Repeated adsorption of phenolic acids on clays

Abstract

The repeated adsorption characteristics of three phenolic acids containing one or two salicylic moieties and with different molecular weights were studied. The phenolic acids used were salicylic acid, 2-hydroxy-3-naphthoic acid (sodium salt) and naphthochrome green, and the adsorbents were on allophanic, halloysitic, and illitic clays. Adsorption characteristics were measured for changing pH values and concentrations. The clay samples were previously washed with ammonium chloride or with calcium chloride solutions. The amounts of salicylic acid adsorbed abruptly increased at relatively high pH values after the second repetition, which occurred on the surfaces of allophanic clay washed by ammonium chloride instead of calcium chloride. The 2-hydroxy-3-naphthoic acid (sodium salt) and the naphthochrome green were, however, largely retained under low pH conditions, so the participation of ionic exchange, hydrogen bonding and/or physical bonding were suggested. The pH and concentration (C,M) dependency of the retention ( n, μM/g) on allophanic clay obeyed the following linear multiple regression equation: log( n) = a pH + b log(C) + c pH × log(C) + d pH 2 + e where a, b, c, d and e were coefficient constants for a combination of a phenolic compound and a clay sample at each repetition.