DSpace at VNU: Mobilization of arsenic and iron from Red River floodplain sediments, Vietnam

Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 74 (2010) 3367–3381 www.elsevier.com/locate/gca Mobilization of arsenic and iron from Red River floodplain sediments, Vietnam Dieke Postma a,*, Søren Jessen a, Nguyen Thi Minh Hue b, Mai Thanh Duc b, Christian Bender Koch c, Pham Hung Viet b, Pham Quy Nhan d, Flemming Larsen a a Dept of Geochemistry, National Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen, Denmark b Research Centre for Environmental Technology and Sustainable Development (CETASD), Hanoi University of Science, VNU, 334-Nguyen Trai, Thanhxuan Dist., Hanoi, Viet Nam c Dept of Basic Sciences and Environment, University of Copenhagen, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark d Hanoi University of Mining and Geology, Dong Ngac Dist., Hanoi, Viet Nam Received 25 September 2009; accepted in revised form 19 March 2010; available online 30 March 2010 Abstract Sediments from the Red River and from an adjacent floodplain aquifer were investigated with respect to the speciation of Fe and As in the solid phase, to trace the diagenetic changes in the river sediment upon burial into young aquifers, and the related mechanisms of arsenic release to the groundwater Goethite with subordinate amounts of hematite were, using Moăssbauer spectroscopy, identied as the iron oxide minerals present in both types of sediment The release kinetics of Fe, As, Mn and PO4 from the sediment were investigated in leaching experiments with HCl and 10 mM ascorbic acid, both at pH From the river sediments, most of the Fe and As was mobilized by reductive dissolution with ascorbic acid while HCl released very little Fe and As This suggests As to be associated with an Fe-oxide phase For oxidized aquifer sediment most Fe was mobilized by ascorbic acid but here not much As was released However, the reduced aquifer sediments contained a large pool of Fe(II) and As that is readily leached by HCl, probably derived from an unidentified authigenic Fe(II)-containing mineral which incorporates As as well Extraction with ascorbic acid indicates that the river sediments contain both As(V) and As(III), while the reduced aquifer sediment almost exclusively releases As(III) The difference in the amount of Fe(II) leached from river and oxidized aquifer sediments by ascorbic acid and HCl, was attributed to reductive dissolution of Fe(III) The reactivity of this pool of Fe(III) was quantified by a rate law and compared to that of synthetic iron oxides In the river mud, Fe(III) had a reactivity close to that of ferrihydrite, while the river sand and oxidized aquifer sediment exhibited a reactivity ranging from lepidocrocite or poorly crystalline goethite to hematite Mineralogy by itself appears to be a poor predictor of the iron oxide reactivity in natural samples using the reactivity of synthetic Fe-oxides as a reference Sediments were incubated, both unamended and with acetate added, and monitored for up to months The river mud showed the fastest release of both Fe and As, while the effect of acetate addition was minor This suggests that the presence of reactive organic carbon is not rate limiting In the case of the river and aquifer sediments, the release of Fe and As was always stimulated by acetate addition and here reactive organic carbon was clearly the rate limiting factor The reduced aquifer sediment apparently can sustain slower but prolonged microbially-driven release of As The highly reactive pools of Fe(III) and As in the river mud could be due to reoxidation of As and Fe contained in the reducing groundwater from the floodplain aquifers that are discharging into the river Deposition of the suspended mud on the floodplain during high river stages is proposed to be a major flux of As onto the floodplain and into the underlying aquifers Ó 2010 Elsevier Ltd All rights reserved * Corresponding author Tel.: +45 38142784 E-mail address: diekepostma@gmail.com (D Postma) 0016-7037/$ - see front matter Ó 2010 Elsevier Ltd All rights reserved doi:10.1016/j.gca.2010.03.024 3368 D Postma et al / Geochimica et Cosmochimica Acta 74 (2010) 3367–3381 INTRODUCTION The widespread contamination of groundwater with arsenic in the floodplain sediments from SE Asia has been extensively documented and the associated health risks are a matter of deep concern (Yu et al., 2003; Polya et al., 2005; Ahmed et al., 2006; Berg et al., 2007) In order to alleviate this threat it is of great importance to elucidate the processes controlling the mobilization of arsenic Overall, our conceptual understanding of arsenic mobilization into the groundwater is as follows Sediments (sand and mud) are transported by the rivers from the Himalayan mountain range to the flood plains The sediments are deposited on the floodplain and develop aquifers and aquitards, which become anoxic and start to release arsenic to the groundwater Most researchers ascribe the mobilization of arsenic to the reduction of As-containing Fe-oxides or Fe-oxyhydroxides, henceforth collectively called Fe-oxides, by organic carbon oxidation (Nickson et al., 1998, 2000; McArthur et al., 2001; Dowling et al., 2002; Harvey et al., 2002; Swartz et al., 2004; Postma et al., 2007) However, the more detailed mechanisms that are involved remain unclear One possible scenario is that arsenic initially is present as As(V) within the Fe-oxide crystal lattice and during reductive dissolution of the Fe-oxide the arsenate ions are released The liberated arsenate ion may be reduced to arsenite either in aqueous solution or at the surface of the Fe-oxide In addition, both arsenate and arsenite can adsorb or desorb to the surface of the remaining Fe-oxide (Dixit and Hering, 2003; Stachowicz et al., 2008) An alternative scenario is that both arsenate and arsenite initially are present at the surface of the Fe-oxide and as the surface of the Fe-oxide diminishes due to reduction, the surface species must go into solution Pedersen et al (2006) co-precipitated trace amounts of arsenate with Fe-oxides and found for ferrihydrite that most As(V) coordinated to the surface, whereas for goethite a large part was incorporated in the crystal lattice Pedersen et al (2006) and Tufano and Fendorf (2008) showed that, during the reductive dissolution of an As-containing-Fe-oxide, the releases of As and Fe are non-stoichiometric These scenarios indicate a large number of possible pathways for arsenic mobilization, which even for simple synthetic systems are poorly understood Also, the role of microbes in the reduction of arsenate and the coupling between the processes of Fe(III) and arsenate reduction are extensively discussed (Akai et al., 2004; Islam et al., 2004; van Geen et al., 2004; Polizzotto et al., 2006; Heimann et al., 2007; Lear et al., 2007; Sutton et al., 2009) Finally, other mineral species in the sediments like carbonates (Roma´n-Ross et al., 2006; Alexandratos et al., 2007; Sø et al., 2008) or silicates (Goldberg and Glaubig, 1988; Chakraborty et al., 2007) may also adsorb arsenic, thereby further complicating the process of arsenic release into the groundwater Hydrogeological pathways may be part of the mobilization scenario as well Polizzotto et al (2008) suggested that most arsenic in the 6000-year-old Mekong delta aquifer is mobilized in surface soil layers and is subsequently transported down through the sandy aquifer In this case the retardation of aqueous arsenic species becomes important On the other hand, Postma et al (2007) observed in a very young Red River aquifer that the mobilization of arsenic did occur within the aquifer There is a clear need for a more mechanistic insight into the water–sediment interactions that are involved in the mobilization of arsenic into groundwater As a first step a better understanding of the solid phase speciation of As and Fe in the sediments should be sought Our current knowledge is based on bulk sediment analysis (Kocar et al., 2008), sequential extraction data (Dowling et al., 2002; Akai et al., 2004; Swartz et al., 2004), and spectroscopic techniques like EXAFS and XANES (Polizzotto et al., 2006; Itai et al., 2010) Currently, there is not much information concerning the mineralogy of the Fe-oxides in the Holocene aquifer sediments to which As is supposed to be associated (Akai et al., 2004; Polizzotto et al., 2006; Rowland et al., 2008) How the solid phase data are to be interpreted in terms of mobilization of As is also not clear The two approaches so far employed are desorption of As from the sediment in the laboratory (Polizzotto et al., 2006) or the field (Harvey et al., 2002), and sediment incubation studies (Islam et al., 2004; van Geen et al., 2004; Gault et al., 2005; Anawar et al., 2006; Radloff et al., 2007) Although such experiments have demonstrated the release of arsenic from the sediment, the mechanisms involved have not been well defined The objective of this study is to improve our understanding of the mobilization of arsenic from the Holocene aquifer sediments into groundwater Our field site is located along the Red River, Vietnam, where previous work has shown extensive mobilization of arsenic in the floodplain aquifer (Postma et al., 2007) We compare river and aquifer sediments in order to detect diagenetic changes in the sediment composition that may reveal the processes involved The methods employed, range from mineralogical investigations using Moăssbauer spectroscopy, extractions over time in ascorbic acid and HCl and finally sediment incubation The leaching experiments with ascorbic acid and HCl give new insight concerning the kinetics of iron and arsenic mobilization, provide data about the redox speciation of arsenic in the solid phase and allow the determination of the reactivity of the Fe(III) pool in the sediment Finally, incubation experiments explore the release of Fe and As on a larger time scale under close to in situ conditions METHODS 2.1 Sediment sampling The sediments were collected at our field site along the Red River, 30 km upstream of Hanoi (Postma et al., 2007) The site is located between the river and the dyke and is subject to seasonal flooding Aquifer sediments were sampled and stored as described by Postma et al (2007) Results are presented for one sample from the oxidized zone (6.6–7.5 m depth) and one from the reduced zone (9.5–10.0 m depth) of the sand aquifer Aquifer sediments from 15 m depth have been dated to be around 460 years old (Larsen et al., 2008) and consequently the aquifer sedi- Mobilization of arsenic ment used for experiments here must be even younger The river sand and mud were sampled nearby at the same low energy bend of the river as surficial sediments in shallow water River and aquifer sediments were stored in O2impermeable Al-laminate bags In this state the samples were transported to Denmark where they were kept refrigerated at 10 °C 2.2 Mineralogy of fine size fractions All handling of the samples was done inside an anoxic glove box The fine size fractions (