Sawdust-Assisted Phytoremediation: Boosting White Mustard ( <i>Sinapis alba</i> ) Growth and Cadmium Uptake from Contaminated Soil

Heavy metal-contaminated soils are a common problem that negatively affect the entire trophic chain. Therefore, solutions are needed that remove heavy metals from the soil, or allow their immobilization in the soil, limiting availability to plants. This research verified whether sawdust added to soil, contaminated with heavy metal ions, supports phytoremediation (bioaccumulation of metals in the plant) or sorbs heavy metals in the soil (immobilization), thus limiting their availability to plants. In batch equilibrium studies, spruce, pine, and oak sawdust were characterized for their physicochemical and biosorption properties, demonstrating varying capacities for the removal of heavy metal ions (Cd-(II), Cr-(III), Cu-(II), Zn-(II)), primarily through surface complexation and ion-exchange related to functional groups (e.g., amino, carboxyl, and phenolic). Among the metal ions tested, the most effective sorption by sawdust occurred in the case of Cd-(II) ions. The bath equilibrium study was supported by modeling calculations and biosorbent analysis using ICP-OES, XRF, FTIR, and SEM-EDS techniques. However, in pot tests using Cd-contaminated soil, the addition of all types of sawdust did not significantly limit the bioaccumulation of cadmium in the above-ground biomass of white mustard (<i>Sinapis alba</i>). Instead, it enhanced the phytoextraction of cadmium by the tested plant. Spruce sawdust was the most effective. Specifically, the cadmium bioaccumulation factor in white mustard, grown with the addition of spruce sawdust, increased by more than 30%, as compared to the unamended control soil. The addition of all of the types of sawdust tested to contaminated soil also positively impacted plant growth, evidenced by increased dry mass, plant length, and chlorophyll content, as compared to control. The research carried out showed that a former waste from wood processing can be used as a low-cost material for soil phytoremediation.