pH dependence of the binding interactions between humic acids and bisphenol A - A thermodynamic perspective

Overlooked interaction between redox-mediator and bisphenol-A in permanganate oxidation

Research efforts on permanganate (Mn(VII)) combined with redox-mediator (RM), have received increasing attention due to their significant performance for bisphenol-A (BPA) removal. However, the mechanisms underpinning BPA degradation remain underexplored. Here we show the overlooked interactions between RM and BPA during permanganate oxidation by introducing an RM-N-hydroxyphthalimide (NHPI). We discovered that the concurrent generation of MnO2 and phthalimide-N-oxyl (PINO) radical significantly enhances BPA oxidation within the pH range of 5.0-6.0. The detection of radical cross-coupling products between PINO radicals and BPA or its derivatives corroborates the pivotal role of radical cross-coupling in BPA oxidation. Intriguingly, we observed the formation of an NHPI-BPA complex, which undergoes preferential oxidation by Mn(VII), marked by the emergence of an electron-rich domain in NHPI. These findings unveil the underlying mechanisms in the Mn(VII)/RM system and bridge the knowledge gap concerning BPA transformation via complexation. This research paves the way for further exploration into optimizing complexation sites and RM dosage, significantly enhancing the system's efficiency in water treatment applications.

2D Ni-Co bimetallic oxide nanosheets activate persulfate for targeted conversion of bisphenol A in wastewater into polymers

The selective removal of targeted pollutants from complex wastewater is challenging. Herein, a novel persulfate (PS)-based advanced oxidation system equipped with a series of two-dimensional (2D) bimetallic oxide nanosheets (NSs) catalysts is developed to selectively degrade bisphenol A (BPA) within mixed pollutants via initiating nonradical-induced polymerization. Results indicate that the Ni0.60Co0.40Ox NSs demonstrate the highest catalytic efficiency among all Ni-Co NSs catalysts. Specifically, BPA degradation rate is 47.34, 27.26, and 9.72 times higher than that of 4-chlorophenol, phenol, and 2,4-dichlorophenol in the mixed solution, respectively. The lower oxidative potential of BPA in relation to the other pollutants renders it the primary target for oxidation within the PDS activation system. PDS molecules combine on the surface of Ni0.60Co0.40Ox NSs to form the surface-activated complex, triggering the generation of BPA monomer radicals through H-abstraction or electron transfer. These radicals subsequently polymerize on the surface of the catalyst through coupling reactions. Importantly, this polymerization process can occur under typical aquatic environmental conditions and demonstrates resistance to background matrices like Cl- and humic acid due to its inherent nonradical attributes. This study offers valuable insights into the targeted conversion of organic pollutants in wastewater into value-added polymers, contributing to carbon recycle and circular economy.

Inhibition mechanisms of biochar-derived dissolved organic matter to triclosan photodegradation: A remarkable role of aliphatics

Endocrine disrupting chemicals like triclosan (TCS) have been thought to be an emergent environmental pollutant. The ubiquitous dissolved organic matter (DOM) is able to interrelate with TCS and hamper its phototransformation. However, how the components in DOM can inhibit the photodegradation of DOM/TCS complex is largely unknown. Herein, we discovered that TCS photodegradation with biochar-derived DOM (BDOM) was interfered by both binding affinity and reactive oxygen species (ROS) productivity. BDOM can not only stimulate TCS photodegradation by producing ROS, but also inhibit the removal of TCS through the interactions between BDOMs and TCS. The quantification of BDOM's impact on TCS photodegradation revealed that BDOM hampered TCS removal with the proportion of -7.95 to -11.24% at pH 8.5, but strengthened it to 13.20% at pH 7.0. Binding process was more easily to inhibit TCS photodegradation in molecular form, while anionic TCS photodegradation was dominated by ROS productivity. Different inhibition mechanisms were involved in TCS photodegradation depending on the components of BDOMs. The hydroxyls and aromatic carbonyls might have hindered the attack of ROS on the phenolic hydroxyl of TCS via hydrogen bond interaction or π-π electron donor-acceptor interaction. Through hydrophobic interaction, the mobile aliphatics could greatly shield TCS to prevent ROS attack by wrapping or twining TCS, playing a significant role in inhibiting TCS removal. Results from this present study can afford a new viewpoint in elucidating the function of BDOMs in the phototransformation of organics and decrease the spread of antibiotic resistance genes.

UV/persulfate processes for the removal of total organic carbon from coagulation-treated industrial wastewaters

Sulfate radical-based oxidation processes were investigated to understand the relationship between persulfate (PS) consumption and total organic carbon (TOC) removal from industrial wastewater under various PS concentrations. First, the degradation and mineralization of Bisphenol A (BPA) (initial concentration: 11 mg/L) were investigated in ultraviolet (UV)/PS systems. Complete degradation was achieved within 30 min of UV irradiation, and 41%-72% TOC removal was achieved at PS concentrations of 200 and 400 mg/L. The consumed concentration of S2O82- and generated concentration of SO42- increased gradually to similar levels. The ratio of the PS consumption to TOC removal based on the mass concentration (mg/L) was 14.5 and 23.2 at 180 min for 200 and 400 mg/L of S2O82-, respectively. Three types of coagulation-treated industrial wastewater from metal-processing, food-processing, and adhesive-producing plants were obtained, and TOC removal was analyzed using the same UV/PS systems (initial TOC concentration: 100 mg/L). The TOC removal rates ranged from 16.9% to 94.4% after 180 min of UV irradiation at PS concentrations of 1,000, 2,000, 4,000, and 8,000 mg S2O82-/L. Despite the higher TOC removal at higher PS concentrations, the PS activation efficiency decreased significantly as the PS concentration increased. Only approximately 30%-40% activation efficiency was achieved at a PS concentration of 8,000 mg/L. In this study, the ratio of PS consumption to TOC removal ranged from 20.6 to 43.9.

Remediation potential of magnetic biochar in lead smelting sites: Insight from the complexation of dissolved organic matter with potentially toxic elements

Magnetic biochar has been widely used in potentially toxic elements (PTEs) polluted soils due to its magnetic separation capability and synchronous immobilization for multiple metals. However, the contribution of magnetic biochar to soil dissolve organic material (SDOM) and its binding behavior with PTEs needs to be further clarified prior to its remediation application on lead smelting sites. In this study, multi-spectral techniques of excitation-emission matrix (EEM) fluorescence spectroscopy and two-dimensional FTIR correlation spectroscopy (2D-FTIR-COS) were used to explore the evolution characteristics of SDOM in the lead smelting site under the remediation of magnetic biochar, and to further analyze its affinity and binding behavior with Pb and As. Results showed that magnetic biochar significantly increased SDOM content and decreased Pb and As available content. EEM and parallel factor analysis (EEM-PARAFAC) and Self-Organizing map analysis showed that humus-like and aromatic DOM increased and microbial-derived SDOM decreased after magnetic biochar cultivation. Furthermore, 2D-FTIR-COS correlation spectroscopy analysis indicated that BDOM had a stronger binding affinity to Pb, while SDOM has a stronger binding affinity to As. The binding sequences of different DOMs to PTEs varied greatly, the carboxyl and amide groups of SDOM and BDOM showed a remarkable and rapid response. Our results enhance the insights of magnetic biochar on soil function and PTEs remediation potential, providing novel information for its environmental remediation application.

Bisphenols-A Threat to the Natural Environment

Negative public sentiment built up around bisphenol A (BPA) follows growing awareness of the frequency of this chemical compound in the environment. The increase in air, water, and soil contamination by BPA has also generated the need to replace it with less toxic analogs, such as Bisphenol F (BPF) and Bisphenol S (BPS). However, due to the structural similarity of BPF and BPS to BPA, questions arise about the safety of their usage. The toxicity of BPA, BPF, and BPS towards humans and animals has been fairly well understood. The biodegradability potential of microorganisms towards each of these bisphenols is also widely recognized. However, the scale of their inhibitory pressure on soil microbiomes and soil enzyme activity has not been estimated. These parameters are extremely important in determining soil health, which in turn also influences plant growth and development. Therefore, in this manuscript, knowledge has been expanded and systematized regarding the differences in toxicity between BPA and its two analogs. In the context of the synthetic characterization of the effects of bisphenol permeation into the environment, the toxic impact of BPA, BPF, and BPS on the microbiological and biochemical parameters of soils was traced. The response of cultivated plants to their influence was also analyzed.

Unraveling the interaction of bisphenol A with collagen and its effect on conformational and thermal stability

Evidence suggests the association of bisphenol A (BPA) with increased collagen (COL) expression in the development of fibrosis. Ultraviolet and fluorescence spectra on collagen-BPA interaction showed that 100 ng/ml of BPA initiated loosening of protein backbone through unfolding with exposure of tyrosine residues resulting in an intermediate "Molten Globule" state, which later aggregated with 1 μg/ml of BPA indicated with an apparent red-shift. Conformational changes with CD and ATR-FTIR showed disappearance of negative band with broadening and shifting of peptide carbonyl groups. Light scattering findings with TEM images presented initial dissolution followed by unordered thick fibrillar bundles with 30 μg/ml BPA. The complex was pH sensitive, with calorimetric thermogram revealing increased thermal stability requiring 83°C to denature. Hydrogen bonds of 2.8 Å with hydrophobic interactions of BPA in all grooves of collagen molecule with same pattern and binding energy (-4.1 to -3.9 kcal/mol) confirmed the intensity of aggregate formation via in-silico docking.

Photodegradation of clindamycin by the dissolved black carbon is simultaneously regulated by ROS generation and the binding effect

As an essential source of the natural dissolved organic matter (DOM), dissolved black carbon (DBC) plays a vital role in the photodegradation of organics; however, there is rare information about the DBC-induced photodegradation mechanism of clindamycin (CLM), one of the widely used antibiotics. Herein, we discovered DBC-generated reactive oxygen species (ROS) stimulated CLM photodegradation. Hydroxy radical (•OH) could directly attack CLM by OH-addition reaction, the singlet oxygen (¹O₂) and superoxide (O₂^•-) contributed to the CLM degradation by transforming to •OH. In addition, the binding between CLM and DBCs inhibited the photodegradation of CLM by decreasing the concentration of freely dissolved CLM. Binding process inhibited CLM photodegradation by 0.25-1.98% at pH 7.0 and 6.1-41.77% at pH 8.5. These findings suggest that the photodegradation of CLM by DBC is simultaneously regulated by the ROS production and binding effect between CLM and DBC, benefiting the exact evaluation of the environmental impact of DBCs.

The mechanism of p-nitrophenol degradation by dissolved organic matter derived from biochar

Recently, p-nitrophenol (PNP), a common organic environmental pollutant, has been reported to be degraded by biochar. Although the degradation mechanism of PNP by biochar has been explored, the role of biochar-derived dissolved organic matter (BDOM) in PNP degradation remains unclear. Two BDOM samples were prepared in this study, and their PNP degradation performance was analyzed. BDOM5 (prepared at 500 °C) exhibited higher PNP degradation ratio than BDOM7 (prepared at 700 °C). The extent of PNP degradation per unit of BDOM5 and BDOM7 reached 9.54 and 4.19 mg/mg, respectively. Gas chromatography-mass spectrometry (GC-MS) analysis showed that both oxidative and reductive processes contributed to the PNP degradation by BDOM. Compared with BDOM7, the higher PNP removal of BDOM5 was due to the higher electron exchange capacity. Furthermore, hydroxyl radicals (OH) played a critical role in the oxidative degradation process of PNP by BDOM. This study sheds light on the phenomenon of PNP degradation by BDOM and these results may be useful for accurately assessing the environmental impact of biochar application.

Insights into the influence of Fe(III) on the interaction between roxarsone and humic acid using multi-spectroscopic techniques

The influence of Fe(III) on the interaction between roxarsone (ROX) and humic acid (HA) was investigated by multi-spectroscopic techniques. The fluorescence quenching experiment indicated that the fluorescence intensity of HA-ROX was quenched by Fe(III) through a static quenching process. Synchronous fluorescence spectra provided further information concerning the competitive combination between ROX and Fe(III) for HA. The results of the dialysis equilibrium experiment confirmed the existence of Fe(III) (0.05-0.1 mmol/L) promotes the combination of HA and ROX. Binding mechanisms were further characterized by FTIR spectroscopy, and the carboxyl functional group is involved in the binding process of HA/Fe/ROX. In addition, acidic and neutral conditions are more conducive to the combination of ROX and HA/Fe than alkaline conditions. The above discussion is of great significance in understanding the environmental fate of ROX under the coexistence of Fe(III) and HA.

Spectral change of dissolved organic matter after extracted by solid-phase extraction and its feasibility in predicting the acute toxicity of polar organic pollutants in textile wastewater

Spectroscopic parameters can be used as proxies to effectively trace the occurrence of organic trace contaminants, but their suitability for predicting the toxicity of discharged industrial wastewater with similar spectra is still unknown. In this study, the organic contaminants in treated textile wastewater were subdivided and extracted by four commonly-used solid-phase extraction (SPE) cartridges, and the resulting spectral change and toxicity of textile effluent were analyzed and compared. After SPE, the spectra of the percolates from the four cartridges showed obvious differences with respect to the substances causing the spectral changes and being more readily adsorbed by the WAX cartridges. Non-target screening results showed source differences in organic micropollutants, which were one of the main contributors leading to their spectral properties and spectral variations after SPE in the effluents. Two fluorescence parameters (C1 and humic-like) identified by the excitation emission matrix-parallel factor analysis (EEM-PARAFAC) were closely correlated to the toxicity endpoints for Scenedesmus obliquus (inhibition ratios of cell growth and Chlorophyll-a synthesis), which can be applied to quantitatively predict the change of toxicity effect caused by polar organic pollutants. The results would provide novel insights into the spectral feature analysis and toxicity prediction of the residual DOM in industrial wastewater.

Soil humic acid and arsenite binding by isothermal titration calorimetry and Dynamic Light Scattering: Thermodynamics and aggregation

The arsenite-humic acid binding process was investigated using Isothermal Titration Calorimetry (ITC), Dynamic Light Scattering and Laser Doppler Electrophoresis techniques. The ITC data were successfully (R² = 0.996-0.936) interpreted by applying the MNIS model, enabling thermodynamic parameters to be determined. The MNIS model was adjusted to the arsenite-HA binding process assuming that hydrogen bonding is the dominant type of interaction in the system. Negative enthalpy change values indicated the arsenite-HAs binding as an exothermic process. Negative ΔG values (-(26.83-27.00) kJ mol^-1) pointed out to spontaneous binding reaction, leading to the formation of the arsenite-HA complexes. The binding constant values ((7.57-5.02) 10⁵ M^-1) clearly demonstrate pronounced binding affinity. As ΔS values are obviously positive but close to zero, and ΔH>ΔS, the reaction can be considered enthalpy driven. Reaction heats and ΔH values (-(18.96-15.64) kJ mol^-1) confirmed hydrogen bonds as the most ascendant interaction type in the arsenite-HA complex. Negative zeta potential values (-45 to -20 mV) had shown that arsenite-HA aggregates remained negatively charged in the whole molar charge ratio range. The HAs' aggregate size change is evident but not particularly pronounced (Z_av = 50-180 nm). It can be speculated that aggregation during the titration process is not expressive due to repulsive forces between negatively charged arsenite-HA particles. Thermodynamic and reaction parameters clearly indicated that arsenite-HA complexes are formed at common soil pH values, confirming the possible influence of humic acids on increased As mobility and its reduced bioavailability.

Characterization of dissolved black carbon and its binding behaviors to ceftazidime and diclofenac pharmaceuticals: Employing the molecular weight fractionation

As the ubiquitous component of the aquatic environment, dissolved organic matter (DOM) readily bind with residual pharmaceutical contaminants (PCs) and influence their environmental behaviors. However, the binding mechanisms between dissolved black carbon (DBC), a vital part of the natural DOM pool, and PCs were poorly researched. In this study, the bulk DBC was divided into four fractions in molecular weight (MW) via an ultrafiltration system, and the properties of DBC and their binding interaction with two kinds of typical PCs (ceftazidime (CAZ) and diclofenac (DCF)) were explored concretely. The results showed that low MW component was the main contributor to bulk DBC, and the aromaticity increased with the increase of MW. The categories of chemical structures and fluorescent substances in different MW DBC were similar. Multispectral techniques showed that the oxygen-enriched compounds in DBC had the higher affinity to CAZ/DCF. The -NH-, -COOH, -NH₂ groups in CAZ molecules appeared to form the hydrogen bond with DBC. Fluorescence quenching experiments were analyzed, and the binding mechanisms were specifically expounded from the thermodynamic perspective. The fluorophore of fulvic acid-like compounds (FA) were quenched by both static and dynamic quenching mechanisms, while only static quenching occurred for humic acid-like compounds (HA). For bulk DBC, the hydrogen bond and van der Waals force were the major forces in the HA-CAZ system, while the hydrophobic force made the primary contribution to the HA-DCF system, which might be ascribed to the higher hydrophobic nature of DCF. Notably, with the increase of HA MW, the main binding mode of HA-CAZ/DCF changed from hydrophobic force to hydrogen bond and van der Waals force gradually, which also directly proved that various noncovalent interactions co-driven the binding processes. Our findings are beneficial to better assess the fate of DBC and PCs and the corresponding complexes in the aquatic environment.

Prediction of polarity-dependent environmental behaviors of humic substances (HS) using a HS hydrophobicity index based on hydrophilic interaction chromatography

A variety of analytical methods have been applied to describe the properties of aquatic humic substances (HS). However, there are only a few methods available to probe HS hydrophobicity because of the heterogeneous character of HS. In this study, hydrophilic interaction chromatography (HILIC) equipped with a UV detector was employed to describe the heterogeneous distribution of HS with respect to its hydrophobicity and to suggest a representative HS hydrophobicity index. To this end, various mobile phases were explored to achieve the optimal separation capability of HILIC. The highest resolution was obtained with a mobile phase comprising acetonitrile and water at a ratio of 70:30 (v:v). A calibration curve was successfully constructed using eight different HS precursor compounds, which allowed for the successful conversion of the retention time (RT) into the octanol-water partition coefficient (K_ow) (log K_ow = -2.83 × (RT) + 17.44, R² = 0.950). Several possible HS hydrophobicity indices were derived from the HILIC chromatogram. Among those, the weight-average log K_OW value exhibited the strongest negative correlation with the well-known polarity index, (O + N)/C ratios, of seven reference HS samples. This new HILIC-based index (i.e., average log K_OW) also presented a good relationship with the HS binding coefficients with pyrene as well as the extent of HS adsorption onto kaolinite at a given solution chemistry (i.e., at a high ionic strength and a neutral pH), both of which are known to be largely governed by the hydrophobic nature of HS. This study demonstrated that the average K_OW value based on HILIC is an intuitive and robust HS hydrophobicity index to fully represent the heterogeneous distribution of hydrophobicity within a bulk HS and could be applied to predict many environmental behaviors related to HS hydrophobicity or HS polarity.

Comparison of the binding interactions of 4-hydroxyphenylpyruvate dioxygenase inhibitor herbicides with humic acid: Insights from multispectroscopic techniques, DFT and 2D-COS-FTIR

4-Hydroxyphenylpyruvate dioxygenase (HPPD) inhibitor is one of the important herbicides to solve the problem of weed control. With the widespread and continued use of HPPD inhibitor (HPPDi) herbicides, it may inevitably put pressure on the environment. Humic acid (HA) can effectively interact with pesticides through sorption or covalent bond formation and promote the degradation of pesticides, which can reduce the risk of pesticides in the environment. In the present study, the interactions of four HPPDi herbicides (sulcotrione, tembotrione, topramezone and mesotrione) with HA were reported and comparative assessment of the binding using multispectral technology, density functional theory (DFT) calculation and two-dimensional correlation spectroscopy (2D-COS). Time-resolved measurements and the Stern-Volmer constant at different temperature verified that HPPDi can bind with HA through the static quenching mechanism. From the thermodynamic parameters, the interaction force between HA and sulcotrione, tembotrione, topramezone and mesotrione was provided by electrostatic force. DFT, binding constant and three-dimensional (3D) fluorescence peak variation all indicated that the order of the binding ability of the four HPPDi and HA was mesotrione > tembotrione > sulcotrione > topramezone. According to dynamic light scattering (DLS), pH 7 is most conducive to the formation of HA-HPPDi complexes. Fourier transform infrared spectroscopy (FTIR) and 2D-COS showed that HA combined with HPPDi through aromatic C-H, CO and C-X, and the first binding group to HA was almost all CO. Sulcotrione, tembotrione, topramezone and mesotrione quench the endogenous fluorescence of HA by a static quenching mechanism and bind to HA through electrostatic interaction to form a complex. These results provide important insights into the combination of environmental pollutants with HA.

New insights into the interactions between humic acid and three neonicotinoid pesticides, with multiple spectroscopy technologies, two-dimensional correlation spectroscopy analysis and density functional theory

The widespread use of neonicotinoid pesticides in agricultural production has caused pressure on the environment. In the present work, the interactions between humic acid (HA) and three neonicotinoid insecticides, dinotefuran, clothianidin and nitenpyram, were investigated by using multiple spectroscopy techniques combined with two-dimensional correlation spectroscopy analysis and density functional theory (DFT). Dinotefuran, clothianidin and nitenpyram could quench the endogenous fluorescence of HA through a static quenching process dominated by hydrogen bonds and van der Waals forces. According to the revised Stern-Volmer equation and DFT calculation, the binding abilities of the three pesticides with HA were ranked as dinotefuran < clothianidin < nitenpyram. The results of dynamic light scattering showed that neutral conditions were more conducive to the combination of HA and dinotefuran, clothianidin and nitenpyram. Through Fourier transform infrared spectroscopy (FTIR) combined with two-dimensional correlation analysis (2D-COS), the functional group with the strongest binding ability in the HA-dinotefuran, HA-clothianidin and HA-nitenpyram system was CH, CO and CO, respectively. The work will help to further understand the behavior of neonicotinoid pesticides in the environment.

Effects of endogenous and exogenous dissolved organic matter on sorption behaviors of bisphenol A onto soils

The transport of organic contaminants in groundwater might be greatly affected by coexistence of dissolved organic matter (DOM) from different sources. In this study, the effects of endogenous and exogenous DOMs (referred to as DOM_en and DOM_ex, respectively) on sorption behavior of bisphenol A (BPA) onto two reference soils were investigated by batch experiments and microscopic characterization. The results showed that BPA sorption onto soils was dominated by soil organic matter content and affected by DOM properties. The effect of DOM_en on BPA sorption was also related to the inorganic components of the two soils. The decrease of organic matter content reduced the sorption capacity of fluvo-aquic soil. However, because the content of available inorganic components in black soil was high, after removing DOM_en, more inorganic sites were exposed to increase the sorption capacity. In addition, DOM_en could form complexes with BPA in solution, thus the removal of DOM_en promoted BPA sorption onto black soil. Under the experimental conditions, contribution of DOM_ex to the total sorption of BPA onto both soils was not more than 30%. Results of dialysis experiments and soil sorption experiments indicated that effects of coexisting DOM_ex on BPA sorption was related to the affinity of DOM_ex to soils and complexation of BPA and DOM_ex. Since the affinity of DOM_ex to fluvo-aquic soil was relatively low, the complex of BPA and DOM_ex in solution was the main inhibition mechanism for BPA sorption. For black soil, higher complexation proportion of BPA with DOM_ex adsorbed onto soil which promoted BPA sorption onto soil. The findings are of significance for understanding the co-migration of DOM with BPA through soils.

Regulation mechanisms of humic acid on Pb stress in tea plant (Camellia sinensis L.)

Though the interaction between humic acid (HA) and heavy metals has been widely reported, the effects of HA on the toxicity of heavy metals to plants are still in debate. In this study, the regulation mechanisms of HA on Pb stress in tea plant (Camellia sinensis L.) was investigated through hydroponic experiments, and the experimental results were explained by using transmission electron microscope (TEM), scanning transmission X-ray microscopes (STXM) and isobaric tags for relative and absolute quantitation (iTRAQ) differential proteomics. Significant alleviation of Pb stress was found with HA coexistence. TEM results showed that HA greatly mitigated the damage of cells caused by Pb stress. Compared with sole Pb treatment, the addition of HA increased the contents of pectin and pectic acid in the cell wall by 10.5% and 30.5%, while arabinose (Ara) and galactose (Gal) decreased by 20.5% and 15.9%, respectively, which were beneficial for increasing Pb adsorption capacity of the cell wall and promoting cell elongation. Moreover, iTRAQ differential proteomics analysis proved that HA strengthened the antioxidant system, promoted the synthesis of cell wall, and stabilized protein and sulfur-containing substance metabolism in molecular level. Notably, the concentration of calcium (Ca) in the cell wall of HA coexistence treatment was 47.4% higher than Pb treatment. STXM results also indicated that the distribution of Ca in the cell wall was restored with the presence of HA. This might promote the formation of the egg-box model, thus alleviating Pb stress in cells. Our results reveal the regulation mechanisms of HA on Pb detoxification in plants and provide useful information for improving the safety of agricultural products.

Occurrence, environmental implications and risk assessment of Bisphenol A in association with colloidal particles in an urban tropical river in Malaysia

Phase distribution of emerging organic contaminants is highly influential in their presence, fate and transport in surface water. Therefore, it is crucial to determine their state, partitioning behaviour and tendencies in water environments. In this study, Bisphenol A was investigated in both colloidal and soluble phases in water. BPA concentrations ranged between 1.13 and 5.52 ng L⁻¹ in the soluble phase and n.d-2.06 ng L⁻¹ in the colloidal phase, respectively. BPA was dominant in the soluble phase, however, the colloidal contribution ranged between 0 and 24% which implied that colloids can play a significant role in controlling BPA’s transportation in water. Urban and industrial areas were the main sources of BPA while forest areas displayed lower levels outside the populated domains. pH levels were between 6.3 and 7.4 which might have affected BPA’s solubility in water to some extent. The particle size distribution showed that the majority of the particles in river samples were smaller than 1.8 µm in diameter with a small presence of nanoparticles. Zeta potential varied between − 25 and − 18 mV, and these negative values suggested instability of particles. Furthermore, BPA was positively correlated with BOD, COD and NH₃–N which might indicate that these organic compounds were released concurrently with BPA. RQ assessment showed low levels of risk towards algae and fish in the study area.

Photochemical Generation of Methyl Chloride from Humic Aicd: Impacts of Precursor Concentration, Solution pH, Solution Salinity and Ferric Ion

Methyl chloride (CH₃Cl) is presently understood to arise from biotic and abiotic processes in marine systems. However, the production of CH₃Cl via photochemical processes has not been well studied. Here, we reported the production of CH₃Cl from humic acid (HA) in sunlit saline water and the effects of the concentration of HA, chloride ions, ferric ions and pH were investigated. HA in aqueous chloride solutions or natural seawater were irradiated under an artificial light, and the amounts of CH₃Cl were determined using a purge-and-trap and gas chromatography-mass spectrometry. CH₃Cl was generated upon irradiation and its amount increased with increasing irradiation time and the light intensity. The formation of CH₃Cl increased with an increase of HA concentration ranging from 2 mg L⁻¹ to 20 mg L⁻¹ and chloride ion concentration ranging from 0.02 mol L⁻¹ to 0.5 mol L⁻¹. The photochemical production of CH₃Cl was pH-dependent, with the highest amount of CH₃Cl generating near neutral conditions. Additionally, the generation of CH₃Cl was inhibited by ferric ions. Finally, natural coastal seawater was irradiated under artificial light and the concentration of CH₃Cl rose significantly. Our results suggest that the photochemical process of HA may be a source of CH₃Cl in the marine environment.