3.1. Effects of Temperature and Catalyst Concentration
The temperatures and volumes of the 25% H2
obtained and employed using the catalyst concentration in solution of 25, 50, 100, and 150 mmol L−1
are shown in Table 2
shows that all conditions were adequate to degrade more than 70% of the initial resin masses. Given that our findings are based on temperature control by the flow rates of the reactants into the reactor, the results for such analysis should consequently be treated with the utmost caution. The contrasting values of temperature as regards the catalyst concentrations of 50, 100, and 150 mmol L−1
reflects the difficulty in maintaining the temperature without an external heater. For instance, the contrast at observed temperatures, e.g., 60 ± 6 °C (50 mmol L−1
O), and 53 ± 1 °C (100 mmol L−1
O) does not mean that an inferior concentration of catalyst generated more energy than a superior one, but indicates a difficulty regarding the control of the flow rates of the reactants.
The lowest concentration of FeSO4
O (25 mmol L−1
) was not sufficient to increase the reaction rate and generate enough heat to reach and maintain the temperature around 60 °C, which affected the reaction. On the other hand, the residual mass of the reactions conducted with the highest concentration indicates that an excess of catalyst may interfere negatively. The solutions of 50 and 100 mmol L−1
were the most effective, degrading about 100% of the resins. A similar result was obtained by the authors of [3
], who used 180 mL of 50% H2
to completely degrade 20 g of a mixed resin. Total Organic Carbon (TOC) was studied to evaluate the efficiency of the reaction. Figure 1
shows the behavior of TOC as a function of time.
Smaller amounts of catalyst showed a progressive decrease in TOC since the beginning. On the other hand, in larger quantities, an increase in TOC content was observed within 1 h of reaction, followed by a decrease to the end. TOC was influenced by the catalyst concentration, interfering at the beginning of the degradation process.
This behavior can be explained by the excess of Fe2+
ions that can act as “scavenger” of free radicals HO•, as shown in Equation (2), reducing the availability of these radicals in the reaction process, and delaying the degradation. Besides, the ion exchange resin is initially solubilized, increasing the TOC and finally decreasing as long as the soluble organics are oxidized in carbon dioxide [3
The constant rate of the radicals with Fe2+
is 3.2 × 108
, much higher than the Fenton reaction with 76 L mol−1
], as previously shown in Equations (1) and (2).
TOC removal efficiency was evaluated as the control parameter of the reaction. Their values were compared with the degradation of IER in %. Degradation of IER (%) and TOC removal are shown in Table 3
As anticipated, TOC is an efficient parameter for determining resin degradation rates. The degradation reactions were not efficient with 25 and 150 mmol L−1
O solutions, since the stoichiometric ratios were not appropriate. These inappropriate ratios were excess of H2
or catalyst concentration. It is common knowledge that excess H2
, as concerns rate constants and the target compound, can negatively interfere with process efficiency, whereas inappropriate concentrations of H2
may act as a HO• scavenger [17
pH measured over time decreased from 7 to 3 during the oxidation process and no significant difference was observed for the different conditions employed in the experiments.
indicates that the reaction with 25 mmol L−1
O showed, after 15 min, a high DO concentration compared to the concentrations of 50 and 100 mmol L−1
O, reaching its maximum value in 45 min.
The increase in the DO concentration may indicate an excess of H2
compared to the ferrous sulfate concentration, showing that there is a strong kinetic element conducting the oxygen/time profile of the reaction system studied [18
As observed in Equation (7), the excess of the oxidant can compete for the hydroxyl radicals, generating the radical HO2
•. These radicals act to oxidize Fe2+
and reduce Fe3+
as shown in Equations (5) and (6), resulting in increased dissolved oxygen formation compared to other catalyst concentrations. The concentration of 150 mmol L−1
O also resulted in a high concentration of DO, as shown in Figure 2
. The increase of Fe2+
leads to higher formations of Fe3+
and hydroperoxyl radical as shown in Equation (5). The hydroperoxyl radical is responsible for the increase in O2
concentration, as shown in Equation (6).
The untreated cationic and anionic resins and the mixture of both post Fenton reactions were analyzed by Fourier Transform Infrared Spectrometry (FTIR). Figure 3
, Figure 4
and Figure 5
illustrate the spectra.
It has been suggested [6
] that the degradation of anionic and mixed resins is much harder to achieve than cationic resins and the reasons were presented by the authors.
Spectrum analysis was performed and identification of groups and chemical bonds are shown in Table 4
The main components of the cationic and anionic resins were identified, as the organic compounds of sulfur, found as sulfonic acid at 1220 cm−1
, 1126 cm−1
, and 679 cm−1
, with strong intensity. The amines (from the anionic resins) were identified on the quaternary amine N–CH3
bond at 1370 cm−1
, 1176 cm−1
and the C–N bond, also on quaternary amines, from the N–H bond at 1598 cm−1
with strong intensity. This method was chosen because it is one of the most practical ways of showing that the residual resins may contain a large amount of carboxyl functionality. However, carboxyl groups are known to be very reactive with hydroxyl radicals produced by the Fenton reaction. The analysis of the residual resin (after Fenton’s reaction) showed the characteristic peak of the O–H bond at 3503 cm−1
, besides the strong presence of carboxylate ion at 1683 cm−1
, because the oxalic and the formic acids could have been formed during the reaction. According to the literature [4
], the production of these acids is a qualitative parameter to verify the degradation of aliphatic and aromatic rings, respectively. This result can be directly compared to that obtained in the TOC content analysis. FTIR indicated that most of the groups identified in the spectra of the individual resins disappeared after Fenton treatment. These groups in solution may have degraded until transformed into CO2
and water, while the remaining solid had organic acids. The presence of these organic acids resulted in the presence of carbon contents in the TOC analyzes at the end of the experiments. Remaining non-degraded resins would be easily mineralized since these organic acids are little resistant to the attack of hydroxyl radicals. Further data collection would be needed to determine the organic compounds, since IR-spectroscopy alone is not a definitive means to identify such compounds.
3.2. Immobilization with Portland Cement
Cement specimens were prepared with different suspension/cement (s/c) ratios in order to establish the appropriate ratio. The s/c ratios were 0.28, 0.30 and 0.35 s/c. The 0.30 and 0.35 ratios were considered inadequate, since a volume of segregated water of more than 0.5% of the total was observed after 24 h. These values were above the limit established by Brazilian regulations [16
]. As a result, the s/c ratio of 0.28 was selected for studies of the specimens at different pH. Good workability during the molds and no segregation were observed for all mixtures before the final setting time. Table 5
shows the setting time and the axial compressive strength for different pH.
All specimens reached the setting time of up to 8 h and this is sufficient for an industrial scale mixing [19
]. A review of the literature on this matter [20
] indicated that different types of cement were applied to immobilize the spent resins. In [20
], the authors employed slag cement, and the compressive strength of the solidified final wastes was greater than 7.35 MPa. On the other hand, [21
] applied sulfoaluminate cement, and the results showed a great compressive strength, 20 ± 2 MPa.
In this study, the acidic solutions presented the highest axial compressive strength (Table 5
). Furthermore, these specimens would be the only ones accepted by the Brazilian regulations [22
], since the minimum value is established at 10 MPa. This result can be explained by the high concentration of sulfate available in the acid suspensions and precipitated in the neutral and basic forms.
According to [23
], the sulfate reacts with the calcium aluminate to form ettringite (Ca6
O). The ettringite fills the pores of the cemented material, increasing its volume and resistance. In addition, the reaction occurs during the hydration of the cement and the ettringite undergoes decomposition generating monosulphaluminate. The monosulphaluminate is generated after the entire sulfate use [21
]. Consequently, the high concentration of sulfate ions in the acid solution may have prevented this conversion, resulting in a product with higher resistance. The sulfate is added in all commercial cement as gypsium (CaSO4
) and aims to delay the setting time and allow the workability. The absence of gypsium causes the cement to harden almost instantly in contact with water [19
]. Therefore, increasing the sulfate concentration in acid suspensions also resulted in an increase in the setting time.
Part of the remaining suspensions of resin degradation was evaporated and concentrated to 40% solids content. The axial compression strength was measured after 3 days of cure, resulting in 9 ± 1 (MPa), similar to the minimum permitted in the regulation, 10 MPa [22
]. It is known that cement hydration is a process that occurs as a function of time and interferes in mechanical resistance. Therefore, it is possible that these values, after 28 days, are superior to those permitted in regulation. Finally, further work is required to evaluate sulfate resistant cement in the immobilization process. Moreover, a kinetic study is necessary, which can be used for process optimization and scale-up.