Equilibrium,KineticandIsothermStudiesofDye AdsorptionfromAqueousSolutionbyRecyclableAmine- FunctionalizedSilicaCoatedMagneticNanoparticles Muhyminul Islam, [a] Year Hossain Manna, [a] Mohammad Ahasanur Rabbi, [b] Abdur Rahman, [a] Hasan Ahmad, [a] O. Thompson Mefford, [c] Mahbubor Rahman,* [a] and Ashraful Alam* [a] The evaluation of the adsorption capacity of amine-functional- ized silica coated iron oxide magnetic particles (Fe 3 O 4 /SiO 2 -NH 2 ) as a highly recyclable adsorbent for Congo Red (CR) and Eriochrome Black T (EBT) azo-dyes from water is reported. The Fe 3 O 4 /SiO 2 -NH 2 were prepared by a simple co-precipitation method of iron oxide (Fe 3 O 4 ) nanoparticles followed by silica coating on oleic acid coated Fe 3 O 4 . A series of investigation was performed to assess the various experimental parameters like contact time (0–360 min), pH (3–10), stirring speed (0–500 rpm), adsorbent dosages (0.02–0.12 g) and initial concentrations (0.02–0.15 mg mL  1 ) of dyes. According to the Langmuir iso- therm, the maximum adsorption capacities of CR and EBT on Fe 3 O 4 /SiO 2 -NH 2 particles were found to be 211.86 mgg  1 and 81.98 mg g  1 , respectively. In comparison, the pseudo-second order model provided adsorption capacities of 200.40 mg g  1 for CR and 80.91 mgg  1 for EBT. Additionally, the maximum adsorption amounts of CR and EBT on Fe 3 O 4 /SiO 2 -NH 2 particles were 179.37 mg g  1 and 75.43 mg g  1 , achieved within 300 min and 180 min at pH 3, respectively. The dye-loaded particles were recoverable for CR and EBT removal by simple regener- ation with 0.1 molL  1 NaOH solution to use the particles until more than five cycles. 1.Introduction Water pollution caused by a variety of hazardous inorganic and organic pollutants is a major environmental concern for humans and ecosystems. [1] Dye is one of the major sources of water pollution which is widely utilized in the textile, [2] cosmetic, [3] and pharmaceutical industries. [4] Most used organic dyes are Congo Red (CR) and Eriochrome Black T (EBT). [5] Following discharge of unused dyes into the water bodies, the biological activity of marine life, especially plants and aquatic organisms, is being greatly hampered by reducing oxygen levels and sunshine penetration. [6] Numerous methods developed to purify dye wastewater are flocculation, [7] photocatalysis, [8] adsorption, [9] and membrane filtration. [10] Among these, adsorption is a promising approach because of its efficacy, low cost, lack of byproduct formation, multiple reuses, and ease of application. [11] To purify dye wastewater, a variety of adsorption materials activated carbon/chitosan/Carica papaya seeds, [12] zeolites, [13] clay minerals, [14] resin, [15] functionalized polymers, [16] and carbon nanotubes [17] have been applied. Some of these adsorbents are difficult to fabricate, require complex synthetic methodologies, time-consuming separation process and often produces secondary wastes. Nanomagnetic materials are particularly inspiring and appropriate is use as adsorbent as they have large specific surface area, easy separation by external magnetic field, and recyclability. [18] Recently, iron oxide magnetic nanoparticles (IONPs) have drawn a lot of attention as adsorbents in waste- water treatment owing to their unique properties like super paramagnetic, surface-to-volume ratio, higher surface area, and simple physical separation. [19] Saha etal. investigated the removal of dyes containing hydroxyl ( OH) groups using IONPs. The adsorption capacity of EBT (~67%), bromophenol blue (~ 68 %), bromocresol green (~ 38 %), and fluorescein (~ 45 %) was higher on the iron oxide surface as compared to methyl red (~ 7%), methylene blue (~5%), and methyl orange (~4%), which does not have any hydroxyl ( OH) groups. [20] Afkhami etal. studied the adsorption of CR by maghemite (γ-Fe 2 O 3 ) nanoparticles [21] and observed much higher adsorption capacity (208.33 mg g  1 ) compared to many other adsorbents. However, native or bare IONPs often tend to aggregate due to large surface area and dipole-dipole interactions. [22] This reduces the total specific surface area, a prime requisite for achieving high adsorption efficiency. The poor thermodynamic stability of bare IONPs in suspension also greatly affects recycle ability and thus prevent economic sustainability for long-term application as adsorbents in water treatment plant. In core-shell systems, silica (SiO 2 ) is often used as a support material which helps IONPs to become stable under specific circumstances. Furthermore, it allows them easy modification opportunity with different func- [a] Md. M. Islam, Y. H. Manna, Md. A. Rahman, H. Ahmad, Md. M. Rahman, Md. A. Alam Polymer Colloids and Nanomaterials Research Lab, Faculty of Science, Department of Chemistry, Rajshahi University, Rajshahi 6205, Bangladesh E-mail: mrchem@ru.ac.bd aashraful41@ru.ac.bd [b] M. A. Rabbi BCSIR Laboratories Rajshahi, Bangladesh Council of Scientific and Industrial Research, Rajshahi 6205, Bangladesh [c] O. T. Mefford Department of Materials Science and Engineering, Clemson University, Clemson SC 29634-0971, USA Supporting information for this article is available on the WWW under https://doi.org/10.1002/slct.202402270 ChemistrySelect 2024, 9, e202402270 (1 of 16) © 2024 Wiley-VCH GmbH ChemistrySelect www.chemistryselect.org Research Article doi.org/10.1002/slct.202402270