J. O. Onyatta

Chemical speciation and bioavailability index of cadmium for selected tropical soils in Kenya

Determination of various chemical forms of a metal in soils is important to evaluate its mobility and bioavailability. Little is known on the chemistry of Cd species of variable charge soils in the tropics which account for a large proportion of the worlds arable land. A study was conducted to investigate the chemical speciation of particulate-bound Cd and its availability index of selected Kenyan soils varying widely in physicochemical properties. Surface and subsurface soils were collected from the main agricultural areas in Kenya. Cadmium in the surface soils was present mainly in the metal–organic complex-bound form which accounted for 25.0 to 45.8% with an average of 37.1% of the total Cd present in the soils. The average amount of each particulate-bound Cd species in the surface soils followed the order: metal–organic complex-bound (0.026 mg kg−1)>residual (0.021 mg kg−1)>crystalline Fe oxide-bound (0.011 mg kg−1)>organic-bound (0.007 mg kg−1)>amorphous mineral colloid-bound (0.003 mg kg−1)>easily reducible metal oxide-bound (0.002 mg kg−1). In the subsurface soils, Cd was present mainly in the residual form which accounted for 20.0 to 48.7% with an average of 43.4% in the 40–70 cm depth and 23.1 to 50.0% with an average of 37.5% in the 70–100 cm depth of the total Cd present in the respective subsoils. Cadmium was neither present in exchangeable nor in carbonate-bound forms in the soils studied. Both the total Cd and the Cd availability index generally decreased from the topsoil to the subsoil and varied with soil type. Cadmium availability index was taken as ammonium acetate-acetic acid–ethylene diamine tetraacetic acid (AAAc–EDTA) extractable Cd. Statistical analysis of the Cd availability index with different particulate-bound Cd species of the soil profiles showed that the Cd availability index was most significantly correlated with the metal–organic complex bound-Cd (r=0.960, p=1.53×10−8). The information obtained from this study is fundamental in establishing a soil Cd database and should be of significance in interpreting the importance of Cd speciation in Cd bioavailability in tropical soils.

KINETICS OF CADMIUM RELEASE FROM SELECTED TROPICAL SOILS FROM KENYA BY LOW-MOLECULAR-WEIGHT ORGANIC ACIDS

&NA; The low‐molecular‐weight organic acids (LMWOAs) commonly present in root exudates may influence the mobility and bioavailability of Cd in soils through the formation of soluble Cd‐organic complexes in the soil rhizosphere. However, little is known about the dynamics of Cd released by LMWOAs from highly weathered tropical soils where large amounts of phosphate fertilizers are applied to correct phosphorus deficiency. Cadmium is a contaminant of phosphate fertilizers. The release of Cd from selected tropical soils treated with the Idaho monoammonium phosphate (MAP) fertilizer or the Cd perchlorate‐added MAP chemical reagent by LMWOAs (10−3 M and 10−2 M) was investigated at 25°C and at an ionic strength of 0.1 M NaNO3 solution. The LMWOAs used in this study were acetic, citric, fumaric, malic, oxalic, and succinic acids. The surface soils used in this study were obtained from main agricultural areas in Kenya varying widely in physicochemical properties. The amounts of Cd released from the natural and the treated soils varied with the soils, the treatments, and the nature of the LMWOA. The amount of Cd released from the soils in the presence of LMWOAs also increased with the log stability constant values of the Cd‐LMWOA complexes, indicating that Cd was brought into solution by LMWOAs as Cd‐LMWOA complexes. A parabolic diffusion equation provided the best fit to the Cd released by LMWOAs during the short reaction period of 0.25 to 1 h. The overall diffusion coefficient values of the Cd released from the natural soils and the treated soils by LMWOAs varied with the soil type and the nature of the LMWOAs. The results showed that the LMWOAs enhanced the rate of Cd released from the soils, especially in the monoammonium phosphate‐treated soils. The continuous release of Cd from the soils by renewal of LMWOAs indicates the sustaining power of the soils to replenish the Cd labile pool of the soils. The results also indicate that LMWOAs commonly present in the root exudates play a vital role in the mobilization of Cd in tropical soils and, hence, may influence its uptake by the plants.

Distribution of applied cadmium in different size fractions of soils after incubation

The size fraction of soils is one of the important factors that influence the retention of heavy metals. The sorptive properties of soils for heavy metals are principally associated with clay and silt-size fractions. Phosphate fertilizers that are applied to highly weathered tropical soils contain a wide concentration range of cadmium (Cd) as an impurity. Tropical soils contain kaolinite and oxides of Al, Fe, and Mn, which have the ability to sorb Cd. However, the distribution of Cd in different size fractions and the chemical speciation of particulate-bound Cd in the clay size fractions when introduced to soil and allowed to incubate at field moisture capacity merits attention. Cadmium was, therefore, applied to selected surface Kenyan soils varying widely in physicochemical properties to investigate its distribution in different soil particle size fractions and the speciation of particulate-bound species in clay size fractions after incubation. The Cd content in different particle fractions was analyzed by graphite furnace atomic absorption technique after HF-HClO4 digestion. The particulate-bound Cd species were investigated using chemical sequential extraction method. The study showed that clay size fraction of the natural and the Idaho monoammonium phosphate (MAP)-fertilizer or the Cd perchlorate-added MAP chemical reagent-treated soils contained the highest amount of the total Cd. However, silt and sand fractions of the treated soils also retained appreciable amounts of Cd. Speciation studies revealed that metal-organic complex-bound Cd was the most predominant compared to other particulate-bound Cd species in the clay size fractions of the soils treated with Idaho MAP fertilizer or the Cd perchlorate-added MAP chemical reagent. The distribution of total Cd in the different soil particle size fractions and the speciation of particulate-bound Cd in the clay size fractions varied with the soil type. The results indicate that clay size fractions can retain Cd making it less available; however, the influence of farming practices, which affect Cd mobility, should not be overlooked.

Validation of the Polyacrylic acid/Glassy Carbon Differential Pulse Anodic Stripping Voltammetric Sensor for Simultaneous Analysis of Lead(II), Cadmium(II) and Cobalt(II) ions.

Electrochemical sensor validation ensures that the determination procedure employed for a specific test is suitable for its intended use. Accuracy and precision, linearity, range, limits of detection and quantification validation parameters were determined for the Polyacrylic acid/Glassy Carbon Differential Pulse Anodic Stripping Voltammetric Sensor used for the simultaneous analysis of Pb(II), Cd(II) and Co(II) in accordance with the guidelines of the ISO/IEC 17025. The recovery degree for accuracy were in the range of 101% to 110% for Pb(II), 100% to 105% for Cd(II) and 93% to 104% for Co(II). The precision was found to be less than 10% for ten determinations. Linear concentration range were also investigated and found to lie in the range of 125 – 7.8μM Pb(II), 16 – 2μM Cd(II) and 2 – 0.125mM Co(II). Limits of detection also were found to be 0.9nM Pb, 1.9mM Cd and 11.0μM Co and limits of quantitation were 3.0nM Pb, 6.3nM Cd and 36.7μM Co. The effects of foreign substances like Cu 2+ , SO4 2, K + , Na + , Cl , NH4 + and O2 were found to have no significant effect on the electrochemical responses of the three heavy metals. These results confirms that this sensor can provide accurate, reliable and consistent results for the determination of Pb(II), Cd(II) and Co(II) heavy metals in diverse samples.

The Impact of Urban Activities on Heavy Metal Distribution and Bioavailability Index in Selected Tropical Urban Soils

The distribution and bioavailability of heavy metals in the environment is of particular concern because of their potential toxicity to the ecosystem. A study was conducted to investigate the impact of informal industries (commonly known in Kenya as Jua kali industries) on the heavy metal distribution and bioavailability indices in selected tropical urban soil samples from Nakuru town, Kenya. The study revealed that both the total metal contents and the bioavailability indices varied with the soil site and depended upon the intensity of industrial activities (painting, oil spills from engine overhauls, deposited wastes) on the sites. The informal industrial sites had higher levels of heavy metal content than the non-industrial sites, indicating that the informal industrial activities in these areas contributed to the elevated amounts of heavy metals. The mean heavy metal content in the informal industrial sites was highest for Zn followed by Fe, Pb, Mn and Cu. In the non-industrial sites, the trend was the same; however, lower values were obtained. The amounts of heavy metal extracted varied with the nature of the extractant. AAAc-EDTA extracted the highest amounts of the metals both from the industrial and nonindustrial sites. The AAAc-EDTA extractable metal could be taken as the bioavailability index of the metals for the soils studied. The study is of significance in developing regulations for setting up informal industrial sheds in relation to micro-urban farming.