recovered from the carbon in that work may have resulted from reactions similar to those reported here.Reactions of chlorine and organic compounds on activated carbon have several environmental implications. The quiñones and polyphenols formed on the carbon surface are of interest since they are toxic to aquatic plants (19) and participate in redox reactions in the environment. Quinones have also been shown to be toxic to blue-green algae (20) and to inhibit the photosynthesis of green algae (21).The columns in these experiments were run for only a short period of time, and many oxidized compounds were found in the inlet to the bed. It would be expected that these compounds could be displaced further into the column during the life of the bed and eventually pass into the effluent. Although outside the scope of the present study, it would be interesting to observe the conditions under which displacement could occur and the concentrations and biological properties of the effluent organic compounds.

The fluoride content of both sound and white-spot enamel was determined in deciduous teeth collected from an anti-caries clinical trial which compared a 0.243% NaF dentifrice, a 0.4% SnF2 dentifrice, and a placebo. A microdrill enamel biopsy technique was developed for the analysis of white spots. No correlations were found between caries increment scores and the fluoride content of sound enamel. The enamel in white spots from users of the 0.243% NaF dentifrice had a mean fluoride content higher than that from users of the 0.4% SnF2 dentifrice. There was a negative correlation between fluoride found in white-spot enamel and caries increment scores. It is hypothesized that fluoride affects the caries rate by its control over the rate ofremineralization in previously demineralized areas.

~~The ion-exchange isotherm for Ca2+ exchange with sodium zeolite A was determined at 25 OC by using a novel solid uptake analytical scheme and a Langmuir adsorption model, resulting in an improved value for the thermodynamic equilibrium constant. Ion-exchange isotherms and approximate thermodynamic equilibrium constants were also determined for calcium zeolite A exchange with Pb2+, Cd2+, and Cu2+. Pb2+ and Cd2+ exchanges were found to be reversible, whereas Cu2+ exchange was not, probably due to Cu(OH),(s) precipitation upon or within the zeolite.Ion exchange was attempted but found to be irreversible with C$+, Hg2+, Fe3+, and A13+. Except for Cr3+, these ions caused zeolite A structure degradation, probably through extensive proton exchange. Cr3+ exchanged partially and then formed a precipitate. Data obtained in this work provide an improved understanding of the possible environmental water effects of zeolite A.

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