Adsorption of Co ( Ii ) Ion from its Aqueous Solution Using Hydrogel Beads as Adsorbent

In this study one of the super absorbent hydrogel beads was used for adsorb Co(II) ion from aqueous solution. The adsorption capacity of the adsorbent is presented and the time required to reach a maximum capacity of bead (112.5 mg/g) form Co (II) ion 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 ,Freundlich, Dubinin and Temkin. The Langmuir isotherm model gave the highest R value of 0.9998. 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 R value of 0.992 .


Introduction
The elimination of heavy metals from waters and waste waters is important to protect public health metal removal [1].The adsorption of heavy metal ions on different adsorbent have been studied extensively in order to find a specific adsorbent for each ion to be used in the treatment of wastewater in the environment [2] .Cobalt, one of the common toxic metals affecting the environment, is present in the waste water of nuclear power plants and many other industries such as mining, metallurgical, electroplating, paints, pigments and electronic.High levels of cobalt may affect several health troubles such as paralysis, diarrhea, low blood pressure, lung irritation and bone defects.The standard level of cobalt in drinking water is 2_g l −1 , but values up to 107_g l −1 have been reported.One of the adsorbent which is widely used for the removal of cobalt and other heavy metals is activated carbon .The application of this adsorbent is limited on the commercial considerations, due to the relative high cost associated with it separation.[3].The commonly used procedures for removing metal ions from effluents include filtration,

Experimental Apparatus:
Atomic absorption spectrophotometer (AAS) type (AURORA, A1200 -Canada) was used to determine Co(II) ions concentration.A metrohm E. 63222 pH meter (Switzerland), fitted with metrohm combined glass electrode was calibrated according to conventional methods and used to adjust the pH of the solution in all experiments.Sartorius BL 210 S (Germany), max.210 g, D 0.1 mg, was used for hydrogel beads and chemicals weighing.A Vernier caliper with 0.01 mm measuring accuracy was used for measurement of the diameter of the hydrogel beads.

Chemicals and solution:
Commercial hydrogel beads (3.60 mm diameter and 0.0400 g weight) were used for metal ion adsorption in this study.All other chemicals used throughout this study were of analytical reagent grade and were purchased from Aldrich Chemical Company (Germany).A 1000 ppm aqueous solution of Co(II) ion were prepared from hydrated metals chloride salt.More dilute solutions of metal ions were prepared from stock solution by simple dilution with distilled water.

Preparation of calibration graph and linearity study:
For determining the linearity, a series of solutions have different metal ion concentrations were prepared by simple dilution of stock solutions.The absorbance of these solutions was measured.The calibration graph was obtained by plotting absorbance versus known concentrations in ppm.adsorption process was determined by AAS and the Co (II) capacity at each time value were calculated according to the equation below [9]: Where Q is the capacity of adsorption at a time (t) or at equilibrium (mg/g), Co and Ce are the initial and remained (at t or at equilibrium) concentrations of Co ion (ppm), V is the volume of metal ion 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 Figure 2).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 (112.5)mg/g of Co (II) from aqueous solution.

Effect of initial concentration:
Adsorption equilibrium and isotherm studies were estimated by varying the metal ion concentration.A 25 ml solution of (50 -350 ppm) metal ion concentration was used at pH = 6.5.The solutions were left at room temperature for 24 hours and the remained metal ion concentration was determined using AAS measurements.The results obtained (Table 3 and

Effect of temperature
The adsorption studies were conducted at four different temperatures (5 -30 ºC).The obtained results (Table 5) reveal that the adsorption of Co (II) ion 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 [11] as shown in Fig. 5.Where Qe, Qt are the amount of metal ion adsorbed (mg/g) at equilibrium and time t respectively.k1and k2 are the rate constant of pseudofirstorder (hr -1 ) and pseudosecondorder (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 (0.9922 ) for Co(II) ion .Figure 6,7 shown the straight plots of Log (Qe -Qt) vs. t and t / Qt vs. t, respectively.Where R is the universal gas constant, T is the absolute temperature and Kc (L/g) is the standard thermodynamic equilibrium constant.The thermodynamic parameters can be calculated from the slop and intercept of the Ln Kc vs. 1/ T plotting (Figure12), the results obtained are tabulated in Table 9, which reveals that the removal process is endothermic with increase of randomness at the solid/ solution interface occur in the internal structure.

Vol: 13
No:3 , July 2017 DOI : http://dx.doi.org/10.24237/djps.1303.296CP-ISSN: 2222-8373 E-ISSN: 2518-9255 chemical precipitation ,ion exchange [4],chemical coagulation , flocculation , ion exchange , reverse osmosis ,membrane technologies and solvent extraction [5,6].These processes may be ineffective or expensive, especially when the heavy metal ions are present in high concentrations.In this study the batch mode studies, the dynamic behavior of the adsorption was investigated on the effect of initial metal ion concentration, temperature, adsorbent dosage and pH.The thermodynamic parameters were also evaluated from the adsorption measurements .The Langmuir, Freundlich and Temkin adsorption isotherms, adsorption Kinetics were calculated from experimental data.[7.8].

Figure3
Figure3) reveals, that the adsorbed metal ion quantity was increased as the initial concentration of metal ion was increased until reach the maximum capacity of the hydrogel beads.At low concentration the hydrogel bead does not reach the maximum capacity, and remained concentration is very low, while at high concentration the hydrogel bead reach its maximum capacity, so that the remained concentration is high.The adsorption percentage calculated as below:

Fig 5 .
Fig 5. Relationship between temperature Vs.metal quantity