Rice straw is a kind of low-cost biosorbent. Through mechanical crushing, pyrolysis, incineration, and citric acid (CA) modification, it could be converted to rice straw powder (Sp), biochar (Sb), ash (Sa), and modified rice straw (Ms) accordingly. Using rice straw as an adsorbent, the influence of pH value (2, 4, and 9), initial Cd(II) concentration (0, 200, and 800 mg/L), and ionic strength (0, 0.2, to 0.6 mg/L) on the adsorption capacity for Cd(II) were examined with three replicates, and the relevant mechanisms were explored using Fourier transform infrared (FTIR) technology. Results showed that the modifications could improve the adsorption capacity of Cd(II) by changing their chemical structures. The products (Sb and Sa) of the pyrolysis and incineration of rice straw contained fewer hydroxyl and alkyl groups, but more Si–O groups. Citric acid modification removed a portion of silica in rice straw, increased its carboxylic content, and made more –OH groups exposed. Compared with Sp, Sb, Sa, and Ms were more likely to act as π donors in the Cd(II) sorption process and exhibited more carboxyl binding. The bands of C = C, –O–CH₃, and the O–H, carboxyl, Si–O–Si or Si–O groups were involved in the Cd(II) sorption process. The adsorption amount of Cd(II) by the four adsorbents increased with the increase in the pH value of the solution and the initial Cd(II) concentration. Affected by pH in a solution, ion exchange, surface complexation, and precipitation were the major adsorption mechanisms. Further, under the influence of the initial Cd(II) concentration, electrostatic attraction played a leading role. With no interference by ionic strength, all the adsorbents had the greatest adsorption amount of Cd(II), and the intensity of O–H vibration was also the weakest; ion exchange was the most important mechanism in this process. Regardless of the influencing factors, Sa, with the greatest specific surface area, had an absolute advantage in the adsorption capacity of Cd(II) over Sp, Sb, and Ms.

The contribution of microbial residues formed on lignin and cellulose to the formation of humus (HS) was investigated. The microbial residues formed by Aspergillus niger (A. niger) in the cultures of cellulose and lignin in a fluid medium were structurally characterized by elemental analysis, differential thermal analysis (DTA), FTIR spectroscopy and CP/MAS ¹³C NMR spectroscopy. Compared to cellulose itself, the microbial residue from cellulose contains more aromatic compounds and N-containing compounds and fewer carbohydrates and carboxylic compounds. A. niger improved the thermal stability and aromaticity of the cellulose. However, compared with that on lignin, more N-containing compounds, carbohydrates and carboxylic acid derivatives and less aromatic material were found in the microbial residue from lignin. Regardless of whether the carbon source was cellulose or lignin, A. niger utilized the N in the fluid medium to synthesize its own cells, and eventually, they could transfer the N into the microbial residue; in addition, the O-alkyl species dominated over the alkyl and aromatic compounds in the microbial residue. Although the molecular structures of the components of the microbial residue from lignin tended to be simpler, they were more alkylated, more hydrophobic and less aliphatic than those from cellulose. During culture with A. niger, the cellulose underwent degradation and then a polymerization, which led to an increased degree of condensation but a lower degree of oxidation, providing essential precursor substances for HSs formation. However, lignin underwent oxidative degradation. The microbial residue from lignin had a lower degree of condensation and a higher degree of oxidation.

The Maillard reaction is a type of nonenzymatic browning process and is an important pathway for the formation of humic-like substances (HLSs). Glucose is one of the three crucial precursors for the Maillard reaction, and a change in glucose concentration can inevitably affect the humification pathway, thereby regulating the composition and quality of HLSs. To verify the scientific hypothesis, the method of liquid shake-flask culture was adopted. Both catechol and glycine with fixed concentrations were added to a phosphate buffer including δ-MnO₂, and only the concentration of glucose was adjusted in the sterile culture system. The obtained supernatant fluid and dark-brown residue were collected dynamically through the centrifugation method. The E₄/E₆ ratio and total organic C (TOC) of the supernatant fluid, the humus composition, and FTIR spectra for the dark-brown residue, and the elemental composition of humic-like acid (HLA) extracted from the dark-brown residue were analyzed to reveal the effect of varying glucose concentrations on the abiotic humification pathways for the Maillard reaction and the characteristics of relevant products under abiotic processes. The results reveal that (1) the exogenous addition of glucose at different concentrations simplifies the molecular structure in the supernatant fluid, and the TOC content is decreased to varying degrees, among which the addition of 0.24 mol/L glucose leads to the formation of simpler organic molecules in the supernatant compared to that for the other treatments, and the addition of 0.03 mol/L glucose shows the largest decrease in TOC content; (2) Under the coexistence of glycine and catechol, C_HLA treated with the addition of glucose at different concentrations shows an upward trend in the course of the culture, which is significantly higher than that obtained for the CK control. The addition of 0.12 mol/L glucose results in the largest increase in C_HLA. During the culture period, the structure of HLA molecules from each treatment first become complex and then gradually become simpler. Finally, the molecular structure of HLA treated with different concentrations of glucose becomes more complex, but the structure of HLA molecules from the CK control tends to be simplified. The addition of glucose can improve the condensation degree of HLA molecules, among which the addition of 0.12 mol/L glucose shows the most significant effect. With increasing exogenous glucose concentration, the number of N-containing compounds in the HLA molecules further decreases, while the number of O-containing functional groups increases. (3) The greater the concentration of glucose added, the higher the proportion of aromatic C structures in the dark-brown residue. During this process, the Mn-O bond lattice vibration of the δ-MnO₂ layered structure is greatly enhanced. The organic molecules in the dark-brown residue and δ-MnO₂ are bound to each other through intermolecular hydrogen bonding. The C_HLA/C_FLA ratio for each treatment increases to varying degrees after the culture period, indicating that the addition of glucose is more conducive to the improvement of humus quality than the CK control, among which the addition of 0.12 mol/L glucose shows the best effect.

The Maillard reaction is a type of nonenzymic browning process, and it is also an abiotic humification process of sugars, amino acids and phenols catalyzed by δ-MnO₂. It is considered to be one of the possible pathways for the formation of humic-like substances (HLS). The change in the ratio of the Maillard precursors inevitably affects the chemical characteristics of HLS, among which the effect of amino acids concentration on the humification pathway and HLS formation has not yet been reported. In view of this, the glucose, glycine and catechol were chosen as tested objects for the present study, and the method of liquid shake-flask culture was adopted. Both catechol and glucose with fixed concentrations were added into a phosphate buffer solution (pH 8.0) containing δ-MnO₂, and only the glycine concentration was adjusted in the sterile culture system. The supernatant solution and dark-brown residue were collected dynamically within 360 h through the centrifugation method. The E₄/E₆ ratio and total organic C (TOC) of the supernatant solution, the C content of the humic-like acid (C_HLA), C_HLA/C_FLA (C content of fulvic-like acid) ratio and FTIR spectra of the dark-brown residue and the E₄/E₆ ratio and atomic ratio of humic-like acid (HLA) extracted from the dark-brown residue were systematically analyzed to reveal the effect of different glycine concentrations on the abiotic humification pathways and the characteristics of related products from the Maillard reaction under abiotic processes. The results showed that (1) Under the influence of the addition of different glycine concentrations, the structure of organic molecules in the supernatant after culture tended to be simplified, and the addition of three lower concentrations (0, 0.03 and 0.06 mol/L) of glycine made the E₄/E₆ ratios increase by 100.4%, 57.7% and 33.0%, respectively, and obtained a simpler structure of organic molecules in the supernatant than that of 0.12 and 0.24 mol/L glycine, which made the E₄/E₆ ratios increase by 5.6% and 18.0%, merely. After culture, the TOC content in the supernatant solution of each treatment decreased to varying degrees, and the addition of Maillard precursors effectively inhibited the loss of TOC in the supernatant solution, especially the addition of glycine at a concentration of 0.06 mol/L, which only reduced the TOC content by 0.1%. (2) The greater the concentration of glycine added, the higher proportion of aromatic C structure existed in the dark-brown residue. O-containing functional groups from the dark-brown residue and δ-MnO₂ were bound to each other through hydrogen bonding, and (3) During the culture process, the C_HLA treated with the addition of Maillard precursors was significantly higher than that of the CK control. Compared with the result at 0 h, the addition of higher concentrations of glycine (0.12 and 0.24 mol/L) were more conducive to the formation of HLA, making the C_HLA increase by 666.2% and 422.7%, which were much more than these results for 256.6%, 282.2% and 360.0% from three concentrations of glycine (0, 0.03 and 0.06 mol/L) at the end of culture. After culture, the structure of HLA molecules treated by the addition of Maillard precursors became more complex, and the overall performance showed that the higher the concentration of added glycine, the more complex the HLA molecules became. Under the coexistence of glucose and catechol, the addition of glycine could promote abiotic condensation and improve the condensation degree of HLA molecules, among which the addition of 0.12 mol/L glycine had the most significant effect. Compared with the CK control, the addition of Maillard precursors could achieve a higher increase in the C_HLA/C_FLA ratio, which was more beneficial to the improvement of humus quality.

The polyphenol–Maillard reaction is considered one of the important pathways in the formation of humic-like substances (HLSs). Glucose serves as a microbial energy source that drives the humification process. However, the effects of changes in glucose, particularly its concentration, on abiotic pathways remain unclear. Given that the polyphenol–Maillard reaction requires high precursor concentrations and elevated temperatures (which are not present in soil), gibbsite was used as a catalyst to overcome energetic barriers. Catechol and glycine were introduced in fixed concentrations into a phosphate-buffered solution containing gibbsite using the liquid shake-flask incubation method, while the concentration of glucose was controlled in a sterile incubation system. The supernatant fluid and HLS components were dynamically extracted over a period of 360 h for analysis, thus revealing the influence of different glucose concentrations on abiotic humification pathways. The results showed the following: (1) The addition of glucose led to a higher degree of aromatic condensation in the supernatant fluid. In contrast, the supernatant fluid without glucose (Glu0) and the control group without any Maillard precursor (CK control group) exhibited lower degrees of aromatic condensation. Although the total organic C (TOC) content in the supernatant fluid decreased in all treatments during the incubation period, the addition of Maillard precursors effectively mitigated the decreasing trend of TOC content. (2) While the C content of humic-like acid (C_HLA) and the C_HLA/C_FLA ratio (the ratio of humic-like acid to fulvic-like acid) showed varying increases after incubation, the addition of Maillard precursors resulted in a more noticeable increase in C_HLA content and the C_HLA/C_FLA ratio compared to the CK control group. This indicated that more FLA was converted into HLA, which exhibited a higher degree of condensation and humification, thus improving the quality of HLS. The addition of glycine and catechol without glucose or with a glucose concentration of 0.06 mol/L was particularly beneficial in enhancing the degree of HLA humification. Furthermore, the presence of glycine and catechol, as well as higher concentrations of glucose, promoted the production of N-containing compounds in HLA. (3) The presence of Maillard precursors enhanced the stretching vibration of the hydroxyl group (–OH) of HLA. After the polyphenol–Maillard reaction of glycine and catechol with glucose concentrations of 0, 0.03, 0.06, 0.12, or 0.24 mol/L, the aromatic C structure in HLA products increased, while the carboxyl group decreased. The presence of Maillard precursors facilitated the accumulation of polysaccharides in HLA with higher glucose concentrations, ultimately promoting the formation of Al–O bonds. However, the quantities of phenolic groups and phenols in HLA decreased to varying extents.

The objective of this study was to examine the structure changes in humic acids (HAs) in Chernozem after the application of different types of organic materials (OMs) under an indoor simulation condition for plastic mulched drip irrigation, measured with Fourier transform infrared (FTIR) spectroscopy. The biotechnological extract of fulvic acid (BFA), decomposed sheep manure (M), corn straw pellets (Ps) and corn straw powder (Pr) were used as the four OMs for testing, and they were applied to Chernozem at the same amount of actual material; three nitrogen (N) levels (no N, low N, and high N supply) were applied to each type of (OMs), separately. The total culture period was set to 90 days and soil sampling was taken at 0, 30, 60 and 90 days, respectively. The results showed that different types of OMs exerted different effects on Chernozem based on the FTIR spectra of HAs. The application of M combined with high N supply was the best way to fertilize Chernozem, under which the H-bonded OH groups and aromatic compounds were enhanced, resulting in increased soil carbon (C) sequestration; while the carbohydrates in HAs was easily consumed as microbial energy substance. The HAs from the Chernozem amended with BFA became more aliphatic, simpler and younger. High N supply was beneficial for increasing the complexity of HAs from Chernozem amended with Ps, but was not conducive to soil cation retention. Within a short time of incubation, the application of Pr combined with high N was detrimental to the C sequestration in Chernozem, and inhibited the consumption of carbohydrates by microorganisms.