Effects And Mechanism Of Separating Sediment Endogenous Nitrogen Via Electrokinetic Drainage Of Pore Water

Water eutrophication has expanded into a global problem,especially in tropical developing countries,and the urgency of water environment remediation has become increasingly prominent.Nitrogen is one of the main factors that limit the primary productivity of lake ecosystems,and it plays an important role in the regulation of algae growth.The load of nitrogen released from sediments into the water body was almost equivalent to the load of external input.The presence of excess nitrogen could keep the lake in a eutrophication state for decades,thereby delaying the recovery of eutrophic waters.With the gradual control of the external input,how to effectively control the release of internal nitrogen from the sediment has become a crucial point in the treatment of lake eutrophication.Pore water is the main carrier for the release of endogenous nitrogen in the sediment.According to research,the nitrogen content in the pore water could reach dozens of times of that in the overlying water.In this study,indoor nitrogen-contaminated sediment remediation experiments has been conducted via pore water electrokinetic drainage equipment,with novel electrokinetic geosynthetics（EKG）as electrode.The experiments were carried out under two conditions,without overlying water and with overlying water.The nitrogen removal effect,the nitrogen concentration of the overlying water and the change characteristics of the nitrogen release flux at the sediment-water interface have been explored.The characteristics of the migration and transformation of different forms of nitrogen in the sediment during the electrodynamic dehydration process were also analyzed.In addition,the mechanism of electrodynamic dehydration to separate nitrogen from the sediment and the regulation mechanism of nitrogen release at the sediment-water interface have been explored in this study.The main work content and understanding of this paper are as follows:1)In the continuous energization mode without overlying water,the pore water drainage volume,nitrogen removal and energy consumption under voltage gradients of 0 V/cm,0.25V/cm,0.5 V/cm and 1 V/cm were monitored.The results showed that,the electroosmosis and electromigration effected stronger and the cumulative drainage volume of pore water and the cumulative removal of nitrogen were greater with the voltage gradient got greater.Under a voltage gradient of 1 V/cm,the total drainage volume of pore water and the total nitrogen removal reached 7806 m L and 577.30 mg,an increase of 19.80%and 88.38%respectively compared with the control group.2)Under a voltage gradient of 1 V/cm without overlying water,the two modes of intermittent energization（12h On/12h Off）and continuous energization were used to carry out the remediation experiments via pore water electrokinetic drainage equipment.The results showed that the contribution rates of electroosmotic flow to the total pore water drainage in intermittent and continuous energization modes are 19.57%and 17.98%,respectively.The electroosmotic coefficient in the intermittent energization mode was about 2.22 times that of the continuous energization mode,and the electroosmotic flow penetration ability performed well in intermittent energization mode.The removal rates of NH₄⁺-N in the sediment under the intermittent and continuous energization modes were 40.41%and 39.27%,respectively,and the removal rates of NO₃^—-N are 25.82%and 27.94%,respectively.In conclusion,the intermittent energization mode was a highly effective mode,and its cumulative energy consumption accounted for only 63.09%of the continuous energization.3)In the presence of overlying water,a weak electric field of 0.5 V/cm was applied through the mode of intermittent energization（4h On/4h Off）to conduct electrokinetic dehydration experiment.The results showed that the flux of DTN at the sediment-water interface has been effectively regulated by this method.During the remediation,the total emission flux of DTN was-402.08 mg N/m²,so that the overlying water appeared as a“sink”of DTN as a whole.After 80 h of the experiment,DTN release reached equilibrium,and the time to equilibrium was shortened by 89.99%compared with the control group.4)The overall proportion of the contents of different extracted nitrogen in the transferable total nitrogen（TTN）in the sediment was presented as:weak acid-exchange nitrogen（WAEF-N 36.87%）>ion-exchange nitrogen（IEF-N 30.67%）>strong oxidant-exchange nitrogen（SOEF-N）19.56%)>strong alkali-exchange nitrogen（SAEF-N 12.90%）.Sediment TTN mainly existed in the form of WAEF-N,and its proportion rose to 39.63%after remediation.Under a voltage gradient of 0.5 V/cm,the electric dehydration experiment was carried out in the mode of intermittent energization,and the results showed that the removal rate of DTN in the sediment IEF-N,WAEF-N,SAEF-N and SOEF-N were 27.07%,7.29%,14.56%and 4.54%.On the whole,this remediation technology had the best removal effect on the most easily released IEF-N in the sediment,reducing the nitrogen release potential of the sediment.