Distribution And Removal Characteristics Of Polychlorinated Biphenyls(PCBs) In Municipal Sewage Treatment Process

Persistent organic pollutants?POPs?is referred to as"one of the biggest environmental challenges facing the world"by UNEP.With persistence,bioaccumulation,high toxicity and half-volatility,POPs may do incalculable harm to human and ecological system.Polychlorinated biphenyls?PCBs?is one of the most representative POPs.Due to a large number of production and application in various industrial fields in the past,it widely exists in various environmental media,including municipal sewage.PCBs is a huge hidden trouble to human health and safety,no mater in water environment diffusing from municipal sewage or in recycle water.Seven kinds of PCBs?PCB28,PCB52,PCB101,PCB118,PCB138,PCB153,PCB180?recommended by EPA were as the target.By solid phase extraction?SPE?,soxhlet extraction?SOX?and gas chromatography-mass spectrometry?GC-MS?,the present existence situation and pollution level of PCBs in municipal sewage and sludge in Beijing were investigated.As the same time,the distribution of PCBs in the sewage and sludge during sewage treatment process were analyzed.Removal performance and effect of PCBs in three sewage treatment processes?AO,A/A/O,SBR?were compared.In addition,one kind of powdered activated carbon?PAC?and four kinds of carbon nanotubes?SWCNT,MWCNT-8,MWCNT-30,MWCNT-50?were adopted as adsorbents to study adsorption characteristics of PCBs from aqueous solution.In additon,the experimental data were fitted by two adsorption kinetics models?pseudo-first-order and pseudo-second-order?and two adsorption isotherm models?Langmuir and Freundlich?.The results showed that:?1?The concentrations of?PCBs in sewage and activated sludge were both at medium level compared with other areas at home and abroad,which were 18.67⁵6.28 ng?L^-1 and43.13¹76.41 ng?g^–1 respectively.?2?During sewage treatment processes,PCBs were removed mainly depending on the adsorption effect of solid suspended particles and activated sludge.Primary sedimentation tank made a good contribution to the removal of PCBs,and oxic stage the main part among secondary treatments,while high chlorinated PCBs may be degraded to lower chlorinated congeners in anaerobic stage.Sedimentation stage of SBR had higher removal effect to PCBs than secondary sedimentation tank of A/O or A/A/O.Compared with influent,the proportion of low chlorinated PCBs increased and the proportion of high chlorinated PCBs decreased in secondary effluent.The proportion of high chlorinated PCBs in primary sludge is higher,and the distribution of PCBs in secondary sludge was similar to that in the influent.?3?PAC and CNTs could both achieve rapid adsorption of PCBs,and the adsorption effect of different PCBs monomers was different.The adsorption of PCBs on PAC achieved equilibrium after 100min,and the adsorption of PCBs on CNTs achieved equilibrium after80min.In contrast,CNTs reached adsorption equilibrium earlier,and the equilibrium adsorption capacity was larger.?4?Pseudo first order and pseudo second order kinetics models could fit the adsorption process of PCBs on PAC as well as CNTs.Adj-R² of pseudo first order model were above0.95,and Adj-R² of pseudo second order model were above 0.98..The fitting effect of pseudo second order model was better,indicating that adsorption of PCBs on PAC and CNTs were complex multi-step processes.?5?Freundlich and Langmuir model can well describe the adsorption isotherm of PCBs on PAC and four CNTs.Langmuir Adj-R² were more than 0.98,while Freundlich Adj-R² were more than 0.99,so that Freundlich showed better fitting.Consistent with the conclusion of adsorption kinetics,the maximum adsorption capacity of SWCNT for PCBs in water was the largest,followed by MWCNTs,and PAC was the smallest.