Dyes are colorants that chemical structures predefine the impart color with some degree of permanence [1-2]. Synthetic organic dyes consist the largest portion of coloring substances and have attracted increasing interest over the last decade due to their indispensable role in printing, dyeing, battery manufacturing and textile industries [3-4]. Malachite green (MG) is a representative triphenylmethane cationic dye that can exist as dye salt, carbinol or pseudobase . Due to its high soluble and stable characteristics in water, there is a large body of experimental evidence showing teratogenic, mutagenic and carcinogenic effects on human cells [6-7]. Moreover, the wastewater containing MG is considered to be one of the most challenging industrial wastewaters for purification . Therefore, seeking an effective separation route has always been a longstanding objective of chemists. Several separation methods including ozonation, coagulation-flocculation, ion-exchange, photodegradation and advanced oxidation processes have been extensively studied to address the practical separation of MG. The aforementioned methods are yet hindered by complex manufacturing processes, high operational costs and additional harmful by-products . Besides these approaches, adsorption is one widely adopted method for the purification of dye-containing industrial wastewater .
A number of carbonaceous materials such as activated carbon (AC), activated carbon fiber (ACF), graphene, graphene oxide and reduced graphene oxide are gaining immense interest owing to their porous structure and high adsorption capacity. The presence of ample adsorption sites and negatively charged oxygen-containing functional groups (-OH, -COOH, C-O/C=O) in carbonaceous materials may promote a favorable adsorption environment of cationic dyes [11-13]. ACs are known as porous carbonaceous material having irregularly shaped particles in powder or granular forms . ACs have suitable features for the aimed application, however, adsorbate molecules have to pass macropores and mesopores first before entering micropores. ACFs are introduced to overcome deficiencies of ACs, since adsorbent molecules can be directly reached to micropores. It is worth underlining that ACF is also a low-cost and easy-collectible adsorbent compared to graphene-based adsorbents. Despite all the advantages of ACFs, their use has been limited due to relatively low adsorption capacity. Hence, surface modification is of paramount importance for ACF-based adsorbents to compete with advanced carbonaceous materials. With untiring efforts, researchers have succeeded in preparing ACFs under strong acids to increase the number of oxygen-containing functional groups [15-16]. Although thus-prepared ACFs have experienced an inevitable decrease in surface areas, increased oxygen-containing functional groups have resulted in high adsorption capacity of heavy metals via the ion-exchange mechanism. Nevertheless, introducing strong acids may be insufficient in this case, because the MG adsorption capacity can be hampered by an acidic environment .
Here, we report an industrially applicable MG adsorption technique using CO2-ACFs. We highlight that the primary culprit of global warming is an essential driving force for achieving high surface characteristics of ACFs, verified by experimental evidence and theoretic calculations. A fairly low oxygen-containing functional groups were introduced during CO2 activation. A significant advantage here of using CO2 is in the greater adsorption capacity of MG without any surface decrease, which is not possible with pre-defined surface modification techniques. The pore-growth phenomenon is further demonstrated via CO2-surface reaction. With these features in hand, CO2-ACF absorbents may lead to new opportunities in wastewater separation of synthetic organic dyes with cationic molecules.