Amorphous Oxide Fractions Research Articles

Systematic evaluation of bulk cyanide leach (BCL) parameters affecting extractability of metals from soils

ABSTRACT Partial extraction techniques for analysis of soil and sediment samples have become widely used in geochemical exploration. One such technique is bulk cyanide leach (BCL) which is used in gold exploration and, along with Au, also extracts other elements such as As, Co, Cu, Mn, Mo, Ni and Zn, which are known pathfinders of Au and base metal mineralization. As with all partial extractions, different extraction conditions are likely to affect the recovery of elements obtained by a given BCL protocol. Some of these conditions, such as pH, may be driven by soil properties and thus jeopardize correct interpretation of BCL data; however, such effects have not yet been quantified. We therefore systematically altered key extraction parameters (soil mass, soil: solution ratio, time, degree of agitation, and pH) and measured the effects on BCL extraction of Au, As, Co, Cu, Mn, Mo, Ni and Zn on a single homogeneous soil from mineralized terrain. In addition, the potential effects of soil organic carbon on BCL extraction efficiency was evaluated using a soil amended with different amounts of natural charcoal. Sequential selective extractions were conducted on the soil before and after BCL extraction to better understand the extraction of these elements under BCL conditions. Gold recovery by BCL was found to be insensitive to most differences in extraction conditions, within the solution parameters typically expected in BCL protocols. Charcoal addition to soil, however, significantly reduced gold extractability. Extraction time and pH had the greatest effect on the extraction of all other elements. Increasing pH from 9.8 to 10.4 increased arsenic concentrations by 62%, but decreased Mn and Mo extraction by c . 50%. The extraction of Au, As and Mo was rapid with c . 60% being dissolved within an hour, but Co was only 20% extracted (compared with a 48-hour extraction). The erratic concentrations of Cu, Mn and occasionally Zn suggest their use as pathfinders for mineralization using BCL extracts may be unadvisable. Preliminary metal fractionation results suggest that Au is quantitatively extracted by BCL from a pool not accessible to the other selective extractants used; other elements are removed by BCL from exchangeable/adsorbed or amorphous oxide fractions. This work emphasizes the importance of controlling extraction conditions for bulk cyanide leach assays. The occurrence of charcoal in soils is common even below surface horizons, so that care must be taken in interpretation of BCL gold assays where charcoal is likely to be present.

Assessment of Contaminant Retention in Constructed Wetland Sediments

The A-01 wetland treatment system (WTS) was designed to remove metals from an industrial effluent at the Savannah River Site, Aiken, SC. Sequential extraction data were used to evaluate remobilization and retention of Cu, Pb, Zn, Mn, and Fe in the wetland sediment. Remobilization of metals was determined by the Potentially Mobile Fraction (PMF) and metal retention by the Recalcitrant Factor (RF). The PMF, which includes water soluble, exchangeable, and amorphous oxides fractions, is the contaminant fraction that has the potential to enter into the mobile aqueous phase under fluctuating environmental conditions. PMF values were low for Cu, Zn, and Pb (13–27 %) and high for Fe and Mn (62–70 %). The RF, which includes crystalline oxides, sulfides or silicates and aluminosilicates, is the ratio of strongly bound fractions to the total concentration of elements in sediment. RF values ranged from 73–87 % for Cu, Zn, and Pb, indicating high retention in the sediment and from 30–38 % for Fe and Mn, indicating low retention. Contaminant retention, which is determined by solid phase metal speciation, determines the potential mobility and bioavailability of captured metals in wetland sediments; hence, their likelihood of being released if chemical, physical, or biological conditions within the wetland change.

Fractionation of soil cadmium from some new zealand soils

The concentrations and forms of soil cadmium (Cd) in 12 different New Zealand topsoils were investigated using a sequential fractionation procedure. Total soil Cd concentrations were low and ranged between 0.03 μg g‐1 to 1.34 μg g‐1 and were highly correlated with total soil phosphorus (r2=0.85, P<0.01). Results indicated that there was a wide range in the concentrations of Cd associated with individual soil fractions and large variations between soils. On average for all soils, the smallest proportion of Cd was in exchangeable forms, i.e., 3%, with 12% in the crystalline oxide fraction, 13% in the amorphous oxide fraction and the greatest proportion of Cd associated with the organic 34% and residual 38% fractions. There was evidence to show that a soil extractant which is commonly used to predict plant uptake of Cd from soils, i.e., 0.04 M ethylene diamine terra acetic acid (EDTA), extracts Cd from both exchangeable and organic forms of soil Cd.

Effect of lime and organic matter on distribution of zinc, copper, iron, and manganese in acid soils

Liming is essential in highly leached acid soils for increasing soil fertility as it promotes congenial chemical and biological environment. Application of organic matter along with lime has also been found beneficial for crop productivity in such soils. As external addition of organic matter provides easy source of organic carbon (C), it proliferates heterotrophic microorganisms. Addition of such amendments is expected to have profound effect on the availability of cationic micronutrients which are present in low amount. An experiment was undertaken at B.C.K.V., West Bengal, India to study the effect of lime and cellulose (as organic C source) additions on the transformation of zinc (Zn), copper (Cu), iron (Fe), and manganese (Mn) in two soils with different cropping history. Results indicated that in tea garden soil, metals were mobilized into organically complexed (in case of Zn and Mn) and amorphous Fe oxides bound (in case of Zn, Cu, and Fe); whereas in field cropped soil these were mobilized into ‘gel’ hydrous oxides and amorphous oxides fractions (except Cu) due to lime application. Lime decreased readily available water‐soluble plus exchangeable forms of metals in both the soils. Providing external organic C source to tea garden soil did not result any significant changes in the micronutrient fractions. In field cropped soil, external organic C addition resulted mobilization of metals mainly into organically complexed and amorphous oxides bound fractions from water‐soluble plus exchangeable and crystalline Fe oxides bound fractions. Significant interaction was observed between lime and organic C treatments on the transformation of Cu‐fractions in field cropped soil. In presence of added lime, added organic C mobilized Cu into potentially plant available organically complexed and amorphous Fe oxides fraction from relatively inert fractions. Whereas, organic matter, in absence of added lime immobilized Cu from these active fractions into inert residual fraction.