Abandoned smelting sites in karst terrain pose a serious environmental problem due to the complex relationship between specific hydrogeological elements and heavy metal contamination. This review combines work from across the globe to consider how karst-specific features (i.e., rapid underground drainage, high permeability, and carbonate mineralogy) influence the mobility, speciation, and bioavailability of “metallic” pollutants, such as Pb, Cd, Zn, and As. In some areas, such as Guizhou (China), the Cd content in the surface soil is as high as 23.36 mg/kg, indicating a regional risk. Molecular-scale analysis, such as synchrotron-based XAS, can elucidate the speciation forms that underlie toxicity and remediation potential. Additionally, we emphasize discrepancies between karst in Asia, Europe, and North America and synthesize cross-regional contamination events. The risk evaluation is complicated, particularly when dynamic flow systems and spatial heterogeneity are permanent, and deep models like DI-NCPI are required as a matter of course. The remediation is still dependent on the site; however, some technologies, such as phytoremediation, biosorption, and bioremediation, are promising if suitable geochemical and microbial conditions are present. This review presents a framework for integrating molecular data and hydrogeological concepts to inform the management of risk and sustainable remediation of legacy metal pollution in karst.

Nano zero-valent iron (nZVI) particles have received much attention in environmental science and technology due to their unique electronic and chemical properties. While sulfate radical-based advanced oxidation processes (SR-AOPs) activated by nZVI show promise for mono-chlorobenzene (MCB) degradation, their efficiency is severely limited by surface oxidation of nZVI and Fe³⁺ accumulation. This study aims to enhance the nZVI/persulfate (PS) system using L-cysteine (Cys) to achieve effective MCB removal. The work involved synthesizing nZVI via borohydride reduction, followed by comprehensive characterization and batch experiments of the Cys/nZVI/PS degradation system of MCB were carried out to evaluate the key influencing factors and analyze the reaction mechanism of Cys-enhanced MCB degradation. Under optimal conditions (0.1 g/L nZVI, 3 mM PS, 0.1 mM Cys, pH 3), 92.6% of MCB was degraded within 90 min—an 18.7% improvement compared to the Cys-free system. Acidic pH promoted Fe²⁺ release and significantly enhanced degradation, while HCO₃⁻ strongly inhibited the process. Mechanistic studies revealed that sulfate radicals (SO₄^•−) played a dominant role, and Cys served as an electron shuttle that facilitated the Fe³⁺/Fe²⁺ cycle and enhanced Fe⁰ conversion, thereby sustaining PS activation. This study demonstrates that Cys effectively mitigates the limitations of nZVI/PS systems and provides valuable insights for implementing efficient SR-AOPs in treating chlorinated organic contaminants.

In arid and semi-arid regions, groundwater is the main domestic and agricultural irrigation water source, supporting regional water security and agricultural development. Under persistent human interference, groundwater pollution has worsened in the Wudaoxian area of Lanzhou New Area. In this study, 44 groundwater samples (13 unconfinedand 31 confined groundwater samples) were analyzed. Multiple approaches including Piper trilinear diagrams, Gibbs plots, ion ratio analysis, correlation analysis and factor analysis were adopted to reveal groundwater hydrogeochemical characteristics and formation mechanisms, and distinguish the influences of natural processes and anthropogenic activities. The Water Quality Index (WQI), United States Salinity Laboratory (USSL) diagram and Wilcox diagram were further used to evaluate groundwater suitability for drinking and irrigation. Hydrogeochemical results show that local groundwater is weakly alkaline, and unconfined and confined groundwater share similar Na-Cl-dominated hydrochemical types. Rock weathering, evaporation-concentration and ion exchange jointly control groundwater chemistry, whereas agricultural fertilization and pesticide application cause aquifer N pollution. WQI results suggest 81.82% of groundwater is fit for drinking. Nevertheless, irrigation assessment via SAR and %Na indicates that 92.31% of unconfined groundwater and 64.51% of confined groundwater are unsuitable for irrigation, with 6.45% marginally usable only for salt-tolerant crops. We recommend popularizing water-saving irrigation to curb water quality degradation. This study provides references for rational groundwater utilization and targeted protection.