Study On The Technical Conditions Of High-fluoride Groundwater Treated By Induced Crystallization

Fluorine is an important trace element in human body which can be used to synthesize human bones and teeth.Long-term drinking appropriate concentration of fluoride water is beneficial to human health.The World Health Organization suggests that the appropriate concentration of fluoride in drinking water is 0.5-1.5 mg/L.The current treatment methods of high fluoride groundwater exist many problems,such as complex process,high operation and management costs,etc.In response to this phenomenon,this study proposes the use of induced crystallization for the treatment of high fluoride groundwater.In this experiment,fluorapatite mixed with calcite was used as seed crystals,and the mass ratio of seed crystals,dosage of seed crystals and precipitants,reaction time and stirring speed,the influence of organic matter（humic acid）and seeds reusability were discussed.Then,use the same composition of phosphate rock powder as seed crystals to further explore its fluoride removal effect.Finally,according to the reaction results and test results to analyze the mechanism of fluoride removal by induced crystallization.The main conclusions are as follows:（i）Experimental design the initial fluoride concentration was 9.5 mg/L（0.5 mmol/L）,and the calcium fluoride/calcite mixture was used as seed crystals,CaCl₂ and Na H₂PO₄ was used as precipitant to carry out the experiments.The optimum control conditions were as follows: the mass ratio of calcium fluoride and calcite was 4:1,the dosage of seed crystals was 6 g/L,the molar ratio of Ca₂ +,PO₄³-and F-was 8:4:1,the stirring speed was 100r/min,the reaction time was 1h.Under this condition,the effluent fluoride concentration can be reduced to below 1.0 mg/L which meets the drinking water fluoride standard,the effluent p H is around 7.And the water samples containing 30 mg/L within the humic acid has no effect on the experiments.Seed crystals have some persistence and can be regenerated by adding calcite.（ii）Experimental design the initial fluorine concentration was 9.5 mg/L（molar concentration was 0.5 mmol/L）,the phosphate rock powder was used as seed crystals,calcium chloride and sodium dihydrogen phosphate was used as precipitant,experimental results were as the following: The optimum dosage of phosphate rock powder was 10 g/L,and the molar ratio of phosphorus and fluoride is 3 mmol/L,the stirring speed was 100 r/min,the reaction time was 1 h.In the conditions of 10:1,8:1 and 5:1 of the calcium/fluoride molar ratio,the fluoride can be reduced to 1.5 mg/L below which meet the World Health Organization recommended drinking water fluoride standards.At the same time,we also learned that in the range of 4-10 of initial pH does not affect the reaction results.The presence of 10 mmol / L of Cl-,SO₄²-,NO₃-,NO²-in water respectively,has no effect on the experiment.Containing 1mmol / L HCO₃-has no effect on the experiment,while 10 mmol / L,the influence is serious,effluent fluoride concentration is not up to standard.（iii）Via the comparison of scanning electron microscopy before and after the reaction.It was found that the calcium ions,dihydrogen phosphate ions and the fluoride ions in the water undergo a crystallization reaction on the surface of the seed crystals formed a needle-like crystal structure,after the energy spectrum and X-ray diffraction analysis shows that the crystallization was Fluorapatite.According to the experimental results and the test results analysis reaction mechanism seems to be that fluorapatite seed crystals provided an active site for the crystallization of the newly formed Fluorapatite during the reaction,and the calcite added has played a role in adjusting the pH of the solution and maintaining the calcium saturation.