Report No DWI0229

Feb 1981

Concentrations of potentially toxic elements in sewage sludges, and soils receiving sludge, are monitored by Water Authorities to prevent addition of excess amounts of metals to agricultural land Much of the error in estimating the concentrations of metals in these materials is probably incurred in sampling but it is important to know the extent of analytical error and to identify methods which give the most accurate results The WRC has therefore undertaken an inter-laboratory study for the D0E to compare the results of determinations of metals made by different laboratories on prepared samples of sewage sludges and soil The study has involved more than one hundred laboratories, of which ninety have submitted results. The results have shown that the accuracy of determinations of Cd, Cu, Ni, Pb and Zn in sludges was usually acceptable (MPD < ± 15% but determinations of Cd and Ni at the lower concentrations of interest proved difficult Determinations of Cr in both sludges and soils and determinations of all metals in soils were subject to comparatively high errors (MPD > ± 15%) Digestion procedures which used sulphuric acid produced anomalous results for Pb and Cr Dry ashing was associated with comparatively high errors of precision. There was li ttle difference between the other wet digestion methods tested except that digestion with a mixture of nitric and perchloric acid recovered more metals (except Cu) from the soil sample than the other procedures The results suggested that adequate determinations of metals (except Cr) in sludge can be made by procedures involving wet digestion (without sulphuric acid) followed by atomic absorption spectrophotometry. More work is needed to improve methods for the determination of Cr and low levels of Cd and Ni on sludges, and of a11 elements in soils. This is of importance if concentrations of metals in soils are used to monitor applications of sludge to agricultural land. A comparison of methods for determining extractable Cu, Ni and Zn in soil showed that an EDTA method was substantially more accurate than an acetic acid method. Limited results were obtained for the part of the exercise concerned with Mo, Hg, As, Se, F and B. These elements are subject to guidelines where sludge is used on land and further work is clearly needed on methods for their determination in both sludges and soils.

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