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Adsorption and Release of Contaminants Onto Engineered Nanoparticles

Mason Tomson
Rice University, Houston, TX

Little is known about the environmental fate of manufactured nanomaterials. Sorption is often the most important fate mechanism for environmental contaminants. We have proposed to test four hypotheses in this research: (1) that carbon nanostructures have a high capacity for sorption/desorption hysteresis with polynuclear aromatic hydrocarbons and other common organic contaminants; (2) that the sorption capacity of inorganic nanomaterials for heavy metals is the same as the corresponding bulk crystals, when corrected for surface area; (3) that sorption of naturally occurring humic materials and surfactants to metal oxide and carbon nanomaterials will diminish the sorption capacity of heavy metals on oxides and increase the sorption of hydrocarbons on carbon nanomaterials; and (4) that the transport of nanoparticles in soils, sediments, and porous medial will be vastly greater than the corresponding colloids or bulk materials. In the first year of this project, we have successfully characterized the sorption/desorption behavior of both organic and inorganic contaminants to nanomaterials and the fate and transport of naphthalene (an organic contaminant) and colloidal C60 particles in soil column. The researchers have attended several national conferences and published 12 peer reviewed papers (or accepted for publication) in the last year and submitted one U.S. patent application.

Environmental Fate of Carbon Nanomaterials

Many soils or sediments contain various forms of carbonaceous materials such as coals, kerogens, and black carbons, which have been reported to have high affinity for hydrophobic organic contaminants. C60 might have similarly important environmental impact as to other forms of carbon (e.g., black carbons). Although C60 is virtually insoluble in water, nano-C60 particles (a term used to refer to underivatized C60 crystalline nanoparticles, stable in water for months, mean diameter ~100 nm in this study) can be formed in water simply by stirring, or by dissolving C60 in non-polar solvents, mixing into water, followed by the removal of the solvents. In this study, the adsorption and desorption of naphthalene and 1,2-dichlorobenzene with aqueous nano-C60 particles prepared by both methods was investigated. To compare the adsorptive property of C60 with other carbons, e.g., naturally occurring organic carbon in soils, and activated carbon, adsorption and desorption of naphthalene with Anocostia River sediment (foc = 3.7%) and activated carbon particles (commercial, and nano-activated carbon particles prepared in our lab) was also conducted and compared with that of C60.

Results show that adsorption to nano-C60 particles is similar to that of nano-activated carbon particles, and is stronger that that of soil organic carbon. Desorption hysteresis was observed for naphthalene desorption from all three forms of carbon tested: C60, activated carbon, and soil organic carbon. A two-compartment sorption model was used to describe naphthalene adsorption and desorption with these three forms of carbon, where and are solid-water distribution coefficients for the first and the second compartment; (g/g) is defined as a maximum sorption capacity for the second compartment; and the factor f (0 "T f "T 1) denotes the fraction of the second compartment that is filled at the time of exposure. Data of naphthalene adsorption and desorption with C60, activated carbon, and soil organic carbon were fitted with this two-compartment model very well, indicating that there may be a common pattern for the adsorption and desorption of naphthalene with these three forms of carbon. Therefore, it may be possible to predict the properties of nano-carbons, such as C60, from that of other carbon forms. This finding may have important environmental significance.

It has been reported that nano-C60 particles are toxic to fish cells and cultured human cells; thus, people may be more concerned about the potential exposure to C60 if it is mobile in water. One might expect that C60 would not enter groundwater in great quantities due to the insolubility of C60 in water. However, water-stable nano-C60 particles can be formed in water by several simple methods, as discussed above, indicating that C60 might be mobile in groundwater. Besides, dissolved organic matter in groundwater has been reported to significantly enhance the partition of neutral organic contaminants into water and thus enhance the transport of those contaminants. It is unknown whether the release of C60 and other nanosized carbonaceous nanomaterials into aqueous environments will have the similar effect. Therefore, it is of central importance to investigate the C60 transport in water/sediment, and its effect on contaminant transport.

In this work, the transport of nano-C60 particles through a sandy soil column (foc = 0.0027%) was investigated for the first time. Nano-C60 particles showed limited mobility at typical groundwater Darcy velocity. At the Darcy velocity of 3 ft/d, the maximum nano-C60 particles breakthrough was only 47 percent, and an unexpectedly high deposition of nano-C60 to the soil column was observed shortly after the peak value, probably indicating an irreversible sorption of nano-C60 particles on the soil column. It might be because the accumulation of nano-C60 on the collector surface served as favorable sites for subsequent nano-C60 deposition. Nano-C60 particles were more mobile in the soil column at higher flow velocities (e.g., 30 and 90 ft/d). A model developed for the transport of colloids in porous media by Yao et al. and Melia et al. was used to describe nano-C60 particles transport in the soil column. The theoretical single collector efficiency, the particle attachment efficiency, and the maximum particle travel distance were calculated for nano-C60 particles transport at different flow velocities. Experimental results showed that at the flow velocity of 30 ft/d, nano-C60 particles could travel 68 cm through the soil column before 99.9 percent of the particles were immobilized; while at the flow velocity of 90 ft/d, nano-C60 particles could travel 1.32 m through the soil column before 99.9 percent of the particles were immobilized. Spiked release of nano-C60 particles was observed repeatedly on flow resumption following a few days period of flow shut-in. Spiked release of nano-C60 particles was also observed during the change of flow velocities. This observed phenomenon may have broad environmental significance.

The transport of naphthalene through the same soil column with 0.18 percent of nano-C60 particles deposited was measured. The observed retardation factor for the naphthalene breakthrough curve with the Lula/0.18 percent-nano-C60 column was about 13, indicating that nano-C60 particles deposited in the soil column adsorbed naphthalene similarly to soil organic carbon.

Adsorption and Desorption of Heavy Metal and Arsenic to Metal Oxide Nanoparticles

The objectives of the current study were to investigate the effect of particle size on adsorption and desorption of typical environmental pollutants (i.e., arsenic and cadmium) onto metal oxide bulk crystals versus nanoparticles (anatase and magnetite), and to examine the competitive sorption between naturally occurring humic materials and heavy metals. In collaboration with Dr. Colvin's group through the support of the Center of Biological and Environmental Nanotechnology (CBEN) at Rice University, several laboratory-synthesized magnetite was also studied. On a surface-area basis, cadmium adsorptions to different sized anatase nanocrystals are similar. The maximum adsorption densities for arsenic to magnetite are also similar for commercially prepared large magnetite crystals (300 nm) and magnetite nanoparticles (20 nm). Surprisingly, the adsorption capacity for arsenic to laboratory-synthesized magnetite (11.72 nm) is significantly higher than the commercially available bulk and nanocrystals. Laboratory-synthesized magnetite nanoparticles can remove approximately hundred times more arsenic than lager commercial materials. With respect to desorption, the cadmium desorption from both particle sizes appeared to be completely reversible; however, arsenic was not readily released from magnetite nanoparticles, presumably because the binding of the adsorbed arsenic results in the formation of highly stable iron-arsenic complexes. The presence of NOM decreased adsorption of both arsenic species to magnetite nanoparticles. NOM in the solution probably competes with arsenite and arsenate for surface sites on magnetite nanoparticles. With joint support from CBEN, these results are currently applied to test the efficiency of magnetite nanoparticles in removing arsenic from drinking water. A U.S. patent application has been submitted.

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