Novel insights into antimony mobilization in different high- antimony aquifers from the molecular signatures of dissolved organic matter

The crucial role of the fluorescent components of dissolved organic matter (DOM) in controlling antimony (Sb) mobilization in groundwater has been confirmed. However, the molecular signatures contributing to Sb enrichment in DOM remain unknown. This study aims to investigate the origins and molecular compositions of DOM in different high-Sb aquifers (Sb-mining and no-Sb-mining aquifer), as well as compare different molecular signatures of DOM and mechanisms for Sb migration. The findings showed that Sb concentrations in Sb-mining aquifer exhibited a positive correlation with lignin- and tannin-like molecules characterized by high O/C and low H/C ratios, indicating an increased abundance of aromatic components with higher Humification Index and SUV-absorbance at 254 nm, compared to no-Sb-mining aquifer. Correspondingly, the complexation and competitive adsorption were considered as the predominate formation mechanisms on Sb enrichment in Sb-mining aquifer. In addition, high abundances of bioreactivity DOM may facilitated the migration of Sb via electron transfer and competitive adsorption in native no-Sb-mining aquifer. The outcomes of this investigation offer novel insights into the mechanism on Sb enrichment influenced by DOM at the molecule level.

Impact of compost methods on humification and heavy metal passivation during chicken manure composting

Livestock manure is one of the main sources of heavy metals (HMs) in agricultural soil. So, it is necessary to reduce its bioavailability before used as organic fertilizer. In this study, the passivation effect of HMs and the evolution of dissolved organic matter (DOM) during four composting processes were explored. Results showed that different composting methods had a great effect on HMs passivation rate and humification degree. HMs were released during the thermophilic phase, and were bound by resynthesized humus during the cooling period. The best passivation effect of HMs was found in FV + T treatment, the passivation rate of Cu, Zn, Cd and Pb reached 63.80%, 34.07%, 86.54% and 45.14%, respectively, then followed by the treatment of NV + T and SC. UV-Vis spectra and excitation-emission matrix (EEM) spectra indicated that humus precursors were produced during thermophilic phase and the accumulation of humus mainly occurred in cooling period. This study can be used as a theoretical support for the safe utilization livestock manure.

Insight into interactions of heavy metals with livestock manure compost-derived dissolved organic matter using EEM-PARAFAC and 2D-FTIR-COS analyses

Dissolved organic matter (DOM), as the most active ingredient in compost, directly determines the speciation and environmental behavior of HMs. Here, the binding properties of DOM derived from chicken-manure compost (CHM), cow-manure compost (COM) and pig-manure compost (PIM) with HMs were explored by analyses of Fluorescence excitation-emission matrix parallel factor (EEM-PARAFAC) and two-dimensional correlation Fourier transform infrared spectroscopy (2D-FTIR-COS). Results showed that the binding characteristics vary with origin of DOM and type of HMs. The fulvic-like component dominated the transformation of HMs speciation, and CHM-DOM had higher affinity with HMs and greater risk causing pollution due to its higher aromaticity, molecular weight and distribution of fluorescent components. Moreover, Cu(II) can efficiently bind to DOM with the stability constants (log k_M) ranging from 4.53 to 5.38, followed by Pb(II) (3.34-3.57), whereas Cd(II) can hardly bind to DOM. The amide and polysaccharide were the predominant sites for HMs binding in CHM-DOM, and polysaccharide and phenolic in COM-DOM, while phenolic and amide in PIM-DOM, respectively. Although the proportion of protein-like components and non-fluorescent polysaccharides in DOM were low, their role in HMs binding should not be ignored. In brief, the environmental risk caused by livestock manure compost may originate from interactions between DOM and HMs.

Chemodiversity of water-extractable organic matter in sediment columns of a polluted urban river in South China

Dissolved organic matter (DOM) in sediments of polluted rivers significantly contributes to oxygen consumption and river blackening and odorization. However, the chemodiversity of DOM at different depths or river reaches is poorly known. Here, we studied the storage and molecular-level signatures of water-extractable organic matter (WEOM) in the sediment column (0-100 cm) of the upper, middle, and lower mainstream of Maozhou River (a polluted river in Shenzhen, China, with 40 years of urbanization) using optical spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry. The sediment WEOM level increased from upstream to downstream. The relative abundances of sulfur-containing surfactants in all sediment WEOM were higher than those previously reported for surface water DOM. The WEOM in surface sediment had higher aromaticity, molecular size, and nominal oxidation state of carbon and greater signals from anthropogenic inputs than did deep sediment at the upper and middle mainstream sites. However, these characteristics varied little between surface and deep sediments at the lower mainstream site, probably due to intensive surface water and pore water interactions. The sediment WEOM at 0-40 cm in the middle mainstream showed a greater anthropogenic signature (e.g., more surfactant and dissolved black carbon contributions) than any other sediment. We demonstrate strong anthropogenic impacts on the surface sediment over decades of urbanization.

Spectroscopic and molecular characterization of biochar-derived dissolved organic matter and the associations with soil microbial responses

Dissolved organic matter (DOM) released from biochar can influence the microbial community structure, but the inherent mechanism associated with the structure of biochar-derived DOM remains insufficiently elucidated. In this study, the spectroscopic characteristics and molecular structures of biochar-derived DOM were studied, and the microbial responses to biochar-derived DOM were explored. With increasing biochar pyrolysis temperature (PT), the molecular weight and proportions of aliphatic and fulvic acid-like compounds in the biochar-derived DOM decrease along with an reduction in the amount of DOM released from the biochars, but the proportions of combustion-derived condensed polycyclic aromatics and humic acid-like and soluble microbial byproduct-like compounds increased. Accordingly, the humification index, H/C and (O + N)/C values also decreased. The spectroscopic characteristics of biochar-derived DOM were distinct from those of natural substrates. Moreover, the DOM extracted from biochar raw materials contained a high proportion of aliphatic compounds, while the DOM derived from high-PT biochars (500 °C) had similar characteristics to fulvic acid-like and soluble microbial byproduct-like compounds. The microbial abundance and community structure varied in different DOM solutions. The relative abundances (RAs) of eight genera (e.g. Dyadobacter, Sphingobacterium and Novosphingobium) had significantly positive correlations with the content of aliphatic compounds, while RAs of seven genera (e.g. Methylotenera, Acinetobacter and Reyranella) had significant positive correlations with the content of high-aromatic combustion-derived condensed polycyclic aromatics. These results are helpful for obtaining a deep understanding of the potential influences of various types of biochar-derived DOM on terrestrial and aquatic microbiology.

Effects of intercropping mulch on the content and composition of soil dissolved organic matter in apple orchard on the loess plateau

Dissolved organic matter (DOM) is an important parameter that reflects soil fertility and quality. In this study, the effects of intercropping perennial ryegrass and straw mulch on the content, composition and spectral characteristics of soil DOM in orchards in arid and semi-arid areas were investigated by three-dimensional fluorescence excitation-emission matrix (EEM) combined with parallel factor analysis (PARAFAC). Three treatments were applied in a 6-year-old apple orchard on the Loess Plateau: (1) clean tillage (CT); (2) intercropping perennial ryegrass mulch (RE); (3) intercropping straw mulch (CS). Soil samples (0-10, 10-20, 20-40, 40-60, 60-80, 80-100 cm) were collected from different treatments. The total soil organic carbon (SOC) and dissolved organic carbon (DOC) content were ranked as CS> RE> CT, and decreased with soil depth. In addition, the vertical distribution of DOC with different soil depth was caused by organic matter input but not leaching. Three humic-like components and two protein-like components were identified by EEM-PARAFAC. Fluorescence spectra showed that humic-like compounds were the dominant fractions in soil DOM in the CS treatment. Fluorescence Index (FI) indicated that the proportion of plant residues and soil organic matter derived DOM ranked as CT> RE> CS and gradually decreased with soil depth. Our results highlight the key role of orchard intercropping mulch in improving the content and compositions of soil DOM through different forms of organic inputs and provide new theoretical support for the evaluation of soil fertility in orchards.

Characterization of dissolved organic carbon in rainwater of an urban/coastal site in Mediterranean area

Concentration and optical characteristics of dissolved organic matter were studied in rainwater in the urban/coastal city of Thessaloniki, Northern Greece for 2-yr sampling period (2014-2016). The concentration of DOC ranged from 0.33-24.5mg/L with higher values measured in spring-summer period. Higher aromaticity and fluorescence intensity was observed in winter. Chromophoric organic matter represents a significant fraction of DOC that is highly correlated with fluorescence during cold period. Three factor spectral profiles of fluorescence were elucidated, with peaks at protein-like and humic-like area at different intensities. Fluorophores at shorter wavelengths are more susceptible to changes. DOC showed negative relationship with precipitation height, particularly during autumn and spring suggesting washout effect. NMR spectra showed the dominance of aliphatic protons in rainwater. Levoglucosan, sucrose and arabitol were determined in rainwater at concentrations <0.07-2.2μg/L, <0.03-5.1μg/L and <0.03-2.1μg/L, respectively showing impact of biomass combustion and biogenic emissions.

Sources, composition, and spectroscopiccharacteristics of dissolved organic matter extractedfrom sediments in an anthropogenic-impacted riverin Southeastern China

The fluorescence and ultraviolet spectroscopic characteristics of dissolved organic matter extracted from sediments in an anthropogenic-impacted river (Hao River) in Southeastern China were analyzed in the current study. Excitation-emission matrix (EEM) fluorescence combined with parallel factor analysis (PARAFAC) was conducted to analyze the sources and composition of dissolved organic matter (DOM). The average dissolved organic carbon (DOC) concentrations of all samples at depths of 0-10, 10-20, and 20-30 cm were 317, 319, and 319 mg kg^-1 dry weight, respectively. Four components were identified from the fluorescence spectra that consisted of three humic-like components (C1, C2, and C3) and one protein-like component (C4). Component C1 was derived from terrestrial plants and soils. Component C2 in the anthropogenic-impacted Hao River was derived from anthropogenic pollution such as wastewater. Component C3 comprised refractory particles derived from terrestrial plants. Component C4 was autochthonous and produced by the aquatic ecosystem. With the exception of site 4, all sites could be assigned to one of two groups. Group 1 consisted of sites 1, 2, 3, and 5, and group 2 consisted of sites 6, 7, and 8. Our result showed that the sediment in group 2 was more anaerobic than in group 1. Consequently, it could be concluded that this river was polluted by both point and non-point pollution. The terrain slopes cause that water flows from the Yangzi River into group 1. The dilution effect in group 1 resulted in less accumulation of DOM in the sediment of group 1 than group 2.