The photocatalytic reduction of NO3- to N2 with ilmenite (FeTiO3): Effects of groundwater matrix.

This work analyzes the role of natural groundwater, as well as the effect of HCO3-, Ca2+, Mg2+, K+, SO42- and Cl- concentrations, upon the photocatalytic nitrate reduction using ilmenite as catalyst and oxalic acid as hole scavenger. The nitrate removal and the selectivity towards N2 are significantly limited compared to previous experiments using ultrapure water matrix. Calcium (Ca2+), bicarbonate (HCO3-) as well as pH are claimed as the major controlling factors related to the process yield. Thus, Ca2+ promotes the formation of insoluble oxalate microcrystals, reducing the amount of hole scavenger available. The presence of HCO3- leads to a steeply increase in the pH value, favoring the adsorption onto the ilmenite surface of ions OH-instead of NO3-, NO2- and C2O42. The aforementioned issues are overcome by working with C2O42-/NO3- ratio well above the stoichiometric one, that also maintains the pH value in an acid range. A completed depletion of the starting NO3-, the no detection of either NO2- or NH4+ in the aqueous phase, and a selectivity towards N2 above 95% were achieved using two times the stoichiometric dose.

Diclofenac photodegradation with the Perovskites BaFeyTi1-yO3 as catalysts.

Perovskite oxides BaFeyTi1-yO3, with y = 0, 0.6, 0.8 and 1, were prepared by ceramic (CM) and complex polymerization methods (CPM) and utilized in UV-LED (365 nm) photocatalytic degradation assays of 25 mg L-1 diclofenac (DIC) model solutions. BaTiO3-CM was also used in the photocatalytic degradation test of a real mineral water for human consumption spiked with 2 mg L-1 DIC. The XRD patterns of the synthesized perovskites showed cubic structure for those prepared by CPM, with distortions of the cubic lattice to hexagonal or tetragonal when prepared by CM, except for BaTiO3. All the perovskites showed good catalytic activity, higher than photolysis, except BaFeO3-CM that presented similar results. BaTiO3-CM and CPM and BaFeO3-CPM were also utilized in UV-LED photocatalytic DIC degradation assays with peroxydisulfate addition. BaFeO3-CPM and BaTiO3-CPM showed better ability to persulfate activation, but the highest mineralization degree was obtained with BaTiO3-CM. This last perovskite was also able to perform DIC degradation in a real matrix. The studied oxides show potentiality for photocatalytic degradation of organic compounds, with or without persulfate addition. A degradation mechanism is proposed.

Coupled heat-activated persulfate - Electrolysis for the abatement of organic matter and total nitrogen from landfill leachate.

This work analyzes the viability of a coupled heat-activated persulfate (PS) and electro-oxidation treatment toabatetheorganic matter and nitrogen from ahigh polluted landfill leachate (5500 mg L-1 TOC; 5849 mg L-1 TN, pH: 8.4). These characteristics makes PS as a suitable oxidant to deal with the recalcitrant organic matter. Under the optimal conditions (70 °C and 60% of the stoichiometric amount of PS), around 60% of the initial organic load was mineralized. On the contrary, the nitrogen removal was below 20%. A subsequent electrolytic stage using Ti/IrO2-TaO2 anode at 175 mA cm-2 and 0.42 M NaCl during 60 min, led to overall organic matter and nitrogen removal above 85% and 90%, respectively, with energy requirement of 38 kWh per kg of nitrogen removed. In this sense, the combined process achieves a significant reduction in terms of energy consumption, up to one fifth in relation to sole electrolysis. These results confirm the feasibility of this combined process to treat landfill leachate.

Evaluation of photoassisted treatments for norfloxacin removal in water using mesoporous Fe2O3-TiO2 materials.

We report the synthesis of mesoporous TiO2 and mesoporous Fe2O3-TiO2 catalysts by using a structure-directing-surfactant method, their characterization and their employment as photocatalysts for norfloxacin degradation in aqueous solution. The main findings show that in the presence of both O2 and H2O2, Fe-containing mesoporous titania (Fe2O3-TiO2), with iron percentages between 1 and 3 wt%, exhibited norfloxacin degradation rates more than 60% greater than otherwise identical mesoporous titania without iron. Furthermore, the activity of the mesoporous composite catalysts also exceeds that of titania when illuminated with 405 nm light-emitting diodes. Iron loading improved the photocatalytic activity for norfloxacin degradation with values of apparent reaction rate constants of 0.037 min-1 and 0.076 min-1 with 1 and 3 surface wt.% of iron, respectively. An optimum of activity was found with the 3 wt% Fe2O3-TiO2 catalyst. Under these conditions, 10 mg/L of norfloxacin is reacted essentially to completion and 90% of total organic carbon conversion was obtained within 120 min of reaction. This higher organic carbon conversion degree was reached due to the photo-oxidation of short-chain organic acids. The high activity of the as-synthesized mesoporous composites is attributed to the additional iron phase which led to the different reactions for H2O2 decomposition, but also due to the improvement in light absorbance. Finally, the activity of the most active catalyst was found to be stable over multiple sequential runs, which was related to a negligible amount of iron leaching (<0.1%) from these materials.

Coupled fenton-denitrification process for the removal of organic matter and total nitrogen from coke plant wastewater.

This work assesses the feasibility of applying a Coupled Fenton-Denitrification (CFD) process for the treatment of wastewater from a coking plant. This highly toxic effluent is characterized by comparable carbon and nitrogen contents and it is usually released into the treatment system at well above room temperature. Recalcitrant organic matter can be easily removed in a first step using Fenton treatment. Working at 50 °C, pH0: 3, and a wastewater obtained from a coking plant, the stoichiometric amount of H2O2 relative to COD and a H2O2/Fe2+ weight ratio of 50, around 60% of carbon load was mineralized whereas H2O2 was completely depleted. However, no changes were observed in the total nitrogen content. A subsequent denitrification stage led to an additional 80% TOC (overall above 90%) and 75% Total Nitrogen removal. This was done in a batch bioreactor at room temperature over 72 h, using a 40-day pre-acclimated denitrifying biomass. These results point to the possibility of designing a combined chemical oxidation and biological treatment to deal with complex effluents containing refractory organic matter including high concentrations of nitrogen species.

Landfill leachate treatment by sequential combination of activated persulfate and Fenton oxidation.

This work assesses the feasibility of sequential persulfate and Fenton oxidation for the decolorization and mineralization of landfill leachate (5600 mg L-1 TOC; pH0: 8.6) in a continuous batch-recirculation system. Firstly, it was analyzed the role of the operational conditions upon the persulfate activation evaluating the effects of electrolysis, ilmenite (FeTiO3) as a source of Fe(II) and UV-LED (at 365 nm). The studied variables include current density (j) (50-200 mA cm-2), persulfate dose (46.8-234 mM) and mineral concentration (500-1500 mg L-1). The increase in j enhanced the hypochlorite generation and PS conversion to SO4- and, consequently, decolorization efficiency increasing the penetration of light through the solution and the photoreduction of Fe(III) to Fe(II) in the FeTiO3 surface. The combined electrolysis/FeTiO3/UV-LED showed synergetic effect compared to the individual processes, achieving mineralization around 53% under the optimum operating conditions (1 g L-1 of FeTiO3, using 234 mM of PS at 200 mA cm-2 under UV-LED radiation). The subsequent Fenton oxidation once the pH decreased up to around 3, led to overall mineralization above 90% after 480 min, confirming the suitability of this combined treatment to deal with recalcitrant and highly colored effluents.

Activated carbon as catalyst for microwave-assisted wet peroxide oxidation of aromatic hydrocarbons.

This paper addresses the removal of four aromatic hydrocarbons typically found in petrochemical wastewater: benzene (B), toluene (T), o-xylene (X), and naphthalene (N), by microwave-assisted catalytic wet peroxide oxidation (MW-CWPO) using activated carbon (AC) as catalyst. Under the studied conditions, complete pollutant elimination (B, 1.28 mM; T, 1.09 mM; X, 0.94 mM; and N, 0.78 mM) was achieved, with more than 90% TOC removal after only 15-min reaction time, working at 120 °C, pH0 = 3, AC at 1 g L-1, and H2O2 at the stoichiometric dose. Furthermore, in the case of toluene, naphthalene, and xylene, the hydroxylation and breakdown of the ring is very rapid and toxic intermediates were not detected. The process follows two steps: (i) pollutant adsorption onto AC followed by (ii) adsorbed compounds oxidation. Thus, MW-CWPO with AC as catalyst appears a promising way for a fast and effective process for B, T, X, and N removal in aqueous phase.

Electro activation of persulfate using iron sheet as low-cost electrode: the role of the operating conditions.

This work assesses the role of the operational conditions upon the electro-activation of persulfate (PS) using sacrificed iron electrode as a continuous low-cost Fe2+ source. An aqueous phenol solution (100 mg L-1) was selected as model effluent. The studied variables include current density (1-10 mA cm-2), persulfate concentration (0.7-2.85 g L-1), temperature (30-90°C) and the solution conductivity (2.7-20.7 mS cm-1) using Na2SO4 and NaCl as supporting electrolyte. A mineralization degree of around 80% with Na2SO4 and 92% in presence of NaCl was achieved at 30°C using 2.15 g L-1 PS at the lowest current density tested (1 mA cm-2). Besides PS concentration, temperature was the main variable affecting the process. In the range of 30-70°C, it showed a positive effect, achieving TOC conversion above 95% (using Na2SO4 under the previous conditions) along with a significant increase in iron sludge, which adversely affects the economy of the process. A lumped and simplified kinetic model based on persulfate consumption and TOC mineralization is suggested. The activation energy obtained for the TOC decay was 29 kJ mol-1. An estimated operating cost of US$ 3.00 per m3 was obtained, demonstrating the economic feasibility of this process.

Nanoscale Fe/Ag particles activated persulfate: optimization using response surface methodology.

This work studied the bimetallic nanoparticles Fe-Ag (nZVI-Ag) activated persulfate (PS) in aqueous solution using response surface methodology. The Box-Behnken design (BBD) was employed to optimize three parameters (nZVI-Ag dose, reaction temperature, and PS concentration) using 4-chlorophenol (4-CP) as the target pollutant. The synthesis of nZVI-Ag particles was carried out through a reduction of FeCl2 with NaBH4 followed by reductive deposition of Ag. The catalyst was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Brunauer-Emmett-Teller (BET) surface area. The BBD was considered a satisfactory model to optimize the process. Confirmatory tests were carried out using predicted and experimental values under the optimal conditions (50 mg L-1 nZVI-Ag, 21 mM PS at 57 °C) and the complete removal of 4-CP achieved experimentally was successfully predicted by the model, whereas the mineralization degree predicted (90%) was slightly overestimated against the measured data (83%).

Improving the Fenton process by visible LED irradiation.

The effect of irradiation with visible light-emitting diode (LED) light on the efficiency of Fenton oxidation is investigated using phenol as the target compound (100 mg/L). The H2O2 dose and temperature are tested as operating variables with the aim of minimizing consumption of the reagents. At 50 °C, 10 mg/L Fe2+, and 60 % of the stoichiometric H2O2 amount, phenol was completely oxidized into CO2, H2O, and short chain organic acids, with oxalic acid completely degraded. Up to 95 % mineralization was achieved. This high efficiency can be attributed to the effect of LED radiation on the quinones/Fe2+/Fe3+/H2O2 cycle, which significantly increases the reaction rate, as well as on the photodecomposition of the iron complexes formed along the oxidation process, which also enhanced mineralization.

Mineralization of naphtenic acids with thermally-activated persulfate: The important role of oxygen.

This study reports on the mineralization of model naphtenic acids (NAs) in aqueous solution by catalyst-free thermally-activated persulfate (PS) oxidation. These species are found to be pollutants in oil sands process-affected waters. The NAs tested include saturated-ring (cyclohexanecarboxylic and cyclohexanebutyric acids) and aromatic (2-naphthoic and 1,2,3,4-tetrahydro-2-naphthoic acids) structures, at 50mgL(-1)starting concentration. The effect of PS dose within a wide range (10-100% of the theoretical stoichiometric) and working temperature (40-97°C) was investigated. At 80°C and intitial pH=8 complete mineralization of the four NAs was achieved with 40-60% of the stoichiometric PS dose. This is explained because of the important contribution of oxygen, which was experimentally verified and was found to be more effective toward the NAs with a single cyclohexane ring than for the bicyclic aromatic-ring-bearing ones. The effect of chloride and bicarbonate was also checked. The former showed negative effect on the degradation rate of NAs whereas it was negligible or even positive for bicarbonate. The rate of mineralization was well described by simple pseudo-first order kinetics with values of the rate constants normalized to the PS dose within the range of 0.062-0.099h(-1). Apparent activation energy values between 93.7-105.3kJmol(-1) were obtained.

Electrochemical oxidation of landfill leachate in a flow reactor: optimization using response surface methodology.

Response surface methodology based on Box-Behnken (BBD) design was successfully applied to the optimization in the operating conditions of the electrochemical oxidation of sanitary landfill leachate aimed for making this method feasible for scale up. Landfill leachate was treated in continuous batch-recirculation system, where a dimensional stable anode (DSA(©)) coated with Ti/TiO2 and RuO2 film oxide were used. The effects of three variables, current density (milliampere per square centimeter), time of treatment (minutes), and supporting electrolyte dosage (moles per liter) upon the total organic carbon removal were evaluated. Optimized conditions were obtained for the highest desirability at 244.11 mA/cm(2), 41.78 min, and 0.07 mol/L of NaCl and 242.84 mA/cm(2), 37.07 min, and 0.07 mol/L of Na2SO4. Under the optimal conditions, 54.99% of chemical oxygen demand (COD) and 71.07 ammonia nitrogen (NH3-N) removal was achieved with NaCl and 45.50 of COD and 62.13 NH3-N with Na2SO4. A new kinetic model predicted obtained from the relation between BBD and the kinetic model was suggested.

Case study of the application of Fenton process to highly polluted wastewater from power plant.

This work investigates the application of Fenton process to the treatment of a highly polluted industrial wastewater resulting from the pipeline cleaning in a power plant. This effluent is characterized by a high chemical oxygen demand (COD>40 g/L), low biodegradability and quite a high iron concentration (around 3g/L) this coming from pipeline corrosion. The effect of the initial reaction temperature (between 50 and 90 °C) and the way of feeding H2O2 on the mineralization percentage and the efficiency of H2O2 consumption has been analyzed. With the stoichiometric amount of H2O2 relative to initial COD, fed in continuous mode, more than 90% COD reduction was achieved at 90 °C. That was accompanied by a dramatic improvement of the biodegradability. Thus, a combined treatment based on semicontinuous high-temperature Fenton oxidation (SHTF) and conventional aerobic biological treatment would allow fulfilling the COD and ecotoxicity regional limits for industrial wastewaters into de municipal sewer system. For the sake of comparison, catalytic wet air oxidation was also tested with poor results (less than 30% COD removal at 140 °C and 8 atm oxygen pressure).

Influence of plants on the reduction of hexavalent chromium in wetland sediments.

This work addresses the effect that plants (Typha latifolia and Carex lurida) have on the reduction of Cr(VI) in wetland sediments. Experiments were carried out using tubular microcosms, where chemical species were monitored along the longitudinal flow axis. Cr(VI) removal was enhanced by the presence of plants. This is explained by a decrease in the redox potential promoted by organic root exudates released by plants. Under these conditions sulfate reduction is enhanced, increasing the concentration of sulfide species in the sediment pore water, which reduce Cr(VI). Evapotranspiration induced by plants also contributed to enhance the reduction of Cr(VI) by concentrating all chemical species in the sediment pore water. Both exudates release and evapotranspiration have a diurnal component that affects Cr(VI) reduction. Concentration profiles were fitted to a kinetic model linking sulfide and Cr(VI) concentrations corrected for evapotranspiration. This expression captures both the longitudinal as well as the diurnal Cr(VI) concentration profiles.