Succinic Acid Removal and Recovery from Aqueous Solution Using

Article pubs.acs.org/IECR

Succinic Acid Removal and Recovery from Aqueous Solution Using Hydrotalcite Granules: Experiments and Modeling Nick Schöwe,* Karlheinz Bretz, Torsten Hennig, Stefan Schlüter, and Görge Deerberg Department of Process Engineering, Fraunhofer Institute for Environmental, Safety, and Energy Technology UMSICHT, Osterfelder Straße 3, 46047 Oberhausen, Germany S Supporting Information *

ABSTRACT: Hydrotalcites (layered double hydroxides) are an alternative for anion removal due to their adsorption and ion exchange properties, e.g., in biotechnology. The applicability of powdered hydrotalcite in columns under dynamic conditions is impossible due to the small particle size and the low specific density. Therefore, the material was granulated for application as inorganic adsorbent in commercially available chromatographic columns. The calcination of a granulated hydrotalcite/betonite water mixture led to an open-porous material with a specific surface area of 153 m2/g. The average adsorption capacity of the hydrotalcite granules for succinic acid in column experiments was estimated at 0.4 mol/kg over 12 succeeding ad- and desorption cycles (pH 6.5). Up to 98% of the bound succinic acid could be recovered. The granules were characterized concerning their pore structure and adsorption equilibrium. The obtained results were included in a mathematical model simulating adequately the breakthrough behavior of the system. anion species by surface adsorption and anion exchange.11 They can exhibit a positive surface charge as well as exchangeable counterions in the interlayer domain.12 Different aliphatic or aromatic organic anions like carboxylate,13 oxalate,14 sulfonate,13,15 or terephthalate13 were already incorporated to balance the charge in the interlayer space. In addition, a large number of inorganic oxyanions were examined for their adsorption characteristics with various LDHs,16 e.g., for defluoridation of drinking water17 or for removal of phosphate from aqueous solutions.18,19 Hydrotalcites are available on a large scale; e.g., they are used as stabilizers in polyvinyl chloride.20 They could serve as a costefficient alternative for commercially available anion exchange resins due to their adsorption capacities. The main advantages of hydrotalcites are their resistance to biofouling as well as high temperature treatments.11 Moreover it could be an interesting and economical alternative for introduction in separation processes used in commercially available chromatographic columns. Isolation of organic acids in aqueous solution by adsorption with hydrotalcite granules as inorganic adsorbent was tested using succinic acid as an example for a biotechnologically produced carboxylic acid. Succinic acid production using Anaerobiospirillum succiniciproducens (pH-optimum of 6.5) is limited by the buildup osmotic pressure due to the acid production. Therefore, a maximum succinic acid production of 269.3 mmol/L can be achieved.5 A continuous in situ removal of the succinic acid would reduce the osmotic pressure and increase the productivity as shown by Meynial-Salles et al.6

1. INTRODUCTION Biotechnology counters the increasing crude oil scarcity by offering new options to substitute petroleum-based chemicals. Many organic acids can be produced by biotechnological cultivation of microorganisms, such as acetic, citric, lactic, and succinic acid. Due to their biotechnological producibility and various possibilities for the derivation of other chemicals, succinic acid has been identified as a possible future bulk chemical in several studies.1−4 Nevertheless, a main inhibitory effect for the biotechnological succinic acid production is the build-up osmotic pressure during fermentation5,6 which requires the development of methods for integrated continuous product removal.6 Beside the main product, unwanted byproducts are produced in many cases. As a consequence, the purification difficulties of the target product are considerably higher. Nowadays the downstream processing takes from around 50−80% of the total production costs of bioproducts.1,7,8 Hence it is necessary to minimize the production and purification costs for the biotechnological process to be competitive with petrochemical production. The purification of carboxylic acids includes typically several steps and is performed today by precipitation, extraction, membrane processes, crystallization, and chromatographic techniques like sorption and ion exchange.9 The present study focuses on the adsorption of organic acids in aqueous solutions with the aim to replace current sorption methods like ion exchange resins. Typically problems with many classical ion exchange resins are their lack of selectivity10 and biofouling. Recently a new, considerable interest in the use of layered double hydroxides (LDH), also known as hydrotalcite-likecompounds, evolved with the removal of anions from aqueous solutions. LDH can be structurally described by the stacking of positively charged layers, neutralized by hydrated organic or inorganic host anions in the interlayer domain. LDHs can bind © 2015 American Chemical Society

Received: Revised: Accepted: Published: 1123

October 31, 2014 January 8, 2015 January 8, 2015 January 8, 2015 DOI: 10.1021/ie504306p Ind. Eng. Chem. Res. 2015, 54, 1123−1130

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Industrial & Engineering Chemistry Research

(50−55 mm bed length) and primed with distilled water. Afterward the column was sealed and shaked manually to receive a uniform settling of the bed. The adsorption was performed with volume flow rates of 0.25−5.0 mL/min using a Watson Marlow 120 U pump (Falmouth, U.K.). Before each run the column was conditioned by washing with demineralized water until a neutral pH was achieved. For determining adsorption and desorption properties under dynamic conditions the breakthrough experiments were accomplished using 84.7 mmol/L succinic acid (≥99%, Merck KGaA, Darmstadt, Germany) solution adjusted to pH 6.5. The effluent of the column was fractionated (5 mL aliquots) and the succinic acid concentration determined. The succinic acid was recovered using a solution containing NaOH (1 mol/L) and NaCl (1 mol/L) at 293 K. Afterward the column was rinsed with demineralized water until the pH was neutral (pH ∼ 7) to finish the cycle. 2.4. Analytical Methods. Succinic acid concentration was estimated by acidification with 1 mol/L HCl to pH < 2, so that the analyte is present in undissociated form. On the basis of the absorption of its carboxy groups, succinic acid concentration was measured via the absorbance at the wavelength of 215 nm using Suprasil 100-QS cuvettes (Hellma GmbH & Co. KG, Mühlheim, Germany) and Shimadzu UV-2102 PC spectrophotometer (Shimadzu, Duisburg, Germany).

The aim of the present study was the development of a sorption process under dynamic conditions in laboratory scale with the potential possibility of in situ adsorption in downstream processing. For better understanding and simulation of the system the obtained results and further data (e.g., column and adsorbent properties) have been introduced in a mathematical simulation model.

2. MATERIALS AND METHODS 2.1. Granulation. 85 wt % of Hydrotalcite EXM 2264 (Clariant International Ltd., Moosburg, Germany) was mixed with 15 wt % bentonite (Montigel F, Clariant International Ltd., Moosburg, Germany) to obtain a homogeneous mixture. Demineralized water of 61 wt % was added to the mixture, and afterward the resulting paste was extruded and cut into granules of 1.3 mm diameter and 2 to 3 mm length. The granules were calcined in a muffle furnace (L15/11/P 320, Nabertherm GmbH, Lilienthal, Germany) with a heating rate of 1 K/min until 773 K was achieved. The temperature was maintained for 3 h. 2.2. Characterization and Adsorption Isotherms. The measurement of the nitrogen adsorption isotherm at 77 K was performed with Sorptomatic 1900 (Thermo Fisher Scientific, Waltham, U.S.A.). Prior to the measurements, the samples were outgassed for 3 h at 473 K in vacuum. The specific surface area was calculated from nine measurements recorded in the relative pressure range of 0.05 to 0.3 according to the BET equation in DIN 66131.21 Succinic acid isotherms were carried out with 500 mg of the hydrotalcite granulate at pH 6.5 using succinic acid concentration in the range of 84.7 to 338.7 mmol/L. The pH value was adjusted with NaOH. The granulate was incubated with 10 mL of the succinic acid solution at 298 K in a gently shaking water quench (GFL 1086, Burgwedel, Germany) for adsorption equilibrium (5 h, succinic acid concentration in the supernatant remained constant). The liquid was removed by filtration, and the succinic acid concentration was estimated using a spectrophotometer (Shimadzu UV-2102 PC, Shimadzu, Duisburg, Germany). Afterward, the amount of isolated succinic acid was calculated by the mass balance equation: * = Y SA

3. RESULTS The applicability of hydrotalcite powder in continuous systems like chromatographic columns is not possible due to small particle size and low specific density. Therefore, to overcome this obstacle, the material was granulated using bentonite as binding agent. In accordance with the literature22 calcination experiments were performed in a range of 673−873 K. In this temperature range, water and carbonate are expulsed from the interlayer space of hydrotalcite. The carbonate expulsion leads to an increased binding (intercalation) of organic and inorganic anions in the interlayer space to balance the positive surface net charge.23,24 Temperatures above 773 K are not suitable for the granulation, as the structure of hydrotalcite can be irreversibly damaged at higher temperatures.25 Series of temperature experiments were performed in order to ensure the physical26 and chemical stability of the produced hydrotalcite granules. The screening revealed a temperature optimum for calcination of 773 K, were the best physical and chemical stability was achieved. The stability was tested in a pH range of 2.7−14 (423.4 mmol/L succinic acid solution and desorption solution consisting of 1 mol/L NaOH). In accordance with the literature16 it was observed that the adsorption of succinic acid tends to decrease with increasing pH. Nevertheless, for biotechnological application the optimal pH value is 6.5, e.g., for succinic acid production using Anaerobiospirillum succiniciproducens.5 In this process a succinic acid concentration of 269.3 mmol/L was achieved.5 These values were used as reference for selected parameters in this study. 3.1. Characterization. The specific surface area, pore structure, and distribution of the calcined hydrotalcite granules were characterized by the Brunauer−Emmett−Teller (BET),21 Berrett−Joyner−Halenda (BJH),27 and Horvath−Kawazoe (HK)28 models. About 75% of the micropore volume is located in the range of 0.2−0.6 nm (Figure 1), whereas the mesopore volume shows a well-balanced range for single pore

V (cSA,0 − ceq) ma

(1)

where cSA,0 and ceq are the initial concentration [mol/L] and the equilibrium concentration of succinic acid, respectively. Further, ma is the amount of the calcinated adsorbent [kg] and V is the volume of the adsorptive solution [L]. Modeling the adsorption process requires an adequate method for the mathematical description of the adsorption equilibrium. The Langmuir and Freundlich models were used, since they give an acceptable description of the experimental data for most adsorbance processes. From the adsorbed succinic acid amount at the different equilibrium concentrations (YSA * ) the maximum capacity Ymax can be extrapolated by the isotherm at the point where the adsorbed amount remains constant with increasing concentration. 2.3. Column Adsorption. Column adsorption was performed in a plastic column (B. Braun Melsungen AG, Melsungen, Germany) with an inner diameter of 15 mm and a length of 75 mm. The column was prepared for adsorbent with a 1.5 mm glass wool layer at the bottom. On average approximately 2.5 g of granulate was added in the column 1124

DOI: 10.1021/ie504306p Ind. Eng. Chem. Res. 2015, 54, 1123−1130

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specific surface area. The analysis of pore size distributions and the N2-sorption isotherms showed that the pore sizes are mainly in the meso- and macrorange. This led to the assumption that organic acid molecules have a good accessibility through the granules. 3.2. Adsorption Isotherms. The experiments were conducted at concentrations of 84.7−338.7 mmol/L succinic acid, and the pH of the feed solution was adjusted to pH 6.5. The pH of all solutions increased to pH 8.5−9.0 at adsorption equilibrium. In Figure 3, the adsorption capacity of calcined hydrotalcite granules for succinic acid at given concentrations in equilibrium

classes from 1.7 to 20.0 nm and does not exceed a volume fraction of about 8%.

Figure 1. Micropore size distribution of hydrotalcite granules using the HK model for calculation from a single nitrogen adsorption isotherm from p/p0 0 to 0.2.28

The micropore volume (Vmicro) was 0.06 cm3/g. The values for the mesopore volume (Vmeso) of 0.25 cm3/g and the total pore volume (Vpore) of 0.29 cm3/g lie close together and indicate that the majority of the pore size is present in the meso-range. The hydrotalcite granules N2 adsorption−desorption isotherms (Figure 2) are based on the classification by Sing et al.29

Figure 3. Succinic acid adsorption isotherms of hydrotalcite granules at 298 K. Points are experimental equilibrium data, and the lines are a model fit. Contact time: 5 h, volume per batch: 10 mL, initial concentrations: 84.7−338.7 mmol/L succinic acid solution (pH 6.5).

is shown. The best fitting is obtained by the Freundlich model (R2 = 0.984) with a = 0.2521 and n = 0.4085, n * = acSA Y SA

(2) 2

but also the Langmuir model (R = 0.948) * = Ymax · Y SA

bcSA 1 + bcSA

(3)

gives a suitable description of the experimental data for representing the adsorption equilibrium. The extrapolated maximum adsorption capacity from the Langmuir isotherm is approximately Ymax = 1.47 mol/kg (b = 0.0675 L/mol). 3.3. Column Adsorption Experiments. The calcined hydrotalcite granules are also applicable under dynamic conditions. In order to survey the operating adsorption capacity in column experiments a succinic acid solution was loaded on a column packed with hydrotalcite granules. Adsorption experiments with volume flow rates between 0.25 and 5.0 mL/min (velocity range 8.5−169.8 cm/h) were conducted to determine the optimal volume flow rate. The optimal volume flow rate was determined at 0.25 mL/min where a maximum adsorption capacity of 0.437 ± 0.011 mol/kg (Figure 4) was achieved (contact time: 2.67 h). The pH of the solution was determined at the outlet of the column. An initially steep increase of the pH value was observed with a subsequent plateau phase as a function of volume flow rate. Subsequently, the pH value reapproaches asymptotically to the initial value. The average pH of effluent increased from pH 6.5 to pH >10. The reversibility of binding was shown by Das et al.18 and Kuzawa et al.19 As a result, hydroxide-containing eluents in

Figure 2. N2-sorption isotherms of hydrotalcite granules for calculation of specific surface area according to the BET model.21

type IV. The initially steep increase (at p/p0 >0) of the adsorption isotherm emerges from micropores and quickly turns into saturation at p/p0 0.02. Associated with capillary condensation taking place in the mesopore structures, the hysteresis loop is the characteristic feature of these sorption isotherms. The hysteresis loop was further specified as type H330 and does not exhibit any limiting adsorption at high p/p0. This indicates the presence of macropores. According to Rouquerol et al.30 type H3 loops are usually given by adsorbents containing slit-shaped pores or aggregates of platy particles. The latter can be derived from the layered structure of hydrotalcites. The BET surface area was calculated with 153 m2/g. The results show that the calcination of the granulated hydrotalcite/betonite water mixture led to an open-porous material with a wide range of pore distribution and a high 1125


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Figure 5. Schematic diagram of the column. The real flow direction of the column is top down. Molecular flow (Ṅ ), load capacity (Y), pressure (P), height of the bed of granules (H), height of the column (L), column internal diameter (D).

• based on the relatively large particles, loading and concentration gradients in the particles must be considered explicitly • the model considers film and pore diffusion (or alternatively only pore diffusion) as mass transfer resistance for the adsorptive The following conservation equations were formulated for the adsorber:

Figure 4. Amount of succinic acid ad-/desorbed from a 84.7 mmol/L succinic acid solution (pH 6.5, 40 mL) and desorbed with 40 mL of an aqueous solution containing NaOH (1 mol/L) + NaCl (1 mol/L) at volume flow rates of 0.25−5.0 mL/min.

Darcy’s law u=−

combination with sodium chloride are most suitable for desorption on hydrotalcites tested in batch elution experiments. The optimal concentration of the desorption solution was elicited in preliminary testing. Thus, succinic acid adsorbed on the hydrotalcite granules was effectively desorbed with 40 mL of alkaline NaCl solution at a volume flow rate of 0.25 mL/min (Figure 4). It is shown in Figure 4 that it was possible to detect up to 98% of the adsorbed succinic acid (0.428 ± 0.006 mol/ kg) in a desorption solution consisting of NaOH (1 mol/L) and NaCL (1 mol/L). In cyclical experiments the ad-/desorption capacity as well as the physical and chemical stability of the granules were tested. Therefore, 12 ad-/desorption cycles were performed using the above-mentioned conditions. In all cycles, the succinic acid capacity was above 95% of the initial adsorption capacity. Further thermal regeneration of the granules was tested at 773 K (calcination parameters). In the process succinic acid can be entirely expulsed from hydrotalcite granules as a result of thermal decomposition. 3.4. Modeling and Simulation. The equations describing the adsorption process consist basically of a mass balance for the adsorptive in the liquid phase and a further mass balance for the adsorptive in the pore space of the adsorbent phase. The mass transfer between the two phases was supplemented by the exchange relations. Since the temperature for the high volumetric heat capacity of the liquid phase during adsorption remains nearly constant, there was no need for the formulation of an independent energy balance. For the modeling of the liquid-phase adsorption process, the work of Bathen and Breitbach31 was used as base literature with references to further literature and detailed information. In addition to that, equations were used from Schieferstein et al.32 and adapted to the liquid-phase adsorption process. The following postulates were made for the mathematical approach (cf. Figure 5): • cylindrical geometry with a gradient formulation in axial and radial directions • the bed of granules being described by porosity and particle properties • a laminar creep-flow in the pore space of the bed (“Darcy-flow”) with axial and radial diffusion effects

K (∇P + gρL ) ηL

(4)

Mass balance for liquid phase (continuity equation) ∂ (ϕρL ) + ∇·(ρL u) = −SL ·MSA ∂t

(5)

Component balance for succinic acid in the liquid phase ∂ eff (ϕcSA,L) + ∇·( −δSA,L ∇cSA,L + ucSA,L) = −SL ∂t

(6)

Component balance for succinic acid (as adsorptive and adsorbate) ∂cSA,P ⎞ 1 ∂ ⎛ 2 ∂ (ϕPcSA,P) − 2 ⎟ = − SP ⎜rP ϕPδSA,P ∂t ∂rP ⎠ rP ∂rP ⎝

(7)

1 ∂ ⎛ 2 ads ∂YSA ⎞ ∂ (ρS YSA ) − 2 ⎟ = SP ⎜rP ρ δSA,P ∂rP ⎠ ∂t rP ∂rP ⎝ S

(8)

Density of the liquid phase ⎛ ρ ⎞ ρL = ρW + cSA,LMSA ⎜⎜1 − W ⎟⎟ ρSA ⎠ ⎝

(9)

Darcy’s law was used (see eq 2), which allows a linear coupling between velocity and pressure gradient for the calculation of the velocity field in the adsorber. The velocity field can hereby be replaced in all equations by a pressure gradient. The equation system 4−8 is suitable for determination of the dependent solution variables P, cSA,L, cSA,P, and YSA. Assuming that liquid and adsorbent phases are in adsorption equilibrium inside the adsorbent (YSA = YSA * (cSA,P)), the following three partial differential equations arise for P, cSA,L, and cSA,P from combining eqs 7 and 8. ϕ

1126

⎛ K ⎞ ∂ρL ∂P − ∇·⎜⎜ρL (∇P + gρL )⎟⎟ ∂P ∂t ⎝ ηL ⎠ ∂ρ ∂cSA,L = −SL ·MSA − ϕ L cSA,L ∂t

(10)


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Industrial & Engineering Chemistry Research ϕ

∂cSA,L ∂t

⎛ ⎞ K eff − ∇·⎜⎜cSA,L (∇P + gρL ) + (δSA,L ∇cSA,L)⎟⎟ ηL ⎝ ⎠

= − SL

For the adsorption equilibrium the measurement series from 3.2 (Figure 3) was used for adaptation by the Langmuir and Freundlich equations. The effective diffusion in the passed through bed is dependent on the direction given by Poling et al.:33

(11)

⎛ ρ dY * ⎞ ∂cSA,P 1 ∂ ϕP⎜⎜1 + S SA ⎟⎟ − 2 ϕ ∂ c t d rP ∂rP ⎝ SA ⎠ P ⎛ ⎛ * ⎞ ∂cSA,P ⎞ ρ ads dY bs ⎜⎜rP 2ϕ ⎜⎜δSA,P + S δSA,P ⎟=0 ⎟⎟ P ϕP dcSA ⎠ ∂rP ⎟⎠ ⎝ ⎝

eff δSA,L

(12)

The balance equations are supplemented by initial and boundary conditions as well as various constituent equations including adsorption equilibrium, diffusion coefficient, and mass transfer coefficient. Initial and boundary conditions for the dependent variables P, cSA,L, and cSA,P are given as follows:

0 cSA,L = cSA,L

(15)

0 cSA,P = cSA,P

(16)

(17)

P(z = 0) = Pu

(18)

̇ in NSA,L in

∂z

∂cSA,L ∂r ϕP

(26)

βSA,L → Sd p δSA,L

= 1.6(2 +

Shlam 2 + Shturb 2 )

Shlam = 0.664Reϕ 0.5Sc1/3

(19)

Shturb =

Reϕ = (20)

A

Sc =

(z = 0) = 0

(r = 0) =

τP

ShLS =

⎞ ⎛ eff ∂cSA,L (z = H ) + uz(z = H )cSA,L(z = H )⎟ −⎜ −δSA,L, z ∂z ⎠ ⎝

∂cSA,L

(25)

(27)

with

D⎞ ∂P ∂P ⎛⎜ (r = 0) = r= ⎟=0 2⎠ ∂r ∂r ⎝

=

(24)

The tortuosity factor summarizes all intraparticle diffusion transport resistances. Surface diffusion of the adsorbate in the particles is neglected (δads SA,P = 0). The diffusion film is considered as an external mass transfer resistance, and the mass transfer coefficient between the liquid phase and the particles can be estimated on the basis of the analogy between heat and mass transfer. According to this analogy, in the formula for calculating the heat transfer coefficient, the Nusselt number is replaced by the Sherwood number and the Prandtl number by the Schmidt number, respectively:34

Boundary conditions in Ṁ −ρL uz(z = H ) = inL A

B

δSA,L

δSA,P =

Initial conditions (14)

2 dp

⎞ uz 2 + ur 2 ⎟ ⎟ ⎟ uz 2 + ur 2 ⎟⎟ ⎠

Here δSA,L is the diffusion coefficient for succinic acid in the free liquid mixture. The diffusion coefficient for diffusion in the pores of the particles arises from the diffusion coefficient for the free liquid diffusion by division with the tortuosity factor τP:

(13)

P = P0

1 − ϕ )δSA,L +

dp

2 ⎛ ⎛ dp ⎞ ⎞ B = 4.6⎜⎜2 − ⎜1 − 2· ⎟ ⎟⎟ D⎠ ⎠ ⎝ ⎝

* ⎞ ∂cSA,P ρ 6 ⎛ ads S dY SA ⎟⎟ ϕP⎜⎜δSA,P + δSA,P dP ⎝ ϕP dcSA ⎠ ∂rP

⎛ dp ⎞ ⎜rP = ⎟ 2⎠ ⎝

1 − ϕ )δSA,L +

with

The local amount of succinic acid adsorbed is given by the flow at the adsorbent surface: SL = (1 − ϕ)

⎛ ⎜(1 − ⎜ =⎜ ⎜⎜(1 − ⎝

∂cSA,L ⎛ D ⎞⎟ ⎜r = =0 2⎠ ∂r ⎝

(21)

0.037Reϕ0.8Sc 1 + 2.443Reϕ−0.1(Sc 2/3 − 1)

ρL d p ϕηL

uz 2 + ur 2

(29)

(30)

ηL ρL ·δSA,L

(31)

The permeability K is determined by permeability experiments. No distinction is made between the flow directions. 3.4.1. Implementation and Simulation of the Model. The results of the above-described experiments were included as a database in the development of the specific adsorbent simulation model. The used data is shown in Supporting Information Table S1, where the deposited pore diameter of 19.4 nm describes an average value of the (meso)pore analysis. To validate the simulation model described above, the characteristic of a breakthrough curve at a volume flow rate of 0.25 mL/min was simulated under the same boundary and

(22)

* ⎞ ∂cSA,P ⎛ ρ 6⎛ D ⎞⎟ ads S dY SA ⎜r = ⎜⎜δSA,P + δSA,P ⎟⎟ · P ⎝ ϕP dcSA ⎠ ∂rP dp ⎝ 2⎠

⎛ ⎛ d p ⎞⎞ = βSA,L → SaP⎜⎜cSA,L − cSA,P⎜rP = ⎟⎟⎟ 2 ⎠⎠ ⎝ ⎝

(28)

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and validation of the model, the effect of volume flow rate on breakthrough was simulated. Thus, breakthrough curves with volume flow rates between 0.15 and 3.0 mL/min were simulated and compared with selected experimental data from analysis of the outflow (Figure 7). Even at volume flow rates of 1.0 mL/min and 3.0 mL/min, the shape and the time scale of the curves of the experimentally determined data points are well displayed. No variable from the model was manipulated, so the model curves were not fitted to the experimental data. As can be shown in Figure 7, higher volume flow rates lead to more rapid breakthrough, and the influence of the volume flow rate can be simulated in pure succinic acid breakthrough curves.

initial conditions as in the experiment. Therefore, the model was implemented in the FEM-Software COMSOL Multiphysics, version 4.3b (Comsol AB, Stockholm, Sweden) (Figure 6).

4. DISCUSSION The aim of the present study was the development of a sorption process under dynamic conditions in laboratory scale. The separation of succinic acid from aqueous solution by adsorption was tested using hydrotalcite granules as inorganic adsorbent. In column experiments, hydrotalcite is only useable by a granulation of the powdered material. The principal applicability of the material for adsorption purposes has already been described.11,16 Several factors like pH, competitive anions, temperature, calcination, or particle size can influence oxyanion adsorption.16 Therefore, a direct comparison with other adsorbents is not possible, unless the identical LDH was used under identical experimental conditions. Davison et al.10 have estimated that a minimum capacity of >0.4 mol/kg is required for industrial application of succinic acid separation. In the present study, an operating adsorption capacity of 0.437 ± 0.011 mol/kg (Figure 4) was achieved. The surface area (BET) of the determined hydrotalcite granules was 153 m2/g. For pure hydrotalcites the specific surface area is a function of intercalated anions. In the literature, the surface area (BET) is mentioned with less than 100 m2/g but also above.35,36 However, a large specific surface area is not directly equivalent to a large adsorption capacity of hydrotalcites, as their properties are dependent on their ratio of diand trivalent cations.37 A comparison of the batch and the column experiment at the same succinic acid feed solution (84.7 mmol/L, pH 6.5) shows a difference between the values for batch adsorption (∼0.80 mol/kg, Figure 3) and column adsorption (0.437 ± 0.011 mol/ kg, Figure 4). Possible reasons are that in the batch system succinic acid has more time to reach also adsorption sites deeper in long pores by diffusion (batch contact time: 5 h, column contact time: 2.67 h). Besides that, the conditioning of the column before each run, carried out by washing the granules with demineralized water, finalizes the rehydration process of hydrotalcite11,38 which might have further influence on operating adsorption capacities and kinetics, respectively. The rehydration process of hydrotalcite in batch adsorption starts with the contact of the feed solution and adsorptive will be directly transported deep into the pores. These reasons lead to the assumption that in the dynamic system succinic acid is mainly adsorbed at the outer surface in the parts of the pores that are more easily accessible. The surveyed operating adsorption capacity in column experiments can amount to a large or small proportion of the total capacity and depends on a number of process variables such as volume flow rate. The adsorption interactions generally depend on the pH, the charge, and the charge density of the surface of LDH. Usually, the adsorption of oxyanions on hydrotalcite tends to increase with

Figure 6. Breakthrough curve of a 84.7 mmol/L aqueous succinic acid solution in a column packed with hydrotalcite granules using a flow rate of 0.25 mL/min (squares). The line is a simulated breakthrough curve. τ is the calculated hydrodynamic residence time.

The comparison of the experimental data points with the progress of the simulated breakthrough curve shows that the shape and the time scale of the curve is well displayed. Regarding the physical complexity of the model, the differences are small. A better adaption of the measured points is possible by fitting with the suitable model parameter, but this was not done here for physical reasons. Experimental data points above a specific level (cf. Figure 6 and Figure 7) cannot be detected by UV spectrophotometer

Figure 7. Breakthrough curves of succinic acid solution (84.7 mmol/ L) in a column packed with hydrotalcite granules using flow rates of 0.25 mL/min (squares), 1.0 mL/min (triangles), and 3.0 mL/min (circles). The lines represent simulated breakthrough curves at various flow rates (0.15 to 3.0 mL/min).

(quantification limit). At this point, the difference of the concentrations of succinic acid in the effluent and in the feed solution is too low for a reliable quantification. The concentration of succinic acid in the effluent of the column was fractionated (5 mL aliquots), and the succinic acid concentration was determined. The calculated hydrodynamic residence time (τ = 21.7 min) is very close to the simulated breakthrough shown in Figure 6. For the further application 1128


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Industrial & Engineering Chemistry Research decreasing pH.16 At adsorption equilibrium, the average pH of effluent in column experiments (pH >10) was higher than in batch experiments (pH 8.5−9). This leads to the assumption that the pH value can have influenced the amount of adsorbed succinic acid in batch and column adsorption and can be one reason for the difference between the results. It should be considered for the column experiments that several ad- and desorption cycles were taken. In detail, after the first desorption step, bound succinic acid anions were exchanged by hydroxide anions. These hydroxide anions will be again exchanged on their part through succinic acid anions in the next adsorption step. Hence, there is an inverse diffusion of succinic acid and hydroxide anions through mutually substituted and displaced molecules, where several mixture equilibria are passed through. The released hydroxide anions form sodium hydroxide solution which results in an (local) increase of the pH value. This leads to the observed initially steep increase of the pH value (pH >10) at the outlet of the column. The difference of batch and column adsorption capacities arises due to the increase of pH of the solution in the pores. This influences the equilibrated adsorption capacity and is especially the case for small pores where not all hydroxide anions are entirely removed yet from the pores in the effluent solution. At higher pH, the succinic acid adsorption may be affected due to the increasing competitive effect of hydroxide anions for adsorption on hydrotalcite.16 Altogether, several of these factors can be the cause of the differences and must therefore be taken into account. The experimentally determined basic data were implemented in a simulation model. The Langmuir and Freundlich models give a suitable description of the experimental data for representing the adsorption equilibrium. With the model described in this study, the effect of volume flow rates on breakthrough curves was accurately predicted and simulated without a manual fitting of breakthrough curves of experimental data. The model was successfully confirmed by selected measured breakthrough curves. Concerning this the shape and the time scale of the curve is well displayed. The simulated breakthrough (Figure 6) is close to the calculated hydrodynamic residence time (τ = 21.7 min). The small deviation can be explained by the different calculations of the free volume of the column. For calculation of the hydrodynamic residence time the free volume of the column was determined by experiments while the free volume of the column is accurately calculated in the simulation from deposited basic data. Nevertheless, the prediction of the performance of the hydrotalcite granules is possible, if accurate equilibrium data for the adsorbent and the conditions at the adsorber inlet are exactly known. To enhance the model application range, further basic data of other adsorbates can be implemented.

desorption solution at room temperature. Further studies should focus on survey of selectivity to determine the suitability of the adsorbent for online sorption processes like removal and recovery of organic acids, e.g., in biotechnology (in situ adsorption in downstream processing for process intensification). The event of competition for the adsorption of succinic acid on hydrotalcite granules by other organic acids such as acetic acid, which can be produced in biotechnological processes with microorganisms, should be examined. The simulation model was successfully validated on experimental data so the prediction of the performance of the hydrotalcite granules is possible. Furthermore, it should be examined whether the implementation of further basic data for other adsorbates like organic (e.g., lactate) or inorganic (e.g., phosphate) anions can enhance the model application range.

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ASSOCIATED CONTENT

* Supporting Information S

Table S1 showing additional information on column parameters and properties of the adsorbent used in the model. Tables S2 and S3 showing additional information on symbols and indices. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +49 208 8598-1266. E-mail: nick.schoewe@umsicht. fraunhofer.de. Notes

The authors declare no competing financial interest.

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REFERENCES

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5. CONCLUSIONS The utilization of hydrotalcite in commercially available chromatographic columns is possible by granulation of the powdered material. The study has shown that the hydrotalcite granules can be an appropriate applicable alternative (under dynamic conditions), e.g., for ion exchange resins for the adsorption of organic acid from aqueous solution. The adsorption capacity of calcined hydrotalcite granules for succinic acid at equilibrium concentrations were determined wherein the Langmuir and Freundlich models give a suitable description of the experimental data. It was observed in column experiment, that it is possible to recover bound succinic acid in 1129


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Succinic Acid Removal and Recovery from Aqueous Solution Using

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