FINAL REPORT - Mechanisms of CCl4 Retention and Slow Release in Model Porous Solids and Sediments Page: 4 of 23
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The amount and type of naturally occurring organic carbon can contribute significantly to
adsorptive capacity of soils for volatile organic compounds, for soils with relatively high
organic contents.11 If the organic content is low, sorption to mineral grains likely
becomes the dominant mechanism. Organic matter in soils can range from relatively
recent materials, such as fulvic and humic acids, through various intermediate materials,
to highly condensed organics classified as kerogen.12 Also, it has been shown that vadose
zone soils may contain significant amounts of soot particles, presumably from fires.1 All
of these materials provide strong adsorption sites for volatile organic contaminants.
Humics tend to form rather loose structures, whereas highly condensed materials like
kerogen form tight structures, with considerable heterogeneity. It has been postulated
that it is these highly condensed, naturally occurring organics that are associated with
slow sorption kinetics.1214 Where considerable soot particles are present, it has been
found that most of the organic contaminants are associated with these particles. Again,
most studies have been conducted on real soils under water saturated conditions, where it
is difficult to separate out effects due to moisture, pore size, and the nature and location
of the naturally occurring organic materials.
Scope of the work reported here includes preparation and characterization of porous silica
particles with narrow pore size distributions in the mesoporous range, followed by
measurements of equilibrium isotherms and release rates for carbon tetrachloride, under
dry conditions and with varying amounts of water present.
Mesoporous silica particles were prepared by the method described by Zhao et al.4 and
characterized for pore size distribution, pore volume, surface area and particle size
distribution. Currently, particles have been prepared containing narrow pore size
distributions in the diameter range (2-6) nm.
Data are reported in the current work for three different particles, one using Tween 40 as
a structure-directing template, and two using Pluronic P123 (the first at 35 0C, and the
second at 35 0C followed by 80 0C, as described in Ref. 4). Herein, these particles are
called T-27, P-47 and P-55, respectively, the leading letter indicating the surfactant
employed in the preparation, and the numbers roughly indicating the pore size in
angstroms. Standard nitrogen adsorption and desorption isotherms at 77 K were obtained
for each type of particle, with pore size distributions estimated by the method of Miyata
et al.6 Sufficient carbon tetrachloride adsorption and desorption isotherm data were
obtained to confirm the nitrogen results. Particle size distributions were measured in a
centrifugal automatic particle size distribution analyzer.
The sorption system used in this investigation involves a magnetically coupled
microbalance, designed and manufactured in Bochum, Germany. The apparatus allows
careful adsorption/desorption studies over considerable ranges of temperature (-80 to
+250) 0C, pressure (vacuum to 350 bar) and gas phase composition in a highly controlled
environment. The microbalance is used for direct mass determination, rather than relying
on PVT (pressure, volume, temperature) relationships to calculate mass adsorption or
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Miller, Dr. Reid C. & Peyton, Dr. Brent M. FINAL REPORT - Mechanisms of CCl4 Retention and Slow Release in Model Porous Solids and Sediments, report, December 11, 2006; United States. (digital.library.unt.edu/ark:/67531/metadc888424/m1/4/: accessed September 25, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.