1. Introduction
Water remediation is recommended for the safety of human health and the environment. The type of material plays an important role in the adsorption process. It was underlined that the cost of adsorbents and the ability of adsorbents to be reused for a number of adsorption/desorption cycles are key parameters for their practical applications in wastewater treatment [
1,
2]. Apart from many materials, geopolymer materials obtained from wastes are a viable alternative for wastewater treatment. For example, for the removal of cobalt, lead, nickel, and cadmium ions, adsorbents based on pyrophyllite mine waste-based geopolymer [
3] and a geopolymer from dolochar ash [
4] were proposed. Nayak and co-workers [
5] prepared hydroxyapatite synthesized from egg shells and used it for fluoride removal.
Copper is the third most utilized metal in the world [
6,
7], being used in various industries [
8]. These industries must treat their effluent before discharging it [
9]. Although copper is an essential element for organisms [
10], when consumed in excess, it shows deleterious effects such as irritation of eyes, nose, and mouth, stomachache, lung cancer, and neurotoxicity [
11].
Liu and co-workers [
6] provided a review focusing on methods used for the removal of copper ions from wastewaters, such as physicochemical techniques (e.g., membrane separation, ion exchange, chemical precipitation, electrochemistry, adsorption) and biological techniques (e.g., biosorption, bioprecipitation, biomineralization).
Currently, the most applied method used for copper ion removal from wastewater is based on adsorption techniques. The method can be applied to industrial wastewater with high copper content. The scientific literature reported copper concentrations in wastewater ranging from approximately 2.5 mg/L to 10,000 mg/L [
6]. For example: plating (silver) wastewater 3–900 mg/L, brass mills wastewater 4–888 mg/L, copper mills wastewater 19–800 mg/L, copper sulphate manufacture wastewater 433 mg/L, copper smelting wastewater 200–3500 mg/L [
12,
13]. Adsorbents that can be used for copper removal are: carbon nanocomposites [
14], NTA (nitrilotriacetic acid)-silica gel [
15], Azadirachta indica powder [
16], sodium hydroxide (NaOH)-treated rice husk [
17], modified hematite (α-Fe
2O
3) iron oxide-coated sand [
18], imidazothiazole Schiff base functionalized silica [
19], modified activated carbon [
20], alkaline earth metal-based metal-organic frameworks [
21], coal gangue [
22], and geopolymers [
23,
24].
By burning coal in a thermal power plant, an industrial solid waste named ‘fly ash’ is obtained [
25,
26,
27]. Though known to have negative impacts on the environment, fly ash, when treated with an alkali reagent, creates a new class of materials called ‘geopolymers’ [
28,
29,
30,
31]. The common alkali types, such as NaOH, KOH, NaOH/Na
2SiO
3 and KOH/Na
2SiO
3 can be used in geopolymer synthesis [
32,
33,
34]. The study of geopolymer materials as potential adsorbents for the treatment of copper-contaminated waters is gaining popularity [
35,
36].
For example, Mužek and co-workers [
37] prepared an adsorbent for copper ion removal using a type F fly ash mixed with NaOH and Na
2SiO
3 solutions. Four initial concentrations and three temperature values were investigated. The results demonstrate that the prepared geopolymer adsorbent shows excellent ability for copper ion removal. Al-Harahsheh and collaborators [
38] synthesized a fly ash-based geopolymer using the alkali activator NaOH. Their data demonstrated that at a pH of 6 and 25 °C, the maximum adsorption capacity of the prepared material was 96.8 mg/g. By increasing the temperature to 45 °C, the maximum adsorption capacity increased to 152 mg/g. The study performed by obtaining glassy ceramic materials and co-workers [
33] offered information regarding the preparation of geopolymers using a fly ash collected from Indonesia, treated with four alkali reagents, as adsorbents for copper ion removal. The results show that the treated geopolymers have enhanced adsorption capacities compared to unmodified fly ash. Purbasari and collaborators activated fly ash with 10 N NaOH solution and Na-silicate solution [
39]; according to the results, the material is suitable for copper ion removal.
Roviello and co-workers have applied, for the first time, hybrid geopolymeric foams for the removal of different ions, such as Pb
2+, Cd
2+, Cu
2+, and Zn
2+; the results show that the materials are effective in the adsorption of the targeted ions [
40].
Harja and collaborators [
41] conducted a study focused on the treatment of copper-contaminated waters by geopolymer materials derived from fly ash. All the developed adsorbents were obtained by treating a locally sourced fly ash with NaOH using different synthesis conditions and synthesis methods at mild temperatures (<100 °C) for a short period of time. The new products have a significant influence on the reduction of the negative impact of fly ash on the environment (related to its storage). In addition, copper-contaminated waters were treated. The study revealed that the prepared adsorbents show good removal efficiencies. The mentioned study was continued with a principal research objective of establishing the best experimental process conditions in order to obtain higher removal efficiency. Thus, to optimize the working parameters used for copper ion removal (i.e., pH, adsorbent dosage, contact time), our research team studied the neuro-evolutionary method, which incorporates neural modeling and genetic algorithm optimization [
42].
The scientific literature indicates several optimization methods for the removal of metal ions from wastewater by adsorption, such as the Taguchi approach, Plackett–Burman Design, and Response Surface Methodology (based on three-level full factorial design, Box–Behnken design, central composite design, or Doehlert design) [
43,
44,
45]. The optimization of copper ion removal was investigated by Response surface methodology [
44], and the optimization of lead ion removal was investigated by Box-Behnken design [
45]. Bayuo and co-workers [
43] present in their critical literature review the Response surface optimization and modeling in heavy metal removal from wastewater. The optimal conditions for the copper adsorption process can be established by using the Taguchi method [
46,
47]. This method is applied to optimize various processes in a wide range of areas, being able to find an optimized design configuration for multifactorial conditions. Compared with other optimization methods, the approach of ranking the controllable factors that influence the analyzed process allows a better visualization of the optimal conditions and requires much less experimental data. The major advantages of the Taguchi method are: (i) keeping the experimental cost to a minimum because a small number of trials are carried out; and (ii) reducing the time of experimental studies and establishing the most effective parameter that influences the process [
48,
49,
50].
The Taguchi method is based on the realization of an orthogonal matrix that distributes the variables in a balanced way, and the experimental results are converted into a signal/noise ratio (S/N), which describes the level of dispersion and the degree of optimization in relation to the desired value [
51]. The term ‘signal’ represents the desired value (mean) for the output characteristic, while the term ‘noise’ represents the undesired value (standard deviation) [
48,
49,
52]. For example, if there are six controllable factors at three levels, a fully classical factorial design must use a number of 36, i.e., 729 experiments, to establish optimal conditions that characterize the process. In this case, the Taguchi method uses an L27 orthogonal matrix, which reduces the number of experiments to 27 [
49,
53].
The study highlights the possibility of using fly ash-based geopolymer materials for copper ion removal and gives detailed information regarding the process optimization insights from Taguchi and ANOVA statistical methods. The Taguchi method was utilized to establish the optimal conditions for the copper ion removal process by adsorption technique [
54,
55]. The ANOVA analysis (general linear model) was used to determine the contribution of each working parameter on the removal efficiency [
56,
57]. Five process variables were considered for the present research: type of adsorbent, solution pH, adsorbent dose, initial copper concentration, and reaction time. To the best of our knowledge, there are no data regarding the optimization conditions of copper ion removal using Taguchi and ANOVA methods for geopolymer materials synthesized from two different fly ash sources.
The following aspects are the strong points that make this study novel for a wide public: (1) the starting material, fly ash, does not involve any costs for purchase; (2) the inexpensive and simple preparation technique (the materials do not require more chemicals and energy); (3) a one-step process for the synthesis of fly-ash-based geopolymers is foreseen, which ensures nearly 100% recovery.
4. Conclusions
This study explores the fly-ash geopolymers prepared from two fly ashes (local and imported types) in mild conditions of temperature as new potential low-cost and environmentally friendly materials for copper ion removal. The performance of synthesized materials was investigated at different operating parameters. To obtain the optimal conditions for the adsorption process, the Taguchi method was used, while the ANOVA analysis (general linear model) was used to determine the contribution of each working parameter on the removal efficiency.
The factor that most influences the adsorption process is the type of adsorbent used, followed by the solution pH, the reaction time, the adsorbent dose, and the initial copper ion concentration. The results of the ANOVA analysis confirm the results obtained by the Taguchi method, indicating the same order of influence of the controllable factors on the adsorption process. A good correlation coefficient between the values was found (R2 = 0.981), which indicates good accuracy for the prediction of the optimization method by Taguchi method.
Newly designed adsorbent, CenNa, can contribute to clean water having a high removal performance of 99.71%. Further, based on the promising results obtained in the present study, we are looking for in-depth research having the objectives: (i) a detailed characterization including other techniques, (ii) isotherms/kinetics/thermodynamics evaluation, (iii) characterization of the loaded material for the suggestion of an adsorption mechanism, (iv) regeneration for multiple cycles using various desorption agents.
For an industrial plant, the technological parameters and especially the controllable ones are the most important. The management and regulation of these parameters are essential for the proper development of the industrial process, while the pollutant retention mechanism became secondary. That is the reason why the optimization methods, used in industrial practice, focus on controllable parameters.
The adsorbents containing heavy metal ions can be easily regenerated, after which, due to the predominant mineral part, they can be used in the cement industry, or for obtaining glassy ceramic materials. Future studies will be focused on these aspects as well.
Therefore, the study can draw the attention of researchers and the removal of other types of pollutants from wastewaters can be investigated by using the findings and fly ash-based geopolymer materials synthesized by our proposed methods.