Polyacrylonitrile-based fiber modified with thiosemicarbazide by microwave irradiation and its adsorption behavior for Cd(II) and Pb(II)

Highlights

• Thiosemicarbazide modified fibrous adsorbents were prepared by MW irradiation.

• Compared with conventional method, MW irradiation showed higher efficiency and yield.

• The PAN_MW–TSC fiber exhibited high adsorption capacities for Cd(II) and Pb(II).

• The PAN_MW–TSC fibers can be readily regenerated by 0.5 M HNO₃ solution.

Abstract

A novel thiosemicarbazide modified adsorbent (PAN_MW–TSC) based on polyacrylonitrile fiber was successfully synthesized under microwave irradiation, which was applied for the uptake of Cd(II) and Pb(II) from aqueous solution subsequently. Microwave irradiation method is a new approach to achieve the modification and it turns out that just a 30 min process is enough for the anchoring of functional groups in the fiber matrix. The surface characterization was performed by fourier transform infrared spectroscopy (FT–IR), scanning electron microscopy (SEM) elemental analysis (EA) and thermogravimetric analysis (TGA), indicating that the modification was successfully accomplished. Batch adsorption experiments including equilibrium isotherms, kinetics and the effects of pH and temperature on the adsorption of Cd(II) and Pb(II) were systematically studied. Among three kinetic models, the pseudo-second-order kinetic model provides the best correlation for the process. The nonlinear resolution of the Langmuir isotherm equation has been found to show the closest fit to the equilibrium date. Thermodynamic parameters, involving △G, △H and △S were also calculated from graphical interpretation of the experimental data, which suggest that metal ions adsorption onto PAN_MW–TSC fibers is spontaneous and exothermic. Regeneration of PAN_MW–TSC fibers loaded with metal ions was efficiently done with 0.5 M HNO₃, by which the investigated adsorbent could be used reproductively for five times with a small decrease in sorption capacity. The feasible preparation of PAN_MW–TSC fibers with high adsorption capacities opens a new perspective in the potential application for wastewater treatment.

Introduction

Nowadays, with the rapid development of industry, heavy metals have been abundantly discharged into the environment through industrial wastewater, which has become a worldwide environmental threat due to their non-biodegradability and carcinogenicity [1], [2]. Discharged toxic metals typically including cadmium and lead could be accumulated into human body through food chains, causing serious diseases and disorders even at low concentrations [3], [4]. Cadimum, for example, is associated with a higher incidence of nephrotoxic effects while lead causes kidney damage, anaemia and detriment to the reproductive system. Thus, it is critical to remove Cd(II) and Pb(II) from wastewater in the perspective of environment and health. Several technologies and processes have been developed and applied for the removal of heavy metal ions from different aqueous solutions, such as chemical precipitation, ion exchange, membrane separation and electrochemical treatment [5], [6], [7]. Whereas, few satisfactory removal results could be achieved by the above-mentioned techniques on account of their low efficiency, high cost, disposal of sludge and sometimes might bring secondary pollution into the environment. Comparatively, adsorption technology with chelating ligands has received considerable attention due to its remarkable simplicity, high efficiency and availability of different adsorbents. Among various absorbents such as chitosan [8], activated carbon [9], silica nanocomposite and magnetite particles [10], [11], the absorbents based on polyacrylonitrile fiber (PANF) possess many desirable superiorities in solvent resistance, thermal and mechanical stability. The raw PANF shows no metal binding ability towards metal ions, nevertheless, the ability can be activated subsequently by modified with manifold low-molecular weight ligands [12], [13]. Xiong et al. [14]. reported that the 2-amino-1,3,4-thiadiazole functionalized PANF containing N and S atoms can be used for the selective adsorption of Hg(II) from mixed metal solutions. Zhao et al. [15]. found that phosphorylate PAN nanofiber mat exhibit high adsorption performance towards Pb²⁺, Cu²⁺, Ag⁺ and Cd²⁺ ions. According to the principle of hard and soft acids and bases, sulfur-containing groups are regarded as soft bases and therefore it is reasonable to use sulfur functionalized PANF to interact with such soft acids like Cd(II) and Pb(II) through a coordinate bond in the side chain of the polymer.

In recent years, microwave (MW) irradiation, as an alternative method, has been utilized in graft copolymerization in substitution of conventional heating methods due to its ability to ensure rapid and uniform heating in chemical reaction [16]. Basically, MW irradiation can penetrate the material and supply energy while heating can be initiated by transferring electromagnetic energy to thermal energy efficiently which results in volumetric heating [17]. Consequently, the processing time of the reaction was reduced and the product quality was enhanced. In our recent report, the iminodiacetic acid (IDA) functionalized chelating fiber was prepared by MW irradiation method which displayed a high adsorption efficiency for Cu(II) and Hg(II) [18].

In the present study, we prepared a sulfur-bearing adsorbent based on PANF in two step graft modification process through MW irradiation method. The obtained fibrous material was systematically explored as adsorbent for removal of Cd(II) and Pb(II) from aqueous solution by batch adsorption experiments. To further understand the adsorption process, the adsorption kinetics, isotherms and the thermodynamic properties of the adsorption of Cd(II) and Pb(II) on the synthetic fiber were also clarified. Moreover, the recovery of Cd(II) and Pb(II) and reusability of the adsorbent were investigated.