Enhanced Effectiveness And Mechanism Of Fe²⁺/Persulfate System With Hydroxylamine

In recent years, advanced oxidation technologies（AOTs） based on the activation of peroxymonosulfate（PMS） and peroxydisulfate（PDS） have gained increasing attention for the generation of reactive oxygen species such as sulfate radical（4SO-·） and hydroxyl radical（ ·OH）. Owing to the advantages of cost effectiveness and high activity, Co²⁺ and Fe²⁺ have been widely used as the activator of PMS and PDS to generate 4SO-· and ·OH for the degradation of organic contaminants in water treatment. Unfortunately, the activation of PMS and PDS with Co²⁺ and Fe²⁺ have some intrinsic drawbacks, including the addition of toxic cobalt ions, the slow transformation from Fe³⁺ to Fe²⁺, and the accumulation of ferric oxide sludge, which limit their widespread application. Therefore, the spectrophotometric methods for the rapid determination of PMS and PDS were proposed in this thesis, which were based on the degradation of organic contaminants by the activation of PMS and PDS with Co²⁺ and Fe²⁺. Additionally, hydroxylamine（HA） have been introduced into Fe²⁺/PMS and Fe²⁺/PDS systems to alleviate the intrinsic drawbacks, including the narrow p H range, the slow transformation from Fe³⁺ to Fe²⁺, and the accumulation of ferric oxide sludge.Fistly, owing to the rapid generation of 4SO-· and ·OH in Co²⁺/PMS, Fe²⁺/PMS and Fe²⁺/PDS systems, methyl orange（MO） and N,N-diethyl-p--phenylenediamine（DPD） have be used as the indicators for the rapid determination of PMS and PDS with the developed Co²⁺/PMS-MO, Fe²⁺/PMS-DPD and Fe²⁺/PMS-DPD spectrophotometric methods. The effects of reaction time, initial solution p H, initial MO or DPD concentration, initial Co²⁺ or Fe²⁺ concentration, and the concentrations of common coexisting foreign species on the determination of PMS and PDS with the developed spectrophotometric methods w ere investigated. The developed spectrophotometric methods have been used for the determination of PMS and PDS in four different types of practical samples（lake water sample, groundwater sample, river water sample and a tap water sample）, and the results were as satisfactory as that obtained by the classical iodometric method, which indicates that there were no significant differences between the developed spectrophotometric methods and the classical iodometric method.Secondly, HA was introduced into Fe²⁺/PMS and Fe²⁺/PDS systems for the degradation of benzoic acid（BA）. The influences of Fe²⁺ concentration, PMS or PDS concentration and initial solution p H on BA degradation were investigated and compared in Fe²⁺/PMS and Fe²⁺/PDS systems with or without the addition of HA. The effects of common reducing agents(HA, Na2S2O3, Na HSO3, Na NO2 and ascorbic acid（vitamin C, VC） on BA degradation were also investigated in Fe²⁺/PMS and Fe²⁺/PDS systems. The results showed that BA degradation was improved and p H range was extended with the addition of HA into Fe²⁺/PMS and Fe²⁺/PDS systems. Meanwhile, BA degradation in Fe²⁺/PMS and Fe²⁺/PDS systems with the addition of HA was much more higher than that in Fe²⁺/PMS and Fe²⁺/PDS systems with the addition of Na2S2O3, Na HSO3, Na NO2 and VC.Thirdly, the enhanced effectivenesses of Fe²⁺/PMS and Fe²⁺/PDS systems with the addition of HA were investigated using BA as the probe compound. The influences of HA concentration, initial BA concentration, common coexisting foreign species in practical samples, including the inorganic negative anions of Cl-, NO3-, SO42-, Br O3- and PO43-, the inorganic positive caions of Na+, K+, NH4+, Ca2+ and Mg2+, the organic compounds of formaldehyde, formic acid, acetic acid, oxalic acid and ethylene diamine tetraacetic acid（EDTA）, the natural organic matter of humic acid, and the water background of lake water, groundwater, river water, and tap water, on BA degradation were investigated in Fe²⁺/PMS and Fe²⁺/PDS systems with the addition of HA. The results showed that Fe²⁺/PMS and Fe²⁺/PDS systems with the addition of HA were well effective to tolerate the interferences of common coexisting foreign species in practical samples.Lastly, the enhanced mechanisms of Fe²⁺/PMS and Fe²⁺/PDS systems with the addition of HA were investigated using BA as the probe compound. Variations of Fe³⁺ concentration, PMS or PDS concentration, and the concentrations of HA and its end products were investigated in Fe²⁺/PMS and Fe²⁺/PDS systems with HA. Meanwhile, the primary reactive radicals for BA degradation were identified. The results showed that the addition of HA into Fe²⁺/PMS and Fe²⁺/PDS systems accelerated the transformation from Fe³⁺ to Fe²⁺ and the decomposition of PMS or PDS. Moreover, HA was gradually degraded to N2, N2 O, 2NO-, and 3NO-, while the environmentally friendly gas of N2 was considered as its major end product. The inhibition effects of tert-butyl alcohol（TBA） and methanol（Me） on the degradation of BA and nitrobenzene（NB）, proved that both 4SO-· and ·OH were the major reactive radicals for BA degradation in Fe²⁺/PMS system with HA, while only ·OH was the major reactive radical for BA degradation in Fe²⁺/PDS system with HA. The experiment of electron spin resonance（ESR） using 5,5-dimethyl-1-pyrolin-N-oxide（DMPO） as the radical scavenger further proved that the generation of 4SO-· and ·OH was accelerated with the addition of HA into Fe²⁺/PMS system, and the generation of ·OH was accelerated with the addition of HA into Fe²⁺/PDS system.From aforementioned results and discussions, it was concluded that the Co²⁺/PMS-MO, Fe²⁺/PMS-DPD and Fe²⁺/PDS-DPD spectrophotometric methods, based on the rapid degradation of MO and DPD in Co²⁺/PMS, Fe²⁺/PMS and Fe²⁺/PDS systems, were developed for the rapid, simple, and accurate determination of PMS in aqueous solutions. Additionally, the addition of HA into Fe²⁺/PMS and Fe²⁺/PDS systems significantly accelerated the transformation from Fe³⁺ to Fe²⁺, and subsequently accelerated the generation of reactive radicals and the degradation of organic contaminants. Meanwhile, the addition of HA into Fe²⁺/PMS and Fe²⁺/PDS systems could extend the optimum p H range for the degradation of organic contaminants.