remediation of water contamination using catalytic technologies

Applied Catalysis B: Environmental 41 (2003) 15–29

Remediation of water contamination usingcatalytic technologies

Gabriele Centi, Siglinda Perathoner∗Department of Industrial Chemistry and Engineering of Materials, University of Messina, and INSTM,

Consortium for the Science and Technology of Materials, Salita Sperone 31, 98166 Messina, Italy

Received 12 October 2001; received in revised form 4 July 2002; accepted 4 August 2002

Abstract

Remediation of contaminated ground and underground water is becoming a critical issue in Europe and worldwide. Wediscuss here the role of catalysis in water remediation, with reference to two specific examples of catalytic water remediationtechnologies: (i) the elimination of nitrate and pesticides from water contaminated as a result of agricultural practices and (ii)the conversion of methyltert-butyl ether (MTBE) in contaminated underground water. Of particular interest is a technologybased on catalytic membranes for remediation of water contaminated by nitrate, which offers various advantages with respectto conventional technologies. Using a Pd-Cu-based catalytic membrane, a reaction temperature below 15◦C, a mixed 4:1CO2:H2 feed and controlling bulk solution pH by HCl addition, it is possible to obtain a nitrate conversion higher than 80%even with ammonium ion formation below 0.5 ppm, i.e. the maximum concentration allowed to meet the requirements fordrinking water quality. In MTBE conversion in contaminated underground water, acid zeolites with suitable pore structures(channel structure and pore openings) such as H-ZSM-5 and H-BEA can be used as catalytic permeable reactive barriers forin situ remediation. These zeolites not only act as adsorbents for both MTBE and its reaction products, but also effectivelycatalyze the hydrolysis of MTBE tot-butyl alcohol (TBA) and methanol (MeOH) which then can be rapidly biodegraded byindigenous microorganisms.© 2002 Elsevier Science B.V. All rights reserved.

Keywords:Remediation; Water contamination; Catalytic technologies; Catalytic membrane; Nitrate removal; MTBE

1. Introduction

Recent reports of the European EnvironmentAgency (EEA) evidenced that the increasing waterpollution and deterioration of aquatic habitats areseverely hampering the use of water for human con-sumption and wildlife[1]. Inadequate amounts ofwater or poor water quality create a conflict betweenhuman demand for water and wider ecological needs.Water pollution may be classified according to point

∗ Corresponding author. Fax:+39-090-391518.E-mail address:[email protected] (S. Perathoner).

and diffuse sources. While for the former varioustechnological options are available, fewer solutionsare possible in the case of diffuse pollution. Reme-diation of groundwater contaminated by agriculturalpractices (nitrate and pesticides, in particular), leakageof non-biodegradable compounds from undergroundfuel tanks and pipelines, and accidental spills andleaking of cleaning solvents and degreasers is thusbecoming a major problem worldwide[2].

In many areas of the world the simultaneous (i) low-ering of water tables, (ii) increasing use of fertilizersand pesticides and (iii) contamination by chemical andnon-chemical products has significantly reduced the

0926-3373/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0926-3373(02)00198-4

16 G. Centi, S. Perathoner / Applied Catalysis B: Environmental 41 (2003) 15–29

fraction of fresh water which can be used for humanpurposes. As a consequence, it is estimated that themarket for water remediation technologies, e.g. tech-nologies to treat contaminated water which can bringit to drinking water quality for human use, will doublein the next 5–10 years. There are thus social, ecolog-ical and economic driving forces which stimulate thedevelopment of new water remediation technologies.

Current available technologies are often inadequateto meet economic and ecological demands, but inaddition the commercial technologies often requirelarge centralized treatment units. In many cases, wellsserve small local communities which cannot be con-nected to centralized water treatment units. It is thusnecessary to develop technologies which are compact,transportable and easily manageable. Catalysis playsa central role in developing this kind of application.

Discussed here are two aspects of the use of cat-alysts to improve water remediation technologies: (i)the elimination of nitrate and pesticides from watercontaminated as a result of agricultural practices and(ii) the conversion of methyltert-butyl ether (MTBE)in contaminated underground water.

2. Experimental

2.1. Catalyst preparation

A Pd-Cu/TiO2 powder catalyst (5:2:93 weightratio) was prepared in three steps. First the puretitania was prepared by the sol–gel method, withdrop-by-drop addition of a H2O–CH3COOH solutionto a Ti-isopropoxide solution in anhydrous ethanol(1:5). The gel was aged, washed, slowly dried inan oven at 90◦C and then calcined at 500◦C. XRDcharacterization of the resulting material indicatedthe presence of only crystalline TiO2 (anatase). Thesurface area is about 60 m2/g. In the second step, Cuwas added to the titania by incipient wet impregna-tion using a solution of Cu2+-acetate solubilized ina solution of acetic acid and ethanol. After drying,palladium was added in a third step using the samemethodology and Pd-acetate as the reagent. Afterdying, the catalyst is calcined slowly increasing thetemperature up to the final value of 520◦C (3 h).Then, the catalyst was reduced in a flow of H2 for 2 hat 135◦C.

A tubular membrane catalyst (Pd-Cu/TiO2/Al2O3)was prepared starting from commercial macroporousalumina tubes (from HITK e.V., Germany) that werethen coated with a titania membrane after prior clean-ing and degreasing of the tube. The inner diameter ofthe tube is about 4 mm and the macroporous aluminawall thickness about 1 mm. Tubes of 10 cm lengthwere used for the tests. The membrane was preparedby repeated dip-coating using a sol-type solution oftitanium-isopropoxide prepared as described above.The dip-coating was carried out in a dehumidified airatmosphere. The estimated thickness of the membranelayer is 10 mm. After aging at room temperature andcareful drying up to a temperature of 150◦C, the tubu-lar membrane was calcined at 500◦C. After impreg-nation with Cu and then Pd salts as described above,the tubular membrane was dried, calcined in air at500◦C for 3 h and finally reduced in hydrogen flow at135◦C. The estimated total amount of the Pd-Cu/TiO2active phase for the whole 10-cm long tube is about100 mg.

Commercial zeolites in the acid form were usedfor the MTBE conversion: (a) HZ525 (H-ZSM-5,SiO2/Al 2O3 = 25 from Alsi-Penta), (b) HBEA25(H-Beta, SiO2/Al 2O3 = 25 from Zeolyst), (c) HZ580(H-ZSM-5, SiO2/Al 2O3 = 25 from Zeolyst), (d)HMOR15 (H-Mordenite, SiO2/Al 2O3 = 15 fromAlsi-Penta) and (e) HY30 (H-Faujasite, SiO2/Al 2O3 =30 from Zeolyst).

2.2. Catalytic tests

2.2.1. Reduction of nitrateThe apparatus for the catalytic tests in the reduc-

tion of nitrate is schematically shown inFig. 1. Theapparatus allows operation under different configu-rations and the use of catalysts in different forms:powder, pellets or tubular membrane. Catalytic testswith the catalyst in the form of powder were madeat room temperature in a slurry batch reactor (SBR)equipped with a pH control/regulation apparatus, N2inlet to allow prior deoxygenation of the solution,H2 inlet, outlet and sampling port as well as a mix-ing system. The catalyst is suspended in deionizedwater (100 ml), saturated with hydrogen and then aconcentrated KNO3 solution is added in the amountnecessary to reach a 50 mg/l NO3

− concentration.Samples were periodically taken from the suspension,

G. Centi, S. Perathoner / Applied Catalysis B: Environmental 41 (2003) 15–29 17

Fig. 1. Scheme of the apparatus used for the catalytic tests of nitrate reduction with illustrations of the different ways of using the reactorand apparatus.

filtered and analyzed in terms of the NO3−, NO2

−and NH4

+ concentrations by ion chromatography.Catalytic activity tests on tubular membrane sam-

ples were made using two different techniques:

(i) A recirculation technique in which the nitrate so-lution was flushed over the tubular catalysts andrecirculated through a reservoir. H2 or a mixtureof CO2/H2 is fed through the catalytic membrane.Typical LHSV through the membrane reactor is370 h−1.

(ii) A continuous flow technique in which the solutionis continuously flushed over the tubular catalystand H2 or a mixture of CO2/H2 is fed through themembrane. Typical LHSV through the membranereactor with continuous flow is 60 h−1.

2.2.2. Methyl tert-butyl ether conversionTests of MTBE conversion using 0.5 g of catalyst

were made in static conditions in a batch-type reactorwith 50 ml of an aqueous solution containing MTBE(in a typical concentration of 1000 mg/l).

The analysis of MTBE was made by gas chro-matography using a mass quadrupole detector (tracefrom Thermoquest). A CP Porabond Q Fused Silica(25 m×0.32 mm) column and a three step temperatureramp were used. The MTBE,t-butyl alcohol (TBA)and methanol (MeOH) concentrations were followedby recording the intensity of the signal at mass 73(MTBE), 59 (TBA) and 31 (MeOH), after checkinglinearity of the response in the target concentrationrange.

3. Results and discussion

3.1. Nitrate reduction

3.1.1. Advantages and when to use a catalytictechnology for water remediation after nitratecontamination

Excess intake of nitrate ions can be harmful, be-cause nitrate can be easily reduced to nitrite in the


intestines causing serious health problems, particu-larly for infants (blue baby syndrome). Furthermore,nitrites are the precursors to the carcinogenic ni-trosamines as well as to otherN-nitroso compounds.Present European Union limits for nitrate and nitriteconcentrations in drinking water are 50 and 0.1 mg/l,respectively, but new European guidelines set the valuefor nitrate at 25±5 mg/l. Several methods to eliminatenitrates in drinking water are available at present. Thetwo most used methods are (i) physicochemical pro-cesses (ion exchange and membrane techniques), and(ii) biological processes (heterotrophic or autotrophictechniques, the latter being the most widely used).

The alternative possibility is to employ catalytic re-duction of nitrates[3–16]. Nitrate ions are reduced ona supported Pd-Me catalysts, where Me is Cu, Sn orother transition metals, using H2 as the reductant. Thetechnology allows a virtually waste free process, and

Table 1Comparison of the characteristics of technologies for water remediation after nitrate contamination

Technology Ion exchange Reverse osmosis Biological denitrification Catalytic reduction

Fate of nitrate Adsorbed andconcentrated

Concentrated in awaste stream

Transformed to N2 Transformed to N2

Waste Waste brine Waste brine Bacteria sludge NoneChemical additives Sodium chloride Sulphuric acid and

baseEthanol and phosphoricacid

H2

Percentage of efficiencyin water purification

85–98% 75–80% 98% 98–100%

Flexibility in variableoperations

Medium Medium Low High

Energy use Low High Medium LowSpace requirements Limited Limited High LowMovable Yes Yes No YesManageability Good Good Poor GoodType of operations Periodic

regenerationContinuous Continuous Continuous

Sensitivity to deactivation Medium High High MediumAutomatic control Simple Simple Complex SimpleStart up time Immediate Immediate Up to 1 month ImmediateMonitoring required Little Little Intensive MediumSelectivity of the process Low Low High HighOdors No No Yes NoNoise Some High None NoneIndicative costa ( /m3) 0.15–0.25 0.4–0.6 0.2–0.3 0.25–0.55Sensitivity of costs to

scale-downMedium High High Low

Multipurpose useb None Depending onmolecular size

Somec Highly effective

a Estimated costs for a 1000 m3/d process.b Removal of pesticides and other halogenated compounds.c Sensitivity to pesticides.

can be scalable from small to large application. Thereis no noise production and the energy requirementsare relatively low.

An overview of the advantages and comparativecharacteristics of these technologies for water remedi-ation after nitrate contamination is reported inTable 1.An estimation of the costs for the technologies is alsogiven, but it must be remembered that the cost variesconsiderably depending on the amount of water to betreated, the composition of the water, etc. The sensi-tivity to down-sizing is also given, in light of the par-ticular need for technologies to treat relatively smallvolumes of water.

When choosing a technology to be used for waterremediation after nitrate contamination and to be usedin a small community or agricultural area, catalyticreduction offers better balanced advantages in termsof eco-compatibility, manageability and multiple use


(for example, ease of movement from one well to an-other), although the cost may be slightly higher. Fur-thermore, the same technology can be applied to thesimultaneous removal of pesticides and halogenatedcompounds, while the other technologies cited aboveare not effective or of little effect. When water reme-diation requires the contemporaneous removal of ni-trate and halogenated compounds, and it is necessaryto treat small water sources (less than 100–200 m3/d),the catalytic technology appears to be the preferablesolution.

3.1.2. Questions to be resolved in the use of catalytictechnology for water remediation

The technology of catalytic nitrate reduction is stillnot at a commercial level due to some drawbacks: (i)ammonium ions form as by-product of the reductionin an amount higher than the allowed limit (0.5 mg/lin EU countries), (ii) use of a catalyst in the form ofsuspended powder causes problems in catalyst recov-ery, and water contamination by suspended particlescontaining noble metals (limits for noble metals indrinking water are very low), and (iii) reactor cost andsafety of operations.

The chemistry of the reaction[3–16] can be sum-marized as follows:

2NO3− + 5H2 → 4H2O + N2 + 2OH− (1)

NO3− + 4H2 → H2O + NH4

+ + 2OH− (2)

The reduction of nitrate ions (which pass throughthe intermediate formation of nitrite ions) produceshydroxide ions which cause an increase in pH favoringthe rate of reaction (2) over reaction (1). Although theincrease in pH in the bulk of the solution can be com-pensated for by addition of an acid, the high reactionrate of nitrate reduction may cause a local increase inpH near to the catalytic site. Indeed, the local pH is de-termined by the high rate of hydroxide ion formationand low rate of diffusion or transport far from the cat-alyst surface. When large catalytic particles are used(some mm as those necessary for fixed bed reactoroperations in a continuous process), the presence ofintraparticle limitations on back-diffusion of hydrox-ide ions make even more critical the question of localincreases in pH and the related increase in the rate ofreaction (2) over reaction (1).This causes lowering ofthe selectivity to N2 and an increase in ammonium ion

concentration. While the effect of pH on the selectiv-ity of nitrate conversion has been reported in the lit-erature[3–18], the distinction between the pH of thebulk of the solution and local pH near to the catalystsurface is not clear. This concept is demonstrated inthe following section.

An additional problem regards the low solubility ofH2 in water. The Pd must operate in a reduced state tobe selective, but competition can exist between rate ofcatalyst surface oxidation by nitrite or nitrate ions andreduction by H2, a faster reaction but one limited bythe low H2 solubility. Higher than atmospheric pres-sures can improve H2 solubility, but an autoclave re-action must be used which considerably increases thecosts of the technology. This aspect is also not clearin the literature and will be discussed in the followingsections.

In order to overcome these limitations, a new con-figuration based on a tubular membrane reactor (TMR)can be used (seeFig. 1) [4,17]. The liquid phase isphysically separated from the gas phase by using atubular porous ceramic membrane. Hydrogen gas dif-fuses from the inner side of the tube through the porouswall to the catalyst which is deposited on the outer sur-face (in direct contact with the water), where the reac-tion takes place. The gas flow through the membranecan be controlled by adjusting the pressure differentialbetween the gas side and the liquid side, thus, provid-ing a simple method for tuning the catalytic activity.

The membrane is based on commercial macrop-orous alumina tubes having the requested mechanicalproperties and coated with a thin layer of a suitable ox-ide (primary support: alumina, titania, zirconia), act-ing both as the membrane layer and as the support forthe catalytically active components. The active com-ponent is a Pd-Me catalyst, where Me= Cu, Ce, Sn,and the Pd:Me ratio is in the 2–4 range, in agreementwith literature data[3–17].

This TMR design offers various advantages overslurry type or fixed bed reactor configurations: (i) itimproves the three-phase contact (H2/nitrate in wa-ter/solid catalyst), thus, avoiding diffusion problemsand providing a positive influence on the selectivityof the reaction, (ii) it allows fine tuning of the reac-tion rate to meet variable operating conditions (nitrateconcentration, rate of water flow, etc.) by simplymodifying the pressure differential across the diffusermembrane, (iii) it avoids the formation of suspended


particles in water, catalyst failure by attrition, etc.,(iv) it allows an easy scale-up of the technology dueto the absence of moving parts, etc., and (v) it lim-its safety problems by having separate H2 and waterflow. Although preliminary water disinfection by UVradiation is necessary, deactivation caused by growthof biomass over the catalyst may be expected. Theuse of a ceramic-based membrane allows easy regen-eration by oxidation as discussed later, while the useof hollow-fiber membranes filled with fine-grainedcatalysts (another possible technological solution tothe previously mentioned problems) does not allowthis regeneration thus preventing their practical use[18,19]. Furthermore, the use of TMR offers ad-vantages in terms of robustness, manageability andpossibility of automatic control, important aspects fora water remediation technology to be used by peoplewho are not experts.

3.1.3. Role of the control of pHAs outlined above, the pH has a relevant influence

on the catalytic behavior in nitrate reduction, but lessclear is the role of bulk versus local pH and the re-lationship with the presence of intraparticle diffusioneffects. In order to clarify this aspect, a series of testsusing different particle dimensions were carried outand the effect of pH change on the catalytic perfor-mances of powder versus membrane type catalysts wasevaluated.

Reported inFig. 2a is the effect of the change inpH on the catalytic performance of a fine powder-typePd-Cu/TiO2 catalyst. The tests were made using aslurry batch reactor (SBR) configuration. During thesetests, the pH was corrected by adding HCl in order tomaintain the pH nearly constant (pH±0.4). There is amaximum in activity for a pH around 4. With increas-ing pH there is a nearly linear increase in the formationof ammonium ions and nitrite. It should be remem-bered, as a reference, that starting from a 100 mg/l ni-trate solution and assuming 75% conversion of nitrate(e.g. residual nitrate 25 mg/l), the selectivity to ammo-nium ions should be lower than 0.7% and that to nitritelower than 0.13% to meet current EU regulations.

In order to compare the results with those which canbe obtained using a catalytic membrane under compa-rable fluidodynamic conditions, the effect of pH on thebehavior of the membrane-type catalyst was studiedby immersing the membrane in the same batch reactor

used for the tests with the catalyst in the form of pow-der (Fig. 2a), controlling bulk solution pH in a similarway by addition of HCl, but feeding H2 through themembrane. The results for the catalytic membrane arereported inFig. 2b. The comparison of the results withthose ofFig. 2a(catalyst in the form of fine powder)evidence a similar general trend. With increasing pHthere is a maximum in the activity centered near pH4, and the formation of nitrite and ammonium ions in-creases with increasing pH. There is thus a multipleeffect of pH: it decreases the rate of reaction both ofnitrate and nitrite reduction and at the same time pro-motes the parallel reaction of ammonium formation.

Although clear indications on the reasons for thiseffect are not give in the literature[3–18], it may betentatively attributed to the modification of the doublelayer at the solid–liquid interface consequent to thechange in pH which modifies both the diffusivity ofspecies through this layer and the surface charge on thenoble metal. When the Pd surface becomes charged byadsorbed hydroxyl species, it is inactive towards thereduction of nitrate or nitrite which cannot be adsorbedon it. In these conditions, the effective rate of reactionof nitrate or nitrite conversion observed depends onthe competition between the rate of hydroxylated Pdsurface reaction with H2 to form again the active sur-face and rate of readsorption on Pd of hydroxide ions.Therefore, it may be expected that the rate depends onthe local availability of H2 for the reaction with thePd surface and the local concentration of OH− (localpH). When H2 is feed through the membrane, the ef-fectiveness of the first reaction increases and for thisreason the decrease in the rate of nitrate conversionwith increasing pH is much less dramatic than whenusing fine powder particles of the catalyst. In addition,the formation of nitrite and ammonium ions is muchlower.

The second parameter which influences the catalyticbehavior is the local pH (concentration of OH−) whichis influenced by the rate of diffusion of hydroxide ionsfar from the catalyst surface. This rate is reasonablyinfluenced by the presence of intraparticle diffusionlimitations. In order to verify this aspect, reported inFig. 3 is the comparison of the catalytic behavior ofthe Pd-Cu/TiO2 catalyst using fine and larger (about1 mm) particle dimensions. The results reported inFig. 3 clearly show a decrease in the rate of reactionof nitrate reduction using the larger particles, because


Fig. 2. Effect of pH of the water solution on the catalytic performance of a Pd-Cu/TiO2 catalyst in nitrate reduction at room temperature.(a) Slurry batch-type reactor tests with the catalyst in the form of fine powder (0.1 mm or less). Catalyst, 56 mg. (b) Tests using a catalyticmembrane with the same composition as the catalyst used in (a) tests. The membrane was immersed in the same batch-type reactor usedfor (a) tests. In both cases, the pH of the bulk of solution was corrected by HCl additions. Inlet nitrate concentration: 50 mg/l. The reactiontime refers to 50 min.


Fig. 3. Comparison of the behavior of a Pd-Cu/TiO2 catalyst in nitrate reduction at room temperature, using different catalyst particles.Slurry batch-type reactor. The pH was maintained at a value close to 5 by addition of HCl. Inlet nitrate concentration: 50 mg/l.

the reaction is limited by intraparticle diffusion bothon the reactant and products. In particular, the lattereffect determines the local increase in pH with theconsequent higher formation of nitrite and ammoniumions, in agreement with the data shown inFig. 3.

Therefore, using a catalytic membrane instead ofthe catalyst in the form of pellets has the benefits ofbetter three phase contact and control of pH, whichare reflected in improved reactivity and selectivity inthe reduction of nitrate.

3.1.4. Influence of the operating conditionsControl of local pH is an important parameter to

improve catalyst performance in the reduction of ni-trate, but controlling bulk solution pH adding an acidis not enough, as evidenced in the data ofFig. 2. Analternative method to control the increase of pH isadding a component which has a buffer effect, but tak-ing into account that the added component should notalter the final quality of the water (for drinking pur-poses, for example). CO2 is an ideal solution, becausethe CO3

=/HCO3 equilibrium has a buffer effect neara pH value of 5 and is compatible with drinking waterquality. One of the advantages of the use of membrane

is that can be feed a mixture of CO2/H2 and thereforeCO2 is feed directly at the catalyst/solution interface.

Fig. 4 reports the effect of the characteristics of thefeed through the membrane (pressure and composi-tion of the CO2/H2 mixture) on the performance of aPd-Cu/TiO2/Al2O3 catalytic membrane in the reduc-tion of nitrate at room temperature. In these tests noHCl was added to control the pH.

The results show a relevant influence of the feedcharacteristics through the membrane on the catalyticperformance. When the amount of CO2 feed is notenough to control the pH (condition 1; in these teststhe pH of the solution increases from the initial valueof around 5 to over 9) the membrane is not veryactive, there is a drastic increase in the formation ofnitrite ions and the ammonium ion selectivity is thehighest. When a large excess of CO2 in the feed isused (condition 2), the formation of ammonium andnitrite ions decreases considerably, but the activity innot very good, due probably to insufficient H2 in thefeed. Maintaining the total pressure across the mem-brane as for condition 2, but changing the CO2/H2molar ratio in the feed (condition 3), results in aconsiderable increase in the activity, while the nitrite


Fig. 4. Reduction of nitrate at room temperature in a batch tubular membrane reactor as a function of the pressure and composition ofthe feed sent through the membrane using a Pd-Cu/TiO2/Al2O3 catalytic membrane. (a) Change in the residual nitrate concentration as afunction of time. (b) Dependence of the selectivity to nitrite and ammonium ion (on an mg/mg basis) as a function of the nitrate conversion.No addition of HCl to control the pH. Characteristics of the feed sent through the membrane: condition 1: 3 bar pressure, CO2/H2 = 0.2;condition 2: 1.5 bar pressure, CO2/H2 = 5; condition 3: 1.5 bar pressure, CO2/H2 = 2.


formation remains unchanged and the formation ofammonium ions only slightly increases. In the tests inconditions 2 and 3, an increase in pH from the start tothe end of the tests in noted, but it is no higher thanabout 1.0–1.5 units of pH.

These data show that good control of the pH is pos-sible using a CO2/H2 feed, even when no acid is addedto the solution to compensate for the increase in pHduring the reaction. The behavior depends consider-ably on the feed composition, but in the preferableconditions the formation of ammonium and nitrite ionsis still too high for application. The combined controlof pH by CO2 and acid addition are thus necessary.

The results of this kind of experiments areshown in Fig. 5 which reports the behavior of aPd-Cu/TiO2/Al2O3 catalytic membrane in the reduc-tion of nitrate at room temperature in continuous

Fig. 5. Reduction of nitrate at room temperature in a continuous tubular membrane reactor as a function of the time on stream.Pd-Cu/TiO2/Al2O3 catalytic membrane. In both cases a CO2/H2 = 3 feed (total pressure 2 bar) was sent through the membrane, but incondition A the reactor inlet pH was 3 (pH of the reactor outlet solution 4.3) while no correction of the pH was made for condition B(the pH increased from 5.2—inlet reactor—to 6.3—outlet of the reactor). Contact time in the TMR: 52 min.

TMR experiments. In the tests in condition B nocorrection of the pH was made, while in the tests incondition A the pH of the inlet solution was loweredto about 3. In both cases, a CO2/H2 mixture was fedthrough the membrane.

Various conclusions can be drawn from the resultsreported inFig. 5:

• The conversion of nitrate rapidly reaches a station-ary state and then remains very stable for long times.There is no effect of the solution inlet pH on the sta-bility of the catalytic behavior and a minimal effecton the conversion of nitrate.

• Differently from the conversion of nitrate, a longtime is required for the formation of nitrite and am-monium ions to reach a steady-state condition. Amaximum in their formation is noted after 5–8 h of


time on stream and more than 60 h are necessary toreach the final value. The solution inlet pH does notinfluence this effect. It should be noted that differ-ently from the tests reported inFig. 4, a continuousreactor configuration was used in these tests (seeFig. 1). The present data do not allow an explana-tion for the long time required to reach stationaryactivity, but probably it is related to a progressivein situ modification (reduction) of the Pd-Cu ac-tive species. It should be noted that there was noevidence for the loss of these metals from the cat-alyst during this time and that the phenomenon isreversible. When the liquid and CO2/H2 feeds arestopped and the membrane catalyst is left exposedto air, a similar evolution of the catalytic behaviorduring a further consecutive test was found.

• When the pH of the reactor inlet liquid feed is de-creased to 3 (condition A, instead of the naturalvalue close to 5—condition B), there is a relevantlowering of the formation of ammonium ions whichat the stationary condition reaches a value of around1 ppm. In both conditions, the formation of nitriteions is negligible, reaching values below 0.1 ppm instationary conditions.

Fig. 6. Effect of the temperature of the liquid feed on the performance in the reduction of nitrate in a continuous tubular membranereactor as a function of the time on stream. Pd-Cu/TiO2/Al2O3 catalytic membrane. CO2/H2 = 4 fed through the membrane (total pressure2.5 bar); no correction of pH.

The combined control of pH by CO2 and acid ad-dition therefore makes it possible to reach high valuesof nitrate conversion together with low formation ofammonium and nitrite ions. However, the ammoniumformation is still higher than the requested 0.5 ppmvalue for drinking water quality. Control of the reac-tion temperature is an additional parameter to analyze.

While room temperature tests (20–22◦C) seem themost appropriate for water remediation in surfacesources, the typical temperature of underground wa-ter is lower (5–15◦C). Therefore, it is reasonable toconsider a water remediation technology which oper-ates with water temperatures in the 12–15◦C rangein order to consider also the heat dispersion to theenvironment during the process. At lower tempera-tures the solubility of H2 increases and thus a positiveeffect may be expected.

The results of the tests when the temperature ofthe water was decreased from 20–22 to 12–15◦C arereported inFig. 6. At the lower temperature, there isa limited decrease in the activity in nitrate reduction(conversion decreases from about 98 to 90%), but aconsiderable decrease in the formation of ammoniumions which in the stationary conditions reached a value


below 0.5 ppm. On the contrary, there is no influenceon nitrite formation which remains very low. Also thetime to reach stationary conditions becomes shorter.

The effect of the reaction temperature in promotingselectivity is probably not a true kinetic effect, becauseit is unlikely that a decrease of a few degrees canhave a so remarkable effect in decreasing the rate ofammonium ion formation. Tentatively, the effect isrelated to a change in the in situ nature of the Pd-Cucatalyst surface which requires further studies to beanalyzed.

3.1.5. Conclusions on the use of a technology basedon catalytic membrane for water remediation afternitrate contamination

Under optimal reaction conditions in terms of reac-tion temperature, composition of the CO2/H2 feed andcontrol of the pH of the solution, it is possible to meetthe requirements for the application of the technologyusing catalytic membranes to water remediation forthe production of drinking water.

As shown inFig. 5, the catalytic membrane ex-hibits a stable activity, although when the membraneis left in contact with the water solution in the ab-sence of the hydrogen feed deactivation was noted,mainly due to oxidation of the Pd surface. The cata-lyst may be reactivated, however, by in situ reduction,although a long time is again necessary to reach sta-tionary activity, similarly to that reported inFig. 5.When it occurs, deactivation due to biomass growthover the membrane can be eliminated by calcinationfollowed by reduction of the catalytic membrane.Experimented showed good reproducibility of datain regenerated catalytic membranes. When tap waterinstead of distilled water was used to prepare thesolution containing the nitrate ions, only a minimalchange in the catalytic performance was noted.

Although further studies are necessary to scale-upthe technology and optimize the membrane composi-tion [20], the laboratory tests indicate good prospectsfor using a technology based on catalytic membranein water remediation after nitrate contamination. Pre-liminary tests with the addition of halogenated hydro-carbons such as tri- and tetra-chloro ethylene to thenitrate solution indicated that these compounds canbe efficiently converted by catalytic hydrodechlori-nation (more than 90% conversion) in the conditionsof nitrate reduction, in agreement with literature data

indicating the activity of palladium-based catalysts fortreating groundwater contaminated with halogenatedhydrocarbons by hydrodechlorination with dissolvedhydrogen[24].

Therefore, the technology based on catalytic mem-brane can be applied to water remediation after bothnitrate and halogenated hydrocarbon or pesticidecontamination.

3.2. Conversion of MTBE in contaminatedunderground water

3.2.1. Catalytic permeable reactive barriers (CPRBs)MTBE is widely used gasoline additive, but be-

cause of its contamination of both ground and surfacewater, its use in California has been banned recently.MTBE water contamination derives from leaking ofunderground fuel tanks and pipelines, tank overfill-ing and faulty construction at gas stations[21]. Whileother gasoline components are characterized by goodbiodegradability and low water solubility (thus gaso-line release shows a fast “natural attenuation” andlow migration index), MTBE is very soluble in wa-ter (therefore, highly mobile in ground water systems)and resistant to biodegradation. Furthermore, a longincubation time is usually necessary before the startof microbiological degradation[22].

Several technologies have been proposed, someof which have been shown to be practical such asair stripping (which, however, requires a secondarytreatment to eliminate MTBE from the stripping air),advanced oxidation (based on ozone or peroxides, butwhich may form dangerous products such as alde-hydes) and activated carbon (which needs periodicregeneration), and some of which are expensive, likethose based on membrane[23]. Bio-remediation byinoculation of specialized bacteria whose growth inthe presence of MTBE can promotes degradation ofthe MTBE is possible, but doubt exist regarding thelong term eco-compatibility of this operation, espe-cially for the purpose of treating contaminated wellsfor the production of drinking water.

These technologies can be use for water remediationof contaminated wells, but permeable reactive barri-ers (PRBs)[25–28]are emerging as a very interestingalternative for both organic and inorganic water con-tamination. The main advantage of a reactive barrieris the passive nature of the treatment and therefore the


Fig. 7. Schematic drawing of the concept of permeable reactive barrier (PRB).

low cost. Once installed, the barrier takes advantageof the in situ groundwater flow to bring the contami-nants in contact with the reactive material. Examplesof possible configurations of a PRB for groundwatertreatment are shown inFig. 7. The reactive materialto be used in the barrier depends on the type of con-taminants being treated. The most common reactivemedium used so far has been granular iron. In all cases,stoichiometric reagents for the PRB have been stud-ied and applied up to now, while the use of catalyticmaterials has not been considered.

Due to the limited oxygen content of undergroundwater and low temperature, catalytic oxidative con-version is not effective. However, the conversion ofMTBE to TBA and MeOH can be catalyzed by acidcatalysts such as zeolites which also act at the sametime as adsorbents. The advantage of this conversionis that TBA has a higher biodegradability than MTBEand shorter induction time necessary for adaptation ofthe microorganisms and the start of microbial activ-ity in MTBE degradation[29]. PRBs based on zeo-lite are thus potentially very interesting, because cancombine a high adsorption capacity (thus the neces-sity for smaller PRBs) together with an acid catalyticaction in promoting degradation of MTBE to more

Table 2Comparison of the behavior of different zeolites as PRB at room temperature

Zeolite ResidualMTBE (mg/l)

TBA insolution (mg/l)

MeOH insolution (mg/l)

MTBE and/or productsadsorbed (mg/l)

HZ525 1415 102 102 480HBEA25 437 22 22 1541HZ580 340 267 267 1393HY30 1920 – – 86HMOR15 1970 – – 25

Initial MTBE concentration: 2000 mg/l. Residence time: 120 h. Natural pH.

biodegradable chemicals which then can be convertedin situ by microorganisms that can grow on the barrieritself.

3.2.2. Performance of different zeolites as CPRBThe performances of different commercial zeolites

were evaluated in r.t. static adsorption/conversion testsusing a volume of the solution to volume of the zeo-lite ratio around 70. This ratio is considerably higherthan that typically used in PRBs[25], but allowsthe effectiveness of the zeolite to be evaluated undermore severe conditions. Under less severe conditions(lower ratio between water to zeolite volumes) it is notpossible to clearly differentiate between the differentzeolites. Tests were made changing the MTBE initialconcentration in the solution in the 1000–4000 mg/lrange, the higher expected range for MTBE in plumesderiving from a leaking underground fuel tank (LUFT)[30]. The comparison of the behavior of selected ze-olites in the treatment of solutions containing about2000 mg/l MTBE is reported inTable 2. The data referto the results observed after 120 h, a typical residencetime for plumes passing through PRBs[25]. Blanktests in the absence of zeolite indicated the absenceof change in the concentration of MTBE without the


Table 3Effect of the initial concentration of MTBE on the behavior of HZ580 zeolite at room temperature

Initial MTBEconcentration (mg/l)

Residual MTBE (mg/l) TBA in solution (mg/l) MeOH in solution (mg/l) MTBE and/or productsadsorbed (mg/l)

1000 78 212 248 7002000 340 267 267 13934000 1694 510 283 1796

Residence time: 120 h. Natural pH.

zeolite, due for example to absorption on the wall ofthe reactor or photochemical conversion.

The nature of the zeolite, the Si/Al ratio and theacidity has a considerable influence on the activity.BEA and ZSM-5 zeolites show good activity, whichhowever depends considerably on the Si/Al ratio asshown by the comparison of the two samples withthe ZSM-5 structure (HZ525 and HZ580), while Yand Mordenite zeolites are completely inactive. Theamount of formed products of reaction (TBA andMeOH) is always lower than the expected amount ob-tained from MTBE conversion, although their detec-tion in the aqueous phase increases as the Si/Al ratiodecreases (compare HZ580 with HZ525 and HBEA25).The missing amount is due to the adsorption of bothMTBE and the alcohols on the zeolites. IR charac-terization of the zeolite after reaction confirms thepresence of relevant amounts of the three compoundsin the zeolite, result confirmed also by the analysisof total organic carbon (TOC) on the solid (zeolite).Blank tests using the zeolites in the completely ex-changed sodium form indicate virtually no MTBEadsorption. This result indicates that it is not MTBEitself, which may remain adsorbed on the zeolite,but that the lack of carbon balance noted in the testsis due to the adsorption of the products of reaction(TBA and MeOH). They are then slowly released.

The zeolites show a high rate of MTBE hydrolysis,but the products of reaction tend to remain adsorbed onthe zeolite. This is a positive aspect, because their slowrelease to the aqueous solution favors their degradationby indigenous microorganisms.

The effect of the concentration of MTBE in solutionis shown inTable 3for the HZ580 zeolite. The time ofreaction is 120 h as inTable 2. It should be noted thatvalues of MTBE in solution in the 2000–4000 mg/lrange can be found only in the case of large amounts ofleakage from underground fuel tanks and pipelines and

which extends only a few meters from the undergroundtank. The zeolite layer can be used, however, also asa protective catalytic layer around underground tanks.In this case, it is necessary to evaluate the performanceusing relatively high MTBE concentrations, but alsoin the case of PRB data tests with solutions in the2000–4000 mg/l MTBE range are indications obtainedfor the most severe possible conditions.

Above a concentration of MTBE in solution ofaround 4000 mg/l, the efficiency of zeolite in adsorp-tion/conversion of MTBE rapidly drops, at least usingthe cited ratio (around 70) between volume of solu-tion and volume of zeolite. This is reasonable due tothe saturation of the adsorption capacity of zeoliteswhich may be estimated in these tests to be around4 g/l, a quite high value indicating that small catalyticbarrier can be used. This is a considerable advantagein comparison with other types of protective barrierssuggested in the literature. It should also be men-tioned that after the MTBE adsorption/conversiontests when the zeolite is filtered, washed and put indistilled water, a slow release of TBA and MeOHonly is observed. This is positive because with timethe MTBE adsorbed on the zeolite (as such or hy-drolyzed to TBA and MeOH) is released in the formof the corresponding alcohols which are most easilybiodegraded by indigenous microorganisms. There-fore, self-regeneration of the zeolite occurs with time,differently from stoichiometric reactants.

4. Conclusions

Remediation of contaminated ground and under-ground water is becoming a critical issue, but alsocan be a good marketing opportunity. Although het-erogeneous catalysts have rarely been considered inwater remediation technologies, they can offer good


opportunities to develop new or improved technolo-gies for water remediation.

Two examples have been discussed here: (i) theelimination of nitrate and pesticides from water con-taminated as a result of agricultural practices and (ii)the conversion of MTBE in contaminated undergroundwater. A technology based on catalytic membranes forremediation of water contaminated by nitrate offersvarious advantages with respect to conventional tech-nologies. Two of the reaction parameters which wereshown to considerably influence the performance werethe reaction temperature and the local pH, the latterbeing controlled both by feeding a CO2/H2 mixturethrough the membrane and acidifying the solution. Thecontrol of all parameters allows drinking water qualityto be obtained, although further studies are necessaryto both scale-up the technology and more thoroughlyinvestigate the possibility of further improvement bymodification of the catalytic membrane compositionand preparation method.

In MTBE conversion in contaminated undergroundwater it is shown that acid zeolites with suitable porestructures (channel structure and pore openings) suchas H-ZSM-5 and H-BEA could be applied as CPRBsfor in situ remediation. These zeolites effectively cat-alyze the hydrolysis of MTBE to TBA and MeOHwhich can then be rapidly biodegraded by indigenousmicroorganisms, and also act as adsorbents for bothMTBE and the products of reaction.

Acknowledgements

Dr. F. Luck (ANJOU Recherche-Vivendi Water,Maisons-Laffitte, France) and Dr. M. van Donk(CH2MHill, Madrid, Spain) are gratefully acknowl-edged for the useful discussion and information aboutthe comparison of the various technologies for nitrateremoval from drinking water. Part of the work wascarried out with financial support from the EU withinthe contract BRPR-CT97-0420 DENITROCAT.

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remediation of water contamination using catalytic technologies

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