Studies of oxidation of elemental S in soils showed that thiosulfate and tetrathionate are produced during S oxidation in soils. In this work, unamended and elemental S‐amended soils were incubated (30°C) under aerobic conditions for various times. The amounts of S2O3‐, S4O6‐, and SO4‐S produced during incubation were determined. Results showed that S2O32‐ was produced within the first few days of incubation and that S4O62‐ accumulated in some soils. The rate of S oxidation increased with increasing incubation temperature (5, 15, and 30°C) and with increasing the rate of S application (50, 100, and 200 µg S/g soil). For 100 µg S/g soil, the rates of oxidation of elemental S in 10 Iowa surface soils ranged from 39 to 75 µg S/g soil after incubation at 30°C for 70 days; rates were more rapid in alkaline soils than in acid soils. There was little change in pH of soils even when the S application rate was increased from 50 to 200 µg S/g soil. The rate of S oxidation was lower in air‐dried soils than in field‐moist soils.
The uptake of copper by water hyacinth (Eichorniu crassipes) was studied using solution culture techniques in the greenhouse. The bioassays indicated that the uptake of copper was a direct function of its speciation. For example, only free Cu2+ was absorbed by the plant in the presence of strong ligands such as EDTA and humic acid. Other ligands tested (fulvic acid, amino acids and simple organic acids), however, failed to suppress the uptake of copper even when the free Cuz+ in solution initially was negligibly small. Copper could have been taken up directly by the plant as various complexes or in the free Cu2+ form after dissociation from the ligands prior to uptake. Complete inhibition of copper uptake in the presence of humic acid, in contrast to fulvic acid, suggests that the chemical stability constants of these complexes with copper were not correlated to the metal's bioavailability. The potential of Eichornia for removing heavy metals from wastewaters is demonstrated by its tremendous capacity to bioaccumulate copper. Depending on the presence or absence of ligands, from 200 to more than 2,400 pg Cu g-' dry matter could be taken up by the plant.
The nature of H+ and Cu 2 + binding by soil-derived humic (HA) and fulvic (FA) acid was characterised using potentiometric titrations. The experimental data obtained showed that the derived proton balance equation was valid and capable of describing proton consumption by both polyelectrolytes. HA was found to be more acidic and more reactive as shown by its lower equivalent weight compared to FA. Acid consumption by HA during titrations was little affected at ionic strength (J..I.) up to 0.1 M although it was enhanced at higher J..I.. Displacement of protons by Cu 2 + resulted in a nonlinear sigmoidal pattern suggesting the formation of different Cu-HA chelates, or existence of sites that differed in their affinities for Cu on the ligand. Different concentrations of added Cu appeared to favour one or both mechanisms, although the titration method could not differentiate which of the probable mechanisms was more dominant at a specific level of Cu added. Similar values were obtained for conditional stability constants using either the equation of Scatchard or Ruzic.
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