Biochar filters reduced the toxic effects of nickel on tomato (Lycopersicon esculentum L.) grown in nutrient film technique hydroponic system

• Biochar as a filter for wastewater irrigation was tested with NFT.
• Biochar reduced the toxic effects of nickel (Ni2+) to tomato growth.
• Biochar filters reduced dysfunctions of cell organelles in root and shoot cells.
• Biochar filters minimized the nutrient reductions in roots, shoots, and fruits.

This work used the nutrient film technique to evaluate the role of biochar filtration in reducing the toxic effects of nickel (Ni2+) on tomato growth. Three hydroponic treatments: T1 (control), T2 (with Ni2+), and T3 (with Ni2+ and biochar) were used in the experiments. Scanning electron microscopy equipped with energy dispersive X-ray spectroscopy and Fourier transform spectroscopy was used to characterize the pre- and post-treatment biochar samples. The results illustrated that precipitation, ion exchange, and complexation with surface functional groups were the potential mechanisms of Ni2+ removal by biochar. In comparison to the control, the T2 treatment showed severe Ni-stress with alterations in cell wall structure, distortions in cell nucleus, disturbances in mitochondrial system, malformations in stomatal structure, and abnormalities in chloroplast structure. The biochar filters in T3 treatment reduced dysfunctions of cell organelles in root and shoot cells. Total chlorophyll concentration decreased by 41.6% in T2 treatment. This reduction, however, was only 20.8% due to the protective effect of the biochar filters. The presence of Ni2+ in the systems reduced the tomato fruit yield 58.5% and 31.9% in T2 and T3, respectively. Nickel concentrations reached the toxic limit in roots, shoots, and fruits in T2, which were not observed in T3. Biochar filters in T3 also minimized the dramatic reductions in nutrients concentration in roots, shoots, and fruits, which occurred in T2 treatment due to the severe Ni-stress. Findings from this work suggested that biochar filters can be used on farms as a safeguard for wastewater irrigation.