Passive sampling

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A firefighter wearing a silicone passive sampler on an elastic necklace, shaped like a dog tag Silicon firefighter tag.jpg
A firefighter wearing a silicone passive sampler on an elastic necklace, shaped like a dog tag

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. [1] 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, [2] [3] [4] [5] while some passive samplers can detect hazardous substances in the air. [6] [7] [8]

Contents

Theory and application

The underlying principle of passive sampling is the flow of contaminant molecules or ions from the sampling medium (air or water) onto a collecting medium (the passive sampler), due to Fick's first law of diffusion and, depending on the passive sampler, a greater binding affinity of contaminants with the collecting medium as compared to the sampling medium. As a result, contaminants concentrate on the collecting medium over time until they reach equilibrium with the surrounding medium. The use of passive sampling provides time-averaged concentrations of contaminants over the sampler's deployment period. [1]

It is important to distinguish passive sampling from active sampling, which has the same underlying principle but employs moving parts, such as pumps, to force the sampling medium onto a collecting medium. [9] Passive sampling solely relies on molecular diffusion and the sorbing or binding of contaminants to agents in the samplers, which is why passive sampling is also called diffusive sampling. [6]

Passive sampling is also distinct from grab sampling, which is the collection of an air, water, or soil sample to analyze directly for contaminants. These samples represent a single point in time and provide information about contaminant concentration at one point in time, unlike passive sampling devices or organisms.

Many different kinds of passive samplers have been developed and have sampled many different contaminants, including:

Passive sampling in water

Several kinds of passive sampling devices exist for monitoring pollutants present in water. In addition to these devices, organisms, such as mussels, living in the environment also "passively sample" contaminants (bioaccumulation) and can be used to monitor water pollution (biomonitoring). [11]

Chemcatcher

Chemcatcher can passively sample inorganic pollutants (metals) and a wide range of organic pollutants. It is composed of a single-use disc, with or without a membrane, sealed into a plastic support. Types of receiving phases and membranes vary greatly, depending on the target chemicals to be sampled. Time-averaged water concentrations of many chemical pollutants can be determined as long as sampling rate and the water flow rate are known. [2] [3]

Diffusive gradients in thin films (DGT)

Diffusive gradients in thin films (DGT) samplers passively sample ionic trace metals, as well as antibiotics, oxyanions, bisphenols, and nanoparticles in different configurations. They are composed of plastic pistons and caps, with a window that exposes a binding gel, diffusive gel, and filter membrane to the sampling water. They can be used in both freshwater and marine environments, as well as in the water located between freshwater and marine sediment particles, called pore water or interstitial water. Once the mass of accumulated contaminants on the DGT sampler is known, the DGT equation (based on Fick's law) can be used to calculate the time averaged water concentration of contaminants. [10]

Microporous polyethylene tubes (MPT)

Microporous polyethylene tubes (MPT) attempt to mitigate the flow-dependency of other kinetic passive samplers such as Chemcatcher and POCIS by introducing a thicker membrane. [12] The diffusive polyethylene layer prevents the thickness of the water-boundary layer (which is affected by flow) from dominating diffusion. [13] The tube is filled with sorbents depending on the chemicals or chemical groups being sampled and has been successfully used to sample glyphosate, AMPA, illicit drugs and pharmaceuticals and personal care products. [13] [14]

Peepers

Peepers are passive diffusion samplers used for metals in freshwater and marine sediment pore water, so they can be used to find areas that may have metal-contaminated sediments. Peepers are plastic vessels filled with clean water and covered in a dialysis membrane, which allows metals in sediment pore water to enter the water inside the peeper. [15] They are usually placed deep enough into sediment to be in an anoxic environment, in which metals will be soluble enough to sample. [16] If the peepers are deployed long enough so the sediment pore water and contained peeper water reach equilibrium, they can accurately provide metal concentrations in sampled sediment pore water. [15]

Polar organic chemical integrative sampler (POCIS)

Polar organic chemical integrative samplers (POCIS) sample polar organic contaminants with a log octanol-water partition coefficient (Kow) value that is less than 3. Examples of these types of chemicals include polar pesticides, pharmaceuticals, illicit drugs, flame retardants, and drug metabolites. The POCIS is composed of variable numbers of solid sorbent discs attached to a support rod and encased in a metal cage, and has two possible sorbent configurations, the pesticide-POCIS and pharmaceutical-POCIS. As long as the amount of water passing over the sampler is known, polar organic contaminant water concentrations can be calculated after extracting sorbed contaminants from a POCIS. [5]

Semipermeable membrane devices (SPMDs)

Semipermeable membrane devices (SPMDs) passively sample nonpolar organic contaminants with a log octanol-water partition coefficient (Kow) value greater than 3. Examples of these types of chemicals include polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs), chlorinated pesticides, dioxins, and furans. SPMDs are composed of sealed plastic tubing filled with triolein, in which nonpolar organics are very soluble and which serves as a representation of the fatty tissues of aquatic organisms. The tubing is then weaved between metal rods and enclosed in a metal cage. The sampler can be made in varying lengths of tubing for different applications, since sampling rate depends on the surface area of tubing exposed to the water. As long as the amount of water passing over the sampler is known, nonpolar organic contaminant water concentrations can be calculated after extracting contaminants from a SPMD. [5]

A 7.5 centimeter SLMD, filled with a 1:1 mixture of Kelex-100 and oleic acid. Stabilized liquid membrane device (SLMD).jpg
A 7.5 centimeter SLMD, filled with a 1:1 mixture of Kelex-100 and oleic acid.

Stabilized liquid membrane devices (SLMDs)

Stabilized liquid membrane devices (SLMDs) passively sample ionic metals in freshwater. They are made of low-density polyethylene plastic tubing sections that are sealed on both ends and filled with an equal mixture of oleic acid and metal chelating agent. They work by interacting with calcium and magnesium ions in freshwater, which forms a hydrophobic film on the outside the SLMD plastic membrane in which the chelating agent can bind to metals in the sampling water. [4] They have been deployed for up to month-long periods in the field, alone or covered by a plastic tube housing to mediate water flow. [16] Metal weight accumulated by a SLMD over its deployment period can be calculated and divided by the SLMD deployment time to get an average metal weight accumulated per time unit, but currently, no method has been developed to convert this to an average metal concentration. In addition, SLMD sampling rates greatly vary with water flow rate, which plastic housings can be used to control. [4] [16]

Passive sampling in air

Passive sampling can also be accomplished for contaminants in the air, including airborne particles and hazardous vapors and gases. This can be done with man-made devices or with biomonitoring organisms, such as lichens. [7] [17]

Sorbent tubes

Sorbent tubes are passive samplers for volatile organic compounds (VOCs). They are glass tubes filled with adsorbing materials, such as charcoal or silica gel, which the air to be sampled passes through. The adsorbing materials remove VOCs from the air that passes through them, and the VOCs can be desorbed and analyzed. Air concentrations can be calculated using the amount of air that flowed through the sorbent tube and the amount of contaminants desorbed. [8]

Advantages

Contaminant concentrations from passive sampling reflect average contamination throughout the sampler deployment time, meaning the sample will capture contaminant concentration fluctuations over the whole deployment period. Traditional grab sampling does not do this, since collected samples only represent a single moment in time and multiple grab samples must be taken to observe variation in contaminant concentrations over time. [1] This integrative sampling method can also can result in the detection of chemicals present at such low concentrations that they would be undetected in a grab sample, due to concentration of the chemicals on the sampler over time. As a result, passive sampling has the potential to be a less time-intensive, less expensive and more accurate sampling method than grab sampling.

In addition, passive samplers are often easy to use and deploy, have no pumps or moving parts, and do not require electricity, since they rely on the molecular diffusion of contaminants or binding of contaminants to agents within the samplers, unlike active sampling. [6] They may also be inexpensive and simple to construct, such as SLMDs, which only require sealed plastic tubing and two chemical components. [4]

Passive sampling may also more accurately reflect metal concentrations that are bioavailable to organisms than other sampling methods. For example, the SPMD sampler uses a semipermeable membrane and triolein (a triglyceride), both of which mimic toxicant uptake by organism fatty tissue. [5] However, this depends on the type of passive sampler used, since some samplers, such as peepers, rely solely on molecular diffusion, [15] which may be too simple to be reflective of the complex processes of contaminant uptake in an organism.

Disadvantages

Since passive sampling provides information about average contaminant concentrations, all possible concentrations over the sampler deployment time are included in this average value. However, there is no way of finding out the complete range of contaminant concentrations over the deployment time at a single site with only passive sampling. [1] If high and low concentrations of contaminants throughout the sampling period are needed, other sampling methods should be used in conjunction with passive sampling.

Not all passive samplers have universally accurate ways to convert contaminant masses accumulated into water concentrations, which are used in government regulation, such as the United States Environmental Protection Agency Clean Water Act. [18] With some samplers, as with the DGT, this can be done using equations developed for the samplers, but not all samplers have these. Passive sampler deployment time is also limited depending on the sampler's capacity; for example, SLMDs have been deployed for month-long periods, but may reach saturation and stop sampling much sooner if metal concentrations and water flow rates are high enough. [4] However, this issue is avoidable if literature on the relevant passive sampler is examined for background information about sampler capacity and ideal deployment times prior to deployment.

Related Research Articles

Bioaccumulation is the gradual accumulation of substances, such as pesticides or other chemicals, in an organism. Bioaccumulation occurs when an organism absorbs a substance at a rate faster than that at which the substance is lost or eliminated by catabolism and excretion. Thus, the longer the biological half-life of a toxic substance, the greater the risk of chronic poisoning, even if environmental levels of the toxin are not very high. Bioaccumulation, for example in fish, can be predicted by models. Hypothesis for molecular size cutoff criteria for use as bioaccumulation potential indicators are not supported by data. Biotransformation can strongly modify bioaccumulation of chemicals in an organism.

Water quality Chemical, physical, and biological characteristics of water based on the standards of its usage

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, extend of water pollution and condition of drinking water. Water quality has a significant impact on water supply and oftentimes determines supply options.

Dialysis (chemistry) Process of separating molecules

In chemistry, 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.

Environmental chemistry Scientific study of the chemical and phenomena that occur in natural places

Environmental chemistry is the scientific study of the chemical and biochemical phenomena that occur in natural places. It should not be confused with green chemistry, which seeks to reduce potential pollution at its source. It can be defined as the study of the sources, reactions, transport, effects, and fates of chemical species in the air, soil, and water environments; and the effect of human activity and biological activity on these. Environmental chemistry is an interdisciplinary science that includes atmospheric, aquatic and soil chemistry, as well as heavily relying on analytical chemistry and being related to environmental and other areas of science.

Soil contamination Pollution of land by human-made chemicals or other alteration

Soil contamination, soil pollution, or land pollution as a part of land degradation is caused by the presence of xenobiotic (human-made) chemicals or other alteration in the natural soil environment. It is typically caused by industrial activity, agricultural chemicals or improper disposal of waste. The most common chemicals involved are petroleum hydrocarbons, polynuclear aromatic hydrocarbons, solvents, pesticides, lead, and other heavy metals. Contamination is correlated with the degree of industrialization and intensity of chemical substance. The concern over soil contamination stems primarily from health risks, from direct contact with the contaminated soil, vapour from the contaminants, or from secondary contamination of water supplies within and underlying the soil. Mapping of contaminated soil sites and the resulting cleanups are time-consuming and expensive tasks, and require expertise in geology, hydrology, chemistry, computer modeling, and GIS in Environmental Contamination, as well as an appreciation of the history of industrial chemistry.

Green nanotechnology refers to the use of nanotechnology to enhance the environmental sustainability of processes producing negative externalities. It also refers to the use of the products of nanotechnology to enhance sustainability. It includes making green nano-products and using nano-products in support of sustainability.

Environmental monitoring Monitoring of the quality of the environment

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.

Biosorption is a physiochemical process that occurs naturally in certain biomass which allows it to passively concentrate and bind contaminants onto its cellular structure. Biosorption can be defined as the ability of biological materials to accumulate heavy metals from wastewater through metabolically mediated or physico-chemical pathways of uptake. Though using biomass in environmental cleanup has been in practice for a while, scientists and engineers are hoping this phenomenon will provide an economical alternative for removing toxic heavy metals from industrial wastewater and aid in environmental remediation.

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

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.

Nanoremediation is the use of nanoparticles for environmental remediation. It is being explored to treat ground water, wastewater, soil, sediment, or other contaminated environmental materials. Nanoremediation is an emerging industry; by 2009, nanoremediation technologies had been documented in at least 44 cleanup sites around the world, predominantly in the United States. In Europe, nanoremediation is being investigated by the EC funded NanoRem Project. A report produced by the NanoRem consortium has identified around 70 nanoremediation projects worldwide at pilot or full scale. During nanoremediation, a nanoparticle agent must be brought into contact with the target contaminant under conditions that allow a detoxifying or immobilizing reaction. This process typically involves a pump-and-treat process or in situ application.

Diffusive gradients in thin films Environmental chemistry technique

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.

Equilibrium partitioning Sediment Benchmarks (ESBs) are a type of Sediment Quality Guideline (SQG) derived by the US Environmental Protection Agency (EPA) for the protection of benthic organisms. ESBs are based on the bioavailable concentration of contaminants in sediments rather than the dry-weight concentration. It has been demonstrated that sediment concentrations on a dry-weight basis often do not predict biological effects. Interstitial water concentrations, however, predict biological effects much better. This is true because the chemical present in the interstitial water is the uncomplexed/free phase of the chemical that is bioavailable and toxic to benthic organisms. Other phases of the chemical are bound to sediment particles like organic carbon (OC) or acid volatile sulfides (AVS) and are not bioavailable. Thus the interstitial water concentration is important to consider for effects to benthic organisms.

Bioavailability, in environmental and soil sciences, represents the amount of an element or compound that is accessible to an organism for uptake or adsorption across its cellular membrane. In environmental and agricultural applications, bioavailability most often refers to availability of contaminants, such as organic pollutants or heavy metals, in soil systems and is also used frequently in determining potential risk of land application of sewage sludge or other inorganic/organic waste materials.

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, PCBs, PBDEs, dioxins and furans as well as hydrophobic waste-water effluents like fragrances, triclosan and phthalates.

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.

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.

Workplace exposure monitoring is the monitoring of substances in a workplace that are chemical or biological hazards. It is performed in the context of workplace exposure assessment and risk assessment. Exposure monitoring analyzes hazardous substances in the air or on surfaces of a workplace, and is complementary to biomonitoring, which instead analyzes toxicants or their effects within workers.

A diffusion tube is a scientific device that passively samples the concentration of one or more gases in the air, commonly used to monitor average air pollution levels over a period ranging from days to about a month. Diffusion tubes are widely used by local authorities for monitoring air quality in urban areas, in citizen science pollution-monitoring projects carried out by community groups and schools, and in indoor environments such as mines and museums.

References

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