Dissolved Organic Matter Sources and Consequences for Iron and Arsenic Mobilization in Bangladesh Aquifers

Abstract

Iron (Fe) and dissolved organic matter (DOM) cycling have been implicated in arsenic mobilization via microbially mediated Fe oxide reduction. To evaluate the sources and multiple roles of DOM in Bangladesh aquifers, we conducted spectroscopic analyses on various types of surface water and groundwater samples from a site representative of aquifer chemistry and hydrology. Surface water contained humic substances with oxidized quinone-like moieties and high concentrations of labile microbially derived DOM. In contrast, in shallow groundwater where dissolved iron and arsenic concentrations were high, the quinone-like moieties of humic substances were more reduced, with less abundant labile DOM than that of surface water. Instead, DOM at these depths was characterized by terrestrial (plant/soil) signatures. A sediment microcosm experiment demonstrated that Fe(II) and terrestrially derived DOM were released from sediment over time. The results provide new evidence to support a dual role of natural DOM in Bangladesh aquifers (1) as a labile substrate for Fe- and humic-reducing bacteria and (2) as an electron shuttle via humic substances to enhance microbial iron reduction. Fluorescence index, amino acid-like fluorescence, and redox index may serve as useful indicators of the type of DOM likely to be involved in Fe solubilization and potentially As mobilization reactions.

The impacts of different long-term fertilization regimes on the bioavailability of arsenic in soil: integrating chemical approach with Escherichia coli arsRp::luc-based biosensor

“…It has been reported that the application of inorganic or organic fertilizers can increase the content of organic carbon in soil and maintain soil productivity (Liu et al 2013;Yang et al 2011). Additionally, it has been confirmed that As mobilization is associated with the concentration of dissolved organic carbon ND not determined in soil, which is due to the organic ligands of dissolved organic carbon (DOC) that facilitate the release of inactive As (F4 fraction) to active soluble As (F1 fraction) through competitive sorption or through solution complexation (Cao et al 2003;Harvey et al 2002;Jackson et al 2006;Mladenov et al 2010). In addition, Fe oxyhydroxides precipitated by microbes have a strong adsorption capacity to As in soil (Harvey et al 2002;Mladenov et al 2010;Yang et al 2002); Consequently, the enhancement of soluble As could also lead to an increase of As bound to Fe oxyhydroxides which belonged to F2 fraction of As .…”

“…Additionally, it has been confirmed that As mobilization is associated with the concentration of dissolved organic carbon ND not determined in soil, which is due to the organic ligands of dissolved organic carbon (DOC) that facilitate the release of inactive As (F4 fraction) to active soluble As (F1 fraction) through competitive sorption or through solution complexation (Cao et al 2003;Harvey et al 2002;Jackson et al 2006;Mladenov et al 2010). In addition, Fe oxyhydroxides precipitated by microbes have a strong adsorption capacity to As in soil (Harvey et al 2002;Mladenov et al 2010;Yang et al 2002); Consequently, the enhancement of soluble As could also lead to an increase of As bound to Fe oxyhydroxides which belonged to F2 fraction of As . Given that the organicbound As was negligible in polluted soil (Sarkar et al 2007), only exchangeable As (F1 fraction) and Fe-Mn-oxide-bound As (F2 fraction) were considered to be potential bioavailable As as determined by BCR-SEPs in this research.…”

“…6 Correlation analysis of BCR-SEP-determined As (F1 + F2) and biosensor-determined As Fig. 7 Comparison of BCR-SEPs-determined As (F1, F2) with the biosensor-determined As at the three As addition levels: 0 mg/kg, 50 mg/kg, and 100 mg/kg Jackson et al 2006;Mladenov et al 2010). Although there is no direct conclusion to prove that As bound with organic carbon is available by biosensor cell, however, interactions between organic carbon and As can be influenced by various factors such as pH, redox potential, and other competing ions in aquatic environments, and it could cause the secondary release of As to aquatic environments and the rise of bioavailable As (Wang and Mulligan 2006).…”

“…However, the application of long-term fertilization increased organic carbon concentration (Liu et al 2013;Yang et al 2011) and the content of phosphorus (P) (Ge et al 2008;Zheng et al 2008a) and changed the acidity of soil depending on the soil conditions (Dong et al 2012;Zheng et al 2008b). Meanwhile, the release of metal including As from the solid phase of soil was influenced by organic carbon based on the formation of soluble metal-organic complexes (Cao et al 2003;Grafe et al 2002;Harvey et al 2002;Jackson et al 2006;Mladenov et al 2010;Wang and Mulligan 2006). pH and P also have been confirmed as major factors impacting the release of As by the mechanism of competitive anion exchange (De Brouwere et al 2004;Grafe et al 2002;Signes-Pastor et al 2007;Wang and Mulligan 2006;Yang et al 2002;Zhao and Stanforth 2001).…”

Strong chemical evidence for high Fe(II)-colloids and low As-bearing colloids (200nm–10kDa) contents in groundwater and flooded paddy fields in Bangladesh: A size fractionation approach

“…Moreover, As in soil inherited from increasing irrigation practices during the dry season with As-rich groundwater becomes of interest because of its potential transfer to plants such as rice (Meharg and Rahman, 2003). It is well recognized that high Fe and As concentrations observed in groundwater in Bangladesh result from reductive dissolution of Fe (oxyhydr)-oxides due to bacterially-induced anoxic conditions (van Geen et al, 2004;Mladenov et al, 2010). For paddy fields irrigated with As-enriched groundwater, it is worth noting that the As content of the rice-submerging water and in soil water obviously depends on the As content of the irrigation water (Norra et al, 2005;van Geen et al, 2006;Roberts et al 2010) but no direct relationship has been observed (Garnier et al, 2010).…”