A magnetically recyclable carboxyl-functionalized chitosan composite for efficiently removing methyl orange from wastewater: Isotherm, kinetics, thermodynamic, and adsorption mechanism

In this study, a kind of magnetically recyclable adsorbent for dyes was synthesized by grafting diethylenetriamine pentaacetate acid (DTPA) to the composite of Fe₃O₄ microspheres and crosslinked chitosan (CS). The microstructures, molecular structure, crystal structure, and magnetic hysteresis loops of the chitosan matrix adsorbent before and after grafting was characterized. The results suggested that DTPA was covalent bonded with the composite of Fe₃O₄ microspheres and chitosan. The modified composite has larger specific surface area and can realize rapid solid-liquid separation. Batch experiments were conducted to optimize the parameters affecting the adsorption of methyl orange (MO). The adsorption process could be better described by pseudo-second-order kinetics model and Langmuir isotherm equation, and its saturated adsorption capacity of the modified adsorbents was 1541.5 mg·g^-1 at 25 °C, which was 1.40 times of that the unmodified adsorbent (1104.1 mg·g^-1). The obtained values of the thermodynamic parameters indicated that the adsorption was a spontaneous process. The regeneration experiment proved the stability and reproducibility of the adsorbent even after five cycles of adsorption-desorption. The primary adsorption mechanism was electrostatic interaction and hydrogen bonding. The adsorbent could be potentially applied for removing dyes from wastewater in wide pH of range, especially acid wastewater.