L. M. Kozak

Speciation of particulate-bound Cadmium of soils and its bioavailability

A modified sequential chemical extraction procedure was developed for partitioning particulate Cd into eight fractions: exchangeable, carbonate-bound, metal–organic complex-bound, easily reducible metal oxide-bound, organic-bound, amorphous mineral colloid-bound, crystalline Fe oxide-bound, and residual. Results of experimental data on 16 surface soils of Saskatchewan, widely varying in physico-chemical properties, indicate the presence of little exchangeable Cd. Cadmium in these soils was predominantly in the form metal–organic complex-bound, accounting for 31–55%, with an average of 40%, of the total Cd present in the soils. The average relative abundance of the different forms of Cd present in these soils is in the order: metal–organic complex-bound (0.107 mg kg–1) > carbonate-bound (0.052 mg kg–1) > residual (0.042 mg kg–1) > organic-bound (0.035 mg kg–1) > crystalline Fe oxide-bound (0.016 mg kg–1) > easily reducible metal oxide-bound (0.010 mg kg–1) > amorphous mineral colloid-bound (0.002 mg kg–1). Statistical treatment of the Cd availability index, measured as ammonium hydrogencarbonate–diethylenetriaminepentaacetic acid (ABDTPA)-extractable Cd, with different particulate-bound Cd species showed high correlation (r= 0.916, p= 6 × 10–7) of the Cd availability index with the metal–organic complex-bound Cd. The beta coefficients obtained from the multiple regression analyses have given an insight into the importance of Al–organic complex-bound Cd species in estimating the bioavailability of Cd in these soils. The relationship of the metal–organic complex-bound Cd and the mobility and bioavailability of soil Cd merits in-depth research in explaining the toxicity and food chain contamination of Cd in the environment.

A new soil test method for the determination of plant-available cadmium in soils

Abstract A new soil test procedure using 1M NH4Cl was developed for the extraction of plant‐available cadmium (Cd) from soils. Five grams of soil is weighed into a 50‐mL polyethylene vial to which 30 mL of 1M NH4Cl solution is added. The soil suspension is then shaken on a horizontal shaker for 16 h at 25°C at 180 cycles per min. The suspension is then centrifuged at 2,500g for 5 min and the supernatant filtered through a 0.45 μm nitrocellulose filter under vacuum. Cadmium in the extract is then determined at 228.8 nm on a graphite furnace equipped atomic absorption spectrophotometer. A highly significant correlation was observed between the natural logarithm (In) of 1M NH4Cl‐extractable Cd in soils and the Cd content in the grain of durum wheat (Triticum turgidum var. durutn L.) grown on the same soils (r = 0.974, p = 3.8 x 10‐7). In comparison with several commonly used extradants, such as ABDTPA, CaCl2, NH4OAc, and NH4NO3, the 1M NH4Cl‐extracted Cd from soils was found to be a better index of Cd availa...

Sorption and desorption kinetics of cadmium from soils: Influence of phosphate

The mobility and availability of heavy metals is controlled by sorption-desorption characteristics of the soils. There is much literature available about the sorption characteristics of heavy metals by soils. However, the influence of ionic environments on the desorption kinetics of heavy metals, pa

Microwave digestion technique for the determination of total cadmium in soils

Abstract A rapid method is described for the determination of total cadmium (Cd) in soils. The soil samples and standard sediment samples were digested in a microwave oven using concentrated HNO3 and also by the conventional HF‐HC1O4 digestion in platinum crucibles. The Cd content in the extracts was determined using graphite furnace atomic absorption spectro‐photometry (GFAAS). Both digestion methods produced comparable quantitative values for total Cd of standard reference materials obtained from the National Institute of Standards and Technology and a series of surface and subsurface soil samples. Thus, the microwave digestion technique was shown to be an accurate and rapid method for digesting soil samples prior to total Cd analysis.

Biogeochemistry of soil cadmium and the impact on terrestrial food chain contamination

ABSTRACT Cadmium has been recognized to be a highly toxic element, but it was not until recently that concern has been expressed about the possible effects on human health of long-term exposure to low concentrations of this element. The discovery that Cd pollution from a basic metal mining operation could cause serious illness and possibly death has led to public anxiety as well as medical interest. There are many reviews on the chemistry, biochemistry, and biology of soil cadmium. However, to assess the impact of Cd pollution on food chain contamination and ecosystem health, major biogeochemical pathways, particularly those in the rhizosphere, have to be elucidated, and gaps in our knowledge must be identified for future research planning. This chapter addresses the biogeochemistry of soil Cd and the impact of Cd pollution on terrestrial food chain contamination through the rhizosphere. Knowledge of the total content of Cd does not imply comprehensive knowledge of its chemical behavior; rather, it is the chemical speciation of Cd that influences Cds chemical reactivity, mobility, bioavailability, and toxicity in the ecosystem. Therefore, it is essential to investigate the chemical speciation of Cd in soils and sediments, and especially in the rhizosphere, which is the bottleneck of terrestrial food chain contamination. The fractionation of metal–organic complex-bound Cd species is a recent innovation in sequential extraction schemes. The metal–organic complex-bound Cd is the most common among the particulate-bound Cd species of surface soils in temperate and tropical regions. Cadmium present as metal–organic complex-bound species is especially enriched in the rhizosphere soils after application of phosphate fertilizer. The importance of metal–organic complex-bound Cd fractions in assessing phytoavailability of soil Cd has been demonstrated, and thus merits attention.

COMBINED EFFECTS OF TANNATE AND AGEING ON STRUCTURAL AND SURFACE PROPERTIES OF ALUMINUM PRECIPITATES

The influence of organics on the crystallization of Al precipitates has been well documented. However, the effects of organics and ageing on the transformation and structural configuration of Al precipitates in relation to their surface and charge properties are not fully understood. This study investigated the structural, microporous and surface properties of Al precipitates formed under the influence of tannate and ageing. The Al precipitates were synthesized at an initial Al concentration of 7 × 10−3 M, an OH/Al molar ratio (MR) of 3.0, and initial tannate/Al MRs of 0, 0.001, 0.01 and 0.1, and aged for 1, 10 and 40 days. As indicated by a decrease in gibbsite and bayerite and an increase in the oxalate-extractable Al contents, the non-crystalline precipitates increased with the increase of the initial tannate/Al MR. This observation is in accord with the X-ray diffraction and Fourier transform infrared (FTIR) data. The impact of tannic acid on the nature of the Al precipitates is also reflected in the increase of the contents of the pyrophosphate-extractable Al, which is indicative of organically bound Al. This observation is in agreement with the increase in the intensity of characteristic FTIR absorption bands of tannate and the organic C and adsorbed water contents. The decrease in the crystallinity of Al precipitates with increase in the tannate/Al MR resulted in the development of microporosity, increase in BET specific surface area and decrease of the average pore diameter and point of zero salt effect (PZSE). The FTIR absorption bands characteristic of tannate of the Al precipitates became weaker with ageing, in accord with the ageing-induced decrease in the contents of organic C and pyrophosphate-extractable Al. Ageing drastically decreased the BET specific surface area of the Al precipitates formed in the absence of tannate but this effect was less conspicuous for the products formed at the tannate/Al molar ratio of 0.1. The ageing-induced change in the PZSE of the Al precipitates formed both in the absence and presence of tannate was not significant. The results accomplished in this study are of fundamental significance to our understanding of the combined effects of organics and ageing on structural configuration of hydrolytic precipitates of Al in relation to their microporosity, surface and charge properties in the environment.

Impact of long-term cultivation on the status of organic matter and cadmium in soil

Research has suggested a link between the bioavailability of soil Cd and total soil organic matter. However, some research suggested a negative relationship between total soil organic matter and bioavailable soil Cd while other research suggested a positive relationship. This study investigated the relationship between soil Cd and both the quantity and quality of soil organic matter as influenced by long-term cultivation. Two Orthic Chernozemic surface soil samples, one from a virgin prairie and the other from an adjacent cultivated prairie, were collected from each of 12 different sites throughout southern Saskatchewan, Canada. The samples were analyzed for total organic C, total Cd, Cd availability index (CAI), and pH. The nature of the soil organic matter was investigated with 13C Cross Polarization Magic Angle Spinning Nuclear Magnetic Resonance spectroscopy (13C CPMAS NMR). The total soil Cd, CAI, and total soil organic C of the cultivated soils were significantly lower than those of the virgin soils...

Distribution of cadmium in selected soil profiles of Saskatchewan, Canada: Speciation and availability

The distribution of Cd with depth, its particulate-bound speciation and availability index were studied in nine typical soil profiles of Saskatchewan, Canada. In the Ap horizons, Cd was predominantly in the metal-organic complex-bound form, accounting for about 38.8%, on average, of the total Cd present in the soils. The carbonate-bound Cd and the metal-organic complex-bound Cd accounted for 33.5 and 12.9%, respectively, on average, of the total Cd present in the B horizons. In the Ck horizons, Cd was predominantly in the carbonate-bound form, accounting for about 70.9%, on average, of the total Cd present in the soils. The M NH4Cl-extractable Cd of the soils, which is a measure of availability index of soil Cd, generally decreased with depth. Statistical treatment of the M NH4Cl-extractable Cd with different particulate-bound Cd species of various horizons in the soil profiles showed its highest correlation (P = 2.0 × 10−9, n = 42) with the amount of the metal-organic complex-bound Cd species, indicating...

KINETICS OF CITRATE-INDUCED SELENITE DESORPTION FROM MONTMORILLONITE AS AFFECTED BY COMPLEXATION WITH HYDROXYALUMINUM AND HYDROXYALUMINOSILICATE IONS

The citrate-induced desorption kinetics of pre-adsorbed Se from montmorillonite (Mt) and its complexes with hydroxyaluminum (HyA-Mt) and hydroxyaluminosilicate (HAS-Mt) were studied. The mole fraction of pre-adsorbed Se released in 24 h from different clays followed the trends of Mt>> HAS-Mt> HyA-Mt with a significant increase with elevated citrate concentration in the desorbing solution. In contrast, the amount of Se adsorbed per unit mass of different clays followed a clear-cut opposite trend of HyA-Mt>HAS-Mt>>Mt. The Se desorption kinetics in different systems indicated multiple rate characteristics; where an initial fast reaction (0.25–2 h) was followed by a slow reaction (2–16 h). Of the six different kinetic models tested (zero-, first-, and second-order; power function, Elovich, and parabolic diffusion), the second-order rate equation showed the overall best fit to the fast and slow desorption kinetic data from the clays. Based on second-order rate constants, the rates of the mole fraction of Se desorption by citrate from different clay systems at 298 K followed the order Mt > HAS-Mt >> HyA-Mt. For both fast and slow reaction, the rates of desorption increased proportionally with the level of citrate. Replicate experiments conducted across a range of temperature (288–318 K) yielded Arrhenius parameters that followed the order HyA-Mt > HAS-Mt >> Mt. Considering that a lower mole fraction of Se desorption after a particular reaction period, slower desorption kinetics, and a greater activation energy of desorption are the indices of increased adsorption bond strength, the results clearly indicate that HyA- and HAS-interlayering and coatings on Mt not only augmented its Se adsorption affinity and capacity, but also increased the adsorption bond strength. Silication in HyA not only reduced the Se adsorption capacity, but also weakened the adsorption bond strength. This establishes a significant role of HyA/HAS-interlayering and coating on Mt in influencing the rate of citrate-induced release of Se. Reduction in surface-positive potential following citrate adsorption on the clay surface, a direct ligand exchange between Se and citrate, and structural dissolution are possible mechanisms responsible for citrate-induced Se desorption in the presentstudy.

CARBONATE-INDUCED STRUCTURAL PERTURBATION OF Al HYDROXIDES

The chemistry of Al transformation has been well documented, though little is known about the mechanisms of structural perturbation of Al precipitates by carbonates at a molecular level. The purpose of the present study was to investigate the structural perturbation of Al precipitates formed under the influence of carbonates. Initial carbonate/Al molar ratios (MRs) used were 0, 0.1, and 0.5 after aging for 32 days, then the samples were analyzed by X-ray absorption near edge structure spectroscopy (XANES), X-ray diffraction (XRD), Fourier-transform infrared absorption spectroscopy (FTIR), and chemical analysis. The XRD data were in accord with the FTIR results, which revealed that as the carbonate/Al MR was increased from 0 to 0.1, carbonate preferentially retarded the formation of gibbsite and had relatively little effect on the formation of bayerite. As the carbonate/Al MR was increased to 0.5, however, the crystallization of both gibbsite and bayerite was completely inhibited. The impact of carbonate on the nature of Al precipitates was also evident in the increase of adsorbed water and inorganic C contents with increasing carbonate/Al MR. The Al K- and L- edge XANES data provide the first evidence illustrating the change in the coordination number of Al from 6-fold to mixed 6- and 4-fold coordination in the structural network of short-range ordered (SRO) Al precipitates formed under the increasing perturbation of carbonate. The fluorescence yield spectra of the O K-edge show that the intensity of the peak at 534.5 eV assigned to σ* transitions of Al-O and O-H bonding decreased with increasing carbonate/Al MR. The XANES data, along with the evidence from XRD, FTIR, and chemical analysis showed clearly that carbonate caused the alteration of the coordination nature of the Al-O bonding through perturbation of the atomic bonding and structural configuration of Al hydroxides by complexation with Al in the SRO network of Al precipitates. The surface reactivity of an Al-O bond is related to its covalency and coordination geometry. The present findings were, therefore, of fundamental significance in understanding the low-temperature geochemistry of Al and its impacts on the transformation, transport, and fate of nutrients and pollutants in the ecosystem.