**7. Discussion**

The present study concerned a novel approach of the phytoremediation process: phytoreme‐ diation followed by heavy metal extraction. The researches concerned first the analysis of soil to reveal the necessity of remediation, then researches concerned the species used for phytor‐ emediation and methods of processing the resulted biomass. The treatment of biomass enriched with heavy metals led to the recovery of these elements as alloy, which can be economically exploited. Also, the resulted ash was characterized as material with low heavy metal concentration, which can be used in agriculture or can be disposed without the risk of toxicity.

Analysis of soil samples from the vicinity of metallurgical plant of Targoviste indicated an exceeding of alert threshold for Cu, Sn, Pb, Mn and Cr, with metal concentration levels that could be harmful for both the agricultural and industrial soil [49]. According to these aspects and because the land was used as pasture, the soil needed interventions for decreasing the heavy metal concentration. The best way to remediate this land was the use of wild growing perennial plants to extract heavy metals from soil and the resulted biomass to be mowed and disposed as hazardous material. The wild growing plants have the advantage of ecological adaptability and low costs for starting and maintaining the crop [2].

Metal concentration was evaluated for seven perennial grasses, regarding the bioaccumulation capacity of these species, to establish the most effective species to be used in remediation of studied area. Results indicated heavy metal concentrations for *Lolium perenne* similar to results from previous articles: chromium and copper concentration showed values similar to the results presented by Arienzo et al., 35 and 70 mg/kg for Cr and Cu respectively [50]. Instead, the results of present study indicated 2 and 6 times higher concentrations for Pb and Zn when compared with plants from unpolluted sites and similar results when compared with plants from metallurgical site [50]. The same values of Pb and Zn concentrations were obtained also by Bidar et al. [51] when have researched the behavior of *Trifolium repens* and *Lolium perenne* growing in a heavy metal contaminated field. This study indicated that the two metals, Pb and Zn, are accumulated predominantly in roots (269.98 mg/kg for Pb and 1511.18 mg/kg for Zn), and in lower concentrations in shoots (45.65 mg/kg for Pb and 218.15 mg/kg for Zn) [51]. The results of soil phytoremediation can be improved by the addition of chelates such as EDGA which lead to a 2 times increasing of heavy metal uptake by plant [52].

For a correct evaluation of the most efficient species to be used in phytoremediation, the bioaccumulation factor should be investigated because it indicated the metal accumulation ability of each species by comparing the metal concentration in plant with the metal content of the soil [7]. The results of present study indicated that, despite some species showed a high metal concentration in shoots, the metal uptake was caused by the high metal content of soil, not by the accumulation capacity of that species [2]. Only some of the studied species accu‐ mulated heavy metals in higher concentration than the metal content of soil (BF > 1) (Table 3).

For effective results of the phytoremediation, the process should be implemented for a long period of time, with many growing seasons followed by mowing of plants [7]. Therefore, from a phytoremediation process implemented at commercial scale will result huge quantities of plant biomass which should be treated as hazardous material because of the contamination with heavy metals [53]. The leaching test showed that the composting of this biomass is not efficient, because by composting process are formed soluble organic compounds that enhanced metal solubility and availability for plants [53]. Based on these requirements, the present study aimed the development of a novel method of thermal treatment of the biomass from phytor‐ emediation to reduce the initial volume to 5.1%, by drying and incineration. A similar percent (2 – 5%) of mass reduction was indicated by Ghosh and Singh [53], after biomass combustion. Thereby, the much smaller quantities of ash can be properly disposed, but still with the risk of heavy metal toxicity. Also, the resulted ash can be used in phytomining process to recover the saleable heavy metals [53].

**7. Discussion**

**Metal**

**Metal concentration in leaching**

\* 2% of Mn deposit is probably because of anodic dissolution

**Table 7.** Results of heavy metal extraction from *Lolium perenne,* by electrolysis

**solution (mg/l)**

328 Environmental Risk Assessment of Soil Contamination

toxicity.

The present study concerned a novel approach of the phytoremediation process: phytoreme‐ diation followed by heavy metal extraction. The researches concerned first the analysis of soil to reveal the necessity of remediation, then researches concerned the species used for phytor‐ emediation and methods of processing the resulted biomass. The treatment of biomass enriched with heavy metals led to the recovery of these elements as alloy, which can be economically exploited. Also, the resulted ash was characterized as material with low heavy metal concentration, which can be used in agriculture or can be disposed without the risk of

**Metal concentration in cathode deposit (mg/kg)**

**before after µg/5 g ash g per ha**

**Cu** 11.20 7.40 445.92 0.31 0.13 **Zn** 30.45 22.40 946.39 0.66 0.27 **Sn** 13.70 11.45 260.91 0.18 0.07 **Pb** 1.25 0.55 < LD - - **Co** < LD < LD 142.31 0.10 0.04 **Ni** 49.60 23.35 3076.37 2.15 0.88 **Mn** 35.5 16.935 2222.486 1.56\* 0.63 **Cr** 79.55 17.70 7312.619 5.12\*\* <2.09

\*\* The results were probably contaminated because of high content of Cr in the stainless steel (14%) Weight of metal deposition – 0.7 mg; fresh biomass per hectare – 40 t; percentage of ash – 5.1%

**Recovered metal**

Analysis of soil samples from the vicinity of metallurgical plant of Targoviste indicated an exceeding of alert threshold for Cu, Sn, Pb, Mn and Cr, with metal concentration levels that could be harmful for both the agricultural and industrial soil [49]. According to these aspects and because the land was used as pasture, the soil needed interventions for decreasing the heavy metal concentration. The best way to remediate this land was the use of wild growing perennial plants to extract heavy metals from soil and the resulted biomass to be mowed and disposed as hazardous material. The wild growing plants have the advantage of ecological

Metal concentration was evaluated for seven perennial grasses, regarding the bioaccumulation capacity of these species, to establish the most effective species to be used in remediation of studied area. Results indicated heavy metal concentrations for *Lolium perenne* similar to results from previous articles: chromium and copper concentration showed values similar to the results presented by Arienzo et al., 35 and 70 mg/kg for Cr and Cu respectively [50]. Instead,

adaptability and low costs for starting and maintaining the crop [2].

In 1994, the development of novel methods for metal recovery from ash resulted from the plant incineration have been referred as future research, known as extraction from bio-ore [54]. The recovered metals could reveal the secondary potential of hyperaccumulating species, the economic value besides the environmental benefits.

In the incineration process, the organic matter is destroyed, releasing metals as oxides [53]. At higher temperature, over 600ºC, the most toxic metals became volatile [37], and because of this reason the thermal treatment should not exceed this temperature [2]. Thus, by the incineration of plant biomass, the metal concentration is increasing by ash mass and the volatilization should be avoided. A very important factor that influenced the metal recovery efficiency was the leaching solution, which determined the metal solubility according with the pH of leachate. The use of nitric acid and sulfuric acid as leaching reagent had a positive influence on the nickel and chromium solubility. The lowest solubility showed tin and copper. The experimental results showed that the methods used for thermal treatment of plant biomass and the leaching method were efficient and a quantity of 0.7 mg of alloyed (Ni, Cr and Mn was majority) metals was obtained. For a selective recovery of heavy metals is necessary that the electrochemical process should be conducted according to the differences of deposition potential of each metal [42, 43].

The balance of phytoremediation and heavy metal extraction indicated that the thermal treatment followed by ash leaching and electrolysis was an efficient method of metal extraction from the phytoremediation by-products – the plant biomass. By this process could be recov‐ ered saleable heavy metals and the waste resulted from phytoremediation was a heavy metal low-concentration material, without toxicity risk.
