Effective adsorption of nickel (II) with Ulva lactuca dried biomass: isotherms, kinetics and mechanisms

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.

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).

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.

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.

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).

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).

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.

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).

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.

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).

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).

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.

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).

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 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).

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⁺.

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).