Liguang Wang

Flotation of marine microalgae: Effect of algal hydrophobicity

This study aims to understand the underlying reasons for the poor flotation response of marine microalgae. The flotation performance and hydrophobicity of a freshwater microalga (Chlorella sp. BR2) were compared to those of a marine microalga (Tetraselmis sp. M8) at different salinities in the presence of a cationic collector, tetradecyl trimethylammonium bromide. It was found that microalgal hydrophobicity played a more important role than salinity in determining the flotation performance.

Anomalous thickness variation of the foam films stabilized by weak non-ionic surfactants

The constant thickness (H) of metastable free films of various non-ionic surfactant solutions was measured at surfactant concentrations less than the critical micelle concentrations or solubility limits with fixed 5x10(-5) M sodium chloride (NaCl) serving as the background electrolyte. The surfactants include n-pentanol, n-octanol, methyl isobutyl carbinol (MIBC), polypropylene glycol (PPG-400), tetraethylene glycol monooctyl ether (C(8)E(4)), and tetraethylene glycol monodecyl ether (C(10)E(4)). H was interferometrically measured. For each surfactant in this study, the H-versus-surfactant-concentration curve finds a peak at a concentration around 5x10(-6)-1x10(-5) M and a valley at a higher concentration. The measured H values were compared to those predicted from the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, which considers solely the contribution from electrostatic double-layer repulsion with van der Waals attraction being neglected in the present work. In determining the double-layer repulsion, the ionic strength was determined from the electrolytic conductivity measurement of the film-forming solutions and the surface potential was estimated from the zeta-potential measurement of air bubbles. It was found that the DLVO theory failed to explain the thickness variance with surfactant concentration, implying that additional non-DLVO attractive forces might be required to explain the experimental results. Finally, the possible origins of these attractive forces were discussed.

Effective harvesting of low surface-hydrophobicity microalgae by froth flotation.

Microalgae harvesting by air flotation is a promising technology for large-scale production of biofuel, feed and nutraceuticals from algae. With an adherence-to-hydrocarbon method and two different types of flotation cells (mechanically agitated cell and Jameson cell), microalgal surface hydrophobicity and bubble size were identified to be critical for effective froth flotation of microalgae. Freshwater alga Chlorella sp. BR2 showed naturally a high hydrophobicity and an ideal response to flotation. However, many marine microalgae possess a low surface hydrophobicity and are thus difficult to harvest. This paper shows that a step-wise optimization approach can substantially improve the flotation of a low surface hydrophobicity marine microalga, Tetraselmis sp. M8, to near full recovery with an enrichment ratio of 11.4.

Effect of pH and NaCl concentration on the stability of surfactant-free foam films.

The stability of surfactant-free foam films was studied in NaCl solutions of varying concentration and pH using the thin film pressure balance (TFPB) technique. In pure water, it was not possible to produce foam films due to weak film elasticity and the strong attractive hydrophobic force in foam films, despite the presence of strong repulsive double-layer force. In the presence of a very small amount of an electrolyte, however, the hydrophobic force was dampened, allowing metastable foam films to form. As the NaCl concentration was raised above 10(-6) M, the film stability diminished as a result of double-layer compression. The TFPB technique was also used to measure the equilibrium film thicknesses (H(e)) in 10(-5) M NaCl solutions of varying pH; H(e) reached a maximum of approximately 130 nm at pH 6.0-7.3, and decreased on either side of this pH range as a result of the increased ionic strength caused by the HCl and NaOH added to control the pH. The hydrophobic force in surfactant-free foam films was maximum at pH 7.3, where the concentration of electrolytes, including that of H(2)CO(3) species, was minimum.

The effects of acid hydrolysis on protein biosurfactant molecular, interfacial, and foam properties: pH responsive protein hydrolysates

The success of hydrolysis in improving the functional foaming properties of surface-active proteins is usually attributed to three factors: decreased molecular size; increased hydrophobicity; and microchemical changes, specifically deamidation of glutamine and asparagine. Studying these individual factors is difficult using naturally-occurring proteins, as hydrolysate products are complex mixed systems, and the mechanisms of foam stabilization are likewise complex. To address this complexity we report studies of a recombinant protein (DAMP4) which comprises four peptide surfactant (DAMP1) molecules connected by acid-labile amino acid (Asp-Pro) linkers. Hydrolysis of DAMP4 under conditions of low pH and high temperature produced h-DAMP1, a mixture of deamidated variants of the chemically-synthesized DAMP1 peptide surfactant. By examining foaming performance of these molecules, we are able to isolate the effects of molecule size (DAMP1 vs. DAMP4) and deamidation (h-DAMP1 vs. DAMP1). Molecule size had little effect on foaming for the conditions studied. However, deamidation completely changed foaming behaviour, most likely due to alteration of interfacial charge structure (through deamidation of glutamine to glutamic acid) and consequent effects on thin-film stability. Good foaming was observed only at pH values away from the isoelectric points (pI) of the biomolecules where an electrostatic barrier to film rupture can occur. The addition of Zn2+ to DAMP4, h-DAMP1 and DAMP1 caused visible aggregation under all conditions, which assisted in stabilising foams only in situations where a net charge would be expected.

Raman Spectroscopic Studies of Clathrate Hydrate Formation in the Presence of Hydrophobized Particles

In the present work, Raman spectroscopy was used to study the structure of water molecules in the vicinity of glass particles with different hydrophobicity, immersed in water and in tetrahydrofuran and cyclopentane hydrates. The glass particle surfaces were clean (hydrophilic), coated with N,N-dimethyl-N-octadecyl-3-aminopropyl trimethoxysilyl chloride (partially hydrophobic), or coated with octadecyltrichlorosilane (hydrophobic). The Raman spectra indicate that, prior to nucleation, water molecules in the vicinity of hydrophobic surfaces are more ice-like ordered than those in the bulk liquid or near either hydrophilic or partially hydrophobic surfaces. Furthermore, the degree of hydrogen-bond ordering of water observed prior to hydrate nucleation, as measured by the ratio of the inter- and intramolecular Raman OH bands, was found to have an inverse relationship with the mean induction time for hydrate formation. Following hydration formation, no significant difference in the water molecule structure was observed in the hydrate phase based on their Raman OH bands, irrespective of surface hydrophobicity. These observations made with Raman spectroscopy provide the foundations for a quantitative link between hydrate nucleation promotion and water-ordering near solid surfaces, which could enable direct comparisons with results from corresponding molecular dynamics simulations.

Effects of Film Elasticity and Surface Forces on the Stability of Foams and Lamellae Films in the Presence of Non‐ionic Surfactants

This paper describes the basic factors affecting the stability of the foam films produced in the presence of nonionic surfactants such as n‐pentanol, n‐octanol, methyl isobutyl carbinol (MIBC), and polypropylene glycol (PPG). We used a model developed by Wang and Yoon (Colloids and Surfaces A: Physicochem. Eng. Aspects 282–283, 84–91 (2006)) to calculate the Gibbs elasticity of the lamellae films. In addition, we used the thin film pressure balance (TFPB) technique to measure film thicknesses. The results were used to determine the disjoining pressures in the thin lamellae (foam) films formed between air bubbles, which are relevant forces governing the final drainage stage of foam films before reaching either equilibrium or rupture. The film elasticity and surface forces data were then compared with the foam stabilities measured in the present work in the presence of the various surfactants. It was found that foam stabilities are controlled both by film elasticity and by surface forces, the relative contr...

Flotation separation of limonite from quartz with sodium oleate: effects of limonite dissolution and addition of sodium hexametaphosphate

The utilisation of limonite ore is potentially important to China as the high quality iron resource, such as magnetite and hematite of high grade, are rapidly depleting, but many detrimental impurities, such quartz and calcite etc, are difficult to upgrade to make suitable concentrates for the blast furnace. The flotation behaviour of limonite, quartz and their mixture by using sodium oleate as collector were studied systematically in this paper. Single-mineral flotation tests showed that the flotation recovery of limonite was much higher than that of quartz in a broad range of pH. However, flotation tests for the mixture of limonite and quartz showed rather poor mineral separation efficiency with the flotation recovery of both minerals being high in froth products. The flotation separation efficiency of limonite from quartz was substantially improved by adding sodium hexametaphosphate (SHMP) as a depressant for quartz flotation. To elucidate these phenomena, the solution species calculation of limonite dissolution and the measurements of iron ions content were carried out. It was found that iron ions or its species released into the pulp from the dissolving of limonite could effectively activate the flotation of quartz but have little influence on the flotation of limonite. In the presence of SHMP, iron ions or its species lost their activating power on quartz flotation which caused by the chemical reaction between SHMP and iron ions or its species, and then removed iron ions or its species from quartz surface into the solution.

Flotation separation of limonite from calcite with sodium oleate: effects of calcite dissolution and addition of sodium pyrophosphate

ABSTRACT Limonite is now considered as an important iron ore resource in China due to the rapidly depleting magnetite and haematite reserves. The separation of limonite from calcite by flotation is a problem to be solved, and it was investigated systematically in this paper. Flotation tests show that the recoveries of limonite and calcite are close to 95% within the pH range of 7.5–10.0 using 3 × 10−4 mol/L sodium oleate as collector. However, the flotation of limonite was depressed by calcium ions generated from the dissolution of calcite in the flotation of a mixture of limonite and calcite. The effects of depression on limonite can be enhanced by adding sodium pyrophosphate. Hence, the separation of calcite from limonite can be finished by reverse flotation. Under the test condition, the limonite–calcite mixture with Fe grade of 28.45% was upgraded to a concentrate with 50.5% Fe and 91.50% Fe recovery.