## 3 Results and Discussion
### Removal of RB-19  
Figure 7 shows the removal of RB-19 by free Lac, immobilized Lac, and carrier (P_p_PD/Fe\({}_{3}\)O\({}_{4}\) nanocomposite without Lac) versus time. For immobilized Lac, the removal rate increased rapidly during the first 1 h of treatment and reached 80%. In comparison, the removal efficiency of dye by free Lac only reached 20%. For the carrier, approximately 40% dye removal was obtained in the first 1 h of incubation, and subsequently no significant increase was found. The primary charge on the surface of P_p_PD/Fe\({}_{3}\)O\({}_{4}\) nanocomposite was positive at pH 4. Thus, the adsorption of dye on the carrier was partly attributed to ionic electrostatic attractions between the positively charged sites of the carrier surface and the negative sulfonyl (\(-\)SO\({}_{3}\)) groups of dye molecules (Figure 1). Additionally, other adsorption mechanisms such as hydrogen bonding may play a partial role, which may be due to the interactions of groups on the surface of carrier with groups of the RB19 molecule [39]. The dye removal efficiency of immobilized Lac (80%) was clearly higher than that of free Lac under the same conditions, and was even higher than the sum of dye removal by free Lac and the carrier (60%), which can be attributed to two causes. The first is the dye adsorption by the carrier that contributes to  
Figure 6: (**a**) Storage stability; (**b**) thermal stability; and (**c**) operational stability of free and immobilized Lac; (**d**) Recovery of immobilized Lac from the solution using a magnet. -type removal. The second is increments in solution conductivity brought about by the immobilization of Lac on P_p_PD/Fe\({}_{3}\)O\({}_{4}\) nanocomposite, which may be an important factor affecting the removal efficiency. Figure 8 shows that the current of the immobilized Lac-P_p_PD/Fe\({}_{3}\)O\({}_{4}\) nanocomposite was higher than that of the carrier without Lac, suggesting that electron transfer occurs between Lac and the P_p_PD fiber. Figure 9 shows the schematic illustration of the interaction mechanisms between Lac, P_p_PD fiber, and RB-19. It can be speculated that the conducting P_p_PD/Fe\({}_{3}\)O\({}_{4}\) fiber mat may act as a \"molecular wire\" and provide effective channels for electron transfer. After the RB-19 molecules were adsorbed to the P_p_PD/Fe\({}_{3}\)O\({}_{4}\) nanocomposite, the carrier may promote electron transfer from the redox centers of Lac to the dye and enhance the catalytic activity of Lac [40; 41]. Similar results have been reported for catalase covalently bound to electrospun nanofiber meshes filled with carbon nanotubes [40]. It was also reported that the conducting carbon nanotubes were able to promote direct electron transport from redox enzymes such as ascorbate oxidase, peroxidase, and Lac [38; 40; 41; 42; 43; 44; 45; 46]. Therefore, P_p_PD/Fe\({}_{3}\)O\({}_{4}\) nanocomposite may also be suitable for enzyme immobilization, especially for redox enzymes. Figure 8: Cyclic voltammetry curves of the P_p_PD/Fe\({}_{3}\)O\({}_{4}\) and Lac-P_p_PD/Fe\({}_{3}\)O\({}_{4}\) nanocomposites. Figure 7: Removal of dye by free Lac, immobilized Lac, and carrier versus time. The reusability of immobilized enzymes is important because of its influence on processing costs in wastewater treatment industries [47]. Figure 10 shows that when the immobilized enzyme was subjected to repeated use and recovery cycles, for the first two cycles, the percentage of dye removal by Lac-P\(p\)PD/Fe\({}_{3}\)O\({}_{4}\) remained the same (90%), and then the recovery efficiency decreased gradually with every cycle, reaching 43% at the eighth cycle. The decreased dye removal efficiency may be related to several factors. First, the enzyme may be inactivated or inhibited by the accumulation of dye degradation products [48]. Second, mass transfer limitations may be induced by dye or metabolite adsorption to the P\(p\)PD/Fe\({}_{3}\)O\({}_{4}\) carrier [49; 50].