Surface modification of silica-coated magnesium ferrite nanoparticles (MgFe 2 O 4 @SiO 2 NPs) by 3-aminopropyltriethoxysilane (APTES) shows enhanced selectivity for the removal of Congo Red (CR) from both single and binary aqueous dye solutions. Before coating the surfaces of amine-functionalized magnesium ferrite nanoparticles (MgFe 2 O 4 −NH 2 NPs) with silica, control studies of the adsorption of cationic, neutral, and anionic dyes were performed using both single and binary dye systems. The studies found that the MgFe 2 O 4 −NH 2 nanoadsorbent favors the adsorption of indigo carmine (IC) and CR in single dye solutions (>90% removal efficiencies). However, MgFe 2 O 4 −NH 2 NPs preferentially adsorb CR in binary dye solutions. Interestingly, the selectivity of CR over IC depends on the initial concentration of IC/CR in the IC/CR binary systems. A further enhancement in the selective removal of CR in both single and binary dye solutions was achieved by coating the MgFe 2 O 4 −NH 2 NPs with silica followed by modification with APTES (i.e., APTES-modified MgFe 2 O 4 @SiO 2 NPs). The highly selective adsorption capacity for CR on the APTES-modified MgFe 2 O 4 @SiO 2 nanoadsorbent was attributed to the mixture of polar functional groups (i.e., −OH and −NH 2 ) on the surface of the nanoadsorbent, which facilitates adsorbent−adsorbate interactions such as electrostatic and hydrogen-bonding interactions, which are amplified for CR with its more numerous polar functional groups (i.e., amine, azo, and sulfonate groups). From the results, the APTES-modified MgFe 2 O 4 @SiO 2 nanoadsorbent offers an effective, inexpensive, and reusable/sustainable system for the selective removal and remediation of Congo Red from wastewaters.

In t his study, solid silica nanoparticles (sSiO2 NPs) were chemically etched using sodium hydroxide solution as an etchant to synthesize porous silica nanoparticles (pSiO2 NPs). Etchant dosage and etching time were optimized to obtain the optimum etching condition providing the effective removal of paraquat (PQ). High removal efficiency of PQ by the synthesized pSiO2 NPs was obtained over 90% using 11.1 mL of 1.25 M NaOH and 12 h for the etching process. SEM and TEM images showed that the porosity of pSiO2 NPs increased with increase of the etchant dosage and etching time. The increment of porosity of pSiO2 NPs enhanced the PQ removal efficiency. FTIR result indicated that the characteristic functionalities of silica remain after the etching process. After optimum condition of etching obtained, the adsorption behavior of PQ was investigated. Several key factors influencing the adsorption efficiency, i.e., initial solution pH, initial concentration, and adsorption time were optimized. The maximum removal efficiency of PQ (~98%) by the pSiO2 nanoadsorbent was obtained using 100 mg L-1 of PQ solution at pH ~7 within 5 minutes. The maximum adsorption capacity (qmax) of the pSiO2 NPs for the PQ removal was 65.7 mg g-1. The pSiO2 nanoadsorbent is effective adsorbent for the PQ removal due to the development of a facile synthetic method for adsorbent preparation, rapid adsorption process, and comparable qmax value with other PQ adsorbents.

Magnesium ferrite (MgFe 2 O 4 ) and silica-coated MgFe 2 O 4 nanoparticles were grafted with poly(cysteine methacrylate) (i.e., MgFe 2 O 4 /PCysMA and MgFe 2 O 4 @SiO 2 /PCysMA nanocomposites) to study pH-tunable adsorption and enhanced capacities for the adsorption of anionic indigo carmine (IC) and cationic methylene blue (MB) dyes. Several characterization techniques (i.e., XRD, FTIR, TGA, ζ potential analysis, VSM, FE-SEM, TEM, N 2 adsorption−desorption isotherm, and XPS) indicated successful syntheses of these nanocomposites. The adsorption behaviors of the dyes demonstrated that the PCysMA-modified nanoadsorbents could selectively adsorb either IC or MB from either single-component or binary dye systems if the initial pH of the dye solution was tailored appropriately (i.e., pH ∼2 for IC and pH ∼10 for MB). The selective adsorption of these dyes was proposed by the electrostatic attractions of the nanoadsorbents and the dyes. Adsorption isotherms also showed enhanced capacities of MgFe 2 O 4 and MgFe 2 O 4 @SiO 2 NPs for the adsorption of IC and MB after grafting with PCysMA. Interestingly, the MgFe 2 O 4 @SiO 2 /PCysMA nanoadsorbent provided highly pH-selective adsorption and large increases in the capacities for the adsorption of IC and MB, which were attributed to the amounts of PCysMA grafted onto different magnetic substrates. The coating of silica on the surfaces of magnetic nanoparticles provided a higher amount of 3-methacryloxypropyltrimethoxysilane, promoting the polymerization of CysMA monomer. Recycling tests indicated that high efficiencies (∼80%) for the adsorption of IC and MB by the PCysMA-modified nanoadsorbents were obtained after five adsorption−desorption cycles. These key findings showed that the MgFe 2 O 4 /PCysMA and MgFe 2 O 4 @SiO 2 /PCysMA nanocomposites exhibited excellent pH-tunable adsorption of anionic and cationic dyes, easy magnetic separation, good reusability, and high stability. Specifically, the MgFe 2 O 4 @SiO 2 /PCysMA nanocomposite offers highly pH-selective adsorption and high adsorption capacities for dyes, demonstrating promising and alternative nanoadsorbents for applications in wastewater treatment and sensors.