Step 5 – Refining the themes

Arunrat and Sukjit (2014) reported on the preparation of activated carbon from Thailand rice husk. The aim of the research was to make value-added activated carbons of rice husk and to study the optimum conditions for gasoline adsorption using the rice husk activated carbon as adsorbents. The 20 g of rice husk samples were carbonized at 200 and 400 °C for 1 hr in a muffle furnace in order to produce charcoal. The sample was

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crushed with blender and sieved to a size smaller than 850 μm to obtain the charcoal of rice husk (CRH). The rice husk was subjected to impregnation in 1 dm³ of 3 M H₃PO₄ at 80 °C for 3 hr. After that, it was washed with distilled water until its pH value was 7. The sample was dried at 100 °C for 24 hr. The dried samples were carbonized at 450 and 700

°C for 2 hr in a muffle furnace. The charcoal was crushed and sieved to a size smaller than 850 μm to obtain the activated carbon of rice husk (ACRH). The activated carbon produced from rice husk at a temperature of 450 °C, has the highest adsorption capacity.

According to gasoline adsorption study, the optimum conditions were 0.1 g of activated carbon, 70 °C of adsorption temperature and 30 minutes of adsorption time. Physical characterization of the activated carbon obtained was performed by scanning electron microscopy (SEM). BET surface area was determined. The results present that the activated carbon of rice husk possesses a high apparent surface area (SBET = 336.35 m2/g). They thus encourage the use of activated carbon of rice husk as an adsorbent for the qualitative analysis of gasoline in order to apply for gasoline sampling in the arson case and to reduce the analysis cost from commercial adsorbent.

Esra (2015) reported on the production of low-cost activated carbon from rice husks by chemical activation using zinc chloride (ZnCl2) at 700 °C in N₂ atmosphere. Rice husk was milled to a particle size of around 1 mm. It was then washed thoroughly with distilled water and then dried in air at 100 °C in an oven for 24 h. 150 g of this air dried biomass was mixed in a beaker with varying concentrations of ZnCl₂ solution, which resulted in impregnation ratios of 1:1, 1:2, 1:3, 1:4 (weight of biomass to be impregnated / weight of reagent). The slurry was then dried in an oven at 105 °C. The impregnated sample was pyrolyzed in a stainless steel fixed bed reactor. The system was heated at a rate of 7 °C/min to 700 °C, and held at that temperature for 2 h. The reactor was continuously purged with nitrogen at a flow rate of 10 mL/min. After pyrolysis, the furnace was cooled down to room temperature in a nitrogen atmosphere. The sample was boiled with 200 mL of 10% HCl solution for 1 h, filtered and washed with hot water and finally cold water to remove the chloride ions and other inorganic contaminants.

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Activated carbon samples were dried at 110 °C for 24 h and weighed. They were then characterized using Scanning electron microscopy (SEM) photos, X-ray diffractometer (XRD), Brunauer-Emmett-Teller (BET) surface analysis and Fourier transform infrared spectra (FTIR) methods. The BET surface area of the highest yield activated carbon was found to be 922.319 m²/g. Results showed that carbonization impregnation ratio hasa significant effect on the surface area and pore structure of the prepared activated carbon.

The whole process can be summarized in the following steps; activation of the rice husk, carbonization, washing, preparing the RHC/Zn mixtures and characterization.

Hanum et al, (2017) evaluated the effects of carbonization time and temperature on activated carbon production from rice husk and its application for lead (Pb) adsorption in car battery wastewater. Rice husks were repeatedly washed using distilled water to remove existing impurities on the surface and dried in an oven at 110 °C for 3 hours.

After that, 25 g dried rice husk was carbonized in the furnace at 400, 450, 500, 550, and 600 °C for 90, 120, and 150 minutes. Then, it was impregnated with hydrochloric acid 5% (v/v) at carbon to acid ratio of 1:10 (w/v) for 24 hours. Afterwards, it was filtered and oven dried at 110°C for 3 hours, followed by sieving to 100 meshes. The result indicated that the maximum carbon yield of 49.33% was obtained at carbonization temperature of 500 °C and carbonization time of 150 minutes. The activated carbon contained 4.86%

moisture, 30.04% ash, and 15.76% volatile matter. The adsorption capacity was found to be 0.56731mg/g with percentage removal of 54.85%.

Korobochkin et al, (2016) reported on the production of activated carbon from rice husk from Delta of the Red River in Vietnam. At the first stage, carbonization of a rice husk was carried out to obtain material containing 43.1% carbon and 25 % silica with a specific surface area of 51.5 m²/g. The process of carbonization of a rice husk was carried out in the Flow Reactor with a volume capacity of 500 cm3 at 600 °C. Aprioristic information about the process was obtained by Differential Thermal Analysis. After separating of silica (the second stage), the specific surface area of the product increased to 204 m2/g and the silica content decreased to 1.23% by weight as well. The most

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important stage in the formation of the porous structure of the material is the activation.

The products with the high specific surface area in the range of 800-1345 m2/g were obtained by activation of carbonized product with water vapour or carbon dioxide at temperatures of 700 °C and 850 °C, with varying the flow rate of the activating agent and activation time. The best results were achieved by activation of carbon material with water vapour at the flow rate of 0.08 dm3/min per 500 g of material and the temperature of 850 °C.

Khu et al, (2014) reported on the production of activated carbon derived from rice husk by NaOH activation and its application in supercapacitor. Firstly, the rice husks were washed with water to remove dirt and other contaminants, oven-dried at 110 ^oC for 12h, grounded and sieved to fractions with an average particle size of 1.0 mm. Then, the prepared husks were carbonized at 4001C under nitrogen flow (300 mL min1) for 90 min. The resulting samples were impregnated with NaOH (weight ratio 1/3) and dried at 120 ^oC for 12 h. Heating at 4001C for 20 min under nitrogen atmosphere at a flow rate of 300 mL/min was followed; thereafter the temperature was raised to the pre-determined temperatures (650–8001C) at a heating rate of 101 ^oC and maintained at the final temperature for 60 min to activate the obtained material. Finally, the activated product was grounded, neutralized by 0.1 M HCl solution and washed several times with hot distilled water to a constant pH (6.6–7.0). The washed activated carbon samples were dried under vacuum at 1201C for 24 h and stored in a desiccator. The specific surface area of the AC sample reached 2681 m²/g under activation temperature of 8001C. The AC samples were then tested as electrode material; the specific capacitance of the as-prepared activated carbon electrode was found to be 172.3 F/g using cyclic voltammetry at a scan rate of 5 mV s1and 198.4 F/g at current density 1000 mA/g in the charge/discharge mode.

Williams and Jennifer (2016) carried out a research that was aimed at the development of activated Carbons from Corn Cobs and the assessment of their efficiency for removing heavy metals like lead Pb, Cadmium Cd, mercury Hg, Arsenic. The Corn Cobs were

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washed thoroughly, dried at 11⁰C in an oven till constant weigh was achieved. Pyrolysis – activation reactor was used at 900⁰C temperature. The Pyrolysis derived char kept at 900⁰C for 60 minutes and allowed to cool. The second step involved physical activation using steam as the activated agent at interval hours from 1hr, 2hr, 2.5hr ana d 3hrs at flow rate of 0.2mol/hlg. BET Method was used to determine the Specific surface area.

Micropore Volume was determined using the DubininRadushkevich (DR) equation to the nitrogen adsorption Isotherms of the activated Carbons of each sample. The BET surface area of the activated Carbons ranged between 510.10m8/g ad 631.15m²/g for various durations ranging from 1hr to 3hr which are well within range reported in the literature from various precursors. Micropore volumes of the derived activated carbons ranging between 0.25-0.6cm³/g were recorded. The metal ions in the waste water analyzed during this research were Pb²⁺, Cu²⁺, Cd²⁺ with initial concentrations 1.57, 1.87 and 0.69mg/L.

The Pb²⁺ Concentration in solution decreased sharply and adsorbed up to 99.6% after 30minutes, 97.1% within 1hr, 98.6% at 3hr. The Cu²⁺ Concentration in solution 1.87mg/l to less than 0.03mg/L in 15min, attaining 98.4-99.0% adsorption. The Cd²⁺Concentration from the waste water at initial concentration 0.69mg/L decreased sharply in the first 15min, absorption of 99.7% was achieved after 45minutes.

Ketcha et al, (2012) reported on the preparation and characterization of activated carbons obtained from maize cobs by zinc chloride activation. The maize cobs were washed, dried in open air in order to remove residual water, crushed and sieved into different sizes of particles, one part of 1.25mm – 1.75mm, which is the soft part while the hard part is 1.75mm above in size. The soft sample was activated with 10% of zinc chloride Zncl₂ for 60minutes and 24hours respectively which the other part were activated with solid Zncl₂ (2g, 3g and1g respectively) for 60 minutes. The activated samples were weighed in desiccators and dried at a temperature of 120⁰C for 24 hours. The activating agent was mixed with the matter by agitating for one hour to ensure access of Zncl₂ Carbonization was carried out using continuous steel pipe which can reach a temperature of 1200⁰C, with a regulatory device of temperature at time (TC) the speed of heating was maintained

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at a temperature of 10⁰C/min. The duration of impregnation was maintained at 1hour and maximum temperature of 500⁰C for all the samples washing with a solution of HCL 1%

then Hot distilled water and finally, cold distilled water drying follows with oven (120⁰C). The resultant activated carbon samples of corn cobs were characterized using SEM analysis using field Emission Gun at 15-20KV, the detection unit was about 100%

and the penetration 14m. Physical absorption was carried out using Nitrogen N₂ on the activated carbon surface. The pH of each sample was measured and found to be between 2 and 12.5. The results show that the cob used, the residence time, quantity and the state of the activating agent affected the activated carbon produced. the hard part showed the most significant properties with a BET surface area of 701.68m²/g and a porous volume of about 0.39cm³/g. On the other hand, samples obtained from the soft part of the cob gave low specified surface area (0.43-11.6²m²g/g) and pore volume (0.00028-0.11cm³/g).

The experimental results indicated that this method of preparation shows that activated carbon is non-corrosive and can be used for purification of water.

Mohammad and Zamam (2016) prepared and utilized corn cob Activated Carbon for Dyes Removal from Aqueous Solutions. The Corn Cobs were cut/sieved to the size of 1mm, washed, dried for 2 hours at 110⁰C. It was then soaked with 0.85% solution of H₃PO₄ for 2hours. Heating of the soaked corncob took place for another 2hours in an oven at 400⁰C. The activated carbon was washed with distilled water until pH reached 6.

Finally, the produced activated carbon was dried at110⁰C for two hours. The Activated Carbons so produced were used for the adsorption of methylene blue dye from aqueous solution. Batch processes were conducted to study the effects of solution pH, Contact time, adsorbent close, agitation speed and initial dye concentration. The optimum value for methylene blue dye adsorption was: solution pHs of 6 and 7, Contact times of 8 and 5hr, adsorbent dosage of 1.5, and 0.5g, agitation speed of 200 and 250rpm and initial dye concentration of 50mg/L. Two isotherm models, Freundlich and Langmuir fitted well with the experimental data found from batch processes with R² of 0.952 and 0.992. The maximum adsorption capacities of 16.12 and 30.95mg/g were obtained by commercial

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activated carbon and corncob activated carbon respectively. The chemically activated corncob shows gold adsorption capacities in comparison with commercial activated carbon.

Nethaji et al, (2013) investigated the efficiency of activated carbon preparation from a precursor corn cob bio-mass, magnetized by magnetite nanoparticles (MCC AC) and used for the adsorption of hexavalent chromium from aqueous solution. The adsorbent was characterized by SEM, TEM, XRD, VSM and zero point charge. The iron oxide nanoparticles were of 50mm sizes and the saturation magnetization value for the adsorbent was 48.43 emu/g. Adsorption was maximum at pH of 2. Isotherm data were modeled using Langmuir, Freundlich and Temkin isotherm. The prepared MCCAC had a heterogeneous surface. The maximum monolayer adsorption capacity was 57.37 mg/g.

Kinetic studies were carried out and the data fitted the pseudo-second order equation. The mechanism of the adsorption process was studied by incorporating the kinetic data with intra-particles diffusion model, Bangham equation and Boyd plot. The adsorption was by chemisorptions and the external mass transfer was the rate determining step. A micro-column was designed and the basic micro-column parameters were estimated.

Shuxioing et al, (2016) reported on the preparation of activated carbon from corn cob and its adsorption behaviour Cr (VI). Optimization was carried out on the preparation of activated carbons from corn cob. The Cr(VI) adsorption capacity of the produced activated carbons was also evaluated. The impact of adsorbent dosage, contact time, initial solution pH and temperature were studied. The results showed that the produced corn activated carbon had a good Cr(VI) adsorptive capacity. The theoretical maximum adsorption was 34.48mg g^-1 at 298k. The Brunnet-Emmet-Teller surface area and iodine adsorption value of the produced activated carbon were 924.9m²/g and 1,188mg/g respectively. Under an initial Cr(VI) concentration of 10mg/L and original solution pH of 5.8, an adsorption equilibrium was reached after 4hr, with the Cr(VI) removal efficiency ranging from 78.9 to 100% as the adsorbent‘s dosage increased from 0.5 to

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0.7g/L. The kinetic and equilibrium data agreed well with the Langmuir Isotherm model.

The equilibrium adsorption capacity improved with increment of the temperature.