Study On Treatment Of Micro-polluted Water By Potassium Ferrate

Currently, water resources in many countries of the world, including our country, are polluted by some normal pollutants, just like ammonia nitrogen and organic pollutants. The conventional water treatment processes have lower ability to remove these normal pollutants in micro-polluted water. The pretreatment processes and advanced treatment processes usually have the defects of complex operation, higher costs and secondary pollution even though they can strengthen the treatment effect. According to the problem that potassium ferrate cannot be industrially produced in large scale, this thesis used hypochlorite oxidation method to synthesis potassium ferrate. Taking model micro-polluted water prepared in laboratory as treatment object, this paper mainly researched the effect of ammonia removal by potassium ferrate and its influencing factors; Taking practical micro-polluted water as treatment object, this paper researched the treatment effect of micro-polluted water with two different operation modes: direct oxidation-coagulation-precipitation by potassium ferrate and peroxidation by potassium ferrate respectively, and its influencing factors. This thesis also compared the treatment effect of different operation modes and summarized proper operation parameters which are suitable for practical engineering. The results of these researches indicate that:①Hypochlorite oxidation method can stably synthesis the purple-black crystal of potassium ferrate. After measuring the infrared absorption spectrum and UV-Vis absorption spectrum of this purple-black crystal, this paper confirm that the purple-black crystal is potassium ferrate. The research indicates that the yield of hypochlorite oxidation method to synthesis potassium ferrate is 40.49%. The purity of potassium ferrate is 91.81%, which is determined by chromite titration. The stability of potassium ferrate could be significantly influenced by the value of pH. The higher the pH is, the better stability is achieved.②This research treated model micro-polluted water which contained ammonia nitrogen by potassium ferrate. The results show that removal of ammonia would decrease at first and then increase as long as the value of pH increases. When pH=11, the removal of ammonia would achieve the highest level. Under the condition of alkalinity, the removal of ammonia would increase as long as the reaction time increase. Influence of the dose of potassium ferrate on ammonia removal effect is little. The higher the initial concentration of ammonia is, the better effect of ammonia removal would be achieved. However, whatever the initial concentration of ammonia is, ammonia removal still stay in a low level.③This research treated practical micro-polluted water with the operation mode of oxidation-coagulation-precipitation by potassium ferrate. The results show that both the value of pH and the dose of potassium ferrate have significant influence on treatment effect. For ammonia nitrogen, under the condition of acidity and neutrality, the concentrations of ammonia in treated water samples are higher than that in untreated water samples. And the concentrations of ammonia in treated water samples would increase as long as the dose of potassium ferrate increases. Under the condition of alkalinity, the removal of ammonia mostly comes from function of ammonia stripping. For organic pollutants and turbidity, the removal effects under the condition of acidity are better than that under the condition of alkalinity and neutrality. When pH=3 or pH=5, the turbidity in water samples treated with filtration is lower than the standard limited value of turbidity regulated in Standards for drinking water quality (GB5749-2006). The relationship between the removal of these two kinds of pollutants and the dose of ferrate accords with model of Slogistic well. When the dose reaches 40mg/L (11.31mg/L as Fe) or more, the increment of removal of permanganate index (CODMn) and turbidity becomes slow. After comprehensive consideration, the suitable operation condition is that the value of pH is 5 and the dose of potassium ferrate is 40~50mg/L (11.31mg/L~14.14mg/L as Fe).④In the experiment of practical micro-polluted water treatment with the operation mode of peroxidation by potassium ferrate, the value of pH in peroxidation phase has little influence on the treatment effect. The concentrations of ammonia in the treated water samples are higher than that untreated water samples. However, compared with the operation mode of directly using polymeric ferric sulfate (PFS), the operation mode of peroxidation by potassium ferrate could improve the removal effect of organic pollutants and turbidity. The best dose of potassium ferrate in the peroxidation phase is 5mg/L (1.41mg/L as Fe). The longer the reaction time of peroxidation phase is, the better treatment effect is achieved. After comprehensive consideration, the suitable peroxidation time is 10~15min.⑤The comparison of the effect of different operation modes to treat micro-polluted water indicate that the operation mode of peroxidation by potassium ferrate can obtain the best micro-polluted water treatment effect. Under this operation mode, one group of suitable operation parameters are that in the peroxidation phase, the reaction time is 10~15min, the dose of potassium ferrate is 5mg/L (1.41mg/L as Fe) and the value of pH is 7. When the dose of PFS is 60mg/L, the removal of CODMn is 35.38% and the removal of turbidity after precipitation treatment is 59.15%.