versión On-line ISSN 0717-9707
J. Chil. Chem. Soc. v.48 n.3 Concepción sep. 2003
J. Chil. Chem. Soc., 48, N 3 (2003) SSN 0717-9324
STUDY OF CHEMICAL SPECIATION IN SEDIMENTS: AN APPROACH TO VERTICAL METALS DISTRIBUTION IN RAPEL RESERVOIR (CHILE).
1 Departamento de Ingeniería Geográfica, Facultad de Ingeniería,
Universidad de Santiago de Chile, Santiago, Chile.
2 Facultad de Química y Biología, Universidad Santiago de Chile, Santiago, Chile.
(Received: December 12, 2002 Accepted: May 9,2003)
A sequential chemical extraction method, following a Tessier's protocol has been applied to sample sediment from lake Rapel (34S), an hydroelectric reservoir. This study is a first approach of chemical speciation in this sediment. The four elements determined in the extracts were Cu, Zn, Mn and Fe. Analysis of the extracts was carried out by flame atomic absorption spectrometry (FAAS) and X-Ray fluorescence. The sample sediment was extracted using a core from 1 cm to 40 cm and the level concentration range were: Fe (19850 mg/g - 43899 mg/g), Mn (752 mg/g - 1269 mg/g), Cu (149 mg/g - 478 mg/g) and Zn (224 mg/g - 596 mg/g). Cu was associated to organic matter and carbonate fraction, Zn was associated to oxides Fe-Mn and Fe and Mn were bonding to oxides. All the metals show significant concentration in residual fraction except Cu.
Keywords: sediment, sequential extraction, metal speciation, trace metal, Rapel reservoir
The many human activities in the last decades have contributed to increase the metal mobility in the environment. Heavy metals are one of the main pollution factors in aquatic systems because some metals are persistent and potentially dangerous to biota 1. The metals are transported by particulate matter to the sediment but this is not necessarily the final fate of metallic species because a fraction of these can be released to column water when the physicochemical conditions have changed 2. The behavior and concentration of trace metals in sediment can play a relevant role in detecting sources, degree of pollution and distribution mechanisms in aquatic environment. The total concentration of the metal is relevant, nevertheless the bioaccumulation, availability, reactivity and mobility are determined by metal chemical form making a chemical speciation study necessary 3.
Trace metals may be distributed among many components of the sediment and may be associated with them in different ways. Chemical speciation is defined as distribution of an individual chemical element among different species or groups. In the last twenty years many studies related to sequential extraction schemes have been carried out 4,5,6,7,8. We selected the Tessier's method since it allows suitable estimation about characteristics of metal association 9.
The goal of this study is to characterize the Fe, Cu, Mn and Zn level in a sediment core sampling from Rapel power reservoir localized in a zone with significative impact of tourism, agriculture and metallic Cu mining (El Teniente mine, CODELCO-Chile Division, located to 90 km from Rapel reservoir).
This study is the first approach to determine sediment chemical speciation in a vertical sampling in Alhué (sub-basin), a branch of Rapel reservoir. The Alhué sub-basin began to receive an effluent from copper mining settling ponds via the Alhué stream.
Study site and sampling
Rapel Reservoir (3410'S, 7129'W) is located in central Chile, Region VI, 140 km southwest of Santiago (Figure 1). The reservoir is supplied by Cachapoal River, Tinguiririca River and Alhué stream and it generates hydroelectric power. The reservoir is approximately 27 kilometers long. Maximum depth is 87 meters and average depth 8 meters. The reservoir has three branches: the north-west branch is the deepest and contains the dam wall. The southern branch is formed after the confluence of Cachapoal and Tinguiririca river. The north-eastern branch is formed by Alhué stream (Figure 1). Alhué basin shown an estimated renewal rate of 14 months, significantly slower than other zone of the reservoir 10.
|Fig. 1. Rapel reservoir morphology and morphometric parameters. A=82.7 km2, V=0.667 km3 and DL=6.94 20.|
The main physical characteristics are summarized in Table I.
Bottom sediments were collected during winter 1998 with a small Phleger corer 11 modified using either gravity coring or scuba. 0.5 cm fraction sediment was obtained between 1-40 cm and stocked in glass vials for further analyses.
a) Extraction Procedures and reagents
Samples were prepared for metals analysis from lyophilized (Lab conco Freezone 4.5) sediments and refrigerated to 5°C for later analysis.
The total content of Fe, Mn, Zn and Cu in sediment samples was determined on 0.1000 ± 0.0001 g dry sample. That was digested by 7.0 mL HF and 1.0 mL Aqua Regia in acid digestion method bomb (Parr), to 100°C (12 h); disgregated solution was solubilized in 100.0 mL H3BO3 1M (Merck, s.p.). Analysis was duplicated.
The metal speciation was made by Tessier's sequential chemical extraction using 0.5000 ± 0.0001 g of sediment:
a.- Exchangeable fraction: the sediment was extracted at room tem perature for 1 hour with 6.0 mL of 1 M Mg Cl2 (pH=7) with con tinuous agitation.
b.- Bond to carbonate: the residue from (a) was leached at room tem perature with 6.0 mL of 1 M NaOAc/HOAc (pH=5) with continu ous agitation.
c.- Associated with Fe-Mn Oxides: the residue from (b) was extracted with 15.0 mL of 0.04 M NH2OH.HCl in 25 % acetic acid; the experiments were performed at 96 ± 2°C during 6 hours with occa sional agitation.
d.- Associated with organic matter:
To the residue from (c) were added 2.3 mL of 0.02 M HNO3 and 3.8 mL of 30 % H2O2 (pH 2 with HNO3) and the mixture was heated to 85 ± 2°C for 2 hours and with occasional agitation; a second 2.3 mL aliquot of 30% H2O2 (pH=2) was then added and the sample was heated again to 85 ± 2°C for 3 hours with agitation. After cooling, 3.8 mL of 3.2 M NH4OAc in 20 % HNO 3 (v/v) was added and the sample was diluted to 20.0 mL and agitated continuously for 30 min.
e.-Residual fraction: the residue from (d) was digested with 7.5 mL of HF and 1.5 mL of HClO4 to near dryness (5 hours); then a second addition of 7.5 mL of HF and 0.75 mL of HClO4 was made and again the mixture was evaporated to near dryness. Finally the residue was dissolved in HCl 30% and diluted to 25.0 mL HClO4
All reagents were Merck Suprapure grade. Double deionized water (2 µS/cm) was used in preparing stock solutions. Extraction were carried out directly in polyethylene tubes. Residues were separated from supernatant by centrifugation (Sorvall Econospin) during 45 min.
Metal contents of the five fraction and total metal contents in solution was determined by Atomic Absorption Spectroscopy (Perkin Elmer model 2380). Semiquantitative estimation of total metals in dried sediment was carried out by X-Ray Fluorescence (Siemens SRS 3000).
RESULTS AND DISCUSSION
Table II shows the total metals content in sediment of Rapel reservoir and relative standard deviation (Rsd %). Of the all metals examined in the sediment sampling from Rapel reservoir. Fe is very high concentrated that Mn, Cu and Zn. Concentration in superficial sediment sample (1 cm) shown the following distribution for Fe, Mn, Cu and Zn: 37304 µg/g, 1249 µg/g, 292 µg/g and 285 µg/g, respectively.
This values are lightly high that previous values measured in 1994 spring season for the same sampling station (I. Vila personal communication) (Fe: 23461 µg/g, Mn: 771 µg/g and Cu: 113 µg/g; Zn was not measured).
Results in Table III represent the elementary analysis from X-Ray Fluorescence. These values show that Mn, Cu and Zn are lower than 1 % and O and Si are the highest concentrations.
Sequential Chemical Extraction
Figure 2 shows Tessier's method 9 results obtained in sediments from Rapel reservoir. These values represent a vertical distribution for each analyzed metal for each extracted fraction.
|Fig. 2. Vertical distribution of Mn, Zn, Cu and Fe in different extraction fraction. F1 (exchangeable fraction), F2 (bond to carbonate fraction), F3 (associated with Fe and Mn oxides) and F4 (associated with organic matter fraction). F1 (#), F2 (! ), F3 (%) and F4 (%).|
The results in Figure 2 demonstrate that Cu is the highest concentration in carbonate and organic matter fractions. Fe is bounded to Mn - Fe oxide and residual fraction.
Mn is bounded to the exchangeable, Fe-Mn oxide and residual fractions. Zn does not indicate a clear tendency.
The metal concentrations are represented by profile metal concentration versus depth. In Figure 2, F1, F2, F3 and F4 correspond to each extracted fraction.
Fe, Cu and Zn in exchangeable fraction are the lowest in the studied sediment. The low concentration is probably due to the fact that metals in this form can easily be absorbed and utilized by organisms in aquatic environment 12. Similar results have been published in many studies 13,14,15,16,17.
The relative percentage range for Mn is 9 % to 38 % is probably due to Mn(II). The Mn(II) chemical oxidation kinetics 18 is slower than Fe(II) in the aquatic systems. The precipitation of manganese is favored through autocatalytic effect over pH 8.5 19. The epilimnion pH values in Alhué basin varied between 8 to 9 20. Under this conditions, manganese probably could be transferred to water column when the ionic strength force is modified 9. Figure 2 shows that manganese concentration level is 100 ppm (0 10 cm) and it is increased to the deepest fractions. The mine tailing is not source of manganese, so the natural origin is the could be probably source of this metal.
Bonding carbonate fraction
The concentration of Fe and Zn bonding carbonate are the lowest in this fraction. Table II shows that Zn concentration is very low. Otherwise the relative percentage of Cu and Mn are 12 % - 18 % and 13 % - 25 %, respectively according to many studies 9,13,21. The concentration of Cu and Mn increases due to depth (Figure 2). pH measured in the interface sediment-water was 7.10 (data not show) which could explain the copper presence in minerals compounds, such as malachite and azurite 19.
Bond to Fe and Mn oxidates
Cu concentration is low compared to other metals analyzed, Figure 2. The range of relative percentage to Cu, Zn, Fe and Mn are 3 % to 18 %, 3 % to 40%, 18 % to 28 % and 14 % to 40 %, respectively. The surface of Fe and Mn oxides have special affinity with the cations to natural pH 9,17,19,22. The high level concentration of Fe and Mn can be explained by the precipitation effect of Fe-Mn oxyhydroxides in water. Under the strong oxidizing conditions and with pH natural values, Fe2+ can be transformed into Fe3+ rapidly and precipitated as Fe oxyhydroxide 19. Also, Mn separates out from water as Mn oxyhydroxide and is transported into the sediments.
The tendency of the evolution of the metals profiles by depth is not regular. In this fraction the concentration of iron and manganese oxyhydroxides are higher than other extracted fractions.
Bond organic matter
Iron and manganese level concentration are the lowest (4 %-14 % and 2 %-9 %, respectively) in accordance with the highlight in the last paragraph. Zinc shows a significant distribution in the sampling according to the tendency of other studies 16,17,22.
The range of relative percentage of Cu is 38 % to 78 %, similar to other studies 9,13,22.The fulvic and humic acids and other source of organic matter with complexant properties explain the high level concentration of Cu associated to this fraction. The stability constant of Cu-organic matter complex is higher than other ions with similar oxidation state 23,24,25,26. Hydrological conditions, an increase incoming nutrients from human activities and input of mine tailings (copper sulfate) could be explained by the high concentration level of bond Cu-organic matter in some fraction of sediment profile in Rapel reservoir 20.
Fe, Mn and Zn shown (Table IV) the relative concentration range between 30 % - 70 % in the residual fraction in all samples sediment analyzed. Fe reaches a level as high as 70 %. These values show over 50 % of the total concentration metals in the sediment sampling, according to many studies 13,16,22. The main source of these metals could come from mineralogical conditions of the soil in according to mineralogical characteristics of Alhué soil (allophane, illite, halloysite and quartz); the chemical analysis of Alhué soil (Table V) show presence Fe and Mn oxides. X-ray Fluorescence in sediments show that Fe is ones majority (8.7%) element and Mn distribution is low (0.24%) while Cu and Zn are present in lowest concentrations (0.07 % and 0.02 %, respectively). Otherwise qualitative analysis by X-ray diffraction show that composition is formed by: quartz, muscovite, albite, reyerite and gismondine.
Table V. Chemical analysis of Alhué soil 27.
nd: not detected
Repeatability and Accuracy
Repeatability was estimated using three independent replicate samples, Table VI, where are shown copper analysis results which range between 0% to 10%. Copper concentration is near 0.03% then the precision is low when the concentration approaches the detection limit.
The analytical accuracy was tested using Natural Matrix Certified Reference Material (NMCRM, Resource Technology Corporation). The Table VII shows Fe, Mn, Cu and Zn concentrations and recovery percentages in the sum of the five fractions and in the total digestion sample. Fe and Mn shown recovery percentage near to 100 %; Cu and Zn shown recovery percentage near to 130%, these metals are present in trace level in NMCRM sample.
F1 exchangeable fraction; F2 , bond to carbonates fraction; F3 associated with Fe and Mn oxides; F4, associated with organic matter fraction; F5, residual fraction. S1, S2 y S3 samples
|a represents the sum of the five fractions
b represents the total metal concentration
nd: not detected
The vertical distribution of copper, iron, manganese and zinc was studied in a representative sediment core from Alhué basin in Rapel reservoir. The study was carried out using sequential chemical extraction. All examinated metals are present in the different fractions. Iron is the most concentrated metal in sediments. Residual fraction was significant (about 50 %) for Fe, Mn and Zn. Iron concentration is one magnitude order larger than Mn and two order magnitude in order to Cu y Zn. Manganese is associated to oxides and it is the only metal that show exchangeable fraction that could be explained by the mobilization to water column. Copper is mainly associated to carbonate and organic matter. Comprehension of the metals role in this sediment requires additional studies specially related to pollution from mining activities.
This work has been supported by DICYT (Universidad de Santiago de Chile), project 010012PK. We express our gratitude to Dr. I. Vila for her helpful contribution.
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