Effect of Gamma Radiation on the Adsorption Capacity and Regeneration / Desorption of Sulphate and Phosphate Ions Using Smart Hydrogel.

Co-polymeric hydrogels containing poly (Acrylamide /Epichlorohydrine) P(AAm/EPI) with different acrylamide and Epichlorohydrine content were fabricated by gamma radiation at different irradiation doses as adsorbent materials for wastewater treatment. The mechanisms of radiation-induced crosslinking of hydrogel in aqueous solution has been evaluated. The gel contents and the swelling/diffusion kinetic parameters were evaluated at different irradiation doses, and the result conrm a non-chian mechanism. The shape, surface morphology, and porosity of P (AAm/ Epi) hydrogel were characterized by scanning electron microscope (SEM). Adsorption experiments were carried out for the removal of sulphate and phosphate ions from wastewater using P(AAm/EPI) hydrogels as adsorbent materials. The isotherm data were analyzed by Freundlich equation. The equilibrium isotherm results show a better tting (R2 > 0.9) to the Freundlich model for all anions. The calculated regeneration eciency (%) values of sulphate and phosphate ions found to be ranged between 63.2 (%) and 46 (%).The relatively higher regeneration eciency (%) and keeping the hydrogels its shape without any deformation promising to use the same hydrogels further times which decrease the economic cost.


Introduction
Industrial water, even natural water, is often contaminated by toxic, or sometimes, carcinogenic impurities, causing ecological disequilibrium and dreadful health disorders in humans. The major sources of water pollution can be attributed to discharge of untreated sanitary and toxic industrial wastes, and run off from agricultural elds, and so on. The majority of pollutants discharged in wastewaters can be toxic to aquatic life and can cause natural waters to be un t as potable water sources. There are several causes of water pollution organic, inorganic as well as municipal, industrial, and agricultural (1).
Sulphate is a common constituent of many natural waters and wastewaters, and is sometimes present in high concentrations. Industrial wastewaters are responsible for most anthropogenic emissions. The damage caused by sulphate emissions is not direct, since sulphate is a non-toxic compound. However, high sulphate concentrations can unbalance the natural sulphur cycle.
Phosphorus as an essential element for the growth of living organisms is widely used in the areas of agriculture and industry. However, excessive discharge of phosphate into water bodies such as lakes, lagoons, rivers and inland seas, may cause detrimental eutrophication. Thus, it is important to control the concentration of phosphorous in water to avoid eutrophication. Phosphorus is found as phosphate (PO 4 3− ) in nature and present as orthophosphate, polyphosphate and organic phosphate in water. In addition, phosphates are also found abundant in the wastewater from laundering agents for examples shampoo and detergents (2,3). Several techniques, such as chemical oxidation, biological treatment, coagulation, photocatalytic degradation and adsorption, have been explored to remove the contaminants from wastewater. The most reliable and effective method for removal of pollutants is adsorption technique. Many kinds of adsorbents have been reported, such as zeolites, activated carbons, agricultural by-products, clays, biomass and polymeric materials as hydrogels (4).
In recent years a great deal of interest has been observed in relation to the applicability of the polymers for removal separation and puri cation of pollutants from contaminated water. The water holding capacity of the hydrogels arise mainly due to the presence of hydrophilic groups, amino, carboxyl and hydroxyl groups, in the polymer chains. The water holding capacity of the hydrogel is dependent on the number of the hydrophilic groups and crosslinking density.
Polyacrylamide is highly water-absorbent, forming a soft gel when hydrated, used in such applications as in materials with electrolyte properties based on the ionized amide groups. Polyacrylamide P(AAm) hydrogels and their derivatives were the subject of many studies. P(AAm) hydrogels containing a hydrophilic group has been used for the adsorption of some heavy metal ions, (5) anions and basic dyes, (6,7).

Materials
Epichlorohydrin (EPI) and Acrylamide (AAm) were received from Merck, (Germany) with 99% purity and were used without further puri cation.
Gamma radiation source: The samples were irradiated with Co 60 Indian research irradiator gamma chamber with activity 9100 curies which is modi ed to a dose rate ranging from 0.99 to 0.85 Gy/sec. The irradiation chamber was constructed by the National Center for Radiation Research and Technology (NCRRT), Atomic Energy Authority of Egypt (AEAE).
Gelation procedure: The prepared hydrogels obtained in cylindrical shapes were cut into 2-3 mm, dried and weighted (W i ).
Then, the hydrogels were extracted in deionized water for 6 hours at 80 o C, and were occasionally shaken. The insoluble part corresponding to the cross-linked sample was dried and weighed (W d ). The gelation % was calculated by the following equation: Where W i is the weight of dried hydrogel before extraction and W d is the cross-linked weight of samples after the extraction of residual homopolymer and monomer.
Swelling studies and diffusion characteristics evaluation of the hydrogels: To study the swelling properties of P (AAm/Epi) hydrogels, a pre-weighed cylindrical shaped hydrogels were soaked in 50 ml of distilled water then weighted at different time intervals to a constant weight. At every measurement, the excess water on the hydrogel discs was removed super cially by lter paper and then weighted. The swelling (%) in each case was calculated as follows: where W o and W s are the weights of the sample in the dry state and swollen state at time (t), respectively (8-10).

Scanning electron microscopy (SEM)
The swollen freeze dried hydrogels were prepared using freeze dryer (Modulyo Jencons' Scienti c Limited, England). A JEOL-JSM-5400 Scanning electron microscope-Japan was used for investigating the pore structure and morphology of different hydrogels. The swollen hydrogel samples were freezedried and coated with gold for SEM observation.
Testing nutrient ions removal by the hydrogel A known weight dry hydrogels were soaked in xed volume of sulphate and phosphate anion solutions with known concentration (C o ) for 24 hours. The remaining concentrations after equilibrium (C q ) were determined using colorimeter (HACH DR/890, USA).

Sulphate (SO 4 -2 ) ions determination
Sulfate ions in the sample react with barium in the SulfaVer 4 Sulfate Reagent to form insoluble barium sulfate. The amount of turbidity formed is proportional to the sulfate concentration. The SulfaVer 4 also contains a stabilizing agent to hold the precipitate in suspension (11,12).

Phosphate (PO 4 -3 ) ions determination
The procedure is based on a UV catalyzed oxidation of phosphonate to orthophosphate. Range may be as low as 0 to 2.5 mg/L or as high as 0 to 125 mg/L. Phosphonate is converted to orthophosphate during the UV digestion. Both the sample and the blank will develop color if orthophosphate is present in the sample. The increase in color in the sample is proportional to the phosphate produced in the digestion. (13)

Adsorption capacity
The adsorption capacity Q, (mg/g hydrogel) was calculated depending on the amount of anions uptake (in mg) by unit mass (in g) of the hydrogels using the following equation (14): where, C o and C q are the initial and the residual concentration of anions at equilibrium, respectively, m is the weight of hydrogels and V is the volume of the anions solution.

Regeneration/ desorption
Experiments were performed to determine whether the P (AAm/Epi) hydrogels could be regenerated after having bound the anions from wastewater. The anion saturated P(AAm/Epi) hydrogel was treated with 50 ml of 1 N NaOH solution for 24 hours after the anion removal experiment in wastewater. The hydrogel was removed from the waste solution consisting of Na 2 SO 4 and Na 3 PO 4 that were formed during the treatment. The hydrogel was re-washed with deionized water to remove any residual Na 2 SO 4 and Na 3 PO 4 that remained. The hydrogel was then dried in an oven to prepare it for reuse in a new anion removal experiment. (15) Results And Discussion and Epi equal 1.26 x10 -3 mol/g acrylamide. When P (AAm/ Epi) was prepared by different gamma irradiation doses from 5 to 35 kGy, gelation percent increased from 88.87% to 93.18% by increasing the irradiation dose from 5 kGy reaching maximum at 25 kGy then level off at 35 kGy.
These results may be attributed to the increase in the available free radical concentration and higher degree of crosslinking of the hydrogel structure during copolymerization process by irradiation (16). This may be also due to; at low dose, the network consists of polymer chains jointed through multifunctional junctions with no or very few closed cycles, thus forming giant molecules with branches and entanglements. When the radiation dose was increased beyond a certain value, the polymer chains would be crosslinked and a gel is then obtained (17). The leveling off of gelation (%) of P (AAm/ Epi) hydrogel can be attributed to the degradation of the hydrogel by the effect of irradiation dose. It is known that the gelation percent increases with the irradiation dose, but in such case, the rate of radiation degradation may be equal to the rate of radiation crosslinking, as a result the gelation levels off as irradiation dose increases (18).

Effect of irradiation dose on swelling percent
The in uence of swelling time on the swelling percent for P(AAm/ Epi) hydrogels prepared at different irradiation doses is shown in Figure (2). It can be seen that, for all the investigated samples, the water absorbency of the hydrogels highly increased as the swelling time increased. The results show that the swelling percent of all hydrogels reached more than 500 % within 5 h only. On the other hand, the swelling of the hydrogels decreased as the radiation dose increased. It was reported that the increase in the total irradiation dose increases the concentration of free radicals generated from acrylamide molecules and the crosslinking capability of the AAm molecules. Such increment in the degree of crosslinking reduces the free volume available for swelling by increasing the tightness of the network structure that hinders the relaxation of the polymer chains and the mobility of the swelling medium, consequently it lowers the hydrogel swelling (19).

Kinetics Study
Swelling/Diffusion mechanism Absorption of water from the environment into the hydrogels system, changes the dimensions and physicochemical properties of the system. The diffusion of water into the hydrogels was classi ed into three different types based on the relative rates of diffusion and polymer relaxation (20). These classi cations of the diffusion of water into the hydrogels are:

i-Case I or Fickian Diffusion
Case I or Fickian diffusion occurs when the rate of diffusion is much less than that of relaxation. When the hydrogels swells in the water, the swollen gel follows Fick's law. Thus, the rate of swelling by Case I systems is dependent on t 1/2 and the diffusion constant n=0.5.

ii-Case II Diffusion
Case II diffusion (relaxation-controlled transport) occurs when diffusion is very rapid compared with the relaxation process. In Case II systems, diffusion of water through the gel is rapid compared with the relaxation of polymer chains. Thus, the rate of water penetration is controlled by the polymer relaxation (n=1).

iii-Non-Fickian or Anomalous Diffusion
Non-Fickian or anomalous diffusion occurs when the diffusion and relaxation rates are comparable. Swelling depends on two simultaneous rate processes, water migration into the hydrogel and the relaxation of polymer chains (0.5< n <1).
In the above equation, F are fraction of swelling percent due to the water uptake, M t is the adsorbed water at time t and M ∞ is the adsorbed water at equilibrium, k is the swelling constant, and n is the diffusion constant. This equation was applied to the initial stages of swelling of P(AAm/EPI) at different AAm contents and irradiation doses. The plots of (ln F) versus (ln t); where t is the time of swelling, are shown in Figures (5and 6).
The values of the exponent (n) and swelling constant (k) were calculated from the slope and intercept of the lines, respectively. The values are presented in Table (1) as a function of different irradiation dose.
Diffusion constant values of these hydrogels indicate that the water penetration is by non-Fickian transport mechanism because diffusion constant (n) (0.5315 -0.6998) values had fallen between 0.5 and 1.0 Table ( 1). It was reported that the non-ckian or anomalous diffusion mechanism where 0.5< n <1 indicates that the diffusion and relaxation rates are comparable Ritger et al., (1987) (21). The entrance of water molecules into the polymer follows non-Fickian and depends on two simultaneous rate processes, water migration into the hydrogel and the relaxation of polymer chains. The diffusion coe cient (D) of cylindrical polymer was calculated from equation (6) (22,23).
Here (D) is the apparent diffusion coe cient for the transport of the penetrate (water) into the hydrogel, (t) is the swelling time (h) in hours, and (r) is the radius of the cylindrical polymer sample. For hydrogels, the graphs of swelling ratio versus t 1/2 are plotted and are shown in Figures (3 and 4). The linearity obtained in the rst stage of the process corresponds to the values of the swelling ratio. The apparent diffusion coe cient (D) can be calculated from the slope of the straight line (24). The values of the diffusion coe cient of P (AAm/Epi) hydrogel are listed in Tables (1). It can be noticed that, the correlation coe cient (R 2 ) for all investigated hydrogels ranges from 0.984 to 0.991 indicating a better tting of the experimental data.
Scanning electron microscopy of the produced hydrogel The shape, surface morphology, and porosity of P (AAm/ Epi) hydrogel at different Epi content have been examined by SEM as ( Figure 5). From the gure it can be noticed that, the increase of Epi content leads to their accumulation on the pores of the hydrogel Further, increase of Epi content in the hydrogels was not only observed on the surface but also within the holes of the hydrogel pores (25,26).

Evaluation of equilibrium adsorption isotherm
Adsorption is one of the most common methods used in wastewater treatment. The main advantages of adsorption are; high e ciency, good selectivity, moderately high removal performance, cost-effectiveness and easy regeneration process of adsorbents. Isotherms studies can describe how the adsorbates interact with adsorbents, affording the most important parameter for designing a desired adsorption system. The adsorption isotherms of sulphate and phosphate ions on all investigated hydrogels as a function of acrylamid contents are shown in Figures (6 and 7). From the gures it can be clearly noticed that, the adsorption isotherms indicate that the adsorption capacity of the investigated anions into the hydrogels decreases with increasing the acrylamide content. The obtained results may be attributed to the high crosslinking of the hydrogels obtained at high monomer content which lead to decreasing the diffusion of the anions e uents into the hydrogels (27)(28)(29). Figures (6 and 7) also indicate that, the adsorption capacity of the investigated anions into the hydrogels decreases with increasing the Epi content from 1.26x10 -3 mol/g AAm to Epi 1.56x10 -3 mol/g AAm then to Epi 1.86x10 -3 mol/g AAm. The observed decrease of the adsorption capacity by increasing the Epi contents may be due to the increase of the crosslinking which increases the tightness of the network structure that hinders the diffusion and mobility of the anions into the hydrogels (30,31). From Figures (6  and 7), it can be noted that, the adsorption capacity of sulphate anion onto hydrogels with different Epi content was higher than that of phosphate anion, where the adsorption capacity order is SO 4 2-> PO 4 -3 .

Determination of the removal percentages of the tested nutrients
The evaluation of the removal percent of sulphate and phosphate by P (AAm/Epi) hydrogels at different AAm and Epi content are listed in Table (2) according to equation .

Removal (%) =[ (Co -Ce ) / Co ]x 100 (6)
Where Co and Ce are the initial and equilibrium anions concentration (mg/l), respectively. From the table it can be noticed that the removal percentage of sulphate and phosphate increased by decreasing AAm content and it showed also that, the removal percentage decreased by increasing Epi content from1.26x10 -3 to 1.86 x10 -3 . These nding may be attributed to the high crosslinking obtained by increasing AAm and Epi contents. In comparison, it was found that, the removal percentage of sulphate ions is more than that of phosphte ions (32).
In a comparison, the maximum adsorption capacity of sulphate ions using barium-modi ed blast-furnace slag geopolymer (Ba-BFS-GP) was up to 119 mg/ g (32) and that using modi ed rice straw was up to 74.76 mg/ g (33). In this work the maximum adsorption capacity of sulphate ions using p(AAm/Epi) hydrogels was up to 220 mg/g .

Freundlich isotherm study
The adsorption isotherms of sulphate, phosphate, nitrate and nitrite ions are shown in Figures (8 and 9). The adsorption isotherms indicate that the adsorption capacity for all investigated anions are clearly affected by the acrylamide content. The isotherm data were analyzed by Freundlich equation, where the Freundlich model is employed to describe heterogeneity. The equilibrium isotherm results were found to be best tted by Freundlich isotherm model (33).
log qe = log k f + 1/n log Ce (7) where qe is mg adsorbate per gram of dry adsorbent, Ce is the equilibrium anion concentration (mg/l), and the Freundlish empirical constants k f and n referred to the ability of polymer adsorbent and a nity of adsorbat solution, respectively.
The Freundlich isotherm constants and R 2 values for different anion solutions are given in Table (3). The experimental data show a better tting (R 2 > 0.9) to the Freundlich model for all anions. From Table (3) it can be noticed that 1/n values indicate that the adsorption process of sulphate is chemical, while that of phosphate anion is physical. On contrary, 1/n values of sulphate anion is nearly close to unity, so the adsorption process is linear (34,35).

Regeneration process
Desorption and reusability of P (AAm/ Epi) hydrogels Desorption process using the same P (AAm/ Epi) hydrogel was carried out as mentioned in the experimental section. The procedure describes "regeneration cycle" that involves an anions adsorbents experiment followed by a release experiment and a second removal experiment. The adsorption capacity of sulphate and phosphate anions after regeneration were evaluated and represented in gures (10 and 11). The gures showed that, the adsorption capacity of P (AAm/ Epi) hydrogels toward different anions are clearly affected by the acrylamide and Epi contents, where the adsorption capacity was found to decrease as the acrylamide content increases. It can also be noticed that, the adsorption capacity are in the same manner but with lower values than before regeneration which indicated that the hydrogel keep its characteristic without any deformation (15 -36).
Removal percentages after the regeneration process Table (4) shows the removal precent of sulphate and phosphate ions with P (AAm/ Epi) hydrogels at different acrylamide and Epichlorohydrin contents after regeneration. The calculated removal percentages were found to increase by decreasing the AAm and Epi contents. The removal percent of sulphate by P (AAm/ Epi) hydrogels was higher than that of other anions (32 -37).

Freundlich isotherm study after the regeneration process
The adsorption isotherms for the different anions tested are shown in Figures (12 and 13) The adsorption isotherms indicated that the adsorption capacity for all investigated anions are clearly affected by the acrylamide contents. The isotherm data were analyzed by Freundlich equation, where the Freundlich model was employed to describe heterogeneous. (32).
The Freundlich isotherm constants (n and k) and R 2 values for different anion solutions are listed in Table ( 5). The experimental data shows tting of over a R 2 of 0.82 to the Freundlich model for all anions.
The higher values of 1/n (Table 5) indicate that the adsorption process of all anions are physical. These results may be attributed to the high crosslinking process that may lead to the formation to more dense and sound network structure, which would hinder the release of anions from the hydrogels into the e uent medium during the regeneration process (31-33).

The regeneration e ciency
The regeneration e ciency percent has been calculated to evaluate the removal (%) after regeneration process comparing with that before regeneration. The regeneration e ciency (%) can be calculated by the following equation: Regeneration e ciency (%) = (Removal (%) after regeneration / Removal (%) before regeneration) x100 (8) The calculated regeneration e ciency (%) values found to be ranged between 63.2 (%) and 46 (%).The relatively higher regeneration e ciency (%) and keeping the hydrogels its shape without any deformation promising to use the same hydrogels further times which decrease the economic cost.    Figure 1 Effect of irradiation dose on gelation percent at acrylamide content 20% and epiclorohydrin 1.26 x10-3 mol/g acrylamide.

Figure 2
Effect of different irradiation doses on swelling percent at acrylamide content 20% and Epi 1.26 x10-3 mol/g acrylamide.

Figure 4
Effect of irradiation dose on the swelling ratio at acrylamide content 20 % and Epi 1.26 x10-3 mol/g acrylamide.  Effect of AAm and Epi content on the adsorption capacity (Q) toward PO4 ions at irradiation dose 20 kGy.

Figure 8
Adsorption isotherm of sulphate ions by P (AAm/Epi) hydrogels at different acrylamide and Epi contents and 20 kGy. Adsorption isotherm of phosphate ions by P(AAm/ Epi) hydrogels at different acrylamide and Epi contents and 20 kGy.

Figure 10
Effect of AAm and Epi contents on the adsorption capacity (Q) toward SO4 ions at irradiation dose 20 kGy after regeneration.

Figure 11
Effect of AAm and Epi contents on the adsorption capacity (Q) toward PO4 ions at irradiation dose 20 kGy after regeneration.