Transformation pathways of arsenic species: SRB mediated mechanism and seasonal patterns

Reductive dissolution of As-bearing iron oxides: Mediating mechanism of fulvic acid and dissimilated iron reducing bacteria

Fulvic acid (FA) and iron oxides often play regulating roles in the geochemical behavior and ecological risk of arsenic (As) in terrestrial ecosystems. FA can act as electron shuttles to facilitate the reductive dissolution of As-bearing iron (hydr)oxides. However, the influence of FA from different sources on the sequential conversion of Fe/As in As-bearing iron oxides under biotic and abiotic conditions remains unclear. In this work, we exposed prepared As-bearing iron oxides to FAs derived from lignite (FAL) and plant peat (FAP) under anaerobic conditions, tracked the fate of Fe and As in the aqueous phase, and investigated the reduction transformation of Fe(III)/As(V) with or without the presence of Shewanella oneidensis MR-1. The results showed that the reduction efficiency of Fe(III)/As(V) was increased by MR-1, through its metabolic activity and using FAs as electron shuttles. The reduction of Fe(III)/As(V) was closely associated with goethite being more conducive to Fe/As reduction compared to hematite. It is determined that functional groups such as hydroxy, carboxy, aromatic, aldehyde, ketone and aliphatic groups are the primary electron donors. Their reductive capacities rank in the following sequence: hydroxy> carboxy, aromatic, aldehyde, ketone> aliphatic group. Notably, our findings suggest that in the biotic reduction, Fe significantly reduction precedes As reduction, thereby influencing the latter's reduction process across all incubation systems. This work provides empirical support for understanding iron's role in modulating the geochemical cycling of As and is of significant importance for assessing the release risk of arsenic in natural environments.

Release characteristics and risk assessment of volatile sulfur compounds in municipal wastewater treatment plants

In recent years, the malodorous gases generated by sewage treatment plants have gradually received widespread attention due to their sensory stimulation and health hazards. The emission concentration, sensory evaluation and health risk assessment of volatile sulfur compounds (VSCs) were all explored in two municipal wastewater treatment plants (WWTPs) with oxidation ditch and anaerobic/oxic treatment process, respectively. The VSCs concentration showed the highest amount in the primary treatment unit in both the two WWTPs (73.3% in Plant A and 93.0% in Plant B), while the H2S took the main role in the composition of VSCs. However, H2S took a larger percentage in Plant A (84.5% ∼ 87.0%) rather than Plant B (61.2% ∼ 83.5%), which may be due to the different operating conditions and sludge properties in different treatment process. Besides, H2S also gained the first rank in the sensory evaluation and health risk assessment, which may cause considerable sensory irritation and health risk to workers and surrounding residents. Furthermore, the influencing factor analyses of VSCs emission showed that the temperature of water and air, ORP of sludge made the greatest effect on VSCs release. This study provides theoretical and data support for the research of VSCs emission control in WWTPs.

Arsenic biogeochemical cycling association with basin-scale dynamics of microbial functionality and organic matter molecular composition

Geogenic arsenic (As)-contaminated groundwater is a sustaining global health concern that is tightly constrained by multiple interrelated biogeochemical processes. However, a complete spectrum of the biogeochemical network of high-As groundwater remains to be established, concurrently neglecting systematic zonation of groundwater biogeochemistry on the regional scale. We uncovered the geomicrobial interaction network governing As biogeochemical pathways by merging in-field hydrogeochemical monitoring, metagenomic analyses, and ultrahigh resolution mass spectrometry (FT-ICR MS) characterization of dissolved organic matter. In oxidizing to weakly reducing environments, the nitrate-reduction and sulfate-reduction encoding genes (narGHI, sat) inhibited the dissolution of As-bearing iron minerals, leading to lower As levels in groundwater. In settings from weakly to moderately reducing, high abundances of sulfate-reduction and iron-transport encoding genes boosted iron mineral dissolution and consequent As release. As it evolved to strongly reducing stage, elevated abundance of methane cycle-related genes (fae, fwd, fmd) further enhanced As mobilization in part by triggering the formation of gaseous methylarsenic. During redox cycling of N, S, Fe, C and As in groundwater, As migration to groundwater and immobilization in mineral particles are geochemically constrained by basin-scale dynamics of microbial functionality and DOM molecular composition. The study constructs a theoretical model to summarize new perspectives on the biogeochemical network of As cycling.

Influence of Sulfate Reduction on Arsenic Migration and Transformation in Groundwater Environment

The sulfate-reducing bacteria-mediated reduction process is considered to be an important mechanism affecting arsenic migration and transformation in anaerobic environments. To investigate the effect of sulfate-reducing bacteria in a high-arsenic aquifer on arsenic migration and transformation, the typical sulfate-reducing bacteria Desulfovibrio vulgaris was selected for micro-cosmic experiments to simulate a groundwater environment with or without sulfate amendment. The reduction of Fe(III) and As(V) by Desulfovibrio vulgaris was identified, and Fe(III) and As(V) were reduced in both sulfate-free and sulfate-containing systems. However, the addition of 1 mM sulfate significantly enhanced Fe(III) and As(V) reduction. Compared with no sulfate addition, 1 mM sulfate increased the reduction rates of Fe(III) and As(V) by 111.9% and 402.2%, respectively. The sulfate process mediated by Desulfovibrio vulgaris also remarkably promoted arsenic release in sediments. These results indicated that sulfate concentration should be considered when sulfate reduction is used as a remediation method for arsenic pollution in groundwater.

Variations of arsenic forms and the role of arsenate reductase in three hydrophytes exposed to different arsenic species

In order to understand the mechanisms of arsenic (As) accumulation and detoxification in aquatic plants exposed to different As species, a hydroponic experiment was conducted and the three aquatic plants (Hydrilla verticillata, Pistia stratiotes and Eichhornia crassipes) were exposed to different concentrations of As(III), As(V) and dimethylarsinate (DMA) for 10 days. The biomass, the surface As adsorption and total As adsorption of three plants were determined. Furthermore, As speciation in the culture solution and plant body, as well as the arsenate reductase (AR) activities of roots and shoots, were also analyzed. The results showed that the surface As adsorption of plants was far less than total As absorption. Compared to As(V), the plants showed a lower DMA accumulation. P. stratiotes showed the highest accumulation of inorganic arsenic but E. crassipes showed the lowest at the same As treatment. E. crassipes showed a strong ability to accumulate DMA. Results from As speciation analysis in culture solution showed that As(III) was transformed to As(V) in all As(III) treatments, and the oxidation rates followed as the sequence of H. verticillata>P. stratiotes>E. crassipes>no plant. As(III) was the predominant species in both roots (39.4-88.3%) and shoots (39-86%) of As(III)-exposed plants. As(V) and As(III) were the predominant species in roots (37-94%) and shoots (31.1-85.6%) in As(V)-exposed plants, respectively. DMA was the predominant species in both roots (23.46-100%) and shoots (72.6-100%) in DMA-exposed plants. The As(III) contents and AR activities in the roots of P. stratiotes and in the shoots of H. verticillata were significantly increased when exposed to 1 mg·L^-1 or 3 mg·L^-1 As(V). Therefore, As accumulation mainly occurred via biological uptake rather than physicochemical adsorption, and AR played an important role in As detoxification in aquatic plants. In the case of As(V)-exposed plants, their As tolerance was attributed to the increase of AR activities.

Time-dependent changes of arsenic and its selected forms in a hydroponic experiment with Quercus robur L

Mutual transformations of particular As forms are dynamic. Therefore hydroponic experiments need to account for this variation. For this reason, the aim of the study was to determine the time-dependent changes of As_total and selected forms of this metalloid (As(III), As(V), DMA or the sum of other organic forms) in modified Knop's solution and organs (root, low and high stem) of 2-year-old Quercus robur L. seedlings within a 33-day long hydroponic experiment. The results indicate the varying speed of As uptake and transport to aerial plant parts. A decrease in contents of As forms in organs of seedlings exposed to individual As forms varied, which indicates simultaneous transformations of As forms both in Knop's solution and plant organs. The obtained results indicate the need to analyse the main forms of As both in the nutrient solution and plant organs to assess the actual effectiveness of As phytoextraction by plants. It is necessary because, as demonstrated in this work, the addition of a specific As form does not mean that the capacity of a given plant relative to this form specifically is assessed. Capsule: The form of arsenic added to the medium undergoes dynamic changes affecting the phytoextraction of this metalloid in Quercus robur L. organs.