Effect of weathering on mobilization of biochar particles and bacterial removal in a stormwater biofilter

A systematic review of biochar aging and the potential eco-environmental risk in heavy metal contaminated soil

Biochar is widely accepted as a green and effective amendment for remediating heavy metals (HMs) contaminated soil, but its long-term efficiency and safety changes with biochar aging in fields. Currently, some reviews have qualitatively summarized biochar aging methods and mechanisms, aging-induced changes in biochar properties, and often ignored the potential eco-environmental risk during biochar aging process. Therefore, this review systematically summarizes the study methods of biochar aging, quantitatively compares the effects of different biochar aging process on its properties, and discusses the potential eco-environmental risk due to biochar aging in HMs contaminated soil. At present, various artificial aging methods (physical aging, chemical aging and biological aging) rather than natural field aging have been applied to study the changes of biochar's properties. Generally, biochar aging increases specific surface area (SSA), pore volume (PV), surface oxygen-containing functional group (OFGs) and O content, while decreases pH, ash, H, C and N content. Chemical aging method has a greater effect on the properties of biochar than other aging methods. In addition, biochar aging may lead to HMs remobilization and produce new types of pollutants, such as polycyclic aromatic hydrocarbons (PAHs), environmentally persistent free radicals (EPFRs) and colloidal/nano biochar particles, which consequently bring secondary eco-environmental risk. Finally, future research directions are suggested to establish a more accurate assessment method and model on biochar aging behavior and evaluate the environmental safety of aged biochar, in order to promote its wider application for remediating HMs contaminated soil.

Performance of wood waste biochar and food waste compost in a pilot-scale sustainable drainage system for stormwater treatment

Sustainable drainage system (SuDS) for stormwater reclamation has the potential to alleviate the water scarcity and environmental pollution issues. Laboratory studies have demonstrated that the capacity of SuDS to treat stormwater can be improved by integrating biochar and compost in the filter media, whereas their performance in scaled-up applications is less reported. This study examines the effectiveness of a pilot-scale SuDS, bioswale followed by bioretention, amended with wood waste biochar (1, 2, and 4 wt.%) and food waste compost (2 and 4 wt.%) to simultaneously remove multiple pollutants including nutrients, heavy metals, and trace organics from the simulated stormwater. Our results confirmed that SuDS modified with both biochar (2 wt.%) and compost (2 wt.%) displayed superior water quality improvement. The system exhibited high removal efficiency (> 70%) for total phosphorus and major metal species including Ni, Pb, Cd, Cr, Cu, and Zn. Total suspended solids concentration was approaching the detection limit in the effluent, thereby confirming its capability to reduce turbidity and particle-associated pollutants from stormwater. Co-application of biochar and compost also moderately immobilized trace organic contaminants such as 2,4-dichlorophenoxyacetic acid, diuron, and atrazine at field-relevant concentrations. Moreover, the soil amendments amplified the activities of enzymes including β-D-cellobiosidase and urease, suggesting that the improved soil conditions and health of microbial communities could possibly increase phyto and bioremediation of contaminants accumulated in the filter media. Overall, our pilot-scale demonstration confirmed that the co-application of biochar and compost in SuDS can provide a variety of benefits for soil/plant health and water quality.

Transport and release behaviors of PFOA in saturated and unsaturated porous media with biochar amendment

The widespread usage of perfluorooctanoic acid (PFOA) in daily consumer products and its high mobility in porous media have resulted in ubiquitous contamination of PFOA in the natural environment. Developing techniques to immobilize and inhibit the transport of PFOA thus is critical to reduce its potential risks. In present study, biochar, one type of environmental-friendly material produced from cellulose, was utilized in porous media to test its addition on inhibiting the transport and release of PFOA before and after aging process. We found that although PFOA had high mobility in saturated/unsaturated porous media, biochar addition could significantly inhibit PFOA transport in porous media with different saturations due to its high adsorption capacity towards PFOA. The inhibited transport of PFOA by biochar also held true in solution with copresence of natural organic matter and in actual river water. Moreover, we found that negligible PFOA was released from porous media with biochar amendment even after exposure to freeze-thaw/dry-wet treatment. PFOA adsorbed onto biochar could be completely desorbed and the biochar could be reused for subsequent cycles after desorption. Clearly, amendment of porous media with biochar would be a feasible and cost-effective method to immobilize PFOA in natural environment and reduce its environmental risks.

Highly variable removal of pathogens, antibiotic resistance genes, conventional fecal indicators and human-associated fecal source markers in a pilot-scale stormwater biofilter operated under realistic stormflow conditions

Green stormwater infrastructure systems, such as biofilters, provide many water quality and other environmental benefits, but their ability to remove human pathogens and antibiotic resistance genes (ARGs) from stormwater runoff is not well documented. In this study, a field scale biofilter in Southern California (USA) was simultaneously evaluated for the breakthrough of a conservative tracer (bromide), conventional fecal indicators, bacterial and viral human-associated fecal source markers (HF183, crAssphage, and PMMoV), ARGs, and bacterial and viral pathogens. When challenged with a 50:50 mixture of untreated sewage and stormwater (to mimic highly contaminated storm flow) the biofilter significantly removed (p < 0.05) 14 of 17 microbial markers and ARGsin descending order of concentration reduction: ermB (2.5 log(base 10) reduction) > Salmonella (2.3) > adenovirus (1.9) > coliphage (1.5) > crAssphage (1.2) > E. coli (1.0) ∼ 16S rRNA genes (1.0) ∼ fecal coliform (1.0) ∼ intl1 (1.0) > Enterococcus (0.9) ∼ MRSA (0.9) ∼ sul1 (0.9) > PMMoV (0.7) > Entero1A (0.5). No significant removal was observed for GenBac3, Campylobacter, and HF183. From the bromide data, we infer that 0.5 log-units of attenuation can be attributed to the dilution of incoming stormwater with water stored in the biofilter; removal above this threshold is presumably associated with non-conservative processes, such as physicochemical filtration, die-off, and predation. Our study documents high variability (>100-fold) in the removal of different microbial contaminants and ARGs by a field-scale stormwater biofilter operated under transient flow and raises further questions about the utility of human-associated fecal source markers as surrogates for pathogen removal.

Natural aging of expanded shale, clay, and slate (ESCS) amendment with heavy metals in stormwater increases its antibacterial properties: Implications on biofilter design

Aging is often expected to decrease the pathogen removal capacity of media because of exhaustion of attachment sites by adsorption of co-contaminants and dissolved organics. In contrast, the adsorption of metals naturally present in stormwater during aging could have a positive impact on pathogen removal. To examine the effect of adsorbed metals on pathogen removal, biofilter media amended with expanded clay, shale, and slate (ESCS) aggregates, a lightweight aggregate, were exposed to metals by intermittently injecting natural stormwater spiked with Cu, Pb, and Zn, and the capacity of aged and unaged media to remove Escherichia coli (E. coli), a pathogen indicator, were compared. Metal adsorption on ESCS media decreased their net negative surface charge and altered the surface properties as confirmed by zeta potential measurement and Fourier-Transform Infrared Spectroscopy (FTIR) analysis. These changes increased the E. coli adsorption capacity of aged media compared with unaged media and decreased overall remobilization of attached E. coli during intermittent infiltration of stormwater. A live-dead analysis confirmed that the adsorbed metals inactivated attached E. coli, thereby replenishing the adsorption capacity. Overall, the results confirmed that natural aging of biofilter media with adsorbed metals could indeed have a net positive effect on E. coli removal in biofilters and therefore should be included in the conceptual model predicting long-term removal of pathogens from stormwater containing mixed pollutants.

Behavioral intentions of rural farmers to recycle human excreta in agriculture

Considerable progress has been made in developing human excreta recovery pathways and processes for maximum nutrient recovery and contaminant elimination. The demand segment has often been ignored as an area for future research, especially during the technology development. The findings from the few published articles on social acceptance show missing and inconclusive influence of demographic, sociological, and economic farmer-characteristics. This study endeavours to close this gap by using the social psychological theories, technology adoption theories and the new ecological paradigm to investigate the factors that influence the behavioral intentions of rural farmers to recycle human excreta in agriculture. Study findings show that social acceptance was driven by awareness, religiosity, income, source of income, and environmental dispositions. Perceived behavioral control represents a potential barrier to human excreta reuse. The study recommends the demographic, cultural, sociological, and economic mainstreaming of dissemination strategies of circular bioeconomy approaches within the context of agricultural innovation systems.

Biological Layer in Household Slow Sand Filters: Characterization and Evaluation of the Impact on Systems Efficiency

Schmutzdecke, the biofilm formed on the top of the sand bed in household slow sand filters (HSSF) is a key factor for the filters’ high efficiency in removing particles and microorganisms from water. This paper aims to investigate the extracellular polymeric substances composition (carbohydrates and proteins), biomass, dissolved oxygen, and microbial community in two types of HSSFs and identify a correlation between them and their efficiency. A continuous- and an intermittent-HSSF (C-HSSF and I-HSSF) were studied to treat river water for 48 days. Their efficiencies for bacteria (E. coli and total coliforms), turbidity, and apparent color removals were analyzed. Results clearly showed an increase of carbohydrates (from 21.4/22.5 to 101.2/93.9 mg·g−1 for C-/I-HSSF) and proteins (from 34.9 to 217/307.8 mg g−1 for C-/I-HSSF), total solids (from 0.03/<0.03 to 0.11/0.19 g L−1 for C-/I-HSSF), dissolved oxygen depletion inside the filter (6.00 and 5.15 mg L−1 for C- and I-HSSF) and diversity of microorganisms over time, pointing out the schmutzdecke development. A clear improvement on the HSSFs’ efficiency was observed during operation, i.e., E. coli removal of 3.23 log and 2.98 log for total coliforms, turbidity from 60 to 95%, and apparent color from 50 to 90%.

Improved removal performance of Gram-negative and Gram-positive bacteria in sand filtration system with arginine modified biochar amendment

Bacterial removal by sand filtration system is commonly inefficient due to the low bacterial adsorption capacity of sand. To improve the bacterial removal performance, biochar fabricated at different temperatures (400 °C, 550 °C and 700 °C) and arginine modified biochar were added into sand filtration columns as filter layers (0.5 and 1 wt%). Addition of biochar into sand columns could improve the removal efficiency for both Escherichia coli and Bacillus subtilis under both slow (4 m/day) and fast (240 m/day) filtration conditions. Bacterial removal efficiency in sand columns with the addition of biochar fabricated at 700 °C were higher than those fabricated at 400 °C and 550 °C due to its best bacterial adsorption capacity. Modification of biochar with arginine could further improve the bacterial removal performance. Specifically, complete bacterial removal (1.35 × 10⁷ ± 10% cells/mL) could be achieved under both slow and fast filtration conditions in sand columns with 1 wt% arginine functionalized biochar amendment. The enhanced bacterial adsorption capacity mainly contributed to the increased bacterial capture performance in columns with addition of arginine-modified biochar. Bacteria more tightly bounded with arginine-modified biochar than bulk biochar. Moreover, complete bacterial removal with the copresence of 5 mg/L humic acid in suspensions was acquired in columns with addition of 1 wt% arginine-modified biochar. Efficient bacterial removal in actual river water, multiple filtration cycles as well as longtime injection duration (100 pore volumes injection) was also obtained. The results of this study demonstrated that arginine-modified biochar had great potential to treat water contaminated by pathogenic bacteria.

Biochar-based bioretention systems for removal of chemical and microbial pollutants from stormwater: A critical review

Biochar has been increasingly used as a filter medium in engineered low impact development systems (e.g., bioretention systems) for decontamination of urban stormwater and management of hydrology. This review paper critically analyzes the performance of biochar-based biofiltration systems for removal of chemical and microbial pollutants present in urban runoff. Biochar-amended biofiltration systems efficiently remove diverse pollutants such as total nitrogen (32 - 61%), total phosphorus: (45 - 94%), heavy metals (27 - 100%), organics (54 - 100%) and microbial pollutants (log₁₀ removal: 0.78 - 4.23) from urban runoff. The variation of biofiltration performance is due to changes in biochar characteristics, the abundance of dissolved organic matter and/or stormwater chemistry. The dominant mechanisms responsible for removal of chemical pollutants are sorption, ion exchange and/or biotransformation, whereas filtration/straining is the major mechanism for bacteria removal. The pseudo-second order and Langmuir isotherm are the best models that describe the kinetics and chemical equilibrium of pollutants, respectively. This critical review provides the fundamental scientific knowledge for designing highly efficient biochar-based bioretention systems for removal of diverse pollutants from urban stormwater. The key knowledge gaps that should be addressed in future research include long-term field-scale bioretention study, development of novel methods for filter media regeneration/reuse, and dynamics of filter media microbial communities.

Iron amendments minimize the first-flush release of pathogens from stormwater biofilters

First flush or the first pore volume of effluent eluted from biofilters at the start of rainfall contributes to most pollution downstream because it typically contains a high concentration of bacterial pathogens. Thus, it is critical to evaluate designs that could minimize the release of bacteria during a period of high risk. In this study, we test the hypothesis of whether an addition of iron-based media to biofilter could limit the leaching of Escherichia coli (E. coli), a pathogen indicator, during the first flush. We applied E. coli-contaminated stormwater intermittently in columns packed with a mixture of sand and compost (70:30 by volume, respectively) and iron filings at three concentrations: 0% (control), 3%, and 10% by weight. Columns packed with a mixture of sand and iron (3% or 10%) without compost were used to examine the maximum capacity of iron to remove E. coli. In columns with iron, particularly 10% by weight, the leaching of E. coli during the first flush was 32% lower than the leaching from compost columns, indicating that the addition of iron amendments could decrease first-flush leaching of E. coli. We attribute this result to the ability of iron to increase adsorption and decrease growth during antecedent drying periods. Although the addition of iron filings increased E. coli removal, the presence of compost decreased the adsorption capacity: exposure of 1 g of iron filings to 1 mg of DOC reduces E. coli removal by 8%. The result was attributed to the alteration of the surface charge of iron and blocking of adsorption sites shared by E. coli and DOC. Collectively, these results indicate that the addition of sufficient amounts of iron media could decrease pathogen leaching in the first flush effluent and increase the overall biofilter performance and protect downstream water quality.

Stabilization of dissolvable biochar by soil minerals: Release reduction and organo-mineral complexes formation

Biochar has two existing forms in the moist soil environment, free dissolvable biochar (particle size < 0.45 μm) and undissolvable particles (particle size > 0.45 μm). The release and decomposition of dissolvable biochar from bulk biochar particles is a primary C loss pathway in biochar-amended soils, which would be reduced by their interactions with soil minerals. Most previous studies focused on the effect of feedstock types and pyrolysis conditions on dissolvable biochar stability, while few studies researched the interaction between dissolvable biochar and soil components, for instance the soil minerals, and its effect on the stability of dissolvable biochar. In this study, bentonite and goethite were selected as model soil minerals because of their differences in structure and surface types: negatively charged 2:1 type phyllosilicate (bentonite) and positively charged crystalline mineral (goethite). Dry-wet cycling was conducted to determine the effect of these two minerals on the release of dissolvable biochar from walnut shell-derived biochar particles. The stability of dissolvable biochar was measured by chemical oxidation and biodegradation. Both soil minerals reduced the release of dissolvable biochar by over 34% with the presence of Ca²⁺. Mechanisms of "Ca²⁺ bridging", "ligand exchange" and "van der Waals attraction" contributed to the formation of dissolvable biochar-bentonite complexes, and Ca²⁺ promoted dissolvable biochar inserting into bentonite interlayer space, expanding d-spacing from 1.25 nm to 1.55 nm. However, "Ca²⁺ bridging" barely formed on goethite because of charge repulsion, indicating that the dissolvable biochar was bound with goethite mainly by "van der Waals attraction" and "ligand exchange". Due to organo-mineral complexes formation, the chemical oxidation extent of dissolvable biochar was reduced by 22.8-36.5%, and the biodegradation extent was reduced by 72.7-85.0%, since the soil minerals are more effective to prevent the dissolvable biochar from being biodegraded. This study proved soil minerals and Ca²⁺ were beneficial for enhancing biochar stability, these observations assisted in assessing the biochar ability for long-term carbon sequestration.

The Role of Biochar in Regulating the Carbon, Phosphorus, and Nitrogen Cycles Exemplified by Soil Systems

Biochar is a carbon-rich material prepared from the pyrolysis of biomass under various conditions. Recently, biochar drew great attention due to its promising potential in climate change mitigation, soil amendment, and environmental control. Obviously, biochar can be a beneficial soil amendment in several ways including preventing nutrients loss due to leaching, increasing N and P mineralization, and enabling the microbial mediation of N2O and CO2 emissions. However, there are also conflicting reports on biochar effects, such as water logging and weathering induced change of surface properties that ultimately affects microbial growth and soil fertility. Despite the voluminous reports on soil and biochar properties, few studies have systematically addressed the effects of biochar on the sequestration of carbon, nitrogen, and phosphorus in soils. Information on microbially-mediated transformation of carbon (C), nitrogen (N), and phosphorus (P) species in the soil environment remains relatively uncertain. A systematic documentation of how biochar influences the fate and transport of carbon, phosphorus, and nitrogen in soil is crucial to promoting biochar applications toward environmental sustainability. This report first provides an overview on the adsorption of carbon, phosphorus, and nitrogen species on biochar, particularly in soil systems. Then, the biochar-mediated transformation of organic species, and the transport of carbon, nitrogen, and phosphorus in soil systems are discussed. This review also reports on the weathering process of biochar and implications in the soil environment. Lastly, the current knowledge gaps and priority research directions for the biochar-amended systems in the future are assessed. This review focuses on literatures published in the past decade (2009–2021) on the adsorption, degradation, transport, weathering, and transformation of C, N, and P species in soil systems with respect to biochar applications.

Conventional and amended bioretention soil media for targeted pollutant treatment: A critical review to guide the state of the practice

Bioretention systems are widely used green infrastructure elements that utilize engineered bioretention soil media (BSM) for stormwater capture and treatment. Conventional bioretention soil media, which typically consists of sand, sandy loam, loamy sand or topsoil amended with compost, has limited capacity to remove and may leach some stormwater pollutants. Alternative engineered amendments, both organic and inorganic, have been tested to supplement BSM. Yet, municipalities and regulatory agencies have been slow to adopt these alternative amendments into their design specifications, partly because of a lack of clear guidance on how to select the right amendment to treat a target stormwater contaminant under highly variable climatic conditions. This article aims to provide that guidance by: (1) summarizing the current design BSM specifications adopted by jurisdictions worldwide, (2) comparing the performance of conventional and amended BSM, (3) highlighting advantages and limitations of BSM amendments, and (4) identifying challenges for implementing amendments in field conditions. The analysis not only informs the research community of the barriers faced by stormwater managers in implementing BSM amendments but also provides guidelines for their adoption by interested agencies to comply with existing regulations and meet design needs. This feedback loop could catalyze further innovation in the development of sustainable stormwater treatment technologies.

Towards a mechanistic understanding of particle shrinkage during biomass pyrolysis via synchrotron X-ray microtomography and in-situ radiography

Accurate modelling of particle shrinkage during biomass pyrolysis is key to the production of biochars with specific morphologies. Such biochars represent sustainable solutions to a variety of adsorption-dependent environmental remediation challenges. Modelling of particle shrinkage during biomass pyrolysis has heretofore been based solely on theory and ex-situ experimental data. Here we present the first in-situ phase-contrast X-ray imaging study of biomass pyrolysis. A novel reactor was developed to enable operando synchrotron radiography of fixed beds of pyrolysing biomass. Almond shell particles experienced more bulk shrinkage and less change in porosity than did walnut shell particles during pyrolysis, despite their similar composition. Alkaline pretreatment was found to reduce this difference in feedstock behaviour. Ex-situ synchrotron X-ray microtomography was performed to study the effects of pyrolysis on pore morphology. Pyrolysis led to a redistribution of pores away from particle surfaces, meaning newly formed surface area may be less accessible to adsorbates.

Effects of ageing on the surface characteristics and Cu(ii) adsorption behaviour of rice husk biochar in soil

The properties of rice husk biochar during the ageing process in soil and the resulting impacts on sorption capacity with respect to Cu(ii) were assessed. Rice husk-derived biochar was placed in fabric bags and buried in a plastic incubator filled with soil for 0–240 d. The aged biochar was then characterised and its sorption capacity compared with control (unaged) biochar in batch sorption experiments. The structural composition and morphology of the biochar before and after ageing were analysed based on element composition, scanning electron microscopy (SEM) coupled with energy X-ray dispersive spectroscopy (EDS), diffuse reflectance infrared Fourier transform infrared spectroscopy (DRIFTS), and X-ray photoelectron spectroscopy (XPS). The concentration of O, atomic O/C ratios, and carboxyl and hydroxyl functional groups increased at the surface of the biochar during ageing, which together indicated oxidation. Within the biochar particles, O/C ratios progressively increased towards their outer surfaces. Furthermore, ageing for more than 120 d facilitated Cu(ii) sorption as oxygen-containing groups were able to develop. The maximum adsorption capacity (qm) of biochar increased by 1.24 ∼ 1.32 times after ageing in the soil for 240 d. It is suggested that biochar surface properties were gradually altered during environmental exposure and the aged rice husk biochar showed increased performance in Cu(ii) adsorption. However, the performance of aged biochar as a soil remediator or conditioner will be affected by the ageing process and interactions among different soil components. As such, further research is required to evaluate these complex effects.

Size-dependent biochar breaking under compaction: Implications on clogging and pathogen removal in biofilters

Breaking of biochar during compaction of amended soil in roadside biofilters or landfill cover can affect infiltration and pollutant removal capacity. It is unknown how the initial biochar size affects the biochar breaking, clogging potential, and contaminant removal capacity of the biochar-amended soil. We compacted a mixture of coarse sand and biochar with sizes smaller than, similar to, or larger than the sand in columns and applied stormwater contaminated with E. coli. Packing columns with biochar pre-coated with a dye and analyzing the dye concentration in the broken biochar particles eluted from the columns, we proved that biochar predominantly breaks under compaction by disintegration or splitting, not by abrasion. Increases in biochar size decrease the likelihood of biochar breaking. We attribute this result to the effective dissipation of compaction energy through a greater number of contact points between a large biochar particle and the adjacent particles. Most of the broken biochar particles are deposited in the pore spaces of the background geomedia, resulting in an exponential decrease in hydraulic conductivity of amended sand with an increase in suspended sediment loading. The clogging rate was higher in the columns with small biochar. The columns with small biochar also exhibited high E. coli removal capacity, partly because of an increase in bacterial straining at reduced pore size after compaction. These results are useful in selecting appropriate biochar size for its application in soils and roadside biofilters for stormwater treatment.

Generation, Resuspension, and Transport of Particulate Matter From Biochar-Amended Soils: A Potential Health Risk

Large-scale soil application of biochar is one of the terrestrial carbon sequestration strategies for future climate change mitigation pathways, which can also help remove and sequester pollutants from contaminated soil and water. However, black carbon emissions from biochar-amended soils can deteriorate air quality and affect human health, as the biochar particles often contain a higher amount of sorbed toxic pollutants than the soil. Yet, the extent and mechanism of inhalable particulate matter (PM10) emission from biochar-amended soils at different wind regimes have not been evaluated. Using wind tunnel experiments to simulate different wind regimes, we quantified particulate emission from sand amended with 1-4% (by weight) biochar at two size fractions: with and without <2-mm biochar. At wind speeds below the threshold speed for soil erosion, biochar application significantly increased PM10 emission by up to 400% due to the direct resuspension of inhalable biochar particles. At wind speeds above the threshold speed, emission increased by up to 300% even from biochar without inhalable fractions due to collisions of fast-moving sand particles with large biochar particles. Using a theoretical framework, we show that particulate matter emissions from biochar-amended soils could be higher than that previously expected at wind speeds below the erosion threshold wind speed for background soil. Our results indicate that current models for fugitive dust emissions may underestimate the particulate matter emission potential of biochar-amended soils and will help improve the assessment of biochar emission from amended soils.

Compaction conditions affect the capacity of biochar-amended sand filters to treat road runoff

Amending roadside soil with adsorbents such as biochar can help remove pollutants from road runoff. To maintain soil stability, the roadside soil requires compaction. However, it is unknown how compaction conditions affect the capacity of biochar-augmented roadside biofilters to infiltrate stormwater and remove pollutants. This work examines the effect of compaction conditions on the release of biochar particles disintegrated during compaction, and the change in their capacity to infiltrate stormwater and remove E. coli. The net loss of biochar particles by mobilization with stormwater was insignificant compared to the biochar remained in the filters. The initial release of biochar particles in wet-compacted biochar columns was greater than that in dry-compacted biochar. The results revealed that compaction can affect the release of biochar particles in a series of three-step processes: generation of particles by disintegration of large biochar under compaction, diffusion of particles deposited near grain walls to bulk pore water, and transport and retention of particles in constricted pore paths based on pore water connectivity. Under similar conditions, compost columns released more particles than biochar columns, suggesting biochar is more stable than compost under compaction. E. coli removal in wet-compacted columns was greater than removal in dry-compacted columns, owing to greater pore path connectivity in wet-compacted columns. These results indicate that addition of moisture during compaction can increase contaminant removal, initial particle release, and infiltration capacity of biochar-augmented sand filters for road runoff treatment. The results would help develop design guidelines for roadside stormwater treatment systems that require compaction of filter media.

Sulfide reduction can significantly enhance transport of biochar fine particles in saturated porous medium

The release of fine particles from biochar materials applied in the environment may have important environmental implications, such as mobilization of environmental contaminants. In natural environments biochar fine particles can undergo various transformation processes, which may change their surface chemistry and consequently, the mobility of the particles. Here, we show that sulfide reduction can significantly alter the transport of wheat-straw- and pine-wood-derived biochar fine particles in saturated porous media. Counterintuitively, the sulfide-reduced biochar particles exhibited greater mobility in artificial groundwater than their non-reduced counterparts, even though reduction led to decrease of surface charge negativity and increase of hydrophobicity (from the removal of surface O-functional groups), both should favor particle deposition, as predicted based on extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory. Using transport experiments conducted in single-cation background solutions containing K⁺, Mg²⁺ or Ca²⁺ under different pH conditions, we show that the surprisingly greater mobility of sulfide-reduced biochar particles was attributable to the removal of surface carboxyl groups during reduction, as this markedly alleviated particle deposition through cation bridging, wherein Ca²⁺ acted as the bridging agent in linking the surface O-functional groups of biochar particles and quartz sand. These findings show the critical roles of surface properties in dictating the mobility of biochar fine particles and call for further understanding of their transport properties, which apparently cannot be simply extrapolated based on the findings of other (engineered) carbonaceous nanomaterials.

Transport and transformation of Cd between biochar and soil under combined dry-wet and freeze-thaw aging

We quantified the transport and transformation of Cd in historically contaminated soil (OS) and artificially contaminated soil (NS), treated with 3% (w/w) rice straw biochar prepared at 400 °C (BC400) and 700 °C (BC700) under combined dry-wet and freeze-thaw cycles for 72 days simulating the natural aging process of 8 years. An improved three-layer mesh experiment was developed to simulate the natural situation in field. The result showed that the total Cd concentration increased in the biochar but decreased in the soil, suggesting that Cd was transported from the soil into the biochar during the aging process. The total Cd concentration in BC400 treated with both soils was higher than that in BC700 treated with both soils, however, BC700 displayed stronger ability on immobilizing Cd than BC400 because the Tessier exchangeable Cd fraction in BC700 treated both soils was lower than that in BC400 treated with both soils. The average Tessier exchangeable Cd fraction in the soil and biochar decreased in all treatments during the aging process, indicating that Cd tended to be more stable in the soil for a long term. The result also showed that biochar could immobilizate Cd by decreasing the Tessier exchangeable Cd fraction of soil and biochar, and the quantitative contributions of biochar and soil to Cd immobilization were different in OS and NS treated with BC400 and BC700. The biochar contribution to the reduction in Tessier exchangeable Cd fraction accounted for 40-85% in NS treated with BC400 and 54-82% in NS treated with BC700. However, in OS treated with biochar, the biochar contribution accounted for nearly 100%, and soil had almost no contribution. In summary, OS did not contribute to Cd immobilization, while NS contributed nearly 50% to Cd immobilization, and BC700 was more effective in immobilizing Cd than BC400.

Removal of E. coli from stormwater by bioretention system: parameter optimization and mechanism

Four bioretention simulation columns were used to study the removal effects and influencing factors of Escherichia coli (E. coli) in stormwater. The mechanism of E. coli removal in the bioretention system was also analyzed. The results show that the removal effects of the four new composite filters are better than that of the conventional filter. The specific surface area and porosity of the filter may be the key factors affecting the removal effect; the increase of the filter depth is beneficial to the removal of E. coli; the area is conducive to the removal of E. coli. Excessive depth of the submerged zone will reduce the E. coli removal effect; drying will reduce the E. coli removal effect, but it can be restored by rewetting.

Waste-derived compost and biochar amendments for stormwater treatment in bioretention column: Co-transport of metals and colloids

Bioretention systems, as one of the most practical management operations for low impact development of water recovery, utilize different soil amendments to remove contaminants from stormwater. For the sake of urban sustainability, the utilization of amendments derived from waste materials has a potential to reduce waste disposal at landfill while improving the quality of stormwater discharge. This study investigated the efficiency of food waste compost and wood waste biochar for metal removal from synthetic stormwater runoff under intermittent flow and co-presence of colloids. Throughout intermittent infiltration of 84 pore volumes of stormwater, columns amended with compost and biochar removed more than 50-70% of influent metals, whereas iron-oxide coated sand was much less effective. Only a small portion of metals adsorbed on the compost (< 0.74%) was reactivated during the drainage of urban pipelines that do not flow frequently, owing to abundant oxygen-containing functional groups in compost. In comparison, co-existing kaolinite enhanced metal removal by biochar owing to the abundance of active sites, whereas co-existing humic acid facilitated mobilization via metal-humate complexation. The results suggest that both waste-derived compost and biochar show promising potential for stormwater harvesting, while biochar is expected to be more recalcitrant and desirable in field-scale bioretention systems.

Post-treatment of paint industry effluents by filtration using Andropogon biochar (Andropogon gayanus Kunth cv. Planaltina)

This study evaluates the filtration potential of the biomass obtained from Andropogon grass (Andropogon gayanus Kunth cv. Planaltina) that was converted to biochar by pyrolysis. The biochar is used in filtration systems for the post-treatment of paint industry effluents. The biomass is characterized by elemental analysis (CHSN-O), determination of specific compounds (cellulose/hemicellulose/lignin), FTIR, and SEM. The produced biochar is characterized by SEM, TGA, and surface area analysis. The efficiency of the filters is evaluated by the following parameters: color, turbidity, removal of total solids (suspended and sedimentable), chemical oxygen demand (COD), and metals (Al, Cu, Zn, Co, Cd, and Cr(VI)). Over 99% removal of aluminum, cadmium, and hexavalent chromium is achieved. Moreover, almost 100% of COD and solids are removed, whereas turbidity and color are reduced by over 90%.

Fecal indicator bacteria and virus removal in stormwater biofilters: Effects of biochar, media saturation, and field conditioning

Stormwater biofilters are used to attenuate the flow and volume of runoff and reduce pollutant loading to aquatic systems. However, the capacity of biofilters to remove microbial contaminants remains inadequate. While biochar has demonstrated promise as an amendment to improve microbial removal in laboratory-scale biofilters, it is uncertain if the results are generalizable to the field. To assess biochar performance in a simulated field setting, sand and biochar-amended sand biofilters were periodically dosed with natural stormwater over a 61-week conditioning phase. Impact of media saturation was assessed by maintaining biofilters with and without a saturated zone. Biochar-amended biofilters demonstrated improved Escherichia coli removal over sand biofilters during the first 31 weeks of conditioning though media type did not impact E. coli removal during the last 30 weeks of conditioning. Presence of a saturated zone was not a significant factor influencing E. coli removal across the entire conditioning phase. Following conditioning, biofilters underwent challenge tests using stormwater spiked with wastewater to assess their capacity to remove wastewater-derived E. coli, enterococci, and male-specific (F+) coliphage. In challenge tests, biochar-amended biofilters demonstrated enhanced removal of all fecal indicators relative to sand biofilters. Additionally, saturated biofilters demonstrated greater removal of fecal indicators than unsaturated biofilters for both media types. Discrepant conclusions from the conditioning phase and challenge tests may be due to variable influent chemistry, dissimilar transport of E. coli indigenous to stormwater and those indigenous to wastewater, and differences in E. coli removal mechanisms between tests. Mobilization tests conducted following challenge tests showed minimal (<2.5%) observable mobilization of fecal indicators, regardless of media type and presence of a saturated zone. While our results emphasize the challenge of translating biochar’s performance from the laboratory to the field, findings of this study inform biofilter design to remove microbial contaminants from urban stormwater.

Role of metal modified water treatment residual on removal of Escherichia coli from stormwater runoff

Extensive studies have been conducted on bioretention filter media applied in best management practices for stormwater runoff treatment. To date, more reported studies are focused on pollutants elimination such as suspended solids and nutrients. There has been limited research on pathogen removal from stormwater runoff. More focused studies on pathogen removal are therefore required if the intended stormwater is harvested for indirect potable use. In this study, water treatment residuals (WTR), a recycled biofilter media was surface-modified with metals to assess its potential for E. coli removal from stormwater runoff. To achieve this goal, four types of modified WTRs, prepared using iron, copper, platinum, and silver as antibacterial agents, were tested in parallel batch tests. After the cost-effectiveness evaluation among the four modified WTRs for bacterial removal, Fe₂O₃- and CuO-WTRs were shortlisted for further mechanism and stability studies. Stable antibacterial performances (E. coli log removal of 0.58 ± 0.04 and 0.90 ± 0.04, respectively) were achieved using the Fe₂O₃- and CuO-WTRs under intermittent synthetic and natural stormwater runoff conditions. No significant metal leaching was observed over prolonged continuous treatment. The experimental results showed the bio-adsorption onto the surface modified Fe₂O₃- and CuO-WTR was a key mechanism for E. coli removal followed by E. coli inactivation at solid-liquid interface caused by the antibacterial effect of metal coatings (where CuO was reported to have higher biotoxicity than Fe₂O₃). These findings clearly suggested the potential of CuO-modified WTR for pathogen removal in stormwater treatment practices.

Polymer-clay composite geomedia for sorptive removal of trace organic compounds and metals in urban stormwater

Functionalized polymer-clay composites were developed and characterized as engineered geomedia for trace contaminant removal during infiltration of urban runoff. Montmorillonite clays were functionalized with either poly(diallyldimethylammonium) chloride (PDADMAC) or poly(4-vinylpyridine-co-styrene) (PVPcoS) to enhance organic compound sorption using a simple, scalable synthesis method. Seven representative trace organic compounds and six trace metals were employed to assess the performance of the polymer-clay composites relative to biochar (i.e., an adsorbent proposed for similar purposes) in batch sorption and column studies under simulated stormwater conditions. Contaminant and geomedia electrostatic and hydrophobic interactions, and the presence of natural organic matter (NOM) affected sorption. In batch studies, polymer-clay composites exhibited similar performance to biochar for perfluoroalkyl substance removal, but had lower affinity for polar pesticides and tris(2-chloroethyl) phosphate. Oxyanion removal was greatest for positively-charged PDADMAC-clay composites (particularly Cr[VI]), while PVPcoS-clay composites removed over 95% of Ni, Cd, and Cu. NOM decreased removal of all organic compounds, but increased trace metal removal on clay composites due to sorption of NOM-complexed metals. Polymer-clay composite-amended columns best removed oxyanions, while biochar-amended columns exhibited superior removal for all trace organics. At 3 wt% geomedia-sand loading, clay composites exhibited significantly higher saturated hydraulic conductivity than biochar, which is advantageous when clogging is a concern or when rapid infiltration is needed. Under typical urban stormwater conditions, the clay composites will remove contaminants for at least 20-30 years before regeneration or replacement is needed.

Contrasting impacts of pre- and post-application aging of biochar on the immobilization of Cd in contaminated soils

Biochar has been used for immobilizing heavy metals in soils due to its abundant surface functional groups and mineral components. However, as carbonaceous material, biochar in soils is susceptible to natural aging, which could alter its structural properties and ability to retain heavy metals. In this study, the impacts of pre- and post-application aging on the properties of dairy manure and sawdust biochars and the significance of different mechanisms of complexation with surface functional groups and mineral precipitation for Cd immobilization in soils were investigated. The simulated natural aging including 25 dry-wet cycles and 25 freeze-thaw cycles decreased the pH of biochar from 8.2 to 10.7 to 7.5-9.7 while increased the release of dissolved organic carbon from 4.1 to 10.9 to 5.9-21.3 mg L^-1, O/C ratios from 0.09 to 0.30 to 0.17-0.33, and O-containing functional groups, especially -OH and -C=O groups. New minerals such as CaC₂O₄ and MgO·MgCO₃ were formed during freeze-thaw cycles. The capacity of pre-aged biochars to immobilize Cd in soils decreased from 57-70% to 53-63%, compared to that of fresh biochar, which was mainly due to decrease of CdCO₃ or Cd₃(PO₄)₂ precipitation as evidenced by XRD and MINTEQ modeling. By contrast, post-aging of biochar application in soil with dry-wet and freeze-thaw cycling had little effect on the biochar's alkalinity while increased the O/C ratios from 0.10 to 0.24 to 0.15-0.27 and intensity of O-containing functional groups. The immobilization capacity of biochars for Cd in soil increased from 44-68% to 59-73% due to the enhanced surface complexation with O-containing groups. In short, biochar subjected to pre-aging had a reduced capacity to immobilize Cd, while biochars with post-aging in soil increased its Cd immobilization capacity as a soil amendment.

Formation and Physicochemical Characteristics of Nano Biochar: Insight into Chemical and Colloidal Stability

Nano biochar (N-BC) attracts increasing interest due to its unique environmental behavior. However, understanding of its formation, physicochemical characteristics, and stability of N-BC is limited. We therefore examined N-BC formation from bulk biochars (B-BCs) produced from peanut shell, cotton straw, Chinese medicine residues, and furfural residues at 300-600 °C. Carbon stability and colloidal processes of nano peanut shell biochars (N-PBCs) were further investigated. N-BCs formed from pore collapse and skeleton fracture during biomass charring, breakup due to grinding, and sonication. Amorphous fraction in B-BCs was more readily degraded into N-BCs than graphitic component. The sonication-formed N-PBCs contained 19.2-31.8% higher oxygen and fewer aromatic structures than the bulk ones, leading to lower carbon stability, but better dispersibility in water. Heteroaggregation of N-PBCs with goethite/hematite destabilized initially and then restabilized with increasing concentrations of N-PBCs. Compared with stacked complexes of N-PBCs-hematite, the association of goethite with N-PBCs could form interlaced heterostructures, thus shielding positive charges on goethite and causing greater heteroaggregation. These findings are useful for better understanding the formation of N-BCs and their environmental fate and behavior in soil and water.

Plenty of room for carbon on the ground: Potential applications of biochar for stormwater treatment

Low impact development (LID) systems are increasingly used to manage stormwater, but they have limited capacity to treat stormwater-a resource to supplement existing water supply in water-stressed urban areas. To enhance their pollutant removal capacity, infiltration-based LID systems can be augmented with natural or engineered geomedia that meet the following criteria: they should be economical, readily available, and have capacity to remove a wide range of stormwater pollutants in conditions expected during intermittent infiltration of stormwater. Biochar, a carbonaceous porous co-product of waste biomass pyrolysis/gasification, meets all these criteria. Biochar can adsorb pollutants, improve water-retention capacity of soil, retain and slowly release nutrients for plant uptake, and help sustain microbiota in soil and plants atop; all these attributes could help improve removal of contaminants in stormwater treatment systems. This article discusses contaminant removal mechanisms by biochar, summarizes specific biochar properties that enhance targeted contaminants removal from stormwater, and identifies challenges and opportunities to retrofit biochar in LID to optimize stormwater treatment.

Surface-modified biochar in a bioretention system for Escherichia coli removal from stormwater

Bioretention systems have been recommended as one of the best management practices for low impact development for water recycling/reuse systems. Although improvement of the stormwater quality has been reported regarding pollutants eliminations such as suspended solids and heavy metals, a substantial removal of indicator bacteria is required for possible non-potable reuse. This study investigated the efficiency of wood biochar with H₂SO₄-, H₃PO₄-, KOH-, and amino-modifications for E. coli removal from synthetic stormwater under intermittent flow. The H₂SO₄-modified biochar showed a specific surface area of 234.7 m² g^-1 (approximately double the area of original biochar), whereas a substantial reduction in surface area was found with amino-modified biochar. The E. coli removal (initial concentration of 0.3-3.2 × 10⁶ CFU mL^-1) by modified biochars as filter media was very promising with, for example, over 98% removal efficiency in the first 20 pore volumes of stormwater infiltration and over 92% removal by the end of the second infiltration cycle. Only a small portion of E. coli attached on the modified biochars (<0.3%, except KOH- and amino-modified biochars) was remobilized during the drainage phase of intermittent flow. The high removal capacity and stability against drainage were attributed to the high surface area, porous structure, and surface characteristics (e.g. hydrophobicity and O-containing functional groups) of the biochars. Thus, the H₂SO₄-modified biochar appeared to give the best treatment performance.

E. coli Surface Properties Differ between Stream Water and Sediment Environments

The importance of E. coli as an indicator organism in fresh water has led to numerous studies focusing on cell properties and transport behavior. However, previous studies have been unable to assess if differences in E. coli cell surface properties and genomic variation are associated with different environmental habitats. In this study, we investigated the variation in characteristics of E. coli obtained from stream water and stream bottom sediments. Cell properties were measured for 77 genomically different E. coli strains (44 strains isolated from sediments and 33 strains isolated from water) under common stream conditions in the Upper Midwestern United States: pH 8.0, ionic strength 10 mM and 22°C. Measured cell properties include hydrophobicity, zeta potential, net charge, total acidity, and extracellular polymeric substance (EPS) composition. Our results indicate that stream sediment E. coli had significantly greater hydrophobicity, greater EPS protein content and EPS sugar content, less negative net charge, and higher point of zero charge than stream water E. coli. A significant positive correlation was observed between hydrophobicity and EPS protein for stream sediment E. coli but not for stream water E. coli. Additionally, E. coli surviving in the same habitat tended to have significantly larger (GTG)₅ genome similarity. After accounting for the intrinsic impact from the genome, environmental habitat was determined to be a factor influencing some cell surface properties, such as hydrophobicity. The diversity of cell properties and its resulting impact on particle interactions should be considered for environmental fate and transport modeling of aquatic indicator organisms such as E. coli.

Particulate matter emissions from biochar-amended soils as a potential tradeoff to the negative emission potential

Novel carbon sequestration strategies such as large-scale land application of biochar may provide sustainable pathways to increase the terrestrial storage of carbon. Biochar has a long residence time in the soil and hence comprehensive studies are urgently needed to quantify the environmental impacts of large-scale biochar application. In particular, black carbon emissions from soils amended with biochar may counteract the negative emission potential due to the impacts on air quality, climate, and biogeochemical cycles. We investigated, using wind tunnel experiments, the particulate matter emission potential of a sand and two agriculturally important soils amended with different concentrations of biochar, in comparison to control soils. Our results indicate that biochar application considerably increases particulate emissions possibly by two mechanisms–the accelerated emission of fine biochar particles and the generation and emission of fine biochar particles resulting from abrasion of large biochar particles by sand grains. Our study highlights the importance of considering the background soil properties (e.g., texture) and geomorphological processes (e.g., aeolian transport) for biochar-based carbon sequestration programs.