Electrochemical properties of humic acid and its novel applications: A tip of the iceberg

Kinetic considerations on the antioxidant properties of humic substances: An electrochemical approach

Humic substances (HS) play a key role in a variety of environmental redox processes including the reduction of organic and inorganic pollutants. However, their complex and heterogeneous composition makes the characterization of their redox activity challenging. In this work, electrochemical methods (cyclic and square-wave voltammetry) were used to study the kinetics of the redox reaction between HS and the mediator 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) under pseudo-first order conditions. ABTS was used to mediate the electro transfer from redox active moieties (phenolic and quinonoid) to the glassy carbon working electrode (WE), since the direct analysis of HS is harmed by slow electron transfer kinetics at the WE surface. This study introduces a practical method for determining kinetic constants for the reaction between ABTS and HS by leveraging the catalytic behaviour of the ABTS oxidative peak in the presence of HS, since a clearly defined peak can be measured in the anodic scan at 0.57 V. Both ABTS and HS samples were individually electrochemically characterized over a range of pH and concentration. It was observed that the redox state of HS from different origins could be quantified by using cyclic voltammetry in solutions containing small concentrations of ABTS. Furthermore, second-order homogeneous constants (k'f) were able to be calculated and were in the range of 0.90-4.03 L s-1 g-1. Considering that these differences can be attributed to different HS samples and different HS types (humic and fulvic acids), the obtained results support the relevance of the proposed approach for an accurate assessment of reaction kinetics between HS and radical species.

The competition of humic acid aggregation and adsorption on clay particles and its role in retarding heavy metal ions

Humic acid (HA) is of great importance in controlling the fate of heavy metals (HMs), however, the pivotal influence of HA aggregation within the HA-clay-HM ternary system on retarding HM mobility remains obscure. This study performed molecular dynamics simulations to delve into the consequences of HA aggregation on the environmental behavior of Cd2+ and Pb2+ (0.1-0.6 M) in the co-existence of illite particles. HA can readily aggregate into clusters, adhering to the illite surface or freely dispersing in the solution. These HA clusters significantly modulate HM mobility, contingent upon their location, arrangement, and interaction with illite. Consequently, HA exhibited a pronounced retardation effect on HM migration, stemming from the competition between HA aggregation and its adsorption on illite. Additionally, the retardation effect of HA aggregation was more obvious for Cd2+ (as compared to Pb2+), owing to its stronger interaction with the functional groups of HA. These findings contribute to the development of potential HA-based strategies for remediation of heavy metal-contaminated sites.

Application of earthworm can enhance biological power generation and accelerate sulfamethoxazole removal in agricultural soils

Microbial degradation plays a crucial role in removing sulfonamides from soil, enhancing sulfamethoxazole (SMX) remediation. To further augment SMX removal efficiency and mitigate the transmission risk associated with antibiotic resistance genes (ARGs), this study proposes a novel approach that integrates micro-animals, microorganisms, and microbial fuel cell (MFC) technology. The results showed that earthworm-MFC synergy substantially reduces SMX content and ARGs abundance in soil. The introduction of earthworms enhances humus content, facilitating electron transfer within MFC and consequently improving current generation. Furthermore, electrical stimulation applied to earthworms led to increased protein secretion and enhanced antioxidant system activity, thereby accelerating SMX degradation. Earthworms also foster MFC-associated bacterial growth and SMX-degrading bacteria proliferation, augmenting MFC treatment efficacy. This synergistic effect significantly augmented the overall efficacy of MFC treatment for antibiotics. Overall, integrating earthworm activity with MFC technology effectively optimizes electricity generation and enhances pollutant removal.

Unraveling the Role of Humic Acid in the Oxidation of Phenolic Contaminants by Soluble Manganese Oxo-Anions

Humic acid (HA) is ubiquitous in natural aquatic environments and effectively accelerates decontamination by permanganate (Mn(VII)). However, the detailed mechanism remains uncertain. Herein, the intrinsic mechanisms of HA's impact on phenolics oxidation by Mn(VII) and its intermediate manganese oxo-anions were systematically studied. Results suggested that HA facilitated the transfer of a single electron from Mn(VII), resulting in the sequential formation of Mn(VI) and Mn(V). The formed Mn(V) was further reduced to Mn(III) through a double electron transfer process by HA. Mn(III) was responsible for the HA-boosted oxidation as the active species attacking pollutants, while Mn(VI) and Mn(V) tended to act as intermediate species due to their own instability. In addition, HA could serve as a stabilizer to form a complex with produced Mn(III) and retard the disproportionation of Mn(III). Notably, manganese oxo-anions did not mineralize HA but essentially changed its composition. According to the results of Fourier-transform ion cyclotron resonance mass spectrometry and the second derivative analysis of Fourier-transform infrared spectroscopy, we found that manganese oxo-anions triggered the decomposition of C-H bonds on HA and subsequently produced oxygen-containing functional groups (i.e., C-O). This study might shed new light on the HA/manganese oxo-anion process.

Hydrothermal carbonization of Chinese medicine residues: Formation of humic acids and combustion performance of extracted hydrochar

Aiming at the sustainable management of high-moisture Chinese medicine residues (CMR), an alternative way integrating hydrothermal carbonization (HTC), humic acids (HAs) extraction and combustion of remained hydrochar has been proposed in this study. Effect of HTC temperature, HTC duration, and feedwater pH on the mass yield and properties of HAs was examined. The associated formation mechanism of HAs during HTC was proposed. The combustion performance of remained hydrochar after HAs extraction was evaluated. Results show that the positive correlation between hydrochar yield and HAs yield is observed. According to three-dimensional excitation emission matrix (3D EEM) fluorescence intensity, the best quality of HAs is achieved with a yield of 8.17 % at feedwater pH of 13 and HTC temperature of 200 °C. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy analyses show abundant aromatic and aliphatic structure as well as oxygenated functional groups in HAs, which is like commercial HAs (HA-C). Besides, in terms of comprehensive combustion index (CCI), HTC can improve the combustion performance of CMR, while it becomes a bit worse after HAs extraction. Higher weighted mean apparent activation energy (Em) of hydrochar indicating its highly thermal stability. HAs extraction reduces Em and CCI of remained hydrochar. However, it can be regarded a potential renewable energy. This work confirms a more sustainable alternative way for CMR comprehensive utilization in near future.

Microbially driven Fe-N cycle: Intrinsic mechanisms, enhancement, and perspectives

The iron‑nitrogen (FeN) cycle driven by microbes has great potential for treating wastewater. Fe is a metal that is frequently present in the environment and one of the crucial trace elements needed by microbes. Due to its synergistic role in the microbial N removal process, Fe goes much beyond the essential nutritional needs of microorganisms. Investigating the mechanisms behind the linked Fe-N cycle driven by microbes is crucial. The Fe-N cycle is frequently connected with anaerobic ammonia oxidation (anammox), nitrification, denitrification, dissimilatory nitrate reduction to ammonium (DNRA), Feammox, and simultaneous nitrification denitrification (SND), etc. Although the main mechanisms of Fe-mediated biological N removal may vary depending on the valence state of the Fe, their similar transformation pathways may provide information on the study of certain element-microbial interactions. This review offers a thorough analysis of the facilitation effect and influence of Fe on the removal of nitrogenous pollutants in various biological N removal processes and summarizes the ideal Fe dosing. Additionally, the synergistic mechanisms of Fe and microbial synergistic N removal process are elaborated, covering four aspects: enzyme activity, electron transfer, microbial extracellular polymeric substances (EPS) secretion, and microbial community interactions. The methods to improve biological N removal based on the intrinsic mechanism were also discussed, with the aim of thoroughly understanding the biological mechanisms of Fe in the microbial N removal process and providing a reference and thinking for employing Fe to promote microbial N removal in practical applications.

Chemical Source Profiles and Toxicity Assessment of Urban Fugitive Dust PM2.5 in Guanzhong Plain, China

Urban fugitive dust is a significant contributor to atmospheric PM2.5 and a potential risk to humans. In 2019, both road dust and construction dust were collected from four cities, including Xi'an, Xianyang, Baoji, and Tongchuan, in Guanzhong Plain, China. Elements, water-soluble ions, and carbonaceous fractions were determined to establish the chemical source profile. High enrichment degrees of Se, Sc, Cl, and Zn in both road dust and construction dust indicated that the industrial system and energy consumption influenced Guanzhong Plain strongly. According to the coefficient of divergence, the two datasets within Xianyang and Tongchuan were similar. Combined with the chemical profile, road dust was affected by more stationary emission sources than construction dust in Xi'an, while biomass burning and vehicle exhaust contributed more to road dust than construction dust in Baoji. Moreover, the health risk of heavy metal was assessed, and corresponding influencing factors were identified. Road dust in all cities showed a non-negligible non-carcinogenic risk for children. Ingestion and inhalation were the main exposure pathways to which As and Co contributed the most, respectively. The land-use regression model revealed that the first-class road in a 100 m radius impacted all high-risk level metals, and the commercial building material and enterprises weakly influenced Co and Pb, respectively.

A new tool for the determination of humic substances in natural waters: Pulsed voltammetry approach

Humic substances (HS) in natural waters can be determined with a new, simple and sensitive method based on their influence on the background current in a differential pulse - adsorptive cathodic stripping voltammetry. The proposed method, termed PB-HS (pulsed background - humic substances) is discussed in detail, including its application in natural samples from the Krka River estuary. The method was additionally compared with absorbance measurements as well as with the typical electrochemical HS quantification in natural waters based on HS complexation with molybdenum (Mo). A good correlation between methods was observed, with PB-HS showing slightly better sensitivity to humic compounds than classical spectrophotometry. Higher HS concentrations measured with the Mo-method may be due to the enhanced hydrophobicity reached at pH 2 that is required by the method. Advantages of the proposed PB-HS method, compared to existing voltammetric methods for HS quantification, are that it does not require any reagent addition (except buffer) and that it can be used at the natural pH of water as well as in a wide salinity range, which is crucial for its application in estuarine waters.