Effect of Strong Acids on Red Mud Structural and Adsorption Properties

12 13 The removal of fluoride using red mud has been improved by acidifying red mud with 14 hydrochloric, nitric and sulphuric acid. This investigation shows that the removal of fluoride 15 using red mud is significantly improved if red mud is initially acidified. The acidification of 16 red mud causes sodalite and cancrinite phases to dissociate, confirmed by the release of 17 sodium and aluminium into solution as well as the disappearance of sodalite bands and peaks 18 in infrared and X-ray diffraction data. The dissolution of these mineral phases increases the 19 amount of available iron and aluminium oxide/hydroxide sites that are accessible for the 20 adsorption of fluoride. The removal of fluoride is dependent on the charge of iron and 21 aluminium oxide/hydroxides on the surface of red mud. Acidifying red mud with 22 hydrochloric, nitric and sulphuric acid resulted in surface sites of the form ≡ SOH2 and 23 ≡ SOH. Optimum removal is obtained when the majority of surface sites are in the form 24 ≡ SOH2 as the substitution of a fluoride ion doesn’t cause a significant increase in pH. This 25 investigation shows the importance of having a low and consistent pH for the removal of 26 fluoride from aqueous solutions using red mud. 27