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International Journal of Modern Chemistry, 2014, 6(1): 28-47 International Journal of Modern Chemistry ISSN: 2165-0128 Florida, USA Journal homepage: www.ModernScientificPress.com/Journals/IJMChem.aspx Article
Adsorption of Zinc and Chromium ions from Aqueous Solution onto Sugarcane Bagasse Temitope Sola Fasoto *, Jacob Olalekan Arawande, Akinyinka Akinnusotu Department of Science Laboratory Technology, Rufus Giwa Polytechnic, P.M.B 1019 Owo, Ondo State, Nigeria * Author to whom correspondence should be addressed; E-Mail: [email protected]; Tel.: +08033550244. Article history: Received 16 December 2013, Received in revised form 25 January 2014, Accepted 27 January 2014, Published 11 February 2014.
Langmuir isotherms respectively was the best for Freundlich isotherm from the equilibrium studies. ASB, MSB and USB are considered as potential adsorbents for the removal of Zn(II) and Cr(VI) ions from dilute aqueous solutions. Keywords: adsorption; heavy metals; activated carbon; equilibrium; sugarcane bagasse; zinc; chromium.
stone and nutshells , peanut hulls , nut shells  and corn cob catalyzed by potassium salts  have been investigated. Sugarcane (Saccharum officinarum) is a grass that is harvested for its sucrose content. After extraction of sugar from the sugarcane, the plant material that remains is termed bagasse. Currently, the bagasse production in the United States is about 8.6 million tons per year. About 1.4 million tons are available for other uses as most of the bagasse is burned to produce steam power . Bagasse is cheap, readily available, and has high carbon content . There is therefore the need to transform this waste to valuable, useful, enhanced products which would promote the environment and the scientific community in the search for more cost effective adsorbents than the use of conventional adsorbent (activated carbon). Therefore, the objective of this study is to investigate the possibility of using sugarcane bagasse (SB) as an adsorbent and to compare the impact of activation and EDTA modification on the adsorption capacity of the SB for the removal of Zn(II) and Cr(IV) ions from aqueous solution.
sizes. 2.3. Preparation of Activated Sugarcane bagasse (ASB) About 5 g of the modified adsorbent was introduced into washed, dried and pre-weighed crucibles. This was introduced into a furnace at 500 oC for 5 min and poured from the crucibles into a bath of ice block. The excess water was filtered and the bagasse was sun dried. This process was repeated until a reasonable amount of carbonized bagasse was obtained . The carbonized bagasse was washed using 10% HCl followed by addition of hot water and rinsed with distilled water to remove surface ash and residual acid respectively . The fine solids were sun dried and further oven dried at 105 oC for one hour. The activated bagasse obtained was air-dried and first sieved through a 2.000 mm mesh and then through 1.000 mm, 0.710 mm, 0.500 mm, 0.250 mm, 0.125 mm and base (0.125 passed) to give respective particle sizes . 2.4. Preparation of Solutions (Adsorbate) The stock solutions of Zn(II) and Cr(VI) metal ions were prepared by dissolving ZnSO 4.7H2O and K2Cr2O7 in distilled water. The resulting solutions were serially diluted to obtain different concentrations of Zn(II) and Cr(VI) metal ions solutions. The main characteristics of these ions are summarized in Table 1.
Table 1. Main characteristics of metal ions studied Property
used for titration, Vb is the volume of thiosulphate (cm3) used for blank titration and Ma is the mass of adsorbent used in gram .
2.5.2. Moisture content About 3 g of each adsorbent was weighed into washed, dried and weighed petri-dish. This was dried in an oven at 105 oC for 2 days. The dried adsorbent was cooled in desiccators for 1 h. To ascertain constant weight, the adsorbent was monitored at intervals of thirty minutes. The percentage moisture content was obtained using equation 2. Loss in weight on drying (g) Moisture (%) = 100 Initial sample weight (g) Percentage dry matter (%) was calculated using equation 3 below. Oven dry weight (g) Dry Matter (%) = 100 Initial sample weight (g) The analysis was carried out in triplicate and the mean was recorded.
2.5.3. Bulk density The tapped density of the adsorbents was determined by a tapping procedure by using a 50 mL graduated plastic cylinder . The cylinder was tapped on the work-bench until the volume of the sample stop decreasing. The mass and volume were recorded and density calculated using equation 4 . ρ = Mass/Vol. occupied
2.5.4. Ash content determination The crucible was preheated in a furnace at about 500 oC, allow cooling in a desiccators and its weight was taken at relatively contact value. Oven dried sugarcane bagasse was transferred into the crucible and placed in a furnace at 500 oC for about 3 h. The sample was removed and allowed to cool to constant weight in desiccators. The percentage ash content was calculated using the equation below. Ash weight (g) Ash Content = 100 (5) Oven dry weight (g) 2.5.5. pH measurement The 1% solution of the sample was made using distilled water. The pH of the supernatant was obtained after 1 h using a pre-calibrated pH meter (Meltler Toledo pH meter).
2.5.6. Determination of porosity of adsorbents (based on swellings procedure) The 0.5 g adsorbent was dispersed in 20 mL water (V1) in a graduated centrifuge tube. This was centrifuged for 30 min at 1000 rpm using centrifuge (80-1 Table Top low speed). The resulting volume was obtained as V2 and recorded . The porosity was obtained using equation 6. Porosity = V1/V2
2.6. Adsorption Studies of Sugarcane Bagasse Adsorbents Adsorption experiments were conducted using different mass of ASB, MSB and USB as adsorbents in 250 mL conical flasks of various concentrations of Zn(II) and Cr(VI) solutions prepared from the stock solution. The conical flasks containing the adsorbent were agitated for 10 min at the highest speed on a HJ-3D constant temperature magnetic stirrer, allowed to settle for maximum adsorption, filtered and the filtrate analyzed for heavy metals using Buck 210 VGP flame atomic absorption spectrophotometer (FAAS). The specified parameters used for this study were: mass of adsorbent = 0.5 g, pH = 6, particle size =125 mm passed, initial adsorbate concentration = 50 mg/L, contact time = 120 min and temperature = 30 oC (except the varied parameter). The pH range was varied between 2.0 to 12.0 using either dilute HCl or NaOH solution to obtain the desired pH value on a Metler Toledo pH meter to determine the effect of pH on the sorption capacities of various adsorbents considered. Temperature of 30 oC, 40 oC, 50 oC and 60 oC at the specified condition was used to investigate the effect of temperature on adsorption capacity of the different adsorbents (ASB, MSB and USB) on the removal of Zn(II) and Cr(VI) ions from aqueous solution. The effect of dosage on the adsorption capacity of the adsorbents was investigated at different dosages in the range 0.1 – 0.9 g/L. Impact of particle sizes on the adsorption potentials of the different adsorbents was investigated using 1.000 mm, 0.750 mm, 0.500 mm, 0.125 mm retained and 0.125 mm passed particle sizes. Contact time also was studied in the range between 20 - 120 min. Adsorption isotherms were studied using Freundlich, Temkin and Langmuir isotherms models at various initial concentrations of the studied metal ions in the range of 20 - 100 mg/L. The amount of the metal adsorbed (qe) and the percentage metal ion removed (% adsorbed) for Zn(II) and Cr(VI) ions was calculated using equations 7 and 8 . V (Co – Ce) qe = 100 M
(Co – Ce)
100 (8) Co where qe is the amount of adsorbate ion adsorbed in milligram per gram of the adsorbent, Co is the % Adsorbed =
initial concentration of the metal ion before the adsorption process, Ce is the equilibrium concentration of the metal ion in the filtrate after adsorption process, M is the mass in gram of the adsorbents, and V is the volume of the solution in mL.
3. Results and Discussion 3.1. Physicochemical Analysis of Sugarcane Bagasse The physicochemical parameters of sugarcane bagasse adsorbents are presented in Table 2. The iodine adsorption number (IAN) is a measure of the adsorption of iodine from an aqueous solution onto the adsorbents. It is a measure of pore structure and an indication of the total surface area for adsorption. Adsorbents with higher iodine number value should perform better in removing small sized contaminants and are classified as effective activated carbon. Higher value suggests high degree of activation .
Table 2. Physicochemical parameters of the ASB, MSB and USB Parameters
adsorption . The lower pH value of ASB when compared to that of MSB and USB suggests ASB as better adsorbent, since the lower pH adsorbent between pH 7 to 4 adsorb more. Bulk density is a factor that affects the length of the filtration cycle of activated carbon . The bulk density values for ASB, MSB and USB were 0.235 g/cm3, 0.254 g/cm3 and 0.269 g/cm3, respectively. The variation in bulk density can be attributed to adsorbents treatments. The adsorbent with least bulk density has its particles occupied least of the available volume space, allowing more space between particles and consequently highest porosity and adsorption capacity. The ash content values of the adsorbents were within the range (0.0 – 6.0%) reported by Manocha . The lower ash content values (1.57, 2.69 and 2.80) for ASB, MSB and USB respectively compared to 5.9 reported for Vitex doniana nut  was an indicator that ASB, MSB and USB are better adsorbents since the ash serves as interference during the adsorption process [25, 26]. Highly porous adsorbents have larger surface areas and possess high adsorption capacities . The higher porosity value (0.882) for ASB indicated that it has higher adsorption capacity when compared to MSB and USB with 0.812 porosity value. The physicochemical analysis indicated that the order of adsorption capacities was ASB > MSB > USB. Therefore, ASB, MSB and USB possess higher degrees of adsorption and high affinity for small sized contaminants. 3.2. Batch Adsorption Studies 3.2.1. The effect of initial solution pH The effect of changes in pH on the adsorption of Zn(II) and Cr(VI) ions onto bagasse adsorbents at the specified condition is presented in Fig. 1.
The adsorption potential of the sugarcane bagasse adsorbents in removing Zn(II) and Cr(VI) metal ions from aqueous solution depends on pH value and this also is related to the ionic state of the adsorbate and nature of the adsorbent (modified or activated ) used. Zn(II) ions uptake level increased from 53.87 to 68.07%, 55.86 to 74.92% and 71.46 to 91.14% for USB, MSD and ASB respectively while Cr(VI) ions uptake level increased from 54.74 to 69.14%, 54.63 to 76.02% and 74.94 to 95.38% for USB, MSD and ASB respectively when the solution’s pH value was increased from 2 to 8. There was very slight decrease in the adsorption potential of the adsorbents for Zn(II) and Cr(VI) when the solution pH value was increase above 8. This is line with studies on tea factory waste . Binding sites are protonated or positively charged in low pH causing repulsion between the metal cation and the adsorbents while at a higher pH value, binding sites may start deprotonating thereby exposing various chemical functional groups to metal binding .
3.2.2. Effect of temperature The adsorption experiments of Zn(II) and Cr(VI) metal ions onto unmodified (USB), modified (MSD) and activated sugarcane bagasse (ASB) adsorbents were run to study the effect of temperature variation at 30, 40, 50 and 60 oC at the specified condition. Fig. 2 shows that the adsorption potential of the adsorbents increased as temperature increases. The increase in entropy of the system causes more interactions between the number of metal ions present in the solution and the adsorbents active sites by increasing the rate of bombardments of metal ions onto the available adsorbent surface area [5, 26].
Adsorbent dosage was varied from 0.1 g to 0.9 g under the specified condition. Fig. 3 shows that increased adsorbent loading leads to increase in the percentage metal ions adsorbed. The adsorption of Zn(II) and Cr(VI) attained the maximum uptake levels for unmodified (USB), modified (MSD) and activated sugarcane bagasse (ASB) at adsorbent dosage of (0.5 g with 67.82% adsorbed and 0.7 g with 67.93% adsorbed), (0.7 g with 71.88% and 0.9 g with 72.48% adsorbed) and (0.9 g with 89.98% adsorbed and 0.9 g with 95.02% adsorbed) for Zn(II) and Cr(VI) adsorbed respectively. The increase in the percentage removal with increase in the adsorbent dosage is due to the increase in the number of adsorbent active sites. Decrease in adsorption potential noticed at higher adsorbent dosage mass may be due to overloading of the adsorbent on the adsorbate resulting in less total adsorbent active surface area available to metal ions and an increase in diffusion path length .
Figure 4. Effect of particle sizes on the sorption of Zn(II) and Cr(VI) ions from aqueous solution by various sizes of unmodified (USB), modified (MSD) and activated sugarcane bagasse (ASB). 3.2.5. Effect of contact time Contact time between the adsorbent and the adsorbate is a highly significant parameter in the adsorption processes. Fig. 5 shows that adsorption increases with increase in time of contact due to the availability of more time for metal ions to make an attractive complex with the adsorbents. Initial removal occurs immediately as soon as the metal and adsorbents came into contact and further increase in contact time did not increase the uptake much due to decrease of the easily available active sites for the binding of metal ions as equilibrium sorption is attained.
Figure 5. Effect of contact time on adsorption of Zn(II) and Cr(VI) metal ions adsorbed unto unmodified (USB), modified (MSD) and activated sugarcane bagasse (ASB). 3.2.2. Effect of initial metal ion concentration The effect of initial metal ion concentrations on percentage metal adsorbed is show in Table 3.
The concentration in the range between 20 to 100 mg/L for the metal ions was investigated under the specified condition. The removal of metal ions by unmodified (USB), modified (MSD) and activated sugarcane bagasse (ASB) was found to increase with increase in initial adsorbate concentration . The percentage adsorption of the metal ions increases with the initial concentration from 20 mg/L to 40 mg/L and the increase gradually decreases from 60 mg/L to 100 mg/L.
Table 3. Amount (percentage) of Zn(II) and Cr(VI) metal ions adsorbed at equilibrium by unmodified (USB), modified (MSD) and activated sugarcane bagasse (ASB) at various concentrations of the adsorbate solution Zn(II) Cr (VI) Adsorbent Initial Conc. (mg/L) %Adsorbed qe (mg/g) %Adsorbed qe (mg/g) USB
by Freundlich isotherm based on the correlation coefficient values obtained. This means that the adsorption of the metal ions onto unmodified (USB), modified (MSD) and activated sugarcane bagasse (ASB) was more physical, heterogeneous and multilayer in nature. The order of fitness was Freundlich > Temkin > Langmuir.
3.3.1. Freundlich isotherm The isotherm model is defined as equation 9 below : qe = KFCe 1/n
Its re-written form is given as: log qe = log KF + 1/n log Ce
In Eq. 10, qe is the amount of the metal ions adsorbed at equilibrium (mg/g), Ce is the equilibrium concentration of the metal ion in solution (mg/L), KF and n are Freundlich constant and intensity factor, respectively. The values of n and KF are calculated from slope and intercept of plots of log qe versus log Ce (Figs. 6-8), which are presented in Table 4.
Table 4. Freundlich adsorption isotherm parameters for the removal of Zn(II) and Cr(VI) ions by activated (ASB), modified (MSB) and unmodified sugarcane bagasse (USB) Adsorbent
Figure 7. Freundlich adsorption isotherm for Zn(II) and Cr(VI) adsorption by modified sugarcane bagasse (MSB).
Figure 8. Freundlich adsorption isotherm for Zn(II) and Cr(VI) adsorption by unmodified sugarcane bagasse (USB).
3.3.2. Temkin isotherm Temkin isotherm assumes that heat of adsorption decrease linearly with the adsorption onto the surface at a particular temperature and the adsorption is characterized by a uniform distribution of binding energies. Temkin isotherm is expressed in linear form by the following equation : qe = B lnA + B lnCe (11) RT B= b where B is related to the heat of adsorption, T (K) is the absolute temperature, R is the universal gas constant (8.3143 J/mol), b indicates the adsorption potential of the adsorbent (J/mol), A is the equilibrium binding constant (L/mg). The parameters for the Temkin model are obtained from the plot of qe versus lnCe (Figs. 9-11). The values for these parameters are given in Table 5.
Figure 11. Temkin adsorption isotherm for Zn(II) and Cr(VI) adsorption by unmodified sugarcane bagasse (USB). 3.3.3. Langmuir adsorption isotherm The Langmuir adsorption isotherm model assumes that adsorption takes place at specific homogeneous sites within the adsorbent : qmax bCe qe = 1 + bCe The linear form of the Langmuir isotherm model is: 1/qe = 1/( qmax bCe) + 1/qmax
where qe is the amount of adsorbate adsorbed per gram of dried adsorbent at equilibrium (mg adsorbate/g of dried adsorbent), qmax is the constant relating to the maximum amount of adsorbate ion bound per g of adsorbent for a monolayer (mg/g), b is Langmuir constant or adsorption coefficient or the adsorption affinity (L/mg) for binding of adsorbate on the adsorbent sites, and Ce is equilibrium (residual) adsorbate concentration in solution after sorption (mg/L). A plot of 1/q e vs 1/Ce should be a straight line with an intercept as 1/qmax and a slope as 1/(qmax b). Langmuir isotherms plots are presented in Figs. 12-14. The values of constants qmax and b can be calculated, and were reported in Table 6. From Table 6, Langmuir isotherms for the adsorption of Zn(II) and Cr(VI) ions onto sugarcane bagasse based on the correlation coefficient, qmax (mg/g) and b(L/mg) follow the order of ASB < MSB < USB. This suggests the formation of monolayer of Zn(II) > Cr(VI) ions onto the outer surface of various adsorbents favours USB > MSB > ASB adsorbents.
Figure 14. Langmuir adsorption isotherm for Zn(II) and Cr(VI) adsorption by unmodified sugarcane bagasse (USB).
4. Conclusions Based on the above study the following conclusions were drawn: (1) Particle sizes, pH, contact time, metal ions concentration, adsorbent dosage and temperature had remarkable effects on the metal uptake level of the adsorbents. (2) Activated carbon prepared from sugarcane bagasse was a good adsorbent for the removal of Zn(II) and Cr(VI) from aqueous solution. (3) Adsorption capacity was ASB > MSB > USB, and adsorbents prepared from sugarcane bagasse were found to be promising adsorbents for the removal of Zn(II) and Cr(VI) ions from aqueous solutions. (4) The equilibrium data based on correlation coefficients could be best explained by Freundlich isotherm, which suggest that the adsorption of the metal ions onto unmodified (USB), modified (MSD) and activated sugarcane bagasse (ASB) was more physical, heterogeneous and multilayer in nature.
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