Sorptive separation of phenolic compounds from wastewater
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This study explores the potential of using PEBA as a sorbent for removing phenolic compounds from wastewater. The sorption isotherms of phenol, 4-chlorophenol (4-CP), 4-nitrophenol (4-NP), 4-methylphenol (4-MP) and catechol from their respective single solute solutions were studied based on the Linear, Langmuir and Freundlich models. The Freundlich model was shown to be suitable to represent the equilibrium sorption of all the phenolic compounds in the PEBA sorbent, and the sorption capacity of these phenolic compounds in PEBA is in the order: catechol<phenol<4-MP<4-NP<4-CP. Thermodynamic analysis revealed that phenol sorption in PEBA was a spontaneous exothermic process. The sorption kinetics was studied using PEBA sorbent in the form of a flat membrane with well-defined dimensions to help identify the mechanism and rate controlling step of the sorption process. The kinetic data were fitted with the pseudo-first and -second order models as well as the diffusion model. The pseudo-second order model was shown to fit the experimental data better than the pseudo-first order model, and an oversight in the model fitting with regard to equilibrium sorption capacity in prior work was discussed and corrected. Sorption of phenol, 4-CP, 4-NP, 4-MP and catechol in PEBA was represented by the pseudo-second order model, while mass diffusion inside the sorbent was not negligible for thick membranes. Multi-solute sorption isotherms and kinetics were determined for binary and quinary solute systems. The competitive Freundlich model and the IAST model were fitted to the equilibrium data. It was found that the competitive Freundlich model was adequate to represent isotherms for all binary solute systems studied; however, there was a considerable deviation between the model predictions and the experimental data in the quinary solute system. Strongly sorbed solutes tended to inhibit the sorption of weakly sorbed solutes. The sorption competition was shown to affect the sorption kinetics of individual solutes, and such effect was found to be related to the molecular size of the sorbate component and its affinity to the sorbent. Chemical elution, thermal regeneration and vacuum-assisted thermal regeneration were proposed and studied for regeneration of PEBA sorbent exhausted with the phenolic compounds. Ethanol, methanol and NaOH solution were all effective regenerants for the sorbent, and no significant change in the sorption characteristics was observed for a number of sorption-regeneration cycles. Thermal regeneration (90°C for 2 h) was also effective for regenerating PEBA exhausted with phenol and 4-MP. When exhausted with 4-MP, the PEBA sorbent could be regenerated using vacuum-assisted thermal regeneration (80°C for 3 h at a vacuum pressure of 5 kPa), and highly concentrated solutions (≈70,000 ppm) of 4-MP were collected. PEBA fibres were prepared and immobilized in a packed-bed for column sorption studies. The effects of inlet feed concentration, flow rate, fibre diameter and flow interruption on the breakthrough were evaluated. The BDST, Clark and Yoon-Nelson models were fitted to the experimental data. It was shown that the BDST model described the breakthrough curve adequately when the solute in the effluent was at lower concentrations (0<C/C0<0.15); while over a broader range of effluent concentration (0<C/C0<0.90), the Clark model fitted the breakthrough curve better than Yoon-Nelson model. The sorption capacities of phenolic compounds in the column were found to be comparable to those determined in batch sorption studies. Complete regeneration of the exhausted column was achieved using NaOH and there was no change in breakthrough characteristics after column regeneration.
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Shuixiu Lai (2017). Sorptive separation of phenolic compounds from wastewater. UWSpace. http://hdl.handle.net/10012/11813