We have synthesized, in this work, zero valent iron (ZVI) nanoparticles to improve the efficiency of degradation of phenolphthalein catalyzed by ozone in aqueous solution. The Fe nanoparticles were obtained using the pulsed plasma in liquid (PPL) method with water as the liquid medium. Such nanoparticles have a mean size of 12 nm and are composed of ~80% Fe0, while the rest are a mixture of Fe+2 and Fe+3 oxides. The degradation of phenolphthalein was carried on a glass reactor injecting a constant amount of ozone and introducing different concentrations of Fe nanoparticles to the system. When using pure ozone, the percentage of degradation of phenolphthalein measured by colorimetry after one hour of reaction was 84%. However, when Fe nanoparticles are used, such percentage can be as high as 98% in 50 minutes of reaction. Furthermore, the degradation rate constant was 0.0334 min−1 with only ozone and it can be as high as 0.0733 min−1 with Fe nanoparticles. Finally, the total mineralization of phenolphthalein was obtained by total organic carbon (TOC) determinations. It is shown that when using only ozone, we obtained a percentage of mineralization of 49% and 96% when using the highest concentration of Fe nanoparticles.
Because of the increasing human activity, wastewaters have become a high-priority issue since the produced contaminants can reach oceans, rivers, underground water, and agricultural soil. This results in population exposure to hazardous substances that lead to public health threat. Among these dangerous substances, we find organic molecules that can be very toxic, carcinogenic, and difficult to eliminate. For these contaminants, it is not enough to use conventional wastewater techniques such as biological, adsorptive, or partial decomposition, since the intermediate products are sometimes worse than those of the original molecule.
One of the novel ways to treat water containing biorefractory or difficult to remove compounds is by using advanced oxidation processes (AOPs), which depends on the hydroxyl radical formation. This radical (•OH) is considered highly reactive, a powerful oxidant, which is easily formed and ready to react leading harmless products [
AOPs are an attractive alternative to reach complete degradation (mineralization) of organic molecules to H2O, CO2, and inorganic salts. The Fenton reaction is a well-known AOP that has been used for many decades and is still under studies because of low-cost and low-temperature and pressure operation [
A novel way to improve the Fenton reaction is by introducing ozone to the system which can tackle the degradation issue by two approaches: first, ozone itself can directly oxidize organic molecules and second, it also can react with iron in solution to produce the desired hydroxyl radicals [
In the last decade, Fe-containing compounds in the nanometric scale have been used in the reactions mentioned above [
To overcome this problem, in this work, we propose the use of pulsed plasma in liquid to produce pure, uncovered ZVI nanoparticles and use them in the degradation of phenolphthalein, which is widely used as an acidic-basic indicator, and because of its intense color at low concentrations, it is a suitable model molecule to easily follow its degradation. By using this method, we avoid the presence of any coating on the particle surface increasing the efficiency of the AOP because the Fe atoms are readily exposed to the molecules involved in the oxidation process [
As mentioned, the method used in this work to produce ZVI nanoparticles was pulsed plasma in liquid (PPL). Briefly, 40 A of AC current is applied to two rods of iron (
A 2 L glass reactor was used for the experiments (Figure
Schematics of the reactor used in the phenolphthalein degradation experiments.
500 mL of water solution (60 ppm) of phenolphthalein was introduced to the reactor, and the pH was adjusted to 3 with H2SO4 1 M. Then, the ozone was injected and immediately after, the nanoparticles were introduced. We used three different amounts of ZVI nanoparticles: 30, 60, and 90 mg as well as only ozone for comparison. The reaction was monitored for 60 minutes taking a small sample (2 mL) each 10 minutes to monitor the phenolphthalein degradation. The phenolphthalein concentration was measured by light absorption. Each sample taken from the reactor was adjusted to a pH of 13 with a solution of NaOH 1 M to achieve the characteristic purple color of phenolphthalein in basic solution and measured with a HACH DR/5000 Uv-Vis spectrophotometer at a wavelength of 550 nm. Total organic carbon (TOC) was measured with a TOC-LCPH/CPN Shimadzu total organic carbon analyzer after the end of each experiment to study the mineralization of the phenolphthalein. Each experiment was repeated three times, and the results presented are the average of the three measurements.
Figure
(a) TEM image of the ZVI nanoparticles. (b) Diffraction pattern of ZVI nanoparticles, the green Miller indexes are for metallic iron and the red indexes are for iron oxide.
Mossbauer spectroscopy of the Fe nanoparticles (a) before the phenolphthalein degradation experiments and (b) after the phenolphthalein degradation experiments.
The percentage of degradation of phenolphthalein against time is presented in Figure
Percentage of degradation of phenolphthalein with different doses of ZVI nanoparticles.
From these reactions, we can observe that pure ozone is capable of forming the oxidative •OH radical and therefore the observed degradation of phenolphthalein.
On the other hand, also from Figure
Equations (
Since not only the final percentage of degradation is affected but the reaction time too, we followed the kinetics of the reaction with the following equation:
By considering the concentration of ozone constant throughout the experiment because of its abundance compared to that of the other reactants, we have
Then, we consider a first order reaction (
We have used (
Phenolphthalein degradation rate constants and TOC results.
Degradation rate |
% of TOC | |
---|---|---|
Ozone only | 0.0334 | 49 |
30 mg ZVI NPs | 0.0441 | 86 |
60 mg ZVI NPs | 0.0721 | 89 |
90 mg ZVI NPs | 0.0733 | 96 |
The colorimetric method used to determine the concentration of phenolphthalein is applied only to measure the degradation of the molecule. However, it is impossible to know the extent to complete mineralization. For this purpose, we performed measurements of total organic carbon (TOC) at the end of each experiment and the results are shown in Table
In previous works, we have studied two methods for the degradation of phenolphthalein in an aqueous solution. The first one is an electrochemical method which requires acidic conditions (pH around 3) to reach a 71% Chemical Oxygen Demand (COD) removal. The other one is the use of ozone which needs basic conditions to obtain a 25% COD removal. This difference in results can be explained in terms of the hydroxyl radical production, which is faster in the electrochemical methods; furthermore, less reactions are required to produce the radicals as compared to those with the use of ozonation [
We have synthesized zero valent iron nanoparticles using the pulsed plasma in liquid method. The nanoparticle mean diameter is 12 nm having the majority of their Fe atoms in the metallic phase as evidenced by Mossbauer spectroscopy. We have used these nanoparticles to enhance the degradation of phenolphthalein by ozonation at different particle concentrations. When using only ozone, the degradation measured by colorimetry was 84% at 60 minutes; however, when ZVI nanoparticles are introduced to the reactor, the degradation is as high as 98% with 90 mg of NPs. The degradation rate constant was also calculated, and it was found that when using Fe nanoparticles, the value of this constant can be twice as big as the one obtained with only ozone. Finally, the incorporation of Fe to the system leads to an almost complete mineralization (96%) of phenolphthalein while it is only 49% for only ozone.
The authors declare that they have no conflicts of interest.