Organic Compounds (Dyes, Pesticides, Pharmaceuticals, Industrial Solvents) Removal
The removal of dyes from wastewaters is considered an environmental challenge and an effective removal process is their adsorption onto agricultural wastes. Dyes can be classified to anionic, cationic and non-ionic dyes. Cationic dyes are basic dyes while anionic dyes are direct, acid and reactive dyes (Salleh et al., 2011). Dye adsorbents derived from agricultural wastes can be classified either to biosorbents or pyrolyzed/activated carbons.
An agricultural waste (deoiled soya) was used for the removal of a synthetic azo dye, carmoisine A, from aqueous solutions. Carmoisine A was the primary red coloring material for foods such as jams and preservatives for many years. Batch experiments, kinetic studies and column operations took place so as to understand dye adsorption onto deoiled soya (Gupta et al., 2009). The biosorption of textile dyes such as Everdirect Orange-3GL and Direct Blue-67 onto rice husk was also studied. A factorial experimental design technique was applied to study the effect of initial dye concentration, biosorbent dose and pH at five levels. pH was found to be the most significant parameter for both dyes (Safa and Bhatti, 2011). Other studies examined the adsorption of methylene blue (MB) on garlic peel (Hameed and Ahmad, 2009) or sesame hull (Feng et al., 2011) or tea waste (Uddin et al., 2009). A batch process was carried out, examining the contact time, MB initial concentration, optimal pH and temperature for the three adsorbents. Adsorption isotherms were modelled with the Langmuir, Freundlich and Temkin equations. The equilibrium data were well represented by the Langmuir isotherm for the removal of MB dye from aqueous solutions onto sesame hull and tea waste and by the Freundlich isotherm for the removal of MB dye from aqueous solutions onto garlic peel. The kinetic data were analyzed using pseudo-first and pseudo-second order equations. According to the results, pseudo-second order model described well the kinetic adsorption data of MB removal onto all adsorbents. Similar studies examined the removal of methylene blue and malachine green dyes from aqueous solutions onto potato (Solanum tuberosum) plant wastes (Gupta et al., 2011) and waste materials of Daucus carota plant, i.e., carrot stem powder and carrot leaves powder (Kushwaha et al., 2014). The effect of physicochemical parameters was investigated. The adsorbents were characterized through Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). Adsorption kinetics and isotherm studies were also studied. Direct and acid dyes such as C.I.Direct red 80 (DR80), C.I.Direct red 81 (DR81), C.I.Acid blue 92 (AB92), C.I.Acid red 14 (AR14), removed from aqueous solutions by the application of Soy Meal Hull (SMH) (Arami et al., 2006). The material was characterized by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The results have shown that SMH presents functional groups, i.e., hydroxyl, amine and carbonyl groups and adsorption capacities equal to 178.57, 120.48, 114.94 and 109.89 mg/g for DR80, DR81, AB92 and AR14, respectively. Consequently, SMH can be characterized as a low-cost, natural adsorbent for dye removal from wastewaters.
Pyrolyzed or activated carbons can be prepared from a variety of raw materials especially agricultural solid wastes. The main characteristics of a carbon material so as to be used in adsorption applications are high specific surface area, adequate pore size distribution and activated sites, i.e., chemical groups containing oxygen or other heteroatoms. Coconut tree flower carbon (CFC) and Jute fibre carbon (JFC) were studied for their adsorption abilities on Reactive red dye (RR) (Senthilkumaar et al., 2006). Kinetic studies have shown that pseudosecond order equation describes well the dye adsorption while isotherm data were fitted well to Langmuir equation indicating that the adsorption capacities were equal to 181.9 and 200 mg/g for CFC and JFC, respectively. The overall rate of RR adsorption appeared to be controlled by chemisorption. Activated carbons, which derived from apricot and cherry stones, almond shells and grape seeds through steam pyrolysis (Gergova et al., 1994), or pine sawdust (PS), rose seed (RS), and cornel seed (CS) through activation (A?ikyildiz et al., 2014) were characterized by N2 adsorption and methylene blue adsorption. The results have shown that the iodine and methylene blue numbers increased with the increase of treatment temperature and soak time for activated carbons derived from apricot stones. The final pore structure of activated carbons depends on the initial raw materials, resulting to microporous structure of activated carbons derived from apricot stones and to meso- and macroporous structure of activated carbons derived from cherry stones and grape seeds while activated carbons derived from almond shells presented an intermediate structure. As far as activated carbons derived from pine sawdust, rose seed, and cornel seed may concern, they presented high specific surface areas equal to 1825, 1265 and 1355 m2/g and methylene blue indexes were found around 300, 297 and 299 mg/g for PS, RS and CS, respectively. Similar studies concerning the adsorption of basic red 46 dye on activated carbons derived from wild olive stone (Boumaza et al., 2012) led to high adsorption yields.
There are several procedures available for pesticides removal from water and wastewaters such as photocatalytic degradation, ultrasound combined with photo-Fenton treatment, advanced oxidation processes, aerobic degradation, electrodialysis membranes, ozonation and adsorption through carbons. Activated carbon was prepared from banana stalk by KOH and CO2 activation as adsorbent for the removal of pesticides, i.e., 2,4- dichlorophenoxyacetic acid (2,4-D) and bentazon (Salman et al., 2011). Kinetic adsorption data were better described by pseudo-second order model while equilibrium adsorption data were fitted well to Freundlich model. Higher adsorption capacity was presented to 2,4-D (196.33 mg/g) than bentazon (115.07 mg/g).
Pharmaceuticals have been detected at high concentration levels in aquatic environments. Clofibric acid (CA) and carbamazerine (CBZ) are drugs that used widely in medicine and were found in wastewaters and groundwater. A new biosorbent from waste rice straw was employed to remove both pharmaceuticals from aqueous solutions (Liu et al., 2013). The adsorption of both pharmaceuticals followed pseudo-second order equation. Intraparticle diffusion was the rate limiting step. Equilibrium adsorption data were fitted well to non-linear Freundlich isotherm. Maximum adsorption capacities were equal to 126.3 and 40 mg/g for CA and CBZ, respectively.
Industrial solvents have a wide use during the production of adhesives, resins, corrosion inhibitors, textile water-repellents. Pyridine is an organic liquid with high solubility. Agricultural waste, e.g coconut coir, was activated by H3PO4 for the removal of pyridine (Ahmed et al., 2014). The adsorption process took place in a batch/column system examining kinetics, isotherm modelling, error and thermodynamic analysis. The adsorbent presented acidic and basic functional groups, i.e., hydroxyl, carboxylic acid and bounded water molecules. The specific surface area was equal to 1254.67 m2/g. The adsorptive capacity was 107.18 and 140.94 mg/g in batch and in column system, respectively. Adsorption kinetic and equilibrium data described well with the pseudo-second-order equation and Langmuir model, respectively. Thermodynamic parameters have shown the endothermic and spontaneous nature of the process. Another study examined the adsorption of trichloroethylene, which is an industrial degreaser, on zerovalent iron/activated carbon adsorbent (Su et al., 2013). The adsorbent derived from coir pith through pyrolysis. Such adsorbents can effectively dehalogenate the chlorinated compounds in water.
Other organic compounds such as phenols and substituted phenols were also removed using activated carbons derived from coconut shell (Singh et al., 2008), olive stones (Ioannou and Simitzis, 2009) and apricot stone shells (Daiffullah and Girgis, 1998). The results indicated that the activated carbons derived from agricultural solid wastes can be used as potential adsorbent for phenols in wastewaters. The removal of oil from water can be achieved by a chemically modified agricultural by-product such as barley straw (Ibrahim et al., 2010). The adsorption capacity was equal to 576 mg/g at 25 oC. Several factors influenced the adsorption process, i.e., temperature, solution pH, particle size, adsorbent.