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.

This study examined the role of intermittent illumination/dark conditions coupled with MnO 2-ammendments to regulate the mobility of As and Fe in flooded arsenic-enriched soils. Addition of MnO 2 particles with intermittent illumination led to a pronounced increase in the reductive-dissolution of Fe(III) and As(V) from flooded soils compared to a corresponding dark treatments. A higher MnO 2 dosage (0.10 vs 0.02 g) demonstrated a greater effect. Over a 49-day incubation, maximum Fe concentrations mobilized from the flooded soils amended with 0.10 and 0.02 g MnO 2 particles were 2.39 and 1.85-fold higher than for non-amended soils under dark conditions. The corresponding maximum amounts of mobilized As were at least 92 % and 65 % higher than for nonamended soils under dark conditions, respectively. Scavenging of excited holes by soil humic/fulvic compounds increased mineral photoelectron production and boosted Fe(III)/As(V) reduction in MnO 2-amended, illuminated soils. Additionally, MnO 2 amendments shifted soil microbial community structure by enriching metal-reducing bacteria (e.g., Anaeromyxobacter, Bacillus and Geobacter) and increasing c-type cytochrome production. This microbial diversity response to MnO 2 amendment facilitated direct contact extracellular electron transfer processes, which further enhanced Fe/As reduction. Subsequently, the mobility of released Fe(II) and As(III) was