The Use of Natural Zeolites in Active Barrier
Systems for Subaqueous In-Situ Capping
of Contaminated Sediments.
Patrick H Jacobs
Department of Environmental Science and Technology, Technical
University Hamburg-Harburg, D-21073 Hamburg, Germany. e-mail: p.jacobs@tu-harburg.de
Abstract
Sub-aqueous in-situ capping (ISC) of contaminated sites is a relatively new method that has become an attractive option for isolating contaminated sediments from the environment to prevent contaminants from being released into the surface water and, possibly, into the food chain (Zeman 1994). As a typical “passive“ in-situ technique, there are no operational costs following the installation of the barrier. Active barrier systems (ABS), i. e. ISC-layers consisting at least partly of one or more reactive materials can extend the concept by chemically immobilising contaminants that are released from contaminated sediments and that are transported advectively through the sediment/water interface. Natural zeolites have been found to meet the requirements for potential ABS materials according to their physical, chemical, and economic properties. However there is a lack of extensive data to predict the long-term behaviour of a zeolite based ABS.
1 Introduction
Remediation of contaminated sediment sites often proves technically
difficult and costly. Consequently, manifold efforts have been made during the
last few decades to develop alternative non-removal remediation techniques. In-situ capping (ISC) uses the
sub-aqueous placement of a covering layer over a contaminated sediment in order
to prevent contaminants from being released into the surface water. The inhibition
of contaminant release is based on (1) stabilisation of the sediment, (2)
physical isolation and (3) chemical isolation of the sediment from the
overlying water body (Palermo et al.
1998, Azcue et al. 1998).
ISC-projects carried out to date were particularly relying on stabilisation and
physical isolation by employing clean sandy or silty material that in some
cases is combined with geotextile or cobble layers. In contrast, the concept of
active barrier systems (ABS) (Jacobs and Förstner 1999) aims to emphasise the
chemical isolation by using capping layers that consist at least partly of one
or more reactive components capable of demobilising contaminants. Hereby, the
long-term efficiency of a cap shall be achieved even in case of notable ground
water seepage inducing advective contaminant transport through the sediment/water
interface (Figure 1).
Natural zeolite minerals,
namely clinoptilolite, chabazite, mordenite, and phillipsite, are effective
materials for reactive barriers due to their favourable physical and chemical
properties along with their abundant occurrence in nature (Jacobs and Förstner
1999). Zeolite minerals can be defined as infinite three-dimensional frameworks
of silicate tetrahedra with an open microporous “honeycomb” structure. The
charge deficiency due the isomorphic substitution of silica by aluminium is balanced
by readily exchangeable alkaline and alkaline earth metal cations situated
within the micropores. Their unique ion exchange properties resulting from this
structural pattern makes them particularly well suited for the retention of
heavy metal cations.
To assess the long-term
efficiency of a zeolite-based BAS, it is a prerequisite to determine all
parameters controlling the cation exchange processes in the cap involving heavy
metals. Thus, the role of equilibria and kinetics of ion-exchange, as well as
the role of humic substances and colloidal solid phases and their impact on
heavy-metal retention are discussed in this paper.
2 Material and Methods
Four different high-grade
zeolitic rocks have been investigated: Ash-Meadows ClinoptiloliteTM
(ARC), Bowie Chabazite (GSA Rsources), phillipsite (TSM 190, Steelhead
Specialty Minerals), and mordenite (Zeobon). In the following, it will referred
to these materials with the zeolite mineral name only. To achieve the
homo-ionic sodium form of the zeolites, they have been washed in 0.1 mol L-1
sodium nitrate solution (S:L-ratio 10 g : 1 L) in five successive
cycles followed by one cycle with de-ionised water prior to use. To attain a
constant water content of the zeolite material, it was stored in a closed
container above a saturated sodium chloride solution providing a constant
vapour content.
All laboratory column
experiments where carried out in Perspex columns (h = 100 mm,
Ø = 61 mm) filled with a mixture of 3.6 g zeolite in
360 g acid washed quartz sand. Batch experiments where carried out in
1000 mL HDPE-flasks shaken continuously in a overhead shaker at 14°C ±0.5
3 Results and Discussion
The result of the batch
experiment in figure 2 reflects the highly selective exchange of lead compared
to copper, cadmium and zinc. Moreover, it reveals that ion exchange processes
are quite slow, since the exchange rate is limited by the diffusion within the
microporous cavities of the zeolite crystals rather than by the exchange
reaction itself. Due to these slow kinetics it must be assumed that there will
be an impact of high seepage rates on the efficiency of a zeolite-based cap.
This assumption is underpinned by a column experiment depicted in
figure 3. A clinoptilolite/sand-filled column is run with a 10 mol L-1
lead acetate solution at 6, 9, and 12 mL h-1 (corresponding
to 10‑4 cm s-1). A pure sand column was
run as a reference. The break through occurs in order of decreasing flow rate.
Another
column experiments shows the effect of humic acid and colloidal particles on
the lead retention in three identical clinoptilolite/sand column (figure 4).
Lead nitrate was dissolved in (1) a solution of natural mineral colloids (280
mg L-1 water-extractable colloidal solid matter) derived from
uncontaminated aquifer material and stabilised by addition of humic acid, (2) a
solution of 40 mg L-1 humic acid (Fluka) and (3) deionised
water.
While lead
retention was decreased in the presence of humic acid it was fully inhibited by
the colloidal particles. Although this experiment was run with relatively high
flow rates (90 mL h‑1) it shows the possible impact
of colloidal particles and humic substances on heavy-metal transport though a
zeolite barrier.
4 Conclusions
The goal of
further research work must be to develop a computer model allowing for all relevant
transport and retention mechanisms as a tool to predict the long-term
efficiency of a cap. According to the presented experimental results there is a
strong need to establish an extensive data base on speciation as well as
equilibria and kinetics of surface reaction as ion exchange and colloidal
phases. While speciation data of dissolved salts are compiled in the data files
of customary computer programs and ion exchange data are readily obtainable
from laboratory experiments there is a substantial gap concerning the last
characterisation of colloidal particles and their behaviour in natural systems.
References
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Jacobs, P.H., Förstner, U. (1999): Concept of sub-aqueous capping of
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zeolites. – Water Res., 33: 2083-2087.
Palermo, M., Maynord, S., Miller, J., Reible, D. (1998): Guidance for In-Situ Subaqueous
Capping of Contaminated Sediments. – EPA 905-B96-004, Great Lakes National
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Zeman, A.J. (1994): Subaqueous capping of very soft contaminated
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list of figure captions
Figure 1: In-situ capping of a contaminated sediment site.
Figure 2: Kinetics of simultaneous exchange of Pb (A), Cu (B), Cd (C), and Zn (D) on natural phillipsite from a solution containing 1 mmol L-1 of each metal.
Figure 3: Breakthrough curves of lead in a clinoptilolite/sand column at different flow rates.
Figure 4: The impact of humic acid (40 mg L-1) and colloidal solid phases (280 mg L‑1) on lead transport through a zeolite-sand (1:100) filled column. The presence of humic acid accelerates the break through notably and in presence of colloidal particles no lead retention is observed.