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

Abstract

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 (Rg) 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, Rg grew from 3.6 to 6.1 (± 0.5) nm, and for the system containing 10(-5) M phosphate, Rg 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.

Fractal aggregation and disaggregation of newly formed iron(iii) (hydr)oxide nanoparticles in the presence of natural organic matter and arsenic

“…The arsenic content of the nanoparticles in the ternary system was measured to be 14.5 ± 1.5 mol% As, which is higher than the 8.1 ± 2.3 mol% As observed in the system without NOM. 4 This composition is also reflected in the zeta potential measurements ( Table 1). The zeta potential of ironIJIII) (hydr)oxide precipitates on quartz powder was 29.7 ± 3.2 mV for the system with AsIJV) + NOM, compared with 32.3 ± 3.7 mV for the system with NOM only.…”

“…Detailed information on data fitting can be found in Section S2 in the ESI † and in our previous publications. 4,5,10,14 In addition, the invariant can be calculated from the scattering intensity using the following formula: 4,5,10,14 (1) The invariant is proportional to the total nanoparticle volume, and thus these quantitative values can be used to compare heterogeneous precipitation amounts between the different reaction systems. Analysis of GISAXS data was carried out using the GISAXS-SHOP macro in Igor Pro (v 6.22 A, WaveMetrics, Inc., OR), which is available at APS beamline 12-ID-B.…”

“…The electron density contrast in the FeIJIII) + AsIJV) system was calculated to be 1.2 times that of the FeIJIII) only system. 4 While this can prevent an absolute quantitative comparison, the scattering intensity difference between these two systems is several times larger than the contrast difference. Thus, we can conclude that the total volume of particles in the FeIJIII) + AsIJV) system is larger than the FeIJIII) only system.…”

“…Thus, although the two NOM-containing systems have the same NOM content, the water content of the ternary system is less. This may be because particles which contain FeIJIII) + AsIJV) without NOM have a water content of 22 wt%, 4 and so the water content in the ternary system will be in between that of the FeIJIII) + AsIJV) system and that of the FeIJIII) + NOM system if the components are interacting with FeIJIII) exclusively. Fig.…”

“…15 These electrostatic forces can also promote ironIJIII) (hydr)oxide interactions with other negatively charged water components such as natural organic matter (NOM). 4 In regions where arsenic contamination of ground and surface waters is a concern, such as Bangladesh, 9 NOM is also ubiquitously present at concentrations ranging from 0-10 mg C (carbon) per L in surface water and 0-2 mg C per L in groundwater. 16 Therefore, it is important to fully understand how the presence of arsenic, together with NOM, will impact ironIJIII) (hydr)oxide precipitation.…”