**4. Discussion**

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 chosen.

The targeted heavy metals were lead and nickel.

For a rational discussion and comparison easiness, the results for removal efficiency are summarized in Table 2.


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

By these values, some conclusions can be evidenced:


To summarize, the ZVI nanoparticles synthesized using green leaves demonstrated a good performance to remove lead and nickel from polluted soil, better than those obtained from chemical synthesis.

To this purpose, different particle properties could play opposite roles. The chemically synthesized particles showed higher BET surface area of approximately 15 against 6–13 m<sup>2</sup> /g of the others. Therefore, the adsorption should occur to a larger extent. However, as a consequence of this property, they were also highly subjected to oxidation, as shown by the TEM images, reducing their removal efficiency.

Examining the residual heavy metal concentration, when chemically synthesized particles were adopted, a decreasing trend was continuous, whereas in the other tests, two-step process could be identified: the first one, lasting about 8–10 days, with a drastic decrease of residual concentration, followed by further removal at a slower rate until the end of the test.

A similar trend was found by other authors:

• Gil-Diaz et al. [32] studied in-field brownfield remediation polluted with arsenic and mercury using commercial nZVI particles for long times (32 months) and achieved an initial sharp reduction of the pollutants, followed by a rather constant residual concentration probably due to interferences of organic matter that can form stable complexes on the particle surface, especially at acidic pH and limiting the particle removal efficiency. Examining Figures 3–5, this occurs for nickel

removal in Soil B, where pH is moderately acidic (5.32) and the organic matter content is high (about 5%). This is evident for nickel, whereas the pH influence is not appreciable for lead. One hypothesis can be the different solubility product constant, KPS, for Ni(OH)<sup>2</sup> and Pb(OH)2, which is equal to 6 × 10−<sup>16</sup> and 1.4 × 10−20, respectively. A rough calculation can provide the saturation concentration of the heavy metals at the tested pH values. For lead, at saturation its concentration is lower than the initial concentration in both instances. Therefore, lead hydroxide also precipitates at an acidic pH. Heavy metal removal by nZVI particles is not only based on precipitation. For lead, this phenomenon could be more influential than the others (adsorption, coprecipitation, oxidation/reduction);


From this, it is evident that the findings of the present study are widely supported by similar studies, even if performed with different operative conditions of pollutants, particle origin, and concentration, soil properties, etc.

These findings also evidence the need to continue the study and clarify the many features still pending to apply the process optimally and achieve high efficiency in sustainable terms.

**Author Contributions:** Conceptualization, S.S.; methodology, S.S. and Y.R.S.; validation, S.S.; formal analysis, S.S. and F.C.; investigation, N.F.; resources, S.S.; writing—original draft preparation, S.S.; writing—review and editing, S.S. and F.C.; supervision, S.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** The first author (N.F.) acknowledges the financial support provided by The Institution of Engineers (India), Kolkata, India under the R & D Grant-in-aid Scheme (Grant No. RDPG2016004).

**Conflicts of Interest:** The authors declare no conflict of interest.
