Retention of Cobalt by Pure and Foreign-Element Associated Goethites

Abstract

Abstract--Retention studies of the cobalt-goethite system were carried out using synthetic, star-shaped and lath-shaped pure, AI-, Cd-, Cu-and Si-associated goethites. Aluminium and Si are commonly occurring foreign elements in natural goethites. The goethites were prepared by coprecipitating Fe and the foreign element under controlled conditions and characterized by X-ray diffraction, transmission electron microscopy, specific surface area determination and 2 M HC1 extraction. The foreign-element associated goethites contained ~3, ~5 and ~9 mole % A1, ~4 mole % Cd and ~3 mole % Cu incorporated by isomorphous substitution but only ~0.4 mole % of probably occluded Si. Crystal size and shape but also number of defects and domains, and hence specific surface area, unit-cell dimensions and reactivity towards 2 M HC1, exhibited great variability among the goethites. Accordingly the amounts of Co sorbed from initially 10 -7 M Co in 0.1 M Ca(NO3)2 in relation to pH (3-8) and reaction time (2-504 h) were very different for the eight gnethites. The affinity of Co is highest for Cd-and lowest for Cu-gnethite. These samples also form the extremes regarding time-dependent sorption with Cu-goethite showing the smallest and Cd-goethite the largest increase in sorption with increasing reaction time. The Co uptake was not caused by precipitation Co(III) oxides due to Co(II) oxidation, since oxygen exclusion during sorption had no effect on the amount of Co sorbed. The amounts of sorbed Co extracted by 2 M HC1 decreased with increasing sorption time but 40--87% of sorbed Co remained unextracted after 48 h, most in Cu-goethite and least in lath-shaped pure goethite. The strong retention suggests Co uptake by diffusion into micropores and fissures resulting from structural defects and intergrowths. The diffusion coefficients range from 3.10-19 to 6.10 -17 cm2/s with the highest values for A1-and Si-associated gnethites emphasizing the importance for Co immobilization, and hence availability, of foreign-element associations in goethite.

Retention of cobalt by an oxisol in relation to the content of iron and manganese oxides

“…For various goethites the time-dependent sorption and reduced extractability of sorbed Co and other heavy metals were attributed to diffusion into micropores and binding onto interior sites (Bruemmer et al, 1988;Gerth et al, 1993;Bibak et al, 1994). The same interpretation may be applied to the soil samples, since the extraction patterns for the soil samples and goethite are similar (Table 2).…”

“…Goethite: Synthetic goethite was prepared from ferric nitrate solution as described by Bibak et al (1994). Fifty mL of 1 M Fe(NO 3 ) 3 solution was titrated to pH 4.5 by 1 M NaOH.…”

“…kaolinite, to a slower reacting but stronger binding component (Mn oxides) could, however, also explain the larger ApH^. Therefore, although diffusion of Co and other heavy metals into micropores appears to be wellsupported for synthetic goethites (Bruemmer et al, 1988;Gerth et al, 1993;Bibak et al, 1994), the importance of this mechanism in relation to the observed timedependent Co sorption by soil is unresolved.…”

“…Sorption of Co and other heavy metals is strongly pHdependent and increases at increasing pH exhibiting the so-called sorption edge (McKenzie, 1980;Benjamin and Leckie, 1981;Barrow, 1985;McLaren et al, 1986;Borggaard, 1990). For Fe oxides Co sorption increases and extractability of sorbed Co decreases with increasing sorption time because of Co diffusion into micropores (Bibak et al, 1994). However, although Co sorption by soils appears to be timedependent (McLaren et al, 1986), the contribution of the various soil components to this time-dependency is unclear as is the relative importance of the components on soil retention of Co.…”

Sorption of Heavy Metals by Inorganic and Organic Components of Solid Wastes: Significance to Use of Wastes as Low-Cost Adsorbents and Immobilizing Agents

“…It is commonly noted that there is an increased retention (a decreased rate of desorption or decreased extractability) of heavy metals on surfaces of Fe, Al, and Mn oxides. Soils and sediments with increased retention time (Backes et al, 1995;Bibak et al, 1995;McKenzie, 1980;Padmanabham, 1983;Schultz et al, 1987). The term used to indicate increased retention of metals with aging is fixation, and the reactions thought responsible are often termed slow reactions.…”