Complete long-term monitoring of greenhouse gas emissions from a full-scale industrial wastewater treatment plant with different cover configurations.

The greenhouse gas (GHG) emissions from wastewater treatment plants (WWTPs), consisting mainly of methane (CH4) and nitrous oxide (N2O), have been constantly increasing and become a non-negligible contributor towards carbon neutrality. The precise evaluation of plant-specific GHG emissions, however, remains challenging. The current assessment approach is based on the product of influent load and emission factor (EF), of which the latter is quite often a single value with huge uncertainty. In particular, the latest default Tier 1 value of N2O EF, 0.016 ± 0.012 kgN2O-N kgTN-1, is estimated based on the measurement of 30 municipal WWTPs only, without involving any industrial wastewater. Therefore, to resolve the pattern of GHG emissions from industrial WWTPs, this work conducted a 14-month monitoring campaign covering all the process units at a full-scale industrial WWTP in Shanghai, China. The total CH4 and N2O emissions from the whole plant were, on average, 447.7 ± 224.5 kgCO2-eq d-1 and 1605.3 ± 2491.0 kgCO2-eq d-1, respectively, exhibiting a 5.2- or 3.9-times more significant deviation than the influent loads of chemical oxygen demand (COD) or total nitrogen (TN). The resulting EFs, 0.00072 kgCH4 kgCOD-1 and 0.00211 kgN2O-N kgTN-1, were just 0.36% of the IPCC recommended value for CH4, and 13.2% for N2O. Besides, the parallel anoxic-oxic (A/O) lines of this industrial WWTP were covered in two configurations, allowing the comparison of GHG emissions from different odor control setup. Unit-specific analysis showed that the replacement of enclosed A/open O with enclosed A/O reduced the CH4 EF by three times, from 0.00159 to 0.00051 kgCH4 kgCOD-1, and dramatically decreased the N2O EF by an order of magnitude, from 0.00376 to 0.00032 kgN2O-N kgTN-1, which was among the lowest of all full-scale WWTPs.

Membrane fouling performance of Fe-based coagulation-ultrafiltration process: Effect of sedimentation time.

Pre-coagulation is commonly used with ultrafiltration (UF) to alleviate the membrane fouling. Compared to conventional coagulation-sedimentation-UF (CSUF) processes, the direct coagulation-UF (CUF) processes are widely believed to perform better due to the formation of a looser cake layer. It is however shown in this study that not only the density of a cake layer, but also its thickness as well, can affect the membrane fouling behavior, which therefore are influenced by both the sedimentation time and flocs characteristics. Herein, the membrane fouling performance of Fe-based coagulation-UF process was systematically investigated with different sedimentation times. A critical threshold of 30 min was observed at the lab-scale: if shorter than that, the membrane fouling depended mainly on the cake layer density, and thus CUF outperformed CSUF; but when the sedimentation time was over 30 min, the cake layer thickness turned to be the dominant factor, thereby resulting in CSUF performing better. Furthermore, it was shown that the critical sedimentation time was decided by flocs characteristics. A lower water temperature induced the formation of irregular flocs with a lower fractal dimension, and the corresponding cake layer exhibited an almost identical density with increasing sedimentation time. In this regard, CSUF processes were constantly superior to CUF as the cake layer thickness decreased. On the other hand, a critical sedimentation time reappeared because of the higher floc fractal dimension under acidic conditions. This work showed for the first time that the membrane fouling of CSUF was up to the sedimentation time, and it was possible to outperform CUF if the sedimentation time exceeded a critical threshold. Such a finding is crucial to the future development of coagulation integrated UF processes.

Toward Precision Measurement and Manipulation of Single-Molecule Reactions by a Confined Space.

All chemical reactions can be divided into a series of single molecule reactions (SMRs), the elementary steps that involve only isomerization of, dissociation from, and addition to an individual molecule. Analyzing SMRs is of paramount importance to identify the intrinsic molecular mechanism of a complex chemical reaction, which is otherwise implausible to reveal in an ensemble fashion, owing to the significant static and dynamic heterogeneity of real-world chemical systems. The single-molecule measurement and manipulation methods developed recently are playing an increasingly irreplaceable role to detect and recognize short-lived intermediates, visualize their transient existence, and determinate the kinetics and dynamics of single bond breaking and formation. Notably, none of the above SMRs characterizations can be realized without the aid of a confined space. Therefore, this Review aims to highlight the recent progress in the development of confined space enabled single-molecule sensing, imaging, and tuning methods to study chemical reactions. Future prospects of SMRs research are also included, including a push toward the physical limit on transduction of information to signals and vice versa, transmission and recording of signals, computational modeling and simulation, and rational design of a confined space for precise SMRs.

Electrocatalytic Oxidation of Tris(2-carboxyethyl)phosphine at Pyrroloquinoline Quinone Modified Carbon Nanotube through Single Nanoparticle Collision.

Inspired by the addition-elimination catalytic mechanism of natural pyrroloquinoline quinone (PQQ) containing proteins, PQQ-modified hybrid nanomaterials have been increasingly developed recently as biomimetic heterogeneous electrocatalysts. However, up until now, no existing electrochemical approach was able to assess the intrinsic catalytic activity of PQQ sites, impeding the design of efficient PQQ-based electrocatalysts. Herein, in this work, we introduced a new method to calculate the turnover frequency (TOF) of any individual PQQ functional group for electrocatalytic oxidation of tris(2-carboxyethyl)phosphine (TCEP), through the study of single PQQ-decorated carbon nanotube (CNT) collisions at a carbon fiber ultramicroelectrode by chronoamperometry. The core advantage of this approach is being able to resolve the number of PQQ catalytic sites grafted on each individual CNT, so that the charge of any CNT collision event can be accurately translated into the intrinsic activity of the respective PQQ functional groups. The resulting collision-induced current responses clearly showed that the functionalization of CNTs with PQQ could indeed enhance its catalytic performance by 3 times, reaching a TOF value of 133 s-1 at 1.0 V vs Ag/AgCl. Such a single CNT collision technique, which is proposed for the first time in this work, can open up a new avenue for studying the intrinsic (electro)catalytic performance at a molecular level.

Highly efficient photocatalytic H2 evolution from water using visible light and structure-controlled graphitic carbon nitride.

The major challenge of photocatalytic water splitting, the prototypical reaction for the direct production of hydrogen by using solar energy, is to develop low-cost yet highly efficient and stable semiconductor photocatalysts. Herein, an effective strategy for synthesizing extremely active graphitic carbon nitride (g-C3N4) from a low-cost precursor, urea, is reported. The g-C3N4 exhibits an extraordinary hydrogen-evolution rate (ca. 20,000 µmol h(-1) g(-1) under full arc), which leads to a high turnover number (TON) of over 641 after 6 h. The reaction proceeds for more than 30 h without activity loss and results in an internal quantum yield of 26.5% under visible light, which is nearly an order of magnitude higher than that observed for any other existing g-C3N4 photocatalysts. Furthermore, it was found by experimental analysis and DFT calculations that as the degree of polymerization increases and the proton concentration decreases, the hydrogen-evolution rate is significantly enhanced.

Enhanced Photocatalytic Degradation of Perfluorooctanoic Acid by Mesoporous Sb2O3/TiO2 Heterojunctions.

Perfluorooctanoic acid (PFOA), a typical perfluorinated carboxylic acid, is an emerging type of permanent organic pollutants that are regulated by the Stockholm Convention. The degradation of PFOA, however, is quite challenging largely due to the ultra-high stability of C-F bonds. Compared with other techniques, photocatalytic degradation offers the potential advantages of simple operation under mild conditions as well as exceptional decomposition and defluorination efficiency. Titanium dioxide (TiO2) is one of the most frequently used photocatalysts, but so far, the pristine nanosized TiO2 (e.g., the commercial P25) has been considered inefficient for PFOA degradation, since the photo-generated hydroxyl radicals from TiO2 are not able to directly attack C-F bonds. Mesoporous Sb2O3/TiO2 heterojunctions were therefore rationally designed in this work, of which the confined Sb2O3 nanoparticles in mesoporous TiO2 framework could not only tune the band structure and also increase the number of active sites for PFOA degradation. It was found that, after loading Sb2O3, the absorption of UV light was enhanced, indicating a higher efficiency of light utilization; while the band gap was reduced, which accelerated the separation of photo-generated charge carriers; and most importantly, the valence band edge of the Sb2O3/TiO2 heterojunction was significantly lifted so as to prevent the occurrence of hydroxyl radical pathway. Under the optimal ratio of Sb2O3-TiO2, the resulting catalysts managed to remove 81.7% PFOA in 2 h, with a degradation kinetics 4.2 times faster than the commercial P25. Scavenger tests and electron spin resonance spectra further revealed that such improvement was mainly attributed to the formation of superoxide radicals and photo-generated holes, in which the former drove the decarboxylation from C7F15COOH-C7F15 •, and the latter promoted the direct electron transfer for the conversion of C7F15COO--C7F15COO•. The Sb2O3/TiO2 photocatalysts were highly recyclable, with nearly 90% of the initial activity being retained after five consecutive cycles, guaranteeing the feasibility of long-term operation.

Influence of Electrolyte Concentration on Single-Molecule Sensing of Perfluorocarboxylic Acids.

Perfluorocarboxylic acids (PFCAs) are an emerging class of persistent organic pollutants. During the fabrication process, it is unavoidable to form PFCA homologs or isomers which exhibit distinct occurrence, bioaccumulation, and toxicity. The precision measurement of PFCAs is therefore of significant importance. However, the existing characterization techniques, such as LC-MS/MS, cannot fully meet the requirement of isomer-specific analysis, largely due to the lack of authentic standards. Single-molecule sensors (SMSs) based on nanopore electrochemistry may be a feasible solution for PFCAs determination, thanks to their ultra-high spatiotemporal resolutions. Hence, as a first step, this work was to elucidate the influence of electrolyte concentration on the four most critical indicators of nanopore measurements, and furthermore, performance of nanopore SMSs. More specifically, three of the most representative short-chain PFCAs, perfluoropentanoic acid (PFPeA), perfluorohexanoic acid (PFHxA) and perfluoroheptanoic acid (PFHpA), were adopted as the target analytes, aerolysin nanopore was employed as the sensing interface, and 2, 3 and 4 M KCl solutions were used as electrolytes. It was found that, when the concentration of KCl solution increased from 2 to 4 M, the conductance of aerolysin nanopore increased almost linearly at a rate of 0.5 nS per molar KCl within the whole voltage range, the current blockade of PFPeA at -50 mV increased from 61.74 to 66.57% owing to the enhanced steric exclusion effect, the maximum dwell time was more than doubled from 14.5 to 31.5 ms, and the barrier limited capture rate increased by 8.3 times from 0.46 to 3.85 Hz. As a result, when using 4 M KCl as the electrolyte, over 90% of the PFPeA, PFHxA and PFHpA were accurately identified from a mixed sample, and the calculated limit of detection of PFPeA reached 320 nM, more than 24 times lower than in 2 M KCl. It was thus clear that tuning the electrolyte concentration was a simple but very effective approach to improve the performance of nanopore SMSs for PFCAs determination.

Simultaneous Removal of Multiple Heavy Metal Ions from River Water Using Ultrafine Mesoporous Magnetite Nanoparticles.

The exploration of simultaneous removal of co-existing or multiple pollutants from water by the means of nanomaterials paves a new avenue that is free from secondary pollution and inexpensive. In the aquatic environment, river water contains a mixture of ions, which can influence the adsorption process. In this respect, removing heavy metal ions becomes a true challenge. Here, four heavy metal ions, namely, Pb2+, Cd2+, Cu2+, and Ni2+, have been successfully removed simultaneously from river water using ultrafine mesoporous magnetite (Fe3O4) nanoparticles (UFMNPs) based on the affinity of these metal ions toward the UFMNP surfaces as well as their unique mesoporous structure, promoting the easy adsorption. The individual removal efficiencies of Pb2+, Cd2+, Cu2+, and Ni2+ ions from river water were 98, 87, 90, and 78%, respectively, whereas the removal efficiencies of the mixed Pb2+, Cd2+, Cu2+, and Ni2+ ions were 86, 80, 84, and 54%, respectively, in the same river water. Thus, the data clearly indicate the complex removal of heavy metal ions in multi-ion systems. This study has demonstrated the huge potential of UFMNPs to be effective for their use in wastewater treatment, especially to simultaneously remove multiple heavy metal ions from aqueous media.

Single Nanoparticle Electrochemistry.

Experimental techniques to monitor and visualize the behaviors of single nanoparticles have not only revealed the significant spatial and temporal heterogeneity of those individuals, which are hidden in ensemble methods, but more importantly, they have also enabled researchers to elucidate the origin of such heterogeneity. In pursuing the intrinsic structure-function relations of single nanoparticles, the recently developed stochastic collision approach demonstrated some early promise. However, it was later realized that the appropriate sizing of a single nanoparticle by an electrochemical method could be far more challenging than initially expected owing to the dynamic motion of nanoparticles in electrolytes and complex charge-transfer characteristics at electrode surfaces. This clearly indicates a strong necessity to integrate single nanoparticle electrochemistry with high-resolution optical microscopy. Hence, this review aims to give a timely update of the latest progress for both electrochemically sensing and seeing single nanoparticles. A major focus is on collision-based measurements, where nanoparticles or single entities in solution impact on a collector electrode and the electrochemical response is recorded. These measurements are further enhanced with optical measurements in parallel. For completeness, advances in other related methods for single nanoparticle electrochemistry are also included.

Real-time monitoring of electrochemical reactions on single nanoparticles by dark-field and Raman microscopy.

Nanoparticles (NPs) play a central role in a wide range of electrochemical applications. One of the ultimate goals for nano-electrochemistry is to establish the structure-activity relationship (SAR) of NPs, so that they can be rationally designed and synthesized. However, it has remained a critical challenge until now, despite the tremendous efforts that have been made. This is largely because most ensemble characterization methods cannot resolve the significant static and dynamic disorder among the individual NPs and their respective active sites. The recently developed single NP electrochemical methods, including both collision and immobilization, opened up a radically new and effective way to uncover such heterogeneity. More importantly, it has also been increasingly recognized that coupling electrochemistry with operando optical microscopy is of great benefit to elucidate the dynamic SAR as well as the underlying reaction mechanisms. Herein, this frontier article aims to provide a timely update on the recent progress of using dark-field and Raman microscopy to probe the single NP electrochemistry in real time.