PH and grain-size variation in leaching tests with bricks made of harbour sediments compared to commercial bricks

Bricks made of 50% wt. harbour sediments from Bremen, Germany, harbour sediment and nine commercial bricks made of common raw materials were leached in various experiments. The harbour sediment is polluted with heavy metals, e.g. Zn, Cd, Pb and organic compounds, e.g. tributyltin. To assess the environmental impact in the potential use of sediment bricks we consider the influence of pH (4-11) and grain size (50-30 000 microm), the two prime variables in the life-cycle of the bricks. Leachability of trace contaminants increased at acidic pH values and remained low at neutral and alkaline pH values. Leachability increased for smaller grain sizes in relation to the increasing specific surface areas. Grain sizes below 63 microm showed reverse effects for V, Cr, Ni, As, Sr, Mo and Pb due to sorption on the sample material or freshly precipitated phases such as barite, anhydrite or cuprous ferrite. A grain-size fraction of 125-1000 microm was selected in the leaching tests in order to compare different brick types. In general, the leachability of heavy metals from the sediment brick was in the upper range of the commercial bricks. At a temperature of 1050 degrees C thermal treatment of harbour sediments led to an immobilization of most trace contaminants. Chromium, V, As and Mo became even more mobile after thermal treatment, but the enhanced mobilization of V, As and Mo differed strongly among the bricks compared.