The use of highly effective sources of zinc (Zn) in alkaline agricultural soils is essential to achieve crop biofortification, maintain crop quality, and avoid potential environmental risks. This research examines the efficacy of environmentally friendly Zn complexes (citric acid, CIT and glycine, GLY) compared to a traditional source (ZnSO₄) for the lettuce cultivation in alkaline soil. The effectiveness of Zn sources was assessed based on the concentration of total and soluble Zn, plant biomass, and contents of photosynthetic pigments. The soil Zn status was also evaluated. While all Zn sources (Zn-GLY, Zn-CIT, and ZnSO₄) showed positive effects on lettuce growth, Zn-GLY exhibited the highest efficacy. This source exhibited increases of 230%, 502%, 296%, and 409% over the control in Zn concentration in young and mature leaves, soluble Zn, and Zn uptake, respectively. Zn-GLY also resulted in a 371% increase in soil exchangeable Zn concentration, compared to the control treatment. Our findings indicate that Zn-GLY could replace the traditional ZnSO₄ treatment, as it achieved high Zn biofortification of lettuce and a high concentration of Zn available in the medium-long term in the soil. The beneficial effect of the chelating agent GLY on plant chlorophyll and carotenoid contents is also remarkable.

Wastes from spent batteries are a secondary source of raw materials. To ensure this, it is mandatory to design sustainable and low-cost processes. In the case of alkaline and zinc–carbon-based batteries, the high content of Zn and Mn makes them of interest in the development of fertilizers. The main objective of this research is to study the fertilizers production from spent zinc-based batteries, using sulfuric acid, citric acid (CIT) and glycine (GLY) solutions as leaching agents. Leaching with glycine at alkaline pHs shows a high selectivity of Zn over Mn, whereas the use of citric and sulfuric solutions leads to recoveries of Zn and Mn. Solutions with the highest Zn recoveries were tested in sand columns. Commercial ZnSO₄ heptahydrate was used as a control. For sulfuric acid, two solutions (H₂SO₄ 2M and 0.25M) were used. The elution of leached Zn and Mn in sand columns depended on the solution added. The Zn-Mn-CIT treatment showed a slight but steady increase in the leachates, reaching 70% and 75% of the total leached Zn and Mn, respectively, in the medium term. The Zn-Mn-H₂SO₄ 2M and ZnSO₄ treatments showed a similar behavior in Zn release. Both Zn-Mn-GLY and Zn-Mn-H₂SO₄ 0.25M treatments showed similar amounts of leached Mn in the medium term (77% of total leached Mn), differing in the leached Zn. Solutions from the leaching of spent black mass batteries, especially Zn-Mn-CIT or Zn-Mn-GLY, showed promising behavior as fertilizer from the point of view of Zn and Mn availability as nutrients.

This study investigates the current application and ageing effects of various Zn sources on acidic Mediterranean soil. Two successive lettuce crops were grown in soil fertilised with 0, 15, 30, 60, and 140 mg Zn kg⁻¹ using commercial ZnO nanoparticles, Zn complex, and Zn sulphate. Plant growth, Zn biofortification, dietary implications, human health, and the soil Zn status were evaluated. Zinc bioavailability was influenced by the source, application rate, and chemical ageing. The bioavailability of Zn in the soil increased from 4.60 to 66.7 compared to the control treatment. Zinc applied in the form of ZnSO₄ was the most bioavailable form in the first year of cultivation. Advanced specialty fertilisers such as ZnO nanoparticles and Zn-lignosulfonate, along with the conventional fertiliser ZnSO₄, demonstrated a residual effect allowing effective Zn uptake by plants in the second crop. Zn concentrations in lettuce leaves were 3.33–34.6 times higher than the control treatment. Application of 30 mg Zn kg⁻¹ and higher of commercial ZnO nanoparticles, Zn complex, and Zn sulphate heptahydrate resulted in some toxicity. Higher application rates of these sources may pose a potential risk to the population, as indicated by the health risk index. These Zn sources represent a promising alternative for enhancing plant growth and providing a sustained release of Zn in several successive crops, making them a potential alternative to conventional fertilisers. Their unique properties can optimise nutrient management strategies and promote sustainable crop production.