adjusted to pH 2.00 which gives the best results. At this pH,
hydrogen ion compete with extra aluminum ions and get exchange
in the soil releasing thereby maximum aluminum ions which is
responsible for the coagulation process. Second factor might be
partial dissolution of the soil releasing aluminum ions for the
coagulation process. Additionally, the trend depicted in Fig. 4
a decrement of percentage of COD and color removal when the
pH of the solution exceeds pH 2.00, and an increase in the COD and
color when compared the raw leachate sample. The dissolution of
metal ions increases at lower pH range and becomes deficient
when the pH falls in the alkaline range, thus competition between
the electrolyte cations and metals adsorbed to the colloidal particles
may occur due to the increment of ionic strength of soil solution
(lower pH) (Lombnæs et al., 2008). Therefore, the effectiveness
of the dissolution of metal ions from soil enhances the treatment
efficiency of the landfill leachate. The COD and color removal of the
leachate by the lateritic soil increases up to pH 2.00 and then
decreases with the increasing of pH as shown in Fig. 4. After pH
4.00, the trend of COD and color removal become similar and show
negative values. This may be accounted for the change in surface
charge which is zero at pH 4.00 (Chairidchai and Ritchie, 1990) and
become positive after that. In acidic medium especially at pH 2.00,
the humic substances are insoluble in water (Thurman and
Malcolm, 1981) and do remain associated with the soil imparting
negative charge to the soil and contributing in the coagulation
process. With the increase in pH, the solubility of humic substances
increases resulting thereby decrease in surface charge of the soil
which is responsible for coagulation. Though lateritic soil might
release the humic substances and color which can increase the TOC