Abstract:
Nanoscale iron(III) (oxyhydr)oxide minerals, typically ranging from 1 to 100 nanometers in size, are common constituents of natural environments. Among these, goethite and hematite (hereafter referred to as iron(III) (oxyhydr)oxides) are especially abundant and geochemically significant in soils and sediments. These minerals occur across a wide range of particle sizes and are susceptible to microbial reduction, affecting the fate and mobility of trace elements, nutrients, and environmental contaminants. However, the size-dependent reduction behavior of iron(III) (oxyhydr)oxides in single and mixed mineral systems remains poorly understood.
Antimony (Sb) has been declared as a priority environmental pollutant, owing to its chronic toxicity and associated health risks. In natural waters, Sb predominantly exists in the +V and +III oxidation states, with Sb(V) prevailing under oxic to slightly reducing conditions, while Sb(III) is stable under reducing conditions. The mobility and fate of both species are tightly linked to sorption and coprecipitation reactions with iron(III) (oxyhydr)oxides. However, critical questions remain unclear, (i) the impact of the particle size and co-existing cation (e.g., Ni2+) on Sb(V) adsorption capacity and retention mechanisms (ii) how Sb(V)-coprecipitated iron(III) (oxyhydr)oxides of different particle sizes behave under microbial Fe(III) reduction, particularly in terms of Sb speciation and distribution.
To address these knowledge gaps, we synthesized [Sb(V)-containing] goethite and hematite of varying particle sizes, then investigated (i) reduction kinetics and extents of goethite and hematite using microbial and mediated electrochemical reduction, (ii) Sb(V) adsorption capacity and retention mechanisms through Sb(V) adsorption and Sb(V)–Ni2+ co-sorption experiments (iii) reductive dissolution behavior of Sb-coprecipitated iron(III) (oxyhydr)oxides and resulting Sb mobilization using Shewanella oneidensis MR-1.
We firstly found that small particles were preferentially reduced relative to their large counterparts in both single and mixed mineral systems, regardless of microbial or electrochemical treatments. The observed differences were attributed to the combined effect of higher thermodynamic favorability and greater surface availability. In mixed mineral systems, small particles were reduced slightly faster, whereas large particles were reduced notably slower and less extensively than solely predicted from single mineral systems. Specifically, when reduced alone, small particles showed Fe(III) reduction rate constants that were 1.5- to 3.6-fold higher than large particles, while when reduced together, the reduction rate constants for small particles were 6- to 21-fold higher than the rate constants for large particles.
Our findings also revealed that (i) Sb(V) adsorption capacities ranged from 2.5 to 55.7 mg/g, with higher affinity observed for smaller particles and for goethite relative to hematite, (ii) the presence of Ni2+ markedly enhanced Sb(V) adsorption onto both goethite and hematite, with adsorption capacities increasing by 16–89% as particle size decreased, (iii) the retention mechanism for Sb(V) adsorption changed from being dominated by edge-sharing inner-sphere complexes to enhanced electrostatic interactions and the likely formation of ternary surface complexes in the presence of Ni2+. These findings demonstrate that nanoparticulate goethite and hematite have an even greater potential for Sb(V) removal than previously expected, particularly in complex systems containing multiple coexisting ions.
Lastly, our findings revealed that (i) Sb(V) was incorporated into nanoscale iron(III) (oxyhydr)oxides to varying extents (Sb: Fe ratios ranging from 2:100 to 8:100), with no clear correlation between the extent of coprecipitation and mineral type or particle size. (ii) The presence of Sb(V) significantly inhibited both the extent and kinetics of microbial Fe(III) reduction across all samples of varying particle sizes, with suppression extent ranging from 25 to 80%. (iii) Sb release was limited (<10% of total Sb for goethite and <4% of total Sb for hematite), exhibited incongruence with the reductive dissolution of Fe, and showed only negligible Sb(V) reduction to Sb(III) across all sample conditions. These collective results indicate that the incorporation of Sb(V) into goethite and hematite may enhance the stability of these iron(III) (oxyhydr)oxides and serve as an effective sink for Sb immobilization.
Overall, this research advances the understanding of size-dependent reactivity of iron(III) (oxyhydr)oxides in processes of Fe(III) reduction, Sb(V) sorption, and Sb(V) coprecipitation. Collectively, these findings offer new insights into the role of nanoparticulate iron(III) (oxyhydr)oxides in mediating environmental redox reactions, enhancing antimony mitigation in multi-ion systems, and promoting the long-term sequestration of Sb under reducing conditions.
Nanoparticles