microbially mediated clinoptilolite regeneration in a multifunctional permeable reactive barrier...

Microbially Mediated ClinoptiloliteRegeneration in a MultifunctionalPermeable Reactive Barrier Used toRemove Ammonium from LandfillLeachate Contamination: LaboratoryColumn EvaluationT H O M A S V A N N O O T E N , L U D O D I E L S ,A N D L E E N B A S T I A E N S *

Flemish Institute for Technological Research (VITO),Separation and Conversion Technologies, Boeretang 200,2400 Mol, Belgium

Received December 21, 2009. Revised manuscript receivedMarch 25, 2010. Accepted March 31, 2010.

This study focuses on multifunctional permeable reactivebarrier (multibarrier) technology, combining microbial degradationand abiotic ion exchange processes for removal of ammoniumfromlandfill leachatecontamination.Thesequentialmultibarrierconcept relies on the use of a clinoptilolite-filled buffercompartment to ensure a robust ammonium removal in caseof temporary insufficientmicrobialactivities.Aninnovativestrategywas developed to allow in situ clinoptilolite regeneration.Laboratory-scale clinoptilolite-filled columns were first saturatedwith ammonium, using real landfill leachate as well assynthetic leachates as feed media. Other inorganic metalcations, typically present in landfill leachate, had a detrimentalinfluence on the ammonium removal capacity by competingfor clinoptilolite exchange sites. On the other hand, the metalshad a highly favorable impact on regeneration of the saturatedmaterial. Feeding the columns with leachate deprived fromammonium (e.g., by microbial nitrification in an upgradientcompartment), resulted in a complete release of the previouslysorbed ammonium from the clinoptilolite, due to exchangewithmetalcationspresent intheleachate.Thereleasedammoniumis then available for microbial consumption in a downgradientcompartment. The regeneration process resulted in a slightlyincreasedammoniumexchangecapacityafterward.Thedescribedstrategy throws a new light on sustainable use of sorptionmaterials for in situ groundwater remediation, by avoiding theneed for material replacement and the use of externalchemical regenerants.

IntroductionDue to an ongoing increase in waste generation and a limitedwaste incineration capacity, landfilling is worldwide still themost common way of solid waste disposal (1). Extensiveamounts of landfill leachate, generally enriched in organicmatter, ammonium (NH4

+), and inorganic ions, are generateddue to rainwater infiltration and moisture release from thewaste (2). Leachates can lead to large groundwater con-tamination plumes, covering several to hundreds of hectares,

if they are not properly collected and treated (3). A sequentialmultifunctional permeable reactive barrier (multibarrier) wasrecently proposed by Van Nooten et al. (4) as an innovativeand semipassive in situ technology for remediation ofleachate-polluted groundwater and for direct leachate treat-ment during the aftercare period of old confined landfills(Supporting Information (SI) Figure S1). The multibarrierconcept, combining different reactive materials and con-taminant removal processes, was successfully demonstratedin a laboratory-scale column experiment for the removal ofammonium, adsorbable organic halogens, chemical oxygendemand, and toxicity from leachate originating from the 40-year-old Belgian landfill site Hooge Maey. This paper focuseson the ammonium removal concept, involving a combinationof microbial degradation (nitrification-denitrification) andabiotic ion exchange processes. Ammonium is microbiallyconverted to nitrate and nitrite (together called NOx

-) in afirst sand-filled nitrification compartment, equipped withdiffusive oxygen emitters and inoculated with diluted nitrify-ing sludge (SI Figure S1). A second compartment is filledwith granular clinoptilolite to remove remaining ammoniumconcentrations by ion exchange. The material is of specialinterest due to its low cost, the relative simplicity ofapplication and operation (5), and the suitability for use inPRBs (6). In this way, complete ammonium removal can beensured by abiotic processes in case of insufficient microbialactivity in the former compartment (e.g., during the start-upphase of the multibarrier, or as a consequence of ammoniumshock loads, variations in toxicity levels, and seasonaltemperature changes). The NOx

- formed in the nitrificationcompartment is microbially reduced to harmless nitrogen ina downgradient sand-filled denitrification compartment, fedwith an external carbon source and inoculated with diluteddenitrifying sludge.

To remain cost-efficient and competitive with conven-tional treatment technologies, a multibarrier must preserveits semipassive character, and rely on sustainable processesand materials with a high longevity. The major drawback ofion exchange materials such as clinoptilolite, however, is theneed for periodical replacements of the material aftersaturation, which renders the multibarrier technology lesspassive and less economically favorable. In addition, otherinorganic cations typically present in landfill leachate (e.g.,K+, Na+, Ca2+, Mg2+) may compete with NH4

+ for cationexchange sites and accelerate saturation, thereby reducingthe ammonium removal capacity of the clinoptilolite (7, 8).Disposal of ammonium-saturated clinoptilolite in the landfillwould create a vicious circle and would not lead to an effectiveremoval of ammonium from the landfill. Chemical regen-eration of saturated clinoptilolite (usually with a concentratedbrine solution) involves high operation and reagent costs,and still requires proper treatment of the ammonium-concentrated spent regenerant (9, 10). This study, however,describes a strategy which allows in situ regeneration ofammonium-saturated clinoptilolite mediated by nitrifyingbacteria, thereby avoiding the need for clinoptilolite removaland chemical regenerants. To become accessible to thebacteria, ammonium must first desorb and diffuse from thenanoscale clinoptilolite pores to the particle surface or intosolution (10). Ammonium desorption can occur due to shiftsin exchange equilibria when ammonium-poor leachate isflowing through the saturated clinoptilolite, as a result ofsufficiently high ammonium removal rates in the upgradientnitrification compartment (4). The released ammonium cansubsequently be degraded in another downgradient nitri-fication compartment as presented in SI Figure S1. In this

* Corresponding author phone: +3214335179; fax: +3214580523;e-mail: [email protected].

Environ. Sci. Technol. 2010, 44, 3486–3492

3486 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 9, 2010 10.1021/es9038616 2010 American Chemical SocietyPublished on Web 04/13/2010

way, the clinoptilolite compartment should be consideredas a buffer compartment which ensures ammonium removalwhen microbial activity is temporary insufficient, and whichautomatically regenerates when microbial activity is restored.

To investigate this microbially mediated regenerationpotential, laboratory-scale clinoptilolite-filled columns wereset up. The columns were first fed with ammonium-richlandfill leachate to determine the sorption capacity of thematerial. In a second stage, after saturation, the columnswere installed after a nitrification column to receive am-monium-free leachate. It was determined to which extentthe ammonium could be desorbed from the material. Thistwo-stage process was then repeated to evaluate the restoredammonium sorption capacity of the material after regenera-tion. In parallel, the same experiment was performed withtwo different synthetic leachates to determine the impact ofcompeting cations present in the leachate on the sorptionand regeneration capacity of the clinoptilolite.

Experimental SectionDescription of the Column Setup. An overview of the columnsetup is given in SI Figure S2. Five glass columns (height, 10cm; inside diameter, 2.4 cm) were dry-filled with granularclinoptilolite (Rota Mining, 1.0-2.0 mm, porosity 0.60) andflushed with CO2 for 30 min to promote total saturation withthe feed solution afterward. Three columns were operatedunder identical conditions and fed with leachate originatingfrom the 40-year-old Belgian landfill Hooge Maey. Theleachate was collected from a drainage well located in aconfined part (39 ha) of the landfill which is in themethanogenic phase, containing a heterogeneous mixtureof industrial and municipal wastes (4). Leachate was collectedin December 2008 and analyzed according to standardmethods. An overview of the physical and chemical char-acteristics is presented in Table 1. In general, the leachateis characterized by a near-neutral pH (7.6) and elevatedconcentrations of NH4

+ (244 ( 9 mg N L-1; 17.4 ( 0.6 meqL-1), AOX (1.9 mg Cl L-1), and dissolved organic carbon (DOC;

130 mg L-1). Major metal cations include potassium (K+: 260mg L-1; 7 meq L-1), sodium (Na+: 840 mg L-1; 37 meq L-1),calcium (Ca2+: 160 mg L-1; 8 meq L-1), and magnesium (Mg2+:110 mg L-1; 9 meq L-1). During the experiment, the landfillleachate was kept under nitrogen atmosphere in 5 L bottles.A fourth column was fed with a synthetic leachate preparedin Milli-Q water, containing ammonium (as NH4Cl) and themajor metal cations (Na+ as NaCl, K+ as KCl, Mg2+ as MgCl2,and Ca2+ as CaCl2) at concentrations similar to the landfillleachate (Table 1). A fifth column was fed with a syntheticleachate containing only ammonium (as NH4Cl) in Milli-Qwater (Table 1). The synthetic leachates were kept aerobicallyin 5 L bottles during the experiment. All the leachates werepumped in an upward flow through the clinoptilolite-filledcolumns at a flow rate of 4.2 ( 1.2 mL h-1, correspondingto a flow velocity of 0.37 ( 0.11 m day-1.

After saturation of the clinoptilolite material, the columnsfed with landfill leachate were periodically installed after anitrificationcompartmenttoreceiveammonium-freeleachate.The nitrification compartment was performed in a poly-acrylate column (height, 66 cm; inside diameter, 4 cm) andhas been described in detail by Van Nooten et al. (4). Briefly,the column was filled with coarse sand (1-2 mm) andinoculated with a 2-fold diluted sludge sample originatingfrom the aerated nitrification compartment of the landfillwastewater treatment plant. Oxygen was slowly released tothe column by using three diffusive oxygen emitters placedin series, each consisting of a 2 m long silicon tubing (2.0mm i.d., 4.0 mm o.d.) looped around a PVC cylinder (FigureS2). One end of the coils was connected via a manifold to apressurized gas cylinder, and the other end was connectedto a venting valve which opened eight times per day for 1min. By pressurizing the silicon tubing (0.2-0.5 bar), oxygencould diffuse through the tubing walls and dissolve into thelandfill leachate due to the imposed chemical gradient. Theoutlet port of the nitrification column was connected to acapped 12 mL vial. The column effluent was pumped fromthis vial into the clinoptilolite-filled column. The columns

TABLE 1. Chemical Composition of the Landfill Leachate and the Synthetic Leachates

columns 1-3 column 4 column 5landfill leachate synthetic leachatea synthetic leachatea

NH4+ mg N/L 244 ( 9 251 ( 9 250 ( 6

meq/Lb 17.4 ( 0.6 17.9 ( 0.6 17.9 ( 0.4AOX mg Cl/L 1.9DOC mg/L 130TOC mg/L 140TIC mg/L 380evaporation residue mg/L 3900total hardness mmol/L 6.6

K mg/L 260 280meq/L 6.7 7.2

Na mg/L 840 840meq/L 36.5 36.5

Ca mg/L 160 160meq/L 8.0 8.0

Mg mg/L 110 120meq/L 9.1 9.9

Al µg/L 51Ba µg/L 350Fe µg/L 2000Cu µg/L 2.5Pb µg/L <5.0Mn µg/L 390Si µg/L 17000Ag µg/L <5.0Zn µg/L 24S µg/L 100000

a Synthetic leachates were prepared in Milli-Q water. b Milliequivalents per liter.

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receiving synthetic leachate were switched over to anequivalent ammonium-free synthetic medium after satura-tion of the clinoptilolite was observed. After desorption ofammonium from the clinoptilolite was evaluated, this two-stage process was repeated. The whole column system wasoperated in a cooling room at a representative groundwatertemperature (12 °C).

Sampling Procedures and Analysis. Periodically, liquidsamples (∼5 mL) were collected from the influent bottlesand the outlet of the columns with glass sampling syringes.Ammonium, nitrate, and nitrite were analyzed spectropho-tometrically using Spectroquant cell tests (Merck). Dissolvedoxygen (DO) concentrations of samples originating from thenitrification column were determined with a Clark-typeelectrode model 781/781b oxygen meter (Strathkelvin In-struments). Larger sample volumes (∼100 mL) were takenafter certain time intervals to analyze metal concentrationsaccording to Dutch Standard Methods NEN-EN-ISO/IEC17025 (CMA/2/I/B.1) (11). Column effluents were continu-

ously collected in plastic bottles and weighed at eachsampling event to determine the mass flow through thecolumns.

ResultsAmmonium Adsorption Capacity. Ammonium removal inthe clinoptilolite columns was calculated by subtractingammonium influent concentrations with effluent concen-trations. During the first 4 days of operation, >97.5% of theincoming ammonium (∼244 mg N L-1; ∼17.4 meq L-1) wasremoved from the landfill leachate after passage throughcolumns 1-3 (Figure 1A). During this period, 0.41 L waspumped through the columns, which is corresponding to 18pore volumes (PVs). In the period thereafter, effluentconcentrations gradually increased indicating a breakthroughof the clinoptilolite material. After 16 days of operation (64PVs), the material was completely saturated, exhibiting atotal ammonium removal capacity of 6.62 mg N (0.47 meq)per g clinoptilolite. Ammonium removal occurred similarly

FIGURE 1. Influent and effluent ammonium concentrations for the columns fed with landfill leachate (A), synthetic leachatecontaining metal cations (B), and synthetic leachate without metal cations. (C), The sorption and regeneration capacities arevisualized by the shaded and the dotted areas, respectively.

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in the column fed with synthetic leachate containing am-monium (∼251 mg N L-1; ∼17.9 meq L-1) together with metalcations (column 4; Figure 1B). After an initially almostcomplete ammonium removal (99.4%), a gradual break-through could be observed after 13.5 PVs. Complete satura-tion was reached after 77 PVs, resulting in a total ammoniumremoval capacity of 6.34 mg N (0.45 meq) per g clinoptilolite,which is only slightly lower than the capacity determined forthe condition fed with landfill leachate. In contrast, a clearlyhigher sorption capacity could be observed for the columnreceiving the synthetic medium, containing only ammonium(∼250 mg N L-1; ∼17.9 meq L-1) and no metal cations (column5; Figure 1C). The ammonium removal was >96.8% duringthe first 41 PVs, followed by a slow breakthrough. After 180PVs, still 17.4% of the incoming ammonium could beremoved, indicating an ammonium removal capacity of morethan 17.40 mg N (>1.24 meq) per g clinoptilolite.

In addition to ammonium concentrations, influent andeffluent samples were regularly analyzed for concentrationsof major metal cations (Figure 2). The metal cations includedK+, Na+, Ca2+, and Mg2+, and were present in the landfillleachate and the metal-containing synthetic leachate atinfluent concentrations of approximately 7, 37, 8, and 9 meqL-1, respectively. After passage through the columns, bothleachates showed decreased effluent concentrations of K+

and Na+, indicating a partial retention of these cations onthe clinoptilolite material (Figure 2A and B). Similar to theammonium, the removal of K+ from the leachates decreasedin time. The Na+ effluent concentrations, however, did notshow a clear trend. Roughly estimated, ∼0.20 meq K+ and∼0.08-0.14 meq Na+ were removed per g clinoptilolite fromboth metal-containing leachates. On the other hand, thecolumn effluents contained increased Ca2+ concentrations,indicating that the adsorbed cations (NH4

+, K+, and Na+)were primarily exchanged with calcium cations present inthe clinoptilolite crystal structure. As expected, the releaseof Ca2+ decreased with the decreasing adsorption of othercations in time. Mg2+ cations were neither retained norreleased in the columns, as effluent concentrations remained

similar to influent concentrations. Column 5 was fed with asynthetic medium, containing only ammonium and no metalcations. Effluent samples contained primarily calcium cations(up to 13.5 meq/L) and to a lesser extent also the other metalcations (<2.3 meq L-1), indicating that NH4

+ was primarilyexchanged with Ca2+ (Figure 2C). Similar to the otherleachates, calcium release was proportional to ammoniumretention on the clinoptilolite. The total cation mass in theinfluent and effluent samples of the three different leachatesis presented in Figure 2. Mass balance calculations agreedreasonably well, with a maximal difference between influentand effluent total cation concentrations of 7.3%.

Ammonium Desorption Capacity. After 150 PVs, theammonium-saturated columns (1 and 2) fed with landfillleachate were periodically installed after the nitrificationcolumn to receive its ammonium-poor (3.1 ( 2.2 mg NL-1) effluent leachate. During the complete operationperiod, 98.9 ( 0.9% of the incoming ammonium wasmicrobially oxidized in the nitrification column, resultingin the formation of primarily nitrate and to a minor extentnitrite (SI Figure S3). Similarly, the ammonium-saturatedcolumn receiving the metal-containing synthetic leachate(column 4) was switched over to an equivalent ammonium-free synthetic medium after 122 PVs. Feeding the columnswith ammonium-free media resulted in ammonium-richcolumn effluents, indicating ammonium desorption fromthe clinoptilolite (Figure 1A and B). The release ofammonium was gradually decreasing in time, similarlyfor both metal-containing leachates. After the effluentconcentrations were reduced to approximately 45 mg NL-1, the columns (1 and 4) were again fed with the originalammonium-containing leachates. At this time, 5.08 mg N(0.36 meq) and 5.72 mg N (0.41 meq) per g clinoptilolitewere released from the columns fed with landfill leachateand the metal-containing synthetic leachate, respectively,corresponding to 77 and 90% of the initially adsorbedammonium. Column 5, fed with synthetic medium con-taining only ammonium and no metal cations, wasswitched over to an ammonium-free medium (i.e., Milli-Qwater) after 185 PVs when it was not yet completelysaturated. In contrast to the metal-containing leachates,only 0.31 mg N per g clinoptilolite was released, which iscorresponding to 1.8% of the initially adsorbed ammonium(Figure 1C).

The release of previously sorbed ammonium from thecolumns receiving metal-containing leachates occurred inparallel with the adsorption of metal cations from theleachates, indicating the exchange of ammonium cationswith metal cations in the clinoptilolite crystal structure (Figure3A and B). Metal cation uptake occurred similarly for thelandfill leachate and the synthetic metal-containing leachate.At the first sampling event, ammonium was primarilyexchanged with sodium, and to a minor extent with potassiumand calcium. Adsorption of potassium increased in time whileadsorption of sodium and calcium decreased, even resultingin a slight desorption at the latest sampling event. Magnesiumeffluent concentrations remained similar to influent con-centrations, indicating neither a release nor an uptake ofmagnesium cations in the columns. No metal cations couldbe detected in the effluent of the column fed with Milli-Qwater (column 5) (Figure 3C).

Second Adsorption and Desorption Periods. The two-stage process described above was repeated to evaluate towhich extent the ammonium sorption capacity of the materialwas restored after regeneration. Feeding the clinoptilolitecolumns again with the original ammonium-containingleachates resulted in adsorption of ammonium (Figure 1). Inthe columns fed with landfill leachate (column 1) and themetal-containing synthetic leachate (column 4), 78-81% ofthe incoming ammonium was removed during 11 PV’s (Figure

FIGURE 2. Overview of ammonium and metal cationconcentrations, initially measured in the three differentleachate influents (black bars) and measured in the columneffluents after four different time intervals during the firstadsorption period (other bars). The number of pore volumes(PVs) run at the moment of sampling is given in the legends,together with the total cation concentrations (meq/L).


1A and B). In the period thereafter, ammonium effluentconcentrations gradually increased in both columns, indi-cating a breakthrough of the clinoptilolite material similarto the first adsorption period. Complete ammonium-satura-tion of columns 1 and 4 was reached, respectively, 103 and111 PVs after the columns were switched over to the originalammonium-containing media. During this period, 5.23 mgN (0.37 meq) and 6.16 mg N (0.44 meq) per g clinoptilolitewas adsorbed in columns 1 and 4, respectively, correspondingto 103% and 108% of the ammonium that was released in thepreceding desorption period. Adsorption of ammonium fromthe leachates occurred in parallel with the release of metalcations from the clinoptilolite crystal structure (Figure 4). Ata first sampling event, ammonium was exchanged withsodium and calcium. At a later sampling event, ammonium

was exchanged with potassium. Similar to the previousadsorption and desorption periods, no magnesium cationswere exchanged. In column 5, where no substantial am-monium release was observed during the desorption period,only 28% of the incoming ammonium was removed whenthe column was fed again with the original ammonium-containing medium without metal cations. During the next203 PVs, ammonium effluent concentrations graduallyincreased to 234.5 mg N L-1, which is corresponding to 94%of the ammonium influent concentrations. Taking intoaccount the slight ammonium release during the desorptionstep, 20.6 mg N (1.47 meq) per g clinoptilolite was adsorbedin column 5 during the complete operation period, i.e. bothsorption steps.

After ammonium-saturation of columns 1 and 4 (after330 and 311 PVs, respectively), the columns were fed againwith equivalent ammonium-free leachates during a seconddesorption period (Figure 1A and B). Similarly to the firstdesorption period, the previously sorbed ammonium wasreleased from the clinoptilolite and column effluent con-centrations gradually decreased during 159-161 PVs toalmost zero (<2.6 mg N L-1). During this period, 6.85 mg N(0.49 meq) and 7.28 mg N (0.52 meq) per g clinoptilolite werereleased from the columns fed with landfill leachate and themetal-containing synthetic leachate, respectively.

DiscussionUnlike common permeable reactive barriers (PRBs) (12), amultibarrier combines different reactive materials andcontaminant removal processes, required to remove acomplex mixture of pollutants. Multibarrier applications havebeen recently reported for the remediation of various mixedgroundwater contamination plumes (13–15) and the treat-ment of landfill leachate contamination (4). This study focuseson an innovative and sustainable multibarrier concept forthe removal of ammonium from landfill leachate. Ammoniumis released from landfill waste mainly by decomposition ofproteins, and is the most significant leachate contaminanton the long-term (2, 3). Nitrification-denitrification processesare sensitive to variable parameters, including temperature,O2 concentrations, and toxicity levels, among others (16–18),and cannot continuously guarantee a sufficient ammoniumremoval under environmental conditions. The complemen-tary use of clinoptilolite as an ion exchanger, typicallyunaffected by these parameters, is therefore beneficial toensure a robust ammonium removal.

Ammonium Exchange Capacity and Impact of Metals.Clinoptilolite [theoretical formula: (Na+,K+)6(Al6Si30O72) ·20H2O]is a naturally occurring zeolite mineral consisting of analuminosilicate framework with loosely held exchangeablecations (typically Na+, K+, and Ca2+) (19). Because of its porousand rigid structure, and its high affinity for NH4

+, heavy metals(e.g., Cu2+, Ag+, Zn2+, etc.), and radioactive cations (Cs+, Sr2+)(20), the mineral has been considered as a suitable PRB material(4, 6, 21). Clinoptilolite has a theoretical cation-exchangecapacity (CEC) of 2.2 meq g-1, although the effective CEC isusually lower due to the presence of mineral impurities anddepending on experimental conditions, such as the initialaqueous cation concentration and the cationic site distributionwithin the zeolite (19, 22). The clinoptilolite used in this studyexhibited a total CEC of 1.47 meq g-1, in the presence of asynthetic leachate containing only ammonium. This is corre-sponding very well to the CEC reported by Cooney et al. (8) (1.5meq g-1) who used the same medium in a batch experiment,though with a higher ammonium concentration (645 mg NL-1).

Feeding the columns with media containing metalcations resulted in a more than 3 times lower totalammonium exchange capacity (0.45-0.47 meq g-1), whichindicates a strong competition effect of the metal cations

FIGURE 3. Overview of ammonium and metal cationconcentrations, initially measured in the three differentleachate influents (black bars) and measured in the columneffluents after three different time intervals during the firstdesorption period (other bars). The number of pore volumes(PVs) run at the moment of sampling is given in the legends,together with the total cation concentrations (meq/L).

FIGURE 4. Overview of ammonium and metal cationconcentrations, initially measured in the landfill leachate andthe synthetic metal-containing leachate, and measured in thecolumn effluents after two different time intervals during thesecond adsorption period. The number of pore volumes (PVs)run at the moment of sampling is given in the legends, togetherwith the total cation concentrations (meq/L).


for exchange sites on the clinoptilolite. This is in agreementwith literature data where up to 10-30% reduced am-monium removal capacities were reported, due to thepresence of metal cations in wastewaters (7, 22). Clinop-tilolite generally exhibits a very high preference forammonium over calcium, magnesium and sodium ions,but not over potassium (5, 19). The selectivity order forammonium and potassium is temperature dependent, witha preference for ammonium at 4 °C and a preference forpotassium at 40 °C, due to changing thermodynamicequilibrium constants (23). Metal cation analyses per-formed in this study revealed that, in addition to am-monium, primarily potassium and sometimes sodium wereretained from the leachates by the clinoptilolite. However,estimations of the total amount of retained metal cationsindicate that competition for cation exchange sites mayonly have partially (∼30-35%) accounted for the observedammonium exchange capacity reduction. The contributionof many other compounds present in the landfill leachate(e.g., organic material) may seem plausible to fully explainthe reduced capacity. However, similar observations weremade in the presence of the synthetic leachate, containingno other substances than the four metals. Therefore, themetal cations have, in addition to occupying exchangesites, likely another yet unidentified effect on the am-monium removal capacity of the clinoptilolite.

In Situ Regeneration Potential. Typically, only a fractionof a zeolite’s CEC is utilized before the breakthroughconcentration of ammonium is reached in a dynamicexchange system such as flow-through columns (20). Indeed,a tardily progressing breakthrough was observed in thecolumns. Considering the regulatory target limit of the landfillwastewater treatment plant (40 mg N L-1) (4) as a practicalbreakthrough concentration, significantly reduces the usableexchange capacity of the clinoptilolite. In the presence of asynthetic leachate containing only ammonium, effluentconcentrations reach the regulatory target limit when only0.64 meq g-1 is retained on the clinoptilolite, which iscorresponding to 44% of the total exchange capacity. Metalcations, typically present in landfill leachate (3), even reducethe useable exchange capacity to 0.31 meq g-1, whichemphasizes the need for frequent replacements of saturatedclinoptilolite material. However, in contrast to this apparentdetrimental influence, the presence of metal cations in theleachates is highly wanted and even required to allow in situregeneration of the clinoptilolite. Ammonium was desorbedfrom the saturated clinoptilolite when landfill leachate,deprived from ammonium by microbial nitrification in anupgradient compartment, was flowing through the column.Our results indicate a complete ammonium desorption asa consequence of the exchange with primarily sodium andpotassium ions from the landfill leachate. Similar observa-tions were made for the synthetic metal-containing leachate,while no substantial ammonium desorption could be ob-served in the presence of the synthetic leachate without metalcations. The regeneration process did not negatively affectthe ammonium exchange capacity of the clinoptilolitematerial. On the contrary, the process resulted in a slightcapacity increase, likely due to a changed cationic sitedistribution within the zeolite, which was shifted more inthe direction of a sodium form (8, 24). It is well-known thatsome of the zeolite cations can be removed less easily thanothers, because of a lower mobility and stronger bondingforces within the structure of the material. Clinoptilolite istherefore often preconditioned with a concentrated NaClsolution to improve its effective exchange capacity byconversion to a homoionic form of more easily removablesodium ions (19, 24).

Regeneration of ammonium-saturated zeolites by nitrify-ing bacteria has been reported earlier by other researchers.

Semmens and Goodrich (10) observed that 80% of previouslysorbed ammonium could be microbially consumed ininoculated batch experiments. Lahav and Green (9) dem-onstrated a two-phase reactor system where ammonium wasfirst removed from wastewater on a chabazite-filled column.In a second phase, ammonium was desorbed using a cationcontaining regenerant solution, followed by microbial deg-radation in a fluidized bed reactor mode with the chabaziteacting as a biofilm carrier. This study is the first to describein detail the potential of microbially mediated clinoptiloliteregeneration in a multifunctional PRB, thereby throwing newlight on the sustainable use of sorption materials for in situgroundwater remediation. Particularly for this kind of semi-passive treatment technologies, it is important that frequentreplacement of reactive materials can be avoided to remainsustainable and cost-efficient. The combinational use ofnitrifying microbial activity and ion exchange on clinoptilolitenot only ensures a robust ammonium removal in case oftemporary insufficient microbial activities, but also allowsin situ regeneration of clinoptilolite once the microbial activityrecovers. The released ammonium is then available formicrobial consumption in a downgradient compartment. Amajor advantage of the described strategy is that no externalregenerant solution has to be used, due to the typically highsalt concentrations in landfill leachate plumes (3). As analternative to the sequential concept, the two differentammonium removal processes can be performed in a mixedmultibarrier configuration where clinoptilolite and nitrifyingbacteria are combined in one compartment (25). However,a single nitrification compartment without clinoptilolite isstill needed downgradient from the mixed compartment tooxidize the ammonium released from the most downgradientpart of the clinoptilolite, and to ensure ammonium-freemultibarrier effluents.

AcknowledgmentsThis work was cofunded by the EU Life project MULTIBAR-DEM (LIFE06 ENV/B/000359). We thank N. Hermans fortechnical support to this study, G. Ayvazoglu (Rota Mining)for kindly providing the clinoptilolite material, and all thepartners of the MULTIBARDEM project for the fruitfuldiscussions.

Supporting Information AvailableSchematic overviews of the multibarrier concept and thecolumn setup, graph showing microbial ammonium removalin the nitrification column. This material is available free ofcharge via the Internet at http://pubs.acs.org.

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