Adsorption of organophosphates from solution by porous organosilicates: Capillary phase-separation

• Adsorption of organophosphorous pesticides using hierarchical sorbents.
• New binding behavior in hierarchical, mesoporous organosilicate materials.
• Capillary phase-separation of paraoxon from aqueous solution in presence of sorbent.
• Target binding shows good agreement with Freundlich–BET isotherm.

The adsorption characteristics for paraoxon, diazinon, and structurally related compounds by hierarchical diethylbenzene-bridged organosilicate sorbents are reported here. Adsorption behavior was found to deviate from the expected Langmuir–Freundlich binding isotherm. Instead, capillary phase-separation within the pores of the sorbent structure occurred when presented with these targets in aqueous solution. This behavior has not been widely reported, but it is similar to the pore condensation observed in gas adsorption studies. The behavior is a result of the pore sizes, the availability of interaction sites on the pore walls, and the miscibility of the targets (solutes) with water (solvent). Adsorption of targets from aqueous solution was observed to follow a hybrid Freundlich–BET binding isotherm. Adsorption of the targets from octanol solutions was observed to follow the Langmuir–Freundlich binding isotherm as was adsorption of more soluble targets such as p-nitrophenol. With the increasing numbers of applications described for mesoporous materials, this phenomenon will likely be more frequently encountered. The range of conditions utilized in characterization of sorbents should be considered carefully.

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