The sleeping giant: A polymer View on humic matter in synthesis and applications (2023)

This study set out to explore the impact of lignite on preserving organic matter and promoting the formation of humic acid (HA) during chicken manure composting. Composting test was carried out for control (CK), 5% lignite addition treatment (L1), 10% addition treatment (L2) and 15% addition treatment (L3). The results demonstrated that lignite addition effectively reduced the loss of organic matter. The HA content of all lignite-added groups was higher than that of CK, and the highest was 45.44%. L1 and L2 increased the richness of bacterial community. Network analysis showed higher diversity of HA-associated bacteria in L2 and L3 treatments. Structural equation models revealed that reducing sugar and amino acid contributed to the formation of HA during CK and L1 composting, while polyphenol contributed more to the HA formation during L2 and L3 composting. Furthermore, lignite addition also could promote the direct effect of microorganisms on HA formation. Therefore, the addition of lignite had practical significance to enhance compost quality.

Gas emitted from landfills contains a large quantity of volatile organic compounds (VOCs) and semi-volatile organic compounds (SVOCs), some of which are carcinogenic, teratogenic, and mutagenic, thereby posing a serious threat to the health of landfill workers and nearby residents. However, the global hazards of VOCs and SVOCs in landfill gas to human health remain unclear. To quantify the global risk distributions of these pollutants, we collected the composition and concentration data of VOCs and SVOCs from 72 landfills in 20 countries from the core database of Web of Science and assessed their human health risks as well as analyzed their influencing factors. Organic compounds in landfill gas were found to primarily result from the biodegradation of natural organic waste or the emissions and volatilization of chemical products, with the concentration range of 1×10^-1–1×10⁶ μg/m3. The respiratory system, in particular, lung was the major target organ of VOCs and SVOCs, with additional adverse health impacts ranging from headache and allergies to lung cancer. Aromatic and halogenated compounds were the primary sources of health risk, while ethyl acetate and acetone from the biodegradation of natural organic waste also exceeded the acceptable levels for human health. Overall, VOCs and SVOCs affected residents within 1,000m of landfills. Air temperature, relative humidity, air pressure, wind direction, and wind speed were the major factors that influenced the health risks of VOCs and SVOCs. Currently, landfill risk assessments of VOCs and SVOCs are primarily based on respiratory inhalation, with health risks due to other exposure routes remaining poorly elucidated. In addition, potential health risks due to the transport and transformation of landfill gas emitted into the atmosphere should be further studied.

The widely existed humic acid (HA) with abundant redox-active groups has been considered to play an important role in biogeochemistry in sediments and soils. Recent studies reported that HA showed great performance in terms of electron transfer capacity (up to HA_EDC = 94 mmol e⁻/mol C, HA_EAC = 42 mmol e⁻/mol C). Since HA is widely available, inexpensive and environmentally friendly, the electrochemistry of HA has been explored to apply in many fields, such as environmental remediation, detection sensor and energy storage. Whereas, these prospective applications of HA and their electrochemical principles were lack of a comprehensive summary. In this review, the electrochemical properties and the prospective electrochemical applications of HA were summarized. Simultaneously, the existing problems like shortages of traditional electrochemical characterization of HA, and future research directions about HA electrochemistry were prospected. This review provides a deeper understanding of HA electrochemistry, and also inspires ideas for environmental remediation, detection sensor and energy storage by exploring the potential application values of HA.

Humus formation of composting is crucial for improving organic fertilizer quality. Biochar has been found to be a low-cost, eco-friendly and effective promoter for humus formation, while the mechanisms underlying the biochar-driven humus formation during composting remain unknown. The present study was aimed to explore the effect of biochar addition on humus formation by integrated analyses of fungal communities, functional genes and metabolic features during composting using the advanced techniques of fluorescence spectrometry, high throughput sequencing, metagenomics, and metabolomics. The efficiencies of fulvic acid and humic acid formation were increased by 17.4 % and 39.4 %, respectively, under biochar addition relative to the control at the mature phase. Biochar addition significantly increased temperature, lignocellulose degradation and the activities of urease, peroxidase, manganese peroxidase and lignin peroxidase, and altered the fungal community composition and diversity with the latter significantly correlated with humic acid content. The altered fungal community subsequently resulted in the stimulation of fungal function of Wood Saprotroph and expression of CAZyme genes at the thermophilic and mature phases. Ultimately, biochar addition significantly enhanced the Xenobiotics metabolism (Bisphenol metabolism and Quinone biosynthesis) and Amino acid metabolism (Tryptophan metabolism and Tyrosine metabolism), which were responsible for humus formation via producing metabolic compounds such as quinolines, phenolics and aromatic amino acids at the mature phase. Our findings suggest that biochar addition can effectively stimulate humus formation by selectively regulating fungal communities and metabolic features during composting, so that benefits the improvement of soil fertility and health upon the organic fertilizer application.

The humification behavior of lignocellulosic components (i.e., cellulose, hemicellulose and lignin) under acid-alkali two-step hydrothermal treatment was investigated with particular interest of revealing the contribution of single components as well as their interaction to hydrothermal humic acid (HHA). Lignin with its rich aromatic structures, had the highest HHA contribution (51.8 $\pm$ 1.0wt%), while cellulose and hemicellulose only contributed 8.8 $\pm$ 1.6wt% and 20.1 $\pm$ 4.4wt% HHA yields, respectively. The conversion pathway of these components toward humification indicated that the hydrochar (HC) rather than the hydrothermal solution (HS) had higher humification capability under alkaline hydrothermal conditions because the hydrochar-derived HHA yields of 40.8 $\pm$ 2.0wt% (cellulose), 66.2 $\pm$ 6.4wt% (hemicellulose) and 55.4 $\pm$ 6.1wt% (lignin) were achieved. In addition, co-hydrothermal treatment of cellulose, hemicellulose and lignin under this process improved the HHA production ascribing to the increased HC yield (23.1–53.1%). The chemical characteristics of HHA products from lignocellulosic components (cellulose, hemicellulose, lignin) and their combinations were similar to the acknowledged natural humic acids. The hydrothermal humification of biowaste mainly passes through a top-down pathway (i.e., HHA forms from the oxidation of large fragments of hydrochar) as well as two bottom-up pathways (i.e., the soluble chemicals conduct the polymerization reaction for HHA production and react with the fragments from HC to synthesize the HHA). This study filled the lack of knowledge in the field of biowaste humification.

Humic acids (HAs) coupled with humic-reducing microorganisms (HRMs) can facilitate contaminants reduction. Molecular-weight (MW) of HA governs the chemical and HRMs behavior. However, MW of HAs with chemical characteristics linking to HRMs in different wastes composting have never been investigated. Results from the HPSEC-UV analysis showed that composting significantly increased weight-average molecular weight (M_w) of HA with a broad range from 675Da to 27983Da, and governing heterogeneous chemical characteristics. In proteinaceous composts, MW<4000Da of HAs were greatly related to alkyl and carbonyl, while MW>20000Da of HAs were presented by oxygen–nitrogenous functional groups, methyl, and alkyl groups. For cellulosic composts, MW<1500Da and 4000–10000Da of HAs were characterized by aliphatic ethers and aromatic groups. MW>20000Da of HAs were constructed by phenols, methoxy and nitrogen functional groups. In lignocellulosic composts, MW>20000Da of HAs were only characterized by aromatic groups. Furthermore, seven groups of HRMs adapted to the heterogeneous chemical characteristics within HAs ranked by MW were recognized. Consequently, the possible routes that composting properties response to the connections of HRMs-chemical structures-MW of HAs in proteinaceous, cellulosic and lignocellulosic composts were constructed, respectively. Our results can help to develop the fine classification-oriented approach for recycling utilization of organic wastes.

Pedosphere, Volume 30, Issue 2, 2020, pp. 159-167

Humic substances (HS), which are defined as a series of highly acidic, relatively high-molecular-weight, and yellow to black colored substances formed during the decay and transformation of plant and microbial remains, ubiquitously occur in nature. Humic substances represent the largest stable organic carbon pool in terrestrial environments and are the central characteristic of the soil. However, the validity of the HS concept and the justification of their extraction procedure have been recently debated. Here, we argue that the traditional humic paradigm is still relevant. Humic substances are distinctive and complex because the extracted HS formed during the humification are chemically distinct from their precursors and are heterogeneous among soils. By reviewing the concept, formation pathways, and stabilization of HS, we propose that the key question facing soil scientists is whether HS are soil microbial residues or unique synthesized compounds. Without revealing the distinctiveness of HS, it is impossible to address this question, as the structure, composition, and reactivity of HS are still poorly known owing to the heterogeneity and geographical variability of HS and the limits of the currently available analytical techniques. In our view, the distinctiveness of HS is fundamental to the soil, and thus further studies should be focused on revealing the distinctiveness of HS and explaining why HS hold this distinctiveness.

Science of The Total Environment, Volume 722, 2020, Article 137815

Humic acids (HA) play an important role in the distribution, toxicity, and bioavailability of metals in the environment. Humic-like acids (HLA) that simulate geochemical processes can be prepared by NaOH aqueous extraction from hydrochars produced by hydrothermal carbonization (HTC). HLA can exhibit properties such as those found in HA from soils, which are known for their ability to interact with inorganic and organic compounds. The molecular characteristics of HLA and HA help to explain the relationship between their molecular features and their interaction with metallic species. The aim of this study is to assess the molecular features of HA extracted from Terra Mulata (TM) and HLA from hydrochars as well as their interaction with metals by using Cu(II) ions as a model. The results from ¹³C NMR, elemental analysis, FTIR, and UV–Vis showed that HA are composed mostly of aromatic structures and oxygenated functional groups, whereas HLA showed a mutual contribution of aromatic and aliphatic structures as main constituents. The interactions of HA and HLA with Cu(II) ions were evaluated through fluorescence quenching, in which the density of complexing sites per gram of carbon for interaction was higher for HLA than for HA. Furthermore, the HLA showed similar values for stability constants, and higher than those found for other types of HA in the literature. In addition, the average lifetime in both humic extracts appeared to be independent of the copper addition, indicating that the main mechanism of interaction was static quenching with a non-fluorescent ground-state complex formation. Therefore, the HLA showed the ability to interact with Cu(II) ions, which suggests that their application can provide a new approach for remediation of contaminated areas.

Environmental Technology & Innovation, Volume 23, 2021, Article 101688

Recent studies have shown a new way of producing humic-like substances (HLS) from hydrochar prepared by hydrothermal carbonization of biomass. However, studies of HLS extracted from process water (PW) produced in hydrothermal carbonization process to the best of our knowledge are non-existent. In this work, hydrothermal carbonization of sugarcane bagasse and vinasse was performed under two concentrations of H₂SO₄ (1 and 4% w:w) followed of extraction of HLS from PW (HLS 1% and HLS 4%). The interaction of these compounds with Cu(II) ions revelated a complexation with high conditional stability constant (K ranged from 5.0 to 5.4) and high complexing capacity (CC${}_{\mathrm{c}}$ ranged from 3.1 to 3.$9\mathrm{\mu }\mathrm{mol}$ mg⁻¹). Elemental analysis and ash content showed that HLS 1% and HLS 4% both have high content of carbon, oxygen and inorganic compounds. In addition, the H/C molar ratio suggests that both HLS have aliphatic nature and O/C molar ratio acidic functional groups. FTIR analysis showed great variety of functional groups and also the presence of inorganic compounds. Excitation–Emission​ fluorescence matrices applied with complexation studies showed that interaction of HLS and Cu(II) occurs with two humic-like components. Data on Time Resolved Fluorescence Spectroscopy (TRFS) suggests that the mechanism of interaction between both HLS and Cu(II) occur by complexation. Furthermore, the infrared bidimensional analysis indicates that the binding sites for HLS 1% were in affinity order: COOCO> CH aliphatic, while for HLS 4% was: CH aliphatic> CO.

Science of The Total Environment, Volume 708, 2020, Article 135000

Inspired by the presence of anthropogenic organic matter in highly fertile Amazonian Dark Earth (ADE), which is attributed to the transformation of organic matter over thousands of years, we explored hydrothermal carbonization as an alternative for humic-like substances (HLS) production. Hydrothermal carbonization of sugarcane industry byproducts (bagasse and vinasse) in the presence and absence of H₃PO₄ afforded HLS, which were isolated and compared with humic substances (HS) isolated from ADE in terms of molecular composition and maize seed germination activity. HLS isolated from sugarcane bagasse hydrochar produced in the presence or absence of H₃PO₄ comprised both hydrophobic and hydrophilic moieties, differing from other HLS mainly in terms of phenolic content, while HLS isolated from vinasse hydrochar featured hydrophobic structures mainly comprising aliphatic moieties. Compared to that of HLS, the structure of soil-derived HS reflected an increased contribution of fresh organic matter input and, hence, featured a higher content of O–alkyl moieties. HLS derived from lignocellulosic biomass were rich in phenolics and promoted maize seed germination more effectively than HLS comprising alkyl moieties. Thus, HLS isolated from bagasse hydrochar had the highest bioactivity, as the presence of amphiphilic moieties therein seemed to facilitate the release of bioactive molecules from supramolecular structures and stimulate seed germination. Based on the above results, the hydrothermal carbonization of lignocellulosic biomass was concluded to be a viable method of producing amphiphilic HLS for use as plant growth promoters.

Journal of Hazardous Materials, Volume 435, 2022, Article 129070

Humic substances (HS) as representative natural organic matters and the most common organic compounds existing in the environment, has been applied to the treatment and remediation of environmental pollution. This review systematically introduces and summarizes the redox activity of HS for the remediation of environmental pollutants. For inorganic pollutants (such as silver, chromium, mercury, and arsenic), the redox reaction of HS can reduce their toxicity and mobilization, thereby reducing the harm of these pollutants to the environment. The concentration and chemical composition of HS, environmental pH, ionic strength, and competing components affect the degree and rate of redox reactions between inorganic pollutants and HS significantly. With regards to organic pollutants, HS has photocatalytic activity and produces a large number of reactive oxygen species (ROS) under the light which reacts with organic pollutants to accelerate the degradation of organic pollutants. Under the affection of HS, the redox of Fe(III) and Fe(II) can enhance the efficiency of Fenton-like reaction to degrade organic pollutants. Finally, the research direction of HS redox remediation of environmental pollution is prospected.

Chemical Engineering Journal, Volume 394, 2020, Article 124832

This publication presents a simple and low-cost hydrothermal humification (HTH) treatment of sewage sludge (SS) together with alkali ash (AA) and biomass for simultaneous implementation of heavy metal removal, nutrient recovery (P) and ash refining. The H/C and O/C atomic ratio plots obtained from elemental analysis demonstrate that dehydration and decarboxylation under hydrothermal conditions are elemental reactions leading to sludge/biomass decomposition and artificial humic matter formation. Introduction of plant biomass into sludge-derived samples and adjustment of KOH or alkali ash (AA) mass effectively improve the recovery of P element, realizing high contents of dissolved phosphorus (DP) (from 7045 to 10075mg/L) at appropriate pH values (6.5 to 7.7). ICP-AES results indicate the drop of Cr and Cd content below detection limit together with a sharp decrease of the elements Cu (from 0.07 to 0.46mg/g), Zn (from 0.15 to 0.98mg/g) and Pb (from 0.067 to 0.142mg/g) after HTH treatment in sludge-derived liquid products and the heavy metal elements enriched in sludge-derived solids can be recovered into industrial salts by subsequent treatment. Pot planting experiments are conducted to investigate the P-availability in both sludge-derived liquids and solids (after treatment of heavy metal recovery) for promotion of plant growth. A higher proportion of shoot-to-root weight (62.1% versus 46.2%) and preserved moisture contents (84.7% versus 83.7%) when compared to the control groups demonstrate the effect of the presence of more nutrients after addition of sludge-derived liquid products. This work could provide a smart, energy utilization and sustainable fertilization route for planting growth.