Characterization and Greenhouse Trial of Zn Bio-Chelates Derived from Spent Coffee Grounds

The conversion of spent coffee grounds (SCG) into hydrochars has been the subject of extensive research in recent years, aimed at evaluating their potential for biofortifying foods and mitigating the plant toxicity linked to SCG. This study aimed to assess the physicochemical characterization and the impact of incorporating both activated (ASCG and AH160) and functionalized SCG (ASCG-Zn), as well as SCG-derived hydrochars (AH160-Zn), on cucumber yield and plant zinc content. The following physicochemical properties were analyzed: specific surface area, pH and electrical conductivity, polyphenols, and nuclear magnetic resonance. The by-products activated and functionalized with zinc were applied to cucumber crops grown in a greenhouse across multiple harvests. The activation of both SCG and H160 reduced the specific surface area of the particles. However, when these by-products were functionalized, their Zn content increased significantly, up to 7400 ppm. Concerning polyphenol content, the activated products showed levels ranging from 3.5 to 4.9 mg GAE/g. Regarding cumulative production, the treatments that showed the highest yields were the by-products activated and functionalized with Zn reaching 25 kg. Incorporating these by-products notably raised the Zn content in cucumbers, reaching 0.1 mg Zn per 100 g of fresh weight. The activated by-products demonstrated the highest Zn utilization efficiency.