Different Arsenate and Phosphate Incorporation Effects on the Nucleation and Growth of Iron(III) (Hydr)oxides on Quartz

Iron­(III) (hydr)­oxides play an important role in the geochemical cycling of contaminants in natural and engineered aquatic systems. The ability of iron­(III) (hydr)­oxides to immobilize contaminants can be related to whether the precipitates form heterogeneously (e.g., at mineral surfaces) or homogeneously in solution. Utilizing grazing incidence small-angle X-ray scattering (GISAXS), we studied heterogeneous iron­(III) (hydr)­oxide nucleation and growth on quartz substrates for systems containing arsenate and phosphate anions. For the iron­(III) only system, the radius of gyration (R_g) of heterogeneously formed precipitates grew from 1.5 to 2.5 (±1.0) nm within 1 h. For the system containing 10^–5 M arsenate, R_g grew from 3.6 to 6.1 (±0.5) nm, and for the system containing 10^–5 M phosphate, R_g grew from 2.0 to 4.0 (±0.2) nm. While the systems containing these oxyanions had more growth, the system containing only iron­(III) had the most nucleation events on substrates. Ex situ analyses of homogeneously and heterogeneously formed precipitates indicated that precipitates in the arsenate system had the highest water content and that oxyanions may bridge iron­(III) hydroxide polymeric embryos to form a structure similar to ferric arsenate or ferric phosphate. These new findings are important because differences in nucleation and growth rates and particle sizes will impact the number of available reactive sites and the reactivity of newly formed particles toward aqueous contaminants.