Effect of surface modification of activated carbon by cold plasma on the adsorption capacity of Rhodamine B

The large amounts of the dyestuffs used in industries are toxic and whenever are released into wastewaters can lead to environmental and health problems. One of the most commonly used dyes in industries is Rhodamine B. It is widely used in printing, leather, paper and textile industries and is harmful to both human beings and animals. Thus, it is necessary to treat contaminated wastewater prior to discharging into water streams. Among all conventional physical, chemical and biological methods, adsorption is a very simple and effective method for removing dyes from wastewater even at low concentration. Activated carbons (ACs) are among the most widely used efficient absorbents which can remove these pollutants from aqueous phase and cause to prevent damages to the environments. Specific surface area, pore volume, pore-size distribution and the nature of surface are the main factors that influence AC performance. The surface modifications result in the change in the surface reactivity, chemical, physical and structural properties. Based on the type and chemical nature of the dyes, the surface of AC can be modified to enhance the affinity toward the desired contaminants. Depending on the application, there are different methods to modify AC, which can be classified as chemical, physical and biological modification. Plasma treatment is an environmental friendly method, which can introduce functional groups onto material surfaces without changing the bulk properties of the substrate.

Modification of AC surfaces by means of plasma is a well-known method to enhance its adsorption capacity. Hence, in this research AC was prepared using chemical activation method from sound (SBAC) and rotted (RBAC) beech wood (Fagus orientalis) as precursors. The impregnation was carried out by ZnCl2 with the ratio of 75 percent to precursor mass and carbonization was performed at activating temperature of 400 ºC.  The potential of AC treatment by plasma was then examined to introduce oxygen-containing groups onto AC surfaces and to enhance its efficiency to remove a basic dye, Rhodamine B, from aqueous solution. After carbonization step, surface modification of activated carbons was then conducted by dielectric barrier discharge (DBD) plasma in a parallel- plate reactor under different conditions of voltages (2, 4, and 6 kv) and treatment time(20, 30, and 40 min). Specific surface area, average pore diameter and pore volume of ACs were characterized from the results obtained by N2 adsorption at 77 K. In order to evaluate structure changes in plasma modified-ACs the FT-IR spectroscopy and scanning electron microscopy (SEM) were also used. Then, the capability of modified-ACs in removing of Rhodamine B was assessed and was compared with untreated ones. The batch adsorption tests have been carried out in the laboratory by contacting a 100 ml of dye aqueous solution with 0.1g of AC. The adsorption capacity of Rhodamine B in aqueous solution was measured using UV-Vis spectrometer at 555 nm.

The results indicated that the iodine number of ACs prepared from sound wood (SBAC) was less than RBAC that indicate more development of micropores in RBAC. Activated carbons from sound wood exhibited more specific surface area (1538 m2g-1), total pore volume (0.649 cm3g-1) and less average diameter (1.69 nm) in comparison with RBAC. As it was expected, chemical activation of precursors by ZnCl2 could successfully develop micropores through dehydrating reactions. It was found that adsorption capacity of untreated ACs prepared from rotted beech wood was higher in comparison with sound wood ACs. This may be attributed to differences in chemical compositions of precursors, which results in different pore structures at the same preparing conditions. It was also revealed that the adsorption capacity of ACs for removal of Rhodamine B can be improved by the surface modification of ACs using plasma treatment. Although, voltage changes showed no significant influence on ACs adsorption capacity, but maximum gain of 20% obtained at 2 kv with plasma treatment time of 2 min. Destruction of micropores walls and probable blocking of pores due to developing new functional groups may be a main cause of adsorption capacity loss in ACs, which  modified by high voltage plasma. Raising heat of plasma because of increasing treatment time up to 30 min impaired dye removal by plasma modified ACs. The results of FTIR spectroscopy showed that oxygen-containing groups on ACs surfaces increased due to cold plasma treatment and changed slightly in acidic nature. A few bands were observed at around 1250, 1570, 1750 and 3440 cm-1 for each spectrum which indicated qualitatively that these carbon surfaces contains a mass of phenols, carboxylic anhydrides and a small quantity of carboxylic acids. This confirms the role of surface acid oxygen containing groups in contributing adsorption of Rhodamine B from aqueous medium. The enrichment of surfaces with hydroxyl groups results in improved interaction between Rhodamine B and adsorbent surfaces via electrostatic attraction and Van der Waals forces.  In addition, a comparison of spectra of treated ACs with unmodified AC showed that use of cold plasma in presence of air to modify the carbon surface could not generate new types of surface chemical groups. It is proposed that adsorption of Rhodamine B on plasma-modified ACs is based on physisorption.  The results of analyzing SEM images showed significant degradation of wood microstructure during impregnation with ZnCl2 as well as carbonization process under N2 inert atmosphere. However, it was observed from SEM images that morphology of ACs remained unchanged during plasma treatment. Although, in some cases the deposits effect of plasma caused to smooth surface of ACs sample.

It is concluded from this research that both rotted and sound beech wood as a lignocellulosic materials has good potential in preparing ACs with great performance using ZnCl2 as activation agent. The results demonstrated that any variations in structure and compositions of lignocellulosic precursors with the same sources due to biological degradation result in different adsorption behavior of activated carbons. It was also found that cold plasma treatment under optimum conditions of voltage and treatment time could be a good surface modification method to enhance the adsorption capacity of ACs without any significant morphological changes to remove dyes such as Rhodamine B from aqueous phase.