Passive sampling for the measurement of freely dissolved concentrations (Cfree) of organic pollutants in sediment pore water has emerged as a very promising approach, but in situ measurements are complicated by slow mass transfer of strongly hydrophobic compounds.
This results in passive sampling deployment periods that are often 4 to 8 weeks, a time period that can delay rapid site-decision-making and jeopardize measurement successes due to sampler loss, biofouling, and other logistical challenges. Additionally, slow mass transfer of the more hydrophobic compounds, even when measured using an 8-week deployment, can result in uncertain or highly variable results. Previous work has shown through laboratory experiments that periodic vibration greatly enhances sampling rates for passive samplers in sediment, reduces the required deployment times by at least 2 to 4 times, and improves the accuracy of estimating the pore water concentrations.
Approach/Activities. Our project has developed a robust platform incorporating a vibration device that can be adapted to an existing commercially-available vibrating passive sampler. The materials and production costs for the platform is inexpensive (< $50-100 per unit) and attachable to currently-available commercial passive sampling devices (e.g., SP3TM samplers) in various configurations. The initial design operates with a user-replaceable battery and a microcontroller, mounted on a 5- by 7-cm circuit board, with the components housed in a small waterproof housing that has been tested to water pressures at 30 m depth. We evaluated the field performance of the devices using SP3TM polyethylene passive samplers impregnated with Performance Reference Compounds (PRCs) in a marine sediment for a time period of 7 to 14 days in order to demonstrate the device performance and evaluate the increase in sampling rates due to the vibration (via compassion to results the standard SP3TM device design, which is not vibrated).
Results/Lessons Learned. We observed that periodic vibration of the passive samplers with the frequency of 4 h pause and 5 s pulse, greatly enhanced the mass transfer of PCBs/PRCs into/out of the passive samplers compared to the static deployment even after 7 days. This represents a marked improvement over typical deployment times for passive samplers, which are often in the range of 28 to 56 days. The initial design remained stable and waterproof, and there was no need to recharge the battery or service the device during deployment, allowing for an efficient and effective field program. In addition, as shown in previous research, closer approaches to equilibrium improves the accuracy of Cfree measurements, especially for the more hydrophobic compounds. The shorter deployment times, which can be reduced to approximately 7 to 14 days, reduces the risk of loss and destruction of the passive samplers during the field deployments. Overall, this technology is mature for application, improves the accuracy of Cfree measurements, and reduces the costs and risks associated with in situ deployments.