Sorption of Organic Acids by Pure Clay
Minerals in Aqueous Solution
L. HEMPHILL, Associate Professor
W. S. SWANSON, Graduate Student
School of Civil Engineering
Oklahoma State University
Stillwater, Oklahoma
INTRODUCTION
The properties and characteristics of sorption reactions have long been recognized as playing a vital role in waste treatment. One group of materials generally associated with sorption reactions are the clay minerals. These materials are
universally distributed in nature and often found in the suspended solids fraction of
raw waters. Many of the physico-chemical properties of the clay minerals, e. g. ,
ion-exchange capacity and sorption affinity are well known; however, the role
these materials play in an aquatic system is unknown. It is the purpose of this
paper to: 1) briefly review the physical basis of sorption reactions, 2) describe a
laboratory procedure for measuring sorption uptake of soluble organic acids by
clay minerals, and, 3) show how the structural properties of clay minerals influence acid sorption in an aqueous system.
Before considering interface sorption reactions, it is well to review the basis
of these reactions. The subject of sorption is not new. It was described in 1773
by Scheele and in 1785 by Lowitz, (1). Both of these early investigators were interested in sorption of a gas by solids. Although the subject of sorption was studied
in detail by many investigators, it remained for J. W. Gibbs (1875) to place the
subject on a theoretical basis. In his classic study, Gibbs theorized that sorption
was a surface phenomena in which the concentration of substances at an interface
are different from those in the dispersed phase. This theory of separation produced
by a soUd interface was incorporated in an expression derived from the original
Gibbs free energy equation. In 1916 Langmuir further refined the subject of sorption at a solid gas interface by showing that the rapid decrease in intermolecular
forces with distance indicated that the sorbed gas layer was only one molecule
thick (2). This relationship, specific for solid-gas interfaces was accepted and
provided impetus for comprehensive quantitative study. In 1938 Brunauer, Emmett
and Teller showed that the original theory of Langmuir could be used to determine the surface area of powered materials (3).
The soUd-gas interface sorption relationships provide a model of study for
sorption at a solid-liquid interface; however, these relationships and even some of
the basic assumptions involved are not always valid in solid-liquid systems. To a
large extent, the solid-gas relationships are based on simple systems; i.e., single
sorbate and single sorbents at constant temperature and volume. Aqueous systems
in nature are by no means Umited to these or similar restrictions, rather the sorbents and sorbates may be present in large number. Perhaps it is for these reasons
that the subject of solid-liquid interface reactions has received little study.
Clay minerals have long been recognized as useful and unique materials;
however, the specific characterization and identification of these materials was
not estabUshed until recently. In the 1930's, X-ray diffraction techniques, together with chemical analysis, established that the clay minerals are specific
materials; continued study by X-ray diffraction, differential thermal analysis and
radioactive tagged ion-exchange techniques has shown the relationship between
structure and physico-chemical properties. In general, many of the clay minerals
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