Ultrasonic-assisted removal of cationic and anionic dyes residues from wastewater using functionalized triptycene-based polymers of intrinsic microporosity (PIMs)

In this work, a series of hypercrosslinked polymers of intrinsic microporosity (HCP-PIMs), namely nitro-triptycene (TRIP-NO2), amino-triptycene (TRIP-NH2), sulfonated-triptycene (TRIP-SO3H) and hydrocarbon-triptycene (TRIP-HC), are employed for the adsorption of organic dyes from wastewater. The materials show the efficient removal of cationic (malachite green, MG) and anionic (methyl orange, MO) dyes. The adsorption parameters herein investigated include the initial pH, the adsorbate concentration and the contact time, with the aim to elucidate their effect on the adsorption process. Furthermore, the adsorption kinetic and isotherms are studied, and the findings suggest the results fit well with pseudo-second-order kinetics and Langmuir model. The reported maximum adsorption capacity is competitive for all the tested polymers. More specifically, TRIP-SO3H and TRIP-HC exhibit adsorptions of similar to 303 and similar to 270 mg g(-1) for MG and MO, respectively. The selectivity toward cationic and anionic dyes is assessed by mixing the two dyes, and showing that TRIP-HC completely removes both species, whereas TRIP-NO2, TRIP-NH2 and TRIP-SO3H show an enhanced selectivity toward the cationic MG, compared to the anionic MO. The effect of the type of water is assessed by performing ultrasonic-assisted adsorption experiments, using TRIP-SO3H and TRIP-HC in the presence of either tap or seawater. The presence of competing ions and their concentrations is evaluated by ICP-MS. Our study shows that tap water does not have a detrimental effect on the adsorption of both polymers, whereas, in the presence of seawater, the performance of TRIP-HC toward MO proved to be more stable than MG with TRIP-SO3H, which is probably due to a larger concentration of competing ions. Comparison between ultrasonic-assisted and magnetic stirring adsorption demonstrates that the former exhibits a greater efficiency. This seems due to a more rapid mass transfer, driven by the formation of high velocity micro-jets, acoustic microstreaming and shock waves, at the polymer surface. Reusability studies show a good stability up to five adsorption-desorption cycles.