Yanbiao Liu

A graphene-based electrochemical filter for water purification

A novel graphene-based electrochemical filter with carbon nanotubes as conductive binders was developed for water purification. Ferrocyanide (Fe(CN)64−) was used as a model compound to study electron transfer mechanisms in the electrochemical filter. A 70% : 30% ratio of graphene : carbon nanotube was optimal for electrochemical oxidation of Fe(CN)64−, and electrooxidation rates increased linearly with increasing concentration of influent Fe(CN)64−. The results of chronoamperometry and normal pulse voltammetry indicated that mass transfer increased up to 15-fold in the electrochemical filter as compared to a batch electrooxidation system. Finally, the efficiency of graphene-based filters for electrooxidation of organic pollutants was evaluated with three selected organic compounds. The oxidation rates increased with increasing anode potential and reached maximum removal rates of 0.010 mol h−1 m−2 (88% removal), 0.064 mol h−1 m−2 (93% removal), and 0.014 mol h−1 m−2 (87% removal) at an applied anode potential of 0.8 V (vs. Ag/AgCl) for tetracycline, phenol, and oxalate, respectively. Overall, the results exemplified the advantages of contaminant removal using a graphene electrode in a flow-through system and demonstrated the potential of using graphene-based electrochemical filters for water purification.

Nitrogen-doped graphene nanosheets as reactive water purification membranes

Oxidation of organic pollutants by sulfate radicals produced via activation of persulfate has emerged as a promising advanced oxidation technology to address various challenging environmental issues. The development of an effective, environmentally-friendly, metal-free catalyst is the key to this technology. Additionally, a supported catalyst design is more advantageous than conventional suspended powder catalysts from the point of view of mass transfer and practical engineering applications (e.g. post-use separation). In this study, a metal-free N-doped reduced graphene oxide (N-rGO) catalyst was prepared via a facile hydrothermal method. N-rGO filters were then synthesized by facile vacuum filtration, such that water can flow through nanochannels within the filters. Various advanced characterization techniques were employed to obtain structural and compositional information of the as-synthesized N-rGO filters. An optimized phenol oxidative flux of 0.036 ± 0.002 mmol·h–1 was obtained by metal-free catalytic activation of persulfate at an influent persulfate concentration of 1.0 mmol·L–1 and filter weight of 15 mg, while a N-free rGO filter demonstrated negligible phenol oxidation capability under similar conditions. Compared to a conventional batch system, the flow-through design demonstrates obviously enhanced oxidation kinetics (0.036 vs. 0.010 mmol·h–1), mainly due to the liquid flow through the filter leading to convection-enhanced transfer of the target molecule to the filter active sites. Overall, the results exemplified the advantages of organic compound removal by catalytic activation of persulfate using a metal-free catalyst in flowthrough mode, and demonstrated the potential of N-rGO filters for practical environmental applications.

Rapid adsorption removal of arsenate by hydrous cerium oxide–graphene composite

Arsenic contamination has posed a health risk to millions of people around the world. In this study, a novel adsorbent, hydrous cerium oxide modified graphene (GNP-HCO), was synthesized for arsenic removal from aqueous solution. In the kinetics study, >88% of the equilibrium adsorption capacity of arsenate (As(V)) can be achieved within the initial 20 min. Such a rapid adsorption rate showed its promising potential towards actual application. The experimental data was better described by the Langmuir isotherm model, and the maximum adsorption capacities were 62.33 and 41.31 mg-As g−1 at pH 4.0 and 7.0, respectively, which are much higher than many modified carbon-based adsorbents previously reported. Phosphate appeared to be the most severe competitive interferent on arsenic adsorption. Furthermore, the adsorptive removal of arsenic from surface water matrix was also evaluated and the results demonstrated that only 15 mg L−1 adsorbent was required to reduce the arsenic concentration from 100 μg L−1 to <10 μg L−1. X-ray photoelectron spectroscopy (XPS) analysis indicated that the major chemical state of cerium (Ce) element in the adsorbent was +IV and the hydroxyl group might be involved in the adsorption process.

Electrochemical wastewater treatment with carbon nanotube filters coupled with in situ generated H2O2

Electrochemically active carbon nanotube (CNT) filters can effectively adsorb and oxidize chemical compounds in the anode, but the role of a cathode in electrochemical filters beyond a counter electrode has not been thoroughly investigated. In this study, a novel wastewater treatment system was developed to combine both adsorption and oxidation in the CNT anode and additional oxidation with in situ generated hydrogen peroxide (H2O2) in the CNT cathode. The impacting factors, treatment efficiency, and oxidation mechanism of the system were systematically studied. The results demonstrated that H2O2 flux could be affected by the electrode material, cathode potential, pH, flow rate, and dissolved oxygen (DO). The maximum H2O2 flux of 1.38 mol L−1 m−2 was achieved with C-grade CNT at an applied cathode potential of −0.4 V (vs. Ag/AgCl), a pH of 6.46, a flow rate of 1.5 mL min−1, and an influent DO flux of 1.95 mol L−1 m−2. Additionally, phenol was used as a model aromatic compound to evaluate the removal efficiency of the system and its oxidation rate was directly correlated with H2O2 flux. H2O2 was likely reacting with a phenol species that was anodically activated to a radical form, since H2O2 alone cannot remove phenol efficiently. Furthermore, electrochemical polymer formation via phenolic radical chain reactions may also contribute to 13% of phenol removal. A stable phenol removal efficiency of 87.0 ± 1.8% within 4 h of continuous operation was achieved with an average oxidation rate of 0.059 ± 0.001 mol h−1 m−2. The developed electrochemical CNT filtration system coupled with in situ generated H2O2 is a new application of carbon nanotube filters and can be used as an effective wastewater treatment system to remove organic pollutants or as a promising point-of-use wastewater treatment system.

Development of electro-active forward osmosis membranes to remove phenolic compounds and reject salts

Forward osmosis (FO) is a promising membrane technology with good salt selectivity and high water permeability. However, it shows limited rejection of certain small organic molecules such as phenolic compounds. Here we developed a composite membrane resulting from the integration of FO with an electro-oxidation process to achieve both effective removal of phenolic compounds (>92% at 2.5 V) and good salt rejection (>98% for Na2SO4). In addition, this composite membrane can be readily integrated into the currently used membrane system. The design presented in this study could provide an advanced solution to the critical demand for water and be used in the next generation of water treatment technologies.

Anaerobic biodegradation and decolorization of a refractory acid dye by a forward osmosis membrane bioreactor

In this study, the feasibility of utilizing an anaerobic osmotic membrane bioreactor (OMBR) for the treatment of a refractory acid dye, Lanaset red G.GR, is demonstrated. The experimental results show that an increased sludge concentration and reversed salt accumulation exacerbate membrane fouling, which leads to flux decline. The excellent rejection performance of the forward osmosis (FO) membrane and salt accumulation could lead to a reduction of microbial activity and an increase in soluble microbial product (SMP) and extracellular polymeric substance (EPS) contents. These consequences will affect the OMBR performance. Moreover, the FO membrane demonstrated a limited rejection of aniline-type intermediates. These overall findings suggest that the OMBR process is a good option for the treatment of dyeing wastewater. Further improvements on the membrane materials and membrane surface properties to alleviate fouling, salt reverse osmosis as well as the remaining color issues are still necessary before practical application becomes possible.

Correlating microbial community structure with operational conditions in biological aerated filter reactor for efficient nitrogen removal of municipal wastewater.

In this study, the combination of strengthen circulation anaerobic (SCA) and biological aerated filter (BAF) reactor was employed to treat municipal wastewater. Different reflux percentages or gas/water ratios were selected for evaluating the removal performance of contaminants in SCA-BAF system and sequential nitrification and denitrification process in BAF reactor. In general, reflux percentage (200%) and gas/water ratio (3:1) were a relatively suitable operational condition for BAF reactor. The COD, NH3-N, TN concentrations of effluents collected from BAF reactor varied in the ranges of 18-80, 0.2-7.2, 9.1-33.0 mg L-1, respectively. A higher NO3-N concentration in effluents of BAF reactor resulted from the lack of organic carbon resource in wastewater. High throughput sequencing analysis indicated that different nitrification and denitrification bacteria thrived in the BAF reactor. The DO, NO2-N and NO3-N concentrations showed a strong correlation with Nitrospira and Nitrosomonas in bacterial samples outlet (c and e) under gas/water ratio of 3:1.

Ligand-Free Nano-Au Catalysts on Nitrogen-Doped Graphene Filter for Continuous Flow Catalysis

In this study, the authors rationally designed a high-performance catalytic filter for continuous flow catalysis. The catalytic filter consisted of ligand-free nanoscale gold (nano-Au) catalysts and nitrogen-doped graphene (N-rGO). The Au catalyst was fabricated in situ onto a pre-formed N-rGO support by the NaBH4 reduction of the Au precursor, and the size of the nano-Au was fine-tuned. A hydrothermal pretreatment of graphene oxide enriched nitrogen-containing species on the surface of two-dimensional graphene supports and enhanced the affinity of Au precursors onto the support via electrocatalytic attraction. The nano-Au catalysts acted as high-performance catalysts, and the N-rGO acted as ideal filter materials to anchor the catalysts. The catalytic activity of the as-designed catalytic filter was evaluated using 4-nitrophenol (4-NP) hydrogenation as a model catalytic reaction. The catalytic filters demonstrated superior catalytic activity and excellent stability, where a complete 4-nitrophenol conversion was readily achieved via a single pass through the catalytic filter. The as-fabricated catalytic filter outperformed the conventional batch reactors due to evidently improved mass transport. Some key operational parameters impacting the catalytic performance were identified and optimized. A similar catalytic performance was also observed for three 4-nitrophenol spiked real water samples (e.g., surface water, tap water, and industrial dyeing wastewater). The excellent catalytic activity of the nano-Au catalysts combined with the two-dimensional and mechanically stable graphene allowed for the rational design of various continuous flow catalytic membranes for potential industrial applications.

Treatment of Typical Organic Pollutants in Textile Wastewater by Direct Contact Membrane Distillation

Direct contact membrane distillation technology was considered as a promising and efficient technology for the treatment of textile wastewater. In this study, hydrophobic polytetrafluoroethylene and polyvinylidene fluoride membranes for the treatment of selected model compounds of textile wastewater (e.g., phenol, aniline, and sulfanilic acid) were explored comparatively in a bench-scale direct contact membrane distillation technology test unit. The effect of various operational parameters including temperature, flow rate, and concentration on the rejection performance was investigated systematically. The results indicated that an increased feed temperature and a faster cross flow velocity contributed positively to the direct contact membrane distillation performance. Limited rejection for phenol and aniline was witnessed, which can be due to their relatively lower boiling point. A > 99% of sulfanilic acid rejection was obtained under the same conditions. Furthermore, the polytetrafluoroethylene membrane always presented enhanced performance compared with the polyvinylidene fluoride samples. In brief, the direct contact membrane distillation process could be potentially used as a promising technique for the treatment of textile wastewater.

Treatment of industrial dyeing wastewater with a pilot-scale strengthened circulation anaerobic reactor

We developed a pilot-scale strengthened circulation anaerobic (SCA) reactor (with an effective volume of 27 m3) and applied to the treatment of industrial textile wastewater. The treatment performance and the working mechanism were studied systematically and the key operational parameters were identified. The results demonstrated that a stable and excellent chemical oxygen demand removal efficiency of 62.7% and a maximum chromaticity removal efficiency of 73.5% were obtained at an optimal reflux ratio of 4. Interestingly, the bio-degradability was evidently improved after the SCA reactor treatment. The high throughput sequencing analysis indicated that the diversity of the bacteria or archaebacteria before the treatment was slightly higher than that after the treatment, which may be attributed to the production of certain toxic intermediates and/or characteristic pollutants during the treatment. Enzyme activity test and COD removal show that numerous microorganisms still maintained active in the anaerobic granular sludge even in a severe environment.