Catalytic Oxidation Removal Of Ammonium And Arsenic From Groundwater By Active Composite Manganese Oxide Film: Mechanisms And Performance

Human activity has exerted a tremendous influence upon the global nitrogen cycle,which greatly result in the ammonium pollution of the groundwater and surface water.Excessive ammonium in water environment can lead to the consumption of dissolved oxygen,eutrophication of water body,toxicity to fish and human health,which has become a serious problem threatening the safety of drinking water.In addition,arsenic pollution in groundwater has also become a major threat to global drinking water.In the world,more than 100 million people are faced with the problem of arsenic contamination in groundwater.Therefore,the removal ammonium and arsenic from water has been an important subject in drinking water area.In this paper,an iron-manganese oxide active filter media?short for?active oxide film??with high ammonium catalytic oxidation activity started up by oxidation-reduction method was adapted.The mechanism of ammonium catalytic oxidation by active active oxide film was investigated.The active filter media was applied for the simultaneous removal of iron/manganese/ammonium/arsenic from groundwater and the mechanism of arsenic removal by the active oxide film was also explored.The main research results and conclusions are as follows:?1?To further explore the mechanism of ammonium catalytic oxidation by the active oxide film.Put forward three steps of ammonium catalytic oxidation reaction kinetics of the active oxide film:?1?ammonium adsorption;?2?ammonium catalytic oxidation by active oxide film;?3?the desorption of reaction products?e.g.,NO₃^-?.On this basis,with the aid of photoelectron spectroscopy?XPS?techniques to further explore the mechanism of active oxide catalytic oxidation of ammonium:the active components of Mn?III?and Mn?IV?in active oxide film react with ammonium directly.Mn?II?produced by the above reaction should be re-oxidized to active oxide film by self-catalysis reactions.At the same time,Techniques such as the scanning electron microscopy?SEM?,X-ray diffraction?XRD?,Raman spectroscopy?RAMAN?were used to explore the structural evolution of the oxide film.Research results show that the active oxide film has the structure characteristics with low crystallinity,small crystal size,low manganese dioxide polymerization degree and low surface negative charge.?2?The effect of water quality during the start-up period on the activity of the manganese oxide was preliminarily proved.Two types of source water?groundwater and surface water?were used as the feed waters for the filtration trials.Although the same initiating method was used,the filter feed with groundwater exhibited much better ammonium removal performance than the filter feed with surface water.In order to find out the cause of different oxide film activity,XRD,SEM and XPS were used to explore the effect of water quality on the structure of the oxide film.The results of XRD analysis showed that the intergrowth of birnessite-like Ca manganese oxides and feitknechtite was the diagnostic feature in differentiating active and inactive oxide film.Mn 3p XPS analysis showed that the average manganese valence of in inactive oxide film was higher than that in active oxide film.Mn?IV?and Mn?III?were identified as the predominant oxidation state of the manganese oxides in surface water and groundwater,respectively.Combined the analysis of composition composition of oxide film and the aluminum content in the influent and effluent water,we preliminary confimed that the presence of polyaluminum ferric chloride?PAFC?seemed to be responsible for the failure of the formation of active manganese oxide.?3?The morphology and structure of manganese oxides in active oxide film were proved.In this study,two oxide films forming in different conditions of oxide film were compared.The active oxide film was adopted from pilot-scale water treatment plant and the natural filter media were adopted from large water treatment plant.XRD,SEM,EDX,XPS and Zeta potential were used to characterize the physical and chemical properties of the two oxide films.The XRD analysis shows that both of the oxide films contain two phase structure,namely,birnessite and buserite.In addition,a new MnO₂phase?PDF#72-1982?was also found in the natural oxidation film.The SEM micrograph of active filter media presents plate structures and consists of stacked particle.The SEM micrograph of natural filter media shows honeycomb structures.Compared with the active oxide film,the activity of ammonium catalytic oxidation for natural oxide film was poor.It may attribute to its low content of trace elements?Ca,Mg?,lower crystallinity,and low Mn?III?and lattice oxygen content.In addition,the presence of?-Fe₂O₃ and MnCO₃ in the natural oxidation film hinders the catalytic oxidation activity.?4?A new process for simultaneous removal of iron,manganese,ammonium and arsenic in groundwater was proposed and the mechanism of arsenic removal by the active oxide film was explored.A pilot-scale filter system was used to inveatigated the dffect of simultaneous removal of iron,manganese,ammonium and arsenic by active oxide film.During the running period,the filter column always maintains a higher removal activity of iron/manganese/ammonium,but there was an apparent penetration of arsenic.Using sodium bicarbonate solution can effectively regenerate the exhausted filter media.In addition,the kinetic process of arsenic removal by active oxide film was was studied by batch experiment.The mechanism of arsenic removal was proposed in this paper.The process of As?III?removal by the active oxide film was an oxidation coupled with sorption approach and followed the pseudo secondary dynamic model.As?III?in the aqueous solution,are oxidized to As?V?on the surfaces of the active oxide film.The converted As?V?are attracted to the surfaces of the active oxide film and bound with the active sites?-OH groups?,through weak intermolecular H-bondings.Finally they are bound with the surfaces eliminating water molecules.