| Nitrobenzene (NB), one of typical recalcitrant organic compounds, is reportedas possible mutagens, teratogens or carcinogens and has been listed in58China’spriority control organic pollutants. As the electrophilic effect of nitryl decreases theelectron density of the benzene, NB is only to a limited extent degraded in aerobicbiological processes. An effective strategy is to transform NB to aniline (AN) first,which is considerably easier mineralized than NB. The electrochemical reduction ofNB is an effieicent and controlable approach, however, the conventional processsuffer from the requirement of special electrode materials or noble metal catalysts toachieve the selective transformation of NB to AN, which limited its application inpractice. Recent develped bioelectrochemical system (BES) is proposed as aprespective process in wastewtaer treatment, since bacteria, as catalyst, hold theinherent advantages of low-cost, self-regeneration and evironment-friendly. Thepresent study demonstrated bacteria can use the cathode as the sole electron donor toselective reduce NB to AN. The introduction of organic carbon to biocathode furtherenhanced this selective transformation capability. Based on above findings, thestudy went on developing novel process that enabled energy internal loopcompensation by recovery energy from aniline at bioanode. The improtant role ofoxygen in the process was demonstrated and the involved mechanism was thandiscussed.When cathode was served as sole electron donor, the transformation efficiencyand rate from NB to AN was dramatically increased with microbial catalysis.93%of AN formation efficiency was achieved in biocathode at the cathode potential of-0.4V, which was6.16times as high as that obtained in abiotic cathode. In addition,the maximum accumulation of toxic nitrosobenzene (NOB), the intermediate duringNB reduction to AN, was decreased by38.7%in biocathode compared to that inabiotic cathode. Cyclic voltammetry (CV) revealed that an unidentified redoxcompound with the midpoint potential around-0.315V could be responsible for theelectron transfer from cathode to bacteria. Based on the apparent first-order kineticmodel, biocathode was suggested to both catalyze the reduction of NB and NOB.The corresponding apparent first-order kinetic constant was increased by62%and100%, respectively. Moreover, the community of the inoculum and the biocathodesample was analyzed based on the pyrosequencing. The results indicated microbialcommunity was changed dramatically under the evolutionary pressure provied bycathode severved as sole electron donor. The predominant bacteria in biocathodecommunity consisted of Rhizobium sp., Leucobacter sp., Achromobacter sp., Mycobacterium sp., Dysgonomonas sp. and Pseudomonas sp.. The functions ofselective nitrobenzene reduction, carbion fixiation and the possible catodicextracellular electron tranfer in the microbial community might feature the catalyzedselective reduction of NB to AN in biocathode.Biocathode enrichied with the present of organic carbon (glucose) showedbetter performance on selective reduction of NB, in which NOB was rarelyaccumulated. Cyclic voltammetry revealed NB reduction peak was positively shiftedby70mV with the present of organic carbon, indicating the enhanced performancewas not only caused by the extra electrons donation from glucose but also theimprovement of bioelectrocatalytic activity.16S rRNA based analysis of the biofilmon the cathode indicated that the cathode was dominated by an Enterococcus species(occupied74.7%of the library) closely related to Enterococcus aquimarinus LMG16607. In BES coupled with bioanode and biocathode, the apparent first-orderkinetic constant (kNB) was increased with the increase of applied voltage, butconsumed more energy and decreased the current efficiency. kNBwas also increasedby introducing more glucose in to catholyte, kNBincrement caused by net incrementof glucose in lower concentration range (≤200mg/L) was5.1times as high as thatin higher glucose concentration range (≥200mg/L). To avoid the excess increase ofCOD in wastewater, the suggested concentration of added organic carbon was200mg/L, which resulted in AN formation efficiency over97%at all tested appliedvoltages (0.15V~0.25V) and averagely increased the apparent first-order kineticconstant by52±6%compared to that without organic carbon.The recovery of electron and energy from AN was found to be facilitated byexposing bioanode to air, which was then further demonstrated to be depending onthe present of oxygen in limited oxygen donation test. Oxygen content in the anodehead space was found to impact AN degradation and the elelctrons recoverysignificantly. The highest output current (0.155mA) and maximum power density(3.32±0.4W/m3) was observed when70%of gas in anode headspace was composedby oxygen at the beginning. However, columbic efficiency decreased with theoxygen concentration increasing from10.32±1.36%to4.61±1.11%in the testedinitial oxygen content conditions (21%~100%). Based on the results of GC-MS andthe intermittent open circuit experiment, the possible mechanism using AN as thesole electron donor in present of oxygen was suggested as following. AN was firstaerobicly converted to organic acids by microbes in the anolyte and located at theouter layer of anodic biofilm, which was then uptook by electrochemical activebacteria located at the inner layer of anodic biofilm and produce current underanoxic condition. CV indicated that anodic extracellular electron tranfer wasfulfilled by certain microbial secreted soluble electron mediator, which has tworeversible redox peaks with the mid-point potentials of-0.027V and0.063V, respectively.To achieve the energy internal loop compensation, the process of NB reductionwith electrons partial fed-back from AN oxidation was proposed. The saved externalelectron donor and internal loop compensated energy was estimated as22%~53%and17%~40%, respectively, based on the experiment results and the thermodynamicand stoichiometric calculation. |
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