Tao

Landfill leachate treatment by a coagulation–photooxidation process

This experimental study was conducted to investigate the effect of treatment of landfill leachate by a coagulation-photooxidation process. The effects of different dosages of coagulant and different pH values on the coagulation processes were compared. The effect of different concentrations of sodium oxalate (Na(2)C(2)O(4)) on the treatment process was also studied after the coagulation was performed using FeCl(3).6H(2)O. The experimental results show that in the pH range of 3-8, the lower the pH value, the higher the efficiency of the treatment. A 24% removal of COD (chemical oxygen demand, mg(O(2)) x l(-1)) can be attained by the addition of 1000 mg x l(-1) FeCl(3). A 31% removal of COD can be attained after 4h of irradiation alone, and a 64% removal of COD can be attained after 4h irradiation at pH 3 with the addition of 500 mg x l(-1) FeCl(3).6H(2)O.

Photodegradation of propranolol by Fe(III)-citrate complexes: kinetics, mechanism and effect of environmental media.

Photogeneration of HO was optimized in Fe(III)-citrate solution within the pH range of 3.0-9.0 to investigate its photoreactivity at neutral pH without the addition of H(2)O(2) under simulated sunlight. The generation of HO decreased with increasing pH within the range of 6.0-9.0 at the Fe(III)-to-citrate ratio of 10:50 (10(-6)M). However, when the concentration of citrate increased to 1.5 × 10(-4)M, the formation rate of HO increased in the order of pH 9.0<3.0<7.0<4.0<5.0. The pH-dependent HO production was governed by the stability of Fe(II)/Fe(II)-citrate and the amount of O(2)(-) in the solution. Propranolol can be efficiently photodegraded in Fe(III)-citrate system at pH 7.0 with pseudo-first-order constant 3.1 × 10(-4)s(-1). HO was verified to be the main reactive oxygen species (ROS) responsible for the photodegradation of propranolol. The presence of metal ions inhibited the Fe(III)-cit-induced photodegradation in the order of Mn(2+)>Cu(2+)>Ca(2+)>Mg(2+). Both humic acid (HA) and fulvic acid (FA) markedly suppressed the degradation of propranolol. Moreover, the iron in Fe(III)-citrate system was reused by a simple addition of citrate to the reaction solution. By GC-MS analysis, the photoproducts of the propranolol were identified and the degradation pathway was proposed. This work suggests that Fe(III)-citrate complexes are good alternative for the advanced treatment of organic pollutants at neutral pH in aqueous solution.

Photodegradation of selected β-blockers in aqueous fulvic acid solutions: Kinetics, mechanism, and product analysis

The photodegradation of the widely used β-blockers atenolol and metoprolol were investigated in the presence of fulvic acid (FA) under simulated sunlight. Both atenolol and metoprolol undergo indirect photodegradation in the FA solutions. The triplet excited state of FA ((3)FA(∗)) was verified to be main reactive species responsible for the photosensitized degradation of β-blockers. An electron transfer mechanism for the interaction between β-blockers and (3)FA(∗) was proposed on the basis of a series of experiments. Magnetic property of metal ions exhibited significant impact on photosensitized degradation. Diamagnetic metal ions such as Mg(2+), Ca(2+), Zn(2+), and Al(3+) negligibly affected the degradation. In contrast, paramagnetic metal ions including Mn(2+), Cu(2+), Fe(3+), and Cr(3+) markedly inhibited the reactions in the order of Cr(3+) < Fe(3+) < Cu(2+) < Mn(2+). The inhibition was related to the complexation ability with FA. By LC-ESI-MS/MS analysis, deisopropyl-atenolol (metoprolol) was identified as the main photosensitized product. The degradation pathways of β-blockers involving electron transfer processes were proposed. This finding strongly suggests that (3)FA(∗) was important reactive species for the degradation of β-blockers in natural waters.

Photodegradation of parabens by Fe(III)-citrate complexes at circumneutral pH: matrix effect and reaction mechanism.

The photodegradation of four parabens including methyl-, ethyl-, propyl-, and butyl-paraben in the presence of Fe(III)-citrate complexes under simulated sunlight was investigated. The degradation of parabens increased with decreasing pH within the range of 5.0-8.0 at the Fe(III)-to-citrate ratio of 10:150 (μM). The addition of low-molecular-weight carboxylic acids showed different effects on the photodegradation of methylparaben. The low-photoreactive carboxylic acids inhibited the photodegradation of methylparaben in the order of formic acid>succinic acid>acetic acid>malonic acid. In contrast, oxalic acid enhanced the photodegradation and exhibited appreciable synergistic effect with Fe(III)-citrate at concentration higher than 500 μM. Up to 99.0% of substrate was degraded after 30 min at pH6.0 in the Fe(III)-citrate-oxalate system. The various fractions of fulvic acid inhibited the photodegradation of methylparaben. The inhibition increased with increasing nominal molecular weight of fractionated fulvic acid. Moreover, the photodegradation of methylparaben was inhibited in natural waters in the order of Liangzi Lake<Donghu Lake<Changjiang River≈Seawater. The photoproducts of methylparaben were identified by GC-MS analyses and the degradation pathway was proposed.

Photoproducts of tetracycline and oxytetracycline involving self-sensitized oxidation in aqueous solutions: effects of Ca2+ and Mg2+.

Tetracyclines constitute one of the most important antibiotic families and represent a classic example of phototoxicity. The photoproducts of tetracyclines and their parent compounds have potentially adverse effects on natural ecosystem. In this study, the self-sensitized oxidation products of tetracycline (TC) and oxytetracycline (OTC) were determined and the effects of Ca2+ and Mg2+ on self-sensitized degradation were investigated. The Ca2+ and Mg2+ in the natural water sample accounted for enhancement (pH 7.3) and inhibition (pH 9.0) of photodegradation of TC and OTC due to the formation of metal-ions complexes. The formation of Mg2+ complexes was unfavorable for the photodegradation of the tetracyclines at both pH values. In contrast, the Ca2+ complexes facilitated the attack of singlet oxygen (1O2) arising from self-sensitization at pH 7.3 and enhanced TC photodegradation. For the first time, self-sensitized oxidation products of TC and OTC were verified by quenching experiments and detected by LC/ESI-DAD-MS. The products had a nominal mass 14 Da higher than the parent drugs (designated M+14), which resulted from the 1O2 attack of the dimethylamino group on the C-4 atom of the tetracyclines. The presence of Ca2+ and Mg2+ also affected the generation of M+14 due to the formation of metal-ions complexes with TC and OTC. The findings suggest that the metal-ion complexation has significant impact on the self-sensitized oxidation processes and the photoproducts of tetracyclines.

Photolysis of chlortetracycline in aqueous solution: kinetics, toxicity and products.

The aqueous photodegradation of the widely used antibiotic chlortetracycline (CTC) was investigated under simulated sunlight. The quantum yield of photodegradation increased from 3.3 x 10(-4) to 8.5 x 10(-3) within the pH range of 6.0 to 9.0. The presence of Ca2+, Fe3+, and NO(-3) enhanced the photodegradation of CTC, whereas Mg2+, Mn2+, and Zn2+ inhibited the degradation with the order Mn2+ > Zn2+ > Mg2+ at pH 7.3. The monovalent cations (Na+ and K+) had negligible effect on the photolysis of CTC. Fulvic acid (FA) decreased the photodegradation of CTC due to light screening effect. Hydrogen peroxide (H2O2) was formed concurrently with direct photodegradation of CTC. The generation rate of H2O2 increased from 0.027 to 0.086 micromol/(L x min) when the pH ranged from 6.0 to 9.0. The CTC solution was about three-fold more toxic to the Photobacterium phosphoreum bacteria after irradiation, suggesting that the photoproducts and H2O2 formed in the CTC solution exhibited high risk on the bacteria. By LC-ESI(+)-MS, the photoproducts of CTC were identified. The direct photodegradation of CTC was involved in hydroxylation and N-demethyl/dedismethyl processes. The main photoproducts included the iso-CTC analog containing hydroxyl groups (m/z 511.4 and 495.4), and the N-demethyl/dedismethyl products of the photoproduct m/z 495.4 (m/z 481.3 and 467.4). In addition, the photochemical dechlorination of CTC led to tetracycline (m/z 445.5).

Photodegradation of hexabromocyclododecane (HBCD) by Fe(III) complexes/H 2 O 2 under simulated sunlight

Hexabromocyclododecane (HBCD) is a globally produced brominated flame retardant used primarily as an additive flame retardant in polystyrene and textile products. Photodegradation of HBCD in the presence of Fe(III)-carboxylate complexes/H2O2 was investigated under simulated sunlight. The degradation of HBCD decreased with increasing pH in the Fe(III)-oxalate solutions. In contrast, the optimum pH was 5.0 for the Fe(III)-citrate-catalyzed photodegradation within the range of 3.0 to 7.0. For both Fe(III)-oxalate and Fe(III)-citrate complexes, the increase of carboxylate concentrations facilitated the photodegradation. The photochemical removal of HBCD was related to the photoreactivity and speciation distribution of Fe(III) complexes. The addition of H2O2 markedly accelerated the degradation of HBCD in the presence of Fe(III)-citrate complexes. The quenching experiments showed that ·OH was responsible for the photodegradation of HBCD in the Fe(III)-carboxylate complexes/H2O2 solutions. The results suggest that Fe(III) complexes/H2O2 catalysis is a potential method for the removal of HBCD in the aqueous solutions.

Photodegradation kinetics, products and mechanism of timolol under simulated sunlight

The photodegradation of β-blocker timolol in fulvic acid (FA) solution was investigated under simulated sunlight. The triplet excited state of FA ((3)FA(*)) and singlet oxygen ((1)O2) were the main reactive species responsible for the degradation of timolol in the aerated FA solutions. Both dissolved oxygen and iodide ions (I(-)) are the efficient quenchers of (3)FA(*). The photodegradation was drastically accelerated after removing the dissolved oxygen. The presence of I(-) inhibited the photosensitized degradation of timolol in the deoxygenated FA solutions, whereas the role of I(-) in the reaction was concentration-dependent in the aerated solutions. The other halide ions such as chloride (Cl(-)) and bromide (Br(-)) exhibited less effect on the photodegradation of timolol in both aerated and deoxygenated solutions. By LC-DAD/ESI-MS/MS analysis, the photoproducts of timolol in both aerated and deoxygenated FA solutions were identified. Electron transfer interaction occurred between (3)FA(*) and amine moiety of timolol, leading to the cleavage of C-O bond in the side chain and oxidation of the hexatomic ring. These findings suggest the photosensitized degradation was a significant pathway for the elimination of timolol in natural waters.

Waste activated sludge treatment based on temperature staged and biologically phased anaerobic digestion system

The concept of temperature staged and biological phased (TSBP) was proposed to enhance the performance of waste-activated sludge anaerobic digestion. Semi-continuous experiments were used to investigate the effect of temperature (35 to 70 degrees C) as well as the hydraulic retention time (HRT) (2, 4 and 6 days) on the acidogenic phase. The results showed that the solubilization degree of waste-activated sludge increased from 14.7% to 30.1% with temperature increasing from 35 to 70 degrees C, while the acidification degree was highest at 45 degrees C (17.6%), and this was quite different from the temperature impact on hydrolysis. Compared with HRT of 2 and 6 days, 4 days was chosen as the appropriate HRT because of its relatively high solubilization degree (24.6%) and acidification degree (20.1%) at 45 degrees C. The TSBP system combined the acidogenic reactor (45 degrees C, 4 days) with the methanogenic reactor (35 degrees C, 16 days) and the results showed 84.8% and 11.4% higher methane yield and volatile solid reduction, respectively, compared with that of the single-stage anaerobic digestion system with HRT of 20 days at 35 degrees C. Moreover, different microbial morphologies were observed in the acidogenic- and methanogenic-phase reactors, which resulted from the temperature control and HRT adjustment. All the above results indicated that 45 degrees C was the optimum temperature to inhibit the activity of methanogenic bacteria in the acidogenic phase, and temperature staging and phase separation was thus accomplished. The advantages of the TSBP process were also confirmed by a full-scale waste-activated sludge anaerobic digestion project which was an energy self-sufficient system.

Multivariate-Parameter Optimization for Photodegradation of Tetracycline by Fe(III)-Citrate Complexes at Near-Neutral pH

AbstractTetracycline antibiotics have been extensively used in pharmaceutical and veterinary therapies, and they are widely present in natural waters. Although Fe(III)-citrate complexes are known to be highly photoreactive, the optimal conditions for treatment of pollutants at near-neutral pH are not fully understood. In this study, the photochemical removal of tetracycline (TC) in the Fe(III)-citrate solution under simulated sunlight was investigated. A central composite design was employed to explore the effects of the initial concentrations of TC (0–90xa0μM), Fe(III) (0–40xa0μM), citrate (0–400xa0μM), and pH (5.0–9.0) on the photodegradation of TC in aqueous solution. Five levels of each factor were included in the design. The photodegradation of TC in the Fe(III)-citrate solution was observed to follow pseudo-first-order kinetics. According to the multivariate effects analysis, the optimal photodegradation condition was achieved at pH 6.9 and [Fe(III)]0/(citrate)0=33.8/324 (μM), with the corresponding rate ...