Simultaneous and Sequential Extraction Protocols as Tools for Determination of Zinc Bioavailability in Dried Anaerobic Sludge

Abstract

Production of unsettleable sewage sludge with high water content is one of the problems of intensification of industrial activities and environmental protection. Sewage sludge with low toxic metals concentrations can be utilized as fertilizer and soil conditioner in agriculture. For determination of metal bioavailability, a wide range of extraction protocols and fractionation analyses can be used. We studied the distribution and quantified the leaching and bioavailability of zinc from dried anaerobic sludge by simultaneous, multi-step and three different sequential extraction protocols. For determination of zinc, the galvanostatic stripping chronopotentiometry (SCP) and electrothermal atomic absorption spectrometry (ETAAS) were used. The distribution of Zn in sequential extraction protocols was determined using a fivestep chemical fractionation procedures (BCR, Tessier and Van Hullebusch protocols). The potential bioavailability (0.9% NaCl, 0.1 mol/dm3 HCl, 0.1 mol/dm3 HNO3, 0.1 mol/dm3 CH3COOH, 0.1 mol/dm3 Na2EDTA, 0.1 mol/dm3 CaCl2, 0.1 mol/dm3 MgCl2, 0.1 mol/dm3 (NH4)2C2O4.H2O extraction) and pseudo total (aqua regia extraction and ETAAS analyses) content of Zn in sludge was determined. The amount of aqua regia extractable zinc in sludge samples was 650 ± 12 mg/kg (d.w.). We found out that the zinc was extractable from anaerobic sludge in first hour of contact time for all tested agents. Zinc was extracted with highest efficiency by 0.1 mol/dm3 (NH4)2C2O4.H2O, 0.1 mol/dm3 HCl and 0.1 mol/dm3 Na2EDTA. Sequential extraction protocols showed that the maximum extractable amount of zinc 126.3 ± 2.6 mg Zn / kg d.w. was bound to organic matter and sulfides. High concentrations of zinc in residual fractions were leachable under extraction conditions of strong acids only.