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Sediment has been considered to be one of the most important mercury methylation sites, but recent studies have demonstrated a new site that is relevant, i.e. the roots of floating aquatic macrophytes, where high methylation is observed. The effects of temperature, pH and electric conductivity on net mercury methylation were studied in the roots of the water‐hyacinth Eichhornia crassipes of a freshwater coastal lagoon (Lagoinha, RJ, Brazil). Root samples were incubated, over three days, with 203HgCl2 addition, at different temperatures (10–90 °C), pH values (3–8) and different electrolytic solutions (KClO4, KCl and CaCl2, at 1, 5, 10, 25 and 50 mM, ranging between 18 and 760 µS cm−1). 203Hg‐labeled methylmercury (Me203Hg) was extracted in toluene, after acid leaching, and measured by β‐counting. Up to 35% of mercury added was converted to MeHg Methylation increased from 10 to 35 °C, and decreased thereafter. The process was completely inhibited at 90 °C. At pH values of 6 and 7 methylation was stimulated and a significant decrease was verified at pH 8. Increasing KClO4 concentrations led to a significant decrease in the methylation rates, while for KCl and CaCl2 solutions only a slight decrease was observed. Copyright © 1999 John Wiley & Sons, Ltd.

Competitive oxyanion adsorption can increase the mobility of toxic trace elements in soils leading to groundwater contamination and a potential threat to the environment. We conducted a study to determine how phosphate fertilization might affect arsenate mobility in soil. The competitive nature of sorbed phosphate on arsenate mobility was investigated using laboratory soil columns constructed from an aggregated Oxisol. The soil was incubated with four levels of phosphate (0–48.6 mmol P kg−1) before being leached with 0.66 mmol KH2AsO4 L−1 in 0.05 M KCl. Arsenate mobility was greatly enhanced by treatment with increasing amounts of phosphate. The As breakthrough curves (BTCs) were divided into two phases. The initial phases of arsenate interaction was consistent with ligand exchange of Me‐OH+2 groups, where Me is the coordinating metal atom at the oxide surface. This phase was characterized by the constant pH of the effluent. The second phase was consistent with ligand exchange of As with Me‐OH groups and surface‐completed P. The asymmetry of the As BTCs suggested that the physical structure of the matrix influenced the convective flow path of the As and subsequently its reaction and movement through the soil.

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