| Qingwan mineral water is of high quality that was identified and approved for development by the National Ministry of Geology and Mineral Resources.Its characteristics are having high degree of mineralization and high concentration of free carbon dioxide,and also having beneficial micro ingredients for example metasilicic acid,strontium and lithium.But the concentration of fluoride in mineral water is constantly 1.6-5.4 mg·L-1(F-)that is above the limit value(1.5 mg-L-1)of the national standard of drinkable mineral water(GB 8537-2008).For a better development and use of this high-quality geological resource in Guangxi,defluoridation methods for Qingwan mineral water were firstly studied in this paper;to find out the main fluorine species in mineral water,analytical methods for five fluorine species including free fluoride(HF and F-),Ca-F,Mg-F,Fe-F and Al-F in mineral water were also discussed.The main points of research work were as follows.1.The ability of reducing fluoride and the effect on beneficial microelements of light magnesia,basic magnesium carbonate and four kinds of strong acid cation-exchange fibers modified by Ca2+,Al3+,La3+,Zr4+ were preliminarily investigated and compared.The results showed that fluoride could hardly be absorbed by fibers modified by Ca2+and Al3+,so they had no ability of reducing fluoride;fibers modified by La3+or Zr4+ had poor defluoridation ability and brought in a large number of La3+ or Zr4+;light magnesia had strong ability of reducing fluoride,but had a heavy loss of the beneficial microelements B,Si,Sr and Ca,and the pH value of treated mineral water was 9.0?10.0 that was greatly different with that of original mineral water(pH 6.7?7.0),which heavily affected the taste.At room temperature,basic magnesium carbonate had obvious effect of reducing fluoride and a less loss of B,Si,Sr and Ca,while the pH value of treated mineral water was 7.4?7.5.The research indicated that defluoridation by basic magnesium carbonate was well feasible.2.In the further research of static defluoridation by basic magnesium carbonate,the effect of reducing fluoride and loss amount of beneficial microelements were two indexes,and the effect of liquid temperature,amount of basic magnesium carbonate,oscillation and static time of water samples on reducing fluoride and residual magnesium in treated mineral water and stability of all kinds of elements were analyzed.The study showed that just considering effect of reducing fluoride,the effect of reducing fluorine was the best(0.04 mg·L-1 F-)when 0.4 g of basic magnesium carbonate was put into 25 mL of mineral water,and water temperature was controlled at 55 0C,oscillating 2 h and standing 8 h;synthetically considering effect of reducing fluoride and loss amount of beneficial microelements,the effect of reducing fluoride was obvious(0.30 mg·L-1 F-)and the loss of beneficial microelements was less when liquid temperature was controlled at 40?,while other conditions were the same as above.The concentration of fluorine in mineral water treated in above two conditions both conformed to the national standard,and the concentration of residual magnesium was 50?60 mg·L-1 that was lower than the amount of magnesium(150 mg?L-1)suggested by WHO.In mineral water treated in the above latter conditions(named the best comprehensive condition of static defluoridation),the concentration of fluoride was 0.30±0.01 mg·L-1 during 30 days of storing time and the change of concentration of other elements was all small.3.In the research of dynamic defluoridation by basic magnesium carbonate,the effect of reducing fluoride,the pH value of treatment fluid,loss amount of beneficial microelements and the volumn of water treated by unite mass of basic magnesium carbonate were four indexes that were considered,and effect of column temperature,amount of basic magnesium carbonate,flow rate of effluent liquid and treatment fluid collection time on reducing fluoride was investigated.Besides,the residual magnesium in treated mineral water was also determined.The result indicated that the best comprehensive condition of dynamic defluoridation was controlling column temperature at 55?,treating 12 mL of mineral water with 0.4 g of basic magnesium carbonate,treatment fluid flowing at the rate of 0.20 mL min-1 and collecting treatment fluid during 1 h.In the mineral water treated in this condition,the concentration of fluorine was 0.72 mg·L-1 that conformed to the national standard and the pH value was 7.5,and the loss of beneficial microelements was little,while the concentration of residual magnesium was 40 mg·L-1 that was that was lower than the value suggested by WHO.The volumn of treatment water in above-mentioned defluoridation condition was half of that of static defluoridation.The volumn of treatment water could be 24 mL that was very close to that of static defluoridation,and the concentration of fluorine was 1.16 mg·L-1 that conformed to the national standard,and the loss of beneficial microelements,the pH value of treated mineralwater and the concentration of residual magnesium were all close to the measured value of the best comprehensive condition of dynamic defluoridation when the time of collecting treatment fluid was lengthened to 2 h and other conditions were kept constant.The result also showed that when the volumn of water treated by unite mass of basic magnesium carbonate in dynamic and static method were very close,and the time needed for the concentration of fluoride to conform to the national standard,the ability of retaining beneficial microelements and concentration of residual magnesium were considered,dynamic method was superior to static method but energy consumption of dynamic method was higher than that of static method.4.In the analysis and research of five fluorine species in mineral water,it was proposed that various TISAB were chosen for demasking different complexed fluorine species,so the concentration of these species were respectively determined.The simulated mineral water containing fixed concentration of total fluorine and a certain amount of Ca2+,Mg2+,Fe3+,Al3+and calcium fluoride saturated solution were analyzed,which showed this method was feasible.The experimental result indicated that at the pH value of 5.0-6.0,the concentration of free fluoride in mineral water could be determined with TISABO(acetic acid-sodium acetate-sodium chloride).The total concentration of free fluoride and Fe-F in mineral water could be determined with TISAB I(VC-acetic acid-sodium acetate-sodium chloride).The total concentration of free fluoride,Ca-F and Mg-F in mineral water could be determined with TISAB II(glycine-acetic acid-sodium acetate-sodium chloride).The total concentration of free fluoride,Ca-F,Fe-F and Al-F in mineral water could be determined with TISABIII(EGTA-acetic acid-sodium acetate-sodium chloride).The total concentration of free fluoride,Ca-F,Mg-F,Fe-F and Al-F in mineral water could be determined with TISAB?(glycine-citric acid-acetic acid-sodium acetate-sodium chloride).On the basis of above results,the concentration of free fluoride,Ca-F,Mg-F,Fe-F and Al-F could be calculated by subtraction.The result also showed that in water samples containing high concentration of Ca2+ and Mg2+ such as Qingwan rruneral water,the concentration of total fluorine determined with TISAB? was higher than that determined according to GB/T 8538-2008,because TISAB in national standard method could not eliminate the effect of Ca-F and Mg-F on determining the concentration of total fluorine.Otherwise,a large number of bicarbonate radical in Qingwan mineral water restrained the formation of Ca-F,which caused that the concentration of Ca-F in Qingwan mineral water was below that of simulated water sample that had the same amount of calcium with Qingwan mineral water and no bicarbonate radical. |
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