Re-collectable and recyclable epichlorohydrin-crosslinked humic acid with spinel cobalt ferrite core for simple magnetic removal of cationic triarylmethane dyes in polluted water

Spinel cobalt ferrite (CoFe2O4) was successfully functionalized with humic acid (CoFe2O4-HA) via hydrothermal method. In order to prevent detachment, the humic acid layer on the CoFe2O4-HA surface was crosslinked with epichlorohydrin to obtain CoFe2O4-HA-ECH. Synthesized adsorbents were then tested for their ability to remove of basic fuchsin (BF), methyl violet 2B (MV), and malachite green (MG) from aqueous solutions. Particle size analysis, vibrating sample magnetometry, scanning electron microscopy, X-ray diffractometry, Fourier transform infrared spectrophotometry, thermogravimetric analysis, and ζ-potential analysis were conducted to characterize the as-synthesized adsorbents. The effects of adsorption parameters including pH, contact time, initial dye concentration, temperature, and ionic strength were explored. The kinetics data fitted well with a pseudo-second order model with Coefficient of Determination (R2) ≥ 0.998, Chi-squared (χ2) ≤ 0.171, and Average Relative Error (ARE) ≤ 3.443, suggesting that adsorption is the rate-limiting step. The Langmuir isotherm model provided R2 ≥ 0.999, χ2 ≤ 0.025, and ARE ≤ 0.891, indicating that adsorption occurs on a single layer on a homogenous surface. The maximum adsorption capacities of CoFe2O4-HA-ECH were 96.494, 62.627, and 48.74 μmol · g-1 for BF, MV, and MG, respectively, which were 10 times higher than those of CoFe2O4. Thermodynamic studies suggested that the adsorption processes are spontaneous-endothermic. Furthermore, CoFe2O4-HA-ECH can be re-collected and recycled for up to 10 cycles.