Enhanced flocculation of oil-in-water emulsions by hydrophobically modified chitosan derivatives

Abstract

Flocculation efficiency of hydrophobically modified (HM) chitosan derivatives in comparison with commercial cationic polyacrylamide flocculant and unmodified (UM) chitosans varying in molecular weights and acetylation degrees has been evaluated in sodium dodecyl sulfate (SDS)-stabilized and surfactant-free oil-in-water emulsions at pH 4–9. It was found that due to the synergetic effect of cationic and hydrophobic functionalities HM-chitosans provide superior phase separation in SDS-stabilized systems at twice- and four-fold lower doses than UM-chitosan and cationic polyacrylamide, respectively. For this type of emulsions, pH had little influence on the efficiency of phase separation. In surfactant-free emulsions with low surface charge density of oil droplets, hydrophobic substitution had weaker effect on chitosan flocculation performance. Complete phase separation was reached only at pH > 7.5, and at UM and HM chitosan doses considerably higher than it was required for surface charge neutralization. This suggests that in surfactant-free emulsions hydrophobic interactions could not compensate a dramatic decrease of electrostatic attraction between polymers and droplet surface, thus, flocculation occurred mainly through the “sweep floc” process in contrast to the “charge patch” mechanism in SDS-stabilized systems.

Introduction

Growing concern for the environmental safety promotes a sustained improvement in waste treatment technologies and development of new materials, which should meet up-to-date strict requirements on the quality of discharge water in various industrial fields. Oily wastes represent one of the main challenges not only in petroleum and petrochemical industry but also in food, cosmetic and pharmaceutical production. Although qualitative and quantitative composition of oily wastes may vary from one industrial field to another, significant part of oil is always present in the emulsified form, which is often ultimately difficult to separate from the water phase. Due to the very strong negative influence of emulsified oil on the efficiency of mechanical and membrane separation, adsorption and filtration, breaking of oil-in-water emulsions remains a crucial step in most technological setups of oily wastes treatment.

Various chemicals can be applied to enhance oil and water separation through neutralization of the surface charge of the emulsion droplets leading to emulsion destabilization and agglomeration of oil droplets into the big flocs. Traditionally, sulfuric acid alone or in combination with salts of iron and aluminum has been used for this purpose. Nevertheless, such disadvantages as required pH control and demand for special equipment with high corrosion resistance for acidic treatment as well as formation of high volumes of difficult to dewater iron and aluminum hydroxides sludge together with today's strict limits for aluminum content in discharge water have resulted in extensive substitution of inorganic coagulants by polymeric flocculants and demulsifiers [1]. However, despite a great variety of commercially available flocculants, there is still need for improvement of their efficiency and development of products for special applications. Hydrophobic modification of synthetic polyelectrolytes, which can be performed through chemical grafting [2] or copolymerization procedure [3], was suggested as one of the possibilities to enhance polymers performance in oil recovery [4] and oil and grease removal [5], [6]. Extension of this approach to biopolymers seems rather promising for the development of non-toxic and biodegradable demulsifiers and flocculants, which can be efficiently applied for destabilization of oil-in-water emulsions in food and pharmaceutical industries, and biotechnology.

Recently, it was shown that, depending on concentration, natural aminopolysaccharide chitosan can act in oil-in-water emulsions both as flocculant [7] and emulsifier [8]. One can expect that incorporation of hydrophobic substitutes into the chitosan backbone through known synthetic routes [9], [10] can provide an additional tool to control polysaccharide behavior in emulsion systems, particularly, to increase its efficiency in oil and water separation due to the synergetic effect of cationic and hydrophobic functionalities. To estimate the potential of amphiphilic chitosans as flocculants and evaluate the role of hydrophobic groups in the mechanism of emulsion destabilization, flocculation properties of chitosan and its hydrophobically modified (HM) derivatives varying in substitution degree were investigated in two types of oil-in-water emulsions—surfactant-free and stabilized with anionic surfactant, sodium dodecyl sulfate (SDS).