Titanium Dioxide Nanoparticles Induced an Enhanced and Intimately Coupled Photoelectrochemical-Microbial Reductive Dissolution of As(V) and Fe(III) in Flooded Arsenic-Enriched Soils

Nanosized titanium dioxide (TiO2) is a naturally existing nanoscale semiconducting mineral, and its co-occurrence with microbes may elicit differential environmental effects. In this study, the impacts of TiO2 nanoparticles (NPs) on the reductive dissolution of As(V) and Fe(III) from flooded arsenic-enriched soils were examined under intermittent illumination and dark conditions. The amendment with TiO2 NPs under intermittent illumination resulted in the highest As/Fe reduction among all amendments. In the amendment with TiO2 NPs, the maximum concentrations of Fe(II) derived from intermittent illumination and dark treatments were nearly 2.1- and 1.7-fold higher than the soils amended with acetate alone under dark conditions (36.5 ± 4.5 mg/L), respectively, and nearly 1.6- and 1.2-fold higher than the increased As(III) concentrations (8175.2 ± 125.5 μg/L) detected under the same conditions. However, the removal of total organic carbon derived from the amendment with acetate-TiO2 NPs under intermittent illumination was only 0.8 times that of the amendment with acetate alone under dark conditions. Because TiO2 NPs are highly responsive to sunlight, more photoelectrons supplied from intermittently illuminated soils were separated synchronously by accompanying them with the capture of photoholes by humic/fulvic acids; thereafter, the photoelectrons participated in As(V)/Fe(III) reduction. In addition, the electrical conductivities of TiO2 NPs-supplemented soil particles were nearly 1.6-fold higher than that of nonsupplemented samples, thereby enabling a long-distance electron transfer. Moreover, the amendment with TiO2 NPs with intermittent illumination resulted in an increase to the abundances of several metal-reducing bacteria in the soil microbial community, e.g., Bacillus, Thermincola, Pseudomonas, and Clostridium, correspondingly boosting the involved microbial degradation of organic substrates to supply more bioelectrons for As(V)/Fe(III) reduction. The findings have an important implication on the understanding of the role of nanosized minerals in the biogeochemical cycling of metal pollutants.