Influence of pH on the release and chemical fractionation of heavy metals in sediment from a suburban drainage stream in an arid mine-based oasis

Purpose

The objectives of this study were to explore the influences of pH on the release of Cu, Zn, Cd, Pb, Ni, and Cr in sediments derived from the upstream, middle, and downstream reaches of Dongdagou stream in Gansu Province, Northwest China, and to examine the fractionation changes of heavy metals in the sediments after reaching their release equilibrium under different pH conditions.

Materials and methods

Sediment samples were obtained using a stainless steel grab sampler to collect the uppermost 10 cm of sediment from the channel bed. The pH-dependent release experiment was conducted in the solid-to-liquid ratio of 1:20 at different pH values (2, 4, 6, 8, 10, and 12) at room temperature. The total Cu, Zn, Cd, Pb, Ni, and Cr concentrations in the sediments were digested using an acid digestion mixture (HNO₃ + HF + HClO₄) in an open system. Metal fractionation of selected sediments was obtained using the Tessier sequential extraction procedure. Heavy metal concentrations in the samples were determined using atomic absorption spectrophotometry.

Results and discussion

The mean concentrations of heavy metals in sediments decreased in the following order: Zn (1676.67 mg kg^?1) > Pb (528.65 mg kg^?1) > Cu (391.34 mg kg^?1) > Cr (53.48 mg kg^?1) > Ni (34.27 mg kg^?1) > Cd (11.53 mg kg^?1). Overall, the solubility of Cu, Zn, Cd, Pb, and Ni decreased with increasing pH, and they were strongly released at pH 2. Moreover, the solubility of Cr increased with increasing pH, and its release was highest at pH 12. After reaching the release equilibrium of heavy metals under different pH conditions, the percentages of organic Cu, Zn, Cd, and Fe-Mn oxyhydroxide Pb decreased, compared to their initial fractions. The residual fractions of Ni and Cr were dominant, regardless of pH.

Conclusions

The average concentrations of Cu, Zn, Cd, and Pb in sediments were highly elevated compared with the soil background values in Gansu Province, China. The results of this pH-dependent release experiment showed that the release behaviors of Cu, Zn, Pb, and Cr followed an asymmetric V-shaped pattern, whereas Cd and Ni followed an irregular L-shaped pattern. The changes in the release of heavy metals in sediments were related to their redistribution between chemical fractionations.

     

The effects of pH,redox, and salinity on metal release from a contaminated sediment

Levels of total Hg were determined in the muscle tissue of Helicolenus dactylopterus (mean=0.29 ug.g⁻¹ ±0.025 S.E.; range=0.041.10 ug.g⁻¹) and Pontinus kuhlii (mean=0.16 ug.g⁻¹ ±0.092 S.E.; range=0.05-0.50 ug.g⁻¹) caught in the Azores between August 1989 and May 1990. Mercury concentrations were related to sex, length, weight, age, growth and condition of both fish species. There were highly significant positive correlations between Hg levels and these variables. In P. kuhlii a significant sex-related difference in Hg levels was found; the rate of Hg accumulation is significantly faster in females than in males and mean Hg levels were higher in females. Relationships describing Hg vs size and age dependence were determined for both species and sexes and patterns and rates of Hg accumulation were discussed in relation to a number of biological and ecological factors with influence on it. A comparison of Hg content of Azorean and Mediterranean populations of H. dactylopterus was conducted and its suitability as an indicator in pollutant monitoring of benthic compartment of marine ecosystems is discussed.     

Rate controlling processes in the release of radium-226 from uranium mill tailings

Uranium mill tailings (a pyritic quartz conglomerate ore) obtained from a Canadian operation were leached according to a modification of the method recommended by I.A.E.A. for the leaching of radioactive ions from solidified wastes. A static leach test procedure was chosen to provide information to permit an assessment of hazard due to release of Ra-226 to the environment. Narrow range size fractions (53 to 75 μm) of both fresh material obtained directly from a tailings line and material weathered for approximately 10 to 15 yr in a tailings basin were leached for up to 70 days. Distilled water and an acidic synthetic effluent (pH 2.5) were chosen as leachants. Two sequential desorption processes were observed, one before and the other after a leaching period of approximately 10 to 15 days. In the later and more significant stage of desorption, the leaching behaviour could be described in terms of a diffusion mechanism. Significantly greater quantities of ²²⁶Ra were leached over relatively short leaching periods by the acidic effluent; thus the quality of the effluent and percolating water in a tailings disposal area would markedly affect the leachability of Ra from tailings. It was also found that the quantities of ²²⁶Ra leached were significantly influenced by the solid: liquid ratio at low leaching volumes.     

Influence of algal blooms on the efficacy of La/Al-based phoslock in the control of phosphorus release from sediment in shallow lakes: a microcosm study

Journal of Soils and Sediments - One of the most significant problems in eutrophic lakes is the presence of algal blooms, which may affect phosphorus (P) inactivation agents (PIAs) in the control...     

Potential contaminant release from agricultural soil and dredged sediment following managed realignment

Purpose

Laboratory experiments were conducted to examine the potential for metal (Cu, Ni and Zn) and herbicide (simazine, atrazine and diuron) release from agricultural soil and dredged sediment in managed realignment sites following tidal inundation.

Materials and methods

Column microcosm and batch sorption experiments were carried out at low (5?practical salinity units, psu) and high (20?psu) salinity to evaluate the changes in the partitioning of metals and herbicides between the soil/sediment and the aqueous phase, and the release of metals and herbicides from soil/sediment to the overlying water column.

Results and discussion

For both the metals and herbicides, the highest contaminant loads were released from the sediment within the first 24?h of inundation suggesting that any negative impacts to overlying water quality in a managed realignment scheme will be relatively short term following tidal inundation of soil and sediment. The release of metals was found to be dependent on a combination of salinity effects and the strength of binding of the metals to the soil and sediment. In the case of the herbicides, salinity impacted on their release. Particulate organic carbon was found to control the binding and release of the herbicides, highlighting the importance of assessing soil and sediment organic matter content when planning managed realignment sites.

Conclusions

Our research demonstrates that metals and herbicides may be released from contaminated sediments and agricultural soils during initial periods of flooding by seawater in managed realignment sites.     

Soil and sediment geochemistry of the iron quadrangle,Brazil the case of arsenic

Soils and sediments around gold ore deposit and mining sites in the Iron Quadrangle present positive As anomalies (median concentrations > 100 mg kg^-1) and wide ranges (< 20 to > 2000 mg kg^-1) even in densely populated areas. These anomalies can be related to geological structures, to hydrothermal ore deposits and to their continuous exploitation over the past three centuries. The paper presents new data on both As geochemistry and soil and sediment geochemistry in general, and discusses consequences of the encountered anomalies, their reasons and potential effects and how they compare with current screening and threshold values. While surface soil As-values are reproducible at a given site, sediment anomalies show a strong seasonal variation that can be explained by tropical hydrological variances. Suggestions are presented on how to minimize the As-load in densely populated areas that might otherwise pose a potential health risk.     

Potential effects of food addition to sediment on test conditions in sediment toxicity tests

Purpose  

Standardized sediment toxicity assays often employ periodic additions of uncontaminated food to sustain energy and growth requirements of the test organisms. Consequently, selective feeding on this uncontaminated food may reduce exposure to sediment particles containing the test substance. To address this issue, some standard guidelines propose to add food to the sediment before spiking with the test substance to account for multiple exposure routes, including ingestion of contaminated food. The present study focused on the influence of different feeding regimens and compositions of the aqueous medium on water quality (ammonia concentrations) and test organism development.     

Effect of reducing conditions on P sorption of soils

Abstract

The phosphate sorption (P _sor) capacity of soils increased when the soils were reduced (Willet and Higgins, Aust. J. Soil Res., 16, 319–326, 1978). The present study aimed at the elucidation of this mechanism using Na₂S₂O₄ and 5 different soils. The P _sor of the 5 soils increased with the addition of a small amount of Na₂S₂O₄. Fe(II) was released from the soils with the addition of the same small amount of Na₂S₂O₄. Furthermore, when the amount of FeCl₂ corresponding to the amount of Fe(II) released along with the small amount of Na₂S₂O₄ was added, the P _sor of the soil increased. However, the P _sor of the lowland soils, of which the hydrous Fe oxide content was lower than the others, decreased when the amount of Na₂S₂O₄ addition was increased up to 150–200 g kg^?1. Based on these results, the following process is inferred for the increase in the P _sor of the soils when they are reduced. Hydrous Fe oxide in soil takes the form of very fine, high-density particles and reacts with P mainly on their surface. When a small amount of Na₂S₂O₄ is added, the hydrous Fe oxide is partially reduced, dissolved and finally re-precipitates with P by oxidation with O₂ from the air during the experiment.     

Use of batch leaching tests to quantify arsenic release from excavated urban soils with relatively low levels of arsenic

Purpose

In Japan, the excavated soils produced from constructions projects, which contain relatively low levels of arsenic (As), are considered as a potential concern if they could release significant amount of As to the environment. The aim of this study was to investigate the As leaching from excavated alkaline soils and, in particular, the influence of drying methods, pH of extracting solution, and consecutive washing on As leaching from these soils.

Materials and methods

Four excavated alkaline soils obtained from different construction sites in Tokyo, Japan, were used in this study. The soils were pretreated by three drying methods (air-dried, 40 °C-dried, and freeze-dried). Sequential extraction procedure was applied to partition As into five operationally defined chemical fractions. Batch leaching tests (initial pH-controlled leaching test and consecutive washing test) were conducted to investigate the As release under different leaching conditions.

Results and discussion

The As contents in the four soils were 9.22, 79.4, 6.75, and 11.7 mg kg^?1, respectively, and As was primarily associated with the residual phase. Arsenic leaching was strongly dependent on the extracting solution pH values. Strongly acidic extracting solution (pH 2) led to circumneutral leachates and limited As mobility, whereas the strong alkaline-extracting solution (pH 12) greatly enhanced the As release from these soils. The consecutive washing test results revealed a long-term release of As from these excavated soils. The pollution potential indices (PPIs) were successfully used to evaluate the pollution threat of As leaching from excavated soils. In addition, different drying methods resulted in variations in the short- and long-term release of As from these excavated soils.

Conclusions

The results revealed that the soil pretreatment and the leaching conditions should be considered if we want to use batch tests for the contamination assessment of excavated urban soils from construction projects. Different drying methods and single extraction may lead to misestimation of the As pollution level. High extraction efficiency with strong alkaline-extracting solution (pH 12) reveals that it could potentially be used to wash As from excavated alkaline soils.

     

Effect of environmental conditions on polychlorinated biphenyl transformations and bacterial communities in a river sediment

Purpose  

The aim of this study was to evaluate polychlorinated biphenyl (PCB) removal in relation to the associated bacterial community composition in Ohio River sediments (USA) using field and laboratory approaches.     

Influence of automatic dishwashings and scrubbings on release of lead from glazed ceramicware

Use of lead glazes on ceramicware and the release of lead from finished glazes are reviewed. Single units of ceramicware with initial lead leach levels from less than 0.1 to 470 micrograms/mL were subjected to multiple automatic dishwashings with intermediate scrubbing of the ware by plastic fiber pads. Ware was periodically leached with 4% acetic acid solution at room temperature for 24 h, and the solutions were analyzed for lead. Release of lead under these conditions is discussed. Results indicated that neither passive leachings nor multiple dishwashings and scrubbings can predict future lead-release characteristics of glazes and decorations.     

Zn and Pb release of sphalerite (ZnS)-bearing mine waste tailings

Background, aim, and scope  Contaminated mine drainage water has become a major hydrogeological and geochemical problem. Release of soluble metal contaminants and acidity from mining sites can pose serious chemical risks to surface and groundwater in the surrounding environment, and it is an important socio-economic factor addressed by working groups like SUITMA Morel and Heinrich (J Soils Sediments 8:206–207, 2008). The release of Zn and Pb from sulfide-bearing flotation residues of a small scale mine in Western Germany is investigated with focus on metal transfer to soil solution. Total contents of the soil material as well as soil water sampled with suction cups were analyzed. The influence of pH on leaching behavior was investigated with pH_stat tests. Isotopic analyses helped assessing seepage water velocity. The aim of this study was the assessment of the environmental behavior of zinc and lead caused by the weathering of sulfide-bearing mine tailings. Especially, we address in this paper the dissolution of sphalerite (ZnS) in contrast to the well-known dissolution processes of pyrite (FeS₂). Materials and methods  Total metal contents of the soil samples were analyzed by energy-dispersive X-ray fluorescence spectroscopy, total C concentration was measured using a CHNS elemental analyzer. X-ray diffraction (XRD) spectra were recorded from powdered soil samples. Soil water was sampled in nylon suction cups. Electrical conductivity (EC), pH, and temperature of the soil water samples were measured in the field immediately after sampling. Major anions (F, Cl, NO₂, NO₃, SO₄) were analyzed by ion chromatography, major cations (Ca, Na, K, Li) were analyzed by flame photometry, heavy metals (Zn, Pb, Fe, Mn, and Mg) by flame atomic absorption spectrometry. Tritium was analyzed by liquid scintillation counting (LSC), ¹⁸O and ²H were analyzed by isotope ratio mass spectrometry (IRMS). pH_stat tests were performed at four different pH values between 2 and 5. Results  Total Zn contents of the soil samples averaged 10 g kg⁻¹, Pb contents averaged 2.5 g kg⁻¹, Fe 22 g kg⁻¹, S 8.0 g kg⁻¹, and total carbon 4.0 g kg⁻¹. Below 2-m depth, soil samples had neutral pH values. Toward the surface, pH decreased down to pH 5.4 in P1 and P3, and to pH 5.9 in core P2, respectively. Dissolved contents of major ions (Mg, Ca, K, SO₄, and HCO₃) in the soil solution increased with depth. Metal concentrations (Fe, Mn, Zn) decreased with depth. The solution pH was neutral to slightly alkaline in samples below 2 m and slightly acidic (pH 6) at 1 m depth. Tritium values are around 7 TU and correspond to modern rain, i.e., after 1975. Stable isotope values plot on the global meteoric water line. The pH_stat tests provide two kinds of information, the acid neutralization capacity after 24 h (ANC24) and the release of metals depending on pH. The ANC24 increases linearly with decreasing pH from about 60 mmol(eq) kg⁻¹ at pH 5 to about 460 mmol(eq) kg⁻¹ at pH 2. Zn and Fe release show a strong increase with decreasing pH to 126 and 142 mmol(eq) kg⁻¹, respectively. Pb release increases at pH <4 and Mn release at pH <5, both to about 10 mmol(eq) kg⁻¹. Discussion  With an average of 10 g kg⁻¹, this field site is highly enriched in Zn. In the oxidized topsoil, Zn concentrations are significantly lower than in the anoxic subsoil. The distribution pattern of total Zn contents and soil pH values indicate that the topsoil, which is prone to oxidation and acidification, is already depleted in Zn. Only in soil core P2, Zn (and Fe) contents in the topsoil were higher than in the subsoil. Oxidation of the sulfidic material leads to redistribution into mobilizable species. High soil water concentrations (10 to 15 mg L⁻¹) can be found at acidic pH. The dominant Zn species in the soil solution is Zn²⁺. At neutral pH, Zn concentrations are below 0.001 mg L⁻¹. During the soil passage, the contaminated seepage water enters the anoxic subsoil with pH buffering carbonates. Results indicate that Zn is immobilized there. However, when the acid neutralization capacity is exhausted, a breakthrough of dissolved Zn to the groundwater has to be expected. Lead averages 2.5 g kg⁻¹ inside the flotation dump. In contrast to Zn, the first centimeters of the oxidized topsoil with high TOC contents show higher Pb contents than the anoxic subsoil. About 80% of the cation exchange capacity in the topsoil is occupied by Pb. In contrast to Zn, Pb is not abundant as aqueous species at slightly acidic pH. Values lower than pH 4 are necessary to mobilize Pb in higher amounts, as pH_stat experiments confirm. Hence, Pb is not expected to be leached out until the buffer capacity of the soil is exhausted. Conclusions  The environmental fate and behavior of Zn and Pb in the flotation dump is strongly depending on pH and redox conditions. Oxidation of sphalerite leads to a transfer of Zn from immobile to easily mobilizable species. Sulfide oxidation leads to an acidification of the topsoil where the buffer capacity is already exhausted due to the leaching of carbonates. At acidic pH, Zn is transferred to the aqueous phase and leached to the subsoil where soil pH is neutral. Electron supply and the buffer capacity of the material are found to be the main factors controlling the mobility of Zn. In contrast, the transfer of comparable amounts of Pb to the aqueous phase requires pH values <4. Since Pb is enriched in the topsoil, not leaching to the groundwater, but direct uptake (e.g., children, animals) and uptake by plants is the highest environmental risk. If the acidification of the soil proceeds with the same rate as in the last 40 years, it will reach the bottom of the tailing in about 200 years and a breakthrough of metals to the groundwater has to be expected. Recommendations and perspectives  The behavior of the different metals and their environmental impact depends on the different metal properties as well as on external conditions, e.g. pH, redox conditions, buffer capacity, and groundwater recharge. To assess the future release of metals from a flotation dump it is crucial to determine the main processes leading to acidification, the buffer capacity, and heavy metal binding forms. The release of heavy metals to the groundwater could be prevented by liming or other buffering techniques de Andrade et al. (J Soils Sediments 8:123–129, 2008). Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Influence of pH on the release of NO and N2O from fertilized and unfertilized soil

Summary NO and N₂O release rates were measured in an acidic forest soil (pH 4.0) and a slightly alkaline agricultural soil (pH 7.8) after the pH was adjusted to values ranging from pH 4.0 to 7.8. The total release of NO and N₂O during 20 h of incubation was determined together with the net changes in the concentrations of NH ₄ ⁺ , NO ₂ ^– and NO ₃ ^– in the soil. The release of NO and N₂O increased after fertilization with NH ₄ ⁺ and/or NO ₃ ^– ; it strongly decreased with increasing pH in the acidic forest soil; and it increased when the pH of the alkaline agricultural soil was decreased to pH 6.5. However, there was no simple correlation between NO and N₂O release or between these compounds and activities such as the NO ₂ ^– accumulation, NO ₃ ^– reduction, or NH ₄ ⁺ oxidation. We suggest that soil pH exerts complex controls, e.g., on microbial populations or enzyme activities involved in nitrification and denitrification.     

The release and fate of nitrogen from catch-crop materials decomposing under field conditions

The release and fate of nitrogen from ¹⁵N-labelled perennial ryegrass (Lolium perenne L.) and white mustard (Sinapis alba L.) catch crops were studied in field microplots. The initial decline in ¹⁵N-labelled organic N, after incorporation of the material in early December, was more rapid from mustard containing 2.6% N than from ryegrass containing 1.4% N. After 9 months of decomposition, the residual organic ¹⁵N from the two materials declined at the same rate; the average decay constant for the following 2 years of decomposition was 0.30 a^?1. After 33 months of decomposition, 23% and 34% of the mustard and ryegrass ¹⁵N, respectively, was recovered in organic residues in the topsoil. Seven per cent of the ryegrass N was leached below 45 cm in micro-lysimeters during the winter following incorporation. Three spring barley (Hordeum vulgare L.) crops, which succeeded ryegrass incorporation, accumulated 19%, 4% and 2%, respectively, of the ryegrass N in the above-ground plant parts. Perennial ryegrass swards recovered a total of 26% of the ryegrass and 22% of the mustard catch-crop N within 2 years. After 2 years of decomposition in unplanted soil, 82% of the ryegrass N was accounted for. The ¹⁵N that was not accounted for may be present in the 10–45 cm depth, or it may have been lost by denitrification.     

Constraining release of pollutants from anoxic bottom sediment via water-lifting aeration in a source water reservoir,East China

Journal of Soils and Sediments - Seasonal hypoxia in water bodies can increase levels of reduced chemical species in the hypolimnion as they are released from anoxic bottom sediments. Water-lifting...     

Influence of foam structure on the release kinetics of volatiles from espresso coffee prior to consumption

The relationship between the physical structure of espresso coffee foam, called crema, and the above-the-cup aroma release was studied. Espresso coffee samples were produced using the Nespresso extraction system. The samples were extracted with water with different levels of mineral content, which resulted in liquid phases with similar volatile profiles but foams with different structure properties. The structure parameters foam volume, foam drainage, and lamella film thickness at the foam surface were quantified using computer-assisted microscopic image analysis and a digital caliper. The above-the-cup volatile concentration was measured online by using PTR-MS and headspace sampling. A correlation study was done between crema structure parameters and above-the-cup volatile concentration. In the first 2.5 min after the start of the coffee extraction, the presence of foam induced an increase of concentration of selected volatile markers, independently if the crema was of high or low stability. At times longer than 2.5 min, the aroma marker concentration depends on both the stability of the crema and the volatility of the specific aroma compounds. Mechanisms of above-the-cup volatile release involved gas bubble stability, evaporation, and diffusion. It was concluded that after the initial aroma burst (during the first 2-3 min after the beginning of extraction), for the present sample space a crema of high stability provides a stronger aroma barrier over several minutes.     

Influence of strawberry yogurt composition on aroma release

The primary objective of this study was to determine how yogurt ingredients affect aroma release in the mouth during eating. A model strawberry flavor consisting of ethyl butanoate, ethyl 3-methylbutanoate, (Z)-hex-3-enol, 2-methylbutanoic acid, 5-hexylhydro-2(3H)-furanone, and 3-methyl-3-phenylglycidic acid ethyl ester was added to unflavored, unsweetened yogurt that had different added sweeteners and hydrocolloids. In all, 12 yogurt formulations were examined to determine the effects of gelatin, modified food starch, pectin, sucrose, high-fructose corn syrup, and aspartame on aroma release. Aroma release was monitored by breath-by-breath analysis (proton-transfer reaction-mass spectrometry) during eating of the test yogurts. Results showed aroma release of the ethyl butanoate, (Z)-hex-3-enol, and ethyl 3-methylbutanoate to be suppressed by sweeteners, with 55 DE high-fructose corn syrup having the greatest effect. Addition of thickening agents had no significant effect on the aroma release profiles of the compounds under study.     

Effect of chemical composition on the release of nitrogen from agricultural plant materials decomposing in soil under field conditions

Summary In two field experiments, plant materials labelled with ¹⁵N were buried separately within mesh bags in soil, which was subsequently sown with barley. In the first experiment, different parts of white clover (Trifolium repens), red clover (T. pratense), subterranean clover (T. subterraneum), field bean (Vicia faba), and timothy (Phleum pratense) were used, and in the second, parts of subterranean clover of different maturity. The plant materials were analysed for their initial concentrations of total N, ¹⁵N, C, ethanol-soluble compounds, starch, hemicellulose, cellulose, lignin, and ash. After the barley had been harvested, the bags were collected and analysed for their total N and ¹⁵N. In the first experiment the release of N was highest from white clover stems + petioles (86%) and lowest from field bean roots (20%). In stepwise regression analysis, the release of N was explained best by the initial concentrations of lignin, cellulose, hemicellulose, and N (listed according to decreasing partial correlations). Although the C/N ratio of the plant materials varied widely (11–46), statistically the release of N was not significantly correlated with this variable. The results of the second experiment using subterranean clover of different maturity confirmed those of the first experiment.     

Effect of aggregation on the adsorption of phosphorus onto air-dried sediment in contrasting shear flow conditions

Purpose

Fine sediments are usually collected in situ and air-dried for adsorption experiments, which may lead to particle aggregation and thus have a significant effect on phosphorus (P) adsorption under dynamic conditions. The main purpose of this study was to investigate the changes of aggregates due to drying with shear stress and the effects on the adsorption of P onto air-dried sediments under different shear rates after re-wetting.

Materials and methods

Sediment samples were collected from an alluvial river. Fine wet sediments (<31 μm) were wet-sieved and air-dried, and some air-dried sediments were further sonicated and served as the control. The grain size distribution of the three sediment samples (wet, dried, and sonicated) was measured to evaluate the particle aggregation level. The P sorption capacity of wet and dried sediments was determined by batch equilibrium experiments. The change of aggregate size with shear stress was investigated for dried and sonicated sediments. Sorption equilibrium experiments were performed to investigate the effect of shear stress on the P sorption with and without change of particle aggregation level, respectively.

Results and discussion

Fine particles agglomerated into larger aggregates during the drying process, resulting in a significant increase in the aggregate size. The sorption capacity was lower in aggregated sediment than in original wet sediment. Aggregate size in dried sediment decreased with the increase of shear rate, leading to an increase in the surface area and available adsorption sites, whereas the particle size of sonicated sediment was hardly affected. Accordingly, the P sorption amount of dried sediment increased with increasing shear rate, whereas that of sonicated sediment showed no significant change with shear rate after all sediments were suspended. There was a significant curvilinear correlation between aggregate size and P sorption amount for dried sediments, thus indicating that the P sorption amount increased significantly with decreasing aggregate size.

Conclusions

Sediment aggregation is an important factor affecting P adsorption besides the amount of suspended sediments and the exchange between suspended and bed sediments under dynamic conditions. The P equilibrium adsorption amount increases with shear stress for air-dried sediment. The effect of particle aggregation on the amount of P sorbed onto sediments should not be ignored, and thus, more attention should be paid to the pretreatment of sediment samples in the sorption experiments under dynamic conditions.