Over the last two decades, many studies have been performed to characterize the environmental behaviour of linear alkylbenzene sulphonate (LAS), one of the major ingredients of synthetic detergents. In fact, the fate, effects, behaviour and sorption of
LAS in different soils have established a good foundation for understanding its interactions [1-2]. However, few reports analyse how desorption processes occur. In recent years, high loads of treated wastewater or sludge, which can contain high concentrations of LAS, have been applied to agricultural areas, and therefore migration of these contaminants could affect groundwater quality.
d
, where
Cads is the sorbed concentration of a solute (moles/kg of solid) and C is the concentration in solution (moles/L of solution)
Convection-dispersion equation
Adsorption affects
Retardation Factor Distribution Coefficient

C
 t

D
 2
C
 x
2
 

C
 x the convection term

C
 t

1

1


 

 gr
K d

C
 x
R

1

1



 gr
K d
Kd
Where:

(velocity),

(porosity),
 gr
(density of grain)
Obtained graphically with experimental results
Determined in reactive transport experiment
[7] is a computer program for one-dimensional reactive transport calculations designed by the U.S. Geological Survey. The user-friendly interface is useful for simulations of many practical problems in hydrogeochemistry.
To study the physicochemical desorption of LAS, two laboratory experiments with columns containing 100 % sand (Test I) and 75% sand – 25 % soil (Test II) have been conducted. The experimental set-up consisted of a cylindrical stainless steel column filled with soil and connected to a HPLC pump [3]. Additional details of specific LAS experiments are reported elsewhere [4,5].
Ion concentration
(mg/L)
Test Ca
2+
Na
+
K
+
Mg
2+
Cl
-
SO
4
2-
HCO
3
-
CaCO
3
: 38.3%. Organic Carbon: 0.78% sand: 23.6%, silt: 38.0% and clay: 38.4%
I 125 90 3.3 45 230 70 280
II 90 100 2.6 40 250 80 290
Sea sand, purified (Merck)
12.1% C
10
LAS, 34.1% C
11
LAS, 30.6% C
12
LAS and 23.2% C
13
LAS, donated by PETRESA.
Samples injected in a HPLC. Stationary-phase:Lichrospher 10
 m 100RP-8(25x0.46) Teknokroma
Mobile phase: MeOH/H
2
O (85/15)+0.5M NaClO
4
·H
2
O Flow: 0.8 mL/min . UV detector (254 nm)
Thermostated stainless steel column: 22.4cm length, 2.5cm internal diameter
(25ºC). Column connected to a HPLC pump (Shimadzu LC 9A)
Experimental set-up: column experiments were performed in laboratory scale.
as SOLUTION_MASTER_SPECIES. Initial SOLUTION_SPECIES and initial sorbed species are quantified to start the simulation.
PHREEQC allows for several options in the simulation of surface reactions. However, in this case we defined
for different species (no surface definitions are needed)
Tebes-Steven et al., [8] defined
for solution species by the rate equation:
R i
 
K m

 C i

C ads
K d

 where Ci is either LAS homologue (mol/L) and Cads their sorbed concentration (mol/kg sediment), Km is the transfer coefficient (hr -1 ) and Kd is the distribution coefficient (L/kg). The values of the coefficients are given in the following table:
Km (h
-1
) Kd (L/kg)
Test C
10
C
11
C
12
C
13
C
10
C
11
C
12
C
13
I
II
0.25
0.25
1
1
4
4
4
4
0.10 0.36 1.30
0.45 3.36 9.70
4.90
23.0
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0 10
SAND COLUMN DESORPTION
20 30
Time (h)
40 50 60
Sim C10LAS
Exp C10LAS
Sim C11LAS
Exp C11LAS
Sim C12LAS
Exp C12LAS
Sim C13LAS
Exp C13LAS
70
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0
75% SAND-25% SOIL COLUM N DESORPTION
50 100 150
Time (h)
200 250
Sim C10LAS
Exp C10LAS
Sim C11LAS
Exp C11LAS
Sim C12LAS
Exp C12LAS
Sim C13LAS
Exp C13LAS
300 350
SAND COLUMN DESORPTION
Sim C10LAS
Sim C11LAS
Sim C12LAS
Sim C13LAS
75 % SAND-25 % SOIL COLUMN DESORPTION
Sim C10LAS
Sim C11LAS
Sim C12LAS
Sim C13LAS
The experimental breakthrough curves (with CaCl
as tracer) were obtained prior to the LAS desorption experiments. Hydrodynamic column parameters were obtained using ACUAINTRUSION [6], designed with Visual Basic 6.0 (Microsoft®).
Test
I
II
Porous medium
100% sea sand
75% sea sand +25% soil
Q u
(mL/min) (cm/h) t m
(h)
Pe=vL/D
0.50
0.50
5.71 1.78
5.74 1.93
156
117

 v=u/

0.45 12.6
0.49 11.58
(cm/h)
D
(cm
L
2
/h)
1.82
2.21

(cm)
0.144
0.191
1.E-08
1.E-08
8.E-09
6.E-09
4.E-09
2.E-09
0.E+00
0 10 20 30
Time (h)
40 50 60 70
7.E-08
6.E-08
5.E-08
4.E-08
3.E-08
2.E-08
1.E-08
0.E+00
0 50 100 150 200
Time (h)
250 300 350
A continuous 0.5 mL/min in-flow of filtered and sterilised tap water containing 5 ppm LAS was injected into both columns for several days until the concentration at the outlet was close to that of each homologue (C
, C
, C
, C
) injected. Formaldehyde was included to avoid growth of bacteria and hence microbiological biodegradation.
The desorption experiments started when tap water without LAS was injected, and the effluent was collected in small proportions, at first every 20 minutes/sample, and later every 100 minutes/sample.
LAS samples were analysed by HPLC using a UV detector (254 nm). Experimental results from sand columns showed that the concentration of different homologues, in general, decreases sharply within a relatively short time, whereas the experiment with sand and soil exhibited more dispersive spreading.
Two continuous LAS desorption experiments have been carried out in columns containing 100 % sand and 75% sand – 25 % soil. Experimental results from sand columns showed that the concentration of different homologues, in general, decreases sharply within a relatively short time, whereas the experiment with sand and soil exhibited more dispersive spreading. PHREEQC was applied in both cases assuming convective-dispersive transport and kinetic sorption reaction. Distribution coefficients, determined earlier using experimental data, are larger in tests employing soil (greater sorption). Transfer coefficients, which increase with homologue chain length, were kept constant during the two tests. Simulated results are in accord with experimental data.
Calculated sorbed homologue concentrations are greater in tests employing soil and a longer desorption time is expected.
[1] Jensen J., 1999. The Science of Total Environment, 226, 93-111.
[2] Verge C., Moreno A, Bravo J. and Berna J. L., 2001. Chemosphere, 44, 1749-1757.
[3] Gomis, V., Boluda, N. and Ruiz, F., 1997. J. Cont. Hydrol., 29, 81-91.
[4] Boluda N., León V. M., Prats D. and Chorro M.C., 2005. 10th Med. Congress of Chem. Eng.
[5] Boluda N., Cases, V., León, V.M., Gomis, V. and Prats, D., 2007. Hidrol. y aguas subt., 22. IGME. Spain.
[6] Boluda Botella, N., Gomis, V. and Pedraza, R., 2006. 1st SWIM-SWICA. Cagliary (Italy).
[7] Parkhurst, D.L. and Appelo, C.A.J., 1999. U.S. Geological Survey. Water Res. Inv. Report 99-4259, 312 pp.
[8] Tebes-Stevens, C., Valocchi A.J., VanBriesen J.M., Rittmann B.E., 1998. J. of Hydrol., v. 209, p. 8-26.