Remediation Of Toxic Cr(?) And/or As(?)-bearing Wastewaters And Investigation Of Their Reaction Mechanisms

Cr?VI?and As?III?are widely distributed in many natural aquatic environments and a variety of relevant industrial effluents.Due to their hypertoxicity,carcinogenicity and stong mobility,Cr?VI?and As?III?are stipulated as priority pollutants.Hence disposal of waste containing Cr?VI?/As?III?pollutants becomes a well-recognized environmental issue of the modern world and till now,many traditional and advanced approaches have been applied for the remediation of Cr?VI?/As?III?-contaminated effluents or sites.In spite of this,these approaches still need to be upgraded for more cost-effective and environmental compatibility.In addition,further studies are still required to inspect the reaction mechanisms of many remediation technologies.In regard to these problems,this study firstly investigated the reaction mechanism occurring in Cr?VI?/sulfite reaction system and examined the applicability of upgrading this reaction system for other inorganic/organic contaminants oxidation.Electron spin resonance and fluorescence spectroscopy were utilized for active radicals detection to probe the underlying reaction mechanisms.The experimental results demonstrate that highly oxiditive species,i.e.,SO₄^·-and^·OH,were produced in Cr?VI?/sulfite reaction system and can oxidize sulfite to sulfate,leading to the excess consumption of sulfite.When inorganic/organic pollutants were introduced into the above reaction system,inorganic/organic pollutants can be successfully oxidized,which retarded the consumption of sulfite and therefore enhanced the reduction of Cr?VI?.Generally,this work provides a new?waste control by waste?strategy for the oxidation of organic/inorganic pollutants,co-occurring with sulfite oxidation and Cr?VI?reduction.At the same time,this upgraded process effectively alleviates the environmental risks,making the process of sulfite reducing Cr?VI?more environmentally favorable.In addition,this study also examined the influence of polycarboxylates on the redox conversions of As?III?and Cr?VI?and provided insight into the role of the interaction between polycarboxylates and chromium species in the electron transfer between Cr?VI?and electron donors.In Cr?VI?/As?III?/oxalate reaction system,oxalate can catalytically drive the redox conversions of Cr?VI?and As?III?.Besides,oxalate can also act as electron donor for Cr?VI?reduction at pH<4.Many other polycarboxylates,such as malic acid,citric acid,tartaric acid,2-picolinic acid,EDTA and nitrilotriacetic acid,can also evidently initiated As?III?oxidation and Cr?VI?reduction.The affinity of above polycarboxylates to chromium species?i.e.,Cr?VI?and Cr?V??probably assists in expanding the coordination sphere of Cr?V?/Cr?VI?and leads to the formation of octahedral activated complexes,which is more favorable for the reduction of Cr?VI?to Cr?III?with octahedral configuration.Based on the above mechanism,oxalate was demonstrated to be an effective activator for scrap zero-valent aluminum reducing Cr?VI?.Furthermore,this study evaluated the effects of polycarboxylates on the reduction of Cr?VI?,the consumption of reductants and the productions of active radicals in Cr?VI?/sulfite and glow discharge plasma/Cr?VI?reaction systems.It was found that the affinity of polycarboxylates?i.e.,malic acid,citric acid,tartaric acid and oxalate?to chromium species?i.e.,Cr?VI?and Cr?V??can enhance the reduction of Cr?VI?to Cr?III?in both above reaction systems.Besides,the polycarboxylates can also act as the electron donors for Cr?VI?reduction,which is dependent on the energies of the highest occupied molecular orbital(E_HOMO)of these polycarboxylates.High values of E_HOMO were favorable for the electron transfer reaction between the polycarboxylates and high valent chromium species.In Cr?VI?/sulfite system,oxalate can improve the productions of active radicals,i.e.,SO₄^·-and^·OH,while others?i.e.,malic acid,citric acid and tartaric acid?inhibited the formations of SO₄^·-and^·OH.As for glow discharge plasma/Cr?VI?reaction system,the presence of polycarboxylates resulted in the evident inhibition of^·OH production.Finally,this study examined the applicability of glow discharge plasma process for simultaneous removal of Cr?VI?/As?III?,or simultaneous arsenic oxidation and immobilization in one process.In glow discharge plasma system,there was a beneficially synergistic effect between Cr?VI?reduction and As?III?oxidation.H₂O₂ generated in-situ in glow discharge plasma can reduce Cr?VI?and produce highly oxidizing^·OH that was responsible for As?III?oxidation.Reaction between As?III?and^·OH in turn partially avoided the re-oxidation of Cr?III?by^·OH to Cr?VI?,thus facilitating the net conversion of Cr?VI?.The glow plasma/Fenton reaction system can also lead to the oxidation As?III?and simultaneous arsenic immobilization in one process.In glow discharge plasma system,Fenton reaction can enhance the formation of^·OH and simultaneously lead to the production of Fe?IV?species,which both accounted for the oxidation of As?III?.The formed ionic As?V?rapidly coprecipitated with Fe?III?ions or was adsorbed on the ferric oxyhydroxides with the formation of amorphous ferric arsenate-bearing ferric oxyhydroxides.In general,this glow plasma/Fenton reaction system was of great interest for engineered systems concerned with the remediation of arsenic containing wastewater.