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Fig. 18.14 Examples of irreversible adsorption: a hysteresis isotherm, immediately after first
sorption; b conditioning effect, hysteresis after repetitive sorption-desorption cycles (Pignatello
2006 )
Irreversible adsorption may occur as a result of hysteresis or by a conditional
effect. Hysteresis is exhibited by nonuniqueness of adsorption-desorption curves
within an adsorption isotherm ( Sect. 5.8 ), and conditioning results from enhanced
sorption following repeated experiments on the same sample after a previous
adsorption-desorption cycle (Pignatello 2006 , Fig. 18.14 ). In thermodynamic
terms, a solute concentration cannot be matched to multiple sorbed concentrations,
and as a consequence, irreversible sorption may arise following generation of
metastable states.
Examples of irreversible retention of pesticides on geosorbents are illustrated
below for both polar and nonionic compounds.
Cationic pesticides Diquat (1,1 0 -ethylene-2,2 0 -dipyridylium dibromide) and
Paraquat (1,1 0 -dimethyl-4,4 0 -bipyridinium dichloride) are cationic herbicides used
broadly for crop protection. They may contaminate the land surface and subse-
quently pollute the subsurface zone and groundwater after being leached by rain or
irrigation water. Retention and release of these herbicides—charged organic
molecules—on and from clay minerals have been the subject of many investiga-
tions during the second half of the last century. The complete adsorption of diquat
and paraquat by bentonite and kaolinite, as measured by the absence of detectable
bipyridyl in solutions equilibrated with these minerals, was observed by Weber
et al. ( 1965 ). The extent of adsorption reaches the limit of the mineral cation
exchange capacity (Knight and Tomlinson 1967 ). Weber and Weed ( 1968 ) con-
firmed that diquat and paraquat are adsorbed by montmorillonitic and kaolinitic
clay minerals to approximately their cation exchange capacity. Adsorption-
desorption experiments exhibit a hysteresis process. After desorption caused by
leaching with 1 MBaCl 2 , about 20 and 95 % of each of the herbicides was retained
on kaolinite on montmorillonite, respectively. Knight and Denny ( 1970 ) explained
the high retention of paraquat on dioctahedral montmorillonite by interlamellar
absorption; the clay thickness of the silicate sheet expanded from 9.6 to 12.6 Å.
Hysteresis in the adsorption and desorption of diquat and paraquat may occur
also when they are in competition with an additional ion for the adsorbing sites of
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