Bioelectrochemical Systems From Extracellular Electron Transfer to Biotechnological Application Edited by Korneel Rabaey, Largus Angenent, Uwe Schroder and Surg Keller & Publishing London New York TECHNISCHE INFORMATION SBIBLIOTHEK UNIVERSITATSBIBLIOTHEK HANNOVER
Contents Foreword List of Contributors xix xxi 1 BIOELECTROCHEMICAL SYSTEMS: A NEW APPROACH TOWARDS ENVIRONMENTAL AND INDUSTRIAL BIOTECHNOLOGY 1 1.1 Fuel cells and bio-electricity 1 1.2 Underlying principles 5 1.2.1 Microorganisms and current 5 1.2.2 Microbial communities in BESs 6 1.2.3 From microbial metabolism to electrical current 7 1.3 Measuring and Defining performance 8 1.3.1 Measuring potentials 9 1.3.2 Rate based performance indicators 10 1.3.3 Efficiency based performance indicators 10 1.4 A plethora of applications 11 1.5 Acknowledgements 12 References 13 2 MICROBIAL ENERGY PRODUCTION FROM BIOMASS 17 2.1 Biomass: solar energy stored in organic material 17 2.2 The energy content of biomass 20 2.3 Bio-alcohol production from biomass 22 2.4 Anaerobic methanogenic digestion: waste stabilization plus renewable energy source 24 2.4.1 Process performance 24
vi Bioelectrochemical Systems 2.4.2 The microbiology of methanogenesis 26 2.4.3 The importance of extracellular electron transfer in AD 27 2.4.4 Application of anaerobic digestion 29 2.4.4.1 Anaerobic Digestion (AD) for solid waste 29 2.4.4.2 AD for wastewater treatment 30 2.4.4.3 Overall benefits and constraints of anaerobic digestion 32 2.5 Bio-hydrogen production from biomass 34 2.6 Future perspectives 35 References 36 3 ENZYMATIC FUEL CELLS AND THEIR COMPLEMENTARITIES RELATIVE TO BES/MFC 39 3.1 Introduction 39 3.2 Similarities between types of microbial and enzymatic biofuel cells 43 3.2.1 Bioreactor design 44 3.2.2 In-situ bioreactor style 44 3.2.3 Catalyst in anolyte solution 45 3.2.4 Immobilized catalyst and/or mediator 46 3.2.5 Direct electron transfer catalysts 46 3.3 Catalyst sources for MET and DET systems 47 3.4 Comparison of properties of microbial and enzymatic fuel cells 48 3.5 Enzymes employed in enzymatic biofuel cells 49 3.6 Deep and/or complete oxidation of fuel 52 3.7 Conclusions 52 3.8 Acknowledgements 53 References 53 4 SHUTTLING VIA SOLUBLE COMPOUNDS 59 4.1 Introduction 59 4.2 Redox shuttles 61 4.3 Early experiments 62 4.4 Exogenous redox mediators 63 4.4.1 Artificial mediators 63 4.4.2 Natural redox mediators in the subsurface environment 64
Contents vii 4.5 Endogenous redox mediators 65 4.5.1 Known microbially produced redox mediators 67 4.5.1.1 Phenazines 67 4.5.1.2 Flavins 67 4.5.1.3 Quinones 68 4.5.1.4 Cytochromes and soluble enzymes 69 70... 4.5.1.5 Melanin 69 4.5.1.6 Other mediators 69 4.5.2 Unidentified endogenous mediators 70 4.6 Methods for identification of soluble redox shuttles 4.6.1 Potentiostat-controlled electrochemical cells 71 4.6.2 Environmental conditions 71 4.6.3 Batch experiments 71 4.6.4 Media formulation 71 4.6.5 Electrochemical methods 71 4.6.6 Medium change 72 4.6.7 Chemical structure of the mediator 72 4.7 Relevance of soluble redox mediators shuttle to microbial metabolism 72 4.8 Soluble redox shuttles in bioelectrochemical devices 74 4.8.1 Microbial fuel cells 74 4.8.1.1 Biosensors 74 4.8.1.2 Electrodes modified with redox mediators 75 References 75 5 A SURVEY OF DIRECT ELECTRON TRANSFER FROM MICROBES TO ELECTRONICALLY ACTIVE SURFACES 81 5.1 Introduction 81 5.2 - Extracellular electron transfer microbial connections 82 5.2.1 Localized sites for membrane associated EET 83 5.2.1.1 Shewanella cytochromes 83 5.2.1.2 Geobacter cytochromes 85 5.2.2 Bacterial nanowires 87 5.2.2.1 Geobacter nanowires 88 5.2.2.2 Shewanella nanowires 88 5.2.2.3 Nanowires produced by other microorganisms 90
viii Bioelectrochemica! Systems 5.2.3 Nanowire characterization 90 5.2.3.1 Composition 91 5.2.3.2 Regulation 91 5.2.3.3 Conductivity 92 5.2.3.4 Function 93 5.2.3.5 Prevalence 93 5.3 Ecological significance of extracellular electron transfer 93 References 95 6 GENETICALLY MODIFIED MICROORGANISMS FOR BIOELECTROCHEMICAL SYSTEMS 101 6.1 Introduction 101 6.2 Extracellular respiration in Shewanella Oneidensis and Geobacter Sulfurreducens 102 6.3 Scientific motivation for heterologous gene expression 105 6.4 Methods and challenges for heterologous gene expression in E. coli 107-6.5 Biotechnological applications designing the 'super bug' 110 6.5.1 The 'super bug' for BES applications 110 6.5.2 The 'super bug' for bioremediation applications 112 6.6 Closing remarks 113 6.7 Acknowledgements 113 References 113 7 ELECTROCHEMICAL LOSSES 119 7.1 Introduction 119 7.2 Individual electrochemical losses 120 7.2.1 Activation polarization 121 7.2.1.1 Means to decrease the activation polarization 122 7.2.2 Ohmic polarization 122 7.2.2.1 Means to decrease the ohmic polarization 124 7.2.3 Concentration polarization (Mass transfer and reaction polarization) 125
'internal Contents ix 7.2.3.1 Means to decrease the concentration 7.2.4 Reactant crossover - polarization 127 currents' 127 7.2.4.1 Means to decrease internal current losses 128 7.2.5 The ph splitting between anode and cathode 129 7.2.5.1 Means to prevent the ph splitting 129 7.3 Methods 129 7.3.1 Experimental strategies for the recording of polarization plots 129 7.3.1.1 Current interrupt technique 130 7.4 Conclusions 131 References 132 8 ELECTROCHEMICAL TECHNIQUES FOR THE ANALYSIS OF BIOELECTROCHEMICAL SYSTEMS 135 8.1 Cyclic voltammetry for the study of microbial electron transfer at electrodes 137 8.1.1 Introduction 137 8.1.2 Turnover vs. non-turnover voltammetry experiments 140 8.1.2.1 General considerations 140 8.1.2.2 Voltammetry in the presence of substrates 141 8.1.2.3 Voltammetry in the absence of substrates 145 8.1.2.4 Concluding remarks 148 References 148 8.2 Importance of Tafel plots in the investigation of bioelectrochemical systems 153 8.2.1 Introduction 153 8.2.2 Use of Tafel plots for performance evaluation of microbial fuel cells 156 8.2.2.1 Tafel plots for monitoring the electrocatalytic activity of anode materials toward microbial consortia 157 8.2.2.2 Tafel plots for examining charge transfer with microbial pure cultures 162
X Bioelectrochemical Systems 8.2.2.3 Estimating the maximum power production from Tafel plots 163 References 165 8.3 The use of electrochemical impedance spectroscopy (EIS) for the evaluation of the electrochemical properties of bioelectrochemical systems 169 8.3.1 Introduction 169 8.3.2 Instrumentation and experimental approach 170 8.3.3 Display and analysis of EIS data 172 8.3.4 Determination of key electrochemical parameters from impedance spectra 175 8.3.5 Applications of electrochemical impedance spectroscopy in the study of MFCs 176 8.3.5.1 Electrochemical characterization of anode and cathode properties 176 8.3.5.2 Determination and analysis of the internal resistance Rm 179 8.3.6 Conclusions 181 References 181 9 MATERIALS FOR BES 185 9.1 Introduction 185 9.1.1 Electrode specific surface areas and material costs 187 9.2 Electrode materials for MFCs 187 9.2.1 Anode 187 9.2.2 Cathode 189 9.2.3 Membranes 193 9.3 Other materials 197 9.3.1 Current collectors 197 9.3.2 Wires, resistors and loads 197 9.4 Materials for microbial electrolysis cells 198 9.5 Conclusions and outlook 200 References 201 10 TECHNOLOGICAL FACTORS AFFECTING BES PERFORMANCE AND BOTTLENECKS TOWARDS SCALE UP 205 10.1 Introduction 205
Contents xi 10.2 Design constraints as determined by wastewater application 207 10.2.1 Footprint and energetic efficiency 207 10.2.2 Effect of conductivity 210 10.2.3 Effect of buffer capacity 212 10.2.4 Membrane separator or not 212 10.3 Design constraints as determined by scale up 213 10.3.1 Scale up and voltage losses 213 10.3.2 Hydrodynamics and mechanics 215 10.4 Costs and choice of materials 215 10.4.1 Material properties and costs 215 10.4.2 Anode 216 10.4.3 Cathode 217 10.4.4 Membranes 217 10.5 Overcoming design constraints 218 10.5.1 Constraints and solutions 218 References 220 11 ORGANICS OXIDATION 225 11.1 Introduction 225 11.2 Respiratory oxidation to carbon dioxide 228 11.3 Fermentation at microbial fuel cell anodes 231 11.4 Syntrophy between fermenters and anodophiles 234 11.5 Methanogens compete for fermentation products 236 11.6 Electrocatalytic oxidation of fermentation products... 237 11.7 Summary 238 References 239... 12 CONVERSION OF SULFUR SPECIES IN BIOELECTROCHEMICAL SYSTEMS 243 12.1 Introduction 243 12.2 Properties of sulfur species 244 12.2.1 Elemental sulfur 244 12.2.2 Sulfide and polysulfides 244 12.2.3 Sulfate and other oxyanions 245 12.2.4 Relationship of electrochemical potential and ph for sulfur species in aqueous systems 245 12.3 Existing sulfide and sulfate removal technologies 247 12.3.1 Sulfide removal technologies 247
Bioelectrochemical Systems 12.3.1.1 Physicochemical processes 248 12.3.1.2 Biological technologies 248 12.3.2 Sulfate removal technologies 249 12.3.3 Evaluation of existing technologies 249 12.4 Abiotic electrochemical removal of aqueous sulfide 250 12.4.1 Introduction 250 12.4.2 Spontaneous sulfide oxidation and electricity generation 252 12.4.3 Final product of sulfide oxidation 252 12.4.4 Properties of electrodeposited sulfur 254 12.5 Removal of aqueous sulfide in BES 256 12.5.1 Introduction 256 12.5.2 Sulfide oxidation in a biotic cell 257 12.6 Outlook 258 References 259 CHEMICALLY CATALYZED CATHODES IN BIOELECTROCHEMICAL SYSTEMS 263 13.1 Introduction 263 13.2 Oxygen Reduction Reaction (ORR) 265 13.2.1 Introduction 265 13.2.2 Oxygen reduction catalysts 267 13.2.2.1 Platinum 267 13.2.2.2 Transition metal macrocycle based catalysts 268 13.2.2.3 Metal oxides 268 13.2.2.4 Enzymes 269 13.2.3 MFC cathode configurations 269 13.2.3.1 Aqueous cathodes 269 13.2.3.2 Air cathodes 269 13.3 Hydrogen Evolution Reaction (HER) 270 13.3.1 Introduction 270 13.3.2 Hydrogen evolution catalysts 274 13.3.2.1 Platinum 274 13.3.2.2 Nickel 276 13.3.2.3 Tungsten carbide 276 13.3.2.4 Enzymes 277 13.3.3 MEC cathode configurations 277
Contents xiii 13.3.3.1 Aqueous cathodes 277 13.3.3.2 Gas diffusion cathodes 278 13.4 Future possibilities 279 References 280 14 BIOELECTROCHEMICAL REDUCTIONS IN REACTOR SYSTEMS 285 14.1 Introduction 285 14.2 Aerobic biocathodes 286 14.3 Anoxic and anaerobic biocathodes 289 14.4 Electron transfer in biocathodes 294 14.5 Limiting factors 297 14.6 Outlook 298 14.7 Acknowledgements 299 References 299 15 BIOELECTROCHEMICAL SYSTEMS (BES) FOR SUBSURFACE REMEDIATION 305 15.1 Bioremediation of contaminated soils and aquifers 305 15.2 Chemical vs. electrochemical strategies of electron delivery 306 15.2.1 Chlorinated hydrocarbons 309 15.2.2 Inorganic pollutants 315 15.3 Outlooks, perspectives, and challenges towards field applications 319 References 322 16 FUNDAMENTALS OF BENTHIC MICROBIAL FUEL CELLS: THEORY, DEVELOPMENT AND APPLICATION 327 16.1 Introduction 327 16.2 Fundamental principles of sediment reduction-oxidation chemistry 328 16.3 Principles of design and approaches to testing Benthic Microbial Fuel Cells (BMFCs) 329 16.4 Anode material and design 330 16.5 Cathode materials and design 332 16.6 Performance and practical considerations of BMFC designs 333
xiv Bioelectrochemical Systems 16.7 Microbial ecology of BMFCs 335 16.8 Factors governing power output 338 16.9 Scaling and environmental variability in BMFCs 340 16.10 Commercial viability of BMFCs 341 References 343 17 MICROBIAL FUEL CELLS AS BIOCHEMICAL OXYGEN DEMAND (BOD) AND TOXICITY SENSORS 347 17.1 Introduction 347 17.1.1 Dissolved oxygen probe-based BOD sensors 348 17.1.2 Photometric BOD sensors 348 17.1.3 Titration and respirometric sensors 349 17.1.4 Electrochemical BOD sensors with mediators 349 17.2 The mediator-less microbial fuel cell 351 17.2.1 Electrochemically-active bacteria 351 17.2.2 Enrichment of an electrochemically-active bacterial community 352 17.2.3 Microbiology of a mediator-less MFC 353 17.2.4 Optimization of MFC performance 353 17.3 Design and performance of an MFC used as BOD sensor 355 17.3.1 MFC to measure BOD values higher than 10 mg/l 356 17.3.1.1 MFC design 356 17.3.1.2 Enrichment and operation 357 17.3.1.3 Performance 358 17.3.2 MFC to measure BOD values lower than 10 mg/l 359 17.3.2.1 Background 359 17.3.2.2 Oligotrophic sensor design and performance 359 17.3.3 BOD determination of samples containing oxygen and nitrate 360 17.3.3.1 Oxygen and nitrate reduce current and coulombic efficiency 360 17.3.3.2 Use of respiratory inhibitors 360 17.4 MFC as a toxicity sensor 361 17.5 Conclusions 361 17.6 Acknowledgements 361 References 362
Contents xv 18 FEEDSTOCKS FOR BES CONVERSIONS 369 18.1 Introduction 369 18.2 Defined substrates utilized by BES 372 18.2.1 Volatile fatty acids and other fermentation end products 372 18.2.2 Soluble carbohydrates, amino acids and xenobiotics 376 18.3 Complex substrates and wastewaters utilized by BES 377 18.3.1 Cellulosic feedstocks 378 18.3.2 Chitin 379 18.3.3 Domestic wastewater 379 18.3.4 Simulated and actual industrial wastewaters 379 18.4 Other aspects of feedstock composition 381 18.5 Feedstocks and BES integration in wastewater treatment processes 383 18.6 Conclusions 387 18.7 Acknowledgements 388 References 388 19 INTEGRATING BES IN THE WASTEWATER AND SLUDGE TREATMENT LINE 393 19.1 Introduction 393 19.2 BES as the single biological treatment unit (A) or followed by an activated sludge system as a polishing step (B) 396 19.3 Preacidification of organic wastewater before BES (C) 398 19.4 Anaerobic digesters for sludge stabilization followed by BES (D) 399 19.5 Generating caustic in the cathode of BES to control anaerobic digester ph (E) 401 19.6 Denitrificaton in the cathode of BES to remove nutrients from water (F) 402 19.7 Generating chemical reagents at cathodes for treatment purposes (G) 403 19.8 Outlook 404 19.9 Acknowledgements 405 References 405
SMALL convection, xvi Bioeleclrochemical Systems 20 PERIPHERALS OF BES - SCALE YET FEASIBLE (DEMONSTRATED) APPLICATIONS 409 20.1 Introduction 409 20.2 Artificial symbiosis 410 20.3 Microbial fuel cells and their configurations 411 20.3.1 Definition of peripherals 411 20.3.2 Bridging the power divide 412 20.3.3 Minimal peripheral requirements for continuous and autonomous operation 415 20.3.4 Complexity in stacks 416 20.3.5 Microbial Electrolysis Cells (MECs) that transform organic feedstocks into other types of energy (hydrogen or methane) but require input of electrical power in the process 419 20.3.6 Microbial Electrolysis Cells (MECs) that consume electrical power to drive useful reactions (e.g. denitrification) 420 References 420 21 TOWARDS A MATHEMATICAL DESCRIPTION OF BIOELECTROCHEMICAL SYSTEMS 423 21.1 Introduction 423 21.2 Mathematical modelling 424 21.2.1 Model characteristics 425 21.2.1.1 Mechanistics vs. empirism 425 21.2.1.2 Dynamic vs. stationary models 426 21.2.1.3 Level of segregation/aggregation 426 21.2.2 How do model characteristics affect the model user? 427 21.3 BESs modelling objectives 427 21.4 Key elements for BESs modelling 429 21.5 Existing BESs models 429 21.6 Current challenges in BESs modelling 436 21.6.1 Bioelectrode kinetics 437 21.6.2 Electron transfer mechanisms 439 21.6.3 Microbial activity: bioenergetics and kinetics 440 21.6.4 Mass transport - diffusion and migration 443 21.6.5 Biofilm and spatial modelling 444
focus Contents xvii 21.7 BESs modelling perspectives 445 21.8 Acknowledgements 446 References 446 22 OUTLOOK: RESEARCH DIRECTIONS AND NEW APPLICATIONS FOR BES 449 22.1 BES research - on the application 449 22.2 Fundamental research directions 450 22.2.1 Understanding bioelectrochemical process fundamentals 450 22.2.2 Practically inspired fundamental research areas 452 22.3 Applied research opportunities 453 22.3.1 Contributions and limitations of current research activities 453 22.3.2 BESs for wastewater treatment? 455 22.3.3 Is power the best product from BESs? 456 22.4 Potential new BES applications 457 22.4.1 Novel options for cathodic reductions 457 22.4.2 Novel options for anodic oxidations 459 22.5 BES Integration into practical applications 459 22.6 Concluding thoughts on the future of BES 461 References 462 Index 467