Driving forces of Ce(III) oxidation to Ce(IV) onto goethite

Iron (Fe) oxyhydroxides are major phases that may control the cerium (Ce) behavior in the Earth's Critical Zone. However, understanding Ce behavior with Fe oxyhydroxides remains uncompleted. Especially, if thermodynamic calculations suggest that Fe(III) is not a sufficiently strong oxidant, several studies reported the presence of Ce(IV) onto Fe oxyhydroxides. In this study, multiple approaches, including modeling and X-ray absorption spectroscopy, deciphered the driving forces of Ce(III) oxidation to Ce(IV) onto goethite. Results showed that oxidized Ce occurred onto goethite with a Ce(III)/Ce(IV) ratio depending on the Ce concentration in the solution ([Ce]tot). The percentage of Ce(IV) onto goethite ranged from 20% to 50%, linearly increasing with [Ce]tot. Comparable observation with a redox-inert Al-hydroxide (gibbsite), allowed to rule out the importance of Fe(III) redox reactivity as the main driver of Ce(III) oxidation. Instead, thermodynamic calculations suggested that surface precipitation of Ce(IV)-hydroxides, whose formation is favored with increasing [Ce]tot, was an important driving force of the redox reaction. Because the goethite surface seemed to stabilize more strongly Ce(IV) than Ce(III) surface species than does gibbsite, differences in binding mechanisms of Ce(III) and Ce(IV) onto different mineral surfaces have been suggested to play a role on Ce redox speciation.