The characterization of dissolved organic matter extracted from different sources and their influence on cadmium uptake by Microcystis aeruginosa

Investigation of dissolved organic matter's influence on the toxicity of cadmium to the cyanobacterium Microcystis aeruginosa by biochemical and molecular assays

Rapid economic development has increased the accumulation of dissolved organic matter (DOM) and heavy metals in aquatic environments. In addition, Microcystis aeruginosa can cause the outbreak of cyanobacteria bloom and can produce microcystin, which poses a threat to human water safety. Therefore, this study analyzed the biochemical and molecular assays of DOM (0, 1, 3, 5, 8, 10 mg C L-1) extracted from four different sources on the toxicity of cadmium (Cd) to M. aeruginosa. The results showed that the addition of different concentrations of DOM from sediment, biochar, and humic acid alleviated the toxicity of Cd to M. aeruginosa. But the addition of rice hulls DOM at high concentrations (8 and 10 mg L-1) significantly reduced the normal growth and metabolic activities of M. aeruginosa. DOM from four different sources promoted the expression level of microcystin-related gene mcyA and the production of microcystin-leucine-arginine (MC-LR), and mcyA was positively correlated with MC-LR. DOM from biochar, sediment, and humic acid were able to bind Cd through complexation. The results will help to understand the toxic effects of heavy metals on toxic-producing cyanobacteria in the presence of DOM, and provide certain reference for the evaluation of water environmental health.

Applying fulvic acid for sediment metals remediation: Mechanism, factors, and prospect

Fulvic acid (FA) has been shown to play a decisive role in controlling the environmental geochemical behavior of metals. As a green and natural microbial metabolite, FA is widely used in environmental remediation because of its good adsorption complexation and redox ability. This paper introduces the reaction mechanism and properties of FA with metals, and reviews the progress of research on the remediation of metal pollutant by FA through physicochemical remediation and bioremediation. FA can control the biotoxicity and migration ability of some metals, such as Pb, Cr, Hg, Cd, and As, through adsorption complexation and redox reactions. The concentration, molecular weight, and source are the main factors that determine the remediation ability of FA. In addition, the ambient pH, temperature, metal ion concentrations, and competing components in sediment environments have significant effects on the extent and rate of a reaction between metals and FA during the remediation process. Finally, we summarize the challenges that this promising environmental remediation tool may face. The research directions of FA in the field of metals ecological remediation are also prospected. This review can provide new ideas and directions for the research of remediation of metals contaminants in sediments.

Impacts of iron amendments and per-fluoroalkyl substances' bio-availability to the soil microbiome in wheat ecosystem

Per-fluoroalkyl substances (PFASs) have become ubiquitous in farmland ecosystems and pose risks to agricultural safety, and iron is often applied to farmland soils to reduce the availability of pollutants. However, the effects of iron amendment on the availability of PFASs in the soil and on the soil microbiome are not well understood. Here, we investigated the responses of wheat soil containing PFASs to iron addition using a 21-day experiment. Our results showed that iron amendment enhanced PFAS availability (p < 0.05) and stimulated superoxide dismutase (SOD) activity in the wheat soil (p < 0.05), but iron amendment decreased the activities of soil catalase (CAT) and peroxidase (POD) (p < 0.05). Soil bacterial community was more structurally stable than fungal community in response to iron addition, while species' pools were more stable in fungi than in bacteria (p < 0.05). Finally, PFPeA's availability in the wheat soil was the most important abiotic factors driving community succession of iron-cycling bacteria (p < 0.05). These results highlighted the potential interactions among PFASs' availability and microbial iron cycling in wheat farmland soil ecosystems and provided guidance in farmland environmental conservation and management.

Prediction of Cd Accumulation in Wheat (Triticum aestivum L.) and Simulation Calculation of Lime or Zn Fertilizer Remediated Soil

Soil Cd contamination to wheat raise wide concerns over food safety. It is essential to find the key factors affecting Cd accumulation in wheat and to establish a predictive model. The effects of pH, Zn, Ca, and DOM on the accumulation of Cd in wheat were investigated using hydroponic experiments. The results showed that Zn was the most important factor inhibiting Cd uptake in wheat. Models were developed to predict the Cd contents in wheat tissues based on the ion concentration. Meanwhile, the available Cd contents in soil were predicted using a geochemical multi-surface model (MSM) which is suitable for various soils and conditions. The combination of the hydroponic accumulation model and MSM exhibits good predictions of wheat-Cd (R² = 0.822-0.862, RMSE = 0.317-0.533). The results of this study can quantitatively predict the accumulation of Cd in wheat and provide a reference for soil remediation and safe wheat production.

Adsorption of Heavy Metal Ions Copper, Cadmium and Nickel by Microcystis aeruginosa

To investigate the treatment effect of algae biosorbent on heavy metal wastewater, in this paper, the adsorption effect of M. aeruginosa powder on heavy metal ions copper, cadmium and nickel was investigated using the uniform experimental method, Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and TG-DSC comprehensive thermal analysis. The experimental results showed that the initial concentration of copper ion solution was 25 mg/L, the temperature was 30 °C, the pH value was 8 and the adsorption time was 5 h, which was the best condition for the removal of copper ions by algae powder adsorption, and the removal rate was 83.24%. The initial concentration of cadmium ion solution was 5 mg/L, the temperature was 35 °C, the pH value was 8 and the adsorption time was 4 h, which was the best condition for the adsorption of cadmium ion by algae powder, and the removal rate was 92.00%. The initial nickel ion solution concentration of 15 mg/L, temperature of 35 °C, pH value of 7 and adsorption time of 1 h were the best conditions for the adsorption of nickel ions by algae powder, and the removal rate was 88.67%. The spatial structure of algae powder changed obviously before and after adsorbing heavy metals. The functional groups such as amino and phosphate groups on the cell wall of M. aeruginosa enhanced the adsorption effect of heavy metal ions copper, cadmium and nickel. Additionally, M. aeruginosa adsorption of heavy metal ions copper, cadmium, nickel is an exothermic process. The above experiments show that M. aeruginosa can be used as a biological adsorbent to remove heavy metals, which lays a theoretical foundation for the subsequent treatment of heavy metal pollution by algae.

Quantifying the bioaccumulation of Pb to Chlorella vulgaris in the presence of dissolved organic matters with different molecular weights

Dissolved organic matter (DOM) is ubiquitous in natural waters which exhibits obvious effects on the toxicity of heavy metals. However, information on the toxicity of heavy metals in the presence of DOMs with different molecular weights (MWs) was still unclear. In this study, Suwannee river humic acid (SRHA) and algae-derived organic matter (ADOM) were selected as typical terrestrial and microbial DOMs, with the bulk DOMs fractionating into high MW (HMW-, 1 kDa ~ 0.45 μm) and low MW (LMW-, < 1 kDa) fractions to explore the MW-dependent heterogeneities in the bioaccumulation of Pb to Chlorella vulgaris. Results showed that, regardless of DOM types, the LMW fraction exhibited more acidic groups and humic-like substances than the HMW counterparts. Presence of bulk DOM can decrease the bioaccumulation of Pb, while the specific effects were MW- and type-dependent. The LMW-SRHA enhanced the bioaccumulation of Pb while the HMW counterpart alleviated the effects. However, both the HMW- and LMW-ADOM can reduce the bioaccumulation of Pb to C. vulgaris. Moreover, the correlation analysis showed a significant positive correlation between the content of phenolic-OH and the adsorbed/internalized amounts of Pb, demonstrating that the phenolic-OH played a critical role in altering the bioaccumulation of Pb. The results obtained in this study suggest that distribution of MWs, number of acidic functional groups, and metal complexation capacity within DOM pool should be considered for the eco-environmental risk assessment of heavy metals in aquatic environments.

Correlative distribution of DOM and heavy metals in the soils of the Zhangxi watershed in Ningbo city, East of China

In peri-urban critical zones, soil ecosystems are highly affected by increasing urbanization, causing probably an intense interaction between dissolved organic matter (DOM) and heavy metals in soil. Such interaction is critical for understanding the biogeochemical cycles of both organic matter and heavy metals in these zones. However, limited research has reported the correlative distribution of DOM and heavy metals at high seasonal and spatial resolutions in peri-urban critical zones. In this study, 160 soil samples were collected from the farmland and forestland of Zhangxi watershed, in Ningbo, eastern China during spring, summer, fall and winter four seasons. UV-visible absorption and fluorescent spectroscopy were used to explore the optical characteristics of DOM. The results indicated a mixture of exogenous and autogenous sources of DOM in the Zhangxi watershed, while DOM in farmland exhibited a higher degree of aromaticity and humification than that in forestland. Fluorescent results showed that humic acid-like, fulvic acid-like and microbial-derived humic-like fractions were mostly affected by seasons. The distribution of heavy metals was affected mainly by land-use changes and seasons. Correlation analysis between heavy metals and DOM characteristics and components suggested that aromatic and humic substances were more favorable in binding with EDTA extractable Ni, Cu, Zn and Cd. The bioavailable Cd and Pb decreased due to binding with humic fractions, indicating its great effects on the bioavailability of Cd and Pb. Overall, these findings provide an insight into the correlative distributions of DOM and heavy metals in peri-urban areas, thereby highlighting their biogeochemical cycling in the soil environment.

The spectral characteristics and cadmium complexation of soil dissolved organic matter in a wide range of forest lands

The quality and quantity of dissolved organic matter (DOM) greatly controls the fate of heavy metals. The characteristics of DOM and its interaction with metals are essential for the metal ecological risk assessment of soils. In this study, the DOM spectral characteristics of representative forest soils and the complex capacities between fluorescent DOM components and cadmium (Cd) were analyzed. Functional groups, such as carboxylic acids, alcohols and phenols, were determined by FT-IR analysis. Chromophoric DOM, fluorescent DOM and dissolved organic carbon (DOC) concentrations exhibited strong correlations with each other, indicating that variations of DOC could be well explained by Chromophoric DOM or fluorescent DOM due to high correlation coefficients. The spectral slope ratio was in the range of 0.85-5.90, implying an abundance of heavy macromolecular humic acids, peptides, and polycondensates. The absorbance spectral at 254 nm (SUVA₂₅₄) strongly correlated with SUVA₂₆₀ (r = 0.992, P < 0.01), indicating that hydrophobicity closely related with aromatic structure, and aromatic groups could be broadly hydrophobic. Fluorescence indices were from 1.62 to 2.21 and biological index values ranged from 0.54 to 1.14, where the DOM was mainly sourced from mixed terrestrial and autogenous inputs in most sites. Four universal fluorescence components were identified and characterized by fluorescence EEM-PARAFAC, including two humic-like (components 1 and 2), one tyrosine-like (components 3) and one fulvic-like (components 4) component. Both components 3 and 4 showed fluorescence quenching with increasing Cd concentrations, while components 1 and 2 had no evident change in fluorescence intensity. The logK₃ and logK₄ values ranged from 4.41 to 5.29 and 4.71 to 5.54, respectively, with most logK values of component 3 for Cd binding being smaller than that of component 4, thus, indicating that the fulvic acid substances exhibited stronger and more stable interactions with Cd than protein-like components.

Differences in bioaccumulation of Ni and Zn by microalgae in the presence of fulvic acid

In the presence of dissolved organic matter, the mechanism of algal bioaccumulation of different metals is complex, and its significance goes far beyond the alga-metal binary system. In the presence of 10 and 20 mg L^-1 fulvic acid (FA), the maximum tolerance concentrations of Chlorella pyrenoidosa to Ni were 0.25 and 0.26 mmol L^-1, and to Zn were 0.62 and 0.68 mmol L^-1, respectively. Within the maximum tolerance concentration ranges, the bioaccumulation behaviors of Ni and Zn were systematically compared in the presence of FA. The presence of FA shortened the adsorption equilibrium time and decreased the maximum bioaccumulation capacity of Ni and Zn. The bioaccumulation mechanism of Ni by C. pyrenoidosa was more inclined to monolayer adsorption, while the bioaccumulation mechanism of Zn was more inclined to multilayer adsorption. More details were revealed after the bioaccumulated metals were separated into adsorption and internalization states by 0.01 M EDTA elution. The presence of FA decreased more adsorbed Zn than the adsorbed Ni, due to the different competitive roles of FA in the ternary system of Ni and Zn, but the presence of FA increased the internalized Ni might due to the stronger complexation of Ni-FA. This research indicated that algae had unique bioaccumulation mechanisms for different metals in the presence of FA, which is of great significance to accurately evaluate the ecological risk posed by heavy metals.

Effect of dissolved organic matter on the phytoremediation of Cd-contaminated soil by cotton

Dissolved organic matter (DOM) assists in the phytoremediation of heavy-metal-contaminated soils, but the effect of synergistic remediation of DOM on plants is unclear. This study investigated the effect of two DOM sources (cotton straw (CM) DOM and farmyard manure (FM) DOM) on cadmium (Cd) accumulation in Cd-contaminated soil by cotton and evaluated the phytoremediation effect of DOM. The results showed that adding DOM reduced the available nitrogen and increased organic matter, available phosphorus and available potassium. Applying DOM increased the proportions of Cd acid soluble fractions and reduced the proportions of Cd residual fractions by 1-7%. DOM application increased root length, root surface area and root volume compared to the control and had a promoting or inhibiting effect on cotton biomass, depending on the soil Cd concentration. Furthermore, applying DOM improved the Cd content and bioconcentration factor of cotton. The lower the molecular weight, hydrophilic components and aromaticity of DOM, the more conducive to Cd accumulation is in cotton. The correlation and random forest analyses also showed that CM showed high remediation potential. According to our study, DOM can improve the phytoremediation efficiency of cotton, especially in low-concentration contaminated soils. This study provides a basis for applying DOM in the phytoremediation of Cd-contaminated soils.

Spectroscopic characterization of dissolved organic matter from macroalgae Ulva pertusa decomposition and its binding behaviors with Cu(II)

Dissolved organic matter (DOM) from macroalgae is regarded a crucial source of autochthonous DOM in coastal ocean. In the present study, the characteristics of DOM from the macroalgae Ulva pertusa decomposition (U. pertusa-DOM) and its binding behaviors with Cu(II) using multiple spectroscopic techniques and chemometric analyses. The labile U. pertusa-DOM could be consumed and transformed by microorganisms. The absorption spectroscopic descriptors indicate that the hydrophobicity, aromaticity, and molecular weight of the U. pertusa-DOM increase during the 27-day incubation period. Fluorescence excitation-emission matrix spectroscopy combined with parallel factor analysis suggests that the relative abundance of the protein-like component (C1) (96.10-84.96%) sequentially decreases, whereas the humic-like components (C2) (2.16-9.73%) and (C3) (1.75-5.31%) in the U. pertusa-DOM increase with the U. pertusa decomposition. The Cu(II) binding properties of the U. pertusa-DOM are dependent on the decomposition time. The order of the conditional stability constant (logK_M) is C2 > C1 > C3. The complexation capacity (f) of C1 is higher than those of C2 and C3 at a specific time. Synchronous fluorescence spectroscopy coupled with two-dimensional correlation spectroscopy reveals that the microbial degradation could accelerate the Cu(II) binding to humic-like fractions in the U. pertusa-DOM. These findings will help us better understand the biogeochemical behaviors of macroalgal DOM and heavy metal in coastal ecosystems.

Molecular Sorption Mechanisms of Cr(III) to Organo-Ferrihydrite Coprecipitates Using Synchrotron-Based EXAFS and STXM Techniques

Ubiquitous organo-ferrihydrite coprecipitates (OFC) significantly affect the mobility and availability of Cr in soil through sorption, but the underlying sorption mechanisms remain unclear at the molecular level. Due to the potential formation of OFC in agricultural soils with returned crop straws, we synthesized OFC with rice/rape straw-derived carbon (C) sources and different loadings. The molecular sorption mechanisms of Cr(III) to the synthesized OFC under different conditions were investigated by Cr K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy and scanning transmission X-ray microscopy (STXM). Cr(III) sorption by OFC decreased with increasing C loading and decreasing pH, regardless of C sources. Moreover, inhibition of Cr(III) sorption to OFC with high C loading occurred when ionic strength (IS) increased, suggesting the presence of outer-sphere complexed Cr(III). EXAFS analysis revealed that more Cr(III) were bound to ferrihydrite of the OFC at a relatively high pH, and organically bound Cr(III) enhanced when increasing C loading and decreasing IS. STXM analysis strongly suggested that C loading reduced Cr(III) sorption through blocking the binding sites on the ferrihydrite, which overwhelmed Cr(III) retention by the direct binding of Cr(III) to carboxyl of the particulate organic matter (OM) and OM coated on the Fh fractions of the OFC. These findings facilitated the comprehensive understanding of the sorption mechanisms of Cr(III) to OFC at the molecular level, which will assist the prediction of Cr(III) mobility in soils, particularly for Cr(III)-contaminated agricultural soils with the application of crop straws.

Effect of Serratia sp. K3 combined with organic materials on cadmium migration in soil-vetiveria zizanioides L. system and bacterial community in contaminated soil

Phytoremediation is an economical strategy to harvest cadmium (Cd) from contaminated soil, but the efficiency of phytoremediation was affected by many factors. This study investigated the potential of Serratia sp. K3 (K3) assisted with straw biogas residue (SBR) or leavening fertilizer (LF) on improving the Cd migration efficiency and micro-environment in soil-vetiveria zizanioides L. system. The results showed that the acid soluble Cd in soil was increased by 2.83-29.79% in treatments compared with control (CK). In addition, Cd accumulation in the roots and shoots of vetiveria zizanioides were significantly enhanced by the combination of K3 and SBR/LF. Especially, the translocation factor of Cd increased by 21.53-62.37% in groups with K3 compared with the groups without K3, correspondingly. Furthermore, SBR/LF effectively changed bacterial community structure, and improved bacterial abundance. Relative abundance of functional genes related with carbohydrate/energy/amino acid metabolism were increased in groups of SBRB/LFB rhizosphere compared with CK. These results provide insight into the change of phytoremediation efficiency and soil bacterial communities in the vetiveria zizanioides rhizosphere after inoculation. This study may provide a promising method for improving phytoremediation in Cd contaminated soil.

Sulfur controlled cadmium dissolution in pore water of cadmium-contaminated soil as affected by DOC under waterlogging

Cadmium (Cd) precipitation and dissolution in pore water is associated with dissolved organic carbon (DOC)-induced reduction-oxidation of sulfur (S) under waterlogging and is vital for controlling the bioavailability in paddy soil. A 120-day soil incubation experiment, including application of sulfur (S, 30 mg kg^-1) and wheat straw (W, 1.0%) alone or in combination (W + S) into Cd-contaminated paddy soil under waterlogging, was conducted to investigate the dynamic of dissolved Cd and its relationship with DOC, S^2-, Fe²⁺, pH, Eh and pe + pH in soil pore water. The results showed that the lowest dissolved Cd concentration was observed in the W + S-treated soil pore water among all treatments when the soil Eh remained at lower values during the period of 15-60 days of incubation, which could be attributed to CdS precipitation and/or co-precipitation of Cd absorbed by FeS₂ because of the reduction in sulfur. The application of S resulted in a Cd rebound in the pore water irrespective of W addition when the Eh began to increase from its lowest values during the period of 45-75 days of incubation, and SOB genera were observed in the S added soil. This could be attributed to re-dissolution of the precipitated Cd in soils under the SOB-driven oxidation of sulfide such as CdS and FeS₂. In conclusion, DOC-driven reduction-oxidation of sulfur controls Cd dissolution in the pore water of Cd-contaminated paddy soil under waterlogging conditions. Further studies are required to investigate the interaction of sulfur and SOM-induced DOC on Cd bioavailability in rice-planted paddy soils.

Relating Cd2+ binding by humic acids to molecular weight: A modeling and spectroscopic study

Molecular weight (Mw) is a fundamental property of humic acids (HAs), which considerably affect the mobility and speciation of heavy metals in the environment. In this study, soil humic acid (HA) extracted from Jinyun Mountain, Chongqing was ultra-filtered into four fractions according to the molecular weight, and their properties were characterized. Complexation of cadmium was investigated by titration experiments. For the first time, Langmuir and non-ideal competitive adsorption-Donna (NICA-Donnan) models combined with fluorescence excitation-emission matrix (EEM) quenching were employed to elucidate the binding characteristics of individual Mw fractions of HA. The results showed that the concentration of acidic functional groups decreased with increasing Mw, especially the phenolic groups. The humification degree and aliphaticity increased with increasing Mw as indicated by elemental composition analysis and FT-IR spectra. The binding capacity of Cd²⁺ to Mw fractions of HA followed the order UF1 (<5kDa)>UF2 (5-10kDa)>UF4 (>30kDa)>UF3 (10-30kDa). Moreover, the distribution of cadmium speciation indicated that the phenolic groups were responsible for the variations in binding of Cd²⁺ among different Mw fractions. The results of fluorescence quenching illustrated that the binding capacity of Cd²⁺ to Mw fractions was controlled by the content of functional groups, while the binding affinity was largely influenced by structural factors. The results provide a better understanding of the roles that different HA Mw fractions play in heavy metal binding, which has important implications in the control of heavy metal migration and bio-toxicity.

Impacts of wheat straw addition on dissolved organic matter characteristics in cadmium-contaminated soils: Insights from fluorescence spectroscopy and environmental implications

Incorporating straw in soil can change the dissolved organic matter (DOM) content and also affect the mobility and solubility of soil pollutants. However, few studies have focused on the spectral features of DOM released by soil after amendment with crop residues. In this study, excitation-emission matrix (EEM) fluorescence and UV-visible absorbance spectroscopy were used to monitor the quality and quantity of the DOM derived from wheat straw incorporation in the soil. We found remarkable changes in the dissolved organic carbon (DOC) concentration (0.10-0.34 g kg^-1). But the mean DOC concentrations were lower in the cadmium (Cd²⁺) added treatments compared with the control treatment due to additional Cd²⁺. The specific UV-visible absorbance at 254 nm was positively correlated with Cd²⁺ content, thereby suggesting a strong relationship between aromatic materials and heavy metals. Three fluorescent components were identified by EEM fluorescence combined with parallel factor analysis (PARAFAC) modeling, i.e., UVC humic-like, fulvic acid-like, and tryptophan-like. The relative abundances of these three components and dynamic variations in fluorescent indices predicted the changes in DOM composition during the wheat straws decomposition process. Therefore, the findings obtained in this study can be utilized to evaluate the variations in DOM for soil incorporated with straws in order to support agricultural production and overcome the heavy metal pollution.