In-situ technologies for groundwater treatment: the case of arsenic - In-Situ Remediation of Arsenic-Contaminated Sites - page 25

Yuan and Chiang (2008) tested comparatively EK in the same soil under four different operating
conditions: (i) groundwater only, and adding (ii) cetylpyridinium chloride, (iii) citric acid and (iv)
EDTA. Systems with additives showed better performance than the groundwater alone. The best
performance was observed in the EK-EDTA system with a gradient voltage of 3.3 V m − 1 , and
at the same time, there was a decrease in the electroosmotic permeability. The results indicated
that the removal efficiency depends more on the mechanisms of electromigration rather than on
the electroosmosis mechanisms. Indeed, the intense electroosmotic flow in the cathode caused a
delay in the electromigration of As towards the anode. The amount of As collected in the anode
compartment was 2.4 times higher than the amount collected in the cathodic compartment.
Leszczynska and Ahmad (2006) studied the electrokinetic removal of the pesticide known as
copper and chromium arsenate (CCA). Studies were made with artificially contaminated kaolin
using a DC power source. The authors compared the behavior of the system with and without the
addition of reagents (NaOH and NaOCl). Doped systems behave more efficiently with removal
of 74.4% and 78.1% for NaOH and NaOCl, respectively.
An As EK treatment plant was built in Loppersum, a small town in Northern Netherlands
(Lageman, 2005). The soil at the site had a clayish nature and was contaminated with As
with concentrations of around 400-500 mg kg − 1 to a maximum depth of 2 m. The source of the

contamination was Na 2 HAsO 4 7H 2 O, a compound used in wood preservation. There were two

contaminated areas. Initially, a potential gradient of 40 V m − 1 was applied and then decreased
to 20 V m − 1 , with a current density of 4 A m − 2 (the total cross sectional area was 110 m 2 ). Ten
monitoring sites were established. After 65 days, approximately 75% of the area had already a
concentration below the target figure of 30 mg kg − 1 . In short, they treated 250 m 3 of soil with an
average As concentration of 115 mg kg − 1 and a maximum concentration of 500 mg kg − 1 . The final
average concentration was 10 mg kg − 1 and the maximum of 29 mg kg − 1 . The energy consumption
was 150 kWh t − 1 . The duration of operation was 80 days of 18 hours, and the EK removed 38 kg
of As, while the remaining 14 kg were removed by excavation.

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1.9 IN-SITU CHEMICAL TREATMENT

In-situ chemical treatment consists of the direct injection of an oxidant in the subsoil (normally
potassium permanganate or oxygen) promoting oxidation of As(III) to As(V), which then copre-
cipitates with iron oxides. All the set of in-situ chemical treatment technologies have in common
the injection of a chemical reagent within an aquifer upstream of the contaminated site. This reacts
with the contaminant and transforms it into a harmless form. Eventually, a closed loop can be used
by pumping a certain volume of groundwater downstream the site and use it for reinjection. It is
important to increase in the groundwater the velocity through the contaminated area by increasing
the hydraulic gradient obtained by the actions of injection (water table elevation) and extraction
(low water table). On the other hand, it is important to monitor the chemical transformation of
the contaminant by chemical reaction within the treatment area.
In the case of As, the most efficient technology involves the introduction in the aquifer of
two solutions: one of hydrogen peroxide and another one of ferric chloride. Hydrogen peroxide
oxidizes As(III) to As(V) according to the following reaction:
HAsO 2 + 2H 2 O → H 3 AsO 4 + 2H + + 2e −

(E 0

= 0 . 56 V) (1.18)

Ferric chloride stabilizes As(V) by coprecipitation in the form of ferric arsenate and other
insoluble precipitates. The precipitation reaction is:
Fe 3 + + AsO 3 4 ( K ps = 5 . 7 × 10 − 21 ) → FeAsO 4 (1.19)

Both solutions can be injected sequentially. As the oxidation reaction by the peroxide has a
rather rapid kinetics, FeCl 3 can be injected shortly thereafter.
This technology was used in the remediation of a site in Tacoma (Ipsen et al ., 2005b),
Washington, USA, where the groundwater contamination was caused by sodium arsenite. The