Sorption Behaviors and Mechanisms of Cadmium on Polypropylene, Bamboo Biochar, and Rice Husk Ash in Aqueous Solutions

This study investigated the adsorption behavior and mechanisms of cadmium ions (Cd²⁺) from aqueous solutions on virgin polypropylene (PP₀), 1-to-3-week aged polypropylene (PP₁, PP₂, and PP₃), bamboo biochar (BBC), and rice husk ash (RHA). Adsorption kinetic and isotherm experiments were conducted to evaluate the performance of each material. Fourier-transform infrared spectroscopy and Brunauer–Emmett–Teller analyses revealed the presence of hydroxyl and carboxyl functional groups and microporous structures, respectively, on the adsorbent surfaces. The pH of the point of zero charge analysis indicated that the surfaces of PP₀-PP₃ carried net negative charges at a pH solution of 7.0±0.1, whereas BBC and RHA exhibited net positive surface charges under the same conditions. The adsorption kinetics of Cd by PP₀, PP₁, PP₂, PP₃, and BBC followed the pseudo-second-order model (R² = 0.951, 0.963, 0.997, 0.996, and 0.999, respectively), suggesting chemical adsorption as the dominant mechanism. In contrast, the adsorption by RHA was best described by the pseudo-first-order model (R² = 0.990), indicating a physical adsorption process. Isotherm analysis showed that the Freundlich model provided the best fit for PP0-PP3, indicating multilayer adsorption on the surfaces. For RHA, the Langmuir model was more suitable, with a maximum adsorption capacity of 815.02 µg/g, implying monolayer adsorption on the surface. Overall, BBC and RHA demonstrated significantly higher cadmium adsorption capacities compared to PP₀-PP₃, likely due to their much greater surface areas, 42 to 56 times higher than that of the PP. Importantly, this study presents a comparative framework evaluating the sorption behavior of aged polypropylene microplastics in relation to biomass-based adsorbents, focusing on surface transformations during environmental aging and their interactions with heavy metals. These findings confirm the performance of agricultural waste-derived adsorbents and the emerging environmental risks posed by co-contamination from aged microplastics and heavy metals in aquatic systems.