**1. Introduction**

In recent years, agricultural use of the anaerobic digestate as organic fertilizer has aroused extensive public criticism due to its increased heavy metal (HM) contents, [1, 2]. Consequently, some novel approaches are developed to treat organic materials and avoid the negative effect of HMs to environment [3, 4]. Comparing with the fresh crops, their biogas residues after anaerobic digestion retained main fibrous texture which is essential for soil texture and fertility. On the other hand, organic materials such as digestate, manure, compost, have been reported to effectively reduce the availability of heavy metals in contaminated soils. This effect can be explained by the enhanced contents of organic matter in amended soil associated with the improvement of the biological, microbiological and biochemical properties of contaminated soils amended with organic materials [5, 6]. Organic amendments decreased

significantly metal availability in soil, due to the binding of metals to organic matter as metal-organic complexes. The addition of 45 t ha−1 of organic ammendaments dry matter, led to an increase biomass yield in a mix of perennial grasses and straw cereals, belonging to the Leguminosae (*Trifolium pratense*) and Gramineae family (*Dactylis glomerata, Lolium perenne, Agropyron repens*), cultivated in polluted area of Copsa Mica (Romania). The contents of Cd and Pb in plants treated with organic amendments were significantly decreased [7]. The Pb concentrations in plants from field treatments with organic amendments were below the threshold for green fodder (40 mg kg−1) according with Directive 2002/32/EC on undesirable substances in animal feed [8]. Administration of organic materials as amendments to contaminated soils is used in many cases with in order to improve soil fertility, improve vegetation on polluted lands and decrease availability of toxic substances for the plants cultivated on marginal lands [9, 10]. The addition of organic amendments to contaminated soils can affect bioavailability of heavy metals by forming metal oxides or carbonates associated with organic matter, which reduce the bioavailable forms to more stable fractions [11]. In addition to these effects, organic amendments are known to improve other soil characteristics such as water and nutrient holding capacities or aeration in the soil particles.

Sorghum crop is selected in this work as a tool with multiple purpose. It is applied as a green cover for un-used, or not-properly used polluted soil; being a robust, drought tolerant and low-demanding for nutrients plant, can deliver important yields of biomass and sugars for industrial purpose; intensive cultivation can be a tool to extract pollutants from soil; residues generated along cascade biorefining of sorghum biomass are regarded as heavy metals carriers, which can be delivered by the biorefinery in concentrated form.

The proposed circular approach proposed in this work consists in processing sorghum crop in cascade. Biorefinery will primarily process sugars to liquid biofuelsbiochemicals and sorghum crops harvested from the HM polluted area (high or low polluted) is considered as feedstock for an industrial scale biorefinery. The harvested biomass (highly polluted and low polluted) from the envisaged area is used as feedstock in biorefinery for liquid biofuels, anaerobic digestion and thermal decomposition (pyrolysis or combustion) applied in cascade:

### **Biorefinery – Anaerobic digestion – Thermal decomposition**

Thermal decomposition (combustion-pyrolysis) is performed to concentrate HM from digestate obtained by processing biomass from very polluted areas, primarily envisaged to carry HM in high concentration.

The aim of this work is to design a truly circular production process for bioenergy production that simultaneous serves as a contaminated land rehabilitation strategy. Thus, our approach will provide a means to address past wrong doings, yield a new opportunity for already polluted lands to be used sustainably and offer an example of how value chains can be made self-sustainable.

In our approach, we start from the paradigm "*considering the plants as the greenest battery able to accumulate and store solar energy, simultaneously delivering organic matter as feedstock for bio-based economy and cure the environment*". It all starts with unique and wonderful ability of the photo-autotrophic living organisms (plants) to convert CO2, H2O and sun energy (by simultaneously emitting free O2) into C6H12O6 (simple sugars)– the molecule used as basic energy source in all living organisms and consequently the main energy carrier in bio-based economy. Plants have also the ability to grow in a wide range of environments, including those affected by humans, and can therefore re-introduce "lost" molecules in natural cycles. We just need to understand

how to harness these organisms, their metabolism and to find them the right place in a sustainable bioeconomy.

Circular bioeconomy is about integration and inter-connection. We propose to connect several processes and technologies in a comprehensive, sustainable and circular bioeconomic value chain with the objective to offer a method of biological remediation of environment affected by humans that integrates a complete biorefinery of plants to deliver products and eco-service at the same time by smart management of bioresources.
