Phosphorus release potential and pollution characteristics of sediment in downstream Nansi Lake,...

RESEARCH ARTICLE

Phosphorus release potential and pollution characteristics ofsediment in downstream Nansi Lake, China

Zhijian LI1,2, Qinyan YUE (✉)1, Baoyu GAO1, Yanwen WANG1, Qing LIU3

1 Shandong Key Laboratory of Water Pollution Control and Resource Reuse,School of Environmental Science and Engineering, Shandong University, Jinan 250100, China

2 School of Light Chemistry and Environment Engineering, Shandong Polytechnic University, Jinan 250353, China3 Weishan Environment Protection Bureau, Weishan 277600, China

© Higher Education Press and Springer-Verlag Berlin Heidelberg 2011

Abstract The research aimed to evaluate present andpotential phosphorous pollution due to high sedimentaryphosphorus load and release from sediment, when externalphosphorus was reduced in downstream Nansi Lake.Pollution load of the sediment and overlying water wasinvestigated. Kinetics and isotherms of adsorption/releaseof sedimentary phosphorus were studied to determineequilibrium phosphate concentration (EPC0) and releasepotential. Kinetics of phosphorus adsorption on sedimentand release from sediment were well described by both thepseudo-first-order rate equation and the pseudo-second-order rate equation, but more appropriate to the pseudo-second-order rate equation with the adsorption/releasecapacity more close to the measured values, suggestingthat the processes were chemically rate controlled anddependent on adsorption capacity. Soluble reactivephosphorus (SRP) sorption isotherms on sediment werebest fitted by the modified Langmuir model indicating amonolayer adsorption. By comparing EPC0 and SRP ofwater, the status (adsorption, releasing or in equilibrium) ofsediment phosphorus could be determined. The sedimentsat site S1, S3, S4, S5, and S7 where the EPC0s were greaterthan the SRPs, had a potential to release phosphorus intothe water column. However, those sediments at S9, S10,and S12, where the EPC0s were approximately equal to theSRPs, were in impermanent equilibrium with overlyingwater in status of phosphorus, the sediments can be likelyto release phosphorus to the water column once theequilibrium was broken. Therefore, sedimentary phos-phorus can be a secondary pollution source in downstreamNansi Lake.

Keywords Nansi Lake, sediment, phosphorus, release

potential, equilibrium phosphate concentration

1 Introduction

In May 2007, a large scale blue and green alga occurred inTai Lake, China, causing a water supply crisis in thesurrounding cities, which aroused further attention to lakerecovery and management. Eutrophication of lowlandrivers and lakes, linked to nutrient enrichment (particularlyof nitrogen and phosphorus), is of global concern overcatchments that supply agriculture and population-denseplaces [1–3]. Phosphorus (P), often as the limiting nutrientfor aquatic plants, plays a significant role in theeutrophication processes [3,4]. Polluted sediments arefrequently regarded as an internal contaminant source inaquatic systems [5,6]. Sediments act as a sink where P canbe stored, and become a potential source of P for overlyingwater and biota [7]. P may accumulate in lake sedimentsduring heavy loading periods and release from sedimentsinto the overlying water after the external loading isconsiderably reduced, such as during eutrophicationcontrol or clean water inflow. The released P sustains theeutrophication processes, and it cycles between overlyingwater and underlying sediments through algal growth,organic deposition, decomposition, and release [8,9].Some sedimentary P can be released at certain periods ofthe year, which under certain conditions contributes to lakewater eutrophication [10].Nansi Lake is the largest freshwater lake in Shandong

Province. Today, it functions as a reserve lake on the EastRoute of the South-to-North Water Diversion Project.According to the “water pollution prevention planning ofthe South-to-North Water Diversion Project (east route) ofShandong Section”, the water quality of the lake should bebetter than the Grade III of the “China EnvironmentalQuality Standard for Surface Water (GB3838-2002)” [11].

Received September 5, 2010; accepted January 26, 2011

E-mail: [email protected], [email protected]

Front. Environ. Sci. Engin. 2012, 6(2): 162–170DOI 10.1007/s11783-011-0313-7

I.e. for water quality, it requires total phosphorus (TP)£0.2 mg$L–1, for lake and reservoir water, TP£0.05mg$L–1.Because external sources of P pollution due to industrialdrainage have been stopped, it is especially necessary toevaluate internal sources of P pollution due to inner P loadand potential release from sediment.In recent years, some researches were conducted on

downstream Nansi Lake, but many of the researches wereon the water quality and sediment pollutants distribution[12,13]. Few studies focused on the nutrients release andits dynamics and isotherm, especially for the downstreamlake, where the transferred water from Yangtze Riverpasses first. Moreover, due to the wide and open watersurface area in downstream Nansi Lake, pollutants such asphosphorus and nitrogen are quite easy to deposit in thesediment [14,15]. The previous research showed that therewere nutrients accumulation and eutrophication due to therelease of nutrients in Nansi Lake to different extents[13,16]. Study on P species indicated that potential activespecies accumulated in surface sediment can arouse Prelease greatly [15,16].Large quantities of soluble inorganic phosphorus in

waterways are the most common cause of eutrophication[17], like soluble reactive phosphorus (SRP). In recentdecades, the equilibrium phosphate concentration (EPC0)has been utilized in many investigations for predicting thebehavior of a sediment or soil and characterizing thephosphorus status of sediments and soils [18,19].EPC0 measurements provide an indication of whether

sediments have potential to take up SRP from the riverwater or release SRP into the river water; EPC0 is theconcentration of SRP which, when placed in contact withthe sediment, produces no change in SRP in solution over a24 h period [3,20–22]. When EPC0> SRP in thesurrounding water, the sediment has potential to releaseSRP to the water column and when EPC0<SRP, thesediment has potential to take up SRP from the watercolumn. If the EPC0 is close to the SRP in the water column(�20%), the bed sediment and the river water are deemedapproximately in equilibrium with respect to SRP [18,20].The purpose of this study is to investigate P load in

sediment and P behaviors of the sediment-water interface,and to describe the kinetics and isotherm of the Padsorption and release of the sediment, and further toevaluate P release potential according to EPC0 measured inthe sorption experiments. All the work can providereference and guide for water pollution prevention andlake ecological management.

2 Materials and methods

2.1 Study area

Nansi Lake (116°34′–117°21′E, 34°27′–35°20′N) lies inthe south-west of Shandong, and is the largest freshwater

lake in Shandong Province, China. The lake consists offour connected lakelets, which are Nanyang Lakelet,Dushan Lakelet, Zhaoyang Lakelet, and Weishan Lakelet.Of them, Weishan Lake and part of Zhaoyang Lakelet arelocated in the lower reaches. A shallow lake with anaverage depth of 1.46 m [13], Nansi Lake is 126 km longby 5–26 km wide, and covers a water area of approxi-mately 30453 km2, with 53 inflow rivers. The downstreamLake covers an area of 531.17 km2 and the circumferenceis 130 km with more than 5 main inflow rivers. Nansi Lakeplays a significant role in the local agriculture, fishery,industry, and entertainment. Large amounts of waste waterand sewage are discharged into the lake each year. Industrywastewater is the main pollution source and about 300000 tindustry wastewater flows into Nansi Lake per day.

2.2 Sampling

The study was undertaken mainly in downstream NansiLake and its inflows. Handed corers were used to obtainthe sediment samples between November 2007 and lateMay 2008 and the sampling sites were coded as S1 to S12in time sequence. The sample locations were shown inFig. 1, of which, site S7 to S12 are located in the lake; siteS1 to S6 are located at the entrances from the six importantrivers to the lake which are in populous district. Most ofthem are national or important controlled monitoringpoints. To make comparison with the downstream, onesample numbered S1 was selected from the upstream.Correspondingly, the overlying water close to the sedi-ment-water interface was sampled for water qualityanalysis. Sediment samples were kept in ice and quicklytaken to the laboratory, where they were first air-dried atroom temperature, then dried in an oven at 80°C, andfinally sieved through a 100 mesh sieve. Sulphuric acidwas added to the water samples to ensure pH levels below2, and all samples were subsequently kept frozen at 4°C forfuture analysis.

2.3 Sample analysis and quality control

SRP concentrations in the supernatants and water sampleswere determined by the molybdenum blue method. Samplesolutions were mixed with ammonium molybdate, anti-monyl potassium tartarate, and ascorbic acid in an acidicmedium, and the absorbance of the phosphomolybdenumblue complex was measured colorimetrically at 700 nmafter 15 min using a Visual-spectrophotometer (722ESpectrum, Shanghai spectral instrument Co., LTD, Shang-hai). Sediment pH was measured by a pH glass electrode(PHS-25CW microcomputer type PH/mV tester, ShanghaiLida instrument plant, Shanghai) in a 1∶2.5 (w/v)sediment-to-distilled water suspension. Sediment TP wasmeasured by treating the sample at 120°C for 30 min viaacid potassium peroxodisulfate oxidation method [23], andthen P concentrations in the supernatant of the extraction

Zhijian LI et al. Phosphorus release potential and pollution characteristics of sediment 163

were analyzed using the molybdenum blue method [23].The water samples or the filtrate from sediment samples

were analyzed for P contents using 722E spectrophot-ometer with the detection limits of 0.01 mg$L–1 for P. Forensuring quality control, the standards were always re-analyzed at the end of each batch of analyses. Blank anddrift standards were performed after 20 determinations tomaintain instrument calibration and recovery efficiencies.The analyses results were only acceptable when themeasured standard concentrations were within 95%–105% of the certified value.

2.4 Phosphorus adsorption/release kinetics

Kinetics describes the solute sorption rate which controlsthe residence time of sorbate sorption at the solid–solutioninterface. Therefore, kinetics is useful and favorable topredict the rate at which pollutant is removed from aqueoussolutions.Dried sediment samples (1.0 g) were added in two 250

mL acid-washed conical flasks with 100 mL 0.01 mol$L–1

KCl solution. Two initial P concentrations were kept at2.0mg$L–1 and 0mg$L–1. The experiment with initial 0 mg$L–1

phosphorus represented a P release process. In accordancewith the actual pH of downstream Nansi Lake, the pHvalues of the slurries were maintained at 7.5–8.0. Afteradding two drops of 0.1% chloroform to inhibit microbial

activities [24], the flasks were aerated by pure nitrogen(99.99%) for about 1 h until the DO was below 2 mg$L–1, and then were sealed and shaken in an incubator at200 rpm, 25°C� 0.2°C for 48 h. A 10 mL suspension wascollected at 1, 2, 5, 8, 10, 24, 30, 32, and 48 h, andcentrifuged immediately at 5000 rpm for 10 min todetermine P, and then was analyzed for P release contentaccording to the relevant method of Wang et al. [25–27].The kinetics curves were obtained by plotting the uptakeand release contentQ (mg$kg–1) of phosphorus versus time(h) respectively.It is generally believed that the process of adsorption can

include the following stages: physical adsorption due tosurface charge or via electrostatic and ionic interactions,external mass transfer (namely fluid film diffusion),intraparticle transport within the adsorbent (aimed por-ous-adsorbent particles); and chemiadsorption (reactionkinetics at phase boundaries), where the rate of adsorptionis usually controlled by the kinetics of bond formation[28,29].To investigate the adsorption mechanism of phosphorus

onto sediment, the experimental data were modeled usingthe first-order rate equation of Lagergren [30], the pseudosecond-order rate equation [31] as shown below by Eq. (1)and Eq. (2).

lnðQe –QtÞ ¼ lnQe – k1t, (1)

Fig. 1 Sketch of sediment and water sampling sites

164 Front. Environ. Sci. Engin. 2012, 6(2): 162–170

t

Qt¼ 1

k2$Q2eþ t

Qe, (2)

where Qe and Qt is the amount (mg$kg–1) of adsorbed P onthe sediment at equilibrium and time t (h); k1 (h

–1) and k2(kg$mg–1$h–1) are the rate constants of the first-order andsecond-order adsorption. Plots of ln(Qe –Qt) versus t givesa straight line for pseudo-first order kinetics, which allowscomputation of the sorption rate constant k1.For Eq. (2), when t approaches 0 the biosorption rateQt /

t becomes the initial sorption rate h ¼ k2$Q2e .

If the pseudo-second-order kinetics is applicable, theplot of t/Qt versus t gives a linear relationship, whichallows computation ofQe, k2 and hwithout having to knowany parameter beforehand [32].

2.5 Phosphate adsorption isotherm

Sorption experiments were carried out according to themethods of House et al. [33], with full details provided byJarvie et al. [20]. Dried sediment samples (1.0 g) wereadded in triplicate to a series of 50 mL acid-washedcentrifuge tubes containing 30 mL of 0.01 mol$L–1 KClsolution containing ten different initial soluble P concen-trations (0, 0.02, 0.04, 0.1, 0.2, 0.4, 0.6, 0.8, 1.0, and 2.0mg phosphorus L–1 as KH2PO4). The values of pH for theslurries were maintained at 7.5–8.0 in accordance with theactual pH of downstream Nansi Lake. After adding twodrops of 0.1% chloroform to inhibit microbial activities[24], the tubes were shaken in an incubator (SHZ-82A,Jintan city medical instrument plant, Jiangsu Province) at200 rpm, 25°C� 0.2°C for 24 h, centrifuged immediatelyat 5000 rpm for 10 min(LG10-2.4A centrifuge, Beijingmedical centrifuge plant, Beijing), and finally filteredthrough 0.45 GF/C filter membrane (0.45μm GF/C, Xinyapurification devices factory, Shanghai) for P analysis.The adsorption isotherm, that is, the plot of the

equilibrium solid-phase concentration (Qe) versus theequilibrium liquid-phase concentration (Ce), is oftendescribed by the Langmuir and Freundlich equations.EPC0 was calculated by fitting an isotherm to the data

using a least squares method [34], when Qe = 0, then Ce =EPC0 [22].

3 Results and discussion

3.1 Phosphorus load in sediment and overlying water indownstream Nansi Lake

Chemical and physical characteristics of overlying water intwo consecutive lakes and inflows at downstream NansiLake provided in Table 1. It was observed that the pH ofwater sample ranged between 7.66 and 9.14, indicatingslightly alkaline to medium alkaline. Chemical oxygendemand (COD) in downstream Nansi Lake was between25.4 and 61.2 mg$L–1. As shown in Table1, rivers sufferedmore heavily contamination from P and COD than lakesdid. Average TP in the inflows was 3–5 times that ofWeishan Lake and Zhaoyang Lake. The pollution extentfollowed the order: inflow rivers>Weishan Lake>Zhaoyang Lake. According the monitoring records, theaverage DO was 7.06 mg$L–1 in winter and 2.37 mg$L–1 insummer. DO was relatively low in summer due to moreexhaust of DO at high temperatures.TP load in sediment and overlying water in downstream

Nansi Lake was shown in Table 2. There was considerableenrichment of phosphorus in sediment of inflows andlakes. The concentrations of sediment TP varied from415.17 to 658.32 mg$kg–1 in winter, and from 424.9 to670.06 mg$kg–1 in summer. Most of the locations showeda medium contamination by P (with TP content 500–1000mg$kg–1), according to the previous study [35]. It found aheavy P accumulation in sediment at site S3, S9, S10, andS11. For overlying water, the concentrations of TP rangedfrom 0.019 to 1.15 mg$L–1, indicating an eutrophicationlevel to different extent, based on the Grading Standards ofLake Eutrophication [36,37]. Location S2 and S6 sufferedfrom the most sever water contamination of P with TPabove 0.5 mg$L–1. SRP varied dramatically from 0.003 to0.487 mg$L–1. The water quality ranged from Grade III tofar worse than Grade V. Compared with in winter, most

Table 1 Chemical and physical characteristics of overlying water in downstream Nansi Lake

Zhaoyang Lake Weishan Lake inflow rivers

average range average range average range

TP/(mg$L–1) 0.15 0.085–0.261 0.20 0.036–0.731 0.75 0.398–1.367

TDP/(mg$L–1) 0.06 0.04–0.093 0.15 0.019–0.418 0.41 0.111–1.03

SRP/(mg$L–1) 0.04 0.023–0.051 0.06 0.005–0.188 0.34 0.065–0.837

DO/(mg$L–1) 11.45 10.99–12.99 11.80 10.28–12.48 10.91 5.84–17.75

COD/(mg$L–1) 32.38 30.2–34.0 34.3 29.6–40.9 36.98 25.4–61.2

conductivity/(µS$cm–1) 1084.33 1079–1095 1030.86 987–1090 1287.20 1093–1563

temperature/°C 12.03 11.9–12.2 12.93 12.1–13.7 14.08 12.98–16.2

pH 8.63 8.58–8.71 8.72 8.19–9.14 8.11 7.66–8.76


water TP tended to increase, while sediment TP tended todecrease in summer suggesting a release of TP into theoverlying water. It is probably due to higher temperature,lower DO, and rainfall dilution in summer, which mayaccelerate release of P from sediment into overlying water.

3.2 Adsorption and release kinetics

The kinetics curve of adsorption and release of phosphoruson the sediment was shown in Fig. 2. The kinetic curveshowed an initial phase of quick adsorption, completedwithin 2 h, followed by a slower stage. The adsorptionreached equilibration in 24 h, while P release reachedequilibrium in 5 h. The similar results were also observedin the sediments of Tai Lake [38] and the Han River [39].To investigate the rule and mechanism of adsorption andrelease, kinetics models were used to fit the experimentdata.The constants and correlation coefficients (R2) of Eq. (1)

and Eq. (2) were given in Table 3, and significance was setto P£0.05. As shown in Table 3, the kinetics ofphosphorus adsorption on sediment was well fitted bypseudo-second-order rate equation (R2 = 0.9986) as well aspseudo-first-order rate equation (R2 = 0.9836). Similarly,the kinetics of release of P was well modeled by thepseudo-first-order rate model (R2 = 0.9999) and pseudo-second-order rate model (R2 = 0.9971).Although the two rate models had similar correlation

coefficients, the pseudo-second-order rate model was moreproper to elucidate sediment P sorption and release foradsorption/adsorption capacity, Qe (141.04 mg$kg–1 and– 1.74 mg$kg–1 in Table 3) predicted by the pseudo-

second-order rate was more close to the measured results(138.80 and – 0.58 mg$kg–1 in Fig. 2).Besides, compared with the pseudo-first-order equation,

the pseudo-second-order equation has the followingadvantages: it allowed an effective sorption capacity; thesorption capacity and rate constant of pseudo-second-orderand initial sorption rate can be all determined from theequation without knowing any parameter beforehand [40].Analysis of the experimental data showed good

agreement with the pseudo-second-order kinetic model,which was reflected in the extremely high correlationcoefficient of determination (> 0.99), thus indicated thatthe adsorption and release of P were chemically ratecontrolled [41].Therefore, pseudo-second-order rate law expression was

applicable in the phosphorus adsorption process, whichdemonstrated how the rate depended on the sorptioncapacity rather than the concentration of the sorbate [42].The pseudo-second-order model is based on the

assumption that the rate-limiting step which may bechemical sorption or chemisorption involving valencyforces through sharing or exchange of electrons betweensorbent and sorbate, provides the best correlation of thedata [43]. The aforementioned two models basicallyinclude all steps of adsorption such as external filmdiffusion, adsorption, and internal particle diffusion, sothey are pseudo models [43].

3.3 Adsorption isotherm of the sediment and the EPC0

All studied sediments tended to release phosphorus at lowinitial SRP concentration. The same phenomenon was

Table 2 Total phosphorus level in surface sediment and overlying water in downstream Nansi Lake

sample sites TP (winter in 2007) TP (summer in 2008) SRP (summer in 2008)

sediment/(mg$kg–1) overlying water/(mg$L–1) sediment/(mg$kg–1) overlying water/(mg$L–1) overlying water/(mg$L–1)

S1 510.69 0.143 624.99 0.112 0.127

S2 526.93 1.150 502.08 0.747 0.030

S3 658.32 0.107 629.22 0.079 0.003

S4 630.19 0.019 424.90 0.215 0.009

S5 415.17 0.059 441.58 0.117 0.005

S6 554.53 0.504 532.86 0.960 0.44

S7 539.67 0.111 432.80 0.147 0.006

S8 484.09 0.031 460.12 0.034 0.487

S9 614.07 0.036 601.25 0.090 0.005

S10 582.26 0.280 670.06 0.090 0.004

S11 595.31 0.105 629.83 0.079 0.208

S12 526.54 0.105 564.47 0.083 0.003

minimum 415.17 0.019 424.90 0.034 0.003

maximum 658.32 1.150 670.06 0.960 0.487

mean 553.15 0.221 542.85 0.229 0.111


found in previous studies [44,45]. The characters ofphosphorus adsorption were analyzed by adsorptionisotherm models. SRP concentration in solution atequilibrium (concentration after 24 h) was plotted againstthe mass of adsorbed dry sediment per kg phosphorus. TheEPC0s of 12 sediments at different locations weremeasured by the batch sorption experiment and simulta-neously, three representative sediments at the importantmonitoring sections, i.e. S7, S8, and S11, were simulatedto the adsorption isotherm of phosphorus on sediment(Fig. 3). Of them, sample S7 and sample S8 are located inthe national-controlled sections, and sample S11 are closeto the city-controlled monitoring section. According to theprevious study, the annual comprehensive pollutionevaluation indexes at S7, S8, and S11 were quite high [46].The modified Langmuir model, Freundlich model, and

linear model were used to describe adsorption isotherm ofP on the sediments. The three models are shown below[44,47].

Q ¼ kLbC

1þ kLC– a, (3)

Q ¼ kFCn, (4)

Q ¼ mC –Ni, (5)

where C (mg$L–1) is the SRP concentration in solution at

time, t (taken as 24 h). Q is P sorption capacity (mg$kg–1

dry weight). n, kL and kF are constants related to thebinding strength. a is the y-intercept which describes the Preleased at low initial SRP concentration in the overlyingwater and b is the asymptote of the Langmuir model. Theasymptote of the Langmuir model is referred to as thesorption capacity [46]. The parameters kL, kF,n, b and a canbe obtained by nonlinear curve-fitting. The m is thesorption constant that describes the linear isotherm, whichis a measure of the phosphate sorption efficiency ofsediments (L$kg–1). Ni is the y-intercept which describesthe phosphate released at low initial phosphate concentra-tions. When Q = 0 then C (24 h) = EPC0, or theconcentration of SRP in contact with sediment that leadsto no net change in SRP in solution over a 24 h period[3,21,22,33]. EPC0 is the x-intercept of the linear modeland can be calculated by the regression equations. Tofurther investigate P sorption and difference of P sorption,stepwise multiple regression analyses were preformed.P sorption parameters kL, b and a, calculated using the

modified Langmuir model, KF and n (calculated using theFreundlich model) and P sorption parameters m, Ni andEPC0 (obtained by linear model) were reported in Table 4.The parameter kL, kF, b and n represented to a certainextent phosphate sorption on sediments, while m and Ni

also represented the sorption ability [44].The coefficient of determination, R2, has been used to

determine the relationship between the experimental dataand the isotherms in most studies. The results shown inFig. 3 and Table 4 indicated that the P sorption isothermson three sediment samples were better described by themodified Langmuir model (R2 = 0.98–0.99) than byFreundlich (R2 = 0.92–0.95) or the linear model (R2 =0.88–0.94) because of its higher R2 value. The result wassimilar to that of Wang [9]. It suggested that the adsorptionprocess was single molecular layer adsorption within a lowconcentration range, and the maximum adsorption contentswere 178.82 mg$kg–1 for S7, 236.21 mg$kg–1 for S8, and206.23 mg$kg–1 for S11 respectively. The Freundlichparameters were kF = 168.65, 434.33, 173.83, n = 0.77,0.077, 0.996 for S7, S8, and S11, respectively. The value ofn< 1 indicated a favorable adsorption process [48].The Langmuir equation was originally developed to

describe individual chemical adsorbents, and was applic-able to physical adsorption (monolayer) in a lowconcentration range [49]. The Freundlich equation is an

Fig. 2 Phosphorus adsorption and release kinetics on the sediment

Table 3 Adsorption and release kinetics constants with kinetics models (P£0.05)

kinetics equation initial P concentration /(mg$kg–1)

Qe/(mg$kg–1) k1/h–1 k2/(k$mg–1$h–1) R2

pseudo-first-order 2 (adsorption) 132.5 1.10 - 0.9836

pseudo-first-order 0 (release) – 3.374 0.252 - 0.9999

pseudo-second-order 2 141.04 - 0.01 0.9986

pseudo-second-order 0 – 1.74 - 0.28 0.9971


empirical approach for adsorbents with very unevenadsorbing surfaces. This model is applicable to adsorptionof a single solute system in a fixed range of concentration.The Freundlich equation is generally suitable for high- andmedium-concentration environments and is not suitable forlow-concentration environment because it does not meetthe requirements of Henry’s law [49].It has been known for a long time that soluble P interacts

strongly with soils and sediments, the reactions being acombination of sorption/desorption and mineral precipita-tion/dissolution [22]. In rivers, the interaction of P withsuspended and bed-sediment largely determines theconcentration of SRP in the water [20]. Sediments interactwith P in the water column and as such play a pivotalecological role in buffering water column P concentrations,and thus the P moved downstream. Specifically, sedimentsoften adsorb P until an EPC0 is obtained [10,33,50,51].Hence, once concentration gradient between EPC0 andSRP is enough, sediment will adsorb or release P towardthe water column.Based on this, EPC0 was calculated and compared with

SRP in the overlying water at 12 sample sites (Fig. 4) so asto determine the state of sediment. At the sample site S1,S3, S4, S5, and S7, EPC0> SRP, the sediment hadpotential to release P to the water column; while at S2,S6, S8, and S11, EPC0< SRP, the sediment had potential

to take up P from the water column; and at S9, S10, S12,EPC0&SRP, the bed sediment and water were approxi-mately in equilibrium with respect to SRP. In addition,from the fit result (Table 4) on adsorption isotherm of thesediment at three control monitoring sites, S7, S8, and S11,the absorption capacity followed the order ofS8>S11> S7, and the EPC0 of S7 was – 0.02 mg$L–1,where the negative value indicated a release potential.However, the EPC0 of S8 and S11 was positive, suggestingan adsorption tendency, which was in accord with thestatus of sediment P based on the relationship betweenEPC0 and SRP.

4 Conclusions

The result denoted that most sediment in downstreamNansi Lake suffered certain P pollution to some extent. TPloads in sediment and overlying water in downstreamNansi Lake were quite high and aroused a risk of seriouslake eutrophication. Under certain conditions, especially insummer season, or when the equilibrium between sedimentand water column was broken, the sediment had potentialto release P into overlying water. Therefore, these stationsand their vicinity areas should be monitored, and somepreventive measures should be taken when necessary.

Fig. 3 Adsorption isotherm curve fitting by modified Langmuir model

Table 4 Adsorption isotherm parameters fitting by modified Langmuir, Freundlich, and liner model (P£0.05)

site Langmuir Freundlich liner model

kL/(L$mg–1)

b/(mg$kg–1)

a/(mg$kg–1)

R2 kF n R2 EPC0

/(mg$L–1)m

/(L$kg–1)Ni

/(mg$kg–1)R2

S7 3.08 178.82 13.23 0.98 168.65 0.77 0.95 – 0.02 197.22 – 3.4 0.93

S8 16.08 236.21 54.30 0.99 434.33 0.077 0.92 0.01 595.67 – 7.87 0.88

S11 1.91 206.23 15.00 0.98 173.83 0.996 0.94 0.02 186.32 3.37 0.94

Fig. 4 Relationship between SRP and EPC0 at 12 sample sites


The adsorption and release kinetics and isotherm canwell describe the adsorption and release capacity andfurther to predict the sediment pollution dynamic andtendency. The EPC0 method can effectively predictsediment status, which could be demonstrated by thelaboratory and field data, for example, the change of theSRP in overlying water and that of EPC0 in sediment at aperiod of investigation. It will provide significant informa-tion for developing corresponding countermeasures onlake or catchment protection and management.

Acknowledgements This research was financially supported by ShandongNatural Science Foundation Project “Research on Prediction Model of theRelease of pollutants in Nansi Lake (No.Y2007B18)” and “Shandong High-tech Project (2007GG20006003)”. The authors are grateful to WeishanEnvironment Protection Bureau for providing conveniences and MissLaura Shen from New York and Mr Yang Zhao for their help with thelanguage.

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