Porewater Redox Species, pH and pCO2 in Aquatic Sediments

Chapter 10

Porewater Redox Species, pH and pCO in Aquatic Sediments: Electrochemical Sensor Studies in Lake Champlain and Sapelo Island 2

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Wei-Jun Cai , Pingsan Zhao , StephenM.Theberge , Amy Witter , Yongchen Wang , and George Luther, III

Downloaded by UNIV LAVAL on July 12, 2016 | http://pubs.acs.org Publication Date: February 14, 2002 | doi: 10.1021/bk-2002-0811.ch010

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Department of Marine Sciences, University of Georgia, Athens,GA30602 College of Marine Studies, University of Delaware, Lewes,DE19958 2

This paper reports millimeter depth resolution microelectrode-basedporewater profiles of O , Mn , Fe , pH and pCO in sediments from Lake Champlain in the northeasternUSand from a creek bank in Sapelo Island in the southeastern US. Such fine scale profiles of multiple redox species measured together with pH and pCO have not been reported previously for lake or salt marsh creek bank sediments. This paper discusses the relationship between redox reactions and the porewater pH values based on micro-profiles and diagenetic mechanisms from both fresh and salt water systems. The microelectrode data clearly show that the very sharp pH minimum is a result of Mn , Fe and NH oxidation at the zone near the O penetration depth as well as CO release during organic matter decomposition. In the freshwater sediment, an overlapping ofO and Mn profiles is observed indicating a close coupling between O usage and Mn oxidation. This is not the case in the marine system except when biological disturbance is serious. A laboratory 2+

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© 2002 American Chemical Society Taillefert and Rozan; Environmental Electrochemistry ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

189 experiment supports an earlier hypothesis that an alternative Mn oxidation mechanism such as via NO reduction may be important in marine systems unless biological irrigation and /or resuspension bring Mn in direct contact with O . 2+

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Introduction On the surface of the earth, solar energy is captured in organic matter and free oxygen is released as a result of photosynthesis. This thermodynamically unfavorable process (increase in Gibbs free energy) provides energy for all the spontaneous, but slow redox reactions between the ultimate reductant, organic carbon, and the ultimate oxidant, free oxygen (/). The decomposition process of dead organic matter is the driving force for most elemental cycling in aquatic sediments. Organic matter (OM) decomposition reactions and other physical and biological changes that occur at surface sediments after deposition are called early diagenesis of sediments (2). Redfield (3) has determined the average elemental ratio of C, Ν, Ρ in living organic matter (marine phytoplanktons) as (CH 0)io6(NH3) (H P0 ). During organic matter decomposition, oxygen is consumed and other elements are released according to the ratio, 0 /C/N/P = 138/106/16/1 (4, J). However, sediment organic matter often has a higher C to Ν or C to Ρ ratio due to the preferential loss of Ν and Ρ during early diagenesis. A recent summary shows that lake sediments may have a wide range of C/N ration with an average about 12/1 ( 106CO + 106/2H S + 16NH + H P 0 => 106/2CO + 106/2CH + 16NH + H PQ 2

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NOTE: O M = (CH 0) (NH3)B(H P04)C or (CH O) 6(NH ), (H3PO4)i with a Redfield 2

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ratio. FeOOH is used to represent reactive Fe-oxides. H 0 is omitted. 2

In freshwater sediments, the redox sequences with depth are not necessarily same. It appears that in Lake Sempach, central Switzerland, FeOOH, S0 " M n 0 reductions occur simultaneously (75). In marine sediments, 0 and " are the dominant oxidants whereas in freshwater systems 0 respiration CH4 generation are dominant pathways for OM decomposition. During this sequential OM decomposition, a number of reduced chemicals can be generated. These reduced species can then be oxidized when encountered with 0 or other oxidants (e.g. Mn0 ). Such reactions are called secondary redox reactions (12) and are summarized in Table II. It is seen from Table II most of the secondary redox reactions generate protons and thus should reduce porewater pH. An interesting feature of the secondary redox reactions is that the reduced species (electron donors) produced at depth in anoxic environments usually diffuse up to the more oxidized environments in surface sediments. These oxidized species can then serve as oxidants again for anaerobic respiration of OM; thus forming redox cycles. In addition to these redox reactions, precipitation and dissolution reactions also affect porewater pH (Table III). the and S0 and

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Taillefert and Rozan; Environmental Electrochemistry ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

191 Table II. Oxidation of Reduced Diagenetic By-Products (Secondary Redox Reactions) +

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N H | + 20 => N0 " + H 0 + 2H M n + 0.5O + H 0 => Mn0 + 2H Fe + 0.25O + 1.5H 0 FeOOH + 2H H S + 2 0 =>S0 " + 2H CH + 20 r>C0 +2H 0 Fe + 0.5MnO + H 0 => 0.5 M n + FeOOH + H* H S + Mn0 + 2H => M n + S° + 2H 0 C H + S0 " + 2H => H S + C 0 + 2H20 2

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Table III. Precipitation and Dissolution Reactions 2+

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Fe + H S " o F e S + H Mn + C 0 " o M n C 0 Fe + C 0 " « F e C 0 CaC0 ο Ca + C0 " 2+

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The top few centimeters of sediments are often the zone of very rapid microbial mineralization of labile OM. 0 is depleted quickly in the top few cm of sediments as a result of primary and secondary redox reactions (75, 75-75). However, oxidation of reduced species such as Mn , Fe , N H and C H by 0 are a major reason for extremely sharp 0 profiles that may exist in lake and coastal marine sediments (19). An important result of the secondary redox reactions is the formation of a sharp pH minimum zone around the 0 penetration depth as opposed to a broad pH increase in the zone below it. This very sharp pH minimum was largely unknown before several microelectrodes were applied. Revsbech and Jorgensen (20) demonstrated such sharp pH changes at the 0 /H S boundary in a microbial mat. Recently such a sharp pH minimum around the redox boundary was measured in a few coastal marine sediments (21-24). In order to correlate the pH minimum to redox reactions, sensors are required for detecting redox species with 0.5 to 1 millimeter depth resolution or better. For example, the depth of M n appearance was often used to indicate the 0 penetration depth in marine systems (25). However, it was found that the depth of M n appearance was 1.5 cm deeper than the 0 penetration depth in the California Borderland Basin (75, 26). Due to the relatively coarse spatial resolution of porewater M n profiles, the issue whether the depth of M n appearance can indicate 0 penetration depth was not resolved (17). In Lake Sempach, a porewater study using in situ peepers with only a depth resolution of 2

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192 5-15 mm did not reflect the rapid processes occurring at the sediment surface (18). For example, the 0 profile could not be discerned, and the depths of M n and Fe appearance appeared to start at the sediment-water interface while other evidence indicated 0 penetration to a depth of more than 10 mm (13). Our understanding of biogeochemical processes near the sediment-water interface has been limited seriously by the ability to accurately measure such sharp gradients of porewater constituents with fine depth resolution without significantly disturbing the sediment environments during core recovery and processing. Such disturbances include temperature and pressure artifacts (27), gas (i.e., 0 and C0 ) loss to or exchange with the atmosphere or N gas in a glove bag, and other effects (28). The situation has improved greatly since the introduction of microelectrodes, which allow us to measure the concentration of solutes at a millimeter (or sub-mm) scale and in situ (0 :(29, 30); 0 /pH/H S: (31, 32); 0 /pH/pC0 :(27); 0 /Mn/Fe/HS: (33); new pC0 : (34, 35)). For example, a voltammetry sensor for 0 /Mn /Fe /S(II) measurements became available only recently (33, 36). It soon became a powerful tool for redox chemistry studies in aquatic sediments, particularly when combined with pH microelectrodes (23). With this sensor, Luther et al. (36) showed clearly that 0 and M n do not overlap in most marine sediments. This paper will demonstrate the correlation between porewater pH profiles and redox reactions with examples from Lake Champlain sediments and sediments from a salt marsh creek bank at Sapelo Island. We will also compare the similarities and differences of diagenetic processes in freshwater and salt marsh systems. 2+

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Experiment section

Preparation of Microelectrodes Gold amalgam glass electrodes were constructed as described in (33) with modifications to insure a waterproof seal. Briefly, the end of a 15 cm section of 4 mm-diameter glass tubing is heated in a small flame and the tip pulled to a diameter of less than 0.4 mm for a length of about 3-5 cm. A non-conductive epoxy is used as a fill. In this work, we used West System 105 epoxy resin and 206 hardener to form a high-purity, optical-grade, nonconductive fill. The epoxy is injected into the glass, which contained the gold wire that was previously


193 soldered to the conductor wire of the BNC cable but which was not sealed at the tip. The epoxy has a moderate setting time (~1 hr) and drains slowly through the open tip. On setting, the epoxy seals the tip and the top end can be refilled with epoxy. Then the top end is coated with Scotchkote (3M) electrical coating and Scotchfil (3M) electrical insulation putty. After final setting of the epoxy, the tip is sanded and polished. Once cooled the excess glass is sanded away with 400 grit sandpaper on a polisher to expose the gold wire. Once constructed each electrode surface is polished and plated with Hg by reducing Hg(II) from a 0.1 Ν Hg / 0.05 Ν H N 0 solution, for 4 minutes at a potential of -0.1 V, while purging with N . The mercury/gold (Au/Hg) amalgam interface is conditioned using a 90-second -9 V polarization procedure in a 1 Ν NaOH solution. The electrode is then run in linear sweep mode from -0.05 to -1.8 V versus a Saturated Calomel Electrode (SCE) or Ag/AgCl electrode several times in oxygenated seawater to obtain a reproducible 0 signal. The preparation and properties of the neutral carriers-based PVC liquid membrane pH microelectrodes and the pC0 microelectrode using this pH microelectrode as an internal sensor are detailed in references (34, 37, 38). The tip diameter of the pH microelectrode is about 15 μπι. The pC0 microelectrode consists of an outer pipette with a gas-permeable silicone membrane in the very tip and an inner pH microelectrode positioned at the center of the outer pipette and at a distance of - 5-30 μηι from the silicone membrane. The internal filling solution is 2 mM NaHC0 and 0.5 M NaCl. The tip diameter of the pC0 microelectrode is 100-250 μπι. Three C 0 gas standards (0.1%, 0.5% and 2.5% balanced with N ) are used for calibration. Due to the high impedance of the microelectrodes and the electrical noises in an average lab, laboratory measurements of micro-profiles are normally conducted in a Faraday cage. A typical setting of pH and pC0 measurements is illustrated in Figure 1. The pH measurement has a standard deviation of 0.01-0.02 units while the pC0 microelectrode has a standard deviation of 3-5%. Evaluation of the electrode performance in marine sediments can be found in (24). Fine scale porewater DIC profiles calculated from pH and pC0 microelectrode in situ measurements compared well with the coarse scale DIC profile measured in extracted porewater (24). Comparison of a pH microelectrode with a glass minielectrode showed that pH profiles measured by both electrodes were in reasonable agreement. However, the down core pH signal changes measured by the microelectrode were sharper than that by the mini-electrode. This was interpreted as a mixing of the sediment particles by the larger glass minielectrode (tip diameter =2-3 mm) (24). 3


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Figure 1. Illustration of microelectrode measurements in a Faraday cage. The motor and the slider can be controlled manually or by a program outside the Faraday cage. The two-channel pH meter is connected to a computer.

Site Description and Sample Collections Lake Champlain is located in the Northwest of New Hampshire, USA. A study of the role of intermediate oxidants (Mn(IV)/Fe(III)/N0 7S0 ") on rates and pathways of carbon cycling was conducted at sites 19 (location: main lake, 44°28.26' Ν and 73°17.95; water depth 100 m) and 21 (location: Burlington Bay, 44°28.49' Ν and 73°13.90; water depth 15 m) in the middle part of the Lake in summer 1997. Bottom water temperature in site 19 and site 21 were 12.6 and 24.0 °C respectively. This paper concentrates on the results collected with microelectrodes. Two other papers (39, 40) will summarize the entire project including numerical modeling, rate and flux measurements, and laboratory manipulation of sediment cores with several electron acceptors. 2

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195 In both sites, undisturbed sediments were collected by a non-contaminating box core system (AC-6, Fabau Inc., Massena, NY) operating from a boat. The 6"x6" corer box was subcored with a 4" inner diameter PVC corer. The cores were store in coolers filled with water from the site and were brought back to the lab at the Ecosystem Center, Marine Biological Laboratory at Woods Hole, MA. Cores were kept at in situ temperature. Porewaters were also collected using a whole core squeezer modified from (41). Porewaters were expressed directly into syringes and had no contact with air. Salt marsh creek bank sediment cores were collected from a tidal creek near the University of Georgia Marine Institute on Sapelo Island, GA, southeastern US (see (42) for a description of the site and the area). This sample was visibly rich in Fe-oxides with yellowish coloration around roots and crab burrows. Another distinguishing feature of the creek bank sediment was severe biodisturbance by fiddler crab activities and many crab burrow openings. The biogeochemistry of these sediments has been studied by several workers (see refs in (42)). Our microelectrode study was conducted when P. Van Cappellen and T. DiChristina conducted a biogeochemical and microbial study program in the same marsh-creek system (43, 44). Intact cores were collected and pH, pC0 , 0 , Mn , Fe and S(-II) profiles were measured with microelectrodes immediately in the field within a few hours of sample collection. 2

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Results

Lake Champlain Several major redox species, pH and p C 0 micro-profiles were measured in the Lake Champlain sediments during August 1997 (Figure 2 and Figure 3). 0 was depleted to undetectable level (-1 μΜ) at around 6-mm depth. M n appeared right around the 0 penetration depth. There was a slight spatial overlap between 0 and Mn " profiles. Fe increased slowly below the 0 minimum depth and a spatial separation of a few mm between the appearance of Fe and the 0 penetration depth exists at both sites. The major difference between the two sites was the relative shape of the M n and Fe profiles. For site 21 (15 m water depth and 24 °C), porewater M n concentrations were higher than the Fe concentrations while for site 19 (100 m water depth and 2

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Figure 2. Profiles of redox species measured with a Au~Hg microelectrode (site 21, Lake Champlain).



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Figure 3. Profiles of redox species measured with a Au-Hg microelectrode (site 19, Lake Champlain).


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12.6 °C) porewater M n profiles leveled off quickly at a concentration below 40 μΜ. However, porewater Fe concentrations at site 19 were much higher than the M n concentrations and were also much higher than Fe concentrations at site 21. The Fe profile sharply decreased below 3.5 cm depth at site 19. S(-II) was not detected (

Porewater Redox Species, pH and pCO2 in Aquatic Sediments

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