Introduction

As the largest industrial source of wastewater pollution, the uncontrolled discharge of papermaking wastewater has seriously affected the sustainability of today’s society1. Lignin, a complex phenolic polymer, is regarded as one of the most difficult pollutants to be normally degraded and removed in papermaking wastewater2. Currently, the main methods used to treat lignin in water are alkaline precipitation3, acid precipitation4, flocculation5, biological method6, and adsorption method7. In view of interaction mechanisms, the alkaline precipitation, acid precipitation, and flocculation methods are mainly based on the “precipitation mechanism”. Still, the lignin removal efficiency will reach a limit at the equilibrium of “precipitation-dissolution”, so the lignin removal efficiency is limited. Moreover, the precipitation technologies also have the characteristics of expensive drug costs and large amount of wastes residue. The biological method uses the metabolism of microorganisms in vivo to decompose organic matter and certain degradable inorganic substances in water and convert them into non-polluting small-molecule metabolites, thus achieving the purpose of wastewater purification8,9,10,11,12. However, the degradation efficiency of lignin in water is only 60-70% by the microbial bacteria now-cultivated, which still needs further improvement13,14,15. Adsorption generally has a wide application potential as an important technical way for the deep treatment of wastewater, which should be more capable of achieving further removal of pollutants at lower concentrations in water16,17,18,19,20,21. Nevertheless, when being used to remove lignin in water, adsorption is generally used for the deep treatment after acid precipitation, alkali precipitation, flocculation, or biological treatment22,23,24,25. It is usually difficult to be used directly for treating lignin in water, probably because the polymeric skeletal structure and large spatial site resistance of lignin itself prevent the mutual adsorption from occurring between it and conventional adsorbents. At the same time, how to realize the resourceful reuse of waste residue after lignin treatment is still a key issue that needs a further solution. Therefore, viewing from the “root cause”, a more effective interaction mechanism to improve the removal of lignins in the purification of papermaking wastewater and to realize the resourceful reuse of its waste should be a deep problem worth further studying.

In the preliminary work, we focused on the integration of interaction mechanism innovation and wastewater purification, to launch a series of researches for developing polyquaternium water-purification materials26,27,28,29,30. It was found that some of the polyquaternium gel adsorbent materials, when being used for purifying papermaking wastewater, could remove 3.89-8.28 times of their own mass of lignins per unit mass26,27. This desirable effect could be attributed to the adsorption of lignins by the skeletons of the gel materials and the self-condensation effect of lignins inside the skeleton spaces (Fig. 1). These successful prior work had clearly indicated that the polyquaternium gel materials could show the potential abilities for super-efficient removal of lignins from papermaking wastewater, because of the adsorption synergistic “inner skeleton-space” effects. However, our next work is still to continuously search for the new breakthroughs for further improving the lignin removal abilities of polyquaternium gel materials and realize the better resourceful reuse of wastes.

Fig. 1
figure 1

A diagram for proposing new ideal in this work based on our previous original work.

Full size image

In this work, we hypothesized that introducing a good skeleton space design in the area outside the gel skeleton could further improve the lignin removal efficiency. In accordance with this assumption, the polyquaternium gel skeleton was modified through a new chain segment technology, to further construct an “effective control region between the chain segments” on the outside of the gel skeleton (Fig. 1). Therefore, a chain segment polyquaternium gel composite water-purification material (SGPQG/SGPQ) was designed to be synthesized, which contained a highly cross-linked, insoluble chain-segment grafting polyquaternium gel skeleton adsorbent component (SGPQG) and a micro-crosslinked, soluble chain-segment grafting polyquaternium flocculant component (SGPQ). As a result, coordinating the chain-segment modification on gel skeleton expanded the effect of “inner skeleton space”, by building an integrated “skeleton space” effect mechanism, to further improve the lignin removal efficiency of the obtained material.

Results and discussions

Optimization of synthesis conditions and control of the structural compositions of SGPQG/SGPQ

In view of the designed synthesis route, SGPQG/SGPQ composite water-purification material was obtained by the cross-linking copolymerization of CSC crosslinkers and DMAC monomers. Both a highly cross-linked, insoluble chain-segment grafting polyquaternium gel skeleton adsorbent component (SGPQG), and the micro-crosslinked, soluble chain-segment grafting polyquaternium flocculant component (SGPQ) were simultaneously formed in the reaction system due to differing cross-linking degrees. This is because the specific function of a product is controlled by its specific microstructure, which is further influenced by the preparation process and structural composition. Therefore, the optimization of the synthesis conditions of SGPQG/SGPQ composite water-purification materials and the control of the structural compositions are the key issues that need to be addressed as a priority.

First, a four-factor, three-level orthogonal experiment [L9(3)4 orthogonal experiment] was designed with the reaction system of CSC crosslinker and DMAC monomer mass ratio of 1/1 as the modular reaction. The effects of different reaction conditions, such as the reaction temperature (factor A), the monomer concentration (factor B), the initiator (APS) dosage (factor C), and the reaction time (factor D), on the synthesis of SGPQG/SGPQ materials were separately investigated (Supplementary table 1). The lignin removal ability by each obtained product was used as the evaluation index. When the reaction temperature was 60 °C, the concentration of monomer was 55%, the initiation dose of APS was 3%, and the reaction time was 5.0 h, the corresponding SGPQG/SGPQ product had the highest removal percentage (R%) of lignin from water, which could be used as the optimized reaction conditions for the synthesis of SGPQG/SGPQ material.

Subsequently, the mass ratios of CSC crosslinkers to DMAC monomesr in the reaction system were varied to adjust the structural compositions of the resulting SGPQG/SGPQ products under the optimized synthesis reaction conditions obtained above, and that results of obtained SGPQG/SGPQ products are listed in Supplementary table 2. The results showed that when the mass ratio of CSC crosslinker to DMAC monomer was 20/80 during the reaction, the resulting SGPQG/SGPQ product had the highest removal percentage (R%) of lignin, and thus could be used as a target SGPQG/SGPQ product with the optimized structural composition.

With the concept of a parallel synthesis reaction, almost all the reaction materials had been converted into the useful water treatment materials without wastes. Thus, the synthesis process was economical and environmentally friendly.

SGPQG/SGPQ performances and serial effects for removing lignin from water

Following the experimental method in “performance analysis of SGPQG/SGPQ treatment with lignin in water” section, the effects of SGPQG/SGPQ materials on lignin removal at the different treatment conditions were measured to evaluate the water-purification performances and serial effects for treating lignin from water.

SGPQG/SGPQ performances and differential impacts of each component

According to the results in Fig. 2a, the lignin removal percentages increased with the increase in dosages of SGPQG/SGPQ materials. When the dosages of SGPQG/SGPQ materials were 0.005 g or more, the lignin removal percentage reached above 90.2%, which was significantly higher than the level of the existing materials. Based on the relationships between the dosages of SGPQG/SGPQ materials and the lignin removal percentages (R%), the mass value (Q’, mg•g-1) of lignin removal per unit mass of SGPQG/SGPQ material was further calculated to be 10,157.71 mg•g-1. That is, each gram of SGPQG/SGPQ material can remove 10,157.71 mg of lignin. The ability for the SGPQG/SGPQ to remove lignin from the water was greatly improved by 1.23–50.55 times compared to the similar water-purification materials obtained from the previous work (Table 1).

Fig. 2: SGPQG/SGPQ performances and differential impacts of each component.
figure 2

a The effect of different dosages of SGPQG/SGPQ materials on lignin removal. b The adsorption results of SGPQG skeleton towards lignin in water. c Isothermal adsorption curve of SGPQG skeleton towards lignin in water. d The flocculation results of SGPQ flocculant components towards lignin in water. e The differentiated impact analysis of the SGPQG/SGPQ composite material versus the separate SGPQG skeleton component and SGPQ flocculant component. (*) RSGPQG/SGPQ%: the lignin removal percentage of SGPQG/SGPQ composite material, RSGPQG%: the removal percentage of the SGPQG skeleton component, RSGPQ%: the removal percentage of the SGPQ flocculant component, ∆R%: the extra increase on the lignin removal percentage.

Full size image
Table 1 Comparing the superiority of SGPQG/SGPQ with the serial existing materials for removing lignin from water*.
Full size table

The SGPQG/SGPQ composite water-purification materials were washed thoroughly to separate the water-insoluble SGPQG skeleton components (60% by mass) and the water-soluble SGPQ flocculant components (40% by mass).

The effect of each component on the lignin removal was further carried out as follows:

First, it was found that the SGPQG skeleton component had a significant adsorption function to remove lignin from water, and the adsorption results were shown in Fig. 2b. According to the average values of the equilibrium adsorption capacities (qe) at the saturated adsorption states (i.e., at the dosages being 0.001–0.009 g), the adsorption capacity of SGPQG skeleton components for lignin was 7507.66 mg•g-1, which is 1.89 times higher than that of similar material model without the chain segmentation (i.e., the PPG material) obtained in the pre-work26 (Supplementary Fig. 1), indicating that the chain segment modification of polyquaternium gel played an efficient role in improving the removal of lignin.

Secondly, the corresponding isothermal adsorption curve of SGPQG skeleton components towards lignin was established, which was observed to be consistent with the theoretical type III isothermal adsorption curve (Fig. 2c), and suggesting that the strong mutual interactions occurred between the adsorbates. This may be due to the fact that the penetration of lignin into the interior of the SGPQG skeleton leads to the further self-condensation.

Further more, it was observed that the SGPQ flocculant component also achieved some lignin removal effect by producing flocculation precipitation with lignin in water (Fig. 2d). In the case, the lignin removal percentages varied with the dosages of SGPQ flocculant components. When the SGPQ dosages were below 40 mg•L-1, the lignin removal percentages were slightly decreased with the increase in SGPQ dosages, possibly because the SGPQ dosages were so low that the SGPQ flocculant components couldn’t interact efficiently with the lignins to form the flocculation precipitations. However, they dissolved in water to increase the absorbance in a similar manner to lignin at the measurement of the lignin concentration in solution by UV-Vis Spectra technology. Therefore, the measured lignin concentration residual in solution were higher than the actual one, resulting in an apparent decrease of the lignin removal percentage measured. Subsequently, when the SGPQ dosages were increased from 40 mg•L-1 to 50 mg•L-1, the lignin removal percentage was clearly increased and reached the highest value, possibly because the state at the SGPQ dosage of 50 mg•L-1 was charge neutralization, so that the cationic SGPQ flocculant components could produce the most efficient electrostatic interactions with the anionic sulfonate groups of lignins, to forming the insoluble precipitations for removing lignins from water. However, when the SGPQ dosages were above 50 mg•L-1, the lignin removal percentages were slightly decreased with the increase of SGPQ dosages again, possibly due to the SGPQ overdose, so that the superfluous SGPQ could adhere to the surface of lignin-containing precipitations to improve their hydrophily and make them highly dispersed with fine particles in water, also resulting in a decrease in lignin removal percentage.

Comparing the differentiated impact of SGPQG/SGPQ composite water-purification material with each separate component in lignin removal efficiency, an unexpected phenomenon of an extra increase on the lignin removal percentage (∆R%) was observed (Fig. 2e). For example, when a 50 mL, 800 mg•L-1 lignin solution was treated by 0.005 g of SGPQG/SGPQ composite water-purification materials, the corresponding lignin removal percentage (R SGPQG/SGPQ %) was 90.2% (as shown in Fig. 2a). By decomposition analysis, the removal percentage (R SGPQG %) of the 60% SGPQG skeleton component (0.003 g) treating with the same 50 mL, 800 mg•L-1 lignin solution was 55.56% (Fig. 2b), and the removal percentage (R SGPQ %) of the 40% SGPQ flocculant component (0.002 g) treating with the same amount of lignin solution was 16.34% (Fig. 2d). Calculations according to Eq. 1 below revealed an extra differential increment (∆R%) on the lignin removal percentage compared to the treatment result superposition of SGPQG/SGPQ versus each separate component towards lignin:

$$\Delta R\% = R_{SQPQG/SQPQ}\% - (R_{SQPQG}\% + R_{SQPQ}\% ) = 90.20\% - (55.56\% + 16.34\% ) = 18.30\%$$
(1)

The phenomenon of extra increase on the lignin removal percentage indicates that the removal of lignin by the SGPQG/SGPQ composite water-purification material is not a simple result superposition of the SGPQG skeleton component and the SGPQ flocculant component treatment, but may be due to an additional synergistic effect occurring between the SGPQG skeleton component and the SGPQ flocculant component. For example, in the region between the chain segment and chain segment outside the material skeleton, the aggregation of flocculation particles or the self-condensation of lignin macromolecules is promoted, which is beneficial to additionally improve the removal efficiency of lignin from water.

Kinetics-thermodynamics behaviors of SGPQG/SGPQ treatment with lignins

The results in Fig. 3a showed that 0.005 g of SGPQG/SGPQ material could reach the equilibrium of treatment reaction (lignin removal percentage of 95.49%) after treating 50 mL of 800 mg•L-1 lignin solution for 3.0 h. Calculated from Eq. 4, the average lignin removal rate of SGPQG/SGPQ material before reaching the reaction equilibrium (i.e., within 3.0 h of the treatment reaction time) was 1561.6 mg·g-1·h-1, which was 1.85 and 3.34 times higher than those of the pre-products PPG and SHPCG [according to the same calculation method, the lignin removal rates of PPG and SHPCG obtained in the pre-work were 840.1 mg·g-1·h-1 and 467.8 mg·g-1·h-1, respectively (Fig. 3b). For the SGPQG skeleton component, its average lignin removal rate was calculated as 1209 mg·g-1·h-1 (Fig. 3b), which was improved by 1.4 times compared to the similar structure of PPG without the chain segments, indicating that the chain segment modification of polyquaternium gel does improve the lignin removal rate.

Fig. 3: Kinetics behaviors of SGPQG/SGPQ treatment with lignins.
figure 3

a The effect of different time on SGPQG/SGPQ and SGPQG treatment with lignin. b Comparing the lignin removal rates of SGPQG/SGPQ and SGPQG to PPG and SHPCG obtained in the pre-works. The results of the Ct data of SGPQG/SGPQ treatment with lignins to fit the serial reaction kinetics equations: c fitting the zero-order reaction equation, d the first-order reaction equation, e the second-order reaction equation, and f the third-order reaction equation. The results of the adsorption data of SGPQG skeleton components towards lignins to fit the serial adsorption kinetics equations: g Fitting the pseudo-first kinetics model, h the pseudo-second kinetics model, i the intraparticle diffusion model, and j the particle diffusion model.

Full size image

The related Ct data of SGPQG/SGPQ treatment with lignins at different interaction time “t” were used to fit the serial reaction kinetics equations, to investigate the interaction kinetics behaviors, and the results are shown in Fig. 3c–f. The results showed that the correlation coefficients (R) for fitting the equations were decreased with the increase of reaction orders, and that of the zero-order reaction equation fitted well with the highest of 0.99, indicating that the SGPQG/SGPQ treatment with lignins followed the zero-order reaction. This meant, the process of SGPQG/SGPQ treatment with lignins was none of the lignin concentrations, and the SGPQG/SGPQ material itself would play a critical role in the lignin removal.

The adsorption data of SGPQG skeleton component as adsorbent towards lignin at different times were substituted to fit the serial adsorption kinetics equations (i.e., the pseudo-first kinetics equation, pseudo-second kinetics equation, intraparticle diffusion equation, and particle diffusion equation), and the results are further shown in Fig. 3g–j. The adsorption data fitted well all the selected adsorption kinetics equations (the correlation coefficients were above 0.92), indicating that the adsorption of SGPQG skeleton component towards lignins followed a multitrack adsorption process, also suggesting that the SGPQG skeleton component had a good affinity for lignin.

Supplementary Fig. 2a shows the treatment results of SGPQG/SGPQ towards lignins at different temperatures, which indicated that the lignin removal percentages derived from the SGPQG/SGPQ treatment increased with the increase in treatment temperatures. This might be due to the kinetic energies of lignin molecules which increased with the increase in treatment temperatures, and permeated insides the material skeletons thus interacted more efficiently. The treatment data of SGPQG/SGPQ towards lignins were further used to fit the thermodynamic equations (Eqs. 13–15), and the results showed that the enthalpy change (∆H) of SGPQG/SGPQ treatment towards lignins was 55.69 kJ•mol-1, with the entropy change (∆S) was 189.28 J•mol-1•K-1 (Supplementary Fig. 2b). The Gibbs free energy ∆G ≤ 0, of the treatment interaction was spontaneous. According to Gibbs free energy equation, i.e., ∆G = ∆H-T•∆S, the critical temperature of SGPQG/SGPQ treatment towards lignins was calculated as 21.0 °C. This meant that the treatment of SGPQG/SGPQ towards lignins would be spontaneous if the treatment temperatures were above 21.0 °C, which could fully meet the normal application requirements.

Micro-structure transformations of SGPQG/SGPQ treatment with lignins

According to the experimental method in “kinetics-thermodynamics experiments of SGPQG/SGPQ treatment with lignin in water” section, optical microscopy, Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) were selected to observe the micro-structural transformations of SGPQG/SGPQ before and after treating lignin, thus achieving some useful information to further confirm the interaction mechanisms of SGPQG/SGPQ treatment with lignin from the perspective of micro-structures.

Optical microscopy analysis

The morphological change patterns of the visible substances during the lignin solution, e.g., SGPQG skeleton component adsorption, SGPQ flocculation, and SGPQG/SGPQ combination treatment were systematically observed by optical microscopy, and the results are shown in Supplementary Figs. 3–6, respectively.

The results showed that no significant particulate matter was observed in the lignin solution (Supplementary Fig. 3), indicating that lignin was adequately dissolved in the solution. When the SGPQ flocculant component was treated with the lignin solution, some small particle-like materials were gradually produced in the mirror image as the interaction time increased (Supplementary Fig. 4), indicating that an effective flocculation-precipitation effect was produced between the SGPQ flocculant component and lignin. When the SGPQG skeleton components as adsorbents were treated with lignin solution, the interior of the skeleton was gradually penetrated by brown lignin as the interaction time increased (Supplementary Fig. 5), indicating that lignin had fully penetrated into the interior of the SGPQG skeleton (adsorbed or self-condensation occurred within the skeleton). During the SGPQG/SGPQ combination treatment with the lignin solutions, on the one hand, the interior of the skeleton of this material is gradually permeated by brown lignin, and adsorption or self-condensation occurs within the skeleton. On the other hand, the material gradually produced more and more larger granular material outside the skeleton and aggregated towards the skeleton (Supplementary Fig. 6). This confirmed that the area outside the SGPQG/SGPQ skeleton (the area between the chain segment and the chain segment) was favorable for promoting the further aggregation of particle substances.

FT-IR analysis

The molecular structure changes of SGPQG/SGPQ composite water-purification materials and SGPQG skeletal component materials before and after treatment with lignin were compared and analyzed by Fourier transform infrared spectroscopy (FT-IR), and the results are shown in Fig. 4a, b.

Fig. 4: Micro-structure transformations of SGPQG/SGPQ treatment with lignins.
figure 4

a FT-IR analysis of SGPQG/SGPQ material and SGPQG skeleton component. b FT-IR analysis of SGPQG/SGPQ material and SGPQG skeleton component after treating lignin. c XRD analysis of SGPQG/SGPQ material and SGPQG skeleton component. d XRD analysis of SGPQG/SGPQ material and SGPQG skeleton component after treating lignin. SEM analysis of SGPQG/SGPQ material before (e) and after (f) treating lignin.

Full size image

Figure 4a showed the FT-IR spectra comparisons of the SGPQG/SGPQ composite water purification material and the remaining SGPQG backbone component. The results showed, the characteristic absorption peaks of C-N linkage at 966 cm-1, C-OH linkage at 1113 cm-1, C-C linkage at 1398 cm-1, C-Cl linkage at 1475 cm-1, and -CH2- linkage at 2926 cm-1, displaying a polyquaternium-based structure with the poly(epichlorohydrin-dimethylamine) chain segment. Moreover, for the remaining SGPQG backbone component after elution, the intensity of the characteristic absorption peaks at 966-1646 cm-1 was significantly decreased, especially the intensity decrease of the characteristic absorption peak of C = C linkage at 1646 cm-1 was more obvious. This could be explained as follows: The soluble SGPQ components were sufficiently separated from the SGPQG/SGPQ composite water-purification material system after elution, resulting in the weakening of the intensity of the corresponding characteristic absorption peaks of the remaining SGPQG skeleton components, also suggesting that the soluble SGPQ component was insufficiently crosslinked (i.e., slighly crosslinked), to contain the higher contents of C = C linkages.

After lignin treatment, the intensities of the characteristic absorption peaks of SGPQG skeleton at 1037-1628 cm-1 (-C-O-, -C = C-, etc) were observed to be basically the same as those of SGPQG/SGPQ (Peak 1in Fig. 4b). Especially, the intensities of the characteristic absorption peaks of SGPQG skeleton at 2875 cm-1 (=C-H) and 2926 cm-1 (-CH2-) became stronger than those of SGPQG/SGPQ (Peak 2 in Fig. 4b). This is probably due to the presence of a large amount of lignin in the wastes, which led to the re-enhancement of the intensity of the corresponding characteristic absorption peaks, and further indicates that the SGPQG skeleton component has a strong lignin removal effect.

XRD and SEM analysis

The X-ray diffraction (XRD) analysis showed, compared to the remaining SGPQG skeleton components, there were two new diffraction peaks being 34.79° (Peak 1) and 48.68° (Peak 2), respectively, existing in the XRD curve of SGPQG/SGPQ composite water-purification material (Fig. 4c), which could be attributed to the SGPQ components. It also indicated that the detachment of the soluble SGPQ component after elution resulted in the disappearance of the corresponding characteristic XRD diffraction peaks. After treating lignins, the XRD diffraction peak outlines of the SGPQG/SGPQ composite water-purification material and the SGPQG skeleton component material residue were similar, but there were still some sharp peaks existing in the SGPQG/SGPQ XRD curve (Fig. 4d). This result indicated that the presence of a large amount of additional lignin in the slag weakens the difference in diffraction peak properties between the SGPQG/SGPQ composite water-purification material and the SGPQG skeletal component material, so that the SGPQG/SGPQ composite water-purification material and the SGPQG skeletal component material had similar XRD peak outlines. The sharp peaks in the XRD curve of SGPQG/SGPQ after treating lignin suggested the existence of other lignin-formed substances. All of the information on XRD analysis confirmed that the SGPQG/SGPQ had achieved a strong lignin removal effect.

Further, scanning electron microscopy (SEM) analysis showed that the surface of the original sample of SGPQG/SGPQ showed a relatively even and smooth morphology (Fig. 4e). In contrast, the shape of the sample after this material treated with lignin was uneven and rough, there were signs of substance accumulation on the surface layer, and some gaps were filled in full (Fig. 4f). This proved that some of the new substances (that is, lignin) was transferred to the inside of the gel skeleton, while the other parts were flocculated and accumulated to the outside of the SGPQG skeleton.

EDS energy spectroscopy analysis

On the basis of SEM analysis, the further quantitative elemental analysis was coupled with EDS energy spectroscopy, and the results are shown in Fig. 5. The results showed that the material skeleton was irregularly shaped and contained a large amount of N, Cl, and O elements (Fig. 5a), which could attributed to the quaternary ammonium cations (-N+, Cl-) and the hydroxyl -OH of the poly(epichlorohydrin-dimethylamine) chain-segment in the material system. The EDS plots in Fig. 5b showed that the treatment of lignin contained a large amount of S elements (derived from lignin molecules) inside the material skeleton, while the Cl elements (derived from quaternary ammonium salt ions) were substantially reduced. This result implied that the quaternary ammonium ions inside the material backbone had undergone effective electrostatic binding (ion exchange) with the lignin molecules. That was, the lignin molecules had sufficiently penetrated into the interior of the material skeleton to interact effectively. According to the EDS plot of material skeleton surface after treatment with lignin (Fig. 5c), the surface contained a large amount of S elements (from the sulfonic acid of Lignin), a large increase of O elements (derived from the hydroxyl group of lignin), and a decrease of Cl elements (from the quaternary ammonium ion), which indicated that SGPQG/SGPQ had sufficient interaction with lignin on the outside of its skeleton, resulting in a large aggregation of lignin on the outside of material skeleton. Therefore, EDS energy spectroscopy further confirmed that lignin underwent the effective aggregation effects both inside and outside the material backbone.

Fig. 5: EDS energy spectroscopy analysis of SGPQG/SGPQ before and after treating lignin.
figure 5

a EDS energy spectroscopy analysis of SGPQG/SGPQ before treating lignin. b EDS energy spectroscopy analysis of inner SGPQG/SGPQ after treating lignin. c EDS energy spectroscopy analysis of SGPQG/SGPQ surface after treating lignin.

Full size image

XPS analysis

The changes in binding energies of key elements during lignin treatment by SGPQG/SGPQ were monitored by the X-ray photoelectron spectroscopy (XPS) analysis technique, and the results are shown in Supplementary Fig. 7. The results showed that the content of Cl 2p elements (derived from quaternary ammonium ions) in lignin significantly decreased from 6.13% to 0.43% after SGPQG/SGPQ treatment with lignin, while the content of S 2p elements (derived from the sulfonic acid ions of Lignin) increased significantly from 0.06% to 1.5% (Supplementary table 3 versus Supplementary table 4), which might be due to the SGPQG/SGPQ treatment with lignin, mutual electrostatic binding (ion exchange) occurred during the process. Meanwhile, the content of the O element (hydroxyl group derived from lignin) was also appropriately increased to 19.63%, further confirming that effective interaction between SGPQG/SGPQ and lignin occurred. Overall, the results of XPS and with the preliminary EDS energy spectroscopy were in general agreement.

Constructing effect mechanism model of SGPQG/SGPQ for removing lignins in water

Several simulation experiments (the kinetics, thermodynamics and differentiated effect experiments, etc) in “SGPQG/SGPQ performances and serial effects for removing lignin from water” section and various modern instrumental analysis (i.e., optical microscopy, FT-IR, SEM, XRD, and XPS) in “micro-structure transformations of SGPQG/SGPQ treatment with lignins” section, had clearly given us a lot of useful information, to infer the effect mechanism model for SGPQG/SGPQ to remove lignins from water. That was, an integrated skeleton-space effect mechanism for SGPQG/SGPQ removal towards lignins in water. The detailed logic derivation diagram could be seen in Fig. 6a.

Fig. 6: Constructing effect mechanism model of SGPQG/SGPQ for removing lignins in water.
figure 6

a Logic derivation diagram. b Interaction mechanism model image for SGPQG/SGPQ treatment with lignin.

Full size image

This integrated skeleton-space effect mechanism of SGPQG/SGPQ removal towards lignins could be further described as follows: When applied, the SGPQG/SGPQ composite water-purification material would release two parts of water-purification materials such as the SGPQG skeleton component and SGPQ flocculant component. For the SGPQG skeleton, lignin molecules could permeate inside the inner region, and parts of lignins were electrostatically adsorbed with the SGPQG skeleton. The other parts of lignins would self-condense in the inner region. Between the inter-chain segment regions outside the SGPQG skeleton surface, it was favorable for the SGPQ flocculant component to form flocculation precipitation with lignins and macroparticle aggregation or to promote the self-condensation of lignin macromolecules. In conclusion, the “integrated skeleton-space” effect mechanism based on the SGPQG/SGPQ skeleton inside and outside the skeleton (i.e., between the inter-chain segment regions) could be essential for the super-efficient removal of lignin from water.

Compared to the existing inner skeleton-space effects discovered in our previous works26,27, the outer skeleton-space effects had been additionally expanded in this new integrated skeleton-space effect mechanism. The mechanism model could be intuitively drawn as shown in Fig. 6b.

Resourceful reuse of wastes from SGPQG/SGPQ treatment with lignins

The SGPQG/SGPQ wastes after treating lignins [referred to as S/S waste, Fig. 7a were directly used for adsorption treatment of dyeing wastewater, thus realizing the resourceful reuse of wastes.

Fig. 7: Resourceful reuse behaviors of wastes from SGPQG/SGPQ treatment with lignins.
figure 7

a The original photos of S/S wastes. bThe adsorption results of S/S wastes towards the dyes of Reactive Scarlet 3BS. c Optical microscope observation of the S/S wastes after adsorbing lignins in water. d SEM analysis of S/S wastes after adsorbing lignins. e Comparing FT-IR analysis of S/S wastes before and after adsorbing dyes. f Comparing XRD analysis of S/S wastes before and after adsorbing dyes.

Full size image

0.001 g-0.010 g of S/S wastes were added to 50 mL, 100 mg•L-1 of Reactive Scarlet 3BS dye solution at 30 °C with continuous stirring and adsorption for 48 h, and the results were shown in Fig. 7b. The results showed that 99.27% of the dyes in water was removed by adsorption when the dosage of S/S wastes were 0.006 g, and the water became clear. When the addition dosage of S/S wastes were greater than 0.006 g, its adsorption and removal percentage of dyes in water did not change much, indicating that it was in the state of excess adsorption capacity. The reverse indicated that when the dosage of S/S wastes residue were lower than 0.006 g, it was in the state of saturation adsorption.

Based on the average value of the equilibrium adsorption capacity of S/S wastes for dye adsorption in the saturated adsorption states (that was, the amount of S/S wastes were 0.001-0.006 g), the maximum adsorption capacity of S/S wastes for the dye of Reactive Scarlet 3BS was calculated as 799.04 mg•g-1, which was higher than most of similar polyquaternium adsorbent materials and 443.9 times higher than the commonly-used activated carbon (Supplementary Table 5).

Optical microscopy analysis showed that the gel skeleton of the S/S wastes further darkened and full in color after dye adsorption (Fig. 7c), probably because the colored dyes were adsorbed into the skeleton of the S/S waste, resulting in a large amount of dyes filling the skeleton.

SEM analysis showed that the sample surface of the S/S wastes was smoother, and the skeleton of the S/S wastes was full (Fig. 7d), which confirmed the presence of new substances (i.e., the dyes) adsorbed into the skeleton of S/S wastes.

FT-IR analysis (Fig. 7e) showed, after adsorbing the dyes, the intensities of the characteristic absorption peaks of S/S wastes at 1037 cm-1 (Peak 1, superposition of -C-N- absorption), 1211-1628 cm-1 (Peak 3, superposition of -N = N-, -C-Cl, and -C = C- absorption), and 2951 cm-1(Peak 5, superposition of -CH2- absorption), became stronger than those of the S/S wastes before adsorbing the dyes, which might be caused by the superposition of similar absorption peaks of adsorbed dyes. Meanwhile, after adsorbing the dyes, there were further two new absorption peaks at 1119 cm-1 (Peak 2, -SO3-) and 2359 cm-1(Peak 4, -C = N-) attributing to the adsorbed dyes in the FT-IR curve of the S/S wastes. Therefore, the FT-IR analysis confirmed that the S/S wastes had produced the efficient adsorption interactions with the dyes.

XRD analysis (Fig. 7f) showed the similar diffraction peaks between the S/S wastes before and after adsorbing the dyes, indicating that there was no change in the state of mutual combinations of internal substances of S/S wastes before and after adsorbing the dyes.

On the basis of SEM analysis, the results were further coupled with the EDS energy spectrum for the elemental quantification, as shown in Fig. 8a. The results showed that the elemental Cl content within the S/S waste skeleton was further reduced after dye adsorption (compared with the previous Fig. 5b), which might be due to the effective electrostatic binding (ion exchange) of the sulfonic acid anion (-SO3-) of the adsorbed dye with the remaining quaternary ammonium ion (-N+) in the S/S waste. Based on this interaction, the dyes were effectively adsorbed into the S/S waste skeleton and removed from water.

Fig. 8: Deep analysis of interaction behaviors for S/S wastes to adsorb the dyes.
figure 8

a EDS energy spectroscopy analysis of S/S wastes after adsorbing lignins. XPS analysis results of the S/S wastes after adsorbing the dyes: b Survey scanning image, (c) scanning the binding energy of C 1 s, (d) scanning the binding energy of O 1 s, (e) scanning the binding energy of Cl 2p, (f) scanning the binding energy of N 1 s, (g) Scanning the binding energy of S 2p. h The interaction mechanism model of the S/S wastes used for adsorbing the colored dyes.

Full size image

In addition, the XPS analysis technique monitored the microstructural changes of S/S wastes before and after dye adsorption, and the results were shown in Fig. 8b–g. The results showed that the elemental content of Cl 2p in the S/S waste decreased from 0.43% to 0.19% after adsorption of the dye, while the content of S 2p also increased to 1.96% (Supplementary Table 6), which was consistent with the results of the preliminary EDS energy spectrum analysis.

Generally, the above-mentioned serial analysis results had clearly given the useful information to confirm the adsorption behaviors of S/S wastes towards dyes, which could be described in detail as follows: When the S/S wastes adsorbed the dyes, the dyes could easily penetrate into the material skeleton because the space volume of the dye molecule was smaller than that of polymeric lignin molecule. It replaced the parts of lignins that were previously bound to the material skeleton in electrostatic forms and were further adsorbed by ion exchange (electrostatic binding) with the material skeleton, while the replaced polymeric lignins had still strongly inter-molecular forces with the material skeleton and attached to the material skeleton without being dislodged. The interaction mechanism model for the adsorption of S/S wastes towards dyes could be intuitively drawn as shown in Fig. 8h.

Extending application of SGPQG/SGPQ in purification of a complex wastewater and the real papermaking wastewater

According to the experimental methods in “Extending application of SGPQG/SGPQ in purification of a complex wastewater” section, the SGPQG/SGPQ materials were extended to treat the complex wastewater solution containing 800 mg•L-1 of lignins and 100 mg•L-1 of Reactive Scarlet 3BS dyes, and the results are shown in Fig. 9(a, b).

Fig. 9: Application of SGPQG/SGPQ in purification of complex wastewater and real papermaking wastewater.
figure 9

a Effects of different time on SGPQG/SGPQ treatment with the complex wastewater. b Effects of different dosages of SGPQG/SGPQ materials on treatment of the complex wastewater. (c) Application results of SGPQG/SGPQ materials in treatment of the real papermaking wastewater.

Full size image

Figure 9a showed the effects of different time on SGPQG/SGPQ treatment with the complex wastewater. The results showed, the tend of the lignin and dye removal was similar for tracking changes over treatment time, indicating that the lignin and dye removal was almost synchronous when treating the complex wastewater, instead of removing lignin first and then dyes as shown in the previous separate treatment experiments.

The effects of different dosages of SGPQG/SGPQ materials on the complex wastewater treatment (Fig. 9b) showed, when 0.001-0.010 g of SGPQG/SGPQ materials were added to treat the complex wastewater, the lignin removal percentages were 58.8-83.2%, and the dye removal percentages were 13.4-96.0%. Based on the relationships between the dosages of SGPQG/SGPQ materials and the lignin or dye removal percentages (R%), the mass value (Q’, mg•g-1) of lignin removal per unit mass of SGPQG/SGPQ material was calculated as 7561.50 mg•g-1, which was slightly lower than the Q’ value (10157.71 mg•g-1) when SGPQG/SGPQ treated the pure lignin solution. Meanwhile, the Q’ value of dye removal was calculated to be 750.14 mg•g-1, which was near to the adsorption capacity of S/S waste adsorption towards the dyes (799.04 mg•g-1). This might be explained as follows: When treating the complex wastewater, the dyes could be prematurely permeated into the skeleton of SGPQG/SGPQ before the lignin could be fully done, which prevented the space effects of SGPQG/SGPQ material for treating or accommodating lignin.

Considering the above-mentioned results of SGPQG/SGPQ treatment towards the complex wastewater, it further confirmed from the reverse, the application order of SGPQG/SGPQ material for treating lignin and dyes was crucial, and the space effects were also necessary for SGPQG/SGPQ material to realize the super-efficient lignin removal. Only when the SGPQG/SGPQ material was first used to treat the lignin and then the waste was subsequently used to treat the dyes, the super-efficient lignin removal and the highly-efficient reuse of waste would be realized.

In addition, the SGPQG/SGPQ material (0.14 g) was further used to treat a real papermaking wastewater (1.0 L), and the lignin removal percentage was 91.9% |(Fig. 9c), indicating that the SGPQG/SGPQ material could be popularized to the practical application for treating papermaking wastewater.

Methods

Materials

3-Chloro-2-hydroxypropylmethyldiallylammonium chloride (CMDA): Self-prepared according to the previous synthesis processes reported in one of our contributions29. Dimethyldiallylammonium chloride (DMAC, industrial purity) was purchased from Shandong Luyue Chemical Co., Ltd (China) and treated by vacuum distillation before usage. Ammonium persulfate (APS, AR) was purchased from Tianjin Jinhai Chemical Co., Ltd (China) and directly used without any treatment. Poly(epichlorohydrin-dimethylamine) (PED, AR) was purchased from Wuxi Yatai United Chemical Co., Ltd (China) and directly used without any treatment. Lignin (AR) was purchased from Hefei BASF Biotechnology Co., Ltd (China) and directly used without any treatment.

Synthesis of chain segment crosslinker (CSC)

First, 1.0 g of poly(epichlorohydrin-dimethylamine) (PED), 1.248 g of CMDA reagent, 0.25 g of NaOH, and 2 mL of deionized water were homogeneously mixed to form the reaction solution. Subsequently, the reaction solution was heated to 60 °C to run a covalent reaction for 3.0 h. Upon completion of the reaction, the reaction solution was neutralized by 1/1 (v/v) HCl solution to pH=7 and then concentrated by distillation under reduced pressure to obtain the new chain segment crosslinker (CSC) solution, which was stored in reserve. The synthesis route of CSC crosslinker was shown as Supplementary Fig. 8.

Synthetic method of SGPQG/SGPQ

First, CSC crosslinker and co-polymerization monomer DMAC were prepared into a monomer solution with a mass fraction of 55% at a mass ratio of 20/80, and ammonium persulfate (APS) initiator accounting for 3% (w/w) of the monomer mass was added to form the reaction solution. Then, the reaction solution was warmed up to 60 °C for 5.0 h to obtain the optimized structure of the SGPQG/SGPQ composite water-purification material product, which was fully dried at 100 °C for 8.0 h and crushed with a mortar, to be stored in reserve. The synthesis route of SGPQG/SGPQ product was shown as Supplementary Fig. 9.

Performance analysis of SGPQG/SGPQ treatment with lignin in water

First, 800 mg•L-1 lignin solution was prepared to simulate paper wastewater as the study object. Next, 0.001-0.010 g of SGPQG/SGPQ composite water purification material was added to 50 mL of the above lignin solution, respectively, and treated with continuous stirring at 30 °C for 48 h. Further, the treated solution was filtered, and the absorbance of lignin solution before (A0) and after (At) treatment was measured by UV-Vis spectrophotometer. The lignin removal percentage R% was calculated by referring to the method in Eq. 2.

$$R\% = \frac{{A_0 - A_t}}{{A_0}} \times 100\%$$
(2)

Then, the mass of lignin removed by per unit mass of SGPQG/SGPQ composite water-purification material (Q’, mg•g-1) was further calculated based on the relationship between the amount of SGPQG/SGPQ composite water purification material added and the lignin removal percentage (R%).

Differential impact experiment of each component of SGPQG/SGPQ

Firstly, 3.0 g of SGPQG/SGPQ composite water-purification material was weighed and placed in 90 mL of deionized water and eluted with stirring at 50 °C for 48 h. After repeating the elution for three times, the water-insoluble SGPQG skeleton component and the water-soluble SGPQ flocculant component could be separated, and the percentage of each component in the SGPQG/SGPQ composite water-purification material system was calculated.

Secondly, under the same conditions, the differentiated effects of SGPQG/SGPQ composite water purification materials, SGPQG skeleton component materials, and SGPQ flocculant component materials on lignin removal were investigated, and the functional roles of each component in lignin removal were determined through the comparison of the differentiated effects. The experimental method for each material treatment with lignin in water was shown as described in section “Performance analysis of SGPQG/SGPQ treatment with lignin in water”. Similarly, the lignin removal percentages (R%) of each component could be calculated as the previous Eq. 2.

Moreover, the isothermal adsorption of SGPQG skeletal component materials when functioning as adsorbents were investigated, and the experimental procedure was as follows: The 0.001–0.010 g of SGPQG skeleton component materials were added into 50 mL 800 mg•L-1 lignin solution, respectively, and adsorbed at 30 °C with constant stirring for 48 h. After reaching the adsorption equilibrium, the solution was filtered. The concentration (Ce) of residual lignin in the solution was measured by UV-Vis Spectra, and the equilibrium adsorption capacity (qe) of SGPQG towards lignin could be calculated as in Eq. 3.

$$q_e = \frac{{(C_0 - C_e) \bullet v}}{m}$$
(3)

Where: v is the volume of lignin solution (mL); C0 is the initial concentration of lignin solution (mg•L-1); m is the mass of SGPQG skeleton component material added (g).

Kinetics-thermodynamics experiments of SGPQG/SGPQ treatment with lignin in water

The effect of lignin removal treated with SGPQG/SGPQ composite water purification materials at different time was investigated: 0.005 g of SGPQG/SGPQ composite water purification material was added to 50 mL, 800 mg•L-1 of lignin solution with continuous stirring action for 0.5-5 h. The average rate (AR) of lignin removal treated with SGPQG/SGPQ composite water purification material was calculated by measuring the absorbance (At) corresponding to residual lignin in water at each treatment time point by UV-Vis spectrophotometer according to the method shown in the following equation:

$$AR = \frac{{\left( {A_0 - A_t} \right) \bullet C_0 \bullet V}}{{A_0 \bullet m \bullet t}}$$
(4)

Where: C0 is the concentration of lignin solution before treatment (800 mg•L-1), V is the volume of the selected lignin solution, and m is the pure mass of SGPQG/SGPQ composite water purification material used, and t is the treatment time.

Similarly, at each treatment time interval (t), the lignin concentration (Ct) residual in the solution could be also measured by the UV-Vis spectrophotometer. The related Ct data at time “t” were used to fit the the serial reaction kinetics equations, e.g., the zero-order reaction equation (Eq. 5), the first-order reaction equation (Eq. 6), the second-order reaction equation (Eq. 7), and the third-order reaction equation (Eq. 8), to investigate the interaction kinetics behaviors of SGPQG/SGPQ treatment with lignins in water.

$$C_t = C_0 - k_0t$$
(5)
$$\ln \frac{{C_0}}{{C_t}} = k_1t$$
(6)
$$\frac{1}{{C_t}} - \frac{1}{{C_0}} = k_2t$$
(7)
$$\frac{1}{{C_t^2}} - \frac{1}{{C_0^2}} = k_3t$$
(8)

Moreover, when functioning as adsorbents, the adsorption kinetics processes of SGPQG skeletal component materials towards lignins were further carried out as follows: 0.008 g of SGPQG skeletal component materials were immersed into 50 mL 800 mg•L-1 lignin-containing solution at 30 oC to be stirred for 0.5-7.0 h, respectively. At each adsorption time interval (t), the lignin concentration (Ct) residual in the solution was timely measured by the UV-Vis Spectra technology, which could be used to further calculate the adsorption capacity (qt) of SGPQG skeletal component materials towards lignins at time “t”. The related adsorption data were used to fit the the serial adsorption kinetics equations [i.e., the pseudo-first kinetics equation (Eq. 9), pseudo-second kinetics equation (Eq. 10), intraparticle diffusion equation (Eq. 11), and particle diffusion equation (Eq. 12)], to investigate the adsorption kinetics behaviors of SHPCG towards lignins in water.

$$\log \left( {q_e - q_t} \right) = \log q_e - \frac{{k_1t}}{{2.303}}$$
(9)
$$\frac{t}{{q_t}} = \frac{1}{{k_2q_e^2}} + \frac{t}{{q_e}}$$
(10)
$$q_t = k_it^{1/2} + x_i$$
(11)
$$\ln \left( {1 - \frac{{q_t}}{{q_e}}} \right) = - k_pt$$
(12)

Finally, the effect of SGPQG/SGPQ treatment with lignin at different treatment temperatures was further carried out as follows: 0.005 g of SGPQG/SGPQ samples were added to treat 50 mL 800 mg•L-1 lignin-containing solution at 30-60 oC, while the other treatment conditions were similar to those of the above-mentioned processes. After reaching the treatment equilibrium at the corresponding adsorption temperature, the lignin concentration (Ce) residual in the solution could be measured by the the UV-Vis Spectra technology, which could be used to further calculate the concentration (Cs) of the lignins which had been removed into the material skeleton. The related data at each treatment temperature were used to fit the the serial thermodynamics equations as shown in Eqs. 13–15, respectively, achieving the corresponding thermodynamics parameters [i.e., equilibrium distribution constant Kc (Eq. 13), Gibbs free energy (∆G) (Eq. 14), enthalpy change (∆H) and entropy change (∆S) (Eq. 15)], to investigate the thermodynamics behaviors of SGPQG/SGPQ treatment with lignins in water.

$${\it{K}}_{\it{c}} = \frac{{{\it{C}}_{\it{s}}}}{{{\it{C}}_{\it{e}}}}$$
(13)
$$\Delta G = - RT\ln K_c$$
(14)
$$\ln K_c = - \frac{{\Delta H}}{{RT}} + \frac{{\Delta S}}{R}$$
(15)

Microstructure analysis of SGPQG/SGPQ treatment with lignin

The microstructural changes of SGPQG/SGPQ materials before and after treatment with lignin were compared and analyzed by optical microscopy, Fourier transforms infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), and other instrumental analysis to obtain relevant patterns.

Reuse of SGPQG/SGPQ wastes for purifying dyeing wastewater

First, 100 mg•L-1 dye solution of Reactive Scarlet 3BS was prepared to simulate dyeing wastewater as the study object. Next, 0.001 g-0.010 g of SGPQG/SGPQ wastes after treating lignins, were added to 50 mL of the above dye solution at 30 °C with continuous stirring and adsorption for 48 h. Further, the treated solution was filtered, and the absorbance of dye solution before (A0) and after (At) treatment was measured by UV-Vis spectrophotometer. The dye removal percentage R% was also calculated according to the previous Eq. 2, to evaluate the ability of SGPQG/SGPQ wastes reused for purifying dyeing wastewater.

Extending application of SGPQG/SGPQ in purification of a complex wastewater

First, a complex wastewater solution containing 800 mg•L-1 of lignins and 100 mg•L-1 of Reactive Scarlet 3BS dyes was prepared as the study object. Next, 0.001–0.010 g of SGPQG/SGPQ materials were added to 50 mL of the above complex solution, respectively, and treated with continuous stirring at 30 °C for 48 h. Further, the treated solution was filtered, and the absorbance of both lignins and dyes residue in solution before and after treatment were measured by UV-Vis spectrophotometer. Both the lignin and dye removal percentages could be calculated according to the previous Eq. 2, for investigating the application ability of SGPQG/SGPQ in purification of complex wastewater.

In addition, the effect of SGPQG/SGPQ treatment with the complex wastewater at different time was investigated: 0.01 g of SGPQG/SGPQ material was added to 50 mL of the complex wastewater solution with continuous stirring interaction for 1.0–48 h. The UV-Vis spectrophotometer technology was selected to detect the concentration changes of lignins and dyes residue in solution, to achieve the respective effects of lignin and dye removal percentages at different treatment time.