Effects of phosphate on REY adsorption by goethite: Insights into REY enrichment and release in marine iron oxyhydroxides during early diagenesis

Deep-sea REY (lanthanides and yttrium) can be initially scavenged by nano ferromanganese (oxyhydr)oxides via ion adsorption and lattice substitution, while subsequent aging and partial recrystallization induced by reduction during diagenesis usually cause release and redistribution of REY. Previous studies find positive correlations among REY, goethite (α-FeOOH) and phosphorus/phosphate in some marine (micro)nodules, but the mechanism responsible for these correlations is yet unclear. The interaction between iron oxyhydroxides and metal oxyanions has been studied extensively, but no study focuses on interactions between iron oxyhydroxides, REY, and phosphate. This study investigates the immobilization and release mechanisms of REY by synthetic goethite at macro- and molecular scales by comprehensive usage of ad/desorption experiments, inductively coupled plasma mass spectrometry, extended X-ray absorption fine structure (EXAFS) spectroscopy, and density functional theory calculations. The results indicate that goethite exhibits an adsorption preference for heavy REY, displaying obvious lanthanide tetrad effects. Phosphate (0.02 mM ≤ CP ≤ 0.2 mM) however, greatly enhances REY adsorption by forming ternary complexes, particularly light REY. EXAFS interpretations further confirm the formation of ternary complexes in La-adsorbed samples, and that phosphate can link trivalent REY and goethite on the surface. Moreover, some CeIII is further oxidized to CeIV with the help of dissolved oxygen. These observations illuminate how phosphate contributes to rare earth enrichment during early diagenesis. As goethite recrystallization is common in hypoxic environment, we further monitored REY behavior during recrystallization and find that previously adsorbed La/Ce and phosphate are released and combined to form LaPO4/CePO4 precipitates. Importantly, the CeIV reduced during recrystallization indicates that the positive Ce anomaly is likely to pass down, potentially explaining why a positive Ce anomaly is observed in diagenetic nodules and apatite at lower depth. Our study provides insights into the role of phosphate in REY-adsorption on goethite and possible mechanisms of REY enrichment, and desorption during early diagenesis, corresponding with REY patterns observed in marine iron oxyhydroxides.