Shasha Liu

Characteristics and degradation of carbon and phosphorus from aquatic macrophytes in lakes: Insights from solid-state (13)C NMR and solution (31)P NMR spectroscopy.

Water extractable organic matter (WEOM) derived from macrophytes plays an important role in biogeochemical cycling of nutrients, including carbon (C), nitrogen (N) and phosphorus (P) in lakes. However, reports of their composition and degradation in natural waters are scarce. Therefore, compositions and degradation of WEOM derived from six aquatic macrophytes species of Tai Lake, China, were investigated by use of solid-state (13)C NMR and solution (31)P NMR spectroscopy. Carbohydrates were the predominant constituents of WEOM fractions, followed by carboxylic acid. Orthophosphate (ortho-P) was the dominant form of P (78.7% of total dissolved P) in the water extracts, followed by monoester P (mono-P) (20.6%) and little diester P (0.65%). The proportion of mono-P in total P species increased with the percentage of O-alkyl and O-C-O increasing in the WEOM, which is likely due to degradation and dissolution of biological membranes and RNA from aquatic plants. Whereas the proportion of mono-P decreased with alkyl-C, NCH/OCH3 and COO/N-C=O increasing, which may be owing to the insoluble compounds including C functional groups of alkyl-C, NCH/OCH3 and COO/N-C=O, such as aliphatic biopolymers, lignin and peptides. Based on the results of this study and information in the literature about water column and sediment, we propose that WEOM, dominated by polysaccharides, are the most labile and bioavailable component in debris of macrophytes. Additionally, these WEOMs would also be a potential source for bioavailable organic P (e.g., RNA, DNA and phytate) for lakes.

Effects of monovalent and divalent metal cations on the aggregation and suspension of Fe3O4 magnetic nanoparticles in aqueous solution.

There has been limited research investigating how the mechanisms of aggregation of magnetic nanoparticles (MNPs) are affected by inorganic ions. In this study, Na+, Mg2+, Ca2+, Sr2+ and Ba2+ were selected to systematically study the aggregation mechanisms of Fe3O4 MNPs. The results indicated that divalent cations more significantly affected the stabilities of MNPs than Na+ at low concentrations (e.g., 0.1mM) in a decreasing order of Ba2+>Sr2+>Ca2+>Mg2+>Na+. Extended DLVO theory did not offer a satisfactory explanation for the above difference due because it ignores specific ion effects. It was also found that the initial adsorption ratios of these metals by Fe3O4 MNPs were linearly proportional to the hydrodynamic diameter (HDD) of Fe3O4 MNPs before aggregation occurred. In addition to the valence states, the hydration forces and ionic radii of the metal cations were proposed to be other factors that significantly affected the interactions of metal cations with Fe3O4 MNPs based on the excellent linear relationships of the HDD of Fe3O4 MNPs and these three factors. Moreover, a bridging function of divalent cations might develop after aggregation occurred based on the increases in their adsorption amounts and intensities for binding oxygen-containing functional groups. In addition, an increase in the positive ζ potential of MNPs was observed with the addition of divalent cations until 10.0mM at a pH of 5.0, which potentially enhances the resistance of MNPs to aggregation in aquatic systems compared with Na+. Consequentially, the effects of metal cations on the aggregation of MNPs are determined by the hydration forces, valance states, ionic radii and bond types formed on the MNPs. Thus, the specific ion effects of these cations should be considered in predicting the environmental behaviors of specific nanomaterials.

Characterization of plant-derived carbon and phosphorus in lakes by sequential fractionation and NMR spectroscopy.

Although debris from aquatic macrophytes is one of the most important endogenous sources of organic matter (OM) and nutrients in lakes, its biogeochemical cycling and contribution to internal load of nutrients in eutrophic lakes are still poorly understood. In this study, sequential fractionation by H2O, 0.1M NaOH and 1.0M HCl, combined with (13)C and (31)P NMR spectroscopy, was developed and used to characterize organic carbon (C) and phosphorus (P) in six aquatic plants collected from Tai Lake (Ch: Taihu), China. Organic matter, determined by total organic carbon (TOC), was unequally distributed in H2O (21.2%), NaOH (29.9%), HCl (3.5%) and residual (45.3%) fractions. For P in debris of aquatic plants, 53.3% was extracted by H2O, 31.9% by NaOH, and 11% by HCl, with 3.8% in residual fractions. Predominant OM components extracted by H2O and NaOH were carbohydrates, proteins and aliphatic acids. Inorganic P (Pi) was the primary form of P in H2O fractions, whereas organic P (Po) was the primary form of P in NaOH fractions. The subsequent HCl fractions extracted fewer species of C and P. Some non-extractable carbohydrates, aromatics and metal phytate compounds remained in residual fractions. Based on sequential extraction and NMR analysis, it was proposed that those forms of C (54.7% of TOC) and P (96.2% of TP) in H2O, NaOH and HCl fractions are potentially released to overlying water as labile components, while those in residues are stable and likely preserved in sediments of lakes. These results will be helpful in understanding internal loading of nutrients from debris of aquatic macrophytes and their recycling in lakes.

Cation-induced coagulation of aquatic plant-derived dissolved organic matter: Investigation by EEM-PARAFAC and FT-IR spectroscopy

Complexation and coagulation of plant-derived dissolved organic matter (DOM) by metal cations are important biogeochemical processes of organic matter in aquatic systems. Thus, coagulation and fractionation of DOM derived from aquatic plants by Ca(II), Al(III), and Fe(III) ions were investigated. Metal ion-induced removal of DOM was determined by analyzing dissolved organic carbon in supernatants after addition of these metal cations individually. After additions of metal ions, both dissolved and coagulated organic fractions were characterized by use of fluorescence excitation emission matrix-parallel factor (EEM-PARAFAC) analysis and Fourier transform infrared (FT-IR) spectroscopy. Addition of Ca(II), Fe(III) or Al(III) resulted in net removal of aquatic plant-derived DOM. Efficiencies of removal of DOM by Fe(III) or Al(III) were greater than that by Ca(II). However, capacities to remove plant-derived DOM by the three metals were less than which had been previously reported for humic materials. Molecular and structural features of plant-derived DOM fractions in associations with metal cations were characterized by changes in fluorescent components and infrared absorption peaks. Both aromatic and carboxylic-like organic matters could be removed by Ca(II), Al(III) or Fe(III) ions. Whereas organic matters containing amides were preferentially removed by Ca(II), and phenolic materials were selectively removed by Fe(III) or Al(III). These observations indicated that plant-derived DOM might have a long-lasting effect on water quality and organisms due to its poor coagulation with metal cations in aquatic ecosystems. Plant-derived DOM is of different character than natural organic matter and it is not advisable to attempt removal through addition of metal salts during treatment of sewage.

Using solid (13)C NMR coupled with solution (31)P NMR spectroscopy to investigate molecular species and lability of organic carbon and phosphorus from aquatic plants in Tai Lake, China.

Forms and labilities of plant-derived organic matters (OMs) including carbon (C) and phosphorus (P) were fundamental for understanding their release, degradation and environmental behaviour in lake ecosystems. Thus, solid 13C and solution 31P nuclear magnetic resonance (NMR) spectroscopy were used to characterize biomass of six aquatic plants in Tai Lake, China. The results showed that carbohydrates (61.2% of the total C) were predominant C functional group in the solid 13C NMR spectra of plant biomass, which may indicate high lability and bioavailability of aquatic plants-derived organic matter in lakes. There was 72.6–103.7% of the total P in aquatic plant biomass extracted by NaOH–EDTA extracts. Solution 31P NMR analysis of these NaOH–EDTA extracts further identified several molecular species of P including orthophosphate (50.1%), orthophosphate monoesters (46.8%), DNA (1.6%) and pyrophosphate (1.4%). Orthophosphate monoesters included β-glycerophosphate (17.7%), hydrolysis products of RNA (11.7%), α-glycerophosphate (9.2%) and other unknown monoesters (2.1%). Additionally, phytate, the major form of organic P in many lake sediments, was detected in floating plant water poppy. These inorganic P (e.g. orthophosphate and pyrophosphate) and organic P (e.g. diester and its degradation products) identified in plant biomass were all labile and bioavailable P, which would play an important role in recycling of P in lakes. These results increased knowledge of chemical composition and bioavailability of OMs derived from aquatic plants in lakes.

Protonation-dependent heterogeneity in fluorescent binding sites in sub-fractions of fulvic acid using principle component analysis and two-dimensional correlation spectroscopy

Heterogeneous distributions of proton binding sites within sub-fractions of fulvic acid (FA3-FA13) were investigated by use of synchronous fluorescence spectra (SFS), combined with principle component analysis (PCA) and two-dimensional correlation spectroscopy (2D-COS). Tryptophan-like, fulvic-like and humic-like materials were observed in SFS. Tyrosine-like materials were identified by use of SFS-PCA analysis. Combined information from synchronous-asynchronous maps and dissociation constants (pKa) was used to describe heterogeneity of binding sites for protons within each sub-fraction. Heterogeneous distributions of proton binding sites were observed in fulvic-like, humic-like, tryptophan-like, and tyrosine-like materials of five sub-fractions and even in the single fulvic-like materials in FA3 and tryptophan-like materials in FA9 and FA13. Values of pKa of sub-fractions ranged from 2.20 to 5.29, depending on associated wavelengths in synchronous-asynchronous maps and use of the modified Stern-Volmer equation. The larger values of pKa (4.17-5.29) were established for protein-like materials (including tryptophan-like and tyrosine-like materials) in comparison to those (2.20-3.38) for humic-like and fulvic-like materials in sub-fractions. Sequential variations of 274nm (pKa 4.15-5.29)→360-460nm (pKa 2.78-2.39) for FA5-FA13 revealed that binding of protons to tryptophan-like materials appeared prior to humic-like/fulvic-like materials. In FA9, protons were preferentially binding to tryptophan-like materials than tyrosine-like materials. In FA3, protons were preferentially binding to humic-like materials than fulvic-like materials. Relative differences of values of pKa for fluorescent materials within each sub-fraction were consistent with sequential orders derived from asynchronous maps. Such an integrated approach, SFS-PCA/2D-COS, has superior potential for further applications in exploring complex interactions between dissolved organic matter and contaminants in engineered and natural environments.

Molecular characterization of macrophyte-derived dissolved organic matters and their implications for lakes

Chemical properties of whole organic matter (OM) and its dissolved organic matter (DOM) extracted from three types of dominant macrophytes in Lake Dianchi were comparatively characterized using elemental analysis, UV, 3D-EEM and 13C NMR spectroscopy and their implications for lakes were discussed. Ratios of C/N and C/P were least in the floating water hyacinth and submerged sago pondweed, while total dissolved nitrogen (TDN) and phosphorus (TDP), dissolved organic nitrogen (DON) and phosphorus (DOP) were greatest in those species. In emergent species, C/N, C/P, DON and DON/TDN were less in leaves than in their corresponding shoots. The specific UV absorbance at 254nm (SUVA254) and 280nm (SUVA280) of extracts were in the range of 0.50-1.96L/mgC·m and 0.40-1.48L/mgC·m. Both SUVA values were greater in leaves than those in shoots. 3D-EEM spectra showed only a single fulvic-like fluorescence in leaves of emergent macrophytes. In contrast, protein-like peak were observed in spectra of floating and submerged species, as well as the shoot DOM of emergent species. Solid-state 13C NMR demonstrated that leaves had greater percentage of recalcitrant alkyl C and aromatic C, while shoots were rich in labile carbohydrates. The overall characterization works suggested that macrophyte-derived DOM has less aromatic constituents than do DOM in natural waters and soil leachates. Also OM and DOM derived from shoots had greater contents of protein-like and carbohydrate materials, while leaves were rich in aromatics. Floating and submerged plants possessed potential to not only accumulate excess N and P, but also for returning them to the lake. Shoots of riparian and emergent species were also an important source of nutrients. Thus, macrophyte biomass should be a great concern in nutrient regulation in Lake Dianchi.

Algal uptake of hydrophilic and hydrophobic dissolved organic nitrogen in the eutrophic lakes

Dissolved organic nitrogen (DON) derived from sediments plays an active role in biogeochemical cycling of nutrients in aquatic ecosystems. Sediments from four eutrophic lakes were studied using three-dimensional fluorescence excitation-emission matrix (3DEEM) spectra and supelite XAD-8 macroporous resin separation to investigate the bioavailability of hydrophilic and hydrophobic DON to algae (Microcystis flos-aquae (Wittr.) Kirchner). The results showed that the average loss of DON was <6.0% after dividing DON into hydrophilic and hydrophobic components, demonstrating the utility of XAD-8 resin separation in the study of DON components from lake sediments. The 3DEEM analysis showed that hydrophobic and hydrophilic DON comprised humic- and protein-like materials, respectively. During the incubation period, the bioavailability of hydrophilic DON, which accounted for 59.3%-80.4% of total DON, stimulated algal growth, suggesting that hydrophilic DON was the primary source of organic nitrogen for algae. In contrast, hydrophobic DON increased algal density by only 31.8% of that observed for hydrophilic DON, and had a small (accounted for 20.0%-26.6% of total DON) effect on algal growth over the short-term. The significant differences in algal growth between the two types of DON suggested that they should be considered separately in the eutrophic lake restorations.

Removal of antimonate (Sb(V)) and antimonite (Sb(III)) from aqueous solutions by coagulation-flocculation-sedimentation (CFS): Dependence on influencing factors and insights into removal mechanisms

This study investigates the effects of different influence factors on the removal of inorganic Sb species using coagulation-flocculation-sedimentation (CFS) and establishes the mechanism of the process. Thus, the influence of pH, initial Sb concentrations, coagulant dosages and competitive matters on Sb(V) and Sb(III) removal via CFS with polymeric ferric sulfate (PFS) was investigated systemically. Competition experiments and characterization methods, including X-ray diffraction (XRD), energy dispersive spectrometry (EDS), and X-ray photoelectron spectroscopy (XPS), were performed to determine the mechanisms of the process. The main conclusions included: (i) Optimum Sb removal was observed at a pH range of 4-6 and dosages of 4 × 10-4 mol/L and 8 × 10-5 mol/L for Sb(V) and Sb(III), respectively. Additionally, both Sb(V) and Sb(III) removal could be inhibited by the presence of phosphate and humic acid (HA). (ii) A higher priority was observed for the removal of Sb(III) over Sb(V). (iii) After excluding precipitation/inclusion/occlusion, coprecipitation involving chemical bonding played a significant role in both Sb(V) and Sb(III) removal, and electrostatic force served another significant role in Sb(V) removal. The Sb(V) and Sb(III) contamination in real contaminated waters was successfully removed using PFS via CFS process. The results of this study provide insights into the removal mechanisms of inorganic Sb species via CFS.

Using dual isotopes and a Bayesian isotope mixing model to evaluate sources of nitrate of Tai Lake, China

AbstractIdentification and quantification of sources of nitrate (NO3–) in freshwater lakes provide useful information for management of eutrophication and improving water quality in lakes. Dual δ15N- and δ18O-NO3– isotopes and a Bayesian isotope mixing model were applied to identify sources of NO3– and estimate their proportional contributions to concentrations of NO3– in Tai Lake, China. In waters of Tai Lake, values for δ15N-NO3– ranged from 3.8 to 10.1‰, while values of δ18O ranged from 2.2 to 12.0‰. These results indicated that NO3– was derived primarily from agricultural and industrial sources. Stable isotope analysis in R called SIAR model was used to estimate proportional contributions from four potential NO3– sources (agricultural, industrial effluents, domestic sewage, and rainwater). SIAR output revealed that agricultural runoff provided the greatest proportion (50.8%) of NO3– to the lake, followed by industrial effluents (33.9%), rainwater (8.4%), and domestic sewage (6.8%). Contributions of those primary sources of NO3– to sub-regions of Tai Lake varied significantly (p < 0.05). For the northern region of the lake, industrial source (35.4%) contributed the greatest proportion of NO3–, followed by agricultural runoff (27.4%), domestic sewage (21.3%), and rainwater (15.9%). Whereas for the southern region, the proportion of NO3– contributed from agriculture (38.6%) was slightly greater than that contributed by industry (30.8%), which was similar to results for nearby inflow tributaries. Thus, to improve water quality by addressing eutrophication and reduce primary production of phytoplankton, NO3– from both nonpoint agricultural sources and industrial point sources should be mitigated. Graphical abstractᅟ