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Chemical Engineering Journal 168 (2011) 493–504

Contents lists available at ScienceDirect

Chemical Engineering Journal

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review of emerging adsorbents for nitrate removal from water

mit Bhatnagara,∗, Mika Sillanpääb

LSRE–Laboratory of Separation and Reaction Engineering, Departamento de Engenharia Química, Faculdade de Engenharia da Universidade do Porto (FEUP),ua Dr. Roberto Frias, 4200-465 Porto, PortugalFaculty of Technology, Lappeenranta University of Technology, Patteristonkatu 1, FI-50100, Mikkeli, Finland

r t i c l e i n f o

rticle history:eceived 21 November 2010eceived in revised form 23 January 2011

a b s t r a c t

Nitrate, due to its high water solubility, is possibly the most widespread groundwater contaminant inthe world, imposing a serious threat to human health and contributing to eutrophication. Among severaltreatment technologies applied for nitrate removal, adsorption has been explored widely and offers

ccepted 26 January 2011

eywords:ater treatment

nionsitrate removal

satisfactory results especially with mineral-based and/or surface modified adsorbents. In this review, anextensive list of various sorbents from the literature has been compiled and their adsorption capacitiesfor nitrate removal as available in the literature are presented along with highlighting and discussing thekey advancement on the preparation of novel adsorbents tested for nitrate removal.

© 2011 Elsevier B.V. All rights reserved.

dsorbentsorption capacities

. Introduction

In many parts of the world, groundwater serves as the soleource of drinking water in rural communities and urban areas.owever, in recent years, increased industrial and agriculturalctivities have resulted in the generation of toxic pollutants suchs inorganic anions, metal ions, synthetic organic chemicals whichave increased public concern about the quality of groundwaters.

norganic anions are of great importance since these are toxic andarmful to humans and animals at very low concentrations (ppb).s there are usually no organoleptic changes in drinking waterue to the presence of trace levels of toxic inorganic anions, it

s therefore possible that some of them may remain undetected,hereby increasing the possible health risks [1]. A number of inor-anic anions have been found in potentially harmful concentrationsn numerous drinking water sources [1–4]. Of these, nitrate (NO3

−)s of prime concern on a global scale. Nitrate is a naturally occur-ing ion in the nitrogen cycle that is the stable form of N forxygenated systems. It can be reduced by microbial action intoitrite (NO2

−) or other forms. The NO2− ion contains N in a rel-

tively unstable oxidation state. Chemical and biological processesan further reduce nitrite to various compounds or oxidize it to
O3
−.Nitrate, due to its high water solubility [5], is possibly the most

idespread groundwater contaminant in the world, imposing aerious threat to drinking water supplies and promoting eutroph-

∗ Corresponding author.E-mail addresses: amit [email protected], [email protected] (A. Bhatnagar).

385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.cej.2011.01.103

ication [6,7]. The presence of elevated concentrations of NO3− in

potable water has become a serious concern worldwide over therecent decades [8–12]. The increase in NO3

− levels can be linkedto several kinds of human activities especially the intensive useof fertilizers in agriculture, which have led to the higher NO3

−

contamination of ground and surface water sources [13]. Nitratedoes not readily bind to the soil causing it to be highly susceptibleto leaching. Point and non-point sources of NO3

− contaminationinclude agricultural and urban runoff, disposal of untreated sani-tary and industrial wastes in unsafe manner, leakage from septicsystems, landfill leachate, animal manure, NOx air stripping wastefrom air pollution control devices.

High NO3− concentrations in drinking water sources can lead

to a potential risk to environment and public health. High NO3−

concentrations are known to stimulate heavy algal growth thuspromoting the eutrophication in water bodies. After ingestion ofplants or water high in NO3

−, acute poisoning may occur within30 min to 4 h in cattle. Thus, the problem occurs quickly and oftenthe cattle are observed to be normal one day and found dead thenext day [14]. An early symptom is salivation followed by frequenturination. Soon after, the cattle exhibit difficult breathing, increasedrespiratory rate, and dark brown or “chocolate” colored blood andmucous membranes [15]. The animals then become weak, reluctantto move, and have convulsions before they die [15]. If pregnant cat-tle receive a dose that is not quite deadly, they may abort soon after
recovering [15].
In humans, increasing NO3− concentrations in drinking water

causes two adverse health effects: induction of “blue-baby syn-drome” (methemoglobinemia), especially in infants, and thepotential formation of carcinogenic nitrosamines [6,7]. Recent

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494 A. Bhatnagar, M. Sillanpää / Chemical Engineering Journal 168 (2011) 493–504

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tudies [16 and references therein] have shown that excess NO3−

n drinking water may also be responsible for causing diverse kindsf cancers in humans. Ward et al. [17] reviewed the epidemiologicvidence for the linkages between drinking water NO3

− and theisk of specific cancers, adverse reproductive outcomes, and otherealth outcomes in the context of the current regulatory limit foritrate in drinking water. Nitrate contaminated water supplies havelso been linked to outbreaks of infectious diseases in humans [18].iterature survey revealed that NO3

− ion also causes diabetes [19]nd is a precursor of carcinogen.

Keeping with the view that serious health problems are asso-iated with excess NO3

− concentrations in drinking water, variousnvironmental regulatory agencies including the U.S. Environmen-al Protection Agency (U.S. EPA) have set a maximum contaminantevel (MCL) of 10 mg/L of NO3

− in drinking water [20]. Nitrateontaminated water must be treated properly to meet applicableegulations.

. Technologies for the removal of nitrate from water

The most commonly used treatment methods to remove/reduceO3

− include chemical denitrification using zero-valent iron (Fe0)21–25], zero-valent magnesium (Mg0) [26], ion exchange (IX)27–29], reverse osmosis (RO) [30], electrodialysis (ED) [31], cat-lytic denitrification [32] and biological denitrification [33]. Worldealth Organization (WHO) has suggested biological denitrifica-

ion and IX as nitrate removal methods, while IX, RO, and EDre approved by US EPA as Best Available Technologies (BAT) toreat NO3

− contaminated water [34,35]. However, current availableechnologies for NO3

− removal have their own strength and limi-ations and are found to be expensive, less effective and generate
dditional by-products. Nevertheless, these traditional technolo-ies do not solve the problem related to the excess of NO3
− inhe environment; in turn, they generate NO3

− concentrated wastetreams that pose a disposal problem due to the high saline con-ent [33,36]. BATs are relatively expensive [31] and moreover, cause

ate removal technologies.

process complexity to be used in in situ application for direct decon-tamination of groundwater [37].

Zero-valent iron (ZVI) has been extensively studied for its abilityto reduce different contaminants including NO3

− in groundwater[21,38–42]. However, this technology has some limitations as dis-cussed by various researchers in different articles. For example,Cheng et al. [21] reported that the main disadvantages of NO3

−

reduction using ZVI are ammonium production and the pH con-trol requirement (by initial pH reduction or use of buffer). Whenapplying ZVI in an in situ remediation technique for NO3

− removal,these disadvantages are more critical [43]. Furthermore, biologicaldenitrification processes are difficult to apply to inorganic wastew-ater treatment because additional organic substrates are requiredto serve as electron donors [44]. Fig. 1 presents an overview of someof the technologies used for NO3

− removal from water [45,46].

3. Removal of nitrate from water using adsorption process

Adsorption process is generally considered better in water treat-ment because of convenience, ease of operation and simplicity ofdesign. Further, this process can remove/minimize different typesof organic and inorganic pollutants from the water or wastewa-ter [47–59] and thus it has a wider applicability in water pollutioncontrol [60]. Adsorption technology has been found successful inremoving different types of inorganic anions, e.g., fluoride [61–63],nitrate [64–66], bromate [67–69], perchlorate [70–72], from watersby using various materials as adsorbents. It should be noted herethat selection of appropriate material for the removal of specifictypes of anions is important to achieve optimum removal rates.Various conventional and non-conventional materials from dif-
ferent origins have been assessed for the removal of NO3
− fromwater, as will be discussed in the following sections in this paper.However, no overview of the sorption potentials of the various con-ventional and non-conventional adsorbents examined so far fornitrate removal has been published.

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A. Bhatnagar, M. Sillanpää / Chemica

This review focuses on the potential of various adsorbents forO3

− removal from water and wastewater. A summary of rele-ant published data (in terms of adsorption capacities, applicabledsorption isotherm models and kinetic models) with some of theatest important findings, and a source of up-to-date literature isresented and the results have been discussed. For informationertaining to detailed experimental methodology and conditions,eaders are referred to the full articles listed in the References.

.1. Removal of nitrate from water by carbon based adsorbents

Activated carbon is generally considered as a universal adsor-ent for the removal of diverse types of aquatic pollutantsspecially organic pollutants. However, it shows poor adsorptionowards anionic pollutants. Only a few studies are available report-ng the sorption of NO3

− by activated carbon. Afkhami et al. [73]tudied the effects of functional groups on the adsorption of NO3

−

nd NO2− by carbon cloth. The carbon cloths were chemically

tched in 4 M H2SO4 solution after deionization cleaning proce-ure and used for the adsorption of NO3

− and NO2− from water

amples at nearly neutral (pH ∼ 7) solutions. It was suggested [73]hat treatment of carbon cloth with acid produced positive sitesn the carbon cloth, by protonation of surface –OH groups causedn increase in electrostatic adsorption of anions. The dramaticncrease in the adsorption of anions by treatment of C-cloth withcid was attributed to the strong electrostatic interaction betweenhe negative charge of anions and positive charge of the surface. Thedsorption capacity of acid treated carbon cloth for NO3

− and NO2−

as 2.03 and 1.01 mmol/g, respectively. These values were muchigher than those obtained for distilled water treated carbon cloth0.38 and 0.05 mmol/g for NO3

− and NO2−, respectively). The effect

f competing ions was found to be negligible on the adsorption.Powdered activated carbon (PAC) and carbon nanotubes (CNTs)

ere used for the removal of NO3− from aqueous solution [74]. The

O3− adsorption capacity of CNTs was found to be higher than PAC

nd decreased above pH 5. The equilibration time for maximumO3

− uptake was 60 min. Adsorption capacity of the PAC and CNTsas found to be 10 and 25 mmol NO3

−/g adsorbent, respectively.Commercial granular activated carbon (GAC) (produced from

oconut shells by steam activation) was chemically activated withnCl2 and examined for NO3

− removal [75]. The optimal condi-ions were selected by studying the influence of process variablesuch as chemical ratio and activation temperature. Experimentalesults reveal that chemical weight ratio of 200% and activationemperature of 500 ◦C was found to be optimum for the maximumemoval of nitrate from water. The lower adsorption of NO3

− withAC prepared at 400 ◦C carbonization temperature was attributed

o the inadequacy of heat energy generated at low carbonizationemperature for any substantial evolution of volatile matters essen-ial for pore development. Furthermore, at 500 ◦C, more volatile

atters were released progressively during carbonization, therebyesulting in the development of some new pores, and hence thedsorption of NO3

− increased progressively. The decrease in thedsorption of NO3

− with further increase in carbonization tempera-ure to 600 ◦C might be due to a sintering effect at high temperature,ollowed by shrinkage of the char, and realignment of the carbontructure, which resulted in reduced pore areas as well as volume.he comparison between untreated and ZnCl2 treated GAC indi-ated that treatment with ZnCl2 had significantly improved thedsorption efficacy of untreated GAC. The adsorption capacity ofntreated and ZnCl2 treated coconut GACs were found to be 1.7 and
0.2 mg/g, respectively. The higher uptake of NO3
− in ZnCl2 treatedAC was attributed to the increased microporosity and formationf zinc oxide in macro- and mesopores in ZnCl2 treated GAC, whichesulted in the enhanced NO3

− adsorption. The adsorption behav-or of NO3

− was investigated from aqueous solution using activated

eering Journal 168 (2011) 493–504 495

carbon (AC) prepared from coconut shells and charcoal (CB) pre-pared from bamboo [76]. The maximum removal of NO3

− by theprepared adsorbents occurred at equilibrium pH 2–4, and was fit-ted well with Langmuir model. The adsorption capacity for AC andCB were reported as 2.66 × 10−1 mmol/g and 1.04 × 10−1 mmol/g,respectively.

The adsorption effectiveness of bamboo powder charcoal (BPC)in removing NO3

− from water has been investigated [77]. The BPCwas prepared by heating the bamboo powder in an electric furnaceat 900 ◦C for 1 h. The calculated uptake values of BPC and com-mercial activated carbon (CAC) at 10 ◦C were 1.25 and 1.09 mg/g,respectively. The results show that the adsorption effectiveness ofBPC for NO3

− was higher than that of CAC regardless of the concen-tration of NO3

− (0–10 mg/L), and temperature (10–20 ◦C).The adsorption of NO3

− including other inorganic anions(bromate, chlorate, chloride, iodate, perchlorate, sulfate, and (di-hydrogen) phosphate) was studied using activated carbon F400 atpH 4 and concentration range from 0.1 to 1.0 mM [78]. The adsorp-tion density of 0.29 mmol/g was observed for nitrate. Among all theanions studied, only nitrate showed competitive adsorption withperchlorate. It was suggested that perchlorate and nitrate prefer thesame surface sites in the adsorption process. Iron oxide-dispersedactivated carbon fibers were also used for NO3

− adsorption [79].As revealed from the literature, surface modification of carbon-based sorbents has found to increase sorption when compared toadsorbents without surface modification.

3.2. Removal of nitrate from water by clay adsorbents

Clays are hydrous aluminosilicates broadly defined as thoseminerals that make up the colloid fraction (<2 �m) of soils, sedi-ments, rocks and water [80] and may be composed of mixtures offine grained clay minerals and clay-sized crystals of other miner-als such as quartz, carbonate and metal oxides [81]. Clays play animportant role in the environment by acting as a natural scavengerof pollutants by taking up cations and anions either through ionexchange or adsorption or both [81]. Calcium bentonite was modi-fied by acid thermoactivation with HCl (2N) and H2SO4 (2N and 4N),and investigated for NO3

− removal from aqueous solutions [82].Calcium montmorrillonite activated by HCl showed better NO3

−

removal capacity, up to 22.28%. It was explained that an ionic inter-change took place between chlorine (due to the hydrochloric acidtreatment) and nitrate ions. The ionic exchange was also confirmedby the presence of KCl in the clay residue. The Brunauer-Emmet-Teller (BET) area measurements showed no direct relation betweenthe surface area and the nitrate removal capacity. Nitrates adsorp-tion in clays was further confirmed by using infrared spectroscopy.

Xi et al. [83] prepared the surfactant modified clay minerals andevaluated them for NO3

− adsorption. It was found that untreatedQueensland (QLD) bentonite did not remove NO3

− from the solu-tion, and kaolinite also showed similar behavior for NO3

− removal.However, halloysite was found to remove ca. 0.54 mg NO3

− pergram of clay. As all these untreated clays showed poor adsorp-tion capacities for nitrate ions, these clays were modified withnonfunctional surfactant hexadecyltrimethylammonium bromide(HDTMA) in 2 or 4 cation exchange capacity (CEC) and the removalcapacities of these materials were found to greatly improve. Amongall these organoclays, HDTMA modified QLD-bentonite showed thebest result: H-B-2CEC and H-B-4CEC could remove 12.83 mg and14.76 mg NO3

−/g of organoclay, respectively. But HDTMA modifiedkaolinite and halloysite were found to remove less NO3

− compared
to H-B samples (e.g., H-K-2CEC, H-H-2CEC and H-H-4CEC removed1.54, 1.78 and 1.93 mg NO3
−/g of clay, respectively). When the con-centration of HDTMA was increased to 4 CEC, the removal capacitywas increased to 4.87 mg/g on H-K-4CEC. It was explained by thefact that the CEC of QLD-bentonite (66.67 meq/100 g) was higher

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96 A. Bhatnagar, M. Sillanpää / Chemica

han that of kaolinite and halloysite (9.78 and 10 meq/100 g, respec-ively). Therefore, after modification with HDTMA, more surfactant

olecules were exchanged and attached on the surface of QLDentonite than that of other two clays.

.3. Removal of nitrate from water by layered-doubleydroxides/hydrotalcite-like compounds/hydroxyapetite asdsorbents

Hydrotalcite-like compounds, also known as layered doubleydroxides (LDHs), constitute an important class of inorganic mate-ials with desirable properties to remove anionic pollutants fromater [15]. Zn-Al-Cl LDH was synthesized by co-precipitationethod and was characterized using various instrumental tech-

iques [15]. It was further tested towards NO3− removal from

ater. The removal of NO3− was found to be 85.5% under neu-

ral conditions, using 0.3 g of LDH in 100 mL of NO3− solution

aving initial concentration of 10 mg/L. Adsorption kinetic studyevealed that the adsorption process followed first order kinetics.dsorption data were fitted to Langmuir isotherm. The Langmuirorption capacity was found to be 40.26 mg/g. The percentageemoval was found to decrease gradually with increase in pH andhe optimum pH was found to be 6. The presence of competitivenions reduced the NO3

− adsorption in the order of carbon-te > phosphate > chloride > sulfate. The Zn-Al-Cl LDH exhibited lowesorption and poor regeneration.

LDHs with different kinds of metal ions (Mg-Al, Co-Fe, Ni-Fe,nd Mg-Fe) in the brucite layers were prepared by Tezuka et al.84] and their anion exchange properties were studied by measure-

ents of distribution coefficient (Kd) and ion exchange capacity.he basal spacing of LDHs varied depending on the kind of metalons in the brucite layer. A relatively high Kd value for NO3

− ionsas observed on Ni-Fe type LDHs, and a markedly high Kd value

or the hydrothermally-treated Ni-Fe type LDHs (Ni-Fe (HT)), pre-ared at 120 ◦C. These high Kd values correlated with the basalpacing of 0.81 nm observed for this sample, where the inter-ayer distance (0.33 nm) is suitable for the stable fixing of NO3

−

ons (ionic size = 0.33 nm). A chemical analysis study showed al−/NO3

− ion-exchange mechanism for NO3− adsorption on Ni-

e (HT). The NO3− uptake by Ni-Fe (HT) was nearly constant

NO3/Fe = 0.7) over a pH range between 5 and 10, which was alsoupported by the adsorption mechanism of Cl−/NO3

− ion exchange.he Ni-Fe (HT) could remove NO3

− ions from seawater effectivelyNO3

− uptake = 168 �mol/g) even though seawater consisted largemount of coexisting anions (2.3 mM of carbonate ions, 14 mM ofulfate ions, 550 mM of chloride ions, etc.).

The selective adsorptive properties of Ni-Fe layered doubleydroxide (LDH (Ni-Fe)), containing Ni and Fe metal atoms in each

ayer were studied for NO3− removal from seawater [85]. LDH (Ni-

e) with Cl− in the interlayers was synthesized by co-precipitationt constant pH. It showed a higher Kd for NO3

− than for othernions (HPO4

2− and SO42−). The prepared LDH (Ni-Fe) was also

tudied for its potential for NO3− removal by batch method using

O3− enriched seawater (NO3

− concentration: 40 �mol/dm3). Thequilibrium was achieved in 4 h with adsorption data followinghe Freundlich model. The maximum NO3

− uptake was found toe 0.33 mmol/g when LDH (Ni-Fe) (0.10 g) was added to NO3

−

nriched seawater (1 dm3), corresponding to the removal of 83% ofhe NO3

− from seawater. The pH dependence of NO3− adsorption

howed a maximum NO3− uptake at around pH 8. Dissolutions of Ni

nd Fe from LDH (Ni-Fe) was less than 0.6% for Ni and less than 0.1%
or Fe at pH 8, indicating that LDH (Ni-Fe) was sufficiently stable ineawater.
The sorption of NO3− on calcined hydrotalcite-type compounds

t 550 ◦C (HT550), 650 ◦C (HT650), and 850 ◦C (HT850) from watert 25 ◦C has been studied by Socías-Viciana et al. [86]. The influence

neering Journal 168 (2011) 493–504

of the temperature was also investigated for the sample calcined at850 ◦C by studying the sorption process at 10 and 40 ◦C. The exper-imental sorption data were fitted to the Langmuir model. Sorptioncapacities of the samples ranged from 61.7 g/kg (HT550 at 25 ◦C) to147.0 g/kg (HT850 at 40 ◦C). The values for the removal efficiencyranged from 70.5% for HT550 at 25 ◦C to 99.5% for HT850 at 40 ◦C.The sorption experiments showed that at higher calcination tem-perature (850 ◦C), the removal of NO3

− was greater. The increasein the temperature from 10 to 40 ◦C for sample HT850 also tendedto increase the sorption of NO3

− from 63.3 g/kg to 147 g/kg and thecorresponding removal efficiency ranged from 71.5 to 99.5%. Thevariation of amount adsorbed of NO3

− and removal efficiency withthe heat treatment applied to the hydrotalcite seemed to be relatedto the sorption mechanisms of the NO3

− ions; i.e., related to theaccess by the NO3

− anions to the recovered layered structure of thecalcined hydrotalcite once rehydrated, by occupying the locationsof the carbonate anions initially present in the synthesized sample(HT). It was noted that for the calcined samples, and according tothe characterization data discussed by the authors, a change in thehydrotalcite structure was observed, mainly due to the loss of CO2and the formation of amorphous Mg1−xAlxO1+x/2 mixed oxide. There-hydration of this mixed oxide lead to the incorporation of NO3

−

in the interlayer space, so the sample recovered its original struc-ture. As the calcination temperature increased to 850 ◦C, a greaterloss of CO2 occurred, thus increasing the amount of NO3

− adsorbed.The adsorption of NO3

− by various layered double hydroxides(LDHs), such as Mg-Al and Zn-Al, was investigated by Hosni et al.[87]. The samples were identified as [MII3P10], where M representsthe divalent cation used to prepare the materials. For example,Mg3P10 stands for the precipitate prepared with Mg2+ as the diva-lent cation and Mg/Al molar ratio of 3, at pH 10. The nature andcontent of divalent cations in LDHs showed a strong influence onthe adsorption process. Calcined Mg-Al LDH with an Mg/Al molarratio of 3.0 showed higher adsorption capacity compared to othercalcined LDHs. The sorption capacity of Mg3P10-500 (activated byheating at 500 ◦C) was found to be 35 mg/g, and that of Zn3P10-500was 20 mg/g. This difference in sorption capacity was explainedby the fact that the nature of the divalent cation in LDH has astrong influence on the adsorption process. The removal of NO3

−

was found to take place via an adsorption process followed by areconstruction of the calcined material. Authors concluded thatthe quantity of NO3

− removed (35 mg/g), which corresponds to56 meq/100 g, was small and it was explained by the fact thatthe CO3

2− coming from the dissolution of atmospheric CO2 dis-placed NO3

−. The carbonate is attributed to their divalent character,their relative small ionic radius, and the strong hydrogen bond thatoccurred between NO3

− and the brucite-like sheets.A laboratory study was conducted to investigate the ability of

Mg-Al-Cl hydrotalcite-like compound for the removal of NO3− from

synthetic NO3− solution [88]. The removal of NO3

− was 87.6% underneutral condition, using 0.3 g of adsorbent in 100 mL of nitratesolution having an initial concentration of 10 mg/L. The percent-age removal was found to gradually decrease (from 87.6% to 69.9%)with an increase in pH (from 6 to 12) and the optimum pH forremoval of nitrate was 6. The equilibrium was established within40 min. The effect of other anions was also studied and it was foundthat the anions reduced the NO3

− adsorption in the order of car-bonate > phosphate > chloride > sulfate. A regeneration study of thematerial with 1–4% NaCl was also carried out and it was foundthat the Mg-Al-Cl hydrotalcite could not be easily regenerated andreused. The percentage regeneration was <2%. It was therefore con-
cluded that once HTlc-nitrate is formed, it is difficult to regenerate.The efficiency of hydroxyapatite (HAP) for NO3
− removal from syn-thetic NO3

− solution was studied by the same workers [89]. Theresults of pH study revealed that there was an increase in the per-centage removal for increase in pH from 2 to 6 but further increase

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n pH beyond 6, no increase in removal was observed. The effectf competing anions reduced the NO3

− adsorption in the orderf chloride > carbonate > sulfate > phosphate. It was suggested thatH− ions were exchanged by NO3

− ions.

.4. Removal of nitrate from water by zeolite adsorbents

Natural zeolites are hydrated aluminosilicate minerals of aorous structure with valuable physicochemical properties [90].eolites have been widely used as adsorbents in separation andurification processes and remain a promising technique in envi-onmental cleaning processes [90]. Besides their natural form,urface modified zeolites have also been tested for the removal ofater pollutants. Surface modifications of natural zeolite were per-

ormed by coating it with a chitosan layer [91]. The chitosan coatedeolite (Ch-Z) was protonated with either sulfuric or hydrochlo-ic acid and tested for its suitability to capture NO3

− from watert 20 and 4 ◦C. It was found that protonation with hydrochloriccid resulted a higher maximum NO3

− exchange capacity whenompared to sulfuric acid. The surface characterization of the Ch-Zas performed using scanning electron microscopy (SEM), Fourier

ransform infrared spectrometry (FTIR), thermogravimetric analy-is (TGA) and nitrogen adsorption tests. The results of these testshowed evidence of chitosan coating onto zeolite particles. Ch-Z hascomparable ion exchange capacity to other weak anion exchang-rs with a NO3

− ion exchange capacity of 0.74 mmol NO3−/g

protonated with HCl).Nitrate adsorption kinetics were determined in batch-wise

xperiments on a nonfunctional surfactant-modified zeoliteSMZ), prepared by treatment of a clinoptilolite sample byDTMABr (HDTMA+ being the hexadecyltrimethylammoniumation) [92]. The influence of various parameters, namely ini-ial NO3

− concentration (0.08–8.06 mmol/L), liquid/solid weightatio (L/S = 5–50 mL/g) and presence of Cl−, SO4

2− and HCO3−

ompeting anions (at the same equimolar concentration equal to.61 mmol/L) were studied. The equilibrium time for NO3

− uptakeas short (0.5–1 h), with a maximal removal value (Rmax.) at equi-

ibrium. Rmax. was found to decrease significantly to 58% and 40%,espectively, for higher concentrations (4.83 and 8.06 mmol/L). Thenal removal rate was ≥80%, with liquid/solid weight ratios ≤10,

nitial NO3− concentrations in the range 0.08–2.42 mmol/L (corre-

ponding to 5–150 mg/L). The sorption equilibrium data were inood agreement with the Langmuir isotherm model. On the otherand, the Rmax. value decreased with the increase in adsorbentass. Under the studied experimental conditions, the presence

f competing anions did not change the NO3− Rmax. value, but

lowed down the exchange kinetics and the increasing affinityrder towards the SMZ was found as follows: Cl− � HCO3

− #O4

2− < NO3−. Other studies have also examined the feasibility

f zeolites for nitrate removal from water [93,94]. Experimentalesults [93] showed that shallow-well water (with NO3

− concen-ration of 74–288 mg/L), after 1 h. mixing with 5 g of 0.315 mmarticle-sized zeolite and after 30 min. sedimentation, NO3

− con-entration stayed the same without any reduction. However, theame shallow-well water (with 1–10 mg/L ammonium ion con-entration) mixed with 5 g of 0.315–0.63 mm particle-sized zeolitehowed ammonium ion removal efficiency of 72–86%, revealinghat zeolite particles were not found suitable for NO3

− sorptionrom water solutions, but 0.315–0.63 mm particle-sized zeoliteould be a useful sorbent for NH4

+ removal from water. Hexade-yltrimethyl ammonium bromide (HDTMABr) surfactant modified
eolite (SMZ) were prepared by Masukume et al. [94] and evalu-ted as a potential adsorption media for NO3
−removal from water.t was found that surfactant modification of zeolite resulted in aignificant increase in the adsorption capacity (∼11.5 mg/g) of thedsorbent.

eering Journal 168 (2011) 493–504 497

Surfactant modified zeolites with different coverage types wereprepared by loading the cetylpyridinium bromide (CPB) onto thesurface of the natural zeolites (NZ) [95]. The adsorption behaviorof NO3

− on SMZ was investigated. NZ and SMZ with monolayerCPB coverage were found inefficient for the removal of NO3

− fromaqueous solution. However, SMZ with patchy bilayer or bilayer CPBcoverage was efficient in NO3

− removal, and the NO3− adsorp-

tion capacity of SMZ increased with its CPB loading. Based onthe Langmuir isotherm model, the predicted maximum monolayerNO3

− adsorption capacity for SMZ7 (SMZ sample with CPB loadingamounts of 409 mmol/(kg NZ)) was found to be 9.36 mg/g. Anionicexchange and electrostatic attraction were the main mechanismsresponsible for the adsorption of NO3

− onto SMZ7. The presenceof chloride, sulfate or bicarbonate ions in solution slightly reducedthe NO3

− adsorption efficiency for CPB modified zeolite.

3.5. Removal of nitrate from water by chitosan adsorbents

Chitin and chitosan-derivatives have gained wide attention aseffective biosorbents due to low cost and high contents of aminoand hydroxyl functional groups which show significant adsorp-tion potential for the removal of various aquatic pollutants [96].Chitosan hydrobeads were prepared by Chatterjee and Woo [97]and examined for the adsorption of NO3

−. The maximum adsorp-tion capacity was 92.1 mg/g at 30 ◦C. Intraparticle diffusion wassuggested to play a significant role at the initial stage of the adsorp-tion process. Nitrate adsorption was found to increase with adecrease in the pH of the solution which was explained by thefact that a decrease in the pH of the solution resulted in moreprotons being available to protonate the chitosan amine group.This resulted in an enhancement of NO3

− adsorption by the chi-tosan beads due to increased electrostatic interactions between thenegatively charged NO3

− group and the positively charged aminegroup. Above pH 6.4, an appreciable amount of NO3

− adsorptionby chitosan beads indicated the involvement of physical forces.The equilibrium adsorption capacity decreased by increasing thetemperature from 30 to 50 ◦C which was explained due to eitherthe damage of active binding sites of the adsorbent or increasingtendency to desorb NO3

− ions from the interface to the solution.However, increase in temperature from 20 to 30 ◦C reduced thenitrate adsorption, which was attributed to an increase of themobility of the NO3

− ions and a swelling effect within the internalstructure of chitosan beads. Desorption of NO3

− from the loadedbeads was accomplished by increasing the pH of the solution to thealkaline range, and a desorption ratio of 87% was achieved aroundpH 12.0.

The same authors [98] also reported the adsorption of NO3−

onto chitosan beads modified via crosslinking with epichlorohy-drin (ECH) and surface conditioning with sodium bisulfate. Themaximum adsorption capacity was found at a cross-linking ratioof 0.4 and a conditioning concentration of 0.1 mM NaHSO4. Itwas reported that ECH mainly cross-links chitosan beads usingthe –OH group of chitosan and did not interact with the cationicamine groups of chitosan during the cross-linking. The maximumvalue for the equilibrium adsorption capacity of cross-linked chi-tosan beads was found at a 0.4 cross-linking ratio, and furtherincrease in this ratio slightly reduced the uptake value. Thus, anincrease in the cross-linking ratio did not reduce the availableadsorption sites but increased the steric hindrance for diffusionthrough the chitosan beads. The conditioning of chitosan beadswith NaHSO4 increased its equilibrium adsorption capacity because
the amine groups of chitosan were protonated by the H+ pro-duced from the dissociation of NaHSO4 during conditioning. Themaximum adsorption capacity was 104.0 mg/g for the conditionedcross-linked chitosan beads at pH 5, while it was 90.7 mg/g fornormal chitosan beads. The NO3
− adsorption was found to be

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trongly pH dependent, and the maximum NO3− removal was

ound at pH 3. The high adsorption capacities in acidic solu-ions (pH 3–5) were due to the strong electrostatic interactionsetween adsorption sites and NO3

−. The mean adsorption energiesbtained from the Dubinin-Radushkevich (D-R) model indicatedhat physical electrostatic forces were involved in the adsorptionrocess. The feasibility of NO3

− removal by cross-linked chitosanel beads was also investigated by Jaafari et al. [99–100]. Pro-onated cross-linked chitosan gel beads were prepared [99] andxamined for NO3

− removal. The sorption capacity was foundo be dependent on pH and was maximal at pH of between 3nd 5. Chloride and sulfate did not show interference but fluo-ide slightly lowered the NO3

− sorption. Increasing pH to 12 withaOH allowed desorption of NO3

− without losing the effectivenessf the sorbent. The same workers extended their study to simu-ate and design the operation of a fixed bed adsorber [100]. It waseported by the authors that daily water required for 5–10 peopleould be produced with an adsorber of 0.2 m diameter and 1.5 mength.

.6. Removal of nitrate from water by agricultural wastes asdsorbents

The use of agricultural waste materials is an attractive options it combines the reuse of waste materials in the remediation ofater and wastewaters. Different agricultural wastes have been

tudied for the removal of NO3− from aqueous solutions. Orlando

t al. [101] investigated the feasibility of lignocellulosic agriculturalaste materials (LCM), sugarcane bagasse (BG) and rice hull (RH)

fter converting them into weak base anion exchangers and furthervaluated their potential for NO3

− removal from water. Pure cellu-ose (PC) and pure alkaline lignin (PL) were also used as reference

aterials to elucidate possible reactivity in LCM. Epoxy and aminoroups were introduced into BG, RH, PC and PL substrates afterhe reaction with epichlorohydrin and dimethylamine in the pres-nce of pyridine and an organic solvent N,N-dimethylformamideDMF). It was found that amino group incorporation into celluloseecreased with the presence of water in the reaction mixture and

ncreased with the reaction time and presence of a catalyst (pyri-ine). The highest maximum NO3

− exchange capacity and yieldsf the prepared exchangers was obtained from PL (1.8 mmol/g and12.5%), followed by BG (1.41 mmol/g and 300%), PC (1.34 mmol/gnd 166%) and RH (1.32 mmol/g and 180%). The proposed syntheticrocedure was found effective in modifying PL, PC and LCM result-

ng in a higher yield and NO3− removal capacity.

Anionic sorbent using wheat straw was prepared and examinedor NO3

− removal from aqueous solution by Wang et al. [102]. Theven-dried and sieved (150–250 �m) raw wheat straw (RWS) wasrosslinked with epichlorohydrin and dimethylamine. The resultsndicated that the yield of the prepared anionic sorbent, the totalxchange capacity, and the maximum adsorption capacity were50%, 2.57 mEq/g, and 2.08 mmol/g, respectively. In the presence ofmixed ion solution, the preferential adsorption of the anions was

n the following order, SO42− > H2PO4

− > NO3− > NO2

−. The capac-ty of NO3

− adsorption was reduced by 50%. Desorption and reusexperiments showed that about 90% of the adsorbed NO3

− ionsould be desorbed from modified wheat straw (MWS) anionic sor-ent using 30 mL of 0.1 M NaOH.

In another study [103], the raw wheat residue (RWR) wasodified by epichlorohydrin in the presence of pyridine and the

dsorption kinetics were investigated in batch experiments. The
ignificant increase in the zeta potential (from −35 mV to 40 mV)nd total exchange capacity (TEC) (from 0.25 mEq/g to 2.57 mEq/g)f modified wheat residue (MWR) was observed after chemicalreatment which greatly enhanced the anion adsorption capacity.he results showed that the MWR had greater anion adsorbing

capacity. The maximum adsorption capacity of RWR and MWRwere 0.02 mmol/g and 2.08 mmol/g, respectively.

Almond shell activated carbon impregnated by Zn◦ and ZnSO4were used as adsorbent with a particle size of 10–20 mesh forNO3

− removal by Rezaee et al. [104]. Experimental data showedthat modified activated carbon by Zn◦ and ZnSO4 was more effec-tive than virgin almond activated carbon for NO3

− removal. It waspostulated that after modification of activated carbon, macrop-ores were filled by Zn◦ and ZnSO4 and micropores were formed.Furthermore, zinc present at carbon surface as ZnO increased pos-itive charge of activated carbon which resulted increased NO3

−

adsorption. The maximum NO3− removal was 64–80% and 5–42%

for modified activated carbon and virgin activated carbon, respec-tively. Maximum removal was ca. 16–17 mg NO3

− per g activatedcarbon for impregnated activated carbon.

The effectiveness of wheat straw charcoal (WSC) and mustardstraw charcoal (MSC) as adsorbents for the removal of NO3

−-N fromwater has been investigated [105]. Wheat straw charcoal and mus-tard straw charcoal were prepared by taking clean and air dried(for 24 h) straw which was sieved (sieve size was ASTM—7–10) andwere heated in a muffle furnace at 300 ◦C for 1 h and 30 min. Thenthe charcoal was washed by using double distilled water to removeany color due to the presence of carbon particles. Commercial acti-vated carbon (CAC) from Eureka Forbes Limited was used as astandard for comparison. The calculated values of amount adsorbedof WSC, MSC and CAC at 15 ◦C were 1.10, 1.30 and 1.22 mg/g,respectively, which showed that the adsorption effectiveness ofMSC was higher than that of WSC and CAC used in this experi-ment.

The nitrate removal was evaluated using a fixed-bed columnpacked with amine-crosslinked wheat straw (AC-WS) [106]. Solid-state 13C NMR and zeta potential analysis validated the existenceof crosslinked amine groups in AC-WS. Raman shift of the nitratepeaks suggested the electrostatic attraction between the adsorbedions and positively charged amine sites. The column sorptioncapacity of the AC-WS for nitrate was 87.27 mg/g in comparisonwith the raw WS of 0.57 mg/g. Nitrate sorption in column wasaffected by bed height, influent nitrate concentration, flow rate andpH, and of all these, influent pH demonstrated significant effect onthe performance of the column. HCl solution (0.1 mol/L) demon-strated its high desorption rate for the regeneration of AC-WS. Inaddition, the sorption-desorption process indicated the excellentregeneration capacity of AC-WS with little loss (5.2%) in its initialsorption capacity when repeatedly used.

A new inorganic/sugar beet pulp composite material was pre-pared from sugar beet pulp (SBP) after loading with zirconium(IV)ions [107]. The prepared anion exchanger material was exam-ined for its ability to remove sulfate and NO3

− from water. Theeffect of contact time, anions concentration, temperature, and pHon the adsorption capacity of Zr(IV)-loaded SBP was studied. Themaximum adsorption capacity of Zr(IV)-loaded SBP was about114 mg/g and 63 mg/g for sulfate and NO3

−, respectively. In addi-tion, the effect of the regeneration of the Zr-loaded SBP after anionremoval was also studied. The results of anion adsorption tests andenergy dispersed X-ray (EDX)-SEM showed that zirconium ionswere strongly bound to the carboxylate groups of SBP constituents,especially pectins, and were not leached as a result of regeneration.

Activated carbon was prepared from sugar beet bagasse bychemical activation using ZnCl2 and the prepared activated car-bon was used to remove NO3

− from aqueous solutions [108]. Themaximum specific surface area of the activated carbon was about
1826 m2/g at 700 ◦C and at an impregnation ratio of 3:1. It wasdiscussed by the authors that increasing the carbonization temper-ature increased the evolution of volatile matters from the precursor,leading to the increase in the pore development, and creating newpores. Maximum removal (41.2%) was achieved at pH 3. The maxi-

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3a

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um adsorption capacity increased from 9.14 to 27.55 mg/g as theemperature increased from 25 to 45 ◦C.

Kinetic and thermodynamic studies were carried out for thedsorption of NO3

− from aqueous solutions onto modified beetesidue and for desorption from the sorbent to the solution in batchxperiments [109]. The beet residue was modified by epichlorohy-rin in the presence of pyridine. The experiments were conducted

n the presence and absence of ultrasound. It was found thatore than 90% of NO3

− was removed in <2 min from the solu-ion. Results indicated that the adsorption of NO3

− in the presencef ultrasound was higher at lower temperature (10 ◦C) and it wasower at higher temperatures with respect to the control methodwithout ultrasound). The maximum capacity was higher in con-rol method (57.12–85.9 mg/g) than in the presence of ultrasound48.4–86.16 mg/g). This behavior could be explained by the acous-ic cavitation (formation, growth, and collapse of the cavity). Theritical conditions produced during the cavitation could reduce theorbed species from the sorbent and lead to the lower amount ofO3

− uptake. In the case of desorption study, the amount of des-rption was higher in the presence of ultrasound than in its absencet different applied temperatures.

.7. Removal of nitrate from water by industrial wastes asdsorbents

Only a few industrial wastes have been tested as adsorbentsor NO3

− removal. Cengeloglu et al. [110] reported the removal ofO3

− from aqueous solution by using the original (water washednd dried) and activated (water washed and HCl treated) red mud.he NO3

− adsorption capacity of activated red mud was found toe higher than that of the original form and decreased above pH
. Adsorption capacity of the original and activated red mud wasound to be 1.859 and 5.858 mmol NO3
−/g red mud, respectively.he increase in sorption capacity in red mud after acid treatmentas attributed to the leaching out of the sodalite compounds dur-

ng acid treatment, which are expected to hinder the adsorption

Fig. 2. List of different adsorbents used fo

eering Journal 168 (2011) 493–504 499

by blocking the available adsorption sites for nitrate in untreatedred mud. The equilibrium time of nitrate sorption was 60 min. Themechanism for NO3

− removal was explained by considering thechemical nature of red mud and the interaction between metaloxides surface and NO3

− ions. Other industrial wastes (e.g., slag, flyash) have been examined with other adsorbents and their resultshave been discussed under Section 3.8.

3.8. Removal of nitrate from water by miscellaneous adsorbents

Sepiolite activated by HCl, slag and powdered activated carbonwere tested as adsorbents for NO3

− removal [111]. The equilib-rium time was found to be 30, 45, 5 min for sepiolite, powderedactivated carbon and activated sepiolite, respectively. The mosteffective pH for nitrate removal was 2 for powdered activated car-bon, however, pH did not affect nitrate removal significantly forother adsorbents. Sepiolite activated by HCl showed the highestpotential (38.16 mg/g) for NO3

− removal which was explained dueto increase in surface area after activation as the proton (H+) of theacids was replaced by part of the Mg2+ ions located in the octa-hedral sheet during acid activation. Furthermore, more carbonatesin sepiolite were partially decomposed leading to new pores andfresh surfaces.

In order to increase the positive charge on the surface, sepiolitewas modified by treatment with nonfunctional surfactant dode-cylethyldimethylammonium (DEDMA) bromide [112]. After mod-ification, it was found that maximum NO3

− adsorption occurred atpH 2.0. The results indicated that the surfactant-modified sepiolitewas more effective (453 mmol/kg) than the unmodified sepiolite(408 mmol/kg) for NO3

− removal. Dried chinese reed (Miscant-hus sinensis), a fast growing plant, was used as a model biomass
for the development of anion exchangers using a quaternizationagent, N-(3-chloro-2-hydroxypropyl)trimethylammonium chlo-ride (CHMAC), and a cross-linking agent, epichlorohydrin byNamasivayam et al. [113]. The adsorption capacity of the cross-linked and quaternized chinese reed for NO3
− was found to be

r the removal of nitrate from water.

500 A. Bhatnagar, M. Sillanpää / Chemical Engineering Journal 168 (2011) 493–504

Table 1Adsorption capacities and other parameters for the removal of nitrate by different sorbents.

S. no. Adsorbent Amount adsorbed Concentration range Contact time Temperature pH Ref.

1. H2SO4 treated carbon cloth 2.03 mmol/g 115 mg/L 60 min 25 ◦C ∼7.0 [73]2. Powdered activated carbon 10 mmol/g – 60 min 25 ◦C <5.0 [74]3. Carbon nanotubes 25 mmol/g – 60 min 25 ◦C <5.0 [74]4. Untreated coconut granular activated

carbon1.7 mg/g 5–200 mg/L 2 h 25 ◦C 5.5 [75]

5. ZnCl2 treated coconut granularactivated carbon

10.2 mg/g 5–200 mg/L 2 h 25 ◦C 5.5 [75]

6. Coconut shell activated carbon 2.66 × 10−1 mmol/g – – 303 K 2–4 [76]7. Bamboo charcoal 1.04 × 10−1 mmol/g – – 303 K 2–4 [76]8. Bamboo powder charcoal 1.25 mg/g 0–10 mg/L 120 h 10 ◦C – [77]9. Halloysite 0.54 mg/g 100 mg/L 17 h Room temperature 5.4 [83]

10. HDTMA modified QLD-bentonite 12.83–14.76 mg/g 100 mg/L 17 h Room temperature 5.4 [83]11. Calcined hydrotalcite-type compounds 61.7–147.0 g/kg 12.7–236 mg/L 24 h 25 ◦C – [86]12. Layered double hydroxides 20–35 mg/g 0–1000 mg/L 4 h 21 ◦C ∼8.5 [87]13. Chitosan coated zeolite 0.6–0.74 mmol/g 10–3100 mg/L 72 h 4 ◦C and 20 ◦C – [91]14. Chitosan hydrobeads 92.1 mg/g 1–1000 mg/L 1440 min 30 ◦C 5.0 [97]15. Chitosan beads 90.7 mg/g 25–1000 mg/L 24 h 30 ◦C 5.0 [98]16. Conditioned cross-linked chitosan

beads104.0 mg/g 25–1000 mg/L 24 h 30 ◦C 5.0 [98]

17. Pure alkaline lignin 1.8 mmol/g 1–30 mg/L 48 h 30 ◦C – [101]18. Sugarcane bagasse 1.41 mmol/g 1–30 mg/L 48 h 30 ◦C – [101]19. Pure cellulose 1.34 mmol/g 1–30 mg/L 48 h 30 ◦C – [101]20. Rice hull 1.32 mmol/g 1–30 mg/L 48 h 30 ◦C – [101]21. Raw wheat residue 0.02 mmol/g 50–500 mg/L 150 min 23 ± 2 ◦C 6.8 [103]22. Modified wheat residue 2.08 mmol/g 50–500 mg/L 150 min 23 ± 2 ◦C 6.8 [103]23. Impregnated almond shell activated

carbon16–17 mg/g 10–50 mg/L 120 min 20 ◦C 6.2 [104]

24. Wheat straw charcoal 1.10 mg/g 0–25 mg/L 10 min 15 ◦C – [105]25. Mustard straw charcoal 1.30 mg/g 0–25 mg/L 10 min 15 ◦C – [105]26. Commercial activated carbon 1.22 mg/g 0–25 mg/L 10 min 15 ◦C – [105]27. Zr(IV)-loaded sugar beet pulp 63 mg/g – 24 h 25 ◦C 6.0 [107]28. Chemically modified sugar beet

bagasse9.14–27.55 mg/g 10–200 mg/L – 25–45 ◦C 6.58 [108]

29. Original and activated red mud 1.859 and 5.858 mmol/g 5–250 mg/L 60 min Room temperature 6.0 [110]30. Sepiolite activated by HCl 38.16 mg/g 100 mg/L 5 min – – [111]31. Unmodified sepiolite 408 mmol/kg – – – – [112]32. Surfactant-modified sepiolite 453 mmol/kg – – – 2.0 [112]

10 3 ◦

10

7difu

aMvetab

TK

33. Cross-linked and quaternized chinesereed

7.55 mg/g

34. Ammonium-functionalizedmesostructured silica

46.0 mg/g

.55 mg of anion per g of the anion exchanger. The pH effect andesorption studies confirm that removal of NO3

− occurred throughon exchange on the quaternized biomass. The presence of sul-ate, fluoride, phosphate, and perchlorate considerably lowered theptake of nitrate.

Adsorption of NO3− and monovalent phosphate anions from

queous solutions on ammonium-functionalized mesoporousCM-48 silica was investigated [114]. The adsorbent was prepared

ia a post-synthesis grafting method, using aminopropyltri-thoxysilane, followed by acidification in HCl solution to converthe attached surface amino groups to ammonium moieties. Thedsorbent was determined to be effective for the removal ofoth anions. The removal of NO3

− was maximum at pH <8,

able 2inetic studies of nitrate on different adsorbents.

S. no. Adsorbent

1. Carbon cloth2. Untreated coconut granular activated carbon3. ZnCl2 treated coconut granular activated carbon4. Chitosan hydrobeads5. Modified wheat straw6. Raw wheat residue7. Modified wheat residue8. Zr(IV)-loaded sugar beet pulp9. Chemically modified sugar beet bagasse

10. Modified beet residue11. HCl activated sepiolite

–40 mg/dm 10 min 25 C 5.8 [113]

0–700 mg/L 60 min 5 ◦C <8.0 [114]

whereas phosphate removal was maximized at 4 < pH < 6. Max-imum removal of 71% and 88% was obtained for NO3

− andphosphate, respectively, using an adsorbent dose of 10 g/L. Des-orption of both anions was rapidly achieved within 10 min, using0.01 M NaOH. Regeneration tests showed that the adsorbentretained its capacity after five adsorption–desorption cycles.

Ammonium-functionalized MCM-41, MCM-48 and SBA-15mesoporous silica materials were synthesized via post-synthesis

−
grafting [115] and their efficiency to remove NO3 and phos-phate anions in aqueous solutions was investigated. The adsorbentsshowed high adsorption capacities reaching 46.5 mg NO3
−/g and55.9 mg H2PO4

−/g under the operating conditions explored. Themesoporous silica functionalized via post-synthesis grafting meth-

Applicable kinetic model Reference

First-order [73]Pseudo-second-order [75]Pseudo-second-order [75]Pseudo-second-order [97]First-order [102]Pseudo-second-order [103]Pseudo-second-order [103]Pseudo-first-order [107]Pseudo-second-order [108]Pseudo-second-order [109]Second-order [111]

A. Bhatnagar, M. Sillanpää / Chemical Engineering Journal 168 (2011) 493–504 501

Table 3Adsorption isotherm studies of nitrate on different adsorbents.

S. no. Adsorbent Applicable isotherm model Reference

1. Carbon cloth Langmuir [73]2. Powdered activated carbon Freundlich [74]3. Carbon nanotubes Freundlich [74]4. Untreated coconut granular activated carbon Langmuir [75]5. ZnCl2 treated coconut granular activated carbon Langmuir [75]6. Bamboo powder charcoal Langmuir [77]7. Calcined hydrotalcite-type compounds Langmuir [86]8. Layered double hydroxides Langmuir [87]9. Chitosan hydrobeads Langmuir [97]

10. Chitosan beads Langmuir [98]11. Conditioned cross-linked chitosan beads Langmuir [98]12. Pure alkaline lignin Langmuir [101]13. Sugarcane bagasse Langmuir [101]14. Pure cellulose Langmuir [101]15. Rice hull Langmuir [101]16. Modified wheat straw Freundlich [102]17. Raw wheat residue Freundlich [103]18. Modified wheat residue Freundlich [103]19. Wheat straw charcoal Langmuir [105]20. Mustard straw charcoal Langmuir [105]21. Commercial activated carbon Langmuir [105]22. Zr(IV)-loaded sugar beet pulp Langmuir [107]

or

sabNiwN

TT

23. Chemically modified sugar beet bagasse24. Modified beet residue25. Original and activated red mud26. HCl activated sepiolite

ds exhibited higher performances in terms of percentage pollutantemoval and adsorption capacities.

Nitrate removal from paper mill industry wastewater wastudied [116] with fly-ash, raw and heat-activated sepiolite asdsorbents. Equilibrium time was found to be 1 h for both adsor-
ents. It was observed that pH played an important role in theO3
− adsorption process, both ionizing the compounds and mod-fying sorbent surfaces. It was noticed that heat-activated sepiolite

as more effective than raw sepiolite and fly-ash to removeO3

−.

able 4hermodynamic studies of nitrate sorption on different adsorbents.

Adsorbent Temperature (◦C) �G (

Zn/Al chloride layered double 20 −0.9hydroxide 25 −1.5

30 −2.135 −2.740 −3.245 −3.850 −4.4

ZnCl2 treated coconut granular activated carbon 10 −4.025 −3.045 −2.7

Thermally activated Mg/Al chloride hydrotalcite-like 20 −0.9compound 25 −1.5

30 −2.135 −2.740 −3.245 −3.850 −4.4

Surfactant modified zeolite 15 −19.25 −19.35 −20.

Chitosan hydrobeads 20 −16.30 −17.40 −17.50 −17.

Sugar beet bagasse carbon 25 −20.35 −24.45 −31.

Langmuir [108]Langmuir [109]Langmuir [110]Freundlich [111]

Six species of trees in the Cedar and Bald Cypress familieswere selected and samples of heartwood, sapwood and bark wereobtained [117]. The wood samples were pretreated with a weakacid solution (pH 3) and distilled water (control). During theexperiment, liquid absorbed, tannin leaching, tissue density and
sorption of NO3
− from solution data were recorded. Samples thatproved most effective for NO3

− removal were also evaluated forre-release of sorbed NO3

− back into water. Nitrate removal fromwater solutions were most effective with incense cedar bark (22%removal/water pre-treatment, 28% removal/acid pre-treatment)

kJ/mol) �H (kJ/mol) �S (kJ/mol·K) Reference

87 to −1.789 13.832 to 16.392 0.0672 to 0.07346 [15]60 to −2.29433 to −2.79806 to −3.30380 to −3.80853 to −4.31326 to −4.817

1 −32.33 [75]5 −13.16 −33.935 −32.4

87 to −1.789 10.361 to 12.912 0.055 to 0.069 [88]60 to −2.29433 to −2.79806 to −3.30380 to −3.80853 to −4.31326 to −4817

6 [95]9 −13.7 20.50

343 47.92 [97]006 −2.302 48.53313 47.96846 48.12

75 [108]18 133.62 0.51615

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4

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nd Port-Orford cedar bark (33% removal/water pretreatment, 30%emoval/acid pre-treatment).

The NO3− adsorption onto a cement paste, a cured mixture of

ement and water, has been observed and is well described bylinear isotherm with the coefficient of 43.6 L/kg [118]. Cement

aste column was used by Park et al. [119] to remove some anionsncluding NO3

−. Recently, the feasibility of nano-alumina for NO3−

emoval from aqueous solutions has been explored [120]. The max-mum sorption capacity of nano-alumina for NO3

− removal wasound to be ca. 4.0 mg/g. Maximum NO3

− removal occurred at equi-ibrium pH ∼ 4.4. Nitrate sorption was affected by the presence ofhloride, sulfate and carbonate anions.

Fig. 2 summarises various sorbents which have been used soar for the removal of nitrate. Furthermore, a summary of adsorp-ion capacities of various adsorbents for nitrate removal fromater has been presented in Table 1. A perusal of Table 1 reveals

hat hydrotalcite-type compounds/layered double hydroxides andhemically modified adsorbents have been found promising fornhanced removal of nitrate from water. Tables 2–4 representinetic studies, adsorption isotherms and thermodynamic stud-es of nitrate sorption onto various adsorbents. In most studies,seudo-second-order kinetic model was found to fit well with theata. Furthermore, nitrate sorption by various adsorbents is char-cterized by Freundlich and Langmuir isotherm models. Negativeibbs free energy change (�G) indicates the feasibility and spon-

aneous nature of the adsorption process, while positive enthalpyhange (�H) represents endothermic nature of an adsorption. Inome cases, negative values of enthalpy change were reportedndicating the exothermic nature of the process. Positive entropyhange (�S) denotes the affinity of the adsorbents and increas-ng randomness at solid–solution interface during the sorption ofitrate on active sites of the adsorbents, while negative value ofntropy change in some cases indicates that degree of freedomecreases at solid/liquid interface during the adsorption.

. Conclusions and future perspectives

Nitrate is ubiquitous in the environment, leading to humanxposure and causing potential impact on human health and pro-oting ecological disturbances such as eutrophication. Among

everal water treatment technologies for removing NO3−, adsorp-

ion has been widely explored. This review compiles a list ofeveral materials which have been explored as adsorbents for NO3

−

emoval. Double layered hydroxides/hydrotalcite-type compoundsnd modified chitosan show higher uptake of NO3

− compared tother conventional adsorbents. Agricultural wastes, after surfaceodification, have also been explored for NO3

− removal and, inome cases, these wastes show an appreciable potential for NO3

−

emoval. However, during chemical treatment of adsorbent, onehould not ignore the cost factor. Low production cost with higheremoval efficiency would make the process economical and effi-ient. More research is needed to properly optimize the conditions.s revealed from the literature reviewed, only a few industrialastes have been examined for NO3

− removal, however, these canerve as potential sorbents for NO3

− sorption as they are gener-lly characterized by positive surface charge thereby increasing theorption of negatively charged NO3

− anion.Selection of a suitable adsorbent media for NO3

− removal fromater generally depends on several factors including, (1) the range

f initial NO3− concentrations, (2) other competing ions and their

oncentration in water, (3) optimization of adsorbent dose, (4)djustment of pH in water, and (5) proper operation and main-enance. Therefore, selection of a suitable adsorbent for effectiveO3

− removal is a complex task. A particular sorbent which showsigher uptake of NO3

− in the laboratory, may fail in field condi-


tions. Thus, the selection of the appropriate technology/sorbentmedia can be tedious. Besides these, some other issues, suchas assessment of efficacy of sorbents for NO3

− removal undermulti-component pollutants, mechanistic modeling to correctlyunderstand the sorption mechanisms, investigation of these mate-rials with real industrial effluents, and continuous flow studiesshould also be conducted in detail. Last but not the least, it would beworthwhile to investigate the reusability of the spent adsorbentsas only a few studies are available in literature. More research isneeded in the field of regeneration and finally for the environmen-tally safe disposal of NO3

−-laden adsorbents.

Acknowledgments

We wish to thank all the anonymous reviewers whose com-ments/suggestions have significantly improved the quality ofthis manuscript. Amit Bhatnagar acknowledges his post-doctoralscholarship (FCT-DFRH-SFRH/BPD/62889/2009) supported by thePortuguese Foundation for Science and Technology (FCT).

References

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