[Basic Thermodynamics By Mk Muralidhara Pdf 28 [EXCLUSIVE]
Gildo Santiago
This research was aimed to study the adsorption of Alizarin Red S (ARS) dye using graphene oxide (GO) as an adsorbent compared with bare graphite powder (BGP). For optimum conditions, the effects of the initial concentration of ARS, solution pH, adsorbent dosage, and contact time were investigated in detail. The optimum conditions for this work were consisted of 350 mg/L initial concentration of ARS with 0.02 mg adsorbent at pH 2.0. The adsorption equilibrium was completely reached within 30 min. The maximum adsorption capacity of GO was 88.50 mg/g which was higher than that of BGP (34.13 mg/g). The adsorption kinetics well fitted using a pseudo second-order kinetic model. The intraparticle diffusion model described that the intraparticle diffusion was not the only rate-limiting step. In thermodynamics diversion, changes in free energy (DG), enthalpy (DH) and entropy (DS) were also evaluated. The overall adsorption process was exothermic and spontaneous in nature. The adsorption isotherms for GO and BGP fit well with the Langmuir and Freundlich models, respectively. It is, therefore, evident that the as-prepared GO can be used as a high potential adsorbent for the anionic dye and it can be reused for fourth time of adsorption.
Textile dyeing process is an important source of an environmental pollution. One of the most problems of textile wastewater in addition to both toxic and carcinogenic nature is color effluent. Particularly, alizarin red S (ARS) is one of anionic dyes which is widely used for dyeing textile materials. The removal of ARS is crucial process from both economical and environmental points of view1. Various techniques for the removal of ARS from wastewater have been studied over the years, such as co-precipitation2, photocatalysis3, gliding arc discharge4, Fenton and Fenton like process1, electrochemical treatment5, fungal degradation6, Photocatalysis7 and adsorption8-9. Among those techniques, adsorption has been found to be promising process superior to the other techniques for dye wastewater treatment in many advantage terms including operational cost, relative simplicity of design, easier operation and insensitivity to toxic matters10. This technique is relied on interactions between adsorbent and adsorbate.
Various adsorbents that have been studied for the removal of ARS including multiwalled carbon nanotubes11, cynodon dactylon12, mentha waste13, Citrullus lanatus Peels14, mustard husk10, magnetic chitosan15, alumina16, activated clay modified by iron oxide8, activated carbon and other carbon-based materials17-19. However, new adsorbents are recently developed to possess high capacity, larger specific surface area and high selectivity because some adsorbents are still low efficiency or adsorption capacity. One of the adsorbents of choice is graphene oxide, which is a new kind of carbon-based materials that draws increasing attentions in recent years. It consists of non-specific functional groups (carbonyl, hydroxyl and epoxide) on their surface providing anchor sites for both dye and metal ion complexation20. The abundance of the functional groups on the GO surface exhibits a capability of GO for both cationic and anionic pollutants adsorption such as methylene blue20, congo red21, acridine orange22, malachite green23, methyl orange24, methyl violet, rhodamine B and orange G25 and lead26. However, our review has revealed that no research concerns on the removal of ARS by adsorption using graphene oxide as an adsorbent. Hence as a novel point of view, the use of graphene oxide for the ARS removal from aqueous solution is extensively studied.
In this study, the GO was chemically produced from graphite materials and applied to remove ARS from aqueous solution. An evaluation of its potential use as the adsorbent for ARS removal in a batch adsorption study was compared with its bare graphite. Both of Langmuir and Freundlich isotherms, adsorption kinetics and thermodynamics parameters were investigated in details to find out the adsorption capacity for ARS removal from aqueous solution.
The adsorption experiment was carried out by using standard batch method in the aqueous suspension of graphene oxide at temperature of 30, 40, 50 and 60C. In this work, the experiments were performed to observe the effects of solution pH, initial concentration of adsorbate, adsorbent dosage, contact time and temperature. For the adsorption study, 25 mL of the dye solution of desired concentration and a fixed amount of graphene oxide were mixed and taken into a 125-mL conical flask. The flask was then agitated on an orbital shaker (approximately 200 rpm) under optimum conditions. After the equilibrium established, the suspension adsorbent was separated from the solutions by centrifugation for 5 min and the supernatant solution was determined spectrophotometrically at 422 nm. All experiments were conducted in triplicate under the same conditions. The adsorption capacity (qe, mg/g) of the dye at an equilibrium state was calculated using the following equation:
Desorption experiment was performed by a batch process under similar conditions using 350 mg/L dye concentration with 0.02 g adsorbent. The desorption process was carried out by shaking the dye loaded adsorbent in 25 mL of three eluents with various pH solutions including sodium hydroxide, hydrochloric and deionized water. After shaking (about 200 rpm) for 3 h at ambient temperature, the adsorbent solid was separated from the solution by centrifugation for 5 min at 5000 rpm, and then the dye concentration was determined for the desorption extent. The percentage of desorption was calculated using the following equation:
The effect of an initial concentration for the adsorptive removal of Alizarin Red S (ARS) is very important parameter for the adsorption study because it can overcome all mass transfer restrictions of ARS between the aqueous and the solid phases, as shown in Fig. 1 showing plot of an equilibrium adsorption capacity of GO versus the initial concentration of ARS. It is clear that the adsorption capacity increases with an increasing of the initial concentration of ARS. In principle, the initial adsorbate concentration provides the necessary driving force to overcome the resistance to the mass transfer of dye between aqueous phase and solid phase of the adsorbent. In addition, the increasing of initial ARS concentration also enhanced the interactions between ARS molecules and the GO. Moreover, a variation in the extent of the adsorption may also be due to the fact that at the initial stage all active sites of the adsorbent surface are vacant and the dye concentration gradient was relatively high. The maximum adsorption capacity of this adsorbent at 350 mg/L of ARS concentration is 62.55 mg/g. A similar effect of the adsorbate concentration on the ARS removal by mustard husk was previously reported10.
The effect of the adsorbent dosage on the adsorption of Alizarin Red S was investigated by varying the GO from 0.02-0.20 g under other fixed conditions, as shown in Fig. 3. It is evident that the adsorption capacity of GO sorbent dramatically decreases with the increasing of the adsorbent dosage in the range of 0.02-0.04 g and then further slightly decrease. High adsorption capacity is seemed to limit with 0.02 g GO and it slightly decreases down at higher dosages. This may be attributed to the decrease in total adsorption surface area available to the counter ionic dye, resulted in an overlapping or an aggregation of the adsorption sites29. Thus, 0.02 g of GO was preferably used due to that reasonable adsorption.
To study the effect of contact time between GO adsorbent and ARS dye solution on the adsorption feature, the dye solution (350 mg/L) was treated with 0.02 g GO for various intervals of time ranging of 5-60 min with 200 rpm constant agitation. From Fig. 4, it is obviously shown that the rate of the adsorption drastically increases at the initial period of the contact time and gradually slows down with time until the equilibrium adsorption reaches its state in the range of 30-60 min. The equilibrium state was established within 30 min. At the initial stage (15 min), the rate of dye adsorption may be due to an available number of active sites on the GO surface. Their adsorption amount on the adsorbent drastically increases and is normally controlled by the diffusion process from the bulk solution to the adsorbent surface. In the final stage, the adsorption amount of the dye is likely an attachment of the controlled process due to less available sorption sites.
In order to study the effect of temperature on the adsorption of ARS onto GO, three basic thermodynamic parameters, the Gibbs free energy change (DG), entropy change (DS) and enthalpy change (DH) were calculated using the following equations:
The obtained thermodynamic parameters are shown in Table 1. Under the steady-state reaction condition, the negative DG indicates the spontaneity of the ongoing adsorption process30. The negative value of DS indicates a tendency to lower disorder at the solid-solution interface during the adsorption31. In addition, the negative DH indicates that the dye adsorption using GO is the exothermic nature, the low values of DH give clearly evidence that the interactions between the dye and its adsorbent were rather weak32. At higher temperatures the thickness of the boundary layer decreases due to the increase in tendency of the dye molecules to escape from the adsorbent surface to its bulk solution, resulting in decrease in the adsorption capacity as the temperature increases33.
The kinetic adsorption data were processed to understand the nature of the adsorption phenomenon in terms of the order of the rate constant. Kinetic data were applied with both pseudo-first order kinetic model and pseudo-second-order kinetic model.
The pseudo first-order equation describes adsorption in solid-liquid systems based on the sorption capacity of solids. It has been proposed that only one ion of the dye is sorbed onto one sorption site on the GO surface. The linearized form of the pseudo first-order models can be written by following eq. (6)11, 14.
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