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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: [e-mail])

**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: [e-mail]; [e-mail])

***Université de Toulouse; INSA,UPS; LPCNO, 135 avenue de Rangueil, F-31077 Toulouse, France / CNRS; LPCNO, F-31077 Toulouse, France

(E-mail: [e-mail])
Clearly label i.e. Table 1 or Figure 1 (no full stop)

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.

REFERENCES

Collins G.L. and Jameson G.J. (1976). Experiments on the flotation of fine particles – the influence of particle size and charge, Chemical Engineering Science, 31, 985-991.

Fukui Y. and Yuu S. (1980). Collection of submicron particles in electro-flotation, Chemical Engineering Science, 35, 1097-1105

Han M.Y., Kim M.K, Ahn H.J. (2006). Effects of surface charge, micro-bubble size and particle size on removal efficiency of electro-flotation, Water Sci. and Tech., 53(7), 127-132.

Lien C.Y., Liu J.C. (2006). Treatment of Polishing Wastewater from Semiconductor Manufacturer by Dispersed Air Flotation, J. of Environmental Engineering, 132 (1), 51-57.

Mishchuk N.A., Koopal L.K., Dukhin S.S. (2001). Microflotation Suppression and Enhancement Caused by Particle/Bubble Electrostatic Interaction, Journal of Colloid and Interface Science, 237, 208-223.

Nguyen A.V., George P., Jameson G.J. (2006). Demonstration of a minimum in the recovery of nanoparticles by flotation: Theory and experiment, Chem. Eng. Sci., 61, 2494-2509.

Rulyov N.N. (1999). Application of ultra-flocculation and turbulent micro-flotation to the removal of fine contaminants from water, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 151, 283-291.

Rulyov N.N. (2001). Turbulent microflotation : theory and experiment, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 192, 73-91.

Schubert H. (2005). Nanobubbles, hydrophobic effect, heterocoagulation and hydrodynamics in flotation, International Journal of Mineral Processes, 78, 11-21.

Tourbin M., Frances C. (2007). A Survey of Complementary Optical and Acoustic Methods for the Characterization of Dense Colloidal Silica Dispersions, Particle and Particle System Characterization, 24(6), 411-423.