Adsorption removal of low-molecular-weight uremic toxins from simulated spent dialysate using various low-cost mixed-matrix membrane stacks

Background: High-flux hemodialysis (HD) is currently the main clinical treatment for the patients with end-stage renal disease at a frequency of 2–4 times a week. The treatment of spent dialysate becomes another issue from the viewpoint of water pollution. Moreover, the regeneration of spent dialysate is one of the biggest challenges in wearable artificial kidney devices. Consequently, to develop a potential and cheap method for possible removal of the cleared small uremic toxins is desired. Methods: In this study, a series of low-cost adsorbents of activated carbon (AC), zeolite ZSM-5 (ZO), zeolite HSZ-840 (ZH), and graphene oxide (GO) were incorporated into cellulose acetate (CA) or polyethersulfone (PES) polymer to prepare porous mixed-matrix membranes using non-solvent-induced phase inversion. These membranes were screened and used as a membrane stack for the removal of low-molecular-weight uremic toxins including urea, creatinine, and p-cresol from simulated spent dialysate. Conditions for preparing membranes (composition of polymer solution and non-solvent bath) were first optimized from their morphologies and their physicochemical and textural properties were characterized. Significant Findings: Based on the amount of each toxin adsorbed at its existing concentration in simulated spent dialysate (2300 mg/L urea, 150 mg/L creatinine, and p-cresol 50 mg/L), three membranes of AC in CA, ZH in CA, and GO in PES were selected evaluate the potential for the removal of three toxins studied. Dynamic closed-loop tests using the membrane stack comprising of 70 wt% ZH/CA, 70 wt% AC/CA, and 40 wt% GO/PES in order (total membrane area 176.6 cm²) revealed a maximum removal of 13.8 % urea, 21.8 % creatinine, and 82.6 % p-cresol from 0.5 L of spent dialysate within 4 h. Four repeated adsorption-desorption cycles demonstrated that the prepared membranes were reusable and had the potential for the removal of small uremic toxins from spent dialysate.