**4. Experimental research on the accumulation of heavy metals over time in fresh lettuce**

Experimental research focused on growing and monitoring a lettuce crop over a 68-day period, the optimal vegetation period is 45–50 days [50]. The lettuce culture was carried out this time again in a controlled environment, in pots and in the greenhouse. The soil was contaminated with zinc sulfate solution in three concentrations: 1.5%, 3.0% and 4.5%, respectively, prepared solutions similar to those in the previous experiment.

The soil was homogenized with each solution before planting the lettuce seedlings in pots, as shown in the **Figure 4**. One kg of soil was homogenized with 250 ml solution, under the same experimental conditions, without further addition. 11 samples were placed for each concentration of the soil in zinc, harvesting being made at distances of about seven days each.

The crop has also developed in the greenhouse, the environmental conditions being favorable for the development of plants and having slight variations.

Sampling of the vegetal samples was done in time up to 68 days after planting (**Figure 5**) and each time the lettuce was harvested, the soil sample was taken from the pot, after it was homogenized.

The determination of zinc from the contaminated soil and from the whole fresh lettuce plant (root and leaves) was performed by the spectrophotometric method (atomic absorption in the flame) [27, 28].

The variation in the zinc content of lettuce is shown graphically in **Figure 6**. It is generally observed that the concentration of zinc in lettuce leaves increases for each of the concentrations of contaminated soil.

**Figure 4.** *Planting the lettuce seedling.*

**Figure 5.**

*Pots with plants from the three types of crops harvested at several.*

**Figure 6.** *Variation in time of the concentration of heavy metal (Zn) in lettuce.*

At certain time periods, experimental data show a decrease in zinc in plants. If the phenomenon is real, and there was no error in the measurement, it can be concluded that the removal of metal from the plant was natural, in this regard, new experiments are indicated to be able to determine and which factors influenced this process.

If we count the zinc concentrations obtained in plants, we notice that: for soil contaminated with 4.5%, 7 of the 11 crops have the highest concentration of zinc; for soil contaminated with 3.0%, 3 of the 11 crops have the highest concentration of zinc; for soil contaminated with 1.5%, 1 of the 11 crops have the highest concentration of zinc. This observation is also supported by the average lettuce harvest, which recorded 228.9 mg/kg, for the crop developed in soil contaminated with 1.5% Zn, 315.3 mg/kg, for the crop developed in soil contaminated with 3.0% Zn si 357.9 mg/kg, for the crop developed in soil contaminated with 4.5% Zn.

The conclusion is that after the concentration of the soil in zinc the order of accumulation of metal in the lettuce is: 4.5% > 3.0% > 1.5%.

*Evaluate the Impact of Soil Contamination on Vegetables and Fruits DOI: http://dx.doi.org/10.5772/intechopen.110445*

**Figure 7.** *Variation in time of the concentration of heavy metal (Zn) in soil.*

The variation in the zinc content of lettuce is shown graphically in **Figure 7**. It is observed that, over time, the concentration of zinc oscillates, even slightly increasing. Again, it is questioned whether the phenomenon is real or whether measurement errors have occurred. If in the lettuce it can be believed that the metal could not be absorbed by the plant and that the mass has increased, in the case of soil, this conclusion is hard to say, although some soil chemicals can pass into plants as well.
