Biodegradation of
LAS Benzene Rings in Activated Sludge
R. D. SWISHER, Senior Research Group Leader
Inorganic Chemicals Division
Monsanto Company
St. Louis 63166
INTRODUCTION
Linear alkylbenzene sulfonate (LAS) has served as the surfactant in commercial detergent formulations in several countries during the past few years. It has
replaced the poorly biodegradable tetrapropylene alkylbenzene sulfonate which
was formerly used. The change has resulted in substantial decreases in the foaming properties and methylene blue active substances (MBAS) of sewage treatment
effluents and the receiving rivers, because of the facile biodegradation of LAS by
microorganisms.
It is regrettable that even before LAS had had a chance to prove itself-- even
before the change was completed --we began to hear calls for the detergent industry to make yet another change -- calls to replace LAS by some"supersoft"
surfactant. Perhaps we may anticipate a decline in these calls with continuing
accumulation of evidence from the field showing the success of LAS. The present
work may also contribute to that decline since it answers one particular criticism
that has heen made against LAS; namely, that in its molecular structure there is
a benzene ring which for some reason might resist biodegradation.
First of all, it must be emphasized that such incompletely degraded fragments
of the original LAS molecule cannot constitute any particular hazard even if they
are present, because it has been shown experimentally that the fish toxicity of LAS
disappears along with its MBAS response upon biodegradation (1). Fragments must
necessarily be present during the biodegradation of any complex molecule; it does
not fly apart into its component atoms in a single step, but rather undergoes a long
sequence of consecutive reactions, each one changing the molecule slightly. LAS
is no exception. Temporary presence of intermediate biodegradation products has
been demonstrated (2). Tne biochemical reactions involved in their formation
and subsequent disappearance have been investigated, and the evidence for the
eventual complete degradation of the entire molecule has been reviewed (2,3).
Bacterial attack on the LAS molecule begins at the end of the alkyl chain remote from the benzene ring, so the earlier intermediate degradation products must
inevitably contain intact benzene rings. But these are destroyed at a later stage.
This is not particularly surprising, since biodegradation of the benzene ring in
general is well known --so much so that no references need be cited. A wide
variety of benzene derivatives, including several of the amino acids and other
molecules directly involved in life processes, are degraded and also synthesized by
living organisms. There is also ample evidence for biodegradation of the ring in
sulfonated benzene derivatives. This has been proved both by Warburg respirometry
(4, 5) and by ultraviolet (UV) spectrophotometry (6) for sodium benzene sulfonate,
toluene sulfonate and other short chain derivatives. Ring degradation in LAS itself
has been indicated by infrared (7) and UV spectrophotometry (8,9) and by isolation
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