Application of Marine Mollusk Shells (Meretrix lusoria) as Low-Cost Biosorbent for Removing Cd²⁺ and Pb²⁺ Ions from Aqueous Solution: Kinetic and Equilibrium Study

Abstract

The present work aims to evaluate the applicability of mollusk (Meretrix lusoria) shells as a biosorbent for toxic metal ions (Cd²⁺ and Pb²⁺) following the batch mode biosorption procedure. Some well-known analytical methods have been used to characterize the biosorbent such as a scanning electron microscope (SEM), an energy dispersive X-ray (EDX), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction. The mechanism of metal ion biosorption was examined using various analytical techniques. Therefore, an evaluation of operating factors such as contact time, pH, initial concentration of metal ions, biosorbent dose, and temperature was performed. The results obtained in this investigation indicated that the optimum conditions for the biosorption of Cd⁺² and Pb⁺² ions are as follows: pH = 6; contact times of 90 min; and the 20 mg/L of initial [M²⁺]. And a biosorbent dosage of 1.0 g/100 mL for each metal ion solution was also determined. The maximum removal efficiency results were 90.6% for Cd⁺² and 91.5% for Pb⁺² at pH 6.0. The biosorption isotherm was investigated using three forms of linear equilibrium (Freundlich, Langmuir, and Temkin models). Kinetic studies were also conducted to determine the equilibrium time for the biosorption of the studied metals utilizing the pseudo-second-order, pseudo-first-order, and intraparticle diffusion model. The data indicate that the biosorption kinetics of Cd²⁺ and Pb²⁺ follow the pseudo-second-order models. According to the present study, it can be identified that the shell of Meretrix lusoria is a suitable biosorbent for Cd²⁺ and Pb²⁺ ions and can contribute to their removal from environmentally polluted water.

1. Introduction

An environment is a place where all living and non-living things are interconnected together with minimal interference in the lives of others. Nevertheless, with the advent of industrial urbanization, revolution, and increasing technological development, human activities have led to a significant and appreciable increase in toxic contaminants in the ecosystem. Global awareness of the crisis related to environmental contamination problems is dispersal, which has become the focus of attention around the world, outside any region or country. Heavy metals are known for their persistence in the environment. They are not easily biodegradable and can remain in the environment for extended periods once released. This persistence contributes to their potential to accumulate and cause harm over time [1,2]. Heavy metal pollution caused by elevated concentrations of metal ions in industrial wastewater, drinking water, food, and air can have adverse health effects on humans as well as aquatic organisms [3]. These elements can be released from sediments into the environment due to changes in environmental conditions. These changes can remobilize previously buried heavy metals, making them available for uptake by organisms, and then transferred through the food chain. Many studies have investigated the effects of heavy metals on human health and the environment, showing the toxicity, health risks, and mechanisms of heavy metal pollution [4,5,6,7]. When heavy metals are ingested by humans, especially in children, this can lead to severe health problems, either through contaminated water or food, and they can accumulate in the body over time.

Some metals such as zinc and copper are essential trace elements that play vital roles in several biological functions. However, excessive intake of these metals can lead to toxic effects on human health. Lead and cadmium, on the other hand, are considered non-essential trace metals and are harmful even at low concentrations. The contamination of natural water resources by these metal ions is a major global concern. These pollutants can have serious health and environmental consequences. These metals can accumulate in fish and other aquatic organisms, which can then be consumed by humans, leading to the bioaccumulation of these toxins in the food chain [8,9,10]. There is an urgent need to tackle water pollution using low-cost and effective methods.

It is important to note that the selection of a biosorbent, biosorption system design, and specific wastewater properties all play an important role in the success of the treatment process. Researchers and engineers continue to study and develop new biosorbent materials and technologies to improve the efficiency and applicability of biosorption in wastewater treatment [11]. The selection of the most appropriate biosorbent depends on various factors, including the specific heavy metal contaminants in the wastewater, the pH of the solution, and cost considerations. Researchers continue to explore and develop new biosorbents and improve existing materials to improve their efficiency and selectivity for heavy metal removal. The choice of biosorbent is often determined by the unique requirements of a particular water treatment application [12]. Biosorption has received the greatest attention among the different methods; this technology has become an alternative to the traditional methods of dating, as it is characterized by its low cost, ease of processing and operation, and recovery of heavy metals.

Biosorption is a promising tool for the elimination of contaminants as textile dyes, nutrients, heavy metals, and organic matter, which have significant risk to the aquatic ecosystems. A biosorbent is prepared from the biomass of naturally abundant waste because of its high biosorption capacity and cost-effective source of raw materials; the use of available natural materials in industrial processes is an important way not only to reduce waste generated in its natural state in the present and in the future, but also to treat environmental pollutants. The process of using naturally available materials as biosorbents is less expensive compared to other methods [13]. During the previous period, interest increased in using many natural wastes that have the property of a biosorbent material, in treating wastewater with high efficiency in removing pollutants, and natural wastes such as cotton seed husks [14] and sugarcane plant waste [15], and the use of eggshells, dry algae, and activated carbon from agricultural waste [16]. Many low-cost sorbents, such as algae, fungi, bacteria, agricultural by-products, chitosan, zeolite, microorganisms, clays, and waste products from industrial processes (fly ash, coal, oxides), have been studied for their biosorption capacity towards water contaminants [17].

There is great interest from environmental organizations in the disposing of mollusk shells as waste materials. However, due care should be given to the use of mollusk shell as a waste material for use as a biosorbent due to its composition and availability. Marine aquaculture is an important industrial sector in northwestern Spain, where processing facilities generate large quantities of shell waste representing more than 80,000 tons per year [18].

Recently, marine components such as mollusk shells have been used because of their natural properties. It has been found to have a high ability to remove pollutants and is environmentally friendly, low-cost, and available. Oysters, snails, and shrimp shells are among the marine materials that have attracted great interest from researchers for their use as biosorbents for pollutants, because they are low-cost, readily available, and biodegradable [19,20,21]. The shell is usually the exoskeleton of invertebrates and is usually composed of calcium carbonate (CaCO₃) or chitin, which facilitate the biosorption of pollutants onto their active surfaces [20,22]. Recycling and reusing mollusk shells has various applications including their use as building materials to produce concrete. Seashells are materials with high potential to become a partial substitute for cement and a filler in concrete. The ability to use and recycle mollusk shell waste is important in civil works, and the engineering application component of the integrated waste management scheme [23,24]. Yoon et al. [25] investigated the chemical and mechanical properties of crushed oyster shell as a plant fertilizer. Kwon et al. [26] studied the pyrolysis of waste oyster shells under specific conditions to solve water enrichment problems by converting shell material into a sustainable reagent to efficiently remove phosphate from wastewater and control eutrophication processes. Ortiz Olivares et al. [27] investigated the potential of an eco-friendly strategy to prepare a stable heterogeneous catalyst from natural waste (Gastropods Mollusca) and functionalized it after stabilization with tri-sodium phosphate to obtain an appetite-like catalyst.

The present work aims to highlight the applications of using mollusk shells (Meretrix lusoria) as an environmentally friendly means of biosorption and cost-effective method for heavy metal removal (lead and cadmium) in an aqueous solution. In addition, the equilibrium, thermodynamic, and kinetic behaviors of the biosorption process of Cd²⁺ and Pb²⁺ ions on mollusk shells were studied.

2. Materials and Methods

2.1. Materials

The material utilized in this study is the shells of Meretrix lusoria mollusks (Figure 1), which were bought from fish markets in Jeddah City. The species of mollusk collected were identified. The studied mollusk was classified as follows: [Bivalvia (class), Venerida (order), Veneridae (family), Meretrix (genus), Meretrix lusoria (species)]. Meretrix lusoria is an important shellfish species in Asian waters [28,29].

Short description: It was found that the outer surface of the shells appears in a few colors (light brown, white, and gray, as well as beige and cream colors); the interior is smooth white with light brown spots. It appears in a triangular shape, full of growth lines, and the size is 60 mm in length (Figure 1). One important ecological role of bivalves, including Meretrix lusoria, commonly known as the common Asian clam, which is indeed one of the most consumed mollusks in Taiwan, is their ability to serve as bio-indicators and accumulate organic and metallic pollutants from the water and sediment. It is a species of saltwater clam that belongs to the family Veneridae and is native to the coastal regions of the Indo-Pacific; it falls under the class Bivalvia in the phylum Mollusca, which is characterized by the presence of two hinged shells that enclose the soft-bodied animal within. The predominant component of clam shells is calcium carbonate crystals. The study conducted by Thangaraj et al. [30] suggests that the chemical composition and physical structure of the shell may be affected by environmental changes, such as water temperature or nutrient availability.

2.2. Scientific Hypothesis

The scientific hypothesis is that natural wastes and remains, whether marine or terrestrial, are valuable because they have natural characteristics that can play an important role in treating, removing, and disposing environmentally polluting elements. There is great interest by environmental organizations in the disposing of mollusk shells as waste materials (mollusk, Meretrix lusoria shell powder). Therefore, there is widespread use of mollusk shell as a waste material for use as a binary biosorbent due to its composition and availability, especially since these materials as biosorbents have become easy to study regarding their components and identifying properties using modern tools and devices in addition to the well-known analytical methods such as a scanning electron microscope (SEM), an energy dispersive X-ray (EDX), and Fourier transform infrared spectroscopy (FTIR)) X-ray diffraction, zeta potential, BET, and particle size. Through this study, there are several factors that play an important role in removing metal ions from the aquatic environment, such as the functional groups that were identified through FTIR, as well as the particle size, surface area, surface charge, and mineral composition of this material.

2.3. Preparation of Adsorbents for Analysis

One kilogram of fresh shells of uniform sizes was obtained. The collected shells were first boiled to separate the soft clam tissue from the shells. The shells were washed thoroughly to remove unwanted organic matter and any remaining residues or contaminants and placed in commercial Clorox bleach with 5% sodium hypochlorite for one week to re-move suspended matter [31], and then the shells were rinsed with DIW (deionized water) and dried at room temperature until the weight of the mollusk shell stabilized after losing water. The cleaned shells were hot air-dried. To create a uniform and easily manageable substance for further processing, the dried shells were crushed in a high-speed mill; the crushed shells were then ground using an agate mortar into a fine powder form. The grinding process was carried out several times, and then the sieving process was carried out to reach the required particle size. The dried products of the biosorbent were sieved using a nylon sieve to the desired particle size, passing through a 230-mesh sieve (pore size: 63 μm). The crushed cortex (<63 µm) included outer, middle, and inner cortex sample sites. The powder was placed in a hot air oven at a temperature of 105 °C for an hour, and the dry powder was packed in a plastic bag to conduct the required studies on it.

2.4. Biosorbent Characterization

The shell sample powder was analyzed utilizing Fourier Transform Infrared Spectrophotometry (FTIR), X-ray diffraction (XRD), and Scanning Electron Microscopy (SEM) analysis instruments, which were supplied by the Central Laboratory, Desert Research Center, Ministry of Agricultural Land Reclamation. FTIR spectra of the functional group were characterized by ATR-FTIR Spectroscopy, THERMO NICLOT, 50, Oregon USA. A clear disk was used to compress the powdered shell powder, which was ground at a mixture ratio of 1:200 mg with KBr, in preparation for the analysis. There was a resolution of 4 cm⁻¹ for the FTIR spectra and measurements of transmittance scans were taken within the 400–4000 wavenumber (cm⁻¹) range. After collecting baseline data for the blank, we removed it from every spectrum. SEM was used to investigate the surface and structural morphology of the shell powder sample, which was characterized using high resolution, and analysis experiments were performed on a FEI Quanta FEG 250, Oregon, USA.

XRD is a non-destructive procedure that offers detailed evidence about the crystal chemical composition, physical properties, and structure of a material. The process depends on the positive interference between monochromatic X-rays and a crystalline material. Experiments were performed using PHASER, D2 2ndGen, BRUKER, Billerica, MA, USA, with CuKα (copper K-alpha) radiation at 20 mA and 40 kV. The diffraction form was collected at a scan rate of 0.02° per second at 2θ fluctuating from 20° to 60°. The concentrations of metals (Cd⁺² and Pb²⁺) were analyzed utilizing ICP-MS (Inductively Coupled Plasma-Mass Spectrometer, NexION 300D (Perkin Elmer, USA)). A zeta potential and particle size analyzer (Malvern Zetasizer Nano ZS90, Cambridge, UK) was used for a particle size distribution analysis.

2.5. Methods of Biosorption Experimentation

Batch biosorption experiments were conducted in a 250 mL conical flask containing 100 mL of the solution containing the studied metal ions (Cd⁺² and Pb²⁺). An individual metal stock solution of Cd⁺² and Pb²⁺ (1000 mg/L) was prepared in deionized distilled water using metal salts [Pb (NO₃)₂ and Cd (NO₃)₂.4H₂O], and then the number of working solutions at different concentrations was obtained by diluting the stock solutions with DDW in 100 mL flasks. The important factors such as contact time, metal ion concentration, temperature, biosorbent dosage, and pH were systematically examined and optimized through individual experiments to ascertain the efficiency of metal sorption.

2.5.1. Effect of pH Value

To assess the role of pH in the biosorption of metal ions (Pb²⁺ and Cd²⁺) on a powder shell surface, different pH levels were used. The pH range varied from 2 to 8 at room temperature. The Pb²⁺ and Cd²⁺ solutions at an initial concentration of 50 ppm were prepared. The pH of these solutions was regulated using 0.1 N hydrochloric acid and 0.1 N sodium hydroxide. Subsequently, 100 mL of each metal solution was brought into contact with 0.2 g of shell powder. The mixture was shaken for 2 h at 300 rpm. The solution was centrifuged at 5000 rpm for 10 min; after that, the supernatant was collected. The residual concentrations of the specific metals in the supernatant were analyzed.

2.5.2. Effect of Contact Time

The purpose of this experiment was to determine the optimum contact time required for the maximum biosorption of Pb²⁺ and Cd²⁺ by the shell powder. By examining how the contact time affects the biosorption process, the contact time varied between 5, 10, 20, 30, 40, 50, 60, 70, 90, and 100 min. The experiments were conducted at a constant temperature of 25 °C. For each metal ion, ten solution vials were prepared, adjusted to pH 6 using hydrochloric acid and sodium hydroxide. In each vial, 100 mg of shell powder was mixed with 100 mL of the corresponding metal ion solution, which had a concentration of 50 mg/L.

2.5.3. Impact of Sorbent Dosage

The impact of biosorbent dosage on the biosorption amount of Pb²⁺ and Cd²⁺ was studied using a separate conical flask at 25 °C and pH 6.0 by varying the amounts of shell powder from 0.10 to 1.0 g/100 mL and the initial concentration of metal ions was 20.0 mg/L. In each flask, the shell powder was mixed with 100 mL of the metal ion solution.

2.5.4. Influence of Initial [M²⁺]

To evaluate the effect of the Cd²⁺ and Pb²⁺ concentration on the process of biosorption, 5, 10, and 20 mgL⁻¹ for each ion concentration were studied at constant pH = 6.0, with 0.2 g L⁻¹ biosorbent for 90 min; 100 mL of aqueous solutions of [M²⁺] was added in previously prepared concentrations. The mixture was stirred with a magnetic stirrer for 90 min at 250 rpm; 1 mL of the sample was taken from the reaction solution after 10, 20, 30, 40, 50, 60, 70, 80, and 90 min; and the experiment was performed at 25 °C.

2.5.5. Effect of Temperature

The temperature effect on the removal of Pb²⁺ and Cd²⁺ by shell powder was studied by changing the temperature between 20 and 60 °C. A 50 mL volume of the metal ion solution with initial metal ion concentrations of 20 mg/L was placed in a conical flask (100 mL). In total, 1.0 g of the biosorbent per liter was then added for 90 min to the solution to obtain a suspension. The suspension was adjusted to pH 6.0. The flasks were agitated for 3 h. Subsequently, the suspension was subjected to centrifugation at 5000 rpm/min for a duration of 10 min. The resulting liquid above the sediment was carefully collected in sterile test tubes. The content of metals was measured at every temperature. The metal ion concentrations in the filtrate for each batch were measured utilizing a flame atomic biosorption spectrophotometer, AAnalyst200 (Perkin Elmer, USA).

2.6. Data Analysis

The equilibrium biosorption capacity and the removal of Pb²⁺ and Cd²⁺ ions were calculated utilizing the following equations:

$${\mathrm{q}}_{\mathrm{e}}={\displaystyle \frac{\left({\mathrm{C}}_{\mathrm{o}}-{\mathrm{C}}_{\mathrm{e}}\right)\ast \mathrm{V}}{\mathrm{m}}}$$

$$\mathrm{\%}\mathrm{R}\mathrm{e}\mathrm{m}\mathrm{o}\mathrm{v}\mathrm{a}\mathrm{l}=\left({\displaystyle \frac{{\mathrm{C}}_{\mathrm{o}}-{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{C}}_{\mathrm{o}}}}\right)\ast 100$$

The variables C_o and C represent the initial and residual [M²⁺] (mg/L), respectively. V represents the volume of the biosorption process in an aqueous solution (L). m represents the mass of the dry biosorbent (gm). q_e represents the metal biosorption capacity (mg/g).

The biosorption kinetics and isotherm models are shown in

Table 1. Biosorption Kinetics and isotherm models.

Biosorption Models	Equations	Parameters
Kinetics
Pseudo-first-order model [32,33]	$\ln \left({\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}}\right)={\mathrm{l}\mathrm{n}\mathrm{q}}_{\mathrm{e}}-{\mathrm{k}}_1$	qe and qt are the values of amount biosorbed/unit mass at any time t at equilibrium condition. k1: equilibrium rate constant for pseudo-first-order biosorption.
Pseudo-second-order model [34,35]	${\displaystyle \frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{t}}}}={\displaystyle \frac{1}{{\mathrm{k}}_2{\mathrm{q}}_{\mathrm{e}}^2}}+{\displaystyle \frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{e}}}}$	k2: equilibrium rate constant for pseudo-second-order biosorption.
Intraparticle diffusion model [36]	${\mathrm{q}}_{\mathrm{t}}={\mathrm{k}}_{\mathrm{i}\mathrm{d}}{\mathrm{t}}^{1/2}+\mathrm{C}$	qt: capacity of biosorption at any time t. kid: rate constant of intraparticle diffusion (mg/g min1/2), C: the film thickness.
Isotherm
Langmuir [37]	${\displaystyle \frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{q}}_{\mathrm{e}}}}=\left({\displaystyle \frac{1}{{\mathrm{q}}_{\mathrm{m}\mathrm{a}\mathrm{x}}}}\right)\ast {\mathrm{K}}_{\mathrm{L}}+{\mathrm{C}}_{\mathrm{e}}/{\mathrm{q}}_{\mathrm{m}\mathrm{a}\mathrm{x}}$	qe: the capacity of biosorption at equilibrium (mg/g), qmax: the maximum capacity of biosorption at single-layer coverage (mg/g). Ce: the [M2+] at equilibrium (mg/L), KL: the intensity of biosorption (L/mg).
Freundlich [38]	$\ln {\mathrm{q}}_{\mathrm{e}}={\mathrm{l}\mathrm{n}\mathrm{K}}_{\mathrm{f}}+{\displaystyle \frac{1}{{\mathrm{n}}_{\mathrm{f}}}}\mathrm{L}\mathrm{n}{\mathrm{C}}_{\mathrm{e}}$	Kf: biosorption capacity constant (mg/g), nf: Freundlich affinity constant, biosorption intensity of the solid biosorbent.
Temkin and Pyzhev [39]	${\mathrm{q}}_{\mathrm{e}}={\mathrm{B}}_{\mathrm{T}}{\mathrm{l}\mathrm{n}\mathrm{A}}_{\mathrm{T}}+{\mathrm{B}}_{\mathrm{T}}\mathrm{L}\mathrm{n}{\mathrm{C}}_{\mathrm{e}}$	AT: the equilibrium binding constant (L/min), BT: constant correlated with the heat of biosorption process.

; the biosorption isotherms are measured using the Langmuir, Freundlich, and Temkin models.

3. Results and Discussion

3.1. Characterization of Adsorbent

3.1.1. SEM Analysis

SEM provides an image of the sample surface through scanning it with a high-energy beam of electrons [40]. The interactions between electrons and the atoms of the sample generate signals that carry information regarding the surface’s topography, morphology, and composition. The SEM analysis of shell powder before metal ion biosorption revealed pores; a rough, orthorhombic crystal structure; and a highly heterogeneous surface (Figure 2A). These characteristics indicate a high-ability surface for biosorption [41], confirming the morphology found naturally in almost all mollusk shells. Figure 2B,C show the SEM micrographs of shell powder after the biosorption of Pb²⁺ and Cd²⁺, respectively. The surface texture of shell powder was completely changed after the biosorption of [M²⁺]. The holes were filled with the [M²⁺] and the surface heterogeneity decline and the pores appear to be smooth [42]. As shown in Figure 2, shell powder indicates that the crystal lattice in the sample is regular. SEM observations of the shell powder before and after the biosorption experiment revealed slight morphological changes. These differences are attributed to the effect of metal biosorption. A surface analysis typically employs a combination of SEM and energy-dispersive X-ray spectroscopy (EDX) techniques to achieve precise spatial resolution. In this study, the EDX technique was utilized to evaluate the elemental composition of the shell powder (

Table 2. Chemical elements observed from energy dispersive X-ray (EDX) data for molusk (Meretrix lusoria) shell powder before and after biosorption of lead and cadmium ions.

	Before Biosorbent		Cd after Biosorbent			Pb after Biosorbent
Element	Mass %	Atom %	Element	Mass %	Atom %	Element	Mass %	Atom %
C	6.23 ± 0.43	11.03 ± 0.25	C	2.34 ± 0.22	4.27 ± 0.20	C	6.11 ± 0.43	10.58 ± 0.42
O	45.11 ± 0.50	62.12 ± 0.12	O	44.35 ± 0.20	62.55 ± 0.32	O	42.31 ± 0.50	60.25 ± 0.08
Ca	44.25 ± 0.22	24.52 ± 0.51	Ca	40.09 ± 0.27	21.72 ± 0.26	Ca	39.25 ± 0.37	21.22 ± 0.47
Al	0.41 ± 0.13	0.35 ± 0.03	Al	0.32 ± 0.13	0.28 ± 0.10	Al	0.30 ± 0.22	0.25 ± 0.14
			Cd	0.43 ± 0.38	0.19 ± 0.05	Pb	0.38 ± 0.25	0.14 ± 0.07

). Atomic percentages are quantified along with the peak area for components in the tested sample. Figure 3A illustrates the elemental composition of the shell as determined by the EDX analysis, revealing calcium (Ca), oxygen (O), and carbon (C) as the primary elements, with aluminum (Al) present as a minor element. The predominance of these elements was consistent with the predominant CaCO₃ mineral form. Figure 3B,C depict the biosorption of metal ions on the biosorbent.

3.1.2. X-ray Diffraction (XRD)

It is a technique widely utilized in materials science and molecular biology for its ability to provide detailed information about the atomic-level structure of materials. It enables the characterization of materials, identification of crystallographic orientations, and analysis of variations in crystal parameters, thereby facilitating structural analyses of various substances [43]. In this study, XRD was employed to investigate the phase structure of the prepared Meretrix lusoria shell powder sample. As shown in Figure 4, the XRD pattern exhibited sharp and strong peaks, indicating a well-crystalline structure. The peaks were observed at specific 2θ angles, corresponding to various crystallographic planes, such as (111), (021), (002), (012), (200), (031), (112), (130), (211), (122), (221), (041), (132), (113), and (231). Notably, distinct diffraction peaks characteristic of calcite or vaterite were not detected, suggesting that the Meretrix lusoria shell powder predominantly consists of the aragonite phase.

3.1.3. FTIR

It is a powerful technique for characterizing the composition and chemical interactions within mollusk shells. It provides valuable information about the functional groups included in shell formation. The FTIR spectra of the shell powder before and after metal ion biosorption are compared in Figure 5. The FTIR spectra of shell powder were obtained to analyze the bending and stretching vibrations of the functional groups involved in the biosorption of the biosorbent material. Hence, the FTIR spectral analysis demonstrates the existence of negatively charged functional groups. The FTIR spectra analysis of shell powder displays dissimilar peaks at 3495, 2923, 2859, 2643, 1451, 1083, 983, 855, 707, and 559 cm⁻¹. The strong absorbance band at 3495 cm⁻¹ may be assigned to the presence of an alcohol hydroxyl group (-OH). The absorbance band at 2923 cm⁻¹ is due to the C-H stretching vibration. The presence of –CH₂ stretching vibrations can be attributed to the band appearing at 2859 cm⁻¹. The strong peak at 1643 cm⁻¹ displays the presence of a carbonate group. The peak observed at 1451 cm⁻¹ may be assigned to the presence of –CH₂ stretching vibrations. The strong peak at 1080 cm⁻¹ indicates the presence of a phosphate group. The strong peak at 983 cm⁻¹ is owing to C–O stretching vibration. Also, the strong band at 559 cm⁻¹ coincides with the O–P–O bending vibration of the P group. Absorbance at 983 cm⁻¹ was due to C–O stretching vibrations, which were shifted at 980 cm⁻¹ after the Pb²⁺ sorption. Therefore, the FTIR analysis proved the presence of –CO₃ in the shells, which was observed to interact with Pb²⁺ ions during its biosorption onto the shell powder. FTIR spectra showed that the vibration bands were shifted by 3 to 25 cm⁻¹ because of the biosorption process of Pb ions on the surface of the aragonite shell powder (Figure 5). This agrees well with previous studies [44], in which they observed the biosorption peak of calcite at 875 cm⁻¹ and the biosorption peaks of aragonite at 1082 cm⁻¹ and 857 cm⁻¹ of CO₃²⁻. The FTIR spectra before and after Pb²⁺ biosorption presented discrete peaks at 706, 980, and 1455 cm⁻¹, which revealed that -CO₃ groups were the main elements of molluscan shells. Also, these strong peaks at 706 cm⁻¹ and 1455 cm⁻¹ indicated the presence of an aragonite phase [45]. The result revealed that the infrared spectra of CO₃^2– ions involved four distinct vibration modes in CaCO₃. These modes included stretching vibrations that are symmetrical and bending vibrations that are out-of-plane of O-CO. The molecule exhibits two degenerate asymmetric stretching vibrations and two degenerate in-plane bending vibrations. The spectrum shows that the frequencies 706 cm⁻¹ (A) and 707 cm⁻¹ (B) correspond to the in-plane bending vibrations of CO₃^2– ions. The peaks observed at 850 cm⁻¹ (A) and 855 cm⁻¹ (B) correspond to the bending vibrations of O-C-O in CO₃^2– ions, which are stimulated by infrared radiation. The symmetrical stretching vibrations of the C=O bond in CO₃^2– ions occur at 1079 cm⁻¹ (A) and 1083 cm⁻¹ (B). The bands at 1455 cm⁻¹ (A) and 1451 cm⁻¹ (B) correspond to the asymmetric stretching vibration of CO₃^2– ions. The biosorption peaks observed are specific to aragonite [46]. The stretching vibration of C-H can be observed at 2859 cm⁻¹ (A and B) and 2927 cm⁻¹ (A) and 2923 cm⁻¹ (B).

3.1.4. Particle Size

The particle size distribution of marine mollusk (Meretrix lusoria) shell Powder occurs in a variety of different sizes as Shown in Figure 6; Average size Is 231.0 nm. Removal efficiency is increased with the smallest particle size of marine mollusk (Meretrix lusoria) shell powder (affected by the specific surface area of the media). The particle size diameter is effective for removing metal ions from the solution.

3.1.5. Zeta Potential

Zeta potential is a general measure of the amount of charge of particles in dispersion. If all the particles in suspension have a large negative or positive zeta potential, then they will tend to repel each other and there will be no tendency for the particles to come together. In the current study, zeta potential is used to evaluate the surface properties of the shell powder synthesized from Meretrix lusoria as biosorbent material in the biosorption process. The result of zeta potential values of shell powder is illustrated in Figure 7. It was observed to be −25.8 mv, which represents a relatively highly negative charge. This high negative potential shows long-term stability without agglomeration.

3.1.6. BET Analysis

The surface area properties of mollusk shell powder (Meretrix lusoria) were investigated using a multipoint BET surface (Brunauer–Emmett–Teller); mollusk shell powder was characterized by a N₂ biosorption test at 77 K. In total, 100 mL/min of dry nitrogen was introduced into the sample tube to avoid the contamination of the clean surface. The sample tube was then removed, and the sample weighed. The results of BET surface area were 3.1795 m² g⁻¹; free space was 18.188 cm³ (Figure 8).

3.2. Optimization of Biosorption Parameters

3.2.1. Effect of pH

The solution pH has a great influence on the heavy metal ion biosorption, which can affect the distribution of the ionization degree and surface charge of the biosorbent, in addition to the existence form and solubility of biosorbent ions [47,48]. With increasing the pH value from 2 to 8, the removal efficiency of shell powder on the studied metal ions (Pb²⁺ and Cd²⁺) gradually increased and became stable when the pH reached 6, and then a sudden decrease was observed with a further increase in the pH value (Figure 9). The maximum elimination occurred at pH 6.0 for both Cd⁺² and Pb⁺². The results revealed that the elimination % of [M²⁺] increased from 62% to 90.6% for Cd⁺² and from 57% to 91.5% for Pb⁺² with raising the pH from 2.0 to 6.0 and decreased to 75% and 78% for Cd⁺² and Pb⁺², correspondingly, at a pH value above 6.0. In an acidic medium, calcium carbonate of the shell is decomposed and reacts with H⁺ in the solution, which decreases the adsorbed active sites on the shell and prevents the formation of carbonate–metal complex precipitation. As the pH increases, the H⁺ ion concentration in a solution decreases, which enables heavy metal ions to combine with freer CO₃²⁻, resulting in the formation of carbonate precipitation. At the same time, when the pH increases, the electrostatic repulsion between the metal cations and biosorbent surface decreases due to the simultaneous increase in the negative charge on the biosorbent surface. Another result of this technique is an augmentation in biosorption capability. When the pH values increase, metal cations gradually convert into hydroxide structures, resulting in a reduction in the concentration of soluble cations [49]. The effectiveness of removing heavy metals can be improved by exploiting the functional groups (-OH and C=O) present in the shell powder, in addition to the strong coordination interactions between metal cations [50]. During the early stage, referred to as electrostatic biosorption, metals form bonds with the many functional groups found on the surfaces of the biosorbent, resulting in the rapid binding of ions to these surfaces [51]. When the metal ion being examined entirely covers the surface of the biosorbent, an equilibrium condition is reached. However, the effectiveness of the biosorption process declines as more surface-active regions become occupied. Nevertheless, as M²⁺ gradually covered the surface of the biosorbent, it proved challenging to fill the remaining vacant areas because of the repulsive forces between the liquid and solid phases of M²⁺ [52].

3.2.2. Effect of Contact Time

Under the buffer condition maintained at pH 6 for each metal, the impact of contact time on the metal biosorption efficiency of the generated biosorbent was investigated over various time intervals from 5 to 100 min (Figure 9). The significant increase in removal efficiency with extended contact time, ranging from 55 to 87% for Cd²⁺ and 61 to 91% for Pb²⁺, underscores the importance of allowing sufficient time for the biosorption process to unfold effectively. This logical progression is consistent with the basic principles of biosorption kinetics, where longer contact times allow for more extensive interaction between biosorbents, ultimately leading to higher biosorption efficiency.

3.2.3. Effect of Biosorbent Dose

The biosorbent dosage is a crucial parameter in biosorption processes, influencing the effectiveness of the removal of contaminants from a solution. The consequence of the biosorbent dosage on the heavy metal was studied at an initial concentration of 20 mg/L for 90 min at 25 °C. The effect of the shell powder biosorbent dose was investigated by the variation of the dosage from 0.1 to 1.0 g. Figure 9 shows that the removal efficiency (%) of Cd²⁺ and Pb²⁺ ions increased rapidly as the increase in the shell powder dosage with biosorption efficiency ranged from 55% to 85% for Cd²⁺ and from 67% to 90% for Pb²⁺. The maximum biosorption of Cd²⁺ and Pb²⁺ was obtained for the biosorbent dose of 1.0 g/100 mL. This phenomenon can be attributed to the increased availability of exchangeable sites or surface area, which leads to a more efficient uptake of Cd²⁺ and Pb²⁺ [40].

3.2.4. Effect of Initial Concentration

The impact of the initial concentration of Pb²⁺ and Cd²⁺ on biosorption was examined using the following optimal conditions: a period of 90 min, a pH of 6, a swirling speed of 300 rpm, a biosorbent dose of 1.0 g, and varying concentrations of Cd²⁺ and Pb²⁺ ranging from 5 to 30 mg/L (Figure 9). It was noticed that there was the same biosorption behavior of Cd²⁺ and Pb²⁺ on the shell with a decrease in removal efficiency from 72 to 46% for Cd²⁺ and from 85 to 64% for Pb²⁺. The removal of metal ions was higher at the beginning, probably attributed to larger surface area of the shell powder being available at the beginning of the biosorption of Cd⁺² and Pb⁺². At excessively high concentrations, the adsorbed sites may quickly become saturated, limiting further biosorption, and possibly reducing the overall efficiency of the process. Therefore, the correct balance is critical to achieve optimal biosorption performance.

3.2.5. Temperature Effect

Temperature is a critical factor that greatly impacts on the biosorption of metal ions on a biosorbent. The thermal effect on Cd²⁺ and Pb²⁺ biosorption on the shell powder was studied every 5 °C, covering a temperature range of 20–60 °C, under a 50 mg/L metal ion concentration, pH=6, and contact time of 90 mins. The data revealed that the biosorption of Cd²⁺ and Pb²⁺ increases with an increase in the temperature (Figure 9). The result showed that the most suitable biosorption temperature is 40 °C, with maximum biosorption around 80.18% and 85.4% for Cd⁺² and Pb⁺², respectively. The increased biosorption with temperature indicated endothermic biosorption. Furthermore, this can also be ascribed to a rise in the quantity of active sites of biosorption, resulting from the heat degradation of internal bonds on the surface of the adsorbent [53].

3.3. Modelling of Biosorption

3.3.1. Biosorption Isotherm Models

Temkin, Freundlich, and Langmuir are three of the linear isotherm models that were used to assess the biosorption data. The correlation coefficients (R²) were used as indicators of the agreement between the respective isotherm model and the experimental data. Best fitting is achieved when the correlation coefficient is around 1 [54].

The Langmuir isotherm assumes that biosorption occurs at specific homogeneous, energetically active sites on a biosorbent surface, with a finite biosorption capacity [55]. The model equation with parameter definitions is stated in

Table 3. Isotherm parameters for biosorption of Pb²⁺ and Cd²⁺ ions on Meretrix lusoria as biosorbent.

Kinetic Models	Parameter	Pb2+	Cd2+
Langmuir	qmax (mg/g) KL (L/g) R2	98.039 23.667 0.9699	63.694 12.083 0.9761
Freundlich	Kf (mg/g)/(L/mg)1/n n 1/n R2	2.0083 1.2475 0.8016 0.9712	1.8482 1.1490 0.8703 0.9891
Temkin	AT (L/g) B (J/mol) R2	1.1488 10.689 0.9735	1.4082 10.128 0.9583

. Langmuir constants were determined from the plot of 1/q_e vs. 1/C_e (Figure 10). At a temperature of 25 °C, the regression coefficients were 0.9699 for Pb²⁺ and 0.9761 for Cd²⁺. The regression coefficients for the studied metal ions approached 1 and therefore the data fit well with the Freundlich model (R² = 0.9712 and 0.9891 for Pb²⁺ and Cd²⁺, respectively).

The Freundlich isotherm model is indeed useful for describing biosorption processes in various environmental and industrial applications. It is particularly advantageous for its ability to account for the heterogeneous nature of the adsorbent surfaces and the varying energy of biosorption sites. Khayyun and Mseer [56] observed that the Freundlich isotherm model described the biosorption process for copper removal on a limestone adsorbent with a high coefficient of determination, R², better than the Langmuir isotherm model, and for a low initial concentration of heavy metal. The order of biosorption appears to be Pb²⁺ > Cd²⁺ based on the computed qe values (Table 3). Variations in binding energies and the stability of the resultant metal complexes could be the cause of this tendency [57,58].

The Freundlich model is used to express the multilayer biosorption isotherm for heterogeneous (rough and multisite) surfaces [40,59]. Similarly, the experimental data demonstrated a good fit to the Freundlich isotherm. The K_f values (Table 3) were 1.8482 mg/g for Cd²⁺ and increased to 2.0083 mg/g for Pb²⁺, indicating a nearly identical removal capability of the studied ions on the biosorbent. Furthermore, the n values for all the examples under study are higher than 1, indicating the presence of heterogeneity in the adsorbent surface. This suggests that the ions may be easily separated from the solution and that the biosorption process is effective [60,61]. Thus, the Freundlich isotherm shows the well-fitting of biosorption data for the studied metal ions as compared to the Langmuir isotherm.

In addition, the data were adjusted to conform to the linear Temkin isotherm, which considers the ways in which the biosorbent and bio-sorbate interact (Table 3). The Temkin isotherm is derived from the premise that the heat of biosorption for each molecule in the layer falls in a linear manner as the surface coverage increases. The isotherm is valid for the processes of biosorption on a heterogeneous biosorbent surface of a solid as well as for the bio-sorbate liquid. The values of b_T and A_T, for Pb²⁺ and Cd²⁺, were determined by analyzing the slope and intercept of q_e vs. ln C_e plots, respectively (Figure 10).

3.3.2. Kinetic Study

The efficiency of a biosorbent is mostly determined by its biosorption kinetics. It describes the rate of solute uptake at the solid–solution interface and gains more insight into the mechanisms [40,59]. Kinetic models are the most flexible type of models. They generally use differential equations to represent the detailed mechanisms of reactions and enable a detailed calculation of reaction flows. The goal in this study is to find out the best kinetic model that describes the biosorption process, and to infer kinetic parameters and obtain appropriate insight and results for use in designing and treating contaminated water for a reuse procedure.

The biosorption of Cd²⁺ and Pb²⁺ on the Meretrix lusoria was investigated utilizing three kinetic models: the intraparticle diffusion model, the pseudo-first-order model, and the pseudo-second-order model. The kinetic model equations for the biosorption of the metal ions under study are provided in

Table 4. Kinetics parameters calculated for the biosorption of Pb²⁺ and Cd²⁺ ions by Meretrix lusoria.

Kinetics	Parameter	Pb2+	Cd2+
qe (experimental) (mg/g)		9.16	7.91
Pseudo-first-order model	K1(min−1) qe (mg/g) R2	−0.0234 5.4712 0.8517	−0.0115 1.8789 0.2962
Pseudo-second-order model	K2 (g/mg/min) qe (mg/g) R2	0.0236 11.5606 0.9991	0.0068 8.4530 0.9731
Intraparticle diffusion model	C (mg/L) kdif (mg/g min1/2) R2	3.4747 0.7508 0.9594	4.0237 0.4876 0.8340

, together with the corresponding computed parameters in Table 4, and Figure 8 illustrates the weak correlation coefficient values for the pseudo-first-order model (R² = 0.8517 and 0.2962 for Pb²⁺ and Cd²⁺, respectively), indicating a lack of linearity that suggests that the model does not accurately represent the complete range of contact time [62]. Furthermore, an accurate evaluation of the validity of the kinetic model may be determined by comparing the empirically measured q_e with the estimated q_e [40,60].

During this research, the estimated q_e significantly deviated from the experimentally measured q_e values (Table 4). Thus, the biosorption of metal ions does not obey the pseudo-first-order kinetic model.

The application of the pseudo-second-order kinetic model (Table 4) demonstrated a noteworthy consistency in linearity across the whole contact period, as evidenced by the strong correlation coefficients (Figure 11). Furthermore, the pseudo-second-order kinetic model indicates that the computed equilibrium biosorption capacities (q_e) nearly match experimental values, indicating a strong agreement between model predictions and empirical findings [40]. The finding highlights the kinetic model of pseudo-second-order confidence and effectiveness in modeling the biosorption behavior of the examined metal ions. In most cases, while the pseudo-second-order model provides valuable insights into the overall kinetics of the biosorption reaction, it does not explain the rate-determining step. To further investigate this aspect, the Weber and Morris model [36] was employed to assess the probability of intraparticle diffusion (Table 4). The higher the values of the constant C, the greater the influence of the boundary layer on the biosorption process [63]. The occurrence of intraparticle diffusion as the rate-limiting step in the biosorption process is confirmed by observing a straight line passing through the origin in the plot of qt versus t^0.5 [64]. Table 4 reveals positive constant C values throughout the experimental period, indicating a significant contribution of surface diffusion to the rate-limiting step. Furthermore, the observed straight lines in the plot, which do not pass through the origin, provide evidence for the occurrence of intraparticle diffusion as a determining factor in the biosorption process. The biosorption process of a biosorbent involves multiple distinct processes [65]. When examining the movement of solute particles from the liquid phase to the solid phase, the boundary layer emerges as the primary element affecting the biosorption process. Consequently, the findings suggest a complex biosorption mechanism in the present study [40]. The findings indicate that the rate of biosorption of metal ions follows the order of Pb²⁺ > Cd²⁺. The ionic size and valence state of metal ions are key parameters that determine their biosorption behavior onto Meretrix lusoria. Understanding these characteristics is essential for enhancing the biosorption process and developing effective techniques for treating wastewater with an objective of removing heavy metal pollutants from water solutions. One possible explanation for this pattern is that the ions in the solution have different sizes when they are hydrated. Pb²⁺ biosorbs onto the Meretrix lusoria more quickly than Cd²⁺ because of its greater ionic radius. In addition, Pb²⁺ ion biosorption occurred more often than Cd²⁺ ions, suggesting that the lead ions may have a stronger attraction to the biosorbent surface’s binding sites or a larger charge density.

3.3.3. Thermodynamics of Biosorption

The effect of temperature changes on the biosorption of Cd⁺² and Pb⁺² was investigated on powdered marine mollusk (Meretrix lusoria) shells dried from an aqueous solution for 90 min, and the initial concentrations were 10 mg/L for Cd²⁺ and Pb²⁺ ions. The biosorbent dose is 1.0 g/100 mL and the temperature ranges between 20, 25, 30, 35, 40, 50, and 60 °C. Through the experimental data, the thermodynamic parameter values involving enthalpy change (ΔH°), Gibbs free energy change (ΔG°), and entropy change (∆S⁰) were designed to evaluate the nature and feasibility of the biosorption process by using the following equations [66,67]:

k_d = q_e/C_e

lnk_d = ∆S°/R – ΔH^o/RT

∆G° = ΔH° –T∆S° = −RT ln k_d

The terms k_d, ΔS°, ΔH°, and ∆G° express the biosorption distribution coefficient, entropy change, enthalpy, and Gibbs free energy; q_e is the concentration of Cd²⁺ and Pb²⁺ in the equilibrium on the biosorbent, Meretrix lusoria powder (mg/g); and T and R represent the temperature and the gas constant in T(K) and 8.314 J/mol K, respectively. The values of the representative thermodynamics’ variables of ΔH° and ∆S° were estimated from the slope and intercept values, respectively, from the plot between ln k_d and 1/T. The results of the parameters of thermodynamics for Cd²⁺ and Pb²⁺ biosorption onto Meretrix lusoria powder are given in

Table 5. Thermodynamic parameter values of Cd²⁺ and Pb²⁺ at different temperatures using adsorbent (marine mollusk, Meretrix lusoria) shells.

Metals	T (°C)	T (K)	Kd	∆G°	∆H°	∆S°	R
				(kJ/mol)	(kJ/mol)	(J/molK)
Cd2+	20	293	20.997	−7.415	−0.3668	4.3031	0.9998
	25	298	21.486	−7.599
	30	303	21.990	−7.787
	35	308	22.728	−8.000
	40	313	23.163	−8.179
	50	323	23.690	−8.513
	60	333	24.290	−8.832
Pb2+	20	293	15.688	−6.699	−0.2995	3.7800	0.9984
	25	298	16.002	−6.863
	30	303	16.408	−7.054
	35	308	16.660	−7.196
	40	313	16.789	−7.338
	50	323	17.301	−7.653
	60	333	17.7886	−7.973

and displayed in Figure 12. The negative values of enthalpy (ΔH°; −0.3668 and −0.2995 kJ/mol for Cd²⁺ and Pb²⁺, respectively) obtained referred to the exothermic nature of the process. Likewise, the positive entropy (∆S°; 4.3031 and 3.7800 J /mol K for Cd²⁺ and Pb²⁺, respectively) favors the complexation and stability of sorption. The data of the variation in the ∆G° during the temperature range were calculated as presented in Table 5. The results reveal that ∆G° data ranged from −7.415 kJ/mol at 293 K to −8.832 kJ/mol at 333 K for Cd²⁺ and a range of −6.699 kJ/mol at 293 K to −7.973 kJ/mol at 333 K for Pb²⁺ biosorbed on the biosorbent Meretrix lusoria powder. The negative values of ∆G° confirmed the feasibility of the biosorption process and the spontaneous nature of biosorption, and the degree of spontaneity of the reaction increases with increasing temperature. The positive ∆S° suggests the increased randomness at the solid/liquid interface during the biosorption of Cd²⁺ and Pb²⁺. Furthermore, when Pb²⁺ ions are absorbed, the solvent molecules that were originally adsorbed are displaced by Cd²⁺ and Pb²⁺ ions. This displacement results in an increase in translational entropy for the solvent molecules, which outweighs the decrease in entropy caused by the biosorbent ions. As a result, the system exhibits a higher degree of randomness.

3.3.4. Application of the Green-Synthesized Adsorbents to Remove Cd²⁺ and Pb²⁺ from Real Water Samples

Real natural water samples were collected from two different locations in different salinities, directly from the areas south of Al-Madinah Al-Munawarah (groundwater), representing fresh water, and the Gulf of Aqaba Red Sea (seawater), Saudi Arabia. Water samples were used to study the biosorption behavior and the effect of the matrix on the biosorption process. The physicochemical properties of water samples were examined (

Table 6. Hydrochemistry properties, major components, and nutrient salt conventions.

	Hydrochemistry Properties			Major Constituents (mg/L)				Nutrient Salts (mg/L)
Real samples	PSU	pH	DOM (mg/L)	Na+	K+	Ca2+	Mg2+	SO42 (g/L)	NO3	NO2	PO4	SiO4
Groundwater	4.25	7.58	1.18	665	20	486	165	0.95	0.079	0.012	0.037	0.033
Gulf of Aqaba, Saudi Arabia	41.54	8.24	2.78	10786	305	742	2325	2.13	1.15	0.016	0.084	0.096

). The results of water samples showed differences in the studied parameters. The pH value ranged from 7.58 to 8.24. Water salinity ranged from 4.25 to 41.54 PSU. DOM values were slightly high in Gulf of Aqaba Red Sea water (2.78 mg/L) and decreased to 1.18 mg/L in the groundwater. The concentration levels of the major interfering ions Na⁺, K⁺, Ca²⁺, Mg²⁺, and SO₄²⁻ (in mg/L) were 665, 20,486, 165, and 0.95 in the groundwater, while they were 10,786, 305, 742, and 2325 mgL⁻¹ in Gulf of Aqaba Red Sea water, respectively. The low nutrient concentrations were found in groundwater (Table 6). The concentrations of nutrients NO_3, NO_2, PO₄, and SiO₄ were 0.079, 0.012, 0.037, and 0.033 mg/L, respectively, for groundwater, while they were 1.15, 0.016, 0.084, and 0096, mg/L, respectively, for saline water.

The applicability of mollusk (Meretrix lusoria) shell powder sorbents in seawater and groundwater was studied using a multistage micro-column technique for the removal of Cd²⁺ and Pb²⁺. One liter of the water sample was measured for the initial metal ion concentration and spiked with 10 and 20 µg/L for Cd²⁺ and Pb²⁺, respectively. Spiked water was passed three times through a small glass column packed with 100 mg of the shell powder sorbent under a constant flow rate (5 mL/min).

Table 7. Removal of Cd²⁺ and Pb²⁺ from real water samples using mollusk (Meretrix lusoria) shell powder as biosorbent.

Spiking	Cd2+ (%Removal)			Pb2+ (%Removal)
Initial Metal (C0)	C0 = 10 µg/L			C0 = 20 µg/L
Water sample	Run 1	Run 2	Run 3	Run 1	Run 2	Run 3
Groundwater	93.81	91.75	90.44	90.86	90.87	91.77
seawater (Gulf of Aqaba, Saudi Arabia)	89.11	87.25	88.35	88.96	90. 28	91.45

shows the efficiency of the sorbent used in this study to remove metal ions. The removal efficiency of Cd²⁺ ranges between 90.44 and 93.81% for groundwater, while it ranges between 87.25 and 89.11% for seawater based on the three replicates on the multistage micro-column system. On the other hand, the removal efficiency of Pb²⁺ was high, ranging from 90.86 to 91.77% in the groundwater sample, and ranging from 88.96 to 91.45% in seawater. This means that the interfering ions in these samples influenced the biosorption.

The lower removal percentage of Cd²⁺ and Pb²⁺ may be due to different impurities and higher concentrations of the main components (Table 7). These ions can clog the biosorption sites on the biosorbent and thus reduce the biosorption of metal ions. This indicates the competitive mechanism of biosorption on active binding sites that is affected by the ionic size, solute layers, and effective nuclear charge of the ions in the medium. It was clearly observed that there was a slight difference in the Pb²⁺ removal efficiency on mollusk shell powder in various water salinities. It must be emphasized that selective biosorption between study areas is challenging for many reasons such as differences in salinity, extent of organic matter that may trap heavy metals, pH, interfering ions, surface areas [M²⁺], and other contaminants present. It is remarkable that the high removal rate of Cd²⁺ and Pb²⁺ by mollusk shell powder sorbents from real water samples shows that mollusk shell powder is a suitable sorbent for removing metal ions from the aquatic environment.

4. Conclusions

The present study highlights the possibility of using marine mollusk (Meretrix lusoria) shell powder biowaste as a biosorbent due to ease of operation and processing for recovery of heavy metals, and a low cost to treat metal ion contamination. It has promising properties for use in environmental treatment to remove heavy metals (Cd⁺² and Pb⁺² ions) from polluted wastewater. The FTIR analysis confirmed the presence of –CO₃ in the shells, which was reacting with Pb²⁺ ions during their bioadsorption on the shell powder. SEM imaging provided visual observations of the shell powder before and after the biosorption experiment that revealed slight morphological changes related to the effect of metal biosorption. In addition, the XRD analysis of the biosorbent showed the presence of calcium carbonate (CaCO₃), which mostly consists of the aragonite phase. The equilibrium description and kinetic and thermodynamic behavior of the biosorption process were studied. The Freundlich isotherm model described the biosorption process for the removal of Cd⁺² and Pb⁺² ions better than the Langmuir isotherm model. Overall, this study showed that mollusk (Meretrix lusoria) shell powder can remove lead and cadmium ions from aqueous solutions, and reusing mollusk shells in such processes can significantly reduce the metal ions present in the environment; the removal efficiency of these metals reached about 90% in groundwater and seawater samples.