Owing to their high surface area and superior adsorption properties, spirobifluorene PIMs namely, PIM-SBF-Me (methyl) and PIM-SBF-tBu (tert-butyl) were used for the first time for the removal of methylene blue (MB) dye from wastewater. Spirobifluorene PIMs are known to have large surface area (can be up to 1100 m2/g) and have been previously used mainly for gas storage applications. Dispersion of the polymers in aqueous solution was challenging due to their extreme hydrophobic nature leading to poor adsorption efficiency of MB. For this reason, cationic (CPC), anionic (SDS) and nonionic (Brij-35) surfactants were utilized and tested with the aim of enhancing the dispersion of the hydrophobic polymers in water and hence improving the adsorption efficiencies of the polymers. The effect of surfactant type and concentration was investigated. All surfactants offered a homogenous dispersion of the polymers in the aqueous dye solution, however, the highest adsorption efficiency was obtained using an anionic surfactant (SDS) and this seems due to the predominance of electrostatic interaction between its molecules and the positively charges dye molecules. Furthermore, the effect of polymer dosage and initial dye concentration on MB adsorption were also considered. The kinetic data for both polymers were well described by pseudo-second-order model, while Langumir model better simulated the adsorption process of MB dye on PIM-SBF-Me and Freundlich model was more suitable for PIM-SBF-tBu. Moreover, the maximum adsorption capacities recorded were 84.0 and 101.0 mg/g for PIM-SBF-Me and PIM-SBF-tBu, respectively. Reusability of both polymers was tested by performing three adsorption cycles and the results substantiate that both polymers can be effectively reused with insignificant loss of their adsorption efficiency (%AE). These preliminary results suggested that incorporation of a surfactant to enhance the dispersion of hydrophobic polymers and adsorption of organic contaminants from wastewater is a simple and cost-effective approach that can be adapted for many other environmental applications.

The removal of MB from water using both PIM-SBF-Me and PIM-SBF-tBu was evaluated by performing a series of experiments. Initially, 1-10 mg of both polymers was added to 10 mL of 5.0–25.0 mg/L MB solution in the presence of a surfactant. After mixing for 100 min, the remaining concentration of MB in the solution was determined by measuring the change λ_max = 662 nm in a quartz cuvette using UV-Vis spectrometer. The adsorption efficiency (AE) for MB on both polymers was estimated using Equation 1 ⁸:

%AE = (C_o-C_t)C_o× 100                                                                                                    (1)

Where C_o and C_t represent the initial MB concentration and the MB concentration at time t (mg/L), respectively. The amount of MB adsorbed per unit mass of PIM-SBF-Me and PIM-SBF-tBu was calculated using Equation 2:

q_t=(C₀-C_t)Vm                                                                                                                  (2)

Where V and m the volume of solution (L) and the mass of the polymers added to the MB solution (g), respectively. At equilibrium, the equation can be modified:

q_eq=(C₀-C_eq)Vm                                                                                                              (3)

Where q_eq and C_eq are the amount of MB adsorbed per gram of the polymer (mg/g) and the concentration of MB at equilibrium (mg/L), respectively.

All adsorption isotherms and kinetic measurements were performed using the optimized conditions of adsorbent dosage and initial dye concentration. In brief, the optimum amount of the polymer was added into 10 mL of MB dye solution. The mixture was magnetically stirred, and the concentration of dye in the solution was determined at definite time intervals and analysed spectrophotometrically without filtration.

The instrumentation used for the characterisation for polymers are reported in the paper 

Bezzu, C.G., Carta, M., Ferrari, M.-C., Jansen, J.C., Monteleone, M., Esposito, E., Fuoco, A., Hart, K., Liyana-Arachchi, T., Colina, C.M., 2018. The synthesis, chain-packing simulation and long-term gas permeability of highly selective spirobifluorene-based polymers of intrinsic microporosity. Journal of Materials Chemistry A 6(22), 10507-10514.

Additionally, to assess the adsorption and its kinetics UV-Vis analysis was carried out using Agilent Cary 5000 Scan UV-Vis-near-infrared spectrometer. The adsorption of MB on PIM-SBF-Me and PIM-SBF-tBu was investigated using Jasco FTIR-630 FTIR spectroscopy. The polymer, surfactant micelles and polymer-micelle size distribution was determined by photon correlation spectroscopy using a Zetasizer Nano ZS (Malvern Instruments, UK), with a frequency doubled DPSS Nd: YAG laser with output power of 50 mW operating at a wavelength of 532 nm and an angle of 175. All the measurements were performed at room temperature and for each sample three measurements were carried out in disposable cuvettes. The refractive index and viscosity of the solvent were taken into consideration when measurements were taken.