Failure-Aware Explainable Decision Support for Circular Sulfate Adsorbent Design under Hydrochemical Domain Shift

This study develops a failure-aware, explainable environmental decision-support framework for circular sulfate adsorbent design and asks whether predictive transportability and engineering usefulness remain aligned under hydrochemical domain shift. A 58-sample Semarang groundwater dataset served as the development domain. Random forest achieved internal AUROC 0.710 ± 0.165, and a target-free matrix-complexity index (MCI) combined EC, Ca²⁺, Mg²⁺, Cl⁻, and HCO₃⁻ into four treatment regimes. Using the fixed Semarang model, discrimination was retained in the 25-sample Bayat basin (AUROC 0.794; balanced accuracy 0.762) but was lost in a larger cross-country Mekong challenge (n = 118; AUROC 0.491; 95% CI 0.367–0.617). In contrast, the fixed-reference MCI remained strongly associated with sulfate in Mekong (rs = 0.699; 95% CI 0.587–0.782), with 73 of 78 high-sulfate events in the complex-matrix domain. To assess consistency between the engineering interpretation and independent material evidence, open-access sulfate-adsorption studies were synthesized at study level. Zirconium modification increased reported pomelo-peel-biochar qmax from 1.02 to 35.21 mg/g (34.5-fold), while CaCl₂-modified kelp carbon retained substantial reported kinetic uptake in lake water (569.37 mg/g versus 624.39 mg/g in simulated water). The combined evidence supports a workflow in which external model validity governs predictive use, whereas transparent hydrochemical regimes and independent experimental evidence guide capacity, selectivity, stability, and functionalization priorities.