Sorption of 2,4-dichlorophenoxy propionic acid by organo-clay complexes

Wyoming montmorillonite (SWy-1, MON), Charng-Bin (SCB) soil montmorillonite, and W.R. Grace vermiculite (VER) have been used as adsorbents to prepare organo-clays. Sorption of 2,4-dichlorophenoxy propionic acid (2,4-DP) by organo-clays is greater after adding of hexadecyltrimethyl-ammonium (HDTMA) to 200% CEC of the clay than after adding the same amount of HDTMA to 100% CEC of clay (HDTMA–clay). The Freundlich adsorption isotherm describes 2,4-DP sorption by HDTMA–clays (Kf=1.828–11.474 l kg−1, nf=l.130–1.454, R=0.919–0.944) quite well. Low surface charge montmorillonite can allow a high concentration of 2,4-DP to penetrate into the interlayer with low residual charge on the wedge. Thus, with increasing 2,4-DP concentration, the basal d-spacings of HDTMA-montmorillonite complexes increased from 2.15 to 2.71 nm. However, increasing the concentration of 2,4-DP did not result in further swelling of vermiculite owing to the high surface charge. In general, HDTMA–montmorillonite (O-MON) sorbed more 2,4-DP than HDTMA–soil montmorillonite (O-SCB) and HDTMA–vermiculite (O-VER). Thus, the interlayer sorption of 2,4-DP exhibited a critical level, beyond which further expansion of the clays does not occur. The equilibrated pH in the suspension affected the adsorption by HDTMA–clay complexes. The maximum 2,4-DP adsorption occurs at a pH level of approximately 3 corresponding to pKa=3.0 of 2,4-DP in the adsorption–pH curve.