The adsorption and photocatalytic activities of AgZnO/POMs nanocomposites were studied by removing BM from aqueous solution. Figure 7a is the UV-vis absorption spectra of BM solution containing the AgZnO/POMs nanocomposites at different intervals. Figure 7b shows a comparative study for removing BM in the presence of (1) POMs, (2) AgZnO and (3) AgZnO/POMs nanocomposites, in which, the ordinate is C/C0, where C is the corresponding concentration of BM at different time intervals, and C0 is the original concentration of BM. It can be observed in combination with Figure 7(a) and (b), the absorption peak strength of BM gradually decreases in 0-30 min, remaining unchanged in 30-50 min for reaching adsorption equilibrium under stirring in the dark, then after 50 min, decreases with the increase of UV-light irradiation, indicating the adsorption and photocatalysis activities of AgZnO /POMs nanocomposites. In Figure 7b, POMS only showed adsorption, while the adsorption and photocatalysis of AgZnO hybrid nanoparticles were relatively weak, when the two are combined, AgZnO/POMs nanocomposite has enhanced adsorption effect relative to AgZnO hybrid nanoparticles, and shows photocatalytic effect relative to POMS, and finally achieves 94% removal rate of BM in aqueous solution. Figure 7c shows a comparative histogram of the removal of BM by POMs, AgZnO, and AgZnO/POMs nanocomposites under UV-light and Vis irradiation, respectively. No matter under UV or visible light irradiation, the photocatalytic-adsorbent AgZnO/POMs qualifies higher removal efficiency than the adsorbent POMs and photocatalyst AgZnO. The removal rate of AgZnO/POMs for removing BM is 94%, which is much higher than that of POMs (54%) and AgZnO (73%) under UV-light irradiation.
The N2 adsorption-desorption isotherms of AgZnO nanoparticles and photocatalytic-adsorbent AgZnO/POMS nanocomposites were determined using the automatic physical/chemical adsorption apparatus. In Figure 8, both samples showed typical type IV isotherms, indicating the presence of mesoporous structures [42]. According to the analysis results of relative position and height of hysteresis loops (Figure 8), the specific surface area (BET) of AgZnO nanoparticles (Figure 8a) is 28.682 m2/g, The BET of AgZnO/POMs nanocomposite (Figure 8b) is 33.535 m2/g. The results indicate that the AgZnO/POMS nanocomposites obtained by the combination of the two have higher specific surface area, which corresponds to the enhanced adsorption performance of the composite under dark conditions.
The pseudo-first-order and pseudo-second-order kinetic models were used to fit the experimental data of AgZnO/POMs nanocomposites.
In the (1) and (2), q0 is adsorption amount at t = 0, qe is equilibrium adsorption amount, qt is adsorption amount at time t, k1 and k2 are the pseudo-first-order and pseudo-second-order kinetic rate constants, respectively.
Table 1. Kinetic correlation coefficients (R2) fitting parameters.
|
Pseudo-first-order
|
Pseudo-second-order
|
R2
|
R2
|
Dark
|
0.3471
|
0.9997
|
UV light
|
0.9380
|
0.9736
|
The kinetic plots of removing BM by AgZnO/POMs nanocomposites are shown in Figure 9 and the results are shown in Table 1. The R2 of pseudo-second-order model were 0.9997 and 0.9736 under dark and UV-light respectively which were higher than that of pseudo-first-order model, 0.3471 and 0.9380 under dark and UV-light respectively, indicating that both the adsorption process and the photocatalysis process of removing BM by AgZnO/POMs nanocomposites followed the pseudo-second-order kinetics. The results demonstrate that the removal rate of AgZnO/POMs nanocomposites is mainly due to the chemical adsorption and electron transfer ability of the composites [27, 43].
When AgZnO/POMs nanocomposites were excited by UV light, the photogenerated e− and hole (h+) will be produced by ZnO. Ag acts as an electron acceptor, by which chemisorbed molecular oxygen reacts with photogenerated e− to form superoxide radicals (˙O2-), facilitating the trapping of photogenerated e−, and thus the separation efficiency of the photogenerated e− and h+ is improved. The h+ in the valence band of ZnO react with hydroxyl groups to form hydroxyl radicals (˙OH), the ˙OH is a strong oxidant for removing the organic chemicals. The h+, ˙OH and ˙O2- produced in the process of photocatalysis are crucial substances for BM removal, in addition to the adsorption between the adsorbent and the dye molecules [19, 27, 44]. As a result, the removal efficiency of AgZnO/POMS nanocomposites was greatly improved by the combination of AgZnO and POMs into a whole through nanoengineering. The photocatalytic-adsorbent AgZnO/POMs nanocomposites are expected to be a new type of dye removers which can remove aromatic organic dyes from water pollution more efficiently, especially for BM, which is difficult to remove.
To investigate the reproducibility of the nanocomposites for removing BM, we collected and washed the AgZnO/POMs nanocomposites. The collected nanocomposites were used to remove BM via five repeated experiments under the same reaction conditions. As shown in Figure 11a, the removal rate of BM in AgZnO/POMs nanocomposites decreased by only 7.0% (from 94.0% to 87.0%) after five cycles, the slight reduction might correspond to the loss of AgZnO/POMs nanocomposites during washing. Figure 11b shows that the FTIR spectrum of the AgZnO/POMs nanocomposites before and after BM removal are similar. It could be proved that the nanocomposites have the well stability and light corrosion of resistance.