Implementation of fluidized-bed Fenton as tertiary treatment of nitro-aromatic industrial wastewater

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Abstract

In this study, the fluidized-bed Fenton (FBF) process was evaluated as a tertiary treatment of the second effluent from a nitro-aromatic industrial wastewater treatment plant. The soluble iron removal performances of four carriers, including quartz sand, construction sand, activated carbon, and zeolite, were examined. For the carriers, the results showed that a large surface area was available for the iron removal due to the heterogeneous nucleation, while a smooth surface contained less mesoporous seemed favorable for the consecutive iron removal because of the brittle iron oxide thickness and wash off effect. The essential variables for analysis include the initial pH and the molar ratio of [Fe2+]/[H2O2]. The results showed that the COD and SUVA254 removal efficiencies, including the utilization ratio of H2O2 on COD removal, had little difference with the increase of initial pH, suggesting that the adaptive pH range of FBF can extend from 2.5 to 7.4. However, high pH caused iron removal via homogeneous nucleation not by heterogeneous nucleation, resulting in the reduction. Under the sufficient H2O2 addition, the removal performances improved when the molar ratio of [Fe2+]/[H2O2] increased to 0.625, but a high iron addition led to a negative effect on iron removal due to the resolvable phenomenon by excess H2O2. An actual engineering project illustrated that FBF progress was a useful and cost-effective method for tertiary treatment.

Introduction

Wastewaters from nitro-aromatic industrial plants are significant sources of environmental contamination (Sun et al., 2015, 2013). This type of wastewater is toxic, carcinogenic, and non-biodegradable, which can pose a serious threat to environmental safety and human health (Oconnor and Young, 1989; Nielsen and Christensen, 1994; Sun et al., 2011). In most cases, the effluent chemical oxygen demand (COD) after its biological treatment is between 120–300 mg/L, which is still higher than the National Integrated Wastewater Discharge Standards (GB8978−2002, COD < 60 mg/L). Therefore, technologies that proposed to enhance COD removal should be developed.

Physical-chemical technologies came as a foremost choice by the researchers. Among many other technologies, advanced oxidation processes (AOPs) have shown great potential in tertiary treatment due to the high efficiency in refractory compounds removal (Balciogu and Otker, 2003; Masomboon et al., 2009; Anotai et al., 2011; Ramin et al., 2019). The Fenton process, which is typical AOPs, has been proved feasible owing to the high efficiency, cost-effectiveness, and environmental benignity (Canizares et al., 2009; Tisa et al., 2014). However, some of the limitations of Fenton include a narrow pH range, excessive Fe(II/III) sludge, and high iron loss to the environment (Chou and Huang, 1999; Sun et al., 2019). In especial, the treatment cannot be discharged with the iron ions if above China Union limits (Sun et al., 2015).

There is a growing effort toward addressing the drawbacks. The use of heterogeneous catalysts, the application of fluidized bed technology are examples of such efforts. Fluidized-bed, which introduced a carrier substrate in a fluidized bed reactor, has many advantages: high mass transfer rate and turbulence between phases (catalysts and effluent) (Chou et al., 1999, 2004; Boonrattanakij et al., 2011; Anotai et al., 2012; Aghdasinia et al., 2016). In the fluidized-bed Fenton (FBF) process, ferric ions generated from the Fenton reaction can precipitate and deposit onto the surface of the carrier, leading to the reduction of floppy iron sludge. This makes it much easier to reduce subsequent handling and disposal costs (Anotai et al., 2009). Furthermore, the deposited iron oxides can act as a heterogeneous catalyst enhancing the overall removal efficiency of pollutants (Bello et al., 2017; Farshchi et al., 2018).

Several researchers have illustrated the application of FBF in real industrial wastewater treatment. For example, Boonrattanakij et al. (2018) used FBF to treat the wastewater from screw manufacture and reached 80 % COD reduction in 40 min. The monoethanolamine (MEA) and phosphate from thin-film transistor liquid crystal display wastewater was treated using FBF: the phosphate removal efficiency was 45 % and MEA removal efficiency was 76 % (Sun et al., 2013). Xing et al. (2020) evaluated the feasibility and safety of papermaking wastewater for the use as an ecological water supplement after the treatment by FBF. Results showed that FBF improved the conventional water quality indicators and reduced the toxicity. In general, FBF is usually considered as a pre-treatment due to the high concentration of pollutants: the influent COD range is from 300 to 12,000 mg/L (Oturan and Aaron, 2014; Matira et al., 2015; Cai et al., 2020). However, little information about the FBF process applied as a tertiary treatment is known yet. It is interesting to discuss the performance of FBF when the influent COD is low (100 mg/L).

In addition, previous researches have pointed out that iron removal efficiency varies dramatically depending on the reaction conditions (Chou et al., 2004; Boonrattanakij et al., 2011). Anotai et al. (2018) illustrated the factors, such as initial pH, fluidized carrier quantity, and bed expansion, affected iron removal significantly. However, few studies have attempted to investigate the role of the carrier in iron removal performance. For example, Su et al. (2011) compared the effect of Al2O3 and SiO2 in the removal of color and COD from textile effluents. The authors thought that SiO2 was a better carrier than Al2O3 because the COD removal efficiency of SiO2 was more than Al2O3 by about 25 %. Similarly, Ratanatamskul et al. (2011) studied the effect of Al2O3, SiO2, white, brown, and colored gravels as carriers on the removal of 2, 6-dimethylaniline. The authors reported that Al2O3 was the best carrier due to high removal efficiency. These studies merely considered the organic pollutant removal performances of different carriers without providing further insights into the iron removal, which is the most obvious advantage over the conventional Fenton process. Thus, data of iron removal are needed so that to optimize the performance of both pollutants and iron removal of the FBF process.

The present study was an attempt to illustrate the iron removal performances of different kinds of carriers, including quartz sand, construction sand, activated carbon, and zeolite. Iron removal behaviors in the absence of organics pollutants were compared and the underlying reasons were confirmed. Then actual wastewater from a nitro-aromatic industrial wastewater treatment plant (COD was about 100 mg/L) was disposed of using the FBF process. The effect of initial pH and the molar ratio of [Fe2+]/[H2O2] were systematically studied to evaluate the COD removal efficiency, the SUVA254 removal efficiency, the utilization ratio of H2O2 on COD removal, and the iron removal efficiency. Furthermore, an actual engineering project was built and worked for 35 days to evaluate the feasibility and economy of the FBF process as tertiary treatment.

Section snippets

Materials

H2O2 (30 %), FeSO47H2O, H2SO4 (98 %), and NaOH were supplied from Sinopharm Chemical Reagent Co., Ltd. All these chemicals were reagent grade (AR) and used without further purification. The carriers, such as quartz sand, construction sand, activated carbon, and zeolite (in the diameter of 0.4−0.6 mm) were used as fluidized carriers. Before packed in the reactor, the carriers were soaked in the 0.1 mmol/L HCl for 24 h, subsequently washed with deionized water, and finally dried at 105 ℃, as

Iron removal performances of different carriers

To investigate the effect of different carriers on the iron removal performances in the FBF system, batch experiments using quartz sand, construction sand, activated carbon, and zeolite as carriers were set up. The same amount of Fenton’s reagents (Fe2+ and H2O2) were added simultaneously into the FBF, and other variables, including pH, fluidized carrier quantity, and bed expansion, were kept constant. As shown in Fig. 2(a), within 240 min, the total iron removal efficiencies of the four

Conclusions

The actual implementation of Fenton processes as water treatment technology faces as one of the most relevant challenges to choose as the most suitable stage fit in existing water treatment. In this study, actual chemical wastewater collected from biological treatment effluent, which influent COD was 100 mg/L, was dealt with by the FBF process. The iron removal performances of four carriers were examined. The high surface area of carrier enhanced the removal rate via heterogeneous nucleation,

CRediT authorship contribution statement

Liang Sun: Methodology, Writing - original draft, Formal analysis, Funding acquisition. Han Jiang: Investigation. Yuxuan Zhao: Investigation. Xiaoyan Deng: Investigation. Ke Shen: Investigation. Yan Li: Supervision, Formal analysis, Funding acquisition, Writing - review & editing. Minge Tian: Investigation.

Declaration of Competing Interest

None.

Acknowledgements

We gratefully acknowledge the generous support provided by the National Natural Science Foundation of China (NO. 51408295), Key Research and Development Project of Shandong Province (NOs. 2017GSF217013, 2018GSF117007, 2019CSF109080), Natural Science Foundation of Shandong Province (NO. ZR2019BEE047).

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