Characteristics of Natural and Urban Soils - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 112

Some antibiotics, e.g. the macrolides, are usually very poorly soluble in water
but some are completely water-soluble or dissociate at typical soil pH values. Once
adsorbed to the soil's solid phase, desorption often shows a hysteresis, meaning
the adsorption curve and desorption curves are different, with the latter showing
incomplete desorption.
There is a range of chemical behaviours described in Thiele-Bruhn ( 2003 ),
Hamscher et al. ( 2004 ), Hamscher et al. ( 2005 ) and summarised by Thiele-Bruhn
( 2003 ). Some important examples of chemical behaviours are:

• The epimers and metabolites of various antibiotics can react in soil quite

differently from the original substances.

Expandable three-layer 4 clay minerals possessed much stronger adsorptive power
to aminoglycosides, tetracyclines and tylosin than did the non-expandable illite
and the two-layer mineral kaolinite. Such adsorption could involve inter-layer
adsorption.

•

• Soil organic matter also strongly sorbs antibiotics, but it also depends on the

amount and its composition.

• The rate of adsorption of most antibiotics in soils is fast, with a specific time

frame mentioned of several hours for efrotomycin and sulfonamides.

• Antibiotic potency is generally reduced by adsorption and hence desorption

reactivates it. Even so, sorption does not totally and necessarily eliminate

antimicrobial activity.

• Mobility and transport in the soil of antibiotics depends obviously on their water

solubility so that those that have low solubility are strongly retarded. To the
extent that there are large pores and preferential flow paths, antibiotics can move
rapidly.

• Degradation of antibiotics in soils can rarely take place through hydrolysis (an

abiotic process) and oxidative decarboxylation and hydroxylation (enzymatic
processes). It has been found that adding manure or sludge with high numbers
of micro-organisms can speed up biodegradation of the antibiotics.

• The degradation of most xenobiotics occurs faster and more completely when the

soil is aerobic than when it is not, and ciprofloxacin, for example, did not degrade
at all under anaerobic conditions.

A number of authors reported concentrations of antibiotics found in soils, mostly
in soils that were fertilised with treated sludge from sewerage systems (Table 2.3 ).
However, it should not be forgotten that antibiotics will occur naturally in

soils.

4 Clay minerals have layered crystal structures comprising octahedral sheets of Al(OH) 3 and
tetrahedral sheets of Si 2 O 5 . In some, the sheets can move apart to let other ions or molecules
enter.