3.1 Characterization of adsorbents
Functional group analysis was studied by fourier transform infrared analysis (FTIR) for CA beads, CHS/ALG beads, MIPs, MIRs and their initial components, showed the disappearance of initials and formation of new product with different properties, because of functional group changes, figures (1, 2 and 3). It was noticed that both imprinted and nonimprinted polymers have similar peaks, with those of nonimprinted more intense due to the absence of any self-assembly and random distribution of functional monomers without any cavity formation as in imprinted ones .
Surface morphology was studied for both molecularly imprinted polymers and resins (together with their nonimprinted powders).
Molecularly imprinted polymers (figure 4) showed formation of cavities in the polymerization process in presence of AZX template (figure 4, a), however, in absence of it, no cavities were constructed (figure 4, b).
Molecularly imprinted resin SEM images showed the large particle size due to the presence of silica gel core. However, these particles were smaller in MIR (figure 5, a) than in NIR (figure 5, b), due to the imprinting reaction that allowed resin to be arranged around template molecules.
The XRD pattern of graphene oxide (GO) showed two peaks at angles 11.079°θ (67.8 count) and 42.56°θ (25.1 count), together with the fingerprint region of FTIR were confirmed its formation (figure 6).
3.2 Optimization of elimination conditions
Time and pH for optimum adsorption and hence elimination of some strobilurins, were studied. Effect of pH on elimination was studied over the pH range 4-11 for both alginate and chitosan/alginate beads as they dissolve in highly acidic solutions. For alginate beads, it was noticed that after 4 hrs of shaking in room temperature at pH 10, the capacity of elimination was 210 mg/g for AZX. However, chitosan/alginate beads showed elimination capacity of 480 and 1140 mg/g at pH 4 and 10, respectively. After one day, adsorption capacity was decreased.
Capacity of MIP to eliminate AZX from aqueous solutions ranging from pH 2 to 11, was studied. A great improvement in elimination capacity (3290-3760 mg/g) was proved at pH 2-8, after reaching equilibrium. Few traces of AZX were detected after one day of shaking, however it was almost diminished after shaking for two days with MIP. After days, the substrate remained bounded to MIP adsorbent, thus elimination capacity was maintained over a large period.
Regarding MIR, its capacity of PYR elimination was comparable to that of MIP at pH 2-8. After equilibrium, it reached (3280-3320 mg/g) by shaking for one day and only few traces of PYR were detected after the second day. No decrease in elimination capacity by time.
Figure 7 shows the effect of pH on elimination of AZX from aqueous solution. Azoxystrobin solubility (6 mg/L) in water, together with its behavior in binding with different types of adsorbents studied, showed that the adsorption mechanism depends on Van der Waals forces without any ionic interaction .
Molecularly imprinted resin also showed a great elimination of PYR pesticide, (figure 7), due to its superhydrophilic nature that helped much more in adsorption of PYR in aqueous medium.
For photocatalytic degradation, it was reported that MIR needed 18 hrs to activate photocatalysis of TRI (after equilibrium). This period was reduced to 12 hrs after addition of GO that accelerated the chemical degradation process induced by the same visible light source under the same conditions of stirring and room temperature.
3.3 Evaluation procedures
The developed method for spectrophotometric analysis used in evaluation was characterized and validated in table 1.
Table 2 showed the calculated capacity of elimination (Qe) for the four types of adsorbents at different pH ranges, and for MIR alone and with GO. Also, imprinting factor (IF) and selectivity (α) were calculated for both imprints (MIP and MIR). It showed that the addition of chitosan to alginate, increases the solidification of calcium alginate to form cavities ready for binding with adsorbate either in acidic or basic pH, in comparison with chitosan alone that works in basic pH only. In addition, more functional groups e.g. NH2 were added to increase the binding capacity . This explains the better elimination capacity by chitosan/alginate beads rather than alginate alone.
Table 2 also showed values of imprinting factor (IF) for MIP and MIR, that reflected the success of imprinting procedures in synthesis of them. Selectivity coefficient (α) was calculated and explained the higher selectivity of both MIR and MIP.
In case of MIP, the addition of hydroxypropyl β-cyclodextrin to other monomers, built up a cup-like cavity that form multiple inclusion complexes with AZX, thus improving its elimination from wastewater greater than alginate and even chitosan/alginate beads. Also MIP could withstand different ranges of acidic pH, in addition to stability of adsorbate binding with time. However, the presence of silica core in MIR particles, made them arranged in a bed that offered a large adsorption surface area and was easily filtered or even may be packed in a column for wastewater purification, if needed.
Molecularly imprinted resins (MIR) under visible light, forms centers for charge carriers that serve as electron acceptors in photocatalysis . Addition of graphene oxide to MIR increased their ability for photocatalytic degradation of TRI in visible light, this was attributed to the enhancement of charge transfer driven by the honeycomb SP2 network structure of GO .
All these advantages made the molecular imprinting technique superior in wastewater purification (physically or chemically) than natural adsorbents e.g., chitosan and alginate.
3.4 Application on real samples
As the measured pH of the real samples was neutral to slightly acidic, alginate and chitosan/alginate beads failed to eliminate AZX. However, MIP and MIR succeeded to eliminate AZX and PYR, with elimination capacity of 3600 mg/g and 3000 mg/g, respectively.
Molecularly imprinted resin helped in photodegradation of TRI in wastewater samples with elimination capacities of 5100.3 mg/g and 5200 mg/g after addition of GO, respectively.