4-(2-Pyridylazo)-resorcinol-functionalized polyacrylonitrile fiber through a microwave irradiation method for the simultaneous optical detection and removal of heavy metals from water†

Sheng Deng , GuangShan Zhang * and Peng Wang *
State Key Laboratory of Urban Water Resource and Environment, School of Environment, Harbin Institute of Technology, Harbin 150090, China. E-mail: gszhanghit@gmail.com; pwang73@vip.sina.com

First published on 12th February 2018

---

---

Water impact Heavy metal ion detection and removal are among the most important issues in the environment. A fibrous material with these two functions has been prepared by microwave irradiation, which is fast, environmentally friendly, and highly efficient. Both colorimetric and adsorption abilities were systematically studied. Such a material has huge potential in the sensing and removal of heavy metal ions from water.

The heavy metal pollution problem in water is widely considered to be one of the most important issues nowadays. Heavy metal pollution, such as Hg²⁺, Cu²⁺, Pb²⁺, Ni²⁺, etc., commonly originates from anthropogenic activities including coal and gold mining, fossil fuel combustion, rubber and fertilizer industries, oil refining and solid waste incineration.^1–5 Many ions can have very toxic effects even at very low concentrations, as they can accumulate through the food chain and increase on a large scale. Moreover, Hg²⁺ ions can be transformed into methylmercury by microorganisms, which presents even more harmful effects to human beings.

Heavy metal ion detection in aqueous solution is usually performed by atomic absorption/emission spectrometry or inductively coupled plasma mass spectrometry, all of which require sophisticated instruments, complex experimental procedures and large financial expenses. Therefore, a simple, sensitive and reliable method for the on-site detection and removal of heavy metal ions in aqueous media is important in the environmental aspect. A variety of sensors, such as colorimetric sensors, fluorometric sensors, molecular sensors and electrochemical sensors, were developed in the last few years.^6–9 Among these, colorimetric sensors are widely investigated due to their direct and naked-eye detection potential. Heavy metal colorimetric sensors are commonly prepared based on hybrid materials,¹⁰ gels,^11,12 fibrous materials¹³ and nanomaterials.^14–20 However, the fabrication of these materials is still a challenge because of the tedious synthesis route, costly primary materials, low resulting mechanical strength and long reaction times which waste lots of energy.

4-(2-Pyridylazo)-resorcinol (PAR) is an azo dye widely used as a colorimetric reagent for metal ions such as Hg²⁺, Cu²⁺, Pb²⁺, Ni²⁺, Zn²⁺, Cd²⁺ and Co²⁺. By incorporating PAR onto the surface of fibrous materials, the PAR moieties could then be easily accessible for trace heavy metal ions in aqueous solution.^13,21 However, for the solid phase modification of PAR, the low grafting rate and long reaction time have been the hindrances to satisfactory performance. Microwave (MW) irradiation is a novel method for the tailor-made manipulation of polymer properties without changing the polymer backbone, and greater flexibility and improved product quality and properties could be obtained compared with conventional heating.^22–24 In this regard, a PAR-anchored fibrous material which acted as a dual-functional polymer for the colorimetric detection and adsorption of heavy metal ions was prepared by a microwave irradiation method. The polymer exhibited fast detection kinetics and high sensitivity, as well as good adsorption capacity for heavy metal ions. Thus, this PAR-grafted dual-functional material is unique and has huge potential applications in heavy metal sensing and water purification.

The dual-functional polymer was successfully prepared by grafting PAR onto polyacrylonitrile (PAN) fibers under MW irradiation through two modification steps: amination and the Mannich reaction (Fig. S1†), and details are explicated in the ESI.† The polymer was named PAN_MW-PAR. Fig. S2† shows the major components of the MW reactor. For comparison, conventional heating was also applied for the preparation of colorimetric fibers and the obtained fibers were named PAN_CV-PAR. The parameters and results of these two methods are presented in Table 1. The microwave irradiation method shows a superior advantage over conventional heating in the aspect of reaction time, grafting rate and elongation percentage. The high efficiency of MW irradiation might be due to its unique heating way and mass transfer characteristics. Generally, the types of solvents play an important role in MW reaction because of their different abilities to convert electromagnetic energy into heat at a given frequency and temperature. The loss factor named dielectric loss tangent (tanδ) indicated the ability of the material to be polarized and heated. Solvents with high tanδ values are more likely to provide more rapid heating, and thus reduce side reactions while boosting the grafting efficiency. The tanδ values for ethylene glycol and water are 1.35 and 0.123, respectively. In our previous studies, we have found that water has a good effect on the amination of PAN fibers. Thus in the present study, water was still chosen as the solvent for amination reaction, and a 39.1% grafting rate was achieved in 0.5 h, as shown in Table 1. The grafting rate under MW irradiation is almost 3 times that of conventional heating, while the reaction time is dramatically shortened. Additionally, PAR is immobilized through the Mannich reaction, which is widely considered to be needed under high temperature. The high loss factor and boiling point of ethylene glycol enable the reaction to be performed at 160 °C and this temperature could be obtained in only 32 s in the MW reactor. Consequently, this reaction is implemented in 0.33 h with a 31.5% grafting rate. The mechanical strength of the modified fibers is characterized by the elongation percentage and higher values indicate a more preserved mechanical strength property. As shown in Table 1, the elongation percentage of PAN_MW-PAR fibers is almost 4 times higher than that of PAN_CV-PAR fibers. The enhanced mechanical properties might due to the uniform volumetric heating characteristic of MW irradiation.

---

The colorimetric effects on a variety of heavy metal ions of PAN_MW-PAR and PAN_CV-PAR are shown in Fig. 1. As illustrated in Fig. 1a, the PAN_MW-PAR fibers exhibit a significant color change from red to black when they encounter Hg²⁺, Pb²⁺, Cu²⁺ and Ni²⁺ ions. Moreover, a weak color change from red to purple of PAN_MW-PAR fibers for Cr³⁺, Cd²⁺ and Zn²⁺ ions is observed, which is in agreement with the findings for PAN_CV-PAR fibers for Hg²⁺, Pb²⁺ and Cu²⁺ ions, as shown in Fig. 1b. Meanwhile, no obvious color change could be observed with the naked eye in the presence of Cr³⁺, Cd²⁺, Zn²⁺ and Ni²⁺ ions for PAN_CV-PAR fibers.

These results clearly indicate that the colorimetric display of PAN_MW-PAR fibers is more sensitive than that of PAN_CV-PAR fibers, with the PAN_MW-PAR fibers showing an obvious naked-eye detection ability for Hg²⁺, Pb²⁺, Cu²⁺ and Ni²⁺ ions. The higher grafting rate of PAR onto the modified fiber is the main reason for the better performance of PAN_MW-PAR fibers than PAN_CV-PAR fibers. It is worth mentioning that all fibers prepared present no color change for light metal ions such as Ca²⁺, Mg²⁺, and Al³⁺. Due to the distinct colorimetric effect of PAN_MW-PAR fibers towards Hg²⁺, Pb²⁺, Cu²⁺ and Ni²⁺, these four ions were chosen as the target ions in the following experiments.

The effect of pH on the colorimetric behaviour of PAN_MW-PAR fibers was investigated. The pH range investigated for Hg²⁺ and Ni²⁺ ions was from 2–10, while it was 2–6 for Cu²⁺ and Pb²⁺ ions. The reason for this is that copper and lead will form a precipitate over pH = 6 at this concentration. According to the results of pH influence shown in Fig. 2, PAN_MW-PAR fibers demonstrate excellent colorimetric sensitivity towards Hg²⁺ ions at a concentration of 10 mg L⁻¹ in a wide pH range (pH = 2–10) by exhibiting identical red to black color changes. The wide pH range recognition towards Hg²⁺ might be due to the high affinity effect between PAN_MW-PAR fibers and Hg²⁺. The full coverage of pH range has a significant value for the practical applications of PAN_MW-PAR fibers for the treatment of Hg²⁺ in wastewater. As for nickel, copper and lead ions, the red to black change is found to be at pH values of 6–7, 4–6 and 5, respectively, while a slighter color change from red to purple is observed at pH values of 8–10, 2–3 and 4, respectively. At low pH values, the less effective colorimetric sensitivity may be due to the protonation of nitrogen or hydroxyl on the surface of PAN_MW-PAR fibers, resulting in poor sensing performance because of the electrostatic repulsion force.²⁵

The sensitivity of PAN_MW-PAR fibers for heavy metal ions was tested at different response times and the results are presented in Fig. 3. The rapid naked-eye detection response for Hg²⁺ ions is found to be only 5 s at a concentration of 10⁻² mol L⁻¹, and a remarkable color change from red to black was observed. In comparison, the color change of PAN_MW-PAR fibers in the presence of Cu²⁺, Pb²⁺ and Ni²⁺ can be clearly viewed in 5 min, 30 min and 60 min, respectively. These results indicate that PAN_MW-PAR fibers offer a higher sensitivity towards Hg²⁺ than towards Cu²⁺, Pb²⁺ and Ni²⁺.

To gain insight into the colorimetric properties of PAN_MW-PAR and its metal ion complexes, the fibers were ground into powder and the UV-vis spectra of the dry powder were recorded with BaSO₄ as the background. The results are shown in Fig. 4.

A typical adsorption spectrum of metal-free PAN_MW-PAR fibers is observed at 420 nm, and the same absorbance value is detected when complexed with Ca²⁺, Mg²⁺ and Al³⁺ ions, as illustrated in Fig. 4a. This result coordinates with the no color change results of PAN_MW-PAR fibers towards these three metal ions. After binding with Hg²⁺, Cu²⁺, Pb²⁺ and Ni²⁺ ions, the fibers give a distinct color change and the absorbance peak at 420 nm disappeared while new peaks appear. Meanwhile, the maximum absorbance value of Hg²⁺ and Cu²⁺ ions is different from the other two metal ions and the λ_max is observed at 520 nm, as presented in Fig. 4b, while the λ_max for Pb²⁺ and Ni²⁺ ions is detected at 540 nm. Commonly, four types of electronic transition mechanisms are believed to take place in coordination compounds, including metal-centered (MC), ligand-centered (LC), ligand-to-metal charge transfer (LMCT) and metal-to-ligand charge transfer (MLCT) transitions.²⁶ The variation in the aspects of valence electron structures, ionic radii and unoccupied orbitals within different metal ions could cause different ligand field transitions or charge transfers, which might explain why the absorption peaks of PAN_MW-PAR fibers complexed with different metals are slightly different.

The PAN_MW-PAR fibers provide excellent naked-eye detection for Hg²⁺ and Fig. 5 shows the color changes in different concentrations of Hg²⁺ between 0.5 mg L⁻¹ and 20 mg L⁻¹. The PAN_MW-PAR fibers display distinguishable color changes from red to dark purple/black upon increasing the Hg²⁺ concentration. These color changes could be clearly identified with the naked eye or a digital camera. Moreover, Fig. S3† shows the absorbance intensity of PAN_MW-PAR fibers after being immersed in Hg²⁺ solution, and the intensity at 520 nm increases gradually with increasing Hg²⁺ concentration. A linear relationship between the adsorption intensity and Hg²⁺ concentration over the range of 0.5–20 mg L⁻¹ is also illustrated in Fig. S4.† The naked-eye detection limit of PAN_MW-PAR fibers for Hg²⁺ is further calculated to be 35 μg L⁻¹. Although this value is larger than the maximum permissible concentration (2 μg L⁻¹) of Hg²⁺ in drinking water, according to United States Environmental Protection Agency (EPA),²⁶ it can still be applied as a Hg²⁺ sensor in some specific wastewater such as electroplating effluent and sudden Hg²⁺ pollution in water.

Additionally, the PAN_MW-PAR fibers not only exhibit superior colorimetric performance towards heavy metals ions but also possess good adsorption capacities. The influence of pH is investigated initially and Fig. 6 shows the sorption of Hg²⁺, Cu²⁺, Pb²⁺ and Ni²⁺ on PAN_MW-PAR fibers as a function of pH ranging from 2–9.

In these experiments, the metal concentration used was 200 mg L⁻¹. The pristine PAN fibers present no adsorption ability for metal ions, as reported in our previous studies.²⁵ As illustrated in Fig. 6, after the surface modification, the adsorption uptake of PAN_MW-PAR fibers increased significantly. In all instances, the adsorption affinity as a function of pH shows a similar profile that the adsorption amount increases sharply over the pH range from 2 to 7 before a plateau is observed with a further increase in pH. The optimum pH condition for Hg²⁺, Cu²⁺, Pb²⁺ and Ni²⁺ uptake is obtained at pH 7, 6, 5 and 6, respectively, which coincides with the pH effect on the colorimetric monitoring of these four ions, as mentioned before. This result indicates that the colorimetric effect of PAN_MW-PAR fibers on metal ions has a positive correlation with the adsorption amount onto the modified fibers. The maximum adsorption capacity of PAN_MW-PAR fibers for Hg²⁺, Cu²⁺, Pb²⁺ and Ni²⁺ is observed at 167.65 mg g⁻¹, 100.73 mg g⁻¹, 74.55 mg g⁻¹ and 65.48 mg g⁻¹, respectively. At pH below 6, the protonation of PAR groups occurs on the modified fibers and the surface is positively charged. Therefore, the repulsive electrostatic interaction between PAN_MW-PAR fibers and the metal ions is significant as these four ions exist in their ion form. In addition, the competitive adsorption effect with hydrogen ions under lower pH further decreases the adsorption uptake amount. The increase in solution pH results in weakened electrical repulsion force and metal ions are more accessible to be adsorbed onto PAN_MW-PAR fibers. However, a further increase in pH results in the formation of precipitates, and the adsorption amount decreases accordingly.

The isothermal adsorption experiments were performed in metal ion solutions ranging from 5 mg L⁻¹ to 250 mg L⁻¹ without adjusting the pH values. The adsorption isotherm results of PAN_MW-PAR fibers for Hg²⁺, Cu²⁺, Pb²⁺ and Ni²⁺ ions are shown in Fig. 7. As depicted in Fig. 7, the PAN_MW-PAR fibers show better affinity for Hg²⁺ compared to Cu²⁺, Pb²⁺ and Ni²⁺ ions, which is in accordance with the colorimetric sensitivity results illustrated previously. The affinity of these four ions towards PAN_MW-PAR fibers is in the order Hg²⁺ > Cu²⁺ > Pb²⁺ > Ni²⁺, and the maximum adsorption capacities are 181.2 mg g⁻¹, 116.9 mg g⁻¹, 77.07 mg g⁻¹ and 70.33 mg g⁻¹, respectively. The Langmuir isotherm gives good fitting results for the adsorption data (ESI†), thus the chemical adsorption mechanism is believed to be responsible for the adsorption process. PAR was anchored onto the fibers through the Mannich reaction, by which a Schiff base was firstly formed between the aminated fibers and formaldehyde, and then reacted with the α-H moiety of PAR.²⁷ Due to the electron donate effect of phenolic hydroxyl, the α-H is located in the 2° position of benzene group. In this regard, the pyridine nitrogen atom, the azo nitrogen close to the resorcin ring, and the o-hydroxyl group of the PAR molecule could all exert effects to form two stable 5-membered chelate rings with metal ions,^28,29 as illustrated in Fig. 8.

In order to be successfully applied in wastewater treatment, the dual-functional fibers should be reversible for the colorimetric detection and adsorption of heavy metal ions. Several reagents were investigated for the regeneration of PAN_MW-fibers, including 0.01 M H₂SO₄, 0.1 M EDTA and thiourea. The results show that 0.1 M EDTA enables the complete and rapid regeneration of PAN_MW-fibers, while 0.01 M H₂SO₄ and 0.1 M thiourea are less effective in regeneration (ESI†). This can be attributed to the high stability of metal ion–PAN_MW-PAR fiber complexes.

The re-adsorption results presented in Fig. 9 reveal that the regenerated fibers could still extract these four metal ions after 10 times of usage and the their adsorption capacities allowed them to retain more than 80% of their original amount for Hg²⁺, Cu²⁺, Pb²⁺ and Ni²⁺. As shown in Fig. 10, the differences of the PAN_MW-PAR fiber–Hg²⁺ complex in terms of color and UV-vis spectrum can barely be distinguished after 10 times of usage, and the naked-eye detection performance towards Hg²⁺ still responds effectively and rapidly. These results prove that the dual-functional colorimetric adsorbent is a promising environmental material for heavy metal detection and elimination from wastewater.

Conclusions

In conclusion, we have developed a fiber-based environmental functional material that shows a unique two-in-one capability to remove and optically detect toxic heavy metal ions. A microwave irradiation method was applied for the surface grafting process and it exhibited a superior advantage over conventional heating in terms of reaction time, grafting rate and colorimetric effect. The dual-functional material could optically detect heavy metal ions in aqueous solution and especially exhibits high sensitivity and wide pH applicability for Hg²⁺ compared to other heavy metal ions. To the best of our knowledge, this is the first example of a PAR-immobilized polymer that preserves both excellent colorimetric sensitivity and good adsorption capability for Hg²⁺ ions. Additionally, this novel developed material could also act as an indicator when combined with other adsorbents by giving an optical color change when the saturated adsorption of the adsorbent is achieved. Thus, this facile synthesis technology and dual-functional material could broaden the applications of absorbents for heavy metal remediation in water.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (51678185), the State Key Laboratory of Urban Water Resource and Environment (Harbin Institute of Technology) (2017DX11), and the Technological Innovation Talent Special Fund of Harbin, China (2016RAQXJ175).

Notes and references

1. X. Wu, S. Chen, J. Guo and G. Gao, J. Cleaner Prod., 2018, 170, 399–406 CrossRef .
2. S. Rajeshkumar, Y. Liu, X. Zhang, B. Ravikumar, G. Bai and X. Li, Chemosphere, 2018, 191, 626–638 CrossRef CAS PubMed .
3. L. Chen, S. Zhou, Y. Shi, C. Wang, B. Li, Y. Li and S. Wu, Sci. Total Environ., 2018, 615, 141–149 CrossRef CAS PubMed .
4. L. Gao, J. Chen, C. Tang, Z. Ke, J. Wang, Y. Shimizu and A. Zhu, Environ. Sci.: Processes Impacts, 2015, 17, 1769–1782 CAS .
5. F. Zan, S. Huo, B. Xi, J. Su, X. Li, J. Zhang and K. M. Yeager, J. Environ. Monit., 2011, 13, 2788–2797 RSC .
6. J. Gong, T. Zhou, D. Song, L. Zhang and X. Hu, Anal. Chem., 2010, 82, 567–573 CrossRef CAS PubMed .
7. Y.-R. Kim, R. K. Mahajan, J. S. Kim and H. Kim, ACS Appl. Mater. Interfaces, 2010, 2, 292–295 CAS .
8. A. Kumar Shaily and N. Ahmed, Ind. Eng. Chem. Res., 2017, 56, 6358–6368 CrossRef CAS .
9. X. Lu, Z. Guo, M. Feng and W. Zhu, ACS Appl. Mater. Interfaces, 2012, 4, 3657–3662 CAS .
10. T. Liu, G. Li, N. Zhang and Y. Chen, J. Hazard. Mater., 2012, 201–202, 155–161 CrossRef CAS PubMed .
11. X. Zhou, J. Nie and B. Du, ACS Appl. Mater. Interfaces, 2015, 7, 21966–21974 CAS .
12. S. Sengupta and R. Mondal, J. Mater. Chem. A, 2014, 2, 16373–16377 CAS .
13. X. Liu, X. Liu, M. Tao and W. Zhang, J. Mater. Chem. A, 2015, 3, 13254–13262 CAS .
14. Y. Li, Y. Wen, L. Wang, J. He, S. S. Al-Deyab, M. El-Newehy, J. Yu and B. Ding, J. Mater. Chem. A, 2015, 3, 18180–18189 CAS .
15. Y. Li, L. Wang, Y. Wen, B. Ding, G. Sun, T. Ke, J. Chen and J. Yu, J. Mater. Chem. A, 2015, 3, 9722–9730 CAS .
16. Y. Li, L. Wang, X. Yin, B. Ding, G. Sun, T. Ke, J. Chen and J. Yu, J. Mater. Chem. A, 2014, 2, 18304–18312 CAS .
17. Y. Si, X. Wang, Y. Li, K. Chen, J. Wang, J. Yu, H. Wang and B. Ding, J. Mater. Chem. A, 2014, 2, 645–652 CAS .
18. L. Zhang, H. Chang, A. Hirata, H. Wu, Q.-K. Xue and M. Chen, ACS Nano, 2013, 7, 4595–4600 CrossRef CAS PubMed .
19. C. Díez-Gil, R. Martínez, I. Ratera, A. Tárraga, P. Molina and J. Veciana, J. Mater. Chem., 2008, 18, 1997–2002 RSC .
20. R. Das, C. D. Vecitis, A. Schulze, B. Cao, A. F. Ismail, X. Lu, J. Chen and S. Ramakrishna, Chem. Soc. Rev., 2017, 46, 6946–7020 RSC .
21. Z. Hua, B. Yang, W. Chen, X. Bai, Q. Xu and H. Gu, Appl. Surf. Sci., 2014, 317, 226–235 CrossRef CAS .
22. L. Hu, G. Zhang, Q. Wang, Y. Sun, M. Liu and P. Wang, Chem. Eng. J., 2017, 326, 1095–1104 CrossRef CAS .
23. D. Gao, Q. Hu, H. Pan, J. Jiang and P. Wang, Bioresour. Technol., 2015, 193, 507–512 CrossRef CAS PubMed .
24. S. Deng, G. Zhang, S. Liang and P. Wang, ACS Sustainable Chem. Eng., 2017, 5, 6054–6063 CrossRef CAS .
25. S. Deng, G. Zhang, S. Chen, Y. Xue, Z. Du and P. Wang, J. Mater. Chem. A, 2016, 4, 15851–15860 CAS .
26. J. Ghasemi, H. Peyman and M. Meloun, J. Chem. Eng. Data, 2007, 52, 1171–1178 CrossRef CAS .
27. N. D. Rudd, H. Wang, E. M. A. Fuentes-Fernandez, S. J. Teat, F. Chen, G. Hall, Y. J. Chabal and J. Li, ACS Appl. Mater. Interfaces, 2016, 8, 30294–30303 CAS .
28. N. A. Yusof and M. Ahmad, Spectrochim. Acta, Part A, 2008, 69, 413–418 CrossRef PubMed .
29. P. A. Jeronimo, A. Araujo, M. C. S. M. Montenegro, C. Pasquini and I. Raimundo, Anal. Bioanal. Chem., 2004, 380, 108–114 CrossRef CAS PubMed .

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ew00045j

This journal is © The Royal Society of Chemistry 2018