Feasibility of Pd-catalytic onto nontronite mineral and H 2 O 2 produced from H 2 and O 2 to degrade organic contaminants

A novel electro-Fenton process based on Pd-catalytic production of H2O2 from H2 and O2 has been studied and examined recently for altering and degrading organic contaminants in different types of wastewaters. The study endeavors to synthesize an integrated catalyst by loading Pd onto the clay mineral,nontronite, (Pd/NTm) so that H2O2 and Fe(II) ‏ can be produced simultaneously in the electrolytic system. In an undivided electrolytic cell, rhodamine B, a probe organic contaminant, is degraded by 93% within 60 min under conditions of 50 mA, 1 g/L Pd/NTm, pH 3 and 20 mg/L initial concentration. A distinct mechanism, reductive dissolution of solid Fe(III) in nontronite by atomic H chemisorbed on Pd surface, is responsible for Fe(II) ‏ production from Pd/NTm.


Introduction
The electro-Fenton (EF) process has been widely used to treat wastewater-containing dyes, herbicides, antibiotics, and landfill leachate because of its convenience and strong oxidation ability [1][2][3][4]. In the EF process, Fe 2+ is commonly obtained by the addition of ferrous salts, the reduction of Fe 3+ , or the oxidation of a sacrificial iron anode [5][6][7][8][9], while H 2 O 2 is in situ generated via the electrochemical reduction of O 2 on the cathode, which can avoid the long distance transportation of H 2 O 2 . The commonly used cathodes include graphite, activated carbon fiber (ACF), carbon sponge, and so on [1,7,10]. However, these cathodes could only produce H 2 O 2 by the two-electron reduction of molecular oxygen [11]. This single H 2 O 2 generation way would restrict the efficiency of the EF process. Recently, a novel E-Fenton process, termed Pd-based E-Fenton, was developed based on Pdcatalytic production of H 2 O 2 from electro-generated H 2 and O 2 . For instance, Yuan et al. developed a new Pd-based EF process to produce H 2 O 2 via the reaction of electro-generated H 2 and O 2 on the Pd catalyst [12][13][14].Water electrolysis produces O 2 and H + on the anode (Reaction 1), while produces H 2 and OH on the cathode (Reaction 2). Under acidic conditions, H 2 O 2 is produced from the combination of H 2 and O 2 on the surface of Pd catalyst (Reaction 3) [15,16]. In the presence of Fe 2+ , •OH is produced by (Reaction 4). This process has shown high performance on degrading many organic contaminants including trichloroethyelene [15], phenol [12] and toluene [17] in wastewater and groundwater.
In addition, because of wide spread occurrence of iron-bearing clays and clay minerals in soils and sediments, several studies have examined the reactivity of structural Fe 2+ in chemically reduced montmorillonite, nontronite, illite, vermiculite, and kaolinite towards heavy metals reduction and immobilization [18][19][20]. Many types of clay minerals are ubiquitous in the environment and often contain iron in their structure, and they are the most plentiful and chemically active parts of the surface mineral world of earth [21].Structural Fe(II) produced from chemical and microbial reduction of iron-bearing clay minerals has been shown to reduce a variety of contaminants through taking part in the redox reactions [22][23][24], and the Fe(II)/Fe(III) valence state is thought to be an important redox buffer across an extensive domain of redox conditions [25][26][27]. It has been shown in previous studies that reduced clay (containing structural Fe(II)) can reduce and mutate the contaminants fate and mobility such as chlorinated solvents [27][28][29],nitroaromatic compounds [30 ], and organic compounds [26] 2H RhB + •OH → Oxidation products (5) In this study, a new integrated catalyst of Pd/Nontronite mineral (Pd/NTm) containing both Pd and Fe is synthesized and characterized. The performance of this new catalyst on simultaneously producing H 2 O 2 and Fe 2+ is evaluated for the Pd-based E-Fenton degradation of organic contaminants. Using Rhodamine B (RhB) as a probe organic contaminant‫‬and studying the effect of solution pH‫.‬The mechanisms of production of Fe 2+ and reactive oxidizing species (ROS) will be elucidated. It is mainly aimed to provide an integrated catalyst for the simultaneous production of H 2 O 2 and Fe 2+ in the Pd-based E-Fenton process.

Research Method and Materials 2.1Chemicals
Nontronite (

2.2Synthesis of Pd/NTm catalyst
Nontronite was reduced chemically using the sodium citrate, bicarbonate, and dithionite (CBD) method as explained by Stucki [30]. 0.5g of nontronite in a flask, then adding 0.2g of sodium dithionate (Na 2 S 2 O 4 ), 20mL of deionized water, and 10mL of CB buffer salts (1 M sodium bicarbonate and 0.9 M sodium citrate in 24:1 mixing rate ), and later put in water bath at 70 ºC for 4 hrs. After one hour cooling, the reductive nontronite was centrifuged and washed by 1MNaCl and finally washed five times by D.I. water.

Pd/NTm catalyst Characterization by XRD
The finely grinded particles were characterized by X-ray diffraction (XRD) on a D8-FOCUS X-ray diffractometer with Cu K radiation (Bruker AXS., Germany). The analysis was carried out at 40 KV and 40 mA at the scanning step size of 0.01° and step time of 0.05 s. To detect any mineralogical changes as a result of chemical reduction and reaction with (RhB), XRD was performed for unreduced and reduced clay minerals.Clay mineral were prepared on petrographic slides and air-dried overnight at 30 ºC inside a glove box incubator.

2.4Degradation Experiments
The degradation was carried out in a conventional undivided electrolytic cell. An MMO mesh was used as the anode and cathode spaced in parallel position to sustain water electrolysis. For each test, 200 mL of 20 mg/L (RhB) solution was transferred into a glass beaker (250 mL), and 1 g/L Pd/NTm was added. The solution was mechanically stirred using a Teflon-coated magnetic stirring bar. The reaction temperature was 25 1 ºC. A constant electric current of 50 mA was applied with a cell potential of about 12 V. Solution pH was adjusted to neutral by addition of dilute 1 M H 2 SO 4 and 1 M NaOH before electrolysis and was not adjusted during the process. The effects of initial solution pH (3and 7) was studied to investigate the acidity and neutral conditions. About 1 mL of aqueous solution was taken out at predetermined time intervals for analysis of (RhB). (RhB) samples were analyzed by a spectrometer (UV-1800 PC, Shanghai Mapada Spectrum Instrument Co., Ltd.) at detection wavelengths of 550 nm.

XRD Characterization
XRD spectra of the novel Pd/NTm catalyst is exhibited in Fig. 1. It is evident that a series of characteristic diffractive peaks of crystal materials can be observed at certain (2θ). As shown in Figure, XRD patterns showed that the Hanford sediments were dominated by quartz, albite, muscovite, mica, illite, nontronite, and goethite. The Pd/NTm catalyst XRD pattern results show that diffraction peaks of unreduced nontronite appear at (2θ) of 8º peaks that attributed to the crystal of Fe(III), meanwhile the XRD profiles for chemical reduced nontronite showed peaks around 28 º which denoted by (*) are indicative of crystalline of Fe(III). Firstly, a preliminary experiment was implemented by using a raw and reductant clay mineral (nontronite) to study its ability to reduce the (RhB) concentrations in wastewaters as well as the E-Fenton application. The results elucidated that the removal efficiency of (RhB) was higher in case we use reductant nontronite than using raw nontronite in E-Fenton process that because of the ferric produced from the reduction process on the clay mineral. From the figure we can conclude that under neutral conditions of pH 7 the removal efficacy could be more by uploading the mineral by Pd-catalytic which enhance the removal process by its activity on E-fenton and ferrous iron produced by the reluctant nontronite that finally transferred to ferric which is easily removed by precipitation process. The figure revealed that the E-Fenton process is inefficient to degrading such types of organic contaminants used in this study represented by (RhB). Figure (1) show that Pdcatalytic onto nontronite mineral and H 2 O 2 produced from H 2 and O 2 to degrade (RhB) at an efficiency of 71% within 60minutes at pH 7 .

3.3Acidic Conditions
The experiments were implemented in another conditions which different to that used in first part of the results to explain the effect of acidity on the overall removal of (RhB) which explained in Figure (2).  Figure (3) illustrated that when the (RhB) solution has an acidic nature then this will lead to enhance the removal efficiency of (RhB) to more than 93% within one hour. The enhancement was got from the increment of ferric production in the nontronite reduction process that generated. After reduction process a blue green color was observed for reductive nontronite that can give an index that reduction was successful chemically, hereafter in synthesizing process it was changed to a greenish yellow color after supporting Pd as a result of oxidation. The figure accentuates that 93% of (RhB) was degraded by the improved Pd-based E-Fenton process using the Pd/ NTm catalyst within 60 min under conditions of 50 mA, 1 g/L Pd/ NTm, pH 3 and (20mg/L) (RhB) initial concentration.  Atomic H chemisorbed by Pd loaded on nontronite is a strongly reducing agent, which may reduce the solid Fe(III) in nontronite to Fe(II). Pd is the active component for the production of atomic H and H 2 O 2 so that (RhB) degradation significantly more with Pd loadings on nontronite than raw nontronite.This is ascribed to the production of more H 2 O 2 by supporting Pd on nontronite, which significantly contributed to the generation of •OH radicals. Therefore, •OH radicals can be supposed to be the predominant reactive oxidizing species for (RhB) degradation in this process.

Conclusion
This study demonstrates the generation of H 2 O 2 in the presence of O 2 using Pd-based catalyst and H 2 . The production of low concentrations of •OH slightly contributes to (RhB) oxidation under neutral and acidic conditions. In particular, when Fe(II) is present, H 2 O 2 efficiently decomposes to •OH resulting in a shift in dominant pathway from heterogeneous hydrodechlorination to homogeneous oxidation. This shift is most pronounced at high Fe(II) concentrations and low pH values. These findings may contribute to understanding the nature for organic contaminants degradation by Pd-catalytic hydrodechlorination in the presence of O 2 , especially when Fe(II) at a level of ppm is present. In the Pd-containing electrolytic system, oxidation of organic contaminants in general and especially (RhB) in the presence Fe(II) is proved to be significantly more effecting than hydrodechlorination in the absence Fe(II) under weak acidic conditions.The conclusions derived were set as, the organic contaminants represented by rhodamine B in this research were efficiently degraded by the addition of Pd/NTm into an undivided electrolytic cell under conditions of 50 mA, 1 g/L Pd/NTm, pH 3 and 20 mg/L initial concentration. As obvious, Pd/NTm catalyzed the production of H 2 O 2 from anodic O 2 and cathodic H 2 . However, a distinct mechanism of Fe(II) ‫‬ production, the reductive dissolution of solid Fe(III) in nontronite by atomic H chemisorbed on Pd surface, was revealed.