Effect of strong acids on red mud structural and fluoride adsorption properties

• Acid activated red mud has improved adsorption compared to untreated red mud.
• Phase composition of acid activated red mud has been determined.
• Fluoride successfully removed using acid activated red mud.
• Fluoride adsorption on iron and aluminium oxy/hydroxide compounds found in red mud.
• Sulphuric acid activated mud highest capacity due to increased SOH2+ surface sites.

The removal of fluoride using red mud has been improved by acidifying red mud with hydrochloric, nitric and sulphuric acid. The acidification of red mud causes sodalite and cancrinite phases to dissociate, confirmed by the release of sodium and aluminium into solution as well as the disappearance of sodalite bands and peaks in infrared and X-ray diffraction data. The dissolution of these mineral phases increases the amount of available iron and aluminium oxide/hydroxide sites that are accessible for the adsorption of fluoride. However, concentrated acids have a negative effect on adsorption due to the dissolution of these iron and aluminium oxide/hydroxide sites. The removal of fluoride is dependent on the charge of iron and aluminium oxide/hydroxides on the surface of red mud. Acidifying red mud with hydrochloric, nitric and sulphuric acid resulted in surface sites of the form SOH2+ and SOH. Optimum removal is obtained when the majority of surface sites are in the form SOH2+ as the substitution of a fluoride ion does not cause a significant increase in pH. This investigation shows the importance of having a low and consistent pH for the removal of fluoride from aqueous solutions using red mud.

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