Synergistic effects of potassium-silicon-calcium mineral fertilizer combined with rice husk biochar on the immobilization of Cd and Pb in soil

Abstract

The combined application of mineral fertilizer and biochar significantly improves the passivation of heavy metal-contaminated soil, surpassing the effects of individual application. This study has reinforced the validation of their passivation competence as soil remediation agents by examining the multifaceted role of potassium-silicon-calcium mineral fertilizer combined with rice husk-based biochar generated at different pyrolysis temperatures. The soil leaching column experiment, conducted based on the adsorption experiments, has facilitated our scrutiny of the passivation impacts of cadmium (Cd) and lead (Pb) when introducing different proportions of mineral fertilizers and biochar into the soil. These results indicate that biochar's adsorption efficiency for Cd and Pb is significantly improved at escalated pyrolytic temperature conditions in a single solution. The biochar generated at 700°C (C700) renders adsorption effectiveness of approximately 84.24% for Cd and 99.74% for Pb. Biochar conspicuously registers superior adsorption efficiency towards Pb relative to Cd. The mineral fertilizer, which achieves an adsorption efficiency of 97.76% for Cd, is identified as the main adsorbent for Cd, although its competence is slightly lower compared to C700 for Pb adsorption. Within a complex solution, biochar and mineral fertilizer show reduced Cd and Pb adsorption levels compared to single solutions. There is a keen competition for adsorption surfaces witnessed between Cd and Pb, with Pb's distribution coefficient (K_d) notably outpacing that of Cd. The isothermal adsorption analyses depict that the mineral fertilizer follows the Langmuir model for Cd adsorption, while C700 conveys the Freundlich model for Pb adsorption. The soil leaching column experiment's results signify that the composite passivation agents significantly outperform the individual passivation agents in efficiency. The combined application of biochar and mineral fertilizer minimizes the cumulative leaching of Cd and Pb, with the optimal soil remedy proportion for heavy metal contamination being 7∶3. In practical application, mindful consideration should be accorded to the deployment ratios of different passivation agents.