Suction Cup Materials and their Potential to Bias Trace Metal Analyses of Soil Solutions: A Review

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Abstract
We reviewed the effects of physical and chemical characteristics of filter materials on trace metal concentrations in soil solutions to evaluate their suitability for porous sections of suction cups. Among the materials described in the literature, e.g. ceramics, Al2O3, sintered Ni or glass, and various plastic filters, a great variation in bubbling pressure, pore size, permeability, chemical composition and stability, CEC, and trace metal dissolution/precipitation or adsorption/desorption processes was found. A low adsorption capacity suggests plastic materials are suitable, provided the bubbling pressure is high enough. All other materials have disadvantages in one or more properties, e.g. release of trace metal impurities into the soil solution, clogging of pores by precipitation of previously dissolved compounds, or sorption of trace metals. Many reported experiments on sorption of trace metals by various materials are not relevant because of the unrealistic high metal concentrations of the test solutions. Our experiments with realistic concentrations revealed that sorption processes during the extraction may alter trace metal concentrations in soil solutions significantly. Ceramic or Al2O3 cups are commonly used, but they adsorb appreciable amounts of trace metals. Therefore, we conclude that metal fluxes and budgets based on such data have to be interpreted with caution. To improve data comparability, detailed description of suction cup materials in the method sections of papers would be required. Moreover, more development and testing with plastic materials should be done, aimed at the optimization of both physical and chemical properties.