Rare earth elements (REE) are essential components of equipment used for renewable energy, green technologies, and more traditional sectors such as chemical catalysis and metallurgy. Interest has been growing in alternative REE sources to supplement current ore sources and related refining, which have historically supplied the bulk of REE oxides. This study investigated the capability of adsorbent silica particles functionalized with three different ligands – phosphonoacetic acid (PAA), N,N-bisphosphono(methyl)glycine (BPG), and diethylenetriaminepentaacetic dianhydride (DTPADA) – to selectively extract REE from acidic to circumneutral aqueous brine solutions in equilibrium conditions. Maximum REE removal from 0.5 M NaCl solutions was displayed at pH 7 for PAA, pH 2 for DTPADA, and in both acidic and basic conditions for BPG functionalized materials. The REE adsorption performance for functionalized materials was largely unimpeded by the presence of competing ions (Ca, Mg, Zn, Fe, Al). Tests with real brines (I ∼ 3 M) showed >90% efficiency in REE recovery, which improved at higher temperatures (up to 100 °C). Effective elution of REE was accomplished with 0.7 N HNO₃, and performance of the adsorbents improved with additional usage cycles.

Bauxite residue (red mud), an alkaline slurry from alumina refining, is produced in large volumes and disposed of in large surface impoundments. The objectives of this study were to measure the extent of neutralization of bauxite residue by carbon dioxide as a function of C O2 partial pressure and to determine the geochemical reactions responsible for carbon sequestration. Bauxite residue was exposed to carbon dioxide (C O2 ) at partial pressures ranging from 10−3.5 to 1 atm and the residue pore-water pH was measured until a steady state pH was achieved. Using pure C O2 ( PC O2 =1 atm) , a steady state pH of 7.7 was established in 1 day while it took 9 days to reach a pH of 9.8 at PC O2 = 10−3.5 atm . The pH rebounded to 9.9 after 1 day when bauxite residue first neutralized at PC O2 of 1, 0.1, or 0.01 atm for 10 days was subsequently exposed to PC O2 = 10−3.5 atm , indicating that the pH change during short-term carbonation (t⩽10 days) was due to reactions of carbonic acid and O H− in the pore water. A l...

The objective of this study was to investigate the mechanisms and extent of bauxite residue (red mud) neutralization at the residue surface in field impoundments as a result of long-term reaction with atmospheric CO2 and addition of amendments to promote vegetation. The results showed that carbonation from atmospheric carbon dioxide reduces the pH of red mud from 12.5 to about pH 9.3 at the surface of storage cells, with the depth of neutralization dependent on the age of the stored residue (up to 1.2 m in 35 years). The presence of vegetation further lowered residue pH to about pH 8.5, with the depth of neutralization dependent on depth of root penetration. Sewage and yard-waste amendments accelerated neutralization and the establishment of vegetation and further lowered the residue pH to about pH 6.7, likely due to organic acid leaching. For vegetated areas, the root density (g roots/g soil) was proportional to the extent of neutralization of residue, with root density higher in near-surface residue tha...