Arsenic Removal using oxidative media and nanofiltration

74 DECEMBER 2008 | JOURNAL AWWA • 100 :12 | PEER-REVIEWED | MOORE ET AL

oxidative media and nanofiltration

Nanofiltration (NF) is a promising drinking water treatment technology for arsenic

removal; however, most of the research on NF treatment of arsenic has used synthetic

water. In this investigation, a pilot membrane system treated groundwater naturally

contaminated with arsenic to test the performance of two NF membranes and one

reverse osmosis (RO) membrane, both with and without oxidizing pretreatment using

manganese dioxide (MnO2). The arsenic concentration in the groundwater was ~ 40

µg/L, mostly present as arsenite, a neutral species. Without the oxidizing pretreatment,

the two NF membranes provided almost no removal whereas the RO membrane

provided ~ 25 to 50% arsenic removal, depending on operating conditions. Following

pretreatment with MnO2, the treated arsenic concentration dropped to < 4 µg/L (90%

removal) for all three membranes. The substantially improved performance for these

negatively charged membranes was attributed to the oxidation of the neutrally charged

arsenite to negatively charged arsenate. These results indicate that where arsenite is

present, facilities with RO or NF processes can dramatically enhance their arsenic

removal by adding a membrane-compatible oxidation step such as MnO2 filtration.

rsenic is a common, naturally occurring element. Arsenic is widely recognized as a poison, and exposure over the long term has been shown to lead to numerous health problems including skin lesions, neurological and cardiovascular damage, and cancer (Ng et al,

2003; Mandal & Suzuki, 2002). Reports of arsenic-contaminated water at concentrations > 50 µg/L are common (Mandal & Suzuki, 2002; Smedley & Kinniburgh, 2002; Frey & Edwards, 1997) and highlight arsenic contamina-tion problems around the world.

Drinking water is a common pathway for human exposure to arsenic. To limit the risk of cancer in the United States, the US Environmental Protection Agency (USEPA) has established a maximum contaminant level of 10 µg/L (USEPA, 2001); similarly Canada recently lowered its maximum allowable concentration to 10 µg/L (Health Canada, 2006). For a water treatment technology to be successful, it must produce treated water that meets or exceeds these criteria.

Arsenic can take many forms, but inorganic forms are most prevalent in groundwater (Smedley & Kinniburgh, 2002; Chen et al, 1999; Brandhuber

BY KENNETH W. MOORE,

PETER M. HUCK,

AND STEVE SIVERNS

Arsenic removal usingThe experimental apparatus

included a membrane treatment

unit with feed pressure supplied

by a multistage centrifugal pump

and controlled by a programmable

logic controller.

A

fi ltration

2008 © American Water Works Association

MOORE ET AL | 100 :12 • JOURNAL AWWA | PEER-REVIEWED | DECEMBER 2008 75

& Amy, 1998). The protonation and deprotonation of inorganic arsenic is controlled by pH. At typical pH values for drinking water (between 6 and 9), only three forms of arsenic are thermodynamically possible: two anionic species and one neutral species (Figure 1). The oxidized form, arsenate [As(V)], is found as anionic spe-cies (HAsO4

2– or AsO43–) whereas the reduced form,

arsenite [As(III)], is found as a neutral species (H3AsO3). This important characteristic of inorganic arsenic is exploited by many arsenic treatment techniques.

Several broad categories of treatment options are available to remove arsenic from drinking water. The first is adsorption, which requires media with a high adsorption capacity and an affinity for arsenic. Iron-oxide-coated media, activated alumina, and ion exchange resins are commonly used but are complicated by the need for media regeneration (Chwirka et al, 2000) or disposal. The second broad category is chem-ical precipitation. Iron- or aluminum-based coagulants are added to form hydroxides that either co-precipitate with arsenic or adsorb arsenic. The iron or aluminum hydroxide precipitates can then be removed through settling or filtration (Edwards, 1994). Finally, mem-brane treatment can also be used. Membranes are semi-permeable and allow the passage of water but are impermeable to various other constituents, depending on their pore size and chemical properties and the prop-erties of the substance. An application in which mem-

branes are commonly used is the treatment of hard or brackish water for potable use. Membranes are a prom-ising technology for arsenic treatment because of their simple operation and minimal chemical requirements compared with other technologies.

Studies (e.g., Brandhuber & Amy, 1998) have shown that nanofiltration (NF) and reverse osmosis (RO) mem-branes typically remove 100% of As(V) but only 60–80% of As(III). The limited As(III) removal provided by mem-brane filtration is not insignificant. Compared with As(V), As(III) is more toxic and more mobile in the envi-ronment (Smedley & Kimbrough, 2002; Pontius et al, 1994). For membrane filtration to be a successful treat-ment method, it must remove both As(III) and As(V).

To exploit the ability of membrane filtration to remove As(V), several researchers have suggested that a pretreatment step that oxidizes the As(III) to As(V) would improve the performance of membrane filtration (Seidel et al, 2001; Oh et al, 2000; Kartinen & Martin, 1995), but the authors of the current research are not aware of any investigations that have tested this hypoth-esis. Oxidation of arsenic using manganese dioxide (MnO2) has been explored by other researchers (Man-ning et al, 2002; Ghurye & Clifford, 2001), but use of this as an arsenic oxidation step has never been com-bined with membrane filtration.

Several oxidants are capable of oxidizing As(III) to As (V). Chlorine and potassium permanganate are effective

The author is shown

changing one of the

three types of filters

used in this study

(left). The filters

were chosen

to represent an

array of filtration

capabilities

and energy

requirements.



and rapid (Ghurye & Clifford, 2001), but NF and RO membranes have limited chlorine tolerance and potas-sium permanganate requires a feeding apparatus. Ultra-violet light and ozone are also effective but require large doses (Ghurye & Clifford, 2001) and are costly to imple-ment. MnO2 is an inexpensive, solid oxidizing media that has been shown to oxidize As(III) under a variety of conditions (Bissen & Frimmel, 2003; Manning et al, 2002; Tournassat et al, 2002; Bajpai & Chaudhuri, 1999; Nesbitt et al, 1998; Scott & Morgan, 1995; Driehaus et al, 1995; Moore et al, 1990; Oscarson et al, 1983), including flow-through column configurations (Ghurye & Clifford, 2001; Bajpai & Chaudhuri, 1999; Driehaus et al, 1995). Because of the inert nature of MnO2 media and its long life, simple operation, and compatibility with oxidant-sensitive membranes, MnO2 was chosen as the companion oxidant for the membrane filtration.

Many authors have defined the stoichiometric reaction between As(III) and MnO2 (Scott & Morgan, 1995; Moore et al, 1990; Oscarson et al, 1983) as shown in Eq 1:

MnO2 + H3AsO3 + 2H+ → Mn2+ + H3AsO4 + H2O (1)

A more recent study (Nesbitt et al, 1998) has shown that an intermediate form of manganese exists, and the reaction proceeds in two steps as shown in Eq 2:

2MnO2 + H3AsO3 → 2MnOOH* + H3AsO4

2MnOOH*+ H3AsO3 → 2MnO + H3AsO4 + H2O (2)

The effect of oxidation on arsenic removal by mem-brane treatment can be attributed to charge. As the charge of a membrane increases, the concentration of co-ions at the membrane surface decreases, and the con-centration of counter ions increases. Other factors being equal, the concentration of an ion at the surface of the membrane determines its flux through the membrane. Thus, for negatively charged membranes, the more nega-tively charged a species is, the better it will be rejected. Because the As(III) species is uncharged at the usual pH values of water, it will not be well rejected by negatively charged membranes. Once oxidized to As(V), the arsenic species has a charge of –2, which greatly reduces its con-centration at the membrane surface and its mobility

within the membrane and thus in -creases its rejection.

The current research sought to demonstrate that the introduction of a membrane-compatible arsenic oxi-dation step enables NF or RO sys-tems to treat waters contaminated with As(III) and meet the necessary regulatory requirement of 10 µg/L.

MATERIALS AND METHODSApparatus. Three membranes

were selected for evaluation. Mem-brane 11 is designed to provide re -moval of divalent ions, color, and total organic carbon (TOC). Mem-brane 22 is a tighter membrane de -signed to provide removal of diva-lent ions and some monovalent ions, color, and TOC. Membranes 1 and 2 are both NF membranes. Membrane 33 is a low-energy RO membrane designed to provide removal of both monovalent and divalent ions. The three membranes represent an array of filtration capabilities and energy require-ments (Table 1).

This research used prototype 200-mm (8-in.) spiral-wound mem-branes, all designed by the same company,4 which has a patent pending on the prototype mem-branes used. The prototype mem-

H3AsO30

1,200

800

400

FIGURE 1 Eh–pH diagram for inorganic arsenic

H3AsO30

H2AsO3–

0

–400

–800

20

15

10

5

0

–5

–10

–15 0 2 4 6 8 10 12 14

Eh

—m

V

pe

pH

H3AsO40

H2AsO4–

HAsO42–

HAsO32–

AsO33–

AsO43–

As(V) As(III)

Eh

—m

V

pe

Adapted from Smedley & Kinniburgh, 2002

As(III)—arsenite, As(V)—arsenate



brane has a modified feed spacer that creates a virtual five-stage membrane system within a single membrane element. This allows the element to achieve high recov-eries, while operating within the manufacturer’s recommended pa -rameters. Although this membrane was de veloped for whole home filtration, using such a membrane for piloting work ensures that the results can be replicated with full-scale systems.

Figure 2 shows a schematic of the setup and apparatus used. The membrane was housed in a stan-dard single-element, 8-in. mem-brane housing.5 The feed pressure was supplied by a multistage cen-trifugal pump6 able to produce 26 L/m at 700 kPa. All piping on the membrane system was schedule 80 polyvinyl chloride.

The membrane filtration sys-tem was controlled by a program-mable logic controller (PLC).7 The process conditions were mon-itored using on-line flow, pres-sure, conductivity, pH, and temperature sensors. The membrane operating conditions were logged by the PLC every 5 s.

The flux and recovery of the system were controlled using manually operated valves and monitored using on-line instrumentation. The recovery was varied from 70 to 85%, and the flux was varied between 15 and 25 L/m2/h. This represented a range of design conditions that would be expected for the average NF or RO plant operating on a groundwater feed. The experiment was conducted as a 22 factorial experiment, with center point replication. The order of the four experimental condi-tions was chosen randomly except for the center point condition (recovery of 77.5%, flux of 20 L/m2/h), which was investigated first and last.

For each experimental run, the membrane filtration system was allowed to operate for a minimum of 1 h before sampling. Each experimental run lasted between 3 and 4 h, during which the operating conditions were monitored and charted to verify that the operating conditions were stable. The performance of the mem-brane was compared with the results predicted by the manufacturer’s software,8 to confirm that the mem-branes were operating as predicted by the manufacturer (Moore, 2000).

New membranes were flushed (operated at 0% recov-ery) for 10 min at 20 L/min to remove any residual manufacturing chemicals or shipping preservatives.

Before any experiments, the membranes were conditioned by operating them at the center point condition (recovery = 77.5%, flux = 20 L/m2/h) for a minimum of 10 h. At the end of each experiment, the membrane was flushed for 10 min with 20 L/min of feed water to remove any concentrate from the membrane.

When not in operation, the membranes were stored in sealed plastic bags after being flushed with a solution of 5 mg/L of sodium metabisulfite (Na2S2O5). Na2S2O5 is used to scavenge oxygen, thereby limiting the growth of aerobic bacteria and preserving the membranes between experiments. Na2S2O5 can make the charge of the membrane more negative; however, within the pH range of 6 to 9, the influence of Na2S2O5 is not signifi-cant (Childress & Elimelech, 1996).

The MnO2 media9 used in this research contained 79–80% MnO2 by weight. In this study, the media size used was a –8/+20 mesh; the media were therefore between 0.8 mm and 2.3 mm. The porosity of the media was approximately 50%.

To validate the arsenic-oxidizing capabilities of MnO2, a pilot column was used. The pilot column was made from a clear acrylic pipe (100 mm in diameter and 1.5 m in length) and was loaded with 0.96 m of MnO2 on top of 100 mm of garnet sand. When the MnO2 was used in combination with membrane filtration, a larger fiberglass–reinforced plastic filter (with an internal diam-eter of ~ 430 mm) was used. As with the pilot column,

Salt Zeta Rejection* Permeability* Potential† Membrane Description % L/m2/h/bar mV

Membrane 1 “Loose” NF 40–60 13 –5 to –16

Membrane 2 “Tight” NF 85–95 6.7 –15 to –30

Membrane 3 “Low-energy” RO 99 7.1 –10 to –25

NF—nanofiltration, RO—reverse osmosis

*Source: Dow Liquid Separations, 2004†Source: Krueger, 2004; Manttari et al, 2004

TABLE 1 Characteristics of membranes used in experiments

Flow rate, pressure,

conductivity, pH


conductivity

Membrane


conductivity

Pump Prefilter Sample

Sample

Sample

Valve 1

Valve 2

Well water

Concentrate

Permeate

FIGURE 2 Membrane filtration system



the filter was loaded with 0.965 m of MnO2 on top of 100 mm of garnet sand.

Before operation, the MnO2 media were backwashed at a flux of 86 m/h (as recommended by the manufac-turer). The filter was backwashed until no discoloration was observed in the backwash water, a process that took between 2.5 and 3 h. The filter was then operated for 200 bed volumes to condition the filter and equilibrate arsenic adsorption capacity of the MnO2 (Ghurye & Clifford, 2001; Bajpai & Chaudhuri, 1999; Driehaus et al, 1995). Before each experiment, a minimum of 5 bed volumes were treated to allow the MnO2 to acclimatize to the hydraulic conditions (Ghurye & Clifford, 2001).

Sampling methodology. Quantifying As(V) and As(III) concentrations is challenging. Unless an arsenic separa-tion and preservation procedure is performed when the sample is taken, it is likely that the speciation of the arsenic will change before analysis. To prevent measure-ment errors caused by shifts in speciation, separation of the various forms of arsenic was performed in the field immediately after sampling. The separation of the sam-ple into aliquots of suspended arsenic [As(Suspended)], As(III), and As(V) analysis was performed using a field separation method similar to those described in the literature (Bednar et al, 2004; Lee et al, 2000; Miller et al, 2000; Edwards et al, 1998; Ficklin, 1983). Separa-tion of the As(Suspended) was performed by passing an aliquot of the sample through a 0.45-µm filter.10 The

filtered aliquot was separated into As(III) and As(V) using an anion exchange cartridge.11 Prefiltering the sample ensured that the anion exchange media were not prematurely exhausted by arsenic co-precipitates (Bednar et al, 2004; Lee et al, 2000; Edwards et al, 1998). Nitric acid was used for preser-vation. It was chosen over hydrochloric acid because it does not introduce addi-tional chloride, which might have inter-fered with the arsenic analysis (Edwards et al, 1998).

All arsenic concentrations were deter-mined using inductively coupled plasma/mass spectrometry (ICP/MS) according to method 200.8 (USEPA, 1991) by a private laboratory12 in Canada.

In the laboratory, the concentration of As(Suspended) was determined by digesting the filters with nitric and hydrochloric acids before ICP/MS anal-ysis. The total dissolved arsenic concen-tration [As(Dissolved)] was determined from the filtered aliquot and represented the combined concentration of As(III) and As(V). The As(III) concentration was determined from the aliquot that

passed through the ion exchange cartridge. The As(V) concentration was determined by subtracting the As(III) concentration from the As(Dissolved) concentration.

The minimum reporting limit (MRL) for suspended arsenic was 0.05 µg per filter. Because 100 mL of the sample water was filtered, this was equivalent to an MRL of 0.5 µg/L. None of the samples taken was reported to have As(Suspended) concentrations > 0.5 µg/L. These nondetect values were treated as 0 µg/L in the subsequent data analysis.

The MRL for the As(Dissolved) and As(III) concentra-tions was 1 µg/L. Some samples were reported as having concentrations of “< 1” µg/L. During the analysis of the results, these values were treated as having 1 µg/L of arsenic, which was a conservative estimate.

All samples were taken in triplicate, and each sample was analyzed once in order to estimate a 95% confidence interval. This 95% confidence interval is shown in Tables 1–5 and shown with error bars in Figures 3–5.

Site description. The research was conducted in Virden, Man., Canada, during fall 2004. The water was supplied to the pilot plant by the town’s wells located 6 km north-east of the town of Virden, in the valley of the Assini-boine River. The groundwater in Virden contains 38–44 µg/L of arsenic (60 samples), made up entirely of dis-solved arsenic; analyses were performed for particulate arsenic but none was detected. Raw water was sampled six times daily during the investigation to monitor any

Experiments were

conducted in the

town of Virden

(Man., Canada)

well pump house.



shifts in arsenic concentration. No statistically significant shifts (� = 0.05) were found during the study. Results of the sampling are shown in Figure 3. The majority of the arsenic in Virden’s groundwater was found in the reduced form, i.e., As(III), in concentrations ranging from 34 to 41 µg/L. As(V) concentrations were smaller, ranging from 3 to 7 µg/L.

Table 2 shows the average values (on the basis of 22 samples) for other raw water parameters mea-sured over the period of the experi-ments. For a groundwater, Virden water is relatively high in TOC, alkalinity, total dissolved solids, ammonia, and turbidity and rela-tively low in hardness.

RESULTSMembrane filtration without preoxi-

dation. The goal of this first experi-ment was to verify the arsenic removal of the RO and NF membranes used. The authors also investigated how membrane type and membrane oper-ating conditions affected the amount of arsenic removed.

For membranes 1 and 2, the arse-nic rejection (i.e., the percentage of arsenic removed by the membrane) was low. For both membranes, the 95% confidence interval included zero under most operating condi-tions. Thus, under the conditions tested, membranes 1 and 2 did not provide significant arsenic removal.

Membrane 3 was able to achieve arsenic rejections as high as 51% under certain conditions (flux of 25 L/m2/h, recovery of 70%). The effect of changing the operating conditions for this membrane showed that increasing the flux or decreasing the recovery increased the rejection of arsenic. This is consistent with accepted membrane operating theory, i.e., increasing the flux increases the pure water flow through the membrane without increasing the solute flux significantly, and decreasing the recovery decreases the solute flux. Both result in increased solute rejection.

The total arsenic concentration of the membrane permeate indicated the level of removal provided by each membrane (Table 4). The total arsenic concentration in the mem-

brane 1 permeate varied from 44 to 46 µg/L, depending on the operating conditions. The total arsenic concen-tration of the water treated using membrane 2 was not noticeably different from that treated with membrane 1. The total arsenic concentration in the water treated by membrane 3 was consistently lower than that treated

50

40

30

20

10

0

Phase A Phase B Phase C Phase D Phase E

Ars

enic

Co

nce

ntr

atio

n—

µg

/L

Total arsenic As(III) As(V)

FIGURE 3 Arsenic concentrations of Virden (Man.) well water during experimental work

As(III)—arsenite, As(V)—arsenate, MnO2—manganese dioxidePhase A: membrane 1 filtration studyPhase B: membrane 2 filtration studyPhase C: membrane 3 filtration studyPhase D: MnO2 validation studyPhase E: membrane filtration with arsenic preoxidation

Experimental were carried on Sept. 11, 2004, to Nov. 4, 2004; concentrations were measured daily (six replicate samples). Error bars are 95% confidence intervals.

Parameter Average Range

Turbidity—ntu 0.9 0.3–1.2

TOC—mg/L C 15.0 13.8–15.9

Alkalinity—mg/L as CaCO3 650 640–680

pH 8.3 7.9–8.4

Sodium—mg/L 409 392–420

Chloride—mg/L 236 230–244

TDS—mg/L 1,100 1,080–1,130

Hardness—mg/L as CaCO3 45.6 42.2–47.9

Iron—mg/L 0.58 0.48–0.64

NH3—mg/L as N 1.02 0.35–1.79

CaCO3—calcium carbonate, NH3—ammonia, TDS—total dissolved solids, TOC—total organic carbon

TABLE 2 Average concentrations of various parameters during experimental work



by membranes 1 and 2. Under all conditions, membrane treatment alone was insufficient to meet the regulatory target of 10 µg/L.

The operating condition of flux of 20 L/m2/h and recovery of 77.5% was repeated at the start and end of each experimental run to detect experimental drift. The results of the duplicate experimental points (shown in Tables 3 and 4) showed that performance essentially did not change throughout an experimental run.

The performance of the relatively “loose” NF mem-branes tested was poorer than suggested in the litera-ture. An earlier study using an older version of mem-brane 1 reported 50% rejection (Brandhuber & Amy, 1998), and arsenic rejection by membrane 1 was expected to be similar in the current research. However, results showed arsenic rejection using membrane 1 to be close to zero. The rejection of arsenic by membrane

2 was also much less than suggested by the literature. Other researchers cited doctoral thesis work that reported 90% rejection of As(III) using a membrane 2 product (Vri-jenhoek & Waypa, 2000).

The performance of the low-pressure RO membrane 3 was con-sistent with performance data for similar membranes. Previous work using a similar membrane13 re -ported its salt passage was 0.4%, less than half that of membrane 3 (1%), and the As(III) passage using this membrane was roughly half that of membrane 3 (Sato et al, 2002; Oh et al, 2000).

The differences between the results reported in the literature and the arsenic passage observed in the current study likely are related to the chemistry of the feed water. All of the work cited previously was con-ducted with synthetic waters or natural waters spiked with arsenic. Compared with the Virden water, which was found to have cation and anion sums of ~ 20 meq/L, these synthetic waters or natural waters had a low ionic strength. It is likely that the added ionic strength of Virden’s water reduced the rejection of arsenic.

Validation of preoxidant. The goal of this second experiment was to validate the arsenic oxidation capa-bilities of MnO2 and its rate of oxi-dation. Five trials were conducted in which the empty bed contact time

(EBCT) was varied between 1 and 8 min. The particulate arsenic, total dissolved arsenic, As(III), and As(V) con-centrations were measured after MnO2 treatment.

Particulate arsenic was not detected during the exper-iment. As shown in Figure 4, the arsenic in the feed water was primarily in the reduced form, As(III). After 1 min in the presence of MnO2, the primary form of arsenic was the oxidized form, As(V). At an EBCT of 2 min, the oxidation was nearly complete as the As(III) concentration dropped below detection levels. The con-sistent total arsenic concentration indicated that little arsenic was removed by the MnO2 filter. The concentra-tion of As(III) shown in the figure does not drop to zero because any nondetect results were interpreted conser-vatively as 1 µg/L. On the basis of these results, a con-tact time of 7 min was used as a conservative run time for the final experiment.

Recovery—% Flux Membrane L/m2/h 70 77.5 85

Membrane 1 25 43 ± 7 µg/L 41 ± 8 µg/L

20 46 ± 5 µg/L, 44 ± 4 µg/L

15 42 ± 5 µg/L 44 ± 4 µg/L

Membrane 2 25 43 ± 3 µg/L 44 ± 1 µg/L

20 40 ± 4 µg/L, 43 ± 2 µg/L

15 41 ± 1 µg/L 40 ± 2 µg/L

Membrane 3 25 21 ± 2 µg/L 27 ± 1 µg/L*

20 26 ± 0 µg/L, 28 ± 0 µg/L

15 26 ± 4 µg/L 33 ± 0 µg/L

*The condition of a flux of 25 L/m2/h and 85% recovery was not achievable. This experiment was carried out at a flux of 22 L/m2/h and 82% recovery.

TABLE 4 Total arsenic concentration of permeate from each membrane under various operating conditions

Recovery—% Flux Membrane L/m2/h 70 77.5 85

Membrane 1 25 1 ± 22 7 ± 18

20 –6 ± 13, –2 ± 20

15 6 ± 20 –2 ± 20

Membrane 2 25 4 ± 8 0 ± 6

20 9 ± 10, 3 ± 7

15 7 ± 4 9 ± 6

Membrane 3 25 51 ± 9 36 ± 10*

20 39 ± 9, 35 ± 9

15 39 ± 12 23 ± 17

*The condition of a flux of 25 L/m2/h and 85% recovery was not achievable. This experiment was carried out at a flux of 22 L/m2/h and 82% recovery.

TABLE 3 Total arsenic rejection under various operating conditions



Membrane filtration with arsenic preoxidation. The goal of the third experiment was to investigate the benefit of combining MnO2 pretreatment with membrane filtra-tion. An MnO2 filter was set up with the same hydrau-lic characteristics as the validation experiment but sized to provide the needed flow rate to the membrane filtra-tion apparatus. All the membranes were operated at a flux of 20 L/m2/h. The MnO2 filter was operated at an EBCT of 7 min, which provided ample time for oxida-tion of the As(III) to As(V), as determined from the previous experiment.

The effect of MnO2 pretreatment was quite dramatic. As noted previously, without pretreatment, membranes 1 and 2 were unable to reject any arsenic whereas mem-brane 3’s rejection capacity was ~ 50% at best (Table 3). Following pretreatment with MnO2, arsenic rejec-tion increased dramatically. Figure 5 shows the overall rejection of the combined MnO2–membrane system at differing recov-eries, using different membranes. Under both high and low recovery conditions, all three membranes removed the majority of the arsenic present (Table 5). The total arsenic concentrations of the treated water were < 4 µg/L.

The effect of oxidation on arsenic removal can be attributed to charge. Because the As(III) species that was predominant in experiment 1 is uncharged at pH values near 8 (as in this water source), it was not well rejected by the membranes used, all of which are negatively charged. Once oxidized to As(V), the arsenic has a charge of –2, which greatly reduced its concentration at the membrane surface and thus increased its rejection.

Once the arsenic was in the oxi-dized form, As(V), membrane perfor-mance was similar to that in the lit-erature (Sato et al, 2002, Oh et al, 2000; Vrijenhoek & Waypa, 2000; Brandhuber & Amy, 1998). On aver-age, the arsenic rejection by mem-brane 2 was slightly greater than that of membrane 3. This was not ex -pected because membrane 3 is a tighter membrane and able to remove more dissolved solids than membrane 2 can. The more negative surface charge of membrane 2 may account for this difference. However, the dif-ference in arsenic removal between membranes 2 and 3 was not statisti-

cally significant (Figure 5); therefore, the improved per-formance of membrane 2 may not be real.

CONCLUSIONKey findings. Pilot-scale research examined the removal

of naturally occurring arsenic (present at a concentration of ~ 40 µg/L) by membranes with and without a preoxi-dation step. Results led to the following conclusions:

• Without the preoxidation step, the NF membranes tested (membranes 1 and 2) removed statistically insig-nificant amounts of arsenic from the groundwater. This was attributed to the fact that most of the arsenic was present as As(III), a neutral species.

• Without preoxidation, the low-pressure RO mem-brane (membrane 3) removed significantly more arsenic than the NF membranes. None of the membranes tested was able to meet the regulatory target of 10 µg/L.

50

40

30

20

10

0 0 1 2 3 4 5 6 7 8

EBCT—min

Co

nce

ntr

atio

n—

µg

/L

Total arsenic As(III) As(V)

FIGURE 4 Arsenic speciation as a function of EBCT

As(III)—arsenite, As(V)—arsenate, EBCT—empty bed contact time

Error bars are 95% confidence intervals.

Recovery—%

Membrane 1 Membrane 2 Membrane 3

Treatment Stage 70 85 70 85 70 85

Feed well water—µg/L 41 ± 2 35 ± 7 40 ± 2 41 ± 2 41 ± 2 41 ± 2

MnO2 treatment—µg/L 35 ± 4 37 ± 5 35 ± 5 38 ± 2 36 ± 2 39 ± 1

MnO2 and membrane 3 ± 1 0 ± 3 1 ± 0 1 ± 1 1 ± 0 2 ± 0 treatment—µg/L

MnO2—manganese dioxide

TABLE 5 Total arsenic concentrations at different stages of treatment



• Use of an MnO2 filtration column provided effective arsenic oxidation. At EBCTs greater than 2 min, complete oxidation of As(III) to As(V) was observed although the total concentration of arsenic remained unchanged.

• Combining MnO2 preoxidation with RO and NF membranes significantly enhanced the removal of arsenic. All three membranes achieved arsenic rejection greater than 90% and reduced treated water arsenic concentra-tions to < 5 µg/L once the preoxidation step was intro-duced. This confirmed suggestions in the literature that a preoxidation step would improve the arsenic rejection of NF and RO membranes.

Recommendations. Treatment facilities that use mem-brane filtration (e.g., to treat hard or brackish water) should investigate the use of MnO2 as a pretreatment alternative if additional arsenic removal is required. The addition of this low-maintenance oxidation step, which is compatible with chlorine-intolerant membranes, will allow existing membrane treatment facilities to meet the regulatory target of 10 µg/L of arsenic.

In addition, this work identified issues that merit fur-ther research:

• The experiments conducted in this research were short term (eight weeks). The long-term operational stability of membrane filtration and MnO2 oxidation processes is unknown. Fouling will influence the charge and other sur-face properties of the membrane (Makdissy et al, 2007), enhancing or hindering its arsenic rejection performance depending on the nature of the foulants. The operational lifetime of an MnO2 arsenic-oxidizing bed should be quan-tified. Potential performance problems such as increased head loss associated with long-term MnO2 operation should also be investigated.

• Membrane type was shown to have a significant influ-ence on the passage of arsenic. A mechanistic investigation of the membrane characteristics (e.g., charge, permeability) that most influence arsenic passage would be valuable.

ACKNOWLEDGMENTFunding for this project was provided by the Natural

Sciences and Engineering Research Council of Canada in the form of an Industrial Research Chair at the University of Waterloo (Ont.) and Zenon Membranes Solutions. Current chair partners include American Water Canada Corp.; Brightwell Technologies Inc.; the Canadian cities of Brantford, Guelph, Hamilton, Ottawa, and Toronto; Conestoga-Rovers & Associates Ltd.; EPCOR Water Services; the Ontario Clean Water Agency; PICA USA Inc.; RAL Engineering Ltd.; the region of Durham (Ont.); the regional municipalities of Niagara and Waterloo (Ont.); Stantec Consulting Ltd.; and Zenon Membrane Solutions.

ABOUT THE AUTHORS Kenneth W. Moore (to whomcorrespondence should be addressed) is a project technologist at CH2M HILL, Level 3, 5 Queens Rd., Mel-bourne, VIC 3004, Australia;[email protected] has BS and MS degrees in engi-neering from the University of Water-

loo, Ont., Canada. He has more than nine years of experience in the water treatment industry and is cur-rently focusing on desalination and water reuse proj-ects. The research described here was undertaken while he was a research engineer for Zenon Environ-mental in Oakville, Ont., and simultaneously com-pleting his postgraduate degree. Peter M. Huck is a professor and Natural Sciences and Engineering Research Council of Canada Chairholder in Water Treatment at the University of Waterloo Department of Civil and Environmental Engineering. Steve Siverns is vice president of product development at EnviroTower Inc., Toronto, Ont.; at the time of this research, he was director of desalination technology at Zenon Membrane Solutions in Oakville.

FOOTNOTES1NF270, Dow Liquid Separations, Midland, Mich.2NF90, Dow Liquid Separations, Midland, Mich.3XLE, Dow Liquid Separations, Midland, Mich.4Zenon Membrane Solutions, Oakville, Ont., Canada5PRO-8-300EP, Protec, Vista, Calif.6CHN2-60, Grundfos, Oakville, Ont., Canada7Micrologix 1500, Allen Bradley, Milwaukee, Wis.8Reverse Osmosis System Analysis (ROSA), Dow Liquid Separations,

Midland, Mich.9Pyrolox, American Minerals, King of Prussia, Pa.10Swinnex® 25 filter holders, Millipore, Billerica, Mass.11Sep-Pak® Plus Accell Plus QMA cartridge, Waters Corp. Milford,

Mass.12RPC, Fredericton, N.B., Canada.13ES10, Nitto Denko, Fremont, Calif.

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100

95

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85

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70

65

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55 50

Ars

enic

Rem

oval

—%

70 85 Recovery—%

FIGURE 5 Total arsenic rejection after MnO2 pretreatment

Membrane 1 Membrane 2 Membrane 3

MnO2—manganese dioxide

Error bars are 95% confidence intervals.



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