
figure 1

SEM images of CCRMs

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The CCRMs were then characterized by FTIR. As shown in Fig. 2, the disappearance of the adsorption peak at 1637 cm−1 (–C(CH3) = C) in CCRMs presented the successful polymerization of DAGMA through suspension polymerization. The adsorption peaks at 1450 cm−1 were attributed to the stretching vibration of aromatic ring carbon skeleton. The peak at 752 cm−1 was assigned to the stretching vibrations of C–H and CH = CH2 in the aromatic ring. The characteristic peaks at 1379 cm−1 corresponded to in-plane bending of O–H. These results confirmed that DAGMA was introduced into the polymer microspheres. In addition, the adsorption peak at 3436 cm−1 resembled the characteristic stretching vibration of -OH in CCRMs. The adsorption peak at 1723 cm−1 was assigned to the stretching vibration of C = O bond. The peak observed at around 1162 cm−1 was attributed to the bending vibration of C–O bond, while the peak at around 1567 cm−1 was assigned to carboxyl groups [25]. Based on these results, the attachment of various functional groups on the CCRMs surface was verified.

Fig. 2
figure 2

FTIR spectra of DAGMA monomer and CCRMs

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The thermal stability and degradation behavior of CCRMs were evaluated by TGA. As shown in Fig. 3, CCRMs exhibited a one-stage decomposition. CCRMs were initially decomposed from 341.1 °C and ended at 480 °C, indicating that CCRMs have a moderately high thermal stability.

Fig. 3
figure 3

TGA and DTG thermograms of CCRMs

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The specific surface area and average pore size of the CCRMs were investigated by BET with typical N2 adsorption–desorption isotherms (Fig. 4). According to the standard BET method, the specific surface area of CCRMs was measured to be about 20.82 m2/g, which is larger than that of some other microspheres [26, 27]. The isotherms of the CCRMs exhibited typical type-II behavior with a hysteresis loop in the relative pressure range of 0.7–1.0, implying the presence of pore structures in the CCRMs. Furthermore, the pore diameter showed a broad size distribution ranging from mesopores (3–50 nm) to macropores (> 50 nm), which was consistent with the results of SEM.

Fig. 4
figure 4

Nitrogen adsorption–desorption isotherm obtained at 77 K for the CCRMs

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Adsorption performance

Adsorption capacity of CCRMs for different dyes

To verify the adsorption characteristics of CCRMs, three different microspheres originated from VMA, St or IBOMA were also used to adsorb MB in parallel. The results are summarized in Fig. S2 (in supplementary material). It was found that CCRMs have the highest adsorption capacity. This could be explained by π–π stacking interactions existing between MB molecules and the three-ring phenanthrene skeleton structure of the rosin in CCRMs [28].

For further evaluation of the adsorption property of CCRMs, we carried out its adsorption test on the anionic dyes (e.g., AO, MO, CR) and cationic dyes (e.g., RhB, CV, MB), respectively. The concentration of dyes was uniformly controlled at 500 mg/L and the temperature was set at 30 °C. The adsorption results are shown in Fig. 5. It was found that CCRMs exhibited higher adsorption for MB and CV with an equilibrium adsorption capacity of 441 and 493 mg/g, respectively. However, the adsorption capacity of MO, AO, CR and RhB was only 0, 12.4, 0 and 128.36 mg/g, respectively. We hypothesized that high selective adsorption property is derived from the three-ring phenanthrene skeleton structure of the rosin in CCRMs, which is likely to enhance π–π stacking interactions existing between MB (or CV) and microspheres. The high selective adsorption property of CCRMs for MB and CV may result from their inherited characters and abundant micropores. In addition, CV and MB are both cationic dyes having amine groups in their structure [29] (Fig. S3, in supplementary material), which could facilitate the adsorption by negatively charged surfaces of CCRMs through the electrostatic interaction with cationic dyes, although RhB is cationic dye having amine group. The space steric hindrance resulted from the lateral chains (long alkyl chains of RhB are close to N+ centers) weakened the electrostatic attraction and π–π stacking interactions [27]. These results confirmed our hypothesis. Herein, the adsorption model of CCRMs was established with MB and MV as the target pollutant in the following experiments.

Fig. 5
figure 5

Adsorption capacities of different dyes on CCRMs

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Effect of pH

The pH of an adsorption solution is a decisive factor for adsorption capacity of adsorbent [30]. Therefore, the effect of pH on the adsorption performance of MB (or CV) was evaluated. The dye concentrations were set to 500 mg/L MB solution or 600 mg/L CV solution at 30 °C. Under this concentration condition, all dyes solutions could achieve saturated adsorption based on their adsorption isotherms and the obtained results are presented in Fig. 6. It was found that the adsorption capacity substantially decreased as the pH decreased from 10 to 2. Particularly, the absorbance dropped drastically at pH below 4. This phenomenon is probably due to the presence of high concentration of H+ ions, which promote the protonation of functional groups. At acidic pH, –COO− will protonate the cationic to form –COOH2+. In addition, MB and CV are cationic dyes. As a result, the electrostatic attraction between CCRMs and positively charged cationic dyes have decreased under acidic condition, leading to the restriction of the cationic dye adsorption [31]. A similar trend has been reported for the pH effect on the CV and MB adsorption in the previous reports [32, 33].

Fig. 6
figure 6

Effect of pH on MB and CV adsorption

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Equilibrium isotherm models of dyes

To better understand the surface properties of CCRMs and the adsorption behavior, the adsorption isotherms of MB and CV were established by the initial concentrations varied from 50 to 1000 mg/L at three temperatures of 20, 30 and 40 °C for 24 h. Figure 7a, c shows the effects of initial concentration and temperature on the MB and CV adsorption capacity of CCRMs. With the increase of initial concentration, the adsorption capacities of two dyes reflect a significant increasing trend. The results indicated that the adsorption process is highly dependent on initial dye concentration, which can be ascribed to the quicker diffusion rate of MB and CV in the higher initial concentration solution, resulting in the higher adsorption capacity.

Fig. 7
figure 7

a Adsorption isotherms of MB onto CCRMs. b Langmuir isotherm model of MB onto CCRMs. c Adsorption isotherms of CV onto CCRMs. d Langmuir isotherm model of CV onto CCRMs

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Different adsorption isotherm models were used to describe the experimental equilibrium data obtained from the study of the absorption of MB and CV. The equations and parameters of Freundlich, Langmuir, Redlich–Peterson and Dubinin–Radushkevich isotherm models are listed in Table S1. They were also applied to examine the experimental data (Fig. 7b, d, and Fig. S4a–f) and the isotherm parameters are summarized in Table 1. It can be clearly observed that the linear correlation coefficients (R2) of the Langmuir model were higher than those of other models for both dyes, suggesting that the Langmuir isotherm model might be more suitable to describe the homogeneous adsorption process of MB and CV on CCRMs. Therefore, it can be concluded that MB and CV adsorption on CCRMs may be a monolayer adsorption process.

Table 1 Fitted isotherm parameters and their goodness of fit to the adsorption data
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According to the Langmuir isotherm model, CV theoretical maximum adsorption capacity (qmax) was calculated to 602.4 mg/g, which is close to the experimental value. In addition, when the temperature increased from 20 to 40 °C, the qmax increased from 546.5 to 598.8 mg/g, indicating that the adsorption process was endothermic. However, it was found that the CCRM adsorption capacity for MB gradually decreased from 512.8 to 497.5 mg/g with increasing temperature, indicating that the adsorption process is exothermic in nature. These results are in agreement with previous reports [29]. A comparison of adsorption capacities of MB and CV on CCRMs with counterparts is shown in Tables S2 and S3. It was found that the maximum qe values can reach 516.33 and 600.71 mg/g for MB and CV when the initial concentration is 1000 mg/L, which are larger than that of many other previously reported polymeric adsorbents (Table S2). Moreover, comparative study of different rosin-based adsorbents was also carried out and the results are summarized in Table S3. It was found that the synthesized CCRMs showed the higher adsorption capacity.

Thermodynamic of adsorption

The enthalpy change (∆H°), entropy change (∆S°) and Gibbs free energy change (∆G°) as the most important thermodynamic parameters, were employed to describe the adsorption process of the two dyes on CCRMs. As shown in Table 2 and Fig. S5, ∆G° values are all negative (−27.68, −28.25, −28.47 kJ/mol for MB at 20, 30 and 40 °C, and −27.79, −30.97, −32.88 kJ/mol for CV at 20, 30 and 40 °C, respectively). These results confirmed the spontaneous nature and the feasibility of the adsorption [12]. The ΔH° value for the adsorption process of MB on CCRMs is − 16.81 kJ/mol, indicating the exothermic nature of the adsorption of MB. The ∆S° was positive, but ∆G° was still negative, suggesting that the MB adsorption process was mainly driven by the entropy effect [34]. Meanwhile, the positive values of ΔH° revealed that the adsorption of CV dyes on CCRMs was an endothermic process, which is agreed with the effect of temperature. The ΔH° value for the adsorption procedures of CV on CCRMs is 48.66 kJ/mol, indicating that chemisorption dominated the adsorption procedures. The positive ∆S° showed the increased disorder at the solid–solution interface during adsorption, which enhanced the contact opportunities between the dye molecules and CCRMs [35].

Table 2 Thermodynamic parameters for adsorption of MB and CV on CCRMs
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Adsorption kinetics

As shown in Fig. 8a, b, the adsorption rate of MB and CV decreased gradually with increasing adsorption time. This can be explained by the adsorption of dye molecules which initially occurred on the outer surface of adsorbent. The dye was then diffused into the internal pores of the adsorbent. Moreover, compared to the lower initial dye concentration, it took longer to achieve equilibrium for higher initial dye concentration.

Fig. 8
figure 8

Non-linear fits of pseudo-first-order and pseudo-second-order kinetic models to (a) MB and (b) MV adsorption at concentrations of 300 and 500 mg/L (T = 30 °C)

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To further investigate the adsorption process of MB and CV on CCRMs, the experimental data were analyzed using pseudo-first-order (Eq. 3) and pseudo-second-order kinetic models (Eq. 4) [36]:

$$q_{t} = q_{{\text{e}}} \left( {1 - {\text{e}}^{{ - k_{1} t}} } \right)$$
(3)
$$q_{t} = \frac{{k_{2} q_{e}^{2} t}}{{1 + k_{2} q_{e} t}}$$
(4)

where qt (mg/g) indicates the adsorption capacity of dyes at time t (min) and qe (mg/g) presents equilibrium. The constants k1 (min−1) and k2 (g mg−1 min−1) are the constant rate of pseudo-first order and pseudo-second order, respectively. The adsorption kinetics curves are shown in Fig. 8a, b and the corresponding kinetic parameters of the two models are summarized in Table 3. It was found that the experimental data for both adsorptions of the MB and CV are better consistent with the pseudo-second-order adsorption kinetic models resulting from their better R-squared. Moreover, the experimental value (qe = 299.79 mg/g, 493.72 mg/g) was fitted well with theoretical equilibrium adsorption (qe = 300.29 mg/g, 500.7 mg/g). This result indicated that the adsorption process could be well described by the pseudo-second-order model. Therefore, the chemical adsorption was regarded as the rate-determining step controlling the process of adsorption of MB and CV on CCRMs in terms of the pseudo-second-order kinetic model [37].

Table 3 Kinetic parameters for adsorption of MB and CV on CCRMs
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Possible adsorption mechanism of the CCRMs

We assumed that the dye molecules were attached on the surface of the adsorbent as a monolayer and were dominated mainly by chemisorption according to adsorption kinetics and isotherm models. Since CV and MB are cationic dyes having amine groups in their structure, they are prone to dissociate into Cl− and dye+ in aqueous solutions, by which they can interact with the anionic sites on the CCRMs through their cationic nitrogen sites.

As shown in Scheme 2, no significant change in the morphology of CCRMs before and after adsorption of MB and CV was observed except that the color became a dye color. The FTIR spectra of CCRMs show the change in the adsorption peaks after MB or CV adsorption (Fig. 9). When CV and MB were adsorbed into CCRMs, new adsorption peaks were observed at 1358 and 1328 cm−1, corresponding to -C–N-stretching, and peak at 1122 cm−1 was clearly observed that was assigned to −COO ↔ N. These results verified the interaction between the protonated nitrogen of the MB, CV and the COO− of CCRMs. The possible interactions between CCRMs and these two dyes are schematically illustrated in Scheme 2. The π–π stacking interactions also could occur between the aromatic ring of dyes molecule and the aromatic ring of CCRMs [31].

Scheme 2
scheme 2

Interactions between the CCRMs, CV and MB

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Fig. 9
figure 9

FTIR spectra of CCRMs, after MB and CV adsorption

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Conclusion

We demonstrated the successful synthesis of several CCRMs porous structures through a convenient and efficient suspension polymerization method. The incorporation of DAGMA into polymer microspheres provided CCRMs with excellent adsorption ability, which is favorable for the isolation of cationic MB and CV dyes from wastewater. The effect of pH on CV and MB adsorption showed that the adsorption of these two dyes is highly dependent on initial solution pH. The adsorption isotherm fitted well to the Langmuir model. The maximum adsorption capacity obtained from the non-linear form of Langmuir isotherm was observed to be 602.4 mg/g for MV at 40 °C, and 512.8 mg/g for MB at 20 °C. The remarkable adsorption performance can be attributed to the π–π stacking, electrostatic interactions and the structure properties of CCRMs. Kinetic studies confirmed that the adsorption of MB and CV could be described by non-linear form of pseudo-second-order kinetics, suggesting that chemisorption of CV and MB occurs on the CCRMs. These results demonstrated that the CCRMs are new adsorbents, and can be considered as a potential competitor for the removal of cationic dyes in terms of high efficiency, low cost and ecofriendly nature.