Comparison of Physicochemical Properties of Fly Ash Precursor, Na-P1(C) Zeolite–Carbon Composite and Na-P1 Zeolite—Adsorption Affinity to Divalent Pb and Zn Cations

Considering the growing needs of environmental remediation, new effective solutions should be sought. Therefore, the adsorbed amounts of heavy metal ions, such as lead(II) and zinc(II), on the surface of high-carbon fly ash (HiC FA), zeolite-–carbon composite (Na-P1(C)) and pure zeolite (Na-P1), were investigated. The applied solids were characterized using the following techniques: XRD, SEM-EDS, TEM, porosimetry, SLS, electrophoresis and potentiometric titration. The heavy metal concentration in the probes was determined by applying ICP-OES spectroscopy. Adsorption/desorption and electrokinetic measurements were performed in the systems containing one or two adsorbates. The obtained results indicated that Pb(II) ions are adsorbed in larger amounts on the investigated solid surface due to the molecular sieving effect. The largest adsorption capacity relative to lead(II) ions was observed for pure Na-P1 zeolite (407 mg/g). The simultaneous presence of Pb(II) + Zn(II) mixed adsorbates minimally affects the amount of adsorbed Pb(II) ions and causes a significant decrease of Zn(II) ion adsorption (in comparison with analogous systems containing single adsorbates). It was also shown that all solids can be efficiently regenerated using hydrochloric acid. Thus, the selected pure zeolite can be successfully applied in soil remediation or other purifying technologies as an effective Pb(II) adsorbent.


Introduction
Human activities and their interactions with the environment, such as mining, oil and gas quarrying, have a significant impact on the progressive destruction and pollution of natural ecosystems [1]. Increased soil exploitation related to attempts to provide food for the still significantly increasing world population led to soil degradation [2,3]. Therefore, it is important to look for as many solutions as possible to minimize the results of adverse processes occurring in soil. Organic amendments, such as livestock manure, biosolids, pulp and paper mill by-products, etc. [4], have been used successfully. Biochar amendments, green waste compost and triple superphosphate (TSP) as well as clay minerals, such as kaolinite [5], were applied to reduce the bioavailability of heavy metals in soils [6,7]. Another way to improve soil quality is zeolites, which have been successfully used as an additive for the composting of organic solid waste. This allowed for a shorter composting period, a reduction in greenhouse gas and ammonia emissions, as well as a reduction in the total amount and bioavailability of heavy metals. The use of zeolite-modified compost contributed to better yields, water retention and reduced loss of nutrients [8].
The preparation of the materials was started by obtaining the Na-P1(C) zeolite-carbon composite. Fly ash, the starting material from the Janikowo Thermal Power Plant resulting from the conventional combustion of hard coal, was used in this procedure. Due to its electromagnetic separation, the obtained high-carbon product (HiC FA) could be used to produce a carbon-zeolite composite. The final product was created in a typical zeolitization process based on the hydrothermal reaction of fly ash with an aqueous solution of sodium hydroxide [29,30].
The Na-P1(C) material was prepared on a technological line for the synthesis of ash zeolites. At the first stage, 20 kg of high-carbon ash was mixed with 90 dm 3 of 3 M NaOH solution. Then the whole mixture was stirred with a mechanical stirrer at a temperature of 90 • C for 24 h. At the final stage the obtained product was washed, rinsed and dried at 105 • C. This reaction also resulted in the production of a waste solution rich in silicon and aluminum coming from the dissolution of the ash aluminosilicate glaze and sodium coming from sodium hydroxide [31,32]. This waste was a substrate for the synthesis of the second type of material, which was pure Na-P1 zeolite without ash residue. It was prepared on a technological scale using the following steps: 40 dm 3 of waste solution was mixed with 10 dm 3 of 2 M aqueous NaOH solution, where 80 g of aluminum foil was dissolved. The mixture was left for 48 h at 100 • C. In the next stage, the obtained product was filtered, washed and dried at a temperature of 105 • C. The materials prepared in this way were characterized in terms of mineralogy, structure and texture.

Methods
The solids characterization was performed as follows. The phase composition of the solids was characterized by X-ray diffraction in the Panalytical XPert Pro MPD(Eindhoven, The Netherlands) apparatus with a copper lamp (CuK α = 1.54178 Å). The angular range was 5-65 • 2Θ with a step of 0.02 • 2θ lasting 5 s. The X'Pert Highscore software ver. 4.1 (Almelo, The Netherlands) was used to process the diffraction data. The mineral phases were identified using the PDF-2 release 2010 database formalized by JCPDS-ICDD.
The Panalytical Epsilon 3 spectrometer, equipped with an X-ray tube Rh 9 W, 50 kV, 1 mA, 4096 channel spectrum analyzer and a high-resolution semiconductor SDD detector cooled by a Peltier cell, was used to determine the elemental composition. The obtained results considered the LOI (loss of ignition).
The Malvern Mastersizer 3000 apparatus allowed the estimation of the grain size through the phenomenon of laser diffraction. The experiment was carried out in the aquatic environment in the HYDRO EV attachment.
The FEI Quanta 250 FEG scanning electron microscope equipped with an EDS attachment from the EDAX company (Mahwah, NJ, USA) allowed the morphological analysis. The experiment was carried out on the samples sprayed with a conductive carbon layer at an accelerating voltage of 15 keV.
The HR/TEM analysis of the Na-P1 sample was conducted on an electron microscope, Titan G2 60-300 kV (FEI Company, Hillsboro, OR, USA), applying an accelerating voltage of the electron beam equal to 300 kV.
The ASAP apparatus supplied by Micromeritics Instrument Corporation was used to measure textural parameters (the test was carried out using a low-temperature nitrogen adsorption/desorption isotherm in liquid nitrogen at a temperature of 77 K (−194.85 • C) in the range of relative pressures p/p 0 ranging from 1.5 × 10 −7 to 0.99). In turn, the analysis of the shape of nitrogen vapor adsorption isotherms was characterized by IUPAC classification [33,34]. Samples for N 2 adsorption were prepared by degassing at 300 • C for  12 h under reduced pressure in the degassing port. Then the samples were degassed at the analytical port just before analysis for 4 h at 300 • C.
The adsorbed amount determination of heavy metal ions on the surface of HiC FA, Na-P1 and Na-P1(C) was based on a static method and the following equation (Equation (1)) [35]: where c ads -the heavy metal adsorbed concentration (the difference in the heavy metal concentration in the system before and after its adsorption), V-the suspension volume, and m-the solid weight. The samples were prepared using 0.003 g of HiC FA, Na-P1 or Na-P1(C), which was added to the solutions containing the supporting electrolyte (0.001 M NaCl) and the selected heavy metal ions with the concentrations of 10, 50, 70, 100, 150 and 200 ppm (total volume of solution was 10 cm 3 ). In the case of mixed adsorbate systems, the concentration of both ions was 100 ppm. The pH of all samples was set to the value of 5 using 0.1 M HCl, 0.1 M NaOH and a pH meter supplied by Beckman (Brea, CA, USA). After the adsorption process, which lasted 3 h, the solid was separated from the solution by paper filters. The time of the adsorption process was established based on the kinetics study. The concentration of ions in supernatants was determined using optical emission spectrometry with inductively coupled plasma (Thermo Scientific iCAP™ 7200 ICP-OES analyzer). Based on the obtained results, the adsorption isotherms of Zn(II) and Pb(II) on the surface of HiC FA, Na-P1 and Na-P1(C) were prepared. The experimental data were fitted to the selected theoretical models, i.e., Langmuir (Equation (2)) and Freundlich (Equation (3)) [36,37]: where K F and K L -the Freundlich [mg/g (mg/L) −1/nF ] and Langmuir [dm 3 /mg] parameters, respectively, q e -the equilibrium adsorption capacity [mg/g], C e -the equilibrium liquid phase concentration [mg/dm 3 ], q m -the maximum adsorption capacity in a Langmuir model [mg/g], and n-the Freundlich constant related to adsorption intensity. The samples for adsorption kinetics study were prepared by adding 0.003 g of the selected adsorbent to 10 cm 3 of solutions containing the supporting electrolyte (0.001 M NaCl) and heavy metal ions with a concentration of 100 ppm. Then the pH was adjusted to the value of 5 and adsorption was performed for 10, 30, 60, 90, 120 and 180 min. After that step, the solids were separated by paper filters and the concentration of heavy metal ions in supernatants was determined by the ICP-OES technique. The modeling of the results was made using the equations of pseudo-first order (Equation (4)) and pseudo-second order (Equation (5)) [38][39][40][41]: where q e -the adsorbed amount at equilibrium [mg/g], q t -the adsorbed amount after time 't' [mg/g], and k 1 [1/min] and k 2 [g/mg·min]-the equilibrium rate constants. Desorption measurements began with the preparation of samples containing a supporting electrolyte (0.001 M NaCl), 0.003 g of HiC FA, Na-P1 or Na-P1(C) and heavy metal ions at concentrations of 100 ppm (total volume of solution was 10 cm 3 ). Then, the pH value was adjusted to 5 and the adsorption was carried out for 3 h. The solid was separated from the solution by paper filters and then 10 cm 3 of 0.1 M HCl or NaCl was added to the separated solids. The desorption process lasted 1 h, after which the solid was separated again using paper filters and the concentration of ions in supernatants was determined where C d -the amount of desorbed heavy metal ions, and C a -the amount of absorbed heavy metal ions. Potentiometric titration was used to determine the surface charge density (σ 0 ) of high-carbon fly ash, zeolite and its carbon composite, without and with adsorbates. The values of this parameter, as a function of solution pH, were calculated by the difference between the volumes of titrant added to the suspension and the supporting electrolyte solution (with a defined pH value) using the computer program "titr_v3" and the following equation (Equation (7)) [42]: where c b -the base concentration, F-the Faraday constant, m-the solid mass in the suspension, S-the specific surface area of the solid, ∆V-the difference in the volume of base that must be added to adjust the pH of the suspension and supporting electrolyte to the specified value. The titration set consisted of a Teflon vessel containing the solution connected to an RE 204 thermostat (Lauda), an automated microburet Dosimat 765 (Metrohm) and a computer, glass and calomel electrodes (Beckman Instruments) and a PHM 240 pH meter (Radiometer) controlling the pH values. The examined suspensions were prepared by adding 0.4 g of HiC FA, 0.4 g of Na-P1 or 0.02 g of Na-P1(C) to 50 cm 3 of supporting electrolyte (0.001 M NaCl). The samples were titrated with 0.1 M NaCl solution in the pH range changing from 3 to 11. One by one, the supporting electrolyte itself, suspensions without adsorbates and suspensions with one or two adsorbates were titrated. The heavy metal ion concentration was 10 ppm.
Measurements of the electrophoretic mobility (u e ) were performed to determine the zeta potential (ζ) of the particles, without and with all adsorbates, using the Henry equation (Equation (8)) [43]: where ε-the dielectric constant, ε 0 -the electric permeability of vacuum, ζ-the zeta potential, η-the solution viscosity, and f [κα]-the Henry function. The suspensions were prepared by adding 0.01 g of HiC FA, Na-P1 or Na-P1(C) to 200 cm 3 of the supporting electrolyte solution without and with one or two heavy metal ions, and then sonicated for 3 min. Next, the solution was divided into parts, and the pH of each of them was adjusted to a specific pH value (ranging from 3 to 10). The electrophoretic mobility measurements were carried out at an ion concentration of 10 ppm using Nano ZS Zetameter (Malvern Instruments).

Characteristics of Adsorbents
The chemical composition of high-carbon fly ash, carbon-zeolite composite and pure zeolite is shown in Table 1.
The chemical composition of all adsorbents is dominated by silicon (29.72%, 18.05% and 49.8% for HiC FA, Na-P1(C) and Na-P1, respectively) and aluminum (14.11%, 10.07%; 18.05% for HiC FA, Na-P1(C) and Na-P1, respectively). In addition, there are negligible amounts of CaO (0.31-3.67%), Fe 2 O 3 (0.51-9.12%) and Na 2 O (0.61-8.11%). Fly ash and composite are also characterized by a high carbon content, ranging from 29.65% to 44.49%. Table 2 presents the distribution of individual grain fractions of all adsorbents.  One clearly dominant fraction was not observed in high-carbon fly ash, whereas the three main fractions 20-50, 50-100 and 100-250 µm, amounting to 27.0, 28.6 and 26.4%, respectively, were noted. The remaining fractions (equal to over 17%) are 2-20 µm (13.7%) and 250-500 µm (4.1%). The Na-P1 material is dominated by the 20-50 µm fraction, the volume content of which is 39.6%. The second is the 2-20 µm fraction (25.2%). The total volume of the particles from the particle size range of 2-100 µm is 78.1%. Obviously, particles with a diameter greater than 500 µm (3.6%) appear, which is most likely due to the fact that particles with smaller diameters agglomerate. This is typical for this type of material. In the case of the Na-P1(C) material, the total content of the 2-100 µm fraction is 78.3%, which is almost the same as for Na-P1 zeolite. However, in this case fractions 2-20, 20-50 and 50-100 µm are distributed almost evenly (25.3%, 24.7% and 28.4%, respectively) and thus, similarly to fly ash, it is difficult to unambiguously indicate the most dominant one. Additionally, the presence of particles with a diameter of 100-250 µm (18.3%) is observed, which is probably the result of the deposition of single zeolite particles on the carbon fragments, naturally increasing their size. Figure 1 presents the curves of particle size distribution obtained for the examined adsorbents.
The particle size distribution curve obtained for the HiC FA fly ash was monomodal with a very wide peak with a maximum diameter of about 80 µm and height equal to 6.8%. The particle size distribution curves for Na-P1 and Na-P1(C) were bimodal, with easily noticeable differences in peak maxima. In the case of Na-P1, a clear maximum was observed for particles with a diameter of approx. 30 µm and a height of about 7.5%. The second maximum was noted for particles with a diameter of 400 µm (1.8%). This is consistent with the observations made for individual factions. In contrast, for the Na-P1(C) material, the maximum of the main peak was observed for particles with a diameter of about 70 µm. This peak was about 6.8%. Importantly, the peak maximum coincides perfectly with the one observed for the HiC FA fly ash (the raw material from which Na-P1(C) was obtained). The maximum of the smaller peak was near particles with a diameter of 2.5 µm. It was 1.8%, similarly to Na-P1. It can also be observed that the main peak is broad and not symmetrical, so that the smaller peak is not perfectly separated. Again, this perfectly reflects the fraction-specific observation indicating the absence of a dominant fraction (very broad main peak). The particle size distribution curve obtained for the HiC FA fly ash was monomodal with a very wide peak with a maximum diameter of about 80 μm and height equal to 6.8%. The particle size distribution curves for Na-P1 and Na-P1(C) were bimodal, with easily noticeable differences in peak maxima. In the case of Na-P1, a clear maximum was observed for particles with a diameter of approx. 30 μm and a height of about 7.5%. The second maximum was noted for particles with a diameter of 400 μm (1.8%). This is consistent with the observations made for individual factions. In contrast, for the Na-P1(C) material, the maximum of the main peak was observed for particles with a diameter of about 70 μm. This peak was about 6.8%. Importantly, the peak maximum coincides perfectly with the one observed for the HiC FA fly ash (the raw material from which Na-P1(C) was obtained). The maximum of the smaller peak was near particles with a diameter of 2.5 μm. It was 1.8%, similarly to Na-P1. It can also be observed that the main peak is broad and not symmetrical, so that the smaller peak is not perfectly separated. Again, this perfectly reflects the fraction-specific observation indicating the absence of a dominant fraction (very broad main peak).
Morphological analyses of high-carbon fly ash are presented in Figure 2. Figure 3 shows the morphology of the Na-P1(C), whereas Figure 4, that of Na-P1.  Morphological analyses of high-carbon fly ash are presented in Figure 2. Figure 3 shows the morphology of the Na-P1(C), whereas Figure 4, that of Na-P1. The particle size distribution curve obtained for the HiC FA fly ash was monomodal with a very wide peak with a maximum diameter of about 80 μm and height equal to 6.8%. The particle size distribution curves for Na-P1 and Na-P1(C) were bimodal, with easily noticeable differences in peak maxima. In the case of Na-P1, a clear maximum was observed for particles with a diameter of approx. 30 μm and a height of about 7.5%. The second maximum was noted for particles with a diameter of 400 μm (1.8%). This is consistent with the observations made for individual factions. In contrast, for the Na-P1(C) material, the maximum of the main peak was observed for particles with a diameter of about 70 μm. This peak was about 6.8%. Importantly, the peak maximum coincides perfectly with the one observed for the HiC FA fly ash (the raw material from which Na-P1(C) was obtained). The maximum of the smaller peak was near particles with a diameter of 2.5 μm. It was 1.8%, similarly to Na-P1. It can also be observed that the main peak is broad and not symmetrical, so that the smaller peak is not perfectly separated. Again, this perfectly reflects the fraction-specific observation indicating the absence of a dominant fraction (very broad main peak).
Morphological analyses of high-carbon fly ash are presented in Figure 2. Figure 3 shows the morphology of the Na-P1(C), whereas Figure 4, that of Na-P1.  SEM micrographs of HiC FA show spherical structures of aluminosilicate glaze, irregular forms of quartz and mullite, unburned carbon fragments and carbon sinters with embedded aluminosilicate glaze spheres. In the SEM micrographs of Na-P1(C), zeolite crystals are very well formed, creating lamellar aggregates with diameters ranging from 2 to 5 µm. These crystals are formed in the carbon ash zone as well as on fragments of aluminosilicate glaze. They have the characteristics of single clusters or mutually overgrown aggregates. Figure 3 shows the chemical analysis performed using EDS on the zeolite crystal (point 1) and the carbon fragment (point 2). In the first case, the dominant elements were aluminum, silicon, sodium, magnesium and oxygen, which are part of the zeolite, whereas in the second case, chemical analysis showed the presence of unburned carbon on which zeolite crystals were formed. In addition, some part of the carbon in the sample also came from its preparation (carbon sputtering). As was mentioned above, Figure 4 shows the morphology of pure Na-P1 zeolite (with no ash residue). The zeolite crystals are very well formed, ranging in size from 6 µm to 12 µm. This picture also shows the EDS analysis performed on zeolite crystals (points 1 and 2). In both cases, the dominant elements were silicon, aluminum, sodium and oxygen, and small amounts of calcium, potassium and magnesium. The presence of carbon was associated with the sample preparation. To obtain more information about the Na-P1 structure, HR/STEM images were taken (Figure 5a,b).     tals are very well formed, ranging in size from 6 μm to 12 μm. This picture also shows the EDS analysis performed on zeolite crystals (points 1 and 2). In both cases, the dominant elements were silicon, aluminum, sodium and oxygen, and small amounts of calcium, potassium and magnesium. The presence of carbon was associated with the sample preparation. To obtain more information about the Na-P1 structure, HR/STEM images were taken (Figure 5a,b).  Figure 5a shows a fragment of a larger zeolite aggregate about 5 μm in size. On the other hand, Figure 5b shows a cross section of the zeolite crystal. In the microarea, one can observe pores as brighter places and their walls as darker ones. The structure of the analyzed material is characterized by well-ordered, cylindrical and regular micropores.
Diffractograms of the phase composition of high-carbon fly ash, Na-P1(C) composite and pure Na-P1 zeolite are shown in Figure 6.  Figure 5a shows a fragment of a larger zeolite aggregate about 5 µm in size. On the other hand, Figure 5b shows a cross section of the zeolite crystal. In the microarea, one can observe pores as brighter places and their walls as darker ones. The structure of the analyzed material is characterized by well-ordered, cylindrical and regular micropores.
Diffractograms of the phase composition of high-carbon fly ash, Na-P1(C) composite and pure Na-P1 zeolite are shown in Figure 6.   Textural parameters (S BET -specific surface area, S micro -micropore area, V t -total pore volume, V micro -micropore volume, and D-average pore diameter) of high-carbon fly ash, zeolite and zeolite-carbon composite are presented in Table 3. These results indicated that all adsorbents had a poorly developed surface. They contained mesopores of average diameter in the range 5.66-6.93 nm. Figure 7 shows the nitrogen adsorption-desorption isotherms at liquid nitrogen temperature of the studied materials. It can be observed that the Na-P1(C) material is characterized by the highest nitrogen adsorption, which is confirmed by the textural parameters of these materials presented in Table 3. These results indicated that all adsorbents had a poorly developed surface. They contained mesopores of average diameter in the range 5.66-6.93 nm. Figure 7 shows the nitrogen adsorption-desorption isotherms at liquid nitrogen temperature of the studied materials. It can be observed that the Na-P1(C) material is characterized by the highest nitrogen adsorption, which is confirmed by the textural parameters of these materials presented in Table 3.   Table 4 the kinetics and isotherm parameters are presented.    Table 4 the kinetics and isotherm parameters are presented.   Table 4 the kinetics and isotherm parameters are presented.   The performed measurements allowed us to determine the time after which the amount of adsorbate on the solid surface did not change. As can be seen in Figure 8a,b, this time for the fly ash, zeolite and composite differs. Among the systems containing Pb(II) ions, the equilibrium was established the fastest in the case of Na-P1, i.e., after 30 min. For Na-P1(C) and HiC FA, this time was equal to 60 min. On the other hand, among the system containing Zn(II) ions, the equilibrium was reached after 120 min in all the studied systems. Thus, it can be concluded that in systems containing Pb(II) ions, the time required to reach equilibrium is shorter than in the case of systems containing Zn(II) ions.

Adsorption Capacity of HiC FA, Zeolite-Carbon Composite and Pure Zeolite Relative to Zn(II)/Pb(II) Ions in the Single Adsorbate Systems
The observed adsorption capacities varied between selected solids, as can be seen in Figure 8c,d. Pb(II) cations are apparently better absorbed on fly ash, zeolite and its carbon composite than Zn(II) cations. The adsorbed amount of Pb(II) on the Na-P1 exceeds twice the adsorbed amount of this element on the Na-P1(C) surface, and 2.5 times the adsorbed amount on the HiC FA one. For an initial Pb(II) concentration of 150 ppm, the amount of adsorbed heavy metal is 407.34 mg/g for Na-P1, 205.00 mg/g for Na-P1(C) and 78.67 mg/g for HiC FA. Zn(II) is absorbed less efficiently. The difference in adsorption level between Zn(II) and Pb(II) cations is best seen in the case of Na-P1, where the amount of the adsorbed Pb(II) is 3.5 times larger than Zn(II). For an initial Pb(II) concentration of 150 ppm, the amount of absorbed Pb(II) ions is 407.34 mg/g, whereas the amount of absorbed Zn(II) ions is 119.33 mg/g. The significantly higher adsorption of Pb(II), compared to Zn(II), can be explained by the different affinity of these ions for zeolite, which is the result of various radii and energies of hydration of these ions. The molecular sieve effect, which is characteristic of systems containing synthetic zeolites, can explain this phenomenon. It assumes that the smaller the radius of the hydrated cation and the higher its hydration energy are, the more difficult ion penetration into the zeolite pores is. As a result, the participation of the cation in ion exchange is prevented. In the systems studied, Pb(II) has a much larger radius of the hydrated ion, which simplifies its diffusion into the pores of the zeolite and effective ion exchange with, e.g., Mg and Ca cations. As a result, a considerable increase in Pb(II) adsorption is observed [44,45].
The experimental kinetics parameters were better fitted to the pseudo-second-order model. This model assumes that the rate-controlling step is a chemical reaction-exchange or sharing of electrons between heavy metal ions and adsorbents, resulting in the formation of chemical bonds (covalent or ionic) between them. So, the Zn(II) and Pb(II) adsorption involved chemisorption. The good fit is indicated by the value of the squared correlation coefficient R 2 , shown in Table 4 [46]. Analysis of theoretical and experimental data led to the conclusion that the adsorption of both heavy metal ions is slightly better fitted to the Langmuir model than to the Freundlich one. Thus, in the systems, a monolayer of uniform energy is formed [47,48]. The analysis of the obtained results showed that the simultaneous presence of Pb(II) and Zn(II) ions did not significantly affect the time of establishing the adsorption equilibrium in the studied systems. Furthermore, the amount of adsorbed Pb(II) ions on the HiC FA, Na-P1, Na-P1(C) surfaces did not change significantly since Zn(II) ions were added to the system. On the other hand, in the case of Zn(II) adsorption, the addition of Pb(II) ions to the system resulted in approximately a threefold decrease in the adsorbed amount of zinc. The reduced amount of zinc adsorbed in the mixed systems compared to the single ones may be caused by competition for adsorbents' active sites and the aforementioned effect, resulting in a higher affinity of Pb(II) ions for three-dimensional zeolite structures. Figure 9a,b show adsorption kinetics of Zn(II) and Pb(II) ions on the HiC FA, Na-P1, Na-P1(C) surfaces in systems containing both ions simultaneously. In turn, Figure 10a,b present a comparison of the absorbed amount of Pb(II) or/and Zn(II) on the high-carbon fly ash, zeolite and zeolite-carbon composite surfaces in the single and mixed systems.  The analysis of the obtained results showed that the simultaneous presence of Pb(II) and Zn(II) ions did not significantly affect the time of establishing the adsorption equilibrium in the studied systems. Furthermore, the amount of adsorbed Pb(II) ions on the HiC FA, Na-P1, Na-P1(C) surfaces did not change significantly since Zn(II) ions were added to the system. On the other hand, in the case of Zn(II) adsorption, the addition of Pb(II)  Figure 9a,b show adsorption kinetics of Zn(II) and Pb(II) ions on the HiC FA, Na-P1, Na-P1(C) surfaces in systems containing both ions simultaneously. In turn, Figure 10a,b present a comparison of the absorbed amount of Pb(II) or/and Zn(II) on the high-carbon fly ash, zeolite and zeolite-carbon composite surfaces in the single and mixed systems.  The analysis of the obtained results showed that the simultaneous presence of Pb(II) and Zn(II) ions did not significantly affect the time of establishing the adsorption equilibrium in the studied systems. Furthermore, the amount of adsorbed Pb(II) ions on the HiC FA, Na-P1, Na-P1(C) surfaces did not change significantly since Zn(II) ions were added to the system. On the other hand, in the case of Zn(II) adsorption, the addition of Pb(II) Figure 10. Amounts of Pb(II) and Zn(II) ions adsorbed on HCFA, Na-P1 and Na-P1(C) in the single (a) and mixed (b) systems at pH 5.   Table 5 shows the changes in the pH pzc (pzc-point of zero charge) values of the adsorbent particle in the examined systems of adsorbates.

Changes in Surface Charge
As it results from the presented potentiometric titration curves, at the pH value of 5, i.e., under the conditions in which the adsorption study was conducted, the surfaces of the all adsorbents assumed a positive charge. The point of zero charge indicated the pH value at which the charge of the solid surface equals zero. At a pH higher than the pH pzc of the solid, its surface takes a negative charge, and correspondingly, at a pH lower than the pH pzc , the surface of the solid becomes a positive charge [49]. Despite the unfavorable electrostatic repulsion at pH 5, adsorption of cations occurs on positively charged solids due to the unique structure of zeolites. They can boast a three-dimensional system of channels and chambers of strictly defined molecular dimensions, which gives zeolites the properties of molecular sieves and enables the occurrence of ion exchange. Moreover, as proved by kinetics studies, chemical bonds between simple metal ions and the solid surface are formed [50][51][52][53].
of zinc. The reduced amount of zinc adsorbed in the mixed systems compared to the single ones may be caused by competition for adsorbents' active sites and the aforementioned effect, resulting in a higher affinity of Pb(II) ions for three-dimensional zeolite structures.  Table 5 shows the changes in the pHpzc (pzc-point of zero charge) values of the adsorbent particle in the examined systems of adsorbates. Figure 11. Surface charge density of HiC FA (a), Na-P1 (b) and Na-P1(C) (c) without and with one or two heavy metal ions. Figure 11. Surface charge density of HiC FA (a), Na-P1 (b) and Na-P1(C) (c) without and with one or two heavy metal ions. The point of zero charge is 9.5 for HiC FA, 9.6 for Na-P1, and 8.8 for NaP1(C). For all adsorbents, this parameter decreases after adding Pb(II) ions, respectively, to 9.2 for HiC FA, 9.1 for Na-P1 and 8.5 for Na-P1(C) and after the addition of Zn 2+ ions to 9 for HiC FA and Na-P1, and to 8.4 for Na-P1(C). Moreover, in a system containing both heavy metal ions, pH pzc decreases most noticeably, for HiC FA and Na-P1 to 8.6, and for Na-P1(C) to 8.3. This means that the adsorption of heavy metal ions affects the surface charge of the adsorbents, causing the pH pzc to shift towards lower values, which is a commonly observed phenomenon. Adsorption of small ions (cations in this case) creates oppositely charged sites on the surface of the adsorbent. It manifests itself in a decrease of the solid surface charge density and, consequently, in a shift of pH pzc position towards smaller pH values. Such a phenomenon may be described by the following equations (Equations (9)-(11)) [54,55]:

Changes in Surface Charge
where Me is a Pb or Zn atom.  Table 6 shows the changes in pH iep (iep-isoelectric point) values of the adsorbent particles, depending on the composition of the studied system. Table 6. Comparison of the pH iep values of the HiC FA, Na-P1, Na-P1(C) particles in the absence or presence of heavy metal ion/ions.

Solid
Na-P1 Na-P1(C) HiC FA Without adsorbates - The pH at which the charge of a slipping plane area is equal to zero is called the isoelectric point (pH iep ). This plane is the interface separating the stiff part of the liquid "attached" to the surface of the solid (specifically and electrostatically bound water, ions and hydrated ions) from the diffuse part of the solution. Whether the pH iep takes on positive or negative values is determined by the predominance of ions carrying a positive or negative charge in this plane. At pH values higher than the solid pH iep , the zeta potential becomes positive values, and at pH values lower than pH iep , negative ones [56,57]. Materials 2021, 14, x FOR PEER REVIEW 16 of 20 Figure 12. Zeta potential of the HiC FA 9(a), Na-P1 (b) and Na-P1(C) (c) without and with one or two heavy metal ions. Figure 12. Zeta potential of the HiC FA 9 (a), Na-P1 (b) and Na-P1(C) (c) without and with one or two heavy metal ions.
The pH iep values of the adsorbent particles in the studied systems are not in the range of the tested pH values. This indicates that pH iep is below the value of 3. This applies, for example, to systems with Na-P1, both without adsorbates and with one or two adsorbates, as well as to Na-P1(C) (in systems with Zn(II) ions and Pb(II) and Zn(II) simultaneously) and HiC FA (without adsorbates and with Zn(II) ions). The isoelectric point of Na-P1(C) is located at pH about 4 and decreases to the value 3.2 with the addition of Pb(II) ions. For suspensions containing HiC FA, the most significant decrease in zeta potential is observed. It is manifested by a change in pH iep value from 3.2 for the system with Pb(II) ions to 3 for the system with Zn(II) and Pb(II) ions.
The specific increase in the zeta potential at pH values in the range 8-10 can be described by the effect of charge reversal, which occurs especially in the systems with Zn(II) ions. The charges of counterions in the inner part of the electrical double layer exceed the charge of ions on the solid surface, resulting in the same charge on the outer part and on the surface. The effect caused by Zn(II) ion adsorption is also called overcharging or overloading of the electrical double layer [57].
3.6. Desorption Degree of Zn(II) and Pb(II) from HiC FA, Na-P1 and Na-P1(C) Surfaces The pH at which the charge of a slipping plane area is equal to zero is called the isoelectric point (pHiep). This plane is the interface separating the stiff part of the liquid "attached" to the surface of the solid (specifically and electrostatically bound water, ions and hydrated ions) from the diffuse part of the solution. Whether the pHiep takes on positive or negative values is determined by the predominance of ions carrying a positive or negative charge in this plane. At pH values higher than the solid pHiep, the zeta potential becomes positive values, and at pH values lower than pHiep, negative ones [56,57].
The pHiep values of the adsorbent particles in the studied systems are not in the range of the tested pH values. This indicates that pHiep is below the value of 3. This applies, for example, to systems with Na-P1, both without adsorbates and with one or two adsorbates, as well as to Na-P1(C) (in systems with Zn(II) ions and Pb(II) and Zn(II) simultaneously) and HiC FA (without adsorbates and with Zn(II) ions). The isoelectric point of Na-P1(C) is located at pH about 4 and decreases to the value 3.2 with the addition of Pb(II) ions. For suspensions containing HiC FA, the most significant decrease in zeta potential is observed. It is manifested by a change in pHiep value from 3.2 for the system with Pb(II) ions to 3 for the system with Zn(II) and Pb(II) ions.
The specific increase in the zeta potential at pH values in the range 8-10 can be described by the effect of charge reversal, which occurs especially in the systems with Zn(II) ions. The charges of counterions in the inner part of the electrical double layer exceed the charge of ions on the solid surface, resulting in the same charge on the outer part and on the surface. The effect caused by Zn(II) ion adsorption is also called overcharging or overloading of the electrical double layer [57].  The analysis of the data presented in Figure 13 leads to the conclusion that hydrochloric acid is a better desorbing agent than sodium base. This trend is present in all sus- The analysis of the data presented in Figure 13 leads to the conclusion that hydrochloric acid is a better desorbing agent than sodium base. This trend is present in all suspensions studied, both for single and mixed systems, regardless of the adsorbate or adsorbent used. The most efficient desorption from the surface of zeolite and zeolite-carbon composite in single systems took place with Pb(II) ions, whereas the greatest desorption from HiC FA surface occurred in systems containing Zn(II) ions. In mixed systems, these trends were similar.

Conclusions
Two types of sorbents were obtained from the waste that is fly ash-a zeolite-carbon composite and pure zeolite. Both zeolite and its carbon composite showed significantly better adsorption capacities than their precursor, fly ash. This is a result of their better textural and morphological parameters obtained due to the conversion of fly ash (zeolitecarbon composite) and waste solution (pure zeolite). Pb(II) ions were adsorbed more efficiently than Zn(II) ones and their adsorption occurs fast. The largest adsorption takes place on the Na-P1 surface (407.34 mg/g for Pb(II)), whereas the smallest on the HiC FA one (78.67 mg/g for Pb(II)). Bounding of all adsorbates occurs through chemisorption and ion adsorption following the Langmuir model. In mixed systems, Pb(II) ions adsorb more efficiently due to the molecular sieving effect. In the mixed systems of adsorbates, the presence of Zn(II) ions minimally influences the adsorbed amount of Pb(II) ions. On the contrary, the addition of Pb(II) ions to the system results in a threefold decrease in the adsorbed amount of Zn(II) ions. The addition of divalent heavy metal cations decreases the pH pzc . More efficient desorption of heavy metal ions (reaching 80%) occurs with the use of hydrochloric acid (in comparison with sodium base). Both the composite and pure zeolite, due to the method of their synthesis, may be an alternative to commercial sorbents in terms of heavy metal ion removal from aqueous solutions. Such use of fly ash is in line with the current ecological trends.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.