A novel green cycle for value-added utilization of waste zinc based adsorbent

Abstract

A novel green cycle for transformation and regeneration of waste zinc based adsorbent was proposed, in which nano CuS/ZnS anode material and nano ZnO with high purity were obtained. The CuS/ZnS@C was used as anode in lithium ion battery with the first discharge capacity 802.7 mA h g⁻¹ and 454.3 mA h g⁻¹ after 38 charge–discharge cycles.

---

H₂S is mainly derived from the process of natural gas exploration, coal gasification, petroleum refining, etc. It will cause not only serious corrosion of metal pipes and equipment and poisoning of the catalyst, but also serious environmental pollution and even endanger human survival. Therefore, it is necessary to carry out desulfurization and purification treatment in those industries which produce H₂S gas.

Adsorption of H₂S by an adsorbent is one of the commonly used methods for desulfurization. Adsorbents can be classified into two types: carbon materials and metal oxides.¹ Activated carbon (AC) is the most common adsorbent material for gas or vapour adsorption.² But research shows that the adsorption capacity in dry and anaerobic environments is very low (only 3 mg g⁻¹ or 0.09 mmol g⁻¹).³ The working temperature for metal oxide adsorbents can be 25–1600 °C, and the concentration of H₂S can be reduced to 10 ppm,⁴ which causes it to be widely studied by scholars all over the world. In recent years, the researches on metal oxide adsorbents was mainly focus on pure^5–11 or composite oxides^12–17 such as FeO,⁵ ZnO,⁶ CuO,⁷ MnO,⁸ CoO,⁹ CeO₂,¹⁰ CaO,¹¹ and their composites with carbon materials.^18,19 SiO₂, Al₂O₃, Y₂O₃, ZrO₂, TiO₂, and many other refractory oxides do not absorb H₂S, but these oxides can be used as an inert support for adsorbents.²⁰

Zinc based adsorbent is a kind of fine desulfurization agent, which is used in the removal of low content of H₂S under medium or high temperature conditions. The concentration of H₂S in purified air can reach below 14 mg m⁻³ using ZnO adsorbent at 300 °C. But the regeneration of zinc based adsorbent is very difficult. There are two main reasons: first, the active surface of adsorbent will be significantly reduced because of sintering at high temperature; second, ZnSO₄ is easily formed after low temperature regeneration.^21,22 Therefore, zinc based adsorbent is extremely easy to lose efficacy and be scrapped. Enterprises have to pay a high cost for that every year. Last, waste desulfurizers are toxic and hazardous chemicals. Long-term exposedness will lead to air pollution. At the same time, harmful substances in those rain-swept waste agents dissolve into ground, flowing into rivers, lakes and seas, which causes pollution of soil, water and ecological damage. This shows that the waste desulfurization agent must be promptly replaced and properly disposed of.

There are many processing methods for waste desulfurizers in different enterprises such as incineration, deep buried method, chemical treatment (including chemical inhibition, pickling, cleaning with high pH solvent, neutralization with lime, etc.), but all of these methods will cause high cost and secondary environmental pollution from SO₂ emission.

The main component of waste zinc based adsorbent is ZnS and other metal sulfides. In recent years, the metal sulfide semiconductor has attracted much attention due to its excellent optical, electrical and magnetic properties. The CuS is one of the important semiconducting transition metal sulfides, which is not only a good semiconductor material, but also has excellent optical, electrical, and other physical and chemical properties, and herein has good prospects in the application of visible-light photocatalysts and high-energy electrode materials for lithium ion batteries, supercapacitors, etc.^23–25

Considering all these factors of excellent electrochemical and photocatalytic properties of CuS, and the significant solubility difference in aqueous solution between ZnS and CuS (K_SP of ZnS and CuS are 1.2 × 10⁻²³ and 8.5 × 10⁻⁴⁵ respectively), a new technology for transformation and regeneration of waste zinc based adsorbent was proposed in this paper, with the directly transforming of ZnS into CuS or other metal sulfides with lower solubilities than that of ZnS as the starting point (as shown in Fig. 1).

Fig. 1 Schematic diagram of the new technology for transformation and regeneration of waste zinc based adsorbent.

It is a novel green cycle for value-added utilization of waste zinc based adsorbent, which include:

(1) Waste zinc based adsorbent which mainly containing ZnS is put into aqueous metal salt solution and transformed into metal sulfide which solubility is much lower than ZnS and which can be used as electrode material for lithium-ion battery, supercapacitor or as photocatalyst.

(2) Zinc ion in the aqueous solution after filtration reacts with aqua ammonia for regenerating nano ZnO absorbent.

(3) The solution after regenerating can be evaporated to obtain chemical fertilizer such as ammonium sulfate.

Until now, studies for regeneration of waste zinc based adsorbent focus on the oxidation processes²⁶ in which ZnS will be changed into ZnO with the emission of SO₂. In such processes, high temperature (700 °C) will decrease the activity of ZnO and pollutant is produced. The advantages of the cycle we proposed are the value-added utilization of the waste and the totally regeneration of the adsorbent with zero emission in the whole process.

The waste zinc based absorbent was obtained from the Kailuan (Group) Limited Liability Corporation. Nano CuS was synthesized by cation exchange method from waste zinc based adsorbent.

The reaction conversion (x) was calculated as follow:

x = C/C₀

In which, C is the concentration of Cu²⁺ in solution which was determined by ULTIMA2 inductively coupled plasma emission spectrometer, C₀ is the initial concentration of Cu²⁺.

It was found that when CuSO₄ was used as the copper source, the initial atom ratio of Cu : Zn was 1 : 1 in the boiling solution (C₀ is 0.5 mol L⁻¹), the reaction conversion can reach as high as 97.41%.

The influences of reaction conditions such as copper source (such as Cu(NO₃)₂, CuSO₄, CuCl₂ and Cu(OAc)₂) were discussed. The component of the product was analysed by D/MAX2500PC X-ray diffractometer (XRD). The surface morphology was observed by S-4800 field scanning electron microscopy (SEM). It was found that Cu(NO₃)₂ was not stable during the experiment, which was easy to decompose. The solubility of Cu(OAc)₂ at room temperature was too low to formulate solution with a desired concentration. So the experimental results for these two copper salts were unacceptable. Fig. 2 was the XRD and SEM diagram for nano CuS synthesized using CuCl₂ or CuSO₄. There are two diffraction peaks which is the hexagon phase of CuS and wurtzite ZnS in Fig. 2a, respectively. When the copper salt was CuCl₂, almost all diffraction peaks can be indexed to the hexagon phase of CuS. However, when the copper salt is CuSO₄, Fig. 2b showed that the intensity of ZnS diffraction peaks was relatively strong. These results demonstrated a decrease in conversion rate. Fig. 2c and d showed the SEM images of the products obtained from CuCl₂ and CuSO₄, the morphology was nanolayered and nanosheet, respectively. The results showed that higher conversion and more uniform particle size and morphology of nano CuS can be obtained using CuCl₂ as copper source.

Fig. 2 XRD (a and b) and SEM (c and d) diagram for nano CuS synthesized using 0.5 mol L⁻¹ CuCl₂ (a and c) or CuSO₄ (b and d) aqueous solution at 100 °C for 6 h.

When the reaction conversion of CuS was close to 100%, the supernatant was used to prepare the nano ZnO based adsorbent. Precipitating agent (NH₃·H₂O) was added into the supernatant (mainly containing zinc ion), then precipitated precursor was obtained and sintered in muffle furnace to produce nano zinc oxide based adsorbent. Fig. 3 shows the XRD and SEM of nano ZnO (prepared from zinc ion) and the waste zinc based adsorbent. It can be seen from Fig. 3a and c that the waste zinc based adsorbent mainly composed of ZnS and hexagonal ZnO can be produced from zinc ion. The very little amount of Zn₅(OH)₈(NO₃)₂ shown as Fig. 3a may caused by the uncompleted decomposition of precursor. It can be seen from Fig. 3b and d that the size of ZnO was smaller than ZnS.

Fig. 3 XRD (a and c) and SEM (b and d) diagram for nano ZnO (prepared from zinc ion) (a and b) and the waste zinc based adsorbent (c and d).

In the study of electrochemical performance, we used acetylene as carbon source to coat nano CuS/ZnS from copper sulfate with carbon, which was used as the anode material of lithium ion battery. The cycle performance was tested by galvanostatic charge–discharge. The prepared nano CuS/ZnS from CuSO₄ coated with carbon was used as active material. Active material, acetylene black and PVDF in weight ratio of 75 : 15 : 15 was mixed and coated on copper foil. The coated foil was vacuum dried at 120 °C for 24 hours, then lithium-ion battery was assembled in a dry argon glove box. The electrolyte is 1 mol L⁻¹ LiPF₆/EC + DEC + DMC (volume ratio is 1 : 1 : 1), and the electrode membrane is polypropylene microporous membrane (Celgard2400). Galvanostatic charge–discharge was tested at the current density of 100 mA g⁻¹ and the voltage range is 0.01 V to 2.5 V. The results show that, the first discharge capacity is 802.7 mA h g⁻¹, the discharge capacity is kept at 454.3 mA h g⁻¹ after 38 cycles (as shown in Fig. 4), which show that it has excellent electrochemical properties.

Fig. 4 Cycle performance of nano CuS/ZnS@C used for anode of lithium-ion battery.

In conclusion, we proposed a new technique in this paper for transformation and regeneration of waste zinc based adsorbent. Nano copper sulfide was prepared by cation exchange method from waste desulfurizer. Zinc-based waste desulfurizer can be fully recovered and utilized with no secondary environmental pollution. The nano CuS/ZnS@C anode material for lithium ion batteries has excellent electrochemical properties. It can be also used in supercapacitors and the visible light photocatalytic treatment of organic waste water. It will provide a new way and method to realize the environmental friendly regeneration and value-added utilization of waste zinc based adsorbent.

Acknowledgements

This work received financial support from the Kailuan (Group) Limited Liability Corporation (Grant No. KL [2014]403), and the Science and Technology Department of Hebei Province (Grant No. 15213816).

Notes and references

1. A. D. Wiheeb, I. K. Shamsudin, M. A. Ahmad, M. N. Murat, J. Kim and M. R. Othaman, Rev. Chem. Eng., 2013, 29, 449–470.
2. J. Köchermann, J. Schneider, S. Matthischke and S. Rönsch, Fuel Process. Technol., 2015, 138, 37–41.
3. Y. Xiao, S. Wang, D. Wu and Q. Yuan, J. Hazard. Mater., 2008, 153, 1193–1200.
4. T. H. Ko, H. Chu and L. K. Chaung, Chemosphere, 2005, 58, 467–474.
5. H. L. Fan, T. Sun, Y. P. Zhao, J. Shangguan and J. Y. Lin, Environ. Sci. Technol., 2013, 47, 4859–4865.
6. C. Zuber, M. Husmann, H. Schroettner, C. Hochenauer and T. Kienberger, Fuel, 2015, 153, 143–153.
7. D. Liu, S. Chen, X. Fei, C. Huang and Y. Zhang, Ind. Eng. Chem. Res., 2015, 54, 3556–3562.
8. J. Wang, B. Liang and R. Parnas, Fuel, 2013, 107, 539–546.
9. L. R. Pahalagedara, A. S. Poyraz, W. Song, C. H. Kuo, M. N. Pahalagedara, Y. Meng and S. L. Suibet, Chem. Mater., 2014, 26, 6613–6621.
10. S. Zhao, L. Ling, B. Wang, R. Zhang, D. Li, Q. Wang and J. Wang, J. Phys. Chem., 2015, 119, 7678–7688.
11. M. Husmann, T. Kienberger, C. Zuber, W. Jong and C. Hochenauer, Ind. Eng. Chem. Res., 2015, 54, 5759–5768.
12. Z. B. Huang, B. S. Liu, F. Wang and R. Amin, Appl. Surf. Sci., 2015, 353, 1–10.
13. D. Liu, W. Zhou and J. Wu, Chem. Eng. J., 2016, 284, 862–871.
14. F. Yin, J. Yu, S. Gupta and S. Wang, Fuel Process. Technol., 2014, 117, 17–22.
15. B. Zeng, H. Yue, C. Liu, T. Huang, J. Li, B. Zhao, M. Zhang and B. Liang, Energy Fuels, 2015, 29, 1860–1867.
16. X. Zhang, X. Zheng, P. Han, Z. Liu and L. Chang, J. Energy Chem., 2015, 24, 291–298.
17. M. Wu, C. Hu, Y. Feng, H. Fan and J. Mi, Fuel, 2015, 146, 56–59.
18. D. A. Bhandari, N. Bessho and W. J. Koros, Chem. Eng. Sci., 2013, 94, 256–264.
19. J. Wang, F. Ju, L. Han, H. Qin, Y. Hu, L. Chang and W. Bao, Energy Fuels, 2014, 29, 488–495.
20. M. D. Dolan, A. Y. Ilyushechkin, K. G. McLennan and S. D. Sharma, Asia-Pac. J. Chem. Eng., 2012, 7, 1–13.
21. E. Sasaoka, M. Hatori and N. Sada, Ind. Eng. Chem. Res., 2000, 39, 3844–3848.
22. M. Flytzani-Stephanopoulos, M. Sakbodin and Z. Wang, Science, 2006, 312, 1508–1510.
23. Y. Wang, X. Zhang, P. Chen, H. Liao and S. Cheng, Electrochim. Acta, 2012, 80, 264–268.
24. H. Peng, G. Ma, J. Mu, K. Sun and Z. Lei, Mater. Lett., 2014, 122, 25–28.
25. K. A. Ann Marya, N. V. Unnikrishnana and R. Philip, Mater. Res. Bull., 2015, 70, 321–327.
26. C. Zuber, M. Husmann, H. Schroettner, C. Hochenauer and T. Kienberger, Fuel, 2015, 153, 143–153.

---