1. Introduction

Ever increasing industrialization to meet the growing demand mankind enhance the release of wastewater creating environmental pollution[1,2]. Presence of heavy metal ion in wastewater is a worldwide problem[3,4]. Polyacrylonitrile (PAN) is one of the excellent economical polymer having plenty of end functional cyano group available for further modification[5-12]. Although pristine polymer is not effective for heavy metal removal but chemical modification with various functional group make is one of the most useful fiber material metal ion adsorptions. Thus, ethylenediaminetetraacetic acid (EDTA), phosphonic, phosphorylated and thiosemicarbazide functional group are linked to the PAN fiber for the removal of Pb²⁺, Hg²⁺, Cd²⁺, Ag⁺, Zn²⁺, Cu²⁺, Ni²⁺, Co²⁺, Ca²⁺ and Mg²⁺ heavy metal ions (Fig. 1). Composite PAN fiber is another class of modified polymer fiber used for the removal of heavy metal ions from waste water.

In this review we focused mainly on the PAN fiber prepared by electrospinning or already available fiber available commercially and end functionalized by grafting or crosslinking method for application of heavy metal ion adsorption. Another section deals with the composite PAN fiber for cleaning of wastewater.

2. Polyacrylonitrile Fiber

2.1. Functionalization of polyacrylonitrile fiber

Polyacrylonitirile fiber (PANF) with approximately 10 μm diameter was crosslinked by diethylenetriamine (DETA) in microwave reactor at 200 MW for 20 min [13]. Further the cross-linked PANF was treated with sodium sulfide in phosphate buffer to transform to thio group in microwave reactor at same power but for short time duration of 5 min. Successful preparation PAN-Thio fiber was confirmed by Fourier transform-infrared spectroscopy (FT-IR), X-ray power diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The adsorption efficiency of PAN-Thio was group was checked by dipping 0.1 g fiber in 100 mL solution ion at 7.0 pH. The target ion of separations is Hg²⁺ and Cd²⁺ in which the Pb²⁺, Cu²⁺, Ni²⁺ and Zn²⁺ interfering ion are present in the solution to check its efficiency. The adsorption efficiency of Hg²⁺ system is of the order of Hg²⁺ ˃ Pb²⁺ ˃ Cu²⁺, ˃ Zn²⁺ ˃ Ni²⁺ that has adsorption capacity of 218.7 mg g^-1, 52.42 mg g^-1, 25.71 mg g^-1, 12.94 mg g^-1 and 8.5 mg g^-1 respectively. Adsorption of Cd²⁺ ion is similar the Hg²⁺ but the in case of order Ni²⁺ is higher than Zn²⁺. Distribution coefficient (K_d) value of Hg²⁺ is higher than Zn²⁺ due to Pearson’s hard-soft-acid-base theory (HSAB). pH of the solution has immense effect on the adsorption of the metal ion and at ˂ 3 pH Hg²⁺ is present but above it HgCl₂, (HgCl₂)₂, Hg(OH)⁺, HgOHCl are there. At lower pH ionic species has higher hydration than undissociated molecule which reduce the adsorption rate of the hydrated entities. So, at higher pH of 7 the adsorption of Hg²⁺ and Cd²⁺ are highest.

Polyacrylonitrile fiber was amine functionalized by treatment with triethylene tetramine[14]. It was further modified with phosphonic acid group. The successful modification was studied by FTIR, XRD and SEM. The modified fiber shows excellent adsorption of Hg²⁺ ion which 400% higher than the unmodified one. Among Pb²⁺, Hg²⁺, Cd²⁺, Ag⁺, Zn²⁺, Cu²⁺, Ni²⁺, Co²⁺, Ca²⁺ and Mg²⁺ ions the best performance is against Hg²⁺. Selectivity of the phosphonic modified PAN fiber was checked by placing in a mixed ion solution with concentration of individual 1.5 × 10^-5 mol L^-1. It shows highest selectivity for Hg²⁺ ion. Solution pH affect a lot on the adsorption properties of the metal ion. When pH is lower than 4, there is poor adsorption of metal ion due to the interference of the large amount of proton surrounding the modified PAN fiber. In the pH range of 5~9 highest adsorption of 1.8 mmol g^-1. Same group modified PAN fiber by refluxing with triethylene tetramine in deionized water. Further it was refluxed with phosphrous acid and formaldehyde in presence of ethanol to have end functional phosphonic acid group [15]. The end functional group showed highest selectivity towards Hg(II) ions in presence of Pb(II), Cd(II), Ag(I), Zn(II), Zn(II), Cu(II), Ni(II), Co(II), Ca(II) and Mg(II). It has very wider range operating pH range from 3 to 11 leading to vast flexibility in its application. Metal ion adsorption process follow pseudo second order kinetic model. Langmuir model of adsorption indicate adsorption of 358 mg g^-1. The reusability of the modified is more that ten times leading to its commercial potential.

Hu et al. prepared PAN nanofiber by electrospinning method. It was crosslinked by hydrazine hydrate and aminated by diethylenetriamine[16]. The amine functional group was phosphorylated by paraformaldehyde and phosphrous acid. Functionalization of the nanofiber leads to the presence of 5.45 mg of phosphrous per gram of PAN nanofiber. Adsorption properties of the nanofiber was checked by adsorption isotherm and adsorption kinetics. At 6 pH metal ions shows highest adsorption on the phosphorylated PAN nanofiber mat. Pb²⁺, Cu²⁺ and Ag⁺ obey Freundlich isotherm model while Cd²⁺ follows Langmuir isotherm model. Adsorption kinetics Pb²⁺, Cu²⁺ and Ag⁺ and Cd²⁺ follows pseudo second order kinetics. The modified nanofiber mat shows better regeneration ability and about 80% of the initial adsorption efficiency at the end of third cycle. Another group used similar technique to prepare phosphorylated PAN nanofiber prepared by electrospinning method and studied removal of Cu(II), Ni(II), Cd(II) and Ag(I) ions[17]. The highest adsorption efficiency is 92.1, 68.3, 14.8, and 51.7 mg g^-1 for Cu(II), Ni(II), Cd(II) and Ag(I) ions respectively as estimated by Langmuir model. Modified nanofiber showed good reusability even after four cycles.

PAN fiber was grafted with thiosemicarbazide in two steps[18]. In the first step the fiber is crosslinked with diethylenetriamine by microwave process and followed by refluxing with thiosemicarbazide in DMF at 125°C. Grafted fiber was characterized with FTIR, SEM and TGA. Cd(II) and Pb(II) metal ions are tested for adsorption study. Chemically modified PAN fiber shows highest adsorption at 6.4 pH due the presence of the sulfur and nitrogen at the end of the chain of the adsorbent. Pseudo second order was followed for adsorption kinetics. According to Langmuir model the best adsorption for Cd(II) was 1.47 mmol g^-1 and 1.01 mmol g^-1 for Pb(II) ions. Thermodynamics studies showed that the adsorption metal ions are spontaneous and exothermic process. Modified PAN fiber can be reused for five times with very less decrease in the adsorption efficiency.

Polyacrylonitrile was electrospun by electrospinning method and crosslinked with ethylenediamine (EDA) [19]. It was further modified with polyethylenediami- netetraacetic acid (EDTA). Modification was confirmed by FTIR, thermogravimetric analysis (TGA) and SEM. Targeted metal ion for the batch adsorption study are Cd(II), Cr(III) and Cr(VI). Effect of pH contact time and metal ion concentration was studied in this report. Maximum adsorption was observed for Cr(VI) in acidic pH but in case of Cr(III) and Cd(II) it is basic medium. It takes 1.5 h for Cd(II) to reach equilibrium adsorption on the modified PAN nanofiber. The nanofiber can be reused for four times with efficiency above 90% for Cr(VI). PAN fiber membrane was cut into pieces and dipped in alkali solution to convert to acid functional group[20]. Then it was dipped in diazo resin (DR) and washed for few times and again dipped in ethylenediaminetetraacetic acid (EDTA) for 24 h and washed cleanly (Fig. 2). It was exposed to UV light to cross link EDTA with DR and DR with PAN. The modified membrane was used to adsorb Cu(II), Pb(II) and Hg(II) from wastewater. Cu(II) adsorption by five modification layer on PAN fiber membrane shows highest value of 50.3 mg g^-1. Water flux with two modification layer has the best flux of 1,085.94 g m^-1 h^-1. The amount of copper adsorption on the three layers modification is 43.5 mg g^-1 when the solution concentration is 50 mg L^-1. Cu(II) metal ion adsorption PAN fiber membrane follow the Langmuir model. At 6.5 pH, 37.4 mg g^-1 of Cu(II) ions are adsorbed on the three modified layer membrane. At 65°C, the same membrane adsorbs 46.2 mg g^-1. PAN nanofiber was prepared by electrospinning method and coated with polydopamine to have hydroxyl end group[21]. Then 2-bromoisobutyryl bromide was linked to this hydroxyl functionalized PAN nanofiber which acts as macroinitiator and further grafted to poly (glycidyl methacrylate) (PGMA) to prepare PAN@PGMA graft copolymer. The graft copolymer was treated with thiourea and then ring opening was performed by treatment with NaHS to synthesize PAN@SH nanofiber membrane. Successful transformation was checked by FTIR, XPS and SEM. Range of pH in which the nanofiber membrane shows best performance for Hg(II) adsorption is 5.5~6. The process of adsorption was explained by pseudo-second- order model. Hg(II) ion concentration within the range of 50 to 200 mg L^-1 reached an adsorption equilibrium within 25 mints. Langmuir isotherm model was observed by isotherm date. The performance of the nanofiber membrane was intact up to 90% till three times.

PAN nanofiber was prepared by electrospinning method[ 22]. PANF was functionalized with hydroxyl amine hydrochloride to synthesize amdoxine PANF. Adsorption of Pb(II) and Cu(II) metal ions are until 80 h. The adsorption process follows Langmuir isotherm. The maximum adsorption capacity of Cu(II) is 52.7 mg g^-1 and 265.65 mg g^-1 for Pb(II).

Zhao et al. compared the Cr(VI) metal ion adoption performance of branched polyethylenimine grafted PAN nanofiber with normal PAN fiber[23]. It showed that nanofiber grafted PAN membrane has highest adsorption of 637.46 mg g^-1. Grafted membrane has can be reused even after 10 cycles. By batch adsorption or dynamic filtration the concentration of Cr(VI) ion can be reduced to 0.05 mg L^-1. The grafted PAN membrane retains great fibrous morphology after adsorption-regeneration cycles, which indicate bPEO-EPAN’s great stability and durability (Fig. 3 and Fig. 4).

2.2. PAN composite fiber

Mg/Al layered double hydroxide intercalated with ethylenediamine tetraacetic acid (EDTA) are prepared with co-precipitation method[24]. Layered double hydroxide (LDH) was mixed with PAN solution and electrospun to fabricate LDH@PAN composite nanofiber membrane. XRD, XPS, Extended X-ray absorption fine structure (EXAFS) spectroscopy studies were performed to characterize the nanofiber composite. EDTA has the advantage of excellent chelating agent and higher surface area of the composite nanofiber has the advantage for metal ion adsorption. Adsorption properties of Cu(II) metal ions studied in this article. The highest adsorption capacity of composite PAN nanofiber membrane was 120.77 mg g^-1. In the pH range of 3~6 maximum metal ion adsorption took place. With the increase in the temperature metal ion adsorption increases. Equilibrium adsorption data follow pseudo second order model.

Polyacrylonitrile/Prussian blue (PB) nanofiber was prepared by electrospinning method[25]. About 5 to 10 wt% of PB powder was mixed with PAN solution in prepared in DMF and electrospun at an electric field of 20 kV and 15 cm between needle and collector plate. PAN/PB nanofiber was characterized by FTIR. Surface area was determined by Brunauer-Emmett-Teller (BET) from nitrogen adsorption-desorption isotherms. Surface area increased from 7.7 to 28.7 m² g^-1 for PAN and PAN/PB(10) composite nanofiber respectively. Higher the surface area better is the metal ion adsorption. Maximum adsorption of cesium ion by PAN/PB composite nanofiber is 0.14 mmol g^-1.

Polyacrylonitrile/magnetite (Fe₃O₄) composite nanofiber was prepared in two steps. Solution of PAN and ferric chloride was prepared in DMF and electrospinning was performed[26]. Composite PAN nanofiber was treated with iron sulfate followed by treatment with alkali at 90°C to transform to magnetite nanocomposite fiber. The performance of Pb(II) metal ion adsorption by the composite fiber was tested. Batch mode was followed for adsorption study and seen that it follows Langmuir adsorption isotherm model. The highest adsorption capacity for Pb(II) metal ion is 156.25 mg g^-1. Another group reported the removal of arsenate (AsO^3-) ion by superparamagnetic iron oxide nanoparticle (SPION) loaded PAN composite nanofiber[27]. PAN nanofiber synthesized by electrospinning was hydrolyzed by alkali (Fig. 5). Carboxylic functional group present on the surface of nanofiber is active group that interact with SPION when dipped in a SPION containing solution. At pH 3 the highest arsenate adsorption on the SPION immobilized PAN nanofiber is 851.7 mg g^-1 of adsorbent.

3. Conclusions

Environmental pollution is ever increasing which is a growing area health concern. Wastewater treatment is important area of research to design cost effective process. Polyacrylonitrile nanofiber can be prepared by electrospinning and modified easily to prepare metal ion adsorbent. Nanofiber has the highest surface area that enhance the efficiency of the material as well as linking it to various chelating agent makes it more effective. Nanocomposite PAN fiber are another type of material which further efficient for the removal of heavy metal ions.