activated alumina has good physical properties, high strength, non-toxic, and tasteless. The specific surface area of about 320 m2/g makes the activated alumina have a large contact area, thus having a good ion exchange capacity. The pore volume above cm3/g 0.4 makes it have a high adsorption capacity, because it is easy to regenerate, has a long life, and is relatively cheap. Among various fluorine removal methods, it is an effective, economical and practical method. Activated alumina to remove fluoride ions is divided into two effects, adsorption and ion exchange.
Adsorption Principle of 1. Activated Alumina
Activated alumina defluorination agent is a kind of adsorption material, high specific surface area and high pore volume are the basic characteristics of adsorption materials, activated alumina has a certain adsorption of impurities, activated alumina for molecules smaller than its own pore size, have a certain adsorption capacity. Fluorine in water mainly exists in the ionic state. In the process of removing fluorine, the ionic state of fluorine is mainly removed from the water. The adsorption of activated alumina accounts for a small part. The most important thing is the ion exchange capacity of activated alumina.
The chemical formula for physical adsorption and ion exchange of activated alumina:
Al2O3 Al2(SO4)3 XH2O + 6F-= Al2O3 2 AlF3 XH2O +3SO42-
2. Activated Alumina
1. The fluorine contained in drinking water mainly exists in an ionic state, and its solubility in water is much greater than the allowable fluorine content (1mg /L). Therefore, the fluorine removal process is mainly to remove ionic fluorine from water. Activated alumina is a typical amphoteric compound. Under acidic conditions, activated alumina is an anion exchanger, and its selection order for anions in water is: OH->PO43->F->SO42- >Cl-.
2. Activated alumina regenerated with aluminum sulfate is in the process of defluorination. However, the fact that the sum of the exchanged SO42-ion milligram equivalents and the removed F-and HCO3-ion milligram equivalents is roughly equal indicates that the principle of defluorination of activated alumina is mainly the mechanism of ion exchange. The activated alumina is treated with aluminum sulfate or sulfuric acid to convert it into R2SO4 type. According to the ion exchange sequence, the F-ion reliable exchange potential in the water transforms SO42-ions from the R2SO4 of the activated alumina, thereby removing F-ions in the water.
3. In the process of defluorination, cations in water are not removed together with F-ions. In the experiment, the following formula holds: 2NaF + R2SO4, 2RF + Na2SO4. This shows that the adsorption performance of activated alumina for fluorine removal is secondary. Fluoride removal with R2SO4 type ion exchanger of activated alumina,
4. After the activated alumina exchange is saturated, it is regenerated with aluminum sulfate. The regeneration process relies on the high concentration of the regenerant to overcome the selectivity, so that the above reaction formula is carried out to the left, so that the activated alumina is regenerated. It can also be regenerated with 1% concentration of NaOH. After regeneration, the filter layer is alkaline and must be acidified to make the filter layer acidic in order to remove F-ions in the water.
5. Experiments show that lowering the pH value of raw water to about 5.5 is the best defluorination environment. Foreign fluoride removal devices mostly adjust the pH value of raw water to between 5.5 and 6.5. Not only is the fluoride removal effect good, but it is also more economical. In order to save the cost of acidification process in China, the pH value of raw water is usually lowered by a small range, generally controlled at 6.5~7.0.