Songhu Yuan

Pd-catalytic in situ generation of H2O2 from H2 and O2 produced by water electrolysis for the efficient electro-fenton degradation of rhodamine B.

A novel electro-Fenton process was developed for wastewater treatment using a modified divided electrolytic system in which H2O2 was generated in situ from electro-generated H2 and O2 in the presence of Pd/C catalyst. Appropriate pH conditions were obtained by the excessive H+ produced at the anode. The performance of the novel process was assessed by Rhodamine B (RhB) degradation in an aqueous solution. Experimental results showed that the accumulation of H2O2 occurred when the pH decreased and time elapsed. The maximum concentration of H2O2 reached 53.1 mg/L within 120 min at pH 2 and a current of 100 mA. Upon the formation of the Fenton reagent by the addition of Fe2+, RhB degraded completely within 30 min at pH 2 with a pseudo first order rate constant of 0.109 ± 0.009 min(-1). An insignificant decline in H2O2 generation and RhB degradation was found after six repetitions. RhB degradation was achieved by the chemisorption of H2O2 on the Pd/C surface, which subsequently decomposed into •OH upon catalysis by Pd0 and Fe2+. The catalytic decomposition of H2O2 to •OH by Fe2+ was more powerful than that by Pd0, which was responsible for the high efficiency of this novel electro-Fenton process.

Electrolytic Manipulation of Persulfate Reactivity by Iron Electrodes for Trichloroethylene Degradation in Groundwater

Activated persulfate oxidation is an effective in situ chemical oxidation process for groundwater remediation. However, reactivity of persulfate is difficult to manipulate or control in the subsurface causing activation before reaching the contaminated zone and leading to a loss of chemicals. Furthermore, mobilization of heavy metals by the process is a potential risk. An effective approach using iron electrodes is thus developed to manipulate the reactivity of persulfate in situ for trichloroethylene (TCE) degradation in groundwater and to limit heavy metals mobilization. TCE degradation is quantitatively accelerated or inhibited by adjusting the current applied to the iron electrode, following k1 = 0.00053·Iv + 0.059 (-122 A/m(3) ≤ Iv ≤ 244 A/m(3)) where k1 and Iv are the pseudo first-order rate constant (min(-1)) and volume normalized current (A/m(3)), respectively. Persulfate is mainly decomposed by Fe(2+) produced from the electrochemical and chemical corrosion of iron followed by the regeneration via Fe(3+) reduction on the cathode. SO4(•-) and ·OH cocontribute to TCE degradation, but ·OH contribution is more significant. Groundwater pH and oxidation-reduction potential can be restored to natural levels by the continuation of electrolysis after the disappearance of contaminants and persulfate, thus decreasing adverse impacts such as the mobility of heavy metals in the subsurface.

An integrated catalyst of Pd supported on magnetic Fe3O4 nanoparticles: Simultaneous production of H2O2 and Fe2+ for efficient electro-Fenton degradation of organic contaminants

A novel electro-Fenton process based on Pd-catalytic production of H2O2 from H2 and O2 has been proposed recently for transforming organic contaminants in wastewaters and groundwater. However, addition of Fe(II) complicates the operation, and it is difficult to recycle Pd catalyst after treatment. This study attempts to synthesize an integrated catalyst by loading Pd onto magnetic Fe3O4 nanoparticles (Pd/MNPs) so that H2O2 and Fe(2+) can be produced simultaneously in the electrolytic system. In an undivided electrolytic cell, phenol, a probe organic contaminant, is degraded by 98% within 60 min under conditions of 50 mA, 1 g/L Pd/MNPs (5 wt% Pd), pH 3 and 20 mg/L initial concentration. The degradation rate peaks at pH 3, increases with increasing Pd loading and electric current and decreases with increasing initial concentration. A distinct mechanism, reductive dissolution of solid Fe(III) in Fe3O4 by atomic H chemisorbed on Pd surface, is responsible for Fe(2+) production from Pd/MNPs. The efficiency of phenol degradation can be sustained at the same level for ten times of repeated treatment using the Pd/MNPs catalyst. The variations of main crystal structure and magnetic property of catalysts are minimal after treatment, but low concentrations of Pd leached, which needs further evaluation.

Efficient degradation of TCE in groundwater using Pd and electro-generated H2 and O2: a shift in pathway from hydrodechlorination to oxidation in the presence of ferrous ions.

Degradation of trichloroethylene (TCE) in simulated groundwater by Pd and electro-generated H(2) and O(2) is investigated in the absence and presence of Fe(II). In the absence of Fe(II), hydrodechlorination dominates TCE degradation, with accumulation of H(2)O(2) up to 17 mg/L. Under weak acidity, low concentrations of oxidizing •OH radicals are detected due to decomposition of H(2)O(2), slightly contributing to TCE degradation via oxidation. In the presence of Fe(II), the degradation efficiency of TCE at 396 μM improves to 94.9% within 80 min. The product distribution proves that the degradation pathway shifts from 79% hydrodechlorination in the absence of Fe(II) to 84% •OH oxidation in the presence of Fe(II). TCE degradation follows zeroth-order kinetics with rate constants increasing from 2.0 to 4.6 μM/min with increasing initial Fe(II) concentration from 0 to 27.3 mg/L at pH 4. A good correlation between TCE degradation rate constants and •OH generation rate constants confirms that •OH is the predominant reactive species for TCE oxidation. Presence of 10 mM Na(2)SO(4), NaCl, NaNO(3), NaHCO(3), K(2)SO(4), CaSO(4), and MgSO(4) does not significantly influence degradation, but sulfite and sulfide greatly enhance and slightly suppress degradation, respectively. A novel Pd-based electrochemical process is proposed for groundwater remediation.

Electrocatalytic activity of Pd-loaded Ti/TiO2 nanotubes cathode for TCE reduction in groundwater.

A novel cathode, Pd loaded Ti/TiO2 nanotubes (Pd-Ti/TiO2NTs), is synthesized for the electrocatalytic reduction of trichloroethylene (TCE) in groundwater. Pd nanoparticles are successfully loaded on TiO2 nanotubes which grow on Ti plate via anodization. Using Pd-Ti/TiO2NTs as the cathode in an undivided electrolytic cell, TCE is efficiently and quantitatively transformed to ethane. Under conditions of 100 mA and pH 7, the removal efficiency of TCE (21 mg/L) is up to 91% within 120 min, following pseudo-first-order kinetics with the rate constant of 0.019 min(-1). Reduction rates increase from 0.007 to 0.019 min(-1) with increasing the current from 20 to 100 mA, slightly decrease in the presence of 10 mM chloride or bicarbonate, and decline with increasing the concentrations of sulfite or sulfide. O2 generated at the anode slightly influences TCE reduction. At low currents, TCE is mainly reduced by direct electron transfer on the Pd-Ti/TiO2NT cathode. However, the contribution of Pd-catalytic hydrodechlorination, an indirect reduction mechanism, becomes significant with increasing the current. Compared with other common cathodes, i.e., Ti-based mixed metal oxides, graphite and Pd/Ti, Pd-Ti/TiO2NTs cathode shows superior performance for TCE reduction.

A Three-electrode Column for Pd-Catalytic Oxidation of TCE in Groundwater with Automatic pH-regulation and Resistance to Reduced Sulfur Compound Foiling

A hybrid electrolysis and Pd-catalytic oxidation process is evaluated for degradation of trichloroethylene (TCE) in groundwater. A three-electrode, one anode and two cathodes, column is employed to automatically develop a low pH condition in the Pd vicinity and a neutral effluent. Simulated groundwater containing up to 5 mM bicarbonate can be acidified to below pH 4 in the Pd vicinity using a total of 60 mA with 20 mA passing through the third electrode. By packing 2 g of Pd/Al(2)O(3) pellets in the developed acidic region, the column efficiency for TCE oxidation in simulated groundwater (5.3 mg/L TCE) increases from 44 to 59 and 68% with increasing Fe(II) concentration from 0 to 5 and 10 mg/L, respectively. Different from Pd-catalytic hydrodechlorination under reducing conditions, this hybrid electrolysis and Pd-catalytic oxidation process is advantageous in controlling the fouling caused by reduced sulfur compounds (RSCs) because the in situ generated reactive oxidizing species, i.e., O(2), H(2)O(2) and OH, can oxidize RSCs to some extent. In particular, sulfite at concentrations less than 1 mM even greatly increases TCE oxidation by the production of SO(4)(•-), a strong oxidizing radical, and more OH.

Efficient reduction of Cr(VI) in groundwater by a hybrid electro-Pd process

Pd-catalytic process is effective for reducing a wide range of contaminants in groundwater. However, limited attention is paid to Cr(VI) reduction presumably due to the weakly oxidizing potential of Cr(VI) under circumneutral conditions. In this study, a new concept of in situ reducing Cr(VI) in groundwater by a hybrid electro-Pd process with automatic pH adjustments is proposed and justified. In an undivided electrolytic cell, Cr(VI) at 5 mg/L is reduced by 95% within 30 min under conditions of pH 3, 1 g/L Pd/Al2O3 and 20 mA current. Reduction of Cr(VI) increases with decreasing pH and increasing current and Pd/Al2O3 dosage. Inhibition of anodic O2 is significant but decreases with drop of pH. Atomic H is assigned as the predominant reactive species contributing to Cr(VI) reduction. Although H2O2 is effective for reducing Cr(VI), its production on Pd surface is completed inhibited by the presence of Cr(VI). The concept is ultimately justified using a specially configured three-electrode column. Cr(VI) is effectively reduced to Cr(3+) in the locally acidic Pd zone, and Cr(3+) is then precipitated in the downstream neutral zone. This hybrid electro-Pd process could be potentially applied in the in situ remediation of Cr(VI)-contaminated groundwater.

Efficient degradation of contaminants of emerging concerns by a new electro-Fenton process with Ti/MMO cathode

The performance of a new electro-Fenton process with Ti-based mixed metal oxides (Ti/MMO) cathode, a dimensionally stable electrode, is evaluated for degrading contaminants of emerging concerns (CECs) in aqueous solutions. Bisphenol A (422μgL(-1)) is completely degraded in an undivided cell using Ti/MMO as the cathode in the presence of 6.9mgL(-1) Fe(2+) within 20min under conditions of pH 4 and 25mA. Both bisphenol A degradation and H2O2 production increase with decreasing solution pH and increasing current. Ti/MMO cathode is effective for reducing O2 to H2O2 and regenerating Fe(2+) from Fe(3+). OH radicals are validated to be the predominant reactive oxygen species (ROS) contributing to bisphenol A degradation. This new electro-Fenton process is also effective for degrading bisphenol A, triclosan and ibuprofen even at a relatively high concentration of 5mgL(-1). The partial removals of total organic carbon suggest a moderate extent of mineralization. The transformation pathways of the three CECs are proposed based on the intermediates identified by HPLC and GC-MS, showing that CECs are mainly transformed to nontoxic aliphatic acids. This study demonstrates that Ti/MMO can be used as the cathode in the electro-Fenton process for degrading CECs at trace levels in waters.

Production of Abundant Hydroxyl Radicals from Oxygenation of Subsurface Sediments

Hydroxyl radicals (•OH) play a crucial role in the fate of redox-active substances in the environment. Studies of the •OH production in nature has been constrained to surface environments exposed to light irradiation, but is overlooked in the subsurface under dark. Results of this study demonstrate that abundant •OH is produced when subsurface sediments are oxygenated under fluctuating redox conditions at neutral pH values. The cumulative concentrations of •OH produced within 24 h upon oxygenation of 33 sediments sampled from different redox conditions are 2-670 μmol •OH per kg dry sediment or 6.7-2521 μM •OH in sediment pore water. Fe(II)-containing minerals, particularly phyllosilicates, are the predominant contributor to •OH production. This production could be sustainable when sediment Fe(II) is regenerated by the biological reduction of Fe(III) during redox cycles. Production of •OH is further evident in a field injection-extraction test through injecting oxygenated water into a 23-m depth aquifer. The •OH produced can oxidize pollutants such as arsenic and tetracycline and contribute to CO2 emissions at levels that are comparable with soil respiration. These findings indicate that oxygenation of subsurface sediments is an important source of •OH in nature that has not been previously identified, and •OH-mediated oxidation represents an overlooked process for substance transformations at the oxic/anoxic interface.

Electrochemically Induced Dual Reactive Barriers for Transformation of TCE and Mixture of Contaminants in Groundwater

A novel reactive electrochemical flow system consisting of an iron anode and a porous cathode is proposed for the remediation of mixture of contaminants in groundwater. The system consists of a series of sequentially arranged electrodes, a perforated iron anode, a porous copper cathode followed by a mesh-type mixed metal oxide anode. The iron anode generates ferrous species and a chemically reducing environment, the porous cathode provides a reactive electrochemically reducing barrier, and the inert anode provides protons and oxygen to neutralize the system. The redox conditions of the electrolyte flowing through this system can be regulated by controlling the distribution of the electric current. Column experiments are conducted to evaluate the process and study the variables. The electrochemical reduction on a copper foam cathode produced an electrode-based reductive potential capable of reducing TCE and nitrate. Rational electrodes arrangement, longer residence time of electrolytes and higher surface area of the foam electrode improve the reductive transformation of TCE. More than 82.2% TCE removal efficiency is achieved for the case of low influent concentration (<7.5 mg/L) and high current (>45 mA). The ferrous species produced from the iron anode not only enhance the transformation of TCE on the cathode, but also facilitates transformation of other contaminants including dichromate, selenate and arsenite. Removal efficiencies greater than 80% are achieved for these contaminants in flowing contaminated water. The overall system, comprising the electrode-based and electrolyte-based barriers, can be engineered as a versatile and integrated remedial method for a relatively wide spectrum of contaminants and their mixtures.