6.3.3 State of R&D
Extensive literature reviews on the diverse technologies and nanoparticles used for water treatment can be found in: Theron et al. (Theron et al., 2008) and Savage and Diallo (Savage and Diallo, 2005).
Photocatalysis (mainly by TiO2) is an effective method for the degradation of pollutants in water and for water disinfection. A comprehensive review of photocatalytic nano-TiO2 for environmental applications is found in Kwon et al.(Kwon et al., 2008). This review also includes information on the mechanism of degradation and the special properties of nano-TiO2 (compared to micro-TiO2).
To avoid free nanoparticles in water, TiO2 particles can be immobilised on a substrate e.g. by dip coating, spray coating or electrophoretic coating (Byrne et al., 1998). Alternatively TiO2 thin films can be produced on FTO glass substrates and polypropylene fibres by sol-gel routes and plasma processing (Rickerby and Morrison, 2007).
McMurray et al. (McMurray et al., 2005) have tested electrochemically assisted photocatalysis (EAP) for the break down of formic acid under UVA and UVB irradiation on nanocrystalline TiO2 films. They found that the rate of formic acid oxidation under UVB irradiation was 30% greater as compared to UVA irradiation. However, a cost analysis of the process would be required to determine the economic viability of employing UVB sources. McMurray et al. conclude that electrochemically assisted photocatalysis may prove beneficial in large-scale reactors where mass transfer limitations exist. In a further experiment, Byrne et al. (Byrne et al., 2002) added a second cathode compartment to the cell for the simultaneous photocatalytic oxidation of formic acid and the recovery of copper ions from solution. Formic acid was degraded at the TiO2 photoanode and copper metal was recovered at the copper mesh cathode with high efficiency (Byrne et al., 2002).
McMurray et al. (McMurray et al., 2006) further investigated the photochemical degradation of the herbicide atrazine on nanocrystalline TiO2 films under UVA and UVB-irradiation. Atrazine removal followed first order kinetics and the rate was dependent upon catalyst loading up to an optimum loading (above which a decrease in the degradation rate was observed). The maximum apparent quantum yield for the photocatalytic degradation was higher under UVB (0.59%) compared to UVA (0.34%).
TiO2 was also tested for its efficiency in removing the estrogenic activity of several potent and environmentally relevant steroid estrogens (Coleman et al., 2004). Coleman et al. found that photocatalysis over titanium dioxide was equally effective at removing the estrogenic activity of all three steroid substrates tested in aqueous solutions with a 50% reduction in estrogenicity within 10 min. They concluded that TiO2 photocatalysis is a promising technology for the treatment of water contaminated with endocrine disrupting chemicals.
Priya and Madras (Priya and Madras, 2006)analysed the degradation of nitrobenzene by nTiO2. The photocatalytic degradation rates of combustion synthesized Tio2 were considerably higher compared to that of Degussa P-25. The kinetics of photocatalytic degradation of aliphatic carboxylic acids by nTiO2 was analysed by Chen et al.(Chen et al., 2009).
Navagemi et al. analysed the solar photocatalytic degradation of organic compounds (Nagaveni et al., 2004a) and various dyes such as methylene blue (MB), remazol brill blue R (RBBR) and orange G (OG) (Nagaveni et al., 2004b) by combustion synthesized nano-TiO2. The initial degradation rates with combustion synthesized nano TiO2 was 20 times higher for RBBR, 4 times higher for MB and 1.6 times higher for OG compared to Degussa P-25 TiO2. They attribute the enhanced photocatalytic activity of the combustion synthesized catalyst to the crystallinity, nano-size, large amount of surface hydroxyl species and reduced band-gap. The degradation of dyes was also investigated by other authors (Andronic and Duta, 2008; Comparelli et al., 2004; Thompson et al., 2009; Wang et al., 2009; Zhang et al., 2009) (Shi et al., 2009; Tian et al., 2009). Hsieh et al. (Hsieh et al., 2010) report that the photocatalytic activity of nTiO2 combined with nZVI was superior to those of nZVI and nano neutral TiO2 sol (measured by the reductive decolourization of acid Black-24).
Modification of TiO2 with noble metals has shown significant promise in increasing the activity of titania for a variety of catalytic processes. Orlov et al. investigated the photocatalytic performance of TiO2 doped with gold nanoparticles (Orlov, 2006; Orlov et al., 2004). The photocatalytic degradation of methyl tert-butyl ether (MTBE) by gold-modified TiO2 (with a gold particle size of smaller or equal to 3 nm) exhibited a threefold rate enhancement compared to unmodified TiO2 (Orlov et al., 2007). Orlov et al. also funcitonalized mesoporous (pore size 2-50nm) molecular sieves with TiO2 for the decomposition of organic pollutants in air and water (Orlov et al., 2006). They found that such materials exhibit a significant activity for oxidation of organic pollutants. Anandan et al. 2008 (Anandan et al., 2008) modified TiO2 with Ag for the photocatalytic degradation of the textile dye Acid red 88. Wang et al. (Wang et al., 2008)investigated the efficiency of Au-loaded TiO2 (Au/TiO2) as a sonocatalyst. They found that the catalyst Au/TiO2 (0.5wt%) greatly accelerated both the discoloration and total organic carbon (TOC) removal of azo dyes such as orange II, ethyl orange, and acid red G compared to bare TiO2 and nano-Au catalyst.
Sonophotocatalysis was also studied by Vinu and Madras. They found that the rate of sonophotocatalytic degradation of anionic dyes and the reduction of total organic carbon was higher compared to the individual photo- and sonocatalytic processes.
TiO2 can also be modified with a second semiconductor, dyes, N, C or S which allows the semiconductor to be activated even by visible light (Dong et al., 2008; Liu et al., 2006; Obare and Meyer, 2004). The water purification potential of N-doped TiO2 was evaluated by studying the photodegradation of Acid Orange 7 (AO7) and E coli (Liu et al., 2006). N-doped TiO2 demonstrated superior photocatalytic activities compared to common Degussa P25 particles in both chemical compound degradation and bactericidal reactions (Liu et al., 2006).
A recently published study analysed the photocatalytic efficiency of quantum-sized ZnO by means of the degradation rate of reactive brilliant blue X-BR in aqueous solution (Su et al., 2008). The experimental results indicated that the photocatalytic property of the ZnO was excellent.
Hilal et al. (Hilal et al., 2004) summarize different applications and research on nanofiltration (NF) membranes in a comprehensive overview.
Transport processes of the solute in the pore is dominated by diffusion, although convective transport is significant for organic nitrogen compounds (Lee and Lueptow, 2001). Electromigration contributes negligibly to the overall solute transport in the membrane (Lee and Lueptow, 2001).
Diverse nanofiltration membranes are tested for their efficiency in removing different compounds and particles. Several authors have shown that NF can successfully remove viruses and bacteria (Jacangelo et al., 1997; Laurent et al., 1999; Yahya et al., 1993). Other papers discuss the removal of sulphate (Andrew, 2001), nitrate (Lee and Lueptow, 2001; Ratanatamskul et al., 1998), lead (Jakobs and Baumgarten, 2002) and chromate (Hafiane et al., 2000).
Yuan et al (Yuan et al., 2008) describe a self-assembly method for constructing thermally stable, nonwoven nanowire membranes that exhibit controlled wetting behaviour ranging from superhydrophilic to superhydrophobic. These membranes can selectively absorb oils up to 20 times the material's weight in preference to water, through a combination of very strong hydrophobicity and capillary action. They can also separate similar organic solvents such as benzene and toluene. The membranes can easily be recycled many times. Yuan et al. propose their new material for applications in the removal of organics, particularly in the field of oil spill cleanup. As the economic and toxicological arguments about the use of manganese oxide are owing, it is though not sure whether the material will find commercial applications (Lahann, 2008).
Schorr (Schorr, 2007) found that the surface area of ceramic filter materials can be greatly increased (10-50 times) by the growth of nanomaterials such as manganese oxide, iron oxide and/or copper (oxide) within the pores. These nanoenhanced membranes can effectively remove phosphates, heavy metals, lead, arsenic and other pollutants.
Visvanathan et al. (Visvanathan et al., 1998) investigated the effects of different parameters on the performance of nanofiltration for removal of trihalomethane precursors (THMPs). Higher pressure, THMP concentration and suspended solids increased the rejection only negligibly. On the other hand the presence of divalent ions reduced the rejection capacity. Generally rejection was found to be greater than 90% for a precompacted membrane.
Different authors found that monovalent ions like nitrates are rejected to a lower extent (Lee and Lueptow, 2001; Molinari et al., 2001; Ratanatamskul et al., 1998). Lee and Lueptow (Lee and Lueptow, 2001) showed that urea as small organic compound has an even lower rejection than ionic compounds such as ammonium, nitrate and nitrite. They thus highlight the important role of electrostatic interaction in rejection whereas the molecular weight and chemical structure of nitrogen compounds appear to he less important.
Molinari et al. (Molinari et al., 2001) compared the performance of RO and NF concerning the separation efficiency of pollutants like silica, nitrate, manganese and humic acids (HA). The mean rejections of the NF membrane were lower than for the RO membrane, and equal to 35%, 6%, 80%, 35% respectively. Mn2+ rejection was the highest due to the positive charge of the NF membrane. Sankir et al. (Sankir et al., 2010)studied the removal of Cr(VI) by a nanocomposite copolymer membrane. At an acid pH, removal efficiencies of up to 99.5% could be achieved.
Molinari et al. (Molinari et al., 2001) also compared membrane washing agents and found NH3 aqueous solution to be the best washing substance for membrane cleaning. Washing only with water was not able to remove Mn2+ and Cu2+ ions. Washing with NaOH lead to the precipitation of insoluble hydroxides, like Mn(OH)2, and consequent plugging of membrane pores.
Several studies were conducted to assess the cleaning efficiency of NF membranes with industrial wastewater. Afonso and Yanez (Afonso and Yanez, 2001) investigated the performance of NF in the treatement of fish meal wastewater and found that NF reduced the organic load and partially desalinated the wastewater which made water reuse possible. Jakobs and Baumgarten (Jakobs and Baumgarten, 2002) studied the removal of lead from nitric acid solutions from the etching process. They achieved a recycling rate of up to 90% which not only reduces the need for fresh acid for the etching process, but also the alkali needed for neutralizing the waste acid stream could be drastically reduced. As an added benefit the nitrate load of the remaining wastewater was also decreased.
Tang and Chen (Tang and Chen, 2002) studied the recovery of electrolyte solution and the rejection of colour from wastewater produced by the textile industry. At low pressure of up to 500kPa, relatively high fluxes were obtained, with an average dye rejection of 98% and NaCl rejections of less than 14%. Thus, a high quality of reuse water could be recovered. Even after a number of cycles, the membrane did not foul irreversibly, with an overall mean waterflux recovery of 99%. Similar results were found by Voigt et al. (Voigt et al., 2001) with a TiO2-NF membrane and Weber et al. (Weber et al., 2003) with a K-NF membrane. Voigt et al. tested their membrane in a pilot plant for its efficiency in decolouring of textile wastewater. The pilot plant tested with 30 types of different coloured wastewater over a period of 6 weeks. Depending on the composition of the dyes, decolouring rates of 70-100% were obtained with running costs around €0.25-1.4 per m3. Weber et al. (Weber et al., 2003) further compared the efficiency of different membranes regarding the treatment of textile wastewater, alkaline solutions from bottle washing machines, and pickling bath solutions. They found that the permeability rates of a ceramic membrane are clearly superior to those of polymer nanofiltration membranes.
Anbia and Lashgari synthesised a new nano sorbent by modifying mesoporous silica with amino-groups. They found significant adsorption for 2-chlorophenol and 2,4,6-trichlorophenol (Anbia and Lashgari, 2009).
Nanofiltration can also be used for the treatment of landfill leachate(Li et al., 2010). Yangali-Quintanilla et al. have developed a model to predict the rejection of contaminants such as pharmaceuticals and pesticides(Yangali-Quintanilla et al., 2010). Another studied investigated the rejection of estrone by nanofiltration and reverse osmosis membranes(Jin et al., 2010).
Several scientists investigate the combination of different membranes/techniques to optimize the results of rejection. Tan and Ng (Tan and Ng, 2010)developed a new hybrid (FO-NF) membrane for the desalination of seawater and achieved good results. Chung et al. (Chung et al., 2010)tested a RO-NF hybrid membrane system for the removal of selenate and chromate from wastewater. The results showed that the rejection of solutes decreases with increasing the recovery due to the increase in osmotic pressure. The combination of coagulation-flocculation and nanofiltration was found to be very effective for dye removal from textile wastewater(Riera-Torres et al., 2010). Efficencies of up to 98% were achieved.
A time-dependent analysis of membrane fouling and cleaning after the treatment of pharmaceutical wastewater was carried out by Wei et al. (Wei et al., 2010). They found that at the initial stage of NF process, the deposition of sulfate and calcium carbonate were the main cause for membrane fouling. At the later stage, complex organic foulants containing carboxyl acid, amide, and alkyl halide functional groups also deposited onto the membrane surface and gradually formed a densely packed fouling layer. For cleaning the the following agents were successfully applied: NaOH (pH 11) < HCl (pH 2) < citric acid (pH 2) < EDTA (10 mM). After cleaning with EDTA for 60 min, the membrane flux recovery ratio reached 99.0% and the SEM image and element content of the membrane cleaned by EDTA were quite similar to those of the virgin membrane.
At the 12th Aachener Membrane Colloquium 2008 several presentations on nanofiltration were held. Among them a presentation by Roehricht et al. (Roehricht et al., 2010)which analysed the retention of pharmaceuticals by nanofiltration in wastewater treatment plants. They found that diclofenac was retained by 65% while the removal of Carbamazepine was not efficient.
Metal/Metal oxides/Metal compounds
Nanoscale metals and metal oxides can be used for the absorbance of metals (Nowack, 2008; Rickerby and Morrison, 2007). Pacheco et al. (Pacheco et al., 2006) describe the removal of cadmium ions from simulated industrial wastewater using sol-gel structured nanoparticles of silica and alumina. The results show that it is possible to reduce the cadmium concentration from 140 ppm to less than 5 ppb using Si-Al particles. Al-Si particles were significantly less efficient. Deliyanni et al. (Deliyanni et al., 2007) conducted experiments to investigate zinc removal from diluted aqueous solutions (i.e. effluent) by sorption onto synthetic nanocrystalline akaganeite which was shown to be a promising inorganic adsorbent.
Zhang et al. applied nano-Al2O3 as novel sorbent for the removal of thallium from aqueous solution. Batch experiments were carried out to investigate its adsorption properties. The removal percentage of thallium by the sorbent increased with increasing pH from 1 to 5. (Zhang et al., 2008). Other authors compared the performance of nano- and nonnano-catalytic electores for the decontamination of municipal wastewater (Chang et al., 2009).
Active agents like manganese oxide can be used to change the valance state of metal ions in water (e.g. arsenic oxide from 3+ to 5+ which can then be more easisly removed (Schorr, 2007)). Iron and other metal oxides (somethimes in combination with metals) can also adsorb heavy metals and radionuclides (Schorr, 2007). Pollutants are caged in zeolite-like structures (Schorr, 2007). Iron oxide has also been used to break down organics, pesticides etc. into non-hazardous compounds (Schorr, 2007) and to effectivly remove phosphate. Reactions can often be enhanced through the addition of other nonomaterials such as copper (oxide) (Schorr, 2007). Zelmanov and Semiat investigated the phenol oxidation kinetics in water using iron (III)-oxide nano-catalysts (Zelmanov and Semiat, 2008).
A few studies investigate metal oxide-CNT composites for the removal of metals and anions (see also chapter on groundwater remediation). Peng et al. (Peng et al., 2005a) synthesized among others carbon nanotubes-iron oxides magnetic composites as adsorbent for removal of Pb(II) and Cu(II) from water.
Oxidized, hydroxlated as well as modified CNTs are good adsorbents for metal ions and also for organic compounds. Yang and Xing though highlight that the adsorption of of PAHs on CNTs is reversible (Yang and Xing, 2007). Gotovac et al. found that purification of CNT improved the absorption (Gotovac et al., 2006).
Efficient absorbtion has been found for various metal ions such as Cu, Ni, Cd and Pb as well as for different organic compounds such as dioxin, PAH and DDT (Li et al., 2001; Liang et al., 2005; Nowack, 2008; Peng et al., 2005a; Peng et al., 2005b). The adsorption capacity was found to be proportional to the cylindrical external surface (Yang and Xing, 2007). In their study neither the inner cavity nor the interwall space of MWCNT contributed to the adsorption.
Fullerenes were found to be only weak sorbants for many types of organic substances (absorbtion capacitiy max. 60%) (Ballesteros et al., 2000). But they showed significant efficiency in the removal of organometallic compounds(Ballesteros et al., 2000). In general, the adsorption of organic compounds on fullerenes depends greatly on the disperion state of C60 which is insoluble in water if not modified (Nowack, 2008).
Nanostructured boron-doped diamond (BDD)
Haenni in (Rickerby and Morrison, 2007) reports that BDD has the largest electrochemical window of all known electrode materials and is considered today as a really new and very versatile electrode material. BDD can be grown artificially by Chemical Vapor Deposition (CVD) with in-situ boron doping on silicon. The nano-diamond crystal shape in these solutions is in the order of 2-10nm. Gandini et al. (Gandini et al., 2000) showed that BDD has an outstanding production capacity of very strong chemical oxidizing agents such as hydroxyl radicals which can effectively oxidize pollutants in wastewater regadless of water turbidity (some three-5 fold faster than conventiaonal chlorine dosing). BDD electrodes have the advantage of self-cleaning by reversing the current polarity.
Based on electrodes from BDD, the DiaCell®-Systems were developed and are today installed for water disinfection and conservation e.g. in spas and swimming pools as well as for electro-oxidation of industrial wastewater, hazardous effluents and landfill leachates. It is effectively possible to incinerate electrochemically all dissolved even very refractory aromatics, aliphatics and heterorganics, like phosphonates, sulfonates, pesticides, EDTA, as well as some inorganic components, like cyanides, ammonia etc. COD (oxycould be reduced by 95-100%. Biodegradability could also be improved for tannery waste water with 16'000ppm COD and 4'000ppm of ammonia and N-containing organics. All ammonia and Ncontaining organics are transformed by 1/3 to nitrate and 2/3 direct to N2-gas. The electrochemical incineration of refractory and biocide organics with diamond electrodes is a highly efficient electrochemical advanced oxidation process (AOP) working without any addition of chemicals with no residues as well as independent of colors and/or turbidity.
Figure 1: Haenni in (Rickerby and Morrison, 2007)
Magnetic nanoparticles can easily be separated from water by applying a magnetic field. Pollutants adsorbed to magnetic nanoparticles can so be easily removed from water and the nanoparticles are recovered. Examples of magnetic NP are magnetite Fe3O4, maghemite (y-Fe2O3) jocobsite (MnFe2O4) which can be used for the removal of chromium(VI) (Hu et al., 2005a; Hu et al., 2006; Hu et al., 2004; Hu et al., 2005b). Hu et al. (Hu et al., 2004) found the adsorption process of Cr(VI) to magnetite to be pH and temperature dependent. The adsorption capacity increased with rising temperature. In a later work with jacobsite nanoparticles, Hu et al. (Hu et al., 2005b) showed that the equilibrium time for Cr(VI) adsorption onto modified MnFe2O4 nanoparticles was as short as 5 min. They further found that EDTA and SO42- inhibited the adsorption of Cr(VI) over a pH range from 2-10, whereas NH4+ enhanced the uptake of Cr(VI) at pH greater than 6.5. Regeneration of the jacobsite and Cr(VI) was possible without compromising on the adsorption capacity or changing the valence respectively. Also maghemite was shown to be an effective adsorbant of Cr(VI) (Hu et al., 2005a). The adsorption reached the equilibrium within 15 min and was independent of the initial Cr-concentration. The maximum adsorption occurred at pH 2.5. Competition with other ions such as Na+, Ca2+, Mg2+, Cu2+, Ni2+, NO3-, and Cl- was ignorable, which illustrated the selective adsorption of Cr(VI) from wastewater. Regeneration studies verified that the maghemite nanoparticles, which underwent six successive adsorption–desorption processes, still retained the original metal removal capacity.
Assessing the toxicity of pure magnetite and palladium/magnetite nanoparticle upon human skin and human colon cell lines as well as a cell line from rainbow trout gills, Hildbrand et al. (Hildebrand et al., 2010)found only minor effects on the cells and therefore support the use of such particles in wastewater treatment (e.g. removal of hallogenated organic pollutants).
Hu et al. (Hu et al., 2006)further investigated the adsorption effectivity of maghemite nanoparticles (10nm) for the selective removal of toxic heavy metals (Cr(VI), Cu(II), and Ni(II)) from electroplating wastewater. The adsorption process was found to be highly pH dependent. The adsorption of heavy metals reached equilibrium within 10 min. Regeneration studies indicated that the maghemite nanoparticles undergoing successive adsorption–desorption processes retained original metal removal capacity. The authors suggest that the adsorption of Cr(VI) and Cu(II) could be due to electrostatic attraction and ion exchange, and the adsorption of Ni(II)could be as a result of electrostatic attraction only.
Magnetic NP can also be coupled with other substances such as CNT. Jin et al. (Jin et al., 2007) functionalized water-soluble multiwalled carbon nanotubules (MWNTs) with magnetic Fe nanoparticle and studied the removal of aromatic compounds in water and subsequent recovery.
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