Single and Binary Equilibrium Studies for Ni²⁺ and Zn²⁺ Biosorption onto Lemna gibba from Aqueous Solutions

Abstract

The biosorption ability of Lemna gibba for removing Ni²⁺ and Zn²⁺ ions in aqueous batch systems, both individually and simultaneously, was examined. The influences of solution pH and initial single and binary metal concentrations on equilibrium Ni²⁺ and Zn²⁺ biosorption was explored. The optimal solution pH for Ni²⁺ and Zn²⁺ biosorption was 6.0, for both the single and binary metal systems. Ni²⁺ and Zn²⁺ biosorption capacities increased with increasing initial metal concentrations. The presence of Zn²⁺ ions more adversely affected the biosorption of Ni²⁺ ions in the binary metal systems than vice versa. The single and binary biosorption isotherms of Ni²⁺ and Zn²⁺ revealed that L. gibba’s affinity for Zn²⁺ ions was higher than that for Ni²⁺ ions. The Redlich–Peterson and Freundlich isotherm models fit well to the experimental equilibrium data of Ni²⁺ ions, whereas Redlich–Peterson and Langmuir models better described the equilibrium data of Zn²⁺ ions in single metal systems. The modified Sips isotherm model best fit the competitive biosorption data of Ni²⁺-Zn²⁺ on L. gibba. FTIR analyses suggest the involvement of hemicellulose and cellulose in the biosorption of Ni²⁺ and Zn²⁺. The presence of Ni²⁺ and Zn²⁺ on the L. gibba surface was validated by SEM-EDX.

1. Introduction

Soil, surface water, and groundwater pollution by divalent nickel (Ni²⁺) and zinc (Zn²⁺) ions can occur by natural processes, but is predominantly due to anthropogenic activities [1], especially the Ni²⁺- and Zn²⁺-contaminated discharges from industrial and commercial processes, including mining; mineral processing; electroplating; production of paints, batteries, coins, and electronic commodities; and many other applications [2,3].

Ni²⁺ and Zn²⁺ are essential minerals for some living beings [4]. However, at high concentrations, they are of serious environmental, public health, human well-being, and economic concern due to their toxic effects on cell physiology and metabolism, which can decrease the viability of living organisms or even cause their death [5]. Human exposure to high Ni²⁺ and Zn²⁺ levels has been associated with serious acute and chronic health effects. Ni²⁺ induces hematotoxic, inmunotoxic, hepatotoxic, pulmonary toxic, nephrotoxic, and embryotoxic effects, allergic reactions, and contact dermatitis. In addition, Ni²⁺ impairs the natural balance of essential minerals and is a carcinogenic and teratogenic agent [6,7]. Zn²⁺ may cause nausea, anemia, vomiting, diarrhea, depression, and lethargy, and circulatory, liver, renal, and neurological disorders [4,8]. Furthermore, it has been reported that the toxicity of binary mixtures of Ni²⁺ and Zn²⁺ is higher than expected from that of individual heavy metals in the mixture. Ni²⁺ and Zn²⁺ exert additive or synergistic negative effects on physiological and biochemical processes of organisms [1,9,10]. Therefore, it is crucial to effectively remove Ni²⁺ and Zn²⁺ ions from industrial and domestic wastewater in order to guarantee that the quality of the treated wastewater meets the recommended standards for its intended use, and to protect aquatic and terrestrial environments and species, human health, welfare, and safety.

Biosorption technology offers potential advantages over traditional methods for the bioremediation of heavy metal-contaminated industrial and domestic wastewater, including an enhanced efficiency for removing heavy metals from diluted or concentrated aqueous solutions, faster removal rates, low operating costs and capital investment, high flexibility, lack of secondary pollution, ease of operation, simplicity of design, and the possibility of recovering the heavy metals and regenerating the biosorbents [11,12]. The competitive advantage of this technology increases when cheap, green, abundant, and widely available biosorbents are used [4].

However, most of the heavy metal biosorption studies have been performed in single metal component systems, and comparatively fewer studies have been conducted in multi-metal component systems. However, wastewater produced by many anthropogenic activities contains several heavy metal ions and other chemical species that may hinder the biosorption process [13,14]. Therefore, studies on the biosorption of heavy metals in multi-element systems are necessary and crucial to understand real systems because these systems resemble more closely the chemical compositions and physicochemical characteristics of industrial effluents than the single-metal systems [13].

Lemna gibba is a free-floating aquatic angiosperm plant. It is a member of the Araceae family and the Lemnoideae subfamily, also known as duckweed. L. gibba grows in tropical and temperate zones throughout a wide range of pH, temperature, and nutrient levels [9]. L. gibba is found abundantly throughout the year in many freshwater systems, such as lakes, lagoons, ponds, stream pools, slow flowing rivers, and stagnant waters [15,16,17]. Duckweeds are considered troublesome because they reproduce rapidly, thereby quickly covering the surfaces of water bodies with a dense layer of small plants. This prevents sunlight penetration and oxygen transfer, affecting the photosynthetic and respiratory activities of aquatic organisms [18]. Nevertheless, L. gibba is one of the most productive plants on Earth [15,19] and therefore represents a valuable biomass resource for a variety of useful practical applications.

A promising alternative use for this readily available biomass of L. gibba is as a biosorbent for removing toxic, inorganic, and organic contaminants from water and wastewaters. This is because the cell wall of Lemnaceae has carbohydrates (cellulose, hemicellulose, etc.), proteins, lipids, lignin, etc., which contain a variety of chemical functional groups, such as carboxyl, hydroxyl, amino, phosphate, sulfate, etc., which can act as binding sites for the biosorption of organic and/or inorganic contaminants [15,20]. Furthermore, L. gibba can be used effectively in several biosorption-desorption cycles of contaminants, thereby decreasing process treatment costs [21]. Additionally, L. gibba requires minimal processing (washing, drying, grinding), thereby reducing treatment costs. Likewise, L. gibba has no economic value; thus, the costs and risks involved in treatment are vastly reduced.

Nevertheless, there is scant information regarding heavy metal biosorption from aqueous solutions by L. gibba [15]. Moreover, to the best knowledge of the authors, no information exists regarding the biosorptive removal of Ni²⁺ and Zn²⁺ ions from aqueous solutions in single and binary metal systems by L. gibba.

The present work aimed to assess the potential performance of L. gibba for the individual and simultaneous biosorption of Ni²⁺ and Zn²⁺ ions, and to model the single and binary equilibrium biosorption of Ni²⁺ and Zn²⁺ of the duckweed. Furthermore, scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) and Fourier transform infrared spectroscopy (FTIR) analyses were conducted to confirm the biosorption of Ni²⁺ and Zn²⁺ ions onto L. gibba.

2. Materials and Methods

2.1. Ni²⁺ and Zn²⁺ Stock and Test Solutions

Some 40 mM Ni²⁺ and Zn²⁺ stock solutions were produced by accurately weighing the required amount of NiSO₄·6H₂O (J.T. Baker^®, Monterrey, Mexico; purity >99.1%) or ZnSO₄·7H₂O (J.T. Baker^®, Monterrey, Mexico; purity >99.9%) and dissolving it in distilled deionized water. For the single and binary metal biosorption experiments, different initial concentrations of each heavy metal ion were obtained by diluting the stock Ni²⁺ and Zn²⁺ solutions.

2.2. Preparation of Biosorbent

L. gibba was harvested in Xochimilco Lake, Mexico City, Mexico. The macrophyte was exhaustively washed with tap and distilled deionized water to completely remove aquatic plants, other than L. gibba, dead insects, garbage, and debris. The duckweed samples were then dried at 60 ℃ until reaching a dry weight constant; thereafter, the macrophyte was ground using a hammer mill, and the obtained powder was sifted through U.S. ASTM sieves to collect the fraction with particle sizes ranging from 0.3 to 0.5 mm, which was employed further in biosorption experiments.

2.3. Proximate Chemical Analysis of Biosorbent

Proximate chemical analysis of L. gibba samples was performed in triplicate, following the procedures described in the AOAC handbook [22]. We applied the Soxhlet method to quantify ether extract (EE). Total protein (TP) was determined using the Kjeldahl method, with 6.25 as the conversion factor from total nitrogen to total protein. Crude fiber (CF) was analyzed in terms of the loss in ignition of dried lipid-free residues, following digestion with H₂SO₄ and NaOH standard solutions. Ash (A) was verified by burning samples in a muffle furnace at 600 ℃, until the weight was constant. Nitrogen-free extract (NFE) was quantified as the difference between dry matter and the sum of ash, total protein, ether extract, and crude fiber (Equation (1)):

$$\%NFE=100-\%A-\%TP-\%EE-\%CF$$

(1)

2.4. Biosorption Studies

Biosorption tests were conducted to examine the effects of metal solution pH and the initial concentrations of Ni²⁺ and Zn²⁺ ions on equilibrium metal biosorption by L. gibba. Previous kinetic studies showed that equilibrium times for Ni²⁺ and Zn²⁺ biosorption were less than 48 h (data not shown). Thus, contact time between L. gibba and Ni²⁺ or Zn²⁺, or mixtures of Ni²⁺ and Zn²⁺ solutions, of 48 h was used in equilibrium biosorption studies.

All single and binary equilibrium biosorption tests were conducted in Erlenmeyer flasks (500 mL) containing 1 g (dry weight) L⁻¹ of L. gibba biomass and 100 mL of metal solution, with a constant agitation (Cole Parmer^®, Vernon Hills, IL, USA) of 180 rpm and a temperature of 20 ℃.

In the single metal biosorption systems, the effects of the solution pH level on L. gibba Ni²⁺ and Zn²⁺ biosorption were explored in metal solutions at 2 mM initial Ni²⁺ or Zn²⁺ concentration, with solution pH values ranging from 1.0 to 6.0. Similar experiments covering the same range of solution pH values were performed to explore the influence of metal solution pH levels on the simultaneous biosorption of Ni²⁺ and Zn²⁺ ions onto L. gibba, using an equimolar initial concentration of 2 mM of both heavy metals. Metal solution pH values higher than 6.0 were not assayed to prevent the precipitation of insoluble Zn(OH)₂ in the single and binary metal biosorption experiments [23]; 0.1 M NaOH or HCl solutions were used to adjust the pH of each monometal and bimetal solution to the desired value.

To investigate the influences of initial Ni²⁺ or Zn²⁺ concentration on their removal by L. gibba in single-metal systems, the initial concentrations of Ni²⁺ or Zn²⁺ ions were varied over the 0.2–30 mM range (0.2, 0.4, 1, 2, 4, 6, 10, 20, and 30 mM). For the two-metal systems, a series of tests were conducted with a constant concentration of the first metal within the interval from 0.2 to 10 mM (0.2, 0.4, 1, 2, 4, 6, and 10 mM), while varying the concentration of the second metal (0.2, 0.4, 1, 2, 4, 6, and 10 mM). A total of 49 different bimetallic systems were assayed combining the two heavy metals studied. In the two-metal systems, no experiments were performed at initial concentrations of each heavy metal greater than 10 mM due to interferences in the quantification of the two metals.

The flasks were taken out from the shaker bath (Cole Parmer^®, Vernon Hills, IL, USA) at the end of the batch biosorption experiments, and the biosorbent was separated from the aqueous solutions by filtration (Whatman^® grade 42, St. Louis, MI, USA). Filtrates were further used to quantify residual Ni²⁺ and/or Zn²⁺ concentrations.

Control experiments without L. gibba biomass were carried out simultaneously using the same conditions as those employed for the Ni²⁺ and/or Zn²⁺ biosorption tests to determine the loss of heavy metal ions by precipitation and/or adsorption onto the glass wall. No statistically significant differences were encountered in the Ni²⁺ and/or Zn²⁺ concentrations of the controls without L. gibba biomass, thereby corroborating that the removal of Ni²⁺ and/or Zn²⁺ ions in the experiments using L. gibba was caused solely by the macrophyte biosorbent.

The equilibrium Ni²⁺ and Zn²⁺ biosorption capacities (q_e, mmol g⁻¹) were quantified according to the following equation (Equation (2)):

$$q_e=\frac{C_0-C_e}{M}$$

(2)

where C₀ and C_e are the initial and equilibrium Ni²⁺ or Zn²⁺ concentrations of the liquid phase (mmol L⁻¹), respectively; and M is the L. gibba concentration (g L⁻¹).

The quantitative simultaneous biosorption effect of both the heavy metals on L. gibba was evaluated according to global index of behavior (ξ), and ratio of biosorption capacities (R_q), which were estimated as follows:

$${\xi }_{Ni}=100\frac{{\left({{\displaystyle \int }}_{C_{0Ni}=0}^{C_{0Ni}=C_{0Ni}}{q}_{eNi}dCoNi\right)}_b-{\left({{\displaystyle \int }}_{C_{0Ni}=0}^{C_{0Ni}=C_{0Ni}}{q}_{eNi}dCoNi\right)}_m}{{\left({{\displaystyle \int }}_{C_{0Ni}=0}^{C_{0Ni}=C_{0Ni}}{q}_{eNi}dCoNi\right)}_m}$$

(3)

$${\xi }_{Zn}=100\frac{{\left({{\displaystyle \int }}_{C_{0Zn}=0}^{C_{0Zn}=C_{0}Zn}{q}_{eZn}dCoZn\right)}_b-{\left({{\displaystyle \int }}_{C_{0Zn}=0}^{C_{0Zn}=C_{0Zn}}{q}_{eZn}dCoZn\right)}_m}{{\left({{\displaystyle \int }}_{C_{0Zn}=0}^{C_{0}Zn=C_{0Zn}}{q}_{eZn}dCoZn\right)}_m}$$

(4)

$$R_{qNi}=\frac{q_{eNi,b}}{q_{eNi,m}}$$

(5)

$$R_{qZn}=\frac{q_{eZn,b}}{q_{eZn,m}}$$

(6)

where ξ_Ni and ξ_Zn are the global indexes of behavior for Ni²⁺ and Zn²⁺ biosorption at different initial concentrations of Zn²⁺ and Ni²⁺, respectively; (q_eNidC₀Ni)_m and (q_eNidC₀Ni)_b are the variations of the equilibrium biosorption capacity for Ni²⁺ over initial Ni²⁺ concentration in single and binary metal experiments, respectively; (q_eZndC₀Zn)_m and (q_eZndC₀Zn)_b are the variations of the equilibrium Zn²⁺ biosorption capacity over initial Zn²⁺ concentration in single and binary metal experiments, respectively; q_eNi,m and q_eZn,m are the equilibrium Ni²⁺ and Zn²⁺ biosorption capacities in the single metal systems respectively; and q_eNi,b and q_eZn,b are the equilibrium Ni²⁺ and Zn²⁺ biosorption capacities in the binary metal systems, respectively.

The experimental curves represented by (q_eNidC₀Ni)_m, (q_eNidC₀Ni)_b, (q_eZndC₀Zn)_m, and (q_eZndC₀Zn)_b were integrated using scientific and statistical software (GraphPad Prism 8.4, GraphPad Software, San Diego, CA, USA, 2020). ξ = 0 indicates that the biosorption of one heavy metal is not affected by the other heavy metal present in the binary metal solution; if ξ > 0, that signifies an improvement in the biosorption of a heavy metal in the bimetal system as compared to the monometal system (i.e., positive or synergic effect); and if ξ < 0, that denotes a diminution in the biosorption of a heavy metal in the bimetal system compared to the monometal system (i.e., negative or antagonist effect). Furthermore, R_q = 1.0 indicates that the biosorption of each heavy metal is independent (i.e., non-interaction between Ni²⁺ and Zn²⁺ ions); if R_q > 1.0, that indicates that the biosorption of one heavy metal is enhanced by the presence of the other heavy metal (i.e., synergic effect); and if R_q < 1.0, it denotes that the biosorption of one heavy metal is suppressed by the other heavy metal present in the metal mixture (i.e., antagonist effect).

2.5. Modeling of Single and Binary Equilibrium Biosorption Isotherms of Ni²⁺ and Zn²⁺ Ions

Modeling experimental equilibrium biosorption isotherm data is an essential tool for trustworthy derivation of design parameters, the comparison of quantitative biosorption performances of distinct biosorption systems, and for diverse conditions in each biosorption system. These equilibrium biosorption isotherm parameters frequently provide some understanding of the affinity and surface characteristics of the biosorbent, in addition to the interactions between the biosorbent and adsorbate, and are consequently required to understand the biosorption mechanism [24]. Therefore, it is critical to establish the most suitable biosorption isotherm model that can be used for optimizing the operation conditions and designing of future large-scale biosorption facilities [24].

2.5.1. Mono-Component Isotherm Models

Equilibrium data for biosorption of single components can be commonly interpreted by the Freundlich, Langmuir, Redlich–Peterson, and Sips isotherms.

Freundlich Isotherm Model

The Freundlich model describes non-ideal, multi-layer biosorption onto heterogeneous surfaces, indicating that biosorption sites are neither independent nor equivalent [25,26]. The Freundlich isotherm model is provided below (Equation (7)):

$$q_e=K_F{C}_F{}^{\frac{1}{n_F}}$$

(7)

where the Freundlich parameters 1/n_F and K_F are relative indicators of intensity and capacity of biosorption, respectively [25].

Langmuir Isotherm Model

This isotherm model has the following assumptions: (1) each biosorption site can adsorb only one molecule of adsorbate, and consequently, only one layer of biosorbed molecules can be formed on the biosorbent surface; (2) there are no interactions between biosorbed species; (3) all the biosorption sites are identical; (4) the capability of a molecule to be biosorbed onto a given biosorption site is not dependent on the occupancy of its neighboring sites; and (5) the biosorptive forces are similar to those found in chemical interactions [26,27]. The Langmuir equation can be expressed as follows (Equation (8)):

$$q_e=\frac{q_{max}{B}_L{C}_e}{1+B_L{C}_e}$$

(8)

where q_max is the theoretical maximum biosorption capacity of the biosorbent; q_e is the biosorption capacity at the equilibrium; C_e is the equilibrium adsorbate concentration in the liquid phase; and B_L is the Langmuir constant that represents the reciprocal of the adsorbate concentration at which half saturation of the biosorbent is reached [25].

Sips Isotherm Model

The Sips equation is a hybrid model of the Freundlich and Langmuir isotherms, used extensively to interpret equilibrium biosorption data. It has the following form (Equation (9)) [28]:

$$q_e=\frac{q_{max}{B}_S{C}_e{}^{n_S}}{1+(B_S{C}_e{}^{n_S})}$$

(9)

where q_max is the maximum biosorption capacity; B_s is the Sips constant; and n_s is the Sips parameter that characterizes the system’s heterogeneity.

Redlich–Peterson Isotherm Model

The Redlich–Peterson isotherm equation is shown in Equation (10) [27]:

$$q_e=\frac{K_{RP}{C}_e}{1+(A_{RP}{C}_e{}^{b_{RP}})}, 0\le b_{RP}\le 1$$

(10)

where K_RP, A_RP, and b_RP are the model parameters. This model reduces to the Langmuir isotherm when b_RP = 1, to the Freundlich isotherm at high concentration of the adsorbate, and to a linear isotherm at low surface coverage.

2.5.2. Multi-Component Isotherm Models

In the case of binary metal or multi-metal systems, as is representative of industrial wastewaters, there may be interference and competition between different components for biosorption binding sites, resulting in a more complex mathematical description of the dynamic equilibrium state [29]. Therefore, the isotherm models proposed for single-component biosorption equilibrium have been modified and extended to represent the binary component and multi-component biosorption equilibria over a wide range of solution concentrations [30]. However, despite its great significance for the design of treatment systems, the equilibrium modeling of binary component and multi-component biosorption has largely been neglected [29].

Seven different multi-component isotherm models were applied to describe the simultaneous biosorption of Ni²⁺ and Zn²⁺ ions onto L. gibba, namely, non-modified competitive Langmuir, modified competitive Langmuir, modified Langmuir with an interaction factor, extended Freundlich, non-modified competitive Redlich–Peterson, modified Redlich–Peterson with interaction factor, and modified Sips isotherm models.

Non-Modified Competitive Langmuir Isotherm Model

This model has the same assumptions as the monocomponent Langmuir isotherm equation and can be expressed for components Ni²⁺ and Zn²⁺ in a mixture of both heavy metals as follows [31]:

$$q_{eNi}=\frac{q_{maxNi}{B}_{Ni}{C}_{eNi}}{1+B_{Ni}{C}_{eNi}+B_{Zn}{C}_{eZn}}$$

(11)

$$q_{eZn}=\frac{q_{maxZn}{B}_{Zn}{C}_{eZn}}{1+B_{Ni}{C}_{eNi}+B_{Zn}{C}_{eZn}}$$

(12)

where q_eNi and q_eZn are the equilibrium biosorption capacities of Ni²⁺ and Zn²⁺ in the bimetal system; C_eNi and C_eZn are the equilibrium Ni²⁺ and Zn²⁺ concentrations in binary metal solution; B_Ni and B_Zn are the constants of the Langmuir model for Ni²⁺ and Zn²⁺ biosorption in monometal systems; and q_maxNi and q_maxZn are the theoretical maximum biosorption capacities of Ni²⁺ and Zn²⁺ in the single metal systems, respectively [32].

Modified Competitive Langmuir Isotherm

Assuming the active biosorption sites are distributed uniformly on the biosorbent surface, and all the adsorbates in the liquid phase compete for the same biosorption sites of the biosorbent, the Langmuir model for single-component systems was extended for multicomponent systems as follows [30]:

$$q_{eNi}=\frac{q_{maxNi}{}^{\prime }{B}_{Ni}{}^{\prime } C_{eNi}}{1+B_{Ni}{}^{\prime }{C}_{eNi}+B_{Zn}{}^{\prime }{C}_{eZn}}$$

(13)

$$q_{eZn}=\frac{q_{maxZn}{}^{\prime }{B}_{Zn}{}^{\prime }{C}_{eZn}}{1+B_{Ni}{}^{\prime }{C}_{eNi}+B_{Zn}{}^{\prime }{C}_{eZn}}$$

(14)

The parameters q_maxNi′, q_maxZn′, B_Ni′, and B_Zn′ can be obtained from the fitting of Equations (13) and (14) with the experimental equilibrium biosorption data of Ni²⁺ and Zn²⁺ in the binary metal system [30].

Modified Langmuir Isotherm with an Interaction Factor η

In multicomponent biosorption systems, the biosorbent affinity for the adsorbates changes due to biosorbent-adsorbate interactions and the competition of the heavy metal ions species for biosorption binding sites. Considering these effects, an interaction term (η) for each adsorbate has been introduced into Equations (11) and (12) to yield Equations (15) and (16) [31].

$$q_{eNi}=\frac{q_{maxNi}{B}_{Ni}\frac{C_{eNi}}{{\eta }_{Ni}}}{1+B_{Ni}\frac{C_{eNi}}{{\eta }_{Ni}}+B_{Zn}\frac{C_{eZn}}{{\eta }_{Zn}}}$$

(15)

$$q_{eZn}=\frac{q_{maxZn}{B}_{Zn}\frac{C_{eZn}}{{\eta }_{Zn}}}{1+B_{Ni}\frac{C_{eNi}}{{\eta }_{Ni}}+B_{Zn}\frac{C_{eZn}}{{\eta }_{Zn}}}$$

(16)

where η_Ni and η_Zn are characteristic of Ni²⁺ and Zn²⁺ ions and dependent on the concentrations of the other components in the solution [30]. These characteristic parameters can be obtained from the experimental competitive biosorption data of Ni²⁺ and Zn²⁺ ions.

Extended Freundlich Isotherm

The single-component Freundlich equation has been extended to represent the binary component biosorption system, and is expressed according to Equations (17) and (18) for each heavy metal of the binary mixture of adsorbates [30]:

$$q_{eNi}=\frac{K_{FNi}{C}_{eNi}{}^{\frac{1}{n_{FNi}}+x_{Ni}}}{C_{eNi}{}^{x_{Ni}}+y_{Ni}{C}_{eZn}{}^{z_{Ni}}}$$

(17)

$$q_{eZn}=\frac{K_{FZn}{C}_{NZn}{}^{\frac{1}{n_{FZn}}+x_{Zn}}}{C_{eZn}{}^{x_{Zn}}+y_{Zn}{C}_{eNi}{}^{z_{Zn}}}$$

(18)

where K_FNi, K_FZn, n_FNi, and n_FZn are Freundlich parameters that are obtained from the corresponding single-component Freundlich isotherm models; whereas x_Ni, y_Ni, z_Ni, and x_Zn, y_Zn, and z_Zn are the competitive Freundlich biosorption parameters for Ni²⁺ and Zn²⁺ ions, respectively.

Modified Sips Isotherm

The single-component Sips isotherm model can be extended to describe the biosorption in binary-component systems [32]:

$$q_{eNi}=\frac{q_{maxNi}{B}_{sNi}{C}_{eNi}{}^{n_{sNi}}}{1+B_{sNi}{C}_{eNi}{}^{n_{sNi}}+B_{sZn}{C}_{eZn}{}^{n_{sZn}}}$$

(19)

$$q_{eZn}=\frac{q_{maxZn}{B}_{sZn}{C}_{eZn}{}^{n_{sZn}}}{1+B_{sNi}{C}_{eNi}{}^{n_{sNi}}+B_{sZn}{C}_{eZn}{}^{n_{sZn}}}$$

(20)

where the modified Sips model parameters q_maxNi, q_maxZn, B_sNi, B_sZn, n_sNi, and n_sZn are obtained from binary component biosorption data [32].

Non-Modified Competitive Redlich-Peterson Isotherm

The competitive Redlich–Peterson isotherm models for binary component biosorption systems are given in Equations (21) and (22):

$$q_{eNi}=\frac{K_{RPNi}{C}_{eNi}}{1+A_{RPNi}{C}_{eNi}{}^{b_{RPNi}}+A_{RPZn}{C}_{eZn}{}^{b_{RPZn}}}$$

(21)

$$q_{eZn}=\frac{K_{RPZn}{C}_{eZn}}{1+A_{RPNi}{C}_{eNi}{}^{b_{RPNi}}+A_{RPZn}{C}_{eZn}{}^{b_{RPZn}}}$$

(22)

where K_RPNi, K_RPZn, A_RPNi, A_RPZn, b_RPNi, and b_RPZn are the single-component Redlich–Peterson model parameters for the Ni²⁺ and Zn²⁺ ions components [33].

Modified Redlich-Peterson Isotherm with Interaction Factor η

The competitive Redlich–Peterson isotherm model can be extended with an interaction factor η, in a manner analogous to that of the modified Langmuir isotherm model, and can be expressed for components Ni²⁺ and Zn²⁺ in a bimetal system [34] as follows:

$$q_{eNi}=\frac{K_{RPNi}\frac{C_{eNi}}{{\eta }_{Ni}}}{1+A_{RPNi}{\left(\frac{C_{eNi}}{{\eta }_{Ni}}\right)}^{b_{RPNi}}+A_{RPZn}{\left(\frac{C_{eZn}}{{\eta }_{Zn}}\right)}^{b_{RPZn}}}$$

(23)

$$q_{eZn}=\frac{K_{RPZn}\frac{C_{eZn}}{{\eta }_{Zn}}}{1+A_{RPNi}{\left(\frac{C_{eNi}}{{\eta }_{Ni}}\right)}^{b_{RPNi}}+A_{RPZn}{\left(\frac{C_{eZn}}{{\eta }_{Zn}}\right)}^{b_{RPZn}}}$$

(24)

where η_Ni and η_Zn are the correction coefficients for Ni²⁺ and Zn²⁺ ions estimated from binary metal biosorption data; and K_RPNi, K_RPZn, A_RPNi, A_RPZn, b_RPNi, and b_RPZN are derived from the corresponding single-component biosorption data [34].

2.6. Determination of Isotherm Parameters and Statistical Analysis

Batch biosorption experiments were conducted in triplicate to confirm the reproducibility of the results and for statistical analysis. The data reported herein represent the mean values ± standard deviations. Biosorption data of Ni²⁺ and Zn²⁺ were analyzed by variance analysis and Tukey’s test (p < 0.05), using GraphPad Prism 8.4 (GraphPad Software, San Diego, CA, USA, 2020).

The isotherm parameters of all the single-metal and binary-metal models were obtained by nonlinear regression analysis using the software MATLAB^® (version 2020a; The Mathworks, Inc., Natick, MA, USA, 2020). The standard deviation of the residuals (RMSE and Sy.x), the sum of squares error (SSE), and the coefficient of determination (R²) were used to determine the adequacy and accuracy of the fit of the various isotherm models assayed with the experimental data. Small RMSE, Sy.x, SSE values, and a value of R² near 1.0 indicate a better description of experimental biosorption data by a particular model [28].

2.7. FTIR Analysis

FTIR spectroscopy was employed to gain information about the main chemical functional groups present on the L. gibba surface, and to detect possible modifications of active biosorption sites on the L. gibba surface during the biosorption of Ni²⁺, Zn²⁺, or the binary mixture of Ni²⁺ and Zn²⁺ ions.

L. gibba (1 g L⁻¹) samples were mixed with aqueous solutions of 10 mM Ni²⁺, 10 mM Zn²⁺, or with 10 mM solutions of equimolar mixtures of Ni²⁺ and Zn²⁺ ions at pH 6.0 ± 0.1 for 24 h, with constant agitation at 180 rpm and 20 ± 1 ℃ to saturate the active biosorption sites with Ni²⁺, Zn²⁺, or with both Ni²⁺ and Zn²⁺ ions. The resulting suspensions were subsequently centrifuged for 5 min at 5000 rpm to pellet the L. gibba biomass. The pellets collected were washed thoroughly with distilled deionized water to remove the unbound heavy metals. After centrifuging the resulting suspensions, the obtained heavy metal-loaded L. gibba biomass was dried at 105 ℃ to remove any retained water which could interfere with the detection of hydroxyl functional groups on the L. gibba surface [35].

Finely ground raw (i.e., not loaded with Ni²⁺ and/or Zn²⁺ ions) and heavy metal-loaded L. gibba samples were mixed with dried potassium bromide (Merck™, Germany; spectroscopic grade) in a ratio of 1:5 and examined by diffuse reflectance FTIR spectroscopy in a Perkin–Elmer FTIR spectrophotometer (Spectrum 2000, Perkin-Elmer, Inc., Waltham, MA, USA). FTIR spectra data were collected over the wavenumber range of 4000–400 cm⁻¹, at a resolution of 4 cm⁻¹, for a total of 16 scans.

2.8. SEM and SEM-EDX Analysis

The surface structures of L. gibba samples, prior to and following saturation with Ni²⁺ and/or Zn²⁺ ions, were studied using a scanning electron microscope (SEM; FEI Quanta Dual Beam Instrument, FEI Company, Hillsboro, OR, USA) with an accelerated voltage of 15 kV. The analysis of chemical elements was conducted by energy dispersive X-ray spectrometry (EDX) using an Apollo X detector (EDAX, AMETEK, Inc., Berwyn, PA, USA). For the analysis, samples remained uncoated at low vacuum conditions to improve electron conductivity and image quality.

2.9. Analytical Techniques

Ni²⁺ and Zn²⁺ concentrations in liquid solution were assessed colorimetrically using a UV-Vis Evolution 201 (Thermo Fisher Scientific, Whaltman, MA, USA) spectrophotometer, following the methodology outlined in the Hach Water Analysis Handbook [36]. Ni²⁺ and Zn²⁺ concentrations were determined from calibration curves established for each heavy metal with a minimum of seven concentration levels.

3. Results and Discussion

3.1. Proximate Composition of L. gibba

The analysis results of the proximate chemical composition of L. gibba are as follows (on a dry matter basis): total carbohydrates, 46.57 ± 1.5%; total proteins, 26.29 ± 0.51%; total ashes, 19.04 ± 0.41%; and total lipids, 8.10 ± 0.05%. It is apparent that L. gibba is rich in carbohydrates and proteins and has a low fat content. These findings concur with those reported for the Lemnaceae by Schmid and Landolt [37] and Landesman [38]. The polymeric compounds present in L. gibba biomass have many chemical functional groups that can be binding sites for the biosorptive removal of toxic metal ions from aqueous solutions.

3.2. Influences of Metal Solution pH on the Individual and Simultaneous Biosorption of Ni²⁺ and Zn²⁺ onto L. gibba

The pH of a metal ion solution is among the crucial environmental factors that affect heavy metal biosorption from liquid solutions due to its ability to influence the chemical speciation, degree of ionization, and aqueous solubility of heavy metals. Metal ion solution pH also influences competition with other heavy metal ions present in the liquid phase and the dissociation degree of chemical functional groups present on the surface of biosorbents [11,12].

To find the optimum value of solution pH for Ni²⁺ and Zn²⁺ biosorption onto L. gibba for the single metal and binary metal systems, biosorption tests with the solution pH varying from 1.0 to 6.0 were performed. The influences of the pH of the aqueous metal solution on the equilibrium Ni²⁺ and Zn²⁺ biosorption capacities for the single and binary heavy metal systems are illustrated in Figure 1.

It is apparent that the equilibrium biosorption capacities of Ni²⁺ and Zn²⁺ ions increased as the solution pH increased and reached its highest value at pH 6.0, both for the mono-metal and bi-metal systems. This biosorption behavior may be due to the fact that at low solution pH levels, the active binding sites available on the L. gibba surface are mostly protonated, resulting in electrostatic repulsion between the Ni²⁺ and Zn²⁺ cationic species and the positive electrical surface charges of L. gibba, and the competition between H⁺ ions and Ni²⁺ and Zn²⁺ ions for occupancy of the binding sites. Contrastingly, the extent of Ni²⁺ and Zn²⁺ biosorption increases with increasing solution pH presumably due to a decrease in competition between H⁺ ions and Ni²⁺ and Zn²⁺ ions for the biosorption binding sites, and also due to an increase in the number of negative electrical charges on the L. gibba surface, which results in a greater electrostatic attraction between the L. gibba surface and Ni²⁺ and Zn²⁺ ions [11].

Previous studies have reported an optimal solution pH of 6.0 for Ni²⁺ and Zn²⁺ biosorption in single metal systems [39,40,41]. However, to the best knowledge of the authors, no works have investigated the influence of pH of the metal solution on the simultaneous biosorption of Ni²⁺ and Zn²⁺ ions from an aqueous solution.

The capacities of Zn²⁺ biosorption were higher than those of Ni²⁺ at pH values greater than 3.0 and 4.0, for the single metal and binary metal systems, respectively. At solution pH values of 5.0 and 6.0, no statistically significant difference was observed in Zn²⁺ biosorption capacities between single and binary metal systems, which indicates that Zn²⁺ biosorption onto L. gibba was not significantly affected by the presence of Ni²⁺ ions at the studied conditions. Contrastingly, a noticeable decrease in biosorption capacity of Ni²⁺ was observed when Zn²⁺ ions were present in the metal solution with pH values between 3.0 and 6.0.

The results show that the solution pH of 6.0 is most suitable for the single and binary Ni²⁺ and Zn²⁺ biosorption onto L. gibba. Consequently, all subsequent experiments were conducted with a solution pH of 6.0.

3.3. Influence of Initial Metal Concentration in the Single and Binary Biosorption Systems of Ni²⁺ and Zn²⁺ onto L. gibba

The influences of initial concentrations of Ni²⁺ and Zn²⁺ on the L. gibba equilibrium biosorption capacities of the heavy metals in monometal and bimetal systems are displayed in Figure 2 and Figure 3, respectively. The capacity of metal biosorption of L. gibba increased as the initial concentration of Ni²⁺ or Zn²⁺ in single (Figure 2) and binary metal systems increased (Figure 3).

These results can be attributed to the fact that the greater the initial concentration of Ni²⁺ or Zn²⁺ ions, the greater the number of Ni²⁺ or Zn²⁺ ions in the aqueous metal solution, which results in a greater number of interactions between the metal ions and the L. gibba biomass. Additionally, a higher initial Ni²⁺ or Zn²⁺ concentration also causes metal concentration gradient increases, resulting in a faster transfer of Ni²⁺ or Zn²⁺ ions from the bulk of the liquid phase to the L. gibba surface due to an enhanced mass transfer coefficient, along with providing the driving force to overwhelm the mass transfer resistances of Ni²⁺ or Zn²⁺ ions between the liquid phase and the L. gibba biomass [11,12].

Figure 2 shows that L. gibba’s Zn²⁺ biosorption capacity was higher than Ni²⁺ biosorption capacity up to a 15 mM initial metal concentration in the single metal systems. At higher initial metal concentrations, the Ni²⁺ and Zn²⁺ biosorption capacities were similar. However, the initial slope of the curve that relates Zn²⁺ biosorption capacity as a function of initial Zn²⁺ concentration was higher than the initial slope of the curve relating biosorption capacity of Ni²⁺ ions to initial Ni²⁺ concentration (Figure 2), which indicates that L. gibba biomass has a greater affinity for Zn²⁺ ions than for Ni²⁺ ions.

Similarly, the results of simultaneous biosorption of Ni²⁺ and Zn²⁺ ions also show that the biosorption of a heavy metal was affected to a differing extent by the other heavy metal present in the metal solution, and that L. gibba biosorption capacity for Zn²⁺ ions was higher than that for Ni²⁺ ions (Figure 3).

Regardless of the initial concentration of Ni²⁺ assayed, the L. gibba Ni²⁺ biosorption capacity at equilibrium decreased with the increasing of initial Zn²⁺ concentration (Figure 3a). Similarly, the global behavior index for the biosorption of Ni²⁺ ions decreased from −2.1% to −60.3% as the initial concentration of Zn²⁺ ions increased from 0.2 to 10 mM (

Table 1. The global index of behavior for Ni²⁺ biosorption (ξ_Ni) at different initial Zn²⁺ concentrations.

Initial Zn2+ Concentration (mM)	ξNi (%)
0.2	−2.096
0.4	−8.474
1	−19.157
2	−30.772
4	−43.319
6	−52.371
10	−60.335

). This indicates that the greater the initial Zn²⁺ concentration, the higher the adverse effect on Ni²⁺ biosorption.

Furthermore,

Table 2. The ratio of Ni²⁺ biosorption capacity (R_qNi) at different initial Zn²⁺ concentrations.

C0Zn (mM)	C0Ni (mM)
	0.2	0.4	1	2	4	6	10
0.2	0.69	0.68	0.79	0.98	0.99	0.99	0.98
0.4	0.52	0.42	0.67	0.90	0.95	0.92	0.94
1	0.43	0.31	0.61	0.76	0.86	0.84	0.79
2	0.26	0.24	0.49	0.63	0.74	0.74	0.68
4	0.12	0.10	0.36	0.54	0.61	0.59	0.55
6	0.02	0.07	0.24	0.40	0.55	0.50	0.47
10	0.01	0.05	0.16	0.30	0.44	0.44	0.39

C_0Ni = Initial Ni²⁺ concentration; C_0Zn = Initial Zn²⁺ concentration.

displays that R_qNi values are far less than 1.0 at almost all initial Zn(II) concentrations, indicating that Zn²⁺ interfered with Ni²⁺ biosorption onto L. gibba. These results show that Zn²⁺ inhibited biosorption of Ni²⁺ ions.

Similarly, Figure 3b and global behavior index data (

Table 3. The global index of behavior for Zn²⁺ biosorption (ξ_Zn) at different initial Ni²⁺ concentrations.

Initial Ni2+ Concentration (mM)	ξZn (%)
0.2	−0.127
0.4	−0.374
1	−2.171
2	−2.507
4	−8.548
6	−12.079
10	−16.536

) of Zn²⁺ biosorption in the presence of Ni²⁺ ions revealed that the initial concentrations of Ni²⁺ in the range of 0.2 to 2.0 mM have a far lower adverse effect on Zn²⁺ biosorption onto L. gibba than initial Ni²⁺ concentrations higher than 2.0 mM.

Furthermore, the inhibitory influence of Ni²⁺ on Zn²⁺ biosorption was only evident when the initial concentration of Ni²⁺ was greater than the equimolar ratio (

Table 4. The ratio of Zn²⁺ biosorption capacity (R_qZn) at different initial Ni²⁺ concentrations.

C0Ni (mM)	C0Zn (mM)
	0.2	0.4	1	2	4	6	10
0.2	0.93	0.96	0.96	1.00	1.00	1.00	1.00
0.4	0.50	1.00	0.98	0.97	1.00	1.00	0.99
1	0.32	0.88	0.96	0.98	0.95	1.00	1.00
2	0.24	0.68	0.94	0.99	0.94	1.00	1.00
4	0.19	0.61	0.74	0.80	0.91	0.97	0.99
6	0.04	0.48	0.54	0.71	0.89	0.95	1.00
10	0.02	0.33	0.44	0.61	0.80	0.94	1.00

C_0Ni = Initial Ni²⁺ concentration; C_0Zn = initial Zn²⁺ concentration.

).

Hence, these results confirm that L. gibba biomass has a higher preference for Zn²⁺ ions than for Ni²⁺ ions, and that Zn²⁺ biosorption was almost free from interferences from Ni²⁺ ions at concentration ratios lower than or equal to the equimolar ratio, at the reported working conditions.

The higher biosorption preference of L. gibba for Zn²⁺ ions over Ni²⁺ ions could be attributed to the higher ionic radius (Zn, 74 pm; Ni, 69 pm) and atomic weight (Zn, 65.4; Ni, 58.7) of Zn²⁺ than of Ni²⁺. Generally, the higher the ionic size and atomic weight of a heavy metal, the higher the affinity of a biosorbent for the heavy metal [42,43]. Consequently, the preference of a biosorbent for biosorbing the heavy metal will be maximum [44]. This may be due to the fact that the motion of a heavy metal has a greater atomic mass generating a greater energy of momentum, which can facilitate the binding of the heavy metal to the biosorbent by increasing the probability of interaction between the binding sites of the biosorbent and the heavy metal, leading to a higher biosorption preference by a biosorbent [44].

A higher affinity for the biosorption of Zn²⁺ ions than for Ni²⁺ ions has been previously reported [45].

3.4. Isotherms and Their Modeling

3.4.1. Single-Metal Biosorption Systems

The equilibrium biosorption isotherm provides important information on the interactions between the biosorbent and the adsorbate (e.g., heavy metal) and is consequently needed to understand the mechanism of biosorption [25]. Figure 4 presents the experimental biosorption isotherm of Ni²⁺ (Figure 4a) and Zn²⁺ (Figure 4b) ions at pH 6.0 and at 20 ℃, for the single metal systems.

It is evident that the biosorption capacities at the equilibrium of Ni²⁺ and Zn²⁺ increased as the equilibrium concentrations of Ni²⁺ and Zn²⁺ ions in the solution increased. The initial slopes of the Ni²⁺ and Zn²⁺ biosorption isotherms steadily decrease with increasing equilibrium metal concentrations because metal ions have more difficulty finding vacant active biosorption sites with the progressive covering of the L. gibba surface. This dynamic indicates a progressive occupation of L. gibba’s active sites by Ni²⁺ or Zn²⁺ ions [11]. In addition, the initial slope of the Zn²⁺ isotherm was steeper than that of the Ni²⁺ isotherm, which is indicative of a higher affinity of L. gibba biomass for Zn²⁺ ions than for Ni²⁺ ions.

Furthermore, the analysis of biosorption equilibrium data is essential to establish the most suitable isotherm equation that characterizes precisely the process of biosorption, which can also be employed to optimize the design and operating conditions of a biosorption system [11,25].

Isotherm models with two (Freundlich and Langmuir) and three (Redlich–Peterson and Sips) parameters were assessed for their ability to fit the experimental data of Ni²⁺ and Zn²⁺ equilibrium biosorption onto L. gibba. The biosorption parameters for each metal ion obtained from the non-linear regression analysis of the isotherm models employed in this work are shown in

Table 5. Isotherm model parameters for Ni²⁺ and Zn²⁺ biosorption in single metal systems.

Model	Ni2+	Zn2+
Langmuir
qmax (mmol g−1)	0.982 ± 0.074	0.852 ± 0.022
BL (L mmol−1)	0.231 ± 0.053	0.875 ± 0.112
R2	0.965	0.9885
SSE	0.029	0.011
Sy.x	0.060	0.0352
RMSE	0.057	0.0333
Freundlich
KF (mmol g−1) (mmol L−1)−1/nF	0.253 ± 0.007	0.382 ± 0.028
nF	2.587 ± 0.069	3.705 ± 0.388
R2	0.997	0.961
SSE	0.002	0.037
Sy.x	0.017	0.064
RMSE	0.016	0.061
Sips
qmax (mmol g−1)	2.849 ± 0.744	0.974 ± 0.040
Bs (mmol L−1)-nS	0.095 ± 0.026	0.677 ± 0.061
nS	0.476 ± 0.03	0.710 ± 0.051
R2	0.999	0.997
SSE	0.001	0.003
Sy.x	0.011	0.019
RMSE	0.010	0.017
Redlich–Peterson
KRP (L g−1)	1.886 ± 0.523	1.234 ± 0.098
ARP (mmol L−1)-BRP	6.456 ± 2.038	2.028 ± 0.222
bRP	0.655 ± 0.012	0.885 ± 0.011
R2	0.999	0.999
SSE	0.001	0.001
Sy.x	0.010	0.011
RMSE	0.009	0.010

, together with the values of R², Sy.x, RMSE, and SSE. Likewise, Figure 4 displays the isotherm curves predicted by the four isotherm models.

The Freundlich, Redlich-Peterson, and Sips isotherm equations best described Ni²⁺ biosorption data, as evidenced by the lowest Sy.x, RMSE, and SSE and the highest R² values of these models. Nonetheless, the Sips model was not capable of predicting the experimental maximum Ni²⁺ biosorption capacity (q_max = 0.91 mmol g⁻¹). Therefore, the Sips model is unsuitable for interpreting the experimental equilibrium isotherm for the biosorption of Ni²⁺ ions onto L. gibba. The Freundlich model parameter n_F, which is related to the biosorption intensity, was found to be 2.59. This value is within the range of 1 to 10, which suggests that Ni²⁺ biosorption by L. gibba is favorable at the conditions being studied [11].

Furthermore, according to the error function (R², RMSE, Sy.x, and SSE) values, the Langmuir, Redlich–Peterson, and Sips isotherm equations better represented the experimental equilibrium data of Zn²⁺ biosorption onto L. gibba than the Freundlich model. However, the Sips model was unable to adequately predict the maximum equilibrium Zn²⁺ biosorption capacity obtained experimentally (0.88 mmol g⁻¹). Thus, the Sips model is not appropriate for describing the biosorption isotherm of Zn²⁺ ions onto L. gibba. Contrastingly, the Langmuir model predicted a maximum biosorption capacity of Zn²⁺ ions of 0.852 mmol g⁻¹, which closely matched the experimental value. Similarly, the exponent value of the Redlich–Peterson (b_RP = 0.885) model was close to 1.0. In this case, the Redlich–Peterson model is transformed to the Langmuir equation [28].

Considering these results, it is concluded that the Freundlich and Redlich–Peterson models provided the most suitable representations of the biosorption isotherm of Ni²⁺ ions by L. gibba, whereas the Langmuir and Redlich–Peterson models best described the equilibrium biosorption pattern for Zn²⁺ biosorption.

Furthermore, the values of the constants B_L of Langmuir equation and n_F of Freundlich equation for Zn²⁺ biosorption are higher than those for Ni²⁺ biosorption. These results confirm that L. gibba has a greater affinity for Zn²⁺ ions than for Ni²⁺ ions, as the higher the values of B_L and n_F, the higher the affinity between the adsorbate and biosorbent [30,40].

3.4.2. Binary-Metal Biosorption Systems

For the purpose of representing the equilibrium data of binary biosorption of Ni²⁺ and Zn²⁺ on L. gibba in a suitable graphical method, three-dimensional biosorption surfaces were built, plotting the capacity of Ni²⁺ or Zn²⁺ biosorption at equilibrium (z-axis) versus the Ni²⁺ and Zn²⁺ concentrations at equilibrium (x- and y-axis). Figure 5 displays the three-dimensional biosorption isotherm surfaces of the Ni²⁺-Zn²⁺ bimetal system.

3D plots confirmed that biosorption of Ni²⁺ ions was negatively influenced to a great extent by Zn²⁺ ions present in aqueous solution (Figure 5a), whereas the presence of Ni²⁺ in the liquid phase had a less pronounced influence on Zn²⁺ biosorption (Figure 5b). This difference can be attributed to the competition between Ni²⁺ and Zn²⁺ ions for the same active biosorption sites on the L. gibba biomass surface and to the higher affinity of L. gibba for Zn²⁺ than for Ni²⁺. Moreover, these results clearly show that an antagonistic competitive effect occurs in the biosorption of each single heavy metal ion (Figure 5a,b), particularly at the high concentration range.

Table 6. Isotherm model parameters for Ni²⁺ and Zn²⁺ biosorption in binary metal systems.

	Ni2+	Zn2+		Ni2+	Zn2+
Non-modified competitive Langmuir			Modified Sips
qmaxNi	0.982 ± 0.074		qmax Ni	0.750 ± 0.048
qmaxZn		0.852 ± 0.022	BS Ni	0.488 ± 0.063	0.306 ± 0.115
BLNi	0.231 ± 0.053	0.231 ± 0.053	nS Ni	0.903 ± 0.048	0.970 ± 0.165
BLZn	0.875 ± 0.112	0.875 ± 0.112	qmax Zn		0.764 ± 0.018
R2	0.935	0.985	BS Zn	0.941 ± 0.151	1.202 ± 0.123
SSE	0.122	0.067	nS Zn	0.785 ± 0.050	1.195 ± 0.059
Sy.x	0.045	0.033	R2	0.988	0.989
RMSE	0.044	0.032	SSE	0.022	0.050
			Sy.x	0.019	0.029
Modified Langmuir with interaction factor η			RMSE	0.018	0.028
qmaxNi	0.982 ± 0.074
qmaxZn		0.852 ± 0.022	Non-modified competitive Redlich-Peterson
BLNi	0.231 ± 0.053	0.231 ± 0.053	KRP Ni	1.886 ± 0.523
BLZn	0.875 ± 0.112	0.875 ± 0.112	ARP Ni	6.456 ± 2.038	6.456 ± 2.038
η Ni	1.076 ± 0.049	1.237 ± 0.161	bRP Ni	0.655 ± 0.012	0.655 ± 0.012
η Zn	1.809 ± 0.206	1.041 ± 0.046	KRP Zn		1.234 ± 0.098
R2	0.960	0.986	ARP Zn	2.028 ± 0.222	2.028 ± 0.222
SSE	0.075	0.063	bRP Zn	0.885 ± 0.011	0.885 ± 0.011
Sy.x	0.035	0.032	R2	0.903	0.440
RMSE	0.034	0.031	SSE	0.182	2.526
			Sy.x	0.055	0.207
Modified competitive Langmuir			RMSE	0.054	0.200
qmax Ni′	0.675 ± 0.02
qmax Zn′		0.832 ± 0.015	Modified Redlich–Peterson with interaction
BNi′	0.523 ± 0.045	0.194 ± 0.026	factor η
BZn′	0.74 ± 0.066	0.908 ± 0.063	KRP Ni	1.886 ± 0.523
R2	0.985	0.986	ARP Ni	6.456 ± 2.038	6.456 ± 2.038
SSE	0.028	0.061	bRP Ni	0.655 ± 0.012	0.655 ± 0.012
Sy.x	0.021	0.032	KRP Zn		1.234 ± 0.098
RMSE	0.021	0.031	ARP Zn	2.028 ± 0.222	2.028 ± 0.222
			bRP Zn	0.885 ± 0.011	0.885 ± 0.011
Extended Freundlich			η Ni	1.019 ± 0.04	28.292 ± 6.56
KF Ni	0.253 ± 0.007		η Zn	0.382 ± 0.036	0.868 ± 0.054
nF Ni	2.587 ± 0.069		R2	0.984	0.983
KF Zn		0.382 ± 0.028	SSE	0.031	0.077
nF Zn		3.705 ± 0.388	Sy.x	0.022	0.035
xNi	0.687 ± 0.067		RMSE	0.022	0.034
yNi	0.759 ± 0.075
zNi	0.823 ± 0.055
xZn		2.142 ± 0.285
yZn		0.074 ± 0.027
zZn		1.249 ± 0.211
R2	0.980	0.978
SSE	0.037	0.095
Sy.x	0.025	0.040
RMSE	0.024	0.039

reports the calculated parameter values of the different multicomponent isotherm models assayed in this work for the binary biosorption of Ni²⁺ and Zn²⁺ ions onto L. gibba, together with the corresponding error functions (i.e., R², Sy.x, RMSE, and SSE values).

The comparative analysis of the error function values indicates that several of the applied models can be used for the estimation of the biosorption behavior of Ni²⁺-Zn²⁺ binary mixtures on L. gibba. However, the modified Sips model gives the best fitting for this binary metal system. The agreement between experimental results and those calculated by the modified Sips model is shown in Figure 5 (points and mesh surfaces, respectively). The modified Sips model predicted maximum capacities for Ni²⁺ and Zn²⁺ biosorption of 0.75 and 0.764 mmol g⁻¹, respectively, for the binary metal system.

3.5. FTIR Analysis

FTIR spectroscopy analyses were performed on raw (i.e., not loaded with Ni²⁺ or Zn²⁺ ions), Ni²⁺-loaded, Zn²⁺-loaded, and (Ni²⁺ + Zn²⁺)-loaded L. gibba samples to determine the chemical functional groups present on the L. gibba surface that might play a key role in the biosorptive removal of Ni²⁺ and Zn²⁺ ions from the liquid phases. To achieve this purpose, the FTIR spectrum of raw biosorbent was compared with that of Ni²⁺-loaded, Zn²⁺-loaded, and (Ni²⁺ + Zn²⁺)-loaded L. gibba (Figure 6).

The abundant infrared absorption bands within the range of 4000–400 cm⁻¹ in the FTIR spectrum of raw L. gibba is indicative of the complex chemical composition of its surface. The broad FTIR band at 3800–2800 cm⁻¹, with a minimum at 3294 cm⁻¹, corresponds to the stretching vibration of hydrogen-bonded –OH and –NH functional groups. This infrared absorption band is associated with the vibrations of the linked -OH groups in lignin, cellulose, and hemicellulose, which are present in Lemnaceae, and water adsorbed on the surface of lignocellulosic biomass [35,46]. Likewise, the -NH stretching vibration band in the FTIR spectrum of raw L. gibba indicated the presence of amino acids in the duckweed biomass, which are the building blocks of proteins. These polymeric compounds constitute approximately 29.26% of the dry biomass of L. gibba.

At approximately 2921 cm⁻¹, the absorption band is attributed to the stretching vibration of CH₂ and C-H functional groups found in lignin [35]. The absorption bands at approximately 1656 cm⁻¹ and 1544 cm⁻¹ are indicative of C=O stretching and N-H bending of amide I and amide II functional groups, respectively [35]. The C-O stretching of cellulose, hemicellulose, and proteins, and the -SO₃ stretching vibration of hemicelluloses, were detected at 1448 and 1240 cm⁻¹, respectively [35]. The absorption band of C-H vibration of cellulose and hemicellulose at approximately 1321 cm⁻¹ [35,47] was also found in the FTIR spectrum of raw L. gibba. In addition, the FTIR band at 1092 cm⁻¹ was assigned to the vibrations of O-H and C-O-C groups of polysaccharides [41].

After loading Ni²⁺, Zn²⁺, or (Ni²⁺ + Zn²⁺) ions onto the raw L. gibba biosorbent, the FTIR spectra displayed several changes related to the polysaccharides present in the biosorbent. Among these changes was the shifting of the absorption band from 1448 to 1434, 1433, and 1418 cm⁻¹ in the FTIR spectra of Ni²⁺-loaded, Zn²⁺-loaded, and (Ni²⁺ + Zn²⁺)-loaded L. gibba, respectively. Furthermore, the FTIR band at 1092 cm⁻¹ shifted to 1104, 1107, and 1104 cm⁻¹ in the FTIR spectra of Ni²⁺-loaded, Zn²⁺-loaded, and (Ni²⁺ + Zn²⁺)-loaded biosorbent, respectively. Likewise, the absorption band at 3294 cm⁻¹ shifted to 3353, 3351, and 3310 cm⁻¹ in the FTIR spectra of Ni²⁺-loaded, Zn²⁺-loaded, and (Ni²⁺ + Zn²⁺)-loaded L. gibba, respectively. It is well known that shifts in FTIR bands after metal biosorption are suggestive of interactions between the biosorbent’s functional groups and metal ions [35]. Furthermore, another important change was the disappearance of the absorption band at 1321 cm⁻¹ in the absorption spectra of Ni²⁺-loaded, Zn²⁺-loaded, and (Ni²⁺ + Zn²⁺)-loaded L. gibba. Considering the aforementioned assignments, these changes clearly denote the involvement of hemicellulose and cellulose of L. gibba in the biosorption of Ni²⁺ and Zn²⁺ ions.

3.6. SEM-EDX Analysis

SEM was employed to examine the physical morphology of the L. gibba surface before and after metal biosorption. SEM micrographs (Figure 7) displayed differences in surface morphology between the raw (Figure 7a) and the heavy metal-loaded L. gibba (Figure 7b–d).

The raw biosorbent showed a relatively regular surface morphology. Contrastingly, the morphology of the biosorbent surface changed after metal biosorption, becoming heterogeneous and smooth. These results are in agreement with other studies that have also found differences between the surface morphology of metal-unloaded and metal-loaded biosorbents [48,49].

Results of EDX analysis showed the presence of carbon, oxygen, nitrogen, sodium, magnesium, silicon, sulfur, chlorine, and potassium, and the absence of nickel and/or zinc on the L. gibba surface before interacting with heavy metals (Figure 7a). However, new Ni (Figure 7b), Zn (Figure 7c), and (Ni + Zn) (Figure 7d) peaks were observed on the biosorbent surface after interacting with Ni²⁺, Zn²⁺, and (Ni²⁺ + Zn²⁺) ions.

Furthermore, SEM and elemental mappings of Ni²⁺ and Zn²⁺ on a particular surface spot after heavy metal biosorption are provided in Figure 8.

Bright spots denote the presence of nickel (Figure 8a,c) or zinc (Figure 8b,d) on the L. gibba surface. It is apparent that the biosorbed Ni and Zn ions were not distributed uniformly on the L. gibba surface, indicating that only some chemical functional groups present on the L. gibba surface were capable of biosorbing the divalent metal ions from the aqueous solutions. Therefore, SEM-EDX analysis confirmed the biosorption of Ni²⁺ and Zn²⁺ ions onto L. gibba.

4. Conclusions

This study demonstrates that L. gibba is an effective and promising biosorbent for the simultaneous removal of Ni²⁺ and Zn²⁺ ions from liquid solutions. Solution pH value has a significant influence on Ni²⁺ and Zn²⁺ biosorption by L. gibba, both in the single and in binary metal systems. The maximum levels of divalent heavy metal biosorption occurred at pH 6.0. In both the single and binary metal biosorption systems, the affinity of L. gibba for Zn²⁺ ions was greater than for Ni²⁺ ions. In the binary metal biosorption systems, it was observed that Zn²⁺ ions exhibited a strong antagonistic effect on Ni²⁺ biosorption. Contrastingly, the interference of Ni²⁺ ions on Zn²⁺ biosorption was less significant. Redlich–Peterson and Freundlich models were found to better fit Ni²⁺ equilibrium data, whereas Redlich–Peterson and Langmuir models provided the best fit of the Zn²⁺ equilibrium data in single metal biosorption systems. The simultaneous biosorption of Ni²⁺ and Zn²⁺ ions onto the L. gibba was adequately and satisfactorily described by the modified competitive Sips model. FTIR results suggest that Ni²⁺ and Zn²⁺ ions bind to L. gibba’s polysaccharides, and SEM-EDX analysis validated the presence of the heavy metals on the surface of duckweed. Both Ni²⁺ and Zn²⁺ ions could be desorbed easily from heavy metal-loaded L. gibba using acid solutions, and facilitate heavy metal recovery and biosorbent regeneration.