Cun Liu

Key Role of Persistent Free Radicals in Hydrogen Peroxide Activation by Biochar: Implications to Organic Contaminant Degradation

We investigated the activation of hydrogen peroxide (H2O2) by biochars (produced from pine needles, wheat, and maize straw) for 2-chlorobiphenyl (2-CB) degradation in the present study. It was found that H2O2 can be effectively activated by biochar, which produces hydroxyl radical ((•)OH) to degrade 2-CB. Furthermore, the activation mechanism was elucidated by electron paramagnetic resonance (EPR) and salicylic acid (SA) trapping techniques. The results showed that biochar contains persistent free radicals (PFRs), typically ∼ 10(18) unpaired spins/gram. Higher trapped [(•)OH] concentrations were observed with larger decreases in PFRs concentration, when H2O2 was added to biochar, indicating that PFRs were the main contributor to the formation of (•)OH. This hypothesis was supported by the linear correlations between PFRs concentration and trapped [(•)OH], as well as kobs of 2-CB degradation. The correlation coefficients (R(2)) were 0.723 and 0.668 for PFRs concentration vs trapped [(•)OH], and PFRs concentration vs kobs, respectively, when all biochars pyrolyzed at different temperatures were included. For the same biochar washed by different organic solvents (methanol, hexane, dichloromethane, and toluene), the correlation coefficients markedly increased to 0.818-0.907. Single-electron transfer from PFRs to H2O2 was a possible mechanism for H2O2 activation by biochars, which was supported by free radical quenching studies. The findings of this study provide a new pathway for biochar implication and insight into the mechanism of H2O2 activation by carbonaceous materials (e.g., activated carbon and graphite).

Degradation of organic dyes via bismuth silver oxide initiated direct oxidation coupled with sodium bismuthate based visible light photocatalysis.

Organic dye degradation was achieved via direct oxidation by bismuth silver oxide coupled with visible light photocatalysis by sodium bismuthate. Crystal violet dye decomposition by each reagent proceeded via two distinct pathways, each involving different active oxygen species. A comparison of each treatment method alone and in combination demonstrated that using the combined methods in sequence achieved a higher degree of degradation, and especially mineralization, than that obtained using either method alone. In the combined process direct oxidation acts as a pretreatment to rapidly bleach the dye solution which substantially facilitates subsequent visible light photocatalytic processes. The integrated sequential direct oxidation and visible light photocatalysis are complementary manifesting a > 100% increase in TOC removal, compared to either isolated method. The combined process is proposed as a novel and effective technology based on one primary material, sodium bismuthate, for treating wastewaters contaminated by high concentrations of organic dyes.

Manipulation of Persistent Free Radicals in Biochar To Activate Persulfate for Contaminant Degradation

This study investigated the effects of metals (Fe3+, Cu2+, Ni2+, and Zn2+) and phenolic compounds (PCs: hydroquinone, catechol, and phenol) loaded on biomass on the formation of persistent free radicals (PFRs) in biochar. It was found that metal and phenolic compound treatments not only increased the concentrations of PFRs in biochar but also changed the types of PFRs formed, which indicated that manipulating the amount of metals and PCs in biomass may be an efficient method to regulate PFRs in biochar. These results provided direct evidence to elucidate the mechanism of PFR formation in biochar. Furthermore, the catalytic ability of biochar toward persulfate activation for the degradation of contaminants was evaluated. The results indicated that biochar activates persulfate to produce sulfate radicals (SO4•-) and degraded polychlorinated biphenyls (PCBs) efficiently. It was found that both the concentration and type of PFRs were the dominant factors controlling the activation of persulfate by biochar and that superoxide radical anions account for 20-30% of sulfate radical generation in biochar/persulfate. This conclusion was supported by linear correlations between the concentration of PFRs consumed and the formation of SO4•- and between λ (λ=[formed sulfate radicals]/[consumed PFRs]) and g-factors. The findings of this study provide new methods to manipulate PFR concentration in biochar for the transformation of contaminants and development of new alternative activators for persulfate-based remediation of contaminated soils.

Transformation of Sulfamethazine by Manganese Oxide in Aqueous Solution

The transformation of the sulfonamide antimicrobial sulfamethazine (SMZ) by a synthetic analogue of the birnessite-family mineral vernadite (δ-MnO(2)) was studied. The observed pseudo-first-order reaction constants (k(obs)) decreased as the pH increased from 4.0 to 5.6, consistent with the decline in δ-MnO(2) reduction potential with increasing pH. Molecular oxygen accelerated SMZ transformation by δ-MnO(2) and influenced the transformation product distribution. Increases in the Na(+) concentration produced declines in k(obs). Transformation products identified by tandem mass spectrometry and the use of (13)C-labeled SMZ included an azo dimer self-coupling product and SO(2) extrusion products. Product analysis and density functional theory calculations are consistent with surface precursor complex formation followed by single-electron transfer from SMZ to δ-MnO(2) to produce SMZ radical species. Sulfamethazine radicals undergo further transformation by at least two pathways: radical-radical self-coupling or a Smiles-type rearrangement with O addition and then extrusion of SO(3). Experiments conducted in H(2)(18)O or in the presence of (18)O(2)(aq) demonstrated that oxygen both from the lattice of as-synthesized δ-MnO(2) and initially present as dissolved oxygen reacted with SMZ. The study results suggest that the oxic state and pH of soil and sediment environments can be expected to influence manganese oxide-mediated transformation of sulfonamide antimicrobials.

Impact of woodchip biochar amendment on the sorption and dissipation of pesticide acetamiprid in agricultural soils.

Pyrolysis of vegetative biomass into biochar and application of the more stable form of carbon to soil have been shown to be effective in reducing the emission of greenhouse gases, improving soil fertility, and sequestering soil contaminants. However, there is still lack of information about the impact of biochar amendment in agricultural soils on the sorption and environmental fate of pesticides. In this study, we investigated the sorption and dissipation of a neonicotinoid insecticide acetamiprid in three typical Chinese agricultural soils, which were amended by a red gum wood (Eucalyptus spp.) derived biochar. Our results showed that the amendment of biochar (0.5% (w/w)) to the soils could significantly increase the sorption of acetamiprid, but the magnitudes of enhancement were varied. Contributions of 0.5% newly-added biochar to the overall sorption of acetamiprid were 52.3%, 27.4% and 11.6% for red soil, paddy soil and black soil, respectively. The dissipation of acetamiprid in soils amended with biochar was retarded compared to that in soils without biochar amendment. Similar to the sorption experiment, in soil with higher content of organic matter, the retardation of biochar on the dissipation of acetamiprid was lower than that with lower content of organic matter. The different effects of biochar in agricultural soils may attribute to the interaction of soil components with biochar, which would block the pore or compete for binding site of biochar. Aging effect of biochar application in agricultural soils and field experiments need to be further investigated.

Efficient transformation of DDTs with Persulfate Activation by Zero-valent Iron Nanoparticles: A Mechanistic Study

In this study, persulfate (PS) activation by nano-Fe(0) was used to degrade dichlorodiphenyltrichloroethane (DDT), and the mechanism of this process was elucidated with EPR, GC-MS and free-radical quenching studies. It was found that DDT was degraded efficiently in PS/nano-Fe(0), and GC-MS analysis showed that benzoic acid, benzyl alcohol, dichlorobenzophenone and 2,2-bis(p-chlorophenyl)-ethane were the dominant products of DDT degradation, while only dechlorination products (DDD and DDE) were observed in nano-Fe(0) without persulfate. EPR results showed that persulfate activation by nano-Fe(0) led to the production of more sulfate radicals and hydroxyl radicals, which accounted for DDT degradation. But the free radical quenching studies suggested that the addition of ethanol to PS/nano-Fe(0) favored the reductive dechlorination of DDT, which was ascribed that the formed ethanol radical (CH(CH3)OH) enhanced the reductive dechlorination of DDT. Furthermore, the nano-Fe(0) loading not only affected the degradation efficiency of DDT, but also influenced the intermediate product distribution of DDT degradation in the PS/nano-Fe(0) process.

Rapid and extensive debromination of decabromodiphenyl ether by smectite clay-templated subnanoscale zero-valent iron.

Subnanoscale zerovalent iron (ZVI) synthesized using smectite clay as a template was utilized to investigate reduction of decabromodiphenyl ether (DBDE). The results revealed that DBDE was rapidly debrominated by the prepared smectite-templated ZVI with a reaction rate 10 times greater than that by conventionally prepared nanoscale ZVI. This enhanced reduction is plausibly attributed to the smaller-sized smectite-templated ZVI clusters (∼0.5 nm) vs that of the conventional nanoscale ZVI (∼40 nm). The degradation of DBDE occurred in a stepwise debromination manner. Pentabromodiphenyl ethers were the terminal products in an alkaline suspension (pH 9.6) of smectite-templated ZVI, whereas di-, tri-, and tetrabromodiphenyl ethers formed at the neutral pH. The presence of tetrahydrofuran (THF) as a cosolvent at large volume fractions (e.g., >70%) in water reduced the debromination rates due to enhanced aggregation of clay particles and/or diminished adsorption of DBDE to smectite surfaces. Modification of clay surfaces with tetramethylammonium (TMA) attenuated the colsovent effect on the aggregation of clay particles, resulting in enhanced debromination rates. Smectite clay provides an ideal template to form subnanoscale ZVI, which demonstrated superior debromination reactivity with DBDE compared with other known forms of ZVIs. The ability to modify the nature of smectite clay surface by cation exchange reaction utilizing organic cations can be harnessed to create surface properties compatible with various contaminated sites.

Kinetics, intermediates and acute toxicity of arsanilic acid photolysis.

Arsanilic acid (4-amino phenyl arsenic acid, ASA) is widely used in poultry production as feed additives, while most of ASA in the feed is excreted in the animal manure and released into the environment. However, the environmental behaviors of ASA were not well understood. In the present study, the photolysis behaviors of ASA and the toxicity of its metabolites to luminescent bacterium were studied. The results showed that ASA could be photodegraded and this process was strongly affected by solution pH, humic acid and dissolved oxygen. Upon UV irradiation for 360 min, ASA could be completely eliminated, but the reduction of total organic carbon (TOC) was not significant. In addition, NH4(+) ions and inorganic arsenic including arsenite and arsenate were identified as the predominant end-products. The conversion of ASA included both direct and indirect photolysis involving radicals, and its possible photolysis pathways were proposed on the basis of the identified intermediates. Unfortunately, higher adverse effects of the conversion products of ASA on bacteria were observed during the photolysis reaction. The results of present study might be helpful for assessing the environmental persistence and risks of ASA.

Probing the microscopic hydrophobicity of smectite surfaces. A vibrational spectroscopic study of dibenzo-p-dioxin sorption to smectite

The interaction of dibenzo-p-dioxin (DD), from aqueous suspension, with smectite was investigated using in situ vibrational spectroscopy (FTIR and Raman), structural and batch sorption techniques. Batch sorption isotherms were integrated with in situ attenuated total reflectance (ATR)-FTIR and Raman spectroscopy and X-ray diffraction. Sorption isotherms revealed that the affinity of DD for smectite in aqueous suspension was strongly influenced both by the type of smectite and by the nature of the exchangeable cation. Cs-saponite showed a much higher affinity over Rb-, K- and Na-exchange saponites. In addition, DD sorption was found to depend on clay type with DD showing a high affinity for the tetrahedrally substituted trioctahedral saponite over SWy-2 and Upton montmorillonites. A structural model is introduced to account for the influence of clay type. Raman and FTIR data provided complementary molecular-level insight into the sorption mechanisms. In the case of Cs-saponite, the selection rules of DD based on D(2h) symmetry were broken indicating a site-specific interaction between DD and intercalated Cs(+) ions in the interlayer of the clay. Polarized in situ ATR-FTIR spectra revealed that the molecular plane of sorbed DD was tilted with respect to the clay surface which was consistent with a d-spacing of 1.49 nm. Finally, cation-induced changes in both the skeletal ring vibrations and the asymmetric C-O-C stretching vibrations provided evidence for site specific interactions between the DD and exchangeable cations in the clay interlayer. Together, the combined macroscopic and spectroscopic data show a surprising link between a hydrophilic material and a planar hydrophobic aromatic hydrocarbon.

Effect of iron oxide reductive dissolution on the transformation and immobilization of arsenic in soils: New insights from X-ray photoelectron and X-ray absorption spectroscopy

The geochemical behavior and speciation of arsenic (As) in paddy soils is strongly controlled by soil redox conditions and the sequestration by soil iron oxyhydroxides. Hence, the effects of iron oxide reductive dissolution on the adsorption, transformation and precipitation of As(III) and As(V) in soils were investigated using batch experiments and synchrotron based techniques to gain a deeper understanding at both macroscopic and microscopic scales. The results of batch sorption experiments revealed that the sorption capacity of As(V) on anoxic soil was much higher than that on control soil. Synchrotron based X-ray fluorescence (μ-XRF) mapping studies indicated that As was heterogeneously distributed and was mainly associated with iron in the soil. X-ray absorption near edge structure (XANES), micro-X-ray absorption near edge structure (μ-XANES) and X-ray photoelectron spectroscopy (XPS) analyses revealed that the primary speciation of As in the soil is As(V). These results further suggested that, when As(V) was introduced into the anoxic soil, the rapid coprecipitation of As(V) with ferric/ferrous ion prevented its reduction to As(III), and was the main mechanism controlling the immobilization of As. This research could improve the current understanding of soil As chemistry in paddy and wetland soils.