Evaluation of sample preparation methods for the detection of total metal content using inductively coupled plasma optical emission spectrometry (ICP-OES) in wastewater and sludge
Physics and Chemistry of the Earth
Department of Applied Chemistry, University of Johannesburg, P.O. Box 17011, Doornfontein, South Africa
Heavy metal contamination exists in aqueous wastes and sludge of many industrial discharges and domestic wastewater, among other sources. Determination of metals in the wastewater and sludge requires sample pre-treatment prior to analysis because of certain challenges such as the complexity of the physical state of the sample, which may lead to wrong readings in the measurement. This is particularly the case with low analyte concentration to be detected by the instrument. The purpose of this work was to assess and validate the different sample preparation methods namely, hot plate and microwave-assisted digestion procedures for extraction of metal ions in wastewater and sludge samples prior to their inductively coupled plasma optical emission spectrometric (ICP-OES) determination. For the extraction of As, Al, Cd, Cr, Cu, Fe, Mn, Ni, Pb, Zn, three acid mixtures, that is, HNO<inf>3</inf>/H<inf>2</inf>O<inf>2</inf>, HNO<inf>3</inf>/HClO<inf>4</inf>/H<inf>2</inf>O<inf>2</inf> and aqua regia+H<inf>2</inf>O<inf>2</inf>, were evaluated. Influent wastewater spiked with the SRM (CWW-TM-B) was used for the optimization of acid mixtures affecting the extraction procedure. After sample digestion, the filtration capabilities of cellulose-acetate filter paper and the acrodisc syringe filter with the pore size of 0.45μm were compared. In terms of performance, acrodisc syringe filter in terms of the improved recoveries obtained, was found to be the best filtration method compared to the filter paper. Based on the analytical results obtained, microwave-assisted digestion (MAD) using aqua regia+H<inf>2</inf>O<inf>2</inf> mixture was found to be the most suitable method for extraction of heavy metals and major elements in all the sample matrices. Therefore, MAD using aqua regia+H<inf>2</inf>O<inf>2</inf> mixture was used for further investigations. The precision of the developed MAD method expressed in terms of relative standard deviations (% RSD) for different metals was found to be <5%. The limits of detection (LOD) and limits of quantification (LOQ) ranged from 0.12% to 2.18μgL-1 and 0.61% to 3.43μgL-1, respectively. The accuracy of the developed method (MAD using aqua regia+H<inf>2</inf>O<inf>2</inf>) was verified by analyzing two SRMs (CWW-TM-A and CWW-TM-B) and the obtained results were in agreement with certified values with recoveries ranging from 80% to 104% for CWW-TM-A and 84% to 102% for CWW-TM-B. The accuracy of the developed method was verified also by the recovery test in the spiked sludge samples. The accuracy and spike recovery test for different metal ions were in the range 80-104% and 92-106%, respectively. The developed method was applied for extraction of the As, Al, Cd, Cr, Cu, Fe, Mn, Ni, Pb, Zn in environmental samples, namely wastewater and sludge. © 2015 Elsevier Ltd.
Aluminum; Electric discharges; Heavy metals; Inductively coupled plasma; Iron compounds; Lead removal (water treatment); Light emission; Manganese; Manganese removal (water treatment); Metal ions; Metal recovery; Metals; Mixtures; Nickel; Nitric acid; Optical emission spectroscopy; Plasma diagnostics; Plasma torches; Plate metal; Pore size; Recovery; Sludge digestion; Spectrometry; Syringes; Wastewater; Wastewater treatment; Zinc; Heavy metal contamination; Hot plates; Inductively coupled plasma optical emission spectrometric; Inductively coupled plasma-optical emission spectrometry; Microwave assisted digestion; Relative standard deviations; Sample preparation methods; Sludge; Extraction; concentration (composition); extraction method; filter; heavy metal; inductively coupled plasma method; sampling; sludge; wastewater