Performance of ceramic nanofiltration membrane for desalination of dye solutions containing NaCl and Na2SO4
Introduction
Most dyes are produced by chemical synthesis which also produces byproduct salts (typical NaCl and Na2SO4) as impurities [1] due to salt-generation reactions, solution neutralization by acid/base or salt addition for promoting a chemical reaction [2], [3]. To obtain the dye as a solid, it is common to add electrolyte (usually NaCl) to the dye solution in order to precipitate the dye, known as the salting-out process. However, the purity of the dye product is low and could contain salt as high as 30% [1], [4]. This results in a product of much lower colour intensity relative to pure dye and varying dyeing characteristics [5].
Nanofiltration (NF) based process are widely used for dye desalting and purification as alternative separation techniques to the conventional salting-out processes [6]. Most of the commercially available NF membranes are made of polymeric materials and have high retention of multivalent anions (e.g. sulfate) and organic molecules with molecular weights above 300 g/mol, and low retentions to most monovalent anions (e.g. chloride) [3], [7], which are suitable for purification and concentration of dye products containing monovalent salts. For example, commercial DK (Osmonics) NF membrane was used for desalination and purification of reactive black 5 and 96.0% rejection of dye and 21.1% retention for salt (NaCl) were observed [8], but with high rejection of Na2SO4 (> 98%) [9].
With the development of ceramic membrane, more ceramic NF membranes are commercially available [10]. Several researches were focused on dye wastewater treatment and separation of dye and salt using ceramic NF and ultrafiltration (UF) membranes [11], [12]. Ceramic membranes are made of a mixture of diverse mineral oxide (Al2O3, ZrO2, TiO2 and SiO2), and compared with polymeric membranes, they are characterized with the high thermal, chemical and mechanical resistance, outstanding separation characteristics and higher lifespan [13], [14]. More important, ceramic NF and UF membranes showed low rejection of NaCl and Na2SO4 [15], [16], [17], showing great promises for desalination of dye. Majewska-Nowak et al. [18], [19] studied the performance of three single-channel ceramic membranes (MWCO of 1 kDa, 15 kDa, and 50 kDa) for nine anionic organic dyes with molecular weights ranging from 327 to 1060 Da with or without salts. The results indicated that all the membranes provides 95– 99% retention of high-molecular-weight organic dyes (> 700 Da) without salt, while the membranes with MWCO of 15 kDa and 150 kDa showed a high rejection of Direct Black DB (> 95%) with the presence of 10 g·L− 1 NaCl. Da et al. [11] prepared a tubular ceramic NF membrane with molecular weight cut-off of 800 Da for separation of fluorescent brightener and NaCl and the fluorescent brightener recovery of 99% was obtained while the NaCl removal was > 98% at a temperature of 60 °C.
Membrane performance was significantly affected by the dye-salt-membrane interaction in the membrane application for separation of dye and salt [12]. Several phenomenon including charge screening [20], change of aggregation state of dye [21], [22], salting-out [23], [24] and interaction between dye molecules and membrane [12] were reported in dye-salt-membrane system which influence the flux of membrane and rejection of solutes.
In this study, a tubular ceramic NF membrane was compared with two commercial spiral-wound organic NF membrane elements (DK and DL) for rejection of salts and six dyes with different molecular weight and charge. The effect of different solution environment (salt type, salt content, dye concentration, and pH) on the dye state and performance of ceramic membrane were discussed. The dye-salt-membrane interactions were discussed and their effect on membrane performance was investigated.
Section snippets
Chemical and feed solutions
Four anion dyes Eriochrome black T (Shenyang Le Heng Technology Co., Ltd. (PRC)), Reactive brilliant blue KN-R (Shanghai Eighth Dyestuff Chemical Plant), Reactive black 5 (Zhejiang Runtu Co., Ltd. (PRC)) and Evercion red H-E7B (Everlight Chemical) and two cationic dyes Methylene blue (Aladdin) and Basic Green 4 (Aladdin) with different molecular weight were used in this study as shown in Table 1. λmax of dye was obtained with dye concentration in the range of Beer's law. NaCl of analytical
Membrane performance for different pure dyes and salts
The membranes (DK, DL and ceramic NF membrane) were evaluated using different pure dye and salt solutions. As shown in Fig. 2, six dyes with different molecular weights (from 300 to 1800 Da) and charge were used in this study. It is evident from Fig. 2 (a) that the ceramic membrane showed a high flux compared with organic NF membranes (DK and DL). The properties of polymeric nanofiltration membranes lie between those of non-porous RO membranes (where transport is governed by a solution-diffusion
Conclusions
A ceramic nanofiltration (NF) membrane with MWCO of 900 Da and two commercial NF DK and DL membranes were used in this study for desalination of dye solutions and ceramic NF membrane showed be suitable for desalination of dye solutions containing NaCl and Na2SO4 simultaneously due to the satisfactory dye rejection and low rejection of both NaCl and Na2SO4. The effect of salt content and type, CNaCl/CNa2SO4, dye concentration, and pH of solution on performance of ceramic NF for desalination of
Acknowledgements
Financial supports from the National Natural Science Foundation of China (No. 21125629, 21306079, and 21276124), the Natural science fund for colleges and universities in Jiangsu Province (13KJB530005) and the Jiangsu Province Scientific Supporting Project (No. BE2015695), the Foundation from State Key Laboratory of Materials-Oriented Chemical Engineering (ZK201313).
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