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Adsorption of 2,4-Dichlorophenoxyacetic Acid by an Andosol

^a Biodiversity Division, National Institute for Agro-Environmental Sciences (NIAES), 3-1-3 Kan-nondai, Tsukuba, Ibaraki 305-8604, Japan
^b Environmental Research Center, 3-1 Hanare, Tsukuba, Ibaraki 305-0857, Japan

View larger version (7K): [in this window] [in a new window]   Fig. 1. Chemical structure of 2,4-dichlorophenoxyacetic acid (2,4-D).

View larger version (17K): [in this window] [in a new window]   Fig. 2. Scheme of 2,4-D adsorption on soils. A: 2,4-D adsorption on active surface hydroxyls of metal (hydr)oxides by an ion exchange reaction (electrostatic interaction). B: 2,4-D adsorption on metal (hydr)oxides by a ligand exchange reaction which replaces an active surface hydroxyl with a carboxyl group of 2,4-D molecule, forming a strong coordination bond. C: 2,4-D adsorption on metal-humate complexes by a ligand exchange reaction. M denotes metals such as Al and Fe.

View larger version (20K): [in this window] [in a new window]   Fig. 3. Kinetics of 2,4-D concentration changes in aqueous solution in the presence of an Andosol (untreated UG5 soil) at equilibrium pH 4.5, 5.0, 5.5, 6.0, and 6.5. Initial 2,4-D concentration, 22.6 µM (5.0 mg L–1); the amount of soil, 50 g L–1 (oven-dry basis); background electrolyte, 0.01 M CaCl2. First, a relationship between 2,4-D concentration in aqueous solution in the presence of untreated UG5 soil and equilibrium pH was drawn at each reaction time (6 to 7 plots between pH 4 and 7), and then 2,4-D concentration at equilibrium pH 4.5, 5.0, 5.5, 6.0, and 6.5 at each reaction time was read from the relationship and plotted on this figure. The equilibrium pH value increased 0 to 0.5 units with reaction time depending on the initial pH value.

View larger version (19K): [in this window] [in a new window]   Fig. 4. The amount of 2,4-D adsorbed on untreated UG5 soil (•), UG5-SOM (), UG5-SOM-AM (), and UG5-SOM-FM () in the presence of 0.01 M CaCl2 as a function of equilibrium pH. Reaction time, 4 h; initial 2,4-D concentration, 22.6 µM (5.0 mg L–1); the amount of soil, 50 g L–1 (oven-dried and untreated soil basis). Had the soil adsorbed all 2,4-D in this experiment, the amount of 2,4-D adsorption would have reached 0.452 mmol kg–1.

View larger version (14K): [in this window] [in a new window]   Fig. 5. Effects of background electrolyte concentrations (•, 0.01 M CaCl2; , 0.1 M CaCl2) on the amount of 2,4-D adsorption on untreated UG5 soil as a function of equilibrium pH. Reaction time, 4 h; initial 2,4-D concentration, 22.6 µM (5.0 mg L–1); the amount of soil, 50 g L–1 (oven-dry basis). Had the soil adsorbed all 2,4-D in this experiment, the amount of 2,4-D adsorption would have reached 0.452 mmol kg–1.

View larger version (31K): [in this window] [in a new window]   Fig. 6. Comparison of adsorption isotherms of (A) 2,4-D and those of (B) phosphate on untreated UG5 soil. (A) The amount of soil, 50 g L–1 (oven-dry basis); background electrolyte, 0.01 M CaCl2; reaction time, 4 h. First, a relationship between 2,4-D adsorption and equilibrium pH was drawn at each initial 2,4-D concentration (6 to 7 plots between pH 3.5 and 7.5), and then 2,4-D adsorption at pH 4.0, 5.0, 6.0, and 7.0 at each initial 2,4-D concentration was read from the relationship and plotted on this figure. (B) The amounts of phosphate adsorption on untreated UG5 soil were calculated based on Hiradate and Uchida (2004).

View larger version (25K): [in this window] [in a new window]   Fig. 7. Comparison of 2,4-D adsorption ability among metal-humate complexes. (A) humic acid (HA) was complexed in the presence of 0.01 M CaCl2 (•), 0.1 M CaCl2 (), 0.01 M FeCl3 (), and 0.01 M AlCl3 (). (B) Comparison between Al-humate complex (humic acid was complexed in the presence of 0.01 M AlCl3, ) and Al hydroxides (0.01 M AlCl3 was precipitated by adding NaOH in the absence of HA, ). Reaction time, 4 h; initial 2,4-D concentration, 22.6 µM (5.0 mg L–1); humic acid added, 2.0 mg; COOH added, 8.14 µmol; Al or Fe added, 50 µmol; Ca added, 50 or 500 µmol.