Chemistry Reference
In-Depth Information
more developed nations have learned by painful and expensive experience, the
ability of an ecosystem to absorb and degrade waste products such as surfactants
can significantly affect the potential usefulness of a given material.
Of particular importance are the effects of surfactants on groundwater and waste
treatment operations. Although it could be technologically possible to physically or
chemically remove almost all residual surfactants completely from effluent streams,
the economic costs would undoubtedly be totally unacceptable. The preferred way
to address the problem is to allow nature to take its course and solve the problem by
biodegradation mechanisms.
Biodegradation may be defined as the removal or destruction of chemical com-
pounds through the biological action of living organisms. Such degradation in sur-
factants may be divided into two stages: (1) primary degradation, leading to
modification of the chemical structure of the material sufficient to eliminate any
surface active properties and (2) ultimate degradation, in which the material is
essentially completely removed from the environment as carbon dioxide, water,
inorganic salts, or other materials that are the normal waste byproducts of biologi-
cal activity. Years of research indicate that it is at the first stage of primary degrada-
tion that the chemical structure of a surfactant molecule most heavily impacts
biodegradability.
Some of the earliest reports on the biodegradability of synthetic surfactants
were made in England, where it was observed that linear secondary alkyl sulfates
(LAS) were biodegradable, while the alkylbenzene sulfonates (ABS) in use were
much more resistant to biological action. It was soon found that the distinction
between the LAS and ABS surfactants was not nearly as clear as first suggested.
Specifically, it was determined that the biodegradability of a particular ABS sample
depended to a large degree on the source, and therefore the chemical structure, of
the sample. Early producers of ABS surfactants in England used either petroleum-
derived kerosene or tetrapropylene as their basic raw material, without great
consideration for the structural differences between the two. As a result, great varia-
bility was found in the assay of materials for determination of biodegradability.
In fact, the materials derived from propylene showed little degradation while the
nominally identical materials based on the kerosene were much more acceptable.
The difference, of course, lay in the degree of branching in the respective alkyl
chains.
In 1955 and 1956 it was suggested that the resistance of tetrapropylene-derived
ABS surfactants to biodegradation was a result of the highly branched structure of
the alkyl group relative to that of the kerosene-derived materials and the LAS mate-
rials. As a result of extensive research on the best available model surfactant com-
pounds and analogs, it was proposed that the nature of the hydrophobic group on
the surfactant determined its relative susceptibility to biological action, and that the
nature and mode of attachment of the hydrophile were of minor significance.
Research using an increasingly diverse range of molecular types has continued to
support those early conclusions.
Although the chemical basis of surfactant biodegradation continues to be studied
in some detail, leading to more specific generalizations concerning the relationship
