The impact of microbial colonization on cadmium adsorption by rice husk biochar: microorganism-dependent outcomes in bioretention systems

Abstract

The use of microorganisms and biochar, individually, has been proposed as a promising strategy for the bioremediation of heavy metals in contaminated water bodies. However, the impact of their combined application, specifically through the immobilization of microorganisms on biochar, on the efficiency of cadmium (Cd) removal and the physicochemical properties of water has not yet been fully assessed, creating uncertainty regarding their effectiveness in decontamination processes. The objective of this research was to evaluate the effect of the physical adsorption capacity for cadmium on rice husk biochar following activation with strains of Trichoderma harzianum, Chlorella sp., and Pseudomonas sp. Methods: Vertical flow modules were constructed in a factorial design: bioretention systems (M0: biochar, M1: biochar + Chlorella sp., M2: biochar + Pseudomonas sp., M3: biochar + Trichoderma harzianum) × initial cadmium concentration (C1: 3 mg·L−1, C2: 6 mg·L−1, C3: 15 mg·L−1). After 24 h, the efficiency of Cd removal varied significantly according to the system and the initial concentration. Results: For water with [Cd] ≤ 6 mg·L−1, pure biochar (M0) exhibited the highest removal efficiency (99.17 ± 0.41% at 6 mg·L−1). For higher concentrations, M1 showed notably enhanced and stable performance, achieving a removal rate of 99.22 ± 1.06%, surpassing M0 (94.35 ± 6.29%) in stability and effectiveness at high loading. In contrast, M2 and M3 systems exhibited lower efficiencies (75.07 ± 12.73% and 77.79 ± 12.22%, respectively, at 15 mg·L−1). Discussion: The results indicate that the outcome of microbial colonization depends critically on the microorganism used. For M2 and M3, removal rates were inversely proportional to the initial Cd concentration, suggesting early saturation of active sites and/or possible ionic competition for nutrient residues. The scaling up of M1 using conventional fertilizers represents a technically and economically sustainable alternative. For M2 and M3, future works should evaluate the incorporation of post-colonization conditioning stages (e.g., washing, pH adjustment) to mitigate potential ionic competition and restore the adsorptive capacity of the biochar.