Abstract

This study aimed to evaluate the Ni2+ ions adsorption capability of Ulva lactuca. The isotherms, kinetics and mechanisms for the adsorption of Ni2+ from aqueous solution by Ulva lactuca were also investigated. Influencing factors including initial pH, initial Ni2+ concentration, biomass, contact time were examined. The results indicate that the maximum Ni2+ adsorption capacity of 38.28 mg/g was obtained at pH 5, initial Ni2+ concentration 250 mg/L, biomass dosage 0.5 g/L and contact time 30 min. The adsorption can be well fitted with Langmuir isotherm, and the kinetics were well described by the pseudo-second-order model. The parameters of thermodynamics verified that Ni2+ adsorption on Ulva lactuca was a spontaneous and endothermic process. Analyses of FT-IR, SEM-EDS and XPS indicate that carboxyl and hydroxyl groups on the surface of biomass are involved in Ni2+ adsorption. The dried biomass of Ulva lactuca can be a cost-effective and eco-friendly adsorbent for the removal of Ni2+ from wastewater.

INTRODUCTION

Heavy metal pollution has been a growing concern as a result of rapid industrialization worldwide (Tatsuki et al. 2017). Many heavy metals cause a great threat to human and other living organisms because of their carcinogenicity (Ghasemi et al. 2011), teratogenicity (Anastopoulos & Kyzas 2015), mutagenicity and biomagnification (He & Chen 2014; Kumar et al. 2015) in the food chain. Nickel (II) ions are frequently discharged into the environment from industries such as mineral processing, battery manufacturing, steam-electric power plants and electroplating (Aksu et al. 2002; Liu et al. 2009; Geetha et al. 2016). It can result in different types of disease, e.g. pulmonary fibrosis, renaledema, skin dermatitis, and gastrointestinal distress (Bulgariu & Bulgariu 2012). Therefore, it is necessary to remove Ni2+ from wastewater before discharge.

Conventional techniques for removing heavy metals from water include chemical precipitation (Tang et al. 2017), ion exchange (Jayakumar et al. 2015), solvent extraction, filtration (Sarı et al. 2011), membrane technologies (Oliveira et al. 2011) and electrochemical treatment (Tuzen & Sari 2010). But these proposed approaches are usually expensive and not eco-friendly, especially in the treatment of low concentration heavy metal effluents. In recent years, adsorption techniques with some natural biomaterials, including agricultural waste products and seaweeds, were reported to be utilized as adsorbents for the removal of heavy metals from the wastewaters (Vijayaraghavan et al. 2006). Among these materials, macroalgal biomass has been proven to be a promising adsorbent in removing heavy metals because of its high adsorption capacity and low cost (Akbari et al. 2015). The high adsorption capacity could be mainly attributed to its biochemical constitutions, especially its cell wall, which contains functional groups (such as amino, carboxyl and hydroxyl) that play an important role in the adsorption process (Daneshvar et al. 2017; Vijayaraghavan et al. 2017). Marine macroalgae can be divided into several sub-groups such as brown algae (Phaeophyta), red algae (Rhodophyta) and green algae (Chlorophyta). Different types of algae have been tested for removal of heavy metals with different effectiveness (Mohammad et al. 2014; Gutiérrez et al. 2015; Ahmad et al. 2016). It has been shown that the inactive biomass may be even more advantageous than active one in the removal of heavy metals because of no biotoxicity concerns and no requirements of growth media (Abdel-Aty et al. 2013). Therefore, non-living alga has become one of the popular choices among various adsorbents for disposing the heavy metals from effluents (Nath & Ray 2015).

Ulva lactuca, a green tide alga, widely grew in the southern coast of China with a large biomass. However, only a small portion of Ulva lactuca is used for food or forage. The biomass of Ulva lactuca was used for Cu2+, Mn2+ and Cr3+ adsorption (Zeroual et al. 2003; Amany et al. 2007; Andrew et al. 2007). But little attention has been paid to use the algal material as adsorbent for Ni2+ removal (Hanan 2008). In the present study, we aimed to test the feasibility of U. lactuca as an adsorbent for removing Ni2+. The influencing factors, including initial pH, initial Ni2+ concentration, sorbent biomass and contact time, on the adsorption performance were examined. In addition, to gain a better understanding of the adsorption mechanisms, the isotherms, kinetics and thermodynamics were also investigated. The surface structure and functional groups were characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) analyses.

MATERIALS AND METHODS

Chemicals and reagents

All chemicals and reagents used were of analytical grade and purchased from Guangzhou Huaxin Reagent Co. Ltd, Guangdong, China. Standard solution (1,000 mg/L) of Ni2+ was prepared with nickel nitrate (Ni(NO3)2). The working solution was serially diluted from standard solution daily. The deionized water used in the study was obtained from a deionized (DI) water generator (YL05-20 L 75G, Shenzhen, China).

Preparation of adsorbent

Biomass of U. lactuca was collected from Nan Ao Island of Shantou, Guangdong, China. The biomass was cleaned by washing repeatedly with deionized water for removing impurities, and then dried in oven at 80 °C for 6 h. Finally, the dried biomasses were crushed, sieved and stored at room temperature in an airtight container for future use.

Batch adsorption experiments

The batch experiments on Ni2+ adsorption were conducted in a temperature-controlled shaker. In order to determine the optimal adsorption conditions, initial pH (2–8), biomass dosage (0.5–5 g/L), initial Ni2+ concentration (0–400 mg/L), temperature (288 K, 298 K, 308 K) and contact time (0–250 min) were tested. Solutions of 1 M HCl and 1 M NaOH were used for pH adjustment, and all the experiments were conducted in triplicate.

The concentration of Ni2+ was detected by inductively coupled plasma-atomic emission spectroscopy (ICP-AES), and the adsorption capacity of Ni2+ was calculated according to the following equation (Lou et al. 2015):
formula
(1)
where qe (mg/g) is the adsorption capacity of Ni2+at equilibrium, C0 and Ce (mg/L) are the initial and equilibrium concentration of Ni2+ in the solution, respectively. V (L) is the volume of the aqueous solution, and W (g) is the mass of adsorbent.

Adsorption isotherms

Adsorption isotherm is one important method to illuminate the mechanism of the adsorption systems, which describes how adsorbate interacts with the adsorbent under the equilibrium state (Yeslié et al. 2011). In this study, the two most often used adsorption isotherms, namely Langmuir and Freundlich models, were adopted to describe the adsorption process. The Langmuir model defines the monolayer sorption, and could be described by the following equation (Bakatula et al. 2014; Li et al. 2017a):
formula
(2)
where qmax is the maximum biosorption capacity (mg/g) and b is the constant of Langmuir isotherm which is closely related to the energy of adsorption.
The Freundlich model describes the nonideal adsorption on heterogeneous surface as well as multilayer adsorption; its equation is listed below (Zhang et al. 2015).
formula
(3)
where Kf (mg/g) is a constant related to the adsorption capacity, n is the constant related to the adsorption intensity.

Adsorption kinetics

In order to evaluate the adsorption rate of Ni2+ adsorption, two widely accepted kinetic models, pseudo-first-order and pseudo-second-order were applied to fit the adsorption capacity over the reaction time. The linear equation of pseudo-first-order model is expressed in the following formula (Shroff & Vaidya 2011).
formula
(4)
where qt (mg/g) is the adsorption capacity at time t, and k1 (h−1) is the rate constant. The formula of pseudo-second-order model can be expressed as the following equation (Daneshvar et al. 2012).
formula
(5)
where k2 (g/mg/h) is the second-order rate constant of the adsorption process.

Thermodynamic studies

The thermodynamics for the adsorption of Ni2+ onto Ulva lactuca were studied at 288 K, 298 K and 308 K, respectively. The thermodynamic parameters change of enthalpy (ΔH0), entropy (ΔS0) and Gibbs free energy (ΔG0) can be calculated from the Equations (6)–(8). (Mitrogiannis et al. 2015; Ioannis & George 2016).
formula
(6)
formula
(7)
formula
(8)
where Kd is the equilibrium adsorption distribution constant (Kd = qe/Ce), R is the gas constant (8.314 J/mol/K) and T is absolute temperature in Kelvin.

Characterization of U. lactuca adsorbent

FT-IR spectra (ALPHA-T, Bruker, Germany) were obtained to explore the surface functional groups presented in U. lactuca adsorbent in the range of 400–4,000 cm−1. SEM-EDS (QUANTA200, USA) was used to investigate the morphological characteristics of adsorbent before and after adsorption of Ni2+ (Cho et al. 2013).

The XPS of adsorbent before and after Ni2+ adsorption were obtained by the Theta Probe Angle-Resolved XPS System (hv = 1,486.6 eV, Kratos Axis Ultra, Japan) which was used to identify the element and compounds of U. lactuca. Correction of the deviation of the binding energy (BE) owing to the relative surface charging was performed with the C 1 s level at BE of 284.6 eV as an internal standard. The fitting of all XPS spectra was conducted using the software XPSPEAK4.1 (Li et al. 2017a, 2017b).

RESULTS AND DISCUSSION

Influence of pH

Many studies have shown that the pH value of solution is one of the key parameters in the adsorption of metal ions which affects the metal binding sites of the sorbent and the metal ion chemistry in aqueous solution (Long et al. 2017; Yang et al. 2017; Zhang et al. 2018). In this study, the influence of initial pH on Ni2+ adsorption was conducted by varying the solution pH from 2 to 8 (Figure 1). The adsorption capacity for Ni2+ was low at pH less than 3; this could be due to the competition of H+ over Ni2+ to bond the adsorption sites, causing the decrease in Ni2+ adsorption performance (Long et al. 2017). With the pH increase of the solution, more negatively charged on the surface of U. lactuca can be available for the ion-exchange of Ni2+, resulting in the increase of adsorption for Ni2+. As the pH increased the adsorption capacity reached a plateau and no longer increased. This may be explained by the formation of soluble hydroxylated complexes of the metal ions and their competition with the active sites on U. lactuca, and finally resulting in no increase upon adsorption capacity (Chen et al. 2008; Daneshvar et al. 2017).

Figure 1

Influence of solution pH on the adsorption of Ni2+ by U. lactuca.

Figure 1

Influence of solution pH on the adsorption of Ni2+ by U. lactuca.

Influence of initial Ni2+ concentration

The adsorption capacity increased with the increased Ni2+ concentration (Figure 2). This is due to the fact that the increasing initial concentration produces driving force to overcome the mass transfer resistance of Ni2+ between the aqueous and solid phase. Then, when the Ni2+ concentration further increased from 250 to 400 mg/L, there was no obvious variation on the adsorption efficiency, which may be attributed to the fact that maximum adsorption capacity was reached at the high Ni2+ concentration. Similar observations have been found by Tounsadi et al. (2015) during biosorption of Cd (II) and Co (II) on Glebionis coronaria L. biomasses.

Figure 2

Influence of initial concentration of Ni2+ on the adsorption by U. Lactuca.

Figure 2

Influence of initial concentration of Ni2+ on the adsorption by U. Lactuca.

Influence of biosorbent dosage

The biosorbent dosage was also an important parameter affecting the adsorption capacity as well as the removal efficiency. As shown in Figure S1 (available with the online version of this paper), the adsorption capacity decreased gradually as the biomass dosage increased, due to the fact that the adsorption became saturated with increasing adsorbent dosage. The available binding sites for limited adsorbate are excessive when the biomass is in high dosage, and thus leads to the decrease in adsorption capacity during the adsorption process (Christoforidis et al. 2015; Chen et al. 2008).

Influence of contact time

The amount of adsorbed Ni2+ increased rapidly during the initial 30 min, and was kept constant when the contact time was further increased (Figure 3). This may be related to the availability of adsorption sites on the biomass surface. At first, adsorption capacity increased quickly owing to sufficient active sites in the beginning; afterward, these active binding sites were gradually occupied (Akbari et al. 2015). Therefore, adsorption capacity was kept invariable in the latter adsorption stage.

Figure 3

Influence of contact time on the adsorption of Ni2+ by U. lactuca.

Figure 3

Influence of contact time on the adsorption of Ni2+ by U. lactuca.

Adsorption isotherms

For all tested temperatures, the Langmuir model better described the adsorption process with a higher R2 value of 0.9853, 0.9861 and 0.9819 than the Freundlich model (Table 1 and Figure S2, available online). The maximum adsorption capacity reached 38.28 mg/g at 308 K, and the constant b was 0.0347, respectively. It should be noted that the data obtained by both isotherms demonstrate that the higher temperature was more beneficial to the adsorption under the tested conditions, which might indicate the endothermic nature during the adsorption process (Masoumeh et al. 2015).

Table 1

Isotherm constants for the adsorption of Ni2+ by U. lactuca at different temperatures

Isotherm modelParameterTemperatures
288 K298 K308 K
Langmuir qmax(mg/g) 29.02 32.19 38.28 
b(L/mg) 0.0203 0.0211 0.0347 
R2 0.9853 0.9861 0.9819 
Freundlich Kf (mg/g) 1.857 1.334 1.363 
2.399 1.745 1.722 
R2 0.9669 0.9534 0.9569 
Isotherm modelParameterTemperatures
288 K298 K308 K
Langmuir qmax(mg/g) 29.02 32.19 38.28 
b(L/mg) 0.0203 0.0211 0.0347 
R2 0.9853 0.9861 0.9819 
Freundlich Kf (mg/g) 1.857 1.334 1.363 
2.399 1.745 1.722 
R2 0.9669 0.9534 0.9569 

Adsorption kinetics and thermodynamics

The R2 value (0.999) of the pseudo-second-order kinetic model was higher than that (R2 = 0.926) of pseudo-first-order (Table 2 and Figure S3, available online). This suggests that the pseudo-second-order kinetic model was better in describing the kinetics of Ni2+ adsorption. Similar results have been documented for various types of algae adsorbent reported by Pang et al. (2011).

Table 2

Kinetics constants for the adsorption of Ni2+ by U. lactuca at different temperatures

Kinetics modelParameterTemperatures
288 K298 K308 K
Pseudo-first-order qe (mg/g) 10.47 16.34 22.96 
K1(1/min) 0.1798 0.1833 0.1864 
R2 0.9142 0.9287 0.9368 
Pseudo-second-order qe (mg/g) 25.23 29.36 35.82 
K2(g/mg/min) 0.0619 0.0648 0.0723 
R2 0.9872 0.9913 0.9979 
Kinetics modelParameterTemperatures
288 K298 K308 K
Pseudo-first-order qe (mg/g) 10.47 16.34 22.96 
K1(1/min) 0.1798 0.1833 0.1864 
R2 0.9142 0.9287 0.9368 
Pseudo-second-order qe (mg/g) 25.23 29.36 35.82 
K2(g/mg/min) 0.0619 0.0648 0.0723 
R2 0.9872 0.9913 0.9979 

The negative value of ΔG0 at three temperatures (Table 3) indicated that the adsorption of Ni2+ was spontaneous (Henriques et al. 2015). The positive values of ΔH0 showed the endothermic nature of the biosorption. The positive values of ΔS0 suggest increased randomness at the solid/solution interface during the adsorption of Ni2+ on the biomass of U. lactuca.

Table 3

Thermodynamic parameters of Ni2+ adsorption by U. lactuca at different temperatures

T (K)△G0(kJ/mol)△H0(kJ/mol)△S0(J/mol K)
288 −48.61   
298 −52.28 37.53 0.287 
308 −88.86   
T (K)△G0(kJ/mol)△H0(kJ/mol)△S0(J/mol K)
288 −48.61   
298 −52.28 37.53 0.287 
308 −88.86   
Table 4

SEM-EDS quantitative elemental analysis of U. lactuca before and after the adsorption of Ni2+

Before elementW%A%After elementW%A%
C K 46.96 54.59 C K 45.79 53.38 
O K 50.73 44.27 O K 52.10 45.60 
Mg K 1.06 0.61 Mg K 0.86 0.49 
S K 1.09 0.48 S K 1.02 0.44 
Ca K 0.16 0.06 Ca K 0.15 0.05 
Ni K Ni K 0.09 0.02 
Totals 100.00  Totals 100.00  
Before elementW%A%After elementW%A%
C K 46.96 54.59 C K 45.79 53.38 
O K 50.73 44.27 O K 52.10 45.60 
Mg K 1.06 0.61 Mg K 0.86 0.49 
S K 1.09 0.48 S K 1.02 0.44 
Ca K 0.16 0.06 Ca K 0.15 0.05 
Ni K Ni K 0.09 0.02 
Totals 100.00  Totals 100.00  

Characterization of the adsorbent

FT-IR analysis

The broad and strong peaks around 3,404 cm−1 were assigned to the –OH groups (Xu et al. 2016) (Figure 4). The bands near 1,652 cm−1 could be attributed to the stretching vibration of C=O groups. The band near 1,252 cm−1 was assigned to C-O stretching (Castro et al. 2017). After Ni2+ adsorption, the stretching vibration bands of –OH groups were shifted to 3,384 cm−1 for Ni2+-loaded biomass, the peaks of C=O and C-O groups were shifted to 1,639 cm−1 and 1,236 cm−1, respectively. This indicates that the chemical interactions between Ni2+ and –OH or –NH, C=O and C-O groups of the biomass mainly occurred in the adsorption. The similar FT-IR results were reported for the adsorption of Ni2+ on other algae species reported by Castro et al. (2017).

Figure 4

FT-IR spectra of U. lactuca before (a) and after (b) adsorption of Ni2+.

Figure 4

FT-IR spectra of U. lactuca before (a) and after (b) adsorption of Ni2+.

SEM-EDS analysis

Before adsorption, the surface of the adsorbent is rough and uneven, surface ridge and void structure decreased after adsorption of Ni2+ (Figure 5), as element contents summarized in Table 4. After adsorption, a new peak appears with Ni2+ characterization, and the content Ni2+ increased by 0.09%, which indicates the successful capture of Ni2+ on to the adsorbent (Ramrakhiani et al. 2017).

Figure 5

SEM-EDS photos and spectra of U. lactuca before (a) and after (b) adsorption of Ni2+.

Figure 5

SEM-EDS photos and spectra of U. lactuca before (a) and after (b) adsorption of Ni2+.

XPS analysis

Figure 6 shows the new characteristic peaks of Ni 3 s and Ni 2P3, which further implies that Ni2+ was successfully adsorbed onto the surface of U. lactuca. To investigate the mechanism of Ni2+ adsorption on U. lactuca, the XPS spectra of the C1s, N1s and O1s are analyzed and summarized in Figure S4 (available online). Before adsorption, peaks appeared at 284.8 eV, 285.2 eV and 287.6 eV (Figure S4(a) and S4(b)), which correspond to the functional groups C-C/C-H, C=O/O-C-O and C-O, respectively (Zhang et al. 2017). After adsorption, the peak area of the C-C/C-H and C=O/O-C-O decreased from 37.7% to 35.71%, and from 53.9% to 46.03%, respectively. The peak area of the C-O group increased, indicating that the three functional groups of C-C/C-H, C=O/O-C-O and C-O were involved in the adsorption of Ni2+. The N1S spectra occurred with the characteristic peak of the group N-C = O/NH2 at 400 eV before adsorption, and a new peak appeared at 400.6 eV after adsorption (Figure S4(c) and S4(d)). In addition, the peak area of the group N-C = O/NH3 decreased to 91.1%, showing that the functional group of N-C = O/NH3 is an active site for Ni2+ adsorption. Figure S4(e) and S4(f) show that the peak area of group O-H increased from 91.28% to 93.7% after adsorption, but the peak area of group C = O/C-O decreased from 8.72% to 6.3%. The group C = O/C-O had a small effect on Ni2+ adsorption, which is consistent with the report by Yang et al. (2017).

Figure 6

XPS spectra survey of U. lactuca before and after adsorption of Ni2+.

Figure 6

XPS spectra survey of U. lactuca before and after adsorption of Ni2+.

CONCLUSIONS

The dried biomass of U. lactuca is found to be an effective, cost-efficient and eco-friendly adsorbent for the removal of Ni2+ from aqueous solutions. Maximum adsorption capacity was obtained at pH 5, after 30 min of contact time.

The adsorption of Ni2+ can be well described by the Langmuir isotherm model, and the maximum adsorption capacity was 38.28 mg/g. The kinetic studies reveal that the adsorption process fitted the pseudo-second-order kinetic model better than the pseudo-first-order. The calculated values of ΔG and ΔH thermodynamic parameters imply that adsorption process of Ni2+ by U. lactuca is spontaneous and endothermic. In addition, higher temperature was more favorable to adsorption process. Characterization by FT-IR, SEM-EDS and XPS techniques demonstrates that the functional groups of −OH, N-H, C = O and C-O on the surface of the adsorbent were proven to take part in the adsorption of Ni2+.

ACKNOWLEDGEMENTS

We gratefully acknowledge the financial support provided by National Natural Science Foundation of China (41376156), Guangzhou Science and Technology Program (201510010204, 201804010281), Guangdong innovation platform characteristic innovation project (2016KTSCX106), Guangzhou city science and technology project (201804010281), the Project of Education Bureau of Guangzhou City (1201620219) and the high level university construction project (regional water environment safety and water ecological protection).

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Supplementary data