Anionic azo dyes removal from water using amine-functionalized cobalt–iron oxide nanoparticles: a comparative time-dependent study and structural optimization towards the removal mechanism

The current study is aimed at synthesizing and characterizing magnetic cobalt–iron oxide nanoparticles (CoFeNPs) functionalized with two different amino reagents, hydrazine and dodecylamine, resulting in CoFeNPs1 and CoFeNPs2, respectively. Both types of cobalt–ferrite nanoparticles were investigated for the removal of six different negatively charged azoic dyes (Amaranth, Acid Orange 7, Naphthol Blue Black, Reactive Orange 16, Acid Orange 52 and Reactive Red-P2B) from water, and their removal efficiency was compared as a function of different factors such as time, type of anchored amine, size of CoFeNPs and structure of the dye. CoFeNPs were successfully characterized by FT-IR spectra, AFM, SEM-EDS, surface charge (ζ-potential) and thermal analysis. CoFeNPs1 revealed 44.5–82.1% dye removal at equilibrium (attained within 28–115 min) with an adsorptive capacity (q(e)) of 5.4–13.5 mg g(−1) observed under unoptimized conditions (temp. 30 °C, adsorbent dose 0.67 g L(−1), pH 6, dye concentration 20 μmol L(−1)). Use of CoFeNPs2 significantly enhanced the removal of each dye (percent dye removal 68.0–98.9%, q(e) 6.6–23.5 mg g(−1)) compared to CoFeNPs1 under similar conditions. From a comparative structural study, a larger size, more complex structure, hydrophobic character and greater number of phenyl SO(3)(−) groups among the tested dyes facilitated their removal by CoFeNPs2, while all of these structural factors were negatively related to dye removal by CoFeNPs1. CoFeNPs2 showed some dye aggregation along with adsorption, while in the case of CoFeNPs1, only adsorption was observed as confirmed by FT-IR and UV-visible spectral studies. Dye removal data in all cases was in best compliance with pseudo-second order kinetics in comparison to pseudo-first order or the Elovich model, where film diffusion was a dominant phenomenon compared to intra-particle diffusion. Adsorption isotherms, thermodynamics and reusability of the CoFeNPs were studied selecting Reactive Orange 16. Adsorption equilibrium was best fitted to the Langmuir isotherm. ΔG° and ΔH° indicated spontaneous and exothermic adsorption. Amine-functionalized CoFeNPs are recommended as potential cost-effective adsorbents with excellent reusability that could be applied efficiently for rapid and selective dye removal from textile effluents considering the size, structure, charge and number of S atoms in the target azo dyes.