Pascal Guiraud

Silica nanoparticles separation from water: Aggregation by cetyltrimethylammonium bromide (CTAB)

Nanoparticles will inevitably be found in industrial and domestic wastes in the near future and as a consequence soon in water resources. Due to their ultra-small size, nanoparticles may not only have new hazards for environment and human health, but also cause low separation efficiency by classical water treatments processes. Thus, it would be an important challenge to develop a specific treatment with suitable additives for recovery of nanoparticles from waters. For this propose, this paper presents aggregation of silica nanoparticles (Klebosol 30R50 (75nm) and 30R25 (30nm)) by cationic surfactant cetyltrimethylammonium bromide (CTAB). Different mechanisms such as charge neutralization, depletion flocculation or volume-restriction, and hydrophobic effect between hydrocarbon tails of CTAB have been proposed to explicate aggregation results. One important finding is that for different volume concentrations between 0.05% and 0.51% of 30R50 suspensions, the same critical coagulation concentration was observed at CTAB=0.1mM, suggesting the optimized quantity of CTAB during the separation process for nanoparticles of about 75nm. Furthermore, very small quantities of CTAB (0.01mM) can make 30R25 nanosilica aggregated due to the hydrophobic effect. It is then possible to minimize the sludge and allow the separation process as greener as possible by studying this case. It has also shown that aggregation mechanisms can be different for very small particles so that a special attention has to be paid to the treatment of nanoparticles contained in water and wastewaters.

Surface-modified microbubbles (colloidal gas aphrons) for nanoparticle removal in a continuous bubble generation-flotation separation system

The treatment of nanoparticle (NP) polluted aqueous suspensions by flotation can be problematic due to the low probability of collision between particles and bubbles. To overcome this limitation, the present work focuses on developing an enhanced flotation technique using the surface-functionalized microbubbles - colloidal gas aphrons (CGAs). The CGA generator was adapted to be air flow rate controlled based on the classical Sebba system; thus it could be well adopted in a continuous flotation process. Cetyl trimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) were employed for CGA creation. Positively surface-charged CTAB-CGAs (∼44.1xa0μm in size) and negatively surface-charged SDS-CGAs (∼42.1xa0μm in size) were produced at the optimum stirring speed of 8000xa0rpm. The half-life of CGAs varied from 100xa0s to 340xa0s under the tested conditions, which was largely sufficient for transferring CGAs from bubble generator to flotation cell. The air flow led to less stable CTAB-CGAs but apparently enhanced the stability of SDS-CGAs at higher air flow rates. In the presence of air flow, the drainage behavior was not much related to the type of surfactants. The continuous CGA-flotation trials highlighted the effective separation of silica nanoparticles - the removal efficiencies of different types of SiO2 NPs could reach approximately 90%-99%; however, at equivalent surfactant concentrations, no greater than 58% of NPs were removed when surfactants and bubbles were separately added into the flotation cell. The SiO2 NPs with small size were removed more efficiently by the CGA-flotation process. For the flotation with CTAB-CGAs, the neutral and basic initial SNP suspension was recommended, whereas the SDS-CGAs remained high flotation efficiency over all investigated pH. The good performance of CGA-flotation might be interpreted: most of the surfactant molecules well covered/coated on the surfaces of stable CGAs and thus fully contacted with NPs, resulting in the efficient utilization of surfactants.

Analysis of cutting-oil emulsion destabilization by aluminum sulfate

ABSTRACT The destabilization mechanism of the high stable cutting-oil emulsion by aluminum sulfate (Al2(SO4)3) was investigated since it can affect properties of aggregates and following separation units. Al2(SO4)3 dosage and pH were key factors in the destabilization. The effective separation occurred when precipitated Al(OH)3 is dominated at the neutral pH of 6.5–7.0. The best separation can be achieved when solid flocs were formed at 1.0u2005mM, which exceeded the dosage from the critical coagulation concentration (CCC) of 0.75u2005mM. Two different mechanisms were proved for the emulsion destabilization depending upon the Al3+ concentration under this pH range. The first mechanism was the adsorption of Al(OH)3 on surface of oil droplets, which led to the droplet coalescence. By increasing the Al3+ dosage, the sweep flocculation by Al(OH)3 precipitates occurred. Al3+ dosage for effective destabilization was increased in accordance with oil concentration. The formation of aluminum hydroxide precipitates in bayerite structure was affirmed by analyzing elemental composition and crystalline structure of flocs from the destablization.