Interpretation of toxicity test results may be hampered when doubt exists about the actual exposure concentration. Processes that are responsible for differences between the nominal and the actual concentration in aqueous test systems may include sorption, precipitation, volatilization, chemical and biological degradation, and uptake into biological or test tissue. In this study, the use of a poly(dimethylsiloxane) (PDMS) film containing the test compound is introduced as a versatile technique for partition controlled delivery of hydrophobic compounds to aqueous toxicity tests. Two methods developed produced preloaded films, having toxicant added to the PDMS prepolymer solution before film deposition and curing, and postloaded films, which are created by the addition of toxicant in a solvent to an already-polymerized PDMS film. Preloaded films were generally more easily prepared, may better accommodate larger molecules, and have a higher capacity than postloaded films. Postloaded films provided film-solution partition coefficients with higher precision and allowed for the use of films from stock and thus for a more portable technique. Chemical analysis showed that equilibrium between films and the aqueous solution was established within 7-10 min and was maintained for a suite of aromatic compounds (log Kow ranging from 2.8 to 6.1). The reliability of the film technique was demonstrated by application to the Microtox bacterial toxicity tests of solutions of polycyclic aromatic hydrocarbons (PAHs).
Field observations on the phosphate distribution in the Ems estuary show a nonconservative concentration gradient, with high summer values in the middle reaches. Experiments indicate the existence of two antagonistic mechanisms that can regulale the phosphorus cycle. Calcite is formed at sea and in the estuary and transported onshore and to the upper reaches of the estuary like other suspended matter. During this transport, part of the phosphate adsorbs onto calcite. Because pH is lower in the upper reaches of the estuary, some of the calcite dissolves and consequently some of the phosphate desorbs. Other suspended matter components, such as clay minerals and iron oxyhydroxidc can also adsorb phosphate. The conditions in the upper reaches of the estuary (lower salinities and pH) are particularly favorable for these minerals to adsorb the phosphate that is available there as a result of calcite dissolution. Thus, during transport upstream, part of the calcitebound phosphate fraction of the suspended matter switches to adsorption sites of suspended noncalcite minerals, producing the nonconservative concentration gradient in phosphate along the estuary. This process of phosphate release may be stimulated by high carbon dioxide production resulting from the biological mineralization of organic matter in the middle and upper reaches of the estuary. The processes of sediment transport and phosphate adsorption and desorption favor the accumulation of marine phosphates as well as retention of fluvial phosphates in the estuary.
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