Competitive effect and mechanism analysis of spectrophotometric detection of Ca2+ in metal cations-containing coal slurry water system

Ca 2+ is an important ion in industrial wastewater such as coal slurry water. The concentration of Ca 2+ directly affects the water quality of recycled water, the physicochemical properties of particle surfaces, and the effectiveness of coal beneficiation. However, the determination of Ca 2+ in coal slurry water is influenced by other coexisting metal ions (M: K + , Na + , Mg 2+ , and Al 3+ ) in the system. The complexation color development – visible spectrophotometry (CCDVS) has been proven to be a rapid and accurate way to determine Ca 2+ . 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, Mg 2+ : 0–20 mg/L, Al 3+ : 0–0.05 mg/L) and valence states (monovalent, divalent, trivalent) on the accurate determination of Ca 2+ using the CCDVS method. All ions are interested in the indicator, with the light absorption intensity order of Al 3+ (Abs = −1.783) > Mg 2+ (Abs = −1.723) > Ca 2+ (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 Ca 2+ , 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 Mg 2+ concentration increases to 15 mg/L, the increase in Abs is 0.078, attributed to the stronger absorption of Mg 2+ occupying the adsorption sites of Ca 2+ . On the contrary, at industrial concentrations, the influence of K + (0–20 mg/L) and Al 3+ (0–0.05 mg/L) on Ca 2+ 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 Ca 2+ for the same adsorption sites, and the reactivity of metal ions with the indicator is as follows Al 3+  > Mg 2+  > Ca 2+  > K +  > Na + . Therefore, this study provides technical and theoretical support for the development of highly selective Ca 2+ detection methods, aiding in the rational control of chemical agent dosages.