of IONPs is decreased from 50 - 100 nm to 15 - 20 nm for both compositions (PPG:TDI 1:2 and 1:1.75). This behavior can be attributed to the higher surface area provided with the smaller size particles and the weaker effect of self-aggregation of the nanoparticles   . Additionally, by allowing more contact time between the arsenic species and the adsorbent surface, a higher removal capacity can be achieved due to the filling of the available binding sites on the surface.
To study the effect of shape on the adsorption capacity, foam samples with a
Figure 3. Cumulative mercury intrusion vs. pore size diameter. (a) PPG:TDI 1:2; (b) PPG:TDI 1:1.75.
Figure 4. (a) Arsenic removal capacity of composition ratio 1:2 (PPG:TDI) Cube; (b) Arsenic removal capacity of composition ratio 1:1.75 (PPG:TDI) Cube, with two size ranges of IONPs; 15 - 20 nm and 50 - 100 nm.
granular shape were prepared with 12 wt% loaded IONPs using both PPG:TDI composition ratios and IONP sizes; under similar exposure conditions of the first stage. Figure 5 illustrates the removal capacity of granular PU-IONPs adsorbents.
The outcomes of second batch of experiments reveal an increase in the adsorption capacity of both compositions (PPG:TDI 1:2 and 1:1.75) and IONP sizes (15 - 20 nm and 50 - 100 nm) compared to the first stage at both exposure times. Foam samples with the granular form provide more contact sites on the surface of adsorbent than the cubic one; therefore, more arsenic species can be trapped by an adsorption mechanism. The increase in the As removal capacity, for all samples, is calculated and listed in Table 2.
Figure 5. (a) Arsenic removal capacity of composition ratio 1:2 (PPG:TDI) granular; (b) Arsenic removal capacity of composition ratio 1:1.75 (PPG:TDI) granular, with granular shape and two size ranges of IONPs; 15 - 20 nm and 50 - 100 nm.
Table 2. The effect of granular shape on the removal capacity of arsenic.
Varying the shape of foam from a cubic to a granular form affects the removal capacity of the adsorbent. It can be noticed that the increase ranges, approximately, between 20% in the case of PPG:TDI (1:2), 15 - 20 nm IONPs, and 6 hr contact time, and 100% in the case of PPG:TDI (1:1.75), 50 - 100 nm IONPs, and 24 hr contact time. Also, the difference in removal capacity for both PPG:TDI compositions, when they were used as cubic shape compared to granular, is eliminated. In other words, the effect of the difference in the foam cellular structure for both compositions is degraded by altering the adsorbent shape from cubic to granular.
3.3. Contact Time and Adsorption Kinetics Studies
The effect of contact time on the removal capacity of arsenic was studied in the range of 3 hr to 24 hr exposure time. One gram foam samples of three different types of adsorbents; (a) PPG:TDI ratio (1:2)-Cube, (b) PPG:TDI ratio (1:1.75)-Cube, and (c) PPG:TDI (1:1.75)-Granular were used. Figure 6 shows the time
Figure 6. Effect of contact time on the removal capacity of As, using (a) PPG:TDI ratio (1:2)-Cube; (b) PPG:TDI ratio (1:1.75)-Cube; and (c) PPG:TDI ratio (1:1.75)-Granular.
profile of arsenic adsorption on PU-IONPs nanocomposite with an initial concentration of 100 ppb.
The experimental outcomes indicate that the uptake of As increases with time. However, the rate of adsorption was rapid in the first 12 hr after which the rate slowed down as the equilibrium state was approached. The highest removal capacity occurred at 24 hr for all adsorbents; 45.29%, 37.14%, and 27.16% removal capacities were achieved for PPG:TDI (1:1.75)-Granular, PPG:TDI ratio (1:2)-Cube, and PPG:TDI (1:1.75)-Granular; respectively.
In order to examine the kinetic mechanism which controls the adsorption process, several kinetic models like Lagergren pseudo-first-order  and pseudo-second-order  were tested to interpret the experimental data. The integrated linear pseudo-first-order rate equation can be represented as:
where qe is the amount of As adsorbed (mg/g) at equilibrium, qt is the amount of As adsorbed (mg/g) at any time “t”. K1 is the pseudo-first-order rate constant (hr−1). The plot of log(qe − qt) vs. t gives a linear representation of Lagergren pseudo-first-order as illustrated in Figure 7. The values of K1 were obtained from the slope of log(qe − qt) vs. t plots.
Figure 7. Pseudo-first-order kinetics for (a) PPG:TDI ratio (1:2)-Cube; (b) PPG:TDI ratio (1:1.75)-Cube; and (c) PPG:TDI ratio (1:1.75)-Granular.
The linear form of pseudo-second-order rate equation is represented by:
where qt is the amount of As adsorbed (mg/g) at any time “t”, qe is the amount of As adsorbed (mg/g) at equilibrium. K2 is the pseudo-second-order rate constant (g/mg∙hr−1). The experimental data plotted against 1/qt vs. 1/t is shown in Figure 8; K2 and qe were calculated from the slope and intercept of these plots. Table 3 summarizes the calculated values of K1, K2, qe, and R2 for both kinetic models.
The evaluation of the best fit kinetic models was made based on R2 values. The calculated values of R2 for the pseudo-second-order are higher than the pseudo-first-order. Hence, the second order kinetic model better represented the adsorption kinetics, suggesting that the adsorption process is more likely to be a chemisorption. The adsorption behavior may involve valence forces through the sharing of electrons between arsenic and the adsorbent  . Furthermore, previous investigations support that a second order kinetic model correlates well with the experimental data of arsenic adsorption     . Adsorbent (a); PPG:TDI ratio (1:2)-Cube fits better than adsorbent (b); PPG:TDI (1:1.75)-Cube, while Adsorbent (c); PPG:TDI (1:1.75)-Granular fits better than adsorbent (b) and (a); respectively.
Figure 8. Pseudo-second-order kinetics for (a) PPG:TDI ratio (1:2)-Cube; (b) PPG:TDI ratio (1:1.75)-Cube; and (c) PPG:TDI ratio (1:1.75)-Granular.
Table 3. Kinetic models rate constants (K1) and (K2).
The present study introduces a new bulk modified nanocomposite material by using IONPs impregnated in PU foam for arsenic removal. Adsorption batch experiments were performed to investigate the effect of IONPs size on the removal capacity of the adsorbent foams. Foam samples with a smaller IONPs size range (15 - 20 nm) achieved higher removal capacity compared to a size range of 50 - 100 nm. In addition, the foam shape effect is evaluated; granular adsorbents exhibit a higher removal capacity compared to cubic adsorbents. The increase percentage ranged between 20% in the case of PPG:TDI (1:2), 15 - 20 nm IONPs, and 6 hr contact time, and 100% in the case of PPG:TDI (1:1.75), 50 - 100 nm IONPs, and 24 hr contact time. The kinetic data correlates well with a pseudo-second-order-kinetic model. Adsorbents with a PPG:TDI ratio (1:2) fit better than adsorbents with a PPG:TDI ratio (1:1.75). Adsorbents with a granular form fit better than adsorbents with a cubic form. The proposed system of the nanocomposite foam offers a potential for the removal of arsenic with higher capacity at lower costs than conventional arsenic removal systems.
We would like to thank Dr. Steven Hardcastle and Mr. Daniel Kaminski at the Advanced Analysis Facility (AAF) for their assistance in the foam characterization. In addition, we would like to thank Dr. Subhashini Gunashekar for her assistance in the foam synthesis.
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