Competitive microbial and geochemical processes controlling the fate of arsenic [electronic resource]
Keywords: Environmental Earth System Science

Within soil environments, reducing conditions generally promote release of As into the aqueous phase, while oxidizing conditions favor As immobilization. While many past studies have examined and illustrated the role of reduction and oxidation processes individually, there is currently a lack of information on competitive redox or coupled processes that may occur within soils. Furthermore, the combined effects of these chemical processes in soils is complicated by the complex physical structure within which these reactions take place. Accordingly, this dissertation focuses on defining the competitive reactions controlling As transformation and partitioning within the physically and chemically heterogeneous structure of soils. Within soils and sediments, redox gradients resulting from mass transfer limitations lead to competitive reduction-oxidation reactions that drive the fate of As. In Chapter 2, As reduction-oxidation dynamics are investigated using a diffusively-controlled system using a Donnan reactor where birnessite and Shewanella sp. ANA-3 are isolated by a semi-permeable membrane through which As migrates. Initially, As(III) is rapidly oxidized to As(V) by birnessite; however, a rapid decline in the rate of As(III) oxidation was observed owing to passivation of the birnessite surface, where high [Mn(II)] combined with increasing [CO32-] from microbial respiration leads to the precipitation of rhodochrosite, which eventually passivates the Mn oxide surface, inhibiting further As(III) oxidation. These findings show that despite the initial capacity of birnessite to rapidly oxidize As(III), the synergistic effect of intense As(V) reduction by microorganisms and the buildup of reactive metabolites capable of passivating reactive mineral surfaces will produce (bio)geochemical conditions outside of those based on thermodynamic predictions. Mass transfer limitations within soils also promote the formation of competitive sorption interfaces, where As mobility can be affected by multiple sorbents. Manganese and Fe oxides are ubiquitous solids in terrestrial systems that have high sorptive capacities for As. Although numerous studies have characterized the effects of As adsorption onto Fe and Mn oxides individually, the fate of As within mixed systems representative of natural environments is unresolved. In Chapter 3, I examined As(III) oxidation and competitive retention of As on goethite and birnessite by employing the Donnan reactor. It was found that As(V) is preferentially partitioned onto goethite due to higher sorption affinity compared to birnessite. Furthermore, reactive transport modeling demonstrates that the amount of aqueous As available is controlled by the sorption capacity of the goethite surface, which when saturated, leads to increased aqueous As concentrations. These findings show that Mn oxides in soils act as a temporary sorbent of As, but operate primarily as strong oxidants responsible for transformation of As(III) to As(V), which can then strongly adsorb on, and is ultimately immobilized by, the surrounding Fe oxide matrix. The aggregate-based structure of soils imparts physical heterogeneity that gives rise to variation in microbial and chemical processes that may influence the speciation and retention of As. To examine the impact of distributed redox conditions on the fate of As in aggregated soils, I imposed various redox treatments upon constructed soil aggregates composed of ferrihydrite- and birnessite-coated sands presorbed with As(V) and inoculation with Shewanella sp. ANA-3. In Chapter 4, I reveal that diffusion-limited transport allows reducing conditions to persist in the interior of the aggregate when aerated treatments are imposed, causing As, Mn, and Fe to migrate from the reduced aggregate interiors and become immobilized at the aerated exterior region. Upon transition to anoxic conditions, pulses of As, Mn and Fe are released into the advecting solution outside of the aggregate. These results demonstrate the importance of considering redox conditions and the physical complexity of soils in determining the As dynamics, where redox transitions can either enhance or inhibit As release due to speciation shifts in both sorbents and sorbates. The physical and chemical heterogeneity of soils is accompanied by the great biological diversity that influences many of the chemical reactions controlling As transformation. Extensive flooding during monsoon seasons in many regions of South and Southeast Asia, such as Cambodia, creates anoxic soil conditions that favor anaerobic microbial metabolic processes, including microbial As(V) respiration. Few studies have successfully amplified arrA without prior enrichment and factors influencing sequence diversity are currently unknown. In Chapter 5, amplification of a highly conserved functional gene encoding dissimilatory As(V) reductase, arrA, was used as a molecular marker to detect the genetic potential for As(V) respiration in environmental samples. I demonstrated successful amplification, cloning, and sequencing of 223 novel arrA gene sequences from Cambodia soils without prior enrichment/stimulation, collectively forming a clade that is phylogenetically distinct from existing sequences available. Application of permutational MANOVA demonstrates that As and depth variables are most strongly associated with variations in arrA sequences. These findings demonstrate the potential for using biogeochemically and ecologically relevant functional genes to understand operative geochemical processes.