Abundant and diverse functional groups of adsorbents are essential for their adsorption performances. Herein, we report a strategy to construct highly efficient ammonia nitrogen adsorbents by installing multiple ion-exchange complexation coordination-hydrogen bonding sites onto covalent organic frameworks (COFs). As a proof of concept, we prepared a COF (TpPa-SO3H) via a modified mechanical grinding synthetic method and then obtained a sulfonated COF bearing single Cu sites (TpPa-SO3Cu0.5) by post-loading. Benefiting from the highly exposed active sites and ordered COF channels, TpPa-SO3Cu0.5 exhibited the highest adsorption kinetics among reported ammonia nitrogen adsorbents proven by the highest pseudo-second-order adsorption rate constant (k2) of 8.97 g mg–1 min–1 with its maximum adsorption capacity (30.45 mg N g–1) higher than most adsorbents (<0.001–0.994 g mg–1 min–1 and 0–25 mg N g–1). Furthermore, TpPa-SO3Cu0.5 exhibited excellent adsorption selectivity with its selective coefficient 328 times as high as that of TpPa-SO3H in real water (10 mg N L–1, pH = 10). It also showed good stability and recyclability with a high ammonia recycle ratio (95.1%) after 5 adsorption–regeneration cycles. These findings pave a new way to develop unique COFs as platforms for ultrafast and selective pollutants in water and wastewater treatment.

Subcritical hydrothermal liquefaction (HTL) has exhibited significant potential in the treatment of antibiotic fermentation residue (AR), but the synchronous recycling of the multiple nutrients in AR by HTL has not been considered previously. A comprehensive understanding of the transformation mechanism of nutrients is the prerequisite of their recycling. Results showed that 85.8–88.9% of phosphorus (P) was enriched in hydrochar by adsorption to Al/Fe (hydro)oxides or precipitation as Ca/Mg minerals, while most nitrogen (N) was released into the liquid phase due to the poor affinity between metal cations and nitrogen species. Moreover, the distribution and evolution of nutrients were controlled by temperature. Increasing temperature promoted the production of the recyclable NH4+–N and apatite-P. Above 220 °C, the increased apatite-P was mainly derived from the transformation of the unstable non-apatite inorganic-P. Further, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) analysis indicated that the dissolved organic-N with N3–4 as the prominent components (180–220 °C), especially proteins, were converted into NH4+–N and stable lignins/carboxyl-rich alicyclic molecule (CRAM)-like compounds via a series of depolymerization, deoxidation, deamination, and cyclization reactions as the temperature increased. Undoubtedly, this work will guide the directional regulation of the distribution and speciation of AR-derived nutrients, and have important implications for optimizing nutrients’ recycling and sustainable biowaste management.

Compared to a conventional membrane bioreactor (MBR) with a porous microfiltration (MF) membrane (MBRMF), a nanofiltration (NF)-based MBR (MBRNF) shows highly attractive features such as high permeate water quality. However, the practical applications of MBRNF are often hindered by the high driven pressures and severe membrane fouling. To address these two critical issues, this study investigated the feasibility of operating MBRNF at an ultralow flux (ULF, e.g., <5 L·m–2·h–1) toward the reuse of municipal wastewater in a single step. We operated the ULF MBRNF at a flux of 2 L·m–2·h–1 and benchmarked its performance against a conventional system using a MBRMF followed by a subsequent NF treatment (MBRMF + NF, both at a constant flux of 20 L·m–2·h–1). The results show that the ULF MBRNF achieved substantial removal of most pollutants, with low negative impacts of ultralow fluxes on pollutant rejections. Besides, the ultralow-flux operation led to a very low fouling rate (0.18 kPa·d–1). More importantly, the ULF MBRNF reduced carbon emissions by 45.2% compared with the MBRMF + NF, mainly due to less energy consumption by pumping. Our findings highlight the simplicity and great potential of ULF MBRNF for wastewater reuse.

Fouled nanofiltration membranes from lead and lag positions of a 2-stage pilot-scale direct potable reuse plant treating secondary municipal wastewater effluent in El Paso, Texas were thoroughly characterized after 9 months of operation to elucidate the role of silicon moieties on fouling. X-ray photoelectron (XPS), energy dispersive X-ray (EDS), and Fourier transform infrared (FTIR) spectroscopies identified silicon oxides/silica as dominant foulants, especially on the lag element. Gas chromatography/mass spectroscopy also identified organosilicon moieties such as linear- and cyclosiloxanes on the lead element. Extensive siliceous fouling was accompanied by calcium, aluminum, and magnesium scaling as well as deposition of bioorganic materials thereby modifying membrane surfaces that resulted in irreversible productivity loss. None of the three selected cleaning agents (sodium dodecyl sulfate and NaOH, EDTA and NaOH, and HCl), either singly or in combination satisfactorily restored water permeability of the membrane. EDTA performed better in the lead element (where bioorganic fouling prevailed), and HCl was more effective in the lag element (where mineral scaling accompanied silicon oxides). This suggests the express need for novel antiscalants to reduce organosilicon/silicon oxide deposition and the necessity of harsher cleaning agents/regimens specifically targeting silicon (e.g., hydrofluoric acid or ammonium bifluoride).

Knowledge of the spatial distribution of heavy metals is indispensable for successful risk analysis of contaminated sites. The common practice is to obtain soil samples for spatial interpolation through site investigation, which generally involves preliminary and detailed surveys. In this study, we propose an information entropy-based site investigation (IESI) method in which an optimal design step is implemented to guide soil sampling at the detailed survey stage. Two types of information entropy (i.e., relative entropy and Shannon entropy) are used to design the optimal sampling strategy. The results show that, within the IESI method, relative entropy is superior to Shannon entropy in guiding soil sampling. Combined with ordinary kriging, the IESI method outperforms conventional surveys for hypothetical and actual heavy metal-contaminated sites as it can identify new polluted and clean areas. For quantitative comparisons, the IESI method coupled with ordinary kriging, logarithmic ordinary kriging, and universal kriging with linear and quadratic trends can improve the interpolation accuracy by 16–43% at the actual heavy metal-contaminated site. Upon further examination of the IESI method, informative sampling points are mainly distributed around the polluted areas identified by the preliminary survey with soil pollution probabilities between 0.75 and 0.95. This work provides an effective tool for delineating the spatial distribution and valuable insights into identifying encryption areas at heavy-metal sites.

Advanced oxidation processes (AOPs) have a broad range of potential applications in the treatment of emerging refractory emerging pollutants. However, due to the presence of highly reactive substances such as free radicals that are difficult to capture, it is challenging to investigate the mechanism of AOPs at the elementary reaction level. The conventional methods, such as electron spin resonance (ESR), free radical quantification, and free radical quenching, are plagued by systematic issues that have led to bottlenecks in the field of AOP studies. The development of computational chemistry theory and computer performance provides a new method to study the mechanism of AOPs through density functional theory (DFT) calculation. Due to its excellent cost–performance benefit, DFT calculations for aperiodic small molecules have become popular in the field of AOPs. In this paper, a comprehensive review is presented on the applications of DFT calculations for predicting active sites and exploring reaction selectivity and oxidant activation mechanisms. A systematic classification of methods related to molecular descriptors and transition states is provided. Furthermore, some current research issues are identified, and future development prospects and challenges are discussed.

Phosphate removal is a crucial objective in wastewater engineering to reduce harmful environmental impacts like eutrophication. Adsorption, a low-cost and efficient process for phosphate abatement, primarily relies on trapping phosphate on low-solubility solid surfaces. Metal-based materials, due to their abundance, low cost, environmental friendliness, and chemical stability, are considered the most promising phosphate adsorbents. However, the synthesis of appropriate adsorbents is complex and time-consuming. In addition, the diverse textural properties, the presence of various metals, and the selection of adsorption parameters make it challenging to the underlying mechanism of phosphate adsorption. In this study, we compiled a data set including 1800 data points mined from 128 peer-reviewed papers and adopted machine learning (ML) to systematically evaluate phosphate adsorption concerning textural properties, metal compositions, and adsorption parameters. We applied three different tree-based algorithms, including random forest (RF), decision trees (DTs), and extreme gradient boosting (XGBoost), to guide the design of adsorbents and predict the phosphate adsorption performances. Among the three algorithms, RF showed the best predictive performance with a high R2 of 0.984 and a low root-mean-squared error (RMSE) of 0.650. Feature importance, based on the Shapley values, demonstrated the contributions of adsorbents’ textural properties (e.g., surface area), adsorption parameters, and metal types in the order of precedence of phosphate adsorption, providing critical insights into guiding adsorbents design and synthesis for phosphate adsorption applications.

The dissociation of H2O molecules plays an important role in producing protons for promoting hydrogenation reduction of CO2. Therefore, developing excellent photocatalysts to promote the dissociation of H2O molecules and reveal the effect mechanism is the key research direction to enhance CO2 reduction efficiency. In this work, we synthesized BiSbO4 with oxygen vacancies (OVs) to investigate the photocatalytic mechanism of CO2 conversion and H2O dissociation. Experimental results indicate that the introduction of OVs into BiSbO4 can enhance the absorption capacity of light, narrowing band gaps and impeding the recombination of photogenerated carriers. Importantly, the existence of OVs in BiSbO4 can promote the adsorption and activation of H2O molecules via the stronger covalent interaction, which facilitates the dissociation of H2O molecules to generate more protons and lowers the energy barriers of •COOH to improve the efficiency of CO2 reduction. This study reinforces our understanding of the principle of photocatalytic CO2 reduction and the important role of H2O dissociation in photocatalytic CO2 conversion.

The destruction of per- and polyfluoroalkyl substances (PFAS) is critical to ensure effective remediation of PFAS contaminated matrices. The destruction of hazardous chemicals within incinerators and other thermal treatment processes has historically been determined by calculating the destruction efficiency (DE) or the destruction and removal efficiency (DRE). While high DEs, >99.99%, are deemed acceptable for most hazardous compounds, many PFAS can be converted to other PFAS at low temperatures resulting in high DEs without full mineralization and the potential release of the remaining fluorocarbon portions to the environment. Many of these products of incomplete combustion (PICs) are greenhouse gases, most have unknown toxicity, and some can react to create new perfluorocarboxylic acids. Experiments using aqueous film forming foam (AFFF) and a pilot-scale research combustor varied the combustion environment to determine if DEs indicate PFAS mineralization. Several operating conditions above 1090 °C resulted in high DEs and few detectable fluorinated PIC emissions. However, several conditions below 1000 °C produced DEs > 99.99% for the quantifiable PFAS and mg/m3 emission concentrations of several nonpolar PFAS PICs. These results suggest that DE alone may not be the best indication of total PFAS destruction, and additional PIC characterization may be warranted.

It has been widely accepted that oxygen vacancies are critical to catalytic oxidation activities. However, the effects of Ce–O bond strength on the formation of oxygen vacancies and the oxidation rate of styrene remain ambiguous. Herein, a series of CeO2 (CeO2-100, CeO2-140, CeO2-180) were synthesized to uncover the effects of Ce–O bond strength on surface chemical properties and unravel the oxidation mechanism of styrene via comprehensive characterization techniques and theoretical calculations. DFT calculations showed a positive correlation between the Ce–O bond strength and the formation of oxygen vacancies. The CeO2-100 catalyst exhibited a lower styrene degradation temperature (T100 = 223 °C) and the lowest apparent activation energy (Ea = 19.12 kJ/mol). This is due to the fact that weakening the Ce–O bond strength would make it easier to generate oxygen vacancies. More oxygen vacancies facilitate the adsorption of styrene and the formation of surface adsorbed oxygen, thereby accelerating styrene oxidation. In situ DRIFTS demonstrated that more oxygen vacancies can accelerate the oxidation of important intermediate products and further promote the deep oxidation of styrene to CO2 and H2O. Furthermore, the CeO2-100 catalyst showed better activity stability at 223 °C and good water resistance in the presence of 10 vol % water.

Remediation of chromium-contaminated groundwater remains a significant environmental challenge around the world. Herein, we synthesized silicified microscale zerovalent iron (Si-mZVIbm) using a mechanochemical method and demonstrated that the silicate modification could significantly improve the Cr(VI) removal efficiency (up to 37.5-fold) compared with its un-silicified counterpart. Results of atomic force microscopy, scanning transmission electron microscopy, and positron annihilation measurements revealed that silicate acted as a milling lubricant to boost strain within zerovalent iron particles, inducing more plastic deformation and surface defects. The defect-rich silicified surface accelerates the electron transfer and subsequent in situ generation of Fe(II). More importantly, the surface-modified silicate can act as a ligand to coordinate leached Fe(II) ions, thus strengthening the reduction of Cr(VI) via surface-bound Fe(II) and favoring subsequent co-precipitation of Cr(III) and Fe(III) species on Si-mZVIbm surfaces. During column experiments using real Cr-contaminated groundwater, Si-mZVIbm (4 wt % in sand) was able to reduce the Cr(VI) concentration from 2 to 0.05 mg L–1, the World Health Organization drinking water standard for up to 1720 bed volumes with an empty-bed contact time of 5.1 min. These results demonstrate the potential field applicability of Si-mZVIbm in real contaminated groundwater remediation.

The anammox-based technologies are generally inhibited by refractory dissolved organic matter (rDOM), which is ubiquitous in real wastewater. In this study, a novel cost-effective approach, namely, zero-valent iron (ZVI) treatment, was presented to alleviate such inhibitory effects. The results showed that ZVI mitigated the inhibition of fulvic acids (FA) to anammox. Due to the 2 g/L ZVI addition, the nitrogen removal efficiency (NRE) increased from 83.53 to 90.06% when the FA concentration increased from 0 to 160 mg/L in wastewater. Additionally, the co-occurrence network linking Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and 16S rRNA sequencing results revealed the mechanism of ZVI mitigating inhibition by promoting the reduction of nitro and nitroso groups of FA and reshaping the metabolic division of functional bacteria. Hydrolytic acidifying bacteria (e.g., Anaerolineaceae and Ignavibacterium) were enriched for FA degradation, which was ultimately beneficial for denitrifying bacteria and anaerobic ammonium oxidation bacteria (AnAOB). Furthermore, metagenomics revealed that ZVI stimulated multiple nitrogen removal formed by anammox, denitrification, and DNRA by accelerating extracellular electron transfer resorting to FA serving as an electron shuttle and upregulating functional genes encoding electron generation, transport, and consumption processes.

The stability and activity of microbial communities directly diagnose the health of leachate biological treatment processes, and unveiling the ecological drivers of microbial assembly could identify the critical factors resulting from the influent disturbances and improve operational performance. Hereby, we conducted a meta-analysis of microbial diversity, ecological mechanisms based on a full-scale working leachate treatment plant (LTP) around one year, and co-occurrence for shaping their communities to screen the critical microbial responses to the variations of influent loadings in practical operation. A significant difference was observed in the treatment process under the low and high-loading periods (p < 0.001) with 0.71 ± 0.09 and 0.95 ± 0.09 mg CODCr/mg MLSS, respectively, with 60.6%, 95.4%, and 64.1% of CODCr, NH4+-N, and TN removal under the low-loading period. The microbial diversity was 6.75 and 5.68 under the low- and high-loading periods in terms of the Shannon index, which was the most limiting factor influencing the operation performance (r = 0.618). The deterministic processes had a dominant role in the microbial assembly during the high-loading period, and they had little effect on the system performance. Low loadings led to over 20% of microorganism species from five bins dominating from homogeneous selection (HoS, deterministic process) to ecological drift (DR, stochastic processes), as the 2456 observed OTUs were divided into 94 phylogenetic bins, which increased microbial stability with a large average clustering coefficient of 0.192 and 15 connectors based on a complex network structure analysis. CODCr loadings rather than nitrogen loadings were critical to affecting the diversity (r = −0.771), leading to the pollutant removal rates decreasing by 11.4–14.3%. norank_f_Saprospiraceae could be the indicator microorganism for excessive organic loadings, and on-site monitoring of microbial ecology evolution should be developed to provide clues about the operational performance of the leachate treatment process in advance.

Selective ionic separations represent an increasingly important technical area for the strategic interests of the U.S. economy─for example, securing critical minerals and materials and circular economy aspirations that include recovering organic acids from processed biomass. This work disseminates bipolar membrane (BPM) capacitive deionization for selective ionic separations from multicomponent, ionic species mixtures. The selective separations are guided by the Pourbaix diagram and acid–base equilibria principles. BPM capacitive deionization was demonstrated to generate alkaline or acidic process streams depending upon the location of the BPM in the electrochemical cell. The role of system operating parameters, such as the cell voltage, residence time, and feed concentration on effluent stream pH was studied. It was observed that the pH adjustment in BPM-CDI/MCDI (MCDI, membrane capacitive deionization) was more sensitive to the cell voltage when compared to the process stream residence time and salt feed concentration. The BPM-MCDI gave over 6 times higher percentage of copper(II) removal when compared to sodium ion removal from brine mixtures. Finally, BPM-MCDI demonstrated over 40% greater removal for copper ions from brine mixtures and fivefold higher removal for itaconic acid from brine mixtures when benchmarked against a traditional flow-by-MCDI setup.

The recent significant increase in the amount of municipal solid waste (MSW) is a remarkable environmental issue. Pyrolysis is a promising thermal treatment process, as it can reduce MSW and produce some valuable products as well. This study evaluated the potential of the carbon residue (CR) as an efficient H2O2 activator. The CR was produced from the copyrolysis of a mixture of abandoned cartons and plastic wraps, which are becoming increasingly dominant in MSW. The results show an interesting interaction between cartons and PVC, that the introduction of PVC into the carton greatly enhanced the activation rates of H2O2 by CR below the ratio of 0.2:1 (PVC/carton, w/w), whereas higher PVC content lowered its activation ability. EPR signals under different conditions revealed that CR directly activated H2O2 to mainly produce •O2–, which was responsible for pollutant degradation. A mechanistic study suggested that the rates of H2O2 activation were mainly governed by two factors: the amount of oxidative sites on CR and the conductivity of CR, which both were also quantitatively correlated with the activation rates of H2O2. Surface defects and surface-bound radicals were attributed to the two factors, respectively. The introduction of a limited amount of PVC to the carton greatly increased both factors mainly contributed from the generation of HCl during pyrolysis of PVC. In summary, this study may provide some new insights on the catalytic features of CRs, which are generated from the pyrolysis of a mixture of abandoned plastic wraps and cartons.

Plant-derived carbon dots have superior light absorption and intrinsic fluorescence properties. In this work, we have prepared nitrogen-doped carbon dots (N-CDs) from Piper betle leaves using a simple hydrothermal method. The synthesized N-CDs were characterized by various techniques such as high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared, and photoluminescence. The N-CDs further proved to have systemic effects on the growth of strawberries compared with irrigating the strawberry plants with water and regular nutrients. The strawberry plants treated with N-CDs exhibited an increase in chlorophyll content of about 24.7%, which was reflected in increased carbohydrate content of approximately 48.61% compared to control plants. Also, N-CD-treated strawberry plants showed increased secondary metabolites (phenolics) compared to control plants. Moreover, at the end of harvesting, the comparison was reflected in significant amounts of strawberries and in an increase in the leaf area of strawberry plants obtained by the N-CD growth. The results demonstrate that biomass-based betel leave-derived N-CDs can be an effective fertilizer for global agricultural production applications.

Conductive materials have been reported to enhance anaerobic digestion (AD) processes, but their role in the synergistic relationship between syntrophic bacteria and hydrogenotrophic methanogens (HMs) remains unclear yet. This study evaluated the holistic impact of conductive materials in an HMs-dominant anaerobic system (HMs over 95% in the archaeal community). Biochar (BC) and iron powder (IP) were selected as conductive materials with food waste and waste-activated sludge as co-substrates. The addition of IP and BC to HMs-dominant reactors significantly increased the methane yield (11.5–26%) and shortened the digestion period (32–45%) compared with the control. In addition, a novel developed integrated kinetic model revealed the diauxic peak for methanogenesis which could be significantly alleviated by the mediation of conductive materials. Furthermore, the degradation of volatile fatty acids followed a sequential order of acetate–butyrate–propionate. A smoother degradation was observed with the addition of conductive materials, while the control groups exhibited a 10 day lag phase between the degradation of butyrate and propionate. The microbial community analysis showed that BC and IP stimulated diverse syntrophic bacteria, that is, BC-enriched genus Longilinea and Aminicenantales and IP-accumulated Syntrophomonadaceae. Moreover, IP significantly facilitated HMs enrichment. Overall, this study offers new insights into the regulation of conductive materials on microbial pathways of syntrophic and hydrogenotrophic methanogenesis, with potential implications for the development of sustainable and efficient organic waste treatment strategies.

Short- and long-term changes in electricity generating unit (EGU) emissions were observed during COVID-19 public health interventions in the United States. In a generalized synthetic control framework, we employ weekly EGU SO2, NOx, and CO2 emissions data from EPA’s Clean Air Markets Database and location-specific meteorology from 2010 to 2019 to estimate each EGU’s hypothetical business as usual (BAU) emissions throughout 2020. We find that over 60% (covering >50% of total electricity generation) of EGUs saw SO2, NOx, and CO2 emissions increases relative to BAU, with most of the increases occurring in the eastern U.S. We find increases relative to BAU in the March–April stringent lockdown period for SO2, NOx, and CO2 of 44% (4500 tons/week), 23% (2200 tons/week), and 14% (2.3 million tons/week), respectively, with similar results from March to December 2020. We find that EGUs using coal as primary fuels are the main driver of increased emissions due to increased operations, and SO2 emissions increases at coal EGUs led to a 28% increase in PM2.5 related to coal SO2 emissions relative to BAU across March–December. We find increases in SO2 and NOx emissions factors at coal EGUs in 2020 relative to 2019 that likely played a role in these increases, and we identify changes in coal fuel consumption and price that may have played a role.

The presence of selenium vacancies (VSe) in metal selenides enables the activation of peroxymonosulfate (PMS) for efficient water purification. However, the mechanisms of interactions of VSe with PMS and organic pollutant removal are unclear. Hence, we precisely prepared a series of FeSe2@MoO3 composites with VSe for effective activation of PMS for the removal of various organic pollutants. The roles of VSe are explored via density functional theory (DFT) calculations: (i) regulating the electron distribution of Fe and Mo orbitals in FeSe2@MoO3 for enhancing the PMS adsorption and (ii) promoting the conversion of transition metallic redox pairs (Fe3+/Fe2+ and Mo6+/Mo5+/Mo4+). The as-prepared FeSe2@MoO3-8 exhibits excellent catalytic performance via PMS activation in which nearly 100% removal efficiencies of various organic pollutants are achieved within 2–10 min. The quenching experiments, electronic spin resonance (ESR), and probe tests demonstrated that the multiple reactive species like SO4•–, O2•–, •OH, and 1O2 contributed to the removal of 2,4-D. Finally, FeSe2@MoO3-8 was attached to the polyvinylidene difluoride (PVDF) membrane for continuous and efficient removal of 2,4-D, in which the removal efficiency and total organic carbon removal efficiency of 2,4-D were > 90% and >70% within 12 h of operation.

Low-grade residuals such as mine wastes and combustion ash are potential sources of critical metals such as rare-earth elements (REEs). Major challenges in the efficient recovery of REEs are the matrix interferences in the waste extracts that impede subsequent purification steps. This study evaluated feedstock matrix variables such as aqueous aluminum (Al), iron (Fe), and pH for their impact on neodymium (Nd) and erbium (Er) recovery flux by supported liquid membrane (SLM) separations, a type of liquid–liquid extraction method. We initially hypothesized that REE mass transfer would be lower at low [REE]/[Fe] and [REE]/[Al] molar ratios due to increased competition for chelation sites at the membrane interface. However, the results showed that the absolute Fe and Al concentrations, not the molar ratios, controlled Nd and Er mass transfer. The permeability coefficients of Nd and Er were most sensitive to the feedstock concentration of Fe3+ relative to Al3+ and Fe2+. The threshold Fe3+ concentration that resulted in reduced Nd and Er permeability was more than 100 times lower than the concentrations required for Al or Fe2+ to decrease REE permeability. REE recovery rates also increased with increasing pH of the feedstock. Separations performed with excess Fe3+ did not result in observable fouling at the membrane interface. Instead, the pH gradient across the membrane and the relative cation affinity for the chelator were the major drivers of mass transfer. These results provide insights for predicting REE mass transfer rates and SLM separation performance for extractions of low-grade feedstocks.