Mercury removal from water streams through the ion exchange membrane bioreactor concept

Journal of Hazardous Materials 264 (2014) 65–70

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Mercury removal from water streams through the ion exchange membrane bioreactor concept Adrian Oehmen ∗ , Dario Vergel, Joana Fradinho, Maria A.M. Reis, João G. Crespo, Svetlozar Velizarov REQUIMTE/CQFB, Chemistry Dept., FCT, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal

h i g h l i g h t s • • • • •

Mercury removal from water achieved through the ion exchange membrane bioreactor. Mercury removal to levels below the 1 ppb drinking water limit were achieved. >98% removal of Hg achieved, with >98% biologically reduced from Hg(II) to Hg(0). Higher water throughputs (>5 times) achieved after membrane pre-treatment. Minimal contaminated waste was produced = clean environmental technology.

a r t i c l e

i n f o

Article history: Received 28 April 2013 Received in revised form 3 October 2013 Accepted 28 October 2013 Available online 4 November 2013 Keywords: Ion exchange membrane bioreactor (IEMB) Mercury bioremediation Water treatment Donnan dialysis Mixed microbial cultures

a b s t r a c t Mercury is a highly toxic heavy metal that causes human health problems and environmental contamination. In this study, an ion exchange membrane bioreactor (IEMB) process was developed to achieve Hg(II) removal from drinking water and industrial effluents. Hg(II) transport through a cation exchange membrane was coupled with its bioreduction to Hg0 in order to achieve Hg removal from concentrated streams, with minimal production of contaminated by-products observed. This study involves (1) membrane selection, (2) demonstration of process effectiveness for removing Hg from drinking water to below the 1 ppb recommended limit, and (3) process application for treatment of concentrated water streams, where >98% of the Hg was removed, and the throughput of contaminated water was optimised through membrane pre-treatment. The IEMB process represents a novel mercury treatment technology with minimal generation of contaminated waste, thereby reducing the overall environmental impact of the process. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Mercury (Hg) is the heavy metal with the highest toxicity to living cells, and has no beneficial biological function [1]. Hg is a bioaccumulative toxin that attacks the central nervous and endocrine systems, while excessive exposure over long periods of time can result in brain damage and, in extreme cases, death. The dangers associated with mercury poisoning have led to increased international restrictions regarding mercury levels in waterways. Hg contamination in the environment enters into water systems mainly through atmospheric deposition (usually by rainfall) and effluents from industrial processes, where mercury exists primarily as Hg(II) [2]. Hg pollution in waterways is a well known problem, whereby numerous countries have detected mercury at potentially

∗ Corresponding author. Tel.: +351 212948571; fax: +351 212948550. E-mail address: [email protected] (A. Oehmen). 0304-3894/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2013.10.067

harmful levels, including the U.S.A., Japan, Brazil, Indonesia, India, Iraq and China [3–5]. Hg can be spread through aquatic systems at elevated levels even in locations far from industrial emission sources [6]. Indeed, Hg has numerous industrial applications, such as in the chlor-alkali industry. While changes in the operation of chlor-alkali plants have largely de-emphasised the use of Hg, it is still emitted at significant levels from these sources even after the plant is closed or the use of Hg has been eliminated [7]. In addition to the chlor-alkali industry, Hg pollution in ground water has been ascribed to mining activities and even natural sources on occasion [5]. Potentially harmful concentrations of Hg have been previously observed in drinking water supplies [4,5] as well as reservoirs that could otherwise serve as a potential drinking water source [8–10]. The maximum concentration of mercury in drinking water recommended by the World Health Organisation is 1 ppb, and by the US EPA is 2 ppb. Drinking water contamination by Hg can be prevented by either treating the drinking water directly, where Hg is present in lower concentrations, and/or

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by treating industrial wastewater streams, where the Hg concentrations are much higher. Existing treatment approaches for Hg removal include adsorption, ion exchange, reverse osmosis and nanofiltration, chemical precipitation and bioremediation. The main challenges with these technologies in isolation are that: (1) Most treatment processes involve the production of an associated brine solution or waste product that must be effectively disposed of or regenerated. Since final disposal of the Hg removed from these streams involve economic (through disposal as a hazardous waste) and/or environmental (through disposal by other means) considerations, processes that emphasise Hg recovery in a concentrated form are advantageous. (2) Mercury is present in waters as a micropollutant, at levels much lower than other ions that will compete for e.g. chemical precipitant, ion exchange or adsorption sites, increasing the amount of material needed to achieve Hg removal. (3) Chemical or biological treatment processes can cause secondary pollution by chemicals or microbial cells and their by-products, lowering drinking water quality. One possible means of overcoming these challenges is through integrated ion exchange membrane processes, which combine the continuous transport of a charged pollutant with simultaneous treatment of the associated brine solution. This technology has shown excellent performance for the removal of anionic micropollutants from drinking water, such as perchlorate, bromate and arsenate [11–13]. However, the removal of cationic micropollutants (Hg(II)), has not yet been systematically investigated with this process. Furthermore, bioremediation technologies based on Hg(II) reduction to Hg0 through microbial activity have previously been reported [2,14–16], enabling brine treatment through bioreduction. The Hg0 that is produced by microbial reduction diffuses out of the cells and can easily be volatilised onto an adsorbent through aeration gas flow, due to its low solubility in water. Hg0 recovery from off-gas streams through adsorption onto various materials (e.g. activated carbon), has been widely demonstrated in the literature [14,17,18]. The integration of ionic mercury transport through a cation exchange membrane with its bioreduction is henceforth referred to as the ion exchange membrane bioreactor (IEMB) concept. This study investigates the potential of the IEMB process for directly treating contaminated drinking water, and for removing and recovering Hg from concentrated industrial effluents. For this purpose, Hg(II) transport was tested through numerous cation exchange membranes by Donnan dialysis in order to select the most appropriate membrane. Then, the IEMB process was tested at different Hg concentration ranges relevant to drinking water and industrial effluents (ppb to ppm ranges, respectively). The process was then combined with a mixed microbial culture for reducing ionic mercury to Hg0 in order to minimise the quantity of Hg-contaminated material. Finally, the effect of membrane pre-treatment was tested in order to maximise the treated water flow rate achievable through the IEMB system.

2. Materials and methods 2.1. IEMB description The IEMB process implemented in this study is illustrated in Fig. 1. This approach allows the separation of an aerobic mixed microbial culture, in the “Bio” compartment, from the “Water” stream through a membrane barrier, avoiding secondary contamination of the treated water by microbial cells and by-products. The cation exchange membrane is negatively charged, thus excluding similarly charged anions, and permitting the flow of cations. The transport of positively charged Hg(II) – containing species to the

Hg0 Recovery in off-gas Bio Reactor Vessel

Medium

Bio Effluent

Hg contaminated water

Aeration

Treated water

Water Bio Cation-exchange membrane

Fig. 1. Schematic of the IEMB system for Hg(II) removal.

“Bio” compartment is stimulated by supplying harmless cations to the mineral media of the “Bio” compartment (e.g. Na(I)) for counter transport, according to Donnan dialysis principles. The bioreduction of Hg(II) to Hg0 in the “Bio” compartment enables the selective recovery of Hg0 in the off-gas through adsorption onto e.g. an activated carbon filter. 2.2. IEMB membrane selection Five commercially available cation exchange membranes obtained from different manufacturers were tested in order to compare the Hg flux achievable through each membrane under Donnan dialysis conditions. The five membranes were: Ionics CR61-CMP, Nafion 117, Selemion CMV, Fumatech FKL and Fumatech FKE. In each test, both the “Water” compartment and “Receiver” solutions (i.e. microorganisms were not employed in these tests) were fed to a flow cell in single pass mode over a 48 h period, where the water throughput, i.e. the flow rate per membrane area (F/A) ratio, through each channel was 40.6 L m−2 h−1 . The volume of each compartment was 136 mL and the exposed area of the membranes was 11.3 cm2 . Each compartment was continuously stirred at 700 rpm. The initial composition of the contaminated “Water” feed was a 0.03 mM HgCl2 solution in deionised water, while the “Receiver” compartment was fed with a 3 mM NaCl solution in deionised water, in order to provide a high concentration of counterion (Na(I)) for Hg(II) transport. 2.3. IEMB setup and operation The setup of the IEMB system is shown in Fig. 1. A HgCl2 solution in tap water was pumped through one of the channels (where each channel was 0.3 cm high) of a two-parallel flat-plate ion exchange membrane module (with a membrane area of 39 cm2 ) that was continuously re-circulated at 1640 mL min−1 (“Water” Compartment). The Fumatech FKE membrane was directly employed as the membrane (unsupported). Simultaneously, a 0.1 mL min−1 mineral media solution was continuously added to a 700 mL bioreactor, which was continuously re-circulated at 1640 mL min−1 through the other channel of the membrane module in counterflow mode (i.e. the “Bio” compartment). The mineral media fed to the “Bio” compartment was described previously by Oehmen et al. [16], where glucose (120 C-mmol L−1 ) was the sole carbon source fed to the bioreactor. The effluent of the “Bio” compartment was also continuously removed at 0.1 mL min−1 , resulting in a hydraulic retention time of 5 d. In the test carried out for drinking water concentrations, the “Water” feed contained a 12 ppb Hg(II) solution in tap water. The influent water flow rate was maintained at 0.2 mL min−1 , leading to

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Table 1 Ion exchange membrane properties. Membrane

Ion exchange Capacityd (meq g−1 )

Thickness (␮m)

2.2 <0.9 2.4 0.6

626 198 116 127 52 54 46

% Water content

Exp. Ionics CR61-CMP Nafion 117 Selemion CMV Fumatech FKL Fumatech FKEa Fumatech FKEb Fumatech FKEc

>1

± ± ± ± ± ± ±

5 1 1 1 1 1 1

Manuf.

Exp.

Manuf.

560 183 130–150 110–120 50–70 50–70 50–70

52 21 33 33 9.4 ± 1.4 10.3 ± 0.2 10.6 ± 1.1

44 5 22 10 15 15 15

Thickness was measured in triplicate for all membranes (average ± standard deviation is presented). % water content determined in triplicate for Fumatech FKE membranes. a Membrane pre-treated with 0.4 M NaCl solution. b Membrane pre-treated with 0.2 M MgCl2 solution. c Membrane pre-treated with 0.2 M HgCl2 solution. d Ion exchange capacity provided by manufacturer.

an F/A ratio of 3.1 L m−2 h−1 . This test was operated without inoculating the “Bio” compartment, i.e. under Donnan dialysis mode, with a mineral media solution [16] fed at 0.1 mL min−1 in the “Receiver” compartment. The IEMB system was also tested for 5 ppm (0.025 mM) Hg(II) solutions in tap water, in order to assess its capacity to remove and recover Hg from more concentrated streams (e.g. industrial effluents). During these tests, the mixed microbial culture developed and characterised previously in our group [16,19] was used as the inoculum for the “Bio” compartment. Both the mineral media and glucose solutions were fed to the “Bio” compartment in this test to promote Hg(II) bioreduction and cell growth. Initially, the IEMB system was operated at an influent water flow rate of 0.1 mL min−1 , leading to an F/A ratio of 1.5 L m−2 h−1 . Subsequent tests focussed on the effect of the composition of the membrane pre-treatment solution on the IEMB process, during operation at an F/A ratio of 7.7 L m−2 h−1 . The Fumatech FKE membrane was stored in solutions of 0.4 M NaCl, 0.2 M MgCl2 and 0.2 M HgCl2 , respectively, in order to determine the effect of membrane pre-treatment on the flux of Hg through the membrane using an equi-charge basis. For further comparison, an intermediate solution of 0.025 mM of HgCl2 was also tested in the IEMB system. In each of the aforementioned IEMB experiments, 3-5 bioreactor hydraulic retention times were required in order to achieve steadystate operational conditions.

3. Results and discussion 3.1. Membrane selection Five cationic exchange membranes were tested in order to select an appropriate membrane to achieve Hg(II) removal. The results of the Donnan dialysis tests are shown in Fig. 2. The flux of Hg(II) removed from the “Water” compartment (Flux w) exceeded the flux of Hg(II) transferred to the “Receiver” compartment (Flux r) in the case of the Ionics CR61-CMP, Nafion 117, and Fumatech FKE membranes, suggesting that steady-state operational conditions had not been achieved in the 48 h operational period in these cases. While the Hg(II) was fully recovered in the “Receiver” compartment for the cases of the Selemion CMV and Fumatech FKL membranes, these tests revealed that the Hg(II) removed from the “Water” compartment in these cases was lower than that observed with the other 3 membranes. As discussed further below, the differing mechanical properties of each membrane is responsible for the time needed to reach steady-state conditions. The properties of each cation exchange membrane are shown in Table 1. High membrane thickness is usually associated to a higher resistance to ionic transport, which is consistent with the results observed in Fig. 2, where the thickness of the Ionics CR61-CMP membrane was observed to be > 3 times higher than that of the other membranes and also exhibited the lowest Hg(II) flux to the “Receiver” compartment. In general, there was a good correlation between the Hg(II) flux and the membranes with lower thickness

2.4. Analytical procedures and calculations

F Ji = (Ci,in − Ci,f ) A

(1)

where J represents the flux of component i, in this case Hg, and Ci,in and Ci,f are the Hg concentrations in the contaminated and treated water, respectively.

3.5E-08 3.0E-08

Jo (mmol cm-2 s-1)

The Hg analysis in the liquid, solid and gas phases was carried out by atomic absorption spectrometry, as previously described by Oehmen et al. [16], where the detection limit of Hg is <0.2 ppb. The total suspended solids (TSS) and volatile suspended solids (VSS) were determined according to standard methods [20], through 0.45 ␮m filtration of the solids, followed by measurement of the weight after heating to 100 ◦ C, and then 550 ◦ C. The water content of the membranes were carried out as previously described by Delimi et al. [21]. The Hg flux through the ion exchange membranes was assessed through the following mass balance equation:

2.5E-08 2.0E-08

Flux w Flux r

1.5E-08 1.0E-08 5.0E-09 0.0E+00 Nafion Ionics CR61-CMP 117

Fumatech Selemion Fumatech FKE CMV FKL

Fig. 2. Flux of Hg from the water (w) to the receiver (r) compartments during membrane transport tests.

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16 14

Hg (ppb)

12 10 Water Feed

8

Water Effluent

6

Effluent limit (1ppb)

4 2 0 0

20

40

60

80

Time (h)

at higher initial Hg(II) concentrations, it was determined that 7.35 ± 0.15 g Hg m−2 membrane must be loaded onto the membrane in order for breakthrough to the “Receiver” compartment to occur. This result implies that under similar operational conditions as performed for the system with drinking water, a very long time would be required for Hg breakthrough to the “Receiver” compartment. This suggests that for most drinking water treatment applications, ionic mercury bioreduction is likely unnecessary, as the ion exchange membrane essentially acts as a sorbent. The brine from the Donnan dialysis process would only require treatment after Hg breakthrough occurred.

Fig. 3. Mercury removal from drinking water through Donnan dialysis at an F/A ratio of 3.1 L m−2 h−1 .

3.3. Mercury removal from concentrated wastewater streams using the IEMB system

when comparing the results of Fig. 2 and Table 1 for the other membranes as well. As expected, thicker membranes require a higher amount of time to achieve steady-state flux conditions as compared to thinner membranes. The Fumatech FKE and Selemion CMV membranes showed similar Hg(II) fluxes to the “Receiver” compartment (Flux r), while the Fumatech FKE membrane provided a higher Hg(II) flux from the “Water” compartment (Flux w), according to the mass-balance calculations of Eq. (1). Considering that the Selemion CMV membrane was more than twice as thick as the Fumatech FKE membrane, it is likely that the higher ion exchange capacity and water content of the Selemion CMV at least partially counteracted this higher thickness. Higher ion exchange capacities and water contents are usually associated with higher ionic fluxes due to the higher concentration of ion exchange groups, and lower cross-linking of the membrane matrix, respectively [22]. Considering that the Fumatech FKE membrane exhibited high Hg(II) removal from the “Water” to the “Receiver” compartments, as well as appropriate physical properties, it was chosen as the membrane implemented in the integrated system. Concerning the mechanism(s) of Hg(II) transport through the membrane, several possibilities exist. Because of the high stability of the HgCl2 complex, the presence of positively charged Hg(II) species in aqueous solutions, even at low water pH, is greatly hindered. From the three possible cationic species: Hg2+ , Hg(OH)+ and HgCl+ , only the latter exists in amounts that could be transported to an appreciable degree by cation exchange. The calculated fraction of HgCl+ for the solution composition used in our study at pH 5.4 (0.12 ␮M) is far lower than that (17.8 ␮M) of the neutral HgCl2 form (ChemEQL, EAWAG, Switzerland, 2009). Nevertheless, the continuous transport of the positively charged HgCl+ causes a shift in the chemical equilibrium, leading to the formation of additional HgCl+ that can, in turn, be transported across the membrane. This is consistent with the results of Agarwal et al. [23], who suggested that the ionic species HgCl+ was the main species transported across a Nafion 117 membrane. Furthermore, the possibility that Hg(II) transport could also partially occur via molecular diffusion of HgCl2 , in addition to the Donnan counter-ion based exchange mechanism, cannot be excluded.

Hg(II) removal via the IEMB system was then tested at higher initial Hg concentrations (5 ppm), simulating industrial wastewater streams. In this case, the “Bio” compartment of the IEMB process was inoculated with biomass during process operation, in order to convert Hg(II) to Hg0 , and maintain Hg(II) transport through the membrane at high levels. Following an initial membrane preequilibration period, the steady-state experimental results at an F/A ratio of 1.5 L m−2 h−1 are shown in Fig. 4. Greater than 98% Hg removal was achieved from the “Water” compartment, while the Hg effluent from the “Bio” compartment was simultaneously maintained at low levels, largely due to its bioreduction to Hg0 . Indeed, only 8.2% of the Hg fed to the IEMB system was removed via the solid and liquid phases of the “Bio” compartment, implying that >90% was transformed to gaseous Hg0 (Fig. 5). While Hg0 was indeed detected in the gas phase of the “Bio” compartment during this study, the relatively small Hg0 loading capacity of the gold-trap system did not enable the experimental determination of the Hg0 flux. Nevertheless, the conversion of Hg(II) to Hg0 by this same mixed microbial culture was previously demonstrated at levels >80% [16], which agrees well with the low Hg levels determined in the solid and liquid phases of the “Bio” compartment in this study. Furthermore, the biomass growth of the mixed microbial culture was maintained at low levels, leading to <0.1 g VSS L−1 d−1 of Hg-containing sludge that is generated. This shows that the IEMB system can indeed be operated with minimal associated hazardous waste products requiring further disposal. The development of an effective Hg removal technology that can be recovered with minimal production of harmful by-products performed in this study is an important advance regarding the treatment of this toxic heavy metal. Further tests focussed on increasing the Hg flux through the cation exchange membrane in order to increase the throughput of water that can be treated via this process. Different membrane pre-treatment strategies were employed for the Fumatech FKE membrane, in NaCl, MgCl2 , and HgCl2 solutions at identical ionic strengths, in order to test the effect of ion valency on Hg(II) transport. Considering that Hg(II) can exist in both the monovalent (e.g. HgCl+ and Hg(OH)+ ) and divalent (Hg2+ ) forms, the effect of membrane pre-treatment with other monovalent (Na+ ) and divalent (Mg2+ ) cations was studied. These tests were carried out at an F/A ratio of 7.7 L m−2 h−1 , five times higher than the aforementioned IEMB test. Table 2 shows the results obtained in the IEMB tests performed for each pre-treated membrane. It can be observed that the Hg(II) flux through the membranes did not change substantially between pre-treatment with NaCl and MgCl2 , where only low mercury removal was observed. Nevertheless, after pre-treatment of the membrane with a HgCl2 solution, the flux of Hg(II) increased dramatically, by a factor >9. The effect of these different membrane pre-treatments on the physical properties of the Fumatech FKE

3.2. Mercury removal from drinking water The removal of Hg(II) from drinking water was tested using the process shown in Fig. 1 in Donnan dialysis mode. The results are shown in Fig. 3, which confirms that Hg removal to levels below the 1 ppb recommended limit is achievable by the process. It should be noted that Hg was not detected in the “Receiver” compartment, most likely due to the low Hg(II) loading to the system. Therefore, inoculation of the “Receiver” compartment for Hg(II) reduction would only be necessary after Hg(II) breakthrough is achieved. According to the aforementioned Donnan dialysis tests

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100

a

90

4500

900

80

4000

800

3500

700

3000

600

2500

500

2000

400

1500

300

1000

200

500

100

0

Feed - F/A= 7.7 Feed - F/A = 1.5 Eff - F/A=7.7 Eff - F/A=1.5

100

200

300

400

70

Eff - F/A= 7.7

60

Eff - F/A = 1.5

50

Gas - F/A=7.7

40

Gas - F/A=1.5

30 20

Bio Sol+Liq - F/A=7.7

10

Bio Sol+Liq - F/A=1.5

0

0 0

Hg (%)

1000

Hg effluent (ppb)

5000

100

0

500

300

400

500

Time (h)

Time (h)

b

200

5000 4500 4000 3500

Feed - F/A= 7.7

Fig. 5. Comparison between the Hg measured in the effluent of the “Water” compartment with that measured in the solid + liquid phases of the “Bio” compartment and the calculated recovery in the gas phase of the “Bio” compartment. Results presented as the % Hg influent in the “Water” compartment. Steady-state operation at F/A ratios of 7.7 and 1.5 L m−2 h−1 , with the HgCl2 (0.2 M) pre-treated membrane and NaCl (0.4 M) pre-treated membrane, respectively, is presented.

3000 2500

Feed - F/A = 1.5

2000 Bio Sol - F/A=7.7

1500 1000

Bio Sol - F/A=1.5

500 0 0

100

200

300

400

500

Time (h)

5000

1000

4500

900

4000

800

3500

700

3000

600

2500

500

2000

400

1500

300

1000

200

500

100

Hg Bio Liq (ppb)

c

Feed - F/A= 7.7 Feed - F/A = 1.5 Bio Liq - F/A=7.7 Bio Liq - F/A=1.5

0

0 0

100

200

300

400

500

Time (h)

Fig. 4. Steady-state operation at F/A ratios of 7.7 and 1.5 L m−2 h−1 , with the HgCl2 (0.2 M) pre-treated membrane and NaCl (0.4 M) pre-treated membrane, respectively. Comparison between the Hg concentration in the contaminated water (Feed) with (a) the treated effluent (Eff) of the “Water” compartment, (b) the solid phase of the “Bio” compartment (Bio Sol) and (c) the liquid phase of the “Bio” compartment (Bio Liq).

membrane was also investigated, and is shown in Table 1. While the water content of the membrane was similar in each case, a decrease in the membrane thickness was observed after pretreatment with the HgCl2 solution, which may have contributed to the increase in Hg(II) flux observed. An additional test where the Fumatech FKE membrane was pre-treated with a weaker HgCl2 solution corroborated these findings, where an intermediate Hg(II) flux was observed (Table 2). These results suggest that the Hg(II) flux increases after the membrane becomes fully saturated with Hg, enabling higher throughputs of contaminated water to be treated via the IEMB process. It should be noted that the improvement in Hg(II) flux at the higher F/A ratio did not come at the expense of increased Hg in the liquid or solid phases of the “Bio” compartment (see Fig. 4), since the mixed culture was able to reduce the incoming Hg(II) to Hg0 . Indeed, not only was the difference observed between the Hg influent (4343 ± 110 ppb and 4227 ± 186 ppb) and Hg effluent (78 ± 50 ppb and 73 ± 10 ppb) of the “Water” compartment insignificant at the F/A ratios of 7.7 and 1.5 L m−2 h−1 , respectively (>98% of Hg was removed in each case), but so were the liquid (16 ± 12 ppb and 2 ± 3 ppb) and solid effluents (758 ± 253 ␮g g−1 and 1264 ±445 ␮g g−1 ) of the “Bio” compartment during each test. A low biomass growth rate was also maintained at an F/A ratio of 7.7 L m−2 h−1 as compared to 1.5 L m−2 h−1 , which was <0.1 g VSS L−1 d−1 of Hg-containing sludge generated in each case. As shown in Fig. 5, the Hg recovery via the liquid and solid phases of the “Bio” compartment accounted for only 1.4 ± 0.4% of the Hg removed from the water compartment at an F/A of 7.7 L m−2 h−1 , whereas 8.1 ± 5.2% of the Hg was recovered in these phases at an F/A of 1.5 L m−2 h−1 . This implies that the Hg(II) to Hg0 bioreduction kinetics increased at the higher F/A ratio, likely due to the higher Hg(II) loading rate. Thus, the IEMB process can effectively remove Hg(II) from concentrated streams (e.g. industrial effluents),

Table 2 Comparison of the Hg flux through the Fumatech FKE membrane subjected to different pre-treatment solutions. Membrane pre-treatment solution

NaCl (0.4 M) MgCl2 (0.2 M) HgCl2 (0.025 mM) HgCl2 (0.2 M)

Water feed rate

Water side

F/A (L m−2 h−1 )

Influent (ppb)

Effluent (ppb)

Removal (ppb)

Flux (mg m−2 h−1 )

7.7 7.7 7.7 7.7

4721 5611 4556 4348

4270 5140 2720 81

452 471 1836 4267

3.5 3.6 14.1 32.8

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and selectively concentrate it as Hg0 with minimal production of contaminated sludge. 4. Conclusions In this study, Hg(II) removal was achieved from dilute and concentrated water streams through the ion exchange membrane bioreactor (IEMB) concept. The primary conclusions of this work are: • The Fumatech FKE membrane was found to achieve the highest Hg flux. • Hg removal to levels below the 1 ppb drinking water limit were achieved, where the Hg remained adsorbed to the membrane. • The process removed >98% of the Hg from concentrated streams, where >98% of the Hg(II) removed was biologically reduced to Hg0 . • Higher water throughputs are achievable by the process after saturation of the membrane with Hg. • The IEMB process generates minimal contaminated waste, thereby substantially reducing its overall environmental impact. There are several potential advantages towards employing the IEMB process for treating concentrated industrial effluents over conventional Hg(II) bioremediation processes. Firstly, the hydraulic retention time of the “Water” compartment can be controlled independently from the “Bio” compartment, enabling a higher water throughput rate. Indeed, in this study, at an F/A ratio of 7.7 L m−2 h−1 , the retention time was 4.2 h, which was far lower than the retention time necessary to operate the “Bio” compartment (5 d). Also, the ion exchange membrane serves as a means to control the Hg(II) flux to the “Bio” compartment, preventing bacterial exposure towards inhibitory levels of Hg(II) that would reduce the efficiency of bioreduction to Hg0 . Furthermore, the membrane barrier prevents against unwanted releases of Hg-containing sludge to aquatic systems, ensuring a high level of environmental protection associated with the bioremediation process. Acknowledgements The authors wish to thank Susana Serra and Javier Llanos for technical assistance. The financial support by Fundac¸ão para a Ciência e a Tecnologia (FCT), Lisbon, Portugal through projects PEst-C/EQB/LA0006/2011 and PPCDT/AMB/57356/2004 is gratefully acknowledged. References [1] D.H. Nies, Microbial heavy-metal resistance, Appl. Microbiol. Biotechnol. 51 (1999) 730–750. [2] I. Wagner-Dobler, Pilot plant for bioremediation of mercury-containing industrial wastewater, Appl. Microbiol. Biotechnol. 62 (2003) 124–133.

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