Flocculant

Ind. Eng. Chem. Res. 2009, 48, 5085–5092

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Anionic Surfactants Removal by Natural Coagulant/Flocculant Products J. Beltrań-Heredia, J. Sańchez-Martıń,* and C. Solera-Hernańdez Department of Chemical Engineering and Physical Chemistry, UniVersidad de Extremadura, AVda. de ElVas, s/n, 06071, Badajoz, Spain

A new tannin-based coagulant and flocculant agent has been tested on the removal of sodium dodecyl benzene sulfonate (SDBS), a dangerous and pollutant anionic surfactant. It is called Tanfloc and consists of a chemically modified tannin extract from Acacia mearnsii de Wild. Tanfloc has been revealed as an efficient product in anionic surfactant removal. Around 70% of SDBS removal has been achieved with Tanfloc doses of 150 mg · L-1. pH has a negative influence on surfactant removal, while the higher the initial surfactant concentration is the higher q capacity is obtained. Theoretical data adjustment has been carried out according to three different models: Frumkin-Fowler-Guggenheim (FFG), Gu and Zhu (G-Z), and Freundlich (F). Adjustment parameters have been obtained with r 2 levels above 0.90 in all cases. 1. Introduction Surfactants have become a very important group of compounds in modern life. They are present in a large variety of usual and normal products like soaps, detergents, pharmaceuticals, personal care products, etc., but not only: they are also used in chemical industry, “hi-tech” devices, paints, and leather.1 As it can be appreciated, surfactants have achieved a main position in human activity. Attending to the last statistical data, more than 12 M tons per year2 are produced, so surfactants can be considerered as a first importance chemical group. Surfactants dumping into the environment represents a harmful and noxious practice. They may be useful and needed compounds, but they are also considered dangerous and undesireable substances because of their impact on water animal and vegetal life. The main aspects in which surfactants modify environmental equilibrium involve3 groundwater and lakes pollution, pharmaceutical products binding (so pollution activity of these kind of chemical compounds is considerably increased), animal and human toxicity, and biopersistance.4 Due to these reasons, removing surfactants from water flows has become a priority of a large number of researchers. Nowadays, surfactants can be removed by several mechanisms, most of them imply adsorption on activated carbon,5 chemical association,6 or electrochemical removal.7 However, new removal methods should be researched because the impact of surfactants and tensioactives is high enough. Specifically, the risk of bioaccumulation of sulfonated surfactants, such as sodium dodecyl benzene sulfonate (SDBS), has been fully characterized.8,9 Taking these risks into account, the investigation we have developed has been focused on this surfactant. Under tannins denomination there are lots of chemical families. Tannins have been used traditionally for tanning animal skins, but it is possible to find several products that are distributed as flocculants. Tannins come from vegetal secondary metabolytes:10 bark, fruits, leaves. Tannin-rich barks come from trees such as Acacia, Castanea, or Schinopsis. However, it is not needed to search for tropical species: Quercus ilex, suber, or robur have also tannin-rich bark. Tanfloc is a trademark that belongs to TANAC (Brazil). It is a tannin-based product, which is modified by a physicochemical process, and has a high flocculant power. It is obtained from Acacia mearnsii de Wild * To whom correspondence should be addressed. E-mail: jsanmar@ unex.es.

bark. This tree is very common in Brazil and it has a high concentration of tannins. The production process is under intelectual patent law, but similar procedures are widely reported as Mannich base reaction.11 Specific industrial process for Tanfloc is referred by U.S. patent number 6,478,986 B1.12 It involves tannin polymerization by the addition of formaldehyde (37%), ammonium chloride, and commercial hydrochloric acid. The product so obtained under certain temperature conditions has a viscous appearance with 36% of active material. Several references have been found regarding these kind of chemical processes.11,13,14 Most of them are patents, including the specific process for Tanfloc, which is reported.12 The scientific literature refers to a reaction mechanism that involves a tannin mixture, mainly polyphenol tannins whose structure may be similar to flavonoid structures such as resorcinol A and pyrogallol B rings. Similar products have been studied as general flocculants previously.15 Tanfloc has been tested as flocculant in wastewater16,17 and its results are promising. Environmental aspects are considered a primary target to work on, but usually economical and availability criteria are not taken into account when a technical solution is proposed for remediation processes such as surfactants removal. This investigation focuses its interest in advancing in surfactant removal by means of a new process that (a) is cheaper than others such as electrocoagulation; (b) is based on a natural product, so its biodegradability is higher than other coagulants; and (c) uses a coagulant agent that has no need of pH adjustment, so its usage is easier than others. Taking care of environmental subjects may include these and others considerations that make the possibility of becoming clean a universal chance. After a preliminary screening on surfactant-removal ability of several natural agents, this paper aims to characterize an interesting capacity Tanfloc seems to present: surfactant removal. Several anionic surfactants have been tested to be removed by Tanfloc. Among them, we have selected SDBS as a specific target. 2. Materials and Methods 2.1. Buffered Solution. All assays were done in a pH-stable medium. A pH 7-buffered solution was prepared by mixing 1.2 g of NaH2PO4 and 0.885 g of Na2HPO4 in 1-L flask. Assays with different pH were carried out by adjusting this buffered solution

10.1021/ie801913y CCC: $40.75  2009 American Chemical Society Published on Web 04/07/2009

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Ind. Eng. Chem. Res., Vol. 48, No. 10, 2009

to the specific pH by using HCl 0.5 M and NaOH 0.5 M. All reagents were supplied by PANREAC in analytical purity grade. 2.2. Wastewater Samples. Two types of wastewater were collected to test the efficiency of Tanfloc in this kind of matrix. With this scope, SDBS was added to wastewater until a 50 mg · L-1 initial surfactant concentration was achieved in the samples. The first type of water was surface water collected from the Guadiana river at Badajoz (Southwest of Spain) during last winter. Water was used the same day it was collected in order to avoid the loss of its properties, mainly referred to turbidity. The second type was municipal wastewater obtained from the Wastewater Treatment Plant of La Albuera, a little town near Badajoz. This treatment plant was designed some years ago. It receives municipal wastewater from 4000 people. There are no significative influent of industrial wastewater, but some agricultural and livestock farms are present, so such diffuse pollution may occur. The effluent has a moderately low COD charge. Average incoming flow rate is 41.63 m3/h. Water involved in this study is collected after previous big solids separation and before oil and sand separation. The main physicochemical characteristics of these samples are reported in the Supporting Information. 2.3. Natural Coagulant Products Preparation. Apart from Tanfloc, nine natural coagulant products were tested in a preliminary screening. They were prepared in the following way: Moringa oleifera seed extract was obtained as described previously.18-20 Seeds were obtained from SETROPA (Holland). The extraction process was carried out in the following way: seeds were reduced into powder by a domestic mill. A 1 M NaCl (PANREAC) solution was prepared and 5 g of powder were put into 100 mL of it. The NaCl solution with powder was stirred for 30 min time at room temperature (around 25 °C). No pH modification was needed, as natural pH 7 was achieved. Then, the extract was filtered twicesonce through commercial filter paper on Bu¨chner funnel and once again through a fine filtering Millipore system (0.45 µm glass fiber). The result is a clear, milky-like liquid. Cationic starch was supplied by Cargill, USA. It is used as an authorized alimentary supplement. It is presented as powder. Opuntia ficus-indica mucilage was obtained as described previously:21,22 pods of Opuntia ficus-indica were cut and the external layer was removed manually. The internal fraction was milled in a domestic blender (Braun). A 200 g portion of the resultant juice was put into a beaker, and it was filled up to 1 L with distilled water. Then it was kept at 60 °C for 24 h. After this period, the mixture was filtered and concentrated by vacuum evaporation to one-third of the initial volume. Then, it was precipitated with ethanol twice, to achieve a clean, impuritiesfree mucilage. The resultant mix of ethanol and mucilage was dried in a heater at 60 °C for 12 h. The final product presents a green, crystal aspect. Another modified tannin was supplied by Silvateam, S.A., Italy. Its name is SilvaFLOC, and it consists of tannins from Schinopsis balansae (Quebracho colorado) that have been chemically modified in order to introduce a quaternary nitrogen that confers SilvaFLOC its cationic character. Aquachimica Seta, S.A., Brazil, provides two more Acacia mearnsii tannin-based flocculants: AQUAPOL C1 and AQUAPOL S5T. The differences between SilvaFLOC, AQUAPOL C1 and S5T and SilvaFLOC are in the tannin nature (Acacia mearnsii for AQUAPOL and Quebracho for SilvaFLOC) and in the chemical modification, which is under copyright law. TanFLOC and

AQUAPOL C1 are presented as powder, while SilvaFLOC and AQUAPOL S5T are presented as a dense solution. Guar and Karaya gum were supplied by SIGMA. They are presented as powder. AluminumsulfateAl2(SO4)3 · 18H2OwassuppliedbyPANREAC. 2.4. Preliminary Screening of the Efficiency of Tanfloc in Removing Other Anionic Surfactants. Eight anionic surfactants were tested with Tanfloc in order to characterize its behavior as flocculant agent: (i) sodium dodecyl benzene sulfonate (SDBS) C18H29SO3Na; (ii) sodium dodecyl diphenyl ether disulfonate (SDDED) C35H56S2O7Na2; (iii) sodium lauryl sulfate (SLS) C12H25SO4Na; (iv) sodium triethanolamine lauryl sulfate (TEA-LS) C18H40NSO4Na; (v) POE (3.5) sodium lauryl ether sulfate (SLES) C12H25(OCH2CH2)XOSO3Na, where average value of X is 3.5; (vi) sodium dioctyl sulfosuccinate (SDSS) C20H37SO7Na; (vii) sodium lauryl sulfoacetate (SLSA) C14H27SO5Na; (viii) POE sodium nonylphenol sulfate (SNS) C17H28SO5Na. All reagents were suplied by Chem Service Inc., U.S., and they are presented in analytical grade. 2.5. General Surfactant Removal Assay. A 500 mg · L-1 portion of a surfactant stock solution was prepared. Different volumes of this stock solution were put into recipients, and a controlled quantity of coagulant was added. Final volume was reached with pH 7 buffered solution. A slow blade-stirring agitation (30 rpm) was applied for 2 h, until equilibrium was achieved. Kinetic studies (data not shown) and previous studies carried out23 reported this period was enough to guarantee equilibrium. Then, a sample was collected and it was centrifuged. Surfactant removal was determined by visible spectrophotometry. Surfactant Analysis. To analyze surfactant concentration, a method based on methylene blue-anionic surfactant association was used.24 A 5 mL portion of clarified sample was put into a separation funnel. Trichloromethane (PANREAC) (25 mL) and methylene blue solution (25 mL) were added, and the funnel was shaken vigorously. The organic fraction was taken out and put into another separation funnel, in which 50 mL of cleaning solution were added. The funnel was shaken again, and the resultant organic fraction was put into a 25 mL-flask. It was filled up to the mark with trichloromethane, and methylene blue concentration was determined by visible spectrophotometry at 652 nm, with zero made with pure trichloromethane by using a HEλIOS spectrophotometer. The cleaning solution was prepared by putting 43.5 g of NaH2PO4 (Aldrich) into 500 mL of distilled water; 6.6 mL of H2SO4 (PANREAC) 98% w/w was added, and the mixture was diluted up to 1 L. The methylene blue solution was made by adding 30 mg of methylene blue (Aldrich) to 1 L of cleaning solution. 2.7. Surface Tension Determination. The entire investigation was carried out by using a buffered solution prepared as above (section 2.1). Critical micelle concentration (CMC) has been reported as a variable parameter depending on the nature of the bulk solution. High differences have been found among SDBS solutions with little variations in salt content.25 Due to this fact, CMC was determined for SDBS in our particular buffered solution. The technique we have used puts surface tension (γ) and bulk concentration of surfactant in relationship.26 γ was determined by using a photographic method described previously.27 3. Results and Discussion 3.1. Preliminary Screening on Surfactant Removal. Several assays of SDBS removal were done with different natural


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Figure 1. Chemical structures of different surfactants:1 sodium dodecyl benzene sulfonate (SDBS),2 sodium dodecyl diphenyl ether disulfonate (SDDED),3 POE (3.5) sodium lauryl ether sulfate (SLSA),4 sodium lauryl sulfoacetate (SLSA),5 sodium dioctyl sulfosuccinate (SDSS),6 sodium triethanolamine lauryl sulfate (TEA-LS),7 POE sodium nonylphenol sulfate (SNS) and8 sodium lauryl sulfate (SLS).

Figure 2. Screening for surfactant removal ability with several natural products. Initial surfactant concentration ) 50 mg · L-1; flocculant dosage ) 100 mg · L-1; pH ) 7; T ) 20 °C.

agents, as well as with alum. Most of them were based on polysaccharides (starch, guar and karaya gums, and Opuntia ficus mucilage) or proteins (vegetal extracts as Moringa oleifera), and others were tannin-based flocculant agents (Tanfloc, SilvaFLOC, and AQUAPOL). No data were available about surfactant chemical removal as coagulation with tannin-based coagulant agents. A preliminary screening was needed in order to search for an efficient and operative surfactant removal mechanism which would be comparable to alum coagulation efficiency28 or other mechanisms.29 General flocculant dosage of 100 mg · L-1 and surfactant initial concentration of 50 mg · L-1 were applied. The pH of this preliminary screening was adjusted to 7 with buffered solution, and the temperature was set to 20 °C. The SDBS long molecule (Figure 1) presents a benzene ring and a large linear chain on one side and a sulfonate negativelycharged group on the other side. This charged group and the large organic chain make SDBS a rather-expected molecule to be removed by a cationic coagulant agent, such as Tanfloc. Figure 2 shows removal percentages that have been carried out by using different agents. Standard dosage of 100 mg · L-1 of coagulant agent and 50 mg · L-1 of surfactant were fixed.

Experiments were carried out at pH 7 and 20 °C. As it can be seen, every product presents a particular removal activity. Moringa oleifera seed extract presents a very high SDBS removal, being almost 90% for the given conditions. This fact has been reported previously,30 but other undesirable characteristics of the Moringa oleifera treatment, such as the buildup of organic matter, motivate the investigation of other possible agents for surfactant removal.31 Referring to tannin-based flocculant, it is observed that AQUAPOL C-1 and SilvaFLOC work very well, they remove around 75% of surfactant concentration. Both tannin-based flocculants have a natural origin. Polysaccharide gums such as Karaya, Guar, or Opuntia ficus mucilage present a sligh SDBS removal ability, but not high enough. Further studies of varying operating variables may improve this action. Cationic starch is almost as effective as alum. Aluminum sulfate was used in order to compare results from natural coagulant agents. As it can be seen, alum presents a less high SDBS removal activity (around 25%), so in a first approach it is possible to say that it has lower SDBS removal activity than Tanfloc. It is also undesirable in the fact that aluminum intake is under health risk suspect32 derived from its usage as primary coagulant and its environmental bioaccumulation. 3.2. Effectiveness of Tanfloc on Several Anionic Surfactants Removal. To test Tanfloc ability to remove other anionic surfactants, several assays were done with seven other surfactants. General doses of 50 mg · L-1 of both surfactant and flocculant were applied. The pH of this screening was adjusted to 7 with buffered solution and the temperature was set to 20 °C. The results of these experiments are shown in Figure 3. As it can be appreciated, the phenol ring inside sulfonated compounds seems to enhance the removal of such products, so Tanfloc is rather more effective with SDBS and SDDED than with SLS and TEA-LS or sulfosuccinate (SDSS). Although TEA-LS presents a rather long organic chain, it also has a quite amphoteric condition, so cationic trapping is more difficult. An intermediate situation appears with SNS (nonylphenol sulfated) and with SLSA (sulfoacetate). It may be due to the similar length of the carbon chain in both cases. Regarding

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Figure 3. Screening for Tanfloc removal ability of different anionic surfactants. Initial surfactant concentration ) 50 mg · L-1; flocculant dosage ) 50 mg · L-1; pH ) 7; T ) 20 °C.

Figure 4. SDBS removal with different Tanfloc dosages. Initial surfactant concentration ) 50 mg · L-1; pH ) 7; T ) 20 °C.

SLES, which has no sulfonic groups, a high removal percentage is presented due surely to the extremely long chain including an additional polyethoxylation (POE). Structures are shown in Figure 1. 3.3. Influence of Variables. 3.3.1. Tanfloc Dosage. An experimental series of tests was performed to determine flocculant dosage influence on surfactant removal. A fixed dose of 50 mg · L-1 of surfactant was evaluated to be removed with different doses of Tanfloc. As it can be appreciated in Figure 4, final surfactant concentration tends to decrease as Tanfloc dose increases. However, it is observed that process efficiency arrives to a maximum, and higher doses of extract does not achieve lower surfactant concentrations. There is a residual surfactant concentration that is not possible to remove through this flocculation process and seems to be about 10 mg · L-1. This can be due to the existence of an “equilibrium surfactant concentration” which is highly difficult to remove, as reported previously.33 The adsorption capacity q tends to be lower as flocculant dose becomes higher. This is a normal and previsible effect according to the definition of q (eq 1). 3.3.2. pH. pH values were varied between 4 and 10, in order to determine its influence on surfactant removal. The experimental data series is shown in Figure 5. As it can be appreciated a fixed dose of 50 mg · L-1 of Tanfloc with a surfactant dose of 50 mg · L-1 tends to be less effective as pH becomes higher. Surfactant anionic character should not be dramatically reduced by lowering the pH, while the cationic form of Tanfloc would

Figure 5. Influence of pH in SDBS removal. Initial surfactant concentration ) 50 mg · L-1; flocculant dosage ) 50 mg · L-1; T ) 20 °C.

be higher at acidic pH. Electrostatic attraction between Tanfloc cationic chains and negatively-charged surfactant active centers is reinforced. In addition, links to hydrophobic chains would be enhanced.33 So both effects should explain this behavior by modifying pH. The modification of pH also affects the removing capacity (see section 3.5 below) which is represented by the q parameter. It tends to be lower with the increasing of pH as well. 3.3.3. Temperature. To appreciate temperature influence on the surfactant removal process, a series of experiments was carried out over a range of temperatures. Experiments at 10, 20, 30, and 40 °C were performed. A loss of efficiency in surfactant removal is reported by varying temperature from 20 to 40 °C (data in Supporting Information). It may be because temperature has an effect on micellization, so the behavior of the surfactant is modified and micelles tend to be more difficult to remove. Micellization and demicellization phenomena constitute two effects that lead to an equilibrium between the fraction of surfactant that is under micelle form and the fraction of free surfactant. Higher temperatures may affect this equilibrium.34 3.3.4. Surfactant Dosage. Initial surfactant dosage was varied between 10 and 200 mg · L-1. CMC is inside the range we are working in, which has been determined by several authors to be between 73235 and 1400 mg · L-136 depending on ionic strength. But in our case, the CMC is determined to be around 60 mg · L-1. A fixed dosage of 50 mg · L-1 of Tanfloc was applied in order to evaluate the effect of increasing initial surfactant dosage. Results can be seen in Figure 6 which shows the removal efficiency (%) versus initial surfactant concentration (mmol · L-1). The percentage tends to increase as surfactant initial concentration increases up to 0.15 mmol · L-1, very close to the calculated CMC, then it remains constant around 40% of removal. This fact means that the removing capacity of this flocculant tends to increase as initial surfactant concentration increases. Removing surfactant from the bulk solution seems to be a combined process of flocculation and adsorption onto flocs, so q capacity is increased as flocculant mass/surfactant concentration ratio decreased up to a limit, once CMC is achieved, which is determined in the following sections (see sections 3.5.2 and 3.5.3). 3.4. Removal of SDBS in Wastewater Matrix. One of the main uses Tanfloc may present involves its incorporation in a wastewater treatment process. In this sense, a loss of efficiency is expected to occur if the coagulation and flocculation mech-


Figure 6. Influence of initial surfactant concentration in SDBS removal. Flocculant dosage ) 50 mg · L-1; pH ) 7; T ) 20 °C.

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Figure 8. Critical micelle concentration for SDBS in the buffered solution: pH ) 7; T ) 20 °C.

By combining data series from sections 3.3.1 and 3.3.4 it is possible to look for a theoretical model that should explain Tanfloc-surfactant interaction phenomena. First, adsorption capacity (q) has been determined, defined as q)

Figure 7. Effect of Tanfloc on water samples. Flocculant dosage ) 50 mg · L-1; initial surfactant concentration ) 50 mg · L-1; T ) 20 °C.

anisms take place in a rather complex matrix, such as wastewater, according to its variable origin. To prove Tanfloc is operative with this kind of polluted water, assays with two different wastewater samples were carried out. The initial surfactant concentrations were about 50 mg · L-1, as was the dose of Tanfloc. Removal percentages of surfactant are showed in Figure 7. As it can be seen, the removal of SDBS with the same dose of Tanfloc tends to be lower as the matrix in which surfactant removal is taking place tends to be more and more complex. A removal efficiency of near 60% in the case of distilled water changes into 40% in the case of river water and finally arrives to 25% in the case of municipal wastewater. Surely the coagulant and flocculant ability of Tanfloc is shared with turbidity removal, which is significant in both cases (data not shown). 3.5. Theoretical Adsorption Modeling. The interaction between surfactants and natural polymers (polysaccharides, proteins, etc.) has been studied for many years because its importance in the success of product formulations in many areas (pharmaceuticals, cosmetics, food processing, etc.). Although the basic mechanisms of surfactant-polymer interaction are reasonably well understood, researchers still disagree about process at the molecular level, although it is generally accepted that interactions may occur between individual surfactant molecules and the polymer chain, or in the form of surfactant-polymer aggregate complexes (micellar or hemimicellar interactions).

(C0 - Cl)V W

(1)

where C0 is initial surfactant concentration (mmol · L-1), Cl is equilibrium surfactant concentration in bulk solution (mmol · L-1), V is the volume of solution (L), and W is Tanfloc mass (g). The basic forces controlling surfactant-polymer interactions are van der Waals and dispersion forces, hydrophobic effects, dipolar and acid-base interactions, and electrostatic interactions. The importance of each type will vary with the nature of the surfactant and the polymer. To evaluate the specific CMC of SDBS in our particular solution, surface tension has been measured in different samples of SDBS-distilled water mixes at pH equal to 7 at 20 °C. Two linear segments appear before and after CMC (see Figure 8). Literature is quite ambiguous in this sense, and several and different references have been found.34,35,37 As it can be appreciated in Figure 8, the behavior of q is radically different before and after the CMC point is crossed. Once it has happened, the original increasing path of q turns dramatically into a decreasing way. It is surely caused by the general appearance of micelles inside the bulk solution, which have a particularly different sorption way onto flocs. To model surfactant removal, we will attend just to the first stage, as the second one refers to a completely different mechanism.38 According to this, the following arguments refer just to the first stage of the process, that is, up to CMC. Figure 9 shows adsorption capacity values versus equilibrium surfactant concentration for those experiments carried out varying the Tanfloc dosage and initial surfactant concentration, at the same temperature (20 °C) and pH (7). As it is observed, an S-shaped curve is presented, with a slight increasing of q at low values of Cl. q values increase quickly along the intermediate range of Cl between 0.05 and 0.12 mmol · L-1. Then, they keep on increasing and presumably they arrive to an asymptotic value, which corresponds to q∞ as it is explained below. This kind of curve has been thoroughly studied by researchers.39 When a polymer is added to a surfactant solution, it is often observed that processes such as micellization appear to begin

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θl ) Clk12 exp(χ12θl) 1 - θl

(2)

where θl is the ratio between the adsorption and the maximum adsorption: θl )

q q∞

(3)

k12 is the adsorption constant, being a measure of the interaction between surfactant and polymer surface, and χ12 is the Flory-Huggins parameter,44 defined as χ12 )

Figure 9. Equilibrium data and models adjustments. Table 1. Fitting Models Parameters parameter

model

equilibrium surfactant concentration in bulk solution initial surfactant concentration adsorbate amount total volume adsorption capacity ((C0 - Cl)V)/W Freundlich adsorption order Freundlich adsorption constant limiting adsorption ratio q/q∞ Flory-Huggins interaction parameter adsorption constant limiting adsorbed surfactant Gu and Zhu adsorption constant Gu and Zhu adsorption order

symbol

units

reference

-1

F, GZ, FFG

Cl

mmol · L

F, GZ, FFG

C0

mmol · L-1

F, GZ, FFG F, GZ, FFG F, GZ, FFG

W V q

g L mmol · g-1

F

nf

none

F

kf

L /(g · mmol

FFG

θl

none

50

FFG

χ12

none

40

FFG FFG, GZ GZ

k12 q∞ kg

L · mmol-1 mmol · g-1 (mmol · L-1)-ng

40,50 40,47 47

GZ

ng

none

47

49

)

θl ) kg · C nl g 1 - θl

49

at surfactant concentrations below the CMC of the surfactant in the absence of the polymer. In many cases, a complex aggregate structure is formed in association with the polymer at a lower concentration of surfactant.40 This concentration is known as critical aggregation (or association) concentration (CAC) and varies with the nature of the polymer. The difference between both concentrations may vary by a factor of 10-1000 in some cases.1 A simple model that has been used to describe the adsorption of surfactants is the regular behavior model.41 For dilute solutions, this model simplifies to the Frumkin-FowlerGuggenheim (FFG) equation42,43

(4)

where NA is the Avogadro’s number, z is the number of the nearest neighbors to a central surfactant molecule, and ε11, ε12, and ε22 are the pairwise interaction potentials. In this model k12 and χ12 should be considered as adjustable parameters expressing the affinity for the surface and the lateral interactions in the adsorbed layer, respectively. Zhu and Gu45 proposed a very simple model for adsorption of surfactants assuming that the adsorbed layer is composed of surfactant aggregates. A surfactant aggregate is formed on the surface before stable aggregates are formed in solution. The model considers that these aggregates are stabilized by the presence of the surface. This model leads to the following:

49 nf -1

nf

NAz [(ε12 - 0.5)(ε11 + ε22)] RT

(5)

where ng is the number of monomers in the surfactant aggregate and kg is the Gu and Zhu constant for the studied model. Taking into account the definition of θl, eq 5 becomes q ) q∞kg

C nl g

(6)

1 + kgC nl g

This equation is reduced to the Langmuir equation for ng ) 1. In eq 6, if the term kgC nl g is much lower than 1, the derived expression is known as the Freundlich equation: q ) kf C nl f

(7)

where kf is the Freundlich adsorption constant and its value is given by kf ) q∞kg

(8)

Equations 2, 6, and 7 lead to three models that have been studied: Freundlich (F), Frumkin-Fowler-Guggenheim (FFG), and Gu and Zhu (GZ) models. Parameter values and statistics summary for the three models are shown in Table 2. Table 1

Table 2. Parameter Values and Statistical Summary model

expression

F

q ) kfClnf

FFG

θl/(1 - θl) ) Clk12 exp(χ12θl)

GZ

q ) q∞kg(Clng/(1 + kgClng))

parameters values

r2

kf ) 11.22 nf ) 0.80 q∞ ) 3.06 k12 ) 3.69 χ12 ) 2.88 q∞ ) 3.93 kg ) 46.92 ng ) 1.62

0.92

ln q ) nf ln Cl + ln kf

0.88

0.91

ln[(θl/(1 - θl))/Cl] ) ln k12 + χ12θl

0.91

0.98

ln (q/(q∞ - q)) ) ng ln Cl + ln kg

0.93

linearization

linear expression r2


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Figure 10. Predicted versus experimental q data.

shows different parameters that have been used in these modelizations. 3.5.1. Freundlich Model. As it is observed in Figure 9, the Freundlich model works well when the curve does not present a clear final zone of saturation. That is, this model just explains the first part of the adsorption phenomena. Due to this reason, it is possible to fit almost every obtained data and to find a rather well-fit curve by excluding the last points that correspond to adsorption saturation.46 To prove the validity of the Freundlich model, a plot of ln q versus ln Cl was carried out (data not shown). The adjusted r2 of this linearization was 0.88 (Table 2). Considering nonlinear data fit, the Freundlich equation fits rather well, conducting to a value of 11.22 (L0.8 · mmol0.2g-1) for kf and 0.80 for nf (r2 ) 0.92). 3.5.2. Frumkin, Fowler, and Guggenheim Model. The FFG model42 is used when adsorption from dilute solution is being studied. With this condition, surfactant concentration usually appears far from CMC.40 It is considered a simplification from a general model41 in which several parameters are included. The FFG equation is presented in eq 2. By carrying out a nonlinear fit, it is possible to determine values of χ12, k12, and q∞, this last parameter needed for the θl calculation. This nonlinear fit conducts to a χ12 value of 2.88, k12 value of 3.69 L · mmol-1 and q∞ value of 3.06 mmol · g-1. Taking the logarithms of both sides of eq 2 and then rearranging the terms yield ln

θl / (1 - θl) ) ln k12 + χ12θl Cl

(9)

Equation 9 is a linear expression, so it is possible to correlate data from q and Cl into a linear model. As it can be seen in Table 2, r2 determination coefficient is high enough again, so it is possible to conclude this model fits well to the present situation. It can be appreciated also in Figure 10, where predicted q is represented versus experimental q. As it can be seen, an r2 determination coefficient equal to 0.98 is achieved. 3.5.3. Gu and Zhu Model. Gu and Zhu47,48 proposed a twostep adsorption model for various types of S-shaped adsorption non-Langmuir isotherms. The first step implies adsorption of surfactant molecules as individual molecules or ions. The second step leads to an adsorption increase as surface aggregates form through interaction of the hydrophobic chains of the surfactant molecules with each other. The physical meaning of this theoretical model may be found in the fact that the adsorption process appears accompanied by some kind of flocculation process, as floc formation is observed in the experimental assay. This may be due to the hemimicellar formation hypothesis.1,40

Mathematically, the GZ model is expressed by eq 6. Figure 9 shows nonlinear experimental data fit and it is possible to observe a very good r2 determination coefficient in Table 2 (0.98). 4. Conclusions This investigation has revealed the following conclusions: • Among several natural products, Tanfloc, a commercial tannin-based flocculant has been found to be an anionic surfactant-removal agent in aqueous solutions. Around 70% of elimination was reached for SDBS in the most cases. For other surfactants this elimination could be different. Inside the operational values of pH, temperature, and flocculant dosage, it has been reported that • as pH increases, efficiency of the process decreases, due surely to the cationic character of the Tanfloc and to the fact that at acidic pH hydrophobic links are enhanced; • temperature negatively affects the surfactant removal process; • initial surfactant concentration has a positive effect by improvinig the q capacity of Tanfloc, while the percentual removal of SDBS remains almost stable. • Tanfloc is effective if used in wastewater effluents, such as polluted surface water or municipal wastewater, but it is less efficient. • Freundlich, Frumkin-Fowler-Guggenheim, and Gu-Zhu adsorption models fit rather well to experimental data, so surfactant removal phenomena can be explained through adsorption hypothesis. The experimental data fit better to the Gu and Zhu hypothesis, second to Freundlich’s, and last to Frumkin-Fowler-Guggenheim’s, according to r2 determination parameter value. It is possible to prove the goodness of these fits by showing graphics of predicted versus experimental data. Acknowledgment This investigation has been supported by the Programa de Iniciacioń a la Investigacioń, Universidad de Extremadura, oriented modality, Banco Santander subprogram, and by the Comisioń Interministerial de Ciencia y Tecnologıá (CICYT) CTQ 2007-60255/PPQ project as well as Junta de Extremadura under PRI-07A031 project. The authors also thank Prof. Montanero and his research team for his support and help in the determination of surface tension. Supporting Information Available: Temperature studies, surfactants characterization, wastewater characterization. This material is available free of charge via the Internet at http:// pubs.acs.org.

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ReceiVed for reView December 12, 2008 ReVised manuscript receiVed January 23, 2009 Accepted March 9, 2009 IE801913Y

Flocculant

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