Activated carbon production from plum pit shells for oily wastewater treatment

Activated carbon is an adsorbent with high porosity (60–75%) and a high specific surface area (500–2,200 m²/g) (Heidarinejad et al. 2020). It is widely used in technologies for environmental protection, industries that use liquid-gas recovery and separation processes. Various parameters are usually considered to optimize the AC production, including the temperature, steam to carbon dioxide ratio, and steam-gas mixture pressure.

The utilization of the renewable and low-cost precursors of activated carbon production has attracted the interest of researchers worldwide to reduce the cost of the production (Shimada et al. 2000). Therefore, many activated carbon studies have been conducted on agricultural waste, including walnut shells, olive pits, coconut shells, acorn shells, pistachios shells, cotton stalks, seeds of Elaeagnus angustifolia, almond shells, date palm seeds, rice hulls, pecan shells, kenaf, hazelnuts and rice straw. The use of various materials and technologies of chemical and thermal activation allowed to obtain activated carbons with different characteristics and properties.

Activation of the walnut shell at a temperature of 900 °C in a CO₂ flow and a duration of 90 min allowed a maximum specific surface area of 1,011 m²/g and a pore volume of 0.65 cm³/g to be obtained. In this case, a homogeneous structure of micropores was obtained, where the ratio V_micro/V is more than 80% (Wu et al. 2018). Tsai et al. (2019) prepared activated carbon from cocoa husks at various activation temperatures from 600 to 900 °C and times from 0 to 120 min, at which the surface area increases from 800 to 1,400 m²/g. Wickramaarachchi et al. (2021) obtained activated carbon from Mango Seed using KOH solution. The obtained activated carbon has a high specific surface area of 1,943 m²/g, a total pore volume of 0.397 cm³/g. SEM images show that activated carbon has a developed microporous structure. In activated carbon obtained from charcoals after short-term thermal oxidation with air, the surface area of charcoal/biochar increased significantly up to 80 times (Xiao et al. 2020). Activated carbon chemical activation with zinc chloride followed by carbonization was obtained from the shell of the Sterculia alata nut (Mohanty et al. 2006). The temperature (700–800 °С) and time (60–90 min) of carbonization had a significant effect on the specific surface area of 712 m²/g and the micropore volume of 0.542 cm³/g. In another study (Wahby et al. 2011), activated carbon was obtained from olive pits at carbonization temperatures ranging from 400 to 850 °C, followed by thermal activation in a flow of CO₂. The obtained activated carbon had developed micropores with diameters of less than 0.56 nm.

Furnace designs that use different heating modes and carbon transfer mechanisms play an important role in producing high-quality charcoal, so potential users face the challenge of determining which furnace is the best choice for a particular application.

In the process of obtaining activated carbon, a rotating reactor with steam-gas activation was used, and a coconut shell was used as the raw material (Zhao & Chen 2020). The proposed design of the reactor led to friction between particles and anisotropy of particles, which increased the efficiency of mixing the shell in the furnace. In another study (Zhao et al. 2019), a hot rod reactor was used for oxidative pyrolysis. The process of pyrolysis of rice straw and the formation of a developed porous structure were influenced by the temperature and oxygen concentration. The known furnace used for heat treatment of carbon-containing materials (Zhukov et al. 1998) contained a body with refractory insulation. It contained a cylindrical retort and heating elements. The stirring device made in the form of plates was evenly spaced along the length of the retort, and the angle of their inclination was 45–60°. The disadvantage of this method is the amount of energy consumption of the process of manufacturing activated carbons through direct contact with the flame of a gas burner. The retort of the furnace is carbonized and often fails, so it requires replacement or repair. There is a furnace for processing carbonaceous materials (Mukhin et al. 2002) that contains a body with fireproof insulation. It has a cylindrical retort with a stirring device inside. The heating elements are gas burners, and the furnace chamber is equipped with a branch pipe for removing heating gases. The disadvantage of this furnace is its low productivity, which does not meet the modern requirements for heat treatment of carbon-containing materials.

Analysis of the current state of technologies and methods for producing activated carbon has made it possible to identify shortcomings of the existing technologies and methods for producing activated carbons from various raw materials; such shortcomings include multistage processes, high energy costs, the use of various toxic, high-cost chemical activators that have corrosive effects on equipment and the complexity of the technological process.

In factories that produce edible oils and fruit canned products, the plum pit shell is a large-tonnage waste. Widespread use in South Kazakhstan, low cost and high adsorption properties, as well as new methods of regulating the porous structure, suggest their economic feasibility as an activated carbon source in South Kazakhstan, for the provision of raw materials, a number of enterprises engaged in fruit processing, such as Food Factory Blago LLP, Capital LLP, GoidenerPfeii LLP, Murat MC, MBI MC, Altyn Alma LLP, Shymkent N.N. LLP, Saryagash-Aman LLP, Business-Shymkent LLP, Nur-Tilek LLP, SIPAN Company LLP, Asia Duran LLP, Alma LLP, Nur-Amir LLP, GlobusPlus LLP. At these companies, waste from pits exceeds a thousand tons per season. The developed technology will make it possible to utilize the resulting waste of fruit processing (Zhenisbekovna et al. 2016).

In this regard, the purpose of the study was to develop an activation mode of the plum pit shells for producing activated carbon with high adsorption capacity, to experimentally substantiate the furnace design and identify patterns in the adsorption of pollutants from a water stream using the developed activated carbon.

Fruit farms are widespread in the Central Asian region (Zhenisbekovna et al. 2016). The plum pit shell is a large-tonnage waste of oil plants that process the raw pits in order to obtain pharmacopoeial edible oil from the pit kernels at InnovTechProduct LLP. For further research, we selected the plum pit shell, which is one of the most common and porous pits.

On the technological line of InnovTechProduct LLP, on the equipment, a pit is knocked out of a plum, then the pit is crushed and divided into two components: the pit kernel and the pit shell. Further, the pit shell enters the thermal activation.

To solve the problems posed, the research methods were chosen based on previous studies.

Analytical equipment was used; a specific surface analyzer (TriStar 3000, Micromeritics, SorbiN.4.1) was used in this work. Elemental analysis was carried out on a spectrometer (VARIAN 820-MS) according to the methodical instructions ‘Methods for determining the content of chemical elements in biological samples on a mass spectrometer with inductively coupled plasma VARIAN-820MS’. Photographs of the porous structure were obtained on a raster electron microscope (JSM-6490LV) according to the methodical instructions ‘Studies of the microstructure of the initial and final products in standard and low vacuum modes on a raster electron microscope JSM-6490LV’. To study the effect of thermal activation processes on the structural-sorption and physicochemical characteristics of the inner peach shell, classical and modern physicochemical research methods (Torosyan 2010) were used to obtain the full characteristics of the objects of study, which can be used to judge the conformational state of molecules, properties and the presence of various functional groups and the adsorption properties of the adsorbent.

To carry out the research, a furnace for the heat treatment of carbon-containing materials was developed. The gas supply and volume were controlled by the gas supply system: the gas pressure was controlled to 1.5–2 kPa. The temperature in the retort was regulated by a gas burner at 7 m³/h, and that in the steam generator and in the activation, the oven was regulated by a gas burner at 10 m³/h. In the retort, the temperature was determined by a thermocouple to be 1,300 °C. Superheated steam was generated in a steam generator at a pressure of 0.2–0.4 MPa. The retort was loaded with 200 kg of the original plum pit shells. The output of activated carbon after activation was 30% – 60 kg.

The method for producing activated carbon is as follows. The shells of plum pits are carbonized at a temperature of 700 °C in an inert gas atmosphere in a carbonization furnace. The plum pit shells are placed in the retort of the activation furnace. Then, an activating agent, a mixture of carbon dioxide and water steam in a ratio of 90:10, is fed into the activation furnace. The process temperature is maintained at approximately 700 °C, and the duration of the process is 1 h.

The problem is solved by the fact that the furnace for the heat treatment of carbon-containing materials (Yessenbek et al. 2020) contains a body with refractory thermal insulation made of heat-resistant kaolin fiber, a cylindrical retort with a stirring device located inside it, heating elements made from gas burners, devices for loading and unloading and branch pipes for the introduction of gaseous reagents. In this invention, the gas burner is located in the front sliding cover in front of the steam sprayer, while additional heating of the furnace is not required, and its operation can begin immediately after loading. The branch pipe for the outlet of gaseous reactants is connected to the exhaust gas afterburner, and the supplied steam is additionally superheated in the exhaust gas afterburner.

The furnace developed for the heat treatment of carbon-containing materials includes a body with refractory thermal insulation from heat-resistant kaolin fiber as well as a cylindrical retort located inside it with a stirring device.

The furnace for the heat treatment of carbonaceous materials operates as follows. A heating gas is supplied to the exhaust gas afterburner 8 through auxiliary burner 9. Then, steam with an initial temperature of 150 °C is supplied through the branch pipe to this device and passed through a spiral cylindrical spring 10, where it is heated to 700 °C and fed to steam injector 6, where CO₂ is used to obtain a steam-gas mixture. The flame of the heating gas from main burner 7, passing through steam injector 6, heats the steam-gas mixture to a temperature of 700 °C. Then, the steam-gas mixture is fed into rotating drum 1. With the help of electric motor 11 and rotation device 12, drum 1 is set in rotation on fixed support 13. Furthermore, the carbon-containing raw material is fed through inlet pipe 3 to drum 1 and is activated by the steam-gas mixture supplied from steam injector 6. After the activation process, the resulting activated carbon is discharged through rear sliding chamber 4 and discharge pipe 5. The off-gases of the steam-gas mixture from drum 1 are discharged through the off-gas afterburner 8. The front 2 and rear 4 cameras can be removed to gain access to the inside of drum 1 for cleaning and maintenance. In the off-gas afterburner 8, the off-gases of the steam-gas mixture are thermally destroyed and utilized. As a result, there is no need for additional bulky gas-cleaning equipment to protect the environment. The steam heated to 600 °C from exhaust gas afterburner 8 is fed into steam injector 6, where due to the direct flame of main burner 7, the steam-gas mixture can be heated to 900 °C without the need for additional heating of the steam-gas activation furnace. The resulting activated carbon is discharged from the activation furnace and crushed to the required fraction.

For the developed drum-type steam-gas activation furnace (Figure 1), a method for producing activated carbon was developed, aimed at increasing the volume of sorbing pores, increasing the adsorption capacity and decreasing the load on technological equipment (Satayev et al. 2021). The method for producing activated carbon is characterized by low economic costs and high environmental and technological indicators due to the absence of aggressive chemical reagents in the activation zone that have a corrosive effect on equipment and increase the cost of the material obtained. Additionally, the activated carbon obtained due to the use of water steam has a higher oxidizing capacity; when it is activated with carbon dioxide, has a high volume of sorbing pores and high adsorption activity with respect to compounds dissolved in water. Table 1 shows the characteristics of the obtained activated carbon.

Thus, the presented experimental results indicate the effectiveness of heat treatment of the plum pit shell, which has a large volume of micropores and a moderately developed transitional porosity that ensures the intensity of diffusion of the adsorbate into the adsorbent grain.

From the table and figures, it can be seen that in comparison with the known activated carbon in the proposed volume of micropores is higher and equal to 0.65 cm³/g, and the volume of mesopores is much less and equal to 0.15 cm³/g. To purify water, the maximum volume of micropores and the minimum volume of mesopores and macropores need to be developed.

The characteristics of the activated carbon formed from the plum pit shells (proposed) and comparisons with known adsorbents: A2PS-1, KAU-1, SCN-4 (Skubiszewska-Zieba et al. 2011), walnut shells (ZK) (Anurov et al. 2011), plum shells (PC) (Anurov et al. 2011), A2PS-5, KAU-2, SCN-5 (Skubiszewska-Zieba et al. 2011), apricot shells (Strelko et al. 1996), and walnut shells (Zabihi et al. 2010). It is seen that the known activated carbons obtained by chemical activation of ZnCl₂ have low values of the total pore volume: (Carbon A) (Zabihi et al. 2010) - 0.426 cm³/g; (Carbon B) (Zabihi et al. 2010) 0.387 cm³/g. When activated with water steam at temperatures from 250 to 400 °C, it leads to a slight increase in micropores: SCN-5 (Skubiszewska-Zieba et al. 2011) (temperature 250 °C) – 0.185 cm³/g; Plum kernels (A2PS-5) (Skubiszewska-Zieba et al. 2011) (temperature 250 °C) – 0.44 cm³/g; (A2PS-1) (Skubiszewska-Zieba et al. 2011) (temperature 250 °C) – 0.508 cm³/g; KAU-2 (Skubiszewska-Zieba et al. 2011) (temperature 350 °C) – 0.28 cm³/g; SCN-4 (Skubiszewska-Zieba et al. 2011) (temperature 350 °C) – 0.157 cm³/g; KAU-1 (Skubiszewska-Zieba et al. 2011) (temperature 350 °C) – 0.33 cm³/g. Activation at higher temperatures up to 800 °C in an inert gas flow also leads to the insignificant formation of micropores: (PK) (Anurov et al. 2011) (temperature 800 °C, nitrogen) – 0.20 cm³/g; Apricot kernels (Strelko et al. 1996) (temperature 850 °C, NaOH) – 0.42 cm³/g. The proposed activated carbon has a maximum micropore volume: plum pit shells (temperature 780 °C, CO₂ – 20%, H₂O – 80%) – 0.65 cm³/g.

To determine the sorption capacity of the activated plum pit shells, oil products were taken as the research object, which are one of the main substances that pollute wastewater and inland water bodies (Rozhkova1 et al. 2021).

The method was developed for the production of activated carbon from the plum pit shells using the mixture of carbon dioxide and water as an activating agent in the ratio of 80:20 at the activation temperature of 700 °C and the process duration of 1 h.

The volumes of macro, meso and micropores of the obtained activated carbon were determined depending on the activation temperature.

The experimental research on the adsorption of hexane- and chloroform-extracted oil products by the activated plum pit shells confirmed the possibility of increasing the oil products’ adsorption by 1.5 times. The value of adsorption of petroleum products depending on the dose of activated carbon and the equilibrium concentration of petroleum products is found.

The developed technology for obtaining the activated carbon from the plum pit shells was tested in the developed new drum-type steam-gas activation furnace and implemented in InnovTechProduct LLP.

The plum pit shell is a multi-tonnage waste product of canning factories, and in this regard, a preliminary analysis of the physicochemical properties and steam-gas activation of the adsorption capacity of activated carbon and a decrease in its cost is relevant.

All relevant data are included in the article.

The authors declare there is no conflict.