Device for photocatalytically treating water, method for preparing it and its use

Abstract

Described is a device for photocatalytically treating water, comprising a substrate having at least one surface onto which a photocatalyst for treating water is immobilised with an acrylic adhesive and a binder, a method for preparing the device, its use for removing contaminants and an apparatus for removing contaminants from water containing this device.

Description

[0001] Device for photocatalytically treating water , method for preparing it and its use

[0002] The present invention relates to a device for photocatalytically treating water, a method for preparing the device , its use for removing contaminants from water and an apparatus for removing contaminants from water containing the device according to the present invention .

[0003] Typically, the removal of organic contaminants from water has been undertaken via contaminant capture methods such as filtration or flocculation, or biological digestion utilising bacteria . The constraints surrounding capture technology is the accumulation of contaminants removed from water forming a waste stream . Disposal of this waste can be both costly and difficult and may pose further environmental harm . The constraints surrounding biological treatment is the large amount of carbon sources (e . g . methanol ) required to feed the bacteria as well as the intensive labour in operating these treatment systems . Additionally, these conventional water purification methods often do not successfully manage chemically and microbiologically synthetic organic compounds which may be dissolved in water at low concentrations , such is often the case with industrial and agricultural wastewater and textile dyes .

[0004] There is a void in the industrial water treatment market for a technology which can completely mineralise organic contaminants during treatment with low operational effort and cost . In response to this void, advanced oxidation processes (AOPs ) have emerged over the last decade . Many AOPs involve the use of a consumable such as ozone or hydrogen peroxide as well as utilising mercury based UVC light with high energy consumption . AOPs using a combination of UV light and nanoparticle metal oxides has over the past 20 years emerged as a more sustainable water treatment method as there are much less consumables . The main constraint to this more sustainable treatment method is the re-collection of the nanoparticles from the water following treatment . Filtration at a nano scale tends to result in fouling of membranes , blockages and releases of nanoparticles to the environment .

[0005] The challenge for the industry has been to find a suitable way to immobilise the nanoparticles on to a substrate whilst still managing to maintain a photocatalytically viable amount of catalyst as well as to make physical contact with the target contaminants in the water so as to complete the water treatment obj ectives .

[0006] Therefore , in view of the above prior art the technical problem underlying the present invention is to provide a device , wherein nanoparticles used as photocatalysts for treating water are immobilised on to a substrate whilst still managing to maintain a photocatalytically viable amount of catalyst as well as to make physical contact with the target contaminants in the water so as to complete the water treatment obj ectives .

[0007] This has been achieved by the subj ect-matter of the independent claims . Preferred embodiments are defined in the dependent claims .

[0008] According to the present invention, there is provided a device for photocatalytically treating water, comprising a substrate having at least one surface onto which a photocatalyst for treating water is immobilised with an acrylic adhesive and a binder .

[0009] As pointed out above , processes for treating water for removing contaminants are already known in the prior art , wherein photocatalysts , in particular nanoparticle metal oxides , are used . The device according to the present invention can comprise those photocatalysts for treating water already known from the prior art mentioned above . Thus , the skilled person does know the chemical constitution of the photocatalysts for treating water as well as to obtain them .

[0010] The photocatalyst for treating water is immobilised onto the at least one surface of the substrate with an acrylic adhesive and a binder . By the term "immobilised" it is meant that the acrylic adhesive and the binder function in such a way that the photocatalyst for treating water is maintained on one place . Furthermore , the term "immobilised" does also mean that the photocatalytic surface for treating water is not completely covered or embedded but it is ensured that a sufficient catalytic activity is maintained and that sufficient contact with the water to be treated and the electromagnetic waves is provided .

[0011] In one embodiment , the device according to the present invention further comprises reduced graphene oxide . Reduced graphene oxide is commercially available . The utilisation of the reduced graphene oxide reduces electron / hole recombination and therefore facilitates the use of the device at an industrial scale . It increases the contaminant mineralisation rate constant by approximately 8-fold .

[0012] In one embodiment , the photocatalyst for treating water comprises a metal oxide , for example the metal oxide is selected from the group consisting of titanium dioxide (Ti02 ) , zinc oxide ( ZnO) and indium oxide ( In2Oa) .

[0013] In a further embodiment the photocatalyst for water treatment is present as nanoparticles . The term "nanoparticles" as understood according to the present invention can mean particles of matter having 1 to 500 nm, in particular 1 to 100 or fibers and tubes that are less than 100 nm in only two directions . Transmission Electron Microscopy (TEM) can be used to determine particle size .

[0014] Acrylic adhesives are generally known and commercially available . In one embodiment according to the present invention, the acrylic adhesive is directly provided onto the surface of the substrate or alternatively the acrylic adhesive is provided onto the surface of the substrate in the form of a double-sided acrylic tape , for example a double-sided acrylicbased hydrophilic nano-tape . The advantage of using a doublesided tape is the easy commercial availability, affordability and ease of use .

[0015] The binder can be a polymeric compound having a molecular weight ranging from 400 to 124000 . In one embodiment of the device according to the present invention, the binder is selected from polyethylene glycol ( PEG) , in particular with a molecular weight ranging from 400 to 6000 , and polyvinyl acetate ( PVA) , in particular with a molecular weight ranging from 85000 to 124000 . Molecular weight in particular of PEG and PVA can be determined via aqueous size exclusion chromatography (aqueous SEC) . By using a binder, the adhesion to the acrylic adhesive is improved .

[0016] The weight ratio of photocatalyst for treating water to binder is greater than 1 , i . e . an excess of photocatalyst for treating water referred to the binder is present , wherein the upper limit of the ratio can be 10 : 0 . 1 . In one embodiment , the ration of photocatalyst for treating water to binder is 10 : 0 . 1 for PEG and 6 : 1 for PVA . Adding too much binder may block the pores within the catalyst and therefore reduce the adsorption of the contaminant onto the catalyst ' s surface . Adding less binder may still be effective , however the binding of the catalyst to the acrylic adhesive will be reduced resulting in less re-usability of the catalyst .

[0017] The device according to the present invention contains a substrate . The substrate is in particular suitable for applying the photocatalyst for treating water, the acrylic adhesive and the binder . Furthermore, the substrate is suitable to be moved in water to be cleaned and to be irradiated with curing light , for example UV (LED) light . In a further embodiment of the device according to the present invention, the substrate is selected from the group consisting of glass , plastics , for example polyacrylates and polymethylmethacrylates , and metal . These substrates have the above properties and it is in particular easy to provide the device according to the present invention in apparatuses for cleaning water .

[0018] The device according to the present invention is described in the following in more detail referring to some specific embodiments .

[0019] The device according to the present invention can use an acrylic adhesive tape to physically immobilise photocatalysts for treating water, like photocatalytic metal oxide nanoparticles , onto a stable substrate (e . g . glass , plastic or metal ) , which then can be moved through the contaminated water using for example a motorised water wheel style mechanism to improve contact with any organic contaminants present in the water and facilitate adsorption on to the photocatalytic surface . The photocatalytic surfaces can be irradiated with any curing light suitable for the photocatalyst for treating water, in particular with energy efficient UV light , for example UV LED light which can result in the creation of hydroxide radicals on the photocatalytic surface which mineralise organic contaminants via reduction reactions , so that water can be effectively treated .

[0020] The device according to the present invention can involve the immobilisation of for example photoactive metal oxides using an acrylic adhesive tape and PEG or PVA as a binder on a solid substrate and then attaching this substrate into a wheel type mechanism which moves the substrate through contaminated water under UV irradiation .

[0021] Metal oxides (e . g . titanium dioxide , zinc oxide and indium oxide ) as thin film coatings and powder dispersions are commonly used in scientific research for mineralisation of various organic contaminants from water . When such photocatalytically active metal oxides are exposed to UV light , they tend to have a high oxidation potential and are able to mineralise organic contaminants to carbon dioxide , mineral acids , hydrogen and water . The present invention focuses on in particular photoactive metal oxides which are biologically and chemically inert , inexpensive and easily sourced . As an energy source , the present invention can utilise UVA LED light which is both energy efficient and mercury free . The photocatalytic process is initiated by the photogeneration of hole / electron pairs in the semiconductor by absorption of a UV light of a specific wavelength equal to or greater than the band gap of the metal oxide used . The band gap of the metal oxides is also lowered by the doping of the metal oxides with other highly conductive metal oxides . Where electrons and holes do not recombine , they initiate redox reactions with electron donor and acceptor species adsorbed onto the catalyst surface . Two forms of mineralisation may then occur . One is through the formation of hydroxyl radicals which are highly reactive and able to oxidise organic molecules , and the other is the reaction between oxygen molecules with negative electrons forming a superoxide anion, which can form hydrogen peroxide and hydroxyl radicals which then also oxidise organic molecules .

[0022] To overcome the issues with the prior art attempts at treating wastewater with metal oxide based AOPs , the present invention utilises a novel adhesion mechanism to physically bond for example metal oxide nanoparticles and dopants onto a substrate which is then rotated through the water containing organic contaminants requiring treatment . The double mechanism of adhesion and movement overcomes any potential issues of loss of nanoparticles into the water, allows for a large surface area of photocatalyst relative to the volume of water being treated, and facilitates both mixing of the pollutant water and improved contact with the actual contaminant molecules present in the water .

[0023] Subj ect-matter of the present invention is as well a method for preparing a device for photocatalytically treating water, in particular the device as defined above , comprising the following steps :

[0024] (a) providing an acrylic adhesive layer onto at least one surface of a substrate , and

[0025] (b) coating the acrylic adhesive layer with a dispersion or solution of photocatalyst and a binder . The dispersion medium or the solvent can be water .

[0026] As pointed out above , the device according to the present invention can also comprise reduced graphene oxide ( rGO) . The rGO can be provided in that it is applied in the solution / dispersion together with the photocatalyst and the binder .

[0027] With this method, the device according to the present invention as described above is obtainable . The materials , which can be used in the method according to the present invention, in particular the photocatalyst , the acrylic adhesive and the binder, are described above in detail in connection with the device according to the present invention . In order to avoid unnecessary repetitions , it is therefore referred to the above in its entirety .

[0028] Specifically, in order to immobilise nanoparticles constituting the photocatalyst , an acrylic adhesive layer can be applied to a substrate then the adhesive can be coated with a for example homogenised dispersion of metal oxide nanoparticles in pure water (H2O) and for example polyethylene glycol ( PEG) as a binder while the adhesive is still wet . The PEG acts as a binder and improves the adhesion to the acrylic adhesive . The acrylic adhesive can then be gelled and bind the nanoparticles , for example as a layer, to its surface . After coating the wet adhesive , the surface of the adhesive and its coating can be heated to a temperature of 100 °C and 140 °C, in particular about 120 ° C in for example air, to evaporate the water used to make the dispersion . The adhesive can be left to fully dry (that is dried until wight consistency is achieved) in air at room temperature and then the coated substrate can be washed twice under sonication to remove any unbound nanoparticles .

[0029] A number of substrates coated with the adhesive layer and nanoparticles can then be fitted to a cylindrical wheel mechanism which can be rotated by for example a motor through the contaminated water whilst also being irradiated by UVA LED light . The UVA LED lights and wheel or wheels can be housed in a reactor, for example a stainless steel reactor, and water can be fed in to an inflow for example on the bottom of the reactor and out through an outflow for example at the top of the reactor . The reactor can then be operated either in a batch setup, a continuous flow system or a side loop batch system ( recirculating) . This allows for the flexibility to run the reactors in series or parallel depending on the concentration of contaminants to be treated and wastewater flow rates of the treatment requirements .

[0030] Furthermore, the present invention relates to the use of the device according to the present invention for removing contaminants from water . For this use , the device according to the present invention can be moved through the water containing the contaminants and simultaneously the device according to the present invention can be irradiated with for example UV light . Further details for the use according to the present invention can be taken from the above description of the device and the method of its production .

[0031] Moreover, the present invention relates to an apparatus for cleaning water containing the device according to the present invention . Such a device is construed in such a sense that on the one hand the device according to the present invention can be moved through water and that on the other hand the device according to the present invention can be irradiated with UV light . One example of a suitable apparatus is exemplified below in the figures .

[0032] In the following, the present invention is illustrated by the example and the figures . It is explicitly pointed out that neither the example nor the figures shall be construed to limit the invention thereto .

[0033] Fig . 1 shows wheels provided with the device of the present invention .

[0034] Fig . 2 shows a reactor, which can contain the wheels of Fig . 1 . Fig. 3 is a top view showing the wheels of Fig. 1 in the reactor of Fig. 2.

[0035] In the Figures, the same parts have the same reference numerals .

[0036] Fig. 1 shows three wheels (1) containing the device according to the present invention comprising a substrate coated with metal oxide immobilised with an acrylic nano-tape inserted into wheels which turn. This turning of the wheels can be accomplished by a DC motor (2) driving the wheels attached by drive pulleys and belts.

[0037] Fig. 2 exemplifies a reactor (7) which can contain the wheels (1) of Fig. 1. The reactor (7) comprises a reactor body (3) , which can be made for example of stainless steel, provided with an outflow (4) , for example a DN65 threaded pipe, an inflow (5) , for example a DN25 threaded pipe, and a lid (6) .

[0038] Fig. 3 is a top view showing the wheels (1) of Fig. 1 in the reactor body (3) of Fig. 2. This plan view also shows UV LED lights (8) irradiating the substrate from all sides and initiating the photocatalytical reaction.

[0039] Example

[0040] Preparation of Nanocomposite Catalyst

[0041] The Ti02 / rG0 (reduced graphene oxide) / PEG nanocomposite catalyst was prepared via a direct and hydrothermal method.

[0042] To prepare the photocatalytic surface, firstly 13 g of Ti02 nanopowder (consisting of 80% anatase and 20% rutile) was added to 90mL of ultrapure water and sonicated for 15 minutes to form a TiC>2 dispersion. A second solution was made by adding 0 . 2 g of rGO to l OmL of ultrapure water and sonicated for 15 minutes to form a rGO dispersion . The rGO dispersion was added dropwise to the iO2 dispersion during sonication followed by further sonication for 15 minutes, resulting in a homogenised solution of rGO at a 1 . 5% wt of TiO2 . The homogenised solution was then transferred to a PTFE-lined stainless steel autoclave and heated at 180 ° C for 6 hours . The solution was then allowed to cool overnight prior to opening .

[0043] Coating of the Substrate

[0044] An acrylic sheet was used as a substrate for the coating . The substrate was cleaned with acetone once , rinsed with distilled water, cleaned twice with isopropanol then rinsed with distilled water and left to dry upright at 60 °C for 30 minutes . Double-sided acrylic-based hydrophilic nano-tape was then applied onto the acrylic substrate and this was then left for 72 hours to form a strong bond .

[0045] 2mL of polyethylene glycol ( PEG) with a molecular weight of 600 was added to the previously homogenised TiO2 / rGO dispersion and the TiO2 / rGO / PEG solution was then sonicated for 15 minutes . The backing foil was removed from the double-sided tape and the TiO2 / rGO / PEG solution was then sprayed onto the exposed adhesive side of the tape using an ultrasonic spray unit at a distance of 20cm until visual coverage of the tape was achieved . After spraying, a heat gun set to 120 °C was used to evaporate visible excess water from the sprayed tape . Spraying and drying was then repeated until the substrate had been sprayed and dried a total of 3 times . After the final spray and dry, the substrate was left in a drier at 40 °C and regularly weighed until it reached its constant weight ( i . e . no further weight was lost ) .

[0046] Photocatalytic Organic Contaminant Mineralisation The coated substrate / s were then inserted into a wheel mechanism within the reactor ( see figures ) . The coated substrate / s were then physically moved through the water containing the contamination, whilst being irradiated with 365nm UVA LED light .

Claims

1. Claims1 . Device for photocatalytically treating water, comprising a substrate having at least one surface onto which a photocatalyst for treating water is immobilised with an acrylic adhesive and a binder .2 . The device according to claim 1 , wherein the device further comprises reduced graphene oxide .3 . The device according to claim 1 or 2 , wherein the photocatalyst for treating water comprises a metal oxide .4 . The device according to any of the preceding claims , wherein the photocatalyst for water treatment is present as nanoparticles .5 . The device according to any of the preceding claims , wherein the acrylic adhesive is directly provided onto the surface of the substrate or wherein the acrylic adhesive is provided onto the surface of the substrate in the form of a double-sided acrylic tape .6 . The device according to any of the preceding claims , wherein the binder is selected from polyethylene glycol ( PEG) and polyvinyl acetate ( PVA) .7 . The device according to any of the preceding claims , wherein the ratio of photocatalyst for treating water to binder is greater than 1 .8 . The device according to any of the preceding claims , wherein the substrate is selected from the group consisting of glass , plastic, and metal .9 . Method for preparing a device for photocatalytically treating water, in particular the device as defined in any of the preceding claims , comprising the following steps :(a) providing an acrylic adhesive layer onto at least one surface of a substrate , and(b) coating the acrylic adhesive layer with a dispersion or a solution of a photocatalyst and a binder .10 . The method according to claim 9 , wherein the solution or dispersion of step (b) further comprises reduced graphene oxide .11 . The method according to claim 9 or 10 , wherein the photocatalyst for water treatment comprises a metal oxide , which for example can be selected from the group consisting of titanium oxide , zinc oxide and indium oxide .12 . The method according to claim 9 to 11 , wherein the photocatalyst for water treatment is present as nanoparticles .13 . The method according to any of claims 9 to 12 , wherein the acrylic adhesive is directly provided onto the surface of the substrate or wherein the acrylic adhesive is provided onto the surface of the substrate in the form of a double-sided acrylic tape .14 . The method according to any of claims 9 to 13 , wherein the binder is selected from polyethylene glycol ( PEG) and polyvinyl acetate ( PVA) .15 . The method according to any of claims 8 to 12 , wherein the ratio of photocatalyst for treating water to binder is greater than 1 .

16. The method according to any of claims 9 to 15, wherein the substrate is selected from the group consisting of glass, plastic, and metal.

17. Use of the device defined in any of claims 1 to 8 for removing contaminants from water.

18. Apparatus for removing contaminants from water containing the device defined in any of claims 1 to 8.

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