Emerging organic pollutants (EOCs) are known as micro-organic compounds, which are defined in Directive 2013/39/EU [1
]. There is a wide range of compounds that are EOCs: drugs, pesticides, hormones, etc. [2
]. These compounds are found in the environment, due to the increase in their consumption and/or production by humans [3
2-chloro-4-ethylamino-6 isopropylamino-s-triazine, known as simazine (CAS No. 122-34-9), is a selective herbicide [6
]. This substance is used to eliminate the weeds in different types of crops [7
]. Simazine applied to soil as an herbicide has contributed to surface and groundwater contamination due to its leaching tendency [8
]. The presence of simazine in the soil–water system is considered an environmental hazard due to its estrogenic effect [9
The United States Environmental Protection Agency (USEPA) considers simazine an endocrine disruptor and a possible carcinogen for humans and animals [11
]. The lethal concentration 50 (LC50) for fish varies from units to hundreds of milligrams per liter [14
]. Several researchers have detected the presence of simazine in water. Kim and Homan [15
] detected the presence of simazine in the waters of Erie’s Lake. Li et al. [16
] identified simazine in the Taizi River waters in concentrations of up to 1150 ng L−1
. The USEPA detected its two main metabolites (desisopropyl-s-atrazine and diaminochlorotriazine) in soil and groundwater samples in California [17
]. The European Union has established a regulation that limits herbicide residues in drinking water to 0.1 μg L−1
for individual herbicides, and 0.5 μg L−1
for the sum of herbicides [18
], while the USEPA has established this limit at 4 µg L−1
Conventional wastewater treatment plants (WWTPs), based mainly on biological processes, are not specifically designed for EOCs reduction. By aerobic or anaerobic biological treatment in WWTPs, simazine is only partially eliminated; therefore, biodegradation is not the most effective technology for its removal. In addition, the presence of this type of contaminants has a negative effect on the survival and reproduction of the microorganisms that allow the water treatment [20
]. There are different studies in which simazine has been detected in the effluents of WWTPs [22
]. Bueno et al. [23
] detected concentrations of 105 and 1242 ng L−1
in the effluents of the WWTPs that treated urban and industrial wastewater in the city of Almeria, Spain.
Active carbon (AC) is used as an adsorbent for pesticides and other compounds in natural water and wastewater. ACs are highly porous and have a wide range of pore sizes. There are two main types of carbon: powdered activated carbon (PAC) and granulated activated carbon (GAC). PAC is most applicable for inclusion in those systems that already have a number of processes including tanks for mixing, precipitation or sedimentation and filtration, PAC is mainly added directly to water from WWTPs with an average contact time of 30 min to 4 h. While GAC is used in filter beds, it can be used in small residential treatment systems (individual supply), or for large volumes in commercial units, such as water treatment for community supply systems. Bataller et al. [24
] reported that, in drinking water, the current trend is to incorporate post-ozonization to interozonization, followed by granular-activated carbon or biofiltration, with the aim to remove pesticides, biodegradable dissolved organic carbon and ozonation by-products. The EPA recommends the use of GAC for atrazine pesticide removal [25
]. Lladó et al. [26
] tested three different types of activated carbons to eliminate carbamazepine and atrazine in water. Gardi et al. [27
] studied the elimination of 3 g L−1
of simazine with 1 g L−1
of GAC (Hydraffin 30N); they found a reduction of 38% at a contact time of 10 min. In other studies, novel porous carbon composites were used; these adsorbents were applied to farmland waters, achieving improvements in the adsorption of triazine group herbicides present in the soil [28
In general, the AC’s adsorption effectiveness depends on their physicochemical characteristics such as: surface, size and number of pores, functional groups, etc.; and also depends on the physicochemical properties of the contaminant such as: molecular size, hydrophobicity, polarity, functional groups, etc. [29
]. In the Water Treatment Plants (WTP) located in Benidorm (Alicante, Spain) [30
], PAC is used to reduce color and odor in the treatment of natural water that the plant captures before distribution to the public supply network. As per Di Bernardo and Dantas [31
], it cannot be generalized that any type of carbon (powder or granular) can adsorb any undesirable organic substance from the water. Therefore, it is essential to carry out laboratory tests, to obtain previous knowledge of the main characteristics of the different types of active carbon and to make the appropriate selection of the active carbon, analyzing the removal of specific substances.
Ozonization is effective in the degradation of a wide range of organic contaminants (pharmaceuticals, synthetic substances, pesticides and herbicides, etc.) [28
]. The degradation of organic substances is achieved mainly in two ways: by direct reaction with molecular ozone, or by indirect reactions with free radicals. Ozone treatment has been applied for the reduction of natural organic matter [32
], inactivation of microorganisms [34
] and reduction of EOCs in waters [35
]. Simazine is considered a low oxidation microcontaminant; its reduction is greater when both direct and indirect reaction ways are used, the indirect one being the most sensitive. Mathon et al. [37
] applied a dose of ozone of 1.6 g O3
to an initial concentration of simazine of 9000 ng L−1
; in this study they obtained a concentration of simazine of 6000 ng L−1
after 800 s of treatment. This means a reduction of 33%. Rate constants of reactions of ozone and hydroxyl radical with simazine were found to be 8.7 M−1
and 2.1 × 109
, respectively [38
]. For example, carbamazepine had a reaction rate constant of ozone (KO3
) and hydroxyl radical (KOH
) of 3.0 × 105
and 8.8 × 109
, respectively [39
], while the reaction rate constants of metoprolol were 2.0 × 103
and 7.3 × 109
, respectively [40
Beltrán et al. [41
] studied the by-products generated during the oxidation process of simazine with ozone. The main oxidation by-products detected in this study were: 2-chloro−4-acetamide-6-ethylamino-s-triazine (CDET), 4.6-diamine-2-hydroxy-s-triazine (OAAT) and 2.4.6-trihydroxy-s-triazine or cyanuric acid (OOOT). These by-products could in some cases be as hazardous as simazine, which would require further treatment.
In recent years, combined treatment processes are being used to remove pollutants from water [42
]. Bernal Romero del Hombre Bueno [43
] studied at laboratory scale the elimination of simazine in wastewater, using a combination of several treatments. For an initial simazine concentration of 7 µg L−1
, a reduction of 25% was obtained when they applied a membrane bioreactor (MBR) treatment, and around 43% when they used an Upflow Anaerobic Sludge Blanket (UASB) treatment. With the combined treatment of MBR + Nanofiltration (NF), they obtained reductions of greater than 90%. When they used the MBR + Reverse osmosis (RO), they observed a reduction of 97%. The combined treatment of MBR + Ozone (O3
) reached yields above 92%. The simazine removal rate with the UASB + MBR treatment, to treat a dose of contaminant of 2.9 µg L−1
, was 72%. In this research, the combination of three treatments was also studied: The UASB + MBR + NF treatment achieved a percentage of 90% for an initial concentration of simazine of 1 µg L−1
. When the combined treatment was UASB + MBR + IO, they obtained a 97.2% reduction when the initial simazine dose was 0.3 µg L−1
]. Flores et al. [44
] studied the photocatalytic degradation of simazine using zinc oxide/graphene oxide composite materials under visible light irradiation. In this study, they achieved percentages above 80% depending on the catalyst dose and pH range. Catalkaya and Kargi [45
] investigated the removal of simazine from aqueous solution by Fenton’s reagent oxidation. The optimal H2
/Fe (II)/simazine ratio to obtain the maximum pesticide removal (100%) was 55/15/3 (mg L−1
No bibliographic references have been found that discuss the reduction of simazine with carbon followed by ozone or vice versa. For this reason, the main objective of this research was to study the effect of the combination of active carbon followed by ozone and vice versa, to achieve the maximum reduction of simazine. For this purpose, an initial dose of contaminant of 0.7 mg L−1 was chosen. In the first part of the research, the optimal dose of carbon and ozone was determined. Two types of active carbon were used (PAC and GAC). Finally, for the combined treatments AC/O3 and O3/AC, the optimal time that allowed a reduction of 90% of simazine was determined.
In view of the results obtained by the isotherm of N2 adsorption at 77 K, it indicates that GAC has more micropores than PAC; however, GAC has less mesopores. The SEM images reflect the similarity on the surface of the coals, while the TEM images make the difference by showing a higher number of micropores in the GAC carbon. Therefore, although GAC has a greater volume of micropores, it also has fewer mesopores on its surface, which makes it difficult for simazine to access the active center and be adsorbed, in contrast to PAC.
In the PAC adsorption experiments, simazine shows a rapid reduction in the first 30–45 min. With the lowest and highest dose of PAC studied (4 mg L−1 and 20 mg L−1) for an initial simazine concentration of 0.7 mg L−1, a reduction of 6% and 79% is achieved for a contact time of 60 min. In the same way, but with a contact time of 24 h, a reduction of 36% and 89% is achieved, respectively. When GAC was used, simazine did not show significant reductions above 10% until after 540 min. With the lowest and highest dose of GAC (4 mg L−1 and 20 mg L−1), for an initial simazine concentration of 0.7 mg L−1 and 24 h of contact, a reduction of 3% and 28% was achieved, respectively. When the time was increased to 7 days of contact, the percentages obtained were 38% and 96%, respectively. Therefore, adsorption treatments for simazine achieve better reduction performance against contact time when treated with Pulsorb PWX HA than with Calgon Filtrasorb 400.
In the oxidation experiments, for the lowest and highest dose of O3 studied (5.6 mg L−1 and 24 mg L−1), an initial simazine concentration of 0.7 mg L−1 and a contact time of 20 min, reduction percentages of 47% and 93% were achieved, respectively. O3 oxidation achieves higher reduction percentages with less contact time than when treating with PAC and GAC, but the oxidation of simazine with O3 generates a series of by-products with negative effects for the aquatic environment and the health of living beings.
With the combined AC/O3 experiments, in order to achieve a simazine reduction greater than 90%, an average contact time of 38 min for PAC/O3 and 3 days for GAC/O3 was required.
Whilst in the experiments where the treatment was initiated with O3 and terminated with PAC or GAC, the time was 40 min and 4 days, respectively. These differences are due to the fact that the application of O3 generates a series of by-products that compete for the active center of the carbon, hindering the adsorption of simazine.
Both treatments are favorable for simazine reduction, taking into account the characteristics of the adsorbent or oxidant with the dose and contact time for each one. However, to achieve a reduction of more than 90% of simazine, any of the GAC combination treatments would not be optimal, since it requires a high treatment time due to the characteristics of the activated carbon, unless it increases the doses of GAC to achieve the adsorption of a greater number of simazine molecules in less time. The best combination treatment would be the one that uses PAC (PAC/O3 and O3/PAC). The O3/PAC treatment is recommended, because the contact time to achieve a reduction of more than 90% of simazine is similar to that of PAC/O3, and, at the end of the treatment with activated carbon, the adsorption of the by-products of the ozonization of simazine would be achieved, avoiding the negative effects to the aquatic environment and the health of any living being.
In conclusion, the individual treatments studied to reduce simazine present in the water show to be effective, except for the treatment with GAC for its low reduction in short times, while for the combined treatments, the best option is the O3/PAC reaching optimal reductions in low time. Considering that the concentration of the contaminant in this study (mg L−1) is higher than the concentration detected in aquatic environments, and also that the maximum concentration of simazine allowed in drinking water in the European Union is 0.1 μg L−1, the results obtained by the combined treatment O3/PAC, allow us to propose that this could be a treatment to be considered for the reduction of simazine in natural water for human consumption.
For future researches, the study could be extended to other EOCs present in the aquatic environment and include real water to determine the effectiveness when other substances are also present in the water.