Decontamination methods primarily are used to reduce cross contamination, especially when ground water sampling devices are not dedicated and are moved from well to well. A survey the various decontamination procedures required by state and other regulatory agencies found:
- there are a number of published decontamination
- there is a lot of disparity between these protocols
- there has been almost no systematic study on the effectiveness of the various procedures
Although decontamination procedures vary considerably in their methodology, most utilize some type of aqueous cleaning method and often use solvent cleaning as a final or additional rinse. A typical protocol for decontaminating ground water sampling devices that have been used to sample organic solutes is as follows: Wash with detergent, rinse with tap water, rinse with (high quality) organic solvents, rinse with some type of high-quality water (e.g., distilled, deionized or organic-free reagent water) and air dry. In addition, steam cleaners or high-pressure washing systems sometimes are recommended for decontaminating sampling devices, especially to remove gross contamination such as dirt and oils.
Aqueous cleaning is used to first remove gross contamination and particles. Water acts as a solvent medium for contaminants that are soluble in water and as a dispersal medium for substances that do not dissolve in water but can be carried in suspension. A surface active agent (or surfactant), such as detergent, commonly is added to improve the wetting ability of the cleaning solution and aid cleaning by separating the contaminant from the solid surface and then keeping the contaminants in suspension, thus preventing redeposition.
With steam cleaning, pressure developed in the steam boiler imparts a high velocity to a mixture of water droplets and steam, which is directed from a nozzle onto the target surface. Detergent and heat from the steam weaken the bonds between the dirt and surface while the high velocity of the water droplets has sufficient power to remove debris from the surface. Depending upon the contaminant, steam also can facilitate volatilization and hydrolysis and may aid in the removal of subsurface contaminants. Problems associated with steam cleaning include workers being burned, corrosion of metal surfaces, warping of some polymers and liberation of toxic vapors. However, from what we have been able to determine, most steam cleaners used for decontaminating ground water sampling devices actually are high-temperature, high-pressure washing systems.
With pressurized-water washing systems, high-pressure pumps produce a stream of water rather than the small droplets produced by steam cleaning. The advantages of this type of cleaning over steam cleaning are that there is increased force available, the energy requirements are lower because water does not have to be converted to steam, polymeric materials are less likely to be degraded, and there is less likelihood of being burned.
Organic solvent rinses are used to remove any residual contaminants by dissolving them. Generally speaking, polar solvents dissolve polar contaminants and nonpolar solvents dissolve nonpolar contaminants. Because water is a very polar solvent, nonpolar solvents typically are used to remove nonpolar organic contaminants (e.g., oils, tars) that have not been removed previously by aqueous cleaning.
Recommended organic solvents vary with the particular protocol but typically include acetone, hexane or methanol. In most protocols, these solvents are recommended without any regard to the type of contaminants, and it should be noted that among the three solvents mentioned, only hexane is relatively nonpolar. Obviously, any organic solvent that is used as a rinsing agent should not be one of the target analytes or interfere with chemical analyses.
There are a number of problems associated with using organic solvents. These can include flammability, toxicity, disposal (although recycling can reduce this problem) and spillage, which can cause additional contamination problems on-site. Also, many polymers (e.g., thermoplastics) are degraded by various organic solvents. In addition, all polymers sorb some organic chemicals, and these contaminants may be subsequently desorbed, thereby contaminating a water sample.
Factors Affecting EfficiencySeveral factors affect how readily a sampling device can be decontaminated. These include the type of sampling device (e.g., pump vs. bailer), the materials to be decontaminated and physical characteristics of the organic contaminant, such as its aqueous solubility, volatility and propensity to adsorb on or absorb into materials used in the sampling device. Contact time and the degree of initial contamination on the surface also are critical factors. Cleaning a surface that has been exposed to trace-level organic contaminants is much different from cleaning a surface exposed to neat (pure product) hydrocarbons or organic solvents. Other types of contaminants, such as grease and oil, also may affect removal.
Several studies have shown that nonpermeable surfaces, such as glass and stainless steel, do not tend to sorb more hydrophilic organic solutes. However, there are reports that these surfaces sorb more hydrophobic contaminants, such as polychlorinated biphenyls (PCBs). Most likely, these losses are due to adsorption rather than absorption.
There has been relatively little study of desorption of hydrophobic organic contaminants from nonpermeable surfaces. The pesticide permethrin is much more readily desorbed from glass than from polyvinyl chloride (PVC), polyethylene (PE) or fluoropolymer (Teflon) surfaces. It has been proposed that decontamination of nonpermeable surfaces, such as metals and glass, should involve removing only surface contaminants, such as any residual film (either wet or dry) that is left on the surface when the sampling device is removed from the well, and any highly hydrophobic contaminants that may have adsorbed to the surface.
In contrast, several studies have shown that polymeric materials, such as well casings and the tubings used in ground water sampling pumps, sorb substantial quantities of some organic contaminants from aqueous solutions. Most of these studies agree that these organic compounds have diffused into the polymer matrix, i.e., absorption has occurred. The rate and extent of sorption varies. Generally, flexible materials tend to be much more sorptive. As an example, one study found that after only 10 minutes, sorption of low-ppm levels of benzene by flexible PVC was approximately 35 percent, while losses were less than 1 percent for solutions exposed to rigid PVC.
For polymers exposed to low concentrations of organic solutes, diffusion of the organic molecule in the polymer is considered to be concentration independent, and at slightly higher concentrations (activities), diffusion is considered to be concentration dependent. However, when glassy amorphous polymers (e.g., rigid PVC) are subjected to neat (undiluted) chemicals or very high concentrations (i.e., approaching the compound’s aqueous solubility) and the chemical is a solvent or swelling agent of the polymer, then diffusion increases several orders of magnitude. The polymer will be seriously degraded, and the sampling device will no longer be useful.
It has been proposed that decontaminating permeable materials should involve more than removing surface contaminants if desorption of absorbed contaminants is significant. Unfortunately, there have been only a few studies that have examined desorption of organic contaminants from polymeric materials. Those studies found that the compounds that were present in the greatest concentrations following desorption were not the same compounds that had been sorbed the most rapidly or to the greatest extent. It’s been noted that the smaller molecules are more readily desorbed, and this is attributed to the fact that diffusion is more rapid for smaller molecules.
Decontamination EfficiencyAlthough not many studies have examined the efficiency of the various decontamination protocols, three studies demonstrate the impact that the type of contaminants, the level of contamination or the materials being decontaminated can have on decontamination effectiveness.
One study found that polyethylene tubing was harder to decontaminate than Teflon tubing (the actual type was not specified). These tubings had been exposed to ppb levels of a suite of volatile organic compounds (VOCs) and were decontaminated by pumping deionized water through them.
Another study compared the effectiveness of distilled water rinses for removing seven pesticides from a PVC bailer. It utilized a brief contact time (1 minute) to represent the time required to take a bailed sample in a shallow well or gravelly aquifer. With one exception (dimethoate), the researchers found a reasonably good correlation between the effectiveness of this procedure and the analyte’s solubility in water. They found that the most hydrophilic contaminants were removed from the bailer with no residual carryover with just one rinse, while the most hydrophobic analytes had residual carryover after six rinses.
The third study looked at decontaminating a stainless steel bladder pump after it had been used to sample VOCs. The pump was cleaned by steam cleaning the outside of the pump and tubing, and then circulating hot (120 F, 49 C) aqueous detergent (1%) solution through the system, followed by ambient temperature rinse water. The pump was contaminated by pumping at least five sampling pump/tubing assembly volumes of contaminated water. Detectable levels of trichloroethylene (TCE) were found in the final rinse water when the pump was used to sample the two wells with the highest TCE concentrations, but not when the pump was used to sample wells with lower concentrations.
Because it was concluded that there have been almost no systematic scientific studies on decontamination of ground water sampling devices, studies were initiated to determine the effectiveness of various decontamination protocols. These studies were designed to consider the type of contaminants, concentration of contaminants, materials being decontaminated and contact times.
The Initial FindingsIn the first phase of the studies, three materials commonly used in sampling devices (rigid PVC, PTFE and stainless steel) were exposed to a test solution containing either VOCs or pesticides and then tested using various decontamination protocols. These chemicals were selected because they ranged from being relatively hydrophilic to relatively hydrophobic.
It was found that all the organic contaminants tested could be readily removed from the nonpermeable stainless steel surfaces with a hot detergent wash and hot water rinse. However, as expected, the permeable polymeric materials were much less readily decontaminated. Ease of decontamination was a function of the analyte, the rigidity or sorptive nature of the polymer and the contact time.
Detergent washing and rinsing with cold water was not effective for removing VOCs from the more sorptive PTFE test pieces or from the rigid PVC test pieces that had been exposed to the test solution for the longer sorption/desorption regime. However, VOCs were readily removed from the PVC test pieces by washing with a hot detergent solution and rinsing with hot water. For the more sorptive PTFE, additional oven drying (220 F) was necessary for effective decontamination. Apparently, oven-drying speeds diffusion of the VOCs out of the polymer.
Pesticides were readily removed from most of the polymeric materials by using a hot detergent wash and hot water rinse. (A cold-detergent wash procedure was not tested.) LDPE was the exception using this method if the exposure times were longer (24 hours). Detergent washing, followed by oven-drying, substantially improved removal of these contaminants (by a factor of 10), with only low concentrations of one pesticide still detected. LDPE tubing was the most sorptive of the tubing materials tested and absorption within the polymer matrix appears to be quite significant. Again, it appears that oven-drying speeds diffusion of the sorbed organic chemicals out of the polymer.
This study also showed that solvent rinsing did not aid in the removal of the VOCs from the more sorptive PTFE surfaces. While solvent rinsing did slightly improve removal of the pesticides from LDPE tubing, oven-drying was much more effective. It is concluded that a considerably cumbersome, expensive and hazardous step (i.e., solvent rinsing) serves no useful purpose and thus could be eliminated from all decontamination protocols.