Structural response of humic acid upon binding with lead: A spectroscopic insight

Variations in fluorescence properties of humic acids from calcareous soils amended with different swine manures in a long-term soil experiment

Molecular properties of soil humic acid (HA) can play an important role in the mechanisms regulating plant nutrient availability. This study explores how the structure of HA is altered by long-term treatment with different forms of swine manure and how these changes may influence nutrient availability. Liquid swine manure (LSM), solid swine manure (SSM), and swine manure compost (SMC) were applied to a calcareous soil over 17 years in a long-term soil fertility study. HA was extracted from site soil samples and analyzed using fluorescence spectroscopic techniques, including a Cu2+ quenching experiment, in order to assess differences in the structure and functionality of the soil organic matter (SOM) resulting from these different treatments. Emission spectra of the SSM-HA and SMC-HA are similar, while the LSM-HA is distinct. Procedures such as parallel factor analysis (PARAFAC) decomposition of emission-excitation matrices showed that structures in the SSM-HA and SMC-HA samples have lower complexity, whereas the structures of LSM-HA are of higher complexity. Interactions with Cu2+ at different pH levels indicate that the LSM-HA shows more dynamic conformational changes as well as stronger interactions and higher quenching efficiency compared to the other treatments. Conversely, SMC-HA demonstrates relatively stable binding constant (Ka) values across different pH levels. The binding constants and quenching efficiency of SSM-HA are significantly affected by changes in pH. This study shows distinct structural characteristics of HA formed under different manure management systems and provides valuable insights into how these variations may impact nutrient dynamics in soils.

Insight into the binding characteristics of dissolved organic matter(DOM)and Fe(Ⅱ)/Mn(Ⅱ): Based on the spectroscopic and dialysis equilibrium analysis

Dissolved organic matter (DOM) plays an important role in metal migration and transformation within inland surface waters. In our study, spectroscopic and dialysis equilibrium analysis were combined to characterize the binding properties between DOM and Fe(II)/Mn(II). Four different type of DOM including two commercial DOM: humic acid、fulvic acid, and two natural dissolved organic matter collected from macrophyte-dominant region (MDR) and algae-dominated region (ADR) of Taihu Lake. Steady state/time resolved fluorescence spectroscopy indicated that the fluorescence intensity of DOM was quenched by Fe(II)/Mn(II) through a static quenching process. The adsorption isotherm shows that the adsorption capacity of DOM from Taihu Lake for metal ions is significantly higher than that of commercial humic acid. Simultaneously, the combination of MDR and Fe(II) has the highest adsorption capacity at 110.950 mg/g among all combinations. Furthermore, the Pseudo-second-order kinetic model and Elovich model were found to be superior in describing the adsorption process, with chemical adsorption controlling the rate of the adsorption reaction. The results of this study show that potentially toxic elements (PETs) pollution in eutrophic shallow lakes may become more serious due to the excessive expansion of algae dominant regions and the reduction of macrophyte dominant regions. In addition, risk analysis and assessment of PETs should consider the contribution of metal binding capabilities.

The binding effects and mechanisms of dissolved organic matter (DOM) on the fate of mercury in sludge anaerobic digestion combined with thermal hydrolysis

Dissolved organic matter (DOM) plays an important role in regulating the fate of mercury (Hg), e.g., mobility, bioavailability, and toxicity. Clarifying the role of DOM in binding Hg in the treatment processes of sewage sludge is important for relieving Hg contamination risks in land applications. However, the impacts of DOM on Hg binding in sewage sludge are still unclear. In this study, we investigated the evolution of Hg and its speciation in full-scale sludge anaerobic digestion (AD) with thermal hydrolysis. The role of DOM in binding Hg(II) was further analyzed. The results showed that AD with thermal hydrolysis led to an increase in the Hg content in the sludge (from 3.72 ± 0.47 mg/kg to 10.75 ± 0.16 mg/kg) but a decrease in Hg mobility (the mercury sulfide fraction increased from 60.56 % to 79.78 %). Further adsorption experiments revealed that at equivalent DOM concentrations, DOM with a low molecular weight (MW<1 kDa) in activated sludge, DOM with a medium molecular weight (1 kDa 5 kDa) in both anaerobically digested sludge and conditioned sludge showed high binding amounts of Hg(II), with 1372.54, 535.28, 942.09 and 801.51 mg Hg/g DOM, respectively. Parallel factor analysis (PARAFAC) and fluorescence quotient (FQ) results showed that tryptophan-like and tyrosine-like substances had high binding affinities for Hg(II). Furthermore, X-ray photoelectron spectroscopy (XPS) indicated that the reduced organic sulfur contained in the DOM was potentially bound to Hg through the interactions of Hg-S and Hg-O. These results indicated that DOM may play special roles in regulating Hg speciation. The association between DOM and Hg(II), such as the significant positive correlation (p < 0.05) between the dissolution rate of Hg(II) and release of tryptophan-like substances during thermal hydrolysis, suggested the potential way for removing Hg from sludge.

Association of tetrabromobisphenol A (TBBPA) with micro/nano-plastics: A review of recent findings on ecotoxicological and health impacts

Despite the diverse research into the environmental impact of plastics, several stones have yet to be unraveled in terms of their ecotoxicological potential. Moreover, their detrimental impacts have become terrifying in recent years as the understanding of their tendency to associate and form cohorts with other emerging contaminants grew. Despite the hypothesis that microplastics may potentially adsorb organic pollutants, sequestering and making them not bioavailable for enhanced toxicity, evidence with pollutants such as Tetrabromobisphenol A (TBBPA) defers this assertion. TBBPA, one of the most widely used brominated flame retardants, has been enlisted as an emerging contaminant of serious environmental and human health concerns. Being also an additive to plasticware, it is not far to suspect that TBBPA could be found in association with micro/nanoplastics in our environment. Several pieces of evidence from recent studies have confirmed the micro/nanoplastics-TBBPA association and have exposed their compounded detrimental impacts on the environment and human health. This study, therefore, presents a comprehensive and up-to-date review of recent findings regarding their occurrence, factors that foster their association, including their sorption kinetics and isotherms, and their impacts on aquatic/agroecosystem and human health. The way forward and prospects for future studies were presented. This research is believed to be of significant interest to the readership due to its relevance to current environmental challenges posed by plastics and TBBPA. The study not only contributes valuable insights into the specific interaction between micro/nanoplastics and TBBPA but also suggests the way forward and prospects for future studies in this field.

Nanoscale Insights into the Interaction Mechanism Underlying the Adsorption and Retention of Heavy Metal Ions by Humic Acid

The mobility and distribution of heavy metal ions (HMs) in aquatic environments are significantly influenced by humic acid (HA), which is ubiquitous. A quantitative understanding of the interaction mechanism underlying the adsorption and retention of HMs by HA is of vital significance but remains elusive. Herein, the interaction mechanism between HA and different types of HMs (i.e., Cd(II), Pb(II), arsenate, and chromate) was quantitatively investigated at the nanoscale. Based on quartz crystal microbalance with dissipation tests, the adsorption capacities of Pb(II), Cd(II), As(V), and Cr(VI) ionic species on the HA surface were measured as ∼0.40, ∼0.25, ∼0.12, and ∼0.02 nmol cm-2, respectively. Atomic force microscopy force results showed that the presence of Pb(II)/Cd(II) cations suppressed the electrostatic double-layer repulsion during the approach of two HA surfaces and the adhesion energy during separation was considerably enhanced from ∼2.18 to ∼5.05/∼4.18 mJ m-2. Such strong adhesion stems from the synergistic metal-HA complexation and cation-π interaction, as evidenced by spectroscopic analysis and theoretical simulation. In contrast, As(V)/Cr(VI) oxo-anions could form only weak hydrogen bonds with HA, resulting in similar adhesion energies for HA-HA (∼2.18 mJ m-2) and HA-As(V)/Cr(VI)-HA systems (∼2.26/∼1.96 mJ m-2). This work provides nanoscale insights into quantitative HM-HA interactions, improving the understanding of HMs biogeochemical cycling.

Unveiling changes in the complexation of dissolved organic matter with Pb(II) by photochemical and microbial degradation using fluorescence EEMs-PARAFAC

Dissolved organic matter (DOM) is very important in determining the speciation, behaviors, and risk of metal pollutants in aquatic ecosystems. Photochemical and microbial degradation are key processes in the cycling of DOM, yet their effects on the DOM-Pb(II) interaction remain largely unknown. This was studied by examining the complexation of river DOM with Pb(II) after degradation, using fluorescence quenching titration and excitation-emission matrices-parallel factor analysis (EEMs-PARAFAC). Three humic-like and two protein-like components were identified, with strong removals of humic-like components and decreasing average molecular weight and humification degree of DOM by photo- and photo-microbial degradation. The changes in humic-like abundance and structure resulted in notable weakening of their interaction with Pb(II). The tryptophan-like C2 was also mainly removed by photo-degradation, while the tyrosine-like C3 could be either removed or accumulated. The Pb(II)-binding of protein-like components was generally weaker but was enhanced in some degradation groups, which might be related to the lowering competition from humic-like components. The binding parameters correlated significantly with the DOM indices, which were dominated by photo-degradation for humic-like components but by seasonal variations for the tyrosine-like component. These results have implications for understanding the key mechanisms underlying the variability of the DOM-metal interaction in aquatic environments.

Effect of microplastics on the binding properties of Pb(ii) onto dissolved organic matter: insights from fluorescence spectra and FTIR combined with two-dimensional correlation spectroscopy

Heavy metal cations are a typical type of inorganic pollutant that has persistent distribution characteristics in aquatic environments and are easily adsorbed on carriers, posing serious threats to ecological safety and human health. Some studies have shown that the coexistence of dissolved organic matter (DOM) and microplastics (MPs) promotes the adsorption of heavy metal cations, but the mechanism of promoting the adsorption process has not been thoroughly studied. In this study, the effect of polystyrene microplastics (PSMPs) on the binding properties of Pb2+ onto humic acid (HA) in aquatic environments was investigated by spectral analysis and two-dimensional correlation (2D-COS) analysis. When PSMPs co-existed with HA, the adsorption capacity of Pb2+ increased. On the one hand, Pb2+ is directly adsorbed on HA through the mechanism of complexation reaction, ion exchange and electrostatic interaction. On the other hand, Pb2+ is first adsorbed on PSMPs by electrostatic action and indirectly adsorbed on HA in the form of PSMPs-Pb2+ owing to the interaction between HA and PSMPs, which increases the adsorption amount of Pb2+ on HA. This study is significant for studying the migration and regression of heavy metal cation contaminants when PSMPs co-exist with DOM in an aqueous environment.

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.

The changes in the physicochemical properties of calcareous soils and the factors of arsenic (As) uptake by wheat were investigated after the cessation of effluent irrigation for nearly 20 years

It is not known what the buffering capacity of soils and arsenic (As) enrichment by crops is for calcareous agricultural soils after the end of long-term effluent irrigation. In this study, changes in soil physicochemical properties and factors of influencing As uptake by wheat were investigated in agricultural soils where sewage irrigation had been ceased for nearly 20 years. The results showed that the content of CaCO₃ and pH in soil increased compared to the period before the cessation of sewage irrigation, but remained below the soil background value. Furthermore, CaCO₃ is by far the main buffering substance in agricultural soils and indirectly contributes to the increase in pH. The As concentration in the soil was 36.4 ± 34.8 mg/kg, which was 0.56-10.28 times and 0.28-5.18 times higher than the soil background and risk screening values, respectively, but showed a decreasing trend. pH and Fe dissolution were the main reasons for the lower As concentration in the soil. Total As in soil was a better predictor of As in wheat, and soil electrical conductivity (EC) and soil organic matter (SOM) promoted As uptake by wheat. The competitive uptake of As by dissolved Si was an important reason for the mismatch between As concentrations in soil and wheat. This study highlighted the key issues of As transport transformation in soil-wheat systems after cessation of effluent irrigation, using agricultural soils, and provided a reference for soil risk management in agricultural soils in mining areas.

Separation of the overlapped humic acid and BGP characteristic peaks using two-dimensional correlation fluorescence-UV-vis spectroscopy

Fluorescence spectroscopy has been widely used to detect polycyclic aromatic hydrocarbons (PAHs) in the environment. However, the interference of coexisting humic acids (HA) in the environment poses a great challenge to the qualitative and quantitative detection of PAHs using fluorescence spectroscopy. In this study, the spectral properties of benzo [ghi] perylene (BGP) and HA were investigated based on fluorescence and UV-vis spectroscopy combined with two-dimensional (2D) correlation analysis. Under the external disturbance of HA concentration, the homo-2D (fluorescence, UV-visible) correlation and hetero-2D fluorescence-UV-visible correlation spectral characteristics of the mixed samples of HA and BGP were studied, and the effect of HA on the fluorescence of BGP was investigated. It can be inferred that the fluorescence peak at 478 nm come from BGP, and the fluorescence peaks at 442 nm and 533 nm, UV absorption peak at 233 nm come from HA. Meanwhile, asynchronous two-trace two-dimensional (2T2D) fluorescence correlation slice spectra at 533 nm were obtained. The slice spectral intensity at 478 nm was extracted to quantify the BGP concentration in mixture. The results showed that the slice spectral intensity and BGP concentration had a good linear relationship with the coefficient of determination R² = 0.96. This research provides a way to further study the separation method of HA and PAHs or explore the correction method of the effect of HA on PAHs.

Insight into the interaction between trimethoprim and soluble microbial products produced from biological wastewater treatment processes

Soluble microbial products (SMPs), dissolved organic matter excreted by activated sludge, can interact with antibiotics in wastewater and natural water bodies. Interactions between SMPs and antibiotics can influence antibiotic migration, transformation, and toxicity but the mechanisms involved in such interactions are not fully understood. In this study, integrated spectroscopy approaches were used to investigate the mechanisms involved in interactions between SMPs and a representative antibiotic, trimethoprim (TMP), which has a low biodegradation rate and has been detected in wastewater. The results of liquid chromatography-organic carbon detection-organic nitrogen detection indicated that the SMPs used in the study contained 15% biopolymers and 28% humic-like substances (based on the total dissolved organic carbon concentration) so would have contained sites that could interact with TMP. A linear relationship of fluorescent intensities of tryptophan protein-like substances in SMP was observed (R²>0.99), indicating that the fluorescence enhancement between SMP and TMP occurred. Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy indicated that carboxyl, carbonyl, and hydroxyl groups were the main functional groups involved in the interactions. The electrostatic and π-π interactions were discovered by the UV-vis spectra and ¹H nuclear magnetic resonance spectra. Structural representations of the interactions between representative SMP subcomponents and TMP were calculated using density functional theory, and the results confirmed the conclusions drawn from the ¹H nuclear magnetic resonance spectra. The results help characterize SMP-TMP complexes and will help understand antibiotic transformations in wastewater treatment plants and aquatic environments.

Interactions between lead(II) ions and dissolved organic matter derived from organic fertilizers incubated in the field

This work was to study composition characteristics and the subsequent effect on the lead (Pb) binding properties of dissolved organic matter (DOM) derived from seaweed-based (SWOF) and chicken manure organic fertilizers (CMOF) during a one-year field incubation experiment using the excitation-emission matrix-parallel factor (EEM-PARAFAC) and two-dimensional correlation spectroscopy (2DCOS) analysis. Results showed that high aromatic and hydrophobic fluorescent substances were enriched in CMOF-derived DOM and SWOF-derived DOM and enhanced over time. And phenolic groups in the fulvic-like substances for SWOF-derived DOM and carboxyl groups in the humic-like substances for CMOF-derived DOM had the fastest responses over time, respectively. Moreover, both non-fluorescent polysaccharides and fluorescent humic-like substances or fulvic-like substances with aromatic (C=C) groups first participated in the binding process of Pb to SWOF-derived DOM on day 0 and 180 during the lead binding process. In contrast, humic-like substances associated with aromatic (C=C) and phenolic groups gave a faster response to Pb binding on day 360. Regarding CMOF-derived DOM, the fulvic-like substances associated with aromatic (C=C) and carboxylic groups displayed a faster response to Pb ions on day 0. Nonetheless, polysaccharides and humic-like associated with phenolic groups had a faster response on days 180 and 360. It is noteworthy that the polysaccharides, which participated in Pb binding to CMOF-derived DOM, posed a higher risk of Pb in the environment after 360 days. Therefore, these findings gave new insights into the long-term applications of commercial organic fertilizers for the amendment of soil.

Critical evaluation of the interaction between fluorescent dissolved organic matter and Pb(II) under variable environmental conditions

Dissolved organic matter (DOM) can strongly influence the behavior and risk of metal pollutants in aquatic ecosystems. However, a comprehensive study on the effects of DOM level and environmental factors on the binding of DOM with Pb(II) is lacking. This study examined the DOM-Pb(II) interaction in the river water under variable DOM level, pH, and major ions, using fluorescence excitation-emission matrices-parallel factor analysis (EEMs-PARAFAC). Four humic-like and one protein-like component were identified, and the abundant humic-like components showed higher Pb(II)-binding fractions (f) than the protein-like component. The f of PARAFAC components decreased while the conditional stability constants (logK_M) increased for the diluted DOM, indicating the influence of DOM level on its metal binding. The DOM-Pb(II) interaction was sensitive to changes in pH, with generally higher f and lower logK_M at the alkaline condition due to changes in the DOM conformation. The addition of major ions significantly decreased the fluorescence quenching by Pb(II), due to competitive effects and potential DOM conformation changes at elevated ions. Overall, our results show that the DOM-Pb(II) complexation is highly dependent on both the DOM properties and environmental factors, which have implications for optimizing the experimental conditions and for comparing the results in different environments.

Comparing the Pb(II) binding with different fluorescent components of dissolved organic matter from typical sources

Dissolved organic matter (DOM) is important for determining the speciation, environmental behavior, and effects of metal pollutants in aquatic environments. However, little is known about the difference between DOM from natural and anthropogenic sources for binding Pb(II). This study examined the Pb(II) binding with DOM from four typical sources including river, leaf litter leachate, and the influent and effluent of a wastewater treatment plant, using fluorescence quenching titration and excitation-emission matrices-parallel factor analysis (EEMs-PARAFAC). Four humic-like and one protein-like fluorescent components were identified, with much higher protein-like fraction and lower humification degree for the influent than for other sources. In the river water and leaf litter leachate, the abundant humic-like components were quenched by 6-17% while the protein-like component kept stable (2-4%) by the addition of Pb(II). In contrast, the influent DOM showed stronger fluorescence quenching of the protein-like component (46%) with higher conditional stability constant and binding fraction of fluorophore than the humic-like components (15-21%). The effluent DOM displayed weak quenching for all fluorescent components (4-6%) and thus weak complexation with Pb(II), indicating notable changes in the chemical composition and metal-binding affinity of DOM by wastewater treatments. These results demonstrated significant impacts of DOM source and chemical composition on its Pb(II) complexation properties, which have implications for understanding the interactions between DOM and heavy metals.

Insight into the effects of biological treatment on the binding properties of copper onto dissolved organic matter derived from coking wastewater

Biological treatment can remove more than 89.8% of total organic carbon (TOC) and 94.4% of fluorescent dissolved organic matter (DOM) in the coking wastewater, thereby affecting the migration, transformation and bioavailability and binding characteristics of heavy metals (HMs). The results of parallel factor analysis (PARAFAC) show that protein-like materials accounted for 97.53% in the coking wastewater DOM, a large number of humic-like substances are produced and accounted for more than 55.40% after biological treatment. A new spectral data processing method, the 1/n-th power transformation after two-dimensional correlated spectroscopy (2D-COS) in combination with synchronous fluorescence spectra (SFS), can identify small features obscured by strong peaks, and reveal more binding sites as well as preserve the sequential order information. The result indicates that the preferential bonding of Cu(II) is at 306 nm (protein-like) for coking wastewater DOM, and at 514 nm (humic-like) for effluent DOM. The C-O group of esters and alcohols can preferentially complexate with Cu(II) in the coking wastewater and effluent DOM. The log K_M values of PARAFAC components with Cu(II) are in the range of 3.59-5.06 for coking wastewater DOM, and in the range of 4.80-5.64 for the effluent DOM. Log K_M values for protein-like materials with Cu(II) are higher than these for fulvic- and humic-like substances. Humic-like substances can form more stable complexes with Cu(II) in the effluent DOM. Biological treatment increases the chemical stability of DOM-Cu(II) complexes, thereby further reducing the environmental risk of Cu(II).

Multi-spectroscopic investigation of the molecular weight distribution and copper binding ability of dissolved organic matter in Dongping Lake, China

The properties and metal-binding abilities of dissolved organic matter (DOM) rely on its molecular weight (MW) structure. In this study, the spatial differences of DOM in compositions, MW structures, and binding mechanisms with copper (Cu²⁺) in Dongping Lake were investigated by applying excitation-emission matrix combining parallel factor analysis (EEM-PARAFAC), synchronous fluorescence (SF) spectra, two-dimensional correlation spectra (2D-COS), and Fourier transform infrared (FTIR) spectra. The EDOM for the entrance of the Dawen River and PDOM for the macrophyte-dominated region were divided from DOM of Dongping Lake based on hierarchical clustering analysis (HCA) and principal component analysis (PCA) and were size-fractioned into MW < 500 kDa and <100 kDa fractions. According to EEM-PARAFAC, Dongping Lake was dominated by tryptophan-like substances with MW < 500 kDa. The concentration of PDOM was higher than that of EDOM (p < 0.05). 2D-COS showed that protein-like components preceded humic-like components binding to Cu²⁺ regardless of sample type (215 nm > 285 nm > 310-360 nm). The Cu²⁺ binding capacity of DOM exhibited specific differences in space, components, and molecular weights. The humic-like component 1 (C1) and tryptophan-like component 4 (C4) of PDOM showed stronger binding abilities than those of EDOM. Endogenous tryptophan-like component 4 (C4) had a higher binding affinity for Cu²⁺ than humic-like components (logK_a: C4 > C1 > C2) in PDOM irrespective of MW. Humic-like components with MW < 500 kDa displayed higher binding potentials for Cu²⁺. FTIR spectra showed that the main participants of DOM-Cu complexation included aromatic hydrocarbons, aliphatic groups, amide Ⅰ bands, and carboxyl functional groups. This study provides spatial-scale insights into the molecular weight structure of DOM in influencing the behavior, fate, and bioavailability of heavy metals in lakes.

FT-IR and synchronous fluorescence two-dimensional correlation spectroscopic analysis on the binding properties of mercury onto humic acids as influenced by pH modification and sulfide addition

Mercuric Hg²⁺ ion forms strong complexes with dissolved organic matter (DOM) in natural waters. The complexation of Hg²⁺ by sulfhydryl groups of DOM was regarded as the main mechanism for Hg²⁺-DOM interactions, particularly in anoxic sulfur and DOM-rich environments. In the present study, the influences of pH and sulfide addition on the molecular structure of Hg²⁺-DOM complexes and the characteristics of Hg²⁺ binding to DOM were investigated using FT-IR and synchronous fluorescence two-dimensional correlation spectroscopic analysis. Results showed that, during the Hg²⁺ binding process, the aromatic hydrogen CH in humic acids (HA) gave the fastest responses to pH perturbation and the S-reacted HA (S-HA) exhibited different reaction patterns from the unreacted HA. In S-HA, the esters/alcohols CO and carboxyl CO gave the fastest responses to Hg²⁺ binding. In the process of S-HA binding to Hg²⁺, the protein-like fractions including proteins, amino acids or monoaromatics played the leading role. Sulfide addition of HA enhanced the reactivity of small molecular weight compounds with low aromaticity and improved the binding ability of protein-like fractions to Hg²⁺. These findings provide a better understanding of the interaction mechanisms between Hg²⁺ and DOM at a molecular level and have important environmental implications in Hg²⁺ biogeochemical transformation, transport and cycling.

Effect of biochar-derived DOM on the interaction between Cu(II) and biochar prepared at different pyrolysis temperatures

The structure and composition of biochar-derived dissolved organic matter (DOM) at different pyrolysis temperatures differed significantly, affecting the environmental geochemical behavior of heavy metals (HMs). Herein, the binding properties of Cu(II) onto walnut-shell DOM were investigated using spectroscopic methods. The results showed that the DOM at low pyrolysis temperatures (300 °C and 500 °C) showed higher Cu(II) affinity than that at high pyrolysis temperature (700 °C). There was a preferential Cu(II) binding with fulvic-like substances (360 nm) at 300 °C, and with protein-like materials (275 nm) at 500 °C and 700 °C. The C-O group of alcohols, ethers, and esters showed preferential binding with Cu(II) at 300 °C and 700 °C pyrolysis temperatures. However, preferential bonding of Cu(II) to the C-O stretching vibration and O-H bending vibration of carboxyl was exhibited at 500 °C pyrolysis temperature. Pyrolysis temperature played a crucial role in the release of biochar-derived DOM and in the migration and bioavailability of HMs. Meanwhile, the adsorption effect of Cu(II) increased by 11.2% for biochar at 300 °C, and decreased by 15.0% and 61.1% for biochar at 500 °C and 700 °C, respectively, after the removal of DOM, suggesting that the presence of DOM influenced the adsorption behavior of biochar towards Cu(II).

Effects of photo-irradiation on mercury binding to dissolved organic matter: Insights from FT-IR and synchronous fluorescence two-dimensional correlation spectroscopy

The binding interactions between Hg and dissolved organic matter (DOM) affect the fate and transport of Hg in the aquatic environment. Here, we investigate the effects of photo-irradiation on the binding characteristics of dissolved organic matter with Hg(II) using FT-IR and synchronous fluorescence two-dimensional correlation spectroscopy (2D-COS). Results showed that the binding of Hg(II) onto humic acid (HA) followed the order of humic-like fraction > fulvic-like fraction > protein-like fraction and photo-irradiation did not affect this order. The binding affinity of each site within the fluorescent fraction was affected by the photoreaction patterns. Pre-irradiation of HA before Hg(II) binding changed its structures and binding ability. UV irradiation showed a more obvious effect on Hg(II)-HA complexes than solar irradiation, and UV irradiation enhanced the reactivity of aromatic groups of HA. The amine or amide N-H of HA played a leading role in binding with Hg(II) in the dark, whereas the aromatic amine C-N became dominant after UV irradiation. In fulvic acid (FA), the aromatic hydrogen C-H played a leading role in Hg(II) binding in the dark, but solar irradiation promoted the binding ability of polysaccharide C-O and the carboxyl CO became dominant after UV irradiation. The response sensitivity of the fulvic-like fraction to Hg(II) was higher than that of the protein-like fraction in FA. Multiple types of sites binding to Hg(II) were verified in the fulvic-like fraction and protein-like fraction of FA. These findings provide new insight into photo-induced structural changes of DOM upon Hg binding.

Applying synchronous fluorescence spectroscopy conjunct second derivative and two-dimensional correlation to analyze the interactions of copper (II) with dissolved organic matter from an urbanized river

Heavy metal speciation and distribution is significantly influenced by dissolved organic matter (DOM) exhibited in ecosystems, particularly in urbanized rivers. Synchronous fluorescence spectroscopy (SFS) conjunct second derivative and two-dimensional correlation spectroscopy (2D-COS) was devoted to characterizing interactions of DOM-copper (II). Three typical water samples were collected from Baitapu River. Only protein-like fluorescence (PLF) and fulvic-like (FLF) were identified from the SFS. Stability constant (log K) values of PLF complexes with copper (II) varied from 4.277 to 5.833, and proportion of binding fluorescent materials (f) were 0.054-2.640. The log K values of FLF complexes with copper (II) varied from 3.996 to 4.243, while the f values were 0.001-0.036. Obviously, PLF had much stronger complexing capacity than FLF. There were four obvious peaks in the principal component analysis and second derivative fluorescence spectroscopy (SDFS), i.e., tyrosine-like (TYLF), tryptophan-like (TRLF), microbial humus-like (MHLF) and FLF. The log K values of TYLF and TRLF complexes were 4.899-5.907 and 4.598-5.831, respectively, which were similar to those from PLF. The log K values of MHLF complexes varied from 4.311 to 5.760, and the f values were 0.261-8.688. The log K values of FLF complexes were ranged from 4.598 to 5.831, which were higher than those deduced from the SFS. Interestingly, by the SDSF, PLF was divided into TYLF and TRLF, which increased the parameters values from DOM-copper (II) complexes. 2D-SFS-COS revealed that the TRLF was more susceptive response to copper (II) appended than TYLF, MHLF, and FLF. Moreover, TYLF and TRLF could priorly interact with copper (II). The SDSF conjunct 2D-COS could be effective approaches for insight into the complexing heterogeneity of DOM with copper (II). The study could present a support to preventing heavy metals and organic pollution in urbanized rivers.

Adsorption behaviour and interaction of organic micropollutants with nano and microplastics - A review

Nano/microplastics (NPs/MPs) and organic micropollutants are contaminants exerting serious threats to aquatic ecosystems, which are further aggravated through their interactions. Organic micropollutants can adsorb on the surface of NPs/MPs, enter to the digestive systems of aquatic organisms with NPs/MPs, and desorb from the surface inside the organism. Consequently, the migration behaviour of organic micropollutants is significantly affected increasing their risk to accumulate in the food chain. Therefore, understanding the adsorption interactions between NPs/MPs and organic micropollutants is critical for evaluating the fate and impact of NPs/MPs in the environment. This review article provides an overview about the role of NPs/MPs as (temporary) sinks for organic micropollutants but also as primary sources of organic micropollutants through the leaching of plastic additives. Specifically, the following aspects are discussed: adsorption/desorption mechanisms (e.g., hydrophobic partitioning interaction, surface adsorption by van der Waals forces or hydrogen bonding, and pore filling), influencing environmental factors (e.g., pH, salinity, and dissolved organic matter), leaching of plastic additives from NPs/MPs, and potential ecotoxicological effects arising from the interactions of NPs/MPs and organic micropollutants.

Determination of lead ions in fish and oyster samples using a nano-adsorbent of functionalized magnetic graphene oxide nanosheets-humic acid and the flame atomic absorption technique

This study aims at the functionalization of magnetic graphene oxide nanosheets and the binding of humic acid as a lead complex ligand. Graphene oxide nanosheets possess a large surface area and various carboxylic acid groups which can be activated easily by activating agents. Therefore, they are suitable to be used for the extraction of heavy metals. To have a better process of extracting lead ions, magnetic graphene oxide was used in this research. Humic acid, as a lead metal complex agent, has an amine functional group which can be bound to modified graphene oxide from one side. The process of constructing the nano-adsorbent proposed for the preconcentration of lead ions as well as its characterization was studied by infrared spectroscopy (IR), ultraviolet-visible spectroscopy (UV-visible), field emission scanning electron microscopy (FESEM), and vibrating sample magnetometry (VSM). The designed nano-adsorbent was tested to measure lead ions in simulated and real samples of sea water, fish, and oysters. The detection limit obtained in the simulated samples was 0.07 μg/L, and the linear range was 0.2-12 μg/L. The apparatus used to measure the ions was a flame atomic absorption device. In the analysis of the real samples, the values obtained through flame atomic absorption were compared with those obtained through furnace atomic absorption. The proposed technique is advantageous due to being cheap, precise, and sensitive for the trace measurement of lead ions.

Characterization and biogeochemical implications of dissolved organic matter in aquatic environments

Dissolved organic matter (DOM) is viewed as one of the most chemically active organic substances on earth. It plays vital roles in the fate, bioavailability and toxicity of aquatic exogenous chemical species (e.g., heavy metals, organic pollutants, and nanomaterials). The characteristics of DOM such low concentrations, salt interference and complexity in aquatic environments and limitations of pretreatment for sample preparation and application of characterization techniques severely limit understanding of its nature and environmental roles. This review provides a characterization continuum of aquatic DOM, and demonstrate its biogeochemical implications, enabling in-depth insight into its nature and environmental roles. A synthesis of the effective DOM pretreatment strategies, comprising extraction and fractionation methods, and characterization techniques is presented. Additionally, the biogeochemical dynamics of aquatic DOM and its environmental implications are discussed. The findings indicate the collection of representative DOM samples from water as the first and critical step for characterizing its properties, dynamics, and environmental implications. However, various pretreatment procedures may alter DOM composition and structure, producing highly variable recoveries and even influencing its subsequent characterization. Therefore, complimentary use of various characterization techniques is highly recommended to obtain as much information on DOM as possible, as each characterization technique exhibits various advantages and limitations. Moreover, DOM could markedly change the physical and chemical properties of exogenous chemical species, influencing their transformation and mobility, and finally altering their potential bioavailability and toxicity. Several research gaps to be addressed include the impact of pretreatment on the composition and structure of aquatic DOM, molecular-level structural elucidation for DOM, and assessment of the effects of DOM dynamics on the fate, bioavailability and toxicity of exogenous chemical species.

Effects on the complexation of heavy metals onto biochar-derived WEOM extracted from low-temperature pyrolysis

Biochar-derived water-extractable organic matter (WEOM) was obtained under low-temperature pyrolysis (300 °C) using corncob as raw material. WEOM may affect the mobility and bioavailability of soil heavy metals (HMs) through complexation when biochar was used for soil HM remediation. Herein, the characteristics of complexation between HMs (Cr(III) and Cu(II)) and biochar-derived WEOM were investigated by using spectroscopic techniques in conjunction with parallel factor (PARAFAC) analysis and two-dimensional correlation spectroscopy (2D-COS). Six components were identified by PARAFAC modeling, in which protein-, fulvic- and humic-like components accounted for 48.86%, 25.63% and 25.51%, respectively. A nonlinear model was employed to determine the conditional stability constant (K_M) and total ligand concentration (C_L) of WEOM-HM complexes. The log K_M values were in the range of 4.02-5.04 for WEOM-Cr(III) and 4.04-6.58 for WEOM-Cu(II). The 2D-COS in conjunction with log-transformed synchronous fluorescence spectroscopy (SFS) suggested that WEOM components were preferentially complexed with HMs in the following order: 433/270, 433/335, 496/270, 496/335, 370/335, 433/402, 496/402, 335/290, 402/290 for Cr(III), and 290/280, 390/280, 433/280, 496/280, 433/335, 496/335, 390/335, 433/420, 496/402, 335/290, 316/290 for Cu(II). The results of 2D-FTIR-COS suggested a preferential bonding of Cr(III) to the C-N group of alkyl, and Cu(II) to the CO group of alcohols, ethers and esters. Meanwhile, the CO group of ethers and the CN group of alkyl indicated preferential susceptibilities for the addition of Cr(III) and Cu(II) at different concentrations. In addition, protein-like components had remarkably higher total ligand concentration (C_L) than fulvic- or humic-like components.

A meta-analysis about the accumulation of heavy metals uptake by Sedum alfredii and Sedum plumbizincicola in contaminated soil

Sedum alfredii and Sedum plumbizincicola typically have high heavy metal (such as Zn and Cd) accumulation capacities with fast growth rates and relatively high Pb tolerance in contaminated soils. We compared the accumulation characteristics of heavy metals in Sedum species through meta-analysis. Furthermore, we analyzed the effects of soil organic matter (SOM) and soil pH on Cd, Pb and Zn accumulation by S. alfredii and S. plumbizincicola and the correlation between various metals. Results showed that the accumulations of Cd and Zn in shoots were higher than that of roots, but Pb accumulated in roots more than shoots. Moreover, there is a significant positive correlation between the accumulation of Zn and Cd in shoots. We found that the heavy metal accumulation rate in shoots was higher with lower soil pH. Sedum species had the highest Cd adsorption capacity in 20-30 g/kg SOM and the highest Zn adsorption capacity in SOM less than 20 g/kg. The accumulation rate of Cd in shoots of S. plumbizincicola was increased with exposure time, while the accumulation rate of Zn was slightly decreased.

Lead release and species transformation of commercial minium pigments in aqueous phase under UV-irradiation

This study reports the lead ions release and species transformation of minium pigment under UV-irradiation in aqueous phase. The effects of fulvic acid (FA) and pH on lead ions release were investigated. Lower pH and higher FA concentration facilitate the release of lead ions. During photoreaction, electron donors (FA) are provided to scavenge photogenerated holes and enhance the charge separation, leading to the increased lead ions release. When significant amounts of FA exist in the water, they will complex with the released lead ions preferentially. Then, lead ions react with CO₂ and proton resulting in the formation of insoluble Pb₃(CO₃)₂(OH)₂ after FA consumed. Correspondingly, the dissolved lead ion concentration in water showed a trend of increasing first and then decreasing. Based upon characterization and experimental results, lead ions release and lead species transformation mechanisms of minium were eventually speculated. This research is helpful for better understanding the environmental behavior and risk of semiconductor pigments in natural water.

Applying synchronous fluorescence and UV-vis spectra combined with two-dimensional correlation to characterize structural composition of DOM from urban black and stinky rivers

UV-visible spectroscopy and synchronous fluorescence spectroscopy (SFS) combined with two-dimensional correlation spectroscopy (2D-COS) were applied for extracting fluorescence components, tracing organic functional groups, and revealing variations of dissolved organic matter (DOM) in Puhe River. Water samples were collected from the mainstream and two tributaries (Nanxiaohe River and Huangnihe River). DOM in three rivers was composed of protein-like fluorescent (PLF), microbial humus-like fluorescent (MHLF), fulvic-like fluorescent (FLF), and humic-like fluorescent components, which were relative to aromatic groups, phenolic groups, carboxylic groups, and microbial products. The PLF and MHLF were dominated in DOM fractions in the rivers, and the average content of the PLF was the highest in Nanxiaohe River. Humification degree of DOM was the highest in Puhe River, followed by Huangnihe River and Nanxiaohe River. However, molecular mass of DOM in Puhe River was the lowest, followed by Huangnihe River and Nanxiaohe River. Based on the 2D-COS of the SFS and UV-visible spectra, the variation order of DOM fractions in Puhe River was PLF → MHLF → FLF, and the PLF was consistent with the phenolic groups, aromatic groups, and carboxylic groups, but the adverse trend with the microbial products. The variation order in Nanxiaohe River was MHLF → PLF → FLF, and the MHLF was consistent with the aromatic groups, phenolic groups, carboxylic groups, and microbial products. The variation order in Huangnihe River was MHLF → PLF → FLF too, and the PLF was consistent with the carboxylic groups and aromatic groups. The results of the present study demonstrate that UV-visible spectroscopy and SFS combined with 2D-COS are useful methods to characterize structural composition of DOM from urban black and stinky rivers so as to investigate their pollution status.

Wood vinegar enhances humic acid-based remediation material to solidify Pb(II) for metal-contaminated soil

Heavy metal lead is a typical widespread potentially toxic element (PET) contamination due to their extensive and wide applications in industrial processes. The development of cost-effective methods for preventing potentially toxic element lead residues from soil into food is thus highly desirable. A new type of humic acid-based remediation material (HA/WV) incorporating humic acid salt (HA), biochar powder (BC), and wood vinegar (WV), which is a cheap and environmentally friendly industrial by-product from charcoal processing, was prepared and evaluated. The results showed that 0.10 g remediation material HA/WV with a mass ratio of 1:1 was added to 1 kg surface soil of 0-20 cm from agricultural land contaminated by 300 mg Pb²⁺, the reduction ratio of available Pb in soil can reach 61.4%. Especially, wood vinegar can enhance the reduction ratio of available Pb by at least 14.7% over without wood vinegar. Furthermore, according to the analysis of adsorption interaction and the electrostatic attraction between Pb(II) and oxygen-containing functional groups on HA/WV are the dominant mechanisms responsible for Pb(II) sorption. The wood vinegar liquid can improve the oxygen-containing group in HA/WV, which can enhance the complexation of remediation materials and Pb(II) ion.

Advances in the characterization and monitoring of natural organic matter using spectroscopic approaches

Natural organic matter (NOM) is ubiquitous in environment and plays a fundamental role in the geochemical cycling of elements. It is involved in a wide range of environmental processes and can significantly affect the environmental fates of exogenous contaminants. Understanding the properties and environmental behaviors of NOM is critical to advance water treatment technologies and environmental remediation strategies. NOM is composed of characteristic light-absorbing/emitting functional groups, which are the "identification card" of NOM and susceptive to ambient physiochemical changes. These groups and their variations can be captured through optical sensing. Therefore, spectroscopic techniques are elegant tools to track the sources, features, and environmental behaviors of NOM. In this work, the most recent advances in molecular spectroscopic techniques, including UV-Vis, fluorescence, infrared, and Raman spectroscopy, for the characterization, measurement, and monitoring of NOM are reviewed, and the state-of-the-art innovations are highlighted. Furthermore, the limitations of current spectroscopic approaches for the exploration of NOM-related environmental processesand how these weaknesses/drawbacks can be addressed are explored. Finally, suggestions and directions are proposed to advance the development of spectroscopic methods in analyzing and elucidating the properties and behaviors of NOM in natural and engineered environments.

Adsorption of ciprofloxacin to functionalized nano-sized polystyrene plastic: Kinetics, thermochemistry and toxicity

Plastic debris is ubiquitous in aquatic systems and has been proven vehicles for the transport of various pollutants including trace organic compounds. Nanoplastics have large specific surface area and hydrophobic characteristics and therefore are capable of adsorbing other organic or inorganic chemicals from the environment. Antibiotics, as another class of emerging contaminants, have raised significant research concern in recent years as they pose threats to the ecosytems and human health. Nevertheless, little information is available on the adsorption behaviors of antibiotics onto nano-sized plastics. The toxicity of combined nanoplastics and antibiotics is also largely unknown. In this study, the physicochemical and thermodynamic interactions between representative nanoplastics, which containing a carboxyl functional group of polystyrene nanoplastics (PS-COOH), and typical antibiotic, i.e., ciprofloxacin (CIP) were investigated in a batch adsorption experiment. The specific thermodynamic correlation function of PS-COOH combined with CIP was obtained through isothermal titration microcalorimetry (ITC) analysis. The adsorption kinetics and isotherm of CIP on PS-COOH closely fit the pseudo-second-order kinetic model (r² = 0.99) and Freundlich isotherm (r² = 0.99). The ITC results showed that the adsorption reaction of PS-COOH with CIP was a spontaneous exothermic reaction. The adsorption of antibiotics on nanoplastics may aggravate the negative impacts of these two pollutants on aqueous ecosystems, and we hypothesized that would be reflected in the survival rate of model organism of Caenorhabditis elegans when exposed to this combination. This work used a mechanistic approach to unravel the adsorption behavior of antibiotics on nanoplastics and shed light on their potential impact on aquatic ecosystems.

Strong sorption of two fungicides onto biodegradable microplastics with emphasis on the negligible role of environmental factors

Microplastics have attracted much attention in recent years because they are able to interact with other pollutants including pesticides, with implications for the potential risks to biota. However, the sorption behavior of pesticides on microplastics, especially on biodegradable microplastics which are promising alternatives to conventional polymers, has been insufficiently studied. In this study, triadimefon and difenoconazole were selected as model triazole fungicides, and their sorption behavior on a typical biodegradable microplastics (PBS: polybutylene succinate) and two conventional polyethylene (PE) and polyvinyl chloride (PVC) microplastics was investigated with batch experiments in an aqueous solution. PBS presented the highest sorption capacity for triadimefon (104.2 ± 4.8 μg g^-1) and difenoconazole (192.8 ± 2.3 μg g^-1), which was 1.8- and 1.3-fold that on PE and 4.4- and 7.4-fold that of PVC, respectively. The results of sorption kinetic and isotherm modeling were better fit by a pseudo-second order model and linear model, respectively. More importantly, the effects of environmental factors (pH, salinity and dissolved organic matter) on the sorption behavior were investigated. Fungicide sorption on PBS was generally not affected by salinity, pH or dissolved organic matter. However, in contrast, salinity and dissolved organic matter both significantly decreased sorption on PE and PVC. The results showed that not only the sorption capacities of biodegradable microplastics but also their responses to environmental factors are quite different from those of conventional microplastics. This finding highlights the importance of the role played by biodegradable microplastics in the accumulation and transportation of organic pollutants.

The application of two-dimensional correlation spectroscopy for the binding properties of heavy metals onto digestate-derived DOM from anaerobic digestion of chicken manure

Anaerobic digestate has been widely used for agricultural activities as an organic fertilizer product. Dissolved organic matter (DOM) derived from anaerobic digestate plays a key role in the speciation, bioavailability and ultimate fate of metals that is related to agriculture and food safety as well as the soil environment. Hence, the binding properties of Cu, Pb and Zn with digestate DOM are investigated using two-dimensional correlation spectroscopy (2D-COS) in combination with ultraviolet absorption, synchronous fluorescence spectra (SFS) and Fourier transform infrared (FTIR) spectroscopy. The 2D absorption COS shows that the DOM at 200 nm is most susceptive with the addition of Pb, followed by Zn and Cu. The log-transformed absorption spectra can also obtain more valuable signals than that from conventional absorption spectra. The 2D-SFS-COS indicates that protein-like peak is more sensitive to the variation of the concentration of metal ions, and fulvic-like substances can preferentially interact with the three heavy metals (HMs). The 2D-FTIR-COS reveals that Cu(II) and Zn(II) ions can be bonded preferentially to the N-H of secondary amide (II), and phenolic OH groups shows a favorable binding with Pb(II). Humic-like peaks with Cu(II) and Zn(II) obtains relatively higher log K_M values than fulvic- and protein-like substances. However, the proportion of initial fluorescence (f) for DOM-Cu(II) and DOM-Zn(II) decreased with an increase in wavelength. Protein-like materials have more fluorescent substances that can combine with Cu(II) and Zn(II). This study provides a guide for understanding the geochemical behavior of metal ions in agricultural soils when anaerobic digestate is applied as an organic fertilizer product.

Degradation characteristics of dissolved organic matter in nanofiltration concentrated landfill leachate during electrocatalytic oxidation

Nanofiltration concentrated landfill leachate (NCLL) is produced during the integration process of biodegradation and nanofiltration, containing a large amount of recalcitrant dissolved organic matter (DOM). In this work, electrocatalytic oxidation technology was employed to degrade DOM in NCLL and spectroscopic technology was applied to explore the structural changes. The results showed that under the optimal experimental condition (pH = 2.0, NaCl concentration = 0.7%, Fe₂(SO₄)₃ concentration = 0.8%, the retention time = 6 h), the removal rates of COD, TOC, and UV₂₅₄ were 99.0%, 57.4%, 99.3% respectively. Ultraviolet-Visible (UV-Vis) spectral analysis showed that aromatic CC can be effectively degraded by electrocatalytic oxidation, resulting in decreases of aromaticity and molecular weight in NCLL. Two fluorescent components (terrestrial humic-like substances and fulvic-like substances) were identified in NCLL by parallel factor analysis, which can be effectively removed by electrocatalytic oxidation with removal rates of 99.9% and 90.5%, respectively. In addition, through two-dimensional correlation spectroscopic analysis, the sequence of structural changes of the DOM in NCLL was confirmed: unsaturated double bonds → fulvic-like components/aromatic structures → terrestrial humic-like components. These spectral characterization techniques can provide a deep understanding of the degradation pathways of DOM and provide new insights for the treatment of NCLL.

Effects of digestate DOM on chemical behavior of soil heavy metals in an abandoned copper mining areas

The influence of digestate dissolved organic matter (DOM) on chemical behavior of soil heavy metals (HMs) in an abandoned copper mining areas was explored by fluorescence quenching titration and heavy metal extracting experiment. Five fluorescent components were obtained from digestate DOM by PARAFAC model combined with the EEM data. The stability constant (log K_M) values were in the range of 4.95-5.53, 5.05-5.29, 5.21-6.00, and 4.12-4.75 for DOM-Cr(III), DOM-Cu(II), DOM-Fe(III) and DOM-Pb(II) complexes, respectively. Alcohols, ethers and esters in digestate DOM were preferentially combined with Fe(III), Cu(II) and Zn(II). However, phenolic hydroxyl groups were more likely to combine with Cr(III) and Pb(II). The speciation distribution of HMs indicated that mining resulted in a higher concentration of Cu(II) in the grassland soil (GS) than those in the agricultural soil (AS) and forest land soil (FS). Fe-Mn oxides and organic forms of Pb(II) increased dramatically due to mining. Digestate DOM extraction can increase the content of Cr(III), Fe(III) and Pb(II), and decrease the content of Cu(II) and Zn(II) in the AS, GS, and FS. However, the contents of HMs in the mining soil (MS) and slag soil (SS) decreased due to the application of digestate DOM, except for Cu(II) in the SS.

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.

XPS and two-dimensional FTIR correlation analysis on the binding characteristics of humic acid onto kaolinite surface

The stabilization and preservation of soil organic matter have been attributed to the strong reactive sites of mineral surfaces that cause physical isolation and chemical stabilization due to the organic-mineral interface. However, much of the micro-scale knowledge about interactions between organic ligands and minerals largely remains at the qualitative level, and neglects the heterogeneity of functional groups of organic matter. Here, we report the use of molecular-scale technologies of two-dimensional FTIR Correlation Spectroscopy (2D-FTIR-CoS) and X-ray Photoelectron Spectroscopy (XPS) to directly measure the binding processes of humic acid (JGHA) groups onto kaolinite surface. The spectroscopy results showed that the carboxylate groups, aliphatic OH and aromatic structure participate in the binding of JGHA on kaolinite surface. The carboxylic and phenolic hydroxyl interact with kaolinite surface through the interfacial COAl/Si bonds. Kaolinite prefers to adsorb C-groups at pH 4.0 and O-groups at pH 8.0. The interaction of COO^- group at 1566 cm^-1 of JGHA leads to the formation of inner-sphere complex first and then outer-sphere complex with increasing contact time. The interaction of COOH group at 1261 cm^-1 with the AlOH²⁺ of kaolinite was could be ascribed to ligand exchange and/or electrostatic attraction, whose contribution was evaluated to be 13.90%, 7.65% and 0% at pH 4.0, 6.0 and 8.0, respectively. These results of molecular binding provide quantitative mechanistic insights into organic-mineral interactions and expound the effect of functional groups of HA on binding mechanisms, and thus bring important clues for better understanding the mobility and transformation of land‑carbon including mineral-bound carbon.

Cultivation substrata differentiate the properties of river biofilm EPS and their binding of heavy metals: A spectroscopic insight

River biofilms inevitably serve as recipients of heavy metals including copper (Cu) and cadmium (Cd) following their introduction in fluvial systems. Nevertheless, the effects of cultivation substrata on the characteristics of river biofilm extracellular polymeric substances (EPS) and the binding behaviors of heavy metals on biofilms remain unclear. Integrating spectroscopic methods with chemometric analyses, we explored the binding behaviors of Cu(II) and Cd(II) onto biofilm EPS cultivated from two representative substrata at the molecular level. Chemical analysis revealed that biofilm cultivated on polyethylene (PE) pieces contained more non-fluorescent protein fractions, whereas EPS from periphyton grown on mineral, i.e., cobblestones was richer in aromatic fractions and polysaccharides. Excitation-emmision matrix combined with parallel factor analysis suggested a stronger interaction between fluorophores in periphytic EPS with Cu(II) compared to fluorophores in plastic biofilm EPS. Integrated use of infrared spectroscopy and two-dimensional correlation analyses revealed that, during the heavy metal binding processes, the amines and phenolics in plastic biofilm EPS gave the fastest responses to metal binding. While the amides and the aliphatic fractions in periphytic EPS showed a preferential binding to heavy metals. This study differentiates the effects of cultivation substrata on structuring the biofilm EPS characteristics and offers new insights into the environmental behaviors of heavy metal discharge into fluvial systems in river biofilm matrix.

Experimental and modeling study of proton and copper binding properties onto fulvic acid fractions using spectroscopic techniques combined with two-dimensional correlation analysis

Fulvic acid (FA) significantly influences the bioavailability and fate of heavy metals in environments, while its acid-base characters and metal binding processes are still unclear. Here, spectroscopic techniques combined with multiple models (e.g., NICA-Donnan model) and two-dimensional correlation spectroscopy (2D COS) were applied to explore the proton and copper binding properties of FA sub-fractions (FA₃-FA₁₃). The charge densities, average contents of carboxylic and phenolic groups, average dissociation constants pKa₁ and pKa₂ of sub-fractions ranged 0-16 meq∙g∙C^-1, 5.03-9.58 meq∙g∙C^-1, 2.52-4.67 meq∙g∙C^-1, 4.15-4.33 and 8.52-9.72, respectively. FA sub-fractions had a relatively narrow distribution of carboxyl group and a broad distribution of phenolic group. FA sub-fractions also exhibited roughly two phenolic hydroxyl groups per every 1-3 phenyl rings. Differential absorbance spectra (DAS) derived Gaussian bands were associated to the inter-chromophore interactions, the changes of molecular conformations and functional groups with copper addition. Differential spectra slopes (DSlope_{275-295&325-375}) were more significant with higher copper concentration and copper amounts bonded to carboxylic groups. UV-Vis and fluorescence spectra with 2D heterospectral COS revealed the copper binding heterogeneities and sequential orders of chromophores and fluorophores, quantitatively confirming by the order of conditional stability constants (log K_Cu: 4.64-5.56). Salicylic-/polyhydroxyphenolic, hydroxyl and amino groups were strongly associated to the basic units for fluorophores. Sequential changes followed the order of humic-like→fulvic-like materials for FA₃/FA₅, humic-like→fulvic-like→tryptophan-like materials for FA₇, and humic-like→tryptophan-like→fulvic-like→tyrosine-like materials for FA₉/FA₁₃. Spectroscopic techniques combined with various models (especially for 2D COS) are beneficial to elucidate the binding heterogeneity and sensitivity for metal-organic matters at the functional group level.

Diagnosis of the unexpected fluorescent contaminants in quantifying dissolved organic matter using excitation-emission matrix fluorescence spectroscopy

Dissolved organic matter (DOM) is widely present in aqueous environments and plays a significant role in pollutant mitigation and transformation. So far, excitation-emission matrix (EEM) fluorescence spectroscopy coupled with parallel factor analysis (PARAFAC) has been widely applied to quantify fluorescent DOM. However, this approach fails to provide accurate concentration of DOM when fluorescent contaminants exist. In this work, a new method, prior linear decomposition (PLD), is developed to solve this problem by introducing prior information, i.e., EEMs of DOM, into data decomposition. First, EEM of humic acid (HA) with different numbers of random Gaussian peaks are tested to confirm the robustness of PLD. The percentages for the relative errors within 5% are found to be 97.7% and 69% using PLD and PARAFAC, respectively. Then, the determination of mixture of HA with several contaminants is performed, validating the feasibility of DOM quantification and capability of contaminant diagnosis using PLD for synthetic water samples. Finally, DOM-containing natural water samples collected from a polluted lake, river and wastewater treatment plant (WWTP) are measured. The testing results confirm that PLD provides an accurate result with less evaluated error than PARAFAC and the EEMs of the contaminants can be inferred precisely. This work clearly demonstrates that PLD offers a robust approach for quantifying fluorescent DOM, which is of great significance in both natural and engineered aqueous environments.

Investigating the binding properties between antimony(V) and dissolved organic matter (DOM) under different pH conditions during the soil sorption process using fluorescence and FTIR spectroscopy

Antimony (Sb) is listed as a priority pollutant by European Union and U.S. Environmental Protection Agency. However, reports on its environmental behavior, particularly the sorption process in soil are still limited. In this paper, Sb(V) was selected as the sorbate and the black soil as the sorbent. The initial sorption rate (k₂q_e,cal²) was calculated to be 0.1254 mg g^-1∙min^-1 and the maximum sorption amount (q_m) 57.33 mg g^-1. Once the dissolved organic matter (DOM) was removed from the soil, the values of k₂q_e,cal² and q_m went down to 0.1066 mg g^-1∙min^-1 and 19.01 mg g^-1, respectively. These results suggested that the existence of DOM significantly influenced the mass transfer rate and sorption amount of Sb(V) in soil. In order to find out the reason why DOM exerted such an influence, the binding interaction mechanism between Sb(V) and DOM was investigated under different pH values. The protein-like and humic-like substances as well as the functional groups of CO, phenol hydroxyl, C-O, C-H, C-X and sulfur/phosphorus contributed to the formation of DOM-Sb(V)-complexes under pH of 7.0, in which the humic-like substance and the functional groups containing oxygen showed higher binding affinity for Sb(V) than protein-like substance and other functional groups, respectively. The protein-like substance and some functional groups disappeared under pH of 4.0 and 10.0. Alkaline condition resulted in a bigger impact on reducing the number of functional groups than acid condition. It can be concluded that the strongest binding interaction occurred at pH of 7.0 then followed by 4.0 and 10.0. This paper might be helpful to further studying the environmental behavior of Sb(V) in soil.

Binding characteristics of heavy metals to humic acid before and after fractionation by ferrihydrite

The fractionation of humic substances (HS) at the mineral and water interface can change the constituents and reactivity of HS, but there is still a lack of the understanding of the effects of HS fractionation on the binding characteristics of heavy metals to HS. In this study, the binding characteristics of five heavy metals (Cd, Cu, Ni, Pb, and Zn) to humic acid (HA) before and after adsorption by ferrihydrite were investigated by employing two-dimensional correlation spectroscopy (2D COS) integrated with synchronous fluorescence and Fourier transform infrared (FTIR) spectroscopies. 2D COS analyses of the fluorescence results indicated that the susceptibility of the fluorescence of humic-like fraction to heavy metals significantly decreased after the adsorption of HA by ferrihydrite, which may be due to the fact that humic-like components were preferentially adsorbed by ferrihydrite. However, the fractionation processes did not alter the metal binding sequence and affinity to different HA components. 2D COS analyses of the FTIR results suggested that fractionation processes decreased the susceptibility of COO^- groups to heavy metals, and changed the metal binding sequence to polysaccharides C-O and aryl groups, with the exception of Pb. Furthermore, model calculations showed that the binding ability of heavy metals to both humic-like and fulvic-like fractions decreased after the adsorption of HA by ferrihydrite. The results of this study contribute to predicting heavy metal behavior in the environment.

Characterizing Properties and Environmental Behaviors of Dissolved Organic Matter Using Two-Dimensional Correlation Spectroscopic Analysis

Dissolved organic matter (DOM) exists ubiquitously in environments and plays critical roles in pollutant mitigation, transformation, and organic geochemical cycling. Understanding its properties and environmental behaviors is critically important to develop water treatment processes and environmental remediation strategies. Generalized two-dimensional correlation spectroscopy (2DCOS), which has numerous advantages, including enhancing spectral resolution and discerning specific order of structural change under an external perturbation, could be used as a powerful tool to interpret a wide range of spectroscopic signatures relating to DOM. A suite of spectroscopic signatures, such as UV-vis, fluorescence, infrared, and Raman spectra that can be analyzed by 2DCOS, is able to provide additional structural information hiding behind the conventional one-dimensional spectra. In this article, the most recent advances in 2DCOS applications for analyzing DOM-related environmental processes are reviewed, and the state-of-the-art novel spectroscopic techniques in 2DCOS are highlighted. Furthermore, the main limitations and requirements of current approaches for exploring DOM-related environmental processes and how these limitations and drawbacks can be addressed are explored. Finally, suggestions and new approaches are proposed to significantly advance the development of 2DCOS in analyzing the properties and behaviors of DOM in natural and engineered environments.

Characterization of interactions between a metabolic uncoupler O-chlorophenol and extracellular polymeric substances of activated sludge

Metabolic uncouplers are widely used for the in-situ reduction of excess sludge from activated sludge systems. However, the interaction mechanism between the metabolic uncouplers and extracellular polymeric substances (EPS) of activated sludge is unknown yet. In this study, the interactions between a typical metabolic uncoupler, o-chlorophenol (oCP), and the EPS extracted from activated sludge were explored using a suite of spectral methods. The binding constants calculated for the four peaks of three-dimensional excitation-emission matrix fluorescence were in a range of 1.24-1.76 × 10³ L/mol, implying that the tyrosine protein-like substances governed the oCP-EPS interactions. Furthermore, the results of Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and ¹H nuclear magnetic resonance indicated that the carboxyl, carbonyl, amine, and hydroxyl groups of EPS were the main functional groups involved in the formation of the oCP-EPS complex. The results of this study are useful for understanding the interactions between metabolic uncouplers and the EPS of activated sludge as well as their fates in biological wastewater treatment systems.

Removal of ammonia-nitrogen in wastewater using a novel poly ligand exchanger-Zn(II)-loaded chelating resin

In this study, a novel poly ligand exchanger-Zn(II)-loaded resin was designed to effectively remove ammonia-nitrogen (NH₃-N) from wastewater. The surface morphology and structure of the Zn-loaded resin were characterized using scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) and Fourier transform infrared spectroscopy (FTIR), respectively. SEM shows the surfaces of the Zn(II)-loaded resin were rough and nonporous and EDS demonstrated that Zn²⁺ was loaded onto the resin successfully. In addition, the combination form of Zn(II) with NH₃-N adsorption reagent was revealed by FTIR spectra; the complex could be R-N-R-O-Zn-O-R-N-R and R-N-R-(O-Zn)₂. The kinetics and equilibrium of the NH₃-N adsorption onto the Zn(II)-loaded resin has been investigated. The effects of pH, reaction time, and temperature on NH₃-N removal from wastewater by Zn(II)-loaded resin were investigated, and the results showed that the maximum adsorption capacity reached 38.55 mg/g at pH 9.54 at 298 K in 240 min. The adsorption ability of the modified resin decreased with an increase in temperature. Moreover, the NH₃-N adsorption followed a pseudo-second-order kinetic process. The kinetic data demonstrated that the adsorption process might be limited by a variety of mechanisms. The study can provide the scientific foundation for the extensive application of a novel poly ligand exchanger-Zn(II)-loaded resin to remove NH₃-N from wastewater.