Gang Ran and
Qibin Li*
Faculty of Geosciences and Environmental Engineering, Southwest Jiaotong University, Chengdu 611756, China. E-mail: liqb@home.swjtu.edu.cn
First published on 14th August 2019
A significant amount of biorefractory organic wastewater is generated during the production of dinitrodiazophenol (DDNP). In this study, ultraviolet light (254 nm) that was coupled with the Fenton (UV-Fenton) process was applied to treat refractory organics in DDNP industrial wastewater. The effects of key parameters (i.e., H2O2 dose, Fe2+ dosage, and initial pH) on the treatment efficacy for DDNP industrial wastewater by the UV-Fenton process was investigated systematically. Alcohol quenching experiments were carried out to identify reactive oxygen species in the UV-Fenton process. The treatment efficacy and degradation characteristics of refractory organics were studied and compared by using control experiments. Increasing H2O2 and Fe2+ doses could lead to improved treatment results to a different extent. A more intense reaction and better treatment results were achieved by using the UV-Fenton process at lower pH conditions. Under optimal conditions of H2O2 dose = 7.5 mL L−1, Fe2+ dosage = 0.05 mM, and initial pH = 5.0, the pseudo-first order constants k for chemical oxygen demand removal and color number removal were 0.18 min−1 and 1.24 min−1, and the chemical oxygen demand and color number removal efficiencies were 74.24% and 99.94%, respectively. The treatment results for the UV-Fenton process were better than other processes under the same conditions, and a significant synergetic effect was observed for the UV-Fenton process. Alcohol quenching experiments indicated that the predominant reactive oxygen species in the UV-Fenton process was the hydroxyl radical (·OH). Because more ·OH was produced, the UV-Fenton process exhibited a much better treatment performance in degrading and destroying organic structures (i.e., benzene rings, –NO2, and –NN–). Furthermore, the biodegradability indicated by the biological oxygen demand/chemical oxygen demand ratio was improved considerably to 0.48 from 0.054. The good treatment performance by UV-Fenton allowed for a more efficient electrical energy consumption compared with the UV and UV-H2O2. This study provides a theoretical reference for DDNP industrial wastewater treatment by using the UV-Fenton process.
Treatment methods for DDNP industrial wastewater and other explosive production wastewater include the adsorption method, persulfate activation process, combined Fe0/air and Fenton process, electrolysis method, and Fe/Cu-enhanced ozonation process.2–7 Satisfactory treatment results were obtained by using all of these methods. However, drawbacks still remain in these processes, such as desorption wastewater, increase of salinity of the treated effluent, iron sludge production, and heavy metal leaching. These methods can treat DDNP industrial wastewater, but some shortcomings restrict their application. In summary, a rapid and efficient method for DDNP industrial wastewater treatment is required.
Advanced oxidation processes (AOPs) have attracted widespread attention in wastewater treatment and groundwater remediation.8–16 AOPs utilize mainly the produced reactive oxygen species (ROS) to oxidize refractory organic matters. ·OH (oxidation potential of 2.8 V) can oxidize substrates non-selectively and result in rapid organic pollutant reaction (second reaction rate constant of 107 to 1010 L (mol s)−1).16–20 Many technologies can produce ·OH, including the Fenton method, UV-H2O2, and the UV-Fenton process.21–26 Among the methods, the UV-Fenton process is a developed process based on the traditional Fenton process. More ·OH can be produced by Fe2+ catalyzing H2O2 under UV conditions, and the utilization efficiency of H2O2 will also be improved.24,27,28 Fe3+ can be transformed to Fe2+ by UV to catalyze H2O2.24,28 The UV-Fenton process is very efficient in organic wastewater treatment.29,30 However, treatment of DDNP industrial wastewater by using the UV-Fenton process has rarely been reported.
This study applied the UV-Fenton process to the treatment of refractory organics in DDNP industrial wastewater. The objectives of this study were to (1) study the effects of H2O2 dose, Fe2+ dosage, and initial pH on treatment efficacy for DDNP industrial wastewater by the UV-Fenton process; (2) confirm the advantage of the UV-Fenton process by carrying our control experiments under the same optimum conditions; (3) identify the predominant ROS in the UV-Fenton process and its contribution to organics degradation; (4) reveal the degradation characteristics of refractory organics in DDNP industrial wastewater by using ultraviolet-visible spectroscopy (UV-Vis); and (5) investigate the improvement in biodegradability presented by the biological oxygen demand/chemical oxygen demand ratio and electrical energy consumption of the UV-Fenton process. This study aimed to provide useful and theoretical reference for DDNP industrial wastewater treatment.
(1) |
A pseudo-first-order model was used to characterize the removal rate of COD and CN according to the operational variables (i.e., H2O2 dose, Fe2+ dosage, and initial pH) according to eqn (2) where Ct and C0 represent the COD or CN concentration at time t min and at time 0 min, respectively; t (min) is the reaction time; and k (min−1) is the pseudo-first-order constant.
(2) |
The synergetic effect (SE) was evaluated according to eqn (3).32 A higher SE (>1) indicates a stronger synergetic effect. In eqn (3), SE represents the synergetic effect; k(combination process) is the pseudo-first-order constant k in the UV-Fenton process; and ∑k(single process) is the sum of the k values in the UV, H2O2, and Fe2+ processes.
(3) |
The electrical energy consumption (EEC) was evaluated according to eqn (4),33 where P (kW) is the UV lamp power; t (min) is the reaction time; V (L) is the volume of wastewater; Ci and Cf are the initial and final COD concentrations, respectively; and 60 converts min to h.
(4) |
Fig. 2 Effect of H2O2 dose on (a) COD removal, (c) CN removal, and k for (b) COD and (d) CN. Conditions: Fe2+ dosage = 0.05 mM and initial pH = 5.0. |
As shown in Fig. 2, the treatment efficiencies increased with an increase in H2O2 dose. When the H2O2 dose increased to 10 mL L−1 from an initial 2.5 mL L−1, the COD and CN removal efficiencies increased from 52.31% and 79.91% to 76.28% and 99.94% at 60 min, respectively, and the kCOD and kCN increased from 0.11 and 0.27 min−1 to 0.18 and 1.29 min−1, respectively. Reaction rate k for COD and CN increased more slowly. A higher H2O2 dose can enhance the treatment efficacy of the UV-Fenton process. ·OH will be produced from UV and Fe2+-catalyzing H2O2 (eqn (5) and (6)).29,34,35 However, an overdose of H2O2 will also react with ·OH and compete with organic substrates that react with ·OH, which results in a less significant increase in treatment efficacy. Therefore, 7.5 mL L−1 will be a suitable H2O2 dose to avoid wasting oxidant.
(5) |
(6) |
Fig. 3 Effect of Fe2+ dosage on (a) COD removal, (c) CN removal, and k for (b) COD and (d) CN. Conditions: H2O2 dose = 7.5 mL L−1 and initial pH = 5.0. |
As shown in Fig. 3, the removal efficiencies of COD and CN and kCOD and kCN increased with an increase in Fe2+ dosage. At 60 min, the COD and CN removal were 78.98% and 99.99%, respectively, when the Fe2+ dose was 0.075 mM. The kCOD and kCN reached their highest values of 0.19 and 1.53 min−1, respectively. In the UV-Fenton process, Fe2+ can catalyze H2O2 to produce ROS, which degrades most organic matters. The generated Fe(OH)2+ can be transformed to Fe2+ and ·OH in UV light (eqn (7)),29,35 and a better treatment efficacy can be obtained. Some research has indicated that Fe2+ of an extremely high concentration will capture ROS and reduce the removal efficiency,25,26,36 however, this result was not found in this study. Above all, 0.5 mM Fe2+ can be selected as an optimal parameter.
(7) |
Fig. 4 Effect of initial pH on (a) COD removal, (c) CN removal, and k for (b) COD and (d) CN. Conditions: H2O2 dose = 7.5 mL L−1 and Fe2+ dosage = 0.05 mM. |
As shown in Fig. 4, the best treatment efficiency was obtained when the initial pH was 3. The removal efficiencies of COD and CN were 82.65% and 99.99%, and those for the kCOD and kCN were 0.22 and 1.63 min−1, respectively, at an initial pH of 3 and a reaction time of 60 min. An increase in pH leads to a decrease in treatment efficiency. When the initial pH reached 11, the COD removal efficiency decreased to 54.03% and likewise, the kCOD and kCN decreased considerably to 0.10 and 0.47 min−1. The inhibition effect of the high pH condition may be contributed to the decomposition of H2O2, and iron colloids will be generated in an alkaline environment. However, the final CN removal efficiency was high for a pH range of 3 to 11, possibly because the –NN– group will be destroyed by alkali and partially by UV light. Thus, a high CN removal efficiency was obtained for a wide range of initial pH values.
The single process, as shown in Fig. 5, yielded much lower treatment efficiencies. The COD removal efficiencies of single Fe2+, single H2O2, and single UV were 18.38%, 21.62%, and 22.46, respectively. The CN removal efficiencies of single Fe2+, single H2O2, and single UV were 23.02%, 27.30%, and 30.42%, respectively. Likewise, the kCOD and kCN value in the three single processes was significantly low. In the single Fe2+ and single UV lacking in oxidant, the main contribution to the removal of organics was iron colloids or UV light.37,38 In single H2O2 without catalysis, ROS production was limited. Hence, the single process cannot achieve a satisfactory treatment results for the above reasons.
The three binary processes (i.e., UV-Fe2+, Fenton, and UV-H2O2) showed an enhanced treatment efficiency to a different extent. The most significant process, the UV-H2O2 process, yielded 61.68% COD removal, 83.07% CN removal, 0.13 min−1 of kCOD and 0.26 min−1 of kCN at a 60 min reaction time. The SE of the UV-H2O2 process was 3.53 for COD removal and 4.34 for CN removal, which is higher than that of the UV-Fe2+ and Fenton. A lack of oxidant is the main reason for the lower treatment efficiency of the UV-Fe2+ compared with the UV-H2O2. Compared with Fe2+, a better catalysis effect will be achieved by the UV and more ·OH will be produced. Thus, the UV-H2O2 process showed a much better treatment efficiency than that of the Fenton process. In the UV-Fenton process, the removal efficiencies of the COD and CN were 74.24% and 99.94%, and the kCOD and kCN were 0.18 and 1.24 min−1, respectively. The treatment results and reaction rate of UV-Fenton were higher than that of other control processes. The SE for COD and CN in the UV-Fenton process were 3.19 and 14.24, respectively, which indicated a strong synergetic effect in the UV-Fenton process. In summary, the treatment efficiency of the UV-Fenton was satisfactory and this process provided an efficient treatment method for DDNP industrial wastewater.
As shown in Fig. 6, after adding TBA to the UV-Fenton and UV-H2O2 processes, a noticeable inhibition effect was obtained. At a reaction time of 40 minutes, the CN removal efficiencies of UV-Fenton and UV-H2O2 decreased to 61.65% and 49.38% from 99.36% and 70.04%, respectively, and kCN decreased from 1.24 and 0.26 min−1 to 0.17 and 0.12 min−1, respectively. Therefore, ·OH was important in the two processes. In addition, in terms of whether CN removal or kCN change, a stronger inhibition effect was found in the UV-Fenton process. The results indicated that more ·OH was produced in the UV-Fenton process. UV and Fe2+ together can better catalyze H2O2 and more ROS was produced.
As shown in Fig. 7a and b, after the single H2O2 and Fenton processes for 60 min, the effluent still showed a strong absorbance and the absorbance from 350 nm to 450 nm was relatively strong. Therefore, the single H2O2 and Fenton processes cannot degrade and destroy refractory organics in DDNP industrial wastewater effectively. As shown in Fig. 7c and d, a much more considerable decrease of absorbance can be observed in the UV-H2O2 and UV-Fenton processes. The absorbance of effluents in the UV-H2O2 and UV-Fenton processes at 60 min was close to 0. The results prove that recalcitrant organics that contain a benzene-ring, nitro group, and diazo group in DDNP industrial wastewater in the UV-H2O2 and UV-Fenton processes can be degraded significantly. In addition, a faster and more thorough decrease of absorbance was presented by the UV-Fenton process, which can be attributed to the fact that the UV-Fenton process can produce more ·OH during the early stage of reaction and therefore exhibited a higher treatment efficiency than the UV-H2O2.
As shown in Fig. 8, the biodegradability of untreated DDNP industrial wastewater is only 0.054, whereas the effluent biodegradability increased to a different degree. After Fenton, UV-H2O2, and UV-Fenton treatment, the effluent biodegradability was 0.11, 0.33, and 0.48. The results indicate that these processes can degrade the refractory organics with different efficacies. Some biorefractory substances in treated effluent were degraded and transformed to biodegradable organics, especially from UV-Fenton treatment of DDNP industrial wastewater.
As shown in Fig. 9, the EEC of the three processes increased with reaction time. In the single UV process, the EEC increased from 48.97 kW h L−1 at 5 min to 339.86 kW h L−1 at 60 min. At 60 min, the EEC of the UV-H2O2 and UV-Fenton were 90.07 kW h L−1 and 63.67 kW h L−1, which are much lower than that of the single UV. Therefore, the UV-Fenton process showed a remarkable economic advantageous over UV and UV-H2O2. Electricity savings of 162.08 kW h and 19.27 kW h will be achieved for every liter of DDNP industrial wastewater that was treated by the UV-Fenton process.
This journal is © The Royal Society of Chemistry 2019 |