Zeolite Fluoride Removal Process

Fluoride is the necessary element for human health. Drinking water with certainconcentration of fluoride can prevent dental caries and enhance the bone, but water containingexcessive fluoride concentration may result in dental and skeletal fluorosis. In China there areseveral ten millions people suffering from endemic disease caused by extra intake of fluoridefrom drinking water. It is important to investigate the methods of fluoride removal from drinkingwater. In this paper, fluoride removal by activated zeolite was studied experimentally using batchreactor and continuous-flow column, in order to find the best conditions of activation andregeneration. The adsorption capacity and the empty-bed contact time for effective fluorideremoval were also studied.In the experiment of zeolite activation, raw materials were smashed and shieved andparticles of 0.8-0.9mm were prepared. The particles were soaked in 2% NaOH solution for about12-24 h, and then in 1% KAl(SO₄)₂ solution for about 30h .The volume ratio of solution to zeoliteis 1:1. For the regeneration of exhausted zeolite, 0.5%-2% KAl(SO₄)₂ solution was used, and thesoaking period was about 20h followed by rising with clean water.There are many factors influencing the adsorption capacity The adsorption capacitydecreases with pH, as pH varies from 4 to 9,and also decreases as temperature increases. Largeradsorption capacity is obtained with the longer contact-time. The adsorption isotherm of zeoliteconforms to Langmuire equation as q_e=0.068c_e/(1+0.0868c_e), and the rate of the adsorptionconforms to Banerm equation as dq/dt=0.0018(q_e-q)/t^{0 22}.Contact-time is a vary important factor in the continuous-flow column experiment. Theeffect of fluoride removal is good, as the empty-bed contact time is controlled as 20 to 40minutes .The pull-down speed of adsorption curve in the continuous-flow column is related to theconcentration and flow rate of the raw water. It is show that the adsorption capacity of zeolite isimproved after repeating regeneration. The treated water meets the drinking water qualitystandards. The experiment results have shown that activated zeolite can be used for fluorideremoval from drinking water at a low cost as other adsorbents such as activated alumina andbone-charActivated alumina was used as adsorbent for a household scale appliance for fluorideremoval, as the appliance was used two times per day ,the adsorbent could be used for 72 timeswithout regeneration. The exhausted activated alumina can be then regenerated and used again.The cost for fluoride removal is very lower.