Adsorption of hydroxamic acid ligands for improved extraction of rare earth elements from monazite ores

Muchu Zhou; Luke D. Gibson; Diana Stamberga; Md Faizul Islam; Ilja Popovs; Jeffrey D. Einkauf; Nikki A. Thiele; Robert L. Sacci; Benjamin Doughty; Vera Bocharova; Vyacheslav S. Bryantsev

doi:10.1016/j.jcis.2025.138887

Adsorption of hydroxamic acid ligands for improved extraction of rare earth elements from monazite ores

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Abstract

Efficient separation of rare earth element (REE) ores via froth flotation requires the development of novel ligands with enhanced adsorption capacity and selectivity. To realize these advances, understanding the mechanisms underlying interactions between the ligand and mineral surfaces is essential. This study systematically evaluates the adsorption behavior of alkyl- and aromatic alkyl-substituted hydroxamic acid ligands on monazite surfaces using complementary spectroscopic techniques, including UV–visible (UV–vis) spectroscopy, Raman spectroscopy, infrared spectroscopy, and vibrational sum frequency generation (SFG) spectroscopy, together with the ab initio molecular dynamics (AIMD) simulations. Among the studied ligands, octanohydroxamic acid (OHA) and 4-ethoxy-N,2-dihydroxybenzamide (EDHBA) exhibit high adsorption capacity under basic pH (8–10) by forming multilayers on the surface. OHA has a higher equilibrium adsorption capacity compared to EDHBA, but it forms a less stable multilayer susceptible to disruption in the presence of interfering ions. AIMD results show that OHA adopts a single stable chelating geometry, while EDHBA exhibits multiple binding modes involving distinct interactions with La surface atoms and phosphate-bound oxygens, resulting in more complex adsorption kinetics. The variations in surface binding and intermolecular interactions observed between alkyl and aromatic molecules influence the differences in adsorption kinetics, equilibrium adsorption capacities on the mineral surface, and their flotation performance. This work provides valuable insight into the adsorption mechanism of ligands at mineral interfaces, which is crucial for guiding the design of new ligands with enhanced separation performance.

Original language	English
Article number	138887
Journal	Journal of Colloid and Interface Science
Volume	702
DOIs	https://doi.org/10.1016/j.jcis.2025.138887
State	Published - Jan 15 2026

Funding

The present research was supported by the Critical Materials Innovation Hub funded by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Advanced Materials and Manufacturing Technologies Office. This research used resources of the Oak Ridge Leadership Computing Facility at the Oak Ridge National Laboratory, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC05-00OR22725. This research also used resources of the National Energy Research Scientific Computing Center (NERSC), a Department of Energy Office of Science User Facility using NERSC award BES-ERCAP0020739. Potentiometric titration work by M.F.I. was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division. Notice: This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan ( http://energy.gov/downloads/doe-public-access-plan ).

Keywords

Ab initio molecular dynamics (AIMD)
Adsorption
Hydroxamic acid ligand
Isotherm
Kinetics
Monazite
Spectroscopy

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10.1016/j.jcis.2025.138887

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Zhou, M., Gibson, L. D., Stamberga, D., Islam, M. F., Popovs, I., Einkauf, J. D., Thiele, N. A., Sacci, R. L., Doughty, B., Bocharova, V., & Bryantsev, V. S. (2026). Adsorption of hydroxamic acid ligands for improved extraction of rare earth elements from monazite ores. Journal of Colloid and Interface Science, 702, Article 138887. https://doi.org/10.1016/j.jcis.2025.138887

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title = "Adsorption of hydroxamic acid ligands for improved extraction of rare earth elements from monazite ores",

abstract = "Efficient separation of rare earth element (REE) ores via froth flotation requires the development of novel ligands with enhanced adsorption capacity and selectivity. To realize these advances, understanding the mechanisms underlying interactions between the ligand and mineral surfaces is essential. This study systematically evaluates the adsorption behavior of alkyl- and aromatic alkyl-substituted hydroxamic acid ligands on monazite surfaces using complementary spectroscopic techniques, including UV–visible (UV–vis) spectroscopy, Raman spectroscopy, infrared spectroscopy, and vibrational sum frequency generation (SFG) spectroscopy, together with the ab initio molecular dynamics (AIMD) simulations. Among the studied ligands, octanohydroxamic acid (OHA) and 4-ethoxy-N,2-dihydroxybenzamide (EDHBA) exhibit high adsorption capacity under basic pH (8–10) by forming multilayers on the surface. OHA has a higher equilibrium adsorption capacity compared to EDHBA, but it forms a less stable multilayer susceptible to disruption in the presence of interfering ions. AIMD results show that OHA adopts a single stable chelating geometry, while EDHBA exhibits multiple binding modes involving distinct interactions with La surface atoms and phosphate-bound oxygens, resulting in more complex adsorption kinetics. The variations in surface binding and intermolecular interactions observed between alkyl and aromatic molecules influence the differences in adsorption kinetics, equilibrium adsorption capacities on the mineral surface, and their flotation performance. This work provides valuable insight into the adsorption mechanism of ligands at mineral interfaces, which is crucial for guiding the design of new ligands with enhanced separation performance.",

keywords = "Ab initio molecular dynamics (AIMD), Adsorption, Hydroxamic acid ligand, Isotherm, Kinetics, Monazite, Spectroscopy",

author = "Muchu Zhou and Gibson, \{Luke D.\} and Diana Stamberga and Islam, \{Md Faizul\} and Ilja Popovs and Einkauf, \{Jeffrey D.\} and Thiele, \{Nikki A.\} and Sacci, \{Robert L.\} and Benjamin Doughty and Vera Bocharova and Bryantsev, \{Vyacheslav S.\}",

note = "Publisher Copyright: {\textcopyright} 2025",

year = "2026",

month = jan,

day = "15",

doi = "10.1016/j.jcis.2025.138887",

language = "English",

volume = "702",

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T1 - Adsorption of hydroxamic acid ligands for improved extraction of rare earth elements from monazite ores

AU - Zhou, Muchu

AU - Gibson, Luke D.

AU - Stamberga, Diana

AU - Islam, Md Faizul

AU - Popovs, Ilja

AU - Einkauf, Jeffrey D.

AU - Thiele, Nikki A.

AU - Sacci, Robert L.

AU - Doughty, Benjamin

AU - Bocharova, Vera

AU - Bryantsev, Vyacheslav S.

PY - 2026/1/15

Y1 - 2026/1/15

N2 - Efficient separation of rare earth element (REE) ores via froth flotation requires the development of novel ligands with enhanced adsorption capacity and selectivity. To realize these advances, understanding the mechanisms underlying interactions between the ligand and mineral surfaces is essential. This study systematically evaluates the adsorption behavior of alkyl- and aromatic alkyl-substituted hydroxamic acid ligands on monazite surfaces using complementary spectroscopic techniques, including UV–visible (UV–vis) spectroscopy, Raman spectroscopy, infrared spectroscopy, and vibrational sum frequency generation (SFG) spectroscopy, together with the ab initio molecular dynamics (AIMD) simulations. Among the studied ligands, octanohydroxamic acid (OHA) and 4-ethoxy-N,2-dihydroxybenzamide (EDHBA) exhibit high adsorption capacity under basic pH (8–10) by forming multilayers on the surface. OHA has a higher equilibrium adsorption capacity compared to EDHBA, but it forms a less stable multilayer susceptible to disruption in the presence of interfering ions. AIMD results show that OHA adopts a single stable chelating geometry, while EDHBA exhibits multiple binding modes involving distinct interactions with La surface atoms and phosphate-bound oxygens, resulting in more complex adsorption kinetics. The variations in surface binding and intermolecular interactions observed between alkyl and aromatic molecules influence the differences in adsorption kinetics, equilibrium adsorption capacities on the mineral surface, and their flotation performance. This work provides valuable insight into the adsorption mechanism of ligands at mineral interfaces, which is crucial for guiding the design of new ligands with enhanced separation performance.

AB - Efficient separation of rare earth element (REE) ores via froth flotation requires the development of novel ligands with enhanced adsorption capacity and selectivity. To realize these advances, understanding the mechanisms underlying interactions between the ligand and mineral surfaces is essential. This study systematically evaluates the adsorption behavior of alkyl- and aromatic alkyl-substituted hydroxamic acid ligands on monazite surfaces using complementary spectroscopic techniques, including UV–visible (UV–vis) spectroscopy, Raman spectroscopy, infrared spectroscopy, and vibrational sum frequency generation (SFG) spectroscopy, together with the ab initio molecular dynamics (AIMD) simulations. Among the studied ligands, octanohydroxamic acid (OHA) and 4-ethoxy-N,2-dihydroxybenzamide (EDHBA) exhibit high adsorption capacity under basic pH (8–10) by forming multilayers on the surface. OHA has a higher equilibrium adsorption capacity compared to EDHBA, but it forms a less stable multilayer susceptible to disruption in the presence of interfering ions. AIMD results show that OHA adopts a single stable chelating geometry, while EDHBA exhibits multiple binding modes involving distinct interactions with La surface atoms and phosphate-bound oxygens, resulting in more complex adsorption kinetics. The variations in surface binding and intermolecular interactions observed between alkyl and aromatic molecules influence the differences in adsorption kinetics, equilibrium adsorption capacities on the mineral surface, and their flotation performance. This work provides valuable insight into the adsorption mechanism of ligands at mineral interfaces, which is crucial for guiding the design of new ligands with enhanced separation performance.

KW - Ab initio molecular dynamics (AIMD)

KW - Adsorption

KW - Hydroxamic acid ligand

KW - Isotherm

KW - Kinetics

KW - Monazite

KW - Spectroscopy

UR - https://www.scopus.com/pages/publications/105015492703

U2 - 10.1016/j.jcis.2025.138887

DO - 10.1016/j.jcis.2025.138887

M3 - Article

C2 - 40945293

AN - SCOPUS:105015492703

SN - 0021-9797

VL - 702

JO - Journal of Colloid and Interface Science

JF - Journal of Colloid and Interface Science

M1 - 138887

ER -

Adsorption of hydroxamic acid ligands for improved extraction of rare earth elements from monazite ores

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