novel advanced oxidation process systems for water ... · development is reported of two novel...

1

NOVEL ADVANCED OXIDATION PROCESS SYSTEMS FOR WATER PURIFICATION ndash PROGRESS AND PROSPECTS FOR LARGE SCALE APPLICATIONS

A J Karabelas K V Plakas V C Sarasidis S I Patsios

Laboratory of Natural Resources and Renewable Energies (NRRE) Chemical Process and Energy Resources Institute (CPERI)

Centre for Research and Technology-Hellas (CERTH) Thermi-Thessaloniki 57001 Greece

Keywords Advanced Oxidation Processes electro-Fenton hybrid photocatalysis-membrane systems Presenting author email karabajcpericerthgr

Abstract Development is reported of two novel systems for effective water treatment (without addition of oxidants or reject streams) relying on different Advanced Oxidation Process (AOP) concepts An electro-Fenton system using a special cell can electro-generate H2O2 in situ thus facilitating the ensuing production of potent radicals (eg bullOH H2O2) the latter are capable of oxidizing refractory organic compounds This cell (a flow-through ldquofilterrdquo ) is a stack of anodiccathodic electrode pairs with the electro-Fenton process occurring on iron-impregnated carbonaceous cathodes Tests with the pharmaceutical diclofenac show rapid removal (gt95) due to enhanced pollutant electro-adsorption on electrodes and subsequent oxidation A hybrid photocatalysis ndash UF membrane separation system based on the catalytic activity of UV-irradiated semi-conductor nanoparticles can degrade recalcitrant pollutants also through formation of radicals This system involves coupling a suspended-TiO2 particle photocatalysis reactor with an ultrafiltration membrane module for particle separation Very good performance is shown in degrading model pollutants (sodium alginate humic acids and diclofenac) Although total mineralization is not achieved the observed complete destruction of aromatic rings eliminates the pollutants potential toxic effects Successful continuous operation of the hybrid system is due to automatic periodic membrane back-washing effectively controlling fouling Considering the attributes of these methods an assessment is made of process requirements for several classes of potential applications mainly characterized by pollutant type and purified water usage leading to RampD priorities which would yield results necessary for reliable system scale-up and optimization

1 Introduction

In various water sources (surface and ground waters effluents for reuse) assorted refractory organic compounds are encountered which are either anthropogenic [1] or of natural origin (eg Natural Organic Matter ndash NOM [2]) The former category comprises a great variety of synthetic compounds including pharmaceuticals pesticides personal care products that are incriminated for adverse health effects [3] Moreover during chlorination of drinking water NOM can act as a precursor for the formation of Disinfection By-Products (DBP) since the latter pose a serious health hazard several countries take measures to ensure control of DBP and NOM [2] Conventional methods to remove these pollutants are either ineffective or inefficient and thus costly additionally they often ldquotransferrdquo the pollution problem from water to another medium (eg absorbents) requiring further handling Extensive research on Advanced Oxidation Processes (AOP) [4] has provided the background for developing effective water treatment systems with no need for external addition of costly and potentially dangerous oxidants and no reject streams This paper deals with development of two such systems based on different AOP concepts

A novel electro-Fenton system The coupling of AOP with electrochemistry has led to electro-chemical AOP which have attracted significant attention showing remarkable effectiveness for decontamination of wastewater polluted with toxic compounds including pharmaceutical residues [5] The most popular is the Electro-Fenton (EF) method on which various related processes [6] are based The EF reaction involves continuous electrogeneration of Fenton reactants (H2O2+Fe2+Fe3+) for the formation of hydroxyl radicals in accordance with the classical Fenton reaction [(Eq (2)] thus increasing the efficacy of the related processes

O2 + 2H+ +2e- rarr H2O2 (1) Fe2+ + H2O2 + H+

rarr Fe3+ + H2O + bullOH (2) Fe3+ + e- rarr Fe2+ (3) These reactions occur on the cathodic electrode whereas water oxidation takes place on the anode H2O rarr 12O2 +2H+ + 2e- (4)

The majority of the related electrochemical cells have the form of simple stirred tank reactors with oxygen (or air) bubbled close to the cathodic electrode A few researchers have designed and tested batch recirculation flow systems

2

where water flows between the anodecathode electrodes (undivided cells) or from the anodic to the cathodic compartment (cells divided by a glass frit or membrane so as to alleviate the different reactions taking place in the two compartments) [2] Iron ions are either added to the contaminated water (homogeneous EF) or embedded onto suitable electrode materials (heterogeneous EF) to catalyze H2O2 electrogeneration for production of oxidizing OH

In view of the EF potential a flow-through electrochemical cell has been designed and constructed in the form of a ldquofilterrdquo consisting of a pair of anodic and cathodic electrodes made of carbonaceous materials In this device fast O2 reduction and significant electro-synthesis of H2O2 take place the latter depending solely on the dissolved O2 and the oxygen generated by water oxidation over the anode The reaction of H2O2 with ferric ions or iron-based nanoparticles impregnated in the cathode (heterogeneous EF) produces oxidizing agents (bullOH HO2

bull) which are consumed in reactions involving organic matter degradation and Fe2+ regeneration This paper reports on the key issue of H2O2 production rate by the novel ldquofilterrdquo and on its efficiency for degrading the pharmaceutically active compounds (PhAC) diclofenac (DCF) carbamazepine (CMZ) ibuprofen (IBU) and sulfamethoxazole (SMX)

A hybrid photocatalysis ndash UF membrane separation system based on the catalytic activity of UV-irradiated semi-conductor nanoparticles permits degradation of recalcitrant compounds on the catalyst surface and by hydroxyl radicals near the particles Heterogeneous photocatalysis employing UV-A irradiation and semiconductor catalyst particles -commonly TiO2- has a widely demonstrated efficiency for degrading a broad range of organic substances [7-9] However the successful application of suspended TiO2 photocatalytic treatment of water streams is constrained by some technical challenges mainly related to the effective recovery of catalyst particles after water treatment [8] A promising approach for separation and reuse of suspended TiO2 is the Photocatalytic Membrane Reactor (PMR) concept [10-11] involving the coupling of photocatalysis with a membrane separation process [12] Two novel systems are described which involve coupling the suspended-type TiO2 nanoparticle photocatalysis process with UF membranes permitting a continuous steady state operation The PMR systems are used to treat water containing Sodium Alginate (SA) Humic Acids (HA) and DiCloFenac (DCF) [13-16] selected as representative compounds of NOM and PhAC frequently encountered in water sources The degradation rate and the removal efficiency of the aforementioned compounds are assessed as a function of several key process parameters such as TiO2 loading feed-water pH pollutant concentration and intensity of UV-A irradiation The operation of the PMR is also evaluated with regard to the performance of the UF membrane module Special attention is paid to the attainment of steady-state operation with constant degradation rates and controlled membrane fouling phenomena

2 Experimental Work

21 Electro-Fenton system

211 Electrode materials and characterization The reduction of oxygen to produce hydrogen peroxide with a high yield occurs only on certain cathodic materials including carbon because of the low oxygen solubility in water and the slow mass transport The use of porous high-surface carbon-based electrodes favors high mass transfer rates of dissolved oxygen and the simultaneously occurring reactions (1) and (3) thus significantly enhancing EF process efficiency Four different carbon materials were tested as anodic andor cathodic electrodes Three of them were of carbon fiber type two were provided by the University of Alicante Spain and the third was purchased from MAST Carbon Intern Ltd (UK) Another carbon electrode was made of powdered carbon compressed to form discs Several analytical tools were employed for characterization of electrode specific surface area porosity surface structure and conductivity ie XRD N2 adsorption desorption porosimetry SEM [17] All materials are micro-porous exhibit low electrical resistance (130-520Ω) their characteristics indicative of an especially effective support for dispersingembedding iron ions andor nano-particles The effective dispersionimpregnation of ferric ions in the above materials was tested using as Fe source three different iron salts (Fe(NO3)39H2O FeCl3 FeCl36H2O) and FeOFe2O3 nanoparticles (magnetite) for different FeC ratios (10 30 50 70 ww) The ICP results of aqueous samples withdrawn during the synthesis of the Fe-cathodic electrodes display an increased embeddingdispersion of iron on the CF-1410 carbon fibers especially when treated with anhydrous or hydrated ferric chloride salts or magnetite nanoparticles also shown in the scanning electron microscope (SEM) images of Figure 1

212 Electro-Fenton ldquofilterrdquo and experimental procedure

The experimental setup and the novel electro-Fenton ldquofilterrdquo specially designed for this work are presented in Figure 2 details are provided in a recent publication [17] The capability of different electrode materials to electrogenerate

3

Figure 1 Scanning electron microscope (SEM) images of new and treated with FeCl36H2O or FeOFe2O3 carbon fiber specimens (carbon fiber with a specific surface area 1410m2gr and thickness ~2mm-CF-1410)

Figure 2 Schematic representation of a) the experimental setup and b) the electro-Fenton ldquofilterrdquo [3]

H2O2 through reaction (1) was studied by electrolyzing 005M Na2SO4 solutions at constant potential in the absence of cathodes impregnated with iron particles ie both anode and cathode were of the same carbon material Controlled-potential electrolysis was used for the optimization of H2O2 electro-generation rate in relation to potential and electrode material Linear sweep voltammetry (LSV) tests were also performed as an alternative tool for assessing the range of potential over which H2O2 is generated at suitable rates and the respective results are described elsewhere [17] PhAC removal by the electro-Fenton ldquofilterrdquo was studied by using a pair of anodecathode electrodes with embedded ferric ions andor iron based nanoparticles on the cathodic carbon fiber The respective experimental protocol includes the following steps

I Filtration of pure water in continuous mode for one hour and measurement of the iron catalyst removal from the cathodic electrode (ICP measurements)

II Batch recirculation of the electrolytic solution (Na2SO4+single PhAC) with no current applied until the two carbon electrodes are saturated with the tested PhAC (stabilization of the respective bulk concentration)

III Initiation of the electrolysis under different working conditions (electric potential feed pH dissolved oxygen concentration etc) and study of PhAC decay with time

Analyses H2O2 concentration was determined spectro-photometrically (UV-1700 Pharmaspec Shimadzu) after the samples were filtered through 045 microm PTFE Millipore membrane Total organic carbon values were determined

CF-1410 CF-1410FeCl36H2O CF-1410FeOFe2O3

b) a)

4

with a Shimadzu TOC analyzer whereas the temporal variation of PhAC concentration was followed by reversed-phase HPLC using a Shimadzu (LC-10AD VP) liquid chromatograph fitted with a XTerra MSC (Waters) column coupled with a UVVis detector (SPD-10AVP) selected at 270nm (DCF) 285nm (CMZ) 2215nm (IBU) and 266nm (SMX) The determination of PhAC in concentrations of few microgL involves a pre-concentration by solid-phase extraction (Discoveryreg DSC-18 SPE Tubes 500mg 3 mL) and application of an internal standard for checking the recovery of the method developed Aromatic intermediates were identified by GCndashMS using an Agilent Technologies system composed of a 7890A GC fitted with a HP-5MS polar column and a 5975C mass spectrometer The pH of electrolytic solutions was adjusted to desired values by H3PO4 and NaOH dilute solutions

22 Hybrid photocatalysis ndash UF membrane separation system

221 Materials and methods Titanium dioxide particles (Aeroxidereg P25 Degussa-Evonik) were used as photocatalyst The TiO2 nano-particles (75 anatase and 25 rutile) have an average primary particle size of 21 nm In aqueous dispersions TiO2 particles tend to aggregate and form fairly large agglomerates of size ranges depending on various parameters [12] Organic compounds used as typical pollutants include Sodium Alginate Humic Acids and Diclofenac purchased from Sigma-Aldrich Feed solutions were prepared using Deionized Water (DW) in the case of SA and HA tests and

Figure 3 Schematic diagrams of the novel (a) PMR1 and (b) PMR2 systems

Ground Water (GW) without chlorination in DCF tests Calcium chloride dehydrate (CaCl2middotH2O) was added in DW to obtain 1 mM Ca2+ feed solution concentration The pH of the suspension in the PMR was adjusted by adding either H2SO4 or NaOH The UF membranes made of hydrophilized Polyvinylidene fluoride (PVDF) with a nominal pore size of 004 microm were provided by Zenon Environmental Inc (GE Power amp Water) DCF concentration was determined through reversed-phase HPLC using a Shimadzu (LC-10AD VP) liquid chromatograph UV254 absorbance measurements were taken with a UVVis (UV-1700 Shimadzu) spectrophotometer SUVA254

(a)

(b)

5

measurements defined as SUVA254 = (UV254TOC) x 100 quantify the degradation of HA aromatic rings that are closely linked with the potential of organic matter for undesirable DBP formation

222 PMR laboratory pilost

Two laboratory scale pilot PMR systems (PMR1 and PMR2) were designed and constructed in this Laboratory in Figures 3 schematic diagrams of these systems are provided PMR1 of total effective volume 9 L was comprised of a cylindrical tank with a submerged UF membrane module of surface area 047 m2 UV-A irradiation was provided by three 30W black light blue lamps hydraulically connected in series The reaction temperature was controlled at 20oC and the mean permeate flux was kept constant at ~14 L(m2middoth) Air was supplied into the membrane tank through a coarse bubble aerator PMR2 of total effective volume 3 L was comprised of a jacketed cylindrical vessel (photocatalytic reactor) made of anodized aluminium connected to a Plexiglas vessel (membrane vessel) where a custom-made UF membrane module (of surface area 0097 m2) was submerged Four borosilicate glass tubes in the form of sleeves closed at the immersed end were fixed and properly sealed on holes of the top flange of the reactor Four 24W black light lamps were employed as UV-Α light source of the system The operating permeate flux was kept constant at approx 15 L(m2middoth) Air was supplied by a small tube placed at the center of the photoreactor In both PMR systems a piston pump was used to withdraw permeate at the same time averaged flow rate as that of the feed thus maintaining constant the working volume of the PMR whereas an online pressure transducer was used to monitor the Trans-Membrane Pressure (TMP) which allows to assess the fouling behaviour of the membrane module Moreover an automated periodic backwashing operation was implemented to mitigate membrane fouling Details on constructionoperation of these special PMR systems are provided elsewhere [13-16]

3 Results and discussion


Figure 4 depicts H2O2 accumulated in the recycled solution as a function of electrolysis time at the optimum cathodic potentials for the four different carbon materials employed it is evident that the CF-1410 and CFm-1005 electrodes exhibit the best performance Specifically a remarkable increase of current efficiency is observed with time when the CF-1410 electrodes are used with an average steady-state value at ~70 (Fig 4b) and a steady concentration of accumulated H2O2 ~107 mgL (Fig 4a) H2O2 electro-generation is not necessarily proportional to the current intensity since the highest H2O2 concentrations were measured for low current densities with CF-1410

Figure 4 a) H2O2 concentration and b) current efficiency as a function of electrolysis time at optimum cathodic potentials for the four different carbon materials CF-1371 and CF-1410 at 13VAgAgCl CFm-1005 at 10VAgAgCl CCB-470 at 05VAgAgCl Solutions of 005M Na2SO4 pH 3 recirculation liquid flow 300 mLmin temperature 25 oC anodic and cathodic electrodes of the same material

electrodes Probably a large fraction of electricity is consumed by side reactions as evidenced by the decreasing current with time for all other materials Considering Ohmrsquos law this phenomenon is attributed to a gradually increasing resistance in the system since a constant potential is applied to the ldquofilterrdquo The limit concentrations are reached when the reactions become mass transfer controlled Any further increase of the current will lead to parasitic reactions of H2O2 consumption which in turn can cause a reduction of the electrical yield

0

10

20

30

40

50

60

70

80

0 50 100 150 200 250 300 350 400Electrolysis time (min)

CE

()

CF-1410CFm-1005CCB-470CF-1371

0

2000

4000

6000

8000

10000

12000


H2O

2 (micro

gL

)

CF-1410CFm-1005CCB-470CF-1371

a) b)

6

A series of experiments with a pair of anode and cathode CF-1410 electrodes exhibited a significant H2O2 electro-generation at low controlled electrode potential regardless of the feed water pH (Figure 5a) On the contrary H2O2 electro-generation was significantly influenced by the electrolyte concentration probably due to the lower current densities recorded for the less conducting solutions (Figure 5b) Considering that significant production of H2O2 is obtained for a single pair of electrodes even in the case of low ionic strength feed waters research is carried out on the possible IR drop compensation due to treated water by investigating system design and parameter modifications including a reduced inter-electrode distance andor an increased current density (under galvanostatic conditions)

Figure 4 Effect of a) solution pH and b) solution ionic strength (different Na2SO4 concentrations) on hydrogen

Figure 5 Effect of a) solution pH and b) solution ionic strength (different Na2SO4 concentrations) on hydrogen peroxide electrogeneration as a function of electrolysis time Constant potential 13 VAgAgCl recirculation liquid flow-rate 300 mLmin temperature 25 oC CF-1410 anodic and cathodic electrodes

Table 1 summarizes results of DCF degradation as a function of different Fe-cathodic CF-1410 electrodes The percentage removal of DCF and TOC correspond to the stabilized concentrations of DCF at the end of electrolysis runs (3-24 hours) with reference to the initial pollutant concentration after the pre-saturation of the two carbonaceous electrodes with DCF The total charge consumed (Q) is calculated using the values of the electric current and the electrolysis time recorded during each experiment (VersaStat software) As shown in Table 1 the FeC content of the cathode and the iron source used play a significant role in the ldquofilterrdquo efficiency to degrade diclofenac An increase of FeC ratio results in higher degradation efficiencies with the optimum initial ratio being 30 it is noted that a higher Fe content eg 50 FeC did not improve the DCF degradation efficiency or the TOC mineralization capacity of the ldquofilterrdquo (data not shown here) Furthermore the hydrated ferric chloride appeared to be the best iron source for the purposes of this work An enhanced electro-sorption of DCF onto the two carbon electrodes is reflected in the reduced concentration of diclofenac during the electrolysis with pure CF-1410 electrodes In turn the reduced H2O2 concentrations and the identification of aromatic intermediates in the bulk solution (obtained in GCMS analyses) are evidence of electro-Fenton activity when iron catalysts are embedded on the cathodic electrode Although total mineralization is not achieved (TOC removal ~50) the observed destruction

of aromatic rings of DCF eliminates their potential toxic effects

Table 2 summarizes results of the four PhAC removals as a function of Fe-CFm-1005 carbon fibers of 30 FeC wt that have been placed below the cathodic electrode (thus minimizing the corrosion of the metallic current carriers that are in contact only to the cathodic CFm electrode) The percentage removal of PhAC and TOC is determined using the respective stabilized concentration data at the end of the electrolysis runs (24-72 hours) with reference to the initial pollutant concentration after the pre-saturation of the three carbonaceous electrodes (anode cathode and FeC electrode) An enhanced electro-sorption of all PhACs onto the three carbon electrodes is reflected in the sharp reduction of the bulk solute concentration during the first 2 hours of electrolysis Moreover the reduced H2O2 concentrations and the identification of aromatic intermediates in the bulk solution (obtained in GCMS analyses) atlonger electrolysis times are evidence of electro-Fenton activity The degradation and mineralization efficiency of the filter varies depending on the target compound the reduced TOC mineralization is attributed to organic intermediates formed during the electrolysis (as evidenced by GCMS analysis) which appear to resist further degradation at the specific conditions (solution pH constant potential 1VAgAgCl) of the present experiments

0

2000

4000

6000

8000

10000

12000


H2O

2 (micro

gL)

0050M Na2SO40025M Na2SO40010M Na2SO4

0

2000

4000

6000

8000

10000

12000


H2O

2 (micro

gL

)

pH 3

pH 5

pH 7

a) b)

7

Table 1 Results of electro-Fenton degradation of diclofenac (DCF) (pH 3 005M Na2SO4 recirculation liquid flow 50 mLmin temperature 25 oC 13 VAgAgCl) Anode CF-1410

Adsorption of DCF Electrolysis of DCF Electrodes - Cathode minitialDCF

(g) mads DCF

(gDCFgCF-1410) Q

(Coulomb)

DCF TOC

Cathode CF-1410 0069 0041 1923 372 -

Cathode CF-1410 FeC 10 (FeCl3) 0085 0079 1856 550 45


Cathode CF-1410 FeC 10 (FeCl36H2O) 0073 0064 11550 730 322


Cathode CF-1410 FeC 30 (FeOFe2O3) 0101 0010 64606 627 359

Table 2 Results of electro-Fenton degradation of PhAC (anode CFm-1005 cathode CFm-1005 + Fe-CFm-1005 30 FeC physical pH 005M Na2SO4 recirculation liquid flow 50 mLmin temperature 25 oC)

Adsorption Electrolysis PhAC Cinitial (mgL) (gPhACgCFm-1005) PhAC TOC

Diclofenac (DCF) 43 0051 718 292

Carbamazepine (CMZ) 196 (x2)a 0025 680 201

Ibuprofen (IBU) 484 0061 736 670

Sulfamethoxazole (SMX) 396 0057 871 349 a Recirculation of feed solutions of the same initial concentration (considering CMZ water solubility)


321 Photocatalytic degradation of Humic Acids

It should be stressed that in all tests the TMP remained practically constant for the long-time test period (usually ~48h) which proves that membrane fouling was insignificant thereby allowing a steady state continuous PMR operation The main results regarding HA photocatalytic degradation are summarized in Figure 6 It appears that there is an optimum pH near 55 where the HA mineralization rate reaches a maximum for both HA feed concentrations (5 and 10 mgL HA) this maximum rate is 956 and 1413 mgTOCh respectively The same trend holds for the HA mineralization efficiency that varies from 498 to 739 and from 496 to 626 for 5 and 10 mgL HA feed concentration respectively The effect of pH on the HA mineralization rate may be attributed to the complicated interplay of several factors including (i) the ζ-potential of the TiO2 particle surface (ii) the agglomeration of TiO2 particles and (iii) the formation potential of hydroxyl radicals [18] The iso-electric point for Degussa P25 TiO2 is at pH ~63 and the catalyst surface is positively charged at acidic conditions thus promoting

Figure 6 Effect of pH on the HA (a) mineralization rate and (b) mineralization efficiency for two HA feed-water concentrations Conditions UV-A radiant power 151 W TiO2 concentration gL backwashing mode ldquo19rdquo Numbers in parentheses at the base of the bars denote the measured time averaged pH

(a) (b)

8

adsorption and subsequent oxidation of negatively charged molecules such as HA However TiO2 particles tend to agglomerate under acidic conditions [10] and the specific surface area of catalyst agglomerates is reduced negatively affecting the photo-oxidation rates The backwashing mode employed does not seem to affect the effectiveness of the photocatalytic mineralization process Indeed although two tests were carried out under two different backwashing modes (19 and 115) with otherwise identical operating conditions the HA estimated mineralization efficiencies and mineralization rates were very close (ie 956 and 947 mgTOCh) Concerning the reduction of the DBP formation potential of water streams containing HA the PMR1 exhibits excellent performance The feed SUVA254 values vary between 473 and 644 m-1(mgL) whereas the permeate SUVA254 is below 036 m-1(mgL) except for one test where it is quite higher [124 m-1(mgL)] Thus the overall SUVA254 removal efficiency is very high (gt 95 in almost all cases) ie consistently higher than the corresponding HA overall removal efficiency

322 Photocatalytic degradation of Sodium Alginate

The SA degradation data depicted in Figure 7 show the important effect of catalyst concentration indeed an increase of TiO2 loading up to ~1 gL leads to a systematic increase of the SA mineralization rate whereas the mineralization efficiency reaches a maximum at ~075 gL TiO2 The maxima of mineralization rate and efficiency are ~104 mgTOCh and ~75 respecively these values are close to those for HA photo-oxidation under similar conditions One might have expected this trend since the increase of the catalyst concentration leads to an increase of the active sites on TiO2 surface ie the surface area of the TiO2 available for degradation However further increase of TiO2 concentration appears to have a negative effect on percentage TOC removal This particular trend is usually attributed [10] to possible UV-A light blocking resulting from the increased turbidity of the higher TiO2 suspension concentration It is interesting to note that experiments performed with the same concentration of photocatalyst but under different backwashing frequencies exhibit practically the same mineralization efficiency

Figure 7 Effect of TiO2 concentration on SA (a) mineralization rate and (b) mineralization efficiency Conditions UV-A radiant power 177 W pH 67 plusmn 04 mean SA feed concentration mgTOCL backwashing mode ldquo15rdquo

323 Photocatalytic degradation of Diclofenac

The reported series of experiments was carried out with ~2 mgL DCF feed concentration and three different TiO2

concentrations ie 03 05 and 075 gL Figure 8 presents the DCF degradation and TOC percentage removal for various TiO2 loadings and three different initial DCF concentrations in the reactor [DCFbulk] Mineralization efficiency denotes the complete degradation of both DCF molecule and its partially oxidized fragments The PMR system reaches steady state operation after approx 1-2 h and can achieve more than 95 DCF degradation in almost all cases whereas the mineralization efficiency varies between 397 and 69 additionally it seems to achieve a maximum at (near optimum) parameter values pH ~ 6 and TiO2 concentration of 05 gL

4 Concluding remarks - Prospects

The results with the novel EF ldquofilterrdquo- type device using electrodes made of porous carbonaceous materials shows that significant H2O2 electro-generation occurs at low controlled electrode potential regardless of pH andor

(a) (b)

(gL) (gL)

9

Figure 8 Effect of (a) TiO2 concentration and (b) DCFbulk concentration on DCF degradation and mineralization efficiency Conditions UV-A radiant power 197 W backwashing mode ldquo19rdquo average feed-water TOC ~2 mgL

ionic strength of feed water The effective embedding of catalytic iron on the cathode facilitates Fenton reactions generating strong oxidizing species capable of degrading recalcitrant micro-pollutants The enhanced pollutant electro-adsorption on the carbon electrodes and ensuing oxidation by utilizing only electricity highlight the great potentialities of the proposed EF ldquofilterrdquo as an environmentally friendly and effective technology The PMR system employing suspension-type catalyst nano-particles possesses similar attractive attributes with demonstrated capability to operate continuously at steady state Both systems are considered most appropriate for treating potable water as well as various effluents with recalcitrant micro-pollutants that would otherwise require conventional treatments (ie addition of oxidants andor absorbents) with all their economic and environmental disadvantages To further develop these systems for applications the following main issues are identified that need particular attention

Electro-Fenton ldquofilterrdquo-type system

bull Electrode materials long-term stability of catalytic iron embedded into carbonaceous materials

bull EF ldquofilterrdquo optimum design morphology type and arrangement of stack of anodecathode pairs

bull Appraisal of electrical energy requirements design of autonomous systems powered by renewable energies

It should be pointed out that these main issues are to a large extent interrelated For instance as outlined above to

optimize H2O2 electro-generation one may have to deal with IR drop compensation through system design and

operating parameter modifications including optimization of inter-electrode distance in connection with applied

current density Moreover there is scope to investigate alternative modes of applying electricity to the EF ldquofilterrdquo

Long term ldquofilterrdquo stability and overall good performance is related to system cost effectiveness

Photo-catalytic Membrane Reactor system

bull Photocatalytic reactor design in relation to UF membrane module reactor morphology for particular UV sources

bull UV light source artificial sources solar light

bull Catalyst type and life-time catalyst activated in particular range of UV light spectrum catalyst deactivation

Here again these important issues are intimately interrelated As significant progress (eg [19]) has been made in

obtaining catalysts that enable solar light utilization appropriate photocatalytic reactor configurations should be

developed However to cope with the problem of intermittent solar light availability hybrid schemes involving

solar-artificial light combinations should be likely pursued inevitably such schemes would impact on the photo-

catalytic reactor design if continuous operation is desirable Regarding artificial light utilization other aspects that

need attention are the electricity cost as well as the heat dissipated by the conventional UV lamps The novel Light

Emitting Diodes (LED) appear to have advantages over the conventional UV lamps in relation to electric energy

efficiency long-term performance stability and reduced heat dissipation although their cost is relatively high

The above brief account suggests that significant RampD is required to further develop the AOP-based systems

Research along these lines is performed in the authorsrsquo Laboratory and related patent applications have been filed

(a) (b)

0

20

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100

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20

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100

030 050 075

UV254

TOC

DC

F d

egra

dat

ion

eff

icie

ncy

(

)

TiO2 concentration (gL)

DC

F m

ineralization

efficiency (

)

0

20

40

60

80

100

0

20

40

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100

000 250 800

UV254

TOC

DC

F d

egra

dat

ion

eff

icie

ncy

()

DC

F m

ineralization

efficiency (

)

DCFbulk

concentration (mgL)

10

Acknowledgements

Financial support by the Gen Secretariat for Research amp Technology Ministry of Education through the programme EPAN-IIESPA ldquoSYNERGASIArdquo project No 09-SYN-42-630 for the electro-Fenton system development is gratefully acknowledged Other project partners are the Laboratory of Inorganic Materials at CPERI ndash CERTH and TEMAK SA mainly collaborating on development of electrode materials and pilot system respectively

References 1 Frimmel FH Abbt-Braun G Heumann KG Hock B Lundemann HD and Spiteller M Refractory Organic

Substances in the Environment Wiley-VCH Weinheim 2001

2 Barrett SE Krasner SW and Amy GL Natural organic matter and disinfection byproducts-characterization and control in drinking water in ACS Symposium Series vol 761 American Chemical Society 2000

3 Schwarzenbach RP Escher BI Fenner K Hofstetter TB Johnson CA von Gunten U and Wehrli B The challenge of micropollutants in aquatic systems Science 313 (2006) 1072-1077

4 Comninellis C Kapalka A Malato S Parsons SA Poulios I Mantzavinos D Perspective Advanced oxidation processes for water treatment advances and trends for RampD J Chem Technol Biotechnol 83 (2008) 769ndash76

5 Sireacutes Iand Brillas E Remediation of water pollution caused by pharmaceutical residues based on electrochemical separation and degradation technologies A review Env Int 40 (2012) 212ndash229

6 Brillas E I Sireś I Oturan MA Electro-Fenton process and related electrochemical technologies based on Fentonrsquos reaction chemistry Chem Rev 109 (2009) 6570minus6631

7 Gaya UI and Abdullah AH Heterogeneous photocatalytic degradation of organic contaminants over titanium dioxide a review of fundamentals progress and problems J Photochem Photobiol C 9 (2008) 1-12

8 Chong MN Jin B Chow CWK and Saint C Recent developments in photocatalytic water treatment technology a review Water Res 44 (2010) 2997-3027

9 Liu S Lim M Fabris R Chow C Drikas M and Amal R TiO2 photocatalysis of natural organic matter in surface water Impact of trihalomethane and haloacetic acid formation potential Environ Sci Technol 42 (2008) 6218-6223

10 Mozia S Photocatalytic membrane reactors (PMRs) in water and wastewater treatment A review Sep Purif Technol 73 (2010) 71-91

11 Molinari R Borgese M Drioli E Palmisano L and Schiavello M Hybrid processes coupling photocatalysis and membranes for degradation of organic pollutants in water Catal Today 75 (2002) 77-85

12 Ho DP Vigneswaran S and Ngo HH Photocatalysis-membrane hybrid system for organic removal from biologically treated sewage effluent Sep Purif Technol 68 (2009) 145-152

13 Sarasidis VC Patsios SI and Karabelas AJ A hybrid photocatalysisndashultrafiltration continuous process The case of polysaccharide degradation Sep Purif Technol 80 (2011) 73-80

14 Patsios SI Sarasidis VC Karabelas AJ A hybrid photocatalysis - membrane continuous process for degradation of refractory organic matter Sep Purif Technol 104 (2013) 333ndash341

15 Karabelas AJ Sarasidis VC and Patsios SI The effect of UV radiant power on the rate of polysaccharide photocatalytic mineralization Chemical Engineering Journal 229 (2013) 484-491

16 Sarasidis VC Plakas KV Patsios SI and Karabelas AJ Investigation of diclofenac degradation in a continuous photo-catalytic membrane reactor Influence of operating parameters (2013) submitted for publication

17 Plakas KV Karabelas AJ Sklari SD Zaspalis VT Toward the development of a novel electro-Fenton system for eliminating toxic organic substances from Water Part 1 In situ generation of hydrogen peroxide Ind Eng Chem Res (2013) doi 101021ie400613k

18 Konstantinou IK and Albanis TA TiO2-assisted photocatalytic degradation of azo-dyes in aqueous solution kinetic and mechanistic investigations Appl Catal B 49 (2004) 1-14

19 Mboula VM Hequet V Andres Y Pastrana-Martinez LM Dona-Rodriguez JM Silva AMT Falaras P Photocatalytic degradation of endocrine disruptor compounds under simulated solar light Water Research 47 (2013) 3997-4005

2

where water flows between the anodecathode electrodes (undivided cells) or from the anodic to the cathodic compartment (cells divided by a glass frit or membrane so as to alleviate the different reactions taking place in the two compartments) [2] Iron ions are either added to the contaminated water (homogeneous EF) or embedded onto suitable electrode materials (heterogeneous EF) to catalyze H2O2 electrogeneration for production of oxidizing OH

In view of the EF potential a flow-through electrochemical cell has been designed and constructed in the form of a ldquofilterrdquo consisting of a pair of anodic and cathodic electrodes made of carbonaceous materials In this device fast O2 reduction and significant electro-synthesis of H2O2 take place the latter depending solely on the dissolved O2 and the oxygen generated by water oxidation over the anode The reaction of H2O2 with ferric ions or iron-based nanoparticles impregnated in the cathode (heterogeneous EF) produces oxidizing agents (bullOH HO2

bull) which are consumed in reactions involving organic matter degradation and Fe2+ regeneration This paper reports on the key issue of H2O2 production rate by the novel ldquofilterrdquo and on its efficiency for degrading the pharmaceutically active compounds (PhAC) diclofenac (DCF) carbamazepine (CMZ) ibuprofen (IBU) and sulfamethoxazole (SMX)

A hybrid photocatalysis ndash UF membrane separation system based on the catalytic activity of UV-irradiated semi-conductor nanoparticles permits degradation of recalcitrant compounds on the catalyst surface and by hydroxyl radicals near the particles Heterogeneous photocatalysis employing UV-A irradiation and semiconductor catalyst particles -commonly TiO2- has a widely demonstrated efficiency for degrading a broad range of organic substances [7-9] However the successful application of suspended TiO2 photocatalytic treatment of water streams is constrained by some technical challenges mainly related to the effective recovery of catalyst particles after water treatment [8] A promising approach for separation and reuse of suspended TiO2 is the Photocatalytic Membrane Reactor (PMR) concept [10-11] involving the coupling of photocatalysis with a membrane separation process [12] Two novel systems are described which involve coupling the suspended-type TiO2 nanoparticle photocatalysis process with UF membranes permitting a continuous steady state operation The PMR systems are used to treat water containing Sodium Alginate (SA) Humic Acids (HA) and DiCloFenac (DCF) [13-16] selected as representative compounds of NOM and PhAC frequently encountered in water sources The degradation rate and the removal efficiency of the aforementioned compounds are assessed as a function of several key process parameters such as TiO2 loading feed-water pH pollutant concentration and intensity of UV-A irradiation The operation of the PMR is also evaluated with regard to the performance of the UF membrane module Special attention is paid to the attainment of steady-state operation with constant degradation rates and controlled membrane fouling phenomena

2 Experimental Work


211 Electrode materials and characterization The reduction of oxygen to produce hydrogen peroxide with a high yield occurs only on certain cathodic materials including carbon because of the low oxygen solubility in water and the slow mass transport The use of porous high-surface carbon-based electrodes favors high mass transfer rates of dissolved oxygen and the simultaneously occurring reactions (1) and (3) thus significantly enhancing EF process efficiency Four different carbon materials were tested as anodic andor cathodic electrodes Three of them were of carbon fiber type two were provided by the University of Alicante Spain and the third was purchased from MAST Carbon Intern Ltd (UK) Another carbon electrode was made of powdered carbon compressed to form discs Several analytical tools were employed for characterization of electrode specific surface area porosity surface structure and conductivity ie XRD N2 adsorption desorption porosimetry SEM [17] All materials are micro-porous exhibit low electrical resistance (130-520Ω) their characteristics indicative of an especially effective support for dispersingembedding iron ions andor nano-particles The effective dispersionimpregnation of ferric ions in the above materials was tested using as Fe source three different iron salts (Fe(NO3)39H2O FeCl3 FeCl36H2O) and FeOFe2O3 nanoparticles (magnetite) for different FeC ratios (10 30 50 70 ww) The ICP results of aqueous samples withdrawn during the synthesis of the Fe-cathodic electrodes display an increased embeddingdispersion of iron on the CF-1410 carbon fibers especially when treated with anhydrous or hydrated ferric chloride salts or magnetite nanoparticles also shown in the scanning electron microscope (SEM) images of Figure 1

212 Electro-Fenton ldquofilterrdquo and experimental procedure

The experimental setup and the novel electro-Fenton ldquofilterrdquo specially designed for this work are presented in Figure 2 details are provided in a recent publication [17] The capability of different electrode materials to electrogenerate

3









b) a)

4






(a)

(b)

5









0

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30

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80


CE

()

CF-1410CFm-1005CCB-470CF-1371

0

2000

4000

6000

8000

10000

12000


H2O

2 (micro

gL

)

CF-1410CFm-1005CCB-470CF-1371

a) b)

6







0

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10000

12000


H2O

2 (micro

gL)


0

2000

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10000

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H2O

2 (micro

gL

)

pH 3

pH 5

pH 7

a) b)

7



(g) mads DCF

(gDCFgCF-1410) Q

(Coulomb)

DCF TOC

Cathode CF-1410 0069 0041 1923 372 -










Ibuprofen (IBU) 484 0061 736 670






(a) (b)

8










(a) (b)

(gL) (gL)

9


























(a) (b)

0

20

40

60

80

100

0

20

40

60

80

100

030 050 075

UV254

TOC

DC

F d

egra

dat

ion

eff

icie

ncy

(

)


DC

F m

ineralization

efficiency (

)

0

20

40

60

80

100

0

20

40

60

80

100

000 250 800

UV254

TOC

DC

F d

egra

dat

ion

eff

icie

ncy

()

DC

F m

ineralization

efficiency (

)

DCFbulk

concentration (mgL)

10

Acknowledgements






















3









b) a)

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(a)

(b)

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0

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80


CE

()

CF-1410CFm-1005CCB-470CF-1371

0

2000

4000

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12000


H2O

2 (micro

gL

)

CF-1410CFm-1005CCB-470CF-1371

a) b)

6







0

2000

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10000

12000


H2O

2 (micro

gL)


0

2000

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8000

10000

12000


H2O

2 (micro

gL

)

pH 3

pH 5

pH 7

a) b)

7



(g) mads DCF

(gDCFgCF-1410) Q

(Coulomb)

DCF TOC

Cathode CF-1410 0069 0041 1923 372 -










Ibuprofen (IBU) 484 0061 736 670






(a) (b)

8










(a) (b)

(gL) (gL)

9


























(a) (b)

0

20

40

60

80

100

0

20

40

60

80

100

030 050 075

UV254

TOC

DC

F d

egra

dat

ion

eff

icie

ncy

(

)


DC

F m

ineralization

efficiency (

)

0

20

40

60

80

100

0

20

40

60

80

100

000 250 800

UV254

TOC

DC

F d

egra

dat

ion

eff

icie

ncy

()

DC

F m

ineralization

efficiency (

)

DCFbulk

concentration (mgL)

10

Acknowledgements






















4






(a)

(b)

5









0

10

20

30

40

50

60

70

80


CE

()

CF-1410CFm-1005CCB-470CF-1371

0

2000

4000

6000

8000

10000

12000


H2O

2 (micro

gL

)

CF-1410CFm-1005CCB-470CF-1371

a) b)

6







0

2000

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8000

10000

12000


H2O

2 (micro

gL)


0

2000

4000

6000

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10000

12000


H2O

2 (micro

gL

)

pH 3

pH 5

pH 7

a) b)

7



(g) mads DCF

(gDCFgCF-1410) Q

(Coulomb)

DCF TOC

Cathode CF-1410 0069 0041 1923 372 -










Ibuprofen (IBU) 484 0061 736 670






(a) (b)

8










(a) (b)

(gL) (gL)

9


























(a) (b)

0

20

40

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80

100

0

20

40

60

80

100

030 050 075

UV254

TOC

DC

F d

egra

dat

ion

eff

icie

ncy

(

)


DC

F m

ineralization

efficiency (

)

0

20

40

60

80

100

0

20

40

60

80

100

000 250 800

UV254

TOC

DC

F d

egra

dat

ion

eff

icie

ncy

()

DC

F m

ineralization

efficiency (

)

DCFbulk

concentration (mgL)

10

Acknowledgements






















5









0

10

20

30

40

50

60

70

80


CE

()

CF-1410CFm-1005CCB-470CF-1371

0

2000

4000

6000

8000

10000

12000


H2O

2 (micro

gL

)

CF-1410CFm-1005CCB-470CF-1371

a) b)

6







0

2000

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12000


H2O

2 (micro

gL)


0

2000

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12000


H2O

2 (micro

gL

)

pH 3

pH 5

pH 7

a) b)

7



(g) mads DCF

(gDCFgCF-1410) Q

(Coulomb)

DCF TOC

Cathode CF-1410 0069 0041 1923 372 -










Ibuprofen (IBU) 484 0061 736 670






(a) (b)

8










(a) (b)

(gL) (gL)

9


























(a) (b)

0

20

40

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80

100

0

20

40

60

80

100

030 050 075

UV254

TOC

DC

F d

egra

dat

ion

eff

icie

ncy

(

)


DC

F m

ineralization

efficiency (

)

0

20

40

60

80

100

0

20

40

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100

000 250 800

UV254

TOC

DC

F d

egra

dat

ion

eff

icie

ncy

()

DC

F m

ineralization

efficiency (

)

DCFbulk

concentration (mgL)

10

Acknowledgements






















6







0

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H2O

2 (micro

gL)


0

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H2O

2 (micro

gL

)

pH 3

pH 5

pH 7

a) b)

7



(g) mads DCF

(gDCFgCF-1410) Q

(Coulomb)

DCF TOC

Cathode CF-1410 0069 0041 1923 372 -










Ibuprofen (IBU) 484 0061 736 670






(a) (b)

8










(a) (b)

(gL) (gL)

9


























(a) (b)

0

20

40

60

80

100

0

20

40

60

80

100

030 050 075

UV254

TOC

DC

F d

egra

dat

ion

eff

icie

ncy

(

)


DC

F m

ineralization

efficiency (

)

0

20

40

60

80

100

0

20

40

60

80

100

000 250 800

UV254

TOC

DC

F d

egra

dat

ion

eff

icie

ncy

()

DC

F m

ineralization

efficiency (

)

DCFbulk

concentration (mgL)

10

Acknowledgements






















7



(g) mads DCF

(gDCFgCF-1410) Q

(Coulomb)

DCF TOC

Cathode CF-1410 0069 0041 1923 372 -










Ibuprofen (IBU) 484 0061 736 670






(a) (b)

8










(a) (b)

(gL) (gL)

9


























(a) (b)

0

20

40

60

80

100

0

20

40

60

80

100

030 050 075

UV254

TOC

DC

F d

egra

dat

ion

eff

icie

ncy

(

)


DC

F m

ineralization

efficiency (

)

0

20

40

60

80

100

0

20

40

60

80

100

000 250 800

UV254

TOC

DC

F d

egra

dat

ion

eff

icie

ncy

()

DC

F m

ineralization

efficiency (

)

DCFbulk

concentration (mgL)

10

Acknowledgements






















8










(a) (b)

(gL) (gL)

9


























(a) (b)

0

20

40

60

80

100

0

20

40

60

80

100

030 050 075

UV254

TOC

DC

F d

egra

dat

ion

eff

icie

ncy

(

)


DC

F m

ineralization

efficiency (

)

0

20

40

60

80

100

0

20

40

60

80

100

000 250 800

UV254

TOC

DC

F d

egra

dat

ion

eff

icie

ncy

()

DC

F m

ineralization

efficiency (

)

DCFbulk

concentration (mgL)

10

Acknowledgements






















9


























(a) (b)

0

20

40

60

80

100

0

20

40

60

80

100

030 050 075

UV254

TOC

DC

F d

egra

dat

ion

eff

icie

ncy

(

)


DC

F m

ineralization

efficiency (

)

0

20

40

60

80

100

0

20

40

60

80

100

000 250 800

UV254

TOC

DC

F d

egra

dat

ion

eff

icie

ncy

()

DC

F m

ineralization

efficiency (

)

DCFbulk

concentration (mgL)

10

Acknowledgements






















10

Acknowledgements






















novel advanced oxidation process systems for water ... · development is reported of two novel...

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