Chromium Ion Accumulations from Aqueous Solution by the Eichorinia crassipes Plant and Reusing in the Synthesis of Cr- Doped ZnO Photocatalyst

Department of Materials Science and Engineering, Adama Science and Technology University, Adama, Ethiopia Department of Applied Chemistry, Adama Science and Technology University, Adama, Ethiopia Department of Materials Science and Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan College of Materials Engineering, Fujian Agriculture and Forestry University, Fuzhou 350002, China


Introduction
Currently, the disposals of industrial effluents without proper treatment are the major threat and the utmost research areas in the world [1,2]. Mainly, organic and inorganic wastes released from different sectors such as textile industries, dyeing, paintings, and other sources are the potential causes of water pollutions [3,4]. Hence, proper controls of pollutants discharged from different sources with appropriate removal techniques should be the primary activity [5][6][7][8]. One of the techniques used in the wastewater treatment technology is photocatalysis. In the photocatalysis process, semiconductors such as ZnO can be used due to the ability to absorb the light, nontoxicity, and stability [9][10][11]. However, the activity of the ZnO semiconductor on the visible light source is not good due to its larger band gap energy (3.2 eV) [12,13]. Due to this reason, different modifications have been employed to enhance the photocatalytic activity performances of the ZnO semiconductor [14,15].
Currently, reports on a ZnO-based photocatalyst are available [15][16][17][18]. For example, the prepared ZnO from Acremonium potronii for dye degradation was reported by Ameen et al. [19]. The hexavalent chromium and dye degradations were also performed by using ZnO under UV light [20]. However, ZnO alone is not effective under visible light irradiation. Due to this reason, a lot has been done on the photocatalytic performance improvements of ZnO [21,22]. Among the reported performance improvements of ZnO are doping with metal [23,24], nonmetals [25], and ZnO combination with other low band gap energy semiconductors [26]. Specifically, Cr-doped ZnO photocatalysts were also synthesized and used for organic dye degradation [27,28]. However, further catalytic performance improvements of ZnO by using a green synthesis method are also needed. For this purpose, the biological renewable plant materials can be used as a template to prepare oxide semiconductorbased photocatalysts [29,30]. Among the biological renewable unwanted plant materials resources is Eichhornia crassipes.
Recently, Eichhornia crassipes is used for pollutant removal [31,32]. For example, pollutant removal via phytoremediation techniques using water hyacinth from wastewater was studied by Rezania et al. [33]. The removal of Pb 2+ , Zn 2+ , and Ni 2+ , with water hyacinth fiber, was also reported [34]. Additionally, the cadmium ion, Cu 2+ , and chromium (VI) ion heavy metal removal with water hyacinth-based biochar, chitosan materials, and water hyacinth roots, respectively, were also tested and showed good removal efficiencies [35][36][37]. Moreover, recycling performance of the heavy metals after accumulation on water hyacinth was also studied [38]. In our previous report, the cobalt ion accumulations onto Eichhornia crassipes for the purpose of cobalt doping on ZnO were also studied [39]. However, there was no report on the chromium ion accumulations onto water hyacinth and recycled for organic pollutant degradations.
Herein, the water hyacinth was immersed into chromium aqueous solution to collect the chromium ion on the surface of the plant tissue. The chromium ion-containing plant was mixed with a zinc precursor to prepare Cr-doped ZnO after calcinations in air. The remained amount of chromium ion in the aqueous solution was measured. The Cr-doped ZnO powder catalyst characterizations were performed. The catalytic performance of Cr-doped ZnO was tested. Here, the chromium ion removal efficiency of water hyacinth from aqueous solution was evaluated. Moreover, the reusing of the accumulated chromium ion on the plant as a dopant for ZnO preparation in the decontamination of organic pollutants was also studied systematically. Pure ZnO only was prepared and tested in the degradation of MB dye for comparison purpose. It is suggested that the Cr-doped ZnO performance could have better catalytic activity than pure ZnO due to widening the visible light absorption, electron/hole separation, and improving the porosity of the materials as a result of Eichhornia crassipes after calcination.

Preparation of the Water Hyacinth.
The Eichhornia crassipes plant was brought from Lake Koka, Ethiopia. The plant was washed with water to remove dusts from the surface. It was dried at room temperature, crushed, and stored for further use.

Preparation of Cr-Doped
ZnO. The crushed powder plant (2 g) was added into a chromium solution at different days (1, 2, 4, 6, and 8) in separate flasks. Then, the Eichhornia crassipes having chromium ion on the surface was separated. The supernatant solution was tested with a microwave plasma atomic emission spectrometer (MP-AES) (Agilent, Germany) to examine the remaining chromium ion content in the solution. For comparison purpose, the initial concentration of chromium ion (before soaking of the plant) was also measured by MP-AES. The plant having chromium ion was dried and mixed with 80 mL of zinc nitrate (0.5 M) precursor. Then, the sample was stirred and heated at 80°C for 2 h, followed by adjusting the pH to 10. Then, the solution was centrifuged, washed, and dried followed by calcination in air at 600°C for 3 h.

2.4.
Characterizations. X-ray diffraction (XRD) was illustrated by Shimadzu XRD-7000. The field emission scanning electron microscopy (FESEM, JSM 6500F, JEOL) was used for morphology analysis. ESCALAB 250 for X-ray photoelectron spectroscopy (XPS) analysis was used to determine the oxidation states in the powder. The diffuse reflectance spectroscopy (DRS) was evaluated using a JASCO V-670UV-Vis spectrophotometer. The photoluminescence behaviors of the materials were characterized with a JASCD FB-8500 fluorescence spectrophotometer. Fourier transform infrared (FTIR) analysis was performed with Spectrum 65 FT-IR (PerkinElmer) using KBr pellets. The concentration of the pollutants before and after photocatalytic reactions was measured by using a Shimadzu-3600 Plus spectrophotometer.   Journal of Nanomaterials 2.5. Photocatalytic Activities. MB dye was used as a pollutant to test the photocatalytic performance of the prepared samples according to the reported literature with modification [18]. In the degradation test, Cr-doped ZnO (25 mg) was mixed into 10 ppm of 100 mL dye solution. Adsorption/ desorption experiment was performed for 30 min under stirring in the dark condition. Then, a light source (visible light) was on after 30 min adsorption/desorption reaction with continuous stirring. Finally, 5 mL of the aliquot sample was taken out at 15 min intervals of time and used for analysis after centrifugation. For comparison purposes, the degradation of the pollutant was performed in the presence of pure ZnO and blank (without catalyst) with the same procedure as mentioned above.

Results and Discussion
After soaking the Eichhornia crassipes plant in the chromium ion solution for different days, the plant materials accumulated with chromium ion was separated and the    Journal of Nanomaterials remaining aqueous solution in the sample flasks was subjected to MP-AES measurement. For comparison purpose, the initial chromium solution was also measured. The initial concentration of chromium ion was 1725.00 mg/L. However, after chromium ion accumulations, the concentrations in the solution were decreased which indicated that the Eichhornia crassipes plant tissue has the ability to accumulate the chromium ion. Table 1 shows the initial concentration and remained and accumulated amount of chromium ion on water Eichhornia crassipes tissue for each day.
The XRD analysis was performed for the powder samples. Figure 1 . 036-1451). There were no other peaks observed from chromium oxides or any other related phases in which the prepared samples were ZnO. The results showed that the structure of ZnO crystal was not affected by adding Cr atoms which indicates that the Zn sites were substituted by Cr in the lattices as it is reported Cr-doped ZnO [40]. Moreover, in the presence of the water hyacinth plant accumulated with chromium ion, the XRD was shifted to the higher angle as it is illustrated from Figure 1(b) in the 31-37°range. This magnificent shifting could be due to the incorporation of Cr 3+ into the ZnO lattice. On the other hand, it is expected that Cr 3+ ions can replace Zn 2+ ions due to the smaller radius (0.063 nm) of Cr 3+ ions as compared to the radius (0.074 nm) of Zn 2+ ions [41]. Figure 2 indicates the SEM images at lower (Figure 2(a)) and higher (Figure 2(b)) resolution and EDS (Figure 2(c)) analysis for sample D1, respectively. As it is indicated in the SEM image, smaller and well-dispersed spherical nanoparticles were observed. The SEM showed that the presence of water hyacinth in the preparation process could prevent the agglomeration and reduces the particle size which also indicates better catalytic performance. Moreover, EDS analysis also indicates Zn, Cr, and O elements in the sample which confirms the preparation of Cr-doped ZnO.
The FTIR analysis was also performed for further exploring the structural properties of the samples. Figure 4 indicates the FTIR data for pure ZnO and Cr-doped ZnO samples. The FTIR spectra exhibiting a band about 628 cm −1 for the Cr-doped ZnO sample could be assigned to O-Cr-O [40]. The absorption peaks showed at~3457 and 1643 cm −1 indicate the stretching and bending vibration mode of O-H as a result of water molecule adsorbed on the prepared sample surface. Moreover, the FTIR peak at 473 cm −1 indicates the stretching vibrational mode of the Zn-O bond [39,48]. The results indicate that the Crdoped ZnO catalyst was synthesized successfully.  Journal of Nanomaterials The absorption properties of pure ZnO and Cr-doped ZnO catalysts were studied as shown in Figure 5(a). The pure ZnO had an absorption edge at about 395 nm. After Cr doping, the absorption edge was shifted to 424 nm. Moreover, the absorption of Cr-doped ZnO becomes relatively higher than that of pure ZnO in the visible region. The band gaps for pure ZnO and Cr-doped ZnO were also calculated and shown as 3.2 and 3.13 eV, respectively ( Figure 5(b)). The result indicates that the presence of Cr as a dopant in the ZnO lattice plays the major role for enhancing the catalytic activity. On the other hand, the Cr-doped ZnO performance could have better catalytic activity than pure ZnO due to widening the visible light absorption and electron and hole separation [49].
The photoluminescence (PL) analysis was also performed as shown in Figure 6. The PL measurements were obtained at 325 nm excitation wavelength. The decrease in the PL band intensity was observed in the Cr-doped ZnO. It is reported that the higher the intensity band in the PL analysis, the higher the electron and hole recombination rate of the samples [7]. It is observed that the PL intensity of Crdoped ZnO is smaller than that of pure ZnO, indicating that Cr doping has the ability to increase the defects and oxygen vacancies in the ZnO sample [50]. It is also known that any compound having a lower PL intensity band and a long lifetime will have a good photocatalytic activity since the separation of photogenerated electron and hole pairs will be easy [51].
The catalytic activities of the prepared powders were checked in the MB dye degradation as shown in Figure 7. Figure 7(a) indicates the UV-visible absorption spectral measurements at different interval reaction times for sample D1. Moreover, Figure 7(b) shows the C t /C o ratio plots for D1, D2, D4, D6, D8, pure ZnO, and blank (without catalyst) samples. As it is indicated from Figure 7(b), the Cr-doped ZnO accumulated for one day (D1) degrades 80% of MB dye. However, the D2, D4, D6, D8, and pure ZnO samples degrade only 74, 76, 79, 73, and 25%, respectively. On the other hand, without the addition of catalysts (blank), there was no significant degradation of MB dye within 90 min irradiation. It is illustrated that the Cr ion incorporated into the ZnO lattice from chromium-accumulated Eichhornia crassipes with optimum amount enhances the photocatalytic performance. Figure 7(c) indicates a plot for ln ðC t /C o Þ with irradiation time in the MB dye degradation. The degradation indicates the pseudo-first-order kinetics which is also similar to the literature report [52]. The rate constants of D1, D2, D4, D6, D8, and pure ZnO samples were also calculated and shown as 0.0149, 0.0133, 0.0125, 0.0142, 0.0117, and 0.0024 min -1 , respectively. As it is shown from the result, the maximum rate constant was obtained from the D1 sample in which the D1 was the best catalyst in the MB degradation. Moreover, the ratio constant (K) for the D1 sample was also calculated and estimated to be 0.596 min -1 g -1 . Moreover, the catalytic performance was also compared with literature as shown in Table 2. It is suggested the catalytic performance is comparable with literature reports. Figure 8 indicates the mechanism for MB dye oxidation with the Cr-doped ZnO catalyst. As it is reported, the photocatalyst absorbs the irradiated light to generate electron and hole pairs. When semiconductor photocatalysts are subjected to light irradiation, there will be excitation of elections into the conduction band (CB). The photogenerated holes will be remained in the VB [57]. When ZnO is doped with chromium, the electrons will be trapped and the recombination of the hole and electron could be suppressed as it is also reported in metal-doped ZnO [15,58]. The electron-hole pairs on the surface of the photocatalyst pass through a series of oxidation-reduction reactions affecting the organic pollutant molecules. For example, the photogenerated electrons can interact with adsorbed O 2 , on the surface of the sample, and generate a superoxide radical anion ( • O 2 ). Moreover, due to the presence of oxygen vacancies as it is illustrated from the XPS results, there will be better absorption of O 2 which is a source of superoxide radical anion [51]. The photogenerated holes can also interact with hydroxyl ion (OH -) or H 2 O to generate hydroxyl radical ( • OH) [15,27,59]. The reactive oxygen species resulting from the reaction will be interacted with MB dye and degrades into smaller degradation products [39,59]. Figure 8 illustrates the mechanism for MB dye oxidation with the Cr-doped ZnO catalyst.

Conclusion
The Cr-doped ZnO was prepared via the chromium ion accumulation with the Eichhornia crassipes plant for one, two, four, six, and eight days and mixed with a zinc precursor. In this report, the removal of chromium ion and reusing it again for the purpose of a dopant in the ZnO photocatalyst synthesis were illustrated. The prepared materials were also tested for MB dye degradation. The chromium ion accumulated for one day and mixed with a zinc precursor followed by calcination (D1) showed the best catalytic performance and degrades 80% of MB dye while D2, D4, D6, D8, and pure ZnO samples degrade only 74, 76, 79, 73, and 25% of MB dye, respectively. Therefore, degradation of the organic pollutants with Cr-doped ZnO with the aid of a plant accumulated with chromium ion was efficient. The catalytic performance enhancement may be as a result of reducing recombination rates of the electrons and hole, the particle size reduction, and porosity improvement resulting from the water hyacinth plant after calcination.

Data Availability
The data used to support the findings of this study are included within the article.

Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.