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International Journal of Water Research Universal Research Publications. All rights reserved

ISSN 2348 – 2710 Original Article

Removal of Lead ion from aqueous solution by Bamboo activated Carbon Masood Akhtar Khan*, Amanual Alemayehu, Ramesh Duraisamy* and Abiyu Kerebo Berekete Department of Chemistry, College of Natural Sciences Arba Minch University, Arba Minch, Ethiopia (East Africa) Corresponding Author’s: e-mail: [email protected], [email protected]; [email protected] Mobile: + 251-938607570; + 251-910171048; + 91-9042725600 Received 11 June 2015; accepted 09 July 2015 Abstract The current study focusing the removal of Pb 2+ from it aqueous solution using activated carbon obtained from bamboo stem has been investigated by batch adsorption method. The results were obtained and indicate that the maximum sorption for lead ion was found at pH 5. The bamboo activated carbon (BAC) dosage reveals better results even at lower metal ion concentrations. Greater adsorption occurs at smaller particle size of adsorbent and at high solution temperature. The results were also confirmed that the adsorption process follows Freundlich isotherm model with a better sorption fit and supported for the multilayer adsorption of Pb2+ ions on BAC. The kinetic model of this study shows a pseudo-second order kinetic model with good correlation coefficient. Thermodynamic parameters such as change in Gibbs free energy, enthalpy and entropy were also evaluated. Thus, these results were reveals the negative free energy changes (ΔG) and positive entropy (ΔS) and enthalpy changes (ΔH) that were recognized the spontaneous and endothermic nature of the adsorption process. © 2015 Universal Research Publications. All rights reserved Key words: Bamboo activated carbon (BAC), Pb2+ions, Adsorption isotherm, Kinetics, Thermodynamics. 1. Introduction Heavy metals are natural components of the Earth's crust, and their concentrations in an aquatic environment have increased due to mining and industrial activities and geochemical processes. They are toxic or poisonous, if avail as more than the recommended enough amounts in water bodies. Heavy metals are common in industrial applications such as the manufacture of pesticides, batteries, mining operations, alloys, plating facilities, textile dyes, tanneries, etc (1). Living organisms require trace amounts of some heavy metals, e.g., iron, copper, and zinc, etc., as they are essential to maintain the metabolism of the human body. But, at higher concentrations of heavy metals can promote poisoning and other hazardous nature because they cannot be degraded or destroyed, and tend to bio accumulate. They pose risks not only to humans but also to other animals and plants because of their extremely toxic effects and have been the main reason behind the number of affliction (2). Heavy metals become toxic when they are not metabolized by the body and accumulate in the soft tissues they can enter the bodies of humans via the food chain, drinking water, air or absorption through the skin.

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Commonly encountered metals include Fe3+, Pb2+, Cu2+, Zn2+, Co2+ and Ni2+ etc. These metals are toxic in both their chemically combined forms as well as the elemental form. These are resulted in heavy metal pollution problems in the eco-system. Toxic metallic compounds not only contaminate the water bodies like seas, lake reservoirs and ponds also enter the underground water in traceable amounts. Unlike the organic pollutants which are biodegradable and the heavy metals are not biodegradable thus making a source of great concern. Exposure of these contaminants present even in low concentrations in the environment can prove to be harmful to the human health. Agricultural development, human health and the ecosystems are all at risk unless water and land systems are effectively managed the availability of heavy metals (3). Nowadays, the important toxic metals with the exponential increase in population, measures for controlling heavy metal emissions into the environment are essential. Lead like heavy metals cause many serious disorders like anemia, kidney disease, nervous disorders, and even death (4). At present lead pollution is considered a worldwide problem because this metal is commonly detected in several industrial wastewaters.

International Journal of Water Research 2015; 5(2): 33-46

In order to solve heavy metal pollution in the environment, it is important to bring applicable solutions. Several treatment technologies such as chemical precipitation, ion exchange, coagulation, bioremediation and sorption/adsorption are available for the removal of heavy metal ions from its aqueous solutions (5). The most commonly known biological method is biosorption using microorganisms and microbial products. Biosorption is a passive non-metabolically mediated process of metal binding by biosorbent. Bacteria, yeasts, fungi, algae and some higher plants are used as biosorbents for the removal of heavy metals (6). Among all these techniques adsorption of heavy metals on solid substrate is preferred because of its high efficiency, easy handling and cost effectiveness as well as availability of different adsorbents (7). Activated carbon (AC) is still the main notable adsorbent for the removal of pollutants from polluted gaseous and liquid streams. The challenge in utilizing activated carbon is, however, to cater to the demands with reasonable costs for end-users. Activated carbon production costs can be reduced by either choosing a cheap raw material for instance using agricultural waste and/or by applying a proper production method (8). Agriculture waste materials are inexpensive and available in large quantities, thus they can be disposed without concerning expensive regeneration process (9). In view of the efficiency and the ease processing of biosorbent from agricultural waste, with which it can be apply for the treatment of waste water containing heavy metals. Therefore, the present study had chosen bamboo stem which is a self-regenerating agricultural products and application of these bamboo activated carbon adsorbent offers highly effective technological means in dealing with the pollution of heavy metals with the requirement of minimum investment. Bamboo is an agricultural product and especially highland bamboo species is botanically known as Yushania alpine was chosen for this study. These highland bamboos grow naturally in ecological zones of the country between 2200-3500 meters above sea level. The coverage of this species in Ethiopia was roughly estimated about 130,000 hectares in 2005. The present study is confirmed that locally activated carbon produced from bamboo stem is a good in adsorbing the Pb 2+ ions from its synthetically prepared aqueous solution. Thus, the optimum removal condition was determined by using the suitable adsorption isotherms and by its related constants. 2. Materials and Methods 2.1 Chemicals, reagents and standard solutions Reagents were used in the present study are analytical grade (AR) such as zinc chloride (97%), phosphoric acid (85%) and sodium hydroxide (97.5%) were obtained from THOMAS BAKER Chemicals Company. Hydrochloric acid (37%) was obtained from Scharlab.S.L Company. 2.1.1 Preparation of adsorbent from bamboo stem Bamboo stem was selected as raw material to prepare the activated carbon to be use as adsorbent for the removal of Pb2+ from its aqueous solution. The bamboo stem was collected from Semen Ari which is located in the southern Nations, Nationalities and peoples’ Region

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(SNNPR), South Omo Zone and which is 589 km south from Addis Ababa (capital city, Ethiopia, East Africa) and 334 km from Arbaminch location with longitude N-06 10 36.1, E- 036 39 47.3 and altitude of 2678 meters above sea level. The collected stem was cut into small pieces, washed thoroughly under running tap water and followed by washing with double distilled water to remove all the dust and any adhering impure particles present on it, and dried under sunlight about three days. The small pieces of dried bamboo stems were kept in muffle furnace and carried out carbonization at 500°C on about two hours to set complete carbonized carbon and allowed to cool into room temperature. The carbonized material was crushed and finally sieved by using automatic sieve shaker D406 with a desired particle size (10) and stored in desiccators for further use. 2.1.2 Chemical Activation of Carbon produced from bamboo The carbonized adsorbent material was weighed separately and poured in to different beakers containing orthophosphoric acid. The content of the beakers was thoroughly mixed until a paste was formed. The pasted sample was transferred into the crucibles and were placed in a carbolite furnace and heated at 800°C about two hours. The activated sample from bamboo stem (BAC) was cooled at room temperature and supernatant acidic solution was decanted. It was repeatedly washed with distilled water until the washing was free from acid (pH is 6-7). Activated carbon was filtered, dried and again activated under thermally in a hot air oven at 105°C upon three hours according to Gimba et.al.,(11). The final product is grounded well and sieved by using automatic sieve shaker D406 with different desired particle size and stored in a glass bottles and kept inside the desiccators for further use as an adsorbent to remove Pb2+ ions from it aqueous solution. 2.1.3 Preparation of synthetic feed Pb2+ solution A stock solution (1000 mg/L) of Pb2+ was prepared by dissolving 1.3557g of PbCl2 in 1000 ml volumetric flask using double distilled water and it is diluted as required for batch adsorption and other experimental studies. It is used as synthetic effluent called adsorbate; a fresh solution of heavy metallic effluent was prepared for every trial, and utilized completely for the entire set of experiments. 2.2 Analytical methods and Instrument were used 2.2.1 Instruments Studies were undergone about the removal efficiency of an adsorbent involves by determination of amount of Pb2+ ions in the effluent solution before and after adsorption takes place. This was done by using Atomic Absorption Spectroscopy (AAS) BUCK SCIENTIFIC MODEL 210 VGP East Norwalk, USA. It is equipped with deuterium arc background, nebulizer and hallow cathode lamp corresponding to metal of interest, and air-acetylene flame was used. The pH of different solutions was measured using H p meter (JENWAY PH meter 3310). Magnetic stirrer with hot plate (Model 04803-02, Cole-Parmer Instrument, U.S.A.) was used for stirring the mixture of adsorbent and


metal ion solution at known time intervals. Then, sample solutions were aspirated in to the AAS instrument and direct readings of total metal ion concentrations were recorded by triplicate measurements on each sample. The amount of metal ion adsorbed was calculated from the difference between the amount of adsorption before and after a certain period of time. 2.2.2 Methods 2.2.2.1 Characterization of adsorbent, BAC The physico-chemical characterization such as pH, surface area, bulk density, ash content, moisture content, volatile content and fixed carbon of the adsorbent was evaluated according to the literatures (12-14). The pH of RHAC was determined taking about 0.5 g of adsorbent into 20 ml of distilled water and the resulting suspension mixture was stirred at 300 rpm for 24 hrs. Thus, the solution was filtered and pH of the filtrate was measured using JENWAY pH meter 3310. Proximate analysis was carried out using thermogravimetric analyzer (Perkin Elmer TGA7, USA) and elemental analysis was performed using Elemental Analyzer (Perkin Elmer series II 2400). 2.2.2.2 Batch adsorption experiments Batch experiments for the removal of Pb2+ was conducted in 250 mL Erlenmeyer flasks by taking 50 mL of three different (30, 60 and 90 mg/L) Pb 2+ solutions. The experiments were carried out at room temperature by shaking a mixture of 1g BAC powder introduced into the metal ion solution containing flask with agitation rate of 200 rpm about 2 hours until equilibrium was reached. After agitation, the residual adsorbent was removed by filtration using filter paper. The experiment was conducted with duplicate under the same conditions and the average results were taken and recorded. The concentration of metal ion in the filtrates as well as in the control samples were determined by using atomic absorption spectroscopy (AAS) spectrometer. The effect of adsorbent dose (4 - 40 g/L), contact time (5 - 240 minutes), feed solution pH (2 - 9), initial concentration of the metal ion (20 – 100 mg/L) and the particle size (150 – 425 m and 1.18 mm) of BAC were investigated by varying any one of the parameters and keeping the other parameters as constant. In addition to this, thermodynamic study was also conducted by varying the temperatures (about 298, 308 and 313 K). The solution pH was adjusted to the desired value by drop wise addition of hydrochloric acid (HCl) or sodium hydroxide (NaOH) solution and the filtrates were analyzed for the influence of p H on metal ion adsorption. The experiments were performed in duplicate and the average result is reported. The uptake of metal ion was calculated using the equation:

Where

Uptake (%) = x 100 = Initial concentration of metal ion (mg/l) = Concentration of metal ion at equilibrium state

(mg/l) 2.2.2.3 Kinetic study of sorption Kinetics of adsorption was determined by analyzing sorptive uptake of the Pb2+ ion from an aqueous solution at different time intervals. For the determination of

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sorption isotherms, metal ion solution of different concentrations was agitated with known amount of sorbents till the equilibrium was achieved at room temperature. Adsorption kinetic experiments (true and pseudo order kinetics) were performed at pH 5 for Pb2+ with initial concentrations of 30 - 90 mg/L solutions with their respective optimum adsorbent doses. Then the residual metal concentrations were measured at different time intervals by taking samples periodically. 2.2.2.4 Study of sorption isotherm The study of adsorption isotherm has been a greater importance in water and wastewater treatment by the batch absorption technique, as they provide an approximate estimate of the monolayer adsorption capacity of adsorbent. The equilibrium isotherm was determined by using different amount of adsorbent ranged 0.2 - 2.0 g of mixed with 50 ml of 30-90 mg/L concentration of Pb2+ solution. This mixture is agitated with the 200 rpm speed for 4 hours, which was sufficient to reach equilibrium. The amount of metal ion adsorbed at equilibrium (q e) was calculated as: Where, V = volume of solution (L) m = mass of adsorbent (g) The equilibrium data for the removal of Pb2+ ions on the adsorbent at room temperature were estimated through testing the Langmuir, Freundlich and Temkin isotherms. 2.2.2.5 Thermodynamic Study The effect of temperature on the sorption characteristics was investigated by taking 30 mg/L and 60 mg/l initial concentrations of Pb2+ solution and 2 g/L of adsorbent dose, at temperatures were ranged from 298 up to 318 K. Increase in temperature does affect the solubility and chemical potential of the sorbate, which can be a controlling factor for sorption. The dependence on temperature of sorption of Pb2+ ions on BAC were evaluated using the equation: Kc =

ln

= = -T S Where, ,Δ , and T are the enthalpy, entropy, Gibbs free energy change and temperature, respectively, R is the gas constant (8.314 J.mol-1.k -1) and is the adsorption coefficient obtained from Langmuir equation. It is equal to the ratio of the amount adsorbed (x/m in mg/g) to the adsorptive concentration in (mg/l). These parameters can be obtained from experiments at various temperatures using the above equations. The values of H and S are determined from the slope and intercept of the linear plots of lnKc versus 1/T (15). In general these parameters indicate that the adsorption process is spontaneous or not and exothermic or endothermic. 3. Results and discussion The present study deals with the removal of lead ion by adsorption on a low cost adsorbent as activated carbon


prepared from bamboo stem agricultural waste material. This is a well-known non-conventional material, which could be employed as an alternative to commercial activated carbon for water and wastewater treatment. This is an endeavor to present data for the design of economical wastewater treatment plant for the removal of metal/metal ions were discharged as effluent from the metal finishing industries. The experimental parameters, which affect the extent of adsorption of pb2+ ion, are reported. The effect of the initial concentration of dye, contact time, dose of adsorbent and pH of the aqueous solution on the removal of Pb2+ by adsorption onto BAC was studied in the present investigation. The various experimental conditions and the related results for the adsorption studies are reported for the discussion about the study. 3.1 Characterization of Adsorbent, BAC The chemical composition, ultimate and proximate analysis of BAC used in the present study was carried out and the data were presented in table.1. Results show good agreement with the earlier reported literatures to work with the precursors of BAC for removal of Pb 2+ from its own aqueous solution. The pH of BAC solution was found to be 6.4 (shown in table.1), which is higher than 6.0 and 4.6 obtained from RH with H3PO4 and FeCl3-H3PO4 activated carbons (16, 17), respectively. The BAC of this present investigation was found to have lower volatile content and higher carbon content, indicating the suitability of BAC for the precursor for the treatment of metallic effluents. The results were shown in good agreement with the reported literatures (16 - 19). Table.1: Physico-chemical Characteristics of activated carbon derived from bamboo stem BAC/H3PO4 Parameter pH Surface area, (m2/g) Bulk density (g/cm3 ) Ash content (%) Moisture content (%) Volatile content (%) Fixed carbon (%)

Value 6.4 807 0.65 5.55 7.6 24.4 62.45

3.2 Batch Adsorption Studies 3.2.1. Effect of pH on the sorption of Pb2+on BAC The effect of pH on the removal of lead was studied, and it is revealed that the solution pH does affect the amount of lead adsorption. The lead uptake was found to be increase with increasing pH, and also shows the removal efficiency does increasing rapidly upon pH starts from 2 to 3 (shown in fig.1). Three different solutions (30 90 mg/L) of Pb2+ were studied, the maximum removal of lead appeared at pH 5 in all three solutions. Therefore, pH 5 was selected as optimum pH for further studies for the removal of Pb2+ from it aqueous solution. The increase in metal removal was observed as pH increases. This may be due to decrease in competition

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between hydronium ions and metal ions for the surface sites. This is also by the decrease in positive surface charge on the adsorbent, which resulted in a lower electrostatic repulsion between the surface and the metal ions and hence uptake of metal ions get increased. A similar theory was proposed earlier (20) for metal adsorption on different adsorbent. It is also supported in an alkaline medium lead ions tend to hydrolyze and precipitate instead of adsorption on adsorbent. It was deteriorated with accumulation of metal ions, and making impossible true adsorption.

Fig.1 Effect of pH on sorption of Pb2+ onto BAC at 298 K; [BAC]-10g/L, Time-120 min. 3.2.2. Effects of contact time of Pb2+ adsorption on BAC Effect of contact time on the removal of lead is illustrated in figure 2. It shows that the removal of lead increased with contact time and it was rapid at initial up to 30 minutes, and then it proceeds at slower rate of increases and finally attained saturation. This behavior suggests that at the initial stage adsorption was takes place rapidly on the external surface of the adsorbent followed by a slower internal diffusion process, which may be the ratedetermining step. The trend in adsorption of Pb 2+suggests that the binding may be through the interactions with the functional groups located on the surface of the carbon. Equilibrium adsorption was established at 60 minutes (for 30 mg/L) and 120 minutes for other studied concentrations of Pb2+ ion solutions respectively. It is clearly shows that the maximum contact time is required for greater uptake of metal ions by BAC is depends on the initial concentrations of metal ion. It is evident that the contact time was fixed at 120 min for the batch experiments to make sure that equilibrium was attained. Thus, the % removal for 30, 60 and 90 mg/L of lead ions upon contact time 120 min. were 96.07 %, 90.2 % and 85.16 %, respectively. The results were demonstrated that at a fixed adsorbent dosage, the amount of adsorbate increased with increasing concentration of Pb2+solution, but the percentage of adsorption was decreased. This is due to at lower concentrations, the ratio of number of metal ions to the available adsorption sites is almost fulfilled and subsequently the adsorption becomes greater. But, at higher concentrations of metal ions, however, the available sites on BAC for adsorption become fewer and subsequently the


Fig.2 Effect of contact time on sorption of Pb2+ onto BAC at 298 K; [BAC]-10 g/L, pH - 5 removal of lead depends on the concentrations of Pb 2+ and decreases with increase in initial Pb2+ concentration was good agreement with reported (21). 3.2.3. Effect of adsorbent dosage onPb2+ As it can be seen from Figure 3, adsorption of Pb 2+ increased from 32.93 % to 97.33 % with increasing adsorbent dose from 4 g/L to 40 g/L, respectively. This is because for a fixed initial metal concentration, while increasing the adsorbent dose provides a greater adsorption sites. On the other hand, the plot of capacity (metal uptake per adsorbent unit) versus adsorbent dose revealed that the capacity was high at low doses and low at greater dose of adsorbent, which shows increase in adsorption with the growth of adsorbent. Similar results were reported (22) in adsorption of Cu2+ and Pb2+ using sawdust and clay as adsorbent respectively. This result can be attributed to the fact that some of the adsorption sites remain unsaturated after the adsorption process. It might be because of formation of particle aggregation, resulting in a decrease in the total surface area and an increase in diffusion path length, which contribute to decrease in amount adsorbed per unit mass. Studies were indicating that the efficiency of (hydroxide) oxides to adsorb heavy metal ions is due to their high surface/mass ratio (23). Even if the up-take of the metal increased by increasing the adsorbent dose, beyond a dose of 20 g/L of BAC, and the rise of the adsorption efficiency is insignificant and the capacity of adsorbent is very low. Therefore, further increase in the dose results the much production of sludge and wastage of material. Thus, 20 g/L of adsorbent dose was taken as an optimum dose for further experiments.

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Fig.3 Capacity and removal efficiency of Pb2+ at different adsorbent dose 3.2.4. Effect of initial concentration of Pb2+ adsorption on BAC The initial metal ion concentration provides an important driving force to overcome all mass transfer resistances of metal ion between aqueous and solid phases. The removal efficiency at a fixed adsorbent dose on the effect of initial concentration of Pb2+ is depicted in figure 4. The capacity of the adsorbent increased significantly even though there is slight decrease in the adsorption efficiency with the increment of initial concentration. The increase of capacity can be due to increment of driving force that is concentration gradient, which causes an increase in the number of metal ions coming in contact with the adsorbent. On the other hand, the number of available adsorption sites in adsorbent is the same for all initial concentrations; thus,


Fig.4 Percent removal of and Pb2+ as a function of initial concentration the initial concentration increases with more number of ions and the same change to be adsorbed and competes the same adsorption sites. This may cause to left many ions without being adsorbed and to decrease the efficiency of the removal upon increases the concentration of Pb 2+ ions. 3.2.5 Effect of particle size of BAC on the adsorption of Pb2+ Particle size of adsorbent is an important factor that affecting the adsorption capacity as it influences the surface area of adsorbent. The effect of particle size on the adsorption of Pb2+ ions was investigated in the range of 150 μm - 1.18 mm. Figure 5 shows that the variation of Pb 2+ uptake with time of different particle size of adsorbent. The results were indicated that increases the uptake of Pb 2+ ion with lower particle size. The higher uptake with in lower particle size was attributed to the fact that smaller particles had larger external surface area compared to larger particles, hence more binding sites were exposed on the surface and thus, leading to higher adsorption capacity since adsorption is a surface process. Apart from that, particles with smaller size also moved faster in the solution compared to larger particles, consequently the adsorption rate was faster. Utilized activated carbon prepared from bamboo waste for the removal of studied metal ion and investigated the uptake of Pb2+ ions at different size of particles (150 μm - 1.18 mm). The results were found that the % removal was increased (shown in figure 5) as in the lower particle size upon 30 mg/L metal ion solution. According to Sekar et.al., (24), larger particles that resist the diffusion to mass transport and most of the internal surface of the particle might not be utilized for adsorption, hence the smaller amount of metal ions were adsorbed.

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Fig.5 Effect of particle size on uptake of Pb2+ ions by BAC 3.3 Equilibrium sorption study Sorption studies describe the interaction of adsorbates with adsorbent, and established equilibrium between adsorbed metal ions and the residual metal ions in solution during the surface sorption. The interaction between adsorbate and adsorbent is characterized using adsorption isotherm models (25). Adsorption isotherms are mathematical models that describe the distribution of adsorbate species among liquid and adsorbent. Based on a set of assumptions, that is mainly related to the heterogeneity or homogeneity of adsorbents, type of coverage and possibility of interaction between adsorbate species (26).


The adsorption equilibrium data is obtained at a fixed initial concentration and varying adsorbent dose have been fit into the linearized Langmuir, Freundlich and Temkin adsorption isotherms. 3.3.1 Langmuir isotherm model (27) The linear Langmuir isotherm model was expressed mathematically as: ------------ (1) =

+

.

experimental condition as well. According to the Langmuir equation the maximum uptake capacity (qm) of Pb2+ ions have 3.30 mg/g (shown in table.2). The Langmuir parameters were also used to predict the affinity of the adsorbent (BAC) surfaces towards the metal ions by using dimensionless constant called equilibrium parameter, RL, which is expressed according to the literature (28). The shape of isotherm is described in terms of R L is shown in table below.

------------- (2)

Where; - Concentration of metal ions at equilibrium (mg/L) - Amount of metal ions adsorbed at equilibrium (mg/g) - Langmuir isotherm constant related to free energy of adsorption (L/mg) - Maximum adsorption capacity (mg/g) In this current study, the plot of 1/q e against 1/Ce gives straight line (seefig.6) with a slope of 1/qmKL and intercept of 1/qm. Figure.6 shows the Langmuir plot of Pb 2+ adsorption on BAC with a correlation coefficient of 0.8996 respectively, which is ≥ 0.828, indicates that the data are fitted in Langmuir isotherm. Thus, values obtained by linear regression correlation coefficient (R2) for Langmuir suggests that monolayer sorption may exist under that

The RL value (0.055) is obtained in the range of 0 and 1, which indicates a favorable isotherm shape for adsorption of Pb2+ ions on BAC. The adsorption capacity (qm - 3.30 mg/g) obtained in this experiment is in agreement with the results were reported (29) in the range of 2.00 - 16 mg/g.

Fig.6 Langmuir isotherm plot for the sorption of Pb 2+ ions Equation 4 could be linearized by taking logarithms as 4.3.2 Freundlich isotherm model followed: The Freundlich isotherm assumes a heterogeneous surface with a non-uniform distribution of heat of log = log + log ------------------ (5) biosorption over the surface and a multilayer biosorption The plot of log qe against log Ce gives a straight can be expressed according to Freudlich.M. (30) model as: line with slope of 1/n and intercept of log KF. This Freundlich type behavior is indicative of the surface = --------------- (4) heterogeneity of the adsorbents, i.e. the adsorptive sites or Where; - Freundlich indicative of relative adsorption capacity surface of the studied adsorbents are made up of small heterogeneous adsorption patches that are homogeneous in of adsorbent themselves (31). n - Freundlich indicative of the intensity of adsorption

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Figure.7 shows the Freundlich isotherm plot of Pb2+ ions adsorption on BAC with a correlation coefficient of 0.935. The greater values of R2 (over than Langmuir isotherm model) indicate the adsorption is favorable for a Freundlich isotherm. The value of Freundlich constant, KF and n obtained for Pb2+ from the plot were 1.468 and 1.485 respectively. It is also noted that the value of 1/n (0.6732) was between 0 and 1 indicating that the sorption of metal ions into the studied adsorbents was favorable. Thus, the results of KF values indicate that the BAC surface is heterogeneous in the long range, but may have short range of uniformity. Also, the ‘n’ values lying in the range of 1 to 10, reveals the favorability of sorption (n  1) of all Pb2+ ions according to Chen, et al., (32). Fig.8 Temkin isotherm plot for the sorption of Pb 2+ ions on BAC The isotherm constants and correlation coefficients for all three isotherms of studied metal ion adsorption are presented in table 2: Table 2 Isotherm model constants and correlation coefficients for adsorption of Pb2+ ions on BAC

Fig.7 Freundlich isotherm plot for the sorption of Pb2+ions on BAC 4.3.3 Temkin Isotherm This isotherm clearly takes into account the interactions between adsorbing species and the adsorbate. It assumes that (i) the heat of adsorption of all the molecules in the layer decreases linearly with coverage due to adsorbate–adsorbent interactions, and (ii) adsorption is characterized by a uniform distribution of binding energies up to some maximum binding energy (32). The Temkin isotherm has been used in the form as follows: =

ln (KT

) ------------------------- (6)

The linearized form of the above equation has the following form, which can be plotted as q e against ln Ce to determine the isotherm constants BT, and KT from slops and intercepts (fig. 8), respectively. =

ln KT +

ln Ce --------------- (7)

BT = ----------------------------- (8) = BTlnKT + BT ln ------------------- (9) Where, BT and bT are Temkin constants KT is Temkin adsorption potential (L/g)

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The results shown in table.2 revealed that the Freundlich isotherm model achieved best fit with the equilibrium adsorption data, which have highest correlation coefficient value (R2) of Pb2+ is 0.9350. It indicates the multilayer adsorption nature of this metal ion takes place on BAC. The adsorption capacity (KF) of the adsorbent of Pb2+ had a value of 0.68 mg/g respectively. 3.4 Adsorption kinetic study The study of adsorption kinetics in wastewater treatment is important as it not only provides valuable insight into the reaction pathways and the mechanism of sorption reactions, but also describes the solute uptake rate, which in turn control the residual time of sorbate uptake at the solid-solution interface (33). The kinetic data was obtained from the adsorption of Pb2+ ions on BAC. This was studied by includes the common kinetics such as Zero, first, second, third order, pseudo-first order, pseudo-second order and Intraparticle diffusion models. The best fit model was selected based on the linear regression correlation coefficient (R2). The R2 values of zero, first, second and third order kinetics does not come under recommended (shown in table.3). So, the corresponding kinetic adsorption plots are not shown. 3.4.1 Pseudo-first order kinetic model The pseudo-first order kinetic model assumes that the rate of occupation of sorption sites is proportional to the number of unoccupied sites. The pseudo-first order equation was expressed according to Lagergren (34). Where (mg/g)

= ( ) --------- (10) - amount of metal ions adsorbed at equilibrium - amount of metal ions adsorbed at time t

(mg/g) International Journal of Water Research 2015; 5(2): 33-46

- pseudo first order rate constant ( ) The equation applicable to experimental results are generally differs from a true first order equation in two ways: the parameter k1(qe − qt) does not represent the number of available sites, and the parameter log q e is an adjustable parameter which is often not found in equal to the intercept of a plot of log (qe − qt) against t, whereas in a true first-order sorption reaction, log qe should be equal to the intercept of log(qe − qt) against t. In order to fit equation 10 to the experimental data, the equilibrium sorption capacity qe must be known. The pseudo first order kinetics of Pb2+ was studied for different concentrations of Pb2+, results were obtained and presented in figure 9.

Fig.9 Pseudo first order plots of Pb2+ sorption on BAC

Figure 9 showed the linear plots of log (q e-qt) against t at initial metal ion concentration of 30 mg/L, 60 mg/L and 90 mg/L. The k1 and values were determined from the slope and intercept of the linear plots respectively and given in table 3. Table 3 reveals the values of , experimental and calculated values of qe, as well as the R2 values for the pseudo-first order kinetic plots. As can be seen, also the R2 values obtained from the plots were high. The calculated values of qe were far lower than the corresponding experimental data obtained. This suggested that a poor fit between the kinetics data and the pseudo-first order model. Theses results were confirmed that this adsorption system is not follows a pseudo first order reaction. 3.4.2 Pseudo-second order kinetic model The adsorption kinetics may also be described by a pseudo second order. The pseudo second order is based on the assumption that the rate limiting step may be chemical sorption involving valence forces through sharing or exchange of electrons between heavy metal ions and adsorbent. The pseudo-second order kinetic rate equation was used in this study according to Gupta, et al., (35). = + t ------------------- (11) Where h = k2qe2 (mg/g min) is the initial sorption rate. The pseudo second order kinetic model was studied with different concentrations and the results were described in figure 10. The figure does give the linear plots of t/qt against t at all studied concentrations of Pb2+ solutions. The values of qe and h were calculated from the slope and intercept of the respective plots, and also calculated the k2 for each plots are presented in table.3.

Fig.10 Pseudo second order plots of Pb2+ sorption on BAC Table 3 reveals that all three linear plots with different initial concentrations of Pb2+ (shown in fig.10) have R2 values of 1. This indicates that the kinetics data fitted perfectly well with the pseudo second order model. In addition to the high values of R2, the calculated qe values also almost agreed well with the experimental data obtained from the pseudo second order kinetics. From table 3 also observed that the values of ‘h’ increased from 7.64 to 18.52 when the initial concentration

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of Pb2+ ions increased from 30 mg/L to 90 mg/L respectively. This was because the higher the initial concentration of Pb2+ ions, the greater chances of collision with the binding sites of adsorbent and hence leads to a higher initial sorption rate. The values of k2 was observed as higher than the corresponding values of k1. So, the pseudo second order kinetic model assumed as the best fit for this adsorption studies and also the sorption rate is proportional to the square of number of unoccupied sites


(36). In addition, the values of k2 get decreases from 1.02g/mg min to 0.28 g/mg min with increasing the initial concentration of Pb2+. This is occurred because at higher concentration of metal ions, the competition for surface active sites was high and consequently lower sorption rates are obtained. The similar result was reported earlier (37). The pseudo-second order kinetic model was also

reported to fit well with the kinetics data from studies of a number of authors, including the adsorption of Cd 2+ ions on pomelo peel (37), adsorption of Cu2+ ions on Tectona grandis leaves (38), adsorption of Pb2+ ions on pumpkin seed shell activated carbon (39), adsorption of Ni2+ions on potato peel, and adsorption of Cr6+ ions on cooked tea dust (40).

Table 3 The adsorption rate constant and correlation coefficient (R2) of adsorption kinetics model in zero, 1 st, 2nd, and 3rd orders and pseudo first and second orders at different [Pb 2+] at constant pH - 6, and and 40 g/l of adsorbent dose at 298K. Initial Zero-order First-order Second-order Third-order conc. K (mg/l) 30 0.0048 0.4916 0.0015 0.5134 4.9130 0.5331 3.1902 0.5502 60 0.0086 0.4548 0.0012 0.5021 1.8477 0.5443 5.5542 0.5910 90 0.0013 0.5861 0.0011 0.6323 1.1078 0.6760 2.0839 0.7233 Pseudo first-order Pseudo second – order R2 qe (mg/g) qe (mg/g) K1 K2 h 2 R (min-1) (g/mg) (mg/mg) Exp. Calc. Exp. Calc. 30 0.0234 2.736 0.990 0.99 1.02 2.736 2.741 7.6394 1 60 0.016 5.418 0.828 0.8279 0.4665 5.418 5.426 13.7363 1 90 0.0106 8.055 0.971 0.9713 0.2843 8.055 8.071 18.5185 1 by only a film diffusion. Also the value of the intercept at the different concentrations gives an idea about the 3.4.3 Intra-particle Diffusion Adsorption is a surface phenomenon, but the thickness of the boundary layer. If the intercept become adsorbate may also diffuse into the interior pores of the larger confirms the thicker the boundary layer (41). adsorbent, which may influence the rate of the reaction. However, some factors have been attributed to be Thus the result also analyzed in terms of intraparticle responsible for the rate determining step of adsorption of diffusion model to investigate whether the intraparticle particular adsorbate. The factors that assign rate diffusion is the rate controlling step or not in adsorption of determining step mechanism as film diffusion or external lead ion on bamboo activated carbon. According to transport mechanism have been reported to be poor mixing, Vadivelan and Kumar (41), sorption mechanisms small particle size, and dilute concentration of the between solid-liquid solution systems follow certain stages: adsorbate and high affinity of the adsorbate for the movement of solutes to the exterior surface of adsorbent adsorbent. The factors that include good mixing, large which implies boundary surface diffusion (external mass particle size, high concentration of adsorbate and low transfer or film diffusion) that the movement of solute from affinity of adsorbate for adsorbent assigned intraparticle external surface of the adsorbent which is intraparticle diffusion mechanism as the rate determining step (43). diffusion. Consequently the prepared bamboo activated carbon has The amount of metal ion sorbed per unit mass of high affinity for the metal ion and as such followed film adsorbents, qt at any time t, was plotted as a function of diffusion mechanism. square root of time, t 1/2. The diffusion model can be expressed by following equation. =Ø+ √t ----------- (12) Where, qt is the amount adsorbed (mg/g) at time t and kip (mg/g min1/2) is the intra-particle diffusion rate constant which was obtained using the equation and Ø is intraparticle diffusion constant, i.e. intercept of the line (mg/g). If plot of qt versus t 1/2 gives a straight line that pass through the origin, then it suggests that the intra-particle diffusion contributes predominantly in the rate-determining step (42). Figure 11 depicts the linearity of plot between q t (amount adsorbed) vs. time, t1/2 that does not pass through the origin. The values obtained from the intercept (5.19 – 7.623) of the intraparticle diffusion kinetic model at different adsorbate concentrations of Pb 2+ are not the same and did not pass through the origin which indicates the intra-particle diffusion is not the rate controlling step. It Fig.11 Plot of Qt versus (time)1/2 for the adsorption of Pb2+ implied that the adsorption process of Pb 2+ was controlled on BAC

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Table 4 Kinetics Parameter of intra-particle diffusion models for Pb2+ sorption on BAC. Initial con.(mg/l)

Kip (g/mg/min1/2)

Ø (mg/g)

30

0.0173

5.1931

0.6355

60

0.0173

5.1931

0.6355

90

0.0254

7.7226

0.7625

The relatively higher initial rates K2 (shown in table 3) with the large intercepts (Ø in mg/g) of the linearized intraparticle plots (shown in table 4) which are almost the same with the (exp), suggest that the process was larger surface adsorption (22). It is also possible to suggest that from the parameters, Kip and Ø values. Moreover, it can be observed that the linearity of intra-particle diffusion could not be applicable for the whole time interval of the adsorption process and also is higher than Kip, which may indicate that the overall adsorption process can be represented better by pseudo second order. 3.5. Thermodynamics study of adsorption of Pb (II) onto BAC Thermodynamic parameters G, H and S can be obtained from the studies of Pb 2+ adsorption from aqueous solution of BAC at various temperatures (at 298 – 318K) using recommended empirical equations. The values of H and S are determined (15) from the slope and intercept of the linear plots of lnKc versus 1/T shown in fig.12. In general these parameters indicate that the

adsorption process is spontaneous or not and exothermic or endothermic. The standard enthalpy change (Hº) for the adsorption process is: (i) Positive value indicates that the process is endothermic in nature. (ii) Negative value indicates that the process is exothermic in nature and a given amount of heat is evolved during the binding metal ion on the surface of adsorbent. This could be obtained from the plot of percent of adsorption (Efficiency %) vs. Temperature (T) present in fig.13. It shows that the percent adsorption increase with increase temperature; this indicates for the endothermic processes and the opposite is correct (15). The positive value of (Sº) indicate an increase in the degree of freedom (or disorder) of the adsorbed species. This can be also seen from the positive value of ΔH0 for metal ion adsorption, that is the endothermic nature of the adsorption according to the calculated data presented in table 6 for a given temperature range. This result is in agreement with the findings of other researchers for Cu2+ adsorption on surfactant modified montmorillonite, and for lead on kaolinite, montmorillonite and Celtek clay (44).

Fig.11 Vant’Hoff plot of lnKc versus 1/T at temperatures range of 298-318 K

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Fig. 13 Effect of temperature on the adsorption efficiency of Pb2+ Table.5 Thermodynamic parameters that are computed from the linearized plot of ln kc versus 1/T at the temperatures range of 298-318 K. Metal ion

Temperature (K) 298

G (KJ/mol ) -1.1322

Pb2+

308 318

-1.9358 -7.1146

H (kJ/mol)

S (J/k.mol )

91.963

306.37

From the entropy value of metal ion adsorption (see table 5) could be observed that the metal ions in aqueous solution were in more stable arrangement, since stability is associated with an ordered distribution than those in the adsorbed state. So the rise in temperature might have positive contribution to enhance the adsorption efficiency by causing the increased collision between the metal ions and the surface sites (44). The result shown table 5 is in an acceptable range of H, indicates the favorability of physisorption. It is very clear that from the results, physisorption is much more possible for the adsorption of lead ion. The positive values of H also indicate the endothermic nature of adsorption. The negative value of ΔG indicates the feasibility and spontaneous nature of the adsorption process and more negative which indicates that the adsorption process becomes more spontaneous with rise in temperature, which favors the adsorption process. In other words that the adsorption process is spontaneous and the degree of spontaneity increases with increasing the temperature (45). The value of ΔS can be used to describe the randomness during adsorption process; the positive value of ΔS reflected the affinity of the adsorbent for particular heavy metal ions and confirms the increased randomness at the solid–solution interface during adsorption (45).

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4. Conclusions The isotherm, Kinetics and thermodynamics of batch adsorption of Pb2+ ions from it aqueous solution using activated carbon prepared from South Ethiopian based bamboo has been investigated and drawn following conclusions:  Adsorption capacity of adsorbate had seen to decrease with increasing adsorbent dose while the efficiencies increased. In addition, a decrease in efficiency of adsorbent was observed with increasing initial metal ion concentration.  The adsorption process follows Langmuir, Freundlich and Temkin isotherms but a better sorption fit of Pb 2+ ions by bamboo activated carbon (BAC) using Freundlich isotherm model was obtained. It indicates a multilayer formation over a surface of the material with the correlation coefficient of (R2) of 0.935 and the maximum adsorption capacity determined is 0.686 mg of Pb2+ ions adsorbed per g of BAC was obtained.  Adsorption kinetics was modeled using true and pseudo order kinetics and intra-particle diffusion models. The kinetic data obtained from this study fitted well with the pseudo-second order model. Also the sorption profiles derived based on the pseudo second order kinetic model showed a good agreement with the experimental curves and the pseudo second order kinetic reaction is the rate controlling step with some intra particle diffusion taking place.  The determined negative free energy changes (ΔG) and positive entropy change (ΔS) indicate the feasibility and spontaneous nature of the adsorption process. The positive value of enthalpy change (ΔH) suggests that the adsorption process was an endothermic. Finally activated carbon produced from bamboo demonstrated that they are a promising adsorbent derived from


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Source of support: Nil; Conflict of interest: None declared

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