Page 224 - Soil Degradation, Conservation and Remediation

6.2 Sources of Soil Pollutants 213

precipitated due to alkaline conditions. In addition, the introduction of weak acids
such as acetic acid into the cathode well/reservoir can lower the soil pH near
the cathode regions, thereby preventing precipitation of the contaminants. The
contaminants are allowed to transport into the cathode well/reservoir for subsequent
removal by either electroplating or pumping aboveground.

Chemical Remediation

Heavy metals occur as dissolved ions or bound to colloidal particles in soils. They

are either precipitated or flocculated and separated. Precipitation is a process
where ionic components transform into a non-soluble phase. In fl occulation, small
undissolved solids of colloidal size are aggregated into larger solid fl ocks.

Precipitates and flocculates are then separated mechanically from the fl uid by
sedimentation, centrifugation, or fl otation.

Precipitation

The most important chemical parameters in the precipitation process are pH and
concentration of metal ions. Generally, heavy metals tend to be present in ionic form
at low pH levels and precipitate when pH is raised. Heavy metals can be precipitated

as insoluble hydroxides, sulfides, carbonates, and others by adding precipitants
(digested sludge, Fe salts, calcium hydroxide, Al salts, etc.). They are then separated
by different solid/liquid separation techniques. The precipitation of metal sulfi des
has been proven to be very effective. Adsorptive materials such as iron oxides,
clay minerals, zeolites, and organic matter are sometimes used to immobilize the
metals in soil.

Ion Exchange

Ion exchange is a matrix or resin laden with dissociable counter ions. Common ion
exchangers are made of interlaced polystyrene and polyacrylate or condensation
resins made from phenol and formaldehyde. When a solution containing dissolved
metal ions are passed through the resin column, the metal ions are exchanged with
adsorbed ions on resin surfaces. For regeneration, acids such as HCl and H 2 SO 4 and
alkaline solution (NaOH) are used. Wastewater produced in the process may be
treated for precipitation or flocculation of metal ions.

Chelation
Chelating agents such as ethylenediaminetetraacetic acid (EDTA), nitriloacetic acid
(NTA), diethylenetriaminepentaacetic acid (DTPA), and S , S - ethylenediaminedi-
succinic acid (EDDS) can form strong metal–ligand complexes and are thus highly
effective in remediating heavy metal-contaminated soils (Norvell 1984 ; Elliott
and Brown 1989 ; Kim and Ong 1998 ). Among these chelators, NTA is a Class II