Characteristics and Properties of Coals Obtained from Rice Husk: Production, Physicochemical Properties, Denitration


Preparation of activated carbons

The detailed method of preparation for the composite is described in [111],

In brief, the initial AC, designated as CRH-I, was obtained from rice husk by carbonization at 700°C in inert atmosphere followed by chemical activation with potassium carbonate at 950°C. In order to form oxygen and nitrogen-containing groups on the surface, CRH-I sample was first ozonated at 120-124°C (24 h) and then heat treated in the atmosphere of ammonia at 350°C giving AC product CRH-II. In an alternative modification route, CRH-I sample was first oxidized with concentrated nitric acid, washed until neutral pH and dried, then soaked in urea solution, dried and heat treated in inert atmosphere at 950° to produce AC material CRH-Ш. All samples were washed with hot distilled water until neutral pH was reached.

Activated carbons characterization

For CRH characterization, we used the following techniques.

Scanning electron microscopy is a microscopy imaging technique to provide high-resolution images of sample structure. The particle morphology of the above-mentioned activated carbons was acquired by using field emission (FE-SEM, Zeiss Sigma at high vacuum and 5.00 kV EHT] equipped with an energy- dispersive X-ray spectroscopy (EDX] detector to confirm the elemental composition of samples.

Low-temperature nitrogen adsorption analysis was used to supply quantitative data of the material porosity and adsorptive properties. BET surface area (SBET] values were calculated from adsorption data. The pore volume and size distribution were calculated using the QS density functional theory (QSDFT] for slit/cylindrical pores [112, 113]. The total pore volume was evaluated by summation of microporous (Kmi) and mesoporous (Уте) volumes. The mean pore diameter, DP, was calculated from DP = 4 VT/S [114], where VT is the total volume of pores.

Differential scanning calorimetry (DSC] is a technique for measuring the energy required to obtain a near zero temperature difference between a test sample and an inert reference material, which are both subjected to an identical heating, cooling or constant temperature cycle. The results were obtained using DSC (Perkin Elmer 6000 simultaneous thermal analyzer] in heating range of 50-500°C at a controlled heating rate of 10°C/min.

Thermogravimetric analysis (TGA] is a method of thermal analysis in which changes in physical and chemical properties of carbon materials can be measured as a function of increasing temperature (with constant heating rate], or as a function of time (with constant temperature and/or constant mass loss]. The thermal analysis was performed using a TGA analyzer Q500 (ТА Instruments] coupled with mass spectrometer (TGA-MS]; measurements were carried out on 5 mg of sample at a heating rate of 10°C/min in a nitrogen atmosphere and a gas flow rate of 120 mL/min; weight losses and corresponding temperatures were recorded and analyzed.

XPS analyses were performed to obtain a deeper understanding of the degree of chemical modification of the CRH surface. In X-ray photoelectron spectroscopy (XPS), the energy and number of electrons released from the surface of a sample owing to X-ray irradiation is measured. This information is used to determine the chemical state of elements in the uppermost layer of the material and the semi-quantitative elemental composition of the layer to the depth of 10 nm. X-ray photoelectron spectroscopy was performed using an ESCALAB 250 Xi system (Thermo Scientific) with a monochromated X-ray source of A1 Кос X-rays (1486.6 eV), a hemispherical electron energy analyzer, and a magnetic lens and a video camera for viewing the analysis position. Wide scans (step size 1 eV, pass energy 200 eV) and narrow scans (step size 0.1 eV, pass energy 20 eV) of the Nls (binding energy, BE -399 eV), Cls (BE -285 eV), Ols (BE-531 eV), and Si 2p (BE -100 eV) regions were acquired from three separate areas for each carbonaceous sample. Data were transmission function corrected and analyzed using Thermo Avantage Software (Version 5.952) using a smart background. Survey scan data were acquired from three areas on each samples and standard deviation were calculated from these data.

Acid-base (Boehm) titration has been proposed as one of the simplest methods for the characterization of surface properties that can be used to identify surface functional groups of different carbons. Surface functionality on the AC samples was assessed via modified Boehm titration procedure [115-118] with pH meter Orion 410A (Thermo Scientific) equipped with a Glass Combination Electrode (BDH) and a titration step of 0.1 titrant volume. In each case 0.100 g of activated carbon was shaken with 20 mb solution of 0.025 M NaOH or 0.05 M HC1 in 0.1 M NaCl overnight. Then extracts were back-titrated with appropriate acid/base to determine the surface functionality, analysis was performed in triplicates for each sample. The total number of carboxylic, phenolic and lactone groups are neutralized by sodium hydroxide. The number of basic sites is calculated from the amount of HC1 required in the titration. Values were expressed as milliequivalents per gram of sample. To analyze the adsorption capacity of synthesized samples, an analytical grade of sodium nitrate (NaN03, Sigma-Aldrich) and DI water were used to prepare stock solutions of the nitrate anion (NO)). Nitrate anion uptake was studied by a batch technique at room temperature (25±0.5°C). A known quantity of sample (0.05 g) was added to 50 ml aqueous solutions (5 or 15 ppm) and shaken continuously (at 100 rpm, for 24 h).

Ion chromatography method was selected for determination of dissolved ions in water. Device Ion Chromatography system, Dionex ICS-1100 (Thermo Fisher Scientific) was employed to determine nitrate concentration; and a standard ion chromatography method for analysis of inorganic anions in water and wastewater [119] was selected for the analysis. Prior to the analysis, all eluents were filtered through the regenerated cellulose syringe filter (0.2 pm) to prevent a chromatographic column clogging. Each experiment was triplicated under identical conditions. The adsorption amount at time (t), qt(mg/g), was calculated using the following equation:

where qt (mg/g) was the adsorbent capacity, C0 and Ct (mg/L) were the initial and equilibrium NO) concentrations in the aqueous solution; F(L) was the volume of the experimental solution and m(g) was the mass of dry adsorbent used.

Iodine value is a relative indicator of the porosity of activated carbons [120]. It does not necessarily correlate with the carbon adsorption capacity with respect to other adsorbates. First, to determine the concentration of the iodine solution, the blank titration was performed. The conical flask with 15 (25) mL of aqueous solution of iodine (0.05N) was titrated with 0.1N solution of sodium thiosulfate until disappearance of the brown color of the iodine solution. The starch was added at this point as an indicator, and the titration was continued slowly until disappearance of the blue color (neutralization point), in order not to over titrate. At the end of the experiment, the concentration of the iodine solution was calculated by the following equation:

0.3 g of each of the activated carbons (CRH) were placed in a beaker with 30 mL of 0.05N aqueous solution of iodine. These solutions were stirred on the magnetic stirrer in light protected conditions. After 1.5-2 h, the unreacted iodine was titrated with sodium thiosulfate solution as in the case with blank titration. The adsorptive capacities of the activated carbons were determined by Eq. (12.1).

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