A new method has been developed for the immobilisation of humic acids on silica. After using 3-aminopropyl-dimethyl-ethoxysilane to prepare aminopropyl silica, humic acids can be irreversibly bound to the aminopropyl silica. The remaining amino groups on the silica are successfully end-capped using a CH3COONa-EDC method. The products have iso-electric points at low pH and are stable in a wide range of pH.The sorption isotherms of paranitrophenol (PNP) onto immobilised Aldrich humic acid show that pH and salt concentration do not affect the sorption of PNP at pH<6, but at higher pH values the sorbed amount decreases strongly. The PNP sorption can be described with a competitive Langmuir equation extended with electrostatic interactions. The strong decrease in sorption at high pH is due to both the electrostatic repulsion between the negatively charged HA and the negative PNP species and the fact that the concentration of the neutral PNP species decreases.
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