Size/Shape – Property Relationships of Non - Carbonaceous Inorganic Nanoparticles and their Environmental Implications - Environmental and Human Health Impacts of Nanotechnology - page 85

Houben, 2003 ; Torrent et al. , 1987). The interpretation of results from these studies
is often complicated by variation in iron oxide phase amongst samples of different
sizes. Further studies are needed to specifi cally examine whether there are any
intrinsic size dependent effects present in iron oxide nanoparticles of the same
phase, independent of surface area. One example of such a study concerns the
oxidation rate of Mn 2+ to Mn 3+ by different sizes of - Fe 2 O 3 (haematite) (Madden

and Hochella, 2005). Surface normalized oxidation rates showed that 7 nm platelets
catalyzed this oxidation over an order of magnitude more quickly than 37 nm
platelets. It was suggested that quantum effects probably did not play a strong role,
due to strong localized bonding in haematite. However, previous calculations and
measurements indicated that the surface oxygen atoms on nanoparticulate hema-
tite would have increased Lewis basicity, enabling them to donate electron density
to Mn 2+ to catalyze the reaction (Noguera et al. , 2002 ).

α

3.4 Size Effects in Nanoparticle Sorption Processes

In nature, the sorption of metals and organics to inorganic surfaces (mineral sur-
faces) can greatly infl uence their mobility (Hochella et al. , 2008 ; Kretzschmar and
Schafer, 2005). As inorganic nanoparticles offer a large amount of surface area
relative to their volume or weight, it is expected that they would participate in
sorption phenomena. Indeed, heavy metals and radionuclides have been found
associated with nanoscale colloids in natural water (Hochella et al. , 2005a, 2005b ;
Hochella and Madden, 2005; Kersting et al. , 1999) and drinking water (Wigginton
et al. , 2007). The sorptive properties of nanoparticles have caught the attention
of chemists and engineers, who are interested in using them for environmental
remediation (Yavuz et al. , 2006 ; Jeong et al. , 2007 ; Yuan, 2004 ).
Predicting the sorption behaviour of nanoparticles is of interest when consider-
ing both natural nanoparticles and the accidental or purposeful release of synthetic
nanoparticles into natural systems. Studies have shown that on a per-mass basis,
nanoparticles sorb more than their bulk counterparts (Zhang et al. , 1999 ; Yean
et al. , 2005 ; Waychunas et al. , 2005 ; Madden et al. , 2006 ; Gao et al. , 2004 ; Giammar
et al. , 2007). Also, size effects (independent of surface area) are expected for
nanoparticle sorption. Experiments have shown that for nanoparticulate titanium
dioxide and - Fe 2 O 3 , the point of zero charge, or the pH at which the particles have

zero charge, is shifted with respect to size (Guzman et al. , 2006 ; He et al. , 2008 ). The
surface energy of nanoparticles is likely to vary with size (Zhang et al. , 1999 ). Also,
nanoparticle structures are often different from those of the bulk, displaying
lowered atomic coordination and higher disorder (Rockenberger et al. , 1997, 1998 ;
Hamad et al. , 1999 ; Marcus et al. , 1991 ; Aruguete et al. , 2007 ); different crystalline
phases may be favoured in the nanoscale as opposed to the bulk (Dinega and
Bawendi, 1999). All of these phenomena could presumably alter sorption capacity
and affi nity.
Studies of sorption size dependence show varying results. Examples of these are
summarized in Table 3.1. Many of these studies have fi t their data to the Langmuir
adsorption equation (McBride, 1994, Drever, 1997):

α