Synthesis of AC from OFA consists of two steps. In the first step, 10 g of as received raw FA (Rabigh power plant, Saudi Arabia) was mixed with 200 mL of an acid mixture comprises of 20% H2SO4 , 40% HNO3 , and 40% H3PO4 (Panreac, Spain) in a round bottom flask. The resultant mixture was constantly heated for 4 h under total reflux conditions.