Recycling Galvanic Sludge to Produce Geopolymer Modified with Algae

Abstract

A new group of geopolymers based on galvanic sewage sludge was synthesized using algae. The sorption properties of the obtained geopolymer materials toward Cu(II), Mn(II), Pb(II), and Zn(II) ions in aqueous solutions were investigated. Algae have good adsorption properties with respect to heavy metals and bind to them irreversibly. Their addition to the geopolymer mass results in a geopolymer in which the C-(N)-A-S-H gel is dominant in the structure, as shown by SEM analysis. Analysis of the FTIR spectra of the geopolymer obtained with the addition of algae before and after sorption of the studied metal ions showed the presence of bands characteristic of geopolymers, proving at the same time that the introduction of algae increases the negative charge on the surface of the geopolymer and the chemisorption of heavy metal ions. The resulting geopolymer material shows excellent removal efficiency for all ions tested, respectively, Cu(II)—96.9, Mn(II)—98.9, Pb—99.7, and Zn(II)—99.5. The sorption process under experimental conditions follows the Langmuir isotherm model. The kinetics of the process are described by a pseudo-second-order kinetic equation, which confirms the contribution of functional groups to the binding of the ions of the studied metals.

1. Introduction

Increasing energy demand, ever-increasing waste generation, and the slow depletion of extractable natural resources are three of the most important issues that the world will have to face in the coming decades [1]. In the past, the most common way to dispose of waste was landfilling. Nowadays, the waste management system mandates waste reduction, reuse, material and/or energy recycling, and disposal of pre-treated waste [2]. Waste should be considered not only as a source of material and energy recovery, but also as an environmental tool focused on the 3R (Recycle, Reuse, and Reduce) concept. This concept considers energy-efficient and economic management in relation to natural resources, minimization of industrial solid waste, and its impact on human health and the environment. Geopolymer materials, which minimize the use of natural resources as raw materials while maximizing the use of industrial waste [3], are to some extent a realization of this concept. Geopolymers are considered environmentally friendly materials primarily due to their energy consumption, low temperatures during production, and low CO₂ emissions [4]. Geopolymers can be obtained by polycondensation from a variety of materials, whether natural (e.g., metakaolin), anthropogenic (e.g., fly ash), or synthetically produced (e.g., sol–gel method) [5,6,7]. The required feature of these materials is a high alumina and silica content to allow reaction with an alkaline reagent (sodium or potassium hydroxide and sodium or potassium silicate), which acts as an activator for the geopolymerization reaction. The most commonly used mechanism details several reaction steps, namely [8,9] the following:

• Dissolution of aluminosilicate materials in a concentrated alkaline solution (KOH or NaOH);
• Diffusion of dissolved silicon and aluminum from the particle surface into the intermolecular space;
• Polycondensation of oligomers and formation of an aluminosilicate gel phase;
• Hardening of the gel phase following polymerization and stabilization of the semicrystalline structures.

In the initial stage, hydroxyl ions react with the surface of aluminosilicate and break the covalent bonds of Si-O-Si, Si-O-Al, and Al-O-Al, and aluminum and silicon monomers are formed (e.g., [Al(OH)⁴⁻], [SiO(OH)³⁻]). Silicon and aluminum ions are released into solution and aluminum ions are released more readily [3]. The higher the pH, the higher the concentration of hydroxyl ions, and the rate of the dissolution reaction is faster [5]. The rate of dissolution is also influenced by the type of alkali metal cation, as indicated by the ionic radius, a lower value of which (in the case of the Na ion) leads to faster formation of stable ionic pairs with the spatial oligomers present in solution, which are formed by condensation of the monomers. It has also been found that a higher silicon concentration in the starting substrate increases the concentration of released ions [10]. The next step is the main condensation process, during which an aluminosilicate gel is formed. Initial bonding begins when the rate of aluminosilicate condensation exceeds the rate of dissolution [11]. In the further course of polycondensation, a three-dimensional lattice structure is formed by the combination of tetrahedral Si-O and Al-O [5]. It exhibits a negative charge due to the difference in valence between Al and Si, which is electrically balanced by base cations (Na⁺ or K⁺). The aluminosilicate network plays an important role in immobilizing heavy metals contained in various types of industrial waste. With regard to solid wastes, among others, fly ash [12], metakaolin [13], blast furnace slag [14], and galvanic sludge [15], the immobilization process can be realized as a direct synthesis of geopolymers using these wastes. The immobilization of the waste occurs through solidification/stabilization, and the immobilization of heavy metals occurs primarily through physical encapsulation, physical adsorption, or chemical adsorption [16]. Although the process is simple and efficient, moreover, from a sustainability point of view, the resulting geopolymer matrices are able to sequester heavy metals, their migration into water and soil, i.e., secondary environmental pollution, remains an issue of concern.

Geopolymers have exceptional adsorption capacities and are used for the removal of inorganic substances—heavy metals, organic substances—organic dyes, and pharmaceutical substances, among others [17]. Many new adsorbents based on geopolymers are being developed to enhance their adsorption capacity. Modifications refer to the structural form of the adsorbent (increase in surface area, optimization of the pore structure, and number of adsorption sites) [18,19] and the conditions of the adsorption process (temperature, initial concentration, pH) [20]. A new direction for the optimization of geopolymer-based adsorbents is the use of auxiliary materials in the form of, for example, carbon nanotubes (CNTs) [21], graphene oxide [22], sodium dithiocarbonate [8], diatomite [23], lignin [24], and alginate [25]. The resulting hybrid geopolymer materials are an effective solution for optimizing the adsorption properties of geopolymers [3]. They structurally represent porous materials, geopolymer foams [26], chemically modified materials, geopolymer composites, loaded zeolites [27], algae clays [4], and nanomaterials [28].

The research performed in this thesis represents a new direction for the optimization of geopolymer adsorbents because of the use of a support material in the form of algae of the genus Chlorella vulgaris. This type of algae has good adsorption properties with respect to heavy metals, which are bound in the cell wall in an irreversible manner. This is due to the specific structure of the algal cell wall, which contains polysaccharide-linked proteins, or glycoproteins. These compounds contain functional groups (amine, carboxyl, sulfate, and hydroxyl) that play an important role in the sorption process. The biosorption process by algae is determined by the pH value. It has a decisive influence on the solubility of metal ions and the total charge of the biosorbent because protons can be adsorbed or released depending on the type of functional groups present on the surface of the algal cell wall. Biomass concentration is also an important parameter during metal uptake. At a given equilibrium concentration, the biomass takes up more metal ions at lower than at higher cell densities. Electrostatic interactions between cells can be an important factor that influences the relationship between the biomass concentration and metal sorption. The lower the biomass concentration in suspension, the higher the metal/biosorbent ratio and the concentration of metal adsorbed by the sorbent will be [29].

The research carried out within the scope of this thesis included the synthesis of a new hybrid adsorbent, representing a galvanizing sludge-based geopolymer using algae of the genus Chlorella vulgaris, and the adsorption studies of Cu(II), Mn(II), Pb(II), and Zn(II) ions on the algae-modified geopolymer material. The specific composition of the algae may facilitate a more permanent chemical bond and the formation of a durable structure during curing, the geopolymer mass, which was carried out at room temperature.

2. Materials and Methods

2.1. Materials

For geopolymer synthesis, a mixture of solid components was used, consisting of an electroplating sludge obtained from the Koelner Screw Factory (Łańcut, Poland) and algae of the genus Chlorella vulgaris (Bioplanet S.A., Leszno, Poland) in a weight ratio [g/g] of 1:1. Electroplating sludge is formed during two production processes, that is, etching and electroplating. During pickling, metal components are treated with sulfuric(VI) acid; after pickling, they are vigorously rinsed with water to completely remove H₂SO₄ and iron(II) sulfate residues and then sent to the electroplating plant. The acidified water is sent to a wastewater treatment plant. Solid and semi-liquid contaminants constitute the electroplating sludge, which contains cations of various metals (e.g., Mg, Si, Ca, Cr, Fe, Cu, and Zn) in its composition, which makes it classified as particularly hazardous waste for the environment [30]. For a detailed physicochemical characterization of the plating sludge used for geopolymer synthesis, see [31].

Algae contain, among others, lipids, carbohydrates, and proteins in their composition. In addition, many micro- and macronutrients, which occur in them in highly bioavailable forms, are expressed as complex or organometallic compounds (bromine, zinc, iodine, magnesium, manganese, copper, and iron).

An alkaline solution was also used to synthesize the geopolymer material. The alkaline solution was prepared by mixing sodium silicate with SiO₂/Na₂O = 2.4–2.6 molar modulus and 1.45–1.48 g/cm³ density (Chempur, Piekary Śląskie, Poland) with solid NaOH (Chempur, Poland).

2.2. Methods

2.2.1. Geopolymers Preparation

A solid mixture composed of 50 weight parts of sand and 50 weight parts of galvanic sewage sludge with algae was prepared. Both the galvanic sludge and the algae were dried to a solid mass before mixing. The mixture was then dry blended for approximately 10 min on a Vibramax 100 laboratory shaker (Heidolph, Schwabach, Germany) to obtain a homogeneous mixture. The optimal molar module SiO₂/Na₂O of the aqueous solution of sodium silicate equal to 1.28, which was obtained by mixing sodium silicate with solid NaOH in the 2:1 ratio, was used for the synthesis of geopolymers. The ingredients were mixed together and then left for about 40 min to equalize the concentrations and reach room temperature of the solution. Then, a solid mixture (1:1), i.e., the galvanic sewage sludge with the addition of sand thoroughly mixed, was added to the alkaline solution. It was prepared with a mixing time of 30 min. The geopolymerization reaction was carried out with a 50:50 ratio between the volume of the alkaline solution and the mass of the solid mixture [cm³/g]. When a homogeneous plastic consistency was obtained, the mixture was inserted into cylindrical molds made from a silicon material. The samples were molded by applying manual pressure; after that, the samples were shaken for 15 min on a Vibramax 100 laboratory shaker (Heidolph, Germany) to release air bubbles. The samples were dried at room temperature under atmospheric pressure. To avoid errors during the analyses, three control samples were prepared, eliminating the possibility of accidental phenomena influencing the end results [32].

Scanning electron microscopy (SEM) was used to characterize the surface of the geopolymers, using a scanning electron microscope equipped with a microarea chemical analysis (EDS) attachment, model S-3400 N (HITACHI, Tokyo, Japan). As a preliminary study of the resulting algal geopolymer material, the following were also determined: the cation exchange capacity (CEC) by the spectrophotometric method using methylene blue [33], the point of zero electrical charge (PZC) by the suspension method [34], and the total concentration of the acids and base centers by acid–base titration.

2.2.2. Sorption Experiments

The sorption of Cu(II), Mn(II), Pb(II), and Zn(II) ions in algal-modified geopolymer material was carried out in single-component systems at room temperature, using a sample weight [g] to solution volume [cm³] ratio of 1:50 for times ranging from 5 min to 24 h. All experiments were carried out without correction for the pH value of the solutions. Before experiments, the geopolymers obtained were treated with an aqueous solution of 0.01 M NH₄NO₃ to remove unreacted Na⁺ ions from their structure. The following solutions of salts of the studied metals ZnCl₂, MnCl₂, CuCl₂, Pb(NO₃)₂ were used in the sorption experiment, with the initial concentrations given in turn: 0.01, 0.05, 0.1, and 0.2 mol/dm³. For this purpose, 1.00 g of powdered geopolymer sample was added to 50 cm³ of solutions of the corresponding metal salts and thoroughly shaken for 24 h on a Vibramax 100 laboratory shaker (v = 300 rpm). After this time, the separation phase was started by centrifugation (t = 5 min, v = 2000 rpm) [35]. The precipitates obtained were dried at ambient temperature and infrared absorption spectra were recorded in the fundamental range of 4000–400 cm⁻¹ with a resolution of 2 cm⁻¹ using an FTIR spectrometer (Bruker, Bremen, Germany). The concentrations of Cu, Mn, Pb, and Zn in the obtained solutions were determined by Flame Atomic Absorption Spectrometry (FAAS) using the Perkin Elmer 3100 model spectrometer (PerkinElmer, Waltham, MA, USA). Each measurement was carried out with three repetitions with a relative standard deviation (RSD) < 5%.

The equilibrium adsorption capacity (q_e; mg/g), the adsorption coefficient (A%), and the distribution coefficients (K_d; dm³/g) were calculated as follows (1), (2), and (3):

$$q_e={\displaystyle \frac{{(C}_0-C_e)V}{m}}$$

(1)

$$A={\displaystyle \frac{C_0-C_e}{C_0}}·100\%$$

(2)

$$K_d={\displaystyle \frac{q_e}{C_e}}$$

(3)

where

C₀ and C_e—initial and equilibrium concentrations of the metal ion (mg/dm³);

m—amount (g) of geopolymer samples;

V—volume of solution (dm³).

The Langmuir isotherm model (4) was used to determine the maximum sorption capacity (q_max):

$$q=q_{max·}{K}_L·c_e/\left(1+K_L·c_e\right)$$

(4)

where

q—ion content in solid phase [mg/g s.m.];

K_L—constant value [dm³/g];

c_e—equilibrium concentration of the metal ion in solution [mg/dm³];

To determine the kinetics of the process, fitting the experimental data to the kinetic equation models, pseudo-first-order (5) and pseudo-second-order (6) were used [36] as follows:

$$\ln \left(q_e-q_t\right)=-k_1·t+\ln q_e$$

(5)

$${\displaystyle \frac{t}{q_t}}={\displaystyle \frac{t}{q_e}}+{\displaystyle \frac{1}{k_2·{q_e}^2}}$$

(6)

where

q_e—equilibrium adsorption capacity [mg/g];

q_t—number of adsorbed metal ions [mg/g];

k₁—rate constant [1/min];

k₂—rate constant [(g-min)/mg];

t—time [min].

3. Results

3.1. Physicochemical Characteristics of the Geopolymer

In the spatial structure of geopolymers, the outer surface and the inner pore region can be distinguished. Figure 1 shows photographs of a sample of the resulting algal geopolymer material at different magnifications. The morphological microstructure of the geopolymer is complex, which is typical of geopolymer materials [37]. SEM images show aggregates and agglomerates of various sizes with irregular shapes, whose surface is porous and shows the presence of microcracks and nanotracks [38].

Individual grain sizes ranged from less than 10 μm to more than 300 μm. Compositional analyses by energy dispersive X-ray spectroscopy (EDS) showed the presence of silicon, aluminum, and oxygen in the geopolymer matrix and Na, K, Ca, Mg, Cr, Mn, Fe, Co, Ni, Cu, Zn, and Pb within the inclusions (Figure 2). Generally, the sewage sludge samples are characterized by a highly changeable chemical composition. As shown in the study [32], the spread of chemical composition of grains is significant. Single grains of sewage sludge can be built from a small number of compounds or can represent only one component. The EDS analyses evidenced the presence of all study elements in all the examined areas, indicating a uniform distribution of metals in the obtained geopolymer. However, the EDS analyses evidenced the presence of oxygen, silicon, calcium, chromium, iron, and zinc in all the examined areas, and the presence of manganese and copper in selected areas, indicating a non-uniform distribution of metals in the sewage sludge [32].

In the resulting material, similar to other alkali-activated materials, there is a coexistence of calcium silicate hydrates with sodium and aluminum substitutions (C-(N)-A-S-H) and sodium aluminosilicate hydrates (N-A-S-H), which are essential products of repolymerization [16]. The presence of MgO, as well as, for example, MnO₂ and TiO₂, reduces dissolution and phase formation and can alter the gel microstructure [39].

Table 1. Atomic percentage of select elements at each study point by EDS method.

Study Point	Ca	O	C	Na	Al	Si	Ca/Si	Al/Si	Na/Si
1	1.8 (±0.1)	36.0 (±0.4)	48.4 (±0.3)	5.0 (±0.0)	0.1 (±0.0)	3.3 (±0.0)	0.55	0.03	1.52
2	0.3 (±0.1)	43.6 (±0.4)	43.4 (±0.2)	2.3 (±0.0)	0.0 (±0.0)	9.8 (±0.0)	0.03	-	0.23
3	0.9 (±0.1)	48.8 (±0.4)	32.8 (±0.2)	11.6 (±0.1)	0.0 (±0.0)	3.5 (±0.0)	0.07	-	3.31
4	1.1 (±0.1)	49.4 (±0.4)	30.6 (±0.2)	8.7 (±0.1)	0.0 (±0.0)	7.0 (±0.0)	0.16	-	1.24
5	2.0 (±0.1)	49.9 (±0.4)	29.7 (±0.2)	8.8 (±0.1)	0.1 (±0.0)	5.5 (±0.0)	0.36	0.02	1.60
6	2.6 (±0.1)	49.2 (±0.4)	30.9 (±0.2)	7.2 (±0.1)	0.2 (±0.0)	4.3 (±0.0)	0.60	0.05	1.67
7	0.7 (±0.1)	34.0 (±0.5)	55.9 (±0.3)	4.2 (±0.0)	0.1 (±0.0)	2.1 (±0.0)	0.33	0.05	2.00

shows the mass ratio of selected elements in the study point detected by EDS.

The presence of Si, Al, Ca, and Na in the study points labeled 1, 5, 6, and 7 suggests the presence of C-(N)-S-H and N-A-S-H gels (symbol description: C = CaO, N = Na₂O, A = Al₂O₃, S = SiO₂, H = H₂O). The remaining spots (2, 3, and 4) did not show aluminum, suggesting the absence of a N-A-S-H gel phase [40]. The Ca/Si ratio is less than one at each tested point, so there is little depolymerization of the silicate chains. It is significantly lower than the Na/Si ratio, which is in the range of 0.2 to 3.3. This indicates that the C-(N)-A-S-H gel is dominant in the structure of the algal geopolymer material obtained [37,39].

To determine the surface properties of the geopolymer, the point of zero electrical charge (PZC) was determined—Figure 3. This is the pH value for which the sum of the surface charges (positive and negative) is zero. For the geopolymer sample tested, its value is 13, indicating that the surface of the sample is alkaline in nature and has the ability to adsorb cations [34].

The overall concentration of acid and basic centers was also determined by the difference between the amount of reagent (HCl or NaOH) added and their amount after the reaction. A lower concentration of acid centers (0.00146 mol/g) was found than of base centers (0.00199 mol/g), confirming the alkaline nature of the surface of the resulting geopolymer. The concentration of acid and base centers in geopolymers is crucial to their chemical and mechanical properties. The acid centers in geopolymers are usually associated with the presence of silanol (Si-OH) and aluminum (Al-OH) groups. These groups can react with water and other compounds, which affect the surface properties and reactivity of geopolymers. Alkali centers are associated with the presence of metal ions such as Na⁺, K⁺, and Ca²⁺, which are introduced into the geopolymer structure during the synthesis process. Their presence can affect the structural stability and mechanical properties of geopolymers. In general, the properties of geopolymers can change depending on the concentration of acid and base centers [12]. Geopolymers exhibit a negatively charged surface, so that positive metal ions can be adsorbed by them. The literature reports on this subject are mainly concerned with the sorption of metals, for example, As, Au, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, and Zn; nonmetals—boron, fluorine; chemical compounds such as NH₃, NO_X, SO_X, phenols, toluene, and polychlorinated aromatic hydrocarbons; and dyes [17,18,19].

In order to estimate the ability of the algal geopolymer material to adsorb cations from aqueous solution, the cation exchange capacity (CEC) value was determined. The value of this parameter corresponds to the number of functional groups that can be involved in the binding of heavy metal cations. The resulting average CEC value for the tested sample of geopolymer material with algal addition is 47.4 [mmol/100 g dry mass]. Clay minerals are often used in the production of geopolymers; hence, the CEC value obtained can be related to this group of minerals. Individual clay minerals are characterized by different CEC values, ranging from low, for kaolinites (3 ÷ 15 mmol/100 g), to high, for smectites (80 ÷ 150 mmol/100 g). Illites and chlorites show intermediate properties (10 ÷ 40 mmol/100 g) [41]. Thus, it can be assumed, on the basis of the determined CEC value, that the obtained hybrid geopolymer material with algal addition has a fairly high cation exchange capacity.

3.2. Sorption Properties

A summary of the sorption parameters investigated is presented in

Table 2. Summary of sorption parameters for the sorption process of the tested metals on a geopolymer with the addition of algae.

Metal Ion	t [min]	ce [mg/dm3]	qe [mg/g]	A [%]	Kd [dm3/g]
Cu(II)	5	54.0	29.15	91.50	0.54
	15	50.0	29.28	92.13	0.59
	30	44.0	29.72	93.08	0.68
	60	48.0	29.20	92.45	0.61
	120	38.0	29.85	94.02	0.79
	300	40.0	29.96	93.71	0.75
	1440	20.0	30.89	96.85	1.54
Mn(II)	5	48.0	134.47	98.25	2.80
	15	50.0	135.25	98.18	2.71
	30	520	135.00	98.11	2.60
	60	46.0	136.18	98.33	2.96
	120	44.0	134.62	98.40	3.06
	300	40.0	136.00	98.54	3.40
	1440	30.0	136.12	98.91	4.54
Pb(II)	5	48.0	161.78	99.27	3.37
	15	56.0	161.90	99.14	2.89
	30	56.0	162.13	99.14	2.90
	60	60.0	160.81	99.08	2.68
	120	48.0	161.54	99.27	3.37
	300	48.0	161.30	99.27	3.36
	1440	36.0	163.89	99.45	4.55
Zn(II)	5	340.0	2064.91	99.18	6.07
	15	358.0	2054.92	99.14	5.74
	30	280.0	2071.26	99.32	7.40
	60	322.0	2075.20	99.22	6.44
	120	300.0	2056.79	99.28	6.86
	300	254.0	2079.47	99.39	8.19
	1440	538.0	2064.51	99.70	3.84

, from which it can be seen that the value of the adsorption coefficient and the equilibrium concentration of the metal ions investigated increases with time. Already after 5 min of conducting the experiment, more than 90% of the metal is bound by the algal geopolymer material. The sorption equilibrium for zinc, manganese, and lead ions is practically established after 15 min. The adsorption efficiency expressed by the adsorption coefficient for the investigated metal ions on the algae-added geopolymer after 24 h is equal to, respectively, Zn(II) 99.7, Pb(II) 99.5, Mn(II) 98.9, and Cu(II) 96.9%. Taking into account the calculated values of the sorption capacity, the studied metal ions form the following series: Zn(II) > Pb(II) > Mn(II) > Cu(II). The values of the ionic potentials [pm⁻¹], for individual ions, are as follows: [Φ (Zn²⁺) = 2.7 × 10⁻²], [Φ (Pb²⁺) = 1.68 × 10⁻²], [Φ (Mn²⁺) = 2.3 × 10⁻²], and [Φ (Cu²⁺) = 2.7 × 10⁻²]. All are greater than the sodium cation potential [Φ (Na⁺) = 9.8 × 10⁻³], indicating that the test metal ions can replace Na⁺ ions to balance the negative charge on the surface of the algal-supported geopolymer [42]. Analogous results for the sorption of lead ions were demonstrated in [43], where an alkali-activated porous material based on metakaolin solid waste and recycled aluminum scrap was used for adsorption, and in [44], where self-supporting geopolymers obtained from fly ash-based zeolite foam using saturated steam were used. The use of a bentonite-based geopolymer with Fe₃O₄ nanoparticles [45] for the adsorption of Zn and Pb ions, or a composite geopolymer based on solid waste (drinking water treatment residue and granulated blast furnace slag) [46], also confirms the achievement of close to 100% removal of the mentioned ions from aqueous solutions. Regarding Cu and Pb ions, porous geopolymer foams obtained from industrial (blast furnace slag) and municipal (drinking water treatment residue) wastes were used [47]. The results obtained are also consistent with those obtained in this work, which undoubtedly demonstrates the possibility of using the geopolymer material obtained with the addition of algae in the adsorption of heavy metals.

3.3. Spectroscopic Analyses

In order to identify the surface functional groups present on the surface of the algal-modified geopolymer, the synthesized geopolymer material obtained before and after the sorption process of each ion tested was analyzed by FTIR. It is a very useful technique to provide information on the chemical bonds that make up the molecular units that comprise C-(N)-A-S-H and N-A-S-H gels [48]. A summary of the spectroscopic spectra obtained is shown in Figure 4.

The spectral bands of the analyzed samples are consistent with the geopolymer spectra characteristics presented by other authors—the characteristics of the individual bands are summarized in

Table 3. Characteristics of individual bands before and after adsorption of the ions studied from aqueous solutions.

Absorption Bands Position [cm−1]		Types of Vibration	References
Before Adsorption	After Adsorption
3427	3427 Pb(II) and Mn(II) 3343 Zn(II) 3392 Cu(II)	Stretching vibrations of the OH groups	[52]
1650	1650 all metal ions	Tensile vibrations of the H-O-H	[52]
1450	1400 Pb (II) 1430 Mn(II) 1450 Cu (II) and Zn(II)	Asymmetric and symmetric stretching vibrations of the CO group	[53]
1260–1020	1260–1020 Pb(II), Zn(II), and Mn(II) 1085–1020 Cu(II)	Stretching vibrations of Si-O(Si) and Si-O(Al) binding	[48]
870–800	870–800 all metal ions	Stretching vibrations of [AlO4]− groups	[54]
800–500	800–500 all metal ions	Asymmetric vibrations of Si-O(Si) and Si-O(Al) binding and internal vibrations	[55]
450–470	450–470 all metal ions	Bending vibrations Si-O-Si	[55]

. From the comparison of the presented FTIR spectra of geopolymers obtained after the adsorption of Cu(II), Mn(II), Pb(II), and Zn(II) ions, it was observed that the recorded changes are related to two areas: 3600–3200 cm⁻¹, associated with OH stretching vibrations; 1450–1400 cm⁻¹, associated with CO stretching vibrations and 1100–500 cm⁻¹, coming from Si-O(Si) and Si-O(Al) vibrations and internal vibrations characteristic for aluminosilicates. A shift in these ranges was observed to lower the wave numbers for all of the ions studied (Table 3). A key indicator that accounts for the formation of new geopolymer structures is a shift toward lower wave numbers in the band from 1100–1000 cm⁻¹ [48]. These may be a direct result of coordination reactions between Si-O-Al and test metal ions, which lead to changes in surface charge, resulting in a shift in the bands on the FTIR spectrum [16]. It is noteworthy that there is a single peak at around 870 cm⁻¹, which is characteristic of asymmetric stretching of the [AlO₄]⁻ groups in the Al-O-Si bonds. Its presence is characteristic of the C-(N)-A-S-H phase and indicates an advanced condensation process of the [SiO₄]⁴⁻ anion and a high content of the amorphous phase [49]. The relatively large half-widths of the most intense bands appearing at 3420 and 970 cm⁻¹ draw attention. This confirms the high degree of amorphousness in the C-(N)-A-S-H phase [14,50]. Furthermore, in the spectra shown in Figure 4, there are also pseudo-lattice vibrations over the tetrahedral structural units that appear in the range 600–800 cm⁻¹ [51]. Their presence, in turn, confirms the sorption of the ions of the metals under study by the algal geopolymer material used. The above-mentioned oscillations show different intensities, which are due to the fact that some of the ions were introduced into the aluminosilicate structure of the geopolymer, while others precipitated in the form of hydroxides. It can be seen in Figure 4 that the band, which describes the stretching vibration originating from the OH group, is most intense for the geopolymer sample after the sorption of Zn(II) and Mn(II) ions and for the geopolymer sample before the sorption process.

In contrast, the weakest signal was recorded in the range of about 3400 cm⁻¹ of the geopolymer spectrum after the sorption of Pb(II) ions. The range of recorded changes is a direct result of the different behavior of the individual metal ions in aqueous solutions and the possibility of forming aqua complexes. The intensity of the band, originating from the stretching vibration from the C-O group, is highest for the geopolymer after the sorption of Pb(II) ions and lowest for the geopolymer sample before the sorption process. A variable signal can also be observed for the stretching vibration coming from the Si-O-Si group. The strongest signal comes from the spectrum obtained after the sorption of Cu(II) ions, the weakest from the spectrum of the geopolymer sample before sorption, while the sorption of Zn(II), Mn(II), and Pb(II) ions gives a very similar spectrum in this range. The visible changes in the position and intensity of individual bands indicate that during the adsorption of Cu(II), Mn(II), Pb(II), and Zn(II) ions on the geopolymer based on galvanic sewage sludge with algae, bonds between functional groups, i.e., O-H and C-O with cations of the above-mentioned metals, are formed. The alkaline functional groups present on the surface of the geopolymer with algae are capable of sorption by replacing H⁺ ions with metal cations in the form of simple Mn⁺ or complex ions such as M(OH)⁽ⁿ⁻¹⁾⁺. Their amount at alkaline pH values is high, allowing an increase in the negative charge on the geopolymer surface and chemisorption of the tested heavy metal ions [48].

4. Discussion

The main property of C-(N)-A-S-H gels is the very high surface charge density. The surface of the particles consists of silicate chains. The silicate tetrahedral has oxygen atoms on each vertex. If the oxygen is not bonded to another tetrahedron or is not coordinated by calcium ions, it then forms a silanol group (Si-OH). At the high pH used to obtain alkali-activated materials, most silanol groups become ionized, according to reaction (R1), resulting in a negative surface charge [54].

≡Si-OH + OH⁻ → ≡SiO⁻ + H₂O

(R1)

In addition to the high-density negative surface charge, another important property of the C-(N)-A-S-H gel phase is its porosity [56]. The sorption of heavy metals by sorbents obtained from the dead biomass of organisms occurs mainly through ion exchange. Metal cations bind to functional groups (carboxyl, hydroxyl, amine, and phosphate) present in structural polysaccharides and nucleic acids of microbial cells through exchange with protons or other bound metal cations [29].

The relationship between the amount of adsorbed metal ions on the geopolymer and their concentration in solution at equilibrium is described by the linear form of the Langmuir isotherm. The results of fitting the sorption equilibrium to the linear form of the Langmuir isotherm are shown in Figure 5. The Langmuir isotherm model was used to determine the maximum sorption capacity (q_max). The values obtained were found to be in general agreement with those obtained under experimental conditions and are, respectively, Zn(II) 2064, Pb(II) 164, Mn(II) 136, and Cu(II) 31 mg/g. The observed differences in values with respect to this parameter are, among other things, due to the magnitude of the negative surface charge of the aluminosilicate lattice, resulting from heterovalent substitutions of silicon and aluminum in the octahedral positions.

Bibliographic references for the sorption capacity values obtained for the tested metal ions are provided in

Table 4. Sorption capacities of different sorbents based on the literature data.

Metal Ion	Type of Sorbent	qe [mg/g]	References
Cu(II)	Geopolymer with algae addition	29.00	In this study
	Geopolymer based on metakaolin	53.93	[13]
	Geopolymer based on metakaolin with zeolite addition in ratio 1:3	28.38	[57]
	Geopolymer based on metakaolin with zeolite addition in ratio 3:1	43.16	[57]
	Geopolymer based on fly ash	7.00	[58]
	CaCO3	99.76	[59]
Mn(II)	Geopolymer with algae addition	135.00	In this study
	Geopolymer based on metakaolin	72.30	[60]
	Geopolymer based on volcanic ash	192.00	[61]
	Glycine-modified chitosan	71.40	[62]
Pb(II)	Geopolymer with algae addition	161.00	In this study
	Geopolymer based on metakaolin	100.79	[13]
	Geopolymer based on fly ash	253.80	[13]
	Geopolymer based on metakaolin with zeolite addition in ratio 1:3	136.51	[57]
	Geopolymer based on metakaolin with zeolite addition in ratio 3:1	261.22	[57]
Zn(II)	Geopolymer with algae addition	2060.00	In this study
	Geopolymer based on metakaolin	30.52	[57]
	Geopolymer based on metakaolin with zeolite addition in ratio 1:1	30.79	[57]
	Geopolymer based on metakaolin with zeolite addition in ratio 1:3	18.71	[57]
	Geopolymer based on metakaolin with zeolite addition in ratio 3:1	35.88	[57]

. According to this table, the obtained value of the sorption capacity for Cu(II) ion is higher than that for the fly ash-based geopolymer sample and comparable to the value obtained for the metakaolin-based geopolymer doped with zeolite in a ratio of 1:3. For the manganese(II) ion, only the volcanic ash-based geopolymer has a higher q_e value. A lower sorption capacity for Pb(II) is shown by the metakaolin-based geopolymer and the metakaolin-based geopolymer doped with zeolite in a ratio of 1:3. On the contrary, the results obtained for the sorption of the Zn(II) ion show that the algal-based geopolymer has the highest sorption capacity, significantly exceeding the results published by other authors.

In order to determine the kinetics of the sorption process, we fitted the obtained experimental data to pseudo-first- and pseudo-second-order kinetic equation models. A summary of the parameters of both kinetic equations is provided in

Table 5. Values of pseudo-first- and pseudo-second-order kinetic equations for geopolymer with algae addition.

Metal Ion	Kinetic Equations
	Pseudo-First-Order			Pseudo-Second-Order
	k1 [1/min]	qe [mg/g]	R2	k2 [g/mg·min]	qe [mg/g]	R2
Cu(II)	0.0019	1.70	0.9798	1.74·10−4	30.96	1.00
Mn(II)	0.0005	1.70	0.2977	3.60·10−4	136.99	1.00
Zn(II)	0.0042	3.18	0.9413	7.88·10−5	163.93	1.00
Pb(II)	0.0007	1.45	0.0469	3.01·10−5	2000.00	1.00

. The obtained values of the kinetic parameters indicate that the sorption of all tested ions is best described by the pseudo-second-order equation. The pseudo-second-order model assumes that the rate-limiting step in the process is chemisorption, indicating that the adsorption of metal ions on the geopolymer occurs through chemisorption [63]. The literature reports [19,57,64] confirm that the metal ion adsorption process in geopolymers obtained from waste materials and aluminosilicate minerals occurs according to the pseudo-second-order model. The obtained rate constants are equal to, respectively, 1.74 × 10⁻⁴ 1/min for Cu(II), 3.60 × 10⁻⁴ 1/min for Mn(II), 7.88 × 10⁻⁵ 1/min for Zn(II), and 3.01 × 10⁻⁵ 1/min for Pb(II) at 25 °C. Based on the pseudo-second-order equation, the equilibrium concentrations for the studied ions were calculated, which are, respectively, Cu(II)—34.47, Mn(II)—138.02, Pb(II)—2086.64, and Zn(II)—172.27 mg/g. The calculated values are close to the experimental values, which confirms their reliability.

An intramolecular diffusion model was also used to determine the adsorption rate, which is described by Equation (7):

$$q_t=k_p·t^{0.5}+c$$

(7)

where

k_p—intramolecular diffusion rate constant [mg/(g·min^0.5)];

c—thickness of the boundary layer [mg/g].

The results for individual ions are shown in Figure 6. The resulting relationships q_t = f(t^0.5) are not rectilinear, so that adsorption occurs not only by intramolecular diffusion but also in its course, e.g., adsorption in microporous structures, transport of ions in mesopores. The stage that controls the speed of the process is the slowest stage, that is, the boundary layer or intramolecular diffusion. The greater the thickness of the boundary layer, the greater the influence of this layer on the adsorption process [64].

In today’s industrial environment, clean manufacturing is of paramount importance. To achieve this, byproducts such as industrial waste and inexpensive, readily available components such as algae, whose biomass can come from, for example, the cultivation of microalgae on wastewater, are used. This work is in line with this trend, as the hybrid materials obtained with the addition of algae can be used for wastewater treatment in the heavy metal range. In this context, the proposed solution can be a sustainable and ecological method of wastewater treatment. In addition, the use of algae, e.g., from microalgae cultivation in wastewater for the production of geopolymers, can also be used for bioremediation and the circular economy.

5. Conclusions

The galvanic sludge-based geopolymer material obtained with the addition of algae represents a new hybrid material that can be used, for example, in the adsorption of heavy metals from aqueous solutions. It is an example of a green technology due to the use of byproducts, such as industrial waste, as raw materials. Importantly, it shows high sustainability potential for industrial applications. The addition of algae as functional auxiliary materials may be a new direction in the optimization of geopolymer-based adsorbents.

Based on the adsorption studies performed on Cu(II), Mn(II), Pb(II), and Zn(II) ions on geopolymers obtained from galvanic sewage sludge and algae, the following conclusions can be drawn:

• The C-(N)-A-S-H gel is predominant in the structure of the obtained geopolymer material with the addition of algae. Its presence is an essential factor that influences the mechanism, properties, and durability of the geopolymer material obtained.
• The adsorbent used shows the high adsorption efficiency of Cu(II), Mn(II), Pb(II), and Zn(II) ions in the studied model systems, respectively, 96.9—Cu(II), 98.9—Mn(II), 99.7—Pb(II), and 99.5%—Zn(II).
• The adsorption process is well described by the Langmuir isotherm. The equilibrium parameters indicate that the geopolymer used has a good adsorption capacity and that the process occurs by chemisorption.
• The adsorption process efficiency and equilibrium concentration of metal ions were found to increase with time. The adsorption efficiency of the studied metal ions on geopolymers can be written in the following series: Pb(II) < Zn(II)< Mn(II) < Cu(II).
• The maximum sorption capacities for the studied metal ions are equal to, respectively, Cu(II)—29, Mn(II)—135, Pb(II)—161, and Zn(II)—2060 mg/g.
• The adsorption of the investigated Cu(II), Mn(II), Pb(II), and Zn(II) ions on geopolymers obtained from galvanic sewage sludge and algae follows the pseudo-second-order model.
• The analysis of the FTIR spectra of the geopolymer after adsorption of the studied metal ions indicates the involvement of functional groups, i.e., O-H and C-O in the adsorption process, which confirms that the adsorption process of Cu(II), Mn(II), Pb(II), and Zn(II) ions occurs mainly by a chemical sorption mechanism.

The resulting geopolymer material with the addition of algae shows sustainability potential, so it would be advisable to assess the sustainability of the environmental, economic, and social benefits of introducing algae biomass into the geopolymer structure.