Effect of hydrophobicity of amphiphile molecule on adsorption behavior onto various adsorbents and its interface characteristics

Abstract

Pharmaceutical contaminations in surface water and groundwater are the environmental emerged problems. Most of pharmaceuticals are amphiphile molecules that contain both polar and non-polar parts within the molecules. To understand the fundamental process of adsorption of these amphiphile molecules, a homologous series of alcohols with varied alkyl chain length from butanol to octanol was used as representatives of these molecules in which the degree of hydrophobicity was systematically varied. The adsorption behavior was studied in term of adsorption isotherm at 25 ํC on each pure media, i.e., alumina, silica, powder activated carbon and AMBERLITE XAD 761. In addition, the surface tension of the alcohol solution at the air/liquid interface, the interfacial tension at water/alcohol interface and the contact angle of alcohol solution on surfaces were studied to in order to correlate with the adsorption behavior. The results showed that the alcohols adsorb onto powder activated carbon surface and AMBERLITE XAD 761 but not onto alumina and silica surfaces. On powder activated carbon, the adsorptions follow the Langmuir adsorption isotherm. The adsorption coefficients which were normalized by specific surface area are 1.98, 7.27, 29.70, 81.70 and 794.00 mL/sq.m. for butanol, pentanol, hexanol, heptanol, and octanol, respectively. On AMBERLITE XAD 761, the adsorption isotherms were expressed by the Freundlich adsorption isotherm by these following equations: q = 0.9738 (g/g XAD 761) (L/g)[superscirpt N] x Ce[superscript 0.5052]; q = 1.2201 (g/g XAD 761) (L/g)[superscript N] x Ce[superscript 1.3269]; and q = 0.4288 (g/g XAD 761) (L/g)[superscript N] x Ce[superscript 1.5911] for butanol, hexanol, and octanol, respectively. As the degree of hydrophobicity increases (or longer the chain length of alcohols), the adsorption onto activated carbon and AMBERLITE XAD 761 surfaces as well as the surface tension and the interfacial tension increase. Therefore, the degree of hydrophobicity affects the adsorption onto organic surfaces, the surface tension and the interfacial tension but no significant effect is found in the contact angle. The adsorption of amphiphile molecules onto powder activated carbon (non-polar surface) is hydrophobic interaction while onto AMBERLITE XAD 761 (hydrophilic surface) are both hydrophobic interaction of alkyl chain and hydrophilic (specific) interaction of hydroxyl functional group onto hydrophilic surfaces. Moreover, the surface tension and interfacial tension of amphiphile molecules (alcohol molecules) correlate to the adsorption behavior onto nonpolar surface (powder activated carbon)