Production of Adsorbents either Biosorbents or Pyrolized/Activated Carbons from Agricultural Wastes
In a biosorption process, biosorbents, which derived from agricultural wastes, need little processing so as to increase their adsorption ability including rinsing with boiling and cold water, drying and sieving. Biosorbents reduces production cost by using a cheap raw material and eliminate energy costs correlated with thermal treatment as it happens to activated carbons (Salleh et al., 2011). Deoiled soya was washed with deionized water, dried, diluted to H2O2 to remove organic impurities and sieved (Gupta et al., 2009). Rice husk was washed with distilled water to remove dust and impurities and then dried in sunlight and in an oven at 60oC, ground and sieved to different sizes (Safa and Bhatti, 2011). Similar procedures followed with tea wastes but before the dry process they were boiled with distilled water to remove caffeine, tannin and other dyes (Uddin et al., 2009). The same procedure applied to sesame hull (Feng et al., 2011), garlic peel (Hameed and Ahmad, 2009) and potato plant wastes (Gupta et al., 2011) leading to the preparation of biosorbents appropriate for dye removal. As far as the preparation of biosorbents for heavy metal and ions removal may concern, raw materials such as tomato wastes (Yargic et al., 2014), peanut hull (Zhu et al., 2009), and rice straw, rice bran, rice husk, hyacinth roots (Singha and Das, 2013) need washing, drying and sieving but there is also an intermediate stage in which they were subjected to low acid or alkali treatment, e.g., 0.1 N NaOH, H2SO4, HCL, HNO3, so as to remove color.
On the contrary, the general process to produce activated carbons is based on the carbonization and activation of the raw materials. There are many raw materials appropriate for the production of activated carbons. Two main categories follows: the first one includes grade low coal such as lignite, waste pulp solution, various agricultural wastes and the second category includes synthetic resins and fibers (Simitzis and Ioannou, 2011, Xuefei et al., 2009). The procedure of carbonization takes place in furnaces with a gradual increase of temperature under a continuous flow of an inert gas such as nitrogen. After the carbonization process of the raw material, an activation process follows so as to ameliorate the pore volume and new porosity will be created. Activation process may be achieved either physically (thermal process) or chemically. Activated carbon presents an extended microporosity increasing their specific surface area and consequently their adsorptive ability. Activated carbons are divided to two basic forms: the granular activated carbon (GAC) and the powdered activated carbons (PAC) (Salleh et al., 2011). Agricultural wastes such as apricot stones, almond shells, cherry stones and grape seeds were carbonized by steam pyrolysis at 600-700oC (Gergova et al., 1994). Other wastes such as pine sawdust, rose seed and cornel seed were dried, crushed and sieved. Wastes were diluted to ZnCL2 solution and stirred for 2h. The mixture was dehydrated in an oven and then pyrolyzed up to 800 oC under nitrogen flow. Activated carbon was washed with 3M HCL solution by heating at 90oC, then filtered, rinsed by distilled water and dried (A?ikyildiz et al., 2014). Similar procedure followed with wild olive stones (Boumaza et al., 2012) and walnut shells (Martinez et al., 2006).