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The reactions of cadmium with calcium carbonate surfaces

SUMMARYThe reaction of cadmium with calcite was studied with 10−6–10−2M Cd2+ using 4g CaCO3 and 50 ml solution. The calcite‐water suspension was equilibrated for 2 d before adding Cd(NO3)2. The CaCO3 surface has a high affinity for Cd and at < 1 μmol g−1 a linear adsorption isotherm (C‐type) describes the reaction, whereas precipitation of CdCO3 on calcite predominates at higher Cd additions. In the latter case pH decreased and solutions were undersaturated with respect to CaCO3 indicating that CdCO3 prevented the CaCO3 surface from reacting with the acidity generated from CdCO3 precipitation. The data for low Cd additions suggests that an ideal surface solid solution is formed between CdCO3 and CaCO3 and the Thorstenson and Plummer equation expressing the pIAP values of surface solid solutions is valid. Equilibrium was attained for low Cd additions in several minutes, whereas precipitation was a slow process; at the changeover point distinction between the two processes is difficult. The reacting surface area calculated from the maximum adsorption of Cd is 0.5‐1.0 times that measured by the BET method.The Cd‐reacting surface areas of four natural calcareous minerals were much less than those found by the BET method which includes the surfaces of other minerals present. The Thorstenson and Plummer equation may be a useful way to relate solution composition and carbonate‐adsorbed Cd in contaminated soils.

Intracellular and Extracellular Uptake of Cadmium by the Moss Rhytidiadelphus squarrosus

Intra- and extracellular Cd uptake were investigated in the moss Rhytidiadelphus squarrosus. Intracellular Cd uptake displayed Michaelis–Menten kinetics and showed that the moss had a high uptake rate and high affinity for Cd. Extracellular Cd binding capacity was also high. The anion used had little effect on Cd uptake to either location but both intra- and extracellular Cd uptake were reduced by equimolar concentrations of Ca, Mg or Zn, although inhibition was not wholly competitive. Intracellular Cd uptake was not significantly affected by the supply of energy generating reserves but, like extracellular Cd uptake, showed altered characteristics following growth under controlled laboratory conditions. The results are compared with data on Cd uptake by the lichen genus Peltigera.

Kinetics of cadmium and zinc absorption by rice seedling roots

The present experiment in which intact roots of rice seedlings were used was conducted primarily to determine the reason for the high accumulation of Cd in roots compared with the absorption isotherms reflecting Cd and Zn uptake in roots. The amounts of the metals accumulated in the roots increased linearly with time within 1 h. The absorption isotherms of Cd and Zn in the roots showed that Cd was more rapidly absorbed than Zn at low medium concentrations up to about 0.6 µm, while the reverse pattern was observed at concentrations higher than 0.6 µm. From the Lineweaver-Burk equation, a dual pattern of transport mechanism was clearly observed for both Cd and Zn at the concentration range of 0.089 to 8.9 µm. The phenomenon of more rapid absorption of Cd than Zn at low concentrations was ascribed to a lower Km, namely higher affinity of Cd, while the more rapid absorption of Zn than Cd at higher concentrations was ascribed to the considerably higher Vmax of Zn.

Cadmium soil sorption at low concentrations: I. Effect of time, cadmium load, pH, and calcium

Sorption of Cd at low concentrations onto two Danish soils (loamy sand, sandy loam) was examined in terms of kinetics and governing factors. From an environmental point of view soil sorption of Cd is a fast process: More than 95% of the sorption takes place within 10 min, equilibrium is reached in 1 hr, and exposures up to 67 wk did not reveal any long term changes in Cd sorption capacities. The soils have very high affinity for Cd at pH = 6.00 (10−3 M CaCl2) exhibiting distribution coefficients in the order of 200 to 250 (soil Cd concentration/solute Cd concentration). However, the sorption isotherms describing the distribution of Cd between soil and solute are slightly curvelinear. In the pH-interval 4 to 7.7, the sorption capacity of the soil approximately increases 3 times for a pH increase of one unit. Increasing the Ca concentration from 10−3 to 10−2 M reduces the sorption capacity of the sandy loam to one third.

Minimal role of metallothionein in decreased chelator efficacy for cadmium

Chelator efficacy in Cd poisoning drops precipitously if therapy is not commenced almost immediately after exposure. Metallothionein (MT), a low-molecular-weight metal-binding protein with high affinity for Cd, may be important for this phenomenon. To more fully assess this role of MT in the acute drop in chelator efficacy following Cd poisoning, rats were injected iv with radioisotopic Cd (1 mg/kg as CdCl 2; 50 μCi/kg) followed by diethylenetriaminepentaacetic acid (DTPA; 90 mg/kg ip) at various times (0, 15, 30, 60, and 120 min) after Cd. The percentage to the Cd dose remaining in major organs 24 hr following Cd was determined. Although DTPA reduced Cd content in the various organs when given immediately after Cd, the chelator was ineffective at all later times. Increases in hepatic and renal MT did not occur until 2 hr after Cd, and did not coincide with the earlier drop in chelator efficacy. Blockade of MT synthesis by actinomycin D treatment (1.25 mg/kg, 1 hr before Cd) failed to prolong the chelators effectiveness. Furthermore, newborn rats have high levels of hepatic MT which had no effect on the time course of chelator effectiveness since DTPA still decreased Cd organ contents if given immediately following Cd but had no effect if given 2 hr after Cd. Therefore, it appears that MT does not have an important role in the acute decrease in efficacy of chelation therapy for Cd poisoning. The quick onset of chelator ineffectiveness may be due to the rapid uptake of Cd into tissues which makes it relatively unavailable of chelation.