Comparative Study on Adsorption of Mn ( II ) from Aqueous Solutions on Various Activated Carbons

The adsorption of Mn(II) on indigenously prepared activated carbons (IPAC) from Bombax malabaricum, Pithecelobium dulse, Ipomea batatas and Peltaforum ferraginium have been studied. The effects of various experimental parameters have been investigated using batch adsorption technique. The extent of Mn(II) removal increased with decrease in initial concentration of the Mn(II), particle size of the adsorbent and increased with increase in contact time, amount of adsorbent used and the initial pH of the solution. Adsorption data were modeled using Freundlich and Langmuir adsorption isotherms and first order kinetic equations. The kinetics of adsorption was found to be first order with regard to intraparticle diffusion rate. The results indicate that such carbons could be employed as low cost adsorbents in waste water treatment for the removal of Mn(II).


Introduction
Manganese(II) is present in water supplies as a result of natural processes involving both catchments and erosion.The dissolution of Mn(II) containing sediments and minerals at or near the sediment water interface 1,2 .Occasionally, discharges also may be source of Mn in water 3 .Mn(II) in surface waters is a micronutrient but elevated concentrations are toxic to fish and humans and impair drinking water quality 4,5 .Mn(II) concentrations in excess of the drinking water standard can result in the formation of oxide deposits in pipelines, discoloration of water and laundry and impart an unpleasant metallic taste 6,7 .So, the determination of Mn in public and industrial water is important for the environment and for human health.In drinking water sources, the secondary maximum 8 contaminant level (SMCL) for Mn(II) must not exceed 0.05 mg/L.
Many studies have been conducted on Mn(II) removal in the past, including those designed to evaluate chemical dynamics, experiment with packed columns of limestone, and evaluate passive treatment systems [9][10][11] .Microbial remediation efforts include designed wetlands, microbial bioreactors and pellets of mixed microbial cultures [12][13][14][15] etc. Pransenjit Mondal 16 reported that Mn can be removed by 41% by using granular activated carbon (GAC).Dinesh Mohan 17 reported that Mn is removed by using lignite up to 25.84%.M.M.Nassar 18 reported that removal of manganese ion by adsorption on palm fruit bunch and maize cobs was in the range of 79-50%.M. Saad 19 reported that manganese is removed from wastewater by using sulphurized activated carbon obtained from burnt date stones.
In the present water management study the author prepared some adsorbent materials from agricultural wastes and an attempt was given to remove water soluble manganese by adsorption with prepared activated carbons.

Standard manganese solution preparation
A stock solution of manganese solution used in this study was prepared by dissolving an accurate quantity of 1.000 g of manganese metal in 50 mL of 6 N HNO 3 and dilute to one litre 1000 µg/mL standard Mn(II) solution.Other concentrations were prepared from this stock solution by dilution.Fresh dilutions were used for each experiment.All the chemicals used were in analytical grade.

Analytical method
Manganese concentrations of the solutions were determined by using an Atomic absorption spectrophotometer (ECIL AAS4103/AAS4127) with an air -acetylene flame oxidizing (lean, blue) and with light source of hallow cathode lamp.Deuterium back ground correction was used and the spectral slit width was 0.2 nm.The working current and wave length were 5.0 mA and 279.5 nm, respectively.The instrument response was periodically checked by using standard metal solution.

Sensitivity
For the standard conditions described above, the sensitivity about 0.04 µg/mL Mn(II) for 1% absorption.A standard containing 1 µg/mL Mn(II) will typically give an absorbance reading of about 0.11 absorbance units (about 40% absorption).

Linear working range
For the standard conditions described above the working range for Mn(II) is linear up to concentrations of approximately 60 mg/L in aqueous solution.
Procedure for the adsorption of Mn(II) using the prepared carbons as adsorbents 50 mL of standard manganese solution (10 mg/L) is pipetted out into a conical flask.0.1 g/L of one of the prepared carbons is added and is stirred at 240 rpm mechanically for 30 min.The solution is filtered through Whatmann No. 42 filter paper.The metal ion concentration in the sample after equilibrium determined from the standard curve.
Comparative Study on Adsorption of Mn(II) 695 The same procedure has been adopted for the experiments carried out by varying (1) adsorbent type Bombax malabaricum, Pithacelobium dulce, Ipomoea batatas and Peltophorum ferrugineum (BMC, PLDC, IBC and PFC) (2) Initial pH of the standard metal ion solution (ranging from 5 to 9), (3) agitation time (ranging from 10 -90 min), (4) dose of adsorbent (ranging form 1.0 to 20.0 g/L and (5) initial concentration of the standard metal ion solution (ranging from 5 to 25 mg/L).

Results and Discussion
The adsorption experiments were carried out at different experimental conditions and the results are discussed below.

Effect of initial concentration
The effect of initial concentration of Mn(II) on the extent of removal of Mn(II) (in terms of percentage removal) on various adsorbents viz., BMC, PLDC, IBC and PFC was studied and the relevant data are given in Table 1.The percentage removal was found to decrease exponentially, while the amount adsorbed increased exponentially with the increase in initial concentration of Mn(II).This indicates that there exist reductions in immediate solute adsorption, owing to the lack of available active sites required for the high initial concentration of Mn(II).Similar results have been reported in literature on the extent of metal ions 20 .The adsorption capacities of IPACs are high and therefore they could be employed as low cost adsorbents for the removal of Mn(II).The percentage removal of Mn(II) by BMC is 99.3.Hence, among the IPACs BMC is the best low cost adsorbent material.

Adsorption isotherms
The adsorption data were analyzed with the help of the following linear forms of Freundlich and Langmuir isotherms 21 : Freundlich isotherm: log (qe) = 1/n log (C e ) + log (K f ) (1) Langmuir isotherm: 1/qe = (1/ a) + (1/ b a Ce) (2) where, log k f is roughly a measure of the adsorption capacity and 1/n, is an indicator of adsorption effectiveness; qe is the amount of Mn(II) adsorbed per unit mass of adsorbent (mg/g), a and b are the Langmuir constants which are the measures of monolayer (maximum) adsorption capacity (mg/g) and energy of adsorption (g/ L), respectively.The values of Freundlich and Langmuir parameters were obtained respectively, from the linear correlations between the values of (i) log q e and log C e and (ii) (C e /q e ) and C e .The adsorption isotherms parameters along with the correlation coeffcients are presented in Table 2.The observed statistically significant (at 95% confidence level) linear relationships as evidenced by the r-values (close to unity) indicate the applicability of these two adsorption isotherms and the monolayer coverage on adsorbent surface.
Freundlich and Langmuir isotherm plots are shown in Figures 1 and 2. The monolayer adsorption capacities of the adsorbents are found to be of the order: BMC> PLDC > IBC > PFC All the four IPACs are observed to possess high adsorption capacity and hence they could be employed as low-cost adsorbents as alternatives to commercial activated carbon (CAC), for the removal of Mn(II).Further, the essential characteristics of the Langmuir isotherm can be descried by a separation factor R L ; which is defined by the following Eq 22 : where, C is the initial concentration of Mn(II) (in ppm or mg/L) and b is the Langmuir constant (in g/L).The value of separation factor R L , indicates the nature of the adsorption process as given below:

Effect of contact time
The effect of contact time on the amount of Mn(II) adsorbed was investigated at the optimum initial concentration of Mn(II) (Table 1) and the data is presented in Table 3.
The extent of removal (in terms of q e ) of Mn(II) by these activated carbon (AC) was found to increase, reach a maximum value with increase in contact time.The relative increase in the extent of removal of Mn(II) after 50 min of contact time is not significant and hence it is fixed as the optimum contact time.
In batch type adsorption systems, monolayer of adsorbate is normally formed on the surface of adsorbent 23 and the rate of removal of adsorbate species from aqueous solution is controlled primarily by the rate of transport of the adsorbate species form the exterior/outer sites to the interior sites of the adsorbent particles 24,25 .

Kinetics of adsorption
The two important physical and chemical aspects for parameter evaluation of the sorption process as a unit operation are kinetics and the equilibria of sorption.Kinetics of sorption, described the solute uptake rate, which in turn governs the residence time of sorption reaction, is one of the important characteristics defining the efficiency of sorption.
The removal of Mn(II) is very rapid initially and decreases markedly before equilibrium is reached.The rate constant (k) is determined using the following Helffrich model (equation1).
The rate constant (k) can be determined using Helffrich equation ln [1-U (t)] = kt (4) Where, U (t) = (Ci -Ct)/Ci-Ce) Ci, Ct, Ce are the concentrations (mg/L) of adsorbent samples initially at any time't', and at equilibrium respectively.The straight line plotted between ln [1-U (t)] and't' indicates the applicability in aqueous system that follows reversible first order.The Kinetic of adsorption of Mn(II) and various AC have been studied by applying the following kinetic equations proposed by Lagergren.
Lagergren equation: log (qe-q) = log qe -(k ad .t) / 2.303 The values of first order rate constants and correlation coefficient (r-value) are given in Table 4.All the linear correlations were found to be statistically significant (as evidenced by r-values close to unity) at 95% confidence level and indicate the applicability of these kinetic equations and the first order nature of the adsorption process of Mn(II) on various adsorbents.The value of k calculated from the Helffrich model are found to be close to that computed from Lagergren equation for any given adsorbent.The rate of adsorption k is found to be high in BMC (-0.0081 min 1 ) and low in CFC (-0.0028 min 1 ).Lagergren plots, Helffrich model plots are shown, respectively, in Figures 3 and 4.

Intraparticle diffusion model
The adsorbate species are most probably transported from the bulk of the solution in to the solid phase through Intraparticle diffusion/transport process, which is often the rate limiting step in many adsorption processes; especially in a rapidly stirred batch reactor 23,26 .The possibility of Intraparticle diffusion was explored by using the Intraparticle diffusion model 27 .
where, qe is the amount of Mn(II) adsorbed per unit mass of the adsorbent (in mg / g) at time t; kp and c are respectively the Intraparticle diffusion rate constant (mg/g.minutes - 1/2 ) and the intercept.The values of amount of Mn(II) adsorbed have been correlated with the t 1/2 (minutes 1/2 ) for various adsorbents.This has resulted in linear relationship as evidenced by the r-values (Table 4), which indicate the existence of intraparticle Time minutes diffusion process.The values of Intraparticle diffusion rate constant (kp) calculated for various adsorbents are also reported in Table 4.The calculated value of kp for BMC is maximum (0.1765) and is minimum (0.1301) for PFC, which indicate that the Intraparticle diffusion process is more significant in BMC system than in PFC system.The values of intercept (C) give an idea of boundary layer thickness, i.e., the larger the intercept, the greater the boundary layer effect 28 .The values of intercept decrease in the order BMC > PLDC > IBC > IFC.
Further confirmation of the occurrence of Intraparticle diffusion has been obtained from the correlation of the values of log (% removal) and log (time) which has also resulted in linear relationships.This indicates that the process of Intraparticle diffusion takes place in the adsorption process.The values of slope calculated for different adsorbents have been found to be widely divergent from the value of 0.5, which corresponds to Intraparticle diffusion as the rate determing step 28,29 .The divergence of slope values from 0.5 indicates that besides Intraparticle diffusion process, there may be other processes controlling the rate of adsorption, all of which may be operating simultaneously.The linear relation between log (time) versus log (percent removal) is shown in Figure 5. Intraparticle diffusion plots for the removal of Mn(II) by adsorption on various adsorbents are shown in Figure 5 respectively.

Effect of dose of adsorbent
The effect of dose of adsorbent on the amount of Mn(II) adsorbed was studied (Table 5).The equilibrium value of amount adsorbed was observed to decrease with increase in dose.The percentage removal of Mn(II) increased with the increase in dose of adsorbent.This may be due to the increase in availability of surface active sites resulting from the increased dose and conglomeration of the adsorbent 30 .The increase in the extent of removal of Mn(II) is found to be insignificant after a dose 3.0 gm/L for BMC, PLDC and CAC for IBC, PFC it is 3.5 g/L which are fixed as the optimum doses of adsorbent.

Effect of initial pH
The effect of initial pH of the Mn(II) solution on the amount of Mn(II) adsorbed was studied by varying the initial pH under constant process parameters.The results are shown in Figure .6.The increase in initial pH increases the amount of Mn(II) adsorbed.In all the cases the percent removal increasing from acid media to basic media but after pH level 7.0 the percent removal is very little.In all the four indigenously prepared carbons the percent removal at pH 9.00 is almost doubled.In case of BMC initial concentration increases percentage removal also increases.This adsorbent shows adsorption capacity from 89.41% (at 5.0) to 99.27% (at 9.0%).In this case % removal at pH value 7.0 is 98.88%.This indicates that percentage removal almost equal after pH value 7.0.In case of PLDC and IBC the percentage removal of metal ion steadily increases from pH value 5.0 to 9.0, so these two adsorbents shows higher adsorption capacity in alkaline media.Again PFC shows similar properties with IBC.

Effect of particle size of IPACs
The effect of particle size of IPACs on the amount of Mn(II) adsorbed was studied.The amount of Mn(II) adsorbed increases with the decrease in particle size of the adsorbent.This is due to the increase in available surface area with the decrease in particle size.The effect of particle size on the amount of Mn(II) by various IPACs.There exists a linear relationship between the amount of Mn(II) adsorbed and particle size, as evidenced by the r-values close to unity (r-value for AC: BMC=0.9188,PLDC=0.9134,IBC=0.9136,PFC=0.9097)..00100.00 99.60 94.30 89.00 0.000 0.040 0.570 1.100 1.0000 0.9960 0.9430 0.8900 15.00 100.00 99.90 95.90 91.00 0.000 0.010 0.410 0.900 0.6667 0.6660 0.6393 0.6067 20.00 100.00 100.00 96.70 92.00 0.000 0.000 0.330 0.800 0.5000 0.5000 0.4835 0.4600 702 K. A. EMMANUEL et al.

Conclusion
Mn(II) is found to adsorb strongly on the surface of carbons.The equilibrium adsorption is practically achieved in 40 min.The results suggest that pore diffusion is more important.Adsorption behaviour is described by a monolayer Langmuir type isotherm.The adsorption process is found to be first order with intraparticle diffusion as one of the rate determining steps.The present study concludes that the IPACs could be employed as low-cost adsorbents as alternatives to CAC for the removal of Mn(II) from drinking water .

Figure 1 .Figure 2 .
Figure 1.Freundlich isotherm for the removal of Mn(II) at various concentrations of Mn(II) solution.

Figure 3 .Figure 4 .
Figure 3. Lagergren model for the removal of Mn(II)

Figure 5 .
Figure 5. Intraparticle diffusion plots for the removal of Mn(II).

Figure 6 .
Figure 6.Effect of adsorbate initial pH on the percentage removal of Mn(II).

Table 2 .
Adsorption isotherm constants for the removal of Mn(II).

Table 3 .
Effect of agitation time on the percent removal of Mn(II) by adsorption with prepared AC.

Table 4 .
Kinetic parameters of various models for the adsorption of Mn(II)