The complexation color development-visible spectrophotometry method for rapid determination of Ca2+concentration in mine wastewater and its mechanism

Ca2+ is an important cation in coagulants used in the treatment of mine wastewater such as coal slime water, and its concentration directly affects the settling and flotation of coal slime water. The rapid determination of Ca2+ concentration is of great significance to guiding industrial production. Herein, the complexation color development-visible spectrophotometry (CCDVS) method was proposed to obtain the concentration information of Ca2+, wherein 1-(1-Hydroxy-4-methyl-2-phenylazo)-2-naphthol-4-sulfonic acid (Calmagite) was used as an indicator and the mechanism of complexation between the indicator and Ca2+ was explored. The strong correlation between spectral data and Ca2+ concentration (R2 = 0.997) indicated that this method could be used for Ca2+ detection, thereby a quantitative model was proposed to predict Ca2+ concentration by using the complexation color development-visible light absorption data. The chemical element and bonding analysis confirmed that Ca2+ was complexed with the sulfonic acid group on the indicator skeleton in the form of Ca(OH)+ intermediate in an alkaline environment. The generated complexation product (-S-O-Ca(OH)+) could stably absorb 620 nm light energy, and its absorption performance remained 99.8% after standing for 60 min. The relative standard deviation of the detection results of Ca2+ in different calcium source solutions was as low as 1.24%, which confirmed the strong calcium source compatibility of the proposed method. More importantly, the successful application of this method in the determination of Ca2+ concentration at different points in industrial coal slime water system proved its advantages of high accuracy (relative error is as low as 5.01%), low cost and simple operation. The LOD of this method is 0.038 mg/L. Therefore, the complexation color development-visible spectrophotometry method can maximize the potential for cost reduction and efficiency improvement in mine wastewater treatment.