Multicomponent Adsorption of Pollutants from Wastewater Using Low-Cost Eco-Friendly Iron-Modified Rice Husk Biochar in the Era of Green Chemistry

Abstract

As pollution escalates, water purification is becoming crucial, with adsorption emerging as an efficient technique. This study aimed to evaluate the effectiveness of iron-modified rice husk biochar as an adsorbent for water pollutants. The biochar was produced at a pyrolysis temperature of 500 °C and modified by FeSO₄·7H₂O. Diverse adsorbent dosages were introduced to simulated wastewater containing aldrin, mercury (Hg²⁺), lead (Pb²⁺), and cadmium (Cd²⁺). The solution was agitated for 60 min, then filtered, and the samples were sent for analysis. The results were promising; aldrin had a removal efficiency of 100%. The removal efficiency for Hg²⁺ ranged from 99.80% to 99.96%, for Pb²⁺ from 88.90% to 99.56%, and for Cd²⁺ from 78.90% to 99.98%. The Freundlich adsorption isotherm best described the mono- and quaternary component systems, while the Langmuir isotherm was the best fit for the binary system. Therefore, iron-modified rice husk biochar shows potential as a sustainable and efficient adsorbent for wastewater treatment.

1. Introduction

Pollutants such as heavy metals and pesticides are hazardous to both human health and the ecosystem [1]. There is a need to address pollution to ensure sustainability. Adsorption is a successful treatment strategy for removing these contaminants from polluted water [2]. Because of its low cost and availability, rice husk biochar in particular is a potential material that can be used as an adsorbent for the removal of these contaminants [3]. It has been discovered that modifying rice husk biochar with iron improves its adsorption capabilities [3]. Adsorption is the process by which contaminants are attracted to the surface of a solid object [4].

Biochar has recently emerged as a viable adsorption medium due to its enormous surface area, porosity, and chemical reactivity [4]. Rice husk biochar is a popular charcoal because it is readily available and affordable [5]. There is a lot of interest in using low-cost, abundant natural materials as heavy metal adsorbents [6]. Among these materials, rice husk has been widely investigated for heavy metals removal due to its high silica content and large surface area [6].

Researchers have modified rice husk biochar with various materials, including iron, to enhance its adsorption capacity towards different contaminants [6].

Iron-modified rice husk biochar has been found to have remarkable adsorption ability towards a variety of pollutants [7]. The addition of iron to rice husk biochar improves its adsorption efficacy for contaminants such as heavy metals, organic pollutants, and nutrients [7]. Iron is a highly reactive material that forms strong bonds with contaminants, and its presence on the surface of rice husk biochar provides additional binding sites for contaminants [7].

The use of iron-modified rice husk biochar for the adsorption of contaminants has various advantages over other materials. Iron-modified rice husk biochar is an environmentally friendly material that is produced from an abundant and inexpensive source. Moreover, the modification of rice husk biochar with iron enhances its adsorption efficiency towards various contaminants, making it an ideal material for the remediation of contaminated environments [7]. The goal of this research is to use iron-modified rice husk biochar as an adsorbent to remove Hg²⁺, Pb²⁺, Cd²⁺, and aldrin from aqueous solutions. This research will concentrate on the optimization of the iron-modified rice husk biochar fabrication process, the assessment of the modified biochar’s adsorption capability for pollutants, and the analysis of the mechanisms involved in the adsorption process.

Research on contaminant removal from aqueous solutions has predominantly centered on single-component adsorption [8,9,10,11,12]. However, in real-world wastewater scenarios, contaminants seldom exist in isolation; they coexist, leading to potential interactions during the adsorption process. Despite the evident complexities introduced by these interactions, there is a notable paucity of comprehensive studies on multicomponent adsorption.

This research gap is concerning. Without robust data on multicomponent systems, the applicability and efficacy of water treatment strategies, based on single-component adsorption data, remain questionable in real-world contexts. Given the global imperative for clean water and the multifaceted nature of wastewater contaminants, there is an urgent need to prioritize and expand research on multicomponent adsorption dynamics. Addressing this gap is not only academically pertinent but also crucial for the development of effective water treatment methodologies.

The exploration of iron-modified biochar also represents a pioneering approach in the realm of adsorbent enhancement. This innovative research not only introduces a novel modification strategy but also offers a critical comparative assessment, underscoring its potential to revolutionize current methodologies in contaminant removal. The systematic evaluation across varied dosages, pH levels, and initial concentrations, spanning mono-, binary, and quaternary systems, offers unprecedented insights into the intricate removal mechanisms and capacities, especially in the context of competitive interactions among coexisting adsorbates. A distinctive feature of this research is the utilization of rice husk biochar, a largely untapped waste biomass, underscoring its remarkable promise as a cost-effective and sustainable adsorbent. Despite the extensive body of research on contaminant removal, focused investigations into aldrin and related organochlorine compounds remain limited. This study’s distinct contribution lies in its demonstration of aldrin’s effective removal using modified biochar. This finding not only fills a notable research gap but also highlights the modified biochar’s potential efficacy against a class of historically understudied pollutants.

2. Methodology

2.1. Rice Husk Biochar Production

Rice husks (RH) were gathered and collected from rice farmers in the Tamale metropolis. The rice husks were subsequently carried to the University of Development Studies, Nyankpala Campus. The rice husks were extensively examined to ensure that no foreign materials or biomass remained. Following that, the rice husk biochar was put in a 20 cm crucible and heated in a muffle furnace at 500 °C with restricted oxygen (O₂) for 60 min. The produced biochar was grounded with a laboratory mortar and pestle before being sieved through a 2 mm sieve.

2.2. Modification of Biochar

The obtained biochar was modified by adding it to a solution of Iron (II) sulphate 7- hydrate (FeSO4.7H2O) at a ratio of 20։1 (biochar Fe w/w) and stirring for 15 min to obtain a homogeneous mixture. The modified biochar was then removed and oven-dried at 40 °C for 48 h. The biochar was then placed in a desiccator to dry and remove moisture. The modifications to the rice husk induce changes in its physical and chemical properties, such as an increased surface area, improved adsorption capacity, and enhanced thermal stability [6]. These modifications can make rice husk more useful for a variety of applications, including as a sorbent for removing pollutants from water.

2.3. Preparation of Stock Solution for Simulated Wastewater

The study was conducted in the Spanish Laboratory at the University for Development Studies’ Nyankpala Campus in Ghana. An accurately weighed 1.68 g of cadmium chloride was dissolved to create the stock solution for the aqueous phase (CdCl2: grade; anhydrous, assay; 99.99%), 1.35 g of mercury chloride (HgCl2: grade; ACS reagent, assay; ≥99.5%) and 1.60 g of lead nitrate (Pb (NO3)2: grade; GR, assay; 99.5%) in deionized water to prepare solutions of 1000 mg/L concentrations of the aqueous phase. To obtain the quantity of compounds containing 1 mg of each heavy metal, the molecular weights of CdCl₂ (183.32), HgCl2 (271.50), and Pb(NO3)2 (331.21) were computed and divided by the atomic weights of Cd²⁺ (122.41), Hg (200.60), and Pb²⁺ (207.20). A 1000 mL volumetric flask was used to make mixes of heavy metal solution. A stock solution of organochlorine (aldrin) in 1000 ppm was obtained from the Ghana Standard Board laboratory and kept in an airtight glass container at a cool temperature of 5 °C. To obtain the desired concentrations, serial dilutions were performed, to obtain maximum concentration limits of the leaching contaminants at different concentration limits. The serial dilutions were prepared using the following equation:

M₁V₁ = M₂V₂

(1)

where M₁ signifies the leaching stock solution concentration; V₁ denotes the volume of leaching stock solution required; M₂ denotes the heavy metal concentration necessary; and V₂ denotes the volume of distilled water to which the leaching stock solution is added. To dilute the aldrin stock, the same formula was employed. Instead of distilled water, acetonitrile solution was used to dilute aldrin to the required concentration. Aldrin was chosen as the solvent since it is insoluble in water but soluble in acetonitrile.

2.4. Adsorption Experiment in Batches

The adsorption experiment was carried out at the Spanish Laboratory of the University for Development Studies. Batches of mono-, binary, and quaternary component systems were used in the experiment. Batch tests were carried out to assess and compare the adsorptive ability of iron-modified rice husk biochar, produced at 500 °C on different pollutants.

The adsorption experiment for aldrin consisted of pipetting 1.00 mg/L of aldrin from the prepared aldrin solution (1 mL of aldrin pipetted into a beaker and 9 mL of acetonitrile pipetted and added to it) into 1000 mL of deionized water in a 1000 mL volumetric. The solution was corked, vigorously mixed, and agitated to ensure consistency. A volume of 100 mL of the solution was then put into a conical flask, containing precisely weighed rice husk biochar (1 g) (the solution’s initial pH would be measured) (

Table 1. Maximum contamination limits of pollutants at varied dosages in aqueous phase.

Batch	Contaminants	Conc. (Onefold) (mg/L) at 1 g of Dosage	Conc. (Threefold) (mg/L) at 5 g of Dosage	Conc. (Fivefold) (mg/L) at 10 g of Dosage
Mono	Hg2+ Pb2+ Cd2+ Aldrin	1.00 1.00 1.00 1.00	3.00 3.00 3.00 3.00	5.00 5.00 5.00 5.00
Binary	Hg2+:Pb2+ Hg2+:Cd2+ Cd2+:Pb2+ Aldrin:Hg2+ Aldrin:Pb2+ Aldrin:Cd2+	1.00:1.00 1.00:1.00 1.00:1.00 1.00:1.00 1.00:1.00 1.00:1.00	3.00:3.00 3.00:3.00 3.00:3.00 3.00:3.00 3.00:3.00 3.00:3.00	5.00:5.00 5.00:5.00 5.00:5.00 5.00:5.00 5.00:5.00 5.00:5.00
Quaternary	Aldrin:Cd2+:Hg2+:Pb2+	1.00:1.00:1.00:1.00	3.00:3.00:3.00:3.00	5.00:5.00:5.00:5.00

Note։ Fold calculations were performed following [14].

). The mixture was then placed on an orbital shaker and tightly adjusted with cotton to prevent the conical flask from breaking and the solution from spilling. The orbital shaker stirred the fluid for 60 min at 160 rpm. Agitation speed and contact time boosted the contaminant adsorptive removal rate via mass transfer resistance and adsorbate and adsorbent surface interaction [13]. The solution was then filtered into 35 mL sample vials using Whatman filter paper. Similar tests, using the same approach, were employed for the full experiment (including binary and quaternary batches), which involved the use of iron-modified and unmodified rice husk biochar at varied doses and contamination limits. The increased adsorbent dosage in this study is significant because it leads to an increase in the number of accessible active sites and pores, which improves adsorbent removal efficiency [14].

In designing the experimental conditions, the selection of contaminant concentrations and adsorbent dosages was rooted in their environmental significance and the need for scalability in real-world scenarios. The concentrations selected are reflective of levels frequently found in polluted water sources, often surpassing recommended limits due to industrial discharges or agricultural runoff. Given this reality, it is imperative to examine high contamination limits to ascertain effective removal. Dosages of 1 g, 5 g, and 10 g correspond proportionally to these concentrations, in order to examine the adsorbent’s linear capacity across varied contamination levels. This proportional increase serves a dual purpose: to assess the performance under typical and “worst-case” conditions, ensuring the data’s relevance and the system’s adaptability to diverse environmental challenges.

2.5. Calculation for Adsorption Capacity Contaminants by Biochar

Equation (1) was used to determine, for each toxic metal at each adsorbent dosage, the equilibrium concentration of the adsorbent and the uptake of the toxic metal, which is represented by the symbol Qe.

The removal efficiency (Qe) was determined to be:

$$\mathrm{Qe}=\frac{Ci-Cf}{M}\times V$$

(2)

As a percentage, adsorption capacity was calculated as:

$$\mathrm{Qe}=\frac{\left(Ci-Cf\right)V}{M}\times 100$$

(3)

where Qe is the adsorption capacity, Ci is the starting concentration of the contaminants, Cf is the finished concentration of contaminants after adsorption, M is the dosage or amount of adsorbent, and V is the volume of the solution [15].

2.6. Adsorption Isotherm Models

A Langmuir isotherm model was used to descend the adsorption of atoms onto homogeneous surfaces that may be crystalline material or a solid that has adsorptive capabilities. This isotherm is often used to describe the type of interaction between the contaminants (such as heavy metals and pesticides) and the adsorbent.

The Langmuir isotherm formula in the linear form is calculated as follows:

$$\frac{Ce}{Qe}=\frac{1}{Kl·Qm}+\frac{Ce}{Qm}$$

(4)

$$\frac{Ce}{Qm}=\frac{1}{KlQmax}+\frac{Ce}{Qmax}$$

(5)

where Qmax (mg/g) is the maximum number of adsorbed molecules on the adsorbent surface at any given time, Kl is the Langmuir constant (L/mg), and Ce is the concentration of the adsorbate at equilibrium (mg/g) [3,15].

This model goes with R_L as the separation factor, which is a dimensionless constant. R_L is expressed as:

$${\mathrm{R}}_L=\frac{1}{1+KlCo}$$

(6)

where KL is the Langmuir constant (mg/g) and Co is the starting concentration of the adsorbate.

Adsorption is deemed unfavorable when the RL exceeds one. When RL = 1, it is linear; when RL = 0, it is irreversible; and ultimately, when 0 < RL < 1, it is advantageous [16,17].

2.7. Freundlich Adsorption Isotherm

The Freundlich isotherm frequently suits adsorption processes that occur on heterogeneous surfaces. The equation for calculating the Freundlich adsorption isotherm is։

Q_e = K_fC_e^1/n

(7)

where Qe stands for the amount of dangerous metal removed at equilibrium per gram of adsorbent (mg g⁻¹), Kf stands for the Freundlich isotherm constant (mg g⁻¹), Ce stands for the equilibrium adsorbate concentration (mg L⁻¹), and 1/n stands for the adsorption intensity. The Freundlich adsorption isotherm model in linear form is calculated as follows:

$$\mathrm{Log}{\mathrm{Q}}_{\mathrm{e}}=\mathrm{Log} {\mathrm{K}}_{\mathrm{f}}+\frac{1}{n} \log {\mathrm{C}}_{\mathrm{e}}$$

(8)

where Kf is the adsorption capacity (L mg⁻¹) and 1/n is the adsorption intensity. The 1/n illustrates how energy is distributed proportionally and how diverse the adsorption sites are. If 1/n is one, the adsorption is typical. If 1/n > 1, it shows cooperative adsorption; if n = 1, it suggests two-phase partitioning that does not depend on concentration [18,19].

2.8. Data Analysis

The effects of contact time, agitation speed, pyrolysis temperature, the aqueous solution’s initial pH, different adsorbent dosages, and maximum contamination limits were investigated. Data obtained from elute analysis were entered into Microsoft Excel in 2019 and imported into RStudio for analysis. The obtained data were used to generate regression curves. The estimated coefficients of determination, slope, and intercept of the regression curves were utilized to construct Langmuir and Freundlich constants and parameters, to further describe the adsorption processes. Tables, charts, and figures were used to portray the analyzed data for simple comprehension and interpretation.

3. Results and Discussion

3.1. Surface Functional Analysis of Iron-Modified Biochar

The surface functional analysis of biochar involves understanding the chemical properties and structures present on the surface of the biochar, which largely determines its reactivity, adsorptive capacity, and interaction with the environment.

3.2. Fourier-Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscopy (SEM) Analysis

The Fourier-transform infrared spectroscopy (FTIR) analysis of the iron-modified rice husk biochar sample suggests the presence of several functional groups. A peak at 3194.44 cm⁻¹ suggests the presence of O–H stretching vibrations, likely originating from the hydroxyl groups associated with cellulose and hemicellulose structures [8]. The peak at 1576.83 cm⁻¹ could correspond to C=C stretching in aromatic structures, which is consistent with known properties of biochar [8]. At 1434.14 cm⁻¹, C–H bending vibrations in methyl and methylene groups are indicated. The peaks at 873.54 and 793.67 cm⁻¹ are likely due to out-of-plane bending vibrations in aromatic compounds, pointing again to the aromatic nature of the biochar [20]. Lastly, the peaks at 694.21 and 617.42 cm⁻¹ suggest the presence of multi-substituted aromatic structures (Figure 1).

SEM images of iron-modified rice husk biochar produced at 500 °C typically show a porous structure with a high surface area (Figure 2). This is because biochar is produced by pyrolysis, a process that breaks down the organic matter in rice husks into smaller molecules. The smaller molecules form pores on the surface of the biochar. The chemical composition of iron-modified rice husk biochar is also influenced by the pyrolysis process. The main components of biochar are carbon, hydrogen, and oxygen [21]. The amount of each component can vary depending on the pyrolysis temperature and time.

In addition to carbon, hydrogen, and oxygen, iron-modified rice husk biochar also contains iron. The iron is typically present in the form of iron oxide nanoparticles [22]. The iron oxide (Fe₃O₄) nanoparticles can be found on the surface of the biochar, or they can be embedded in the biochar matrix. The presence of iron oxide nanoparticles in iron-modified rice husk biochar can improve the properties of the biochar [23]. For example, iron oxide nanoparticles can improve the biochar’s ability to adsorb pollutants. The iron oxide nanoparticles can also improve the biochar’s ability to resist degradation.

In a study [24], biochar samples produced at 350 °C, 500 °C, and 650 °C were examined. They corroborated the superior surface area of the biochar produced at 500 °C (230.91 m²/g). This elevated surface area at high temperatures, particularly around 500 °C and 650 °C, is attributed to intensified pyrolytic reactions, leading to the generation of mesoporous structures in the biochar. The rise in surface area under high-temperature pyrolysis, as observed in our study and others, can be tied to the volatilization of organic materials, resulting in the enhancement of micropore volume, a phenomenon also noted by [14].

3.3. Mono-Component System Adsorption of Contaminants onto Iron-Modified Rice Husk Biochar

This study looked at the adsorption efficiency of Hg²⁺, Pb²⁺, Cd²⁺, and aldrin using different adsorbent doses at different pH levels. The findings revealed that the adsorption efficiency of Hg²⁺ ranged from 99.89% to 99.95%. The adsorption efficiency of Hg²⁺ increased with the increasing adsorbent dosage, increasing the amount of target molecules that can be adsorbed. The adsorbent dosage plays a crucial role in determining the adsorption efficiency of pollutants. The results showed that the highest removal efficiency was obtained at a dosage of 10 g of adsorbent. The highest removal efficiency for aldrin was obtained for all dosages of adsorbent. The results suggest that the optimal dosage of adsorbent varies depending on the type of pollutant. However, beyond a certain point, the removal efficiency may not increase significantly [25]. Also, the saturation of adsorbent sites can result in decreased removal efficiency at lower dosages, since the number of active sites available is limited [26].

Mercury (Hg²⁺) obtained the highest heavy metal adsorption efficiency in the mono-component system, likely due to its high atomic weight/high density, polarizability, electronegativity, and ionic radii. Pb²⁺ adsorption efficiency was investigated using different adsorbent doses (1 g, 5 g, and 10 g) at various pH levels (7.45, 7.47, and 7.51). The maximum removal efficiency was found with 1g of adsorbent and a pH of 7.45. A similar study by Senthilkumaar et al. [27] reported that iron-modified rice husk biochar was an efficient adsorbent for removing mercury from synthetic wastewater, with a maximum removal efficiency of 98.2%. The significance of pH in the adsorption process is supported by a study by Gao et al. [28] which revealed that metal uptake at low solution pH is predominantly regulated by cation exchange, making the adsorbent extremely selective to Hg²⁺. Another study by Zhao et al. [29] emphasized that Hg²⁺, adsorbed onto certain materials, could be effectively desorbed in specific solutions, suggesting the importance of the adsorbent’s properties and the solution’s pH in the desorption process [30]. The results of this study also agree with [31].

Lead (Pb²⁺) is another toxic heavy metal that has the potential to cause major health problems, including harm to the neurological system, kidneys, and reproductive system [32]. Pb²⁺ adsorption efficiency was investigated using different adsorbent doses (1 g, 5 g, and 10 g) at varied pH levels (7.45, 7.47, and 7.51). The adsorption efficiency of Pb²⁺ varied from 95.78% to 97.20% (

Table 2. Mono-component adsorption of contaminants from aqueous solution using iron-modified rice husk biochar as adsorbent.

Pollutant	Adsorbent Dosage (G)	pH	Initial Conc. (mg/L)	Final Conc. (mg/L)	Removal Efficiency (%)
Hg2+	1	7.34	1	0.00111	99.89
	5	7.26	3	0.00214	99.93
	10	7.35	5	0.00233	99.95
Pb2+	1	7.45	1	0.028	97.20
	5	7.47	3	0.098	96.73
	10	7.51	5	0.211	95.78
Cd2+	1	7.67	1	0.072	92.80
	5	7.64	3	0.181	93.97
	10	7.65	5	0.199	96.02
Aldrin	1	7.22	1	0	100
	5	7.25	3	0	100
	10	7.60	5	0	100

). The maximum removal efficiency was found with 1g of adsorbent, with a pH of 7.45. The reduced adsorption efficiency found at larger adsorbent doses might be due to adsorbent site saturation [33]. The adsorption efficiency was generally higher than Cd²⁺ in the present study. Lead has a larger ionic radius than cadmium, which means it has a larger surface area for adsorption [25]. This allows lead to form stronger bonds with adsorbent surfaces than cadmium. Secondly, lead has a higher electronegativity than cadmium. As a result, it is more likely to attract other charged particles, including those found in adsorbent surfaces. This enhances the adsorption efficiency of lead [5]. The results obtained in this study agree favorably with the findings reported by [31,34].

Aldrin adsorption efficiency was 100% for all adsorbent doses. This is due to the adsorbent’s binding affinity and the porous nature of rice husk biochar, which increases the surface area of the biochar, providing more sites for aldrin molecules to attach to. Adsorption is a viable strategy for eliminating aldrin from the environment. This outcome finds support in another study previously carried out [14].

3.4. Multi-Component (Binary and Quaternary) System Adsorption of Contaminants onto Iron-Modified Rice Husk Biochar

Toxic metals may be removed concurrently from several pollutants via multicomponent adsorption, which is more effective, affordable, and sustainable [35]. At a dose of 10 mg/L adsorbent, the maximum adsorption efficiency in the binary component was observed for Hg²⁺ at pH 7.95 and for Pb²⁺ at pH 7.85. This is explained by the adsorbent’s great affinity for the metal ions. At all adsorbent doses, mixtures, and pH levels, Hg²⁺ had a greater adsorption efficiency than Pb²⁺ and Cd²⁺. The creation of Cd²⁺(OH)₂, which makes it less accessible for adsorption, and the presence of other ions in the solution which can compete with Cd²⁺ for adsorption, are both responsible for the reduced adsorption effectiveness of Cd²⁺ at high pH levels [19].

The adsorption efficiency of Pb²⁺ and Cd²⁺ ranged from 88.90% to 99.56% and 99.88% to 99.98%, respectively (

Table 3. Multi-component adsorption efficiency of contaminants using iron-modified rice husk biochar.

Binary Component	Pollutant	Adsorbent Dosage	pH	Initial Conc.	Removal Efficiency
Hg2+–Pb2+
	Hg2+	1	7.85	1	99.80
		5	7.92	3	99.90
		10	7.95	5	99.92
	Pb2+	1	7.85	1	98.80
		5	7.92	3	97.90
		10	7.95	5	97.58
Hg2+–Cd2+
	Hg2+	1	7.96	1	99.88
		5	7.97	3	99.93
		10	8.01	5	99.95
	Cd2+	1	7.96	1	78.90
		5	7.97	3	92.23
		10	8.01	5	93.92
Pb2+–Cd2+
	Pb2+	1	8.07	1	88.90
		5	8.1	3	98.53
		10	8.15	5	99.56
	Cd2+	1	8.07	1	99.88
		5	8.1	3	99.96
		10	8.15	5	99.98
Aldrin–Hg2+
	Aldrin	1	7.8	1	100.00
		5	7.3	3	100.00
		10	7.16	5	100.00
	Hg2+	1	7.8	1	99.89
		5	7.3	3	99.95
		10	7.16	5	99.96
Aldrin–Pb2+
	Aldrin	1	6.8	1	100
		5	6.88	3	100
		10	7.01	5	100
	Pb2+	1	6.8	1	98.80
		5	6.88	3	95.97
		10	7.01	5	98.24
Aldrin–Cd2+
	Aldrin	1	7.71	1	100.00
		5	7.45	3	100.00
		10	7.19	5	100.00
	Cd2+	1	7.71	1	98.20
		5	7.45	3	99.03
		10	7.19	5	99.34
Quaternary Component
Aldrin–Hg2+–Pb2+–Cd2+
Aldrin		1	7.15	1	100.00
		5	7.14	3	100.00
		10	6.76	5	100.00
Hg2+		1	7.15	1	99.88
		5	7.14	3	99.95
		10	6.76	5	99.97
Pb2+		1	7.15	1	98.20
		5	7.14	3	99.27
		10	6.76	5	99.44
Cd2+		1	7.15	1	98.90
		5	7.14	3	99.53
		10	6.76	5	99.46

). Cd²⁺ had a higher adsorption efficiency due to its smaller ionic radius and high charge density. Pb²⁺ had a lower adsorption efficiency due to the formation of saturated complexes on the surface of the adsorbent. The adsorption efficiency of aldrin was 100% in all binary cases. This indicates that the adsorbent used in this study is highly effective in removing aldrin and these heavy metals from water. The pH of the solution played a significant role in the adsorption efficiency of aldrin. This finding is consistent with the findings from Zhu et al. [30].

Aldrin exhibited a high adsorption efficiency of 100% in the quaternary adsorption of aldrin–Hg²⁺–Pb²⁺–Cd²⁺. Hg²⁺, Pb²⁺, and Cd²⁺ showed high removal efficiencies, ranging from 99.88% to 99.97% (Table 3). Aldrin’s hydrophobicity allowed it to strongly interact with the adsorbent surface through Van der Waals forces [9].

3.5. Adsorption Isotherm

3.5.1. Langmuir Adsorption Isotherm

The Langmuir adsorption isotherm model was used to investigate the adsorption of Hg²⁺, Pb²⁺, and Cd²⁺ ions, individually and in binary and quaternary combinations. The maximum adsorption capacities (Qmax) for Pb²⁺ were the highest at 44.2478 mg/g, followed by Cd²⁺ (39.8406 mg/g), and Hg²⁺ (36.4964 mg/g). The separation factors (R_L) were all less than one, and the correlation coefficients (R²) were all greater than 0.97. The Qmax and K_L values of Pb²⁺–Cd²⁺ adsorption were the highest at 114.9425 mg/g, followed by Hg²⁺–Pb²⁺ and Hg²⁺–Cd²⁺, respectively. All binary combinations showed favorable adsorption with R_L values less than one, and R² values greater than 0.96. Aldrin–Hg²⁺ *, aldrin–Pb²⁺ *, and aldrin–Cd²⁺ * showed lower Qmax and K_L values compared to the other binary combinations. The lower Qmax values may have been due to the competition between aldrin and the other ions for adsorption sites on the surface, resulting in reduced adsorption of Hg²⁺, Pb²⁺, and Cd²⁺ (Figure 3). The dimensionless equilibrium (R_L) parameter was close to unity for most of the binary combinations, indicating a favorable adsorption process [36]. The quaternary combinations showed favorable adsorption with R_L values less than one (

Table 4. Langmuir adsorption isotherm for multi-component adsorption of pollutants using iron-modified rice husk biochar as an adsorbent.

Mono-Component	Qmax (mg/g)	KL (mg/L)	RL	R2
Hg2+	36.4964	−7.30 × 10−4	9.99 × 10−1	0.9746
Pb2+	44.2478	−1.77 × 10−2	9.47 × 10−1	0.9977
Cd2+	39.8406	−4.38 × 10−2	7.81 × 10−1	0.9803
Binary Component
Hg2+ *–Pb2+	34.4828	−1.38 × 10−3	9.99 × 10−1	0.9985
Hg2+–Pb2+ *	46.0829	−9.22 × 10−3	9.72 × 10−1	0.9945
Hg2+ *–Cd2+	35.3357	−7.07 × 10−4	9.96 × 10−1	0.9993
Hg2+–Cd2+ *	25.9067	−1.35 × 10−1	8.65 × 10−1	0.9661
Pb2+ *–Cd2+	114.9425	3.45 × 10−2	1.10	0.9804
Pb2+–Cd2+ *	19.0840	1.53 × 10−3	1.01	0.2172
Aldrin–Hg2+ *	28.9017	8.67 × 10−4	1.00	0.9949
Aldrin–Pb2+ *	53.1915	2.66 × 10−3	1.01	0.9766
Aldrin–Cd2+ *	32.1543	1.29 × 10−2	1.06	0.9879
Quaternary Component
Al–Hg2+ *–Pb2+–Cd2+	23.0947	9.24 × 10−4	1.00	0.9826
Al–Hg2+–Pb2+ *–Cd2+	26.6667	1.33 × 10−2	1.04	0.9894
Al–Hg2+–Pb2+–Cd2+ *	38.4615	7.69 × 10−3	1.04	0.9894

Note։ Contaminants bolded with * attached are metals of interest in the multi-component system.

). The correlation coefficients (R²) for all three quaternary combinations were greater than 0.98, indicating good fits to the Langmuir model. The adsorption capacity (Qmax) was generally lower than in the mono-component experiments, which is consistent with the binary adsorption experiments. Aldrin–Hg²⁺ *–Pb²⁺ *–Cd²⁺ * has the lowest Qmax value, likely due to the presence of aldrin. The highest K_L value in all three quaternary adsorption combinations may also be due to increased competition between the four contaminants for adsorption sites, resulting in a lower affinity for the adsorbent. Venkatesan et al. [4], Xiang et al. [12], and Srivastav et al. [37] reported findings that agree favorably with the findings of this study, about the efficacy and the potency of rice husk biochar for the removal of toxic metals [38].

For a detailed breakdown of the Langmuir Adsorption Isotherm data, refer to the Supplementary Data.

3.5.2. Freundlich Adsorption Isotherm

The Freundlich adsorption isotherm model is widely used to study the adsorption of contaminants in aqueous solutions. The mono component results of Hg²⁺, Pb²⁺, and Cd²⁺ in

Table 5. Freundlich adsorption isotherm model parameters for the multi-component adsorption of pollutants from aqueous solution using iron-modified rice husk biochar as adsorbent.

	1/n	N	Kf	R2
Mono-Component
Hg2+	−1.14338	−0.8746	0.261999	0.9773
Pb2+	−2.80505	−0.3565	26.66245	0.9857
Cd2+	−1.65317	−0.6049	18.99328	0.9772
Binary Component
Hg2+ *–Pb2+	1.071926	0.9329	3.486583	0.931
Hg2+–Pb2+ *	−3.25839	−0.3069	25.35712	0.9996
Hg2+ *–Cd2+	−1.11707	−0.8952	0.239828	0.9848
Hg2+–Cd2+ *	−0.79726	−1.2543	10.17185	0.797
Pb2+ *–Cd2+	2.75634	0.3628	193.1079	0.9782
Pb2+–Cd2+ *	0.19668	5.0844	5.57× 1016	0.3796
Al2+–Hg2+ *	−0.73174	−1.3666	0.009361	0.9997
Al2+–Pb2+ *	−3.63504	−0.2751	28.72764	0.8849
Al2+–Cd2+ *	−0.90481	−1.1052	1.163322	0.9972
Quaternary Component
Al–Hg2+ *– Pb2+–Cd2+	−0.51467	−1.943	0.000196	0.8931
Al–Hg2+– Pb2+ *–Cd2+	−0.66046	−1.5141	0.209122	0.9025
Al–Hg2+–Pb2+– Cd2+ *	1.508068	0.6631	4.298332	0.749

Note։ Contaminants bolded with * attached are metals of interest in the multi-component system.

show the values of 1/n, N, K_f, and R² for each contaminant. The values of 1/n for Hg²⁺, Pb²⁺, and Cd²⁺ are −1.14338, −2.80505, and −1.65317, respectively (Table 5). The Kf values for Hg²⁺, Pb²⁺, and Cd²⁺ are 0.261999, 26.66245, and 18.99328, respectively. The R² values for all binary combinations are high, except for Pb²⁺–Cd²⁺ *, with an R² value of 0.3796 (Figure 4).

The low R² value for Pb²⁺–Cd²⁺ * suggests that the Freundlich isotherm may not be the best fit for this binary combination. The high R² values indicate a good fit of the Freundlich isotherm to the experimental data. The higher K_f value for Pb²⁺*–Cd²⁺ indicates a stronger adsorption affinity for the adsorbent material. Quaternary combinations have negative 1/n, low Kf value, and R² values lower than binary components, suggesting a weak fit of the Freundlich model to the data.

Extensive data on the Freundlich Adsorption Isotherm Model Parameters can be found in our Supplementary Data document.

4. Conclusions

This study investigated the removal efficiency of four pollutants (Hg²⁺, Pb²⁺, Cd²⁺, aldrin) at different dosages, initial concentrations, and pH levels by iron-modified rice husk biochar. The adsorption efficiency of aldrin was 100%, while the removal efficiency of heavy metals ranged from 99.80% to 99.96% for Hg²⁺, from 88.90% to 99.56% for Pb²⁺, and from 78.90% to 99.98% for Cd²⁺. The Freundlich adsorption isotherm fitted adsorption in the mono-component systems best, while the Langmuir adsorption isotherm fitted adsorption in the binary and quaternary component systems best. The outcome of the study reveals that eco-friendly iron-modified rice husk biochar can be a good adsorbent material for the removal of contaminants from the aqueous environment.