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Agricultural soils in karst regions present a remarkable paradox where high geochemical background levels of heavy metals correspond with unexpectedly low crop uptake, challenging traditional risk assessment frameworks and limiting agricultural development. To decode this paradox, we investigated the geochemical speciation of cadmium (Cd), nickel (Ni), and zinc (Zn) in soil-rice systems in southwestern China, which collectively constitute the world's largest continuous karst region and represent diverse soil weathering stages. We employed three chemical extraction methods that revealed reactive pools ranking as Cd (58.74 %) > Zn (7.31 %) > Ni (4.65 %) and risk patterns varying with soil type (Andosols > Cambisols/Gleysols > Lithosols), while multi-surface speciation model (MSM) elucidated the underlying mechanisms. We identified a stage activation-contamination model (SACM) that demonstrates how pH-dependent weathering controlled heavy metal distribution among dissolved, surface-active, semi-stable carbonate, and nonactive species, thereby explaining the observed risk patterns. Specifically, in alkaline soils (pH > 7.50), Cd and Zn were primarily humic acid (HA)- and carbonate-bound, while Ni was goethite-bound. As weathering intensified, the reactive pool shifted to more active HA- and hydrous ferric oxide (HFO)-bound species. In acidic soils (pH < 6.50), dissolved and HA-bound species dominated. Both random forest, offering robust predictions using readily available data, and MSM stepwise regression models, providing high accuracy with mechanistic insights, effectively predicted rice risks. This study from speciation and activation model to prediction model clarifies why standardized risk assessments fail in karst regions and offers practical tools for accurate risk evaluation and management in these agricultural environments., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2025 Elsevier B.V. All rights reserved.)

Nanoscale zerovalent iron (nZVI) has garnered significant attention as an efficient advanced oxidation activator, but its practical application is hindered by aggregation and oxidation. Coating nZVI with carbon can effectively addresses these issues. A simple and scalable production method for carbon-coated nZVI composite is highly desirable. The anti-oxidation and catalytic performance of carbon-coated nZVI composite merit in-depth research. In this study, a highly stable carbon-coated core-shell nZVI composite (Fe 0 @RF-C) was successfully prepared using a simple method combining phenolic resin embedding and carbothermal reduction. Fe 0 @RF-C was employed as a heterogeneous persulfate (PS) activator for degrading 2,4-dihydroxybenzophenone (BP-1), an emerging contaminant. Compared to commercial nZVI, Fe 0 @RF-C exhibited superior PS activation performance and oxidation resistance. Nearly 95% of BP-1 was removed within 10 min in the Fe 0 @RF-C/PS system. The carbon layer promotes the enrichment of BP-1 and accelerates its degradation through singlet oxygen oxidation and direct electron transfer processes. This study provides a straightforward approach for designing highly stable carbon-coated nZVI composite and elucidates the enhanced catalytic performance mechanism by carbon layers., Competing Interests: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (© 2024 The Authors.)