WATER-PROCESSING ELECTROCHEMICAL REACTOR

Abstract

A water-processing electrochemical reactor that comprises a cylindrical inner anode (73), an outer tubular cathode (74), an intermediate chamber between the anode (73) and the cathode (74) and being crossed by the water, an outer shell (77) surrounding the cathode (74), a water inlet (71) and a water outlet (78), and a gas inlet (80) and gas outlet (79) connected to the outer shell (77) and to the gas chamber. The cathode surrounds the inner anode (73) and is porous to gas. A gas chamber is defined between the cathode (74) and the outer shell (77). The gas chamber contains a gas comprising oxygen and is at an overpressure that forces the gas through the porous cathode (74).

Claims

1. A water-processing electrochemical reactor comprising: a cylindrical inner anode, wherein the anode is a hollow cylinder with a plurality of openings in the cylinder's side walls, so that the water to be processed is able to flow from the interior of the cylinder to the intermediate chamber through the openings, wherein the anode is a non-soluble anode; an outer tubular cathode that surrounds the inner anode, the cathode being porous to gas; an intermediate chamber between the anode and the cathode, the intermediate chamber being crossed by the water, wherein the distance between the anode and the cathode is below 8 mm; an outer shell surrounding the cathode, where between the cathode and the outer shell a gas chamber is defined, the gas chamber being able to contain a gas comprising oxygen, the gas chamber being able to have the gas at an overpressure that forces the gas through the porous cathode; a water inlet in fluid communication with the interior of the cylindrical anode; a water outlet in fluid communication with the intermediate chamber; and a gas inlet and a gas outlet connected to the gas chamber.

2. The reactor according to claim 1, wherein the anode is microporous, so that the water is able to flow from the interior of the cylinder to the intermediate chamber through the micropores of the anode.

3. The reactor according to claim 1, wherein the anode is of a titanium based metal or mixed-metal oxide.

4. The reactor according to claim 1, wherein the cathode comprises a carbon cloth, coated with carbonaceous powder mixed with hydrophobizing material, preferably polytetrafluoroethylene, and a metallic mesh as current collector, being the carbon cloth and the metallic mesh in contact with each other.

5. The reactor according to claim 4, wherein the coated carbon cloth is catalysed.

6. The reactor according to claim 1, wherein the outer shell is transparent.

7. The reactor according to claim 1, further including a UV source irradiating the intermediate chamber.

8. The reactor according to claim 1, further comprising a photoreactor connected downstream of the water outlet.

9. The reactor according to claim 8, wherein said photoreactor is a UV photoreactor comprising a UV source.

10. The reactor according to claim 8, wherein said photoreactor is a solar photoreactor.

11. The reactor according to claim 1, wherein the distance between the anode and the cathode is between 2 and 3 mm.

12. The reactor according to claim 1, wherein said treatment is a continuous treatment.

13. A process for the treatment of water in a water-processing electrochemical reactor according to claim 1, the process comprising the steps of: supplying a gas comprising O.sub.2 through the gas inlet into the gas chamber, the gas being at an overpressure that forces the gas through the porous cathode and that avoids that the water enters the gas chamber; passing water to be treated through the reactor, entering the water into the interior of the cylindrical anode through the water inlet, passing the water from the interior of the cylindrical anode to the intermediate chamber through the plurality of openings in the cylinder's side walls, and exiting the water from the intermediate chamber through the water outlet; supplying an electric current between the cathode and the anode; and treating the water.

14. The process according to claim 13, further comprising the promotion of an EF process with the H.sub.2O.sub.2 electrogenerated in the reactor.

15. The process according to claim 14, wherein the EF process is catalysed using Fe(II) present in the water, with the generation of ′OH by Fenton's reaction.

16. The process according to claim 15, wherein a source of Fe(II) is added to the water to be treated before entering the reactor.

17. The process according to claim 15, wherein the Fe(II) concentration is between 0.15 mM and 1 mM.

18. The process according to claim 13, wherein said electric current accounts for a cathodic current density between 5 mA cm.sup.−2 and 150 mA cm.sup.−2.

19. The process according to claim 13, wherein the pH of the incoming water is adjusted between 2.8 and 3.0.

20. The process according to claim 13, wherein the treatment is a continuous treatment.

21. The process according to claim 13, wherein said water is wastewater comprising organic matter and said process includes degrading said organic matter through .OH.

22. The process according to claim 13, wherein said treatment is an electrochemical disinfection treatment or a water purification treatment.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0040] Other advantages and features of the invention can be seen from the following description in which the preferred embodiment/embodiments of the invention are described in reference to the attached drawings in a non-limiting manner. In the drawings:

[0041] FIG. 1 is a sketch of the setup with a tubular reactor according to the invention.

[0042] FIG. 2 represents the tubular reactor of FIG. 1.

[0043] FIGS. 3, 4 and 5 show a front, bottom and side view, respectively, of the inlet end of the tubular reactor of FIG. 2.

[0044] FIGS. 6, 7 and 8 show a bottom, front and side view, respectively, of the outlet end of the tubular reactor of FIG. 2.

[0045] FIGS. 9 and 10 show a side and front view, respectively, of the cathode holder of the tubular reactor of FIG. 2.

[0046] FIG. 11 shows a view of a longitudinal section of the transparent outer shell.

[0047] FIG. 12 shows H.sub.2O.sub.2 accumulation, specific conductivity (κ), energy consumption (EC) and current efficiency (CE) determined in different electrolytes.

[0048] Conditions: cathodic current density (j.sub.cath) of 20 mA cm.sup.−2, liquid flow rate of 2 L h.sup.−1, inlet air flow rate of 4 L min.sup.−1, pH 3.0, room T.

[0049] FIG. 13, Concentration of dissolved Fe.sup.2+ and Fe.sup.3+ ions at 40 min, as a function of j.sub.cath in EF and PEF processes. Conditions are: 50 mM Na.sub.2SO.sub.4, 15 mg L.sup.−1 benzotriazole (BTR), ˜5.5 mg L.sup.−1 Fe.sup.2+ (0.1 mM) added at 0 min, liquid flow rate of 2 L h.sup.−1, inlet air flow rate of 4 L min.sup.−1, pH 3.0, room T.

[0050] FIG. 14, (a) BTR and (b) total organic carbon (TOC) abatement by different processes (electro-oxidation (EO), EF and PEF) along time (t), as a function of the liquid flow rate. Conditions are: 50 mM Na.sub.2SO.sub.4, 15 mg L.sup.−1 BTR, ˜5.5 mg L.sup.−1 Fe.sup.2+ (0.1 mM) added at 0 min (in EF and PEF), j.sub.cath of 20 mA cm.sup.−2, inlet air flow rate of 4 L min.sup.−1, pH 3.0, room T.

[0051] FIG. 15, (a) BTR and (b) TOC abatement by different processes (EO, EF and PEF) along time (t). Conditions are: 15 mg L.sup.−1 BTR in urban wastewater, ˜5.5 mg L.sup.−1 Fe.sup.2+ (0.1 mM) in EF and ˜5.5-55.0 mg L.sup.−1 Fe.sup.2+ (0.1-0.5 mM) in PEF added at 0 min, j.sub.cath of 20 mA cm.sup.−2, liquid flow rate of 2 L h.sup.−1, inlet air flow rate of 4 L min.sup.−1, pH 3.0, room T.

DESCRIPTION OF EMBODIMENTS

[0052] This application discloses a novel reactor with ability for degradation and mineralization of organic (or microbiological) toxic contaminants from water. The novel reactor has a tubular air-diffusion cathode for water treatment (preferably in continuous mode), which moreover can be applied in the PEF (and other H.sub.2O.sub.2-based EAOPs) treatment for water decontamination and disinfection at low input current (and hence, low energy consumption).

[0053] A tubular electrochemical reactor devised to carry out EF, PEF or other H.sub.2O.sub.2-based EAOPs is disclosed, which includes: a cylindrical inner anode (a drilled metal oxide membrane anode, which can provide O.sub.2 from simultaneous H.sub.2O oxidation); an outer tubular air-diffusion cathode (to generate H.sub.2O.sub.2) in contact with a tubular metallic mesh (as current collector), as triple-phase boundary electrode; an outer tubular gas chamber (between the cathode and a transparent outer shell). Anode, cathode and gas chamber are assembled in a casing-type, in a concentric fashion; both sides are sealed with screwing inlet and screwing outlet; and the outer packaging shell is installed with inlet and outlet for gas feeding with compressed atmospheric air (from an air pump) and flow rate regulated with a backpressure gauge; and two titanium wires are connected to anode and cathode to supply DC current from a power supply.

[0054] Several process parameters can be adjusted within a wide range: pH value (depending on type of Fenton's catalyst), conductivity (low interelectrode gap that allows operating with solutions of low conductivity), irradiation with UV or sunlight, etc. The whole system includes a reservoir to store the solution to be treated; the tubular reactor for the eletrogeneration of Fenton's reagents (feasibility of Fe(II) electroregeneration as well) connected to the reservoir by a magnetic pump, with flowrate controlled by a flowmeter; an air pump to provide compressed air to the electrochemical reactor outer shell through an inlet, with pressure and flow rate regulated by a backpressure gauge at the outlet; an optional photoreactor (glass tubes irradiated and connected to the outlet of the electrochemical reactor, to operate under continuous PEF conditions; as an alternative, a batch photoreactor to increase the residence time); a reservoir tank connected to the outlet of the photoreactor (or the electrochemical reactor if UV irradiation is not needed).

[0055] The present prototype of electrochemical reactor consists of a central porous titanium-based metal oxide (cylinder) anode and a rolled air-diffusion electrode (i.e., placed coaxially) fed with air from an outer shell (casing). It has a capacity of about 150 mL and can treat up to 10 L h.sup.−1 of water.

[0056] The electrochemical reactor has been operated:

(i) with only H.sub.2O.sub.2 electrogeneration (i.e., EO process with H.sub.2O.sub.2 generation);
(ii) in EF mode (with Fe(II) addition to the solution to be treated); and
(iii) it has also been connected to a photo-Fenton reactor, giving rise to a PEF unit (if desired or needed).

[0057] As will be explained in more detail below, the new reactor is able:

a) to produce H.sub.2O.sub.2 continuously upon current supply and air feeding;
b) to reduce more than 50% metal catalyst need (Fe(II)) typically reported in EF and PEF treatments with air-diffusion cathodes, thanks to regeneration;
c) to completely degrade a model organic pollutant in conductive ultrapure water, in a single passage, upon application of EF process (addition of metal catalyst); and
d) to ensure more than 50% mineralization of solutions with the same model organic pollutant in conductive ultrapure water, in a single passage, upon coupling of the electrochemical reactor with a UV photoreactor (PEF process with UV light or solar photoreactor (SPEF process).

[0058] Dimensions for the Tubular Reactor and Setup

1—Reactor Dimensions:

[0059]

TABLE-US-00001 TABLE 1 Detailed parameters of the reactor Parameter Name Area/cm.sup.2 Cathode 81 Total Anode (16 holes, φ6 mm each) 129 Outer surface 71 Inner surface 58 Volume/mL Total Volume 159 Between cathode and Outer 49 anode surface Inside the anode 110 Distance/cm Anode-Cathode (but it can be 0.9 decreased significantly) Dimensions Anode (inner)/cm L10 × Φ2.5 Anode (outer)/cm L10 × Φ3.0 Cathode/cm L7 (with 4 active parts of W2.9)

[0060] Liquid Flow Rate:

[0061] Continuous system (tests: 1-3 L h.sup.−1). The reactor is intended to work in continuous mode. However, the convenience of recirculation will depend on the desired removal percentage (when the reactor is used for water or wastewater treatment). Therefore, the inclusion of a recirculation can be a preferred option in certain cases.

[0062] Air Flow Rate:

[0063] It is regulated in order to control flooding and humidity in the dry side of the cathode (the one facing the shell). The air pump is connected to the shell inlet through a tube with a valve (in order to minimize excessive air feeding). A tube with a valve is also connected to the shell outlet in order to have enough pressure inside.

[0064] Anode:

[0065] A tubular porous electrode made of a titanium substrate coated with RuO.sub.2 was selected as the anode. As a porous electrode, it provided a large surface area and performance, having superior electrocatalytic activity for direct and indirect oxidation. The anode was drilled to form 16 small holes (φ6 mm, placed perpendicularly facing 4 to 4) to ensure the uniform distribution of solution, avoid pressure problems and allow the interaction between .OH and organic molecules in the reactor volume.

[0066] Cathode:

[0067] A carbon cloth (or carbon paper is also possible) coated with carbon-PTFE dispersion (raw carbon was employed here, but modified carbons could also be used to enhance the electrocatalytic activity to produce H.sub.2O.sub.2 and to reduce Fe(III); and PTFE could be replaced by another polymer to hydrophobize the cathode). The material was rolled, in contact with a conductive metal mesh. Careful sealing must be ensured.

[0068] Catalyst:

[0069] Normally, soluble Fe(II) was used as catalyst (from a salt), but other forms of soluble (i.e., chelated/complexed Fe(II) or Fe(III)) or insoluble (synthetic and natural iron oxides, or other forms of solid iron) iron could be used instead, to perform homogeneous or heterogeneous Fenton processes. These alternatives forms of iron are good to work at natural pH, without needing to adjust pH to 3.0, although pH˜3.0 is the optimum for Fenton's reaction (homogeneous Fenton process) and it is the value selected in this study.

[0070] UV Lamp:

[0071] A photocatalysis unit/reactor with UV lamp, connected to the electrochemical reactor outlet, can be included to perform PEF process. This is based on the occurrence of photo-Fenton reaction (i.e., photoreduction of Fe(III) to Fe(II)), and also promotes the photodecarboxylation of stable complexes of Fe(III) with refractory short-chain aliphatic carboxylates like oxalate. Herein, we choose the lined glass tubes as photo-Fenton reactor. Actually, multiple forms of photo-Fenton reactor can be used instead, such as glass/quartz batch reactors, plate-overflow vessel with a slope, etc. UV light can be replaced by sunlight to give rise to SPEF process.

2—Installation of Reactor

[0072] FIG. 1 shows a sketch of the setup with a tubular reactor according to the invention. It comprises: [0073] (1) Influent tank; [0074] (2) Pump; [0075] (3) Air pump/compressor; [0076] (4) Power supply; [0077] (5) Flowmeter; [0078] (6) Gas valve; [0079] (7) Tubular reactor; [0080] (8) Photoreactor; [0081] (9) Mirror; [0082] (10) UV source; [0083] (11) Effluent tank.

[0084] FIG. 2 shows a detailed view of the tubular reactor. It comprises: [0085] (71) Water inlet; [0086] (72) Wire (electric connection to anode); [0087] (73) Cylindrical inner anode; [0088] (74) Carbonaceous GDE as outer tubular cathode; [0089] (75) Metallic mesh as current collector; [0090] (76) Cathode holder; [0091] (77) Transparent outer shell; [0092] (78) Water outlet; [0093] (79) Gas pipeline as gas outlet. [0094] (80) Gas pipeline as gas inlet.

[0095] The reactor is thus divided into four pieces: [0096] inlet and central anode; [0097] outer rolled cathode (on a holder, to keep the shape); [0098] transparent gas chamber shell; and [0099] outlet.

[0100] Inlet and outlet conic parts are screwed on the shell body.

Experimental Results

[0101] 1. H.sub.2O.sub.2 Production

[0102] The ultimate goal of the reactor is not H.sub.2O.sub.2 production itself. Nonetheless, with this reactor, the continuous H.sub.2O.sub.2 electrogeneration was ensured in different electrolytic media, which is crucial for subsequent application in Fenton-based EAOPs. FIG. 12 shows the effect of electrolyte composition (pure Na.sub.2SO.sub.4 at different concentrations from 5 to 50 mM, mixed sulfate-chloride medium and real wastewater from a municipal wastewater treatment plant) on the H.sub.2O.sub.2 accumulation (fourth bar, right Y axis). The small interelectrode gap allowed that a good H.sub.2O.sub.2 production was feasible even in low conductivity media. On the other hand, the presence of chloride affects the H.sub.2O.sub.2 accumulation, due to the concomitant production of active chlorine at the anode that further reacts with H.sub.2O.sub.2. FIG. 12 also shows the conductivity (κ) of the different electrolytes (first bar, left Y axis) and the effect of electrolyte composition on energy consumption (EC) (second bar, left Y axis) and current efficiency (CE) (third bar, left Y axis).

2. Fe(II) Regeneration

[0103] Second, continuous iron reduction from Fe(III) formed from Fenton's reaction to Fe(II) was ascertained, which can be accounted for the large surface area of the air-diffusion cathode. In FIG. 13, the speciation of iron is depicted, at different current densities (j). A higher current density favored the regeneration. Also, UV photons in PEF process enhanced the regeneration, thus promoting both, electroreduction and photoreduction of Fe(III). These results ensure that, in this reactor, a catalytic amount of dissolved Fe(II) is always present, thus allowing the occurrence of Fenton's reaction.

3. BTR Degradation

[0104] BTR is widely used as corrosion inhibitor. It is not readily (bio)degradable. Hence, it is only partly removed in wastewater treatment plants and a substantial fraction reaches surface water. The inventors carried out the degradation of 15 mg L.sup.−1 BTR solutions by EO, EF and PEF. The j.sub.cath and Fe(II) catalyst concentration (needed in EF and PEF) were optimized, obtaining these values: 20 mA cm.sup.−2 and 0.1 mM (i.e., ˜5.5 mg L.sup.−1) Fe.sup.2+.

[0105] FIG. 14 shows the degradation of BTR solutions by the three processes at different flow rates (1 and 2 L h.sup.−1). In FIG. 14a, BTR removal followed by HPLC; in FIG. 14b, TOC removal. The optimum flow rate depends on the goal: for only pollutant removal, 2 L h.sup.−1 is preferable; if TOC removal is needed, the reactor must be operated at a lower flow rate of 1 L h.sup.−1. The largest combined removals were obtained at 1 L h.sup.−1: 100% BTR removal and 71% TOC removal.

4. BTR Degradation in Urban Wastewater

[0106] BTR was also treated in urban wastewater by the different processes. As shown in FIG. 15a, total BTR removal was also feasible in this medium by PEF, showing a similar rate to that observed in Na.sub.2SO.sub.4 medium thanks to the generation of active chlorine, which counterbalanced the presence of scavengers associated to natural organic matter (NOM) from the real water matrix. The use of 0.2 mM Fe.sup.2+ instead of 0.1 mM contributes to reach a quicker removal in PEF. On the other hand, TOC removal (FIG. 15b) was slightly lower, as expected from the formation of recalcitrant chlorinated by-products. The increase of Fe.sup.2+ concentration to 0.5 mM enhanced the TOC abatement up to 40%. Note that the initial TOC in these experiments was higher than in FIG. 14 due to the contribution of the aforementioned NOM components.

5. Toxicity Assessment

[0107] The toxicity evolution during the treatments in urban wastewater was evaluated via Microtox® method. EC.sub.50 values of different samples allowed concluding that: [0108] The addition of BTR to urban wastewater increased the toxicity markedly, which justifies the need for an effective water treatment. [0109] The EF treatment decreased the toxicity (EC.sub.50 increase), which means that BTR and some of the organic compounds in wastewater were degraded. [0110] The PEF treatment was the most effective to decrease the toxicity (largest increase of EC.sub.50), ending in a relatively harmless solution, which means that most of the potentially toxic organochlorinated compounds are destroyed in PEF.

Acronym List

[0111] BDD boron-doped diamond
BTR benzotriazole
CE current efficiency
EAOP electrochemical advanced oxidation process
EC energy consumption
EF electro-Fenton
EO electro-oxidation
GDE gas diffusion electrode
MWCNT multi-walled carbon nanotube
NOM natural organic matter
ORR oxygen reduction reaction
PEF photoelectro-Fenton
PTFE polytetrafluoroethylene
SPEF solar photoelectro-Fenton
TOC total organic carbon