Use of multivariate analysis for the quantification of arsenic in the presence of phosphates in water treatment plants

MOSQUEIRA S.; BERARDOZZI, E.; GARCÍA EINSCHLAG, F. SEBASTIÁN

Congreso; WCCE11, XI Congreso internacional de Ingeniería Química; 2023

High arsenic concentrations have been found in the groundwater of the Chaco-Pampean plain, with levels exceeding those established by the World Health Organization (10 µg/L)1. In this context, the application of the “Zero-Valent Iron” (ZVI) technique for the remediation of groundwater contaminated with As may be advantageous due to its high efficiency/cost ratio2. Arsenic removal by ZVI is based on complex mechanisms that may involve both chemical reduction processes and adsorption and/or co-precipitation phenomena of the contaminant during the in situ formation of insoluble products generated by the corrosion of metallic iron. Our research group, from the National University of La Plata, has developed a ZVI-based water treatment plant for As removal that is being tested in Pipinas (Buenos Aires province).To evaluate the system’s performance, the arsenic content in the treated water is routinely analyzed. Among the methods for quantifying As in water, the molybdenum blue technique3 is a colorimetric assay with several advantages since it is fast, inexpensive, and does not generate toxic products. However, this technique is also used to determine phosphates, the latter being the main interferent in the As measurement. Because the concentrations of arsenic and phosphate in the groundwater of Pipinas are 168 and 720 µg/L, respectively, it was necessary to develop a simple method to quantify the arsenic content in the presence of phosphate. To this end, the latter spectrophotometric technique was coupled to the multivariate curve resolution by alternating least squares (MCR-ALS) algorithm. Standards with known concentrations of arsenic and phosphate were prepared using different proportions of these species. Tests were carried out with concentrations from 0 to 800 ppb of arsenic and from 0 to 1000 ppb of phosphate. The molybdenum blue technique was applied, the absorption of the samples was recorded in the wavelength range from 400 to 900 nm, and the calibration spectra were analyzed using the MCR-ALS algorithm.Results show that the method developed is suitable for evaluating the treatment plant performance since it can be used for simultaneously quantifying arsenic and phosphate. The arsenic detection and quantification limits of 4 and 12 µg/L, respectively, are below the maximum concentration allowed in the Buenos Aires province (i.e., 50 µg/L). However, since the As quantification limit is slightly higher than that recommended by the WHO (10 µg/L), further research is ongoing to increase the sensitivity of the method.