Early diagenetic behaviour of selenium in freshwater sediments

Abstract

The vertical distributions of dissolved Se species [Se(IV), Se(VI) and organic Se] and diagenetic constituents [Fe(II) and Mn(II)] were obtained in porewater samples of two Sudbury area lakes (Clearwater and McFarlane). The sedimentary concentration profiles of total Se, Se species bound to Fe–Mn oxyhydroxides and to organic matter, and of elemental Se were also determined along with the concentrations of Fe, Mn and S in different extractable fractions. Results indicated that the concentrations of total dissolved Se in porewater samples were very low, varying from around 2.0 nM to a maximum level of 6.5 nM, while the concentrations of total Se species in the solid phase varied between 2 and 150 nmol/g on a dry weight basis. The two lakes showed striking differences in the presence of Se(IV) and Se(VI) at the sediment–water interface (SWI). In Clearwater Lake, Se(VI) was present at this interface and Se(IV) was not detectable, whereas the opposite was found in McFarlane Lake. This suggests that reducing conditions might have existed near the SWI of McFarlane Lake at the sampling time; this hypothesis was confirmed by several other measured chemical parameters. The profiles of total dissolved Se of both lakes suggest upward and downward diffusion of dissolved Se species along the concentration gradients. Assuming that no precipitation occurred at the SWI, the fluxes of dissolved Se species across the SWI in Clearwater and McFarlane lakes were estimated to be 0.108 and 0.034 nmol cm⁻² a⁻¹, respectively. These values do not include the possible losses of volatile Se species due to microbial methylation. In the reducing sediments of both lakes, the formation of elemental Se and pyritic Se were found to be important mechanisms for controlling the solubility of Se in this environment. The main geochemical processes involving Se identified in this study are: the adsorption of Se onto Fe–Mn oxyhydroxides at or near the SWI, the release of adsorbed Se by the reduction of the same oxyhydroxides and the mineralization of organic matter, and the removal of Se from porewaters to form elemental Se and a S mineral phase such as Se–pyrite or pure ferroselite.

Introduction

Selenium is known for its dual essential and toxic character (Frankenberger and Benson, 1994). In most aquatic systems, Se can exist in four oxidation states, +VI, +IV, 0, and −II and in several organic forms (Cutter, 1982, Masscheleyn et al., 1989). The solubility and mobility of the Se species is largely dependent on pH and redox conditions (Masscheleyn et al., 1989, Weres et al., 1989). Thermodynamic calculations indicate that selenate, in the highly soluble form SeO²⁻₄ is the stable form of Se in oxic waters. Selenite SeO²⁻₃ is found in less oxic conditions and can be strongly adsorbed by Fe and Mn oxyhydroxides (Balistrieri and Chao, 1990). The microbial reduction of selenate and selenite to elemental Se (Garbisu et al., 1996, Oremland et al., 1990, Zhang and Moore, 1997) could be an important mechanism for the incorporation and retention of Se in soils and sediments since Se(0) occupies a large region of the Eh-pH stability field. Similarly, the microbial oxidation of Se(0) to selenite and selenate can play a significant role in oxic soils (Dowdle and Oremland, 1998).

In sediments and their adjacent porewaters, Se is also subjected to chemically and/or microbially-mediated oxidation-reduction and methylation reactions often involving conversions between particulate and dissolved phases (Frankenberger and Engberg, 1998). Examples of such processes include the reduction of selenite Se(IV) and selenate Se(VI) to Se(0), the scavenging of Se(IV) by Fe and Mn oxides, the oxidation of organic selenide Se (−II), the precipitation of achavalite (FeSe) or ferroselite (FeSe₂) or the incorporation of Se into solid phases such as pyrite (Belzile and Lebel, 1988, Myneni et al., 1997, Oremland et al., 1990, Tokunaga et al., 1997, Velinsky and Cutter, 1991). Under acidic and/or reducing conditions, ferric selenite may be reduced to elemental Se (Yao and Millero, 1995). Alkaline and oxidizing conditions favour the formation of selenate species, which are soluble and easily transported (Moore, 1991). The distribution of Se(IV) and Se(VI) in marine sediments and their porewaters appears to be related to changes in the redox environment of the sediments. Decrease of Se(IV) and Se(VI) in sediments could also be due to the formation of volatile (methylated) species (Frankenberger and Benson, 1994).

Studies on freshwater Se geochemistry are scarce and it is crucial to understand the geochemical behaviour (diagenesis, solubility, mobility and transport) and biological availability of this potentially toxic element especially in an area that has been so strongly affected by Se pollution (Nriagu and Wong, 1983). The objectives of this study were to improve our knowledge on reactions that involve Se and modify its speciation in lake sediments. In this paper, the speciation of dissolved Se in porewater and the partitioning of solid Se species in sediments are presented for two Sudbury lakes differing in the acidity level and oxic status of their sediment–water interface. Other related parameters such as the dissolved and solid species of Fe, Mn and S were also determined. Based on these data, possible geochemical processes and diffusion mechanisms involving Se in oxic and anoxic freshwater environments are proposed.