The role of macropores in soil and water processes has motivated many researchers to describe their sizes and shapes. Several approaches have been developed to characterize macroporosity, such as the use of tension infiltrometers, breakthrough curve techniques, image‐analysis of sections of soils, and CAT scanning. Until now, efforts to describe macropores in quantitative terms have been concentrated on their two‐dimensional (2‐D) geometry. The objective of this study is to nondestructively quantify the three‐dimensional (3‐D) properties of soil macropores in four large undisturbed soil columns. The geometry and topology of macropore networks were determined using CAT scanning and 3‐D reconstruction techniques. Our results suggest that the numerical density of macropores varies between 13421 to 23562 networks/m3 of sandy loam soil. The majority of the macropore networks had a length of 40 mm, a volume of 60 mm3, and a wall area of 175 mm2 It was found that the greater the length of networks, the greater the hydraulic radius. The inclination of the networks ranged from vertical to an angle of ≈55° from vertical. Results for tortuosity indicated that most macropore networks had a 3‐D tortuous length 15% greater than the distance between their extremities. More than 60% of the networks were made up of four branches. For Column 1, it was found that 82% of the networks had zero connectivity. This implies that more than 4/5 of the macropore networks were composed of only one independent path between any two points within the pore space.
Titanium dioxide supported on microporous zeolites of
type X and Y and on mesoporous molecular sieves
of the MCM41 type was studied for the photocatalytic degradation of
acetophenone in an aqueous medium.
The photoactivity of the supported catalyst is strongly influenced
by the method of titania loading, but less
affected by the temperature at which the sample was calcined. The
highest photoactivity among the supported
catalysts is observed for a support that has a lower Si/Al ratio in the
framework and relatively large pore size.
The measured apparent activation energies for photocatalyzed
consumption of acetophenone for a commercial
TiO2 (P-25) and 10 wt % TiO2-loaded samples of
zeolite A, X, Y, and Al-MCM41 are 4.2−6.7, 16.9, 13.5,
32.2 and 3.6 kJ/mol, respectively. Selective doping by Fe, Mn, and
V into the framework of MCM41
suppresses the photoactivity of the supported titanium dioxide.
The physical state of the titanium dioxide on
the supports is characterized by XRD, adsorption and pore size
analysis, IR, and Raman spectroscopy. All
methods emphasize the small particle or amorphous character of the
TiO2. The normal phase transition to
rutile does not occur at higher temperatures on zeolite supports.
In some cases, very low loadings of TiO2
appear to achieve total absorption of the light entering the reaction
vessel. For the Al-rich MCM41-supported
catalysts, maximal photoactivity can be achieved at low
TiO2 loading (<3%). It appears to absorb 100%
of
light entering the reaction vessel with a much lower TiO2
load than is the case with TiO2 itself. There
is
evidence that the crystallinity of the zeolite is an important factor
in photocatalytic efficiency, but the mechanism
of zeolite participation in reactions remains incompletely
elucidated.
A new approach to the use of high-field instruments for
cross-polarization (CP) magic angle spinning (MAS)
13C
NMR for analysis of humic materials is described.
This
technique consists of using a high sample spinning rate
and a ramp CP pulse sequence, which, among other
advantages, addresses chemical shift anisotropy effects.
The theoretical aspects of high spinning rates on
line
broadening for nonrigid solids are reviewed. Also,
the
ramp CP pulse sequence and its implications for humic
materials are discussed. It is shown that the highest
resolution and most informative spectra can be obtained
via a high-field instrument if the sample is spun at a
rate
higher than 0.25 of the anisotropy of the chemical shift
of the aromatic moieties and a ramp CP pulse sequence
is used. This study raises concerns that previous
methods may have underestimated the amount of aromatic
carbons in humic materials. Because of the
similarities
between humic materials and coals, this work may also
have implications in the CP-MAS 13C NMR approach
to
coals.
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