Modification of stability properties of yttrium(III) oxide particles by simultaneous adsorption of poly(acrylic acid) and poly(ethylene glycol) for possible applications in wastewater treatment
The adsorption, electrokinetic and stability properties of yttrium(III) oxide—Y2O3 in mixed solutions of macromolecular compounds were investigated. The interfacial behavior of poly(acrylic acid)-PAA and poly(ethylene glycol)—PEG in single and binary systems was examined in the pH range 3–10. The polymers used were characterized by different ionic nature—the PAA is an anionic polymer, whereas PEG belongs to the group of non-ionic polymers. Based on the results obtained, the most probable mechanisms for the binding of PAA and PEG macromolecules on the yttrium(III) oxide surface were proposed. In addition, the analysis of adsorption and electrokinetic data enabled explanation of the obtained changes in the stability of Y2O3 suspensions without and in the presence of PAA or/and PEG, as well as determination of the specific stabilization-destabilization mechanisms of the studied systems. It was shown that yttrium(III) oxide modification by mixed adsorption layers of both polymers with different ionic character changes considerably the surface and stability properties of the examined solid suspensions.
Graphical Abstract
×
Notes
Publisher's Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Introduction
Adsorption of polymers, especially those of an ionic nature (polyelectrolytes) on the solid surface is a very complex process and takes place through many different mechanisms (Napper 1983; Farrokhpay 2009; Somasundaran and Runkana 2005; Tripathy 2006; Borostow et al. 2009; Landman et al. 2021; Shvets and Semenow 2013; Svenson and Tomalia 2005; Wiśniewska et al. 2017). Because of the large size of polymeric macromolecules, their high molecular weight, as well as multi-segment structure, adsorption of these compounds is more complex, happens in two steps and takes much longer than for small molecules or simple ions (Wiśniewska 2011). The first step in the adsorption of polymers is the transport of the macromolecules from the bulk phase to the surface phase of the solution, where the initial binding of the polymeric chain to the solid surface occurs. This process undergoes randomly, it can be the result of electrostatic interaction between the polymer and the solid surface, the formation of hydrogen bonds or the hydrophilic-hydrophobic nature of the examined system. In subsequent stages, the macromolecules undergo reconformation, during which the polymer acquires its final conformation on the solid surface (the so called equilibrium conformation) (Clayden et al. 2010). The specific conformation adopted by the macromolecules is the result of many parameters regarding the used polymeric compound (its type, concentration, molecular weight, polydispersity, impurity degree); the properties of the adsorbent (type and concentration of surface groups, value of zero charge and isoelectric points, impurity degree); the properties of the solution (type of supporting electrolyte, pH and ionic strength of the solution); and it also depends on temperature and the presence of a magnetic field. Summary effect of the interaction of the above-mentioned parameters is represented in a certain amount of adsorbed polymer and the thickness and structure of its adsorption layer (Rabek 2008; Morawetz 1970; Kawaguchi and Takahashi 1992; Aubouy and Raphael 1998).
Water-soluble polymers are widely used in various technological processes that are carried out in many fields of human activity, including agriculture (Henriksen et al. 1998), paper industry and mining (Wagberg and Nordqvist 1999; Swerin et al. 1996). They are used both as stabilizers and flocculants (destabilizers) of colloidal suspensions. Depending on the conformation of the polymer, its presence in the system results in a decrease or increase in the stability of such a systems. The possibility for the adsorbed polymer to affect the stability of suspensions finds wide usage in water treatment processes, wastewater treatment, mineral processing, varnishing, pharmaceutical and cosmetic industries (Miller et al. 1999), medicine, as well as agriculture and environmental protection (Szewczuk-Karpisz and Wiśniewska 2016), and many others.
Advertisement
Metal oxide nanoparticles, due to several interesting properties related to their textural and surface characteristics and unique physicochemical features, are used as adsorbents of macromolecular compounds (Somasundaran and Krishnakumar 1997; Walker and Grant 1996). The regeneration abilities of the applied adsorbent is also an important parameter characterizing its practical usage. One of such solids is yttrium(III) oxide (Y2O3), which finds many technological applications. It is characterized by high dielectric constant and high thermal stability. Y2O3, as a rare earth compound, is used in many dopants applied for biological imaging and photodynamic therapies (Dasgupta et al. 2001; Balakrishnan et al. 2015; Kwo et al. 2000). Yttrium(III) oxide is also used as a polarisator, luminophore, biosensor, laser and as a constituent of optoelectronic fields in cancer therapies. In addition, its nanoparticles show desirable antibacterial and antioxidant properties (Xu et al. 1997). Y2O3 is widely used as a component of refractory ceramic materials, liquid reactive alloys and salts in nuclear reactors, providing an excellent protective coating (Lin et al. 2018). The current research shows that yttrium(III) oxide mixed with rare earth compounds (zinc oxide, titanium oxide, cerium oxide) can be applied for solid state lighting purposes, such as LEDs (Wang et al. 2002). Nanoparticles doped with europium are a luminescent materials with red–orange emission, excited by electrons (so-called cathodoluminescence). Published papers on the applications of Y2O3 is a quite sparse and doesn’t fully illustrate the specific mechanisms of this solid interactions with various substances (Lakshminarasappa et al. 2014; Rajakumar et al. 2021). On the other hand, it was proved that Y2O3 shows cytotoxicity and genotoxicity effects on the human cells. Andelman et al. (2009) examined the effects of exposure to different types of Y2O3 nanoparticles on human foreskin fibroblast cells and demonstrated a concentration-dependent (25–500 μg/mL) cytotoxicity by live/dead cell assay. Selvaray et al. (2014) studied the effects of exposure to different concentrations (0–50 μg/mL) and incubation times (10, 24 or 48 h) of yttrium(III) oxide nanoparticles on human embryonic kidney cells. It was demonstrated that the yttrium(III) oxide exposure is associated with increased cellular apoptosis and necrosis in kidney cells. Thus, there is a need for the effective removal of this oxide from wastewaters.
In connection with the above, the main objective of the conducted research was to determine the adsorption, electrokinetic and stability properties of yttrium(III) oxide in the mixed solution of two polymers well-soluble in water, namely, poly(acrylic acid)—PAA and poly(ethylene glycol)—PEG. This selection of components for the adsorption system resulted from their common use, as well as from their high stability and negligible solubility of the solid, allowing the study to be carried out over a wide pH range (3—10). It is worth noting that the mechanisms of polymers adsorption and stability of the yttrium(III) oxide suspension have not previously been studied. Besides, the determination of structure of the mixed adsorption layer of two polymers at the solid-solution interface is not enough clarified yet, since most of the literature reports describe single adsorbate systems. In addition, the issue of suspension stability modification by simultaneous addition of two polymers (Fan et al. 2000; Gregory and Barany 2011; Yu and Somasundaran 1996) are rather rarely undertaken, so there is still the need to the knowledge completing in this field. Equally important is the fact that stabilization of the suspension by both polymers adsorption preserves the large specific surface area for adsorption of small ions and organic molecules in the treatment of contaminated waters.
Experimental
Materials
In the present study, yttrium oxide(III)- Y2O3, manufactured by Sigma-Aldrich, was used as an adsorbent. Before performing the experiments, the oxide was washed with doubly distilled water to remove inorganic impurities, until the supernatant conductivity was below 2 μS/cm. The specific surface area of the yttrium(III) oxide was determined by the porosimetry method using nitrogen adsorption–desorption BET isotherm analysis (Micrometritics ASAP 2405 analyzer) and it was equal to 3.85 m2/g. Y2O3 was characterized by a micropore area of 1.4 m2/g and a total pore volume of 0.0243 cm3/g. The SEM technique (FEI Quanta 250 FEG) was used for determination of surface morphology of the examined solid.
In the experiments, two macromolecular compounds with different ionic character were used as adsorbates: poly(acrylic acid)- PAA manufactured by Sigma- Aldrich and poly(ethylene glycol)- PEG manufactured by Fluka. Both polymers have a weight average molecular weight of 2000 Da. PAA macromolecules, a representative of anionic polymers obtained via acrylic acid polymerization, contain functional carboxyl groups, which dissociate with the increase in solution pH. The pKa value for PAA is about 4.5 (Wiśniewska and Nowicki 2020), so in the pH range below 4.5, undissociated -COOH groups quantitatively predominate over dissociated -COO− ones. As the pH increases above 4.5, the degree of dissociation of the polyelectrolyte increases and at pH around 9 reaches a value close to 1. The characteristic of this process is shown in Table 1. PEG, obtained during the hydrolysis of ethylene oxide, is a typical nonionic polymer and as a consequence its macromolecules have no charge (Mastalerz 1984).
Table 1
The ratio of the concentration of [COOH] groups to the concentration of [COOˉ] groups in PAA macromolecules and the degree of their dissociation (αdys) as a function of solution pH
pH
[COOH]/[COOˉ]
αdys
3
31
0.03
4.5
1
0.5
6
0.031
0.96932
9
0.000031
0.99998
Advertisement
Solutions of polymeric substances were prepared using redistilled water. PAA and PEG solutions were obtained by diluting basic solutions with concentrations of 1 10−3 g/cm3. The following concentrations of polymer solutions were used in the study: 5, 10, 30, 50, 70, 100, 150 and 200 ppm. Additionally, for adsorption measurements 1% hyamine 1622, 20% sodium hydroxide and 1 10−1 g/cm3 mol/dm3 tannic acid solutions were used. The supporting electrolyte was NaCl with concentration of 1 10−2 mol/dm3. In addition, for the examined systems pH adjustment, NaOH and HCl solutions with concentrations varying from 1 10−2 to 1 mol/dm3 were applied.
Methods
Determination of the polymer adsorbed amount on the yttrium(III) oxide surface
Measurements of the adsorbed amounts of PAA and PEG polymers consisted of two steps. The first one involved the preparation of calibration curves, whereas the second one included determination the adsorption capacities using a statistic method. The adsorption process was carried out at pH 3, 6 and 9. All experiments were performed at 25 °C. At the beginning, the calibration curves were obtained. The methodology with hyamine 1622 proposed by Crummett and Hummel (1963) was used for determination of poly(acrylic acid) concentration in the solution. The PAA adsorbed amount on the yttrium oxide(III) surface was calculated from the difference between the polymer concentration in the solution before and after this process. For this purpose, 10 cm3 of PAA solutions with concentration range 5⋅10−6—2⋅10−4 g/cm3 were prepared and next 0.1 g of adsorbent was added. This process was carried out under the continuous shaking conditions for 24 h. The absorbance originating from the white color of solution, indicating PAA-hyamine complex formation, was measured at the wavelength 500 nm after 15 min of the hyamine addition. The UV–VIS spectrophotometer Cary 100 (Varian) equipped with quartz cuvettes was applied. The appropriate pH values of the examined suspensions were adjusted with the pH-meter (Beckman Instruments). To obtain calibration curve for determination of the poly(ethylene glycol) concentration (Nuysink and Koopal 1982), 20 cm3 of tannic acid solution was introduced into six 25 cm3 volume flasks, followed by the addition of 5 cm3 of PEG solution with concentrations varying from 5 10−6 to 2 10−4 g/cm3. The obtained solutions were characterized by different degrees of turbidity. After 15 min, absorbance was measured spectrophotometrically at the wavelength 600 nm.
The possibility of yttrium (III) oxide regeneration by desorption using 1⋅10−1 mol/dm3 HCl and NaOH solutions was also determined.
A single adsorption/desorption result was the average of three repetitions. The reproducibility was better than 5%.
Potentiometric titrations and electrophoretic mobility measurements—electric properties determination
The surface charge density in all examined systems was determined at 25 °C using the potentiometric titration method. For this purpose, the solution of NaCl supporting electrolyte was introduced into a thermostated Teflon vessel (thermostat RE 204, Lauda). Next, the HCl with the concentration of 1 10−1 mol/dm3 was added to obtain the initial pH of the solution in the range of 3–3.5. After equilibrium was established, the electrolyte solution was titrated with NaOH (1 10−1 mol/dm3) using microburette Dosimat 765 (Methrom), and a reference curve (showing the dependence of the supporting electrolyte pH as a function of the added base volume) was prepared. For pH measurements glass and calomel electrodes (Beckman Instruments) were applied. Then, titrations of yttrium(III) oxide–supporting electrolyte system and yttrium(III) oxide −polymer/s systems were carried out. The used solid mass of Y2O3 was 1 g. Numerical calculations of the surface charge density without and in the presence of applied polymers were made using the computer program “Titr-v3” devoted by Prof. W. Janusz (1999). As a result, the solid surface charge density (σ0) as a function of solution pH and point of zero charge (pzc) were determined.
Electrophoretic mobility measurements of examined suspensions were carried out at 25 °C using an immersion dip cell in the pH range of 3–10, and the adsorbent mass 0.07 g. The values of the zeta potential (ζ) were calculated by the special computer program, through the conversion of particle electrophoretic mobility into electrokinetic potential using Henry equation (Ohshima 1994). Mixed oxide suspensions in NaCl solution (1 10−3 mol/dm3) without and in the presence of single polymers or their mixture (100 ppm) for electrophoretic mobility measurements were prepared by adding 0.07 g of yttrium(III) oxide to 250 cm3 of appropriate solution. Each suspension was sonicated for 3 min (Ultrasound XL 2020, Misonix), after this time it was divided into eight parts. The corresponding pH values (3, 4, 5, 6, 7, 8, 9 and 10 ± 0.1) were then adjusted in each series. Electrophoretic mobility of such prepared systems was measured using zetasizer Nano ZS apparatus (Malvern Instruments). Additionally, the size of formed aggregates in the examined suspensions were also determined.
The final result of electrophoretic mobility was the average of five repetitions for a given sample. The reproducibility was better than 3%.
Determination of stability properties of Y2O3 suspensions without and in the presence of polymer/s
The stability measurement of Y2O3 suspension in the NaCl solution (1 10−3 mol/dm3) without and with PAA or/and PEG (100 ppm) were performed at 25 °C using the apparatus Turbiscan LabExpert with the cooling module TLAb cooler (Formulaction). The light beam (λ = 880 nm) was directed to a measuring vial containing the examined system, placed in the constant temperature chamber. Light detection was made in two ways: the light passing through the sample at the angle of 0° in relation to the direction of starting beam was registered by the transmission detector and the light scattered at the angle of 135° was registered by the backscattering detector. For this purpose, the yttrium(III) oxide suspension was prepared by adding 0.1 g of the solid to 20 cm3 of 1 10−3 mol/dm3 NaCl solution. The prepared suspension was subjected to ultrasound for about 3 min and its corresponding pH value was determined, i.e., 3, 6 or 9. The suspension was shaken on the thermostated water bath for 30 min, meanwhile controlling its pH value. Changes in the suspension stability were monitored for 15 h during which single scans were collected every 15 min. The measurement results were obtained in the form of transmission and backscattering intensity curves of light as a function of time. The TSI stability coefficients, the average size of the formed aggregates, the rate of their sedimentation and the thickness of the sediment layer were calculated using special computer program.
Stability measurements were also performed applying the spectrophotometric method by the absorbance changes recording (UV–VIS spectrophotometer Cary 100, Varian) at 500 nm. The procedure of suspensions preparation was analogous to that used in measurement with Turbiscan application. Each suspension was transferred to the quartz cuvette and subjected to cyclic spectrophotometric measurement for 6 h (a single result was recorded every 15 min).
Results and discussion
Adsorption affinity of poly(acrylic acid) and poly(ethylene glycol) to yttrium(III) oxide
Figures 1 and 2 present adsorption isotherms obtained for the PAA and PEG polymers at the three studied solution pH values. Analysis of these data indicates that the adsorbed amounts (Γ) of both polymers depend on the acidic-basic properties of the solution in which the process takes place. They affect not only the sign and density of the surface charge but also the degree of dissociation of anionic PAA macromolecules. The results obtained from potentiometric titrations showed that the point of zero charge (pzc) of Y2O3 is at pH about 7.6, which means that at pH values of 3 and 6 (at which the adsorption process was carried out) the surface of the solid is endowed with the positive charge, whereas at pH 9 it is endowed with the negative one. This strongly influences the binding mechanism of the polymer, especially poly(acrylic acid), whose functional carboxyl groups dissociate with increasing pH (Table 1). The minimal dissociation of PAA at pH 3 of only 3% results in the coiled conformation of adsorbed macromolecules. This conformation makes it possible to achieve significant packing of the adsorption layer, and this is manifested by the highest PAA adsorption. The adsorption affinity of poly(acrylic acid) in systems at pH 6 is also high, whereas at pH 9 it is significantly lower, caused by unfavorable electrostatic repulsion between the negative surface of yttrium(III) oxide and fully dissociated polymer chains. Nevertheless, the amount of adsorbed PAA reaches a maximum value of 2.5 g/m2, which, compared to the amount of PEG adsorption (Fig. 2) obtained under the same pH conditions, is the significant result. Despite the unfavorable electrostatic conditions, the binding of PAA macromolecules on the solid surface takes place, indicating formation of hydrogen bonds between the hydroxyl groups of Y2O3 and the functional groups of the polymer. Previous studies have confirmed that not only neutral surface groups and undissociated carboxyl groups of the polymer, but also charged groups (positive or negative in the case of yttrium(III) oxide and negatively charged groups in the case of poly(acrylic acid)) can be involved in the formation of hydrogen bridges (Kasprzyk-Hordern 2004).
Fig. 1
Adsorption isotherms of PAA on the Y2O3 surface obtained for pH values: 3, 6 and 9; supporting electrolyte concentration: 1 10−2 mol/dm3, initial polymer concentration: 5–200 ppm
Fig. 2
Adsorption isotherms of PEG on the Y2O3 surface obtained for pH values: 3, 6 and 9; supporting electrolyte concentration: 1 10−2 mol/dm3, initial polymer concentration: 5–200 ppm
×
×
For nonionic poly(ethylene glycol), the marked increase in adsorption was observed at pH 3, whereas at pH 6 and 9, it remains quite low, compared to all examined systems. PEG macromolecules contain oxygen bridges in their chains (oxygen atoms have free electron pairs) and terminal hydroxyl groups. The binding of such macromolecules is much less influenced by the sign of the surface charge, and formation of hydrogen bonds are responsible for their adsorption process. The obtained results indicate that the formation of these bonds proceeds the most efficiently at pH 3, which undoubtedly affects the conformation of adsorbed macromolecules. The specific structure of the adsorption layer under these conditions provides the highest level of PEG adsorption on the Y2O3 surface.
Figure 3 shows the results of adsorption tests carried out in the mixed system of two polymers (with initial concentrations of 100 ppm). Analyzing the values of PAA adsorbed amounts from the binary solution (PAA + PEG), it can be concluded that the presence of PEG does not significantly affect this parameter at pH 3, whereas at pH 6, it causes a noticeable Γ increase and at pH 9—its decrease, in comparison to suspensions containing only poly(acrylic acid). The main phenomenon occurring in mixed adsorbate systems is the competition of both types of macromolecules for solid surface sites. The formation of PAA-PEG complexes through the hydrogen bonds between them is also possible. Adsorption of such complexes can lead to the formation of adsorption multilayer and as a result to increase of the polymer adsorption. The obtained data proved that the latter mechanism is more dominant at pH 6, whereas competitive adsorption, leading to the decrease in the PAA adsorbed amount and the simultaneous significant increase in the case of PEG, takes place at pH 9. Under such basic pH conditions totally dissociated PAA chains repeal with negatively charged solid surface and thus nonionic PEG macromolecules show significantly greater adsorption affinity in the mixed adsorbate system. Specific structure of polymeric mixed layer at pH 9, composed of smaller number of PAA macromolecules with stretched conformation (repulsion between oxide surface and PAA negatively charged carboxyl groups) and PEG chains with more coiled conformation assumes relatively high adsorption level of poly(ethylene glycol). The PAA chains can interact with PEG macromolecules through hydrogen bonds and they somehow “capture” them from the solution, increasing their adsorption at pH 9.
Fig. 3
Comparison of the amounts of PAA or/and PEG adsorbed on the yttrium(III) oxide surface in single and mixed adsorbate systems, pH values: 3, 6 and 9; supporting electrolyte concentration: 1 10−2 mol/dm3, initial polymer concentration: 100 ppm
×
An important parameter characterizing a given adsorbent is the possibility of its regeneration. In the examined systems, the desorption of polymers was carried out with the use of hydrochloric acid and sodium base (with concentrations of 1 10−1 mol/dm3) and the obtained results are presented in Table 2. In general, hydrochloric acid (single adsorbate systems) turned out to be the more effective desorbing agent for both tested polymers. The maximum desorption in this case is in the range of 40–45%. On the other hand, in a mixed system of two polymers, both PAA and PEG desorptions are low, regardless of the desorbing reagent used. This can indicate the greater permanence of mixed adsorption layers in this respect and also indicate the presence of stable PAA + PEG complexes in their structure.
Table 2
Desorption values of the examined polymers from the Y2O3 surface obtained for systems at pH 3, 6 and 9; initial polymer concentration: 100 ppm
System
pH
PAA
PEG
Desorption [%]
HCl
NaOH
HCl
NaOH
Y2O3PAA
3
40.2
25.5
–
–
6
23.6
45.5
–
–
9
33.9
26.2
–
–
Y2O3/PEG
3
–
–
31.5
12.8
6
–
–
45.2
11.4
9
–
–
1.6
2.2
Y2O3/PAA + PEG
3
2.0
1.6
11.1
13.1
6
2.4
0.8
2.4
2.5
9
1.6
2.5
2.2
2.5
Binding mechanism of macromolecules at the solid polymer solution interface in terms of changes in surface charge and zeta potential
The analysis of the changes in the surface charge and the zeta potential of the suspension of particles coated with polymer films, in relation to the suspension of particles dispersed in the solution of the supporting electrolyte provides very valuable information regarding the mechanism of polymer binding on the solid surface. Figures 4 and 5 show the dependencies of surface charge density (σ0) and zeta potential (ζ) as a function of solution pH.
Fig. 4
Surface charge density of Y2O3 as a function of solution pH without and in the presence of the polymer/s; supporting electrolyte concentration: 1 10−2 mol/dm3, initial polymer concentration: 100 ppm
Fig. 5
The zeta potential of Y2O3 particles as a function of solution pH without and in the presence of the polymer/s; supporting electrolyte concentration: 1 10−2 mol/dm3, initial polymer concentration: 100 ppm
×
×
The presence of both polymers causes the decrease in the σ0 parameter over the whole studied pH range and the shift of the pHpzc toward lower pH values (from value 7.6 for suspensions without adsorbates to values 7.4; 7.0 and 6.8, respectively, for suspensions with PEG, PAA and PAA + PEG) (Fig. 4). This is mainly due to the desorption of supporting electrolyte ions from the yttrium(III) oxide surface into the by-surface layer, which is caused by the adsorption of nonionic polymer via oxygen atoms containing free electron pairs. An even greater effect is caused by adsorption of ionic poly(acrylic acid) macromolecules. In this case, the accumulation of negative charges in the by-surface layer of the solution originating from the dissociated carboxyl groups of adsorbed PAA chains, and present in segments located in loop and tail structures, dominates the direct binding of these groups on the solid surface. The former effect results in the decrease in the σ0 value, whereas the latter one—increases this parameter. The significant predominance of the former, results in the decrease in surface charge density in the presence of PAA compared to the system without anionic polymer (Wiśniewska and Szewczuk-Karpisz 2013). On the other hand, the mechanisms of the supporting electrolyte ions desorption and the accumulation of -COO− groups in the by-surface layer of the solution cause the greatest reduction of the solid σ0 value in the mixed adsorbate system (PAA + PEG).
The curves of electrokinetic zeta potential as a function of solution pH obtained for analogous systems, and presented in Fig. 5, show the slightly different course. The isoelectric point (iep) of Y2O3 is at pH 8.1, i.e., its value differs slightly from that of pHpzc (by about 0.5 pH units). This difference can be result of the overlapping of the diffusion parts of the electrical double layers (edl) formed on opposite walls in the solid pores (Skwarek et al. 2014). Nonionic PEG causes the noticeable reduction in the suspension ζ potential in the whole range of studied pH valyes, whereas for anionic PAA, the reduction effect is very large, with the zeta potential taking only negative values. In the presence of both polymers, there is observed the further reduction in the zeta potential of yttrium(III) oxide particles. In the examined systems, the resultant value of the electrokinetic potential depends on the superposition of the following effects: the change in the ionic composition of the diffusion layer due to the desorption of supporting electrolyte ions by the adsorbed macromolecules, the shift of the slipping plane from the surface of the colloidal particle resulting from the presence of polymeric films of considerable thickness, and the accumulation of negative charges coming from the functional groups of the adsorbed PAA macromolecules in the slipping plane area (Szewczuk-Karpisz et al. 2018; M’Pandou and Siffert 1987). All of these effects cause more or less changes in zeta potential values. In the case of PAA, all three effects are present (as in the case of the mixed layer of both polymers). On the other hand, in the case of PEG their adsorption undergoes mainly through the hydrogen bridges formation, in which two electronegative atoms (O…H–O) are involved. This phenomenon is therefore accompanied by the desorption of positively charged ions of the supporting electrolyte and there is a slight reduction in the zeta potential in the presence of PEG (Zhivkov and Hristov 2017).
Changes in Y2O3 suspensions stability due to the presence of polymeric adsorption layers with different structure
Figures 6, 7 and 8 show the changes in system absorbance as a function of time for Y2O3 suspensions without and in the presence of polymer/s obtained at pH 3, 6 and 9. Analysis of these data indicates that the yttrium(III) oxide suspension without adsorbates at pH 3 has the lowest TSI stability coefficient among all the examined systems. The TSI coefficient assumes values in the range from 0 to 100. The higher its value, the less stable the system is (Table 3). This is due to the rather high values of surface charge density and zeta potential, which provide effective electrostatic repulsion between the solid particles. Generalizing, it can be said that at pH 3, the addition of polymers causes the slight deterioration of suspensions stability, at pH 6—it has practically no effect, whereas at pH 9, it slightly improves the stability of the examined systems. In the case of PAA, the decrease in the suspension stability at pH 3 can be due to partial neutralization of the positive charge of Y2O3 particles by adsorbed polymeric coils having only few dissociated carboxyl groups, whereas the increase in system stability at pH 9—from the appearing repulsive electrosteric interactions. In the case of PEG, the observed changes in suspension stability are mainly due to the steric interactions leading to the increase in suspension stability or the formation of polymer bridges between colloidal particles (destabilization through bridging flocculation mechanism) (Ostolska and Wiśniewska 2015). In the mixed adsorbate system, the stability mechanism is more complex and usually results from the overlapping of at least two of the above-mentioned effects. For example, the marked decrease in the Y2O3 suspension stability at pH 3 due to the presence of the mixed PAA + PEG adsorption layers (Fig. 6) is manifested by an increase in the size of the formed aggregates (Table 3), which is visible in the obtained SEM images (Fig. 9). Moreover, the stability conditions improvement after the polymers addition is also proved by the decrease in mean sizes of aggregates obtained at pH 9 using zetasizer Nano ZS apparatus and is presented in Fig. 10. This parameter assumes the following values: for Y2O3 without polymers−1084 nm, for Y2O3 with PAA −342 nm; for Y2O3 with PEG −460 nm and for Y2O3 with PAA + PEG −250 nm.
Fig. 6
Changes in absorbance as a function of time obtained for Y2O3 suspensions without and in the presence of the polymer/s at pH 3; supporting electrolyte concentration: 1 10−2 mol/dm3, initial polymer concentration: 100 ppm
Fig. 7
Changes in absorbance as a function of time obtained for Y2O3 suspensions without and in the presence of the polymer/s at pH 6; supporting electrolyte concentration: 1 10−2 mol/dm3, initial polymer concentration: 100 ppm
Fig. 8
Changes in absorbance as a function of time obtained for Y2O3 suspensions without and in the presence of the polymer/s at pH 9; supporting electrolyte concentration: 1 10−2 mol/dm3, initial polymer concentration: 100 ppm
Table 3
Stability parameters of Y2O3 suspensions without and in the presence of the examined polymers obtained at pH 3, 6 and 9; initial polymer concentration: 100 ppm
System
TSI
Thickness of sediment layer [mm]
Size of aggregates [μm]
Velocity of aggregates sedimentation [μm/min]
pH
3
6
9
3
6
9
3
6
9
3
6
9
Y2O3
61.8
82.2
81.3
0.65
0.85
0.75
0.56
0.78
0.82
41.2
78.1
87.9
+ PAA
99.1
86.2
74.1
0.94
0.89
0.47
1.16
0.91
0.58
173.5
107.7
44.3
+ PEG
67.5
82.6
75.6
0.65
0.88
0.86
0.59
0.56
0.71
46.2
76.9
65.0
+ PAA + PEG
74.6
81.2
73.9
0.73
0.78
0.85
0.87
0.74
0.64
53.1
71.5
52.7
Fig. 9
SEM images obtained for Y2O3 without and with mixed adsorption layers of PAA + PEG at pH 9; supporting electrolyte concentration: 1 10−2 mol/dm3, initial polymer concentration: 100 ppm
Fig. 10
Mean sizes of aggregates of Y2O3 particles without and in the presence of the polymer/s obtained by the use of zetasizer;—Y2O3 without polymers;—Y2O3 with PAA;—Y2O3 with PEG;—Y2O3 with PAA + PEG at pH 9; supporting electrolyte concentration: 1 10−2 mol/dm3, initial polymer concentration: 100 ppm
×
×
×
×
×
Other parameters characterizing the stability of the colloidal system (provided in Table 3) remain in relationship with the certain values of the TSI stability coefficients. For most of the systems, a similar trend was observed, namely the increase in system stability (the decrease in the TSI value) is associated with the smaller thickness of the formed sediment layer, the smaller size of aggregates and the lower velocity of their sedimentation to the bottom of the measuring vial.
Conclusions
The adsorbed amounts of anionic PAA and nonionic PEG on the yttrium(III) oxide surface depend on the solution pH, and its highest value for both polymers was obtained at pH 3 (the maximum sorption capacity for PAA is 4.56 mg/m2, whereas for PEG—3.32 mg/m2). Hydrogen bonds, which are also formed under conditions of electrostatic repulsion of adsorbates with the Y2O3 surface, are primarily responsible for the process of macromolecules binding at the solid-solution interface. In the mixed solution of both polymers there is competition for active sites on the metal oxide surface, which in many systems leads to the decrease in the adsorption of both PAA and PEG. The binding of both polymers (PAA—PEG) complexes (via hydrogen bonds between the oxygen bridges of PEG and the carboxyl groups of PAA) can also be present. The more effective desorbing agent for both polymers in single adsorbate systems is hydrochloric acid (compared to sodium base)—the maximum desorption reaches the value of about 45%, whereas in the mixed adsorbate system, the desorption remains low. The presence of the polymeric adsorption Q3layer causes the slight decrease in the stability of suspensions at pH 3, at pH 6—it has practically no effect on this parameter and at pH 9 it slightly improves the stability of the examined systems. The reduction in the suspensions stability at pH 3 can be caused by the partial neutralization of the positive charge of Y2O3 particles by the adsorbed polymeric coils (only in the presence of PAA) and the formation of polymeric bridges leading to the flocculation process. In turn, improvement of yttrium(III) oxide suspensions stability at pH 9 is the result of steric (for PEG) and electrosteric (for PAA) interactions, resulting in reduction of aggregation tendency for particles covered with polymeric films.
Declarations
Competing interests
On behalf of all authors, I state that there is no conflict of interest.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
Publisher's Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Modification of stability properties of yttrium(III) oxide particles by simultaneous adsorption of poly(acrylic acid) and poly(ethylene glycol) for possible applications in wastewater treatment
Authors
Małgorzata Wiśniewska Karolina Herda Teresa Urban Piotr Nowicki Agnieszka Woszuk
