The pH effect on the detection of heavy metals in wastewater by laser-induced breakdown spectroscopy coupled with a phase transformation method

Abstract

Heavy metal particles in water are mainly derived from acidic industrial wastewater and seriously endanger the environment and public health. In this work, the pH effect on the detection of toxic metals in wastewater by laser-induced breakdown spectroscopy coupled with a phase transformation method (LIBS-PT) was investigated. Heavy metals of cadmium (Cd) and chromium (Cr) were selected as examples. The results showed that the presence of acids in wastewater inhibited the spectral enhancement of LIBS-PT on a metal substrate. This was mainly due to the presence of the salt floccule formed by the reaction of an acid with a metal substrate on the substrate surface. The floccule content increased as pH decreased, and the corresponding substrate ablation threshold increased. Therefore, more laser energy was used for ablation, resulting in reduced laser energy for ionization and reduced electron density. Eventually, spectral intensity decreased as electron density decreased. However, there was no significant change in plasma temperature. Meanwhile, the determination coefficients (R²) of Cd and Cr were all above 0.99 under the optimal pH 6.5 and on the optimal zinc (Zn) substrate. Limits of detection (LoDs) of 0.0089 mg L⁻¹ and 0.0006 mg L⁻¹ for Cd and Cr were obtained, respectively. The LoDs of Cd and Cr elements met the sewage discharge standard of China. The results indicated that the detection sensitivity of heavy metal elements in acidic wastewater can be significantly improved by optimizing the pH value of the solution using LIBS-PT.