Semipermeable membrane devices

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SPMDs, or semipermeable membrane devices, are a passive sampling device used to monitor trace levels of organic compounds with a log Kow > 3. SPMDs are an effective way of monitoring the concentrations of chemicals from anthropogenic runoff and pollution in the marine environment because of their ability to detect minuscule levels of chemical. The data collected from a passive sampler is important for examining the amount of chemical in the environment and can therefore be used to formulate other scientific research about the effects of those chemicals on the organisms as well as the environment. Examples of commonly measured chemicals using SPMDs include: PAHs (Polycyclic aromatic hydrocarbons), PCBs (polychlorinated biphenyls), PBDEs (polybrominated diphenyl ethers), dioxins and furans as well as hydrophobic waste-water effluents like fragrances, triclosan and phthalates.

Contents

To take into consideration short-lived pulses of contaminants found in surface water, passive samplers may be used to monitor and record these pulses that would otherwise be missed. SPMDs can accumulate contaminants from the water column because of triolein (glyceryl trioleate) comprising the lipid membrane that is housed within the canister. [1] However they are most successful in accumulating trace chemicals in surface water with a calculable flow. The amount of chemical measured using an SPMD is related to the surface area of the sampling device therefore using a larger SPMD increases the amount of chemical sampled.

Deployment

SPMDs can be deployed within a wide range of water bodies, although flowing shallow water is preferable. These devices need to be secured to nearby structures to allow the SPMD to remain in a fixed position in its environment. To aid in the stability of SPMDs, various ways can be employed including attaching the device to a buoy system, an anchor, a boat, or structures/debris in shallow water. For the best possible data to be collected using passive samplers, some degree of stability and the ability of the SPMD to be stationary are required. As long as there are openings on the canister of the device, there is no particular way that the SPMD needs to be facing while deployed. [2] Depending on the type of analysis that will be conducted, the extract of the sampler one or more may be needed to be deployed. SPMDs normally are deployed up to 30 days in the field, depending on how much accumulation of trace chemicals occurs in the passive sampler itself.

Flowing water

The advantage of deploying SMPDs into flowing water like streams or rivers is that these systems will increase the volume of water sampled. Areas of extremely high flow should be avoided however, as they present a danger to the integrity of the SPMD by way of floating debris (rocks, sediment or wood) and can move the device downstream. If the stream or river has suspended solids flowing through at regular intervals it may be advantageous for the device to be deployed with most of the openings facing away from the direction of flow. Placing the SPMD canister behind an obstacle in flowing water may also reduce the amount of suspended solids that interact with the device in these types of systems. [2]

Lakes and oceans

The SPMD can be deployed in areas with low rates of flow or even in deep water areas. To ensure the safety and correct deployment of the SPMD within deep areas, it is important to attach the device to an anchor as well as a flotation device. To retrieve SPMDs from the field, a boat or a diver may be required, depending on the depth of the canister. An SPMD can concentrate chemicals from the water as well as the sediments in the water column, therefore it is important to know the composition of the sediment and benthic surface before deployment. To reduce interference from chemicals of unwanted sources, anchoring the SPMD at certain depths (e.g. higher for muddy sediments in aquatic systems) can be very beneficial. [2]

Biological obstructions

Bio-films

Bio-films may grow on the canister which reduces the amount of contaminant collected due to membrane pores being covered.

Organisms

In marine systems a common problem involving barnacles growing on and in the canister can occur which can moderately to greatly reduce the amount of contaminant collected, as well as make it difficult to retrieve the devices.

Advantages

The advantages of working with an SPMD passive sampler as opposed to a normal field test with an organism are that SPMDs are able to be deployed in extremely toxic waters that might be too toxic for an organism to live in or just not inhabited by sessile filter feeders. [3] The design of the SPMD makes it so that it imitates the process of accumulating contaminants the way a mussel or oyster would, but without the issues of mortality or metabolizing any contaminants that may be present. [3] They can also be deployed for a long period of time and can account for surge runoff events, chemical spills or other abnormal pollution events. The physical structure of a SPMD with its stainless steel covering protects it and allows it to be suspended on a sessile anchor in the water column. [3] The main advantage of the SPMD over a different passive sampler called the polar organic chemical integrative sampler (POCIS) is that SPMDs will detect contaminants that have not fully dissolved in the water. [3]

Disadvantages

Although SPMDs have been around since the mid-1990s, they are still relatively new in the toxicology world and are still being studied as reliable forms of data collection. Because they are sessile, they don’t always paint an accurate picture of the environment because there are many organisms in the water that are mobile and can move away from contamination.

Since the SPMD is made of a low-density polyethylene membrane, it is transparent to UVa and UVb waves. And unfortunately, chemicals that are sensitive to light, like PAH’s, can degrade before correct concentrations are measured.

SPMDs are designed to accumulate low level concentrations of chemicals and those that are exposed to air for more than 30 minutes can concentrate airborne pollutants. Surface waters that are covered with oils or other layers must be disturbed, and the water cleared before the SPMD can be placed into the water otherwise false data collection will commence [1] .

In addition, while an SPMD can account for surge events of contaminants, it is difficult to determine when this event took place during the sampling period because the SPMD does not track time [3] . Another large disadvantage is that an SPMD will not be able to detect contaminants that readily dissolve in water, whereas a device like POCIS can. [3]

Applications

Data analysis

Two types of information are provided by passive samplers: the concentrations of contaminant inside the sampler and a predicted concentration of the contaminant in the water surrounding the sampler. The concentration of chemical inside the sampler is determined from SPMD dialysis, and the water concentration from a set of calculations based on the dialysis results and sampling methods. [3]

The dialysis method starts with a thorough cleaning of the device to remove salts. It is then submerged in hexane and incubated for 18 to 24 hours. This process is done twice and the hexane from both dialysis periods is combined. The samples are then processed by an analytical chemistry lab to determine the content of the mixture. [3]

As laid out by the USEPA, the dissolved concentration outside the sampler can be predicted by the following equation: 

Cw = Cps/Kow *1000  [4]

Cw is the dissolved concentration in the water (in µg/L); Cps is the concentration of the contaminant in the SPMD (µg contaminant/g sampler); Kow is the phase-partitioning coefficient (L/kg); and 1000 is a multiplier to correct for a change in units. [4] This equation is for general passive samplers, while a more defined equation includes the length of time the SPMD was monitoring:

Cw = Cspmd*Vspmd/ (Rs*t)  [5]

Cw is the concentration of contaminant in the water; Cspmd is the concentration in the SPMD (usually ng/L); Vspmd is the volume of the SPMD (usually L); Rs is effective sampling rate (L/day); and t is the time of deployment (day). [5]

Use in EPA

SPMDs are currently being used by the United States Environmental Protection Agency (USEPA) as a tool to assess management strategies of contaminants in water and sediments. At a Superfund site in South Carolina, three versions of an SPMD was used at a superfund cleanup site to measure PCBs: one was kept in contact with surface sediments; a second suspended in the water-sediment interface; and a third in the water column. [6] In June 2005, a Superfund site in North Providence, Rhode Island deployed SPMDs in six locations. They were set in the water column and sediment for a 27-day exposure to test for TCDD, furans, dioxins, and volatile organic compounds. [7]

Related Research Articles

Dialysis

In medicine, dialysis is the process of removing excess water, solutes, and toxins from the blood in people whose kidneys can no longer perform these functions naturally. This is referred to as renal replacement therapy. The first successful dialysis was performed in 1943.

Semipermeable membrane

Semipermeable membrane is a type of biological or synthetic, polymeric membrane that will allow certain molecules or ions to pass through it by osmosis—or occasionally by more specialized processes of facilitated diffusion, passive transport or active transport. The rate of passage depends on the pressure, concentration, and temperature of the molecules or solutes on either side, as well as the permeability of the membrane to each solute. Depending on the membrane and the solute, permeability may depend on solute size, solubility, properties, or chemistry. How the membrane is constructed to be selective in its permeability will determine the rate and the permeability. Many natural and synthetic materials thicker than a membrane are also semipermeable. One example of this is the thin film on the inside of the egg. Note that a semipermeable membrane is not the same as a selectively permeable membrane. Semipermeable membrane describes a membrane that allows some particles to pass through, whereas the selectively permeable membrane "chooses" what passes through.

Water quality

Water quality refers to the chemical, physical, and biological characteristics of water based on the standards of its usage. It is most frequently used by reference to a set of standards against which compliance, generally achieved through treatment of the water, can be assessed. The most common standards used to monitor and assess water quality convey the health of ecosystems, safety of human contact, and condition of drinking water. Water quality has a significant impact on water supply and oftentimes determines supply options.

Water pollution Contamination of water bodies

Water pollution is the contamination of water bodies, usually as a result of human activities. Water bodies include for example lakes, rivers, oceans, aquifers and groundwater. Water pollution results when contaminants are introduced into the natural environment. For example, releasing inadequately treated wastewater into natural water bodies can lead to degradation of aquatic ecosystems. In turn, this can lead to public health problems for people living downstream. They may use the same polluted river water for drinking or bathing or irrigation. Water pollution is the leading worldwide cause of death and disease, e.g. due to water-borne diseases.

Environmental remediation Removal of pollution from soil, groundwater etc.

Environmental remediation deals with the removal of pollution or contaminants from environmental media such as soil, groundwater, sediment, or surface water. Remedial action is generally subject to an array of regulatory requirements, and may also be based on assessments of human health and ecological risks where no legislative standards exist, or where standards are advisory.

Dialysis (biochemistry) Process of separating molecules

In biochemistry, dialysis is the process of separating molecules in solution by the difference in their rates of diffusion through a semipermeable membrane, such as dialysis tubing.

Hemodialysis Medical procedure for purifying blood

Hemodialysis, also spelled haemodialysis, or simply dialysis, is a process of purifying the blood of a person whose kidneys are not working normally. This type of dialysis achieves the extracorporeal removal of waste products such as creatinine and urea and free water from the blood when the kidneys are in a state of kidney failure. Hemodialysis is one of three renal replacement therapies. An alternative method for extracorporeal separation of blood components such as plasma or cells is apheresis.

Total organic carbon

Total organic carbon (TOC) is the amount of carbon found in an organic compound and is often used as a non-specific indicator of water quality or cleanliness of pharmaceutical manufacturing equipment. TOC may also refer to the amount of organic carbon in soil, or in a geological formation, particularly the source rock for a petroleum play; 2% is a rough minimum. For marine surface sediments average TOC content is 0.5% in the deep ocean, and 2% along the eastern margins.

Microdialysis

Microdialysis is a minimally-invasive sampling technique that is used for continuous measurement of free, unbound analyte concentrations in the extracellular fluid of virtually any tissue. Analytes may include endogenous molecules to assess their biochemical functions in the body, or exogenous compounds to determine their distribution within the body. The microdialysis technique requires the insertion of a small microdialysis catheter into the tissue of interest. The microdialysis probe is designed to mimic a blood capillary and consists of a shaft with a semipermeable hollow fiber membrane at its tip, which is connected to inlet and outlet tubing. The probe is continuously perfused with an aqueous solution (perfusate) that closely resembles the (ionic) composition of the surrounding tissue fluid at a low flow rate of approximately 0.1-5μL/min. Once inserted into the tissue or (body)fluid of interest, small solutes can cross the semipermeable membrane by passive diffusion. The direction of the analyte flow is determined by the respective concentration gradient and allows the usage of microdialysis probes as sampling as well as delivery tools. The solution leaving the probe (dialysate) is collected at certain time intervals for analysis.

Reverse osmosis (RO) is a water purification process that uses a partially permeable membrane to remove ions, unwanted molecules and larger particles from drinking water. In reverse osmosis, an applied pressure is used to overcome osmotic pressure, a colligative property that is driven by chemical potential differences of the solvent, a thermodynamic parameter. Reverse osmosis can remove many types of dissolved and suspended chemical species as well as biological ones (principally bacteria) from water, and is used in both industrial processes and the production of potable water. The result is that the solute is retained on the pressurized side of the membrane and the pure solvent is allowed to pass to the other side. To be "selective", this membrane should not allow large molecules or ions through the pores (holes), but should allow smaller components of the solution (such as solvent molecules, i.e., water, H2O) to pass freely.

Environmental monitoring describes the processes and activities that need to take place to characterize and monitor the quality of the environment. Environmental monitoring is used in the preparation of environmental impact assessments, as well as in many circumstances in which human activities carry a risk of harmful effects on the natural environment. All monitoring strategies and programs have reasons and justifications which are often designed to establish the current status of an environment or to establish trends in environmental parameters. In all cases, the results of monitoring will be reviewed, analyzed statistically, and published. The design of a monitoring program must therefore have regard to the final use of the data before monitoring starts.

T.E. Laboratories (TelLab), based in Tullow, Ireland, holds the global licence to manufacture and sell Chemcatcher®. Chemcatcher® is a passive sampling device for monitoring a variety of pollutants in water. Most monitoring programmes involve the periodic collection of low volume spot samples of water, which is challenging, particularly where levels fluctuate over time and when chemicals are only present at trace, yet toxicologically relevant concentrations. Chemcatcher® is used to measure time-weighted average (TWA) or equilibrium concentrations of a wide range of pollutants in water. This allows the end user to obtain a more representative picture of the chemicals that may be present in the aquatic environment. The Chemcatcher® concept was developed by Professors Richard Greenwood and Graham Mills at the University of Portsmouth, together with colleagues from Chalmers University of Technology, Sweden. The device is patented in a number of countries and the name is a registered trademark in Ireland and the United Kingdom

Bioconcentration is the accumulation of a chemical in or on an organism when the source of chemical is solely water. Bioconcentration is a term that was created for use in the field of aquatic toxicology. Bioconcentration can also be defined as the process by which a chemical concentration in an aquatic organism exceeds that in water as a result of exposure to a waterborne chemical.

Analytical thermal desorption, known within the analytical chemistry community simply as "thermal desorption" (TD), is a technique that concentrates volatile organic compounds (VOCs) in gas streams prior to injection into a gas chromatograph (GC). It can be used to lower the detection limits of GC methods, and can improve chromatographic performance by reducing peak widths.

A polar organic chemical integrative sampler (POCIS) is a passive sampling device which allows for the in situ collection of a time-integrated average of hydrophilic organic contaminants developed by researchers with the United States Geological Survey in Columbia, Missouri. POCIS provides a means for estimating the toxicological significance of waterborne contaminants. The POCIS sampler mimics the respiratory exposure of organisms living in the aquatic environment and can provide an understanding of bioavailable contaminants present in the system. POCIS can be deployed in a wide range of aquatic environments and is commonly used to assist in environmental monitoring studies.

Diffusive gradients in thin films

The diffusive gradients in thin films (DGT) technique is an environmental chemistry technique for the detection of elements and compounds in aqueous environments, including natural waters, sediments and soils. It is well suited to in situ detection of bioavailable toxic trace metal contaminants. The technique involves using a specially-designed passive sampler that houses a binding gel, diffusive gel and membrane filter. The element or compound passes through the membrane filter and diffusive gel and is assimilated by the binding gel in a rate-controlled manner. Post-deployment analysis of the binding gel can be used to determine the time-weighted-average bulk solution concentration of the element or compound via a simple equation.

The stabilized liquid membrane device (SLMD) is a passive, integrative sampler that provides an alternative or complementary approach to the conventional water sampling of aqueous metals. The simple device is composed of nonporous low-density plastic lay-flat tubing, which is filled with a chemical mixture containing a chelating agent and a long chain organic acid. The water-insoluble chelating agent-organic acid mixture diffuses in a controlled manner to the exterior surface of the sampler membrane and binds to environmental metals. In practice, the SLMD provides for continuous sequestration of bioavailable forms of trace metals, such as, cadmium (Cd), cobalt (Co), copper (Cu), nickel (Ni), lead (Pb), and zinc (Zn). The SLMD can also be utilized for in-laboratory preconcentration and speciation of bioavailable trace metals from grab water samples.

The Diamond Head Oil Refinery is a former oil reprocessing facility located in Kearny, New Jersey, United States, that was designated as a Superfund site by the Environmental Protection Agency (EPA). It opened up in 1946, but then stopped production in 1979 and has been inactive since then. The refinery was shut down in 1980 and the EPA designated it as a Superfund site in 1991 due to the discovery of toxic chemicals in the soil and the surface water. This created a dangerous work environment for the workers at the facility. The EPA proposed a clean up plan for the site, but it has yet to take effect. So far, the Diamond Head site is still in the process of being cleaned up. Although cleanup plans were discussed and finalized, the future of the Diamond Head Oil Refinery and its cleanup state is unknown.

Stabilized liquid membrane devices

A stabilized liquid membrane device or SLMD is a type of passive sampling device which allows for the in situ, integrative collection of waterborne, labile ionic metal contaminants. By capturing and sequestering metal ions onto its surface continuously over a period of days to weeks, an SLMD can provide an integrative measurement of bioavailable toxic metal ions present in the aqueous environment. As such, they have been used in conjunction with other passive samplers in ecological field studies.

Passive sampling is an environmental monitoring technique involving the use of a collecting medium, such as a man-made device or biological organism, to accumulate chemical pollutants in the environment over time. This is in contrast to grab sampling, which involves taking a sample directly from the media of interest at one point in time. In passive sampling, average chemical concentrations are calculated over a device's deployment time, which avoids the need to visit a sampling site multiple times to collect multiple representative samples. Currently, passive samplers have been developed and deployed to detect toxic metals, pesticides, pharmaceuticals, radionuclides, polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and other organic compounds in water, while some passive samplers can detect hazardous substances in the air.

References

  1. 1 2 Huckins, James N.; Manuweera, Gamini K.; Petty, Jimmie D.; MacKay, Donald; Lebo, Jon A. (December 12, 1993). "Lipid-containing Semipermeable Membrane Devices for Monitoring Organic Contaminants in Water". Environmental Science & Technology. 27 (12): 2489–2496. Bibcode:1993EnST...27.2489H. doi:10.1021/es00048a028.
  2. 1 2 3 Alvarez, David (2010). Guidelines for the Use of the Semipermeable Membrane Device (SPMD) and the Polar Organic Chemical Integrative Sampler (POCIS) in Environmental Monitoring Studies. USGS.
  3. 1 2 3 4 5 6 7 8 "Semipermeable Membrane Device". NOAA. April 16, 2013.
  4. 1 2 Guidelines for Using Passive Samplers to Monitor Organic Contaminants. EPA. 2012.
  5. 1 2 Huff, Tom (2016). Semipermeable Membrane Device. USGS.
  6. ↑ Foote, Eric. "Using SPMDs to Assess Natural Recovery of PCB-contaminated Sediments in Lake Hartwell, SC: I. A Field Test of New In-Situ Deployment Methods". Soil and Sediment Contamination.
  7. ↑ Superfund Records Center. EPA. 2005.
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