Activated Carbon from Paper Waste as Potential Adsorbents for Methylene Blue and Hexavalent Chromium  
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the most widely used dyes in the world, and around 700,000 tons are produced worldwide every year. Approximately 10-15% is unintentionally lost during use [8]. Chromium (Cr) is mainly found in anthropogenic and natural ecosystems. Due to its unique properties, it is known for its use in various industries, e.g., pyrite ores, the semiconductor industry, mining and smelting, the combustion of coal and wood preservatives, the production of stainless steel, the textile and leather industries, and electroplating and refining of metals [9]. It has oxidation states of +6 to +2, but in water, the common states are Cr(VI) and Cr(III). Small amounts of Cr(III) dissolve in water with a weak migration ability. Cr(VI) species depend on the pH value and its concentration in aqueous solutions. At a pH above 7, chromium exists in water as chromate anion (CrO4\({}^{-}\)); hydrochromatic anions (HCrO4\({}^{-}\)) are more favorable under weakly acidic conditions, and chromic acid (H2 CrO4) dominates under strongly acidic conditions. The dichromate anion species (Cr2O7\({}^{2-}\)) can be observed when the concentration of Cr(VI) is above 1 g/L in pH ranges of 1-6 [10]. Trace amounts of Cr(III) are essential for the human body [11]. However, Cr(VI) is carcinogenic, highly toxic, and not essential for all living organisms [12]. The content of Cr(VI) ions in industrial wastewater can reach 270 mg/L [13]. The information above shows that MB and Cr(VI) should be removed from water bodies since they have negative effects on human health. Various advanced methods such as adsorption, reduction, precipitation, biological, chemical oxidation, photodegradation, chemical degradation, coagulation, and phytoremediation have been used for the removal of pollutants [14]. Adsorption appears to be the preferred method [15]. Many different adsorbents such as layered double hydroxides, zeolites, polymers, fly ash, biomass, graphene oxide, activated carbon, municipal waste, agricultural waste, and industrial waste have been described for their application in the removal of contaminants from water bodies [16]. Activated carbon is by far more advantageous compared to other adsorbents due to its large surface area, pore volume, ease of construction, and high efficiency. However, commercially available activated carbons are expensive due to their high costs for swelling, regeneration, and reactivation processes [17]. Recently, studies have been conducted to produce low-cost and high-efficiency activated carbon using materials with a high carbon content from agricultural, municipal, and industrial waste [18]. To achieve a high capacity to remove pollutants, the properties of the adsorbent such as surface morphology, surface area, functional groups, active sites, a wide range of porosities, and pore volume are usually enhanced [19]. Paper has an annual production of over 300 million tons worldwide and is one of the largest municipal wastes, consisting of newspapers, old magazines, printed paper, and mixed office paper [20]. Wastepaper is a complex material and contains cellulose fibers (containing functional polar groups, such as alcohol and ether) and inorganic fillers, such as kaolin, clay, and calcium carbonate [21]. Paper waste is biocompatible and biodegradable, is produced in large quantities, and is, therefore, readily available [22]. The cellulose-rich nature of paper waste makes it a good potential resource for the production of carbon material for the removal of MB and Cr(VI). The properties of adsorbents that are typically carbon are enhanced by the chemical activation method. The adsorbent is reacted with an oxidizing agent, acid, base, inorganic salt, and organic solvent.