Competitive Adsorption Of Cd Research Articles

Competitive adsorption of Cd, Cu, Hg and Pb by agricultural soils of the Changjiang and Zhujiang deltas in China

Soils can often be contaminated simultaneously by more than one heavy metal. The sorption-desorption behavior of a metal in a soil will be affected by the presence of other metals. Therefore, selective retention and competitive adsorption of the soils to heavy metals can affect their availability and movement through the soils. In this study, the simultaneous competitive adsorption of four heavy metals (Cd, Cu, Hg, and Pb) on ten agricultural soils collected from the Changjiang and Zhujiang deltas, China was assessed. The results showed that the competition affected the behavior of heavy metal cations in such a way that the soils adsorbed less Cd and Hg, and more Pb and Cu with increasing total metal concentrations, regardless of the molar concentration applied. As the applied concentrations increased, Pb and Cu adsorption increased, while Cd and Hg adsorption decreased. The adsorption sequence most found was Pb>Cu>Hg>Cd. The maximum adsorption capacity for the heavy metal cations was calculated, and affected markedly by soil properties. The results suggest that Hg and Cd have higher mobility associated to the lower adsorption and that Pb and Cu present the opposite behavior. Significant correlations were found between the maximum adsorption capacity of the metals and pH value and exchangeable acid, suggesting that soil pH and exchangeable acid were key factors controlling the solubility and mobility of the metals in the agricultural soils.

Competitive Adsorption of Cd and Dyes to Kaolinite

AbstractThe competitive adsorption to kaolinite between Cd(II) and four polyaromatic dyes (9-aminoacridine, 3,6-diaminoacridine, azure A and safranin O) was studied in 5 mM KNO3 at 25°C. Under these conditions, Cd adsorbs to the silica face of kaolinite between about pH 4 and 6.5, but at higher pH, adsorbed Cd is progressively relocated to the crystal edges. In the presence of dye, less Cd adsorbed to kaolinite below pH 7. If sufficient dye was added to saturate the kaolinite surface, Cd adsorption was totally suppressed up to ∼pH 6. At higher pH, Cd followed the characteristic pattern for edge adsorption. In separate experiments 9-aminoacridine and azure A displaced pre-adsorbed Cd from kaolinite. The displacement curves were initially linear, with one Cd ion being displaced for every 13 dye molecules adsorbed at pH 5.5, and one Cd ion for every 35 dye molecules at pH 7.5. The interpretation of these results is that the dyes bind to kaolinite much more strongly than Cd(II) does, but only to the silica face.

Single and competitive adsorption of Cd and Zn onto a granular activated carbon

Single and competitive adsorption of cadmium and zinc onto granular activated carbon DARCO 12–20 mesh has been investigated. This activated carbon has been shown as an effective adsorbent for both metals. Cadmium and zinc removals increased with pH and decreased with molar metal/carbon ratio. Surface precipitation phenomena have been detected for the higher pHs and molar ratios. The adsorption process has been modelled on the surface complexation Triple Layer Model (TLM). For this purpose, the amphoteric nature of the activated carbon has been studied. Single metal adsorption data have been used to calibrate TLM parameters. A dependence of the adsorption constants on pH and molar metal/carbon ratio has been observed, and a correlation for log K ads has been determined. In the competitive system, the removal efficiency of the activated carbon decreased for both metals. The TLM model, using surface complexation constants determined from single adsorption experiments, successfully predicted cadmium and zinc removal from the two metal solutions.

Competitive Adsorption of Heavy Metals by Soils

AbstractThe competitive adsorption of Cd, Cu, Pb, and Zn from 0.025 M NaClO4 solutions onto four soils with differing chemical properties was investigated with batch adsorption experiments. For two mineral soils, adsorption under acidic conditions (pH 5.0) followed the sequence Pb > Cu > Zn > Cd, which corresponds to the order of increasing pK for the first hydrolysis product (e.g., PhOH+) of the metal ions. In contrast, the order of selectivity for two soils containing 20 to 40 g kg−1 organic C was Pb > Cu > Cd > Zn, suggesting that organic matter (OM) increased Cd retention preferentially over that of Zn. When accompanied by a sizable reduction in cation exchange capacity (CEC), extraction of soil OM markedly reduced adsorption of all four metals. However, only Cd and Cu adsorption were appreciably smaller for a soil that maintained a sizable CEC following OM removal. Lead, and possibly Zn, adsorption by soils with substantial inorganic exchange sites may be unresponsive to organic waste amendments. For Cd and Cu, increased soil OM should restrict mobility and bioavailability, at least under acidic conditions where soluble metal complex formation is limited.

Competitive adsorption of cd, cu, zn, and pb on amorphous iron oxyhydroxide

Competitive adsorption of Cd, Cu, Pb, and Zn onto amorphous iron oxyhydroxide (Fe 2O 3 · H 2O (am)) was studied in systems containing pairs of metal adsorbates. The concentration of the more strongly binding metal was 10–100 times that of the other metal in the system. The conditions were such that if the metals competed for the same group of surface sites, adsorption of the weaker-binding metal should have been significantly reduced when the second metal was added to the system. Nevertheless competitive interactions were minimal, indicating that many of the strong binding sites for one metal are not preferred binding sites for other metals. These results support the hypothesis that the Fe 2O 3 · H 2O (am) surface consists of several distinct groups of binding sites. A differential adsorption equilibrium constant is defined and related to the overall average adsorption equilibrium constant. Based on this relationship, the binding constants for a single metal to different surface sites range over at least two orders of magnitude. The differential constant can be used to assess competitive interactions. For instance, when 5 × 10 −5 M Cu and 5 × 10 −7 M Cd compete for sites on Fe 2O 3 · H 2O (am) (1 × 10 −3 M Fe T), less than 2% of the adsorbed Cu binds to “preferred” Cd-binding sites. Failure to account accurately for competitive effects can lead to serious errors when modeling trace metal behavior in natural systems and engineering processes.