Release Of Dissolved Organic Matter Research Articles

Mobility of arsenic in soil amended with biochar derived from biomass with different lignin contents: Relationships between lignin content and dissolved organic matter leaching

• Lignin content is inversely proportional to DOM release from the biochar. • Biochar pyrolyzed sawdust with more lignin extracted less Fe from soil. • Sawdust biochar with less lignin can enhance the As leaching from soil. Biochar is widely used material whose physical and chemical characteristics been widely investigated. Nevertheless, dissolved organic matter (DOM) released from biochar has received relatively little attention. In particular, little research has been conducted to understand the effects of feedstock biomass components on biochar DOM release. To control the amount of DOM released from biochar, this study focuses on the role of lignin, a component of biomass. To this end, samples of sawdust containing different lignin contents and a binary mixture of cellulose and lignin were pyrolyzed at 400 and 700 °C, and then the physico-chemical properties of the resulting biochar, and Fe(II) and As mobility were investigated in arsenic-contaminated soil amended by the biochar. This study showed that lignin is an critical factor in controlling the release of DOM. The amount of DOM released from the sawdust biochar with the lowest lignin content was 33% (400 °C) and 44% (700 °C) lower than those produced from lignin-rich biochar (sawdust without the extraction of lignin). The amount of oxalic acid in DOM decreased with increased lignin content. In addition, As mobility and the transformation of Fe(II) were minimized when lignin-rich biochar was applied to the As-contaminated soil. More importantly, these results suggest that controlling the lignin content of biomass can be universally applied to predict DOM concentrations of the biochars produced.

Influence of agricultural organic inputs and their aging on the transport of ferrihydrite nanoparticles: From enhancement to inhibition

Organic matter effectively regulates nanoparticles transport. However, little is known about the effect of agricultural organic inputs on the transport of ferrihydrite nanoparticles (FHNPs) during aging. In this study, columns were filled with sand mixed with varying proportions of pristine, water-processing, or alkali-processing biochar or swine manure and used to simulate the release of organic matter and changes in surface roughness of sand grains during field aging. The influence of these factors on FHNPs transport was investigated using column experiments. The dissolved organic matter (DOM) (0.008–24.8 mg L−1) released from agricultural organic inputs decreased the zeta potential of the FHNPs from 30.8 mV to 14.6–-48.9 mV and further caused electrostatic repulsion, osmotic repulsion, and elastic-steric repulsion between FHNPs and mixed sand, thus enhancing FHNPs transport. Ferrihydrite nanoparticles transport increased with increasing content of biochar and swine manure due to the increased amount of DOM. However, with the presence of organic inputs, surface roughness up to a certain degree (the increase in specific surface area up to 4.6 m2) became the dominant inhibition factor affecting FHNPs transport. After DOM release, agricultural organic inputs decreased the enhancement of FHNPs transport; with the increase input, their rougher surface gradually increased inhibition of FHNPs transport. The strongest FHNPs retention in the alkali-processing biochar (0.2–10%) or swine manure (1–2%) mixed sand columns indicated that fully aged agricultural organic inputs strongly inhibited FHNPs transport. Our findings provided novel insights into the critical influence of agricultural organic inputs and their aging on FHNPs transport, which changed gradually from enhancement to inhibition gradually.

Release of Photosynthetically Fixed Carbon as Dissolved Organic Matter by Anoxygenic Phototrophic Bacteria

Dissolved organic matter (DOM) release by anoxygenic phototrophic bacteria belonging to diverse taxonomic groups is a ubiquitous process, resulting in 5–20% of the CO2 fixed by photosynthesis leaving the cell. The green photosynthetic bacterium Chlorobaculum limnaeum releases ~10% of the carbon assimilated during autotrophic СО2 fixation. Extracellular DOM release by Cba. limnaeum is a normal function of living cells, since it does not occur in the dark and is inhibited by fluoroacetate, an inhibitor of СО2 fixation via the reducing TCA acids cycle, which is used for CO2 assimilation in this bacterium. In the presence of acetate or pyruvate in the medium, DOM release increases to 30 and 60%, respectively. Nitrogen limitation stimulates DOM release by Cba. limnaeum up to 60% of assimilated СО2. Under these conditions, addition of acetate results in DOM release increasing up to 100%. Therefore, the extracellular release of DOC by anoxygenic phototrophic bacteria can be considered as an important source of substrates for the growth of heterotrophic bacteria in oxygen-free aquatic ecosystems.

Links between size fractionation, chemical speciation of dissolved copper and chemical speciation of dissolved organic matter in the Loire estuary

Environmental contextThe toxicity of copper to aquatic life is highly dependent on its chemical form. In the vineyards of the Loire valley, mixtures of copper compounds are increasingly being used as fungicides. Our study investigating the origin and behaviour of dissolved copper on the land-sea continuum of the Loire advances our understanding of the chemical forms and fate of copper in estuarine systems. AbstractWe present data on the organic speciation of dissolved copper (dCu) in the Loire estuary for the soluble (<0.02µm) and dissolved (<0.45µm) fractions. These results were interpreted according to the distribution of natural organic matter along the estuary. We observed a high concentration of dCu (80nmolkg−1) upstream of the tidal front (S=0.1, where S is the salinity), possibly induced by drainage from vineyards located on the watershed of the estuary, and a concentration typical of coastal seawater at the end of the salinity gradient (3–4nmolkg−1). dCu showed a non-conservative distribution along the estuary with a notably strong decrease in concentration at the very first stage of mixing (S ~ 0.5) that increased again at low salinity (S=4.7). The latter increase in the concentration of dCu was related to the supply of colloidal copper induced by particle desorption in the maximum turbidity zone. The organic speciation of dCu revealed that the complexing capacity of copper ligands (LCu, up to 147 nanomoles of Cu per kilogram of water) exceeded dCu in both the soluble and dissolved fractions, which kept the activity of cupric ions below toxic levels for most unicellular phytoplankton. Humic substances comprised up to 95% of LCu in the continental shelf sample, but their complexing capacity did not account for all of the LCu in the inner estuary. We conducted pseudopolarographic experiments and found two other additional ligand classes: thiol-like and amino-acids/proteins. While humic substances are the dominant component of dissolved organic matter (DOM), the study of DOM suggested the release of colloidal DOM from a wastewater treatment plant. This structure could be a major source of proteinaceous LCu in the system that changes the dCu speciation in the middle of the estuary.

Anthropogenic influences on dissolved organic matter transport in high arsenic groundwater: Insights from stable carbon isotope analysis and electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry

In East and Southeast Asia, the health of over 100 million people is threatened by the consumption of groundwater containing high concentrations of arsenic (>10 μg L−1), which is released from sediments through reductive dissolution of arsenic-bearing iron/manganese oxides. Dissolved organic matter (DOM) is known to play a crucial role in the process of arsenic mobilization in shallow aquifers, and its availability and reactivity are key factors controlling the variation of arsenic concentrations in groundwater. However, it is unclear how human activities influence the transport of DOM and how the transportation affects the DOM molecular properties in high arsenic groundwater. This study provides insights on the sources and molecular compositions of DOM in groundwater from the Jianghan Plain, central China, a newly discovered area with seasonal fluctuations in arsenic concentrations in shallow groundwater. Monitoring of water levels and stable carbon isotope compositions in groundwater from different depths and canal water over a year indicated that terrestrial DOM was the dominant source, accounting for 54.2%–85.5% of groundwater DOM. Electrospray ionization combined with ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry revealed that canal water infiltration transferred aliphatic, tannin-like and leached aromatic DOM from sediments into groundwater. Therefore, groundwater recharge through irrigation using canal water not only inputs terrestrial DOM, but also accelerates the release of sedimentary DOM. Furthermore, carboxylic-rich alicyclic molecule (CRAM)-like DOM that is derived from biomolecules has the highest proportion (60.1%–65.5%) among the identified DOM structures. And, it might be reused in biochemical processes during arsenic mobilization, suggesting a third source of groundwater DOM in addition to canal water and sediments. The findings in this study advance the understanding on transport processes and molecular properties of DOM in high arsenic groundwater under extensive anthropogenic influences.

Pretreatment-promoted sludge fermentation liquor improves biological nitrogen removal: Molecular insight into the role of dissolved organic matter

Waste activated sludge (WAS) can be used as carbon sources to support biological nutrient removal (BNR). In this study, thermal-alkaline (THALK), ozonation (OZN), electrolysis (EC) and NaClO-promoted electrolysis (EC-AOP) were investigated to facilitate WAS solubilization and production of volatile fatty acids (VFAs). EEMF-PARAFAC and FT-ICR-MS were employed to characterize the transformation of dissolved organic matter (DOM) in WAS fermentation liquors at molecular level. THALK achieved the highest fluorescence intensity of C1 protein after pretreatment. Proteins and lipids were the dominant DOM in the pretreated WAS, while the DOM shifted towards substances with higher H/C and lower O/C after fermentation. The BNR results showed that THALK (100%) and EC-AOP (96.9%) outperformed other groups (78.9–90.3%) in terms of NO3-N removal, indicating the significant impact of DOM compositions. Overall, these results demonstrated that THALK and EC-AOP effectively enhanced release of VFAs and DOM, which subsequently improved NO3-N removal efficiency.

Spectroscopy, quality variations, and chemical release characteristics of dissolved organic matter extracted from soil of a wheat field at the North China Plain

Soil organic matter (SOM) property and the desorption effect by the hydrochemical solution during rainfall or irrigation play key roles in soil carbon dynamics. The quantity and molecular characteristics of the dissolved organic matter (DOM) extracted from the soil at a wheat field in the North China Plain were investigated to understand the chemical release characteristic of the SOM and the responsible factors. The amounts of DOM desorbed from the different extraction solutions were of the following order: Na2SO4 (62.1 mg/kg) > NaCl (58.7 mg/kg) > H2O (55.7 mg/kg)≈NaHCO3 (55.6 mg/kg) > CaCl2(46.2 mg/kg). The comparative differences of spectroscopy parameters including the smaller E2/E3, larger SUVA254, higher HIX, and lower BIX and FI of the Na-derived DOM than those of the Ca-obtained DOM indicate that the sodium electrolytes were favorable for the desorption of the organic matter with larger molecular weight, higher aromaticity, and humification degrees representing the more refractory property of the DOM. The good ternary relationship of the soil capacity of exchangeable cation (CEC), HIX of soil DOM, and soil TOC (rCEC-HIX = 0.78 ± 0.07, p < 0.05; rCEC-TOC = 0.82, p < 0.05; rHIX-TOC = 0.81 ± 0.09, p < 0.05) suggests that the binding of exchangeable cation with humus were the main association of organic matter with soil. Soil TOC was highly correlated with DOC concentrations (r = 0.78 ± 0.04, p < 0.05), and both parameters were positively correlated with the HIX of the DOM (rTOC-HIX = 0.81 ± 0.09, p < 0.05; rDOC-HIX = 0.64 ± 0.08, p < 0.05). This result indicates that the soil with higher TOC content tended to release higher concentrations of DOM with humus as the main component. The opposite relationships of the sodium adsorption ratio (SAR) with E2/E3 observed in ultrapure water and sodium electrolytes suggest that the larger SAR contributed to the release of DOM with more macromolecules in the solutions. Our study demonstrated that the quantity and quality of DOM leached from soils depend on the chemistry of extraction solutions, the dominant organic components of SOM as well as the soil properties such as CEC and SAR.

Photo-induced redox coupling of dissolved organic matter and iron in biochars and soil system: Enhanced mobility of arsenic

Dissolved organic matter (DOM) elucidated from biochars enhances the dissolution of iron oxides and reduction of iron. However, given that reduction mechanism of iron (Fe(III)) in the practical biochar applications for soil amendment and environmental remediation have not been fully elucidated, this study laid great emphasis on the photo-induced Fe(II) liberated from DOM-Fe(III) complexes. Thus, pyrolysis of biomass was carried out at 300 °C to maximize DOM release from biochars. Moreover, three different biomass samples (rice straw (R), granular sludge (G) from an anaerobic digester, and spent coffee grounds (C)) were chosen as carbon substrates for biochars preparation. To demonstrate the transformation of Fe(III), 1 and 5 wt% biochar was applied to the clean (S1) and arsenic-contaminated (S2) soil with/without the light. The results indicate that the light condition produces more Fe(II). The amount of Fe(II) accounts for 25.3, 28.6, and 30.7% of total iron under the light with 5 wt% GB, RB, and CB in S1, and 10.6, 13.1, and 13.8% in S2. This study demonstrates that Fe(II) is generated more under ultraviolet irradiation than visible light and dark condition. In addition, a control experiment without biochar showed that DOM plays an important role in the reduction of Fe(III). The mobility of arsenic increased under the light condition since the intermediates of DOM photo-degradation accelerates the dissolution of iron oxides and arsenic competes with DOM for the adsorption. Therefore, there was no significant correlation between the elution of arsenic and the formation of Fe(II) during the reductive dissolution of iron oxide under the light condition.

Impacts of Identified Bacterium Ensifer adhaerens on Microcystis aeruginosa and Subsequent Microcystin Release

The release of microcystin and dissolved organic matter (DOM) during algae-lysing process draws much attention. In this study, Ensifer adhaerens (bacterium r23) was screened for algicidal activity against Microcystis aeruginosa (MA). The effects of dosage (2.04 × 106, 3.90 × 106, and 7.15 × 106 CFU ml−1) and dosing modes (single, double, and quadruple treatments totaling 7.15 × 106 CFU ml−1) on the release of microcystin-LR (MC-LR) and DOM were investigated. Besides cell density, intra/extra-cellular, and total MC-LR were measured during the lysing treatment. The DOM components were analyzed by parallel factor analysis (PARAFAC). Results show that the lowest dosage (2.04 × 106 CFU ml−1) not only stimulated MA growth during the initial 4 days but also triggered the production of more toxins, resulting in higher total MC-LR than the controls on 2–16 days. The higher dosages suppressed MA growth and MC-LR production simultaneously, and the total MC-LR content was substantially lower than those of the controls during the whole experimental period. The total MC-LR for the double and quadruple treatments were lower than the controls after 30-day treatment, but still higher than the single dosage. Both intracellular and extracellular DOM (IDOM, EDOM) of MA had five components, namely tyrosine-like (C1), tryptophan-like (C2 and C3), fulvic acid-like (C4), and humic acid-like (C5) substances. C1 and C3 in the EDOM resulted from MA lysis and increased along with the treatment. C2 in the EDOM came from the nutrient broth and could be consumed by r23. C4 and C5 in the EDOM mainly resulted from the growth of r23.

Organic matter from Arctic sea-ice loss alters bacterial community structure and function

Continuing losses of multi-year sea ice (MYI) across the Arctic are causing first-year sea ice (FYI) to dominate the Arctic ice pack. Melting FYI provides a strong seasonal pulse of dissolved organic matter (DOM) into surface waters; however, the biological impact of this DOM input is unknown. Here we show that DOM additions cause important and contrasting changes in under-ice bacterioplankton abundance, production and species composition. Utilization of DOM was influenced by molecular size, with 10–100 kDa and >100 kDa DOM fractions promoting rapid growth of particular taxa, while uptake of sulfur and nitrogen-rich low molecular weight organic compounds shifted bacterial community composition. These results demonstrate the ecological impacts of DOM released from melting FYI, with wide-ranging consequences for the cycling of organic matter across regions of the Arctic Ocean transitioning from multi-year to seasonal sea ice as the climate continues to warm. Arctic sea-ice melt causes a release of dissolved organic material (DOM) into the surface waters. The increased dominance of first-year ice and DOM release is impacting under-ice bacterial communities.

Experimental evidence reveals impact of drought periods on dissolved organic matter quality and ecosystem metabolism in subalpine streams

AbstractSubalpine streams are predicted to experience lower summer discharge following climate change and water extractions. In this study, we aimed to understand how drought periods impact dissolved organic matter (DOM) processing and ecosystem metabolism of subalpine streams. We mimicked a gradient of drought conditions in stream‐side flumes and evaluated implications of drought on DOM composition, gross primary production, and ecosystem respiration. Our experiment demonstrated a production and release of DOM from biofilms and leaf litter decomposition at low discharges, increasing dissolved organic carbon concentrations in stream water by up to 50%. Absorbance and fluorescence properties suggested that the released DOM was labile for microbial degradation. Dissolved organic carbon mass balances revealed a high contribution of internal processes to the carbon budget during low flow conditions. The flumes with low discharge were transient sinks of atmospheric CO2 during the first 2 weeks of drought. After this autotrophic phase, the metabolic balance of these flumes turned heterotrophic, suggesting a nutrient limitation for primary production, while respiration remained high. Overall our experimental findings suggest that droughts in subalpine streams will enhance internal carbon cycling by transiently increasing primary production and more permanently respiration as the drought persists. We propose that the duration of a drought period combined with inorganic nutrient availability are key variables that determine if more carbon is respired in situ or exported downstream.

Phosphorus mobility in dam reservoir affected by redox oscillations: An experimental study

The internal sedimentary phosphorus (P) load of aquatic systems is able to support eutrophication, especially in dam–reservoir systems where sedimentary P stock is high and where temporary anaerobic conditions occur. The aim of this study therefore is to examine the response of sedimentary P exposed to redox oscillations. Surface sediments collected in the Champsanglard dam–reservoir (on the Creuse River, France) were subjected to two aerobic phases (10 and 12 days) alternated with two anaerobic periods (21 and 27 days) through batch incubations. The studied sediment contained 77 ± 3 μmol/g DW of P, mainly associated with the ascorbate fraction (amorphous Fe/Mn oxyhydroxides). The used sediment was rich in organic matter (OM) (21% ± 1%) with primarily allochthone signature. Our results showed that redox oscillations enhance dissolved inorganic phosphorus release at sediment/water interface. During the first anaerobic stage, the P release was mainly controlled by the dissolution/precipitation of iron minerals. The more pronounced increase of P release during the second anaerobic stage (44%) was due to various mechanisms related to the change in quality of dissolved organic matter (DOM), namely a higher SUVA254 and humification indices. The release of more refractory DOM (rDOM) served to lower the microbial metabolism activity, possibly favored iron oxyhydroxide aggregation and thus limiting iron reduction. In addition, rDOM is able to compete for mineral P sorption sites, leading to a greater P release. In reservoir with predominant allochthone OM input, the release of more aromatic DOM therefore plays an important role in P mobility.

A DEB-based approach of modeling dissolved organic matter release by phytoplankton

Extracellular release of dissolved organic matter (DOM) by phytoplankton is a significant process that drives the microbial loop, providing energy and nutrients to bacteria. A model based on Dynamic Energy Budget (DEB) theory is proposed for describing DOM release by phytoplankton under nitrogen-replete and nitrogen-limiting conditions. The model allows for variable biomass stoichiometry by partitioning biomass into structural mass and a number of reserves masses (C-reserves, N-reserves and generalized reserves), with each pool having a constant chemical composition. In the context of DEB theory, two alternative pathways of DOM release emerge from the theory, without additional assumptions; one relates to growth and lysis of the cells and one to rejection of unprocessed substrates due to stoichiometric constraints. We assume that these pathways correspond to the two conceptual mechanisms of DOM release, i.e., passive diffusion and active exudation respectively. Model parameters were estimated by fitting the model to literature data for the diatom Thalassiosira pseudonana. Model results suggest higher mass-specific growth, dissolved organic carbon (DOC) and nitrogen (DON) release rates during the nitrogen-replete phase of growth, while the percentage extracellular release (DOC release rate over primary production rate) is higher during the nitrogen-limited phase of growth. Moreover, nutrient limitation induces an increase in biomass C:N ratio which is followed by an increase in C:N ratio of the produced DOM. These results are in accordance with a wide range of experimental evidence. The relative contribution of the two release mechanisms affects the quality of DOM produced in terms of elemental and molecular composition and size fractionation, which, in turn, may have implications for the bioavailability of the produced DOM to bacteria. Moreover, the model predicts a close to linear relationship between (log-transformed) primary production and DOC release under nitrogen-replete conditions. Deviations from linearity are related to the prevalence of the active exudation mechanism and the reduction of the primary production rate under nitrogen-limiting conditions.

Leaching of dissolved organic matter from seagrass leaf litter and its biogeochemical implications

Dissolved organic matter (DOM) represents a significant source of nutrients that supports the microbial-based food web in seagrass ecosystems. However, there is little information on how the various fractions of DOM from seagrass leaves contributed to the coastal biogeochemical cycles. To address this gap, we carried out a 30-day laboratory chamber experiment on tropical seagrasses Thalassia hemprichii and Enhalus acoroides. After 30 days of incubation, on average 22% carbon (C), 70% nitrogen (N) and 38% phosphorus (P) of these two species of seagrass leaf litter was released. The average leached dissolved organic carbon (DOC), dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP) of these two species of seagrass leaf litter accounted for 55%, 95% and 65% of the total C, N and P lost, respectively. In the absence of microbes, about 75% of the total amount of DOC, monosaccharides (MCHO), DON and DOP were quickly released via leaching from both seagrass species in the first 9 days. Subsequently, little DOM was released during the remainder of the experiment. The leaching rates of DOC, DON and DOP were approximately 110, 40 and 0.70 µmol/(g·d). Leaching rates of DOM were attributed to the nonstructural carbohydrates and other labile organic matter within the seagrass leaf. Thalassia hemprichii leached more DOC, DOP and MCHO than E. acoroides. In contrast, E. acoroides leached higher concentrations of DON than T. hemprichii, with the overall leachate also having a higher DON: DOP ratio. These results indicate that there is an overall higher amount of DOM leachate from T. hemprichii than that of E. acoroides that is available to the seagrass ecosystem. According to the logarithmic model for DOM release and the in situ leaf litter production (the Xincun Bay, South China Sea), the seagrass leaf litter of these two seagrass species could release approximately 4×103 mol/d DOC, 1.4×103 mol/d DON and 25 mol/d DOP into the seawater. In addition to providing readily available nutrients for the microbial food web, the remaining particulate organic matter (POM) from the litter would also enter microbial remineralization processes. What is not remineralized from either DOM or POM fractions has potential to contribute to the permanent carbon stocks.

Distinct molecular signatures in dissolved organic matter produced by viral lysis of marine cyanobacteria.

Dissolved organic matter (DOM) plays a central role in the microbial ecology and biogeochemistry of aquatic environments, yet little is known about how the mechanism of DOM release from its ultimate source, primary producer biomass, affects the molecular composition of the inputs to the dissolved pool. Here we used a model marine phytoplankton, the picocyanobacterium Synechococcus WH7803, to compare the composition of DOM released by three mechanisms: exudation, mechanical cell lysis and infection by the lytic phage S-SM1. A broad, untargeted analytical approach reveals the complexity of this freshly sourced DOM, and comparative analysis between DOM produced by the different mechanisms suggests that exudation and viral lysis are sources of unsaturated, oxygen-rich and possibly novel biomolecules. Furthermore, viral lysis of WH7803 by S-SM1 releases abundant peptides derived from specific proteolysis of the major light-harvesting protein phycoerythrin, raising the possibility that phage infection of these abundant cyanobacteria could be a significant source of high molecular weight dissolved organic nitrogen compounds.

Dynamic processing of DOM: Insight from Exometabolomics, Fluorescence Spectroscopy, and Mass Spectrometry.

Dissolved organic matter (DOM) in freshwater environments is an important source of organic carbon, supporting bacterial respiration. Frozen environments cover vast expanses of our planet, with glaciers and ice-sheets storing upwards of six petagrams of organic carbon. It is generally believed that DOM liberated from ice stimulates downstream environments. If true, glacial DOM is an important component of global carbon cycling. However, coupling the release of DOM to microbial activity is challenging due to the molecular complexity of DOM and the metabolic connectivity within microbial communities. Using a single environmentally relevant organism, we demonstrate that processing of compositionally diverse DOM occurs, but, even though glacially derived DOM is chemically labile, it is unable to support sustained respiration. In view of projected changes in glacier DOM export, these findings imply that biogeochemical impacts on downstream environments will depend on the reactivity and heterogeneity of liberated DOM, as well as the timescale.

Chemical cleaning-associated generation of dissolved organic matter and halogenated byproducts in ceramic MBR: Ozone versus hypochlorite

This study characterized the dissolved organic matter (DOM) and byproducts generated after the exposure of activated sludge to ozone and NaClO in ceramic MBR. It was found that NaClO triggered more significant release of DOM than ozone. Proteins with the molecular weight greater than 20 kDa and humic acid like-substances were the principal components of DOM generated by NaClO, while ozone was found to effectively degrade larger biopolymers to low molecular weight substances. The results showed that more than 80% of DOM generated by NaClO and ozone could pass through the 0.2-μm ceramic membrane. Furthermore, total organic chlorine (TOCl) was determined to be the principal species of halogenated byproducts in both cases, while the generation of TOCl by NaClO was much more significant than that by ozone. Only a small fraction of TOCl was removed by the 0.2-μm ceramic membrane. More importantly, the toxic bioassays further revealed that the supernatant of sludge suspension and permeate in the MBR with NaClO cleaning exhibited higher developmental toxicity to the polychaete embryos than those by ozone. The results clearly showed that on-line chemical cleaning with ozone should be a more eco-friendly and safer approach for sustaining long-term membrane permeability in ceramic MBR.

Nutrient availability as major driver of phytoplankton-derived dissolved organic matter transformation in coastal environment

Incubation experiments were performed to examine the processing of fresh autochthonous dissolved organic matter (DOM) produced by coastal plankton communities in spring and autumn. The major driver of observed DOM dynamics was the seasonally variable inorganic nutrient status and characteristics of the initial bulk DOM, whereas the characteristics of the phytoplankton community seemed to have a minor role. Net accumulation of dissolved organic carbon (DOC) during the 18-days experiments was 3.4 and 9.2 µmol l−1 d−1 in P-limited spring and N-limited autumn, respectively. Bacterial bioassays revealed that the phytoplankton-derived DOC had surprisingly low proportions of biologically labile DOC, 12.6% (spring) and 17.5% (autumn). The optical characteristics of the DOM changed throughout the experiments, demonstrating continuous heterotrophic processing of the DOM pool. However, these temporal changes in optical characteristics of the DOM pool were not the same between seasons, indicating seasonally variable environmental drivers. Nitrogen and phosphorus availability is likely the main driver of these seasonal differences, affecting both phytoplankton extracellular release of DOM and its heterotrophic degradation by bacteria. These findings underline the complexity of the DOM production and consumption by the natural planktonic community, and show the importance of the prevailing environmental conditions regulating the DOM pathways.

Comparative study of dissolved organic matter generated from activated sludge during exposure to hypochlorite, hydrogen peroxide, acid and alkaline: Implications for on-line chemical cleaning of MBR

Although on-line chemical cleaning has been extensively employed for maintaining the MBR permeability, little attention has been given to the negative impacts of such prevalent membrane cleaning practice. This study thus comparatively investigated the potential release of dissolved organic matter (DOM) from activated sludge upon the exposure to different kinds of frequently-used cleaning reagents, i.e. NaClO, H2O2, HCl and NaOH. It was found that NaClO at 50 and 80 mg L−1 triggered significant release of DOM, while NaOH strongly promoted soluble nitrogen release. However, the DOM generation induced by H2O2 in the range of 0–80 mg L−1 was nearly negligible. The combined analysis by EEM-PARAFAC and LC-OCD-OND further revealed that NaClO-triggered DOM mainly originated from the breakdown of humic substances and other small humics with molecular weight (MW) less than 500 Da. In contrast, proteins and other biopolymers with higher MW highly contributed to DOM induced by NaOH. Most of DOM detected in this study belonged to low molecular weight (LMW) substances, which were not considered readily biodegradable or physically retainable by microfiltration membrane. It appears from this study that DOM generated from suspended activated sludge during membrane cleaning with different chemicals should be taken into serious consideration when water recycle and reuse are concerned.

Sustained production of dissolved organic carbon and nitrogen in forest floors during continuous leaching

The release of dissolved organic matter (DOM) in forest floors is a dynamic component of organic carbon transformations in forest soils. The biogeochemical processes driving the production and release of dissolved organic carbon and nitrogen (DOC and DON) in forest floors are strongly affected by hydroclimatic conditions during rainfall events. We conducted an exhaustive percolation experiment to examine the pool size and sustained production of mobilizable DOC and DON in Oi, Oe and Oa layers of spruce, larch and beech forest floors. The percolation experiment with small reconstructed soil columns was conducted at 5 and 15°C for 25days with a percolation volume equal to 36mm d−1, adding up to a total percolation volume of 900mm. Percolates were collected at an interval of 5days and analyzed for DOC and DON. Simultaneously to the periodic analysis of percolates, the CO2 release was measured. The sustained release of DOM during the last leaching period from day 20 to 25, after 720mm of percolation, was defined as the production rate of DOM. The cumulative release of DOM was large: in case of the 15°C treatment, the total amount of DOM extracted for 25days was on average 1.6% and 2.2% of the total C and N stock, respectively. The largest cumulative release of DOM and CO2 was observed for the beech samples. The ratio of cumulative CO2/cumulative DOC release ranged from 1 to 3 for Oi and Oe samples but was <0.2 for spruce Oa. No changes in DOC/DON ratios as a result of percolation amount were detected. The production rates indicated a rapid replenishment of DOM pools after leaching. The Q10 values for the DOM production rates ranged from 2 to 4 and were similar to those for CO2 production. The positive correlation between DOM and CO2 production rates in Oi and Oe samples highlights the importance of microbial activity for DOM release. The pool of mobilizable DOM in forest floors seems large enough to provide a sustained rate of DOM release throughout precipitation events under field conditions although the actual rate of mobilization may vary in the field, depending on antecedent conditions and the duration and intensity of the rainfall event.