Release studies of benzalkonium chloride from hydrogel in a freshwater environment

The use of underwater optical sensors to monitor pollution and climate change processes has led to the development of robust instruments able to be deployed in lakes and seas for months at a time. However, despite this improvement in their durability they are subject to biofouling on their optical ports resulting in erroneous readings. The use of hydrogel coatings containing the cationic surfactant benzalkonium chloride (BAC) has been shown to prevent the development of biofouling for up to 12 weeks in the marine environment. In this study the use of hydrogel coatings in the freshwater environment was less successful with fouling visible at 2 weeks. In both field and laboratory studies a rapid initial loss of BAC from the hydrogel film was observed. The loss is a combination of diffusive and mass flow but the period from 12 to 50 h appeared to fit to diffusion kinetics and a diffusion coefficient of 7.3 x 10(-8) cm2 s(-1) (13 degrees C) was calculated, an order of 10 times greater than that found in seawater. Subsequently the rate of loss of the residual BAC, for which a diffusion coefficient of 5.7 x 10(-10) cm2 s(-1) (15 degrees C) was measured, was too low to prevent the early stages of biofouling.