Influence of surface coverage of kaolin with surfactant ions on adsorption of ochratoxin A and zearalenone

Abstract

A natural kaolin from Serbia was modified with different amounts of octadecyldimethylbenzyl ammonium (ODMBA) - (25, 50 and 90% of kaolin cation exchange capacity). Samples were denoted as OKR-25, OKR-50 and OKR-90. Several methods (FTIR spectroscopy, thermal analysis, zeta potential measurements, and N2 physisorption) were used for characterization of the organokaolinites. Adsorption of the common mycotoxins - ochratoxin A (OCHRA) and zearalenone (ZEN) by the organokaolinites was investigated at different levels of solid phase in suspension, different initial mycotoxin concentrations and different pH values. The natural kaolin was not effective in binding OCHRA or ZEN. Adsorption of both mycotoxins by organokaolinites increased with increasing amounts of solid phase as well as with increasing levels of surfactant on the kaolin surface. OCHRA and ZEN adsorption by all organokaolinites followed non-linear adsorption isotherms, at pH 3, 7 and 9. The maximum adsorption capacity for OCHRA adsorption was at pH 3 (4.8 mg/g for OKR-25, 26.7 mg/g for OKR-50 and 39.2 mg/g for OKR-90) that was calculated from the Langmuir model. Much lower OCHRA adsorption capacities were found at pH 7 and 9 (from 0.8 mg/g to 6.9 mg/g at pH 7 and from 1.1 mg/g to 4.6 mg/g at pH 9). The following adsorption capacities for ZEN were obtained from the Langmuir isotherms, at pH 3: 4.5 mg/g for OKR-25, 12.0 mg/g for OKR-50 and 13.5 mg/g for OKR-90. At pH 7, adsorption of ZEN was 5.7 mg/g for OKR-25, 15.3 mg/g for OKR-90 and 14. 4 mg/g for OKR-90. At pH 9, ZEN adsorption capacities were 2.4, 14.1 and 8.1 mg/g for OKR-25, OKR-50 and OKR-90, respectively. Thus, at the lowest amount of ODMBA at the kaolin surface, adsorption of ZEN was similar at pH 3 and 7, while a slightly lower value was obtained for its adsorption at pH 9. With increasing amounts of organic phase at the kaolin surface, the adsorption of ZEN was practically independent of pH. Adsorption of both mycotoxins was dependent on the amount of ODMBA ions at the kaolin surface as well as on their forms in solution.