Sorption of trace metals by standard and micro suction cups in the absence and presence of dissolved organic carbon.

Both the bioavailability of a trace metal (TM) in a soil and the risk of leaching to the ground water are linked to the metals concentration in the soil solution. Sampling soil solution by tension lysimetry with suction cups is a simple and established technique that is increasingly used for monitoring dissolved TM in soils. Of major concern, however, is the sorption of TM by the walls of the samplers. Metal sorption by different materials used in suction cups can vary widely, depending also on the chemistry of the soil solution. We compared the sorption of Cu, Zn, Cd, and Pb by different standard-size and micro suction cups in the laboratory at two pH values (4.5 and 7.5 or 8.0) in absence and presence of dissolved organic carbon (DOC). In addition, we investigated the sorption of DOC from different origins by the cup materials. At both pH values, the weakest sorption of all four TMs was exhibited by standard-size suction cups based on nylon membranes and by hollow fibers made from polyvinyl alcohol (PVA). At alkaline pH, borosilicate glass, ceramic materials, and polytetrafluorethylene (PTFE) mixed with silicate were characterized by generally strong sorption of all investigated TMs. In addition, Cu and Pb were strongly sorbed at low pH by PTFE-silicate and a ceramic material used for the construction of standard-size suction cups. On the other hand, sorption of Cu, Zn, and Cd by ceramic capillaries produced from pure aluminum oxide was negligible at low pH. Micro suction cups made of an unknown polymerous tube sorbed Cu strongly, but were well suited to monitor Zn, Cd, and Pb at low pH, and, in the presence of DOC, also at high pH. Major cations (Na+, Mg2+, K+, Ca2+) and anions (Cl-, NO3-, SO4(2-)) were not or very weakly sorbed by all cup materials, except for Mg2+, K+, and Ca2+ by borosilicate glass at pH 7.5. Trace metal sorption by suction cups was generally greatly reduced in the presence of DOC, especially at alkaline pH. The sorption of DOC itself depended on its source. Dissolved organic carbon from leaf litter extracts with a probably large hydrophobic fraction was sorbed more strongly than mainly hydrophilic DOC from a mineral soil solution.