Dependence of Arsenic Fate and Transport on Biogeochemical Heterogeneity Arising from the Physical Structure of Soils and Sediments

Abstract

Reduction of As(V) and Fe(III) is commonly the dominant process controlling the fate and transport of As in soils and sediments. However, the physical structure of such environments produces complex heterogeneity in biogeochemical processes controlling the fate and transport of As. To resolve the role of soil and sediment physical structure on the distribution of biogeochemical processes controlling the fate and transport of As, we examined the biogeochemical transformations of As and Fe within constructed aggregates-a fundamental unit of soil structure. Spherical aggregates were made with As(V)- or As(III)-bearing, ferrihydrite-coated quartz that was fused with agarose and placed in a cylindrical reactor; advective flow of anoxic solutes was then initiated around the aggregates to examine As release from a dual-pore domain system. To examine the impact of biotic As(V) and Fe(III) reduction, constructed aggregates having As(V)-bearing, ferrihydrite-coated quartz inoculated with sp. ANA-3 were placed in flow-through reactors under anoxic and aerated advective flow. Consistent with desorption from advective columns, As(III) is released to advecting water more prevalently than As(V) within abiotic aggregate systems, indicating a greater lability and concomitant enhanced propensity for transport of As(III) relative to As(V). During reaction with , As release to advecting water was similar between anoxic and aerated systems for the first 20 d; thereafter, the anoxic advecting solutes increased As release relative to the aerated counterpart. With aerated advecting solutes, Fe remained oxidized (or was oxidized) in the aggregate exterior, forming a protective barrier that limited As release to the advective channel. However, anaerobiosis within the aggregate interior, even with aerated advective flow, promotes internal repartitioning of As to the exterior region.

Influence of Fe(II) on Arsenic(III) Oxidation by Birnessite in Diffusion-Limited Systems

“…Within diffusion-limited soil environments, steep redox gradients can form, causing reduced compounds from anaerobic respiration within anoxic zones to diffuse toward more oxidized zones. ,,, The interaction between reduced and oxidizing compounds at these interfaces can result in the formation of new mineral phases and alter the reactivity of the original soil constituents. In this study, we examined the effect of birnessite pre-exposure to high and low concentrations of Fe­(II) on As­(III) oxidation using a diffusion-controlled multichamber reactor to simulate soil redox interfaces.…”

“…Manganese­(III/IV) oxides are common soil minerals that have high oxidative capacity even under reducing conditions that can rapidly oxidize As­(III) to As­(V). − The interaction between reduced [e.g., As­(III)] and oxidizing compounds (e.g., Mn­(III/IV) oxides) can occur at redox interfaces which form due to the complex structure of soils, where diffusion-limited transport within soil aggregates leads to the development of anoxic microsites in close proximity to oxic flow paths. − Anaerobic respiration in the interior of aggregates can reductively dissolve Fe­(III) (oxyhydr)­oxides, releasing Fe 2+ (aq) which then diffuses toward the aggregate exterior . The Fe 2+ (aq) can then react with Mn oxides on the oxic aggregate exterior, potentially altering the reactivity of Mn oxides. , …”

Pteris vittata Arsenic Accumulation Only Partially Explains Soil Arsenic Depletion during Field-Scale Phytoextraction

“…The greater abundance of arsenic(III) we found on the surface of a bulk root zone soil aggregate could indicate transport of arsenic(III) from rhizosphere to bulk soil. This redox gradient is contrary to expected reducing conditions typically found in the aggregate interior, not exterior [104]. Soluble species, including arsenic(III) and DOC, could be flushed from rhizosphere to bulk soils when gravitational flow exceeds transpiration flux, for example during irrigation events.…”

Emergent Properties of Microbial Activity in Heterogeneous Soil Microenvironments: Different Research Approaches Are Slowly Converging, Yet Major Challenges Remain

“…Hallett et al (2013) also point out that breakdown of soils by dynamic or static mechanical loading yields different fragmentations of soil aggregates. This dependence of the aggregate size distribution on the operational conditions under which it is measured raises the question of whether aggregates exist in soils in their natural state (Young et al, 2001), calling into question the extensive literature that tries to analyze the influence of aggregate size on various processes, e.g., in terms of the sequestration of OM, the distribution of bacteria, a wide range of geochemical processes, or the release of greenhouse gasses (Ranjard and Richaume, 2001; Jasinska et al, 2006; Nunan et al, 2006; Razafimbelo et al, 2008; Goebel et al, 2009; Pallud et al, 2010; Chivenge et al, 2011; Masue-Slowey et al, 2011, 2013; Blaud et al, 2014; Rabbi et al, 2014, 2016; Ebrahimi and Or, 2015; Jiang et al, 2015; San José Martínez et al, 2015; Sheehy et al, 2015; Hausladen and Fendorf, 2017; Rillig et al, 2017; Zhao et al, 2017; Bocking and Blyth, 2018; Li et al, 2018), and explaining perhaps why some authors have failed to observe anticipated correlations between OM content and aggregation (Razafimbelo et al, 2013). Nevertheless, one might argue that this dependence problem can be alleviated somewhat by standardizing methods, and that, in any event, it does not particularly affect attempts to understand at a very local scale in soils the interactions between pore geometry, chemical composition, and microbial activity.…”