Desalination 271 (2011) 295–300

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Desalination

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Application of modified zeolite for ammonium removal from drinking water

Mingyu Li ⁎,1, Xiaoqiang Zhu, Fenghua Zhu, Gang Ren, Gang Cao, Lin SongDepartment of Environmental Engineering, Jinan University, Guangzhou, 510630, China

⁎ Corresponding author. Tel.: +86 20 85226890; fax:E-mail address: limingyu2000@163.com (M. Li).

1 http://www.jnusci.com.

0011-9164/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.desal.2010.12.047

a b s t r a c t

a r t i c l e i n f o

Article history:Received 29 October 2010Received in revised form 18 December 2010Accepted 20 December 2010Available online 1 February 2011

Keywords:Drinking waterAmmoniumModified zeolite

Clinoptilolite was processed, first, by sodium chloride solution, and then mixed with Na2SiO3 and powderedactivated carbon. The mixture went through extrusion forming and calcination to prepare the Silicate-CarbonModified Zeolite (SCMZ) that was used for removing ammonium from drinking water. The SCMZ wasanalyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD) and thermogravimetry(TG), and itsBET surface was compared with that of natural zeolite. This study was performed using column experiments,and batch tests were performed under a range of conditions to assess the effect of solution pH, filtration rate,initial ammonium concentration and regeneration methods on the performance and capacity of the zeolite inammonium removal. Results indicate that the adsorption capacity of SCMZwas 0.115 mg NH4

+–N/g, when thepH of the solution was 7, filtration rate was 10 m/h and initial ammonium concentration was 5 mg/L, and wasmuch higher than that of natural zeolite. This breakthrough product made from SCMZ filtering media wouldhave high ammonium-removal efficiency when regenerated by three cycles through sodium chloridesolution.

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1. Introduction

Ammonia nitrogen, a major pollutant in surface water, can causeeutrophication and impair self-purification in lakes and rivers. Inrecent years, ammonium concentration, in certain surface watersserving as the source for potable water, is much higher than thepermissible standard, as a large quantity of industrial and municipalwastewater has been discharged into existing water resources.However, the traditional processes of “coagulation–precipitation–filtration–disinfection” in water works are not able to removeammonium effectively. The above mentioned situation has threat-ened the availability of safe drinking water and, thus, human health.Therefore, the question of how to remove ammonium effectively andeconomically from drinking water has become an issue that has to besolved urgently.

Currently, physical–chemical [1,2] (air stripping, break-pointchlorination and ion exchange, etc.) and biological methods [3](biological filter, biological contact oxidation, etc.) are being appliedin ammonium removal. However, there are both advantages anddisadvantages in these methods. For example, the traditional ion-exchange method is limited to application in smaller quantitiesbecause ammonium will be exchanged by other high-valence ionsfirst in water [4]. In addition, its running cost tends to be high.Biological filter, an effective method of ammonium removal, improvesupon traditional processes significantly. However, high costs are

incurred in construction of the filter and there is a higher risk to safetyduring the subsequent processing. Contingency on temperature andclimate conditions constitutes another disadvantage in this process[5]. Compared with the above mentioned methods, high safety andlow cost are two of the attributes that are attracting an increasingfocus on the use of zeolite in water-purification processes [6–9], suchas ammonium removal [10–13], of drinking water. First, naturalzeolite needs to be modified to improve its ion-exchange and sorptionproperties as well as purity, before it can be used to removeammonium effectively. Traditionally, there are different ways toprepare zeolite; these include physical methods, chemical methods[14–17] (acids, alkali and salts of alkaline metals, etc.) and acombination of the two [18]. Different varieties of zeolite from acrossthe world have been studied and several reports on zeolite have beenpublished in the literature in recent years. However, the smalladsorption capability of zeolite for ammonium and lower efficacy arestill a key problem for its wider application in water-purificationprocesses.

In this study, a newfiltermaterialwasprepared by shapingmodifiedzeolite powder into a cylinder [D (diameter)=4 mm, H (height)=8 mm], which was then designated as the Silicate-Carbon ModifiedZeolite (SCMZ). SCMZwas integrated into the conventional processes ofcoagulation–precipitation–filtration–disinfection in water-purificationworks, for the removal of ammonium from drinkingwater. Through themethod of adsorption filtration, ammonium was removed successfullyfrom drinking water by the SCMZ filter. Compared with the naturalzeolite filter, the ammonium removal by the SCMZ filter was moreeffective, and, thus, provided valuable statistical data and guidance forfurther research.

Fig. 1. Experimental set-up of the ammonium adsorption experiments.

Fig. 2. SEM patterns of the SCMZ.

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2. Materials and methods

2.1. Silicate-Carbon Modified Zeolite (SCMZ)

Natural zeolite (clinoptilolite), obtained from the Henan provincein China, was ground and sieved to the specified optimum size(D=74 μm).

The modification process applied in the preparation of SCMZconsisted of the following three steps:

Step 1: The clinoptilolite powder(74 μm) was repeatedly washedwith tap water and then dried at 100 °C for 4 h.

Step 2: The zeolite was dried and 1000 g of the dry zeolite wasdispersed into 1 L sodium chloride solution (2 mol/L) thatwas prepared using tap water and stirring for 12 h. Thesample was repeatedly washed with tap water and thendried at 100 °C for 4 h. Finally, the dried sample was groundand sieved to 74 μm again, and designated as sample A.

Step 3: Sample A, Na2SiO3 and powdered activated carbon weremixed at weight/weight ratio of 100:9:2, and mixed evenly;then, 10% tap water (weight/weight ratio) was added andstirred again. The mixture was shaped into a cylinder(D=4 mm, H=8 mm) by an extrusion method, and thendried at 100 °C for 2 h and calcined in a muffled furnace at500 °C for 2 h, and the SCMZ filter was obtained.

In this study, the BET surface of SCMZ and clinoptilolite were27 m2/g and 21 m2/g. The cation exchange capacity (CEC) of SCMZwas 1.4 meq/g and that of clinoptilolite was 1.2 meq/g.

2.2. Reagent

All chemicals used in this study were analytical grade reagents.Ammonium chloride (99.5%), mercuric iodide (99%), potassiumiodide (99%), hydrochloric acid (36%) sodium chloride (99%,), sodiumhydroxide(99%), sodium silicate (99%), potassium sodium tartrate(99%) and powdered activated carbon were supplied by Damaochemical reagent factory in China. While sample of natural zeolite(Clinoptilolite) with a Si/Al ratio of 5.4 was supplied in Hennan. Theammonium-containing water samples that imitated micro-polluteddrinking water were prepared by the addition of NH4Cl to tap water.

2.3. Experimental methods

The ammonium-containing water sample was pumped from acontainer by using a peristaltic pump and loaded into two separatehydraulically saturated synthetic resin columns simultaneously. Itthen filtered through the prepared column and into a storagecontainer. The columns (D=6 cm, H=110 cm) were filled withSCMZ and natural zeolite (Φ=4 mm). The filtering media was 60-cmhigh with a volume of 1695.6 cm3. Before the adsorption experiment,backwashing was performed while columns were shaking for thepurpose of outgassing. Details of the experimental set-up are asdepicted in Fig. 1. Experimental samples were obtained at regularintervals from the containers, both prior to and following passagethrough columns. These samples were analyzed for the presence ofammonium (determined with Nessler's reagent spectrophotometryEPA350.1) as well as the pH. The pH was adjusted using hydrochloricacid and sodium hydroxide solution, and tested by pH meter.

The adsorbance of ammonium was calculated using followingequation:

Q = vS∫t

0C0−Ctð Þtdt ð1Þ

where Q is the amount of ammonium in the solid phase (mg/g); v isfiltration rate (m/h); S is cross-sectional area of the column (m2); C0

and Ct are the ammonium concentrations before and after passagethrough the columns (mg/L); t is equipment-operation time (h).

The column was break through when Ct/C0=0.9. The adsorbanceof ammonium at the breakthrough point was defined as its adsorptioncapacity.

3. Results and discussion

3.1. Characterization of zeolites

The surface-morphology analysis by SEM reveals that the SCMZhas a rough surface and porous structure with defined channels/cavities. Compared with the SCMZ, the surface of natural zeolite issmooth, and its channels/cavities are smaller than that of the SCMZ(Figs. 2 and 3). Calcination can decompose organic matter and removewater from the surface or pores of zeolites, as well as enlarge porediameter [19]. In this study, a portion of the pore-forming material(powdered activated carbon) was oxidized to CO2 at a hightemperature (500 °C). Small pores were formed when CO2 gas

Fig. 3. SEM patterns of the natural zeolite (NZ).

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escaped during formation of the SCMZ. Salt treatment prior tocalcination could enhance the resistance of the natural zeolite tohigher temperatures, and the pore volume and pore diameterexpanded by such salt treatment would result in an increasedammonium ion-exchange and adsorption activities [18]. Morenumber of channels/cavities were formed, which increased in BETsurface area [20] and offered larger surface area for ammoniumadsorption and ion exchange. In this study, the BET surface of SCMZwas 27m2/g and that of natural zeolite was 21 m2/g.

However, due to thermal stabilities of zeolite crystal lattices, high-temperature treatment could also induce dealumination and dehy-droxylation of zeolites, and change the surface functional groups andthus cause the framework to collapse; this can result in lower surfacearea and pore volume and, therefore, lower adsorption capacity [21–23]. Fig. 4 depicts the XRD patterns of the SCMZ and natural zeolite. Itcan be seen from Fig. 4 that modification brought about significantreduction in relative intensities at 17°, 22.7°, 30 .1°and 31.9°2θ forclinoptilolite. However, as can be seen from diffraction patterns of thesamples, the amount and position of the peak intensities were notaltered. There was no transformation of crystalline structure in themodification. These data show that the SCMZ has certain thermalstability.

Fig. 4. XRD patterns of the SCMZ and natural zeolite (NZ). A, B and C represent clinoptilolite, fchanged after modification.

Fig. 5 depicts the thermogravimetric(TG) curves of the SCMZ andnatural zeolite. Fig. 5 indicates that the weight of Natural zeolite andSCMZ were decreased, and stabilized until 340–350 °C. Both of themhave microporous structure which contained a certain amount ofwater. The water was gasified gradually with increasing temperature,and caused a decline in weight. The weight-loss ratio of SCMZ was6.1% at 340 °C, and that of natural zeolite was 4.09%. The differencewasmaintained steady until 800 °C. Thismay be due to the SCMZ got ahigher poriness after modification, thus, it held more water thannatural zeolite. The more water SCMZ have, the more it lost in thethermogravimetric analysis.

3.2. The effect of pH

In order to analyze the effect of pH on the removal of ammoniumby zeolite, the following tests were carried out according to the abovementioned experimental methods. The ammonium water samplesfiltered through the column;when pHwas adjusted to 5.0, 7.0 and 9.0,respectively; filtration rate was 10 m/h and the initial ammoniumconcentration was 5 mg/L. Fig. 6 shows the results of the process ofammonium sorption onto the SCMZ and natural zeolite at three pHconditions.

Fig. 6 indicates that the SCMZ column could maintain highammonium adsorbance in longer time. The breakthrough points ofthe SCMZ appeared after 28–30 h, and that of natural zeolite columnwas 20–23 h. The effective ammonium-removal time of zeoliteincreased by 34–40% after modification. The adsorption capacity ofthe SCMZ was clearly superior, and it was 2–3 times higher than thatof natural zeolite, when maintained at a specific pH value (Fig. 6).

On the other hand, pH had a light effect on the curve shape,showing a rising trend and breakthrough time. Its effect on theadsorption capacity of zeolites was small as well (Fig. 6). These resultsindicate that natural zeolite and the SCMZ could adsorb ammoniumsteadily at pH 5–9, but that the absorption effect of the SCMZ wasobvious higher than that of natural zeolite.

Some research studies have reported that H3O+ would competewith NH4

+ for ion exchange in a solution at low pH [24], and that thiscould influence the ammonium-removal efficiency of zeolite. How-ever, this study demonstrated that the ammonium-removal efficiencyof both zeolites did not change with the pH range of 5–9. Althoughlittle changes could be seen from Fig. 6 at different pH, they could becaused by ammonia/ammonium liquid phase equilibrium. In addition,a little zeolite might be dissolved at low pH and lead to changes.

eldspars, quartz. The circles for SCMZ show the place where the relative intensities were

Fig. 5. Thermogravimetry(TG) curves of the SCMZ and natural zeolite (NZ). Heating ratewas 10 K/min, N2 (99.999%) was used as purge gas. Fig. 7. The relationship between ammonium adsorbance and filtration time at different

initial ammonium concentrations. The end of each curve was breakthrough point, andthe ammonium adsorbance at breakthrough point was adsorption capacity of zeolite ateach level of initial ammonium concentration.

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3.3. The effect of initial ammonium concentration

In order to analyze the effect of the initial ammonium concentra-tion, the following tests were carried out wherein initial ammoniumconcentration was set at 2.5, 5 and 10 mg/L, respectively, filtrationrate was 10 m/h and pH of the solution was 7.0. Fig. 7 depicts theresults of the process of ammonium sorption onto the SCMZ andnatural zeolite with three specified initial ammonium concentrations,respectively.

In varying initial ammonium concentration, the SCMZ columncould remove ammonium effectively but in a longer duration. Forinstance, the breakthrough point for the SCMZ column was observedafter 47 h, and that of the natural zeolite column was after 32 h, whenthe initial ammonium concentration was 2.5 mg/L (Fig. 7). It wasobserved that the breakthrough time of two zeolites would bereduced, with increased initial ammonium concentration, when theconcentrations ranged from 2.5 to 1.0 mg/L. This indicates that highammonium concentration would add load of zeolite and acceleratebreakthrough. Thus, the volume of the zeolite column should be

Fig. 6. The relationship between ammonium adsorbance and filtration time at differentpH. The end of each curve was breakthrough point, remarked when Ct/C0 ratio was 0.9.The ammonium adsorbance at breakthrough point was adsorption capacity of zeolite atdifferent pH.

enhanced when the ammonium concentration is high as it isnecessary to achieve efficient ammonium removal.

On the other hand, the adsorption capacity of zeolite increaseswith the initial ammonium concentration (Fig. 7). The adsorptioncapacity of the SCMZ was higher than that of natural zeolite at eachinitial ammonium concentration, with the ratio ranging between 2.0and 2.5. For example, when the initial ammonium concentration was5 mg/L, the adsorption capacity of the SCMZ was 0.114 mg/g, and thatof natural zeolite was 0.043 mg/g.

It is evident that the ammonium-removal efficiency and ammoniumadsorbance of the SCMZweremuchhigher than that of natural zeolite ateach level of ammonium concentration.

3.4. The effect of filtration rate

The ammonium water samples filtered through the column whenfiltration rates were adjusted to 8, 10 and 12 m/h, respectively, andinitial ammonium concentration was fixed at 5 mg/L with pH of 7.0.Fig. 8 shows the results of the process of ammonium sorption onto theSCMZ and natural zeolite with three different initial ammoniumconcentrations.

The SCMZ column had a prolonged time of operation comparedwith the natural zeolite columnat eachfiltration rate. The ammonium-removal efficiency breakthrough time of both zeolites reduced whenthe filtration rate increased (Fig. 8). This is because the hydraulicretention time and the contact between water and zeolite reducedwith the increase in filtration rates. This phenomenon is unfavorablefor developing adsorption equilibrium of zeolite. Conversely, theadsorption capacity of the SCMZ and natural zeolite decreased withincrease in the filtration rate. The adsorption capacity of the SCMZwas2.5–3 times higher than that of natural zeolite at the same filtrationrate (Fig. 8). In conclusion, ammonium-removal efficiency andadsorption capacity of the SCMZ were much better than that ofnatural zeolite at each different filtration rate.

3.5. The effect of regenerative methods

The ammonium-removal efficiency of zeolite decreases afteroperating for a long duration. Thus, zeolite should be regeneratedbefore using it again. Therewere three regenerationmethods (Heatingregeneration, Acid regeneration andNaCl regeneration) applied in this

Fig. 8. The relationship between ammonium adsorbance and filtration time at differentfiltration rates. The end of each curve was breakthrough point. The ammoniumadsorbance at breakthrough point was adsorption capacity of zeolite at each level offiltration rate.

Fig. 9. Adsorption capacity of regenerated zeolites.

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study respectively (Table 1). When pH was 7, initial ammoniumconcentration was 5 mg/L and filtration rate was 10 m/h, andregenerated zeolites were compared with fresh zeolites. The “NaClregeneration” method was superior to other methods and theadsorption capacity of the SCMZ was 0.1117 mg/L after “NaClregeneration” and was closest to that of fresh SCMZ, 0.1155 mg/L(Fig. 9). The results of the “heating method” and “acid method” wereinferior to the NaCl regeneration (Fig. 9).

In addition, this study regenerated SCMZ for thrice with 2.0 mol/Lsodium chloride solution, so as to investigate the effect of regenerationtime on adsorption ammonium-removal efficiency. The adsorptioncapacities of regenerated SCMZ were 0.1117 mg/L, 0.1116 mg/L and0.1116 mg/L. The SCMZ regenerated using NaCl maintained adsorptioncapacity at the same level. In summary, “NaCl regeneration” turned outto be the best method compared with heating and acid methods; thus,the SCMZ can be regenerated and used several times.

The following reaction occurred in the process of “NaCl regenera-tion”:

NaClþ NH4 � Zeolite⟺NH4Clþ Na� Zeolite ð2Þ

In the process of regeneration, Na+ exchanged the NH4+ that was

adsorbed on zeolite, and the ammonium-removing property of zeolitewas restored [25]. The SCMZ that was regenerated by three cycles inNaCl solution had high ammonium-removal efficiency with itsadsorption capacity of 0.11047 mg/L close to that of fresh zeolite.

Table 1Methods of zeolite regeneration.

Regenerate methods Step of regeneration

Heating regeneration The breakthrough SCMZ was calcined in a mufflefurnace at 200 °C for 24 h.

Acid regeneration The breakthrough SCMZ was treated with 1.0 mol/L HClsolution for 24 h, repeatedly washed with tap water,and dried at 45 °C for 4 h.

NaCl regeneration The breakthrough SCMZ was treated with 2.0 mol/L NaClsolution for 24 h, repeatedly washed with tap water,and dried at 100 °C for 4 h.

4. Conclusions

(1) Ammonium-removal efficiency of the SCMZandnatural zeolite isrelated to filtration rate and initial ammonium concentration. Inthis study, ammonium-removal efficiency of two zeolites did notchange with pH, with pH in the range of 5–9. The adsorptioncapacity of the SCMZ was much higher than of natural zeoliteunder variable conditions, and the SCMZ column could maintainhigh ammonium adsorbance for longer duration.

(2) In this study, the BET surface of SCMZ was 27 m2/g and that ofnatural zeolite was 21 m2/g. The XRD patterns of the SCMZ andnatural zeolite indicate that no transformation of crystallinestructure was part of the modification. The TG curves showedwater loss difference between SCMZ and natural zeolite.

(3) When regenerated by NaCl solution repeatedly, the SCMZwouldhave high efficiency of ammoniumremoval, e.g., when generatedthree times, its adsorption capacity was 0.11047 mg/L, close tothat of fresh SCMZ zeolite.

(4) Based on the current conventional water-works process, theammonium was actually removed by the placement of theSCMZ into the filtration unit. It is hypothesized that the SCMZwill have a wide range for application as filter material.

Acknowledgements

This study is financially supported by “the Fundamental ResearchFunds for the Central Universities” (No. 21610513), “Projects underScientific and Technological Planning of Guangdong Province” (No.2006A36802001, No. 2007A032400001 and No. 2008A030202010)and “The University-Industry Cooperation Project about WaterTreatment Materials of Guangdong Province”. The authors wouldlike to express their appreciation to Ms. Li Yihan and other colleaguesin Jinan University.

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