A salt and alkali synergy for synthesising active carbons from lignin: porosity development and techno-economic assessment

Abstract

Active carbons (AC) are the most common and efficient adsorbents. Alkali activation (using NaOH or KOH) was found to be a promising method for AC production since it allows gaining large surface area, high yield and controlled pore size. However, the method is not widely used due to high alkali consumption in the production process. To obtain active carbon with a developed porosity at a high yield and a reduced alkali amount, a new method was developed utilizing lignin. A salt (NaCl or KCl) and an alkali (NaOH or KOH) were used simultaneously to facilitate a decrease alkali consumption. To understand the activation mechanism, the roles of salt and alkali were methodically disclosed. The parameters that facilitate the highest surface area, the largest yield and controlled porosity at the minimized alkali amount were defined by the response surface method (RSM). To demonstrate the benefits of the method, a techno-economic assessment (TEA) was accomplished. The produced active carbon with a high surface area was compared to commercial carbon to show the benefit of natural organic matter (NOM) removal in the water treatment process.

This method using a lignin-KOH-KCl mixture allows producing superactive carbon (SAC) with a high surface area of >2900 m2/g at a yield of >28%. Furthermore, the consumption of KOH was significantly reduced to 1 g/g compared to 3–4 g/g in the previous methods. In addition, KOH as well as KCl were recovered after use. The template mechanism shows that macropores are the same size as salt crystals and alkali grains. Microporosity originated from the elemental potassium formation with its subsequent intercalation between graphene layers. Moreover, the simultaneous use of salt and alkali promotes the synergy between them, which further facilitates the reduced amount of alkali. By altering the alkali amount, temperature and time, the size of pores can be altered. The higher the values of the parameters, the more developed the porosity. Based on TEA, the minimum selling price of the SAC produced by this method is 2.93– 4.36 kEUR/t. Also, there are appreciable process savings of 23–55% for NOM removal by SAC in the water treatment process compared to commercial AC.