Coagulation of trace arsenic and cadmium from drinking water using titanium potassium oxalate

Heavy metal contamination in drinking water poses a significant threat to human health. Arsenic (As) and cadmium (Cd) are particularly concerning due to their toxicity and prevalence in various industrial discharges. Traditional coagulation methods using iron and aluminum salts are cost-effective but often insufficient for simultaneous removal of both anionic (As) and cationic (Cd) heavy metals. Advanced technologies like adsorption, ion exchange, and membrane separation offer high efficiency but involve expensive materials and complex operations. This study explores the use of titanium potassium oxalate (K₂TiO(C₂O₄)₂), a water-soluble tanning reagent, as a novel coagulant for the simultaneous removal of As and Cd from drinking water. K₂TiO(C₂O₄)₂ is chosen due to its water solubility and its potential to hydrolyze into hydrous TiO₂, a known adsorbent for heavy metals, and calcium oxalate (CaC₂O₄), which can contribute to the removal of Cd. The study aims to evaluate the efficacy of K₂TiO(C₂O₄)₂ in removing As and Cd compared to traditional coagulants, optimize the coagulation process using response surface methodology (RSM), and elucidate the underlying removal mechanism.

Existing literature highlights the challenges in efficiently removing multiple heavy metals from drinking water. While advanced techniques like adsorption, ion exchange, and membrane filtration offer effective solutions, they are often economically prohibitive for widespread implementation. Traditional coagulation with iron and aluminum salts remains a popular and cost-effective method but demonstrates limitations in simultaneously removing anionic and cationic heavy metals. Research has explored various modified coagulants and alternative materials for enhanced heavy metal removal. For example, studies on Fe-modified mesoporous carbon for arsenic removal and microporous stannosilicate for cadmium removal have shown promising results. The use of titanium-based materials for heavy metal removal has also gained attention, with titanium dioxide (TiO₂) exhibiting effective adsorption capacities for both arsenic and cadmium. However, the application of titanium-based coagulants for simultaneous removal of arsenic and cadmium remains relatively unexplored.

The study employed both experimental and modeling techniques. Coagulation experiments were conducted using tap water spiked with arsenic (As(III) and As(V)) and cadmium (Cd). Different concentrations of K₂TiO(C₂O₄)₂, CaCl₂, and pH levels were tested to evaluate their impact on arsenic and cadmium removal efficiency. The Box-Behnken design (BBD), a response surface methodology (RSM) technique, was used to optimize the coagulation process by determining the optimal combination of factors. The influence of reaction time and temperature on removal efficiency were also investigated. Comparative experiments were performed using conventional coagulants, Al₂(SO₄)₃ and Fe₂(SO₄)₃, to benchmark the performance of K₂TiO(C₂O₄)₂. The flocs formed during the coagulation process were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) to understand the removal mechanism. Geochemical modeling using Visual Minteq 3.1 was utilized to predict arsenic and cadmium speciation under different pH conditions and to support the mechanistic analysis. Atomic fluorescence spectrophotometry (AFS) was used for arsenic and cadmium concentration measurements, while ICP-MS was used for measuring titanium, potassium, and calcium concentrations. Ion chromatography was employed for oxalate analysis.

K₂TiO(C₂O₄)₂ demonstrated superior performance in removing arsenic and cadmium compared to conventional coagulants Al₂(SO₄)₃ and Fe₂(SO₄)₃. A dose of 120 µmol/L K₂TiO(C₂O₄)₂ achieved over 90% removal of both As and Cd, even at initial concentrations ten times higher than the maximum contaminant levels. The removal of As and Cd by K₂TiO(C₂O₄)₂ occurred rapidly within the first 5 minutes of reaction. Temperature had a minor influence on Cd removal but significantly affected As removal, with higher temperatures leading to decreased efficiency. Response surface methodology (RSM) analysis revealed that K₂TiO(C₂O₄)₂ dose and pH were the most significant factors influencing As(III) and Cd removal. The optimal conditions for As(III) removal were identified to be pH 9 and a K₂TiO(C₂O₄)₂ dose of 40 mg/L. Characterization of the flocs revealed the presence of crystalline calcium oxalate (CaC₂O₄) and amorphous hydrous titanium oxide (TiO(OH)₂). Arsenic was primarily removed through complexation with TiO(OH)₂, while Cd removal involved both adsorption onto TiO(OH)₂ and mixed-crystal formation with CaC₂O₄. Geochemical modeling supported the proposed mechanism, confirming the formation of CaC₂O₄ and TiO(OH)₂ as the main solid phases under the experimental conditions. Notably, the use of K₂TiO(C₂O₄)₂ also led to a reduction in water hardness due to the precipitation of calcium ions, potentially offering additional benefits.

The superior performance of K₂TiO(C₂O₄)₂ in removing both anionic and cationic heavy metals is attributed to its dual mechanism of action involving both adsorption onto TiO(OH)₂ and co-precipitation with CaC₂O₄. This contrasts with traditional coagulants like aluminum and ferric salts, which often exhibit limited efficiency for simultaneous removal. The RSM analysis effectively identified the optimal operating conditions, further highlighting the practicality of K₂TiO(C₂O₄)₂ for water treatment applications. The reduction in water hardness resulting from the consumption of calcium ions during coagulation offers an added benefit. The findings suggest that K₂TiO(C₂O₄)₂ presents a viable and efficient alternative for treating drinking water contaminated with multiple heavy metals.

This study demonstrates the effectiveness of K₂TiO(C₂O₄)₂ as a novel coagulant for the simultaneous removal of arsenic and cadmium from drinking water. Its superior performance compared to conventional coagulants, along with the added benefit of water softening, makes it a promising candidate for water treatment applications. Future research could explore the long-term stability and cost-effectiveness of K₂TiO(C₂O₄)₂ in real-world water treatment settings. Investigating the applicability of this coagulant to other heavy metal contaminants and exploring potential modifications to enhance its performance could also be fruitful.

The study primarily utilized tap water spiked with arsenic and cadmium, which might not fully represent the complexity of real-world contaminated water sources. The presence of other ions or organic matter could influence the removal efficiency of K₂TiO(C₂O₄)₂. Further research is needed to evaluate its performance in different water matrices. The long-term stability and potential environmental impact of the coagulant and its byproducts require further investigation.