Effects of organic acids on desorption of phosphate from the surfaces of aluminum hydroxide and complexes

The desorption of phosphate from aluminum (Al) hydroxides by 0.01 mol L KCl in the presence of different organic acids and from Al-OH, Al-P and Al-Oxalate, AlCitrate complexes by organic acids was studied. When phosphate adsorbed on Al(OH)x in the presence and absence of oxalate was desorbed sequentially twice using 0.01 mol L KCl, total desorption rates of phosphate adsorbed in the presence of 0.5-2.0 mmol L 1 oxalate were 4.10%-4.75%. However, in the absence of organic acid, the desorption rate was only 2.05%. The amount of P desorbed by 0.01 mol L KCl increased with increasing concentration of organic acids in adsorption solution. Some phosphate, which could not be desorbed by oxalate, would be desorbed by citrate at the same concentration. When phosphate was desorbed using a series of oxalate and citrate, the total desorption rate of phosphate from Al-oxalate complex was less than that from Al hydroxide, while the rate from Al-citrate complex was the highest. Citrate could release the phosphate from Al-P complex, and the amount of phosphate desorption increased with the increment of citrate concentration used. All these indicated that the mechanism of phosphate desorption involved ligand exchange and dissolution by organic acid. Organic acids increase the desorption of phosphate and improve the efficiency of phosphate in acid soil.