Characteristics of phosphorus sorption at the sediment–water interface in Dongting Lake, a Yangtze-connected lake

Phosphorus in sediments plays an important role in lake eutrophication. This study investigated the phosphorus sorption characteristics of sediments in a river-connected lake and identified the phosphorus source or sink areas in the lake. Sediment samples with different physicochemical characteristics were collected in 2011 and 2013 from Dongting Lake, which is connected to the world’s third longest river, the Yangtze. Batch incubation experiments were conducted to determine the phosphorus sorption characteristics of the sediment samples. The sediment sample from Luzui (D3) exhibited the highest sorption capacity in 2013 (0.75 mg g ) because of its large proportion of fine particles (<63 μm). Results of sorption isotherms illustrated that the modified Langmuir model can effectively describe the sorption isotherms (R> 0.91). The calculated results (equilibrium phosphorus concentration, 0.025–0.032 mg L ) indicated that the estuary from Dongting Lake to the Yangtze River (D1) served as a phosphorus sink because of the large proportion of fine particles and low total phosphorus content in the sediments. The east Dongting Lake (D2 and D3) with a high phosphorus concentration served as a phosphorus source. Yugong Miao (D4) and Nanzui (D5) had a low phosphorus concentration in water because of their location and flow field, respectively. This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY 4.0), which permits copying, adaptation and redistribution, provided the original work is properly cited (http://creativecommons.org/licenses/by/4.0/). doi: 10.2166/nh.2016.206 ://iwaponline.com/hr/article-pdf/47/S1/225/367202/nh047s10225.pdf Wei Huang Kun Wang Ting Wang Shuhang Wang Zhuoma Yangmao Xia Jiang (corresponding author) State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China E-mail: yixinghd6@163.com Hongwei Du Environmental Simulation and Pollution Control Center, South China Institute of Environmental Science, Guangzhou 510655, China


INTRODUCTION
Phosphorus is one of the main nutrients in soil, sediments, The interaction between a river and a lake strongly affects lake water resources, flood, and drought potential and determines the occurrence of other environmental problems in the lake basin. Researchers have studied this phenomenon in recent years. Changes in the discharge or water level of a river can change the blocking force of the river on outflows from the lake (Hu et al. ) and thus affect lake level, water storage, and seasonal variations. A previous study examined and quantified the effects of a dam on river discharge and the influence of such effects on the interactions between rivers and lakes (Guo et al. ). The influence of dam operation and climate change on the interaction between rivers and lakes was evaluated in a previous study (Guo et al. ). These studies have provided important information on the interaction between rivers and lakes. Dongting Lake, which is the second largest freshwater lake in China, is located in Hunan Province The objective of the study is to illustrate the phosphorus sorption characteristic at the sediment-water interface in the river-connected lake and identify the phosphorus source or sink areas in the lake. This study presents useful information that can be utilized in the control of phosphorus release in the sediments of river-connected lakes.

MATERIALS AND METHODS
Site description and sediment collection

Analysis of physicochemical parameters
The total phosphorus (TP) of the sediments was measured using standardized methods and tests (Ruban et al. ).
The phosphorus concentration in the extracted solution or water body was determined with the molybdenum blue method after the solution was filtered (0.45 μm filtration membrane) (Murphy & Riley ). The pH of the sediment was measured in a 1:2.5 (w/v) mixture of sediment with deionized water (Liao et al. ). Organic matter (OM) content was calculated according to the loss on ignition to constant mass (4 h) at 550 W C. The particle size distribution was determined with a particle size analyzer (Hydro2000Mu, Malvern, UK). The main properties of the collected sediments are shown in Table 1.

Statistical analysis
The phosphorus uptake amount in the different sediment samples at each time, Q t (mg g À1 ), was calculated by using a mass balance relationship as follows: where C 0 (mg L À1 ) is the initial liquid-phase phosphorus concentration, C t (mg L À1 ) is the blank corrected concentration of phosphorus at time t, V (L) is the volume of the solution, and W (g) is the mass of dried sediment.
where Q t and Q e are the uptake amounts (mg g À1 ) of phosphorus adsorbed at time point t and equilibrium (mg g À1 ), respectively. K 1 (h À1 ) is the first-order kinetic rate constant, and K 2 is the sorption rate constant of the pseudo secondorder kinetic model (g mg À1 h À1 ).
Modified Langmuir and Freundlich models were used to describe the sorption isotherms. The modified Langmuir where Q e and Q m are the adsorbed amounts of phosphorus in the sediments at equilibrium and the maximum phosphorus uptake amount (mg g À1 ), respectively. C e is the phosphorus concentration in the aqueous phase at equilibrium (mg L À1 ), and K is the affinity parameter (L mg À1 ). NAP where K f is the sorption coefficient (L g À1 ) and m is a constant utilized to measure sorption intensity or surface heterogeneity. K p (L g À1 ) is the affinity parameter of the modified Langmuir model.

Phosphorus sorption kinetics
In this study, sorption kinetics was described with the phosphorus sorption behavior. The experiment results corresponding to the phosphorus sorption of the five sediments from two different years fitting the kinetic equations (Equations (2) and (3) The value of equilibrium sorption capacity using the pseudo second-order model derived through nonlinear regression is presented in Figure 3. In 2013, the sediments exhibited a relatively high equilibrium sorption capacity at an initial phosphorus level of 50 mg L À1 , and had the highest value of 0.75 mg g À1 . The sediments from D1 and D3 showed a higher equilibrium sorption capacity than the other sediments. At the initial phosphorus level of 1 mg L À1 , the equilibrium sorption capacity of the five sediments was lower than 0.06 mg g À1 . In 2011, the sediments exhibited a similar sorption characteristic, and the equilibrium sorption capacity of the sediments was lower than that in 2013.  (Figure 4(a)), the sediment from D5 had the highest sorption rate at three phosphorus concentration levels (1, 5, and 10 mg L À1 ). When the initial phosphorus concentration was increased to 50 mg L À1 , the sediment from D2 exhibited the highest sorption rate (1.4 g mg À1 min À1 ). In 2013 (Figure 4(b)), when the phosphorus concentration was 1 mg L À1 , the sediment from D2 had the highest sorption Downloaded from http://iwaponline.com/hr/article-pdf/47/S1/225/367202/nh047s10225.pdf by guest rate; D1 and D5 exhibited higher sorption rates (7.9 and 8.1 g mg À1 min À1 , respectively) than the other two sediments. When the initial phosphorus concentration was increased, the sorption rate decreased rapidly. Specifically, when the initial phosphorus concentration increased to 50 mg L À1 , the sorption rate decreased to below 0.8 g mg À1 min À1 .

Sorption isotherm of P
Modified Langmuir and Freundlich models were utilized in this study to describe the sediment isotherms. Figure 5 presents the fitting of sorption equations with the modified models. The figure indicates that the equilibrium sorption   However, the overall data were better fitted by the modified Langmuir model than by the modified Freundlich model. Table 2 also provides a good estimate of theoretical sorption maxima Q m and sorption affinity parameter K.
The Langmuir sorption maxima and the sorption affinity parameter ranged from 0.578 mg g À1 to 0.865 mg g À1 and from 0.057 L mg À1 to 0.365 L mg À1 , respectively. D3 in 2011 exhibited the maximum sorption and minimum K.
The calculated results indicated that D4 had the highest EPC 0 and NAP, whereas D1 had the lowest EPC 0 and NAP (Table 2).
Researchers have concluded that the physicochemical properties of sediments significantly influence Q m , NAP, and EPC 0 values. Table 3

DISCUSSION
Phosphorus sorption was rapid in the first few minutes (0 min to 120 min) because the active sorption sites of the sediments were occupied rapidly in the first few minutes.
In the next few hours, the active sorption sites and phos-

Fitting results
Calculated results

Sample
Year  of Dongting Lake. On the contrary, the TP contents in sediments in D2 and D3 were high, and the values of EPC 0 were substantially higher than the SRP concentration of the interstitial water in the eastern part of Dongting Lake. Thus, the two districts had a high phosphorus concentration in water.
D4 is located in the southern part of Dongting Lake. The value of EPC 0 was larger than the SRP concentration of the release of phosphorus from sediments did not lead to eutrophication in the lake. D5, which is the district with the smallest area, is located in the western part of Dongting Lake. The values of EPC 0 in 2011 and 2013 did not vary in D5 and were relatively lower than those in D2, D3, and D4.
However, the values of EPC 0 in both years were higher than the SRP concentration of interstitial water (<0.1 mg L À1 ) (Table 4). This area is the intersection of two large rivers,  Sediments are a potential phosphorus source under certain conditions, especially in a river-connected lake.
Understanding the characteristics of phosphorus sorption at the sediment-water interface and the phosphorus source or sink areas in a lake is crucial to clarify the role of sediments during phosphorus release or sorption. With Dongting Lake as an example, this study revealed the sorption characteristics at the sediment-water interface to provide a theoretical foundation for the control of phosphorus release in sediments.