Utilizing Cement Kiln Dust as an Efficient Adsorbent for Heavy Metal Removal in Wastewater Treatment

Abstract

Cement kiln dust (CKD), a by-product of cement manufacturing, has been largely underutilized despite its potential as an eco-friendly adsorbent for wastewater treatment. This study addresses the knowledge gap regarding CKD’s effectiveness in removing heavy metals from wastewater residuals. A comprehensive experimental program was conducted to optimize key parameters such as the pH (6–9), contact time, sorbent dosage, and initial heavy metal concentrations using a batch equilibrium technique. The results demonstrated that CKD can effectively remove heavy metals, achieving removal efficiencies of 98% for Pb, 94% for Zn, 92% for Cu, and 90% for Cd within just 4 h of treatment. Importantly, CKD not only provided high adsorption efficiency but also resulted in a significant reduction in the formation of hazardous solid sludge, a major concern in traditional wastewater treatment methods. The adsorption data closely matched the Langmuir isotherm model, further validating CKD’s potential as a sustainable, cost-effective solution for reducing heavy metal contamination in wastewater while minimizing the environmental impact.

1. Introduction

Heavy metals, considered widespread pollutants, have been found in a significant volume of wastewater produced by various industries worldwide [1,2]. The effective removal of heavy metals and other toxic pollutants from wastewater is crucial for safeguarding human health and protecting vulnerable ecosystems. As a result, considerable efforts have been dedicated to creating practical and sustainable solutions to tackle this problem [3]. Heavy metals that surpass permissible limits are considered carcinogenic and harmful to the environment, with their effects lingering and eventually becoming apparent in humans, animals, and plants, even at low concentrations [4].

Over the years, researchers and scientists have developed various technological methods of remediation to prevent the escalation of heavy metal contamination. Many techniques could be used to treat these types of wastewaters such as biological, chemical, and physical treatments. However, many of these techniques including novel techniques such as electrochemical techniques still face several limitations, including low metal removal efficiency, limited versatility, complex processes, low technical maturity, challenges with scalability and automation, high environmental impact, and high operational costs [5]. In contrast, the adsorption technique has gained significant attention due to its numerous advantages, making it one of the most effective and widely used methods in wastewater treatment (WWT) applications, including the removal of metals from aqueous media [6,7].

Adsorption techniques offer many advantages, including unburnt carbon, low overall cost, minimal sludge production, and simplified operation procedures [8]. Many authors have used adsorption techniques to remove heavy metals and contaminates using different materials [9] such as clay [10], montmorillonite [11], bentonite [12], glauconite [13], kaolin [14], calcite [15], hydrochar [16], and zeolite [17]. Recently, carbon-based adsorbents, chitosan-based adsorbents, mineral adsorbents, bio-sorbents, and magnetic adsorbents have emerged as highly regarded materials for effectively removing and recovering both harmful and valuable metals from various aqueous environments [9].

A few studies have used cement kiln dust as a valuable adsorbent for heavy metal removal. The cement manufacturing sector typically employs dry methods in the production of cement, resulting in the generation of a large amount from cement kiln dust (CKD) as a side by-product [18]. This CKD is considered both a waste material and a source of pollution [18] and should be managed for environmental security. Management could be used, as will be shown further in the current study, to use this waste in a beneficial manner to mitigate its environmental impact. Some authors have revealed and shown that CKD has a high adsorption capacity and abundant availability as a low-cost by-product of the cement industry and, from this point of view, that CKD could be used to adsorb organics or heavy metals [19]. CKD consists of quartz, a small quantity of gypsum, sodium chloride, and limestone, which serves as its primary component [20]. Moreover, it includes various coagulants that contribute to its exceptional adsorbent characteristics [21]. One of the most common uses of CKD is in the treatment of sewage sludge, where it functions as a chemical conditioner and stabilizer. In comparison to lime and other coagulants and adsorbents, CKD proves to be more cost-effective, presenting itself as a feasible choice for waste treatment while maintaining high performance standards [22].

Numerous research studies have indicated that CKD can be extensively utilized for the elimination of various heavy metals like lead, copper, and cadmium [23]. An extensive experimental investigation has been carried out to explore the different key parameters that impact the efficiency of CKD in removing organics from wastewater [24]. Some findings from other research that used CKD have shown a significant adsorption capacity for simulated sewage sludge at a pH range from 5.5 to 8 [25]. Some research used Langmuir and Freundlich isotherms as models in many studies of CKD adsorption kinetics, largely because of their ability to represent a monolayer formation on the surface [26]. CKD was used in sewage and water treatment applications, where it serves as a chemical conditioner and stabilizer. It is more cost-effective than lime and other coagulants and adsorbents, offering the same performance while helping to reduce waste treatment costs. Ref. [27] investigated the efficiency of COD, color, and heavy metal removal from textile wastewater; the obtained results showed that the adsorption of COD and heavy metals increased as the addition of CKD and water treatment residuals (WTRs) was increased. In addition, it was found that the maximum Langmuir adsorption was higher with WTRs (100.0 mg g⁻¹) compared to CKD (14.3 mg g^–1). Additionally, the Langmuir adsorption capacity was greater with WTRs than CKD. Ref. [24] examined the adsorption efficiency of CKD towards wastewater treatment; the obtained results indicated that CKD was highly effective in removing water contaminants and heavy metals. It demonstrated the significantly higher efficiency of CKD compared to alum as a coagulant. CKD achieved removal efficiencies ranging from 61.2% to 97.2%, while alum achieved removal efficiencies of between 39.7% and 96.8% for all contaminants, following the application of optimal conditions.

From the previous concerns, searching for a better adsorbent material with cost effectiveness and accelerated removal efficiency was suggested and investigated for heavy metal removal. The primary objective of this research project is to examine the adsorption behavior of CKD as an efficient adsorbent, with a specific focus on eliminating heavy metals like Pb, Zn, Cu, and Cd from sewage wastewater. To identify the optimal treatment conditions, a batch experimental program was conducted to monitor various factors, including the pH, contact time, sorbent dosages, and initial concentrations of heavy metals. The optimal operating condition was determined and assessed. Subsequently, the experimental results were also assessed using the Langmuir isotherm model to determine the necessary parameters. This study was conducted by studying many factors with one model to prove the ability of CKD to adsorb the heavy metals from contaminated sewage.

2. Materials and Methods

2.1. Research Methodology Outline

Figure 1 illustrates the research methodology for the wastewater treatment process utilizing CKD adsorbent.

The CKD wastewater treatment process involves several key steps to remove heavy metals and contaminants. First, cement kiln dust (CKD) is added to the wastewater, where its high alkalinity facilitates the precipitation of metal ions as insoluble compounds. CKD’s porous structure allows it to adsorb heavy metals while also promoting to binde suspended solids and organic matter and settle. Finally, treatment analysis is conducted to confirm the reduction in heavy metal concentrations, ensuring the water meets the desired quality standards.

2.2. Cement Kiln Dust (CKD)

CKD was obtained from the Suez Cement Company, located in Cairo, Egypt. Prior to utilization, it underwent a drying process for a duration of 24 h at a temperature of 105 °C. The chemical composition of the CKD was analyzed using Energy Dispersive X-ray Analysis (EDX) with the help of a (Philips ESEM XL 30 FEG) (Philips, Amsterdam, The Netherlands). The results of this analysis are presented in

Table 1. Chemical composition of CKD (%w/w).

Components	Average (%)
SiO2	21.5 ± 1.9
Al2O3	7.4 ± 0.6
Fe2O3	4.5 ± 0.2
CaO	43.5 ± 2.4
MgO	2.8 ± 0.2
K2O	6.3 ± 0.3
Na2O	6.3 ± 0.4
TiO2	1.1 ± 0.1

. Furthermore, scanning electron microscope (S-4800, Hitachi) (Tokyo, Japan) analyses revealed that the dust particles in CKD exhibited a spherical shape, consisting of a calcium carbonate and clay core, along with an alkali coating [28].

2.3. Sewage Sludge Samples

The wastewater samples used in this study were obtained from the Zeinin wastewater treatment facility, located in Giza, Egypt, which serves as a secondary treatment plant. This facility processes large volumes of sewage sludge, with a daily intake of over 20,000 m³ of liquid return activated sludge (RAS). The RAS, with a solids content of approximately 0.5%, is generated as part of the biological treatment process to remove organic pollutants from the sewage. A 75 L sample of this RAS was collected to initiate the experimental work, ensuring that the sludge characteristics accurately represent the typical wastewater residuals found in municipal treatment systems in the region. The sample was carefully analyzed for key parameters such as the pH, heavy metal concentrations, and organic content, providing a realistic and relevant basis for testing the potential of cement kiln dust (CKD) as an adsorbent for heavy metal removal. This approach reflects the representativeness of this study, with the sample reflecting the typical characteristics of sewage sludge from large-scale treatment plants in Egypt.

2.4. Wastewater Adsorption Treatment Experiments

The adsorption of heavy metals (Pb, Zn, Cu, and Cd) on cement kiln dust (CKD) was carried out through batch experiments using 125 mL high-density polyethylene (HDPE) containers, designed to ensure minimal interaction with the adsorbent. The experimental conditions were carefully selected to mimic realistic wastewater treatment scenarios and to optimize the adsorption process.

A specific amount of CKD was equilibrated with 50 mL of heavy metal solutions at known initial concentrations, ranging from 20 to 200 mg/L. This level of heavy metals causes serious health issues. The toxicity produced by the heavy metals may harm the mental and central nervous activities and damage the lungs, liver, kidneys, blood composition, and other organs, and this was mentioned by [29]. Moreover, most of the literature that used the adsorption method for heavy metals used the same level [30]. These solutions were prepared and placed in Erlenmeyer fasks using an orbital shaking incubator (DAIHAN ThermoStable™ IS-30, Seoul, Korea). at a constant temperature of 24 °C. The contact time varied from 5 min to 5 h, chosen based on previous studies indicating that equilibrium adsorption often occurs within this time frame for similar adsorbent systems. The goal was to identify the time required for CKD to reach adsorption equilibrium, ensuring efficient heavy metal removal while preventing the over-exposure of the adsorbent.

The pH range for the experiments was set between 3.0 and 11.0. This wide pH range was selected to evaluate the influence of different pH levels on the adsorption capacity, as pH is a critical factor affecting metal ion speciation and the surface charge of CKD. Acidic conditions (pH < 5) generally enhance the removal of certain metals due to the increased solubility of metal hydroxides, while higher pH values promote metal precipitation and adsorption on the CKD surface. These pH values were chosen to reflect the variability in real wastewater systems, where the pH often fluctuates depending on industrial discharges and natural water characteristics. The pH of the adsorptive solutions was adjusted using hydrochloric acid (HCl), sodium hydroxide (NaOH), and buffer solutions to maintain the required conditions. Following filtration, the solutions were acidified to pH < 2.0 using 1:1 nitric acid (HNO₃) for ICP analysis. The results from these experiments were used to identify the optimal pH, contact time, and sorbent dosage for maximal removal of heavy metals from aqueous solutions.

Sorbent dosages were tested over a range from 0.2 to 2.0 g/L. This range was selected based on a balance between cost effectiveness and the efficiency of metal removal. Lower dosages are more economically feasible, but higher dosages may enhance the adsorption capacity, especially in cases of high contamination. The dosages were chosen to evaluate the adsorbent’s performance at varying concentrations and to identify the optimal dose that would achieve maximum removal efficiency without a significant waste of CKD.

Ionic strength was maintained at 0.01 M to simulate the typical environmental conditions found in wastewater. The use of simulated sewage sludge was crucial for better representing the characteristics of real industrial and municipal effluents. After equilibrium was reached, the adsorbent suspension was separated from the solution through filtration using Whatman filter paper. The remaining concentration of heavy metals in the filtrate was measured inductively coupled plasma optical emission spectrometer (ICP-OES, Agilent 5100) (Santa Clara, CA, USA). Blank experiments were conducted to ensure that metal ion adsorption on the container walls was negligible. These steps were repeated twice for confirmation. A comprehensive analyses were conducted on both raw and treated samples.

2.5. Quality Control Aspect

In order to ensure the precision, dependability, and consistency of the gathered data, each batch experiment was conducted twice, and the average values of the two datasets are provided. If the relative error surpassed the relative standard deviation by a margin of 1.0% or more, the data were deemed inconsistent, and a third experiment was performed until the relative error fell within an acceptable range.

2.6. Removal and Adsorption of Heavy Metals

The percentage of heavy metal removed was determined by the following formula:

R = (C₀ − C) × 100/C₀

(1)

where the following variables are used:

• C₀: initial concentration of heavy metal ions in the test solution, in mg/L.
• C: final equilibrium concentration of the test solution, in mg/L.

2.7. Sorption Isotherm Model

In this research, the sorption equilibrium on CKD is analyzed using the Langmuir isotherm. The equation for the sorption equilibrium is given by

q_e = (q_m K_L C_e)/(1 + K_L C_e)

(2)

Additionally, the Langmuir adsorption isotherm equation can be expressed linearly as

1/q_e = (1/q_m) + (1/q_m K_L) (1/C_e)

(3)

K_L (L/mg) is the Langmuir constant related to adsorption energy, Ce is the equilibrium concentration in mg/L, and Qe is the adsorbate amount adsorbed per unit weight (mg/g).

3. Results and Discussion

The effect of various batch study treatment operational conditions including the pH, contact time, sorbent dosages, and initial concentrations of heavy metals on the wastewater pollutant removal efficiencies are investigated and analyzed in detail in the following sections.

3.1. Impact of pH on Removal of Different Heavy Metals from Simulated Sewage Sludge

The absorption of heavy metals from simulated sewage sludge is greatly influenced by pH. According to the graph in Figure 2, the highest percentage of heavy metal removal based on the pH levels is as follows: Pb—97%; Zn—91%; Cu—88%; and Cd 82%. These results were obtained with a CKD concentration of 1.6 g/L. Generally, the removal of heavy metals increased as the pH levels rose, reaching a peak of over 70% removal at a specific pH. The increase in pH from 5.7 to 8.7 resulted in a more negatively charged CKD surface, which enhanced the electrostatic attraction forces and improved the adsorption of cationic metal ions. However, the impact of pH on heavy metal adsorption varied for each metal. For Pb, the removal was 25% at pH 4 and increased to 97% at pH 6. Zn removal was 35% at pH 5 and increased to 92% at pH 7. Cu removal was 45% at pH 5 and increased to 88% at pH 8. Cd removal increased proportionally with pH, from 50% at pH 6 to 82% at pH 9. The percentage of adsorption tended to peak between pH 6 and 9 before decreasing with further increases in pH. It is commonly believed that the tendency of metal cations to adsorb to oxide surfaces is closely related to their hydrolysis reactions in solution, as suggested by Pang et al. (2011) [31] and supported by Kiran et al. (2019) [21]. Moreover, at a low pH, protons that are present at the surface sites cause a high positive charge density, and consequently, the electrostatic repulsion will be high during the uptake of metal ions, resulting in lower removal efficiency, as mentioned by the literature [32].

3.2. Impact of Contact Time on Removal of Different Heavy Metals from Simulated Sewage Sludge

Figure 3 illustrates the impact of contact time on the percentage of heavy metal removal. The experimental conditions were as follows: pH 7, initial concentration of 40 mg/L, and sorbent dose of 18 g/L. The duration of contact time played a crucial role in this study as it influenced the adsorption kinetics of the adsorbent under specific conditions [33]. It is evident that the adsorption rate of metals, namely Pb, Zn, Cu, and Cd, was relatively rapid during the initial 20, 30, 40, and 60 min, respectively, throughout the entire experimental period. The CKD uptake capacity reached a breakthrough point, with removal percentages exceeding 93%, 89%, 86%, and 80% for Pb, Zn, Cu, and Cd, respectively, after 15, 30, 45, and 60 min. Subsequently, the CKD’s uptake capacity gradually declined. This can be attributed to the limited presence of numerous vacant surface sites that are available for contaminant adsorption. This observation aligns with the proposed mechanism suggested by Ref. [33], who found that significant removal of Mn, Cu, Pb, Cd, Zn, and Cr using CKD could be achieved after a contact time of 30 min.

3.3. Impact of Sorbent Dose on Removal of Different Heavy Metals from Simulated Sewage Sludge

The outcomes regarding the removal of heavy metals through adsorption in relation to the adsorbent dosage are illustrated in Figure 4 under the conditions of pH 7, initial heavy metal concentrations of 80 mg/L, and a contact time of 45 min. The removal percentage of heavy metal ions displays a consistent upward trajectory as the adsorbent dosage increases. Transitioning from a sorbent dose of 0.2 g/L to 2 g/L results in a rise in the heavy metal removal percentage by 62%, 59%, 57%, and 56% for Pb, Zn, Cu, and Cd, respectively. This can be ascribed to the enhanced availability of exchangeable sites or surface area. Furthermore, the percentage of metal ion adsorption on the adsorbent is influenced by the adsorption capacity of the adsorbent towards different metal ions.

3.4. Impact of Initial Concentrations on Removal of Different Heavy Metals from Simulated Sewage Sludge

Figure 5 presents the impact of the initial concentration on the removal of heavy metals by CKD under specific conditions. These conditions include a pH of 7, a sorbent dose of 1.2 g/L, and a contact time of 60 min. The figure clearly illustrates that as the initial heavy metal concentration increases, the percentage removal decreases. It is worth noting that when the initial heavy metal concentration increased from 20 to 200 mg/L, a significant decrease in CKD heavy metal uptake was observed. The percentages for Pb, Zn, Cu, and Cd were 26%, 25%, 33%, and 35%, respectively. This indicates that at lower initial metal ion concentrations, there are enough adsorption sites available for the heavy metal ions. Therefore, the fractional adsorption remains unaffected by the initial metal ion concentration. However, at higher concentrations, the number of heavy metal ions surpasses the available adsorption sites. Consequently, the percentage removal of heavy metals is influenced by the initial metal ion concentration and decreases as it increases. The variation in the percentage removal of different heavy metal ions, even under the same initial metal ion concentration, sorbent dose, and contact time, can be attributed to differences in their chemical affinity and ion exchange capacity in relation to the chemical functional group on the adsorbent’s surface.

The interactions between cement kiln dust (CKD) and metal ions in wastewater involve several adsorption mechanisms, including electrostatic attraction, ion exchange, chemical precipitation, and surface complexation. CKD’s alkaline nature and high surface area enable it to attract positively charged metal ions, such as Pb²⁺, Zn²⁺, Cu²⁺, and Cd²⁺, through electrostatic forces. Ion exchange occurs when metal ions replace calcium ions in CKD, while chemical precipitation forms insoluble metal hydroxides or carbonates, further removing metals from the solution. Additionally, metal ions may form coordination bonds with oxygen atoms on CKD’s surface, enhancing adsorption. These combined mechanisms contribute to CKD’s effectiveness in removing heavy metals from wastewater.

The differences in the heavy metal removal efficiencies for Pb (98%), Zn (94%), Cu (92%), and Cd (90%) using cement kiln dust (CKD) can be attributed to several factors. Pb exhibits the highest removal due to its strong electrostatic interaction with CKD and its tendency to precipitate as Pb(OH)₂ at a higher pH, making it highly available for adsorption. Zn also adsorbs effectively, though its removal is slightly lower due to its more complex hydrolysis and precipitation behavior. Cu and Cd show slightly reduced efficiencies due to their propensity to form stable complexes with other ions and to undergo hydrolysis, limiting their availability for adsorption. The pH range (6–9) and the ionic strength of the solution influence metal speciation and CKD’s surface charge, with higher pH favoring precipitation and enhancing adsorption for metals like Pb and Zn, while complexion at higher pH affects Cu and Cd removal. These factors, along with the adsorbent dosage and the presence of competing ions, collectively explain the observed differences in removal efficiencies.

3.5. Adsorption Isotherm for Removal of Different Heavy Metals from Simulated Sewage Sludge

Table 2. Langmuir constants for the sorption of different heavy metals onto CKD adsorbent.

Heavy Metals	Langmuir Constants
	Qm (mg/g)	KL(L/mg)
Pb	0.011	0.489
Zn	0.024	0.373
Cu	0.271	0.047
Cd	0.041	1.112

presents the Langmuir constants for the sorption of various heavy metals onto the CKD adsorbent at a temperature of 25 ± 2 °C [34]. Furthermore, Figure 6 visually depicts the adsorption of different heavy metals, expressed linearly in the Langmuir adsorption isotherm.

Conversely, in order to verify the precision of the heavy metal removal efficiency data from the experiment based on the Langmuir isotherm model, three statistical measures were chosen to assess the accuracy of the heavy metal data in relation to the Langmuir isotherm model used in the experiment:

I—Relative Mean Absolute Error (MAE)_rel.

The MAE_rel can be estimated as follows:

$$MAE=\left[{\displaystyle \frac{1}{n}}\sum _{i=1}^n\left|Y_{Observied}-Y_{Simulated}\right|\right]$$

(4)

$${\left(MAE\right)}_{rel}={\displaystyle \frac{MAE}{\overline{Y_{Observied}}}}$$

(5)

II—Percent Bias (PBIAS).

The ideal value for PBIAS is zero, and this statistical measure can be calculated as follows:

$$PBIAS=100\ast {\displaystyle \frac{\sum _{i=1}^n\left(Y_{Observied}-Y_{Simulated}\right)}{{\sum }_{i=1}^n{Y}_{Observied}}}$$

(6)

III—Nash–Sutcliffe Efficiency (NSE).

The NSE typically varies between zero and one, with an ideal value of one. The NSE is computed as follows:

$$NSE=1-\left[{\displaystyle \frac{\sum _{i=1}^n{\left(Y_{Observied}-Y_{Simulated}\right)}^2}{{\sum }_{i=1}^n{\left(Y_{Observied}-\overline{Y_{Observied}}\right)}^2}}\right]$$

(7)

Table 3. The statistical evaluation measures.

Heavy Metals	MAErel	PBIAS	NSE
Pb	0.10	0.13	0.95
Zn	0.09	0.11	0.92
Cu	0.07	0.10	0.96
Cd	0.08	0.08	0.94

presents the statistical assessment metrics for the compliance of the observed experiment’s heavy metal removal efficiency data with the Langmuir isotherm model previously employed. The efficiency data for removing heavy metals in this study align well with the Langmuir isotherm model. Nevertheless, a recommended fitness value of zero for the MAPE and PBIAS was observed. Additionally, the NSE results show an optimal compliance value of 1.00. The adsorption data demonstrate a strong agreement with the Langmuir model, supported by high values in various statistical evaluation measures, indicating a robust fit with the model. Our findings follow [25] regarding the Langmuir model and metal adsorption capacity as for CKD. Moreover, a comparison of the maximum metal adsorption capacity achieved with CKD in this study and other similar low-cost and/or waste-based adsorbents is shown in

Table 4. Langmuir monolayer adsorption capacity of the adsorbents for the removal of Zn, Cu, and Cd ions.

Adsorbents	Metal	Langmuir Monolayer Adsorption Capacity, qm (mg/g)	Ref.
Cedar leaf ash	Zn	4.8	[35]
Bael tree leaf powder	Zn	2.1	[34]
Chitosan–PVA blend	Zn	5.9	[36]
Cement kiln dust	Zn	1.9	This study
Unmodified Strychnos potatorum seeds	Cu	8.6	[37]
Raw Caryota urens seeds	Cu	5.0	[38]
Activated alumina	Cu	4.3	[39]
Cement kiln dust	Cu	1.9	This study
Spruce wood	Cd	2.0	[40]
Bael tree leaf powder	Cd	1.8	[41]
Bagasse fly ash	Cd	1.2	[42]
Cement kiln dust	Cd	1.8	This study

.

4. Conclusions

This study demonstrated the effectiveness of cement kiln dust (CKD) as a cost-effective and environmentally friendly adsorbent for removing heavy metals (Pb, Zn, Cu, and Cd) from wastewater. The optimal removal occurred at a pH range of 6 to 9, with removal efficiencies of 98%, 94%, 92%, and 90%, respectively. The adsorption data fitted well with the Langmuir isotherm, suggesting a monolayer adsorption mechanism. The findings highlight CKD’s potential for large-scale wastewater treatment, particularly in industries with heavy metal contamination. CKD offers a sustainable, low-cost solution that can reduce hazardous sludge formation, making it a viable alternative to conventional adsorbents in real wastewater treatment plants.