Sorption mechanisms of phenanthrene, lindane, and atrazine with various humic acid fractions from a single soil sample

A commercial humic acid inhibits benzo(a)pyrene biodegradation by Paracoccus aminovorans HPD-2

Benzo(a)pyrene (BaP) is posing serious threats to soil ecosystems and its bioremediation usually limited by environmental factors and microbial activity. Humic acid (HA), a ubiquitous heterogeneous organic matter, which could affect the fate of environmental pollutants. However, the impact of HA on bioremediation of organic contamination remains controversial. In the present study, the biodegradation of BaP by Paracoccus aminovorans HPD-2 with and without HA was explored. Approximately 87.4 % of BaP was biodegraded in the HPD-2 treatment after 5 days of incubation, whereas the addition of HA dramatically reduced BaP biodegradation to 56.0 %. The limited BaP biodegradation in the HA + HPD-2 treatment was probably due to the decrease of BaP bioavailability which induced by the adsorption of HA with unspecific interactions. The excitation-emission matrix (EEM) of fluorescence characteristics showed that strain HPD-2 was responsible for the presence of protein-like substances and the microbial original humic substances in the HPD-2 treatment. Addition of HA would result in the increase of soluble microbial humic-like material, which should ascribe to the biodegradation of BaP and probably utilization of HA. Furthermore, both the growth and survival of strain HPD-2 were inhibited in the HA + HPD-2 treatment, because of the limited available carbon source (i.e. BaP) at the presence of HA. The expression of gene1789 and gene2589 dramatically decreased in the HA + HPD-2 treatment, and this should be responsible for the decrease of BaP biodegradation as well. This study reveals the mechanism that HA affect the BaP biodegradation, and the decrease of biodegradation should ascribe to the interaction of HA and bacterial strain. Thus, the bioremediation strategies of PAHs need to consider the effects of organic matter in environment.

Assessment of tolerance limits of petroleum residues in soil organic matter: sorption of dichlorobenzene by soil

Soil organic matter can protect plants and microorganisms from toxic substances. Beyond the tolerance limit, the toxicity of petroleum pollution to soil organisms may increase rapidly with the increase of petroleum content. However, the method for evaluating the petroleum tolerance limit of soil organic matter (SOM) is still lacking. In this study, the petroleum saturation limit in SOM was first evaluated by the sorption coefficient (Kd) of 1,2-dichlorobenzene (DCB) from water to soils containing different petroleum levels. The sorption isotherm of dichlorobenzene in several petroleum-contaminated soils with different organic matter content and the microbial toxicity test of several petroleum-contaminated soils were determined. It is found that when the petroleum content is about 5% of the soil organic matter content, the sorption of petroleum to organic matter reached saturation limit. When organic matter reaches petroleum saturation limit, the sorption coefficient of DCB by soil particles increased linearly with the increase of petroleum content (R2 > 0.991). The results provided important insights into the understanding the fate of petroleum pollutants in soil and the analysis of soil toxicity.

Star polymer soil delivery nanoplatform for applying biological agents in the field to control plant rhizosphere diseases

As the main cause of destructive plant diseases, pathogenic oomycete in plant rhizosphere brings about enormous losses to agricultural production. Although chemical pesticides are still one of the most important prevention and control methods for phytopathogens, the usage of chemical pesticides was limited by the 3R (resistance, residue, and rampant) problem. In the early stage of our research, analysis and comparison of the metabolome of resistance to Phytophthora nicotianae and common strain suggested that naringenin might be a highly efficient potential biogenic antimicrobial agent to prevent and control soil rhizosphere diseases. Unfortunately, the bioactivity and absorption capacity of active ingredients in the environment made it unsuitable for field application; thus, for efficient field application of naringenin, the 24 nm-sized naringenin-loaded nano-star-shaped polymerized (NSPs) were prepared with good loading efficiency 37.3% for naringenin. The soil mobility test indicated that NSPs could effectively reduce the adsorption of active ingredients and enhance the mobility of active ingredients in soil. The bacteriostatic test proved that these NSPs had better antimicrobial activity than the naringenin used alone and could efficiently induce the expression of plant resistance phenylpropanoid compounds. Finally, pot and field experiments showed improved control efficiency of NSPs 41% loaded with naringenin. Transcriptome analysis found that a large number of energy-related genes were downregulated in NSPs nematodes, suggesting that disturbed energy-related genes might lead to the disturbance of energy synthesis and metabolism. Naringenin-loaded nano-carriers were used to prevent and control plant disease-causing pathogens in the rhizosphere, which is of great significance to improve the prevention and control effect and reduce the environmental load of these anti-pathogenic agents.

Differences in structure and composition of soil humic substances and their binding for polycyclic aromatic hydrocarbons in different climatic zones

Humic substances (HSs) play important roles in the transport and bioavailability of hydrophobic organic compounds (HOCs) in soils. The sorption of HOCs depends on the compositions and structures of HSs which may differ in different climatic zones, however, the sorption behavior of HOCs by HSs in soils from different climatic zones is poorly understood. In this study, different HS fractions (humic acids-HAs, fulvic acids-FAs and humin-HM) in soils from different climatic zones were extracted and used as sorbents for polycyclic aromatic hydrocarbons (PAHs). The results indicated that HSs (including HA, FA and HM fractions) from colder climatic zones contained more oxygen-containing functional groups and exhibited smaller molecular weight as well as higher aliphaticity and polarity than those from warmer climatic zones. The sorption affinity of HAs at the low given concentration (0.05 Sw) of naphthalene (Nap), phenanthrene (Phe), pyrene (Pyr) and benz(α)anthracene (BaA) from warmer climatic zones to colder ones increased from 26.3 to 43.9, from 36.7 to 114.0, from 125.8 to 388.8, and from 322.5 to 876.1, respectively, and the same trends were obtained for FAs and HM at the same PAH concentration. The results indicated that HSs from colder climatic zones showed higher sorption affinity than those from warmer climatic zones. Moreover, the weighted contributions of FAs, HAs and HM to the overall sorption from different climatic zones were 9.1-28.4%, 13.5-59.2% and 23.4-76.9%, respectively. This indicates FA fraction, a previously neglected component, is also an important contributor to binding PAHs in soils. This study suggests that the difference in sorption behaviors of HOCs to HSs among different climatic zones should be considered when predicting HOC fates and bioavailability in soils.

Biochar derived from hydrolysis of sewage sludge influences soil properties and heavy metals distributed in the soil

Sewage sludge contains a large number of nutrients and dangerous substances, when sludge was processed into sludge hydrochar that was added to the soil, which not only solve the problem of sludge disposal, but also amend the soil and fix pollutants in the soil. However, it was lack of report on the effect of the sludge hydrochar on soil compositions and soil microorganism community structures until now. In the present study, the hydrothermal carbonization method is used to prepare hydrochar from sewage sludge at temperatures of 180 ℃ and 240 ℃ at durations of 6 h and 15 h in this paper. The effects of the prepared sludge hydrochar on soil-derived dissolved organic matter (DOM), the content of total dissolved nitrogen (TDN) and NO₃^--N in soil, and the community structure of soil bacteria and fungi were evaluated. Furthermore, the change rules in heavy metal speciation in soils treated with sludge hydrochar were investigated. With the increase in the preparation temperature and dosage of sludge hydrochar, the main components of DOM changed from soluble microbial byproducts to fulvic acid-like and humic acid-like fractions through UV and fluorescence characterization. The sludge hydrochar prepared at low temperature could significantly increase the contents of TDN and NO₃^--N in the soil. Affected by sludge hydrochar, the dominant phylum of the bacterial community changed from Proteobacteria to Actinobacteria, and the dominant phylum in the fungal community did not change, but its relative abundance increased. Finally, the sludge hydrochar obtained when the carbonization time was 15 h was more beneficial to reduce the total amount and available content of heavy metals in the soil. The study provides a basis for sludge hydrochar application for the soil amendment.

Long-term application of nitrogen fertilizer alters the properties of dissolved soil organic matter and increases the accumulation of polycyclic aromatic hydrocarbons

Soil is a key component of terrestrial ecosystems, as it provides nutrients and energy for all terrestrial organisms and is the site of various physical, chemical, and biological processes. Soil organic matter is particularly important for the role that it plays in element cycling, as well as the adsorption and degradation of soil pollutants. Nitrogen (N) fertilizer is an important nutrient element in the soil microenvironment. Applications of N fertilizer can improve soil quality, but the long-term excessive application of N fertilizer can lead to the deterioration of the soil environment, alter the properties of organic matter, and affect the adsorption and accumulation of soil pollutants. In recent years, several pollutants, especially polycyclic aromatic hydrocarbons (PAHs), have accumulated in farmland soil due to long-term sewage irrigation. However, few studies have examined the response of soil PAHs accumulation to long-term N application, as well as the relationship between this response and changes in soil microenvironmental indicators caused by N application. Here, we conducted field experiments to study changes in soil pH, total organic carbon, and dissolved organic matter (DOM) under long-term N application, as well as their effects on PAHs accumulation. The application of N fertilizer resulted in the aromatization and humification of soil DOM, enhanced the accumulation response ratio (-0.05-0.32) and the amount of PAHs accumulated in soil (more than 30%), and exacerbated the environmental risks of PAHs. Our findings provide new insights that could aid the management and control of PAHs pollution of soil in sewage-irrigated areas.

Particle-scale understanding sorption of phenanthrene on sediment fractions amended with black carbon and humic acid

Black carbon (BC) and humic acid (HA) have been proposed to dominate the sorption behavior of phenanthrene in sediment. Nevertheless, little is known about the sorption mechanism that related to particle-scale by spiking of BC and HA in sediment particle size fractions. In this study, sorption isotherms for phenanthrene were determined in four particle-size sediment fractions (<2 μm, 2-31 μm, 31-63 μm and >63 μm) that amended with BC and HA, or not. The fitting results by Freundlich model indicated that the sediment particle size fractions amended with BC increased the sorption capacity and affinity for phenanthrene. Sediment coarser size fractions (31-63 μm and >63 μm) by spiking of BC contributed higher to sorption capacity factor (K_F) and nonlinearity factor (n) than the finer size fractions (2-31 μm and <2 μm). By contrast, the sediment particle size fractions amended with HA enhanced the sorption distribution coefficient (K_d), but reduced the sorption affinity for phenanthrene. All these phenomena are obviously affected by the distribution of heterogeneous organic matter that related to sediment particle-scale. Results of this work could help us better understand the impact of increased BC and HA content in sediments on the sorption of hydrophobic organic pollutants (HOCs) and predict the fate of HOCs in offshore sediments due to tidal action.

PPCPs and heavy metals from hydrothermal sewage sludge-derived biochar: migration in wheat and physiological response

Once the sludge was directly used in the farmland, it will have a negative impact on human health through the food chain because sludge contains pollutants. Sewage sludge pyrolysis into biochar is an effective way to realize sludge harmless and resourceful utilization. This research used hydrothermal carbonization method to convert sludge into sludge biochar (SLBC) to reduce the types and contents of pharmaceuticals and personal care products (PPCPs) and available heavy metals. Furthermore, migration of the residual caffeine (Caf), acetaminophen (Ace), and heavy metals (Cr, Pb, Cu, Zn) released from the SLBC in the wheat was assessed. The results showed that the levels of Caf, Ace, Pb, Cu, and Zn accumulated in the shoots were lower than the limit regulated by Drug and Food Additive Use Standard in China (Caf: 150 mg/kg; Ace: 2.5 ~ 5 mg/kg; Pb: 0.3 mg/kg; Cu: 10 mg/kg; Zn: 20 mg/kg). The migration of Cr from roots to shoots was also significantly controlled by SBLC. SBLC delayed the germination time of wheat seeds with increasing in hydrothermal temperature, the germination rate and root length showed a decreasing trend. Evans blue and O₂^- fluorescence staining of root tips also confirmed this conclusion. When the wheat was exposed to the low temperature and dose of SLBC, the chlorophyll contents and growth of wheat can be significantly increased; the oxidative damage of cell plasma membrane and net photosynthetic rate were reduced. However, 0.8 g/L of SLBC made plants suffer abiotic stress and caused oxidative damage to plants, and decreased membrane system stability. The study provides some parameters for sludge to realize resource utilization in the agricultural system.

The response of steroid estrogens bioavailability to various sorption mechanisms by soil organic matter extracted with sequential alkaline-extraction method from an agriculture soil

The long-term groundwater contamination risks posed by steroidal estrogens (SEs) in animal-manured agricultural soils are closely associated with the soil organic matter (SOM) content and composition. In this study, the bioavailability of estrone (E1) and 17β-estradiol (17β-E2) under different sorption mechanism in humic acids (HA1 and HA2) and humin (HM) extracted with sequential alkaline-extraction technique (SAET) were examined. These SOMs extracted by SAET showed various properties and sorption characteristics for SEs. The alkyl carbon and condensed SOM increased during SAET, but aromatic carbon decreased and the same trend for polarity. Quick sorption was the major SEs sorption mechanism on HA1 and HA2, which contributed more than 69%; whilst slow sorption rate was about 50% in soil and HM. The logK_oc values were proportional to the TOC of SOM according to Freundlich fitting, and the sorption capacity of sorbent for E1 and 17β-E2 was related to the logK_ow values, indicating that the main mechanism controlling the SEs sorption was hydrophobic interaction. The larger micropore volume of HM and soil was more conducive to the micropore filling of SEs. Meanwhile, the specific sorption of SEs on condensed domain of SOM was the main reason for the strong desorption hysteresis and slow sorption in HM and soil. The SEs degradation rate was positively correlated with the contribution rate of quick adsorption and negatively correlated with the contribution rate of slow adsorption, indicating that the bioavailability of SEs sorbed by hydrophobic interaction was higher than that of micropore filling or specific sorption, which was also the reason for the low bioavailability of SEs in HM and soil. This work confirms the regulation of on-site SOM compositions and their properties on SEs sorption and bioavailability. Characterization of these details is crucial for the improved prediction of long-term risks to groundwater.

Tide-Triggered Production of Reactive Oxygen Species in Coastal Soils

We report an unrecognized, tidal source of reactive oxygen species (ROS). Using a newly developed ROS-trapping gel film, we observed hot spots for ROS generation within ∼2.5 mm of coastal surface soil. Kinetic analyses showed rapid production of hydroxyl radicals (^•OH), superoxide (O₂^•-), and hydrogen peroxide (H₂O₂) upon a shift from high tide to low tide. The ROS production exhibited a distinct rhythmic fluctuation. The oscillations of the redox potential and dissolved oxygen concentration followed the same pattern as the ^•OH production, suggesting the alternating oxic-anoxic conditions as the main geochemical drive for ROS production. Nationwide coastal field investigations confirmed the widespread and sustainable production of ROS via tidal processes (22.1-117.4 μmol/m²/day), which was 5- to 36-fold more efficient than those via classical photochemical routes (1.5-7.6 μmol/m²/day). Analyses of soil physicochemical properties demonstrated that soil redox-metastable components such as redox-active iron minerals and organic matter played a key role in storing electrons at high tide and shuttling electrons to infiltrated oxygen at low tide for ROS production. Our work sheds light on a ubiquitous but previously overlooked tidal source of ROS, which may accelerate carbon and metal cycles as well as pollutant degradation in coastal soils.

Coupled variations of dissolved organic matter distribution and iron (oxyhydr)oxides transformation: Effects on the kinetics of uranium adsorption and desorption

The interactions between dissolved organic matter (DOM) molecules and minerals play significant roles in affecting the fate of carbon and contaminants in soil environment. However, the mechanisms controlling the variations of DOM molecules distribution during the transformation of Fe (oxyhydr)oxides, and the effects of these variations on contaminant behaviors are still largely unknown. In this study, the dynamic variations of DOM properties and distributions, and the kinetics of uranium adsorption on and desorption from Fe (oxyhydr)oxides during the transformation were investigated, employing a combination of Orbitrap mass spectrometry (MS), high-resolution transmission electron microscopy (HR-TEM), and kinetic experiments. Orbitrap MS results indicated that aliphatic molecules and phenolic and polyphenolic molecules with lower O/C values were preferentially released to solution. HR-TEM results indicated that the coprecipitated DOM molecules by ferrihydrite were mainly released to solution rather than sorbed on the newly formed lepidocrocite or goethite during the transformation. Furthermore, the stirred-flow experiment results suggested that soil DOM significantly reduced the adsorption of uranium on, and accelerated the release of uranium from Fe (oxyhydr)oxides, which was ascribed to the changed distribution of DOM molecules and the structure and composition of Fe (oxyhydr)oxides. Our results contribute to predicting contaminant behaviors in soils.

Structural characteristics of sediment humins from South Korean lakes and their phenanthrene binding compared to other carbon sources

Sediment humins are extremely important for binding hydrophobic organic contaminants in rivers and lakes. Nonetheless, little is known about their structure and binding. We, therefore, examined the structure and phenanthrene sorption affinity of sediment humin samples upstream, midstream, and downstream from two artificial lakes in South Korea by using the elemental ¹³C-NMR analysis, Freundlich model, and Langmuir model. The characteristics and phenanthrene sorption affinity of sediment humins were also compared with those of sediment humic acids from similar origins as well as soil humins/humic acids in South Korea from previous studies by using principal component analysis. In both lakes, downstream sediment humins exhibited lower N/C, O/C, and (N + O)/C ratios, lower internal oxidation, and higher aliphaticity due to the presence of long-chain aliphatic compounds generated during anaerobic decomposition. The principal component analysis results also showed that C,H-alkyl, O-alkyl, and polar organic carbon contents were significantly different when comparing the up-mid stream and downstream sediment samples in Daecheong Lake. In addition, midstream sediment humin in Andong Lake presented higher C,H-alkyl and lower polar organic carbon contents compared to those of up-downstream samples. In both lakes, the sorption coefficient and adsorption isotherm linearity were positively correlated with the C,H-alkyl content and negatively correlated with the O-aryl content. Similar to C,H-alkyl and POC, C,H-alkyl, and (N + O)/C had an extremely high correlation coefficient when predicting the sorption coefficient (Freundlich model) and the maximum adsorption capacity (Langmuir model) of sediment humins. Sediment humins had higher C,H-alkyl contents and lower sorption coefficients than those of sediment humic acids and soil humins/humic acids. These findings provide key information for monitoring water quality and polycyclic aromatic hydrocarbon contamination in South Korean lake sediments.

Insight into the hetero-interactions of 4-nonylphenol with dissolved organic matter: multiple spectroscopic methods, 1H NMR study and principal component analysis

Understanding the interactions between heterogeneous dissolved organic matter (DOM) and nonylphenols (NPs) is essential for predicting their behavior and fate in the environment. Herein, we firstly obtained different MW-fractionated humic acids (HAs) using the ultrafiltration method. Afterward, the molecular weight (MW)-dependent interactions of HAs with 4-nonylphenol (4-NP) were analysed by excitation emission matrix (EEM) fluorescence spectroscopy, fluorescence quenching, UV-vis spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy and principal component analysis (PCA). EEM spectra indicated that the quenching mechanism was static. In the binding process, the higher MW fractions showed stronger interaction with 4-NP than the lower MW counterparts, exhibiting a clear MW-dependent interaction heterogeneity. The interaction constants for the 4-NP–HAs system were suppressed as the ionic strength decreased and pH increased, which was especially obvious in the binding of 4-NP to the lower MW-fractionated HAs. The FTIR spectra revealed that hydroxyl and aromatics were involved in the interaction process of HA fractions with 4-NP. It was also found from ¹H NMR that π–π interactions between aromatic rings of 4-NP and MW-fractionated HAs were responsible for the complexation. The correlation analysis and PCA results indicated that aromaticity and MW play important roles in the interaction process and confirmed an obvious interaction heterogeneity among MW-fractionated HAs samples. This work highlighted MW-dependent interaction heterogeneities of HA, which suggested that heterogeneity in MW distribution should be taken into consideration when exploring the fate and biogeochemistry cycling of 4-NP from contaminated environments.

Multiple spectroscopic methods, ¹H NMR study and PCA were used to investigate the heterointeractions of 4-nonylphenol with humic acids.

New insight into the adsorption mechanism of PCP by humic substances with different degrees of humification in the presence of Cr(VI)

Humic substances (HSs) have great retention effects on pentachlorophenol (PCP) migration in subsurface environment, but the adsorption mechanism of PCP by HSs with various aromatic/aliphatic moieties and acidic functional groups in the presence of Cr(VI) is still unclear. In this study, the adsorption mechanism of PCP by undissolved humic acid (HA) and humin (HM) extracted from peat, black soil, lignite and coal was investigated under the presence of Cr(VI). According to the results, HA samples had much lower adsorption capacity for hydrophobic PCP than HM samples due to their higher contents of hydrophilic polar oxygen-containing functional groups. In respect to PCP adsorption mechanism, the molecular unsaturation of HSs associated with humification degree was found to be the determinant instead of polarity. Notably, after reacting with Cr(VI), significant decreasing of PCP adsorption quantities occurred on HSs extracted from lignite and coal with higher degrees of unsaturation (H/C < 0.64), while HSs extracted from peat and black soil with lower degrees of unsaturation (H/C > 0.83) kept almost unchanged, which can be attributed to the much higher reactivity of aromatic domains of HSs for Cr(VI) reduction compared with aliphatic moieties. This indicated that the adsorption mechanism of PCP by HSs with higher and lower degrees of unsaturation might be respectively driven by π-π interaction and hydrophobic interaction. This study highlighted the diverse adsorption mechanisms of PCP on HSs with different degrees of humification, and emphasized the coexisting Cr(VI) only have significant effect on PCP adsorption by HSs with higher humification degrees instead of the lower ones.

Probing the roles of pH and ionic strength on electrostatic binding of tetracycline by dissolved organic matters: Reevaluation of modified fitting model

The binding performance of dissolved organic matters (DOM) plays a critical role in the migration, diffusion and removal of various residual pollutants in the natural water environment. In the current study, four typical DOMs (including bovine serum proteins BSA (proteins), sodium alginate SAA (polysaccharides), humic acid HA and fulvic acid FA (humus)) are selected to investigate the binding roles in zwitterionic tetracycline (TET) antibiotic under various ionic strength (IS = 0.001-0.1 M) and pH (5.0-9.0). The dialysis equilibration technique was employed to determine the binding concentrations of TET, and the influence of IS and pH on binding performance was evaluated via UV-vis spectroscopy, total organic carbon (TOC), and Excitation-Emission-Matrix spectra (EEM), zeta potentials and molecule size distribution analysis. Our results suggested that carboxyl and phenolic hydroxyl were identified as the main contributors to TET binding based on the fourier transform infrared spectroscopy (FTIR) analysis, and the binding capability of four DOMs followed as HA > FA » BSA > SAA. The biggest binding concentrations of TET by 10 mg C/L HA, FA, BSA and SAA were 0.863 μM, 0.487 μM, 0.084 μM and 0.086 μM, respectively. The higher binding capability of HA and FA is mainly attributed to their richer functional groups, lower zeta potential (HA/FA = -15.92/-13.54 mV) and the bigger molecular size (HA/FA = 24668/27750 nm). IS significantly inhibits the binding interaction by compressing the molecular structure and the surface electric double layer, while pH had a weak effect. By combining the Donnan model and the multiple linear regression analysis, a modified Karickhoff model was established to effectively predict the binding performance of DOM under different IS (0.001-0.1 M) and pH (5.0-9.0) conditions, and the R2 of linear fitting between experiment-measured logKDOC and model-calculated logKOC were 0.94 for HA and 0.91 for FA. This finding provides a theoretical basis for characterizing and predicting the binding performance of various DOMs to residual micropollutants in the natural water environment.

Water-induced release of recalcitrant polycyclic aromatic hydrocarbons from soil organic matter during microwave-assisted solvent extraction

Polycyclic aromatic hydrocarbons (PAHs) in soil can be recalcitrant to solvent extraction after aging. We showed in this study that mixing a small amount of water in the extracting solvent during microwave-assisted extraction (MAE) can release recalcitrant PAHs, resulting in significant improvement in the analyzed concentrations. The improvement factor (F) for the total of 16 priority PAHs (∑PAH16) listed by the United States Environmental Protection Agency was 1.44-1.55 for field soils. By comparing the F values for different soil organic components, we demonstrated that the recalcitrant PAHs were primarily associated with biochar, humic acid (HA), and humin (HM), with the F values for ∑PAH16 of 1.94, 6.62, and 4.59, respectively. The results showed that the recalcitrant PAHs comprised a sequestered fraction and a desorption-limited fraction. NMR spectra showed that water worked alone at elevated temperature to promote hydrolysis of biochar and destroy the macromolecular structure, thus causing the release of the otherwise sequestered PAHs during MAE. The substantial reduction in F values for HA and HM after demineralization indicated sequestration of PAHs in organic-mineral complexes, which can be destroyed by hot water treatment. The release of the sequestered fraction was nonselective and independent of compound hydrophobicity. In comparison, the release of the desorption-limited fraction was positively affected by the hydrophobicity of PAHs and was facilitated by the presence of water in the extracting solvent. The results of this study provide important insights into the sequestration and release of recalcitrant PAHs in soil.

Sorption of organic compounds by pyrolyzed humic acids

Humic acids (HAs) are frequently subjected to pyrolysis and carbonization by wildfires, which could significantly change the sorption of organic contaminants and their environmental risks in natural system. In previous studies, sorption of organic compounds was investigated for HAs pyrolyzed at temperature below 330 °C, but not for HAs pyrolyzed at higher temperature. Therefore, in this study, sorption of 22 typical organic compounds by HAs pyrolyzed at a series of temperatures from 300 to 700 °C was investigated. Sorption of organic compounds was dominated by nonlinear partition for HAs pyrolyzed at low temperature (e.g., 300 and 400 °C) due to the aliphatic and nonporous structures of pyrolyzed humic acids (PyHAs), while it was dominated by pore-filling adsorption for HAs pyrolyzed at high temperature (e.g., 700 °C) due to the aromatic and porous structures of PyHAs. For HAs pyrolyzed at moderate temperature (e.g., 450, 500 and 600 °C), both nonlinear partition and pore-filling adsorption were responsible for the sorption of organic compounds. Meanwhile, the contribution of pore-filling adsorption to overall sorption increased but the contribution of nonlinear partition decreased with the increasing pyrolytic temperature of PyHAs, attributed to the structure change of PyHAs from aliphatic and nonporous to the aromatic and porous. Moreover, with the increasing pyrolytic temperature of PyHAs, sorption affinity of organic compounds increased, while the change of sorption capacity could be explained by the decrease of nonlinear partition and the increase of pore-filling adsorption. The obtained results could help to evaluate the transport, bioavailability and health risks of organic contaminants in the environment.

Adsorption of phenanthrene onto magnetic multi-walled carbon nanotubes (MMWCNTs) influenced by various fractions of humic acid from a single soil

In the present study, two magnetic multi-walled carbon nanotubes (MMWCNTs) with different ratios of Fe²⁺/Fe³⁺ were prepared, and the effects of different fractions of dissolved humic acid (DHA) on the adsorption of phenanthrene by multi-walled carbon nanotubes (MWCNTs) and MMWCNTs from the aqueous solution were investigated. The adsorption kinetics of DHA₁ and DHA₄ were best fitted with pseudo-second order model. The adsorption of DHAs on MMWCNTs was weaker than that on MWCNTs, and DHA₁ was easier to adsorb to MWCNTs and MMWCNTs than DHA₄. The phenanthrene adsorption capacities by 1:2:1MMWCNTs and 4:2:1MMWCNTs with higher polar groups and magnetic gradient were less than that of MWCNTs. The pH value had no obvious effect on the adsorption of phenanthrene to MWCNTs loaded with different iron. Additionally, the DHAs could form soluble complexes of DHAs-Fe (II) in solution to reduce the phenanthrene adsorption on MMWCNTs, DHA₁ inhibit more obviously phenanthrene adsorbed onto MWCNTs and MMWCNTs than DHA₄. As for MMWCNTs, the main mechanisms of phenanthrene adsorbed onto it included new adsorption sites formed by π-π interaction and magnetic gradient. In this study, MMWCNTs after adsorbed DHAs had a weaker inhibitory effect on phenanthrene adsorption than MWCNTs, implying that when phenanthrene is adsorbed by DHAs-coated MMWCNTs, the bioavailability and mobility of phenanthrene will be reduced, and it is easy to be removed by the magnet for further processing.

Amendments with pyrolyzed agrowastes change bromacil and diuron's sorption and persistence in a tropical soil without modifying their environmental risk

Knowledge of pesticides fate in tropical soils and how it could be affected by pyrolyzed biomass as amendment is limited. Combining conventional and radiotracer methods, as well as risk assessment tools, the effects of several charred agrowastes on the sorption, persistence, and ecological risk of the herbicides bromacil (BMC) and diuron (DRN) were evaluated in a tropical agricultural soil under laboratory conditions. Pineapple stubble (PS), palm oil fiber (PF), and coffee hull (CH) were charred at 300 (torrefied) and 600 °C (biochar) and applied to the soil at 10 and 20 t ha^-1 rates. The sorption coefficients (Koc) in unamended soil for BMC and DRN were 18.4 and 212.1 L kg^-1, respectively. The addition of torrefied PS and PF caused a 3 to 4-fold increment in BMC sorption and a 3 to 6-fold change in DRN's sorption. The only biochar that affected the sorption was PS that increased DRN's sorption 3.5 times. The application of coffee hull materials had no significant effect. In terms of degradation (half-life, DT50), for unamended soil BMC's degradation (300 days) was limited compared to DRN (73 days). Alternatively, biodegradation (mineralization half-life time, MT50) was 1278 d for BMC and 538 for DRN. While only PF and CH torrefied increased BMC's persistence, all the torrefied affected DRN's persistence. However, despite the observed effects, the predicted ecological risk was not mitigated. Our results highlight the need for scientific evidence on the use of pyrolyzed organic amendments to assess potential benefits and prevent unintended impacts in tropical agroecosystems.

Nonlinear partition of nonionic organic compounds into humus-like substance humificated from lignin

Nonlinear sorption of nonionic organic compounds (NOCs) by soil organic matter (SOM) is a significant behaviour that affecting their distribution, transport and fate in the environment. Sorption of typical NOCs, including phenols, anilines, nitrobenzenes and polycyclic aromatic hydrocarbons (PAHs) by Lig48, a humus-like substance humificated from lignin (the principal component of plant precursors of SOM), is nonlinear and without desorption hysteresis, and interpreted by nonlinear partition mechanism in this study. The positively linear relationship between sorption capacity and water solubility of NOCs is a distinguish characteristic for their nonlinear partition into Lig48. Moreover, the nonlinear partition capacity of NOCs is mainly dependent on the aromaticity of humus-like substances with a positively linear relationship, while the nonlinear partition affinity is mainly dependent on the polarity of humus-like substances with a negatively linear relationship. Competition between phenols, anilines, nitrobenzenes and PAHs was observed for their nonlinear partition into Lig48. In addition to van der Waals force, specific interactions, i.e., hydrogen-bonding and π-π interactions are responsible for the nonlinear partitioning of NOCs into humus-like substances including Lig48. These novel observations are helpful for understanding the nonlinear sorption of NOCs by SOM and elucidating the migration and transport of NOCs in the environment.

Isotope fractionation of micropollutants during large-volume extraction: heads-up from a critical method evaluation for atrazine, desethylatrazine and 2,6-dichlorobenzamide at low ng/L concentrations in groundwater

Micropollutants are frequently detected in groundwater. Thus, the question arises whether they are eliminated by natural attenuation so that pesticide degradation would be observed with increasing residence time in groundwater. Conventional analytical approaches rely on parent compound/metabolite ratios. These are difficult to interpret if metabolites are sorbed or further transformed. Compound-specific stable isotope analysis (CSIA) presents an alternative for identifying degradation based on the analysis of natural isotope abundances in pesticides and their changes during degradation. However, CSIA by gas chromatography-isotope ratio mass spectrometry is challenged by the low concentrations (ng/L) of micropollutants in groundwater. Consequently, large amounts of water need to be sampled requiring enrichment and clean-up steps from interfering matrix effects that must not introduce artefacts in measured isotope values. The aim of this study was to evaluate the accuracy of isotope ratio measurements of the frequently detected micropollutants atrazine, desethylatrazine and 2,6-dichlorobenzamide after enrichment from large water volumes (up to 100 L) by solid-phase extraction with consecutive clean-up by HPLC. Associated artefacts of isotope discrimination were found to depend on numerous factors including organic matter content and extraction volume. This emphasizes the necessity to perform a careful method evaluation of sample preparation and sample pre-treatment prior reliable CSIA.

Application of biochar for the remediation of polluted sediments

Polluted sediments pose potential threats to environmental and human health and challenges to water management. Biochar is a carbon-rich material produced through pyrolysis of biomass waste, which performs well in soil amendment, climate improvement, and water treatment. Unlike soil and aqueous solutions, sediments are both the sink and source of water pollutants. Regarding in-situ sediment remediation, biochar also shows unique advantages in removing or immobilizing inorganic and organic pollutants (OPs). This paper provides a comprehensive review of the current methods of in-situ biochar amendments specific to polluted sediments. Physicochemical properties (pore structure, surface functional groups, pH and surface charge, mineral components) were influenced by the pyrolysis conditions, feedstock types, and modification of biochar. Furthermore, the remediation mechanisms and efficiency of pollutants (heavy metals [HMs] and OPs) vary with the biochar properties. Biochar influences microbial compositions and benthic organisms in sediments. Depending on the location or flow rate of polluted sediments, potential utilization methods of biochar alone or coupled with other materials are discussed. Finally, future practical challenges of biochar as a sediment amendment are addressed. This review provides an overview and outlook for sediment remediation using biochar, which will be valuable for further scientific research and engineering applications.

Fate characterization of bound residues of 14C-Pyraoxystrobin in soils

Formation of bound residues (BR) has generally been considered as a detoxification process for organic contaminants. BR is an indispensable component for risk assessment of pesticides. In this study, BR of ¹⁴C-pyraoxystrobin in three soils cultivated for 100 days were characterized in light fraction (LF), loosely combined humus (LCH), stably combined humus (SCH), humic acid (HA), fulvic acid (FA), and humin. Isotope labeling technique was used to detect the distribution of BR of ¹⁴C-pyraoxystrobin in the six fractions of soil organic matter (SOM). The results showed that the amount of total BR was positively correlated with the SOM content (p < 0.05). The BR of ¹⁴C-pyraoxystrobin in cambisol soil was largest at 31.26 ± 0.04% of the induced radioactivity. During the whole incubation period, the BR of pyraoxystrobin in LCH of the three soils were consistently higher than that in SCH, and the amount of BR in FA was consistently greater than that in HA. The BR of ¹⁴C-pyraoxystrobin bound with humin increased over time. In addition, a degradation product 3-(4-chlorophenyl)-1-methyl-1H-pyrazol-5-ol (M1) from the hydrolysis of pyraoxystrobin was detected in cambisol soil, indicating the bonding of M1 with the HA separated from LCH (HA_LCH) via ester or ether linkages. The results provide new insights into the fate of BR of pyraoxystrobin in soils and may help to develop an understanding for the risk assessment of pyraoxystrobin and other strobilurin fungicides.

The quality of soil organic matter, accessed by 13C solid state nuclear magnetic resonance, is just as important as its content concerning pesticide sorption

The global increase of food production has been achieved mainly through the intensive use of inputs such as pesticides. Once released to the soil, sorption (which could be represented by Freundlich solid-water distribution coefficients - K_F) and degradation are two governing processes that determine the distribution and persistence of pesticides in the environment. In spite of the huge dataset, the only apparent generalisation is the high correlation between K_F and soil organic matter (SOM) content. However, in this work no correlation was observed between K_F and organic C content (OC) and so the obtained K_OC (K_F normalised by OC) spread out in a wide range: 1100 to 11,400 mL g^-1 for abamectin; and 30-150 mL g^-1 for atrazine, both ranges corroborate with data from literature. These high variabilities indicate that other soil components or SOM quality strongly interfere in the pesticide sorption in addition to SOM content. Seeking to estimate the influence of SOM quality in the abamectin and atrazine K_OC values, the humic acids, a fraction of the SOM, was analysed by ¹³C nuclear magnetic resonance spectroscopy (¹³C NMR) and Principal Component (PC) Regression. The first PC of ¹³C NMR spectra presented negative loadings for aliphatic compounds and positive loadings for aryl C, typical of partially oxidised pyrogenic C. Their scores showed strong correlation with the abamectin K_OC values (R² = 0.91, p < 5 10^-8) and weaker with atrazine K_OC (R² = 0.63, p < 0.0001), in addition to a smaller standardised slope: 1.01 for abamectin and 0.76 for atrazine. These results could be explained by the higher hydrophobicity of abamectin, being thus more prone to interact with the polycondensed aryl groups from the pyrogenic C. It is also important to highlight that humic acids are useful proxies for understanding the paramount interaction of SOM with pesticides.

The Impact of Organic Matter on Polycyclic Aromatic Hydrocarbon (PAH) Availability and Persistence in Soils

Polycyclic aromatic hydrocarbons (PAHs) exhibit persistence in soils, and most of them are potentially mutagenic/carcinogenic and teratogenic for human beings but also influence the growth and development of soil organisms. The PAHs emitted into the atmosphere are ultimately deposited (by dry or wet deposition processes) onto the soil surface where they tend to accumulate. Soil organic matter (SOM) plays an important role in the fate and transformation processes of PAHs, affecting their mobility, availability, and persistence. Therefore, the aim of this research was to investigate the influence of SOM fractional diversification (fulvic acids-FA, humic acids-HA, and humins-HN) on PAH availability and persistence in soils. Twenty soil samples (n = 20) were collected from upper horizons (0-30 cm) of agricultural soils exposed to anthropogenic emissions from industrial and domestic sources. The assessment of PAH concentrations included the determination of medium-molecular-weight compounds from the US EPA list: fluoranthene-FLA, pyrene-PYR, benz(a)anthracene-BaA, and chrysene-CHR. The assessment was conducted using the GC-MS/MS technique. Three operationally defined fractions were investigated: total extractable PAHs (TE-PAHs) fraction, available/bioavailable PAHs (PB-PAHs) fraction, and nonavailable/residual PAHs (RE-PAHs) fraction, which was calculated as the difference between total and available PAHs. TE-PAHs were analyzed by dichloromethane extraction, while PB-PAHs were analyzed with a hydrophobic β-cyclodextrin solution. SOM was characterized by total organic carbon content (Turin method) and organic carbon of humic substances including FA, HA, HN (IHSS method). Concentrations of PAHs differed between soils from 193.5 to 3169.5 µg kg^-1, 4.3 to 226.4 µg kg^-1, and 148.6 to 3164.7 µg kg^-1 for ∑4 TE-PAHs, ∑4 PB-PAHs, and ∑4 RE-PAHs, respectively. The ∑4 PB-PAHs fraction did not exceed 30% of ∑4 TE-PAHs. FLA was the most strongly bound in soil (highest content of RE-FLA), whereas PYR was the most available (highest content of PB-PYR). The soils were characterized by diversified total organic carbon (TOC) concentration (8.0-130.0 g kg^-1) and individual SOM fractions (FA = 0.4-7.5 g kg^-1, HA = 0.6-13.0 g kg^-1, HN = 0.9-122.9 g kg^-1). FA and HA as the labile fraction of SOM with short turnover time strongly positively influenced the potential ∑4 PAH availability (r = 0.56 and r = 0.52 for FA and HA, respectively). HN, which constitutes a stable fraction of organic matter with high hydrophobicity and poor degradability, was strongly correlated with ∑4 RE-PAHs (r = 0.75), affecting their persistence in soil.

Importance of the structure and micropores of sedimentary organic matter in the sorption of phenanthrene and nonylphenol

The demineralized fraction (DM), lipid-free fraction (LF), nonhydrolyzable organic carbon fraction (NHC), and black carbon (BC) were isolated from five marine surface sediments, and they were characterized by elemental analysis as well as CO₂ and N₂ adsorption techniques, respectively. The NHC fractions were characterized using advanced solid-state ¹³C nuclear magnetic resonance (NMR) and x-ray photoelectron spectroscopy (XPS). Then, the sorption isotherms of phenanthrene (Phen) and nonylphenol (NP) on all of the samples were investigated by a batch technique. The CO₂ micropore volumes were corrected for the outer specific surface areas (SSAs) by using the N₂-SSA. Significant correlations between the micropore-filling volumes of Phen and NP and the micropore volumes suggested that the micropore-filling mechanism dominated the Phen and NP sorption. Meanwhile, the (O + N)/C atomic ratios were negatively and significantly correlated with the sorption capacities of Phen and NP, indicating that the sedimentary organic matter (SOM) polarity also played a significant role in the sorption process. In addition, a strong linear correlation was demonstrated between the aromatic C and the sorption capacity of Phen for the NHC fractions. This study demonstrates the importance of the micropores, polarity, and aromaticity on the sorption processes of Phen and NP in the sediments.

Correlations and nonlinear partition of nonionic organic compounds by humus-like substances humificated from rice straw

The debate on whether the nonlinear sorption of nonionic organic compounds (NOCs) by soil organic matter (SOM) is captured by nonlinear partition or adsorption has been going on for decades because the used SOM samples are complex mixtures from various precursors with varied humification degrees in natural environment. Therefore, in this study, hydrothermal method was employed to prepare humus-like substances from a sole precursor (i.e., rice straw) with various humification degrees for nonlinear sorption of 25 aromatic compounds, then to have an insight into the underlying mechanisms of the nonlinear sorption of NOCs by SOM. It was observed that the increasing humification degree of humus-like substances, i.e., decreasing in the polarity ((O + N)/C) and increasing in the aromaticity, result in the increase of isotherm nonlinearity and sorption capacity/affinity of NOCs. Sorption capacity of NOCs, obtained by isotherm fitting using Dubinin-Astakhov (DA) model and Dual-Mode (DM) model, are positively correlated with their solubility in water and octanol, indicating the nonlinear sorption could be captured by nonlinear partition mechanism. Specific interactions including hydrogen-bonding interaction and π-π interaction between aromatic structures of humus-like substances and organic molecules could be responsible for the nonlinear partition and the increase of sorption affinity with the enhancement of humification degree. These obtained correlations are valuable for understanding the underlying mechanisms of nonlinear sorption and elucidating the transport of NOCs in the environment.

Influence of organic carbon fractions of freshwater biofilms on the sorption for phenanthrene and ofloxacin: The important role of aliphatic carbons

Sorption to biofilms is thought to be a crucial process controlling the fate of trace organic contaminants in aquatic systems. The organic composition of biofilms is regarded as the determining factor in the sorption mechanism of biofilm organic carbon fractions; however, its role is not well known. Here, the sorption of phenanthrene and ofloxacin was modeled with classic and emerging organic contaminants, respectively, by comparatively investigating nine type of freshwater biofilms cultured in a river, lake, and reservoir in spring, summer, and autumn. The chemical features of the nine biofilms were analyzed using elemental analysis, infrared spectroscopy, X-ray photoelectron spectroscopy, and carbon-13 nuclear magnetic resonance. Results showed that the freshwater biofilms were aliphatic-rich natural amorphous solid substances with O-containing functional groups, and their surface polarity was significantly lower than their bulk polarity. All the isotherms of phenanthrene and ofloxacin sorption by the biofilms were linear. The organic carbon-normalized partition coefficient values for phenanthrene and ofloxacin on the nine biofilms ranged from 91.9 to 364.2 L g^-1 and 3.2 to 43.2 L g^-1, respectively. The van der Waals interaction between a majority of aliphatic carbon (73.4%-83.9%) in biofilms and the two sorbates was much stronger than π-π interactions between a minority of aromatic carbon (12.7%-21.7%) and sorbates. The surface polarity of the biofilms regulated polar interactions including the hydrogen bonding and electron donor-acceptor interactions. Both the aliphatic carbon and surface polarity in the biofilms enhanced the sorption of phenanthrene and ofloxacin. The sorption characteristics and mechanisms of polycyclic aromatic hydrocarbons and antibiotics on biofilms shown in our present and previous studies are different from those of other ubiquitous natural solid materials such as soils and sediments. This study provides insight into the importance of aliphatic carbon fractions of freshwater biofilms for the sorption of classic and emerging organic contaminants.

Oxidative ageing induces change in the functionality of biochar and hydrochar: Mechanistic insights from sorption of atrazine

One attraction of using hydrochar (HC) and biochar (BC) in soil is their intrinsic affinity for organic contaminants. Oxidative ageing is likely to induce changes in physicochemical properties and functionality. To explore the long-term potential trajectories for corn stalk HC and BC to adsorb organic pollutants, we employed HC and BC exposure in 5% H₂O₂ to simulate oxidative ageing and get insights into mechanisms of atrazine adsorption on fresh and artificially aged materials. The physicochemical properties of fresh and aged materials were systematically compared using elemental analysis, SSA, FTIR, XPS and SEM-EDS, alongside K₂Cr₂O₇/H₂SO₄ treatment to assess chemical oxidation stability. Atrazine is a typical herbicide chemical and hydrophobic organic pollutant. Adsorption isotherms of atrazine were used to reveal differences in mechanisms of sorption to BC and HC, by assessment before and ageing. BC freshly produced at 650 °C displayed higher capacity for atrazine sorption than BC produced at 500 °C, with a dominant role for π-π EDA interactions. The sorption capacity of HC freshly produced at 250 °C was higher than for HC produced at 200 °C HC, owing to higher C content and atrazine partitioning into the organic phase. Ageing increased the surface abundance of oxygenated functional groups for BC and HC and diminished bulk aromaticity. After ageing, atrazine sorption by high temperature BC was lower, but for HC it was increased. Such divergent effects must be considered when developing strategies to co-manage contaminants and carbon through the addition of carbonized materials to land.

Sorption of pentachlorophenol and phenanthrene by humic acid-coated hematite nanoparticles

Hematite nanoparticles (NPs) exist naturally and ubiquitously in soil, and they are always associated with soil organic matter by forming organic-inorganic complexes. In this work, hematite NPs coated with peat humic acid (HA_peat) and soil humic acid (HA_soil) were chosen as sorbents for hydrophobic organic contaminants (HOCs) to simulate the sorption processes in soil. Ionizable pentachlorophenol (PCP) and non-ionizable phenanthrene (PHE) were selected as representative HOCs. Compared with sorption isotherms of uncoated hematite NPs, the coating of HA onto the surface of hematite NPs substantially increased its sorption affinity for PCP and PHE by about 1-2 orders of magnitude, and the increasing degree was positively correlated to the HA content. These phenomena emphasized the dominant role of HA in the sorption process. The reduced polarity and the introduction of functional groups contributed to the enhanced sorption of HOCs on HA-coated hematite NPs. Furthermore, HA_peat-hematite NPs showed higher sorption affinity for both PCP and PHE than HA_soil-hematite NPs, which was mainly due to the lower polarity and higher hydrophobicity of HA_peat-hematite NPs. The sorption of PCP and PHE on HA-coated hematite NPs was inhibited obviously with increasing pH values and the pH effect on PCP sorption was more significant than that of PHE, due to the deprotonation of functional groups within adsorbed HA, the loose structure of adsorbed HA and the dissociation of PCP. Our findings elucidated the mechanisms involved in HOCs sorption processes by HA-hematite NPs and provided a theoretical basis for environmental remediation with natural NPs (e.g., hematite NPs).

Sono-advanced Fenton-like degradation of aromatic amines in textile dyeing sludge: efficiency and mechanisms

In this paper, a novel strategy integrating ultrasound (US) with a Fenton-like (zero-valent iron/EDTA/air, ZEA) process was proposed for the removal of the refractory and carcinogenic aromatic amines (AAs) in textile dyeing sludge for the first time. The operating condition was optimized as 1.08 W/cm³ ultrasonic density, 15 g/L ZVI, and 1.0 mM EDTA, which could reach degradation efficiencies of 51.79% in US, 72.88% in ZEA, and 92.40% in US/ZEA system after 90-min reaction. Quenching experiments showed that electron transfer reactions generated by the iron ligands in ZEA brought about various reactive oxidative species (ROS), in which Fe (IV), O₂˙^-, and ˙OH dominated the degradation. US induced sludge disintegration by ultrasonic shear, proven by particle size decrease and supernatant organic matter upsurge, which helps ROS contact with those pollutants in the sludge cavities. Besides, US facilitated the iron redox cycle for oxygen activation by promoting the corrosion of ZVI and stripping considerable ferric ions from sludge iron oxides which were verified by SEM, XRF, and XPS. Graphical abstract.

Environmental and Agricultural Relevance of Humic Fractions Extracted by Alkali from Soils and Natural Waters

To study the structure and function of soil organic matter, soil scientists have performed alkali extractions for soil humic acid (HA) and fulvic acid (FA) fractions for more than 200 years. Over the last few decades aquatic scientists have used similar fractions of dissolved organic matter, extracted by resin adsorption followed by alkali desorption. Critics have claimed that alkali-extractable fractions are laboratory artifacts, hence unsuitable for studying natural organic matter structure and function in field conditions. In response, this review first addresses specific conceptual concerns about humic fractions. Then we discuss several case studies in which HA and FA were extracted from soils, waters, and organic materials to address meaningful problems across diverse research settings. Specifically, one case study demonstrated the importance of humic substances for understanding transport and bioavailability of persistent organic pollutants. An understanding of metal binding sites in FA and HA proved essential to accurately model metal ion behavior in soil and water. In landscape-based studies, pesticides were preferentially bound to HA, reducing their mobility. Compost maturity and acceptability of other organic waste for land application were well evaluated by properties of HA extracted from these materials. A young humic fraction helped understand N cycling in paddy rice ( L.) soils, leading to improved rice management. The HA and FA fractions accurately represent natural organic matter across multiple environments, source materials, and research objectives. Studying them can help resolve important scientific and practical issues.

Changes in atrazine speciation and the degradation pathway in red soil during the vermiremediation process

Atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine) is a triazine herbicide intensively used in agricultural production and is often detected in different environmental matrices at concentrations above the permitted limit. This study investigated the influence of two earthworm species (epigeic Eisenia foetida and endogeic Amynthas robustus) on atrazine speciation and the degradation pathway. Our results revealed that both earthworms significantly accelerated atrazine degradation in a 28-day vermiremediation, and the residual atrazine declined from 4.23 ± 0.21 mg/kg in bulk soils to 0.51 ± 0.29 mg/kg (E. foetida) and 0.43 ± 0.19 mg/kg (A. robustus). By consuming organic matter (from 40.37 ± 1.14 to 36.31 ± 1.55 and 34.59 ± 1.13 g/kg for E. foetida and A. robustus) and neutralizing the soil pH (from 5.37 ± 0.27 to 6.36 ± 0.11 and 6.61 ± 0.30 for E. foetida and A. robustus), both earthworms reduced humus-fixed atrazine and increased the available atrazine. The percentage of available atrazine increased from 8.80 ± 0.21% in bulk soil to 10.30 ± 0.29% and 16.42 ± 0.18% in the vermiremediation treatments. Both earthworms promoted the hydroxyatrazine pathway by consuming soil organic matter and encouraged the deethylatrazine/deisopropylatrazine pathway by neutralizing the soil pH. Our findings unravel a new mechanism of vermiremediation by improving the soil physical-chemical properties and altering the atrazine degradation pathway, providing new insights into the influential factors on atrazine bioremediation in red soil.

Molecular structure-reactivity correlations of humic acid and humin fractions from a typical black soil for hexavalent chromium reduction

Different soil humus fractions are structurally distinct from each other molecularly, however, the relationship between their microscopic molecular structures and the macroscopic reduction of Cr(VI) is still unknown, especially for the humin fraction. In this study, different humus fractions (HA, humic acid; HMi, humin linked to iron oxides; HMc, humin linked to clay; and HMr, humin residue) were sequentially extracted from a typical black soil and well characterized. It was found that HA, HMi and HMc were the same type of humus with similar molecular structures, while HMr was structurally different from the other fractions with a high cellulose content. The removal rate of Cr(VI) in solution decreased with progressive humus fractionation, namely, HA > HMi > HMc > HMr. Based on the two-dimensional correlation spectroscopic analysis (2DCOS) of the FTIR data, the changing functional groups of all humus fractions during reacting with Cr(VI) followed a similar order: carboxyl > phenol > hydroxyl > methyl > methylene. According to the correlation analysis, Cr(VI) reduction rates by different humus fractions were mainly determined by the content of phenol (R² = 0.99) instead of carboxyl (R² = 0.28). Except for HMr, the Cr(VI) reduction rates of different humus fractions were also positively correlated with surface and bulk polarity (R² = 0.98 and 0.99) but not with aromaticity or aliphaticity (R² = 0.21).

Toxicity of GO to Freshwater Algae in the Presence of Al2O3 Particles with Different Morphologies: Importance of Heteroaggregation

The roles of Al₂O₃ particles with different morphologies in altering graphene oxide (GO) toxicity to Chlorella pyrenoidosa were investigated. Algal growth inhibition by GO with coexisting Al₂O₃ particles was much lower than the sum of inhibitions from the individual materials for all the three Al₂O₃, showing the toxicity mitigation by Al₂O₃. The lowest GO toxicity was observed at the concentrations of 300, 150, and 100 mg/L for Al₂O₃ nanoparticles (NPs, 8-10 nm), bulk particles (BPs, 100-300 nm), and fibers (diameter: 10 nm; length: 400 nm), respectively. GO-Al₂O₃ heteroaggregation was responsible for the observed toxicity reduction. GO-induced algal membrane damage was suppressed by the three types of Al₂O₃ due to GO-Al₂O₃ heteroaggregation, and the reduction in intracellular reactive oxygen species generation and physical contact were confirmed as two main mechanisms. Moreover, the exposure sequence of GO and Al₂O₃ could highly influence the toxicity, and the simultaneous exposure of individual GO and Al₂O₃ showed the lowest toxicity due to minimum direct contact with algal cells. Humic acid further decreased GO-Al₂O₃ toxicity due to enhanced steric hindrance through surface coating of GO-Al₂O₃ heteroaggregates. This work provides new insights into the role of natural mineral particles in altering the environmental risk of GO.

Sorption of chlorinated hydrocarbons to biochars in aqueous environment: Effects of the amorphous carbon structure of biochars and the molecular properties of adsorbates

Currently, the role of amorphous carbon structure (ACS) in sorption of chlorinated hydrocarbons (CHs) to biochars remains little known. Therefore, three CHs (1,1,2,2-tetrachloroethane, 1,3,5-trichlorobenzene and γ-hexachlorocyclohexane) with different molecular properties were selected as model adsorbates to investigate the effect of ACS on sorption of CHs to biochars produced at seven different pyrolysis temperatures (300-900 °C). There were two main mechanisms for ACS controlling the sorption of CHs. First, the polar sites on ACS are hydrophilic, CHs with greater polarity could strongly compete with the water molecule for the hydrophilic sites. Second, ACS of low temperature (300-400 °C) produced biochars possessing the natural organic matter (NOM)-like structure occupied some hydrophobic sites on condensed graphitic structure (CGS) of biochars. CHs with great hydrophobicity possibly seized the hydrophobic sorption sites on CGS from the NOM-like structure. Therefore, ACS of biochar was more benefit for sorption of strong polar CHs (1,1,2,2-tetrachloroethane: π^∗ = 0.95; LogK_ow = 2.39) or strong hydrophobic CHs (1,3,5-trichlorobenzene: π^∗ = 0.70; LogK_ow = 4.19) than CHs (γ-hexachlorocyclohexane: π^∗ = 0.68; LogK_ow = 3.72) with relatively low polarity and hydrophobicity. The result reflects that the interaction between NOM and natural black carbon/biochars in soil and water environment possibly plays the similar role in controlling the environmental behavior of various polar or hydrophobic organic pollutants. Moreover, with increasing concentration of adsorbate (C_e), the first mechanism enhanced, while the second mechanism weakened. This study gives a deep insight into the roles of ACS of biochars in controlling the fate and availability of CHs with different molecular properties in environment.

Effect of aging in field soil on biochar's properties and its sorption capacity

Due to its high sorption capacity for different kinds of contaminants, biochar is advocated as a novel remediation strategy for contaminated soils. However, it is not clear how long this extraordinary sorption capacity will be maintained after the biochar is applied to the soil. In this study, a commercial biochar was applied to an agricultural soil, and the sorption of atrazine and phenanthrene on biochar amended soils with different aging periods ranging from 0 to 2 y was investigated. The application of fresh biochar in soil led to an obvious enhancement of the sorption coefficients (K_d) of atrazine and phenanthrene (by 3.13 and 2.93 times at C_e = 0.01 S_w, respectively) compared with the untreated soil. The surface area of biochar first increased and then decreased with aging time. Correspondingly, the sorption of atrazine and phenanthrene on the biochar amended soils first increased and then decreased markedly. Based on the changing trend of the K_d values with aging time, it could be predicted that the sorption capacity of biochar amended soils will decrease to the level of the untreated soil after 2.5 y.

Aging effect of minerals on biochar properties and sorption capacities for atrazine and phenanthrene

Biochars that were produced from pig manure at two different temperatures were incubated with three different minerals to examine the effects of soil minerals on biochar properties and sorption capacities. Biochars freshly mixed with minerals showed dramatic decreases in the sorption of atrazine (maximum decrease by 70.9% at C_e = 0.5 S_w) and phenanthrene (maximum decrease by 69.5% at C_e = 0.5 S_w) compared to unmixed biochars. Following the incubation, minerals were tightly attached to the biochar surface and insert into inner pores, thus changing the elemental composition and surface area of biochars in a manner dependent on the types of biochars and minerals involved. The changes in biochar properties in turn affected biochar sorption capacities. The sorption of both atrazine and phenanthrene by the pig manure biochar produced at 300 °C (BC300) decreased after aging due to an increase in surface hydrophilicity. In contrast, the sorption of atrazine and phenanthrene by BC700 increased after aging with minerals, which could be attributed to the increase in surface area caused by the minerals. However, the sorption capacities of the aged BC700 were still lower than those of the fresh BC700.

Effects of trifluralin on the soil microbial community and functional groups involved in nitrogen cycling

Large amounts of trifluralin are applied each year for weed control; however, its effects on soil microbial communities and functions are unknown. Two agricultural soils, one silty loam and one silty clay were spiked with TFL (0, 0.84, 8.4, and 84 mg kg^-1) and studied the effects using a laboratory microcosm approach. The half-lives were 44.19-61.83 d in all cases. Bacterial abundance increased 1.12-5.56 times by TFL, but the diversity decreased. From the next-generation sequencing results, TFL altered the bacterial community structure, which initially diverged from the control community structure, then recovered, and then diverged again. Linear discriminant analysis effect size indicated that Sphingomonas and Xanthomonadaceae were the predominant species on day 7 and 15 in TFL treatments. N₂-fixing bacteria were initially increased, then decreased, and then recovered, and it was positively correlated with NH₄⁺-N content. Compared with the control, ammonia-oxidizing bacteria were decreased by 25.51-92.63%, ammonia-oxidizing archaea were decreased by 17.12-85.21% (except day 7), and the NO₃^--N concentration was also inhibited. In contrast to bacteria, fungal abundance was inhibited without any observable effects on fungal diversity or community structure. These results suggest that TFL impacts soil bacterial community and alters functional microorganisms involved in soil N processing.

Reduction mechanism of hexavalent chromium by functional groups of undissolved humic acid and humin fractions of typical black soil from Northeast China

Soil organic matters (SOM) have a great retention effect on Cr(VI) migration in subsurface environment, which act as the main electron donors for Cr(VI) reduction; however, Cr(VI) reduction mechanism by different SOM fractions is still unclear, such as undissolved humic acid (HA) and humin (HM). In this study, HA and HM fractions extracted from typical black soil from Northeast China were used to investigate the reaction mechanism between humus functional groups and Cr(VI). According to the results, phenol and hydroxyl were determined as the main electron donors for Cr(VI) reduction by HA and HM instead of carboxyl and carbonyl, which were more likely involved in Cr complexation. Furthermore, Cr(VI) reduction was more dependent on aromatic carbon, rather than aliphatic carbon, and functional groups on the particle surfaces of HA and HM were much more active for Cr(VI) reduction than their interior part. Additionally, HM was found to have a relatively low Cr(VI) reduction capability compared with HA resulting from its high content of cellulose structures that are quite resistant to Cr(VI) oxidation. These results suggest that in the soil environment, undissolved HA tends to play a much more important role than HM in Cr(VI) reduction and retention in the condition that their mass contents are comparable.

Characterization and phenanthrene sorption of organic matter fractions isolated from organic and mineral soils

Sorption of phenanthrene (PHE) to humic acid (HA) and humin (HM) fractions isolated from organic and mineral soils was investigated to better understand sorption processes in varying soil types. Samples were characterized by elemental analysis, X-ray photoelectron spectroscopy, ¹³C nuclear magnetic resonance, and CO₂ adsorption. No clear correlation was found between the distribution coefficient (K_d) and the bulk polarity of the soil organic matters (SOMs). By contrast, PHE K_d values generally increased with increasing surface polarity of the tested SOMs, implying that surface polarity may play a more important role in PHE sorption than the bulk one. The organic carbon (OC)-normalized K_d values (K_oc) of HMs were higher than those of HAs as a result of the higher aliphatic C contents of HMs. For SOMs isolated from mineral soil (MI-SOMs), part of the aliphatic domains may be tightly associated with minerals and were not accessible to PHE molecules, resulting in lower PHE K_oc values of MI-SOMs than the corresponding fractions extracted from the organic soil. This study implies that both chemical characteristics and physical conformation of SOMs are paramount considerations when investigating sorption process of hydrophobic organic compounds in soils.

Dual Role of Humic Substances As Electron Donor and Shuttle for Dissimilatory Iron Reduction

Dissimilatory iron-reducing bacteria (DIRB) are known to use humic substances (HS) as electron shuttles for dissimilatory iron reduction (DIR) by transferring electrons to HS-quinone moieties, which in turn rapidly reduce Fe(III) oxides. However, the potential for HS to serve as a source of organic carbon (OC) that can donate electrons for DIR is unknown. We studied whether humic acids (HA) and humins (HM) recovered from peat soil by sodium pyrophosphate extraction could serve as both electron shuttles and electron donors for DIR by freshwater sediment microorganisms. Both HA and HM served as electron shuttles in cultures amended with glucose. However, only HA served as an electron donor for DIR. Metagenomes from HA-containing cultures had an overrepresentation of genes involved in polysaccharide and to a lesser extent aromatic compound degradation, suggesting complex OC metabolism. Genomic searches for the porin-cytochrome complex involved in DIR resulted in matches to Ignavibacterium/Melioribacter, DIRB capable of polymeric OC metabolism. These results indicate that such taxa may have played a role in both DIR and decomposition of complex OC. Our results suggest that decomposition of HS coupled to DIR and other anaerobic pathways could play an important role in soil and sediment OC metabolism.

Synchrotron infrared microspectroscopy reveals the roles of aliphatic and aromatic moieties in sorption of nitroaromatic compounds to soils

A consensus on the role of organic carbon moieties as the sorptive domains for nonionic organic compounds in soils is lacking due to the extremely complicated compositions of soil matrices. In this study, synchrotron radiation-based infrared microspectroscopy (IMS) was applied to in situ probe the distributions of four nitroaromatic compounds with varying hydrophobicity (namely, 1,3-dinitrobenzene, 1,5-dinitronapthalene, 3-nitrophenanthrene and 6-nitrobenzo[a]pyrene) and their associations with aliphatic and aromatic organic carbon moieties in soils. The technique revealed that both nitro group (NO₂) from the nitroaromatic compounds and organic carbon moieties were unevenly distributed in the soils at the micron scale. The spatial distribution of nitro groups was positively correlated with that of aromatic carbon (C=C) (r>0.804, p<0.01), indicating that the aromatic moieties of soil organic carbon play a key role in sorption of nitroaromatic compounds to soils. Neither nitro groups nor aromatic carbon showed a close relationship with aliphatic carbon (CH) in the spatial distribution in the soils. Meanwhile, the nitro groups from 1,3-dinitrobenzene and 1,5-dinitronapthalene exhibited a significant correlation with clay minerals (OH) in their distributions (r>0.629, p<0.01) in the soils and the correlation became insignificant for the other two compounds with high hydrophobicity. This study for the first time provides micron-scale spectroscopic evidence for the roles of organic carbon moieties in the sorption of nonionic organic compounds to soils.

Rhizospheric effects on atrazine speciation and degradation in laterite soils of Pennisetum alopecuroides (L.) Spreng

Atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine) is a worldwide-used herbicide and often detected in agricultural soils and groundwater at concentrations above the permitted limit, because of its high mobility, persistence, and massive application. This study applied pot experiments to investigate the atrazine contents and speciation during the phytoremediation process by Pennisetum alopecuroides (L.) Spreng. in laterite soils. From the change of the total atrazine and bioavailable atrazine measured by diffusive gradients in thin film (DGT), P. alopecuroides significantly improved atrazine degradation efficiency from 15.22 to 51.46%, attributing to the increasing bioavailable atrazine in rhizosphere. Only a small amount of atrazine was taken up by P. alopecuroides root and the acropetal translocation from roots to shoots was limited. The atrazine speciation was significantly different between rhizosphere and non-rhizosphere, attributing to the declining pH and organic matters in rhizosphere. The relationship between pH and soil-bound/humus-fixed atrazine illustrated the pH-dependant release of the atrazine from soils and the competition between humus adsorption and uptake by P. alopecuroides. The present study reveals the important roles of soil pH and organic matters in atrazine speciation and availability in laterite soils, and provides new insights in the rhizospheric effects on effective phytoremediation of atrazine.

Effects of two ecological earthworm species on atrazine degradation performance and bacterial community structure in red soil

Vermicomposting is an effective and environmentally friendly approach for eliminating soil organic contamination. Atrazine is one of the most commonly applied triazinic herbicides and frequently detected in agricultural soils. This study investigated the roles and mechanisms of two earthworm species (epigeic Eisenia foetida and endogeic Amynthas robustus) in microbial degradation of atrazine. Both earthworms accelerated atrazine degradation performance from 39.0% in sterile soils to 94.9%-95.7%, via neutralizing soil pH, consuming soil humus, altering bacterial community structure, enriching indigenous atrazine degraders and excreting the intestinal atrazine-degrading bacteria. Rhodoplanes and Kaistobacter were identified as soil indigenous degraders for atrazine mineralization and stimulated by both earthworm species. A. robustus excreted the intestinal Cupriavidus and Pseudomonas, whereas Flavobacterium was released by E. foetida. This study provides a comprehensive understanding of the distinct effects of two earthworm species on soil microbial community and atrazine degradation, offering technical supports to apply vermicomposting in effective soil bioremediation.

Sorption mechanisms of neonicotinoids on biochars and the impact of deashing treatments on biochar structure and neonicotinoids sorption

To elucidate the sorption affinity of biochars for neonicotinoid pesticides and the influence of biochar structure on sorption mechanisms therein, 24 biochar samples were obtained by pyrolyzing maize straw and pig manure at pyrolyzing temperatures (PTs) of 200-700 °C and by further deashing them using acids, and the sorption of three typical neonicotinoids, imidacloprid, clothianidin and thiacloprid on untreated and acid-deashed biochars were evaluated. All the biochar samples could efficiently adsorb the three neonicotinoids and multiple mechanisms were involved in sorption. With the increasing PTs, hydrophobic partition sorption increased, but had a declined contribution to the total sorption as revealed by a dual-mode model. Besides hydrophobic partition, specific interactions like cation-π electron donor acceptor (EDA) interactions (only for protonated IMI and CLO) and hydrogen bond and contributed much to the sorption on low-PT (≤500 °C) biochars, while the sorption on those high-PT (>500 °C) biochars mainly depended on pore-filling strengthened by cation-π and p/π-π EDA interactions. Thiacloprid showed stronger sorption on untreated biochars compared to imidacloprid and clothianidin, due to its greater ability to form hydrogen bond and hydrophobic interactions. Acid-deashing treatments increased the relative percentage contents of organic carbon, bulk O, aromaticity and O-containing functional groups, surface area and pore volume of biochars. The ash can bind neonicotinoids by specific interactions but played a negative role in the whole sorption on high-PT biochars by covering the inner sorption sites of organic moieties and blocking the micropores in biochars. The results acquired in the present study will help us to get deep insight in the comprehensive sorption mechanisms of polar pesticides on biochar and the effects of biochar structure.

Comparison between Soil- and Biochar-Derived Humic Acids: Composition, Conformation, and Phenanthrene Sorption

Biochar-derived organic matter (BDOM) plays an important role in determining biochar's application potential in soil remediation. However, little is known about the physicochemical properties of BDOM and its sorption of hydrophobic organic compounds (HOCs). Humic acids (HAs) were extracted from oxidized biochars produced from plant straws and animal manures at 450 °C, and their sorption of phenanthrene, a representative of HOCs, was investigated. The organic carbon recovery of biochar-derived HAs (BDHAs) was 13.9-69.3%. The ¹³C NMR spectra of BDHAs mainly consisted of aromatic and carboxylic C, while those of soil-derived HAs (SDHAs) contained abundant signals in aliphatic region. BDHAs and SDHAs had comparable CO₂ cumulative surface areas. BDHAs were found to exhibit higher phenanthrene sorption than SDHAs. After the removal of amorphous aromatic components, the logK_oc values of BDHAs were significantly decreased, implying that amorphous aromatic C regulated phenanthrene sorption by BDHAs. In contrast, aliphatic moieties dominated phenanthrene sorption by SDHAs, as evidenced by the enhanced sorption after the removal of amorphous aromatics. This study clearly demonstrated the contrasting characteristics and sorption behaviors of BDHA and SDHA, indicating that biochar addition and subsequent weathering could greatly affect native organic matter properties and the fate of HOCs in biochar-amended soils.

Sludge treatment by integrated ultrasound-Fenton process: Characterization of sludge organic matter and its impact on PAHs removal

In this work, the impact of organic matter on the degradation of polycyclic aromatic hydrocarbons (PAHs) in textile dyeing sludge by ultrasound-Fenton process has been studied. Sludge organic matter (SOM) was characterized and the degradation efficiencies of PAHs at various oxidation intensities (Fenton's reagent of 20, 70, and 140mmol/L, ultrasonic densities of 0.36, 0.90, and 1.80W/cm³, and reaction time of 15, 25, and 40min) were determined. The results showed that 75.52-84.40% of PAHs and 16.32-31.13% of SOM had degraded after ultrasound-Fenton treatment, confirming the competitive relation between both of them for degradation. The aliphatic SOM fractions were preferentially oxidized owing to their easily degradable properties, while equimolar amounts of the aromatic moieties would require more oxidant compared to the aliphatic fractions. Correlation analysis demonstrated that SOM with its lower content, stronger polarity, and a higher proportion of labile organic fraction was more favourable for PAHs degradation. In addition, the SOM fractions were decomposed to biodegradable matter after treatment, which further enhance the biodegradability of sludge. This study provides insights into the role of SOM in PAHs removal by AOPs, and confirms that the ultrasound-Fenton treatment could not only effectively degrade PAHs, but also modify SOM.

Dynamic changes in atrazine and phenanthrene sorption behaviors during the aging of biochar in soils

The objective of this study was to elucidate the dynamic changes in the properties of biochar-amended soil and their sorption capacity for typical organic contaminants with increasing contact time between biochar and soil. To do so, biochars that were produced from pig manure at two temperatures were added to two soils, and the sorption behaviors of atrazine and phenanthrene (Phen) on soil-biochar mixtures aged for different times were investigated. Soils freshly amended with biochars showed a dramatic increase in the sorption of atrazine (up to 23.4 times at C _e = 0.01 S _w) and Phen (up to 3.12 times at C _e = 0.01 S _w) compared to the bare soils without biochars. The physicochemical properties of soil-biochar mixtures changed with aging time, which in turn affected the sorption capacity. After the biochar produced at 300 °C (BC300) was aged in soil, the sorption of atrazine and Phen by black soil (BS) and fluvo-aquic soil (FS) both increased by different extents, except the sorption of Phen on BS. However, after the biochar produced at 700 °C (BC700) was aged in soil, the sorption of atrazine on the two soils decreased markedly, which was sill 56.3% higher than that on the original soil, while an opposite trend was observed for Phen on the two soils. The complex change patterns could be due to the different dominant sorption mechanisms for different biochars and chemicals.

Sorption, transport and biodegradation - An insight into bioavailability of persistent organic pollutants in soil

Contamination of soils with persistent organic pollutants (POPs), such as organochlorine pesticide, polybrominated diphenyl ethers, halohydrocarbon, polycyclic aromatic hydrocarbons (PAHs) is of increasing concern. Microbial degradation is potential mechanism for the removal of POPs, but it is often restricted by low bioavailability of POPs. Thus, it is important to enhance bioavailability of POPs in soil bioremediation. A series of reviews on bioavailability of POPs has been published in the past few years. However, bioavailability of POPs in relation to soil organic matter, minerals and soil microbes has been little studied. To fully understand POPs bioavailability in soil, research on interactions of POPs with soil components and microbial responses in bioavailability limitation conditions are needed. This review focuses on bioavailability mechanisms of POPs in terms of sorption, transport and microbial adaptation, which is particularly novel. In consideration of the significance of bioavailability, further studies should investigate the influence of various bioremediation strategies on POPs bioavailability.