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Ecological Engineering 37 (2011) 1515– 1522
Contents lists available at ScienceDirect
Ecological Engineering
j ourna l ho me page: www.elsev ier .com/ locate /eco leng
ffect of temperature on phosphorus sorption to sediments from shallowutrophic lakes
idong Huanga, Lili Fub, Chongwei Jina, Gerty Gielenc, Xianyong Linb, Hailong Wangc, Yongsong Zhanga,∗
Ministry of Education Key Laboratory of Environmental Remediation and Ecosystem Health, College of Environmental and Resources Sciences, Zhejiang University, Hangzhou10029, ChinaZhejiang Provincial Key Laboratory of Subtropical Soil and Plant Nutrition, College of Environmental and Resources Sciences, Zhejiang University, Hangzhou 310029, ChinaScion, Private Bag 3020, Rotorua, New Zealand
r t i c l e i n f o
rticle history:eceived 19 October 2010eceived in revised form 7 April 2011ccepted 9 May 2011
eywords:orptionedimentesorptionractionation
a b s t r a c t
The availability of phosphorus (P) in lakes is dependent on the sorption characteristics of the underlyingsediments. Temperature is a crucial factor affecting the P sorption in sediments. The objective of thisstudy was to evaluate the effect of temperature on sorption of P by sediments from two eutrophic lakes.The study was carried out using short-term batch experiments at 4, 20 and 30 ◦C. Phosphorus sorptionkinetics, isotherms, fractionation and desorption were investigated. The P sorption was dependent onsediment type and temperature (p < 0.001). The Mei sediments showed a higher sorption rate and sorp-tion capacity than Hua sediments. The P sorption kinetics were best described by a pseudo second ordermodel (R2 > 0.97). Activation energies derived from the kinetics rate constant indicated that P sorptiononto the two sediments was controlled by a diffusion process. For both sediments, Freundlich modelfit the P sorption isotherms well and the calculated apparent sorption heat was 6.37 kJ mol−1 for Meisediments and 8.67 kJ mol−1 for Hua sediments. This indicated that P sorption onto both sediments wasendothermic. Adding P significantly increased the soluble and loosely bound P (S/L–P), aluminum-bound
◦
P (Al–P) and iron-bound P (Fe–P) (p < 0.05). The amount of Al–P and Fe–P was markedly higher at 30 Cthan at 4 ◦C (p < 0.05). Subsequent P desorption indicated that adsorbed P was highly labile, in partic-ular for Hua sediment. The degree of P mobility that occurred during sediment sorption was inverselyrelated to the temperature at the time of sorption. A significant relationship (R2 = 0.978) between phos-phorus sorption maximum and oxalate-extractable Fe and Al at different temperatures reflects that theamorphous contents of Fe and Al are responsible for the temperature effect on P sorption.wsislLTiuo
. Introduction
Over the past 30 years, economic development in China, espe-ially in east inshore region, has resulted in a large increase ofomestic wastes and non-point pollution from agricultural prac-ices and urban development. Large quantities of the pollutants areischarged into the surrounding lakes. As a result, eutrophicationas become one of the most serious environmental problems inhese lakes (Jin, 2003). Excessive phosphorus (P) in water is recog-ized as a main factor leading to eutrophication and deterioration
f water bodies, resulting in the bloom of aquatic plants, growthf algae and depletion of dissolved oxygen (Saha et al., 2009). Theake sediments could act as either P source or pool to the overlying∗ Corresponding author. Tel.: +86 571 86971147; fax: +86 571 86049815.E-mail address: [email protected] (Y. Zhang).
(
piahA
925-8574/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.ecoleng.2011.05.006
© 2011 Elsevier B.V. All rights reserved.
ater column. During periods of enhanced external loading, theediments as a pool can take up P, but after reduction of externalnputs the sediments can release adsorbed P and thus become aource of eutrophication. Most of the lakes in east China are shal-ow, such as Taihu Lake with an average depth of 1.89 m, Hongzeake at 1.5 m, and Xuanwu Lake at 1.14 m (Wang et al., 2006).he importance of sediment–water interactions in shallow lakess furthermore enhanced by the high sediment surface:water col-mn ratio, which means that the potential influence of sedimentn lake water P concentrations is stronger than in deeper lakesSøndergaard et al., 2003).
Furthermore, the time frame over which P sorption may takelace could influence P exchange with the sediment. Often, studies
nto the kinetics of P sorption on sediments have been limited to few hours (Appan and Wang, 2000; Jin et al., 2005). Phosphorus,owever, can continue to react with sediment over longer periods.
kinetically rapid equilibrium occurs within a few minutes but
1 ngineering 37 (2011) 1515– 1522
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Table 1The basic properties of the Mei and Hua sediments.
Properties Mei Hua
TP (mg kg−1) 642.39 ± 57.41a 824.19 ± 50.97TN (mg kg−1) 2259.70 ± 46.38 2671.81 ± 90.78OM (%) 3.68 ± 0.15 4.43 ± 0.07CEC (cmol kg−1) 29.99 ± 2.71 7.78 ± 1.90pH 6.92 ± 0.07 7.50 ± 0.10BET surface area (m2 g−1) 16.40 ± 0.27 1.45 ± 0.01Clay (%) 8.33 ± 0.00 2.97 ± 0.01Silt (%) 87.10 ± 0.03 56.70 ± 0.20Sand (%) 4.57 ± 0.03 40.33 ± 0.21Total Fe (g kg−1) 19.47 ± 0.16 13.79 ± 0.50Total Al (g kg−1) 32.48 ± 1.18 14.88 ± 0.67Total Mn (g kg−1) 0.54 ± 0.01 0.38 ± 0.19Total Ca (g kg−1) 3.41 ± 0.14 10.53 ± 0.46
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516 L. Huang et al. / Ecological E
low ones could take days. The ultimate P level in the lake waterherefore would not only depend on P input and measures takeno lower the P level but also be determined by the rate of sorption.redicting the rate at which the P adsorbed onto sediments over
longer time frame is an important factor for the assessment of Pollution and lakes restoration.
In shallow lakes, wind-driven sediment resuspension is a com-on phenomenon (Reddy et al., 1996; Pant and Reddy, 2001) which
esults in direct exchange with the water column and would be aey process to regulate the P level in water due to the immedi-te contact with the overlying water. Sediment resuspension mayncrease the accessibility to P sorption sites and thus be helpfuln immobilizing P from a water column with high P concentra-ion and protecting the lake by sedimentation–deposition andorption–precipitation processes (Li and Huang, 2010). Unlike deepakes where, due to stratification, sediments in the hypolimnion areredominantly isolated from temperature variations on the sur-ace, shallow lakes are usually isothermal and the sediments areusceptible to temperature variations. Therefore, temperature mayave a larger effect on P sorption by sediments from shallow lakes.here are only a few investigations focused on the effect of temper-ture on P sorption by sediments (Redshaw et al., 1990; Jin et al.,005). Those studies mainly investigated the effects of comprehen-ive factors (pH, dissolved oxygen, temperature) on the P exchangecross sediments–water interface. In soil, it was found that P sorp-ion increased with temperature and thus decreased the soluble Praction (Barrow, 1979; Sah and Mikkelsen, 1986b). Furthermore,t higher temperatures a greater proportion of P adsorbed irre-ersibly, thereby decreasing the equilibrium P concentration in soilolution. However, in shallow lakes under eutrophic conditions, theffect of temperature on non-steady-state P sorption kinetics willeed to be investigated in more detail.
Sorption–desorption experiments in soils have been widelysed to investigate the mobility of added P (Dolui and Roy, 2005;air et al., 2009). Previous studies have shown that the freshlydsorbed P remained mostly exchangeable in soil (Vu et al., 2010),lthough P desorption exhibited a hysteresis effect (Lair et al.,009). Investigations to date have mostly focused on P sorption inediments (Pant and Reddy, 2001; Cyr et al., 2009). It is, however,lso necessary to study P desorption in order to assess the mobilityf P in sediment better. This is particularly pertinent to improvehe understanding of lake eutrophication processes. Therefore, thebjectives of this study were to investigate the effect that temper-ture and kinetics have on P sorption and desorption in shallowutrophic lake sediments.
. Materials and methods
.1. Area descriptions
Taihu Lake, being a typical large shallow lake, is one of the fivereatest lakes in the East China plain. With an area of 2, 250 km2
nd an average depth of 1.9 m (Qin et al., 2007), Taihu Lake is one ofhe most well known contaminated large lakes in China (Qin et al.,007). Meiliang Bay, located in northwest of Taihu Lake, is one of theain drinking water sources of the city of Wuxi, Jiangsu province.
t is the most seriously polluted bay in Taihu Lake. The highest andowest temperatures of water are 38 ◦C and 0 ◦C, respectively, withn annual average of 17.1 ◦C.
Another shallow lake, Hua-jia-chi Lake, is an urban lake inangzhou, Zhejiang Province. It has 53, 360 m2 of total area with an
verage water depth of 1.8 m. It is the second largest water bodyn Hangzhou, and is extensively used for recreation. This lake isainly replenished by rainfall with an average annual rainfall of 1,35 mm. This lake has been seriously contaminated by discharged
Twpi
Total Mg (g kg−1) 3.67 ± 0.09 4.26 ± 0.17
a Values are means ± standard deviation, n = 3.
unicipal wastewater. Extensive algal blooms and occurrences ofsh death were observed in the lake. The maximum and minimumater temperatures are 32 ◦C and 2 ◦C, respectively, and an annual
verage temperature of 17.5 ◦C.Due to the very shallow depth of these lakes, wind-driven resus-
ension of bottom sediments has given rise to intensive internalirculation of particles (Zhou et al., 2005).
.2. Sediment sampling and characteristics
Sediment cores (0–15 cm) were collected from Meiliang Bayf Taihu Lake (31◦28′N, 120◦10′E) and Huajiachi Lake (30◦16′N,20◦11′E) with a stainless-steel grab sampler. The samples were
mmediately taken to the laboratory, where they were freeze-dried,round and passed through a 0.149 mm sieve to obtain uniformize (Jin et al., 2005; Zhou et al., 2005). The grain size distributionf samples was analyzed using a Laser grain size analyzer (Malvern,aster size R2000, UK) which separated clay (<0.002 mm), silt
0.002–0.05 mm) and sand fractions (0.05–2 mm) (Jin et al., 2005).ation exchange capacity (CEC) of sediments was analyzed usingDTA–NH4
+ method. Total nitrogen (TN) and total phosphorus (TP)ere measured as ammonium and orthophosphate by colorimetry
fter the digestion of the samples (ISSCAS, 1978). Organic mat-er content (OM) was calculated according to the loss on ignitiono constant mass (4 h) at 550 ◦C. The BET surface area was deter-
ined by the N2 sorption–desorption technique on an ASAP 2010Micrometrics, USA) at liquid N2 temperature (−194 ◦C). The totalontents of metals as Fe, Al, Ca and Mn were measured by induc-ively coupled plasma-atomic emission spectrometry (ICP-AES)
ethod after digestion (Lin et al., 2009). The Fe and Al extractedy ammonium oxalate–oxalic acid at 4, 20 and 30 ◦C were consid-red to be as amorphous iron and aluminum at the correspondingemperature (Danen-Louwerse et al., 1993).
The basic properties of sediment samples showed that specificurface area, cation exchange capacity, clay, silt, iron and aluminumontent were higher in the Mei sediments compared to the Huaediments, while TP, TN, calcium, OM, sand and magnesium con-ent were higher in Hua sediments compared to the Mei sedimentsTable 1).
.3. Sorption kinetics
Phosphorus sorption kinetics was examined at 4, 20 and 30 ◦C.
he above three temperatures are typical for the sample locations,hich corresponded with the transition from subtropics to tem-erate zone. The average temperature of the studied area is 4 ◦Cn winter, 20 ◦C in autumn and spring and 30 ◦C in summer. For
ngineering 37 (2011) 1515– 1522 1517
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L. Huang et al. / Ecological E
oth Mei and Hua sediments, 12.5 g dried sediments were mixedith 1000 mL solution with initial P concentration of 2.0 mg L−1
Appan and Wang, 2000). Two drops of 0.1% chloroform were addedo inhibit bacteria activity (Zhou et al., 2005). Periodically (0.17,.5, 2, 4 and 9 h), samples were taken and then filtered through
0.45 �m membrane immediately to analyze phosphate concen-ration (Murphy and Riley, 1962). Mixing was performed in anrbital shaker (150 rpm). The P adsorbed onto sediment samplesas determined using the difference between the initial and tested
oncentrations.
.4. Sorption and desorption isotherm tests
The sorption isotherms were conducted at 4, 20 and 30 ◦C. Sam-le aliquots of 0.5 g in triplicate were added to a series of 50 mLentrifuge tubes containing 40 mL phosphate solution at P concen-rations of 0, 0.1, 0.2, 0.5, 1, 2, 5 and 10 mg L−1. In this study, wesed deioned water instead of any electrolytes since electrolytesad impact on sorption (Wang et al., 2006). Two drops of chloro-
orm were added to inhibit bacteria activity (Zhou et al., 2005). Theentrifuge tubes were shaken in an orbital shaker with 150 rpm athe constant temperature of 4, 20 and 30 ◦C. After 24 h of equilib-ium, the solutions were centrifuged at 5000 rpm for 8 min and theupernatants were decanted into clean and dried glass bottles, thenhe solution was filtered (0.45 �m) and analyzed for phosphate PZhou et al., 2005). The P adsorbed on sediment samples was cal-ulated using the differences between the initial and equilibriumoncentrations.
After removing the supernatants of the sorption experiment,ach tube was weighed to estimate the volume of the residualolution and account for P entrapped in that solution (Lair et al.,009). Exactly 40 mL deioned water was added to the sedimentsemaining in the centrifuge tubes. The same conditions and pro-edures (temperature, time, and centrifugation) used for sorptiontudy were also used for desorption experiment. The net amount of
sorption onto the sediments was determined from the differencen P sorption and desorption.
.5. P fraction
In order to investigate the distribution of adsorbed P ontohe sediments, the following sequential extractions were per-ormed after equilibrating sediments with an initial 10 mg L−1 Polution. Ammonium chloride (1 mol L−1 NH4Cl) was first usedo remove soluble and loosely bound P (S/L–P). Aluminum-ound P (Al–P) was separated from Fe–P with 0.5 mol L−1
H4F, then iron-bound P (Fe–P) was removed with 0.1 mol L−1
aOH. Reductant-soluble P (RS–P) within the matrices of retain-ng aggregates/minerals was removed with CDB (0.3 mol L−1
odium citrate (Na3C6H5O7·2H2O)/25 g L−1 sodium dithioniteNa2S2O4)/1 mol L−1 sodium bicarbonate) extraction. The Ca–P wasxtracted with 0.25 mol L−1 H2SO4 solutions (Zhang and Kovar,000). The original sediment (without P sorption) was as controlo the sediment with P sorption.
.6. Estimation of sorption parameters
In order to investigate the mechanism of sorption and potentialate controlling steps, pseudo second order rate kinetic model firstsed by Ho et al. (1996) was fitted to the experimental data. Thisodel has been widely used (Tian and Zhou, 2007; Plazinski et al.,
009) and is described in Eq. (1):
t
qt= 1
(k2q2e )
+ t
qe(1)
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1518 L. Huang et al. / Ecological Engineering 37 (2011) 1515– 1522
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Fig. 1. Effect of temperature on phosphorus sorption kinetics
here qt (mg kg−1) is the amount of adsorbed P at time t (h); qe
mg kg−1) is qt at equilibrium status and k2 (kg mg−1 h−1) is theate constant.
The Arrhenius equation (Eq. (2)) may be used to relate theate coefficient (k2) of sorption to temperature (Mezenner andensmaili, 2009):
2 = A exp(−Ea
RT
)(2)
here A (kg mg−1 h−1) is a constant; Ea (kJ mol−1) is the activationnergy of sorption; R (8.314 J mol−1 K−1) is gas constant; T (K) isbsolute temperature.
A plot of logarithm of k2 and the reciprocal of temperature couldive the value of Ea which is listed in Table 2.
Sorption isotherm data were also fitted using the Langmuir (Eq.3)) and Freundlich (Eq. (4)) sorption models:
1(q + q0)
= 1(qmaxkL)
× 1c
+ 1qmax
(3)
n(q + q0) = ln kF + b × ln c (4)
here q (mg kg−1) is the amount of equilibrium adsorbed P atfter 24 h sorption; q0 (mg kg−1) is represented by the fractionf oxalate-extractable P (see Section 2.2) in sediments prior toorption (Lai and Lam, 2009); qmax (mg kg−1) is the P sorptionaximum, kL (L mg−1) is Langmuir sorption constant; c (mg L−1)
s equilibrium P concentration after 24 h sorption; kF (L kg−1) isreundlich sorption constant and b is an empirical constant (b < 1).
Thermodynamic parameters of the sorption of P on the sedi-ents, including �H0, apparent heat of sorption, and �S0, entropy
hange were calculated by the use of Van’t Hoff’s equation (Eq. (5))Sah and Mikkelsen, 1986b; Jin et al., 2005):
n kF = �S0
R− �H0
RT(5)
here kF is the Freundlich isotherm sorption constant as suggestedn Eq. (4); �S0 (kJ mol−1 K−1) is the entropy change; �H0 (kJ mol−1)s the heat of sorption; R and T are the same meanings with param-ters in Eq. (2).
The plot of ln kF vs.1/T according to Eq. (5) resulted in strait lineith a slope �H0/R and an intercept �S0/R. By substituting the
alue of R, �H0 and �S0 were calculated (Table 2). The �G0, change
f Gibbs energy could be calculated with the following equationEq. (6)):G0 = �H0 − T �S0 (6)
st(0
e two sediments (error bar means standard deviation, n = 3).
here �G0 (kJ mol−1) is the change of Gibbs energy; the otherarameters are listed in Eq. (5).
.7. Statistical analysis
The least significant difference (for p < 0.05) (Fisher’s protectedultiple comparisons) was calculated for each P fraction from
one-way analysis of variance (ANOVA) using SPSS 18.0 forindows (2009 SPSS Inc.). The effects of sediment type and tem-
erature on P sorption (initial 10 mg L−1 P) and fractions werenalyzed using a two-way ANOVA with SPSS 18.0 (2009 SPSS Inc.).
. Results
.1. Phosphorus sorption kinetics and thermodynamics
The P sorption rose sharply during the first hour of incubationnd was then followed by a slow sorption stage in both Mei and Huaediments (Fig. 1). However, the Mei sediments adsorbed more Pnd adsorbed P at a faster rate than the Hua sediments (Fig. 1).
Fitting the pseudo second order rate kinetic model (Eq. (1))o the kinetics data of Fig. 1 resulted in a good model fit, asas evident from R2 that ranged from 0.970 to 1.000. The modelarameters qe and k2 increased with temperature (Table 2) indi-ating that increasing temperature enhanced the P sorption. Thenitial sorption rate expressed as k2q2
e (Ho et al., 1996) were63, 357, and 538 mg kg−1 h−1 in Mei sediments and 12, 61, and81 mg kg−1 h−1 in Hua sediments when temperature increasedrom 4 to 30 ◦C. The Ea, �H0 and �S0 were 2.71 × 10−2, 6.37 × 10−2
nd 7.36 × 10−2 kJ mol−1 for P sorption in Mei sediments and theyere 7.67 × 10−2, 8.67 × 10−2 and 7.60 × 10−2 kJ mol−1 in Hua sed-
ments. The �G0 ranged from −14.02 to −15.92 kJ mol−1 in Meiediments while it ranged from −12.36 to −14.33 kJ mol−1 in Huaediments when temperature rose from 4 to 30 ◦C (Table 2).
.2. Sorption isotherms at various temperatures
The P adsorbed in the Mei sediments was much greater than inua sediments at all levels of added P (Fig. 2). The amounts of P
orption increased with equilibrium P concentration and tempera-ure. A Freundlich model described the P sorption isotherms betterR2 values of 0.956–0.999) than a Langmuir model (R2 values of.686–0.915) (Table 2).
L. Huang et al. / Ecological Engineering 37 (2011) 1515– 1522 1519
ffecte
3
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Fig. 2. The phosphorus sorption isotherms of the two sediments a
.3. Changes in sediment P fractions following P sorption
A significant amount of S/L–P was observed in sediment after sorption in comparison with the control sediment (Table 3). Thel–P and Fe–P were markedly higher at 30 ◦C than those at 4 ◦C
p < 0.05), while RS–P and Ca–P were almost constant after P sorp-ion (Table 3). The proportion (%) of the adsorbed P recovered inach P fraction was shown in Table 4. A large proportion of thedsorbed P was recovered in S/L–P, Al–P and Fe–P fraction (Table 4).n average, the recovery of S/L–P decreased with temperature. Inontrast, the recovery of Al–P and Fe–P increased with tempera-
ure (Table 4). It was found that the S/L–P, Al–P and Fe–P variedignificantly with sediment type and temperature (p < 0.05) andl–P and Fe–P were also affected by the interaction of sedimentwt3
able 3he distribution of various phosphorus (P) fractions (mg kg−1) in the control (original) Mifferent temperatures.
Sediment P fraction Control
Mei S/L–P 1.8 ± 0.8a
Al–P 43.4 ± 2.2a
Fe–P 206.3 ± 1.9a
RS–P 173.1 ± 44.7a
Ca–P 145.8 ± 6.4a
Hua S/L–P 33.4 ± 14.9a
Al–P 83.7 ± 2.3a
Fe–P 48.7 ± 1.8a
RS–P 159.9 ± 25.5a
Ca–P 402.2 ± 22.0a
ote: Initial phosphorus concentration for sorption was 10 mg L−1; S/L–P is defined as sos iron bound phosphorus; RS–P as reductant soluble phosphorus; and Ca–P as calcium bollowed by the same letter in a row are not significantly different at p ≤ 0.05.
able 4he recovery of each phosphorus (P) fraction as % of adsorbed P in the two sediments at d
Sediment Temperature (◦C) S/L–P
Mei 4 46.1
20 37.5
30 32.6
Hua 4 67.6
20 61.0
30 44.0
ote: The recovery of the adsorbed P in each P fraction was calculated by the differencnd loosely bound phosphorus; Al–P is aluminum bound phosphorus; Fe–P is iron bounhosphorus.
d by the temperature (error bar means standard deviation, n = 3).
ype × temperature (Table 5). In addition, Ca–P was varied withediment type ((p < 0.05) (Table 5)).
.4. Phosphorus desorption
The amounts of desorbed P from sediments increased withhe amounts of P adsorbed (Fig. 3). However, the P desorptionecreased with temperature (Fig. 3). The net P sorption (sorptioninus desorption) by the two sediments increased with the level of
sorption. At the highest level of added P (10 mg L−1), the amount
as greater in Mei sediments for all temperatures (from 58% at 4 ◦Co 60% at 30 ◦C) than in Hua sediments (from 14% at 4 ◦C to 34% at0 ◦C) (Fig. 3).
ei and Hua sediments and in these two sediments after P sorption incubated at
4 ◦C 20 ◦C 30 ◦C
83.8 ± 2.0bc 85.2 ± 1.8b 82.5 ± 1.2c97.1 ± 7.3b 121.8 ± 4.2c 131.7 ± 1.8d
237.9 ± 16.9b 261.3 ± 8.4c 275.6 ± 1.0c180.8 ± 32.5a 171.9 ± 22.0a 177.5 ± 16.9a145.3 ± 1.3a 147.6 ± 0.8a 149.0 ± 1.9a
62.1 ± 2.1b 66.7 ± 3.1b 63.8 ± 3.1b89.9 ± 1.9b 99.2 ± 3.0c 109.7 ± 1.7d52.5 ± 2.9b 56.1 ± 2.3c 60.5 ± 0.6d
165.0 ± 3.9a 156.1 ± 5.4a 163.1 ± 23.8a404.4 ± 3.3a 403.0 ± 5.3a 408.4 ± 7.1a
luble and loosely bound phosphorus; Al–P as aluminum-bound phosphorus; Fe–Pound phosphorus. Numbers are expressed as means ± standard deviation. Means
ifferent temperatures.
Al–P Fe–P RS–P Ca–P
30.2 17.8 4.3 −0.335.3 24.8 −0.5 0.835.6 28.0 1.8 1.3
14.7 8.9 11.9 5.228.5 13.5 −6.9 1.437.7 17.1 4.6 8.9
e between the P recovered in a given P fraction and its control. S/L–P is solubled phosphorus; RS–P is reductant soluble phosphorus; and Ca–P is calcium bound
1520 L. Huang et al. / Ecological Engineering 37 (2011) 1515– 1522
Table 5Significance levels of effects of sediment type and temperature and their interactions on phosphorus (P) sorption and fractions.
Main effect and interaction P sorption S/L–P Al–P Fe–P RS–P Ca–P
Temperature 0.000 0.032 0.000 0.000 0.929 0.220Sediment 0.000 0.000 0.000 0.000 0.317 0.000Temperature × sediment 0.027 0.331 0.001 0.005 0.997 0.673
Note: The initial P sorption concentration is 10 mg L−1. The value over 0.05 is not significant. S/L–P is soluble and loosely bound phosphorusl Al–P is aluminum boundp ; and
4
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hosphorus; Fe–P is iron bound phosphorus; RS–P is reductant soluble phosphorus
. Discussion
.1. P sorption kinetics
The sorption of P has been extensively regarded as a multipleinetic process involving at least two processes at different rates:1) a fast initial sorption step, followed by (2) a slower reactiontage (Lopez et al., 1996; Lai and Lam, 2009). The P sorption in Meiediments was quick and was mostly completed in the first stage<0.5 h). This was similar to the results reported by Wang et al.2006). Lopez et al. (1996), however, observed a P sorption rate of0 h obtained in their sediment study. Due to the smaller amountf P sorption in Hua sediment, any variation in the tested P equi-ibrium concentrations will have a relatively larger effect on theuantity of sorption determined and thus may cause a large stan-ard deviation of sorption. The differences between Mei and Huaediment in Fig. 1 were probably in part due to the sediment charac-eristics and initial P levels used (Plazinski et al., 2009). The pseudoecond order kinetics has been widely used to study P sorption (Tiannd Zhou, 2007). Also in this study, this model described the P sorp-ion onto the Mei and Hua sediments well (R2 > 0.97), thus enablinghe prediction the P sorption in sediments with time. Values ofa were positive for both sediments, indicating the presence of annergy barrier in the sorption process. The energy barrier in thistudy, however, was smaller than in other studies which reportedctivation energy levels of 8.9 and 34.5 kJ mol−1 for soil (Sah andikkelsen, 1986a) and 32.74 kJ mol−1 for hydroxide-eggshell waste
Mezenner and Bensmaili, 2009). Low activation energies are char-cteristic for diffusion-controlled processes (Nollet et al., 2003),uggesting that P sorption onto Mei and Hua sediments wereiffusion-controlled.
hm1o
ig. 3. Effect of temperature on phosphorus desorption by the two sediments (q denotesn sediments).
Ca–P is calcium bound phosphorus.
.2. Phosphorus sorption isotherm and thermodynamics
The P sorption capacity at equilibrium (qe) of both sedimentsncreased with temperature, indicating that the sorption of P wasavored at higher temperatures (p < 0.001) (Table 5). Increasingemperature also increased the sorption rates. This was consistentith P sorption on the hydroxide-eggshell waste (Mezenner andensmaili, 2009) and P sorption on soil (Sah and Mikkelsen, 1986a).
High temperatures facilitated the P sorption at all levels ofdded P confirming that the P sorption onto the sediments wasndothermic reaction. This was also observed by Jin et al. (2005)nd Mezenner and Bensmaili (2009) in sediments and hydroxide-ggshell waste. The P sorption is also significantly influenced byediment type (p < 0.001) (Table 5). A great many studies havehown that P sorption was mainly correlated with clay, metalhydr)oxides contents and surface area (Lopez et al., 1996; Wangt al., 2006; Luo et al., 2009). Similarly, the differences of P sorp-ion between Mei and Hua sediments could be explained by thebserved differences in clay, metal (hydr)oxide contents and sur-ace area (Table 1). Sorption isotherms were useful in summarizingarge amounts of sorption data and illustrating the relationshipetween aqueous P concentration and the amounts of sorption byediments. For the aquatic sediments of Mei and Hua, however,
large amount of native P was already present in the sedimentriginally. Therefore, in order to model P sorption accurately, theative P present in sediments will also need to be accounted for
n a sorption isotherm model (Appan and Wang, 2000). It was
ypothesized that the available P sorption sites in sediment wereainly supported by oxalate-extractable Al and Fe (Richardson,985; Danen-Louwerse et al., 1993; Zhou et al., 2005). The previ-usly adsorbed oxalate-extractable P was regarded as a measure
the phosphorus adsorbed from solution, q0 is the amount of oxalate-extractable P
ngineering 37 (2011) 1515– 1522 1521
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L. Huang et al. / Ecological E
f occupied sorption sites. Therefore, the q0 in Eqs. (3) and (4)as replaced by the oxalate-extractable P fraction according to the
tudy of Lai and Lam (2009).The two sorption isotherm models fit to the P sorption data dif-
ered in the assumptions made about the P sorption. The Langmuirodel assumed the occurrence of monolayer sorption with uni-
orm energy in homogeneous surface, while the Freundlich modelssumed the heterogeneous surface of sorption and affinity fororption decreased exponentially with increasing saturation of theurface (Dolui and Roy, 2005). Fitting of both models to the P sorp-ion data indicated that the Freundlich (R2 = 0.957–0.999) fittedetter than the Langmuir model (R2 = 0.686–0.915) for both sed-
ments but in particular for the Hua sediment. This indicated thathe sediment P sorption mechanism was more complex than mono-ayer sorption onto homogeneous surface. As indicated in othertudies (Sah and Mikkelsen, 1986b; Lai and Lam, 2009). However,rrespective of which model was fitted, the P sorption (pseudo)
aximum increased with increasing temperature.The values of thermodynamic parameters generally depend
n the sorption characteristic of a sediment (Sah and Mikkelsen,986b). The �H0, �S0 and �G0 were higher in Hua sedimentshan those in Mei sediments. However, the variation was not large.he positive values of �H0 for the sorption of P, indicated that theorption process for both sediments was endothermic. The �H0
btained in this study were observed to be higher than those in soil0.146–0.269 kJ mol−1) (Sah and Mikkelsen, 1986b) but lower thanhose in hydroxide-eggshell waste (81.84 kJ mol−1) (Mezenner andensmaili, 2009), which suggested that the �H0 was highly materi-ls dependent. The negative values of �G0 (Table 2) reflected thathe sorption of P onto sediments was feasible and spontaneous.he positive value of �S0 indicated an increased randomness athe sediment/water interface occurred in the sorption process.
.3. Distribution of P fractions
After P sorption onto the sediments, increased concentrationsf extractable S/L–P, Al–P and Fe–P were recovered from bothediments in comparison with the control sediments (p < 0.05)Table 3). The increments of P sorption caused by elevating tem-eratures (from 4 ◦C to 30 ◦C) could be explained by the increase ofl–P and Fe–P in sediments (Table 3). This concurred with Pennnd Warren (2008), who reported that the reaction of P withluminum was endothermic. Furthermore, amorphous iron andluminum (hydr)oxides (oxalate-extractable) have been reportedo play a major role in P retention (Zhou et al., 2005). Significantorrelation (R2 = 0.978) (Fig. 4) between oxalate-extractable Fe–Pnd Al–P with qmax at 30, 20 and 4 ◦C in the two sediments indi-ated that increased sorption onto iron and aluminum (hydr)oxidesikely contributed to the favorable effects of temperature on P sorp-ion. Furthermore, Stumm and Morgan (1981) reported �H0 was8.7 kJ mol−1 for the reaction: Fe3+ + PO4
3− → FePO4. The reactionas therefore endothermic. Overall, it demonstrated the positive
ole of iron and aluminum content in P sorption with increasingemperature even though sorption in sediments may be very com-lex.
The greater accumulation of S/L–P fraction in Hua sediment thann Mei sediment following P sorption (Table 4) reflected the effectsf different sediment properties on P transformation (p < 0.05)Table 5). The considerably higher level of P recovery in S/L–P frac-ion of treated sediment than the control sediment indicated thathe adsorbed P into this fraction remained highly exchangeable
Table 4). In addition, the large increase of Al–P and Fe–P in thedded P sediment compared to the control sediment suggested thatarge amounts of newly added P remained in these two pools. Alu-inum and iron could strongly adsorb P due to their high specific
at(s
ig. 4. The relationship between the phosphorus sorption maximum (qmax) andmorphous iron and aluminum at different temperatures.
urface areas (Danen-Louwerse et al., 1993). The Al–P and Fe–Pere regarded as moderately labile P pools (Zhou et al., 2001).lmost no adsorbed P was recovered in RS–P and Ca–P fractions
Table 3 and Table 4), suggesting that these fractions did not con-ribute to the sorption. The results were different from those of Vut al. (2010), who reported an accumulation of freshly applied P ina–P forms. This inconsistency is probably due to the different frac-ionation method and incubation time used (4–42 d in their study).t was stated that the freshly added P was quickly adsorbed onto theurface of minerals at the initial stage and then gradually diffusedo firmly held forms (Barrow, 1979). This suggests that transfer ofdsorbed P into Ca–P (acid-extractable) form may require a longime.
In the present study, sterile conditions were employed to avoidicrobial activity. Organic P as a potentially significant P fractionas not measured due to the absence of microbes and short-termeriod of incubation (24 h), which would limited the turnover of
in to organic pool. Moreover, Vu et al. (2010) found only a smallmount of the freshly applied P was detected in the organic P poolnder non-sterile conditions. They stated that the inorganic P poolainly dominated P dynamics. Furthermore, organic anions were
ot thought to retain much P by itself because they are normallyegatively charged (Guppy et al., 2005). Therefore, we determinedhe inorganic P fraction other than organic P fraction as Lin et al.2009) did in their study.
.4. Desorption of adsorbed P
Desorption of freshly adsorbed P was hysteretic in the Meiediments, which indicated that the P was at least partly irre-ersible adsorbed (58% at 4 ◦C to 60% at 30 ◦C under the highest Pevels added). The recovery of the various extractable P fractionsn Table 4 indicated that a higher proportion of adsorbed P wasoosely bound in sediment. These results were in agreement withhe study of Vu et al. (2010) who demonstrated a large proportionf adsorbed P could remain exchangeable using isotopic labelingechniques. It was suggested that the added P was rapidly trans-ormed into labile pools then progressively into less labile poolsVu et al., 2010). However, the P desorption data in Hua sedimentsuggested that most of P adsorbed could be released to the water
gain (Fig. 3), indicating a high mobility of the freshly adsorbed P inhe sediments. The P desorption expressed by the total adsorbed Pq + q0) further indicated that P was adsorbed more firmly onto theediments at higher temperature. The P would therefore be less
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522 L. Huang et al. / Ecological E
abile in sediments when sorption occurred at a higher temperatureTables 3 and 4).
. Conclusions
The P sorption in sediments varied with sediment type andemperature. Increasing the temperature accelerated the rate of
sorption in sediments. Additions of P were retained by sedi-ents in labile S/L–P and moderately labile Al–P and Fe–P pools.
urthermore, the significant accumulation of S/L–P after P sorp-ion indicated that the freshly adsorbed P was rapidly transferredo this fraction. Increasing temperature favored the transforma-ion of adsorbed P into Al–P and Fe–P fractions, suggesting thatediment could retain more P in summer even if it is only tempo-ary. As an important ambient factor, temperature could influencehe transport P between water and sediment. Therefore, increas-ng temperatures due to global warming and periods of unseasonaligh temperatures in recent years could influence P cycling in a
ake significantly. It would thus be prudent to consider the effectf temperature when examining P cycling in sediments in relationo lake water quality.
cknowledgements
This work was financially supported by the Natural Scienceoundation of China (NSFC, 30871590), the Major Research Pro-ram of Zhejiang Province (2008C12061-1) and China Postdoctoralcience Foundation funded project (200803350117).
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