Adsorption And Regeneration Mechanism Of DOM From Powdered Activated Carbon

Difficult to degrade dissolved organic matter(DOM)has been a key challenge in achieving zero discharge in industrial wastewater treatment.Due to the complex composition and nature of DOM and its low concentration in industrial tailwaters,it is extremely difficult to treat advanced oxidation technology,and the use of adsorption technology to capture DOM has become the best choice.Among them,powdered activated carbon(PAC)has once again become an object of interest due to its large specific surface area and well-developed pores.However,there has been a lack of systematic studies on whether the adsorption of DOM by PAC is selective and the sources of variability in the effect of different regeneration methods on the pore structure and surface properties of PAC when actually treating industrial tailwater.In this study,the adsorption of DOM by PAC on coking wastewater reverse osmosis concentrated water(ROC)and the regeneration of PAC by electrochemical oxidation and microwave heating,respectively,were investigated to explore the effects and differences of different regeneration methods on the performance of PAC.The main research contents are as follows.(1)A study of the adsorption of refractory DOM from ROC by PAC revealed that in the early stages of adsorption,PAC relies on its welldeveloped microporous structure and large specific surface area to remove organic matter mainly by physical adsorption;In the later stages of adsorption PAC adsorbs predominantly chemically by virtue of its own oxygen-containing functional groups.The total organic carbon(TOC)content in ROC decreased from 243.1mg/L to 48.2mg/L after PAC adsorption,and the total TOC removal rate reached 80.2%,after which the removal rate no longer increased with the increase of PAC dosing.BET and GC-MS analysis showed that PAC relied mainly on 0.5-1 nm micropores for adsorption,with 0.5-0.7 nm pores being completely blocked after adsorption.PAC has a significant effect on the removal of heterocyclic organics from ROC,but is less effective in the adsorption of phenols and alkanes and olefins,where organics with molecular sizes larger than the best adsorption pore size of PAC 0.5-1nm(squalene,tetradecane,eicosane,etc.)are difficult to remove.The overall adsorption process of PAC is more consistent with the pseudo-second-order kinetics and Freundlich isotherm model,with chemisorption being the main mode of adsorption with multiple sites acting together making it difficult to shed the adsorbed organic matter.(2)The electrochemical oxidation regeneration PAC response surface optimization results show that the order of influence of different factors on PAC regeneration efficiency is electrolyte concentration>electrolysis time>current density,and the interaction between current density and electrolyte concentration is the most obvious.This indicates that indirect oxidation relying on oxidation by-products such as active chlorine and chlorine radicals has a significant effect on the efficiency of PAC regeneration.The results showed that the heterocyclic compounds,polycyclic aromatic hydrocarbons and other organic substances adsorbed by PAC were not completely desorbed after electrolytic regeneration,resulting in an increase in the content of oxygen-containing functional groups on the surface of PAC(from 14.21%to 15.07%after regeneration)and a decrease in the content of nitrogen-containing functional groups(from 2.52%to 2.09%for nitrogen).The ester organic compounds adsorbed by PAC were desorbed and removed gradually with the enhancement of electrolytic oxidation,most of the organic matter is completely degraded,the fulvic acid humus and humic acid humus desorbed in the electrolyte are degraded and rearranged to form new diagenetic compounds during the oxidation process,and the content of TOC in the electrolyte decreased to 25mg/L.During the regeneration process the PAC surface structure is unavoidably destroyed by the strong oxidizing properties of oxidation by-products such as hypochlorite and the specific surface area of PAC is reduced from 720.31 m2/g to 658.44 m2/g.(3)The results of optimizing the PAC microwave regeneration parameters by response surface show that the magnitude of the influence of different factors on PAC microwave regeneration is:microwave power>radiation time>solid to liquid ratio.Among them,the solid-liquid ratio mainly affects the rate of organic matter desorption into liquid phase and direct evaporation,and the interaction with microwave power is the most significant.The results show that the microwave regeneration of PAC is effective and the adsorption capacity of TOC in ROC is still greater than 90%after three cycles of regeneration.During regeneration,nitrogencontaining heterocyclic organic matter was difficult to remove and gradually accumulated in the pores of the PAC.Some amides and esters are difficult to desorb after PAC adsorption,which increases the content of C=O bond on the surface of PAC after repeated cycle regeneration.The results of EEMs spectrum analysis show that DOM accumulates gradually under the influence of PAC’s own pore effect and functional groups in the process of adsorption,and the degree of humifaction increases,and it will form larger flocs after long-term contact with PAC,which is beneficial to the complete removal of pollutants.The results of simulated wastewater showed that the degree of humification of oxygen-containing heterocycles and aromatic hydrocarbons represented by furan and potassium phthalate increased after PAC adsorption,and the fluorescence range of stable organic compounds such as cyan acetamide,2min 4-di-tert-butylphenol and 2min 4-trimethyl arsenide remained unchanged before and after adsorption.(4)In summary,the adsorption of PAC is mainly influenced by the types of surface functional groups and the optimal adsorption pore size for selective adsorption of organic matter.Among the different regeneration methods,electrochemical regeneration of active chlorine and other oxidation by-products in the degradation of DOM at the same time will damage the surface structure of PAC;Microwave regeneration is mainly due to the failure of desorption of organics such as nitrogen-containing heterocycles,which reduces the regeneration efficiency.The regeneration effect of microwave cycle on PAC is the same as that of traditional high temperature regeneration,but it has more advantages in action time and reaction efficiency.