(454d) Macroalgae and Chitosan-Macroalgae Biocomposite As Potential Adsorbents of Water-Soluble Hydrocarbons: Effect of pH, Organic Matter and Ionic Strength

Abstract

Nowadays the recovery of low-molecular aromatic hydrocarbons (HC) released into natural bodies of water continues to be a challenging task. These contaminants bring sever consequences to the environment and to the human health. The oil spill cleanup strategies are primarily design to deal with the heavy fractions accumulated in the water surface. Unfortunately, very limited information is available regarding the treatment of dissolved fractions. Biosorption on macroalgae (Ma) biomass seem to be a potential alternative to overcome disadvantages associated to high-priced costs and complexity of activated carbon production. Three seaweeds of a representative sample of brown, green and red macroalgae (Macrocystis pyrifera, Ulva expansa and Acanthophora spicifera, respectively) were evaluated to remove benzene and toluene from water, which are the most soluble hydrocarbons. Toward this objective, the influences of temperature and ionic strength were also determined. Raw biomasses were characterized by different physical and chemical techniques to assess their potential as biosorbents and the mechanisms involved. The organic and inorganic dissolved fractions were also considered. The characterization techniques mainly identified carbohydrates, lignin and proteins. M. pyrifera biomass registered the highest removal capacities: about 112 and 28 mg·g^-1 for benzene and toluene, respectively, probably attributed to alginate and lignin content. It was found that the hydrocarbons biosorption affinity was not affect up to an ionic strength of 0.6 M, due to the biosorbents pore occlusion by water clusters. For benzene and toluene nonpolar solutes, the biosorption mechanism is an addition of simultaneous interactions between the sorbents and the chemical constituents of the cell wall, presumably by London (dispersion) forces onto â??OH and â??NH active sites from polysaccharides and proteins and hydrophobic interactions from lignin and lipid fractions.

Despite these advantageous properties, the physical characteristics (small particle size, low strength and density) of such biomaterials are not viable for a continuous process operation and make biomass difficult to apply. Therefore the development of innovative low-cost techniques of immobilized biosorbents with special attention to increase their effectiveness in the biosorption process is mandatory. The biopolymer-biopolymer interactions between positive and negative macromolecules can enhance the compatibility and stability of biosorbents, and these new structures are called polyelectrolytes complexes (PEC). The linear amino polycationic compound of chitosan (Ch) possess remarkable quantity of amine and hydroxyl groups that interacts with negatively charged substances with excellent chelation behavior and high affinity toward pollutants like metals and dyes. On the other hand pectin (Pe) is a negatively charged polysaccharide composed mostly by esterified D-galacturonic acid residues linked together by α-(1-4) chain. Pectine molecule is also used in the food industry for its well-known thickening, stabilization and gelling properties.

 The main objective of this research was to explore the performance of Ma and PEC complexes of different proportions of Ma/Ch/Pe on biosorption capacity of the two main soluble hydrocarbons (benzene and toluene) in water. The biocomposite synthesis was optimized by application of the factorial design and response surface methodology. Moreover, a through chemical and physical characterization analysis, by textural properties, potentiometric titrations, elemental content, chemical stability, KBr-FT-IR and TGA analysis, were performed to explain the adsorption mechanisms.