Thursday, 13 July 2006

Adsorption-Desorption Characteristics of Copper, Lead, and Cadmium at Contaminated Levels in Variable Charge Soils.

Zhenli He1, Shen Yu2, Jinyan Yang3, Xiaoe Yang4, Haiping Xu4, and Peter J. Stoffella5. (1) University of Florid, Institute of Food and Agricultural Sciences, Indian River Research and Education Center, 2199 South Rock Road, Fort Pierce, FL 34945, (2) Rutgers University, 14 College Farm Road, New Brunswick, NJ 08901, (3) University of Florida, 2199 South Rock Road, Fort Pierce, FL 34945, (4) Zhejiang University, 268 Kaixun Road, Hangzhou, China, (5) University of Florida, Institute of Food and agricultural Sciences, Indian River Research and Education Center, 2199 South Rock Road, Fort Pierce, FL 34945

Heavy metal contamination of soils has increased in the past few decades due to industrial activities, use of industry byproducts in agriculture, land disposal of wastes, and application of fertilizers and chemicals. Variable charge soils are generally more vulnerable to heavy metal contamination because of their low organic matter content and a small cation exchange capacity as well as being acidic. The movement and bioavailability of heavy metals in most agricultural soils of tropical and subtropical regions is mainly controlled by adsorption-desorption processes. However, the adsorption-desorption behavior of heavy metals at contaminated levels is not fully understood. Heavy metals can be adsorbed onto surfaces of soil colloids through non-specific adsorption (by static electric force) and specific adsorption (formation of chemical bonds between the ion and the surface). Adsorption of copper (Cu2+), lead (Pb2+), and cadmium (Cd2+) in highly weathered variable charge soils involves both mechanisms. A series of studies were conducted to understand the adsorption-desorption behavior of Cu2+, Pb2+, and Cd2+ at contaminated levels in two representative variable charge soils in China, i.e. a REQ soil (clayey, kaolinitic thermic plinthite Aquult) and a RAR soil (clayey, mixed siliceous thermic typic Dystrochrept).The results indicate that adsorption of Cu2+, Pb2+, or Cd2+ resulted in a significant decrease in soil pH due to replacement of H+ and/or Al3+ from soil surfaces. The pH drop varied among the different metals in the order of Pb2+ (1.06-1.13 units) > Cd2+ (0.95-1.13 units)>Cu2+ (0.60-0.80 units) at the loading of 31.5 m mol kg-1 soil. The REQ soil had a slightly greater pH decrease than the RAR soil at the same metal loadings. The moles of H+ released per mole of metal adsorbed in the RAR and REQ soil were 0.611.44 for Pb2+, 0.36-0.55 for Cd2+, and 1.12-2.57 for Cu2+. Adsorption of Pb2+, Cd2+, or Cu2+ in the variable charge soils consists of a fast reaction that lasted from 15 min to 2 h, followed by a slower but longer stage (up to 24 h). The isotherms of Cu2+, Pb2+, or Cd2+ adsorption fit well with some physical-chemical models, such as the Freundlich and the simple Langmuir equation. A maximum adsorption of each individual metal could be obtained from the Langmuir equation for comparing heavy metal holding capacity in different soils. The adsorption of Pb2+, Cd2+, or Cu2+ is generally pH-dependent, increasing with increasing pH. Temperature influenced Pb2+, Cd2+, or Cu2+ adsorption, but in a complicated manner. Presence of competitive cations reduced adsorption of any individual metals. The effects of inorganic anions on adsorption of Cu2+, Pb2+, or Cd2+ were dependent on anion types and apparently related to the altered surface properties caused by anion adsorption and/or the formation of anion-metal complexes. Addition of organic acids such as citric acid or oxalic acid at low concentrations slightly increased adsorption of Cu2+, Pb2+, and Cd2+ in variable charge soils but significantly decreased their adsorption at relatively high concentrations (>0.001 mol L-1), likely because of chelating effects. Desorption of the adsorbed metals in the 0.01 mol L-1 NaNO3 was small for Pb2+, but moderate for Cd2+. The recovery of adsorbed Pb2+ by five successive desorption with 0.01 mol L-1 NaNO3 solution was 0-19%, depending on adsorption saturation, and the corresponding value for Cd2+ was 0-53%, indicating that Pb2+ is more tightly bound in the variable charge soils than Cd2+. Approximately 61 to 95% of the adsorbed Cu2+ was desorbed by five successive extraction using 1 mol L-1 NH4Ac (pH 5.0), indicating that most of the adsorbed Cu2+ is bioavailable within a short time of adsorption.

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