(589f) Efficiency and Feasibility Assessment of Calcined LDH Pyroaurite for Phosphorus Removal from Municipal Wastewaters: Experimental and Simulation-Based Investigation

Phosphorus (P) has been identified as a critical raw material by the European Union due to its crucial role in vegetation growth [1]. However, excessive P presence can lead to harmful phenomena such as eutrophication. To address this, the EU has introduced threshold values, 1 mg L^-1, for P concentrations in municipal wastewater (MWW). Balancing agricultural needs with sustainable environmental practices is challenging, with MWW emerging as a potential solution containing about 7 mg_P L^-1. Current MWW treatment methods for P removal have drawbacks. Chemical precipitation requires expensive reagents and may generate waste, while biological processes are sensitive to environmental conditions and advanced oxidation processes can be energy-intensive. In this context, the adsorption and ion exchange process presents advantages for P removal and recovery. It effectively removes P even at low concentrations, operates in various environmental conditions, and allows for regeneration and reuse of adsorbed P. Additionally, it doesn't require the addition of chemical reagents, reducing the risk of harmful by-products. Furthermore, nutrient removal and recovery via ion exchange have proven economically advantageous compared to traditional biological nutrients removal, which predominates in MWW treatment plants (MWWTPs), with a total cost being lower [2]. Various materials have been experimented with for P removal from MWW, among which Layered Double Hydroxides (LDHs) stand out due to their high adsorption capacity and ability to be regenerated for continued use. LDHs contribute to reducing operational costs and environmental impact in MWWTPs.

Therefore, this study investigates the effectiveness of calcined LDH pyroaurite for P removal from MWWs. It pioneers continuous-flow ion exchange processes tailored for P removal and recovery in real MWW matrices by exploring the adsorption and ion exchange properties of calcined LDH pyroaurite, evaluating its mechanical resistance and chemical stability through repeated cycles of adsorption and desorption, and assessing the performance of the regeneration and P recovery process from the adsorbent material. Additionally, this study aims to scale up the process from a laboratory-scale plant to a pilot-scale plant.

Furthermore, all the experimental data collected were used to simulate the process using the Aspen Adsorption^TM v14 software. Aspen Adsorption^TM accurately simulates solid-liquid phases, optimizing adsorption processes. Its multifunctionality includes advanced equations of state and thermodynamic algorithms, facilitating exploration of various process parameters. Despite limited studies on ion exchange, its potential for efficiency enhancement and sustainability in industrial operations is evident, allowing for a reduction of costs and fostering innovation.

Methods

The main characteristics of LDH pyroaurite are described in [3]. The experimental procedures and methods employed are detailed in [4]. Continuous tests were conducted in a laboratory-scale PVC column measuring 0.013 m in diameter, with the adsorbent material packed to a height of 0.20 m. At the conclusion of preliminary bench-scale testing, the material underwent pilot-plant scale testing in a fully automated facility using a column with a diameter of 0.013 m and a bed height of 0.60 m. All tests were conducted using real MWWs obtained from a MWWTP based in Northern Italy. In the Aspen Adsorption^TM software, input parameters inserted are listed in Table 1 and 2.

Results

Isotherm tests were conducted using real MWWTP effluent as the liquid medium, with an adsorbent concentration of 1 g L^-1. The material was tested in its original state and after repeated BT tests (Figure 1), with isotherms fitted using Langmuir and Freundlich models. The Langmuir model provided the most accurate fit, with model parameters detailed in Table 3. Experimental data indicated favorable isotherms, with high solute adsorption even at low concentrations. The maximum sorption capacity at equilibrium (C_s^∞) was estimated at 20.7 mgP g_{dry adsorbent}^-1 for the virgin material. Differences between isotherms before and after BT tests were minimal and not statistically significant, demonstrating pyroaurite sustained adsorption capacities over multiple cycles. Compared to a hybrid anion exchanger Layne^RT [5], here showed as a benchmark, calcined pyroaurite exhibited superior C_s^∞ and K_eq values, resulting in a steeper and more favorable isotherm.

BT tests for adsorption/desorption were conducted using a packed-bed column fed with effluent. During the adsorption phase, an empty bed contact time (EBCT) of 5 minutes was employed. The results depicted in Figure 2a exhibit remarkable selectivity towards P: all other anions are eluted prior to the appearance of phosphate. Competitive anions exit the column between 8 and 12 hours of operation, returning to inlet concentrations by hour 29. P, on the other hand, begin to elute after approximately 35 hours, indicating the potential for treating substantial MWW within a single adsorption cycle. Pyroaurite demonstrated very good stability, retaining its performances over 4 repeated BT tests, achieving a P adsorption yield of 97% at the breakpoint (BP) 1 mg_P L^-1, with a sorbent utilization efficiency at BP of 67% (Table 4).

The desorption/regeneration procedure was performed eluting the NaOH 0.5 M with EBCT 10 min to regenerate the resin and recover the P rich product (Figure 2b). Phosphates and sulphates co-eluted, with phosphate concentrations reaching about 690 mg L^-1. Chloride desorbed at much lower concentrations, while nitrate levels were minimal. By cutting the P-rich fraction at 28.5 hours, a solution with 210 mg L^-1 of P and 237 of SO₄^2- concentration is obtained, achieving a 34-fold concentration of P, but only 2.4 of SO₄^2-. The P desorption yield resulted to be 65%.

Figure 2a illustrates a comparison between the P adsorption BT curves achieved with bed heights of 20 and 60 cm. The elution sequence remains consistent, with comparable separations between the BT curves of various anions (not depicted). Notably, a slight delay in BT was observed for P during the adsorption phase. A comparison between the performance parameters obtained from the 2 BT tests are shown in Table 4. These results offer promising insights into the feasibility of scaling up the P adsorption process.

The data collected from isotherm and BT tests conducted were employed to perform the simulation of the process with Aspen Asdorption^TM software. The model employed for simulating the BT curve was the Ion Exchange model, which also allows for the simulation of adsorbent regeneration. The choices made to configurate the bed (Table 1) determined the set of equations governing the simulation of the adsorption process. The equation of the isotherm considered, along with its explanation, is provided in Table 1, and the associated parameters inserted as inputs were: the valence ratio between A and B (m) for all ions and the total capacity of the material (Q) (Table 2). Specifically, the value of the Q was obtained experimentally. The equilibrium constants of the ionic compounds (K_AB), the Mass Transfer Coefficient (MCT) and the initial concentration of bicarbonate (HCO₃^2-, not determined experimentally) were estimated using the dedicated function of the software (Table 5). As evident from Figure 2a, a good best-fit was obtained for all ions except for NO₃^-. Indeed, due to its very low concentrations in the MWW, NO₃^- was not included in the objective function that was minimized. The correspondence between simulated and experimental curves confirms the sound choice of the model and input parameters, thereby enabling further parameter adjustments to optimize the process in terms of performance and costs prior to conducting additional experimental tests.

Conclusion

The calcined LDH pyroaurite demonstrated excellent performance in adsorbing P from MWW effluent, as evidenced by favorable isotherms, extreme selectivity for P, and sustained adsorption capacities over multiple cycles. Compared to HAIX Layne^RT, calcined pyroaurite exhibited superior sorption capacity and equilibrium constant values, indicating a more favorable isotherm. The adsorption/desorption BT tests highlighted pyroaurite's remarkable selectivity towards P, with efficient removal of competing anions. Furthermore, the desorption/regeneration procedure successfully recovered P-rich products, achieving a significant concentration increase. These findings suggest the potential for scaling up the P adsorption process using pyroaurite, promising advancements in MWW treatment technologies.

Aspen Adsorption^TM, employing the Ion Exchange model, demonstrated excellent simulation of adsorption processes, guided by experimental data for precise configuration. Despite challenges fitting NO₃^-, agreement with other ions affirmed model efficacy. Estimating equilibrium constants from BT curves enhanced predictability, showcasing Aspen Adsorption^TM 's versatility in system optimization. Next, focus will shift to the simulation of the material regeneration step and process scale-up simulation. Once a robust parameter set is established, optimization aims to boost overall efficiency. This iterative approach holds promise for refining adsorption system design for practical applications.

References

[1] «Critical raw materials - European Commission».

[2] X. Huang, et al., «Economic evaluation of ion-exchange processes for nutrient removal and recovery from municipal wastewater», Npj Clean Water, vol. 3, fasc. 1, pp. 1–10, 2020, doi: 10.1038/s41545-020-0054-x.

[3] F. Cavani, et al., «Hydrotalcite-type anionic clays: Preparation, properties and applications.», Catal. Today, vol. 11, fasc. 2, pp. 173–301, 1991, doi.org/10.1016/0920-5861(91)80068-K.

[4] D. Pinelli et al., «Ammonium recovery from municipal wastewater by ion exchange: Development and application of a procedure for sorbent selection», J. Environ. Chem. Eng., vol. 10, fasc. 6, p. 108829, 2022, doi: 10.1016/j.jece.2022.108829.

[5] D. Pinelli et al., «Regeneration and modelling of a phosphorous removal and recovery hybrid ion exchange resin after long term operation with municipal wastewater», Chemosphere, vol. 286, p. 131581, 2022, doi: 10.1016/j.chemosphere.2021.131581.