Thermodynamics and kinetics studies of Phenol adsorption on to Anchote peel activated carbon adsorbent

Thermodynamic ( ∆𝐺 , ∆𝐻, ∆𝑆) and kinetics (pseudo first, pseudo second) of phenol adsorption were investigated. Anchote peel (coconia Abysinica peel) was carbonized and activated by treating with KOH solution followed by heating in an electrical furnace at 800 ℃ for 2 hrs. Kinetic studies of the data showed that the adsorption follows the pseudo-second-order kinetic model. Thermodynamic parameters showed that adsorption on the surface of APAC was feasible, spontaneous in nature, and exothermic. The equilibrium data better fitted the Freundlich isotherm model. Maximum adsorption efficiencies of Phenol were 97% at optimum pH 6 and optimum contact time 210 min., adsorbent dose 0.25 g and initial conc. 0.025 mg/l respectively. Maximum adsorption capacity of APAC was observed to 43.75 mg/g of Phenol at 25 ℃ and 5 mg/L.

leather, paint, pharmaceutical, coking plant, steel and pesticides industries (Uberoi et al., 1997;Srivastava et al., 2006). The improper discharge of these compounds in water bodies over a long period can cause the deterioration of water environments, while its intake by both human and animals causes liver and kidney damage, central nervous system impairment, diarrhoea and excretion of dark urine (Sarkar and Acharya, 2006). They may be present in waste waters and natural environmental waters. Phenols are introduced to the environment in variety of ways: wastes from paper manufacturing, agriculture, petrochemical industry, coal processing or as municipal wastes [1].The phenolic micropollutants generally include phenol derivatives [2]. Due to its toxicity at higher concentration, phenol considered as water pollutants. Concentration of phenolic compounds can possibly get increased in drinking water. Therefore, an easily applied, sensitive and selective method is required for monitoring of phenolic compounds concentration in environmental waters. In the past decade a variety of analytical methods were proposed for determination of phenol in natural environmental waters and waste waters. The most widely used are gas chromatography [5][6][7], high performance liquid chromatography [8][9][10]. These methods are important to determine phenolic compounds, however they often considered complicated and expensive equipment.
Many efforts have been made to develop the simple and effective methods for determination of phenols [3,16]. UV-visible spectrophotometer instrument used in determination of the total concentration of phenol. There are many methods available for the removal of pollutants from polluted water. The most important are chemical and physical methods involves coagulation combined with flotation and filtration, electro flotation, electro kinetic coagulation, irradiation and electro chemical processes. The application of such methods is cost expensive and is unaffordable for large scale treatment of water. Therefore, more economical means adsorption for the removal of pollutant has been required because it is an effective and simplicity of design. In this study adsorption capacity and adsorption efficiency of Anchote (Coccinia abyssinica) peel activated carbon adsorbent was studied.

PREPARATION OF ADSORBENT
The Adsorbent, Anchote peel were collected and cut into small sizes and allowed to be air dried. The dried adsorbent was washed by distilled water and allowed to be dried and further Oven dried at 105 ℃. The well dried sample was grinded and then sieved. The sieved adsorbent was soaked in 25% (W/V) of KOH solution. The mixture was stand for 3 hr for agitation. Then oven dried for 12 hr at 105 ℃.The dried sample was carbonized at a temperature of 800℃ for two hr and the carbonized sample was cooled. The activated carbon product was treated with 5%HCl solution followed by washing with distilled water until the pH of the washing solution become neutral. Then the carbonization product "activated carbon" was kept in plastic container and from which applied for adsoption study. [10] Proximate analysis of APAC adsorbent [11,12]

Fourier Transform Infrared (FTIR) Characterization of Adsorbent
The FT-IR spectroscopic characteristics of the Activated carbon Anchote peel (Activated carbon coconia abysinica peel) and Phenol sorbed Anchote Peel activated carbon are shown in Table 2. The type and net charge of functional groups bonded to the carbon surface is important in understanding the mechanism of adsorption of ionic adsorbate on activated carbons. FT-IR spectrum is an essential tool to identify the surface functional groups which can contribute significantly to enhance adsorption efficiency of the activated carbon.

Adsorption and study of parameters influence adsorption
Study of the adsorption of Phenol on adsorbent (activated carbon prepared from Anchote peel (coconia Abysinica peel) have been performed. Adsorption parameter pH, contact time, initial metal ion concentration, adsorbent dosage, and temperature were studied.

Effects of pH
This is an important parameter on which the adsorption of Phenol is strongly depended. It was carried out by varying a pH between (1-12) and by contacting 0.25 g of adsorbent with 0.025 mg/l Phenol solution.
The mixture was shaken for 210 min. Then was filtered using whatman No 42 filter paper.
The filtrate was analyzed using UV-Vis spectrophotometry.Then the optimum pH was determined as the pH with the highest adsorption efficiency of phenol. As it is shown in Figure 1, maximum adsorption was observed at pH 6 of phenol. Lower adsorption of adsorbents at maximum pH values may be due to the competition of the adsorbate ions and OHto be adsorbed on the surface of the adsorbents.

Effects of contact time
Adsorption of Phenol was studied at various adsorption times (35, 70 The adsorption efficiencies of phenol increases with increase in contact time and maximum efficiency occurred at 210 min then it becomes decreases. The initial rapid rate of adsorption may be due to the availability of Vacant surfaces of the adsorbents. The later slow adsorption rate could be due to the electrostatic hindrance caused by already adsorbed adsorbate species and the slow pore diffusion of the ions.

Study of effect of initial concentration and adsorbent dose
Was done by introducing Phenol concentration (0.00625, 0.0125, 0.025, 0.05, 0.1mg/L) and then mixed with 0.25 g APAC and shaken for 210 min at optimum temperature 25 ℃ keeping other variables constant. After adsorption time was completed the AC were removed from the solution and the final concentration of metal ion in each solution was determined using UV-Vis spectrophotometer.The adsorption of phenol ion at different adsorbent doses in the range of 0.15 -0.55 g was investigated. This was carried out by adding a weighed mass of AC adsorbent to 0.025 mg/L Phenol at optimal pH and contact time by keeping other variables constant. After adsorption time was completed, the solution was filtered and concentration of residual metal ion in each solution was determined using UV-Vis Spectrophotometer. Then the effect of adsorbent dose on the adsorption of adsorbate ion is shown in Figure 5 below. The adsorption efficiencies increase with increase in adsorbent dose. Because, contact surface of adsorbent particle increase for adsorbate and reach a saturation level at high doses. Then mass transfers of phenol ion from bulk liquid to the surface of the solids become resisted.  Because of the chemical interaction between adsorbate and adsorbents and the increased rate of intra particle diffusion of ions into the pores. But it was shown decreasing after 40 o C because of desorption caused by an increase of the available thermal energy that may damage the active binding sites of adsorbents. Higher temperature induces higher mobility of the adsorbate causing desorption.

Adsorption Isotherms
Indicate how the adsorbed molecules distribute between the liquid and the solid phases when the adsorption process reaches an equilibrium state.
Langmuir and Freundlich isotherms have been widely used to explain the phenomena of adsorption. Langmuir isotherm is based on the principle that only a single adsorption layer exists on adsorbents. Freundlich isotherm is an indicator of the extent of heterogeneity of the adsorbents surface. In this work, the experimental data was fitted to both isotherms. However, it well fitted to the Freundlich isotherm as indicated by the higher R 2 values (Table 3). This show the heterogeneity of the adsorbent surface.
The adsorption is favorable since n values are more than unity and the values of RL are between zero and one.

Adsorption Kinetic Studies
Pseudo-first and pseudo-second order kinetics are used to explain mechanism of adsorption.
Experimental results showed rapid initial adsorption rate followed by a slower rate because initially, adsorption sites are open and metal ions interact easily with the sites. Slow adsorption was due to slower diffusion of adsorbate into the interior of the adsorbent.  (Table 4) below showed, pseudo-second order fits better to experimental data than pseudo-first order

Adsorption thermodynamic study
The thermodynamic parameters that help us to understand the nature of the adsorption are ΔG, ΔH and ΔS. Their calculated results are indicated in Table 5 below. ions are readily desolvated and thereby their adsorption becomes more favorable.

Collection of water sample
The study was conducted at Wollega University Chemistry laboratory, East Wollega Figure   8, Shows the overall view of the Gimbi town in West Wollega, and the sampling sites. 142 Technium Vol. 10, pp.131-145 (2023) ISSN: 2668-778X www.techniumscience.com

Analysis of Phenol
Study was conducted at wollega University Chemistry laboratory, East Wollega.
Water samples were distilled using simple distillation and distillate was collected. The distillate was quantitatively transferred to a separately funnel; extracted using isobutyl acetate yield orange colour distillate. The absorbance of the extract was measured at a wavelength of 510 nm using UV-Visible spectrophotometer. Stock solution (Standard) and blank were analyzed by exactly the same procedure for calibration and accuracy.

Concentration of phenol in analyzed water sample
Study has been conducted and concentration level of total phenol was determined. The analysis were done as mean ±SD of three replicates.

Conclusion
In this study, locally available, less costly and environmentally friendly adsorbent Anchote peel (Cocinia abysinica peel ) were investigated for its efficiency and suitability in removing Phenol at low concentrations by preparing activated carbon from Anchote peel (Coccinia abyssinica peel ) by chemical activation method. Parameters pH, contact time, adsorbent dose, initial concentration, and temperature were investigated. Maximum adsorption efficiency was 97% for phenol at pH 6 and optimum contact time 210 min., adsorbent dose 0.25 g and initial conc. 0.025 mg/l respectively. Maximum adsorption capacity of APAC was observed to 43.75 mg/g of Phenol at 25 o C and 5 mg/L.
The adsorption was found to be strongly influenced by contact time and pH, and initial metal ion concentration. The adsorption kinetic data were modeled using the pseudo-first order and pseudo-second order kinetic equations. Pseudo second order model gave better R 2 values confirming suitability of the model for describing the adsorption system. Both Freundlich and Langmuir models can be used to fit the data and estimate model parameters, but the overall data is slightly better fitted by Freundlich isotherm reflecting surface heterogeneity of APAC.
Thermodynamic parameters (∆H, ∆S and ∆ at 293 k, 298 k, 303 k, 308 K and 315 K have also been calculated and it has been found that the adsorption was favorable, spontaneous and exothermic in nature. The positive values of the entropy change suggest that the increased randomness at solid-liquid interfaces during the adsorption of metal ions and Phenol onto adsorbent.
The method was simple, cost effective and environmental friendly. Therefore, it will help to enable us to use adsorbent environmentally friendly and in easily operational methods.