Evaluating the properties of deinking paper sludge from the Mediterranean area for recycling in local areas as a soil amendment and to enhance growth substrates

Recycling of paper generates large quantities of paper sludge that is rich in organic matter and can be exploited to counteract organic matter loss in agricultural soils, especially in some geographical areas, such as the Mediterranean. In order for deinking paper sludge (DPS) to be recycled, chemical and physical characterizations are required along with an analysis of possible contaminants. DPS from Italy and Tunisia, resulting from various deinking processes, were analyzed and compared in terms of their chemical properties (pH, OM, total N, C/N ratio, mineral nutrients, and trace metals), surface chemistry [diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and Fourier-transform infrared (FTIR) spectroscopy], and thermal stability [thermogravimetric analysis (TGA)]. The aim was to improve their use as amendments in soils or growth substrates. Elemental analysis revealed higher carbon concentrations in the Tunisian DPS and similar nitrogen contents, high mineral nutrient levels, and potentially toxic trace element levels below the limits allowed for soil amendment. No evidence was found for the presence of organic pollutants, pathogens, or toxic heavy metals. Surface chemical analysis revealed comparable materials rich in aromatic, phenolic, aliphatic, and polysaccharide structures along with clay minerals and carbonates. The thermal analysis revealed similar decomposition temperatures for the organic matter. Practical implications of the results are discussed, highlighting the similar properties of sludge following ink removal by different processes and the need to integrate the N content in order to be able to employ sludge as an amendment in soil and growing media. Given the similarity of the compositions of the materials, appropriate DPS management is key to improving soil fertility, reducing paper waste disposal, and implementing a circular economy in the Mediterranean, where climate change is having a significant impact.