Currently, water shortage and quality are great concerns for humans around the world (1). Water shortage affects more than 40% of the worldwide contamination, and this problem will become even more significant with climate alteration (2). 4-Nitrotoluene (4NT) includes one benzene ring attached to nitro and methyl groups. It may actually be carcinogenic in humans or categorized as a significant contaminant based on its toxicity. Therefore, it is essential to develop operative and efficient methods to remove damaging aromatic pollutants from the environment and waste.
Ozonation can be used as a public method for the sterilization and elimination of tastes and odors from water. Furthermore, ozonation is used for the oxidation of organic pollutants
to obtain minor biodegradable molecules. However, the efficiency of ozone is highly reliant on the chemistry of organic contaminants and amount of O3
and produced hydroxyl radicals. Ozone selectively reacts with organic combinations and functional groups that have high electron density, including activated aromatic systems, double bounds, reduced sulfur species, and non-protonated secondary and tertiary amines with second-order rate constants. Nevertheless, one of the main disadvantages of ozonation is the production of oxidation by-products from matrix compounds and micro-contaminants, such as N-nitrosodimethylamine and bromate, which can be sometimes toxic (3).
Ozonation has great significance in water treatment due to its disinfection ability and high oxidation potential of ozone; nonetheless, heterogeneous catalytic ozonation is preferred to single ozonation based on economic and higher removal proficiencies. Therefore, in recent years, there have been studies carried out on heterogeneous catalytic ozonation systems for increasing the efficiency of ozonation. The degradation of many organic mixtures has been investigated using different heterogeneous catalytic methods, including ozonation with activated carbons, zeolites/O3
(5), and Al2
Still, there are many polemics concerning investigating the mechanisms of these methods (6-7). For instance, according to some studies, adsorption was proposed as an essential stage in the catalytic ozonation method (8); however, based on other studies, it was suggested that adsorption may negatively affect the catalytic ozonation process. In addition, the impact of the catalyst is uncertain.
Based on the results of previous studies, aqueous ozone can be degraded by catalysts in order to produce hydroxyl radicals (9); however, other studies recommended that catalysts act as adsorbents for both contaminants and ozone to enable surface reactions (10-11). Consequently, it is crucial to identify the catalytic ozonation process to develop this process from laboratory to bench, demo, and industrial scale. Based on the literature, there have been a limited number of studies conducted on the role of MnO2
in ozonation reactions, especially about the ozone decomposition reaction, indicating a severe lack of information in this regard. The present study aimed to investigate the degradation of 4NT
as an aromatic pollutant by the MnO2
/ Clinoptilolite (CP)/O3
process and effect of pH, initial concentration of 4NT, amount of MnO2
, and kinetics of reaction for higher degradation of 4NT.
All 4NT, hydrogen chloride (HCl), sodium hydroxide (NaOH), potassium iodide (KI), and sodium thiosulfate (Na) were reported as reagent grades and provided by Merck
(Merck, the United States). An ozone generator (ARDA Company, Iran) produced ozone by feeding through dry oxygen. Moreover, all the reagents were utilized as received without further purification, and distilled water was applied all over the current study.
A semi-batch (batch for MnO2
and 4NT and continuous for ozone) reactor was used in order to carry out the experiments. The pure oxygen, from a pressurized capsule, was entered into an ozone generator (214V and 0.39A; ARDA Company, Iran). A 2-liter capacity reactor was equipped with a water-flow jacket connected to a thermostat (BW20G model; Korea) for the adjustment of temperature to 25ºC in all the procedures (Figure 1). The pH was calculated by pH Meter PT-10P Sartorius Instrument A Company in Germany. The progress in the degradation of 4NT was recorded by high-performance liquid chromatography (HPLC; Knauer, Germany) equipped with a spectro-photometer (Platm blue, Germany).
A reverse-phase column was filled with 3 μm Separon C18
with a length of 150 mm and diameter of 4.6 mm. The isocratic technique was adopted with adjusted pH to 2.5 by means of orthophosphoric acid and a solvent mixture of acetonitrile and deionized water (60:40% v/v) at a flow rate of 1 ml/min at room temperature. The suspension was centrifuged and filtered for the collection of the catalyst particles in all the experiments.
Catalytic ozonation experiments
In each run, about 2 L of the aqueous solution, including 75 mg/l of 4NT and nano-MnO2
, were thoroughly mixed in the reactor. A combination of O3
was formed by an ozone generator and entered from the bottom of the reactor by a porous diffuser in order to mix well, saturate the solution with O3
, better transfer mass, and react between ozone, MnO2
, and pollutant. The concentration of gaseous ozone was estimated by the iodometric method through the application of 2% neutral buffered potassium iodide for ozone trapping and sodium thiosulfate as a titrant (12). The flow rate of the O3
mixture remained constant at 0.3 L min-1
according to the literature and initial experiments, with an ozone concentration of 12.45 mg/L-1
. In order to calculate the amount of consumed ozone, the reactor outlet gas was bubbled through a KI (2%w) tamponed solution for the determination of not reacted ozone with the reaction of the potassium iodide solution with the excess ozone according to the following equation:
+ 2KI + H2
O → I2
+ 2KOH + O2
Standard sodium thiosulphate titrated the produced iodine in the presence of starch regarded as an indicator. The amounts of not reacted and reacted ozone were determined, and the value of ozone in tail gas was equivalently obtained. The residual of ozone in an aqueous solution was calculated by a spectrophotometer through the adoption of the indigo method (13).
particles were dispersed and suspended in the solution as the ozone gas entered the reactor. At different intervals, the withdrawal and filtration of the samples were conducted for the removal of MnO2
particles. The 4NT concentration was specified using a spectrophotometer at 230 and 280 nm regarded