Insight into the enhancement effect of humic acid on microbial degradation of triclosan in anaerobic sediments

Humic acid-supported nanoscale zero-valent iron for sustainable cadmium remediation and crop safety in farmland soil

The immobilization efficacy of most existing stabilizers for cadmium (Cd)-contaminated soil is highly sensitive to ecological and environmental fluctuations. These fluctuations can induce the reactivation of Cd, thereby restricting the long-term applicability of stabilization technologies. In this study, we synthesized a humic acid (HA)-supported nanoscale zero-valent iron (nZVI) particle (nZVI@HA). The roles of nZVI@HA in Cd sequestration and rice production were investigated throughout the entire rice life cycle using pot experiments with separate and combined applications of pristine nZVI and HA, as well as varying dosages of nZVI@HA. The application of 500-1000 mg/kg nZVI@HA significantly reduced the available Cd content to 37.85 %-55.24 % of that in the control group at 140 days. Analysis of the dynamic relationship between Cd species and solid-phase iron revealed that HA modification increased electronegativity and inhibited Fe0 oxidation, enabling nZVI@HA to passivate Cd through electrostatic attraction and complexation. Furthermore, the grain Cd content decreased from 0.78 to 0.16-0.19 mg/kg, whereas the grain yield increased from 3.59 to 6.54-8.24 g/plant after treatments with 500-1000 mg/kg nZVI@HA. The stimulatory effects of nZVI@HA on both soil ammonia nitrogen (NH4+-N) content and iron plaque formation were identified as specific functions. Additionally, 16S rRNA sequencing indicated that the simultaneous promotion of Cd sequestration and safe crop production relied on the nZVI@HA-facilitated coupling process of microbial iron-metabolism, Cd species transformation, and nitrogen cycling.

Enhanced microbial degradation of hexabromocyclododecane in riparian sediments through regulating flooding regimes

Hexabromocyclododecane (HBCD), a persistent halogenated organic pollutant, has been commonly detected in river sediments, especially in riparian zones, but strategies for promoting its microbial degradation remain insufficiently explored. This study hypothesized that regulating the flooding regime of sediments could accelerate microbial degradation of HBCD in riparian zones and evaluated the underlying mechanisms. Results showed that, compared with high-frequency flooding-drying or no alternations, the low-frequency flooding-drying alternation (6 weeks of flooding and 6 weeks of drying, 6F:6D) significantly promoted microbial degradation of HBCD. This may be due to changes in sediment redox potential under the 6F:6D regime, facilitating the sequential reductive debromination and aerobic degradation process of HBCD. The abundances of organohalide-respiring bacteria (Dehalococcoides spp. and Dehalogenimonas spp.) were always high in the 6F:6D regime, irrespective of flooding or drying periods. Furthermore, the complex bacterial co-occurrence patterns, specific ecological clusters, and potential keystone species including the genera Methylibium, Nitrospira, and Dehalococcoides, may play important degradative roles of HBCD in the 6F:6D regime. Overall, microbial degradation of HBCD can be promoted under low-frequency flooding-drying alternation regulated by hydraulic structures, providing an effective and eco-friendly strategy for ecological restoration.

Invisible threats from typical endocrine disrupting compounds in estuarine environments caused by continuing seawater incursion: In-situ evidence of bio-geochemical processes captured by diffusive gradients in thin films

Continued seawater incursion significantly affects the fate of pollutants in coastal estuaries, yet understanding of the in-situ behavior of endocrine-disrupting compounds (EDCs) in these areas remains limited. The distribution, transport and microbial response of two model EDCs, bisphenol A (BPA) and nonylphenol (NP), in three estuarine zones of slight (SZ), moderate (MZ) and complete (CZ) seawater incursion were investigated in-situ. Results showed seawater incursion reshaped the environmental gradients of the coastal estuaries on a spatial scale. Varying salinity gradient and tidal hydrodynamic conditions altered the dependence of EDCs on organic carbon, and promoted the release of accumulated EDCs from estuarine sediments resulting in the lowest residues of BPA (2.74 ± 0.76 μg/kg) and NP (10.25 ± 5.86 μg/kg) in the MZ. The resupply potential of BPA (R = 0.171 ± 0.058) and NP (R = 0.107 ± 0.015) from sediment to porewater was significantly higher in the SZ than in other zones (p < 0.001), due to both higher contaminant accumulation in this zone and inhibited resupply in MZ and CZ caused by seawater incursion. Furthermore, seawater incursion significantly reduced the microbial community diversity in the CZ (p < 0.001), being dominated by Vibrio (67.00 ± 1.13 %), and accordingly weakened the ability to transform organic matter in this region. Based on predicted sea level rise and the transport characteristics of EDCs under increased seawater incursion, it is estimated that the cumulative additional release of BPA and NP in the estuary will reach 1.8 and 1.5 tons by 2100, respectively. In order to mitigate the risk of additional estuarine EDCs release due to seawater incursion, increasing vegetation cover, strict monitoring, and climate policy interventions may be effective strategies.

Simultaneous removal of nitrate, copper, carbamazepine, and calcium from micropolluted water by fulvic acid through promotion of denitrification and microbial-induced calcium precipitation: Performance and mechanism

In this paper, a study of the denitrification strain Cupriavidus sp. W12 was conducted to remove copper (Cu2+), carbamazepine (CBZ), and calcium (Ca2+) by microbial-induced calcium precipitation (MICP) after adding fulvic acid (FA). After the addition of 20 mg/L FA, the removal efficiencies of nitrate (NO3--N), Cu2+, CBZ, and Ca2+ reached 100.0 %, 98.7 %, 96.6 %, and 73.6 %, correspondingly and there was no accumulation of nitrite (NO2--N). FA stimulated the growth of strain W12, improved electron transfer activity, and facilitated the conversion of gaseous nitrogen. The research revealed that FA might enhance microbial activity and result in a more dense and porous structure of the biological precipitate. Cu2+ and CBZ were removed by co-precipitation and adsorption. As the initial report of FA promoting MICP to remove complex pollutants, this paper offers a theoretical foundation for NO3--N, Cu2+, CBZ, and Ca2+ remediation in micropolluted water.

The promotion of the polycyclic aromatic hydrocarbons degradation mechanism by humic acid as electron mediator in a sediment microbial electrochemical system

To enhance the removal efficiencies of polycyclic aromatic hydrocarbons (PAHs) in sediments and to elucidate the mechanisms by which microbial electrochemical action aids in the degradation of PAHs, humic acid was used as an electron mediator in the microbial electrochemical system in this study. The results revealed that the addition of humic acids led to increases in the removal efficiencies of naphthalene, phenanthrene, and pyrene by 45.91%, 97.83%, and 85.56%, respectively, in areas remote from the anode, when compared to the control group. The investigation into the microbial community structure and functional attributes showed that the presence of humic acid did not significantly modify the microbial community composition or its functional expression at the anode. However, an examination of humic acid transformations demonstrated that humic acid extended the electron transfer range in sediment via the redox reactions of quinone and semiquinone groups, thereby facilitating the PAHs degradation within the sediment.

Adding carbon sources to the substrates enhances Cr and Ni removal and mitigates greenhouse gas emissions in constructed wetlands

Constructed wetlands for wastewater treatment pose challenges related to long-term operational efficiency and greenhouse gas emissions on a global scale. This study investigated the impact of adding peat, humic acid, and biochar into the substrates of constructed wetlands and focused on Cr, and Ni removal, greenhouse gas emissions, and microbial communities in constructed wetlands. Biochar addition treatment achieved the highest removal efficiencies for total Cr (99.96%), Cr (VI) (100%), and total Ni (91.04%). Humic acid and biochar addition both significantly increased the heavy metal content in wetland plant Leersia hexandra and substrates of constructed wetlands. Further analysis of microbial community proportions by high-throughput sequencing revealed that biochar and humic acid treatments enhanced Cr and Ni removal efficiency by increasing the abundance of Bacteroidetes, Geobacter and Ascomycota. Humic acid addition treatment reduced CO2 emissions by decreasing the abundance of Bacteroidetes and increasing that of Basidiomycota. Peat treatment decreased CH4 emissions by reducing the abundance of the Bacteroidetes. Biochar treatment increased the abundance of the Firmicutes, Bacteroidetes, Proteobacteria as well as Basidiomycota, resulting in reduced N2O emissions. Biochar and humic acid treatments efficiently removed heavy metals from wastewater and mitigated greenhouse gas emissions in constructed wetlands by modifying the microbial communities.

Humic substances mediated superior photochemical pollutant conversion on defective TiO2 in environmentally relevant matrices: The key roles of oxygen vacancy in surface interactions, oxidant activation and radical generation

Ubiquitous humic substances usually exhibit strong interfering effects on target pollutant removal in advanced water purification. This work aims to develop a photochemical conversion system on the nonstoichiometric TiO2 for pollutant removal in environmentally relevant matrices. In this synergistic reaction system, the redox-reactive humic substances and defective oxygen vacancies can serve as the organic electron transfer mediator and the key surface reactive sites, respectively. This system achieves a superior pollutant degradation in real surface water at low oxidant concentrations. Reactive oxygen vacancies on the TiO2 surface and sub-surface are of considerable interest for this photochemical reaction system. By engineering defective oxygen vacancies on high-energy {001} polar facet, the surface and electronic interactions between tailored TiO2 and humic substances are greatly strengthened for the promoted electron transfer and oxidant activation. Rendered by the strong surface affinity and molecular activation, defective oxygen vacancies thermodynamically and dynamically promote reactive chain reactions for free radical formation, including the selective O2 reduction to ·O2- and the H2O2 activation to ·OH. Our findings take new insights into environmental geochemistry, and provide an effective strategy to in-situ boost the humic substances-mediated water purification without secondary pollution. ENVIRONMENTAL IMPLICATION: Humic substances are widely distributed in aquatic environment, thus playing important roles in environmental geochemistry. For example, humic substances can achieve good surface adsorption through electrostatic adsorption, ligand exchange and electronic interactions with typical TiO2 to form reactive ligand-metal charge transfer complexes for pollutant degradation. Inspired by the unique properties of surface and sub-surface oxygen vacancies, the defective TiO2 was designed to refine the humic substances-mediated photochemical reactions. A superior reactivity was measured for pollutant degradation. Our findings provide an effective strategy to boost naturally photochemical decontamination in environmentally relevant matrices.

Mitigating phthalate toxicity: The protective role of humic acid and clay in zebrafish larvae

This research study aimed to explore the mitigating effects of humic acid and clay on the toxicity induced by three different phthalates (DBP, DEP, DEHP) on zebrafish larvae growth. Prolonged exposure to DBP resulted in a concerning 87.33% mortality rate, significantly reduced to 7.3% when co-administered with humic acid. A similar reduction in mortality was observed for the other two phthalates (DEP and DEHP). Additionally, the introduction of phthalates with humic acid, clay, or their combination led to a significant decrease in the malformation rate in larvae. High-Performance Liquid Chromatography (HPLC) analysis of phthalates in treatments revealed a noteworthy decline in their concentration when combined with humic acid and clay. This suggests a reduced bioavailability of phthalates to larvae, aligning with diminished toxicity, lower mortality, fewer malformations, and improved organ development, as well as less oxidative stress. Furthermore, measurements of larval length and morphological scoring affirmed the protective role of humic acid and clay in promoting the normal growth of zebrafish. This study underscores the potential of environment modulators, such as humic acid and clay, as effective bioremediation agents against phthalate toxicity. The generation of reactive oxygen species (ROS), indicative of oxidative stress, was markedly higher in larvae treated solely with phthalates compared to the control. Conversely, larvae treated with a combination of phthalates and humic acid or clay exhibited a significant decrease in ROS generation, signaling a decline in oxidative stress. Histopathological analysis of adult fish subjected to various treatments revealed significant damage to vital organs like the liver and intestine when treated with phthalates alone. However, when phthalates were introduced with humic acid, clay, or both, the morphology closely resembled that of the control, reinforcing the protective role of humic acid and clay in zebrafish development against administered phthalates.

Application of dissimilatory iron-reducing bacteria for the remediation of soil and water polluted with chlorinated organic compounds: Progress, mechanisms, and directions

Chlorinated organic compounds are widely used as solvents, but they are pollutants that can have adverse effects on the environment and human health. Dissimilatory iron-reducing bacteria (DIRB) such as Shewanella and Geobacter have been applied to treat a wide range of halogenated organic compounds due to their specific biological properties. Until now, there has been no systematic review on the mechanisms of direct or indirect degradation of halogenated organic compounds by DIRB. This work summarizes the discussion of DIRB's ability to enhance the dechlorination of reaction systems through different pathways, both biological and biochemical. For biological dechlorination, some DIRB have self-dechlorination capabilities that directly dechlorinate by hydrolysis. Adjustment of dechlorination genes through genetic engineering can improve the dechlorination capabilities of DIRB. DIRB can also adjust the capacity for the microbial community to dechlorinate and provide nutrients to enhance the expression of dechlorination genes in other bacteria. In biochemical dechlorination, DIRB bioconverts Fe(III) to Fe(II), which is capable of dichlorination. On this basis, the DIRB-driven Fenton reaction can efficiently degrade chlorinated organics by continuously maintaining anoxic conditions to generate Fe(II) and oxic conditions to generate H2O2. DIRB can drive microbial fuel cells due to their electroactivity and have a good dechlorination capacity at low levels of energy consumption. The contribution of DIRB to the removal of pesticides, antibiotics and POPs is summarized. Then the DIRB electron transfer mechanism is discussed, which is core to their ability to dechlorinate. Finally, the prospect of future work on the removal of chlorine-containing organic pollutants by DIRB is presented, and the main challenges and further research directions are suggested.