Simultaneous removal of inorganic and organic pollutants from multicomponent solutions by the use of zeolitic materials obtained from fly ash waste

The used adsorbents: zeolite-carbon composites (Na-X(C), Na-P1(C)) were prepared by the hydrothermal reaction of aqueous sodium hydroxide solution (NaOH) and by-product of coal combustion generated in the Janikowo Thermal Power Plant—high-carbon fly ash (HC FA). Then, pure synthetic zeolites (Na-X, Na-P1) were obtained from the post-reaction waste rich in silicon and aluminum. The last adsorbent investigated in this paper was the precursor of the above-mentioned structures – HC FA (Panek et al., 2017; Wdowin et al., 2014; Franus et al., 2014; Bandura et al., 2021). The textural parameters of porous materials obtained with an ASAP 2020 apparatus (Micromeritics Instrument Corporation, Norcross, GA, USA) are shown in Table 1. The detailed characteristics of composition as well as structural, textural and surface properties of the examined materials were presented in our previous papers (Panek et al., 2021a, 2021b).

All measurements were performed in 0.001 mol/dm³ NaCl solution, which was applied as the supporting electrolyte to ensure the same ionic strength in the investigated samples.

Poly(acrylic acid) (PAA) (Aldrich) with an average molecular weight of 240,000 and the non-steroidal anti-inflammatory drug diclofenac (DCF) (Sigma-Aldrich) were used as anionic adsorbates for the experiments conducted. Both substances have carboxyl groups of weak acidic properties which dissociate with the increasing pH. This is reflected in the pK_a values – 4.15 for DCF and 4.5 for PAA (Jodeh et al., 2016). Lead nitrate(V) and zinc nitrate(V) (Sigma-Aldrich) were used as the source of lead(II) and zinc(II) ions, respectively. Their concentration was 100 ppm. In the desorption study, 0.1 M sodium hydroxide (NaOH) and 0.1 M hydrochloric acid (HCl) were used as desorbing agents.

Adsorption and desorption were carried out in the following systems, analogous for each solid (here on the example of Na-X zeolite): (1) Na-X + DCF, (2) Na-X + DCF + Pb, (3) Na-X + DCF + Zn, (4) Na-X + PAA, (5) Na-X + PAA + Pb, (6) Na-X + PAA + Zn. The adsorbed amounts of Pb/Zn and DCF were studied at 25 °C using a static method, based on the change in specified adsorbate concentration before and after the adsorption process (Jutakridsada et al., 2016). The samples were prepared by adding 0.01 g of the appropriate solid to 10 cm³ solution containing 100 ppm Pb or Zn and 50 ppm PAA or DCF, as well as NaCl as the supporting electrolyte. The same method was used to prepare the samples with the Pb(II) and Zn(II) ions in single systems. In this case 0.003 g of the appropriate porous material was weighed and added to analogous amounts of solutions to obtain final suspensions. After the sample preparation, the pH was adjusted to the value of 5 using NaOH and HCl with concentration of 0.1 mol/dm³. This pH value was selected to avoid the formation of metal hydroxides. Usually, at pH close to 6, the Zn(OH)₂ and Pb(OH)₂ species begin to form. The adsorption process was carried out under constant shaking conditions (Unimax 1010, Heidolph, Schwabach, Germany) for 24 h. After the process completion, the solids were separated from the solutions. In the case of heavy metal ions, this was done by centrifugation (310 b, Precision Mechanics, Poland), while in the case of diclofenac, by paper and syringe filters. The concentration of heavy metals in the obtained supernatants was determined using inductively coupled plasma-optical emission spectrometry (Thermo Scientific iCAPTM 7200 ICP-OES analyzer, Waltham, MA, USA). In turn, the DCF concentration was determined using high-performance liquid chromatography (Ultimate 3000, Dionex) equipped with DAD detector.

In the desorption study, the solids with adsorbed substances were used. Desorption was carried out in the solution of 0.1 mol/dm³ NaOH or 0.1 mol/dm³ HCl for 1 h, under shaking conditions.

Electrokinetic measurements, potentiometric titrations and stability tests were performed in the following systems, analogous for each solid (here on the Na-X zeolite example): (1) Na-X, (2) Na-X + DCF, (3) Na-X + DCF + Pb, (4) Na-X + DCF + Zn, (5) Na-X + PAA, (6) Na-X + PAA + Pb, (7) Na-X + PAA + Zn. In order to determine the isoelectric point (iep) and zeta potential (ζ) of the investigated suspensions, electrophoretic mobility (U_e) measurements were conducted. Initially, a stock suspension was prepared by adding 0.005 g of the appropriate solid to 100 cm³ of a solution containing NaCl and PAA/DCF with concentration 50 ppm or/and Pb²⁺/Zn²⁺ ions with concentration of 10 ppm. Then, the obtained suspension was sonificated for 3 min and divided into 8 samples. After the pH adjustment to the selected value (changing in the range 3–10), the electrokinetic experiments were performed on the samples using Nano ZS zetameter (Malvern Instruments, Cambridge, UK) equipped with immersion dip cell (Oshima, 1994).

On the other hand, in order to determine the point of zero charge (pzc) and the surface charge density (σ₀) of the investigated suspensions, potentiometric titrations were conducted. The automatically controlled kit containing thermostated Teflon vessel, glass and calomel electrodes (Beckman Instruments), PHM 240 pH meter (Radiometer), laboratory stirrer, RE 204 thermostat (Lauda), Dosimat 765 automatic microburette (Metrohm) and computer equipped with the special program "titr_v3" were applied. The program enabled calculation of surface charge density as a function of solution pH, based on the difference in volume of the titrant added to the examined solution and the supporting electrolyte providing the specified pH value (Janusz, 1999). Potentiometric titration studies were conducted in the samples containing 0.03 g of Na-X, 0.075 g of Na-X(C), 0.4 g of Na-P1, 0.2 g of Na-P1(C) or 0.4 g of HC FA in 50 cm³ of solution containing supporting electrolyte. The PAA/DCF concentration was 50 ppm, whereas that of heavy metal ions was 10 ppm.

A spectrophotometric method was applied to determine the stability of the investigated suspensions. Cary 100 UV–VIS spectrophotometer (Varian) was used for this purpose. The samples were obtained by adding 0.025 g of the selected solid to 10 cm³ of solution containing supporting electrolyte, DCF/PAA with concentration 50 ppm and Pb/Zn with concentration 100 ppm. The samples were sonificated for 3 min before the addition of adsorbates. The absorbance was measured over time at wavelength 500 nm (Wiśniewska et al., 2021a, b). Each measurement was conducted for 1 h and the absorbance values were recorded every 2 min.

High-carbon fly ash (HC FA) generated in the combustion of hard coal was the starting material for the synthesis of zeolitic materials. It was composed of silicon (30%), carbon (30%), aluminum (13.5%), iron (8.6%) and small amounts of potassium (2.3%), calcium (3.7%), sulfur (1.2%), magnesium (1.5%), sodium (0.6%) and titanium (1%). The mineral composition of HC FA included quartz, mullite, hematite, unburned carbon and amorphous aluminosilicate glaze created during rapid cooling of ash performed after the coal combustion process. Complex mineral composition as well as the presence of unburned carbon contributed to developed specific surface area (S_BET) of HC FA equal to 46 m²/g.

Zeolite-carbon composites, NaX(C) and NaP1(C), were created as a result of hydrothermal reaction of high-carbon fly ash with aqueous sodium hydroxide solution. For Na-P1(C), 20 kg of HC FA and 90 dm³ of 3 M NaOH were used, the temperature was 90 °C, and the reaction was conducted for 24 h. For Na-P1(C), 25 kg of HC FA and 90 dm³ of 3 M sodium hydroxide solution were applied, the temperature was 80 °C, and the reaction lasted 48 h. The separation of the solid phase after the zeolite-carbon composites synthesis led to the formation of silicon and aluminum-rich liquid waste, which derived from dissolved aluminosilicate glaze of HC FA. This solution was used to obtain pure NaX and NaP1 zeolites in another hydrothermal reaction. For Na-P1, 40 cm³ of waste solution, 10 cm³ of aqueous NaOH solution and 80 g of aluminum foil were used, the temperature was 100 °C, and the process lasted 48 h. For Na-X, 50 cm³ of waste solution, 50 cm³ of aqueous NaOH solution and 450 g of aluminum foil were used, the temperature was 70 °C, and the reaction was conducted for 24 h. The pure zeolite material had a monophasic structure with no ash residues.

The phase composition of zeolitic materials was checked using the XRD (X-ray diffraction) technique. The presence of zeolite NaX phase was confirmed by dhkl = 7.21; 5.05; 4.11; 3.20; 2.91; 2.69; 2.53; 2.38; 1.97 Å for NaX and NaX(C). In turn, the presence of zeolite Na-P1 phase was confirmed by dhkl = 14.44; 8.84; 5.74; 3.81; 3.34; 2.89 Å for Na-P1 and Na-P1(C). Apart from the zeolite phase, the diffraction patterns of Na-X(C) and Na-P1(C) also showed the components constituting the ash residue. They were dominated by mullite (dhkl = 5.39; 3.41; 2.78; 2.58 Å), quartz (dhkl = 4.25; 3.34; 2, 45; 2.28; Å) and amorphous aluminosilicate glaze in the form of a raised background on the diffractogram in the angular range of 15–30° 2Θ.

The chemical composition of zeolitic materials was dominated by silicon (41%, 28%, 49%, 18% for Na-X, Na-X(C), Na-P1, Na-P1(C), respectively), aluminum (23%, 14%, 18%, 10% for Na-X, Na-X(C), Na-P1, Na-P1(C), respectively), sodium (3.5–8%), iron (0.5–9.3%) and small amounts of other ingredients such as magnesium, calcium, potassium, and titanium. In the case of the zeolite-carbon composite, the carbon content was also high (31.5% and 44.5% for Na-X(C) and Na-P1(C), respectively).

The porous nature of zeolitic materials translated into their developed specific surface area, which was equal to 728 m²/g, 272 m²/g, 27 m²/g and 70 m²/g for Na-X, Na-X(C), Na-P1 and Na-P1(C), respectively (Table 1).

Figures 1 a, b present the adsorbed amounts of Pb(II) and Zn(II) ions with and without DCF and PAA. In the case of zeolites and their composites, the adsorption of Pb(II) ions is significantly reduced in the presence of organic compounds. It is slightly lower in the systems with PAA than in the systems with DCF. In turn, the adsorption of Pb(II) on the carbon precursor is reduced in the system containing DCF, whereas the presence of PAA results in a slight increase in the Pb(II) adsorbed amount. Similarly, the adsorption of zinc(II) ions on zeolites and their composites, but also on the carbon precursor, decreases with the addition of organic compounds to the system. In the case of Na-X and Na-P1, the decrease in the adsorbed amount of ions is slightly greater in the presence of PAA than in the presence of DCF. On the other hand, in the case of Na-X(C), Na-P1(C) and HC FA, the adsorption of Zn(II) ions in the presence of PAA is greater than after the addition of DCF. All above changes may result from different mechanisms of adsorption and accompanying phenomena. In such complex systems, both competition for adsorbent active sites and the formation of complexes between adsorbed molecules and ions occur certainly. As was mentioned above, DCF and PAA are characterized by anionic nature, whereas the other adsorbate in the mixed system are in the form of divalent metal cations. Under the conditions in which the adsorption study is carried out (pH 5), the solids assume positive charge, which favors DCF/PAA adsorption and limit Pb(II)/Zn(II) binding. In the examined mixed systems, the formation of complexes between two types of adsorbate with opposite ionic character can also be present. This phenomenon contributes to the reduction of the adsorption of heavy metal ions. Moreover, the adsorption of big organic molecules and small inorganic ions can lead to the blockade of solid active sites by the first ones. Then, the entrance to the pores is inaccessible to heavy metal ions. The radius of hydrated Pb ions is 0.401 nm, and the radius of hydrated Zn ions is 0.430 nm (Oter and Akcay, 2007). On the other hand, the hydrodynamic radius (r_h) of PAA is about 2.7 nm (Wiśniewska et al., 2013a), and the r_h of most drugs is in the range of 0.5–5 nm (Axpe et al., 2019). The blockade of ions can occur during the adsorption of organic substance-metal complexes – both of intramolecular and intermolecular structure The affinity of these complexes to the surface is not as high as that of organic molecules. Heavy metal ions neutralize negative charge of DCF or PAA and, as a consequence, the complexes formed are not attracted to the surface of zeolitic materials effectively.

The measured adsorbed amounts of both inorganic and organic pollutants on zeolitic materials can be considered satisfactory. In the single and multicomponent systems, they were higher than those determined for other adsorbents. The comparison of adsorption capacities of HC FA, prepared zeolitic materials and other adsorbents described in the literature was made in the form of (Table 2).

Percentage desorption of pollutants caused by HCl and NaOH in the examined systems containing lead(II) and zinc(II) ions with and without DCF or PAA is shown in Figs. 2a and 2b. Such desorbing agents are the most commonly used substances for adsorbent regeneration. The previous studies indicated that the desorption of inorganic and organic pollutants from different adsorbents using water was minimal (Szewczuk-Karpisz et al., 2022; Tomczyk and Szewczuk-Karpisz, 2022). In the examined systems, considerably higher desorption occurs when HCl is used as the desorbing agent. Such tendency is observed for both single and mixed solutions with organic compounds. In most systems, greater desorption occurs in the samples in which adsorption was rather small, that is, in those in which the binding of metal ions to the surface of porous materials is weak. The suspensions containing Zn(II) ions are characterized by higher desorption in comparison to that occurred in analogous systems with Pb(II) ions.

The comparison of adsorbed amounts of diclofenac with and without Pb(II) and Zn(II) ions is presented in Fig. 1c. In the systems containing Na-X and Na-X(C), the addition of Pb(II) and Zn(II) ions noticeably increases the adsorption of the drug molecules. For both materials, the presence of zinc(II) increases the adsorption of zinc(II) to greater extent than lead(II). In contrast, in the systems containing HC FA, Na-P1 and Na-P1(C), the addition of heavy metals causes decrease in the DCF adsorption. In the case of Na-P1 and its composite, this decrease is more visible with Pb(II) ions than with Zn(II) ions, whereas in the case of HC FA, this trend is opposite. As it was mentioned in the previous section, the adsorption of pollutants in their mixed systems is driven by various factors. The favored binding of DCF in the systems containing heavy metal ions may be caused by electrostatic attraction between them and the solid surface as well as by the formation of DCF-Pb(II) or DCF-Zn(II) complexes. In the examined solutions, the creation of intermolecular complexes, in which one divalent cation interacts with two drug molecules, is more probable. The hydrophobic forces and hydrogen bonds can also be responsible for diclofenac adsorption on the surface of zeolitic materials (Li et al., 2021). The slight decrease in adsorption due to the addition of heavy metals to the systems containing Na-P1, Na-P1(C) and HC FA may also be related to the formation of complexes. The mean pore sizes of these solids are in the range of 5.7–6.9 nm (Table 1), which allows effective adsorption of DCFs with a size of 0.97 × 0.98 nm and area of 0.52 nm² (Sotelo et al., 2014). The intermolecular complexes of DCF with Pb(II) or Zn(II) ions can be too large to effectively penetrate the pores.

The comparison of percentage desorption of diclofenac caused by hydrochloric acid and sodium hydroxide from the surface of the examined materials with and without heavy metal ions is given in Fig. 2c. In the single systems of adsorbates, more efficient desorption occurs with NaOH, whereas in the mixed ones, HCl proves to be the preferable desorbing agent. This tendency is not observed in the systems containing Na-X and lead(II) ions as well as in the systems of Na-P1(C) or HC FA with zinc(II) ions. In these 3 cases, slightly more efficient desorption takes place in the NaOH presence. In the single systems, where diclofenac is the only adsorbate, NaOH is a better desorbent due to higher affinity of the base for the acidic carboxyl groups present in the drug molecule. In the mixed systems, this affinity is not so significant due to the neutralization of carboxyl groups by heavy metal ions.

The effect of organic compounds and heavy metal cations on surface charge density (σ₀) of Na-P1(C) is presented in Figs. 3a and 3b. Similar tendencies were obtained for all investigated solids. The results of the potentiometric titration are presented as a function of solution pH value. These curves allowed determination of the points of zero charge (pzc) of the examined solids. The pzc parameter indicates the pH value, at which the concentrations of positively and negatively charged surface groups are the same (σ₀ = 0). At pH > pzc, the solid surface charge is negative, and at pH < pzc, it is positive. Under the conditions in which adsorption is conducted (pH 5), the investigated solid assumes positive sign of charge. This means that the adsorption of both diclofenac and poly(acrylic acid) is favored electrostatically due to their anionic character. The opposite tendency occurs for the cations of lead(II) and zinc(II), which are repelled from positively charged surface of zeolitic materials.

The addition of DCF and PAA decreases surface charge density of the solids, which is also accompanied by lowering of the pzc value. The points of zero charge of the examined suspensions are summarized in Table 3. In the case of the Na-P1 zeolite, there is a decrease from a value of 9.5 to 8.6 after the addition of DCF and to 7.3 after the addition of PAA. Furthermore, the addition of Pb(II) or Zn(II) ions to the Na-P1 suspension results in even greater decrease in this parameter. For the system of DCF and Pb(II), pzc is equal to 8.5, for the system of DCF and Zn(II), 8.3, for the system of PAA and Pb(II), 7.1, whereas for the system of PAA and Zn(II), 6.9. This is most likely caused by the combination of effects accompanying the adsorption of metal cations and anionic macromolecular compounds. The described above complexes’ formation is very important here (Wiśniewska et al., 2021a, b). The addition of zinc(II) or lead(II) ions alone usually decreases surface charge density of the solids. This is due to the interaction of these cations with hydroxyl groups of zeolitic materials resulting in the formation of additional number of negatively charged surface groups (Fijałkowska et al., 2019, 2020a, 2020b; Skwarek et al., 2014). But, in the case of adsorption of large anionic molecules including long-chain polymers or drug molecules, the decrease in surface charge density is related to the existence of negatively charged groups belonging to PAA or DCF in the near-surface layer of the solution. They are not directly bound to the surface of porous materials and thus other interactions, besides electrostatic forces, can affect their adsorption (Nibou et al., 2010). In summary, the effect of adsorbate on surface charge density of the examined solids is strictly dependent on the adsorbate type, i.e., ionic character and molecular size, as well as it is closely associated with the formation of adsorbates complexes.

The changes in zeta potential (ζ) of the Na-P1 zeolite as a function of solution pH value in the single systems of DCF or PAA and the mixed ones containing also Pb(II) or Zn(II) ions are presented in Figs. 4a and 4b. All tendencies discussed in the systems of Na-P1 are similar to those occurring for other investigated adsorbents. For Na-P1, it is impossible to determine the isoelectric point (iep). This parameter indicates the pH value, at which the concentrations of positively and negatively charged ions in the area of slipping plane are the same (ζ = 0). The zeta potential of the Na-P1 particles assumes only negative values, that is, negatively charged groups prevail in the slipping plane area. An increase in zeta potential is observed after the DCF addition to the system (in the pH range of 3–7). The addition of Pb or Zn cations to the Na-P1 suspension induced smaller increase in zeta potential than DCF. In contrast, the addition of PAA as well as PAA together with Pb(II) or Zn(II) decreases the zeta potential values. This decrease is smaller in the mixed systems containing also metal cations. All noted changes in the ζ parameter are associated with the adsorbates’ complexation described in the previous subchapters. The passage of electrolyte counter ions (Na⁺) from the surface layer to the diffusion part of the electrical double layer (edl) is responsible for the increase of the zeta potential after the DCF addition. In contrast, the decrease caused by the addition of PAA is related to the presence of negatively charged carboxyl groups of the polymer in the slipping plane area. The addition of PAA reduce the zeta potential also due to the shift of slipping plane area from the solid by adsorbed macromolecules (Wiśniewska et al., 2015; M’Pandou and Siffert, 1987).

Electrokinetic potential is also considered as parameter determining the stability of colloidal systems. A stable suspension has an absolute ζ value in excess of 30 mV. Nevertheless, this rule refers only to the systems containing small inorganic ions.

The stability of suspensions is directly related to the structure of electrical double layer. It varies according to the proportion of ions present in the specific layers (of Stern, Helmholtz etc.). The stability of the examined suspensions was investigated by spectrophotometric method. Figures 5a, b, c, d, e present the absorbance over tome for the systems of HC FA, Na-X, Na-X(C), Na-P1 and Na-P1(C) with and without adsorbates. At the pH close to pzc, the systems exhibit decreasing stability, which is visible in the absorbance curves. Their analysis indicated that in most cases the stability decreases after the addition of adsorbates. The most stable suspension without additives is noted for the Na-P1(C) composite (Fig. 5e). The stability deteriorates considerably in the systems containing PAA + Zn(II) and DCF + Zn(II) (especially over time). For the other systems investigated, the stability decreases with the addition of adsorbates, although a slight increase is sometimes observed in the systems containing only DCF or DCF with metals. Various effects can occur in such complex solutions. Polymer bridges can be formed when one long chain of the polymer adsorbs onto at least two solid particles. This phenomenon is called flocculation and leads to formation of sedimenting aggregates (Gelardi and Flatt, 2016). The segments of polymer chain create ‘loops’ and ‘tails’ on the solid surface, and the bridging is possible when their length exceeds the range of electrostatic repulsive forces occurring between the particles. Additionally, the surface cannot be entirely covered by the adsorbed polymer. In the mixed adsorption layers, the steric and electrosteric mechanisms are mainly responsible for the suspension stabilization (Wiśniewska et al., 2013b). The most unstable system proved to be the one with Na-P1 zeolite without and with the addition of heavy metals and/or diclofenac or poly (acrylic acid). The lack of adequate stabilization of suspensions results in more efficient separation of the solid with the adsorbates from the solution. This phenomenon was observed in the examined systems and thus it can be considered that the zeolites and its carbon composites form easily separable suspensions (Wiśniewska et al., 2021a, b; Riddick, 1968).