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Applying nanotechnology to water treatment

(Nanowerk Spotlight) While everybody talks about oil prices, water scarcity and water pollution are two increasingly pressing problems that could easily and quickly surpass the oil issue. Renewable energy sources can substitute for fossil fuels – but freshwater can't be replaced. This makes the ability to remove toxic contaminants from aquatic environments rapidly, efficiently, and within reasonable costs an important technological challenge. Nanotechnology could play an important role in this regard. An active emerging area of research is the development of novel nanomaterials with increased affinity, capacity, and selectivity for heavy metals and other contaminants. The benefits from use of nanomaterials may derive from their enhanced reactivity, surface area and sequestration characteristics.
We have addressed this issue extensively in two previous Spotlights: "Nanotechnology and water treatment" and "Water, nanotechnology's promises, and economic reality".
Numerous nanomaterials are in various stages of research and development, each possessing unique functionalities that are potentially applicable to the remediation of industrial wastewater, groundwater, surface water and drinking water. The main goal for most of this research is to develop low-cost and environmentally friendly materials for removal of heavy metals from water. A recent example is a novel low-cost magnetic sorbent material for the removal of heavy metal ions from water, developed by scientists in China, who coated iron oxide magnetic nanoparticles (Fe3O4 – magnetite) with humic acid (HA). The coating greatly enhanced material stability and heavy metal removal efficiency of the nanoparticles.
"Recently, magnetic nanoparticles were used for this purpose as these materials with the adsorbed heavy metals can be easily recovered by utilizing magnetic separation" Dr. Gui-Bin Jiang explains to Nanowerk. "However, bare magnetite nanoparticles in aqueous systems are very much susceptible to air oxidation and are easily aggregated, which resulted in reduced saturation magnetization and adsorption capacity for metals. In our recent paper, we describe the development of a novel material, humic acid coated Fe3O4 magnetic nanoparticles, to resolve theses problems."
removal of heavy metals with the humic acid coated magnetic iron nanoparticles
Scheme of the removal of heavy metals with the humic acid coated Fe3O4 magnetic nanoparticles. (Reprinted with permission from American Chemical Society)
Jiang, a professor at the Research Center for Eco-Environmental Sciences of the State Key Laboratory of Environmental Chemistry and Ecotoxicology, Chinese Academy of Sciences in Beijing, together with his colleagues Jing-Fu Liu and Zong-Shan Zhao, found that coating Fe3O4 magnetic nanoparticles with humic acid can 1) greatly enhance the stability of dispersed nanoparticles by preventing their aggregation; 2) maintain the saturation magnetization by avoiding their oxidation; and 3) enlarge the adsorption capacity for some heavy metals by making use of the abundant carboxylic acid and phenolic hydroxyl functional groups of humic acid to complex with heavy metal ions.
The scientists published their findings in the August 14, 2008 online edition of Environmental Science & Technology ("Coating Fe3O4 Magnetic Nanoparticles with Humic Acid for High Efficient Removal of Heavy Metals in Water").
Jiang notes that bare magnetite nanoparticles are very much susceptible to air oxidation and are easily aggregated in aqueous systems. "We were aware of recent research which indicated that humic acid has high affinity to Fe3O4 nanoparticles, and that sorption of HA on the Fe3O4 nanoparticles enhanced the stability of nanodispersions by preventing their aggregation," he says. "It is also well-known that HA, which is abundant in natural aqueous systems, has a skeleton of alkyl and aromatic units that attach with carboxylic acid, phenolic hydroxyl, and quinone functional groups. These functional groups have high complex capacity with heavy metal ions and HA has therefore been applied to remove heavy metal ions from water."
In previous research it was also found that the adsorption capacity for metal ions with the complexes of HA and iron oxides was larger than that with the respective iron oxides and HA alone. The Chinese researchers therefore hypothesized that by coating Fe3O4 magnetic nanoparticles with HA they could develop a very effective sorbent material for the removal of heavy metals from water.
For removal of metals from freshwater, 50 mg of the coated iron oxide nanoparticles – prepared by a co-precipitation procedure with cheap and environmentally friendly iron salts and HA – was added into 100 mL of water. Then the magnetic Fe3O4/HA with sorbed heavy metals were separated from the mixture with a permanent hand-held magnet.
To test the effect of time on the leaching of Fe3O4/HA components and the sorbed heavy metals, Jiang and his colleagues resuspended Fe3O4/HA laden with heavy metals in de-ionized water. The results indicated that the leaching of both the sorbed heavy metals and the material components were negligible.
"We also found that the removal of heavy metals by Fe3O4/HA was not affected by the environmentally relevant parameters such as pH, salinity and dissolved organic matter" says Jiang. "Furthermore, the proposed procedure was very efficient as the sorption equilibrium was reached in less than 20 min, and the Fe3O4/HA with adsorbed heavy metals can be simply recovered from water with magnetic separations at very low magnetic field gradients within a few minutes."
This research is a good example of the ways nanotechnology can be used to take substances that are abundant in nature (like iron oxide and humic acid) and use them to synthesize novel, highly effective adsorbent materials that are low-cost and have no adverse effect on the environment. The result will not only be more effective and efficient water treatment processes but also lower overall costs.
By Michael is author of two books by the Royal Society of Chemistry: Nano-Society: Pushing the Boundaries of Technology and Nanotechnology: The Future is Tiny. Copyright © Nanowerk

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