Molecular Weight Humic Substances Research Articles

What Is the Molecular Weight of "High" Molecular Weight Dissolved Organic Matter?

The use of ultrafiltration to isolate high molecular weight dissolved organic matter (HMWDOM) from seawater is a fundamental tool in the environmental organic chemist's toolbox. Yet, important characteristics of HMWDOM relevant to its origin and cycling, such as its molecular weight distribution, remain poorly defined. We used diffusion-ordered NMR spectroscopy coupled with mixed-mode chromatography to separate and characterize two major components of marine HMWDOM: acylpolysaccharides (APS) and high molecular weight humic substances (HS). The molecular weights (MWs) of APS and HS both fell within distinct, narrow envelopes; 2.0-16 kDa for APS and 0.9-6.5 kDa for HS. In water samples from the North Pacific Ocean the average MW of both components decreased with depth through the mesopelagic. However, the minimum MW of APS was >2 kDa, well above the molecular weight cutoff of the ultrafilter, suggesting APS removal processes below 2 kDa are highly efficient. The MW distribution of APS shows only small variations with depth, while the MW distribution of HS narrowed due to removal of HMW components. Despite the narrowing of the MW distribution, the concentration of HS did not decrease with depth between 15 and 915 m. This suggests that HMW HS produced in surface waters was either degraded into lower MW compounds without significant remineralization, or that HMW HS was remineralized but replaced by an additional source of HS in the mesopelagic ocean. Based on these results, we propose potential pathways for the production and removal of these major components of HMWDOM.

The effect of surfactant on biosolubilization of weathered coal

AbstractWeathered coal is a type of coal with barren utility value, which is exposed on the surface for a prolonged time or shallowly buried in coal seams, and the physicochemical properties of weathered coal have changed compared with high‐quality coal. China is also a country with large reserves of weathered coal, therefore, the value utilization of weathered coal has received tremendous scientific attention. Herein, the main purpose is to explore the combined effect of surfactants and microorganisms on bioconversion of weathered coal to produce humic substances. Tween‐80, Phanaerochaete chrysosporium (J1) and Trametes versicolor (J2) were selected as surfactants and engineering strains to convert weathered coal. 3D fluorescence spectra were used to analyze the species and contents of humic acids during the biosolubilization process of weathered coal. Results revealed that both J1 and J2 exhibited good performance on weathered coal biosolubilization (increasing 10% and 14% compared to the control, respectively). The intensity of weathered coal biosolubilization was significantly increased by 15% and 12% when J1 and J2 were applied in combination with Tween‐80. And the addition of Tween‐80 also distinctly reduced the molecular weight of humic substances. In summary, surfactant plays an important role in bioconversion of weathered coal, while molecular weight of humic substances was different that weathered coal biosolubilization under the co‐action of J1 and Tween‐80 increased and J2 with Tween‐80 was opposite. In short, this project might provide a new strategy for exploring the biodegradation of weathered coal.

Effects of cornstalk mulching and rainfall intensity on the quantity and quality of dissolved organic carbon in runoff: A field rainfall simulation at the plot scale

AbstractUnderscoring how crop mulching changes the dissolved organic carbon (DOC) in runoff is essential to understanding the sustainable application of cornstalk mulching. This study examined the runoff DOC quantity and quality in plots with and without cornstalk mulching under various rainfall intensities in a representative steep fallow land of southwestern China. Field rainfall simulations within a 60 min duration were conducted on the triplet plots (1.5 m × 1 m for a single plot) with bare soils under 30‐ and 90‐mm h−1intensities and with cornstalk mulching (air‐dried whole plants with 60% coverage and weight 1.65 kg per plot) under 90 mm h−1intensity, respectively. The concentration and loss of runoff DOC and its quality variables, including SUVA254(aromaticity of DOC), C:C (proportion of colored humic substances in DOC), and E4:E6 (fulvic acid to humic acid in DOC), are determined. Results showed that runoff DOC concentration and loss were both significantly higher at 90 mm h−1intensity than at 30 mm h−1intensity (p < 0.05), simultaneously, they were 52.6% and 53.2% higher in mulched plots than in bare plots (p < 0.05), respectively. Runoff SUVA254and C:C in bare plots were 90% and 88.9% higher in 30 mm h−1intensity than in 90 mm h−1intensity (p < 0.05), respectively. Whereas, E4:E6 of runoff was 12.3% higher in mulched plots than that in bare plots (p < 0.05). Our findings imply that cornstalk mulching enhances the export of runoff DOC with a decline in aromaticity and the molecular weight of humic substances.

Microbial metabolic responses and CO2 emissions differentiated by soil water content variation in subarctic tundra soils.

Recent rapid air temperature increases across the northern-latitude tundra have prolonged permafrost thawing and snow melting periods, resulting in increased soil temperature (Ts) and volumetric soil water content (SWC). Under prolonged soil warming at 8°C, Alaskan tundra soils were incubated in a microcosm system and examined for the SWC differential influence on the microbial decomposition activity of large molecular weight (MW) humic substances (HS). When one microcosm soil (AKC1-1) was incubated at a constant SWC of 41% for 90 days (T = 90) and then SWC was gradually decreased from 41% to 29% for another T = 90, the initial HS was partly depolymerized. In contrast, in AKC1-2 incubated at a gradually decreasing SWC from the initial 32% to 10% for T = 90 and then increasing to 27% for another T = 90, HS depolymerization was undetected. Overall, the microbial communities in AKC1-1 could maintain metabolic activity at sufficient and constant SWC during the initial T = 90 incubation. In contrast, AKC1-2 microbes may have been damaged by drought stress during the drying SWC regimen, possibly resulting in the loss of HS decomposition activity, which did not recover even after re-wetting to an optimal SWC range (20-40%). After T = 90, the CO2 production in both treatments was attributed to the increased decomposition of small-MW organic compounds (including aerobic HS-degradative products) within an optimal SWC range. We expect this study to provide new insights into the early effects of warming- and topography-induced SWC variations on the microbial contribution to CO2 emissions via HS decomposition in northern-latitude tundra soil.

Distribution of Molecular Weight of Humic Substances Isolated from Soils of Tallgrass Temperate Rainforests (Chernevaya Taiga)

The boreal forests of Russia are one of the largest forest areas on the planet. As a result of climate change, the rate of carbon sequestration and stabilization of organic matter are important indicators of environmental conservation. To understand mechanisms of stabilization and the structure of soil organic carbon, the molecular-weight (MW) distribution of humic acids (HAs) in soils of the central deciduous-forest zone of boreal forests (Chernevaya taiga, transitional ecotone forest, coniferous forest) was studied. Analysis of the MW distribution of HAs was conducted using size-exclusion chromatography. This approach allowed us to obtain the distribution of high-, medium-, and low-molecular fractions of HAs. It has been revealed that the content of the low-MW fraction prevails over the content of the medium- and high-MW fractions of HAs, which indicates the process of stabilization in soil organic matter. The accumulation of low-molecular-weight fraction occurs with the aromaticity increase in HAs, which indicates that HAs with a relatively high proportion of aromatic fragments have smaller hydrodynamic radius and a lower MW. It has been statistically substantiated that the low-molecular-weight fraction correlates with the content of aromatic compounds and carboxyl structural fragments of HAs, which indicates the resistance of the soil organic matter of Chernevaya taiga to biodegradation.

Characterization of humic substances isolated from a tropical zone and their role in membrane fouling

Humic substances (HS) are heterogeneous organic polymers that show monotonous properties. HS are present at different proportions in the terrestrial environment, and they implicate the performance of water desalination plants. Recently electrodialysis self-reversal (EDR) technology has been used to treat excess salinity, hardness and fluoride in HS rich groundwater in Sri Lanka (~3 mg L−1, dissolved C). Due to the high organic content of HS in groundwater, the EDR ion exchange membranes are prone to fouling. To unravel the fouling mechanisms, humic acid (HA) and fulvic acid (FA) fractions were isolated and fractionated from the groundwater and characterized spectroscopically. All membrane fouling experiments were conducted for HA and FA contaminated water using a lab-scale EDR setup operated under batch mode. Sequential desorption experiments were carried out using distilled water, 2% (w/w) NaCl, and 0.3% (w/w) sodium dodecyl sulfate to probe variations among two fouling mechanisms. Similarities were found in aromatic and aliphatic carbon contents, humification degree, and molecular weight of humic substances. Higher proportions of carboxylic and phenolic groups were found in FA compared to HA. Ester groups in HA facilitate P, S, and Si migration within organic moieties. The molecular environs in the vicinity of carboxyl functional groups led different fouling mechanisms. HA fouling was found irreversible compared to FA fouling. The subtle differences between fouling mechanisms will help implement appropriate pretreatments to ensure the durability of EDR membranes.

How the Supramolecular Nature of Lignohumate Affects Its Diffusion in Agarose Hydrogel.

Lignohumate, as an industrially produced analog of natural humic substances, is studied from the point of view of its diffusion properties. This work focuses on its permeation ability, important in agricultural and horticultural applications, connected with its penetration into plant organs as leaves and roots. The hydrogel based on agarose was used as a model material for the diffusion of lignohumate. Two types of experiments were realized: the diffusion of lignohumate in the hydrogel diffusion couple and the diffusion of lignohumate from its solution into hydrogel. The diffusion coefficient of lignohumate in the hydrogel was determined and used for the modelling of the time development of concentration profiles. It was found that the model agrees with experimental data for short times but an accumulation of lignohumate in front of the interface between donor and acceptor hydrogels was observed after several days. The particle size distribution of lignohumate and changes in the E4/E6 ratio used as an indicator of molecular weight of humic substances were determined. The results showed that the supramolecular structure of lignohumate can react sensitively to actual changes in its environs and thus affect their mobility and permeability into different materials. A filtration effect at the interface can be observed as an accompanying phenomenon of the re-arrangement in the lignohumate secondary structure.

Application of heat-activated peroxydisulfate pre-oxidation for degrading contaminants and mitigating ultrafiltration membrane fouling in the natural surface water treatment

Membrane fouling is posing a critical obstacle limiting the widespread application of ultrafiltration (UF). Among the numerous membrane foulants, natural organic matter (NOM) is one of the most problematic since it exists ubiquitously in natural waters and can cause severe membrane fouling. This study investigated the removal of NOM in surface water and the mitigation of membrane fouling using heat-activated peroxydisulfate (PDS) as a pretreatment for UF process. The results demonstrated that the NOM was efficiently removed, with ultraviolet absorbance (UV254) and dissolved organic carbon (DOC) decreasing by approximately 71% and 52%, respectively, at a PDS dose of 0.8 mM within 60 min (80 °C). The chromatograms of high performance size exclusion chromatography (HPSEC) indicated that some high molecular weight humic substances with a peak at approximately 10 kDa were oxidized to low molecular weight organic matters distributed in the range of < 100 Da during the pretreatment process. Moreover, three-dimensional fluorescence parallel factor analysis (PARAFAC) indicated that humic-like substances were much more easily degraded by heat-activated PDS pretreatment than protein-like substances. These results indicated that some unsaturated NOM fractions were first degraded and then mineralized to carbon dioxide during pretreatment. Meanwhile, the destroyed structure of humic substances might hinder its binding with high valence cations to reduce the possibility of high valence cations deposited on the membrane surface, thereby reducing membrane fouling. Therefore, membrane fouling could be significantly mitigated due to the shifts of NOM concentration and structure by heat-activated PDS pretreatment in the surface water treatment.

Soil organic carbon storage and quality are impacted by corn cob biochar application on a tropical sandy loam

Humic substances, which are integral components of total organic carbon (TOC), influence soil quality. The study aimed to investigate whether humic and non-humic fractions exhibit early, consistent, and measurable changes and affect TOC sensitivity and storage in a tropical sandy loam soils amended with corn cob biochar. There were four treatments with four replicates established in a randomized complete block design. Composite soil samples were taken from plots without biochar (CT), from plots incorporated with 15 t biochar ha−1 (BC-15), and 30 t biochar ha−1 without or with phosphate fertilizer (BC-30 and BC-30+P). The TOC, and humin, humic acid (HA), and fulvic acid (HA) fractions of soil organic carbon were determined for each treatment. The optical densities (400–700 nm) were measured on the soil-free extracts by spectrophotometry; the densities measured at 465 and 665 nm were used to calculate the E465/E665 ratios. The BC-30 and BC-30+P plots recorded the highest TOC, humin, humic acid (HA), and fulvic acid (FA) contents with respect to the lowest in the CT. The total exchangeable carbon stratification was significantly higher in all the biochar-treated plots relative to the CT. Spectral analysis showed higher values of E465/E665 (5.02 and 5.15) in the CT and BC-15-treated soils, respectively, compared with the BC-30 and BC-30+P-amended soils with E465/E665 ratios of 2.76 and 2.98, respectively. Corn cob biochar applied to a tropical sandy loam: • increased the concentrations of HA and FA and led to increased stratification of TOC, with a stronger effect on HA compared with FA; • significantly lowered E465/E665 at the high biochar application rate of 30 t ha−1, implying the dominance of high molecular weight humic acid-like substances, and increased degree of aromaticity of the TOC.

Effects of fouling on separation performance by forward osmosis: the role of specific organic foulants.

In this study, forward osmosis (FO) membranes and fouling solutions were systematically characterized to elucidate the effects of organic fouling on the rejection of two pharmaceutically active compounds, namely, sulfamethoxazole and carbamazepine. Municipal wastewater resulted in a more severe flux decline compared to humic acid and sodium alginate fouling solutions. This result is consistent with the molecular weight distribution of these foulant solutions. Liquid chromatography with organic carbon detection analysis shows that municipal wastewater consists of mostly low molecular weight acids and neutrals, which produce a more compact cake layer on the membrane surface. By contrast, humic acid and sodium alginate consist of large molecular weight humic substances and biopolymers, respectively. The results also show that membrane fouling can significantly alter the membrane surface charge and hydrophobicity as well as the reverse salt flux. In particular, the reverse salt flux of a fouled membrane was significantly higher than that under clean conditions. Although the rejection of sulfamethoxazole and carbamazepine by FO membrane was high, a discernible impact of fouling on their rejection could still be observed. The results show that size exclusion is a major rejection mechanism of both sulfamethoxazole and carbamazepine. However, they respond to membrane fouling differently. Membrane fouling results in an increase in sulfamethoxazole rejection while carbamazepine rejection decreases due to membrane fouling.

Combining Coagulation/MIEX with Biological Activated Carbon Treatment to Control Organic Fouling in the Microfiltration of Secondary Effluent.

Coagulation, magnetic ion exchange resin (MIEX) and biological activated carbon (BAC) were examined at lab scale as standalone, and sequential pre-treatments for controlling the organic fouling of a microfiltration membrane by biologically treated secondary effluent (BTSE) using a multi-cycle approach. MIEX gave slightly greater enhancement in flux than coagulation due to greater removal of high molecular weight (MW) humic substances, although it was unable to remove high MW biopolymers. BAC treatment was considerably more effective for improving the flux than coagulation or MIEX. This was due to the biodegradation of biopolymers and/or their adsorption by the biofilm, and adsorption of humic substances by the activated carbon, as indicated by size exclusion chromatography. Coagulation or MIEX followed by BAC treatment further reduced the problematic foulants and significantly improved the flux performance. The unified membrane fouling index showed that the reduction of membrane fouling by standalone BAC treatment was 42%. This improved to 65%, 70%, and 93% for alum, ferric chloride and MIEX pre-treatment, respectively, when followed by BAC treatment. This study showed the potential of sequential MIEX and BAC pre-treatment for controlling organic fouling and thus enhancing the performance of microfiltration in the reclamation of BTSE.

Coupling effects of abiotic and biotic factors on molecular composition of dissolved organic matter in a freshwater wetland

In this study, temporal and spatial variations in five defined molecular size fractions of dissolved organic matter (DOM) were examined for a well preserved wetland (Upo Wetland) and its surrounding areas, and the influencing factors were explored with many biotic and abioic parameters. For each DOM sample, the five size fractions were determined by size-exclusion chromatography coupled with organic carbon detector (SEC-OCD). For 2-year long monthly monitoring, bio-polymers (BP), humic substances (HS), building blocks (BB), low molecular-weight (LMW) neutrals, and LMW acids displayed the median values of 264, 1884, 1070, 1090, and 11μg-CL−1, respectively, accounting for 6.2%, 41.7%, 24.5%, 26.4%, and 0.4% of dissolved organic carbon (DOC). The dominant presence of HS indicated that terrestrial input played important roles in DOM composition of the freshwater ecosystem, which contrasted with coastal wetlands in other reports. Both seasonal and periodic patterns in the variations were found only for HS and BB among the size fractions. It was also notable that the sources of HS were seasonally shifted from aquagenic origin in winter to pedogenic origin in summer. The correlations among the size fractions revealed that BB and LMW neutrals might be degradation products from HS and humic-like substances (HS+BB), respectively, while LMW acids, from LMW neutrals. Principle component analysis revealed that the humic-like substances and the aromaticity of DOM were associated with temperature, chlorophyll a, phosphorous, and rainfall, whereas the other fractions and the molecular weight of HS were primarily affected by solar irradiation. Significant correlations between DOM composition and some biotic factors further suggested that DOM may even affect the biological communities, which provides an insight into the potential coupling effects of biotic and abiotic factors on DOM molecular composition in freshwater wetlands.

Comparison of extractable soil carbon and dissolved organic carbon by their molecular characteristics

The aim of this study was to compare the molecular characteristics of extractable soil carbon from biotopes of the Rašeliník watershed and dissolved organic carbon (DOC) from Rašeliník creek, by using the 0.1M pyrophosphate, 0.01M CaCl2 and Britton-Robinson buffer as extraction agents. The molecular weight Mr and weight-average molecular weight Mw, determined by the low pressure size exclusion chromatography, increased in the following sequence: humic substances (HS) in CaCl2 &lt; aquatic HS &lt; HS in sodium pyrophosphate ≤ HS in a buffer. Elution curves of all humic substances were characteristic by two peaks with predominant low-molecular fraction. Mr and Mw values of aquatic humic substances were 5.9 and 7.9 kDa, respectively, and proportion of this low-molecular fraction reached 97%. This corresponds to the fact, that the main fraction of HS in surface waters constitute fulvic acids. Using soil extraction in CaCl2 we obtained the fraction of organic carbon similar to the humic substances contain in DOC. Differences in quantity of humic substances extracted from soils among CaCl2 (mean 0.42 ± 0.39), Britton-Robinson buffer (34.9 ± 11.2) and sodium pyrophosphate (293.2 ± 113.4) were statistically significant. The A465/A665 ratio negatively correlated with molecular weight of humic substances.

Application of Fenton’s reagent as a polishing step for removal of UV quenching organic constituents in biologically treated landfill leachates

When landfill leachate, with or without biological pretreatment, is discharged to publically owned treatment works (POTWs), it can interfere with the installed treatment facilities. Biological treatment is ineffective for the removal of some of the bio-refractory organic matter, including UV254 quenching substances. Fenton’s reagent treatment for biologically treated landfill leachates is examined in this study as a polishing step to make landfill leachates acceptable to POTWs. The optimum conditions for the Fenton’s reagent treatment are explored. The molecular weight and hydrophobic–hydrophilic nature based fractions of the Fenton’s treated leachate samples are analyzed to provide insight into the leachate fractions targeted by the Fenton’s reagent. The results indicate that Fenton’s reagent can act as a good compliment to biological treatment as it can remove leachate fractions which are widely considered to be bio-refractory. It exhibited good UV254 absorbance removal by removing larger molecular weight humic substances and thus, can help solve the UV254 quenching problem due to leachates discharged to POTWs.

Humic substances in surface waters of the Ukraine

The results of studies of humic substances in water bodies of different types (lakes, rivers, and reservoirs) are given. It is shown that concentration of these natural organic compounds varies in a wide range of values (from 1.2 to 126.5 mg L−1) due to different sources of their formation. The highest concentrations of humic substances are characteristic for rivers of the Pripyat’ River basin flowing through the wetland. As we move from the north to the south, the content of humic substances is reduced. So, in the Kakhovka Reservoir, closing the Dnieper cascade of reservoirs, the concentration of humic substances is almost thrice as low as in the Kiev Reservoir, which is at the head of the cascade. Seasonal changes of humic substances concentration and the reasons for these changes are discussed. The prevailing fraction in the composition of humic substances is represented by fulvic acids, the content of which reaches 80.8–94.8% of the total. The results of studies of the molecular weight distribution of humic substances and the reasons for changes in the ratio of their individual fractions, depending on the detection method (spectrophotometric and fluorescence methods), are considered. The values of the number average (M n) and weight average (M w) molecular weight of humic substances and the degree of polydispersity are calculated. It is shown that M w varies seasonally. In spring and summer it is lower, but significantly increased in autumn. The reason for this phenomenon is degradation of high-molecular fractions of humic substances under the influence of UV light of solar radiation and increased microbiological activity during the summer season. As a result of these processes high-molecular fractions of humic substances are transformed into fractions with lower molecular weight, which become predominant.

Luminescence of Speleothems: A Comparison of Sources and Environments

Calcite speleothems are typically coarsely crystalline, colored various shades from brown through orange and white, and strongly luminescent. For most speleothems, the color is due to higher molecular weight humic substances incorporated in the calcite crystal structure, while the luminescence is mainly due to lower molecular weight fulvic acids. The fine scale banding of luminescence intensity has importance as a climatic indicator. A suite of eighteen speleothems spanning a range of geologic and climatic settings and a range of colors from deep brown to nearly white were selected for detailed characterization and spectroscopic measurements. Spectra were measured on solid samples and on solutions prepared by dissolving the speleothems in dilute HCl. The luminescent emission appears as a single broad band with peak wavelength varying from 390 to 450 nm. The excitation spectra are typically more complicated, with several maxima, and show more locality-to-locality variation. The emission bands shift to longer wavelengths as the excitation bands move to longer wavelengths, indicating that a mixture of molecular species is being selectively excited. The spectra of the solutions are similar but not identical to the spectra of the crystalline solids. The decay time of the luminescence (phosphorescence) is in the range of 0.5 to 0.7 second. Comparison of speleothem spectra from caves in different climatic settings and of speleothem spectra from the same cave indicate that each speleothem produces spectra characteristic of specific overlying soils and pathways through the epikarst and the vadose zone. No features were discovered that characterize regional scale geologic or climatic settings.

Effects of Ozonation on Molecular Weight Distribution of Humic Substances and Coagulation Processes – A Case Study: The Úzquiza Reservoir Water

In this article the influence of preozonation on the effectiveness of NOM removal via coagulation processes will be studied (focusing on the influence of the calcium hardness) as well as changes in MW (molecular weight) distribution of humic substances caused by ozonation. Additionally, THMFP removal in both ozonation and preozonation-coagulation processes is assessed. Three different types of water have been used in this study: a natural water from the Úzquiza Reservoir (Burgos, Spain), a synthetic water prepared using natural fulvic acids extracted from the Úzquiza Reservoir and a synthetic water prepared using a commercially supplied humic acid. Molecular weights of humic substances were determined using high-performance size exclusion chromatography (HPSEC); average molecular weights calculated for the unozonated humic substances are 4500 Da for the commercial humic acids and 1000 Da for the natural fulvic acids extracted from the Úzquiza Reservoir. Preozonation shifted the molecular weight distribution of humic substances (both humic and fulvic acids) towards lower average molecular weight values. For the natural water from the Úzquiza Reservoir (with low levels of calcium hardness and hydrophobic fraction (humic substances) being the main fraction of NOM), preozonation has a negative effect on the effectiveness of the coagulation process for NOM removal: the percentages of TOC removal via coagulation decrease with increasing ozone dosage; the maximum TOC removal (33%) is achieved for the unozonated water. Also for this water, ozonation reduced 5–25% of THMFP with ozone doses varying from 0.25 to 2.5 mg O3/L. A preferential THMFP removal, that is to say, higher reduction in THMFP (43%) relative to TOC (28%) is achieved by the coagulation-flocculation process; this also occurs when preozonation is used, independently of ozone dosage.

Assessing desorption resistance of PAHs in dissolved humic substances by membrane-based passive samplers

Binding of hydrophobic organic compounds (HOCs) to humic substances (HS) has significant influence on their fate and bioavailability in an aquatic environment, yet very little is known on the desorption behavior of HOCs in dissolved HS. In this study, triolein-embedded cellulose acetate membranes (TECAMs) were used as passive samplers in dissolved HS-containing solutions to extract the free polycyclic aromatic hydrocarbons (PAHs). The partitioning coefficients ( K OC) of PAHs in dissolved HS from different sources were determined. Then the rapidly desorbable fractions ( f rd) of HOCs in dissolved HS were quantified by a quasi-equilibrium model. Results show that the magnitude of K OC of PAHs correlated strongly with the O/C atomic ratio and molecular weight of HS. The PAHs with lower K OW values desorbed almost reversibly from an aquatic fulvic acid, a sediment humic acid (HA), a lignite HA, and Aldrich HA. However, for strongly hydrophobic chrysene binding to the lignite HA, some extent of desorption resistance was found and attributed to sorption hysteresis as well as steric hindrance. The present study illustrated that the quality of HS and the property of solute may play key roles in determining the desorption reversibility.

Hydrophobicity and molecular weight of humic substances on ultrafiltration fouling and resistance

Humic substances are the major foulant during ultrafiltration (UF) of wastewater. This study evaluates the effects of hydrophobicity and fractionated humic substances on UF fouling and permeation resistance. A commercial humic acid (HA) obtained from Aldrich was subjected to DAX-8 resin for fractionation of hydrophobic and hydrophilic fractions. These fractions were fractionated further into different molecular weight groups using gel filtration chromatography. The hydrophilic fraction exhibited the greatest flux decline, revealing that hydrophobicity decreases fouling. Since the size of particle fractions was significantly smaller than the pore size of membranes, fouling was greatest for the largest fraction with a 100-kDa membrane and the smallest fraction with a 10-kDa membrane. Severe fouling was due to adsorption and pore blocking. For the first 300 min of filtration, the fouling rate was high in all fractions and is more for 100-kDa than 10-kDa membrane due to greater hydraulic resistance of the HA deposit on the membrane surface. The effect of resistances was also investigated. Each operationally defined resistance depended on membrane pore size, pressure, and HA characteristics. For hydrophobicity and the molecular weight effect, the hydrophilic fraction has the strongest resistance. This study suggested that strong resistance is responsible for irreversible fouling and that is primarily due to pore adsorption and pore blocking.

A field study of advanced municipal wastewater treatment technology for artificial groundwater recharge

Field studies were conducted to investigate the advanced treatment of the municipal secondary effluent and a subsequent artificial groundwater recharge at Gaobeidian Wastewater Treatment Plant, Beijing. To improve the secondary effluent quality, the combined process of powdered activated carbon adsorption, flocculation and rapid sand filtration was applied, which could remove about 40% dissolved organic carbon (DOC) and 70% adsorbable organic halogens. The results of liquid size exclusion chromatography indicate that in the adsorption unit the removed organic fraction was mainly low molecular weight compounds. The fractions removed by the flocculation unit were polysaccharides and high molecular weight compounds. The retention of water in summer in the open recharge basins resulted in a growth of algae. Consequently, DOC increased in the polysaccharide and high molecular weight humic substances fraction. The majority of the DOC removal during soil passage took place in the unsaturated area. A limited reduction of DOC was observed in the aquifer zone.