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Journal of Environmental Chemical Engineering xxx (2013) xxx–xxx

G Model

JECE 238 1–9

Design of a multi-stage stirred adsorber using mesoporous metal oxides forherbicide removal from wastewaters

Domenico Pirozzi a,*, Filomena Sannino b

a Dipartimento di Ingegneria Chimica, Universita di Napoli ‘‘Federico II’’, P.le Tecchio, 80, 80125 Napoli, Italyb Dipartimento di Agraria, Universita di Napoli ‘‘Federico II’’, Via Universita 100, 80055 Portici, Napoli, Italy

A R T I C L E I N F O

Article history:

Received 14 August 2013

Received in revised form 25 November 2013

Accepted 4 December 2013

Keywords:

Sorption

Multistage batch stirred adsorber

Mathematical modeling

Herbicides

Mesoporous metal oxides

A B S T R A C T

Stirred adsorbers offer a number of advantages in comparison to fixed bed systems, as they reduce mass

transfer resistances, pressure drops and fouling of the sorbent particles. Single-, double- and triple-stage

adsorbers were designed for the adsorption of two herbicides, 6-chloro-N,N0-diethyl-1,3,5-triazine-2,4-

diamine (Simazine) and (4-chloro-2-methylphenoxy)acetic acid (MCPA), on two mesoporous metal

oxides, aluminum oxide (Al2O3) and iron(III) oxide (Fe2O3).

The equilibrium data were described by the Freundlich model (for Simazine) and Langmuir model (for

MCPA). The sorption kinetics was analyzed adopting pseudo-first (for MCPA) and the pseudo-second (for

Simazine) order models. Based on the kinetic and equilibrium equations selected, a mathematical model

describing single and multi-stage sorption systems was developed to predict the minimum amount of

adsorbent required to remove a given fraction of herbicide from a fixed volume of wastewater.

The replacement of a single-stage system with a two-stage system results in a significant reduction

(from 78 to 91%) of the overall required sorbent amount that balances the higher cost of the two-stage

plant. On the contrary, the adoption of a three-stage system allows only a limited reduction (from 24 to

41%) of sorbent in comparison to a two-stage system having the same total volume.

� 2013 Published by Elsevier Ltd.

Contents lists available at ScienceDirect

Journal of Environmental Chemical Engineering

jou r n al h o mep ag e: w ww .e lsev ier . co m / loc ate / jec e

26272829303132333435363738394041424344

Introduction

The widespread use of pesticides with potential toxic,carcinogenic and mutagenic effects causes the extended contami-nation of soils and water bodies. The recent EC directive fordrinking water allows a maximum residue of 0.1 mg L�1 for anactive ingredient and 0.5 mg L�1 for the total pesticide load.

So far, different methods have been studied to remove pesticidesfrom waters, including membrane technology (ultrafiltration,nanofiltration, and reverse osmosis), solid phase extraction, ozoneand chemical oxidation, Fenton degradation, photocatalysis andadsorption on various supports [1]. Adsorption is the mostwidespread technology used to deal with purification of watercontaminated by pesticides, due to the flexibility in design andregeneration of adsorbents, the low maintenance costs, the highefficiency, and the easiness of operation.

This study deals with the adsorption of two herbicides, namely6-chloro-N,N0-diethyl-1,3,5-triazine-2,4-diamine (Simazine) andthe (4-chloro-2-methylphenoxy)acetic acid (MCPA), both causing

4546474849

* Corresponding author. Tel.: +39 081 7682274; fax: +39 081 2391800.

E-mail addresses: dpirozzi@unina.it, domenico.pirozzi@unina.it (D. Pirozzi),

fsannino@unina.it (F. Sannino).

Please cite this article in press as: D. Pirozzi, F. Sannino, Design of a

herbicide removal from wastewaters, J. Environ. Chem. Eng. (2013),

2213-3437/$ – see front matter � 2013 Published by Elsevier Ltd.

http://dx.doi.org/10.1016/j.jece.2013.12.013

serious environmental concerns due to their persistence in theenvironment. The MCPA is a post-emergence phenoxy acidherbicide extensively used to control annual and perennial weedsin cereals, grasslands, trees, and turf. Its different forms, includingthe free acid, salts and esters, are all soluble in water and releasethe acid as the active ingredient [2]. The Simazine is a synthetic s-triazine herbicide used for pre-emergence control of broad-leafweeds and annual grasses in agricultural and non-crop fields [3,4].The recalcitrance of s-triazines against chemical and biologicaldegradation has led to their accumulation in the environment [5].The Simazine is the second most commonly detected pesticide insurface and groundwater in the USA, Australia and Europe [6].

Two mesoporous metal oxides, aluminum oxide (Al2O3) andiron(III) oxide (Fe2O3), have been adopted as sorbent materials.Mesoporous materials have been shown [7–9] to offer largeadsorption capacity, good selectivity and improved recoverabilityfor the removal of toxic compounds from aqueous solutions.

In this paper, a batch stirred system is designed for theadsorption of MCPA and Simazine, on the metal oxides Al2O3 andFe2O3. Batch stirred adsorption systems are attracting increasinginterest, due to the advantages offered in comparison to the fixedbed processes [10]. First of all, the mixing increases the adsorptionrate, reducing the mass transfer resistances [11]. In addition, thepressure drop in a stirred plant is very low in comparison to the

multi-stage stirred adsorber using mesoporous metal oxides for http://dx.doi.org/10.1016/j.jece.2013.12.013

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121122123124125126

Abbreviations

a constant in the linear isotherm model

(L kg�1)

b constant in the linear isotherm model

(mmol kg�1)

c0 initial concentration of herbicide

(mmol L�1)

c1 herbicide concentration at the end of the

incubation (mmol L�1)

ceq, c0eq, c00eq, c0 00eq equilibrium concentration of pesticide

(mmol L�1)

K constant of Freundlich model

[(mmol kg�1)/(mmol L�1)1/n]

k constant of Langmuir model (L mmol�1)

k2 constant in the pseudo second-order

kinetic model (kg mmol�1 min�1)

Ka constant of sorption in the pseudo first-

order kinetic model (min�1)

M, M0, M00, M0 00 weight of sorbent (kg)

n constant of Freundlich model (dimension-

less)

q amount of herbicide sorbed at time t

(mmol kg�1)

qe amount of herbicide sorbed at equilibri-

um (mmol kg�1)

R removed fraction of herbicide (%)

t time (min)

V sorption plant volume (L)

x0 initial amount of pesticide on sorbent

particles (mmol kg�1)

x1 amount of pesticide on sorbent particles

at the end of incubation (mmol kg�1)

xeq, x0eq, x00eq, x00 0eq amount of pesticide on sorbent particles

at the equilibrium (mmol kg�1)

xm maximum amount of pesticide sorbed in

the Langmuir model (mmol kg�1)

D. Pirozzi, F. Sannino / Journal of Environmental Chemical Engineering xxx (2013) xxx–xxx2

G Model

JECE 238 1–9

ed bed, and the binding and the fouling of the sorbent particlese reduced. Nevertheless, only a limited number of papers haveen so far focused on the design of stirred adsorption systems.ese papers concern the adsorption of dyes [11–18], heavy metals9], phenol [20], toluene [21], Pb2+ and Cd2+ ions [22,23], arsenate4], phosphate [25]. As far as we know, no papers have been so farblished concerning the design of stirred adsorption systems fore removal of herbicides.Several authors, dealing with stirred systems in which the

eatment time is not long enough to reach an equilibrium state,ve analyzed the sorption kinetics to determine the minimum

eatment time [14,15,22–27]. When the sorption equilibrium isached within a short time, or expensive sorbents are to be used,

the sorption capacity of the sorbent is small, an alternativeproach is usually followed, aimed at exploiting the equilibriumta to determine the minimum mass of adsorbent required tomove a given amount of herbicide from a given volume ofastewater [12,13,16,17,28].

In the present paper, this latter approach was followed. Themoval capacity of the oxides was described using the Langmuird the Freundlich equation for MCPA and Simazine sorption,spectively. Based on the isotherm equations selected, a

Please cite this article in press as: D. Pirozzi, F. Sannino, Design of aherbicide removal from wastewaters, J. Environ. Chem. Eng. (2013

mathematical model was developed to describe single, doubleand triple-stage sorption systems.

Materials and methods

Materials

The (4-chloro-2-methylphenoxy)acetic acid (MCPA) and the 6-chloro-N,N0-diethyl-1,3,5-triazine-2,4-diamine (Simazine) werepurchased from Sigma–Aldrich (99.0% purity). All solvents wereof HPLC grade (Carlo Erba). All other chemicals were obtained fromSigma–Aldrich. g-Aluminum (Al2O3) and iron(III) (Fe2O3) nano-sized oxides were purchased from IoliTec Nanomaterials (Denzlin-gen, Germany; 99.9 and 99.5% purity for Al2O3 and Fe2O3,respectively).

Physical analysis of Al2O3 and Fe2O3

The specific surface area (SSA) of Al2O3 and Fe2O3 wascalculated by the BET method (24). N2 adsorption–desorptionisotherms at 77 K were obtained by a Micromeritics Gemini II 2370apparatus. The detailed analyses are described in the SupportingInformation.

Analytical determination of herbicides

MCPA and Simazine were analyzed by an HPLC apparatus(Agilent 1200 Series), equipped with a DAD and a ChemStationAgilent Software. The detailed analyses are described in theSupporting Information.

Sorption equilibrium and kinetics

Stock solutions of herbicides were prepared by dissolving100 mg of MCPA in 500 mL of 0.03 M KCl (final concentration1000 mmol L�1) or 2 mg of Simazine in 500 mL of 0.03 M KCl (finalconcentration 20 mmol L�1).

The sorption studies of Simazine and MCPA on the selectedoxides were conducted in batch conditions. The experimentsconducted with Simazine were performed at a solid/liquid ratio of0.5 mg/mL, obtained by adding 10 mg of Al2O3 or Fe2O3 to a finalvolume of 20 mL, with an herbicide concentration of 10 mmol L�1,at pH values of 6.5 (Al2O3) and 3.5 (Fe2O3). The experiments withMCPA were performed at a solid/liquid ratio of 0.1 mg/mL,obtained by adding 2.0 mg of Al2O3 or Fe2O3 to a final volumeof 20 mL, with an herbicide concentration of 10 mmol L�1, at pHvalues of 4.0 (Al2O3) and 3.5 (Fe2O3).

Sorption isotherms were obtained using initial herbicideconcentrations ranging from 0.50 to 10.69 mmol L�1 for Simazine,and from 0.05 to 200 mmol L�1 for MCPA. After incubation in arotatory shaker at 30 8C, the samples were centrifuged at 7000 rpmfor a fixed time. The amount of herbicide sorbed on the oxides wascalculated as the difference between the quantity of herbicideinitially added and that present at the equilibrium. Blanks of bothherbicides in 0.03 M KCl were analyzed in order to check herbicidestability and sorption to vials.

The herbicide removal (%) was calculated by the followingbalance equation:

R ¼ c0 � c1

c0(1)

where c0 is the initial concentration of herbicide (mmol L�1) and c1

the herbicide concentration at the end of the incubation(mmol L�1).

Kinetic tests were conducted at 30 8C. After centrifugation, thesupernatants were analyzed as described above.

multi-stage stirred adsorber using mesoporous metal oxides for), http://dx.doi.org/10.1016/j.jece.2013.12.013

127

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140141142143144145146

147148149150151152153154155156157

158159160161162163164165166167168169170171

172173174175176177

178179

Fig. 1. Sorption isotherms of: Simazine on Al2O3 and Fe2O3 (a); MCPA on Al2O3 and

Fe2O3 (b). The isotherms obtained with MCPA are taken from a previous work

(Addorisio et al., 2010).

D. Pirozzi, F. Sannino / Journal of Environmental Chemical Engineering xxx (2013) xxx–xxx 3

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Statistical analysis of the data

All the experiments were performed in triplicate and therelative standard deviation was in all cases lower than 4%.

Results and discussion

Modeling equilibrium and kinetics data

The selection of the isotherm models is described in theparagraph S1 (Supplementary Materials). The isotherms describingthe equilibrium partition of Simazine on both Al2O3 and Fe2O3,reported in Fig. 1(a), were correlated using the Freundlich model:

xeq ¼ K � c1=neq (2)

where xeq is the amount of herbicide sorbed (mmol kg�1), ceq is theequilibrium concentration of herbicide (mmol L�1), K

[(mmol kg�1)/(mmol L�1)1/n] and n (dimensionless) are constants

Table 1Sorption (a) and kinetic (b) parameters, appearing in the Eqs. (2)–(6), respectively.

Herbicide Sorbent Model

(a) Sorption parameters

Simazine Al2O3 (2)

Simazine Fe2O3 (2)

MCPA Al2O3 (3)

MCPA Fe2O3 (4)

(b) Kinetic parameters

Simazine Al2O3 (6)

Simazine Fe2O3 (6)

MCPA Al2O3 (5)

MCPA Fe2O3 (5)

Please cite this article in press as: D. Pirozzi, F. Sannino, Design of a

herbicide removal from wastewaters, J. Environ. Chem. Eng. (2013),

that give estimates of the sorptive capacity and intensity,respectively [25].

The adoption of Freundlich model for the Simazine adsorptionisotherms is in agreement with many recent papers [29–32],though in some cases the Langmuir model is preferred [33].

The isotherm describing the MCPA sorption on Al2O3, shown inFig. 1(b), was better described by the Langmuir model:

xeq ¼xm � k � ceq

1 þ k � ceq(3)

where xeq is the amount of herbicide sorbed (mmol kg�1), ceq is theequilibrium concentration of MCPA (mmol L�1), k is a constantrelated to the binding energy (L mmol�1), and xm is the maximumamount of herbicide adsorbed (mmol kg�1) [34].

The MCPA sorption tests on Fe2O3 yielded a more complexsorption isotherm, also shown in the Fig. 1(b): at lower equilibriumconcentrations the amount of sorbed MCPA increased linearly,whereas at higher equilibrium concentrations a Langmuir-typebehavior (horizontally shifted from the origin) was observed. Theanalytical description adopted for this curve is as follows:

ðceq < 8:3 mmol L�1Þ xeq ¼ a � ceq þ b

ðceq� 8:3 mmol L�1Þ xeq ¼xm � k � ðceq � 8:3Þ1 þ k � ðceq � 8:3Þ

8<: (4)

where a and b are the slope and the intercept of the linear isothermmodel; k and xm are the Langmuir-type isotherm constants.

The values of parameters appearing in models (2)–(4), obtainedby nonlinear regression, are reported in the Table 1(a).

The adoption of the Langmuir model for MCPA sorptionisotherm is in disagreement with some data obtained withdifferent materials [35–39], though in some cases the Langmuirmodel is preferred [40] or both the Freundlich or Langmuirisotherms provide adequate fits for the sorption data [41].

The selection of the kinetic models is described in the paragraphS2 (Supplementary Materials). The MCPA sorption kinetic curves(not shown) were best described by a pseudo first-order kineticmodel [15]:

logðqe � qÞ ¼ logqe �Kat

2:303(5)

where qe and q are the amounts of herbicide sorbed (mmol kg�1) atequilibrium and at time t, respectively, Ka is the rate constant ofsorption (min�1) and t is the time (min).

The Simazine sorption kinetics (not shown) was best describedby a pseudo second-order kinetic model [15]:

t

q¼ 1

k2 � q2e

þ t

qe

(6)

where k2 is the rate constant of sorption (kg mmol�1 min�1).

R2 Parameters estimates

0.992 k = 168 mmol kg�1; n = 0.444

0.998 k = 166 mmol kg�1; n = 0.596

0.980 xm = 2.01 � 105 mmol kg�1; k = 5.05 � 10�3 L mmol�1

0.975 xm = 5.42 � 104 mmol kg�1; k = 0.452 L mmol�1

a = 752 kg L�1; b = 418 mmol L�1

0.979 k2 = 2.85 � 10�5 kg mmol�1 min�1; qe = 6098 mmol kg�1;

0.996 k2 = 9.03 � 10�4 kg mmol�1 min�1; qe = 1695 mmol kg�1;

0.996 Ka = 0.348 min�1; qe = 21,649 mmol kg�1;

0.970 Ka = 0.0539 min�1; qe = 16,336 mmol kg�1;

multi-stage stirred adsorber using mesoporous metal oxides for http://dx.doi.org/10.1016/j.jece.2013.12.013

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240

241242

243244245246247

Fig. 2. Schematic layout of batch sorption plants: single-stage (a), double-stage (b) and triple-stage (c). HERB = herbicide; SOLV = solvent; SORB = sorbent.

D. Pirozzi, F. Sannino / Journal of Environmental Chemical Engineering xxx (2013) xxx–xxx4

G Model

JECE 238 1–9

The estimates of parameters of models (5) and (6), withference to MCPA and Simazine sorption on both mesoporousides, are reported in the Table 1(b). Using these results, alatively short time (<90 min) was estimated to be required fore amount of sorbed herbicide (q) to reach the 99% of theeoretical equilibrium value (qe), whatever the sorption systemnsidered. Consequently, the mathematical elaboration wassed on the equilibrium data, with the aim of determining theinimum amount of sorbent required to remove a given amount ofrbicide from a given volume of wastewater (or the maximumlume of wastewater removed with a fixed mass of adsorbent), inder to reduce the operating costs of the sorption plant.

sign of a single-stage batch system for Simazine sorption

The schematic layout of a single-stage batch sorption plant isscribed in the Fig. 2(a). A sorption system of volume V (expressed

L) is designed to reduce the amount of herbicide from an initialncentration c0 (mmol L�1) to a final concentration c1 (mmol L�1),ing a weight M (kg) of sorbent. Meanwhile, the loading of sorbedrbicide increases from x0 (mmol kg�1) to x1 (mmol kg�1). Theass balance for the system in Fig. 2(a) is:

� ðc0 � c1Þ ¼ M � ðx1 � x0Þ (7)

Once the equilibrium conditions are achieved the balance (7)comes:

� ðc0 � ceqÞ ¼ M � ðxeq � x0Þ (8)

Substituting the sorption isotherms of Simazine (2), the Eq. (8)ay be rewritten as:

� ðc0 � ceqÞ ¼ M � ðK � c1=neq � x0Þ (9)

If a target concentration ceq is fixed, the Eq. (9) can be solved totermine the required amount of sorbent (M) as a function of V, c0,:

¼ V � ðc0 � ceqÞðK � c1=n

eq � x0Þ(10)

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The Fig. 3(a) reports the amounts of Al2O3 required to removegiven fractions of Simazine from an initial concentration of20 mmol L�1. Because the sorption isotherm of Simazine onAl2O3, displayed in Fig. 1(a), shows an upward concavity(Freundlich-type with n < 1), small increases in the Simazineremoval fraction lead to significant increases in the requiredamount of sorbent. For instance, the amounts of Al2O3 required toremove 70%, 80%, 90% and 95% of the Simazine from 1000 L ofsolution are 1.47, 4.19, 22.5 and 113.0 kg, respectively.

A similar behavior was observed when adsorbing the Simazinewith Fe2O3, as shown in Fig. 3b. In this case, higher amounts ofadsorbent are required to remove a given amount of Simazine, dueto the lower capacity of Fe2O3. Nevertheless, when 95% Simazine isto be removed, the results obtained with Al2O3 and Fe2O3 arecomparable, due to the similarity of the corresponding sorptionisotherms at lower concentrations of Simazine.

The Fig. 4 (curves a and b) describes the effect of initialconcentration of Simazine on the amount of oxides required toobtain a 90% herbicide removal (V = 1000 L). A significantreduction of the required amount of oxides can be observed asthe initial Simazine concentration increases. This result is due tothe upward concavity of the isotherm curves, corresponding to afaster increase of the equilibrium concentration of sorbed Simazine(xeq) as higher values of the herbicide concentration in the liquidphase (ceq) are adopted.

Design of a single-stage batch system for MCPA sorption

Substituting the sorption isotherms of MCPA on Al2O3 (Eq. (3)),the Eq. (8) may be rewritten as:

V � ðc0 � ceqÞ ¼ M � xm � k � ceq

1 þ k � ceq� x0

� �(11)

Again, once a target concentration ceq is fixed, the Eq. (11) canbe solved to determine the required amount of sorbent (M) as afunction of V, c0, x0:

M ¼ V � ðc0 � ceqÞxm � k � ceq=1 þ k � ceq � x0

(12)

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Fig. 3. Amounts of sorbent required in a single-stage sorption plant as a function of the herbicide removal (%) and the volume of the liquid phase: Simazine on Al2O3 (a),

Simazine on Fe2O3 (b), MCPA on Al2O3 (c), MCPA on Fe2O3 (d). The initial herbicide concentration is c0 = 20 mmol L�1.

Fig. 4. Amounts of sorbent required in a single-stage sorption plant (V = 1000 L) as a

function of the initial concentration of herbicide, to obtain a 90% herbicide removal.

Simazine on Al2O3 (*), Simazine on Fe2O3 (*), MCPA on Al2O3 (&), MCPA on Fe2O3

(&).

D. Pirozzi, F. Sannino / Journal of Environmental Chemical Engineering xxx (2013) xxx–xxx 5

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The sorption isotherm of MCPA on Fe2O3 is described by adiscontinuous function (4). The function (4) was coupled with thebalance (8), to obtain a system of two algebraic equations in twounknown variables, namely M and xeq. A suitable algorithm wasimplemented to find M and xeq once the parameters V, c0, x0 arefixed.

The Fig. 3(c) and (d) report the amounts of Al2O3 and of Fe2O3,respectively, required to remove a given fraction of MCPA, from aninitial concentration of 20 mmol L�1. These results confirm that thesorption capacity of Fe2O3 is significantly lower than that of Al2O3,even when MCPA is adsorbed. A comparison between the Fig. 3(a)and (b) and Fig. 3(c) and (d) demonstrates that the MCPA requiresmuch lower amounts of sorbent than Simazine.

The Fig. 4 describes the effect of initial concentration of MCPA(curves c and d) on the amount of oxides required to obtain a 90%herbicide removal (V = 1000 L). The results obtained with MCPAare significantly different from these pertaining to Simazine (seeFig. 4, curves a and b). The required amount of Al2O3 remainsalmost constant as the initial MCPA concentration increases (curvec). When using Fe2O3 (curve d), the required amount of solid doesnot show a definite behavior, due to the discontinuity of thecorresponding sorption isotherm, shown in Fig. 1(b).

Design of a two-stage batch system for Simazine sorption

A two-stage batch sorption system is shown in the Fig. 2(b). Theamount of Simazine in the first stage, containing a volume V(L) ofsolution and a weight M0 (kg) of sorbent, is reduced, once theequilibrium is achieved, from an initial concentration c0

(mmol L�1) to the concentration c0eq (mmol L�1). The loading of

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herbicide removal from wastewaters, J. Environ. Chem. Eng. (2013),

sorbed solute increases from x0 (mmol kg�1) to x0eq (mmol kg�1). Inseries with the first stage, a second stage is considered, containingthe same volume of solution V and a weight M00 (kg) of freshadsorbent. Under the equilibrium conditions, the Simazineconcentration is reduced from c0eq to c00eq (mmol L�1), and theloading of sorbed herbicide increases from x0 to x00eq (mmol kg�1).

multi-stage stirred adsorber using mesoporous metal oxides for http://dx.doi.org/10.1016/j.jece.2013.12.013

284

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307308309310311312313314315316317318319320321322323324325326327328329330331332333

Fig. 5. Amounts of sorbent required in a double-stage sorption plant for a 95% herbicide removal from an initial herbicide concentration c0 = 20 mmol L�1. Simazine on Al2O3

(a), Simazine on Fe2O3 (b), MCPA on Al2O3 (c), MCPA on Fe2O3 (d). The volume of each stage is assumed to be 500 L.

D. Pirozzi, F. Sannino / Journal of Environmental Chemical Engineering xxx (2013) xxx–xxx6

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The following two mass balances describe the first and thecond stage, respectively:

� ðc0 � c0eqÞ ¼ M0 � ðx0eq � x0Þ (13)

� ðc0eq � c00eqÞ ¼ M00 � ðx00eq � x0Þ (14)

Using the sorption isotherm of Simazine (2) to express x0eq and

q, the Eqs. (13) and (14) can be rewritten as:

� ðc0 � c0eqÞ ¼ M0 � ðK � c0eq1=n � x0Þ (15)

� ðc0eq � c00eqÞ ¼ M00 � ðK � c00eq1=n � x0Þ (16)

Once the final concentration (c00eq) and the weight of sorbent ine first stage (M0) are fixed, Eqs. (15) and (16) can be solved totermine, respectively, the equilibrium concentration of the 1st

age (c0eq) and the required amount of sorbent in the second stage00) as a function of V, c0, x0.The Fig. 5(a) shows the total amounts of Al2O3 (I and II stage)

quired to remove the 95% of the Simazine, from c0 = 20 mmol L�1

c00eq ¼ 1 mmol L�1, adopting a volume V = 500 L for each stagee. a total volume of 1000 L). Obviously, increases in the Al2O3

ed in the first stage (M0) led to reduced amounts of Al2O3 in thecond stage (M00). An optimum (minimum) value in the total

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amount of Al2O3 (Mtot = M0 + M00) was observed under the followingconditions: M0 = 3.84 kg, M00 = 6.34 kg, Mtot = 10.2 kg, as reported inthe Table 2(b). Under the optimum condition found, the sorbentamount in the first stage was lower (M0 < M00). This result disagreewith Li and co-workers [17], that found, using a Langmuir-typeisotherm, an optimum conditions with similar adsorbent amountsin both stages. This difference can be explained observing that, dueto the upward concavity of the Freundlich-type isotherm curve ofSimazine, an higher value of M00 leads to a better efficiency of thesorption system.

When considering a single-stage system of 1000 L, to achieve asimilar reduction (95%) of the Simazine concentration, a muchhigher amount of sorbent (113 kg) is required, as shown inTable 2(a). The adsorbent amount saved by adopting a two-stagebatch adsorber increase as the Simazine percentage removalincreases. This result agrees with that obtained by Li and co-workers [17].

The results obtained simulating a double-stage system withFe2O3, reported in Fig. 5(b), show a similar trend in comparison tothose obtained with Al2O3, though the amounts of Fe2O3 requiredare higher. The minimum value of the total required amount ofFe2O3 is obtained under the following conditions: M0 = 5.75 kg,M00 = 7.82 kg, Mtot = 13.6 kg, as reported in Table 2(b). Again, theoptimum conditions are obtained using an higher sorbent amountin the second stage (M0 < M00).

Even when using Fe2O3 the adoption of a double-stage sorptionsystem reduces significantly the required amount of adsorbent. As

multi-stage stirred adsorber using mesoporous metal oxides for), http://dx.doi.org/10.1016/j.jece.2013.12.013

334

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360361

362363364365366367

Table 2Optimum (minimum) amounts of sorbents required for a 95% herbicide removal in single-stage (a), double-stage (b) and triple-stage (c) sorption plants, from an initial

herbicide concentration c0 = 20 mmol L�1 to a final concentration c00eq ¼ 1 mmol L�1.

Herbicide Sorbent Mtot (kg)

(a) Single-stage sorption plant. Volume: 1000 L

Simazine Al2O3 113

Simazine Fe2O3 114

MCPA Al2O3 12.9

MCPA Fe2O3 16.2

Herbicide Sorbent M0 (kg) M00 (kg) Mtot = M0 + M00 (kg)

(b) Double-stage sorption plant. Volume of each stage: 500 L

Simazine Al2O3 3.84 6.34 10.2

Simazine Fe2O3 5.75 7.82 13.6

MCPA Al2O3 1.19 1.23 2.42

MCPA Fe2O3 1.80 1.71 3.51

Herbicide Sorbent M0 (kg) M00 (kg) M00 0 (kg) Mtot = M0 + M00 + M00 0 (kg)

(c) Triple-stage sorption plant. Volume of each stage: 333.3 L

Simazine Al2O3 1.30 2.00 2.73 6.03

Simazine Fe2O3 2.24 2.76 3.74 8.74

MCPA Al2O3 0.631 0.615 0.572 1.82

MCPA Fe2O3 0.280 0.991 1.013 2.28

D. Pirozzi, F. Sannino / Journal of Environmental Chemical Engineering xxx (2013) xxx–xxx 7

G Model

JECE 238 1–9

a matter of facts, using a single-stage system of 1000 L, a solidamount of 114 kg would be required to reduce the Simazineconcentration from 20 mmol L�1 to 1.0 mmol L�1 (Table 2a).

A cost analysis of different configurations of stirred adsorbers,described in the Supplementary Material, was carried out. Theresults demonstrated that, when metal oxides are used to adsorbSimazine, the adoption of a double-stage adsorber improves theeconomic balance of the process (see Table S4), due to thesignificant reduction of the required amount of adsorbent.

This reduction is less significant when MCPA is to be removed.As a matter of facts, when using MCPA, lower amounts of metaloxides are required, and the single-stage adsorber is the leastexpensive configuration, as shown in Table S4.

The Fig. 6 (curves a and b) describes the effect of the initialconcentration of Simazine on the amount of oxides required toobtain a 90% herbicide removal from a double-stage sorption planthaving a volume V = 500 L for each stage (i.e. a total volume of1000 L). The results reflect the trend observed in the case of thesingle-stage plant (Fig. 4, curves a and b), confirming that areduction of the required amount of oxides can be observed as the

368369370371372373374375376377378379380381382383384385386387388389

Fig. 6. Amounts of sorbent required in a double-stage sorption plant

(V = 500 L + 500 L) as a function of the initial concentration of herbicide, to

obtain a 95% herbicide removal. Simazine on Al2O3 (*), Simazine on Fe2O3 (*),

MCPA on Al2O3 (&), MCPA on Fe2O3 (&). The volume of each stage is assumed to be

500 L.

Please cite this article in press as: D. Pirozzi, F. Sannino, Design of a

herbicide removal from wastewaters, J. Environ. Chem. Eng. (2013),

initial Simazine concentration increases, due to the upwardconcavity of the isotherm curves.

Design of a two-stage batch system for MCPA sorption

The Al2O3 amounts required for MCPA sorption in a two-stagesystem were calculated substituting the sorption isotherm ofMCPA on Al2O3 (3) in the mass balances (13) and (14):

V � ðc0 � c0eqÞ ¼ M0 �xm � k � c0eq

1 þ k � c0eq

� x0

!(17)

V � ðc0eq � c00eqÞ ¼ M00 �xm � k � c00eq

1 þ k � c00eq

� x0

!(18)

Once the final concentration (c00eq) and the weight of sorbent inthe first stage (M0) are fixed, Eqs. (17) and (18) can be solved todetermine the equilibrium concentration of the Ist stage (c0eq) andthe required amount of sorbent in the second stage (M00) as afunction of V, c0, x0. A similar procedure can be followed tocalculate the Fe2O3 amounts required for MCPA sorption, using theisotherm (4) in place of the isotherm (3).

The Fig. 5(c) shows the amounts of Al2O3 required for a 95%reduction of the MCPA content of liquid phase, adopting a totalvolume of 1000 L (V1 = V2 = 500 L). The minimum amount of Al2O3

required is Mtot = M0 + M00 = 2.42 kg (M0 = 1.19 kg, M00 = 1.23 kg) asshown in the Table 2(b). This result confirms that MCPA requires asignificantly lower amount of sorbent in comparison to Simazine.In this case, the optimum condition is obtained with similaramounts of sorbent in the first and the second stage (M0 � M00). Thisis due to the asymptotic character of the Langmuir-type isothermcurve obtained for MCPA sorption on Al2O3. It is worth noting thatLi and co-workers [17], using a Langmuir-type isotherm, found anoptimum conditions with similar adsorbent amounts in bothstages, as well.

When using a single-stage system of 1000 L to remove a similaramount of MCPA, a solid amount of 12.9 kg is required, as shown inTable 2(a). Again, the adoption of a double-stage system allows asignificant reduction of the sorbent amount in comparison to thesingle-stage system. It is interesting to observe that, as a sorbentwith lower adsorbing capacity (in this case, the Simazine) is used,

multi-stage stirred adsorber using mesoporous metal oxides for http://dx.doi.org/10.1016/j.jece.2013.12.013

390 th391 sin392 w393

394 M395 ob396 am397

398 ob399 M

400 of401 95402 so403 Ag404

405 co406 a

407 to408 am409 co410 (c411 re412 th

413 De

414

415 co416 so417 co418 (m419 (m420 lo421 (m422 se423

424 st

V

425426

V

427428

V

429430431

432 us433 re

V

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440 th441 so442 2n443 th444

445 on446 10447 th448 re449 c0 0e

450451452453454455456457458459460461462463464465466467468469470471472473474475476477478479480481482483484485

486

487488489490491

Fig. 7. Amounts of Al2O3 required in a triple-stage sorption plant for a 95% Simazine

removal from an initial herbicide concentration c0 = 20 mmol L�1 to a final

concentration c00eq ¼ 1 mmol L�1. The volume of each stage is assumed to be 333.3 L.

D. Pirozzi, F. Sannino / Journal of Environmental Chemical Engineering xxx (2013) xxx–xxx8

G Model

JECE 238 1–9

e ratio between the required amounts of sorbent pertaining togle- and double-stage adsorber is higher. These results agree

ith those obtained from the literature [17].The Fig. 5(d) describes the amounts of Fe2O3 required for a 95%

CPA removal in a double-stage sorption system. A similar trend isserved in comparison to that obtained with Al2O3, though theounts of Fe2O3 required to remove the herbicide are higher.The minimum value in the total required amount of Fe2O3 is

tained under the following conditions: M0 = 1.80 kg,00 = 1.71 kg, Mtot = 3.51 kg, as shown in Table 2(b). For the sake comparison, it is worth noting that, carrying out the removal of a% fraction of MCPA in a single-stage system of 1000 L, a lowerlid amount of is required (16.2 kg), as reported in Table 2(a).ain, this result is in agreement with the literature data [17].The Fig. 6 (curves c and d) describes the effect of initial

ncentration of MCPA on the amount of oxides required to obtain90% herbicide removal in a double-stage sorption plant with atal volume of 1000 L (V1 = V2 = 500 L). Again, the required

ount of Al2O3 remains almost constant as the initial MCPAncentration increases (curve c), whereas, when using Fe2O3

urve d), the required amount of solid shows a minimum,flecting the discontinuity of the corresponding sorption iso-erm.

sign of a three-stage batch sorption system

A three-stage batch sorption system is described in Fig. 2(c). Wensidered three stages containing the same volume V(L) oflution, and different weights M0, M00 and M00 0 (kg) of sorbent. Thencentration of Simazine is reduced in the first stage from c0

mol L�1) to c0eq (mmol L�1), in the second stage from c0eq to c00eq

mol L�1), in the third stage from c00eq to c00 0eq (mmol L�1). Theading of sorbed solute increases from x0 (mmol kg�1) to x0eq

mol kg�1) in the first stage, from x0 to x00eq (mmol kg�1) in thecond stage, from x0 to x0 00eq (mmol kg�1) in the third stage.The following mass balances describe the first, second and third

age, respectively:

� ðc0 � c0eqÞ ¼ M0 � ðx0eq � x0Þ (19)

� ðc0eq � c00eqÞ ¼ M00 � ðx00eq � x0Þ (20)

� ðc00eq � c000eqÞ ¼ M00

0 � ðx0 00eq � x0Þ (21)

In the case of the Simazine sorption, the isotherm (2) can beed to express x0eq, x00eq, x00 0eq, and the Eqs. (19)–(21) can bewritten as:

� ðc0 � c0eqÞ ¼ M0 � ðK � c0eq1=n � x0Þ (22)

(23)V � ðc0eq � c00eqÞ ¼ M00 � ðK � c00eq1=n � x0Þ

� ðc00eq � c000eqÞ ¼ M00

0 � ðK � c000eq1=n � x0Þ (24)

Once the final concentration (c0 00eq) and the weights of sorbent ine first two stages (M0 and M00) were fixed, Eqs. (22)–(24) can belved to determine the equilibrium concentrations of the 1st andd stages (c0eq and c00eq) and the required amount of sorbent in theird stage (M00 0) as a function of V, c0, x0.The results reported in the Fig. 7 refer to the Simazine sorption

Al2O3 in a three-stage sorption plant having a total volume of00 L (i.e. a volume V = 333.3 L for each stage). The Fig. 7 reportse total amounts of Al2O3 (Mtot = M0 + M00 + M00 0) required for a 95%duction of the Simazine concentration, from c0 = 20 mmol L�1 to0q ¼ 1 mmol L�1, as a function of the sorbent amounts in the 1st

Please cite this article in press as: D. Pirozzi, F. Sannino, Design of aherbicide removal from wastewaters, J. Environ. Chem. Eng. (2013

stage (M0) and in the 2nd stage (M00). An absolute minimum value ofMtot = 6.03 kg is observed under the following conditions:M0 = 1.30 kg, M00 = 2.00 kg, M00 0 = 2.73 kg, as reported inTable 2(c). This value is significantly lower than the Al2O3 amount(113 kg) required to obtain a similar Simazine removal with asingle-stage system of 1000 L, as shown in Table 2(a). Nevertheless,it is only moderately reduced in comparison to the Al2O3 amount(10.2 kg) required with a two-stage system having a total volumeof 1000 L, as shown in Table 2(b).

The reduction of sorbent amount achieved replacing a two-stage system with a three-stage system is not sufficient to justifythe higher design complexity, as well as the higher plant andexercise costs. As a matter of the facts, the results of the costanalysis, reported in the Table S4, demonstrate that, whatever theherbicide used, the adoption of a triple-stage adsorber does notimproves the economic balance of the process.

It is worth noting that, under the optimum conditions, theamounts of Al2O3 required in the three stages are increasing(M0 < M00 < M00 0), confirming that due to the upward concavity ofthe Freundlich-type isotherm curve, lower solid loadings in theearly stages lead to a better efficiency of the sorption system. Asubstantially similar behavior is observed for the Simazinesorption on Fe2O3, as shown in the Table 2(c).

The Table 2(c) also reports the optimum values of M0, M00 andM00 0 found when MCPA is adsorbed on Al2O3 in a three-stagesorption plant. In this case, substantially similar amounts ofsorbent are required in the three stages (M0 � M00 � M00 0). This trendagrees with the results obtained in the Literature, when using aLangmuir-type isotherm [17]. This is due, as already observed inPar. 3.5, to the asymptotic character of the Langmuir-type isothermcurve of MCPA on Al2O3.

The optimum conditions for the MCPA sorption on Fe2O3 in athree-stage sorption plant, reported in the Table 2(c), show a M0

value significantly lower than M00 and M00 0. This result can beexplained by the discontinuous behavior of the isotherm curve ofMCPA on Fe2O3, shown in Fig. 1(b).

Conclusions

The design of single- and multiple-stage sorption plants wascarried out with reference to two sorbent oxides (Al2O3 and Fe2O3),and two herbicides displaying different behaviors at equilibrium:the Simazine, with Freundlich-mode sorption isotherms, and theMCPA, with Langmuir-mode sorption isotherms.

multi-stage stirred adsorber using mesoporous metal oxides for), http://dx.doi.org/10.1016/j.jece.2013.12.013

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5595605615625635645655665675685695705715725735745755765775785795805815825835845855865875885895905915925935945955965975985996006016026036046056066076086092610611612613614615616617618619620621622623624625626627628629630631632633634635636637638639640641

D. Pirozzi, F. Sannino / Journal of Environmental Chemical Engineering xxx (2013) xxx–xxx 9

G Model

JECE 238 1–9

Under the operating conditions adopted, the amount of Al2O3

required to carry out a given separation was usually lower than theamount required if using Fe2O3, though the design of the single-stage sorption systems was affected by the sorption isothermshape. As a matter of facts, due to the upwards concavity of thesorption isotherms of Simazine, small increases in the Simazineremoval fraction led to significant increases in the requiredamount of sorbent. On the contrary, when MCPA sorption wasconsidered, increases in MCPA removal fraction corresponded tolower increases in the required amount of sorbent.

The minimum required adsorbent amount was calculatedunder different conditions, as relatively short residence times wererequired to achieve the sorption equilibrium. In all cases examined,the adoption of a double-stage system led to a large reduction(from 78% to 91%) of the required amount of sorbent. A costanalysis demonstrated that the double-stage adsorber representsthe best configuration to adsorb Simazine. On the contrary, and thesingle-stage adsorber is the least expensive configuration whenMCPA is to be removed.

The replacement of a double-stage system with a triple-stagesystem having the same total volume allowed a lower reduction(from 24% to 41%) of the required sorbent amount. As aconsequence, whatever the herbicide considered, the three-stageadsorber resulted to be the most expensive configuration.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.jece.2013.12.013.

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