Preliminary experiments of flotation for the development of a process
treatment for the removal of nanoparticles from liquid wastes
M. Tourbin*, Y. Liu**, S. Lachaize*** and P. Guiraud**
*Université de Toulouse ; INP, UPS, CNRS ; Laboratoire de Génie Chimique ; 4 allée
Emile Monso, BP 44362, F-31432 Toulouse Cedex 4
(E-mail: Mallorie.Tourbin@ensiacet.fr)
**Université de Toulouse; INSA,UPS,INP; LISBP, 135 avenue de Rangueil, F-31077
Toulouse, France / INRA, UMR792 Ingénierie des Systèmes Biologiques et des
Procédés, F-31400 Toulouse, France / CNRS, UMR5504, F-31400 Toulouse, France
(E-mail: yaliu@insa-toulouse.fr; Pascal.Guiraud@insa-toulouse.fr)
***Université de Toulouse; INSA,UPS; LPCNO, 135 avenue de Rangueil, F-31077
Toulouse, France / CNRS; LPCNO, F-31077 Toulouse, France
(E-mail: slachaiz@insa-toulouse.fr)
Abstract
The recovery of nanoparticles from wastewaters will be an important challenge in the near future because of
the rapid development of nanotechnology. Indeed, this type of particles will inevitably be found in growing
quantities in the industrial and domestic wastes, and thus possibly in water resources. Nanoparticles differ
from classical particles by their size, smaller by several orders of magnitude, and their specific properties due
to their high surface over volume ratio. Few studies were yet pursued on the subject and the specific
properties of the nanoparticles could induce the inefficiency of classical water treatments processes among
which principally coagulation and flotation processes.
The objective of this work is thus to develop a specific treatment technique by flotation or a combined
coagulation-flotation process with the suitable additives for nanoparticles. Preliminary experiments of
coagulation and flotation are performed with the colloidal silica suspension Klebosol® 30R25 (Rohm and
Haas, France) and the addition of salt (AlCl3) or surfactant (CTAB). The physicochemical properties of the
suspensions (pH, particle size and turbidity) are followed during the colloids destabilisation.
Keywords. Nanoparticles – aggregates – flotation – coagulation – wastewater.
INTRODUCTION
The conventional coagulation treatments used in industries are not adapted to the recovery of nano-scale
particles because, inter alia, of the large amount of coagulant leading to a bulky sludge volume. Then, in the
last years some papers referring to flotation processes eventually combined with coagulation for the
separation of SiO2 nanoparticles from water have been published.
The example of the treatment of chemical and mechanical polishing effluents by dissolved air flotation (DAF)
has been tested by Lien and Liu (2006). The parameters governing the efficiency of the particles’ collection
by bubbles are the particle and bubble sizes and charges, and the hydrophobicity of the surfaces of particle
and bubble (Fukui and Yuu, 1980). Many works have shown the significant effect of colloidal forces on the
capture of micro and nanoparticles (Collins and Jameson, 1976; Fukui and Yuu, 1980; Mishchuk, 2001; Han,
2002; Schubert, 2005) and that applies for colloidal silica (Lien and Liu, 2006; Nguyen et al., 2006). The rate
of flotation drastically depends on the charge of both the bubble and the particle: Lien and Liu (2006) brought
out the importance of the addition of a suitable collector because a better flotation can be observed if the
particles surface and the bubble interface experiences opposite charges. The bubble to particle size ratio is
the other key-parameter. Nguyen et al. (2006) highlighted that there is a size of particle for which there is a
minimum of the collection efficiency. Underneath this size, the efficiency increases because of the Brownian
diffusion and the colloidal forces that control the collection of particles. With larger particles, the interception
and collision mechanisms predominate. With bubbles of typical average diameter of 150 µm, their
experimental and numerical results show the collection efficiency to have a minimum at a particle size in
order of 100 nm. Rulyov (1999, 2001) showed that an effective recovery of submicron particles can be
achieved with the use of 40mm in diameter microbubbles. But producing submicronic or ''nano'' bubbles
remains very difficult, electroflotation being one possible solution (Fuki and Yuu, 1980; Schubert, 2005; Han
et al., 2006).
METHODS
In this stydy, Klebosol 30R25 colloidal silica suspension (Rohm and Haas Electronic Materials, France) was
provided with an initial solid content of about 15.3% v/v and a monodispersed particle size distribution
around 27 nm. The diluted suspension at 0.15% was prepared with or ultra pure water and it has been
assumed that the suspension remains stable over a large concentration range (Tourbin and Frances, 2007).
The physicochemical measurements were focused on the pH, the particle size by dynamic light scattering,
the surface charge of the particles by the measurement of the zeta potential and their concentration by the
analysis of the turbidity of the samples.
The experiments of dissolved air flotation were carried out in the tanks of a flottatest (Orchidis, France) with
or ultra pure water pressurized at 6 atm, after verification of the good mixing of the suspension with the
air/bubbles solution injected.
RESULTS
For coagulation, as expected, the higher the valence of the cation, the lower the critical coagulation
concentration and the more efficient the nanoparticles aggregation are, that is why for flotation the
experiments were made with AlCl3. Moreover, a monitoring of the physicochemical properties of the treated
suspensions, especially of the pH, is important. As a matter of fact, different chemical species can be formed
by the cation as a function of the pH and depending on the species, the addition of salt can actually stop the
aggregation of the particles. The surfactant CTAB, which acts by steric destabilisation, also gives an
important aggregation of the particles but it increases more the turbidity of the treated water than the salts
do.
Consequently, treatments by flotation were carried out with both additives with their concentrations being
optimized to form smaller aggregates to allow a good floatability if they latter are captured by bubbles of
about 30-70 µm in diameter. With both additives, depending on the concentration, an aggregation of the
particles is observed but a flotation of the aggregates was only observed in the case of the use of the
surfactant CTAB at a concentration of 6.10-4mol/L-1.
CONCLUSION
The removal of nanoparticles from water is not easy because of the difficult choise of the suitable additive
and its concentration to induce an aggregation but to form sufficiently small aggregates that will be easily
float by micronic bubbles to give a relatively clear water. We actually are going into details because more
additives and more concentrations have to be tested, and in an optimized experimental device to find good
conditions for the removal of suspended silica nanoparticles in water.
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