Introduction

2. Materials and Methods

3. Results and Discussions

3.1. Characterization of polymeric materials

The polymeric materials, dried under vacuum condition, without and with zerovalent iron nanoparticles (A400 and A400-nZVI), were characterized using different techniques, such as: Fourier Transforms Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), EDAX spectrometer, Thermogravimetric analysis (TGA), and X-ray Diffraction (XRD). Also, the dried polymeric materials A400-nZVI, was used as polymeric adsorbent for removal of nitrate ions from a simulated groundwater, in order to demonstrate the adsorption capacity.

3.1.1. SEM-EDAX

Surface morphology of the polymeric materials was performed through a Scanning Electron Microscope (SEM) (Quanta 650 FEG, FEI Company, Hillsboro, USA), equipped with an EDAX spectrometer (175.6 eV resolution) (Oxford Instruments, Hillsboro, USA). Scanning electron microscopy (SEM) images (magnification of 5000 x) and Energy Dispersive X-ray Spectroscopy (EDAX) spectra of polymeric materials, A400 and A400-nZVI are indicated in Figure 3a,c.

The SEM image of resin-supported nZVI shows that the iron nanoparticles are uniformly deposited on the resin and confirms the successful attachment of nZVI on the resin (Figure 3c).

The SEM images (Figure 4e,f) clearly show that the Fe⁰ nanoparticles are presented and dispersed on the A400-nZVI surface.

EDAX analysis of the two forms of polymer resins in the figure showed that Fe⁰ was distributed on the surface of the resin, the nanoparticles were discrete and dispersed on the surface, indicating that A400 represented a suitable surface for dense and uniform deposition of nanoparticles of Fe⁰.

As suggested, the precharged nZVI particles will block some internal pores or reduce/narrow the pores. On the other hand, they could offer a more accessible surface and thus increase the specific surface.

The iron nanoparticles deposited on A400-nZVI did not aggregate (the unfavorable characteristic of magnetization and aggregation was eliminated), and the nZVI particles were also dispersed inside the surface of the porous polymeric hosts.

It is shown in the SEM and EDAX images of A400 and A400-nZVI that A400 presented a typical porous structure. This result suggests that nZVI was uniformly immobilized/fixed in A400 resin (Figure a). Compared to A400, a more abundant pore structure was observed on the surface of A400-nZVI (Figure b), where the surface is very dense and compact.

3.1.2. FTIR

The infrared spectra of the polymeric materials were registered on a FTIR spectrophotometer (PerkinELMER, Ltd, London, United Kingdom) in the transmittance mode equipped with a Golden Gate unite. Data were performed in the wavenumber range between 4000 and 650 cm⁻¹, with a resolution of 4 cm⁻¹.

Figure 4 is the FTIR spectrum of the spectra for A400 and A400-nZVI before and after contact with ${NO}_3^{-}$.

New peaks at 2322 cm⁻¹ and 2231 cm⁻¹ appeared in the spectrum of fresh A400-nZVI compared to A400-nZVI loaded with ${NO}_3^{-}$, which could be attributed to bending vibration of B–O which was the residual compound after iron reduction using NaBH₄ (Figure ). Thus, the distribution of nZVI in A400-nZVI does not result from the distribution of ammonium groups in A400, but is possibly related to NaBH₄ diffusion into the polymer phases during the formation of nZVI. A similar phenomenon was also observed by Gasparovicova´ et al. (Gasparovicova´ et al., 2007).

The presence of a band at the wave number 2231 cm^-1 is due to the –C≡N bond (stretching vibration) of the nitrile group, the group has no characteristic bands. The appearance of a peak at 2923cm^-1 may be due to the group – CH₂– (acrylic) stretching vibration, asymmetric, C-H bond (reticulated resin).

The peak at 1381.58 cm⁻¹, 1379.55 cm⁻¹and 1336.61 cm⁻¹ (O-H bending vibration) indicated the presence of surface hydroxyls on nZVI. After adsorption, a portion of hydroxyl groups reacted, and this peak moved to a lower wave number (1336.61 cm⁻¹) and increased in intensity. The fact that the 1379 cm⁻¹, 1346 cm⁻¹ peak does not move to a higher but a lower wavenumber indicates that reduction is occurring and not adsorbing. No additional peaks appeared in the spectra loaded with ${NO}_3^{-}$ compared to fresh A400-nZVI.

The bands at 827 cm⁻¹ corresponding to Fe-O, Fe-O-Fe oxygen-iron stretching vibration of Fe2O3 and Fe3O4 in the spectra, indicating that the nZVI particles were partially oxidized.

3.2. Kinetics Evaluation

3.2.1. Experimental determination of curves in dynamic regime and calculation of kinetic parameters of the process

The experimental data are presented in the form of logistic curves under the specified working conditions as a variation of the N concentration in the effluent as a function of time for the three types of flow Q₁= 14 mL/min, Q₂= 16,8 mL/min, Q₃= 28 mL/min, denoted by P1, P2 and P3 (sample P1, P2 and P3 Tabel 1).

The experimental data on the basis of which the curves will be drawn are presented in table 6.1.

Table 1. Experimental data regarding the variation of the absolute concentration of ${\mathrm{N}\mathrm{O}}_3^{-}$ ions as a function of time for the three types of flows (Table 1), for C₀ = 100 mg/L.

Table 1. Experimental data regarding the variation of the absolute concentration of ${\mathrm{N}\mathrm{O}}_3^{-}$ ions as a function of time for the three types of flows (Table 1), for C₀ = 100 mg/L.

P1		P2		P3
Timp (h)	${\mathit{C}}_{\mathit{N}{\mathit{O}}_3^{-}}$ (mg/L)	Timp (h)	${\mathit{C}}_{\mathit{N}{\mathit{O}}_3^{-}}$ (mg/L)	Timp (h)	${\mathit{C}}_{\mathit{N}{\mathit{O}}_3^{-}}$ (mg/L)
0	68,75	0	71,2	0	76,5
3	67,2	6	70,9	4	75,3
24	62,3	25	69,5	26	74,5
38	60,9	41	68,2	39	73,1
48	58,1	45	67,9	47	72,8
60	57,2	58	66,2	61	71,6
66	55,8	68	66,0	65	70
69	52,6	71	65,7	68	68,7
72	50,79	79	52,5	74	66,1
84	31,36	83	34,6	83	51,8
96	11,1	92	13,4	99	34,1
130	5,76	128	6,8	130	17,1

The experimental data from Table 1 regarding the ionic and redox exchange process in the fixed layer for the three types of flow rates.

Figure 5. The variation of nitrate concentration in the samples to be analyzed as a function of time at the three operating flow rates.

Figure 5. The variation of nitrate concentration in the samples to be analyzed as a function of time at the three operating flow rates.

By testing the A400-nZVI barrier, a reduction of the nitrate concentration to a value close to 0 was achieved within 5 days, with effluent recirculation every day. For a more in-depth investigation of the nitrate reduction process in the column system, a graph was made showing the nitrate removal efficiency as a function of the number of effluent recirculations (Figure 1), which was reached in 5 days. It is noteworthy that the maximum nitrate reduction occurs during the seventeenth to seventeenth circulation (Cir16 to Cir17).

For the verification and interpretation of the data, a logistic mathematical model was used, which represents an important tool widely used to solve several practical problems, both in the design phase and in the operation phase of the BRP, such as:

-

determining the thickness of the reactive medium layer along the barrier

-

determination of the standing time of the water,

-

the evaluation of the economic potential as a result of the adequate design of the operating parameters, which cannot be modified during the remediation

The logistic curve is a growth-limited curve that has the shape of an oblong S-shaped curve. It characterizes an evolution that starts from the lower asymptotic limit with the zero value, and finally reaches an upper limit. At first the evolution proceeds slowly, similar to simple exponential growth. Then the growth accelerates, meanwhile it passes through an inflection point, after which the growth slowly decreases, thus the evolution asymptotically heading towards the upper limit.

Figure 6. Logistic curves compared to the experimental points for the three samples.

Figure 6. Logistic curves compared to the experimental points for the three samples.

It should be noted that the removal efficiency is progressive, but not very accelerated at the beginning of the process, this fact would be attributed to two processes, first are the redox processes, in which the iron in the Fe⁰ state reacts with nitrogen ions, and on as soon as the exchange positions are freed, ion exchange processes appear simultaneously, which will contribute to accelerating the removal of the nitrogen ion.

From the examination of the curves represented for the three samples in relation to the position of the experimental points, it is found that the removal kinetics of nitrogen ions can be described mathematically precisely enough by logistic equation models.

During the tests, nitrite was not detected in most samples and no accumulation was detected (Figure 7). This suggests that denitrification by the acceptor A400-nZVI was rapid and nitrite ions were rapidly converted to ammonium after generation. Ammonium concentration was also not detected and this can be attributed to adsorption by the ion exchange resin. However, considering the low regulatory limit of ammonium, it is important to monitor the concentration of this ion, especially in heavily contaminated sites.

Based on the data in table 6.1. and using relation (1)-(2) the characteristic parameters of the first-order kinetic model, t_1/2 and k, are calculated, under dynamic conditions for the three types of flows (Table 2).

The cumulative mean percentage of nitrate removal observed for each run indicates that the half-life for nitrate degradation is about three days and more than 90% of the NO₃^- has been degraded after about 5 days.

The rate of worsening of the remediation conditions of N-polluted groundwater will be all the more alert the longer the half-life of N is and vice versa. The half-life t_1/2 of the N content varies depending on the working conditions, regarding the variation of the infiltration rate and the precipitation regime.

3.3. Calculation of the yield (η) of nitrogen ion removal

The removal efficiency (η) of the nitrogen ion from the water is determined. A graph is drawn of the yield (η) of nitrogen ion removal over time. A graph is drawn to express the formation of the nitrate and ammonium ions over time.

$$\eta =\frac{C_i-C_f}{C_i}·100$$

(3)

where:

• C_i – concentration of nitrogen ions from the initial sample, mg/L
• C_i – the concentration of nitrogen ions removed from the residual water, mg/L

Table 3. The removal efficiency (η) of the nitrogen ion from synthetic water.

Table 3. The removal efficiency (η) of the nitrogen ion from synthetic water.

η1, %	η2, %	η3, %
31,5	28,8	23,5
32,8	30,6	25,5
37,7	31,8	27,2
39,1	33,9	29,9
41,9	35,3	31,3
42,8	37,6	33,1
44,2	38,9	34,3
47,4	42,6	36,5
49,21	47,5	43,7
68,64	65,4	48,2
88,9	86,6	65,9
94,24	93,2	82,9

3.4. Calculation of barrier parameters

3.4.1. Hydraulic conductivity

To calculate the hydraulic conductivity (k) Darcy's law is used:

$$Q=k\ast \frac{A}{L}(k_2-k_1)$$

$$A=\frac{{\pi d}^2}{2}=\frac{\mathrm{3,14}\ast 3^2}{2}=\mathrm{14,13}{cm}^2$$

$k_2-k_1$= the hydraulic gradient = 7

L = column length = 100 cm

$$k=\frac{Q\ast L}{A\left(k_2-k_1\right)}$$

It operates at three flow rates Q₁, Q₂, Q₃, so we will have to calculate 3 hydraulic conductivities:

$$k_1=\frac{Q\ast L}{A\left(k_2-k_1\right)}=\frac{14\ast \mathrm{1,66}\ast {10}^{-7}\ast 100\ast {10}^{-2}}{\mathrm{14,13}\ast {10}^{-3}\ast 7\ast {10}^{-2}}=0.23\ast {10}^{-2}\frac{m}{s}$$

Q from [$\frac{mL}{min}]$ is converted to [$\frac{m^3}{s}]$

$$\begin{gathered} 1\frac{mL}{min}=\frac{{10}^{-6}}{60}=\mathrm{1,66}\ast {10}^{-7}\frac{m^3}{s} \\ {k}_2=\frac{\mathrm{16,8}\ast \mathrm{1,66}\ast {10}^{-7}\ast 100\ast {10}^{-2}}{\mathrm{14,13}\ast {10}^{-3}\ast 7\ast {10}^{-2}}=0.28\ast {10}^{-2}\frac{m}{s} \\ {k}_3=\frac{28\ast \mathrm{1,66}\ast {10}^{-7}\ast 100\ast {10}^{-2}}{\mathrm{14,13}\ast {10}^{-3}\ast 7\ast {10}^{-2}}=0.47\ast {10}^{-2}\frac{m}{s} \\ \overline{k}=\frac{k_1+k_2+k_3}{3}=\frac{0.23\ast {10}^{-2}+0.28\ast {10}^{-2}+0.47\ast {10}^{-2}}{3}=0.32\ast {10}^{-2}\frac{m}{s} \end{gathered}$$

3.4.2. Flow rate through the section, or filtration rate

$$\begin{gathered} \mathrm{v}=\frac{\mathrm{Q}}{\mathrm{A}} \\ {v}_1=\frac{Q_1}{A}=\frac{14\ast \mathrm{1,66}\ast {10}^{-7}}{\mathrm{14,13}\ast {10}^{-3}}=\mathrm{1,64}\ast {10}^{-4}\frac{m}{s} \\ {v}_2=\frac{Q_2}{A}=\frac{\mathrm{16,8}\ast \mathrm{1,66}\ast {10}^{-7}}{\mathrm{14,13}\ast {10}^{-3}}=\mathrm{1,97}\ast {10}^{-4}\frac{m}{s} \\ {v}_3=\frac{Q_3}{A}=\frac{28\ast \mathrm{1,66}\ast {10}^{-7}}{\mathrm{14,13}\ast {10}^{-3}}=\mathrm{3,27}\ast {10}^{-4}\frac{m}{s} \end{gathered}$$

3.4.3. Barrier Permeability (K)

$$K=\frac{d_m^2\ast {\phi }^3}{180{(1-\phi )}^2}$$

${\mathrm{d}}_{\mathrm{m}}$= the modular diameter of the particles (it will be calculated based on the texture of the soil and the barrier)

φ = porosity = 0.4

$$\begin{gathered} {\mathrm{d}}_{\mathrm{m}}=\frac{66\ast \mathrm{0,2}}{100}+\frac{21\ast \mathrm{0,02}}{100}+\frac{\mathrm{5,6}\ast \mathrm{0,02}}{100}+\frac{7.4\ast \mathrm{0,02}}{100}+\frac{1.2}{100}=0.148{mm}^2 \\ K=\frac{{0.148}^2\ast {0.4}^3}{180{(1-\mathrm{0,4})}^2}=\mathrm{2,17}\ast {10}^{-5}{mm}^2=\mathrm{2,17}\ast {10}^{-7}{m}^2 \end{gathered}$$

Table 4. BRP Parameters.

Table 4. BRP Parameters.

Q, m3/s	k, m⋅s-1	v, m⋅s-1	K, m2
Q1	0.23⋅10-2	1.64⋅10-4	2.17⋅10-7
Q2	0.28⋅10-2	1.97⋅10-4
Q3	0.47⋅10-2	3.27⋅10-4

Contaminant removal with BRP occurs mainly in the zone of reactive media and in some cases, down the gradient of the barrier, depending on the type of media used.

Some of the reactive media remove contaminants by direct contact, while other reactive media modify/involve biogeochemical processes in the treatment area, thus ensuring favorable conditions for immobilization of contaminants or (bio)degradation. The primary objective of BRP, regardless of the design used, is to bring contaminants into the reactive zone where they can be destroyed or immobilized.

The average resin diameter was ∼1.2 mm. The size is related to the permeability of the media, this parameter being very important for the successful operation of a permeable reactive barrier. The material selected should minimize constraints on groundwater flow and the particle size should not be excessively small. The permeability of the doped nZVI was higher than that of the aquifer (~10-3 cm/s). This suggests that the created material can be used as a reactive medium for permeable reactive barriers.

3.5. Adsorption Mechanism

Nitrate is a stable and chemically unreactive species that occurs as part of the nitrogen cycle. Nitrate (NO₃^–) is the most oxidized form of combined nitrogen for oxygenated systems (equations (4.1) and (4.2)).

However, nitrate can be reduced by microbial and chemical processes to nitrite, ammonia, nitrogen oxides and nitrogen gas under various environmental conditions (equation (4.3)

$$4{NH}_4^{+}+7{\mathrm{O}}_2\to 4{NO}_2^{-}+4{\mathrm{H}}^{+}+6{\mathrm{H}}_2\mathrm{O}$$

(4.1)

$$2{NO}_2^{-}+{\mathrm{O}}_2\to 2{NO}_3^{-}$$

(4.2)

$${NO}_3^{-}\to {NO}_2^{-}\to \mathrm{NO}\to {\mathrm{N}}_2\mathrm{O}\to {\mathrm{N}}_2$$

(4.3)

Nitrate contamination of groundwater is governed by natural biogeochemical processes that control nitrate leaching into groundwater. In the root zone of the topsoil, nitrogen fixation and nitrification processes make nitrogen available to plants in the form of ammonium (NH₄⁺) and nitrate. Nitrate reduction can occur through plant uptake, mineralization-immobilization processes, volatilization, runoff losses, and denitrification. (Huno, 2018)

These processes, either independently or in combination, limit the flow of nitrate to groundwater. However, nitrate ions, unlike positively charged NH₄⁺ (which are bound to negatively charged soil particles), are weakly bound and can percolate beyond root zones into the water table of the groundwater aquifer environment. Microbial denitrification is common in soils with more than 60% pore saturation, where oxygen for microbial respiration is limited. Microbial reduction of nitrate ion is a respiratory process that uses nitrate or nitrite as the terminal electron acceptor under conditions of limited oxygen supply.

The nZVI reaction is a nitrate reduction reaction to ${NO}_2^{-}$ (nitrite), NH₄⁺ (ammonium) and N₂ (nitrogen). ZVI is oxidized to Fe (II). The main chemical reaction equations of denitrification are summarized here:

Fe⁰ + 2 H₂O → Fe²⁺ + H₂ + 2 OH^-

(4.5)

$$5{\mathrm{Fe}}^0+2{NO}_3^{-}+12{\mathrm{H}}^{+}\to {\mathrm{Fe}}^{2+}+{\mathrm{H}}_{2+}6{\mathrm{H}}_2\mathrm{O}$$

(4.6)

$$4{\mathrm{Fe}}^0+2{NO}_3^{-}+10{\mathrm{H}}^{+}\to 4{\mathrm{Fe}}^{2+}+4{NH}_4^{+}+3{\mathrm{H}}_2\mathrm{O}$$

(4.7)

$${\mathrm{Fe}}^0+2{NO}_3^{-}+4{\mathrm{H}}^{+}\to {\mathrm{Fe}}^{2+}+2{NO}_2^{-}+2{\mathrm{H}}_2\mathrm{O}$$

(4.8)

$$6{\mathrm{Fe}}^{2+}+{NO}_2^{-}+8{\mathrm{H}}^{+}\to 6{\mathrm{Fe}}^{3+}+{NH}_4^{+}+2{\mathrm{H}}_2\mathrm{O}$$

(4.9)

Figure 8. Scheme of the nitrate reduction mechanism with nZVI. (Rohard, 2020).

Figure 8. Scheme of the nitrate reduction mechanism with nZVI. (Rohard, 2020).

The core of the nZvi particle consists mainly of zero-valent iron and provides the power to reduce reactions with environmental contaminants. The coating is mostly iron oxides/hydroxides formed from the oxidation of zero-valent iron.

To date, ZVI applications have mainly focused on the electron-donating properties of ZVI. Under ambient conditions, ZVI is quite reactive in water and can serve as an excellent electron donor, making it a versatile remedial material. (Li, 2006)

There are problems with these reactions, first of all: under neutral conditions, corrosion of nZVI produces iron oxides. These products provide a barrier layer on the nZVI particles, which decrease the reaction rate due to a decrease in mass transfer. The second aspect is represented by the main products of the reaction: nitrites and ammonium are also contaminants. (Rohard, 2020)

4. Conclusion

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