Characterization of the prepared hydrogels
Effect of irradiation dose on gelation percent
Figure (1) shows the effect of different irradiation dose on gelation percent at acrylamide content 20% 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 influence 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).
4.1.3. Kinetics Study
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 classified into three different types based on the relative rates of diffusion and polymer relaxation (20). These classifications 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).
The swelling/diffusion kinetic parameters, as diffusion coefficients (D), swelling constant (k), diffusion constant (n) at maximum equilibrium swelling were calculated using the dynamic swelling values for P(AAm/Epi) hydrogels by equation (4):(21,22).
F = (M t /M ∞) = k tn (4)
In the above equation, F are fraction of swelling percent due to the water uptake, Mt 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-fickian 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 coefficient (D) of cylindrical polymer was calculated from equation (6) (22,23).
Here (D) is the apparent diffusion coefficient 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 t1/2 are plotted and are shown in Figures (3 and 4). The linearity obtained in the first stage of the process corresponds to the values of the swelling ratio. The apparent diffusion coefficient (D) can be calculated from the slope of the straight line (24). The values of the diffusion coefficient of P (AAm/Epi) hydrogel are listed in Tables (1). It can be noticed that, the correlation coefficient (R2) for all investigated hydrogels ranges from 0.984 to 0.991 indicating a better fitting 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 figure 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).
Adsorption study of P (AAm/ Epi) hydrogels
Evaluation of equilibrium adsorption isotherm
Adsorption is one of the most common methods used in wastewater treatment. The main advantages of adsorption are; high efficiency, 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 figures 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 effluents into the hydrogels (27-29).
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 SO42- > PO4-3.
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 finding 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-modified blast-furnace slag geopolymer (Ba-BFS-GP) was up to 119 mg/ g (32) and that using modified 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 fitted by Freundlich isotherm model (33).
log qe = log kf + 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 kf and n referred to the ability of polymer adsorbent and affinity of adsorbat solution, respectively.
The Freundlich isotherm constants and R2 values for different anion solutions are given in Table (3). The experimental data show a better fitting (R2 > 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).
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 figures (10 and 11). The figures 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).
184.108.40.206. 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 R2 values for different anion solutions are listed in Table (5). The experimental data shows fitting of over a R2 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 effluent medium during the regeneration process (31-33).
220.127.116.11. The regeneration efficiency
The regeneration efficiency percent has been calculated to evaluate the removal (%) after regeneration process comparing with that before regeneration. The regeneration efficiency (%) can be calculated by the following equation:
Regeneration efficiency (%) = (Removal (%) after regeneration / Removal (%)
before regeneration) x100 (8)
Table (6) shows results of sulphate and phosphate ions removal with P (AAm/ Epi) hydrogels at different AAm and Epi contents. From the table it can be noticed that, the removal of sulphate and phosphate ions is slightly affected by the AAm and Epi contents (32).
The calculated regeneration efficiency (%) values found to be ranged between 63.2 (%) and 46 (%).The relatively higher regeneration efficiency (%) and keeping the hydrogels its shape without any deformation promising to use the same hydrogels further times which decrease the economic cost.