| Ferrate(?)(abbreviated as Fe(?))has raised increasing interest in water treatment and is a promising potential water treatment chemical because of its multifunction as disinfectant,oxidant,flocculating agent,adsorbent and aggregation aid.Here,the preparation of Fe(?)and its application in removing heavy metal,organic pollutants from aqueous solution and the disintegration of waste activated sludge by newly prepared alkaline ferrate(NPAF)solution was investigated.Firstly,the preparation process of Fe(?)was optimized in this study on the basis of previous studies.Alkaline solution of hypochlorite was prepared through the addition of potassium hydroxide into sodium hypochlorite solution.Hypochlorite solution with effective chlorine concentration of 105 g/L could be obtained when the dosage of KOH is 9 mol/L and mixing temperature is 0 ?.With the optimization of various factors such as reaction ratio,temperature,type of iron salts,operation process of filtration and washing,67% yield of ferrate solution could be produced after the 60 min reaction of hypochlorite oxidant and ferrous sulfate with ratio of 4:1 at 20 ?.Finally,a ferrate chemical with 90% purity could be produced after the process of crystallization,filtration and washing.Thallium is highly toxic to mammals and relevant pollution emergencies are increasing world-widely.Here,the removal of trace Tl from aqueous solution with Fe(?)has been investigated systematically in this study for the first time.Over 90% of Tl(1 ?g/L)was removed after 5 min reaction by applying 2.5 mg/L of K2 Fe O4.The removal of Tl with Fe(?)shows a p H dependent behavior and has a better performance in neutral or basic conditions.No apparent impact on the Tl removal was observed with the coexistence of K+,Na+,Ca2+,Cl-and HCO3-.However,the presence of humic acid(HA)significantly inhibits the removal of Tl with Fe(?).The influence of several factors on the release of captured Tl from ferric particles was investigated.It was found that the ion strength showed no apparent influence on the desorption of Tl and elevation of temperature from 20 ? to 30 ? reduced the Tl removal ratio by 10%.Also,the decrease of solution p H from 7.0 to 3.5 decreased the Tl removal ratio by 30%.A 10 mg/L dosage of hydroxylamine hydrochloride would induce over 60% of the captured Tl to desorb from the ferric particles.The operation procedure and parameters for the removal of Tl by Fe(?)were also investigated and optimized.It was found that 1min of Fe(?)preoxidation followed with 20 min of poly aluminium chloride flocculation and 20 min of sedimentation,resulted in approximately 98% removal ratio of total Tl.Besides the removal of trace Tl from aqueous solution,the Fe(?)pre-oxidation coupled with poly aluminium chloride flocculation could simultaneously eliminate chemical oxygen demand and turbidity from raw water,with the concentrations of residual Fe and Al ions below relevant standards.The removal performance of trace Tl by in-situ formed and exsitu formed ferric particles was examined respectively,and the results revealed the important role of the ferric nanoparticle reduced from Fe(?).Surface characterization of ferric particles was determined by scanning electron microscope and X-ray photoelectron spectroscopy to disclose the function mechanism.It is speculated that the removal of Tl could be attributed to the combination of adsorption and co-precipitation processes.Indole is considered as a potential precursor of the nitrosamine disinfection byproducts due to its nitrogen-containing heterocyclic ring.Here,the elimination of trace indole from aqueous solution by Fe(?)and the degradation pathway were investigated deeply.Indole could be effectively degraded by the oxidation of Fe(?).The apparent second order rate constants between Fe(?)and indole in weakly acidic and neutral conditions were 692 ± 26 M-1s-1 and 186 ± 16 M-1s-1 respectively,indicating a relatively higher reactivity in acidic and neutral conditions.The existence of Ca2+,Mg2+,NTA and low concentration of ethylene diamine tetraacetic acid would enhance the degradation of indole by Fe(?).Comparatively,the presence of pyrophosphate and high concentration of ethylene diamine tetraacetic acid suppressed the oxidation of indole by Fe(?).Both high concentration(2.5 ?M)and low concentration(0.5 ?M)of indole in real water could be effectively removed by Fe(?).Some oxidation products were identified with mass spectrometry and compared with standard chemicals to assure the accuracy and a tentative reaction pathway was proposed.The biodegradability of the oxidation products assessed by assimilable organic carbon shows that low dosage of Fe(?)([Fe(?)] : [indole] < 6 : 1)could decreese the biodegradability whereas high dosage of Fe(?)([Fe(?)]:[indole] > 6:1)has no apparent influence.Based on the efficient removal of thallium and indole by Fe(?),this study further investigates the direct utilization of newly prepared alkaline ferrate solution into the disintegration of waste activated sludge,to expand the application field of ferrate.Because of the direct usage of newly prepared alkaline ferrate solution,the process of crystallization,filtration,washing and drying during the preparation of Fe(?)could be avoided,greatly simplifying the operation procedure and lowering the cost.Further,both the oxidation by Fe(?)and alkaline hydrolysis could be obtained in such single dose of NPAF solution.The disintegration of sludge by Fe(?)alone,alkaline alone and newly prepared alkaline ferrate solution was studied comparatively.The results demonstrated that 700 mg Fe(?)/L dosage of the newly prepared alkaline ferrate solution could effectively disrupt sludge structure and simultaneously improved the sludge settleability and dewaterablity by >50% and 7% respectively,while the single use of Fe(?)or alkaline shows no significant improvement.The concentrations of soluble chemical oxygen demand,total organic carbon,total nitrogen and total phosphorus in the supernatant were increased with the application of newly prepared alkaline ferrate solution or alkali alone,while the single use of Fe(?)oxidation has a relatively slight influence in sludge disintegration and little increase of dissolved substance in the treated sludge was observed.Besides the destruction of the sludge structure and cell wall,the extracellular or intracellular macromolecular matters were also oxidized by Fe(?)during the treatment by newly prepared alkaline ferrate solution,which was evidenced by the scanning electron microscope,excitation-emission matrix spectroscopy and the analysis of molecular weight distribution.Comparatively,little degradation of macromolecular substances by alkali alone was observed.The variation of Fe(?)concentration during the disintegration of sludge was determined and the results showed that the presence of alkali in newly prepared alkaline ferrate solution improved the effective concentration of Fe(?)as well as the aggregation and settling performance of the in-situ formed ferric particles,resulting the highly efficient volume reduction and dewatering of the sludge.Furthermore,the application of newly prepared alkaline ferrate solution on sludge has an excellent performance on the odor control. |
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