Total phosphorus release from bottom sediments in flowing water

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2012,24(4):589-594 DOI: 10.1016/S1001-6058(11)60281-3

TOTAL PHOSPHORUS RELEASE FROM BOTTOM SEDIMENTS IN FLOWING WATER* ZHANG Kun, CHENG Peng-da, ZHONG Bao-chang, WANG Dao-zeng Shanghai Institute of Applied Mathematics and Mechanics and Shanghai Key Laboratory of Mechanics in Energy Engineering, Shanghai University, Shanghai 200072, China, E-mail: [email protected]

(Received September 23, 2011, Revised January 10, 2012) Abstract: In this paper, the bottom of the Dianshan Lake was selected as a test sample. The dynamic release of contaminated sediments into the overlying water column was experimentally investigated in an open water channel under different hydrodynamic conditions. The experimental results indicate that the Total Phosphorus (TP) release process can be divided into three stages: rapid release, slow release and equilibration release. In the initial release stage the measured TP concentration changes along the depth. The TP concentration near the sediment-water interface is higher than that near the water surface, but the TP concentration becomes uniform along the depth after 3 h. The dynamic release of re-suspension sediment pollutants is about 6 times higher than the static release of sediment-water interface. There are three main types of release mechanism: diffusion release, re-suspended pore water mixing release and re-suspended particles desorbing release. Key words: Total Phosphorus (TP) concentration, sediment re-suspension, overlying-water, release mechanism

Introduction Lake eutrophication has become a serious environmental problem in China, especially for shallow lakes in the middle and lower reaches of the Yangtze River Basin[1]. As a major nutrient for aquatic ecology, phosphorus has been recognized as the most critical nutrient limiting lake productivity[2-4]. The sources of phosphorus can be categorized into external and internal ones. The phosphorus concentration in the overlying water of lakes, which comes from sediment, is regarded as a major component of the internal source. Such release from bottom sediment may have a significant impact on water quality and may result in continued eutrophication. Relevant environmental factors mainly include temperature, pH value, redox potential, etc.[5-8]. An increase in pH can release P from its binding with ferric complexes due to the competition * Project supported by the National Natural Science Foundation of China (Grant Nos. 10972134, 11032007), the Shanghai Program for Innovative Research Team in Universities. Biography: ZHANG Kun (1979- ), Female, Ph. D. Corresponding author: WANG Dao-zeng, E-mail: [email protected]

between hydroxyl ions and the bound P ions. The higher the temperature in the water column above the sediment, the greater fluxes of phosphorus release[9]. Most experimental studies in recent years have focused on static release of phosphorus[10,11]. Because of the incompleteness of the dynamic release research, contaminated sediment released under hydrodynamic condition is a growing area of focus. Laboratory experiments such as oscillating grid[12], annular tank and open water channel have been conducted to study the contaminated sediment release regularity under conditions of flowing water[13]. By using the annular tank test, the law of sediment suspension and release were simulated by different disturbance forces[14]. The Dianshan Lake is an important source of water supply in Shanghai, but has been reported to be suffering from eutrophication and blue-green algae bloom, especially in summer and autumn. The eutrophication is usually caused by higher concentrations of nutrients such as nitrogen and phosphorus. Sediments may lead to an increase in phosphorus and nitrogen concentrations, contributing to the total amount of these nutrients in almost comparable proportion with other external inputs or sources including atmospheric dry/wet deposition and urban runoff[15,16]. In this paper, the bottom sediment of the Dianshan Lake

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Fig.1 Flow chart of the water channel

was selected as a test sample. The present study aims to assess the dynamic release properties of contaminated sediment and the impact on the water quality of the overlying water column under different hydrodynamic conditions.

1. Experiment 1.1 Experimental setup The experiments were carried out in an open flume with a rectangular test section of 4.5 m in length, 0.2 m in width and 0.4 m in height. The overlying recirculated water of the channel was supplied from a rectangular water tank with the volume of 1.5 m3. The required water flow velocity (0.05 m/s-0.15m/s) and water depth ( H = 0.1 m ) of the overlying water-body were attained by suitably adjusting both the rotating speed of a frequency-controlled centrifugal water pump and the opening width of a tail gate located at the end of the channel flume. The sketch of experimental setup is shown in Fig.1.

nts in the experiments was generally close to that of the uppermost centimeters of the sediments in the Dianshan Lake. The sampling locations in the Dianshan Lake are shown in Fig.2. 1.2 Measurements During the course of the experiments, three sampling sections of the water samples were set downstream from the inlet of the flume, and seven sampling points at heights of 0.005 m to 0.1 m above the sediment-water interference were arranged in each section. Samples of the overlying water were taken to measure the Total Phosphorus (TP) concentration in the laboratory for different values of flow velocity u . A micropropeller velocimeter was used to measure and supervise the velocity of water flow. In the present experiments, the release properties of the contaminants of the sediments with time could be determined from the measured contaminant concentrations C of the overlying water column. The sampled solution was filtered through a 0.45 µm GF/C filter membrane. The filtrate was taken for TP analysis using the molybdenum blue/ascorbic acid method (GB11893-89).

Fig.3 Locations of sampling points in flume (m) Fig.2 Sampling locations in Dianshan Lake

Core samples of sediment were collected in situ from the Dianshan Lake and then transported immediately to the laboratory. The range of sediment size distribution is 3×10–5 m-5×10–5 m. A layer of sediment about 0.05 m thick was gently and uniformly laid on the bottom of the flume before the experiments started and the permeability and sorting of the sedime-

2. Results and analysis 2.1 Vertical profiles of TP concentration in overlying water column In order to study the vertical distribution of TP concentration in flume, seven sampling points were arranged in each section. The schematic is shown in Fig.3. The first sampling point was 0.005 m above the

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sediment-water interface, while other points were 0.01 m, 0.015 m, 0.02m, 0.03 m, 0.05 m, and 0.07 m respectively above the interface.

velocity can enhance the amount of contaminant release from bottom sediment. This phenomenon becomes clearer in the mid and later stages of test. Higher velocity causes stronger convection at the water-sediment interface, which results in increasing amount of release from polluted sediment. In low flow velocity situation, only the surface layer contaminants of the sediment are released into the overlying water. In the mid and later stages, it is very difficult for the bottom layer contaminants of the sediment to move to the surface layer. Hence, they have little influence on water quality.

Fig. 4 Measured variations in TP concentration with depths ( D ) and time

Figure 4 shows the measured variations in TP concentration with time and depth in the overlying water column. It can be seen from the curve that in the initial stage of release, the TP concentration changes along the depth. The TP concentration at 5×10–3 m above the sediment-water interface is 0.213 mg/L, and the concentration decreases as depth increases. The TP concentration at 0.07 m above the sediment-water interface is decreased to 0.143 mg/L. The difference in concentrations of contaminated sediment and water column would normally cause phosphorus to move from the more concentrated (sediment) area to the less concentrated area (water column). As time goes on, the differences in concentration diminish gradually. After 3 h, it can be seen that there is little difference vertically. In the conditions of flowing water, the timedependent concentration varies under the co-action of convective and molecular diffusion (without sediment particle re-suspension). Matter exchange between the overlying water and pore water of bottom sediment causes the release of sediment contaminants into the overlying water. In general, because the concentration containing pollutants of overlying water is lower than that of pore water, the contaminants are constantly released into the overlying water from pore water. 2.2 Effect of flow velocity on TP release quantity To investigate the effect of overlying water flow velocity on the contaminant release under the steady condition of bottom sediment (i.e., the so-called dynamic water flow and static sediment), the values of u = 0.05 m/s, 0.1 m/s, and H = 0.1 m were selected for tests. Figure 5 shows the variations in TP release quantity with time in the overlying water column for H = 0.1 m . It can be observed that at the same depth ( H = 0.1 m ), the TP release quantity increases as flow velocity increases, which indicates that higher flow

Fig.5 Variations in TP release quantity with time for H = 0.1 m

The TP release process can be divided into three stages: rapid release, slow release and equilibration release. The period of 0 h-10 h is the rapid release stage. In this stage, the TP concentration increases exponentially over time. In the mid stage, the difference in TP concentration becomes smaller and the release rate becomes slower, so it is called the slow release stage. The later stage is called equilibration release, as the TP concentration remains substantially unchanged. 2.3 Comparison of static release and re-suspension release of the sediment pollutants under hydrodynamics The contaminants in sediment affect water quality through static sediment release and re-suspension release under hydrodynamic conditions. Static sediment conditions usually occur only at the low flow velocity. However, in natural lakes or rivers, runoff, tide flow and shipping could cause bottom sediment re-suspension. Accordingly, the contaminant release test about re-suspended sediment was carried out in the state of flowing water. The conditions of u = 0.1 m/s, 0.15 m/s, and H = 0.1 m were selected in the test. During the experiments, it was discovered that the sediment remains resting when the velocity is 0.1 m/s and the release of pollutants from sediment is mainly through sediment-water interface release, which is the so-called static release with dynamic water flow. When velocity is increased to 0.15 m/s,

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the hydrodynamic action causes the incipient motion of bottom sediment and the sediment re-suspension.

the mechanism of pollutant release from bottom sediment under the hydrodynamic conditions was divided into the following two processes: static release near the sediment-water interface, and dynamic release of re-suspension of sediment. The static release could be further divided into two cases: flowing water and still water in which the sediment remained resting.

Fig.6 Comparison of static release and re-suspension release of sediment pollutants under hydrodynamic action

By comparison, the variation in TP concentration with time for the static sediment and re-suspension sediment under hydrodynamic action is also given for H = 0.1 m , as shown in Fig.6. It can be observed that the dynamic release by the re-suspension of sediments is much more remarkable than the static release. The TP concentration of re-suspension release is about 6 times higher than that of static release. Another important result obtained in the test is that under the situation of re-suspension, sediment particles together with pore water enter into the overlying water. Differences in P concentrations normally cause the contaminants to move from pore water to water column. On the other hand, desorption of phosphorus at the re-suspension sediment particle surface also affects water quality. There is a peak value point in the measured TP concentration curve of the overlying water column at 3 h. According to the theory of diffusion, the TP concentration of the water should increase until a balance is achieved. However, the measured concentration decreased mildly after 3 h and kept decreasing to a new balance, which may be caused by experimental setup. The re-suspended sediment and water column are transported to the water tank and then enter the open channel via water pump during test. In the circulating flow a portion of re-suspended sediment sank to the water tank, which caused a reduction of the source of pollution and led to the slightly low equilibration level. 2.4 The mechanisms of pollutant release from bottom sediment A conservative tracer and a reactive tracer (NaCl and KH2PO4) were used as the contaminants in bottom sediment in experiments. The release mechanism of polluted sediment was studied in detail. Through experimental study, the mechanism of pollutant release from sediment was revealed. In general,

Fig.7 Comparison of static and re-suspension releases

As can be seen from Fig.7, the conservative tracer NaCl was used to simulate the pollutant release by diffusion and pore water, the reactive tracer KH2PO4 was used to simulate the pollutant release by re-suspended particle desorbing. When the polluted bottom sediment particles re-suspend under the hydrodynamic conditions, the re-suspended sediment with pore water would enter into overlying water column and release pollutants and impact water environment, this is called polluted sediment dynamic release. The dynamic release include two ways: the pore water containing pollutant exchanging with overlying water and the re-suspend polluted sediment particles desorbing contaminants into overlying water. The pore water release contaminant mainly occurred in the initial stage and the release process is instantaneous. In this process, the release quantity is not only larger but also the release rate is higher than that in other stages. At the same time, the molecular diffusion also exists, but its influence is relatively slight. Due to the release of the re-suspended particles, the concentration of TP in the overlying water grew somewhat. In the mid and later stages of the test, the concentration of TP increased with the time. The release amount is larger than that in the initial stage, as shown in Fig.7. This suggests that the re-suspended particle release make up the majority of pollutants. Particles desorption release dominates over the later stage. Therefore, we can see three main release mechanisms: diffusion release on the sediment-water interface, re-suspended pore water mixing release and re-suspended particles desorbing release. The influence of various release mechanisms on sediment impact level for water quality is shown in Table 1.

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Table 1 Influence of various release mechanisms on sediment impact level for water quality Time

Static release (sediments static) NaCl

KH2PO4

Mechanism

Molecular diffusion

Re-suspended pore water

Re-suspended particles

Molecular diffusion

Re-suspended pore water

Re-suspended particles

Initial stage

Large

None

None

Large

None

None

Mid stage

Large

None

None

Large

None

None

Later stage

Large

None

None

Large

None

None

Time

Dynamic release (sediments re-suspension) NaCl

KH2PO4

Mechanism

Molecular diffusion

Re-suspended pore water

Re-suspended particles

Molecular diffusion

Re-suspended pore water

Re-suspended particles

Initial stage

Small

Large

None

Small

Middle

None

Mid stage

Large

None

None

Small

None

Large

Later stage

Large

None

None

Small

None

Middle

3. Conclusions The characteristics of contaminant release from bottom sediments into the overlying water-body have been experimentally investigated in an open water channel under different flow velocity conditions. The following conclusions can be reached: (1) Without external pollution sources, polluted sediment has an influence on the overlying water quality. Contaminant release is caused by re-suspension of sediment and difference in concentration between the water column and the contaminated sediment. In this way, the concentration of contaminants in the overlying water increases. (2) In the initial release stage, the measured TP concentration changes along the depth. The concentration near the sediment-water interface is approximately 1.5 times that of the concentration near the water surface, but the distinction decreases after 3 h. (3) Flow velocity contributes to the release of sediment contaminants. In steady hydrodynamic conditions, at a given depth, the TP concentration of overlying water column increases as the flow velocity increases. The TP release process can be divided into three stages: rapid release, slow release, and equilibration release. (4) Compared to the static release, the effect of water quality of re-suspension release is greater, and sediment particles are likely to act as an important source in the whole release process. In the case of

static release, pollutants in the sediment mainly affect the water quality through the exchange between overlying water and pore water, while in the case of resuspended sediment the re-suspended pore water and the re-suspended particle play a more important role. The amount of re-suspension release is much larger than that of static release. (5) There are three mechanisms of pollutant release from bottom sediments: diffusion at the sediment-water interface, pore water entering overlying water mixing release and re-suspended particles desorbing release. When the sediment is resting whatever under the static water or the running water, only pollutant diffusion at the sediment-water interface is observed. When sediment re-suspension occurred, re-suspended pore water and re-suspended particle play a more important role.

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