Gypsum scaling in forward osmosis: Role of membrane surface chemistry
Graphical abstract
Introduction
Forward osmosis (FO), an osmosis-driven membrane process, could potentially advance desalination and wastewater reuse. FO utilises the osmotic pressure of a highly concentrated draw solution as the driving force to transfer water from the feed solution to the draw solution through a dense polymeric membrane. FO has demonstrated a much lower fouling propensity and higher fouling reversibility than RO, which was attributed to the lack of applied hydraulic pressure [1], [2], [3], [4]. Consequently, FO is widely used to treat low quality feedwater, including landfill leachate [5], anaerobic digester concentrate [6], activated sludge solution [7], [8], and municipal wastewater [9], [10], [11].
The core of FO membrane has advanced from asymmetric cellulose triacetate (CTA) membrane to polyamide thin-film composite (TFC) membranes because of the excellent mass transfer properties of polyamide [12], [13], [14]. Indeed, the TFC membrane not only produces higher water permeability but also exhibits better contaminant rejections in comparison with the CTA membrane. For example, the TFC membrane achieved rejections of four pharmaceuticals above 95%; in contrast, the CTA membrane exhibited varying rejections of these compounds from 65% to 95% [15]. This better performance of the TFC membrane was further demonstrated by substantially higher rejections of neutral contaminants than the CTA membrane [16].
Membrane surface chemistry of CTA and TFC membranes are markedly different. Unlike the CTA membrane abundant with hydroxyl functional groups, the TFC membrane is characterised by a high density of carboxylic acid functional groups, which results in potentially high fouling propensity. Previous knowledge from RO membrane fouling demonstrated that carboxylic functional groups enabled the formation of calcium bridging between the membrane surface and a wide range of organic foulants, and consequently increased organic fouling. Wu et al. [17] found that the carboxylic functional groups on RO membrane exhibited the highest initial alginate adsorption rate in seawater desalination. This higher adsorption was further revealed by measuring the alginate – membrane surface intermolecular force [18], which increased with higher density of carboxylic functional groups.
Role of membrane surface chemistry in FO membrane fouling is conflicting and not well understood. Limited investigations were conducted to compare FO membrane fouling behaviour between CTA and TFC membranes with different surface chemical functionalities. For instance, more severe gypsum scaling of the TFC membrane was observed in comparison with the CTA membrane [19], [20], which was attributed to stronger adhesion force measured by atomic force measurement (AFM). In contrast, negligible difference in water flux decline was observed between the TFC and CTA membranes during silica scaling. In addition, the adhesion force measurement by AFM also showed stronger hydrogen bonding between silica and the CTA membrane abundant with hydroxyl functional groups [20]. As a result, the role of membrane surface chemistry on FO fouling is not straightforward and necessitates systematic investigation.
In this study, we investigated the role of FO membrane surface chemistry on gypsum scaling using CTA and TFC membranes. Membrane surface chemistry – surface charge and surface functional groups – were characterised. Gypsum scaling of the CTA and TFC membranes was conducted in a real-time observation setup and was quantified in terms of water flux decline, gypsum surface coverage and gypsum crystal morphology. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy were used to capture changes of membrane surface chemistry on the CTA and TFC membranes during gypsum scaling, thereby elucidating the role of membrane surface chemistry on gypsum scaling in FO process. We provided, for the first time, a time-resolved gypsum scaling profile of CTA and TFC membranes in FO process. The real-time observation, microscopic imaging as well as comprehensive membrane surface chemistry characterization constituted compelling experimental evidence to elucidate the scaling mechanism.
Section snippets
FO membranes
An asymmetric cellulose triacetate (CTA) and a polyamide thin-film composite (TFC) forward osmosis (FO) membrane were employed in this study. The CTA membrane was composed of a cellulose triacetate layer with an embedded woven support mesh [21], [22]. The TFC membrane was made of a thin selective polyamide active layer on top of a porous polysulfone support layer [23], [24].
Real-time observation FO setup
A transparent acrylic FO membrane cell coupled with microscopic observation enabled real-time observation of gypsum
Membrane surface chemistry properties
CTA and TFC membranes possess markedly different surface chemistry properties (Fig. 1). The nature of the functional groups of CTA membrane was identified by the shifts in the binding energy of the deconvoluted XPS peak spectra [27], [28] as –C–H-(284.6 eV) and –C–OH (286.2 eV) (Fig. 1A); while in contrast, that of the TFC membrane was revealed as –C–H-(284.6 eV), –C=O (286.7 eV), and –COOH (288.9 eV) (Fig. 1B) [29], [30], [31]. Indeed, the elemental survey by XPS analysis also showed the presence
Conclusion
Results reported here demonstrated that membrane surface chemistry played an important role in gypsum scaling in FO process. Gypsum scaling, characterised by water flux decline and gypsum crystal surface coverage using real-time observation technique, became more severe for both CTA and TFC membranes as the initial water flux increases. However, at the same initial water flux, the TFC membrane was subjected to more severe gypsum scaling than the CTA membrane in terms of water flux and gypsum
Acknowledgements
M. X. thanked Victoria University for the award of a Vice Chancellor Early Career Fellowship. Dr. Yichao Wang (Royal Melbourne Institute of Technology) was thanked for the discussion and technical assistance in XPS analysis.
References (42)
- et al.
Chemical and physical aspects of organic fouling of forward osmosis membranes
J. Membr. Sci.
(2008) - et al.
Organic fouling of forward osmosis membranes: Fouling reversibility and cleaning without chemical reagents
J. Membr. Sci.
(2010) - et al.
Fouling control in a forward osmosis process integrating seawater desalination and wastewater reclamation
J. Membr. Sci.
(2013) - et al.
Comparison of fouling behavior in forward osmosis (FO) and reverse osmosis (RO)
J. Membr. Sci.
(2010) - et al.
Forward osmosis for concentration of anaerobic digester centrate
Water Res.
(2007) - et al.
The forward osmosis membrane bioreactor: A low fouling alternative to MBR processes
Desalination
(2009) - et al.
Membrane fouling and process performance of forward osmosis membranes on activated sludge
J. Membr. Sci.
(2008) - et al.
Membrane contactor processes for wastewater reclamation in space: Part I. Direct osmotic concentration as pretreatment for reverse osmosis
J. Membr. Sci.
(2005) - et al.
Characterization of novel forward osmosis hollow fiber membranes
J. Membr. Sci.
(2010) - et al.
Synthesis and characterization of flat-sheet thin film composite forward osmosis membranes
J. Membr. Sci.
(2011)
Rejection of pharmaceuticals by forward osmosis membranes
J. Hazard. Mater.
Relating rejection of trace organic contaminants to membrane properties in forward osmosis: measurements, modelling and implications
Water Res.
Studying the impact of RO membrane surface functional groups on alginate fouling in seawater desalination
J. Membr. Sci.
Silica scaling and scaling reversibility in forward osmosis
Desalination
Forward osmosis: principles, applications, and recent developments
J. Membr. Sci.
Influence of membrane support layer hydrophobicity on water flux in osmotically driven membrane processes
J. Membr. Sci.
Pilot demonstration of the NH3/CO2 forward osmosis desalination process on high salinity brines
Desalination
Standard methodology for evaluating membrane performance in osmotically driven membrane processes
Desalination
Role of pressure in organic fouling in forward osmosis and reverse osmosis
J. Membr. Sci.
Probing the nano- and micro-scales of reverse osmosis membranes—A comprehensive characterization of physiochemical properties of uncoated and coated membranes by XPS, TEM, ATR-FTIR, and streaming potential measurements
J. Membr. Sci.
Effect of membrane chemistry and coating layer on physiochemical properties of thin film composite polyamide RO and NF membranes: I. FTIR and XPS characterization of polyamide and coating layer chemistry
Desalination
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