Insight into enhanced adsorption of Congo red by petal-like MgAl2O4: Effect of dehydroxylation

This paper reports a facile treatment to enhance the capacity of petal-like MgAl₂O₄ for Congo red (CR) adsorption. The influence of dehydroxylation treatment on the adsorption performance of petal-like MgAl₂O₄ is investigated. FT-IR, XPS, TG, XRD and FE-SEM are employed to analyze the differences between samples without and with dehydroxylation. Mechanism on the adsorption capacity improvement of CR by MgAl₂O₄ after dehydroxylation is investigated systematically. Results show that the adsorption process conforms well to the pseudo-second-order kinetic model and Langmuir isotherm model. The adsorption mechanism study shows that the adsorption of CR on petal-like MgAl₂O₄ is mainly due to chemisorption including Lewis acid-base interaction and electrostatic attraction. It is observed that the maximum adsorption capacity of petal-like MgAl₂O₄ after dehydroxylation reaches 3264.54 mg/g, much higher than sample without dehydroxylation (591.72 mg/g). Most Lewis acid sites of petal-like MgAl₂O₄ (Mg²⁺ and Al³⁺) without dehydroxylation process are occupied by hydroxyl groups. It is not conductive to combine with –NH₂ and –SO₃^2- in CR molecular, resulting in the deterioration of adsorption performance. The activity of Mg²⁺ and Al³⁺ in MgAl₂O₄ can be recovered greatly after the removal of hydroxyl groups, and it is favorable to the adsorption between MgAl₂O₄ and CR.