Adsorption Properties and Mechanism of Copper Ions from Wastewater by Lessonia nigrescens and Lessonia nigrescens Residue

Abstract

Given the advantages of readily availability, low cost, convenient operation, and large adsorption capacity, brown seaweed has been studied extensively as a biosorbent for heavy metal remediation from aqueous media. Herein, raw Lessonia nigrescens and brown seaweed residue, a waste product from the manufacturing of alginate from L. nigrescens, were employed as low-cost and renewable adsorption materials for effective copper removal in wastewater streams. The influences of temperature, sample loadings, adsorption time, initial metal ion concentrations, and pH on the efficiency of the metal ions adsorption process were investigated. The thermodynamics and kinetics of Cu (II) adsorption for both the raw seaweed and seaweed residue were studied in order to determine the maximum removal efficiency and capacity. The characterization of the seaweed and seaweed residue before and after copper adsorption with SEM, FTIR, EDS, etc., coupled with the thermodynamics study, confirmed the ion exchange mechanism involved in the adsorption process. Under optimal conditions, the removal efficiencies were 75% and 71% for L. nigrescens and seaweed residue, respectively, and the adsorption capacities can reach 12.15 mg/g and 9.09 mg/g within 10 min for L. nigrescens and seaweed residue, respectively. The slight reduction in removal efficiency was because the active ion exchange sites were partially removed during the alginate extraction. The comparable metal ion removal efficiency between raw seaweed and seaweed residue suggesting the L. nigrescens residue is viable as bio-adsorbent and potential for industrial applications in adsorption process. The results provided a novel way to upgrade seaweed biomass in a biorefinery concept.

1. Introduction

Nowadays, the unavoidable use of heavy metals has led to water and soil pollution, becoming an increasingly urgent environmental problem [1,2]. Due to their high toxicity and resistance to environmental metabolism, even at low concentrations, heavy metals can be accumulated by aquatic organisms in water, leading to biomagnification in the food chain and ultimately affecting the safety of underwater organisms as well as human beings [3,4]. For example, lead poisoning can damage the nervous system [5], and exposure to arsenic can lead to cognitive impairments and growth retardation [6,7]. Copper is not poisonous as a metal and is also an essential trace mineral, which is extremely important for human health. However, Cu is toxic when forming compounds, such as copper sulfate, copper carbonate, and copper sub-acetate [8]. Excessive exposure to Cu can also influence human health and stimulate the digestive system, causing abdominal pain, vomiting, and potential liver damage. It is reported that long-term consumption of water with excessive copper content can lead to cirrhosis and cardiovascular diseases [9,10,11,12]. Many instances of copper toxicity are often associated with accidental consumption or installation of contaminated water sources; therefore, it is important to remove Cu from the environment. Various techniques, such as ion exchange [13], membrane filtration [14], chemical precipitation [15], and electrochemical methods [16] have been applied to remove Cu²⁺ from contaminated water. However, these methods are normally associated with high costs, potential secondary pollution, and complex operations, making them difficult to apply in practical applications [17,18]. In comparison, the bioadsorbent and bioadsorption process, with its characteristics of simple operation, low cost, strong adsorption capacity, and absence of secondary pollution, is the preferred option for heavy metal removal [19,20,21].

Lessonia nigrescens is a type of brown seaweed that is mainly distributed along the Chilean coast in the South Pacific [22]. This seaweed can grow up to several meters in length, with the longest specimens reaching up to 200 to 300 m. Due to its large size, rapid growth rate, and high alginate content, Lessonia nigrescens is widely used for the production of alginates [23]. Alginate is an important sustainable and renewable natural gelling material that has been widely applied in various fields, such as food, medicine, and cosmetics. The global market for alginate has continuously risen in recent years and is estimated to be over USD700 million in 2025 [24]. Depending on the seaweed species, harvesting season and extraction parameters, the yield of alginate is normally between 17–40%. Unavoidably, significant amount of seaweed residue is generated every year which is discarded as waste [25]. However, these seaweed residues are of great value because some functional groups such as -OH, -SO₃H, and -COO⁻ are remaining. These functional groups can potentially act as reducing agent or chelating agent for various applications including heavy metal adsorption. Therefore, using seaweed residue as heavy metal bioadsorbent is one of the important approaches to enhance the economic value of seaweed and research on the use of algal residue as heavy metal adsorbent is increasing [26].

The research conducted by Héctor A. Cid et al. on the adsorption of Cu²⁺ by brown seaweed revealed that the seaweed captured Cu²⁺ from solution via surface interactions involving different functional groups such as carboxyl, hydroxyl, sulfonic acid, and amide groups. These interactions occur via ion and coordination bonds, ligand polymorphism, and rearrangement of cell wall rigidity to adsorb metal ions [27]. Davis studied the extraction and purification of alginates from macroalgae and concluded that the main factors influencing heavy metal adsorption are the content and composition of alginates, with particular emphasis on the presence of glucuronic acid residues [28]. Carvalho discovered that seaweeds depleted of alginates still exhibit excellent metal adsorption capabilities. This suggests that although the extraction of alginates from brown algae can hinder metal ion adsorption, in some cases, alginates are not the main component responsible for the adsorption of heavy metal ions, and the alginate extraction procedure may destroy the algal cell wall, which will then expose other functional groups and therefore beneficial for heavy metal removal [29].

This study aims to explore the potential of Cu²⁺ adsorption by utilizing L. nigrescens particles after extracting alginates. The adsorption ability of Cu²⁺ by L. nigrescens particles and seaweed residue after extracting alginates from L. nigrescens was compared. Adsorption parameters such as pH value, initial concentration of Cu²⁺, adsorbent dosage, adsorption time, and temperature were considered to investigate their impacts on the adsorption process. Additionally, elemental analysis, spectroscopic analysis, and adsorption isotherm and kinetic models were used to explore the mechanism of Cu²⁺ adsorption by L. nigrescens particles and seaweed residue.

2. Materials and Methods

2.1. Raw Materials and Chemicals

The brown seaweed, Lessonia nigrescens, was kindly supplied by Bright Moon Seaweed Group, Qingdao, China, and was washed with tap water before drying in the oven at 60 °C until constant weight. The dried seaweed was then ground into particles (1 mm × 1 mm) and stored in dry, sealed containers. 27 mg DDTC-Na (C₅H₁₀NNaS₂·3H₂O, Rhawn) was dissolved in 500 mL water as a chromogenic agent. Copper stock solution (5000 mg L⁻¹) was prepared by dissolving 0.3911 g CuSO₄·5H₂O (Gaojing chemical plant) into 50 mL water, and then different concentrations of copper solutions (0–500 mg L⁻¹) were obtained by diluting copper stock solution as appropriate. Na₂CO₃ was purchased from Rhawn. Ammonia solution was purchased from Rhawn.

2.2. Residue Seaweed Preparation

The L. nigrescens particles (2 g) were mixed with 150 mL Na₂CO₃ solution (15 g L⁻¹). The mixture was stirred at 200 rpm under magnetic stirring for 60 min at 60 °C, and then the mixture was filtered to separate the liquid and solids. Filtered L. nigrescens residue was washed with Milli Q water and dried in the oven at 60 °C until constant weight.

2.3. Characterization of Original Seaweed Particles and the Seaweed Particles after Cu²⁺ Adsorption

The size and morphology of the L. nigrescens particles were visualized by scanning electron microscopy (SEM, Zeiss Sigma VP HD). Molecular information was characterized by FTIR. FTIR spectra were recorded using Thermo Scientific Nicolet iS20 scanning (Thermofisher, Waltham, MA, USA) from 4000–400 cm⁻¹. All spectra were collected with 32 scans and analyzed using Opus 7.0 software. The presence and related content of metals on the seaweed surface were analyzed using Energy-Dispersive X-ray Spectroscopy (EDS).

2.4. Cu²⁺ Removal by L. nigrescens Particles and Seaweed Residue

L. nigrescens particles or seaweed residue (50 mg) were mixed with 5 mL of different concentrations of Cu²⁺ solutions (12.5, 50, 100, 200, 400, 800 mg L⁻¹) for the effect of the initial Cu²⁺ concentration investigation. The mixture was stirred at 200 rpm under magnetic stirring for 60 min and then filtered to separate the liquid and solids. 1 mL of the supernatant was mixed with 3 mL of the DDTA-Na solution for color development. After incubation at room temperature for 5 min, the absorbance of 452 nm was recorded using UV-Vis Spectrometer (Jinghua, 723, Shanghai Jinghua Technology Instrument Co., Ltd., Shanghai, China). Calibration curves were obtained using 1 mL of Cu²⁺ solutions (12.5, 25, 50, 100, and 200 mg L⁻¹) under the above procedure. Adsorption of copper by L. nigrescens can be calculated by:

$${\mathrm{q}}_{\mathrm{t}}=\frac{({\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{t}})\mathrm{V}}{\mathrm{W}}$$

(1)

where C₀ is the initial concentration of the metal ion (mg L⁻¹); C_t is the concentration of the metal ion in the liquid phase after adsorption (mg L⁻¹); V is the volume of the solution (L); and W is the mass of adsorbent applied (g). The resulting solid seaweed residue was washed with distilled water three times and dried at 60 °C until constant weight for further analysis.

The mixture of L. nigrescens particles (50 mg) and 5 mL of Cu²⁺ solutions (200 mg L⁻¹) were stirred at 200 rpm for different contact times (1, 3, 5, 10, 15, 30 min) for the effect of contact time investigation. After stirring, the mixture was filtered to separate the liquid and solids. The subsequent analyses and calculations of metal ion concentrations of the supernatants were the same as described above.

The L. nigrescens particles (50 mg) were mixed with 5 mL of Cu²⁺ solutions (200 mg L⁻¹) at different pH values (4, 5, 6, 7, 8) for the effect of pH investigation. The pH of the Cu²⁺ solution was adjusted with a diluted ammonia solution. The mixture was under magnetic stirring for 10 min at 200 rpm and then filtered to separate the liquid and solids. The subsequent analyses and calculations of metal ion concentrations of the supernatants were the same as described above.

The L. nigrescens particles (50 mg) were mixed with 5 mL Cu²⁺ solution (200 mg L⁻¹). The mixture was under magnetic stirring for 10 min at different temperatures (30, 40, 50, 60 °C) and then filtered to separate the liquid and solids for the effect of temperature investigation. The subsequent analyses and calculations of metal ion concentrations of the supernatants were the same as described above.

Different qualities of the L. nigrescens particles (12.5, 25, 50, 100, 200, 400 mg) were mixed with 5 mL Cu²⁺ solution (200 mg L⁻¹) for the effect of L. nigrescens quality investigation. The mixture was under magnetic stirring for 10 min at 200 rpm and then filtered to separate the liquid and solids. The subsequent analyses and calculations of metal ion concentrations of the supernatants were the same as described above.

2.5. Adsorption Isotherms

Experimental data of Cu²⁺ adsorption by L. nigrescens particles and seaweed residue were fitted to Langmuir and Freundlich models to describe the adsorption isotherms. The Langmuir equation can be expressed as:

$$\frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{q}}_{\mathrm{e}}}=\frac{1}{{\mathrm{q}}_{\max }{\mathrm{K}}_{\mathrm{L}}}+\frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{q}}_{\max }}$$

(2)

where q_e is the equilibrium removal capacity (mg/g), q_max is the amount of heavy metal adsorbed in a complete monolayer (mg/g), C_e is the equilibrium concentration of metal ions (mg/L), and k_L is the Langmuir constant (L/mg). q_max and K_L can be determined from a linear plot according to Equation (3).

The Freundlich Equation can be expressed as:

$$\ln {\mathrm{q}}_{\mathrm{e}}=\ln {\mathrm{K}}_{\mathrm{F}}+\frac{1}{\mathrm{n}}\ln {\mathrm{C}}_{\mathrm{e}}$$

(3)

where q_e is the equilibrium removal capacity (mg/g), C_e is the equilibrium metal ion concentration in the supernatant (mg/L), K_F is the Freundlich constant, and 1/n is the adsorption intensity. K_F and 1/n can be determined from a linear plot according to Equation (3).

2.6. Adsorption Kinetics

The pseudo-second-order kinetic parameters using rate-controlling steps of chemical adsorption can be described by Equation (4).

$$\frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{t}}}=\frac{1}{{\mathrm{K}}_2{\mathrm{q}}_{\mathrm{e}}^2}+\frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{e}}}$$

(4)

where q_t is the adsorption capacity at time t (mg/g), k₂ is the pseudo-second-order rate constant pseudo-second-order kinetic model (g/mg min), and q_e is the adsorption capacity at equilibrium (mg/g).

The pseudo-first-order kinetic model considers that the occupancy rate of adsorption sites is directly proportional to the number of unoccupied sites. It can be written as:

$$(\frac{{\mathrm{dq}}_{\mathrm{t}}}{\mathrm{dt}})={\mathrm{k}}_1({\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}})$$

(5)

where q_t is the adsorption capacity at time t (mg/g), q_e is the adsorption capacity at equilibrium (mg/g), and k₁ is the pseudo-first-order rate constant (g/mg min).

The Elovich model describes the kinetic process of gas-chemical adsorption on the surface of adsorbents. The specific form of the Elovich diffusion model is described as follows:

$${\mathrm{q}}_{\mathrm{t}}=\frac{1}{\mathrm{\beta }}\ln (\mathrm{\alpha }\mathrm{\beta })+\frac{1}{\mathrm{\beta }}\mathrm{lnt}$$

(6)

where q_t is adsorption capacity at time t (mg/g). α and β are the initial adsorption rate and desorption constant, respectively.

The intraparticle diffusion model was established by Weber and Morris to explore the internal diffusion mechanism and diffusion process. The specific form of the intraparticle diffusion model is described as follows:

$${\mathrm{q}}_{\mathrm{t}}={\mathrm{k}}_{\mathrm{di}}{\mathrm{t}}^{\frac{1}{2}}+\mathrm{C}$$

(7)

where C is the constant of the boundary layer (mg/g). k_di is the intraparticle diffusion rate (mg/g min^0.5). q_t is the adsorption capacity at time t (mg/g).

2.7. Adsorption Thermodynamics

The thermodynamic behavior of the adsorption of Cu²⁺ on L. nigrescens was evaluated, and the thermodynamic parameters, including the Gibbs free energy change (ΔG⁰), enthalpy change (ΔH⁰), and entropy change (ΔS⁰), were determined according to the following Equations (8) and (9):

$$\mathrm{K}={\mathrm{q}}_{\mathrm{e}}/{\mathrm{C}}_{\mathrm{e}}$$

(8)

$$\Delta {\mathrm{G}}^0=-\mathrm{RTlnK}$$

(9)

$$\mathrm{lnK}=\frac{\Delta {\mathrm{S}}^0}{\mathrm{R}}-\frac{\Delta {\mathrm{H}}^0}{\mathrm{RT}}$$

(10)

where R is the universal gas constant, 8.314 J·mol⁻¹·K⁻¹; T is the Kelvin temperature (K); q_e is the adsorption capacity of the adsorbent at the equilibrium (mg g⁻¹); C_e is the residual Cu²⁺ concentration at equilibrium (mg L⁻¹).

3. Result and Discussion

3.1. Characterization of the Seaweed Particles

The SEM images of the Lessonia Nigrescens particles, the L. nigrescens particles after Cu²⁺ adsorption, the seaweed residue, and the seaweed residue after Cu²⁺ adsorption are shown in Figure 1a–d. According to Figure 1a, the intact cell structure can be observed from the L. nigrescens particles, and the cell diameter of L. nigrescens is approximately 20–30 μm. For the seaweed residue (Figure 1c), the cellular structure is more crowded and wrinkled, with a significant reduction in cell diameter. This is because alginates are important constituents of seaweed cell walls, providing them with structural strength and stability. When alginates are extracted and the cells lose their support, the cell walls are collapsed and fold, resulting in wrinkling. However, after one cycle of Cu²⁺ adsorption, significant damage to the cell structure of both L. nigrescens particles and seaweed residue was observed, with almost no observable cell structure remaining (Figure 1b,d).

FTIR spectroscopy has been frequently used to study the adsorption mechanisms between heavy metals and brown seaweed with respect to metal-carboxylate coordination [30], metal-phenolics coordination [31], metal-amino group coordination [32], etc., by detecting the vibrational frequency changes of carbonyl groups (-CO), hydroxyl groups (-OH) and amino groups (-NH₂). The FTIR spectra of the L. nigrescens particles, the L. nigrescens particles after Cu²⁺ adsorption, the seaweed residue and the seaweed residue after Cu²⁺ adsorption are shown in Figure 2a. The peaks at 3431 cm⁻¹ and 2924 cm⁻¹ were associated with O-H stretching vibration and C-H antisymmetric stretching vibration [33,34]. The peak at 1033 cm⁻¹ was due to the C-O-C stretching vibration from the carbohydrates in algal cell wall. The peaks centered around 1624 cm⁻¹ and 1417 cm⁻¹ were due to the asymmetric COO⁻ and symmetric COO⁻ stretching vibrations [35,36], mainly attributed by the naturally occurring alginates in the algal cell wall. Alginates are believed to be one of the most important biomaterials that involved in heavy metal adsorption in brown seaweed via ion exchange or formation of metal-alginate complex. From Figure 2b, some changes of the spectra that related to the COO⁻ stretching vibration can be observed before and after Cu²⁺ adsorption for both L. nigrescens particles. Specifically, the peak at 1417 cm⁻¹ shifts to 1416 cm⁻¹ for L. nigrescens particles, and the peak at 1624 cm⁻¹ changes to 1623 cm⁻¹ for both L. nigrescens particles and seaweed residue. This may be attributed to the association of metal chelates formed by the adsorption of Cu²⁺ by Lessonia Nigrescens particles and seaweed residue in a “pseudo bridged” unidentate arrangement, where one carboxylate group interacts with the metal ion while another forms a hydrogen bond with adjacent hydroxyl groups. The formation of hydrogen bonds can lead to an elongation of the C=O bond length, and the vibrational frequency of chemical bonds is inversely proportional to the square root of the bond length [37,38]. The peak at 819 cm⁻¹ is because of the presence of mannuronic acid [39,40,41]. By comparing the peaks around 817 cm⁻¹ in seaweed residue and L. nigrescens particles, it was found that the peak intensity in the seaweed residue was reduced (Figure 2a). This was attributed to the partially extraction of alginate, resulting in a small amount of residual alginate within the residue.

Important cell wall components such as alginates and sulfated fucans in brown seaweed are ion exchangers for heavy metal adsorption, in which cations such as Na⁺, Mg²⁺, K⁺, and Zn²⁺ are substituted by heavy metal ions. Thus, in this study, EDS was used to characterize the elemental compositions and concentrations of L. nigrescens particles and seaweed residue surfaces before and after Cu²⁺ adsorption for a better understanding of the adsorption mechanism. Figure 3 summarizes the changes in the content of elements of the L. nigrescens particles and the seaweed residue before and after adsorption detected using EDS. From Figure 3, it can be observed that C and O were the main elements of both the L. nigrescens particles and seaweed residue, attributing to the carbohydrates in the algal cell wall. Na, K, Mg, and Ca were the primary metallic elements, and Cu was not detectable in either the L. nigrescens particles or the seaweed residue before adsorption. After adsorption, there were 1.28% and 0.39% Cu²⁺ on the L. nigrescens particles and the seaweed residue surface, respectively. According to the batch adsorption experiments described in Section 3.2.3, removal percentage and removal capacity were 65.85%, 40.09%, and 26.34 mg/g, 16.04 mg/g, respectively. The disproportionality of the experimental data suggest a different adsorption mechanism of L. nigrescens particles and seaweed residue, where the adsorption of L. nigrescens particles was more likely to occur on the cell surface, while the seaweed residue was more likely to be co-adsorbed on the surface and inside the cell.

Additionally, the surface of the L. nigrescens particles shows a significant amount of Na and Cl, which probably due to the sodium chloride was not fully removed when washing (Figure 3a). Additionally, the significant increase in carbon and oxygen content for L. nigrescens particles after adsorption was also due to the removal of sodium chloride, resulting in the proportional increase in other elements. A complete disappearance of Zn was also observed for L. nigrescens particles after adsorption, suggesting Zn was totally replaced by Cu during adsorption. Compared to L. nigrescens particles, the seaweed residue lacks Zn on the cell surface due to the extraction of alginate. The reduction in Na and K in the seaweed residue after adsorption indicated that ion exchange occurred mainly among these elements (Figure 3b) [42].

3.2. Cu²⁺ Adsorption by the L. nigrescens Particles and Seaweed Residues

In order to further investigate the differences and mechanisms of Cu²⁺ adsorption by the L. nigrescens particles and seaweed residues, as well as the potential of seaweed residue as adsorbents for heavy metals, batch adsorption experiments were conducted on the L. nigrescens particles and seaweed residue for Cu²⁺ adsorption.

3.2.1. Effect of Different Contact Time

The adsorption equilibrium time is an important parameter in the adsorption process. It plays an important role in understanding as well as controlling the dynamic behavior of adsorption, and most importantly, the fast adsorption process is beneficial for industrial scale-up. Figure 4 shows the variation of Cu²⁺ uptake with time for L. nigrescens particles. The adsorption rate of L. nigrescens particles was extremely rapid within the first 10 min. When the initial concentration of Cu²⁺ was 200 mg/L, approximately 12.15 mg of copper ions were adsorbed per 1 g of L. nigrescens particles in the initial 10 min, and the removal percentage reached 60.78%. After 10 min, the adsorption speed significantly slowed down, with only 12% increase in the removal efficiency for the next 50 min (Figure 4). The rapid metal uptake speed by seaweed agrees with other findings. For example, Jia et al. [43] studied chromium adsorption using different types of seaweeds, including kelp, sea lettuce, wakame, and laver. The removal efficiencies after 1 h were found to be 95.4%, 93.6%, 99.1%, and 57.2%, respectively, demonstrating the rapid heavy metal adsorption processes of seaweed.

3.2.2. Effect of Seaweed Loading

The effect of different amounts of L. nigrescens particles on adsorption efficiency and capacity of Cu²⁺ adsorption was also investigated (Figure 5). With the increase of L. nigrescens particle mass from 0.0125 g to 0.4 g, the adsorption efficiency was improved from 43.6% to 81.2%. This was because as the amount of L. nigrescens particles increased, the number of active sites on the adsorbent also increased. However, a significant decrease in removal capacity was observed with the increasing amount of L. nigrescens particles, which was probably due to the fact that with the amount of seaweed increased, the mixture was more difficult to be homogeneously dispersed with stirring so that longer stirring time may be required to reach adsorption equilibrium.

3.2.3. Effect of Initial Cu²⁺ Concentration

To study the effect of initial Cu²⁺ concentration on the amount of metal uptake by both the L. nigrescens particles and seaweed residue, the adsorption behaviors with Cu²⁺ concentrations increasing from 100 mg/L to 800 mg/L were investigated (Figure 6). From Figure 6, we can conclude that L. nigrescens particles and seaweed residue showed similar adsorption patterns with the increasing of the initial Cu²⁺ concentrations. According to the results, it can be observed that as the initial Cu²⁺ concentration of the solution increased, the adsorption capacity continuously increased while the removal efficiency decreased. When the initial concentration of Cu²⁺ increased from 100 mg/L to 800 mg/L, the adsorption capacity increased from 6.23 mg/g to 37.58 mg/g. This was because as the Cu²⁺ concentration increases, the unoccupied adsorption sites start interacting with Cu²⁺, leading to an increase in the adsorption capacity until the adsorbent approaches saturation.

Langmuir and Freundlich isotherm models are commonly used to describe the adsorption equilibrium and analyze the adsorption mechanisms [44,45,46]. The Langmuir adsorption isotherm model assumes that adsorption occurs on a monolayer surface or a homogenous surface with identical positions. The Freundlich adsorption isotherm model assumes that adsorption occurs on a heterogeneous surface with multiple layers, and the adsorption capacity increases infinitely with an increase in Cu²⁺ concentration [47]. The adsorption isotherms and the isotherm modeling data are shown in Figure 7 and

Table 1. Adsorption isotherm modeling data.

Adsorbent	Model	Parameter	Value	R2
L. nigrescens particles	Langmuir	KL (L mg−1)	0.0025	0.9988
		qmax (mg g−1)	77.165
	Freundlich	KF (mg g−1) (L mg−1)1/nF)	0.9878	0.9686
		N (-)	1.6693
Seaweed residue	Langmuir	KL (L mg−1)	0.0041	0.99147
		qmax (mg g−1)	31.908
	Freundlich	KF (mg g−1) (L mg−1)1/nF)	0.8039	0.9869
		N (-)	1.7242

. From Figure 7a,b, it can be observed that compared to the Freundlich model (R² = 0.9686), the adsorption of Cu²⁺ by L. nigrescens particle is better described by the Langmuir model (R² = 0.9988), suggesting that L. nigrescens particles primarily underwent monolayer adsorption when the copper concentrations were ranging from 200 mg/L to 800 mg/L. From Figure 7c,d, it can be concluded that both the Langmuir model (R² = 0.9915) and the Freundlich model (R² = 0.9869) fit well for seaweed residue, suggesting a more complicated adsorption mechanism compared to L. nigrescens particles. The maximum Langmuir monolayer adsorption capacity of the L. nigrescens particles and seaweed residue, q_max (mg/g), are 77.2 and 31.9, respectively (Table 1). The differences could be attributed to the lack of adsorption sites for seaweed residue due to the alginate extraction.

3.2.4. Effect of Temperature

The influence of temperature on the adsorption capacity of metal ions is mainly manifested via its impact on the chemical structure of the adsorbent surface and the physicochemical properties of the solution [48]. As shown in Figure 8a, the adsorption efficiency of the L. nigrescens particles for Cu²⁺ significantly increased with the increasing of temperature. At temperatures of 30 °C, 40 °C, 50 °C, and 60 °C, the adsorption efficiencies were 63.54%, 64.03%, 67.01%, and 67.54%, respectively. For seaweed residue, the adsorption efficiencies of 200 mg/L Cu²⁺ at temperatures of 30 °C, 40 °C, 50 °C, and 60 °C were 50.78%, 53.97%, 58.24%, and 62.73%, respectively. The increasing of temperature is advantageous to the present adsorption processes since higher temperature activates the metal ions for enhancing the adsorption at the coordinating sites of the mineral.

The thermodynamic behavior of Cu²⁺ adsorption on L. nigrescens particles and seaweed residue was determined by calculating the thermodynamic parameters, including the changing of Gibbs free energy (ΔG⁰), the enthalpy (ΔH⁰) and the entropy (ΔS⁰) according to Formulas (8)–(10) [49,50]. Table 1 summarizes the thermodynamic parameters obtained from the intercepts and slopes of the Va, not the Hoff plot (Figure 8c,d). A positive value of ΔH⁰ indicates that the adsorption of Cu²⁺ by the L. nigrescens particles and seaweed residue is an endothermic process, which is consistent with the isotherm data showing an increase in adsorption capacity with temperature. The negative values of Gibbs free energy for Cu²⁺ adsorption by seaweed and seaweed residue in the studied temperature range demonstrate the adsorption processes occurred spontaneously. When comparing the ΔG values of the same temperature between seaweed and seaweed residue adsorption processes, L. nigrescens particles always showed more negative values (

Table 2. Calculated Thermodynamic Parameters.

L. nigrescens Particles				Seaweed Residue
T (K)	ΔG0 (kJ·mol−1)	ΔH0 (kJ·mol−1)	ΔS0 (J·mol−1·K−1)	ΔG0 (kJ·mol−1)	ΔH0 (kJ·mol−1)	ΔS0 (J·mol−1·K−1)
303	−1.2886	6.5673	25.9681	−0.0787	13.5019	44.6797
313	−1.5129			−0.4143
323	−1.8369			−0.8937
333	−2.0978			−1.4161

), indicating it is more thermodynamically favorable for L. nigrescens particles than seaweed residue. Adsorption entropy (ΔS⁰) represents the degree of disorder at the interface between the adsorbent and the adsorbate during the adsorption process. A value of ΔS⁰ for seaweed residue was larger than L. nigrescens particles, which means that the degree of randomness in the solid/solution interface of L. nigrescens particles was larger than that of seaweed residue.

3.2.5. Effect of pH

The pH value of the initial solution is an important factor that influences the bioadsorption process, which affects the type and number of functional groups on the seaweed or seaweed residue surfaces [51,52]. Thus, the adsorption experiments were conducted by adjusting the pH within the range of 4–8 while keeping other parameters constant.

For the L. nigrescens particles (Figure 9a), When the pH was between 4–6, the adsorption efficiency increased with the increase in the initial pH. However, when the pH was between 6–8, the adsorption efficiency rapidly decreased with the increase in pH. At low pH, the functional groups on the cell surface undergo protonation, which reduces the adsorption possibility of heavy metal cations from the solution [53]. Additionally, the high concentration of free hydrogen ions in the solution competes with the heavy metal ions for the adsorption sites on the seaweed and seaweed residue surfaces [54]. Under weak acidic conditions, the concentration of H⁺ in the solution is decreased, and the material surface exposes more active adsorption sites that can be occupied by Cu²⁺ [55]. When the solution becomes alkaline, the carboxyl group undergoes an acid-base neutralization reaction, resulting in the formation of corresponding salts and water. The decrease in carboxyl groups leads to a reduction in adsorption sites; hence, the adsorption capacity of L. nigrescens particles was decreased when the pH values were 7 and 8. By comparison of seaweed residue and L. nigrescens particles, smaller changes of adsorption capacity were observed at high pH values (from 6 to 8) (Figure 9b), indicating different adsorption mechanisms of seaweed residue and the carboxyl groups were not the major functional groups that were chelating Cu²⁺ for adsorption.

FT-IR was used to investigate the changes of carboxyl groups of seaweed and seaweed residue after Cu²⁺ adsorption at different pH. From Figure 10a,b, it can be observed that carboxyl groups form coordination complexes with metal ions and undergo hydrogen bonding, resulting in changes in bond length and bond angles, which in turn cause shifts in wave numbers. Within the range of pH studied, the peak shape and peak position are almost the same. Similarly, Fuks et al. studied the Mn-alginate complex by FT-IR and found that pH did not influence the peak position and shape [56].

3.2.6. Regeneration of the Adsorbents and Cu²⁺ Re-Adsorption

Figure 11 shows the adsorption efficiency of both L. nigrescens particles and seaweed residue during three adsorption cycles. From the results, we can conclude that a significant reduction in removal percentage was observed for L. nigrescens particles from the first adsorption cycle to the second adsorption cycle, and a slight reduction in removal percentage was observed from the second adsorption cycle to the third adsorption cycle. After three cycles, the adsorption efficiency was reduced from 60.8% to 40.8% for L. nigrescens particles. On the other hand, the removal efficiency was reduced from 45.5% to 38.8% after three adsorption cycles for seaweed residue. The reason for observing a greater adsorption efficiency decrease in L. nigrescens particles than seaweed residue for the second and third cycles may be because some of the adsorption sites of L. nigrescens particles were destroyed during the first adsorption cycle. Thus, the seaweed residue has the potential to be used in industrial wastewater remediation.

3.3. Kinetic Modeling

Adsorption kinetic data are important to predict the optimum operating conditions in experimental adsorption processes and help to design the adsorption system. Information such as adsorption mechanisms and potential rate-controlling steps can be provided by kinetic study [57]. Hence, in this study, kinetic models, including the Pseudo-first model, Pseudo-second order model, Elovich model, and Intraparticle diffusion model, were conducted by fitting the time adsorption curve. The kinetic fitting plots and parameters are concluded in Figure 12 and Figure 13 and

Table 3. Kinetic model parameters for Cu biosorption on L. nigrescens particles.

Model		Model
Pseudo-first-order		Elovich
k1 (min−1)	0.131	α (mg/g min)	20.5102
R2	0.981	β (g/mg)	0.3725
		R2	0.988
Pseudo-second-order		Intraparticle diffusion
qe (mg/g)	15.129	kd1 (mg/g min0.5)	3.221
k2 (g/mg min)	0.028	R2	0.984
R2	0.997	kd2 (mg/g min0.5)	0.861
		R2	0.991

. The R² for the Pseudo-first model, Pseudo-second model, Elovich model, and Intraparticle diffusion model are 0.981, 0.997, 0.988, and 0.991, respectively.

Important information such as adsorption rate and desorption rate can be obtained from the Elovich model by determining the initial adsorption rate α (mg/g min) and the desorption constant β (g/mg) [58]. According to Figure 12 and Table 3, the initial adsorption rate α was 20.5102 mg/g min, and the desorption constant β was 0.3725 g/mg in this study. Figure 12b shows the experimental data had good agreement with the theoretical data from the Elovich model.

Figure 12c represents the plot of q_t versus t^0.5, and two linear stages for Cu²⁺ adsorption could be clearly observed by the intraparticle kinetic model. The kinetic parameters shown in Table 3 confirm such a two-stage linear relationship. The first stage was the rapid one (K_d1 = 3.221), whereas the second stage was much slower (K_d2 = 0.861), which was probably because the first stage represents the diffusion of Cu²⁺ to the algal surface, whereas the second stage represents intraparticle diffusion. However, neither of these two linear stages passes through the origin, indicating that although the intraparticle diffusion was involved in the adsorption process, it was not the only rate-limiting mechanism.

In the case of the pseudo-second-order kinetic model, the adsorption efficiency is assumed to be directly proportional to the number of available sites, and it is independent of the initial concentration of the adsorbate [59]. The adsorption data of Cu²⁺ onto the L. nigrescens particles was fitted using the Pseudo-second order model (Equation (4)). The linear plot of t/q_t against t can give the pseudo-second-order adsorption rate constant k from the slope and qe from the intercept (Figure 13a). The correlation coefficient (R²) was found to be 0.997, indicating that the adsorption kinetics of Cu²⁺ onto the L. nigrescens particle can be well-fitted using the Pseudo-second order model (Figure 13). Thus, the efficiency-limiting step of the Cu²⁺ adsorption onto L. nigrescens particles process may be dominated by chemical adsorption processes, where the formation of metal-organic compounds and ion exchange are the rate-controlling steps [60].

4. Conclusions

This article investigates the adsorption processes and influencing factors by controlling the adsorption conditions, including time, temperature, pH, and amount of biomass of Cu²⁺ removal from wastewater by brown seaweed L. nigrescens particles and seaweed residue. It was found that the seaweed residue, after extraction of alginates, and the L. nigrescens particles exhibited effective removal of copper from aqueous solutions. The L. nigrescens particles and seaweed residue can achieve adsorption capacities of 12.15 mg/g and 9.09 mg/g, respectively, within 10 min. The study successfully validated that the adsorption of L. nigrescens particles and seaweed residue follows the mechanism of monolayer adsorption and chemical adsorption process by fitting the experimental data to a pseudo-second-order kinetic model and Langmuir isotherm. Although slightly lower removal efficiency and removal capacity have been observed for seaweed residue, there is still a potential for it to be used as a bioadsorbent because of its considerable Cu²⁺ adsorption capacity. Overall, brown seaweed residue is worth further investigation as an environmentally friendly heavy metal adsorbent on a large scale.