Effects of Aging on Adsorption of Tetracycline Hydrochloride by Humin

To explore the effects of "aging", an environmental factor, on adsorption of tetracycline hydrochloride (TC) by humin (HM), this paper coats HM surface with ferric hydroxide precipitate to simulate the aging process. The research findings indicate that compared with fresh HM, aged HM (HM-Fe) displays an accelerated adsorption rate and higher adsorption capacity on TC. With an initial concentration of 20 mg·L-1, TC's equilibrium adsorption capacity on HM and HM-Fe is 4.6 and 5.3 mg·g-1, respectively, whereas the corresponding initial adsorption rate is 0.036 and 0.132 mg·g-1·min-1. The pseudo-second-order kinetic model and Freundlich adsorption isotherm model could adequately simulate the adsorption process of TC by HM and HM-Fe, suggesting the occurrence of chemical adsorption and multimolecular layer adsorption between TC and HM and HM-Fe. Based on ΔAbs deduced from Job's calculation, it can be assumed a complex reaction occurs between the iron element on the HM-Fe surface and TC, which acts as a sort of bridge in strengthening the adsorption of TC by HM-Fe. The aforesaid findings may provide subsequent further study on environmental behaviors of TC in the soil with both fundamental theories and a scientific basis.

Effect of Humin and Chemical Factors on CO2-Fixing Acetogenesis and Methanogenesis

Acetogenesis and methanogenesis have attracted attention as CO₂-fixing reactions. Humin, a humic substance insoluble at any pH, has been found to assist CO₂-fixing acetogenesis as the sole electron donor. Here, using two CO₂-fixing consortia with acetogenic and methanogenic activities, the effect of various parameters on these activities was examined. One consortium utilized humin and hydrogen (H₂) as electron donors for acetogenesis, either separately or simultaneously, but with a preference for the electron use from humin. The acetogenic activity was accelerated 14 times by FeS at 0.2 g/L as the optimal concentration, while being inhibited by MgSO₄ at concentration above 0.02 g/L and by NaCl at concentrations higher than 6 g/L. Another consortium did not utilize humin but H₂ as electron donor, suggesting that humin was not a universal electron donor for acetogenesis. For methanogenesis, both consortia did not utilize extracellular electrons from humin unless H₂ was present. The methanogenesis was promoted by FeS at 0.2 g/L or higher concentrations, especially without humin, and with NaCl at 2 g/L or higher concentrations regardless of the presence of humin, while no significant effect was observed with MgSO₄. Comparative sequence analysis of partial 16S rRNA genes suggested that minor groups were the humin-utilizing acetogens in the consortium dominated by Clostridia, while Methanobacterium was the methanogen utilizing humin with H₂.

Humin Assists Reductive Acetogenesis in Absence of Other External Electron Donor

The utilization of extracellular electron transfer by microorganism is highly engaging for remediation of toxic pollutants under “energy-starved” conditions. Humin, an organo-mineral complex of soil, has been instrumental as an external electron mediator for suitable electron donors in the remediative works of reductive dehalogenation, denitrification, and so forth. Here, we report, for the first time, that humin assists microbial acetogenesis as the extracellular electron donor using the electron acceptor CO2. Humin was obtained from Kamajima paddy soil, Japan. The anaerobic acetogenic consortium in mineral medium containing CO2/HCO3− as the inorganic carbon source used suspended humin as the energy source under mesophilic dark conditions. Retardation of acetogenesis under the CO2-deficient conditions demonstrated that humin did not function as the organic carbon source but as electron donor in the CO2-reducing acetogenesis. The consortium with humin also achieved anaerobic dechlorination with limited methanogenic activity. Total electron-donating capacity of humin was estimated at about 87 µeeq/g-humin. The metagenomic sequencing of 16S rRNA genes showed the predominance of Firmicutes (71.8 ± 2.5%) in the consortium, and Lachnospiraceae and Ruminococcaceae were considered as the CO2-reducing acetogens in the consortium. Thus, microbial fixation of CO2 using humin introduces new insight to the holistic approach for sustainable treatment of contaminants in environment.

Anaerobic Dechlorination by a Humin-Dependent Pentachlorophenol-Dechlorinating Consortium under Autotrophic Conditions Induced by Homoacetogenesis

Anoxic aquifers suffer from energy limitations due to the unavailability of organic substrates, as dictated by hydrogen (H2) for various electron-accepting processes. This deficiency often results in the accumulation of persistent organic pollutants, where bioremediation using organic compounds often leads to secondary contamination. This study involves the reductive dechlorination of pentachlorophenol (PCP) by dechlorinators that do not use H2 directly, but rather through a reduced state of humin-a solid-phase humic substance-as the extracellular electron donor, which requires an organic donor such as formate, lactate, etc. This shortcoming was addressed by the development of an anaerobic mixed culture that was capable of reductively dechlorinating PCP using humin under autotrophic conditions induced by homoacetogenesis. Here, H2 was used for carbon-dioxide fixation to acetate; the acetate produced was used for the reduction of humin; and consequently used for dechlorination through reduced humin. The 16SrRNA gene sequencing analysis showed Dehalobacter and Dehalobacterium as the possible dechlorinators, while Clostridium and Oxobacter were identified as the homoacetogens. Thus, this work contributes to the development of an anaerobic consortium that balanced H2 dependency, where efficiency of humin reduction extends the applicability of anaerobic microbial remediation in aquifers through autotrophy, syntrophy, and reductive dechlorination.

Demonstration of Chemical Distinction among Soil Humic Fractions Using Quantitative Solid-State 13C NMR

Humic substances (HS) are vital to soil fertility and carbon sequestration. Using multiple cross-polarization/magic-angle spinning (multiCP/MAS) NMR combined with dipolar dephasing, we quantitatively characterized humic fractions, i.e., fulvic acid (FA), humic acid (HA), and humin (HM), isolated from two representative soils (upland and paddy soils) in China under six long-term (>20 years) fertilizer treatments. Results indicate that each humic fraction showed chemical distinction between the upland and paddy soils, especially with much greater aromaticity of upland HMs than of paddy HMs. Fertilizer treatment exerted greater influence on chemical natures of upland HS than of paddy HS, although the effect was less than that of soil type. Organic manure application especially decreased the percentages of aromatic C in the upland HAs and HMs compared with the control. We concluded that humic fractions responded in chemical nature to environmental conditions, i.e., soil type/cropping system/soil aeration and fertilizer treatments.

Assessment of Multiorigin Humin Components Evolution and Influencing Factors During Composting

Humin (HM) is a complex mixture of molecules produced in the different biological processes, and the structural evolution of HM in the agricultural wastes composting are not well-known. Elucidating and comparing the structural evolution during livestock manure (LMC) and straw wastes (SWC) composting can help one to better understand the fates, features, and environmental impacts of HM. This study exploits excitation emission matrix-parallel factor (EEM-PARAFAC), two-dimensional correlation spectroscopy (2D-CoS), hetero-2DCoS, and structural equation model (SEM) to compare the fate of the HM. We fit a three-component EEM-PARAFAC model to characterize HM extracted from LMC and SWC. The results show that the HM evolution has a significant difference between LMC and SWC. As a result, the opposite change tendency and different change order of HM fluorescent components determine the different synthesis formation and evolution mechanisms. The diverse organic matter composition and dominant microbes might be the reason for the different evolution mechanism. Based on these results, a comprehensive view of the component changes of HM in the composting process is obtained. Furthermore, the superior potential of such an integrated approach during investigating the complex evolution in the environment was also demonstrated.

Humin as an External Electron Mediator for Microbial Pentachlorophenol Dechlorination: Exploration of Redox Active Structures Influenced by Isolation Methods

Humin (HM) has been reported to function as an external electron mediator (EEM) in various microbial reducing reactions. In this study, the effect of isolation methods on EEM functionality and the chemical/electrochemical structures of HM were examined based on the correlation between dechlorination rates in the anaerobic HM-dependent pentachlorophenol (PCP)-dechlorinating consortium and the chemical/electrochemical structures of HM. A lack of PCP dechlorination activity suggested no EEM function in the HM samples prepared as a soluble fraction in dimethyl sulfoxide and sulfuric acid (which did not contain any electric capacitance). Other HM samples exhibited EEM functionality as shown by the dechlorination activity ranging from 0.55 to 3.48 (µmol Cl⁻) L⁻¹d⁻¹. The comparison of dechlorination activity with chemical structural characteristics suggested that HM with EEM functionalities had predominantly aliphatic and carbohydrate carbons with the partial structures C=O, O=C–N, and O=C–O. EEM functionality positively correlated with the proportion of O=C–N and O=C–O, suggesting an association between peptidoglycan structure and EEM functionality. The lack of detection of a quinone structure in one HM sample with EEM functionality and a negative correlation with aromatic or C=C carbon suggested that the mechanism containing quinone structures is a minor component for the functionality of EEM.

[Effects of Biochar Pyrolyzed at Varying Temperatures on Soil Organic Carbon and Its Components: Influence on the Composition and Properties of Humic Substances]

Application of biochar (BC) is an important way to increase soil organic carbon sequestration. At the same time, the effect of BC on fractions and properties of soil humic substances is concerned. A laboratory experiment was conducted to study the influences of BC pyrolyzed at different temperatures on the composition and properties of humic substances. The modified method for the extraction and fractionation of humic substances was adopted in this work. The carbon (C) contents of Humin (Hu), Humic acids (HA), Fulvic acids (FA) were analyzed by the thermal oxidation of K₂Cr₂O₇ and TOC analyzer, and the optical properties of HA and FA were measured by using spectrophotometer. The results showed that the increasing temperature (from 300 to 600℃) decreased like-humic substances (LHS) from 10.93 g·kg^-1 to 0.26 g·kg^-1, while the structure of theLHS tended to be complicated. Compared with control treatment (CK), the addition of BC produced under 400℃ increased the contents of HA and FA (after 240 d incubation a lower FA content was found in treatments) and increased as BC application rate increased, after 360 d of incubation, BC300 and BC400 significantly increased by 69.93% and 48.75% for HA (P<0.05), while decreased by 1.35% and 5.19% for FA. Higher contents of HA and FA were found in soil samples amended with BC prepared at above 400℃ only during the initial period of 3-10 d of incubation and increased as BC application rate increased, at the end of the incubation, the contents of HA and FA significantly decreased by 34.38%, 44.48% in BC500 treatments and 42.84%, 49.27% in the BC600 treatments (P<0.05). During the incubation, the addition of BC significantly increased the contents of Hu (P<0.05), and the treatments amended with BC500 were the highest. The addition of BC decreased the relative contents of HA and FA, while increased the relative content of Hu, indicating that the proportion of relatively stable organic carbon in the soil was increased. The ratio of HA/FA (H/F) varied between 0.88 and 2.52 and increased with decreasing pyrolysis temperature and increasing BC application. A significantly lower color tonal coefficient (ΔlgK) and E₄/E6 values in treatments amended with BC produced at temperatures above 400℃, indicating that higher temperatures derived BC complicated the structure of soil humic substance, while an opposite rend was observed in treatments amended with BC produced at temperatures under 400℃. Considering the improvement of the stability of organic carbon, when the BC products were applied to the Loutu soil, 500℃ was the optimal temperature for preparing apple-derived BC not only because it could significantly increase the content of inert soil organic carbon, but also improve the quality of the soil as a result of enhancing the degree of soil humification.