Montmorillonite-anchored magnetite nanocomposite for recovery of ammonium from stormwater and its reuse in adsorption of Sc3+

The treatment of stormwater to remove and recover nutrients has received increasing interest. The objective of this study was to develop a novel adsorbent that is easy to handle, has good adsorption capacity, and is economical to use. A novel nanocomposite of montmorillonite (MT)-anchored magnetite (Fe3O4) was synthesised by co-precipitation as an adsorbent for ammonium. The MT/Fe3O4 nanocomposite had pore sizes (3–13 nm) in the range of narrow mesopores. The dispersion of the anchored Fe3O4 was confirmed by transmission electron microscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy (XPS). The nanocomposite exhibited higher affinity towards ammonium than the original MT. The Langmuir isotherm model was found to be the most suitable model to explain the ammonium adsorption behaviour of the nanocomposite. The maximum adsorption capacity for ammonium was 10.48 mg/g. The adsorption mechanism was a combination of ion exchange and electrostatic interaction. In an authentic stormwater sample, the synthesised adsorbent removed 64.2% of ammonium and reduced the amount of heavy metal contaminants including Mn, Ni, Cu and Zn. Furthermore, the ammonium loading on MT/Fe3O4 during adsorption functionalised the adsorbent surface. Additionally, the spent nanocomposite showed potential for rare earth elements (REEs) adsorption as a secondary application, especially for the selective adsorption of Sc3+. The versatile application of montmorillonite-anchored magnetite nanocomposite makes it a promising adsorbent for water treatment.


Introduction
Urban living is replacing rural land use, increasing the size of impervious areas, reducing the availability of arable land, reducing infiltration and leading to increased surface runoff during storm events. Stormwater mobilises pollutant 1 3 55 Page 2 of 14 loads (e.g. suspended solids, nutrients, metals, and oxygen-demanding matter) and transports them to receiving water bodies such as rivers, lakes and ponds [1][2][3]. Nutrients (ammonium and phosphate salts) accumulating in the aquatic environment result in eutrophication and algal blooms, causing the death of aquatic life and degradation of water quality [4]. Ammonium is conventionally removed by biologic processes through nitrification and denitrification. Then, it is finally converted to N 2 but with the presence of by-products such as nitrate [5][6][7]. Moreover, the ammonium content in stormwater varies throughout the year depending on the season and amount of precipitation; this has a negative impact on the biologic treatment. An abiotic process such as adsorption can recover ammonium with low energy consumption as well as deal with the fluctuations in ammonium concentration since adsorbents work efficiently with high concentrations of pollutants as well as trace amounts of contaminants [8][9][10]. Adsorption is also simple to execute and has low energy requirements [11]. For practical applications, adsorbents should be of low cost with high efficiency and easy recoverability.
Clay minerals are inexpensive, non-toxic natural materials which are widely used in the ceramic, paper, rubber, plastics, cosmetics and medicine industries. Among them is montmorillonite (MT), a one-dimensional crystal of aluminosilicate layers with interlayers of alkali metal and earth cations-typically Na + , Ca 2+ , and Mg 2+ . It is a potential adsorbent owing to its large surface area, stable chemical properties and high cation exchange capacity [12,13]. It was reported as an adsorbent with or without modification for the removal of copper (Cu 2+ ) [14] and ammonium [15]. The net negative charge on the structure of MT can attract and capture cations, making it a natural candidate for ammonium adsorption.
Recently, many efforts have been made for synthesis of adsorbent material by the incorporation of magnetic nanoparticles. As one of the popular inorganic nanoparticles [16] for wastewater treatment, magnetite (Fe 3 O 4 ) possesses a high adsorption capacity and a fast adsorption rate. Furthermore, its magnetic feature allows for easy recovery after use [17,18]. It was recently investigated as an adsorbent for the removal of cadmium (Cd 2+ ) [19]. Additionally, an ammonium-pillared MT/Fe 3 O 4 nanocomposite was synthesised for caesium (Cs + ) removal from water and soil [20]. Moreover, the semiconductor properties of magnetite provide possible photocatalytic reactions-such as the degradation of organic pollutants-as an extra merit [21][22][23][24].
To the best of our knowledge, the use of the nanocomposite form of MT and Fe 3 O 4 as adsorbents for ammonium recovery has not been reported. In this study, MT/Fe 3 O 4 nanocomposites were synthesised using the co-precipitation method which consumes little power and is environmentally friendly [25] as a low-cost, non-toxic, easy-to-handle adsorbent for ammonium recovery in stormwater treatment. The characterisation of adsorbents-adsorption performance under various dosages, contact time, initial pH of ammonium solutions, initial concentration of ammonium solutions, kinetics, isotherm models, and stormwater treatment-has been studied thoroughly. Likewise, the ammonium-loaded nanocomposite was applied as an adsorbent for rare earth elements (REEs) recovery, especially for the selective adsorption of Sc 3+ .

Synthesis of MT/Fe 3 O 4 nanocomposites
The method for the synthesis of Fe 3 O 4 and MT/Fe 3 O 4 nanocomposite was adapted from previous studies with minor modifications [20]. First, 1.8 g of FeCl 2 ·4H 2 O was dissolved in 150 mL of deionised water. Then, MT was added to the solution in an ultrasonic bath for 10 min, and the mixture was stirred and heated in an oil bath for 20 min. Next, 2.7 g of FeCl 3 ·6H 2 O was dissolved in the mixture and stirred for 30 min under N 2 flux at 80 °C, then 50 mL of NaOH (1.6 mol/L) solution was pumped into the mixture solution at a constant rate (0.5 or 2 mL/min) and stirred under N 2 atmosphere for times varying from 2 to 4 h. After the reaction, the suspension was centrifuged and washed thoroughly in ethanol and water. Finally, the solid product was dried in a hot air oven (Termaks) for 24 h at 60 °C.

Characterisation of adsorbent
MT/Fe 3 O 4 nanocomposites synthesised in different conditions were thoroughly characterised using different techniques. The phase identification was done by X-ray Diffraction (XRD) in a PANalytical X-ray diffractometer using Co Kα irradiation at λ = 1.79 Å with 2θ ranging from 5° to 80° at 40 kV and 40 mA. The magnetic features of the synthesised adsorbents were characterised using a vibrating sample magnetometer (VSM, Princeton Measurements Micromag Model 3900); the field applied ranged from − 1.2 T to 1.2 T. Transmission electron microscopy (TEM) was conducted using a Hitachi HT-7700 at 100 kV. Its morphological and elemental characteristics were evaluated through scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) in a Hitachi S-4800 microscope at 20 kV. Surface functional groups were identified by Fourier transform infrared spectroscopy (FTIR) using the Bruker Vertex 70 model in a spectra range of 4000-400 cm −1 . X-ray photoelectron spectroscopy (XPS) was done using a Thermo Fisher Scientific ESCALAB 250Xi (Thermo Fisher Scientific, UK), with an X-ray source of monochromatic Al Kα (1486.6 eV). The specific surface area and pore size distribution were determined by the N 2 Adsorption-desorption Isotherm (Tristar® II Plus). Samples were degassed at 60 °C under N 2 overnight and analysed using liquid nitrogen (77 K). The surface charge was investigated through a Surface Zeta Potential (ζ) study using a Malvern Zeta sizer Nano ZEN350 model; a 2 mg sample was dispersed in 10 mL of deionised water, and the pH was adjusted to 1-10 by HCl and NaOH.

Batch adsorption experiments for ammonium removal
The stock solution of ammonium was prepared by dissolving NH 4 Cl in deionised water as 1,000 mg/L. The working solutions with desired concentrations were diluted from the stock solution. All the adsorption experiments were conducted using 15 mL tubes by mixing 10 mL of working solutions with known amounts of adsorbent. The tubes were shaken in an orbital shaker (IKA KS 4000 ic control) at a speed of 300 rpm for a given time at 25 °C or 35 °C. After shaking, the mixture was filtered using a 0.45 µm cellulose acetate syringe filter and analysed through Ion Chromatography (IC, Shimadzu; Shodex IC YS-50 column, column oven temperature 40 °C, 4 mM methanesulphonic acid eluent, pump flow 1 mL/min) for the concentrations of ammonium. The iron leaching from the adsorbents under different initial pH was analysed by inductively coupled plasma-optical emission spectrometry (ICP-OES, Thermo iCAP 6300 series) to determine the concentrations of iron in the treated solution after adsorption. The effects of adsorbent dosage, contact time, initial concentration and initial pH of working solutions were studied. The initial pH of working solutions was investigated in a range of 2 to 12 and adjusted by adding HCl (1 M and 0.1 M) and NaOH (1 M and 0.1 M). Adsorption experiments were also performed using real stormwater to investigate the efficiency of MT/Fe 3 O 4 nanocomposites.
The amount of ammonium adsorbed was calculated from the mass balance, assuming constant liquid phase density: where q e (mg/g) is the adsorbed amount, V L (L) is the volume of liquid phase, m ads (g) is the mass of adsorbent, and C 0 and C e (mg/L) are the ammonium concentrations initially and at equilibrium, respectively. The removal efficiency RE was calculated by the following equation:

Preliminary adsorption experiments
Several batches of nanocomposite adsorbents were prepared by varying the amount of MT, the reaction time, and the flow rate of NaOH addition as listed in Table 1. The table also shows the preliminary characterisation of the surface area using the BET method. Preliminary adsorption tests were conducted to determine the adsorbent batch with the highest uptake of NH 4 + . The initial NH 4 + concentration was 50 mg/L, and the solid-toliquid phase ratio was kept low (1:500 g/L) to avoid adsorbing all of ammonium which would yield only small differences in uptake between the adsorbent batches. Both the original MT and the prepared Fe 3 O 4 showed limited removal efficiency of NH 4 + at 4.90% and 2.52%, respectively, while the synthesised adsorbents were able to remove NH 4 + with various efficiencies ranging from 16.58% to 37.20%.
The performance of the synthesised adsorbents with respect to NH 4 + adsorption increased with retention time and the amount of MT used during synthesis. A possible reason for this is that the longer retention time ensures better dispersion of Fe 3 O 4 on the surface of the MT. The higher flow rates of NaOH pumped to the reaction lead to higher specific surface area (S BET ), but the NH 4 + uptake ability changes depending on the amount of MT. The obvious improvement of the NH 4 + uptake ability of MT after the modification indicates that the addition of iron creates and stimulates new active sites. However, excessive Fe 3 O 4 may cover partial active sites resulting in lower removal efficiency of NH 4 + . As shown in Table 1, MT/Fe 3 O 4 604 has the highest adsorption towards NH 4 + . Therefore, further studies were conducted with sample MT/Fe 3 O 4 604 and reaction conditions were optimised for the enhancement of ammonium removal efficiency. The spent (ammonium loaded) adsorbent was then named MT/Fe 3 O 4 604N.

Structure and magnetic properties
The The magnetic properties of the prepared MT/Fe 3 O 4 products were evaluated by VSM, as shown in Figs. 1b and S2. All of the products exhibited superparamagnetic properties with extremely narrow hysteresis loops [33,34]. Moreover, sample MT/Fe 3 O 4 517 (Fig. S2) had a relatively high saturation magnetisation, which is favourable for magnetic separation. The Fe 3 O 4 aggregated to larger sizes due to the relatively high amount of it, resulting in low removal efficiency of MT/Fe 3 O 4 517. It was not selected as the sample to be further studied since the more important property is the high adsorption capacity. Although the magnetic moments of the products are relatively low, their magnetic feature can accelerate separation in the after-adsorption processes.

Morphology and composition analysis
The TEM images showed the sheet-like nature and small number of rod-like structures of MT (Fig. 2a) as well as the  (Fig. 3c), except for that the surface of MT/ Fe 3 O 4 604N (Fig. 3d) was less covered by Fe 3 O 4 particles which was corresponded by the iron leaching phenomenon (Fig. 6a).
The surface composition was analysed by XPS, as shown in Fig. 4a-b. Peaks at 724.9 eV and 711.4 eV-as two typical spectra of Fe 2p 1/2 and Fe 2p 3/2 , respectively-confirmed the existence of Fe 3 O 4 in the as-synthesised adsorbents [36,37]. The Mg 1s peak was present in the spectrum of MT, but barely showed after modification, indicating that Fe took the place of Mg. As expected, N 1s appeared in the spectrum of MT/Fe 3 O 4 604N after adsorption, as evidence of the existence of NH 4 + on the surface of the MT/Fe 3 O 4 nanocomposite. The characteristic peak of Si 2p verified the composition of MT [20]. The binding energy of Si 2p increased by 1.  (Fig. S3), coming from the NaOH used in the modification. During the adsorption process, Na was released to the aqueous solution, so there was a low-intensity peak of Na 1s in the used adsorbent (MT/Fe 3 O 4 604N).
The infrared spectra of MT, Fe 3 O 4 and the adsorbent before and after the adsorption of NH 4 + are shown in Fig. 5a. The band at 544 cm −1 in the spectra of Fe 3 O 4 corresponds to the characteristic band of magnetite, which is attributed to the mixture of Fe 2+ and Fe 3+ in the sample [38,39]. The characteristic strong bands at 1,031 and 449 cm −1 of MT are assigned to the stretching of Si-O. The Si-O bands are present in the spectra of modified samples at around 1000 cm −1 . The bands showed at around 3600 and 1630 cm −1 are related to the stretching and bending of hydroxyl groups, respectively [40,41]. The shifting of Si-O and hydroxyl groups from MT to MT/Fe 3 O 4 604 indicated that the loaded Fe 3 O 4 encountered these groups on the surface of MT. One new band appeared in the spectra of the after-adsorption sample at 1,443 cm −1 due to the deformation of NH 4 + , indicating that chemisorption of NH 4 + happened during the contact [15]. The bands at around 520 cm −1 are attributed to the vibrations of the Si-O-Al structure [20].

Specific surface area and pore size analysis
According to IUPAC classification, the N 2 adsorption-desorption isotherm of MT/Fe 3 O 4 604 (Fig. 5b) is a Type IV physisorption isotherm and Type H3 hysteresis loop, given by non-rigid aggregates of plate-like particles; in this case, it is montmorillonite, with slit-shaped pores. The Kelvin equation-based Barrett-Joyner-Halenda (BJH) method would underestimate the pore size for narrow mesopores, so the non-local-density functional theory (NLDFT)-based method was applied to obtain a more reliable assessment of pore size distribution [42]. As shown in Fig. 5b, the pore sizes of MT/Fe 3 O 4 604 are distributed in the range of narrow mesopores, which are in the range of 3-13 nm. The particle size of MT/Fe3O4604 was analysed by Zeta sizer, found to  be 609.4 nm in diameter. The physical dimensions of the pores suggest a good molecular sieving effect, which indicate that the nanocomposite can be used as a filling material to be dispersed into polymer matrices of membranes for a better separation performance in water treatment [43,44]. The S BET was calculated by the Brunauer-Emmett-Teller (BET) method and listed in Table 1. The raw MT has the highest S BET of 245.99 m 2 /g, while all the MT/Fe 3 O 4 nanocomposites have lower S BET . There is no clear trend of surface area among the prepared adsorbent samples based on the MT amount for the reaction, but the flow rate of NaOH affected the S BET in a such a way that the higher flow rate (2 mL/min) led to a higher S BET than that of the lower flow rate (0.5 mL/min) when the MT amount and retention time were the same. This is possibly due to the behaviour of Fe 3 O 4 aggregates on the MT surface; more Fe 3 O 4 was formed inside the pores of MT at a lower rate. The results also suggested that the adsorptive property need not always be positively correlated to the specific surface area. It is noteworthy that the best adsorption capacity was obtained with MT/Fe 3 O 4 604 even though the highest S BET was attained with MT/Fe 3 O 4 522. The main difference is higher MT/Fe ratio and longer reaction time in the synthesis of MT/Fe 3 O 4 604.

Adsorption mechanism
A series of adsorption equilibrium studies were performed with initial NH 4 + concentrations of 10 to 80 mg/L at 25 and 35 °C. These are typical ambient temperatures in many geographic regions. Higher temperatures were not studied because the high energy requirement for stormwater heating was not economic. The concentrations of NH 4 + in stormwater vary throughout seasons. Therefore, concentrations ranging up to 80 mg/L were appropriate. NH 4 + uptake increased with increasing initial concentration of NH 4 + (Fig. 6b), meaning that some of the available active sites remain unoccupied at lower NH 4 + concentrations. Similar results were observed in other studies on the adsorption of NH 4 + by clay and Fe 3 O 4 materials [45,46]. Additionally, higher equilibrium loading was achieved at 35 °C than at 25 °C, which suggests that the adsorption process is endothermic.
The pH of the NH 4 + solution influences the adsorption process. Therefore, the adsorption of ammonium ions by MT/Fe 3 O 4 604 was also investigated in different pH ranges. 50 mg/L NH 4 + solution was adjusted to an initial pH of 2, 4, 6, 7, 8, 10, and 12, respectively, and contacted with MT/Fe 3 O 4 604 through shaking at 300 rpm for 60 min at 25 °C. As shown in Fig. 6a, the abundant H + ions inhibited the adsorption of NH 4 + in highly acidic conditions (pH 2) through competitive adsorption [47] while the adsorption amounts of NH 4 + under other pH conditions are affected to some extent. This may have been because the added adsorbents affected the pH of the solutions, resulting in a reduction in the effect of the initial pH of the NH 4 + solutions on the adsorption behaviour.
A possible issue with the nanocomposite adsorbent is dissolution of iron. A high solid-to-liquid phase ratio of 1:4 was used to observe this phenomenon even if leaching was low. As observed in Fig. 6a, iron leaching was significant at pH 2 (51.3 mg/L) but negligible at pH ranging from 4 to 10 (< 0.85 mg/L). Leaching of iron increased again under alkaline conditions (pH 12, 1.43 mg/L). Therefore, we conclude that the nanocomposite adsorbents are stable at the pH range of their most likely applications.
The cations in the MT framework contributed to the removal of NH 4 + by MT/Fe 3 O 4 nanocomposite through ion   [51].
Based on the analysis above, the mechanism (Scheme 1) of the uptake of NH 4 + by the MT/Fe 3 O 4 nanocomposite may be pictured as the following reactions, which contribute to ion exchange and electrostatic interaction, respectively. The sodium ions (Na + ) existing in the interlayers of MT/Fe 3 O 4 nanocomposite exchange with NH 4 + during the adsorption procedure, as shown in Eq. (3). The MT/Fe 3 O 4 nanocomposite exhibits a negative charge at a pH that is higher than 5. Thus, NH 4 + can be captured through electrostatic interaction, Eq. (4). (3)

Adsorption isotherms
The adsorption isotherms were studied by correlating them with Langmuir, Freundlich, and Fowler-Guggenheim isotherm models [52][53][54][55]. The Langmuir isotherm [56] is written as where C e (mg/L) is the concentration of NH 4 + in equilibrium, q e (mg/g) and q m (mg/g) are the amount of NH 4 + adsorbed in equilibrium and the maximum uptake, respectively, and K L (L/mg) is the Langmuir constant.
The equation for Freundlich isotherm [57] is where n represents the heterogeneity factor related to the distribution of interaction energies of adsorption sites, and K f (mg/g·(L/mg) 1/n ) is the Freundlich constant. The Fowler-Guggenheim isotherm (Eq. 7) assumes an energetically homogeneous adsorbent surface like the Langmuir isotherm but includes an additional parameter to describe interactions between adsorbed species. Such interactions can be positive or negative (i.e. lead to increased or decreased surface concentration on the adsorbent). While the model is not explicit, solving Eq. (7) for the equilibrium loading q e is straightforward with standard numerical methods.
In Eq. (7), K FG and q m have the same meaning as the corresponding parameters in the Langmuir isotherm. χ describes the lateral interactions between adsorbed molecules.

Scheme 1 Mechanism of ammonium adsorption on MT/Fe 3 O 4 nanocomposite
To avoid artefacts arising from linearising the isotherm models and data, the parameters of the isotherm models were fitted with the nonlinear least squares method. The best-fit values are listed in Table 2, and plots for adsorption at 25 °C are shown in Fig. 7b. The Langmuir isotherm model was best model to explain the NH 4 + adsorption at 25 °C, indicating monolayer adsorption to a finite number of adsorption sites. The Freundlich model predicts unreasonably high ammonia uptake at low concentrations. The Fowler-Guggenheim model, which includes interaction between adsorbed molecules, gives a somewhat higher R 2 than the Langmuir isotherm. However, it fails to predict the increase of loading beyond 8 mg/g in the high concentration range. The lateral interaction parameter, χ = 2.43, is quite large, suggesting a strong attractive interaction between adsorbed species. Considering that ammonium is charged, it is unlikely that this is the case. We therefore concluded that the improved fit is only due to the additional degree of freedom and that the model is overparameterised.
The maximum Langmuir adsorption capacity was found to be 10.48 mg/g for MT/Fe 3 O 4 604 towards NH 4 + . This capacity is comparable to or better than reported for other clay-based materials (Table 3). For example, a capacity of 12.5 mg/g for NH 4 + on montmorillonite-biochar composites has been reported [58]. On the other hand, capacities of 1.54 mg/g and 1.38 mg/g have been reported for montmorillonite nano-clay and natural vermiculite, respectively [59]. Significantly higher capacity (40.4 mg/g) has been reported for NH 4 + uptake in montmorillonite, but the liquid phase concentration was tenfold that in this study [15]. At conditions comparable to this study, however, the uptake in montmorillonite was slightly lower than here (q e = 7 mg/g at C e = 60 mg/L) [15].

Adsorption kinetics
The adsorption kinetics were studied in a batch adsorber using 2.5 g/L adsorbent dosage and initial NH 4 + concentrations of 30 mg/L, 50 mg/L, and 80 mg/L. As shown in Fig. 8, the NH 4 + uptake reaches a high level within 5 min for 30 mg/L, and within 3 min for 50 and 80 mg/L. The batch method is not well suited for such fast adsorption; the data are not accurate enough for discrimination between kinetic models. Nevertheless, the intra-particle diffusion model that assumes Fickian diffusion in a homogeneous medium with constant boundary conditions was applied to check if    [15] intra-particle diffusion resistance could explain the results. The loading as a function of time was calculated from Eq. (8) where q and q eq (mg/g) are the amount adsorbed at time t and at equilibrium, and D' (1/s) is the mass transfer parameter that includes the diffusion coefficient and the (unknown) Sauter mean diameter of the particles. The infinite series was truncated at j = 30, which is more than sufficient as the series converges rapidly (e.g. below 1e-17 with j = 15 at t = 0.1 min). As observed in Fig. 8, this model fits reasonably well to the data, suggesting that the intra-particle diffusion is the sole rate-limiting step [60,61]. The mass transfer parameter D' was found to increase with increasing initial concentration (and thus loading). Values of 0.15 1/s, 0.40 1/s and 0.60 1/s were obtained with initial concentrations 30 mg/L, 50 mg/L and 80 mg/L, respectively. Since ammonium adsorption is strongly nonlinear, this suggests that diffusion may take place also in the adsorbed phase.

Ammonium removal from authentic stormwater
Application of the adsorbent to remove ammonium from an authentic stormwater was studied at laboratory scale. The operating conditions and suitable adsorbent dosing were first determined in preliminary tests. The adsorbent dosage was varied from 1 to 5 g/L, studied using 50 mg/L NH 4 + solution, shaking at 300 rpm for 120 min at 25 °C. The removal efficiency of NH 4 + increased with increasing dosage due to the increased active sites for NH 4 + adsorption which increased the uptake amount. As shown in Fig. 9, the removal efficiency was improved by more than 10 percentage points from 1 to 3 g/L, but less than 5 percentage points from 3 to 5 g/L. This is typical for Langmuir-type adsorption and originates from the decrease in the equilibrium concentration when the adsorbent dosage increases. Considering the cost and removal efficiency, 2.5 g/L was selected as the optimal dosage for a single-contact adsorption process; this was used in the subsequent adsorption studies.
The adsorption efficiency of MT/Fe 3 O 4 604 was investigated for real stormwater collected from the landfill factory Metsäsairila Oy in Mikkeli, Finland, on August 19, 2019. The sample was filtered and used as a working solution directly for adsorption at conditions of 2.5 g/L dosage, shaking at 300 rpm for 1 h at 25 °C. The contents of the stormwater were examined by ICP-OES for Na, Mg, Al, K, Ca, Cr, Mn, Fe, Co, Ni, Cu, and Zn; all these elements were detected except Cr and Co, which were under the detection limit. NH 4 + removal efficiency reached 64.2% as shown in Fig. 10. Simultaneously, the concentrations of Mg, K, and Ca in the stormwater were reduced significantly. Heavy metals, including Mn, Ni, Cu, and Zn, were diminished to zero or nearly zero. Na as well as a small amount of Al were released by the adsorbent, which confirms the role of ion exchange mechanism. As previously discussed, Fe leaching was low since the pH of stormwater was 6.54. Overall, the new adsorbent recovered NH 4 + well in the presence of other metallic ions during treatment of stormwater. The additional benefit of eliminating heavy metals such as Mn, Ni, Cu, and Zn to extremely low concentrations (in current case ≤ 0.1 mg/L, which is lower than the detection limit of ICP-OES) is notable.

Potential of reusing NH 4 + loaded adsorbents in REE recovery
To improve the economics of adsorptive ammonium removal from stormwater, we studied the reuse of the spent (ammonium loaded) adsorbent in a secondary application. Utilising the ammonium ion-loaded adsorbents for selective recovery of valuable elements avoids the generation of chemical waste from the regeneration of adsorbents. To this end, the adsorbents were first loaded with 50 mg/L NH 4 + solution; then, the uptake of REEs from aqueous solution was studied. The REEs used for the study include scandium (Sc), yttrium (Y), and all the lanthanides except promethium (Pm). The adsorption study was conducted at conditions of 2.5 g/L dosage, 20 mg/L working solution, shaking at 300 rpm for 1 h at 25 °C.
As observed in Fig. 11, the used MT/Fe 3 O 4 nanocomposite achieved almost 100% removal efficiency for all 16 REE ions in the single-REE containing solutions. In the mixture of all REEs, the adsorbent showed selective affinity towards Sc 3+ . The results indicate that the spent (ammonium loaded) MT/Fe 3 O 4 nanocomposite possesses the potential for REE recovery as a secondary application, especially for selective adsorption of Sc 3+ among other REEs. However, regeneration of the adsorbent to recover REEs was not studied.

Conclusions
In this study, the nanocomposite of montmorilloniteanchored magnetite was synthesised and studied for the removal of ammonium in stormwater. FTIR, XRD and XPS confirmed the successful loading of Fe 3 O 4 onto MT. The Fe 3 O 4 enhanced the adsorption of NH 4 + and provided magnetic features to MT, which accelerated the separation of the adsorbent from water. It was observed that the adsorption was influenced by dosage, contact time, initial concentration, and pH. The adsorption behaviour of NH 4 + was well expressed by pseudo-second-order kinetics and the Temkin isotherm model. Both ion exchange and electrostatic interaction contributed to its affinity for NH 4 + . The adsorbent MT/Fe 3 O 4 604 was able to treat real stormwater, reducing the NH 4 + as well as heavy metal contents. Moreover, the ammonium-loaded nanocomposite possesses the potential for REE recovery as a secondary application, especially for the selective adsorption of Sc 3+ among other REEs. Further study regarding the desorption of Sc 3+ could be conducted to affirm and maximize the recovery potential. This study suggests that MT/Fe 3 O 4 nanocomposites are potential adsorbents for stormwater treatment regarding NH 4 + recovery. Furthermore, the nanocomposites could be utilised as functional material in a membrane or in polymer beads that are better suitable for large scale water treatment operations than powders [62,63].