Batch investigations on P immobilization from wastewaters and sediment using natural calcium rich sepiolite as a reactive material

w a t e r r e s e a r c h 4 7 ( 2 0 1 3 ) 4 2 4 7 e4 2 5 8

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Batch investigations on P immobilization from wastewaters and sediment using natural calcium rich sepiolite as a reactive material Hongbin Yin a,*, Ming Kong a,b, Chengxin Fan a a

State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, 73 East Beijing Road, 210008 Nanjing, China b University of School of the Chinese academy of Sciences, 19 Yuquan Road, 100049 Beijing, China

article info

abstract

Article history:

Phosphorus from wastewaters and sediment flux to surface water represents a major source of

Received 17 February 2013

lake eutrophication. Active filtration and in situ capping (which refers to placement of a

Received in revised form

covering or cap over an in-situ deposit of contaminated sediment) are widely used as a means

20 April 2013

to immobilize phosphorus from wastewaters and sediment, to mitigate lake eutrophication.

Accepted 22 April 2013

There is, however, a need to develop more efficient means of immobilizing phosphorus

Available online 4 May 2013

through the development of binding agents. In this study, natural calcium-rich sepiolite (NCSP) was calcined at a range of temperatures, to enhance its phosphorus removal capacity. Batch

Keywords:

studies showed that the 900
C calcinated NCSP (NCSP900) exhibited excellent sorption per-

Phosphorus

formance, attaining a phosphorus removal efficiency of 80.0%e99.9% in the range of 0.05 mg/L

Lake eutrophication

e800 mg/L phosphorus concentrations with a dosage of 20 g/L. The material displayed rapid

Active filtration

sorption rate (maximum amount of 99.9% of phosphate removal with 5 min) and could lower the very high phosphate concentration (200 mg/L) to less than 0.1 mg/L after 4 h adsorption. It

In situ capping Natural

calcium-rich

sepiolite

was also noted that factors such as pH, competing anions (except HCO3  ) and humic acid, had no effect on phosphorus removal capacity. The sediment immobilization experiment indi-

(NCSP)

cated that NCSP900 had the capacity to transform reactive phosphorus into inert-phosphorus and significantly reduce the amount of algal-bioavailable phosphorus. The excellent phosphorus binding performance of NCSP900 was mainly due to the improvement of point of zero charge (pHPZC) as well as the transformation of the inert-calcium of NCSP to active free CaO during calcination. Phosphorus speciation indicated that phosphorus was mainly captured by relatively stable calcium-bound phosphorus (Ca-P) precipitation, which can account for 80.1% of the total phosphorus. This study showed that NCSP900 could be used as an efficient binding agent for the sequestration of phosphorus from wastewaters and sediment. ª 2013 Elsevier Ltd. All rights reserved.

\ 1.

Introduction

Eutrophication has now become a global environmental problem attracting the attention of scientists and governments around the world. Persistent algal bloom in lakes and

costal zones has caused major ecological, economic and social problems. Measures must therefore be taken to improve aquatic ecosystems that have become degraded due to excessively high phosphorus levels. Phosphorus is considered to be the limiting nutrient for the growth algae in freshwater

* Corresponding author. Tel.: þ86 25 86 86882209; fax: þ86 25 57714759. E-mail addresses: [email protected] (H. Yin), [email protected] (C. Fan). 0043-1354/$ e see front matter ª 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.watres.2013.04.044

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ecosystems. Superfluous phosphorus input into freshwater ecosystems can induce eutrophication and subsequently result in the formation of harmful algal blooms. It thus becomes necessary to reduce the concentrations of phosphorus in receiving water bodies. In certain freshwater ecosystems, phosphorus originates mainly from external loading, such as industrial wastewaters, domestic sewerage and agricultural runoff, as well as internal loading from sediment flux (Xiong and Peng, 2008). Technologies, such as chemical precipitation and biological treatment have been used to remove phosphorus from industrial, household and agricultural wastewaters (Sibrell et al., 2009). This typically requires considerable capital investment and costly maintenance (Kaasik et al., 2008; Sibrell et al., 2009). Active filtration through iron, aluminum or calcium-rich substrates to remove phosphorus from wastewaters has recently been considered an effective treatment method (Rentz et al., 2009; Ko˜iv et al., 2010). These substrates must be inexpensive, readily available, environment-friendly and efficient (Sibrell et al., 2009; Ko˜iv et al., 2010). To date, numerous reactive media have been developed and applied in practice. These include minerals (limestone, opoka, wollastonite, bauxite and zeolites), soils (laterite and marl), industrial byproducts (fly ash, red mud, burnt oil shale and slag materials) and man-made products (lightweight aggregates) (Johansson Westholm, 2006; Cucarella and Renman, 2009; Vohla et al., 2011). In summary, it seems that the calcium-rich materials are the most promising and widely-accepted reactive media used for wastewater treatment (Johansson Westholm, 2006; Cucarella and Renman, 2009; Vohla et al., 2011). Phosphorus has been effectively removed from wastewater through absorption, but mainly through precipitation of chemically-stable phases (Ko˜iv et al., 2010; Renman and Renman, 2010; Claveau-Mallet et al., 2012). There are, however, some disadvantages associated with a number of sorbents currently in use, such as low phosphorus removal efficiency and the use of certain highly pH-dependent natural calcium-rich materials that are only suitable for a small number of applications (Yin et al., 2011a). There is thus a need to modify these promising sorbents in order to expand their sorption performance with respect to phosphate. Simple and easy methods have been encouraged so as to avoid additional costs. For example, the heating of calcium-rich media at high temperature is a common method used to enhance material performance in terms of phosphorus retention capacity. During calcinations, CaO will probably form, which has a more reactive Ca-phase than commonly-existing calcareous minerals such calcite and dolomite (Karaca et al., 2006; Vohla et al., 2011). Sediment dredging and in situ active capping have been used to control internal loading of water bodies and eutrophication in lakes and estuarine waters (Berg et al., 2004; Xiong and Peng, 2008; Lin et al., 2011). There is also a need to develop an efficient and low-cost sediment capping agent that can effectively bind sediment phosphorus and hence suppress its release from sediments (Berg et al., 2004; Xiong and Peng, 2008; Lin et al., 2011). Conventional Ca/Fe-rich materials, which originated from industrial byproducts such as fly ash, red mud and slags, may not be suitable for ameliorating the effects of eutrophication regardless of their excellent

phosphorus removal capacity and efficiency, since such substances may potentially have a toxic effect on aquatic species in lakes (Xiong and Peng, 2008). In contrast, clay minerals that are based on capping agents or adsorbents are normally environmental friendly and also inexpensive. They can thus be used safely in freshwater ecosystems (Berg et al., 2004; Xiong and Peng, 2008; Lin et al., 2011). Sepiolite, a hydrated magnesium silicate clay mineral with a fibrous chain structure, is nontoxic and relatively inexpensive. The main deposits of sepiolite are located in Anatolia in Turkey, Ceelbuur in Somalia, South Central China and Spain with 70% of the world reserves and annual output being approximately 1,300,000 tons (Hrenovic et al., 2010). Structurally, sepiolite consists of a ribbon-like structure that alternates with open channels along fiber axes, which provides sepiolite with good adsorption properties (Hrenovic et al., 2010). In environmental studies, sepiolite has been widely used to absorb heavy metals (Kocaoba, 2009), chloride (Gonza´lezPradas et al., 2005), basic dyes (Tekbas et al., 2009) and cationic surfactants (Sabah et al., 2002). Approximately 10 million tons of sepiolite deposits exist in China (approximately 1/5 of the world’s reserve) (Yin et al., 2011a). The chemical composition of sepiolite varies geographically and the sepiolite in China is characterized by high calcium content, which has been found to have a calcium oxide content in the range of 20.5%e27.1% (Yin et al., 2011a). Previous studies indicated that natural calcium-rich sepiolite (NCSP) from Nanyang (Henan Province) had a phosphorus removal capacity as high as 32.0 mg P/g at acid conditions (Yin et al., 2011a). This contrasts with certain characteristics of NCSP that greatly narrow its usage, such as its pH-dependent characteristics and low phosphorus removal efficiency at low concentrations. A proper activation method is therefore needed to enhance the phosphorus sorption performance of this compound. The objectives of this study were therefore as follows: (1) to test the effect of calcinations (or material pre-treatment) on its P removal capacities; (2) to test the reactive materials in batch experiments for their suitability in terms of immobilization capacity P in both wastewaters and sediment; and (3) to characterize the P retention mechanism on the reactive material. The above-mentioned research would represent a necessary pre-evaluation process for controlling lake eutrophication that results from phosphorus-contaminated water and sediment fluxing.

2.

Materials and methods

2.1.

Materials

The methods used to collect and treat the natural calciumrich sepiolite (NCSP) from Nanyang are based on those used in a previous study (Yin et al., 2011a). The samples were manually ground and sieved through a 100-mesh sieve. The modified NCSP was produced by calcination of NCSP at temperatures of 100e1000
C for 2 h. The modified NCSP was denoted as NCSP100, NCSP200, etc. Surface sediments (0e10 cm) were sampled from the Nanfei River estuary (31.684019 N; 117.399702 E) from the hyper-eutrophic Lake Chaohu, (the fifth largest freshwater

w a t e r r e s e a r c h 4 7 ( 2 0 1 3 ) 4 2 4 7 e4 2 5 8

lake in China) and the Nanfei River, which carries the highest pollutant load among the waters of the Lake Chaohu catchment (Yin et al., 2011b). The collected sediments were homogenized, freeze-dried, crushed, passed through 0.063mm mesh sieves, and stored at 4
C in the dark until further analysis. Sediment P forms were extracted according to the methods first proposed by Hieltjes and Lijklema (1980), with a slight modification, as proposed by Rydin and Welch (1998). A small quantity (1.00 g) of freeze-dried sediment was added to a set of 50 ml centrifuge tubes and sequentially extracted with: (a) 1 M NH4Cl at pH ¼ 7 (Liable-P), (b) 0.11 M Na2S2O4/0.11 M NaHCO3 (Fe-P), (c) 1 M NaOH, and (d) 0.5 M HCl (Ca-P). (Note: the NaOH solution, described in c above, was digested and measured to be NaOH-Tot P. If measured directly, this was NaOH-rP (Al-P). Organic phosphorus (OrgP) was estimated as the difference between NaOH-Tot P and NaOH-rP, respectively.) Residual P (Res-P) was the concentration of TP minus the sum of a to d. Reactive phosphorus was identified as the sum of Liable-P, Fe-P and Org-P (NaOHnrP).

2.2.

Immobilization of P from wastewaters

To identify the optimum adsorbent among the calcined NCSPs, a pre-screening sorption experiment was carried out. 0.50 g of NCSP and the modified NCSPs were added to 25 ml of low (0.5 mg/L and 1 mg/L), medium (30 and 100 mg/L) and high (400 mg/L and 800 mg/L) phosphorus concentration solutions at a pH of 7.0. After equilibration for 24 h in the temperaturecontrolled shaker (25
C), solutions were centrifuged, filtered and measured. The optimal modified NCSP was selected and used for further study. The phosphorus sorption isotherms and kinetics of the optimally-modified NCSP were evaluated by batch experiments. For the sorption isotherms, 0.50 g of optimallymodified NCSP was added to 50-ml polyethylene centrifuge tubes with 25 ml of various phosphorus solutions (0.05e800 mg P/L) at a pH of 7.0. The tubes were then placed on a constant temperature shaker (25
C) for 24 h at 160 rpm, to ensure complete mixing. The solutions were then centrifuged and the supernatants filtered through a 0.45-mm membrane. The resulting solutions were kept at 4
C until further analysis. Analysis of the phosphorus sorption kinetics was carried out at 25
C with an initial phosphorus concentration of 1 mg/L, 50 mg/L and 200 mg/L at a pH of 7.0. 10.0 g of the optimallymodified NCSP was added to 500 ml of a phosphorus solution in three 1-L Erlenmeyer flasks, which were subsequently placed on a rotary shaker at 160 rpm. Periodically, 1 ml of each of the well-mixed aliquots in the flasks was sampled, centrifuged and filtered, as described above. To examine the effect of pH, co-existing anions (SO4 2 , F, NO3  , Cl and HCO3  ), and humic acid, on the phosphorus sorption capacity of the optimally-modified NCSP, a series of batch studies were carried out. For comparison, the same experiments were also performed on NCSP. To obtain the optimal pH values, 1 M NaOH or 0.5 M HCl was used to adjust the pH of the tested phosphorus solution. To determine the coexisting anions and humic acid, 50 mg/L phosphorus with various concentrations of different anions and humic acid, were evaluated. All experiments were carried out in triplicate.

2.3.

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Immobilization of P from sediment

To examine the phosphorus-immobilization capacity of the optimal modified NCSP on the sediment phosphorus, different amounts of modified NCSP were added to the sediment at ratios of 1:20, 1:10 and 1:5. Theoretically, the lowest required ratio of the modified NCSP for complete inactivation of sediment reactive phosphorus is 1:20, according to the results from the batch adsorption experiment and the sediment phosphorus sequential extraction. Two additional ratios (1:5 and 1:10) were added to the experiment, to examine the ratio effect of modified NCSP on sediment reactive phosphorus. The well-mixed sediment and modified NCSP were added simultaneously to 50-ml polyethylene centrifuge tubes with 25 ml deionized water. These tubes were then placed on a constant temperature shaker (25
C) at 160 rpm to ensure complete mixing. After 1, 3, 5, 7 and 10 days of equilibrium, samples were collected and centrifuged. The remaining residue was freeze dried and underwent sequential phosphorus extraction, according to the methods of Rydin and Welch (1998). Algal-available phosphorus (AAP) in the residue was also analyzed according to the method proposed by Zhou et al. (2001).

2.4. Chemical extraction and microcosmic observation of the P-saturated calcined NCSP To increase understanding of the mechanisms associated with phosphorus binding by the optimal calcined NCSP, sequential phosphorus extraction was performed on the phosphorus-saturated modified NCSP. The optimal calcined NCSP (10.0 g) was added to 500 ml of a 400 mg/L phosphorus solution, which was thoroughly mixed in a temperaturecontrolled shaker (25
C) at 160 rpm. The concentration of the solution was analyzed daily. The experiment was terminated when the phosphorus concentration remained constant. The P-saturated calcined NCSP was freeze-dried and sieved through a 100-mesh sieve. For chemical extraction, a method widely used to fractionate P in calcareous soil was used in the present study. The extraction steps were described in detail in a previous study (Yin et al., 2011a). The method includes six operationally-defined phosphorus fractions: dicalcium phosphate (Ca2-P), octacalcium phosphate (Ca8-P), ten-calcium phosphate (Ca10-P), aluminum phosphate (Al-P), iron phosphate (Fe-P) and occluded phosphate (O-P). SEM-EDS was used to characterize the microcosmic pattern changes before and after phosphorus sorption by calcined NCSP.

2.5.

Analytical methods

X-ray fluorescence spectroscopy (XRF) was used to identify the chemical composition of natural calcium-rich sepiolite (NCSP) and the calcined sepiolite. X-ray diffraction (XRD) was used to study the mineralogical composition of the raw sepiolite and the calcined samples. Power samples were measured using a Rigaku X-ray diffractometer (TTRAX3) with Cuka radiation (40 kV and 200 mA) and a Ni filter from 3.0
to 60
with a scan speed of 4.0
/min. The quantitative analysis performed by XRD used internal standards, comparing the reflection from standard substances to the reflection from components.

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i i i h g h

Intensity (CPS)

Scanning electron microscope (SEM) observations and semiquantitative energy dispersive spectrometry (EDS) electron microprobe analyses (Sirion 200) were performed on the powder samples, to assess microcosmic changes. Fourier transform infrared (FT-IR) spectra of the raw NCSP and modified NCSPs were analyzed by a Thermo Scientific Instrument Spectrum (Nicolet 8700) in the wave-number range of 400e4000 cm1. The specific surface area was measured by the N2 adsorption-desorption technique on an ASAP 2020 M þ C (Micrometrics) at liquid N2 temperature (at 197
C). Point of zero charge (pHPZC) was determined by adding 0.5 g of sample to 5 ml N2-sparged DI water that had been shaken for 24 h in a sealed vial, and measured for pH (Tennant and Mazyck, 2007). pHPZC values are the average of triplicate samples. Phosphorus concentration in the solution and extract were measured by the molybdenum blue method with a spectrophotometer at 700 nm (UV-2550).

3.1.1.

XRD analysis

Mineralogical composition of NCSP and the calcined samples were measured by XRD and their patterns are illustrated in Fig. 1. The result indicated that NCSP contained 40e50% sepiolite, 10e15% smectite, 15e20% calcite, 10e20% dolomite and a small quantity of quartz and talcum. The impurity of the sepiolite used in this study was also observed during previous research and was noted to differ greatly when compared with calcium oxide originating from Turkey or Spain (Yin et al., 2011a). Mineral composition remained unchanged when NCSP was calcined at a temperature of 400
C. Considerable changes were observed when the temperature of calcinations exceeded 600
C (Fig. 1). The reflection of sepiolite disappeared when the heating temperature increased to 600
C, which was probably due to a collapse of the crystal structure of NCSP. Smectite and kaolin have greater heat-resisting properties than that of sepiolite. These were removed from NCSP at temperatures of 700
C and 900
C, respectively. Calcium oxide was observed at a temperature of 800
C, which was mainly due to the decomposition of dolomite (Karaca et al., 2006). When the temperature increased to 900
C, the reflection peak of calcium oxide was enhanced remarkably, due mainly to the contribution of calcite decomposition (Yu et al., 2010). The reflection peak of calcium hydroxide was probably the cause of water adsorption during sample storage. Akermanite (Ca2Mg(Si2O7) was observed at a temperature of 1000
C, which may be interpreted as a result of the chemical reaction of calcium oxide, magnesia and silicon dioxide at high temperatures.

3.1.2.

FT-IR analysis

FT-IR is a useful tool, which is widely used in environmental studies, for characterizing specific functional groups present in the crystal lattice (Gustafsson et al., 2008; Navarro et al., 2010). The FT-IR spectrum of NCSP and the thermally activated samples are presented in Fig. 2. The bands at 3689 cm1,

NCSP1000

g

h

NCSP900

NCSP700 NCSP600 NCSP400

f

a b b b

Results and discussion

3.1. Changes of mineral characteristics during calcination

g

i

NCSP800

c 3.

i

ii i i

0

20

db e

d

f f f

40 2 theta (degree)

ff f

NCSP

60

Fig. 1 e XRD patterns of natural calcium-rich sepiolite (NCSP) and the calcined ones (a: smectite; b: sepiolite; c: kaolinite; d: dolomite; e: talcum; f: calcite; g: calcium oxide; h: calcium hydroxide; i: akermanite).

3570 cm1 and 1629 cm1 are characteristic of hydroxyl stretching vibrations of hydrated aluminum-magnesium silicate minerals structural hydroxyl groups (Navarro et al., 2010; Dikmen et al., 2011; Liang et al., 2013). The band at 3689 cm1 corresponds to hydroxyl groups attached to octahedral Mg2þ ions located at the edges of the structural blocks (Navarro et al., 2010; Dikmen et al., 2011; Liang et al., 2013), while another two bands respectively correspond to bound water coordinated to magnesium in the octahedral sheet and zeolitic water in the channel (Navarro et al., 2010; Dikmen et al., 2011; Liang et al., 2013). Heating may cause the three types of water removal from natural sepiolite under different temperature regimes. As indicated in the FT-IR spectrum, the intensity of the bands either disappeared or were attenuated when temperature exceeded 400
C. The bands at 2918 cm1 and 2850 cm1 correspond to the asymmetric and the symmetric stretching of methylene (eCH2e) groups in NCSP, which are either attenuated or disappear when the temperature increases (Chung et al., 2004). The bands at 3676 cm1, 3435 cm1, 2515 cm1, 1797 cm1 and 1425 cm1 can be assigned to CeO stretching modes of the CO3 2 ion, and 873 and 715 cm1 bands correspond to deformation modes of the CeO vibration (Navarro et al., 2010; Ravisankar et al., 2011). These bands have been attributed to carbonated impurities relating to dolomite and calcite, which either disappear or are attenuated when heating temperatures remain above 800
C. This situation was mainly due to the decomposition of these carbonate minerals and transformation to oxide substances. As can been seen from Fig. 2, very sharp bands appeared at 3644 cm1 when the temperature remained above 800
C.

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36 44

NCSP1000

3.1.3.

36 4 4

Transmittance (%)

NCSP900 36 4 4

NCSP800

NCSP700

NCSP600

2918 3689 3676

2850

NCSP400

2 515 1797

3500

753 715 873

1629

3435 3570

4000

small quantity of Al2O3 was detected by XRF. The adsorption band disappeared when the heating temperature exceeded 800
C. This was probably due to transformation to other forms of aluminum.

1425

3000

2500

2000

1500

NCSP 49 4

1016

1000

500

0

Wavenumber (cm-1) Fig. 2 e Infrared spectra of natural calcium-rich sepiolite (NCSP) and the calcined ones.

Physico-chemical parameters analysis

Some physico-chemical parameters of NCSP and the activated products are presented in Table 1. The BET surface area of NCSP was 231.1 m2/mg, which was higher than that of other natural clay minerals such as palygorskite and diatomite (Xiong and Peng, 2008; Gan et al., 2009). As the calcined temperature increased, the BET surface area of the calcined NCSPs decreased sharply. This was probably due to a structural collapse and the presence of decomposition minerals present in NCSP, which caused micropore shrinkage and blockage, resulting in a rapid decrease of the BET surface area as the temperature increased. NCSP was characterized by an acidic surface (pHPZC < 7) which changed to basic properties when the calcined temperature exceeded 600
C (pHPZC > 7). The improved pHPZC of the calcined NCSPs, relative to NCSP, was probably due to newly-formed hydroxyl ligands resulting from the structural collapse of NCSP during calcination.

3.2. Phosphorus immobilization capacity of NCSP and calcined NCSPs from wastewaters 3.2.1.

These are characteristic absorption bands of calcium oxide (Navarro et al., 2010) and are consistent with the XRD data (Fig. 1), which suggests that calcium oxide was observed and that this probably resulted from the decomposition of dolomite and calcite at heating temperatures of between 800 and 1000
C. Bands that are characteristic of silicate minerals are observed between 1400 and 400 cm1 (1212 cm1, 1080 cm1, 979 cm1, 1022 cm1, 470 cm1 and 440 cm1) (Dikmen et al., 2011; Liang et al., 2013). It was noted that some aluminum oxyhydroxides and iron oxyhydroxides also appear in the region of the spectrum between 120 and 950 cm1 (Navarro et al., 2010). The bands characteristic of SieO bonds, which appeared in the samples, indicated that the silicate minerals were highly resistant to high temperature. The band at 753 cm1 corresponds to Al-O stretching vibrations and suggests that aluminum compounds, such as Al2O3, are present in NCSP (Navarro et al., 2010). As can been seen from Table 1, a

Phosphorus sorption capacity evaluation

The phosphorus sorption capacity of NCSP and the calcined NCSPs were evaluated at low (0.5 and 1 mg/L), medium (30 and 100 mg/L) and high (400 and 800 mg/L) phosphorus concentrations. The results indicate that these sorbents have quite different phosphorus removal capacities at the three levels of phosphorus concentrations (Fig. 3a and b). Phosphorus removal capacity remained nearly the same when NCSP was calcined at temperatures not exceeding 600
C. The removal capacity increased remarkably when calcination temperatures exceeded 600
C, reached a maximum at 900
C, and then decreased after reaching 1000
C. As confirmed by the results (summarized as Figs. 1 and 2 and Table 1), a large amount of CaO and a small amount of akermanite were formed at temperatures not exceeding 900
C, which then resulted in a large amount of exchange of Mg and Ca that were present in NCSP900. This means that NCSP900 has the highest phosphorus sorption capacity among the sorbents used. When the

Table 1 e Physical and chemical properties of the natural calcium-rich sepiolite (NCSP) and the calcined ones. Adsorbents

BET surface area (m2/g)

pHPZC

Exchange Ca (mg/g)

Exchange Mg (mg/g)

231.1 38.8 26.5 13.3 12.4 12.9 7.15

6.5 6.9 8.2 9.3 10.8 11.7 11.8

142.1 135.9 153.1 164.5 186.1 245.8 211.7

42.1 4.47 16.7 17.3 16.1 17.7 14.1

NCSP NCSP400 NCSP600 NCSP700 NCSP800 NCSP900 NCSP1000 a Measured by XRF.

SiO2a (%)

Caoa (%)

MgOa (%)

Al2O3a (%)

31.0

22.3 e e 29.1 e 33.7 e

21.4

1.49

e e 37.5 e 39.2 e

e e 22.3

e e 1.54

e 25.1

e 1.62

e

e

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b 0.06

1 mg/L 100 mg/L 800 mg/L

Phosphorus removed (mg/g)

30

20

10

0

c

0.5 mg/L 30 mg/L 400 mg/L

0

200

400

600 800 Temperature (ºC)

0.00

Phosphorus removed (mg/g)

0.8 18

0.6 0.4

9 0.2

0

200 400 600 800 Initial phosphorus concentration (mg/L)

0.0

200

400 600 Temperature (ºC)

800

1000

36

1.2

NCSP 1.0 27 0.8 18

0.6 0.4

9 0.2 0

0

200

400

600

800

0.0

Initial phosphorus concentration (mg/L)

f

2 1

0

Percentage phosphorus removal (100%)

27

Percentage phosphorus removal (100%)

1.0

e

d

1.2

NCSP900

0

0.02

1000

36

0.5 mg/L 1 mg/L

0.04

Phosphorus removed (mg/g)

Phosphorus removed (mg/g)

a

NCSP900 R=0.995

NCSP900 R=0.999 NCSP R=0.997

NCSP

R=0.992

1/Q

Log Q

0 -1 -2 -3 -4 -2

-1

0

1

2

3

Log C

-5

0

5

10 1/C

15

20

25

Fig. 3 e Phosphorus sorption by natural calcium-rich sepiolite (NCSP) and the calcined samples at 25
C with an initial pH value of 7. (a) Phosphorus sorption capacity of NCSP and the calcined samples at low (0.5 and 1 mg/L), medium (30 and 100 mg/L) and high (400 and 800 mg/L) phosphorus concentrations. (b) Phosphorus sorption capacity of NCSP and the calcined samples at low (0.5 and 1 mg/L) phosphorus concentration. (c) Phosphorus sorption by NCSP at different initial phosphorus concentrations (d) Phosphorus sorption by NCSP900 at different initial phosphorus concentrations. (e) Phosphorus adsorption isotherm regression using the linear form of the Freundlich equation. (f) Phosphorus adsorption isotherm regression using the linear form of the Langmuir equation.

temperature increased to 1000
C, phosphorus sorption capacity decreased abruptly. This was probably due to the decrease of CaO, which reacted with SiO2 to form akermanite at high temperatures. The decrease in the phosphorus sorption of NCSP1000 also indicated that akermanite has a lower sorption capacity than CaO. We concluded that NCSP900 exhibited excellent phosphorus sorption performance at the three levels of phosphorus concentrations. This was further investigated in the present study.

3.2.2.

Sorption isotherm

The phosphorus sorption isotherms of NCSP and NCSP900 were evaluated at concentrations between 0.05 mg P/L and 800 mg P/L at 25
C with an initial pH value of 7. The results show that the amount of phosphorus removed from the NCSP900 and NCSP increased linearly with increased initial phosphorus concentration, which ranged from 0.0020 to

33.9 mg P/g, and 0.0018e9.04 mg P/g, respectively (Fig. 3c). The amounts of phosphorus removed by the two adsorbents varied in two different ways. The percentage removed by NCSP900 increased from 80.0 to 98.0% at low concentrations (<0.5 mg P/L), then remained constant, 98.8e99.9% from 1 to 600 mg P/L, then decreased to 84.7% at 800 mg P/L (Fig. 3c). However, the percentage of phosphorus removed by NCSP decreased in an almost-linear fashion with increased initial phosphorus concentrations. The removal efficiency of NCSP ranged from 56.5% to 83.5% at low phosphorus concentrations (<0.5 mg P/L), decreased to 29.0% at 50 mg P/L, then leveled off at 16.7e25.1% at concentrations of 100e800 mg P/L (Fig. 3d). Compared to NCSP, the phosphorus removal efficiency of NCSP900 had increased 106%e596%. As indicated above, this was mainly due to the improvement of pHPZC as well as the transformation of the inter calcium of NCSP to active free CaO during calcination.

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The Langmuir and Freundlich isotherm equations are widely used to describe the sorption processes in sorption experiments. The linear form of the isotherm equations can be written as follows (Langmuir isotherm equation, Eq. (1); and Freundlich isotherm equation, Eq. (2)):

1.2

1mg/L 0.8

NCSP NCSP900

(1)

1 lnqe ¼ lnKF þ lnCe n

(2)

where qe is the phosphate concentration on the sorbent (mg/ g); Ce is the equilibrium phosphate concentration in the solution (mg/L); KL, KF and n are constant parameters of the models; and qm is the maximum sorption capacity of the sorbent at given concentrations (mg/g). The linear fitting curves of NCSP and NCSP900 are presented in Fig. 3e and f. The correlation coefficients of the two samples indicated that the experimental data agreed well with the Freundlich model. Similarly, other calcium-rich adsorbents such as hydrated calcarious oil-shale ash (Kaasik et al., 2008), acid mine drainage sludge (Wei et al., 2008;Sibrell et al., 2009) and calcium-rich palygorskite (Gan et al., 2009) were also found to have a better fit with the Freundlich model than with the Langmuir model, during phosphorus sorption experiments. This was probably due to Ca-P precipitation being the mode of phosphorus removal by calcium-rich materials, therefore resulting in an unsuitable fit to the Langmuir model (Del Bubba et al., 2003;Yin et al., 2011a).

3.2.3.

Sorption kinetics

The reduction, by NCSP and NCSP900, of phosphorus concentrations in the solution as a function of time is shown in Fig. 4. The results indicate that, within 5 min, 92.9%, 99.6% and 99.8% of the phosphorus was removed by NCSP900 from low (1 mg/L), medium (50 mg/L) and high (200 mg/L) phosphorus concentrations. After 1 h adsorption, phosphorus concentrations in the low and medium levels were reduced to 0.013 mg/ L and 0.052 mg/L, respectively, then remained almost constant over the 28 h of contact time. In contrast, the phosphorus concentration in the high level stabilized at 0.076e0.097 mg/L after 4 h of contact time in the experiment. Compared to NCSP900, the time required (w12 h) for NCSP to reach equilibrium at low, medium and high phosphorus concentrations was quite long. The distinct sorption rate among the two sorbents was probably due to the calcium in NCSP900 being more aggressive than that in NCSP. This has also been identified in Figs. 1 and 2 and Table 1. In comparison, phosphorus sorption rate and equilibrium time of NSCP900 is much faster and shorter than that of some calcium-rich sorbents that normally require days to reach equilibrium and have a relatively low removal efficiency (<80%) within a short contact time (<30 min) (Søvik and Kløve, 2005; Kaasik et al., 2008). In contrast, the equilibrium time of NCSP900 was comparable to that of some Fe-based sorbents, which take several hours to reach sorption equilibrium (Rentz et al., 2009). Rapid and efficient removal of phosphorus from the three phosphorus concentration levels suggests that NCSP900 can be considered a suitable adsorbent in eutrophic waters and wastewater treatment facilities. It has been widely accepted that fast

Phosphorus concentrations (mg/L)

0.08

Ce Ce 1 ¼ þ qe qm KL qm

0.04 0.00

50mg/L

40 20

NCSP NCSP900

0.08 0.04 0.00 200

200mg/L

150 0.4

NCSP NCSP900

0.2 0.0 0

200

400

600

800 1000 1200 1400 1600

Time (minutes)

Fig. 4 e Sorption kinetics of phosphorus on NCSP and NCSP900 at low (1 mg/L), medium (50 mg/L) and high (200 mg/L) phosphorus concentrations at 25
C, with an initial pH value of 7.

sorption kinetics plays an important role in the efficiency and field-deployment costs of a sorbent (Chouyyok et al., 2010).

3.2.4.

Matrix effects on P removal capacity

In the present study we evaluated matrix effects, including pH, coexisting anions and humic acid on P removal capacity of NCSP and NCSP900. The phosphorus removal capacities of NCSP and NCSP900 at various pH values (3e10) were evaluated in low (1 mg/L), medium (50 mg/L) and high (200 mg/L) phosphorus concentrations. The results showed that pH values had no influence on the phosphorus removal capacity of NCSP900 at the three levels of phosphorus concentration (Fig. 5a). We found that there was little influence on the phosphorus removal capacity of NCSP at low phosphorus concentrations (Fig. 5b). However, NCSP was found to be considerably influenced at medium and high phosphorus concentrations (Fig. 5b). The phosphorus removed by NCSP was higher in acid solutions (with pH values of 3e6) than in alkaline conditions. These results are in agreement with previous research in which steel slag (Bowden et al., 2009), palygorskite (Gan et al., 2009), modified diatomite (Xiong and Peng, 2008), acid mine drainage sludge (Wei et al., 2008) and other materials were used as sorbents. The pH dependency of phosphate removal is related to the dissolution of cations from the adsorbent, overall charge of the adsorbent, and the polyprotic nature of phosphate (Chouyyok et al., 2010; Gan

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12

NCSP900

b

1mg/L 50mg/L 200mg/L

3

0.05

6

8

2 0.04 0.02 0.00

10

2

4

Initial pH

0.2 M 0.4M 0.6 M

50

0

P+HCO

P+Cl

P+NO

0 P+F

10

f NCSP900

100 80 60 40 20 0

0

5

20 10 15 Humic acid concentration (mg/L)

Percentage phosphorus removed (%)

e

NCSP

P+SO

50

P+SO

8

100

Control

0.2 M 0.4 M 0.6 M

d Percentage phosphorus removed (%)

NCSP900

100

Control

Percentage phosphorus removed (%)

c

Percentage phosphorus removed (%)

6

Initial pH

P+HCO

4

1mg/L 50mg/L 200mg/L

4

P+Cl

2

6

P+NO

6

Phosphorus removed (mg/g)

Phosphorus removed (mg/g)

NCSP

8

9

0.00

10

P+F

a

NCSP

100 80 60 40 20 0

30

0

5 20 10 15 Humic acid concentration (mg/L)

30

Fig. 5 e Matrix effect on phosphorus removal capacity of NCSP and NCSP900. (a) Effect of pH on phosphorus removal capacity of NCSP900 at low (1 mg/L), medium (50 mg/L) and high (200 mg/L) phosphorus concentrations. (b) Effect of pH on phosphorus removal capacity of NCSP at low (1 mg/L), medium (50 mg/L) and high (200 mg/L) phosphorus concentrations. (c) Effect of coexisting anions on phosphorus removal capacity of NCSP900. (d) Effect of coexisting anions on phosphorus removal capacity of NCSP. (e) Effect of humic acid on phosphorus removal capacity of NCSP900. (f) Effect of humic acid on phosphorus removal capacity of NCSP.

et al., 2009; Yin et al., 2011a). Higher pH values (7) can decrease the amount of Mg2þ, Ca2þ and Al3þ leaching from NCSP and cause the surface to carry more negative charges, thereby resulting in increased repulsion of the negativelycharged phosphate, weakening the affinity toward phosphate in the solution (Yin et al., 2011a). The pH-independent characteristic of NCSP900 on phosphorus removal capacity was only found in a few previous studies and was associated with calcium-rich materials. This excellent characteristic of NCSP900 was probably due to a significant improvement to pHPZC, relative to NCSP, resulting from the formation of large alkaline calcareous materials at higher calcined temperatures.

Since the surface charge is known to be negative when the solution pH is above pHPZC, the contrary can also be expected. This suggests that NCSP900 was always positively charged within the pH values tested, so could therefore adsorb phosphorus, irrespective of the influence of pH values. Besides, the more aggressive CaO in NCSP900 would release large amount of Ca2þ, thus capturing phosphate to rapidly precipitate calcium in the solutions. The results indicated that NCSP900 did not only function in acidic conditions during wastewater treatment, but could also function in alkaline eutrophic waters, which is particularly useful for in-situ eutrophication control.

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The effect of coexisting anions and humic acid enhancing phosphorus removal capacity of NCSP and NCSP900 is presented in Fig. 5cef. The results show that the presence of NO3  and Cl had almost no effect on the phosphorus removal capacity of NCSP. Phosphorus sorption capacity of NCSP was not inhibited by low sulfate concentrations (0.2 M) and was largely affected in medium (0.4 M) and high (0.6 M) sulfate concentrations. However, the presence of F, HCO3  and humic acid dramatically affected the phosphorus removal capacity of NCSP, with humic acid having a larger influence. The higher the level of F, HCO3  and humic acid concentrations in solutions, the more inhibited the effect would be. In comparison, NCSP900 exhibited an excellent phosphorus sorption performance and it could selectively adsorb phosphate in the presence of SO4 2 , NO3  , F, Cl and humic acid. However, HCO3  exerted a great influence on the phosphorus sorption capacity of NCSP900. Whereas, it is normally observed in association with high concentrations of NO3  and Cl, but not usually as HCO3  in natural waters or wastewaters (Chouyyok et al., 2010). Thus the phosphorus capacity of NCSP900 is not usually significantly influenced during field application. The highly selective adsorption of phosphorus by NCSP900 was probably due to the style of phosphorus removal. Ko˜iv et al. (2010) have concluded that the homogenous precipitation of Ca-phosphates seems to not be influenced by ionic and organic inhibitors, when using active calcium-rich materials as sorbents during wastewater treatment.

3.3.

Sediment phosphorus immobilization effect

Amending contaminated sediment with active sorbents, which is also referred to as in situ active capping, has proved to 250

control

1:20

1:10

200

1:5

1200

control

1:20

1:10

Labile-P

1:5

Fe-P 800

150

Phosphorus concentration (mg/kg)

be an effective way to inhibit sediment phosphorus release, thereby controlling lake eutrophication (Berg et al., 2004; Xiong and Peng, 2008; Lin et al., 2011). Sorbents with excellent performance in phosphorus-contaminated wastewater treatment plants do not necessarily also function well in terms of binding sediment phosphorus in more natural environments. The immobilization effects of different amounts of NCSP900 on sediment phosphorus are presented in Fig. 6. The results show that NCSP900 can greatly change the composition of sediment phosphorus and the immobilization effect increases with an increased addition of NCSP900. The reactive phosphorus, defined as the fractions of Labile-P, Fe-P and Org-P, all decreased with the addition of NCSP900. The Labile-P fraction, which is readily used by phytoplankton or algae, decreased from 195.3 mg/kg (control) to 113.0 mg/kg (20% addition) after 10 days remediation. The organic-P and Fe-P factions decreased from 1066.2 mg/kg to 793.4 mg/kg and 744.5 mg/kg to 407.9 mg/kg, respectively, after 10 days treatment with a 20% addition. The two forms of phosphorus fractions have the potential for mobilization in sediments and thus have the potential to be released into overlying waters, when environmental conditions change (Rydin and Welch, 1998; Xiong and Peng, 2011). Interestingly, the Al-P fraction also decreases with the addition of NCSP900. Al-P fractions are not redoxsensitive and are consequently relatively stable in lake sediment and are generally regarded as inert phosphorus (Rydin and Welch, 1998; Xiong and Peng, 2011). This probably indicates that the active calcium in NCSP900, which has a sorption advantage compared to reactive aluminum in sediment, could thus transform from Al-P fractions to Ca-P fractions. This is clearly indicated in Fig. 6. It was observed that the Ca-P fractions increased remarkably with the addition of NCSP900

100

400

50 0 900

0

Org-P

600

400

300

200

0 2500

Ca-P

2000

Al-P

600

0 1600

AAP

1200

1500 800 1000 400

500 0

0

1

3

5

7

Time (days)

10

1

3

5

7

Time (days)

Fig. 6 e Immobilization effect of NCSP900 on sediment phosphorus.

10

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50

40

4

30

2

20

10

0

3.4.

Ca2-P

Fe-P

Al-P

Ca10-P

Ca8-P

0 O-P

Phosphorus concentration (mg/g)

P forms Percentage of the TP

Percentage of the total phosphorus (%)

6

mode for controlling the release of sediment phosphorus (Berg et al., 2004; Lin et al., 2011; Xiong and Peng, 2011). A large amount of phosphorus in the sediment was transformed to inert Ca-P fractions, which therefore caused the algal-available phosphorus (AAP) to decrease sharply during NCSP900 treatment (Fig. 6). AAP was generally used to characterize the amount of phosphorus that can be easily used by algae (Zhou et al., 2001; Xiong and Peng, 2011). The results indicate that NCSP900 can be used as an efficient P binding agent in the control of eutrophication in lakes.

Fig. 7 e Phosphate fractionation in discrete chemical forms.

after a 10-day treatment. This study indicated that NCSP900 could not only transform reactive phosphorus, but also the inert Al-P fractions, to more stable Ca-P fractions. The Ca-P fractions are generally recognized as a refractory sedimentary phosphorus fraction because phosphorus is incorporated into the crystal structure of Ca minerals, forming Ca-P minerals such as hydroxyapatite and dicalcium (Xiong and Peng, 2011). The transformation of sediment reactive phosphorus to Ca-P fractions is also considered to be a suitable function

Phosphorus retention mechanism on calcined NCSP

Phosphorus fractionation was performed on P-saturated NCSP900 and the results are presented in Fig. 7. This indicates that calcium-bound phosphorus forms, including Ca2-P, Ca8-P and Ca10-P, accounted for most of the total phosphorus and that Al-P, Fe-P and O-P only accounted for a small part of the total phosphorus. The order of each fraction of P was distributed as follows: Ca10-P (39.3%) > Ca8-P (38.5%) > Ca2-P (12.3%) > Al-P (4.5%) > Fe-P (2.6%) > O-P (2.3%). The relatively high percentage of Ca-P (Ca2-P þ Ca8-P þ Ca10-P ¼ 80.1%) in the P-saturated NCSP900 indicated that phosphorus removal from solution by NCSP900, was primarily through Ca-P precipitation. As suggested by the XRD and FT-IR data (Figs. 1 and 2), the inert calcium (calcite and dolomite) in NCSP could transform to aggressive calcium (calcium oxide) at 900
C

a

b

c

d

Kev Fig. 8 e SEM images and EDS analysis of NCSP and NCSP900. (a) Microstructure of natural calcium-rich (NCSP) before phosphorus adsorption (note the fibrous-like cluster). (b) Microstructure of the 900
C heated NCSP (NCSP900); note the expanded fiber, due to heating. (c) Calcium-phosphate overgrowths on the expanded fiber of NCSP900 after treatment with 400 mg/L phosphorus solution. (d) SEM-EDS analysis spectrum of NCSP900 after phosphorus adsorption.

w a t e r r e s e a r c h 4 7 ( 2 0 1 3 ) 4 2 4 7 e4 2 5 8

calcination. The dissolution of CaO in NCSP900 could quickly release a large quantity of Ca2þ, which then reacts with phosphorus to form various calcium phosphate precipitates. Gan et al. (2009) and Yin et al. (2011a) previously found that the Ca8-P forms accounted for most of the total phosphorus (>50%) during phosphorus sorption experiments when thermal treated palygorskite and natural calcium-rich sepiolite were used as sorbents. Other researchers also found various kinds of Ca-P precipitationsdsuch as amorphous calcium phosphate (ACP), dicalcium phosphate dihydrate (DCPD), octacalcium phosphate (OCP),hydroxyapatite (HAP) and othersdwere formed on calcium-rich sorbents, such as basic oxygen steel slag (Bowden et al., 2009), hydrated oil shale ash (Kaasik et al., 2008), Polonite (Renman and Renman, 2010) and steel slag (Barca et al., 2012), after phosphorus sorption experiments. This indicated that the phosphorus removal mechanism by calcium-rich materials was mainly through the precipitation of Ca-P complexes, but this varied for different forms. In comparison, the Ca-P forms, especially the percentage of Ca10-P form (HAP), was higher, accounting for the total phosphorus content as noted by Gan et al. (2009) and Yin et al. (2011a) in previous findings obtained from phosphorus experiments. As stated previously, HAP is the most stable phosphorus form among the forms of Ca-P precipitation, and phosphorus is difficult to desorb once formed (Kaasik et al., 2008). It should be noted that it generally takes a long time to recrystallize poorly crystallized or amorphous DCPD and OCP into thermodynamically stable HAP (Kaasik et al., 2008). This therefore indicates that the use of NCSP900 is advantageous in terms of environmental safety, compared to other calcium-rich materials, since a more stable Ca-P (HAP) precipitate can be formed during phosphorus contaminated water management. The formation of discrete Ca-P precipitates in NCSP900 can be vividly observed by SEM-EDS. Fig. 8a indicates that the NCSP mainly resembles a fibrouslike cluster prior to phosphorus adsorption. However, it becomes more compact and forms an intumescent fibrous-like cluster when heated at 900
C (Fig. 8b) and the calciumphosphate aggregates overgrow on the surface of NCSP900 after phosphorus adsorption (Fig. 8c and d).

4.

Conclusions

The results of this study suggest that phosphorus removal efficiency and the capacity of natural calcium-rich sepiolite (NCSP) can be greatly enhanced through calcination. Prescreening studies showed that the 900
C heated natural calcium-rich sepiolite (NCSP900) had excellent phosphorus removal efficiency in comparison to other thermally-activated products. Further study suggested that the value of pH, coexisting anions (except HCO3  ) and humic acid had no influence on phosphorus removal efficiency. Moreover, more than 90% of the phosphorus could be removed from solution within 5 min by NCSP900. In addition, it could effectively transform active phosphorus forms (liable-P, Fe-P and Org-P) as well as Al-P into a stable Ca-P form, thereby sharply decreasing the amount of algal bioavailable phosphorus (AAP) in the sediment. The excellent phosphorus binding performance of NCSP900 was mainly due to the improvement of

4257

pHPZC as well as the transformation of the inert-calcium of NCSP to active free CaO during calcination. Sequential extraction indicated that phosphorus removal by NCSP900 was mainly through Ca-P precipitation, which was further confirmed by the SEM-EDS study. This study showed that NCSP900 could be used as an efficient binding agent to sequestrate phosphorus from wastewaters and sediment. It is recommended that further field or pilot experiments should be carried out, so as to assess more practical results.

Acknowledgments This work was jointly supported by the State major project of water pollution control and management (Grant No. 2012ZX 07103-005), Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences (Grant No. NIGLAS2010KXJ01 and NIGLAS2010QD11).

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