Mechanisms by which different polar fractions of dissolved organic matter affect sorption of the herbicide MCPA in ferralsol

Effect of interaction between dissolved organic matter and iron/manganese (hydrogen) oxides on the degradation of organic pollutants by in-situ advanced oxidation techniques

Iron and manganese (hydrogen) oxides (IMHOs) exhibit excellent redox capabilities for environmental pollutants and are commonly used in situ chemical oxidation (ISCO) technologies for the degradation of organic pollutants. However, the coexisting dissolved organic matter (DOMs) in surface environments would influence the degradation behavior and fate of organic pollutants in IMHOs-based ISCO. This review has summarized the interactions and mechanisms between DOMs and IMHOs, as well as the properties of DOM-IMHOs complexes. Importantly, the promotion or inhibition impact of DOM was discussed from three perspectives. First, the presence of DOMs may hinder the accessibility of active sites on IMHOs, thus reducing their efficiency in degrading organic pollutants. The formation of compounds between DOMs and IMHOs alters their stability and activity in the degradation process. Second, the presence of DOMs may also affect the generation and transport of active species, thereby influencing the oxidative degradation process of organic pollutants. Third, specific components within DOMs also participate and affect the degradation pathways and rates. A comprehensive understanding of the interaction between DOMs and IMHOs helps to better understand and predict the degradation process of organic pollutants mediated by IMHOs in real environmental conditions and contributes to the further development and application of IMHO-mediated ISCO technology.

Insights into the effect of hydrochar-derived dissolved organic matter on the sorption of diethyl phthalate onto soil: A pilot mechanism study

Biowaste-derived hydrochar is an emerging close-to-natural product and has shown promise for soil improvement and remediation, but the environmental behavior of the dissolved organic matter released from hydrochar (HDOM) is poorly understood. Focusing on the typical mulch film plasticizer diethyl phthalate (DEP), we investigated the effect of HDOM on the sorption behavior of DEP on soil. The relatively low concentration of HDOM (10 mg L-1, 25 mg L-1) decreases the sorption quantity of DEP on soil, while it increases by a relatively high concentration, 50 mg L-1. The transformation from multilayer to monolayer sorption of DEP on soil occurs as the concentration of HDOM increases. The tryptophan-like substance is the main component of HDOM sorbed to soil, reaching 49.82 %, and results in competition sorption with DEP. The soil pores are blocked by HDOM, which limits the pore filling and mass transfer of DEP, but partitioning is significantly enhanced. The surface functional groups in HDOM are similar to those in soil, and chemical sorption, mainly composed of hydrogen bonding, exists but is not significantly strengthened. We identified the specific impact of HDOM on the sorption of organic pollutants on soil and provide new insights into the understanding of the environmental behavior of hydrochar.

Characteristics of dissolved organic matter (DOM) in Chinese farmland soils under different climate zone types: A molecular perspective

Interactions between dissolved organic matter (DOM) and surrounding environments are highly complex. Understanding DOM at the molecular level can contribute to the management of soil pollution and safeguarding agricultural fields. Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) has enabled a molecular-level understanding of DOM. Accordingly, in this study, we investigated soil samples from 27 different regions of mainland China with various soil types and climatic characteristics. Based on the geographical features of the four typical climatic zones in mainland China (temperate monsoon, temperate continental, subtropical monsoon, and Qinghai-Tibet Plateau climates), we employed high-resolution mass spectrometry to determine the molecular diversity of DOM under different climatic conditions. The results indicated that lignin and tannin-like substances were the most active categories of DOM in the soils. Collectively, the composition and unsaturation of DOM molecules are influenced by sunlight, precipitation, temperature, and human activity. All climatic regions contained a substantial number of characteristic molecules, with CHO and CHON constituting over 80%, and DOM containing nitrogen and sulfur was relatively more abundant in the monsoon regions. The complex composition of DOM incorporates various active functional groups, such as -NO2 and -ONO2. Furthermore, soil DOM in the monsoon regions showed higher unsaturation and facilitated various (bio) biochemical reactions in the soil.

Effect of biodegradable PBAT microplastics on the C and N accumulation of functional organic pools in tropical latosol

Microplastics (MPs) pollution is becoming an emerging global stressor for soil ecosystems. However, studies on the impacts of biodegradable MPs on soil C sequestration have been mainly based on bulk C quantity, without considering the storage form of C, its persistency and N demand. To address this issue, the common poly (butylene adipate-co-terephthalate) (PBAT) was used as the model, and its effects on soil functional organic pools, including mineral-associated (MAOM), particulate (POM) and dissolved organic matter (DOM), were investigated from the novel coupled perspective of C and N stocks. After adding PBAT-MPs, the contents of soil POM-C, DOM-C, and MAOM-C were increased by 546.9 %-697.8 %, 54.2 %-90.3 %, and 13.7 %-18.9 %, respectively. Accordingly, the total C increased by 116.0 %-191.1 %. Structural equation modeling showed that soil C pools were regulated by PBAT input and microbial metabolism associated with C and N enzymes. Specifically, PBAT debris could be disguised as soil C to promote POM formation, which was the main pathway for C accumulation. Inversely, the MAOM-C and DOM-C formation was attributed to the PBAT microbial product and the selective consumption in DOM-N. Random forest model confirmed that N-activated (e.g., Nitrospirae) and PBAT-degrading bacteria (e.g., Gemmatinadetes) were important taxa for soil C accumulation, and the key enzymes were rhizopus oryzae lipas, invertase, and ammonia monooxygenase. The soil N accumulation was mainly related to the oligotrophic taxa (e.g., Chloroflexi and Ascomycota) associated with aggregate formation, decreasing the DOM-N by 46.9 %-84.3 %, but did not significantly change the total N storage and other N pools. Collectively, the findings highlight the urgency to control the nutrient imbalance risk of labile N loss and recalcitrant C enrichment in POM to avoid the depressed turnover rate of organic matter in MPs-polluted soil.

Soil Organic Matter Composition and pH as Factors Affecting Retention of Carbaryl, Carbofuran and Metolachlor in Soil

The majority of studies concerning the environmental behavior of hydrophobic pollutants in soil consider soil organic matter (SOM) content as a main factor influencing chemical retention, whereas the composition of SOM and its individual fraction share are often neglected. In the present paper, carbaryl, carbofuran and metolachlor retention by loamy sand and loam topsoil materials is compared and referred to humic acids (CHA) and the residual carbon (CR) content of SOM. Additionally, the sorption-desorption behavior of agrochemicals in soils was tested at a pH of three to seven. Calculated isothermal parameters point to favorable, spontaneous and physical pesticide sorption. Groundwater ubiquity score (GUS) indexes confirmed the low leaching ability of metolachlor on soils and moderate of carbofuran. The high affinity of carbaryl to CR may explain its pronounced sorption in loam soil and the lowest percolation potential. Carbofuran retention in soils was associated with montmorillonite (Mt) and CR fractions. Meanwhile, metolachlor uptake was related to humic acid and Mt content of the soils. Lower pH enhanced retention of the agrochemicals, except for carbaryl sorption in sandy loam soil. Results of this study highlight that SOM composition and mutual share of individual organic carbon fractions alongside pH may play a crucial role in predicting non-ionic pesticide behavior in soil.

Molecular linkages between chemodiversity and MCPA complexation behavior of dissolved organic matter in paddy soil: Effects of land conversion

Complexation of dissolved organic matter (DOM) plays a crucial role in regulating the fate and risk of agrochemicals. Here, taking a toxic herbicide MCPA (4-chloro-2- methylphenoxyacetic acid) as the target, the effect of land conversion on complexation behavior of DOM to agrochemicals was investigated in paddy soil. Furthermore, the mechanisms were explored in a new perspective of DOM chemodiversity. Soil DOMs were selected from four long-term cropping systems, including paddy field (PF), vegetable field (VF), rice-vegetable rotation (RV) and abandoned land (AL). The results showed that the DOMs in PF and AL were rich in hydrophilic substances (e.g., carbohydrates or protein-like molecules) with low aromaticity. However, after converting PF to VF and RV, abundant aromatic macromolecules and aliphatic alkanes were observed in DOM. Due to those changes in DOM chemodiversity, the binding site and capability of DOM were highest in VF and RV, and were positively correlated with DOM aromaticity, MW, humus and polar groups (e.g., amino). This was because the complexation of "DOM-MCPA" was static binding via ligand exchange and H-bonding among polar groups and hydrophobic interaction among aromatic skeletons. The EEM-PARAFAC confirmed that microbial humic-like substances dominated the complexation of DOM rather than terrestrial humic-like and tryptophan-like matters. The 2D-COS analysis further revealed that the complexation of DOM preferentially occurred in amino, polysaccharide C-O and aliphatic C-H for PF and AL, but in aromatic C=C, amide C=N for RV and VF. In summary, these findings provide molecular insight into the effect of land conversion on DOM complexation activity, which highlight the importance of DOM chemodiversity. These results will contribute to the risk assessments of agrochemicals in paddy soil.

Addition of biodegradable microplastics alters the quantity and chemodiversity of dissolved organic matter in latosol

Dissolved organic matter (DOM) chemodiversity plays an important role in regulating nutrient cycles and contaminant behavior in soil. However, how biodegradable microplastic (MPs) affect the DOM chemodiversity is still unknown, although developing biodegradable plastics are regarded as a promising strategy to minimize the risks of MPs residues in soil. Here, with the common poly (butylene adipate-co-terephthalate) (PBAT) as the model, the molecular effect of biodegradable MPs on soil DOM was explored by adding 0%, 5% and 10% (w/w) of PBAT to tropical latosol, respectively. The results showed that PBAT addition increased microbial activity and exoenzyme activity (e.g., rhizopus oryzae lipase, invertase and cellulose). As a result, the quantity and chemodiversity of soil DOM were changed. The multispectroscopic characterization showed that PBAT addition significantly increased the DOC molecules in soil, including condensed aromatic-like substances and carbohydrates. In contrast, the TDN molecules with high bioavailability and low aromaticity, such as amino acids, were decreased. The multivariate statistical analysis indicated that there were three mechanisms that drove the shift in DOM chemodiversity. Firstly, the degradation of PBAT by rhizopus oryzae lipase facilitated the release of exogenous aromatic molecules. Secondly, PBAT decomposition stimulated the selective consumption of native N-rich molecules by soil microbes. Thirdly, PBAT accelerated the enzymatic transformation of native aliphatic CH_x and cellulose toward humic substances. In addition, concentration effect was also observed in the study that high-concentration PBAT were more likely to trigger the molecular shift in DOM chemodiversity. These findings provided a new insight into the impact of biodegradable MPs on soil DOM chemodiversity at molecular level, which will be beneficial to understanding the fate and biochemical reactivity of DOM in MPs-polluted soil.