distributed water treatment systems offer 
 a potential means of 
 exploiting alternative water sources , including municipal wastewater 
 effluent , roof water , stormwater , and water from shallow aquifers .
unfortunately , alternative water sources often 
 contain trace concentrations of organic contaminants ( e.g. , pesticides , 
 solvents , pharmaceuticals , disinfection byproducts ) . as a result ,
previous attempts to 
 develop point - of - use treatment systems capable of removing trace organic 
 contaminants prior to nonpotable reuse have employed electrochemical 
 processes , but these systems suffer from limitations 
 including the production of toxic byproducts , an inability to remove 
 recalcitrant compounds and high cost of treatment . trace organic contaminants can be removed from water 
 by exposure 
 to hydroxyl radicals ( ho ) in advanced oxidation 
 processes ( aops ) . in full - scale potable water reuse systems ,
trace organic contaminants 
 are frequently removed by addition of a modest concentration of h2o2 ( e.g. , 3 mg / l ) followed by exposure to ultraviolet 
 ( uv ) light .
this approach offers numerous benefits over other aops 
 in terms of energy consumption , reliability , and production of toxic 
 byproducts .
although uv / h2o2 is a well established technology in centralized treatment 
 facilities , challenges associated with the transport and storage of 
 h2o2 make it an impractical solution for distributed 
 treatment systems .
electrochemical production 
 of h2o2 from o2 is an attractive 
 alternative means of producing h2o2 if it can 
 be achieved without the consumption of large amounts of energy or 
 the formation of toxic byproducts .
systems 
 in which oxygen is bubbled into a solution prior to reduction on an 
 electrode surface consume a considerable amount of energy due to the 
 low solubility of oxygen and the need to ensure that it reaches the 
 electrode surface .
bubbling air or pure oxygen into a solution is 
 also an impractical approach for h2o2 production 
 in decentralized systems because it requires pumps and controllers . 

furthermore , the yield of h2o2 from reduction 
 of o2 is often quite low , which greatly increases electricity 
 consumption .
recently , gas diffusion electrodes have been used to generate h2o2 without a need to bubble air or oxygen into 
 a solution .
most of the research in this area has been focused on producing 
 concentrated h2o2 solutions by using highly 
 conductive solutions or organic solvents .
cathodic production of h2o2 for the removal 
 of organics has typically been used for electro - fenton treatment ; 
 however , differences in ph needed for the optimal kinetics of the 
 two reactions ( i.e. , production of h2o2 is most 
 efficient at basic ph values and fenton s reaction is more 
 effective at acidic ph values ) results in inefficient oxidation of 
 trace organics if ph correction is not performed . to produce low concentrations of h2o2 in water 
 immediately prior to an aop
, the cathode must be capable of producing 
 h2o2 in a low ionic strength , poorly buffered 
 solution at circumneutral ph values .
furthermore , increases in ph 
 that occur in the cathode chamber due to consumption of protons must 
 be compensated for by a subsequent treatment process if the water 
 is to be sent into a water distribution system .
the purpose 
 of this study was to evaluate a system that combines 
 in situ electrochemical production of h2o2 followed 
 by uv irradiation and anodic ph adjustment as a cost - effective means 
 of removing trace organic contaminants from water .
this new system , 
 which also inactivates waterborne pathogens and transforms photolabile 
 contaminants through exposure to uv light , can be controlled by varying 
 the production of h2o2 through adjustment of 
 the applied current . to provide insight into the performance of the 
 system under conditions likely to be encountered in distributed water 
 treatment systems , three representative source waters ( i.e. , synthetic 
 surface water , synthetic groundwater , and municipal wastewater effluent ) 

the performance of the system was investigated 
 in terms of contaminant removal and energy consumption .
all experiments were performed at room temperature 
 ( 23  2 c ) with chemicals of reagent grade or higher ( sigma - aldrich , 
 st .
the composition of the waters used is summarized in 
 table s1 , supporting information .
experiments were 
 carried out in a two - chambered parallel plate electrochemical cell 
 consisting of two square perspex frames ( internal dimensions : 8  
 8  1.9 cm ) separated by a cation exchange membrane 
 ( ultrex cmi-7000 , membranes international inc . , ringwood , nj ) .
the 
 frames were bolted together between two square perspex side plates , 
 creating anode and cathode compartments that each had effective volumes 
 of 122 ml ( figure s1 , supporting information ) .
a solid plate was used for the anode frame , while the cathode 
 chamber was bolted with a hollow side plate allowing for one side 
 of the gas diffusion cathode to be exposed to air .
a ti mesh electrode 
 coated with an ir mixed - metal oxide was used as the anode ( dimensions : 
 7.8  7.8 cm ; 1 mm thickness ; specific surface area 1.0 m m , magneto special anodes , netherlands ) . 

the uv reactor consisted of a 1 l brown glass bottle 
 ( veffective = 925 ml ) containing a low - pressure 
 uv lamp ( arc length = 16.5 cm , optical path length = 4.3 cm ) used 
 typically for swimming pool disinfection ( g23 odyssea pool lamp , 9w , 
 odyssea aquarium appliance co. , ltd . ,
the gas diffusion 
 cathode was created by modifying carbon fiber paper ( avcarb p75 t , 
 10  10 cm , fuel cell store , college station , tx ) 
 with a conductive , hydrophobic support layer and a carbon catalyst .
the air - facing side of the cathode was prepared 
 by coating a mixture of 60 wt % ptfe and 30 wt % graphite powder ( 200 
 mesh , alfa aesar , ward hill , ma ) onto one side of the carbon base 
 layer .
the cathode was then air - dried at room temperature , followed 
 by sintering at 350 c for 40 min .
the liquid - facing side was 
 prepared by applying a mixture of 3 ml of propanol with 150 mg of 
 carbon black ( cabot black pearls 2000 , cabot , boston , ma ) and 50 mg 
 of ptfe onto the other side of the carbon base layer .
the cathode 
 was again air - dried at room temperature , followed by sintering at 
 350 c for 40 min .
electrolysis 
 experiments were 
 performed at fixed currents controlled by a multichannel potentiostat 
 ( gamry instruments inc . ,
water entered the cathode 
 compartment operating in a flow - through mode with hydraulic residence 
 times ( hrt ) ranging from 1.5 to 5.0 min ( 12035 l d ) .
cathode effluent was supplied to the uv reactor and then passed 
 through the anode of the electrochemical cell .
the applied charge 
 density ( q , c l ) was expressed 
 as a product of the current density ( i , a m ) , electrode surface area ( a , m ) , and 
 the hydraulic residence time ( t , s ) normalized by 
 the half chamber reactor volume ( v , l):1 source waters were 
 amended with a mixture of ten test compounds each at a concentration 
 of 10 g l. for each experiment , samples 
 were collected prior to the electrochemical cell , after passing through 
 the cathode chamber , after the uv reactor , and after passing through 
 the anode .
at least 3.5 l of the test solution was passed through 
 the system prior to collection of a sample .
1.9 
 ml subsamples to be analyzed for trace organic compounds were mixed 
 with 0.1 ml of methanol to quench radical reactions that could occur 
 prior to analysis .
h2o2 and ph were measured 
 within 5 min , whereas trace contaminants were stored for a maximum 
 of 8 h. experiments quantifying h2o2 production 
 in the varying waters were performed with applied cathodic current 
 densities from 0 to 30 a m under varying flow 
 regimes ( 35120 l d ) .
to assess the long - term 
 cathode performance , 6000 l of 5 mm na2so4 in 
 tap water ( alkalinity = 0.34 mm , [ ca ] = 0.2 mm ) was run 
 through the cell continuously at an applied current density of 15 
 a m at a fixed flow rate of 120 l d. samples were collected daily and analyzed for h2o2 .
the effects of dissolved oxygen concentration on h2o2 production were evaluated by sparging source water 
 with n2 to remove o2 .
h2o2 and free 
 chlorine were measured with a shimadzu uv-2600 spectrophotometer with 
 the titanium(iv ) sulfate method at 405 nm and the n , n - diethyl - p - phenylenediamine ( dpd ) 
 method at 515 nm , respectively .
determination of free 
 chlorine was performed in the absence of h2o2 to eliminate the positive interference of h2o2 with dpd .
the uv absorbance of the 
 four source waters was measured with a shimadzu uv-2600 spectrophotometer . 

dissolved organic carbon ( doc ) and dissolved inorganic carbon ( dic ) 
 were measured using a shimadzu toc - v analyzer .
no3 , cl , and so4 were analyzed using a dionex dx-120 ion chromatograph with an as19 g 
 column .
k , na , ca , and mg were analyzed using a dionex ics-2000 ion chromatograph 
 with a cs12a column .
fluence rate values were determined by chemical 
 actinometry using 10 m atrazine as an actinometer ( 254 = 3860 m cm , 254= 0.046 mol ei , buffered at ph = 8 using 
 a borate buffer ; details of the calculation in the supporting information ) .
conductivity 
 was measured with an ultrameter ii 4p ( myron l company , carlsbad , 
 ca ) .
test compounds were quantified in multiple reaction monitoring 
 ( mrm ) mode with an agilent 1200 series hplc system coupled to a 6460 
 triple quadrupole tandem mass spectrometer ( hplc - ms / ms ) , as described 
 previously .
analytical details and compound 
 specific parameters are provided in the supporting 
 information text and tables s2 and s3 , respectively .
the gas diffusion electrode 
 was polarized cathodically against a ag / agcl reference electrode ( + 0.197 
 v vs she ; basi , usa ) .
the full cell potential between the working 
 ( i.e. , cathode ) and counter ( i.e. , anode ) electrodes was measured 
 in a two - electrode setup . the total system power
( ptotal , w ) is a combination of the uv lamp power ( plamp , w ) and the electrochemical 
 cell power , which can be expressed as a product of the current density 
 ( i , a m ) , cell potential ( vcell ) , and the electrode surface area ( a , m):2
hydrogen peroxide concentrations ( figure 1a ) and production rates ( figure s2 , supporting information ) were determined for an 
 array of charge densities that were achieved from combinations of 
 current densities ( 030 a m ) and cathode 
 chamber retention times ( 1.55 min ) . a linear relationship 
 between h2o2 production and applied charge density 
 was observed at charge densities greater than 50 c l , independent of how the charge was obtained ( i.e. , high current 
 density and short retention times or low current density and long 
 retention times):3where q is the specific 
 charge density applied in c l and [ h2o2 ] is the hydrogen peroxide concentration in mm . for 
 all of the waters
tested , the coulombic efficiency of o2 reduction to h2o2 averaged 88.8  1.8% 
 at charge densities
lower coulombic efficiencies 
 were observed ( note the deviation from the linear fit in figure 1a at charge densities below 50 c l ) . the observed increased coulombic efficiencies at higher charge 
 densities agreed with previously published data for electrochemical 
 synthesis of h2o2 using a gas diffusion electrode 
 composed of a fluorocarbon binder and activated carbon catalyst fed 
 with conductive , alkaline solutions .
production of hydrogen peroxide ( a ) and coulombic efficiency 
 ( b ) 
 as a function of applied charge density ( wwtp : wastewater treatment 
 plant ) .
h2o2 production 
 was independent of the type 
 of source water used despite the substantial variability in the composition 
 of the matrices ( table 1 ) .
this suggests that 
 h2o2 production was not affected by ph , the 
 presence of natural organic matter ( nom ) , dissolved ions , or conductivity 
 over the range of applied charge densities studied .
although h2o2 production was not influenced by influent water 
 quality , the cell potential , and therefore energy consumption , was 
 affected by the conductivity of the source waters .
higher ionic strength 
 waters exhibited lower ohmic resistances and therefore operated at 
 lower cell potentials , thus decreasing their energy consumption .
for 
 a given applied charge , the power required decreased with increased 
 conductivity ( synthetic surface water > synthetic groundwater > 
 wastewater 
 effluent  electrolyte ) ( figure s3 , supporting 
 information ) . even for low conductivity surface water , however , 
 the energy consumption for hydrogen peroxide production at a flow 
 rate of 120 l d was still relatively low ( 0.0180.31 
 kwh m for 5 < i < 30 a 
 m ) , indicating that in situ h2o2 production required much less energy than operation of the 
 uv lamp ( 1.8 kwh m ; calculations provided in the supporting information ) .
the h2o2 production 
 rate increased linearly 
 with applied current density , with a maximum of between 14.4 and 14.8 
 mg h2o2 l min at 30 a m for all of the waters tested ( figure 
 s2 , supporting information ) . in full - scale 
 aop systems ( e.g. , the orange county water district s groundwater 
 replenishment system ) , 3 mg
this 
 concentration can be obtained with the gas diffusion electrode at 
 an applied current density of only 4.14 a m at 
 a hydraulic residence time of 1.5 min . under these conditions ,
this 
 benchtop system could process approximately 120 l of water per day 
 while consuming approximately 1.7 wh to produce h2o2 .
the removal of trace 
 organic contaminants involved direct photolysis , reactions with ho produced by photolysis of h2o2 in the uv chamber , and direct oxidation of contaminants on the anode . 
 at the fluence employed in the uv chamber ( f0  3000 mj cm ) ,
direct photolysis 
 only removed those compounds that exhibited high quantum yields and 
 strong light absorbance at 254 nm ( figure s4 , supporting information ) . among the compounds tested , carbamazepine 
 exhibited the lowest tendency for direct transformation by uv light 
 ( < 30% removal for all matrices ) , while more photoreactive compounds , 
 such as sulfamethoxazole , propranolol , and atrazine , displayed higher 
 removals ( 5599% ) . unlike the variability of the compounds 
 with respect to direct photolysis , the suite of trace organics all 
 reacted with ho at near diffusion controlled rates 
 ( 1010 m s ; table s3 , supporting information ) . as a result of its low reactivity with uv light , carbamazepine 
 removal in the presence of h2o2 and
uv light 
 provided useful information on the transformation of organic contaminants 
 by ho .
the extent of removal of carbamazepine 
 varied among the different matrices ( figure 2 ) .
the presence of ho scavengers explained much 
 of the variability . in the absence of current ( and therefore h2o2 ) , the variability of carbamazepine transformation 
 was predominately influenced by the screening of light , as accounted 
 for by the water factor:4where z is the mixed water 
 body depth ( m ) and  is the attenuation coefficient of the water 
 body .
the water factor , which was primarily influenced by the amount 
 of nom , followed the trend : electrolyte ( 0.998 ) > groundwater ( 0.985 ) 
 > synthetic surface water ( 0.598 ) > wastewater effluent ( 0.508 ) . 

transformation 
 of carbamazepine solely in the presence of uv light varied from 22 
  5% in the electrolyte solution to 15  4% in the wastewater 
 effluent .
the observed removal of carbamazepine in the surface water 
 and wastewater effluent , however , was greater than suggested from 
 the water factor .
this may be explained by the generation of ho and dom * produced from nom sensitization 
 by uv light , which can be significant at uv fluences employed in aops .
removal 
 of carbamazepine as a function of current density for the 
 four types of source waters ( wwtp : wastewater treatment plant ) .
the rate of transformation of 
 trace contaminants increased with 
 current density due to additional ho production 
 that occurred at higher h2o2 concentrations 
 ( tables s4s7 , supporting information ) .
complete carbamazepine transformation was observed after the uv 
 treatment chamber at an applied current density of 5 a m for the electrolyte . for the three representative source waters , 

carbamazepine transformation increased to 98.7  0.6% for groundwater , 
 93.3  1.0% for surface water , and 78.5  1.9% for wastewater 
 effluent as the current increased to 25 a m. organic 
 compounds that have lower reaction rate constants with ho and are not susceptible to direct photolysis will require higher 
 current densities to achieve a similar level of treatment .
the 
 fraction of ho that reacted with the contaminants 
 can be estimated by considering the concentrations and rate constants 
 for reactions of different solutes with ho:5where koh , s and kho , cont are the second order reaction rate constants of scavengers 
 and contaminants with ho , respectively , and [ s ] is the concentration of the scavenger ( e.g. , hco3 , co3 , nom , 
 h2o2 ; tables 1 and s3 , supporting information ) . at increasing 
 h2o2 concentrations
, a trade - off 
 exists between additional transformation of trace organics by ho produced from h2o2 photolysis 
 and greater radical scavenging and light screening by h2o2 . therefore , despite linear 
 increases in h2o2 production with current density , 
 there is a diminishing benefit to the treatment . as h2o2 increased from 0.09 mm ( 4.14 a m ) to 
 0.54 mm ( 25 a m ) , the fraction of ho reacting with contaminants
decreased by 20% , 21% , and 10% for the 
 surface water , groundwater , and wastewater effluent , respectively . 
 at 0.54 mm h2o2 ( 25 a m )
, 
 there was a 4.0% , 4.7% , and 3.8% reduction in direct photolysis rates 
 of contaminants from additional light screening by h2o2 for the surface water , groundwater , and wastewater effluent , 
 respectively ( details of ho branching ratio and 
 direct photolysis calculations are included in the supporting information ) . at ph 8 , approximately 
 6.7% , 6.5% , and 2.9% of ho reacted with the organic 
 contaminants in the uv reactor at an initial h2o2 concentration of 3 mg
l ( 0.09 mm ) for the surface 
 water , groundwater , and wastewater effluent , respectively . at ph 10 , 
 the fraction of ho reacting with trace organic 
 contaminants decreased to 0.9% , 0.6% , and 0.4% for the three source 
 waters , respectively .
the significant decrease in ho reacting with the trace organic contaminants was due to scavenging 
 by carbonate at the higher ph values .
the product of this reaction , co3 , can play a significant 
 role in the transformation of certain organic compounds ( e.g. , propranolol 
 and sulfamethoxazole ; table s3 , supporting information ) . as a result of differences in alkalinity 
 of the different source waters ,
the importance of carbonate scavenging 
 and co3 reactions 
 depends on the source water composition and the applied current density 
 ( i.e. , higher applied currents result in greater ph increases in the 
 cathode chamber ) .
although nitrite is an effective scavenger of ho ( kho , no2 = 6  10 m s ) , less than 0.3% of the generated 
 ho would be scavenged by nitrite at concentrations 
 typically found in nitrified wastewater effluent ( i.e. , 0.1 
 mg l ) .
the formation of halogen radicals from 
 reactions between ho and halide ions ( chloride 
 and bromide ) should only be significant at low ph values and therefore 
 will have a negligible impact on contaminant transformation at the 
 circumneutral and basic ph values observed in this system .
the generation 
 of h2o2 by the cathode consumed protons and 
 increased the solution 
 ph ( reaction 6 ) . in the anode , oxidation reactions 
 produced protons and lowered the solution ph ( reaction 7):6789 for the production of 3 mg 
 l ( 0.09 mm ) of h2o2 at a 
 current density of 
 4.14 a m , approximately 0.20 mequiv l of protons should have been consumed or produced at the cathode 
 and anode , respectively . to maintain electroneutrality , a net migration 
 of protons occurred from the anode chamber to the cathode chamber 
 via the cation exchange membrane .
in addition to protons , cations 
 that were present at higher concentrations ( e.g. , na , 
 ca , mg ) also carried ionic charge through 
 the membrane satisfying electroneutrality in the cathode chamber while 
 creating a proton deficit in the cathode chamber . as a result of differences in buffering among matrices
, 
 the solution 
 ph should have increased more in the cathode chamber for waters with 
 low alkalinity .
the ph in the cathode chamber increased for each of 
 the waters as the current increased from 0 to 25 a m ( figure 3 ) .
the magnitude of ph increase 
 was most pronounced for the electrolyte and surface water ( alkalinity 
 = 0 and 2.45 mm , respectively ) with post - cathode ph values ranging 
 from 10 to 10.5 , while the ph never exceeded 9.9 and 9.2 in the groundwater 
 ( alkalinity = 3.89 mm ) and the municipal wastewater effluent ( alkalinity 
 = 4.97 mm ) , respectively . the ph increases following the cathode resulted 
 in supersaturation with respect to calcite ( caco3(s ) ) in 
 the groundwater ( log si > 1.31 ) , surface water 
 ( log si > 1.58 ) , and wastewater effluent ( log si > 1.09 ) beginning at a current density of 5 a m. this reaction could result in scaling on the cathode 
 or the ion 
 exchange membrane that might eventually affect system performance . 
 loss of caco3(s ) from the system could also result in an 
 overall decrease in ph as water passed through the treatment system . 

for example , if the surface water solution reached equilibrium at 
 the current density needed to produce 3 mg l ( 0.09 
 mm ) h2o2 , 0.74 mmol ( 74 mg ) of calcite would 
 precipitate for each liter of water treated and the ph would have 
 dropped from 9.82 to 7.23 .
on the basis of the observed ph values , 
 it is evident that equilibrium was not achieved .
however , additional 
 research is needed to assess the importance of calcite precipitation 
 to scaling and ph control
. ph change of the source waters prior to entering 
 the electrochemical 
 cell , after passing through the cathode chamber , after the uv reactor , 
 and after the anode as a function of current density ( wwtp : wastewater 
 treatment plant ) . to readjust the solution 
 ph , water leaving the uv reactor was passed 
 through the anode chamber .
if no mineral precipitation occurred in 
 the cathode and uv chambers , the final ph should have been equal to 
 the influent ph .
a slight decrease in ph was observed in all solutions 
 to which current was applied , with greater ph decreases at higher 
 current densities occurring in the least buffered of the three environmental 
 matrices ( i.e. , surface water ) . under the conditions that would likely 
 be used for treatment ( i.e. , 510 a m and 
 short hydraulic residence times ) ,
due 
 to the relatively low molar absorptivity of h2o2 at 254 nm ( 254 = 18.6 m cm ) and the limited residence times in the uv reactor 
 (  = 660 s ) , much of the h2o2 passed through 
 the uv chamber without undergoing photolysis . for solutions with relatively 
 low light screening ( i.e. , electrolyte , synthetic groundwater , and 
 synthetic surface water ) ,
between 40% and 50% of the h2o2 was photolyzed at current densities ranging from 5 
 to 25 a m ( figure 4 and 
 figure s5 , supporting information ) .
as 
 expected , less h2o2 photolysis occurred in municipal 
 wastewater effluent due to light screening .
production of h2o2 in the cathode , residual 
 h2o2 after the uv cell , and residual h2o2 after the anode for the four types of source waters 
 at applied current density of 25 a m. see figure 
 s5 , supporting information , for data on 
 the production and removal of h2o2 over the 
 full range of current densities ( 525 a m ) ( wwtp : wastewater treatment plant ) . in practice
, many centralized treatment plants employ reducing 
 agents ( e.g. , bisulfite ) , chlorine , or activated carbon to remove 
 residual h2o2 before distribution
. the use of activated carbon or the addition 
 of chemicals , however , may be impractical in a distributed treatment 
 system .
partial removal of h2o2 occurred when 
 the solution passed through the anode , especially in the electrolyte 
 solution ( figure 4 and figure s5 , supporting information ) .
anodic removal of h2o2 increased with increasing current density ( figure 
 s5 , supporting information ) . in the nacl 

electrolyte solution , up to 0.12 mm h2o2 was 
 removed at 25 a m. for the three source waters , 
 however , the anode only removed about 25% of the amount removed in 
 the electrolyte .
removal of h2o2 in the 
 anode was attributable 
 to a combination of direct anodic oxidation and reactions with oxidants 
 produced on the anode surface .
for example , oxidation of chloride 
 can result in the production of hypochlorous acid ( hocl ; pka = 7.6 ) ( reactions 8 and 9 ) .
hypochlorite reacts rapidly with hydrogen peroxide 
 under alkaline conditions with the bimolecular rate constant increasing 
 from 196 to 7.5  10 m s from ph 6 to 9 ( see the supporting 
 information for calculation of the ph - dependent bimolecular 
 rate constant):10 although ti - iro2 electrodes 
 have a high electrocatalytic 
 activity with respect to chlorine evolution , only modest concentrations 
 of chlorine were produced in control experiments at varying chloride 
 concentrations due to the short hydraulic residence times and relatively 
 low current densities applied ( table 2 ) . to separate the effects of reactive halogen species from direct 
 electrode oxidation on the removal of h2o2 , 
 experiments were repeated using an inert electrolyte ( i.e. , na2so4 ) ( figure s6 , supporting 
 information ) .
h2o2 removal was independent 
 of applied current density with 37  2 m h2o2 removed from 5 to 25 a m ; a concentration 
 equivalent to the observed h2o2 removal in the 
 anode for the three source waters in figure s5 , supporting information . given the low chloride concentrations 
 ( < 1 mm ) of the simulated surface water and groundwater , it is not 
 surprising that ocl production was low and only 
 a small quantity of h2o2 was removed in the 
 anode .
free chlorine production ( as m 
 cl[i ] ) in the anode chamber as a function of applied current density 
 and chloride concentration .
experiments were performed with a stainless 
 steel cathode to prevent h2o2 formation , which 
 interferes with the chlorine measurement . despite the electrolyte and municipal wastewater effluent 
 having 
 roughly the same concentration of chloride , h2o2 removal was significantly higher in the electrolyte control than 
 in the wastewater effluent , suggesting that reactive halogen species 
 did not play an important role in h2o2 removal 
 in the wastewater effluent matrix .
nom is an effective sink of hocl / ocl ; however , under the experimental conditions used in 
 this study , the half - life of hocl / ocl with respect 
 to its reaction with h2o2 was much shorter than 
 that predicted for nom ( i.e. , 1.39 and 49.3 s , respectively , as described 
 in the supporting information ) . as a result , nom is only a minor sink for hocl / ocl in the presence of h2o2 .
this 
 was consistent with observations from experiments in which h2o2 removal decreased by less than 40% ( 58 m ) when 
 nom was added to a solution containing a fixed concentration of chloride 
 at a current density of 25 a m , suggesting that 
 nom is only partially responsible for the difference in h2o2 removal between the two solutions ( figure s6 , supporting information ) .
the apparent discrepancy 
 between municipal wastewater effluent 
 and the sodium chloride solution may be partially attributable to 
 differences in ph values in the anode chamber . at higher ph values , like those found in the municipal 
 wastewater effluent after anodic treatment
, there is a larger driving 
 force for oxygen evolution compared to cl2 production:1112 experiments conducted in the anode chamber at different ph 
 values 
 confirmed that chlorine production increased substantially as ph dropped 
 from 9 to 7 ( figure s7 , supporting information ) . due to the presence of bicarbonate in the municipal wastewater 
 effluent ,
the anode ph was considerably higher ( i.e. , 8 ) during 
 treatment than in the unbuffered nacl electrolyte , where ph decreased 
 to approximately 6 during anodic treatment .
as a result , considerably 
 more hocl / ocl was produced in the anode chamber 
 when the electrolyte was treated .
total hydrogen peroxide removal 
 in the system ( i.e. , photolysis 
 and anodic loss ) was 48  5% for the simulated groundwater , 49 
  3% for the simulated surface water , and 25  3% for the 
 wastewater effluent for the array of current densities tested . at 
 the current density required to produce 3 mg l ( 0.09 mm ) h2o2 , water leaving the treatment 
 system effluent contained 1.52.3 mg l h2o2 .
although h2o2 does not 
 pose a health risk at these concentrations , its presence in potable 
 water may be undesirable .
in a point - of - use water treatment system , 
 it might be possible to remove the excess h2o2 by passing it through activated carbon or a high surface area catalyst 
 consisting of metal oxide or silver .
alternatively , the efficiency of chloride oxidation 
 in the anode chamber might be improved through the use of three - dimensional 
 or porous electrodes that reduce mass transfer limitations or through 
 the use of more catalytic anode materials . despite 
 only accounting for a small fraction of the total removal 
 observed in the treatment system ,
experiments conducted in the absence of uv exposure 
 with solutions amended with 10 g l of trace 
 organic contaminants indicated that direct anodic oxidation resulted 
 in the removal of up to 20% of certain trace organics ( e.g. , propranolol ) 
 at a current density of 25 a m ( figure s8 , supporting information ) .
oxidation of organic 
 compounds on the anode could be increased through the use of inactive 
 anodes ( e.g. , boron - doped diamond , doped - sno2 , pbo2 ) .
these electrodes , however , have higher capital 
 and operating costs than ti - iro2 electrodes .
although 
 carbamazepine is relatively unreactive with hypochlorite , other compounds 
 ( e.g. , propranolol and sulfamethoxazole ) react with hocl ( table s3 , supporting information ) . however , the presence 
 of h2o2 reduced the importance of reactions 
 between trace organics and chlorine species generated at the anode 
 because chlorine preferentially reacts with h2o2 . as a result ,
nearly all of the observed loss of the test compounds 
 in the anode chamber was due to direct oxidation on the anode surface . 

this observation was consistent with experiments comparing anodic 
 removal of trace organics in the presence and absence of h2o2 ( figure s9 , supporting information ) .
the performance of gas 
 diffusion electrodes can decrease over time due to clogging of the 
 pores by precipitates , fouling with nom as well as charge transfer 
 resistance attributable to the loss of conductive graphite paste . in a long - term trial , cathode performance ( i.e. , 
 h2o2 production at a fixed current density ) 

l of tap water amended with 5 
 mm na2so4 was passed through the system at an 
 applied current density of 15 a m ( figure s10 , supporting information ) .
calcium carbonate scaling 
 due to the elevated ph and migration of calcium ions in the tap water 
 into the cathode chamber was observed on the interior of the cathode . 

additional experiments are needed to assess 
 the importance of scaling and the efficacy of simple descaling approaches 
 ( i.e. , polarization reversal ) over longer time periods and more realistic 
 operating conditions .
the treatment 
 system used 
 electricity to produce the oxidant ( i.e. , h2o2 ) and to convert it into ho ( i.e. , the uv lamp ) . 
 electrical energy per order ( eeo )
is a 
 useful figure of merit for comparing the efficiency and cost of the 
 treatment system with other aops .
eeo is 
 the electrical energy ( in kwh ) required to reduce a contaminant concentration 
 by 1 order of magnitude in 1 m of water:13where p ( kw ) is the electrical 
 power for the electrochemical cell and uv lamp , q ( m h ) is the system flow rate , and c0 and c ( m ) are the initial 
 and final contaminant concentrations . without the production of h2o2 , eeo values ranged 
 from 16.8  0.3 to 28.1  0.2 kwh m order , with larger amounts of energy needed to transform 
 contaminants in waters that contained high concentrations of ho scavengers and chromophores ( i.e. , municipal wastewater 
 effluent and synthetic groundwater ) ( figure 5 ) .
a substantial decrease in eeo occurred 
 when current was applied to the electrochemical cell ( i.e. , from 0 
 to 5 a m ) .
as current density increased from 5 
 to 25 a m , the eeo decreased by less than 11% .
aop system is much more efficient than the use 
 of uv alone and that there is a marginal benefit associated with the 
 production of higher h2o2 concentrations in 
 the cathode because the reactions become less efficient at higher 
 concentrations of h2o2 .
only 814% of 
 the energy demand was attributable to the electrochemical production 
 of h2o2 , with the majority of the energy required 
 for the operation of the low - pressure uv lamp . at a current density 
 of 25 a m ,
an energy requirement of 1.08  
 0.1 to 2.84  0.1 kwh m order was observed , which was similar to results from previous studies 
 of the transformation of trace organic compounds by uv / h2o2 in different source waters .
electrical 
 energy per order ( eeo ) for 
 the removal of carbamazepine as a function of current density ( wwtp : 
 wastewater treatment plant ) .
the high coulombic efficiency and low current densities result 
 in a low - cost means of h2o2 production even 
 for treatment of water with low conductivity . for comparison ,
electrochemically 
 produced h2o2 costs between 0.1 and 0.3 $ kg , while h2o2 produced by the 
 anthraquinone process typically costs between 1 and 2 $ kg .
when considering the lack of a need 
 to transport , store , and handle h2o2 as well 
 as modest capital and operational costs , the modular aop treatment 
 system can be a competitive technology for point - of - use treatment 
 at a household and community level or even for wellhead treatment 
 of trace organic contaminants present in potable water sources . as 
 h2o2 production was directly proportional to 
 current density , the treatment system could be scaled up by using 
 faster flow rates accompanied by higher applied current densities 
 or by increasing surface area of the cathode . to obtain the equivalent 
 contaminant removal after scale - up , optimization of the uv reactor 
 geometry would be required .
additional research is needed to assess 
 long - term system performance under realistic operating conditions .
 hydrogen 
 peroxide ( h2o2 ) is frequently used 
 in combination with ultraviolet ( uv ) light to treat trace organic 
 contaminants in advanced oxidation processes ( aops ) . in small - scale 
 applications , such as wellhead and point - of - entry water treatment 
 systems , the need to maintain a stock solution of concentrated h2o2 increases the operational cost and complicates 
 the operation of aops . to avoid the need for replenishing a stock 
 solution of h2o2 , 
 a gas diffusion electrode 
 was used to generate low concentrations of h2o2 directly in the water prior to its exposure to uv light . following 
 the aop 
 , the solution was passed through an anodic chamber to lower 
 the solution ph and remove the residual h2o2 . 
 the effectiveness of the technology was evaluated using a suite 
 of trace contaminants that spanned a range of reactivity with uv light 
 and hydroxyl radical ( ho ) in three different types 
 of source waters ( i.e. , simulated groundwater , simulated surface water , 
 and municipal wastewater effluent ) as well as a sodium chloride solution . 
 
 irrespective of the source water , the system produced enough h2o2 to treat up to 120 l water d1 . 
 the extent of transformation of trace organic contaminants was 
 affected by the current density and the concentrations of ho scavengers in the source water . the electrical energy per order 
 ( eeo ) ranged from 1 to 3 kwh m3 , with the uv lamp accounting for most of the energy consumption . 
 
 the gas diffusion electrode exhibited high efficiency for h2o2 production over extended periods and did not show a 
 diminution in performance in any of the matrices . 
Introduction
Materials and
Methods
Results