Article
Reference
Contrasting geochemistry of antimony in lake sediments
FILELLA, Montserrat, et al.
Abstract
The geochemical behaviour of antimony in aquatic sediments is directly affected by pH and redox conditions which, in turn, determine the presence of other compounds that can exert a strong control on the mobility of Sb. Our study illustrates how iron and manganese oxides, natural organic matter and sulphides play a role in the geochemistry of this toxic element
FILELLA, Montserrat, et al . Contrasting geochemistry of antimony in lake sediments. Journal de Physique. IV Proceedings , 2003, vol. 107, p. 471-474
DOI : 10.1051/jp4:20030343
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DOI : 10. 1051/jp4 : 20030343
Contrasting geochemistry of antimony in lake sediments
M. Filellal, N. Belzile, Y.-W. Chen and T.-L. Deng2
Department of Chemistry and Biochemistry, Laurentian University, Sudbury, Ontario P3E 2C6, Canada
1 Deparfment of lnorganic, Analytical and Applied Chemistry, University of Geneva, 30 quai Ernest-Ansermet, 1211 Geneva 4, Switzerland
2 Deparlment of Applied Chemistry, Chengdu University
of Technology, Chengdu 610059, China
Abstract. The geochemical behaviour of antimony in aquatic sediments is directly affected by pH and redox conditions which, in tum, determine the presence of other compounds that can exert a strong control on the mobility of Sb. Our study illustrates how iron and manganese oxides, natural organic matter and sulphides play a role in the geochemistry of this toxic element
1. INTRODUCTION
The geochemistry of Sb remains poorly known [1, 2]. Antimony is mostly present in the dissolved phase of natural waters. Thermodynamic calculations predict that Sb exists in the antimonate form, Sb (OH) 6-, under oxidizing conditions and as antimonite, Sb (OH) 3, under reducing conditions at natural pH values and in absence of sulphur [3]. However, numerous studies have demonstrated that real situations often differ from predictions. Sb shows affinity for organic carbon [4] and for Fe and Mn oxides [5, 6], which are also capable of oxidizing Sb (III) into Sb (V) relatively rapidly [7].
We have focused this study on the distribution of Sb species in porewaters and sediments of two Sudbury area lakes characterized by contrasting pH and redox conditions at their sediment-water interface (SWI).
2. METHODOLOGY
. Samples were collected from two lakes located in the Sudbury area, Canada : (i) Clearwater L., an acidie lake (pH changed from 4. 2 in the 1970's to 6. 3 now) characterized by an oxic SWI ; (ii) McFarlane L., a well-buffered alkaline lake (pH = 7. 5), with a SWI with low 02 levels [8, 9].
. Porewaters were sampled with in situ diffusion samplers inserted in sediments and allowed to equilibrate for 2 weeks [8, 10].
. Sediment cores were collected and subsampled in layers before acid digestion (total concentration detennination) and sequential extraction (fractionation) [8, 9].
. Porewaters were analysed for pH, dissolved organic carbon (DOC), dissolved sulphide, dissolved Fe and Mn and dissolved Sb species.
. Solid sediments were analysed for total organic carbon total reducible sulphur (TRS), total and extractable fractions of Fe, Mn and Sb [9, 11].
. Sb species in porewater and sediment were measured by continuos flow hydride generation ato nic t'uorescence spectrometry (FIG-AFS) on a PSAu 5 Millenium Excalibur System followirlg a
previously established procedure Ll l l.
. Statisticai corrélations were catcuiated using data from porewater and sédiment profiles.
c
3. RESULTS AND DISCUSSION
3. 1 The interplay of pH and redox conditions at SWI
The SWI of Clearwater L. is slightly acidie and well oxygenated. Measured pH values ranged from 6. 2 near the surface of the lake to 5. 5 close to the SWI. In McFarlane L., where pH varied from 8. 3 in the surface to 7. 2 at the SWI, the concentration of dissolved O2 dropped from 8. 5 at the surface to approximately 5. 0 mg/L at the SWI. From our previous studies [8] and the appearance of the top sediment layer, conditions of anoxia are strongly suspected at the SWI of MeFarlane L.
Profiles of measured dissolved species are compared in Figure 1. Reducing conditions at the SWI of MacFarlane L. are confirme by significant peaks of dissolved Mn (Fig. lb), dissolved sulphide (Fig. lc) and inorganic Sb (Fig. lb) in the overlying waters. Sb (III) was the dominant species in the porewaters of both lakes [9] but, whereas Sb (III) could be detected in the overlying waters of McFarlane L., this reduced Sb form was below the detection limit in the overlying waters of Clearwater L. The presence of dissolved Fe and Mn was observed in both lakes. However, their presence may be due to different reasons. While in McFarlane L. it may be explained because the low oxygen levels do not favour the precipitation of these oxyhydroxides, in Clearwater L. it might be due to the acidic conditions prevailing at the SWI of this lake.
The presence of Sb (III) at thé SWI of MacFarlane L. can be considered as a direct effect of the more reducing conditions of the MacFarlane SWI as compared to that of Clearwaters L.
0. 0 2. 0 4. 0 0. 0 0. 5 1. 0 1. 5 2. 0 0 10 20 30 40
10 10 10
5 5 5
0-... 0-0--- E-5--5--5
-10-10 -10-
Uiss. Fe-15---<O>--'ss. Fe -15 Inorg. Sb--a--Inorg. Sb-15
Díss Mn'--i>-McF
Diss Mn A Diss. Mn M cF IV
C -20-20- (b)-20 1
-25- (c) Sb (nM)
Sb (nM)
Fe&Mn/100 (pM) Fe & Mn/100 (pM) Diss. Sulfide (pM)
Figure 1. Comparison between dissolved Fe, Mn and inorganic Sb across the sediment-water interface of (a) Clearwater L. and (b) McFarlane L., and (c) dissoived sulphide in both lakes. The original concentrations of dissolved Fe and Mn were divided by
100 to maintain the same concentration scale.
3. 2 Effeet of the presence of other compounds 3. 2. 1 Fe and Mn oxyhydroxides
Fractions of sediments were obtained by sequential extraction. This procedure allows the separation and measurement of an oxidized fraction (identified here as M-Ox). In Figure 2, the observed surficial enrichment of Fe and Mn oxyhydroxides in Clearwater L. is characteristic of an oxygenated SWI and is in
strong contrast with the situation in McFarlane L. In this lake, there was no visible sediment oxidized layer. as it was confirmed by the profiles ofFe-Ox and Mn-Ox (Fig. 2b).
The affinity of Sb for Fe and Mn oxyhydroxides, already reported in the literature [5, 61, was also confirme in our study, particularly in Clearwater L. (Fig. 2a), by a strong statistical correlation between total Sb or Sb-Ox and total Fe or Fe-Ox. Positive statistical correlations also existed between dissolved species of Sb and dissolved Fe and, to a lesser extent, between Sb species and dissolved Mn in Clearwater L. (Fig. 2a).
Because Fe and Mn oxyhydroxides coexist in the same sediment, it becomes difficult to determine whether there is a preferential association of Sb with any of these two oxides, particularly when considering that concentrations of Fe-oxyhydroxides are 40-50 times higher in surficial sediments than their Mn counterpart.
The simultaneous increase of dissolved Fe, Mn and Sb in porewaters of Clearwater L. (Fig. la) can be related to the dissolution of Fe and Mn oxyhydroxides under reducing conditions, with the simultaneous release of sorbed Sb.
0. 0 1. 0 2. 0 3. 0 4. 0 0. 0 0. 5 1. 0 0 0
-5-5
10-10-
0-15-o-Fe-Ox--15--o-Fe-Ox i--Sb-Ox
- 20-
(a) (b) -25(a)-25
Sb-Ox/ 10 (nmol/ g) Sb-Ox/ 10 (nmol/ g) Mn-Ox/10 (Mmolig) Mn-Ox/100 (pmol/g)
& Fe-Ox (mmollg) & Fe-Ox (mmol/g)
Figure 2. Comparison between the oxidized fractions of Sb, Mn and Fe in sediments of (a) Clearwater L. and (b) McFarlane L.
The original concentrations of Sb-Ox and Mn-Ox were divided by] 0 and 10 or 100, respectively, to maintain the same concentration scale.
3. 2. 2 Sulphides
The formation of Sb sulphides has been extensively studied in laboratory (see ref. 2 for a detailed discussion). In natural sediments, a serious competition for sulphide can be expected from several cations, including Fe2+ present at much higher concentrations than most trace elements. In this study, strong statistical correlations were found between total Sb and TRS (r = 0. 88), especially in McFarlane L., where dissolved sulphide and total reducible sulphide were much more abundant than in Clearwater L.
Moreover, thermodynamic calculations using the JESS program [3] predict the formation of the soluble form SbS2-under reducing conditions at the pH values of the two lakes (calculations made in the absence of trace element competition). This would then suggest that the dissolved concentration of Sb under such conditions is likely controlled by sorption on the Fe sulphide that is known to be formed in the sediment.
A non-identifie fraction of Sb in reducing sediments was indeed highly coirelated with TRS. This renforces te hypothesis of a control of Sb dissolved concentration by amorphous Fe sulphide or pyrite as it has been suggested for As [12] and Se [8].
The simultaneous presence of Sb (V) and dissolved sulphide was observed în surficial sediments of both lakes (results not shown, see ref. 9). Thenllodynamic calculations [1] predict the formation of SbS4'-, which is favoured at pH values between 6 and 8. The presence of Sb (V) in sulphidic waters has been
reported in the literature [13] and the occurrence of dissolved sulphide in oxygenated waters explained by thé formation of stable metal sulphide complexes [14, 15].
3. 2. 3 Natural organic matter
Experimental evidence for complexation of Sb with natural organic matter (NOM) is rarely mentioned in the literature [2]. In a previous study, we have identified a fraction of dissolved Sb associated with NOM.
In this work, major portions of Sb in porewater were present in a refractory fraction, partially or entirely made of NOM [9]. In Clearwater L., good statistical correlations were observed between DOC and dissolved Sb species in porewaters. There was no such evidence of Sb-NOM association in the sediments, where the fractions of Sb extracted with an acidic solution of H202 (to extract organie matter) were very small in both lakes.
4. CONCLUSIONS
Profiles of Sb species in porewaters and sediments of the two study lakes confirmed the influence of pH and redox potential and showed the effect of Fe and Mn oxyhydroxides, natural organic matter and sulphides on the solubility and mobility of the trace element.
Aeknowledgements
The authors acknowledge the financial support received from the Natural Sciences and Engineering Research Council of Canada and Questron Technologies Corporation.
References
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