Kaizhong Li

Ion-adsorption-type rare earth ores (IRE-ores) are a vital source of rare earth elements globally. The percolation properties of the ore body significantly determine the ease of rare earth extraction using in situ leaching processes. This paper investigates the interactions among clay particles in ion-adsorption-type rare earth ores within aqueous solutions, utilizing methods such as mineral particle aggregation and settling and acid–base adsorption on mineral surfaces. Based on these analyses, this paper elucidates the influence of solution properties on the percolation process of ion-type rare earth ores during leaching. The results indicate that the electrostatic attraction between minerals in aqueous solutions is pronounced, enhancing the interaction and resulting in a notable instability of the aggregates. The aggregation of minerals impacts the viscous effect of water bound to the mineral surface on the solution, thereby affecting the percolation rate. Extremely low concentrations of SO₄²⁻ and CH₃COO⁻ can act as surface modifiers, reducing the electrostatic attraction between particles and consequently increasing the leaching percolation rate.

Naproxen (NPX), as a typical pharmaceutical and personal care product (PPCP), is frequently detected in water and is difficult to remove effectively by conventional methods. Layered double hydroxides, as highly efficient photocatalysts, have not been used in photocatalysis of PPCP. In this study, CuZnAl-layered double hydroxide (CuZnAl-LDH) was prepared and its performance in the photocatalytic degradation of NPX under UV light illumination was investigated. The results indicated that the NPX photodegradation is considerably enhanced by CuZnAl-LDH with an efficiency of 98.25% in the optimal conditions, which was obviously more than that under pure light condition (efficiency less than 30%). In addition, the CuZnAl-LDH photocatalyst exhibited superior photocatalytic stability confirmed by the analyses of the catalyst structure, morphology, and cycle experiments. Moreover, by the comparison of optical properties of ZnAl-LDH and CuZnAl-LDH, it was found that Cu²⁺ can broaden light absorption and facilitate the separation of photogenerated charges electrons (e⁻) and holes (h⁺), and in the CuZnAl-LDH system, the formation mechanism of active species complies with the mechanisms of the ligand-to-metal charge transfer (LMCT) from the 2p orbital of oxygen to the 3d orbital of M²⁺ and of the d–d transition of Cu²⁺ ions of the CuO₆ unit. Besides, ESR and scavenging tests revealed that the photocatalytic process was enhanced by the synergistic effect of h⁺, ˙O₂ ⁻, and ˙OH, which displayed the sequence of h⁺ > ˙O₂ ⁻ > ˙OH. Finally, the intermediate products were identified by LCMS-IT-TOF, and no intermediates with greater toxicity than NPX were produced. The possible degradation pathway of NPX was proposed involving the decarboxylation, demethylation, and hydroxylation reactions and ring-opening based on intermediates. These results are expected to provide new data on the photodegradation of naproxen.

Rare earths dissolved in carbonate solutions exhibit a metastable state. During the period of metastability, rare earths dissolve stably without precipitation. In this paper, neodymium was chosen as a representative rare earth element. The effects of additional NaCl and CO₂ on the metastable state were investigated. The metastable state can be controlled by adding NaCl to the Na₂CO₃ solution. Molecular dynamics studies indicated that the Cl⁻ provided by the additional NaCl partially occupied the coordination layer of Nd³⁺, causing the delayed formation of neodymium carbonate precipitation. In addition, the additional NaCl decreased the concentration of free carbonate in the solution, thereby reducing the behavior of free contact between carbonate and Nd, as well as resulting in the delay of Nd precipitate formation. Consequently, the period of the metastable state was prolonged in the case of introduction of NaCl. However, changing the solution environment by introducing CO₂ can destroy the metastable state rapidly. Introduction of CO₂ gas significantly decreased the CO₃²⁻ content in the solution and increased its activity, resulting in an increase of the free CO₃²⁻ concentration of the solution in the opposite direction. As a result, the precipitation process was accelerated and the metastable state was destroyed. It was possible to obtain a large amount of rare earth carbonate precipitation in a short term by introducing CO₂ into the solution with dissolved rare earths in the metastable state to achieve rapid separation of rare earths without introducing other precipitants during the process.