Key Takeaways & Executive Findings
- •• • At optimized conditions (pH 12, 2.0% thallium removal agent, 1.0% multi-effect auxiliary agent), thallium concentration was reduced from 9.58 mg·L−1 to 4.31 μg·L−1, achieving a removal efficiency of 99.955% and meeting the ≤5 μg·L−1 discharge standard (GB 13456-2012). • • Thermodynamic simulation revealed that at pH 9–10, Tl+ is the dominant species; under oxidizing conditions, [TlCl4]− forms at pH < 8.1, while at pH > 8.1, Tl2O3(s) and TlClO3(aq) coexist, guiding pH control for optimal removal. • • The combined sulfide precipitation-coagulation-flocculation process simultaneously removed Cu, Zn, and Cd, addressing the challenge of multi-metal coexistence in high-salinity wastewater. • • The identification of KxTlyCl phases confirms lattice substitution between Tl+ and K+ as an auxiliary removal mechanism, enhancing understanding of thallium immobilization in complex matrices.
Abstract
Sintering ash washing wastewater from steel plants is characterized by high salinity, high chloride content, high thallium load, and coexistence of multiple metals, posing significant treatment challenges. This study employed thermodynamic simulation to elucidate the speciation and transformation of thallium in such wastewater, and systematically investigated a combined process of sulfide precipitation coupled with coagulation-flocculation. The results showed that at pH 9–10, thallium predominantly existed as Tl+. Under oxidizing conditions, the stable complex anion [TlCl4]− dominated at pH < 8.1, while at pH > 8.1, a mixed system of solid Tl2O3 and dissolved TlClO3 coexisted. Under optimized conditions (pH 12, 2.0% thallium removal agent, 1.0% multi-effect auxiliary agent), the thallium concentration in the wastewater decreased from an initial 9.58 mg·L−1 to 4.31 μg·L−1, meeting the stringent discharge limit of ≤5 μg·L−1. Concurrent removal of Cu, Zn, and Cd was achieved. The primary removal mechanism was sulfide precipitation, with lattice substitution between Tl+ and K+ serving as an auxiliary pathway. This study provides a practicable technical route for advanced treatment of high-chloride, high-thallium industrial wastewater.
1. Introduction
Thallium (Tl) is a highly toxic trace metal with toxicity exceeding that of lead, cadmium, and mercury, and comparable to arsenic. Its bioaccumulative nature poses severe risks to ecosystems and human health even at trace concentrations. China, as a major producer and consumer of thallium-bearing minerals, faces significant contamination challenges, particularly in steel manufacturing. During sintering, thallium volatilizes and enriches in flue dust, which is subsequently washed, generating wastewater with thallium concentrations as high as 4.0–8.4 mg·L−1. Conventional treatment methods, such as hydroxide precipitation or simple sulfide precipitation, often fail to achieve the stringent discharge limit of ≤5 μg·L−1 due to the high salinity, high chloride content, and the presence of stable thallium-chloride complexes.
This study addresses the bottleneck by integrating thermodynamic speciation analysis with a combined sulfide precipitation-coagulation-flocculation process. The approach not only achieves deep thallium removal to sub-ppb levels but also enables simultaneous removal of coexisting heavy metals (Cu, Zn, Cd). The identification of lattice substitution between Tl+ and K+ provides new insights into thallium immobilization mechanisms, offering a robust technical pathway for treating high-chloride, high-thallium industrial wastewater in steel metallurgy.
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WANG Yunyan, CHEN Qingbin, OUYANG Rui, FU Jie, TONG Tianxing, KE Yong, YANG Bentao, ZHANG Xuekai, SUN Zhumei (2026). Speciation Analysis and Advanced Removal of Thallium from Washing Wastewater of Sintering Machine Head Ash. Environmental Chemistry. https://doi.org/10.7524/j.issn.0254-6108.2026031103
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Frequently Asked Questions
What is the maximum chloride concentration that this process can handle while maintaining thallium removal efficiency below 5 μg/L?
The study tested wastewater with chloride concentration as high as 152.1 g·L−1. Under optimized conditions (pH 12, 2.0% thallium removal agent, 1.0% multi-effect auxiliary agent), the effluent thallium concentration was 4.31 μg·L−1, meeting the discharge limit. This indicates the process is effective at extreme chloride levels, though performance at even higher concentrations remains to be validated.
How does the presence of high chloride ions affect the speciation of thallium and the efficiency of sulfide precipitation?
Thermodynamic simulation showed that at pH < 8.1, thallium forms stable [TlCl4]− complexes, which are less amenable to sulfide precipitation. At pH > 8.1, these complexes decompose, allowing thallium to precipitate as Tl2S. Therefore, maintaining pH above 8.1, ideally at 12, is critical to overcome the chloride interference and achieve efficient removal.
What is the cost implication of using 2.0% thallium removal agent and 1.0% multi-effect auxiliary agent compared to conventional treatment?
The paper does not provide a detailed cost analysis. However, the process achieves deep removal to 4.31 μg/L, which is significantly lower than typical conventional methods. The use of relatively low reagent dosages (2.0% and 1.0%) suggests potential cost-effectiveness, but a full economic assessment would require scale-up studies.
Can this process be scaled up to treat large volumes of wastewater in a continuous industrial operation?
The study was conducted at laboratory scale. The combined process of sulfide precipitation and coagulation-flocculation is well-established in industrial wastewater treatment, and the identified mechanisms (sulfide precipitation and lattice substitution) are scalable. However, pilot-scale studies are necessary to address issues such as sludge handling, mixing efficiency, and continuous operation stability.
What is the fate of the thallium-containing sludge generated from this process?
The paper does not detail sludge management. Typically, sulfide precipitates are stable and can be disposed of in secured landfills or further processed for metal recovery. The presence of KxTlyCl phases suggests potential for thallium recovery, but this requires further investigation.
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