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In recent years, the removal of heavy metals from water and wastewater systems remains a challenging issue globally. Adsorption technology is widely used for the removal of heavy metals from aquatic systems due to several advantages including its simple operation, low cost and high efficiency. Biochar, which is an environmental benign and low-cost adsorbent, is increasingly used for remediation of heavy metal pollution in aquatic systems. This review critically analyzes recent developments on the biochar-based adsorptive removal of heavy metals from water and wastewater. Biochar is effective for the removal of several toxic metals (As, Cr and Mn) from groundwater and drinking water. Biochar also shows promising performance for the removal of various heavy metals (Cr, Pb, Cu, Cd and Zn) from wastewater. The heavy metal adsorption capacity of the biochar largely depends on the water/wastewater chemistry (pH, competing ions and initial concentrations of adsorbates), contact time between adsorbates and the adsorbent, adsorbent dose and environmental parameters (temperature). The adsorption isotherms and kinetics can be well explained by the Langmuir and pseudo-second order models, respectively. The key mechanisms which drive the heavy metal adsorption onto biochar include precipitation and ion exchange. Biochar-based adsorptive decontamination of heavy metal pollution is sustainable due to low environmental impacts, low energy consumption and low-costs. The key knowledge gaps and future research directions for improvement of biochar-based sorption capacities are highlighted. This review would help for the development of effective biochar-based adsorption technology for the removal of heavy metals from aquatic environments. [Display omitted] • Recent progress on biochar-based removal of heavy metals is critically analyzed. • Biochar is a low-cost, low-energy demand and environmentally friendly adsorbent. • Biochar is effective for heavy metal removal from water and wastewater systems. • Langmuir isotherm and pseudo-second order kinetic well explain the adsorption data. • Key mechanisms for heavy metal removal include surface precipitation and ion exchange. [ABSTRACT FROM AUTHOR]

Graphene aerogels (GAs) are increasingly being recognized as high performance multifunctional materials to tackle our current and emerging environmental concerns. In order to extend the application potential of GAs, herein we have successfully synthesized nitrogen (N) and sulfur (S) codoped GAs (NSGAs) via a simple, scalable, and inexpensive approach. Owing to their large specific surface area (up to 132 m2 g−1), profound porosity, superior mechanical properties, and coexistence of N and S atoms with tunable atomic content and bonding configurations, the as-prepared NSGAs demonstrated exceptional absorption capacity toward a broad spectrum of oils and organic solvents, with an average absorption rate many folds higher than conventional absorbents. Further, the NSGAs exhibited excellent photocatalytic activity for the decomposition of recalcitrant organic compounds under visible light illumination due to pronounced synergistic coupling effect between the heteroatoms. Specifically, after 5 h of exposure to visible light, a degradation efficiency of over 99% was observed and more than 84% of the total organic carbon was eliminated. Radical trapping experiments revealed that superoxide anion radicals are the predominant oxygen reactive species driving the photocatalytic reactions. More importantly, the mineralization byproducts did not pose any significant antibacterial activity, illustrating the environmentally benign nature of these macroscale photocatalysts. Image 1 • N/S codoped graphene aerogels (NSGAs) are prepared by facile hydrothermal method. • NSGAs can rapidly absorb oils and different types of organic solvents. • NSGAs induce near complete mineralization of a recalcitrant dye under visible light. • More than 80% of the total organic carbon is eliminated by NSGAs. • Coupling of N and S dopants synergistically enhance the application potential of GAs. Versatile nitrogen and sulfur codoped graphene aerogels were synthesized with unique geometrical configurations that show potential for a wide range of environmental applications. [ABSTRACT FROM AUTHOR]

• Reduction in the exposure to BC and UFPs was assessed with three types of vegetation barriers (VBs). • All three configurations of VBs reduced the exposure of pedestrians to BC and UFPs. • Three-layered hedges of varying heights showed higher BC and UFPs reductions. • PM with diverse morphology and elemental composition was found on leaves of VBs. • This is the first study of its kind conducted in a tropical Asian city. This study investigated the effectiveness of vegetation barriers (VBs) planted along roadsides in a highly urbanized tropical Asian city-state to mitigate pedestrians' exposure to airborne particles (PM) of traffic origin for the first time. Mobile and fixed-site measurements of black carbon (BC) and ultrafine particles (UFPs) were conducted with three types of VBs (green walkway covered with creepers; hedgerow; and three-layer hedges with varying heights) using portable PM devices. The three-layered hedges showed higher BC and UFPs reductions (the average reduction of about 60%) than the other two configurations (20–40%). Deposition of PM with diverse size, morphology and elemental composition onto VBs leaves plays an important role in reducing PM exposure concentration, providing health benefits to pedestrians involved in active mobility on roads. Our findings provide a scientific basis for formulating appropriate guidelines for selecting and installing VBs to reduce the inhalation exposure of pedestrians to PM of traffic origin. [Display omitted] [ABSTRACT FROM AUTHOR]

• Nature-based systems (NbS) can be retrofitted into the existing urban environment. • NbS show good performance for stormwater hydrology management and treatment. • Roadside vegetation acts as a biofilter to capture airborne environmental pollutants. • NbS provide ecosystem services, improve public health, and contribute to circularities. • Long-term studies are needed to develop guidelines for NbS installation and operation. In recent years, increasing attention has been given to the application of Nature-based systems (NbS) for mitigation of environmental pollution in cities. NbS namely bioretention systems, green roofs and constructed wetlands are widely adopted in urban environments for stormwater hydrology management and treatment, while roadside vegetation is utilized for mitigating air pollution of local traffic origin. Both bioretention systems and wetlands are capable of removing diverse pollutants such as total suspended solids, nutrients (nitrogen and phosphorous) and potentially toxic elements from runoff. Green roofs can act as a sink, or a source of pollutants to roof runoff outflow depending on the composition of engineered media used in support of vegetation growth. A holistic understanding of NbS in terms of their wide range of ecosystem services, multi-functionality nature, and contribution to circularity in cities is currently lacking. NbS can improve public health by providing clean air to breathe and natural waters of better quality for various purposes, but this application is not fully elucidated with an integrated approach involving both air and water quality assessments. For effective use of NbS, fundamental laboratory studies on the relative importance of phytoremediation versus bioremediation, laboratory studies to assess their actual performance under diverse weather conditions, and numerical simulations of hydrology and airflow should be conducted in tandem. Furthermore, it is important to develop guidelines for the design, installation, operation and maintenance of NbS for effective removal of environmental pollutants in the context of sustainable urban development. [Display omitted] [ABSTRACT FROM AUTHOR]

• Carbon foam was obtained from biowaste by a self-foaming approach. • The aromatic segments identified by 2D NMR enhance the foaming process. • Hydrothermal and pyrolysis processes increase graphitic carbon in carbon foam. • Carbon foam shows significantly improved CO 2 uptake. • The favorable CO 2 uptake is attributed to the hierarchical 3D porous carbon. Thermal transformation of biowaste to carbon foam provides an economically attractive and eco-friendly strategy for biowaste recycling. Herein, we report a novel self-foaming approach involving modified hydrothermal carbonization (HTC) and pyrolysis to produce carbon foam from biowaste. Notably, biowaste-derived carbon foam consists of foam structure and multi-porous structure, originated from the self-forming process of aromatic segments during modified HTC and pyrolysis processes. Based on characterization results, biowaste-derived carbon foam possesses hierarchical micropore, mesopore and macropore with increasing sp2 hybridized carbon atoms. We demonstrate the potential applications of this material for CO 2 capture. Biowaste-derived carbon foam exhibits 5.0-fold and 4.8-fold CO 2 uptake at 35 °C and 50 °C, respectively, compared to those of pristine biowaste. The recycling of biowaste to carbon foam for CO 2 capture contributes to decarbonization efforts worldwide by a synergistic way, including the photosynthesis of atmospheric CO 2 to biomass, carbon sequestration in carbon foam and CO 2 capture by carbon foam. [Display omitted] [ABSTRACT FROM AUTHOR]

Graphene, by virtue of its unique molecular structure and many interesting properties, is receiving considerable attention as an adsorbent for separation and purification of gases. In this study, graphene nanosheets were subjected to heat treatment in the temperature range of 200–800 °C under flowing N 2 to improve their textural characteristics (surface area, pores size, and total pore volume) for efficient removal of CO 2 from flue gases. The resulting graphene materials showed highly ordered structure, large surface area (up to 484 m 2 g −1 ) and interconnected hierarchical pore networks with narrow pore size distribution in the large-micropore/small-mesopore range. As a consequence, the heat-treated graphene samples demonstrated significantly greater CO 2 uptake capabilities (up to 2.89 mmol g −1 ) compared to pristine graphene (0.81 mmol g −1 ) at 0 °C and 1 bar. More importantly, the materials displayed rapid adsorption kinetics with ultrahigh selectivity for CO 2 over N 2 , as well as stable and readily reversible adsorption/desorption cycling behavior. The isosteric heat of adsorption had an unusual dependence on surface loading because of the presence of attractive intermolecular forces between the adsorbed quadrupolar CO 2 molecules. These findings demonstrate for the first time that thermal treatment at high-temperatures can have a positive influence on the single component CO 2 adsorption characteristics of graphene sheets and should be explored further as an effective strategy in the design and development of graphene-based porous solid adsorbents for CO 2 abatement. [ABSTRACT FROM AUTHOR]

One of the key challenges in photocatalysis is the suppression of electron-hole pairs' recombination, which reduces the photocatalytic degradation efficiency of promising materials. In this study, a novel heterostructured photocatalyst (In(OH) 3 /NGAs) was synthesized by coupling indium hydroxide (In(OH) 3) with nitrogen-doped graphene aerogels (NGAs). A comprehensive evaluation of the photocatalytic degradation performance of In(OH) 3 /NGAs was conducted using para -nitrophenol (p -NP) as a model pollutant under visible light. Results show that impressive photocatalytic activities of In(OH) 3 /NGAs are mainly due to their strong synergistic effects of the heterojunction formed between In(OH) 3 and NGAs , and the active nitrogen doping of GAs. The synergistic effects result in a sizable bandgap and thus enhance the visible light absorption capability. The favorable formation of photo-generated holes and hydroxyl radicals is responsible for the efficient photocatalytic degradation of p -NP, leading to 98% removal of total organic carbon. Additionally, a comparative evaluation of the photodegradation performance of In(OH) 3 /NGAs with that of another indium-based heterostructured material (In 2 O 3 /g-C 3 N 4 (graphitic carbon nitride) shows the superior performance of the former photocatalyst which is attributed to its unique molecular configuration and chemical bonds. This work reveals how a rational combination of NGAs with In(OH) 3 can lead to efficient photocatalytic degradation of recalcitrant organic pollutants such as p -NP. [Display omitted] • In-based N-doped graphene aerosols (NGAs) and g-C 3 N 4 composites were synthesized. • Photocatalytic degradation of p-NP by In-NGAs and In g-C 3 N 4 was studied. • In-NGAs demonstrated better photocatalytic performance than In g-C 3 N 4. • The performance of In-NGAs was optimized to maximize their photocatalytic potential. • Insights into the photocatalytic degradation of p-NP by In-NGAs are provided. [ABSTRACT FROM AUTHOR]

Black shale (BS) deposits found in Europe (Kupferschiefer type) contain base (e.g., copper and aluminium) and precious (e.g., gold and silver) metals and also PGE (platinum group elements), but also high contents of organic matter that potentially impede with metal recovery by conventional techniques. The aim of the present investigation was to recover copper (Cu) from BS using microbial leaching. BS deposits were collected from a mining site in Lubin, Poland which consisted of 14 ± 2 mg/g of Al, 13 ± 2 mg/g of Cu, 10 ± 1 mg/g of Fe and relatively significant amounts of Mn and Zn. Several chemolithotrophic bacteria (Leptospirillum ferrooxidans, Leptospirillum ferriphilum, Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans, and Sulfobacillus thermosulfidooxidans) were evaluated for their ability to extract Cu selectively from BS with a 2% pulp density. Among these bacteria, the iron oxidizing L. ferrooxidans, L. ferriphilum, At. ferrooxidans were identified as the potential candidates, and their Cu extraction efficiencies were 62% ± 3%, 55% ± 3%, 25% ± 2%, respectively. Using a mixed population of these microorganisms, the bioleaching process was performed at different pulp densities of BS and their corresponding Cu leaching efficiency was evaluated. Following the batch experiments, the bioleaching process was scaled-up to a 5 liter laboratory scale custom-designed Air Up-lift Bioreactor (AUBR). The Cu extraction could be increased to > 70% at a higher pulp density (10% wt/v) under the experimental conditions set at pH 2.0 and 37o C with the aeration rate of 150 liter per hour for 15 days of continuous bioleaching operation. [ABSTRACT FROM AUTHOR]

Abstract: The polyurethane foam (PUF) disk-based passive air samplers (PAS), mounted inside two aluminium bowls to buffer the air flow to the disk and to shield it from precipitation and sunlight, were used for the collection of atmospheric POPs in Singapore during April 2008eJune 2008. Data obtained from PAS measurements are compared to those from active high-volume air sampling (AAS). Single factor ANOVA tests show that there were no significant differences in chemical distribution profiles between actively and passively collected samples(PAHs, F=3.38×10-8) < Fcritica l= 4.17 with p>0.05; OCPs, F=2.71×10-8< Fcritical = 4.75 with p > 0.05). The average air-side mass transfer coefficient (kA) for PAS, determined from the loss of depuration compounds such as 13C6-HCB (1000 ng), 13C12 - 4,4’ DDT (1000 ng) and 13C12-PCB 101 (1000 ng)spiked on the disks prior to deployment, was 0.12 ± 0.04 m s-1. These values are comparable to those reported previously in the literature. The average sampling rate was 3.78 ± 1.83 m3 d- 1 for the 365cm2 PUF disk. Throughout the entire sampling period (∼68 d), most of the PAHs and all OCPs exhibited a linear uptake trend on PAS, while naphthalene, acenaphthylene, acenaphthene and fluorene reached the curvilinear phase after the first ∼30 d exposure. Theoretically estimated times to equilibrium (teq) ranged from around one month for Acy to hundreds of years for DB(ah)A. Sampling rates, based on the time integrated active sampling-derived concentrations and masses collected by PUF disks during the linear uptake phase, were determined for all target compounds with the average values of 2.50 m3 d-1 and 3.43m3 d-1 for PAHs and OCPs, respectively. More variations were observed as compared to those from th depuration study. These variations were most likely due to the difference of physicochemical properties of individual species. The validation and calibration work conducted for PAS in this study clearly indicates that it would be feasible to use such an economic PAS for routine measurements of POPs in the tropical atmosphere at a large network of sampling locations. [Copyright &y& Elsevier]

Abstract: The occurrences of increased atmospheric nitrogen deposition (ADN) in Southeast Asia during smoke haze episodes have undesired consequences on receiving aquatic ecosystems. A successful prediction of episodic ADN will allow a quantitative understanding of its possible impacts. In this study, an artificial neural network (ANN) model is used to estimate atmospheric deposition of total nitrogen (TN) and organic nitrogen (ON) concentrations to coastal aquatic ecosystems. The selected model input variables were nitrogen species from atmospheric deposition, Total Suspended Particulates, Pollutant Standards Index and meteorological parameters. ANN models predictions were also compared with multiple linear regression model having the same inputs and output. ANN model performance was found relatively more accurate in its predictions and adequate even for high-concentration events with acceptable minimum error. The developed ANN model can be used as a forecasting tool to complement the current TN and ON analysis within the atmospheric deposition-monitoring program in the region. [Copyright &y& Elsevier]

[Display omitted] • Elevated PM 2.5 and BC levels were observed in home and transport microenvironments. • High UFP levels were observed in recreation microenvironments. • Long-term exposure to toxic trace elements contained in PM 2.5 presents health risks. • Potential health risks were reduced while using the portable air cleaner. Assessment of integrated personal exposure (PE) to airborne particulate matter (PM) across diverse microenvironments (MEs) over 24 hours under different exposure scenarios is necessary to identify appropriate strategies to improve urban air quality and mitigate the health effects of PM. We carried out a collaborative study in a densely populated city-state (Singapore) to assess the integrated PE to fine particles (PM 2.5), ultrafine particles (UFPs) and black carbon (BC) across diverse indoor and outdoor urban MEs, estimate related health risks and make suitable recommendations for healthy living in cities. Two volunteers with different lifestyles participated in the study by tracking their PE to particulate air pollution and the time-activity patterns over 24 hours using portable PM monitoring devices and recording their whereabouts using GPS coordinates. Home, transport and recreation (i.e., food court) MEs represented pollution hotspots of PM 2.5 (21.0 μg/m3), BC (3.4 μg/m3) and UFP (33.0 × 103 #/cm3), respectively. Among the different modes of transport used by the participants (walking, cycling, e-scooter, mass rapid transport (MRT), bus, car and taxi), the air pollutants had elevated concentrations while commuting by public transport (bus and MRT) as well as during active modes of transport (walking and cycling). Air-conditioned cars and taxis, equipped with air filtration systems, represented the lowest PE. The health risk assessment revealed that there are potential carcinogenic risks associated with the long-term exposure to elevated levels of PM 2.5 -bound toxic trace elements. These risks can be mitigated with the introduction of low-carbon and active modes of transport in place of internal combustion engines and the use of indoor air pollution exposure mitigation devices. [ABSTRACT FROM AUTHOR]

• Personal exposure to PM 2.5 and BC in Danang(Vietnam) and Singapore was assessed. • Exposure to PM 2.5 and BC by commuters was much higher in Danang than Singapore. • Exposure to PM 2.5 and BC in homes should also be considered. • Heavy-duty vehicles and motorcycles are the major contributors to PM in both cities. • Health effects of motorized transport can be avoided by switching to active mobility. The present study investigated the personal exposure (PE) of commuters to fine particles (PM 2.5) and black carbon (BC) in two cities: Singapore and Danang (Vietnam) with different socio-economic conditions. The PE measurement was conducted during a multimodal round trip from Singapore to Danang and back with various on-road transport modes in urban micro-environments within each city by using portable devices. Real-time assessment of PE revealed that more than 60% of the integrated inhaled dose of PM 2.5 by commuters to occurred at airport concourses and transit micro-environments to/from the aeroplane by apron bus. Whereas for BC, the transit by apron bus made a major contribution (> 50%) to the total integrated exposure. The PE to PM 2.5 and BC in transport micro-environments was an order of magnitude higher in Danang compared to Singapore while using various on-road modes of transport in each city. Elevated concentrations of airborne particles in Singapore and Danang were significantly contributed by heavy-duty diesel vehicles and motorcycles, respectively. Moreover, a reduction in years of life expectancy is likely to occur among urban commuters while using motorized transport compared to active mobility (cycling). [ABSTRACT FROM AUTHOR]

• B/N codoped graphene aerogels (BNGAs) are prepared by one-pot hydrothermal method. • BNGAs can effectively photodegrade bisphenol A under visible light. • Over 88 % total organic carbon is removed by BNGAs within 3 h. • BNGAs can also remove pathogens such as Escherichia coli. • BNGAs can be recycled and reused repeatedly with no significant loss in efficiency. It is widely acknowledged that doping of carbon materials by multi-elements with different electronegativities can result in unique electron-donor properties and novel functionalities because of the strong synergistic interaction among the dopant atoms. In this study, boron and nitrogen codoped graphene aerogels (BNGAs) are synthesized and their photocatalytic activity towards decomposition of bisphenol A (BPA) under visible light irradiation is systematically examined. The BPA molecules are rapidly adsorbed onto the 3D interconnected pore system of the BNGAs under dark conditions, and eventually mineralized upon exposure to visible light, indicating the synergy between adsorption-enrichment and photocatalysis during degradation of BPA. Notably, almost 96 % of BPA is removed and over 88 % of total organic carbon is eliminated by the as-prepared BNGAs. More importantly, the BNGAs can retain approximately 92 % of their initial activity even after repeated cycling. In addition, the BNGAs display great potential for the disinfection of harmful pathogens like Escherichia coli , with a photocatalytic decontamination rate of 1.2 × 103 CFU h−1 g cat −1. In view of their attractive multi-functional performance, the as-developed BNGAs merit further consideration for eliminating emerging organic contaminants and pathogens from freshwater sources. [ABSTRACT FROM AUTHOR]

Light absorbing carbonaceous aerosols (LACA) consisting of black carbon (BC) and brown carbon (BrC) have received considerable attention because of their climate and health implications, but their sources, characteristics and fates remain unclear in Southeast Asia (SEA). In this study, we investigated spatio-temporal characteristics of LACA, their radiative properties and potential sources in Singapore under different weather conditions. Hourly BC concentrations, measured from May 2017 to March 2018, ranged from 0.31 μg/m3 to 14.37 μg/m3 with the mean value being 2.44 ± 1.51 μg/m3. High mass concentrations of BC were observed during the south-west monsoon (SWM, 2.60 ± 1.56 μg/m3) while relatively low mass concentrations were recorded during the north-east monsoon (NEM, 1.68 ± 0.96 μg/m3). There was a shift in the Absorption Ångström exponent (AAE) from 1.1 to 1.4 when the origin of LACA changed from fossil fuel (FF) to biomass burning (BB) combustion. This shift is attributed to the presence of secondary BrC in LACA, derived from transboundary BB emissions during the SWM. Lower AAE values were observed when local traffic emissions were dominant during the NEM. This explanation is supported by measurements of water-soluble organic carbon (WSOC) in LACA and the corresponding AAE values determined at 365 nm using a UV–vis spectrophotometer. The AAE values, indicative of the presence of brown carbon (BrC), showed that photochemically aged LACA contribute to an enhancement in the light absorption of aerosols. In addition, spatio-temporal characteristics of BC in the intra-urban environment of Singapore were investigated across diverse outdoor and indoor microenvironments. High variability of BC was evident across these microenvironments. Several air pollution hotspots with elevated BC concentrations were identified. Overall, the results stress a need to control anthropogenic emissions of BC and BrC in order to mitigate near-term climate change impacts and provide health benefits. Image 1 • Black carbon showed diurnal and seasonal variations due to changes in its sources. • Presence of both primary and secondary brown carbon was observed. • Light absorption by brown carbon is linked with the composition of organics. • Contributions from biomass burning to BC and BrC dominated during the dry season. • Key sources of BC in Singapore were construction sites, temples and bus stops. This paper provides insights into the concentration levels, diurnal variations, sources and radiative properties of carbonaceous aerosols over an urban environment in Southeast Asia. [ABSTRACT FROM AUTHOR]