*2.4. Pollution of Soil Samples and Nanoparticle Addition*

The heavy-metal-contaminated soil was prepared by adding Ni or Pb solution to the soil. For nickel, a solution was prepared by dissolving 4.785 g of NiSO4·7H2O in 1 L of double-distilled water. Similarly, the lead solution was made by dissolving 1.615 g of Pb(NO3)<sup>2</sup> in 1 L of double-distilled water. Spiking of the soil was conducted to achieve a target metal concentration around 250 mg/kg of soil and, at the same time, considering the solution water content as a contribution to meet the soil field capacity. The initial heavy metal concentration was constantly verified by analysis.

The addition of nanoparticles was conducted on parts of the initial sample to optimize the distribution. Each soil sample and nanoparticle dose were divided into five parts. Then, each soil part was mixed with one particle part. Finally, these 5 quotas were transferred to a 1 L beaker and monitored for 30 days. Two dosages were tested, namely 0.1 and 0.2 g/kg of soil. *Processes* **2020**, *8*, x FOR PEER REVIEW 4 of 12

Similarly, the lead solution was made by dissolving 1.615 g of Pb(NO3)2 in 1 L of double-distilled

#### *2.5. Remediation of Soil Samples* water. Spiking of the soil was conducted to achieve a target metal concentration around 250 mg/kg

The remediation of heavy-metal-contaminated soil was studied at room temperature for 30 days. Along each test period, Pb and Ni residual concentration was monitored by AAS analysis on the extract derived from the 5 g sample leaching with HNO<sup>3</sup> (see Section 2.3), and the effect of concentration and time on soil was studied. of soil and, at the same time, considering the solution water content as a contribution to meet the soil field capacity. The initial heavy metal concentration was constantly verified by analysis. The addition of nanoparticles was conducted on parts of the initial sample to optimize the distribution. Each soil sample and nanoparticle dose were divided into five parts. Then, each soil part was mixed with one particle part. Finally, these 5 quotas were transferred to a 1 L beaker and

All the analyses were replicated twice. The average results will be shown without error bars and standard deviations because two replicates cannot support a reliable statistical analysis. monitored for 30 days. Two dosages were tested, namely 0.1 and 0.2 g/kg of soil. *2.5. Remediation of Soil Samples* 

The residual concentration at time t = 30 days was used to calculate the removal efficiency: The remediation of heavy-metal-contaminated soil was studied at room temperature for 30 days.

$$\text{Heavy metal removal} \left\| \begin{aligned} \text{°C}\_{i} &= \text{C}\_{i} - \text{C}\_{f} \\ \text{°C}\_{i} &= \text{C}\_{i} \end{aligned} \right\| \times 100 \tag{1}$$

×100 (1)

Ci − Cf Ci

where: C<sup>i</sup> = initial heavy metal concentration (mg/kg of soil), C<sup>f</sup> = final heavy metal concentration (mg/kg of soil). All the analyses were replicated twice. The average results will be shown without error bars and standard deviations because two replicates cannot support a reliable statistical analysis. The residual concentration at time t = 30 days was used to calculate the removal efficiency:

Heavy metal removal % =
