PHOSPHATE ADSORPTION BY HUMIC/FE-OXIDE MIXTURES AGED AT PH-4 AND PH-7 AND BY POORLY ORDERED FE-OXIDE

The effect of the addition of humic substances on phosphate adsorption by poorly ordered Fe-oxide aged at pH 4 or 7 for 56 days was investigated at pH 6.2 in a 0.01M CaCl2 matrix. The adsorption maximum of Fe-oxide without humics prepared at pH 6 was reduced by about 38% within 55 days. One day after addition of humic substances, P adsorption by Fe-oxide was reduced at low equilibrium concentrations (< 300 mu mol P/l). Over the course of the reaction between humic substances and Fe-oxide, P adsorption increased. In mixtures of Fe-oxide and humic substances, the P adsorption maximum increased by about 40 (mu mol P/mmol Fe) within the same period. Aging of the Fe oxide/humics mixtures for 56 days at pH 4 led to an adsorption maximum of 333 and to 294 at pH 7 (mu mol P/mmol Fe). The increase of adsorption caused by humic substances can be attributed to the inhibition of Fe-oxide crystallization and to organic complexing of Fe by humic substances. The importance of Fe complexing for P adsorption is shown by the increase in pyrophosphate extractable Fe which amounts to 35% and < 20% of total Fe at pH 4 and pH 7, respectively, after 56 days of reaction. The higher P adsorption maximum of Fe-oxide/humics mixtures at pH 4 in comparison to pH 7 is assumed to be due to the higher amount of organically complexed Fe.