Elsevier

Chemical Geology

Volume 288, Issues 3–4, 25 September 2011, Pages 162-177
Chemical Geology

Cycling of trace metals and rare earth elements (REE) in acid sulfate soils in the Plain of Reeds, Vietnam

https://doi.org/10.1016/j.chemgeo.2011.07.018Get rights and content

Abstract

Trace metals and rare earth elements (REE) are subject to intensive chemical cycling in acid sulfate soils. The oxidation of pyrite bearing deposits and the subsequent acidification mobilizes a variety of trace metals and REE. In this study solid phase and pore water data of three cores from the Plain of Reeds in the Mekong River Delta, Vietnam, were analyzed. Distinct authigenic enrichments of trace metals and REE were observed in the solid phase of these cores. Thermodynamic modeling was employed to elucidate the controls on trace metal and REE mobility and the governing factors for the formation of these authigenic enrichments.

The formation of REE-enrichments in these settings is clearly controlled by pH and the availability of potent sorbents. Under suitable conditions trace metals such as e.g. zinc, nickel and cobalt are likely bound as trace metal monosulfides in the enrichment zones. The solid phase and pore water data and the results of a thermodynamic model suggest a possible sequential precipitation of trace metal sulfides. However, the incorporation of trace metals into iron sulfides cannot be ruled out. In the case of lead and arsenic, the formation of Pb-As-jarosite proved to be a further major sink.

These insights provide a better understanding of the controls and governing factors for trace metal and REE mobility in acid sulfate soils and will therefore help to assess impacts on soil and water.

Highlights

► Cycling of trace metals and rare earth elements in acid sulfate soils. ► Distinct authigenic enrichments of trace metals and REE in the subsoil. ► Vertical separation between trace metal- and REE-enrichments. ► Governing factor for the formation of REE-enrichments is the pH. ► Primary control on the formation of trace metal enrichments is sulfide formation.

Introduction

The Mekong River Delta in South Vietnam is one of the major Asian delta systems. Vast areas of the modern delta plain are characterized by acid sulfate soils. About 40% (1.6 million ha) of the soils in the Mekong River Delta are either severely acidic or will turn acidic upon aeration (Tuong, 1993, Husson, 1998, Tri and van Mensvoort, 2004). Acid sulfate soils generally develop from pyritic sediments. When pyrite bearing deposits are exposed to ambient air, e.g. by a decrease in regional sea level, by artificial drainage or excavation, pyrite is oxidized. During oxidation, acid is released (van Breemen, 1973, Madsen et al., 1985, Ritsema et al., 2000):FeS2(s) + 15/4O2 + 7/2H2O  Fe(OH)3(s) + 2 SO42  + 4 H+.

If the amount of acid released exceeds the acid neutralizing capacity of the soil, the pH drops sharply to values between 2 and 4 (e.g. Dent, 1986, Ritsema et al., 2000). This oxidation and acidification promotes the mobilization and leaching of various metals from the soil (Åström and Bjorklund, 1995, Minh et al., 1997a, Åström and Spiro, 2000).

Since the release and export of metals from acid sulfate soils can have severe impact on the environment, it is important to understand the processes involved in the intensive (trace) metal cycling in these soils. The cycling of trace metals and REE in these systems is governed either directly or indirectly by acidification, leaching and changes in the redox state. Therefore, a variety of sources and sinks have to be considered. The complex behavior of trace metals (Åström, 1998, Sohlenius and Öborn, 2004, Welch et al., 2007, Burton et al., 2008, Johnston et al., 2010) and rare earth elements (Åström, 2001, Åström and Corin, 2003, Welch et al., 2009) in acid sulfate soil settings has been the focus of several studies. Åström, 1998, Fanning et al., 1986 show that trace metals are leached from the oxidized topsoil and form authigenic enrichments in a transition zone between oxidized acidic topsoil and reduced non-acidic subsoil. Åström (2001) observed a similar behavior for lanthanum (La).

In this study, acid sulfate soil profiles from the Plain of Reeds (northeastern Mekong River Delta, Vietnam) were studied. The solid phase composition, pore water data and thermodynamic modeling were used to elucidate the governing factors for the distribution of trace metals and REE in theses cores and to address the following questions:

  • 1.

    Which source to sink mechanisms control the trace metal and REE mobility in this environment?

  • 2.

    What are the controls on the development of authigenic solid phase enrichments of trace metals and REE?

  • 3.

    Is it possible to discern the various governing factors for enrichments of trace metals and REE within the soil profiles?

Understanding the processes that govern the cycling of trace elements is essential for the assessment of risks posed by acid sulfate soils. This study of a highly variable system shall contribute to this understanding and provide insights for the assessment and management of acid sulfate soils.

Section snippets

Geology and hydrology of the study area

The study area is located in the north-eastern part of the Vietnamese Mekong River Delta (Fig. 1-A). The modern delta body overlies a palaeolandscape of Pleistocene age (Brinkman et al., 1993, Nguyen, 1993, Dent and Pons, 1995). The onset of the delta development is dated into the mid-Holocene (Stanley and Warne, 1994, Hori and Saito, 2007, Tamura et al., 2009). During this early phase of development, the regional sea level in this area was ca. 2–4.5 m above modern level (mid-Holocene sea level

Core characteristics

The visual core descriptions of the cores THD, THT and TLD are shown in Fig. 2.

Acknowledgments

We would like to thank S. Pape, S. Siemer and K. Enneking for technical support in the lab. Furthermore, we would like to thank the local Vietnamese authorities who supported us during fieldwork and two anonymous reviewers for their comments. This study was funded by the Senator of Environment, Construction, Traffic and Europe of the City of Bremen within the frame of the Economic Action Fond — Section Ecology (WAP) and by the Deutsche Forschungsgemeinschaft (DFG, project HA4317\2-1).

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