LIANG Wen, WANG Ze, JIAO Zeng-xiang

College of Mechanics and Materials, Hohai University, Nanjing 210098, China, E-mail: yclwvean@163.com

WAN Jun

State Key Laboratory for Turbulence and Complex System, Department of Mechanics and Aerospace Engineering, Peking University, Beijing 100871, China

Adsorption of phosphorus in sediment re-suspension under sudden expansion flow conditions*

LIANG Wen, WANG Ze, JIAO Zeng-xiang

College of Mechanics and Materials, Hohai University, Nanjing 210098, China, E-mail: yclwvean@163.com

WAN Jun

State Key Laboratory for Turbulence and Complex System, Department of Mechanics and Aerospace Engineering, Peking University, Beijing 100871, China

(Received July 5, 2012, Revised November 10, 2012)

Based on the study of hydraulic characteristics of the sudden expansion water flow of an annular flume, this paper determines the vertical velocity distribution and the turbulence intensity distribution in the mainstream and the recirculation regions to analyze the basic features of this flow field. The adsorption of the phosphorus in the sediment is studied by adding the bacteriostatic agent. The results show that the flow speed in the mainstream region is higher than that in the recirculation region. However, the turbulence intensity in the recirculation region increases more than that in the mainstream region. The adsorption of the phosphorus in the sediment includes the physisorption and the biosorption, and the former is stronger than the latter. With the biosorption in the phosphorus removal process, the phosphorus released by the sediment is mainly completed by the poly-P bacteria in the anaerobic condition. The adsorption of the phosphorus in the sediment in the mainstream region of a sudden expansion water flow is strong and stable, whereas the adsorption in the sediment in the recirculation region is largely fluctuated.

sudden expansion flow, sediment, phosphorus, adsorption effect

Introduction

The rapid urbanization process leads to a serious problem related with the discharge of industrial wastewater, sanitary waste, and agricultural nonpoint source pollutants into water bodies, with an accelerated eutrophication[1-3]. The eutrophication of natural water bodies has become a focal issue in current environmental studies[4-6]. It is caused by the prevalence of algae and organic substances resulting from an increase in nutrient substances. The predominant nutrient substance is the phosphorus, followed by the nitrogen, the carbon, and trace elements or vitamins[7-9]. The phosphorus concentration in a lake is a key indicator of the lake productivity and the eutrophication level. The phosphorus is one of the basic nutrient salts in an aquatic ecosystem and the uppermost restricted nutrilite in a freshwater lake[10-12]. One way to inhibit the eutrophication is to control the concentration of the phosphorus in the water. However, after the exogenous phosphorus is effectively controlled, the lake eutrophication may persist due to the pollution from the endogenous phosphorus. Therefore, the control of the endogenous phosphorus is a major environmental concern[13-15].

In this paper, the hydraulic characteristics of the sudden expansion flow of an annular flume are analyzed, to reveal the physical adsorption and the biosorption of the phosphoeus in the sediment by adding a bacteriostatic agent.

1. Materials and methods

The overlying water in the annular flume is prepared by adding the phosphorus into the distilled water. The Total Phosphorus (TP) is one of the indicators of the lake eutrophication. Table 1 presents the relationship between the TP and the evaluation order for the eutrophication[16]. To better simulate the eutrophication, the phosphorus content in the water sample isset between the middle eutrophication (＞23μg/L) and the high eutrophication (＜110μg/L).

Table 1 TP indicators and grading standards of lake eutrophication

The sediment is taken from Gonghu Bay of Taihu Lake. The breakstone and sundries are removed from the sediment after air drying.

Fig.1 Annular flume

In this research, an improved annular flume with an outer diameter of 0.56 m and an inner diameter of 0.3 m (Fig.1) is used. A channel of 0.13 m wide with a sudden expansion section measuring 0.21 m is formed. The annular flume has a maximum working depth of 0.25 m and a capacity of 70 L. The transmission adjustments can generate a water flow with speed of 0.05 m/s to 0.5 m/s. In addition, a traveling rest is attached to fix the speed measurement device. Four sampling points are selected, namely, #1, #2, #3, and #4, which are the leading portion (#1) and the posterior segment (#2) of the mainstream region, and the flow reducing section (#3) and the incremental section (#4) of the recirculation region. The sediment is 0.05 m thick and the water sample is 0.22 m deep. The plectrums with sizes of 1.5 mm and 7 mm are selected, with rotation speed of 17 r/min.

A 3-D Ultrasound Doppler Velocimeter produced by the US SonTek (SonTekADV) is used to measure the local velocity in the mainstream (upturned probe) and the recirculation (Birdseye probe) regions.

The test is carried out as follows: (1) The flume collects the water samples in four sites with water depths H =0 m, 0.05 m, 0.10 m, and 0.15 m. The sampling starts at the time of 0 h, 0.1 h, 0.5 h, 1 h, 1.5 h, 2 h, 3 h, 6 h, 12 h, 24 h and 48 h after the device is turned on. (2) The flume is closed to collect the water samples in four sites with water depths of H= 0 m, 0.05 m, 0.10 m, and 0.15 m. The sampling starts at the time of 72 h and 96 h after the device is turned on, that is, 24 h and 48 h after the perturbation device of the flume is closed, respectively.

2.Results and discussions

2.1 The hydraulic characteristics of sudden expansion flow

2.1.1 Vertical velocity distribution

The velocity distributions in the mainstream and the recirculation regions are shown in Fig.2. The vertical velocity distribution is nonlinear and is a semiarc skew distribution. The velocity is distributed in a descending order: the leading portion in the mainstream region > the posterior segment in the mainstream region > the incremental section > the flow-reducing section of the recirculation region. The maximal value takes place below the water surface.

Fig.2 Vertical distribution of velocity

2.1.2 Turbulence intensity distribution

If u, v and w are the longitudinal, the transverse, and the vertical fluctuating velocities, respectively, the turbulence intensities of the open-channel flow can be defined as

The longitudinal and the vertical turbulence intensities can be obtained based on the experiment as follows[17]:where u*and v*are the friction resistance velocity, Du,Dv,λuand λvare the empirical constants, y is the coordinate in the water depth direction and h is the water depth.

Fig.3 Turbulence distribution of intensity

Figure 3 shows the turbulence intensity distributions in the mainstream and the recirculation regions. In the mainstream region, the turbulence intensity distribution in #1 is in the same form as that in #2. The longitudinal and the transverse turbulence intensities decrease with the increase of the water depth. The maximal value takes place below the water surface. The vertical turbulence intensity increases with the increase of the water depth. Notably, the turbulence intensity distribution on the #3 flow-reducing section in the recirculation region is in a fluctuation manner. The turbulence intensities in all directions on the #4 incremental section of the recirculation region are in a linear distribution with the water depth. It is shown that the velocity is larger in the mainstream region than in the recirculation region. However, the difference in the turbulence intensities between the two regions is minimal, that is, the turbulence intensity in the recirculation region increases more than that in the mainstream. As compared with the longitudinal and transverse turbulence intensities, the vertical turbulence intensity in the recirculation is smaller. In addition, the longitudinal and transverse turbulence intensities are approximately 1.66-fold of the vertical turbulence intensity.

2.2 Adsorption of phosphorus in the sediment

2.2.1 Physical adsorption

The bacteriostatic agent is added into the sudden expansion annular flume to analyze the physical adsorption of the phosphorus in the sediment. The Dissolved Total Phosphorus (DTP) in the water in the flume is 0.069 mg/L before the perturbation.

Figure 4 shows the gradual changes in the DTP content at the depth of 0 m, 0.05 m, 0.10 m and 0.15 m in the mainstream and recirculation regions. The phosphorus removal of the sediment in the recirculation region is better than that in the mainstream. When the perturbation starts, the, DTP content decreases rapidly. However, as the perturbation time increases, the DTP content barely changes. In the mainstream and the recirculation regions, the adsorption of the phosphorus in the sediment is stable. When the perturbation stops, the DTP content in the water increases gradually with the standing time.

The physical adsorption of the phosphorus in the sediment is mainly due to the adsorption of the deposited matter in the sediment. To form different shapes of the phosphorus in the water, it is combined with iron, carbon, aluminum, manganese and other metal ions in the deposited matter. The temperature, the pH value, the dissolved oxygen content, the oxidation reduction potential, and other factors also affect the physical adsorption of the deposited matter and the phosphorus in the sediment. The iron phosphorus is themost unstable, so it is taken as an example. When there is sufficient dissolved oxygen, the sediment is in the oxidation state. The ferric iron ion is combined with the phosphorus to be deposited in the form of ferric phosphate. Under the anaerobic state, the sediment is in the reduction state (i.e., the ferric iron ion is reduced to the ferrous ion). The colloidal iron is changed into the water-soluble ionic iron to remove the phosphorus from the iron in the water[18]. The perturbation of the annular flume rapidly increases the dissolved oxygen in the water. The sediment is suspended after the perturbation to adsorb the phosphorus in the water for the phosphorus removal. The dissolved oxygen decreases with the increase of the depth, thus the removal effect of the phosphorus in the sediment decreases as well. Meanwhile, there is more pulsating movement of the posterior segment in the mainstream region #2 than that of the leading portion #1. Thus, the concentration of the dissolved oxygen in #2 is higher than that in #1 and the phosphorus removal of the sediment in #2 is more significant than that in #1. When the perturbation ceases, the dissolved oxygen in the sediment gradually decreases, the sediment turns to the reduction state, and the phosphorus enters the water from the adsorption state. Therefore, the DTP content increases when the perturbation stops.

Fig.4 Effect of physical adsorption of sediment on DTP content in the mainstream and recirculation regions at different depths

The sediment in the recirculation region has a better physical removal effect on the phosphorus because the turbulence intensity in that region increases faster than in the mainstream. The turbulence improves the transfer speed of the dissolved oxygen towards greater depths, and makes the sediment to remain in the oxidation state. The phosphorus is adsorbed by the deposited matter in the sediment. Meanwhile, the amount of the sediment in the recirculation region increases due to a slower flow rate. In addition, the increase of the sediment in a unit volume enlarges the contact between the phosphorus and the sediment, resulting in the improved phosphorus removal. When the perturbation ceases, the dissolved oxygen in the sediment gradually decreases. The sediment is in the reduction state, and the phosphorus enters the water from the adsorption state. Therefore, the DTP content in the water gradually increases.

The physical adsorption of the phosphorus in the sediment decreases with the increase of the water depth, with the simultaneous decrease of the dissolved oxygen in the sediment. The sediment is in the oxidation state while the dissolved oxygen is in a high level, and the phosphorus is adsorbed by the sediment. The sediment is in the reduction state under the anaerobic condition, and the phosphorus enters the water from the adsorption of the sediment.

2.2.2 Biosorption effect

The experiment described above is repeated with no bacteriostatic agent added into the sudden expansion annular flume. The DTP in the water is 0.069 mg/L before the perturbation.

Figure 5 shows the gradual changes of the DTP content with depths of 0 m, 0.05 m, 0.10 m and 0.15 m in the mainstream and recirculation regions. The sediment in the mainstream exhibits better and more stable biological adsorption of the phosphorus than that in the recirculation region. The DTP content rapidly decreases after the perturbation starts. With the longer perturbation time, the DTP content in the mainstream region barely changes while that in the recirculation region is unstable. When the perturbation stops, the DTP content in the water increases with the increase of the standing time.

Indeed, the biological effect (i.e.,＜50%) on the phosphorus removal of the sediment can be seen even without a bacteriostatic agent in the mainstream region. At the beginning of the perturbation, the dissolved oxygen in the sediment increases. Meanwhile, the polyphosphate-accumulating organism is aerobic and ideal for the phosphorus adsorption. Thus, the phosphorus is rapidly adsorbed due to the oxidation state of the sediment. The rapid decrease of the phosphorus content in the water is caused by the simultaneous physical and biological effects of the sediment. When the perturbation stops, the sediment gradually turns into the anaerobic state. At the same time, the polyphosphate-accumulating organism is anaerobic and suitable for the phosphorus release. Thus, the phosphorus in the adsorption state is released into the water because of the reduction state of the sediment. The sediment in the recirculation region shows an unstable and poor phosphorus removal effect because the high turbulence intensity in this region increases the amount of the sediment. At the start of the perturbation, there is enough dissolved oxygen in the sediment to metabolize the polyphosphate-accumulating organism, which at that time is aerobic and ideal for the phosphorus adsorption. When the perturbation continues, the dissolved oxygen in the sediment is in a declining curve because it is consumed by the polyphosphate-accumulating organism, almost anaerobic at this point and ready for the phosphorus release. However, with the continuance of the perturbation, the dissolved oxygen is continuously added into the sediment, now aerobic and ideal for the phosphorus adsorption. The process is repeated and it is shown that the biological adsorption of the phosphorus in the sediment changes with the perturbation time. The physical adsorption of the phosphorus in the sediment is stronger than the biological adsorption. Moreover, when the biosorption is involved in the phosphorus removal, the phosphorus released by the sediment is mainly due to the biosorption effect in the anaerobic state. The sediment in the sudden-expansion flow mainstream region has a strong and stable adsorption effect for the phosphorus. In contrast, the adsorption of the phosphorus in the sediment in the recirculation region is unstable.

Fig.5 Effect of biosorption of sediment on DTP content in the mainstream and recirculation regions at different depths

3. Conclusions

(1) The velocity of the flow in the mainstream region is significantly different from that in the recirculation region, with a higher velocity of the flow in the former region. However, the turbulence intensity in the recirculation region increases more than that in the mainstream region.

(2) The adsorption of the phosphorus in the sediment consists of the physical and biological adsorptions. The effect of the physical adsorption is stronger than that of the biological adsorption. Furthermore, when the biosorption participates in the phosphorus removal, the phosphorus released by the sediment is mainly due to the biosorption effect in the anaerobic condition.

(3) The sediment in the sudden expansion flow of the mainstream region shows a strong and stable adsorption of the phosphorus. In addition, the adsorption of the phosphorus in the sediment in the recirculation region is largely fluctuated.

[1] RABALAIS N. N., TURNER R. T. and DIAZ R. J. et al. Global change and eutrophication of coastal waters[J].ICES Journal of Marine Science,2009, 66(7): 1528-1537.

[2] FEUCHTMAYR H., MORAN R. and HATTON K. et al. Global warming and dutrophication: Effects on water chemistry and autotrophic communities in experimental hypertrophic shallow lake mesocosms[J].Journal of Applied Ecology,2009, 46(3): 713-723.

[3] QIN B. Lake eutrophication: Control countermeasures and recycling exploitation[J].Ecological Engineering,2009, 35(11): 1569-1573.

[4] CAO Cheng-jin, ZHENG Bing-hui and CHEN Zhen-lou et al. Eutrophication and algal blooms in channel type reservoirs: A novel enclosure experiment by changing light intensity[J].Journal of Environmental Sciences,2011, 23(10): 1660-1670.

[5] ZHANG Ming-liang, SHEN Yong-ming and GUO Yakun. Development and application of a eutrophication water quality model for river networks[J].Journal of Hydrodynamics,2008, 20(6): 719-726.

[6] WAN Jun, WANG Ze and QIAN Shu-qin. Distribution of bioavailable phosphorus between overlying water and SPM under abrupt expansion condition[J].Journal of Hydrodynamics,2011, 23(3): 398-406.

[7] GAO Li, ZHOU Jian-min and YANG Hao et al. Phosphorus fractions in sediment profiles and their potential contributions to eutrophication in Dianchi Lake[J].Environmental Geology,2005, 48(7): 835-844.

[8] SEPPALA J., KNUUTTILA S. and SILVO K. Eutrophication of aquatic ecosystems a new method for calculating the potential contributions of nitrogen and phosphorus[J].The Intenational Journal of Life Cycle Assessment,2004, 9(2): 90-100.

[9] WANG Gui-qin, ZHANG Dong-ming and CHEN Yong et al. Harm and countermeasure of eutrophication[J].Journal of Jilin Agricultural University,2000, 22(Special): 116-118, 112(in Chinese).

[10] JIANG X., JIN X. and YAO Y. et al. Effects of biological activity, light, temperature and oxygen on phosphorus release processes at the sediment and water interface of Taihu Lake, China[J].Water Research,2008, 42(8-9): 2251-2259.

[11] MCDOWELL R. W., SHARPLEY A. N. Phosphorus solubility and release kinetics as a function of soil test P concentration[J].Geodeama,2003, 112(1-2): 143-154.

[12] BECHMANN M. E., BERGE D. and EGGESTAD H. O. et al. Phosphorus transfer from agricultural areas and its impact on the eutrophication of lakes–two long-term integrated studies from Norway[J].Journal of Hydrology,2005, 304(1-4): 238-250.

[13] ROOS L., LEON P. M. L. and JAN G. M. R. Prediction of phosphorus mobilisation in inundated floodplain soils[J].Environmental Pollution,2008, 156(2): 325-331.

[14] KU Xiao-ke, LIN Jian-zhong. Fiber orientation distributions in slit channel flows with abrupt expansion for fiber suspensions[J].Journal of Hydrodynamics,2008, 20(6): 696-705.

[15] SUN Shu-juan, HUANG Sui-liang and SUN Xue-ming et al. Phosphorus fractions and its release in the sediments of Haihe River, China[J].Environmental Sciences,2009, 21(3): 291-295.

[16] YU Lu, LI Fan-xiu and HUANG Yu. Idear point of decision–making applied in lake eutrophication evaluation[J].Journal of Yangtze University (Natural Science Edition),2011, 8(5): 16-18(in Chinese).

[17] NEZU I., RODI W. Open-channel flow measurements with a laser doppler anemometer[J].Journal of Hydraulic Engineering,ASCE,1986, 112(5): 335-355.

[18] XIA Xue-hui, DONG YE Mai-xing and ZHOU Jianmin et al. Geochemistry and influence to environment of phosphorus in modern sediment in Dianchi Lake[J].Acta Sedimentologica Sinica,2002, 20(2): 416-420(in Chinese).

10.1016/S1001-6058(13)60344-3

* Project supported by the National Basic Research Development Program of China (973 Program, Grant No. 2008CB418203).

Biography: LIANG Wen (1987-), Male, Master

s: WANG Ze, E-mail:daoshiwz@126.com