Biologically active carbons are highly porous biomass carbon materials, which are promising carbon absorbers in the field of ecological restoration, agriculture and environmental protection due to their specific surface area (2500–3000 m2/g) [1–3].
Activated carbon applications are divided into two categories: gas phase and liquid phase applications. The liquid phase of carbon has a pore diameter close to 3 nm or larger, while the carbon pore diameter in the gas phase is mainly in the range of 1-2.5 nm. Also, the speed of diffusion of molecules in the liquid phase is much slower than in the gas phase [4]. Refining sugar and corn, improvement of taste, color and smell of water, purifying urban sewage, removal of impurities in the food and beverage industry, use in pharmaceutical processes, electroplating industry are the main and important applications that are used in the liquid phase of activated carbon. Solvent recovery, catalyst, gas storage, energy storage in the capacitor, diesel or gasoline emission control, flue gas control, protection against hazardous atmospheric pollutants, etc. are also gas phase applications [5–8].
Activated carbon can be defined as a warped lattice of defective hexagonal carbon layers with many irregular atoms. These irregular atoms on the surface of active carbon play a key role in the discussion of surface chemistry [9]. The heterogeneous nature of activated carbon is due to the phenomena that occur during burning and activation on the surface. The heterogeneous character of activated carbon is determined by the activation method and the type of raw materials [10].
There are two physical and chemical activation processes to prepare activated carbon. In physical activation, some gases such as water vapor, carbon dioxide, oxygen or their mixture, boron vapor and phosphorus are used under annealing. In chemical activation, activating reagents such as potassium hydroxide, potassium carbonate, sodium carbonate, magnesium chloride and some acids such as phosphoric acid, sulfuric acid, aluminum chloride, sodium chloride and zinc chloride are used. Therefore, the activation method is necessary for carbon black porosity and increasing the surface area of activated carbon [11–12].
Agricultural waste such as coconut shell [13], almond [14], hazelnut [15], olive kernel [16], coffee grounds [17], pomegranate wood [18], palm kernel [19], and walnut shell [20] are materials are used for the synthesis of activated carbon due to their suitable carbon structure and cheapness.
Specific surface area, pore size and surface functional groups are the basic factors for the adsorption rate of activated carbons. The adsorption characteristics of activated carbons depend on raw materials and preparation conditions regarding their effectiveness in different applications. In physical activation, reactions occur between the activator agents and carbon, as a result of which the oxygen in the activator gases is burned and comes out of the carbon structure in the form of inactive parts such as carbon dioxide and carbon monoxide. Activation is done in two stages: in the first stage, the carbon is burned and the connected pores between the main carbon plates are opened, and then in the second stage, the surface of the crystalline plates is prepared for contact with the activating gases. The burning process in the crystal is not completely uniform because the speed of the reaction is different in different areas of the surface that are exposed to the activating gases. If the carbon burning process is not done correctly at the beginning of the work, new pores cannot be formed. Therefore, parts of the primary crystal structure that are non-uniform cause the formation of a series of pores and an almost uniform porous structure. With the deterioration of the walls of porous neighbors, which of course are smaller, the total porosity increases and also larger pores are formed [21].
Activation with steam and carbon dioxide usually occurs in the temperature range of 800 to 1000°C with a series of heterogeneous reactions. The activation temperature must be carefully selected and adjusted, since lower temperatures have a negative effect on the reaction kinetics due to endothermic reactions, the temperature must be high in this process. In the kinetic control zone, a reaction occurs on the inner surface of the carbon, which removes the carbon from the pores of the walls and increases the expansion and enlargement of the pores. But at very high temperatures, reactions occur that are controlled, and in this situation, the reactions occur on the outer surface of the carbon particles. When oxygen or air is used as an activating gas, due to their high activity, the reaction with carbon becomes very fast, which results in uncontrolled combustion. This combustion causes losses on the carbon surface and creates a large amount of surface oxides. Due to these problems, activation with steam and carbon dioxide is more preferable than activation with oxygen and air [22, 23]. The exothermic reactions that occur during activation with steam, carbon dioxide and oxygen are summarized in the form of reactions (1) to (5):
H2O + Cx → CO + H2 + Cx−1 (1)
2 H2O + Cx → CO2 + 2 H2 + Cx-1 (2)
CO2 + Cx → 2 CO + Cx−1 (3)
O2 + Cx → 2 CO + Cx−2 (4)
O2 + Cx → CO2 + Cx−1 (5)
On the other hand, the adsorption power of physical activated carbon is determined by the chemical nature of porous carbon and the concentration of oxidizing gases. The amount of carbon activation in this process depends on the amount and type of minerals in the primary raw material and coal [24]. Also, the uncontrolled increase of combustible process followed by the development of uncontrolled pores has an effect on the quality of activated carbon and reduces its performance in production. In addition to the fact that the two processes of carbonization and activation are carried out at very high temperatures, it consumes a high cost in terms of energy.
Activation is done in the temperature range of 550°C to 1200°C and in the presence of gases such as CO2 and water vapor. In this method, at the beginning, carbon materials are converted to gas with steam, which is known as the water-gas reaction:
C + O2 → CO2 + 94 Kcal (6)
The physical activation process based on water vapor is as follows:
C + H2O → CO + H2 ∆H = 29 kcal/mol (7)
CO + H2O → CO2 + H2 ∆H = -10 kcal/mol (8)
At the same time, the required amount of air enters the reaction so that the coal does not burn. Activated carbons with this method usually have small pores. In the gas method, because the reaction temperature is high, heavy hydrocarbons or other compounds formed during the carbonization process fill the pores of the carbon and become volatile and are swept away with the incoming gases. As a result, its contact surface increases significantly. Steam activation is more important from an economic point of view regardless of operational considerations.
Unlike physical activation, chemical activation at lower temperatures can provide highly porous products. Chemicals remaining in porous carbon can be removed by effective washing [25]. In chemical activation, phosphoric acid is one of the most common activating agents that can combine with organic substances in biological waste and form phosphate and polyphosphate bridges. In addition, some of these phosphate groups remain on the carbon surface after washing. In activation with phosphoric acid, pyrolysis is often performed at a temperature of 400°C to 700°C [26].
According to the studies of Li et al. [27], the activation mechanism of the raw material using H3PO4 can be done during steps (9) to (14) in three different temperature ranges:
in the range of 100°C to 400°C:
2H3PO4 → H4P2O7 + H2O (9)
3H3PO4 → H5P3O10 + 2H2O (10)
nH3PO4 → Hn+2PnO3n+1 + (n-1) H2O (11)
in the range of 400°C to 700°C:
Hn+2PnO3n+1 → P4O10 + H2O (12)
P4O10 + 2C → P4O6 + 2 CO2 (13)
in the range of 700°C to 800°C:
P4O10/ P4O6 + CHx → PH3 + CO2/CO (14)
All these processes occur in the pyrolysis stage of organic compound. During activation, carbon dioxide is usually preferred over steam because of its low reactivity (ΔH > 0), which allows control of oxidation rates to produce uniform porosity.
In physical adsorption, the adsorbed substances are held on the adsorbent surface by weak van der Waals forces and electrostatic forces that exist between the adsorbed species and the adsorbent surface. While in chemical absorption, the forces are not weak and absorption occurs only on the active parts of the absorbent surface. Chemical absorption includes a series of electron exchanges that create chemical bonds on certain parts.
A comparison between the electronic energy and the interaction between these two types of adsorptions has been made, which is obvious that the interaction energy in chemical adsorption is stronger than the interaction energy in physical adsorption. Most of the discriminating properties between physical and chemical absorption are related to the formation of single or multiple layers in the adsorbent, activation energy, reversibility and the important feature of absorption relative to the surface, these are the main differences between physical and chemical absorption [28].
Absorption capacity and absorption kinetics are directly related to the porous structure of the absorbent, and the porous structure of the absorbent is affected by the raw materials and its production methods [29]. Many studies have been done on the synthesis method and different properties of activated carbon.
Zhang et al. [30] developed an H3PO4 activated hydrochar derived from peanut hull and showed that chemical modification greatly increases the adsorption capacity of hydrochar compared to acetone and cyclohexane. Pam et al. [31] used palm kernel shell chemically to synthesize activated carbon and investigated the effect of process variables (ratio of H3PO4 to PKS) and carbonization temperature on microstructure performance, morphology, pore structure and adsorption properties for methylene blue and iodine. Haghbin et al. [32] prepared highly-porous activated carbon from date palm bark by chemical activation method with phosphoric acid and after characterization confirmed that date palm bark can be considered a cheap and promising precursor for the commercial preparation of activated carbon. Rugayah et al [33] produced granular activated carbon from palm kernel shell using carbonization and steam activation systems on a commercial scale. They determined that palm kernel shell is a suitable material for the production of activated charcoal due to its low ash content, but high carbon and volatile content. Hidayu et al [34] prepared activated carbon from palm kernel shell by chemical activation to absorb β-carotene in crude palm oil and showed that activation of palm kernel shell carbon using zinc chloride (ZnCl2) with mass saturation ratio 1:1 for 24 hours and carbonization at 500°C for 90 minutes shows the highest percentage of β-carotene removal (69%). Boulika et al [35] synthesized activated carbon from almond shell using H3PO4 as a chemical activator and room vacuum decomposition as a physical activator. They studied the optimum adsorption efficiency of crystal violet dye on the produced activated carbon. Mohan et al [36] prepared magnetic and non-magnetic low-cost activated carbons from almond shells, characterized them, and used them to remove 2,4,6-trinitrophenol from water. Osobamiro et al [37] synthesized activated carbons from almond and groundnut shells by chemical activation using phosphoric acid and zinc chloride. The results of the physicochemical properties of their produced activated carbons have shown that the activated carbon produced can be used as an adsorbent in the removal of organic pollutants in wastewater.
These cases are only a small number of reports on the synthesis of activated carbon using almond shell and palm kernel with physical and chemical activation methods with phosphoric acid.
Due to the easier synthesis method and compatibility with the environment, methods of synthesis, physical and chemical characterization and electrochemical properties of activated carbons are still considered for mass production. Tuning or engineering the energy gap of the synthesized activated carbon is of particular importance to investigate the physical and electrochemical properties; Because one of the effective factors in regulating the energy gap can be the activation method or the use of different precursors [38]. Therefore, in this study, after the synthesis of activated carbon using almond shell and palm kernel by physical and chemical activation methods with H3PO4, the effect of the type of activation (physical and chemical method) on the structural and optical properties of the synthesized activated carbon are compared and investigated.