Surface and Textural Heterogeneity of Fresh Hydrous Ferric Oxides in Water and in the Dry State

Abstract

Fresh hydrous ferric oxides (HFO) are formed by precipitation of Fe3+ solution or ferric polycations initially formed at very acidic pH at different neutralization ratios n = 2 and 2.5. At pH 6.5 the precipitates are particles with a positive surface charge. The IEP and the surface charge measured in NaCl electrolyte vary with n. But in all cases IEP or ZPSE are lower than 8.3. The adsorption of sodium dodecane sulfonate molecules at 20°C allows us to compare the different samples formed by precipitation from n = 0, n = 2, and n = 2.5 solutions and named "n = 0," "n = 2," and "n 2.5." Using the two-dimensional adsorption on heterogeneous surfaces theory of Cases's group (Langmuir 8, 1251 (1992)), it is possible to compare the different solids relatively to the affinity of surfactants for the surfaces. The energetic heterogeneity decreases in the following order, "n = 2" > "n = 2.5" > "n = 0." The adsorption isotherms present several plateaus. The evolution of ζ vs Ce mimics the evolution of Qa vs Ce. This indicates that the adsorption sites are mainly located on external surfaces as adsorption within micro- or mesopores would correspond to a constant zeta potential. The available specific surface area at pH 6.5 can be derived from the adsorption isotherms. The plateaus of the isotherms corresponding to ζ = 0 mV. The available specific surface area S can then be calculated using the equation S=Qa · Na · A, where Qa is the adsorbed amount (mol/g), Na is Avogadro's number, and A is the molecular area in the hydrated crystal state (A = 20.8 Å2). The values of S are 163, 134, and 139 m2/g for "n = 0," "n = 2," and "n = 2.5," respectively. DRIFT spectroscopy carried out on precipitates covered by various amounts of surfactants show that they are adsorbed on positively charge sites which are the = Fe - OH(H) singly coordinated sites located on edges of the double chain of βFeOOH-like particles. Continuous Ar gas adsorption technique on "n = 0," "n = 2," and "n = 2.5" freeze-dry samples shows that pore sizes of 6 Å are intercrystal pores present in βFeOOH solids and also that dry samples are microporous with a specific surface area of 300 m2/g. The large discrepancies between textural properties of HFO in water and in a dry state are certainly due to the possibility of HFO swelling in water.