1 ^\ '1
Measurement of redox species in estuarine sediments using
voitammetric micro electrodes
(Rehoboth Bay, Delaware USA)
Part of the annual study of redox cycling in the sediments of Rehoboth Bay.
Author: Ing. R . E . T r o u w b o r s t December, 1999
Supervisors: Prof. D r . G . W . L u t h e r H I
Graduate School of Marine Studies University of Delaware
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D r . G . J . de L a n g e Department: Geochemistry Faculty: E a r t h Sciences University of Utrecht • *r Universiteit UtrechtIng. R . E . Trouwborst Groenhovenweg 175 2803 D D Gouda Phone: +0031-182-570175 Email: [email protected] Student number: 9707565 Prof. D r . G . W . Luther I I I
Graduate School o f Marine Studies University o f Delaware 218 Cannon Laboratory 700 Pilottown Road Lewes D E 19958-1298 Phone: +302-645-4208 Fax: +302-645-4007 Internet: [email protected] Dr. G . J . de Lange University o f Utrecht Faculty: Earth Sciences Department: Geochemistry Budapestlaan 4 3584 CD Utrecht Phone: +0031-30-2535034 Fax: +0031-30-2535030 Email: [email protected]
Summary
This research project is conducted during a three months visit at the Graduate School o f Marine Studies, University Delaware. The project is part o f the annual study o f redox cycling in the sediments o f Rehoboth Bay by the research group o f Professor Luther UI.
The main goal is to come to a better understanding considering the behaviour o f redox species and phosphate in a shallow water bay .
During this project the recently developed solid-state A u - H g microelectrodes were
succesfuily used to analyze the concentration profiles o f oxygen, H2S, FeS, Fe(III),Fe(II) and manganese (II) in the sediment porewater. The profiles provide useful information f o r the interpretation o f the measured porewater and solid phase data.
The profiles o f the ammonium and phosphate concentration in the porewater indicate the occurrence o f an organic rich layer i n the upper part o f the sediment. This layer o f organic matter is likely the result o f high productivity during the summer. B y degradation o f this organic matter, produced nutrients phosphate and ammonium are released to the overlying water in August. The degradation o f organic matter seems to directly influence the profiles o f the oxidized agents, O2, manganese and iron oxides.
The pyrite concentration coincides w i t h the organic matter concentration in both the cores sampled in August and October. Even though a high amorphous iron concentration was measured in the upper part of the sediment, no distinct ratio between the occurrence o f amorphous iron oxides and adsorbed phosphate was found.
The occurrence o f conditions o f eutrophocation is most likely to happen in August, because o f the observed flux o f nutrients during that month. Nutrients are transported to the overlying water and are available for macroalgae.
T A B L E O F CONTENTS
1. I N T R O D U C T I O N 6
1.1 Context o f this research project 6
1.2 Theory 6 1.3 Project goal 9
2. M A T E R I A L A N D M E T H O D S 10
2.1 Sampling 10 2.2 The analysis o f oxygen, manganese, Fe(II), H2S, FeS en Fe(III)
in porewater 11 2.2.1. The construction and calibration o f the sohd-state
gold-amalgam microelectrode 11 2.2.2. The microelectrode analysis o f oxygen, manganese, Fe(II),
H2S, FeS and Fe(III) 12 2.3 Porewater collection and analysis 13
2.3.1 Fe^"*" and Fe^"^ and phosphate in the porewater 13
2.3.2. A m m o n i u m analysis 13
2.4 Solid phase 14 2.4.1. The analysis o f Acid Volatile Sulphur ( A V S )
and pyritic sulphur 14 2.4.2. The analysis o f phosphate and iron f r o m amorphous
hon oxides on solid phase samples (ascorbic acid extraction) 15 2.4.3. The analysis o f iron f r o m amorphous iron oxides
-t-crystalline iron oxides on solid phase samples
(dithionite extraction). 16
3. R E S U L T S 17
3.1 Porewater analysis 17
-3.1.1 Measurement o f the porewater composition by a
solid-state microelectrode 17 3.1.2 The porewater nutrient profiles and p H profiles o f cores
sampled i n respectively August, September and October 19
3.2 Solid phase analysis 20 3.2.1 A c i d volatile sulphur (AVS) and pyritic sulphur analysis 20
3.2.2 The analysis o f phosphate and iron f r o m amorphous iron oxides and ii-on f r o m amorphous + crystalline iron oxides on sohd phase
samples (ascorbic acid and dithionite extractions) 22
3.3. The monthly change in NO3', N H / , P04^' and Fe^* concentration
of the water overlying the sediment during 1999. 24
4. D I S C U S S I O N 25 4.1 Porewater 25 4.2 Solid phase 26 4.3 Nutrients 28 5. C O N C L U S I O N S 29 6. A C K N O W L E D G E M E N T S 30 7. L I T E R A T U R E 31 A N N E X 34
- Calibration curve phosphate analysis - Calibration curve iron analysis
Calibration curve ammonium analysis Calibration curve A V S and pyrite analysis
1 Introduction
1.1 Context o f this research project
During the academic program o f a study on geochemistry at the University o f Utrecht, I participated i n two marine research projects. That experience encouraged me to conduct research abroad to expand my current knowledge o f marine research and especially the use o f electrochemistry in marine geochemistry studies.
This report describes the contents o f the research project I conducted at the Graduate school o f Marine Studies, University Delaware f r o m 1 October t i l l 24 December 1999. The project is carried out during the study geochemistry, but is conducted outside o f the compulsory
curriculum.
I was given the opportunity to learn how to construct and use the recently developed
analytical technique o f the multi-element gold-amalgam microelectrode (according to Brendel and Luther, 1995). This voitammetric analytical method can be used to measure important environmental and biogeochemical ions such as Fe(II), M n ( I I ) , O2, H2S, T, FeS, Fe(III) and
8203^ in marine as well as in freshwater systems. The microelectrodes are used to do
measurements not only in the laboratory, but also m situ in the water column and in the sediment pore water (Luther et al, 1999). The microelectrodes were also introduced i n other fields o f research, like biocorrosion studies ( X u et al., 1998) as weU as research on marine microbial growth (Dollhopf et al, 1999).
It was a challenge to participate in the research conducted by the research group o f P r o f Luther at the Graduate School o f Marine Sciences, University o f Delaware, because it included gaining experience w i t h new analytical methods and conducting research i n an international context.
1.2 Theory
The input o f large amounts o f nutrients to a shallow estuarine environment can increase the primary productivity and in that manner also increase the flux o f organic matter to the water-sediment interface. The breakdown o f organic matter i n the water-sediment as w e l l as i n the bottom
water results in the decrease o f the oxygen and the production o f nutrients. The available high nutrient concentrations in the surface water can initiate algal blooms resulting i n a decrease o f the species diversity in the estuarine ecosystem, this process is called eutrophocation. The occurrence o f conditions o f eutrophocation depends: the nutrient input and output, mixing o f the water body and in that way supply o f oxygen and the availability o f these nutrients f o r growth. Sediments play an important role i n controlling the availability o f nutrients by adsorption processes as w e l l as by mineral formation. The presence o f large amounts o f
substrate for adsorption processes depends on the balance and reduction/oxidation cycle o f the elements in the sediment. Recent studies indicate that phosphate can be adsorbed on iron oxides surfaces (Haese, 1998, Anschutz et al., 1998). The oxidation and reduction cycle i n the sediment controls the formation and preservation o f iron oxide surfaces and so the phosphate fluxes to the surface water. To study the redox cycle and balance o f nutrients in the sediment and in the surface water, it is important to measure nutrient and redox-species concentrations on the solid phase but also in the porewater with a high resolution. Recently the multi-element solid-state microelectrode has been developed and tested by the research group o f Prof.
Luther (Brendel, 1995, Brendel and Luther, 1995). This analytical technique makes it possible to study dissolved O2, M n , Fe, S(-2) and p H in pore waters o f sediments. The technique can be used to do measurements on sediment cores transported to the laboratory, but the
electrodes can also be mounted on a R O V to perform in situ measurements (Luther et al., 1999).
In marine environments where oxygen is available and the organic carbon accumulation rate is low, the process o f aerobic decomposition dominates. During conditions o f oxygen
depletion other oxidized species w i l l be used for the breakdown o f organic matter. Dependent on the redox potential (E°) O2, NO3", Mn02, Fe(0H)3 and S04^" are successively consumed (Table 1). The same succession can be observed in the sediment because o f the gradient i n redox potential w i t h depth. I n the upper part o f the sediment, oxygen can diffuse in the sediment where it is consumed by the breakdown o f organic matter. Deeper in the sediment also nitrate reduction takes place followed by a zone o f manganese and iron oxide reduction.
Table 1. The primary redox reactions for the breakdown of organic matter in coastal marine sediments (Luther etal, 1999).
138 O2 + (CH2O),06(NH3)i6H3PO4 ^
106 HCO3- + 16 NO3" + HP04^- + 16 H2O + 124 E T (1)
94.4 N03- + (CH2O),06(NH3),6H3PO4 ^
106 HCO3- + 55.2 N2 + HP04^" + 71.2 H2O + 13.6 (2)
236 Mn02 + (CH2O)i06(NH3)i6H3PO4 + 364^^
8 N2 + 106 H C 0 3 ' + HPO/- + 236 Mn^"^ + 260 H2O (3)
424 F e O O H + (CH2O),06(NH3)i6H3PO4 + 756 I T ^
106 HCO3' + 16 NH4* + 424 Fe'^ + HPO4'" + 636 H2O (4) 53 804'-+ (CH2O),06(NH3),6H3PO4
106 H C O j - + 16 NH4^ + 53 H S ' + HP04^" + 39 I T (5)
Tiie brealcdown o f organic matter results in the production o f phosphate, nitrate, bicarbonate and water. I n reactions 1, 2 and 5, protons are produced while consumption o f protons takes place when manganese and iron oxides are being used as electron acceptors. Produced phosphate can be transported by diffusion to the surface water, or get adsorbed on iron and manganese oxides. Especially amorphous iron oxides f o r m a substrate for phosphate
adsorption. High input o f organic matter can result in the reduction o f iron hydroxides and i n that way i n the release o f adsorbed phosphate to the bottom water. That is why it is important to study the reduction o f iron oxides i n detail. Iron i n sediments is often associated with sulphidic compounds (FeS, FeSi), so the relationship o f iron and iron oxides associated w i t h sulphidic compounds must also be investigated.
During this research project microelectrodes w i l l be used to study the behaviour o f redox constituents i n porewater o f a sediment core firom Rehoboth Bay. Solid phase extractions w i l l be conducted to analyze the amount o f iron present as amorphous as well as crystalline iron. Because amorphous iron is assumed to be the most important substrate for phosphate adsorption i n sediments the amount o f adsorbed phosphate to this fraction w i l l be analyzed.
This project is part o f an annual study o f reduction and oxidation processes i n Rehoboth Bay by the geochemistry research group o f P r o f Luther I I I . During this study seasonal changes as well as monthly changes i n the concentration o f constituents i n the porewater and changes i n the concentration o f species associated w i t h the solid phase are analyzed. The study w i l l improve our current knowledge o f the redox processes i n the sediments and possibly explain seasonal changes or an annual cycle. This report describes part o f this annual study, namely reseai-ch conducted on cores taken August 14"*, September 12"' and 13 October 1999.
1.3 Project goal
Come to a better understanding considering the behaviour o f redox species and phosphate i n a shallow water bay and enlarge our knowledge on factors causing conditions o f
2. Materials and metiiods
2.1 Sampling
The sampling site is located in the east of Rehoboth Bay (Fig 1). Rehoboth Bay is a shallow sub-tidal inland bay w i t h a high nutrient loading. The nutrients are mainly originated on the continent and transported by little streams to the surface water o f Rehoboth Bay.
Figure 1. The shallow estuarine Rehoboth Bay with indicated samphng site (x).
The research vessel R / V 'Captain White' owned by the Graduate School o f Marine Studies was used to navigate to the sampling site i n the east o f Rehoboth Bay on 13 October 1999. The sampling site ( 3 8 ° 4 0 ' 2 0 7 " N , 7 5 ° 0 7 ' 8 1 3 " E) was characterized by a shallow water depth of 3.4-3.8 ft (app. l m ) and a salinity o f 29 %o (refractometer). The yellow colored sediment
-was vegetated by green-brown colored seaweeds. A transparant P V C tube (0 = 75 mm) attached to an aluminum bar was pushed into the sediment. The core was closed on top w i t h a dropping plastic lid by pulling the core out o f the sediment. The core was than released f r o m the bar and carefully transported to the laboratory. A t first the microelectrode measurements were conducted as well as the analysis o f the pH-profile. A f t e r the electrode studies, the core was sliced and analyzed for porewater and the solid phase analyses.
2.2 The analysis o f oxygen, manganese, Fe(II), H2S, FeS en Fe(III) in the porewater.
2.2.1 The construction and calibration o f the solid-state gold-amalgam microelectrode.
The microelectrode was constructed as described by Brendel and Luther I I I (Brendel and Luther, 1995). The gold wire tip o f the electrode was pohshed (gold wire diameter = 100(Jm) with 2 different grain size sandpaper (type 240 en 400) and 4 different diamond polishing pastes (subsequently 15, 6, 1 and 0.25 micron). Than a small layer o f mercury was plated on the polished gold wire tip o f the electrode. The electrode was clamped w i t h the tip in a 0.1 M HgN03 -I- 300 Ml concentrated HNO3/100ml deaerated solution. B y applying a potential o f -0.1 V for 4 minutes, mercury is deposited at the polished tip so a transition between gold and the newly formed amalgam-mercury is created. The electrode tip is than cleaned by putting the electrode tip in a I M base solution, while a potential o f -9 V is applied for 90 seconds. The formation o f H2 at the electrode tip takes place and cleans the electrode tip surface.
In order to use the electrode in scientific experiments it is impoitant to calibrate and test the electrode. The oxygen concentration o f aerated seawater water is analyzed by using a linear sweep scan. The signal o f oxygen reacting to peroxide at the tip o f the electrode must equal the signal produced by the peroxide reacting to water. This test can be used to conclude i f the polishing or plating is performed correctly and i f the electrode can be used for scientific measurements. A f t e r this test also the manganese and iron calibration curves were constructed. The pilot-method was used to calibrate the electrode using the results o f the manganese calibration curve.
2.2.2 The microelectrode analysis o f oxygen, manganese, Fe(II), H2S, FeS and Fe(III).
The microelectrode was together w i t h a p H electrode attached to a micromanipulator so that, the p H as well as microelectrode measurements were carried out at the same depth. The microelectrode was connected to an Analytical Instrument Systems Inc. Electrochemical Analyzer model DLK-100. The porewater concentration depth profiles were constructed w i t h a 0.5 millimeter depth resolution.
Table 2, Reactions at the tip of an Au/Hg electrode vs. the saturated calomel electrode. O2 and H2O2 data were collected by linear-sweep voltammetry; all others were collected by square-wave voltammetry. These measurements were conducted with an A I S model D L K - 1 0 0 (according to Luther et al., 1998).
Minimum detection Slope Reaction: Ep ( V ) limit ([iM) ( n A - MM"')
02 + 2 i r + 2 e => H2O2 -0.30 5 0.152 H202 + 2 i r + 2 e => 2 H 2 O -1.30 5 0.152 HS -t-Hg 0 HgS + H*- + 2 e -0.62 <0.2 2..2 Fe^"' + Hg + 2e <=> Fe(Hg) -1.43 15 0.025 Mn^'- + Hg + 2e * f Mn(Hg) -1.55 5 0.070 2 r + 2 H g <=> Hg2l2 + 2e" -0.30 <0.3 3.2 2S203^' + Hg Hg(S203)2"'+ 2 e -0.15 16 0.111 FeS + 2 e +H*- Fe(Hg) + HS' -1.1 Colloidal species -Fe^^' + e Fe^"" -0.25 t o - 0 . 9 Colloidal species
-First the oxygen profile was analyzed in the linear sweep mode. After O2 was no longer detected, the mode was changed to square wave. The settings o f the linear sweep mode were as follows: mode = linear sweep, initial potential - 0 . I V , fmal potential -1.750 V , scan rate 200 mV/s, current range, sensitivity = lOOnA. The square wave measurements were
conducted with the following settings: mode = square wave; frequency = 200 H z ; pulse height = 24 m V ; step increment = 1 m V ; w i t h a condition step o f a potential -0.9 V i f sulphides were detected.
-2.3 Porewater collection and anaiysis
The bottom water above the core was sampled and the sediment was sliced under an argon atmosphere in a glovebag. Porewater was collected by centrifugation o f t h e sliced sediment. Immediately after centrifugation, the received porewater was filtered w i t h a 0.2 [Jm Whatman puradisc filter. Part o f the received porewater was used for the analysis o f the iron and
phosphate concentration and the remaining porewater was stored i n a fireezer (-4°C) f o r ammonium analysis.
2.3.1 Fe ^"^ and Fe ^"^ and phosphate concentration in the porewater.
The soluble iron and phosphate concentration i n the porewater was analyzed by colorimetry. The methods are based on the formation o f a molybdate blue complex for the phosphate analysis and the reaction o f iron with ferrozine for the iron analysis (Stookey, 1970). The molybdate complex was measured by 725 nm and the adsorption o f the ferrozine-iron complex was conducted by 562 nm. Dr. Martial Taillefert conducted the analyses w i t h a Spectronic instruments Inc. 601 (Milton Roy) spectrophotometer.
2.3.2 Ammonium analysis
The ammonium concentration i n the porewater o f the sediment cores sampled in August (08/19/99), September (09/22/99) and October (10/13/99) was analyzed by a flow injection system (FIA) (according to Hall and Aller, 1992). The basic principle o f this method is to inject a sample into a f l o w i n g 0.01 M NaOH stream where ammonium is converted into soluble NHs-gas. The carrier stream is passed over a gas-permeable hydrophobic membrane of teflon to the other side where a flow o f 0.01 H C l streams. This receiving stream is passed through a conductivity meter. The change in the conductivity o f the receiving stream is linear w i t h the concentration o f the transferred NH3 and is used to calculate the ammonium
concentration in the sample. A 20 |Jl sample volume was injected into the system per analysis without preliminary purification o f the porewater.
-2.4 Solid phase
2.4.1 The analysis o f A c i d Volatile Sulphur (AVS) and pyritic sulphur (According to Henneke, 1991)
0.8-1.2 g wet sediment was weighed and put in an argon flushed glass bottle. A syringe was used to add 5 m l o f 3 N H C l to the sediment through a septum. The argon gas flushing the glass bottle was lead through a 1 N N a O H solution to capture the formed H2S. During the experiment a sturer mixed the sediment-IN H C l solution i n the glass bottle. After 1 hour the bisulphide concentration o f the I N NaOH solution was analyzed by polarography.
In preparation o f the analysis of pyritic sulphur, 10 m l o f acetone was added to 1-2 g. wet weighed sediment i n a plastic bottle. The bottles were shaken and the acetone solution was taken away by using a pipette in order to remove the S° f r o m the sample. After that the sediment was dried by 60°Celsius, 0.050-0.100 g dry weight sediment was used for the analysis o f pyritic sulphur. The pyritic sulphur analysis is also based on the measurement o f H2S captured in a I N N a O H solution. Instead o f 1 N H C l , 5 m l o f a 1 N Cr(II) in 1 N H C l solution was added by syringe into the glass bottle. A color change o f t h e sediment/
chromium-solution f r o m blue to slightly green indicated the end o f the reaction, so that after approximately 2 hours the bisulphide concentration was analyzed. A n E G & G PARC model 303 A S M D E electrode was connected to an Analytical Instrument Systems Inc
Electrochemical Analyzer model D L K - 1 0 0 with an A I S , Inc DLKIOO to 303 model # IN-303 interface. The settings o f the electrode were as follows: purge time = 4 minutes, drop size = medium, mode = hanging mercury drop electrode ( H M D E ) . Settings o f t h e Chemical Analyzer were:
Technique = Square Wave Number of steps = 1301 Measurable currents - -5.00 to +5.00 fjA Initial potential - O . I V pulse width = 2.500 ms & -50.0 to +50.0 |JA Final potential = -1.4 V pulse height = 24 m V Current precision = 24.4 nA
Scanrate = 200 mV/s ending potential = -0.1 V Conditioning = -0.1 V for 5 seconds Frequentie = 200 Hz current range = 1 |jA & 10 p A (stir while conditioning) Scan increment = 1 Input gain = 1.0 Equilibrium time = 5 seconds (no stir).
-10-milliliter de-ionized water was purged for 4 minutes by argon gas and 100 |Jl o f t h e 1 M N a O H solution was pipetted into the cell. Three square wave scans were used to measure the HS- concentration. Subsequently 2 more additions o f 100 |Jl were done and every addition was scanned three times. The average obtained signal was calculated and used as a measure o f the bisulphide concentration.
2.4.2 The analysis o f phosphate and iron f r o m amorphous iron oxides on solid phase samples (ascorbic acid extraction according to Anschutz et al, 1998 and Kostka et al., 1994)
The amorphous iron-oxide fraction is defined by Canfield as the fraction iron oxides extractable w i t h ascorbic acid (Canfield, 1989). This fraction o f recent precipitated iron
oxides is seen as the most important substrate for the adsorption o f phosphate and trace metals and is likely found in the upper part o f the sediments. The amorphous iron oxides as well as phosphate adsorbed on amorphous iron oxides were extracted by adding a solution of ascorbic acid. Approximately 0.4 grams o f wet weight sediment was put in a plastic bottle and 10 m l . o f a n ascorbic acid solution was added. The ascorbic acid solution consisted o f 10 g Na-citrate and 10 g bicarbonate in 200 ml. de-aerated de-ionized water. Onto this solution 4 g o f
ascorbic acid was slowly added to a p H o f 8.
The plastic bottles were put in a water bath and shaken at 200 r p m w i t h r o o m temperature. After 24 hours the extracts were filtered and 3 m l o f the filtered (0.2 um) extract was put in a
15 m l . plastic tube. This sample was acidified by adding 10 m l o f a 0.01 M H C l solution i n order to keep iron in solution.
The iron concentration was determined by adding 0.4 m l o f the sample solution to 1.6 m l D I and 2 m l ferrozine solution was added. Also the blanks and standards were put through the same procedure. The ferrozine solution consisted o f 2.5 M ammonium acetate buffer and a ferrozine concentration o f 0,005 M , The solutions were left alone for at least half an hour to allow the color to develop. The absorbance was measured at 562 nm.
The phosphate concentration was analyzed by adding 0.6 m l sample solution to 4.4 m l de-ionized water and 0.8 m l o f an ammonium molybdate solution. A 10 m o l / I ascorbic acid
-solution was added to allow the molybdate-blue complex to develop. The absorbance o f the molybdate blue complex is measured by 885 n m after two hours and is a measure for the phosphate concentration.
2.4.3.The analysis o f iron f r o m amorphous iron oxides + crystalline iron oxides on solid phase samples (dithionite extraction, according to Anschutz et al, 1998 and Kostka et al., 1994).
A l l the iron species possibly reactive w i t h sulphides are called, the 'reactive iron' fraction. This fraction was analytical defined by Canfield as the fraction o f iron compounds extractable by a dithionite solution (Canfield, 1989). This experimental defined fraction has shown to contain large portions o f Fe, as crystalline and amorphous Fe(III)oxides, reactive Fe sillicates such as smectites and a smaller portion o f FeS (Canfield, 1989).
Approximately 0.25 g wet sediment was weighed in a small plastic container and 10 m l o f a dithionite solution was added. The dithionite solution was prepared by dissolving 20 g o f dithionite in 200 m l o f a 0.35 M Na-acetate/ 0.2 M Na-citrate solution made in aerated de-ionized water. The bottles were capped and put on a shaker at 200 r p m f o r two days at room temperature. The extract was then filtered through a 0.2 |Jm. poresize Waltman puradisc filter. Three milliliters o f t h e extract were put in a 15 m l plastic test tube and 10 m l 0.01 M H C l was added. The procedure for analyzing iron coincides w i t h the analysis o f the iron concentration in the ascorbic acid extracts.
16-3. Results
3.1 Porewater analysis
3.1.1 Measurement o f the porewater composition by a sohd-state microelectrode
Figure 2. The concentrations of different species analyzed in the porewater of a sediment core taken fi-om
Rehoboth B a y on 13 October 1999. The analysis is conducted with the solid-state gold-amalgam microelectrode.
Figure 2 shows that oxygen is rapidly depleted i n the top 2 m m o f the sediment and that the decrease of oxygen akeady starts i n the bottom water just above the sediment. The
manganese(II) concentration i n the porewater increases f r o m 4 to 16 m m sediment depth and decreases to zero at 27 m m sediment depth. A slight increase i n soluble manganese
concentration is also seen between 39-51 mm. The soluble ferrous k o n concentration i n the
-porewater coincides w i t h the profile o f the manganese(II) concentration. A strong increase i n the iron(II) concentration from zero to 250 [XM between 11 and 13 m m depth is observed, decreasing to zero at a sediment depth o f 27 mm. The signal o f the soluble ferric iron is low and starts at a depth o f 12 mm, the signal is constant deeper i n the sediment profile. The profile o f t h e FeS concentration shows a zone enriched in soluble FeS at a depth between 12 m m and 21 mm, also a higher FeS concentration is observed at 40 m m depth. The Z S signal represents the concentration o f sulphide (H2S, HS-) and S° (Sg, polysulphides). These
constituents are observed in the porewater ranging between a depth o f 11 and 20 m m w i t h an average concentration o f 16 |JM.
-3.1.2 The porewater nutrient profiles and p H profiles o f cores sampled i n respectively August, September and October.
August (19/8/1999) August (19/8/1999) August (19/8/1999) 20 sediment depth 3 0 1 (mm) P O / ' conc.(uM) 5 0 0 1 0 0 0 1500 0 2 5 NH4* conc.(uM) ' ^4 September (20/9/1999) N H / conc.(uM) sediment 2 0 depth (mm) October (13/10/1999) 5 0 100 150 2 0 0 October (13/10/1999) October (13/10/1999) 0 5 0 100 150 2 0 0 0 2 5 5 0 3-, N H / conc.(uM) P O / c o n c . ( u M ) 6.0 6.5 7.0 7.5 pH(units)
Figure 3. The profiles of p H and nutrients: ammonium and phosphate in the porewater of sediment
cores sampled in August, September and October.
-The ammonium profiles show an increase w i t h depth in the first centimeters o f the sediment (Figure 3). The sediment core sampled in August shows a decrease o f the ammonium
concentration deeper than 40 mm. The core taken in August shows a zone o f phosphate enrichment in the porewater between 20 and 60 m m whereas the phosphate profile o f the core taken in October coincides w i t h the course o f the ammonium concentration. The profile shows an increase between 0 and 15 m m depth and a slow decrease i n phosphate
concentration deeper i n the sediment. The p H profiles (Figure 3) are corrected f o r temperature as well as ionic strength and show a decrease in p H in the top 6 millimeters and a slight
increasing p H w i t h depth.
3.2 Solid phase
Figure 4. The acid volatile sulphur ( A V S ) and pyritic sulphur concentration of two sediment cores, taken respectively in August and in October.
-The acid volatile sulphur concentration i n the core taken i n August shows an increase between zero and approximately 25 m m sediment depth (Figure 4). This increase o f A V S coincides with the zone of pyritic sulphur enrichment i n the upper 25 millimeters o f t h e sediment. The core taken i n October shows also a pyritic sulphur enrichment i n the upper 23 milhmeters o f the sediment core, but the extent o f enrichment is much smaller, approximately 10 [Jmol/g dry weight compared to 90 [Jmol/g dry weight i n August. The amount o f acid volatile sulphur i n the core taken in October is low over the whole profile.
-3.2.2 The analysis o f phosphate and iron from amorphous iron oxides and iron from amorphous + crystalhne iron oxides on solid phase samples (ascorbic acid and dithionite extractions)
August August August
concentration (umol'g ' drywelght sedlnnent)
concentration (urnoi-g ' dryweigiit sediment)
sediment depttn (mm)
concentration (umol'g ' drywelght sediment)
phosphate
(ascorbic acid)
October
concentration (umoi'g'' drywelght sediment)
October
concentration (umol'g"' dryweight sediment)
sediment depth
(mm)
October
concentration (umol'g'' dryweight sediment)
phosphate
(ascorbic acid)
Figure 5. The amorphous iron oxide and phosphate distributions as well as the sum of amorphous and crystalline iron oxides distribution in the solid phase of the sediment cores taken in August and October (analysis conducted in triplicate, error bars indicate standard deviation).
-The concentration o f amorphous iron oxides in the sediment core sampled i n August shows an enrichment i n the upper 22 mm. Compared to the amorphous iron oxide concentration in the core taken in October the enrichment in the core f r o m August is much stronger, as shown i n Figure 5. The phosphate concentration adsorbed to the solid phase is slightly enriched i n the upper 20 millimeters of the sediment core f r o m October. The profile o f phosphate adsorbed on the sohd phase o f the core taken in August shows the same enrichment.
-3.3 The monthly change in NO3", NH4'', P04^' and Fe^"" concentration o f the water overlying the sediment during 1999.
Evolution of Overlying Waters for 1999
Month
Figure 6. The monthly variation in nitrate, ammonium, phosphate and soluble ferrous iron in the overlying
water of Rehoboth Bay during 1999.
The analysis o f nitrate, soluble ferrous iron, phosphate and ammonium i n the water overlying the sediment is shown i n figure 6. The concentration o f the nutrients ammonium and
phosphate in the overlying water is high in August, whereas the Fe^"^ and nitrate concentration decreases dramatically in August. A f t e r the change in concentration in August the course o f the species are stabilizing to previous levels.
-4. Discussion
4.1 Porewater
The decrease o f the oxygen concentration in the top o f the sediment core (Figure 2) can be explained by the reduction o f organic matter, formula 1 Table 1 (Luther I I I et al., 1999; Trouwborst, 1999). The oxidation o f organic matter by oxygen yields much more energy gained by the catalyzing microorganisms compared to other reducing agents. The
microorganisms catalyzing this reaction are thus in favor o f microorganisms using other oxidizing agents i f oxygen is available. This reaction (Table 1 formula 1) also explains the rapid decrease o f p H in the upper part o f the sediment (Figure 3). During conditions o f oxygen depletion other oxidizing species w i l l be used f o r the breakdown o f organic matter. Dependent on the redox potential (EO) successively oxygen, nitrate, manganese oxides, iron oxides and sulphate are consumed (Burdige, 1993). The oxygen profile shows that organic matter is present i n the upper part o f the sediment resulting in the production o f ammonium, phosphate and protons and the consumption o f oxygen.
The occurrence o f soluble manganese(II) between 1 0 - 2 0 m m and iron(II) between 10 and 25 m m indicate the reduction o f manganese oxides and iron oxides due to the breakdown o f organic matter. Table 1 reaction 3 and 4. On the left hand o f these reactions is indicated that protons are consumed during these reactions. These processes are also the reason that the p H in the sediment between 10 m m and 30 mm depth increases. I t is clear that the profile o f soluble ferrous iron and manganese overlap. A reason that Fe ^* seems not to be produced deeper in the sediment as expected by the difference in energy gained (respectively a delta G'^ o f - 3 4 9 k j mol"' o f CH2O for the reduction o f manganese oxides compared to
-114 k j m o f ' o f CH2O for the reduction of ironoxides) can be the precipitation o f iron w i t h HS". The decrease i n Mn^"" concentration in the porewater is possibly the result o f
precipitation o f carbonate phases which incorporate M n and generate CO2 and H2O or to adsorption onto solid phases such as metal-carbonates and sulphides which are present i n the sediment.
The ferric iron concentration in the porewater shown i n Figure 2 at a depth o f 12 m m and deeper in the sediment is low, but constant. The shown l o w signal o f Fe(III) concentration i n
-the porewater can be -the result o f -the ferric iron concentration i n equilibrium w i t h metastable Fe(III) species stabilized by dissolved organic material (Heuttel et al., 1998).
4.2 Solid phase
The occurrence o f H2S in the porewater between 12 and 2 5 m m sediment depth indicate the production o f HS" by the reduction o f sulphate. The produced sulphate results in the formation of FeS as w e l l as the bacterial catalyzed production o f S°. Figure 4 shows that the occurrence of both FeS and S° result in the production o f pyritic sulphur i n a distinct layer between 0 and 25 sediment depth. This layer of pyritic sulphur at the same depth o f the produced Mn^"^ and Fe^"^ by organic matter reduction results in the deduction that an excess amount o f organic matter must be present. Studies show that there is a reasonably good correlation found
between pyritic sulphur and excess organic matter (Berner 1970). Also Henneke et al. found a correlation between the pyritic sulphur concentration and organic carbon content (Henneke et al, 1997).
Organic C H,S Minerals Iron
Organic Sulphur
b: bacteria
Figure 7. A schematic diagram of the pathway by which pyrite is formed in anoxic marine sediments (Henneke et al., 1997).
-The constituents necessary for pyrite formation are shown i n Figure 7. -The hmitations for pyrite formation are: (1) The concentration dissolved sulphide (H2S and HS"), (2) The amount of reactive iron (iron reacting w i t h sulphides, according to Canfield (Canfield, 1989), (3) The available amount Sg or S*^ and (4) The concentration organic C and its decomposition during sulphate reduction. The reactions controlling the formation o f pyrite are:
The profiles of the pyritic sulphur analysis show the occurrence o f pyritic sulphide in the upper part of the sediment and that the conditions for pyrite formation are present. This points out that a sufficient amount o f reactive iron, Sg or S° as weh as organic C, dissolved H2S is present in the upper part o f the sediment or is transported to the upper part o f the sediment (Raisweh and Berner, 1985). This conclusion is confirmed by the data collected by means o f the microelectrodes. Figure 2. The FeS data (intermediate in the formation o f pyrite) show a zone o f enrichment in the porewaters between 10-25 m m sediment depth. FeS is observed in the porewater and is thus not limiting for pyrite formation. FeS enrichment in the upper part, (0-2.5 cm depth) o f the sediments is also found by Kostka and Luther, in their study of saltmarsh sediments (Kostka and Luther I I I , 1995). These saltmarsh sediments are characterized by a high organic carbon loading, a main factor influencing the formation o f pyrite (shown in figure 7). These findings coincide w i t h our data namely a high organic matter concentration in the upper part o f the sediments resulting in the formation o f pyrite.
The occurrence o f ii'on species in the porewater o f the upper part o f the sediment as well as FeS suggests the reduction o f iron oxides by sulphide and/or bacteria to f o r m Fe^"^. Also dissolved Fe^"^ forms as indicated by the microelectrode data. Fe^* can f o r m when organic compounds non-reductively dissolved Fe^"^ solids, equation 8 (Luther et a l , 1992).
FeS + SO (Sg, S/-) O FeS2 (6)
FeS + H2S ^ FeS2 + H2 (7)
FeOOH -I- C6H4(OH)2 ^ {Fe^''[C6H4(0)2]} + 0 H " + H2O (8)
-The production o f this very reactive Fe-^"^ can result i n the dissolution o f pyrite, equation 9.
FeS2 + 14 Fe^^ + 8 H2O ^ 15 Fe'^ + 2 SO4'" + 16 (9)
The results o f t h e ascorbic acid extractions show that sulphate reduction is not as dominant in October as it is in August, because iron is relatively more occurring as reactive iron
(amorphous fraction). This means that the sediment core taken in October is relatively more oxidized in respect to the sediment core taken in August.
4.3 Nutrients
The course of the ammonium and phosphate in the porewater can be explained primarily by the breakdown o f organic matter. The phosphate data, but especially the ammonium data are often used as an indication o f organic matter degradation. I n this case the p H data also confirm this observation as described by the proton production/consumption o f the reactions in Table 1. The Redfield ratio (P/N/C, 1/16/106) gives the relation o f the amount nutrient produced by the breakdown o f organic matter (Passier, 1997). Even though complexation and adsorption behaviour as weU as diffusion influence the comparison, roughly the same relation can be observed in the porewater phosphate : ammonium data between 30 and 50 m m
sediment depth. The strong gradient o f ammonium as w e l l as phosphate in the top o f the sediment profiles is the resuk o f diffusion. I t is clear that the optimum ammonium concentration was found in August decreasing in September and October, as a result o f increased organic matter production and degradation during the summer.
Figure 6 shows that the highest concentrations o f ammonium and phosphate in the overlying water are observed in August. This is clearly the result o f the production o f nutrients by the breakdown o f organic matter. The decrease i n soluble iron(II) and nitrate concentration is also related to the same process, namely nitrate is next to oxygen preferably used as a oxidized agents for the decomposition o f organic matter. When sulphate reduction takes place, the soluble iron is scavenged and bound in sulphur species, eventually resulting i n the formation o f crystalline sulphur species like pyrite.
-5. Conclusions
Solid-state Au-Hg microelectrodes were succesfuily used to analyze the concentration profiles of important redox species in the sediments. These measurements were added to the gathered porewater and solid state results to create a clearer image o f the processes occurring i n the sediment.
The profiles o f t h e ammonium and phosphate concentration i n the porewater indicate the occurrence o f a n organic rich layer in the upper part o f the sediment, This layer o f organic matter is likely the result of high productivity during the summer. The degradation o f organic matter seems to directly influence the profiles o f the oxidized agents, O2, manganese and iron oxides. The pyrite concentration coincides w i t h the organic matter concentration and
decreases due to oxidation of pyrite between August and October. Even though a high amorphous iron concentration was measured i n the upper part of the sediment, no distinct ratio between the occurrence o f amorphous iron oxides and adsorbed phosphate could be found.
The overlying water Fe^"^ concentration drops in August as a result o f the organic matter production and formation of pyrite. The ammonium and phosphate concentrations i n the overlying water show an increase during August as a result o f degradation o f organic matter. Thus implying a flux to f r o m the sediment to the surface water. The occurrence o f conditions o f eutrophocation is most likely to happen i n August, because o f the observed flux o f
nutrients during that month. Nutrients are transported to the overlying water and available for macroalgae.
-6. Acknowledgements
First I want to thank the captain o f the research vessel RA'^ Captain White, Arthur Sundberg. Secondly I want to thank my supervisor Professor Luther I I I f o r giving me the opportunity to visit and learn about the microelectrodes, for his supervision o f the project as well as his critical review o f this manuscript. Thirdly I also want to thank my second supervisor Dr. G.J. de Lange for his supervision and coordination o f the project out o f Utrecht. I also want to thank Dr. T i m Rozan and Dr. Martial Taillefert f o r their advice concerning the analytical methods.
I also want to thank the 'Stichting het Molengraaff Fonds' and the 'Trajectumfonds' for their financial support. Finally I want to thank the students o f the University basketball team and all the students at the Franklin C. Daiber Housing Complex f o r making these three months an unforgettable experience.
-7. Literature
Anschutz, P., Zhong, S. and Sundby, B . (1998) Burial efficiency o f phosphorus and the geochemistry o f iron in continental margin sediments. L i m n o l . Oceanogr., V o l . 43 ( l ) p p . 53-64.
Berner, R.A. (1970) Sedimentary pyrite formation. Amer. J. Sci. V o l . 268, pp. 1-23.
Brendel, P.J. (1995) Development o f a mercury thin film voitammetric microelectrode f o r the determination o f biogeochemically important redox species i n porewaters o f marine and freshwater sediments. Doctoral dissertation. University o f Delaware.
Brendel, P.J. and Luther I I I , G.W. (1995) Development o f a gold amalgam voitammetric microelectrode f o r the determination o f dissolved Fe, M n , O2 and S(n) in porewaters of marine and freshwater sediments. Environ. Sci. Technol., V o l . 29, pp. 751-761.
Burdige, D.J. (1993) The biogeochemistry o f manganese and iron reduction in marine sediments. Elsevier Science Publishers B . V . Amsterdam: Earth Science Reviews, V o l . 35, pp. 249-284.
Canfield, D.E. (1989) Reactive iron in marine sediments Geochim. Cosmochim. Acta, Vol. 53, pp. 619-632.
Dollhopf, M . E . , Kenneth, H . N . , Simon, D . M . , Luther n i , G.W. (1999) Kinetics o f Fe(III) and M n ( I V ) reduction by the Black Sea strain o f Shewanella putrefaciens using in situ Solid-state voitammetric Au/Hg electrodes. Submitted to Marine Geochemistry, September 1999.
Hall, P.O.J., Aller, R. (1992) Rapid, small-volume, flow injection analysis f o r ECO2 and N H / in marine and freshwaters. Limnol. Oceanogr., V o l . 37 (5) pp. 1113-1119.
-Haese, R.R. (1998) The reactivity o f iron (not pubhshed). Department Geochemistry, University o f Utrecht, October 1998.
Henneke E., Luther m , G.W., De Lange, G.J. (1991) Determination o f inorganic sulphur speciation w i t h polarographic techniques: Some prehminary results f o r recent hypersaline anoxic sediments. Marine Geology, V o l . 100, pp. 115-123.
Henneke E., Luther H I , G.W., De Lange, G.J. and Hoefs, J. (1997) Sulphur speciation i n anoxic hypersaline sediments f r o m the eastern Mediterranean Sea. Geochim. Cosmochim. Acta, Vol. 61 (61) pp. 307-321.
Huettel, M . , Ziebis, W., Forster, S. and Luther, G.W. (1998) Advective transport affecting metal and nutrient distributions and interfacial fluxes in permeable sediments. Geochim. Cosmochim. Acta. V o l . 62 (4) pp. 613-631.
Kostka, J.E. and Luther I I I , G.W. (1994) Partitioning and speciation o f solid phase iron in saltmarsh sediments. Geochim. Cosmochim. Acta, Vol. 58 (7) pp. 1701-1710.
Kostka, J.E. and Luther I E , G.W. (1995) Seasonal cycling o f Fe in saltmarsh sediments. Biogeochemistry V o l . 29, pp. 159-181.
Luther I I I , G.W. (1992) Seasonal iron cycling in the sak-marsh sedimentary environment: importance o f ligand complexes w i t h Fe(II) and Fe(III) in the dissolution o f Fe(III) minerals and pyrite, respectively. Marine Chemistry, V o l . 40, pp. 81-103.
Luther i n , G.W., Brendel, P.J and Lewis, B . L . (1998) Simukaneous measurements o f Oa, M n , Fe, r and S(-II) in marine pore waters with a soUd-state vokammetric
microelectrode. Limnol. Oceanogr., V o l . 43 (2) pp. 325-333.
-Luther I H , G.W. (1999) In Situ deployment o f voitammetric, potentio metric and
amperometric microelectrodes f r o m a R O V to determine dissolved 02,Mn, S(-2) and p H in pore waters. Environ. Sci. Technol., V o l . 33, pp. 4352-4356.
Passier, H.F., Luther I E , G.W. and De Lange, G.J. (1997) Early diagenesis and sulphur speciation in sediments o f the Oman Margin, northwestern Arabian Sea. Deep-Sea Research E , Vol. 44 (6-7) pp. 1361-1380.
Raiswell, R. and Berner, R.A. (1985) Pyrite formation in euxinic and semi-euxinic sediments. Am.J.Sci. V o l . 285, pp. 710-724.
Stookey, I.L. (1970) ferrozine, a new spectrophotometric reagent for iron. Anal. Chem. V o l . 4 1 , pp. 779-781.
Trouwborst, R.E. (1999) Nutrient and (trace) metal cycling in estuarine surface sediments (Grevelingen, The Netherlands). Part 1 Nutrient fluxes at the sediment-water interface related to the degradation o f organic matter. Report submitted to the University o f Utrecht, June 1999.
X u , K . , Dexter, S.C, Luther I I I , G.W. (1998) Vokammetric microelectrodes for biocorrosion studies. Corrosion Vol. 54 (10) pp. 814-823.
-Annex 1.
Calibration curve ( l a )
(measuring after lialf an hour)
Calibration c u r v e (2a)
(measuring after 3 hours)
0 50 100 150
Phosphate concentration (uM)
200 250 0 50 100 150
Phosphate concentration (uM)
250 0.5 0.4 • 0.3 0.2 0.1 i 0.0 •
Calibration curve (1a)
R-squared = 0.9996
Y-coeff = 0.01810 ±0.00013
20 0 5 10 15
Phosphate concentration (uM)
25 30
B 0.3 i
i 0.2
0.1
0.0
Calibration curve (2a)
R-squared = 0.9990 Y-coeff = 0.0173 ± 0 . 0 0 0 2
0 5 10 15 20
Phosphate concentration (uM)
25 30
Figure 8. The calibration curves used to calculate the phosphate concentration
-Figure 9. The calibration curves used to calculate the iron concentration in the porewater.
-E O 03 0) Q. •O 0) .N "cö E O c 250 200 ~ 150 100 50
Calibration curve 1, ammonium analysis (0-1000 uM) Regression output:
Constant = O R squared = 0.999 Slope = 0.2122 ±0.0017
200 400 600 800 Standard ammonium concentration (uM)
1000
Calibration curve 2, ammonium anaiysis (0-1000 uM)
E ü O) ca 0) Q. •O m N 15 O c 250 200 ö) 150 100 50 ^ Regression output: 0 Constant = 0 - R squared = 0.997 Slope = 0.232 ± 0.002 200 400 600 800 1000
Standard ammonium concentration (uM)
Figure 10. The calibration curves used to calculate the ammonium concentration in the porewater
-HS'conoenlralion (uM) S l o p e : 0.456 0.457 HS* concentration (uM) HS'concentration (uM) HS" concentration (uM) Slope : 0.101 0.107 HS'concentration (uM)
HS'concentration (uM) HS'concentration (uM)
Figure 11. The calibration curves used to calculate the A V S and pyritic sulphur concenttation
