Mechanism of organic pollutants sorption from aqueous solution by cationic tunable organoclays

A variety of quaternary ammonium salts with different carbon chain lengths were used to modify and convert montmorillonite clay to organoclays. The surface modification attributing to the wettability was investigated using various techniques. The zeta-potential values of all clays showed pH dependency. The prepared organoclays were utilized for phenol (protic) and nitrobenzene (aprotic) adsorption in a batch system, with pH and contact time as variables. The optimized removal for phenol and nitrobenzene within 30 min of equilibrium observed at pH 9.0 and 5.0, respectively. Observed equilibrium data followed the Langmuir monolayer adsorption kinetics with two adsorption sites (outer-layer and interlayer) for purified clay, and third additional sites for organoclays. Kinetic studies revealed that the adsorption of phenol was in the order of mono- > tetra- > di- > benzyl-substituted organoclays, which is similar to the hydrophilicity order. However, the exact opposite trend of adsorption was observed for nitrobenzene. From the performed quantitative study, it is inferred that -OH functional phenol and -NO₂ functional nitrobenzene have an affinity towards the hydrophilic clay surface and hydrophobic surfaces of quaternary ammonium salts as its first preference, respectively. This study brings promising observations and implications to the vital structural-property relationship for selective adsorption of pollutants from aqueous solutions.