Evidence of decoupling between arsenic and phosphate in shallow groundwater of Bangladesh and potential implications

Abstract

Reductive dissolution of iron oxyhydroxides and reduction of arsenic are often invoked as leading causes of high dissolved As levels in shallow groundwater of Bangladesh. The second of these assumptions is questioned here by comparing the behavior As and phosphate (P), a structural analogue for As (V) which also adsorbs strongly to Fe oxyhydroxides but is not subject to reduction. The first line of evidence is provided by a detailed groundwater time-series spanning two years for three wells in the 6–9 m depth range showing removal of As(III) from shallow groundwater during the monsoon without of loss of P. The data indicate a loss of ~90% of the dissolved As from groundwater in the intermediate well relative to a level of 3 μmol/L As predicted by conservative mixing between groundwater sampled from the shallower and the deeper well. In contrast, P concentrations of ~30 μmol/L in the intermediate well closely match the prediction from conservative mixing. Reduction therefore appears to inhibit the release of As to groundwater at this site relative to P instead of enhancing it. A re-analysis of existing groundwater As and P data from across the country provides a broader context for this finding and confirms that, without reduction, elevated concentrations of As would probably be even more widespread in shallow aquifers of Bangladesh. Without providing definite proof, X-ray absorption spectroscopy of sediment from the time-series site and elsewhere suggests that the loss of As from groundwater may be coupled to precipitation of As sulfide. Further study is needed to assess the implications of these observations for shallow aquifers that have been subjected to increased withdrawals for irrigation in recent decades.

Inorganically modified clay minerals: Preparation, characterization, and arsenic adsorption in contaminated water and soil

“…There are two chemical hypotheses, namely arsenopyrite oxidation hypothesis (Mandal et al, 1996) and ferric oxyhydroxide reduction hypothesis (Bhattacharyya et al, 1997), which explains the widespread arsenic occurrence in the groundwater of Bengal delta basin and Bangladesh. The latter hypothesis proved more consistent according to some literature (Sanyal 1999;Aziz et al, 2016). According to this hypothesis, anoxic condition of the aquifers caused the mobilization of arsenic from arsenic bearing minerals into the groundwater.…”

Pyrite Biomineralization and Arsenic Sequestration at a Florida Industrial Site: Imaging and Geochemical Analysis

“…Ironreducing bacteria compete with SRB for organic carbon and they can cause arsenic release (Chapelle and Lovley, 1992). Amendments of nutrients in wells might also cause initial arsenic release because phosphate and nitrate can compete with arsenic for sorbing sites on aquifer minerals (Neumann et al, 2010;Aziz et al, 2016). Fe, SO4, and As levels in affected wells show concurrent decreases in the third week, followed by a decrease in H2S in the 4th week, consistent with SRB mediated precipitation of pyrite and removal of arsenic from groundwater ( Figure 6).…”

Simultaneously quantifying ferrihydrite and goethite in natural sediments using the method of standard additions with X-ray absorption spectroscopy

“…For unknown multi-mineral assemblages, EXAFS-LCF requires proper identification of the minerals present and inclusion of their spectra in fitting. To be consistent with previously published studies (Aziz et al, 2016; Jung et al, 2012; Mihajlov, 2014), ferrihydrite, micro-goethite, hematite, magnetite, Fe-bearing silicates, mackinawite, and siderite were selected as references for the samples derived from the Bangladesh sediments. These references were then evaluated based on their SPOIL values.…”

“…For each sample/reference, parallel EXAFS scans were averaged, normalized with linear pre-edge and quadratic post-edge functions, and converted to k 3 -weighted chi function with a threshold energy (E 0 ) of 7124 eV. Relevant Fe mineral references to be included in linear combination fitting (LCF) were selected based on previously published studies on Bangladesh and Dover aquifers (Aziz et al, 2016; Jung et al, 2012; Mihajlov, 2014; Sun et al, 2016a; Sun et al, 2016b). To be specific, relevant references included ferrihydrite, micro-goethite, hematite, magnetite, Fe-bearing silicates, mackinawite, and siderite (reference spectra are in Figure SA2).…”