Sediment Core Sectioning and Extraction of Pore Waters under Anoxic Conditions

A protocol for sectioning sediment cores and extracting pore waters under anoxic conditions in order to permit analysis of redox sensitive species in both solids and fluids is presented.

Transcript

The overall goal of this procedure is to section sediment cores and extract pore waters from these sediment cores while maintaining the original oxidation state. This method can allow researchers in geochemical fields to examine metal speciation in pore waters and sediments from redox-sensitive areas such as lakes and wetlands. The main advantages of this technique are that it's flexible and adaptable, field portable, inexpensive, and does not disrupt sediment layers.

Demonstrating the procedure will be Rahul Sinha, an undergraduate from my laboratory. Begin preparing the field laboratory by setting up the glove bag. First, clamp the core guidance plate to a work surface.

Position the hole for the core liner extending in front of the counter to ensure it is open. Next, secure the nitrogen tank using a bench clamp. Then, place the glove bag over the plate and run tubing between the bag and the gas regulator.

Extend about eight inches of tubing into the bag. Now, seal the tubing connection at the bag with electrical tape and inside the bag slide a clamp onto the tube but leave the clamp open. Next, cut an X into the bottom of the bag over the hole in the plate then load the bag with the listed items.

Double check the lists, because forgetting an item is very inconvenient. The next task is to set up the core. First, put a lab jack on the floor under the work surface.

The lab jack should have a piece of plywood on it with a hole in the center to stabilize the core. Then, with the core in an upright position, push it through the core guidance plate and through the X in the glove bag. About four to six inches of core should extend above the plate.

Now, while one person supports the core, tape the core to the bag around the X with electrical tape. Then, set up the core extruder. First insert the handle into a six-inch long, two-inch diameter PVC spacer followed by a three-inch long coupling and then another space and another coupling and so forth until the handle is completely covered.

Now, place the core extruder below the core and support it with the jack. The jack should be cranked as low as possible. Next, with a box cutter carefully cut around the bottom of the core cap.

Leave a ring of core cap around the core liner and leave a circular piece of cap against the core materials. Now, one person must reach into the glove bag to secure the core while the other raises the jack. Gradually move the core extruder into the core liner raising the sediment upwards.

It may be necessary to wiggle the extruder a little. Continue raising the core until the sediment and overlying water reaches to the top of the core liner. The last task is to seal the bag.

First, double check that everything is in there. Then, open all the objects in the bag that contain trapped air. Now, ideally with another person's help, purge the glove three times.

First, with the bag as deflated as possible, seal the main opening with a few bungee cords. Then, with an assistant regulating the nitrogen flow at 50 psi, direct the gas around the bag, especially into crevices where air could be trapped. When the bag is highly inflated, turn off the regulator or close the clamp to shut off the gas.

Then, open the main opening and push out as much gas from the bag as possible. Repeat the process of filling the bag with nitrogen, including all crevices, and emptying the bag two more times. Once this is completed, the glove bag should contain very low levels of oxygen.

Finish with a comfortable amount of gas in the glove bag. This procedure is best performed while working in pairs. For this procedure, put a pair of disposable gloves over the built-in gloves in the bag.

Change these gloves whenever they are dirty or torn. First, remove all the standing water from the top of the core using a syringe. Then, filter this water through a 0.45-micron syringe filter.

Next, place a core sectioning ring over the top of the core liner. Then, raise the core until the sediment meets the top of the ring. Inside the bag, secure the core liner so when the core moves the liner does not.

It may be necessary for the operator working inside the glove bag to push down slightly on the core liner so that the core material is at the top of the ring. Next, push a core slicer between the top of the liner and the ring so that the sediment section is above the slicer. This is the first section of core.

With plastic spoons, move the section to a 50-milliliter conical tube and cap it tightly. Wash the tools and repeat the process until the whole core is sectioned and placed into tubes. During the core sectioning process the lab jack may become fully extended.

If this occurs, hold the core in place, lower the jack, and position the PVC pieces to extend the core extruder. Then secure the jack back in place. During this process, the bag may get too deflated or inflated.

Make adjustments by briefly opening the nitrogen gas valve or by briefly opening the main bag opening to release gas. Once the core is sectioned and all the sediment is in centrifuge tubes, these tubes can be removed from the glove bag and the glove bag closed. Then, prepare the collected tubes for centrifugation as explained in the text protocol.

Then, spin them down at 1, 100 g or faster for 20 minutes. After the centrifugation, return the tubes to the oxygen-free environment and introduce some empty tubes for the sample collection. The bag should then be purged again two to three times as before.

Now, open a centrifuged tube and remove the porewater using a syringe with tubing on the end. Then, using a 0.45-micron syringe filter, filter the water into a collection tube. The text protocol explains how to further process the samples.

Using the described protocol, a core was taken from a wetland near New Orleans nine months after the Deep Water Horizons Bill. This wetland was heavily oiled. Using a Hach DR 2700 portable spectraphotometer, the data showed extremely high sulfide concentrations.

Iron concentrations were low and iron in the three plus oxidation state was not detected. So after watching this video you should have a good understanding of how to purge the glove bag of oxygen, extrude the core inside the glove bag to allow sectioning, and separate the porewaters from the sediments. Once mastered, this technique can be done in one to three hours per core, depending on the core length.

While attempting this procedure, it's important to remember to purge the glove bag thoroughly and adjust the core height slowly. Following this procedure methods like iron speciation by ferrousing, arsenic speciation by voltammetry or column separation, and mythylmercury separation can be performed in order to answer questions about the redox state of the system. Don't forget that working with compressed gasses can be extremely hazardous and securing the tank is essential while performing this procedure.

Some sediment samples may be contaminated with hazardous materials and appropriate cautions should be taken.

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Journal (Environment)

Journal (Environment)

Sediment Core Sectioning and Extraction of Pore Waters under Anoxic Conditions

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