Carbonate accelerated transformation of ferrihydrite in the presence of phosphate

• Effects of carbonate and phosphate on ferrihydrite transformation were investigated. • Carbonate can override the retarding impact of P on the ferrihydrite transformation. • Carbonate promoted the formation of more crystalline rhombic hematite particles. • Phosphate and carbonate serve a different role in the cycling of nutrients & metals. In soils and rhizospheres, iron (oxyhydr)oxides and oxyanions like carbonate and phosphate occur ubiquitously and their interaction have important implications for nutrients and metals cycling. An elevated activity of carbonate in soils and sediments (e.g., pCO 2 , ∼2%) above current atmospheric CO 2 (∼0.04%) is observed. The level of agronomic soil test phosphorus (P) in intensively managed agricultural soils can be up to several hundred mg kg −1 . Although it is known that the transformation of iron (oxyhydr)oxides is suppressed by phosphate adsorption, it is unclear how increasing carbonate activities exert the reaction process. Here, the effects of carbonate on the transformation of ferrihydrite were evaluated at pH 7.5 in the presence of phosphate using experimental geochemistry, Fe K-edge X-ray absorption spectroscopy, transmission electron microscopy-energy dispersive X-ray spectroscopy, and Fourier transform infrared spectroscopy. Our results, for the first time, showed that the inhibitory effect of phosphate on ferrihydrite transformation was retarded by carbonate, and it transformed into hematite. Under pCO 2 = 408 ppm (0.213 mM carbonate), an increase in [phosphate] o from 0 to 0.5 mM decreased the transformation rate of ferrihydrite from 0.009 to 0.003 d −1 . However, the suppression was drastically perturbed when carbonate was added to the phosphate-ferrihydrite system. When 11.42 mM carbonate (pCO 2 = 20000 ppmv) was present, the rate increased from 0.009 to 0.033 d −1 without phosphate and from 0.003 to 0.018 d −1 with 0.5 mM of phosphate. Moreover, carbonate promoted the formation of more crystalline rhombic hematite particles, while phosphate modified the morphology of hematite from rhombic to ellipsoidal-like with rough surfaces. The distinctive difference of the influence of carbonate and phosphate on the transformation kinetics, products distribution, and morphologies come from the interaction between carbonate/phosphate and ferrihydrite. Although phosphate formed strong inner-sphere complexes on the ferrihydrite surface, which inhibited the direct contact and dissolution of ferrihydrite, co-adsorbed carbonate still promoted the olation reaction, facilitating the formation of hematite. These results suggest that as an important geochemical process, the increasing activity of carbonate or pCO 2 can override the retarding impact of phosphate on the transformation of ferrihydrite and control the occurrence/crystallization of hematite in the subsurface soils under the scenario of climate change.