Uptake of heavy metals from aqueous media onto the blend of sodium alginate and kernel powder

doi:10.21203/rs.3.rs-5274877/v1

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Uptake of heavy metals from aqueous media onto the blend of sodium alginate and kernel powder

https://doi.org/10.21203/rs.3.rs-5274877/v1

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Heavy metal poisoning is widely recognized as a serious problem for both the environment and human health. The damage caused by these metals has raised concerns for global public health and ecology, prompting a significant focus on developing effective materials for heavy metal removal. This study presents a method for creating an environmentally friendly adsorbent for removing metal ions from aqueous solutions, using sodium alginate/mango seed kernel blend beads (SA/MSK). The goal was to develop a low-cost, beneficial adsorbent by utilizing mango seed kernel (MSK), an agricultural waste product, as a resource to manufacture material for the removal of specific heavy metals. Adsorption, one of the most promising techniques, was employed in this work. Batch studies were conducted to examine the effects of pH, dosage, initial metal ion concentration, adsorbent particle size, and contact time on the percentage removal of Cd²⁺, Cu²⁺, and Pb²⁺ ions. FTIR, SEM, and XRD analyses demonstrated that carboxyl and hydroxyl functional groups were involved in the sorption of Cu²⁺, Cd²⁺, and Pb²⁺ ions. The efficacy of SA/MSK beads in eliminating metal ions from effluent samples showed that the beads were able to remove all three metal ions to varying degrees. Physicochemical and spectroscopic methods revealed that the binding sites involved were ether, carboxyl, hydroxyl, and amine groups. These findings suggest that SA/MSK beads hold great potential for applications in heavy metal removal and could become a sustainable method for removing metal ions from effluent.

Mango seed kernel

heavy metals

adsorption kinetics

sodium alginate

Chemicals are widely used in industries such as paints, electroplating, automobiles, and the electrical sector, releasing various metals into the environment. High concentrations of copper, iron, nickel, mercury, zinc, and cadmium ions are found in industrial and mining drainage, posing serious ecological threats [1]. Aquatic environments, including wetlands, rivers, lakes, and oceans, are vital for sustaining life and providing essential resources. Heavy metals are defined by their release of cations five times denser than water when dissolved. Contamination of surface and groundwater leads to environmental issues as these metals are consumed directly or through the food chain [2]. While some metals are necessary for biological functions, hazardous metals like lead, nickel, and cadmium, or excessive essential metals, can be toxic and accumulate in living cells, threatening all organisms [3]. Organs such as the heart, liver, and nervous system are particularly vulnerable to heavy metal exposure. Preventing heavy metal ion contamination in ecosystems is essential before pollutants enter natural water systems. Techniques like ion exchange, filtration, reverse osmosis, precipitation, adsorption, and electrochemical methods have been developed to remove heavy metal ions. Adsorption is often favored due to its efficiency, raw material availability, ease of use, and cost-effectiveness. Adsorption is particularly useful for removing heavy metal ions from aqueous solutions at low concentrations.

Mango seed kernel, a byproduct of mango fruit production, demonstrates exceptional qualities as a biosorbent for wastewater treatment [5]. The physical properties of the mango seed kernel reveal a porous structure that provides an extensive surface area, essential for adsorption. The chemical properties of the mango seed kernel include hydroxyl (-OH), carbonyl (-C = O), and carboxyl (-COOH) functional groups [6]. These functional groups, identified by Fourier-transform infrared (FTIR) spectroscopy, play a crucial role in binding metal ions through ion exchange or complexation pathways. The mango seed kernel serves as an efficient, economical, and environmentally sustainable biosorbent for the removal of heavy metals and dyes from aqueous solutions [7]. Functional groups found in mango seeds, which are often considered organic waste, make it an inexpensive biosorbent. These groups include hydroxyl, carboxyl, carbonyl, alcohol, amide, and aromatic groups. This offers an environmentally responsible and sustainable method for purifying water. Mango seeds contain cellulose, hemicellulose, lignin, and other functional groups, such as hydroxyl, carboxyl, and phenolic groups, which are effective at binding heavy metals. Research using scanning electron microscopy (SEM) demonstrates the coarse and porous surface of the kernel, enhancing the accessibility of active sites for adsorption [8, 9].

The mango seed kernel can absorb heavy metals such as lead (Pb(II)), chromium (Cr(VI)), and nickel (Ni(II)) [10]. A study indicated an adsorption capacity of 77.5 mg/g for nickel (Ni(II)). Optimal operating conditions for the mango seed kernel include pH: the kernel functions best in slightly acidic to neutral conditions (pH 5–6), which enhances the electrostatic interactions between metal ions and functional groups. Effective adsorption typically occurs within 60 to 120 minutes, depending on the specific pollutant [11].

Polysaccharides like cellulose, starch, chitosan, and sodium alginate (Na-alginate) are becoming increasingly popular as adsorbent options due to their abundance, safety, and affordability. Numerous studies on the use of sodium alginate as an adsorbent have been conducted and published, with promising results. It is a naturally occurring biopolymer derived from brown seaweed, widely utilized in various applications due to its biocompatibility, biodegradability, and gel-forming properties [12]. Sodium alginate has gained popularity as a biosorbent in environmental applications, particularly for the removal of heavy metals and dyes from aqueous solutions. Sodium alginate has been investigated for the removal of several contaminants, including heavy metals (such as lead (Pb²⁺), cadmium (Cd²⁺), zinc (Zn²⁺), copper (Cu²⁺), and chromium (Cr³⁺)), dyes (methylene blue, Congo red, and other textile dyes), and other organic compounds. The presence of surface functional carboxyl and hydroxyl groups facilitates ion exchange and chelation, resulting in the formation of stable beads or hydrogels that enhance adsorption. The gel structure and porous characteristics improve the availability of binding sites, enabling the exchange of Na⁺ ions with metal cations like Pb²⁺, Cu²⁺, and Cd²⁺ [13]. While alginate is effectively used to remove heavy metals from contaminated water, its adsorption capacity, stability, and mechanical strength require improvement for optimal efficiency in water treatment applications [14]. Alginate-based sorbents have been widely applied in water purification systems, but the combination of mango seed kernel powder with sodium alginate has not been extensively studied. Mango seed kernel powder, with its amorphous structure and superior ion exchange capability, holds promise. Therefore, a blend of sodium alginate and mango seed kernel powder beads is being explored as an adsorbent for heavy metal removal.

The study aims to synthesize active polysaccharide matrices, specifically sodium alginate and mango seed kernel powder, to create combined sorbent beads for heavy metal ion removal. The objectives include preparing the blend beads using the precipitation method, followed by an assessment of their physical and chemical properties. Additionally, the study investigates the removal of specific heavy metals from aqueous media and analyzes the data using various sorption models.

2.1. Materials

The chemicals were used in current study were analytical grade. Sodium alginate was bought from Captain PQ Chemical Industries (Pvt.). Nitric acid, sodium hydroxide and calcium chloride were purchased from local market in Pakistan. Cupper nitrate, lead nitrate and cadmium nitrate were acquired from JK Enterprises Limited. Water and other equipments used in this study were present in lab.

2.2. Apparatuses and instruments

The apparatus and instruments used in the laboratory were mechanical shaker to shake the sample, pH meter were use to measure pH of solution, Whatman filter papers No-1 were use to separate the adsorbent and filtrate, to weight digital electronic balances and AAS to observe metal concentration.

2.3. Mango seed kernel powder preparation

Mango seeds were sourced from mangoes purchased at Faisalabad's local market. The endocarps of the mangoes were properly washed with water to remove contaminants, and then sun-dried for an hour to produce a dry mass. The mango seeds were subsequently washed with deionized and distilled water to eliminate dirt particles and other impurities, and dried in an oven at approximately 45°C for 24 h. To obtain the required size fractions, the dried seeds were crushed, pulverized with a mortar and pestle, and then sieved to < 180–500 µm [15]. The mango seed kernel powder was then further dried in an oven at about 50°C for 24 h to increase its surface area, enhancing its potential as a good adsorbent.

2.4. Preparation of SA/MSK blend beads

The synthesis of sodium alginate and mango seed kernel beads entails merging the gelling characteristics of sodium alginate with the bioactive components of mango seed kernel. The procedure employs sodium alginate as a carrier matrix to encapsulate mango seed kernel extract, resulting in bead formation through a cross-linking process with calcium ions (Fig. 1). The materials utilized during the method included a blender, pipette, mango seed kernel extract, sodium alginate, calcium chloride (CaCl2), and distilled water. Two grams of mango seed kernel were added to 100 mL of water and allowed to stir for one hour. Sodium alginate was then added to the mango seed kernel solution in a 2:2 (grams) ratio (kernel to sodium alginate). Utilizing a pipette or syringe, we meticulously introduced the sodium alginate and mango seed kernel extract mixture into the 0.4 g/100 mL calcium chloride solution. Upon interaction, calcium ions cross-link the alginate, resulting in beads that contain the mango seed kernel extract. Subsequently, we gently agitated the calcium chloride solution during bead formation. The beads were allowed to remain in the calcium chloride solution for 24 hours to achieve the desired firmness and encapsulation efficiency. Once the beads had formed and solidified, they were transferred to a strainer and rinsed thoroughly with distilled water to eliminate excess calcium ions [16, 17]. Figure 2 shows the wet and dry beads.

2.5. Stock solutions preparation

For each experiment, analytical-grade chemicals were used. The compounds Pb(NO₃)₂, CuSO₄·5H₂O, and Cd(NO₃)₂·4H₂O were utilized to prepare stock solutions for each metal. To prepare stock solutions at a concentration of 1000 mg/L, the appropriate amounts of the copper, lead, and cadmium salts were dissolved in distilled water. The stock solution was then diluted to the desired concentrations to produce standard solutions for the calibration of instruments. The pH of the solutions was adjusted using 0.1 M NaOH and 0.1 M HCl solutions. All solutions used in these experiments were prepared with distilled water [18].

2.6. Samples preparation of heavy metals for adsorption studies

Samples are prepared as follows prior to the removal of heavy metals from aqueous solutions using sodium alginate-impregnated mango seed kernel blend beads. A portion of the contaminated liquid is collected for analysis. The pH of the solution is adjusted to the optimal range for the existing heavy metal ions. Adequate amounts of sodium alginate/mango seed kernel beads are incorporated into the solution. To facilitate the adsorption of heavy metal ions onto the beads' surface, the solution is agitated for a sufficient duration, varying from several minutes to several hours. The solution is then filtered or centrifuged to eliminate the beads. The beads are cleaned by rinsing them in distilled water [19]. The beads and solution samples are now ready for the metal uptake study.

2.7. Characterization of samples

To determine the functional groups in the adsorbent beads, FT-IR spectroscopy was employed. The FT-IR spectra of the adsorbent were measured in the range of 4000–400 cm⁻¹ before and after the adsorption of Cd²⁺, Cu²⁺, and Pb²⁺ ions. A variety of physicochemical and spectroscopic techniques were utilized to evaluate the beads before and after adsorption of Pb(II), Cd(II), and Cu(II), as well as the Pb(II) and Cd(II)-adsorbed NA-MSK beads, with the aim of elucidating the true adsorption mechanisms and identifying the active binding sites of the beads.

The XRD diffractogram of the SA-MSK beads was recorded using CuKα radiation (λ = 1.54060 Å) at 40 kV and 15 mA over a 2θ range from 10˚ to 80˚ (Bruker, Germany). The elemental properties and surface morphology of the beads were determined using a scanning electron microscope (Nano SEM-450, FEI, USA). Additionally, the functional groups on the surface of the NA-MSK beads were characterized using FTIR spectroscopy in the 400–4000 cm⁻¹ wavenumber range (Thermo Fisher, Nicolet iS10, USA). After filtration of the solutions, the residual concentration in the solution (Ceq, mg metal/L or mmol metal/g) was measured using the ICP-AES technique.

2.8. Statistical analysis

All measurements were conducted in triplicate and data were presented as mean ± standard deviation. Statistical analyses were performed using SAS 9.0 (SAS Institute Inc., Cary, NC), followed by one-way ANOVA and the Tukey test at a 5% significance level.

3.1. Microstructural analysis

An electron microscope was used to study the morphology of the mango seed kernel (Fig. 3). Under the scanning electron microscope (SEM), with an accelerating voltage of 7 kV, the materials were analyzed in bead form. The mango seed powder, combined with sodium alginate beads, forms spherical structures with microspores capable of absorbing different metals. These are SEM micrographs of the SA/MSK blend beads taken before and after the adsorption of Cd(III), Pb(II), and Cu(II) from an aqueous solution.

The sodium alginate beads initially exhibited largely acicular and rod-like structures with large interstitial voids. The bead surfaces displayed some visible pores and spaces. An understanding of the sodium alginate matrix is provided in Fig. 3A [89], which shows a continuous, uniform matrix without any flaws or fissures. It appears that the sodium alginate beads are well encapsulated by the matrix. The spherical SA/MSK beads contain microspores that serve as active sites for the adsorption of metals like Cu, Ni, Cd, Pb, and Cr, enhancing their metal absorption capacity.

The image suggests that the porosity of the Na-alginate-impregnated mango seed kernel beads results in superior adsorption capabilities due to the greater abundance of functional groups in the MSK powder, which enhances the affinity for metal ion absorption. The porosity of the spherical SA/MSK beads is clearly greater than that of Na-alginate alone (Fig. 3B), leading to a larger adsorption capacity. Consequently, the beads are more effective in attracting metal ions. SEM micrographs indicate that these adsorbents have an increased surface-to-volume ratio.

After adsorption, the needle- and rod-like structures were significantly reduced, leading to fewer voids between the components (Fig. 3C, D, and E). The adsorption of metal ions filled many of the spherical particles. Research and observations showed that SA/MSK beads remove metal ions through pore filling and internal surface adsorption. Both chemical and physical adsorption processes were observed on the materials. The SEM study revealed a porous and rugged surface for the SA/MSK beads, which aligns with their primary function of removing toxic metals (Pb, Cu, Cd) from aqueous solutions.

3.2. XRD (X-Ray diffraction analysis)

The experiment was conducted using a wide-angle X-ray diffractometer (Rigaku, Model Ultima IV). The samples, in powdered form, were analyzed to determine the crystalline characteristics of sodium alginate beads infused with mango seed powder. The combination of mango seed powder with sodium alginate beads was studied using CuKα radiation (wavelength (λ) = 1.54056 Å) at 40 kV and 40 mA. The dimensions, morphology, defects, and minute forces within the crystals affect the positioning and configuration of the XRD peaks. The scanning range for the angles 2 theta (2θ) was from 10° to 80°, with a scanning velocity of 0.02°/min (Fig. 4). The X-ray diffractograms of four samples, comprising a blend of mango seed kernel/Na alginate beads prior to adsorption and three samples post-adsorption of Cu, Pb, and Cd metals, are illustrated in Fig. 4.

The extensive X-ray diffraction peak pattern suggests that mango seed kernels predominantly consist of amorphous substances, such as carbohydrates and lipids, while also exhibiting semi-crystalline regions attributed to starch and cellulose. MSK powder shows peaks at 15°, 17°, 18°, and 23° (2θ), which are characteristic of A-type starch crystallinity. Sodium alginate is typically an amorphous polymer (Fig. 4A), with a prominent peak occurring between 15° and 30° (2θ). Sodium alginate exhibits low-impact, sharp peaks due to structural damage to its crystallinity, indicating that the material is predominantly amorphous rather than crystalline [90].

The XRD results showed that the mixture has both amorphous and crystalline structural patterns when sodium alginate beads are blended with mango seed powder. A broad peak centered between 15° and 30° (2θ) signifies the amorphous characteristics of the sodium alginate beads and certain components of the mango seed powder. Crystalline peaks emerged within the 17°–23° (2θ) range. The ability of metal sorption is limited by crystallinity, as metal adsorption is enhanced when the crystallinity of the specimen is reduced, as shown in Fig. 4 (B, C, D).

3.3. Point of zero charge (pHpzc)

The Point of Zero Charge (pHpzc) can be experimentally determined by the strong acid or base addition method, using biosorbents such as sodium alginate, mango seed kernel beads, and their composite. For biosorbents like sodium alginate, the PZC is a crucial characteristic, indicating the pH at which the biosorbent's surface has no net charge. The PZC for sodium alginate typically falls within the pH range of 4 to 6. Sodium alginate contains carboxylate groups that deprotonate at higher pH levels, resulting in a negative surface charge above its PZC. Below the PZC, the surface carries a positive charge, which can influence the adsorption of various ions.

The PZC for biosorbents derived from mango seed kernels generally ranges from pH 3 to 7. The surface functional groups of mango seed kernel biosorbents, including carboxyl, hydroxyl, and phenolic groups, are affected by pH, as shown by a pH meter in Fig. 5.

The findings for the biosorbents are as follows: i) The pHpzc for sodium alginate ranges from 4.5 to 5.5. At this pH, the surface carboxyl groups (-COOH) remain protonated, resulting in no net surface charge [91]. ii) The pHpzc for mango seed kernel beads is approximately 5 to 6, due to the presence of hydroxyl, carboxyl, and phenolic groups on their surface [92]. iii) The pHpzc for the composite is about 5 to 5.5, depending on the mixing ratio.

The salt addition method provides a direct approach for experimentally determining the pHpzc of sodium alginate, mango seed kernel beads, and their composite. Establishing the pHpzc is essential for optimizing pH conditions for maximum adsorption efficiency during biosorption tests, particularly for cationic metal ions such as Pb²⁺, Cu²⁺, and Cd²⁺.

The graph in Fig. 5 illustrates the ΔpH (change in pH) versus the initial pH, used to determine the Point of Zero Charge (pHpzc) for sodium alginate, mango seed kernel beads, and their composite.

3.4. FTIR spectra

Different absorption bands observed in the FTIR spectra of MSK reveal its complex molecular structure. The FTIR spectrum of the MSK corresponded to the following vibrations: O-H stretching vibrations with hydrogen bonding (3356 cm⁻¹), C-H in alkanes (2915 cm⁻¹), C = O in esters (1729 cm⁻¹), C = O in carboxylic salts (1452 cm⁻¹), C-N in amines (1340 cm⁻¹), C-O-C in ethers (1161 cm⁻¹), and C-O in primary alcohols (1022 cm⁻¹), as well as the bending vibrations of N-H in primary amines (1618 cm⁻¹). This indicates that MS powder contains a variety of functional groups, including amine, carboxyl, ester, and hydroxyl groups, as shown in Fig. 6.

The Fourier transform infrared (FTIR) spectra of sodium alginate also displayed multiple prominent peaks. The O-H stretching vibration was observed around 3273 cm⁻¹, and the aliphatic C-H stretching vibration at 2925 cm⁻¹ in Na-alginate. Asymmetric and symmetric C = O vibrations, which are the two main types of COO⁻ stretching, were found at 1593 cm⁻¹ and 1410 cm⁻¹, respectively. The peak at 1295 cm⁻¹ was attributed to C-O stretching vibrations. Additionally, the stretching vibrations of C-O, C-C, and C-O-C were associated with the peak at around 1085 cm⁻¹. The noticeable peak at 1025 cm⁻¹ was caused by vibrations between carbon atoms and carbon ions, as shown in Fig. 6B.

The FTIR spectra of SA/MSK beads revealed the presence of free hydroxyl groups, as suggested by the prominent peak at 3400 cm⁻¹ in the composite beads made from sodium alginate and mango seed kernel powder. The hydroxyl groups of -COOH and intramolecular hydroxyl stretching are indicated by the peaks at 2915 cm⁻¹ and 2868 cm⁻¹, respectively. The stretching of C-O bonds and C-O-C bonding are indicated by the peak at 1015 cm⁻¹. The symmetric stretching of the -COO⁻ group is shown by the peak at 1595 cm⁻¹. The stretching vibration seen at 1600 cm⁻¹ results from ionic bonding with calcium and corresponds to the C = O group.

3.5. Effects of pH on the adsorption of metal ions

The pH of the solution is a key factor that controls the adsorption process, as it directly influences the electrostatic interactions between the adsorbent and adsorbate. An adsorption study was conducted to evaluate the effect of pH. The pH can be easily determined before and after the experiment by checking the point of zero charge (pHPZC), with a pH range from 4.5 to 5.5. The percentage removal of metal ions decreases as the pH increases. The higher removal efficiency at lower pH values is attributed to the electrostatic interactions between the oppositely charged metal ions and the adsorbent surface. At lower pH, the surface is highly protonated. As the pH increases, the level of surface protonation decreases, indicating that the biosorbents made from different beads weaken the adsorption of metal ions (Cu, Pb, Cd).

3.5.1. Adsorption at 4.5 pH

The form in which metal ions exist depends on the pH of the solution. Lead (Pb²⁺), cadmium (Cd²⁺), copper (Cu²⁺) are cations in acidic to neutral conditions. At very low pH, metal ions remain dissolved and protonated, making them more available for adsorption. At the lower pH, the surface is highly protonated. As the pH increase, the surface protonation decreases and identify that the biosorbents of different beads weaken the adsorption of metals ions (Cu, Pb, Cd). At 4.5 pH adsorption efficiency is less favorable.

3.5.2. Adsorption at 5.5 pH

In acidic conditions (low pH 2–3), the functional groups (-COOH) on sodium alginate beads become protonated, leading to reduced adsorption due to the decreased availability of negatively charged binding sites. As the pH increases from 4.5 to 5.5, the carboxyl groups (-COO⁻) deprotonate and become negatively charged, promoting electrostatic attraction between the biosorbent and metal ions (Pb²⁺, Cd²⁺, Cu²⁺).

Mango seed kernels contain cellulose, lignin, and other components that adsorb metal ions through ion exchange and surface interactions. They perform best in slightly acidic to neutral pH conditions, where metal ions are most reactive and the functional groups on the bead surface become available for binding. The blend of sodium alginate and mango seed kernel beads enhances adsorption across a broader pH range due to the combined functionality of sodium alginate (effective at pH 4.5–5.5) and mango seed kernel (effective at pH 5.5-6) (Fig. 7). At low pH, sodium alginate tends to be less effective, but the mango seed kernel can still provide reasonable adsorption capacity. At pH 6 to slightly basic pH 8, both biosorbents exhibit lower adsorption efficiency. Therefore, SA/MSK beads demonstrate better adsorption efficiency at pH 5.5.

3.6. Adsorption kinetics (effect of time) for Pb², Cd³, and Cu²⁺

Both sodium alginate and the blend of SA-MSK beads are effective for the adsorption of Cd²⁺, Cu²⁺, and Pb²⁺; however, Cd²⁺ adsorption takes longer to reach equilibrium compared to Pb²⁺ and Cu²⁺. The blend of SA-MSK beads performs particularly well for Pb²⁺ and Cu²⁺ ions due to strong electrostatic interactions with carboxyl groups.

Initial stage (0–60 min): Fast adsorption occurs due to the abundance of active sites on both sodium alginate and mango seed kernel beads, particularly in their blend. Pb²⁺ and Cu²⁺ are adsorbed more quickly than Cd²⁺, but all metal ions experience rapid initial uptake (Fig. 8).
Equilibrium stage (60–180 min): The adsorption rate decreases as the number of available binding sites diminishes. Equilibrium is reached when the adsorption capacity of the biosorbents is maximized, and no significant further uptake of metal ions occurs. The blend of sodium alginate and mango seed kernel beads reaches equilibrium faster and exhibits a higher adsorption capacity compared to each biosorbent used individually.
Lead (Pb²⁺): Lead ions tend to adsorb quickly in the initial stages, with rapid uptake occurring within the first hour.
Copper (Cu²⁺): Copper adsorption is also rapid initially, with a significant amount being adsorbed within the first hour, although the rate slows down over time.
Cadmium (Cd²⁺): Cadmium ions adsorb slightly slower than Pb²⁺ and Cu²⁺ due to their chemical complexity and the potential for different oxidation states.

The blend of sodium alginate and mango seed kernel beads demonstrates higher efficiency for Cu²⁺ and Pb²⁺ than the individual biosorbents due to the availability of more active sites and better ion exchange capacity. Initially, the process is dominated by the availability of binding sites and the rapid uptake of metal ions. After this initial phase, the biosorbents reach a state of equilibrium where the rate of adsorption slows down, and further adsorption becomes minimal over time. The blend of sodium alginate and mango seed kernel beads reaches equilibrium faster and provides a higher total adsorption capacity for Pb²⁺, Cu²⁺, and Cd²⁺. The graph below illustrates the relationship between the percentage of adsorption (%) and time (t); as time increases, the rate of adsorption decreases.

3.7. Effect of concentration adsorption equilibrium isotherms

The adsorption of metal ions depends on their initial concentration in the solutions. Mass transfer limitations between the solute and adsorbent are overcome by the driving force of the initial concentration (Fig. 9). For biosorbent concentrations ranging from 100 to 300 mg/L, the effects of biosorbent amount on the adsorption of metal ions (Pb²⁺, Cd²⁺, and Cu²⁺) are as follows:

Low Concentration (100 mg/L): The adsorption efficiency at 100 mg/L is lower because fewer heavy metal ions are available for the biosorbents. Competition for active sites among the metal ions is high, so not all metal ions in the solution will be adsorbed. As a result, the percentage removal of metal ions will be lower, and equilibrium will be reached quickly as the biosorbent becomes saturated.
Moderate Concentration (200 mg/L): Increasing the concentration to 200 mg/L improves adsorption efficiency, as more metal ions are available for the active sites on the biosorbents. Metal ion removal increases, leading the solution to approach a higher degree of purification. Equilibrium is still reached relatively quickly, but with more metal ions being adsorbed compared to the lower concentration.
High Concentration (300 mg/L): Fig. 4.10 shows that at 300 mg/L, the highest concentration in this range, there will be a large number of active binding sites on the biosorbents for metal ion attraction, allowing for maximum adsorption efficiency. The percentage removal of metal ions will be higher than at lower concentrations, with the biosorbent absorbing most of the Pb²⁺, Cd²⁺, and Cu²⁺ ions in the solution. However, while the adsorption capacity increases with more biosorbent, diminishing returns may begin to occur at this concentration, as the adsorption process becomes limited by the availability of metal ions in the solution.

3.8. Optimal composition of the prepared SA–MSK beads

The adsorption of Pb²⁺, Cu²⁺, and Cd²⁺ ions by the four prepared SA–MSK gel beads was compared through experiments conducted under optimal conditions. Straight line curves were formed, as shown in Fig. 10, indicating that as the MSK content increased, the adsorption capacity also increased, although the mechanical strength decreased. When the MSK content was 2 g (SA:MSK = 1:1), metal adsorption exceeded 70%.

By combining specific surface area and pore analysis with mechanical strength testing using the squash technique, the SA–MSK beads with a ratio of SA:MSK = 1:1 were found to be superior to the other gel beads in terms of Pb²⁺, Cu²⁺, and Cd²⁺ adsorption, mechanical strength, specific surface area, and pore size. Therefore, all subsequent tests were conducted using the gel beads with the ratio of SA:MSK = 1:1, as shown in Fig. 4.10.

3.9. Stability of the beads

Mango seed kernel-impregnated sodium alginate beads demonstrated high stability in demineralized water over a period of months. However, their stability was also examined under conditions without water. In this scenario, the beads were found to gradually lose approximately 60% of their water content; however, their structural integrity remained intact. After being thoroughly dried in the oven, the ability of the beads to remove metals was evaluated. It was observed that even after being stored for eight months and nearly drying out, the adsorption capacity for Pb²⁺, Cd²⁺, and Cu²⁺ did not decrease.

3.10. Swelling behavior of SA-MSK beads

The swelling behavior of the SA-MSK beads was observed. It was found that as the amount of MSK powder increased in the SA-MSK beads, their swelling behavior significantly increased. Furthermore, the addition of HCl resulted in a reduction in swelling behavior due to the beads becoming more compact in structure, as shown in the SEM images. Various ratios of beads with differing swelling behaviors have been designed for suitable applications.

The swelling behavior of the SA-MSK beads was investigated over three cycles. For this experiment, 1 g of SA-MSK beads was placed in 50 mL of water for 24 hours at 30°C. The air-dried beads were found to have the ability to expand again, and the findings are presented below. After three repetitions, the swelling behavior remained nearly consistent. Therefore, for the SA-MSK beads, the swellability gradually decreased, and after several cycles, the beads could not be completely swollen. The percentage swelling after several attempts is shown in Fig. 11.

3.11. Reuse of SA-MSK beads

Repeated regeneration cycles were employed in a series of batch studies to assess the suitability of polymerized composite beads. The SA-MSK beads were initially combined with 100 mL aqueous solutions containing Pb²⁺, Cu²⁺, and Cd²⁺ for a predetermined period. Afterward, the beads were removed from the solution by filtering and thoroughly washed with distilled water until a neutral pH was achieved. Following the final treatment, 50 mL of distilled water (DW) was added to the modified MSK and SA beads, and the mixture was agitated at 30°C for two hours. According to the results, metals can be recovered at rates exceeding 80% under slightly acidic conditions when the composite beads are desorbed, thanks to the entrapment of metal oxides. Although HCl is a strong desorbing agent for Pb²⁺ and Cd²⁺, it can cause pore distortion in the sorbents. It is generally recognized that continuous treatment with acid over successive cycles reduces the adsorption capacity of the sorbents and eliminates any residual alkalinity from the utilized medium. To assess the capability of the recycled beads for reuse, they were tested for metal removal over an additional two or three regeneration cycles [33].

In this research, the adsorption efficiency of sodium alginate (Na-alginate) and mango seed kernel (MSK) powder blend beads for the removal of Cd²⁺, Pb²⁺, and Cu²⁺ ions from aqueous media was studied. Evaluations were conducted based on the following experimental parameters: solution pH, adsorbent dosage, initial metal ion concentration, and contact time during the adsorption process. The optimal pH for adsorption was determined to be 5 for Cd²⁺, Pb²⁺, and Cu²⁺ ions. The order of metal ion removal efficiency was Cu²⁺ > Pb²⁺ > Cd²⁺ for the blend beads, as binding sites and ion exchange were more readily available for Cu²⁺. As the initial metal ion concentration increased, the amount of metal removed also increased at the optimal pH. The highest metal ion adsorption was observed at pH 5, with an adsorbent dose of 0.2 g, an initial metal ion concentration of 2.5 mg/L, and a contact time of 120 min for each metal ion. The adsorption of Cu²⁺, Pb²⁺, and Cd²⁺ ions onto the blend beads was primarily dependent on pH. Langmuir and Freundlich isotherms were used to analyze the adsorption data over a broad range of adsorbent dosages. Both isotherms fit the interaction between the adsorbent and adsorbate, though the Langmuir isotherm showed a stronger association with metal ion adsorption. From an economic standpoint, reusing the blend beads and recovering the metal ions in subsequent adsorption cycles is likely to be important. The material can be reused in fresh adsorption processes if it exhibits a high desorption potential. Based on the findings, it can be concluded that MSK, an agricultural waste product, shows promise as a low-cost adsorbent for retaining metal ions in aqueous solutions. Additionally, this adsorbent can be considered a viable component in waste management strategies.

Acknowledgements

Not applicable

Funding

This study was not funded

Author Information

Muhammad Hamza Sardar, Department of Applied Sciences, National Textile University, Sheikhupura Road Faisalabad, 37610 Pakistan.

Muhammad Tahir Saddique, Department of Applied Sciences, National Textile University, Sheikhupura Road Faisalabad, 37610 Pakistan.

Contributions

Muhammad Hamza Sardar, contributed to conducting experiments, writing-draft preparation, Muhammad Tahir Saddique, contributed the conceptualization, review and editing. Both authors contributed to the final version of manuscript.

Corresponding author

Correspondence to Muhammad Tahir Saddique, [email protected]

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The author states on behalf of the co-author that there is no conflict of interest between them

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Uptake of heavy metals from aqueous media onto the blend of sodium alginate and kernel powder

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Abstract

Figures

1. Introduction

2. Materials and methods

2.1. Materials

2.2. Apparatuses and instruments

2.3. Mango seed kernel powder preparation

2.4. Preparation of SA/MSK blend beads

2.5. Stock solutions preparation

2.6. Samples preparation of heavy metals for adsorption studies

2.7. Characterization of samples

2.8. Statistical analysis

3. Results and discussion

3.1. Microstructural analysis

3.2. XRD (X-Ray diffraction analysis)

3.3. Point of zero charge (pHpzc)

3.4. FTIR spectra

3.5. Effects of pH on the adsorption of metal ions

3.5.1. Adsorption at 4.5 pH

3.5.2. Adsorption at 5.5 pH

3.6. Adsorption kinetics (effect of time) for Pb², Cd³, and Cu²⁺

3.7. Effect of concentration adsorption equilibrium isotherms

3.8. Optimal composition of the prepared SA–MSK beads

3.9. Stability of the beads

3.10. Swelling behavior of SA-MSK beads

3.11. Reuse of SA-MSK beads

4. Conclusions

Declarations

References

Additional Declarations

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