Investigation of the suitability of remediated dredged sediment as growing media for food crops

In accordance with the principle of the sustainable reuse of environmental resources, and with the final objective to tend towards a circular economy, this PhD thesis deals with the reuse of dredged marine sediment in horticulture. The dredging of sediments is a routine engineering activity carried out to guarantee the navigability of water bodies. Dredging, however, generates a great amount of material usually containing several inorganic and organic pollutants. Accordingly, contaminated sediments are not frequently relocated in other processes and are destined to landfill. Horticulture is essential for human nutrition, but, at the same time, intensive horticulture entails some environmental impacts, partly due to the large use of peat. In fact, the use of peat in nursery and plant growth activities is characterized by environmental implications related to its extraction and long-way transport, no longer sustainable due to the exhaustion of peat bogs. Phytoremediated sediments could be one of the possible new waste-derived soilless substrates proposed for plant cultivation. Phytoremediation can lower the level of contaminants in the sediments, converting them in a fertile matrix. Remediated sediments have been successfully used as an alternative substrate to peat for producing ornamental plants. However, as compared to ornamental plant, recycling dredged sediments for growing food crops rises additional concerns related to human health. As far as is known, no data are available on the quality and safety of the edible products cultivated in remediated dredged sediments. In this thesis, phytoremediated sediments, dredged from Leghorn port, were used alone or mixed with a peat-based commercial substrate as growing media for plant cultivation. As model plants, lettuce, strawberry and pomegranate were chosen as they have different morphology and physiology patterns. The main aim of the present PhD project was to investigate the effect of the sediment on plant yield, quality and safety. The sediments were characterized from a physical, chemical and toxicological point of views. Notwithstanding remediation, sediments showed concentrations of Zn, aliphatic hydrocarbons and polycyclic aromatic hydrocarbons exceeding the limits established by the Italian L.D. 152/2006, regulating the contamination of soil in green areas. However, no evidence of plant contamination by toxic heavy metal was highlighted. Among organic contaminants, only dioxin-like polychlorinated biphenyls were determined in the edible products, at toxic equivalent concentrations fourfold lower than the limit established by the European Union. Plants grown on the sediment-based substrates showed a significant increase in ascorbic acid and sugar (i.e. in strawberries), antioxidant activity and organic acids (i.e. lettuce heads), anthocyanins, ellagitannins and polysaccharides (i.e. in pomegranates) in the edible products. Considering plant productivity, a general decrease of lettuce, strawberry and pomegranate yield grown on pure sediment was detected due to its unsuitable physical properties, whereas the use of mixture sediment–peat and pure peat resulted in the same yield. Nevertheless, in the strawberry case, the plant productivity was increased after the first year of cultivation, indicating an improvement of the physicochemical condition of the sediment. In conclusion, the whole set of data obtained in these experiments (i.e. data regarding the transfer of inorganic and organic contaminants from the substrate to plant, but also the quality and quantity of the final product obtained) indicates that also in case the pure sediment (i.e. the worst scenario in terms of substrate native contamination) is adopted for the soilless growth, the edible products obtained are safe and of good quality.