# Adsorption of Anionic Dye on the Acid-Functionalized Bentonite

## TL;DR

Acid-treated bentonite significantly improves the adsorption of anionic dyes like methyl orange, with up to 8000% increased capacity.

## Contribution

The study demonstrates that acid-functionalized bentonite can enhance anionic dye adsorption through electrostatic interactions.

## Key findings

- Acid treatment increased methyl orange adsorption capacity up to 8000% in acidified bentonite.
- The Sips equation best described the adsorption isotherms for the dyes.
- Acid functionalization had minimal effect on cationic dye methylene blue adsorption.

## Abstract

The efficiency of acid treatment on natural calcium bentonite (natural bentonite) for anionic dye adsorption was investigated using methyl orange (MO) as a probe. Additionally, adsorption experiments were accomplished between the natural bentonite, acidified bentonite, and a cationic dye (methylene blue, MB). Acid functionalization in natural bentonite (RF) was carried out with HCl and H2SO4 acids (RF1 and RF2, respectively). The samples were characterized by chemical analysis, mineralogy, particle size, and thermal behavior with the associated mass losses. The adsorption efficiency of MO and MB dyes was investigated by the effects of the initial concentration of adsorbate (Ci) and the contact time (tc). The acid treatment was efficient for increasing the adsorption capacity of the anionic dye, and the Qmaxexp values measured were 2.2 mg/g, 67.4 mg/g e 47.8 mg/g to RF, RF1 e RF2, respectively. On the other hand, the acid functionalization of bentonite did not significantly modify the MB dye adsorption. The Sips equation was the best fit for the adsorption isotherms. Thus, we found that the acid-functionalized bentonite increases the anionic dye adsorption by up to 8000%. The increased adsorptive capacity of acidified bentonite was explained in terms of electrostatic attraction between the clay surface and the dye molecule.