Adsorption Studies on Magnetic Nanoparticles Functionalized with Silver to Remove Nitrates from Waters

Abstract

This paper presents a novel procedure for the treatment of contaminated water with high concentrations of nitrates, which are considered as one of the main causes of the eutrophication phenomena. For this purpose, magnetic nanoparticles functionalized with silver (Fe₃O₄@AgNPs) were synthesized and used as an adsorbent of nitrates. Experimental conditions, including the pH, adsorbent and adsorbate dose, temperature and contact time, were analyzed to obtain the highest adsorption efficiency for different concentration of nitrates in water. A maximum removal efficiency of 100% was reached for 2, 5, 10 and 50 mg/L of nitrate at pH = 5, room temperature, and 50, 100, 250 and 500 µL of Fe₃O₄@AgNPs, respectively. The characterization of the adsorbent, before and after adsorption, was performed by energy dispersive X-ray spectroscopy, scanning electron microscopy, Brunauer-Emmett-Teller analysis and Fourier-transform infrared spectroscopy. Nitrates can be desorbed, and the adsorbent can be reused using 500 µL of NaOH solution 0.01 M, remaining unchanged for the first three cycles, and exhibiting 90% adsorption efficiency after three regenerations. A deep study on equilibrium isotherms reveals a pH-dependent behavior, characterized by Langmuir and Freundlich models at pH = 5 and pH = 1, respectively. Thermodynamic studies were consistent with physicochemical adsorption for all experiments but showed a change from endothermic to exothermic behavior as the temperature increased. Interference studies of other ions commonly present in water were carried out, enabling this procedure as very selective for nitrate ions. In addition, the method was applied to real samples of seawater, showing its ability to eliminate the total nitrate content in eutrophized waters.

1. Introduction

Industrial and agricultural activities have resulted in an increase of the concentration of nutrients in water. Among them, nitrates are considered as one of the most diffused contaminant, especially because of their high solubility in water and low retention by soil particles [1]. Eutrophication is the most important process of water pollution in aquatic environments such as lakes, rafts, rivers or reservoirs [2]. This process is caused by the excess of nutrients in water, mainly nitrogen and phosphorus, mostly arising from human activity. Moreover, eutrophication affects the quality of waters since they acquire a foul smell as rot increases and the oxygen depletes. The smell of these waters can lead to economic losses (tourism, areas that lose value, among others), respiratory problems and their consumption can cause health problems in the nearby areas [3,4,5].

For these reasons, nowadays, many methods to remove nitrates from water samples have been developed. Among them, procedures based on catalysis [1,6,7,8,9], electrocoagulation [10,11,12] and photocatalysis [13,14] are common.

Approaches for removing nitrates based on biological treatments were also widely used some years ago [1,15,16,17,18]. However, these techniques have been superseded by other methods that lead to better results in terms of removal efficiency, quickness and simplicity. In this sense, methods for removing nitrates based on adsorption lead to high removal efficiencies. Different materials have been used as nitrate adsorbents, such as carbon activated and zeolite [19], amberlite [20], several resines [21,22], composites [23] and microalgae [24]. Nonetheless, these procedures do not achieve the total removal of nitrates in water compared to the work presented in this article.

The use of nanoparticles to remove nitrates in water samples has been studied in recent years, obtaining good results in relation to removal efficiency. However, these procedures are not very fast or easy to carry out, in general [25,26,27,28,29]. By comparison with our approach, these methods involve more tedious experimental processes with higher costs, higher temperatures, or longer times to achieve the maximum adsorption of nitrates, which in no case reaches 100% of the removal of the pollutant. The procedure presented in this work represents a great advance in the elimination of nitrates in water over the aforementioned, presenting numerous advantages such as the total adsorption of high concentrations of nitrates in water in just a few seconds, under mild experimental conditions and using a very low adsorbent dose.

Consequently, in this study we present a novel, very simple and fast method for adsorption of nitrates from aqueous samples using magnetic core functionalized with silver nanoparticles (Fe₃O₄@AgNPs) as an adsorbent. The choice of Fe₃O₄@AgNPs is primarily based on the quick and easy removal of magnetic nanoparticles from aquatic environments using a magnet, as shown in the recent literature [30,31,32]. Moreover, a novel study showed the notable increase in the adsorption efficiency to eliminate phosphates in water when the adsorbent was functionalized with silver nanoparticles [33]. Phosphates and nitrates present similar characteristics, thus the effect of silver on nitrate adsorption resulted positive, in the same way. However, to the best of our knowledge, Fe₃O₄@AgNPs was not previously used as an adsorbent for nitrate removal. Experimental conditions such as pH, temperature, contact time and adsorbent dose have been studied to achieve maximum nitrate removal efficiency. After the adsorption process, Fe₃O₄@AgNPs can be recycled and reused. Interference studies were carried out, providing this process as very selective for nitrate ions in the presence of other ions commonly present in water.

2. Materials and Methods

2.1. Materials

For all the experiments conducted, the solvents and reagents employed were of analytical reagent grade. Pure water obtained with a Millipore system (Millipore, Bedford, MA, USA) was used exclusively. All the solutions were stored in polypropylene or polytetrafluoroethylene vessels. Nitrate solutions were prepared with sodium nitrate purchased from Sigma Aldrich. Nanoparticles were prepared using FeCl₃·6H₂O, FeCl₂·4H₂O and concentrated ammonia solution from Sigma Aldrich. Nitric acid and potassium iodide were purchased from Panreac.

Permanent Nd-Fe-B magnets from Supermagnete, agitator system model AM20-D from Argolab and ultrasound bath model 4820 from PCE instruments were used to prepare the nanoparticles and conduct the experiments. Nitrate concentrations were determined using UV-Vis Nanodrop Spectrophotometer model 2000 (Thermo Fisher Scientific, Madrid, Spain) operating at 220 nm.

2.2. Preparation of Fe₃O₄@AgNPs

Water solution (20 mL) was heated at 80 °C and steadily under nitrogen atmosphere. Then, 0.56 g FeCl₃·6H₂O, 0.2 g FeCl₂·4H₂O and 2 mL of concentrated ammonia solution were added. The mixture was stirred for 10 min. Using a permanent magnet, the particles were separated, and the supernatant solution was rejected. A brown solid was obtained (Fe₃O₄), washed three times with water and suspended in 20 mL of water. Then, a solution of silver nitrate at a concentration of 0.011 g L⁻¹ (5.7 mL) was added and the mixture was stirred for 5 min. Subsequently, the nanoparticles obtained (Fe₃O₄@AgNPs) were washed several times with water using and suspended in 20 mL of water.

Iron oxide-based magnetic nanoparticles are recognized to be good as adsorbents, but magnetic and long-range electrostatic interactions make them aggregate [34]. Their synthesis by modifying the surface with nanoparticles avoids this fact and enhances adsorption efficiency by incorporating new active sites [35,36], thus giving Ag NPs a leading role.

2.3. Nitrates Adsorption Procedure

The adsorption procedure has been studied for different concentrations of nitrates in waters, $C_e$. Specifically, adsorption was carried out using 10 mL of aqueous samples placed in polypropylene tubes at nitrate concentrations of 2, 5, 10 and 50 mg L⁻¹. 200 µL of acetate buffer solution (0.1 M, pH = 5) were added. Then 100, 100, 250 and 500 µL of Fe₃O₄@AgNPs were incorporated to the mixtures for the respective solutions at different nitrate concentration (2, 5, 10 and 50 mg L⁻¹). After shaking at 60 s, the adsorbent was separated by placing the magnet at the bottom of the tube. The supernatant was analyzed by spectrophotometry, showing the total nitrate removal in every solution.

The adsorption capacity of the nitrate adsorbent, known as $q_e$ (mg g⁻¹), was calculated according to the following equation:

$$q_e=\frac{\left(C_0-C_t\right)V}{m}$$

(1)

where $C_0$ (mg L⁻¹) and $C_t$ (mg L⁻¹) are the of nitrate concentrations in aqueous solution at the initial and after removing process. $V$ (L) is the volume of the solution and $m$ (mg) is the mass of the Fe₃O₄@AgNPs.

3. Results and Discussion

3.1. Characterization of Fe₃O₄@AgNPs

In order to check the nitrate adsorption onto the adsorbent surface, Fe₃O₄@AgNPs was characterized before and after adsorption process. Figure 1a shows a scanning electron microscopy (SEM) image of Fe₃O₄@AgNPs, where the well-differentiated lighter structures correspond to Ag on the Fe₃O₄ surface, due to its higher atomic number. The corresponding mapping SEM image also reveals the presence of Ag on the adsorbent, highlighted in blue color and labeled in the plot (Figure 1b).

To confirm the presence of Ag in the adsorbent, Figure 2 shows the energy dispersive X-ray spectroscopy (EDX) pattern for Fe₃O₄@AgNPs associated to the SEM, where the characteristic signal for Ag at 2.984 keV is shown. Additionally, strong characteristic signals for Fe, at 0.705 and 6.398 keV, and for O, at 0.525 keV, indicate the presence of Fe₃O₄.

Figure 3 shows Fourier-transform infrared spectroscopy (FTIR) spectrum for the adsorbent after the adsorption process in order to confirm the presence of nitrate. The characteristic nitrate signals are marked. The vibrational peak for N=O stretching appears at 1600 cm⁻¹, the NO₃ asymmetric stretching is shown at 1380 cm⁻¹, while the NO₃ symmetric stretching peak is observed at 800 cm⁻¹, as supported by the literature [37].

As a complementary result, Figure 4 shows a SEM image after the adsorption process, where the darker structures are associated to nitrates, due to the lower atomic number of N. Additionally, the spectrophotometry performed before the adsorption detects a signal at 220 nm, which is indicative of the presence of nitrates in the aqueous sample. After the removal of the magnetic nanoparticles with the magnet the signal is zero, revealing that the nitrates were adsorbed onto the Fe₃O₄@AgNPs.

A Brunauer-Emmett-Teller test determined the contact surface area of Fe₃O₄@AgNPs, as equal to 116.476 m²/g. One of the roles of Ag NPs is to increase the specific surface area of adsorption, as it was demonstrated by previous studies by the authors [33].

3.2. Effect of pH on Nitrate Removal by Fe₃O₄@AgNPs

The effect of pH on the adsorption of nitrates was investigated by determining the adsorption efficiency within the range [1,2,3,4,5,6,7,8,9], employing nitrate concentrations equal to 2, 5, 10 and 50 mg L⁻¹. The results are shown in Figure 5, which indicates that there is a common pH range, 4–5, presenting the highest adsorption efficiency for all concentrations. The error bars are smaller than the size of the symbols in Figure 5 and the following, not shown.

The point of zero charge (PZC) of the aqueous system is considered as one of the main parameters that affects the behavior of the adsorbents [38]. In general, the influence of pH on anion exchange reaction is principally due to the effect of competitive hydroxyl ions and anions [39]. The PZC of adsorbent Fe₃O₄@Ag is found to lie within the range [6.03–6.7] [40]. For values of pH > PZC, the surface of Fe₃O₄@Ag becomes negatively charged and there is a repulsion force between the adsorbent and the nitrate anion, thus the adsorption capacity decreases. For values of pH < PZC, the surface of the adsorbent becomes positively charged, which results in the electrostatic attraction between the nitrate anion and the surface, and thus, the adsorption efficiency increases. Nevertheless, at very low pH, the electrostatic attraction is drastically reduced due to the ionic strength, which measures the effect of ions in solution on the electrostatic potential [41]. An increase in the ionic strength of the solution lowers the electrostatic interactions, either attractive or repulsive, due to a screening effect of the surface charge caused by the reduction of pH [42], thus shielding electrostatic interactions. For all plots in Figure 5, this effect is regarded from a pH < 4, being the screening reduced as the nitrate concentration decreases, in concordance with recent surface-force measurements showing that the typical screening length increases with ion concentration [43].

The results in Figure 5 suggest that our procedure is especially suitable for treatment plants, where the pH and other factors can be regulated for water treatment. Nevertheless, application to waters in natural ecosystems such as seawater, for which pH has a neutral-basic character [44], also provides a high adsorption efficiency, around 75% for pH between 7 and 8, as depicted in Figure 5b–d. These efficiencies are still high compared to the traditional methods usually employed [1,6,10]. For higher nitrate concentrations, such as those shown Figure 5a, water could be processed in treatment plants.

The adsorption mechanism can be regarded as a physicochemical adsorption. Semi-empirical quantum-chemical studies reveal that orbitals 5s and 5d of Ag could be responsible for the chemisorption of nitrate [45]. A physicochemical process agrees with thermodynamic studies presented in Section 3.7.

3.3. Effect of Adsorbent Dose and Performance Metrics

The effect of Fe₃O₄@AgNPs volume was studied in order to achieve the maximum nitrate efficiency removal in aqueous solutions, for nitrate concentrations equal to 2, 5, 10 and 50 mg L⁻¹. Volumes under study lied in the range of (50, 750) µL. All experiments employed pH = 5 and 60 s of contact time between solution and adsorbent. Figure 6 shows that the maximum adsorption efficiencies for 2 (magenta empty inverted triangles), 5 (blue triangles), 10 (red dots), and 50 (black squares) mg L⁻¹ nitrate concentrations were achieved by using 100, 100, 250 and 500 µL of adsorbent, respectively.

The variable $q_e$ was calculated for each nitrate concentration versus the suspension volume of Fe₃O₄@AgNPs, increasing with the nitrate concentration. For this reason, to evaluate the adsorbent performances by reducing the bias it is appropriate to use the partition coefficient (PC) (mg g⁻¹ μM⁻¹) [33,46], defined as:

$$PC=\frac{q_e}{\frac{C_0}{M}\left(1-MRE\right)}=\frac{q_e}{\frac{C_f}{M}}$$

(2)

where C₀ the initial concentration of pollutant in the liquid sample (mg L⁻¹), C_f the final concentration of the liquid sample, M the molar mass (mg mM⁻¹) and MRE is the maximum removal efficiency. As depicted in

Table 1. Performance metrics of Fe₃O₄@AgNPs employed for adsorption of different concentrations of nitrate species in water.

Optimum Conditions (Temperature (°C), pH)	Initial Nitrate Concentration (mg L−1)	Fe3O4@AgNPs Mass (mg)	Maximum Removal Efficiency (%)	Adsorption Capacity (mg g−1)	Partition Coefficient (mg g−1 µM−1)
25, 5	2	9.5	100	2.11	-
25, 5	2	7	100	2.86	-
25, 5	2	3.5	100	5.71	-
25, 5	2	1.4	100	14.29	-
25, 5	2	0.7	60	28.57	2214.29
25, 5	5	9.5	100	5.26	-
25, 5	5	7	100	4.17	-
25, 5	5	3.5	100	14.29	-
25, 5	5	1.4	100	35.71	-
25, 5	5	0.7	60	71.43	163.16
25, 5	10	9.5	100	10.53	-
25, 5	10	7	100	14.29	-
25, 5	10	3.5	100	28.57	-
25, 5	10	1.4	95	71.43	8857.14
25, 5	10	0.7	95	142.86	17,714.29
25, 5	50	9.5	99	26.32	3263.16
25, 5	50	7	99	35.71	4428.57
25, 5	50	3.5	86	71.43	632.65
25, 5	50	1.4	80	157.57	1107.14
25, 5	50	0.7	80	357.14	2214.29

, the PC diverges when MRE is 100%, and decreases as the MRE does.

Table 1 shows that the PC diverges when MRE is 100% which occurs for every nitrate concentration in an equilibrium situation. At fixed nitrate concentration, $q_e$ increases as the Fe₃O₄@AgNPs mass decreases. At fixed Fe₃O₄@AgNPs mass, $q_e$ increases as the nitrate concentration increases.

3.4. Contact Time Effect

Kinetics of Langmuir-like physicochemical reversible processes are well described employing an efficient kinetic three-parameter model connecting both the equilibrium and kinetic viewpoints [33]:

$$Adsorption efficiency=\left(\alpha -\beta \right)\frac{\left(\beta /\alpha \right)e^{\left(\beta -\alpha \right)\gamma t}}{\left(\beta /\alpha \right)e^{\left(\beta -\alpha \right)\gamma t}-1}+\beta$$

(3)

where $t$ is the contact time for the adsorption process and $\alpha$, $\beta$ and $\gamma$ are characteristic parameters. This model characterizes Langmuir-type adsorption processes matching kinetics and isotherms, unlike other models like the pseudo-first or pseudo-second order [47]. The time of contact between the adsorbent Fe₃O₄@AgNP and the solutions containing nitrate at different concentrations of 2, 5, 10 and 50 mg L⁻¹, was studied for pH = 5 (characterized by Langmuir isotherm, see Section 3.6) and adsorbent volumes equal to 50, 100, 250 and 500 µL, respectively. Contact times needed for achieving the maximum removal efficiency were 15, 30, 45 and 60 s for 2, 5, 10 and 50 mg L⁻¹ nitrate concentrations, respectively. Figure 7 shows the dependence of the adsorption efficiency on the contact time for concentrations of 50 mg L⁻¹ (black dots) and 10 mg L⁻¹ (red triangles). The solid lines represent the fits to the theoretical kinetic model given by Equation (3), showing R² values equal to 0.988 and 0.999, respectively. Reduced ${\chi }^2$ values are equal to 16.774 and 0.196, respectively. All data corresponding to 2 and 5 mg L⁻¹ are equal to the maximum adsorption efficiency, 100%, not represented in Figure 7. Additionally, comparison with pseudo-first order, pseudo-second order, Elovich and interparticle diffusion kinetic models [48] was performed through Bayesian information criterion and Akaike’s information criterion computed in software Origin 2020, both concluding that Equation (3) is more likely to describe the adsorption kinetics.

3.5. Effect of Coexisting Anions on Nitrate Removal

The presence of anions such as sulphates, phosphates, carbonates and chlorides along with nitrates is common in water samples [49,50]. The interference of these ions at different concentrations on nitrate adsorption was studied in this work, by adding them to the aquatic medium before the adsorption process was performed, determining the removal efficiency through spectrophotometry operating at 220 nm. Several scenarios of contaminated water were considered by varying the concentrations of these ions, based on methods from other works [31,32,33,51]. The molar ratio of nitrate/chloride and nitrate/phosphate of 1:5 led to a slight decrease in the nitrate removal efficiency from 100.0% to 93.2% at 10 mg L⁻¹ of nitrate concentration, 250 µL of Fe₃O₄@AgNPs suspension, at room temperature, pH 5 and 1 min of contact time. However, the molar ratio of nitrate/chloride and nitrate/phosphate of 1:1 and 1:3 did not influence on nitrate adsorption. The presence of the other anions, sulphates and carbonates, was tested in the molar ratio P/anion from 1:1, 1:3 and 1:5, finding no significant influence on nitrate adsorption for the same experimental conditions. This fact shows that Fe₃O₄@Ag presents higher affinity towards anions with lower charge density.

3.6. Adsorption Isotherms

Adsorption isotherms characterize the relationship between the equilibrium values of the adsorption capacity, $q_e$ (mg g⁻¹), and the adsorbate concentration, $C_e$ (mg L⁻¹). Langmuir isotherm considers homogenous monolayer adsorption, described by the equation [33]:

$$\frac{1}{q_e}=\frac{1}{q_m}+\frac{1}{K_L{q}_m{C}_e}$$

(4)

where $K_L$ is the Langmuir adsorption constant (L mg⁻¹) and $q_m$ is the maximum adsorption capacity of the adsorbent (mg g⁻¹). This model adequately represents the fit to the experimental data for adsorption of nitrates to Fe₃O₄@AgNPs at pH = 5 and higher. In Figure 8 (left) the dependence of $1/q_e$ on $1/C_e$ at pH = 5 is shown. The fit to the Langmuir isotherm, Equation (4), gives rise to the best $R^2$ and reduced ${\chi }^2$ values, equal to 0.998 and 1.886, respectively. A deeper study reveals that the shape of the isotherm curve is sensitive to the change of pH, and it has been previously reported by some authors that variations in pH conditions can lead to different isotherm behaviors, described by different theoretical models [52]. Freundlich isotherm considers multilayer adsorption and, particularly, affinity between surface sites, which can be achieved when reducing the pH due to the increase in positively charged surface sites [52]. Freundlich equation is [53]:

$$q_e=K_F{C}_e{}^{\frac{1}{n}}$$

(5)

where $K_F$ and $1/n$ are known as the adsorption capacity and the adsorption intensity constants, respectively. Figure 8 (right) shows the dependence of $\log (q_e)$ on $\log (C_e)$ for adsorption of nitrates to Fe₃O₄@AgNPs at pH = 1. The nonlinear fit to Equation (5) gives rise to the best $R^2$ and reduced ${\chi }^2$ values, equal to 0.99997 and 0.05092, respectively. By comparison, the data fit to Equation (4) shows $R^2=0.99982$ and ${\chi }^2=0.32504$, which reveals a better description for the equilibrium behavior at pH = 1 achieved by the Freundlich model. Other isotherms such as Temkin gave rise to worse fits.

3.7. Thermodynamic Analysis

The analysis of adsorption thermodynamics characterizes the nature of the process, i.e., physisorption or chemisorption, through the determination of the standard Gibbs free energy $\Delta G^0$ (kJ mol⁻¹). Additionally, the estimation of the standard free enthalpy, $\Delta H^0$ (kJ mol⁻¹), reveals if the process is exothermic or endothermic, while the standard entropy, $\Delta S^0$ (kJ mol⁻¹ K), gives information about the increase or decrease of order in the solid/liquid interface [33,54]. $\Delta G^0$ values are determined from the equation:

$$\Delta G^0=-RTln\left(K_D\right)$$

(6)

where $R$ is the gas constant, $T$ is the absolute temperature, and $K_D$ is the so-called distribution coefficient:

$$K_D=\frac{q_e}{C_e}$$

(7)

Negative values for $\Delta G^0$ are consistent with a spontaneous adsorption. Values in the range [−20, 0] kJ mol⁻¹ reveal a physisorption process, while [−400, −80] kJ mol⁻¹ correspond to chemisorption [33]. The $\Delta G^0$ interval was determined for the adsorption of nitrates to Fe₃O₄@AgNPs, for $C_e=10$ mg L⁻¹, 250 µL of adsorbent and pH = 5, and ranging temperature from 278 to 343 K. The results show that $\Delta G^0$ lies in the interval [−15.75, −21.03] kJ mol⁻¹, which can be considered as a physicochemical adsorption process [55].

Van’t Hoff equation relates variables $T$, $\Delta H^0$, $\Delta S^0$ and $\Delta G^0$ (through Equation (6)) by the following expression:

$$\ln \left(K_D\right)=\frac{\Delta S^0}{R}-\frac{\Delta H^0}{RT}$$

(8)

In adsorption processes it is usual to find a unique linear dependence between $\ln \left(K_D\right)$ and $1/T$. For the adsorption of nitrates to Fe₃O₄@AgNPs described above, we distinguish three different behaviors. Figure 9 shows the plot of $\ln \left(K_D\right)$ vs. $1/T$ for temperatures ranging from 278 to 343 K. Fitting Equation (8) to the low temperature region (right solid line) gives rise to $\Delta H^0=48.71$ kJ mol⁻¹ and $\Delta S^0=0.23$ kJ mol⁻¹ K, corresponding to an endothermic process with decrease in the order for the solid/liquid interface. The fit to the high temperature region (left straight line) shows $\Delta H^0=-17.96$ kJ mol⁻¹ and $\Delta S^0=0.0088$ kJ mol⁻¹ K, indicative of an exothermic process with slight decrease in the order for the solid/liquid interface. The central $T$ region shows no temperature dependence for $\ln \left(K_D\right)$.

3.8. Desorption and Recycling Nanoparticles

In order to reuse Fe₃O₄@AgNPs for successive adsorption processes, nanoparticles were separated from the aqueous solution using a magnet, washed with a few milliliters of acetone and desorbed by employing 500 µL of NaOH solution 0.01 M. Thereafter, NaOH solution was separated, and the nanoparticles were employed again to remove nitrate (10 mg L⁻¹) from aqueous solution under the experimental conditions described before.

Fe₃O₄@AgNPs can be reused, remaining unchanged for two more cycles, and exhibiting 90% adsorption efficiency after the third regeneration and 81% after the fourth regeneration.

3.9. Adsorption of Nitrates onto Fe₃O₄@AgNPs in Real Seawater Samples

The method introduced in this work for the adsorption of nitrates from water solutions was tested in real seawater samples. Nitrates were analyzed for those samples and, afterwards, their adsorption was performed by using Fe₃O₄@AgNPs under the optimal experimental conditions described above. The results are summarized in

Table 2. Adsorption of nitrates to Fe₃O₄@AgNPs in real seawater samples.

Sample	Absorbance before Adsorption	Absorbance after Adsorption	Removal Efficiency (%)
1	0.37	0.02	94.6
2	0.22	0.01	95.4
3	0.20	0.00	100

.

Removal efficiency values within the range 94.6–100% were achieved, slightly lower than those presented in the experimental procedure because seawater contains other anions at very high concentrations that could interfere in the adsorption process.

4. Conclusions

This work introduces a novel and simple method for the adsorption of nitrate from water samples using magnetic nanoparticles coated with nanomeric Ag. Different experimental conditions were studied showing to be optimal at pH = 5 and room temperature. An adsorption efficiency of 100% was reached in 60 s of contact time for a nitrate concentration equal to 50 mg L⁻¹, employing 500 µL of Fe₃O₄@AgNPs. At lower nitrate concentrations, the contact time needed was just 15 s using 250, 100 and 100 µL of adsorbent for 10, 5 and 2 mg L⁻¹ of nitrate concentration, respectively. Adsorption isotherms and thermodynamic studies revealed interesting behaviors in the removal process. Firstly, a strong pH-dependence for the adsorption equilibrium was found, characterized by the Langmuir model at pH = 5 and Freundlich model at pH = 1. Secondly, thermodynamics revealed a transition from an endothermic to an exothermic physicochemical adsorption as the temperature increased. The nanoparticles can be recycled by applying a few milliliters of a NaOH solution and they can be reused for two additional adsorption cycles, exhibiting no loss of the adsorption properties. In addition, the interference of other ions in the adsorption process was studied, showing that the presence in high concentrations of the species usually found in waters does not affect the adsorption, except for the extreme ratio of nitrate/phosphate of 1:5. Moreover, the procedure was satisfactorily applied to real seawater samples, enabling the approach as a new and very efficient method for the decontamination of eutrophized waters.