Effects of ionic strength on the chromophores of dissolved organic matter

Advanced oxidation process/coagulation coupled with membrane distillation (AOP/Coag-MD) for efficient ammonia recovery: Elucidating biofouling control performance and mechanism

The inadequate understanding of the biofouling formation mechanism and the absence of effective control have inhibited the commercial application of membrane distillation (MD). In this study, an advanced oxidation process (AOP)/coagulation-coupled (Coag) membrane distillation system was proposed and exhibited the potential for MD ammonia recovery (recovery rate: 94.1%). Extracellular polymeric substances (EPS) and soluble microbial products (SMP) components such as humic acid and tryptophan-like proteins were disrupted and degraded in the digestate. The curtailment and sterilizing efficiency of AOP on biofilm growth was also verified by optical coherence tomography (OCT) in situ real-time monitoring and confocal laser scanning microscopy (CLSM). Peroxymonosulfate (PMS) was activated to generate sulfate (SO4•-) and hydroxyl radicals (HO•), which altered the microbial community. After oxidative treatment, 16 S rRNA sequencing indicated that the dominant phylum of the microbial community evolved into Firmicutes. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis demonstrated that free radicals produced by PMS could disrupt cells' signaling molecules and interactions. In conjunction with these analyses, the mechanisms of response to free radical attack by Gram-negative bacteria, Gram-positive bacteria, and fungi were revealed. This research provided new insights into the field of membrane fouling control for membrane technology resource recovery processes, broadening the impact of AOP applications on microbiological response and fate in the environment.

Photochemical processes transform dissolved organic matter differently depending on its initial composition

Dissolved organic matter (DOM) is one of the most important fluxes in the global carbon cycle but its response to light exposure remains unclear at a molecular-level. The chemical response of DOM to light should vary with its molecular composition and environmental conditions while some basic hypotheses are still unclear, such as the balance between photobleaching and photo-humification and the question of oxidative properties. Here we exposed aquatic DOM from diverse freshwaters impacted by different levels of anthropogenic activity and algal exudates to environmentally-realistic light conditions. We found that photobleaching occurred in DOM with relatively high initial humic content producing low H/C molecules, whereas DOM with low initial humic content was humified. DOM pools with relatively high initial saturation and low aromaticity were prone to transform towards more unsaturated molecular formulae and high H/C molecules with a distinct decrease of bioavailability. Photo-transformation was mainly influenced by reactive intermediates, with reactive oxygen species (ROS) playing a dominant role in humification when the initial humus content of DOM was high. In contrast, for algal DOM with high protein content, it was likely that the autoxidation of excited state DOM was more important than indirect oxidation involving ROS. Our results reveal how photo-transformation patterns depend on the initial composition of DOM and provide new insights into the role of photochemical processes in biogeochemical cycling of DOM.

Spectroscopic insights into the binding characteristics of heavy metals to dissolved organic matter in landfill leachate

Landfill leachate is produced in the process of sanitary landfilling, which contains significant amounts of dissolved organic matter (DOM) and heavy metal contaminants. Insights into the interactions between heavy metals and DOM in landfill leachate are beneficial for the understanding of heavy metal fates and optimization of landfill leachate treatment. In this work, the coherent structural changes of landfill leachate DOM during binding with various heavy metals were explored through the integration of molecular spectroscopic methods with chemometrics and statistic correlation analyses. The results indicate that protein substances, phenolic and discrete carboxyl groups in landfill leachate DOM were involved in the complexation with heavy metals, resulting in the formation of conjugated macromolecules/aggregates with high aromaticity and molecular weight/size. The fluorescent protein-like, fulvic acid-like, and humic-like fractions in DOM were engaged in the interaction, which were closely related to phenolic-like and carboxylic-like structure. Compared to membrane concentrates DOM, raw leachate DOM exhibited a higher binding affinity to heavy metals (especially for Cu2+, whilst the weakest was Cd2+). The integrated approach provides useful information in elucidating the binding processes of metals with landfill leachate DOM, including site heterogeneity, binding strength and functional group sequences.

Multiple spectroscopic insights into the interaction mechanisms between proteins and humic acid

Proteins are important constituents of dissolved organic matter (DOM) in aqueous environments, and their interaction with humic acid (HA), another key component of DOM, substantially affects the environmental behaviors of DOM. In this work, the interaction mechanisms between tryptophan-containing proteins and HA were systematically investigated using multiple molecular spectroscopic approaches. The fluorescence quenching tests indicate that bovine serum albumin (BSA) was more readily quenched by HA and the coexisting phenolic, carboxyl, and quinone groups in HA contributed to this process significantly. By comparison, the fluorescence of L-tryptophan (L-Trp) was more stable under the same conditions. Furthermore, with multiple groups in HA, static quenching with the binding constants and the number of sites were calculated in the protein-HA and L-Trp-HA mixtures. In addition, the differential fluorescence spectra, UV‒Vis spectra, and two-dimensional correlation spectroscopy results confirmed that L-tryptophan amino acid could indeed form a complex with HA, while did not lead to fluorescence quenching. Finally, the molecular docking and density functional theory (DFT) simulations highlighted the contribution of multiple residues surrounding the HA groups to their interactions. The direct interaction between the tryptophan residue and HA might not be the prerequisite for the fluorescence response. Therefore, our work provides further insights into protein-HA interactions and implies other reasonable elucidations for further explanation.

Spatiotemporal optical properties of dissolved organic matter in a sluice-controlled coastal plain river with both salinity and trophic gradients

Due to the combined effect of sluices and sea tide, the sluice-controlled coastal plain river would be characterized by both trophic state and salinity gradients, affecting the spatiotemporal optical properties of dissolved organic matter (DOM). In this study, we investigated the spatiotemporal variation of water quality parameters and optical properties of DOM in the Haihe River, a representative sluice-controlled coastal plain river in Tianjin, China. A significant salinity gradient and four trophic states were observed in the water body of the Haihe River. Two humic- and one protein-like substances were identified from the DOM by the three-dimensional fluorescence spectra combined with the parallel factor (PARAFAC) analysis. Pearson's correlation analysis and redundancy analysis (RDA) showed that the salinity significantly affected the abundance of chromophoric DOM (CDOM) but did not cause significant changes in the fluorescence optical characteristics. In addition, the effect of Trophic state index (TSI) on the CDOM abundance was greater than that on the fluorescence intensity of fluorescent dissolved organic matter (FDOM). In the water body with both salinity and trophic state gradients, TSI posed a greater influence than salinity on the CDOM abundance. Our results fill the research gap in spatiotemporal DOM characteristics and water quality variation in water bodies with both salinity and trophic state gradients. These results are beneficial for clarifying the joint influence of saline intrusion and sluices on the DOM characteristics and water quality in sluice-controlled coastal plain rivers.

The photoactivity of complexation of DOM and copper in aquatic system: Implication on the photodegradation of TBBPA

The photoactivity of dissolved organic matter (DOM) has a great impact on the photodegradation of organic pollutants in natural waters. In this study, the photodegradation of TBBPA was investigated under simulated sunlight irradiation in the presence of copper ion (Cu²⁺), dissolved organic matter (DOM) and Cu-DOM complexation (Cu-DOM) to illustrate the effect of Cu²⁺ on photoactivity of DOM. The rate of photodegradation of TBBPA in the presence of Cu-DOM complex was 3.2 times higher than that in pure water. The effects of Cu²⁺, DOM and Cu-DOM on the photodegradation of TBBPA were highly pH dependent and hydroxyl radical(·OH) responded for the acceleration effect. Spectral and radical experiments indicated that Cu²⁺ had high affinity to fluorescence components of DOM, and acted as both the cation bridge and electron shuttle, resulting the aggregation of DOM and increasing of steady-state concentration of ·OH (·OH_ss). Simultaneously, Cu²⁺ also inhibited intramolecular energy transfer leading to the decrease of steady-state concentration singlet oxygen (¹O_2ss) and triplet of DOM (³DOM^⁎_ss). The interaction between Cu²⁺ and DOM followed the order of conjugated carbonyl CO, COO^- or CO stretching in phenolic groups and carbohydrate or alcoholic CO groups. With these results, a comprehensive investigation on the photodegradation of TBBPA in the presence of Cu-DOM was conducted, and the effect of Cu²⁺ on the photoactivity of DOM was illustrated. These findings helped to understanding the potential mechanism of interaction among metal cation, DOM and organic pollutants in sunlit surface water, especially for the DOM-induced photodegradation of organic pollutants.

Freshwater Salinization Syndrome Alters Retention and Release of 'Chemical Cocktails' along Flowpaths: from Stormwater Management to Urban Streams

We investigate impacts of Freshwater Salinization Syndrome (FSS) on mobilization of salts, nutrients, and metals in urban streams and stormwater BMPs by analyzing original data on concentrations and fluxes of salts, nutrients, and metals from 7 urban watersheds in the Mid-Atlantic U.S. and synthesizing literature data. We also explore future critical research needs through a survey of practitioners and scientists. Our original data show: (1) sharp pulses in concentrations of salt ions and metals in urban streams directly following both road salt events and stream restoration construction (e.g., similar to the way concentrations increase during other soil disturbance activities); (2) sharp declines in pH (acidification) in response to road salt applications due to mobilization of H+ from soil exchange sites by Na+; (3) sharp increases in organic matter from microbial and algal sources (based on fluorescence spectroscopy) in response to road salt applications likely due to lysing cells and/or changes in solubility; (4) significant retention (~30-40%) of Na+ in stormwater BMP sediments and floodplains in response to salinization; (5) increased ion exchange and mobilization of diverse salt ions (Na+, Ca2+, K+, Mg2+), nutrients (N, P), and trace metals (Cu, Sr) from stormwater BMPs and restored streams in response to FSS; (6) downstream increasing loads of Cl-, SO4 2-, Br-, F-, and I- along flowpaths through urban streams, and P release from urban stormwater BMPs in response to salinization, and (7) a significant annual reduction (> 50%) in Na+ concentrations in an urban stream when road salt applications were dramatically reduced, which suggests potential for ecosystem recovery. We compared our original results to published metrics of contaminant retention and release across a broad range of stormwater management BMPs from North America and Europe. Overall, urban streams and stormwater management BMPs consistently retain Na+ and Cl- but mobilize multiple contaminants based on salt types and salinity levels. Finally, we present our top 10 research questions regarding FSS impacts on urban streams and stormwater management BMPs. Reducing diverse 'chemical cocktails' of contaminants mobilized by freshwater salinization is now a priority for effectively and holistically restoring urban waters.

Challenges in quantifying and characterizing dissolved organic carbon: Sampling, isolation, storage, and analysis

Despite the advancements in analytical techniques, there are still great challenges and difficulties in accurately and effectively quantifying and characterizing dissolved organic carbon (DOC) in environmental samples. The objectives of this review paper are (a) to understand the roles and variability of DOC along the water continuum; (b) to identify the constraints, inconsistences, limitations, and artifacts in DOC characterization; and (c) to provide recommendations and remarks to improve the analytical accuracy. For the first objective, we summarize the four ecological and engineering roles of DOC along the water continuum from source water to municipal utility, including nutrients and energy sources, controlling the fates of micropollutants, buffering capacity, and treatability and precursors of disinfection byproducts. We also discuss three major challenges in DOC analysis, including spatial and temporal variations, degradability and stability, and unknown structures and formulas. For the second objective, we review the procedures and steps in DOC analysis, including sampling in diverse environmental matrices, isolation of DOC fraction, storage and preservation techniques, and analyses on bulk chemical characteristics. We list and discuss the available options and evaluate the advantages and disadvantages of each choice. Last, we provide recommendations and remarks for each stage: sampling, isolation, storage, and analysis.

An easy spectrophotometric acid-base titration protocol for dissolved organic matter

UV-vis spectrophotometric acid-base titration can characterize dissolved organic matter (DOM) acid-base properties. However, it requires incremental pH adjustment, which make the procedure time consuming and the results subjected to dilution effect. This study brings forth a new approach, referred as the “buffer method” for pH adjustments, by using carefully selected pH-buffers to adjust the pH. This, statistically validated method minimizes the pH adjustment time and lightens the laboratory work load. Chemical product cost associated with this novel method is slightly increased as compared to the previous approach, due to the necessity to use pH-buffers.

• Buffer method: Acid-base titration by using buffer for pH adjustment

• Buffer method validated by statistical means

• Rapid, reliable and economical method

The influence of pH on dissolved organic matter fluorescence in inland waters

Fluorescence is an easily available analytical technique used to assess the optical characteristics of dissolved organic matter (DOM). Despite widespread use, there has been some confusion about how robust fluorescence spectroscopy is to differences in solution pH. Here we assess fluorescence characteristics of three natural water samples and one commercially available standard (Nordic Reservoir) by modifying the pH across a range from 3.5 to 9.0 at 0.5 pH increments. We used two statistical approaches to assess if fluorescence intensity shifted significantly across this pH range. We identified that humic-like and protein-like fluorescence was largely stable within the pH range of 5.5 to 7.5, which represents 80% of Swedish lakes and streams. Likewise, we found that the three commonly used fluorescence indices were robust across the full pH range tested with the exception of the humification index, which had a narrower range of stability. The commerical humic substance sample was highly unstable with changes to pH in the regions of protein-like fluorescence being particularly sensitive. One of our conclusions is that differences in fluorescence intensity in the pH range of 5.5 to 7.5, typical for most inland waters, are generally minor. We recommend adjusting the pH when samples fall outside this region and to be especially careful in interpreting results from commercial humic substances.

Assessing inputs of aquaculture-derived nutrients to streams using dissolved organic matter fluorescence

Salmon aquaculture is an important economic activity globally where local freshwater supplies permit land-based salmon aquaculture facilities to cultivate early life stage salmon. Nitrogen, phosphorus and organic matter in aquaculture effluents contribute to the eutrophication of adjacent and downstream rivers and lakes. This study quantifies the enrichment of nutrients in land-based salmon aquaculture facility effluents compared to receiving waters. We measured nutrient concentrations and dissolved organic matter (DOM) quantity and quality via fluorescence spectroscopy in streams and effluent waters associated with 27 facilities in Chile. We found that facilities added on average 0.9 (s.d. = 2.0) mg-C L^-1, 542 (s.d. = 637) μg-total N L^-1, and 104 (s.d. = 104) μg-total P L^-1 to effluents compared to stream waters. DOM in stream water was enriched in humic-like fluorescence, while aquaculture effluents were enriched in protein-like DOM fluorophores. Principal component and correlation analysis revealed that tryptophan-like fluorescence was a good predictor of total N and P in effluents, but the strength of significant linear relationships varied among individual facilities (r²: 0.2 to 0.9). Agreement between laboratory fluorescence and a portable fluorometer indicates the utility of in-situ sensors for monitoring of both tryptophan-like fluorescence and covarying nutrients in effluents. Thus, continuous in-situ sensors are likely to improve industry management and allow more robust estimates of aquaculture-derived nutrients delivered to receiving waters.

Abatement technology of endocrine-disrupting chemicals (EDCs) by means of enhanced coagulation and ozonation for wastewater reuse

In this study, we have investigated the purification efficiency during enhanced coagulation, ozone oxidation and their combined processes for the removal of trace amounts of EDCs in different DOM matrices. The results indicated that the maximum removal efficiency of EDCs occurred at or near pH 7.0 when measured over a pH range of 4.0-10.0. The addition of natural colloids had a two-part influence. While the floc generated by polyaluminium chloride (PAC) significantly increased in size from 198.0 μm to 290.4 μm with a simultaneous improvement in the removal efficiency of EDCs, the floc size generated by polyferric sulfate (PFS) had no worthwhile change except for a slight decrement. The removal efficiency of EDCs and the decrease in spectral parameters including UVA254, UVA280 and humic-like fluorescence during ozonation processes with and without pre-coagulation were investigated. During the ozonation process, efficient elimination of target EDCs are achieved at low O₃ doses (O₃/dissolved organic carbon (DOC) < 0.2) in different water matrices. The pH-titration differential absorbance spectra technique further demonstrated that the high reactivity of O₃ to EDCs is owing to their phenolic moieties. In addition, when mgO₃/mgDOC ratio reaches to ~0.40, >90% of estrogenic activity was eliminated. In a nutshell, ozonation with pre-coagulation together leads to considerably higher abatement of EDCs and estradiol (E2) equivalent values (EEQ) at the same ozone dosage than ozonation only process for wastewater treatment.

Toward a microscopic model of light absorbing dissolved organic compounds in aqueous environments: theoretical and experimental study

Water systems often contain complex macromolecular systems that absorb light. In marine environments, these light absorbing components are often at the air-water interface and can participate in the chemistry of the atmosphere in ways that are poorly understood. Understanding the photochemistry and photophysics of these systems represents a major challenge since their composition and structures are not unique. In this study, we present a successful microscopic model of this light absorbing macromolecular species termed "marine derived chromophoric dissolved organic matter" or "m-CDOM" in water. The approach taken involves molecular dynamics simulations in the ground state using on the fly Density Functional Tight-Binding (DFTB) electronic structure theory; Time Dependent DFTB (TD-DFTB) calculations of excited states, and experimental measurements of the optical absorption spectra in aqueous solution. The theoretical hydrated model shows key features seen in the experimental data for a collected m-CDOM sample. As will be discussed, insights from the model are: (i) the low-energy A-band (at 410 nm) is due to the carbon chains combined with the diol- and the oxy-groups present in the structure; (ii) the weak B-band (at 320-360 nm) appears due to the contribution of the ionized speciated form of m-CDOM; and (iii) the higher-energy C-band (at 280 nm) is due to the two fused ring system. Thus, this is a two-speciated formed model. Although a relatively simple system, these calculations represent an important step in understanding light absorbing compounds found in nature and the search for other microscopic models of related materials remains of major interest.

Staged assessment for the involving mechanism of humic acid on enhancing water decontamination using H2O2-Fe(III) process

Humic acid (HA) as a natural coordinating agent was employed to modify the Fenton-like process by promoting the redox cycle of Fe(III)/Fe(II) and enhancing the pH tolerance. However, the roles of coordinating stages of HA-Fe(III) and the dynamic changes of iron species remain unclear. In this study, HA was introduced into the H₂O₂-Fe(III) process to investigate the accelerating roles of coordinating stages and systematically reveal the mechanism via the reactive oxygen species (ROS) identification, HA-Fe(III)/Fe(II) redox cycles tracking, electrochemical and kinetic analysis. Results suggested that two reaction stages were separated concerning the enhancement for HA in H₂O₂-Fe(III) process, including coordinating stage (slow rate) and promoting the redox stage (fast rate). HA-Fe(III) was identified as the major contributor, along with hydroxyl radical (·OH) and superoxide radical (·O₂^-) as the dominant ROS with formation rates calculated as 7.0 × 10^-9 and 2.1 × 10^-3 M s^-1 via the steady-state model. Based on the density-functional theory (DFT) calculations and HPLC-MS/MS analysis, three degradation pathways of 2,4-Dichlorophenol were proposed with ten intermediate products identified, and the ecotoxicity was evaluated through Ecological Structure Activity Relationships (ECOSAR) program. This study unveiled the mechanism of HA on enhancing water decontamination via H₂O₂-Fe(III) process in stages.

Insight into metal binding properties of biochar-derived DOM using EEM-PARAFAC and differential absorption spectra combined with two-dimensional correlation spectroscopy

A large amount of biochar-derived dissolved organic matter (BDOM) will be released into the environment with biochars application into repairing soil/water, which may alter the fate and transport of contaminants. In this study, four DOM samples were extracted from cauliflower root biochar (CRBC), reed straw biochar (RSBC), corn stalks biochar (CSBC), and potato stalk biochar (PSBC). Excitation-emission matrix combined with parallel factor (EEM-PARAFAC) analysis, differential absorbance spectra (DAS), and two-dimensional correlation spectroscopy (2D-COS) analysis were applied to explore the complexation property of BDOM with metals. DAS showed sites heterogeneity within the DOM pool for metals complexing. Humic-like and fulvic-like substances were main fluorescent components identified by EEM-PARAFAC. 2D-COS analysis revealed that polysaccharides and aliphatic firstly responded to Pb(II) binding with CRBC-derived DOM and three other biochar-derived DOM, respectively. While aliphatic groups, aromatic N=O, and polysaccharides gave the fastest response to Cu(II) binding with CRBC, RSBC, and the other two biochar-derived DOM, respectively.

Comparison of the properties of standard soil and aquatic fulvic and humic acids based on the data of differential absorbance and fluorescence spectroscopy

This study compared effects of pH, ionic strength and complexation with Mg²⁺ on the chromophores and fluorophores of aquatic and terrestrial NOM exemplified by the standard isolates Suwannee River fulvic and humic acid (SRFA and SRHA) and Pahokee Peat fulvic and humic acids (PPFA and PPHA) provided by the International Humic Substance Society (IHSS). The intensity of the differential spectra of the NOM isolates increased monotonically with pH. These spectra comprised contributions of similar chromophore systems associated with the carboxylic and phenolic moieties. The intensity of SRFA and PPFA fluorescence changed non-monotonically vs. pH indicating that the deprotonation of the phenolic fluorophores decreased their emission yields. Examination of the effects of pH on the slopes of the log-transformed absorbance of NOM showed that the influence of deprotonation on the conformations of PPFA and PPHA molecules was less prominent than those for SRFA but not dissimilar to those of SRHA. Changes of the differential spectra and spectral slopes showed that Mg²⁺/PPFA and Mg²⁺/PPHA complexation was more effected by electrostatic interactions while the involvement of phenolic groups was notable for SRFA. The observed trends highlight similarities and differences in the properties of the chromophores and fluorophores in the standard isolates of soil and aquatic NOM. These results necessitate further systematic comparison of the properties of NOM isolates and those of unaltered NOM.

Effects of fulvic acids on the electrochemical reactions and mass transfer properties of organic cation toluidine blue: Results of measurements by the method of rotating ring-disc electrode

This study examined effects of aquatic and soil natural organic matter (NOM) exemplified by standard Suwannee River fulvic acid (SRFA) and Pahokee Peat fulvic acid (PPFA), respectively, on the electrochemical (EC) reactivity and mass transfer properties of the cationic organic probe toluidine blue (TB) that forms complexes with NOM. EC measurements that were carried out using the method of rotating ring-disc electrode (RRDE) showed that for disc potentials below -0.4 V vs. the standard Ag/AgCl reference electrode, TB molecules undergo EC reduction accompanied by the formation of EC-active products that undergo oxidation at the ring electrode. EC reactions of TB in the range of potentials -0.2 to -0.4 V were determined to involve free TB⁺ cations and TB species adsorbed on the electrode surface. The EC reduction of TB species at the disc potentials < -0.4 V was controlled by the mass transfer of the free TB⁺ cations and TB/NOM complexes to the electrode surface. Formation of TB/NOM complexes caused the mass transfer-controlled TB currents to undergo a consistent decrease. The observed changes were correlated with the extent of TB/NOM complexation and decreases of the diffusion coefficients of TB/NOM complexes that have higher molecular weights (MW) than the free cations. Properties of the intermediates formed upon the reduction of TB⁺ cations were also affected by NOM. These results demonstrate that RRDE measurements of EC reactions of TB or possibly other EC active probes allow probing the complexation of EC-active organic species with NOM and mass transfer properties of NOM complexes and ultimately NOM itself.

Investigation of the deprotonation of tetracycline using differential absorbance spectra: A comparative experimental and DFT/TD-DFT study

Tetracycline is a type of broad-spectrum, naturally occurring antibiotic that leads to several side effects, such as affecting intestinal flora and increase in bacterial resistance. The affinity of tetracycline for abiotic and biotic surfaces and metal ions is closely related to its deprotonation state and charge distribution; however, its deprotonation-protonation property remains unclear. In this study, the hydrolysis of tetracycline was investigated by combining experiments with quantum-chemical calculations. The molecular structure of the probable deprotonation states were optimized by quantum-chemical calculations, and the corresponding absorbance spectra were predicted based on frontier molecular orbital (FMO) theory. The absorbance spectra showed structure-specific features at the different deprotonation states. In addition, changes in tetracycline absorbance spectra in the pH range of 2.00-12.00 was examined by spectroscopic titration. The deprotonation was found to proceed in the order of site 3, 4, 12, and 10, which was identified by comparing the quantum-chemical calculations and experimental differential absorbance spectra (DAS). The results in this study are of great significance for further studies of the transport and fate of tetracycline in the environment.

Spectroscopic response of soil organic matter in mining area to Pb/Cd heavy metal interaction: A mirror of coherent structural variation

Understanding the interaction between heavy metals and soil organic matter (SOM) in mining area is important for the clarification of the environmental behaviors of heavy metals. In this work, the coherence of structural changes of SOM during interaction with Pb²⁺ and Cd²⁺ ions were examined by using UV-vis/fluorescence spectroscopy coupled with correlation analyses. The result showed that phenolic- and carboxylic-like groups of SOM were engaged in the complexation of heavy metals (Pb²⁺ and Cd²⁺) with SOM, resulting in the formation of highly conjugated macromolecules/aggregates and an increase in molecular weight/size. Fluorescent humic-like, fulvic-like, and protein-like species were involved in the binding with Pb²⁺/Cd²⁺ ions, which were closely correlated with phenolic-like and carboxylic-like constitutes. SOM was more favorable to bind with Pb²⁺ ions than Cd²⁺ ions, with a less susceptive of SOM structure to Pb²⁺/Cd²⁺ ions in the mining area compared to those off the mining area under heavy metal stress. These results may provide a new insight for the treatment and remediation of heavy metal-polluted soil in mining area.

Investigation of eluted characteristics of fulvic acids using differential spectroscopy combined with Gaussian deconvolution and spectral indices

The characteristics of subfractions of soil fulvic acid (FA₃, FA₅, FA₇, FA₉, and FA₁₃) using stepwise elution from XAD-8 resin with pyrophosphate buffers were investigated by differential absorption spectroscopy (DAS) and differential fluorescence spectroscopy (DFS) combined with mathematical deconvolution and spectral indices. The log-transformed absorbance spectra (LTAS) exhibited three regions for both acidic-buffer-eluted subfractions (AESF) and neutral-buffer-eluted subfraction (NESF) and four regions for basic-buffer-eluted subfractions (BESF) according to the differences in spectral slopes. The DAS spectra of FA subfractions were closely fitted with seven Gaussian bands with maxima location at 199.66, 216.18 ± 1.50, 246.20 ± 3.85, 285.22 ± 7.26, 345.64 ± 5.30, 389.27, and 307.37 nm, respectively (R² > 0.993). The content of salicylic-like and carboxyl groups in FA subfractions decreased, while the phenolic chromophore increased with elution sequence. From the 11 spectral indices, AESF had greater molecular weight, condensation, polymerization, hydroxyl radical production, humification degree, and terrigenous contribution, as well as contained more conjugated aromatic structures and less N-containing groups than NESF and BESF. The humification degree and humic characters of FA subfractions were closely associated to the aromaticity, molecular condensation, and DOM-metal-bound functional groups. The proper separation of FA into subfractions is beneficial for reducing its complexity and heterogeneity, which helps us to further explore its chemical properties and interactions with various contaminants in soil environments. Graphical abstract.

Tracking metal ion-induced organic membrane fouling in nanofiltration by adopting spectroscopic methods: Observations and predictions

Natural organic matter (NOM) with the size approaching to membrane pore size is commonly considered as the crucial component leading to severe pore blocking and superfluous energy consumption. Aquatic metal ions coexisting with this NOM constituent (target NOM) exert a significant influence on membrane filtration performance; however, little work elucidated their interactions and the impacts on nanofiltration (NF). Therefore, we systematically investigated this issue by titrating three environmentally-relevant metal ions (Al³⁺, Fe³⁺ and Cu²⁺) into the target NOM sample obtained by pre-filtering using NF membrane. Fast spectrophotometric techniques were employed to observe the interactive performance. Results suggested that all metal ions at their critical concentrations caused severe flux decline; Cu²⁺ at a very low concentration of 5 μM, Al³⁺ and Fe³⁺ at 20 μM. NF performance recovered when the concentrations were beyond their critical values, and was improved at excessive concentration when flocs formed. Relationship between spectroscopic characteristics and NF performance was particularly addressed. UV-vis spectrum can be expected to be useful and predictive in membrane fouling control when Al³⁺ or Fe³⁺ presented. However, fluorescence fingerprint was not likely that effective since fluorescence intensity continuously reduced with the increasing metal ion concentration, attributed to their quenching effect on NOM fluorophores.

Monitoring the kinetics of reactions between natural organic matter and Al(III) ions using differential absorbance spectra

This study examined the kinetics of the binding of Al(III) ions by natural organic matter (NOM) exemplified by Suwannee River humic acid (SRHA). This processes was studied for a 5-8 pH range and environmentally relevant concentrations of the system components. Al(III)-NOM interactions were quantified using differential absorbance spectra whose intensity and shape depended on pH and reaction time. In all cases the differential spectra had four bands with maxima located at 245, 275, 320, 380 nm. These bands were assigned to the engagement of the carboxylic-like and/or phenolic-like groups, as well as electrostatic gel in NOM. Several parameters of the absorbance spectra (e.g., spectral slopes of log-transformed spectra in wavelength range 260-270 and 350-400 nm, ΔS_260-270 and ΔS_350-400 respectively) were linearly correlated (R² = 0.98) with concentrations of carboxylic-like groups and total NOM-bound Al(III) ions predicted based on the NICA-Donnan model. The binding of Al(III) ion by NOM at all pHs was modeled assuming the presence of three kinetically distinct sites. This study demonstrates that differential absorbance spectroscopy can be used to quantify the kinetics and mechanisms of NOM-metal ions interactions and monitor them in practically important system including water treatment operations.

Combined effects of dissolved organic matter, pH, ionic strength and halides on photodegradation of oxytetracycline in simulated estuarine waters

Estuarine waters of variable compositions are sinks for many micropollutants. The varying water properties can impact the photodegradation of organic pollutants. In this study, the combined effects of dissolved organic matter (DOM), pH, ionic strength, and halides on the photodegradation of the model organic pollutant oxytetracycline (OTC) were investigated. Suwannee River natural organic matter (SRNOM) was used as a representative DOM. The results showed that the observed photolysis rate constant (kobs) of OTC increased rapidly upon increase of pH. SRNOM induced a 11.0-17.9% decrease of the kobs for OTC. In the presence of SRNOM, the ionic strength and specific halide effects promote OTC photodegradation with a 39.2-84.2% and 7.1-28.8% increase of the kobs, respectively. The effects of SRNOM, ionic strength and halides on OTC photodegradation are pH-dependent. Direct photolysis half-lives (t1/2) of OTC were estimated in view of the more important role of direct photolysis compared to indirect photolysis. The estimated t1/2 values decreased from 187.4-206.6 d to 34.4-36.6 d as the pH increases in the Yellow River estuarine region. The results of this research demonstrate that the photodegradation rate of OTC increases rapidly in the gradient from river water to marine water in estuarine regions.

Dissolved organic matter binding with Pb(II) as characterized by differential spectra and 2D UV-FTIR heterospectral correlation analysis

Dissolved organic matter (DOM) in aquatic environment significantly influences the behavior and fate of heavy metals via binding, complexation and thus changes the metal speciation; however detailed interfacial processes and mechanisms are still unclear. Here, differential absorbance and fluorescence spectra and two dimensional UV-FTIR heterospectral correlation analysis were applied to probe into the Pb(II)-DOM interaction at a wide range of pH and ionic strength (IS). The absorbance of DOM molecules under all conditions increased with metal addition, while the different extents of absorbance variations along the wavelength range in the differential zero-order and log-transformed absorbance spectra indicated the site heterogeneity within the DOM pool for metal binding. Spectral parameters, namely differential fluorescent components 1 and 2 (DFC1 and DFC2) and differential slopes of log-transformed absorbance in the range of wavelength 350-400 nm (DSlope_350-400) were found to be highly correlated with the total amounts of DOM-bound Pb(II) predicted by the NICA-Donnan model, while the differential absorbance spectra at 235 nm (DA₂₃₅) was related to the extent of Pb(II) bound by carboxylic groups. Thus, these parameters are an indicator or proxy for the in situ Pb(II)-DOM interaction extent. Aryl C-H gave the fastest response to Pb(II) binding at lower pH and IS (e.g., pH 4.7 and IS = 0.01 M), followed by carboxyl C=O and polysaccharide C-OH and then chromophoric groups at 265 nm (CDOM₂₆₅). However, the CDOM₂₆₅ bound to Pb(II) prior to aryl C-H and polysaccharide C-OH groups at higher pH and IS (6.0 and 0.1 M, respectively), showing that the binding sequences were highly dependent on solution chemistry. Differential spectra combined with two dimensional UV-FTIR heterospectral correlation analysis can be used as a promising approach to elucidate metal-DOM interaction processes, including site heterogeneity, binding sensitivity and sequence at the functional group level.

Contrasting effects of photochemical and microbial degradation on Cu(II) binding with fluorescent DOM from different origins

Effects of photochemical and microbial degradation on variations in composition and molecular-size of dissolved organic matter (DOM) from different sources (algal and soil) and the subsequent influence on Cu(II) binding were investigated using UV-Vis, fluorescence excitation-emission matrices coupled with parallel factor analysis, flow field-flow fractionation (FlFFF), and metal titration. The degradation processes resulted in an initial rapid decline in the bulk dissolved organic carbon and chromophoric and fluorescent DOM components, followed by a small or little decrease. Specifically, photochemical reaction decreased the aromaticity, humification and apparent molecular weights of all DOM samples, whereas a reverse trend was observed during microbial degradation. The FlFFF fractograms revealed that coagulation of both protein- and humic-like DOM induced an increase in molecular weights for algal-DOM, while the molecular weight enhancement for allochthonous soil samples was mainly attributed to the self-assembly of humic-like components. The Cu(II) binding capacity of algal-derived humic-like and fulvic-like DOM consistently increased during photo- and bio-degradation, while the soil-derived DOM exhibited a slight decline in Cu(II) binding capacity during photo-degradation but a substantial increase during microbial degradation, indicating source- and degradation-dependent metal binding heterogeneities. Pearson correlation analysis demonstrated that the Cu(II) binding potential was mostly related with aromaticity and molecular size for allochthonous soil-derived DOM, but was regulated by both DOM properties and specific degradation processes for autochthonous algal-derived DOM. This study highlighted the coupling role of inherent DOM properties and external environmental processes in regulating metal binding, and provided new insights into metal-DOM interactions and the behavior and fate of DOM-bound metals in aquatic environments.

Storage effects on quantity and composition of dissolved organic carbon and nitrogen of lake water, leaf leachate and peat soil water

This study aimed to evaluate the effects of freezing and cold storage at 4 °C on bulk dissolved organic carbon (DOC) and nitrogen (DON) concentration and SEC fractions determined with size exclusion chromatography (SEC), as well as on spectral properties of dissolved organic matter (DOM) analyzed with fluorescence spectroscopy. In order to account for differences in DOM composition and source we analyzed storage effects for three different sample types, including a lake water sample representing freshwater DOM, a leaf litter leachate of Phragmites australis representing a terrestrial, 'fresh' DOM source and peatland porewater samples. According to our findings one week of cold storage can bias DOC and DON determination. Overall, the determination of DOC and DON concentration with SEC analysis for all three sample types were little susceptible to alterations due to freezing. The findings derived for the sampling locations investigated here may not apply for other sampling locations and/or sample types. However, DOC size fractions and DON concentration of formerly frozen samples should be interpreted with caution when sample concentrations are high. Alteration of some optical properties (HIX and SUVA₂₅₄) due to freezing were evident, and therefore we recommend immediate analysis of samples for spectral analysis.

Interactions between stepwise-eluted sub-fractions of fulvic acids and protons revealed by fluorescence titration combined with EEM-PARAFAC

In aquatic environments, pH can control environmental behaviors of fulvic acid (FA) via regulating hydrolysis of functional groups. Sub-fractions of FA, eluted using pyrophosphate buffers with initial pHs of 3.0 (FA₃), 5.0 (FA₅), 7.0 (FA₇), 9.0 (FA₉) and 13.0 (FA₁₃), were used to explore interactions between the various, operationally defined, FA fractions and protons, by use of EEM-PARAFAC analysis. Splitting of peaks (FA₃ and FA₁₃), merging of peaks (FA₇), disappearance of peaks (FA₉ and FA₁₃), and red/blue-shifting of peaks were observed during fluorescence titration. Fulvic-like components were identified from FA₃-FA₁₃, and protein-like components were observed in fractions FA₉ and FA₁₃. There primary compounds (carboxylic-like, phenolic-like, and protein-like chromophores) in PARAFAC components were distinguished based on acid-base properties. Dissociation constants (pK_a) for fulvic-like components with proton ranged from 2.43 to 4.13 in an acidic pH and from 9.95 to 11.27 at basic pH. These results might be due to protonation of di-carboxylate and phenolic functional groups. At basic pH, pK_a values of protein-like components (9.77-10.13) were similar to those of amino acids. However, at acidic pH, pK_a values of protein-like components, which ranged from 3.33 to 4.22, were 1-2units greater than those of amino acids. Results presented here, will benefit understanding of environmental behaviors of FA, as well as interactions of FA with environmental contaminants.

The One-Sample PARAFAC Approach Reveals Molecular Size Distributions of Fluorescent Components in Dissolved Organic Matter

Molecular size plays an important role in dissolved organic matter (DOM) biogeochemistry, but its relationship with the fluorescent fraction of DOM (FDOM) remains poorly resolved. Here high-performance size exclusion chromatography (HPSEC) was coupled to fluorescence emission-excitation (EEM) spectroscopy in full spectral (60 emission and 34 excitation wavelengths) and chromatographic resolution (<1 Hz), to enable the mathematical decomposition of fluorescence on an individual sample basis by parallel factor analysis (PARAFAC). The approach allowed cross-system comparisons of molecular size distributions for individual fluorescence components obtained from independent data sets. Spectra extracted from allochthonous DOM were highly similar. Allochthonous and autochthonous DOM shared some spectra, but included unique components. In agreement with the supramolecular assembly hypothesis, molecular size distributions of the fluorescence fractions were broad and chromatographically unresolved, possibly representing reoccurring fluorophores forming noncovalently bound assemblies of varying molecular size. Samples shared underlying fluorescence components that differed in their size distributions but not their spectral properties. Thus, in contrast to absorption measurements, bulk fluorescence is unlikely to reliably indicate the average molecular size of DOM. The one-sample approach enables robust and independent cross-site comparisons without large-scale sampling efforts and introduces new analytical opportunities for elucidating the origins and biogeochemical properties of FDOM.

Critical role of natural organic matter in photodegradation of methylmercury in water: Molecular weight and interactive effects with other environmental factors

Photodegradation is the main depletion pathway of methylmercury (MeHg) in surface water. However, the underlying mechanism of MeHg photodegradation is still not well understood. In this study, the critical role of natural organic matter (NOM) from Suwannee River natural organic matter of the International Humic Substance Society, especially its molecular weight, and the impacts of other related environmental factors in MeHg photodegradation were investigated. We observed that MeHg cannot photo-degrade in de-ionized water, excluding the direct photodegradation of MeHg. While either NOM or Fe³⁺ alone induced MeHg photodegradation, co-existing NOM significantly inhibited the Fe³⁺-induced degradation, highlighting the critical and complex role of NOM in MeHg photodegradation. Additionally, MeHg exhibited different photodegradation rates in the presence of molecular weight fractionated natural organic matter (M_f-NOM). More importantly, high concentration of NOM caused light attenuation significantly inhibited the photodegradation of MeHg, which was more significant for high molecular weight M_f-NOM. In the presence of M_f-NOM, MeHg photodegradation was also affected by light quality, pH and co-existing Cl^- and NO₃^-. The study is helpful for a better understanding of the critical role of NOM and other environmental factors on MeHg photodegradation in surface water.

Examination of effects of Cu(II) and Cr(III) on Al(III) binding by dissolved organic matter using absorbance spectroscopy

Effects of Cu(II) and Cr(III) ions on the binding of Al(III) onto Dissolved Organic Matter (DOM) exemplified by Suwannee River Humic Acid (SRHA) at pH 6.0 were quantified in this study using linear and log-transformed SRHA absorbance spectra acquired at varying Al(3+) concentrations and Cu(2+) or Cr(3+) levels. The competition between Al(3+) and Cu(2+)/Cr(3+) for the binding sites in DOM was ascertained by examining the intensity and shapes of the metal-specific differential spectra of DOM. The results indicated that the binding of Al(3+) onto SRHA is little influenced in the cases of in presence of 1.0 and 10.0 μM background Cr(3+) and in presence of 1.0 μM background Cu(2+), but it is significantly depressed in presence of 10.0 μM Cu(2+). Changes of the spectral slope of the log-transformed absorbance spectra in the 350-400 nm wavelength range (S350-400) were unambiguously correlated with the total amount of DOM-bound metals. The concentrations of Me-DOM complexes were determined using the NICA-Donnan Model. The results demonstrate that differential absorbance measurements provide quantitatively interpretable information concerning the nature and mechanisms of metal-DOM interactions and effects of metal cations competition on these processes.

Use of log-transformed absorbance spectra for online monitoring of the reactivity of natural organic matter

This study examined the significance of water quality monitoring parameters obtained via logarithmic transformation of the absorbance spectra of raw and treated drinking water. The data were generated using samples of the influent, settled and filtered water acquired weekly over a six months period at two full scale treatment plants. Examination of the weekly plant samples combined with the data of laboratory fractionation and chlorination experiments showed that the slopes of the log-transformed spectra are correlated with typically reported water quality parameters (e.g., its specific absorbance at 254 nm, SUVA254); yet the determination of spectral slopes is considerably simpler and potentially information-rich. The spectral slopes determined for the range of wavelength 280-350 nm were shown to be correlated with the yields of trihalomethanes (THMs) and haloacetic acids (HAAs). These results support the notion that multi-wavelength monitoring of the absorbance spectra of drinking water and their interpretation via logarithmic transformation constitutes a promising practically implementable approach for online water quality monitoring.