Nanoclays for Water Treatment

Guodong Yuan
Landcare Research

     Full text: Not available
     Last modified: February 15, 2006

Abstract
Abstract

Nanoscale science, engineering, and technology (in brief, nanotechnology) is one of the most exciting and fastest growing science frontiers. Global interest and advance in nanotechnology provide scientists and technologists with new opportunities to find out effective and affordable means for environmental protect and for the remediation of environmental problems from local water eutrophication to regional greenhouse gas emissions. This may be achieved by reducing resource consumption, producing few pollutants, improving energy production and storage efficiency, and remediating contaminated soil, water, and air.

Nanotechnology is more than nano-size and nano-structured machines and electronic devices. Nanoscale phenomena permeate and often control the natural processes. Indeed, the nature has been performing nanotechnology from time immemorial. For example, the photosynthetic conversions of solar energy and atmospheric CO2 into chemical energy and a vast array of biomolecules by living plant cells underpin the occurrence and maintenance of ecosystem and life. In contrast, human beings are only just beginning to mimic nature in making nano-materials and nano-devices. Learning from the nature’s library of amazing stories would help human beings design and produce nanomaterials for beneficial endeavours. The discovery of fullerenes (carbon nanoball) in 1985 and carbon nanotubes in 1991 was decades later than the findings in volcanic ash soils of allophane and imogolite. The former is a group of hydrous aluminosilicate whose primary particles are made up of hallow nano-spherules, whereas the latter is a nano-dimensional aluminosilicate mineral with a hollow tubular particle morphology.

The nano-size or nano-structure of some clay materials offer several advantages that could be exploited for environmental protection and remediation. For example, the large surface area-to-volume ratio of nanoclays could lead to an enhanced reactivity with environmental contaminants; a high proportion of the atoms in nanoclays are at or near surfaces, resulting in many exposed groups and unbalanced surface charge. The surface area, exposed groups, and surface charge of nanoclays lies behind their reactivity towards extraneous molecules and ions through such mechanisms as ion exchange, adsorption, ligand exchange reaction, and precipitation.

Water pollution by nutrients (nitrogen and phosphorus) is becoming a problem in many parts of the world. Eutrifications have been reported in ponds, lakes, rivers and estuaries, reducing water quality, releasing toxins, creating health concerns, and decreasing recreational and tourism values. Besides reducing nutrient inflow to waterbodies through better nutrient management in agricultural system and more effective treatment of urban wastewater, there is not much effective and affordable means to deal with eutrification. Here I reported the propensity and capacity of two natural and modified nanoclays for adsorbing phosphorus in water, and compared the merits and disadvantages for potential commercial applications of the nanoclays in reducing phosphorus concentration in eutrophic waterbodies. The natural nanoclay (α-nanoclay thereafter) was extracted from a deposit in New Zealand, whereas the modified nanoclay is a lanthanum-exchanged montmorillonite (trade name ‘phoslock’), manufactured in China. While both nanoclays were very effective in adsorbing phosphorus in batch experiments and could reduce the equilibrium concentration of phosphorus to the parts per billion level, α-nanoclay adsorbed more phosphorus than phoslock did at high (>100 ppm) equilibrium concentration. Besides being inexpensive and clean (without any chemicals of potential environmental and health concern), α-nanoclay has an obvious advantage over phoslock: non-dispersible in water. From application points of view, this is very desirable property. Because α-nanoclay is nondispersible in water it can be removed from waterbody after its use. Thus, the adsorbed phosphorus leaves waterbody and may be recycled to an agricultural system. In contrast, phoslock disperses in water and becomes slurry. It is not recoverable from water. The adsorbed phosphorus remains in waterbody and would be released when the ambient chemical conditions in the water change.

Environmental application is an important avenue of nanotechnology research. Both of the nanoclays reported here are effective sorbents for phosphorus, and thus may be used for preventive or remediative water treatment. However, by comparing their phosphorus sorption capacities and a number of operational factors (cost, recoverability, and potential side effect), we can conclude that α-nanoclay is better than phoslock for the prevention of water contamination by phosphorus and for the remediation of eutrophic waterbody. It (α-nanoclay) may also serve the purpose of being a benchmark for evaluating the effectiveness and cost of other nanomaterials for water treatment.