2022 Volume 70 Issue 9 Pages 624-627
In this study, we evaluate the desorption or recovery capacity of chromium(VI) ions using desorption solutions containing sodium hydroxide (NaOH) or sodium sulfate (Na2SO4). A complex hydroxide of nickel–aluminum–zirconium (NAZ) was prepared as the adsorbent for the removal of chromium(VI) ions. The results from repeated adsorption/desorption experiments on chromium(VI) ions using NAZ complex hydroxide were evaluated. The desorption percentage of chromium(VI) ions increased with the increase in the concentration of NaOH or Na2SO4 in the desorption solution. The determined optimal concentration of NaOH or Na2SO4 in the desorption solution was 10 mmol/L under the used experimental conditions. After three adsorption–desorption cycles, the recovery percentages of chromium(VI) ions using NaOH and Na2SO4 were 60% (total amounts adsorbed and desorbed were 102 and 61 mg/g, respectively) and 75% (total amounts adsorbed and desorbed were 96 and 72 mg/g, respectively), respectively. Additionally, we confirmed the existence of chromium on the surface of the NAZ complex hydroxide. After three adsorption/desorption cycles, the crystal structure of the NAZ complex hydroxide was maintained. These results indicated the potential of the NAZ complex hydroxide using a desorption solution containing NaOH or Na2SO4 for the recovery of chromium(VI) ions.
Chromium(VI) ions are well known for their toxicity, bioaccumulation, and carcinogenicity to the ecological environment (e.g., water environment) and human beings.1) Therefore, WHO recommends a maximum allowable concentration for chromium of 50 µg/L in drinking water.2) Conversely, chromium is one of the stockpiled metals in Japan due to its relative rarity and valuable applications in electronics. Therefore, the removal (adsorption) and recovery (desorption) of chromium(VI) ions in water are important issues.
Many researchers have focused on the removal technology for chromium(VI) ions from aqueous media by physical, chemical, and biological treatments (e.g., precipitation, reduction, oxidation, ion exchange, solvent extraction, adsorption, reverse osmosis, and electrodialysis).3,4) Adsorption treatment is one of the most useful and promising treatment for the removal of chromium(VI) ions; thus, it has been employed for the purification of wastewater.5) However, useful and efficient recovery treatments for rare metals, including chromium(VI) ions, have not been adequately demonstrated. A recent study reported that research on metal hydroxides/oxides is an important subject for the modern researchers of physicochemical science.1) Although metal hydroxides/oxides have adsorptive properties, in the presence of desorption solution (during desorption treatment), adsorbed materials could be easily eluted and recovered. Therefore, some investigations regarding desorption solutions such as sodium hydroxide (NaOH), nitric acid (HNO3), sulfuric acid (H2SO4), and ethylenediaminetetraacetic acid (EDTA) for desorption have been evaluated in detail.2,6–9) However, the evidence is not sufficient until now.
We have recently reported the preparation of nickel–aluminum–zirconium (NAZ) complex hydroxide as metal hydroxides for the removal of chromium(VI) ions. This material has shown high potential for the adsorption of chromium(VI) ions in aqueous media.10) However, the ability of NAZ complex hydroxide for the sufficient desorption capacity of adsorbed material (e.g., chromium(VI) ions in this study) using desorption solutions has not been shown. This ability will significantly contribute to the recovery of rare metals in Japan.
Therefore, the potential of the NAZ complex hydroxide for the adsorption/desorption of chromium(VI) ions in desorption solutions was investigated. In particular, the effect of the concentration of desorption solution and cycle times on the recovery of chromium(VI) ions was assessed.
A standard solution of chromium(VI) ions (K2Cr2O7 in 0.1 mol/L HNO3) was obtained from FUJIFILM Wako Pure Chemical Corporation (Osaka, Japan). The NAZ complex hydroxide was synthesized using nickel(II) sulfate hexahydrate, aluminum sulfate hydrate, and zirconium sulfate tetrahydrate with a molar ratio of Ni2+/Al3+/Zr4+ = 0.9 : 1.0 : 0.09. The synthesis process and characteristics of the prepared NAZ complex hydroxide are found in a previous report.11) Scanning electron microscopy (SEM), X-ray diffraction (XRD), and elemental distribution experiments were conducted using SU1510 from Hitachi, Ltd. (Tokyo, Japan), Mini Flex II from Rigaku (Tokyo, Japan), and JXA-8530F from JEOL (Tokyo, Japan), respectively.
Adsorption/Desorption Capacity of NAZ Complex Hydroxide on Chromium(VI) IonsAdsorption/desorption experiments were conducted by the following procedures. 50 mL of 100-mg/L chromium(VI) ions solution and 0.1 g of NAZ were mixed and reacted at 100 rpm for 24 h. The adsorption temperature was 25 °C. After adsorption, the sample solution was filtrated using a 0.45-µm membrane filter from Advantec MFS, Inc. (Tokyo, Japan). The adsorption capacity was calculated using the initial and equilibrium concentrations of chromium(VI) ions. Thereafter, the NAZ complex hydroxide was collected and dried for its use in the desorption experiment. Sodium hydroxide (NaOH) or sodium sulfate (Na2SO4) solution at 1, 10, and 100 mmol/L and the collected NAZ complex hydroxide were mixed and reacted at 100 rpm for 24 h with a desorption temperature of 25 °C. The desorption capacity of chromium(VI) ions from the NAZ complex hydroxide was calculated using the initial and equilibrium concentrations of chromium(VI) ions. The appropriate replicates of the adsorption/desorption of chromium(VI) ions experiments were conducted. To elucidate the interaction between chromium(VI) ions and NAZ complex hydroxide, SEM images, XRD spectra, and elemental distribution were evaluated under appropriate conditions. The data are presented in the form of mean ± standard error.
We have reported the adsorption of chromium(VI) ions using NAZ complex hydroxide and adsorption mechanism in a previous study.10) However, the recovery of chromium(VI) ions from NAZ complex hydroxide was not sufficiently demonstrated. We investigated whether the proposed recovery system is useful for the elucidation of the adsorption and/or desorption behavior of chromium(VI) ions on the NAZ complex hydroxide. This study focused on the adsorption/desorption capacity of NAZ complex hydroxide on chromium(VI) ions in NaOH or Na2SO4 solution.
Different NaOH or Na2SO4 concentrations were scrutinized for the chromium(VI) ions desorption capacity of the NAZ complex hydroxide. The effect of concentration of desorption solution on the desorption of chromium(VI) ions is summarized in Fig. 1. The amount adsorbed was approximately 25 mg/g under the used experimental conditions. It was observed clearly that desorption solutions at high concentrations were more efficient than low concentrations. The desorption percentages of chromium(VI) ions using NaOH solutions at 1, 10, and 100 mmol/L were 29.8, 72.8, and 80.9%, respectively. Meanwhile, the percentages for Na2SO solutions at 1, 10, and 100 mmol/L were 38.4, 75.6, and 67.9%, respectively. An adsorption mechanism is the ion exchange between chromium(VI) ions and sulfate ions and/or hydroxide ions in the interlayer region of a NAZ complex hydroxide.10,12) Therefore, a likely desorption mechanism was related to the ion exchange between adsorbed chromium(VI) ions and sulfate ions and/or hydroxide ions in the desorption solution. Moreover, the desorption percentages of chromium(VI) ions using desorption solutions at 10 mmol/L were higher compared to the percentages of 1 mmol/L. However, an efficient and effective desorption capacity using desorption solutions at 100 mmol/L was not obtained under the experimental conditions tested. Therefore, the optimal concentration of NaOH or Na2SO4 in the desorption solutions was 10 mmol/L.
Adsorption condition: Initial concentration of chromium(VI) ions: 100 mg/L, sample volume: 50 mL, adsorbent: 0.1 g, temperature: 25 °C, contact time: 24 h, agitation speed: 100 rpm. Desorption condition: Initial concentration of NaOH or Na2SO4: 1, 10, and 100 mmol/L, sample volume: 50 mL, temperature: 25 °C, contact time: 24 h, agitation speed: 100 rpm.
Subsequently, to confirm repeatedly the use of NAZ complex hydroxide for the recovery the chromium(VI) ions, the recovery and regeneration experiments were conducted with 10-mmol/L NaOH and Na2SO4. These investigations were necessary to manage the solid waste, avoid the production of secondary waste, and improve the recovery of valuable materials.3) The repetition use of NAZ for adsorption/desorption of chromium(VI) ions is summarized in Fig. 2. Chromium(VI) ions adsorbed onto the NAZ complex hydroxide were eluted with both desorption solutions. More than approx. 60 and approx. 75% of the adsorbed chromium(VI) ions were desorbed from the NAZ complex hydroxide using the NaOH and Na2SO4 solutions, respectively. The adsorption/desorption capacities of the NAZ complex hydroxide were basically unchanged during the repeated adsorption–desorption operations under the used experimental conditions. This was except for the 3rd repeated experiment using the NaOH solution. A similar observation was reported using biomass.13) Our results indicated that the NAZ complex hydroxide could be repeatedly used in the chromium(VI) ions adsorption studies without significant loss in its initial adsorption capacity.
Adsorption condition: Initial concentration of chromium(VI) ions: 150 mg/L, sample volume: 100 mL, adsorbent: 0.2 g, temperature: 25 °C, contact time: 24 h, agitation speed: 100 rpm. Desorption condition: Initial concentration of NaOH or Na2SO4: 10 mmol/L, sample volume: 100 mL, temperature: 25 °C, contact time: 24 h, agitation speed: 100 rpm.
Finally, to confirm the characteristics of NAZ complex hydroxide before and after the adsorption/desorption of chromium(VI) ions, we evaluated the SEM images, XRD spectra, and elemental distribution of chromium. The results are summarized in Figs. 3 and 4. The surface of the NAZ complex hydroxide before and after adsorption/desorption was different. These phenomena suggest the possibility that the physicochemical characteristics of NAZ surface was changed by the adsorption/desorption of chromium(VI) ions. Additionally, the intensity of chromium increased after adsorption/desorption (The average intensities of chromium (Cr) before adsorption, using NaOH solution, and using Na2SO4 solution were 0, 0.29, and 0.01, respectively). The concentrations of chromium(VI) ions on the surface of the NAZ complex hydroxide using the NaOH solution were higher than the concentrations in the Na2SO4 solution. The total desorption percentages of chromium(VI) ions using the NaOH and Na2SO4 solutions were 60 and 75%, respectively. The XRD patterns of NAZ complex hydroxide before and after adsorption/desorption are shown in Fig. 4. The crystal structure was maintained after the adsorption/desorption treatment, which indicated that the NAZ complex hydroxide could be regenerated and reused at least three times. In addition, the NAZ complex exhibited adsorption capacity of chromium(VI) ions in aqueous media. However, further studies are needed to elucidate fully the adsorption/desorption mechanism of chromium(VI) ions using the prepared NAZ complex hydroxide.
We presented the recovery of chromium(VI) ions using NAZ complex hydroxide based on adsorption–desorption treatment. We were able to determine the optimal concentration of the desorption solution, which was a solution of NaOH or Na2SO4. Additionally, the results indicated that the prepared NAZ complex hydroxide is reusable for at least three times under the used experimental conditions. The recovery percentages of chromium(VI) ions using NaOH and Na2SO4 solutions were approx. 60 and approx. 75%, respectively. After three times of adsorption/desorption, the NAZ complex hydroxide was not destroyed and its crystal structure was maintained. Therefore, the NAZ complex hydroxide could be regenerated and reused for the recovery of chromium(VI) ions using NaOH or Na2SO4 solution as the desorption solution. This study is useful for the recovery of chromium(VI) ions from aqueous media.
This research was funded in part by Kurita Water and Environmental Foundation (21K004) and the Hattori Hokokai Foundation in Japan.
The authors declare no conflict of interest.
