Uso de baterías alcalinas agotadas para activar carbón de granza de arroz Use of spent alkaline batteries to activate rice husk carbon

Objective: Objective: Use agro-industrial waste and spent alkaline batteries as raw materials to produce activated carbon. Methodology: As a first step, rice husk was calcinated in a nitrogen atmosphere at 300 °C for six hours. Then activated after being soaked in a solution of hydrochloric acid and zinc chloride at 600 °C for two hours under a nitrogen atmosphere. Zinc chloride [1.0 mol L-1] was obtained by treating the inner paste of spent alkaline batteries with hydrochloric acid [3.0 mol L-1]. In contrast, neither potassium hydroxide, extracted from spent alkaline batteries, nor gaseous carbon dioxide produced activated carbon under the same conditions. Pineapple leaf fiber was also tested but failed to produce activated carbon under any of the conditions described. Results: Activated carbon, rich in heterogeneous silica with a surface area of 315 m2 g-1 was produced using rice husk. Conclusions: It is possible to convert rice husk waste into activated carbon with the aid of zinc chloride obtained from discarded alkaline batteries. The use of two wastes to produce activated carbon is not described in the scientific literature. Further tests on the conditions to produce higher surface is required.


Introduction
Activated carbon (AC) includes a wide range of processed carbon-based materials that have a highly developed porosity and an extended interparticulate surface area with an amorphous structure [1]. AC may contain other elements such as oxygen, sulfur, hydrogen, nitrogen, and halogens bonded chemically to the structure. Oxygen is found to be predominant element which exist in the form of functional groups such as carboxyl, carbonyl, phenols, lactone and others [2]. There are three properties of AC that control their adsorption characteristics: the distribution of pore sizes within the carbon, the mean pore size and the composition of the surfaces which contain the porosity. Pore widths typically range from < 1.0 nm to 50 nm, with surface compositions varying from graphitic to oxygenated polar surfaces [3]. According to IUPAC classification, pore with width not exceeding 2.0 nm is considered micropore, from 2 nm to 50 nm are considered mesopores and bigger than 50 nm are macropores [4]. The pore size is affected by the temperature, duration of the activation process and activation method [5]. The smaller the pore the higher the surface area, this can be determined using the Brunauer-Emmett-Teller (BET) method, which measures nitrogen adsorption at different pressures at the temperature of liquid nitrogen. AC is commonly prepared by two basic methods: physical or gas activation and chemical activation. The selection of the method depends upon the starting material and whether a low or high density, powdered or granular carbon is desired [6]. For the chemical method, the carbonized raw material is first impregnated with chemical reagents, such as phosphoric acid, zinc chloride, alkaline hydroxides, and ferric chloride [7] and later activated at temperatures that range from 200 °C to 1000 °C [2]. Activated carbon has many uses: in medicine, it can be used to treat drug overdoses [8], it can be used to remove dyes and metal ions from aqueous solutions [9], remotion of contaminants from soil, air, water [10], gas streams and support for catalysts [11]. Lately, other potential applications have been described by Nadeem et al. [12] and include the use of AC in electrochemical capacitors, lithium ion batteries, solar cells, fuel cells and hydrogen and carbon dioxide storage systems.
Commercial AC normally uses raw materials such as petroleum residues, wood, coconut shells, coal, peat and lignite. Recently, the use of agricultural waste and inexpensive lignocelluloses materials has drawn attention [2]. In the case of Costa Rica, there is potential of using lignocellulose residues from forestry and waste from the production of palm oil and pineapple [13]. Banana [14], rice, sugar cane and coffee waste are also potential sources of carbon [15]. Rice husk, discarded from the rice milling process, has a reported The composition is not homogeneous because the amorphous silica, also refereed as opaline silica, is concentrated in the protuberances and hairs on the outer and inner epidermis of the husk [16].
Along agricultural waste, Costa Rica also deals with domestic and industrial waste, including the waste generated by 2340 tons of batteries imported yearly [17,18]. Table 1 shows that spent batteries are potential source for some of the chemicals used to activate carbon such as potassium hydroxide (KOH) and zinc (Zn) which can be transformed to zinc chloride (ZnCl 2 ) using hydrochloric acid (HCl). The revalorization of waste is of vital importance for a nation that is producing municipal waste at a rate of 390 kg/person-year [20].
The objective of this study was to use two types of waste to generate an AC that has commercial value. It was hypothesized that the chemicals in the alkaline batteries would be capable to activate rice husk and fiber from the pineapple leaves.

Methods and materials
Ten AA size alkaline batteries of different brands were crushed, their paste was collected and washed with one liter of hot distilled water. The liquid was filtered, resulting in a colorless solution with pH 12.9. The remaining black solid, insoluble in water, was washed with 0.1 L of hydrochloric acid solution 3.0 mol L -1 , yielding a straw-colored solution rich in zinc ions (1 mol L -1 ) with traces of iron, manganese and copper.
Prior to activation, all agricultural waste samples were carbonized for six hours at 300 °C in a nitrogen atmosphere flowing at 15 mL/min. Three methods were tested to try to produce AC using the carbonized rice husk and pineapple leaf fiver, one physical and two chemical. The physical method used employed CO 2 as activator for two hours at 600 °C. One of the chemical methods used KOH obtained from the alkaline batteries and the second used HCl and the ZnCl 2 solution also obtained from the alkaline batteries waste as previously described. These solutions were used separately to infuse two calcined samples of two grams each, for eight hours before being activated at 600 °C for two hours under a nitrogen atmosphere.
Only the rice husk samples treated with HCl and ZnCl 2 showed mesopores when observed under a Hitachi S-3700N Scanning Electronic Microscope, this sample was further analyzed using X-ray crystallography and ATR-FTIR spectrophotometry.
Revista     with what was found by Rinawati et al. [21] who asserted that silica must be removed first from rice husk in order to obtain activated carbon. The surface area of 315 m 2 g -1 obtained by BET also confirms surface activation, although areas ten times as big have been reported [6].
XRD spectra in Figure 3, shows a broad diffraction peak centered at 2θ = 22° indicating that the activated carbon contains abundant amorphous silica which is consistent with data from Mon [22]. IR spectra, in  the bulk powder and the surface indicates that the activated carbon surface is hydrophobic in nature due to the absence of the 3434 cm -1 band and the presence of hydrophobic aromatic groups as indicated by the strong bands between 1300 and 1800 cm -1 , the strongest band at 1600 cm -1 corresponds to the stretching of the carbonyl (ʋ C=O) groups present in the hemicelluloses and lignin aromatic groups. These bands are barely noticeable in the bulk IR spectra which has a pronounced O-H band at 3434 cm -1 . The heterogeneous composition of rice husk is consistent with results presented by Alam et al [16].

Conclusions
Zinc chloride prepared from discarded alkaline batteries could be used to activate carbon obtained from rice husk. This is the first time that such procedure has been reported in the scientific literature. Further experiments need to be carried out to study the influence of time, temperature and ZnCl 2 concentrations on pore distribution. It is expected that further pore development could be achieved at higher temperature and increased time of calcination.