EXPERIMENTAL STUDY AND MODELLING OF LINEAR EXPERIMENTAL STUDY AND MODELLING OF LINEAR ALKYLBENZENE SULPHONATE IN SAND AND SOILALKYLBENZENE SULPHONATE IN SAND AND SOIL

Boluda-Botella N., Cases V., Gomis V., León V.M., and Soriano R. Chemical Engineering Department, University of Alicante. Apdo. 99, E-03080 Alicante (Spain). Tel.+34 965903400, ext.2647. Fax:+34 965903826.

E-mail: nuria.boluda@ua.es

INTRODUCTIONINTRODUCTION

11th Mediterranean Congress of Chemical Engineering (EXPOQUIMIA) Barcelona, Spain October 21-24, 2008

MATERIALS AND METHODSMATERIALS AND METHODS

Agricultural soil:Agricultural soil: CaCO3: 38.3%. Organic Carbon: 0.78% sand: 23.6%, silt: 38.0% and clay: 38.4%

LAS Standard:LAS Standard: 12.1% C10LAS, 34.1% C11LAS, 30.6% C12LAS and 23.2% C13LAS, donated by PETRESA.

Commercial Sand:Commercial Sand: Sea sand, purified (Merck)

LAS Analysis: Samples injected in a HPLC. Stationary-phase:Lichrospher 10m 100RP-8(25x0.46) Teknokroma Mobile phase: MeOH/H2O (85/15)+0.5M NaClO4·H2O Flow: 0.8 mL/min . UV detector (254 nm)

Performed Column Experiments: Thermostated stainless steel column: 22.4cm length, 2.5cm internal diameter (25ºC). Column connected to a HPLC pump (Shimadzu LC 9A)

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].

REFERENCESREFERENCES[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.

Experimental set-up: column experiments were performed in laboratory scale.

[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.

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.

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 (C10, C11, C12, C13) 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.

COLUMN TRANSPORT PARAMETERS

The experimental breakthrough curves (with CaCl2 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®).

LAS DESORPTION EXPERIMENTSLAS DESORPTION EXPERIMENTS

SAND COLUMN DESORPTION

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RESULTSRESULTS

REACTION PARAMETERS

Convection-dispersion equation

Ion concentration

(mg/L)

Test Ca2+ Na+ K+ Mg2+ Cl- SO42- HCO3

-

I 125 90 3.3 45 230 70 280

II 90 100 2.6 40 250 80 290

Test Porous medium

Q (mL/min)

u (cm/h)

tm (h)

Pe=vL/D

v=u/ (cm/h)

DL (cm2/h)

(cm)

I 100% sea

sand 0.50 5.71 1.78 156 0.45 12.6 1.82 0.144

II 75% sea

sand +25% soil

0.50 5.74 1.93 117 0.49 11.58 2.21 0.191

xC

x

CD

tC

2

2

xC

tC

dgr

K1

1

Adsorption affects the convection term Retardation Factor

dgrR K

11

Distribution Coefficient

Where: (velocity), (porosity), gr (density of grain)Obtained graphically with

experimental results

SIMULATION CURVES WITH PHREEQC

CONCLUSIONSCONCLUSIONS

Tap water composition: Tap water composition: concentration of major ions (in table)

The Linear distribution coefficient Kd is considered to describe adsorption: Cads = Kd C, where Cads is the sorbed concentration of a solute (moles/kg of solid) and C is the concentration in solution (moles/L of solution)

Determined in reactive transport experiment

Kd

PHREEQC (Versión 2) [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.

LAS homologues are defined 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 kinetic sorption reaction for different species (no surface definitions are needed)

Tebes-Steven et al., [8] defined kinetic sorption for solution species by the rate equation:

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:

d

adsimi

CCKR

K

Km (h-1) Kd (L/kg)

Test C10 C11 C12 C13 C10 C11 C12 C13

I 0.25 1 4 4 0.10 0.36 1.30 4.90

II 0.25 1 4 4 0.45 3.36 9.70 23.0

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.

This graphical user interface calculates the best fit of the experimental data (chloride concentration (mmol/L) versus experimental time (h)) with the analytical solution of the convection-dispersion equation.

SAND COLUMN DESORPTION

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75 % SAND-25 % SOIL COLUMN DESORPTION

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