3.3.1. Remediation by Chemically Synthesized nZVI Particles

The remediation monitoring of lead and nickel using chemically synthesized nZVI particles is presented in Figure 3, where the residual lead and nickel concentration at the dosage of 0.1 g of nanoparticle/kg of soil added to each contaminated soil sample (A and B) is presented. *Processes* **2020**, *8*, x FOR PEER REVIEW 7 of 12

**Figure 3.** Remediation of contaminated soil using chemically synthesized nZVI particles (dosage: 0.1 g/kg of soil): (**a**) lead removal; (**b**) nickel removal. **Figure 3.** Remediation of contaminated soil using chemically synthesized nZVI particles (dosage: 0.1 g/kg of soil): (**a**) lead removal; (**b**) nickel removal.

Considering the system heterogeneity, the experimental data show similar values for the tested soil samples, and on this basis, the heavy metal removal efficiency was calculated as the average value at the end of the experimental run. Considering the system heterogeneity, the experimental data show similar values for the tested soil samples, and on this basis, the heavy metal removal efficiency was calculated as the average value at the end of the experimental run.

After 30 days and with a particle dosage of 0.1 g/kg of soil, the Pb removal efficiency was 21.6% and 18.5% for nickel. It can be observed that in any instance, the contaminant removal efficiency of the nanoparticle is low. This finding could be due to aggregation and/or oxidation of nanoparticles. In general, oxidation of the outer layer of the nanoparticle is mainly due to contact with air, which shrinks the adsorption capability of the nanoparticle. This hypothesis is supported by the SEM and TEM analyses. The SEM images show that the nanoparticles are aggregated, which reduces the surface area of the particle. The TEM images indicate the oxidation of the outer layer of nanoparticles. After 30 days and with a particle dosage of 0.1 g/kg of soil, the Pb removal efficiency was 21.6% and 18.5% for nickel. It can be observed that in any instance, the contaminant removal efficiency of the nanoparticle is low. This finding could be due to aggregation and/or oxidation of nanoparticles. In general, oxidation of the outer layer of the nanoparticle is mainly due to contact with air, which shrinks the adsorption capability of the nanoparticle. This hypothesis is supported by the SEM and TEM analyses. The SEM images show that the nanoparticles are aggregated, which reduces the surface area of the particle. The TEM images indicate the oxidation of the outer layer of nanoparticles.

**Soil A**

**Soil B**

 (**a**) (**b**) **Figure 4.** Remediation of nZVI particles synthesized using neem leaves (dosage: 0.1 g/kg of soil): (**a**)

**0 10 20 30 40**

**Soil A**

**Soil B**

**Time (d)**

The nZVI particles synthesized using neem leaves were tested in the same conditions as the

**Residual Ni concentration**

**(mg/kg of soil)**

3.3.2. Remediation by nZVI Particles Synthesized Using Neem Leaves

lead removal; (**b**) nickel removal.

**0 10 20 30 40**

**Time (d)**

**Residual Pb concentration**

**(mg/kg of soil)**

with Soils A and B at a dosage of 0.1 g of neem synthesized nZVI/kg of soil.

#### 3.3.2. Remediation by nZVI Particles Synthesized Using Neem Leaves The nZVI particles synthesized using neem leaves were tested in the same conditions as the chemically synthesized particles, and the experimental results are shown in Figure 4.

g/kg of soil): (**a**) lead removal; (**b**) nickel removal.

**0 10 20 30 40**

**Time (d)**

value at the end of the experimental run.

**Residual Pb concentration**

**(mg/kg of soil)**

The nZVI particles synthesized using neem leaves were tested in the same conditions as the chemically synthesized particles, and the experimental results are shown in Figure 4. Figure 4a,b shows the residual lead and nickel concentration, respectively, along with the tests with Soils A and B at a dosage of 0.1 g of neem synthesized nZVI/kg of soil.

*Processes* **2020**, *8*, x FOR PEER REVIEW 7 of 12

**Soil A**

**Soil B**

 (**a**) (**b**) **Figure 3.** Remediation of contaminated soil using chemically synthesized nZVI particles (dosage: 0.1

**Residual Ni concentration**

**(mg/kg of soil)**

**0 10 20 30 40**

**Soil A**

**Soil B**

**Time (d)**

Considering the system heterogeneity, the experimental data show similar values for the tested soil samples, and on this basis, the heavy metal removal efficiency was calculated as the average

After 30 days and with a particle dosage of 0.1 g/kg of soil, the Pb removal efficiency was 21.6% and 18.5% for nickel. It can be observed that in any instance, the contaminant removal efficiency of the nanoparticle is low. This finding could be due to aggregation and/or oxidation of nanoparticles. In general, oxidation of the outer layer of the nanoparticle is mainly due to contact with air, which shrinks the adsorption capability of the nanoparticle. This hypothesis is supported by the SEM and TEM analyses. The SEM images show that the nanoparticles are aggregated, which reduces the surface area of the particle. The TEM images indicate the oxidation of the outer layer of nanoparticles.

**Figure 4.** Remediation of nZVI particles synthesized using neem leaves (dosage: 0.1 g/kg of soil): (**a**) lead removal; (**b**) nickel removal. **Figure 4.** Remediation of nZVI particles synthesized using neem leaves (dosage: 0.1 g/kg of soil): (**a**) lead removal; (**b**) nickel removal. *Processes* **2020**, *8*, x FOR PEER REVIEW 8 of 12

Figure 4a,b shows the residual lead and nickel concentration, respectively, along with the tests with Soils A and B at a dosage of 0.1 g of neem synthesized nZVI/kg of soil. Regarding the chemically synthesized particles, Soils A and B performed similarly. After 30

Regarding the chemically synthesized particles, Soils A and B performed similarly. After 30 days, the removal efficiency of lead at a dosage of 0.1 g of neem synthesized nZVI/kg of soil was 26.9%, and for nickel, it was 33.2%, demonstrating better performance with nickel than with lead. days, the removal efficiency of lead at a dosage of 0.1 g of neem synthesized nZVI/kg of soil was 26.9%, and for nickel, it was 33.2%, demonstrating better performance with lead than with nickel. After this, it was decided to test double particle dosage with the same initial heavy metal

After this, it was decided to test double particle dosage with the same initial heavy metal concentration. concentration.

Figure 5 depicts the monitoring with 0.2 g of nZVI particles synthesized using neem leaves per kg of soil. Figure 5 depicts the monitoring with 0.2 g of nZVI particles synthesized using neem leaves per kg of soil.

**Figure 5.** Remediation of nZVI particles synthesized using neem leaves (dosage: 0.2 g/kg of soil): (**a**) lead removal; (**b**) nickel removal. **Figure 5.** Remediation of nZVI particles synthesized using neem leaves (dosage: 0.2 g/kg of soil): (**a**) lead removal; (**b**) nickel removal.

The trend is the same as that achieved with a dosage of 0.1 g/kg of soil. The lead and nickel removal efficiency was observed as 33.3% and 38.2%, respectively. Comparing these values to lower values, it is evident that the improvement is limited in the order of 24% for lead and 15% for nickel. The trend is the same as that achieved with a dosage of 0.1 g/kg of soil. The lead and nickel removal efficiency was observed as 33.3% and 38.2%, respectively. Comparing these values to lower values, it is evident that the improvement is limited in the order of 24% for lead and 15% for nickel.

Examining all the findings, the removal efficiency is not high. This could be due to the low surface area measured by the BET analysis. Agglomeration and oxidation of nanoparticles play a vital Examining all the findings, the removal efficiency is not high. This could be due to the low surface area measured by the BET analysis. Agglomeration and oxidation of nanoparticles play a vital role in

role in the reduction of removal efficiencies, and unfortunately, these phenomena are evidenced by

 **(a) (b) Figure 6.** Remediation of nZVI particles synthesized using mint leaves (dosage: 0.1 g/kg of soil): (**a**)

**0 10 20 30 40**

**Soil A**

**Soil B**

**Time (d)**

**Soil A**

**Soil B**

The particles derived from mint leaves were tested at the same dosage previously used, namely

**Residual Ni concentration**

**(mg/kg of soil)**

3.3.3. Remediation by nZVI Particles Synthesized Using Mint Leaves

0.1 g/kg of soil. Figure 6 reports the monitoring during the 30-day test.

SEM and TEM images, respectively.

lead removal; (**b**) nickel removal.

**0 10 20 30 40**

**Time (d)**

**Residual Pb concentration**

**(mg/kg of soil)**

lead removal; (**b**) nickel removal.

**0 10 20 30 40**

**Time (d)**

concentration.

kg of soil.

**Residual Pb concentration**

**(mg/kg of soil)**

the reduction of removal efficiencies, and unfortunately, these phenomena are evidenced by SEM and TEM images, respectively. surface area measured by the BET analysis. Agglomeration and oxidation of nanoparticles play a vital role in the reduction of removal efficiencies, and unfortunately, these phenomena are evidenced by SEM and TEM images, respectively.

The trend is the same as that achieved with a dosage of 0.1 g/kg of soil. The lead and nickel removal efficiency was observed as 33.3% and 38.2%, respectively. Comparing these values to lower

**0 10 20 30 40**

**Soil A**

**Soil B**

**Time (d)**

 (**a**) (**b**) **Figure 5.** Remediation of nZVI particles synthesized using neem leaves (dosage: 0.2 g/kg of soil): (**a**)

**Soil A**

**Soil B**

*Processes* **2020**, *8*, x FOR PEER REVIEW 8 of 12

Regarding the chemically synthesized particles, Soils A and B performed similarly. After 30 days, the removal efficiency of lead at a dosage of 0.1 g of neem synthesized nZVI/kg of soil was 26.9%, and for nickel, it was 33.2%, demonstrating better performance with lead than with nickel.

After this, it was decided to test double particle dosage with the same initial heavy metal

Figure 5 depicts the monitoring with 0.2 g of nZVI particles synthesized using neem leaves per

**Residual Ni concentration**

**(mg/kg of soil)**

3.3.3. Remediation by nZVI Particles Synthesized Using Mint Leaves 3.3.3. Remediation by nZVI Particles Synthesized Using Mint Leaves

The particles derived from mint leaves were tested at the same dosage previously used, namely 0.1 g/kg of soil. Figure 6 reports the monitoring during the 30-day test. The particles derived from mint leaves were tested at the same dosage previously used, namely 0.1 g/kg of soil. Figure 6 reports the monitoring during the 30-day test.

**Figure 6.** Remediation of nZVI particles synthesized using mint leaves (dosage: 0.1 g/kg of soil): (**a**) lead removal; (**b**) nickel removal. **Figure 6.** Remediation of nZVI particles synthesized using mint leaves (dosage: 0.1 g/kg of soil): (**a**) lead removal; (**b**) nickel removal. *Processes* **2020**, *8*, x FOR PEER REVIEW 9 of 12

The tested soils behaved similarly, probably due to their similar properties. At the end of the test (after 30 days), the removal efficiency was 62.3% (lead removal) and 50.6% (nickel removal). The tested soils behaved similarly, probably due to their similar properties. At the end of the test (after 30 days), the removal efficiency was 62.3% (lead removal) and 50.6% (nickel removal).

In this instance, the particles showed better performance with lead than nickel. In both instances, the values were much higher than those with particles derived using neem leaves. In this instance, the particles showed better performance with lead than nickel. In both instances, the values were much higher than those with particles derived using neem leaves.

Figure 7 shows the removal efficiency at a doubled dosage of particles derived using mint leaves (0.2 g/kg of soil). Figure 7 shows the removal efficiency at a doubled dosage of particles derived using mint leaves (0.2 g/kg of soil).

**Figure 7.** Remediation of nZVI particles synthesized using mint leaves (dosage: 0.2 g/kg of soil): (**a**) lead removal; (**b**) nickel removal. **Figure 7.** Remediation of nZVI particles synthesized using mint leaves (dosage: 0.2 g/kg of soil): (**a**) lead removal; (**b**) nickel removal.

The experimental data do not show relevant differences between the tested soils. The experimental data do not show relevant differences between the tested soils.

area is higher than for neem-derived particles (Figure 1).

The targeted heavy metals were lead and nickel.

**4. Discussion** 

chosen.

summarized in Table 2.

The removal efficiency was higher for lead than nickel, confirming the trend achieved with a lower dosage. After 30 days, 66.1% of the initial lead and 56.1% of the initial nickel was removed. The removal efficiency was higher for lead than nickel, confirming the trend achieved with a lower dosage. After 30 days, 66.1% of the initial lead and 56.1% of the initial nickel was removed.

One reason for the better performance of particles synthesized using mint leaves could be the agglomeration, which is comparatively less than for neem-derived particles, and hence, their surface

This study aimed to provide preliminary results about the performance of nZVI particles derived using green leaves to remediate heavy-metal-polluted soil. At the same time, chemically synthesized nanoparticles were prepared to compare their removal efficiency to those achievable by vegetal-origin nanoparticles. As leaves, *Azadirachta indica* (neem) and *Mentha longifolia* (mint) were

For a rational discussion and comparison easiness, the results for removal efficiency are

**Table 2.** The heavy metal removal efficiency of the tested nZVI particles.

Neem leaves 26.9% 33.3% 33.2% 38.2% Mint leaves 62.3% 66.1% 50.6% 56.1%

Particle dosage

Chemically synthesized 21.6% 18.5%

By these values, some conclusions can be evidenced:

**Particle Origin Pb Removal Efficiency at t = 30 days Ni Removal Efficiency at t = 30 days** 

0.1 g/kg of soil 0.2 g/kg of soil 0.1 g/kg of soil 0.2 g/kg of soil

of the chemically synthesized particles, as can be observed from the TEM images (Figure 2).

One reason for the better performance of particles synthesized using mint leaves could be the agglomeration, which is comparatively less than for neem-derived particles, and hence, their surface area is higher than for neem-derived particles (Figure 1).

When the particles were oxidized on their outer surface, the oxidation rate was lower than that of the chemically synthesized particles, as can be observed from the TEM images (Figure 2).
