Dissolved Solute Sampling Across an Oxic-Anoxic Soil-Water Interface Using Microdialysis Profilers

As a soil scientist, we always need to take soil powder water for analysis. However, it's not very easy, especially when the chemicals in the powder water are very sensitive to the oxygen. This is a new technology.

We call it API sampler. By using the sampler, we can take the soil powder every two milliliter with actual load disturbance to the soil. My students Zhang Sha, Yujia, Liu Ziyan, and Liu Hao are going to demonstrate how to build the sampler and use it to take soil powder water.

Begin by accurately cutting the pristine nano membrane tubes into 33 short tubes, having 58 millimeter lengths. Then cut the polytetrafluoroethylene or PTFE pipe into 66 pipes, having 180 millimeter lengths with a ceramic knife. Next, fully mix the two part of AB epoxy adhesive on any clean plastic plate, and let it stand for 30 minutes until it becomes sticky before applying it to the outer surface of the top of the PTFE pipe.

Ensure that AB epoxy adhesive only covers the four millimeter of the tube and that there is no additional adhesive blocking the tubes. Connect the two PTFE pipes with each nano membrane tube by gently screwing the PTFE pipes into the nano membrane tube to fully assemble all 33 pristine microdialysis samplers. Let the assembled samplers stand overnight to ensure the complete curing and stabilization of the adhesive.

To enhance the hydrophilicity and clean the microdialysis samplers, soak them in ethanol for one hour, followed by ultrasonic cleaning with 2%diluted nitric acid and ultrapure water for 15 minutes each. Check the patency and air tightness of the microdialysis sampler by bubbling in water using a five milliliter syringe. To assemble the microdialysis profiler, use the CAD file to print the pre-designed skeleton using nylon material.

Then hollow out an acid washed PVC container with two parallel slots of five centimeter intervals to match the skeleton size. Use the engraving module in the 3D printer for slotting. Construct a one-to-many connector by stabilizing epoxy adhesive in the shape of a 50 millimeter centrifuge tube cap.

Then insert 33 silicon caps of one centimeter length into the epoxy adhesive before curing and let it stand overnight. Next, remove the one-to-many connector from the tube cap and cut the curate epoxy adhesive using a ceramic knife so that all the silicone cap ends are unobstructed. Rinse the one-to-many connector thoroughly with 2%diluted nitric acid and ultrapure water for 15 minutes each and dry under ambient conditions.

Once dried, connect a three-way valve to the bottom of the tube to serve as a buffering container. Assemble the buffering container by installing a one-to-many connector to a 50 milliliter syringe tube using AB epoxy adhesive assemble the individual microdialysis samplers on the skeleton using a hot meld adhesive, ensuring that each sampler is parallel to the top or bottom edge of the skeleton. Install all 33 microdialysis samplers on the skeleton, ensuring that the 33 samplers on both sides pass through the PVC slots.

Seal the gaps at the skeleton joints and the slots with a AB epoxy adhesive. Next, connect all the samplers to one side of the skeleton to a buffering container via a one-to-many connector valve pre-installed into a 50 milliliter centrifuge tube. Then connect a medical infusion bag prefilled with 18.3 milli ohm water to the buffer container through the three-way valve.

Close all the samplers on the sampling side using silicon caps. Double check the patency and air tightness of each microdialysis sampler by turning the three-way valve, allowing water to flow from the medical infusion bag to the sampler. Then close and turn off all the samplers and the valve on the buffering container.

Before incubating the flooded soil, remove oxygen by degassing the water in the medical infusion bag. Bubbled nitrogen gas overnight in the pathway of the line of high purity nitrogen gas to the medical infusion bag. Using a three-way valve, close the connection between the profiler and the degassed bag.

Then add 450 grams of sieved air dried soil into a PVC container, ensuring five microdialysis samplers remain above the soil surface. Cover the soil surface with tissue before flooding the soil with ultrapure water. Once the soil is flooded five centimeters above the soil surface, remove the tissue.

Once soil incubation has been initialized, immediately purge the system with the preloaded solution. Then flush the sampling system by turning on the connection between the anaerobic bag and the dialysis sampler. Use tenfold the total volume of the sampler when purging each sampler with water.

Once the purging of one sampler is complete, cap it using a clean silicon cap before purging every sampler to establish one flooded soil incubation and sampling system. Next, adjust the anaerobic bag to the height of the water surface, ensuring all the tubes are full of water. If not, remove the cap and lower the tube top, allowing the water to flow from the anaerobic bag.

Close the caps and valves and incubate for seven days with the connection between the anaerobic bag and the dialysis sampler turned off. Before sampling, adjust the water levels in the soil container, the sampling tops, and the anaerobic bag to a similar height to avoid markedly different water potentials. Then turn on the connection between the anaerobic bag and the buffer container.

Remove the cap of the first sampler from top to bottom. Using a pipette, transfer 133 microliters of sample from the sampler to a 0.6 milliliter vial, preloaded with 133 microliters of 2%nitric acid for preservation. During the sampling, observe a slow but uniform flow of water droplets toward the microdialysis sampler in the anaerobic bag observation chamber.

Close the tube top with a silicon cap before moving to the next sampling tube. Repeat this for all 33 samples before turning off the connection between the anaerobic bag and the buffer container. Replenish the flooded water on the sixth day after the sampling.

Calculate the sample volume recovery by weighing the sample vial before and after transferring the poor water sample. Then measure the total dissolved concentrations of elements in the poor water using inductively coupled plasma mass spectrometry or ICPMS. The recovery percentage of the sample volume averaged 101.4%and ranged from 100.2%to 103.6%A slightly higher recovery of the sample volume indicated a water level difference between the anaerobic bag and the top of the sampling tube.

Using the samples across the soil water interface collected on the sixth and seventh day, the total dissolved concentrations of iron, manganese, arsenic, cadmium, copper, lead, nickel, and zinc in the poor water were determined. On the sixth day, the dissolved concentrations of manganese, iron, and arsenic increased along with the soil depth, whereas those of copper and lead decreased with increasing soil depth. However, for cadmium, nickel, and zinc, concentration depth profiles indicated a different pattern, as the dissolved concentrations increased from minus 20 millimeters to deeper locations.

The concentration depth profiles of iron and arsenic at a depth of minus 12 millimeter on the sixth day were significantly higher than the levels on the seventh day. However, the iron and arsenic concentrations were significantly higher from the depths of minus 18 to minus 50 millimeter. For most elements determined except manganese, the dissolved concentrations in the surface water and the even surface soil at minus 15 millimeter depth were significantly lower after aerobic water replenishment.

A concentration peak for lead on the seventh day at approximately minus 10 millimeter depth showed a contrasting pattern to the sixth day. This technique is especially useful researchers who studied biogeochemical micro interface processes. It can narrow down medical confounding factors.

This procedure is applied to flatted soil, meaning that the leakage or intrusion of oxygen will significantly alter the chemical processes unexpected, ensure that all connections are airtight and water degassing is sufficient. Following this procedure, other methods such as carpal liquid chromatography and mass chromatography and special resolution microbial analysis can be performed to bridge the chemical and biological processes. This technique paved the way for researchers to explore new questions of how virus perturbations affect behaviors of the redacted sensor element under a changing environment.