Abstract

The gelation in situ of polymers may be a method for temporarily containing organic solvents and other petroleum derived products, which may enter the subsurface. In order to create gels that are neither toxic to humans nor to the ecosystem, we have investigated on the use of the biopolymer xanthan gum with aluminum as a crosslinking agent. Firstly we have considered xanthan/sodium aluminate solutions, which upon preparation are strongly basic and do not gel. The gelation takes place in acid solutions so that Al(III) ions are released, and is instantaneous. Therefore, a special technique must be used for generating the gel structure in situ. The un-gelled solution must be injected and an acid must be added afterwards. We have investigated on the gelling reaction induced by gaseous carbon dioxide, which is an acid gas. The insufflation of CO₂ above the solution generates a layer of gel whose thickness progressively increases as CO₂ diffuses in the polymer solution. Secondly we have investigated on the use of aluminum chloride as the source of Al(III) ions. Also in this case, due to the full availability of Al(III) ions, the gelling reaction would be instantaneous. Therefore, the solution of AlCl₃ must be added on the top of the xanthan solution after its injection. For both gelling systems we have studied both theoretically and experimentally the rate of formation of the gel layer which progressively thickens after the insufflation of CO₂ or after the addition of the AlCl₃ solution.

Keywords: Containment of spills in soil; Gelation time; Xanthan/Al(III)/CO₂ gel; Xanthan/AlCl₃ gel; Gelled polymer technology

Nomenclature

c

aluminum chloride concentration (mol/cm³)

c⁰

initial concentration of AlCl₃ in the solution (mol/cm³)

c_T

total molar concentration in the liquid phase; c_T ≈ 55.6 (mol/l)

D

diffusivity of carbon atom in the gel (cm²/s)

D_CO2

diffusivity of CO₂ in water at T = 25 °C (cm²/s)

D_S

diffusivity of AlCl₃ in the gel at any time t (cm²/s)

D_S°

diffusivity of AlCl₃ in Xn solution (cm²/s)

G′

shear storage modulus G′ (Pa)

H_CO2

Henry's constant (atm)

L

height of AlCl₃ solution in the vial (see Fig. 6) (cm)

M

molecular weight of AlCl₃ (g/mol)

pH_i

calculated value of pH at the interface upon addition of CO₂

pH₀

initial pH of the xanthan/aluminate solution

P

pressure (atm)

t

time (s)

t^*

time at which the gel boundary stops (s)

Xn

xanthan gum concentration in aqueous solution (ppm)

y_CO2

mole fraction of CO₂ in the gas phase (=1 in our experiments)

Greek symbols

α

adjustable parameter for diffusivity, see Eq. (8)

β

AlCl₃/Xn pseudo stoichiometric coefficient (g of AlCl₃ consumed per g of xanthan gelled)

γ

see Eq. (13)

λ

position of the gel/solution boundary (cm)

ρ

density of Xn solution (g/cm³)

[·]

square brackets indicate concentrations (mol/l)

Subscript

i

at interface

Article Outline

Nomenclature

1. Introduction

2. Ionic equilibria in aluminate aqueous solutions

3. Runs Xn/NaAlO₂/CO₂

3.1. Materials and experimental procedure

3.2. Interpretation of experimental results

3.3. Transient diffusion in the gel

4. Runs Xn/AlCl₃

4.1. Materials and experimental procedure

4.2. Interpretation of experimental results

5. Chemical resistance of the gel

6. Conclusions

Acknowledgements

References