Our modern cities are resource sinks designed on the current linear economic model which recovers very little of the original input. As the current model is not sustainable, a viable solution is to recover and reuse parts of the input. In this context, resource recovery using nature-based solutions (NBS) is gaining popularity worldwide. In this specific review, we focus on NBS as technologies that bring nature into cities and those that are derived from nature, using (micro)organisms as principal agents, provided they enable resource recovery. The findings presented in this work are based on an extensive literature review, as well as on original results of recent innovation projects across Europe. The case studies were collected by participants of the COST Action Circular City, which includes a portfolio of more than 92 projects. The present review article focuses on urban wastewater, industrial wastewater, municipal solid waste and gaseous effluents, the recoverable products (e.g., nutrients, nanoparticles, energy), as well as the implications of source-separation and circularity by design. The analysis also includes assessment of the maturity of different technologies (technology readiness level) and the barriers that need to be overcome to accelerate the transition to resilient, self-sustainable cities of the future.

Orobanche nowackiana Markgr. is the only parasitic flowering plant in Europe which is an obligate serpentinophyte. This plant parasitizes the Ni hyperaccumulator Alyssum murale Waldst. & Kit. which grows in many areas including serpentine slopes of Mt. Komjan (Albania). The aims of the study were to investigate this host/parasite relationship from a biogeochemical point of view in order to evaluate: (1) the metal accumulation in different parts of the parasite and host plants and (2) the effect of Orobanche infection on host growth. Soil and plant samples of A. murale and O. nowackiana were analyzed for elemental concentrations after acid digestion using inductively coupled plasma atomic emission spectrometry (Ca, Mg, Fe, Ni, Cr, Co, Zn, Cu, Mn, Na, Al), colorimetry (for N and P), and flame photometry (for K). Individuals of A. murale, both infected and healthy, showed slight differences in their composition of essential elements (N, P, K) and of some micronutrients. The parasite had higher P, K, Na, and lower N, Ca, Mg, Ni, Zn, Co, Mn and Al concentrations than the host. Orobanche nowackiana is a Ni accumulator with 299 mg kg−1 in the leaves. Hyperaccumulation of Ni by A. murale did not prevent attack by O. nowackiana. After infection there was a decline in Ni concentration and the dry weight of all organs of the host, thus reducing the biomass yield and consequently Ni phytoextraction yield of A. murale. This infection could be a potential threat to the use of A. murale for Ni agromining.

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Populations of medicinal plants growing on serpentines and their respective soils were analyzed for Fe, Ni, Mn, Cr, Co, Cd, Cu, Zn, and Pb using inductively coupled plasma atomic emission spectrometry. Aqua regia extraction and 0.43 M acetic acid extraction were used for the quantification of pseudototal and bioavailable fractions, respectively, of elements in soil and nitric acid digestion for determination of total element content in plants. Screening was performed to (1) document levels of toxic metals in herbs extensively used in preparation of products and standardized extracts, (2) compare accumulation abilities of ferns and seed plants, and (3) estimate correlations between metal content in plants and their soils. The toxic element content of plants varied from site to site on a large scale. The concentrations of Fe and Ni were elevated while those of Cu, Zn, and Pb were close to average values usually found in plants. The highest concentrations for almost all elements were measured in both Teucrium species. Specific differences in metal accumulation between ferns and seed plants were not recorded. The investigated species are not hyperaccumulators but can accumulate toxic elements, in some cases exceeding permissible levels proposed by the World Health Organization and European Pharmacopoeia. The harvesting of medicinal plants from serpentines could be hazardous to humans.