Pre-ozonation for the mitigation of reverse osmosis (RO) membrane fouling by biopolymer: The roles of Ca2+ and Mg2+
Graphical abstract
Introduction
Biopolymer in effluent organic matter (EfOM), comprised of proteins and polysaccharides, etc. (Siembida-Losch et al., 2015), caused severe reverse osmosis (RO) membrane fouling (Li et al., 2007; Tay et al., 2018). Generally, Ca2+ and Mg2+, two common divalent cations in wastewater, had a higher fouling potential than monovalent cations (e.g., Na+ and K+) (Mo et al., 2008) and enhanced the accumulation of biopolymer onto membrane surface by charge neutralization, aggregation and cross-linking, etc. (Li et al., 2011; Seidel, 2002). Mo et al. (2008) observed a denser fouling layer on RO membrane surface caused by bovine serum albumin (a model protein) in the presence of Ca2+. In addition, Ca2+ also resulted in severe RO membrane fouling by alginate (a model polysaccharide) (Ang and Elimelech, 2008) due to the formation of cross-linked gel between adjacent alginates by Ca2+ bridging (“egg-box” structure) (Grant et al., 1973; Lee and Elimelech, 2006; Zhang et al., 2018).
Our latest work (Yin et al., 2020) suggested that pre-ozonation was effective to mitigate RO fouling by model biopolymer (bovine serum albumin and alginate) in the absence of divalent cations, which governed by electrostatic (EL) and hydrophobic (HP) interactions. However, it is still a controversial issue about the impact of pre-ozonation on RO membrane fouling when divalent cations are present. It was reported that more ionisable groups (e.g., carboxyl groups) in organics generated after ozone oxidation (Seidel and Elimelech, 2002) and precipitated with divalent cations (e.g., Ca2+ and Mg2+), which could exacerbate membrane fouling (Xue et al., 2016). Singh et al. (2014) observed that pre-ozonation aggravated RO fouling for treatment of landfill leachate. Brown et al. (2008) also concluded that ozone oxidation at high dose enhanced RO membrane fouling resistance. They all attributed the negative impact of pre-ozonation to the increased particle size of organic matter due to the coagulation of organic matter and divalent cations (Chandrakanth and Amy, 1996). In contrast, several studies showed that pre-ozonation mitigated RO fouling for filtration of surface water and secondary effluent (SE) containing Ca2+ and Mg2+ (Brown et al., 2008; Stanford et al., 2011; Wang et al., 2010). Zhao et al. (2019) found ozone broke the calcium-humic acid complexes and alleviated RO fouling. The opposite results were probably because of the different solution chemistries and oxidation conditions, such as the species and concentration of divalent cations and organics, and ozone doses.
The aim of this project was to offer deep insights into the roles of Ca2+ and Mg2+ in pre-ozonation for the control of reverse osmosis (RO) membrane fouling by biopolymer, which was represented by bovine serum albumin (BSA) and sodium alginate (SA). We examined the effect of ozone dose and divalent cation concentrations on membrane fouling control. To explore underlying membrane fouling mechanism, the physicochemical properties of BSA and SA were analyzed including size, zeta potential, hydrophilic/hydrophobic properties and carboxylic acidity titration. Furthermore, the adsorption of foulants on pristine and fouled membrane surface was applied to study the foulants-membrane and foulants-foulants interactions by in-situ streaming potential measurement, respectively. Finally, membrane autopsy was investigated by scanning electron microscope (SEM) and Attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR). To the best of our knowledge, no published literatures focused on the influence of pre-ozonation on biopolymer fouling of RO membrane in the presence of Ca2+ and Mg2+ to date.
Section snippets
Feed water
The biopolymer solutions (100 mg/L) were prepared by dissolving BSA (≥98%, Sigma, USA) or SA (≥98%, Alfa Aesar, UK) into deionized (DI) water (18.2 MΩ cm) and stirred for 24 h to obtain complete dissolution. Then CaCl2 or MgCl2 (AR-grade, Nanjing Chemical Reagent Co., LTD, China) was used to adjust Ca2+ or Mg2+ concentration in the biopolymer solutions ranging from 0.5 to 2.0 mM. Before pre-ozonation, the pH of all test solutions was adjusted to 7.0 by 0.1 M NaOH (analytical reagent, Nanjing
BSA fouling
The impact of ozone dose on RO fouling by BSA was investigated in the presence of 0.5 mM Ca2+ or Mg2+. As illustrated in Fig. 1a, the permeate flux reduced slightly by approximately 10% before 300 min, but following by dramatic flux decline (i.e., 47%) at the second fouling stage (300–700 min) for filtration of BSA + Ca2+ without pre-ozonation. Compared to Ca2+, Mg2+ led to less BSA fouling with flux loss of approximately 39% as illustrated in Fig. 1b, which was in agreement with the results
Conclusions
For the first time, this study investigated the role of Ca2+ and Mg2+ in pre-ozonation for the mitigation of RO membrane fouling including flux decline and fouling reversibility by model biopolymer (BSA and SA). We concluded that the following results might partially explain the controversial impact of pre-ozonation on membrane fouling control.
- 1.
Pre-ozonation changed the physicochemical properties of BSA and SA as indicated by the increased negative charge, particle sizes, carboxylic acidity and
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This research was financially funded by the National Key R&D Program of China (No. 2016YFE0112300) and the State Key Program of National Natural Science of China (No. 51438008).
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