Article

Diffusion of landfill leachate through compacted natural clays containing small amounts of carbonates and sulfates

Applied Geochemistry (Impact Factor: 2.02). 06/2012; DOI: 10.1016/j.apgeochem.2012.02.032

ABSTRACT The diffusion of a synthetic urban landfill leachate through compacted natural illitic
clays and the role of reactive accessory minerals (carbonates and gypsum) in the
geochemical behavior of major soluble ions are evaluated. The leachate is composed of
NH4
+ and Na+ (0.25 M) balanced by Cl- (0.25 M), acetate (0.1 M) and HCO3
- (0.15 M).
The pH is 7.8 and it is typical for the mature stage of organic matter degradation within
an urban landfill. Laboratory scale diffusion tests were performed during four months
(long term experiment, LT) and two weeks (short term experiment, ST). The ST
experiments were designed to allow the measurement of chloride gradient as diffusion
tracer in the compacted clay. In the LT experiments the chemical gradients were already
absent, but geochemical reactions involving dissolution of gypsum and precipitation of
calcite were observed. Evolution of pore-water chemistry, mineralogy, cation exchange
properties, and specific surface of clays were determined. Numerical simulations were
carried out using the geochemical code RETRASO. Chloride transport, precipitation of
carbonates, pH buffering, and Ca2+/NH4
+cation exchange reactions took place in the
laboratory tests. Apparent Cl- diffusion coefficients were determined by direct modeling
of the ST tests and validated with the LT experiments. These coefficients were
considered in the simulation of transport coupled with experimentally calculated
exchange constants and dissolution/precipitation reactions of gypsum and carbonates.
Sulfate reduction coupled with acetate oxidation has been proven to be relevant in the
sample with significant concentration of sulfate (Bailén). This process also includes an
important precipitation of carbonates (mainly calcite) and produces a decrease of sulfate
in the porewater.

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