Influence of size, morphology, surface structure, and aggregation state on reductive dissolution of hematite nanoparticles with ascorbic acid

Nanoscale iron (oxyhydr)oxide minerals are omnipresent in natural environments and past studies have found that their size, morphology, surface structure, and aggregation state influence chemical reactivity. In this study, we have investigated the influence of these factors on the reductive dissolution of hematite with ascorbic acid using two types of nanoparticles with average diameters of 6.8 ± 0.8 nm and 30.5 ± 3.5 nm, referred to as Hem-7 and Hem-30, respectively, in this paper. Hematite nanoparticles were synthesized by forced hydrolysis of ferric nitrate and characterized with powder XRD, TEM, and BET surface area measurements. Reductive dissolution experiments were conducted at 23.0 ± 0.5 °C and pH 3.35 ± 0.03 in the absence of light under nitrogen. Aqueous [Fe(II)] was measured by the ferrozine assay. High-resolution TEM (HRTEM) revealed that the Hem-7 crystals are pseudo-hexagonal plates and the Hem-30 crystals are rhombohedral. Initial and steady state dissolution rates were determined from batch experiments and compared after surface area (SA) normalization: the initial rates of Hem-7 and Hem-30 are 9.11 ± 2.24 and 4.48 ± 1.62 (10−7 mol m−2 h−1), and the steady state rates are 1.94 ± 0.53 and 1.29 ± 0.36 (10−7 mol m−2 h−1), respectively. These results suggest that both initial and steady state rates of Hem-7 dissolution are faster than the steady state rate of Hem-30, although the differences between the steady state rates of Hem-7 and Hem-30 are within one standard deviation. HRTEM observation of individual crystals and aggregates of Hem-7 reveal that this hematite is defect-free and no preferential etching occurred. Additional TEM measurements indicate that previous to dissolution, Hem-7 is present as both dispersed particles and as aggregates. Dispersed particles dissolve initially before aggregates, which influences the dissolution rate. The Hem-30 hematite has nanoscale surface steps and internal defects, and its dissolution initiates from the steps, defects, or sharp edges of the crystals. In addition, HAADF-STEM (high angle annular dark field – scanning transmission electron microscope) tomography was employed to observe the three-dimensional structures of individual particles and aggregates. This three-dimensional tomography reveals that aggregates of nanoparticles dissolve heterogeneously and holes form on the surface of crystals. This study directly shows the importance of size, surface roughness, defects, crystal morphology and aggregation states on dissolution rates of nanoparticles.