Research on Powdered Activated Carbon Modification Using Chosen Chemical Methods ^†

Abstract

Activated carbon has many applications in the environment, cosmetology, medicine and industry. The surface of each activated carbon can be modified to obtain the desired adsorption properties. Chemical activation can greatly affect the adsorption efficiency, control activity and application of the activated carbon. The aim of the study was to modify the selected activated carbon by chemical and physical methods, while maintaining these parameters so that it could be used in medicine. Powdered activated carbons with higher mechanical strength, large specific surface area and large macro-, meso- and micropore volume were prepared using natural waste wood material. This was followed by the digestion process and the washing of activated carbon. The study results indicate a significant influence of the centrifugal washing of activated carbon on the changes in the ash content and methylene index in the final product.

1. Introduction

The use of activated carbons (AC) both in scientific research and in the design and optimisation of many industrial processes is related to the need for their continuous improvement in quality, as well as the development of production methods [1,2]. Activated carbon has many applications in the environment, cosmetology, medicine and industry. It is used for separation, recovery, total removal or modification of various compounds from water, wastewater, air, medical or cosmetic solutions, body fluids, etc. The surface of each activated carbon can be modified to obtain the desired adsorption properties. Different activation methods can be applied including chemical, physical and thermal activation. Impregnation or washing of the carbon can also be used. Chemical activation can greatly affect the adsorption efficiency, control activity and application of the activated carbon. It is the formation of a special coating of specific cation or anion groups on the surface of an activated carbon grain under the influence of a selected medium. Chemical activation can be carried out in either aqueous solutions and/or gaseous media. Substances used for chemical modification of activated carbons in aqueous washing can be: nitric acid (V), hydrogen peroxide, solution of concentrated nitric acid (V) and sulphuric acid (VI). Among activators of an oxidising nature, chlorine water, sodium chlorate (I), dichromate (VI), potassium, a mixture of chlorate (VII) and fuming nitric acid (V) and solutions of potassium permanganate and ammonium persulphate can be used [3]. Chemical activation in a gaseous environment is carried out with oxygen from air, ozone, water vapour, carbon dioxide, and NOx [3,4]. Physical activation, on the other hand, involves the action of a temperature factor on compounds present on the surface of activated carbon. The functional groups covering the carbon surface decompose with release of CO₂ (acidic coatings) or CO (basic coatings) [4]. There are also other ways to enrich the surface of porous carbon materials. Their impregnation with various inorganic or organic compounds can be used. These can be oils, waxes, or large-molecule compounds. The properties of carbon can also be improved by chemical binding of various ligands, deposition of nanoparticles or colloids of metals and their oxides [5].

Activation of the activated carbon surface is particularly important for carbons used as medical adsorbents. AC has long been used as a therapeutic agent. In medicine, its applications are almost unlimited; from internal application as an oral adsorbent (in tablet or powder form) to out-of-body applications, e.g., in haemoperfusion columns. Powdered activated carbon can improve the therapeutic effect of wound dressing materials, dental fillings or antiseptic fluids [6]. ACs are effective in eliminating toxins from the blood. Due to its unique properties, AC can be used in the treatment of internal health problems such as liver and kidney failure, sepsis and rheumatic disease [7,8].

In view of the above, the aim of the study was to refine the selected activated carbon by chemical and physical methods, while maintaining these parameters so that it could be used in medicine.

2. Material and Methods

A new generation of powder activated carbons with higher mechanical strength, large specific surface area and large macro-, meso- and micropore volume has been prepared using natural waste wood material. The chemical activation process with CH₃COOH and washing of activated carbon were used in carbon production. The average sample of activated carbon before chemical activation had the following properties: Methylene Number (MNo) 41 mL, Ash Content (A) 4.88%, pH 11.7, Iodine Index 141.08 g/100 g of carbon, Phenolic Index 30.24%.

The chemical activation, then the washing and drying process, were carried out using raw activated carbon, condensate solution and acetic acid. The initial concentration of used solution was 1.74%. The condensate solution had a pH of 7.1. After approximately 20 h of chemical activation, the modified carbon pulp suspension was subjected to centrifugation, followed by neutralisation with condensate and finally the drying process. During the chemical activation and washing process, activated carbon samples were taken to determine the Methylene Number (MNo), Ash Content (A), Extinction (E), pH of the aqueous extract and concentration of CH₃COOH after washing.

3. Research Results and Discussion

Samples of primarily activated (in high temperature) and washed pulvered carbons were further chemically activated with 0.5% acetic acid solution and then dried, after which they were tested for Methylene Number (MNo), Extinction (E), pH and Ash Content (A).

Table 1. Properties of modified AC.

Sample	A [%]	pH	MNo	E	Concentration CH3COOH after Washing [%]
Carb/1.5	1.12	6.8	38	0.075	0.003
Carb/3	1.25	6.7	38	0.08	0.0031
Carb/9	1.41	6.7	39	0.08	0.0035

summarises the results of the measurements taken for the chemically activated pulvered carbons. The individual samples were labelled as follows:

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carb/1.5—activated carbon etched with 1.5% CH₃COOH solution

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carb/3—active carbon etched with 3% CH₃COOH solution

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carb/9—activated carbon etched with 9% CH₃COOH solution.

The reached results indicate a significant influence of the centrifugal washing of activated carbon on the changes in the ash content and methylene blue index in the final product. Laboratory tests on the chemical activation of studied carbon using acetic acid solutions with concentrations of 0.5%, 1.5%, 3.0% and 9.0% show that the concentration of CH₃COOH solution in the studied concentration range does not affect the formation of the methylene number of activated carbon in the medical type after digestion and washing.

4. Conclusions

• The presented data show the methylene index to be 2–5 mL lower after demineralisation than in the activated carbon before chemical treatment with acetic acid.
• In all samples of acetic acid-treated activated carbons, lower methylene index was found than in the original corresponding samples before chemical treatment.
• The decrease in MB of activated carbon after demineralisation is due to the different colouration of methylene blue depending on the environment, with the colouration of dilute methylene blue solutions being more intense in acidic environments than in alkaline environments.