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Biographie Due to its versatility and wide range of applications, activated carbon is widely used as contaminant removal media. Recent research have focused on enhancing the effectiveness of activated carbon by modifying their specific properties in order to enable the carbon to develop affinity for certain contaminants. In view of this, a comprehensive list of literatures on chemical, physical and biological modification techniques of activated carbon pertaining to enhancement of contaminant removal from aqueous solutions was compiled and reviewed. Acidic treatment to introduce acidic functional groups onto surface of coal based activated carbon was by far, the most studied technique. It was apparent from the literature survey that the beneficial effects of specific modification techniques on activated carbon adsorption of targeted contaminant species from aqueous solutions were profound, with some studies reported increase of contaminant uptake factors of more than 2. Concurrently, considerable decreases associated with certain contaminant uptakes can also occur depending on the technique used.
Without pure water, it is impossible to survive for any living beings. The ratio of freshwater on our planet is very poor and the demand is increasing with time for the growing population. Furthermore, water is being contaminated by industrial and agricultural activities, pharmaceuticals, technocratic civilization, pesticides, garments, global changes etc. In addition to this, environmental pollution and global warming are swelling due to the greenhouse and harmful gases generated from the dumping and burning of fossil fuel. Addressing these problems, it is necessary to find out the cost-effective and environmental friendly processes to purify the contaminated water and air. Activated carbons (ACs) are one of the best solutions for removing the pollutants from aqueous and atmosphere as it is the carbonaceous materials with a high degree of porosity, well-developed surface area, and distinguished functional groups which are required for elimination of contaminants. The preparations of activated carbon are easy and safe processes, mainly from the pyrolysis or gasification of biomass with heat and/or chemicals. The recycling and regeneration of bituminous coal based activated carbon after use are also essential for resource maintenance and environmental safety. Thus, AC can protect the ecosystem in a double direction by purifying the water and air from the pollutants.

As the world’s population continues to grow, the demand for water increasing by 1% annually (WWAP/UN-Water, 2018). The population increase corresponds to the economic growth and development leading to rise in the use of chemical compounds, industrial compounds, agricultural wastes which pose the risk of polluting the existing water sources (Mohammad Razi, Al-Gheethi, Al-Qaini, and Yousef, 2018; Oladipo, Ifebajo, Nisar, and Ajayi, 2017; Qaisrani, Shams, Zhenren, Reza, and Zainuddin, 2018). It has reported that 663 million people have lacked access to clean water and 2.2 million deaths occurring due to diarrhea caused by inadequate sanitation and ingestion of contaminated potable water (Huang et al., 2018). The river, lake, and groundwater sources are contaminated due to fertilizers, pesticides, antibiotics, dyes, heavy metals from the industry which results in diseases like cancer, skin defect, kidney damage, liver problems, etc. (Ahmad and Danish, 2018). On the other hand, the burning of fossil fuel generates the greenhouse (GHG) and harmful gases like CO2, CH4, H2S, NO2 to the environment which are increasing with current energy demand for the rapid economic development (Ahmed, Abu Bakar, Azad, Sukri, and Mahlia, 2018a). Therefore, it is highly desirable to find the cheap and environmentally friendly adsorbent to get rid of the pollutants from water and air.

Of the above techniques, adsorption is one of the most popular methods used for the elimination of contaminants from wastewater due to its features of cost-effective, easy to operate, environment-friendly, low health risk, and non-destructive process (Balasundram et al., 2017; Oladipo and Gazi, 2015; Oladipo and Ifebajo, 2018). The addition of activated carbon (AC) in the adsorption process aids in the removal of a wide range of contaminants and carcinogenic compounds such as pharmaceuticals, metallic and non-metallic pollutants, dye and even taste and odor from aqueous solutions (Din, Ashraf, & Intisar, 2017). In comparison with other adsorbents like zeolite, clays, and polymers, AC shows better performance and stability in terms of adsorption (Regti, Laamari, Stiriba, and El Haddad, 2017). Recently, the pollutant gas adsorption by activated carbon has been recognized as a promising technology for the attraction mechanism between the pollutant and the surface functional groups (Le-Minh, Sivret, Shammay, and Stuetz, 2018). Production of AC from biomass can be seen as advantageous in two ways, firstly, it can prevent the production of CO2 by fixing the carbon and secondly, the AC can go into the soil naturally (Danish and Ahmad, 2018).

AC is carbonaceous material with an amorphous structure solid which has a high degree of porosity and well-developed surface area with numerous oxygenated functional groups such as carboxylic acids, phenols, carbonyls and lactones (Benedetti, Patuzzi, and Baratieri, 2018). The pores present on the AC surface are of significant importance and they exist in three forms: microspores, mesopores, and macropores (Jodeh, Abdelwahab, Jaradat, Warad, and Jodeh, 2016). Except for macropores that contribute the least contribution, the other pores contribute to the increase of the surface area and their presence on carbons is vital as they are the main source of driving the adsorption of gases and. In addition, functional groups also help promote adsorption capabilities (Elsayed et al., 2017). These distinctive characteristics make AC a versatile substance material that can be used not only as an adsorbent in water and gases but also as a catalyst or co-catalyst for the removal of pollutants from gases, liquids as well as the recovery of chemicals (Afif et al., 2019). Most studies have shown satisfactory results of ACs, specifically the effectiveness of removing organic compounds like dye, phenolic compounds, and inorganic compounds. They are also used in the field of the pharmaceutical industry to get rid of ingested toxins from the human body, recovery of gold, silver, and other metals (Vences-Alvarez et al., 2017). They are used as catalysts and co-catalyst in the gas mask filter company, food industries, chemical industries and automobile pollution control companies (Danish and Ahmad, 2018).

Usually, ACs are produced from finite resources such as coal, lignite, peat, petroleum residue materials that are expensive and required intensive regeneration (Yahya et al., 2018). It is reported that the demand for AC is estimated to increase by up to 2.1 million metric tons by the year 2018 (Maneerung et al., 2016). The high demand and the necessities have led to the discovery of cheaper, environmental-friendly and sustainable resources for the production of AC like biochar from thermal treatment of biomass. The primary sources of AC are agricultural waste, sewage, municipal waste, industrial waste, forestry residue, acacia species, Imperata cylindrica(Ahmed et al., 2018a, Ahmed, Parvaz, Johari, and Rafat, 2018b; Danish and Ahmad, 2018 ). Acacia species are known as the most invasive and they have adverse effects on other trees, and thus, threatens the biodiversity seriously (Hidayat, Abu Bakar, Yang, Phusunti, and Bridgwater, 2018; Islam, Mohamad, and Azad, 2019). A total of 1350 different types of acacia trees are known to exist in the world, which can grow in any climate (Ahmed, Hidayat, et al., 2018c; Radenahmad, Rahman, Morni, and Azad, 2018; Reza et al., 2019 ). Improvements in biofuel technology have developed the new technique to manage these invasive acacia trees to convert them into biochar (Alhinai, Azad, Bakar, and Phusunti, 2018) which can produce wood based activated carbon.

For activated carbon, biochar is the primary source which can be produced from the thermochemical conversion of biomass like pyrolysis or gasification (Odetoye, Abu Bakar, and Titiloye, 2019; Radenahmad et al., 2020). Through slow pyrolysis, a higher amount of biochar can be produced from various types of precursors, typically biomass as a feedstock under a range of processes and operating conditions (Reza et al., 2020). Pyrolysis/gasification is the process to generate biochar, bio-oil, and biogas from biomass which has different properties than the parent biomass (Abu Bakar, 2013; Odetoye, Onifade, AbuBakar, and Titiloye, 2013). Bio-oil can be used as energy after refinement (Odetoye, Afolabi, Abu Bakar, and Titiloye, 2018). Biogas is the combination of different gases that can be used as fuel in a fuel cell (Abdalla et al., 2018; Afif et al., 2016; Afroze et al., 2020b, 2020a, 2019; Radenahmad et al., 2016). Finally, the solid residue is the biochar which can be converted into activated carbon (Morni et al., 2018). Using the same precursor, different types of ACs can be produced by activating agents under different operating conditions (Yusuff, 2019). Activated carbon can be produced by physical, chemical, physiochemical and microwave-assisted activation of biochar and biomass. Physical activation consists of heat and gas (steam, CO2, N2 or mixture), chemical activation is done by chemical agents (acid, base, metal oxide, alkaline metal), physiochemical activation involves heat and chemical and microwave-assisted activation is by microwave radiation (Ao et al., 2018). The activation depends on particle size, retention time, impregnation ratio, procedure configuration, activation period, properties of the precursor, and chemical substances. To further improve the features of AC, researchers have been performing research studies for producing much better AC via loading nanoparticles on the surface of this material which may be used in the removal of organic and inorganic compounds from aqueous solutions more efficiently (Lakshmi et al., 2018).

The present paper reviews the efficiency of different types of activation processes of activated carbon from biomass. The adsorption capacity of contaminants and pollutants from water and air has been described elaborately to get pure water with clean air. The recycling/regeneration and handling process of Saw Dust Activated Carbon is also illustrated.

2. Preparation of activated carbon

For the preparations of shell activated carbon, two basic steps are maintained. The first one is the carbonization and the second one is the activation. Carbonization is done through the pyrolysis/gasification at a higher temperature in an inert atmosphere to produce the biochar (Odetoye et al., 2019). In this stage, the carbon content of the carbonaceous substance was prepared by removing the volatile matter through thermal degradation (Radenahmad et al., 2020). The temperature, the heating rate, the nitrogen gas flow rate and the residence time are the significant parameters in this stage. As the achieved biochar reveals low adsorption ability, an activation process is essential to improve the pore volume, the pore diameter and the surface area (X. Yang et al., 2019). In the activation process, initially, the disorganized carbon was eliminated, exposed the lignin to the activating agents and developed the microporous structure. Finally, the existing pores are widened to a large size by burning of the walls between the pores. This raises the intermediate pores and macro-porosity which reduces the volume of micro-pores. Depending on the type of activation, activation can be a process prior to carbonization or subsequent to carbonization for the elimination of deposited tarry substances in biochar that can help to enhance the porosity and to provide high surface areas for the ACs (Ukanwa et al., 2019). 
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