pH > 3.0) should exhibit negative charges mainly due to
the variable charge from pH dependent surface hydroxyl
sites (Sparks, 1995). It is noted that with the increase of
pH the adsorbed amount of cationic paraquat increased
in response to the increasing number of negatively
charged sites that are available due to the loss of H+
from the surface (Sparks, 1995). From the results in
Fig. 1, this behavior was also demonstrated by pHpzc
(*1.5) of the clay–water system determined in the electrokinetic
experiment.
These negative charges increase with increasing pH,
which is consistent with the results of the increased
amounts adsorbed onto treated diatomaceous earth
with pH under the initial concentration of paraquat at
30 mg/L and 25 Cas shown in Fig. 2. From Fig. 2, it
is evident that the rate of adsorption decreased with time
until it gradually approached a plateau because the equilibrium
concentration was approached in an aqueous
solution. Also, the paraquat adsorption by the base-activated
diatomaceous earth was evidently affected over
the pH ranges of 3–5. The kinetic data obtained from