Urea and uric acid adsorption by nanoporous biomaterials
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Date
2016-05-01
Authors
Cheah Wee Keat
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Abstract
The limitations of the present hemodialysis have led towards the research and development of several wearable artificial kidney prototypes. The most important component of the miniaturised model is the closed-system dialysate, achieved through the utilisation of solid activated carbon as adsorbents. With the application of superior alternative adsorbents, the amount of dialysate required could be reduced due to efficient regeneration. The main motivation for this project is the lack of adsorbent materials selection. Thus, this project aims to synthesise and evaluate three emerging nanoporous biomaterials, i.e. hollow activated carbon fibre (ACF), mesoporous silica (MS) and mesoporous hydroxyapatite (HAp), targeting major uremic toxin constituent urea and uric acid. ACF was obtained though the acid activation route, with variation in acid used; inorganic acids sulphuric, nitric and phosphoric; organic acids acetic and citric. MS and HAp was synthesised through soft templating route using Pluronics surfactant. Results show that amine functionalised MS and sulphuric acid treated ACF performed well in the preliminary urea adsorption capacity evaluation (550 mg/g), as compared to the control commercial activated carbon (CAC) (350 mg/g). Subsequent urea kinetics study revealed better understanding of urea adsorption mechanism by ACF and MS, whereby ACF and MS operate through physisorption and chemisorption respectively. Amine and diamine MS adsorbed more than 30 molecules per nm2 (strong chemisorption interaction) compared to bare MS, CAC and various ACF, which adsorbed less than 10 molecules per nm2 (physisorption). The most important factors which govern adsorption capacity are porosity and suitable surface chemistry,
both which are possessed by amine and diamine MS. The flexibility of surface functionalisation of MS is the basis of subsequent uric acid adsorption kinetics test. Amine functionalised MS is hypothesised to improve uric acid adsorption through acid-amine reaction. Uric acid adsorption by MS did not follow theoretical adsorption curve. Further analysis using MATLAB curve fit revealed that MS underwent simultaneous adsorption-desorption, with initial adsorption rates as high as 20.3 mg/g/s compared to commercial silica gel, with initial adsorption rate of 0.39 mg/g/s. As a conclusion, on a whole, MS and ACF performed better than the benchmarked CAC and commercial silica gel in terms of urea and uric acid adsorption.