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61.
Previous epidemiological studies have shown that dental fluorosis is endemic in the lowland, dry zone of Sri Lanka, which is considered to be an area in which excessive quantities of fluorides are present in the drinking water supplies. It has been found that kaolinitic clay forms a suitable raw material in the defluoridation of water.It is shown that there is a noticeable effect of selenium and media pH on the reactions involved in the interaction of fluoride with clay. In this study, 1 mM fluoride solutions containing SeO
3
2–
(selenite) concentrations of 0 mM, 0.1 mM, 0.5 mM and 1 mM were used in the reactions with kaolinitic clay. The effect of pH was monitored in the range 4 to 8. It was observed that fluoride adsorption was maximum at a pH of 5.6 without either SeO
3
2–
or SeO
4
2–
, the adsorption capacity being 15.2 mol F– g–1 clay. However, when the SeO
3
2–
concentration was increased up to 0.5 mM at this optimum pH, the adsorption capacity reduced to 12.8 mol F– g–1 clay. Monitoring of the effect of SeO
4
2–
and media pH on fluoride adsorption showed that when the SeO
4
2–
concentration increases from zero to 0.1 mM, there is a reduction of fluoride adsorption capacity. However, when the SeO
4
2–
concentration is further increased from 0.1 mM to 1.0 mM, there was an increase in the fluoride adsorption capacity, indicating a more consistent effect of SeO
3
2–
on fluoride-kaolinitic clay interaction than SeO
4
2–
.Fluoride concentrations in drinking water supplies have a marked effect on dental health and the geochemistry of selenium appears to play an important role in the geochemical mobility of fluoride ions. 相似文献
62.
Fuzzi S Facchini MC Orsi G Ferri D 《Environmental pollution (Barking, Essex : 1987)》1992,75(1):75-80
Fog frequency in the Po Valley, Northern Italy, can be as high as 30% of the time in the fall-winter season. High pollutant concentrations have been measured in fog water samples collected in this area over the past few years. The combined effects of high fog occurrence and high pollutant loading of the fog droplets can determine, in this area, appreciable chemical deposition rates. An automated station for fog water collection was developed, and deployed at the field station of S. Pietro Capofiume, in the eastern part of the Po Valley for an extended period: from the beginning of November 1989 to the end of April 1990. Time-resolved sampling of fog droplets was carried out during all fog events occurring in this period, and chemical analyses were performed on the collected samples. Statistical information on fog occurrence and fog water chemical composition is reported in this paper, and a tentative seasonal deposition budget is calculated for H+, NH4+, NO3- and SO4(2-) ions. The problems connected with fog droplet sampling in sub-freezing conditions are also addressed in the paper. 相似文献
63.
This paper documents the rapid expansion and changes in food aid flows to Sub-Saharan Africa up to mid-1985. Trends for Sub-Saharan Africa as a whole and for the more seriously affected countries are examined, as are the comparative experiences of food aid of individual countries in the region. Table 11 lists the Sub-Saharan African countries and indicates the most seriously affected food-short countries, as defined by the WFP/FAO task force. It should be borne in mind that some practical problems exist in compiling data on food aid. These include lack of availability of reliable data, especially of most recent data; lack of common terminology and definitions; and different accounting systems. These data problems are sometimes a source of confusion. However, the basic facts are clear: food aid gradually emerged during the early 1980s as a resource too often of considerable significance to many countries of Sub-Saharan Africa. These trends were only accelerated with the crisis of 1984-1985. These facts provide a point of reference for further analysis of the sources of the crisis, its actual dimensions and consequences. 相似文献
64.
Rate-temperature responses in scyphozoan medusae and polyps 总被引:1,自引:0,他引:1
The effects of temperature on oxygen consumption and spontaneous rhythmic activity have been investigated in various stages of the life histories of 3 species of jellyfish from the Chesapeake Bay, USA. All 3 species clearly show the ability to acclimate positively to temperature change. Thermal sensitivity of metabolism in the winter medusa Cyanea capillata fulva is fairly low at temperature intervals which are experienced in nature. Polyps of the two summer medusae, Chrysaora quinquecirrha and Aurelia aurita, show reduced metabolic sensitivity at temperatures normally accompanying high developmental activity and the onset of strobilation. 相似文献
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