Electrocoagulation Water Treatment Technology Based On Control Of Flocs Structure And Yield

As an environmentally-friendly water treatment technology,electrocoagulation can effectively remove pollutants from wastewater by in-situ generated flocs through adsorption,sweep flocculation,surface complexation reaction and so on.At present,a lot of researches have been done on optimizing the efficiency of specific pollutant removal by electrocoagulation.Since a wide range of pollutants are present in wastewater,it has become tedious to optimize the operation parameters of EC for every single kind of pollutant.In this thesis,we tried to improve the removal efficiency of various pollutants by adjusting flocs structure and yield.Three typical pollutants(heavy metal ions(Ni²⁺),organics（MO）and anions（F^-）)were chosen as the target pollutant.The influence of flocs yield on pollutant removal and the adsorption capacity of different flocs were explored.Suitable operating conditions for different types of pollutants were found by adjusting the flocs.Improving removal efficiency of different contaminants can be achieved based on maximization of flocs yield,which depends on three key operating conditions:aeration rate,initial p H,and extra time after electrolysis.The flocs structure changed with aeration rate.The flocs was mainly composed by green rust（GR）without aeration.Since GR was composed of ferric hydroxide and inorganic anions,flocs yield reached its maximum without aeration.The flocs were not conducive to the formation when the electrolyte was strongly acidic or strongly alkaline,so flocs yield was the highest when initial p H of electrolyte was 5～7.The extra time after electrocoagulation resulted in the dissolution of the flocs,so flocs yield was the highest when there was no extra time.The variation trends of removal efficiency of F^-and MO were consistent with that of flocs yield,indicating that the increment of flocs yield can effectively improve the removal efficiency of anions and organic compounds.However,the flocs adsorption and co-precipitation are both conducive to the removal of heavy metal ions,the variation trends of removal efficiency with the initial p H is inconsistent with that of the flocs yield.The flocs yield determined the removal rate of Ni²⁺under different aeration rate and extra time conditions.Flocs with different structures（GR,Fe₃O₄,α-Fe OOH）could be generated by adjusting the initial dissolved oxygen（DO_i）of wastewater during EC process,which have different adsorption capacities for pollutants.Because in addition to flocs adsorption,the removal mechanisms of pollutants include electrolysis redox,co-precipitations and so on,it is unable to compare adsorption capacity of different flocs for pollutants directly during EC process.Therefore,in this thesis,in-situ wastewater treament which means wastewater is directly treated by EC and ex-situ wastewater treatment which means electrolyte is without pollutant during EC process and pollutants are added to the solution after electrolysis were proposed to explore the adsorption capacity of various flocs to pollutants.The results show that when DO_i<12 mg/L,GR was the main component of flocs.When 12 mg/L<DO_i<16 mg/L,flocs was composed by GR and Fe₃O₄.When 16 mg/L<DO_i<20 mg/L,Fe₃O₄was the main component of flocs.When 20 mg/L<DO_i<24 mg/L,flocs consisted of Fe₃O₄andα-Fe OOH.When DO_i>24 mg/L,α-Fe OOH was the main component of flocs.Through in situ/ex-situ adsorption of pollutants by these flocs,the removal capacity of GR is the best,followed by Fe₃O₄,andα-Fe OOH is the worst.Wastewater containing Ni²⁺and MO could be treated by ex-situ adsorption of flocs,while wastewater containing F^-was poorly treated by ex-situ traetment.Flocs plays an important role during EC process,but high salinity will affect the flocs adsorption effect of pollutants.Electrolytes with high salinity were used to prepare wastewater to study the changes of flocs yield and composition and removal efficiency of pollutants.The results show that the inorganic salt ions(SO₄^2-)in the wastewater could be adsorbed by the flocs during EC process.The removal efficiency of three pollutants decreased with the increment of salinity regardless of treating mode（in-situ or ex-situ）.What’s more,the effect of salinity on the removal of MO was minimal.With the increase of salinity of wastewater,adsorption of pollutants decreased and the adsorption ofSO₄^2-increased,leading to the reduction of the removal efficiency of pollutants.