Journal of Hazardous Materials 283 (2015) 815–823

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Impact of polyacrylamide with different contents of carboxyl groups on the chromium (III) oxide adsorption properties in aqueous solution Małgorzata Wi´sniewska ∗ , Stanisław Chibowski, Teresa Urban Department of Radiochemistry and Colloid Chemistry, Faculty of Chemistry, Maria Curie Sklodowska University, M. Curie Sklodowska Sq. 3, 20-031 Lublin, Poland

h i g h l i g h t s • • • • •

PAM adsorption on Cr2 O3 surface increases with rise of carboxyl groups content. Decrease of PAM adsorption with pH rise was obtained. PAM addition influences the surface properties of Cr2 O3 suspension. At pH 3 and 9 stability deterioration of Cr2 O3 particles covered with PAM occurs. Suspension stability is affected by specific conformation of adsorbed PAM chains.

a r t i c l e

i n f o

Article history: Received 23 August 2014 Received in revised form 21 October 2014 Accepted 25 October 2014 Available online 1 November 2014 Keywords: Anionic polyacrylamide Carboxyl groups content Chromium (III) oxide removal Suspension stability Polymer adsorption

a b s t r a c t The main goal of experiments was determination of solution pH and contents of anionic groups in polyacrylamide (PAM) macromolecules on the stability mechanism of chromium (III) oxide suspension. The spectrophotometry, potentiometric titration, microelectrophoresis, viscosimetry and turbidimetry were applied. They enabled determination of polymer adsorbed amount, surface and diffusion charges of solid particles with and without PAM, thickness of polymer adsorption layer, macromolecule dimensions in the solution and stability of the Cr2 O3 – polymer systems, respectively. It was found that adsorption of anionic PAM decreases and thickness of polymeric adsorption layer increases with the increasing pH. Slightly higher adsorption was obtained for the PAM samples containing a greater number of carboxyl groups. At pH 3 and 9 insignificant deterioration of stability conditions of Cr2 O3 particle covered with polyacrylamide was observed (neutralization of solid positive charge by the adsorbed polymeric chains (pH 3) and single polymeric bridges formation (pH 9)). The electrosteric repulsion between the solid particles covered with PAM layers at pH 6, is the main reason for significant improvement of Cr2 O3 suspension stability in the polymer presence. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Adsorption process of various substances from their solutions on the solid surface [1–5] is a very desirable phenomenon in many practical applications. Macromolecular compounds soluble in water (especially those of ionic character – polyelectrolytes) have proved to be very effective agents changing surface properties of various adsorbents significantly. Adsorption of polyelectrolytes on the metal oxide surface is a very complex process, which is dependent on many factors [6–10].

∗ Corresponding author. Tel.: +48 81 5375622; fax: +48 81 5332811. E-mail address: [email protected] (M. Wi´sniewska). http://dx.doi.org/10.1016/j.jhazmat.2014.10.037 0304-3894/© 2014 Elsevier B.V. All rights reserved.

Research in this area, besides its basic character, is also of great practical importance. In fact, polyelectrolytes are widely used in many technological processes as stabilizers or flocculants of colloidal suspensions [11–20]. The effects of stabilization and destabilization of the suspension by the adsorbed polyelectrolyte are exploited in many fields of human activity, such as industry, agriculture and environment. The phenomenon of flocculation in the polymer presence is applied in purification of industrial wastewaters present in the form of strongly dispersed aqueous suspension. In addition, flocculation is also used in the flotation process of mineral ores enrichment. Using a properly selected macromolecular substance, it is possible to modify solid surface properties (greater separation efficiency). In turn, the polymeric stabilization is used in pharmaceutical, food, cosmetic, paint industries and many others.

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In the present study particular emphasis was placed on the effects of the type and number of functional groups of anionic polyacrylamide (PAM), its molecular weight and solution pH on the conformation of polymer macromolecules in the adsorption layer formed on the dispersed chromium (III) oxide surface. Determination of PAM adsorption mechanism enabled clarification and understanding of Cr2 O3 suspension stability in the polymer presence. PAM is a primary ingredient in many flocculants used in various fields of human activity. It is applied in the oil regeneration process and for reduction of plough land erosion to improve its stability. PAM is also largely used for minimization of inorganic salt concentrations in drinking water [21–25]. The presence of chromium (III) oxide in water is particularly undesirable because of its properties. This oxide is of low-toxicity. However, it has intense green color, which gives water an unsightly appearance and disrupts life processes. It was found that even small amounts of colored water weaken light penetration to the deeper layers, so that the process of photosynthesis is inhibited. Thus, polyacrylamide can be used as an effective flocculent of Cr2 O3 particles contributing to solid removal. On the other hand, chromium (III) oxide is a very popular mineral pigment [26] applied in many branches of industry. In such a case, polyelectrolyte is used as an efficient stabilizer of pigment particles dispersed in water solution, preventing phase separation in the system. Thus, depending upon specific requirements of practical application polyacrylamide can act as a flocculent or stabilizer. Previous reports have indicated that chromium (III) oxide removal can be successfully carried out using other synthetic and natural polymers such as polyacrylic acid [27], polyamino acids (polyaspartic acid, polylysine) [28,29], albumins [30] and bacterial polysaccharide [31]. Extensive researches on the removal of Cr2 O3 by the use of various compounds have been realized among them active carbons [32,33], combined adsorbents of activated carbon and mineral oxides [34] and biosorbents [35,36]. 2. Experimental 2.1. BET surface area of adsorbent The chromium (III) oxide – Cr2 O3 samples (POCh) were used in the experiments. The BET specific surface area of Cr2 O3 was equal to 7.12 m2 /g (low-temperature nitrogen adsorption–desorption isotherm method; Micrometritics ASAP 2405 analyzer). The average pore diameter of alumina was 6.1 nm. The mean grain size of the solid particles was 265 nm (photon correlation spectroscopy; Zetasizer 3000, Malvern Instruments). 2.2. Contents of carboxyl groups in polymer chains The samples of anionic polyacrylamide – PAM (Korona) differing in weight average molecular weight and contents of carboxyl groups were applied as adsorbates (Table 1). Their macromolecules contain two types of functional groups: amide and carboxyl ones (Fig. 1). Using the potentiometric titration method [37] pKa values of applied polymer solutions were determined. Additionally, the degrees of anionic groups ionization (˛) were calculated (Table 2).

PAM 11 000 000 PAM 14 000 000 PAM 14 000 000 PAM 15 500 000

5% 20% 30% 50%

CH

CH2

C

O

NH2

CH C

n

O

O- Na+

m

Fig. 1. Anionic PAM chemical formula.

2.3. Adsorption All measurements were carried out in the solution pH range 3–10 at 25 ◦ C. NaCl (1 × 10−2 mole/dm3 ) was used as the supporting electrolyte. Adsorption measurements were made by the static method in the polymer concentration range 5–150 ppm at pH values: 3, 6 and 9 (±0.1) using 0.2 g of Cr2 O3 . The reaction of polyacrylamide with hyamine proposed by Crummet and Hummel [38] was applied. The solution turbidity was measured after 15 min using the UV–vis spectrophotometer (Carry 1000; Varian) at 500 nm. 2.4. Surface charge density The potentiometric titrations of Cr2 O3 with and without PAM (with the concentrations 1, 10 and 100 ppm) were performed in the thermostated Teflon vessel. The solid surface charge density was calculated with the special program Titr v3 (author W. Janusz). 1.5 g of the alumina was added into the vessel to 50 cm3 of polymer solution in the supporting electrolyte (or only to the supporting electrolyte solution). 2.5. Zeta potential Cr2 O3 samples (without and with PAM) for zeta potential measurements were prepared adding 0.015 g of Cr2 O3 to 500 cm3 of the supporting electrolyte (CNaCl = 1 × 10−2 mole/dm3 ) or PAM (1 ppm) solutions. The electrokinetic potential was measured using the Zetasizer Nano ZS with the universal dip cell and MPT-2 titrator (Malvern Instruments). 2.6. Thickness of the polymer adsorption layer Thickness of the polymer adsorption layers (ı) was determined from the viscosity measurements [39] using a CVO 50 rheometer (Bohlin Instruments). For this purpose the following dependency was applied:

 ı=r

p o



1/3

−1

(1)

where: r – solid particle radius, o – volume fraction of the solid without polymer, p – volume fraction of the solid with polymer. The viscosity measurements in the presence of PAM were made with the volume fraction of Cr2 O3 equal to 0.0072. Table 2 PAM anionic groups ionization.

Table 1 Polyacrylamide characteristic. Symbol

CH2

Polymer

Molecular weight [Da]

Anionic groups content [%]

11 000 000 14 000 000 14 000 000 15 500 000

5 20 30 50

PAM 11 000 000 PAM 14 000 000 PAM 14 000 000 PAM 15 500 000

pKa

5% 20% 30% 50%

3.7 3.7 3.7 3.6

˛ [%] pH 3

pH 6

pH 9

16.6 16.6 16.6 20.1

99.5 99.5 99.5 99.6

99.9 99.9 99.9 99.9

M. Wi´sniewska et al. / Journal of Hazardous Materials 283 (2015) 815–823

Fig. 2. Reduced viscosities of PAM 15 500 000 50% solutions versus polymer concentration.

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Fig. 3. Adsorption isotherms of PAM 11 000 000 5% on the Cr2 O3 surface.

Initrinsic viscosity [] was obtained from the plot r = f(c) by extrapolating the straight line to the concentration of polymer solution (c) equal to zero (example in Fig. 2): [] = lim r

(2)

c→0

These measurements were made using a CVO 50 rheometer (Bohlin Instruments) and within polymer concentration range 50–200 ppm. 2.7. Stability of Cr2 O3 -PAM system The stability measurements of Cr2 O3 -PAM systems were carried out using Turbiscan LabExpert with the cooling module TLAb Cooler. The suspension with 0.4 g of oxide in 20 cm3 of NaCl solution (or PAM solution with the concentration 100 ppm) was prepared. Changes in the suspension stability were monitored for 15 h (single scans were collected every 15 min). The results are presented in the form of intensities of transmission and backscattering as a function of time. The transmission and backscattering data enable calculation of the following stability parameters: the stability coefficient TSI (Turbiscan Stability Index), the thickness of formed sediment, the size of aggregates (flocks) and the rate of aggregates (flocks) migration. These data were calculated using the programs TLab EXPERT 1.13 and Turbiscan Easy Soft from the dependencies:



TSI =

n (x i=1 i

− xBS )2

(3)

n−1

where: xi – average backscattering for each minute of measurement, xBS – average x1 , n – number of scans (repetitions of single measurement during the total time of the experiment), and: V (, d) =

  p − c  gd2 18c

·

1− 3

[1 + 4.6/(1 − ) ]

(4)

where: v – particles migration velocity, c – continuous phase density, p – particle density, d – particle mean diameter, v – continuous phase dynamic viscosity,  – volume fraction of dispersed solid. 3. Results and discussion 3.1. Conformation of PAM macromolecules in the solution and adsorption layer The analysis of the exemplary adsorption isotherms obtained for PAM 11 000 000 5% (Fig. 3) shows that the polyacrylamide adsorption on the Cr2 O3 surface decreases with the rise of solution pH.

Fig. 4. Adsorbed amounts of PAM on Cr2 O3 surface at pH 3 and initial PAM concentration 100 ppm.

The analogous tendencies were observed for all examined polymeric samples. For a given pH value, slightly greater adsorption is obtained, both for higher molecular weight of the polymer, and for a greater number of carboxyl groups in the PAM macromolecules (Fig. 4). The pKa values for the polyacrylamide samples are within 3.6–3.7. The corresponding degrees of carboxyl groups dissociation in the polymer chains varies in the range of 16.6–20.1% at pH 3 up to 99.5–99.9% at pH 6 and 9 (Table 2). Thus, at pH 6 and 9, all anionic groups of the polymer are present in the ionized form and they are a source of negative charge of adsorbing macromolecules. It should be mentioned that the second type of PAM functional groups, namely the amide ones, does not dissociate, so they do not make any contribution to the total charge of polymeric substance. On the other hand, the sign and density of Cr2 O3 surface charge clearly depends on the alkalinity of the solution. The pHpzc point of oxide achieved the value of approximately 5.8. Thus, in the pH range 3–5.8 there are attractive electrostatic interactions in the adsorbent–polymer systems. In turn, in the pH range 5.8–9 the repulsion between the negatively charged PAM macromolecules and the negative active sites in the chromium (III) oxide surface occurs. The degree of adsorbing macromolecules development can be characterized based on the analysis of the values of two parameters: 1/2 r¯2 – the root mean square chain end-to-end distance and R h

– the hydrodynamic radius of the polymer coil in the bulk solution (Table 3). They were calculated based on the following equations:

1/2 r¯2

=

 []M 1/3 F

(5)

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Table 3 Characteristic of linear dimensions of PAM 15 500 000 50% chains in the solution. pH

[] [100 cm3 /g]

3 6 9

1.0 1.95 2.7

1/2 r¯2

[nm]

78.09 97.56 108.74

Rh [nm] 21.04 26.28 29.29

1/2 Rh = f ·

r¯2

(6)

61/2

where: F – universal Flory–Fox constant (F) equal to 2.1 × 1021 [40], independent of the type of polymer and solvent, as well the polymer molecular weight; M – molecular weight of the polymer, [] – initrinsic viscosity of the polymer solution, f – constant for a given polymer-solvent system (for PAM f = 0.66 [41]). Analysis of the macromolecules size in the solution clearly indicates that the degree of their development increases with the increasing solution pH. This is rather obvious because the pH increase causes the growing dissociation of carboxyl groups in the PAM chains, which makes that their repulsion becomes stronger. It refers to repulsive interaction occurring between both the COO− groups located along one macromolecule, and those belonging to different chains. As a result, polyacrylamide conformation in solution becomes more and more stretched, which leads to the increase 1/2 of distance between the ends of polymer chains (increase of (r¯2 ) ) and the rise of the hydrodynamic radius Rh of macromolecules.0 Taking into account different contents of carboxyl groups in the PAM molecules [42], it can be noted that a greater number of these functional groups is, the larger size of the macromolecules in solution is. This is related to the increase of electrostatic repulsion forces between the rising number of negative carboxyl groups in the polymer chains. The conformation of macromolecules adsorbed on the solid surface is somewhat flattened due to the interaction of polyacrylamide with active sites on the chromium (III) oxide surface. This is confirmed by the thicknesses of PAM adsorption layers (ı). The values of ı are given in Table 4. Comparing the hydrodynamic radius of polymeric coils in the PAM 15 500 000 50% solution with respective ı value, it can be concluded that the conformation of adsorbed macromolecules, resulting from the PAM segments interactions with the surface, becomes less stretched. It is reflected in lower ı value in relation to Rh . 3.2. Mechanism of PAM adsorption on the chromium (III) oxide surface Considering all the above information, it can be stated that the highest adsorption of anionic polyacrylamide on the surface of dispersed Cr2 O3 at pH 3 is primarily due to attractive favorable interactions between the positively charged solid surface and PAM chains, whose carboxyl groups are only partially dissociated. The fact that the polymeric macromolecules possess predominantly nonionized COOH groups in relation to those charged ( COO− ) makes their conformation in the solution more coiled (the smallest Table 4 Thickness of PAM adsorption layers (ı) PAM on the Cr2 O3 surface. Polymer

PAM 11 000 000 PAM 14 000 000 PAM 14 000 000 PAM 15 500 000

ı [nm]

5% 20% 30% 50%

pH 3

pH 6

pH 9

5.78 10.49 12.06 15.31

9.34 13.33 16.21 20.37

15.11 18.44 21.01 24.62

linear dimensions of PAM molecules at pH 3). Due to electrostatic attraction with the surface these coils are flattened, but the film formed by them has the highest adsorption packing (compared to the examined higher pH values). In such a case, more PAM macromolecules can absorb on the unit of surface area. This is manifested by not only greater adsorption of macromolecular compound, but also formation of the thinnest adsorption layers consisting of tangled polymer chains. The greater the number of carboxyl groups in the PAM macromolecules is, the thicker adsorption layers are formed (simultaneously with the minimal increase of the adsorbed amounts). The increase of the polymer film thickness is rather clear. This is a result of somewhat looser structure of the adsorbed coils due to stronger repulsion of COO− groups within the macromolecules. However, the slight increase of polymer adsorption shows that PAM carboxyl groups must demonstrate a greater affinity for the surface active sites of Cr2 O3 than amide ones. Apart from electrostatic interactions, hydrogen bonds are present in the studied systems. There are many possibilities for creation of this type of connection – both between neutral and charged groups, in which all types of surface chromium oxide groups, namely CrO− , CrOH2 + and CrOH, can participate. Particularly the neutral hydroxyl groups of the solid play an essential role, mainly due to their several times greater number (compared to the charged ones) [43]. Two kinds of the PAM functional groups – both amide and carboxyl (undissociated and dissociated) are capable of hydrogen bonds formation. The decrease in the polyacrylamide adsorption on the Cr2 O3 surface at pH 6 is due to the fact that, despite the total ionization of carboxyl groups in the PAM chains, there are not favorable electrostatic conditions (near zero surface charge of the adsorbent (pHpzc 5.8)). Furthermore, the significantly extended conformation of adsorbing macromolecules, which are fully ionized, makes that they undergo direct binding to the surface sites through a limited number of their segments. Their vast number is located in the loop and tail structures of the adsorbed chains whose presence affects the observed increase of the formed polymer film thickness. At pH 9, despite very unfavorable electrostatic repulsion between the negative surface and the completely ionized PAM macromolecules, the occurrence of polyacrylamide adsorption is observed. This is the best evidence that it must proceed through the formation of hydrogen bonds. Otherwise, the adsorbed amount of the polymer should be zero. The greatest development of polymer chains in the solution results in the formation of the thickest adsorption layer. It is composed primarily of loops and tails characterized by a considerable length. Additionally, the strong adsorbent-polymer repulsion makes that these structures are mostly located perpendicular to the solid surface. Thus, the formed polymer film has a loose packing, so that it does not cover tightly the particles surface. 3.3. Solid surface charge density in the PAM presence Important information about the mechanism of PAM macromolecules binding to the surface of Cr2 O3 can be obtained by analyzing the changes in the solid surface charge density ( 0 ) in the polymer presence. The greatest effects in  0 changes were obtained for the polymer concentrations 100 ppm (Fig. 5). In such a case, polyacrylamide adsorption caused a significant increase in the surface charge density of Cr2 O3 in the whole range of studied pH. Moreover, the greatest shift of pHpzc of oxide in the presence of polymer with the concentration 100 ppm, relative to the system in its absence, was obtained (Fig. 6). For these reasons, the curves depicting the oxide surface charge density as a function of solution pH without and with polyacrylamide of the concentration 100 ppm (having a different content

M. Wi´sniewska et al. / Journal of Hazardous Materials 283 (2015) 815–823

Fig. 5. Surface charge density of Cr2 O3 in the absence and presence of PAM 15 500 000 50%.

819

Fig. 8. Zeta potential of Cr2 O3 particles without and with PAM.

of  0 value. The total contribution of these two effects gives the experimentally observed surface charge of Cr2 O3 . It seems that the most important factor in the studied systems is the positive surface groups formation as a consequence of anionic polymer adsorption. Considering the effect of carboxyl groups content in the PAM macromolecules, it should be concluded that surface charge change is more complicated. In the pH range 4–8, there is observed the following tendency: the smaller the number of COOH groups in PAM chains is, the higher growth of  0 is. In the pH range 8–11, this relation is reversed. 3.4. Zeta potential of Cr2 O3 particles in the PAM presence

Fig. 6. pHpzc values of Cr2 O3 without and with PAM.

of carboxyl groups) are placed in Fig. 7. Their analysis indicated that the anionic polymer causes increase of surface charge density of Cr2 O3 in the whole range of the examined pH. This is probably related to the presence of negative functional groups in the PAM macromolecules, whose adsorption enforces creation of additional positively charged active sites on the Cr2 O3 surface [44]. On the other hand, the negative carboxyl groups present along the polymer segments directly located in the solution layer adjacent to the surface, containing loop and tail structures, cause the reduction

The proposed adsorption mechanism of polyacrylamide on the mineral oxide surface is confirmed by the results of the zeta potential measurements (Fig. 8). The zeta potential of the Cr2 O3 particles with adsorbed PAM assumes lower values in comparison to the system without the polymer. There is also obtained the shift of solid pHiep to more acidic values, i.e. from about 6 for the adsorbent particles without PAM to pH in the range of 2.8–4.8 for the solid particles coated with the polymeric film. The greater reduction of the zeta potential is, the higher the molecular weight of PAM and the higher content of anionic groups in their chains is. The predominant effect, which is responsible for the electrokinetic behavior of Cr2 O3 particles coated with the polymer layer is the presence of negatively charged functional groups in the adsorbed PAM chains, which are located on the border of the compact and diffuse parts of the electrical double layer (edl), i.e. in the slipping plane. On the other hand, some contribution to the observed reduction in the  potential comes from the shift of the slipping plane from the surface of the colloidal particles due to the macromolecular compound adsorption. Both the effect of slipping plane position changing, and the concentration of COO− in a specific part of edl are dependent on the molecular weight and contents of the anionic groups in the polymer chains. Thus, it is evident that the increase in these parameters is manifested by the greater reduction of the electrokinetic potential of solid particles in the polymer presence. 3.5. Stability of Cr2 O3 suspension in the absence and presence of PAM

Fig. 7. Surface charge density of Cr2 O3 without and with PAM.

A very useful parameter, which allows estimation of the suspension stability is the numerically calculated factor TSI (Turbiscan Stability Index). Its value may vary within the range of 0–100, and the value close to zero is obtained for extremely stable systems. The corresponding data in Table 5 shows that in the PAM absence, the Cr2 O3 suspension has the highest stability at pH 3 (the

820

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Fig. 9. Transmission and backscattering for Cr2 O3 -NaCl at pH 3.

Table 5 TSI values of Cr2 O3 suspensions without and with PAM. System

Cr2 O3 Cr2 O3 – PAM 11 000 000 5% Cr2 O3 – PAM 14 000 000 30% Cr2 O3 – PAM 15 500 000 50%

TSI pH 3

pH 6

pH 9

12.8 14.2 19.4 28.2

61.0 35.3 33.2 33.5

29.3 40.8 41.9 45.5

lowest TSI value of all tested systems). On the other hand, at pH 6 it undergoes significant destabilization. At pH 9 an intermediate state between two extreme pH values is obtained. The addition of polyacrylamide causes noticeable deterioration in the stability of solid particles at pH 3 and 9. In turn, at pH 6 the improvement in stability

of the examined suspensions in the presence of macromolecular compound is observed. The changes in the TSI values correspond to the curves presenting the amount (in %) of light passing through the studied sample (transmission) and backscattered, recorded at various sample levels. The exemplary graphs showing the transmission and backscattering of the selected systems are placed in Figs. 9–12. The values on the x-axis correspond to different heights of the suspension placed in the measurement vial: 0 mm means its bottom, whereas the range of 40–45 mm indicates the level to which the sample was poured in. The individual curves indicated by different colors from pink (t = 0) through different shades of blue and green, and ending in red (t = 15 h) were recorded every 15 min for a total time of measurement established at 15 h. Comparing the curves in Figs. 9 and 10, which relate to the suspension of chromium (III) oxide not containing PAM at pH 3 and 6,

Fig. 10. Transmission and backscattering for Cr2 O3 -NaCl at pH 6.

M. Wi´sniewska et al. / Journal of Hazardous Materials 283 (2015) 815–823

821

Fig. 11. Transmission and backscattering for Cr2 O3 -PAM 15 500 000 50% at pH 3.

clear differences can be seen in their course, which are related to the stability of these different systems. For example, for the relatively stable system at pH 3, the zero transmission level remaining at considerable height of the examined suspension is obtained, with the exception of the upper layer, in which the clarification process starts. This is manifested by the presence of the peak, which reaches the maximum value exceeding 70%. The transmission peak width is equal to the thickness of the clear layer. In the case of relatively unstable system at pH 9, the transmission is maintained at a quite high level of 50–70% over the entire length of the sample, with the exception of the bottom layer, on which sediment is formed (corresponding transmission is 0%). The backscattering curves also differ for these two analyzed systems. The stable suspension at pH 3 is characterized by relatively high backscattering (30%), which in the upper part of the sample decreases during the experiment to about 15% (clarification process). In turn, the presence of a small sediment layer is visible as

the backscattering peak, the width of which indicates its thickness. In the case of significant destabilization of the suspension at pH 6, the backscattering level is low (10–15%) and varies slightly during the measurement time. The formed sediment has also significantly greater thickness than in the former system. Figs. 11 and 12 relate to the systems containing polyacrylamide. Both systems are characterized by the TSI coefficients equal to approximately 30. In their case, there is observed significant thickness of the sediment, which consists of flocks formed with PAM macromolecules and solid particles. Flocks are the loose aggregates of solid particles, that are connected together via long polymer chains. So thick sediments may be related to a not very packed structure of formed flocks in the presence of polymer. As can be seen in Fig. 13, the size of aggregates formed by the Cr2 O3 particles in the PAM absence is the highest at pH 6. In addition, they fall down the fastest (Fig. 14) of all studied systems. This is associated with the smallest suspension stability at pH 6,

Fig. 12. Transmission and backscattering for Cr2 O3 -PAM 11 000 000 5% at pH 6.

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Fig. 13. Aggregate sizes for Cr2 O3 suspensions without and with PAM.

which achieved the highest value of TSI. Due to the anionic polymer addition, there is a significant increase in the size of flocks, whose sedimentation is much faster compared to the dispersions without the polymer (except pH 6). Despite the fact that at pH 6 the flock size is much larger than that of aggregates without polymer, their loose structure may be the reason that they fall down more slowly than the heavy aggregates of solid particles. The obtained results indicate that flocks formed at pH 6 must have the smallest packing relative to those created at pH 3 and 9. In this case, marked improvement of system stability in the polymer presence is obtained, which may indicate steric interactions appearing due to the presence of stabilizing PAM layers. These effects probably cause an increase in the distance between the colloidal particles in formed flocks. For this reason, the dynamics of its sedimentation is not as large as that of the aggregates composed of only solid particles. 3.6. Mechanism of stabilization–destabilization of chromium (III) oxide suspension Taking into account all the experimental data, there can be proposed the most probable stability mechanism of the systems containing polyacrylamide and those without polymer. The Cr2 O3 suspension not containing the polymer is characterized by the greatest stability at pH 3. This is probably due to the electrostatic stabilization of solid particles – the electrical double layer is formed on their surfaces (electrolyte ion adsorption). Under these conditions, the relatively high value of electrokinetic potential of Cr2 O3 particles (approximately 40 mV) is obtained. Such

value is sufficient to ensure the effective repulsion between the solid particles coated with the diffusion layer of electrolyte ions. In turn, at pH 6 the suspension of chromium (III) oxide is the least stable of all investigated systems. The main reason for the observed destabilization is the occurrence of pHiep at pH 6, which means that the zeta potential of the solid particle is equal to zero. This is equivalent to the lack of electrostatic repulsive forces between the solid particles, which greatly facilitates their collisions and the formation of large, rapidly falling aggregates (coagulation process occurs). The system without the polymer at pH 9 exhibits intermediate behavior between that obtained at pH 3 and at pH 6. Under these conditions, the absolute value of  potential of the oxide particles is about 25 mV (Fig. 8), which seems not to be sufficient to provide effective stabilization of the suspension. The addition of anionic polyacrylamide at pH 3 causes slight reduction in Cr2 O3 suspension stability for the carboxyl groups content in the polymer chains 5 and 30%. For PAM containing 50% of these groups, the system stability deterioration is significant. Under these conditions, the highest adsorption of the polymer, which forms the thinnest adsorption layer, is achieved. A small dissociation degree of carboxyl groups in the PAM macromolecules makes that they adsorb in a more coiled form. Such structure of surface layer favors partial neutralization of positive charges of the solid particles by the adsorbed polymer coils having a number of negatively charged groups. As a result, slight decrease in the repulsive forces between the colloidal particles coated with a polymer film causes reduction in the system stability (especially in the case of PAM containing the largest number of anionic groups, i.e. 50%). A different situation occurs at pH 6, at which the polyacrylamide presence results in a significant improvement of suspension stability, compared to the unstable Cr2 O3 system in the electrolyte solution. This is due to the conformational changes of the adsorbed macromolecules, whose functional groups at pH 6 are almost completely ionized, and the formed adsorption layer has larger thickness than at pH 3. Thus, repulsive interactions appear under these conditions. They are derived not only from the steric layer of the adsorbed polymer, but also on the negative charges distributed along the PAM chains. This type of interactions is defined as electrosteric ones. They have major contribution to improvement of the stability of chromium (III) oxide suspension in the PAM presence at pH 6. Significant development of the adsorbed polyacrylamide chains at pH 9 due to the adsorbent–adsorbate electrostatic repulsion is the reason for bridging interactions appearing in the system. Formation of individual polymer bridges was favored, on one hand, by the developed conformation of macromolecules on the surface of colloidal particles and on the other hand, by the leaky coverage of solid surface with the polymer layer (the lowest PAM adsorption). This allows connecting two or more colloidal particles by adsorption of one extended chain on their surfaces. This process is known as bridging flocculation. Thus, the bridging effects cause the noticeable reduction of system stability in the polymer presence. It should be noted that these bridges are not very efficiently formed because after all, this process is considerably limited by the unfavorable electrostatic interactions. 4. Conclusions

Fig. 14. Aggregate migration velocity for Cr2 O3 suspensions without and with PAM.

The stability of the chromium (III) oxide suspension in the presence of anionic polyacrylamide (PAM) with different contents of negatively charged groups ( COO− ) was determined. It has been proved that the adsorbed amount of polyacrylamide on the chromium (III) oxide surface slightly increases with the rising content of the carboxyl groups in the PAM chains. Apart from electrostatic interactions, the hydrogen bridge type interactions

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play an important role in the polymer binding with the solid surface. They are formed mainly between the neutral surface groups CrOH and carboxyl groups of the polymer, which are preferentially adsorbed in comparison to amide ones. A slight decrease in Cr2 O3 suspension stability in the PAM presence at pH 3 is probably due to the neutralization of the positive charge of solid particles by partially ionized carboxyl groups belonging to the adsorbed polymer chains. The increase of polyacrylamide layer thickness at pH 6 leads to the marked improvement of stability properties in relation to the unstable suspension without polymer (electrosteric repulsion). At pH 9 there is slight deterioration in the stability of the suspension containing the polymer. Under these conditions, single polymeric bridges are formed. The bridging flocculation is promoted by the greatest development of adsorbed macromolecules (the thickest adsorption layer). Polyacrylamide with the molecular weight 15 500 000 and the carboxyl groups content 50% proved to be the most effective flocculent of chromium (III) oxide suspension. Acknowledgement The study was supported by the Polish National Center of Science, grant No. 2012/07/B/ST4/00534. References ´ [1] A. Nosal-Wiercinska, Adsorption of cystine at mercury/aqueous solution of chlorate (VII) interface in solutions of different water activity, Cent. Eur. J. Chem. 10 (2012) 1290–1300. ´ [2] A. Nosal-Wiercinska, M. Grochowski, Adsorption of thiourea and its methyl derivatives from chlorate (VII) with varied water activity, Collect. Czech. Chem. Commun. 76 (2011) 265–275. ´ [3] A. Nosal-Wiercinska, G. Dalmata, Adsorption of methonine at mercury/aqueous solution of chlorate (VII) interface; dependence on the supporting electrolyte concentration, Electroanalysis 22 (2010) 2081–2086. ˛ [4] E. Grzadka, Influence of surfactants on the adsorption and electrokinetic properties of the system: guar gum/manganese dioxide, Cellulose 20 (2013) 1313–1328. ˛ [5] E. Grzadka, The adsorption layer in the system: carboxymethylcellulose/ surfactants/NaCl/MnO2 , J. Surfactant Deterg. 15 (2012) 513–521. [6] M. Wi´sniewska, A review of temperature influence on adsorption mechanism and conformation of water soluble polymers on the solid surface, J. Dispers. Sci. Technol. 32 (2011) 1605–1623. ´ ˛ [7] M. Wi´sniewska, A. Nosal-Wiercinska, I. Dabrowska, K. Szewczuk-Karpisz, Effect of the solid pore size on the structure of polymer film at the metal oxide/polyacrylic acid solution interface – temperature impact, Microporous Mesoporous Mater. 175 (2013) 92–98. [8] M. Wi´sniewska, S. Chibowski, T. Urban, Effect of the type of polymer functional groups on the structure of its film formed on the alumina surface – suspension stability, React. Funct. Polym. 72 (2012) 791–798. [9] S. Chibowski, M. Paszkiewicz, M. Wi´sniewska, The influence of surfactant (SDS) on the adsorption properties of polyvinyl alcohol and polyethylene glycol in an alumina/solution system, Adsorpt. Sci. Technol. 20 (2002) 573–582. ˛ J. Patkowski, Influence of a type of electrolyte and [10] S. Chibowski, E. Grzadka, its ionic strength on the adsorption and the structure of adsorbed polymer layer in the system: polyacrylic acid/SiO2 , Croat. Chem. Acta 82 (2009) 623–631. [11] R. Duro, C. Souto, J.L. Gomez-Amoza, R. Martinez-Pacheco, A. Concheiro, Interfacial adsorption of polymers and surfactants: implications for the properties of disperse systems of pharmaceutical interest, Drug Dev. Ind. Pharm. 25 (1999) 817–829. [12] A. Nasu, Y. Otsubo, Effects of polymeric dispersants on rheology and UVprotecting properties of complex suspensions of titanium dioxides and zinc oxides, Colloids Surf. 326 (2008) 92–97. [13] S. Haydar, J.A. Aziz, Coagulation–flocculation studies of tannery wastewater using combination of alum with cationic and anionic polymers, J. Hazard. Mater. 168 (2009) 1035–1040. [14] T. Tripathy, B.R. De, Flocculation: a new way to treat the wastewater, J. Phys. Sci. 19 (2006) 93–127. [15] A.V.P. Gurumoorthy, K.H. Kha, Polymers at interfaces: biological and nonbiological applications, Recent Res. Sci. Technol. 3 (2011) 80–86.

823

[16] T.F. Tadros, Colloids aspects of cosmetic formulations with particular reference to polymeric surfactants, Colloids Interface Sci. Ser. 4 (2008) 1–34. [17] F. Shahidi, J.K.V. Arachchi, Y.J. Yeon, Food application of chitin and chitosans, Trends Food Sci. Technol. 10 (1999) 37–51. [18] F. Siepmann, J. Siepmann, M. Walther, R.J. MacRae, R. Bodmeier, Polymer blends for controlled release coatings, J. Control. Release 125 (2008) 1–15. [19] Y.Z. Jin, Y.F. Zhang, X.P. Chen, H.S. Gao, Application of organic polymeric flocculants in centrifugal dewatering of oil refinery sludge, J. Environ. Sci. (China) 15 (2003) 510–513. [20] F. Kreppel, S. Kochanek, Modification of adenovirus gene transfer vectors with synthetic polymers: a scientific review and technical guide, Mol. Ther. 16 (2007) 16–29. [21] X.C. Zhang, W.P. Miller, Polyacrylamide effect on infiltration and erosion in furrows, Soil Sci. Soc. Am. J. 60 (1996) 866–872. [22] E. Pefferkorn, Polyacrylamide at solid/liquid interfaces, J. Colloid Interface Sci. 216 (1999) 197–220. [23] M.S. Nasser, A.E. James, The effect of polyacrylamide charge density and molecular weight on the flocculation and sedimentation behaviour of kaolinite dispersions, Sep. Purif. Technol. 52 (2006) 241–252. [24] R.E. Sojka, D.L. Bjorneberg, J.A. Entry, R.D. Lentz, W.J. Orts, Polyacrylamide in agriculture and environmental land management, Adv. Agron. 92 (2007) 75–162. [25] K.C. Taylor, H.A. Nasr-El-Din, Water-soluble hydrophobically associating polymers for improved oil recovery: a literature review, J. Petrol. Sci. Eng. 19 (1998) 265–280. [26] G. Anger, J. Halstenberg, K. Hochgeschwender, C. Scherhag, U. Korallus, H. Knopf, P. Schmidt, M. Ohlinger, Chromium Compounds in: Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005. [27] M. Wi´sniewska, K. Szewczuk-Karpisz, Removal possibilities of colloidal chromium (III) oxide from water using polyacrylic acid, Environ. Sci. Pollut. Res. 20 (2013) 3657–3669. [28] I. Ostolska, M. Wi´sniewska, Application of the zeta potential measurements to explanation of colloidal Cr2 O3 stability mechanism in the presence of the ionic polyamino acids, Colloid Polym. Sci. 292 (2014) 2453–2464. [29] I. Ostolska, M. Wi´sniewska, Comparison of the influence of polyaspartic acid and polylysine functional groups on the mechanism of polymeric film formation at the Cr2 O3 – aqueous solution interface, Appl. Surf. Sci. 311 (2014) 734–739. [30] K. Szewczuk-Karpisz, M. Wi´sniewska, Adsorption properties of the albumin–chromium (III) oxide system – effect of solution pH and ionic strength, Soft Matter 12 (2014) 268–276. [31] K. Szewczuk-Karpisz, M. Wi´sniewska, M. Pac, A. Choma, I. Kamieniecka, Sinorhizobium meliloti 1021 exopolysaccharide as a flocculant improving chromium (III) oxide removal from aqueous solutions, Water Air Soil Pollut. 225 (2014) 2052 (13 pp.). [32] V.K. Gupta, I. Ali, Removal of lead and chromium from wastewater using bagasse fly Ash – a sugar industry waste, J. Colloid Interface Sci. 271 (2004) 321–328. [33] V.K. Gupta, I. Ali, V.K. Saini, Defluoridation of wastewaters using waste carbon slurry, Water Res. 41 (2007) 3307–3316. [34] V.K. Gupta, S. Agarwal, T.A. Saleh, Chromium removal combining the magnetic properties of iron oxide with adsorption properties of carbon nanotubes, Water Res. 45 (2011) 2207–2212. [35] V.K. Gupta, D. Pathania, S. Agarwal, S. Sharma, Removal of Cr (VI) onto Ficus carica biosorbent from water, Environ. Sci. Pollut. Res. 20 (2012) 2632–2644. [36] V.K. Gupta, A. Rastogi, Biosorption of heksavalent chromium by raw and acidtreated green alga Oedogonium hatei from aqueous solutions, J. Hazard. Mater. 163 (2009) 396–402. [37] J. Minczewski, Z. Marczenko, Analytical Chemistry, National Scientific Publishing House, Warsaw, 1985. [38] W.B. Crummett, R.A. Hummel, The determination of traces of polyacrylamides in water, J. Am. Water Works Assoc. 1 (1963) 209–219. [39] A. M’Pandou, B. Siffert, Polyethylene glycol adsorption at the TiO2 –H2 O interface: distortion of ionic structure and shear plane position, Colloids Surf. 24 (1987) 159–172. [40] P.J. Flory, Principles of Polymer Chemistry, Cornell University Press, New York, 1953. [41] M.J. Garvey, T.F. Tadros, B. Vincent, A comparison of the volume occupied by macromolecules in the adsorbed state and in the bulk solution, J. Colloid Interface Sci. 49 (1974) 57. [42] M. Wi´sniewska, S. Chibowski, T. Urban, Investigation of removal possibilities of colloidal alumina by flocculation using anionionic polyacrylamide, J. Ind. Eng. Chem. (2014) (in press). [43] S. Chibowski, M. Wi´sniewska, Study of the adsorption mechanism and the structure of adsorbed layers of polyelectrolyte at metal oxide–solution interface, Adsorpt. Sci. Technol. 19 (2001) 409–421. [44] W. Janusz, Electrical Double Layer at the Metal Oxide/Electrolyte Interface in Interfacial Forces and Fields: Theory and Applications Surfactant Science 85, M. Dekker, New York, 1999.