Study On Degradation Characteristics And Mechanisms Of Typical Chlorinated Aromatic Compounds In Ultrasound/zero-value-iron System

Many chlorinated aromatic compounds (CACs) are persistent organic pollutants due totheir resistance to biodegradation and chemical degradation. They are hazardous toenvironment and have a wide range of pollution. The collaborative technology of ultrasonicand zero-valent-iron (US/Fe~0) is an advanced oxidation technology developed in recent years.Its application has been widespread concern in the environment. Many typical chlorinatedaromatic compounds including chloronitrobenzenes (CNBs), chloroanilines (CAs) andchlorophenols (CPs) were degraded in US/Fe~0system in this study. The optimal reactionconditions, the reaction kinetics and degradation mechanism were explored and thequantitative structure-property relationships (QSPR) were researched. As a result, thefollowing main conclusions were obtained:The optimal conditions for p-chloronitrobenzene degradation are pH=3and the ironpowder dosage of2.0g/L. The pseudo-first-order kinetic constants of7CNBs were measuredunder the optimum conditions. By the detection of the intermediate product and thetheoretical calculation, the degradation pathway was speculated for the nitro graduallyhydrogenation and ultimately generating chloroanilines. Meanwhile, other substances as4,4-dichloroazoxybenzene were generated by the intermediates coupling reactions.The Optimal conditions of p-chloroaniline degradation are pH=9, iron dosage of2.0g/Land the initial concentration of0.1mmol/L. Under these conditions, the pseudo-first-orderkinetic constants of7CAs were measured. By the detection of the intermediates and inorganicanion products, the degradation pathways of p-chloroaniline were speculated that they arepyrolysis and hydroxyl radical oxidation.The optimal conditions of2,3,4,6-TeCP degradation are pH=6and the iron dosage of2.0g/L. Under these conditions, the pseudo-first-order kinetic constants of13CPs weremeasured. The degradations of CPs are in conformity with the pseudo-first-order reactionkinetic model and have good correlation.The research of synergy mechanism showed that the ultrasonic cavitation causes the pHvalue reducing, but the addition iron powder can significant stabilize the pH value of system.The dosage iron powder will affect hydroxyl radical and Hydrogen peroxide generation. Thehigher the initial pH value of the solution, the stronger the effect of ultrasonic cavitation andmore hydroxyl radical and Hydrogen peroxide generation. In contrast, the corrosion of ironpowder is stronger and the concentrations of Fe2+and Fe3+are higher in system. The study showed that the chloronitrobenzene degradation is mainly due to the reduction of the ironpowder and the ultrasonic cleaning of the iron powder surface can enhance the role of the ironpowder. However, p-chloroaniline degradation was mainly due to the role of hydroxyl radicalgenerating from ultrasonic cavitations and it can be strengthen with iron powder added intothis system. Ultrasonic pyrolysis is the secondary cause of p-chloroaniline degradation.The QSPR models of CACs, CNBs and CPs degradation in US/Fe~0system wereestablished using partial least squares (PLS), respectively. The QSPR model of CACs showsthat the degradation rate is positively correlated with dipole moment(dipole), final heat offormation(HOF), ionization potential(IP), molecular surface area(Sm), molecular Volume(Vm),molecular weight(Mw) and is negative correlation with the energy of the lowest unoccupiedmolecular orbital (Elumo), the energy of the second lowest unoccupied molecular orbital(Elumo+1), the energy of the second highest occupied molecular orbital(Ehomo-1), the energy ofthe highest occupied molecular orbital(Ehomo), the logarithm of the partition coefficient forn-octanol/water(logKow). It explains the degradation of CACs is mainly due to the radicalreaction and the degradation rate depends primarily on the capability of CACs capturingelectronic. The QSPR model of CNBs shows that the degradation rate is positively correlatedwith Sm, total energy(TE), Vm and is negative correlation with Mw, Elumo+1, molecularhardness (Elumo-Ehomo). It explains that the degradation of CNBs is harder with less chlorineatoms and greater molecular hardness. The QSPR model of CAs shows that the degradationrate is positively correlated with the core–core repulsion energy (CCR), Elumo, Sm, averagemolecular polarizability (polar), Vm, Mwand is negative correlation with HOF. It explains thatthe degradation of CAs is easier with more chlorine atoms, greater CCR and lower Elumo.