Prior to the investigation of the effect of pH on the
paraquat adsorption, zeta potential of spent and treated
diatomaceous earths in the absence of paraquat was
determined by a Zeta Meter System 3.0 (ZETERMETER
Inc., USA). From the data of zeta potential
vs. pH (Fig. 1), it is clear that the isoelectric point or
pH of point of zero charge (i.e., pHpzc) of the diatomaceous
earth–water system was thus determined to be
approximately 1.5. Thus, the surface of these diatomaceous
earths at the experimental conditions (i.e.,