Previous studies have attributed both phytotoxicity and the capacity to mobilize nutrient elements to the presence of polyphenols and melanoidins in spent coffee grounds (SCG) and SCG-hydrochars obtained through hydrothermal carbonization (HTC). This work aimed to evaluate SCG and two SCG-hydrochars obtained at 160 and 200 °C that were functionalized with Zn salts (bio-chelates), to achieve the in vitro biofortification of lettuce. Two application modes were established: (1) a fixed Zn concentration of 10 mg kg⁻¹ of soil and (2) a fixed dose of 0.5% bio-product. Soil alone (control A) and commercial chelates (control B) were used as controls. Outcomes showed that SCG-hydrochars retain the capacity to mobilize Zn compared to SCG. However, the chelating capacity was reduced (Zn: 94%) and the toxicity was significantly increased (p < 0.05) with higher temperatures of HTC (200 °C). Both fresh and dry lettuce weights were less affected at doses of 0.5% of bio-product and registered a maximum increase of 136% of Zn in the plant content. The present study approaches the possibility of using these by-products as bioinorganic fertilizers at subtoxic doses, although more research is needed.

Iron biofortification has been of main interest for tackling iron deficiency anemia, one of the highest prevalence among micronutrient deficiencies (hidden hunger) in developing countries. This study investigated the effect of activated spent coffee grounds (SCG) and its hydrochars at three temperatures (160°C, 180°C, 200°C) as bio-chelates to level up the iron content of lettuce. Four bio-chelates (ASCG-Fe, AH160-Fe, AH180-Fe and AH200-Fe) were obtained by activation and Fe-functionalization. A pot trial was conducted at doses of 0.2% of the bio-chelates on lettuce with two controls: soil without biofortifying agents (control) and a commercial chelate (control-Fe). Outcomes showed no significant differences (p < 0.05) in soil properties nor in plant growth and morphology, indicating absence of phytotoxicity. All bio-chelates enhanced iron content of plants between 41% (AH200-Fe) and 150% (AH160-Fe) compared to control. The best biofortification effect performed by AH160-Fe was similar to control-Fe (169%), also in terms of soil–plant efficiency both products showed the same transfer factor of 0.07. A proportional impact (up to 150%) was seen on the contribution to the recommended daily intake (RDI). Moreover, higher contents of Mn (29%) and Cu (133%) was evidenced in lettuce with the application of ASCG-Fe and AH180-Fe. These findings suggest activated SCG hydrochars, better than SCG, at small (sub-toxic) doses can successfully achieve agronomic iron biofortification.

The transformation of spent coffee grounds (SCGs) into hydrochars has been extensively studied in recent years to explore their potential in biofortifying foods and mitigating the plant toxicity associated with SCGs. This study aimed to evaluate the effects of adding activated (ASCG and AH160) and functionalized SCGs, as well as SCG-derived hydrochars (ASCG-Fe and AH160-Fe), on cucumber production and plant iron content. To achieve this, SCGs and SCG-derived hydrochars activated and functionalized with Fe were incorporated into cucumber crops grown in a greenhouse over multiple harvests. Among the treatments, SCG-Fe proved to be the most promising for cucumber production, yielding an average of 25 kg of cumulative production per treatment across three harvests. Regarding iron content, the average results across all harvests showed that SCGs and functionalized SCG-hydrochars matched the performance of the commercial chelate (0.108 vs. 0.11 mg Fe/100 g fresh weight). However, in subsequent harvests, iron appeared to leach out, with the activated bio-products (ASCG and AH160) leaving the highest iron reserves in the soil. Additionally, the hydrochar activated at 160 °C demonstrated the highest utilization efficiency. In conclusion, the incorporation of SCG residues and second-generation residues (hydrochars) shows promise as agents for biofortifying cucumbers.

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.