# CMPO-Functionalized Silica Sorbents for pH-Tunable Separation and Enrichment of Rare-Earth Elements from Environmental Matrices

## TL;DR

A new silica-based material is developed to efficiently separate and enrich rare-earth elements from complex solutions using pH control.

## Contribution

A pH-tunable CMPO-functionalized silica sorbent is introduced for selective rare-earth element separation and enrichment.

## Key findings

- REEs were successfully eluted with ultrapure water through pH adjustment.
- Packed-bed column studies achieved a 20-fold enrichment of REEs and 30-fold enrichment of neodymium.
- Adsorption followed Langmuir isotherm behavior and pseudo-second-order kinetics.

## Abstract

Rare-earth elements
(REEs) are crucial in many applications, yet
mutual separation is challenging due to their similar chemical behavior.
Octyl-phenyl-N,N-diisobutyl carbamoyl
methyl phosphine oxide (CMPO) is an organophosphorus ligand originally
developed for extracting actinides and lanthanides from spent nuclear
fuel. Here, we report a pH-tunable CMPO-functionalized silica sorbent
for selective REE separation from complex aqueous matrices. A CMPO-associated
silica gel sorbent was synthesized and characterized by Brunauer–Emmett–Teller
(BET) surface area, scanning electron microscopy, and X-ray photoelectron
spectroscopy to confirm the surface functionalization and binding
behavior. Sorbent performance was evaluated by using a synthetic 46-element
solution and a real phosphate rock fertilizer leachate. Notably, REEs
were successfully eluted with ultrapure water, demonstrating reversible
desorption controlled by pH adjustment. Packed-bed column studies
increased the REE mass fraction from 3.6% to 64% (20-fold enrichment),
with up to 30-fold enrichment of neodymium. The adsorption process
follows the Langmuir isotherm behavior and follows pseudo-second-order
kinetics. The uptake capacity of 1 μmol of REEs per 4.2 μmol
of CMPO supports the formation of a predominantly 4:1 ligand:rare
earth element­(III) pseudocomplex. These results demonstrate CMPO-functionalized
silica as a selective, water-elutable, and low-chemical-input platform
for sustainable REE recovery from environmental and industrial sources.