Release and stability of water dispersible biochar colloids in aquatic environments: Effects of pyrolysis temperature, particle size, and solution chemistry

Visualization of phenanthrene effect on biochar colloids transport in porous media

Biochar colloids (BCs) can be used as adsorbent materials for remediation of phenanthrene due to their high specific surface area and other characteristics. Understanding the effects of phenanthrene on the transport of BCs contributes to facilitate the removal of phenanthrene in soil and water habitats. In this work, the influence of phenanthrene on the transport of BCs under different environmental factors (pH, ionic strength (IS), media size) in a one-dimensional sand column was firstly explored together with a real-time visualization system to explore the transport mechanism of BCs in two-dimensional sand tank. The results show that phenanthrene adsorbed on the surface of BCs, shielded its surface charge and reduced the mobility of BCs in porous media. Acidic conditions promoted the agglomeration of BCs and adsorption of phenanthrene, resulting in a 51.03 % decrease in the maximum breakthrough rate of BCs compared to alkaline conditions. The same was true for the high IS condition, where the maximum breakthrough rate of BCs was only 0.95 % at IS = 50. Additionally, there was a substantial and positive correlation between media particle size and BCs mobility. As the quartz sand particle size increased, the maximum breakthrough rates of BCs were 2.67 %, 33.28 %, and 52.27 % in the 1-D experiment, and 0, 13.88 %, and 13.10 % in the 2-D experiment, respectively. The contact area of BCs with the medium expands under the fine particle size condition, leading to a significant decrease in the mobility of BCs at low potentials influenced by phenanthrene. This finding is significant for biochar application in phenanthrene contaminated soil and groundwater remediation.

New Insights into Aggregation Behaviors of the UV-Irradiated Dissolved Biochars (DBioCs) in Aqueous Environments: Effects of Water Chemistries and Variation in the Hamaker Constant

The aggregation behavior of ubiquitous dissolved black carbon (DBC) largely affects the fate and transport of its own contaminants and the attached contaminants. However, the photoaging processes and resulting effects on its colloidal stability remain yet unknown. Herein, dissolved biochars (DBioCs) were extracted from common wheat straw biochar as a proxy for an anthropogenic DBC. The influences of UV radiation on their aggregation kinetics were systematically investigated under various water chemistries (pH, electrolytes, and protein). The environmental stability of the DBioCs before and after radiation was further verified in two natural water samples. Hamaker constants of pristine and photoaged DBioCs were derived according to Derjaguin-Landau-Verwey-Overbeek (DLVO) prediction, and its attenuation (3.19 ± 0.15 × 10-21 J to 1.55 ± 0.07 × 10-21 J after 7 days of radiation) was described with decay kinetic models. Pearson correlation analysis revealed that the surface properties and aggregation behaviors of DBioCs were significantly correlated with radiation time (p < 0.05), indicating its profound effects. Based on characterization and experimental results, we proposed a three-stage mechanism (contended by photodecarboxylation, photo-oxidation, and mineral exposure) that DBioCs might experience under UV radiation. These findings would provide an important reference for potential phototransformation processes and relevant behavioral changes that DBC may encounter.

Properties of biochar colloids and behaviors in the soil environment: Influencing the migration of heavy metals

Biochar pyrolyzed by biomass shows excellent application prospects for heavy metal (HM) remediation, but a part of biochar can be inevitably broken into micro- and nano-sized biochar colloids (BCs) under biological and physicochemical actions in soil. BCs derived in the process of remediation have rough surface, rich elemental species and contents, and multiple functional groups, which are similar to biochar. However, BCs have some unique colloidal properties because of their micro and nano scale size. Due to these properties, BCs exhibit strong mobilities in the soil environment, and the mobilities may be influenced by a combination of colloidal properties of BCs and environmental factors including soil colloids and other soil environmental conditions. In addition, BCs may have affinity effects on HMs through electrostatic adsorption, ion exchange, surface complexation, precipitation/co-precipitation, and redox because of the properties such as large specific surface area, and rich oxygen-containing functional groups and minerals on the surface. This review summarizes the physicochemical and migratory properties of BCs, and the internal and external factors affecting the migration of BCs in the soil environment, and the possible effects of BCs on HMs are high-lighted. This review provides a theoretical basis for the optimization of soil contaminated with HMs after remediation using biochar. Notably, the innovative idea that BCs may influence the presence of HMs in soil needs to be further confirmed by more targeted detection and analysis methods in future studies to prevent the possible environmental toxicities of the lateral and vertical diffusion of HM caused by BCs in soil.

Differential photodegradation processes of adsorbed polychlorinated biphenyls on biochar colloids with various pyrolysis temperatures

Despite the crucial role of photodegradation in the environmental transformation of organic pollutants, the photodegradation process of organic pollutants irreversibly absorbed on biochar colloids (BCCs) remains poorly understood. This study investigated the photodegradation processes and mechanisms of 2,4,4'-trichlorobiphenyl (PCB28) adsorbed on BCCs released from bulk biochars derived from bamboo chips at pyrolysis temperatures of 300, 500, and 700 °C. Results show that BCCs-adsorbed PCB28 could be degraded under simulated solar illumination (95-105 mW·cm-2) but at decreased photodegradation rates compared to the dissolved PCB28. The inhibition effect of BCCs on the PCB28 photodegradation increased with increasing pyrolysis temperature. After adsorptive binding to BCCs, the half-life of PCB28 (0.1 mg/L) was prolonged from 2.65 h for the dissolved PCB28 alone in deionized water to 7.48, 40.67, and 81.82 h in the presence of BCC300, BCC500, and BCC700 (5.0 mg/L), respectively. Mechanistically, the photodegradation of adsorbed pollutants was regulated by the photogenerated free radicals and surface functional groups of the low-temperature BCCs, as well as the defects and direct electron transfer capabilities of the high-temperature BCCs; PCB28 adsorbed on the low-temperature BCCs accepted electrons from persistent free radicals under light illumination, which led to PCB28 dechlorination, followed by ring-opening oxidation through hydroxyl radical attack, ultimately resulting in progressive mineralization; singlet oxygen caused preferential ring opening of adsorbed PCB28 on the high-temperature BCCs, preceding dechlorination. The photodegradation of BCCs-adsorbed PCB28 remained significant though more or less being inhibited under the effects of water pH, ionic strength, dissolved organic matters (humic acid and fulvic acid), and in natural water samples. These findings contribute to a better understanding of the structural properties of BCCs that impact phototransformation processes of adsorbed pollutants and facilitate an accurate assessment of the environmental risk associated with biochar application.

Effects of chemical aging on carbonaceous materials: Stability of water-dispersible colloids and their influence on the aggregation of natural-soil colloid

Although hydrochar and biochar have been used as soil conditioners, there is not a clear understanding of how their properties changes due to aging impacts their colloidal particles behavior on the soil system. From this premise, we produced hydrochar and biochar from the same feedstock (cashew bagasse) and aged with different chemical methods: (i) using hydrogen peroxide, (ii) a mixture of nitric and sulfuric acids, and (iii) hot water. It was analyzed the effects of aging on the stability of the carbonaceous materials (CMs) colloids in aqueous medium with different ionic strength (single systems), as well as the stability of the natural-soil colloid when interacting with biochar and hydrochar colloids (binary systems). A chemical composition (C, H, N, and O content) change in CMs due to the chemically induced aging was observed along with minor structural modifications. Chemical aging could increase the amount of oxygen functional groups for both biochar and hydrochar, though in a different level depending on the methodology applied. In this sense, hydrochar was more susceptive to chemical oxidation than biochar. The effectiveness of chemical aging treatments for biochar increased in the order of water < acid < hydrogen peroxide, whereas for hydrochar the order was water < hydrogen peroxide < acid. While the increase in surface oxidation improved the biochar colloidal stability in water medium at different ionic strengths (single systems), the stability and critical coagulation concentration (CCC) slightly changed for hydrochar. Natural-soil clay (NSC) interactions with oxidized carbonaceous material colloids (binary systems) enhanced NSC stability, which is less likely to aggregate. Therefore, the aging of carbonaceous materials modifies the interaction and dynamics of soil small particles, requiring far more attention to the environmental risks due to their application over time.

Release behavior of fertilizers and heavy metals from iron-loaded sludge biochar in the aqueous environment

In this study, the release behavior of fertilizers (NH4+-N, PO43- and K) and heavy metals (Mn, Zn, Ni, Cu, Pb and Cr) from iron-loaded sludge biochar (ISBC) was investigated to evaluated the feasibility and risks of ISBC as a slow release fertilizer. Their release capacity was significantly enhanced with decreasing initial pH, increasing solid-liquid ratio (RS-L) and rising temperature (p < 0.05). When the initial pH, RS-L and temperature were separately 5 (fertilizers)/1 (heavy metals), 1:5 and 298 K, the final concentrations of NH4+-N, PO43-, K, Mn, Zn and Ni were 6.60, 14.13, 149.4, 53.69, 72.56, and 1.01 mg L-1, while the maximum concentrations of Cu, Pb and Cr were 0.94, 0.77, and 0.22 mg L-1, respectively. Due to the tiny difference between the R2 values, revised pseudo-first-order and pseudo-second-order kinetics models described their release behavior well, suggesting that physical and chemical interactions played an important role. Activation energies greater than 40 kJ mol-1 indicated that the rate-controlling steps of the release of NH4+-N, PO43- and Ni were chemical reactions, while chemical reactions and diffusion together determined the release rates of K, Mn, Zn, Cu, Pb and Cr because their activation energies were in the range of 20-40 kJ mol-1. The increasingly negative ΔG and positive ΔH and ΔS suggested that their release was a spontaneous (except Cr) and endothermic process with an increase of randomness between the solid-liquid interface. The release efficiency of NH4+-N, PO43- and K were in the ranges of 28.21%-53.97%, 2.09%-18.06% and 39.46%-66.14%, respectively. Meanwhile, the pollution index and evaluation index of heavy metals were in the ranges of 33.31-227.4 and 4.64-29.24, respectively. In summary, ISBC could be used as a slow-release fertilizer with low risk when the RS-L was less than 1:40.

Influence of dissolved organic matter, kaolinite, and iron oxides on aggregation and transport of biochar colloids in aqueous and soil environments

The aggregation and transport of biochar colloids (BCs) in the soil and groundwater are critical for applying biochar in the field and assessing long-term environmental risk. This research aimed to study the influence of dissolved organic matter (DOM) with different molecular weights (including humic acid, HA; bovine serum albumin, BSA; deoxyribonucleic acid, DNA) and three minerals (including kaolinite, goethite, and hematite) on the aggregation and transport behaviors of BCs. The adsorption of DOM on the surface of BCs increased the stability, inhibited aggregation, and promoted the transport of BCs. As the molecular weight of DOM increased, the thicknesses of the adsorption layer of HA, BSA, and DNA on BCs surface were 2.2 nm, 5.3 nm, and 5.6 nm, respectively, resulting in increasing steric hindrance and improving the stability and mobility of BCs. Kaolinite also significantly enhanced the stability and mobility of BCs by increasing the electrostatic repulsion. Goethite and hematite quickly combined with BCs through electrostatic attraction, resulting in stronger aggregation and retention of BCs. Compared to hematite, goethite provided more adsorption sites for BCs due to its needle-like shape, so goethite caused a larger heteroaggregation rate. Overall, the presence of DOM with different molecular weights and the minerals with varying surface charges in the soil environment had a significant and distinct impact on the stability, aggregation, and transport of BCs, which advances the knowledge of colloidal biochar fate in the soil and groundwater.

Pyrolysis temperature affects the inhibitory mechanism of biochars on the mobility of extracellular antibiotic resistance genes in saturated porous media

The migration of extracellular antibiotic resistance genes (eARGs) in porous media is an important pathway for ARGs to spread to the subsoil and aquifer. Biochar (BC) has been widely used to reduce the mobility of soil contaminants, however, its effect on the mobility of eARGs in porous media and the mechanisms are largely unknown. Herein, the effects of BCs synthesized from wheat straw and corn straw at two pyrolysis temperatures (300 °C and 700 °C) on the transport of plasmids-carried eARGs in sand column were investigated. The BC amendments all significantly decreased the mobility of eARGs in the porous medium, but the mechanism varied with pyrolysis temperature. The higher temperature BCs had a stronger irreversible adsorption of plasmids and greatly enhanced the attachment and straining effects on plasmids during transport, thus more effectively inhibited the mobility of eARGs. The lower temperature BCs had weaker adsorption, attachment, and straining effects on plasmids, but induced generation of hydroxyl radicals in the porous medium and thereby fragmented the plasmids and hindered the amplification of eARGs. These findings are of fundamental significance for the potential application of BC in controlling the vertical spread of eARGs in soil and vadose zones.

Aggregation kinetics of biochar nanoparticles in aqueous environment: Interplays of anion type and bovine serum albumin

The colloidal particles, especially those at the nanoscale, are the most active part of the pyrogenic carbon (biochar). Increasingly applied biochar has resulted in a large number of biochar nanoparticles (NPs) being released into the environment. The aggregation of biochar NPs affects their environmental behavior and fate. The complex effects of anion type (Cl^-, SO₄^2-) and protein (bovine serum albumin, BSA) on the aggregation of wheat straw biochar (WB) and pinewood biochar (PB) NPs in solutions were investigated by the time-resolved dynamic light scattering method. The critical coagulation concentration (CCC) of WB and PB NPs in Na₂SO₄ solution was higher than their CCCs in NaCl solution, which was consistent with the Hofmeister series that SO₄^2-, a kosmotrope anion, increased the interaction between water molecules, thus enhancing the hydrophobic interactions between biochar NPs in solution and promoting their aggregation, while Cl^-, a chaotropic agent, exhibited the opposite effect. When BSA was added into the solution, BSA was adsorbed on the surface of biochar NPs and BSA corona was formed, which inhibited the aggregation of biochar NPs by inducing steric force. The enhanced stability of biochar NPs by BSA was more significant in NaCl than in Na₂SO₄ solution because BSA corona had a more negatively charged surface and a more steric structure in NaCl solution, thus generating stronger electrical repulsion and steric hindrance. The classical DLVO theory and the XDLVO theory incorporating the steric repulsion (in the presence of BSA) were used to interpret the aggregation and dispersion of biochar NPs. Through this study, we found that anion type indirectly affected the aggregation of biochar NPs by influencing the interaction between water molecules, while the aggregation of BSA-biochar NPs conjugates is mainly influenced by the surface charge and structure of BSA corona.

Pyrolyzed biomass-derived nanoparticles: a review of surface chemistry, contaminant mobility, and future research avenues to fill the gaps

Nanoparticles are abundant in the subsurface, soil, streams, and water bodies, and are often a critical control on elemental speciation, transport and cycling in the natural environment. This review provides an overview of pyrolyzed biomass-derived nanoparticles (PBNPs), their surface properties and reactivity towards aqueous species. We focus specifically on biochar-derived nanoparticles and activated carbon-derived nanoparticles which fall under our classification of PBNPs. Activated carbon-iron (nano)composites are included in some instances where there are significant gaps in literature because of their environmental relevance. Increased use of activated carbon, along with a resurgence in the manufacture and application of biochar for water treatment and soil amendment, has generated significant concerns about the mobility and toxicity of PBNPs derived from the bulk material in environmental applications. Recent examples are discussed to highlight current progress in understanding the influence of PBNPs on contaminant transport, followed by a critical discussion of gaps and future research directions.

Dynamic Evolution and Covariant Response Mechanism of Volatile Organic Compounds and Residual Functional Groups during the Online Pyrolysis of Coal and Biomass Fuels

Volatile organic compound (VOC) emissions from pyrolysis of widely used biomass are expected to increase significantly under the carbon neutrality target. However, the dynamic emissions and evolution mechanism of biomass-VOCs remain unclear, hindered by complex reactions and offline measurements. Here, we propose a novel covariant evolution mechanism to interpret the emission heterogeneities, sequential temperature responses, and evolved correlations of both VOCs and residual functional groups (RFGs) during corn straw (CS), wood pellet (WP), and semibituminous coal (SBC) pyrolysis. An innovative combination of online thermogravimetric-Fourier transform infrared-gas chromatography/mass spectrometry and two dimensional-correlation spectroscopy was applied. The relative percentages of CS/WP-VOCs were higher than those of SBC-VOCs, and most VOCs tended to have relatively small carbon skeletons as the average carbon oxidation state increased. With the temperature increased from low to high during CS/WP pyrolysis, the primary sequential response of VOCs (acids → phenols/esters → alcohols/ethers/aldehydes/ketones → hydrocarbons/aromatics) corresponded to the RFG response (hydroxyl groups → -CH₃/-CH₂-/-CH groups → aliphatic ethers and conjugated ketones). Compared with the relative regularity for CS/WP responses, the gas-solid products from SBC pyrolysis exhibited complex temperature-dependent responses and high oxidation-induced variability. These insights provide favorable strategies for the online monitoring system to facilitate priority removal of coal and biomass fuels-VOCs.

Biochar effectively inhibits the horizontal transfer of antibiotic resistance genes via transformation

The rapid spread of antibiotic resistance genes (ARGs) has posed a risk to human health. Here, the effects of biochar (BC) on the horizontal transfer of ARG-carrying plasmids to Escherichia coli via transformation were systematically investigated. BC could significantly inhibit the transformation of ARGs and the inhibition degree increased with pyrolysis temperature. Rice straw-derived BC showed a stronger inhibitory effect on the transformation of ARGs than that of peanut shell-derived BC from the same pyrolysis temperature. The inhibitory effect of BC from low pyrolysis temperature (300 ℃) was mainly caused by BC dissolutions, while it was mainly attributed to BC solids for high pyrolysis temperature (700 ℃) BC. BC dissolutions could induce intramolecular condensation and even agglomeration of plasmids, hindering their transformation into competent bacteria. The cell membrane permeability was slightly decreased in BC dissolutions, which might also contribute to the inhibitory effect. Plasmid can be adsorbed by BC solids and the adsorption increased with BC pyrolysis temperature. Meanwhile, BC-adsorbed plasmid could hardly be transformed into E. coli. BC solids could also deactivate E. coli and thereby inhibit their uptake of ARGs. These findings provide a way using BC to limit the spread of ARGs in the environment.

Stability of biochar in mineral soils: Assessment methods, influencing factors and potential problems

Amendment of biochar into mineral soils has been reported a promising strategy for carbon sequestration and greenhouse gas mitigation due to its high stability. Currently, most studies on the stability of biochar are mainly focused on the assessment methods and influencing factors. The assessment methods include qualitative evaluation of physical and chemical properties, and utilization of kinetic mineralization models on the basis of laboratory incubation. As a result, these assessment methods are difficult to accurately reflect the real impact of the interaction between biochar and environmental factors. This article reviews the existing assessment methods, influencing factors, and the impact of environmental aging on the stability of biochar. It is found that under the influence of environmental factors, existing assessment methods are likely to overestimate the stability of biochar in mineral soils. Therefore, more emphases should be laid on the analyses of the deficiencies in the existing assessment methods on the stability of biochar in the consideration of practical applications. Long-term field experiment is strongly recommended to establish a more accurate assessment model on biochar stability for the evaluation of its carbon sequestration potential in mineral soils.

Overlooked contributions of biochar-derived dissolved organic matter on the adsorption of Pb (Ⅱ): Impacts of fractionation and interfacial force

Comprehensive understanding of how the release of biochar-derived dissolved organic matter (BDOM) affects the immobilization of heavy metals when biochar (BC) is applied for long-term soil remediation is extremely important. In this study, BCs prepared under different pyrolysis temperatures were fractionated into residual BC (RBC), nano-sized BC (NBC), and BDOM, in order to clarify the contribution of BDOM for lead (Pb(II)) adsorption on BC and to explore the interfacial mechanisms. Results demonstrated that the adsorption capacity (Q_e) of Pb(II) on BC improved from 166.1 to 423.9 mg g^-1 with the increase in the pyrolysis temperature from 350 to 800 °C. The sum of Q_e of Pb(II) on NBC and RBC was lower than that on BC, due to the complexation between BDOM and Pb(II) rather than pH variance and cation exchange. Ultraviolet-visible and fluorescence spectroscopy revealed that fulvic-like substances as well as small molecules with low aromaticity in BDOM underwent favorable association with Pb(II) and got re-adsorbed on RBC. With the increase in the Pb(II) concentration, the contribution of van der Waals interaction for adsorption of BDOM350-Pb complexes was improved, whereas adsorption mechanism in BDOM800-Pb complexes was more dependent on ligand exchange. This study provides mechanistic insights into the impact of BDOM on Pb(II) immobilization, which can provide valuable information for the long-term remediation of Pb-contaminated soils using BC.

Demulsification Treatment of Spent Metalworking Fluids by Metal Cations: The Synergistic Effect and Efficiency Evaluation

In this paper, various metal ions were utilized for the demulsification of spent metalworking fluids discharged from an automobile parts workshop. Five types of metal ions, i.e., Fe3+, Al3+, Fe2+, Ca2+ and Mg2+, combined with coagulant were systematically evaluated, and the synergistic effect as well as the optimum operating conditions were studied. The results indicated that the Ca2+ as well Mg2+ possessed hardly efficiency for the demulsification, on the contrary, Fe2+ reduced the yield of the by-product sludge and lowered the SV30 ratio, and Al3+ boosted the CODCr removal. Furthermore, Fe3+ and Al3+ had a significant synergistic effect to achieve a better transmittance and a higher CODCr/SV30 ratio which revealed that more CODCr was removed, as well as less by-product sludge was generated. For a better demulsification of spent metalworking fluids, the optimum operating conditions were gathered as follows: the dosage of metal ions was 0.08 mol/L with Al3+:Fe3+ ratio was 1.5:1, the reaction pH was 6.00, the reaction time was 18.00 min and the temperature was 323.00 K. Based on this, the CODCr removal, the SV30 ratio and the transmittance and CODCr/SV30 ratio of the spent metalworking fluids were 80.21%, 40.00%, 95.20% and 128.33 mg/mL, respectively. This combined metal ion demulsification method possessed an advantageous minimization of spent metalworking fluids, which greatly benefited the automobile parts workshops in cutting down the operating cost in environmental protection.

Accumulation of sulfamethazine and ciprofloxacin on grain surface decreases the transport of biochar colloids in saturated porous media

The increasing amount of antibiotics entering the environment through manure usage and sludge application from wastewater treatment plants (WWTP) attracts much concern due to their potential threat to ecological security and human health. When biochar, a soil and water amendment, is introduced into the soil for remediation, the antibiotics are usually co-present with the biochar colloids (BC) or pre-accumulated in soils. However, little is known about the effect of antibiotics on the behavior of BC. Column experiments were conducted at three different pH values to study the effect of sulfamethazine (SMT) or ciprofloxacin (CIP) on BC transport. Under certain conditions (co-present in the influent and pre-sorbed on quartz sand), large numbers of cation and zwitterion forms of the less mobile CIP at pH 5 and 7 led to less negatively-charged surface of BC and quartz sand, resulted in higher BC retention compared to the highly mobile SMT. The decrease in BC transport became more significant with a higher amount of SMT or CIP pre-sorption. Therefore, when biochar is applied into soils polluted by antibiotics, the pH-dependency and the loading amount of antibiotics in soil matrix should be paid attention to as they might affect the transport of BC and the related facilitated-contaminants transport.

Co-transport and retention of zwitterionic ciprofloxacin with nano-biochar in saturated porous media: Impact of oxidized aging

While biochar (BC) is used for contaminant remediation (i.e. antibiotics) in the field, geochemical aging can alter its chemical structure, releasing nano-sized BC (NBC, sizes ranging from approximately 200 nm to 500 nm), and further influence the environmental behaviour of antibiotics affiliated with BC. In this study, we comprehensively examined the sorption behaviour of NBCs with and without aging toward ciprofloxacin (CIP), their aggregation performance, and transport behaviour in porous media. The results showed that aging improved the oxygen-containing groups within the NBCs and made their surfaces more negatively charged. The thermodynamic enhancements of specific interactions (i.e. π-π interaction or Coulombic force) with CIP resulted in the enhancement of slow sorption (from 60-64% to 40-58%) and a higher normalised sorption capacity (Q_e). The aggregation of NBCs was affected by changes in individual specific interactions and interfacial forces between the NBCs before and after CIP sorption. Further, aging could enhance the transport of NBCs both in the absence and presence of CIP. In addition to the interaction with the quartz sand surface, the contributions of aggregation and chemical heterogeneity caused by rebalanced specific interactions with CIP, may explain the observed transport behaviours of the aged NBCs in porous media. Additionally, the presence of NBCs, regardless of aging, suppressed the transport of CIP. Thus, mechanisms such as increased sorption sites due to aggregation and competitive sorption between NBCs and CIP, rather than the contribution of co-transport from NBCs, might play an important role in determining the fate of CIP in the natural environment.

Release and sedimentation behaviors of biochar colloids in soil solutions

The release of biochar colloids considerably affects the stability of biochar in environment. Currently, information on the release behavior and suspension stability of biochar colloids in real soil solutions is scarce. In this study, 20 soils were collected from different districts in China and the release behavior of biochar colloids and their suspension stability in soil solutions were systematically examined. The results showed that both pyrolysis temperature and biomass source had important effects on the formation of biochar colloids in soil solutions. The formation amount of biochar colloids from low pyrolysis temperatures (400 °C) (average amount of 9.33-16.41 mg/g) were significantly higher than those from high pyrolysis temperatures (700 °C) (average amount of less than 2 mg/g). The formation amount of wheat straw-derived biochar colloids were higher than those of rice straw-derived biochar colloids probably due to the higher O/C ratio in wheat-straw biochar. Further, biochar colloidal formation amount was negatively correlated with comprehensive effect of dissolved organic carbon, Fe and Al in soil solutions. The sedimentation curve of biochar colloids in soil solutions is well described by an exponential model and demonstrated high suspension stability. Around 40% of the biochar colloids were maintained in the suspension at the final sedimentation equilibrium. The settling efficiency of biochar colloids was positively correlated with comprehensive effect of the ionic strength and K, Ca, Na, and Mg contents in soil solutions. Our findings help promote a deeper understanding of biochar loss and stability in the soil-water environment.

Co-transport of biochar colloids with organic contaminants in soil column

Co-transport of biochar (BC) colloids with coexisting organic contaminants (OCs) in soil involves complex interactions among BC colloids, OCs, and soil particles, which is significant for the environmental application and risk assessment of BC and yet has not been well addressed. This study explored co-transports of three typical OCs (i.e., phenanthrene (PHN), atrazine (ATZ), and oxytetracycline (OTC)) and BC colloids obtained from bulk BCs with different charring temperatures (200-700 °C) and particle sizes (250 nm, 500 nm, and 1 μm) in a soil column of 9 cm in height. Considerable transport of BC colloids alone was observed and the maximum breakthrough concentration (C/C_o) increased from 0.08 to 0.77 as the charring temperature decreased from 700 to 200 °C. The mobilities of PHN, OTC, and ATZ alone were very low but were greatly increased by co-transports with BC colloids, and their maximum C/C_o values were within 0.05-0.33, 0.03-0.44, and 0.05-0.62, respectively, in the absence and presence of various BC colloids. The enhancement effect of BC colloids on the OC transport decreased with increasing charring temperature or particle size of BC colloids. BC colloids mainly acted as a vehicle to facilitate the transport of OCs, and dissolved organic carbon from BC colloids also contributed to the increased mobility of OCs in dissolved form. These findings provide new insights into co-transport of BC colloids and contaminants in soil.