High-Performance Graphene Oxide Nanofiltration Membranes for

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High-Performance Graphene Oxide Nanofiltration Membranes for Black Liquor Concentration Zhongzhen Wang, Chen Ma, Scott Sinquefield, Meisha L. Shofner, and Sankar Nair ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.9b03113 • Publication Date (Web): 30 Jul 2019 Downloaded from pubs.acs.org on August 5, 2019

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ACS Sustainable Chemistry & Engineering

High-Performance Graphene Oxide Nanofiltration Membranes for Black Liquor Concentration Zhongzhen Wang1,2,‡, Chen Ma1,‡, Scott A. Sinquefield2, Meisha L. Shofner2,3, and Sankar Nair1,2*

1

School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA 311 Ferst Dr NW, Atlanta, Georgia 30332-0100, United States 2

3

Renewable Bioproducts Institute, Georgia Institute of Technology, Atlanta, GA 500 10th Street NW, Atlanta, Georgia 30332-0620, United States

School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 771 Ferst Dr NW, Atlanta, Georgia 30332-0245, United States

*

Corresponding author: [email protected]

‡

These authors contributed equally.

1 ACS Paragon Plus Environment

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Abstract Black liquor (BL) concentration by multi-effect evaporation is an extremely energyintensive operation in the kraft pulping cycle. Membranes can significantly save energy in this process, but conventional membranes are strongly challenged by low solute rejections and poor stability in BL, which is a complex mixture containing dissolved lignin, other non-lignin organics, multiple inorganic salts at highly alkaline pH and process temperatures of 70-85°C. Here we describe in detail the fabrication, modification, and characterization of robust and highperformance graphene oxide (GO) nanofiltration membranes for BL concentration. We show that polyethersulfone (PES)-supported GO membranes prepared from chemically reduced GO, and then subjected to high-pressure hydraulic compaction, show excellent chemical and mechanical stability under real BL conditions in comparison to conventional GO membranes. These membranes (referred to as ‘GO-3’ in this work) show near-perfect (>99%) lignin rejection, high total organic carbon (TOC) rejection (up to 93%), and greatly improved inorganic rejections especially for divalent anions that are predominant in BL. Finally, the GO-3 membranes are scaled up on larger PES sheets (~660 cm2 in size) and are operated under realistic cross-flow conditions with real BL feed flow rates as high as 10 L/min at 70°C. The GO-3 membranes show robust performance over more than 1,500 hours (60 days) of continuous operation in multiple cycles of 10-50 bar transmembrane pressures, attaining stable and sustained permeate fluxes as high as 25 LMH and excellent rejection performance equal to that obtained at smaller-scale. The main results of this work have strong implications on the development of membrane processes for BL dewatering and more generally for processing of complex biorefinery feed streams. Keywords: Black liquor; Lignin; Membrane; Graphene Oxide; Nanofiltration


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Introduction The concentration of weak black liquor (WBL) to strong black liquor (SBL) in the kraft paper making process by multi-stage evaporator trains1-2 is very energy intensive.3 Membranebased BL concentration (or dewatering) has long been proposed as a desirable alternative that can provide significant energy savings (> 30%)3, and the advantage of membranes over evaporationbased dewatering are well known in different applications, especially in seawater desalination where membranes have largely replaced evaporation processes.4-6 Specifically, dewatering by a membrane is typically driven by a pressure differential, which costs much less energy than that required to perform a phase change from liquid to vapor during evaporation. The goal of membrane-based dewatering is two-fold: firstly to concentrate BL to around 30-40 wt% TS (total solids) and significantly reduce the evaporator energy load, and secondly to produce processquality water (about 0.1-1 wt% TS) that can be used to make-up the process-quality water that is no longer produced by evaporation and maintain the overall water balance in the kraft process. However, in comparison to typical dewatering membrane applications, the extreme operating conditions of BL processing – high alkaline pH (>12.5), high total solids content (> 15 wt%), and elevated temperature (70-85°C) – lead to unique challenges in development of robust membranes that can also provide high water fluxes and high solids rejections. Additionally, there is a very large molecular weight distribution inherent in the BL composition. A typical kraft BL contains lignin (0.5-10 kDa), other organics (0.2-0.8 kDa), and inorganic salts ( GO-2 > GO-3. This trend is expected, since GO-2 is less hydrophilic than GO-1 due to chemical reduction of hydroxyl, carboxylic, and epoxide functional groups, and furthermore, GO-3 has undergone additional physical compaction.26-27 However, under BL feeds, all the membranes have comparable fluxes. The GO-3 flux is somewhat lower than the GO-1 and GO-2 flux, but the difference is much smaller than the pure water feed. This result indicates that in the case of the high-solids BL feed, the concentration polarization resistance on the surface of the membrane dominates over the transport resistance of the GO layer and the PES support. Figure S9 shows the visual appearance of GO-1, GO-2 and GO-3 membrane permeate samples at different TMPs. At the same TMP, the permeate samples appear more dilute when going from GO-1 to GO3 membrane. Additionally, for each type of membrane the permeates appear more dilute as the TMP increases. At 50 bar TMP, the GO-3 membrane permeate appears nearly colorless.

Figures 3-4 summarize the rejection performance of the three types of GO coupon membranes: total solids (TS), lignin, total organic carbon (TOC), and total inorganics (estimated based on the sulfate ash analysis) as well as the rejections of five individual salts present in BL. From Figure 3, the general rejection trend of the three GO membranes is: GO-3 > GO-2 ≈ GO-1. This behavior is consistent with that of the pure water flux (Figure 2a), as the compaction of the GO-3 membrane improved the selectivity towards smaller molecules and ions. This reduction in effective pore size of the GO-3 membranes has previously been attributed to the narrowing of the “wrinkles” in the rGO laminates after physical compaction27, or the emergence of a more ordered rGO laminate structure by rearrangement of dislocated rGO flakes.26 The TS rejection (Figure 3a)


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(a)

(b)

(c)

(d)

Figure 3. Rejections of the GO-1, GO-2 and GO-3 coupon membranes. (a) Total solids (TS),

(b) Lignin, (c) Total organic carbon (TOC), and (d) Total inorganics. of all the three kinds of GO membranes improved with TMP. The GO-3 membrane showed significantly higher TS rejection at all TMPs, reaching ~70% at 50 bar TMP. The lignin rejection (Figure 3b) of GO-1 membranes reached a plateau of 96.5%, which is consistent with our previous report.11 The lignin rejection improves further with GO-2 (98.5%), and the GO-3 membrane reaches an excellent lignin rejection of ~99.5 % at around 50 bar TMP. Given that the subsequent RO membrane stage will have a molecular weight cut-off (MWCO) < 0.2 kDa, the GO-3 membrane can greatly reduce the potential for membrane fouling in the RO stage. GO-3



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membranes also reach >90% TOC rejection (Figure 3c). Finally, the GO-3 membranes were able to reach total inorganic rejections of ~50% (Figure 3d), which is considerably higher than GO-1 and GO-2 (~40%).

Figure 4 shows the individual salt rejections of the GO-1, GO-2, and GO-3 membranes. In the kraft process cycle, the sulfur-containing divalent salts (Na2S2O3, Na2SO4, and Na2SO3) are reduced to Na2S in the recovery boiler. The Na2S as well as Na2CO3 are be re-dissolved and sent to the lime kiln to regenerate NaOH.30 The resulting Na2S-NaOH solution is referred to as “white liquor” (WL) which is used to treat wood chips at the beginning of the kraft cycle. In the membrane-based dewatering of 15 wt% BL, these salt species should also be rejected as much as possible in the NF and the RO stages so that they proceed to the recovery boiler along with lignin. Here, we find that all the GO membranes can remove a substantial portion of these divalent salts at 30 – 50 bar TMP (Figures 4a-4c). At 50 bar, the rejections are generally in the order Na2S2O3 > Na2SO4 > Na2SO3, as also shown in Figure 4f. This reflects the mechanism of ion rejection by size, as the hydrated ion diameters also follow the same sequence: S2O32- (0.776 nm) > SO42- (0.760 nm) > SO32- (0.736 nm).31-33 The divalent carbonate salt (Na2CO3) also has comparable rejection to the divalent sulfated salts (Figure 4d). The GO-3 membrane continues to show the highest rejections. This is clearly important for the subsequent RO-based membrane treatments (not discussed in this work). It produces an NF permeate with a much lower osmotic pressure (~19 bar when using the GO-3 membrane at 50 bar TMP) than previously reported polymeric and ceramic membranes (38-49 bar), thereby drastically lowering the operating pressure and energy consumption of any downstream RO process. Furthermore, lower salt concentrations in the NF permeate will also increase the rejection of the RO membranes, as known from experimental data


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(a )

(b )

(c )

(d )

(e )

(f )

Figure 4. Salt rejections of GO membrane coupons from a BL (15 wt% TS) feed: (a) Na2S2O3, (b) Na2SO4 (c) Na2SO3, (d) Na2CO3, and (e) NaCl; (f) Summary of salt rejections at 50 bar TMP.



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as well as modeling studies based on the extended Nernst-Plank equations.34-37 This phenomenon was also observed for the GO membranes.24 The GO-3 membrane emerges as a robust and highperformance NF membrane for BL processing. In addition to 99.5% lignin rejection and > 90% TOC rejection, the GO-3 membrane can reject 73% Na2S2O3, 67% Na2SO4, 62% Na2SO3, and 63% Na2CO3 at higher temperatures of 70°C with real BL feeds. The other divalent salt in BL is Na2S, present in equilibrium with monovalent NaHS. As mentioned earlier, these components are more difficult to measure directly and may also undergo slow oxidation to sulfates/sulfites. Therefore, their concentration is instead estimated by subtracting the measured concentrations of the other sulfated salts from the total inorganic content (obtained by sulfated ash analysis). Thus, we estimate a Na2S+NaHS combined rejection of ~ 22% at 50 bar TMP for the GO-3 membrane.

On the other hand, the monovalent Cl- ion has a much smaller hydrated diameter (0.646 nm)31 than the divalent ions and also experiences lower electro-repulsion from the negatively charged GO membrane surfaces. Thus, the NaCl rejection is much lower, and its removal would be accomplished in the downstream RO stage instead. At lower TMPs such as 10 bar, it is noteworthy that the GO-1 and GO-2 membranes exhibited negative rejections towards divalent salts, and all three membranes exhibited negative rejections for NaCl. This phenomenon can be observed in NF membranes when treating mixtures containing both salts and larger charged organic species, or mixtures of monovalent and multivalent salts. The separation of organic dyes from inorganic salts is one such example.38-40 The negative rejections can be explained by Donnan equilibrium at the membrane surface when the feed solution contains non-permeable multivalent organic anions (such as the organic species present in BL). In order to satisfy Donnan equilibrium


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as well as electroneutrality conditions, the salt transport will be enhanced due to an increase in effective salt concentration on the feed side:41

𝑐𝑠 = 𝑐𝑠′ (1 +

𝑣𝐶𝑥′ 0.5 ) 𝑐𝑠′

(1)

Here 𝑐𝑠 is the salt concentration on the feed side inside the membrane (intra-pore concentration), 𝑐𝑠′ is the salt concentration at the interface outside the membrane, v is the valence of the organic anions, and 𝐶𝑥′ is the organic anion concentration at the interface outside of the membrane. The elevated salt concentration at the feed side will increase the salt concentration gradient, leading to enhanced salt transport and thus a low (or even negative) salt rejection. As evident from Eqn. (1), this effect increases with organic anion concentration, and BL contains large concentration of charged organic anions such as lignin and lower-MW organics (Table S1). The divalent salts are less influenced by this effect, due to their higher charge as well as their larger size (which independently allows higher rejection by the sieving mechanism especially in the GO-3

Figure 5. MWCO measurements of GO-1, GO-2, and GO-3 membranes using a set of dye molecules. 15 ACS Paragon Plus Environment


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membranes). At the higher TMPs, the rejection increases with the TMP, which was also in agreement with the conventional polymeric nanofiltration membranes.42

Our experimental results and discussion above also imply that a high organic anion rejection in the NF stage would be useful to improve the salt rejection of the subsequent RO stage. Specifically, a reduction in 𝐶𝑥′ will reduce salt passage through the membrane and allow the salts to be retained for processing in the recovery boiler. However, in other applications wherein one may want to separate organic species from salts, increasing the concentration of organic anions might be a viable solution.43 Based on all our results, it is seen that operation of the GO-3 membrane above 50 bar pressure appears to be a very efficient means of performing NF on BL feeds. In order to estimate the effective pore sizes of the three membranes, MWCO measurements (Figure 5) were performed using 5 different dyes listed in Table S3. The GO-3 membranes showed the highest dye rejections, corresponding to a MWCO of 0.36 kDa in comparison with the MWCOs of GO-1 and GO-2 membranes (0.45-0.5 kDa). The corresponding interlayer effective pore sizes are estimated as 1.26-1.29 nm (GO-1 and GO-2) and 1.18 nm (GO-3) respectively.44 The GO-3 membrane shows a clear reduction in effective pore size that allows enhanced rejections of lignin, other organics, and divalent salts.

Figure 6 shows the SEM microstructures of the three membranes with GO layers of approximately equal thickness as determined from SEM measurements at multiple locations on the samples. The GO-1 and GO-2 membranes (Figures 6a-6b) have thicknesses of 138±16 nm and 137±11 nm, respectively. The GO-3 membrane (Figure 6c) shows moderate densification of the PES support after 48 h of hydraulic treatment high pressure compaction. However, the


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(a)

2 μm

(b)

2 μm

(c)

2 μm

Figure 6. SEM images of (a) as-made GO-1 membranes, (b) as-made GO-2 membranes, and (c) GO-3 membranes after 48 h compaction. thickness of the GO-3 membrane layer (134±9 nm) was similar to the GO-1 and GO-2 membranes. It is clear that the compaction process does not directly decrease the interlayer spacing, since the rGO layer thickness did not decrease. Rather, it appears that more subtle effects such as migration and rearrangement of the rGO flakes take place during the compaction process.26 Additionally, it



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is possible that the compaction process increases the adhesive forces between the rGO layer and the PES support by mechanisms that are not fully understood yet.

Due to the excellent performance characteristics obtained with GO-3 membranes, we proceeded to perform a scale-up study of GO-3 NF membranes using a larger vacuum filtration system, as described in the Experimental Section and in Figure S1. Before the membrane compaction, we also characterized the quality of the intermediate GO-2 membrane coatings. Figure S10 shows a photograph of a scaled-up GO-2 membranes, including 10 locations from which “cut-out” coupons were taken and used for determining membrane thicknesses by SEM. As seen in Figure S11, the GO-2 membrane shows excellent uniformity, with less than 10% difference in membrane thickness between the middle and the outer edges of the membrane sheet. After in situ compaction, the resulting GO-3 sheet membranes (~265 cm2 area rectangular sheet cut from a ~650 cm2 circular sheet) were then subjected to long-term crossflow permeation measurements in a real BL feed at 72°C and 5.6 L/min feed flow rate (about 0.55 m/s feed velocity over the membrane), with repeated cycling over a TMP range of 10-50 bar (Figure S12). In each of the three cycles, the membrane was operated for 3000-4000 mins at each TMP value to ensure steady-state operation. In the first pressure cycle, the membrane generally showed a somewhat lower flux (~ 8 LMH) as compared to the coupon membranes. We speculate that this behavior was (d)lignin and other larger solutes to the membrane surface to create caused by initially fast transport of a concentration polarization layer, followed by slow evolution of the polarization layer to a steadystate structure that allows permeation channels through it. In the second and third cycles, the flux behavior stabilized to the expected behavior. The flux increased with increasing TMP in a manner similar to Figure 2b, and the membrane showed steady and long-lived performance with no


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Figure 7. (a) Steady-state fluxes and lignin rejections at each TMP during the long-term operation of GO-3 membrane in cycles 2 (triangles)-3 (squares) (from Figure S12), (b) Effect of BL feed flow rate on flux and rejection at 30 bar TMP, and (c) Effect of TMP on flux and lignin rejection at the highest BL feed flow rate of 10.3 L/min allowed by the pumping system. 19 ACS Paragon Plus Environment


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evidence of degradation, delamination, or fouling over 1,000 hours (41 days) of continuous operation. Furthermore, Figure S13 shows that the key NF membrane separation parameter, the lignin rejection, remains very stable over the entire 41-day period. Figure 7a summarizes the steady-state flux and lignin rejection behavior obtained at each TMP in cycle 2 and cycle 3. The most interesting region is the operation at 30-50 bar, wherein high fluxes of 15-18 LMH and lignin rejections of 98.5-99% are observed. Next, we investigated the effect of feed flow rate/velocity (i.e., external mass transfer resistances including bulk transport and the polarization layer thickness) at a fixed TMP of 30 bar as shown in Figure 7b. The steady-state flux increased rapidly upon increasing the feed flow rate from 5.6 L/min up to about 9 L/min, and then reached a plateau up until 10.3 L/min (which is the maximum flow rate allowed by our pumping system, and corresponds to a 1 m/s feed cross-flow velocity over the membrane). This behavior is clear evidence of reduction in external mass transfer resistance due to the increased convection over the membrane. A flux of ~21 LMH is achieved with a flow rates of 9 L/min. By this point the GO membrane mass transfer resistance has become dominant over the external mass transfer resistance, and the flux reaches a saturation level. Lignin rejection remains excellent (>99%), as shown by permeate composition measurements at two of the flow rates. We then conducted a final TMP cycling run at the highest flow rate of 10.3 L/min (Figure 7c). At 50 bar TMP, the flux reaches > 25 LMH while maintaining lignin rejection > 99%. Overall, the measurements described above spanned more than 1,500 hours (63 days) of continuous cross-flow operation of the GO-3 NF membrane without any intermittent cleaning or shutdown time, during which it maintained excellent BL concentration performance. At 50 bar TMP, the membrane showed total solids (TS) rejection of 69.5%, total organic carbon (TOC) rejection of 88.5%, and total inorganics (TI) rejection of 56%. While the rejections of individual salts were not measured in this operation, one


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can fully expect the same levels of performance seen in Figures 3-4 based upon the measured fluxes and lignin rejections in Figure 7.

Finally, we attempted to characterize the effect of higher solids concentrations on the flux with the aim of determining whether the membrane can still operate at such conditions. In a spiralwound or tubular module operating at a given TMP, the cross-flow flux of all membranes would certainly decline as the BL proceeds from one end to the other. This is because the BL becomes progressively more concentrated in total solids and the viscosity also increases (i.e., the shear Reynolds number decreases). From a process point of view, this essentially means a larger membrane area required (or operation at higher TMP). It is difficult to study this effect reliably at present in our system, given the limited quantities of BL available and the long measurement times required for steady-state results. However, we prepared a smaller quantity of concentrated (32 wt%) BL by nanofiltration of 15 wt% BL through the GO-3 membranes. For this concentrated feed at a lower flow rate of 5.6 L/min, we measured a significant flux of 6 LMH with no change in the lignin rejection over about 1 hr of permeation. However, this result should not be considered as a true steady-state value or to be directly comparable with the values obtained in Figure 7. For example, as described earlier, Figure S12 also showed relatively low fluxes (7 LMH) in the first cycle of operation before stabilizing at much higher values. More detailed measurements with higher flow rates and longer measurement times are required in a future work.

Conclusions We have investigated in detail the effects of chemical and physical modifications of graphene oxide (GO) membranes on their permeation characteristics and long-term stability during



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membrane-based concentration of kraft black liquor. Three types of membranes, labeled GO-1, GO-2, and GO-3 are studied. In particular, the GO-3 membranes that are fabricated by deposition of chemically reduced GO membranes (GO-2) followed by hydraulic compaction at pressures up to 50 bar, show superior chemical and mechanical stability over GO-2 membranes and GO-1 (conventional GO membranes) in real black liquor at a realistic process temperature of 70°C. The BL nanofiltration characteristics of these membranes are studied to understand the influence of effective pore sizes and external mass transfer resistances (including concentration polarization) on the rejection of multiple types of species present in BL and the molecular weight cut-offs (MWCOs) of the membranes. At 50 bar transmembrane pressure, the GO-3 membrane exhibits excellent (>99%) lignin rejections, up to 93% total organic carbon rejections, as well as improved rejections of the divalent salts Na2S2O3, Na2SO4, Na2SO3 and Na2CO3 present in BL. The GO-3 membranes have a MWCO of 360 Da, which is significantly improved compared to GO-1 and GO-2 membranes (450-500 Da). Furthermore, we have demonstrated the scale-up of GO-3 membranes to larger (~660 cm2) sheets and their stable, robust operation for more than 1,500 hours (60 days) in real BL feed flows as high as 10 L/min at 70°C. As a result, GO-3 membranes are excellent candidates for nanofiltration of BL and potentially other complex biorefinery feedstocks, not only because of their superior performance and high stability, but also because the significantly reduced lignin and organics concentrations in the permeate will minimize membrane fouling and salt rejection loads in subsequent reverse osmosis (RO) membrane stages for further processing of NF permeates and production of process-quality water. The latter type of membranes (RO) will be discussed by us in detail in a forthcoming work.


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Supporting Information All experimental details (materials, GO suspension preparation, membrane fabrication, characterization, permeation measurements, and stream composition analysis), tables of BL composition, schematic of membrane permeation apparatus, conductivity reduction and water flux data for GO membranes. This material is available free of charge at http://pubs.acs.org.

Author Information Corresponding Author * S. Nair: [email protected] Zhongzhen Wang ORCID: https://orcid.org/0000-0001-9763-5077 Sankar Nair ORCID: http://orcid.org/0000-0001-5339-470X

Acknowledgements We acknowledge financial support by the DOE-RAPID Institute (#DE-EE0007888-7-5) and an industry consortium comprising Domtar, Georgia-Pacific, International Paper, SAPPI, and WestRock. Additionally, Z. W. acknowledges support by the Renewable Bioproducts Institute Ph.D. Fellowship. We acknowledge the following individuals (all at Georgia Tech): Prof. E. Reichmanis for access to UV-Vis spectrophotometry; and X. Zeng and M. Buchanan for performing capillary ion electrophoresis, coulometry, and sulfated ash analysis. XPS and SEM characterization was performed at the Georgia Tech Institute for Electronics and Nanotechnology, home to one of the sixteen sites of the National Nanotechnology Coordinated Infrastructure (NNCI), which was supported by the National Science Foundation (Grant No. ECCS-1542174).



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Synopsis We report the fabrication and characterization of robust, high-performance graphene oxide membranes for concentration of kraft black liquor. These membranes have also been scaled up to ~660 cm2 sheets and show excellent nanofiltration characteristics over more than 1,500 hours onstream in real black liquor at 70°C for separation of lignin, organics, and salts.

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