**3. Polluted area of Copsa Mica – Romania, as case study**

Copsa Mica (see **Figure 2**) has for decades been known as" *the most polluted town in Europe*". The main pollutants identified in this area were cadmium, copper, lead and zinc. Moreover, this city was presented in Blacksmith Institute and Green Cross Switzerland Report 2012 - "*World's Worst Polluted Places*" [15] as examples of high cadmium pollution. In Romania there are some critical areas in terms of heavy metal pollution (Baia Mare, Zlatna, Moldova Noua, Copșa Mică). Of these, Copșa Mică area presents the highest risk of interception of heavy metals through locally produced local food, due to the large abundance of agrosystems in the structure of local socioecological systems. The Copsa Mica polluted area can be defined as the surface of land where the pollutant content in the top level of the soil (upper 20 cm) exceeds the alert thresholds defined by Romanian legislation. According to Vrinceanu and Lacatusu [12, 13], this polluted area covers 7040 ha where zinc content in soil is over 300 mg/ kg; 10,320 ha of land where cadmium content in soil is over 3 mg/kg, or 22,565 ha where the lead content in soil exceed 50 mg/kg.

### **Figure 2.**

*Polluted area of Copsa mica (adaptation after: Barbu Horia, Lucian Blaga University of Sibiu [14]).*

In Copsa Mica, the amount of Pb and Cd in vegetable samples exceeded the maximum permissible limits in carrots (median concentration 0.32 mg/kg for Pb and Cd) and in yellow onions (median concentration 0.24 mg/kg for Cd).

The European Commission has recently set new maximum levels for Cd and Pb in a range of food products to improve public health protection, with these measures entering into force from Aug. 30th 2021. Such *actions aim to further reduce the presence of carcinogenic contaminants in food and make healthy food more accessible* — a key aim of Europe's Beating Cancer Plan. Examples of these thresholds include 0.030 milligram per kilogram wet weight (ppm WT) for Cd and 0.10 ppm WT for Pb in stem vegetables; 0.10 ppm WT for Cd and 0.10 ppm WT for Pb in root and tuber vegetables (EU Regulations/2021 [16, 17]). In the case study area, the daily TM intake rates via local vegetable consumption are well above these values, more precisely 2 to 4 times higher for Pb and 5 to 10 times higher for Cd; yielding potential adverse public health effects [18]. Even after 10 years of ceasing production of the nonferrous smelter (production was ceased in 2009), the HM contents in the soil and plants are high inside the polluted area [7]. Such soils, or those with various other components that can be absorbed by crop plants and endanger public health, are recommended for use in the production of non-food crops, ideal for obtaining biomass to be used as a raw material in biorefinery.

Chronic effects in human health mainly result from exposure to low levels of cadmium and are represented by chronic obstructive diseases of the lungs and the renal system. There may also be effects on the cardiovascular and bone system. The fetus, young children, and pregnant women are unanimously recognized as sensitive populations with increased risk of developing adverse effects in chronic exposure to lead, including relatively low concentrations. Another indicator influenced by chronic lead exposure is somatic development, the height and weight can be changed, in the sense of growth delay, of about 1–1.5 years [19].

Over the decades, many options to clean the soils polluted with heavy metals have been considered. Taking into account the problems posed by top layer removal and replacement, chemical washing and many other "hard" methods, "gentle remediation" options have been explored.

Phytoextraction, using plants that can accumulate large amounts of potentially toxic metals in their above ground parts was proposed by (Gosh and Singh, 2005) as a feasible option, but the proposed plants (*Thlaspi caerulescens, Brassica juncea, Ipomoea carnea, Datura innoxia, Phragmytes karka* etc), either were exotic in Europe, raise cultivation problems, deliver smalls yields in terms of feedstock (carbohydrates, biomass) for biorefinery and/or concerns about their invasive potential. On the other hand, common European plants with high productivity have a small metal accumulation capacity [20]. Even several remediation attempts in Copsa Mica area have applied, none of them have been proven particularly successful [21–24] and the local community and farmers have adopted coping strategies, looking for other crops with a low uptake of heavy metals (i.e. excluders, as defined by Baker [25]) (**Figures 3** and **4**).

An important research focused on bioremediation of polluted soil in Copsa Mica area is *The RECARE project funded by the European Commission FP7 Programme, ENV.2013.6.2–4 'Sustainable land care in Europe'. EU grant agreement: 603498*. The research in this project regarding Copsa Mica case was focused on immobilization of these pollutants using different types of inorganic additives, such as zeolitic tuff, bentonite, volcanic tuff, and organic materials such as biosolids, cattle manure [26]. Although the situation is well documented, and there have been some attempts to remediate these soils, until now, no feasible solution for the decontamination of this large region has been found.

When phytoextraction is approached, important attention is dedicated to plants able to accumulate a particular metal from soil with higher efficiency compared with other plants. These plants are defined as hyperaccumulator plants and can accumulate metals in high concentration in some of their tissues - 100-fold or 1000-fold when compared to other plants growing in that soil. The bad news is that such plants are relatively rare, are endemic only in scattered areas around the world, are less adaptive

**Figure 3.** *Area in vicinity of heavy metals smelter.*

*Biorefinery for Rehabilitation of Heavy Metals Polluted Areas DOI: http://dx.doi.org/10.5772/intechopen.109626*

### **Figure 4.** *Miscanthus giganteus (left) and sweet sorghum (right) cultivated on polluted soil in Copsa mica area.*

to the polluted areas where they need to be cultivated and the number of known species are low - less than four hundred species identified for as little as eight polluting metals [27]. The good news is that heavy metals can be extracted by very common plants, even they do not have ability to accumulate metal in high concentrations in their tissues as hyperaccumulator plants. If we take into account plants that produce high amounts of biomass per hectare of land, such as corn (*Zea mays*), sorghum (*Sorghum bicolor*), alfalfa (*Medicago sativa L.*) cup plant (*Silphium perfoliuatum*) [27] or sunflower (*Helianthus annus L.*) [28], the quantity of metals extracted from soil by plants producing high yields even containing lower concentrations of metals, can be more important than quantity of metals extracted by low yielding hyperaccumulator plants. Large quantity of biomass produced by such crops can remove higher amount of metals from the polluted soil even at lower concentrations of metal within the plants compared with low quantity of biomass with high concentrations of metal produced by hyperaccumulator plants [27].

Even high levels of heavy metals have been found in edible plants in the polluted area, people continue to farming and to produce food. There are farms producing different types of crops, cattle, chicken and other animal farms. There is no constraining regarding farming in the polluted area. The people are not informed regarding the risks. Still, in the polluted area there are land owners avoiding cultivation of edible plants. There are several hectares of *Miscanthus giganteus* as energy crops. *Sweet sorghum* has been cultivated as well, as trials in a former research projects, obtaining high yields: 60–100 to/ha of fresh mater, or up to 35 to/ha of biomass dry matter. **Figure 4** is an example of *Miscanthus giganteus* cultures adopted by local land owners to use the polluted soil for energy purpose.

Although is generally accepted that phytoremediation of metals from contaminated soil is possible and several plants have the ability to extract the pollutants from soil [29], the utilization of the biomass containing heavy metals raise important questions. Several approaches for disposal of metals containing biomass include decaying, thermal decomposition including burning or pyrolysis, chemical extraction, or even recovery of precious and semiprecious metals - called phytomining [30]. These technologies applied for processing polluted biomass generates wastes. For example, in

the process of thermal decomposition by pyrolysis of heavy metals polluted biomass, almost all metals accumulate in the char, while by burning, metals can be found both in flue gasses and ash. Consequently, questions are raising regarding disposal of the heavy metals contaminated byproducts/residues such as char or ash. An attractive alternative approach to use heavy metal-polluted soils is to produce sugars crops and high-yielding biomass crops such as sweet sorghum. Sugars extracted from such crops can be converted in biorefineries to a wide range of biochemical; the organic residues resulted in biorefinery to be digested for production of biogas and digestate and to return this digestate in the same polluted soil as a fertilizer. By this circular bioeconomy approach, heavy metals can be confined in the polluted area and the risks of disposal of contaminated residues and further disperse the pollutants is eliminated.

Plant biomass can be used for different energy-recovery techniques, such as anaerobic digestion, incineration, gasification and liquid biofuels production. It is important to be sure that the metal burden, toxic metals such as Cd, in plant biomass will not affect biofuels production [31]. So, it is essential to assess these effects of metals concentrations on biofuels production systems and the design of biotechnological processes (ethanol fermentation, anaerobic digestion for biogas production etc). Moreover, selecting suitable plants is essential, as species accumulating high concentrations of pollution may raise difficulties in conversion processes [31].

Sweet sorghum has been chosen by our team for bioremediation/biorefinery application. This type of plant biomass could be used to supplement the metals needed in fermentation systems and anaerobic digester, contributing to the implementation of circular economic strategies and closing the loop in resource utilization chain.

Why sweet sorghum has been chosen for this study case? Sweet sorghum can be an important source of fermentable sugars for industrial biotechnology. A wide range of bioproducts for industrial, pharmaceutical or agricultural use can be obtained by microbial fermentation processes (alcohols, organic acids, amino acids, proteins, antibiotics, etc.). Sweet sorghum is an annual plant with a short production cycle and can be harvested after about 140–150 days of cultivation. The optimal seeding season is at the end of April - beginning of May and the harvesting can be done in September-October. It can be cultivated as secondary or precursor crops in combination with other short-cycle plants (for example triticale from early spring to June, followed by sweet sorghum for biofuels production). In classical biogas production technology, sugars in sorghum stems are converted in lactic acid during ensiling and by this the biomass is preserved until anaerobic digestion for biogas production.

Several studies indicates a variety of advantages of sorghum bicolor (L.) Moench over other sugar and starchy crops, which make this plant a highly studied energy crop [32]. Exhibiting high tolerance to draught, both sweet and grain sorghum varieties produce high yields even under a wide range of environmental conditions. More than that, studies demonstrates that sorghum can be cultivated on marginal lands and require low inputs [33–35]. In the context of climate change adaptation, temperatures and higher atmospheric level of CO2 may beneficially affect sorghum crops in terms of a higher biomass yield. Sorghum cultures have been previously applied in *phytoextraction* experiments in polluted areas [36–38] and our own research results [39] indicated sweet sorghum as a good alternative for utilizing land polluted by heavy metals for bioethanol production, which would not only avoid food-fuel competition issues, but also provide way forward for land that is uncultivated due to human pollution.

Compared to other energy crops, sorghum has a global potential, being one of the most variable plants in terms of genetic resources, making breeding and development of new cultivars, adapted to different climate zones around the globe, easier [40].



### **Table 1.**

*Relevant features of application of three main crops cultivable in European climate conditions.*

Calculations show that sorghum can be comparable and, in some cases, competitive to sugar cane and corn in terms of sugar and bioethanol output per hectare (**Table 1**), while requiring much less water [42–45].

In this work we propose an integrated process in cascade using as feedstock sorghum biomass produced on heavy metals polluted soil (15 km distance from the smelter – the core of polluted area) having as objective soil remediation. Preliminary results obtained in lab scale [39] recommended sorghum as a crop able to grow on polluted soil and to provide readily fermentable sugars by juice extraction. In this research, metals concentration in sorghum juice are between 0.5 and 1.0 mg·kg−1 for Pb and between 22.7 and 86.2 mg·kg−1 for Zn, while Cd and Cu are not detected. When bagasse resulted after juice extraction is analyzed, the concentration of heavy metals increase as bagasses are pretreated - the average metals levels found in the thermo-chemically pretreated biomass were higher than those in the unpretreated biomass. Concentrations of all four analyzed metals increased after pretreatment: Cd from 3.60 to 4.03 mg·kg−1, Cu from 15.57 to 25.56 mg·kg−1, Pb from 11.24 to

19.38 mg·kg−1 and Zn from 123.50 to 134.19 mg·kg−1. These increased values after pretreatments indicates higher availability of free metals after decomposition of lignocellulosic complex. More than that, the fate of metals was tracked in the biorefinery of bagasse to produce second generation bioethanol and after distillation, portions of Cu and Pb were found in the distilled ethanol, while Cd and Zn remained in vinasse.

In order to define "highly polluted biomass" and "low polluted biomass", we refer to the European legal frame [46, 47]. In this EU legal frame are defined the highest levels of heavy metals accepted in products used as fertilizers and soil improvers (**Table 2**). Assuming that the concentration of metals increases in the solid part of the biomass along cascade processing (regarded as by-products in biorefinery) during treatments (juice pressing, hydrolysis, anaerobic digestion), we expect higher concentration of heavy metals to be found in digestate than in the raw material (sorghum biomass). According to European legal frame above mentioned, contaminants must not exceed the following limit values:

In this respect and according to previous research [39], in average Cd is the main pollutant found in sorghum in the area selected as study case – average concentration of 5 mg/kg dry matter of sorghum biomass, while lead is found in average concentration of 20 mg/kg (under the limit of 120 mg/kg in EU legislation). It is expected to harvest sorghum biomass containing higher concentrations of cadmium and lead closer to smelter and lower in low polluted areas (blue and yellow on the map, **Figure 2**). When the limit of Cd < 1,5 mg/kg DM (EU threshold for fertilizer) is regarded here to define low polluted biomass, the surface delivering very polluted biomass in the map from **Figure 2** includes red and blue zones.

Respecting the above-mentioned legal frame, we propose to developed a new approach for biorefining agricultural feedstock, addressing as case study/main feedstock the biomass obtained from an energetic crop cultivated in marginal lands with reference to industrially polluted areas that take into consideration not only the economy related to the biorefinery products but also to deliver an integrated soil remediation system and create a complete value and social environment that will allow agriculture to bloom again in the selected area.

Regarding extraction of metals from biorefinery by-products, ashes/chars obtained through combustion/pyrolysis of solid digestate fraction can be considered as renewable secondary sources for the recovery of heavy metals. These ashes are usually classified as hazardous material due to their high content of toxic metals and soluble components. The most widespread leaching method is acidic leaching using strong mineral acids as many metal compounds have high solubility at low pH.


### **Table 2.**

*Highest levels of heavy metals accepted in products used as fertilizers and soil improvers according to EU legal framework.*

*Biorefinery for Rehabilitation of Heavy Metals Polluted Areas DOI: http://dx.doi.org/10.5772/intechopen.109626*

### **Figure 5.**

However, due to the alkalinity of the ash large amounts of acid are needed. This problem can be overcome by alternative leaching media (especially organic acids obtained in biorefinery) in order to favor the achievement of high efficiency in the dissolution and in the electrowinning of heavy metals. In this respect, the biorefinery can be an important provider for metals industry. Firstly, redistributing back "lost metals" extracted by plants and delivered through biorefinery by-products and secondly, providing catalysts for metals extraction. Organic acids can be produced in biorefinery by fermentation of sugars and delivered to industry to prepare leaching media for dissolution and in the electrowinning of heavy metals. This can be another aspect of circular (bio)economy proposed here. The schematic approach of the proposed circular economy is presented in **Figure 5**.
