Determination of adsorbed Mn (II) and Cr (III) ions using hydrogel beads and AAS measurements

In this work the hydrogel (Poly acrylic acid) beads used for adsorb Mn (II) and Cr (III) from aqueous solutions. The adsorption capacity of the adsorbents is presented, the time required to reach a maximum capacity of bead (130.62, 123.13) mg/g for Mn (II) and Cr (III) ions respectively was about 24 hr. The initial concentration, temperature, time and pH effect on adsorption process were studied. The experimental data have been analyzed using the Langmuir and Freundlich. The Langmuir isotherm model gave the highest R value of (0.9999 and 0.9992) for Mn (II) and Cr (III) ions respectively. The thermodynamic parameters were studied and calculated. First-order and secondorder kinetic models were used and it is shown that the experimental data was in reliable compliance with the firstorder model with an R value of (0.984 and 0.993) for Mn (II) and Cr (III) ions respectively. The process is very efficient, especially for the removal of pollutants in aqueous solutions and more than 95% of study cations were removed by this adsorbent. The concentration of metal ion in the solutions was measured using AAS. The method was linear with an R of (0.9992 and 0.9989) for Mn and Cr respectively.


Introduction
The water is an essential matter of human and other living organism.Water may be polluted from the effluent of several industries such as chemical industry, electroplating industrial, dye industrial and battery industrial.The heavy metal ions were found to be one of the main pollutants in water [1].The toxic metals can cause accumulative poisoning cancer and brain damage when found above the tolerance levels [2].Metal ions such as cadmium, chromium, copper, lead, zinc, manganese and iron are commonly detected in both natural and industrial considered to be an essential dietary element to the human and mammals [5].
In this study, the performance of hydrogel beads as low cost adsorbers for Mn (II) and Cr (III) adsorption from aqueous solution at high level concentration was investigated.In batched experiments, the influence of pH solution, contact time, temperature and initial concentration of solution were studied.The maximum capacity of the adsorbent, kinetic parameters and thermodynamic parameters using different types of isotherms was calculated from experimental data.

Chemicals and solution
Commercial hydrogel beads (3.60 mm diameter and 0.0400 g weight) were used for metals ions adsorption in this study.All chemicals were of analytical reagent grade from Aldrich Chemical Company (Germany).A1000 ppm aqueous solution of Mn (II) and Cr (III) ions were prepared from hydrated metals chloride salts.

Calibration graph and linearity study
For determining the linearity, a series of solutions have different metals ions concentrations The absorbance of these solutions was measured.The calibration graph was obtained by plotting absorbance versus known concentrations in ppm. Figure 1, illustrate the calibration graph Mn and Cr by Atomic absorption spectroscopy (AAS).The method is linear with an R 2 of (0.9992 and 0.9989) for Mn (II) and Cr (III) respectively.Linearity was determined by the regression analysis.The obtained results were tabulated in Table (1)which shows that the value of tcal is larger than ttab value, and R 2 values are (0.9992 and 0.9989), which indicating that there is a strong correlation between the variation of concentration and response.metals ions concentration after the adsorption process was determined by AAS and the metals ions capacities at each time value were calculated according to the equation below [24]:

Determination of adsorbed Mn (II) and Cr (III) ions using hydrogel beads and AAS measurements
Where Q is the capacity of adsorption at a time (t) or at equilibrium (mg/g), Co and Ce are the initial and remained (concentration at t or at equilibrium) concentrations of metals ions (ppm), V is the volume of metals ions solutions (L), and m is the weight of hydrogel bead used (g).
In the present study, m value equal to 0.0400 g, the adsorbed metal ion concentration was calculated by subtract the remained concentration from initial concentration.The results obtained are illustrated in (Table 2 and Figures 2, 3).The results indicate that the adsorption process take place via two steps.In the first step, the adsorption of metal ion increases rapidly due to the availability of a large number of active sites on sorbent surface.In the second step, the adsorption process became less efficient due to the complete occupation of the surface with the metal ion.The big advantage of this sorbent is the large adsorption capacity (i.e. one hydrogel bead with 40 mg weight adsorbed (130.62 or 123.13) mg/g of Mn (II) and Cr (III) ions respectively from aqueous solution.

Effect of initial concentration
Batch equilibrium experiments were estimated by varying the metal ion concentration.A 25 ml solution of (50 -350 ppm) metals ions concentration was used at pH = 6.5.The solutions were left at room temperature for 24 hours .The results obtained (Table 3 and Figure 4) x 100 ……………………… 2     4, which indicate that the optimized pH for the adsorption of metals ions was (5 -7.5) for Mn (II) and Cr (III) ions.At low pH values, protons were available to protonate all sites on the hydrogel bead surface, therefore, the attraction to cationic ions decrease.The pH value which was chosen for this study at 6.5 (near the pH of deionized water) due to the high degree of deprotonation of the sites in the hydrogel bead surface is occurring at high value of pH [25] and to avoid the precipitation of metal ions as hydroxide.

Effect of temperature
The adsorption studies were conducted at four different temperatures (5 -30 ºC) ,the initial concentration(50-250 mg/l).The obtained results (Table 5) reveal that the adsorption of Mn (II) and Cr (III) ions increases as temperature increases; this may be due to the increase in ion mobility, which may also cause a swelling effect within the internal structure of hydrogel leading to more penetrate of metal ion [26] as shown in Fig. 7.

Adsorption kinetic study
The Lagergren pseudofirstorder and pseudosecondorder equations were used to test the experimental data by application equations (3 and 4) [27]: Where Qe, Qt are the amount of metals ions adsorbed (mg/g) at equilibrium and at any time t respectively.k1and k2 are the adsorption rate constant of pseudofirstorder (hr -1 ) and pseudo secondorder (g/mg.hr).The results obtain are summarized in Table 6, which indicate that the adsorption process follow a pseudofirstorder with a correlation coefficient R 2 value of Where Qmax, Qe are the maximum ion uptake per unit mass of hydrogel bead (mg/g)related to adsorption capacity and capacity at equilibrium (mg/g) respectively, Ce is the equilibrium concentration (ppm), KL and KF are Langmuir and Freundlich constant and n is Freundlich exponents.Therfore, Langmuir parameters calculeated from the slope and intercepts of the linear plots of Ce/Qe versus.Ce gives a stragth line of slope1/ Qmax,and intercept1/KL Qmax while the Freundlich parameters can be calculated from the slope and intercepts of the linear plots of Log Qe vs. Log Ce.It was found from this study that the adsorption of the two metal ions was followed Langmuir ' s isotherm.The value of n is larger than 1, which represents a favorable removal condition.All evaluated parameters are present in Table 7.Where Kc (L/g) is the standard thermodynamic equilibrium constant.The thermodynamic parameters can be calculated from the slope and intercept of the Ln Kc vs. 1/T plotting (Figure 14 and 15), the results obtained are tabulated in Table 8, which reveals that the adsorption process is endothermic and increase of randomness at the solid/ solution interface occur in the internal structure.
To conduct this experiment, 25 ml volumetric flasks each of which contains 25 ml of 100 ppm metal ion solution and one hydrogel bead was used.The pH of solution was adjusted at range (1-7.5) and left at room temperature for 10 hr.The capacity and adsorption percentage were calculated from equation 1 and 2, respectively.The results obtained were tabulated in Table

Figure 5
and 6;  show the relationship between pH with remaining metal ion concentration adsorption percentage and capacity.