Efficiency And Mechanism Of Decomplexation Of Cr??? Complexes And Simultaneous Removal Of Cr As Cr₂O₃ Precipitat By Photo-Oxidation

Due to the extensive use of organic ligands such as EDTA,NTA and citric acid,Cr??? usually exists in stable organic complexes in industrial wastewater such as electroplating,tannery and metal surface treatment.Most available methods,such as ozone oxidation and electrochemical oxidation,can not effectively remove Cr??? -organic ligands,and face the problem of in-process formation of more toxic Cr.Therefore,new strategies are highly urgent for efficient removal of Cr??? complexes from industrial effluents with development a treatment low Cr??? accumulation and simultaneous removal of Cr.Photochemical oxidation has attracted wide attention in wastewater treatment due to its high efficiency,concurrent oxidation and reduction,and enviromental friendliness.Cr??? -hydroxycarboxylic/ammoniacarboxylic complexes are degraded by ligand-to-metal charge transfer?LMCT? under UV irradiation,and simultaneous production of reducing speices such as organic free radicals,O₂^·- and CO₂^·-,are expected to reduce the accumulation of Cr??? formed in the process.Concurrently,the released Cr??? can be precipitatedas?hydr?oxides under neutral and alkaline conditions.Based on this assumption,Cr-EDTA was chosen to study the perfermance and mechanism of decomplexation of Cr-complexes,in-situ reduction of Cr??? and simultaneous removal of Cr during photo-oxidation.The results showed that photo-oxidation could effectively degrade Cr-EDTA with 100% removal within 60 min,and around 84% of total Cr and 78% of TOC were removed within 180 min.The formation of Cr??? increased first and then decreased with the illumination time,indicating that the accumulation of Cr??? was reduced by in-process formed reducing species.The increase of solution initial pH from 6.0 to 10.0 promoted the removal of Cr-EDTA,total Cr and TOC,while the accumulation of Cr??? was less than 1.5 mg/L,but the efficiency was inhibited when pH continued to increase to 11.0-12.0.The increase of initial concentration and light intensity accelerated the degradation of Cr-EDTA.The decrease of dissolved oxygen was beneficial to the removal of Cr-EDTA and total Cr,and could also reduce the accumulation of Cr??? .The coexistence of Cl^-,Cu²⁺ and Ni²⁺ had little effect on the decomplexation of Cr-EDTA,while the addition of NOM,methanol and tert-butanol had obvious inhibitory effect.Photo-oxidation could achieve a good effect on the treatment of industrial wastewater containing complexed Cr with 65%removal of total Cr and 2.05 mg/L Cr??? formation.The addition of Fe²⁺ can effectively remove the residual total Cr and Cr??? after the treatment of photo-oxidation.The decreasing dissolved oxygen?DO? effectively enhanced the removal of total Cr and abated the accumulation of Cr??? during the photo-degradation of Cr-EDTA.When DO is less than 5 mg/L,residual Cr and accumulated Cr??? was less than 1.0 mg/L and 0.1 mg/L,respectively.Compared with non-aerated N₂?DO?8 mg/L?,aerated N₂?DO?5 mg/L?could increase the total Cr removal and the reduction of Cr??? in a wide range of pH 6.0-10.0 and the Cr-EDTA concentration of0.05-0.25 mM,and less affected by coexisting ions and organic matter.The efficiency of treatment of realisitic complexed Cr wastewater under aerated N₂was also better than that of non-aerated N₂,with 83% removal of total Cr and accumulated Cr??? less than 0.22 mg/L.XRD results of precipitates showed that released Cr during photo-oxidation precipitated as Cr₂O₃.The formation of ammonia nitrogen and the maintaining neutral oralkaline pHs during photo-oxidation were in favour of the hydrolysis of Cr³⁺to Cr₂O₃.The variation of FT-IR spectra before and after Cr-EDTA degradation showed that carboxyl group and C-C bond were significantly weakened,suggesting that and thedegradation of Cr-EDTA was a successive decarboxylation processbased on 70-80% TOC mineralization.With the negligible accumulation of Cr??? under aerated N₂?<0.1mg/L?,the C-centered radicals produced from the?-fragmentation of O-centered radicals induced by LMCT may be responsible for in-situ reduction of Cr??? in the photodegradation process,thus resulting in the low accumulation of Cr??? .The decomplexation of Cr-EDTA by photo-oxidation was a stepwise decarboxyltion process involving LMCT followed by hydrolytic precipitation of the released Cr.