Degradation Efficiency And Mechanism Of Phosphonate By UV Assisted Electrochemically Mediated Generation Of Active Chlorine

Phosphorus is not only an important material basis for life activities and industrial and agricultural production,but also a key factor for water eutrophication.In order to slow down and prevent the occurrence of water eutrophication,the restriction of phosphorus emission in the process of human production and life is more and more strict in China.However,the diversity of phosphorus forms increases the difficulty of treatment,and the traditional process for the treatment of orthophosphate has very limited effect on organic phosphonate.With the wide application of wastewater reuse process,phosphonate antiscalants in membrane concentrate become an important source of organic phosphonate in water.Efficiently degrading phosphonate and reducing the risk of eutrophication has been a study hot spot in recent years.In this paper,hydroxyethylidene diphosphonic acid（HEDP）,a typical phosphonate antiscalants in reverse osmosis（RO）process,was taken as the research object.It was proposed to use the chloride ion in the concentrated solution of RO membrane to innovatively combine the UV and in-situ electrogenerated chlorine to degrade HEDP efficiently.This method does not need additional oxidants,and solves the difficulties in storage and transportation and has the advantages of simple operation and relatively low cost.Firstly,we studied the degradation efficiency of phosphonate antiscalants in UV/electrochemically mediated generation of active chlorine system,and determined the key influencing factors in the system.It was found that the degradation of HEDP by UV and electrooxidation had obvious synergistic effect.The results showed that 2.5 m A/cm² was the best current density to degrade HEDP,and increasing or decreasing the current density would reduce the degradation efficiency.The acid condition was more conducive to the degradation of HEDP.When the concentration of Cl^-in the system was too high,it would result in the degradation efficiency of HEDP decreasing.The degradation efficiency of HEDP decreased with the increase of the initial concentration of HEDP,but UV assisted electrochemically mediated generation of active chlorine in general had a good effect on the treatment of various phosphonate.SO₄^2-had no effect on the degradation of HEDP,while CO₃^2-had obvious inhibition effect.Secondly,The free radicals were detected qualitatively and quantitatively by capture experiment and probe experiment.Tert butyl alcohol（TBA）,a free radical scavenger,had obvious inhibition on the degradation of HEDP,and the increase of TBA concentration would strengthen its inhibition on the degradation of HEDP.It could be seen that there were a great number of active substances such as Cl·and·OH in the system.The pseudo first order rate constants k′_NB and k′_BA were 0.051 min^-1 and 0.1754 min^-1.The steady-state concentrations of·OH and Cl·were 2.18×10^-13 M and 9.10×10^-14 M.In addition,based on the chemical reaction and reaction rate constant in UV assisted electrochemically mediated generation of active chlorine system,the kinetic model was established and verified.The fitting curve of active chlorine concentration with time under different reaction conditions was basically consistent with the experimental data.According to the established kinetic model,the degradation mechanism of HEDP by UV synergetic electrogenerated active chlorine mainly included three steps:in-situ electrolysis of chloride ion to produce active chlorine,UV photolysis of active chlorine to produce free radicals and free radical oxidation of HEDP.Finally,the energy consumption and economic benefit of degradation of phosphonate by UV assisted electrochemically mediated generation of active chlorine process were analyzed.The main energy consumption factors of HEDP degradation by UV/electricity to produce active chlorine process were current,p H and initial concentration of HEDP.Compared with other advanced oxidation technologies,UV/electricity to produce active chlorine,UV/Na Cl O,UV/Fenton and UV/persulfate processes could degrade phosphonate efficiently.The degradation efficiency of UV/electricity to produce active chlorine and UV/Na Cl O processes was more than 88%.However,the cost of UV/Na Cl O is 7 times higher than that of UV/electrooxidation electricity to produce active chlorine.UV/electricity to produce active chlorine process was a feasible process for degradation of phosphonate because of its high degradation efficiency,no need to add additional oxidants,no reagent transportation and management steps,simple operation and low energy consumption cost.