Development and application of novel bio-magnetic membrane capsules for the removal of the cationic dye malachite green in wastewater treatment

Abstract

Novel bio-magnetic membrane capsules (BMMCs) were prepared by a simple two-step titration-gel cross-linking method using a polyvinyl alcohol (PVA) and sodium alginate (SA) matrix to control the disintegration of phytogenic magnetic nanoparticles (PMNPs) in an aqueous environment, and their performance was investigated for adsorbing cationic malachite green (MG) dye from water. The prepared BMMCs were characterized by FTIR, powder XRD, SEM, EDX, XPS, VSM and TGA techniques. The findings revealed that the hysteresis loops had an excellent superparamagnetic nature with saturation magnetization values of 11.02 emu g^-1. The prepared BMMCs not only controlled the oxidation of PMNPs but also improved the adsorptive performance with respect to MG dye (500 mg g^-1 at 298.15 K and pH 6.5) due to the presence of a large amount of hydrophilic functional groups (hydroxyl/-OH and carboxyl/-COOH) on/in the BMMCs. The smooth encapsulation of PMNPs into the PVA-SA matrix established additional hydrogen bonding among polymer molecular chains, with improved stability, and adsorptive performance was maintained over a wide range of pH values (3-12). Importantly, the prepared BMMCs were easily regenerated just by washing with water, and they could be re-utilized for up to four (4) consecutive treatment cycles without observing any apparent dissolution of iron/Fe⁰ or damage to the morphology. According to the mass balance approach, an estimated amount of 100 mL of treated effluent can be obtained from 160 mL of MG dye solution (25 mg L^-1) just by employing a 0.02 g L^-1 adsorbent dosage. Finally, a model of BMMCs based on zero-effluent discharge was also proposed for commercial or industrial applications. The prepared BMMCs are greatly needed for improving the water/wastewater treatment process and they can be utilized as an excellent adsorbent to remove cationic pollutants for various environmental applications.

Fig. 1. A schematic diagram of bio-magnetic membrane capsule (BMMC) fabrication by the encapsulation of phytogenic magnetic nanoparticles (PMNPs). (Reproduced from Ali et al. Copyright@2019, with permission from Elsevier Ltd.)

Fig. 2. The Fourier transform infrared (FTIR) spectra of (a) PVA–SA, (b) PVA–SA–PMNPs, (c) PVA–SA–PMNPs after primary cross-linking and (d) fabricated bio-magnetic membrane capsules (BMMCs). (Reproduced from Ali et al. Copyright@2019, with permission from Elsevier Ltd.)

Fig. 3. Conceptual possible cross-linking networks of SA–CaCl₂, PVA–H₃BO₃ and PVA–SA–GA during the fabrication of bio-magnetic membrane capsules (BMMCs). (Reproduced from Ali et al. Copyright@2019, with permission from Elsevier Ltd.)

Fig. 4. Powder X-ray diffraction patterns of (a) PVA–SA–PMNPs and (b) the prepared bio-magnetic membrane capsules (BMMCs) after reaction with primary and secondary cross-linking agents (displaying the amorphous carbon and magnetite structures). (Reproduced from Ali et al. Copyright@2019, with permission from Elsevier Ltd.)

Fig. 5. Scanning electron microscopy (SEM) images of (a) the fabricated bio-magnetic membrane capsules (BMMCs), (b) cross-sectional area of the BMMCs, (c) outer boundary/surface/layer of the BMMCs, (d, e & j) inner surface/core of the BMMCs, (f) real pictorial view of the outer boundary/surface/layer of the BMMCs, (g) real pictorial view of the cross-sectional area of the BMMCs, (h) real pictorial view of the outer boundary/surface/layer of the BMMCs after the sorption of cationic MG dye, and (i) real pictorial view of the cross-sectional area of the BMMCs (after the sorption of cationic MG dye). (Reproduced from Ali et al. Copyright@2019, with permission from Elsevier Ltd.)

Fig. 6. X-ray photoelectron spectrum (XPS) of (a) bio-magnetic membrane capsules (inset table shows the atomic percentages of the elements); (b) high resolution XPS spectrum of Fe 2p; (c) high resolution XPS spectrum of O 1s; (d) high resolution XPS spectrum of C 1s.

Fig. 7. Energy dispersive X-ray (EDX) image of the prepared bio-magnetic membrane capsules (BMMCs). (Reproduced from Ali et al. Copyright@2019, with permission from Elsevier Ltd.)

Fig. 8. Thermogravimetric analysis (TGA) plot of the fabricated bio-magnetic membrane capsules (BMMCs). (Reproduced from Ali et al. Copyright@2019, with permission from Elsevier Ltd.)

Fig. 9. (a) M–H hysteresis loop/vibrating sample magnetometer (VSM) measurement of the prepared PMNPs and bio-magnetic membrane capsules (BMMCs) at a temperature of 300 K; and magnetic separation studies of the prepared BMMCs in (b) an aqueous environment, and (c) freeze-dried conditions using a simple hand-held magnet. (Reproduced from Ali et al. Copyright@2019, with permission from Elsevier Ltd.)

Fig. 10. (a) Effect of various amounts of PMNP encapsulated into BMMCs on the removal efficiency of cationic MG dye, (b) effect of adsorbent/BMMCs dosage on the removal the efficiency of cationic MG dye by BMMCs (dosages = 0.0062–1 g L⁻¹; C_o = 25 mg L⁻¹; contact time = 24 h; replication = 3; standard deviation: ± 1.23); (c) effect of contact time on the removal efficiency of MG dye by BMMCs (dosage = 0.02 g L⁻¹; replication = 3; standard deviation: ± 1.42); (d) effect of initial concentration of MG dye (replication = 3; standard deviation: ± 1.69); (e) effect of reaction temperature (dosage = 0.02 g L⁻¹; replication = 3; standard deviation: ± 2.03); and (f) effect of co-existing cationic pollutants on the removal efficiency of MG dye by BMMCs (dosage = 0.02 g L⁻¹; replication = 3; standard deviation: ± 1.03).

Fig. 11. (a) Point of zero charge (pH_PZC) of PMNPs, PVA–SA-capsules and BMMCs; (b) Effect of solution pH on the removal efficiency of malachite green (MG) by PMNPs, PVA–SA-capsules and BMMCs. Interval of pH (2–12); initial dye concentration: 25 mg L⁻¹; dosage: 0.02 g L⁻¹; solution volume: 40 mL; temperature: 25 °C and agitation time: 120 min; replication = 3; standard deviation: ± 2.29 [figures show the real conditions of the (c) solution after reaction with (d) PMNPs, (e) PVA–SA-capsules and (f) BMMCs].

Fig. 12. (a) Kinetics of malachite green (MG) dye sorption by bio-magnetic membrane capsules (BMMCs) (dosage = 0.02 g L⁻¹; replication = 3; standard deviation: ± 2.83); (b) the linear plot of the pseudo-second-order model of MG dye sorption by BMMCs (dosage = 0.02 g L⁻¹; replication = 3; standard deviation: ± 2.83); (c) intra-particle diffusion kinetic model fit for malachite green (MG) dye sorption by BMMCs (dosage = 0.02 g L⁻¹; replication = 3; standard deviation: ± 2.83); (d) the linear plot of the Langmuir isotherm model for malachite green (MG) dye sorption by BMMCs; (e) sorption isotherm curve for the sorption of cationic MG dye by BMMCs (dosage = 0.02 g L⁻¹; MG concentration = 25–1000 mg L⁻¹; replication = 3; standard deviation: ± 3.09); and (f) thermodynamic plot for the sorption of malachite green (MG) dye by BMMCs (dosage = 0.02 g L⁻¹, C_o = 25 mg L⁻¹; replication = 3; standard deviation: ± 1.49).

Fig. 13. Effect of different concentrations of desorption solutions on the desorption efficiency of malachite green (MG) dye (adsorbents dosage = 0.02 g L⁻¹, MG initial concentration (C_o) = 25 mg L⁻¹; replication = 3; standard deviation: ± 4.67).

Fig. 14. Effect of different feed-to-regeneration ratios (v/v) on the desorption efficiency of malachite green (MG) dye (water as regeneration solution; adsorbent dosage = 0.02 g L⁻¹, MG initial concentration (C_o) = 25 mg L⁻¹; replication = 3; standard deviation: ± 1.59).

Fig. 15. Study of the stability and reusability of BMMCs for consecutive sorption–desorption cycles of malachite green (MG) dye (15 mL of water as regeneration solution; adsorbent dosage = 0.02 g L⁻¹, MG initial concentration (C_o) = 25 mg L⁻¹; replication = 3; standard deviation: ± 3.18).

Fig. 16. Fourier transform infrared (FTIR) spectrum of bio-magnetic membrane capsules (BMMCs) (a) before and (b) after the sorption of cationic malachite green (MG) dye.

Fig. 17. Proposed adsorptive removal mechanism of cationic malachite green (MG) dye by bio-magnetic membrane capsules (BMMCs) and (a) a real pictorial representation of fabricated BMMCs, (b) SEM images of BMMCs, (c) real pictorial representation of the outer surface of the BMMCs after the sorption of cationic MG dye, and (d) a real pictorial representation of the cross-sectional/inner surface of the BMMCs after the sorption of cationic MG dye for a better understanding.

Fig. 18. Proposed conceptual treatment model using bio-magnetic membrane capsules (BMMCs) based on zero-effluent discharge for water/wastewater containing cationic toxic dyes and heavy metal ions.