Ca2+ is an important ion in industrial wastewater such as coal slurry water. The concentration of Ca2+ directly affects the water quality of recycled water, the physicochemical properties of particle surfaces, and the effectiveness of coal beneficiation. However, the determination of Ca2+ in coal slurry water is influenced by other coexisting metal ions (M: K+, Na+, Mg2+, and Al3+) in the system. The complexation color development – visible spectrophotometry (CCDVS) has been proven to be a rapid and accurate way to determine Ca2+. This study investigates the impact and mechanism analysis of factors such as the concentrations of coexisting ions (K+: 0–15 mg/L, Na+: 0–2000 mg/L, Mg2+: 0–20 mg/L, Al3+: 0–0.05 mg/L) and valence states (monovalent, divalent, trivalent) on the accurate determination of Ca2+ using the CCDVS method. All ions are interested in the indicator, with the light absorption intensity order of Al3+ (Abs = −1.783) > Mg2+(Abs = −1.723) > Ca2+(Abs = −1.248) > K+(Abs = −0.268) > Na+(Abs = −0.075) under the same ions concentration (e.g. 16 mg/L). Interestingly, the absorbance of Ca–M mixed solutions shows different trends. For example, adding Na+ decreases the absorbance value of Ca2+, with a decrease in Abs of 0.065 when the Na+ concentration is 2000 mg/L. This indicates a competitive relationship between the complexation color development reaction of Ca and Na due to the same reaction mechanism. However, for Ca–Mg mixed solutions, when the Mg2+ concentration increases to 15 mg/L, the increase in Abs is 0.078, attributed to the stronger absorption of Mg2+ occupying the adsorption sites of Ca2+. On the contrary, at industrial concentrations, the influence of K+ (0–20 mg/L) and Al3+ (0–0.05 mg/L) on Ca2+ detection can be ignored. Complementary characterization (XRD, XPS) and DFT theoretical calculations indicate that complexation color development reaction depends on the adsorption of metal ions on the sulfonic acid sites on the indicator. Other metal cations compete with Ca2+ for the same adsorption sites, and the reactivity of metal ions with the indicator is as follows Al3+ > Mg2+ > Ca2+ > K+ > Na+. Therefore, this study provides technical and theoretical support for the development of highly selective Ca2+ detection methods, aiding in the rational control of chemical agent dosages.
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