Deyang Kong

Kinetic and mechanistic investigations of the degradation of sulfamethazine in heat-activated persulfate oxidation process

Sulfamethazine (SMZ) is widely used in livestock feeding and aquaculture as an antibiotic agent and growth promoter. Widespread occurrence of SMZ in surface water, groundwater, soil and sediment has been reported. In this study, degradation of SMZ by heat-activated persulfate (PS) oxidation was investigated in aqueous solution. Experimental results demonstrated that SMZ degradation followed pseudo-first-order reaction kinetics. The pseudo-first-order rate constant (kobs) was increased markedly with increasing concentration of PS and temperature. Radical scavenging tests revealed that the predominant oxidizing species was SO4·(-) with HO playing a less important role. Aniline moiety in SMZ molecule was confirmed to be the reactive site for SO4·(-) attack by comparison with substructural analogs. Nontarget natural water constituents affected SMZ removal significantly, e.g., Cl(-) and HCO3(-) improved the degradation while fulvic acid reduced it. Reaction products were enriched by solid phase extraction (SPE) and analyzed by liquid chromatography-electrospray ionization-triple quadrupole mass spectrometry (LC-ESI-MS/MS). 6 products derived from sulfonamide S--N bond cleavage, aniline moiety oxidation and Smiles-type rearrangement were identified, and transformation pathways of SMZ oxidation were proposed. Results reveal that heat-activated PS oxidation could be an efficient approach for remediation of water contaminated by SMZ and related sulfonamides.

Thermo activated persulfate oxidation of antibiotic sulfamethoxazole and structurally related compounds

The widespread occurrence of sulfonamides (e.g., sulfamethoxazole) in natural environment has raised growing concerns due to their potential to induce antibiotic-resistant genes. In this study, the degradation of SMX and related sulfonamides by thermo activated persulfate (PS) oxidation was investigated. Experimental results demonstrated that SMX degradation followed pseudo-first-order reaction kinetics. The pseudo-first-order rate constant (k(obs)) was increased markedly with increasing temperature and pH. The presence of bicarbonate manifested promoting effect on SMX degradation while fulvic acid reduced it. Radical scavenging tests revealed that the predominant oxidizing species was SO4(•-) at neutral pH. Aniline moiety in SMX molecule was confirmed to be the primary reactive site for SO4(•-) attack by comparison with substructural analogues. Reaction products were enriched by solid phase extraction (SPE) and analyzed by liquid chromatography-electrospray ionization-triple quadrupole mass spectrometry (LC-ESI-MS/MS). A total of 7 products derived from hydroxylation, sulfonamide S-N bond cleavage, aniline moiety oxidation and coupling reaction were identified, and transformation pathways of SMX oxidation were proposed. Degradation of sulfonamides was appreciably influenced by the heterocyclic ring present in the molecules. Results reveal that thermo activated PS oxidation could be an efficient approach for remediation of water contaminated by SMX and related sulfonamides.

New insights into atrazine degradation by cobalt catalyzed peroxymonosulfate oxidation: kinetics, reaction products and transformation mechanisms.

The widespread occurrence of atrazine in waters poses potential risk to ecosystem and human health. In this study, we investigated the underlying mechanisms and transformation pathways of atrazine degradation by cobalt catalyzed peroxymonosulfate (Co(II)/PMS). Co(II)/PMS was found to be more efficient for ATZ elimination in aqueous solution than Fe(II)/PMS process. ATZ oxidation by Co(II)/PMS followed pseudo-first-order kinetics, and the reaction rate constant (k(obs)) increased appreciably with increasing Co(II) concentration. Increasing initial PMS concentration favored the decomposition of ATZ, however, no linear relationship between k(obs) and PMS concentration was observed. Higher efficiency of ATZ oxidation was observed around neutral pH, implying the possibility of applying Co(II)/PMS process under environmental realistic conditions. Natural organic matter (NOM), chloride (Cl(-)) and bicarbonate (HCO3(-)) showed detrimental effects on ATZ degradation, particularly at higher concentrations. Eleven products were identified by applying solid phase extraction-liquid chromatography-mass spectrometry (SPE-LC/MS) techniques. Major transformation pathways of ATZ included dealkylation, dechlorination-hydroxylation, and alkyl chain oxidation. Detailed mechanisms responsible for these transformation pathways were discussed. Our results reveal that Co(II)/PMS process might be an efficient technique for remediation of groundwater contaminated by ATZ and structurally related s-triazine herbicides.

Cobalt catalyzed peroxymonosulfate oxidation of tetrabromobisphenol A: Kinetics, reaction pathways, and formation of brominated by-products.

Degradation of tetrabromobisphenol A (TBBPA), a flame retardant widely spread in the environment, in Co(II) catalyzed peroxymonosulfate (PMS) oxidation process was systematically explored. The second-order-rate constant for reaction of sulfate radical (SO4(-)) with TBBPA was determined to be 5.27×10(10)M(-1)s(-1). Apparently, degradation of TBBPA showed first-order kinetics to the concentrations of both Co(II) and PMS. The presence of humic acid (HA) and bicarbonate inhibited TBBPA degradation, most likely due to their competition for SO4(-). Degradation of TBBPA was initiated by an electron abstraction from one of the phenolic rings. Detailed transformation pathways were proposed, including β-scission of isopropyl bridge, phenolic ring oxidation, debromination and coupling reactions. Further oxidative degradation of intermediates in Co(II)/PMS process yielded brominated disinfection by-products (Br-DBPs) such as bromoform and brominated acetic acids. Evolution profile of Br-DBPs showed an initially increasing and then decreasing pattern with maximum concentrations occurring around 6-10h. The presence of HA enhanced the formation of Br-DBPs significantly. These findings reveal potentially important, but previously unrecognized, formation of Br-DBPs during sulfate radical-based oxidation of bromide-containing organic compounds that may pose toxicological risks to human health.

Natural Organic Matter Exposed to Sulfate Radicals Increases Its Potential to Form Halogenated Disinfection Byproducts.

Sulfate radical-based advanced oxidation processes (SR-AOPs) are considered as viable technologies to degrade a variety of recalcitrant organic pollutants. This study demonstrates that o-phthalic acid (PA) could lead to the formation of brominated disinfection byproducts (DBPs) in SR-AOPs in the presence of bromide. However, PA does not generate DBPs in conventional halogenation processes. We found that this was attributed to the formation of phenolic intermediates susceptible to halogenation, such as salicylic acid through the oxidation of PA by SO4(•-). In addition, reactive bromine species could be generated from Br(-) oxidation by SO4(•-). Similar in situ generation of phenolic functionalities likely occurred by converting carboxylic substituents on aromatics to hydroxyl when natural organic matter (NOM) was exposed to trace level SO4(•-). It was found that such structural reconfiguration led to a great increase in the reactivity of NOM toward free halogen and, thus, its DBP formation potential. After a surface water sample was treated with 0.1 μM persulfate for 48 h, its potential to form chloroform, trichloroacetic acid, and dichloroacetic acid increased from 197.8, 54.3, and 27.6 to 236.2, 86.6, and 57.6 μg/L, respectively. This is the first report on possible NOM reconfiguration upon exposure to low-level SO4(•-) that has an implication in DBP formation. The findings highlight potential risks associated with SO4(•-)-based oxidation processes and help to avoid such risks in design and operation.

Degradation of roxarsone in a sulfate radical mediated oxidation process and formation of polynitrated by-products

Organoarsenicals such as roxarsone (ROX) are extensively utilized in the poultry industry, and land application of poultry litter is an important route by which arsenics are introduced into the environment. In the present study, degradation of ROX and structurally related nitrophenols by a heat activated persulfate (PS) oxidation process, one in situ chemical oxidation process (ISCO), was systematically explored. The effects of temperature, PS dosage, pH and natural water constituents (i.e., fulvic acid, Cl−) on the degradation of ROX were investigated. Products analysis by solid phase extraction (SPE) and liquid chromatography-electrospray ionization-triple quadrupole mass spectrometry (LC-ESI-MS/MS) revealed that 2,4-dinitrophenol (2,4-DNP) and 2,4,6-trinitrophenol (2,4,6-TNP) were generated as major intermediate products, suggesting denitration–renitration process occurred during SO4˙−-based oxidation of ROX. Interestingly, the formation of polynitration by-products was further confirmed in heat activated persulfate oxidation of nitrophenols. Formation of inorganic arsenics during ROX degradation was measured by inductively coupled plasma-mass spectrometry (ICP-MS). It was evident that the arsenic substituent of ROX was converted to As(V). On the basis of the intermediate products identified, detailed mechanisms and transformation pathways for ROX oxidation were proposed. Results manifest that heat activated PS oxidation could be an efficient approach to treat ROX contamination. However, post-treatment is necessary for complete removal of As(V) to minimize ecotoxicological risks.

Bicarbonate-activated persulfate oxidation of acetaminophen

Persulfate (PS) is widely used as an oxidant for in situ chemical remediation of contaminated groundwater. In this study we demonstrated for the first time that PS could be activated by bicarbonate. Acetaminophen was used as the probe compound to examine the reactivity of PS/bicarbonate system. It was found that acetaminophen could be effectively transformed and the reaction rate appeared pseudo-first-order to the concentrations of both acetaminophen and PS. Radical scavenger tests indicated that neither free radicals (SO4- and HO) nor superoxide (O2-) was responsible for acetaminophen transformation. Generation of singlet oxygen (1O2) was verified using furfuryl alcohol (FFA) as a probe. Formation of 1O2 was further quantified in D2O fortified solution based on kinetic solvent isotopic effect (KSIE) but it was found that 1O2 contributed only 51.4% of the total FFA transformation. The other 48.6% was presumed to be ascribed to the reaction with peroxymonocarbonate (HCO4-). However, the transformation of acetaminophen was mostly due to the reaction with HCO4- but not 1O2. Instead of degradation, HCO4- oxidized acetaminophen via a one-electron abstraction mechanism resulting in the generation of acetaminophen radicals which coupled to each other to form dimers and trimers. HCO4- also hydrolyzed rapidly to form hydrogen peroxide (H2O2) which led to the formation of 1O2, during which O2- was a key intermediates. Because bicarbonate is ubiquitously presented in groundwater, the findings of this research provide important insights into the fundamental processes involved in PS oxidation in subsurface.

Transformation of triclosan by laccase catalyzed oxidation: The influence of humic acid-metal binding process ☆

Laccase is a widely present extracellular phenoloxidase excreted by fungi, bacteria, and high plants. It is able to catalyze one-electron oxidation of phenolic compounds into radical intermediates that can subsequently couple to each other via covalent bonds. These reactions are believed to play an important role in humification process and the transformation of contaminants containing phenolic functionalities in the environment. In this study, we investigated the kinetics of triclosan transformation catalyzed by laccase. It was found that the rate of triclosan oxidation was first order to the concentrations of both substrate and enzyme. Humic acid (HA) could inhibit the reaction by quenching the radical intermediate of triclosan generated by laccase oxidation. Such inhibition was more significant in the presence of divalent metal cations. This is because that binding to metal ions neutralized the negative charge of HA molecules, thus making them more accessible to laccase molecule that is also negatively charged. Therefore, it has greater chance to quench the radical intermediate that is very unstable and can only diffuse a limited distance after being released from the enzyme catalytic center. Based on these understandings, a reaction model was developed by integration of metal-HA binding equilibriums and kinetic equations. This model precisely predicted the transformation rate of triclosan in the presence of HA and divalent metal ions including Ca2+, Mg2+, Cd2+, Co2+, Mn2+, Ba2+, and Zn2+. Overall, this work reveals important insights into laccase catalyzed oxidative coupling process.

Transformation of iodide and formation of iodinated by-products in heat activated persulfate oxidation process

Formation of halogenated disinfection by-products (DBPs) in sulfate radical-based advanced oxidation processes (SR-AOPs) have attracted considerable concerns recently. Previous studies have focused on the formation of chlorinated and brominated DBPs. This research examined the transformation of I- in heat activated PS oxidation process. Phenol was employed as a model compound to mimic the reactivity of dissolved natural organic matter (NOM) toward halogenation. It was found that I- was transformed to free iodine which attacked phenol subsequently leading to iodinated DBPs such as iodoform and iodoacetic acids. Iodophenols were detected as the intermediates during the formation of the iodoform and triiodoacetic acid (TIAA). However, diiodoacetic acid (DIAA) was formed almost concomitantly with iodophenols. In addition, the yield of DIAA was significantly higher than that of TIAA, which is distinct from conventional halogenation process. Both the facts suggest that different pathway might be involved during DIAA formation in SR-AOPs. Temperature and persulfate dose were the key factors governing the transformation process. The iodinated by-products can be further degraded by excessive SO4- and transformed to iodate. This study elucidated the transformation pathway of I- in SR-AOPs, which should be taken into consideration when persulfate was applied in environmental matrices containing iodine.

Degradation of tetrabromobisphenol A in heat activated persulfate oxidation process

Sulfate radicals (SO4˙−) generated by heat activated persulfate were employed to degrade brominated flame retardant tetrabromobisphenol A (TBBPA). Experimental results showed that the reaction rate was first order to the concentrations of both persulfate and TBBPA. The degradation of TBBPA was accelerated with increasing temperature. Radical scavenging tests using ethanol and t-butanol as probes revealed that SO4˙− was the dominant oxidizing species. The degradation efficiency was adversely affected by the presence of humic acid and HCO3−. No significant inhibition of TBBPA degradation was observed in the presence of Cl−. Seven brominated intermediates and products were identified by liquid chromatography-mass spectrometry. Based on this, debromination and electron transfer followed by β-scission were proposed to be the primary pathways of TBBPA degradation. The results of this study suggest that the heat activated persulfate process could be an effective approach to remove TBBPA in aqueous solution.