The toxic and recalcitrant polychlorinated biphenyls (PCBs) adversely affect human and biota by bioaccumulation and biomagnification through food chain. In this study, an anaerobic microcosm was developed to extensively dechlorinate hexa- and hepta-CBs in Aroclor 1260. After 4 months of incubation in defined mineral salts medium amended PCBs (70 mmol·L–1) and lactate (10 mmol·L–1), the culture dechlorinated hexa-CBs from 40.2% to 8.7% and hepta-CBs 33.6% to 11.6%, with dechlorination efficiencies of 78.3% and 65.5%, respectively (all in moL ratio). This dechlorination process led to tetra-CBs (46.4%) as the predominant dechlorination products, followed by penta-(22.1%) and tri-CBs (5.4%). The number of meta chlorines per biphenyl decreased from 2.50 to 1.41. Results of quantitative real-time PCR show that Dehalococcoides cells increased from 2.39 ×105±0.5 × 105 to 4.99 × 107±0.32 × 107 copies mL–1 after 120 days of incubation, suggesting that Dehalococcoides play a major role in reductive dechlorination of PCBs. This study could prove the feasibility of anaerobic reductive culture enrichment for the dehalogenation of highly chlorinated PCBs, which is prior to be applied for in situ bioremediation of notorious halogenated compounds.

Cathodic denitrification using a bioelectrochemical system removes nitrogen at a low C/N ratio, and also harvests energy as electricity. Denitrifying biocathodes were cultured using three electrode systems with nitrate (NO3−) and/or nitrite (NO2−) as electron acceptors. Results showed that denitrification of NOx− in biocathodes exhibit typical enzymatic reaction kinetics and denitrification rate follows the Monod equation, with rmax = 1.33 kg N m−3 d−1, Ks = 5.52 g L−1 for NO3− and rmax = 1.76 kg N m−3 d−1, Ks = 8.09 g L−1 for NO2−, respectively. Optimal cathodic efficiency was obtained at an initial substrate concentration of 0.5 g L−1. A high-throughput sequencing analysis of 16S rRNA gene showed high biodiversity in a denitrifying biocathode and nitrite contributed more to the formation of cathodic microbial community structure. Denitrification functional gene analysis revealed Pseudomonas are effective denitrifiers in a biocathode.

Phosphate is generally used as an effective electrolyte buffer in bioelectrochemical systems, but it is not sustainable. Bicarbonate, if it can replace phosphate as an alternative buffer, may improve the overall economic feasibility of microbial electrolysis cells (MEC) for its application in wastewater treatment and H2 production. In this study, the performance of single-chamber MECs with combo buffers with different PO43− to HCO3− (P/C) ratios was investigated. The results demonstrate that large (about 80%) but not complete replacement of PO43− with HCO3− is feasible, indicating phosphate is necessary to maintain the electric current and the stability of MECs. The current density of MEC with P/C at 20/80 (in mol%) was comparable to that with a full phosphate buffer, and can be kept stable for as long as 1800 h, under 0.5 to 0.9 V applied voltages. Analysis by using molecular approaches, including denaturing gradient gel electrophoresis and DNA sequencing, shows that P/C ratios affect the microbial community structure of the bioanode biofilm, especially on the population of Geobacter, which is a predominant and key exoelectrogenic bacteria to produce an electric current in MEC. The results of this study will broaden the knowledge of the buffer effect and promote the potential applicability of MECs for H2 production.

•Bio-H2 yield from corn stalk was enhanced by integrating dark fermentation and MECs.•Max. H2 yield of 387.1 mL/g-corn stalk was recorded by two-stage process.•H2 recovery and production rate reached 257.3 ml/g-straw and 3.43 ± 0.12 m3/m3 d in MECs.•Energy efficiency and effluent COD removal reached 166 ± 10% and 44 ± 2% in MECs.•MECs played a critical role in generating additional H2 and effluent COD removal.The low conversion efficiency of substrate is one of the main bottlenecks in dark fermentation for bio-H2 production. Herein, an enhanced H2 yield from corn stalk was achieved by integrating dark fermentation and single chamber microbial electrolysis cells (MECs). In the dark fermentation stage, a H2 yield of 129.8 mL H2/g-corn stalk and an average H2 production rate of 1.73 m3/m3 d were recorded at 20 g/L of corn stalk and initial pH 7.0. The effluent from dark fermentation was diluted and further employed as feedstock to generate H2 by MECs. A H2 yield of 257.3 mL H2/g-corn stalk, an HPR of 3.43 ± 0.12 m3/m3 d and an energy efficiency of 166 ± 10% were obtained with the effluent COD of 3995.5 mg/L under 0.8 V applied voltage. During MECs operation stage, about 90 ± 2% of acetate was converted to H2 and the corresponding COD removal reached 44 ± 2% in MECs. Overall, the H2 yield can reach 387.1 mL H2/g-corn stalk by integrating dark fermentation and MECs, which had nearly tripled as against that of dark fermentation.

A biocathode with microbial catalyst in place of a noble metal was successfully developed for hydrogen evolution in a microbial electrolysis cell (MEC). The strategy for fast biocathode cultivation was demonstrated. An exoelectrogenic reaction was initially extended with an H2-full atmosphere to enrich H2-utilizing bacteria in a MEC bioanode. This bioanode was then inversely polarized with an applied voltage in a half-cell to enrich the hydrogen-evolving biocathode. The electrocatalytic hydrogen evolution reaction (HER) kinetics of the biocathode MEC could be enhanced by increasing the bicarbonate buffer concentration from 0.05 mol·L−1 to 0.5 mol·L−1 and/or by decreasing the cathode potential from − 0.9 V to − 1.3 V vs. a saturated calomel electrode (SCE). Within the tested potential region in this study, the HER rate of the biocathode MEC was primarily influenced by the microbial catalytic capability. In addition, increasing bicarbonate concentration enhances the electric migration rate of proton carriers. As a consequence, more mass H+ can be released to accelerate the biocathode-catalyzed HER rate. A hydrogen production rate of 8.44 m3·m−3·d−1 with a current density of 951.6 A·m−3 was obtained using the biocathode MEC under a cathode potential of − 1.3 V vs. SCE and 0.4 mol·L−1 bicarbonate. This study provided information on the optimization of hydrogen production in biocathode MEC and expanded the practical applications thereof.

High-phosphorus hematite flotation wastewater (HHFW) is difficult to treat, because it possesses the properties of high concentration suspended solid (SS) and organic pollutant, high alkalinity and strong color. This study was conducted to investigate the treatment of HHFW using coagulation/flocculation process with the mixture of ferric chloride (FC) and polyaluminum chloride (PAC) as coagulants and polyacrylamide (PAM) as flocculants. Results show that a hybrid coagulant (B4), the mixture of FC/PAC in the Fe/Al molar ratio of 3:7, was found to be the most efficient in the coagulation process. PAM flocculants, such as cationic (PAM-C), non-ionic (PAM-N) and anionic (PAM-A), will improve the turbidity removal efficiency. The dosage of the coagulant B4 and the PAM flocculants was optimized using two statistic tools, central composite design (CCD) and response surface methodology (RSM). According to the analysis of variances (ANOVA), three quadratic models with high regression coefficient (R2) were obtained. Through RSM optimization, a minimum residual turbidity of 0.8 NTU was achieved when B4 and PAM-N were in the dosage of 356 mg L−1 and 5.4 mg L−1, respectively. The concentration of iron, aluminum and COD in the purified HHFW were 0.055 mg L−1, 0.184 mg L−1 and 33.92 mg L−1, respectively. This study demonstrated that FC/PAC hybrid coagulant, coupling with PAM-N flocculant, has a great potential and economically feasible to the treatment of industrial wastewater containing high turbidity and high alkalinity.Graphical abstractResearch highlights▶ Hybrid coagulants exhibit high charge neutralization and turbidity removal capability. ▶ Hybrid coagulants maintain a neutral final pH after coagulation in a HHFW treatment. ▶ The minimum residual turbidity of 0.8NTU was obtained under the optimized condition.