Regeneration of spent carbon nanotube by electrokinetic process with binary metallic oxide electrodes of MnO2/Ti RuO2/Ti, and PbO2/Ti

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Abstract

This study was aimed to investigate the regeneration performance of carbon nanotube saturated with 150 mg L−1 of nonylphenol (NP) by electrokinetic process with binary oxide electrodes (BMOEEK) under potential gradient of 2 V cm−1 and elapsed time of 8 h. Twenty experiments were conducted with four processing fluids of de-ionized water (DW), 0.06 M citric acid (CA), 0.2 M NaOH, and 0.1 M NaCl in a Pyrex glass cylindrical cell (4.2 cm in diameter × 12 cm in length). Two inert electrodes of carbon (C) and titanium (Ti) and three binary metallic electrodes of MnO2/Ti, RuO2/Ti, and PbO2/Ti were investigated in this study. Results were shown that higher EO permeability (3.8 × 10−6 to 10.0 × 10−6 cm2 V−1 s−1) was found in citric acid, NaOH, and NaCl systems than in DI water system (0.6 × 10−6 to 2.5 × 10−6 cm2 V−1 s−1). The regeneration efficiency of CNT was 16.5–29.5%, 27.3–49.2%, 30.8–54.8% and 38.4–69.0% in BMOEEK–DW, –CA, –NaOH, and –NaCl systems, respectively. This might largely be because both OCl- and OH-radical were contributing for NP degradation in BMOEEK–NaCl system and, however, OH radical was the only contributor in other systems. Among the investigated electrodes, the RuO2/Ti electrode has shown the best degradation performance of NP. Results of this study concluded that NP was degraded by BMOEEK system, which was highly related to both processing fluid and binary metallic electrode, rather than removal only by conventional EK system. The innovative BMOEEK technology was promoted to be a sustainable technology.

Highlights

► Three binary metallic electrodes of MnO2/Ti, RuO2/Ti, and PbO2/Ti were investigated in EK systems. ► The oxidant of OCl would be produced to enhanced degradation performance as using NaCl as processing fluid. ► The RuO2/Ti electrode was shown the best degradation efficiency of NP. ► The treatment efficiency was highly related to both processing fluid and binary metallic electrode, rather than only removed by conventional EK system.

Introduction

Carbon nanotubes (CNT) were discovered in 1991, which have made a significant impact on materials science by stimulating the progress in nanotechnology. Nowadays, CNT has been used as a well-performed adsorbent for organic pollutants compared with activated carbon due to its high specific surface area except the cost. An innovative regeneration technology is highly needed to prolong the lifecycle of CNT, which will improve economic efficacy. Common technologies as ultrasonic, microwave irradiation, wet-oxidation, and thermal regeneration are applied for regeneration of spent activated carbon. It was reported that a regeneration efficiency of 64% was found for spent activated carbon saturated with trichloroethylene under 20 kHz ultrasonic treatment for 5 h [1]. The activated carbon saturated with pentachlorophenol was successfully regenerated with 850 W microwave irradiation for 10 min [2], which resulted, however, in 20% reduction of adsorption capacity after regeneration. Sabio et al. [3] reported that 60–70% of regeneration performance of spent activated carbon saturated with p-nitrophenol was achieved by thermal regeneration under 300–850 °C for 3–5 h. The above-mentioned common regeneration technologies were not recommended because they require high energy consumption and reduce adsorption capacity.

Recently, Electrokinetic process, operated under lower temperature with low power consumption, was applied to regeneration of activated carbon. Zhang [4] reported 95% regeneration efficiency of activated carbon saturated with phenol was achieved by an electrokinetic system with 0.1 M NaCl as processing fluid under constant current of 100 mA operated for 5 h. It was also shown that desorption mechanism was found in cathode reservoir and, whereas, oxidation degradation was found in anode reservoir. The processing fluid using elecrokinetic process was the dominant factor affecting regeneration performance. Miguel et al. [5] reported that 99% regeneration efficiency of activated carbon saturated with toluene was achieved with 0.5 M NaOH as processing fluid, which was higher than that by using processing fluid of Na2CO3 and Na2SO4.

Nonylphenol (NP) is a degraded product of nonylphenol polyethoxylate (NPnEO) mode surfactants which accounted for 80% of the non-ionized detergent used [6]. It is widely proliferated in the streams and creeks of country because of illegal dumping into effluent waters of NPnEO surfactants. Since the structure of NP is very similar to animal estrogen, NP possesses the ability to mimic natural hormones by interaction with estrogen receptors. It has been confirmed that, once NP runs into human body, it will decrease male sperm count and interfere normal metabolism [7]. It will also reduce immunity, interfere physiology, and cause breast and testicular cancer when exposures at NP-contaminated environment in a long time [8]. Nonylphenol would also reduce and inhibit estrogen efficacy in animals [9] and zooplankton [10]. The properties of NP were listed in Table 1.

Anode chemical oxidation has been demonstrated a good degradation performance for nonylphenol polyethoxylate, phenol, and chlorinated organics in aqueous phase. Binary metallic oxide electrode, such as MnO2/Ti, PbO2/Ti, RuO2/Ti, and SnO2/Ti were used for innovative application [15], [16], [17], [18], [19]. The advantages of this technology include easy operation, no extra chemical addition, high mineralization efficiency, and low cost. In aqueous electrochemical reaction, the OH radical would be produced from hydrolysis of water as shown in Eq. (1).H2O(aq)  HOradical dot(aq) + H+ + eThe OH radical would react with metallic oxide coated on electrode surface by Eqs. (2), (3), (4), (5) to result in mineralization of pollutants [17]:M + xHOradical dot  MOx + H+ + eMOx + R  ROx + MMOx + H2O  MOx(OHradical dot) + H+ + eMOx(OHradical dot) + R  CO2 + nH+ + MOx + newhere M is the metal coated on electrode surface, R is organic compounds, and n is coefficient.

This study was aimed to investigate the regeneration performance of carbon nanotube saturated with 150 mg L−1 of nonylphenol (NP) by electrokinetic process with binary metallic oxide electrodes (BMOEEK) under potential gradient of 2 V cm−1 and elapsed time of 8 h.

Section snippets

Experimental materials

The investigated carbon nanotube (CNT) was purchased as commercial product, which was classified as multi-walled texture with pHZPC of 2.6, length of 5–15 μm, 95% purity, and less than 3% of amorphous carbon content. Before experiment, CNT was purified with 65% HNO3 solution at 120 °C for 40 min [20] to remove surface metal catalysts, which resulted in BET area of 122 m2 g−1 and pore volume of 0.44 cm3 g−1. The characteristics of CNT were summarized in Table 2.

NP was purchased as regent grade from

Results and discussion

Summary results of regeneration of carbon nanotube in four BMOEEK systems were shown in Table 3 and further discussed in the following sections.

Conclusions

This study demonstrated two important contributions included (a) the electrokinetic process was confirmed to be successfully applied on regeneration of nano-materials without causing adsorbent destructions compared to the common regeneration process, i.e. thermo-regeneration; (b) NP was degraded by BMOEEK system rather than only removed by conventional EK system, which results in BMOEEK technology being a sustainable technology. The important conclusions in this study have been summarized as

Acknowledgements

This research was funded by the National Science Council of Taiwan, Grant # NSC 97-2211-E-390-007. The authors were grateful to the reviewers for their valuable comments.

References (31)

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