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Kinetics of Simultaneous Ammonium and Phosphate Recovery by Natural Zeolite

Kinetics of Simultaneous Ammonium and Phosphate Recovery by Natural Zeolite Article Kinetics of Simultaneous Ammonium and Phosphate Recovery by Natural Zeolite Sandro Pesendorfer * and Markus Ellersdorfer Chair of Process Technology and Industrial Environmental Protection, Montanuniversität Leoben, 8700 Leoben, Austria; markus.ellersdorfer@unileoben.ac.at * Correspondence: sandro.pesendorfer@unileoben.ac.at Abstract: Nowadays, fertilizers containing nitrogen and phosphorus are indispensable for medium and large-scale industrial agriculture. To meet the growing demand of nutrients and reduce the accompanied ecological footprint of primary fertilizer production, processes and technologies for nutrient recovery are necessary and have to be developed. This study represents the basis of an extension of the ion-exchange-loop-stripping process (ILS), which is a combined stripping and ion exchange process using natural zeolite for nitrogen recovery. In batch experiments with a special zeolite filled stirrer, the mechanism and kinetics of simultaneous ammonium and phosphate recov- 3− −1 ery by natural zeolite were determined. Zeolite loadings of 6.78 mg PO4 g were reached and after regeneration, phosphate recovery rates up to 75% of the initial concentration were achieved. The speed of phosphate precipitation is mostly controlled by the pH value of synthetic wastewater. Phosphate removal in simultaneous experiments does not affect ammonium sorption onto zeolite. These findings and the different removal mechanisms of ammonium and phosphate lead to versa- Citation: Pesendorfer, S.; tile applications in wastewater treatment and reveal great potential of natural zeolite in simultane- Ellersdorfer, M. Kinetics of ous nutrient recovery processes. Simultaneous Ammonium and Phosphate Recovery by Natural Keywords: nutrient recovery; wastewater treatment; clinoptilolite Zeolite. ChemEngineering 2021, 5, 68. https://doi.org/10.3390/ chemengineering5040068 1. Introduction Academic Editor: Luis M. Gandía As a result of the constantly growing world population, the demand of organic and Received: 14 September 2021 inorganic fertilizer for food production increases every year [1]. The major macronutrients Accepted: 5 October 2021 nitrogen (N) and phosphorus (P) are produced in energy and resource intensive pro- Published: 8 October 2021 cesses, e.g., the Haber-Bosch synthesis for nitrogen. On the other hand, overfertilization and the resulting eutrophication of surface and subsurface water present a global and Publisher’s Note: MDPI stays neu- persistent environmental problem [2,3]. Nutrient amounts in organic fertilizers like ani- tral with regard to jurisdictional mal manure or biogas digestates are subject to fluctuations in process conditions, thus it claims in published maps and institu- is hard to fertilize plants with the appropriate quantity of nutrients. Additionally, harmful tional affiliations. components in manure and sewage sludge, such as microplastics, germs, antibiotics and heavy metals accumulate in the soil and water. As a result of those problems the direct utilization of organic fertilizers in agriculture like manure, sewage sludge or biogas di- gestates is restricted by legal regulations of the European Commission [4]. Recycling is an Copyright: © 2021 by the authors. Li- essential element in modern industry, hence nutrient recycling by the selective recovery censee MDPI, Basel, Switzerland. of N and P from organic wastewater could be the next step. Nutrient recovery can save This article is an open access article energy in the primary production and prevent the contamination of soil and water by distributed under the terms and con- harmful substances. Industrial scaled processes use the precipitation of struvite to recover ditions of the Creative Commons At- N and P from different wastewater streams [5–7]. Two disadvantages of these precipita- tribution (CC BY) license (http://crea- tion processes are the fixed N:P ratio in struvite and the possibility to separate recovered tivecommons.org/licenses/by/4.0/). ammonium from phosphate. This study shows further development possibilities of the ChemEngineering 2021, 5, 68. https://doi.org/10.3390/chemengineering5040068 www.mdpi.com/journal/chemengineering ChemEngineering 2021, 5, 68 2 of 12 ILS process [8,9] and kinetics of a simultaneous recovery process of ammonium (NH4 ) 3− and phosphate (PO4 ) from aqueous solutions. Ammonium recovery in the ILS process is based on the principle of ion exchange by zeolites especially clinoptilolites, which are micro- and mesoporous-aluminosilicate min- erals. A unique property of natural clinoptilolite is the high selectivity for ammonium adsorption. Its low cost makes it one of the most commonly used zeolites to remove am- monium from wastewater [10–12]. The positively charged ammonium-ion is adsorbed onto the aluminosilicate crystal framework and in exchange the zeolite releases cations 2+ + + 2+ (e.g., Ca , K , Na and Mg ) for charge balance in the aqueous solution. The adsorption + −1 capacity for ammonium on natural zeolite is in the range of 2.7–30.6 mg NH4 g [12]. Few studies investigated the property to remove phosphate from wastewater due to electrostatic attraction, anion exchange or special pretreatment of the zeolite [13–17]. The mechanism of phosphate sorption onto the zeolite is determining the kinetics, capacity and removal rate of dissolved phosphate [18]. Simultaneous N- and P-removal based on phosphate precipitation were performed with synthetic zeolites from fly ash [19–21] and calcined dolomite [22]. Only Lin et al. [23] used natural zeolite with the main mineral cli- noptilolite-Na for simultaneous N- and P-removal in a small scale laboratory setup. Zeo- lite has a very limited mechanical stability, which means in stirred batch processes there is no steady particle size distribution, in case zeolite can move freely. In contrast to others, this study shows batch experiments for simultaneous removal of phosphate and ammo- nium from synthetic wastewaters, in which the zeolite is fixed in a stirrer. The impacts of the pH value and initial concentration of N and P on the kinetics were also investigated. For zeolite regeneration and recovery, precipitated phosphate on the zeolite surface was dissolved in an acidic solution. 2. Materials and Methods 2.1. Materials The zeolite was obtained from a Slovakian deposit and its main mineral is a calcium- bearing clinoptilolite. After crushing and sieving to 0.5–2.5 mm, zeolite was washed with distilled water to remove impurities and small particles formed by mechanical abrasion. Washed zeolite was dried at 105 °C for 24 h and stored in the desiccator. The BET specific 2 −1 surface area of the crushed and washed zeolite is 32.08 m g (nitrogen calibrated, Mi- cromeritics Flowsorb 2300). A total of 30 g of zeolite were filled in a thin, rectangular shaped bag made from polypropylene (PP) net with the dimensions of 62 × 100 mm. The zeolite bag had a thickness of 4–5 mm and was stabilized between two PP frameworks (80 × 116 mm), which were held together by a PP cord. In brief, zeolite is fixed in a self-de- signed stirrer and immersed into synthetic wastewater. The thin layer ensured that the total amount of zeolite is in contact with the synthetic wastewater. Hence, the influence of a potential concentration gradient inside the layer of zeolite grains on the exchange kinet- ics can be neglected. Experiments were carried out in a double jacket glass reactor with an inner diameter of 110 mm and a height of 220 mm. The reactor was tempered to 25 °C by a circulatory cooling unit (Julabo F250). During the experiments the reactor opening was covered with plates (PMMA) to prevent evaporation of the synthetic wastewater. The synthetic wastewaters were a mixture of ammonium di-hydrogen phosphate ((NH4)H2PO4) and ammonium chloride (NH4Cl) dissolved in 2 L of distilled water. To increase the pH to 8 or 9, 1 M sodium hydroxide (NaOH) was added. All used chemicals were of analytical grade. 2.2. Simultaneous Exchange Experiments and P-Regeneration At the beginning of each experiment, the stirrer was dipped into distilled water for 5 min to prevent distortion in the kinetics of the diffusion process due to capillary effects in pores. Exchange experiments were carried out for 24 h with a stirrer speed of 100 rpm. ChemEngineering 2021, 5, 68 3 of 12 The double jacket glass reactor was filled with 1.5 L of synthetic wastewater, which was tempered to 25 °C. At time 0 min, the rotating zeolite stirrer was immersed into synthetic wastewater. After certain instants of time (30 or 60 min), a sample of 5 mL was taken and filtered with a 0.45 µm syringe filter (Rotilabo-PVDF). The filtration ensured a clearly de- fined end of the exchange reaction, because small particles or free moving precipitates could affect the determination of N & P. Preliminary tests have shown that equilibrium is reached between 8 and 20 h. Hence, the samples after 21, 22, 23 and 24 h are defined to determine the equilibrium concentration, which was calculated as the arithmetic mean of these four values in each experiment. To determine the kinetics of desorbed (exchanged) cations of the zeolite, an addi- tional exchange experiment was performed with ammonium only. After the simultaneous removal experiments, the ammonium and phosphate loaded zeolite stirrer was washed with distilled water and drained afterwards. For P-regenera- tion, the precipitated phosphate phases on the zeolite were dissolved in diluted sulfuric acid at pH 2.5. Compared to the exchange experiments, regeneration samples were taken in shorter periods because of faster reaction kinetics. Since the increasing pH slowed down the dissolution, another 2 mL of 1 M sulfuric acid was added after 3 h. P-regeneration 3− −1 ended after 4 to 5 h, when low P-loading (<1 mg PO4 g ) of zeolite was achieved. 2.3. Analytical Methods and Calculations Samples of the solution were diluted according to calibration lines. Afterward, the concentrations of N and P were determined photometrically by using Spectroquant test kits for ammonium (Merck 1.14752.0001) and phosphate (Merck 1.4848.0002). A reference beam photometer (WTW photoLab 7600 UV-VIS) was used in the experiments. The initial concentrations of each synthetic wastewater were determined photometrically in tripli- cate. Temperature and pH value were measured with the pH meter inoLab pH7310. The 2+ + + 2+ desorbed cations (Ca , K , Na , Mg ) were quantified by ICP-OES (ÖNORM EN ISO 11885:2009-11) at the Chair of Waste Processing Technology and Waste Management, Montanuniversiät Leoben. The zeolite loading of ammonium and phosphate was calculated using the difference between initial and temporally decreasing concentration in solution related to the total mass of zeolite (see Equation (1)). (𝑐 − 𝑐 ) ∗ 𝑉 0,𝑒𝑖𝑠𝑐𝑠𝑝𝑒 𝑖 ,𝑒𝑖𝑠𝑐𝑠𝑝𝑒 𝑢𝑡𝑜𝑛𝑙𝑖𝑠𝑜 (1) 𝑞 = 𝑜𝑧𝑒 + −1 3− −1 qi: zeolite loading at t = i (mg NH4 gzeo , mg PO4 gzeo ); mzeo: the amount of zeolite in the stirrer (gzeo); + −1 3− −1 c0,species: the initial concentration of ammonium or phosphate (mg NH4 L , mg PO4 L ); + −1 3− −1 ci,species: concentration at time t of ammonium or phosphate (mg NH4 L , mg PO4 L ); Vsolution: the volume of synthetic wastewater in reactor (L); t: elapsed time (min). 2.4. Scanning Electron Microscope (SEM) Samples of natural zeolite before and after N- and P-removal from synthetic wastewater were sputtered with carbon at the Chair of Resource Mineralogy, Montanuni- versität Leoben, to ensure surface conductivity. The images of the zeolite surface were taken with a field emission gun (FEG) SEM LEO 1525 (Carl Zeiss AG, Oberkochen, Ger- many) at the Erich Schmid Institute of Materials Science (ESI). 3. Results Table 1 gives an overview of the experimental runs (a–f) and their initial parameters which are compared in detail in the following Figures 1–5. Experiments (a1) and (a2) are + −1 ammonium exchange experiments with 259 and 514 mg NH4 L , which represent basic ChemEngineering 2021, 5, 68 4 of 12 experiments to determine and compare the single exchange performances with simulta- neous removal experiments. All experiments were performed at least in duplicate. 14 7.0 6.5 Ammonium abs. Calcium des. 6.0 Potassium des. Magnesium des. Sodium des. Sum of des. ions 5.5 pH 0 5.0 0 2 4 6 8 10 12 14 16 18 20 22 24 Time [h] + 2+ + 2+ + Figure 1. Equivalent concentrations of absorbed (NH4 ) and desorbed (Ca , K , Mg , Na ) cations over time. Line of absorbed ammonium is shown as the exchanged amount of equivalent concen- + −1 tration. Initial concentration: 514 mg NH4 L . 3- PO on zeolite NH on zeolite pH 0 2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 Time [h] Figure 2. Effect of initial ammonium concentration at pH = 8 on P-removal. Initial values of experiments: (b) black solid + −1 3− −1 + −1 3− −1 lines: 544 mg NH4 L , 122 mg PO4 L , pH = 8.00; (c) red dashed lines: 257 mg NH4 L , 126 mg PO4 L pH = 8.02. -1 Zeolite loading [mg species g ] zeo -1 Equivalent concentration [meq L ] pH [-] pH [-] ChemEngineering 2021, 5, 68 5 of 12 3- PO on zeolite 2 + NH on zeolite NH single 1 4 pH 0 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 Time [h] Figure 3. Effect of initial phosphate concentration at pH = 8 on P-removal. Initial values of experiments: black solid lines— + −1 3− −1 + −1 3− −1 257 mg NH4 L . 126 mg PO4 L pH = 8.02; red dashed lines: 267 mg NH4 L . 245 mg PO4 L pH = 8.00; green solid + −1 line: 259 mg NH4 L (ammonium only). 9 9 8 8 7 7 6 6 5 5 4 4 3 3 3- PO on zeolite 2 2 NH on zeolite 1 1 pH 0 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 Time [h] + −1 Figure 4. Effect of initial pH value on P-removal. Initial values of experiments: black solid lines: 257 mg NH4 L . 126 mg 3− −1 + −1 3− −1 PO4 L pH = 8.02, removal rates: P = 61%, N = 65%; red dashed lines: 250 mg NH4 L . 125 mg PO4 L pH = 9.00, removal rates: P = 71% N = 43%. -1 -1 Zeolite loading [mg species g ] Zeolite loading [mg species g ] zeo zeo pH [-] pH [-] ChemEngineering 2021, 5, 68 6 of 12 9 9 8 8 7 7 6 6 5 5 4 4 3 3 3- PO on zeolite NH on zeolite 2 2 pH 1 1 0 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 Time [h] Figure 5. Effect of initial pH value at higher phosphate concentration. Initial values of experiments—black solid lines: 267 + −1 3− −1 + −1 3− mg NH4 L . 245 mg PO4 L pH = 8.00, removal rates: P = 56%, N = 66%; red dashed lines: 263 mg NH4 L . 246 mg PO4 −1 L pH = 9.00, removal rates: P = 34%, N = 46%. Table 1. Overview of initial parameters and zeolite loading at equilibrium and after 120 min. Initial Parameters Zeolite Loading Figure Experiment Ammonium Phosphate pH qN,eq qP,eq qP(120′) + −1 3− −1 + −1 3− −1 3− −1 (g NH4 L ) (mg PO4 L ) (-) (mg NH4 g ) (mg PO4 g ) (mg PO4 g ) 1 a1 514 - 5.47 10.49 - - b 544 122 8.00 12.53 4.78 0.95 c 257 126 8.02 8.32 3.84 0.77 c 257 126 8.02 8.32 3.84 0.77 3 d 267 245 8.00 8.76 6.78 1.60 a2 259 - 8.03 8.30 - - c 257 126 8.02 8.32 3.84 0.77 e 250 125 9.00 5.34 4.43 2.14 d 267 245 8.00 8.76 6.78 1.60 f 263 246 9.00 6.01 4.09 2.67 3.1. Kinetics of Desorbed Cations Figure 1 shows the cation kinetics, i.e., the amount of absorbed NH4 during the single ammonium exchange experiment (a1) in contrast to the type and the number of cations desorbed from the zeolite. Based on the cation kinetics, it is obvious that calcium is the dominant cation in the exchange reaction with untreated natural zeolite. After 4 h, zeolite −1 + −1 2+ −1 + −1 absorbed 9.69 meq L NH4 and released 5.99 meq L Ca , 2.56 meq L K , 0.61 meq L 2+ −1 + Mg and 0.39 meq L Na into the solution. As a result of ammonium sorption, pH in- creased from 5.47 to 6.39 during the experiment (24 h). The decreased ammonium concen- tration in the solution leads to a shifted NH3/NH4 equilibrium, which was readjusted by consuming hydronium ions (Equation (2)). The red solid line in Figure 1 shows the sum −1 of the desorbed cations in meq L . Compared to the line of absorbed ammonium (square indicators), it indicates the overall charge balance of the sorption process. + + 𝑁𝐻 ↔ 𝑁𝐻 + 𝐻 (2) 4 3 -1 Zeolite loading [mg species g ] zeo pH [-] ChemEngineering 2021, 5, 68 7 of 12 3.2. Simultaneous Exchange Experiments (N & P) In the following chapters, the results of simultaneous exchange experiments are de- scribed by comparing two experiments in each figure. Consequently, many of the oc- curred effects will be much easier to detect. Experiment (c) of Table 1 is defined as a stand- ard experiment because, with these initial parameters, the first reproducible higher P- loadings on zeolite were achieved. The subsequent parameter study was based onto these initial parameters. 3.2.1. Effect of Initial Ammonium Concentration on N- and P-Removal The first simultaneous exchange experiments show the impact of initial ammonium concentration on phosphate removal as indicated by experiments (b) and (c) (Figure 2). In contrast to the experiment (a1), pH values decreased in both simultaneous experiments from 8 to 6.75 and 6.86, respectively. Experiment (b) starts with an ammonium concentra- + −1 + tion of 544 mg NH4 L and reached a zeolite N-loading at equilibrium of 12.53 mg NH4 −1 + −1 g (circle indicators). Experiment (c) starts with 257 mg NH4 L and reached 8.32 mg + −1 NH4 g at equilibrium. Hence, higher initial N-concentration results in a higher N-load- ing but does not lead to more efficient N-removal (e.g., removal rate (b) 46% (c) 65%). At the beginning the sorption process, ammonium has a steep progression due to high con- centration imbalance between zeolite and synthetic wastewater (high driving force for ion exchange). Over time, the exchange capacity of zeolite decreases, the sorption process be- comes slower, and the curve flattens between 4 and 7 h. In contrast, phosphate starts in both experiments at the same initial concentration 3− −1 3− −1 (122 and 127 mg PO4 L ) but different zeolite P-loadings (b: 4.78; c: 3.48 mg PO4 g ) and P-removal rates (b: 80%; c: 61%) are reached at equilibrium (square indicators). The dissolution and recovery of the formed phosphate compound starts immediately after im- mersing the zeolite stirrer into diluted sulfuric acid (pH 2.5). After 4 h of P-recovery the 3− −1 zeolite loading stabilized at 0.18 and 0.26 mg PO4 g . At the end, the recovery solution 3− −1 comprises phosphate concentrations of 91 and 71 mg PO4 L , thus 75 and 57% of phos- 2+ phate were recovered, respectively. Significant concentrations of Ca were detected in the + + 2+ recovery solution, whereas K , Na and Mg were below detection limits. 3.2.2. Effect of Initial Phosphate Concentration at pH 8 on N- and P-Removal Figure 3 shows the effect of the initial phosphate concentration on P-removal. Both + −1 experiments start with an ammonium concentration of (c) 257 mg NH4 L and (d) 267 mg + −1 NH4 L with a pH of 8. Ammonium sorption reveals similar progression and removal rates (c: 65%; d: 66%) in both experiments. For comparison, the green curve (triangle indi- cators) represents an exchange experiment (a2) with ammonium only (initial concentra- + −1 tion: 259 mg NH4 L ). Therefore, a simultaneous P-removal has no effect on the ammo- nium exchange. Every ammonium loading line of a simultaneous experiment was ap- proved with an additional ammonium experiment to check if a similar N-loading can be reached on zeolite. 3− −1 3− −1 Initial phosphate concentrations were 126 mg PO4 L and 245 mg PO4 L . Experi- ment (d) has a faster progression in P-removal and reaches higher equilibrium P-loading 3− −1 3− −1 of 6.78 mg PO4 g compared to (c) 3.84 mg PO4 g . Consequently, higher initial phos- phate concentrations lead to higher P-loadings on the zeolite at pH 8. Although the P- removal rate of (d) at 56% is slightly lower than (c) at 61%, substantially more phosphate was removed and recovered in (d). Consequences of a higher P-loading are also seen in acid consumption during regen- eration. The dissolution process of precipitated phosphate compounds in (d) is slower and pH value increases more than in (c), which indicates that more acid is needed to dissolve the higher amount of precipitated phosphate. ChemEngineering 2021, 5, 68 8 of 12 3.2.3. Effect of Initial pH Value on N- and P-Removal Obviously, decreasing pH value is an indicator that hydronium ions participate or have an impact on the precipitation of phosphate. A higher pH value could affect the speed or equilibrium of the reaction positively. Figure 4 shows the standard experiment (c) compared to experiment (e), which has the same initial concentrations of N and P (250 + −1 3− −1 mg NH4 L , 125 mg PO4 L ) but starts at pH 9. There are two clearly visible differences in the results of experiment (c) and (e), which show the impact of higher pH value on simultaneous removal. First, ammonium sorption at equilibrium is significantly lower at higher pH values (e) as the NH3/NH4 equilibrium is shifted to ammonia and less ammonium ions are available for the sorption process. Sec- ond, the progression of phosphate removal is influenced by the pH value, compared to (c), the trend of P-removal in experiment (e) is much more like the N-removal curve, a reaction which starts at a higher speed and slows down when it strives for equilibrium. With 71%, the P-removal rate of (e) is higher compared to (c) with a value of 61%. Regen- eration shows again slower dissolution and higher acid consumption in (e), which indi- cates a higher amount of precipitated phosphate on zeolite. In short, phosphate removal at pH 9 is much faster and more efficient than at pH 8. Contrastingly ammonium sorption is less efficient, due to shifted NH3/NH4 equilibrium. 3.2.4. Effect of Initial pH at Higher Phosphate Concentration on N- and P-Removal Previous experiments showed that high initial phosphate concentration leads to ele- vated zeolite P-loadings in equilibrium (3.2.2). Figure 5 shows the impact of pH on N- and P-removal at high initial phosphate concentration by comparing experiment (d) and (f). Experiment (d) reached the highest P-loading of all experiments at initial phosphate con- 3− −1 centration of 245 mg PO4 L and pH 8. The same initial N and P concentrations were used in experiment (f) at pH 9. As determined in Section 3.2.3 higher pH leads to lower N-loading on the zeolite and overall lower N-removal, which were in (d) 66% and in (f) 46% in state of equilibrium. In the first 3 h P-removal at pH = 9 is faster and more efficient 3− than P-removal at pH = 8. In an equilibrium state, P-loadings of zeolite are 6.78 mg PO4 −1 3− −1 g in (d) and 4.09 mg PO4 g in (f), which reveals the relative P-removal rates of 56% and 34%, respectively. In summary, the following results were obtained from the laboratory experiments: • Calcium is the preferred exchange ion in ammonium sorption; • P removal is a precipitation reaction combined with ammonium sorption; • High initial phosphate concentrations lead to high P-loadings; • Speed of phosphate precipitation is controlled by pH; • Ammonium sorption reduces at pH > 9. 3.2.5. Observations Regarding the Zeolite Surface Two zeolite samples were analyzed by FEG-SEM. The surface of an unloaded natural zeolite is shown in Figure 6a. After simultaneous removal experiments, surface changed significantly. A fine textured crystal phase precipitated all over zeolite surface (Figure 6b). 3− −1 The second sample has a P-loading of 6.59 mg PO4 g , which is equal to the loading of experiment (d). ChemEngineering 2021, 5, 68 9 of 12 (a) (b) Figure 6. FEG-SEM pictures of: (a) unloaded natural zeolite and (b) N & P loaded zeolite with precipitated CaP-phases. 4. Discussion As shown in Figure 1, calcium is the preferred, desorbed cation in ammonium sorp- tion and so, higher initial ammonium concentration leads to higher amounts of desorbed calcium ions. Analysis of the regeneration solution revealed that only calcium and phos- phate were dissolved from the zeolite. Consequently, phosphate and calcium ions form a compound which is soluble in acid solutions. Significant concentrations of other ions such + + 2+ + as NH4 , K , Mg or Na were not found in the regeneration solution, so the formation of magnesium ammonium phosphate (MAP) as a potential P-precipitate can be excluded. A doubled initial ammonium concentration results in a 19% higher P-removal rate at the same initial phosphate concentration. In short, higher initial ammonium concentration potentially leads to higher calcium desorption and further to higher P-removal (Figure 2), correlating to the findings of Lin et al. [23]. Higher initial phosphate concentration affects higher P-loading on zeolite (Figure 3), but the removal rate is lower than in experiment (c). Ammonium sorption reveals the same N-loadings in both experiments (c) and (d). Compared to the single ammonium ex- periment (a2), simultaneous N and P removal has no impact on N-removal. An essential finding in Figure 3 is that P-loading is faster and higher in (d) with an equal calcium avail- ability in case of equal initial ammonium concentrations. Hence, phosphate is the limiting factor in Figure 2b, because a sufficient amount of calcium was desorbed due to higher ammonium sorption. Increasing the initial pH value from 8 to 9 has a considerable effect on the rate of phosphate removal at the beginning of the experiment (Figure 4). Calcium phosphates (e.g., brushite and hydroxyapatite) are highly soluble in acid solutions, which could affect the slightly irregular progression at pH 8 compared to pH 9. Hermassi et al. [20] demon- strated that higher pH value encouraged the formation of hydroxyapatite and a lower pH the formation of brushite. Additionally, Macha et al. [24] detected a solubility minimum for differing calcium phosphates in the range of pH 8. In preliminary tests at pH < 7, it was not possible to precipitate phosphate on zeolite. All these findings lead to the possible chemical reaction (Equation (3)) formulated by Loehr et al. [25] 2+ − − 5𝐶𝑎 + 4 + 3 ↔ 𝐶𝑎 ( ) ↓ +3𝐻 𝑂 (3) 4 5 4 3 2 This reaction is slow between pH 7 and 9. Higher pH values increase the precipitation of calcium phosphates (Figure S1), correlating to Lin et al. [23] A disadvantage of high pH value expresses in a lower ammonium sorption at pH 9, as a result of a shifted NH3/NH4 equilibrium. A further increase in the pH value led to a desorption of gaseous ammonia detected via ammonia warning device and accompanied by the typical strong smell. Un- noticed loss of gaseous ammonia would result in a falsely higher N-loading on zeolite, 𝑃𝑂 𝑂𝐻 𝐻𝑃𝑂 𝑂𝐻 ChemEngineering 2021, 5, 68 10 of 12 due to lower photometrically detected ammonium concentrations in the solution. Hence, pH 9 at 25 °C is the limit for ammonia removal with this laboratory setup to ensure no loss of ammonia. In Figure 5 two significant parameters to reach a fast and high P-loading are com- bined (high pH and high initial phosphate concentration). Compared to experiment (e), phosphate precipitation in (f) is even faster at the beginning (qP(120′) in Table 1: (e) 2.14 and 3− −1 (f) 2.67 mg PO4 g ), due to high initial parameters. At equilibrium state P-loading of (f) is lower than (d) and even lower than (e), though initial phosphate concentration is dou- bled. Desorbed calcium reacts with dissolved phosphate near the zeolite surface and after simultaneous N- and P-removal, the whole surface is covered with precipitated calcium phosphates (Figure 6b). As a result of faster precipitation at pH 9, calcium phosphates probably form a denser layer on the zeolites surface and therefore reduce the area of ion exchange and affect low calcium desorption. The denser layer of calcium phosphate and low ammonium sorption at pH 9 lead to calcium limitations and finally to a low P-removal in experiment (f). No abrasion of zeolite or precipitated calcium phosphates were detected in the reac- tor, which proves the functionality of the constructed stirrer to determine kinetics without affecting the particle size of zeolite. When the stirrer was washed with distilled water be- tween N- and P-loading and P-regeneration, only small losses of phosphate (<0.70 mg 3− −1 PO4 g ) occurred. This loss was detected as the difference between the amount of re- moved phosphate from the synthetic wastewater and the amount of recovered phosphate in regeneration solution. The successful P-removal and regeneration of each experiment was also confirmed by the remaining P-loadings on the zeolite, because solutions were completely exchanged between removal and regeneration and the majority of removed phosphate was found in regeneration solution. After N- and P-loading, a white coating covered the inner bag (pp net) of the stirrer, which could not be removed by brushing or other mechanical stress. Dipping the inner bag into diluted sulfuric acid removed all of the white coating. Chemical analysis of sul- furic acid revealed that the white coating consists of calcium phosphate. The mechanical stability of these calcium phosphates underlines that hydraulic effects or higher flow rates in fixed bed columns do not remove precipitated phosphate from the zeolite surface. 5. Conclusions In this study, natural zeolite was used to recover N and P from synthetic wastewater. The removal process shows great opportunity for a separate recovery of N and P. Exper- iments led to a better understanding of the P-removal mechanism and impacts of different parameters like pH value, ammonium sorption and initial phosphate concentration. A pH value between 8 and 9 reveals high and fast P-removal rates, which enables the applica- tion of a combined N- and P-removal in real wastewaters (e.g., sludge liquor, biogas di- gestates, pig manure), which are in the same range of pH. In the ILS process ammonium is recovered from the zeolite by sodium hydroxide using reversed ion exchange of ammo- nium sorption [26]. As precipitated calcium phosphate only dissolves in acid solutions, ammonium recovery in the ILS process with basic solutions has no impact on zeolites P- loading. On the other hand, separate phosphate recovery under acidic conditions is pos- sible and would be a big advantage for nutrient recovery in order to enable many options for a further utilization of each nutrient. Supplementary Materials: The following are available online at www.mdpi.com//5/4/68/s1, Figure S1: Ammonium and phosphate loading over time at pH 7 / 8 / 9. Author Contributions: Conceptualization, S.P. and M.E.; methodology, S.P.; data curation, S.P.; writing—original draft preparation, S.P.; writing—review and editing, M.E.; visualization, S.P. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by FFG—The Austrian Research Promotion Agency, grant num- ber 864876 (ReNOx2.0). ChemEngineering 2021, 5, 68 11 of 12 Acknowledgments: The authors acknowledge the support during experiments in laboratory and helpful discussions at the office by Manuel Wassertheurer, Thomas Braunsperger and Stefan Nie- dermayer. Furthermore, thanks are also due to Kristina Stocker, Maik Zimmermann and Daniel Kiener for sample preparation and the visualization of the zeolite surface via FEG-SEM. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manu- script, or in the decision to publish the results. Reference 1. World Fertilizer Trends and Outlook to 2022; Food & Agriculture Organization of United Nations: Rome, Italy, 2019; ISBN 978-92-5-131894-2. 2. Withers, P.J.A.; Haygarth, P.M. Agriculture, phosphorus and eutrophication: A European perspective. Soil Use Manag. 2007, 23, 1–4. https://doi.org/10.1111/j.1475-2743.2007.00116.x. 3. Carpenter, S.R.; Caraco, N.F.; Correll, D.L.; Howarth, R.W.; Sharpley, A.N.; Smith, V.H. Nonpoint Pollution of Surface Waters with Phosphorus and Nitrogen. Ecol. 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Water Process Eng. 2019, 32, 100909. https://doi.org/10.1016/j.jwpe.2019.100909. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png ChemEngineering Multidisciplinary Digital Publishing Institute

Kinetics of Simultaneous Ammonium and Phosphate Recovery by Natural Zeolite

Pesendorfer, Sandro; Ellersdorfer, Markus
ChemEngineering , Volume 5 (4) – Oct 8, 2021
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Article Kinetics of Simultaneous Ammonium and Phosphate Recovery by Natural Zeolite Sandro Pesendorfer * and Markus Ellersdorfer Chair of Process Technology and Industrial Environmental Protection, Montanuniversität Leoben, 8700 Leoben, Austria; markus.ellersdorfer@unileoben.ac.at * Correspondence: sandro.pesendorfer@unileoben.ac.at Abstract: Nowadays, fertilizers containing nitrogen and phosphorus are indispensable for medium and large-scale industrial agriculture. To meet the growing demand of nutrients and reduce the accompanied ecological footprint of primary fertilizer production, processes and technologies for nutrient recovery are necessary and have to be developed. This study represents the basis of an extension of the ion-exchange-loop-stripping process (ILS), which is a combined stripping and ion exchange process using natural zeolite for nitrogen recovery. In batch experiments with a special zeolite filled stirrer, the mechanism and kinetics of simultaneous ammonium and phosphate recov- 3− −1 ery by natural zeolite were determined. Zeolite loadings of 6.78 mg PO4 g were reached and after regeneration, phosphate recovery rates up to 75% of the initial concentration were achieved. The speed of phosphate precipitation is mostly controlled by the pH value of synthetic wastewater. Phosphate removal in simultaneous experiments does not affect ammonium sorption onto zeolite. These findings and the different removal mechanisms of ammonium and phosphate lead to versa- Citation: Pesendorfer, S.; tile applications in wastewater treatment and reveal great potential of natural zeolite in simultane- Ellersdorfer, M. Kinetics of ous nutrient recovery processes. Simultaneous Ammonium and Phosphate Recovery by Natural Keywords: nutrient recovery; wastewater treatment; clinoptilolite Zeolite. ChemEngineering 2021, 5, 68. https://doi.org/10.3390/ chemengineering5040068 1. Introduction Academic Editor: Luis M. Gandía As a result of the constantly growing world population, the demand of organic and Received: 14 September 2021 inorganic fertilizer for food production increases every year [1]. The major macronutrients Accepted: 5 October 2021 nitrogen (N) and phosphorus (P) are produced in energy and resource intensive pro- Published: 8 October 2021 cesses, e.g., the Haber-Bosch synthesis for nitrogen. On the other hand, overfertilization and the resulting eutrophication of surface and subsurface water present a global and Publisher’s Note: MDPI stays neu- persistent environmental problem [2,3]. Nutrient amounts in organic fertilizers like ani- tral with regard to jurisdictional mal manure or biogas digestates are subject to fluctuations in process conditions, thus it claims in published maps and institu- is hard to fertilize plants with the appropriate quantity of nutrients. Additionally, harmful tional affiliations. components in manure and sewage sludge, such as microplastics, germs, antibiotics and heavy metals accumulate in the soil and water. As a result of those problems the direct utilization of organic fertilizers in agriculture like manure, sewage sludge or biogas di- gestates is restricted by legal regulations of the European Commission [4]. Recycling is an Copyright: © 2021 by the authors. Li- essential element in modern industry, hence nutrient recycling by the selective recovery censee MDPI, Basel, Switzerland. of N and P from organic wastewater could be the next step. Nutrient recovery can save This article is an open access article energy in the primary production and prevent the contamination of soil and water by distributed under the terms and con- harmful substances. Industrial scaled processes use the precipitation of struvite to recover ditions of the Creative Commons At- N and P from different wastewater streams [5–7]. Two disadvantages of these precipita- tribution (CC BY) license (http://crea- tion processes are the fixed N:P ratio in struvite and the possibility to separate recovered tivecommons.org/licenses/by/4.0/). ammonium from phosphate. This study shows further development possibilities of the ChemEngineering 2021, 5, 68. https://doi.org/10.3390/chemengineering5040068 www.mdpi.com/journal/chemengineering ChemEngineering 2021, 5, 68 2 of 12 ILS process [8,9] and kinetics of a simultaneous recovery process of ammonium (NH4 ) 3− and phosphate (PO4 ) from aqueous solutions. Ammonium recovery in the ILS process is based on the principle of ion exchange by zeolites especially clinoptilolites, which are micro- and mesoporous-aluminosilicate min- erals. A unique property of natural clinoptilolite is the high selectivity for ammonium adsorption. Its low cost makes it one of the most commonly used zeolites to remove am- monium from wastewater [10–12]. The positively charged ammonium-ion is adsorbed onto the aluminosilicate crystal framework and in exchange the zeolite releases cations 2+ + + 2+ (e.g., Ca , K , Na and Mg ) for charge balance in the aqueous solution. The adsorption + −1 capacity for ammonium on natural zeolite is in the range of 2.7–30.6 mg NH4 g [12]. Few studies investigated the property to remove phosphate from wastewater due to electrostatic attraction, anion exchange or special pretreatment of the zeolite [13–17]. The mechanism of phosphate sorption onto the zeolite is determining the kinetics, capacity and removal rate of dissolved phosphate [18]. Simultaneous N- and P-removal based on phosphate precipitation were performed with synthetic zeolites from fly ash [19–21] and calcined dolomite [22]. Only Lin et al. [23] used natural zeolite with the main mineral cli- noptilolite-Na for simultaneous N- and P-removal in a small scale laboratory setup. Zeo- lite has a very limited mechanical stability, which means in stirred batch processes there is no steady particle size distribution, in case zeolite can move freely. In contrast to others, this study shows batch experiments for simultaneous removal of phosphate and ammo- nium from synthetic wastewaters, in which the zeolite is fixed in a stirrer. The impacts of the pH value and initial concentration of N and P on the kinetics were also investigated. For zeolite regeneration and recovery, precipitated phosphate on the zeolite surface was dissolved in an acidic solution. 2. Materials and Methods 2.1. Materials The zeolite was obtained from a Slovakian deposit and its main mineral is a calcium- bearing clinoptilolite. After crushing and sieving to 0.5–2.5 mm, zeolite was washed with distilled water to remove impurities and small particles formed by mechanical abrasion. Washed zeolite was dried at 105 °C for 24 h and stored in the desiccator. The BET specific 2 −1 surface area of the crushed and washed zeolite is 32.08 m g (nitrogen calibrated, Mi- cromeritics Flowsorb 2300). A total of 30 g of zeolite were filled in a thin, rectangular shaped bag made from polypropylene (PP) net with the dimensions of 62 × 100 mm. The zeolite bag had a thickness of 4–5 mm and was stabilized between two PP frameworks (80 × 116 mm), which were held together by a PP cord. In brief, zeolite is fixed in a self-de- signed stirrer and immersed into synthetic wastewater. The thin layer ensured that the total amount of zeolite is in contact with the synthetic wastewater. Hence, the influence of a potential concentration gradient inside the layer of zeolite grains on the exchange kinet- ics can be neglected. Experiments were carried out in a double jacket glass reactor with an inner diameter of 110 mm and a height of 220 mm. The reactor was tempered to 25 °C by a circulatory cooling unit (Julabo F250). During the experiments the reactor opening was covered with plates (PMMA) to prevent evaporation of the synthetic wastewater. The synthetic wastewaters were a mixture of ammonium di-hydrogen phosphate ((NH4)H2PO4) and ammonium chloride (NH4Cl) dissolved in 2 L of distilled water. To increase the pH to 8 or 9, 1 M sodium hydroxide (NaOH) was added. All used chemicals were of analytical grade. 2.2. Simultaneous Exchange Experiments and P-Regeneration At the beginning of each experiment, the stirrer was dipped into distilled water for 5 min to prevent distortion in the kinetics of the diffusion process due to capillary effects in pores. Exchange experiments were carried out for 24 h with a stirrer speed of 100 rpm. ChemEngineering 2021, 5, 68 3 of 12 The double jacket glass reactor was filled with 1.5 L of synthetic wastewater, which was tempered to 25 °C. At time 0 min, the rotating zeolite stirrer was immersed into synthetic wastewater. After certain instants of time (30 or 60 min), a sample of 5 mL was taken and filtered with a 0.45 µm syringe filter (Rotilabo-PVDF). The filtration ensured a clearly de- fined end of the exchange reaction, because small particles or free moving precipitates could affect the determination of N & P. Preliminary tests have shown that equilibrium is reached between 8 and 20 h. Hence, the samples after 21, 22, 23 and 24 h are defined to determine the equilibrium concentration, which was calculated as the arithmetic mean of these four values in each experiment. To determine the kinetics of desorbed (exchanged) cations of the zeolite, an addi- tional exchange experiment was performed with ammonium only. After the simultaneous removal experiments, the ammonium and phosphate loaded zeolite stirrer was washed with distilled water and drained afterwards. For P-regenera- tion, the precipitated phosphate phases on the zeolite were dissolved in diluted sulfuric acid at pH 2.5. Compared to the exchange experiments, regeneration samples were taken in shorter periods because of faster reaction kinetics. Since the increasing pH slowed down the dissolution, another 2 mL of 1 M sulfuric acid was added after 3 h. P-regeneration 3− −1 ended after 4 to 5 h, when low P-loading (<1 mg PO4 g ) of zeolite was achieved. 2.3. Analytical Methods and Calculations Samples of the solution were diluted according to calibration lines. Afterward, the concentrations of N and P were determined photometrically by using Spectroquant test kits for ammonium (Merck 1.14752.0001) and phosphate (Merck 1.4848.0002). A reference beam photometer (WTW photoLab 7600 UV-VIS) was used in the experiments. The initial concentrations of each synthetic wastewater were determined photometrically in tripli- cate. Temperature and pH value were measured with the pH meter inoLab pH7310. The 2+ + + 2+ desorbed cations (Ca , K , Na , Mg ) were quantified by ICP-OES (ÖNORM EN ISO 11885:2009-11) at the Chair of Waste Processing Technology and Waste Management, Montanuniversiät Leoben. The zeolite loading of ammonium and phosphate was calculated using the difference between initial and temporally decreasing concentration in solution related to the total mass of zeolite (see Equation (1)). (𝑐 − 𝑐 ) ∗ 𝑉 0,𝑒𝑖𝑠𝑐𝑠𝑝𝑒 𝑖 ,𝑒𝑖𝑠𝑐𝑠𝑝𝑒 𝑢𝑡𝑜𝑛𝑙𝑖𝑠𝑜 (1) 𝑞 = 𝑜𝑧𝑒 + −1 3− −1 qi: zeolite loading at t = i (mg NH4 gzeo , mg PO4 gzeo ); mzeo: the amount of zeolite in the stirrer (gzeo); + −1 3− −1 c0,species: the initial concentration of ammonium or phosphate (mg NH4 L , mg PO4 L ); + −1 3− −1 ci,species: concentration at time t of ammonium or phosphate (mg NH4 L , mg PO4 L ); Vsolution: the volume of synthetic wastewater in reactor (L); t: elapsed time (min). 2.4. Scanning Electron Microscope (SEM) Samples of natural zeolite before and after N- and P-removal from synthetic wastewater were sputtered with carbon at the Chair of Resource Mineralogy, Montanuni- versität Leoben, to ensure surface conductivity. The images of the zeolite surface were taken with a field emission gun (FEG) SEM LEO 1525 (Carl Zeiss AG, Oberkochen, Ger- many) at the Erich Schmid Institute of Materials Science (ESI). 3. Results Table 1 gives an overview of the experimental runs (a–f) and their initial parameters which are compared in detail in the following Figures 1–5. Experiments (a1) and (a2) are + −1 ammonium exchange experiments with 259 and 514 mg NH4 L , which represent basic ChemEngineering 2021, 5, 68 4 of 12 experiments to determine and compare the single exchange performances with simulta- neous removal experiments. All experiments were performed at least in duplicate. 14 7.0 6.5 Ammonium abs. Calcium des. 6.0 Potassium des. Magnesium des. Sodium des. Sum of des. ions 5.5 pH 0 5.0 0 2 4 6 8 10 12 14 16 18 20 22 24 Time [h] + 2+ + 2+ + Figure 1. Equivalent concentrations of absorbed (NH4 ) and desorbed (Ca , K , Mg , Na ) cations over time. Line of absorbed ammonium is shown as the exchanged amount of equivalent concen- + −1 tration. Initial concentration: 514 mg NH4 L . 3- PO on zeolite NH on zeolite pH 0 2 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 Time [h] Figure 2. Effect of initial ammonium concentration at pH = 8 on P-removal. Initial values of experiments: (b) black solid + −1 3− −1 + −1 3− −1 lines: 544 mg NH4 L , 122 mg PO4 L , pH = 8.00; (c) red dashed lines: 257 mg NH4 L , 126 mg PO4 L pH = 8.02. -1 Zeolite loading [mg species g ] zeo -1 Equivalent concentration [meq L ] pH [-] pH [-] ChemEngineering 2021, 5, 68 5 of 12 3- PO on zeolite 2 + NH on zeolite NH single 1 4 pH 0 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 Time [h] Figure 3. Effect of initial phosphate concentration at pH = 8 on P-removal. Initial values of experiments: black solid lines— + −1 3− −1 + −1 3− −1 257 mg NH4 L . 126 mg PO4 L pH = 8.02; red dashed lines: 267 mg NH4 L . 245 mg PO4 L pH = 8.00; green solid + −1 line: 259 mg NH4 L (ammonium only). 9 9 8 8 7 7 6 6 5 5 4 4 3 3 3- PO on zeolite 2 2 NH on zeolite 1 1 pH 0 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 Time [h] + −1 Figure 4. Effect of initial pH value on P-removal. Initial values of experiments: black solid lines: 257 mg NH4 L . 126 mg 3− −1 + −1 3− −1 PO4 L pH = 8.02, removal rates: P = 61%, N = 65%; red dashed lines: 250 mg NH4 L . 125 mg PO4 L pH = 9.00, removal rates: P = 71% N = 43%. -1 -1 Zeolite loading [mg species g ] Zeolite loading [mg species g ] zeo zeo pH [-] pH [-] ChemEngineering 2021, 5, 68 6 of 12 9 9 8 8 7 7 6 6 5 5 4 4 3 3 3- PO on zeolite NH on zeolite 2 2 pH 1 1 0 0 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 Time [h] Figure 5. Effect of initial pH value at higher phosphate concentration. Initial values of experiments—black solid lines: 267 + −1 3− −1 + −1 3− mg NH4 L . 245 mg PO4 L pH = 8.00, removal rates: P = 56%, N = 66%; red dashed lines: 263 mg NH4 L . 246 mg PO4 −1 L pH = 9.00, removal rates: P = 34%, N = 46%. Table 1. Overview of initial parameters and zeolite loading at equilibrium and after 120 min. Initial Parameters Zeolite Loading Figure Experiment Ammonium Phosphate pH qN,eq qP,eq qP(120′) + −1 3− −1 + −1 3− −1 3− −1 (g NH4 L ) (mg PO4 L ) (-) (mg NH4 g ) (mg PO4 g ) (mg PO4 g ) 1 a1 514 - 5.47 10.49 - - b 544 122 8.00 12.53 4.78 0.95 c 257 126 8.02 8.32 3.84 0.77 c 257 126 8.02 8.32 3.84 0.77 3 d 267 245 8.00 8.76 6.78 1.60 a2 259 - 8.03 8.30 - - c 257 126 8.02 8.32 3.84 0.77 e 250 125 9.00 5.34 4.43 2.14 d 267 245 8.00 8.76 6.78 1.60 f 263 246 9.00 6.01 4.09 2.67 3.1. Kinetics of Desorbed Cations Figure 1 shows the cation kinetics, i.e., the amount of absorbed NH4 during the single ammonium exchange experiment (a1) in contrast to the type and the number of cations desorbed from the zeolite. Based on the cation kinetics, it is obvious that calcium is the dominant cation in the exchange reaction with untreated natural zeolite. After 4 h, zeolite −1 + −1 2+ −1 + −1 absorbed 9.69 meq L NH4 and released 5.99 meq L Ca , 2.56 meq L K , 0.61 meq L 2+ −1 + Mg and 0.39 meq L Na into the solution. As a result of ammonium sorption, pH in- creased from 5.47 to 6.39 during the experiment (24 h). The decreased ammonium concen- tration in the solution leads to a shifted NH3/NH4 equilibrium, which was readjusted by consuming hydronium ions (Equation (2)). The red solid line in Figure 1 shows the sum −1 of the desorbed cations in meq L . Compared to the line of absorbed ammonium (square indicators), it indicates the overall charge balance of the sorption process. + + 𝑁𝐻 ↔ 𝑁𝐻 + 𝐻 (2) 4 3 -1 Zeolite loading [mg species g ] zeo pH [-] ChemEngineering 2021, 5, 68 7 of 12 3.2. Simultaneous Exchange Experiments (N & P) In the following chapters, the results of simultaneous exchange experiments are de- scribed by comparing two experiments in each figure. Consequently, many of the oc- curred effects will be much easier to detect. Experiment (c) of Table 1 is defined as a stand- ard experiment because, with these initial parameters, the first reproducible higher P- loadings on zeolite were achieved. The subsequent parameter study was based onto these initial parameters. 3.2.1. Effect of Initial Ammonium Concentration on N- and P-Removal The first simultaneous exchange experiments show the impact of initial ammonium concentration on phosphate removal as indicated by experiments (b) and (c) (Figure 2). In contrast to the experiment (a1), pH values decreased in both simultaneous experiments from 8 to 6.75 and 6.86, respectively. Experiment (b) starts with an ammonium concentra- + −1 + tion of 544 mg NH4 L and reached a zeolite N-loading at equilibrium of 12.53 mg NH4 −1 + −1 g (circle indicators). Experiment (c) starts with 257 mg NH4 L and reached 8.32 mg + −1 NH4 g at equilibrium. Hence, higher initial N-concentration results in a higher N-load- ing but does not lead to more efficient N-removal (e.g., removal rate (b) 46% (c) 65%). At the beginning the sorption process, ammonium has a steep progression due to high con- centration imbalance between zeolite and synthetic wastewater (high driving force for ion exchange). Over time, the exchange capacity of zeolite decreases, the sorption process be- comes slower, and the curve flattens between 4 and 7 h. In contrast, phosphate starts in both experiments at the same initial concentration 3− −1 3− −1 (122 and 127 mg PO4 L ) but different zeolite P-loadings (b: 4.78; c: 3.48 mg PO4 g ) and P-removal rates (b: 80%; c: 61%) are reached at equilibrium (square indicators). The dissolution and recovery of the formed phosphate compound starts immediately after im- mersing the zeolite stirrer into diluted sulfuric acid (pH 2.5). After 4 h of P-recovery the 3− −1 zeolite loading stabilized at 0.18 and 0.26 mg PO4 g . At the end, the recovery solution 3− −1 comprises phosphate concentrations of 91 and 71 mg PO4 L , thus 75 and 57% of phos- 2+ phate were recovered, respectively. Significant concentrations of Ca were detected in the + + 2+ recovery solution, whereas K , Na and Mg were below detection limits. 3.2.2. Effect of Initial Phosphate Concentration at pH 8 on N- and P-Removal Figure 3 shows the effect of the initial phosphate concentration on P-removal. Both + −1 experiments start with an ammonium concentration of (c) 257 mg NH4 L and (d) 267 mg + −1 NH4 L with a pH of 8. Ammonium sorption reveals similar progression and removal rates (c: 65%; d: 66%) in both experiments. For comparison, the green curve (triangle indi- cators) represents an exchange experiment (a2) with ammonium only (initial concentra- + −1 tion: 259 mg NH4 L ). Therefore, a simultaneous P-removal has no effect on the ammo- nium exchange. Every ammonium loading line of a simultaneous experiment was ap- proved with an additional ammonium experiment to check if a similar N-loading can be reached on zeolite. 3− −1 3− −1 Initial phosphate concentrations were 126 mg PO4 L and 245 mg PO4 L . Experi- ment (d) has a faster progression in P-removal and reaches higher equilibrium P-loading 3− −1 3− −1 of 6.78 mg PO4 g compared to (c) 3.84 mg PO4 g . Consequently, higher initial phos- phate concentrations lead to higher P-loadings on the zeolite at pH 8. Although the P- removal rate of (d) at 56% is slightly lower than (c) at 61%, substantially more phosphate was removed and recovered in (d). Consequences of a higher P-loading are also seen in acid consumption during regen- eration. The dissolution process of precipitated phosphate compounds in (d) is slower and pH value increases more than in (c), which indicates that more acid is needed to dissolve the higher amount of precipitated phosphate. ChemEngineering 2021, 5, 68 8 of 12 3.2.3. Effect of Initial pH Value on N- and P-Removal Obviously, decreasing pH value is an indicator that hydronium ions participate or have an impact on the precipitation of phosphate. A higher pH value could affect the speed or equilibrium of the reaction positively. Figure 4 shows the standard experiment (c) compared to experiment (e), which has the same initial concentrations of N and P (250 + −1 3− −1 mg NH4 L , 125 mg PO4 L ) but starts at pH 9. There are two clearly visible differences in the results of experiment (c) and (e), which show the impact of higher pH value on simultaneous removal. First, ammonium sorption at equilibrium is significantly lower at higher pH values (e) as the NH3/NH4 equilibrium is shifted to ammonia and less ammonium ions are available for the sorption process. Sec- ond, the progression of phosphate removal is influenced by the pH value, compared to (c), the trend of P-removal in experiment (e) is much more like the N-removal curve, a reaction which starts at a higher speed and slows down when it strives for equilibrium. With 71%, the P-removal rate of (e) is higher compared to (c) with a value of 61%. Regen- eration shows again slower dissolution and higher acid consumption in (e), which indi- cates a higher amount of precipitated phosphate on zeolite. In short, phosphate removal at pH 9 is much faster and more efficient than at pH 8. Contrastingly ammonium sorption is less efficient, due to shifted NH3/NH4 equilibrium. 3.2.4. Effect of Initial pH at Higher Phosphate Concentration on N- and P-Removal Previous experiments showed that high initial phosphate concentration leads to ele- vated zeolite P-loadings in equilibrium (3.2.2). Figure 5 shows the impact of pH on N- and P-removal at high initial phosphate concentration by comparing experiment (d) and (f). Experiment (d) reached the highest P-loading of all experiments at initial phosphate con- 3− −1 centration of 245 mg PO4 L and pH 8. The same initial N and P concentrations were used in experiment (f) at pH 9. As determined in Section 3.2.3 higher pH leads to lower N-loading on the zeolite and overall lower N-removal, which were in (d) 66% and in (f) 46% in state of equilibrium. In the first 3 h P-removal at pH = 9 is faster and more efficient 3− than P-removal at pH = 8. In an equilibrium state, P-loadings of zeolite are 6.78 mg PO4 −1 3− −1 g in (d) and 4.09 mg PO4 g in (f), which reveals the relative P-removal rates of 56% and 34%, respectively. In summary, the following results were obtained from the laboratory experiments: • Calcium is the preferred exchange ion in ammonium sorption; • P removal is a precipitation reaction combined with ammonium sorption; • High initial phosphate concentrations lead to high P-loadings; • Speed of phosphate precipitation is controlled by pH; • Ammonium sorption reduces at pH > 9. 3.2.5. Observations Regarding the Zeolite Surface Two zeolite samples were analyzed by FEG-SEM. The surface of an unloaded natural zeolite is shown in Figure 6a. After simultaneous removal experiments, surface changed significantly. A fine textured crystal phase precipitated all over zeolite surface (Figure 6b). 3− −1 The second sample has a P-loading of 6.59 mg PO4 g , which is equal to the loading of experiment (d). ChemEngineering 2021, 5, 68 9 of 12 (a) (b) Figure 6. FEG-SEM pictures of: (a) unloaded natural zeolite and (b) N & P loaded zeolite with precipitated CaP-phases. 4. Discussion As shown in Figure 1, calcium is the preferred, desorbed cation in ammonium sorp- tion and so, higher initial ammonium concentration leads to higher amounts of desorbed calcium ions. Analysis of the regeneration solution revealed that only calcium and phos- phate were dissolved from the zeolite. Consequently, phosphate and calcium ions form a compound which is soluble in acid solutions. Significant concentrations of other ions such + + 2+ + as NH4 , K , Mg or Na were not found in the regeneration solution, so the formation of magnesium ammonium phosphate (MAP) as a potential P-precipitate can be excluded. A doubled initial ammonium concentration results in a 19% higher P-removal rate at the same initial phosphate concentration. In short, higher initial ammonium concentration potentially leads to higher calcium desorption and further to higher P-removal (Figure 2), correlating to the findings of Lin et al. [23]. Higher initial phosphate concentration affects higher P-loading on zeolite (Figure 3), but the removal rate is lower than in experiment (c). Ammonium sorption reveals the same N-loadings in both experiments (c) and (d). Compared to the single ammonium ex- periment (a2), simultaneous N and P removal has no impact on N-removal. An essential finding in Figure 3 is that P-loading is faster and higher in (d) with an equal calcium avail- ability in case of equal initial ammonium concentrations. Hence, phosphate is the limiting factor in Figure 2b, because a sufficient amount of calcium was desorbed due to higher ammonium sorption. Increasing the initial pH value from 8 to 9 has a considerable effect on the rate of phosphate removal at the beginning of the experiment (Figure 4). Calcium phosphates (e.g., brushite and hydroxyapatite) are highly soluble in acid solutions, which could affect the slightly irregular progression at pH 8 compared to pH 9. Hermassi et al. [20] demon- strated that higher pH value encouraged the formation of hydroxyapatite and a lower pH the formation of brushite. Additionally, Macha et al. [24] detected a solubility minimum for differing calcium phosphates in the range of pH 8. In preliminary tests at pH < 7, it was not possible to precipitate phosphate on zeolite. All these findings lead to the possible chemical reaction (Equation (3)) formulated by Loehr et al. [25] 2+ − − 5𝐶𝑎 + 4 + 3 ↔ 𝐶𝑎 ( ) ↓ +3𝐻 𝑂 (3) 4 5 4 3 2 This reaction is slow between pH 7 and 9. Higher pH values increase the precipitation of calcium phosphates (Figure S1), correlating to Lin et al. [23] A disadvantage of high pH value expresses in a lower ammonium sorption at pH 9, as a result of a shifted NH3/NH4 equilibrium. A further increase in the pH value led to a desorption of gaseous ammonia detected via ammonia warning device and accompanied by the typical strong smell. Un- noticed loss of gaseous ammonia would result in a falsely higher N-loading on zeolite, 𝑃𝑂 𝑂𝐻 𝐻𝑃𝑂 𝑂𝐻 ChemEngineering 2021, 5, 68 10 of 12 due to lower photometrically detected ammonium concentrations in the solution. Hence, pH 9 at 25 °C is the limit for ammonia removal with this laboratory setup to ensure no loss of ammonia. In Figure 5 two significant parameters to reach a fast and high P-loading are com- bined (high pH and high initial phosphate concentration). Compared to experiment (e), phosphate precipitation in (f) is even faster at the beginning (qP(120′) in Table 1: (e) 2.14 and 3− −1 (f) 2.67 mg PO4 g ), due to high initial parameters. At equilibrium state P-loading of (f) is lower than (d) and even lower than (e), though initial phosphate concentration is dou- bled. Desorbed calcium reacts with dissolved phosphate near the zeolite surface and after simultaneous N- and P-removal, the whole surface is covered with precipitated calcium phosphates (Figure 6b). As a result of faster precipitation at pH 9, calcium phosphates probably form a denser layer on the zeolites surface and therefore reduce the area of ion exchange and affect low calcium desorption. The denser layer of calcium phosphate and low ammonium sorption at pH 9 lead to calcium limitations and finally to a low P-removal in experiment (f). No abrasion of zeolite or precipitated calcium phosphates were detected in the reac- tor, which proves the functionality of the constructed stirrer to determine kinetics without affecting the particle size of zeolite. When the stirrer was washed with distilled water be- tween N- and P-loading and P-regeneration, only small losses of phosphate (<0.70 mg 3− −1 PO4 g ) occurred. This loss was detected as the difference between the amount of re- moved phosphate from the synthetic wastewater and the amount of recovered phosphate in regeneration solution. The successful P-removal and regeneration of each experiment was also confirmed by the remaining P-loadings on the zeolite, because solutions were completely exchanged between removal and regeneration and the majority of removed phosphate was found in regeneration solution. After N- and P-loading, a white coating covered the inner bag (pp net) of the stirrer, which could not be removed by brushing or other mechanical stress. Dipping the inner bag into diluted sulfuric acid removed all of the white coating. Chemical analysis of sul- furic acid revealed that the white coating consists of calcium phosphate. The mechanical stability of these calcium phosphates underlines that hydraulic effects or higher flow rates in fixed bed columns do not remove precipitated phosphate from the zeolite surface. 5. Conclusions In this study, natural zeolite was used to recover N and P from synthetic wastewater. The removal process shows great opportunity for a separate recovery of N and P. Exper- iments led to a better understanding of the P-removal mechanism and impacts of different parameters like pH value, ammonium sorption and initial phosphate concentration. A pH value between 8 and 9 reveals high and fast P-removal rates, which enables the applica- tion of a combined N- and P-removal in real wastewaters (e.g., sludge liquor, biogas di- gestates, pig manure), which are in the same range of pH. In the ILS process ammonium is recovered from the zeolite by sodium hydroxide using reversed ion exchange of ammo- nium sorption [26]. As precipitated calcium phosphate only dissolves in acid solutions, ammonium recovery in the ILS process with basic solutions has no impact on zeolites P- loading. On the other hand, separate phosphate recovery under acidic conditions is pos- sible and would be a big advantage for nutrient recovery in order to enable many options for a further utilization of each nutrient. Supplementary Materials: The following are available online at www.mdpi.com//5/4/68/s1, Figure S1: Ammonium and phosphate loading over time at pH 7 / 8 / 9. Author Contributions: Conceptualization, S.P. and M.E.; methodology, S.P.; data curation, S.P.; writing—original draft preparation, S.P.; writing—review and editing, M.E.; visualization, S.P. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by FFG—The Austrian Research Promotion Agency, grant num- ber 864876 (ReNOx2.0). ChemEngineering 2021, 5, 68 11 of 12 Acknowledgments: The authors acknowledge the support during experiments in laboratory and helpful discussions at the office by Manuel Wassertheurer, Thomas Braunsperger and Stefan Nie- dermayer. 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Journal

ChemEngineering – Multidisciplinary Digital Publishing Institute

Published: Oct 8, 2021

Keywords: nutrient recovery; wastewater treatment; clinoptilolite

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