Exploring the synergy of CO nanobubbles and biochar as a hydroponic substrate for enhanced carbon and nutrient utilization with a comprehensive health risk assessment.

In this study, a deep-water culture (DWC) hydroponic system integrating carbon dioxide nanobubble (CNB) water and biochar (BC) was explored as a potential substrate for carbon and nutrient management. Lettuce seedlings were cultivated under varying substrates, including tap water (TW) and deionized water (DW) with and without CNB and BC at concentrations of 0.1 or 0.5 g/L. CNB with BC treatments showed significant concentrations of Ca, Cu, Fe, K and Zn in lettuce tissues. BC treatments resulted in decreases of Cu, Zn and Fe in the solution by 24.82 %, 36.78 % and 50.45 %, respectively. CNB treatments appeared to increase Ca in roots by approximately 55 %. The highest Fe in edible parts was observed with CNB_DW_BC0.1 treatment, while for root highest was in TW_BC0.5. However, treatments with CNBs addition counteracted this effect by mobilizing nutrients for plant uptake. Chlorophyll content increased significantly by 125.59 % with CNB_TW_BC0.5, marking the most substantial improvement observed. Furthermore, initial higher concentrations of NH, NO, and NO in TW compared to DW were noted. BC displayed NH availability reduction, particularly at high doses by 14.87 %, while influencing NO and NO inconsistently. CNBs impacted nitrogen dynamics, showing increased NO by 4.78 ppm when combined with BC in TW. The investigation into free radical generation revealed that the CNBs with BC exhibited the weakest signal for •OH, •O suggesting optimal growth conditions. Translocation factor (TF) indicated nutrient absorption efficiency, with BC notably decreasing TF for Cu, Fe, Zn while, CNBs increased TF for Fe and Zn. The order of target hazard quotient (THQ) for CNB_TW, was Zn > Fe > Cu. Moreover, the calculated daily intake rate, THQ and hazard quotient were all below 1, indicating negligible health risks associated with consuming the lettuce grown under the studied conditions.