Mustafa Ghulam, Singh Balwant, Kookana Rai S
Faculty of Agriculture, Food and Natural Resources, The University of Sydney, Ross Street Building, Sydney, NSW 2006, Australia.
Chemosphere. 2004 Dec;57(10):1325-33. doi: 10.1016/j.chemosphere.2004.08.087.
The transport and bioavailability of cadmium is governed mainly by its adsorption-desorption reactions with minerals such as goethite--a common iron oxide mineral in variable charged and highly weathered tropical soils. Soil factors such as pH, temperature, solution Cd concentration, ionic strength and ageing affect Cd adsorption on goethite. The desorption behaviour of Cd from goethite at low concentrations is not fully understood. This study investigates the adsorption-desorption of Cd at low Cd concentrations (Cd adsorbed on goethite from 20 to 300 microM Cd solutions) in Na and Ca nitrate solutions of 0.03 M nominal ionic strengths. Synthetic goethite prepared by ageing a ferric hydroxide gel at high pH and room temperature was used for Cd adsorption and desorption studies. For desorption experiment 10 successive desorptions were made for the whole range of initial Cd concentrations (20-300 microM) in the presence of 0.01 M Ca(NO3)2 or 0.03 M NaNO3 solutions. Cadmium adsorption was found to be higher in Na+ than Ca2+ probably due to the competition of Ca2+ ions with Cd2+ ions for adsorption sites on the surfaces of goethite. The effect of index cation on Cd adsorption diminished with increase in pH from 5.0 to 6.0. Cadmium desorption decreased with increase in pH from 5.0 to 6.0 in both Na and Ca systems. After 10 successive desorptions with 0.03 M NaNO3 at the lowest initially adsorbed Cd approximately 45%, 20% and 7% of the adsorbed Cd was desorbed at pH 5.0, 5.5 and 6.0, respectively. The corresponding desorptions in the presence of 0.01 M Ca(NO3)2 were 49%, 22% and 8%, respectively. The Freundlich parameter, k, based on each progressive step of desorption at different adsorbed concentration increased with increasing desorption step, which may indicates that a fraction of Cd was resistant to desorption. Low Cd desorbability from goethite may be due to its specific adsorption and/or possibly as a result of Cd entrapment in the cracks or defects in goethite structure.
镉的迁移和生物有效性主要受其与针铁矿等矿物的吸附-解吸反应控制，针铁矿是可变电荷且高度风化的热带土壤中常见的氧化铁矿物。土壤因素如pH值、温度、溶液镉浓度、离子强度和老化都会影响镉在针铁矿上的吸附。低浓度下镉从针铁矿上的解吸行为尚未完全了解。本研究调查了在名义离子强度为0.03M的硝酸钠和硝酸钙溶液中，低镉浓度（20至300微摩尔/升镉溶液吸附在针铁矿上的镉）下镉的吸附-解吸情况。通过在高pH值和室温下老化氢氧化铁凝胶制备的合成针铁矿用于镉的吸附和解吸研究。对于解吸实验，在0.01M硝酸钙或0.03M硝酸钠溶液存在下，对整个初始镉浓度范围（20 - 300微摩尔/升）进行了10次连续解吸。发现镉在钠离子存在下的吸附高于钙离子，这可能是由于钙离子与镉离子竞争针铁矿表面的吸附位点。随着pH值从5.0增加到6.0，指示阳离子对镉吸附的影响减弱。在钠和钙体系中，随着pH值从5.0增加到6.0，镉的解吸减少。在最低初始吸附镉的情况下，用0.03M硝酸钠进行10次连续解吸后，在pH值为5.0、5.5和6.0时，分别约有45%、20%和7%的吸附镉被解吸。在0.01M硝酸钙存在下的相应解吸率分别为49%、22%和8%。基于不同吸附浓度下每次解吸的逐步过程的弗伦德利希参数k随着解吸步骤的增加而增加，这可能表明一部分镉抗解吸。镉从针铁矿上的低解吸率可能是由于其特异性吸附和/或可能是由于镉被困在针铁矿结构的裂缝或缺陷中。
