Rapid detection of low-concentration polyacrylamide and study on the influence of ions in solution

The residual polyacrylamide (PAM) in treated mineral processing wastewater affects the quality of recycled water and its potential for reuse. Continuous circulation and accumulation of cations in mineral processing wastewater can also influence the chemical properties and flocculating performance of PAM. In this study, a rapid detection method for low detection limit and concentration of PAM was proposed. The ultraviolet absorption spectra of four types and different molecular weights of PAM were collected in the wavelength range of 190–300 nm. The sensitivity and detection limits of this ultraviolet spectral method of PAM were verified. The effects of different K⁺ and Ca²⁺ concentrations on the detection of PAM concentration were examined. Change in the PAM molecular conformation was analyzed using atomic force microscopy. The findings demonstrated that for the four polymers, the highest absorbance was recorded at 194 nm. There was a strong linear relationship between PAM concentration and absorbance (R²>0.98). The limit of detection was lower than 0.1 mg·L⁻¹. This method showed high detection sensitivity. However, the linear relationship between concentration and absorbance was affected by the type of PAM used. With increasing K⁺ and Ca²⁺ concentrations, the intensity of the absorbance characteristic peak of the PAM solution was reduced. The maximum absorption wavelength was continuously red-shifted. In addition, when the K⁺ and Ca²⁺ concentrations were greater than 50 and 5 mmol·L⁻¹, respectively. The intensity of the characteristic absorbance peak gradually decreased. In deionized water, the surface roughness (R_q) values of APAM-3 and CPAM-12 deposited on SiO₂ and Au substrates were 24.3 and 38.1 nm, respectively. When the K⁺ and Ca²⁺ concentrations increased from 10 and 1 mmol·L⁻¹ to 100 and 10 mmol·L⁻¹, R_q increased from 80.4 and 120.0 nm to 124.0 and 145.0 nm, respectively. In the morphology map, the molecular chains of PAM transformed from linear dendrites to point blocky structures. It was demonstrated that the conformation of PAM chains in solution gradually became curled, resulting in a decrease in the peak intensity of the characteristic UV absorption spectrum. The effect of 10 and 100 mmol·L⁻¹ K⁺ and 1 and 10 mmol·L⁻¹ Ca²⁺ on the conformational change of PAM was essentially the same degree. The effects of typical monovalent and bivalent cations (K⁺ and Ca²⁺) on the molecular conformation of PAM were in alignment with the findings of their UV absorption spectra. In this study, the rapid and accurate quantitative detection of PAM is critical for exploring its migration behavior in mineral processing water treatment.