**2. Heavy metals in soil**

Heavy metals are harmful for the human health, because they tend to accumulate in the living organisms. This bioaccumulation is caused by the high rate of absorption comparing to the rate of metabolism or excretion of the harmful compounds. The emissions from the metallur‐ gical plants are transported by air masses up to 10 km from the pollution source and are deposited on the ground leading to an increase of heavy metal concentration in the upper layer of soil.

In soil, the soluble metals go into the soil solution and can be absorbed or immobilized by plants or can be leached to the deeper layer of soil and to the ground water (Figure 1). Some metals are chemical or physical adsorbed to soil particles. The fate of metals in soil depends on the depth of soil layer, on the erosion processes and on the pH. The heavy metals adsorbed to the soil particles from the upper layer can be subject to the erosion processes and transported by surface waters or by wind. The metals absorbed to deeper soil particles can be subject to microbiological and chemical degradation, can be stabilized by the plant root, or can cross from stable to available forms according with pH. The biological activity influences the speed and rate of pollutants degradation and the clay-humus complex represents an efficient buffer in neuter or alkaline soil reaction [4].

**Figure 1.** The fate of pollutants in soil [2]

ment, and transfer into the food chain. With the decomposition of plant biomass, metals can be washed by rain and transported back into the soil. In order that the phytoremediation process to result in effective outcomes and the level of heavy metals from the environ‐ ment to decrease, not only to move those metals from one area to another, the remedia‐ tion of polluted soils should end with quantitative recovery of metals [3]. The recovery of heavy metals has the advantage of increasing the economic value of the phytoremediation process by transforming this method in a financial self-supporting approach of environmen‐

There have been numerous studies on the phytoremediation process, having examined the species of plants that have greatest ability to accumulate heavy metals, factors affecting results of phytoremediation and the areas to be covered with plants for remediation purpose, but studies on treatment, storage or use of resulting biomass are insufficient. Some studies presented the possibilities of heavy metal recovery from different waste, even from agricul‐ tural waste. The present research aims to put in one sentence the phytoremediation process

The research focused on identifying methods of heavy metal recovery from ash, resulted from the incineration of biomass. The phytoremediation process needs to end up with the heavy metal recovery to obtain (a) de-polluted soil, (b) ash with low content of heavy metals, that can be used as fertilizer in agriculture and (c) amounts of heavy metals that can be recovered in the industry to obtain an economic advantage by financially self-supporting of the phytor‐ emediation process. Because of the lack of researches in this domain, this research was conducted based on the results of those studies that aim to recover metals from different kind

Heavy metals are harmful for the human health, because they tend to accumulate in the living organisms. This bioaccumulation is caused by the high rate of absorption comparing to the rate of metabolism or excretion of the harmful compounds. The emissions from the metallur‐ gical plants are transported by air masses up to 10 km from the pollution source and are deposited on the ground leading to an increase of heavy metal concentration in the upper layer

In soil, the soluble metals go into the soil solution and can be absorbed or immobilized by plants or can be leached to the deeper layer of soil and to the ground water (Figure 1). Some metals are chemical or physical adsorbed to soil particles. The fate of metals in soil depends on the depth of soil layer, on the erosion processes and on the pH. The heavy metals adsorbed to the soil particles from the upper layer can be subject to the erosion processes and transported by surface waters or by wind. The metals absorbed to deeper soil particles can be subject to microbiological and chemical degradation, can be stabilized by the plant root, or can cross from stable to available forms according with pH. The biological activity influences the speed

and the recovery of heavy metals from the phytoremediation by-products.

of waste (from agriculture, sewage sludge, or woods).

**2. Heavy metals in soil**

of soil.

tal remediation.

310 Environmental Risk Assessment of Soil Contamination

The availability of metallic compounds in soil for plants depends on the soil texture, organic matter content of soil, cation exchange capacity, calcium carbonate equivalent and pH. Soil organic substances play an extremely important role because they can delay both the accu‐ mulation and transfer of metals and their movement into the soil. Metal toxicity in soil can be increased or reduced by the organic substances. Soil pH directly influences the availability of metals as soil acidity determines the metal solubility and its ability to move in the soil solution. Concerning the content of phosphorus in soil there are areas where uptake of metals is accelerated or rather diminished due to the presence of high doses of P2O5. In addition, the physiology of the plant species influences accumulation of metals. For example, in the case of cadmium uptake by grain was noted either a competition or a synergism in case of high concentrations of lead in the soil [5].

For soil protection, the limits for various pollutants have been established only under certain conditions and soil parameters. It was not taken into account the fact that on light soils, lowcarbon, there is a strong influence of acid rainfall leading to a strong mobilization and uptake into plants of toxic heavy metals. This does not happen on heavier soils rich in limestone.

The solubility of zinc in soil was studied by Herms and Brummer [6], who demonstrated the extent to which this element is dissolved by increasing acidity of the soil and became available to plants absorption. In a low-zinc soil, a pH value of 5 could lead to a lasting effect of uptake large amounts of zinc, with all the negative consequences that result. Zinc equilibrium in the soil solution is realized at the level of 1200 mg/kg soil and a pH of 7, at level of 100 mg/kg soil and pH of 6 and at a level of only 40 mg/kg at a pH of 5. These levels of equilibrium make the low-zinc soil to release in the soil solution dangerous amounts of this element.
