Analysis of Ion Exchange Isothermal Supersaturation Process for

Article pubs.acs.org/IECR

Analysis of Ion Exchange Isothermal Supersaturation Process for Struvite Production F. Mijangos,*,† A. Celaya,† M. Ortueta,† and D. Muraviev‡ †

Department of Chemical Engineering, Faculty of Science and Technology, University of the Basque Country UPV/EHU, P.O. Box 644, 48080 Bilbao, Spain ‡ Department of Analytical Chemistry, Autonomous University of Barcelona, 08193 Bellaterra, Barcelona, Spain ABSTRACT: This paper describes the production of struvite by the ion exchange isothermal supersaturation (IXISS) process. By stripping magnesium from a weak cationic resin with ammonium phosphate, a highly supersaturated solution is formed. However, it remains stable while it is flowing along the fixed bed, thus avoiding clogging problems and later facilitating salt separation by self-phase segregation. In this paper, the best ion exchanger for avoiding crystallization inside the column has been selected, and the conditions for stable operation have been investigated. In addition, cyclic fixed bed operation has been checked to evaluate resin performance and long-term operation stability. Eluent concentration mainly modulates the degree of supersaturation during the elution, which is a maximum around the concentration peaks of magnesium. Crystalline aggregates could partially block the ion exchanger surface and stop the ion interdiffusion process. Usually, working with 5−18 mM ammonium phosphate at a specific rate of 7 × 10−5 m3/m2·s, the system can be operated safely and no crystallization occurs inside the bed. The amount of eluted magnesium increases slightly over the operation cycles. After bed stabilization, the operation poses no clogging problems, the performance is reproducible, and the crystallization reactor yields 14 g/h of struvite per kg of dry resin. Consequently, the IXISS process could be applied to the recovery of nutrients from domestic wastewaters.

1. INTRODUCTION The technological pathway for the production of nitrogen and phosphorus derivatives is unidirectional. Thus, the flow from sources to natural sinks goes without recycling or recovering, following a linear pathway. Therefore, natural resources (atmospheric nitrogen and phosphate rock) are processed, consumed, and finally spread widely in the environment. Although these compounds appear naturally in the environment, in the past few decades some human activities have significantly altered their cycles, increasing their environmental concentrations. This has led to major accumulations of those compounds in different chemical forms, which can lead to longterm environmental problems that are frequently unforeseen and poorly evaluated and, consequently, may be irreversible. The most important effect related with the nitrogen and phosphorus cycles is that if they are carried by superficial water into rivers and lakes they act as fertilizers for aquatic vegetation. In high concentrations, a eutrophication process ensues in the water. This results in high consumption of oxygen and a reduction of its concentration in aquatic media and does great damage to animals and plants. Ammonium and phosphates can be recovered from domestic wastewaters by promoting reaction 1, which yields the double phosphate of ammonium and magnesium called struvite. This is of great interest as an environmental alternative for the recovery of nutrients.

which allows it to be used in a single dose without damaging or preventing plant growth.1 Fertilizers are better when they have low solubility, e.g., those used in fields or forests, where fertilizers are applied usually once every few years, and it is in these cases that struvite would be useful. Furthermore, struvite is an attractive alternative as a fertilizer for use on crops such as sugar beet, which needs magnesium.2 Its insoluble nature in neutral water prevents eutrophication problems and decreases its infiltration into groundwater, giving it another advantage in its use as a fertilizer.3 However, it should be mentioned that struvite would need to be supplemented with potassium to achieve the NPK (nitrogen, phosphorus, and potassium) requirements, which would inevitably add costs in processing and production.4 In addition, it can be used as a filler in fire-resistant panels and in the production of concrete. If cheap production methods can be developed, it could also be used in detergents, cosmetics, animal feed, and any other application that uses phosphates. Conversely, struvite precipitation in wastewater plants has caused a lot of operational problems by clogging.2,4 Consequently, over the past 10 years attention has been focused on processes for nutrient recovery, and some studies have examined the optimum conditions for phosphorus and ammonium recovery via struvite precipitation.5−7 Most of these studies have been carried out using ion exchangers to catch cations from waters. However, Mijangos 8 managed to

Mg 2 + + NH4 + + HPO4 2 − + 6H 2O → MgNH4PO4 ·6H 2O(s) + H+

Received: Revised: Accepted: Published:

(1)

In this sense, struvite fulfills all the sustainability requirements for use as a slow-release fertilizer due to its low solubility, © 2013 American Chemical Society

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Industrial & Engineering Chemistry Research

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Figure 1. Schematic diagram of the setup used to produce struvite by the IXISS process.

synthesize pure struvite using a multiple fixed bed ion exchange scheme. In their paper the concept of ion exchange isothermal supersaturation (IXISS) was used to recover salt with no supplementary precipitation reaction. In fact, in this IXISS process magnesium is stripped from a weak cationic resin with ammonium phosphate, but the highly supersaturated solution remains stable while it is flowing along the fixed bed, thus avoiding clogging problems. After leaving the column, the chemical environment destabilizes the supersaturated solution, and this facilitates reagentless salt separation and recovery by solid-phase self-segregation. Later, the same authors investigated the kinetics of the process and obtained scientific and technical conclusions about the stability of the supersaturated solution of struvite in and outside the ion exchanger matrix.8 In this field, ion exchange processes should comply with the environmental safety and economic efficiency requirements. To that end, it is necessary to eliminate the principal wastes and reduce the energy and the chemicals consumed in the process. The characteristics and advantages of the ion exchange isothermal supersaturation process were stated by Muraviev.9 By using IXISS-based processes some typical steps of the conventional ion exchange scheme can be eliminated, such as the concentration of the solution after treatment and the recovery of the purified product as well. Moreover, ion exchanger regeneration is direct and the presence of aggressive residues is avoided. On the basis of those conclusions, an experimental study is proposed here for determining the principal operating parameters of the IXISS process, particularly those that affect the production of struvite and the stability of the operation. In this process the stabilization of the supersaturated solution is crucial for industrial feasibility and for the design and scale-up of equipment. Involuntary crystallization inside the column of the IXISS fixed bed tends to be unstable, so the variables and parameters that have the most influence on the stability of the process have also been investigated, with a view toward setting

safe operating conditions. Ammonium phosphate concentration and flow rate are the variables that most influence the degree of local supersaturation achieved inside the column, so they have been selected for this experimental study. In addition, the influence of the number of operating cycles on column operation, struvite productivity, and operation stability has been checked.

2. EXPERIMENTAL SECTION 2.1. Ion Exchange Resin. Operational conditions control crystallization kinetics, but the stability of the supersaturated solution is fundamentally based on the matrix type, properties, and chemical state of the ion exchanger. In previous batch kinetics studies,9 certain commercial resins have been studied with regard to obtaining pure struvite by the IXISS process. From those experiments it has been concluded that the carboxylic functional group and the microporous matrix are optimal for this process. All assays described in this paper have been carried out using a microporous carboxylic resin, Amberlite IRC86 (Rohm and Haas, Barcelona, Spain). This weak acid resin is a gel-type polyacrylic copolymer, which has a total ion exchange capacity for magnesium of QMg = 3.80 mmol/g of dry resin. The resin particles are spherical, with a harmonic mean diameter of 0.580−0.780 mm.10 The resin supplied (H-form) is first water-washed and then conditioned to the sodium form by three consecutive H+/Na+ exchange cycles. Using this procedure, the humidity (vacuum, 60 °C) of the sodium-form resin is uniform and close to 56%, which is the reference state for load measurements (DR, Na-form dry resin). Finally, the sample is loaded with a 0.1M solution of magnesium chloride. After washing with deionized water the Mg-form resin is ready for column experiments.8,9 2.2. Chemicals and Equipment. All chemicals were of reagent grade, supplied by Panreac (Barcelona, Spain), and were used as received, including (NH4)2HPO4, MgCl2.6H2O, and NaOH. Stock solutions were diluted using deionized Milli10277

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Therefore, the NH4+/Mg2+ exchange takes place within the column; i.e., magnesium is eluted from the resin toward the ammonium phosphate solution, giving rise to the metastable supersaturated solution. This elution reaction is the key part of the process, but for proper operation the magnesium phosphate should only precipitate out of the column in the crystallizer and with struvite as the only crystalline species. Depending on the pH and species concentration, several phosphates and polynuclear species may be formed12 in this chemical system. The solution pH causes magnesium species to change from MgHPO4 in acidic media to Mg3(PO4)2 and MgNH4PO4 under alkaline conditions. Other magnesium phosphates could also be found in this system, e.g., newberyite (MgHPO4·3H2O) and Mg3(PO4)·2H2O. So under these conditions several magnesium precipitates could appear in the crystallizer,8 but the proper nutrient ratio is only achieved by the double magnesium ammonium phosphate. To ensure the stability of the system and achieve pure struvite production, the eluent concentration and flow rate, which are the variables with most influence on the overall process throughout the experiment, have been investigated. 3.1. Eluent Concentration. To analyze how the concentration of (NH4)2HPO4 solution affects the stability of the process, four trials were performed in the ion exchange column. Magnesium is eluted from the resin using (NH4)2HPO4 solution at different concentrations (5, 10, 15, 50, and 100 mM), working at a constant flow rate of 1 mL/min. Figure 2 shows the magnesium elution curves for these experiments. As shown, the lower the eluent concentration

Q water (Millipore, Madrid, Spain) to prepare all solutions used in this study. Conventional laboratory methods were used throughout the experiments. The metal concentration in the solution was determined by atomic absorption spectroscopy (Perkin-Elmer 1100B, Madrid, Spain). Each single sample analysis was replicated three times. The results reported are the averaged values. The crystallization starting point was detected by using a laser pointer, the ray from which is easily observed by light scattering if the solution contains suspended particles. 2.3. IXISS Experimental Setup. The experimental setup used in this work consists mainly of a fixed bed ion exchange column, a crystallization reactor, a pH control system, the tanks required to store the solutions, and a peristaltic pump for feeding, as shown in Figure 1. As shown in Figure 1, a (NH4)2HPO4 solution is fed through a peristaltic pump from the reservoir to the top of the ion exchange column, previously filled with 4 g of wet resin, which forms a fixed bed 4.6 cm3. Inside the column Mg2+/NH4+ exchange takes place, producing a front of metastable supersaturated solution of struvite. This solution leaves the column and enters the crystallizer through the bottom opening. When the supersaturated solution from the column reaches the crystallizer it is destabilized by stirring and by the chemical environment. It is then that struvite begins to precipitate spontaneously on the struvite bed. Inside the crystallizer the solution flows upward through a bed of struvite that acts as a nucleation zone, crystallization progresses rapidly in this layer, and after precipitation the remaining (NH4)2HPO4 solution is almost magnesium-free. This solution exits the crystallizer through the overflow outlet. Some studies11 have shown that struvite itself performs better as a nucleation center than sand, providing a higher crystallization rate and improving the subsequent secondary nucleation of the crystals. This solution “absorbs” the excess concentration of free ammonium and phosphate that arrives from the column, but attention should be centered on bulk solution values because the concentration of these ions plays an important role in the crystallization of struvite.3 The pH at the crystallizer tends to decrease because of reaction 1, but it plays a key role in the efficiency and purity of the precipitate obtained.5 In a previous paper,8 Mijangos and co-workers investigated the role of the inlet solution pH and concluded that it was crucial and that it must be controlled and kept constant at 9.5 throughout the reaction. For this purpose we use an autoburette (Mettler Toledo T50, Barcelona, Spain), which features an internal program to maintain the pH constant by adding 0.1 M NaOH. This crystallization reactor operates at the same time as a settling tank, so it is designed to ensure the retention of a minimum size of crystals formed in accordance with the crystallization kinetics. The struvite produced is collected batchwise at the bottom of the crystallizer, whereas the clarified effluent overflows from the crystallizer through a lateral bleed.

Figure 2. Influence of (NH4)2HPO4 concentration on the Mg2+/NH4+ exchange in a fixed bed under supersaturation conditions.

used in the system is, the lower the concentration peak of magnesium is, but the elution time is substantially higher. Consequently, as the degree of supersaturation (Γ, eq 4) peaks around these values, the risk of spontaneous crystallization is also at its highest around these peaks, because nucleation and crystallization kinetics are directly dependent on Γ, as usually described in the relevant literature by eq 3.

3. RESULTS AND DISCUSSION Reaction 2 takes place partly within the ion exchange column. Thus, an ammonium phosphate solution is brought into contact with the ion exchanger previously loaded with magnesium.

φ = k(Γ − 1)n

(3)

The ion exchange column usually operates with no precipitation reaction, so the crystallization rate could not be calculated in these experiments, but from batch experiments it

R 2−Mg + (NH4)2 HPO4 → 2R−NH4 + Mg 2 + + HPO4 2− (2) 10278



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has been measured13 and concluded that, for these conditions, k = 3.8 × 10−6 mmol/L·min and the value for n is close to 1. The degree of local supersaturation can be exactly calculated from experimental results by its own definition (eq 4), but also considering that the concentration of eluent does not change noticeably in the emerging eluent solution. ⎛ [PO 3 −][NH +][Mg 2 +] ⎞ 4 4 ⎟⎟ Γ = ⎜⎜ K ⎝ ⎠ ps

1/3

Table 1. Summary of the Results Obtained for Mg2+/NH4+ Exchange in a Fixed Bed under Supersaturation Conditions (NH4)2HPO4 (mM)

Γmax

bed depletion (bv)

5 10 15 50 100

83 165 250 810 1610

160a 83 45 14 8

⎛ [Mg 2 +] ⎞ ⎟ ≈ ⎜⎜2Co 2 K ps ⎟⎠ ⎝

1/3

(4)

a

12

For these calculations it is assumed that pKps = 13.26. The main effect on supersaturation is thus due to the initial concentration of ammonium phosphate in the eluent, as can be derived from eq 4 and reaction 2. Meanwhile, magnesium concentration is modified by an exponent of 1/3 and therefore contributes less to system stability. The time when this maximum concentration of magnesium is reached is approximately 5 bed volumes, though it decreases slightly with the feed concentration. Trials show that if the (NH4)2HPO4 concentration is decreased, the eluate volume required for the depletion of the bed increases inversely. These values are shown in Figure 3,

Δq (mmol/g DR)

elution yield (%)

0.200 0.287 0.800 0.345 0.398

5.26 7.55 21.05 9.08 10.47

The value is estimated by tendency due to the long tail.

magnesium eluted at a given time (Δq) is calculated by integrating the area under the corresponding elution curve, according to eq 5. Δq =

∫0

V

CMg mR

dV =

Δv mR

∑ Ci i

(5)

In spite of the degree of supersaturation achieved in these experiments and the fact that no precipitations occurred, as expected, it can be seen in Table 1 that the maximum amount of eluted magnesium is obtained by using a higher concentration of (NH4)2HPO4, but only 5−20% of the full magnesium capacity is used to produce struvite. It can be seen in Table 1 that the maximum degree of supersaturation, which is attained at the peak concentration of magnesium, is very high using highly concentrated ammonium phosphate solutions. It is well-known16 that the porous matrix of the ion exchange stabilizes the supersaturated solution, but as a consequence of the extremely high concentration, these solutions are relatively unstable and any vibration or perturbation can make the system start to crystallize. Obviously, the crystallization rate would be very high because it is proportional to the degree of supersaturation, as eq 3 states, and consequently the bed would clog suddenly. Such operational problems do indeed appear from time to time, but it is not always easily observable. In fact, using higher ammonium phosphate concentrations, that is 50−100 mM, the corresponding elution curves are sharper, and they end suddenly, showing no tail. Moreover, the elution yield is clearly lower in spite of the higher eluent concentration. These facts are attributed to a fast precipitation reaction inside the resin bead, blocking its macroporous structure and so stopping the interdiffusion process. However, neither percolation problems nor struvite crystallization was actually observed in these experiments. This is in agreement with the work of Muraviev and co-workers,17 who proposed a mathematical model of the IXISS dynamics, and with experiments that concluded that the ion exchanger surface could be partially blocked by adsorbed precrystalline aggregates. 3.2. Eluent Flow Rate. Furthermore, in these experiments it is observed that, as the reaction proceeds, magnesium precipitation tends to occur randomly inside the column. Obviously, the risk of precipitation increases with eluent and magnesium concentration, as mentioned above. In search of a safe operation zone, some run tests were performed within a narrower range, but in the same conditions as reported for Figure 2, so as to check eluent concentration and flow rate exhaustively. On the basis of the arguments presented above, some assays were carried out to study the effect of concentration on the

Figure 3. Influence of the (NH4)2HPO4 concentration on the volume needed for bed depletion during the Mg2+/NH4+ exchange in a fixed bed under supersaturation conditions.

where it can be observed that the elution volume, and therefore the elution time, increases exponentially. It can be concluded that, in the case of very dilute solutions, the eluent volumes required for full magnesium elution would be operationally nonviable. For this reason concentrations lower than 5 mM are disregarded for effective magnesium elution. When the eluent concentration is increased, the ion exchange rate increases and consequently the time and volume needed to achieve a given elution level decrease. These values are represented in Figure 3. As the experiments are run at constant flow rate, it can be concluded by looking at the shape of the curve that the kinetics of elution closely fit with pseudo-secondorder kinetics. In ion exchange systems, this has been reported by many authors.14,15 Table 1 summarizes the main parameters and results of this set of experiments, which shows the amount of magnesium recovered from the column. With total ion exchange capacity being QMg = 3.80 mmol/g DR for magnesium, the amount of 10279



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stability of the process, restricting the initial concentration range of (NH4)2HPO4 from 5 to 20 mM. The physical appearance of the resin, which is translucent, makes it easy to observe with the naked eye when the struvite precipitates inside the column, because the bead surfaces turn opaque white if struvite starts to precipitate in the resin bed. Due to the high influence of hydrodynamics and solution perturbations on the stability of the supersaturated solution, the effect of flow rate is also considered in this study. Table 2 summarizes the main results of these assays. Table 2. Qualitative Results Concerning the Stability of the Ion Exchange Column for Struvite Precipitation Throughout the Operation eluent concn (mM)

feed flow-rate (mL/min)

residence time (min)

precipitation within the fixed bed

20 19 18 18 18 15 15 15 10 5

1.0 1.0 10.0 5.0 1.0 10.0 1.0 0.5 1.0 1.0

4.60 4.60 0.46 0.92 4.60 0.46 4.60 9.20 4.60 4.60

yes yes yes no no no no no no no

In the experiments with a feed of less than 18 mM (NH4)2HPO4, independent of the flow rate, struvite precipitates spontaneously only after leaving the column, so the supersaturation solution remains stable inside the column. At higher concentrations or flow rates the risk of precipitation within the column goes up, whatever the residence time. On the other hand, for all these runs the particle Reynolds number, based on the specific flow rate, is lower than 100, which confirms that the flow regime is laminar and that hydrodynamic conditions are adequate for avoiding spontaneous crystallization inside the column. At the same time it indicates that the overall ion exchange rate is controlled by the external mass transfer. However, the radial gradient of velocity in these ion exchange columns is usually very large, so local values of the Rep could be dramatically higher in the center of the column, causing the metastable solution to start to crystallize. Figure 4 shows the magnesium elution curves using 15 mM (NH4)2HPO4 as the eluent. There are two trends in these experiments. On the one hand, when a low flow rate is used, magnesium is eluted within lower solution volumes, and higher magnesium concentrations are thus achieved. Figure 4A shows that when the flow is increased the elution peaks of magnesium are clearly lower and the maximum is reached sooner. On the other hand, for high flow rates the elution kinetics should speed up; indeed, Figure 4B shows that in spite of the initially lower concentrations the concentration of magnesium decreases more slowly, so after the initial lag time the kinetics of elution keeps the rate constant during operation. In other words, these two trends measure the contribution of mass transfer to the exchange kinetics. This tendency is described by the specific elution rate, μe, which can be easily calculated from the values reported in Figure 4, using eq 6

Figure 4. Effect of the flow rate on magnesium elution working under supersaturation conditions. Temporal elution curves (A) and magnesium concentration vs throughput volume (B) for assays all run using Amberlite IRC86 (fixed bed of 4.6 mL) and 15 mM (NH4)2HPO4 as eluent. Flow rate (mL/min): 0.5 (×), 1.0 (□), 5.0 (◇), and 10.0 (○).

μe =

1 ⎛ dq ⎞ F ⎛ ∑i Ci ⎞ ⎜− ⎟ ≈ ⎜ ⎟ Q Mg ⎝ dt ⎠ Q Mg ⎝ mR ⎠

(6)

Figure 5 shows the evolution of the specific elution rate as a function of the flow rate. It is observed that the kinetics of the elution of magnesium show an increasing trend when the elution flow increases. This result coincides with those reported by Inglezakis and Grigoropoulou in a paper18 that states that the capacity of fixed bed operation depends on the flow rate. It is clear that under these hydrodynamic conditions the external mass transfer coefficient, that is to say the specific rate, increases almost proportionally to the Reynolds number, as many correlations19,20 state for packed beds in a laminar regime. From Figure 5 it can be concluded that the total amount of eluted magnesium is almost proportional (R2 = 0.993) to the flow rate. But at higher flow rates, the risk of bed destabilization is also higher. The main results for this set of experiments are summarized in Table 3, which shows the Reynolds number, the maximum elution rate of magnesium, and the total amount of magnesium 10280



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Figure 5. Evolution of the elution rate as a function of the eluent flow rate.

Figure 6. Trend in the degree of supersaturation throughout operation at different flow rates (mL/min) using ammonium phosphate 15 mM as eluent. Flow rate (mL/min): 0.5 (×), 1.0 (□), 5.0 (◇), and 10.0 (○).

eluted from the column as a ratio of the total ion exchange capacity. Table 3. Effect of Flow Rate on the Elution of Magnesium by 15 mM (NH4)2HPO4 Solution as a Function of the Flow Rate flow rate (mL/min)

.

ReP × 103

0.5 1.0 5.0 10.0

3.52 7.03 35.20 70.30

2 4 22 44

elution rate (mmol/g DR·min) 3.44 6.25 2.00 3.33

× × × ×

10−3 10−3 10−2 10−2

conclusion would be better supported by modeling the thermodynamics state of the bed for a real physical and chemical description of the system. Thus, it seems that under these conditions the system can be operated safely, and independent of the flow rate, no crystallization occurs inside the bed because in this set of experiments the maximum degree of supersaturation is independent of the flow rate and slightly higher than 300. The critical operation time tends to appear at values of the throughput parameter near to 1. 3.4. Number of Cycles. Using the experimental setup described in Figure 1 and on the basis of the above conclusions, a pilot test was carried out in which successive exchange cycles were performed using a 4.6 cm3 resin bed and 15 mM (NH4)2HPO4 solution with a flow rate of 1 mL/min to determine the stability and reproducibility of the process over several operation cycles, consisting of Mg2+/NH4+ exchange and later magnesium crystallization as struvite. Figure 7 summarizes the results obtained over 15 cycles. The fixed bed did not suffer from clogging in any case, in spite of working under very high supersaturation conditions. Under these conditions the time required to exhaust the resin bed completely in each of these exchange cycles has been estimated as approximately 10−12 h. This time is due to the relatively low feed concentration, which means that the elution curve has long queues. Struvite production is shown in Figure 8 as the proportion of the total capacity of the resin accounted for by stripped magnesium. This assumes8 that all eluted magnesium is precipitated and recovered from the crystallization reactor as pure struvite. Moreover, in these experiments we have observed that the solution emerging from the crystallizer does not produce any precipitate, and magnesium concentration usually averages 200 ppb. As noted, the amount of eluted magnesium increases with the number of cycles. As can be seen in Figure 8, after a few cycles the bed yields the same amount of eluted magnesium and the elution curves overlap after the sixth cycle. It is thus concluded that from cycle 6 onward the bed is properly stabilized and its performance is reproducible.

elution (%) 22.0 21.6 16.7 17.0

3.3. Degree of Supersaturation throughout the Operation. The specific elution rate increases proportionally to the Reynolds number. But at higher flow rates the risk of bed destabilization is also higher. Moreover, the efficiency of elution is improved by using a lower flow rate. Maximum elution of magnesium is around 22% of the total capacity. In conclusion, using lower flow rate improves the efficiency of elution and better guarantees bed stability. In order to find optimal operation conditions, it is crucial to know how the degree of supersaturation changes in the course of the operation, as it plays a key role in bed stability and in the risk of uncontrolled crystallization inside the column. The risks of spontaneous crystallization and the critical operation time can thus be better evaluated by representing the degree of supersaturation versus the dimensionless throughput parameter, Π, a parameter defined by eq 7 to fit the results of the experiment. Π=

(V − εVB) γVB

where γ is the partition coefficient, defined as qρ γ= B Co

(7)

(8)

It can be clearly seen in Figure 6 that, in terms of the dimensionless parameters, the flow rate does not influence strongly the magnesium elution rate, with the supersaturation degree ranging from 200 to 300. This trend continues until the elution rate is stabilized and the generalization in the 10281



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4. CONCLUSIONS The IXISS process can be applied to recover nutrients from wastewaters as struvite. In this process resin regeneration and metal loading take place simultaneously at the elution stage, provided that supersaturation conditions are used. With proper operation, struvite should only crystallize spontaneously after leaving the column. This saves on both working time and reagent consumption: there is no need for an acid−base cycle to condition the resin for the following elution step. The eluent concentration does not change noticeably over the bed, so the main effect on supersaturation is due to the initial concentration of ammonium phosphate. However, local magnesium concentration changes by several orders of magnitude during operation. Consequently, the eluent concentration modulates the system performance, and bed stability is critical around peak concentrations. As the degree of supersaturation is at its highest around these values, the risk of spontaneous crystallization is also greatest there. The elution rate fits into pseudo-second-order kinetics, so the bed depletion time increases with the inverse of the eluent concentration. As a result, concentrations of ammonium phosphate lower than 5 mM are disregarded for effective magnesium elution. Although the porous matrix of the ion exchange stabilizes the supersaturated solution, extremely high eluent concentrations could cause destabilization and the bed could clog suddenly, because the crystallization rate is proportional to the degree of supersaturation. But even in standard operation the elution yield decreases in the range of 15−100 mM. This is probably due to the intermatrix formation of crystalline aggregates that could partially block the ion exchanger surface. Usually, a safe operation zone is assured by working with eluent concentrations below 18 mM. Within an operational range of 5−18 mM, ammonium phosphate at 15 mM gives the maximum elution yield: around 22% of the total capacity. The effect of the flow rate on bed stabilization was investigated using this concentration, taking 0.5−10.0 mL/ min as the operational flow rate. The corresponding particle Reynolds number ensures that the flow regime is laminar, elution clearly controlled by external mass transfer, and hydrodynamic conditions are such that spontaneous crystallization is prevented inside the column. Under this flow regime, the specific elution rate increases almost proportionally to the Reynolds number. But at higher flow rates the risk of bed destabilization is also higher and elution is more efficient when lower flow rates are used. In spite of the fact that the maximum supersaturation degree ranges from 290 to 310 in this set of experiments, the operation of the system is stable, and there is no crystallization inside the bed, whatever the flow rate. Working with ammonium phosphate at 15 mM, at a specific rate of 7 × 10−5 m3/m2·s, the amount of magnesium eluted increases slightly over the operation cycles. After the sixth cycle the bed is properly stabilized, with no clogging problems, and its performance is reproducible. The amount of magnesium eluted stabilizes at around 22.3% of the total capacity. However, the long tail of the elution curves means that the resin bed takes 10−12 h to become completely exhausted. Finally, magnesium is precipitated and recovered from the crystallization reactor, which yields over 14 g of struvite/kg DR·h.

Figure 7. Elution curves of magnesium. Operation conditions: resin, Amberlite IRC86 Mg-form; bed volume, 4.6 cm3; eluent concentration, 15 mM; flow rate, 1 mL/min. Key for symbols: cycle 1 (○), 6 (□), 10 (◇), and 15 (×).

Figure 8. Trend in the total amount of magnesium eluted from the bed throughout the operation cycles. Experimental conditions: resin, full Mg-form; bed volume, 4.6 cm3; eluent concentration, 15 mM; flow rate, 1 mL/min.

The value for eluted magnesium stabilizes at around 22.3% of total capacity; i.e., the fixed bed yields over 14 g/kg DR·h of struvite. This average productivity of the bed is calculated under the assumption that all magnesium becomes pure struvite after crystallization.8 Furthermore, after the first two cycles it is observed that once the solution leaves the column, some crystals of struvite tend to appear in the elbows and in the tubing walls before the crystallizer is reached. But, due either to hydrodynamic dragging or the chemical environment, these few crystals do not grow big enough to block the tube. The operation runs properly in any case. However, this is a very critical point for industrial feasibility, because the immediate consequences of uncontrolled precipitation would be the formation of bottlenecks in pipelines, giving rise to operational problems and a decrease in process efficiency. These results confirm that, in the way in which it is performed here, the operation is sufficiently stable and reproducible. 10282



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(9) Ortueta, M.; Celaya, A.; Mijangos, F.; Muraviev, D. Mg2+/NH4+ ion exchange kinetics under isothermal supersaturated conditions. Solvent Extr. Ion Exch. 2008, 26 (4), 405−419. (10) Amberlite IRC86, Product Data Sheet, Rohm and Haas, PDS 0234A-jan 2008. (11) Wu, Q.; Bishop, P. L. Enhancing struvite crystallization from anaerobic supernatant. J. Environ. Eng. Sci. 2004, 3 (1), 21−29. (12) Olhinger, K. N.; Young, T. M.; Schroeder, D. Predicting struvite formation in digestors. Water Res. 1998, 32 (12), 3607−3614. (13) Celaya, A. Producción de estruvita mediante intercambio iónico en lecho fijo operando en condiciones de sobresaturación isotérmica, Ph.D. Thesis, University of the Basque Country UPV/EHU, Bilbao, Spain, 2011. (14) Atar, N.; Olgun, A.; Wang, S. Adsorption of cadmium(II) and zinc(II) on boron enrichment process waste in aqueous solutions: Batch and fixed-bed system studies. Chem. Eng. J. 2012, 192, 1−7. (15) Huang, L. H.; Sun, Y. Y.; Yue, Q. K.; Yue, Q.; Li, L.; Gao, B. Adsorption of Cd(II) on lotus stalks-derived activated carbon: Batch and column studies. Desalination 2012, 41 (1−3), 122−130. (16) Muraviev, D.; Khamizov, R.; Tikhonov, N. A. Ion exchange isothermal supersaturation. Solvent Extr. Ion Exch. 1998, 16, 151−221. (17) Muraviev, D.; Khamizov, R. K.; Tikhonov, N. A. Peculiarities of the dynamics of ion exchange in supersaturated solutions and colloid systems. Langmuir 2003, 19 (26), 10852−56. (18) Inglezakis, V. J.; Grigoropoulou, H. P. Modelling of ion exchange of Pb2+ in fixed beds of clinoptilolite. Microporous Mesoporous Mater. 2003, 61, 273−282. (19) Helfferich, F. Ion Exchange; McGraw-Hill: New York, 1962; pp 252−257. (20) Wilson, E. J.; Geankoplis, C. J. Liquid mass transfer at very low Reynolds number in packed beds. Ind. Eng. Chem. Fundam. 1966, 5, 9−14.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was economically supported by the Catedra UNESCO Foundation, the University of the Basque Country (Subvención General a Grupos de Investigación UPV/EHU 2010), and the Spanish Ministry of Science and Education (CTQ2006-13088).

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LIST OF SYMBOLS CMg concentration of magnesium in solution (mM) Ci concentration of magnesium in volume i element (mM) Co concentration of eluent (mM) F eluent flow rate (L/min) k crystallization kinetics constant (mmol/min·L) Kps solubility constant of struvite mR mass of dry resin (g) n exponent for crystallization kinetics (eq 3) q resin load of magnesium (mmol/g DR) QMg magnesium total ion exchange capacity (mmol/g DR) t time (min) V throughput volume (mL) VB bed volume (mL) Δq total amount of magnesium eluted (mmol/g DR)

Greek Symbols

Γ degree of supersaturation (eq 4) Π throughput parameter (dimensionless) ε bed void fraction (dimensionless) γ partition coefficient (eq 8) φ crystallization rate (mmol/min·L) μe specific elution rate (min−1) Abbreviations

IXISS ion exchange isothermal supersaturation DR dry resin, Na-form bv bed volume

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REFERENCES

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Analysis of Ion Exchange Isothermal Supersaturation Process for

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