Bioremediation Of Polluted Urban River Using Hybrid Processes Of Macrophyte Sediment Microbial Fuel Cell(mSMFC)

Per the rapid economic development,expansion of industry and the aggregation of human population in urban areas,tons of industrial,livestock and poultry breeding wastewater,as well as domestic sewage,are discharged daily into rivers and lakes resulting in organic matter（OM）,nitrogen,phosphorus,and heavy metal pollution if improperly controlled.Under appropriate p H or redox potential,the pollutants in the sediment move into the surface water by desorption and diffusion to stimulate microbial growth which may cause oxygen depletion.The sediment then becomes polluted,the surface water quality deteriorates,water body turns black,and odorous to affect aquatic life,public health and disrupt the whole aquatic ecosystem.To treat urban river sediment,and surface water,based on the current pollution and the transformation of pollutant status,a macrophyte sediment microbial fuel cell（mSMFC）was proposed and constructed on the efficacy of floating macrophyte-（Limnobium laevigatum,Pistia stratiotes and Lemna minor L.）and electrochemically active bacteria（EABs）,which can be used for river bioremediation as well as bioelectricity generation.The prototype of the mSMFC is similar to the natural aquatic ecosystem because it was not externally stimulated and an aeration pump was inserted in the cathode chamber to create an aerobic environment in the catholyte.The effect of macrophyte growth and dissolved oxygen（DO）concentration at the cathode was studied and operation conditions were determined.To determine the optimum operation conditions,the mSMFCs were compared with the control and the natural degradation process in the open-circuit SMFC,to elucidate the pollutant removal,bioremediation and bioelectricity generation performance of the bioreactors（BR）under feed-batch and continuous flow experiment,separately.The results showed that,after the 120 d operation,the macrophytes in the catholyte of the mSMFC increased in biomass without withering or dying off,P.stratiotes in BR 4 exhibited the highest increase of 29.35±0.1g to achieve a mean maximum DO concentration of 16.85±0.2 mg/L during the continuous flow experiment.The study on organic’s removal revealed that TCOD and TOC removal in all the bioreactors（BR）was not significantly greater than the control BR 2 at the end of the experiment;however,at organic loading rate of 4.77 kg CODm3 d in phase 3 a higher TCOD removal of 80% was achieved in L.minor L.-BR 5.The highest ammonia removal of 93% was observed in BR 4 during the continuous flow;highest nitrate removal of 99% was achieved in L.minor L.-BR 5 and in P.stratiotes-BR 4 during both feed-batch and continuous mode,while nitrite removal of 99% was obtained in all the BRs during the feed-batch and in L.minor L.-BR 5 only during the continuous flow,respectively.The highest phosphorus removal of 94% was reported in L.minor L.-BR 5 during continuous flow,while sulphate removal of 99% was achieved in L.minor L.-BR5 and L.laevigatum-BR 3 during the feed-batch and continuous flow,respectively.The bioremediation of heavy metal’s from the sediment showed that the mSMFC could promote metal bioaccumulation in the dry plant biomass,in the order of Hg > Zn > Cr > As > Pb > Cd.Maximum heavy metal removal from the sediment was 75.31% of Pb in BR4,while Cr in the surface water in all BRs,the Pd in BR 3,BR 4 and BR 5 was completely removed.The As and Cd in all BRs,the Hg in all the BRs except for BR 3,the Pd in BR 1 and BR 2 showed a certain degree of increase.L.minor L.in BR 5 exhibited the highest efficiency in the absorption and bioaccumulation of heavy metals viz.As,Cd,Cr and Hg among the three macrophytes.The study of bioelectricity generation during the SMFC operation revealed a maximum open-circuit voltage of（0.49⁰.89V）in BR 1,maximum electrode potential and voltage output of-0.01/0.89 V and 0.88 V was observed in BR 5 at phase 3 during continuous flow,respectively.Maximum power density,Coulombic efficiency,net energy recovery,and net energy production of 86.06 m W m^-3,24.7%,0.033 k Wh m^-3 and 0.020 k Wh m^-3 was obtained in BR 5 during the feed-batch experiment,respectively.The mechanism of pollutant removal in mSMFC included: removal of nitrogen by aerobic nitrifying bacteria in the surface water by degradation of NH₄ to NO₃^-and diffusion into the sediment to be utilized by denitrifying bacteria,using NO₃^-as the final electron acceptor,N₂ was produced by reduction reaction;the denitrifying bacteria use the electrons produced during the OM degradation to achieve simultaneous OM and nitrogen removal.Furthermore,the metal ions of As³⁺,Cd²⁺,Pb²⁺,and Zn²⁺ because they have low redox potential compared with oxygen serving as a terminal electron acceptor（TEA）,was precipitated on the anode and sediment,they were recovered by bioreduction and biosorption by anaerobic microorganisms on the anode.The higher standard redox potential of Cr³⁺ and the low standard redox potential of Hg²⁺/Hg0 ions with respect to oxygen was reduced via oxidation-reduction reaction at the cathode.Pollutants,OM,rhizodeposits,exudates,and heavy metal ions removal in the mSMFC was achieved through the mechanism of bioelectrochemical oxidation and precipitation at the anode,macrophyte bioaccumulation and oxygen supply at the catholyte to serve as TEA for the oxidation-reduction reaction at the cathode in the surface water.The sequence cluster analysis of the electrodes and the sediment revealed predominant bacterial phyla of Proteobacteria in BR 3（62.86%）;dominate class of Deltaproteobacteria in BR 2（29.20%）and a prevailing genus of Pseudomonas in BR 4（25.18%）during the continuous flow and feed-batch experiment,correspondingly.The Archaeal community analysis presented a large presence of the phylum Euryarchaeota in BR 4（90.91%）and class Methanomicrobia in BR 4（82.87%）;while the genus Methanoregula was abundant in BR 3（35.68%）during the feed-batch and continuous flow experiment,respectively.A comparative study of the BRs performance points out that the mSMFC is a sediment and surface water bioremediation technology for the treatment of polluted aquatic ecosystem with simple structure,easy operation,nonsecondary pollution with efficient pollutant removal and bioelectricity generation.This study proved the efficiency of such a novel mSMFC system may offer a promising avenue in the degradation of OM,removal of heavy metals with bioelectricity generation and serve as an alternative method for in situ bioremediation of polluted aquatic ecosystems.