Abstract
Spent activated carbon (SAC) usually exhibits a low specific surface area due to its high ash contents. In this study, pre-treatments, such as heat and acid treatments, were optimized to improve this feature. The heat pre-treatment did not reduce the ash content, nor did it increase the surface area. Because metallic ions adsorbed in SACs turn into ash upon the heat treatment. In the acid pre-treatment, the volatiles and fixed carbon were increased with decreasing ash contents. In this study, it was found that the surface area increase was correlated with the ratio between fixed carbon and ash. Among the pre-treatment methods, the combined heat and acid pre-treatment method highly increased the ratio, and therefore led to the surface area increase. Additionally, the acid pre-treatment was carried out using different types of acid (organic and inorganic acids) solutions to further improve the surface areas. The organic acid treatment caused a significant structural collapse compared to the inorganic acid treatment, decreasing the surface area. In particular, H3PO4 effectively removed ashes adsorbed on the activated carbon surface and regenerated the exhausted activated carbon. Both the heat and acid pre-treatments before chemical activation resulted in the positive effects such as strong desorption of pollutants and ashes within the internal structure of the activated carbon. Therefore, the regeneration introduced in this study is methodically the best method to regenerate SAC and maintain a stable structure.
Highlights
Drinking water guidelines of the World Health Organization (WHO), which are followed by South Korea [1], define permissible levels of contaminants in water
The Spent activated carbon (SAC) were obtained from a wastewater treatment plant at Seoul in South Korea
The SACs were prepared by four types of pathways, named Case 1, 2, 3, and 4, and heat treatment, acid treatment, and chemical activation were investigated
Summary
Drinking water guidelines of the World Health Organization (WHO), which are followed by South Korea [1], define permissible levels of contaminants in water. Adsorption is one of the most effective methods to remove contaminants from wastewater due to its simple operation, low cost, and high adsorption efficiency. Activated carbon (AC) is commonly used as an adsorbent in water and wastewater treatment facilities because it can adsorb a wide variety of organic and inorganic contaminants due to its porous nature, surface area, and abundant surface functional groups [2,3,4]. Consumption of ACs for industrial use has become an indicator of development and environmental management efficiency. ACs are generally prepared using natural resources, such as wood, coal, nutshells, and petroleum residues [5,6,7]. The price of precursors, especially woods, has recently increased due to restricted logging, which commonly causes environmental impacts. The waste resources were accessed, but Molecules 2020, 25, 4561; doi:10.3390/molecules25194561 www.mdpi.com/journal/molecules
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