A polyurethane (PU) foam composite, loaded with iron oxide nanoparticles (IONPs), was developed for arsenic removal from drinking water at low concentrations. The effect of various synthesis parameters such as the size of IONPs and the foam shape, on the performance of the adsorbents in removing arsenic was investigated. To examine the surface adsorption of arsenic species, Energy Dispersive X-ray Microscopy (EDX) was utilized. Mercury Porosimetry was used to analyze the porosity and density of the PU-IONPs nanocomposites. Atomic Absorption Spectrometry (AAS) was conducted to measure the arsenic concentration in the treated solutions. Kinetic models were applied to determine the mechanisms which control the adsorption process. A pseudo-second-order model was found to be the best fit model for the adsorption data. Experimental results revealed that decreasing the size of IONPs from 50 - 100 nm to 15 - 20 nm yields a higher removal capacity. In addition, granular adsorbents exhibit higher removal capacity compared to cubical shaped adsorbents in the order of 20% - 100%.
Heavy metal pollutants such as Lead, Arsenic, Cadmium, and Mercury are the main sources of water contamination. They have serious toxic effects on humans and living organisms. Arsenic contamination of ground water poses a serious concern in many countries throughout the world, including the United States. The contamination level is variably defined to be greater than 10 µg/L or greater than 50 µg/L by different agencies [
The removal of arsenic species involves a selective separation of arsenate As (V) and arsenite As (III). The conventional treatment methods of arsenic involve a coagulation with ferric chloride or aluminum sulphate coagulants, followed by the separation of insoluble product by settling or by direct filtration through sand beds [
In this study, a new bulk modified nanocomposite material (adsorbent) is developed by loading iron oxide nanoparticles (IONPs) inside a polyurethane (PU) foam matrix. This allows for exploiting the inherent advantages of porous PU foam structures and flexibility, combined with the functionality of the adsorbent nano particles imbedded in the foam media, which facilitate the post treatment step in the filtration system. The ability of iron compounds to react with arsenic species by adsorption and ion exchange mechanisms increases the arsenic removal capacity.
For the synthesis of PU-IONPs nanocomposite, the following raw materials were used as received: toluene di-isocyanate (TDI; 2.4% - 80%, 2.6% - 20%, Alfa Aesar), polysiloxane surfactant (Sigma Aldrich), nitrogen gas (Airgas, O2 free UHP), iron oxide nanoparticles (IONPs; Fe3O4, high purity 99.5%, US Research Nanomaterials Inc.) with two size ranges: 50 - 100 nm and 15 - 20 nm, 18.2 MOhm-cm deionized water, and polypropylene glycol 1200 (PPG; Sigma Aldrich Co. LLC) dehumidified in a vacuum oven at 70˚C. 1 ppm standard arsenic solution, from Inorganic Ventures Co., was diluted to 100 ppb. The solution contained both As (III) & As (V) at pH 6.5.
The experimental setup described in this study is designated in a previous publication [
Many samples were prepared using two PPG: TDI ratios; 1:1.75 and 1:2, and two sizes of IONPs; 15 - 20 nm and 50 - 100 nm. The optimal loading percentage of IONPs was determined to be 12 wt% [
Batch mode experiments were conducted in two stages. In the first stage, 1 g cubic samples with PPG:TDI ratios; 1:1.75 and 1:2, and two sizes of IONPs; 15 - 20 nm and 50 - 100 nm, were soaked in 50 ml of 100 ppb As solution for 6 and 24 hours. The cubes were shaken in neutral solution (pH = 6.5) at 200 rpm and at room temperature (22˚C). The purpose of the first stage is to study the effect of the IONPs size on the arsenic removal capacity. In the second stage, samples with different shapes; i.e., cube and granular, were used in similar conditions of the first stage to investigate the effect of the foam shape on the adsorption capacity. After each batch test, 25 ml of each treated solution was filtered and preserved with 2% HNO3 for AAS analysis. Additionally, three sets of samples were prepared to investigate the adsorption kinetics. Samples with PPG:TDI ratios; 1:1.75 and 1:2, and one foam shape (cube), were stirred under the same conditions, as the previous batches, between 3-hrs and 24-hrs at 3-hr intervals. This stage was aimed to study the effect of PU compositions on the adsorption kinetics. Moreover, foam samples with PPG:TDI ratio of 1:1.75, and different shapes (cube and granular), were used to study the effect of the foam shape on its kinetic behavior.
To enhance the EDX analysis, the foam samples were coated using a gold sputter coater (Denton Desk II Cold Sputter/Etch unit). In
The measurements of porosity and density were conducted for both 1:1.75 and 1:2 PPG:TDI compositions. MicroActive AutoPore IV 9600 was used to measure the nanocomposite foam porosity by applying various levels of pressure to foam samples immersed in mercury.
PPG: TDI | Bulk Density (g/ml) | Total Pore Area (m2/g) | Porosity (%) |
---|---|---|---|
1:2 | 0.925 | 28.48 | 8.26 |
1:1.75 | 1.17 | 40.16 | 10.17 |
pore area, and porosity for both PU compositions.
PU foams with PPG: TDI of 1:1.75 exhibit higher bulk density compared to those with PPG:TDI of 1:2. In addition, the total pore area and porosity were found to be greater in PPG:TDI (1:1.75). This can be attributed to the physical properties of each material component in the mixture such as density and viscosity.
First batch experiments were performed, under the same exposure conditions, to investigate the effect of IONPs size on the As removal capacity of the PU nanocomposite foams. The foam samples were prepared with the optimum percentage of loaded IONPs (12%) using both PPG:TDI composition ratios; 1:2 and 1:1.75, and two size ranges of the IONPs; 15 - 20 nm and 50 - 100 nm. The sorption batch experiments were carried out under two exposure time intervals; 6 hr and 24 hr.
The results shown in
To study the effect of shape on the adsorption capacity, foam samples with a
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.
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
Molar Ratio of (PPG:TDI) | IONPs Size (nm) | Contact Time (hr) | Increase Percentage (%) |
---|---|---|---|
1:2 | 15 - 20 | 6 | 17.35 |
24 | 30.1 | ||
50 - 100 | 6 | 44.32 | |
24 | 40.46 | ||
1:1.75 | 15 - 20 | 6 | 71.68 |
24 | 66.75 | ||
50 - 100 | 6 | 88.89 | |
24 | 100 |
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.
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.
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 [
log ( q e − q t ) = log q e − ( K 1 / 2.303 ) t (1)
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
The linear form of pseudo-second-order rate equation is represented by:
1 / q t = 1 / ( K 2 q e 2 ) t + 1 / q e (2)
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
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 [
Type of Adsorbent | Pseudo first order | Pseudo second order | |||
---|---|---|---|---|---|
K1 (hr−1) | R2 | K2 (g/mg∙hr−1) | qe (mg/g) | R2 | |
(a) | 0.32 | 0.690 | 0.38 | 0.082 | 0.953 |
(b) | 0.16 | 0.749 | 17.01 | 0.011 | 0.793 |
(c) | 0.14 | 0.864 | 3.77 | 0.025 | 0.982 |
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.
Hussein, F.B. and Abu-Zahra, N.H. (2017) Adsorption Kinetics and Evaluation Study of Iron Oxide Nanoparticles Impregnated in Polyurethane Matrix for Water Filtration Application. Journal of Minerals and Materials Characterization and Engineering, 5, 298-310. https://doi.org/10.4236/jmmce.2017.55025