Improvement of alkaline electrochemical characteristics of bauxite residue amendment with organic acid and gypsum

Neutralization of alkaline properties of bauxite residue (BR) by using organic acid and gypsum additions may effectively improve electrochemical properties and alleviate physicochemical barriers to ecological rehabilitation. Mineral acids, citric acid and hybrid acid-gypsum additions were compared for their potential to transform and improve zeta potential, isoelectric point (IEP), surface protonation and active alkaline OH groups, which are critical factors for further improvement of physicochemical and biological properties later. Isoelectric points of untransformed bauxite residue and six transformed derivatives were determined by using electroacoustic methods. Electrochemical characteristics were significantly improved by the amendments used, resulting in reduced IEP and OH groups and decreased surface protonation for transformed residues. XRD results revealed that the primary alkaline minerals of cancrinite, calcite and grossular were transformed by the treatments. The treatments of citric acid and gypsum promoted the dissolution of cancrinite. From the SEM examination, citric acid and gypsum treatments contributed to the reduction in IEP and redistribution of OH groups on particle surfaces. The collective evidence suggested that citric acid and gypsum amendments may be used firstly to rapidly amend bauxite residues for alleviating the caustic conditions prior to the consideration of soil formation in bauxite residue.