New Insights into the Use of Spent Coffee Grounds By-products as Zn Bio-chelates for Lettuce Biofortification

The purpose of this study is to evaluate modified spent coffee grounds (SCG), together with three SCG hydrochars as Zn bio-chelates. These by-products were intended to increase Zn in lettuce (Lactuca sativa) by a biofortification process. SCG and three SCG hydrochars (produced at 160, 180, or 200 °C) were activated with NaOH and functionalized with Zn sulfate. Lettuce was grown for 40 days in soils amended with the described substrates at the rate of 1.75 g kg−1 soil. Plant growth parameters and Zn, Fe, and Mn content in the plant were measured. No significant phytotoxic effect on lettuce growth was detected. In all cases, a significant increase in the Zn content in plant was observed with a maximum of 318.2% for the hydrochar at 180 °C with respect to the control. Both SCG and hydrochars promoted similar Zn levels in plant (0.30–0.35 mg Zn 100 g−1 fresh weight), although lower than those generated by the commercial chelate (0.45 mg Zn 100 g−1 fresh weight). In addition, other micronutrients such as Fe and Mn were also increased in the plant. Zn bio-chelates derived from SCG and SCG hydrochars have biofortified lettuces in Zn and also in Fe and Mn, without significantly limiting plant growth. The behavior of the different bio-products used is similar, and therefore, any of them could be used in Zn biofortification. This is a novel strategy in the reuse of SCG.