Hongna Li

Destination of organic pollutants during electrochemical oxidation of biologically-pretreated dye wastewater using boron-doped diamond anode.

Electrochemical oxidation of biologically-pretreated dye wastewater was performed in a boron-doped diamond (BDD) anode system. After electrolysis of 12h, the COD was decreased from 532 to 99 mg L(-1) (<100 mg L(-1), the National Discharge Standard of China). More importantly, the destination of organic pollutants during electrochemical oxidation process was carefully investigated by molecular weight distribution measurement, resin fractionation, ultraviolet-visible spectroscopy, HPLC and GC-MS analysis, and toxicity test. As results, most organic pollutants were completely removed by electrochemical oxidation and the rest was primarily degraded to simpler compounds (e.g., carboxylic acids and short-chain alkanes) with less toxicity, which demonstrated that electrochemical oxidation of biologically-pretreated dye wastewater with BDD anode was very effective and safe. Especially, the performance of BDD anode system in degradation of large molecular organics such as humic substances makes it very promising in practical applications as an advanced treatment of biologically-pretreated wastewaters.

Scale-up of BDD anode system for electrochemical oxidation of phenol simulated wastewater in continuous mode

Scale-up of boron-doped diamond (BDD) anode system is significant to the practical application of electrochemical oxidation in bio-refractory wastewater treatment. In this study, the performance of a smaller BDD anode (24 cm(2)) system in continuous mode electrochemical oxidation of phenol simulated wastewater was first investigated and well described by the response surface methodology (RSM). Furthermore, the RSM was extended to examine the scale-up feasibility of BDD anode systems with similar configurations. It was demonstrated that both COD degradation efficiency and specific energy consumption could be expected at the same level even as the system was enlarged over 100 times, which implied that BDD anode system could be successfully scaled up through controlling the same retention time, current density, initial COD, and conductivity conditions. Based on this study, a larger BDD anode (2904 cm(2)) system was constructed and systematic measurements were made on its performance in electrochemical oxidation of phenol simulated wastewater. Very good agreement was found between measured and predicted results by RSM. At the optimized conditions, the larger BDD anode system could easily reduce the COD of phenol simulated wastewater from 633 mg L(-1) to 145 mg L(-1) (<150 mg L(-1), National Discharge Standard of China) during 80 min with specific energy consumption only 31 kWh kgCOD(-1).

Comparative electrochemical degradation of phthalic acid esters using boron-doped diamond and Pt anodes.

Phthalic acid esters (PAEs) are a group of endocrine disruptors commonly used as plasticizers. The present study compares the electrochemical oxidation of PAEs at boron-doped diamond (BDD) anode with that at Pt anode. Both the degradation and the mineralization processes of PAEs became much slower when using the Pt anode compared with that using the BDD anode. Moreover, the degradation rates of PAEs decreased at the BDD anode but increased at the Pt anode with increasing alkyl chain length. This was attributed to the different oxidation mechanisms at the two anodes. The BDD electrode has an inert surface that holds a large amount of strong oxidants as free hydroxyl radicals ((*)OH), causing electrophilic attack by (*)OH to be the main reaction. Therefore, degradation of PAEs became slower on the BDD anode due to there being less available electronic energy as the alkyl chain length increased. However, adsorbed oxidants (PtO(x)(+1)) with low oxidation ability tended to form on the surface of the active Pt anode. Therefore, PAEs with longer alkyl chains promote faster degradation owing to their stronger hydrophobicity. Detection of intermediates in the GC/MS tests confirmed the above conclusion, in which oxidation of dimethyl phthalate on BDD occurred on the aromatic ring at first, while the alkyl chain was preferentially attacked on the Pt anode.

Electrochemical oxidation of nitrogen-heterocyclic compounds at boron-doped diamond electrode.

Nitrogen-heterocyclic compounds (NHCs) are toxic and bio-refractory contaminants widely spread in environment. This study investigated electrochemical degradation of NHCs at boron-doped diamond (BDD) anode with particular attention to the effect of different number and position of nitrogen atoms in molecular structure. Five classical NHCs with similar structures including indole (ID), quinoline (QL), isoquinoline (IQL), benzotriazole (BT) and benzimidazole (BM) were selected as the target compounds. Results of bulk electrolysis showed that degradation of all NHCs was fit to a pseudo first-order equation. The five compounds were degraded with the following sequence: ID>QL>IQL>BT>BM in terms of their rates of oxidation. Quantum chemical calculation was combined with experimental results to describe the degradation character of NHCs at BDD anode. A linear relationship between degradation rate and delocalization energy was observed, which demonstrated that electronic charge was redistributed through the conjugation system and accumulated at the active sites under the attack of hydroxyl radicals produced at BDD anode. Moreover, atom charge was calculated by semi empirical PM3 method and active sites of NHCs were identified respectively. Analysis of intermediates by GC-MS showed agreement with calculation results.

Effect of nitro substituent on electrochemical oxidation of phenols at boron-doped diamond anodes

In order to investigate nitro-substitutents effect on degradation of phenols at boron-doped diamond (BDD) anodes, cyclic voltammetries of three nitrophenol isomers: 2-nitrophenol (2NP), 3-nitrophenol (3NP) and 4-nitrophenol (4NP) were studied, and their bulk electrolysis results were compared with phenols (Ph) under alkaline condition. The voltammetric study showed nitrophenols could be attacked by hydroxyl radicals and nitro-group was released from the aromatic ring. Results of bulk electrolysis showed degradation of all phenols were fit to a pseudo first-order equation and followed in this order: 2NP>4NP>3NP>Ph. Molecular structures, especially carbon atom charge, significantly influenced the electrochemical oxidation of these isomers. Intermediates were analyzed during the electrolysis process, and were mainly catechol, resorcinol, hydroquinone, and carboxylic acids, such as acetic acid and oxalic acid. A simple degradation pathway was proposed. Moreover, a linear increasing relationship between degradation rates and Hammett constants of the studied phenols was observed, which demonstrated that electrochemical oxidation of these phenols was mainly initiated by electrophilic attack of hydroxyl radicals at BDD anodes.

Evolution of microbial communities during electrokinetic treatment of antibiotic-polluted soil

The evolution of microbial communities during the electrokinetic treatment of antibiotic-polluted soil (EKA) was investigated with chlortetracycline (CTC), oxytetracycline (OTC) and tetracycline (TC) as template antibiotics. The total population of soil microorganisms was less affected during the electrokinetic process, while living anti-CTC, anti-OTC, anti-TC and anti-MIX bacteria were inactivated by 10.48%, 31.37%, 34.76%, and 22.08%, respectively, during the 7-day treatment compared with antibiotic-polluted soil without an electric field (NOE). Accordingly, samples with NOE treatment showed a higher Shannon index than those with EKA treatment, indicating a reduction of the microbial community diversity after electrokinetic processes. The major taxonomic phyla found in the samples of EKA and NOE treatment were Proteobacteria, Bacteroidetes, Firmicutes and Actinobacteria. And the distribution of Actinobacteria, Cyanobacteria, and Chloroflexi was greatly decreased compared with blank soil. In the phylum Proteobacteria, the abundance of Alphaproteobacteria was greatly reduced in the soils supplemented with antibiotics (from 13.40% in blank soil to 6.43-10.16% after treatment); while Betaproteobacteria and Deltaproteobacteria showed a different trend with their abundance increased compared to blank soil, and Gammaproteobacteria remained unchanged for all treatments (2.36-2.78%). The varied trends for different classes indicated that the major bacterial groups changed with the treatments due to their different adaptability to the antibiotics as well as to the electric field. SulI being an exception, the reduction ratio of the observed antibiotic resistance genes (ARGs) including tetC, tetG, tetW, tetM, intI1, and sulII in the 0-2cm soil sampled with EKA versus NOE treatment reached 55.17%, 3.59%, 99.26%, 89.51%, 30.40%, and 27.92%, respectively. Finally, correlation analysis was conducted between antibiotic-resistant bacteria, ARGs and taxonomic bacterial classes. It was found that sulII was the most representative of many different bacteria among the seven ARGs studied. This is the first report on the changes in microbial communities before and after EKA, and the present results demonstrated that the application of EKA is a useful and effective approach to suppressing both antibiotic resistant microorganisms and ARGs.

Factors influencing the removal of antibiotic-resistant bacteria and antibiotic resistance genes by the electrokinetic treatment

The performance of the electrokinetic remediation process on the removal of antibiotic-resistant bacteria (ARB) and antibiotic resistance genes (ARGs) was evaluated with different influencing factors. With chlortetracycline (CTC), oxytetracycline (OTC), and tetracycline (TC) as template chemicals, the removal of both ARB and ARGs was enhanced with the increase of voltage gradient (0.4-1.2 V cm-1) and prolonged reaction time (3-14 d). The greatest removal (26.01-31.48% for ARB, 37.93-83.10% for ARGs) was obtained applying a voltage of 1.2 V cm-1, leading to the highest electrical consumption. The effect of polarity reversal intervals on the inactivation ratio of ARB followed the order of 0 h (66.06-80.00%) > 12 h (17.07-24.75%) > 24 h (10.44-13.93%). Lower pH, higher current density, and more evenly-distributed voltage drop was observed with a polarity reversal interval of 12 h compared with that of 24 h, leading to more efficient electrochemical reactions in soil. Compared with sul genes, tet genes were more vulnerable to be attacked in an electric field. It was mainly attributed to the lower abundance of tet genes (except tetM) and the varied effects of electrokinetic remediation process on different ARGs. Moreover, a relatively less removal ratio of tetC and tetG was obtained mainly due to the mechanism of the efflux pump upregulation. Both tet and sul genes were positively correlated with TC-resistant bacteria. The efflux pump genes like tetG and the cellular protection genes like tetM showed different correlations with ARB. This study enhances the current understanding on the removal strategies of ARB and ARGs, and it provides important parameters for their destruction by the electrokinetic treatment.