MODIFICATION OF CARBONACEOUS ADSORBENTS WITH MANGANESE COMPOUNDS

Abstract. Four series of samples containing manganese supported carbonaceous adsorbents were prepared. Samples of series AC-B, synthesised from the carbonaceous support with basic surface, were obtained with a yield of 50-60%, while manganese was loaded in amount of 1.44-1.65 % depending on applied method. The samples of series AC-A, synthesised from the carbonaceous support with acidic surface, were obtained with a higher yield (9298%), but with small quantities of loaded manganese. Obtained results reveal the importance of surface chemistry of carbonaceous adsorbents on the manganese loading.


Introduction
It is well known that modifi cation of carbonaceous adsorbents with metal oxides improves their physicochemical characteristics, infl uencing the catalytic activity in redox reactions.A number of catalysts consisted of various transition metal oxides (Co, Ni, Mn, Fe, Cu etc.) on active carbons have been studied for removal/oxidation of hydrogen sulphide, ammonium ions, dyes etc. [1][2][3][4][5].Among these catalysts, manganese oxides supported on active carbons have attracted much interest due to their high catalytic activities.
Various active manganese state and dispersion of particles can be obtained using different precursors and preparation methods [2,6,7].Different quantities of loaded manganese on active carbon have been obtained depending on initial concentration of the precursor solution [7].Thermal treatment (autoclave or furnace, under oxic or anoxic conditions) also are important parameters exploited by researchers [2,6].However, any reports about the dependence between the surface chemistry of carbonaceous adsorbents and amount of loaded manganese are not presented in the literature.
The aim of this work was to highlight the infl uence of surface chemistry of carbonaceous adsorbents on the manganese oxides loading.

Materials
In this study a commercially available activated carbon with basic surface (Norit®), designated as AC-B, and a sample of active carbon with acidic surface obtained by chemical activation method [8], designated as AC-A, have been used.All the chemical reagents used in this study were of analytical grade.

Samples
Four series of samples containing manganese were prepared, supported on carbonaceous adsorbents (particle size 0.8-1.3mm).The carbonaceous adsorbents were impregnated with an aqueous solution of manganese salt (at a solid/liquid ratio equal to 10) for ca.24 h, followed by treatment with an alkaline solution to generate manganese hydroxide within carbon pores.Dried samples (110ºC) were subjected to thermal treatment at different temperatures (300, 450, 600ºC) to obtain manganese oxides.Obtained samples were washed several times with distilled water to remove soluble species and dried at 110ºC.

Characterization methods
Prior characterization measurements the active carbon samples were dried at 110ºC for 3 h.Elemental analysis (C, H, N, Cl, S) was carried out by the Elemental Analysis group of the Institute of Chemistry of Academy of Sciences of Moldova.
The content of metals was determined by atomic absorption spectroscopy (AAS-1N, Laboratory of Atomic Spectroscopy of the Institute of Chemistry of A.S.M.).
The ash content of the samples was determined by burning off the carbon at 700ºC for 2 h.
The pH of the samples has been evaluated by determination of pH value of active carbon suspension (10 g of dried sample/100 mL of distilled water) equilibrated for 24 h [9].
Thermal analysis measurements were performed using a Derivatograph Q-1000 analyzer.The samples were heated from room temperature up to 1000°C in a fl owing air atmosphere (100 mL/min) at a heating rate of 10°C/min.

Results and discussion
Active carbons used for impregnation with manganese ions are very different concerning surface chemistry and ash content.Sample AC-B has higher content of ash due to compounds of calcium, magnesium and iron, and has a basic surface pH (9.5) (Table 1).Sample AC-A has only traces of ash (0.29%, Table 1) and an acidic surface (pH 4.0).The initial samples, as well as the modifi ed by impregnation with manganese salts were subjected to thermal analysis in order to choose the treatment temperatures.Initial sample AC-B is quite stable till around 450 °C and then the sample buns out (Figure 1).TGA curves of the modifi ed samples (AC-B-0, AC-B-A) show an initial weight loss of about 10-14% around 100 ºC, which is related to thermo-desorption of physically adsorbed water.For series of AC-B samples, impregnation with manganese salts leads to the decomposition of the carbonaceous support.Above 325 ºC TGA curves show a drastic decrease in weight of these samples (Figure 1).
Thermo-gravimetric curves (TGA) of AC-A series samples are presented in Figure 2. All samples show generally similar thermal behaviour: weight losses of ca.7-15 % around 100 ºC, most probably due to thermo-desorption of physically adsorbed water; above ca.450 ºC the burning of the carbon skeleton occurs.
Summarizing, the temperatures of 300, 450 and 600ºC have been chosen for thermal treatment of impregnated samples.After thermal treatment obtained samples were washed.Thermal treated samples of series AC-B were obtained with a yield of 50-60% and manganese was loaded in amount of 1.44-1.65 % depending on applied method (Table 2).The samples of series AC-A were obtained with a higher yield (92-98 %), but with small quantities of loaded manganese, and don not contain chloride ions (Table 3).Present work highlights the infl uence of surface chemistry of carbonaceous adsorbents on the manganese oxides loading.Following researches will be focused on the determination of the oxidation state of the supported manganese phases Mn x O y , dispersion of the manganese oxide particles, and evaluation of physical-chemical characteristics of modifi ed adsorbents.

Conclusions
Obtained results reveal the importance of surface chemistry of carbonaceous adsorbents on the manganese loading.Samples of series AC-B, synthesised from the carbonaceous support with basic surface, were obtained with a yield of 50-60% while manganese was loaded in amount of 1.44-1.65%depending on applied method.The samples of series AC-A, synthesised from the carbonaceous support with acidic surface, were obtained with a higher yield (92-98%), but with small quantities of loaded manganese.