Levels of toxic metals in Dutch cucumbers biofortified (Fe-Zn) with chemically-modified spent coffee grounds

Spent coffee grounds (SCGs) are a form of biowaste that can be reused in agronomic biofortification with Zn and Fe to enhance environmental sustainability. Various chemical treatments of SCGs have been explored to reduce their known phytotoxicity. The aim of this study was to evaluate how different chemically modified SCGs used as soil amendments for iron (Fe) and zinc (Zn) biofortification of Dutch cucumbers under greenhouse conditions influence the accumulation of arsenic (As), cadmium (Cd), mercury (Hg), and lead (Pb). Cucumbers grown with SCGs activated with sodium hydroxide (NaOH) (ASCG) and all SCG-based biochelates showed reduced Cd concentrations compared to the control group (p < 0.05). Activated hydrochar derived from SCGs and functionalized with Fe (biochelate: AH160-Fe; 0.027 ± 0.019 μg kg^-1, fresh weight, f.w), or ASCG functionalized with Zn (biochelate: ASCG-Zn; 0.033 ± 0.025 μg kg^-1), reduced Hg levels relative to the control (0.050 ± 0.035 μg kg^-1; p < 0.05). Activated hydrochar produced from SCGs heated at 160 °C (AH160) decreased As concentrations (7.69 ± 1.41 μg kg^-1) compared to commercial Zn and Fe chelates (control-Zn: 9.57 ± 1.70 μg kg^-1 and control-Fe: 8.28 ± 1.38 μg kg^-1, respectively; p < 0.05). AH160 functionalized with Zn (biochelate AH160-Zn) and Fe (biochelate AH160-Fe) reduced As and Hg concentrations (8.51 ± 2.34 and 0.027 ± 0.019 μg kg^-1, respectively) compared to control-Zn and control-Fe (9.57 ± 1.70 and 0.049 ± 0.041 μg kg^-1, respectively; p < 0.05). Cucumbers harvested at later stages exhibited higher As and Cd levels, in contrast to Hg and Pb, which decreased over time. In conclusion, the four biochelates (ASCG-Zn, AH160-Zn, ASCG-Fe, and AH160-Fe) reduced Cd bioavailability in cucumbers (0.120, 0.126, 0.122, and 0.136 μg kg^-1, respectively), with ASCG-Zn and ASCG-Fe showing the most significant effect compared to the control (0.156 μg kg^-1). The tested biochelates present a promising alternative to commercial Zn and Fe chelates, enabling biofortification while simultaneously lowering Cd, Hg, and As levels. In subsequent harvests, As and Cd levels tend to increase, whereas Hg and Pb levels decrease. None of the estimated daily intakes of the analyzed toxic metals (0.0439, 0.0007, 0.0002, and 0.0027 μg day^-1 for As, Cd, Hg, and Pb, respectively) pose a risk to human health.