Transformation of 2-Line Ferrihydrite to Goethite at Alkaline pH

Transformation of exogenous hexavalent chromium in soil: Factors and modelling

There is a great need, but not yet available, for quantitative transformation models and influencing factors of exogenous hexavalent chromium (Cr(VI)) in soil for environmental risk assessment and regulation. Therefore, the transformation processes of exogenous Cr(VI) in 13 soils across China were investigated. The changes in Cr forms in soils spiked with 100 mg kg-1 of Cr(VI) over 90-120 days indicate that both the reduction and immobilization of Cr(VI) occurred with the decrease in availability of Cr(VI) and its reduced counterpart Cr(III). In these processes, soil pH is the controlling factor and pH 6.5 is a critical inflection, where pH < 6.5 promoted the Cr(VI) transformation and decreased the Cr availability. A two-parameter model with a complementary error function for the reduction of Cr(VI) to Cr(III) using soil pH and incubation time was developed with the regression coefficient (R2) of 0.98 and root-mean-square-error (RMSE) of 7.94 %, which was validated using data from other independent literature. This semi-mechanistic model suggests that diffusion process of electrons controlled the Cr(VI) reduction. These results are helpful for understanding the Cr(VI) evolution in soil in a long term and complementing the risk assessment of redox-sensitive metal contaminated soils.

Effects of carbonate on ferrihydrite transformation in alkaline media

Alkaline media widely exist in natural and engineered systems such as semiarid/arid areas, radioactive waste sites, and mine tailings. In these settings, the commonly occurring iron (oxyhydr)oxides differed in their ability to influence the fate of nutrients and contaminants. Due to the substantially increased atmospheric carbon dioxide (CO2) concentration, carbonate stands to increase in these media. However, how increasing carbonate affects the transformation of poorly crystalline iron (oxyhydr)oxides (e.g., two-line ferrihydrite) under alkaline conditions still remains unclear. Here, kinetics of ferrihydrite transformation were evaluated at pH ∼10 as a function of [carbonate] = 0-286 mM using synchrotron-based X-ray and vibrational spectroscopic techniques. The results showed that carbonate slowed down ferrihydrite transformation slightly and suppressed goethite formation, but promoted hematite formation regardless of its concentration. At low carbonate concentration (11.42 mM), the effect of carbonate on product formation was obvious due to the weak inner-sphere complex; however, at high carbonate concentration (80-286 mM), the effect was retarded because of the adsorption equilibrium of carbonate as well as the initial carbonate adsorption followed by desorption. Moreover, carbonate modified the morphology of hematite from rhombic to ellipsoidal to honeycomb and goethite from rod-like to needle-like to spindle-like due to the inner-sphere adsorption-desorption of carbonate and adsorption of hydroxyl ions on reactive sites of iron (oxyhydr)oxides in alkaline media. The results suggest that the concurrently increasing carbonate with enhanced atmospheric CO2 could control the transformation and occurrence of iron (oxyhydr)oxides in natural and engineered environments and have important implications for the biogeochemical cycles of iron and carbon.