Degradation Efficiency And Mechanism Of The Cu(Ⅱ) Activated Sulfite System For Iodinated X-ray Contrast Media

Iodinated X-ray contrast media（ICM）is a kind of synthetic iodinated organic compounds,which is used to enhance the effect of medical imaging and injected（or taken）into human tissues or organs.Although most ICMs have strong hydrophilicity,they are not easy to be absorbed by human body,and more than 90%of them are discharged with human excreta after24 hours.Traditional water treatment processes such as activated sludge process and biofilm process are difficult to remove ICMs,so more effective methods are urgently needed to degrade ICMs.Advanced oxidation processes（AOPs）based on hydroxyl radicals（·OH）and sulfate radicals(SO₄^·-)have a good degradation effect on organic pollutants,so they are widely used in the field of water treatment.Transition metal activated sulfite system is a new advanced oxidation system,which can produce·OH、SO₄^·-and SO₅^·-.In this study,the Fenton-like advanced oxidation system was constructed by using transition metal Cu（Ⅱ）and sulfite（SI）.The degradation of DTZ and IPM was investigated under aerobic and anaerobic conditions respectively.（1）The degradation of DTZ by Cu（Ⅱ）/SI system under aerobic condition was studied.When the initial p H was 9,[Cu（Ⅱ）]=10μmol/L,and[SI]=0.5mmol/L,the DTZ with the initial concentration of 5μmol/L could be degraded by about 60%in 60 minutes in the presence of oxygen.The free radical quenching experiments confirmed that·OH was the main reactive species in DTZ degradation,ESR experiment detected DMPO-SO₃^·-signal,confirmed the path of free radical chain reactions.The experimental results of influencing factors showed that:when the Cu（Ⅱ）dosage increased from 1μmol/L to 100μmol/L,DTZ degradation efficiency increased firstly and then decreased,and reached the maximum when[Cu（Ⅱ）]=10μmol/L.When the SI dosage rose from 0.1 to 1.0mmol/L,DTZ degradation efficiency increasesd first and then decreases,and the best degradation effect was achieved when[SI]=0.5mmol/L.Alkaline condition was good for DTZ degradation and the degradation efficiency of DTZ was the highest at p H9.The presence of Cl^-,HCO₃^-and humic acid could inhibit the degradation of DTZ.Eleven degradation products of DTZ were determined by HPLC-Triple TOF MS.According to the structure analysis of DTZ products,the main degradation paths of DTZ under aerobic conditions were:deamidation hydroxylation,methyl hydroxylation,deiodination/decarboxylation hydroxylation,intramolecular cyclization,hydroxyl oxidation,deacetylation.（2）In neutral water,when[Cu（Ⅱ）]=100μmol/L and[SI]=2.0mmol/L,DTZ could be effectively degraded without oxygen supply（without aeration and agitation）,and more than95%of DTZ could be degraded within 60min.Methanol,tert butanol and N₂ purging had no effect on the degradation of DTZ,but O₂ purging could obviously inhibit the degradation of DTZ.The degradation effect of SO₃^·-was eliminated by experiments.It was determined that the reactive species of DTZ degradation under this condition was Cu（Ⅰ）（and its complexes）,and the Cu（Ⅰ）was detected during the reaction.The degradation curve of DTZ satisfied the first-order kinetics,and the reaction rate constant was 0.050min^-1.With the increase of Cu（Ⅱ）dosage（10-500μmol/L）,DTZ degradation first increased and then remained unchanged,and it was nearly completely degraded when[Cu（Ⅱ）]=100μmol/L.When SI dosage increased from0.5mmol/l to 2.0mmol/l,the degradation efficiency of DTZ increased;when SI dosage increased from 2.0mmol/l to 4.0mmol/l,the final degradation efficiency did not change.The degradation efficiency of DTZ increased first and then decreased with the increasing of p H,and the highest degradation efficiency was when p H=7.HCO₃^-and humic acid had no significant influence on DTZ degradation.Seven DTZ degradation products were determined by HPLC-Triple TOF MS.By the product structure analysis,the main degradation paths of DTZ under the condition of no oxygen supply were obtained:deiodination/dehydrogenation and hydroxylation,dehydration and cyclization,decarboxylation hydroxylation,carbonyl hydrogenation,decarboxylation and hydrogenation,etc.（3）The degradation of IPM by Cu（Ⅱ）/SI system was studied under the conditions of air exposure as aerobic condition and N₂ exposure as anaerobic environment.Through the free radical quenching experiments and factor exclusion experiments,the reactive species of Cu（Ⅱ）/SI system in IPM degradation under aerobic/anaerobic conditions were free radicals(·OH and SO₄^·-)and Cu（Ⅰ）（and their complexes）,respectively.Under the aerobic condition,the degradation efficiency of IPM increased first and then remained unchanged with the increasing of both Cu（Ⅱ）and SI dosages,and increased with the rise of initial p H.Under anaerobic condition,the degradation efficiency of IPM improved first and then remained unchanged with the increasing of Cu（Ⅱ）dosage,and increased with the increasing of SI dosage.When SI concentration≥2.0mmol/l,the degradation efficiency increased slightly;when the initial p H elevated,the degradation efficiency of IPM first increased and then decreased,and reached the maximum at p H7,and the yield of Cu（Ⅰ）was also the largest.With the same concentration of Cu（Ⅱ）and SI（[Cu（Ⅱ）]=50μmol/L and[SI]=1.0mmol/L）,IPM could be effectively degraded by Cu（Ⅱ）/SI system under both aerobic and anaerobic conditions.Cl^-and humic acid could inhibit the degradation of IPM in aerobic condition,but not in anaerobic condition.The degradation of IPM was accompanied by the formation of inorganic iodine.Under aerobic condition,the iodine on IPM molecule trasferred to the form of iodine ions and iodate ions,and iodate ions were the majority.Under anaerobic condition,only iodine ions were formed.In general,the higher the degradation efficiency was,the greater the amount of inorganic iodine would be.The degradation experiments of IPM in lake water and tap water were carried out.The results proved that the removal of IPM with air purging was not ideal,while DTZ degradation rate was much better when purging N₂ whether in the lake or in the tap water and had not much different from the efficiency in pure water.When Cu（Ⅱ）/SI system was used under aerobic condition,it was greatly affected by the background composition of water body.The optimization experiment should be carried out according to the water quality,while this disadvantage can be overcome under anaerobic condition.