Fabrication of highly efficient encapsulated SnO₂@alginate beads as regenerative nanosorbents for anionic dye pollutants removal from aqueous solution

Nano-adsorbents are emerging as potential candidates for the treatment of toxic organic pollutants. In this context, spherical SnO2 nanoparticles (NPs) are synthesized through a facile hydrothermal method without the usage of toxic chemical precursors, which is confirmed through PXRD, TEM, FTIR, and Raman spectroscopy. SEM imaging was used to confirm the spherical morphology of the NPs. These NPs when applied as nano-adsorbents demonstrate the ability to remove anionic dyes from aqueous solution. In particular, 0.1 g/100 mL SnO2 NPs exhibit 95.96% Congo red (CR) removal efficiency from its aqueous solution within an exposure time of 1 min. Primary driving forces of adsorption can be attributed to electrostatic attraction and hydrogen bonding. Raman spectra confirmed the presence of oxygen vacancies which can be a major factor for their high adsorption efficiency. The adsorption process for CR dye fitted felicitously with pseudo-second order kinetics indicating chemisorption and the Langmuir model indicating CR monolayer formation on the nano-adsorbent. The maximum adsorption capacity is estimated to be 961.5 mg g(-1). Finally, the possibility of metal leaching has been addressed through encapsulation of SnO2 NPs within sodium alginate crosslinked by CaCl2, which also exhibited nearly 75% adsorption efficiency after an exposure time of 5 min, with successful regeneration. In addition to adsorption capability toward CR dye, SnO2@SA also exhibited moderate adsorption efficiencies towards eosin yellow, orange G, and methyl orange dyes. This study demonstrates SnO2 NPs as a potential nano-adsorbent for treatment of toxic effluents through a sustainable, facile and economical method.