Dissolved fulvic acids from a high arsenic aquifer shuttle electrons to enhance microbial iron reduction

Abstract

It was demonstrated more than two decades ago that microorganisms use humic substances, including fulvic acid (FA), as electron shuttles during iron (Fe) reduction in anaerobic soils and sediments. The relevance of this mechanism for the acceleration of Fe(III) reduction in arsenic-laden groundwater environments is gaining wider attention. Here we provide new evidence that dissolved FAs isolated from sediment-influenced surface water and groundwater in the Bengal Basin were capable of electron shuttling between Geobacter metallireducens and Fe(III). Moreover, all four Bangladesh sediment-derived dissolved FAs investigated in this study had higher electron accepting capacity (176 to 245μmol/g) compared to aquatic FAs, such as Suwanee River Fulvic Acid (67μmol/g). Our direct evidence that Bangladesh FAs are capable of intermediate electron transfer to Fe(III) supports other studies that implicate electron shuttling by sediment-derived aqueous humics to enhance Fe reduction and, in turn, As mobility. Overall, the finding of greater electron accepting capacity by dissolved FAs from groundwater and other sediment-influenced environments advances our understanding of mechanisms that control Fe reduction under conditions where electron transfer is the rate limiting step.

Design of Gravel‐Sand Filter for Arsenic Removal: A Case Study of Muzaffargarh District in Pakistan

“…Raw water is sustained for two hours in inlet compartment simultaneously exposing to air (oxygen). Therefore As (III) and Fe (II) are oxidized to As (V) and Fe (III) respectively (Ilgen et al, 2017;Kulkarni et al, 2018;Meng et al, 2017). Redox process of As þ3 , Fe þ2 to As þ5 , and Fe þ3 is expected to be completed in two hours (Leupin and Hug, 2005).…”

Evaluation of the Effect of Humic Acids on the Reductive Elution of Fe from Fe₂O₃ in a Saline, Seawater-like Medium

“…HAs were extracted from hardwood bark compost (Sanyo-Bark, Sanyo-Chip Industry, Japan), softwood bark compost (Daishizen, Green-Sogyo Co., Ltd., Japan) and peat powder in Aberdeen shire (River-Import, Japan), and then purified according to IHSS procedures. 18,19) Yields of HAs were evaluated by dividing the HA (g, dw) obtained in a one-time extraction by an amount of initial material taken compost or soil (kg, dw). 1 000 mg L − 1 stock solutions of HAs were prepared by dissolving freeze dried HA powder in 0.04 M aqueous NaOH.…”

“…[13][14][15] Attention has focused on the electron shuttle effect of humic substances which could accelerate the metal reductive reaction in sediments or aquatic environments. [16][17][18] Labile organic substances, such as saccharides, could also contribute to the production of electrons by becoming a microbial energy source. 18) A structural analysis of humic acid (HA) indicated that anaerobic bacteria are activated in the presence of a fertilizer composed of steelmaking slag and compost during a 6-month period of incubation in coastal seawater.…”

“…[16][17][18] Labile organic substances, such as saccharides, could also contribute to the production of electrons by becoming a microbial energy source. 18) A structural analysis of humic acid (HA) indicated that anaerobic bacteria are activated in the presence of a fertilizer composed of steelmaking slag and compost during a 6-month period of incubation in coastal seawater. 19) Based on these findings, it is possible that humic substances can contribute to the elution of Fe, which will then serve as an activator of bacterial growth or promote electron shuttle from bacteria to steelmaking slag in the fertilizer.…”

Investigating Fluorescent Organic-Matter Composition as a Key Predictor for Arsenic Mobility in Groundwater Aquifers

“…2,3,7,[11][12][13] While sedimentary DOM from peat layers, 9,[14][15][16] as well as surfacederived organic carbon, [17][18][19] have been suggested, the source of labile DOM is still a matter of much debate. 20,21 Beyond labile DOM, recent studies also highlight the potential influence of humic and fulvic acids (refractory organic polymers) in As mobilization through electron shuttling, [22][23][24][25] or competition for sorption, 26 and complexation processes. 27,28 Since the chemical composition of DOM defines its reactivity in the environment, 29,30 the ability to track the different chemical fractions of DOM is essential to better understand its role on arsenic mobilization in groundwater.…”