Assessing the sorption of uranium and thorium from simulated solutions using chemically treated biomass of <i>Sargassum aquifolium</i> macroalgae

Mohammad Albayari; Norazzizi Nordin; Rohana Adnan; Fawwaz Khalili; Mazen Nazal

doi:10.4491/eer.2024.004

Environ Eng Res > Volume 29(6); 2024 > Article

Albayari, Nordin, Adnan, Khalili, and Nazal: Assessing the sorption of uranium and thorium from simulated solutions using chemically treated biomass of Sargassum aquifolium macroalgae

Research

Environmental Engineering Research 2024; 29(6): 240004.

Published online: April 16, 2024

DOI: https://doi.org/10.4491/eer.2024.004

Assessing the sorption of uranium and thorium from simulated solutions using chemically treated biomass of Sargassum aquifolium macroalgae

Mohammad Albayari¹, Norazzizi Nordin^1,^†

, Rohana Adnan¹, Fawwaz Khalili², Mazen Nazal³

¹School of Chemical Sciences, Universiti Sains Malaysia, 11800 Gelugor, Pulau Pinang, Malaysia

²Department of Chemistry, Faculty of Science, The University of Jordan, Amman 11942, Jordan

³Applied Research Center for Environment and Marine Studies (ARC-EMS), Research & Innovation, King Fahd University of Petroleum and Minerals (KFUPM), Dhahran 31261, Dammam, Saudi Arabia

^†Corresponding author: E-mail: azzizi@usm.my, Tel: +603 653 4030

Received December 31, 2023 Revised April 7, 2024 Accepted April 15, 2024

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

This study aimed to investigate the potential application of activated Sargassum aquifolium macroalgae (ASAM) as a biosorbent for uranium(VI) and thorium(IV) ions, employing controlled experimental conditions. The parameters examined included pH, biosorbent mass, initial concentration, contact time, and temperature. To enhance sorption characteristics, the raw Sargassum aquifolium macroalgae (SAM) biomass underwent separate pre-treatments using phosphoric acid (H₃PO₄) and potassium hydroxide (KOH). Various models were employed to analyze the kinetic and sorption isotherm data, and thermodynamic parameters were determined to assess the sorption mechanism. The KOH-treated SAM demonstrated a higher capacity for biosorbing U(VI) and Th(IV) compared to the acid-treated biomass. The sorption mechanism was investigated through characterization techniques such as FTIR, SEM/EDS, XRD, Zeta-potential, and BET analysis. The desorption-sorption cycle efficiency of both sorbents was evaluated, indicating that 0.1 mol L⁻¹ HNO₃ exhibited the most efficient desorption reagent for both metal cations over five consecutive cycles. These findings suggest the potential of ASAM as a biosorbent for removing U(VI) and Th(IV) from aqueous solutions. Furthermore, the study demonstrated the remarkable effectiveness of the two activated sorbents in eliminating U(VI) and Th(IV) ions from real wastewater samples.

Keywords: Desorption, Isotherm, Kinetic, Sorption, Thorium, Uranium

Graphical Abstract

Keywords: Desorption, Isotherm, Kinetic, Sorption, Thorium, Uranium

1. Introduction

The increasing industrial and technological applications of rare earth metals have attracted significant attention from environmental scientists and public health experts [1]. The toxic and radioactive properties of rare earth metals present the possibility of environmental and health hazards. The extraction and treatment processes of these metals have the potential to discharge hazardous chemicals and radioactive substances into the surroundings, thereby causing pollution of soil, water, and air [2]. Radionuclides are currently used in the medical treatment of cancer patients, in the production of various drugs, and in the nuclear industry. Together, these activities result in large amounts of radioactive liquid waste showing uranium (VI) contamination [3]. According to the report, effluents contaminated with thorium (IV) and uranium (VI) represent a complex aqueous system characterized by the presence of numerous coexisting cations. As a result, disposal of thorium (IV) and uranium (VI) resources becomes a difficult task [4]. Hence, it is strategically necessary to devise advanced technologies capable of extracting rare earth metals effectively and environmentally from contaminated industrial and aqueous solutions. Many conventional techniques, including solvent extraction, ion exchange, and co-precipitation, have been applied to extract or recover rare earth metals from various environmental samples [5]. Although conventional techniques are effective in extracting rare earth metals, they are associated with limitations such as low loading capacity, high operating costs, and the need for high primary metal concentrations. As a result, biosorption technology has emerged as a prominent means to capture or separate rare earth metals from aqueous solutions. Biosorption is preferred due to its effectiveness, biodegradability, natural abundance, cost-effectiveness, and ease of implementation [5,6].

Biosorption is a naturally occurring phenomenon in which certain biological substances, such as algae, have the ability to bind and remove heavy metals and other pollutants from water. The effectiveness of this process is affected by various factors, including the concentration of uranium and thorium in the solution, temperature, pH level, and the duration of contact between the biomass and the solution [7,8].

Nowadays, there is an increasing push to promote the use of natural sorbents, which are substances of biological origin that include bacteria, fungi, yeast, algae and similar substances. The use of lemon peel [9], Cladophora hutchinsiae, green algae [10], Hedera helix leave [11], sugar beetroot pulp [11], Cystoseria indica, brown algae [7] as a biosorbent for uranium ion removal is described in some scientific papers. An investigation was conducted to study the adsorption and transport characteristics of uranium (VI) using crushed-granite of Korean origin [12].

Sargassum aquifolium macroalgae possesses several advantageous characteristics that render it an excellent biosorbent when compared to other biomaterials. Its wide distribution and abundance in marine environments make it easily accessible for large-scale applications [13,14]. The macroalgae exhibit a notable sorption capacity for various pollutants, thanks to its distinctive cell wall structure and chemical composition abundant in functional groups. These functional groups create ample binding sites for the adsorption of contaminants, particularly heavy metals and radioactive elements [15,16]. Collectively, these attributes underscore the potential of Sargassum aquifolium macroalgae as a sustainable and effective biosorbent for the remediation of pollution. Utilizing Sargassum aquifolium macroalgae for the extraction of Uranium and Thorium from aqueous solutions is an environmentally sustainable, cost-effective, and efficient method [16].

Through chemical activation, the biomass is treated with a chemical solution that enhances its ability to absorb metal ions from the solution. This chemical solution changes the surface properties of the algae, thus increasing their ability to bind to ions effectively and remove them from the water [17–20].

The primary goal of this research is to use an inexpensive biosorbent to remove uranium (VI) and thorium (IV) ions from both simulated solutions and real water samples. To our knowledge, this study represents the initial investigation into the application of acid-base-treated Sargassum aquifolium macroalgae (SAM), referred to as SAM-A and SAM-B, for uranium and thorium adsorption. Furthermore, this study presents a new method to recover uranium and thorium using SAM that has undergone acid and base treatment to enhance its physicochemical properties and increase its adsorption capacity. Furthermore, there is currently a paucity of literature on the recovery of uranium and thorium, which are highly sought after in the magnet industry, using an environmentally friendly and cost-effective SAM adsorbent. Thus, this research makes a valuable contribution to our understanding of the affinity of rare earth metals. To the best of our knowledge, the potential of acid-treated (SAM-A) and base-treated (SAM-B) Sargassum aquifolium macroalgae as a means to effectively remove U(VI) and Th(IV) radioactive elements from water remains unexplored in the existing research.

In this study, we demonstrate the production of ASAM biomass treated with both acid and base, proving its effectiveness as an adsorbent for extracting U(VI) and Th(IV) ions from water solutions. We performed a comprehensive analysis to evaluate the effect of different factors, such as pH, sorbent mass, contact time, and metal concentrations, on the adsorption efficiency. In addition, we examined the kinetics and thermodynamics of the removal process. In addition, we investigated ion recovery, reusability of sorbents, and removal of U(VI) and Th(IV) ions from primary contaminated water samples in order to replicate real-world situations. In this particular investigation, we have the chance to make a valuable contribution to the body of scientific knowledge, advance the adoption of sustainable methods, and potentially bring about positive effects on environmental cleanup and water purification techniques.

2. Experimental

2.1. Materials

All chemicals employed in this investigation were of analytical grade and did not undergo any additional purification steps. Thorium nitrate tetrahydrate (Th(NO₃)₄4.4H₂2O) and uranyl nitrate hexahydrate (UO₂(NO₃)₂.6H₂O) were procured from BDH, while Arsenazo(III) indicator was sourced from Fluka. Sodium chloride, potassium hydroxide, sodium carbonate, sodium hydroxide, EDTA, phosphoric acid, and hydrochloric acid (HCl 37%) were obtained from Sigma Aldrich, and nitric acid (69%), ethanol, and hexane were acquired from TEDIA. Throughout the process of preparing aqueous solutions and cleaning glassware, only deionized water was utilized.

2.2. Preparation of Activated Sargassum Aquifolium Macroalgae

The procedures described in a previously published article were adhered to in order to gather and process the SAM samples [16].

For the activation of the SAM, a cleaned biomass weighing 25.0 g was combined with either 500 mL of 0.50 M phosphoric acid or 0.50 M sodium hydroxide at room temperature. The mixture was stirred continuously for 24 hours. Subsequently, the mixture underwent suction filtration, and the obtained solid was washed with deionized water until the filtrate reached a neutral pH. Afterward, the solid was dried in an oven at 65°C for 24 hours. The resulting products were designated as SAM-A and SAM-B, respectively, and were carefully stored in glass bottles with screw caps. The identical procedure was followed for both products.

2.3. Instrumentation

For sample weighing, an electronic balance (RADWAG® AS 220 R2) was employed. Subsequently, the samples underwent filtration using nylon syringe filters with a diameter of 0.46 cm. To determine the pH of the resulting solutions, an EUTECH pH-meter was utilized. FTIR spectroscopy was carried out using a Nicolet 6700 Thermo Electron Spectrophotometer equipped with a KBr disc. Thermal Gravimetric Analysis (TGA) was performed using a TGA Q500 (TA Instruments, USA) with a heating rate of 20 °C min⁻¹ over a temperature range spanning from 25 to 800°C. X-Ray Diffraction (XRD) analysis was conducted using a Philips X pert PW 3060 operating at 45 kV and 40 mA. Surface morphological studies and elemental composition analysis of the prepared sorbent were performed using a Field Emission-Scanning Electron Microscope (FE-SEM) in combination with Energy Dispersive X-ray Spectroscopy (EDXS) utilizing the Jeol 6700LV apparatus. The samples were agitated using a water bath shaker from Memmert GmbH Instrument Germany. The concentrations of U(VI) and Th(IV) ions were quantified using a METASH V-5100 UV-Vis spectrophotometer within a 1-cm quartz cell. The surface area and porosity of the sorbent were evaluated using an automated gas sorption analyzer (Autosorb iQ Quantachrome USA).

2.4. Preparation of Solutions

A spectrophotometric study was performed to evaluate U(VI) and Th(IV) concentrations before and after biosorption. This analysis used a 0.1% (m/V) solution of the Arsenazo(III) indicator. In order to create the indicator solution, a solution of uranyl nitrate hexahydrate and thorium nitrate tetrahydrate was dissolved in deionized water at a concentration of 200 mg L⁻¹ of each salt. Stock solutions were used to create different concentrations of U(VI) and Th(IV) solutions, ranging from 5.0 to 80.0 mg L⁻¹. The pH of the standard solution was adjusted by using 0.1 M solutions of HCl and NaOH.

2.5. Determination of U(VI) and Th(IV) Ions

To perform spectroscopic analyses, a 25 mL volumetric flask was used to combine 0.5 mL of Arsenazo(III) indicator, 2.0 mL of U(VI) solution, and 10 mL of 0.01 mol L⁻¹ HCl. Deionized water was then added to reach the vial mark. Measurements were performed using a spectrophotometer at a wavelength of 650 nm and a temperature of 25°C. For Th(IV) analysis, the same procedure was repeated, but a 9 mol L⁻¹ HCl solution was used, and all spectroscopic measurements were taken at a wavelength of 660 nm and a temperature of 25°C [16].

2.6. Sorption Experiments

The sorption process was assessed through batch equilibrium mode, taking into account various factors including initial pH, biomass dosage, initial concentration of U(VI) and/or Th(IV), contact time, and temperature. For the experimental procedure, 0.100 g of biosorbent was combined with 50 mL of a 40 mg L⁻¹ U(VI) and/or Th(IV) ion solution at pH 3.0 and 298 K. The mixture was agitated at 150 rpm for a duration of four hours in a water bath. Subsequently, the mixture was filtered using a 0.45 μm filter paper, and the concentration of U(VI) and/or Th(IV) in the filtrate was determined according to the instructions provided in section 2.5. The quantity of sorbed uranium and/or thorium metal, q_e (mg g⁻¹), was computed using Eq. (1).

(1)

q_{e} = \frac{(C_{o} - C_{e}) \times V}{m}

The equation utilized to determine the quantity of U(IV) and/or Th(IV) adsorbed by the sorbent relies on several factors: the initial concentration of U(VI) and/or Th(IV) (C_o, measured in mg L⁻¹), the equilibrium concentration of U(VI) and/or Th(IV) (C_e, measured in mg/L), the volume of the solution (V, measured in liters), and the mass of the biosorbent when dry (measured in grams). These parameters are essential in calculating the extent of U(IV) and/or Th(IV) sorption by the sorbent.

In order to evaluate the effectiveness of sorption (uptake), the sorption efficiency (SE%) was determined by dividing the concentration of U(VI) and/or Th(IV) sorbed at equilibrium (C_sorbed, measured in mg L⁻¹) by the initial concentration of U(VI) and/or Th(IV). This calculation provides a quantitative measure of the sorbent’s ability to absorb U(VI) and/or Th(IV) from the solution (Eq. (2) and Eq. (3)):

(2)

Sorption efficiency (%) = S E % = \frac{C_{s o r b e d} \times 100}{C_{o}}

(3)

C_{s o r b e d} = C_{o} - C_{e}

The impact of pH on the sorption of U(VI) and/or Th(IV) by SAM-A and SAM-B sorbents was investigated through the examination of various initial pH values ranging from 0.5 to 5.0. Prior to the experiment, the pH of the solution was modified using 0.1 M HCl and 0.1 M NaOH solutions.

To calculate the thermodynamic parameters, sorption experiments were carried out at four distinct temperatures: 298 K, 303 K, 308 K, and 313 K.

The kinetics experiments were conducted using all sorbents under optimized conditions, which included a pH of 3.0 and a temperature of 298 K. Each experiment involved combining 0.100 g of activated sorbents with 50 mL solutions containing 40 μg mL⁻¹ of U(VI) and/or Th(IV). The mixture was then agitated at 150 rpm for different time intervals (15, 30, 45, 120, 240, 480, 1440 min) using a water bath. Subsequently, the mixture was filtered using 0.145 μm filter paper, and the concentration of U(VI) and/or Th(IV) in the filtrate was promptly measured according to the procedure outlined in section 2.5.

In order to explore the interaction between uranium and thorium ions and the sorbents SAM’-A and SAM-B, comprehensive investigations were conducted to analyze both the kinetic and equilibrium aspects of the process. The kinetics were assessed using well-established models like the pseudo-first-order and pseudo-second-order models. Meanwhile, the equilibrium data were fitted to different models such as Langmuir, Freundlich, and Temkin models. Linear isotherms, which are mathematical models, were employed to establish a relationship between the solute concentration in a solution and the adsorbed amount of the solute on a solid surface under specific temperature conditions [21,22].

2.7. Sorption from Real Water Sample Experiments

Combined studies were performed using SAM-A and SAM-B sorbents to study the sorption behavior of U(VI) and Th(IV) ions in actual water samples. A total of four separate water samples were collected, consisting of two samples obtained from Nestlé Saudi Arabia (described as clean, drinkable water) and two groundwater samples obtained from Wadi Al Disah, located in the Tabuk region in the south. Part of the Kingdom of Saudi Arabia. Each water sample, with a volume of 50.0 mL, with a concentration of 40.0 mg L⁻¹ of U(VI) or Th(IV) ion, was combined with 0.100 g of SAM-A or SAM-B, in a 100.0 mL glass vial. The mixture was then stirred for 4 hours while maintaining a constant pH of 3.0 and temperature of 25°C. Quantification of metal ions in water samples was performed using a UV spectrophotometer, as described in Section 2.5 of the paper.

2.8. Desorption Experiments

To perform desorption experiments, 100 mg of SAM-A or SAM-B biomass loaded with uranium and/or thorium (40.0 mg L⁻¹) was mixed with 50 mL of 0.1 M solutions of NaCl, Na₂CO₃, EDTA, and HNO₃ and shaken in a water bath for 4 hours at 298 K. After elution, the sorbent biomass was rinsed multiple times with water and then used again in another solution containing 40 ppm of metals for the next sorption cycle. This desorption-sorption cycle was repeated five times using the same biomass and initial concentration of metals in every cycle. The concentration of ions in the solution was measured as described earlier, and the degree of desorption was calculated using Eq. (4):

(4)

Desorption (%) = \frac{d e s o r b e d a m o u n t o f s o l u t e}{a b s o r b e d a m o u n t o f s o l u t e} \times 100

3. Results and Discussion

3.1. Material Characterization

FTIR experiments were conducted to identify the functional groups present on the surface of SAM-A and SAM-B, and their role in metal binding. The FTIR spectra for the SAM-A and SAM-B materials are shown in Fig. 1.

In the FTIR spectrum of SAM-A (Fig. 1), a distinctive peak was detected at 3471 cm⁻¹, indicating the presence of -OH and -NH₂ stretching vibrations. Furthermore, additional peaks appeared at 2885 and 2692 cm⁻¹, which can be attributed to the stretching of C-H bonds in alkanes. Additionally, a band observed at 1789 cm⁻¹ corresponds to the presence of C=C bonds in the aromatic structure [16,23,24].

In Fig. 1, the FTIR spectrum of SAM-B exhibits characteristic peaks. The peaks observed at 2885 and 2692 cm⁻¹ are associated with the stretching of C-H bonds in alkanes. Furthermore, the stretching peak at 1783 cm⁻¹ corresponds to the presence of C=C bonds in the aromatic structure. Medium intensity stretching peaks at 1351 cm⁻¹ suggest the existence of C≡N bonds. Additionally, a medium intensity stretching peak at 914 cm⁻¹ indicates the presence of C-O bonds in ether and alcohol functional groups [16,23]. The FTIR spectra reveals that both SAM-A and SAM-B sorbents possess multiple functional groups that have the potential to serve as sorption sites for U(VI) and Th(IV) ions.

The X-ray diffraction pattern of SAM-A and SAM-B sorbents, derived from activated sargassum aquifolium macroalga, is illustrated in Fig. 2. SAM-A displays two distinct peaks: one at 2θ = 14.845° and a second broad peak at 2θ = 23.742°, both of which are indicative of crystalline cellulose. On the other hand, SAM-B exhibits three sharp peaks at 2θ = 14.228°, 2θ = 22.953°, and 2θ = 29.434°, which are also associated with crystalline cellulose [25].

Both SAM-A and SAM-B display three clearly identifiable weight loss phases in their TGA thermograms when exposed to air and nitrogen. The initial stage involves eliminating volatile solvents and moisture. The second stage arises from the decomposition of the sorbent, while the third stage is the result of oxidation of the sorbent (Fig. 3).

The surface area of the biosorbents was found to be small. The BET surface area of SAM-A and SAM-B were 0.013 m²/g and 0.082 m²/g, respectively.

Fig. 4 (a and b) presents information on the surface morphology of SAM-A and SAM-B materials. SAM-A appears as solid pieces with more cavities, while SAM-B has wavy surface, grooves, and blunt ends that could potentially enhance the sorption process. The particles found on both SAM-A and SAM-B sorbents are of varying sizes and shapes, and are macro-porous. When viewed through a scanning electron microscope, the particles appear to be a combination of both heterogeneous and homogeneous mixtures. These findings are consistent with those reported in a previous study [24]. The identified elemental compositions of SAM-A and SAM-B sorbents are depicted in the EDX spectra of the sorbents in Fig. S1 (a and b) (Supplementary Materials, SM). SAM-A contains 45.6% carbon, 45.4% oxygen, 7.0% nitrogen and 2.0% calcium, whereas SAM-B contains 38.2% carbon, 41.2% oxygen, 8.3% potassium, 6.2% calcium, 3.7% nitrogen, 1.8% sodium and 0.4% magnesium. This suggests that both SAM-A and SAM-B sorbents are primarily composed of carbon and oxygen, which may have originated from organic compounds that contain oxygen-containing functional groups such as hydroxyl and carboxylic acids.

Zeta-potential is a measurement of the electrical potential between a charged surface in motion and a liquid at the point of shear. In the case of both SAM-A and SAM-B sorbents, the zeta-potential is negative, with values of −7.37 mV and −6.17 mV in water at pH 3.0. This indicates that the sorbents have a negative surface charge, making them well-suited for adsorbing positively charged species, including U(VI) and Th(IV) ions [26].

3.2. Effect of pH

The pH level has a notable impact on the chemical species of U(VI) and Th(IV) ions, as well as on the surface properties of both SAM-A and SAM-B sorbents, including protonated, deprotonated, and neutral surface charges. At low pH values (pH 0.5 to 1.5) (Fig. S2 (SM)), the hydronium ion (H₃O⁺) competes with the metal ion for available binding sites, thereby limiting its sorption. Conversely, at higher pH values, the sorption capacity is increased. As illustrated in Figs. 6 (a and b), the maximum sorption capacity was observed for both sorbents at pH levels between 3.0 and 4.0. Fig. S2 (SM) demonstrates that the percentage of U(VI) and Th(IV) ions sorption on both SAM-A and SAM-B sorbents increases as pH levels rise from 0.5 to 3.0. Sorption percentages of U(VI) and Th(IV) reach a maximum of 71.2% and 81.3%, respectively, on SAM-A, and 75.7% and 84.6%, respectively, on SAM-B at pH levels between 3.0 and 4.0 before declining due to hydrolysis of the metal ions. Based on the available data, there is a notable rise in sorption capacity within the pH range of 2.0 to 3.0, followed by a stabilization at higher pH values. Consequently, it is probable that the pKa falls within this particular pH range. To estimate the pKa more precisely, one can identify the pH value where the sorption capacity reaches approximately 50% of its maximum value. This pH value would correspond to the pKa (estimated around 2.3 to 2.4). When the pH level increases, various hydrolyzed forms of U(VI) and Th(IV) ions are generated, including [UO₂(OH)]⁺, [UO₂(OH)₂]²⁺, [(UO₂)₃(OH)₅]⁺, [Th(OH)]³⁺, [Th(OH)₂]²⁺ and Th(OH)₄ [27,28]. However, since insoluble hydroxides may form and interfere with sorption at higher pH levels, they are not considered or recommended [29]. Consequently, further sorption experiments focused on U(VI) and Th(IV) ions sorption at a pH level of 3.0.

3.3. Effect of Sorbent Dosage

The percentage uptake of U(VI) and Th(IV) ions increases as both SAM-A and SAM-B sorbent masses increase, reaching 80% and 86%, respectively, on SAM-A, and 86% and 90%, respectively, on SAM-B at 0.5 g (as shown in Fig. S3 (a and b) (SM). The increased availability of active sites for sorption is the reason behind this phenomenon. As anticipated, the sorption capacity of both SAM-A and SAM-B sorbents diminishes with an increase in sorbent mass. This can be attributed to the reduced number of sorbate ions available in comparison to the abundance of sorption active sites on the surface of the sorbent. Moreover, it was discovered that 0.1 g of both SAM-A and SAM-B sorbents displayed a notable sorption capacity (q_e) for U(VI) and Th(IV) ions, aligning with findings from previous research studies [30,31].

3.4. Effect of Initial Concentration: Sorption Isotherm

Fig.S4 (a and b) (SM) illustrates the impact of U(VI) and Th(IV) concentrations on sorption. The sorption isotherm displays the relationship between the quantity of U(VI) or Th(IV) sorbed on a specific amount of sorbent and its concentration in the solution. With a pH of 3.0, the sorption capacities (q_e) of U(VI) and Th(IV) ions in both SAM-A and SAM-B sorbents increase as the metal ion concentration increases. This indicates that the sorbent has a strong attraction for sorbate at the beginning of the isotherm, especially when the sorbate concentration is low. However, when the concentration increases, the affinity of the sorbents decreases due to the limited availability of sorbent sites [32].

Calculating the sorption parameters is essential for designing the sorption system and understanding its fundamental characteristics. To analyze the experimental data, various equilibrium models were employed, including Langmuir [33], Freundlich [34], and Temkin [35]. To investigate the sorption of U(VI) and Th(IV) ions onto SAM-A and SAM-B sorbents, the most commonly used sorption isotherm models were employed, and their linear forms are presented in Eq. (5–7).

(5)

\frac{C_{e}}{q_{e}} = \frac{1}{b Q_{m a x}} + \frac{C_{e}}{Q_{m a x}}

(6)

ln (q_{e}) = ln (K_{f}) + \frac{1}{n} ln (C_{e})

(7)

q_{e} = \frac{R T}{b_{T}} ln (A_{T}) + \frac{R T}{b_{T}} ln (C_{e})

Eq. (5). contains the Langmuir model’s maximal sorption monolayer capacity (mg g⁻¹), denoted by Q_max, and the Langmuir constant b (dm³ mg⁻¹). The Freundlich model parameter n (dimensionless) and the Freundlich constant K_f (mg g⁻¹)(L mg⁻¹)^1/n are also included (Eq. (6).). Moreover, the Temkin model’s equilibrium binding constant A_T (L g⁻¹) and the Temkin constant b_T (J mol⁻¹), which is associated with the heat of sorption, are presented (Eq. (7).).

The sorption of U(VI) and Th(IV) ions onto SAM-A and SAM-B sorbents is depicted in Fig. S5 (a-c) (SM) and Fig. S6 (a-c) (SM), and the isotherm equilibrium data were subjected to testing and evaluation using various linear forms of isotherm models.

The parameters for the isotherm, as shown in Table 1, were obtained by analyzing the linear form of each model using the slope and intercept (Fig. S5 (SM) and Fig. S6 (SM)). All three models successfully explain the experimental results, but the Langmuir model demonstrates the best fit for the U(VI) and Th(IV) ion experimental isotherm data, as indicated by higher R² values (greater than 0.99 for both SAM-A and SAM-B sorbents). The R² values for the other models also exceed 0.96, suggesting that the sorbent surface comprises a combination of both homogeneous and heterogeneous components, which aligns with SEM observations. Consequently, the sorption of U(VI) and Th(IV) ions involves multiple mechanisms [36].

The determination of the Langmuir constant (b) allowed for the calculation of a separation factor (R_L) (R_L =1/1+bC_o). The R_L values, falling between zero and one, indicate that the sorbent is well-suited for the sorption of U(VI) and Th(IV) ions. The maximum sorption capacities are 22.86 and 25.37 mg g⁻¹ for U(VI) and Th(IV) ions on the SAM-A sorbent, respectively, while for the SAM-B sorbent, the values are 23.59 and 26.33 mg g⁻¹ for U(VI) and Th(IV) ions, respectively. The Freundlich equation yields an n value exceeding two, which suggests that the sorption of both ions onto both SAM-A and SAM-B sorbents is favorable. The positive values of b_T obtained from the Temkin model equation indicate the endothermic characteristics of the sorption process. These values further suggest that the proposed interactions, such as ion exchange and electrostatic contacts, between the U(VI) and Th(IV) ions and the surfaces of both sorbents (SAM-A and SAM-B), are effective in facilitating the sorption process [37].

To evaluate the fitting model, the correlation coefficient (R²) was employed, with the best fit line having an R² value close to one. In order of the highest R² values, the most suitable isothermal model for explaining the sorption of U(VI) and Th(IV) on both SAM-A and SAM-B sorbents is as follows: AM-A and SAM-B: Langmuir > Temkin > Freundlich

Table 2 summarizes the maximum biosorption capacity of various sorbents reported in the previous studies, including both SAM-A and SAM-B (present study) on U(VI) and Th(IV) sorption. SAM-B exhibits a higher maximum sorption capacity for uranium and thorium ions than SAM-A.

3.5. Effect of Contact Time

As depicted in Fig. S7 (a and b) (SM), both SAM-A and SAM-B sorbents exhibited a swift increase in the percentage uptake of U(VI) and Th(IV) ions as the contact time was extended within the first hour, subsequently attaining equilibrium within two hours (maximum percent uptake). This behavior could be attributed to the reduction in available binding sites with continuous contact, ultimately reaching a saturation plateau [38].

Eq. (8 and 9). were employed to examine the impact of time on the sorption process and elucidate the sorption kinetics onto both SAM-A and SAM-B sorbents using the pseudo-first order [33]and pseudo-second order [46].

(8)

ln (q_{e} - q_{t}) = ln (q_{e}) - k_{1} t

(9)

\frac{t}{q_{t}} = \frac{1}{q_{e}^{2} k_{2}} + \frac{t}{q_{e}}

The sorption capacity at a given time is represented by q_t in milligrams per gram (mg g⁻¹), while k₁ denotes the rate constant in the pseudo-first order model (min⁻¹) and k2 represents the rate constant in the pseudo-second order model (g mg⁻¹min⁻¹).

Two kinetic models, the pseudo-first order model and the pseudo-second order model, were employed to determine the best fit for the experimental outcomes of the kinetic study. Linearized kinetics models are presented in Fig. S8 (a–d) (SM), and the sorption kinetics parameters were analyzed from the corresponding linear plots’ slope and intercept. The results of these investigations can be found in Table 3.

Based on the obtained results, the pseudo-second order kinetic model proved to be the most precise in describing the sorption kinetics of U(VI) and Th(IV) ions by the SAM-A and SAM-B sorbents. This conclusion is supported by the high values of the squared correlation coefficient (R₂) and the close agreement between the calculated and experimental q_e values, as presented in Table 3. These findings suggest that the rate-determining step in the sorption process is the chemisorption of metal ions onto the biosorbent surfaces, which is contingent upon the availability of vacant sites [47], while the overall process is physisorption [48]. The value of k₂ (Table 3) reveals that the uptake of Th(IV) by both SAM-A and SAM-B sorbents at pH = 3 is faster in reaching equilibrium than the uptake of U(VI) [26].

3.6. Effect of Temperature

The sorption of U(VI) and Th(IV) ions onto both SAM-A and SAM-B sorbents was examined at temperatures of 298 K, 303 K, 308 K, and 313 K. The thermodynamic parameters can be determined (as shown in Fig. S9 (a & b) (SM) by observing the variation of the thermodynamic equilibrium constant (K_d) with temperature.

Table 4 displays the values obtained for the thermodynamic parameters, namely the free energy (ΔG^o), enthalpy changes (ΔH^o), and entropy change (ΔS^o), which were determined using the Following equation (Eq. (10).):

(10)

K_{d} = \frac{q_{e}}{C_{e}}

The sorption distribution coefficient is represented as K_d, and and correspond to the equilibrium sorption capacity and concentrations of U(VI) and Th(IV) ions on both SAM-A and SAM-B sorbents, respectively. The free energy change (ΔG^o) was determined using the following relation (Eq. (11).):

(11)

Δ G^{o} = - RTln K_{d}

The temperature is represented in Kelvin and the gas universal constant is denoted as R (8.3145 J mol⁻¹ k⁻¹) in the calculation of the enthalpy change (ΔH^o), which is determined using the following equation (Eq. (12).).

(12)

Δ G^{o} = Δ H^{o} - T Δ S^{o}

When Eq. (10–12). are combined, they yield the following linear relationship Eq. (13).

(13)

ln K_{d} = \frac{Δ S^{o}}{R} - \frac{Δ H^{o}}{RT}

The sorption process is thermodynamically favorable and spontaneous, as indicated by the negative values of free energy (ΔG^o) calculated within the temperature range of 298 to 318 K. The absolute values of ΔG^o increased as the temperature increased, as reported in references [47,49]. The endothermic nature of the sorption process for both ions is indicated by the positive values of ΔH^o. This suggests a strong interaction between sorbate and sorbent [45]. The positive entropy value for both ions during the sorption process implies an increase in randomness at the solid/solution interface [50].

3.7. Sorption of U(VI) and Th(IV) Ions from Real Water Samples

Using both SAM-A and SAM-B sorbents, the sorption of U(VI) and Th(IV) ions from real water samples was investigated in a batch experiment. The sorption capacity (qe) and sorption percentage(%) of U(VI) and Th(IV) ions in the water sample solution were determined and recorded in Table S1 (SM). The results indicate that the adsorption efficiency of the actual water samples is comparable to the water samples [51].

3.8. Desorption Experiments

The desorption step is crucial for the sorbent to be regenerated and used in subsequent sorption processes. Fig. S10 (a&b) (SM) displays the percentage recovery values for both SAM-A and SAM-B sorbents using four eluting agents, namely HNO₃, EDTA, Na₂CO₃, and NaCl, at a concentration of 0.1 mol L⁻¹. The results revealed that 0.1 mol L HNO₃ is the most efficient eluting agent for desorbing U(VI) and Th(IV) from both SAM-A and SAM-B sorbents, with the highest recovery percentages of 90.37% and 89.3% for U(VI) and Th(IV) on SAM-A, respectively, and of 87.79% and 88.35% for U(VI) and Th(IV) on SAM-B, respectively. The observed phenomenon can be explained by the existence of H⁺ ions within the solution, leading to an escalation in electrostatic repulsion and competitive interactions between the U(VI) or Th(IV) ions and H⁺ ions. Consequently, this diminishes the driving force between the sorbent and the U(VI) or Th(IV) ion. Additionally, it is possible for HNO₃ to form stable complexes with U(VI) and Th(IV) ions. These findings align with previous literature reports on the subject [26,50,52].

3.9. Sorption Mechanism

Both SAM-A and SAM-B sorbents exhibit various binding mechanisms for the interaction with Uranium(VI) and Thorium(IV) ions. This interaction can be attributed to ion exchange between U(VI) and Th(IV) ions and surface functional groups (such as H⁺, Na⁺, K⁺, and Ca²⁺) present on the biomass of both SAM-A and SAM-B. Additionally, the electrostatic interaction between the negative surface charge and the positive charge of U(VI) and Th(IV) ions contributes to this interaction. The sorption mechanism of U(VI) and Th(IV) radioactive ions on both SAM-A and SAM-B sorbents involves a combination of physisorption and chemisorption. The pseudo-second-order model provides the best description of the kinetic data, while the Langmuir model fits the isotherm data most effectively. The sorption process occurs spontaneously and is favored at lower temperatures.

The results of this study provide valuable knowledge that can inform the creation of effective and environmentally friendly approaches for the remediation of uranium and thorium contamination in water sources. These insights contribute to the development of sustainable strategies that aim to efficiently remove these radioactive elements from contaminated water. This study stands out due to its distinctiveness in several aspects. It places particular emphasis on chemically treated biomass, conducts a thorough assessment of treatment methods, provides a deeper understanding of the underlying mechanisms, evaluates the efficiency of desorption-sorption cycles, and applies the findings to real wastewater scenarios. These unique elements set it apart from other studies on uranium and thorium sorption, making significant contributions to the existing knowledge in this field.

4. Conclusions

The aim of this study was to investigate the potential of utilizing cost-effective and readily available biosorbents derived from activated Sargassum aquifolium macroalgae (SAM-A and SAM-B) powder for the efficient removal of U(VI) and Th(IV) ions from water samples. This research marks the first exploration of both SAM-A and SAM-B as sorbents for U(VI) and Th(IV) ions. Comprehensive characterization of the activated sorbents was carried out using multiple techniques, including FTIR, XRD, TGA, SEM, ESDX, elemental analysis, zeta potential, and BET. Batch sorption experiments were conducted under various experimental conditions to evaluate the effectiveness of SAM-A and SAM-B sorbent powders in removing U(VI) and Th(IV) ions from aqueous solutions. The sorption process was studied under optimal conditions of pH 3.0 and temperature 25°C. To analyze the kinetic data for the sorption of U(VI) and Th(IV) ions using SAM-A and SAM-B sorbents, a pseudo-second equilibrium model was used. Among the different absorption isotherm models, the Langmuir absorption isotherm showed the best fit with the sorption data, allowing the determination of maximum sorption capacities. Thermodynamic analysis indicated that the sorption processes for U(VI) and Th(IV) ions were endothermic and driven by entropy. The sorption of metal ions onto SAM-A and SAM-B sorbents was found to be spontaneous and exhibited a preference for U(VI) and Th(IV) ions. Both sorbents demonstrated excellent reusability for the sorption of U(VI) and Th(IV) ions, with the highest percentage of loaded ions being recovered using a 0.1 mol L⁻¹ HNO₃ solution. Overall, this study makes a novel contribution to the existing literature by investigating the sorption of uranium and thorium through the utilization of chemically treated Sargassum aquifolium macroalgae biomass. It enhances our understanding of the underlying sorption mechanisms, treatment approaches, the efficiency of desorption-sorption cycles, and the applicability of the biosorbent in real wastewater scenarios.

Supplementary Information

eer-2024-004-Supplementary.pdf

Acknowledgements

The authors express their gratitude to the editor and anonymous reviewers for their valuable comments and suggestions, which have significantly enhanced the quality of the manuscript. This study did not receive any financial support from public, commercial, or non-profit organizations.

Notes

Author Contributions

M.A. (PhD. student) conducted all the experiments, analyzed the data and wrote the manuscript.

N.N. (Senior Lecturer) checked the data and revised the manuscript.

R.A. (Professor) revised the manuscript.

F.K. (Professor) revised the manuscript.

M.N. (Assistance Professor) conducted the characterization of sorbent materials.

Conflict of Interest Statement

The authors confirm that they do not have any known financial interests or personal affiliations that could be perceived as having an impact on the research findings presented in this study.

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Fig. 1

FTIR spectra of SAM, SAM-A and SAM-B materials.

Fig. 2

XRD pattern of SAM-A and SAM-B materials

Fig. 3

Thermal gravimetric (TG) of (a) SAM-A and (b) SAM-B materials.

Fig. 4

SEM for both materials, a) SAM-A; and b) SAM-B.

Table 1

Isotherm Table for U(VI) and Th(IV) sorption obtained from the linear regression analysis.

Isotherm		U(VI) SAM-A	Th(IV) SAM-A	U(VI) SAM-B	Th(IV) SAM-B
Langmuir	Q_max	22.86	25.37	23.59	26.33

	B	0.13	0.23	0.19	0.32

	R_L	0.161	0.098	0.116	0.072

	R²	0.9939	0.9978	0.9927	0.9993

Freundlich	K_f	5.45	7.67	7.20	9.04

	1/n	0.36	0.32	0.30	0.31

	N	2.81	3.08	3.30	3.25

	R²	0.9811	0.9666	0.9948	0.9564

Tempkin	A_T	1.00	1.01	1.01	1.02

	b_T	512.68	492.50	559.00	491.65

	R²	0.9901	0.9862	0.9839	0.9860

Table 2

Comparison of maximum adsorption capacity of both SAM-A and SAM-B for U(VI) and Th(IV) ions with those reported in the literature for other biosorbents.

Ion	Biosorbent	pH	Q_max (mg g⁻¹)	Equilibrium Time (min)	Reference
U(V)	Biochar derived from Salvadora Persica branches	4.0	85.71	120	[38]

	Rice husk	3.0	14.86	60	[29]

	Orange peel	4.0	16.12	60	[39]

	Poplar leave	4.0	2.30	300	[40]
	Poplar ranches		0.40	120

	HQ-bentonite	4.0	63.90	15	[41]

	M-HNAC	4.5	688.03	30	[42]

	Hedera helix leave	3.0	3.86	1440	[10]

	SAM	3.0	20.43	120	[16]

	SAM-A	3.0	22.86	120	This study
	SAM-B		23.59

Th(IV)	Biochar derived from Salvadora Persica branches	4.0	84.97	120	[38]

	Rice husk	4.0	24.08	15	[43]

	Rice bran	−0.6	49.30	20	[44]
	Wheat bran		38.70

	HQ-bentonite	3.0	65,44	8	[45]

	TiO₂@SiO₂-CMBC	4.5	710.4	10	[4]

	Hedera helix leave	3.0	5.16	1440	[10]

	SAM	3.0	24.13	120	[16]

	SAM-A	3.0	25.37	120	This study
	SAM-B		26.33

Table 3

Kinetic parameters using linear regression analysis

Ion		U(VI) SAM-A	Th(IV) SAM-A	U(VI) SAM-B	Th(IV) SAM-B
1^st *Order*	q_e Exp.	14.18	16.14	15.08	17.24

	q_e Cal.	10.17	11.34	11.68	11.62

	K₁ (×10⁻²)	5.38	6.55	6.47	6.28

	R²	0.7352	0.7275	0.7661	0.7432

2^nd *Order*	q_e Exp.	14.18	16.14	15.08	17.24

	q_e Cal.	14.48	16.38	15.36	17.48

	K₂ (×10⁻³)	3.02	3.73	3.29	3.75

	R²	0.9991	0.9996	0.9993	0.9997

Table 4

The calculated thermodynamic parameters, at pH 3.0, initial ion concentration 40.0 mg L⁻¹, sorbent mass 0.1 g and contact time 4 hours.

Temp	U(VI) SAM-A			Th(IV) SAM-A

	ΔH°	ΔS°	ΔG°	ΔH°	ΔS°	ΔG°
298	76.88	0.26	−0.96	62.48	0.21	−4.22

303			−2.27			−5.25

308			−3.57			−6.29

313			−4.88			−7.32

*Temp*	*U(VI)SAM-B***			*Th(IV)SAM-B***

	ΔH°	ΔS°	ΔG°	ΔH°	ΔS°	ΔG°

298	83.82	0.29	−2.11	53.43	0.20	−5.94

303			−3.55			−6.93

308			−4.99			−7.93

313			−6.43			−8.93