New Zealand Journal of Marine and Freshwater Research abstracts
Adsorption and transport of cadmium and rhodamine WT in pumice sand columns
Liping Pang
Murray Close
Helen Greenfield
Greg Stanton
Institute of Environmental Science and Research Limited
P.O. Box 29 181
Christchurch, New Zealand
email: Liping.pang@esr.cri.nz
Abstract The transport and attenuation of cadmium (Cd)
and rhodamine WT (RWT) in a pumice sand aquifer media was investigated using
column experiments to study a scenario of point-source contamination. A pore-water
velocity of 1.7-1.8 m/day, which is a typical field groundwater velocity
in a pumice sand aquifer system, was applied to triplicate columns. A pulse
of a solution containing Cd and RWT, together with the conservative tracer
tritiated water (3H2O) at pH = 7, was introduced into the columns.
Experimental results showed that concentration breakthrough curves (BTCs)
of 3H2O were symmetrical and fitted well into an equilibrium model.
In contrast, BTCs of Cd and RWT were asymmetrical with significant tailings
and fitted well with a two-site adsorption/desorption model. The symmetric
3H2O BTCs suggest that physical non-equilibrium was absent in
the experimental system, therefore the asymmetrical BTCs of Cd and RWT were
attributed to chemical non-equilibrium. Modelling results showed that, in
comparison with 3H2O, Cd was apparently retarded by 101-108 times
in pumice sand aquifer media (apparent adsorption coefficient 7.33-9.24 ml/g)
and underwent a mass loss of 20-30% that was probably because of precipitation
of CdCO3. As CdCO3 is extremely insoluble, Cd precipitation
would be irreversible and therefore it would not contribute to the tailing
of the Cd BTCs. The experimental results suggest that the adsorption and
desorption of Cd in pumice sand aquifer media in hydrodynamic conditions
was a kinetic process. Cd desorption rates were two orders-of-magnitude slower
than its adsorption rates. This resulted in a prolonged mean residence time
for Cd in pumice sand aquifer media, which was 10-12 days in the 18-cm-long
columns under a flow velocity of 1.7-1.8 m/day. Since the mean residence
time is only indicative for the arrival of the central of mass in a contaminant
BTC, the time required for the total disappearance of Cd will be much longer
than the mean residence time because of the significantly long tailing of
the BTC. This implies that natural attenuation of Cd from a contaminated
pumice sand aquifer would take a time period from decades to centuries. Batch
isotherm experiments were also carried out to obtain Cd adsorption coefficients
at equilibrium conditions. In comparison with the column results obtained
from non-equilibrium conditions, adsorption coefficients of 20 ml/g obtained
from the batch equilibrium experiments were 2-3 times higher for Cd. This
finding suggests that care should be taken when using batch adsorption isotherm
results to predict field problems because the delay in the appearance of
contaminants in drinking water wells and springs could be overestimated.
In comparison with 3H2O, RWT was retarded 5-7 times and had BTCs
that were significantly more spread out. Hence the use of RWT to indicate
groundwater flow in pumice sand aquifers will underestimate groundwater velocity
and overestimate aquifer dispersivity. About 4-14% of RWT mass was lost during
its transport, probably because of the irreversible adsorption of isomer
2 (one of two major components of RWT) dye onto the aquifer sand.
Keywords heavy metals; adsorption; desorption; pumice
sand aquifer; groundwater; rhodamine WT
M03081; Received 10 November 2003; accepted 8 April 2004; Online publication
date 8 June 2004
New Zealand Journal of Marine and Freshwater Research, 2004, Vol. 38:
367-378
0028-8330/04/3802-0367 © The Royal Society of New Zealand 2004
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