Preparation Of Bentonite Defluoridators And The Study Of Their Defluoridation Properties

Fluoride is the necessary element for human health. Drinking water with certain concentration of fluoride can prevent dental caries and enhance the bone and the teeth, but drinking water with excessive fluoride in the long run may result in dental and skeletal fluorosis. In China, there are several millions of people suffering from endemic disease caused by extra intake of fluoride from drinking water. It is important to investigate the cost-effective methods of defluoridation from drinking water. In this paper, defluoridation by bentonite and its modified products was studied experimentally using batch reactor and/or continuous-flow column. The defluoridation principle of calcium bentonite is found in the experiment. The sorption of fluoride by calcium bentonite can be separated into two parts, the defluoridation principle is analyzed. Calcium bentonite is modified and the bentonite defluoridators are prepared. The main conclusions have been summarized as follows:(1) The sorption isotherms of bentonite defluoridators correlate with Fleundlich isotherm models.(2) Defluoridation by calcium bentonite in lower concentration is mainly due to the ion-exchange. Due to the ion-exchange between Ca2+ from the bentonite and Na+ in the solutions in higher fluoride concentration solutions, with the release of Ca 2+ from bentonite, Fcan be reacted with the released Ca2+, and precipitated out in the form of CaF2. Complementary sorption mechanism may be due to typically high isomorphous charge from bentonite, as it is possible that F is held within the electric double layer of the clay. With the Na+ exchanged with the Ca +, F" as the counter-anion accompanied by Na+ can enter the electric double layer of the clay.(3) The sorption capacity of calcium bentonite is too small and need too much bentonite in the experiment. The calcium bentonite can be modified by sulphuric acid or hydrochloric acid or Zirconium Oxychloride solutions. The defluoridationproperties of the modified products are obviously enhanced. The operation conditionsof the modified bentonite products is poor, the defluoridated water is difficult to clarify. The products is prone to swell in the continuous-flow column, the water is difficult to seepage. They can not be used in the defluoridation process directly.(4) Binder Poly (vinyl alcohol) is selected to granulate the bentonite products. The granulated products have good stabilities in the water. Zirconium-loaded granular bentonite can be used to remove fluoride from waste water and acid treated granulated bentonite can be used to remove the fluoride from drinking water and waste water. The defluoridators can be regenerated by Alum. They have good applied values.(5) Two granular products have good affinity to fluoride in the solution. The sorption capacity is improved after regeneration.(6) The water quality after defluodation meets the Guidelines for Drinking Water Quality except pH. Simultaneously they can also remove other contaminants.(7) The sorption capacity of the granulated acid treated bentonite is 1/6~1/5 that of activated alumina and bone charcoal, and triple of zeolite. The sorption capacity of Zr loaded granuated bentonite is 4 folds of the granulated acid treated bentonite. The capital cost of the granulated acid treated bentonite is lower than that of activated alumina and zeolite and higher than that of bone charcoal, but the running cost of the acid-treated granulated bentoinites is lower than that of activated alumina and bone charcoal. The capital and the running cost of the acid-treated granulated bentoinites are less in the defluoridation of drinking water. Therefore they can be used a cost-effective defluoridators in the defluoridation of drinking water.