Effects of pyrolysis temperature on proton and cadmium binding properties onto biochar-derived dissolved organic matter: Roles of fluorophore and chromophore

Adsorptive immobilization of cadmium and lead using unmodified and modified biochar: A review of the advances, synthesis, efficiency and mechanisms

Biochar is an environmentally friendly adsorbent material with excellent adsorption performance due to its extensive pore structure, large specific surface area, and numerous surface functional groups. It is commonly used to treat inorganic and organic pollutants. In recent years, with increasing focus on controlling soil pollution caused by heavy metals such as cadmium (Cd) and lead (Pb), the potential application of biochar has attracted much attention. This review used Citespace to quantitatively analyze the literature on the application of biochar from 2021 to 2024. It then explains the preparation techniques of unmodified and modified biochar and presents the physical and chemical properties and adsorption capacity of different biochar types for Cd and Pb. It also illustrates and compares the preparation process, modification methods, and adsorption mechanisms of biochar. Additionally, it evaluates the impacts of biochar application on heavy metal removal from rice, wheat, and corn, as well as their yields. This article contributes to the identification of the most effective materials and methods for biochar synthesis. It provides suggestions for remediation of soil heavy metal pollution and yield increase.

Insights into the enhanced adsorption of glyphosate by dissolved organic matter in farmland Mollisol: effects and mechanisms of action

Dissolved organic matter (DOM) is easy to combine with residual pesticides and affect their morphology and environmental behavior. Given that the binding mechanism between DOM and the typical herbicide glyphosate in soil is not yet clear, this study used adsorption experiments, multispectral techniques, density functional theory, and pot experiments to reveal the interaction mechanism between DOM and glyphosate on Mollisol in farmland and their impact on the environment. The results show that the adsorption of glyphosate by Mollisol is a multilayer heterogeneous chemical adsorption process. After adding DOM, due to the early formation of DOM and glyphosate complex, the adsorption process gradually became dominated by single-layer chemical adsorption, and the adsorption capacity increased by 1.06 times. Glyphosate can quench the endogenous fluorescence of humic substances through a static quenching process dominated by hydrogen bonds and van der Waals forces, and instead enhance the fluorescence intensity of protein substances by affecting the molecular environment of protein molecules. The binding of glyphosate to protein is earlier, of which affinity stronger than that of humic acid. In this process, two main functional groups (C-O in aromatic groups and C-O in alcohols, ethers and esters) exist at the binding sites of glyphosate and DOM. Moreover, the complexation of DOM and glyphosate can effectively alleviate the negative impact of glyphosate on the soil. This study has certain theoretical guidance significance for understanding the environmental behavior of glyphosate and improving the sustainable utilization of Mollisol.

Molecular characteristics of organic matter derived from sulfonated biochar

Sulfonated biochar (SBC), as a functional carbon-based material, has attracted widespread attention due to its excellent adsorption properties. The composition of biochar-derived organic matter (B-DOM) is a key factor influencing the migration and transformation of soil elements and pollutants. However, molecular characteristics of sulfonated biochar-derived organic matter (SBC-DOM) are still unclear. In this study, the molecular composition of derived organic matter (DOM) from SBC prepared via one-step carbonization-sulfonation techniques was investigated by Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) and then compared with those of DOMs from rice husk (RH), pyrochar (PYC), and hydrochar (HYC). The results show that the CHOS- and CHONS-containing formulae are predominant in SBC-DOM, accounting for 85% of the total molecular formula number, while DOMs from RH, PYC, and HYC are dominated by CHO-containing formulae. Compared to PYC-DOM and HYC-DOM, SBC-DOM has more unsaturated aliphatic compounds, which make it more labile and easily biodegraded. Additionally, SBC-DOM has higher O/C, (N + O)/C ratios and sulfur-containing compounds. These findings provide a theoretical basis for further research on the application of sulfonated biochar in soil improvement and remediation.

Effect of biochar-derived DOM on contrasting redistribution of chromate during Schwertmannite dissolution and recrystallization

Biochar-derived dissolved organic matter (BDOM), is extensively involved in the recrystallization of minerals and the speciation alteration of associated toxic metals. This study investigates how BDOM extracted from tobacco petiole (TP) or tobacco stalk (TS) biochar influences the speciation repartitioning of Cr(VI) in environments impacted by acid mine drainage (AMD), focusing on interactions with secondary minerals during Schwertmannite (Sch) dissolution and recrystallization. TP-BDOM, rich in lignin-like substances, slowed down the Cr-Sch dissolution and Cr release under acidic conditions compared to TS-BDOM. TP-BDOM's higher O/C component exerts a delayed impact on Cr-Sch stability and Cr(VI) reduction. In-situ ATR-FTIR and 2D-COS analysis showed that carboxylic and aromatic N-OH groups in BDOM could interact with Cr-Sch surfaces, affecting sulfate and Cr(VI) release. It was also observed that slight recrystallization occurred from Cr-Sch to goethite, along with increased Cr incorporation into secondary minerals within TS-BDOM. This enhances our understanding of BDOM's role in Cr(VI) speciation changes in AMD-contaminated sites.

Biochar-derived organic carbon promoting the dehydrochlorination of 1,1,2,2-tetrachloroethane and its molecular size effects: Synergies of dipole-dipole and conjugate bases

The environmental effects of biochar-derived organic carbon (BDOC) have attracted increasing attention. Nevertheless, it is unknown how BDOC might affect the natural attenuation of widely distributed chloroalkanes (e.g., 1,1,2,2-tetrachloroethane (TeCA)) in aqueous environments. We firstly observed that the kinetic constants (ke) of TeCA dehydrochlorination in the presence of BDOC samples or their different molecular size fractions (<1 kDa, 1∼10 kDa, and >10 kDa) ranged from 9.16×103 to 26.63×103 M-1h-1, which was significantly greater than the ke (3.53×103 M-1h-1) of TeCA dehydrochlorination in the aqueous solution at pH 8.0, indicating that BDOC samples and their different molecular size fractions all could promote TeCA dehydrochlorination. For a given BDOC sample, the kinetic constants (ke) of TeCA dehydrochlorination in the initial pH 9.0 solution was 2∼3 times greater than that in the initial pH 8.0 solution due to more formation of conjugate bases. Interestingly, their DOC concentration normalized kinetic constants (ke/[DOC]) were negatively correlated with SUVA254, and positively correlated with A220/A254 and the abundance of aromatic protein-like/polyphenol-like matters. A novel mechanism was proposed that the CH dipole of BDOC aliphatic structure first bound with the CCl dipole of TeCA to capture the TeCA molecule, then the conjugate bases (-NH-/-NH2 and deprotonated phenol-OH of BDOC) could attack the H atom attached to the β-C atom of bound TeCA, causing a CCl bond breaking and the trichloroethylene formation. Furthermore, a fraction of >1 kDa had significantly greater ke/[DOC] values of TeCA dehydrochlorination than the fraction of <1 kDa because >1 kDa fraction had higher aliphiticity (more dipole-dipole sites) as well as more N-containing species and aromatic protein-like/polyphenol-like matters (more conjugate bases). The results are helpful for profoundly understanding the BDOC-mediated natural attenuation and fate change of chloroalkanes in the environment.

Mechanisms of cadmium release from manganese-rich sediments driven by exogenous DOM and the role of microorganisms

Dissolved organic matter (DOM) is a crucial component of natural sediments that alters Cd sequestration. Nevertheless, how different types of DOM fuel Cd mobilization in Mn-rich sediments has not been elucidated. In the present study, four typical DOM, fluvic acid (FA), bovine serum albumin (BSA), sodium alginate (SA), and sodium dodecyl benzene sulfonate (SDBS), were used to amend Cd-contaminated sediment to study their effects on Cd/Mn biotransformation and microbial community response. The results demonstrated that different DOM drive microbial community shifts and enhance microbially mediated Mn oxide (MnO) reduction and Cd release. The amendment of terrestrial- and anthropogenic-derived DOM (FA and SDBS) mainly contributed to enriching Mn-reducing bacteria phylum Proteobacteria, and its abundance increased by 38.16-74.47 % and 56.41-73.98 %, respectively. Meanwhile, microbial-derived DOM (BSA and SA) mainly stimulated the abundances of metal(loid)-resistant bacteria phylum Firmicutes. Accompanied by microbial community structure, diversity, and co-occurrence network shifts, the DOM concentration and oxidation-reduction potential changed, resulting in enhanced Cd mobilization. Importantly, FA stimulated Cd release most remarkably, probably because of the decreased cooperative interactions between bacterial populations, stronger reduction of MnOs, and higher aromaticity and hydrophobicity of the sediment DOM after amendment. This study linked DOM types to functional microbial communities, and explored the potential roles of different DOM types in Cd biotransformation in lake sediments.

How does adsorptive fractionation of dissolved black carbon on ferrihydrite affect its copper binding behaviors? A molecular-scale investigation

Adsorptive fractionation of dissolved black carbon (DBC) on minerals is proven to alter its molecular composition, which will inevitably affect the environment fate of heavy metals. However, the effects of molecular fractionation on the interaction between DBC and heavy metals remain unclear. Herein, we observed that the selective adsorption of ferrihydrite caused molecular changes of DBC from high molecular weight/unsaturation/aromaticity to low molecular weight/saturation/aliphatics. This process accompanied by a retention of carbohydrate and a reduction of oxygen-rich functional groups (e.g., polyphenols and carboxyl) and long carbon chain in DBC. The residual DBC in aqueous phase demonstrated a weaker binding affinity to copper compared to the original DBC. This decrease in binding affinity was primarily attributed to the adsorption of polycyclic condensed aromatic compounds of 200-250 Da, oxygen-rich polycyclic condensed aromatic compounds of 250-300 Da, oxygen-rich non-polycyclic aromatic compounds of 300-450 Da, and non-polycyclic aromatic compounds of 450-700 Da in DBC by ferrihydrite. Additionally, the retention of carbohydrates and aliphatic compounds of 300-450 Da also made a significant contribution. Notably, carboxylic groups rather than phenolic groups were the dominant oxygen-containing functional groups responsible for this affinity reduction. This study has significant implications for understanding of the biogeochemical processes of DBC at soil-water interface and surface water, especially its role in the transportation of heavy metals.

Biochar improved the solubility of triclocarban in aqueous environment: Insight into the role of biochar-derived dissolved organic carbon

Biochar as an effective adsorbent can be used for the removal of triclocarban from wastewater. Biochar-derived dissolved organic carbon (BC-DOC) is an important carbonaceous component of biochar, nonetheless, its role in the interaction between biochar and triclocarban remains little known. Hence, in this study, sixteen biochars derived from pine sawdust and corn straw with different physico-chemical properties were produced in nitrogen-flow and air-limited atmospheres at 300-750 °C, and investigated the effect of BC-DOC on the interaction between biochar and triclocarban. Biochar of 600∼750 °C with low polarity, high aromaticity, and high porosity presented an adsorption effect on triclocarban owing to less BC-DOC release as well as the strong π-π, hydrophobic, and pore filling interactions between biochar and triclocarban. In contrast and intriguingly, biochar of 300∼450 °C with low aromaticity and high polarity exhibited a significant solubilization effect rather than adsorption effect on triclocarban in aqueous solution. The maximum solubilization content of triclocarban in biochar-added solution reached approximately 3 times its solubility in biochar-free solution. This is mainly because the solubilization effect of BC-DOC surpassed the adsorption effect of biochar though the BC-DOC only accounted for 0.01-1.5 % of bulk biochar mass. Furthermore, the high solubilization content of triclocarban induced by biochar was dependent on the properties of BC-DOC as well as the increasing BC-DOC content. BC-DOC with higher aromaticity, larger molecular size, higher polarity, and more humic-like matters had a greater promoting effect on the water-solubility of triclocarban. This study highlights that biochar may promote the solubility of some organic pollutants (e.g., triclocarban) in aqueous environment and enhance their potential risk.

Dissolved biochar fractions and solid biochar particles inhibit soil acidification induced by nitrification through different mechanisms

Biochar can inhibit soil acidification by decreasing the H⁺ input from nitrification and improving soil pH buffering capacity (pHBC). However, biochar is a complex material and the roles of its different components in inhibiting soil acidification induced by nitrification remain unclear. To address this knowledge gap, dissolved biochar fractions (DBC) and solid biochar particles (SBC) were separated and mixed thoroughly with an amended Ultisol. Following a urea addition, the soils were subjected to an incubation study. The results showed that both the DBC and SBC inhibited soil acidification by nitrification. The DBC inhibited soil acidification by decreasing the H⁺ input from nitrification, while SBC enhanced the soil pHBC. The DBC from peanut straw biochar (PBC) and rice straw biochar (RBC) decreased the H⁺ release by 16 % and 18 % at the end of incubation. The decrease in H⁺ release was attributed to the inhibition of soil nitrification and net mineralization caused by the toxicity of the phenols in DBC to soil bacteria. The abundance of ammonia-oxidizing bacteria (AOB) and total bacteria decreased by >60 % in the treatments with DBC. The opposite effects were observed in the treatments with SBC. Soil pHBC increased by 7 % and 19 % after the application of solid RBC and PBC particles, respectively. The abundance of carboxyl on the surface of SBC was mainly responsible for the increase in soil pHBC. Generally, the mixed application of DBC and SBC was more effective at inhibiting soil acidification than their individual applications. The negative impacts of dissolved biochar components on soil microorganisms need to be closely monitored.

Manganese oxides mediated dissolve organic matter compositional changes in lake sediment and cadmium binding characteristics

In sediment environments, manganese (Mn) minerals have high dissolved organic matter (DOM) affinities, and could regulate the changes of DOM constituents and reactivity by fractionation. However, the effects of DOM fractionation by Mn minerals on the contaminant behaviors remain unclear. Herein, the transformations of mineral phases, DOM properties, and Cd(II) binding characteristics to sediment DOM before and after adsorption by four Mn oxides (δ-MnO₂, β-MnO₂, γ-MnOOH, and Mn₃O₄) were investigated using multi-spectroscopic tools. Results showed a subtle structural variation of Mn oxides in response to DOM reduction, and no phase transformations were observed. Two-dimensional correlation spectroscopy based on synchronous fluorescence spectra and Fourier transform infrared spectroscopy indicated that tryptophan-like substances and the amide (II) N-H groups could preferentially interact with Cd(II) for the original DOM. Nevertheless, preferential bonding of Cd(II) to tyrosine-like substances and phenolic OH groups was exhibited after fractionations by Mn oxides. Furthermore, the binding stability and capacity of each DOM fraction to Cd(II) were decreased after fractionation based on the modified Stern-Volmer equation. These differences may be attributed to DOM molecules with high aromaticity, hydrophobicity, molecular weight, and amounts of O/N-containing group were preferentially removed by Mn oxides. Overall, the environmental hazard of Cd will be more severe after DOM fractionation on Mn minerals. This study facilitates a better understanding of the Cd geochemical cycle in lake sediments under the DOM-mineral interactions, and recommends being careful with outbreaks of aquatic Cd pollution when sediments are rich in dissolved protein-like components and Mn minerals.

Pyrolysis Atmospheres and Temperatures Co-Mediated Spectral Variations of Biochar-Derived Dissolved Organic Carbon: Quantitative Prediction and Self-Organizing Maps Analysis

Biochar-derived dissolved organic carbon (BDOC), as a highly activated carbonaceous fraction of biochar, significantly affects the environmental effect of biochar. This study systematically investigated the differences in the properties of BDOC produced at 300-750 °C in three atmosphere types (including N₂ and CO₂ flows and air limitation) as well as their quantitative relationship with biochar properties. The results showed that BDOC in biochar pyrolyzed in air limitation (0.19-2.88 mg/g) was more than that pyrolyzed in N₂ (0.06-1.63 mg/g) and CO₂ flows (0.07-1.74 mg/g) at 450-750 °C. The aliphaticity, humification, molecular weight, and polarity of BDOC strongly depended on the atmosphere types as well as the pyrolysis temperatures. BDOC produced in air limitation contained more humic-like substances (0.65-0.89) and less fulvic-like substances (0.11-0.35) than that produced in N₂ and CO₂ flows. The multiple linear regression of the exponential form of biochar properties (H and O contents, H/C and (O+N)/C) could be used to quantitatively predict the bulk content and organic component contents of BDOC. Additionally, self-organizing maps could effectively visualize the categories of fluorescence intensity and components of BDOC from different pyrolysis atmospheres and temperatures. This study highlights that pyrolysis atmosphere types are a crucial factor controlling the BDOC properties, and some characteristics of BDOC can be quantitatively evaluated based on the properties of biochar.

Adsorption Characteristics and Mechanism of Methylene Blue in Water by NaOH-Modified Areca Residue Biochar

To solve the water pollution problem caused by methylene blue (MB), areca residue biochar (ARB) was prepared by pyrolysis at 600 °C, and modified areca residue biochar (M-ARB) was obtained by modifying ARB with 1.5 mol/L NaOH, and they were utilized to adsorb and eliminate MB from water. The structural characteristics of ARB and M-ARB were examined, and the main influencing factors and adsorption mechanism of MB adsorption process were investigated. The outcomes demonstrated an increase in M-ARB’s specific surface area and total pore volume of 66.67% and 79.61%, respectively, compared with ARB, and the pore structure was more abundant, and the content of oxygen element was also significantly increased. When the reaction temperature was 25 °C, starting pH of the mixture was 10, the initial MB concentration was 50 mg/L, the ARB and M-ARB dosages were 0.07 g/L and 0.04 g/L, respectively, the adsorption equilibrium was achieved at about 210 min, and the elimination rate for MB exceeded 94%. The adsorption behaviors of ARB and M-ARB on MB were more in line with the Langmuir isotherm model (R2 > 0.95) and the quasi-secondary kinetic model (R2 > 0.97), which was characterized by single-molecule layer chemisorption. The highest amount of MB that may theoretically be absorbed by M-ARB in water ranging from 136.81 to 152.72 mg/g was 74.99–76.59% higher than that of ARB. The adsorption process was a spontaneous heat absorption reaction driven by entropy increase, and the adsorption mechanism mainly involved electrostatic gravitational force, pore filling, hydrogen bonding, and π–π bonding, which was a complex process containing multiple mechanisms of action. NaOH modification can make the ARB have more perfect surface properties and more functional group structures that can participate in the adsorption reaction, which can be used as an advantageous adsorption material for MB removal in water.

Insight into complexation of Cd(II) and Cu(II) to fulvic acid based on feature recognition of PARAFAC combined with 2DCOS

Fulvic acid which could govern the environmental geochemistry behavior of heavy metals is considered as the eco-friendly substances for controlling heavy metal pollutants in environment. Knowledge on the individual fulvic acid ligand is crucial to characterize the effect of fulvic acid on the migration and toxicity of metal pollutants. Herein, fulvic acid substances were analyzed by fluorescence quenching associated with parallel factor analysis (PARAFAC). Three components were identified based on PARAFAC. Furthermore, two-dimensional correlation spectroscopy (2DCOS) associated with complexation model were used to elucidate the Cd(II)- and Cu(II)-binding characteristics of the individual fulvic acid ligand. The Cd(II)- and Cu(II)-binding capability and speed of different fulvic acid ligands were revealed and theoretical guidance and technical support were provided for the practical application. The Cd(II) contaminated soil could be amended with high fulvic acid ligands A1 and Y2 containing composting products and the Cu(II) contaminated soil could be amended with high fulvic acid ligands Y1, T1 and A1 containing composting products to control the pollution and improve the soil condition. Based on these excellent results, the different fulvic acid ligands-contaminants-binding properties was characterized for the theoretical supporting of environmental pollution control.