Assessment of metals and anions in tap, river, wastewater, and sludge: comparison of hotplate- and microwave-assisted digestion

Abstract

In this study, the analysis of metals in tap, river, wastewater, and sludge samples was conducted using ICP−OES after hotplate- or microwave-assisted digestion. Both digestion methods produced a good degree of accuracy, indicating their suitability for the analysis of the studied metals in water samples. From method development studies, 100 mL of HNO₃   was found to be the optimum sample volume and acid type for digestion. The average concentrations obtained ranged from 4.9–410.8 µg/L, 5.9–465.0 µg/L, 3.6–425.4 µg/L, 16.1–647 µg/L and 9.7–784 µg/L in tap water, river, influent, effluent, and sludge samples, respectively. All metals were below their maximum permissible limits, with the exception of Mn in all sludge samples and Pb in all tap water, Umhlathuzana River, and Northern Works influent samples.  Comparable recoveries and metal concentrations were obtained by microwave and hotplate methods, suggesting that the cheaper hotplate method can be used as an effective digestion method for daily analysis. Common anion concentrations obtained ranged from 0.03–23.5 mg/L, 0.02–3 064.67 mg/L, and 0.32–175.67 mg/L for tap, river, and wastewater samples, respectively. The anion concentrations were found to be below the maximum acceptable limits indicating no negative health effect on human and aquatic life, with the exception of Cl⁻ and SO₄²⁻ in Amanzimtoti and Northern River water, respectively.