ZnS-mediated trace Tl(I) removal from wastewater: Synergistic pathways and practical implications for wastewater remediation

Thallium (Tl) and its compounds pose significant environmental risks due to their high toxicity and bioaccumulative potential, with industrial discharge standards mandating Tl concentrations below 5 μg/L. This study addresses the underexplored challenge of removing trace Tl(I) (≤1 mg/L) by investigating the efficacy of zinc sulfide (ZnS) as an adsorbent. Batch experiments, complemented by kinetic analyses (pseudo-second-order model), isotherm studies (Langmuir model, Qmax = 34.7 mg/g), and multi-technique characterization, were conducted to evaluate adsorption performance. Results show that ZnS achieves >99.9 % removal of trace Tl(I) at pH 10, reducing effluent concentrations to <0.4 μg/L—well below regulatory limits. Mechanistic investigations reveal a synergistic pathway: electrostatic attraction to negatively charged hydroxylated ZnS surfaces (zeta potential = −15.4 mV at pH 10), surface complexation via ≡ Zn-O-Tl bonds, and gradual formation of crystalline Tl₂S precipitates over prolonged contact. A two-stage treatment strategy, integrating sodium sulfide precipitation with ZnS adsorption, effectively purifies real-world matrices, including Tl-contaminated rainwater (Tl(I) from 352 μg/L to 0.440 μg/L) and high-salt industrial effluent (Tl(I) from 55.0 mg/L to 0.330 μg/L). This work provides experimental evidence for ZnS as a viable adsorbent for trace Tl(I) remediation, supporting the development of cost-effective heavy metal removal technologies.