| As the dramatical increasing of alkaline wastewater with high salinity from metallurgy, pharmaceutical, printing and dyeing, food industry, it has been the focus of wastewater treatment owing to its great damage to environment. However, water treatment technology such as membrane separation, chemical oxidation, electrochemical treatment, biochemical treatment can often show low efficiency in the disposal of alkaline wastewater with high salinity. Large quantities of anions and cations in wastewater lead to high operating pressure on the membrane separation, low selectivity of oxidization rate to organics, and high energy comsumption in electrochemical treatment, respectively. Moreover, the efficiency of the biological treatment is also very low because the extremely high concentration of salinity slows down the metabolization of microorganisms. Futhermore, non-degradable pollutant, only can be oxidated by radicals with high oxidation potential which is generated from Advanced Oxidation Processes (AOPs). High concentration of chloride anions in wastewater will consume large amount of free radicals and other ions will lower the genertation of free radicals in AOPs. Thus, it is not feasible for the most conventional AOPs methods are used to treat alkaline and hypersaline non-degradable wastewater.In this study, we introduce a new method of AOPs in water treatment with alkaline and hypersaline non-degradable wastewater by in-situ synthsis of ferrate. Ferrate can rapidly be disproportionated to ferrate intermediates and free radicals with high oxidation potential, which can react with organic pollutants. We intend to use the high oxidation potential of ferrate intermediates and free radicals in the alkaline and hypersaline wastewater and develop an efficient chemical oxidation method to treat with wastewater.In our work, the oxidation efficiency of an alkaline and hypersaline EDTANa2 solution, as an imitated wastewater, treated by Fenton reaction, PMS/Co system and hypochlorite oxidation was evaluated. The results showed that all oxidation efficiency of conventional AOPs dramatically decrease in the high chloride anion and alkaline environment (pH= 11, the chloride anion concentration is 10000mg/L). While maintaining the oxidant’s theoretical amount of oxygen is 500mg/L, the efficiency of Fenton reaction, PMS/Co oxidation and hypochlorite oxidation decreased from 43.6% to 6%, from 22.8% to 6.8% and from 32% to 18%, respectively when addional chloride anion (10000mg/L) added in imitated wastewater. Thus, it is not feasible for these three conventional AOPs methods to treat alkaline and hypersaline non-degradable wastewater due to the extremlly low oxidation efficiencies.Therefore, the oxidation efficiency of an in-situ ferrate oxidation system, using sodium hypochlorite as oxidant and ferric ions as catalyst, was investigated to treat the imitated alkaline and hypersaline wastewater. The effects of reaction pH, chloride ion concentration, the amount of oxidant, catalyst/oxidant molar ratio, reaction temperature and other factors in in-situ ferrate oxidation system were investigated.The results showed that the ferric ions performed significant catalytic effect under alkaline conditions so that the oxidation efficiency is significantly improved. The oxidation efficiency was enhanced and up to 50% or more as the pH increasing and reduced as high concentration of chloride anion added. However, the oxidation efficiency decreased from 50.4% to 40.8% when the chloride ion concentration increased from 0 mg/L to 20000mg/L.On the other hand, the oxidation efficiency of in-situ ferrate oxidation system with sodium chlorate as oxidant shows a similar advanced oxidation capacity as that of sodium hypochlorite system under the same reaction condition as pH, the amount of oxidant and temperature. It is found that the chloride anions can enhance the oxidation process of in-situ ferrate oxidation system with sodium chlorate as oxidant. And the oxidation efficiency increased 8.8% when 20000mg/L chloride anion was added into the solution.Possible reaction mechanisms of in-situ ferrate oxidation system were investigated. In the sodium hypochlorite system, it is considered that catalyst ferric ions should be oxidized to ferrate(VI) cations by oxidant sodium hypochlorite, and then the ferrate(VI) cations continuously generate Ferrate(V) cations by taking a single electron from the organics or through disproportionation reaction. Ferrate(V) cations could futher oxidized organics and returned to ferric ions as a cycle. Both ferrate and ferrate(Ⅴ) cations were effective oxidants to enhance the oxidation process. In the sodium chlorate system, ferric ions were oxidated to ferrate(Ⅳ) cations by oxidant sodium chlorate, and the ferrate(Ⅳ) cations were disproportionated to ferrate(Ⅵ) ions and ferric ions. Both ferrate(Ⅳ) and ferrate(Ⅵ) cations can degrade organics by radical reaction and return to ferric ions. Meanwhile, chlorate ions and high cencentraion chloride ions may form hypochloride ions in alkaline solution so that hypochlorite in-situ ferrate process also exist in chlorate system and showed stronger oxidation ability in alkaline and hypersaline environment owing to their synergitic effectWe also investigated the in-situ ferrate oxidation system in industrial scale wastewater treatment. The industrial wastewater was composed of organic extractant, metal cations and large amount of chloride ions. Organic extractant was not only the main source of water COD, but also difficult to degradation comparing to aromatic compounds or dyes owing to its phosphate structure consisting of short-chain aliphatic hydrocarbons. A cobalt extraction wastewater, (COD>400mg/L, Cl-> 10000mg/L and pH= 10.0,500-600t/day), has been treated with 4 unit operation including oxidation, flocculation, separation and neutralization. The wastewater treatment processe of the above cobalt extraction includes 6h oxidation by 0.1% sodium chlorate (w/v) and 0.04%(w/v) polymeric ferric sulfate at 50℃ under pH 9.5-10.0, and then being followed by 0.5-1h flocculation with the adding 0.001% polyacrylamide. Moreover, the oxidation process is very robusted. The increase of COD in wastewater can be removed through increased oxidant amount with only slight reduction of the oxidation efficiency. According to a long period of operation, the wastewater treatment by in-situ ferrate oxidation system is fully compatible with the cobalt manufacture processes, and the results were consistent with the laboratory results. The cost of reducing 100mg/L COD per ton wastewater was 2.5～3 yuan. Since in situ ferrate oxidation with sodium chlorate as oxidant is mild, environment-friendly and cost effective technology, and show great advantages in alkaline wastewater treatment with high salinity, it is reasonable to consider in-situ ferrate oxidation system will play important role in the disposal of wastewater with advanced oxidation processes. |
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