Adsorption and structural incorporation of light and heavy rare earth elements on ferrihydrite: implications for phosphate and Cr(vi) sequestration

Abstract

Ferrihydrite (Fh), a metastable iron(oxyhydr)oxide abundant in soils and sediments, plays a pivotal role in regulating phosphorus bioavailability and contaminant Cr(VI) fate, but its rapid phase transformation to low-reactivity crystalline phases limits its long-term functionality. This study investigated how light (Nd³⁺) and heavy (Y³⁺) rare earth elements (REEs) modulate Fh's phase transformation and Cr(VI)/PO₄³⁻ sequestration. The results showed that LREE Nd³⁺ predominantly adsorbed onto Fh surfaces, forming a protective layer that shielded reactive Fe–OH sites and delayed hydroxyl displacement. In contrast, HREE Y³⁺ underwent isomorphic substitution into Fh's octahedral Fe–O framework, inducing lattice distortion and suppressing acicular goethite crystallization by ca. 26%. Both REEs preserved Fh's metastability, and maintained Fe in the oxidized Fe³⁺ state. In addition, both Nd³⁺ and Y³⁺ integration into hematite during transformation slowed Fh conversion to low-adsorptivity crystalline phases. Consequently, after 30 days of aging, REE-doped Fh exhibited significantly enhanced Cr(VI)/PO₄³⁻ sequestration (q_max,Cr(VI) = 19.39 mg g⁻¹ and q_max,P = 43.33 mg g⁻¹ for Fh–Nd, 24.47 and 64.03 mg g⁻¹ for Fh–Y) compared to pristine Fh (q_max,Cr(VI) = 4.91 and q_max,P = 24.03 mg g⁻¹). These enhancements might be driven by prolonged amorphous Fh persistence, REE–Cr(VI)/PO₄ surface precipitation, and REE–O–Fe ternary binding sites. These findings clarify how REE–Fe coupling regulates Cr(VI)/PO₄³⁻ fates in terrestrial ecosystems and also provide insights for designing REE-modified adsorbents for long-term contaminant/nutrient retention in soils or aquatic systems, where Fh's natural transformation would otherwise compromise such efficacy.