report
6.4.3 State of R&D

Metal oxide – CNT

Experiments showed that ceria supported on carbon nanotubes (CeO2-CNTs) is an effective adsorbent for arsenate, and that the adsorption is pH-dependent (Peng et al., 2005). Ca2+ and Mg2+ significantly enhanced its adsorption capacity suggesting that it is a promising adsorbent for drinking water purification. The loaded adsorbent could be efficiently regenerated by diluted NaOH, and achieved a regeneration efficiency of 94%. Di et al (Di et al., 2006) corroborate these results. They prepared ceria nanoparticles supported on aligned carbon nanotubes (CeO2/ACNTs) as a novel adsorbent for Cr(VI) from drinking water. The best Cr(VI) adsorption occured at a pH range of 3.0–7.4.

Li et al. (Li et al., 2001) investigated amorphous aluminiumoxide (Al2O3) supported on carbon nanotubes (Al2O3/CNTs) as adsorbant for fluoride from water. They found that its adsorption capacity is significantly higher than that of the reference materials (AC-300 carbon and c- Al2O3).

Photocatalysis

There are many studies on the efficacy of photocatalytic degradation by nanomaterials (mostly TiO2) and they show that photocatalytic materials are effective against a wide variety of contaminants. The main focus for the use of photocatalysis in drinking water treatment is though clearly disinfection.

The Report from the Workshop on Nanotechnologies for Environmental Remediation (Rickerby and Morrison, 2007) names solar photocatalysis to be the main technology breakthrough for water treatment and purification, particularly in developing regions. Photocatalytic disinfection is an effective method to provide clean drinking water and works also for chlorine resistant organisms and even organic polllutants such as herbicides and pesticides (Rickerby and Morrison, 2007). Photocatalytic applications have been tested on laboratory and pilot scale and commercial systems are already available. The main barrier to widespread commercialisation (e.g. in developing countries) is the need for education of potential customers (Rickerby and Morrison, 2007).

Dunlop et al. (Dunlop et al., 2002) prepared TiO2 electrodes by the electrophoretic immobilisation of TiO2 powder (Aldrich and Degussa P25). They found that the application of an electrical bias to the working electrode increased the rate of disinfection by 40% to 80%. They further studied the photocatalytic inactivation of Clostridium perfringens spores on TiO2 electrodes (Dunlop et al., 2008).

Sichel et al. (Sichel et al., 2007) studied the dependence of photocatalytic water disinfection on solar irradiation conditions under natural sunlight. This dependency was evaluated for solar photocatalysis with TiO2 and solar-only disinfection with three microorganisms. It was shown that once the threshold solar dose is given, the photocatalytic disinfection efficacy is not enhanced by any further increase in irradiation.

Lindstrom et al. (2007 have developed a miniaturised phototocatalytic reactor with an immobilised, highly porous film of anatase titania nanoparticles with a high specific surface area on the walls of the 100 μm wide channels of a glass microfluidic device. Microfluidic reactors are excellent for photocatalysis since a large density of UV photons can be concentrated in the immobilised photoactive layer within a small volume efficiently coupled to the light source (Lindstrom et al., 2007).

BDD (Boron Doped Diamond)

Haenni in (Rickerby and Morrison, 2007) describes the BDD/Si electrodes to allow a very high anodic potential which may be used to produce very efficient oxidants for water treatment and disinfection. Disinfection is achieved without chlorine, independently of water turbidity, and with low by-product potential. For viruses and bacteria (E.Coli, Legionella) inactivation is 3-5 times faster than with conventional chlorine dosing. Destruction of algae, fungi and protozoan is also assured. COD (chemical oxygen demand)/TOC (total organic carbon) is reduced by the production of hydroxyl radicals for the destruction and increased biodegradability of organic pollutants (pesticides, phenols, solvents, PCBs). Based on electrodes from BDD by Gandini et al. (Gandini D et al., 2000), 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.

Metals, Metall oxides and magnetic particles

Stipp et al. in (Rickerby and Morrison, 2007) describe the removal of nickel from drinking water by calcite nanoparticles. They also studied iron oxide (Fe(II)) as effective substance to degrade redox sensitive elements such as chromium and chlorinated solvents.

Sorption on iron (III) oxides such as amorphous hydrous ferric oxide (FeOOH), poorly crystalline hydrous ferric oxide (ferrihydride) and goethite (α-FEOOH) have been found to be effective in removing both As(V) and As(III) (Rave et al., 1998; Sun and Doner, 1998; Wilkie and Hering, 1996; Zeng, 2003).

Nanofiltration

Hilal (Hilal et al., 2004) summarizes 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; Otaki et al., 1998; Urase et al., 1996; Yahya et al., 1993). Other papers discuss the removal of sulphate (Andrew, 2001; Redondo and Lanari, 1997) and nitrate  (Lee and Lueptow, 2001; Ratanatamskul et al., 1998; Redondo and Lanari, 1997). Monovalent ions like nitrates found to have lower extent of rejection (Lee and Lueptow, 2001; Ratanatamskul et al., 1998). Redondo and Lanari (Redondo and Lanari, 1997) further investigated the removal of harndess and organic matter on several pilot plants.

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.

Several studies investigated the combination of NF with other treatment methods such as (SW)RO and multistage flash. Hassan et al. (Hassan et al., 1998) found that  (at a pressure of 22 bars) the NF unit reduced turbidity and microorganisms and removed Ca2+, Mg2+, SO42- and HCO3- by 89.6%, 94.0%, 97.8% and 76.6%,  respectively. Monovalent ions (Cl, Na, K) were rejected by ca. 40% (Hassan et al., 1998). Mohesn et al (Mohesn et al., 2003) showed that NF is efficient for reducing the organic and inorganic substances and allows for a high water recovery of u to 95%. In an integrated system with NF, RO and membrane crystallizer (MC), NF increased the water recovery of the RO unit up to 50% leading to a total recovery of 100%.

In total, the combination of NF with other methods allows producing fresh water from seawater at a 30% lower cost (Al-Sofi, 2001). Hilal et al. (Hilal et al., 2004) highlights the advantages of a NF pretreatment of seawater in desalination: NF pretreatment prevents membrane fouling by removal of turbidity and bacteria, prevents scaling by removal of hardness ions and lowers the requested pressure to operate SWRO. These results were confirmed in a pilot plant study (Hassan et al., 2000).

The removal of arsenic from water has been studied by different authors. Seidel et al. (Seidel et al., 2001) and Brandhuber and Amy (Brandhuber and Amy, 1998)  found that the rejection of the uncharged As(III) species was significantly lower compared to As(V). Waypa et al. (Waypa et al., 1997) found the rejection of both species to be equally effective. The rejection of As(V) decreased sharply when the pH was lowered (Seidel et al., 2001) (Vrijenhoek and Waypa, 2000).  The separation of the uncharged As(III) was independent of pH over the studied pH range (Seidel et al., 2001). Vrijenhoek and Waypa (Vrijenhoek and Waypa, 2000) found arsenic (V) was removed by 60-90% from synthetic feed waters while Brandhuber and Amy (Brandhuber and Amy, 1998) found out that the rejection of As(III) was only about 40%.