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    The influence of zinc and copper fertilizer application on zinc, copper and cadmium concentration in mixed pasture : a thesis presented in partial fulfilment of the requirements for the degree of Master of Applied Science in Soil Science at Massey University
    (Massey University, 1996) Khan, Md. Afiqur Rahman
    There has been considerable debate about the accumulation of cadmium (Cd) in agricultural soils and its subsequent uptake by pasture plants due to phosphate fertilizer application. Ruminants grazing pastures absorb a small fraction of this Cd, and some of this is subsequently accumulated in the liver and kidney. Although tissue accumulation of Cd in grazing livestock is generally small (< 1 mg Cd kg-1 fresh tissue), but any reduction in plant uptake is beneficial in reducing such accumulation further, especially in the kidneys. Uptake of Cd by pasture may be affected by the concentration of other nutrient cations, such as zinc (Zn) and copper (Cu). In addition, since Zn and Cu are complexed by the same metal binding protein (metallothionein) as Cd, a change in the ratio of these nutrients in pasture may also reduce Cd accumulation rates by interfering with Cd accumulation. In order to assess the effects of Zn and Cu on Cd uptake by pasture, a field experiment was conducted, using three pairs of pasture plots with low ( 0.2 mg Cd kg-1) and high (0.6 mg Cd kg-1) background Cd status. Twelve sub-plots (l.44 m2) were laid out in each plot and increasing levels of Zn (0, 5, 15 and 40 kg ha-1) and Cu (0, 2, 5 and l0 kg ha-1) were added as ZnSO4. 7H2O and CuSO4.5H2O respectively. Pasture samples were collected at regular intervals and analysed for dry matter yield, botanical composition and Zn, Cu and Cd uptake. Soil samples were extracted with 0.01M CaCl2 and 0.lM HCl solution to measure the plant available Zn, Cu and Cd. It was found that the plots with a high background Cd status in the soil resulted in a higher Cd concentration in mixed pasture (0.22 mg Cd kg-1 DM) than those with a low background Cd status (0.10 mg Cd kg-1 DM) at the first harvest (after 73 days). The Cd concentration in the mixed pasture was higher during the summer (December) period than in the early spring (September). Application of Zn fertilizer increased the Zn concentration in pasture from 37 to 150 mg kg-1 DM at the first harvest. Excessive amounts of Zn lead to a decrease in DM yield. The growth of pasture was controlled principally by the amount of plant available Zn, which depended on the amount of both added Zn and added Cu. The effect of the added Cu was to increase the toxicity of the addd Zn. Application Cu fertilizer increased the Cu levels from 9 to 16 mg kg-1 DM at the first harvest. The Cu concentration in pasture continued to decrease with time following the addition of fertilizers. The legumes are more tolerant of Cu than grass. The Cu concentration in harvest 4 (after 159 days) ranged from 6.9 to 7.0 mg kg-1 DM in grass and 8.9 to 9.9 mg kg-1 DM in legumes. The Cd concentration in the pasture decreased with increasing Zn concentration in the pasture at the first harvest. The effect of Zn on Cd uptake was more pronounced on plots with a high background Cd status in the soil. The effect of Zn on Cd concentration depends on the external Zn concentration levels. There was no consistent effect of Cu concentration on Cd concentration. The effect of the addition of Cu and Zn in fertilizer was to lower the Cd:Cu and Cd:Zn ratios in the herbage. There was a good relationship between soil available Zn as extracted by 0.1M HCl and Zn concentration in the herbage. A similar observation was obtained for Cu. But there was no consistent relationship between 0.01M CaCl2 extractable Cd and the Cd concentration in pasture. The results indicated that pasture and soil analysis for Cd and Zn may provide useful guides to situations where Cd concentrations in pasture may be decreased by Zn applications.
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    Adsorption of zinc and cadmium by soils and synthetic hydrous metal oxides : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science at Massey University
    (Massey University, 1981) Roberts, Antony Hugh Coleby
    This study involved an investigation into the adsorption ot Zn and Cd by soils and synthetic hydrous metal oxides, and the effect of several factors on the adsorption reactions in controlled laboratory experiments. Initially, in order to determine low concentrations of Zn and Cd in solution, a concentration procedure involving solvent extraction was developed. The procedure utilised a chelating agent, dithizone, and an organic solvent, carbon tetrachloride. Zinc and Cd were back-extracted from the organic solvent into hydrochloric acid to achieve a ten-fold increase in concentration. For soils and Fe gel, adsorption of both Zn and Cd was characterised by an initial, rapid removal from solution which was followed by a much slower, continuing decrease in solution concentration. Isotherms (40 hr) for Zn and Cd adsorption from 3 x 10-2M NaCl by soils, synthetic hydrous ferric oxide (Fe gel) and allophane, were described using two Langmuir equations. Values for the derived free energies of adsorption, for each region of adsorption, were similar for both Zn and Cd on all soils and on Fe gel. Allophane, however, had higher free energies of adsorption than either the soils or Fe gel. For all adsorbents, Langmuir constants derived from adsorption data indicated that the adsorbing surface contained a relatively small number of sites with a high free energy of adsorption and a much larger number of sites with a lower adsorption energy. Iron gel appeared to provide a satisfactory model for describing Zn and Cd adsorption by soils. Amounts of Zn and Cd adsorbed by Fe gel increased as pH increased. Adsorption of Zn and Cd by Fe gel in each Langmuir region appeared to be affected similarly by changes in pH, although the pH50 values (the pH at which 50% of the initial amount added was adsorbed) were higher for Cd than for Zn. Experimental data obtained in the study are consistent with a two-mechanism model for both Zn and Cd adsorption. It is proposed that the first mechanism involves the formation of a bidentate complex. Adsorption of Zn and Cd into this region involves the release of two protons for each Zn or Cd ion adsorbed, resulting in a bond of higher energy than when Zn or Cd is adsorbed by the second mechanism. In this latter case, it is proposed that a monodentate complex is formed with one proton released for each cation adsorbed. In addition to proton release, data in support of the two-mechanism system was obtained in isotopic exchangeability and desorption studies. For example, the mole ratios (i.e., number of protons released for every Zn or Cd ion adsorbed) for Zn and Cd were non-integer values. For Zn the mole ratios ranged between 1.31 and 1.67, and for Cd from 0.80 to 1.12, at pH 6.4. There was no obvious trend in mole ratios for Zn either with increasing amounts of Zn adsorbed or increasing pH, but for Cd mole ratios increased with increasing amounts of Cd adsorbed of increasing pH. The isotopic exchangeability of Zn was similar at all levels of Zn adsorbed, but decreased with increasing pH (pH 5.85 - 6.65) from 58% exchangeability to 27%, possibly due to an increased proportion of more-tightly bound Zn. Cadmium, by contrast, had a lower exchangeability at low levels of Cd adsorbed (55 - 76%) than at higher levels (80 - 85%) but was more exchangeable (55 - 85%) than Zn at equivalent pH values and lower surface coverages, indicating that a greater proportion of adsorbed Cd was less tightly bound compared to Zn. Sequential desorption of Zn by calcium ions followed by copper ions showed that a proportion of Zn (12 - 24%) was retained by the surfaces against desorption, further indicating the presence of two adsorption mechanisms of different binding strength. Only at low levels of adsorbed Zn did the amount of desorbed Zn closely approximate the amounts of Zn calculated (from Langmuir constants) to be in region II. These desorption data, together with the exchangeability data for Zn, point to the possible limitations in the use of Langmuir equations for quantifying the amount of Zn absorbed by each mechanism. The Langmuir isotherm studies, together with proton release, exchangeability and desorption data, indicated two mechanisms of adsorption for Zn and for Cd. However, the greater exchangeability and fewer protons released per mole adsorbed suggest that more Cd is held by the mechanism involving monodentate bonding than is the case for Zn. Zinc or Cd was not observed to be absorbed or "occluded" by synthetic ferric oxide gel or goethite. There was evidence to suggest that Zn and Cd might diffuse into cracks or defects in the crystal structure of natural goethite, developed by grinding. Although some fraction of adsorbed Zn or Cd was non-exchangeable and non-desorbable (by copper) for all three adsorbents, this Zn and Cd was not in the absorbed phase. Long term (up to thirty year) additions of superphosphate fertiliser to three soils did not produce measurable accumulations of Zn or Cd in the soils. The calculated additions agreed well with actual increases measured in total Zn, but not with actual increases in total Cd.