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

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 (qmax,Cr(VI) = 19.39 mg g−1 and qmax,P = 43.33 mg g−1 for Fh–Nd, 24.47 and 64.03 mg g−1 for Fh–Y) compared to pristine Fh (qmax,Cr(VI) = 4.91 and qmax,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.