Comparison of adsorption affinity of ionic polyacrylamide for the surfaces of selected metal oxides

Abstract

The influence of metal oxide type on the adsorption affinity of anionic polyacrylamide for the solid surface was studied. There were selected three metal oxides: chromium(III) oxide, zirconium(IV) oxide and aluminium(III) oxide. The adsorption behaviour of high-molecular weight polymer containing 30% of carboxyl groups was determined based on the results of spectrophotometric, electrokinetic and turbidimetric measurements. The anionic polymer exhibits the greatest adsorption on the surface of chromium(III) oxide whereas PAM affinity for the aluminium(III) oxide is the lowest. The polymer presence has a pronounced impact on the electrokinetic properties of examined suspensions manifesting in changes in pH_pzc and pH_iep values of PAM-containing systems compared to those without polyacrylamide. The anionic PAM addition at pH 6 has the greatest effect on the stability of metal oxide aqueous suspensions.

Introduction

Metal oxides are widely used in many technological processes due to their good mechanical, electrical, optical and physicochemical properties. They find application in the microelectronics to create sensors and piezoelectric devices, in the fuel cells production, in anticorrosion coatings preparation and in catalysis – as catalysts and catalysts carriers (Kantorova and Vesely, 2013; She and Flytzani-Stephanopoulos, 2006; Yu et al., 2016). Additionally, metal oxides, besides activated carbons (Nowicki, 2016; Nowicki et al., 2016) and zeolites (Franus, 2012; Franus et al., 2014), play an important role in adsorptive removal of a great variety of noxious substances – both toxic gases and liquid compounds (Wawrzkiewicz et al., 2015). Another application area of these solids includes production of cosmetics, pharmaceuticals and mineral pigments (Simpson et al., 2011; Vargas-Reus et al., 2012). Due to very good biocompatibility, high mechanical strength and chemical inertness of some metal oxides (especially zirconia) they can be used in implantology in the processes of tissues regeneration (Salinas and Vallet-Regi, 2007; Silva-Bermudez and Rodil, 2013).

The above-mentioned practical applications of metal oxides require often modification of their surfaces by the adsorption process of a specific substance. Macromolecular compounds (polymers) proved to be an excellent tool to achieve the desired surface properties of different solids. Due to the fact that long polymeric chains composed of numerous segments can adopt specific conformations in the adsorption layer formed on the solid surface, a great variety of different hybrid materials can be obtained. There are a lot of parameters which affect macromolecule conformation at the solid–liquid interface (Chibowski and Krupa, 2000; Chibowski and Wiśniewska, 2001; Wiśniewska, 2011). Besides the kind of polymer (ionic, nonionic) and type of metal oxide, the most important are temperature, solution pH, supporting electrolyte presence and its concentration, addition of other low-molecular substances (surfactants, amino acids, dyes) and magnetic field action. Understanding the adsorption mechanism in such systems makes it possible to obtain the desired surface properties of the metal oxide coated by an appropriately selected polymer.

On the other hand, polymeric chains present in the adsorption layers around the solid particles determine the stability of aqueous suspensions of metal oxides (Wiśniewska, 2013). This property is essential for the colloidal systems and is the most important parameter in their characteristics. Ionic macromolecules bounded with the solid surface affect system stability through both steric and electric forces. The steric interactions are a result of spatial hindrance which prevents particles aggregation. Additionally, the charges of functional groups in the adsorbed macromolecules can cause two opposite effects: solid surface charge neutralization (resulting in the suspension stability decrease) or electrosteric stabilization of solid particles.

In the present paper, the comparison of adsorption behaviour of anionic polyacrylamide (PAM) in the aqueous suspensions of three metal oxides was presented and discussed. There were applied the following metal oxides: chromium(III) oxide – Cr₂O₃, zirconium(IV) oxide – ZrO₂ (zirconia) and aluminium(III) oxide – Al₂O₃ (alumina). The adsorption and electrokinetic data enabled not only determination of mechanism of PAM binding with the solid hydroxyl groups of amphoteric character but also explanation of solid suspension stability in the polymer presence. The adsorption properties of other metal oxides in relation to PAM were also studied, for example in the systems containing Fe₂O₃ (Chibowski and Wiśniewska, 2002), MnO₂ (Chibowski et al., 2002) and SiO₂ (Wiśniewska, 2012).

Experimental

In the experiments three metal oxides were used as adsorbates – chromium(III) oxide: Cr₂O₃ (POCh), zirconium(IV) oxide: ZrO₂ (Sigma-Aldrich) and aluminium(III) oxide: Al₂O₃ (Merck). The physicochemical characteristics of these metal oxides are presented in Table 1. The BET specific surface area (S_BET), mean pore size (d_p) and mean pore volume (V_p) of the applied solids were determined using the low-temperature nitrogen adsorption–desorption isotherm method (Micromeritics ASAP 2405 Analyzer), whereas the average grain size (D_g) was obtained using the dynamic light scattering (DLS) method (Mastersizer 2000, Malvern Instruments). The samples for DLS measurement were prepared adding 0.01 g of the appropriate solid to 20 cm³ of double distilled water. The obtained suspension was sonicated for 5 min. The natural pH of such systems changes in the range 5–7.

Table 1.

Physicochemical characteristics of applied metal oxides.

Parameter	Cr2O3	ZrO2	Al2O3
SBET (m2/g)	7.12	21.7	155
dp (nm)	9.3	3.1	6.1
Vp (cm3/g)	0.017	0.168	0.253
Dg (nm)	265	100	496

The anionic PAM (Korona) with the weight average molecular weight 14,000,000 Da and the content of carboxyl groups 30% was applied as an adsorbate. The pK_a value of this polymer being 3.7 was determined using the potentiometric titration method. The dissociation degree (α) of the PAM carboxyl groups was also calculated. At pH 3 α = 16.6%, at pH 6 it assumes the value 99.5%, whereas at pH 9 the dissociation degree is equal to 99.9%.

All measurements were made in the pH range 3–9 at 25℃ in the presence of NaCl with the concentration 1·10⁻²mole/dm³.

Adsorption measurements were performed using the static method. The reaction of PAM with hyamine proposed by Crummet and Hummel (1963) was applied to determine the PAM concentration in the solution after the adsorption process. The PAM solution turbidity was measured using the UV–VIS spectrophotometer (Carry 1000; Varian) at the wavelength 500 nm. The initial concentration of PAM was 100 ppm. The efficiencies of solid surface modification by polymer at pH 3 (the greatest PAM adsorption) were 81% for Cr₂O₃, 38% for ZrO₂ and 87% for Al₂O₃.

The potentiometric titration method (Janusz, 1999) was used for the solid surface charge density determination. The special program Titr_v3 (author W. Janusz) was also applied to this end. The appropriate mass of metal oxide was added into the thermostated (thermostat RE204, Lauda) Teflon vessel containing 50 cm³ of polymer solution in the NaCl electrolyte (or only to NaCl electrolyte solution). Such systems were titrated with NaOH with the concentration 1·10⁻¹mole/dm³ using the automatic burette Dosimat 665 (Metrohm). The changes in the pH values during titration were monitored with the pH-meter 71 (Beckman).

The electrophoretic mobility of metal oxide particles (without and covered with polymer) was measured using the Zetasizer Nano ZS with the universal dip cell and MPT-2 titrator (Malvern Instruments). The Doppler laser electrophoresis technique was applied and the zeta potential value was calculated with the special computer program using the Henry equation (Hunter, 1981).

The stability measurements of metal oxide suspensions in the absence and presence of PAM were performed using the turbidimetry method (Terpiłowski et al., 2015). For this purpose the apparatus Turbiscan Lab^Expert with the cooling module TLAb cooler (Formulaction, France) was applied. TLab EXPERT 1.13 and Turbiscan Easy Soft computer programs enabled calculations of stability coefficients turbiscan stability index (TSI). The limit values of TSI are 0 (very stable suspensions) and 100 (extremely unstable systems).

Results and discussion

The analysis of the data presented in Figure 1 indicated that anionic PAM exhibits the greater adsorption on the chromium(III) oxide surface and the lowest on the aluminium(III) oxide surface. Additionally, for all examined metal oxides the adsorbed amounts of the polymer decrease with the solution pH increase. Such behaviour in the aqueous solution is the result of changes in the sign and value of the solid surface charge with the pH rise and dissociation of the PAM carboxyl groups.

Figure 1.

Adsorbed amounts of PAM on the surfaces of chosen metal oxides at pH 3, 6 and 9.

The source of metal oxide charge is their hydroxyl surface groups ≡ MeOH of the amphoteric character (where Me is the metal atom: Cr, Zr, Al). H⁺ ions were considered as potential-determining ions because they affect the formation of the solid surface charge according to the following acid–base reactions

$$\equiv {\mathrm{MeOH}}^0+H^{+}\to \equiv {\mathrm{MeOH}}_2^{+}$$

(1)

$$\equiv {\mathrm{MeOH}}^0\to \equiv {\mathrm{MeO}}^{-}+H^{+}$$

(2)

Moreover, in the solution of supporting electrolyte the reactions of the surface hydroxyl groups with Na⁺ and Cl⁻ ions are also possible. According to the site-binding model (Janusz, 1999), the surface complexes can be formed in the following way

$$\equiv {\mathrm{MeOH}}_2^{+}{\mathrm{Cl}}^{-}\to \equiv {\mathrm{MeOH}}^0+H^{+}+{\mathrm{Cl}}^{-}$$

(3)

$$\equiv {\mathrm{MeOH}}^0+{\mathrm{Na}}^{+}\to \equiv {\mathrm{MeO}}^{-}{\mathrm{Na}}^{+}+H^{+}$$

(4)

The presence of anionic PAM in the aqueous suspension of metal oxide particles affects also the solid surface charge density according to the reaction (Wiśniewska et al., 2016)

$$\equiv {\mathrm{MeOH}}^0+{\mathrm{PAM}}^{-}+H^{+}\to \equiv {\mathrm{MeOH}}_2^{+}{\mathrm{PAM}}^{-}$$

(5)

The total charge of the solid is the sum of all contributions of the above processes and at specific pH for each metal oxide it assumes the value zero (point of zero charge – pzc). The determined values of pH_pzc points for all examined systems are presented in Figure 2.

Figure 2.

Values of pH_pzc for the metal oxide systems without and with the ionic polymer.

As can be seen in Figure 2 the points of zero charges for Cr₂O₃, ZrO₂ and Al₂O₃ oxides are equal to 5.8, 5.9 and 7.8, respectively. Thus, the greatest adsorption level of anionic PAM at pH 3 is a result of favourable electrostatic attractions between the positively charged solid surface and the slightly dissociated polymeric chains (assuming more coiled conformation at the solid–liquid interface). The PAM adsorption is the lowest at pH 9 due to the repulsion between the totally ionized PAM macromolecules and the negative surface of the solid. Under these disadvantageous conditions the existence of hydrogen bonds between the adsorbent and the adsorbate is demonstrated. Although the charged active solid sites can contribute to formation of these bonds, the neutral ones are mainly responsible for it. This results from the fact that ≡MeOH⁰ groups are much more numerous than ≡MeOH₂⁺ and ≡MeO⁻ ones (Chibowski and Wiśniewska, 2001). The presence of hydrogen bonds in the metal oxide–polymer (containing carboxyl groups) system was proved by our previous studies (Wiśniewska et al., 2009) concerning desorption measurements of radioactive ions caused by macromolecules, as well as calculations from Pradip equation based on zeta potential data and from Langmuir equation based on adsorption data.

The greatest adsorption of anionic PAM on the chromium(III) oxide surface is probably caused by the greatest concentration of solid hydroxyl groups per unit surface area of the metal oxide. In such situation the considerable number of polymeric segments can interact directly with the solid hydroxyl groups and as a consequence, the adsorbed amounts of PAM are the largest on the Cr₂O₃ surface.

Such adsorption mechanism is confirmed by the greatest difference in the pH_pzc values obtained for the Cr₂O₃ suspension in the PAM presence in comparison to the system without polymer (Figure 2). This difference for Cr₂O₃ equals 1.3 (for ZrO₂ it is 0.6 and for Al₂O₃ only 0.3).

The adsorption of anionic polymer containing negatively charged functional groups at the interface enforces the formation of an additional number of positive surface sites (according to reaction (5)). As a consequence, the solid surface charge density of metal oxide increases and the pH_pzc point is shifted towards the higher pH values (Wiśniewska et al., 2015). This shift is the greatest in the case of chromium(III) oxide due to the highest adsorption of anionic PAM on its surface, which is related to more numerous polymeric segments bonded directly with the Cr₂O₃ hydroxyl groups (in train-type structures of adsorbed PAM chains).

The electrokinetic (or zeta) potential is a very important parameter characterizing the solid particle interaction in the aqueous media related to such systems stability. The absolute value of ζ potential is decisive for the about suspension aggregation or stabilization (Jesionowski, 2003; Klapiszewski et al., 2013). As can be seen in Figure 3 the isoelectric points (iep) of metal oxides without the polymer are located at pH values 6, 6.4 and 8.4 for Cr₂O₃, ZrO₂ and Al₂O₃ adsorbents, respectively. Some differences between the pH_pzc and pH_iep values of applied oxides are a result of overlapping of the electrical double layers formed on the mesopores surface existing in the solids structure (Skwarek, 2014, 2015).

Figure 3.

Values of pH_iep for the metal oxide systems without and with the ionic polymer.

The presence of anionic polymer layer around the solid particles results in a significant decrease of the zeta potential values and the considerable shift of the pH_iep to lower pH values. The differences between the pH_iep values obtained for the solid suspension in the PAM presence in comparison to the system without polymer are as follows: for Cr₂O₃ – 2.5, for ZrO₂ – 2.7 and for Al₂O₃ – 5.1. This is a completely opposite tendency compared to that obtained for the surface charge density characteristics and pH_pzc location. The main reason for zeta potential lowering in the anionic polymer presence is the occurrence of negatively charged carboxyl groups of the adsorbed polymer macromolecules in the area of slipping plane around the solid particle (M’Pandou and Siffert, 1987). These groups belong to the polymeric segments located in the loop and tail structures of the adsorbed PAM chains. For this reason these groups are not directly bound with the solid surface and exist in the border of the stiff and diffusion parts of the electrical double layers formed on the solid particle surfaces. The shift of slipping plane due to the polymer adsorption can also play an important role in the zeta potential decrease in the systems under consideration. The more extended conformation (towards the bulk solution) is assumed by the adsorbed macromolecules, the greater shift of slipping plane is observed. The greatest difference between the pH_iep points location (suspension with PAM in comparison to the system without PAM) is obtained for the aluminium(III) oxide. This is another evidence for correctness of the assumed mechanism of anionic PAM adsorption. The lowest polymer adsorption level on the Al₂O₃ surface is associated with more extended conformation of PAM macromolecules at the solid–liquid interface which results from a lower concentration of the hydroxyl groups on the alumina surface. As a consequence, the shift of slipping plane is considerable and the probability of negatively charged PAM carboxyl groups occurrence in this plane area is greater. It leads to the most pronounced modification of the electrokinetic properties of alumina suspension after the anionic PAM addition.

The polymer presence at pH 6 has the greatest impact on the metal oxide suspension stability. The main reason is the fact that at the pH value, which is close or equal to the isoelectric points of all examined solids, the metal oxide particles dispersed in the aqueous media (without PAM) show a strong tendency towards aggregation which is evidenced by high values of TSI stability coefficients (Figure 4) changed from 61 for Cr₂O₃ to 71.6 for Al₂O₃.

Figure 4.

Values of TSI stability coefficients for the metal oxide suspension without and with polyacrylamide at pH 6. TSI: turbiscan stability index.

The anionic PAM addition causes considerable improvement of metal oxide suspension stability and this effect is the greatest for the alumina-containing system – the TSI decrease by 57.8 units (for the Cr₂O₃ system the reduction of TSI is 27.8 and for the ZrO₂ system - 33.2). The adsorption of completely dissociated polymer chains on the solid surface at pH 6 results in appearance of electrosteric repulsion of PAM adsorption layers covered by metal oxide particles. The most extended conformations of the PAM macromolecules on the alumina particles surface provide the most efficient repulsion of solid particles preventing successfully their aggregation.

Conclusions

High-molecular anionic PAM exhibits the greatest adsorption affinity for the surface of chromium(III) oxide. This is probably caused by the greatest concentration of the amphoteric hydroxyl groups on the Cr₂O₃ surface. The evidence of this is the biggest difference in the position of pH_pzc points between the Cr₂O₃ suspension without and with PAM. The polymeric segments (occurring in train structures) directly bonded with numerous ≡CrOH⁰ groups are responsible for the pronounced increase of the chromium(III) oxide surface charge density (pH_pzc shift towards the higher pH values). On the other hand, the more extended conformation of the adsorbed PAM chains in the aluminium(III) oxide system (the lowest adsorption affinity of the polymer for ≡AlOH⁰ surface groups) results in considerable reduction of the suspension zeta potential and shift of pH_iep point towards the more acidic pH values. This is due to both pronounced displacement of slipping plane towards the bulk solution and occurrence of the greatest number of negatively charged PAM carboxyl groups (belonging to the loop and tail structures) in this plane area. The PAM addition to the aluminium(III) oxide aqueous suspension at pH 6 has the greatest impact of the solid suspension stability. The lowest PAM adsorption on the Al₂O₃ surface and the most developed conformation of macromolecules at the solid–liquid interface provide the most efficient electrosteric repulsion between the metal oxide particles covered with the polymeric layers.