Effect of silica particles on cellulose acetate blend ultrafiltration membranes: Part I

Abstract

This paper deals with the preparation of organic–inorganic ultrafiltration (UF) membranes by solution casting followed by phase separation method. The silica (SiO₂) particles were added to the cellulose acetate (CA) polymer with the increment of 10 wt.% from 0 to 40% by weight using N,N-dimethyl formamide (DMF) as polar solvent. The prepared organic–inorganic membranes were characterized for UF performance such as compaction, pure water flux, % water content, membrane hydraulic resistance, molecular weight cut-off (MWCO) and pore statistics. MWCO and pore statistics were investigated using protein solutions of different molecular weights. It is observed that by increasing the concentration of SiO₂ in CA polymer, the MWCO, pore radius, surface porosity and pore density has been increased. The mechanical stability of the CA/SiO₂ blend membranes increased initially and then declined with the addition of inorganic particle above 10 wt.% to the casting solution. The morphological structure was changed with the addition of SiO₂ particles in the casting solution. Further the permeate flux and % rejection of different molecular weight of proteins were investigated which showed an increased protein permeate flux with the decreased solute rejection. Meanwhile, the effect of SiO₂ content in the CA membranes on fouling-resistant ability was studied using BSA solution. The results indicated that increasing the SiO₂ content in the casting solution, the reversible fouling resistance dominated the total fouling resistance thereby improving the fouling resistance ability of the blend membranes.

Introduction

Membrane process at present time is extensively applied in many sectors of the manufacturing industries including water desalination, ultra-pure water production, oil–water separation, production of beverages, electro-coat paint recovery, etc. [1]. Membrane processes have gained more importance than the other separation processes due to low energy consumption, easy scale-up, less or no use of chemicals and no harmful by-product formation.

Ultrafiltration (UF) is the most commonly used membrane separation method used to separate a solution containing desirable and undesirable components. Typical rejected species include sugars, bio-molecules, polymers, and colloidal particles [2]. UF membrane process depends on physical properties of the membrane, such as hydraulic permeability, membrane thickness, process and system variable like system pressure, filtration time and feed concentration.

In general, Cellulose acetate (CA) is one of the best membrane materials and is very important in the field of Reverse osmosis (RO), UF, Gas permeation [3]. Asymmetric UF membranes based on CA were prepared and studied extensively as a function of casting solution composition and membrane formation mechanism [4]. CA membranes has been prepared by many of the membrane researchers and characterized for their compaction, hydraulic permeability and osmotic permeability [5]. CA and its derivative membranes made them suitable as membrane materials because of their moderate flux and moderate salt rejection, has good film forming properties, they are relatively easy to manufacture, cost effective, and highly non-toxic. However, CA is not suitable for more aggressive cleaning, has low oxidation, has poor resistance to chlorine, can be used only in the narrow temperature range (maximum 30 °C) and has poor mechanical strength, the modification of CA gains importance [6].

In practice, CA is blended with other polymers to improve the performance of the membranes [7], [8], [9], [10], [11], [12]. The blend membranes have shown improved permselectivity and permeability than those made of individual polymers. Few researchers have investigated the blend of organic polymer with inorganic materials like alumina, silica, zirconia, titania, etc. [13], [14], [15], [16], [17], [18]. Bottino et al. [16] prepared the novel organic–inorganic membranes formed by uniform dispersion of fine silica particles in the porous matrix of polyvinylidenefluoride (PVDF) and observed that with the increasing concentration of SiO₂ in PVDF, the permeate flux increased with the lowered protein retention. Yan et al. [18] prepared blend membranes based on PVDF and alumina (Al₂O₃) materials by phase inversion processes and characteristic studies like membrane hydrophilicity, porosity, protein retention and surface morphologies were investigated. It was observed that the permeation flux increase of the membrane is attributed to surface hydrophilicity increase due to the hydrophilic inorganic nano-sized Al₂O₃ particles addition. Yang et al. [13] prepared PSf/TiO₂ organic–inorganic composite membranes via phase-inversion process and observed that with the addition of TiO₂ particles to polymer casting solution resulted in the increased pore numbers in the skin layer and the hydrophilicity also improved apparently that enhanced the pure water flux. The mechanical strength of membrane was enhanced through adding inorganic fillers, especially, at 2 wt.% filler concentration, the bursting strength and the breaking strength increased 50% and 26.7%, respectively. Wara et al. [14] reported the fabrication of nano-composite membranes of Al₂O₃ particles in CA by using the solution blending. We have examined consist of inorganic domains that are randomly dispersed with the polymer matrix. The presence of the inorganic phase can also serve to restrict the molecular motions of the polymer chains and to induce an increase in the mean distance (free volume) between polymer chains. We postulate that the restricted molecular motions and a favorable increase in the mean distance between chains can lead to a simultaneous improvement in selectivity and permeability.

Hence, for the present study, SiO₂ particles are blended with CA with an increment of 10 wt.% from 0 to 40 wt.% to obtain membranes with improved quality in terms of UF characteristics such as compaction, pure water flux, percent water content, membrane hydraulic resistance, and in terms of % solute rejection, MWCO, pore statistics, mechanical stability and membrane morphology. The fouling-resistance ability and the recycling of the CA/SiO₂ blend membranes were evaluated through UF experiments using BSA as model protein. Further, increase of SiO₂ in CA is above 40 wt.%, resulted in the turbid form and the polymer phase separation occurred. Hence, the threshold limit of SiO₂ is maintained to a maximum of 40 wt.% in CA blend composition.