Production Of Active Free Radicals And Removal Of Nitrobenzene And Aniline In Soil By Dithionite

Nitrobenzene is a potentially carcinogenic and ecotoxic organic pollutant often released into the environment.Remediation of soil contaminated with nitrobenzene is met with a number of challenges related to the nature of the molecule and matrix effect of the medium.The electron-withdrawing effect of the nitro functional group of nitrobenzene renders bioremediation ineffective.Similarly,sulfate radical-based advanced oxidation is equally affected by the same phenomenon,while the medium matrix effect limits hydroxyl radical effectiveness.Reduction and subsequent oxidation are reported as the most promising technique for the degradation of nitrobenzene.To implement the reduction and subsequent oxidation of nitrobenzene two technical routes;two-stage and single-stage are followed.In a two-stage system,the reduction stage is separated from the oxidation stage,whereas in a single-stage system,both reduction and oxidation can occur concurrently within the same reactor.In this study,we investigated the use of dithionite for a single-stage reduction and subsequent oxidation of nitrobenzene in soil medium under aerobic condition.The use of dithionite for reductive remediation of soil and sediment contaminated with nitroaromatic compounds have ample literature support.We speculate that,treating contaminated soil in a reactor,dithionite may interact with soil to generate oxidative radicals particularly sulfate radical.To the best of my knowledge dithionite has not been used to generate sulfate radical in this manner.If successful,it will be a new source of sulfate radical for the transformation of organic pollutants other than persulfate.This research aims to demonstrate the generation of oxidative radical by following the generation of intermediate of a radical probe.Following this,reduction and oxidation of nitrobenzene in soil is demonstrated.Finally,the degradation of aniline which is the primary reduction product of nitrobenzene is treated using dithionite added multiple times in the reactor.The main results and conclusion are as below.(1)Generation of oxidative reactive radicals in the soil using dithioniteThe generation of oxidative reactive radicals was determined by following the generation of para-hydroxybenzoic acid(p-HBA)from the oxidation of sodium benzoate.During the experiment,soil slurry in a 250 m L open Erlenmeyer flask wrapped in thin foil was shaken at 220 rpm.Each reactor contained 20 m M sodium benzoate and a different concentration of dithionite.Samples were taken at a different time interval,and the concentration of p-HBA determined using HPLC.The result showed that p-HBA generation increased with increasing dithionite concentration from2.5 to 10 m M.A further increase of dithionite to 20 m M led to a lag phase appearance within the first 60 min of the reaction,whereas p-HBA was not generated at 50 and 100m M dithionite.The concentration of dissolved iron in the aqueous phase was strongly controlled by dithionite concentration.Additionally,the p-HBA generated reduced significantly in the presence of 2,2'-Bipyridine(BPY),an Fe(II)chelating agent,and under anaerobic condition indicating that Fe(II)and oxygen were essential for the generation of p-HBA,respectively.Similarly,catalase and Nitro Blue Tetrazolium(NBT)reduced p-HBA generation indicating the possibility of hydrogen peroxide and superoxide as intermediate in the system.In order to demonstrate the dominant radicals,benzoate was used as model contaminant and various concentrations of tert-butanol and ethanol was added in the reactor.While ethanol significantly reduced the degradation of benzoate,tert-butanol did not have noticeable effect.Therefore,it was concluded that sulfate radical was the predominant radical within the system.The influence of essential factors such as p H,temperature and soil dosage were equally investigated.It was shown that p-HBA generated increased as potassium carbonate concentration increased from 0to 5 and 10 m M,whereas at 20 m M,p-HBA generation was suppressed.The influence of temperature within a range of 25 to 45^oC did not show an appreciable effect on p-HBA generation.An increase in soil dosage generally led to the rise in p-HBA generation.An assessment of soil using FTIR,XRD and XPS revealed the iron cycling in the soil during the reaction was essential for radical generation.(2)Reduction and oxidation of nitrobenzene in the dithionite/soil slurry/O₂systemHaving demonstrated the generation of sulfate radical,the reduction and subsequent oxidation of nitrobenzene was demonstrated in chapter four.The experiments were conducted in a similar under the same condition as the radical generation.10 g of soil contaminated with 328.5±0.7 nitrobenzene was used.The concentration of aniline and nitrobenzene in the aqueous phase was monitored at 20 min interval for 120 min.After the 120 min,the soil was filtered from the mixture using a vacuum pump.The residual concentration of pollutants in the soil was determined by extracting the soil residue using 100%methanol.From the result,it was observed that without dithionite,the concentration of nitrobenzene did not change noticeably.However,when dithionite was added,nitrobenzene concentration decreased rapidly by over 93%within 20 min for all dithionite concentration.The concentration of aniline generated was equally measure simultaneously.It was observed that concentration of aniline generated reduced with increasing dithionite concentration.Surprisingly,aniline was not generated within the first 100 min at 50 m M dithionite but rose afterwards while at 100m M dithionite aniline was not generated throughout the period of the experiment.The result of the reduction and subsequent oxidation of nitrobenzene was contradictory to the pattern of radical generation in chapter three.Because radical was not generated at 50 and 100m M dithionite,it was not expected to result in aniline degradation.In addition,the rise in aniline concentration at 50 m M dithionite after about 100 min of disappearance indicated a process other than degradation process was taking place.In order to investigate the process taking place,aniline was used in place of nitrobenzene.The pattern was similar to the observation at 50m M dithionite using nitrobenzene.Aniline concentration fell initially and then began to rise.The depth to which aniline fell and length of time before subsequent rise was dependent on dithionite concentration.Anaerobic experiment showed conducted showed that the phenomenon was promoted under anaerobic condition.When the soil was replaced with quartz the phenomenon did not occur but when Fe(II)was added to the quartz aniline fell and rose as was observed in soil slurry medium.When aniline was replaced with phenol did phenomenon did not occur indicating that it may be unique to aniline.From the result it was proposed that a reversible complex forming between aniline and sulfur dioxide must responsible.Aniline-sulfur dioxide adduct has abundant literature support and it is said to be a Lewis acid-Lewis base complex with the amino group acting as a Lewis base.The sulfur dioxide can be generated as an intermediate in dithionite decomposition.(3)Main factors and mechanism on the degradation of nitrobenzene in the dithionite/soil slurry/O₂systemBecause it was not possible to demonstrate oxidative degradation of aniline due to the unexpected phenomenon,it was speculated that the addition of dithionite in small doses over time would give a more reliable and interpretable result.This speculation was investigated in chapter 5.The generation of radical with multiple dithionite addition was first demonstrated.Following this,the oxidation of aniline in considerable dithionite addition was evaluated.All the experimental conditions and set up was similar to that used for radical generation in chapter 3.From the results,it was observed that addition dithionite in small doses significantly increased the radical generated.With regard to aniline degradation,it was observed that increasing the mass of dithionite added per time,the rate and extent of aniline degradation increased.However,the trend of the aniline was undulating as the mass of dithionite increased.The rate and extent of degradation decreased with increasing initial aniline concentration,perhaps due to limiting radicals or competition with degradation intermediates.The addition of potassium carbonate reduced the rate and extent of aniline degradation,which may be due to competition by carbonate ions.Similarly,increase in soil dosage reduced aniline degradation.Aniline degradation did not occur when soil was replaced with quartz,indicating that soil factor was essential.Surprisingly,under anaerobic condition,aniline concentration fell for the first 3hr at an undistinguishable rate as the aerobic condition,but after 3 hr,the aniline concentration rose to full recovery.While under aerobic conditions,the aniline concentration kept decreasing.Competitive kinetics shows that sulfate radical was the predominant radical within the system,which is similar to the conclusion reached in chapter 3.HPLC-MSMS was used to screen the presence of intermediate.It was discovered that aminophenol and hydrazobenzene were present.Overall,it may be concluded that dithionite can reduce nitrobenzene to aniline and simultaneously generate sulfate radicals in the System.However,it may not be possible to distinguish the oxidative degradation of aniline and the possible complexation reaction of aniline with dithionite.It is speculated that the complexation phenomenon is unique to aniline and only occurs in the presence of Fe(II)and under anaerobic condition or high dithionite concentration.Further studies may be required to have a full understanding of the phenomenon.As an innovation,this study explored a new technology for the removal of nitrobenzene in soil by dithionite.Additionally,it demonstrated the reduction and oxidation mechanism of nitrobenzene from soil by dithionite.