Simultaneous Removal Of Ammonium And Manganese By Activated Backwash Mud Cake Of Groundwater Treatment Plant:Performance And Mechanisms

In recent years,the problem of excessive ammonia and manganese in groundwater has been particularly prominent.How to remove ammonium and manganese economically and efficiently has received much attention from water industry scholars.Activated manganese oxide film prepared by chemical oxidation method can remove ammonium and manganese from water by catalytic oxidation efficiently.However,this method consumes a large amount of strong oxidizing agents,which increases the treatment cost,and the residual agent may pollute the environment.The high concentration of manganese in groundwater causes the groundwater plant mud obtained by dewatering after groundwater treatment also be rich in manganese oxides（Mn O_x）.Therefore,groundwater plant backwash mud cake may be a potential Mn O_xfilm that can be used for ammonium and manganese removal.The conditions for activation of groundwater plant backwash mud cake were optimized,the factors influencing the simultaneous removal of ammonium and manganese by the activated mud cake and the actual operating effects of its coated filter media were systematically investigated.The aim is to investigate the potential application of groundwater plant mud cake as Mn O_xfilm for catalytic oxidation of ammonium and manganese removal,and to provide new insights for improving the treatment efficiency of water plants and achieving resource utilization.The main conclusions are as follows:（1）The method for activating backwash mud cake in groundwater plants was proposed and the related operating conditions were optimized.Through single-factor analysis,the optimal operating conditions for activation were determined as follows:1.5 mg NH₄⁺and 0.5 mg Mn（Ⅱ）per gram of mud cake（dry weight）with 100 m L of groundwater as solvent,and the activation solution should be renewed daily during the activation process,with a maximum catalytic oxidation capacity of 0.243 mg NH₄⁺-N/（g·h）.The structural changes of the activated mud cake were analyzed by SEM,EDS,Zeta potential,BET,XRD,FTIR and XPS.It was found that the surface of the mud cake was negatively charged and contained abundant pores and a large number of oxygen functional groups,which were favorable to the mass transfer process of ammonium and manganese to the surface of the mud cake and consequently to be catalytically oxidized.The safety analysis shows that the activated mud cake does not contain pathogenic microorganisms such as E.coli and there is no risk of heavy metal precipitation.（2）The catalytic oxidation capacity of the activated sludge cake and the factors influencing its removal of ammonia and manganese were investigated.The Mn content of the backwash mud cake was only 31.9%of that of the active film,but its ammonium and manganese removal rate could reach 47.9%-89.7%of that of the active film,indicating that the activated backwash mud cake has high-level catalytic oxidation activity.For simultaneous removal,the manganese concentration has a large effect on the ammonium removal capacity,while the ammonium concentration does not have a significant effect on the manganese.The simultaneous dosing of ammonium and manganese at appropriate concentrations could improve the catalytic oxidation efficiency.The ammonium and manganese removal rates of activated mud cake can be maintained at high levels over a wide p H range（6.0-10.0）,and thus can maintain high activity in natural water.The rates of ammonium and Mn removal were related to the strength and valence of the coexisting cations as well as the ionic radius,and the rate of ammonium oxidation in the presence of 0.01 mol/L Na⁺was 0.270mg NH₄⁺-N/（g·h）.The kinetic analysis shows that the catalytic oxidation processes of ammonium and Mn by activated mud cake are fitted with the first-and second-order power law models,respectively.The activation energies of catalytic oxidation of ammonium and manganese were 10.70 k J/mol and 10.51 k J/mol when the temperatures ranged from 298.15 K to 318.15 K.Therefore,the removal rates of ammonium and manganese by activated mud cakes were slightly affected by temperature changes.（3）The practical application capability of activated mud cake-coated filter media in the column experiment was verified,and the mechanism of simultaneous removal of ammonium and manganese from water by activated mud cake was elucidated.The experimental results from the filter column showed that the activated mud cake significantly improved the ability of the filter media to remove ammonium and manganese from water,although the coating operation caused some damage to the activity of the mud cake.It also transformed the catalytic oxidation products of ammonium into less toxic nitrate and is widely applicable to different water.The filter media could reduce the start-up time to 9 days,and the average removal rates were88.56%and 98.07%for the simultaneous removal of 1.5 mg/L ammonium and manganese,respectively.The SEM,EDS,and XPS analyses showed that an“active manganese oxide film”was formed on the surface of the filter media in the form of a monolayer during the column experiment.The addition of ammonium was helpful to the formation of Mn O_xand the conversion of Mn on the surface of the filter media to higher values,which further accelerated the catalytic oxidation.It is assumed that the simultaneous removal of ammonium and manganese by activated mud cake is mainly through four steps:1)combination of ammonium and manganese;2)autocatalytic oxidation of manganese;3)catalytic oxidation of ammonium by manganese oxide;and4)desorption of reaction products.The total annual cost of activated mud cake-coated filter media is 1.215 RMB/m3,which is a cost-effective and environment-friendly water treatment material.