This work investigates the absorption properties of soluble brown carbon (BrC), extracted in methanol and water, from ambient aerosol (PM₁₀) samples, collected over an urban background site in Mumbai, India. The diurnal variability was investigated in samples collected in the morning (7–11 a.m.) and afternoon (12–4 p.m.) periods. Absorption properties of BrC (in the 300–600-nm wavelength range) were measured in filter extracts of water-soluble organic carbon (WSOC) and methanol-soluble organic carbon (MSOC). WSOC and MSOC accounted for on average 52% and 77%, respectively, of the measured OC, potentially indicating unextracted BrC and rendering these values the lower bound. Compared with afternoon samples, the morning samples of MSOC and WSOC had increased BrC concentrations and absorption coefficients (bₐbₛ365; 40%–65%). The correlation between bₐbₛ365 and EC, ns-K⁺, and NO₃⁻ in the morning samples indicated contributions from primary sources, including both biomass and vehicular sources. The decreased bₐbₛ365 in the afternoon samples was partly explained by mixing layer dilution, accompanied by a reduction in the concentrations of primary aerosol constituents. Furthermore, in the afternoon samples, ¹HNMR spectroscopy revealed the presence of more oxidized functional groups and significantly higher OC/EC and WSOC/OC ratios, indicating the greater aging of afternoon aerosol. The MAC₃₆₅ (m²gC⁻¹) for both WSOC and MSOC extracts decreased significantly by 20%–34% in the afternoon samples compared with the morning samples, indicating degradation in the absorption properties of the particles and potentially a change in the constituent BrC chromophores.

Studies on Fe uptake by phytoplankton have been often conducted using artificial culture media. However, Fe chemistry in freshwater can be influenced by riverine anthropogenic impacts and other factors causing water quality changes. In this study, therefore, Fe uptake in natural (river and reservoir) and effluent waters was investigated for the notorious bloom-forming freshwater cyanobacterium Microcystis aeruginosa. To investigate the Fe uptake mechanism, a short-term incubational assay was conducted in the presence of light, Fe(II) ligand and Fe(III) reductant, with results consistently indicating that unchelated Fe(III) is the major substrate for Fe uptake by M. aeruginosa. Further assays using various freshwater samples indicated that Fe uptake is lower in natural waters compared to that of effluent waters and, interestingly, Fe uptake was found to be limited in natural waters. These results suggest that Fe limitation can be alleviated by the inflow of effluent waters. Statistical analysis with various water quality variables indicated that Fe availability is significantly influenced by concentrations of dissolved Fe and organic matter as well as specific UV absorbance (an index of aromaticity). Overall, findings of this study highlight that watershed anthropogenic activities exert important roles in Fe uptake by freshwater cyanobacteria via alteration of Fe speciation.

Although fertilization plays an important role in determining the contents of soil dissolved organic matters or water-extractable organic matter (DOM, WEOM), knowledge regarding the dynamics, biodegradability, and microbial community shifts of WEOM in response to different fertilization treatments is very limited, particularly in rice–wheat cropping soil. Thus, in the present study, we performed biodegradation experiments using WEOM extracted from samples of soil that had been subjected to four different fertilization treatments: unfertilized control (CK), chemical fertilizer (CF), 50% chemical fertilizer plus pig manure (PMCF), and 100% chemical fertilizer plus rice straw (SRCF). UV spectrum and fluorescence 3D excitation–emission matrix analyses applied to investigate the chemical composition of WEOM revealed that all examined WEOMs were derived from microbial activity and the dominant portion comprised humic acid-like compounds. After the incubation, 31.17, 31.63, 43.47, and 33.01% of soil WEOM from CK, CF, PMCF, and SRCF treatments, respectively, were biodegraded. PMCF- derived WEOM had the highest biodegradation rate. High-throughput sequencing analyses performed to determine the microbial community before and after the incubation indicated that Sphingomonas, Bacillus, and Flavisolibacter were the predominant bacterial genera in the original inoculum derived from the four fertilization treatments. Following biodegradation, we observed that the dominant bacteria differed according to fertilization treatments: Curvibacter (43.25%) and Sphingobium (10.47%) for CK, Curvibacter (29.68%) and Caulobacter (20.00%) for CF, Azospirillum (23.68%) and Caulobacter (13.29%) for PMCF, and Ralstonia (51.75%) for SRCF. Canonical correspondence analysis revealed that, shifts in the microbial community were closely correlated with pH and specific UV absorbance at 254 nm. We speculated that the inherent traits of different WEOM and the properties of soil solutions under different fertilization treatments shaped the soil microbial community structure, thereby influencing the biodegradation of WEOM.

The occurrence and distributions of selected endocrine-disrupting chemicals (EDCs), along with related environmental factors, were investigated in two rivers and six reservoirs in the Pearl River Delta. The vertical profiles of aqueous 4-tert-octylphenol (OP), 4-nonylphenol (NP), and estrone (E1) were constant, with little change in concentration between the surface and the river bottom, while higher aqueous concentrations of bisphenol A (BPA) were found in the bottom layers of the rivers. OP and NP in suspended particulate matter (SPM) were transferred from the surface to the bed layer, ultimately accumulating in the sediment. However, the particulate profiles of BPA and E1 both featured increases from the surface to the bottom layers and attenuation in the river bed. Dissolved oxygen (DO), water temperature, and pH were negatively correlated with the EDC concentrations, and negative relationships between DO and distribution coefficient (Kd) values for OP and NP were found as well. This indicated that these environmental parameters were primarily responsible for the EDC vertical distribution and SPM-water partitioning in the rivers. Positive relationships were observed between chlorophyll a and EDCs in the particulate phase, and the algae/water Kd values for EDCs in reservoirs were comparable to the SPM/water and sediment/water Kd values from the rivers. These results suggest that algae played an important role in regulating the distribution of EDCs in surface waters. Moreover, relationships between UV absorbance and EDCs revealed that π-π interactions were among the dissolved organic carbon (DOC)-EDC binding mechanisms and that DOC fractions with higher degrees of aromaticity and humification possessed higher affinities towards EDCs.

Currently, there is little comparative data on the colloidal stability and the toxicity of ultraviolet (UV)- and visible-light (VL)-transformed graphene oxide (GO). In order to identify this knowledge gap, the physicochemical properties of UV/VL-transformed GO are investigated in detail. Attempts are made to correlate the physicochemical alterations of UV/VL-transformed GO to the observed changes in its colloidal properties and toxicity. The results show that both UV and VL irradiations induce the significant change in the color, UV–vis absorbance, morphology, surface charge, size, oxygen containing functional groups, total of carbon, and photoluminescence properties of GO. The photo-reaction behavior of GO under UV exposure is different from that under VL irradiation in terms of reaction rate, order, and extent. Finally, the UV and VL irradiations show different effects not only on the colloidal stability of GO in the City water and Dongpu Lake water, but also on the toxicity of GO to Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus bacteria. This study clearly shows how the environmental fate and risk of GO are modified by UV and VL irradiations.

In this study, a sensitive and low-cost multi-wavelength spectrophotometric method for the determination of hydrogen peroxide (H₂O₂) in water was established. The method was based on the oxidative coloration of 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonate) (ABTS) via Fenton reaction, which resulted in the formation of green radical (ABTS•⁺) with absorbance at four different wavelengths (i.e., 415 nm, 650 nm, 732 nm, and 820 nm). Under the optimized conditions (CABTS = 2.0 mM, CFₑ²⁺ = 1.0 mM, pH = 2.60 ± 0.02, and reaction time (t) = 1 min), the absorbance of the generated ABTS•⁺ at 415 nm, 650 nm, 732 nm, and 820 nm were well linear with H₂O₂ concentrations in the range of 0–40 μM (R² > 0.999) and the sensitivities of the proposed Fenton-ABTS method were calculated as 4.19 × 10⁴ M–¹ cm–¹,1.73 × 10⁴ M–¹ cm–¹, 2.18 × 10⁴ M–¹ cm–¹, and 1.96 × 10⁴ M–¹ cm–¹, respectively. Meanwhile, the detection limits of the Fenton-ABTS method at 415 nm, 650 nm, 732 nm, and 820 nm were respectively calculated to be 0.18 μM, 0.12 μM, 0.10 μM, and 0.11 μM. The absorbance of the generated ABTS•⁺ in ultrapure water, underground water, and reservoir water was quite stable within 30 min. Moreover, the proposed Fenton-ABTS method could be used for monitoring the variations of H₂O₂ concentration during the oxidative decolorization of RhB in alkali-activated H₂O₂ system.

Pollution of environmental endocrine disruptors has caused increasing concern globally. In the current study, a simple colorimetric method with high sensitivity and good selectivity for 17β-estradiol (E2) detection was developed, which employed gold nanoparticles (AuNPs) as colorimetric probe for specific recognition of shortening DNA aptamer. Visible color change from bright red to violet was observed for aggregation of AuNPs without the protection of DNA aptamer. After optimization, the method exhibited great performance for E2 detection with good linearity between E2 concentrations from 0.2 to 5 nM and the absorbance ratio at 620 and 523 nm, with the limit of detection of 0.1 nM. The method was also successfully applied to E2 determination in different spiked water samples including fishpond water, lake water, and tap water, in which good recoveries from 93.1 to 108.9% and acceptable relative standard deviations from 3.4 to 8.9% were obtained. This technique showed great potential for on-site fast determination of E2 in environmental water samples.

This study aims to investigate the photocatalytic degradation performance, mechanism, and dynamics of methyl orange (MO) which is a widely used organic dye in textile industries as well a hazardous wastewater pollutant. The degradation process was catalyzed by employing a synthesized Fe₃O₄ magnetic nanoparticle (NP) using the coprecipitation method. The structural and morphological properties of the synthesized Fe₃O₄ NPs were investigated by employing XRD, HR-SEM, and XPS, which proved that acquired Fe₃O₄ NPs were in a pure phase. Moreover, the crystallite sizes fall in the range of 28–31.8 nm and were estimated by applying the Scherrer equation on the XRD spectrum as well as calculated independently by applying a statistical approach on the SEM micrographs. The UV–Vis maximum in the visible range at 468.8 nm consists of two absorption frequency bands due to the effect of the hydrogen-bond interaction between water and the azo nitrogens in the MO. A non-monotonic spectral dynamic accompanied by peak wavelength shifts, as well as the absolute signal amplitude and signal area of the MO band, suggests that a cleavage of the azo bond is not the only and/or the dominant process in the photocatalytic oxidization of the MO in a protic solvent. The overall absorbance process is a complicated response to a combination of nonspecific and specific solute-solvent interactions, dipole-dipole interactions, hydrogen-bonding networks, and other possible intermolecular interactions such as hydrophobic/hydrophilic interactions. A bi-exponential decay was found to be the best fitting function to model the decay of the time-dependent electrical conductivity of the MO aqueous solution under photocatalytic oxidization. The Fe₃O₄ NPs exhibited a 98.3% removal of MO within 110 min. Photocatalytic degradation of methyl orange can be modeled to the first-order model with a rate constant k of 0.037 min⁻¹ taking into account the initial concentration of 1175 ppm of MO. The degradation/decolorization efficiency deduced from the low-frequency band of the visible spectra is around 99.4% after 110 min. The real-time degradation/decolorization efficiencies deduced from the overall absorbance maxima and the low-frequency band have a discrepancy of 50.1% at 20 min and 12.3% at 60 min representing the progressive attenuation of the H-bond impact dissociation of MO (degradation/decolorization).

In this study, we investigated the Sb(V) adsorption on ferrihydrite (Fh) at different pH values, in the presence and absence of common competing anions in soil such as phosphate (P(V)) and arsenate (As(V)). Batch adsorption experiments, carried out at pH 4.5, 6.0, and 7.0, showed a greater affinity of Fh towards P(V) and As(V) with respect to Sb(V), especially at higher pH values, while the opposite was true at acidic pH. The capacity of Fh to accumulate greater amounts of phosphate and arsenate in the 6.0–7.0 pH range was mainly linked to the different acid properties of P(V), As(V), and Sb(V) oxyanions. The Sb(V) adsorption on Fh was highly pH-dependent and followed the following order: pH 4.5 (0.957 mmol·g⁻¹ Fh) > pH 6.0 (0.701 mmol·g⁻¹ Fh) > pH 7.0 (0.583 mmol·g⁻¹ Fh). Desorption of antimonate from Sb(V)-saturated Fh, treated with citric and malic acid solutions, was ~equal to 55, 68, and 76% of that sorbed at pH 4.5, 6.0, and 7.0, respectively, while phosphate, arsenate, and sulfate were able to release significantly lower Sb(V) amounts. The FT-IR spectra revealed substantial absorbance shifts related to the surface hydroxyl groups of Fh, which were attributed to the formation of Fe-O-Sb(V) bonds and supported the formation of inner-sphere bonding between Sb(V) and Fh.

In this work, a sunlight-sensitive photocatalyst of nanocubic-like titanium dioxide (TiO₂) and N-doped graphene quantum dots (N-GQDs) is developed through a simple hydrothermal and physical mixing method. The successful amalgamation composite photocatalyst characteristics were comprehensively scrutinized through various physical and chemical analyses. A complete removal of bisphenol A (BPA) is attained by a synthesized composite after 30 min of sunlight irradiation as compared to pure TiO₂. This clearly proved the unique contribution of N-GQDs that enhanced the ability of light harvesting especially under visible light and near-infrared region. This superior characteristic enables it to maximize the absorbance in the entire solar spectrum. However, the increase of N-GQDs weight percentage has created massive oxygen vacancies that suppress the generation of active radicals. This resulted in a longer duration for a complete removal of BPA as compared to lower weight percentage of N-GQDs. Hence, this finding can offer a new insight in developing effective sunlight-sensitive photocatalysts for various complex organic pollutants degradation.