Phosphorus removal from wastewater using mussel shell: Investigation on retention mechanisms

Ecological Engineering 97 (2016) 558–566

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Research paper

Phosphorus removal from wastewater using mussel shell: Investigation on retention mechanisms ˜ a R. Paradelo a,∗ , M. Conde-Cid a , L. Cutillas-Barreiro a , M. Arias-Estévez a , J.C. Nóvoa-Munoz b b b ˜ , E. Álvarez-Rodríguez , M.J. Fernández-Sanjurjo , A. Núnez-Delgado a Universidade de Vigo, Departamento de Bioloxía Vexetal e Ciencia do Solo, Área de Edafoloxía e Química Agrícola, Facultade de Ciencias de Ourense, As Lagoas s/n, 32004 Ourense, Spain b Universidade de Santiago de Compostela, Departamento de Edafoloxía e Química Agrícola, Escola Politécnica Superior, Campus Univ. s/n, 27002 Lugo, Spain

a r t i c l e

i n f o

Article history: Received 21 October 2015 Received in revised form 6 September 2016 Accepted 11 October 2016 Available online 18 October 2016 Keywords: Adsorption Mussel shell Phosphate Precipitation Wastewater

a b s t r a c t Mussel shell is a carbonate-rich by-product that could be recycled in wastewater treatment. In this work, phosphorus removal from aqueous solutions was obtained in a series of batch and column experiments in the laboratory, using a calcined and a finely-ground (non-calcined) mussel shell. Phosphorus removal followed a Freundlich model at high contact times (72 h) and a Langmuir model at lower time (24 h). Phosphorus removal capacity increased with contact time and with P concentration in the solution, while desorption of the retained P was very low (<4%). Calcined mussel shell presented a higher retention capacity than the fine shell, which can be attributed to differences in mineralogy and composition. The process of P removal from aqueous solution showed features that are typical of chemical reactions rather than denoting adsorption; concretely, the percentage of P removed increased with initial P concentration in the solution, thus pointing at a relevant role of precipitation in P removal. The results corresponding to the fractionation of the P retained in the mussel shell after the experiments showed that both mechanisms, adsorption and precipitation, contributed to P removal. © 2016 Elsevier B.V. All rights reserved.

1. Introduction The well-known role played by phosphorus on the eutrophication of surface waters has led to the general implementation of P control measures in municipal wastewater treatment plants. The most common technologies for P removal from wastewater are based on phosphate precipitation and include reaction with FeCl3 , FeSO4 , Al2 (SO4 )3 or lime. Research on the use of low-cost adsorbents for the treatment of P in wastewaters has been gaining attention recently, using materials capable of adsorbing/precipitating phosphates due to their composition. Among these materials, some studies have focused on Fe-rich residuals, such as fly ash, steel slag or bauxite (Huang et al., 2008; Xiong et al., 2008; Callery et al., 2015), and residual carbonates such as crushed rocks and mollusc shells (Baker et al., 1998; Kwon et al., 2004; Mateus et al., 2012). Treatment of wastewater with lime or residual carbonates is commonly used as a low-cost method for P removal,

∗ Corresponding author. E-mail address: [email protected] (R. Paradelo). http://dx.doi.org/10.1016/j.ecoleng.2016.10.066 0925-8574/© 2016 Elsevier B.V. All rights reserved.

based on the reaction of dissolved Ca with phosphate, which leads to the precipitation of hydroxyapatite [Ca10 (PO4 )6 (OH)2 ]. Mussel shells are an abundant residual source of carbonate in regions were shellfish aquaculture and canning industries are well developed, in particular China, Chile or Spain (Barros et al., 2009). In the last decades some factories have implemented industrial processes to transform shells into products rich in carbonates and oxides, achieving appropriate particle size and eliminating residual organic matter, undesirable odors and microorganisms. These factories wash, crush and sieve the shells, and sometimes also perform calcination. Traditionally, mussel shell has been recycled as an amendment to increase the pH of acid soils by farmers in coastal areas, where it has shown to be of agronomic value (Álvarez et al., 2012). Also, shell use for chicken feed has been practiced for decades (Guinotte et al., 1991). Besides, new potential applications are being developed, including use as a low-value material for concrete production (Ballester et al., 2007) or for the treatment of ˜ et al., 2013; Seco-Reigosa polluted soil and waters (Pena-Rodríguez et al., 2014, 2015). Since they are mainly composed of calcium carbonate, the use of mollusc shells for phosphate removal has been recently postulated and investigated by several researchers (Kwon et al., 2004;

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Currie et al., 2007; Lee et al., 2009; Abeynaike et al., 2011; Oladoja et al., 2012). Studies in this direction have produced promising results, and high percentages of P removal from water solutions (often above 90%) have been achieved (Kwon et al., 2004; Lee et al., 2005; Currie et al., 2007). However, the mechanism by which P is retained on mussel shells is still a matter of discussion. In the case of Fe-based materials, Robertson and Lombardo (2011) noted that although P removal is normally described as a monolayer surface complexation reaction (adsorption), a number of studies revealed secondary P removal effects that do not conform to conventional adsorption models. This secondary removal has been attributed to direct precipitation of P minerals (IsenbeckSchroter et al., 1993; Li and Stanforth, 2000). The distinction between adsorption- and precipitation-based P removal is important for designing wastewater treatment systems. For adsorption processes, media replenishment or maintenance is required when adsorption sites are filled. On the other hand, for precipitation reactions, if the associated cation is available the potential exists for more long-lived treatment systems (Robertson and Lombardo, 2011). In the case of mussel shell, P removal by means of precipitation would occur until Ca is exhausted, this is, potentially until the whole mussel shell is dissolved. In this work, we have performed several batch and column experiments using two kinds of mussel shell with different properties, with the objective of studying their potential use as a P removing material, as well as aiming to investigate the mechanisms by which P is removed from aqueous solution when using this material. 2. Material and methods 2.1. Materials Two samples of mussel (Mytilus galloprovincialis Lamarck) shell were used in this study: a finely-ground non-calcined mussel shell (size <2 mm), obtained from Abonomar S.L. (Illa de Arousa, Pontevedra, Spain), and a calcined shell (size <2 mm), supplied by the ˜ Spain). Calcination takes factory Calizamar S.L. (Boiro, A Coruna, place in a rotary kiln at 550 ◦ C for 15 min. Heating the shell above 500 ◦ C, organic matter is burnt out; it also promotes that the crystal structure of calcium carbonate changes from the aragonite polymorph, with an orthorhombic crystal structure, to the calcite polymorph, with a trigonal-rhombohedral structure; and, finally, this heating allows transformation of carbonates to oxides (Currie et al., 2007). A pH-meter (model 2001, Crison, Spain) was used to measure pH in water (10 g of solid sample, with solid:liquid ratio 1:2.5). Total carbon (C), nitrogen (N) and sulphur (S) were measured using an elemental Tru Spec CHNS auto-analyzer (LECO, USA). Total concentrations of Na, K, Ca and Mg were determined using ICP-mass (820-NS, Varian, USA), after nitric acid (65%) microwave assisted digestion. Available P was extracted in 0.5N NaHCO3 as per Olsen and Sommers (1982) and determined by means of UV–vis spectroscopy (UV-1201, Shimadzu, Japan). The main properties of both materials are shown in Table 1. 2.2. Batch experiments For the adsorption studies, 10 mL of 0.01 M NaNO3 solutions containing known concentrations of P (0.03, 0.08, 0.16, 0.32, 0.65, 1.6, 3.2, 4.8, 6.5, 8.0, 9.7, 13 and 16 mmol P L−1 ) were added to 200 mg sample of each mussel shell type in 15-mL polyethylene tubes. Sodium nitrate was used as a background electrolyte in order to provide a constant ionic strength for the experiments. The samples were shaken for 24 or 72 h in an end-over-end shaker at 48 rpm, then centrifuged at 4000 rpm (equivalent to 2665 × g) for 10 min,

559

Table 1 Main properties of the mussel shells employed in the study.

pH Ctotal (g kg−1 ) Stotal (g kg−1 ) Ntotal (g kg−1 ) Catotal (g kg−1 ) Natotal (g kg−1 ) Ktotal (mg kg−1 ) Mgtotal (mg kg−1 ) Polsen (mg kg−1 )

Finely-ground shell

Calcined shell

9.4 114 3.4 2.1 280 5.2 202 981 54

9.2 128 2.1 2.6 399 5.3 503 1968 62

and the supernatant filtered through 0.45-␮m acid-washed filter paper. Two different contact times were chosen because phosphate sorption is known to proceed in two steps: a fast one that is complete in some hours, and a second one, slower, that may take several days to complete (Bohn et al., 2001). The amount of P adsorbed (X) was calculated as the difference between the amount added and the equilibrium concentration in the solution (C). Freundlich (Eq. (1)) and Langmuir (Eq. (2)) models were used to describe the adsorption behavior of P. These equations are expressed as follows: 1

X = KF C n X=

(1)

KL Xm C 1 + KL C

(2)

where X is the amount of adsorbed P (mmol kg−1 ), C is the concentration of P in solution at equilibrium (mmol L−1 ), KL (L mmol−1 ) is a constant related to the energy of adsorption, Xm (mmol kg−1 ) is the maximum adsorption capacity of the sample, and KF (Ln kg−1 mmoln−1 ) and n (dimensionless) are the Freundlich coefficients. For the study of desorption, the centrifuged residues from the adsorption test were weighed to determine the amount of occluded solution and re-suspended in 10 mL of a 0.01 M NaNO3 solution; the suspensions were shaken for 72 h, centrifuged and filtered as described above. This study was carried out only for the highest initial P concentrations (4.8, 6.5, 8.0, 9.7, 13 and 16 mmol P L−1 ). Phosphorus desorption was expressed as percentage of previously adsorbed P. For this calculation, the amount of occluded P (estimated as the difference between the final and initial weights) was taken into account. For the study of retention kinetics, 10 mL of 0.01 M NaNO3 solutions containing 0.16 or 1.6 mmol P L−1 were added to 200 mg samples of each mussel shell type in 15-mL polyethylene tubes. The samples were shaken for 1, 6, 12, 24, 48 or 72 h in an end-overend shaker at 48 rpm, then centrifuged and filtered as described above. The amount of P retained in the solid was calculated as above, and two kinetic models were tested to describe the data, a pseudo first-order model (Eq. (3)), and a pseudo second-order model (Eq. (4)): dq/dt = k1 ·(qe − q)

(3)

dq/dt = k2 ·(qe − q)2 (mmol kg−1

(4) min−1 )

is the P adsorption rate, q where dq/dt (mmol kg−1 ) is the amount of P in the solid phase for a given time t (min−1 ), qe (mmol kg−1 ) is the maximum capacity for P retention under the experimental conditions, k1 is the pseudo first-order reaction rate (min−1 ), and k2 is the pseudo second-order reaction rate (kg mmol−1 min−1 ). Phosphorus concentrations were measured in all the extracts by colorimetric determination of the phosphomolybdic complex (Olsen and Sommers, 1982). All the experiments were carried out in triplicate.

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The models were fitted to the experimental data using the Solver package for Microsoft Excel 2011 for MacOsX.

A relative retardation factor (R) was calculated for each metal (M) with respect to the inert tracer (Br): R = M /Br

2.3. Transport experiments Laboratory column experiments were conducted to analyze the transport of P through both kinds of mussel shell. Solutions of NaH2 PO4 containing total concentrations of 0.5 or 1.5 mmol P L−1 were used for the experiments, with 0.01 M NaNO3 as background electrolyte. The experiments were conducted at 20 ± 1 ◦ C. Vertically oriented columns were constructed in glass tubes, 80-mm long, with 10 mm inner diameter. Mussel shell was added incrementally with depths of 1 cm, followed by gentle tapping of the column to consolidate it. The columns were weighed at this point to determine the mass of shell employed. At the input, the columns were connected to a peristaltic pump (Gilson Minipuls 3, USA) through 1/16-in. PVC tubing. The pump was connected through a three-way valve to the bottles containing deionized water or P solutions. At the output, columns were connected to a fraction collector (Gilson FC 203 B, USA) through 1/16-in. PVC tubing. Experimental conditions for all columns are summarized in Table 2. Once packed, columns were slowly saturated from below with deionized water during a 3-day period, with water entering the columns at 5 mL h−1 as incoming flow. After this period, the valve was switched and the corresponding P solutions were introduced in the columns, with a 5 mL h−1 flow rate maintained for 24 h. Then, the valve was switched and 0.01 M NaNO3 solution was introduced in the columns, for a 48-h period. At this point, the column was weighed again to determine the pore volume. The outflow effluents were sampled every 60 min, both during the incoming P–solution phase and during the incoming NaNO3 solution. pH, Ca and P concentrations were measured in each outflow sample, using a glass electrode for pH (Crison pH-Meter Basic 20, Spain), flame atomic absorption spectroscopy (Thermo Solaar M series spectrometer, USA) for Ca, and the phosphomolibdic complex method for P (Olsen and Sommers, 1982). A parallel experiment was carried out to determine the retardation factor of the solution within the column. With this aim, a 10 mg L−1 solution of KBr was used as a solute tracer. The bromide (Br− ) in the effluent samples was measured in a segmented flow analyser (Bran Luebbe Auto Analyzer 3, Germany). The resulting breakthrough curves (relative P concentration in the effluent vs. elapsed time) were plotted. Relative P concentration (C/C0 ) is calculated by dividing the concentration in the effluent (C) by the concentration in the input solution (C0 ). Curves were further studied using temporal moment analysis as described by Valocchi (1985), Stagnitti et al. (2000) and Kamra et al. (2001). Several parameters used for the description of the breakthrough curves have been calculated using this method. The first normalised moment (␮1 ) is the mean concentration breakthrough time (), and it was calculated as n

 = ␮1 = M1 /M0 where M0 and M1 represent the zeroth and first moments.

The second central moment, 2 , quantifies the variance of the BTC, a measure of the typical spread of the BTC in relation to the mean breakthrough time. The standard deviation, , is given by the square root of the second central moment: √  = 2 Central moments, p , are defined by 1 p = M0

∞





n p

t − 1

C (z, t) /C0 dt

0

where the subscript p = 0; 1; 2; 3. . . represents the zeroth, first, second, third moment, etc.; C0 is the initial solution concentration at time t = 0, and z is the location. The dispersivity, , indicating dispersion of a given element within the column (Schoen et al., 1999; Stagnitti et al., 2000), is calculated as:



 

 = L/2 · 2 / 2



where L is the column length. The third central moment (␮3 ) characterizes the asymmetry of the breakthrough curve and can be used to calculate a nondimensional skewness parameter, S, defined by 3/2

S = 3 /2

The Ca and P concentrations in the effluents, as well as their pH, were used to calculate mineral saturation indices with the assistance of the chemical speciation model Visual MINTEQ 3.0 (Gustafsson, 2010). 2.4. Forms of P retention To study the distribution and forms of P retention, several extractions were carried out on the fine and calcined mussel shell before and after the transport experiments. For this, mussel shell was removed from the columns after the experiment, cut in 1-cmwide slices and dried at 105 ◦ C until a constant weight was reached. Each slice, plus samples corresponding to the initial fine and calcined mussel shell (this is, not submitted to P retention), were analyzed by a sequential extraction scheme, using the protocol that follows: Step 1. One gram of sample was weighed by duplicate in 100mL centrifuge tubes, and shaken for 30 min with 50 mL of 0.5 M NaHCO3 solution at pH 8.5. The suspensions were centrifuged for 10 min at 4000 rpm, and the supernatant decanted into a 100-mL volumetric flask. The solid residue was washed twice with 25-mL portions of saturated NaCl and centrifuged; next, all the supernatants were combined into the same 100-mL volumetric flask. Step 2. 50 mL of 0.25 M H2 SO4 solution were added to the solid residue from the first step and shaken for 1 h. The centrifugation,

Table 2 Working conditions of the mussel shell columns. C0 is the P concentration in the solution; T0 is the P pulse duration expressed as number of pore volumes eluted; Tf is the total number of pore volumes eluted. C0 (mmol P L−1 )

Mass (g)

Bulk density (g cm−3 )

Pore volume (cm3 )

T0

Tf

Finely-ground shell

0.5 1.5

21.58 20.75

1.36 1.47

8.7 6.6

14 18

41 55

Calcined shell

0.5 1.5

21.87 20.24

1.55 1.43

5.2 5.7

23 21

69 64

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Fig. 1. Phosphorus retention curves of fine and calcined mussel shell in batch experiments, and fit to Freundlich and Langmuir models. X: amount of P retained by the solid phase; Ceq : P concentration in solution at equilibrium.

Table 3 Parameters of the Freundlich and Langmuir equations (Eqs. (1) and (2)) fitted to the P retention data for the mussel shells. KF , n: coefficients of the Freundlich equation; KL , Xm : coefficients of the Langmuir equation; R2 : correlation coefficient between experimental and modelled data. t = 24 h

KF (Ln kg−1 mmoln−1 )

n

R2

KL (L mmol−1 )

Xm (mmol kg−1 )

R2

Finely-ground Calcined

128 282

1.3 0.7

0.82 0.71

0.9 0.2

588 1250

0.922 0.655

t = 72 h Finely-ground Calcined

850 891

1.7 2.2

0.82 0.94

– –

– –

– –

decantation, and washing steps were repeated as for the first step, and the supernatants combined in one. Step 3. 50 mL of 0.5 M HCl solution were added to the residue of the previous step and shaken for 1 h. The centrifugation, decantation, and washing steps were repeated as for the first step, and the supernatants combined in one. Step 4. 5 mL of 6 N HCl solution were added to the solid residue of the previous step and shaken for 1 h. No centrifugation was needed in this step since no solid residue remained after the extraction. Phosphorus was determined in all the extracts by the phosphomolibdic complex method. Blanks were also treated using the same procedure. Mass recovery for P (solid material into columns plus leachates) ranged between 98 and 112%. The extraction with 0.5 M NaHCO3 is used to analyze adsorbed phosphates, as the conditions of this extraction do not produce solubilisation of calcium phosphates. These were quantified after extractions of increasing strength, with 0.25 M H2 SO4 and 0.5 M HCl, that differ in their capacity to dissolve calcium carbonate: 0.25 M H2 SO4 only dissolved 10–15% of the mussel shell in a previous test, whereas 0.5 M HCl was able to dissolve more than 90% of the shell (data not shown). The attack with 6 M HCl allowed to obtain total P in the shell, which was in all cases completely dissolved.

2.5. Statistics Linear regression analysis and ANOVA were performed using the R statistical package for MacOSX (R Development Core Team, 2011). 3. Results 3.1. Batch experiments Fig. 1 shows P retention data as a function of P concentration in the solution for both kinds of mussel shell (fine and calcined), for contact times of 24 and 72 h. Retention curves and model parameters (Table 3) were similar for both materials for a contact time of 72 h, whereas at a lower contact time, calcined shell presented the highest P retention. The Langmuir model described adequately the results obtained for finely-ground and calcined shell at 24 h, while the Freundlich model described better than Langmuir the 72 h data (Table 3). These differences are likely due to a kinetic control of phosphate sorption. At the lower contact time, increasing the concentration of phosphate in the solution beyond a certain limit does not increase the amount sorbed due to this kinetic limitation. The apparent maximum of sorption in the curves fits well to

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Fig. 2. Kinetics of P retention onto fine and calcined mussel shell in batch experiments. X: amount of P retained by the solid phase; C0 : P concentration added.

Fig. 3. Breakthrough curves for P solutions in mussel shell columns. C0 is the P concentration in the incoming solution. C/C0 : relative P concentration in the effluent. Table 4 Phosphorus desorption from the mussel shells in the batch experiments. C0 is the P concentration in the solution. C0

P desorbed (%) −1

mmol P L

Fine

Calcined

4.8 6.5 8.1 9.7 12.9 16.1

0 0 0 0 1.0 3.6

0 0 0 0 0 0

the Langmuir equation, that assumes a limited number of sorption sites in a surface. At a longer contact time, in turn, this kinetic limitation would not happen and sorption would continue to increase progressively with increasing P concentration in the solution. The resulting curve is better described then by the Freundlich equation. The amounts of P desorbed from both kinds of mussel shell, expressed as a percentage of the previously-adsorbed P (Table 4), were very low (<4% in all cases). For the calcined shell, no desorption was observed at the P concentrations tested, whereas for

the fine non-calcined shell it was only observed at the highest P concentrations added. Phosphorus retention increased with contact time, as shown in the kinetic experiments (Fig. 2). The absolute amounts and percentages of P retained increased when the highest P initial concentration was used. This is typical of chemical processes. The pseudo-second order model described very well the kinetic data (Table 5), with k2 values around 10−3 –10−4 kg mmol−1 min−1 .

3.2. Transport experiments The breakthrough curves for P in the transport experiments are shown in Fig. 3. Both kinds of mussel shell tested showed a high P retention capacity under the conditions of the transport experiment, as shown by the low C/C0 values, which were always lower than 1. For the two initial P concentrations considered (0.5 and 1.5 mmol kg−1 ), calcined shell showed a higher affinity for P than the finely-ground shell, as shown by the lower C/C0 values. A delay for P breakthrough with respect to the solution (shown by the inert tracer, Br− ) was observed in all cases, what indicates a strong interaction between P and the shell samples into the columns. The

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Table 5 Fitting of kinetic models to experimental data for both kind of mussel shells (fine non-calcined, and calcined). k1 : pseudo first-order kinetic rate; k2 : pseudo second-order kinetic rate; C0 is the P concentration added; R2 : correlation coefficient between experimental and modelled data. C0 (mmol P L−1 )

k1 (min−1 ) −03

R2

k2 (kg mmol−1 min−1 )

R2

−03

Finely-ground

0.16 1.6

1.1·10 8.0·10−05

0.873 0.349

1.3·10 1.5·10−03

0.998 0.9998

Calcined

0.16 1.6

1.3·10−03 4.0·10−04

0.840 0.720

3.6·10−04 1.2·10−04

0.972 0.987

Table 6 Transport parameters derived from the breakthrough curves. Also shown, P sorption capacity of both kinds of mussel shell, as well as total amount of Ca released in the column experiments. C0 is the P concentration in the incoming solution; ␶ is mean concentration breakthrough time; R is retardation with respect to Br− ; ␴ is standard deviation; ␭ is dispersivity; S is skewness. C0 (mmol P L−1 )

P removed (% total)

P removed (mmol P kg−1 )

Ca released (mmol)

␶ (h)

R

␭

␴

S

Finely-ground

0.5 1.5

60 78

1.8 7.3

0.165 0.54

28.1 29.7

1.9 2.0

0.36 0.41

9.8 11.0

−0.03 −0.04

Calcined

0.5 1.5

77 89

2.2 8.5

0.07 0.57

36.4 37.2

2.5 2.6

0.24 0.16

10.3 8.6

−0.05 −0.04

at 1.5 mM (P < 0.001). Overall, the results of the fractionation of the solids into the columns show that P is retained by both adsorption and precipitation mechanisms, since bicarbonate extracts adsorbed P (as it is not able to dissolve calcium phosphate), whereas diluted HCl and H2 SO4 extract precipitated P.

4. Discussion

Fig. 4. Overall content of P in the mussel shell columns, before (C0 = 0) and after (C0 = 0.5, 1.5) the transport experiments. C0 is the P concentration in the incoming solution.

delay was higher in the experiments with the highest P concentration, which is also shown by the values of R and ␶ (Table 6). The shapes of the curves were similar in all cases, also shown by the comparable values for the  and S parameters. The amounts and percentages of P removed were higher for calcined shell than for fine shell, and they increased with P concentration of the input solution (Table 6), showing that the interaction was stronger at higher P concentrations. In agreement with the information obtained from the breakthrough curves, mussel shell total P contents significantly increased with P input (Fig. 4). In comparison to P fractionation in the mussel shell before the transport experiments, bicarbonate-extractable P was the fraction that increased most with P input (being statistically significant, P < 0.001 for the two types of shell), followed by HCl-extractable P (P < 0.001 for the two types of shell). A fourth extraction step was performed with 6 N HCl, but no P was detected in the extracts, so the results are not shown in the figure. The study of P contents within the columns (Fig. 5) showed overall homogeneous distributions of the element, except in some cases for the first two centimetres, where P was concentrated (corresponding to the entry of the P solution). A significant effect of position within the column on P content was observed for calcined mussel shell at both P initial concentrations (P < 0.001), and for the fine shell only

Mussel shell, mainly composed of calcium carbonate, is a potentially suitable material for wastewater treatment, since precipitation with carbonate is commonly employed for P removal from wastewaters (Tchobanoglous et al., 2002). The batch and column retention experiments performed in this work confirmed this potentiality, in agreement with previous results obtained by other authors working with several mollusc shells (Kwon et al., 2004; Lee et al., 2005, 2009; Namasivayam et al., 2005; Currie et al., 2007; Abeynaike et al., 2011). Batch experiments showed that P removal had overall better fitting to a Freundlich adsorption model than to Langmuir’s, what has been previously observed for Hg and As ˜ et al., 2013; Secoadsorption on the same material (Pena-Rodríguez Reigosa et al., 2014). Retention followed a pseudo second-order kinetics, what points to an important role of chemisorption in the process (Ho and Mckay, 1999). Our results show that P retention on mussel shell is a quick process at high P concentrations in the solution, in agreement with several authors that obtained high P removal percentages (often over 90%) after less than one hour of solid-liquid contact (Kwon et al., 2004; Lee et al., 2005; Currie et al., 2007; Abeynaike et al., 2011; Oladoja et al., 2012). In turn, retention is slower at low P concentrations. This increased P retention rate at increasing P concentrations has also been observed by Oladoja et al. (2012). Also important, P removal was highly irreversible, as shown by the very low desorption values, in particular for the calcined shell, that presented the highest retention capacity and the lowest desorption in the batch tests. Low desorption from mussel shell was ˜ et al. (2013) for Hg, and the already observed by Pena-Rodríguez percentages are similarly low to those obtained by Robertson and Lombardo (2011) using Fe oxyhydroxides as sorbent. Although the overall characteristics of P removal were similar for the two materials tested, it was observed that the calcined shell removed more P than the fine (non-calcined) shell, both in the batch and transport experiments. Higher P removal capacity by calcined shell has been observed by Currie et al. (2007) and Kwon et al. (2004), who obtained increases in P removal from 40 to 90%, and from 10% to 60%, when using calcined shell, as compared to

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Fig. 5. Distribution of P within both kinds of mussel shell columns after the transport experiments. Solutions enter the columns from the bottom upwards. C0 is the P concentration in the incoming solution.

the raw non-calcined material. Similar sequences for As and Hg removal capacity were obtained by Seco-Reigosa et al. (2014) and ˜ Pena-Rodríguez et al. (2013). Differences in P removal capacity are a consequence of differences in mineralogy and composition affecting the two kinds of mussel shell. Calcination is known to produce several changes in calcium carbonate that could affect the physical and chemical properties of the mussel shell. First, aragonite, with an orthorhombic crystal structure, is the main form of calcium carbonate in raw mussel shell, but it is transformed into calcite, with a trigonalrhombohedral structure, during calcination at temperatures above 500 ◦ C (Currie et al., 2007; Abeynaike et al., 2011). Second, calcium carbonate is transformed to calcium oxide at 600–800 ◦ C. Different efficacy in P removal has been observed for calcite and aragonite, although aragonite shows higher P retention than calcite (Millero et al., 2001). In our case, calcined shell has consistently shown higher P retention despite having more calcite than aragonite, so the reason must be the presence of calcite. Phosphorus removal by mussel shell may happen by means of two mechanisms: adsorption and precipitation as calcium phosphates (such as hydroxyapatite). The precipitation mechanism depends on the presence and amount of CaO, which is more soluble than CaCO3 , since Ca in solution is necessary for the precipitation of calcium phosphate. For raw shells, because calcium carbonate is relatively insoluble compared

to CaO, it is likely that the main phosphate removal mechanism is by absorption and/or adsorption. In this sense, Lee et al. (2009) observed that, in calcined oyster shell, precipitation is the main mechanism of P retention, it occurs homogenously and with no adsorption of phosphates, while in non-calcined shell, adsorption is the main mechanism. However, if calcination does not achieve full conversion to CaO, both mechanisms can act together, where phosphate could both adsorb and/or precipitate onto the surface (Abeynaike et al., 2011). The higher retention capacity of calcined shell with respect to non-calcined shell can be explained by a higher contribution of the precipitation mechanism in the first case: adsorption removal capacity is limited to the surface area of the particles, whilst precipitation can remove phosphate in much larger stoichiometric amounts. According to the literature, then, P removal should proceed mainly through a precipitation mechanism in calcined shell, and mainly through adsorption in non-calcined shell. Some of our results point at precipitation as the main mechanism for P retention in mussel shell. This is the case of the increment of the percentage of P retention with increasing initial P concentrations, as shown in the kinetic study, and by the reduction of the C/C0 values at higher rates of P in the transport experiments. This does not agree with a dominant adsorption mechanism, in which case C/C0 should increase with increasing P input until a value of 1 is reached (when satura-

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Table 7 Conditions of the simulations with Visual MINTEQ 3 and results for the precipitation of P as hydroxyapatite, referred to both kinds of mussel shell (fine non-calcined, and calcined). C0 is the P concentration in the solution. C0 (mmol P L−1 )

pH

Ca2+ (mmol L−1 )

NaNO3 (mol L−1 )

% P precipitated

Finely-ground

0.5 1.5

8.0 8.0

0.3−0.7 1–6

0.01 0.01

47–71 24–100

Calcined

0.5 1.5

7.5 7.5

0.1–0.5 1–6

0.01 0.01

11–58 40–100

tion of the P retention capacity of the mussel shell into the column is reached), but rather with a chemical reaction process. Besides, Li and Stanforth (2000) observed that, as phosphate concentration in the solution increase, mineral precipitation-dominated P removal becomes more important than adsorption. This would explain the increase of P removal at high P concentrations, as a result of the increasing relevance of precipitation as removal mechanism. Simulations of P speciation under the conditions of the transport experiment performed with Visual MINTEQ 3 (Gustafsson, 2010) support this hypothesis. The chemical conditions existing in the transport experiments (pH, NaNO3 , Ca and P concentrations, Fig. S1 in Supplementary material) favour the precipitation of P under the form of hydroxyapatite (Table 7). However, the different role of precipitation in the calcined shell with respect to the non-calcined shell suggested in the literature was not confirmed by the simulation. The selective extractions performed after the transport experiments point to a significant contribution of adsorption to P removal. For both calcined and fine shell, the P fraction that increased the most was the bicarbonate-extractable, and given that sodium bicarbonate is not able to dissolve calcium phosphate, nor carbonate, this fraction should represent adsorbed P (Figs. 4 and 5). Smaller increments of P were extracted with sulphuric acid and HCl, which are able to dissolve calcium phosphate precipitates, as well as calcium carbonate, so the coexistence of both mechanisms of P retention (adsorption and precipitation), in both calcined and non-calcined mussel shell, is thus suggested. 5. Conclusions The potential use of two kinds of mussel shell (finely-ground non-calcined, and calcined) for the removal of P from wastewater was studied by means of batch and column experiments. Phosphorus removal at high contact times followed a Freundlich model, with low levels of P desorption, and pseudo-second order kinetics. Calcined shell has higher P retention capacity than fine shell, due to differences in mineralogy and composition: conversion of aragonite to calcite and calcium carbonate to calcium oxide during calcination. The process of P removal from aqueous solution shows features that are typical of chemical reactions rather than adsorption, in particular the fact that the percentage of P removed increases with initial P concentration in the solution. Fractionation of the P retained in both kinds of mussel shell after the transport experiments confirmed that direct adsorption and precipitation as calcium phosphate contribute simultaneously to P removal. Acknowledgements This study was funded by the Spanish Ministry of Economy and Competitiveness by means of the research projects CGL201236805-C02-01 and CGL2012-36805-C02-02. It was also partially financed by the European Regional Development Fund (ERDF) (FEDER in Spain). Dr. R. Paradelo holds a Juan de la Cierva postdoctoral contract (JCI-2012-11778).

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