Mechanisms of Synergistic Inhibition of Cadmium Accumulation in Rice by a Corncob-Oyster Shell Co-pyrolyzed Composite Passivator

To reduce Cd accumulation in rice grains and the associated risks to human health, this study aimed to enhance the efficacy of in situ remediation of Cd-contaminated paddy soils.

In situ immobilization using mineral- and carbon-based amendments represents a potentially cost-effective strategy for reducing Cd bioavailability and limiting its transfer to rice grain. However, the effectiveness of individual amendments is commonly constrained by limited sorption capacity, insufficient pH-buffering ability, or inadequate regulation of Cd uptake and translocation at the plant level. In this study, a pot experiment was conducted to evaluate the effectiveness of corncob biochar, pyrolyzed oyster shell, and a corncob-oyster shell co-pyrolyzed composite amendment in mitigating Cd accumulation in rice. The amendments were characterized using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), Brunauer-Emmett-Teller (BET) surface area analysis, and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS). Their effects on soil Cd speciation, Cd retention by root-surface Fe plaque, Cd uptake and translocation within rice plants, plant biomass, and grain Cd accumulation were systematically investigated.

All three amendments decreased the proportion of exchangeable Cd in soil by 8.05%–23.0%, indicating a redistribution of Cd from relatively labile pools toward more stable geochemical fractions. Amendment application also enhanced Cd sequestration by Fe plaque on rice roots by 15.7%–38.8%, reduced the root-to-shoot Cd translocation factor by 12.5%–27.9%, decreased Cd mass fraction in aboveground tissues by 9.17%–23.4%, and increased rice biomass by 19.4%–44.2%. These responses demonstrate that the amendments not only reduced soil Cd availability but also strengthened the sequential barriers controlling Cd transfer from soil to roots and from roots to aboveground tissues. Among the tested materials, the corncob-oyster shell co-pyrolyzed composite amendment exhibited the greatest overall effectiveness, decreasing the grain Cd mass fraction by 57.1% relative to the unamended control. Structural and spectroscopic characterization indicated that the co-pyrolyzed composite retained the carbonaceous framework and oxygen-containing functional groups inherited from corncob biochar, together with the CaCO₃-rich mineral phase derived from oyster shell. The integration of these components generated a heterogeneous carbon-calcium interfacial structure. The carbonaceous fraction provided a porous matrix and oxygen-containing surface ligands capable of retaining Cd through surface complexation and electrostatic interactions. In parallel, the oyster shell-derived Ca-bearing phase supplied alkalinity and reactive mineral surfaces that may promote Cd adsorption, ion exchange, and carbonate-associated precipitation or coprecipitation. The coexistence of these chemically distinct domains within a single composite matrix likely increased both the abundance and diversity of Cd-binding sites while improving the physicochemical conditions required for sustained Cd immobilization. Corncob biochar and pyrolyzed oyster shell provided complementary functions in the soil system. Biochar supplied a porous carbon framework and abundant surface functional groups, although its immobilization performance may be influenced by feedstock properties and aging-induced changes in surface reactivity. Oyster shells, in contrast, was dominated by CaCO₃ and could improve soil acidity buffering and promote mineral-associated Cd retention, but its effectiveness might be constrained by limited accessibility of mineral surfaces and spatial separation from organic sorption domains. Co-pyrolysis integrated these complementary components, thereby improving the dispersion of Ca-bearing mineral particles across the carbon matrix, increasing contact between mineral and organic reactive sites, and reducing aggregation of the individual phases. This coupled structure simultaneously regulated soil-solution chemistry and provided multiple reactive surfaces, thereby enhancing Cd immobilization across heterogeneous microscale environments. In soil, the composite amendment promoted Cd stabilization and decreased the exchangeable Cd pool available for root uptake. At the soil-root interface, amendment application enhanced Cd sequestration by root-surface Fe plaque, thereby reinforcing an additional geochemical barrier between soil Cd pools and root tissues. Although Fe plaque can function as either a source or a sink depending on environmental conditions, the increased plaque-associated Cd observed here indicates that Fe plaque predominantly acted as a retention phase under the experimental conditions. Within the plant, the composite amendment enhanced Cd retention in roots and suppressed its subsequent transfer to aboveground tissues, as reflected by the lower root-to-shoot translocation factors. The coordinated regulation of soil Cd speciation, Fe-plaque-mediated sequestration, root retention, and internal Cd translocation ultimately accounted for the pronounced decrease in grain Cd accumulation. The observed increase in rice biomass further indicates that amendment application mitigated Cd-induced phytotoxicity and promoted plant growth. This growth-promoting effect may be related to decreased exposure to available Cd, improved soil chemical conditions, and the contribution of Ca-containing phases; however, the associated physiological and nutritional responses require further investigation. Importantly, the superior performance of the co-pyrolyzed material cannot be attributed solely to the additive effects of biochar and oyster shell. Instead, the carbon-calcium interfacial architecture generated during co-pyrolysis provides a plausible mechanistic basis for enhanced Cd immobilization. Organic functional groups, porous carbon domains, and Ca-bearing mineral phases may act synergistically to promote Cd immobilization. Such multifunctionality is particularly relevant to paddy soils, where periodic variations in pH, redox conditions, dissolved organic matter, and Fe cycling can destabilize Cd retained through a single mechanism. By distributing Cd among multiple adsorption, complexation, and mineral-associated retention environments while strengthening Fe-plaque-mediated sequestration, the composite amendment may provide more robust control of Cd mobility than either corncob biochar or pyrolyzed oyster shell alone. Moreover, the use of corncob and oyster shell residues offers an additional environmental benefit by converting agricultural and aquacultural wastes into a value-added remediation material.

Overall, this study demonstrates that co-pyrolysis of biomass and shell-derived mineral feedstocks can produce an efficient multifunctional amendment for regulating Cd transfer across the soil-rice system. The corncob-oyster shell co-pyrolyzed composite simultaneously decreased soil Cd availability, strengthened Cd sequestration at the rhizosphere and root interfaces, restricted Cd translocation to aboveground tissues, promoted rice growth, and substantially reduced grain Cd accumulation. These findings demonstrate that the corncob-oyster shell co-pyrolyzed composite is a promising amendment for the in situ remediation of Cd-contaminated paddy soils and for mitigating Cd accumulation in rice grains. Accordingly, this study underscores the importance of integrating soil Cd immobilization with plant uptake and translocation processes when evaluating remediation effectiveness. Further field-scale studies are required to evaluate the long-term stability of immobilized Cd under repeated flooding-drainage cycles, the persistence of the carbon-calcium interface during soil aging, and the agronomic and environmental performance of the amendment under variable field conditions.